ISSN 0288-4534

CODEN: KPPJAD

KONA

Powder andParticle Journal

No.34 (2017)

Hosokawa Powder Technology Foundation
available online-www.kona.or.jp

KONA Powder and Particle Journal No. 34 (2017)

http://www.kona.or.jp

About this Journal

Editorial Board

KONA Powder and Particle Journal is a refereed scientificjournal that publishes articles on powder and particle scienceand technology. This journal has been published annually
since 1983 and is distributed free of charge to researchers,
members of the scientific communities, universities and research libraries throughout the world, by Hosokawa Powder
Technology Foundation (http://www.kona.or.jp/) establishedby Mr. Masuo Hosokawa in 1991.

Editor-in-Chief
J. Hidaka(Doshisha Univ., Japan)
Asian / Oceanian Editorial Board
S. Biggs(Univ. of Queensland, Australia)
X.S. Cai(Univ. of Shanghai for Sci. & Tech., China)
T. Charinpanitkul(Chulalongkorn Univ., Thailand)
Y. Fukumori(Kobe Gakuin Univ., Japan)
K. Higashitani(Kyoto Univ., Japan)
H. Kage(Kyushu Institute of Technology, Japan)
Y. Kang(Chungnam National Univ., Korea)
D.J. Lee(National Taiwan Univ., Taiwan)
J.H. Li(Chinese Academy of Science, China)
H. Makino(CRIEPI, Japan)
S. Matsusaka(Kyoto Univ., Japan)
M. Naito(Osaka Univ., Japan)
K. Nogi(Osaka Univ., Japan)
K. Okuyama(Hiroshima Univ., Japan)
Pradip(TRDDC, India)
M. Rhodes(Monash Univ., Australia)
M. Takahashi(Nagoya Institute of Technology, Japan)
H. Takeuchi(Gifu Pharmaceutical Univ., Japan)
W. Tanthapanichakoon (Chulalongkorn Univ., Thailand)
Y. Tsuji(Osaka Univ., Japan)
C.H. Wang(National University of Singapore)
S. Watano(Osaka Pref. Univ., Japan)
Secretariat
T. Yokoyama(Hosokawa Powder Tech. Foundation, Japan)
L. Cui(Hosokawa Micron Corp., Japan)

The Chinese character “ 粉 ” in the cover is pronounced“KONA” in Japanese, and means “Powder”. The hand writ” is after the late Mr. Eiichi Hosokawa, founder often “
the Hosokawa Micron Corporation (http://www.hosokawamicron.co.jp).

About the Cover
Preventing transfer of pathogens from touch surfaces has
been an extremely important topic of research in reducing
hospital acquired infections. With increasing challenges of
antimicrobial resistance, there is need for new and advanced
materials systems that can disinfect pathogens for long periods of time, without inducing concerns of antimicrobial resistance. TiO2 based photocatalytic system has been used toaddress this need for inactivation of pathogens via visible
light photocatalysis. The proposed mechanism of the self
cleaning surface includes contaminant mediated activation of
the photocatalytic surface. The presence of visible light absorbing contaminant (microbe) triggers a chain of photocatalytic reactions leading to complete oxidation, degradationand inactivation of the microbe. Please read the paper on
pp.234–240.

European / African Editorial Board
Chairman
G.M.H. Meesters(Delft Univ. of Technology, Netherlands)
B. Biscans(Univ. de Toulouse, France)
M.-O. Coppens(Univ. College London, UK)
L. Gradon(Warsaw Univ. of Technology, Poland)
A. Kwade(TU Braunschweig, Germany)
P. Milani(Univ. of Milan, Italy)
J.M. Valverde(Univ. of Seville, Spain)
Secretariat
S. Sander(Hosokawa Alpine AG, Germany)
L. Kneisl(Hosokawa Alpine AG, Germany)
American Editorial Board
Chairman
B.M. Moudgil(Univ. of Florida., USA)
A.J. Hickey(Univ. of North Carolina, USA)
V. M. Puri
(Pennsylvania State Univ., USA)
R.K. Rajamani(Univ. of Utah, USA)
A.D. Rosato(New Jersey Institute of Technology, USA)
L.M. Tavares(UFRJ, Brazil)
Secretariat
C.C. Huang(Hosokawa Micron Intl. Inc., USA)
■ Publication Office

Hosokawa Powder Technology Foundation (http://www.kona.or.jp)(in the headquarters building of Hosokawa Micron Corporation)
1-9, Shodaitajika, Hirakata-shi, Osaka 573-1132, Japan
E-mail: contact_zainq@hmc.hosokawa.com■ Printing Office: Nakanishi Printing Co., Ltd., Japan

Headquarters of Hosokawa Micron Corporation■ Publication Date: 10 January 2017

KONA Powder and Particle Journal No. 34 (2017) <Editorial>
1

Editor's Preface<Review Papers>
3

Recent Progress in Nanoparticle Dispersion
Using Bead Mill

Takashi Ogi, Rizka Zulhijah, Toru Iwaki andKikuo Okuyama
24

Present Advancement in Production of Carbon
Nanotubes and Their Derivatives from
Industrial Waste with Promising Applications
Konrat Kerdnawee, Chompoopitch
Termvidchakorn, Pacharaporn Yaisanga, Jirapat
Pakchamsai, Cheewapon Chookiat, Apiluck
Eiad-ua, Winadda Wongwiriyapan, Weerawut
Chaiwat, Sakhon Ratchahat, Kajornsak
Faungnawakij, Komkrit Suttiponparnit and
Tawatchai Charinpanitkul

44

Aerosol Delivery of siRNA to the Lungs.
Part 2: Nanocarrier-based Delivery Systems

Susanne R. Youngren-Ortiz, Nishant S. Gandhi,
Laura España-Serrano and Mahavir B.
Chougule
70

Toward Low-Temperature Coal Gasification:
Experimental and Numerical Studies ofThermochemical Coal Conversion
Considering the Interactions between
Volatiles and Char Particles

Cheng-Yi Li, Li-Xin Zhang, Shinji Kudo,
Jun-ichiro Hayashi and Koyo Norinaga

Contents

KONA Powder and Particle Journal No. 34 (2017) 

80 Synthesis of Nanoparticles by Laser Ablation:
A Review

Myungjoon Kim, Saho Osone, Taesung Kim,
Hidenori Higashi and Takafumi Seto
91

Deficiencies in Traditional Oral Dosage
Forms and the Emergence of ControlledRelease Powder Manufacturing

Martin G. Teresk, Cory J. Berkland and Nathan
H. Dormer
106

A Review of Advanced Ball Mill Modelling

Luís Marcelo Tavares
125

A Review of the Modeling of High Pressure
Grinding Rolls

Samira Rashidi, Raj K. Rajamani and Douglas
W. Fuerstenau

Contents

KONA Powder and Particle Journal No. 34 (2017) 

141

Liquid Flame Spray—A Hydrogen-Oxygen
Flame Based Method for Nanoparticle
Synthesis and Functional Nanocoatings

Jyrki M. Mäkelä, Janne Haapanen, Juha Harra,
Paxton Juuti and Sonja Kujanpää
155

Small-Scale Particle Interactions Are Having
Significant Effects on Global Fluidized Bed
Behavior

Ray Cocco, Allan Issangya, S.B. Reddy Karri,
Tim Freeman, Heinrich M. Jaeger and Ted M.
Knowlton
168 Advances in Nanoparticle Sizing in
Suspensions: Dynamic Light Scattering andUltrasonic Attenuation Spectroscopy

Wu Zhou, Mingxu Su and Xiaoshu Cai<Original Research Papers>
183

Microstructural Evaluation of ZrB2/ZrO2
Ceramic Powders Prepared by MillingAssisted Magnesiothermic Reduction of
Oxide Raw Materials

Duygu Ağaoğulları, Özge Balcı, M. LütfiÖveçoğlu and İsmail Duman

Contents

KONA Powder and Particle Journal No. 34 (2017) 

197

Comparison of Compression and Material
Properties of Differently Shaped and Sized
Paracetamols

Michal Šimek, Veronika Grünwaldová andBohumil Kratochvíl
207 Nanocrystallization of Al Powder by
Cryomilling Process

Mohammad Hossein Enayati
213 Identification of Optimal Mill Operating
Parameters during Grinding of Quartz with
the Use of Population Balance Modeling

Evangelos Petrakis, Elias Stamboliadis andKonstantinos Komnitsas
224

Changes in Pore Structures of Porous Beds
When Subjected to Vertical Vibration

Rong Yue and Qiang Zhang

Contents

KONA Powder and Particle Journal No. 34 (2017) 

234 Visible Light Photocatalytic Bacterial
Inactivation on Titanium Dioxide Coatings

Vignesh Nandakumar, Zhao Han, Zachary
Fritz, Vijay Krishna, Ben Koopman and Brij
Moudgil
241

Eddy Current Separation of Nonferrous
Metals Using a Variable-Frequency
Electromagnet

Nakul Dholu, James R. Nagel, Dave Cohrs and
Raj K. Rajamani
248 Impact Dispersion Using 2D and 3D
Composite Granular Packing

Surajit Sen, T. R. Krishna Mohan and Mukesh
Tiwari258 Flow of Bi-modal Slurry through Horizontal
Bend

Deo Raj Kaushal, Arvind Kumar, Yuji Tomita,
Shigeru Kuchii and Hiroshi Tsukamoto

Contents

KONA Powder and Particle Journal No. 34 (2017) 

275

Contents

A Study of the Mixing Index in Solid
Particles

Shih-Hao Chou, Yue-Lou Song and Shu-San
Hsiau
282

Characterisation of the Wetting Behaviour ofPoor Wetting Food Powders and the Influenceof Temperature and Film Formation

John J. Fitzpatrick, Justine Salmon, Junfu Ji andSong Miao<Information Articles>290

The 50th Symposium on Powder Technology

292

The 23rd KONA Award(Awardee: Dr. Hisao MAKINO)

293

General Information

(a)

(b)

Some achievements of the research for the 23rd KONA Award: (a) Concept of ultra-low NOx combustion technology and burner. (b) Comparison of unburned fraction against NOx concentration
between developed and conventional burners.(p.292)

KONA Powder and Particle Journal No. 34 (2017) 1–2/Doi:10.14356/kona.2017023

Editorial

Editorʼs Preface
Jusuke Hidaka, Editor-in-Chief
Doshisha University, JapanI have the pleasure of sending you the KONA No.34 Journal which had many interesting review papers and original research reports. Two years ago, this annually published KONA Journal was opened to the J-STAGE (Japan
Science and Technology Information Aggregator, Electronic), which is an international online archive to support
the transmission function of the information of science and technology organized by Japan Science and Technology
Agency belonging to Ministry of Education, Culture, Sports, Science and Technology. It introduced the Open Access policy this year to enable free browsing and reuse by the readers all over the world. As a result, the KONA
Journal has approximately 300 viewers per day and the latest Impact Factor of this journal is 1.67 and is increasing
every year.
The KONA Editorial Committee consists of three editorial boards located in Europe, America and Asia/Oceania,
as shown on the opposite side of the front cover of the Journal. The Committee receives papers submitted by authors
throughout the year. The papers will then be published worldwide after the submission to the Advance Publicationon the J-STAGE passing through the strict reviewing procedures. Among the 22 papers published in No.34, 18 papers have been submitted as Advance Publication and accessed by readers worldwide before the publication of theprinted version. We are looking forward to receiving articles from you for the KONA Journal.
Presently our society is continually seeking a higher quality, where people can live healthy and safely with material wealth and spritual prosperity. In order to achieve this, there is a strong desire to create (/produce) powder materials with a higher quality to solve current problems of our present society with energy, medicine, food etc. and todevelop fine particles with advanced functions. In response to these requirements, powder science and technology
are advancing remarkably, entering the region of “Product Design”.
Powder technology started from the design of processing system to produce powder materials. It was the engineering for designing the unit operations for powder processing like grinding, classification and the like as well as
powder processing systems combining the unit operations. It was thought that the production processes of powdermaterials would consist of powder unit operations and be constructed just by the combination or connection of generally developed powder machines and equipment. It has worked that way in the age of mass production of generalproducts. However, in the present age, confronting the production of high functional materials, it is required to design
microstructure of materials most suitable to achieve the desired functions and the powder processes to realize it
precisely. It is also characterized that the powder processes to produce the advanced functional materials need theprocessing of each specified material and that their individualization and complexity are advancing rapidly.
The KONA Journal is published once a year and the publication frequency is too low to have the research reportpublished. I would like to publish the review papers with high quality concerning the rapidly advancing powder science and technology to contribute for further advancement of powder science and technology in the world. I intend
to make every effort to publish the KONA Journal under this editing policy and would like to ask for your cooperation and contributions.

Copyright © 2017 The Author. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

1

Reviewer Appreciation

I express our gratitude to the reviewers for the publication of this KONA Powder and Particle Journal. Thanks to
the generous contributions of the volunteer reviewers as well as the interest of researchers in our journal, the number of submission to this journal is increasing after the registration on the major international journal platforms andits level is further improving. Because of the limited space of this annual journal, the number of papers publishedthere is regrettably rather restricted. Therefore the papers of good quality need to be selected for the journal andfurther improved and refined by the reviewing.
The editorial team of KONA Journal would like to take this opportunity to gratefully acknowledge the reviewers
here and deeply appreciate their valuable time and professional contributions to this number of KONA Journal. We
apologize to any reviewer who may have inadvertently been omitted from this list. We are sincerely grateful to all
who have been of any assistance to the publication of this journal.

ADACHI, Motoaki

MORI, Yasushige

TAKEDA, Shin-ichi

COPPENS, Marc-Olivier

MOUDGIL, BrijTANAKA, Hidekazu

FUKUMORI, Yoshinobu

OKUYAMA, Kikuo

TAVARES, Luis Marcelo

HICKEY, Anthony

PEUKERT, Wolfgang

TOZUKA, Yuichi

HUANG, Ching-Chung

PURI, Virendra

TSUJI, Yutaka

KANO, Junya

RAJAMANI, Raj

TSUJI, Takuya

KIMATA, Mitsumasa

SAITO, Fumio

WINDOWS-YULE, Kit

MEESTERS, Gabriel

SATOH, Munetaka

YOKOYAMA, Toyokazu

MILANI, Paolo

SUZUKI, Hiroshi

YOSHIDA, Hideto

MIYAHARA, Minoru

TAKARADA, Takayuki

Jusuke Hidaka
Editor-in-ChiefProf. emeritus, Doshisha University
October 2016

2

KONA Powder and Particle Journal No. 34 (2017) 3–23/Doi:10.14356/kona.2017004

Review Paper

Recent Progress in Nanoparticle Dispersion Using Bead Mill †Takashi Ogi*, Rizka Zulhijah, Toru Iwaki and Kikuo Okuyama
1

Department of Chemical Engineering, Hiroshima University, Japan

Abstract
There is great demand for nanoparticles (NPs) dispersed in liquid phases for practical applications of functional
NP materials. However, it is difficult to produce NP dispersions with specific particle sizes, concentrations,viscosities, and purities on an industrial scale (large mass production rate and low energy consumption). In thisreview, we highlight recent developments in NP dispersion using low-energy bead mill. Such processes enable theuse of small beads (7–50 μm). Smaller beads reduce the collision and shear energies of NPs during agitation. This
minimizes NP breakage/damage, and retains the shape and crystallinity of the NPs, which determine the inherent
NP functions. This review starts with a brief explanation of the theory and current status of NP dispersion anddescribes the mechanism and experimental results for low-energy bead mill processes, i.e., using uniaxial, dualaxial, and all-separator bead mills, and selection of dispersing agent. Applications of NP dispersions, including
nanocomposite materials, and methods for dealing with NP dispersion coloration are also discussed, along with
future research directions.
Keywords: nanoparticle, dispersion, bead mill, dispersant, nanocomposite material

1. Introduction
1.1 Importance of NP dispersion
This century, research on the synthesis of nanoparticles(NPs) has attracted much attention because of their advantages compared with bulk materials. When the particle size of a material is reduced to the nanoscale range,quantum size effect and size effect become more important than they are for submicron particles. The former effect shows the specific energy band gap differ from bulk
solid one, and the latter effect produces high surface areas, single-domain sizes, and high surface activities.
These characteristic properties of NPs are important inindustrial applications such as catalysts, magnetic storage,
optical devices, and ultrafine wiring.
NPs have large surface areas and excess surface energy, and therefore they tend to agglomerate to minimize
the energy, which is larger than the kinetic energy caused
by Brownian motion. Cohesion and adhesion forces easilyarise between NPs because of intermolecular forces suchas van der Walls forces, electric dipole interactions, andchemical bonding (hydrogen bonding), which lead to par†1*

Received 30 September 2015; Accepted 24 November 2015
J-STAGE Advance published online 30 January 2016
1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
Corresponding author: Takashi Ogi;
E-mail : ogit@hiroshima-u.ac.jp
TEL : +81-82-424-3765 FAX: +81-82-424-7850

ticle agglomeration and aggregation.
NPs agglomerate and aggregate during synthesis. Agglomerates are loosely attached particles (the so-called
primary particles) by van der Waals, capillary, electrostatic or other physical forces. While, primary particles
held together by chemical/sinter forces (metallic, ionic orcovalent bonds) are termed aggregates. In liquid-phaseprocesses, e.g., precipitation methods, primary particles
easily interact each other and agglomerate. Although pH
control or surfactant addition is usually used to obtainstable and dispersed NPs (Hwang et al., 2008; Lan et al.,2007), such methods also involve drying at high temperature, which usually causes agglomeration. In gas-phaseprocesses, NP agglomerates and aggregates are easily
formed because the high temperature causes sintering.
Agglomerations also occurs during condensation and particle collection in NP powder handling.
Control of the repulsive forces between NPs is therefore necessary in NP dispersion. Dispersion control inaqueous or organic solutions is based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory or excluded
volume effect, respectively.

1.1.1 DLVO theory of dispersion

DLVO theory is based on the van der Waals interaction
force (VA) and electrostatic double layer repulsive force(VR), which is generated by the surface charge of the particles. NP agglomeration and dispersion are explained using DLVO theory as follows.

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

3

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

in the surrounding dispersant because of steric hindrance
or an impediment. In the case of spherical NPs, the closest distance to which two NPs can approach each other isequal to the sum of their radii. Dispersion and agglomeration can therefore be controlled by excluding this volumeusing dispersants. In the case of nanoscale particles, theenergy required to approach the dispersed state from theagglomerated one, in which the NPs are surrounded bydispersant, is also lower than that usually observed forsubmicron particles.

1.2 Current status of NP dispersion

Fig. 1

Cohesion of pair of NPs and potential energy of dispersion. VA: van der Waals interaction force; VR: electrostatic double layer repulsive force; VS: Born repulsive
force; H: particle distance.

Fig. 1 shows the potential energy between particles (VT)as a function of particle distance (H). When the distance
between the particles is small, there is a potential energy
minimum in the VT curve (primary minimum). This distance is approximately 4 Å and it indicates an agglomerated state, which depends on the strength of theinterparticle bonding, e.g., van der Waals interactions,electrostatic hydrogen bonding, and sintering. A secondary minimum potential also occurs when the distance
between the particles is large (secondary minimum),indicating stable particle dispersion. When the potential
barrier (VTmax) between these two minimum potentials is
10–20 times larger than the kinetic energy (kT), the dispersed NPs will be stably suspended in the secondary
minimum state. When VTmax is lower than the kinetic energy, e.g., the zeta potential of the particles is low, the dispersed particles easily revert to the primary minimum
state.
When the particle size is in the nanoscale range, the VA
and VR values become smaller than those of submicronparticles. For example, when the particle size is one-tenththat of the submicron range, the potential becomes onetenth that of the submicron particles. The energy required
to disperse agglomerated NPs is lower than that required
for submicron particles.

1.1.2 Excluded volume effect for dispersion

The dispersion of particles in an organic solvent is controlled by the dispersant and polymers, and dispersion
control is explained based on the excluded volume effecttheory. This theory refers to the phenomenon whereby
two neighboring particles cannot occupy the same space4

As explained above, agglomeration and aggregation are
inevitable in NP synthesis. However, only well-dispersed
NPs can be used for further applications as functional
materials such as nanostructured particles and nanocomposite polymer materials (Ogi et al., 2014). Innovativetechniques for liquid-phase production of well-dispersed
NPs with specific concentrations, viscosities, and purities
are therefore needed for practical applications.
The energy needed to disperse agglomerated particles
to isolated ones consists of physical (mechanical) andchemical (surface modification) energies. The dispersion
process also involves complete wetting of particles, andbreakup of agglomerated/aggregated particles into primaryparticles, followed by particle stabilization. In principle,
agglomerates can be broken up by applying compression
or shear stress forces, and particle stabilization depends
on the electrostatic repulsive force between the particles,or the excluded volume effect, influenced by stabilizing
additives.
Table 1 shows widely used methods for NP dispersion.
Agglomerated NPs are broken up using various machines
by applying compression or shear stresses. Each type ofTable 1 Various dispersion machines used on industrial scale
Dispersion machines

Particle size

Three-roller mill

600 nm–1 μm

Media method-Roller mill

600 nm–1 μm-Ball mill

600 nm–1 μm-Usual bead mill

100 nm–100 μm-Low-energy bead mill

5 nm–1 μm

Media-less method-Wet jet mill

50 nm–10 μm-Sonication

50 nm–5 μm-Cavitation

50 nm–5 μm-Ultra-thin high shear dispersion

50 nm–5 μm

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

machine, e.g., three-roller mill, media-less and media dispersion machines, has their own dispersing characteristics. In practice, they are often used in combination, andstress forces can be created by fluid and solid surfaces, asin media dispersion methods.

1.2.1 Three-roller millParticles are stressed by a shear gradient arising fromthe different speeds of the rollers. For high concentrations
and large agglomerates, the particles are not only stressedby the shear gradient but also by shear stress in the narrow gap between two rollers. In this condition, the stressforce from one roller to the other roller is transferredthrough the particles. The stress intensity increases withdecreasing particles size. (Schilde et al., 2010, 2011).

1.2.2 Media-less dispersion

Particle dispersion is performed using an agitated liquid flow such as sonication, cavitation, or ultrathin highshear dispersion. Such methods are called media-less dispersion. In the case of wet-jet milling, mixing and dispersion are carried out only by mutual collision of particles.
Particles flowing in a slurry in an oblique direction athigh pressure burst and collide. Dispersion by sonication
causes cavitation effects. When a liquid sonicated at highintensities creates vacuum bubbles, which collapse violently, agglomerated particles are separated. Another dispersion technique that involves cavitation force is the useof high-pressure nozzle systems. Collapse of a cavitation
bubble can create very high local pressure and release energy, which can break agglomerated particles. Anothermedia-less method is ultrathin-film high-shear dispersion.
An ultrathin film is formed along the walls of the vessel,
where significant energy from centrifugal force is applied
to break up agglomerated NPs.
This media-less dispersion system is appropriate forelectronic materials such as battery slurries because it is acontamination-free method. However, in the case of NPs,
this method does not give good dispersion because thedifferences between NP movements in liquid and fluid
systems are small because of the low masses of the NPs.
In other words, the force applied to the particles will be
too small to break up agglomerated NPs. Another dispersion technique such as media dispersion is necessary toprovide adequate force to the agglomerated NPs.

1.2.3 Media dispersion

In media dispersion, agglomerated NPs are broken up
with the aid of a medium, which is classified as balls or
beads, based on sizes of 0.1–40 mm and 0.015–0.5 mm,
respectively. A ball mill machine is usually used forcrushing large block particles to produce smaller particles
for dispersion in a liquid. Bead mill are usually used fordispersion of agglomerated NPs (several micrometers)that are much smaller than the bead size. Collisions andshear stress between the moving beads during agitation
cause separation of agglomerated NPs to their primaryparticles, as explained in detail in the next section.
Among dispersion machines, media dispersion is mostwidely used and effectively disperses many types of NPs,
depending on the agglomerated state. According to the
DLVO theory, a low potential energy is usually needed toseparate two particles. If the energy is too high, the NPs
will be damaged during dispersion. In media dispersion
methods, control of the shear stress and collisions istherefore important to provide appropriate energy for dispersion. Bead mill machines usually use a mesh or slit
method for separation of dispersed NPs from the beads,
and therefore the bead size limit is above 0.1 mm, which is
too large with respect to the NP size. This condition resultsin supply of too much energy to the dispersion system.
Our groups have therefore developed low-energy beadmill methods. Unlike the usual machines, a centri-separator
is used for bead separation, and therefore very smallbeads (7–50 μm) can be used for NP dispersion. The energy that the NPs receive from collisions and shear ofbeads can be reduced, and therefore particle breakage/
damage can be minimized. Only small beads can produce
a low dispersion energy. In the following sections, three
types of low-energy bead mill processes are explained indetail. The effects of the experimental conditions, i.e.,
particle shape (spherical, rod, core–shell), rotation speed,
and dispersion time, are also discussed, as well as accompanying phenomena such as dispersion coloring.

2. Low-energy bead mill dispersion
Low-energy bead mill dispersion can be understoodusing the simple model shown in Fig. 2. This model is
only applicable when the primary NP size is smaller than
100 nm and the bead size is smaller than 50 μm. Step (i)shows the initial condition of the beads before NP dispersion. The beads move and rotate during agitation. Step (ii)involves collisions between the beads and NPs and application of stress to the agglomerated NPs. After breakupof the aggregated NPs, the surfaces of the isolated NPs
become active (formation of activated sites). Step (iii) involves stabilization of the isolated NPs by creating a repulsive force, for example, using a dispersant or zeta potential,
to the active sites. The active surface is inhibited by thedispersing agent or zeta potential in the suspension andstable dispersion is achieved. In multicollision phenomena, bead milling affords highly dispersed NPs. This condition can be achieved only when the collision force andshear stress are not too strong. Control of the bead size
and rotation speed (very small size and ~3–8 m/s, respectively) is important. Outside these ranges, the energy pro-5

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 2

NP dispersion mechanism during low-energy bead mill process.

vided to the NPs is too high, which results in NP breakage
and reagglomeration. Our group has developed three typesof bead mill methods, classified based on the provided
dispersion energy level, namely, uniaxial, dual-axial, andall-separator bead mills. These are described in the following sections.

2.1 Uniaxial bead mill
The first low-energy bead mill to be developed was auniaxial bead mill (UAM) (Inkyo et al., 2006). A schematic diagram of a UAM is shown in Fig. 3. The apparatus consists of a 0.15 L vessel, a pump for supplying aslurry, and a slurry tank. The vessel contains a rotor pin
and separator, which are set in the lower and upper portionsof the vessel, respectively. The vessel is enclosed in a cooling jacket to prevent temperature increases in the system,
and is completely sealed from the outside environment.
ZrO2 beads of size 7, 15, 30, and 50 μm are used as the medium (Fig. 4). The ZrO2 beads occupy 65 % of the vessel
volume. The dispersion process is explained as follows.
A slurry containing agglomerated NPs is pumped into
the bead mill vessel and through to the lower part. In thispart, agglomerated NPs interact with the agitated beads,
and the agglomerated NPs are dispersed by collisions andshear stress between the beads. Gradually, the slurryreaches the upper part of the vessel. In this region, separation is performed using a centri-separator, in which thedispersed NPs are separated from the beads by centrifugal force. The beads remain inside the mill, and the NP
slurry is pumped out of the vessel. The product particles
are collected in the slurry tank and the dispersion process
is repeated.

6

Fig. 3

Schematic diagram of UAM.

2.1.1 Dispersion behavior of TiO2 NPs using UAM

This UAM was used for the dispersion of needle-type
TiO2 NPs in water (Inkyo et al., 2006). The effects ofbead size, dispersion time, and rotation speed on particle
breakage and the particle size distribution were investigated; the results are shown in Fig. 5(a). For all the tested
beads except those of size 15 μm, the particle size distribution shifted to smaller sizes, and then gradually shiftedback to larger sizes, indicating that the agglomerated NPs
were dispersed, followed by reagglomeration of the dispersed NPs. Reagglomeration may be caused by activation of the particle surfaces newly generated by the
milling process, followed by interactions among broken
particles. Breakup of the agglomerated NPs significantly
increases the particle concentration in the slurry, which inTakashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 4

SEM images of beads of various sizes: (a) 7, (b) 15, (c) 30, and (d) 50 μm.

turn significantly reduces the characteristic coagulation
time for particles.
When 100 and 50 μm beads were used, no particles weredispersed to primary particles, showing that the collision
force from large beads is too high and damages the particles, followed by reagglomeration of broken particles. The
particle size distribution shifted from a unimodal distribution to a bimodal distribution for processing times longer
than 240 min. This sudden change in the size distribution cannot be explained by any traditional particle agglomeration mechanism. A possible reason for the shift
from a unimodal to a bimodal size distribution is thatthe broken NPs are deposited on the beads and then growincreasingly large as collisions continue to occur. Once thedeposits of broken NPs achieve sufficient size, further collisions may cause these NP deposits to break away fromthe beads, and the broken NPs agglomerate. These results
show that the breakup of agglomerated NPs is eventuallyfollowed by reagglomeration. There is therefore an optimum milling time for each dispersion with each bead size
before reagglomeration occurs, as indicated by the particle size distribution shifting to larger size. In the cases of 30
and 15 μm beads, the agglomerated NPs were completely
dispersed and a sharp peak at 15 nm appeared as the minimum particle size distribution.
The changes in particle size as a function of dispersion
time for various bead sizes are shown in Fig. 5(b). In thisexperiment, spherical TiO2 NPs were also dispersed forcomparison. In general, both needle and spherical TiO2
NPs have the same dispersion behavior.
The particle size gradually decreased as the dispersion
time increased, and reached the primary particle size, indicating that the agglomerated NPs were dispersed to pri-Fig. 5

Effects of bead size and dispersion time on dispersionof TiO2 NPs: (a) needle TiO2 NPs; and (b) sphericalTiO2 NPs. Fig. 5(a): Reprinted with permission fromInkyo et al., 2006. Copyright: (2006) Elsevier B.V.

mary particles. When excessive energy was provided tothe dispersed NPs, the particle size gradually increased,
indicating that the primary particles were damaged andbroken, resulting in reagglomeration of broken particles.
The spherical TiO2 NPs were easier to disperse than theneedle ones. For 8 μm beads only, the agglomerated
spherical TiO2 NPs were dispersed to their primary particles without reagglomeration.

7

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

These results are confirmed by transmission electron
microscopy (TEM) analysis of the NPs before and after
dispersion (Fig. 6). Before dispersion, the TiO2 NP agglomerates were of size 200–300 nm and rod-shaped, likethe primary particles. After dispersion with 15 and 30 μm
beads, well-dispersed rod-shaped TiO2 NPs were obtained. Beads of size 50 and 100 μm had little effect on
the size of the NP agglomerates. Furthermore, the particle
morphology changed from rod-shaped to spherical when
100 μm beads were used.
The detailed effects of rotation speed, bead size, anddispersion time on the dispersibility of rod-type TiO2 NPs
were reported by Tahara et al. (2014). In this study, tworotation speeds (10 and 8 m/s) and various bead sizes (15–50 μm) were used for TiO2 NP dispersion. At the same rotation speeds, the size of the dispersed particles decreased
with increasing dispersion time. The size of the agglomerated NPs decreased rapidly in the initial stage (30 min)and then the particle size changed gradually during dispersion.
At the same rotation speed, larger beads promoted theformation of smaller particles. For the samples dispersed
at a rotation speed of 10 m/s and a bead size of 50 μm, theparticle size decreased until 330 min and then gradually
increased with milling time. This is possibly because ofreagglomeration of broken NPs during milling.
The morphologies of the particles after dispersion using various bead sizes and rotation speeds are shown in
Fig. 7. Various morphologies are observed, depending on
the rotation speed and bead size. At rotation speeds (m/s)/
bead sizes (μm) of 8/15 and 8/30, the TiO2 particles retain
their rod shape after bead milling. However, under otherconditions, the morphology of the TiO2 NPs changed
from rod shaped to ellipsoidal, and the particle size decreased. For the conditions 8/15 and 8/30, TEM images
showed no change in the crystallinity of the particles after
dispersion; this was confirmed by X-ray diffraction(XRD) analysis. These results show that the conditions
8/15 and 8/30 effectively provide low energy for dispersion, producing dispersed agglomerated NPs withoutchanging the particle crystallinity. NP dispersion in an
organic solvent has also been reported (Joni et al., 2009).
Fig. 8 shows scanning electron microscopy (SEM) imagesof TiO2 NPs dispersed in diethylene glycol dimethyl ether(diglyme).
Fig. 9 shows a diagram of the morphological and struc-Fig. 6 TEM images of TiO2 NPs (a) before and (b–e) after dispersion using various bead sizes: (b) 15, (c) 30, (d)50, and (e) 100 μm. Reprinted with permission from Inkyo et al., 2006. Copyright: (2006) Elsevier B.V.

8

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 7

TEM images of TiO2 NPs (a) before and (b–g) after bead milling; rotor speed (m/s)/bead size (μm) (b)8/15, (c) 8/30, (d) 8/50, (e) 10/15, (f) 10/30, and (g) 10/50. Reprinted with permission from Tahara et al.,2014. Copyright: (2014) Elsevier B.V.

Fig. 8SEM images of TiO2 NPs (a) before and (b–d) after bead milling in organic solvent as a function of milling time: (b) 60, (c) 90, and (d) 120 min. Reprinted with permission from Joni et al., 2009. Copyright:(2009) American Chemical Society.

9Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 9Diagram of morphological and structural changes in NPs during bead milling depending on supplied energy. Reprinted with permission from Tahara et al., 2014. Copyright: (2014) Elsevier B.V.

tural changes in NPs during bead milling depending on
supplied energy (high energy or low energy). Low-energy
dispersion defines the condition for bead milling that provide the breakup of agglomerated NPs (not grinding/crushing). High-energy dispersion defines the condition
that damages the NP structure and changes the inherent
properties of the NPs. Although high-energy dispersion
enables rapid breakup of agglomerated NPs, the main
body (the material itself) is damaged and broken (destruction of crystal). As a result, the dispersed solution contains multi-sized particles. Because of the high energy,
this condition cannot be used for fragile materials such as
metal, hollow, and core–shell structured particles. The
milling time is also important. A milling time that is toolong leads to reagglomeration. NPs are typically softly
agglomerated, and therefore the optimum conditions thatenable breakup of agglomerated NPs at the agglomerating
position only are crucial. In low-energy dispersion processes, the slurry contains single-sized NPs with the inherent material properties.
During bead mill dispersion, the physicochemical
properties of the particles may change. These changes arecaused by decreases in the particle crystallinity during
bead mill dispersion. During bead mill dispersion, collisions between beads and agglomerated TiO2 particles
provide energy that can break the main body of the crystal, resulting in production of smaller particles and an
amorphous phase on the crystal surface, and this changes
the properties of the dispersed particles (bottom image in

10

Fig. 9).
Based on the above results, several operating parameters should be considered for providing low-energy dispersion in bead mill processes, including milling time,
temperature, bead and particle sizes, rotation speed, physicochemical properties, and composition of dispersed
media and agglomerated particles. Low energy can beobtained when the milling process is conducted at lessthan 0.50 μW/bead. This condition can be achieved when
the process is conducted below a rotational speed andbead size of 10 m/s and 30 μm, respectively. Deviationfrom these conditions results in reagglomeration, production of broken particles, and impairment of the crystallinity and physicochemical properties of the NPs.

2.1.2 Dispersion of boron nitride NPs using UAM

Hexagonal boron nitride (hBN) NPs are an indispensable material for many industrial applications because of
their properties such as high thermal conductivity, lowthermal expansion, good thermal shock resistance, highelectrical resistance, low dielectric constant and loss tangent, and microwave transparency. When inorganic hBN
NPs are used as inclusion particles for reinforcement, dispersion is very important. Poorly dispersed hBN NPs insuspension will not give nanostructured materials withexcellent properties.
Our group successfully dispersed hBN NPs of primaryparticle size 35 nm using a UAM, as shown in Fig. 10.
Two silane coupling agents, i.e., (3-acryloxypropyl)trime-

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 10 Dispersion of hBN NPs by UAM. (a), (b), and (c) are SEM images, slurry photographs, and XRD patterns of hBN before and after dispersion. Reprinted with permission from Joni et al., 2011. Copyright:(2011) Elsevier B.V.

thoxysilane (APMS) and trimethoxy(propyl)silane, wereused for surface modification to enhance the dispersion
stability (Joni et al., 2011). Optimization of the rotation
speed, slurry flow rate, and dosage of dispersing agent
gave agglomerated hBN NPs that were well dispersed toprimary particles. A highly dispersed hBN suspension,
with a zeta potential below ~ +40 mV, was prepared using
APMS as the dispersing agent. The bead mill process did
not significantly change the crystallinity and chemicalcomposition of the hBN NPs, but enhanced the dispersion
stability (Joni et al., 2011).

2.2 Dual-axial bead millBecause of the limited bead sizes available with UAM,
dual-axial bead mills (DAM) were developed. A schematic diagram of a DAM is shown in Fig. 11. Unlike thecase for a UAM, the separator and rotor pin are separated,
and therefore they can run independently. Details of thisbead mill were reported by Tahara et al. (2011). Use of a
DAM enables the rotation speed to be controlled to belower than the separator speed, allowing the use ofsmaller beads to provide a lower dispersion energy.
A DAM was used for the dispersion of rod-shaped TiO2
NPs. Fig. 12 shows the effect of the rotor pin rotation
speed in the range from 12 to 3 m/s on dispersion using
beads of size 50 μm and separator speed 12 m/s. At a low
rotation speed (3 or 6 m/s), the particle size was reduced
to 30 nm, indicating that the agglomerated NPs were dis-Fig. 11 Schematic diagram of DAM. Reprinted with permission from Tahara et al., 2011. Copyright: (2011) Journal of the Society of Powder Technology, Japan.

persed to the primary particle size without reagglomeration. At a rotation speed higher than 6 m/s, rod-shaped
TiO2 NPs were broken into ellipsoidal shapes and the particle size increased as dispersion continued. Under these
conditions, the TiO2 NPs were damaged and then reagglomeration occurred because of the strong interactions
among active broken surfaces.

11

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 12 Effect of rotation speed on dispersion of TiO2 NPs using DAM. Reprinted with permission from Tahara
et al., 2011. Copyright: (2011) Journal of the Society of Powder Technology, Japan.

2.3 All-separator bead mill
Even a DAM has limitations in the dispersion of morefragile NPs such as core–shell NPs. In the cases of a
UAM and DAM, well-dispersed NPs are crushed in theseparator, in which high energy is needed to separate
small beads from dispersed NPs.
Another type of low-energy bead mill apparatus wastherefore developed, i.e., an all-separator bead mill(ASM). An ASM consists of only a separator, which isaligned from the bottom to the upper part, without a rotor
pin, in the vessel. This new apparatus enables lower-energy
dispersion at lower rotation speeds with smaller beads.
A schematic diagram of an ASM is shown in Fig. 13.
Our group used an ASM to disperse agglomerated core–shell α-Fe/Al 2O3 and α’’-Fe16 N2/Al 2O3 magnetic NPs(Zulhijah et al., 2015). Core–shell α’’-Fe16N2/Al2O3 magnetic NPs, the magnetic material with the highest reported
saturation magnetization, have good potential for theconstruction of high-magnetic-moment rare-earth-freemagnetic materials. However, α’’-Fe16N2 magnetic NPs
have strong magnetic lateral interactions between NPs;
this enables easy NP agglomeration, and decreases theirmagnetic performance after structuring. The dispersion
process is therefore the key to preparing dispersed single-­domain magnetic NPs.
Fig. 14(a) shows TEM images of α’’-Fe16N2 NPs before
and after dispersion using different bead sizes and rotation speeds. When the energy supplied was insufficient tobreak up the agglomerated particles, the particle size wasalmost the same as the initial one. When sufficient energy
was supplied to the agglomerated NPs, the agglomerates
were broken up into primary particles without destroying

12

Fig. 13 Schematic diagram of ASM. Reprinted with permission from Zulhijah et al., 2015. Copyright: (2015)
American Chemical Society.

the core–shell structure. The crystal structure of thecore–shell particle after dispersion was the same as thatof the original NPs, as shown in Fig. 14(b). When the applied dispersion energy was too high, rapid breakup of theagglomerated particles on either the agglomeration
boundary or main body of the particles occurred, resulting in particle destruction.
The magnetic properties of core–shell α’’-Fe16N2/Al2O3
NPs before and after dispersion were also evaluated (Fig.
14c). The saturation magnetization of the particles after

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 14 Dispersion of core–shell α’’-Fe16N2 NPs using ASM. (a) TEM images (from left to right) of core–shell
α’’-Fe16N2/Al2O3 before dispersion, and well-dispersed and reagglomerated broken α’’-Fe16N2/Al2O3);(b) XRD patterns; and (c) magnetic evaluation of particles after dispersion. Black, dotted-red, anddashed-blue hysteresis curves correspond to α’’-Fe16N2/Al2O3 before dispersion, and well-dispersed andaggregated dispersed α’’-Fe16N2/Al2O3 NPs, respectively. Reprinted with permission from Zulhijah etal., 2015. Copyright: (2015) American Chemical Society.

dispersion was similar to that of the particles before dispersion. Different hysteresis was found for the well-dispersed
NPs when magnetization relaxation occurred abruptly
near zero applied field. This indicates spin coupling of theneighboring magnetic NPs by release of the magneticallydecoupled restriction of the high magnetic field, which
can be caused by movement of the particles and magnetic
moment during measurement (Zulhijah et al., 2015). The
morphological changes in the agglomerated core–shell
α’’-Fe16N2/Al2O3 NPs are shown in Fig. 15.
A summary of the characteristics, i.e., mean particle
diameter, concentration, and viscosity, of NP dispersions
prepared by bead milling is shown in Fig. 16.

3. Selection of dispersant
As mentioned above, dispersion in organic solvents is
also controlled by the excluded volume effect. The presence of a dispersant that interacts with the particle surface
controls the excluded volume and prevents interaction between particles. Selection of an appropriate dispersant istherefore crucial for obtaining stable NP dispersions.
However, there is a lack of systematic information on dispersant selection, and selection is based on experienceand experimental results.

During dispersion, agglomerated NPs are broken up
and then the particle surfaces become active sites. An appropriate surfactant will have good attractive forces withthe active sites of the particles and solvent. All of the active sites on the particle surface will then bond with thesurfactant, resulting in repulsive forces between the particles, and these prevent reagglomeration. Fig. 17 shows adiagram of dispersant selection for NP dispersion.
Dispersants can be divided into two classes according
to their chemical structures: polymeric dispersants andsurfactants. The primary differences between polymericdispersants and surfactants are their molecular weights,
stabilization mechanisms, and stabilities. Polymeric dispersants stabilize systems sterically. They have a two-­
component structure, which must meet the following
requirements. (1) They can strongly adsorb on particle
surfaces and have specific anchoring groups. (2) They
contain polymeric chains that sterically stabilize the solvent or solution system.
In contrast, the stabilization mechanism of surfactants
is electrostatic: the polar group forms an electrostatic
double layer and produces a coating around the particles.
Because of the electrostatic stabilization mechanism andchemical structure of surfactants, the final coating tends
to be water sensitive.
The effectiveness of a polymeric dispersant is deter-

13

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 15 Illustration of morphological changes of in agglomerated core–shell α’’-Fe16N2/Al2O3 NPs by low-energy
bead mill dispersion under various dispersion conditions. Reprinted with permission from Zulhijah etal., 2015. Copyright: (2015) American Chemical Society.

Fig. 16 Summary of NP dispersion characteristics. (a) particle mean diameter, (b) viscosity, and (c) particle
concentration

mined by the anchoring groups on the surface. The anchoring groups can be an amine, carboxylic, sulfonic, or
phosphoric acid, or their salts. The crucial requirement isthat the chains are successfully anchored to the particle
surfaces, and particle surfaces are covered with sufficiently
dense chains. The chains must have attractive forces with14

both the particle surfaces and the solvent, in the presenceor absence of a polymer (Takeda et al., 2009).
In general, surfactants are selected based on their polarity and molecular weight, as shown in Fig. 17. The
characteristics of a surfactant depend on its polarity andmolecular weight. An appropriate surfactant for a system

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 17 Selection of dispersant for NP dispersion.

can therefore be identified. The various types of surfactant and their applications in NP dispersion are listed inTable 2.
A summary of the use of bead mills and the dispersant
types used in dispersion of various NPs is shown in Table
3.

4. Optical properties of NP dispersions
4.1 Coloring of NP dispersion
One of the problems with NP dispersions is that their suspensions are colored (Fig. 18). For example, well-dispersed
TiO2 NPs in an organic solvent, i.e., diethylene glycol dimethyl ether, are yellow. This suspension is not suitable
as an alternative material to glass, because such materials
must be colorless and transparent. NP dispersions may becolored because of their complex physicochemical properties (refractive index, bandgap, chemical functions), thedispersing agent, and the solvent.
Fig. 19 shows possible reasons for coloring of NP
dispersions. First, Rayleigh scattering of NPs is strongly
related to the color of the NP suspension. Rayleigh scattering involves the predominantly elastic scattering oflight or other electromagnetic radiation by particles muchsmaller than the radiation wavelength. The intensity I oflight scattered by a small sphere of diameter d and refractive index n from a beam of unpolarized light of wavelength λ and intensity I0 is given by
I  I04

2

1  cos 2  2π   n 2  1   d 
  
  
2R 2     n 2  2   2 

6(1)

where R is the distance to the particle and θ is the scattering angle. In the case of a spherical particle, the scattering
intensity increases in direct proportion to the sixth powerof the particle size and in inverse proportion to the fourth
power of the light wavelength. A shorter wavelength(blue) is easier to scatter than a longer wavelength (red).
Undispersed TiO2 NPs are white as a result of scatteringof all visible wavelengths (sun or room light). In the caseof a highly dispersed NP suspension, the transparency is
increased because light is not scattered. However, if agglomerated NPs are present in the suspension, only blue
light is scattered and yellow light is transmitted as a complementary color. The optical transparency loss of a suspension therefore depends on the average particle size andthe presence of aggregates.
The second possible reason is the inherent properties of
NP materials. The bandgap of rutile TiO2 is 3.0 eV and it
has an absorption edge at around 413 nm (form purple toblue). If the material has lattice defects and impurities in
the crystal structure, impurity levels are formed in the
bandgap, and the material absorbs light of wavelength
longer than 413 nm. This is why blue light is absorbed by
TiO2 NPs and yellow light is transmitted as a complementary color (Fig. 19b). High crystallinity and doping ofTiO2 NPs effectively changes their bandgap (absorptionedge).
The third reason is attributed to the structure and functions of the dispersing agent. In our experiment, KBM
5103(3-Acryloxypropyl-trimethoxysilane) was used for
TiO2 NP dispersion. The double bond in KBM 5103 canreact with oxygen radicals to form carbon–oxygen radical
bonds, as shown in Fig. 19(c). These bonds can cause
chemical coloring of the NP dispersion. Selection of an

15

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Table 2 Various types of dispersant for NP dispersion
Surfactant

Nanoparticles

References

Anionic surfactant

CaCO3(Du L. et al., 2013; Hait S. and Chen Y., 2014)

Carbonate (Na stearate)

FeO, Ag, Cu,(Saterlie M.S. et al., 2012; Wang W. et al., 2014; Zong
B.-Y. et al., 2014)

Phosphate salt

CNC, CoFe2O4, γ-Fe2O3(Kovalenko A. et al., 2014; Salajková M. et al., 2012;
Tang B.Z. et al., 1999)

Sulfonate salt (SD)Ag

(Sondi I. et al., 2003)

Aliphatic amine & Quaternary amine

Laponite,(Li W. et al., 2012)

Aromatic Quaternary amine salt

Ag

(Sondi I. et al., 2003)

Ester type (span)

ZnS(Naskar M.K. et al., 2006)

Ester-ether type (tween)

Fe2O3, acrylamide(Guha S. and Mandal B.M., 2004; Guskos N. et al., 2005)

Ether type (PEG dodecylether)Pt, Ag(Ingelsten H.H. et al., 2001; Sondi I. et al., 2003)

Polyacrylate salt

SiO2, Ag, Fe3O4, BaTiO3(Hajdú A. et al., 2012; Pu Z. et al., 1997; Shen Z.-G. et al.,
2004; Zhang Z. et al., 2000)

Polyurethane, polyester type

Fe3O4

(Ashjari M. et al., 2010)

Poly(N-isopropyl acrylamide)

Ag(Dong Y. et al., 2007)

Perfluorooctanoic acidTiO2

(Yang J. et al., 2011)

Perfluorooctanoate (K, NH4)

Ag, polyampholyte(Fan X. et al., 2006; Thünemann A.F. et al., 2002)

Silicone (silylating agent)TiO2(Ouyang G. et al., 2012)

Propyltrimethoxysilane (TMS)

ZrO2, SiO2, BTO(Luo K. et al., 2008; Nomoto H. et al., 2014; Yang X. and
Liu Z.-H., 2010)

Fe3O4, Au, Ag, Ni(Harish S. et al., 2011; Martínez S. et al., 2006; Zhang Y.
et al., 2002)

ZrO2, Al2O3(Takahashi M. et al., 1992)

TiO2, Fe2O3, ZnS(Warad H. et al., 2005; Zhang Y. et al., 2008)

Cationic surfactant

Nonionic surfactants

Polymer (brushing polymer)

Fluoride surfactant

Coupling agent
Silane type
Methacryloyloxypropyl trimethyl
methoxysilane (MTMS)
Aluminum type
Acetoalkoxy Al diisopropylate
Inorganic phosphate
Hexametaphosphateappropriate dispersing agent, i.e., without double bonds,
is key to avoiding coloration. The use of polymerization
inhibitors, which are also considered to cause coloration,
should also be avoided.

4.2 Decoloring of NP dispersions via UV
irradiation
Joni et al. (2012b) reported a method for decoloring ayellow suspension of rutile needle TiO2 NPs in an organic

16

solvent (diethylene glycol dimethyl ether). Well-dispersed
rod-type TiO2 NPs of diameter and length 15.5 and109 nm, respectively, were successfully prepared using a
UAM. Although the bead-milled TiO2 NPs were well dispersed in the organic solvent, the solution was yellow. The
color was removed by UV irradiation. After dispersion,
the suspension was illuminated under UV irradiation(λ = 254 nm) for 5 h. Fig. 20(a) shows the transmittanceproperties of the dispersed TiO2 NPs before and after UV
irradiation. UV irradiation changed the surfaces of theTakashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Table 3 Examples of bead mill dispersion of NPs
Material

Solvent

Dispersant

Conc. (wt%)

Particle size
(before bead mill)Particle size
(after bead mill)

TiO2

Water

—

10

140

25

TiO2

Diglyme

Trimethoxypropyl silane

1.0–10

1000

15

TiO2

MMA/Toluene

3-Acryloxypropyl
trimethoxysilane

1.0–15> 100000

90–150

TiO2

Neo pentyl glycol
dimethacrylate

Butyl acetate

10

85

50

BaTiO2

Neo pentyl glycol
dimethacrylate

Polymeric dispersant
solsperse

10–20

90

40

BN

Water

3-Acryloxypropyl
trimethoxysilane

0.5

1000–3000

46

Al(OH)3MMA

3-Acryloxypropyl
trimethoxysilane

1.0–10

750

210

SiO2

Methanol

3-Acryloxypropyl
trimethoxysilane

4

1000–10000

12

ZnO

Toluene—1.0

1000–10000

10

ITO

Toluene—1.0

1000–10000

20

ZrO2

Toluene—1.0

1000–10000

20

Fig. 18 Coloring of highly dispersed NP slurries: (a) TiO2 (15 nm); (b) ZrO2 (20 nm); (c) indium-tin oxide (ITO)(20 nm); and (d) BaTiO3 (20 nm).

dispersed TiO2 NPs (1 wt%), indicated by a change in thesolution color from yellow to colorless and transparent(Joni et al., 2012b).
Optical scattering loss is avoided if the particle mean
diameter is less than one-tenth the length of the visible-light wavelength (200–800 nm), typically < 25 nm. In

these experiments, the particle size in the dispersed suspension was 25 nm, and therefore the contribution of theparticle size to optical transparency loss can be neglected.
The main factors responsible for the strong light scattering loss of the suspension are therefore the refractive indices of the particles and the matrix. Surfactant desorption

17

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 19

Possible reasons for coloring of NP dispersion.

in the organic solvent. UV irradiation of the suspension
resulted in matching of the refractive indices of the particles and the organic solvent. Optical scattering loss wastherefore avoided, and the prepared suspension was colorless and transparent.

4.3 Decoloring of NP dispersions via nitrogen
dopingFig. 20 Comparison of photographs and UV-vis transmissions: (a) before and after UV irradiation and (b) bare
(BRB) and nitrogen-doped (NRB) rutile TiO2 NP suspensions (1 wt%). Reprinted with permission from
Joni et al., 2012b, copyright: (2012) Elsevier. B.V.;
Joni et al., 2012a, copyright: (2012) American Chemical Society.

was suggested to be the main cause of changes in the refractive index of the matrix, caused by excess surfactant

18

Another method for preparing colorless suspensions ofTiO2 NPs in organic solvents is nitrogen doping. Joni et
al. reported a study of the synthesis of nitrogen-doped
TiO2 NPs and their dispersion in an organic solvent (diethylene glycol dimethyl ether), with 3-(acryloyloxy)propyltrimethoxysilane as the dispersing agent, using beadmilling (Joni et al., 2012a). Yellow nitrogen-doped TiO2
NP powders were prepared by combustion of a mixture ofrutile TiO2 NPs and urea at 800 °C, and then dispersing
the NPs using a UAM.
Fig. 20(b) shows photographs and the UV-vis transmissions of dispersed solutions of nitrogen-doped and barerutile TiO2 NPs. The bare rutile TiO2 NP suspensions
were yellow. When the materials were doped with nitrogen, the resulting TiO2 suspensions were colorless (Joni etal., 2012a). This change in color and transparency as a result of nitrogen doping of TiO2 NPs can be explained byRayleigh scattering, as follows. According to the Rayleighlaw, if the volume fractions are assumed to be the same,the particle sizes and refractive indices of the particles

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Fig. 21 Applications of dispersed TiO2 NPs. (a) Representation of milling–polymerization process. (b, c, d)
SEM images of TiO2 particles in PMMA–TiO2 nanocomposites after polymerization, cross-sectional
TiO2-dispersed polymer films, and composite polymer microspheres containing dispersed TiO2 NPs,
respectively. Reprinted with permission from Inkyo et al., 2008, copyright: (2008) American Chemical
Society; Takeda et al., 2008, copyright: (2008) The Society of Polymer Science, Japan; and Takeda etal., 2009), copyright: (2009) Chemical Society of Japan.

Fig. 22 Applications of dispersed ATH: (a) SEM images of ATH filler dispersed in PMMA–ATH composites atdifferent magnifications; (b) photographs of slurry at various milling times; and (c) storage moduli of
PMMA–ATH composites and pure PMMA. Reprinted with permission from Joni et al., 2010. Copyright: (2010) Elsevier B.V.

and monomer (solvent) can be used to reduce the scattering loss and improve the transparency. The refractive indices can be controlled by choosing appropriate fillers andsolvents, and through design of the interface. The particle
size has to be considerably less than the wavelength of thevisible light. As explained above, optical scattering loss
can be avoided if the particle domain size (typically< 25 nm) is less than one-tenth of the visible-light wavelength. Ideally, TiO2 should therefore have a diameter< 10 nm to avoid coloring issues.
The suspension of bare TiO2 particles in diethylene
glycol dimethyl ether was not transparent, and was yellow
because of the presence of some large aggregates of TiO2
NPs (> 100 nm). These aggregates led to strong lightscattering in the visible region. In contrast, in the case of
a nitrogen-doped TiO2 NP dispersion, the particles were
smaller and monodispersed, which considerably improved
the transparency, and reduced the color, of the suspension.

19

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

However, a small percentage of nitrogen-doped TiO2 waspresent as aggregates (> 100 nm), indicating that thetransparency was not only dependent on the particle size
but was also affected by the interfacial properties. These
results suggest that nitrogen atoms on the TiO2 surface
also contributed to the lack of color and transparency ofthe suspension.

5. Applications of NP dispersions
Many reports have shown that composite formation
with NPs can alter the thermomechanical properties ofpolymers, such as the elastic modulus, viscosity, andglass-transition temperature, and the optical properties.
NP–polymer composites can be synthesized by dispersing
NPs in the monomer and polymerizing in the presence of
the NPs.

5.1 Poly(methyl methacrylate)-TiO2
nanocomposites
Inkyo et al. (2008) studied the synthesis of poly(methyl
methacrylate) (PMMA)–TiO2 NP composites. TiO2 NPs
were dispersed in MMA monomer using a bead mill, followed by polymerization to form PMMA–TiO2 nanocomposites, as shown in Fig. 21(a). The TiO2 NPs were welldispersed in the MMA monomer, and therefore transparent composites were formed. In contrast, PMMA–TiO2
nanocomposites synthesized from undispersed TiO2 NP
suspensions were opaque.
SEM confirmed that the TiO2 NPs were evenly distributed in the composite, as shown in Fig. 21(b), unlike thecase for a composite comprising undispersed TiO2 NPs.
Well-dispersed TiO2 NPs enhanced the UV-absorbing
properties of MMA and increased its thermal stability.
Because these PMMA–TiO2 nanocomposites are transparent and have high refractive indices, they have goodpotential optical applications.
Our group has also developed TiO2-dispersed nanocomposite polymer films with a high concentration of
TiO2 (65 wt%), as shown in Fig. 21(c) (Takeda et al.,2009). Bead mill dispersion was used to prepare finely
dispersed TiO2 NPs in the polymer; two monomers (neopentyl glycol dimethacrylate or divinylbenzene) and anorganic solvent (butyl acetate) were used as the dispersing
medium. The prepared films (thickness: 5 μm) were transparent (81.5 %) and had a high refractive index (1.85).
These films had smoother surfaces than films prepared bydirect dispersion without bead milling.
Takeda et al. successfully prepared TiO2 nanocomposite microspheres of high concentration (40 wt%) withacrylic monomer, as shown in Fig. 21(d). These TiO2 NPs
were dispersed in butyl acetate by bead milling. The TiO2

20

nanocomposites were prepared by replacement of butyl
acetate with the monomer used as the medium. The TiO2
NPs were close packed and well distributed, without any
vacancies in the microspheres (Takeda et al., 2008).

5.2 PMMA–aluminum trihydroxide
nanocomposites
Aluminum trihydroxide (ATH) is an attractive alternative to halogen-based flame retardants, which give offheavy smoke and hazardous gases during combustion. The
use of these NPs as a filler in polymers such as PMMA,
which is widely used as a shatterproof replacement forglass and in surface coatings, improves the thermal stability and mechanical properties of the polymer. The sizeand dispersibility of the filler (NPs) in the polymer areimportant in obtaining high-performance materials.
Our group used bead mill dispersion to control the size
distribution of an ATH NP filler in MMA. A dispersed
solution was obtained by surface modification of ATH NPs
using a silane-based dispersing agent, (3-acryloxypropyl)
trimethoxysilane (APTMS). In the final product, the ATH
NPs were well dispersed in the PMMA, as confirmed by
TEM and digital photographs (Fig. 22a, b). The mechanical properties and thermal stability of the PMMA–ATH
nanocomposite were also investigated (Fig. 22c). The
storages modulus of the PMMA–ATH nanocomposite in
the glassy state was larger than that of pure PMMA. In
the composite reported by Joni et al. (2010), the storage
modulus increased with increasing amount of ATH NP
filler.

6. Conclusion
NP dispersion is an important technique in the synthesis of functional NP materials for practical applications.
NP dispersion is also essential for NP assembly and structuring. In this paper, recent progress in the development
of bead mill processes for NP dispersion was reviewed.
The bead mill machines described here enable the use ofsmaller beads (5–50 μm) than those normally used. These
small beads have a lower dispersion energy and minimize
damage to the shape and crystal structure of the NPs.
This is a significant advantage and enables NPs to show
their inherent superior material properties. Because of
their capacity of large-scale operating capacities (10–30 L), these low-energy dispersion machines are now
used in many industries. However, the following problems
have become apparent: 1) how to find appropriate dispersants efficiently; 2) how to control coloring phenomena of
NP dispersions; and 3) how to disperse fragile NPs (metal
NPs and hollow NPs) without breaking their structures.
NP dispersion using lower dispersion energies should help

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

to overcome these problems; this would broaden the range
of industrial applications of NPs.

Acknowledgments
This work was supported by JSPS KAKENHI Grant
Numbers 26709061 and 25620164. We also gratefully acknowledge provision of a scholarship (R.Z.) by the Ministry of Education, Culture, Sports, Science and Technology(MEXT) of Japan.

References
Ashjari M., Mahdavian A.R., Ebrahimi N.G., Mosleh Y., Efficient dispersion of magnetite nanoparticles in the polyurethane matrix through solution mixing and investigation of
the nanocomposite properties, Journal of Inorganic andOrganometallic Polymers and Materials, 20 (2010) 213–219.
Dong Y., Ma Y., Zhai T., Shen F., Zeng Y., Fu H., Yao J., Silver
nanoparticles stabilized by thermoresponsive microgel particles: Synthesis and evidence of an electron donor-acceptor
effect, Macromolecular Rapid Communications, 28 (2007)
2339–2345.
Du L., Wang Y., Luo G., In situ preparation of hydrophobic
CaCO3 nanoparticles in a gas–liquid microdispersion process, Particuology, 11 (2013) 421–427.
Fan X., Mcleod M.C., Enick R.M., Roberts C.B., Preparation ofsilver nanoparticles via reduction of a highly CO2-solublehydrocarbon-based metal precursor, Industrial & Engineering Chemistry Research, 45 (2006) 3343–3347.
Guha S., Mandal B.M., Dispersion polymerization of acrylamide: Iii. Partial isopropyl ester of poly (vinyl methyl
ether-alt-maleic anhydride) as a stabilizer, Journal of Colloid and Interface Science, 271 (2004) 55–59.
Guskos N., Typek J., Maryniak M., Roslaniec Z., Petridis D.,
Kwiatkowska M., FMR study of γ-Fe2O3 magnetic nanoparticles in a multiblock poly (ether-ester) copolymer matrix,
Materials Science-Wroclaw-, 23 (2005) 971–976.
Hait S., Chen Y., Optimization of milling parameters on thesynthesis of stearic acid coated CaCO3 nanoparticles, Journal of Coatings Technology and Research, 11 (2014) 273–
282.
Hajdú A., Szekeres M., Tóth I.Y., Bauer R.A., Mihály J., Zupkó
I., Tombácz E., Enhanced stability of polyacrylate-coatedmagnetite nanoparticles in biorelevant media, Colloids andSurfaces B: Biointerfaces, 94 (2012) 242–249.
Harish S., Sabarinathan R., Joseph J., Phani K., Role of pH in
the synthesis of 3-aminopropyl trimethoxysilane stabilized
colloidal gold/silver and their alloy sols and their application to catalysis, Materials Chemistry and Physics, 127(2011) 203–207.
Hwang Y., Lee J.-K., Lee J.-K., Jeong Y.-M., Cheong S.-I., Ahn
Y.-C., Kim S.H., Production and dispersion stability ofnanoparticles in nanofluids, Powder Technology, 186 (2008)
145–153.

Ingelsten H.H., Bagwe R., Palmqvist A., Skoglundh M., Svanberg
C., Holmberg K., Shah D.O., Kinetics of the formation of
nano-sized platinum particles in water-in-oil microemulsions, Journal of Colloid and Interface science, 241 (2001)
104–111.
Inkyo M., Tahara T., Iwaki T., Iskandar F., Hogan C.J.,
Okuyama K., Experimental investigation of nanoparticle
dispersion by beads milling with centrifugal bead separation, Journal of Colloid and Interface Science, 304 (2006)
535–540.
Inkyo M., Tokunaga Y., Tahara T., Iwaki T., Iskandar F., Hogan
C.J., Okuyama K., Beads mill-assisted synthesis of poly
methyl methacrylate (PMMA)-TiO2 nanoparticle composites, Industrial & Engineering Chemistry Research, 47(2008) 2597–2604.
Joni I.M., Balgis R., Ogi T., Iwaki T., Okuyama K., Surfacefunctionalization for dispersing and stabilizing hexagonalboron nitride nanoparticle by bead milling, Colloids andSurfaces A: Physicochemical and Engineering Aspects, 388(2011) 49–58.
Joni I.M., Nishiwaki T., Okuyama K., Isoi S., Kuribayashi R.,
Enhancement of the thermal stability and mechanical properties of a PMMA/aluminum trihydroxide composite synthesized via bead milling, Powder Technology, 204 (2010)
145–153.
Joni I.M., Ogi T., Iwaki T., Okuyama K., Synthesis of a colorless suspension of TiO2 nanoparticles by nitrogen doping
and the bead-mill dispersion process, Industrial & Engineering Chemistry Research, 52 (2012a) 547–555.
Joni I.M., Ogi T., Purwanto A., Okuyama K., Saitoh T., TakeuchiK., Decolorization of beads-milled TiO2 nanoparticles suspension in an organic solvent, Advanced Powder Technology, 23 (2012b) 55–63.
Joni I.M., Purwanto A., Iskandar F., Okuyama K., Dispersion
stability enhancement of titania nanoparticles in organic
solvent using a bead mill process, Industrial & Engineering
Chemistry Research, 48 (2009) 6916–6922.
Kovalenko A., Jouhannaud J., Polavarapu P., Krafft M.P., Waton
G., Pourroy G., Hollow magnetic microspheres obtained bynanoparticle adsorption on surfactant stabilized microbubbles, Soft Matter, 10 (2014) 5147–5156.
Lan Q., Yang F., Zhang S., Liu S., Xu J., Sun D., Synergistic
effect of silica nanoparticle and cetyltrimethyl ammonium
bromide on the stabilization of o/w emulsions, Colloids andSurfaces A: Physicochemical and Engineering Aspects, 302(2007) 126–135.
Li W., Yu L., Liu G., Tan J., Liu S., Sun D., Oil-in-water
emulsions stabilized by laponite particles modified withshort-chain aliphatic amines, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 400 (2012) 44–51.
Luo K., Zhou S., Wu L., Gu G., Dispersion and functionalization of nonaqueous synthesized zirconia nanocrystals via
attachment of silane coupling agents, Langmuir, 24 (2008)
11497–11505.
Martínez S., Moreno-Mañas M., Vallribera A., Schubert U.,
Roig A., Molins E., Highly dispersed nickel and palladium
nanoparticle silica aerogels: Sol–gel processing of tethered
metal complexes and application as catalysts in the mizo-

21

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

roki–heck reaction, New Journal of Chemistry, 30 (2006)
1093–1097.
Naskar M.K., Patra A., Chatterjee M., Understanding the role ofsurfactants on the preparation of ZNS nanocrystals, Journalof Colloid and Interface science, 297 (2006) 271–275.
Nomoto H., Mori Y., Matsuo H., Barium titanate dispersion
obtained by a high pressure methods and light resistantcomposites containing the nanoparticles, Journal of the
Ceramic Society of Japan, 122 (2014) 129–133.
Ogi T., Nandiyanto A.B.D., Okuyama K., Nanostructuring
strategies in functional fine-particle synthesis towards
resource and energy saving applications, Advanced Powder
Technology, 25 (2014) 3–17.
Ouyang G., Wang K., Chen X., TiO2 nanoparticles modified
polydimethylsiloxane with fast response time and increased
dielectric constant, Journal of Micromechanics and Microengineering, 22 (2012) 074002.
Pu Z., Mark J.E., Jethmalani J.M., Ford W.T., Effects of dispersion and aggregation of silica in the reinforcement of poly(methyl acrylate) elastomers, Chemistry of Materials, 9(1997) 2442–2447.
Salajková M., Berglund L.A., Zhou Q., Hydrophobic cellulose
nanocrystals modified with quaternary ammonium salts,
Journal of Materials Chemistry, 22 (2012) 19798–19805.
Saterlie M.S., Sahin H., Kavlicoglu B., Liu Y., Graeve O.A.,
Surfactant effects on dispersion characteristics of copper-­based nanofluids: A dynamic light scattering study, Chemistry of Materials, 24 (2012) 3299–3306.
Schilde C., Kampen I., Kwade A., Dispersion kinetics of nanosized particles for different dispersing machines, Chemical
Engineering Science, 65 (2010) 3518–3527.
Schilde C., Mages-Sauter C., Kwade A., Schuchmann H., Efficiency of different dispersing devices for dispersing nanosized silica and alumina, Powder Technology, 207 (2011)353–361.
Shen Z.-G., Chen J.-F., Zou H.-K., Yun J., Dispersion of nanosized aqueous suspensions of barium titanate with ammonium polyacrylate, Journal of Colloid and Interface Science,
275 (2004) 158–164.
Sondi I., Goia D.V., Matijević E., Preparation of highly concentrated stable dispersions of uniform silver nanoparticles,
Journal of Colloid and Interface Science, 260 (2003) 75–81.
Tahara T., Imajyo Y., Nandiyanto A.B.D., Ogi T., Iwaki T.,
Okuyama K., Low-energy bead-milling dispersions of rodtype titania nanoparticles and their optical properties,
Advanced Powder Technology, 25 (2014) 1492–1499.
Tahara T., Inkyo M., Imajyo Y., Okuyama K., Low energy dispersion of titania nanoparticles by dual axes beads mill
Journal of the Society of Powder Technology, Japan, 48(2011) 198–205.
Takahashi M., Hayashi J., Suzuki S., Ishigure Y., Improvementof the rheological properties of the zirconia/polypropylene
system for ceramic injection moulding using coupling

22

agents, Journal of Materials Science, 27 (1992) 5297–5302.
Takeda M., Tanabe E., Iwaki T., Yabuki A., Okuyama K.,
High-concentration transparent TiO2 nanocomposite films
prepared from TiO2 nanoslurry dispersed by using beadmill, Polymer Journal, 40 (2008) 694–699.
Takeda M., Tanabe E., Iwaki T., Yabuki A., Okuyama K.,
Preparation of nanocomposite microspheres containing
high concentration of TiO2 nanoparticles via bead mill dispersion in organic solvent, Chemistry Letters, 38 (2009)
448–449.
Tang B.Z., Geng Y., Lam J.W.Y., Li B., Jing X., Wang X., Wang
F., Pakhomov A., Zhang X., Processible nanostructured
materials with electrical conductivity and magnetic susceptibility: Preparation and properties of maghemite/polyaniline nanocomposite films, Chemistry of Materials, 11 (1999)
1581–1589.
Thünemann A.F., Wendler U., Jaeger W., Schnablegger H.,
Nanoparticles of polyampholyte-surfactant complexes withperfluorododecanoic acid, Langmuir, 18 (2002) 4500–4504.
Wang W., Song Y., Liu Q., Yang K., Facile synthesis and catalytic properties of silver colloidal nanoparticles stabilized
by SDBS, Bulletin of Materials Science, 37 (2014) 797–803.
Warad H., Ghosh S., Hemtanon B., Thanachayanont C., Dutta J.,
Luminescent nanoparticles of Mn doped ZnS passivatedwith sodium hexametaphosphate, Science and Technologyof Advanced Materials, 6 (2005) 296–301.
Yang X., Liu Z.-H., A kind of nanofluid consisting of surface-­functionalized nanoparticles, Nanoscale Research Letters,
5 (2010) 1324–1328.
Yang J., Zhang Z.-Z., Men X.-H., Xu X.-H., Superoleophobicity
of a material made from fluorinated titania nanoparticles,
Journal of Dispersion Science and Technology, 32 (2011)
485–489.
Zhang Y., Chen Y., Westerhoff P., Hristovski K., Crittenden J.C.,
Stability of commercial metal oxide nanoparticles in water,
Water research, 42 (2008) 2204–2212.
Zhang Y., Kohler N., Zhang M., Surface modification of superparamagnetic magnetite nanoparticles and their intracellular uptake, Biomaterials, 23 (2002) 1553–1561.
Zhang Z., Patel R.C., Kothari R., Johnson C.P., Friberg S.E.,
Aikens P.A., Stable silver clusters and nanoparticles prepared in polyacrylate and inverse micellar solutions, The
Journal of Physical Chemistry B, 104 (2000) 1176–1182.
Zong B.-Y., Ho P., Zhang Z.-G., Ng G.-M., Yao K., Guo Z.-B.,
Polypyrrole: FeOx·ZnO nanoparticle solar cells with breakthrough open-circuit voltage prepared from relatively stableliquid dispersions, RSC Advances, 4 (2014) 58608–58614.
Zulhijah R., Suhendi A., Yoshimi K., Kartikowati C.W., Ogi T.,
Iwaki T., Okuyama K., Low-energy bead-mill dispersion of
agglomerated core–shell α-Fe/Al2O3 and α”-Fe16N2/Al2O3
ferromagnetic nanoparticles in toluene, Langmuir, 31 (2015)
6011-6019.

Takashi Ogi et al. / KONA Powder and Particle Journal No. 34 (2017) 3–23

Author’s short biography
Takashi Ogi
Takashi Ogi is an associate professor in Chemical Engineering at Hiroshima University. He received his PhD degree in Chemical Engineering in 2008 from Hiroshima University. He was a JSPS researcher from 2006 to 2008 and an assistant professor at Osaka
Prefecture University from 2008–2010. His current research interests include synthesisof nanostructured particles materials via liquid phase and spray method and recovery ofrear metal using biosorption. Especially, he focuses on the development of rare earth
free/less nanomaterials, and recovery of rare metal.

Rizka Zulhijah
Rizka Zulhijah is a doctor student at Hiroshima University supervised by Prof. Kikuo
Okuyama. She received her BS and MS in Institut Teknologi Sepuluh Nopember
Surabaya. Her research focus is magnetic nanoparticles. During her doctoral degree she
received research support grant from Hosokawa micron foundation.

Toru Iwaki
Toru Iwaki is a researcher engaged in NEDO project on new magnet developmentfrom magnetic nanoparticles at graduate School of Engineering of Hiroshima University.
He received Dr. Sci. at Hiroshima University in 1973. He was a chemical assistant at the
University before working at Hiroshima R&D Center of Mitsubishi Heavy Industries
1984, Toda Kogyo 1998 and Kotobuki Kogyo 2006. His research area is covering surface
and colloid science for nanomaterial applications.

Kikuo Okuyama
Kikuo Okuyama is a Professor of Chemical Engineering at Hiroshima University. He
received his doctoral degree in 1978 in chemical engineering at Osaka Prefecture University. His research has touched many aspects of aerosol science and technology, fromfundamental investigations on aerosol dynamic behavior to the development of aerosol
measurement equipments with a recent focus on nanomaterial synthesis. Prof. Okuyama
has received numerous honors and awards, including the Fuchs Memorial Award in
2002, and KONA Award in 2007. He has coauthored more than 450 scientific papers, 120
review papers, 50 books/chapters, and 70 patents.

23

KONA Powder and Particle Journal No. 34 (2017) 24–43/Doi:10.14356/kona.2017001

Review Paper

Present Advancement in Production of Carbon Nanotubes
and Their Derivatives from Industrial Waste
with Promising Applications †Konrat Kerdnawee 1, Chompoopitch Termvidchakorn 1, Pacharaporn Yaisanga 1,
Jirapat Pakchamsai 1, Cheewapon Chookiat 1, Apiluck Eiad-ua 2, Winadda Wongwiriyapan 2,
Weerawut Chaiwat 3, Sakhon Ratchahat 4, Kajornsak Faungnawakij 5, Komkrit Suttiponparnit 6
and Tawatchai Charinpanitkul 1*
1

Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Thailand
College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Thailand
3
Environmental Engineering and Disaster Management Program, Mahidol University, Thailand
4
Department of Chemical Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology, Japan
5
National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Thailand
6
Environmental Research and Management Department, PTT Research and Technology Institute, Thailand
2

Abstract
An increase in global consumption has led to an exponential increase in industrial production activities which
inevitably results in overwhelming remain of industrial waste. Consequently it has driven increasing attentions ofresearch and development teams in various countries to propose and investigate novel methodologies to utilize
such industrial waste. Instead of using as alternative energy sources, usage of industrial waste for production ofcarbonaceous nanomaterials has been examined via various routes, such as catalytic pyrolysis, hydrothermal
treatment and so on. Meanwhile, for sustainable and secure continuity of the carbonaceous nanomaterial
production, broad spectra of promising applications have also been examined. Among those emerging
applications, utilization of carbonaceous nanomaterials in pollution control and prevention has been focusedworldwide. Therefore, in this review, relevant research works focusing on catalytic pyrolysis of carbonaceous
industrial waste for carbonaceous nanomaterial production were comprehensively analyzed and summarized. In
addition, promising applications involving with antibiotic removal, spilled oil handling and pollutant gas detection
were also reviewed.
Keywords: carbon, nanomaterial, industrial waste, synthesis, applications

1. Introduction
An increase in global consumption has led to an exponential increase in industrial production activities which
has inevitably resulted in overwhelming remain of industrial waste. Such industrial waste management is the
major consideration, especially plastics waste like poly†

Received 1 October 2015; Accepted 4 November 2015
J-STAGE Advance published online 30 January 2016
1
Payathai Rd., Patumwan, Bangkok 10330, Thailand
2
Chalongkrung Rd., Ladkrabang, Bangkok 10520, Thailand
3
Kanchanaburi Campus, Saiyok, Kanchanaburi 71150, Thailand
42-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan5
111 Thailand Science Park, Paholyothin Rd., Klong Nueng, Klong
Luang Pathumthani 12120, Thailand
6
71 Moo 2, Phaholyothin Rd., Km.78, Sanubtueb, Wangnoi,
Ayutthaya Province 13170, Thailand* Corresponding author: Tawatchai Charinpanitkul;
E-mail : ctawat@chula.ac.th
TEL: +66-2-218-6894

24ethylene, polypropylene, and so on. Consequently it has
driven increasing attentions of research and development
teams in various countries to propose and investigate
novel methodologies to utilize such industrial waste. Instead of using as alternative energy sources to reduce environmental problems from such industrial waste, theirusage for production of carbonaceous nanomaterials has
been examined via various routes, such as catalytic pyrolysis, hydrothermal treatment and so on (Afre et al., 2006;
Liu et al., 2006; Vijayaraghavan and Stevenson, 2007;
Zhang et al., 2008; Charinpanitkul et al., 2009a; Zhuo etal., 2010; Alves et al., 2011; Li et al., 2012; Mishra et al.,2012; Sano et al., 2012; Yang et al., 2012; Bajad et al.,2015).
Since the significant findings of new forms of carbonaceous nanomaterials, including buckminsterfullerene(C60) discovered by Kroto et al. in 1985 and carbon nanotubes (CNTs) reported by Iijima in 1991, considerable re-

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

search and development (R&D) efforts have been madetoward the synthesis and applications of functional carbonaceous nanomaterials with diverse structures and morphologies, such as nanotubes, nanohorns, nanocapsules,
nano-onions, nanofibers, coin-like hollow carbon, andgraphenes (Kroto et al., 1985; Iijima, 1991; Bandow et al.,
2000; Thostenson et al., 2001; Andrew et al., 2002; Dai,
2002; Sano et al., 2003; Li et al., 2003; Jang and Bae,
2004; Sano et al., 2004; Sun and Li, 2004; Xiao et al.,2006; Yao et al., 2007; Yuan et al., 2007; de Volder et al.,2013). Such carbonaceous nanomaterial has become ofinterest for many researchers because of its unique andoutstanding properties. Table 1 would give an outlook of
appropriate method which could provide some specificcarbonaceous nanomaterials (Dai 2002; Popov 2004;
Bazargan and McKay 2012; Iijima, 2012). Each synthesizing technique would provide a variety of carbonaceous
nanomaterials with different advantages and disadvantages with respect to production yield, purity, and handling. For instance, arc discharge method with or without
usage of catalyst would be appropriate for production of
SWCNTs, MWCNTs and fullerenes but scale-up would
be a drawback of this method. Meanwhile, chemical vapor deposition (CVD) technique would be very flexible
for producing various carbonaceous nanomaterials except
carbon nanohorns (CNHs). In term of synthesized products, besides SWCNTs and MWCNTs, graphene and carbon nanohorns (CNHs) have recently become research
topics which also draw attention from many researchteams. Anyway, a large variety of products at different
synthesizing parameters implies that the so-called self-­
assembly reactions are complicated. The detailed mechanism has not been fully understood at the present time.
Therefore, many of recent research and development efforts
have been paid for elucidating the formation mechanismof such carbonaceous nanomaterial with respect to eachcategory because such knowledge could be useful for regulating the production cost and their key characteristics.

Unless their actually economic production processes are
realized, commercial applications of carbonaceous nanomaterial would not reach their full potential (Thostenson
et al., 2001; Dai, 2002; Zhang et al., 2003; Parkansky etal., 2004; Montoro et al., 2005; Kusaba and Tsunawaki,
2006; Guo et al., 2007; Tsai et al., 2009; Lebel et al., 2010;de Volder et al., 2013; Gong et al., 2014). Therefore, there
is still much requirement for development of such carbonaceous nanomaterial production from cheaper and stable
raw materials of various kinds.
Nevertheless, among those nanomaterials, carbon
nanotubes (CNTs) have attracted widespread interest, owing to their unique structures and extraordinary properties in electrical, optical and mechanical properties, which
could lead to many highly promising applications including adsorbents, atomic force microscopy (AFM) tips, catalyst or catalyst supports, electrodes for fuel cell anddye-sensitized photovoltaic cells, field emission displays(FED), hydrogen storage, nano-electronic devices,
strength reinforcing fillers in polymers (Colomer et al.,
2000; Barreiro et al., 2006; Paradise and Goswami, 2007;
Arora and Sharma, 2014). In addition, some of their derivatives, such as metal-hybridizing carbon nanotubes arealso recognized as novel nanomaterials with some promising applications, such as magnetic recording, medical
diagnosis, selective adsorbents and supercapacitors (deVolder et al., 2013). As a result sizable production of CNTs
and their derivatives have been continuously proposed andexamined for more than two decades. As mentioned above
various methods for CNT production would include laser
ablation, arc discharge, chemical vapor deposition (CVD),
electrolysis, and hydrothermal treatment. A remarkableamount of research attempts have been made toward comparison in various aspects, such as production yield, purity, operating cost, and scalability (Auer et al., 1998;
Bandow et al., 2000; Colomer et al., 2000; Serp et al.,
2001; Andrew et al., 2002; Sano et al., 2003; Popov, 2004;
Barreiro et al., 2006; Xiao et al., 2006; Zeng et al., 2006;

Table 1 Comparison of typical synthesizing techniques and carbonaceous nanomaterials
Structure

SWCNT

MWCNT

Fullerene

CNH

Graphene

Arc discharge

x

o

o

o

x

Arc discharge with catalyst

o

o

o

x

x

Laser deposition

x

x

o

o

x

Laser deposition with catalyst

o

o

o

x

x

CVD (Pyrolysis)

o

o

x

x

x

CVD with catalyst

o

o

x

x

o

CVD (Plasma with catalyst)

o

o

o

x

o

TechniqueNote: o represents “applicable”
x represents “not applicable”

25

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Grobert, 2007; Charinpanitkul et al., 2009b; Puengjinda
et al., 2009; Quan et al., 2010; Charinpanitkul et al., 2011).
Generally, all of such CNT production methods would require some similar handling process conditions, such as
high temperature condition for preparing carbon precursors from decomposition of carbon sources following by
the self-assembly processes which could be regulated byvarious operating parameters. With different controllingparameters, different types of CNTs, which are mainly
categorized into single-walled carbon nanotubes (SWCNTs)
and multi-walled carbon nanotubes (MWCNTs), could be
produced (Kim et al., 2003; Qiu et al., 2010; Altalhi et al.,2013).
Among various synthesizing methodologies, as compared in Table 1 chemical vapor deposition (CVD) or
thermal pyrolysis is recognized as an efficient mean because of its advantages in large-scale production withrelatively low operating costs and excellent controllability
in orientation, alignment, length, diameter, purity anddensity of resultant CNTs. Many technical reports haveindicated that the thermal pyrolysis of organometallic
compounds, such as ferrocene, iron phthalocyanine, andnickel phthalocyanine can be employed as either reactants
or catalysts for production of carbon nanotubes and nanocapsules (Afre et al., 2006; Liu et al., 2006; Vijayaraghavan
and Stevenson, 2007; Charinpanitkul et al., 2009a; Zhuo
et al., 2010; Sano et al., 2012; Li et al., 2012; Yang et al.,2012). With the thermal pyrolysis, some metal species,
such as Fe, Ni or Co, in those compounds are well recognized to play an important role as catalyst for CNP growth(Auer et al., 1998; Li et al., 2003; Zeng, et al., 2006; Yuan
et al., 2007; Morales et al., 2013; Nahil et al., 2015). Many
novel methods to incorporate necessary catalyst with carbon sources are also the on-going research issues whichhave been reported by various research teams around theworld. Investigation on tuning of synthesizing parameters
for optimized productions of carbonaceous nanomaterials
has been one of the most concentrating topics for researchteams in many countries.
Meanwhile, it has been well accepted that utilization ofalternative carbon sources available in gas, liquid, or solid
phase by introducing over catalytic substrates or pre-mixing
with catalyst supply would possibly increase production
yield, improve CNT quality, or minimize the amount ofexpensive catalyst usage (Liu et al., 2006; Vijayaraghavan
and Stevenson, 2007; Yuan et al., 2007; Zhang et al., 2008;
Zhang et al., 2008; Quan et al., 2010; Zhuo et al., 2010;
Alves et al., 2011; Sano et al., 2012; Li et al., 2012; Mishra
et al., 2012; Yang et al., 2012; Sawant et al., 2013; Bajad etal., 2015). High purity carbon sources (> 99.9 %), such as
CO, CO2, alcohols, and various hydrocarbons (methane,
ethylene, acetylene, n-butane, hexane, benzene, toluene,
xylene) have been extensively utilized for synthesizing
CNTs with specific know-how relevant to synthesizing

26

and purifying parameters which are synthesizing temperature and pressure, residence time in reactors, ratio ofcarbon source to catalyst and so on. However, such carbon
sources are still expensive and present in limited supply,
resulting in elevating attentions to search for alternative
carbon sources, which include industrial wastes, municipal wastes (i.e. plastics wastes), food wastes, and low value-added industrial by-products (red oil, glycerol, slopoil). Instead of using as alternative energy sources, such industrial wastes or by-products could be considered as oneof good alternatives for production of carbonaceous nanomaterials (Liu et al., 2006; Vijayaraghavan and Stevenson,
2007; Yuan et al., 2007; Zhang et al., 2008; Quan et al.,2010; Zhuo et al., 2010; Alves et al., 2011; Li et al., 2012;
Yang et al., 2012; Altalhi et al., 2013; Acomb et al., 2014;
Gong et al., 2014; Bajad et al., 2015). Table 2 summarizes
some previous works which have made use of industrial
waste as raw material for production of carbon nanotubes(CNTs) and derivatives. It could be clearly observed thatsome specific catalysts would be essentially required for
production of CNTs with different characteristics. Among
operating conditions, synthesizing temperature would
also be a key parameter for converting raw materials
into reactive carbon clusters which would undergo theself-assembly process to generate carbon nanotubes (Leeet al., 2003; Parkansky et al., 2004). Without catalysts,
there is high possibility that the resultant products would
be carbon black or amorphous carbon nanoparticles (Liuet al., 2006; Zhuo et al., 2010; Li et al., 2012). In addition,
because of uncertain compositions, different physical andchemical properties, such industrial wastes or by-products
contain some unknown impurities, such as sulfur whichis expected to severely affect the synthesizing process,poisoning of catalyst, corrosion of reactor wall and so on.
Some counter-measures have been proposed and examined for improving the productivity of carbonaceous
nanomaterials from those industrial wastes.
Therefore, in this review, relevant research works focusing on catalytic pyrolysis or chemical vapor depositionof carbonaceous industrial waste for carbon nanomaterial
production were comprehensively analyzed and summarized. Synthesizing parameters, such as molar ratio of organometallic catalyst to carbon source, types of catalysts
and carbon sources, reaction temperature, were comprehensively examined for improving the synthesis efficiency. Furthermore, some promising applications
involving with antibiotic removal, spilled oil handling andpollutant gas detection were also taken into account.

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Table 2 Some referable works involving with synthesis of CNTs using industrial wastes
Characteristics

Synthesizing parameters
ReactorType

Temperature(°C)

CarrierGas

Co and Fe

QuartzTube

500–900

N2

Petroleumrefining process
waste

Deoiled Asphalt Ferrocene

QuartzTube

700–1200

Ar

Bajad et al.,
2015

Plastic waste

Polypropylene

Ni/Mo/MgO

MuffleFurnace

700–900

Charinpanitkul
et al., 2009

Chemicalprocess waste

Naphthalene

Ferrocene

QuartzTube

600–1000

Zhuo et al.,
2010

Plastic waste

HDPE

304 Stainlesssteel with Co,
Ni or No

Quartz
Tube

800

Sano et al.,2012

Petroleumrefining process
waste

Ethylene

316 StainlessSteel

QuartzTube

700

Li et al.,2012

Petroleumrefining process
waste

Heavy Oil
Residue

Fe, Co, Ni,
Au and Pt

QuartzTube

Yang et al.,2012

Automobile
waste

Scrap Tyre
RubberCo, Mg, Mn,
Al

Morales et al.,2013

Petroleumrefining process
waste

Acetylene

FeAl2O4

SourceFeed

Catalyst

Afre et al.,2006

Chemicalprocess waste

Turpentine Oil

Liu et al.,
2006

ReactionTime

LengthDiameterNR

2.5 nm20–50 minNR

180 nm

10 minNR

2–25 nm

15 minSeveralMicrons> 100 nm

1 min

1–5 μm

15–84 nm

H2

30 min5–80 μm 3–10 nm

900

H2

20 minNR

0.89–
1.19 nm

Quartz
Tube

650

H2

60 minNR

30–50 nm

QuartzTube730

H 2/N260 minNR

2–45 nmN2

Note: NR represents “not reported”

2. Present advancement of carbonaceous
nanomaterial production from industrial
waste
A comprehensive search of international journal publications was carried out using keywords of carbonaceous
nanomaterial, industrial waste, and applications throughthe SCOPUS database. It should be noted that most of
motivations for industrial waste utilization in those previous works are focused on requirement of reducing greenhouse gases (GHGs) in the atmosphere for mitigation of
the global warming effect. It is commonly accepted thatthe combustion of fossil fuels is responsible for 73 % of
CO2 emission thereby partial replacement of fossil fuelsby bioethanol would enable a reduction in the levels of
CO2 emissions. According to the Renewable Fuels Association (RFA), the world bioethanol production in 2008
was approximately 65.6 billion liters (17.3 billion gallons),
representing a 24 % increase in the production than theprevious year (Yuan et al., 2007; Alves et al., 2011).
Among various environmental concerns, managementof plastic waste has posed a serious challenging issue forour society because they make up a significant proportionof municipal waste, typically around 10 wt% (Altalhi etal., 2013). Even though recycling rates for waste plastics
have recently increased there is still a large remaining
amount which ends up by being unsustainably disposedin landfill sites (Zhang et al., 2008; Mishra et al., 2012;
Bajad et al., 2015; Nahil et al., 2015). In almost every
country, the governmental sector has enforced more serious criteria for such industrial waste handling, such as
limitation or avoidance of landfilling while encouraging
promotion of waste management techniques are provided.
In order to develop a more economically competitive usage, it is necessary to develop new methods to use in­dustrial waste as replaced resources. Therefore, one ofpromising alternatives is to utilize such industrial waste
containing carbonaceous content as feed stock for CNT
production. If a feasible process could be developed fortransforming those available carbon-rich wastes into highvalue-added products like CNTs, economic benefits could
be made through simultaneously solving the plastic
wastes and CNT production shortcomings. It seems thatthe amount of published studies attempting to synthesize
CNTs from such industrial wastes were scattered andlimited because most of them were exclusive projects or27

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

announced in limited venues. So far serious issues including the variety and fluctuation of chemical compositions
within such waste have been explored. It seems that thebroad spectra of carbon-containing compounds, such as
CH4, C2H6, C2H4, C2H2, CO, CH3OH, and so on, would
involve as carbon sources for productions of CNPs (Bai etal., 2003; Lee et al., 2003; Chaisitsak et al., 2007; Zhang
et al., 2008; Gong et al., 2014; Bajad et al., 2015). Furthermore, regarding to a common issue on using specific reactor system which would be promising for scalable
production, some typical reports available in the publicdomain could be summarized in Table 2. It should be
noted that various types of raw materials existing in gas,liquid and solid phases could be utilized for CNT synthesis with the assistance of various kinds of catalysts, suchas ferrocene, nickel, cobalt and iron (Afre et al., 2006;
Liu et al., 2006; Vijayaraghavan and Stevenson, 2007;
Charinpanitkul et al., 2009b; Zhuo et al., 2010; Sano etal., 2012; Li et al., 2012; Yang et al., 2012; Morales et al.,2013). Those information would be good evidences thatutilizing chemical vapor deposition or catalytic pyrolysis
is a promising method because of its advantages onpractical scalability and handling (Colomer et al., 2000;
Grobert, 2007; Bazaegan and McKay, 2012; Mishra etal., 2012). In addition, it has been recognized that the industrial CNT manufacturing processes become sustainable and inherently safe though it would be sensible to use
readily-available alternative fuels, such as existing waste
streams. At the moment, since the production of CNTs
has been successfully transferred from laboratory-scale toindustrial-scale, their worldwide market is expected to
grow from $215 million in 2009 to over $9 billion by 2020
(Iijima, 2012; Li et al., 2012; Nahil et al., 2015).

2.1 Gaseous carbon source
Hydrocarbons, such as methane, acetylene, ethylene,
ethane, propane and some of gaseous hydrocarbons havebeen experimentally confirmed as carbon sources for
CNT synthesis (Satishkumar, et al., 1999; Endo, et al.,
2001; Bai, et al., 2003; Emmenegger, et al., 2003; Lee, etal., 2003; Liu, et al., 2003; Kim, et al., 2003; Lu et al.,2005a; Paradise and Goswami, 2007; Morales, et al.,2013). As a category of CVD synthesis, combusting or
pyrolytic synthesis has the advantage of being an exothermic process which such carbon source can be utilized forin situ synthesis by means of decomposition and catalytic
growth of CNTs. The composition of hydrocarbon gasses
from those industrial processes would be varied fromprocess to process. For example, in petrochemical downstream processing, the hydrocarbon gasses from themethane reformer would contain high content of CH4, CO
and CO2, while those from refining units would consist oflarge compositions of BTEX, propane and ethylene. The

28

current strategy for using these hydrocarbon gases is toinstall the carbon-material production unit on-board
where the hydrocarbon sources are produced (Alves et al.,2011; Bazargan and McKay, 2012; Mankhand et al., 2012;
Yang et al., 2012; Acomb et al., 2014). The use of suitablecombustion equipment and catalyst as well as implementation of appropriate operating conditions, such as carbon
source, flow of inert gas, and operating pressure, could
realize controllable synthesis of CNTs with specifiablecharacteristics, such as diameter, length and purity (Liu,et al., 2003; Paradise and Goswami, 2007; Prasek, et al.,2011; Morales, et al., 2013).
Satishkumar et al. (1999) proposed to synthesize
MWCNT bundles using pyrolysis of methane, acetylene
or butane with the presence of ferrocene as catalyst
precursor. The MWCNT bundles associated with nominal diameter in a range 2–13 nm contain iron nanoparticles inside their walls. As a result, such MWCNTs
exhibit ferromagnetic behavior showing low saturation
magnetization when compared to bulk iron. Meanwhile,
Emmenegger et al. (2003) have synthesized MWCNTs
from decomposition of C2H2 over a thin catalytic film ofiron-oxide substrate. C2H2 concentration, time of deposition, temperature and ratio of C2H2 to N2 are regulated tocontrol the resultant MWCNT length. It is found that
Fe2O3 is reduced to Fe3O4 and FeO after contacting with
C2H2:N2 flow, resulting in formation of MWCNTs withtips anchoring on iron carbide clusters. Later, Lee et al.
(2003) could synthesize well-aligned MWCNTs with highpurity by pyrolysis of C2H2 at 800 °C with the presence ofiron (II) phthalocyanine. The synthesized MWCNTs exhibit a bamboo-like structure with good crystallinity. It isalso reported that the growth rate of well-aligned
MWCNTs could be enhanced with an increase in C2H2
concentration. With those typical examples, such pyrolytic method has been recognized as one of the most practical routes appropriate for mass production of MWCNTs.
As a result, most of commercially available MWCNTs aresynthesized by many manufacturers, such as Hyperion
Catalysis International, Inc. (Cambridge, MA). Meanwhile, Carbon Nanotech Research Institute (CNRI), a
subsidiary of Mitsui Chemical has planned to develop alarge scale production of 120 t of MWCNTs per annum.
In addition, Applied Sciences, Inc. (API) and Showa
Denko (SDK), have been successful in a large scale production of MWCNTs with relatively large diameters and awide distribution in the range 70–200 nm. However, it
should be noted that synthesis of those carbon nanomaterials from such gaseous raw materials would possibly be
upset by the unstable supply.

2.2 Liquid carbon source
Instead of gaseous raw materials, various kinds of liq-

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

uid containing carbon atoms, for example acetone, benzene, ethanol, toluene and high-molecular weighthydrocarbons, have been employed for synthesizing carbonaceous nanomaterials with different properties. Somekinds of liquid carbon sources would be intentionally prepared while others would be available as liquid by-products
or wastes received from some specific industrial processes(Ago et al., 2000; Endo et al., 2001; Afre et al., 2006;
Alves et al., 2011; Li et al., 2012). For investigation of formation mechanism, Ago et al. (2000) used a vertical flow
reactor for synthesizing single and multi-walled carbon
nanotubes by the catalytic reaction of colloidal solutionsof metal nanoparticles. They made use of a reverse micelle solution of Co–Mo nanoparticles dissolved in toluene as raw material and injected the solution directly intoa reactor of which temperature was maintained at
1200 °C. Therefore, it could be implied that such liquid
precursor would be vaporized to provide clusters of carbon atoms as building blocks (Sun and Li, 2004; Yuan etal., 2007; Sowichai et al., 2012). The growth of SWCNTs
was observed when the small amount of thiophene wasadded in the colloidal solution (1 wt.%). However, when
they added 10 wt.% of thiophene, only MWCNTs can begrown from the self-assembly process. For mechanism investigation, it was found that carbon nanotubes contained
metal nanoparticles encapsulated at the tip so that the tipgrowth model was discussed.
Meanwhile, in many countries petrochemical industries
have also produced some useful products necessary forour daily life but they also generate some hydrocarbon
wastes which are mainly in liquid form. In general, as-­received heavy hydrocarbon residue which is a highly
aromatic material received as a bottom product after the
distillation of petroleum could also be fed into a furnace
or boiler for the generation of heat or “lighting up” facility in many coal-fired power plants. Certainly, suchby-product of petroleum industries has great potential as
feedstock in making carbon materials because it contains
a large amount of carbon-rich molecules. As a typical example, slop oil which is a residual waste obtained from apetrochemical plant in Thailand was fractionated using alaboratory distilling unit could be separated into somefractions based on their boiling points as shown in Fig. 1.
Light fraction contains hydrocarbons with boiling point
lower than 170 °C while the middle and large factions are
separated at 150–320 and > 300 °C, respectively. It could
be clearly observed that the fractionated hydrocarbons
exhibit different appearance because of their compositions
which could be decomposed for generating carbon clusters
after subject to sufficient thermal energy (Endo et al., 2001;
Andrews et al., 2002; Dai, 2002). Anyway, the main drawback of residual hydrocarbons is its high initial viscosity. In
addition, it usually contains relatively high amounts of pollutants and particularly sulfur which forms sulfur dioxideFig. 1

Physical appearance of each fraction of liquid hydrocarbon waste (slop oil) obtained from petrochemical
process.

upon combustion. Li et al. (2012) reported that the growthtemperature plays an important role in determining theformation of SWCNTs. However, different characteristicsof SWCNTs synthesized from heavy oil when comparedwith the small hydrocarbons would be attributed to theformation process which did not proceed through C2 carbon units. It should also be noticed that effects of impurities in such waste on the synthesis of CNPs would be ofconcern so that many current strategies have been explored by adapting from some existing technologies. For
example, the bottom product of a crude distillation unit ina petroleum refinery process usually contains high contents of sulfur and mercury compounds (Mohammed etal., 2012; Zubaidy et al., 2013; Khairi et al., 2015). Typically, various methods including chemical adsorption, gas
stripping and chemical precipitation are employed for removing mercury from crudes and other hydrocarbon liquids to avoid the problems of poisoning. Meanwhile,sulfur removal with sorbent beds or chemical scrubbing
has also been used with new supporting methods, such as
microwave irradiation, for scavenging the sulfur content
from liquid hydrocarbons in prior to their processing. In
addition, effect of hydrocarbon chain length on the synthesis and the final quality of CNTs is also important issue though it is still ambiguously understood. Basically,
the longer chain hydrocarbons would possibly be decomposed, resulting in formation of a broad spectrum ofsmall hydrocarbons and carbon clusters with different
specific molecular weights. Such hydrocarbons would undergo different reaction routes, resulting in formation ofeither arranged graphitic nanostructures or amorphous
forms (Alberts 1997; Endo et al., 2001; Andrews et al.,
2002; Dai 2002; Prasek et al., 2011; Mishra et al., 2012).
In comparison with other carbon sources, such as methane, acetylene and ethanol, those hydrocarbons withbroad molecular weight distribution would provide both
crystalline and amorphous carbon nanostructures as
could be confirmed by Raman spectroscopic analyses(Lee et al., 2003; Liu et al., 2003; Puengjinda et al., 2009).

29

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Fig. 2

Schematic diagram of reactor system for synthesizing carbonaceous nanomaterials from hydrocarbon feedstock.

Therefore, some pre-treatment would be essential for controlling the final quality or purity of CNPs which are synthesized from those hydrocarbon wastes.
In general, a typical set-up employed for synthesizing
carbonaceous nanomaterials from such hydrocarbons
would consist of a reactor equipped with raw material
feeding and product collection as shown in Fig. 2. Feedstock would be introduced into the reactor by manymeans, such as spraying nozzle, ultrasonic nebulizer oreven static container (Vilatela et al., 2015; Chaisitsak etal., 2007; Charinpanitkul et al., 2009b). Some inert carriergas would be supplied into the reactor for carrying thehydrocarbon precursor which would be vaporized by regulated heating with a designated ramp rate for ensuring
the steady flow of carbon sources into the reaction zone.
The self-assembly reaction would be stimulated by a control of sufficiently high temperature zone within the reactor(Colomer et al., 2000; Barreiro et al., 2006; Charinpanitkul
et al., 2009b; Puengjinda et al., 2009; Sano et al., 2012).
During the formation of carbonaceous nanomaterials
within the reactor, complicated phenomena, such as convective diffusional flow, thermophoresis and decomposition, would play a competitive role in regulating the
production yield and characteristics of the resultant products. As a typical example, SEM micrographs of carbonaceous nanomaterials synthesized from fractionatedhydrocarbon wastes and its mother liquor (slop oil) weredepicted in Fig. 3. It could be clearly observed that synthesized products collected from different location of thereactor exhibit different apparent characteristics. Carbonaceous nanomaterials collected from Zone 1 where temperature was a little bit higher than the boiling point ofslop oil mainly consist of spherical carbon nanocapsules(CNCs) which contain metal nanoparticle inside. Meanwhile, the highest thermal energy supply in Zone 2 would
result in formation of well aligned MWCNTs with rather

30

uniform diameter because of enhanced self-assembly reaction. However, at Zone 3 where the remaining precursors would further undergo the self-assembly reaction but
would be discouraged by the convective flow, resulting information of a mixture of MWCNTs and amorphous carbon nanospheres (CNSs). Further analyses on characteristics of resultant products obtained from different locationin the reactor in cooperation with synthesizing conditions(temperature, ratio of feedstock and catalyst, flow rate ofcarrier gas and so on) would be taken into account for
elucidating the formation mechanism of such carbonaceous nanomaterials. Most of previous works agree with apostulation that metal catalysts which would be intentionally supplied into the reactor would exist in active liquid
form which would adsorb carbon molecular clusters onto
its surface and then catalyze the self-assembly reaction,
resulting in formation of carbonaceous nanostructures
which contain those catalytic metal nanoparticles inside(Bai et al., 2003; Emmenegger et al., 2003; Barreiro et al.,2006; Prasek et al., 2011). However, other concrete evidence from in situ characterization would still be requiredfor clear understanding on such phenomenon.

2.3 Solid carbon source
In general sense, plastic is considered as a nuisancewhich could provide many negative consequences, suchas destruction of mangrove ecosystem due to its non-­dissociation property in nature. Thin plastic bags have norecyclable value and are abandoned or by packing refuse
in it (Tarig et al., 2013). Converting such plastic wastes
into valuable products, such as carbon nanotubes (CNTs)
could be an important and profitable option for industry
and for environmental protection. Recently many researchworks focusing on synthesis of CNTs and other nanostructure derivatives through thermal decomposition ofKonrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Fig. 3

SEM micrographs of CNTs prepared from (a) low boiling point hydrocarbon (LHC), (b) medium boiling pointhydrocarbon (MHC), (c) high boiling point hydrocarbon (HHC) and (d) slop oil under a synthesizing temperature of 900 °C.

plastic wastes, such as polypropylene (PP), low density
polyethylene (LDPE), high density polyethylene (HDPE),
polyacrylate (PC), polyvinyl chloride (PVC), polystyrene(PS) and polyethyleneterephthalate (PET) with the presence of specific catalysts have been reported (Zhang etal., 2008; Quan et al., 2010; Zhuo et al., 2010; Mishra etal., 2012; Yang et al., 2012; Savant et al., 2013; Acomb etal., 2014; Bajad et al., 2015: Nahil et al., 2015). Withinthose previous works, usage of low-cost alternative feedstocks which are post-consumer plastic wastes for CNT
production has been demonstrated. Sufficient concentrations of carbon-containing species (either hydrocarbons
alone or a mixture of hydrocarbon, CO and CO2) as wellas hydrogen and water molecules could be generated fromthe thermal decomposition of those plastic wastes. Thenstand-alone pyrolysis of such plastic wastes or pyrolysis
followed by premixed combustion of resulting gaseous py­
rolyzates leads to CNT growth on catalytic surfaces. For
example, Zhang et al. (2008) used combustion method forsynthesis of CNTs using Ni compound/organic-modified
montmorillonite and polypropylene (PP) as a source.
They reported that with the increment of temperature, PP
began to decompose to form carbon clusters which would
diffuse around Ni nanoparticles, which could facilitate
the growth of CNTs due to root-growing mechanism (Liuet al., 2003). Later, Mishra et al. (2012) have tried to use
PE and MA-PP as precursor for synthesis of CNTs bymixing PE with PP using ferrocene or Ni as catalyst in astainless steel autoclave heated at 700 °C for 100 min.
They reported that CNTs with average diameter of about
160 nm were synthesized with negligible generation of
toxic or corrosive reagents. In addition, effect of synthesizing temperature on catalytic conversion of those plastic
wastes to carbonaceous nanomaterials could be manifested
through changes of catalyst particle size which is intentionally loaded into the synthesizing reactor (Bazargan
and McKay, 2012; Altalhi et al., 2013). As a result, it
should be noted that effluents of such plastic waste treating system are gases with substantial amounts of carbon
monoxide, light hydrocarbons and hydrogen (Acomb etal., 2014). Therefore, it may be possible to utilize suchgaseous by-products for power generation. Some new gas
turbine technologies, such as Siemens G-class Gas Turbines, could work under flexible load conditions whichsuch gaseous by-products with CO content could be effectively controlled (Engelber et al., 2004). In addition, theconversion of such gaseous by-products to reactive chemicals, such as alcohol and liquid hydrocarbons would beachievable by chemical reactions prior to its utilization as
feed materials for CNT production (Andrews et al., 2002).
These previous works would reveal that usage of suchcommodity plastic wastes with appropriate catalysts is apotential mean for synthesizing CNTs with competitive
cost. However, due to the complexity of experimental

31

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

process, exact formation process of CNTs still needs further research.
Meanwhile, because of the increasing demand of electronic applicant usage, printed circuit board (PCB), which
typically consists of 15 % epoxy resin, 30 % glass cloth
filament, 22 % copper coils, metal (Sn, Pb, Fe, Ni, etc.)
and Br, has become a serious social problem. One possible method for recovering both organic and non-organic
fractions from PCB is pyrolysis technique so that someresearch works on conversion of PCB wastes have beenreported (Jie et al., 2008; Quan et al., 2010; Mankhand etal., 2012). It is demonstrated that PCB waste pyrolysis
could provide liquid products containing high concentrations of phenol-group species. Hollow-centered andstraight CNTs with outer diameter of ~338 nm could bedirectly synthesized by pyrolysis of PCB pyrolysis oil using ferrocene as catalyst at 900 °C under nitrogen atmosphere. CNTs with nominal length of several microns
could be observed with the presence of amorphous carbon
nanoparticles (Quan et al., 2010). The external surface ofporous carbonaceous nanomaterials prepared from carbonization of KOH-treated resin at 700 °C is full of cavities with BET specific surface area of 1214 m 2/g andmicropore volume of 0.41 cm3/g, respectively.
Eventually, it should be noted that scaled-up industrial
system has also been explored especially for utilization of
useful energy released during the synthesis of carbonaceous nanomaterials from such industrial wastes. Such
exothermally released energy will offset the energy required to pyrolyze the industrial waste and to preheat thesynthesizing system, resulting in a great improvement of
the overall energy-efficiency. Thus, it would possibly help
minimize the production cost of carbonaceous nanomaterials (Thostenson et al., 2001; Paradise and Goswami
2007; Prasek et al., 2011; de Volder et al., 2013). As mentioned above, comprehensive analyses of synthesized
products within those different synthesizing process
would still be required for ensuring the economically justifiable production of such carbonaceous nanomaterials.

3. Present advancement of carbonaceous
nanomaterial applications
As mentioned above, applications of carbon nanotubes(CNTs) including their derivatives, such as carbon nanocapsules (CNCs), carbon nanohor ns (CNHs) andgraphenes have been an emerging subject of considerable
research attempts owing to their unique mechanical, thermal, electrical and optical properties (Andrews et al.,
2002; Zeng et al., 2006; Paradise and Goswami, 2007;
Iijima, 2012; de Volder et al., 2013). Owing to those advantages, CNTs hold great promise as fundamental building blocks for nanoelectronics, field emitters, drug

32

delivery system, sensors and energy storage devices. For
sustainable and secure continuity of the carbonaceous
nanomaterial production, broad spectra of promising applications have been examined by many research teams
around the world. Therefore, a substantial number ofworks done for exploring what might be the key characteristics of such carbon nanomaterials for each specificrequirement of their usage are available. Using a comprehensive collection of literatures, promising applications
involving with some specific technical requirements, suchas antibiotic removal, spilled oil handling and pollutant
gas detection were summarized in this review.

3.1 Fundamental applications
As fundamental applications, carbonaceous nanomaterials could be employed in their primitive form as they
are produced or in modified form as other composites,
which would require additional processing (Andrews etal., 2002; Yang et al., 2005; Zeng et al., 2006; Xu et al.,2007; Tusi et al., 2010; Iijima, 2012). Applications of suchcarbonaceous nanomaterials with specific purposes would
be metal catalyst support, pollutant removal and so on.
Based on our search through SCOPUS database, it seems
that there are a variety of fundamental applications but
three main categories have drawn attentions of many researchers because of some specific emerging needs fromindustrial and environmental viewpoints (Dai, 2002; deVolder et al., 2013).

3.1.1 Catalyst supports

Industrial sector generally requires robust catalysts
made of precious metal species which are generally embedded onto the surface of some supporting materials.
Among various materials, carbon is recognized as an excellent candidate due to its stability and compatibility(Bazargan and McKay, 2012; Nahil et al., 2015). For example, Yang et al. (2005) prepared spherical carbon particles
or carbon spherules for accommodating Pt nanoparticles
which was employed as electrocatalyst in direct methanol
fuel cells (DMFCs). Their microscopic analyses revealed
that K 2PtCl6 with ethylene glycol could reduce size of thePt nanoparticles with faceted crystalline structure. Aggregation and activity of the Pt nanoparticles are dependent on the surface properties of the carbon spherules. It
should be noted that with cyclic voltammetry and galvanostatic polarization analyses the Pt/HCS catalyst could
exhibit a higher catalytic activity in the electrooxidationof methanol than that of commercial ones. Then Xu et al.
(2007) had also tried to deposit Pt or Pd on carbon microspheres (CMS), which could be used for methanol andethanol oxidation in alkaline media. The results show that
noble metal electrocatalysts supported on carbon microspheres give better performance than that supported on

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

carbon black. It is well known that Pd is not a good electrocatalyst for methanol oxidation, but it shows excellently
higher activity and better steady-state electrolysis than Pt
for ethanol electrooxidation in alkaline media. The results
confirmed a synergistic effect by the interaction between
Pd and carbon microspheres which could exert a great potential in direct ethanol fuel cells application. Meanwhile,
Tusi et al. (2010) examined hydrothermal carbonization
process for preparing composite of CNTs and PtRu
nanoparticles, which were respectively generated fromstarch as carbon source and platinum and ruthenium salts
as catalysts, with the presence of KOH or TPAOH (tetrapropylammonium hydroxide) for pH control. Based oncomprehensive analyses using SEM/EDX, TGA, XRD
and cyclic voltammetry, they reported that carbon PtRu
nanocomposites prepared from TPAOH were more active
for methanol oxidation than those prepared from KOH
due to the pore volume and mesoporous structure. Morerecently Nahil et al. (2015) has reported that the pyrolysis
with catalytic reforming of waste polypropylene with thepresence of Ni-composite catalysts could provide multiwalled carbon nanotubes as a by-product of hydrogenproduction. They found that injection of steam with thepresence of Ni–Mn–Al catalyst and MWCNTs would playa crucial role for achieving an optimal hydrogen production. Because of abundant surface area available for accommodating metal catalysts and the presence functionalgroups, MWCNTs could provide preferable yield of hydrogen as well as promoting decomposition of pollutants,
such as phenol (Sano et al., 2012; Nahil et al., 2015).
With such typical examples, it could be implied that carbonaceous nanomaterials would be a good candidate forsupporting catalyst but clear understanding on their synergetic functions is still under investigation.

3.1.2 Water pollutant removal

Leakage of some chemicals employed in many humanactivities including agricultures and health remediation has
been recognized as one of emerging threats to the world.
Among those pollutants, it happens that antibiotics lead totremendously serious consequence to the eco-system (Luet al., 2005b; Grobert, 2007; Ji et al., 2009; Wang et al.,2009; Sowichai et al., 2012; Zhang et al., 2012). Thoughsome conventional adsorbents, such as activated carbon
or zeolite, are commercially available there are somedrawbacks related to post-handling would draw researchattentions to explore other new candidates. Therefore,
some carbonaceous nanomaterials with special properties
have been developed as novel adsorbent. From a viewpoint of adsorption kinetics, CNTs with ordered pore
structure could make it easier for the diffusion of pollutants to adsorption sites when compared to conventional
activated carbon (AC) with random pore structure. It
should be noted that ACs generally contain micropores,

which are seldom available for the access of relatively
large organic molecules. Lu et al. (2005b) studied the adsorption of trihalomethanes to CNTs and powdered activated carbon (PAC). They reported that CNTs reachedadsorption equilibrium much faster than PACs, which
would be attributed to the different porous structures of
CNTs and PAC. The more uniform pore structure of
CNTs would be beneficial for the diffusion of pollutants
into the inner pores. Based on experimental investigation
on adsorption of tetracycline to CNTs, graphite and activated carbon done by Ji et al. (2009), the adsorption affinity of tetracycline decreased in the order of graphite/
SWNT > MWNT >> AC upon normalization for adsorbent
surface area. These finding results would be attributed to
the weaker adsorption of tetracycline to AC of which adsorption affinity was greatly influenced by the accessibilityof available adsorption sites. Meanwhile, the remarkably
strong adsorption of tetracycline to CNTs can be attributed to the strong adsorptive interactions (van der
Waals forces, π-π EDA interactions, cation-π bonding)with the graphene surface of CNTs.
Then Sowichai et al. (2012) synthesized magnetic carbon nanoparticles (M-CNPs) from a mixture of glyceroland ferrocenes. The synthesized nanomaterials whichmainly consisted of CNTs were used as an adsorbent forremoval of tetracycline dissolved in simulated waste water. It was found that the adsorption capacity on M-CNPs
was strongly dependent upon contact time and adsorption
temperature. The increase in the adsorption temperature
would result in the increase in both adsorption capacity
and rate. However, the adsorption equilibrium would beachieved within 180 min regardless of adsorption temperature. Meanwhile, Zhang et al. (2012) examined theadsorption kinetics of phenanthrene and biphenyl on
granular activated carbon (GAC) and CNTs. They reported that external mass transfer controlled the adsorption of organic compound to CNTs, while intraparticle
diffusion dominated in the adsorption of organic compounds onto ACs. As a result, in well mixed systems,
CNTs are superior to ACs in terms of sorption kinetics.
Based on those typical examples of previous investigation, there are sufficient evidences revealing that regardless of their morphology, carbonaceous nanomaterials
would be a good candidate for water pollution removal.

3.1.3 Spilled oil handlingMore recently spreading news of many incidents ofdamaged pipeline and liner containing hydrocarbons
which were spilled into the environment would clearly reveal that such spilled oil pollution could exert tremendously serious destruction of the eco-system. Spreadingof spilled oil layer on natural water resources usually occurs after the oil discharge from emission sources andstarting contact with water-surface. Rate of spreading

33

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

would be dependent upon oil characteristics, such as surface tension, viscosity, density, speed and environmental
conditions, such as direction of wind, and water currents.
Spreading would increase the oil covering surface area
which would prevent the exposure to the sunlight and air,resulting in serious collapse of the eco-system. Therefore,
development of effective adsorbent technology for handling such problem has gained an increasing interest frommany research teams in many countries (Walkup et al.,
1969; IPIECA, 1991; Albert, 1997; Wei and Molloy 2003;
White and Molloy, 2003; Wang et al., 2007; Ren et al.,2011; Liu et al., 2015).
In general, natural inorganic sorbents, such as clay,
glass, perlite, sand, vermiculite, wool, and volcanic ash
would be employed for adsorbing those spilled oil. Those
adsorbents would be able to absorb from 4 to 20 times
their weight of oil. On the other hand, organic materials
can be used on land and are not adaptable to water use foroil spill cleanup. More recently some synthetic sorbents
includes man-made materials that are similar to plastics,
such as polyurethane, polyethylene, polypropylene, andnylon fibers, which could exhibit superior adsorption capability as much as 70 times their weight of oil. However,those synthetic sorbents possess some drawbacks, especially their reusability and difficulties in their handling
until they are disposed of properly (IPIECA, 1991). It appears that carbonaceous nanomaterials could be suited toadsorb spilled oils that can perpetrate or wick within their
tortious surface (Wang et al., 2007; Ren et al., 2011; Liu etal., 2015). Such adsorbents could be employed likesponges to collect oil by capillary action or suction, attributed to large surface area, the chemical affinity of thesorbents for the spilled oil, and chemical constituents including their porosity, molecular structure and change involume (Ren et al., 2011). Absorbents work best on light,
less viscous oils, while adsorbents work best on heavy,
sticky, more viscous oils. For actual demonstration, theauthors have also employed a freeze drying method toprepare sponge composites made of polyvinyl acetate(PVA) and MWCNTs. Fig. 4 illustrated the appearance of
2 different sponge samples which the first was preparedfrom a mixture PVA and MWCNTs and the other from asimilar mixture with addition of carboxymethyl cellulose(CMC). It could be clearly observed that the sponge prepared from the PVA/MWCNT/CMC exhibited stable porous structure with only insignificant shrinkage. Suchstronger structure could be explained by the evidences
obtained from microscopic analyses as shown in Fig. 5.
With the presence of CMC, MWCNTs were uniformly
dispersed within the PVA matrix, resulting in formationof fibrous network which could endure the external forces
and could recover to its initial dimension after the external forces were released. Finally, with oil adsorption test,
both sponge samples could exhibit impressive perfor-

34

Fig. 4

Appearance of MWCNT sponge prepared by freeze
drying with (a) 1 wt% of PVA and (b) 1 wt% of PVA/
CMC mixture.

mance in collecting spilled oil within their porous structures. Fig. 6 is a typical example of slop oil adsorption
using the PVA/WMCNT/CMC sponges. In good agreement with other previous works, such composite sponge
could exhibit a superior adsorption capability of about
60 times their weight of oil. Therefore, based on theseevidences, it would be considered that carbonaceous
nanomaterials could be a good alternative for many fundamental applications.

3.2 Novel applications
Different from fundamental applications, transformation of carbonaceous nanomaterials into distinguishable
forms would lead to some novel applications which would
hardly be achieved by using their primitive forms. It could
be expected that derivation of novel applications of carbonaceous nanomaterials by integration with other technologies, such as sensor or quantum electronic technology,
would cover much broader perspectives (Bittencourt et al.,2006; Espinosa et al., 2007; Balazsi et al., 2008; Hashishin,
and Tamaki, 2008; Ghasempour and Zad, 2009). In thisreview, some typical examples of novel applications ofcarbonaceous nanomaterials are summarized.

3.2.1 Pollutant gas detection

Various gaseous pollutants have been emitted frommany sources which are related to industrial manufacturing or daily living of human. Preventive actions against
natural or artificial emission of some toxic exhausted gas
have been accomplished by many research teams aroundthe world (Bittencourt et al., 2006; Espinosa et al., 2007;
Balazsi et al., 2008; Hashishin and Tamaki, 2008). With
the excellent conductivity, carbonaceous materials would

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Fig. 5

SEM micrographs of MWCNT sponge prepared with (a) 1 wt% of PVA and (b) 1 wt% of PVA/CMC
mixture.

be hybridized with some semi-conductor nanoparticles
for improving sensing capability. Here are some typicalexamples of previous works which made of carbonaceous
nanomaterials which could detect some toxic gaseous pollutants (NOx, Sox and VOC).
Bittencourt et al. (2006) prepared active layers for gas

sensing applications by adding oxygen plasma functionalised multi-walled carbon nanotubes (MWCNTs) to WO3
and using the drop-coating deposition method. Two different ratios of MWCNTs in WO3 (1/100 and 1/1000) wereconsidered and the response of these sensors towards
toxic gases such as nitrogen dioxide, carbon monoxide

35Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Fig. 6

Typical experiment on spilled oil adsorption using PVA/MWCNTs/CMC songe. (a) spilled oil, (b) spilled oilwith PVA/MWCNTs/CMC songe just before adsorption, and (c) spilled oil after adsorption by PVA/MWCNTs/
CMC songe.

and ammonia was compared with that of WO3 and
MWCNTs gas sensors. It was found that the addition of asuitable quantity of MWCNTs in a WO3 film can lower
the sensor operating temperature to room temperature.
The response of the hybrid films to NO2 could be enhanced when only a few MWCNTs were added into the
WO3 films. Furthermore, hybrid films were able to detectammonia which is very difficult to detect by using pureWO3 and MWCNTs gas sensors, when operated at 150 °C.
Then Espinosa et al. (2007) investigated response to NO2of three different type of metal oxide (SnO2, WO3 or TiO2)
prepared with the presence of a low amount of oxygen-­functionalized MWCNTs. It was reported that the responsiveness towards NO2 of these metal oxide/MWCNT
hybrid films could considerably be improved. Meanwhile,the sensors based on hybrid SnO2/MWCNTs films present
excellent sensitivity towards NO2 when operated at room
temperature. The results suggest that there should be an
optimum amount of carbon nanotubes to be added to eachspecific metal oxide in order to enhance sensitivity.
Later, Hashishin and Tamaki (2008) also prepared
MWCNT-WO3 composite by directly growing MWCNTs
on Au electrode by means of thermal CVD following by
an impregnation by a suspension of H2WO3 on the surfaceof as-grown MWCNTs. After calcinations in argon at
400 °C for 3 h, the composite was fabricated as sensor for
testing its responses to NO2. The MWCNTs-WO3 composite sensor could exhibit good sensor response (Ra/
Rg = 3.8 at 200 °C, whereas Ra is the resistance of sensor
in air and Rg is that in NO2-containing atmosphere). The
sensor response was greatly improved with MWCNT-WO3
composite, comparing with that of MWCNT sensor (Ra/
Rg = 1.05) which would be ascribed to formation of p-njunction, between MWCNT(p) and WO3(n), and thus improvement of NO2 adsorption.

3.2.2 Electrode material

As promising novel applications, electronic devices
have been proposed and developed for various purposes,

36such as supercapacitor, dye-sensitized solar cell, hydrogenfuel cell and so on (Yoon et al., 2000; Soneda et al., 2003;
Liu et al., 2005; Lee et al., 2008; Ramasamy et al., 2008).
In those devices, electrode materials have been considered as a key issue for improving such device performance. Among various carbonaceous materials, such asactivated carbon, disordered carbon, mesoporous carbon
and MWCNTs are under close scrutiny for use as a promising electrode material. For instance, in electrical double
layer capacitors (EDLC), composites involving, carbon or
CNT and RuO2 or MnO2 compounds, have been developed for usage as electrode materials. In some preliminary studies involving MWCNTs and MnO2 composites
focused on optimal composition of MWCNTs and theirrate capability for few cycles, and showed their promising
performance (Yoon et al., 2000; Liu et al., 2005). It is wellrecognized that long cycle and high rate stability of anenergy storage system are very important criteria for its
applications as supercapacitors. However, long cycle performance at a considerably higher charge–discharge current has not been reported so far. Meanwhile, forcounter-electrode of dye-sensitized solar cell application,
carbon based materials, i.e. fullerene and SWCNTs havealso been investigated as cost-effective and simpler alternatives. Catalytic activity towards iodine ion reduction
has been investigated on carbon based counter-electrodes(Soneda et al., 2003). So far, an overall energy conversion
efficiency of such solar cell device has still been belowthat of conventional Pt counter electrode DSSCs. However, it has also been suggested that Pt counter electrodein DSSC could be replaced by carbonaceous nanomaterials without affecting the energy conversion efficiency.
Despite high catalytic activity towards iodine ion reduction, carbonaceous counter-electrode would probably exert some risk to the stability of DSSCs (Ramasamy et al.,2008). Since the electrodes made up of carbonaceous
nanomaterials, prolong exposure in corrosive iodine redox
electrolyte would be expected to result in the detachmentof loosely bounded particles from rest of the electrode,Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

thereby promoting the dark current and degrade the overall device performance. Nevertheless, the stability of carbon counter-electrode in DSSC through optimization ofelectrolyte composition and device fabrication process
would be further explored.

4. Summary
Because of the increasing global consumption, an exponential increase in industrial production activities inevitably results in overwhelming remain of industrial waste.
As a result, increasing attentions of research and development teams in various countries have led to developmentof novel methodologies to utilize such industrial waste
for production of carbonaceous nanomaterials instead ofusing as alternative energy sources. Meanwhile, for sustainable and secure continuity of the carbonaceous
nanomaterial production, broad spectra of promising applications have also been examined. Among those emerging applications, utilization of carbonaceous nanomaterials
in pollution control and prevention has been focusedworldwide. Therefore, in this review, relevant researchworks focusing on catalytic pyrolysis of carbonaceous industrial waste for carbon nanomaterial production werecomprehensively analyzed and summarized. In addition,broad spectra of promising applications involving with
antibiotic removal, spilled oil adsorption, pollutant gas
detection and electrode materials were also reviewed.
Based on comprehensively reviewed literatures, it would
be considered that carbonaceous nanomaterials could be
potentially produced from various industrial wastes andthey would be a good alternative for many fundamental
and novel applications. Linkages between their characteristics and dominating phenomena taking place in those
applications would be important research issues for further exploration.

Acknowledgement
Main support from the Silver Jubilee Fund of Chulalongkorn University through the Climate Change and Disaster
Management Research Cluster (CU 56-357-CC) is gratefully acknowledged. Partial support from the Institutional
Research Grant (IRG5780014) of TRF and Chulalongkorn
University (Contract No. RES_57_411_21_076) is also
acknowledged.

References
Acomb J.C., Wu C., Williams P.T., Control of steam input to thepyrolysis-gasification of waste plastics for improved pro-duction of hydrogen or carbon nanotubes, Appl. Catal. B:
Environ., 147 (2014) 571–584.
Afre R.A., Soga T., Jimbo T., Kumar M., Ando Y., Sharon M.,
Somani P.R. and Umeno M., Carbon nanotubes by spraypyrolysis of turpentine oil at different temperatures andtheir studies, Microporous. Mesoporous Mater., 96 (2006)
184–190.
Ago H., Komatsu T., Ohshima S., Kuriki Y., Yumura M., Dispersion of metal nanoparticles for aligned carbon nanotube
arrays, Appl. Phys. Lett., 77(2000) 79–81.
Alberts P.H., Petroleum and individual polycyclic aromatichydrocarbons. Hoffman D.J., et al. Editors, Handbook of
ecotoxicology, Lewis, Boca Raton (FL), 1997, p. 330–356.
Altalhi T., Kumeria T., Santos A., Losic D., Synthesis of well-­organised carbon nanotube membranes from non-degradable
plastic bags with tuneable molecular transport: Towards
nanotechnological recycling, Carbon, 63 (2013) 423-433.
Alves J.O., Zhuo C., Levendis Y., Tenório J.A.S., Catalytic conversion of wastes from the bioethanol production into carbon nanomaterials, Catalytic conversion of wastes from thebioethanol production into carbon nanomaterials, Appl.
Catal. B: Environ., 106 (2011) 433– 444.
Andrews R., Jacques D., Qian D., Rantell T., Multiwall Carbon
Nanotubes: Synthesis and Application, Acc. Chem. Res.,35 (2002) 1008–1017.
Arora N., Sharma N.N., Arc discharge synthesis of carbon
nanotubes: Comprehensive review, Diam. Rel. Mater., 50(2014) 135–150.
Auer E., Freund A., Pietsch J., Tacke T., Carbons as supports forindustrial precious metal catalysts, Appl. Catal. A: General,
173 (1998) 259–271.
Bai S., Li F., Yang Q.H., Cheng H.M., Bai J.B., Influence offerrocene/benzene mole ratio on the synthesis of carbon
nanostructures. Chem. Phys. Lett., 376 (2003) 83–89.
Bajad A., Tiwarib S., Vijayakumara R., Synthesis and characterization of CNTs using polypropylene waste as precursor,
Mater. Sci. Eng. B., 194 (2015) 68–77.
Balázsi C., Sedlácková K., Llobet E., Ionescu R., Novel hexagonal WO3 nanopowder with metal decorated carbon nanotubes as NO2 gas sensor, Sensors Actuators B: Chemical,
133(2008) 151–155.
Bandow S., Kokai F., Takahashi K., Yudasaka M., Qin L.C.,
Iijima S., Interlayer spacing anomaly of single-wall carbon
nanohorn aggregate, Chem. Phys. Lett., 321 (2000) 514–
519.
Barreiro A., Selbmann D., Pichler T., Biedermann K., GemmingT., Rümmeli M.H., Schwalke U., Büchner B., On the effects
of solution and reaction parameters for the aerosol-assisted
CVD growth of long carbon nanotubes, Appl. Phys. A, 82(2006) 719–725.
Bazargan A., McKay G., A review—Synthesis of carbon nanotubes from plastic wastes, Chem. Eng. J., 195 (2012) 377–
391.
Bittencourt C., Felten A., Espinosa E.H., Ionescu R., Llobet E.,
Correig X., Pireaux J.J., WO3 films modified with functionalised multi-wall carbon nanotubes: Morphological, compositional and gas response studies, Sensors Actuators B:
Chemical, 115 (2006) 33–41.

37

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Chaisitsak S., Nukeaw J., Tuantranont A., Parametric study of
atmospheric-pressure single-walled carbon nanotubes
growth by ferrocene–ethanol mist CVD, Diam. Relat.
Mater., 16 (2007) 1958–1966.
Charinpanitkul T., Sano N., Muthakarn P., Tanthapanichakoon
W., Enhancing effect of monoolein surfactant on carbon
nanoparticle synthesis by arc discharge in liquid, Mater.
Res. Bull., 44 (2009a) 324–327.
Charinpanitkul T., Sano N., Puengjinda P., Klanwan J.,
Akrapattangkul N., Tanthapanichakoon W., Naphthalene as
an alternative carbon source for pyrolytic synthesis of carbon nanostructures, J. Anal. Appl. Pyro., 86 (2009b) 386–
390.
Charinpanitkul T., Tanthapanichakoon W., Regeneration ofactivated carbons saturated with pyridine or phenol using
supercritical water oxidation method enhanced with hydrogen peroxide, J. Ind. Eng. Chem., 17 (2011) 570–575.
Colomer J.F., Stephan C., Lefrant S., van Tendeloo G., Willems
I., Kónya Z., Fonseca A., Laurent C., Nagy J.B., Large­
scale synthesis of single-wall carbon nanotubes by catalytic
chemical vapor deposition (CCVD) method, Chem. Phys.
Lett., 317 (2000) 83–89.
Dai H., Carbon nanotubes: opportunities and challenges, Surf.
Sci., 500 (2002) 218–241.
de Volder M.F.L., Tawfick S.H., Baughman R.H., Hart A.J.,
Carbon Nanotubes: Present and Future Commercial Applications, Science, 339 (2013) 535–539.
Emmenegger C., Bonard J.M., Mauron P., Sudan P., Lepora A.,
Grobety B., Zuttel A., Schlapbach L., Synthesis of carbon
nanotubes over Fe catalyst on aluminium and suggested
growth mechanism, Carbon 41 (2003) 539–547.
Endo M., Kim Y.A., Takeda T., Hong S.H., Matusita T., Hayashi
T., Dresselhaus M.S., Structural characterization of carbon
nanofibers obtained by hydrocarbon pyrolysis, Carbon 39(2001) 2003–2010.
Engelbert C., Fadok J., Fuller R., Lueneburg B., Introducing the
1S.W501G Single-Shaft Combined Cycle Reference Power
Plant, Proceedings of ASME Power, 2004, Baltimore,
Maryland.
Espinosa E.H., Ionescu R., Chambon B., Bedis G., Sotter E.,
Bittencourt C., Felten A., Pireaux J.J., Correig X., Llobet E.,
Hybrid metal oxide and multiwall carbon nanotube films
for low temperature gas sensing, Sensors Actuators B:
Chemical, 127 (2007) 137–142.
Ghasempour R., Zad A.I., Hybrid multiwalled carbon nanotubes
and trioxide tungsten nanoparticles for hydrogen gas sensing, J. Phys. D: Applied Phys., 42 (2009) 165105.
Gong J., Feng J., Liu J., Jiang Z., Chen X., Mijowska E., Wen X.
Tang T., Catalytic carbonization of polypropylene into cupstacked carbon nanotubes with high performances inadsorption of heavy metallic ions and organic dyes, Chemical Engineering Journal, 248 (2014) 27–40.
Grobert N., Carbon nanotubes—becoming clean, Mater. Today,
10 (2007) 28–35.
Guo J., Wang X., Yao Y., Yang X., Liu X., Xu B., Structure ofnanocarbons prepared by arc discharge in water, Mater.
Chem. Phys., 105 (2007) 175–178.
Hashishin T., Tamaki J., Conductivity-Type Sensor Based on

38

CNT-WO3 Composite for NO2 Detection, J. Nanomater.,2008 (2008) 1–4.
Iijima S., Helical microtubules of graphitic carbon, Nature, 354(1991) 56–58.
Iijima S., Nanocarbon Materials: Synthesis and Structure Characterizations, Proceeding of the 2nd Workshop on Nanoscience : Carbon-related Systems and Nanomaterials, NCKU,
Tainan, Taiwan (2012).
IPIECA. A Guide to Contingency Planning for Oil Spill on
Water, Report Series, Vol.2, 1991.
Jang J., Bae J., Fabrication of polymer nanofibers and carbon
nanofibers by using a salt-assisted microemulsion polymerization, Angew. Chemie, 43 (2004) 3803–3806.
Ji L.L., Chen W., Duan L., Zhu D.Q., Mechanisms for strong
adsorption of tetracycline to carbon nanotubes: A comparative study using activated carbon and graphite as adsorbents. Environ. Sci. Technol., 43 (2009), 2322–2327.
Jie G., Shun L.Y., Xi L.M., Product characterization of waste
printed circuit board by pyrolysis, J. Anal. Appl. Pyrolysis,
83 (2008) 185–189.
Khairi A.N.S., Yusof N.A., Abdullah A.H., Mohammad F.,
Removal of Toxic Mercury from Petroleum Oil by Newly
Synthesized Molecularly-Imprinted Polymer, Int. J. Mol.
Sci., 16 (2015) 10562–10577.
Kim N.S., Lee Y.T., Park J.H., Vertically Aligned Carbon Nanotubes Grown by Pyrolysis of Iron, Cobalt, and Nickel
Phthalocyanines, J. Phys. Chem. B, 107(2003) 9249–9255.
Kroto H.W., Heath J.R., O’Brien S.C., Curl R.F., Smalley R.E.,
C60: Buckminsterfullerene, Nature, 318 (1985) 162–163.
Kusaba M., Tsunawaki Y., Production of single-wall carbon
nanotubes by a XeCl excimer laser ablation, Thin Solid
Films, 506 (2006) 255–258.
Lebel L.L., Aissa B., Khakani M.A.E., Therriault D., Preparationand mechanical characterization of laser ablated single-­
walled carbon-nanotubes/polyurethane nanocomposite micro­
beams, Comp. Sci. Technol., 70 (2010) 518–524.
Lee Y.T., Kim N.S., Park J.H., Han J.B., Choi Y.S., Ryu H., Lee
H.J., Temperature-dependent growth of carbon nanotubes
by pyrolysis of ferrocene and acetylene in the range
between 700 and 1000 °C. Chem. Phys. Lett., 372 (2003)
853–859.
Lee W.J., Ramasamy E., Lee D.Y., Song J.S., Performance variation of carbon counter electrode based dye-sensitized solarcell, Solar Energy Mater. Solar Cells, 92 (2008) 814–818.
Li X., Lei Z., Ren R., Liu J., Zuo X., Dong Z., Wang H., Wang J.,
Characterization of carbon nanohorn encapsulated Fe particles, Carbon, 41 (2003) 3068–3072.
Li Y., Wang H., Wang G., Gao J., Synthesis of single-walledcarbon nanotubes from heavy oil residue, Chem. Eng. J.,
211 (2012) 255–259.
Liu B.C., Lee T.J., Lee S.H., Park C.Y., Lee C.J., Large-scale
synthesis of high-purity well-aligned carbon nanotubes
using pyrolysis of iron(II) phthalocyanine and acetylene,
Chem. Phys. Lett., 37 (2003) 55–59.
Liu H.Y., Wang K.P., Teng H.S., A simplified preparation ofmesoporous carbon and the examination of the carbon
accessibility for electric double layer formation, Carbon 43(2005) 559–566.

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Liu X., Yang Y., Lin X., Xu B., Zhang Y., Deoiled asphalt as
carbon source for preparation of various carbon materials
by chemical vapor deposition, Fuel Process. Technol., 87(2006) 919–925.
Liu T., Chen S., Liu H., Oil Adsorption and Reuse Performanceof Multi-Walled Carbon Nanotubes, Procedia Engineering,
102 (2015) 1896–1902.
Lu Y., Zhu Z.P., Liu Z.Y., Carbon-encapsulated Fe nanoparticles
from detonation-induced pyrolysis of ferrocene, Carbon, 43(2005a) 369–374.
Lu C.S., Chung Y.L., Chang K.F., Adsorption of trihalomethanes from water with carbon nanotubes. Water Res., 39(2005b) 1183–1189.
Mankhand T.R., Singh K.K., Gupta S.K., Das S., Pyrolysis ofPrinted Circuit Boards, Inter. J. Metal. Eng., 1(2012) 102–
107.
Mishra N., Das G., Ansaldo A., Genovese A., Malerba M.,
Povia M., Ricci D., Fabrizio E.D., Zitti E.D., Sharone M.,
Sharon M., Pyrolysis of waste polypropylene for the synthesis of carbon nanotubes, J. Anal. Appl. Pyro., 94 (2012)
91–98.
Mohammed A.D., Isah A.G., Umaru M., Ahmed S., Abdullah
Y.N., Comparative study on sulphur reduction from heavy
petroleum- Solvent extraction and microwave irradiation
approach, Inter. J. Energy Environ., 3 (2012) 949–960.
Montoro L.A., Lofrano R.C.Z., Rosolen J.M., Synthesis ofsingle-walled and multi-walled carbon nanotubes by arc-­
water method, Carbon, 43 (2005) 200–203.
Morales N. J., Goyanes S., Chiliotte C., Bekeris V., Candal R.J.,
Rubiolo G.H., One-step chemical vapor deposition synthesis of magnetic CNT–hercynite (FeAl2O4) hybrids withgood aqueous colloidal stability, Carbon, 61(2013) 515–524.
Nahil M.A., Wu1 C., Williams P.T., Influence of metal addition
to Ni-based catalysts for the co-production of carbon nanotubes and hydrogen from the thermal processing of waste
polypropylene, Fuel Proc. Technol., 130 (2015) 46–53.
Paradise M., Goswami T., Carbon nanotubes—Production andindustrial applications, Mater. Desgn., 28 (2007) 1477–
1489.
Parkansky N., Boxman R.L., Alterkop B., Zontag I., Lereah Y.,
Barkay Z., Single-pulse arc production of carbon nanotubes
in ambient air, J. Phys. D: Appl. Phys., 37 (2004) 2715–
2722.
Popov V.N., Carbon nanotubes: properties and application,
Mater. Sci. Eng.: R: Reports, 43 (2004) 61–102.
Prasek J., Drbohlavova J., Chomoucka J., Hubalek J., Jasek O.,
Adam V., Kizek R., Methods for carbon nanotube synthesis—review, J. Mater. Chem., 21 (2011) 15872–15884.
Puengjinda P., Sano N., Tanthapanichakoon W., Charinpanitkul
T., Selective synthesis of carbon nanotubes and nanocapsules using naphthalene pyrolysis assisted with ferrocene, J.
Ind. Eng. Chem., 15 (2009) 375–380.
Qiu J., Chen G., Li Z., Zhao Z., Preparation of double-walledcarbon nanotubes from fullerene waste soot by arc-discharge,
Carbon, 48 (2010) 1312–1320.
Quan C., Li A., Gao N., Synthesis of carbon nanotubes andporous carbons from printed circuit board waste pyrolysis
oil, J. Hazard. Mater., 179 (2010) 911–917.

Ramasamy E., Won Jae Lee W.J., Lee D.Y., Song J.S., Spraycoated multi-wall carbon nanotube counter electrode fortri-iodide reduction in dye-sensitized solar cells, Electrochem. Commun., 10 (2008) 1087–1089.
Ren X., Chen C., Nagatsu M., Wang X., Carbon nanotubes as
adsorbents in environmental pollution management: areview, Chem. Eng. J., 170 (2011) 395–410.
Sano N., Akazawa H., Kikuchi T., Kanki T., Separated synthesis of iron-included carbon nanocapsules and nanotubes bypyrolysis of ferrocene in pure hydrogen, Carbon, 41 (2003)
2159–2162.
Sano N., Charinpanitkul T., Kanki T., Tanthapanichakoon W.,
Controlled synthesis of carbon nanoparticles by arc inwater method with forced convective jet, J. Appl. Phys., 96(2004) 645–649.
Sano N., Hori Y., Yamamoto S., Tamon H., A simple oxidation–
reduction process for the activation of a stainless steel surface to synthesize multi-walled carbon nanotubes and its
application to phenol degradation in water, Carbon 50(2012) 115–122.
Satishkumar B.C., Govindaraj A., Rao C.N.R., Bundles of aligned
carbon nanotubes obtained by the pyrolysis of ferro­cene-hydrocarbon mixtures: role of the metal nanoparticles
produced in situ, Chem. Phys. Lett., 307 (1999) 158–162.
Sawant S.Y., Somani R.S., Panda A.B., Bajaj H.C., Formationand characterization of onions shaped carbon soot fromplastic wastes, Mater. Lett., 94 (2013) 132–135.
Serp P., Feurer R., Kalck P., Kihn Y., Faria J.L., Figueiredo J.L.,
Chemical vapour deposition process for the production ofcarbon nanospheres, Carbon, 39 (2001) 621–626.
Soneda Y., Toyoda M., Hashiya K., Yamashita J., Kodama M.,
Hatori H., Inagaki M., Huge electrochemical capacitance of
exfoliated carbon fibers, Carbon, 41 (2003) 2680–2682.
Sowichai K., Supothina S., Nimittrakoolchai O.U., Seto T.,
Otani Y., Charinpanitkul T., Facile method to prepare magnetic multi-walled carbon nanotubes by in situ co-precipitationroute, J. Ind. Eng. Chem., 18(2012), 1568–1571.
Sun X., Li Y., Colloidal Carbon Spheres and Their Core/Shell
Structures with Noble-Metal Nanoparticles, Angew. Chemie, 43 (2004) 597–601.
Thostenson E.T., Ren Z., Chou T.W., Advances in the scienceand technology of carbon nanotubes and their composites:a review, Comp. Sci. Technol., 61 (2001) 1899–1912.
Tsai Y.Y., Su J.S., Su C.Y., He W.H., Production of carbon nanotubes by single-pulse discharge in air, J. Mater. Proces.
Technol., 209 (2009) 4413–4416.
Tusi M.M., Brandalise M., Verjúlio-Silva R.W.R., Correa O.V.,
Villalba J.C., Anaissi F.J., Neto A.O., Linardi M., Spinacé
E.V., Preparation of PtRu/C electrocatalysts by hydrothermal carbonization using different carbon sources, Proceeding of the 10th Inter. Symp. “Scientific Bases for the Preparation of Heterogeneous Catalysts” edited by GaigneauxE.M., Devillers M., Hermans S., Jacobs P., Martens J., Ruiz
P. (2010)
Vijayaraghavan G., Stevenson K.J., Synergistic assembly ofdendrimer-templated platinum catalysts on nitrogen-dopedcarbon nanotube electrodes for oxygen reduction, Langmuir, 23 (2007) 5279–5282.

39

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Vilatela J., Rabanal M., Cervantes-Sodi F., A Spray Pyrolysis
Method to Grow Carbon Nanotubes on Carbon Fibres, Steeland Ceramic Bricks., J Nanosci. Nanotech., 15 (2015)
2858–2864.
Walkup P.C., Polentz L.M., Smith J.D., Peterson P.L., Study ofequipment and methods for removing oil from harborwaters, Proceedings of Joint Conference on Prevention andControl of Oil Spills, 15–17 December 1969, New York,
237–248.
Wang S.G., Liu X.W., Gong W.X., et al. Adsorption of fulvic
acids from aqueous solutions by carbon nanotubes, J.
Chem. Technol. Biotechnol., 82 (2007) 698–704.
Wang X., Tao S., Xing B., Sorption and Competition of Aromatic Compounds and Humic Acid on Multiwalled Carbon
Nanotubes, Environ. Sci. Technol, 43(2009) 6214–6219.
Wei Q.F., Mather R.R., Fotheringham A.F., Yang R.D., ESEM
Study of Oil Wetting Behaviour of Polypropylene Fibres,
Spill Sci. Technol. Bull., 46 (2003) 780–783.
White I.C., Molloy F., Factors that Determine the Cost of Oil
Spills, Proceeding of International Oil Spill Conference
2003, Vancouver, Canada, 6–11 April 2003.
Xiao Y., Liu Y., Cheng L., Yuan D., Zhang J., Gu Y., Sun G.,
Flower-like carbon materials prepared via a simple solvothermal route, Carbon, 44 (2006) 1589–1591.
Xu C., Cheng L., Shen P., Liu Y., Methanol and ethanol electrooxidation on Pt and Pd supported on carbon microspheresin alkaline media, Electrochem. Commun., 9 (2007) 997–
1001.
Yang R., Qiu X., Zhang H., Li J., Zhu W., Wang Z., Huang X.,
Chen L., Monodispersed hard carbon spherules as a catalyst
support for the electrooxidation of methanol, Carbon 43(2005) 11–16.
Yang W., Sun W.J., Chu W., Jiang C.F., Wen J., Synthesis of car-40

bon nanotubes using scrap tyre rubber as carbon source,
Chinese Chem. Lett., 23 (2012) 363–366.
Yao Z., Zhu X., Li X., Xie Y., Synthesis of novel Y-junction hollow carbon nanotrees, Carbon, 45 (2007) 1566–1570.
Yoon S., Lee J.W., Hyeon T., Oh S.M., Electric Double-Layer
Capacitor Performance of a New Mesoporous Carbon, J.
Electrochem. Soc. 147 (2000) 2507–2512.
Yuan D., Xu C., Liu Y., Tan S., Wang X., Wei Z., Shen P.K.,
Synthesis of coin-like hollow carbon and performance as
Pd catalyst support for methanol electrooxidation, Electrochem. Commu., 9 (2007) 2473–2478.
Zeng Q., Li Z., Zhou Y., Synthesis and Application of Carbon
Nanotubes, J. Nat. Gas Chem., 15 (2006) 235–246.
Zhang H., Ding Y., Wu C., Chen Y., Zhu Y., He Y., Zhong S.,
The effect of laser power on the formation of carbon nanotubes prepared in CO2 continuous wave laser ablation atroom temperature, Physica B: Condensed Matter, 325(2003) 224–229.
Zhang J., Li J., Cao J., Qian Y., Synthesis and characterizationof larger diameter carbon nanotubes from catalytic pyrolysis of polypropylene, Materials Letters, 62 (2008) 1839–
1842.
Zhang S.J., Shao T., Kose H.S., Karanfil T., Adsorption kinetics
of aromatic compounds on carbon nanotubes and activatedcarbons, Environ.Toxic. Chem., 31(2012) 79–85.
Zhuo C., Hall B., Richter H., Levendis Y., Synthesis of carbon
nanotubes by sequential pyrolysis and combustion of polyethylene, Carbon, 48 (2010) 4024–4034.
Zubaidy I.A.H., Tarsh F.B., Darwish N.N., Majeed B.S.S.A.,
Sharafi A.A., Chacra L.A., Adsorption Process of Sulfur
Removal from Diesel Oil Using Sorbent Materials, J. Clean
Energy Technol., 1 (2013) 66–68.

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Author’s short biography
Konrat Kerdnawee
Konrat Kerdnawee obtained her Bachelor in Chemical Engineering from King
Mongkut’s University of Technology Thonburi (KMUTT) in 2012. At the present she isconducting her Ph.D. in Center of Excellence in Particle Technology (CEPT), Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University. Her
research back ground in the field of carbon nanoparticles, such as carbon nanotubes. She
can be synthesized magnetic-carbon nanoparticles using pyrolysis method and study on
application of these materials for removal pollutant in water.

Chompoopitch Termvidchakorn
Chompoopitch Termvidchakorn received her Bachelor in Chemical Engineering from
Chulalongkorn University in 2013. Now she is conducting her Ph.D. in Center of Excellence in Particle Technology (CEPT), Department of Chemical Engineering, Faculty ofEngineering, Chulalongkorn University. Her present research involve to synthesis ofsingle-walled carbon nanohorns and their catalytic performance for sugar conversion.

Pacharaporn Yaisanga
Pacharaporn Yaisanga obtained her Bachelor in Chemical Engineering from Mahidol
University in 2012. At present she is conducting her Master in Center of Excellence inParticle Technology (CEPT), Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University. Her interest is in carbon nanotube composite membrane
synthesis for carbon dioxide capture.

Jirapat Pakchamsai
Jirapat Pakchamsai received his Bachelor in Nano Engineering from Chulalongkorn
University in 2015. During his internship in May-June 2014 at Department of Chemical
Engineering, Kyoto University he performed the synthesis of Fe-single-walled carbon
nanohorns (Fe-SWCNHs) composites via Gas-Injected Arc-In-Water (GI-AIW) method
and studied the influence of Au-Addition on some physical properties of carbon
nanoparticles. He is a member of Center of Excellence in Particle Technology (CEPT),
Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University. His present research is emphasized on the synthesis of single-walled carbon nanohorns via Gas-Injected Arc-In-Water (GI-AIW) method and their application of
antibiotics adsorption.

41

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Author’s short biography
Cheewapon Chookiat
Cheewapon Chookiat will receive his Bachelor in Nano Bachelor in Nano Engineeringfrom Chulalongkorn University in October 2015. During his two-month internship in
2014, he conducted a research on Freeze-Drying technology at Division of Food Scienceand Biotechnology, Department of Agriculture, Kyoto University, Kyoto, Japan. Now he
is a member of Center of Excellence in Particle Technology (CEPT), Department ofChemical Engineering, Faculty of Engineering, Chulalongkorn University. His present
research is emphasized on the synthesis of single-walled carbon nanohorns via Gas-­
Injected Arc-In-Water (GI-AIW) method and their application of antibiotics adsorption.

Apiluck Eiad-ua
Apiluck Eiad-ua is an Assistant Professor of College of Nanotechnology, King Monkut’s
Institute of Technology Ladkrabang (KMITL). He received Bachelor in Chemical Engineering from Prince of Songkla University in 2001, Master in Chemical Engineeringfrom Chulalongkorn University in 2004, and Doctor in Frontier Material from Nagoya
Institute of Technology in 2011, respectively. He had experiences to work in National Institute of Advanced Industrial Science and Technology (AIST), Japan for 2 years andNational Metal and Material Technology Center (MTEC) for 10 months. He joined
KMITL with present research interest on Synthesis and Application of Nanoporous Materials since 2011.

Winadda Wongwiriyapan
Winadda Wongwiriyapan is an Assistant Professor of College of Nanotechnology,
King Mongkuts’s Institute of Technology Ladkrabang. She received her Bachelor, Masterand Ph.D. in Electronic Engineering from Osaka University in 2003, 2005 and 2008,
respectively. She also experienced a one-year postdoctoral fellowship at Institute of Carbon Science and Technology, Shinshu University. Her previous and current research interest has been focused on synthesis of carbon nanomaterials and their applications insensor and energy storage devices.

Weerawut Chaiwat
Weerawut Chaiwat is a Lecturer in Environmental Engineering and Disaster Management Program, affiliated with School of Interdisciplinary Studies, Mahidol University,
Kanchanaburi Campus, Thailand. He received his Bachelor in Chemical Engineeringfrom Chulalongkorn University, Thailand, in 2003, and Master and Ph.D. in Chemical
Engineering from Kyoto University, Japan, in 2007 and 2010, respectively. He also experienced a two-year postdoctoral fellowship at Fuels and Energy Technology Institute,
Curtin University, Perth, Australia, during 2010-2012. His research interest has been focused on thermo-chemical conversion technology of biomass and hydrocarbon wastes tobio-fuels and value-added carbonaceous materials.

42

Konrat Kerdnawee et al. / KONA Powder and Particle Journal No. 34 (2017) 24–43

Author’s short biography
Sakhon Ratchahat
Sakhon Ratchahat received his Bachelor and Master Degree in Chemical Engineeringfrom Chulalongkorn University. During his Master study, he conducted research on synthesis of carbon microspheres from native starch by hydrothermal process. He worked as
research assistant at Center of Excellence in Particle Technology, Chulalongkorn University for 2 years before pursuing his Ph.D. Now he is a Ph.D. student at Sekiguchi laboratory, Graduate School of Science and Engineering, Tokyo Institute of Technology, Japan.
The research is Solar Gasification of Biomass in Catalytic Molten Salt Reactor.

Kajornsak Faungnawakij
Kajornsak Faungnawakij is a principal researcher and a director of Nanomaterials andNanosystems Engineering Research Unit at NANOTEC, NSTDA. He received his B.Eng.
(ChE with first-class honor) from Prince of Songkla University in 2001, and D.Eng.
(ChE) from Chulalongkorn University in 2005. During 2005–2007, he held a researcher
position at Japan Science and Technology Agency, and a postdoctoral fellow at Kyoto
University. He has worked in the fields of nanocatalysis in biorefinery, published > 65 research articles, and filed > 25 patents. He has received some prestigious awards, including a “Distinguished Young Chemist Award 2010” and a “Young Scientist Award 2011”.

Komkrit Suttiponparnit
Komkrit Suttiponparnit is a researcher in Environmental Research and Management
Department, PTT Research and Technology Institute, Thailand. He received his Bachelor Degree in Chemical Engineering from Srinakharinwirot University in 2001 and subsequently pursued his Master and Doctoral Degree in Chemical Engineering from
Kasetsart University in 2003 and 2011, respectively. His present research interest is on
Nanotechnology for Environmental Sustainability, particularly in the areas focusing on
the utilization of petroleum and petrochemical by-products for synthesis of carbon-based
nanomaterials, synthesis of nanocatalysts for pollution treatment, and the development of
nanomaterials for heavy metals sensing in environment.

Tawatchai Charinpanitkul
Tawatchai Charinpanitkul is Associate Professor of Chemical Engineering, and Director of Center of Excellence in Particle Technology, Chulalongkorn University. He received Bachelor in Chemical Engineering from Chulalongkorn University in 1986, andMaster and Doctor in Chemical System Engineering from University of Tokyo in 1989
and 1992, respectively. His research interest is focused on Synthesis and Application of
Carbonaceous and Zinc Nanoparticles, and Biomass Technology. He has published 110
technical papers. In recognition of dedication, he was bestowed Royal Decorations, thehighest being the Knight Grand Cordon (Special Class) of the Most Exalted Order of theWhite Elephant.

43

KONA Powder and Particle Journal No. 34 (2017) 44–69/Doi:10.14356/kona.2017005

Review Paper

Aerosol Delivery of siRNA to the Lungs.
Part 2: Nanocarrier-based Delivery Systems †Susanne R. Youngren-Ortiz 1, Nishant S. Gandhi 1,
Laura España-Serrano 1 and Mahavir B. Chougule 1,2*
1Translational Drug Delivery Research (TransDDR) Laboratory, Department of Pharmaceutical Sciences,
The Daniel K. Inouye College of Pharmacy, University of Hawaii at Hilo, USA
2
Natural Products and Experimental Therapeutics Program, The Cancer Research Center, University of Hawaii at Manoa, USA

Abstract
In this article, applications of engineered nanoparticles containing siRNA for inhalation delivery are reviewed
and discussed. Diseases with identified protein malfunctions may be mitigated through the use of well-designed
siRNA therapeutics. The inhalation route of administration provides local delivery of siRNA therapeutics to thelungs for various pulmonary diseases. A siRNA delivery system can be used to overcome the barriers of pulmonary
delivery, such as anatomical barriers, mucociliary clearance, cough clearance, and alveolar macrophage clearance.
Apart from naked siRNA aerosol delivery, previously studied siRNA carrier systems include those of lipidic,
polymeric, peptide, or inorganic origin. These delivery systems can achieve pulmonary delivery through thegeneration of an aerosol via an inhaler or nebulizer. The preparation methodologies for these siRNA nanocarriersystems will be discussed herein. The use of inhalable nanocarrier siRNA delivery systems have barriers to theireffective delivery, but overcoming these constraints while formulating a safe and effective delivery system willoffer unique advances to the field of inhaled medicine.
Keywords: siRNA, nanocarrier, nanoparticle, aerosolization, pulmonary, lung

1. Introduction
siRNA has potential therapeutic applications in treating‘undruggable’ diseases via post-transcriptional downregulation of target gene expression. The mechanisms ofRNAi have been comprehensively reviewed (Fellmann C.
and Lowe S.W., 2014; Fire A. et al., 1998; Hannon G.J.,
2002; Kim D.H. and Rossi J.J., 2007). The siRNA possessa specific sequence that is complementary with its target
mRNA and induces site-specific cleavage and subsequent
inhibition of intracellular protein synthesis. The synthesisof siRNAs is relatively simple compared to other therapeutic classes because it does not require a cellular expression system, complex protein purification, or refolding
schemes (Amarzguioui M. et al., 2005). A major advantage of siRNA over small molecule drugs or protein therapeutics is that the sequences can be rapidly designed forhighly specific inhibition of the target of interest.
†
1
2

*

44

Received 11 January 2016; Accepted 17 February 2016
J-STAGE Advance Publication online 30 April 2016
200 West Kawili Street, Hilo, Hawaii 96720, USA
Honolulu, Hawaii 96813, USA
Corresponding author: Mahavir B. Chougule;E-mail: mahavir@hawaii.edu
TEL: +1-808-981-4519 FAX: +1-808-981-4520

Pulmonary diseases such as asthma, lung cancer, cystic
fibrosis, pulmonary hypertension, and chronic obstructive
pulmonary disorder (COPD) have potential siRNA therapeutic targets (Amarzguioui M. et al., 2005; Burnett J.C.
and Rossi J.J., 2012; Kanasty R. et al., 2013). However,
due to the high negative charge density and the relatively
large size of the siRNA molecules, naked siRNA molecules are not able to enter cells efficiently (Reischl D. andZimmer A., 2009). Thus, delivery systems for siRNAneed to be developed to successfully protect and deliver
these agents. Nevertheless, siRNA delivery systems arelikely to have instability issues that cause premature release of the nucleic acids, especially with systems that incorporate their cargo through electrostatic interactions.
Pulmonary delivery of siRNA faces major challenges thatinvolve decreased correlation between in vitro and in vivo
experiments, difficulty in translation from animal models
to humans, and administration routes used in animalstudies that are non-applicable for human use (Lam J.K.W. et al., 2012). Another challenge in siRNA therapy is
the possibility of off-target effects induction.
This second part of a two part review article focuses on
the pulmonary route of administration, siRNA loaded
non-viral particulates for pulmonary or nasally inhaleddelivery systems, and preparation techniques for siRNA

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open access article
under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

loaded nanoparticles (Youngren-Ortiz S.R. et al., 2016).
While the previous part covered the rationale for the useof various siRNA delivery systems, this part will focus on
the preparation of siRNA loaded nanocarrier systems including examples of their pulmonary delivery.

2. Non-viral delivery of siRNA to the lungsiRNA is a highly negative charged, hydrophilic, andlarge-sized (approximately 13.3 kDa) macromolecule thatcannot cross biological membranes to reach their target
sites. Viral vectors have previously demonstrated cell uptake and siRNA efficacy, however, major limitations tohuman therapeutic delivery exist, such as uncontrolled viral replication, immunogenicity, tumorigenicity, and toxicity (Thomas C.E. et al., 2003). Due to these concerns,
non-viral delivery systems have been developed and successfully used to deliver siRNA. An ideal siRNA delivery
system should (1) condense siRNA into nanosized particles, (2) protect siRNA from enzymatic degradation, (3)facilitate cellular uptake, (4) promote endosomal escapeto release siRNA to the cytoplasm where the RNA-­
induced silencing complex (RISC) is located, (5) havenegligible effects on gene silencing activity or specificity
and (6) have negligible toxicity (Lam J.K.-W. et al., 2012;
Merkel O.M. and Kissel T., 2011). Non-viral delivery systems include naked siRNA, and delivery vectors such as
lipids, polymers, peptides, and inorganic materials, as
shown in Fig. 1.

2.1 siRNA delivery
2.1.1 Introduction

Unformulated or naked siRNA involves the delivery ofsiRNA without the use of a delivery vehicle or carrier.
Advantages of this strategy include the ease of preparation and the facility of delivery by inhalation, intratracheal, or intranasal routes. A major disadvantage of thisstrategy is that the delivered siRNA is susceptible to poor
cell targeting and uptake, and to degradation within theFig. 1 Schematic of different non-viral siRNA delivery vectors.

airways. As discussed in part 1 of the review article,
the major barriers for the delivery of siRNA to the lunginclude the presence of mucus, alveolar fluid, alveolar
macrophages, and mucociliary clearance. Unfavorable
physicochemical properties of siRNAs (negative charge,
large molecular weight) and instability in plasma (half-life
10 min) also poses major delivery challenges (Ren Y. et al.,2010). Furthermore, after being transported intracellularly
into the lysosomes via endocytosis, siRNA gets degradedin the lysosomes which diminishes the activity of siRNAtherapeutics (Tseng Y.-C. et al., 2009). Since unmodified
siRNA is prone to enzymatic degradation, new methods
such as chemical modification of siRNA have been developed to increase stability. The chemical modification also
improves specificity and potency, and reduces the immune
response and off-target effects (Watts J.K. et al., 2008).

2.1.2 RNA modifications & preparation methods

The local delivery of siRNA is particularly well-suitedfor lung disease and infection therapy (De Fougerolles A.
and Novobrantseva T., 2008). The direct instillation ofsiRNA into the lungs through intranasal or intratracheal
routes results in direct contact with lung epithelial cells.
Non-modified siRNA’s can also induce nonspecific activation of immune system through the Toll-like receptor 7
pathway (Whitehead K.A. et al., 2009a). However, chemical modifications can be introduced into the RNA duplex
structure which can enhance biological stability without
adversely affecting gene-silencing activity and prevent
nonspecific immune activation. Some of the modifications
include incorporation of 2’-O-methyl modifications into
the sugar structure of selected nucleotides within both thesense and antisense strands. The therapeutic efficacy ofdelivered siRNA can also be improved using conjugationof small molecules or peptides to the sense strand ofsiRNA. Several other modifications have been reported toeliminate the off-target effects such as phosphorothioate or
boranophosphate introduction (Gandhi N.S. et al., 2014).
In a recent study, Antagomir-122 was synthesized froma hydroxyproline-linked cholesterol solid support and2’-O-methyl phosphoramidites. On administration amarked decrease in endogenous miR-122 levels in theliver was observed (Krützfeldt J. et al., 2005). This andsuch other newer strategies have shown promise in protecting the siRNA against degradation by endonucleases
thus allowing them to reach their site of action.
There are 3 main methods used for the production ofsiRNA in vitro which includes 1) chemical synthesis, 2) invitro transcription of small RNA’s and 3) in vitro transcription of long RNA’s produced by the digestion with
Dicer enzyme (Aalto A.P. et al., 2007). The in vitro chemical synthesis of siRNAs involves in vitro transcription
and digestion of long dsRNAs by an RNase III family enzyme (e.g. Dicer, RNAse III). These production methods

45

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

require the design of siRNA sequences before siRNA
preparation. Recently, Aalto et al. utilized an in vitro system using the combination of T7 RNA polymerase andRNA-dependent RNA polymerase (RdRP) of bacteriophage f6 to generate siRNA molecules. They further used
an in vivo RNA replication system to produce siRNA.
This system was based on carrier state bacterial cells containing the f6 polymerase complex which can result in
unlimited amounts of siRNA up to 4.0 kb in size. Unmodified or modified siRNA can be prepared with simple reconstitution within normal saline or 5 % dextrose solution
for inhalation delivery. Aerosolization of the siRNA solution can be conducted using a nebulizer. A new class of
naked siRNA has been developed by Hamasaki et al.,termed ribophorin II (PnkRNATM) and nkRNA ® asshown in Fig. 2 (Hamasaki T. et al., 2012). This novel
class of RNAi agents was synthesized as single-stranded
RNA on a solid phase that, following synthesis, self-­anneal into a unique helical structure containing a central
stem and two loops (Merkel O.M. et al., 2014). Furthermore, a naked and unmodified novel RNAi agent, such as
RPN2-PnkRNA, which has been selected as a therapeutictarget for lung cancer when delivered via inhalation resulted in efficient inhibition of tumor growth without any
significant toxicity.

nucleic acids (Moschos S.A. et al., 2011). Ex vivo luminometry elucidated organ associated luciferase knockdown. IVIS imaging of oligonucleotides labeled with Cy5
combined with confocal microscopy was used to determine their biodistribution (Moschos S.A. et al., 2011).
Upon administration, the oligonucleotides underwent fastsystemic distribution by transcytosis and renal clearance.
The kidney and liver uptake of the phosphorothioate
locked nucleic acid antisense oligonucleotides caused
gene knockdown in these organs. However, protein expression was not downregulated in purified lung tissue
cells (Moschos S.A. et al., 2011). The reduced therapeutic
efficacy of free RNA supports observations by anothergroup, who concluded that intratracheally delivered free
TNF-α siRNA post-hemorrhage was inefficient in reducing the symptoms in a septic shock model of acute lunginjury (Lomas-Neira J. et al., 2012). After bleomycin triggered pulmonary fibrosis was induced in mice with transgenic expression of human TGF-β1, 5 mg/kg TGF-β1
siRNA or a scrambled control was delivered intratracheally. This study found that siRNAs sequences sharedby human and rodents successfully knocked down TGF-β1
expression in human derived cell lines and significantly
inhibited pulmonary fibrosis in vivo (D’alessandro-­Gabazza C.N. et al., 2012).

2.1.3 Examples of siRNA delivery

2.2 Lipid-based delivery systems

Luciferase expressing mice were treated by intratracheal administration of 10 nmol siRNA duplex or 10 nmol
phosphorothioate locked antisense oligonucleotide freeFig. 246

Structure of novel PnkRNA (A) and nkRNA (B) RNAitherapeutic agents. Both PnkRNA (A) and nkRNA (B)were prepared as single stranded RNA oligomers thatthen self-anneal as shown by Hamasaki et al. Nucleotides shown in red indicate sense strand of the target(GAPDH), nucleotides shown in violet are the antisense
strand, and nucleotides shown in blue are the loop cassettes. P indicates a proline derivative. Reprinted
through open access agreement under the Creative
Commons Attribution (CC BY) license from Ref.
(Hamasaki T. et al., 2012).

2.2.1 Introduction

Lipid-based delivery systems are commonly used todeliver siRNA in vitro or in vivo (Tseng Y.-C. et al.,2009). Most often, cationic lipids or liposomes are used toform complexes, termed lipoplexes with anionic siRNAthrough spontaneous electrostatic interaction. Commercial siRNA transfection agents are commonly lipid-based
systems, including OligofectamineTM, TransIT-TKO, Lipofectamine® RNAiMAX, and DharmaFECT (Bitko V.
et al., 2005; Heidel J.D. et al., 2007; Tompkins S.M. et al.,
2004; Wang J.-C. et al., 2010). A major challenge of using
lipid-based siRNA delivery systems is their toxicity andnon-specific activation of inflammatory cytokines and interferon responses (Wu S.Y. and Mcmillan N.A., 2009).
Since aerosolization is a high shear stress process, thestability of the liposomes should be monitored since thisprocess may cause physical and chemical changes thatcan lead to early siRNA release and degradation of thesiRNA (Gaspar M.M. et al., 2008). Lipid based delivery
systems for siRNA include liposomes, solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC).
SLN are prepared by replacing the oil of the fat emulsion by a solid lipid or a blend of solid lipids, which makes
the lipid matrix of the SLN solid at room and body temperature (Cavalli R. et al., 1997; Kalariya M. et al., 2005;
Schwarz C. et al., 1994; Siekmann B. and Westesen K.,

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

1994). SLN are composed of 0.1–30 % w/w lipid dispersed
in an aqueous solution of 0.5–5 % w/w surfactant as a stabilizing agent (Pardeike J. et al., 2009; Weber S. et al.,2014). SLN provide physical stability, chemical stability,
controlled release, and low cytotoxicity if appropriate excipients are utilized (Wissing S. et al., 2004). SLN can beproduced without the use of organic solvents and can bescaled up. Disadvantages of SLN are low loading capacityand premature release during storage (Mehnert W. andMaeder K., 2012). These events occur because the low ordered lipid modification of the particle matrix after production transforms to the highly ordered β-modification
during storage. The β-modification is characterized byperfect crystal lattice with few imperfections and therefore, little room is left for siRNA storage. In order toovercome these issues, a second generation of lipid
nanoparticles, the NLC were developed. NLC have a solid
lipid matrix at room and body temperature and consist ofa blend of solid lipid and oil, preferably at a ratio of 70:30
up to a ratio of 99.9:0.1 (Mehnert W. and Maeder K.,
2012; Patel A.R. et al., 2013; Patlolla R.R. et al., 2010).
Through mixing different kinds of lipids, a less orderedmatrix with more room for active compounds is achieved(Mehnert W. and Maeder K., 2012).

2.2.2 Preparation methods

2.2.2.1 Cationic lipoplexes
Cationic lipids or liposomes can spontaneously form
complexes with negatively charged siRNA through electrostatic interactions to form lipoplexes (Fig. 3). Complexation of cationic lipids and siRNA occurs through thenegatively charged siRNA interaction with the positively
charged lipids. This interaction causes neutralization of
the cationic lipids, causing them to aggregate and encapsulate the siRNA (Bochicchio S. et al., 2014). It is necessary to optimize the lipid composition, lipids to siRNAratio and the lipoplexes preparation methods, in order toimprove stability and delivery efficiency while reducing
possible adverse effects. Lipoplexes often have goodtransfection efficiency due to their efficient interaction
with the negatively charged cell membranes as shown in
Fig. 3. However, lipoplexes generally display poor stability and reproducibility when compared to other lipid
based siRNA delivery systems, such as liposomes or SLN(De Fougerolles A.R., 2008). Another disadvantage is thatcationic lipids are typically more toxic than neutral lipids(Dokka S. et al., 2000). To shield the positive charge ofthe cationic lipids, hydrophilic polymers such as PEG
have been used to reduce inflammatory response.
Lipoplexes are not stable in liquid suspension for
long-term storage, as shown by several studies in which
the lipoplexes aggregate (Anchordoquy T.J. et al., 1997;
Felgner P.L. et al., 1995). This leads to the requirement of
preparation immediately prior to administration or to im-

Fig. 3

Cationic liposome and anionic siRNA interaction forming a lipoplex. The cationic lipoplex interacts with thenegatively charged cellular membrane leading to enhanced delivery within the cells via endocytosis.

prove lipoplex stability through the use of high pH diluents, sucrose gradients, and conjugation of PEG to thesurface of lipoplexes (Caplen N.J. et al., 1995; Felgner P.L.
et al., 1995; Gao X. and Huang L., 1996; Hofland H. et al.,
1996; Hong K. et al., 1997). Lyophilization is anothermethod for intervening this issue that has been shown to
inhibit aggregation for long-term storage of nucleic acidformulations (Allison S.D. and Anchordoquy T.J., 2001;
Allison S.D. et al., 2000; Anchordoquy T.J. et al., 1997).
Lyophilization allows for storage at room temperature,and therefore is preferred over freezing due to cost reduction in transportation and storage, and improved stability of
biomolecules resulting from the removal of non-freezable
water associated with most biomolecules (Umrania Y. etal., 1995). Lyophilization can cause liposome fusion andphase separation during the drying and rehydration steps(Crowe J.H. and Crowe L.M., 1988). To overcome these
problems, cryoprotectants, such as carbohydrates, areused. Cryoprotectants limit mechanical damage andrupture of the lipid bilayer, caused by ice crystals, during
the freeze-drying and the rehydration process by maintaining the membrane in a flexible state (Allison S.D. andAnchordoquy T.J., 2000).
2.2.2.2 Liposomes
The basic steps of loaded liposome production include(1) lipid hydration, (2) regulation of liposome size, and (3)removal of non-encapsulated drug (Samad A. et al.,2007).
Lipid hydration can be achieved through mechanicalmethods, organic solvent methods, or detergent removal

47

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

methods (Bochicchio S. et al., 2014; Youngren S. et al.,2013). Mechanical methods for lipid hydration are basedon using a rotating evaporator to form a thin-layer ofphospholipids onto the wall of the rotating vessel. The
layer is hydrated with an aqueous buffer solution that contains the siRNA to load while the vessel rotates. The process forms multilamellar vesicles, which can further besonicated, extruded, or homogenized to form small unilamellar vesicles. Glass beads may be used to optimize theprocess by allowing the creation of thinner lipid films byincreasing surface area. These thinner lipid films allows
for higher hydration efficiency and siRNA entrapment.
Lipid hydration by organic solvent methods requires themelting of lipids within an organic solvent followed byexposure to an aqueous phase that is then separated byevaporation. If the starting point is an organic solvent immiscible with the aqueous phase, then it is possible to use
the reverse-phase evaporation vesicles technique, where
the intermediate state is represented by emulsions. The
lipid solution in an organic solvent with the aqueous buffer is sonicated to obtain a water-in-oil (W/O) emulsion.
Controlled evaporation of the organic solvent leads to theformation of unilamellar vesicles. When most of the organic solvent is removed, a gel is formed, and it has to beagitated to transform into a viscous fluid that contains
liposomes. The gel collapse coincides with the conversionof the W/O emulsion in the liposome form. If the starting
point is an organic solvent miscible with water, then it ispossible to continue with a precipitation stage. An aqueous buffer is added to the miscible organic solvent todilute it, leading to the precipitation of lipids which thenaggregate to form liposomes. The speed of the dilution
passage can be adjusted to obtain desirable liposomal particle size, with faster dilution passages being associatedwith smaller particle size of formed vesicles.
Detergent removal methods for lipid hydration form
micellar structures that engulf more lipids as the detergent is removed to form unilamellar vesicles. This method
is less efficient for low molecular weight compounds thanother lipid hydration methods (Tarkunde S.V. et al., 2014).
Since this method has not been studied for large hydrophilic compounds like siRNA molecules, it is assumed
that liposomal formulation of siRNA using this method is
not suitable (Bochicchio S. et al., 2014).
Size optimization and removal of free siRNA should be
performed following the lipid hydration step. Size of liposomes should be adjusted based on the siRNA that needs
to be incorporated. This may be achieved through (1) extrusion at low or medium pressure through dimensionallydefined pores, (2) fractionation of a heterogeneous population by centrifugation or size-exclusion chromatography, and (3) homogenization to obtain smaller sized
liposomes (Bochicchio S. et al., 2014). Examples of extrusion devices are the Avanti Mini-Extruder and Nano

48

DeBEE high pressure homogenizer shown in Fig. 4.
Other mechanical dispersion methods include sonication,
freeze-thawed liposomes, lipid-film hydration by shaking
or freeze drying, or dried reconstituted vesicles (Çağdaş M.
et al., 2014). The non-encapsulated siRNA can be removed
by dialysis, ultrafiltration via ultracentrifugation, gelchromatography, and ionic-exchange resins (Bochicchio
S. et al., 2014).
Methods of encapsulating siRNA in liposomes include simple mixes of siRNA with pre-assembled liposomes, pre-condensation of siRNAs before liposome
encapsulation, direct hydration of a lipid thin-film layer
with a siRNA solution, and the ethanol dilution method(Bochicchio S. et al., 2014). A pre-assembled liposome
can be complexed with siRNA with a simple mixture
method. This method is not suitable for PEGylated liposomes, since siRNA is unable to penetrate the PEGylated
lipid bilayer efficiently (Gutbier B. et al., 2010). In thiscase, the siRNA would bind to the liposomal surface,which causes premature release of the siRNA from the liposome in vivo. To achieve a siRNA PEGylated liposome
loaded using the simple mixture method, the siRNA andthe non-PEGylated liposomes can be incubated to form
the siRNA liposome complex and subsequent incubation
with PEG-lipids at elevated temperatures. The PEG-lipids
would be inserted within the lipid bilayer, which can beverified by measuring zeta potential (Whitehead K.A. etal., 2009b). An alternative is to covalently bind PEG
chains to the siRNA liposome complexes after simple
mixture loading of the siRNA. An example of this surface
modification is the binding of PEG to the amine functional groups of the cationic cholesterol polyamine(CDAN) component of a liposomes using a pH-sensitive
oxime linkage that decomposes at pH < 5.5 (Carmona S.
et al., 2009). This technique, known as the “ABCD paradigm”, overcomes drawbacks of other PEGylation methods,
in that there is adequate siRNA encapsulation, PEGylation elicits a stealth behavior, and the PEG chain releaseof the liposomes within the endosomal compartments
leads to a pH triggered endosomal escape (Kostarelos K.
and Miller A.D., 2005).
Another technique involves pre-condensation of siRNAwith protamine, a natural cationic polypeptide, along withhigh molecular weight polyanions, such as hyaluronic
acid, at a ratio allowing for negatively charged nanoparticles (Li S.D. and Huang L., 2009). The high molecularweight polyanion is used to provide resistance to reticuloendothelial clearance instead of PEG (Kim S.S. et al.,2010). These complexes are then incubated with cationicliposomes to form the siRNA encapsulated liposomes.
Alternatively, siRNAs can be pre-condensed with calcium
phosphate nanoparticles followed by the cationic liposomal coating and PEG-lipids (Li J. et al., 2010).
The direct hydration of a thin-film lipid layer with

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

Fig. 4

Avanti Mini-Extruder (A), Avanti Mini-Extruder internal schematic (B), and Nano DeBEE high-pressure
homogenizer (C).

siRNA solution allows for encapsulation of siRNA within
the aqueous core of a liposome. Addition of a concentrated solution of siRNA to a dry thin lipid film layer
allows for the incorporation of the siRNAs into the internal cationic space of a lipidic bilayer (Buyens K. et al.,2009). Since siRNA is present during the formation of theliposomes, approximately 50 % of the siRNA is encapsulated as it is uniformly distributed across cationic lipids,
including the internal and external lipid bilayers. An alternative approach involves the formation of a monophasic solution of siRNAs and DOTAP in water/methanol/
chloroform mixture that leads to a perfect complex between the negative charge of siRNA and the positive
charge of the cationic lipids (Podesta J.E. and Kostarelos
K., 2009). Once excess water and chloroform are added, aphase separation occurs. siRNA/DOTAP micelles stay inthe organic phase and the aqueous/methanol phase can bediscarded. The neutral PEGylated lipids are added alongwith water and then the organic solvent can be removedby evaporation. The liposomes can then be extruded forfurther size control (Bochicchio S. et al., 2014).
The ethanol dilution method for directly hydrating thelipid layer is an alternative method of direct hydration.
Cationic PEGylated liposomes are mixed with siRNA in

the presence of a critical concentration of ethanol (approximately 40 %) in the aqueous buffer (Semple S.C. et al.,2001). At this ethanol concentration, liposomes are destabilized and their membrane structural integrity is compromised, allowing siRNA to uniformly penetrate theliposomes and associate with the positive charges on thelipids. Ethanol concentration is pivotal since if it is at an
inadequate level, the siRNAs would not be able to efficiently penetrate the lipid bilayer. If the ethanol concentration is too high, liposomal aggregation would occur.
The ethanol is removed by dialysis or tangential filtration
following siRNA loading, forming the PEGylated cationicliposomes containing approximately 50 % of the originally added siRNA. Stabilized Nucleic Acid Lipid Particles, or SNALPs, is a widely recognized example ofliposomal complexes prepared using the ethanol dilution
method (Morrissey D.V. et al., 2005). A cationic lipid isprotonated at low pH of 4–5 within the ethanol of thecomplex formation buffer, but is neutral when the acidic
buffer is exchanged with a physiological buffer at pH 7.4.
The advantages of cationic lipids are that siRNAs boundto the external surface of the liposome are detached whenneutralized. The released siRNAs can be washed awayfrom the complexes, together with ethanol, during the49Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

buffer exchange procedure. The final complexes can havea final siRNA complexation and encapsulation efficiency
of around 90 %. Other complexes of siRNA/PEGylated
liposomes are prepared similarly to SNALPs only thatthey contain other cationic lipids and termed as Lipidoid
Nanoparticles (Akinc A. et al., 2008; Love K.T. et al.,2010). This process is carried out with a combination ofsiRNAs in aqueous buffer with an equal volume of lipids(cationic, neutral, and PEGylated lipids), dissolved in butanol, the mixture is lyophilized and the lyophilized matrix is rehydrated (Akinc A. et al., 2008; Wu S.Y. andMcmillan N.A., 2009).
2.2.2.3 Solid lipid nanoparticles and nanostructured lipid
carriers
SLN formulations are composed of solid lipids, emulsifiers, and water (Mehnert W. and Maeder K., 2012). A
schematic of SLN preparation detailing the types of components and feed solutions used is shown in Fig. 5. NLC
formulations are composed of solid lipids, oil, surfactant,
and water (Mehnert W. and Maeder K., 2012). Lipids areused in a broad sense to include triglycerides, partial
glycerides, fatty acids, steroids, and waxes. All classes of
emulsifiers have been used to stabilize the lipid dispersion. Particle agglomeration may be efficiently prevented
through the combination of emulsifiers selected in respect
to charge and molecular weight. The use of physiologic
lipids within the lipid matrix decreases the possibility ofacute or chronic toxicity. The choice of emulsifier heavily
depends on the administration route.
Initially, high shear homogenization and ultrasound are
dispersing techniques used for the production of solid lipid
nanodispersions. Despite both methods being common
due to their ease of use, dispersion quality is poor due toFig. 550

Schematic of SLN preparation with encapsulatedsiRNA. Feed solutions 1 and 2 pictured in (a) are sequentially added to the receiver solution pictured in (b).
SLNs form following evaporation of organic solvents,
as shown in (c). Alternatively, siRNA/DOTAP complex
and tristearin are solubilized in CHF, as shown in (d),and then added to the aqueous receiver solution. A
schematic drawing of SLN is presented in (e). Reprinted with permission from Ref. (Lobovkina T. et al.,2011). Copyright (2011) American Chemical Society.

the presence of microparticles and metal contamination.
High pressure homogenization is a reliable and powerful technique for the preparation of SLN. High pressure
homogenization has been used for the production of nanoemulsions for parenteral nutrition; therefore these homogenizers are available from several manufacturers withvarying sizes. The scalability of this technique is highly
efficient. High pressure homogenizers force liquid with
100–2000 bar pressure through a narrow orifice with sizeof approximately a few micrometers. Due to the narrowed
orifice, the fluid accelerates on a very short distance to ahigh velocity of over 1000 km/h. High shear stress andcavitation forces disrupt the particles down to thenanoparticle size range. Typically, the lipid contents are
between 5–10 % and even higher lipid concentrations up
to 40 % have been successfully homogenized to lipid nanodispersions. A preparation step that incorporates drug
into the bulk lipid by dissolving or dispersing the drug inthe lipid melt followed by hot or cold homogenization
techniques are used to prepare SLN.
Hot homogenization, or the homogenization of anemulsion, is performed at temperatures above the melting
point of the lipid. The pre-emulsion of siRNA loaded lipid
melt and the aqueous emulsifier phase is obtained using ahigh shear mixing device, such as an Ultra-Turrax homogenizer. The quality of the pre-emulsion affects thequality of the final product to a high degree since it isdesirable to obtain droplets within the size range of a few
micrometers. The higher temperature leads to a lower
particle size because of the decreased viscosity of the inner phase. However, high temperatures may increase thedegradation of the siRNA or the SLN. The homogenization step may be repeated several times to achieve the desired particle size, keeping in mind that the high pressure
homogenization equipment increases the temperature of
the sample approximately 10 °C for 500 bar. Increasing
the homogenization pressure and number of cycles over
the recommended 500–1500 bar 3–5 cycles, respectively
causes a particle size increase of the lipid nanocarriers
due to particle coalescence as a result of high kinetic energy of the particles. The primary product of hot homogenization is a nanoemulsion due to the liquid state of thelipid. Cooling the nanoemulsion sample to a lower temperature forms the solid particles. Lipid crystallization isslowed due to the small particle size and the incorporationof emulsifiers, therefore the sample may remain as a supercooled melt for several months.
Cold homogenization is a high pressure milling of asuspension of solid lipids. Effective temperature control
and regulation is needed in order to ensure the unmolten
state of the lipid due to the increase in temperature duringhomogenization. Cold homogenization has been developed to overcome temperature induced drug degradation,
drug distribution into the aqueous phase during homoge-

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

nization, and the complexity of the crystallization stepof the nanoemulsion leading to several modification or
supercooled melts associated with hot pressure homogenization. The first preparatory step that includes the dispersing of the drug in the melt of bulk lipid is followed bya rapid cooling step within liquid nitrogen or dry ice. In
this step, the high rate of cooling causes the formation ofa homogenous distribution of the drug within the lipid
matrix. The solid, siRNA containing lipid is then milled
to microparticles. Ball or mortar milling typically resultsin particle sizes of 50–100 micrometers. Low temperatures increase the fragility of the lipid and causes particlecomminution, or the reduction in particle size. The solid
lipid microparticles are dispersed in a chilled emulsifier
solution. Then the pre-suspension is subjected to highpressure homogenization at or below room temperature.
Compared to hot homogenization, generally larger particle sizes and broader size distribution are obtained in cold
homogenized samples. The method of cold homogenization minimizes the thermal exposure of the sample, but it
does not fully avoid it due to the melting of the lipid/
siRNA mixture during the initial step.
SLN can be prepared by solvent emulsification or evaporation techniques by precipitation in o/w emulsions. Lipophilic material is dissolved in a water-immiscibleorganic solvent that is emulsified within an aqueous phase.
Once the solvent evaporates, precipitation of the lipid in
the aqueous phase forms a nanoparticle dispersion.
SLN can be prepared by the dilution of microemulsions. Since there are varying opinions on the structureand dynamics of a microemulsion, a detailed review on
the subject has been published (Moulik S. and Paul B.,
1998). A microemulsion can be envisioned as a two-phasesystem composed of an inner and outer phase, such as o/w
microemulsions. They are prepared by stirring an optically clear mixture at 65–70 °C composed of a low melting fatty acid, an emulsifier, co-emulsifiers, and water.
The hot microemulsion is dispersed in cold water (2–3 °C)
under stirring. Typical volume ratios of the hot microemulsion to cold water are between 1:25 to 1:50. The dilution process is determined by the composition of the
microemulsion. The droplet structure is already containedin the microemulsion, therefore no energy is required toachieve nanoparticle sized particles. The temperature gradient, pH, and the composition of the micro emulsion determine the product quality. High-temperature gradientsfacilitate rapid lipid crystallization and prevent aggregation. The dilution step constrains the lipid to a considerably lower concentration level than high pressure
homogenization based formulations.
Lyophilization increases the chemical and physical stability of SLN over an extended period of time. Transformation of the SLN into a solid form prevents crystal
growth by Ostwald ripening and avoids hydrolysis reac-

tions. Lyophilization is also possible for SLN formation ofdry powders for inhalation or for reconstitution for nebulizer delivery. The solid state of the lyophilized SLN will
have increased chemical and physical stability comparedto the aqueous lipid dispersion. Two additional transformations between the formulations are necessary and may
cause stability problems. The first transformation from
aqueous dispersion to powder involves the freezing of thesample and the evaporation of water under vacuum. Freezing of the sample may cause stability problems due to thefreezing out effect which results in changes in the osmolarity and the pH. The second transformation, resolubilization, involves low water and high particle content and highosmotic pressure that could facilitate particle aggregation.
The lipid content of the SLN dispersion should not exceed 5 % to prevent an increase in particle size because
direct content of lipid particles is decreased in more diluted samples. The protective effect of the surfactant canbe compromised during lyophilization. Diluted SLN dispersions will also have higher sublimation velocities anda higher specific area. The addition of cryoprotectants
will be necessary to decrease SLN aggregation and to obtain a better redispersion of the dry product. Typical cryoprotective agents are sorbitol, lactose, mannose, trehalose,
glucose, and polyvinylpyrrolidone. They decrease the osmotic activity of water and crystallization and favor a
glassy state of the frozen sample. Cryoprotectants areplace holders that prevent contact between discrete lipid
nanoparticles and they interact with polar head groups ofthe surfactants and serve as a pseudo hydration shell.
As an alternative to lyophilization, spray drying can beused to transform an aqueous SLN dispersion into a dry
product (Chougule M. et al., 2007, 2008; Patel G. et al.,2009). Spray drying can cause particle aggregation due tohigh temperatures, shear forces, and partial melting of theparticles, therefore it is recommended to use lipids withmelting points > 70 °C. The incorporation of carbohydrates and low lipid content improve the preservation of
the colloidal particle size during spray drying. The melting of the lipid can be minimized by using ethanol-water
mixtures as a dispersion medium instead of pure water
due to the lower inlet temperatures.

2.2.3 Examples of lipid-based delivery systems

Functional genomic studies to study novel targets forlung cancer or other diseases have commonly utilizedcommercially available cationic lipid transfection rea­gents, including Lipofectamine RNAiMAX (RamachandranS. et al., 2013), Lipofectamine2000 (Cao W. et al., 2012;
Chen G. et al., 2012; Fourtounis J. et al., 2012; Huang B.
et al., 2013; Liu Y. et al., 2013; Matsubara H. et al., 2012;
Nishimura Y. et al., 2014; Rahman M.A. et al., 2013;
Wang Y. et al., 2013; Zhang Y.-P. et al., 2012) and HiPerfect (Fourtounis J. et al., 2012). Other groups have made51

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

lipoplexes composed of phospholipids and cholesterol
with particle size of approximately 100 nm that haveshown higher cellular internalization in lung cancer cells
than with Lipofectamine2000, but had higher cytotoxicity. Oxime ether lipid nanoparticles with particle size of
150–220 nm transfected cells more efficiently with serum
present than conventional liposomal formulations (Biswas
S. et al., 2011).
A myeloid cell leukemia 1 (Mcl1)-specific siRNA administered in a lung metastases model at 0.21 mg/kg wasdelivered by a microsprayer intratracheally four times onalternating days starting at 5 days post intravenous (i.v.)
injection of either B16 or Lewis Lung Carcinoma cells(Shim G. et al., 2013). In this study cationic nanoliposomes with particle size of 200 nm were prepared using
the thin-film hydration technique and were composed of
the various lipid ratio of EDOPC, DOTAP, DOTMA,
DC-Cholesterol, DOPE, and Cholesterol (Shim G. et al.,2013). The thin-films were hydrated with 1 ml of
20 mmol/l HEPES (pH 7.4), and the resulting nanoliposomes were extruded through 0.2 μm polycarbonate
membrane filters three times using an extruder (Shim G.
et al., 2013).
An intranasally delivered liposome formulation containing a novel siRNA sequence against the epithelial sodium channel (ENaC) alpha subunit was developed andevaluated by Clark et al. (Clark K.L. et al., 2013). Liposomes administered intranasally caused significant ENaC
downregulation without TLR3, 8, or 9 activation, thereby
supporting that the effects were induced by RNAi (Clark
K.L. et al., 2013).
An aerosolized liposomal formulation was developedby Mainelis et al., who prepared liposomes composed ofegg phosphatidylcholine: 1,2-dipalmitoyl-sn-­glycero-3phosphatidylcholine:cholesterol containing doxorubicinand antisense oligonucleotides or siRNA targeted against
multidrug resistance-associated protein 1 (MRP1). This
group used these formulations to evaluate the delivery ofa nose-only small animal exposure chamber (Mainelis G.
et al., 2013). Mainelis et al. characterized the aerosolized
particles and found that the change of mean particle size
was insignificant during the 1 hour of continuous aerosolization. It was shown that, of the nebulized formulation,
1.4 % at each port was available for inhalation and thelung retention was higher than in the injected samples(Mainelis G. et al., 2013). The lung tumor model was prepared by intratracheal administration of luciferase expressing A549-luc cells followed by detection of tumor
growth by luminescence imaging. After inhalation treatment with antisense oligonucleotides and doxorubicin
combination, the tumor volumes were 90 % reduced,
compared to 40 % after doxorubicin i.v. injection (Mainelis
G. et al., 2013).

52

2.3 Polymer-based delivery systems
2.3.1 Introduction

Polymers for use in nanoparticle drug delivery systems
should be biocompatible in terms of non-toxicity, non-­antigenicity, biodegradability, and biocompatibility. The
natural polymers that have commonly been used for preparation of polymeric nanoparticles are albumin, chitosan,
dextran, gelatin, lectins, and sodium alginate (Tekade
R.K. and Chougule M.B., 2013; Tekade R.K. et al., 2014;
Youngren S.R. et al., 2013). There are many synthetic
polymers available, while some are still under development. The well-known synthetic polymers include Poly(e-caprolactone) (PECL), polylactide (PLA), polystyrene,
polyglycolide (PGA), Poly(lactide co- glycosides (PLGA),
polyanhydrides, polyorthoesters, polycyanoacrylates,
polycaprolactone, polyglutamic acid, polymaleic acid,poly(N-vinyl pyrrolidone), poly(methyl methacrylate),poly(vinyl alcohol), poly(acrylic acid), polyacrylamide,
polyethylene glycol, and polymethacrylic acid. Polymericnanoparticles include nanocapsules, which contain a reservoir core surrounded by a solid material shell, or nanospheres, which are solid matrix particles (Vauthier C. andBouchemal K., 2009). Polymeric siRNA carriers are capable of swelling due to hydration and release siRNA by
diffusion. Another physicochemical mechanism of releaseis by enzymatic degradation of the polymer at the delivery site, thus releasing the siRNA from the core. The
siRNA may also dissociate causing de-adsorption or release. An advantage of the use of polymers as a siRNAdelivery systems is the possibility of chemical modifications, which include the synthesis of block polymers andcomb polymers or attachment of targeting moieties(Glasgow M.D. and Chougule M.B., 2015). Disadvantagesof polymer delivery systems stem from the use of organic
solvents in production processes that leave residues, polymer cytotoxicity, and obstruct scalability of the production
process. Another disadvantage of peptide based polymer
inhaled delivery systems is their ability to increase theelastic and viscous moduli of lung fluid and mucus, decreasing mucociliary clearance, which can complicate
many diseases states (Innes A.L. et al., 2009; List S.J. etal., 1978; Marriott C. et al., 1982).

2.3.2 Preparation methods

The preparation methods of polymeric nanoparticles
from a polymeric dispersion include emulsification/ solvent diffusion, solvent evaporation, nanoprecipitation,
salting out, dialysis, and supercritical fluid (SCF) technology methods. The preparation methods of polymericnanoparticles from polymerization of monomers are
emulsion (including mini- and micro-), interfacial polymerization, and controlled/living radical polymerization (C/LRP). Ionic gelation and coacervation are methods forSusanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

polymeric nanoparticle preparation from hydrophilicpolymers.
2.3.2.1. Emulsion and diffusion techniques
Emulsions are typically formed by two immiscible
phases and a surface active agent and some form of dispersive force. Polymer precipitation from emulsion droplets can be achieved by removing the polymer solvent
through solvent evaporation, fast diffusion after dilution,or salting out (Vauthier C. and Bouchemal K., 2009).
These methods lead to formation of nanospheres when
performed on simple oil-in-water emulsions. Oil containing nanocapsules can be obtained by adding oil in thepolymer solution composing the emulsion droplets.
Emulsification and solvent diffusion is a modified version of the solvent evaporation method (Niwa T. et al.,1994). The encapsulating polymer is dissolved in a partially water soluble solvent such as propylene carbonate
and saturated with water to ensure the initial thermodynamic equilibrium of both liquids. In order to produce theprecipitation of the polymer and the consequent formationof nanoparticles, it is necessary to promote the diffusionof the solvent of the dispersed phase by dilution with anexcess of water when the organic solvent is partly miscible with water or with another organic solvent in the opposite case. The polymer-water saturated solvent phase isemulsified in an aqueous solution containing stabilizer,
leading to solvent diffusion to the external phase. Resultant nanospheres or nanocapsules will be formed dependent on the oil-to-polymer ratio. The solvent is removedby evaporation or filtration.
This technique allows for high encapsulation efficiencies (> 70 %), high batch-to-batch reproducibility, eliminates the need for homogenization, ease of scale-up, andnarrow size distribution. A disadvantage of this method
is that it requires high volumes of water that has to be
removed from the suspension and the leakage of water-­soluble entrapped therapeutic agent drugs into theexternal saturated-aqueous phase during emulsification
causes a reduction in entrapment efficiency (Reis C.P. etal., 2006). This method has moderate efficiency in theentrapping hydrophilic molecules, therefore it may have
limited applications for siRNA delivery systems.
Emulsion polymerization can be used to prepare
nanoparticles by monomer polymerization. It is a fast andeasily scalable nanoparticle preparation method. The continuous organic phase methodology involves the monomer
dispersion into an emulsion, inverse microemulsion, or
into a non-solvent of the monomer (Nagavarma B. et al.,2012; Reis C.P. et al., 2006). Surfactants or protective soluble polymers were used in the initial stages of polymerization to prevent aggregation. Since this method utilizes
toxic organic solvents, surfactants, monomers, and initiators that can be leached from the formed nanoparticles,

this method has been used sparingly in recent years(Nagavarma B. et al., 2012).
An alternate approach was developed because of theconventional method disadvantages of having non-­biodegradable polymer and difficult procedure. Poly(methylmethacrylate) (PMMA), poly (ethylcyanoacrylate)(PECA), and poly (butylcyanoacrylate) nanoparticles were
produced by a surfactant dispersion within an organic
phase consisting of cyclohexane, n-pentane, and toluene(Nagavarma B. et al., 2012). The monomer is dissolved in
the aqueous continuous phase and surfactants or emulsifiers are not required. Initiation occurs when collision happens between the monomer molecules and the initiatormolecule, which could be an ion or a free radical, within
the continuous phase (Nagavarma B. et al., 2012). Alternatively, the monomer molecule itself can be converted
into an initiating radical by high-energy radiation, such as
g-radiation, ultraviolet, or strong visible light. Polymer
chains form, according to a mechanisms of anionic polymerization, when collision occurs between the initiatedmonomer ions or monomer radicals and other monomer
molecules. Solid particle formation and phase separation
may occur before or after the polymerization reactions(Kreuter J., 1982; Vauthier C. et al., 2003).
The mini-emulsion polymerization method has beendeveloped using a wide range of polymer materials (Arias
J. et al., 2001; Ham H.T. et al., 2006; Ziegler A. et al.,2009). A typical mini-emulsion polymerization formulation contains water, monomer mixture, co-stabilizer,
surfactant, and initiator. The key difference between
emulsion polymerization and mini-emulsion polymerization is the use of a low molecular mass compound as theco-­stabilizer and also the use of a high-shear device.
Mini-emulsions are critically stabilized, require a highshear to reach a stead state, and have an interfacial tension much greater than zero (Rao J.P. and Geckeler K.E.,
2011).
Micro-emulsion polymerization is used to preparenanosized polymeric particles that involves a kineticallydifferent process than other emulsion methods (Mishra S.
and Chatterjee A., 2011). Although these processes bothproduce high molar mass colloidal polymeric particles,
the particle size achievable for micro-emulsion based
techniques is much smaller than for other emulsion based
techniques. For micro-emulsion polymerization, typicallya water soluble initiator, is placed into the aqueous phaseof a thermodynamically stable micro-emulsion containing
swollen micelles (Nagavarma B. et al., 2012). These spontaneously formed thermodynamically stable micelles arewhere polymerization commences. This process relies onlarge quantities of surfactant systems, which have an interfacial tension at the oil/water interface close to zero.
The particles formed are completely coated with surfactant because of this high concentration of surfactant.

53

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

Polymer chains are first formed only in some droplets, as
the initiation cannot be attained in all droplets simultaneously. Then, the osmotic and elastic influence of thechains destabilize the fragile micro-emulsions and typically lead to an increase in the particle size, the formationof empty micelles, and secondary nucleation. Very smalllattices of particle size of 5–50 nm are formed with thepresence of empty micelles. Critical factors of the kinetics
of micro-emulsion polymerization and the resultant polymeric nanoparticle properties are the concentration of
initiator, surfactant, monomer, and reaction temperature(Fessi H. et al., 1989; Nagavarma B. et al., 2012).
2.3.2.2 Solvent evaporation
The first step of the solvent evaporation technique involves the preparation of a polymer solution prepared involatile solvents and emulsions. Ethyl acetate is the solvent of choice because it has a better toxicological profile
when compared to the other widely used solvents, dichloromethane and chloroform (Rao J.P. and Geckeler
K.E., 2011). The primary emulsion is transformed into acolloidal nanoparticle suspension following the evaporation of the solvent from the polymer, which is facilitatedthrough the diffusion of the emulsion’s continuous phase(Nagavarma B. et al., 2012). The conventional methods
involve the formation of primary single emulsions, such as
oil-in-water (o/w) or the formation of double-emulsions,such as (water-in-oil)-in-water (w/o)/w. Nanocarriers
formed through this method utilize high-speed homogenization or ultra-sonication with subsequent solvent evaporation through continuous magnetic stirring or undervacuum. The resultant semi-solid particles may undergo
ultracentrifugation and be washed with distilled water tocollect and purify the nanocarriers. The resultant nanocarriers may then be freeze-dried with a suitable cryoprotectant and stabilizing agent. Particle sizes of thesenanocarriers are dependent on the type and concentrationof stabilizer, homogenizer speed, and polymer concentration. Reduction of particle size can be achieved throughhigh-speed homogenization and ultra-sonication.
2.3.2.3 Nanoprecipitation
Nanoprecipitation, or the solvent displacement method,
involves the precipitation of preformed polymer from anorganic solution and the diffusion of the organic solventin the aqueous medium with or without a surfactant(Barichello J.M. et al., 1999; Fessi H. et al., 1989; Galindo-­
Rodriguez S. et al., 2004; Ganachaud F. and Katz J.L.,
2005). The polymer is dissolved in a water miscible solvent of intermediate polarity, leading to the precipitationof spherical nanocarriers. This phase is injected into a
stirred aqueous solution containing a stabilizer as a surfactant. Polymer deposition on the interface between thewater and the organic solvent, caused by fast diffusion of54

the solvent, leads to instant formation of stable nanocarriers in suspension (Quintanar-Guerrero D. et al., 1998). In
order to facilitate the suspension formation, the first stepis to perform phase separation with a completely miscible
solvent that is also a non-solvent for the polymer (VauthierC. et al., 2003). This nanoprecipitation technique allows
the preparation of nanocapsules when a small volume ofnontoxic oil is incorporated in the organic phase. Considering the oil-based central cavities of the nanocapsules,
high loading efficiencies of hydrophobic drugs are often
reported. This technique spontaneously produces emulsification because of the high diffusion rates in result oflimiting water-miscible solvents (Nagavarma B. et al.,2012). Spontaneous emulsification is not observed if the
coalescence rate of the formed droplets is sufficiently
high due to certain instability when mixed in water ofsome water-miscible solvents. Acetone/dichloromethane
is used to dissolve drugs and increase their entrapment.
However, dichloromethane increases particle size and has
toxicological issues. This method is appropriate for lipophilic drugs; therefore its usefulness for siRNA nanocarriers is limited by low entrapment efficiency.
2.3.2.4 Salting out
The salting out effect causes the separation of a water
miscible solvent from an aqueous solvent (Reis C.P. et al.,2006). This procedure is considered a modified emulsification/solvent diffusion method. Drug and polymer are
first dissolved in a solvent, like acetone, which is thenemulsified with the salting-out agent into an aqueous gel.
Salting-out agents include electrolytes, such as magnesium chloride or calcium chloride, or non-electrolytes,
such as sucrose with a colloidal stabilizer such as polyvinylpyrrolidone or hydroxyethylcellulose (Nagavarma B.
et al., 2012). Induction of nanoparticle formation occurs
when the oil/water emulsion is diluted with enough wateror aqueous solution to enhance the diffusion of acetone
into the aqueous phase (Reis C.P. et al., 2006). Salting out
agent selection is highly influential in the drug encapsulation efficiency (Nagavarma B. et al., 2012). Formed polymeric nanoparticles are purified by cross-flow filtration toremove the solvent and salting out agent (Nagavarma B.
et al., 2012).
An advantage of the salting out method is that it minimizes stress to protein encapsulates (Jung T. et al., 2000;
Lambert G. et al., 2001; Nagavarma B. et al., 2012). The
salting out procedure is useful for heat sensitive substances since it does not require increased temperature.
The multiple washing steps pose a disadvantage when
considering entrapment efficiency. The greatest disadvantage of this method is that it is used exclusively for lipophilic drugs, therefore its application to siRNA is low.

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

2.3.2.5 Dialysis
Nanocarriers with small particle size and narrow particle size distribution can be prepared by dialysis (Fessi H.
et al., 1989; Kostag M. et al., 2010; Rao J.P. and Geckeler
K.E., 2011). Polymer dissolved in an organic solvent isplaced within a dialysis tube with desired molecularweight cut off (Nagavarma B. et al., 2012). Dialysis isthen performed against a non-solvent miscible with theformer miscible solvent. The use of the dialysis membraneor semi-permeable membranes allow the passive transportof solvents to slow down the mixing of the polymer solution with a non-solvent (Nagavarma B. et al., 2012). The
displacement of the solvent inside the membrane is followed by the progressive aggregation of the polymer due
to a loss of solubility and the formation of homogeneous
suspensions of nanoparticles. The mechanism of nanocarrier formation by dialysis method is based on a mechanism similar to that of nanoprecipitation. The solvent usedin the preparation of the polymer solution affects the morphology and particle size distribution of the nanoparticles.
2.3.2.6 Supercritical fluid technology (SCF)
Supercritical fluid technology has allowed for the useof more environmentally friendly solvents for the preparation of polymeric nanocarriers with high purity(Kawashima Y., 2001; York P., 1999). Supercritical fluidand dense gas technology are expected to offer an effective technique of nanoparticle preparation because it
avoids most of the drawbacks of traditional techniques,
such as presence of organic solvent (Nagavarma B. et al.,2012).
The production of nanoparticles using supercritical fluids has two principles. The first is the rapid expansion ofsupercritical solution (RESS). The second principle is the
rapid expansion of supercritical solution into liquid solvent (RESOLV). In traditional RESS, the solute is dissolved in a supercritical fluid, followed by the rapid
expansion of the formed solution across a capillary nozzle
or an orifice into ambient air. The resultant high degreeof super saturation and rapid pressure reduction in the
expansion results in homogenous nucleation and formation of dispersed particles (Nagavarma B. et al., 2012).
Mechanistic studies of different model solutes for the
RESS process indicate that within the expansion jet, both
micrometer-sized and nanometer particles are present(Weber M. and Thies M.C., 2002). A few studies werecarried out on the production of nanoparticles using
RESS. Poly (perfluoropolyetherdiamide) droplets produced from the rapid expansion of CO2 solutions. The
RESS experimental apparatus consists of three major
units: a high pressure stainless steel mixing cell, a syringe
pump, and a pre-expansion unit. A solution of polymer in
CO2 is prepared at ambient temperature. The solution isheated isobarically to the pre-expansion temperature

while pumped through pre-expansion unit before the solution reaches the nozzle. At this point, the supercriticalsolution is now allowed to expand through the nozzle, atambient pressure. The concentration and degree of saturation of the polymer have a considerable effect on theparticle size and morphology of the particles for RESS
(Blasig A. et al., 2002; Chernyak Y. et al., 2001; Lim K.T.
et al., 2005; Sane A. and Thies M.C., 2007).
A RESS modification, known as RESOLV, involves the
expansion of the supercritical solution into a liquid solvent instead of ambient air (Rao J.P. and Geckeler K.E.,
2011; Sun Y.P. et al., 2005). The RESS technique resultsin microscaled sized particles rather than nanoparticles.
In order to overcome this drawback, RESOLV was developed. In RESOLV, the liquid solvent suppresses the particle growth in the expansion jet, thus making it possible toobtain primarily nanosized particles (Meziani M.J. et al.,
2004; Nagavarma B. et al., 2012; Sun Y.P. et al., 2005).
2.3.2.7 Interfacial polymerization
Interfacial polymerization is one of the most establishedmethods for the preparation of polymeric nanoparticles(Gaudin F. and Sintes-Zydowicz N., 2008; Landfester K.
et al., 2010). In interfacial polymerization, there is stepwise polymerization of two reactive monomers or agents,
which are dissolved respectively in two phases (continuous and dispersed), and the reaction takes place at the interface of the two liquids (Karode S. et al., 1998). Hollow
polymeric nanoparticles were synthesized with the use ofinterfacial cross-linking reactions as polyaddition, polycondensation, or radial polymerization (Crespy D. et al.,2007; Danicher L. et al., 2000; Sarkar D. et al., 2005;
Scott C. et al., 2005). Polymerization of monomers at theoil/water interface of an oil-in-water micro-­emulsion produce oil-containing nanocapsules (Fallouh N.a.K. et al.,1986). The organic solvent, which is completely misciblewith water, served as a monomer vehicle and the interfacial polymerization of the monomer was believed to occur
at the surface of the oil droplets that formed during emulsification (Aboubakar M. et al., 1999; Gallardo M. et al.,
1993; Vauthier C. and Bouchemal K., 2009). The use ofacetone or acetonitrile, which are aprotic solvents, wasrecommended for nanocapsule preparation (Nagavarma
B. et al., 2012). Alternatively, protic solvents, such as ethanol, m-butanol and isopropanol, were found to also induce the formation of nanospheres (Nagavarma B. et al.,2012; Puglisi G. et al., 1995). Nanocapsules containing
aqueous water can be formed by the interfacial polymerization of monomers in water-in-oil interface and precipitated out forming the nanocapsule shell (Gasco M. andTrotta M., 1986; Watnasirichaikul S. et al., 2000). Therefore, siRNA containing nanocapsules can be formed using
the interfacial polymerization of monomers in water-inoil microemulsions.

55

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

2.3.2.8 Controlled/living radical polymerization (C/LRP)The primary restrictions of radical polymerization are
due to unavoidable radical to radical termination reactions
and cause a lack of control over the molar mass, the end
functionalities, and the overall macromolecular structure(Nagavarma B. et al., 2012). The C/LRP processes are
based on previous polymerization techniques and opens
new prospects in polymeric nanoparticle preparation(Matyjaszewski K. and Xia J., 2001; Nagavarma B. et al.,2012; Zetterlund P.B. et al., 2008; Zetterlund P.B. et al.,2007). The growing public concern over environmental
impacts and the growth of hydrophilic polymer applications in medicine have sparked interest in C/LRP process.
These factors have given rise to green chemistry techniques and created a demand for environmentally andchemically benign solvents such as water and supercritical carbon dioxide (Nagavarma B. et al., 2012). Industrial
radical polymerization in aqueous dispersed systems andspecifically in emulsion polymerization is widely used.
The primary goal was to control the characteristics of thepolymer to modulate their function, molar mass, molar
mass distribution, and structure. Implementation of C/LRP
in aqueous dispersed systems, results in the formation ofpolymeric nanoparticles with precise particle size and size
distribution control (Braunecker W.A. and Matyjaszewski
K., 2007; Nagavarma B. et al., 2012). Among the available
controlled/living radical polymerization methods successfully and extensively studied methods include nitroxide-mediated polymerization (NMP), atom transfer
radical polymerization (ATRP), and reversible addition
and fragmentation transfer chain polymerization (RAFT)(Dire C. et al., 2008; Min K. et al., 2009; Nicolas J. et al.,2007; Rieger J. et al., 2010; Siegwart D.J. et al., 2009;
Zhou X. et al., 2007). The size of the resultant polymericnanoparticles depends on the type of control agent, control agent concentration, monomer, initiator, and theemulsion type.
2.3.2.9 Hydrophilic polymer ionic gelation/coacervation
Polymeric nanoparticles are prepared by using biodegradable hydrophilic polymers such as chitosan or gelatin
as shown in Fig. 6. In this method, the positive charged
amino groups of the polymer interacts with the negatively
charged triphosphate to form coacervates with a size inthe nanometer range (Calvo P. et al., 1997a; Calvo P. et al.,1997b; Dustgania A. et al., 2008). Coacervates are formed
as a result of electrostatic interaction between two aqueous phases, whereas ionic gelation involves the material
undergoing transition from liquid to gel due to ionic interaction conditions at room temperature (Nagavarma B. etal., 2012).

2.3.3 Examples of polymer-based delivery systems

Chitosan is a generally regarded as safe (GRAS) excip-

56

Fig. 6

Diagram showing the preparation of polymericnanoparticles using the coacervation (desolvation)technique.

ient and a natural polysaccharide (Keefe D., 2011; Mao S.
et al., 2010). However, its applications in siRNA delivery
systems have been hindered due to poor water solubility
and low transfection efficiency (Merkel O.M. et al., 2014).
An inhalable chitosan/siRNA dry powder composed ofunmodified chitosan was prepared using the supercritical
CO2 technique followed by manual grinding in an investigation by Okuda at al. (Okuda T. et al., 2013). Water, ethanol and CO2 solvents were combined in a compressedcolumn (35 °C, 25 MPa). This supercritical CO2 technique
embodies tolerable conditions for the precipitation of
powders capable of reconstitution without siRNA or
nanoparticle degradation. The hydrodynamic particle
sizes and zeta potentials of the developed chitosan particles were shown to be maintained throughout and after
the powder formation process (Okuda T. et al., 2013). Biodistribution of intratracheally administered Cy5.5 labeledsiRNA formulations, including the nanoparticle suspension, the dry powder, and the free/naked siRNA weremonitored. The efficiency of the delivered siRNA wasmeasured in a lung metastases model of colon26/Luc cells
by luciferase downregulation. The dry powder particles
had highest knockdown efficiency, however they had
higher lung clearance than the nanoparticle suspension(Okuda T. et al., 2013). This study is one of the few reports that describe the formulation of an inhalable dry
powder containing siRNA.
Another dry powder formulation prepared using a double emulsion solvent evaporation method was designed by
Jensen et al., where siRNA was entrapped within
DOTAP-modified PLGA nanoparticles (Jensen D.K. etal., 2012; Jensen D.M. et al., 2010). The total concentration of DOTAP and PLGA in chloroform was kept constant and the optimal formulation contained 25 % (w/w)
DOTAP by varying the weight/weight (w/w) percentage
of DOTAP. The particles were spray dried with mannitol
and an aerodynamic size of 3.69 ± 0.18 μm obtained,
which is within the optimal size range for deep lungdeposition, and did not exhibit aggregation or coalescenceSusanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69(Jensen D.K. et al., 2012; Jensen D.M. et al., 2010). During
spray drying, the sugar alcohol remained in its crystalline
state, as determined by X-ray powder diffraction analysis,
and therefore functioned as a stabilizer. The hydrodynamic diameters and cellular uptake remained unchanged
before and after freeze-drying according to statisticalanalysis, which supported the conclusion that spray-­drying is a powerful technique for engineering siRNA
nanoparticle dry powder formulations (Jensen D.K. et al.,2012).
PLGA poly[vinyl-3-(dialkylamino) alkylcarbamate-­covinyl acetate-co-vinyl alcohol]-graft-poly(D,Llactideco-glycolide), (DEAPA-PVA-g-PLGA), that contained
covalently modified positively charged side groups for
siRNA complexation was prepared by Benfer et al.
(Benfer M. and Kissel T., 2012). The H1299 NSCLC cellular uptake of DEAPA-PVA-g-PLGA/siRNA nanoparticles increased after addition of surfactant and wasinternalized mostly by clathrin-mediated endocytosis(Benfer M. and Kissel T., 2012). Merkel et al. also showedthat DEAPA-PVA-g-PLGA/siRNA nanoparticles remainin the lung for extended periods of time when compared
to gold-standard PEI-formulated or free siRNA. Thus,
making the formulation applicable for intratracheal administration (Zheng M. et al., 2012). To prepare the inhalable dry DEAPA-PVA-g-PLGA/siRNA nanoparticle
powder, the freeze-drying protocol was optimized. Cryoprotectants, such as dextrose, hydroxyproxypropyl-­betacyclodextrin, lactosucrose, polyvinylpyrrolidone (PVP),
sucrose, and trehalose can be used to stabilize the nanoparticles. Although particles freeze-dried in the cryoprotectant 10 % glucose were about 150 nm in particle size
after reconstitution, the particles freeze-dried in the cryoprotectant 10 % saccharose were much larger with a particle size of 250 nm. However, the freeze-dried larger
particles showed more similar and efficient cellular uptake when compared to freshly prepared nanoparticles(Zheng M. et al., 2012). It was demonstrated that gene
downregulation efficacy was preserved through thefreeze-drying process as assessed after reconstitution of
the nanoparticles (Zheng M. et al., 2012). These findings
are similar to those of a study by Kasper et al. who
showed that cryoprotectant concentrations of 12 % andhigher can prevent PEI/DNA polyplex aggregation which
retains their transfection efficiency after redispersion(Kasper J.C. et al., 2011).
Luo et al. synthesized salbutamol-modified guanidylated
chitosan and subsequently formed polyplexes targeted tolung smooth muscle cells for potential applications inasthma or COPD (Luo Y. et al., 2012). The formulations
were nebulized with an Aeroneb ® Pro nebulizer (Aerogen,
Galway, Ireland) and then the collected condensate wasadministered intratracheally with a PennCentury microsprayer to transgenic mice expressing enhanced green

fluorescent protein (EGFP) (Luo Y. et al., 2012). The animals were treated daily with 5 μg of siRNA 3 days, and invivo knockdown was quantified by confocal laser scanning microscopy (CLSM) of tissue sections and by Western blot. The targeted formulation achieved a 40 % gene
downregulation (Luo Y. et al., 2012).
Atelocollagen, a highly purified and pepsin-treatedtype I collagen obtained from calf dermis also raised interest for its use in pulmonary delivery of siRNA (Merkel
O.M. et al., 2014). Protein and siRNA polyplexes wereprepared by mixing during a 16h incubation and wereadministrated intratracheally to Wistar or EGFP expressing rats (Liu S. et al., 2012). The therapeutic gene target
was syntaxin4, which regulates the function of mucosal-­
type mast cells and is one of the soluble membrane
N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Following siRNA treatment,
syntaxin4 knockdown significantly decreased granulecontents release from rat mucosal mast cells. Rats weresensitized and challenged with ovalbumin (OVA) following the allergic mouse model protocols to elicit symptoms
of asthma. The rats were then intratracheally instilledwith atelocollagen siRNA complexes once a day for 5 days.
These treatments decreased the rat mast cell protease-II(RMCPII) levels in the bronchoalveolar lavage fluid(BALF) and prevented asthmatic airway constriction.
Therefore, the authors concluded that syntaxin 4 knockdown prevents degranulation and stabilizes mucosal mast
cells (Liu S. et al., 2012).
Spermine, a naturally occurring polymer consisting of
small linear tetra-amines with two primary and two secondary amines, is popular for the delivery of nucleic acids(Merkel O.M. et al., 2014). Endogenous spermines play a
key role in the compact packaging of cellular DNA, thisproperty is exploited artificially in nucleic acid delivery(Eliyahu H. et al., 2006; Jiang H.L. et al., 2011). Due tothe short molecules and the resultant rigidity of spermine,
unmodified spermine does not efficiently compact siRNA(Merkel O.M. et al., 2014; Pavan G.M. et al., 2010). Jiang
et al. prepared glycerol propoxylate triacrylate-spermine(GPT-SPE) and shRNA polyplexes that had a particle sizeof 160 nm and had a zeta potential of +9 mV. Human NSCLC was mimicked using a KrasLA1 model and administered 8 mg of GPT-SPE and 0.8 mg of shRNA (scrambled
or Akt1 shRNA) twice a week for 4 weeks as aerosol (Jiang
H.L. et al., 2011). The authors did not discuss whether the
preparation and delivery processes had an effect on thepolyplex physico-chemical characteristics. In a later
study, GPT-SPE delivered shRNA against the same target.
The same nose-only exposure chamber was used but administered half of the previous dose (0.4 mg) DNA as anaerosol (scrambled or Akt1 shRNA) twice a week over a
timeframe of 4 weeks (Hong S.-H. et al., 2012). Aerosoldelivery of Akt1 shRNA suppressed lung tumor growth57

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

and the authors elucidated the mechanism of action (HongS.-H. et al., 2012). The same group conducted anotherlater study, which used the GTE-SPE polymer for sodium-­
dependent phosphate co-transporter 2b (NPT2b) siRNAdelivery via nose-only inhalation (Hong S.-H. et al.,2013). The animals were treated for four weeks with
0.5 mg of siRNA twice a week. Aerosolized NPT2b
siRNA downregulated NPT2b expression levels as detected via Western blotting, densitometric analysis andqPCR, and significantly increased the pro-apoptotic protein levels. The activation of apoptosis was confirmed in
TUNEL positive cells, and PCNA and VEGF levels weredecreased after knockdown of Akt1 (Hong S.-H. et al.,2013).
Choi et al. synthesized a conjugate of dexamethasone
and PEI (DEXA-PEI) polyplex to target the cellular nucleus via interaction with the glucocorticoid receptor(Choi M. et al., 2013). This concept was previously explored, but the novelty of this approach was induction of
the model of acute lung inflammation by the instillationof baked SiO2 nanoparticles (Choi M. et al., 2013; Mi Bae
Y. et al., 2007). siRNA against macrophage migration inhibitory factor (MIF) was complexed with DEXA-PEI
and particles obtained had a particle size of 355 nm,
whereas PEI/siRNA complexes aggregated to a particle
size of > 600 nm. These particles were administered intratracheally with either 2.5 μg or 10 μg siRNA per animal and were shown to decrease pulmonary inflammationand the MIF mRNA and Muc5ac expression after treatment with conjugated and nonconjugated PEI complexes
72 h post-administration of the polyplexes (Choi M. et al.,2013).

2.4 Peptide-based delivery vectors
2.4.1 Introduction

Protein transduction domains (PTDs) are short aminoacid chains that interact with the plasma membrane thatallows for cellular uptake that can be used for siRNA delivery. Obtaining controlled siRNA cellular delivery
through the use of peptides as carriers can overcome
siRNA delivery constraints, which include poor bioavailability and clinical efficacy. Chemical conjugation of cationic peptide cell penetrating peptides (CPP) to siRNAhas facilitated enhanced siRNA tissue delivery and cellular internalization. The TAT protein from HIV-1 which
was discovered to be responsible for the cellular uptake ofthe virus was of interest of drug delivery scientists. Since
that time, a variety of CPPs have been developed to facilitate the cellular uptake transport of therapeutic molecules
including siRNA (Lam J.K.-W. et al., 2012). Covalent
peptide to siRNA disulfide bond formation attachment, or
non-covalent electrostatic interactions between the siRNAand peptide to form complexes can be used to prepare

58

siRNA peptide conjugates (Lam J.K.-W. et al., 2012).
Their mechanism of action is also expected to vary due tothe sequence diversity of different CPPs. These peptides
function to improve cellular delivery by efficient transport
across the cellular membrane or promote endosomal escape (Endoh T. and Ohtsuki T., 2009). The activities ofdifferent CPPs have been reviewed elsewhere (Deshayes
S. et al., 2005; Meade B.R. and Dowdy S.F., 2007).

2.4.2 Preparation methods

Double stranded siRNA, with or without modification,
can form stable or cleavable peptide conjugates using a
range of different conjugation chemistries (Jeong J.H. etal., 2008). Designing conjugation schemes include selection of linkage chemistry, spacer molecule, and orientation of the siRNA and peptide components (Zatsepin T. etal., 2005). There are two main strategies for synthesizing
siRNA peptide conjugates: the total stepwise solid-phase
synthesis and the solution-phase or solid-phase fragment
coupling of peptides with oligonucleotides that have been
prepared individually on solid supports (Corey D.R.,
2004). Stable linkages may be prepared from either
method, but fragment coupling of peptides is necessary
for cleavable disulfide linkages.
The most common conjugation chemistries have involved fragment coupling in aqueous solution. Peptide
and siRNA fragments are prepared on their own solid
supports by conventional automated solid-phase synthesis
procedures. Each fragment requires a masked functionalgroup to be incorporated during the assembly, which canbe released chemoselectively during a simple deprotonation step. A specific chemical reaction is then initiated inaqueous medium between the functional group on the oligonucleotide and that on the peptide to produce the desired conjugate. An advantage of the fragment-coupling
method is that both components can be purified, for example by HPLC, before conjugation so that the conjugation product can be easily identifiable. This method’s
disadvantage is that the aqueous conditions of conjugation
sometimes need to be modified to maintain the solubility ofboth components and the conjugate product. A reaction ofa cysteine-substituted siRNA with a thioester-­substituted
peptide produces a stable amide linkage through themechanism of native ligation. Another method involves
the reaction of an aldehyde-containing siRNA with acysteine-containing peptide, forming a thiazolidine linkage, or with an aminooxy peptide forming an oxime linkage, or with a hydrazinopeptide forming a hydrazine. An
aldehyde oligonucleotide is unstable and must be generated
by periodate treatment of a cis-diol substituted siRNAjust before conjugation (Hermanson G.T., 2013).
The most straightforward method of fragment conjugation involves the synthesis of the siRNA component
with an alkyl thiol linker on either of the 5’ or 3’-endSusanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

forming a 3’-thiopropyl or a 5’-thiohexyl linker (Turner
J.J. et al., 2006). Following activation with pyridylsulfide,
the siRNA can be coupled to a peptide. These thiol-­functionalized siRNA can be reacted with a cysteine-­
containing peptide to form a cleavable disulfide linkage
or with a bromoacetyl-substituted peptide to form a stablethioether linkage. A maleimide derivative of the peptide can also be prepared for the reaction with a thiol-­substituted siRNA via a Michael-type addition reaction(Hermanson G.T., 2013).

2.4.3 Examples of Peptide-based Deliver Vectors

The PTDs and cell penetrating peptides are small (10–30 amino acid), positively charged molecules that usuallycontain arginine and lysine. Arginine and lysine aminoacids provide primary and secondary amine functionalgroups in their side chains that can be protonated for electrostatic interaction with siRNA and cellular membranes
to enhance the cellular permeability (Merkel O.M. et al.,2014). Delivery of siRNA using a short peptide conductedby Oh et al. who used R3V6 peptides, which were composed of 3 arginines and 6 valines (Merkel O.M. et al.,2014; Oh B. and Lee M., 2014). Ternary complexes wereformed with sphingosine-1-phosphate lyase (S1PLyase), a
recombinant high mobility group box-1 box A peptide(HMGB1A), siRNA , and R3V6 (siS1PLyase/HMGB1A/
R3V6). Premixture of siRNA and HMGB1A avoided aggregation and formed particles with sizes below 200 nm
that elicited an anti-inflammatory effect (Oh B. and Lee
M., 2014). These ternary complexes were tested in vitro
and delivered siRNA into non-phagocytosing LA-4 lungepithelial cells more efficiently than PEI and Lipofectamine, inferring that their uptake mechanisms were notbased on phagocytosis (Merkel O.M. et al., 2014; Oh B.
and Lee M., 2014). Intratracheal administration of the ternary complexes reduced the S1PLyase level efficiently in
an LPS-induced BALB/c ALI model. The cell penetrating
peptides, TAT and double TAT (dTAT) was used forsiRNA delivery by Baoum et al. (Baoum A. et al., 2012).
To form the complexes, siRNA was mixed with the polycations and then CaCl2 was added to the mixture in order
to compact the siRNA and to decrease the hydrodynamic
diameters of the particles (Baoum A. et al., 2012). Following i.v. injection of the complexes, high gene knockdown
was observed in the lung (Baoum A. et al., 2012). These
results suggest that formulating these siRNA complexes
for pulmonary delivery by inhalation, would allow forhigh local concentration while keeping high accumulationin the lungs.

2.5 Inorganic-based delivery systems
2.5.1 Introduction

Various promising inorganic delivery systems havebeen investigated for the delivery of siRNA devised for
diagnostic and therapeutic purposes. They include carbon
nanotubes and metals such as iron oxide, quantum dots,
gold and silica (Chen S. et al., 2014). Mesoporous silica
based nanoparticles (MSN) have been widely investigatedas carriers for siRNA based targeted drug delivery systems (Chen A.M. et al., 2009; Radu D.R. et al., 2004). The
MSN are chemically stable, safe and biodegradable whichmakes it a promising gene delivery carrier. (Borm P. etal., 2006; Finnie K.S. et al., 2008). MSN also possess several advantages over other inorganic carriers, such as capability to encapsulate higher amounts of drugs due tolarge pore volumes and improved stability due to their inorganic oxide framework (Slowing, Ii et al., 2007).

2.5.2 Examples of inorganic-based delivery systems

Taratula et al. prepared MSNs with a 3-mercaptopropyl
modification for labeling with Cy5.5 and with pyridylthiol
for conjugation of a lutein hormone releasing hormone(LHRH) peptide via a PEG spacer and for attaching thiol
modified siRNA (Merkel O.M. et al., 2014; Taratula O. etal., 2011b). The MSNs pores were loaded with either
doxorubicin or cisplatin with two types of siRNA for achemotherapeutic combination therapy, as shown in Fig.
7. The MRP1 and BCL2 siRNA were selected based on
their target mRNA’s role in suppression of pump and nonpump cellular chemoresistance. An orthotopic model oflung cancer was established by intratracheal installationof luciferase expressing A549 cells (Garbuzenko O.B. etal., 2010). Nanoparticle accumulation in the mouse lungs
was detected by whole body fluorescence imaging and revealed that inhalation administration prevented the MSN
systemic circulation delivery and thus limited their accumulation elsewhere in the body (Taratula O. et al., 2011b).
The same group further utilized the modified MSN’s toFig. 7

Co-delivery of siRNA and anticancer agents using
mesoporous silica nanoparticles (MSN) The surface
engineered approach consists of surface bound siRNAand PEG-LHRH.

59

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

co-deliver anticancer drugs [doxorubicin (DOX) or cisplatin (CIS)], suppressor of pump resistance (siRNA targeting MRP-1 mRNA), and suppressor of non-pump cellularresistance (siRNA targeting BCL2 mRNA) using tumor
targeting moiety LHRH peptide. The inhalation deliveryof LHRH targeted MSN-drug complexes carrying both
BCL2 and MRP1 siRNA (LHRH-PEG-siRNA-DOXMSN) showed that 73.6 % of MSN was retained in lungcompared to 5 % when i.v. injected (Taratula O. et al.,2011a). Also, after i.v. administration MSN-based DDS
was found to be accumulated mainly in liver (73 %), kidneys (15 %) and spleen (7 %) while inhalation delivery
resulted in significant reduction in accumulation to only
17 %, 9 % and 1 % in liver, kidneys and spleen respectively.

A RSV vaccine is not available for pediatric patients dueto the potential teratogenicity and reduced effectiveness(Burnett J.C. et al., 2011). RSV may also produce severe respiratory diseases like pneumonia in immunocompromisedadolescents and adults, and in the elderly (Devincenzo J.
et al., 2010). RSV replicates in the outermost layer of the
airway epithelium, including regions lining the nasal passages, trachea, and the bronchioles (Devincenzo J.P.,
2009). The anti-viral efficacy of ALN-RSV01 was demonstrated by the reduced infection rate in healthy patientsand by reducing the daily symptoms in lung transplantation patients (Devincenzo J. et al., 2010; Zamora M.R. etal., 2011). This example clearly shows the advancementthat siRNA brings to clinical care by providing safe andefficacious treatment options.

3. Ongoing clinical trials on aerosolized
siRNA based medicines

4. Conclusions and future directions

ExcellairTM from ZaBeCor Pharmaceuticals (Bala
Cynwyd, Pennsylvania, USA) is an inhaled siRNA for thetreatment of asthma. This siRNA functions as a spleentyrosine kinase (Syk) inhibitor thereby inhibiting thetranscription factors regulated by Syk. Syk is involved in
the signaling from the B-cell receptor and regulates thedownstream signaling cascades that ultimately leads tothe activation of several proinflammatory transcription
factors. Thus, the specific Syk inhibition by ExcellairTM is
designed to reduce the inflammation associated withasthma. In a phase I study, patients with asthma received
the inhaled siRNA therapeutic for 21 consecutive days
(Watts J.K. and Corey D.R., 2010). The drug was well tolerated in all asthma patients, with no serious adverse side
effects. Moreover, 75 % of the patients treated reported
improvement of breathing or reduced rescue inhaler use,
while placebo patients reported no such improvement.
These results helped propel ExcellairTM into a phase II
clinical trials in 2009, however these results are not available (De Backer L. et al., 2015).
Alnylam Pharmaceuticals (Cambridge, Massachusetts,USA) developed a nebulizer and nasal spray delivered
siRNA therapeutic. The ALN-RSV01 siRNA was designed
for the treatment of respiratory syncytial virus (RSV) targeting the nucleocapsid protein is indicated for prophylactic treatment against RSV infections in healthy patients(NCT00496821) and for treating RSV infection in lungtransplant patients (NCT00658086 and NCT01065935).
The ALN-RSV01 siRNA is composed of a double-stranded
RNA duplex with 19 base pairs of complementary and 2-nt
dT overhangs at both 3’ ends (Alvarez R. et al., 2009,
Devincenzo J. et al., 2008).
RSV is the leading cause of hospitalization of infants,
infecting approximately 70 % of infants under a year old.

60

The therapeutic potential of siRNA in the treatment oflung diseases has yet to be fully explored. The use of inhalable nanoparticulate siRNA delivery systems havebarriers to their effective delivery, such as degradation by
RNase, mucociliary clearance, cough clearance, and alveolar macrophage clearance, but overcoming these constraints will offer unique advances to the field. Importantparameters to consider in aerosolized siRNA formulation
design and development are the particle, aerosol, physicochemical, stability, targeted site, and lung physiologicalproperties.
siRNA carrier systems for aerosol delivery include
those of naked siRNA formulation, lipidic, polymeric,peptide, or inorganic origin. Lipid-based delivery systems, such as liposomes or solid lipid nanoparticles havereceived attention for inhaled pulmonary delivery by using endogenously present phospholipids and surfactant-­
like lipids. Polymer-based siRNA delivery systems can becomposed of biocompatible polymers which may bechemically modified to improve cellular delivery, internalization, and intracellular release. Peptide-based siRNAdelivery vectors improve the poor bioavailability andclinical efficacy associated with naked siRNA delivery.
Inorganic-based delivery systems allows for the uniquedelivery of siRNA for diagnostic and therapeutic purposes at higher payloads compared to other delivery systems. Safety of these formulations must be proventhrough various laboratory and clinical studies. In vitro
techniques for characterizing aerosolized pulmonarysiRNA delivery systems include dissolution tests, inertial
cascade impaction, delivered dose uniformity assays, laser diffraction, and laser Doppler velocimetry. Ex vivo
techniques such as the isolated perfused lung model canbe used to identify inhaled particulate distribution. In
vivo techniques like scintigraphy and pharmacokinetic/

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

pharmacodynamics analysis can give even more insightto the distribution, safety, and efficacy of a pulmonary inhaled siRNA nanocarrier. The vigorous work researchers
have completed regarding the local pulmonary inhaleddelivery of siRNA in various models of lung disease. Twoinhalable therapeutic siRNA products have entered clinical trials. More clinically relevant preclinical studies utilizing biocompatible and safe siRNA delivery vehicles
will provide inhalable aerosolized siRNA delivery vehicles for future clinical studies. Overall, recent work focused on inhalation delivery of siRNA for the treatmentof pulmonary disorders has demonstrated the feasibilityof this approach. The rapid progress in this area of research would facilitate the translation to the clinic.

Acknowledgements
The authors acknowledge the support of the National
Institute of General Medical Science of the National Institutes of Health under award number SC3GM109873. The
authors acknowledge Hawai’i Community Foundation,
Honolulu, HI 96813, USA, for research support on lungcancer, mesothelioma, and asthma projects (Leahi Fund
15ADVC-74296) in 2015, 2013, and 2011, respectively.
The authors would like to acknowledge the 2013 George F.
Straub Trust and Robert C. Perry Fund of the Hawai’i
Community Foundation, Honolulu, HI 96813, USA, forresearch support on lung cancer. The authors also acknowledge a seed grant from the Research Corporation of
the University of Hawai’i at Hilo, Hilo, HI 96720, USA,and The Daniel K. Inouye College of Pharmacy, University of Hawai’i at Hilo, Hilo, HI 96720, USA, for providing start-up financial support to their research group. The
authors acknowledge the donation from Dr. Robert S.
Shapiro, MD, Dermatologist, Hilo, HI 96720, USA in
support of development of nanotechnology based medicines.

References
Aalto A.P., Sarin L.P., Van Dijk A.A., Saarma M., Poranen
M.M., Arumae U. and Bamford D.H., Large-scale production of dsRNA and siRNA pools for RNA interference
utilizing bacteriophage phi6 RNA-dependent RNA polymerase, RNA, 13 (2007) 422–429.
Aboubakar M., Puisieux F., Couvreur P., Deyme M. and VauthierC., Study of the mechanism of insulin encapsulation in poly
(isobutylcyanoacrylate) nanocapsules obtained by interfacial polymerization, Journal of biomedical materials
research, 47 (1999) 568–576.
Akinc A., Zumbuehl A., Goldberg M., Leshchiner E.S., Busini
V., Hossain N., Bacallado S.A., Nguyen D.N., Fuller J. andAlvarez R., A combinatorial library of lipid-like materials

for delivery of RNAi therapeutics, Nature biotechnology,
26 (2008) 561–569.
Allison S.D. and Anchordoquy T.J., Mechanisms of protectionof cationic lipid DNA complexes during lyophilization,
Journal of pharmaceutical sciences, 89 (2000) 682–691.
Allison S.D. and Anchordoquy T.J., Maintenance of nonviral
vector particle size during the freezing step of the lyophilization process is insufficient for preservation of activity: Insight from other structural indicators, Journal ofpharmaceutical sciences, 90 (2001) 1445–1455.
Allison S.D., Dc Molina M. and Anchordoquy T.J., Stabilizationof lipid/DNA complexes during the freezing step of thelyophilization process: The particle isolation hypothesis,
Biochimica et Biophysica Acta (BBA)-Biomembranes,
1468 (2000) 127–138.
Alvarez R., Elbashir S., Borland T., Toudjarska I., Hadwiger P.,
John M., Roehl I., Morskaya S.S., Martinello R., Kahn J.,
Van Ranst M., Tripp R.A., Devincenzo J.P., Pandey R.,
Maier M., Nechev L., Manoharan M., Kotelianski V. andMeyers R., RNA interference-mediated silencing of therespiratory syncytial virus nucleocapsid defines a potent
antiviral strategy, Antimicrob Agents Chemother, 53 (2009)3952–3962.
Amarzguioui M., Rossi J.J. and Kim D., Approaches for chemically synthesized siRNA and vector-mediated RNAi, FEBS
Letters, 579 (2005) 5974–5981.
Anchordoquy T.J., Carpenter J.F. and Kroll D.J., Maintenanceof transfection rates and physical characterization of lipid/
DNA complexes after freeze-drying and rehydration,
Archives of biochemistry and biophysics, 348 (1997) 199–
206.
Arias J., Gallardo V., Gomez-Lopera S., Plaza R. andDelgado A., Synthesis and characterization of poly (ethyl2-­cyanoacrylate) nanoparticles with a magnetic core, Journal of Controlled Release, 77 (2001) 309–321.
Baoum A., Ovcharenko D. and Berkland C., Calcium condensed
cell penetrating peptide complexes offer highly efficient,
low toxicity gene silencing, Int J Pharm, 427 (2012) 134–
142.
Barichello J.M., Morishita M., Takayama K. and Nagai T.,
Encapsulation of hydrophilic and lipophilic drugs in PLGA
nanoparticles by the nanoprecipitation method, Drug development and industrial pharmacy, 25 (1999) 471–476.
Benfer M. and Kissel T., Cellular uptake mechanism and knockdown activity of siRNA-loaded biodegradable DEAPAPVA-g-PLGA nanoparticles, European Journal of Pharmaceutics and Biopharmaceutics, 80 (2012) 247–256.
Biswas S., Knipp R.J., Gordon L.E., Nandula S.R., Gorr S.U.,
Clark G.J. and Nantz M.H., Hydrophobic oxime ethers: A
versatile class of pDNA and siRNA transfection lipids,
ChemMedChem, 6 (2011) 2063–2069.
Bitko V., Musiyenko A., Shulyayeva O. and Barik S., Inhibitionof respiratory viruses by nasally administered siRNA,
Nature medicine, 11 (2005) 50–55.
Blasig A., Shi C., Enick R.M. and Thies M.C., Effect of concentration and degree of saturation on RESS of a CO2-soluble
fluoropolymer, Industrial & engineering chemistryresearch, 41 (2002) 4976–4983.

61

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

Bochicchio S., Dalmoro A., Barba A.A., Grassi G. and Lamberti
G., Liposomes as siRNA delivery vectors, Current drug
metabolism, 15 (2014) 882–892.
Borm P., Klaessig F.C., Landry T.D., Moudgil B., Pauluhn J.,
Thomas K., Trottier R. and Wood S., Research strategies
for safety evaluation of nanomaterials, part V: Role of dissolution in biological fate and effects of nanoscale particles,
Toxicol Sci, 90 (2006) 23–32.
Braunecker W.A. and Matyjaszewski K., Controlled/living radical polymerization: Features, developments, and perspectives, Progress in Polymer Science, 32 (2007) 93–146.
Burnett J.C. and Rossi J.J., RNA-based therapeutics: Current
progress and future prospects, Chemistry & biology, 19(2012) 60–71.
Burnett J.C., Rossi J.J. and Tiemann K., Current progress of
siRNA/shRNA therapeutics in clinical trials, Biotechnology Journal, 6 (2011) 1130–1146.
Buyens K., Demeester J., De Smedt S.S. and Sanders N.N.,
Elucidating the encapsulation of short interfering RNA in
pegylated cationic liposomes, Langmuir, 25 (2009) 4886–
4891.
Çağdaş M., Sezer A.D. and Bucak S., Liposomes as potential
drug carrier systems for drug delivery, 2014.
Calvo P., Remuñan-López C., Vila-Jato J.L. and Alonso M.J.,
Chitosan and chitosan/ethylene oxide-propylene oxide block
copolymer nanoparticles as novel carriers for proteins andvaccines, Pharmaceutical Research, 14 (1997a) 1431–1436.
Calvo P., Remunan Lopez C., Vila Jato J. and Alonso M., Novelhydrophilic chitosan polyethylene oxide nanoparticles asprotein carriers, Journal of Applied Polymer Science, 63(1997b) 125–132.
Cao W., De Oliveira Ribeiro R., Liu D., Saintigny P., Xia R.,
Xue Y., Lin R., Mao L. and Ren H., EZH2 promotes malignant behaviors via cell cycle dysregulation and its mRNAlevel associates with prognosis of patient with non-small
cell lung cancer, PloS one, 7 (2012) e52984.
Caplen N.J., Alton E.W., Mddleton P.G., Dorin J.R., Stevenson
B.J., Gao X., Durham S.R., Jeffery P.K., Hodson M.E. andCoutelle C., Liposome-mediated CFTR gene transfer to thenasal epithelium of patients with cystic fibrosis, Nature
medicine, 1 (1995) 39–46.
Carmona S., Jorgensen M.R., Kolli S., Crowther C., Salazar
F.H., Marion P.L., Fujino M., Natori Y., Thanou M. and
Arbuthnot P., Controlling HBV replication in vivo by intravenous administration of triggered PEGylated siRNA-­
nanoparticles, Molecular pharmaceutics, 6 (2009) 706–717.
Cavalli R., Caputo O., Carlotti M.E., Trotta M., Scarnecchia C.
and Gasco M.R., Sterilization and freeze-drying of drugfree and drug-loaded solid lipid nanoparticles, International
journal of pharmaceutics, 148 (1997) 47–54.
Chen A.M., Zhang M., Wei D., Stueber D., Taratula O., Minko
T. and He H., Co-delivery of doxorubicin and Bcl-2 siRNAby mesoporous silica nanoparticles enhances the efficacy ofchemotherapy in multidrug-resistant cancer cells, Small, 5(2009) 2673–2677.
Chen G., Kronenberger P., Teugels E., Umelo I.A. and De GrèveJ., Targeting the epidermal growth factor receptor in nonsmall cell lung cancer cells: The effect of combining RNA

62interference with tyrosine kinase inhibitors or cetuximab,
BMC medicine, 10 (2012) 28.
Chen S., Ni B., Huang H., Chen X. and Ma H., siRNA-loaded
pegylated porous silicon nanoparticles for lung cancer therapy, Journal of Nanoparticle Research, 16 (2014) 1–8.
Chernyak Y., Henon F., Harris R.B., Gould R.D., Franklin R.K.,
Edwards J.R., Desimone J.M. and Carbonell R.G., Formation of perfluoropolyether coatings by the rapid expansionof supercritical solutions (RESS) process. Part 1: Experimental results, Industrial & engineering chemistryresearch, 40 (2001) 6118–6126.
Choi M., Lee M. and Rhim T., Dexamethasone-conjugatedpolyethylenimine/mif siRNA complex regulation of particulate matter-induced airway inflammation, Biomaterials,
34 (2013) 7453–7461.
Chougule M., Padhi B. and Misra A., Nano-liposomal dry powder inhaler of tacrolimus: Preparation, characterization,and pulmonary pharmacokinetics, Int J Nanomedicine, 2(2007) 675–688.
Chougule M., Padhi B. and Misra A., Development of spraydried liposomal dry powder inhaler of dapsone, AAPS
PharmSciTech, 9 (2008) 47–53.
Clark K.L., Hughes S.A., Bulsara P., Coates J., Moores K.,
Parry J., Carr M., Mayer R.J., Wilson P. and Gruenloh C.,
Pharmacological characterization of a novel ENaCα siRNA(GSK2225745) with potential for the treatment of cystic
fibrosis, Molecular Therapy—Nucleic Acids, 2 (2013) e65.
Corey D.R., Synthesis of oligonucleotide-peptide andoligonucleotide-protein conjugates, (Eds), Bioconjugation
protocols, Springer, 2004, pp. 197–206.
Crespy D., Stark M., Hoffmann-Richter C., Ziener U. andLandfester K., Polymeric nanoreactors for hydrophilic
reagents synthesized by interfacial polycondensation on
miniemulsion droplets, Macromolecules, 40 (2007) 3122–
3135.
Crowe J.H. and Crowe L.M., Factors affecting the stability ofdry liposomes, Biochimica et Biophysica Acta (BBA)-­
Biomembranes, 939 (1988) 327–334.
D’alessandro-Gabazza C.N., Kobayashi T., Boveda-Ruiz D.,
Takagi T., Toda M., Gil-Bernabe P., Miyake Y., Yasukawa
A., Matsuda Y. and Suzuki N., Development and preclinicalefficacy of novel transforming growth factor-β1 short interfering RNAs for pulmonary fibrosis, American journal of
respiratory cell and molecular biology, 46 (2012) 397–406.
Danicher L., Frère Y. and Le Calvé A., Synthesis by interfacial
polycondensation of polyamide capsules with various sizes.
Characteristics and properties, Macromolecular Symposia,
Wiley Online Library, 2000.
De Backer L., Cerrada A., Pérez-Gil J., De Smedt S.C. and
Raemdonck K., Bio-inspired materials in drug delivery:
Exploring the role of pulmonary surfactant in siRNA inhalation therapy, Journal of Controlled Release, (2015).
De Fougerolles A. and Novobrantseva T., siRNA and the lung:Research tool or therapeutic drug?, Curr Opin Pharmacol, 8(2008) 280–285.
De Fougerolles A.R., Delivery vehicles for small interfering
RNA in vivo, Human gene therapy, 19 (2008) 125–132.
Deshayes S., Morris M., Divita G. and Heitz F., Cell-penetrating

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

peptides: Tools for intracellular delivery of therapeutics,
Cellular and Molecular Life Sciences CMLS, 62 (2005)
1839–1849.
Devincenzo J., Cehelsky J.E., Alvarez R., Elbashir S., HarborthJ., Toudjarska I., Nechev L., Murugaiah V., Van Vliet A.
and Vaishnaw A.K., Evaluation of the safety, tolerability
and pharmacokinetics of ALN-RSV01, a novel RNAi antiviral therapeutic directed against respiratory syncytial
virus (RSV), Antiviral research, 77 (2008) 225–231.
Devincenzo J., Lambkin-Williams R., Wilkinson T., CehelskyJ., Nochur S., Walsh E., Meyers R., Gollob J. and Vaishnaw
A., A randomized, double-blind, placebo-controlled studyof an RNAi-based therapy directed against respiratory syncytial virus, Proceedings of the National Academy of Sciences, 107 (2010) 8800–8805.
Devincenzo J.P., Harnessing RNA interference to develop neonatal therapies: From nobel prize winning discovery to
proof of concept clinical trials, Early Hum Dev, 85 (2009)
S31–35.
Dire C., Magnet S., Couvreur L. and Charleux B., Nitroxide-­
mediated controlled/living free-radical surfactant-freeemulsion polymerization of methyl methacrylate using apoly (methacrylic acid)-based macroalkoxyamine initiator,
Macromolecules, 42 (2008) 95–103.
Dokka S., Toledo D., Shi X., Castranova V. and Rojanasakul Y.,
Oxygen radical-mediated pulmonary toxicity induced bysome cationic liposomes, Pharmaceutical research, 17 (2000)
521–525.
Dustgania A., Vasheghani Farahani E. and Imani M., Preparation of chitosan nanoparticles loaded by dexamethasone
sodium phosphate, Iranian Journal of Pharmaceutical Sciences, 4 (2008) 111–114.
Eliyahu H., Siani S., Azzam T., Domb A.J. and Barenholz Y.,
Relationships between chemical composition, physicalproperties and transfection efficiency of polysaccharide–
spermine conjugates, Biomaterials, 27 (2006) 1646–1655.
Endoh T. and Ohtsuki T., Cellular siRNA delivery using cell-­
penetrating peptides modified for endosomal escape,Advanced drug delivery reviews, 61 (2009) 704–709.
Fallouh N.a.K., Roblot-Treupel L., Fessi H., Devissaguet J.P.
and Puisieux F., Development of a new process for the
manufacture of polyisobutylcyanoacrylate nanocapsules,
International Journal of Pharmaceutics, 28 (1986) 125–132.
Felgner P.L., Tsai Y.J., Sukhu L., Wheeler C.J., Manthorpe M.,
Marshall J. and Cheng S.H., Improved cationic lipid formulations for in vivo gene therapy, Annals of the New York
Academy of Sciences, 772 (1995) 126–139.
Fellmann C. and Lowe S.W., Stable RNA interference rules forsilencing, Nature cell biology, 16 (2014) 10–18.
Fessi H., Puisieux F., Devissaguet J.P., Ammoury N. and Benita
S., Nanocapsule formation by interfacial polymer deposition following solvent displacement, International journalof pharmaceutics, 55 (1989) R1–R4.
Finnie K.S., Waller D.J., Perret F.L., Krause-Heuer A.M., Lin
H.Q., Hanna J.V. and Barbé C.J., Biodegradability of sol–gel silica microparticles for drug delivery, Journal of
Sol-Gel Science and Technology, 49 (2008) 12–18.
Fire A., Xu S.Q., Montgomery M.K., Kostas S.A., Driver S.E.

and Mello C.C., Potent and specific genetic interference by
double-stranded RNA in caenorhabditis elegans, Nature,391 (1998) 806–811.
Fourtounis J., Wang I.-M., Mathieu M.-C., Claveau D., Loo T.,
Jackson A.L., Peters M.A., Therien A.G., Boie Y. andCrackower M.A., Gene expression profiling following NRF2
and KEAP1 siRNA knockdown in human lung fibroblasts
identifies CCL11/Eotaxin-1 as a novel NRF2 regulated gene,
Respir Res, 13 (2012) 92.
Galindo-Rodriguez S., Allemann E., Fessi H. and Doelker E.,
Physicochemical parameters associated with nanoparticle
formation in the salting-out, emulsification-diffusion, andnanoprecipitation methods, Pharmaceutical research, 21(2004) 1428–1439.
Gallardo M., Couarraze G., Denizot B., Treupel L., Couvreur
P. and Puisieux F., Study of the mechanisms of formationof nanoparticles and nanocapsules of polyisobutyl-2-­
cyanoacrylate, International journal of pharmaceutics, 100(1993) 55–64.
Ganachaud F. and Katz J.L., Nanoparticles and nanocapsules
created using the ouzo effect: Spontaneous emulsification
as an alternative to ultrasonic and high shear devices,
ChemPhysChem, 6 (2005) 209–216.
Gandhi N.S., Tekade R.K. and Chougule M.B., Nanocarrier
mediated delivery of siRNA/miRNA in combination withchemotherapeutic agents for cancer therapy: Current progress and advances, J Control Release, 194 (2014) 238–256.
Gao X. and Huang L., Potentiation of cationic liposome-­
mediated gene delivery by polycations, Biochemistry, 35(1996) 1027–1036.
Garbuzenko O.B., Saad M., Pozharov V.P., Reuhl K.R., Mainelis
G. and Minko T., Inhibition of lung tumor growth by complex pulmonary delivery of drugs with oligonucleotides as
suppressors of cellular resistance, Proceedings of the
National Academy of Sciences of the United States ofAmerica, 107 (2010) 10737–10742.
Gasco M. and Trotta M., Nanoparticles from microemulsions,International journal of pharmaceutics, 29 (1986) 267–268.
Gaspar M.M., Bakowsky U. and Ehrhardt C., Inhaled liposomes–­
current strategies and future challenges, Journal of Biomedical Nanotechnology, 4 (2008) 245–257.
Gaudin F. and Sintes-Zydowicz N., Core–shell biocompatible
polyurethane nanocapsules obtained by interfacial steppolymerisation in miniemulsion, Colloids and Surfaces A:
Physicochemical and Engineering Aspects, 331 (2008) 133–
142.
Glasgow M.D. and Chougule M.B., Recent developments inactive tumor targeted multifunctional nanoparticles for
combination chemotherapy in cancer treatment and imaging, Journal of Biomedical Nanotechnology, 11 (2015)
1859–1898.
Gutbier B., Kube S.M., Reppe K., Santel A., Lange C.,
Kaufmann J., Suttorp N. and Witzenrath M., RNAi-mediatedsuppression of constitutive pulmonary gene expression bysmall interfering RNA in mice, Pulmonary pharmacology& therapeutics, 23 (2010) 334–344.
Ham H.T., Choi Y.S., Chee M.G. and Chung I.J., Singlewall carbon nanotubes covered with polystyrene nanoparticles by63

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

in situ miniemulsion polymerization, Journal of Polymer
Science Part A: Polymer Chemistry, 44 (2006) 573–584.
Hamasaki T., Suzuki H., Shirohzu H., Matsumoto T.,
D’alessandro-Gabazza C.N., Gil-Bernabe P., Boveda-RuizD., Naito M., Kobayashi T. and Toda M., Efficacy of a novel
class of RNA interference therapeutic agents, PloS one, 7(2012) e42655.
Hannon G.J., RNA interference, Nature, 418 (2002) 244–251.
Heidel J.D., Liu J.Y.-C., Yen Y., Zhou B., Heale B.S., Rossi J.J.,
Bartlett D.W. and Davis M.E., Potent siRNA inhibitors ofribonucleotide reductase subunit RRM2 reduce cell proliferation in vitro and in vivo, Clinical Cancer Research, 13(2007) 2207–2215.
Hermanson G.T., Bioconjugate techniques, Academic press,
2013.
Hofland H., Shephard L. and Sullivan S.M., Formation of stablecationic lipid/DNA complexes for gene transfer, Proceedings of the National Academy of Sciences, 93 (1996) 7305–
7309.
Hong K., Zheng W., Baker A. and Papahadjopoulos D., Stabilization of cationic liposome-plasmid DNA complexes bypolyamines and poly (ethylene glycol)-phospholipid conjugates for efficient in vivo gene delivery, FEBS letters, 400(1997) 233–237.
Hong S.-H., Kim J.-E., Kim Y.-K., Minai-Tehrani A., Shin J.-Y.,
Kang B., Kim H.-J., Cho C.-S., Chae C. and Jiang H.-L.,
Suppression of lung cancer progression by biocompatible
glycerol triacrylate–spermine-mediated delivery of shakt1,
International journal of nanomedicine, 7 (2012) 2293.
Hong S.-H., Minai-Tehrani A., Chang S.-H., Jiang H.-L., Lee
S., Lee A.-Y., Seo H.W., Chae C., Beck Jr G.R. and Cho
M.-H., Knockdown of the sodium-dependent phosphate co-­transporter 2b (NPT2b) suppresses lung tumorigenesis,
PloS one, 8 (2013) e77121.
Huang B., Zhou H., Wang X. and Liu Z., Silencing satb1 withsiRNA inhibits the proliferation and invasion of small celllung cancer cells, Cancer Cell Int, 13 (2013).
Innes A.L., Carrington S.D., Thornton D.J., Kirkham S.,
Rousseau K., Dougherty R.H., Raymond W.W., Caughey
G.H., Muller S.J. and Fahy J.V., ex vivo sputum analysis
reveals impairment of protease-dependent mucus degradation by plasma proteins in acute asthma, American journalof respiratory and critical care medicine, 180 (2009) 203–
210.
Jensen D.K., Jensen L.B., Koocheki S., Bengtson L., Cun D.,
Nielsen H.M. and Foged C., Design of an inhalable dry
powder formulation of DOTAP-modified PLGA nanoparticles loaded with siRNA, J Control Release, 157 (2012) 141–
148.
Jensen D.M., Cun D., Maltesen M.J., Frokjaer S., Nielsen H.M.
and Foged C., Spray drying of siRNA-containing PLGA
nanoparticles intended for inhalation, J Control Release,142 (2010) 138–145.
Jeong J.H., Mok H., Oh Y.-K. and Park T.G., siRNA conjugatedelivery systems, Bioconjugate chemistry, 20 (2008) 5–14.
Jiang H.L., Lim H.T., Kim Y.K., Arote R., Shin J.Y., Kwon J.T.,
Kim J.E., Kim J.H., Kim D., Chae C., Nah J.W., Choi Y.J.,
Cho C.S. and Cho M.H., Chitosan-graft-spermine as a gene64

carrier in vitro and in vivo, Eur J Pharm Biopharm, 77(2011) 36–42.
Jung T., Kamm W., Breitenbach A., Kaiserling E., Xiao J. andKissel T., Biodegradable nanoparticles for oral delivery of
peptides: Is there a role for polymers to affect mucosal
uptake?, European Journal of Pharmaceutics and Biopharmaceutics, 50 (2000) 147–160.
Kalariya M., Padhi B.K., Chougule M. and Misra A., Clobetasol
propionate solid lipid nanoparticles cream for effective
treatment of eczema: Formulation and clinical implications,Indian J Exp Biol, 43 (2005) 233–240.
Kanasty R., Dorkin J.R., Vegas A. and Anderson D., Delivery
materials for siRNA therapeutics, Nature materials, 12(2013) 967–977.
Karode S., Kulkarni S., Suresh A. and Mashelkar R., Newinsights into kinetics and thermodynamics of interfacial
polymerization, Chemical engineering science, 53 (1998)
2649–2663.
Kasper J.C., Schaffert D., Ogris M., Wagner E. and Friess
W., Development of a lyophilized plasmid/LPEI polyplex
formulation with long-term stability—a step closer frompromising technology to application, Journal of Controlled
Release, 151 (2011) 246–255.
Kawashima Y., Nanoparticulate systems for improved drugdelivery, Advanced drug delivery reviews, 47 (2001) 1–2.
Keefe D., Agency response letter GRAS notice no. GRN 000397,
2011, FDA (U.S. Food and Drug Administration) <http://
www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/
NoticeInventory/ucm287638.htm> accessed 29.02.2016.
Kim D.H. and Rossi J.J., Strategies for silencing human diseaseusing RNA interference, Nature Reviews Genetics, 8 (2007)
173–184.
Kim S.S., Peer D., Kumar P., Subramanya S., Wu H., AsthanaD., Habiro K., Yang Y.G., Manjunath N., Shimaoka M. andShankar P., RNAi-mediated CCR5 silencing by LFA-1-­
targeted nanoparticles prevents HIV infection in BLT mice,
Mol Ther, 18 (2010) 370–376.
Kostag M., Köhler S., Liebert T. and Heinze T., Pure cellulose
nanoparticles from trimethylsilyl cellulose, Macromolecular symposia, Wiley Online Library, 2010.
Kostarelos K. and Miller A.D., Synthetic, self-assembly ABCD
nanoparticles; a structural paradigm for viable synthetic
non-viral vectors, Chemical Society Reviews, 34 (2005)
970–994.
Kreuter J., On the mechanism of termination in heterogeneouspolymerization, Journal of Polymer Science: Polymer Letters Edition, 20 (1982) 543–545.
Krützfeldt J., Rajewsky N., Braich R., Rajeev K.G., Tuschl T.,
Manoharan M. and Stoffel M., Silencing of microRNAs invivo with ‘antagomirs’, nature, 438 (2005) 685–689.
Lam J.K.-W., Liang W. and Chan H.-K., Pulmonary delivery oftherapeutic siRNA, Advanced drug delivery reviews, 64(2012) 1–15.
Lambert G., Fattal E. and Couvreur P., Nanoparticulate systems
for the delivery of antisense oligonucleotides, Advanced
drug delivery reviews, 47 (2001) 99–112.
Landfester K., Musyanovych A. and Mailänder V., Frompolymeric particles to multifunctional nanocapsules forSusanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

biomedical applications using the miniemulsion process,
Journal of Polymer Science Part A: Polymer Chemistry,
48 (2010) 493–515.
Li J., Chen Y.-C., Tseng Y.-C., Mozumdar S. and Huang L.,
Biodegradable calcium phosphate nanoparticle with lipid
coating for systemic siRNA delivery, Journal of Controlled
Release, 142 (2010) 416–421.
Li S.D. and Huang L., Nanoparticles evading the reticuloendothelial system: Role of the supported bilayer, Biochim Biophys Acta, 1788 (2009) 2259–2266.
Lim K.T., Subban G.H., Hwang H.S., Kim J.T., Ju C.S. andJohnston K.P., Novel semiconducting polymer particles bysupercritical fluid process, Macromolecular rapid communications, 26 (2005) 1779–1783.
List S.J., Findlay B.P., Forstner G. and Forstner J., Enhancementof the viscosity of mucin by serum albumin, Biochem. J,
175 (1978) 565–571.
Liu S., Nugroho A.E., Shudou M. and Maeyama K., Regulationof mucosal mast cell activation by short interfering RNAs
targeting syntaxin4, Immunology and cell biology, 90(2012) 337–345.
Liu Y., Dong Q.-Z., Wang S., Fang C.-Q., Miao Y., Wang L., Li
M.-Z. and Wang E.-H., Abnormal expression of pygopus 2
correlates with a malignant phenotype in human lung cancer, BMC cancer, 13 (2013) 346.
Lobovkina T., Jacobson G.B., Gonzalez-Gonzalez E., Hickerson
R.P., Leake D., Kaspar R.L., Contag C.H. and Zare R.N., In
vivo sustained release of siRNA from solid lipid nanoparticles, ACS Nano, 5 (2011) 9977–9983.
Lomas-Neira J., Perl M., Venet F., Chung C.-S. and Ayala A.,
The role and source of TNF-α in hemorrhage induced
priming for septic lung injury, Shock (Augusta, Ga.), 37(2012) 611.
Love K.T., Mahon K.P., Levins C.G., Whitehead K.A., Querbes
W., Dorkin J.R., Qin J., Cantley W., Qin L.L. and Racie T.,
Lipid-like materials for low-dose, in vivo gene silencing,
Proceedings of the National Academy of Sciences, 107(2010) 1864–1869.
Luo Y., Zhai X., Ma C., Sun P., Fu Z., Liu W. and Xu J., An
inhalable β2-adrenoceptor ligand-directed guanidinylated
chitosan carrier for targeted delivery of siRNA to lung, J
Control Release, 162 (2012) 28–36.
Mainelis G., Seshadri S., Garbuzenko O., Han T., Wang Z. andMinko T., Characterization and application of a nose-only
exposure chamber for inhalation delivery of liposomal
drugs and nucleic acids to mice, Journal of aerosol medicine and pulmonary drug delivery, 26 (2013) 345–354.
Mao S., Sun W. and Kissel T., Chitosan-based formulations fordelivery of DNA and siRNA, Advanced drug delivery
reviews, 62 (2010) 12–27.
Marriott C., Beeson M.F. and Brown D.T., Biopolymer inducedchanges in mucus viscoelasticity, (Eds), Mucus in healthand disease—ii, Springer, 1982, pp. 89–92.
Matsubara H., Sakakibara K., Kunimitsu T., Matsuoka H., Kato
K., Oyachi N., Dobashi Y. and Matsumoto M., Non-small
cell lung carcinoma therapy using mTOR-siRNA, International journal of clinical and experimental pathology, 5(2012) 119.

Matyjaszewski K. and Xia J., Atom transfer radical polymerization, Chemical reviews, 101 (2001) 2921–2990.
Meade B.R. and Dowdy S.F., Exogenous siRNA delivery using
peptide transduction domains/cell penetrating peptides,
Advanced drug delivery reviews, 59 (2007) 134–140.
Mehnert W. and Maeder K., Solid lipid nanoparticles production, characterization and applications, Advanced Drug
Delivery Reviews, 64 (2012) 83–101.
Merkel O.M. and Kissel T., Nonviral pulmonary delivery ofsiRNA, Accounts of chemical research, 45 (2011) 961–970.
Merkel O.M., Rubinstein I. and Kissel T., siRNA delivery to thelung: What’s new?, Adv Drug Deliv Rev, 75 (2014) 112–
128.
Meziani M.J., Pathak P., Hurezeanu R., Thies M.C., Enick R.M.
and Sun Y.P., Supercritical fluid processing technique fornanoscale polymer particles, Angewandte Chemie International Edition, 43 (2004) 704–707.
Mi Bae Y., Choi H., Lee S., Ho Kang S., Tae Kim Y., Nam K.,
Sang Park J., Lee M. and Sig Choi J., Dexamethasone-­
conjugated low molecular weight polyethylenimine as anucleus-targeting lipopolymer gene carrier, Bioconjugate
Chemistry, 18 (2007) 2029–2036.
Min K., Gao H., Yoon J.A., Wu W., Kowalewski T. andMatyjaszewski K., One-pot synthesis of hairy nanoparticles
by emulsion ATRP, Macromolecules, 42 (2009) 1597–1603.
Mishra S. and Chatterjee A., Novel synthesis of polymer andcopolymer nanoparticles by atomized microemulsion technique and its characterization, Polymers for Advanced
Technologies, 22 (2011) 1593–1601.
Morrissey D.V., Lockridge J.A., Shaw L., Blanchard K., Jensen
K., Breen W., Hartsough K., Machemer L., Radka S. andJadhav V., Potent and persistent in vivo anti-HBV activityof chemically modified siRNAs, Nature biotechnology, 23(2005) 1002–1007.
Moschos S.A., Frick M., Taylor B., Turnpenny P., Graves H.,
Spink K.G., Brady K., Lamb D., Collins D. and Rockel T.D.,
Uptake, efficacy, and systemic distribution of naked,
inhaled short interfering RNA (siRNA) and locked nucleic
acid (LNA) antisense, Molecular Therapy, (2011).
Moulik S. and Paul B., Structure, dynamics and transport properties of microemulsions, Advances in Colloid and Interface science, 78 (1998) 99–195.
Nagavarma B., Yadav H.K., Ayaz A., Vasudha L. and Shivakumar
H., Different techniques for preparation of polymericnanoparticles—a review, Asian J. Pharm. Clin. Res, 5(2012) 16–23.
Nicolas J., Ruzette A.-V., Farcet C., Gérard P., Magnet S. andCharleux B., Nanostructured latex particles synthesized bynitroxide-mediated controlled/living free-radical polymerization in emulsion, Polymer, 48 (2007) 7029–7040.
Nishimura Y., Takiguchi S., Ito S. and Itoh K., Evidence that
depletion of the sorting nexin 1 by siRNA promotes HGF-­
induced MET endocytosis and MET phosphorylation in agefitinib-resistant human lung cancer cell line, International journal of oncology, 44 (2014) 412–426.
Niwa T., Takeuchi H., Hino T., Kunou N. and Kawashima Y., In
vitro drug release behavior of D, L-lactide/glycolide copolymer (PLGA) nanospheres with nafarelin acetate prepared65

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

by a novel spontaneous emulsification solvent diffusion
method, Journal of pharmaceutical sciences, 83 (1994)
727–732.
Oh B. and Lee M., Combined delivery of HMGB-1 box a peptide and S1PLyase siRNA in animal models of acute lunginjury, J Control Release, 175 (2014) 25–35.
Okuda T., Kito D., Oiwa A., Fukushima M., Hira D. andOkamoto H., Gene silencing in a mouse lung metastasis
model by an inhalable dry small interfering RNA powder
prepared using the supercritical carbon dioxide technique,
Biological and Pharmaceutical Bulletin, 36 (2013) 1183–
1191.
Pardeike J., Hommoss A. and Mueller R.H., Lipid nanoparticles(SLN, NLC) in cosmetic and pharmaceutical dermal products, International Journal of Pharmaceutics, 366 (2009)170–184.
Patel A.R., Chougule M.B., Townley I., Patlolla R., Wang G.
and Singh M., Efficacy of aerosolized celecoxib encapsulated nanostructured lipid carrier in non-small cell lungcancer in combination with docetaxel, Pharmaceuticalresearch, 30 (2013) 1435–1446.
Patel G., Chougule M., Singh M. and Misra A., Nanoliposomal
dry powder formulations, Methods in enzymology, 464(2009) 167–191.
Patlolla R.R., Chougule M., Patel A.R., Jackson T., Tata P.N.V.
and Singh M., Formulation, characterization and pulmonary deposition of nebulized celecoxib encapsulated nanostructured lipid carriers, Journal of Controlled Release, 144(2010) 233–241.
Pavan G.M., Mintzer M.A., Simanek E.E., Merkel O.M., Kissel
T. and Danani A., Computational insights into the interactions between DNA and siRNA with “rigid” and “flexible”
triazine dendrimers, Biomacromolecules, 11 (2010) 721–
730.
Podesta J.E. and Kostarelos K., Chapter seventeen-engineering
cationic liposome: siRNA complexes for in vitro and invivo delivery, Methods in enzymology, 464 (2009) 343–
354.
Puglisi G., Fresta M., Giammona G. and Ventura C., Influenceof the preparation conditions on poly (ethylcyanoacrylate)nanocapsule formation, International Journal of Pharmaceutics, 125 (1995) 283–287.
Quintanar-Guerrero D., Allemann E., Fessi H. and Doelker E.,
Preparation techniques and mechanisms of formation of
biodegradable nanoparticles from preformed polymers,
Drug Development and Industrial Pharmacy, 24 (1998)
1113–1128.
Radu D.R., Lai C.Y., Jeftinija K., Rowe E.W., Jeftinija S. and
Lin V.S., A polyamidoamine dendrimer-capped mesoporous silica nanosphere-based gene transfection reagent, J
Am Chem Soc, 126 (2004) 13216–13217.
Rahman M.A., Amin A.R., Wang D., Koenig L., Nannapaneni
S., Chen Z., Wang Z., Sica G., Deng X. and Shin D.M.,
RRM2 regulates Bcl-2 in head and neck and lung cancers:
A potential target for cancer therapy, Clinical Cancer
Research, 19 (2013) 3416–3428.
Ramachandran S., Krishnamurthy S., Jacobi A.M., Wohlford-­
Lenane C., Behlke M.A., Davidson B.L. and Mccray P.B.,

66

Efficient delivery of RNA interference oligonucleotides topolarized airway epithelia in vitro, American Journal of
Physiology-Lung Cellular and Molecular Physiology, 305(2013) L23–L32.
Rao J.P. and Geckeler K.E., Polymer nanoparticles: Preparation
techniques and size-control parameters, Progress in Polymer Science, 36 (2011) 887–913.
Reis C.P., Neufeld R.J., Ribeiro A.J. and Veiga F., Nanoencapsulation i. Methods for preparation of drug-loaded polymeric nanoparticles, Nanomedicine: Nanotechnology,
Biology and Medicine, 2 (2006) 8–21.
Reischl D. and Zimmer A., Drug delivery of siRNA therapeutics: Potentials and limits of nanosystems, Nanomedicine:
Nanotechnology, Biology and Medicine, 5 (2009) 8–20.
Ren Y., Kang C.S., Yuan X.B., Zhou X., Xu P., Han L., Wang
G.X., Jia Z., Zhong Y., Yu S., Sheng J. and Pu P.Y., Co-­delivery of as-miR-21 and 5-FU by poly(amidoamine) dendrimer attenuates human glioma cell growth in vitro, J
Biomater Sci Polym Ed, 21 (2010) 303–314.
Rieger J., Zhang W., Stoffelbach F.O. and Charleux B.,
Surfactant-­f ree RAFT emulsion polymerization using poly(n, n-dimethylacrylamide) trithiocarbonate macromolecular
chain transfer agents, Macromolecules, 43 (2010) 6302–6310.
Samad A., Sultana Y. and Aqil M., Liposomal drug delivery
systems: An update review, Current drug delivery, 4 (2007)
297–305.
Sane A. and Thies M.C., Effect of material properties and processing conditions on RESS of poly (l-lactide), The Journalof supercritical fluids, 40 (2007) 134–143.
Sarkar D., El-Khoury J., Lopina S.T. and Hu J., An effective
method for preparing polymer nanocapsules with hydrophobic acrylic shell and hydrophilic interior by inverse
emulsion radical polymerization, Macromolecules, 38 (2005)
8603–8605.
Schwarz C., Mehnert W., Lucks J. and Müller R., Solid lipid
nanoparticles (SLN) for controlled drug delivery. I. Production, characterization and sterilization, Journal of Controlled Release, 30 (1994) 83–96.
Scott C., Wu D., Ho C.-C. and Co C.C., Liquid-core capsules
via interfacial polymerization: A free-radical analogy ofthe nylon rope trick, Journal of the American Chemical
Society, 127 (2005) 4160–4161.
Semple S.C., Klimuk S.K., Harasym T.O., Dos Santos N.,
Ansell S.M., Wong K.F., Maurer N., Stark H., Cullis P.R.
and Hope M.J., Efficient encapsulation of antisense oligonucleotides in lipid vesicles using ionizable aminolipids:
Formation of novel small multilamellar vesicle structures,
Biochimica et Biophysica Acta (BBA)-Biomembranes, 1510
(2001) 152–166.
Shim G., Choi H.-W., Lee S., Choi J., Yu Y.H., Park D.-E., Choi
Y., Kim C.-W. and Oh Y.-K., Enhanced intrapulmonary
delivery of anticancer siRNA for lung cancer therapy using
cationic ethylphosphocholine-based nanolipoplexes, Molecular Therapy, 21 (2013) 816–824.
Siegwart D.J., Srinivasan A., Bencherif S.A., Karunanidhi A., Oh
J.K., Vaidya S., Jin R., Hollinger J.O. and Matyjaszewski K.,
Cellular uptake of functional nanogels prepared by inverse
miniemulsion ATRP with encapsulated proteins, carbohy-

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

drates, and gold nanoparticles, Biomacromolecules, 10(2009) 2300–2309.
Siekmann B. and Westesen K., P234 solid lipid nanoparticles
stabilized by tyloxapol, European Journal of Pharmaceutical Sciences, 2 (1994) 177.
Slowing, Ii, Trewyn B.G. and Lin V.S., Mesoporous silica
nanoparticles for intracellular delivery of membrane-­impermeable proteins, J Am Chem Soc, 129 (2007) 8845–
8849.
Sun Y.P., Meziani M.J., Pathak P. and Qu L., Polymericnanoparticles from rapid expansion of supercritical fluid
solution, Chemistry-A European Journal, 11 (2005) 1366–
1373.
Taratula O., Garbuzenko O.B., Chen A.M. and Minko T., Innovative strategy for treatment of lung cancer: Targeted
nanotechnology-based inhalation co-delivery of anticancer
drugs and siRNA, J Drug Target, 19 (2011a) 900–914.
Taratula O., Garbuzenko O.B., Chen A.M. and Minko T., Innovative strategy for treatment of lung cancer: Targeted
nanotechnology-based inhalation co-delivery of anticancer
drugs and siRNA, Journal of Drug Targeting, 19 (2011b)
900–914.
Tarkunde S.V., Gujar K., Harwalkar M. and Gambhire M.,
Liposome–a novel drug delivery, Int. J. Pure App. Biosci, 2(2014) 92–102.
Tekade R.K. and Chougule M.B., Formulation development andevaluation of hybrid nanocarrier for cancer therapy: Taguchi
orthogonal array based design, BioMed Research International, 2013 (2013) 18.
Tekade R.K., Youngren-Ortiz S.R., Yang H., Haware R. andChougule M.B., Designing hybrid onconase nanocarriers
for mesothelioma therapy: A taguchi orthogonal array andmultivariate component driven analysis, Molecular pharmaceutics, 11 (2014) 3671–3683.
Thomas C.E., Ehrhardt A. and Kay M.A., Progress and problems with the use of viral vectors for gene therapy, Nature
Reviews Genetics, 4 (2003) 346–358.
Tompkins S.M., Lo C.-Y., Tumpey T.M. and Epstein S.L., Protection against lethal influenza virus challenge by RNA
interference in vivo, Proceedings of the National Academyof Sciences of the United States of America, 101 (2004)
8682–8686.
Tseng Y.-C., Mozumdar S. and Huang L., Lipid-based systemic
delivery of siRNA, Advanced drug delivery reviews, 61(2009) 721–731.
Turner J.J., Williams D., Owen D. and Gait M.J., Disulfide conjugation of peptides to oligonucleotides and their analogs,
Current protocols in nucleic acid chemistry, (2006) 4.28.
21–24.28. 21.
Umrania Y., Nikjoo H. and Goodfellow J., A knowledge-basedmodel of DNA hydration, International journal of radiation
biology, 67 (1995) 145–152.
Vauthier C. and Bouchemal K., Methods for the preparation andmanufacture of polymeric nanoparticles, Pharmaceuticalresearch, 26 (2009) 1025–1058.
Vauthier C., Dubernet C., Fattal E., Pinto-Alphandary H. andCouvreur P., Poly (alkylcyanoacrylates) as biodegradable
materials for biomedical applications, Advanced Drug

Delivery Reviews, 55 (2003) 519–548.
Wang J.-C., Lai S., Guo X., Zhang X., De Crombrugghe B.,
Sonnylal S., Arnett F.C. and Zhou X., Attenuation of fibrosis in vitro and in vivo with SPARC siRNA, Arthritis
research & therapy, 12 (2010) R60.
Wang Y., Chen L., Huang G., He D., He J., Xu W., Zou C., ZongF., Li Y. and Chen B., Klotho sensitizes human lung cancer
cell line to cisplatin via PI3k/Akt pathway, PloS one, 8(2013) e57391.
Watnasirichaikul S., Davies N.M., Rades T. and Tucker I.G.,
Preparation of biodegradable insulin nanocapsules from
biocompatible microemulsions, Pharmaceutical research,
17 (2000) 684–689.
Watts J.K. and Corey D.R., Clinical status of duplex RNA,
Bioorg Med Chem Lett, 20 (2010) 3203–3207.
Watts J.K., Deleavey G.F. and Damha M.J., Chemically modified siRNA: Tools and applications, Drug discovery today,
13 (2008) 842–855.
Weber M. and Thies M.C., Understanding the RESS process,Supercritical Fluid Technology in Materials Science andEngineering: Syntheses, Properties, and Applications,(2002) 387–427.
Weber S., Zimmer A. and Pardeike J., Solid lipid nanoparticles(SLN) and nanostructured lipid carriers (NLC) for pulmonary application: A review of the state of the art, European
Journal of Pharmaceutics and Biopharmaceutics, 86 (2014)
7–22.
Whitehead K.A., Langer R. and Anderson D.G., Knocking
down barriers: Advances in siRNA delivery, Nat Rev Drug
Discov, 8 (2009a) 129–138.
Whitehead K.A., Langer R. and Anderson D.G., Knocking
down barriers: Advances in siRNA delivery, Nature
Reviews Drug Discovery, 8 (2009b) 129–138.
Wissing S., Kayser O. and Müller R., Solid lipid nanoparticles
for parenteral drug delivery, Advanced drug delivery
reviews, 56 (2004) 1257–1272.
Wu S.Y. and Mcmillan N.A., Lipidic systems for in vivo siRNAdelivery, The AAPS journal, 11 (2009) 639–652.
York P., Strategies for particle design using supercritical fluid
technologies, Pharmaceutical science & technology today,
2 (1999) 430–440.
Youngren-Ortiz S.R., Gandhi N.S., España-Serrano L. andChougule M.B., Aerosol delivery of siRNA to the lungs.
Part 1: Rationale for gene delivery systems, KONA Powder
and Particle Journal, 33 (2016) 63–85.
Youngren S., Mulik R., Jun B., Hoffmann P., Morris K. andChougule M., Freeze-dried targeted mannosylated selenium-­loaded nanoliposomes: Development and evaluation, AAPS
PharmSciTech, 10.1208/s12249-013-9988-3 (2013) 1–13.
Youngren S.R., Tekade R.K., Gustilo B., Hoffmann P.R. andChougule M.B., STAT6 siRNA matrix-loaded gelatin nanocarriers: Formulation, characterization, and ex vivo proof ofconcept using adenocarcinoma cells, Biomed Res Int, 2013(2013) 858946.
Zamora M.R., Budev M., Rolfe M., Gottlieb J., Humar A.,
Devincenzo J., Vaishnaw A., Cehelsky J., Albert G.,
Nochur S., Gollob J.A. and Glanville A.R., RNA interference therapy in lung transplant patients infected with respi-

67

Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

ratory syncytial virus, Am J Respir Crit Care Med, 183(2011) 531–538.
Zatsepin T., Turner J., Oretskaya T. and Gait M., Conjugates ofoligonucleotides and analogues with cell penetrating peptides as gene silencing agents, Current pharmaceuticaldesign, 11 (2005) 3639–3654.
Zetterlund P.B., Kagawa Y. and Okubo M., Controlled/living
radical polymerization in dispersed systems, Chemical
reviews, 108 (2008) 3747–3794.
Zetterlund P.B., Nakamura T. and Okubo M., Mechanistic
investigation of particle size effects in TEMPO-mediatedradical polymerization of styrene in aqueous miniemulsion,
Macromolecules, 40 (2007) 8663–8672.
Zhang Y.-P., Li W.-B., Wang W.-L., Liu J., Song S.-X., Bai L.-L.,
Hu Y.-Y., Yuan Y.-D. and Zhang M., siRNA against plasminogen activator inhibitor-1 ameliorates bleomycin-in-

duced lung fibrosis in rats, Acta Pharmacologica Sinica, 33(2012) 897–908.
Zheng M., Librizzi D., Kilic A., Liu Y., Renz H., Merkel
O.M. and Kissel T., Enhancing in vivo circulation andsiRNA delivery with biodegradable polyethylenimine-­
graft-polycaprolactone-block-poly(ethylene glycol) copolymers, Biomaterials, 33 (2012) 6551–6558.
Zhou X., Ni P. and Yu Z., Comparison of RAFT polymerizationof methyl methacrylate in conventional emulsion andminiemulsion systems, Polymer, 48 (2007) 6262–6271.
Ziegler A., Landfester K. and Musyanovych A., Synthesis ofphosphonate-functionalized polystyrene and poly (methyl
methacrylate) particles and their kinetic behavior inminiemulsion polymerization, Colloid and polymer science,
287 (2009) 1261–1271.

Author’s short biography
Susanne R. Youngren-Ortiz
Susanne R. Youngren-Ortiz is a Ph.D. candidate the Daniel K. Inouye College of
Pharmacy of the University of Hawai’i at Hilo. She obtained her Bachelor of Science
Degree in Pharmaceutical Sciences with specialization in Industrial and Physical Pharmacy and minors in Chemistry and Statistics from Purdue University. Her dissertation
research project investigates the optimal delivery and the evaluation of cellular effects oftherapeutic nanocarriers for the treatment of asthma. She has authored original research
articles, review articles, and book chapters. She is an active member of the American
Association of Pharmaceutical Scientists (AAPS) and has chaired the University of Hawaii
AAPS Student Chapter.

68Susanne R. Youngren-Ortiz et al. / KONA Powder and Particle Journal No. 34 (2017) 44–69

Author’s short biography
Nishant S. Gandhi
Nishant S. Gandhi is a Ph.D. candidate at the Daniel K Inouye College of Pharmacy
UH Hilo. He has completed his B.Pharm and M.Pharm from University of Pune in India.
He has also worked as a chemist at Aristo Pharmaceuticals in Bhopal, India. He is currently working on developing novel nanotechnology based drug delivery systems for thetreatment of lung cancer in Dr. Chougule’s lab at the DKICP. He has authored publications including original research articles, review articles and book chapters. He is currently an active member of the American Association of Pharmaceutical Scientist (AAPS)
and professional pharmacy fraternity “Phi Delta Chi” Gamma theta chapter of the
DKICP. He has served as the chair of the University of Hawaii AAPS Student Chapter.

Laura Espana-Serrano
Dr. Laura Espana-Serrano is a Research Associate in the Translational Drug Delivery
Research laboratory in the department of Pharmaceutical Sciences of the Daniel K.
Inouye College of Pharmacy at the University of Hawaii at Hilo. She obtained her PhD
in Biochemistry and Molecular Biology from the Complutense University of Madrid,
Spain. She has nine years of experience in the fields of Cellular and Molecular Biology
of Cancer and Psychiatric diseases. She has authored original research publications andreview articles, and she is a member of the National Postdoctoral Association and theAmerican Association of Pharmaceutical Scientists.

Mahavir B. Chougule
Dr. Mahavir B. Chougule is an Associate Professor of the Department of Pharmaceutical Sciences at the Daniel K. Inouye College of Pharmacy, University of Hawaii at Hilo.
He is also Associate Member of Natural Products and Experimental Therapeutics Program, University of Hawaii Cancer Research Center. Dr. Chougule’s translational drugdelivery research laboratory investigates the use of newer combinaiton of therapeutic
agents, siRNA, proteins and targeted nanotechnology based formulations for treatmentof lung cancer, breast cancer, mesothelioma, neuroblastoma, colon cancer and asthma.
Dr. Chougule has 26 publications, 8 patents, 6 review articles, 7 book chapters and 2 editorial articles. His publications have received more than 700 citations with an h-index of
15. He delivered 17 invited talks and 63 scientific presentations at international conferences. He served as a grant reviewer on the NIH, DOD, and international review panels.
He is also serving as a member of editorial boards on 7 international journals including
AAPS PharmSciTech, Drug Development and Industrial Pharmacy, Journal of Nanopharmaceutics and Drug Delivery and Advanced Science, Engineering and Medicine.
Dr. Chougule is a recipient of several awards including recent American Association ofCancer Research Minority-Serving Institution Faculty Scholar in Cancer ResearchAward, USA. He is an active member of American Association of Cancer Research andAmerican Association of Pharmaceutical Scientists.

69

KONA Powder and Particle Journal No. 34 (2017) 70–79/Doi:10.14356/kona.2017008

Review Paper

Toward Low-Temperature Coal Gasification:
Experimental and Numerical Studies ofThermochemical Coal Conversion Considering
the Interactions between Volatiles and Char Particles †Cheng-Yi Li 1,2, Li-Xin Zhang 1,3, Shinji Kudo 4,
Jun-ichiro Hayashi 1,4 and Koyo Norinaga 1,4,*
1Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Japan
China Tianchen Engineering Corporation (TCC), China
3
Xi’an Thermal Power Research Institute Co. Ltd., China
4
Institute for Materials Chemistry and Engineering, Kyushu University, Japan
2

Abstract
A novel triple-bed combined circulating fluidized-bed (TB-CFB) coal gasifier, consisting of a downer (pyrolyzer),a bubbling fluidized bed (gasifier), and a riser (combustor) was proposed for realizing low-temperature coalgasification. Several key thermochemical reactions were extracted from those expected in the downer unit: thereforming of refractory tar in both the gas phase and over the char surface, and the steam gasification of thenascent char. This review highlights our recent progress, both experimental and numerical, in studies ofthermochemical coal conversion including the various reaction processes, by employing a drop-tube reactor thatwell approximates the reaction environment in a downer reactor. This discussion can be utilized in designing TBCFBs and optimizing their operation.
Keywords: coal gasification, simulation, fluidized-bed reactor, tar reforming, drop-tube reactor, reactor modeling

1. Introduction
Coal plays an important role in meeting current worldenergy demands, and should continue to do so in the near
future. The development of techniques for the higher-­efficiency clean utilization of coal, such as coal gasification, is becoming an increasingly active area of research(Li C.Z., 2006). Currently, many coal gasification technologies are based on partial combustion at temperatures
above 1200 °C. High temperature provides significant advantages for realizing the very fast thermochemical conversion of coal into syngas, but also offers the added
expense of chemical energy of the fuel due to heating.
The cold gas efficiency can be raised theoretically further by decreasing O2 consumption while increasing steam
or CO2 consumption, which would result in gasification†

Received 2 December 2015; Accepted 8 February 2016
J-STAGE Advance Publication online 30 April 2016
1,4
Kasuga-koen 6-1, Kasuga, Fukuoka 816-8580, Japan2
No. 1, Jingjin Road, Beichen District, Tianjin 300400, China
3
No. 99 Yanxiang Road, Xi’an 710054, China* Corresponding author: Koyo Norinaga;
E-mail: norinaga@cm.kyushu-u.ac.jp
TEL: +81-92-583-7794 FAX: +81-92-583-7793

70

operations at lower temperature. Low-temperature gasification with higher cold gas efficiency has received widespread attention in recent years (Hayashi et al., 2014).
The low-temperature gasification of coal is conducted
at temperatures well below 1000 °C. Fixed- and fluidized-­
bed gasifiers, as the main reactor systems for low-­temperature gasification, have been developed toward thecommercial production of power. The low temperatures,
however, inevitably result in slower coal conversion intosyngas. The novel Lurgi fixed-bed gasifiers suffer fromhigh tar yields in the product gases (Li X.T. et al., 2004),
and the processing rates are slow compared with entrained- and fluidized-beds (Hobbs et al., 1993). Fluidized
beds now find wide application in coal gasification (Li
X.T. et al., 2004). However, due to their high degree ofsolids mixing and particle entrainment, a single fluidized
bed cannot achieve high solids conversion, as observedwith the U-gas® gasifier (Goyal and Rehmat, 1992).
Therefore, it is necessary to develop a novel gasification
technology with higher cold gas efficiency.
Recently, a triple-bed combined circulating fluidized-­
bed reactor (TB-CFB) system, consisting of a downer(pyrolyzer), a bubbling fluidized bed (BFB, gasifier), anda riser (combustor), as shown in Fig. 1, was proposed

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open access article
under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

sion in the downer reactor of the TB-CFB system, together with some future perspectives.

2. In situ tar reforming process

Fig. 1

Schematic diagram of the triple-bed combined circulating fluidized-bed reactor system.

(Guan, et al., 2010a; Guan, et al., 2010b; Guan et al., 2011;
Fushimi et al., 2011; Zhang, et al., 2013a; Zhang, et al.,2013b). In brief, the downer was added to a dual-bed circulating fluidized reactor system, after the riser (combustor) and before the BFB gasifier. Fresh coal was fed,
mixed with hot circulating solids for the riser, including
recycled char and the circulating medium, and rapidly
pyrolyzed in the downer at around 900 °C. The resultant
tar was simultaneously and rapidly decomposed into syngas and/or coke over the nascent and recycled chars. Then,the tar-free char was separated from the syngas, sent to
the BFB gasifier, and gasified without the negative effectof the volatiles-char interaction. The char gasification andvolatiles reforming are thus expected to be decoupled under this reactor system. The endothermic and exothermic
processes were separated in this system to realize theconcept of low-temperature gasification with high coldgas efficiency.
As one important part of the reactor system, the novel
downer reactor was expected to decompose tar and thus
avoid the detrimental effects of tar on the subsequent chargasification and downstream system operation. After thecoal is fed, coal, char particles, and gases flow concurrently in the downer. Several key thermochemical reactions were extracted from the coal conversion involved in
the downer: the reforming of tar, the steam gasification of
the nascent char, and complex interactions between thevolatiles and char. The focus of this review is to highlight
our recent understanding of thermochemical coal conver-

Tar, an undesirable product of gasification, is defined as
the products heavier than C6 compounds, such as benzene, that can be condensed at ambient temperature. Its
reforming is mandatory for power generation and liquid
production (Hayashi et al., 2014). Liquid scrubbing techniques and catalytic reforming methods suffer from undesirable overall thermal efficiency loss and/or catalyst
deactivation because of the deposition of chlorine/sulfur
compounds and carbon (Abu El-Rub et al., 2004a; Devi,et al., 2002; Simell et al., 1997). Thus, it is necessary todevelop a tar reforming system with high thermal efficiency and without catalyst poisoning or deactivation.
The effective decomposition of tar compounds over biomass/coal-derived chars has been frequently reported(Abu El-Rub, 2008; Gilbert et al., 2009; Krerkkaiwan etal., 2015; Krerkkaiwan et al., 2013; Sun et al., 2011).
Naphthalene, representative of the most refractory compounds among tar constituents, was decomposed almost
completely over chars or charcoals from different biomass/coals within a short gas residence time (less than
0.25 s) in a fixed-bed reactor (Abu El-Rub et al., 2004b;
Fuentes-Cano et al., 2013). Except for benzene and naphthalene, tar constituents were completely decomposedover the coke accumulated in nanoporous alumina withina gas residence time of 0.03 s (Hosokai et al., 2005).
These studies demonstrate the potential of utilization ofchar for tar elimination.
The in situ tar reforming process is one of the key thermochemical phenomena in the downer reactor of the TBCFB system. Matsuhara et al. (2010) used a two-stage
reactor to realize the in situ reforming of nascent tar fromthe rapid pyrolysis of brown coal over char obtained fromthe same coal. The results showed that the released tar
can be decomposed over the char completely at 900 °C.
This indicated the potential for the rapid and complete decomposition of tar over a char surface due to an intensive
tar-char interaction. However, the characteristics of thechar and reaction conditions of the two-stage reactor system are different from those of the downer reactor,
through which coal, char particles, and gases are concurrently flowing. In the downer reactor, the in situ reforming of nascent tar occurs over the nascent char from therapid pyrolysis of the same coal. In addition, it is not possible to use a fixed char bed to realize the decompositionof tar in the continuously operating TB-CFB system.
In our recent study (Zhang et al., 2013b), a drop-tube
reactor (DTR) was employed to simulate reaction environments similar to those in the downer reactor. An

71

Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

Australian brown coal (Loy Yang, hereafter referred to as
LY) was co-fed with char (LYC) or partially gasified char
(GLYC) into the DTR, and the effects of the char properties, char concentration in the blended sample, peak temperature, and feeding rate related to the solid hold-up on
the tar decomposition were investigated.
The yields of heavy tar for in situ reforming at different
concentrations of char are shown in Fig. 2. The results arecompared with predicted values by assuming the independent thermoconversion of coal and char. The results showthat the observed heavy tar yields were 0.4–0.5 molC/100-mol-C-(LY + LYC/GLYC) lower than the corresponding predicted values at whole blending ratios of LY
and LYC/GLYC fed into the DTR. The differences between their yields are direct evidence of nascent tar decomposition through intensive tar-char interactions.
Coking/coke gasification and direct steam reforming are
two current possible mechanisms proposed to explain thephenomenon in which the tar is decomposed over the charsurface (Hosokai et al., 2008; Matsuhara et al., 2010). It is
more reasonable that coking gasification is the main contributor to nascent tar decomposition over a char surface
in non-catalytic tar reforming, compared with the direct
steam reforming of tar, because the direct steam reforming reactions occur to a lesser extent than coking if catalysts are not used. The nascent tar from the pyrolysis of
LY was decomposed by coking over the char from theflash pyrolysis of the same coal, and the char/coke was simultaneously gasified with steam. The steam gasificationof the char/coke created new active sites to maintain thetar coking over the char surface. It is noted that the direct
steam gasification of tar also is an important role of in situ
tar reforming. LY contains abundant Na+ and Ca2+, andthese alkali and alkaline earth metal (AAEM) species thatremain in the resultant char are considered to play cata-Fig. 2

72

Heavy tar yields of in situ tar reforming at different
char ratios on a (LY +LYC, GLYC)-fed carbon basis.
Reprinted with permission from Ref. (Zhang, et al.,2013b). Copyright: (2013) Elsevier B.V.

lytic roles in the steam gasification of char/coke. Thismeans that higher AAEM contents would be favorable formaintaining the coking activity of the char, and thus, tosome extent, the higher coking activity of the char would
promote more tar conversion into soot/coke over the charsurface.

3. In situ steam gasification of chargasification
The steam gasification of char, as a gas-solid thermochemical conversion, is the rate-determining step (Mühlen
et al., 1985) for the entire coal gasification process. The
process has been studied by many researchers to understand the kinetics and mechanisms of both non-catalytic
and catalytic reactions using fixed- (Haga and Nishiyama,1988) and fluidized-bed reactors. H2 is the main inhibitorin the steam gasification of char (Lussier et al., 1998;
Yang and Duan, 1985); the rate of steam gasification in thepresence of H2 is lower by an order of magnitude or morethan in its absence. However, recent studies (Hayashi etal., 2000, 2002) have shown that pyrolysis-derived volatiles, including tar, are much stronger inhibitors than H2in the steam gasification of char, because of the volatiles-­char interactions.
The gasification of AAEM-loaded coals has been investigated. Bayarsaikhan et al. (2005) found that the inherent Na + and Ca 2+ species increased the initial
gasification rate of the nascent char under a steam atmosphere by more than 80 % in a drop-tube/fixed-bed reactor system. Mašek et al. (2009) reported that catalytic
steam gasification proceeded extensively even under conditions in which the gasification of the unloaded coal was
retarded by coexisting tarry compounds and char. In addition, higher AAEM contents have also been shown tobe favorable for maintaining the coking activity of thechar, promoting the in situ steam reforming of tar over thechar surface. Therefore, the in situ steam gasification of
AAEM-loaded char under volatiles-char interactions is
worth studying.
AAEM species are generally loaded on low rank coals
using water-soluble precursors such as NaCl (Takarada etal., 1989; Wu et al., 2005), Na2NO3 (Yamashita et al.,1991), Ca(NO3)2 (Ohtsuka and Tomita, 1986), CaCl 2(Yamashita et al., 1992), Ca(CH3COO)2 (Ohtsuka andTomita, 1986; Tyler and Schafer, 1980), KCl (Liu and Zhu,
1986), or K 2CO3 (Liu and Zhu, 1986; Tristantini et al.,2015). Kajita et al. (2010) reported that Na+ and K+ were irreversibly transferred from the char to the mineral matter(alumina) when contacting with each other at 800 °C. Irreversible reactions between the AAEM and fluidizing
medium/circulating medium not only diminish the catalytic action, but also bring about problems in downstream

Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

operations. For example, the defluidization and agglomeration of bed materials occur in fluidized beds because of
the formation of Na/K silicates with melting point temperatures below 1000 °C. Therefore, Ca-containing materials are the most promising catalyst precursors for thesteam gasification of char. It is noteworthy that a slurry ofcoal and water usually becomes acidic as a result of the
dissolution of organic acidic compounds from brown coal.
CaCO3 and Ca(OH)2 (which become more soluble in theslurry) have also been reported for use as catalysts in thesteam gasification of char (Kim et al., 2013; Ohtsuka andAsami, 1996, 1997; Ohtsuka and Tomita, 1986). Ohtsuka
et al. (1997) reported that the catalytic activity of Ca2+
from CaCO3 was higher than that from Ca(OH)2 in a thermobalance reactor. The reported results, however, wereobtained at a low heating rate; the reactor system could
not provide a heating rate as high as that achieved in thedowner reactor for coal gasification. The higher heating
rate causes more rapid thermal cracking and provides
more active sites on the char particles for steam gasification. AAEM species can better catalyze the gasification of
nascent char, as well as the in situ reforming of volatiles
over the char surface, even in a short reaction time.
Ca-catalyzed gasification is also known to slow as theprocess of char conversion proceeds (Liu and Zhu, 1986;
Ohtsuka and Asami, 1996, 1997). Possible mechanisms(Radovic et al., 1984) include the sintering of CaO or
CaCO3 and crystalline growth.
In our recent study (Zhang, et al., 2013a), inexpensive
materials were selected as catalyst precursors, such as
NaCO3 and Ca(OH)2. The catalytic effects of Na+ andCa2+ from these inexpensive materials on the in situ steam
gasification of char from the rapid pyrolysis of low rank
coal (Adaro coal from Indonesia, hereafter referred to asAD) were investigated.
The net conversions of the primary chars from Na+ andCa2+ loaded ADs (AD-Na and AD-Ca, respectively) withdifferent concentrations of the catalysts after in situ steam
gasification in the DTR are shown in Fig. 3. Here the results with both Na+ and Ca2+ were co-loaded AD (ADCa-Na) were also shown. The net conversion of the
primary char Xchar is defined by
X char (primary char yield)  (char yield from DTR)
primary char yield

100%
Xchar is the difference in the extent of carbon deposition
between the char and steam gasification of char carbon
together with the deposited carbon. Fig. 3 shows Xchar as afunction of temperature. Xchar for AD at 900–950 °C isless than 10 %, and catalysis by the inherent AAEM
species, if any, was insignificant. However, Xchar for thecatalyst-loaded coals was in the range from 15 to 45 %,and the process of steam gasification within 3 s was clear.

Fig. 3 The net conversions of primary char of AD, AD-3.2%Ca,
AD-0.52%Ca, AD-1.5%Na, and AD-3.0%Ca-0.36%Na
after in situ steam gasification at 900–1000 °C in the
DTR. Reprinted with permission from Ref. (Zhang, etal., 2013a). Copyright: (2013) Elsevier B.V.

The Xchar value increases in the order AD < AD-Na <AD-Ca < AD-Ca-Na, indicating that AD-Ca-Na had thehighest reactivity under the current conditions. Someother studies (Ohtsuka and Tomita, 1986; Takarada et al.,1989) reported that the Na in Australian brown coal at
650 °C exerted a smaller catalytic effect than Ca in thesame coal at the same catalyst content (5 %).
The Xchar results for AD-3.2%Ca and AD-0.52%Ca (ofwhich concentrations of Ca are 3.2 and 0.52 wt%, respectively) show that the Ca2+ in the former was more active
than that in the latter at 900 °C, but less active at 1000 °C.
It is generally accepted that the activity of AAEM catalysts increases with increasing loading and then levels off
at a certain point. Increases in the catalyst concentration
tend to bring about increases in the size of the catalytic
particles, coarsening them through agglomeration and/or
coalescence. Thus, a higher Ca content causes more facile
coarsening of the catalyst particles (Cazorla-Amoros etal., 1993; Radovic et al., 1983). In addition, although theinitial Ca content in the AD-0.52%Ca was only about 1/6of that in the AD-3.2%Ca, the activity seemed to beequivalent with that in the AD-3.2%Ca at 950 °C, andthus, even higher at 1000 °C. This means that both thespecific and overall activities of Ca were reduced by increasing the Ca loading on the AD from 0.52 % to 3.2 %for in situ steam gasification at 950–1000 °C.
It is noteworthy that Xchar for AD-3.0%Ca-0.36%Na
was as high as 71 % and much higher than that for AD3.2%Ca, although their Ca contents were nearly equivalent. Compared with catalytic effect of AD-1.5%Na, thedifference in Xchar between AD-3.0%Ca-0.36%Na andAD-3.2%Ca is attributed mainly to either the activity of
Na or synergy due to the coexistence of Na and Ca that
could suppress the coarsening of the Ca catalyst particles.

73Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

4. Kinetic modeling coupled with CFD
approach accounting for volatiles-charinteractions
Coal conversion incorporating the volatiles-char interactions proceeds via a number of complex parallel reactions, and produces a wide range of products such as char,soot, light hydrocarbons, and both single and polycyclicaromatic hydrocarbons. The volatiles-char interaction has
been frequently reported for different reactor systems,
and the results show that the interaction has a significant
role in inhibiting char gasification and promoting nascenttar decomposition. (Hayashi et al., 2014; Li C.Z., 2013).
However, those studies focused on laboratory-based experimental investigations only. Kajitani et al. (2013) proposed a kinetic model incorporating the volatiles-charinteraction, which reproduced the experimental findings
from coal conversion very well, including the effects of the
Na concentration in the char during steam gasification. It
can also be used in designing any industrial fluidized-bed
gasifier in the presence of volatile-char interactions. Although no studies have been performed, the developmentof a mathematical model based on the experimental results for coal conversion, including the volatiles-char interactions in the downer reactor of a TB-CFB system,
should be considered.

4.1 Kinetic model
The lumping component model, an approach in which
complex mixtures with large numbers of compounds are
lumped into smaller groups of pseudo-components, iswidely employed to provide a tractable number of kinetic
equations and estimate kinetic parameters. To date, it has
been mainly applied in investigations of hydrogenation or
the catalytic cracking of heavy oil (Lan et al., 2009;
Theologos and Markatos, 1993).
In our recent study (Li C.Y. et al., 2015), it was employed for the first time to estimate kinetic parameters incomplex coal conversion environments, including volatiles-­char interactions, which were approximated for the
downer reactor of the TB-CFB system. A network among
lumps in the DTR is proposed as shown in Fig. 4. Under
our conditions, it is assumed that the coal sample has already been converted into tar, carbon gases, char, andsoot by primary pyrolysis at the top non-isothermal zoneof the DTR (at 500–900 °C) (Jamil et al., 2004; Sathe etal., 1999). The carbon conversion reactions in the isothermal zone of the DTR were simulated, and the feedstock
was lumped into tar, carbon gases, char, and soot derived
from the primary pyrolysis of the coal. The possible main
reactions in the DTR were considered to be thermal
cracking/steam reforming of the volatiles, gasification,and the volatiles-char interactions. The kinetic parameters

74

Fig. 4

Schematic diagram of the four-lump reaction networkin the DTR coal conversion process accounting for thevolatiles-char interaction. Reprinted with permission
from Ref. (Li C.Y. et al., 2015). Copyright: (2015) Elsevier
B.V.

of the four-lump model were estimated through a mathematical model integrated with a four-order Runge-Kutta
method.

4.2 Kinetic model coupled with a computational
fluid dynamics (CFD) approach
In the downer reactor of the TB-CFB system, coal,char particles, and gases flow concurrently. Gas-solid
two-phase mixed flows are key phenomena and of particular interest for understanding the downer reactor.
Ropelato et al. (2005) proposed an Eulerian-Eulerian approach for predicting fluid dynamics in the downer reactor, with good agreement between the experimental data
and model predictions. Fushimi et al. (2012) also used an
Eulerian-Eulerian method to investigate the degree ofsolid-­solid mixing in the downer unit by simulating thehydrodynamics of hot sand, cold sand, and air, as wellas the heat transfer among them. The simulation results
agreed well with the experimental observations. An
Eulerian-Lagrangian method (CFD-DEM) was applied
by Zhao et al. (2007) to simulate gas-solid flows in a2-­dimensional downer reactor, and the predicted macro-­scale flow structure had good agreement with the experiments. The published works can successfully predict fluid
dynamics in the downer reactor with good agreement between experimental data and model predictions. The data
can be useful, but the models only include numerical simulations of gas-solid flows in the downer reactor for coalconversion without considering the chemical reactions.
Thus, a CFD approach for gas-solid flows combined witha chemical kinetic model for coal conversion that accounts for the volatiles-char interactions in the downerreactor is urgently needed.
An Eulerian-Eulerian model was employed to simulate
the fluid dynamics of gas-solid flow environments approximated for the downer reactor. Six global chemical

Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

reactions with the corresponding kinetics parameters derived from the lumping model were considered in thesimulation. The proposed CFD model gave acceptablepredictions of gas-solid flows with carbon conversion reactions in the DTR in the presence of volatiles-char interactions (Li C.Y. et al., 2015).
The mass fraction distribution of the four lumps obtained from the simulation is illustrated in Fig. 5. In the
carbon thermo-conversion process, in situ reforming of tar
over the char surface occurred, and resulted in the massfraction of tar decreasing gradually from inlet to outlet.
For char particles, steam gasification is a main pathway
for conversion, and their mass fraction also decreases. In
contrast, the mass fraction of carbon gases increases very
quickly along the DTR. This can be explained by the production of carbon gases through tar cracking and the gasification of both char and soot, which occur at faster rates
than that of tar pyrolysis to produce tar.
In addition, the effects of reaction temperature, solid
hold-up, and steam partial pressure on tar consumption
along the DTR are shown in Fig. 6. It is very clear thatabout 85 % of the tar is consumed at a rapid rate near the
DTR entrance zone. Furthermore, the reaction temperature and solid hold-up are two significant factors in tar
decomposition, whereas the steam partial pressure has little effect.

Fig. 5

5. Conclusions
This manuscript describes the potential of the downerreactor for coal conversion, including tar reforming andchar gasification, in a triple-bed combined circulating
fluidized-bed reactor system. A drop-tube reactor was
employed to simulate the reaction environment approximated in the downer reactor, wherein coal, char particles,
and gases are concurrently flowing. Increasing the reaction temperature, feeding rate, and char concentration enhanced the tar decomposition. Inexpensive raw materials
such as Na2CO3 and Ca(OH)2 effected good catalytic performance for the steam gasification of the char. Finally, akinetic model with four lumps (gases, tar, char, and soot)and six reactions encompassing the volatiles-char interactions was developed and successfully integrated with a
CFD approach to simulate the gas/particle flow in the
DTR.

Acknowledgements
This work was in part supported financially by the New
Energy and Industrial Technology Development Organization (NEDO), Japan and MOST-JST, Strategic International Collaborative Research Program, SICORP. In
addition, Li-Xin Zhang and Cheng-Yi Li gratefully acknowledge financial supports given by the China Scholar-

The contours of different lumps derived from the CFD simulation (in run 4, (a) carbon gases; (b) tar; (c)char; (d) soot; Sum of mass fractions of gas phases = 1; Sum of mass fractions of solid phases = 1) Reprinted with permission from Ref. (Li C.Y. et al., 2015). Copyright: (2015) Elsevier B.V.

75

Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

ship Council (CSC Program) and the Kyushu University
Global COE program (G-COE Program). The authors are
grateful to Professor Hsiu-Po Kuo and Dr An-ni Huang at
Department of Chemical and Materials Engineering,
Chang Gung University, Taipei for their valuable suggestions for mixed phase flows.

Nomenclature
AAEM

alkali and alkaline earth metal

AD

Adaro coal

BFB

bubbling fluidized bed

CFDchemical fluid dynamics

DTR

drop-tube reactor

GLYC

partially gasified Loy Yang coal char

LY

Loy Yang coal

LYC

Loy Yang coal char

TB-CFB

triple-bed combined circulating fluidized-bed
reactorXchar

net conversion of the primary char

References

Fig. 676

The effect of reaction conditions on tar consumption as
a function of the distance from the entrance of the DTR:(a) temperature; (b) solid hold-up; and (c) steam partialpressure (Li C.Y. et al., 2015). Reprinted with permission from Ref. (Li C.Y. et al., 2015). Copyright: (2015)
Elsevier B.V.

Abu El-Rub Z., Bramer E.A., Brem G., Review of Catalysts for
Tar Elimination in Biomass Gasification Processes, Industrial & Engineering Chemistry Research, 43 (2004a) 6911–
6919. http://doi.org/10.1021/ie0498403
Abu El-Rub Z., Bramer E.A., Brem G., Modeling of tar reduction in biomass fuelled gasification using biomass char as acatalyst, In Proceeding of Conference and Technology
Exhibition on Biomass for Energy, Industry and Climate
Protection, (2004b) 530–541. http://www.abuelrub.com/
uploads/7/2/4/0/7240057/canada_2004.pdf.
Abu El-Rub Z., Biomass Char As an in-Situ Catalyst for Tar
Removal in Gasification Systems, PhD thesis, Twente University, Enschede, The Netherlands March 2008.
Bayarsaikhan B., Hayashi J., Shimada T., Sathe C., Li C.,
Tsutsumi A., Chiba T., Kinetics of steam gasification of
nascent char from rapid pyrolysis of a Victorian brown
coal, Fuel, 84 (2005) 1612–1621. http://doi.org/10.1016/j.
fuel.2005.02.008
Cazorla-Amoros D., Linares-Solano A., Salinas-Martinez deLecea C., Nomura M., Yamashita H., Tomita A., Local
structure of calcium species dispersed on carbon: influenceof the metal loading procedure and its evolution during
pyrolysis, Energy & Fuels, 7 (1993) 625–631. http://doi.
org/10.1021/ef00041a010
Devi L., Ptasinski K.J., Janssen F.J.J.G., A review of the primary measures for tar elimination in biomass gasification
processes, Biomass and Bioenergy, 24 (2002) 125–140.
http://doi.org/10.1016/S0961-9534(02)00102-2
Fuentes-Cano D., Gómez-Barea A., Nilsson S., Ollero P.,
Decomposition kinetics of model tar compounds over chars
with different internal structure to model hot tar removal inbiomass gasification, Chemical Engineering Journal, 228(2013) 1223–1233. http://doi.org/10.1016/j.cej.2013.03.130
Fushimi C., Guan G., Nakamura Y., Ishizuka M., Tsutsumi A.,
Matsuda S., Suzuki Y., Hydrodynamic characteristics of alarge-scale triple-bed combined circulating fluidized bed,
Powder Technology, 209 (2011) 1–8. http://doi.org/10.1016/

Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

j.powtec.2011.01.018
Fushimi C., Guan G., Nakamura Y., Ishizuka M., Tsutsumi A.,
Suzuki Y., Wang C.-H., Mixing behaviors of cold–hot particles in the downer of a triple-bed combined circulating
fluidized bed, Powder Technology, 221 (2012) 70–79. http://
doi.org/10.1016/j.powtec.2011.12.017
Gilbert P., Ryu C., Sharifi V., Swithenbank J., Tar reduction in
pyrolysis vapours from biomass over a hot char bed, Bioresource Technology, 100 (2009) 6045–6051. http://doi.org/10.
1016/j.biortech.2009.06.041
Goyal A., Rehmat A., Fuel Evaluation for a Fluidized-Bed Gasification Process (U-GAS), In Clean Energy from Waste
and Coal (1992) 58–71. http://doi.org/10.1021/bk-1992-0515.
ch005
Guan G., Fushimi C., Tsutsumi A., Prediction of flow behaviorof the riser in a novel high solids flux circulating fluidized
bed for steam gasification of coal or biomass, Chemical
Engineering Journal, 164 (2010a) 221–229. http://doi.
org/10.1016/j.cej.2010.08.005
Guan G., Fushimi C., Tsutsumi A., Ishizuka M., Matsuda S.,
Hatano H., Suzuki Y., High-density circulating fluidized
bed gasifier for advanced IGCC/IGFC—Advantages andchallenges, Particuology, 8 (2010b) 602–606. http://doi.
org/10.1016/j.partic.2010.07.013
Guan G., Fushimi C., Ishizuka M., Nakamura Y., Tsutsumi A.,
Matsuda S., Wang C.-H., Flow behaviors in the downer of alarge-scale triple-bed combined circulating fluidized bed
system with high solids mass fluxes, Chemical Engineering
Science, 66 (2011) 4212–4220. http://doi.org/10.1016/j.
ces.2011.06.006
Haga T., Nishiyama Y., Influence of structural parameters oncoal char gasification, Fuel, 67 (1988) 748–752. http://doi.
org/10.1016/0016-2361(88)90144-5
Hayashi J.I., Iwatsuki M., Morishita K., Tsutsumi A., Li C.Z.,
Chiba T., Roles of inherent metallic species in secondary
reactions of tar and char during rapid pyrolysis of browncoals in a drop-tube reactor, Fuel, 81 (2002) 1977–1987.
http://doi.org/10.1016/S0016-2361(02)00128-X
Hayashi J.I., Kudo S., Kim H.S., Norinaga K., Matsuoka K.,
Hosokai S., Low-temperature gasification of biomass andlignite: Consideration of key thermochemical phenomena,
rearrangement of reactions, and reactor configuration,
Energy and Fuels, 28 (2014) 4–21. http://doi.org/10.1021/
ef401617k
Hayashi J.I., Takahashi H., Iwatsuki M., Essaki K., Tsutsumi A.,
Chiba T., Rapid conversion of tar and char from pyrolysis
of a brown coal by reactions with steam in a drop-tube
reactor, Fuel, 79 (2000) 439–447. http://doi.org/10.1016/
S0016-2361(99)00179-9
Hobbs M.L., Radulovic P.T., Smoot L.D., Combustion and gasification of coals in fixed-beds, Progress in Energy andCombustion Science, 19 (1993) 505–586. http://doi.
org/10.1016/0360-1285(93)90003-W
Hosokai S., Hayashi J.-I., Shimada T., Kobayashi Y., Kuramoto
K., Li C.-Z., Chiba T., Spontaneous Generation of Tar
Decomposition Promoter in a Biomass Steam Reformer,
Chemical Engineering Research and Design, 83 (2005)
1093–1102. http://doi.org/10.1205/cherd.04101

Hosokai S., Kumabe K., Ohshita M., Norinaga K., Li C.,
Hayashi J.-I., Mechanism of decomposition of aromatics
over charcoal and necessary condition for maintaining its
activity, Fuel, 87 (2008) 2914–2922. http://doi.org/10.1016/
j.fuel.2008.04.019
Jamil K., Hayashi J.I., Li C.-Z., Pyrolysis of a Victorian brown
coal and gasification of nascent char in CO2 atmosphere ina wire-mesh reactor, Fuel, 83 (2004) 833–843. http://doi.
org/10.1016/j.fuel.2003.09.017
Kajita M., Kimura T., Norinaga K., Li C.-Z., Hayashi J.I., Catalytic
and Noncatalytic Mechanisms in Steam Gasification ofChar from the Pyrolysis of Biomass, Energy & Fuels, 24(2010) 108–116. http://doi.org/10.1021/ef900513a
Kajitani S., Tay H.-L., Zhang S., Li C.-Z., Mechanisms andkinetic modelling of steam gasification of brown coal in thepresence of volatile–char interactions, Fuel, 103 (2013)
7–13. http://doi.org/10.1016/j.fuel.2011.09.059
Kim H., Kudo S., Tahara K., Hachiyama Y., Yang H., Norinaga
K., Hayashi J.I., Detailed Kinetic Analysis and Modeling of
Steam Gasification of Char from Ca-Loaded Lignite,
Energy & Fuels, 27 (2013) 6617–6631. http://doi.org/10.
1021/ef401688h
Krerkkaiwan S., Mueangta S., Thammarat P., Jaisat L.,
Kuchonthara P., Catalytic Biomass-Derived Tar Decomposition Using Char from the Co-pyrolysis of Coal and GiantLeucaena Wood Biomass, Energy & Fuels, 29 (2015) 3119–
3126. http://doi.org/10.1021/ef502792x
Krerkkaiwan S., Tsutsumi A., Kuchonthara P., Biomass derived
tar decomposition over coal char bed, ScienceAsia, 39(2013) 511. http://doi.org/10.2306/scienceasia1513-1874.2013.
39.511
Lan X., Xu C., Wang G., Wu L., Gao J., CFD modeling of gassolid flow and cracking reaction in two-stage riser FCC
reactors, Chemical Engineering Science, 64 (2009) 3847–
3858. http://doi.org/10.1016/j.ces.2009.05.019
Li C.Y., Appari S., Zhang L.X., Huang A.N., Kuo H.P., Kudo S.,
Hayashi J.-I., Norinaga K., Modeling of gas/particle flow incoal conversion with a drop tube reactor using a lumped
kinetic model accounting volatiles–char interaction, Fuel
Processing Technology, 138(2015) 588–594. http://dx.doi.
org/10.1016/j.fuproc.2015.06.043
Li C.Z., Importance of volatile-char interactions during thepyrolysis and gasification of low-rank fuels—A review,
Fuel, 112 (2013) 609–623. http://doi.org/10.1016/j.fuel.2013.
01.031
Li C.Z., Special Issue—Gasification: a Route to Clean Energy,
Process Safety and Environmental Protection, 84 (2006)
407–408. http://doi.org/10.1205/psep.ed.0606
Li X.T., Grace J.R., Lim C.J., Watkinson A.P., Chen H.P., Kim
J.R., Biomass gasification in a circulating fluidized bed,
Biomass and Bioenergy, 26 (2004) 171–193. http://doi.
org/10.1016/S0961-9534(03)00084-9
Liu Z., Zhu H., Steam gasification of coal char using alkali andalkaline-earth metal catalysts, Fuel, 65 (1986) 1334–1338.
http://doi.org/10.1016/0016-2361(86)90099-2
Lussier M.G., Zhang Z., Miller D.J., Characterizing rate inhibition in steam/hydrogen gasification via analysis of adsorbedhydrogen, Carbon, 36 (1998) 1361–1369. http://doi.org/10.

77Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

1016/S0008-6223(98)00123-7
Mašek O., Hosokai S., Norinaga K., Li C.Z., Hayashi J.I., Rapid
gasification of nascent char in steam atmosphere during thepyrolysis of Na- and Ca-Ion-exchanged brown coals in a
drop-tube reactor, Energy and Fuels, 23 (2009) 4496–4501.
http://doi.org/10.1021/ef900235e
Matsuhara T., Hosokai S., Norinaga K., Matsuoka K., Li C.Z.,
Hayashi J.I., In-situ reforming of tar from the rapid pyrolysis of a brown coal over char, Energy and Fuels, 24 (2010)
76–83. http://doi.org/10.1021/ef9005109
Mühlen H.-J., van Heek K.H., Jüntgen H., Kinetic studies ofsteam gasification of char in the presence of H2, CO2 andCO, Fuel, 64 (1985) 944–949. http://doi.org/10.1016/00162361(85)90149-8
Ohtsuka Y., Asami K., Ion-Exchanged Calcium from Calcium
Carbonate and Low-Rank Coals: High Catalytic Activity in
Steam Gasification, Energy & Fuels, 10 (1996) 431–435.
http://doi.org/10.1021/ef950174f
Ohtsuka Y., Asami K., Highly active catalysts from inexpensive
raw materials for coal gasification, Catalysis Today, 39 (1997)
111–125. http://doi.org/10.1016/S0920-5861(97)00093-X
Ohtsuka Y., Tomita A., Calcium catalysed steam gasification of
Yallourn brown coal, Fuel, 65 (1986) 1653–1657. http://doi.
org/10.1016/0016-2361(86)90264-4
Radovic L.R., Walker Jr. P.L., Jenkins R.G., Catalytic coalgasification: use of calcium versus potassium, Fuel, 63 (1984)
1028–1030. http://doi.org/10.1016/0016-2361(84)90329-6
Radovic L.R., Walker P.L., Jenkins R.G., Importance of catalyst
dispersion in the gasification of lignite chars, Journal of
Catalysis, 82 (1983) 382–394. http://doi.org/10.1016/00219517(83)90205-1
Ropelato K., Meier H.F., Cremasco M.A., CFD study of gassolid behavior in downer reactors: An Eulerian-Eulerian
approach, Powder Technology, 154 (2005) 179–184. http://
doi.org/10.1016/j.powtec.2005.05.005
Sathe C., Pang Y., Li C.Z., Effects of heating rate and ion-­exchangeable cations on the pyrolysis yields from a Victorian brown coal, Energy and Fuels, 13 (1999) 748–755.
http://doi.org/10.1021/ef980240o
Simell P., Kurkela E., Stahlberg P., Hepola J., Catalytic hot gas
cleaning of gasification gas, Catalysis Today, 27 (1997)
3–68. http://doi.org/10.1016/0920-5861(95)00172-7
Sun Q., Yu S., Wang F., Wang J., Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of
hot char, Fuel, 90 (2011) 1041–1048. http://doi.org/10.1016/
j.fuel.2010.12.015
Takarada T., Nabatame T., Ohtsuka Y., Tomita A., Steam gasification of brown coal using sodium chloride and potassium

78

chloride catalysts, Industrial & Engineering Chemistry
Research, 28 (1989) 505–510. http://doi.org/10.1021/
ie00089a001
Theologos K.N., Markatos N.C., Advanced modeling of fluid
catalytic cracking riser-type reactors, AIChE Journal, 39(1993) 1007–1017. http://doi.org/10.1002/aic.690390610
Tristantini D., Supramono D., Suwignjo R.K., Catalytic Effectof K 2CO3 in Steam Gasification of Lignite Char on Mole
Ratio of H2/CO in Syngas, International Journal of Technology, 6 (2015) 22. http://doi.org/10.14716/ijtech.v6i1.208
Tyler R.J., Schafer H.N.S., Flash pyrolysis of coals: influence of
cations on the devolatilization behaviour of brown coals,
Fuel, 59 (1980) 487–494. http://doi.org/10.1016/0016-2361(80)90175-1
Wu H., Li X., Hayashi J.I., Chiba T., Li C.-Z., Effects of volatile–char interactions on the reactivity of chars from NaClloaded Loy Yang brown coal, Fuel, 84 (2005) 1221–1228.
http://doi.org/10.1016/j.fuel.2004.06.037
Yamashita H., Nomura M., Tomita A., Local structures of metals dispersed on coal. 4. Local structure of calcium species
on coal after heat treatment and carbon dioxide gasification,
Energy & Fuels, 6 (1992) 656–661. http://doi.org/10.1021/
ef00035a018
Yamashita H., Yoshida S., Tomita A., Local structures of metals
dispersed on coal. 3. Na K-edge XANES studies on the
structure of sodium gasification catalyst, Industrial & Engineering Chemistry Research, 30 (1991) 1651–1655. http://
doi.org/10.1021/ie00055a037
Yang R.T., Duan R.Z., Kinetics and mechanism of gas-carbon
reactions: Conformation of etch pits, hydrogen inhibition
and anisotropy in reactivity, Carbon, 23 (1985) 325–331.
http://doi.org/10.1016/0008-6223(85)90118-6
Zhang L.X., Kudo S., Tsubouchi N., Hayashi J.I., Ohtsuka Y.,
Norinaga K., Catalytic effects of Na and Ca from inexpensive materials on in-situ steam gasification of char fromrapid pyrolysis of low rank coal in a drop-tube reactor, Fuel
Processing Technology, 113 (2013a) 1–7. http://doi.org/
10.1016/j.fuproc.2013.03.009
Zhang L.X., Matsuhara T., Kudo S., Hayashi J.I., Norinaga K.,
Rapid pyrolysis of brown coal in a drop-tube reactor withco-feeding of char as a promoter of in situ tar reforming,
Fuel, 112 (2013b) 681–686. http://doi.org/10.1016/j.fuel.
2011.12.030
Zhao Y., Cheng Y., Ding Y., Jin Y., Understanding the hydrodynamics in a 2-dimensional downer by CFD-DEM simulation,
In The 12th International Conference on Fluidization—New
Horizons in Fluidization Engineering, 2007.

Cheng-Yi Li et al. / KONA Powder and Particle Journal No. 34 (2017) 70–79

Author’s short biography
Cheng-Yi Li
Cheng-Yi Li received his PhD by accomplishing a study on modelling and simulation
on reacting flow included in thermochemical conversion process of coal in 2015 at Kyushu
University. He received his bachelor and master degrees at China University of Mining
and Technology (Beijing) and currently working at China Tianchen Engineering Corporation as a process engineer for designing and developing advanced chemical plants indifferent fields, such as coal gasification plant, petrochemical plant and so on.
Li-Xin Zhang
Li-Xin Zhang received his PhD by accomplishing a study on the decomposition characteristics of tar derived from low temperature gasification of brown coal over coexisting
char in 2013 at Kyushu University. He received his bachelor and master degrees at China
University of Mining and Technology (Beijing) and currently working at Xi’an Thermal
Power Research Institute Co. Ltd. (P.R. China) for developing coal-based poly-generation
system and advanced coal power generation system.
Shinji Kudo
Shinji Kudo is an assistant professor at Institute for Materials Chemistry and Engineering, Kyushu University. He obtained his Ph.D. in Chemical Engineering from Kyoto
University (2010) where he studied catalyst and reactor design for hydrogen production
by steam reforming of methanol. He was an assistant professor at Research and Education Center of Carbon Resources, Kyushu University in 2010–2012, and moved to thepresent position in 2013. His current research focus is on thermochemical conversion of
lignocellulosic biomass and lignite for production of syngas, valuable chemicals, carbonaceous materials, and coke.
Jun-ichiro Hayashi
Jun-ichiro Hayashi has been a professor of Institute for Materials Chemistry andEngineering, Kyushu University, since 2009, and also Director of Research and Education
Center of Carbon Resources, Kyushu University, since 2012. He received his doctoral
degree (Engineering) from Kyushu University in 1996. He was an associate professor(1996–2005), and then professor (2005–2009) of Hokkaido University. His major academic field is chemical engineering and chemical reaction engineering. His research area
ranges over upgrading and conversion of carbon resources, carbon/inorganic materials,
and environmental technologies.
Koyo Norinaga
Koyo Norinaga has been an Associate Professor of Institute for Materials Chemistry
and Engineering, Kyushu University since 2009. He received his doctoral degree in 1999
in applied chemistry at Hokkaido University. He was an assistant professor at Tohoku
University (1999–2002), a senior scientist (Humboldt fellow) at Karlsruher Institute ofTechnology (Karlsruhe, Germany, 2002–2006), and an associate professor at Hokkaido
University (2006–2009). His research covers modeling and simulation of chemically reacting flows, including pyrolysis of light hydrocarbons, gasification and combustion ofcoal and biomass, and CVD/CVI of carbon and silicon carbide for composite material
production.

79

KONA Powder and Particle Journal No. 34 (2017) 80–90/Doi:10.14356/kona.2017009

Original Research Paper

Synthesis of Nanoparticles by Laser Ablation: A Review †Myungjoon Kim 1, Saho Osone 2, Taesung Kim 1,3,
Hidenori Higashi 2 and Takafumi Seto 2*
1

School of Mechanical Engineering, Sungkyunkwan University, Korea
Faculty of Natural System, Kanazawa University, Japan
3
SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Korea
2

Abstract
Laser ablation is a method for fabricating various kinds of nanoparticles including semiconductor quantum dots,
carbon nanotubes, nanowires, and core shell nanoparticles. In this method, nanoparticles are generated by
nucleation and growth of laser-vaporized species in a background gas. The extremely rapid quenching of vapor isadvantageous in producing high purity nanoparticles in the quantum size range (< 10 nm). In this review, theformation mechanism of nanoparticles by laser ablation is summarized. Recent progress on the control ofnanoparticle size and the challenges for functional nanoparticle synthesis by advanced laser ablation technology
are then discussed.
Keywords: laser ablation, nanoparticle, quantum dot, nanocarbons

1. Introduction
Nanoparticles have a variety of unique properties thatare not observable in bulk materials (Buzea et al., 2007).
The most critical characteristic of nanoparticles is that theproperties (electrical, optical, magnetic, and so on) depend
strongly on the size and size distribution of the particles(Kim et al., 2014). For example, silicon nanoparticles arephotoluminescent in the visible spectrum at room temperature (Kenemitsu, 1995; Yoshida et al., 1998); thewavelength of the emitted light can be controlled with theparticle size (Orii et al., 2003a). Nano-sized titania particles have also gathered attention as building blocks forphotovoltaic devices (O’regan and Gratzel, 1991) and forphotocatalytic applications (Fujishima and Honda, 1972).
The photochemical properties of titania nanoparticles also
have a strong dependence on particle size as well as on
crystal structure. Many kinds of nanoparticles exhibit
special characteristics (ferromagnetism, paramagnetism,
pinned emission, fluorescence, spin quantum effect, etc.)
when the size of a particle is at the nanoscale level (Zheng
et al., 2004; Kim et al., 2011; Kim et al., 2014). The spe†

Received 25 January 2016; Accepted 22 March 2016
J-STAGE Advance Publication online 30 April 2016
1,3
300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi, 440-746,
Korea
2
Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan* Corresponding author: Takafumi Seto;
E-mail: t.seto@staff.kanazawa-u.ac.jp
TEL: +81-76-234-4815 FAX: +81-76-264-6239

80

cial characteristics are significantly impacted by the sizeand size distribution of the particles.
Various nanoparticle fabrication methods have beendeveloped with the bottom-up approach in the liquid
phase (including sol-gel and chemical reduction) as wellas vapor phase (such as physical/chemical vapor deposition and flame synthesis). Each fabrication method has
advantages and disadvantages. Liquid phase methods are
cost-effective and are used for synthesizing various kindsof nanoparticles with well-controlled structures at the laboratory scale. Vapor phase processes are superior at synthesizing high purity nanoparticles by means of the continuous
flow reactor. In both of the liquid and gas bottom-up
processes, solid nanoparticles are generated from the nucleation of supersaturated species that are prepared by
precursor reactions and/or evaporation of solids. Laser
ablation is a method that utilizes laser (which is an acronym for light amplification by stimulated emission ofradiation) as an energy source for ablating solid target
materials. In this process, extremely high energy is concentrated at a specific point on a solid surface to evaporate light-absorbing material. The term ‘ablation’ refers to
the removal of surface atoms and involves not only a single photon process (breaking the chemical bonds) but also
multiphoton excitation (thermal evaporation). High-purity
nanoparticles can be generated by laser ablation because
the purity of the particles is basically determined by the
purity of the target and ambient media (gas or liquid)
without contamination from the reactor. However, it isdifficult to control size distribution, agglomeration, andCopyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

Fig. 1

Fig. 2

Schematic of particle generation procedure in the laser ablation process.

Laser-induced plume of silicon in low pressure.

crystal structure in the conventional laser ablation processsince nanoparticles are built by random (Brownian) motion of molecules. Therefore, several advanced laser ablation techniques have been developed for fabricating
morphology-controlled nanoparticles. In this review, laser
ablation-based nanoparticle formation processes and theirmechanisms are briefly discussed, followed by a review
of recent studies of laser ablation techniques for synthesizing various kinds of nanoparticles. Finally, advanced
laser ablation processes for synthesizing functional nanomaterials with highly controlled nanostructures are introduced.

2. Nanoparticle formation by laser ablation
2.1 Basic concept of laser ablation
Fig. 1 is a schematic of the nanoparticle formation process by laser ablation. When the laser beam is focused on
the surface of a solid target material in the ambient media(gas or liquid), the temperature of the irradiated spot rapidly increases, vaporizing the target material. The colli-Fig. 3 Typical transmission electron micrographs of laser ablation-­
generated (a) silicon, (b) carbon and (c) surface-oxidized
nickel nanoparticles. (d) Aggregates of nickel nanoparticles as a result of coagulation and necking.

sions between the evaporated species (atom and clusters)
and the surrounding molecules result in excitation of theelectron state coupled with light emission and generationof electrons and ions, forming a laser-induced plasma
plume (Fig. 2). The plasma structures (size of the plumeand its emission spectrum) depend on the target material,
ambient media (liquid or gas), ambient pressure, and laser
conditions. Fig. 3 shows typical transmission electron micrographs of nanoparticles generated by laser ablation ofvarious materials. In general, laser ablation in low-pressure
background gas is preferable for creating a large plume
and works well in generating small particles (Fig. 3(a)).

81

Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

Laser ablation in liquid is employed to confine the plasma
plume in a small region to directly disperse nanoparticles
in the liquid phase (Fig. 3(b)). In any case, the ambient media must be carefully selected because the laser-generated
particles easily react with surrounding molecules to create
complexes such as oxides and other undesirable species(Figs. 3(b) and (c)). Coagulation is another critical phenomena that must be finely controlled in the later stagesof nanoparticle formation. Since laser-generated particles
have a very clean surface, agglomerated particles createchemical bonds at the contact point (neck), which significantly compromise the properties of primary particles(Fig. 3(d), Sakiyama et al., 2004). The low-pressure gas
process is advantageous not only for reducing the size ofthe primary particles but also for preventing coagulation.

2.2 Laser
For the fabrication of nanoparticles of the desired sizeand structure, the selection of a suitable laser system is
one of the most critical decisions. The evaporation rate of
the target material is generally determined by the laserparameters (laser source, wavelength, fluence, pulse
width and frequency), the light absorption efficiency of
the target material, and the condition of the ambient media. Laser energy per unit area on the target material isdefined as fluence of the laser F, which is given as,
F

I
A

(1)

where I [J/pulse] is the laser power and A [m2] is the areaof the laser spot. In the case of silicon nanoparticle formation using the double harmonic wave of the Nd:YAG laser(wavelength of 532 nm), a laser with a fluence (F) of morethan several J/cm 2 is frequently used. The wavelength ofthe laser is another important parameter that determines
the absorption efficiency of the target. The absorption
depth and spot (focusing) area are also influenced by laser
wavelength (Friedrich et al., 1998). In early studies ofnanoparticle synthesis by laser ablation, excimer lasers in
the ultraviolet spectrum (193 nm for ArF, 248 nm forKrF) are often used as a light source (Yoshida et al., 1996;
Shinde et al., 2000). Recently, Q-switch pumped pulsed
YAG (Yttrium Aluminum Garnet) lasers are more commonly used for laser ablation because they do not require
hazardous gases. The wavelength of Nd:YAG laser(1,064 nm for fundamental wave) can be changed by employing nonlinear optical crystals. For example, 532 nm
for second harmonic generation and 355 nm for third harmonic and other frequency mixing are possible. Overall,
it is difficult to attain stable ablation using the fundamental wave because of the weaker light absorption in the infrared region (Lu et al., 2008). Torrisi et al. (2003)
investigated the effect of wavelength on laser ablation of82

solid Cu using a 1064 nm Nd:YAG laser and a 308 nm
XeCl excimer laser. They showed that UV laser is more
efficient at evaporating the copper atoms, even though infrared radiation has higher ionic kinetic energy andhigher plasma temperature. Lu et al. (2008) calculated theevaporation thresholds of Si and estimated them to be
2 × 1010 W/cm 2 for a 266 nm wavelength laser and4 × 1011 W/cm2 for a 532 nm wavelength laser. Short pulse
lasers (nanoseconds) with less energy density have also
been used to generate Si nanoparticles. As an example,
Seto et al. (2001b) generated Si nanoparticles with a 5 ns
pulsed Nd:YAG laser (532 nm) with an average fluence of
16 J/cm 2 (about 3 × 109 W/cm 2). The pulse width of thelaser is also another important parameter that determines
peak energy. Recently picosecond (Lau et al., 2014) andfemtosecond lasers (Fischer et al., 2015) have been applied to enhance the photon absorption efficiency of thetarget surface to break chemical bonding.

2.3 Change in the temperature by laser
irradiation
When the target surface is irradiated with short laser
pulses, the absorbed photon energy is transferred to thesolid lattice as a form of thermal (internal) energy. Heattransfer in the electron and lattices subsystems can becharacterized by the following one-dimensional, two-­
temperature diffusion equations with temperatures ofelectron Te and lattices Ti (Chichkov et al., 1996).
CeTe
Q( z)  (Te  Ti )  S
t
z

(2)

Ti
Ci 
 (Te  Ti )
t

(3)

T
Q( z) SI (t ) A exp( z)
k e e ,z(4)where Ce and Ci are the heat capacities of the electron andlattices subsystems, z is perpendicular direction to theheat surface, Q(z) is the heat flux, γ is the electron-phonon
coupling strength, S is the laser source term, ke is the heat
conductivities of the electron, I(t) is the laser intensity, A
is the absorbance of the sample, and 1/α can be defined as
the optical penetration depth.
Equations (2–4) can be modified for the laser pulse
width. In the case of nanosecond laser pulses, Te is assumed to be equal to Ti (Chichkov et al., 1996) due to theduration of the laser pulse being much larger than the lattice heating time. Then equations (2–4) can be reduced toCiTi  T ko  I (t ) A exp( z)tz  z (5)where, ko is the conventional equilibrium thermal conduc-Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

tivity of a substance.

2.4 Nucleation and particle growth
Since the laser-vaporized materials are very rapidly
quenched from extremely high temperatures (> 5000 K)
to room temperature (usually shorter than 50 μs after thelaser pulse; Wen at al., 2006), nanometer-sized particles
form by supersaturated vapor nucleation at the edge of thelaser plume (Fig. 1(c)). The change in the number concentration of vaporized material (monomer), clusters, andparticles can be given by the general dynamic equation(Pratsinis and Kim, 1989) by considering nucleation, condensation, and coagulation. Numerical simulations of thesolutions to the general dynamic equation have been conducted for predicting the size distribution of nanoparticles
generated by the ablation process (Wen et al., 2007;
Kuroda et al., 2012). It should be noted that there are
significant contributions of electron and ion species on the
nucleation process, i.e. ion-induced nucleation in the laser
ablation process. As a result, the particles generated by
the laser ablation were electrically charged (Seto et al.,2003). However, it is still difficult to reproduce the rapidchange in the size distribution due to nucleation andgrowth of the laser-vaporized materials. To experimentally analyze the particle formation process, advanced
in-situ observation techniques are necessary. Emission
spectroscopy of plumes (Geohegan et al., 1998) is one insitu measurement technique for investigating the dynamics
of particle formation. The mobility analyzing technique
may also be applied to investigate the effect of particle
generation conditions on the size distribution as discussed
later in this review.

3. Synthesis of various kinds of nanoparticles
by laser ablation
3.1 Semiconductor quantum dot
A semiconductor quantum dot is a nanocrystal that exhibits quantum properties such as tunable light emission
driven by the quantum confinement effect (Canham,
2000) and single electron transport phenomena (Tilke etal., 2001). There are some reports on the synthesis of II–
VI and III–V semiconductor quantum dots by laser ablation such as formation of GaAs nanocrystals (Perriere etal., 2001), ZnSe and CdS quantum dots (Anikin et al.,2002), CdSe and CdTe nanoparticles (Ruth and Young,2006) that can be applied to the study of optical imaging
therapy (Wolfbeisref, 2015). However, the laser ablation
process for composite materials is challenged in controlling the stoichiometry and crystallinity in the process
for rebuilding laser-vaporized elements. Group-IV ele-ments such as silicon and germanium and carbon
nanoparticles are single element semiconductors. Silicon
and carbon are abundant (non-rare metal) elements thatexhibit a variety of unique properties when the particle
sizes are in the nanometer range. Therefore, we will focus
on the synthesis of silicon and carbon nanoparticles inthis review.
Table 1 lists laser ablation studies on silicon nanoparticle synthesis. Okada and Iijima (1991) measured the oxidation rate of silicon nanoparticles generated by laser
ablation and found that the oxidation rate drops with decreasing particle size. They used a YAG laser of 12 millisecond pulse width and produced 20–500 nm particles
under pressures of 0.75 to 3 Pa. Yoshida et al. (1996) synthesized silicon crystallites in gas between pressures of 333
to 1330 Pa using an argon-fluoride (ArF) excimer laser(193 nm wavelength, 12 ns pulse width, fluence 1 J/cm2).
They showed mean diameter of particle can be varied as afunction of the He background gas pressure. The mean
diameter calculated from the SEM images were about
23 nm at 333 Pa, 36 nm at 432 Pa, and 52 nm at 665 Pa.
Pressure effects on particle size in laser ablation generation was studied in more detail by Seto et al. (2003)
through particle classification with a low pressure differential mobility analyzer (low pressure DMA, LPDMA,
see later section). Suzuki et al. (2001) investigated thesuitability of LPDMA as a size classification tool for thesilicon nanoparticles generated by laser ablation under
low-pressure conditions. They compared the mode size
and the geometrical standard deviation (GSD) of silicon
nanoparticles with and without LPDMA size classification. The GSD was significantly reduced from 1.6 (as-­prepared) to 1.2 for particles as small as 4 nm afterclassification by LPDMA. The results indicate that sizeclassification is effective for generating the monodispersed nanoparticles that can be used to clarify the relationship between quantum properties and size.
Another important factor affecting the quantum
properties of Si nanoparticles is their crystallinity. Unfortunately, laser ablation tends to generate amorphous agglomerates which have poor (broad) optical properties(Hirasawa et al., 2004). Gas phase annealing is an effective method to improve crystallinity and the shape of the
Si nanoparticles. Fig. 4 shows laser ablation process coupled with gas phase annealing, particle classifying, andparticle measurement systems. Since laser ablation systems have good compatibility with gas flow reactors andother instruments as shown in Fig. 4., particle morphologies (size, shape and crystallinity etc.) can be controlled
in the aerosol phase. Hirasawa et al. (2004) investigatedthe change in the crystallinity of Si nanoparticles by
Raman spectroscopy and TEM as a function of the gasphase annealing temperature. They also employed LPDMA
for size selection and studied the size-dependent crystalli-

83

Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

Table 1 Silicon nanoparticle synthesis studies using pulsed laser ablation method (A: annealing, m: mean, M: mode)
AuthorLaser Source

Pressure

Okada and Iijima 1991

YAG

0.75~3.0 Pa

Yoshida et al. 1996

ArF excimer

333~1330 Pa
333 Pa434 Pa666 PaSeto et al. 2001a

Nd:YAG

666 PaSeto et al. 2001b

Nd:YAG

666 PaSuzuki et al. 2001

Nd:YAG

Particle size
20~500 nm

Aggregates
Aggregates
Aggregates

3~100 nm(m) 23 nm(m) 36 nm(m) 52 nm

DMA

Primary

2~7 nmDMA

Primary(M) 7, 10 nm

Primary(M) 6 nm

DMA

Primary(M) 3.8 nm

DMA
DMA
DMA
DMA
DMA
DMA

Aggregates
AggregatesPrimary
Primary
Primary
AggregatesPrimary
Aggregates~70 nm~ 120 nm(M) 5 nm(M) 4 nm(M) 4 nm(M) > 6 nm
(M) 4 nm(M) > 6 nm

666 Pa666 PaParticle

Seto et al. 2003

Nd:YAG

333 Pa666 Pa280 Pa334 Pa380 Pa380 Pa434 Pa434 PaOrii et al. 2003a

Nd:YAG931 Pa(A) 900 °C, DMA

Primary(M) 3~6 nm

Orii et al. 2003b

Nd:YAG931 Pa(A) 900 °C, DMA

Primary(M) 6.9 nm

Hirasawa et al. 2004

Nd:YAG

1000 Pa
1000 Pa
1000 Pa
1000 Pa(A) 300 °C, DMA(A) 500 °C, DMA(A) 700 °C, DMA(A) 900 °C, DMA

Primary
Primary
Primary
Primary(M) > 14 nm(M) 11.2 nm(M) 10.3 nm(M) 10 nm

Hirasawa et al. 2006

Nd:YAG

1000 Pa

DMA

Primary

7~14 nmFig. 4

Schematic of nanoparticle synthesis process using laser ablation, which is composed of (a) particle generation, (b) gas phase annealing, (c) particle classifying, and (d) particle measurement.

zation process of Si nanoparticles in the gas phase. Based on
their analysis, for Si nanoparticles (< 10 nm), gas phase annealing over 900 °C is effective in improving both crystallinity and shape as a result of sintering. The size-classified,spherical, and single crystal Si nanoparticles generated by84

Additional Process

the method exhibited sharp photoluminescence (PL) thatwas tunable with the size of Si nanoparticles (Orii et al.,2003a, b). The PL spectra of Si nanoparticles showed thatpeak energy increased from 1.34 to 1.79 eV with a drop in
particle size from 7 to 3 nm, and full width at half maxi-

Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

mum (FWHM) of the PL drops to approximately 0.22 eV
as a result of size classification by the LPDMA.

3.2 Metal and oxide nanoparticles
Metal and metal oxide nanoparticles also display various unique features that have potential for electrical,
magnetic, catalytic, and optical applications. Some studies on the synthesis of metal and metal oxide nanoparticles by laser ablation are listed in Table 2. Kawakami etal. (2002) produced gold nanoparticles by laser ablation
under helium gas under pressures of 3 to 90 kPa. They
generated a film by directly depositing the gold nanoparticles on the substrate by generating an aerosol-jet. The
microstructure of the gold film changed from porous tohigh density with increasing pressure. As a result, theelectrical resistivity of the film changed from 10 –1 Ωcm(3 kPa) to 10 –5 Ωcm (50 kPa). Dikovska et al. (2013) alsosynthesized silver nanoparticles by laser ablation underhigh vacuum conditions (~10 –5 torr). They investigated
the influence of Nd:YAG laser wavelength on the size ofgenerated particles. They reported that a shorter wavelength (266 nm) was better at generating smaller particles(4 nm in diameter) and that it exhibited surface plasmon
resonance.
Magnetic nanoparticles from transition metals andmetal alloys have also been synthesized by laser ablation.
Since magnetic properties are strongly influenced by purity, crystallinity, and particle size of the nanoparticles,
advanced laser ablation (Fig. 4) is significantly more capable of controlling nanoparticle morphologies. Sakiyama
et al. (2004) fabricated Ni particles of a select size coatedwith a NiO shell with the laser ablation method. They also
applied the aerosol process, i.e., LPDMA size classification, and gas phase annealing to improve Ni nanoparticle
morphology. A Ni/NiO core shell structure was successfully generated by the oxidation of Ni nanoparticles. Setoet al. (2004, 2005) investigated the magnetic properties of
Ni/NiO nanoparticles generated by this method using a
superconducting quantum interference device (SQUID)
magnetometer at room temperature. Ferromagnetism wasobserved with a film comprising nickel particles larger

Table 2 Metallic nanoparticle synthesis studies using pulsed laser ablation method (A: annealing, m: mean, M: mode, S: shell
thickness)
AuthorLaser Source

Material

Pressure

Additional Process

Particle

Particle size

Shinde et al. 2000

Excimer

FexOy
SrM

266 Pa266 PaCold Finger
Cold Finger

Seto et al. 2001a

Nd:YAG

TiOx

2.6~13.3 kPa

DMAPrimary

10~50 nm

DMA(A) 500 °C, DMA(A) 650 °C, DMA(A) 800 °C, DMA(A) 900 °C, DMA

Primary
Primary
Primary
Primary
Primary(M) 5.9 nm(M) 4.9 nm(M) 4.4 nm(M) 6.9 nm(M) 14.3 nm

Aggregates

10~50 nm,100~1000 nm(M) 8 nm(M) 13 nm(M) 20 nm(M) 30 nm(M) 75 nm(M) 80 nm

30~60 nm15~100 nm

Hirasawa et al. 2002

Nd:YAG

TiOx

1.73 kPa
1.73 kPa
1.73 kPa
1.73 kPa
1.73 kPa

Harano et al. 2002

Nd:YAG

TiOx

101.3 kPa
3 kPa
7 kPa
20 kPa
30 kPa
50 kPa
90 kPa

DMA
DMA
DMA
DMA
DMA
DMA

Primary
Primary
Primary
Primary
Primary
Primary

1 kPa

DMA

Aggregates

1 kPa(A) 400~800 °C, DMA

Primary

5~20 nm(S) 2 nmKawakami et al. 2002

Nd:YAG

Au

Sakiyama et al. 2004

Nd:YAG

Ni/NiO

Seto et al. 2006

Nd:YAG

CoPt

1 kPa

(A) 1000 °C, DMA

Primary(M) 14 nm

Seto et al. 2007

Nd:YAG

CoPt/SiO2

1 kPa(A) 1000 °C, DMA

Primary(M) 7 nm,(S) 2 nmNiu et al. 2012

KrF excimer

FeNi

High vacuum
266 PaPrimary
Aggregates(M) 4 nm(M) 6 nmDikovska et al. 2013

Nd:YAGAg~10 –3 Pa

Primary(M) 4~11 nm

85

Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

than 6.2 nm in core diameter. In contrast, smaller Ni
nanoparticles with core diameters of 3 nm exhibited superparamagnetism. They reported that the formation ofan antiferromagnetic NiO shell is effective for controllingthe manifestation of superparamagnetism in the nanometer-­sized ferromagnetic Ni core.
Another approach for overcoming the superparamagnetic limit is to synthesize highly anisotropic magnetic
alloy nanoparticles such as fct FePt and CoPt. Alloynanoparticles of Co and Pt were synthesized by laser ablation by varying the Co:Pt ratio of the target (Seto et al.,2006). Their EDX results showed that the composition of
the generated particles could be controlled by the target
composition. The magnetization of the laser-synthesized
CoPt particles (83.7 emu/g) was almost double that ofbulk CoPt (44 emu/g). They also observed a weak exchange coupling at the interface between the ferromagnetic core (CoPt) and antiferromagnetic shell (CoO). Setoet al. (2007) also investigated the influence of shell material by synthesizing a non-magnetic SiO2 shell by laser
ablation of a multi-component target comprising Co, Pt
and SiO2. Metal oxide and metal composite nanoparticles
have also been generated by laser ablation. Shinde et al.
(2000) also synthesized iron oxides and strontium ferrite(SrM) with the laser ablation method and evaluated themagnetic characteristics.
Vapor phase synthesis of titania (TiO2) nanoparticles
has been extensively investigated since they have photovoltaic and photocatalytic applications (Harano et al.,
2002; Pallotti et al., 2015). Seto et al. (2001a) synthesized
TiOx particles by Nd:YAG laser ablation of a TiO2 target
in a background gas (He) with pressure ranging from 2.6
to 13.3 kPa. They observed increases in the mobility diameter due to agglomeration as the pressure increased.
High-resolution TEM analysis of the nanostructures of thegenerated revealed that the metallic (Ti) core was covered
with a sub-stoichiometric shell structure with a thicknessof several nanometers. Hirasawa et al. (2002) also synthesized TiOx nanoparticles by laser ablation and investigatedthe effect of aerosol post-annealing temperatures of up to

900 °C. They reported an increase in the primary particle
diameter due to sintering and change in crystallinity from
amorphous form to anatase and rutile, which was confirmed by Raman scattering. They also discussed thenon-stoichiometric phase of TiOx particles such as Ti2O3.
More recently, Tsuji et al. (2012) investigated the effect of
nucleation (particle formation) temperature on the morphology of TiO2 nanoparticles generated by the laser ovenprocess (laser ablation in a tubular furnace).

3.3 Nanocarbons and nanowires
The benefit of non-equilibrium laser ablation nucleation
processes is that it can generate various kinds of crystal
phases and allotropes that are not seen in the normal(equilibrium) synthesis processes. Fig. 5 shows transmission electron micrographs of nanometer scale carbon allotropes (nanocarbons) such as carbon nanotubes (CNTs,
Fig. 5(a)), onion-like carbon (Fig. 5(b)), and nano-­
diamonds (Fig. 5(c)) synthesized by laser ablation. Sincelaser ablation is superior at generating high-purity andnanometer-sized metal particles, it is also suitable as acatalyst for the growth of single walled carbon nanotubes(SWCNTs) on the substrate. For example, Kohno et al.
(2004) generated Co/Mo and Co/Pt alloy nanoparticles bylaser ablation and investigated the ethanol-CVD growthof the CNT on the substrate. In another development, laser ablation in a high temperature flow reactor (laser ovenmethod) has been widely used to generate high-purity
SWCNTs (Puretzky et al., 2000). References on carbon-­related nanomaterial synthesis using gas-phase laser ablation are arranged in Table 3. Compared to conventional
CVD formation on substrates, the laser oven method has
an affinity for generating SWCNTs with low level ofdefects in a continuous flow-type generator.
As an example of nanocarbon generation in a flow-type
generator, a simple and continuous one-step system using
the laser oven method generated a SWCNT aerosol(Klanwan et al., 2010). A graphite target containing Ni
and Co was ablated by pulsed laser in the tubular furnace

Fig. 5 Transmission electron micrographs of nanocarbons generated by laser ablation. (a) Onion-like carbon, (b)carbon nanotubes, and (c) diamond-like nanoparticles.

86Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

at temperatures of 25 to 1080 °C. The generated SWCNTs(< 2 nm in diameter, Fig. 5(a)) were carried by atmospheric pressure nitrogen gas for on-line analysis of theirsize distribution using the scanning mobility particle sizer(SMPS). Other types of carbon allotropes have been synthesized by advanced laser ablation methods. Inoue et al.
(2012) synthesized onion-like carbon nanoparticles (Fig.
5(b)) using a graphite target with a cavity that confined
the generated particles so they can be irradiated by multiple laser beams. As a result of periodic laser beam irradiation of the gas-suspended carbon particles in the cavity,
the phase transition from amorphous carbon nanoparticles
to the orderly concentric graphitic shell structure was
identified. Seto et al. (2014) also showed that the present
method is capable of synthesizing a variety of unique
nanostructured carbons including diamond-like carbon(Fig. 5c). They evaluated the change in the density of the
carbon nanoparticles during the laser-induced phase transition.

4. Summary—Achieving large-scale
production of nanoparticles by laser
ablationIn this review, the formation mechanism and examplesof nanoparticle synthesis by laser ablation were discussed.
As mentioned previously, the properties of the nanoparticles depend strongly on size and size uniformity. Advanced laser ablation methods can generate size- andshape-controlled nanoparticles by post processing (Fig.
4), but that, at the same time, significantly reduces the
production yield. In addition, laser ablation is disadvantaged by high input energy (Kushnir et al., 2008) and thesmall laser-irradiating area for evaporating the target materials. Therefore, laser ablation applications have been
limited to exploring unique properties of new types ofnanoparticles. To achieve large-scale production of
nanoparticles by laser ablation, it is necessary to generate

Table 3morphology-controlled nanoparticles without subsequentpost-processing.
Laser ablation has an affinity for synthesizing anisotropic nanomaterials such as carbon nanotubes (Karthikeyan
et al., 2009), nanorods (Tian et al., 2013) and nanowires(Rao et al., 2003) in the large-scale. In these processes,
high-purity catalysts that are small in size and have good
crystallinity play an important role in the anisotropic
growth of such one-dimensional nanomaterials. In addition, laser ablation has good compatibility with aerosol
flow systems and with the benefit of being capable of continuously producing large quantities of nanoparticles aswell as nanoparticle-based nanomaterials. There are also
several reports on the production of colloidal metal
nanoparticles in a liquid flow system (Muttaqin et al.,2015). Such metal nanoparticles have applications as
bio-markers.
Size classification by LPDMA is advantageous, not only
for obtaining size-classified particles, but also for measuring the size distribution of nanoparticles in the gas phaseprocess. Therefore, it can be implemented as a processmonitor for optimizing process parameters (pressure, flow
rate, temperature etc.). The size distribution of nanoparticles generated by laser ablation can be measured when an
LPDMA is coupled with a particle detector such as anaerosol electrometer (Seto et al., 2001b). Quantum-sizedparticles (< 10 nm) with a relatively narrow size distribution can be generated by laser ablation under low pressure
conditions without post-processing. For this purpose, an
LPDMA/electrometer system can be used for exploring
the optimum conditions to obtain nanoparticles with thedesirable size distribution.

Acknowledgements
The authors are grateful for support by JSPS KAKENHI
Grant Number 15K14200.

Carbon nanoparticle synthesis studies using pulsed laser ablation method in a flow reactor (H: heating, D: diameter, L:
length, M: mode)
AuthorLaser Source

Structure

Pressure

Additional Process

Size

Puretzky et al. 2000

Nd:YAG
XeCl

SWNTs
SWNTs

66.5 kPa66.5 kPa(H) 1000 °C(H) 1000 °C

200 nm/s growth rate

Klanwan et al. 2010

Nd:YAG

SWNTs

101.3 kPa

(H) 25~1080 °C(D) < 2 nm, (L) > 500 nm

Inoue et al. 2012

Nd:YAG

AmorphousOnion-like

101.3 kPa
101.3 kPa

Flat target
Cavity target(M) 83 nm(M) 18 nm

Seto et al. 2014

Nd:YAG

Onion-like
Diamond-like

101.3 kPa

Flat target
Cavity target

87

Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

References
Anikin K.V., Melnik N.N., Simakin A.V., Shafeev G.A., Voronov
V.V., Vitukhnovsky A.G., Formation of ZnSe and CdSquantum dots via laser ablation in liquids, Chemical Physics Letters, 366 (2002) 357–360.
Buzea C., Pacheco I.I., Robbie K., Nanomaterials and nanoparticles: Sources and toxicity, Biointerphases, 2 (2007)
MR17–MR71.
Chichkov B.N., Momma C., Nolte S., Von Alvensleben F.,
Tünnermann A., Femtosecond, picosecond and nanosecond
laser ablation of solids, Applied Physics A, 63 (1996) 109–
115.
Dikovska A.O., Alexandrov M.T., Atanasova G.B., Tsankov
N.T., Stefanov P.K., Silver nanoparticles produced by PLD
in vacuum: Role of the laser wavelength used, Applied
Physics A, 113 (2013) 83–88.
Fischer D., Schwinghammer K., Sondermann C., Lau V.W.,
Mannhart J., Lotsch B.V., Laser ablation of molecular carbon nitride compounds, Applied Surface Science, 349(2015) 353–360.
Friedrich C., Keynton R., Vasile M., Warrington R., Development of a core curriculum in miniaturization technologies,
Journal of Engineering Education, 87 (1998) 567–574.
Fujishima A., Honda K., Electrochemical photolysis of water at
a semiconductor electrode, Nature, 238 (1972) 37–38.
Geohegan D.B., Puretzky A.A., Duscher G., Pennycook S.J.,
Photoluminescence from gas-suspended SiOx nanoparticles
synthesized by laser ablation, Applied Physics Letters, 73(1998) 438–440.
Harano A., Shimada K., Okubo T., Sadakata M., Crystal phasesof TiO2 ultrafine particles prepared by laser ablation ofsolid rods, Journal of Nanoparticle Research, 4 (2002) 215–
219.
Hirasawa M., Seto T., Orii T., Aya N., Shimura H., Synthesis ofsize-selected TiOx nanoparticles, Applied Surface Science,
197–198 (2002) 661–665.
Hirasawa M., Orii T., Seto T., Effect of insitu annealing on
physical properties of Si nanoparticles synthesized bypulsed laser ablation, Applied Physics A, 79 (2004) 1421–
1424.
Inoue A., Seto T., Otani Y., Onion-like carbon nanoparticles
generated by multiple laser irradiations on laser-ablatedparticles, Carbon, 50 (2012) 1116–1122.
Kawakami Y., Seto T., Yoshida T., Ozawa E., Gold nanoparticles and films produced by a laser ablation/gas deposition(LAGD) method, Applied Surface Science, 197–198 (2002)
587–593.
Kim B.H., Lee N., Kim H., An K., Park Y.I., Choi Y., Shin K.,
Lee Y., Kwon S.G., Na H.B., Park J.-G., Ahn T.-Y., Kim
Y.-W., Moon W.K., Choi S.H., Hyeon T., Large-scale synthesis of uniform and extremely small-sized iron oxide
nanoparticles for high-resolution t1 magnetic resonanceimaging contrast agents, Journal of the American Chemical
Society, 133 (2011) 12624–12631.
Kim B.H., Hackett M.J., Park J., Hyeon T., Synthesis, characterization, and application of ultrasmall nanoparticles, Chemistry of Materials, 26 (2014) 59–71.

88

Klanwan J., Seto T., Furukawa T., Otani Y., Charinpanitkul T.,
Kohno M., Hirasawa M., Generation and size classificationof single-walled carbon nanotube aerosol using atmosphericpressure pulsed laser ablation (AP-PLA), Journal of
Nanoparticle Research, 12 (2010) 2747–2755.
Kohno M., Orii T., Hirasawa M., Seto T., Murakami Y., Chiashi
S., Miyauchi Y., Maruyama S., Growth of single-walledcarbon nanotubes from size-selected catalytic metal particles, Applied Physics A, 79 (2004) 787–790.
Kuroda S., Kaihara S., Fujii Y., Kinoshita T., Adachi M., Modeling of particle generation in laser ablation plasma, Journalof Aerosol Science, 50 (2012) 38–56.
Lau M., Haxhiaj I., Wagener P., Intartaglia R., Brandi F.,
Nakamura J., Barcikowski S., Ligand-free gold atom clusters adsorbed on graphene nano sheets generated by oxidative laser fragmentation in water, Chemical Physics Letters,
610–611 (2014) 256–260.
Lu Q., Mao S.S., Mao X., Russo R.E., Theory analysis of wavelength dependence of laser-induced phase explosion of silicon, Journal of Applied Physics, 104 (2008) 083301.
Niu X., Murray P., Sarangan A., Synthesis of Fe–Ni bimetallic
nanoparticles from pixel target ablation: Plume dynamics
and surface characterization, Journal of Nanoparticle
Research, 14 (2012) 1–8.
Okada R., Iijima S., Oxidation property of silicon small particles, Applied Physics Letters, 58 (1991) 1662–1663.
O’regan B., Gratzel M., A low-cost, high-efficiency solar cellbased on dye-sensitized colloidal TiO2 films, Nature, 353(1991) 737–740.
Orii T., Hirasawa M., Seto T., Tunable, narrow-band light emission from size-selected Si nanoparticles produced by pulsed-­laser ablation, Applied Physics Letters, 83 (2003a) 3395–
3397.
Orii T., Hirasawa M., Seto T., Aya N., Onari S., Temperaturedependence of photoluminescence from mono-dispersed Si
nanoparticles, The European Physical Journal D—Atomic,
Molecular, Optical and Plasma Physics, 24 (2003b) 119–
122.
Pallotti D.K., Ni X., Fittipaldi R., Wang X., Lettieri S., VecchioneA., Amoruso S., Laser ablation and deposition of titanium
dioxide with ultrashort pulses at 527 nm, Applied Physics
B, 119 (2015) 445–452.
Puretzky A.A., Geohegan D.B., Fan X., Pennycook S.J.,
Dynamics of single-wall carbon nanotube synthesis bylaser vaporization, Applied Physics A, 70 (2000) 153–160.
Ruth A.A., Young J.A., Generation of CdSe and CdTe nanoparticles by laser ablation in liquids, Colloids and Surfaces A:
Physicochem. Eng. Aspects, 279 (2006) 121–127.
Sakiyama K., Koga K., Seto T., Hirasawa M., Orii T., Formationof size-selected Ni/NiO core−shell particles by pulsed laser
ablation, J. Phys. Chem. B, 108 (2004) 523–529.
Seto T., Kawakami Y., Suzuki N., Hirasawa M., Kano S., Aya
N., Sasaki S., Shimura H., Evaluation of morphology andsize distribution of silicon and titanium oxide nanoparticles
generated by laser ablation, Journal of Nanoparticle
Research, 3 (2001a) 185–191.
Seto T., Kawakami Y., Suzuki N., Hirasawa M., Aya N., Laser
synthesis of uniform silicon single nanodots, Nano Letters,

Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

1 (2001b) 315–318.
Seto T., Orii T., Hirasawa M., Aya N., Fabrication of siliconnanostructured films by deposition of size-selected
nanoparticles generated by pulsed laser ablation, Thin Solid
Films, 437 (2003) 230–234.
Seto T., Koga K., Akinaga H., Takano F., Sakiyama K., Hirasawa
M., Orii T., Laser synthesis and magnetic properties ofmonodispersed core–shell nanoparticles, Applied Physics
A, 79 (2004) 1165–1167.
Seto T., Akinaga H., Takano F., Koga K., Orii T., Hirasawa M.,
Magnetic properties of monodispersed Ni/NiO core−shell
nanoparticles, The Journal of Physical Chemistry B, 109(2005) 13403–13405.
Seto T., Koga K., Akinaga H., Takano F., Orii T., Hirasawa M.,
Laser ablation synthesis of monodispersed magnetic alloy
nanoparticles, Journal of Nanoparticle Research, 8 (2006)
371–378.
Seto T., Koga K., Takano F., Akinaga H., Orii T., Hirasawa M.,
Murayama M., Synthesis of magnetic CoPt/SiO2 nanoparticles, Journal of Physics: Conference Series, 59 (2007) 255–
258.
Seto T., Inoue A., Higashi H., Otani Y., Kohno M., Hirasawa
M., Phase transition and restructuring of carbon nanoparticles induced by aerosol laser irradiation, Carbon, 70 (2014)
224–232.
Shinde S.R., Kulkarni S.D., Banpurkar A.G., Nawathey-Dixit
R., Date S.K., Ogale S.B., Magnetic properties of nanosized
powders of magnetic oxides synthesized by pulsed laser
ablation, Journal of Applied Physics, 88 (2000) 1566–1575.
Suzuki N., Makino T., Yamada Y., Yoshida T., Seto T., Monodispersed, nonagglomerated silicon nanocrystallites, Applied
Physics Letters, 78 (2001) 2043–2045.

Tilke A.T., Simmel F.C., Blick R.H., Lorenz H., Kotthaus J.P.,
Coulomb blockade in silicon nanostructures, Progress in
Quantum Electronics, 25 (2001) 97–138.
Torrisi L., Gammino S., Andò L., Nassisi V., Doria D., Pedone
A., Comparison of nanosecond laser ablation at 1064 and308 nm wavelength, Applied Surface Science, 210 (2003)
262–273.
Tsuji M., Seto T., Otani Y., Effect of surrounding gas temperature on the morphological evolution of TiO2 nanoparticles
generated by laser ablation in tubular furnace, Journal of
Nanoparticle Research, 14 (2012) 674–683.
Wen S.B., Mao X., Greif R., Russo R.F., Radiative cooling oflaser ablated vapor plumes: Experimental and theoreticalanalyses, Journal of Applied Physics, 100 (2006) 053104.
Wen S.B., Mao X., Greif R., Russo R.E., Experimental and theoretical studies of particle generation after laser ablation ofcopper with a background gas at atmospheric pressure,
Journal of Applied Physics, 101 (2007) 123105.
Wolfbeis O.S., An overview of nanoparticles commonly used influorescent bioimaging, Chemical Society reviews, 44(2015) 4743–4768.
Yoshida T., Takeyama S., Yamada Y., Mutoh K., Nanometer—
sized silicon crystallites prepared by excimer laser ablationin constant pressure inert gas, Applied Physics Letters, 68(1996) 1772–1774.
Yoshida T., Yamada Y., Orii T., Electroluminescence of silicon
nanocrystallites prepared by pulsed laser ablation in
reduced pressure inert gas, Journal of Applied Physics, 83(1998) 5427–5432.
Zheng J., Zhang C., Dickson R.M., Highly fluorescent,water-soluble, size-tunable gold quantum dots, PhysicalReview Letters, 93 (2004) 077402.

Author’s short biography
Myungjoon Kim
He received his Bachelor of Science degree in Mechanical Engineering from
Sungkyunkwan University, Korea in 2009. Currently, he is a candidate of combined
master’s and doctorate program in the School of Mechanical at Sungkyunkwan University. His research is focused on the numerical analysis on particle suspended in fluid.

89

Myungjoon Kim et al. / KONA Powder and Particle Journal No. 34 (2017) 80–90

Author’s short biography
Saho Osone
She received her Bachelor of Engineering degree in Chemical Engineering from
Kanazawa University, Japan in 2007. She received her Master of Engineering in Chemical
Engineering from Kanazawa University, Japan in 2009. Currently, she is working at
Takeda Pharmaceutical Company Limited. She is also a doctoral course student at
Kanazawa University. Her research in focused on nanoparticle material synthesis by gas
phase processes and measurement of aerosol nanoparticles.

Taesung Kim
He received his Bachelor of Science degree in Mechanical Engineering from Seoul
National University, Korea in 1994. He received his Master of Science, and Doctor ofPhilosophy degrees in Mechanical Engineering from Minnesota University, USA in
1998, and 2002, respectively. Dr. Kim currently works as professor in the School of
Mechanical Engineering and adjunct professor in the SKKU Advanced Institute of Nano
Technology at Sungkyunkwan University in Suwon, Korea. His research interests include nanoparticle synthesis, development of applications related with bio aerosol,
Chemical Mechanical Polishing, and thin film synthesis.

Hidenori Higashi
He received his Bachelor of Engineering degree in Chemical Engineering from Kyushu
University, Japan in 1994. He received his Master of Engineering, and Doctor of Engineering degrees in Chemical Engineering from Kyushu University, Japan in 1996, and2001, respectively. From 1997 to 2004, he worked as a Research Associate at Kyushu
University. From 2004, he works as an Assistant Professor (2004 to 2015) and as an Associate Professor (from 2016), Faculty of Natural System, Institute of Science and Engineering, Kanazawa University, Japan. His research topics cover broad area insupercritical fluid technology, aerosol science and technology.

Takafumi Seto
He received the Doctoral degree in Engineering from Hiroshima University at 1996.
From 1997 to 2007, he worked at Mechanical Engineering Laboratory (from 2001, National
Institute of Industrial Science and Technology (AIST)). From 2007, he works as an Associate Professor (2007 to 2013) and as a Professor (2013 to now), Kanazawa University,
Faculty of Natural System, Institute of Science and Engineering, Kanazawa University,
Japan. His research topics cover broad area in aerosol science and technology such as
nanoparticle synthesis, classification, transport phenomena, electrostatics, instrumentation, and laser ablation.

90

KONA Powder and Particle Journal No. 34 (2017) 91–105/Doi:10.14356/kona.2017013

Review Paper

Deficiencies in Traditional Oral Dosage Forms and theEmergence of Controlled-Release Powder Manufacturing †Martin G. Teresk 1, Cory J. Berkland 1,2 and Nathan H. Dormer 1*
1
2

Orbis Biosciences, Inc., USA
Department of Pharmaceutical Chemistry, University of Kansas, USA

Abstract
The importance of providing safe and effective delayed- and extended-release oral formulations that can replace
products requiring multiple administrations has been continually cited as an area in need of improvement forpharmaceutical companies. Such controlled release challenges become especially critical when they must be
adapted for paediatrics, those suffering from dysphagia, or patients with specific dosage administration
limitations. More often than not, lack of palatability and taste-masking compound this formulation challenge.
Many particulate approaches show promise, but can be fraught with broad particle size distributions, initial drug
burst, poor drug entrapment efficiency, low drug loading, and limited scalability. Here, we summarize the key
factors that drive formulation development of format-flexible controlled-release oral powders, and the
manufacturing aspects involved with some of the foremost marketed products, including next-generation singlestep layered powder manufacturing (below).
Keywords: powder, particle, microsphere, taste-masking, paediatric, controlled-release

1. The dosage form problem
The importance of providing safe, effective, and provenmedicines for populations with dysphagia, such as children, geriatrics, and those suffering from debilitating illnesses, has been continually cited as an area in need of
improvement for pharmaceutical companies and the providers who administer their product (Bergstrom D. et al.,
2004; Bhardwaj S. and Hayward M., 1996; Cram A. et al.,2013; Dickens D.S. et al., 2008; Engelen L. et al., 2005;
Imai E. et al., 1995; Ivanovska V. et al., 2014; Lopez F.L.
et al., 2015; Matsui D., 2007; Milne C.P. and Bruss J.B.,
2008; Rocca J.G. and Park K., 2004; Sugao H., 1997; Tyle
P., 1993). The widespread lack of dispersed format oral
products, however, forces clinicians and pharmacists to
use alternative solutions to treat their patients that are notalways backed by supporting bioavailability, stability, andsafety studies. Tablets are sometimes administered extemporaneously by crushing the dosage form and mixing
with food or drink. Not only are these delivery methods
inconsistent, they often lead to dosing errors, decreased
†
1
2

*

Received 18 April 2016; Accepted 16 June 2016
J-STAGE Advance Publication online 30 July 2016
Lenexa, KS 66214, USA
Lawrence, KS 66045, USA
Corresponding author: Nathan H. Dormer;
E-mail: nate@orbisbio.com
TEL: +1-913-544-1199

bioavailability or efficacy, and non-adherence because offoul-tasting active pharmaceutical ingredients (APIs)(Jayanthi B. and Manna P., 2011; Osterberg L. andBlaschke T., 2005; Sansom L., 1999; Schier J. et al., 2003).
Due to taste and efficacy concerns, The Institute for
Safe Medical Practices (ISMP) has issued a “Do Not
Crush” list, which highlights over 400 dosage forms thatcannot be compounded due to special controlled-releaseproperties, taste-masking, or API protection (Bergstrom
D. et al., 2004; Bhardwaj S. and Hayward M., 1996;
Dickens D.S. et al., 2008; Engelen L. et al., 2005; Imai E.
et al., 1995; Matsui D., 2007; Milne C.P. and Bruss J.B.,
2008; Rocca J.G. and Park K., 2004; Sugao H., 1997; Tyle
P., 1993). Lack of titratable and palatable formulations affects over half of the global population (under 18 and over
65 years of age) and can subject patients to avoidable adverse events (Jayanthi B. and Manna P., 2011; Osterberg
L. and Blaschke T., 2005; Sansom L., 1999; Schier J. etal., 2003). Of all medication-related hospitalizations that
occur in the United States, between one-third and twothirds are the result of poor medication adherence (MatsuiD., 2007; Milne C.P. and Bruss J.B., 2008). Current dosing regimens for populations with dysphagia or those unwilling to take traditional tablets or capsules suffer because
many current oral formulations fail to simultaneously address the critical aspects discussed below (Griffith R.,
2005; Wright D., 2002).

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

91

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

1.1 Palatability
Many APIs are extremely bitter and coating efforts canbe ineffective or result in an unpleasant mouth feel due toirregular surface features. Approximately half of patientswith organoleptic sensitivities refuse to take their medicine, with the large majority of those reporting bad taste
as the single major reason for non-compliance (Matsui D.,2007). Sweeteners alone are often unable to overcome
the extremely unpleasant taste of many active ingredientsin syrups and suspensions (Bergstrom D. et al., 2004;
Bhardwaj S. and Hayward M., 1996; Dickens D.S. et al.,2008). Moreover, efforts to mask flavors using coatings,
agglomeration, or microencapsulation often result inpoorly-controlled, heterogeneous particle distributions that
result in a gritty or granular mouth feel. Ideally, a highly
palatable dosage form will consist of taste-masking witha smooth mouthfeel while maintaining other controlled
release properties.

1.2 Titratable dosing
Different ages, weights, body mass indices, andmetabolically-impaired individuals require considerable
dosing precision that is not linearly scaled (Milne C.P.
and Bruss J.B., 2008). Because medication errors arecommon in six percent of pediatric hospitalizations, dose
titration is critical, as a “one-size-fits-all” dosing is ineffective in children due to their developmental variability(Cram A. et al., 2013; Ivanovska V. et al., 2014; Lopez
F.L. et al., 2015). and can prove deleterious for geriatricpatients with hepatic or renal impairment.

1.3 Controlled-release kinetics
Achieving controlled-release kinetics with tablets is arelatively simple process, as the size and form factor ofthe dosage form lends to using robust coating methods,
sometimes with several layers (Jayanthi B. and Manna P.,
2011; Sansom L., 1999). Capsules have the advantage ofbeing injection moulded, extruded, or die pressed withgelatines and other controlled-release polymers in a repeatable, high-throughput manner, enabling large dosesof medication in a modest form factor. Tablets are simply
pressed by traditional means, then coated with subsequent
layers of controlled-release components, which makes
translation of specialised functions (e.g. delayed or extended release) simple with a pill format (Jayanthi B. andManna P., 2011; Sansom L., 1999). The major problem,
however, is the size of such dosage forms, which renders
them impracticable to certain patient populations.

92

2. Challenges with traditional oral dosage
forms
Universal technology hurdles exist independently of
drug or indication even for adult populations, and thus
substantially impede development of optimal therapeutics
for those with special dosing considerations. Specifically,for pediatric drugs, companies must meet all standard
pharmaceutical benchmarks (bioavailability, shelf life,
safety, and efficacy) but also consider palatability, dosing
accuracy, and age-appropriate formats. Through the Best
Pharmaceuticals for Children Act, Pediatric Research Equity Act, and EMEA Pediatric Investigation Plans, the industry is either incentivized or in some cases required toperform pediatric studies (2002, 2003, 2006; Cram A. etal., 2013). Internal capabilities for this development arelimited, and many companies do not have the capacity tomake pediatric formulations. This emphasizes the needfor scalable, flexible, cost-effective formulation platforms
for practical, palatable, age-appropriate, and accurate dosing to create safer medications that enhance complianceat any age. While advances in pediatric-centric dosage
formats take to market, adults with dysphagia will also
secure the benefits of research and development efforts
focused on providing alternative formats to historicalsolid oral dosage forms.

2.1 Size burden of tablets and capsules
Data from marketed pharmaceutical products indicatesthat the average size of a controlled- or extended-releasetablet is nearly 1.5 cm in length (Pharmacircle, 2016).
Physiological studies demonstrate that swallowing becomes increasingly difficult when the dimension of theobject being ingested reaches more than 50 % of the oesophageal diameter, which is 2.0 cm for the prototypicaladult.(Harb J., 2015) By this logic, the average controlled-­
release tablet is too large to be swallowed comfortably.
The merits residing in pills and capsules is that they contain the volumetric space to (1) deliver a large payload ofAPI, (2) utilize elaborate controlled-release mechanisms(such as ion-exchange, micro-pumping mechanisms, etc.),and (3) circumvent many shelf stability challenges. These
merits, however, fall short of benefitting a significant
fraction of the world population simply due to the form
factor size.

2.2 Taste and consistency burden of syrups,
suspensions, elixirs, and solutions
Where large tablets and capsules present swallowing
and administration challenges, liquid formats succeed indose titration most of the time. The advantages beyond
ease of dosing are limited in traditional syrups, however.

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

Liquid formats (1) are not extended-release, (2) have little­to-no taste-masking, (3) and can contain API particles
prone to settling and aggregation if not reconstituted
properly prior to administration, which have resulted inrisks to patient safety (Bergstrom D. et al., 2004; Cram A.
et al., 2013; Ivanovska V. et al., 2014; Lopez F.L. et al.,2015; Matsui D., 2007; Milne C.P. and Bruss J.B., 2008).
Recent advances (which will be discussed later in this article) have enabled extended release and taste-masking oforally-administered APIs, but the breadth of application
currently covers less than one percent of marketed drugs.

2.3 Alternatives to traditional dosage forms
As a technical resolution to the large format of controlled-­
release pills and capsules, the foul taste of traditional syrups and suspensions, and the lack of controlled-release
options for APIs tableted and encapsulated in nearly 85 % ofmarketed drugs (Table 1) (Maalouf N., 2013; Vummaneni
V. and Nagpal D., 2012), many pharmaceutical and contract manufacturing organizations (CMOs) are focusing
research and development efforts on controlled-releasepowder formats, which combine the stability of solid oral
dosage forms and dose titration advantages of liquids.
These tablet alternatives address many of the deficiencies
discussed earlier, but can still be fraught with inadequacies such as multiple-step manufacturing and inconsistentparticle sizes.

3. Controlled-release powders
Micro- and nanoparticulate powders are manufactured
with a myriad of processes, but the primary motivation is
integration of controlled-release mechanisms to govern
particle disintegration and API dissolution. The requirement of achieving controlled release universally relies on
physical sequestration of the API via one or more physicochemical mechanisms, which typically requires multiple steps. A powder form factor, however, can present
unique challenges to achieving controlled release coatings
due to (1) the high surface area of particles, (2) irregularsizes of particles within the powder, and (3) the number ofprocess steps required to ensure predictable performanceand reasonable quality of the final product. Though,taste-masking can still be achieved with powders when acoating or other chemical modification is applied. Here,
we discuss the salient manufacturing steps for current andnext-generation dosage forms (Table 2) (Cram A. et al.,2013; Maalouf N., 2013; Vummaneni V. and Nagpal D.,2012).

3.1 Coated drug granules
The most straightforward method for achieving taste-­masking and controlled release with powders employs a
two-step process in which a precursor particle is manufactured by various means, then coated with one or morelayers containing controlled-release materials (Fig. 1).

Table 1 Overall dosage form landscape for most-prescribed medications
Types of Pediatric/
Of MostGeriatric/
Prescribed
Dysphagia-Friendly
Drugs (%)
Formulations
Suspension

13.0

Granules/Powder

2.8

OrallyDisintegrating
Tablet (ODT)

3.0

Chewable Tablet

4.2

Sublingual/Lingual

0.6

Tablet42.7

Capsule18.0

Solution/Syrup

15.0

Elixir0.8

Taste Masking
Methods

coating,
microencapsulation,ion exchange resin,
inclusion
complexes,
granulation,
adsorption,
prodrug, bitterness
inhibitors, multiple
emulsion, gelformation

Disadvantage

Advantage

Major Technologies andCompanies Practicing
in Each Dosage form Space
TPG/Adare: Microcaps®,
Liquitard®. Tris: OralXTTM,
LiquiXRTM Cipla: Sprinkles®,Orbis: Optimμm®

Easy to swallow, can be tastemasked, though most form
factors of these typs are not(see exceptions at right).

TPG/Adare: AdvaTab®,
Diffucaps®. Tris: ODTXRTM.
SPI: Actimask®. Catalent: Zydis®.
Cima: OraSolv®, Durasolv®,
LyocTM
Tris: ChewableXRTM.
SPI: Actimask®
Tris: StripsXRTM

Does not help those Taste-masking
with dysphagia
available
Inherently no taste-masking:drug freely/rapidly available

No swallowing
difficulty for
those withdysphagia

TPG/Adare, Catalent, Capsugel/
Bend Reserach, Allergan/Actavis,
Pfizer, GlaxcoSmithKline, Bayer,
Procter & Gamble, Johnson&Johnson, Bristol Myers Squibb,
AstraZeneca, Impax, SPI,

93

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

Table 2 Various manufacturing technologies that rely on particles and powders as final dosage form
Powder-Based Technology

Manufacuring
Considerations

Form Factor

Proprietor ofTechnology

Flagship Formulations
Approved or In Pipeline

Optimμm®

Single-step microencapsulation,
uniform particle size distribution(100s-1000s μm), scalable,
dosage and format flexible,
enables combination therapy

Microspheres
andMicrocapsules

Orbis
Biosciences

predninsone,
guaifenesin,
ibuprofen

DETERx®

Multi-step production (drug-wax
complex, particle fabrication,
coating), dosage and format
flexible, less uniform size
distribution, abuse deterrent
formulation

Hydrophobic
matrix as
physical or
diffusional
barrierCollegium
Pharmaceutical

oxycodone, oxymorphone,
hyrocodone, morphine,
methylphenidate

LiquiTime®

Multi-step production (granulation, coating, drying),
non-uniform granule size
distribution, size limited(> 100 um), dosage and format
flexible

Traditional
2-Step Coatingof particles

FlamelTechnologies

guaifenesin, ibuprofen

Microcaps® (Used alone andin Liquitard®, AdvaTab®

API particles complexed withcomtrolled-release matrix

Drug-coatedpowderAdare
Pharmaceuticals

tenofovir disoproxil
fumarate, potassium
chloride, paracetamol

Minitablets

Adare
Pharmaceuticals

propranalol,
methylphenidate,
cyclobenzaprine

Minitablets

Cipla

lopinavir, ritonavir

Tris Pharma

amphetamine, codeine/chlorpheniramine
polistirex,
dextromethorphan

Neos
Therapeutics

methylphenidate

ODTCima Labs

loperamide, clozapine,
prednisolone

ODT

SPI
Pharmaceuticals

paracetamol, ibuprofen

ODT

Catalent Pharma
Solutionslorazepam, loratadine,piroxicam, olanzapine

Diffucaps®

Sprinkles®
OralXR+TM (Used alone andin LiquiXRTM,
ChewableXRTM,ODTXRTM, StripsXRTM)
XR Technologies (RDIM,DTRS®, and KCTPTM)

Multi-step production(granulation, coating, drying),
non-uniform granule size
distribution, size limited(> 500 μm), dosage and format
flexible
Multi-step production (resin
priming, drug loading, resinate
wash, drying), requires acidic orbasic drug chemical structure,relies on proprietary resin beads

ORaSolv®, DuraSolv®,
LyocTM
Actimask®

API granules co-lyophilizedwith matrix

Zydis®

Precursor particles can either be (1) finely-milled API
crystals, (2) crystalline or amorphous API co-mixed withinert bases or controlled-release materials, or (3) an entirely
inert core without API. These three types of precursors
can be manufactured by one or more means, which include traditional vibratory methods, congealing/spinningdisk atomization, prilling, hot-melt extrusion (HME) andspheronization, aqueous dispersion, blending/bulking,electrohydrodynamic spraying (EHDS), or spray drying(Ambike A.A. et al., 2005; Cloupeau M. and Prunet-FochB., 1994; Eldem T. et al., 1991; Gharsallaoui A. et al.,2007; Hancock B.C. et al., 2003; Passerini N. et al., 2006;

94

Solid suspension
or ODT

Vehring R., 2007; Yurteri C.U. et al., 2010). Material selection for the precursor particle relies on process capabilities, desired end-product controlled-release properties,
API thermal and oxidative stability and desired physicalproperties (surface features, density, friability, hardness,etc). Once the precursor microparticulate is manufactured, the product, in some instances, can be considered“finished.”
If taste-masking, delayed-release, or stability-enablingproperties are required, the precursor particle advances tosubsequent traditional coating steps using fluidized beds,
Würster coaters, dry polymer coating, spray coating, pan

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

Fig. 1

Powder creation and coating has traditionally been at least a two-step process, but provides great flexibility in end product dosage form and function. Each technique gives a different particle size range, withvibratory techniques creating the largest powders, and spray drying the smallest powders.

coating, or coacervation (Dewettinck K. and Huyghebaert
A., 1999; Gouin S., 2004; Jono K. et al., 2000; Lopez F.L.
et al., 2015; Sastry S.V. et al., 2000). Materials of choicefor the secondary coating steps are selected for reasonscommensurate with precursor particles (i.e. material compatibility, controlled-release behaviour, and stability). The
final dosage form can then be bottled for resuspension attime of use, packaged in sachets or sprinkle capsules,
placed in dissolving tongue strips, co-lyophilized withother materials for orally-disintegrating tablets (ODTs), or
reconstituted in syrup if liquid stability is not an issue(Table 2).
The history of manufacturing controlled-release powders by adding one or more coating steps to API-richcores is no less than colossal, and for this reason we haveprovided a high-level overview with references to detailed
reviews. These techniques are, however, divergent fromstate-of-the art techniques that focus on chemical modifi-

95

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

cation of the API and/or substrate to which it is affixed
via ion exchange resins (Elder D.P., 2005; Fazal U.-R. andKhan S.N., 2012; Pande S.V. et al., 2011). The main advantages that these methods can yield are (1) enabling liquidstability, and (2) deterring abuse of scheduled APIs, such as
opiates and amphetamines. While revolutionary, drug complexation employs a number of manufacturing steps thatfar surpasses that of simple bead layering, and include
and still usually include a final coating step (Dewettinck
K. and Huyghebaert A., 1999; Gouin S., 2004; Jono K. etal., 2000; Lopez F.L. et al., 2015; Sastry S.V. et al., 2000).
Here, we look in detail at two processes.

3.2 Granules with increased lipophilicity
Collegium Pharmaceutical’s manufacturing processachieves extended release with abuse-deterrent properties
via two major steps (1) API solubility reduction and (2)API carrier dispersion. (Eldem T. et al., 1991; Passerini N.
et al., 2006) An optional third step is, unequivocally,
coating for added protection of the particle contents.
Solubility reduction can be achieved by either reducing
the overall charge of the API molecule, forming a salt
between the API and a lipid moiety, forming a complex
between and API and metal cation, complexation withcyclodextrins, or covalently modifying the molecule withester or amide linkages.
Following solubility reduction, the modified API is dissolved in a molten wax or lipid with or without the assistance of co-solvents. Suitable materials include fatty
alcohols, fatty esters, fatty acid glycerides, hydrogenatedoils, and stearates. The final powder is then made via traditional means using congealing, extrusion/spheronization, prilling, or aqueous dispersion (Fig. 1) (Eldem T. etal., 1991; Passerini N. et al., 2006). Optional coatings canthen be added by traditional means used for coated drug
granules with one or more controlled-release polymers fortaste-masking or API protection (Dewettinck K. andHuyghebaert A., 1999; Gouin S., 2004; Jono K. et al.,
2000; Lopez F.L. et al., 2015; Sastry S.V. et al., 2000).

3.3 Ion exchange resin complexation
Similar to Collegium’s DETERx technology, Tris
Pharma and Neos Therapeutic’s technologies also use adrug-tethering concept, but also provide liquid stability
and extended-release. The manufacturing methods between Tris and Neos are very similar (Fig. 2), but are described here in brief. The main premise with these twotechnologies is reliance on an ion-exchange complex between API and inert substrate (Elder D.P., 2005; Fazal
U.-R. and Khan S.N., 2012; Mehta K. et al., 2012; Pande
S.V. et al., 2011).
The first major step includes manufacturing of a resin

96

Fig. 2

Tris, Neos, and Collegium all use ion-exchange typeprocesses for powder precursors, but Collegium disperses the API-complex in lipids and waxes that constitute the particle bulk, whereas Tris and Neos placethin coatings over the API-resin complex. All threetechniques provide controlled-release, taste-masking,and abuse deterrence.

microparticulate with charged moieties. Resin particles canbe created through traditional processes such as suspension polymerisation with polystyrene, methacrylate, or
acrylonitrile salts or variants thereof. Resins can be cationic or anionic in nature, depending on whether the API
to be complexed is weakly or strongly acidic or basic. The
resin must then be primed to ion exchange with the API.
The second step incorporates the ion exchange processduring which the API of interest is ionized by salt removal, then complexed to the resin microparticle precursor by itself or with other additives that enhance stability
or solubility, such as sugars. This process creates a bond
between the API and resin that is more physically andchemically robust than mixing and emulsifying processes
in traditional powder extrusion and layering discussed

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

previously. Following this, the resinate is (1) washed toeliminate unreacted API, then (2) dried for the final formulary step.
Just as in traditional powder-coating methods, a final
step focuses on securing the complexed API to the resinby coating with controlled-release polymers that provide
diffusional control or pH-responsiveness, or layered withlipids and waxes. Following the drug-resin coating, thepowder is ready for final format packaging, which may
include compressing in to ODTs, filling in sachets, titrating into capsules, or bottling into liquid suspensions. Because taste is only perceived when the API is in solution,ion exchange resins provide taste-masking because the
API is sequestered to the inert substrate.

3.4 Next generation powder manufacturing
Developing controlled-release powders, to date, has
employed combinations of manufacturing mechanisms
and complex chemistry, which achieve substantial advantages over traditional pill and capsule formats, enabling
extended- and delayed-release liquid suspensions andpowder sachets, while providing taste-masking as-is, in aliquid constituent, or further compounded into ODTs or
dissolving strips. The major criticisms of these methods,
however, focus on the sheer number of process steps andexcipients with undefined long-term safety and toxicology
profiles (such as polystyrene). Thus, it comes as no surprise that manufacturers are investigating less complex
chemistry and single-step manufacturing methods for
producing controlled-release powders.

Fig. 3One such technology platform, Precision Particle Fabrication technology, also known as Optimμm technology,
comes from Orbis Biosciences. The manufacturing
scheme creates microsphere and microcapsule powders
with high drug loading while maintaining the ability toprocess into a variety of extended-release, delayed-release,and taste-masked formats (Berkland C. et al., 2007a,
2007b; Pack D.W. et al., 2004a, 2003, 2001, 2002, 2004b).
The approach first creates either (1) a homogenous
API-excipient mixture or suspension (i.e. for non-layered
microspheres), or (2) separate core and drug-free shellsolutions (i.e. for coated microspheres) (Fig. 3). Highshear mixing under controlled heat is added, after which
the contents are transferred through heat-traced processlines and sprayed through a nozzle with (1) vibrational
excitation to produce uniform droplets and (2) stabilizing
anti-solvent stream to reduce the diameter of the exiting
jet. The platform is capable of processing a variety of liquids, melts, polymer mixtures, slurries, and solutions ofvarious viscosities and thermal properties. Unlike otherencapsulation techniques, Optimμm’s central advantage
is the ability to, in a single step, manufacture multi-­
kilogram batches of microcapsules (core and coating)having nominal diameters around 100 μm and API content up to 80 % by volume. Controlled-release formulations in the pipeline using this next-generation powder
technology are APIs such as Ibuprofen, Guaifenesin,
Prednisone, and Ritonavir, where the titratable dosing andflexibility allows for unparalleled dissolution matching
against reference listed drugs (RLDs). The simultaneous
advantages of this manufacturing method are (1) produc-

Precision Particle Fabrication Technology, also known as Optimμm technology, creates monodisperse
powders just as traditional vibratory methods, but with (1) highly reduced diameters and (2) single-stepmanufacturing of coatings, which conserves time and resources of historical powder manufacturing
techniques. There is also no need for complex chemistry beyond the controlled-release polymers used.
Depending on the formulation steps used to incorporate the API, abuse deterrence with Optimμm technology is possible, while taste-masking and controlled release are also present.

97Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

tion of uniform, monodisperse powders, (2) single-stepmanufacturing, (3) the ability to control a wide range oftherapeutic classes, (4) no requirement for API modification or complexation, (5) no requirement for processing
solvents, (6) controlled-release with high API content, and(7) particle sizes that are indiscernible to the patient.
Such high API loading and entrapment is made possible because the Optimμm fabrication method can processsuspensions and high viscosity mixtures while shearing
the molten stream to less than 1/5 of the nozzle orifice before layered droplet breakup and hardening.

4. Formulation examples
Here, we will look closely at specific formulation examples representing some of the highlighted techniques
for producing controlled-release powder formulations.
Approaches detailed include hot-melt extrusion pairedwith spinning disk atomization, fluidized bed coating, ion
exchange resins, and precision particle fabrication technology. Depending on the steps involved, either an immediate-, delayed-, or extended-release formulation isachieved.

4.1 Azithromycin
This example outlines manufacture of immediate-­release microspheres containing azithromycin via hot
melt extrusion and spinning disk atomization (Appel L. etal., 2005).
A mixture consisting of 370 g microcrystalline wax,
90 g Pluronic® F87, and 90 g stearyl alcohol was added toa glass beaker in a water bath, and was melted over 60
minutes at 93 °C. Following melting, 450 g azithromycin
dihydrate was added such that the anticipated batch size
was 1.0 kg with a theoretical drug loading of 45 %. This
mixture was mixed for an additional 25 minutes.
Using a gear pump, the molten azithromycin solution
was transferred at 250 mL/min onto the center of a spinning disk atomizer, which was rotating at 8000 rpm
heated to 100 °C. The microparticles solidified mid-air
via convection, were collected in a lined vessel, and determined to have a mean diameter of 190 μm.
Following manufacture, the powder was dissolution
tested with a United States Pharmacopeia (USP) Type II
apparatus (Fig. 4), which demonstrates the immediate-­release nature of the un-coated formulation.

4.2 Cetirizine
This example outlines manufacture of delayed-releasemicrocapsules containing cetirizine via hot melt extrusion, spinning disk atomization, then fluidized bed coat-98

Fig. 4

Dissolution of uncoated azithromycin microspheres
created with spinning disk atomization. Paddle speed
was 50 rpm in a 37 °C vessel with simulated gastricfluid. Image created from tabulated dissolution data in
US 2005/0152982 A1 (Appel L. et al., 2005).

ing.(Appel L. et al., 2012) The manufacture is largely thesame as the previous example, with the exception of adding a second coating step to make delayed-release microcapsules.
A mixture consisting of 750 g Compritol® 888 and62.5 g Pluronic® F127 was added to sealed stainless steeltank, and was melted over 40 minutes at 90 °C with heating fluid circulating through the tank’s jacket. The molten
solution was stirred at 75 rpm for 5 minutes. Following
melting, 250 g cetirizine and 187.5 g of croscarmellose
sodium was added such that the anticipated batch size
was 1.25 kg with a theoretical drug loading of 20 %. This
mixture was stirred for an additional 5 minutes with ahigh speed homogenizer.
Using nitrogen, the tank was pressurized to 103 kPa totransfer the molten cetirizine solution at 145 g/min onto
the center of a spinning disk atomizer, which was rotating
at 5500 rpm heated to 90 °C. The microparticles solidified
mid-air via convection, were collected in a lined vessel.
The cetirizine-containing microparticles were thencoated with a 15 % solution of Surelease® E77050 for 150
minutes in a Glatt GPCG-1 fluidized bed coater equippedwith a Würster column set to 15 mm. Circulating air(~17.9 L/s) and bed temperatures were kept at an averageof 63 °C and 46 °C, respectively, while the coating wasapplied between 3.8 and 7.4 g/min at 220 kPa.
Following manufacture, the powder was dissolution
tested with a United States Pharmacopeia (USP) Type II
apparatus (Fig. 5), which demonstrates the delayed-release nature of the coated formulation.

4.3 Potassium chloride
This example outlines manufacture of extended-releasemicrocapsules containing potassium chloride via coacer-Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

Fig. 5

Dissolution of coated cetirizine microcapsules createdwith spinning disk atomization and Würster coating.
Paddle speed was 50 rpm in a 37 °C vessel with simulated mouth buffer. Image created from tabulated dissolution data in US 8,236,349 B2 (Appel L. et al., 2012).

vation, fluidized bed coating (Powell T.C., 1995).
A mixture consisting of 567 kg of potassium chloride
crystals between 300 μm and 600 μm in diameter werefirst mixed with 3312 L of cyclohexane, 83.5 kg of ethylcellulose, and 58 kg of polyethylene. The contents werethen heated under agitation until all of the components
were in solution. The solution was then cooled to achieve
initial coating of the potassium chloride crystals, filteredunder vacuum, then dried in a fluidized bed, resulting insingle-layer microcapsules less than 850 μm in diameter.
A second coating is applied in a fluidized bed coater witha solution consisting of 6.9 kg of hydroxypropylcellulose
in 35 L purified water, such that the anticipated batch size
was 715 kg with a theoretical drug loading of 79 %.
Following manufacture, the powder was dissolution
tested with a United States Pharmacopeia (USP) Type II
apparatus (Fig. 6), which demonstrates the extended­release nature of the dual-coating formulation.

4.4 Dextromethmorphan
This example outlines manufacture of immediate-­release microparticles containing dextromethmorphan via
ion exchange resins (Hirsh J. et al., 2005).
First, 1.0 kg of an ion exchange resin, Amberlite®
IRP69, was added to 4.75 L of deionized water heated to
90 °C, and was mixed well before adding 300 g of dextromethmorphan HBr. The solution was allowed to mix
for 120 minutes to facilitate binding. Next, the resin particles were isolated via vacuum filtration, washed with 10 Lof heated deionized water, then re-suspended in another3 L of heated deionized water. An additional 300 g ofdextromethmorphan HBr was then added to the 3 L volume under mixing, which continued for an additional 120
minutes.

Fig. 6

Dissolution of single-coated potassium chloride microcapsules created with ion-exchange resins. Paddle
speed was 100 rpm in a 37 °C vessel with purified water. Image created from tabulated dissolution data in
US 5,422,122 (Powell T.C., 1995).

Fig. 7

Dissolution of uncoated dextromethmorphan microparticles created with ion exchange resins. Paddle speed
was 50 rpm in a 37 °C vessel with 0.1 N HCl. At 1
hour, the buffer pH was increased to 6.8. Image createdfrom tabulated dissolution data in US 2005/0181050 A1(Hirsh J. et al., 2005).

Following second stage binding, the reaction mixture
was cooled overnight, filtered, washed three times with
10 L volumes of heated deionized water, and dried at
45 °C until substantially free of water as determined by amoisture analyzer. The resulting microparticles had adrug load of approximately 31 %.
Following manufacture, the powder was dissolution
tested with a United States Pharmacopeia (USP) Type II
apparatus (Fig. 7), which demonstrates the immediate-­release nature of the un-coated formulation.

4.5 Methylphenidate
This example outlines manufacture of an extended-­release microparticle formulation containing methylphe-

99

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

nidate via ion exchange resins and fluidized bed coating(Mehta K. et al., 2012). The manufacture is largely thesame as the previous example, with the exception of adding second and third coating steps to make extended-­release microcapsules. The entire formulation, however,uses single-coated, dual-coated, and drug-free diluent
particles to achieve the extended release profile.
First, 3.1 kg of methylphenidate HCl was solubilized in
80 L of purified water with continuous mixing, before adding 7.7 kg of Amberlite® IRP69. This solution was allowed
to mix 60 minutes before being filtered, rinsed, and dried
until water content was between 3 % and 7 %. The methylphenidate-resin complex was milled and sieved suchthat particles under 410 μm were isolated for the next step.
In a secondary containment system, 657 g of Kollidon®
K30 was dissolved in 2.7 L of purified water, to which
the methylphenidate resin complexes were added under
mixing until the Kollidon® solution gained nearly 8 % inweight. The slurry was dried until the moisture content
was between 15 % and 25 %, milled and sieved a second
time to yield particles under 410 μm, dried until watercontent was between 3 % and 7 %, then milled and sieved
a third time to yield particles under 410 μm. The resulting
product were single-coated particles.
Dual coated particles were manufactured by applying asecond coat to the methylphenidate resin complexes detailed above. A solution consisting of 2.1 kg of purified
water, 56 g triacetin, and 3.4 kg Kollicoat® SR30D was
prepared and fed to a fluid bed processor with Würster
column to coat approximately 3.9 kg of methylphenidate
particles from the previous coating step. This was performed until the particles achieved 30 % weight gain. As
a last step, the dual-coated methylphenidate ion exchange
complex was dried for 5 hours at 60 °C before being
passed for a fourth time through a 410 μm mesh screen.
The third component, diluent microparticles, were prepared by mixing 125 g Poloxamer® 188, 34.4 kg sugar,
965 g sodium citrate, 1.3 kg anhydrous citric acid, 500 g
sodium benzoate, and 200 g sucralose to 1.8 L purified
water in a high shear granulator. The granules were dried
until the water content was below 1.5 %, then milled andsieved through an 850 μm mesh screen.
The entire extended release composition consisted ofstarch, xanthan gum, talc, flavor, and silicon dioxide suchthat the diluent granules, single-coated granules, and dual-­coated granules, approximated 75 %, 1.7 %, and 9.9 % of
the total dry weight, respectively.
Following manufacture, the above components werereconstituted to an equivalent of 60 mg methylphenidatein water, and administered to healthy adults. Mean plasma
concentration of methylphenidate was compared to adults
administered two 30 mg dose of the RLD product (Fig. 8),
which demonstrates the extended-release nature of thedual-coated formulation.

100

Fig. 8

Pharmacokinetics of a suspension of single-coated,
dual-coated, and diluent methylphenidate microcapsules. Plasma concentration was also compared to administration of two 30 mg RLD tablets. The data
demonstrate that the microcapsule suspension maintains plasma concentration of methylphenidate as two
separately-dosed tablets would. Image reproduced
from US 8,287,903 B2 (Mehta K. et al., 2012).

4.6 Prednisone
This example outlines manufacture of taste-masked
microspheres containing prednisone via precision particle
fabrication (Berkland C. and Singh M., 2014, 2015;
Berkland C. et al., 2014). The manufacture is largely thesame as the previous example, with the exception of using
a dual chamber nozzle for simultaneous coating to yielddelayed-release microcapsules.
A mixture consisting of 120 g glyceryl monostearate
and 50 g sorbitan monostearate was melted at 95 °C underconstant stirring in a jacketed glass reactor under constantmixing. Following, 20 g Eudragit® EPO was added until
dispersed. This mixture was allowed to mix for 2 hours,
followed by addition of 10 g of prednisone.
The resulting solution was transferred to a 200 mL
stainless steel syringe loaded onto a Harvard Apparatus
PHD Ultra syringe pump. The syringe was maintained at
80 °C and the contents were injected to primary chamberof a single-barrel stainless steel precision particle fabrication nozzle heated to 80 °C. The flow rate of the prednisone solution was 35 mL/h. such that the anticipated batch
size was 200 g with a theoretical prednisone loading of
5.0 %. A third chamber was fed filtered ambient nitrogenat 3.0 L/min to stabilize the droplet breakup. The nozzle
was subjected to a frequency of 1.0 kHz with an amplitude of 1.0 Volts, peak-to-peak (Vpp).
The microspheres solidified mid-air via convection andwere collected in a stainless steel vessel lined with a
2 mm poly-bag. Resulting particles had a mean size of
180 μm in diameter.
Following manufacture, the powder was dissolution
tested with a United States Pharmacopeia (USP) Type II
apparatus (Fig. 9) and compared to an RLD syrup, which

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

demonstrates the delayed-release nature of the single-stepcoated formulation.

4.7 Guaifenesin
This example outlines manufacture of extended-releasemicrospheres containing guaifenesin via precision particle fabrication (Berkland C. and Singh M., 2014, 2015;
Berkland C. et al., 2014), where a coating step is not included to achieve extended release.
A mixture consisting of 1360 g carnauba wax, 200 gstearic acid, and 20 g ethylcellulose was melted at 95 °C
under constant stirring in a stainless steel mixing vessel.
After the molten mixture was formed, 640 g of guaifenesin was incorporated until a homogenous solution wasformed such that the anticipated batch size was 1.0 kg
with a guaifenesin loading was 32 %.
The resulting solution was pressurized with filtered nitrogen and fed at 1.0 kg/h through a mass flow controller,
to the primary chamber of a stainless steel 12-barrel precision particle fabrication nozzle heated to 95 °C. A secondary chamber was fed filtered ambient nitrogen at 3.6 L/min
to stabilize the droplet breakup. The nozzle was subjected
to a frequency of 1.4 kHz with an amplitude of 1.0 Vpp.
The microspheres solidified mid-air via convection andwere collected in a stainless steel vessel lined with a
2 mm poly-bag. Resulting particles had a mean size of
150 μm in diameter.
Following manufacture, the powder was dissolution
tested with a United States Pharmacopeia (USP) Type II
apparatus (Fig. 10) and compared to an RLD tablet,which demonstrates the extended-release nature of theun-coated formulation.

4.8 Ibuprofen
This example outlines manufacture of extended-releasemicrospheres containing ibuprofen via precision particle
fabrication (Berkland C. and Singh M., 2014, 2015;
Berkland C. et al., 2014), where a coating step is not included to achieve extended release.
A mixture consisting of 3150 g carnauba wax, 250 g
stearic acid, and 100 g ethylcellulose was melted at 95 °C
under constant stirring in a sealed 30 L stainless steel
mixing vessel with high shear homogenizer. After themolten mixture was formed, 1500 g of ibuprofen was incorporated until a homogenous solution was formed suchthat the anticipated batch size was 5.0 kg with an ibuprofen loading was 30 %.
The resulting solution was pressurized with filtered nitrogen and fed at 2.0 kg/h through heat-traced processlines and distributed to five mass flow controllers, feeding
the primary chambers of five stainless steel 12-barrel precision particle fabrication nozzles heated to 95 °C. The

Fig. 9(Top) Dissolution of coated prednisone microcapsules
created with precision particle fabrication. Paddle
speed was 75 rpm in a 37 °C vessel with 0.1 N HCl.
Data are compared to the RLD tablet. (Bottom) Microsphere rupture in vitro after pH change.

101Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

Fig. 11 Dissolution of uncoated 12-hour and 24-hour extended-­release ibuprofen microspheres created with precisionparticle fabrication. Paddle speed was 75 rpm in a
37 °C vessel with 0.05 M SLS. Data are compared to
the RLD tablet.

4.9 RitonavirFig. 10 (Top) Dissolution of uncoated guaifenesin microspheres created with precision particle fabrication.
Paddle speed was 75 rpm in a 37 °C vessel with 0.1 N
HCl. Data are compared to the RLD tablet, where themicrosphere formulation has an F2 (similarity factor)of 97.2. (Bottom) Scanning electron micrograph image of guaifenesin microspheres with a mean size of
150 μm in diameter.

five secondary nozzle chambers were each fed filteredambient nitrogen at 5 L/min/nozzle to stabilize the droplet
breakup. The nozzles were subjected to a frequency of
2.0 kHz with an amplitude of 0.6 Vpp.
The microspheres solidified mid-air via convection inside an enclosed 825 L jacketed stainless steel vessel
chilled with heat transfer fluid to a temperature of 15 °C.
Resulting particles had a mean size of 120 μm in diameter.
A similar formulation was also made on a single-barrel
precision particle fabrication nozzle that with a composition of 45 % carnauba wax, 25 % stearic acid, and 30 %ibuprofen.
Following manufacture, the powders were dissolution
tested with a United States Pharmacopeia (USP) Type II
apparatus (Fig. 11) and compared to an RLD tablet,which demonstrates the extended-release nature of theun-coated formulations.

102

This example outlines manufacture of taste-masked microcapsules containing ritonavir via precision particle fabrication (Berkland C. and Singh M., 2014, 2015; Berkland
C. et al., 2014). The manufacture is largely the same as
the previous example, with the exception of using adual chamber nozzle for simultaneous coating to yielddelayed-release microcapsules.
A mixture consisting of 40 g stearic acid and 10 g Eudragit® EPO was melted at 95 °C under constant stirringin a stainless steel mixing vessel. This mixture would
comprise the microcapsule shell feed.
A second vessel was prepared where 42.5 g of polyethylene glycol 1500 was melted at 80 °C. Following, 7.5 g ofritonavir was incorporated until a homogenous dispersion
was formed. This mixture would comprise the microcapsule core feed.
The resulting solutions were each transferred to 50 mL
stainless steel syringes loaded onto a Harvard Apparatus
PHD Ultra syringe pumps. The syringes were maintained
at 80 °C and the contents were simultaneously injected toprimary and secondary chambers of a single-barrel stainless steel precision particle fabrication nozzle heated to
80 °C. The flow rates of the prednisone-free shell andprednisone-containing core were set to 30 mL/h and
20 mL/h, respectively, such that the anticipated batch
size was 90 g with a theoretical prednisone loading of
6.7 %. A third chamber was fed filtered ambient nitrogenat 2.5 L/min to stabilize the droplet breakup. The nozzle
was subjected to a frequency of 1.0 kHz with an amplitude of 1.0 Vpp.
The microcapsules solidified mid-air via convection
and were collected in a stainless steel vessel lined with aMartin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

more controlled-release layers for a finished product.
Contemporary techniques incorporate chemical modification and sequestration of the API, prior to secondary coating steps. Next-generation techniques eliminate the needfor multiple steps, achieving even coatings while maintaining monodisperse size distributions and high API
content at small overall particle size to enhance palatability and mouth-feel. Though the number of powder alternatives that are available for currently approved
medications is limited, a rising number of pharmaceuticalcompanies and manufacturing organizations are incorporating controlled-release powder manufacturing to theirportfolios to address the growing dosage form problem
for paediatric and geriatric patients. In an age where “bigger is better,” for nearly everything in culture, the opposite is unquestionably becoming true for powders, where“less is more” in terms of dosage form size, manufacturing steps, and process chemistry.

Nomenclature
Fig. 12 Dissolution of coated delayed-release ritonavir microcapsules created with precision particle fabrication.
Paddle speed was 75 rpm in a 37 °C vessel with 0.1 N
HCl for the first 2 minutes, then 0.05 M SLS. Data arecompared to the RLD syrup.

2 mm poly-bag. Resulting particles had a mean size of
250 μm in diameter.
Following manufacture, the powder was dissolution
tested with a United States Pharmacopeia (USP) Type II
apparatus (Fig. 12) and compared to an RLD syrup,
which demonstrates the delayed-release nature of thesingle-step coated formulation.

5. Conclusion
Though traditional oral dosage forms such as pills, capsules, caplets, and tablets work for many individuals, asignificant fraction of the world’s population suffers fromdysphagia, taste sensitivities, or an unwavering avoidance
to taking oral medication of any format. As these patientsare afflicted with acute or chronic illnesses, sometimes alack of format flexibility and dosage options limits treatment, and in the worst circumstances, prevents it.
An emergence in manufacturing of controlled-releasepowders has taken place over the last decade, replacing
large tablets with dispersible and dose-flexible alternatives that can be supplied in sachets, sprinkled in food, or
suspended in liquid. The methods for making controlled
release powders vary, but typically include forming an
API/excipient core precursor then coating with one orDparticle size (μm)

llength (m)mmass (kg)P

pressure (Pa)ttime (s)Δttime durationT

temperature (K)Vvolume (L)μgas (Pa s)ρgdensity (kg m–3)

Vpp

volts, peak-to-peak

References
Best pharmaceuticals for children act, Public Law 107–109,
Washington, DC, 2002.
Ambike A.A., Mahadik K.R., Paradkar A., Spray-dried amorphous solid dispersions of simvastatin, a low Tg drug: In
vitro and in vivo evaluations, Pharmaceutical Research, 22(2005) 990–998.
Appel L., Friesen D.T., Lachapelle E.D., Konagurthu S., Falk
R.F., Reo J.P., Taste-masked drugs in rupturing multiparticulates, Google Patents, https://www.google.com/patents/
US8236349 (2012).
Appel L., Ray R., Newbold D., Lyon D., West J., Friesen D.,
Mccray S., Crew M., Lo J., Controlled release multiparticulates formed with dissolution enhancers, Google Patents,
https://www.google.com/patents/US20050152982 (2005).
Bergstrom D., Mcnally E., Freeman S., The growing pediatrics

103

Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

market, Pharmaceutical Executive, (2004).
Berkland C., Pollauf E., Raman C., Silverman R., Kim K., Pack
D.W., Macromolecule release from monodisperse PLG
microspheres: Control of release rates and investigation of
release mechanism, Journal of Pharmaceutical Sciences, 96(2007a) 1176–1191.
Berkland C., Pollauf E., Varde N., Pack D.W., Kim K., Monodisperse liquid-filled biodegradable microcapsules, Pharmaceutical Research, 24 (2007b) 1007–1013.
Berkland C., Singh M., Sustained release particle formulations,
Google Patents, https://www.google.com/patents/US20140294980 (2014).
Berkland C., Singh M., Taste masking drug formulations, Google Patents, https://www.google.com/patents/WO2015130760
A1?cl=en (2015).
Berkland C., Singh M., Dormer N., Tramadol particle formulations and methods, Google Patents, https://www.google.
com/patents/WO2014205228A1?cl=en (2014).
Bhardwaj S., Hayward M., Palatable pharmaceutical compositions, SmithKline Beecham Corporation, (1996).
Cloupeau M., Prunet-Foch B., Electrohydrodynamic spraying
functioning modes: A critical review, Journal of Aerosol
Science, 25 (1994) 1021–1036.
Cram A., Bartlett J., Heimlich J., Oral multiparticulates as aflexible solid dosage form approach for paediatric use, BioPharm Asia, (2013).
Dewettinck K., Huyghebaert A., Fluidized bed coating in foodtechnology, Trends in Food Science & Technology, 10(1999) 163–168.
Dickens D.S., Sinsabaugh D., Fahner J.B., Characteristics ofpediatric chemotherapy medication errors in a national
error reporting database, Cancer, 112 (2008) 445–446;
author reply 446.
Eldem T., Speiser P., Hincal A., Optimization of spray-dried
and -congealed lipid micropellets and characterization of
their surface morphology by scanning electron microscopy,
Pharmaceutical Research, 8 (1991) 47–54.
Elder D.P., Pharmaceutical applications of ion-exchange resins,
Journal of Chemical Education, 82 (2005) 575.
Engelen L., De Wijk R.A., Van Der Bilt A., Prinz J.F., Janssen
A.M., Bosman F., Relating particles and texture perception,
Physiol Behav, 86 (2005) 111–117.
Fazal U.-R., Khan S.N., Therapeutic applications of ion
exchange resins, in: Inamuddin D. and Luqman M. (Eds),
Ion exchange technology ii: Applications, Springer Netherlands, Dordrecht, 2012, pp. 149–168.
Gharsallaoui A., Roudaut G., Chambin O., Voilley A., Saurel R.,
Applications of spray-drying in microencapsulation of foodingredients: An overview, Food Research International, 40(2007) 1107–1121.
Gouin S., Microencapsulation: Industrial appraisal of existing
technologies and trends, Trends in Food Science & Technology, 15 (2004) 330–347.
Griffith R., Managing difficulties in swallowing solid medications: The need for caution., Nurse Presc, 3 (2005) 201–203.
Hancock B.C., Colvin J.T., Mullarney M.P., Zinchuk A.V., Pharmaceutical powders, blends, dry granulations, and immediate-release tablets, Pharmaceutical technology, (2003)

104

64–80.
Harb J., Why so many pills are too big to swallow: And why it’ssafe to crush or cut up some, but not others., (2015)
Hirsh J., Fleming A., Rariy R., Dosage forms using drug-loaded
ion exchange resins, Google Patents, https://www.google.
com/patents/US20050181050 (2005).
Imai E., Hatae K., Shimada A., Oral perception of grittiness,
Journal of Textural Studies, 26 (1995) 561–576.
Ivanovska V., Rademaker C., Van Dijk L., Mantel-Teeuwisse A.,
Pediatric drug formulations: A review of challenges andprogress, Pediatrics, 134 (2014).
Jayanthi B., Manna P., Per oral extended products – an overview, J App Pharm Sci, 1 (2011) 50–55.
Jono K., Ichikawa H., Miyamoto M., Fukumori Y., A review ofparticulate design for pharmaceutical powders and theirproduction by spouted bed coating, Powder Technology,
113 (2000) 269–277.
Lopez F.L., Ernest T.B., Tuleu C., Gul M.O., Formulation
approaches to pediatric oral drug delivery: Benefits andlimitations of current platforms, Expert Opinion on Drug
Delivery, 12 (2015) 1727–1740.
Maalouf N., Developing patient-centric drug formulations tomeet patient needs, (2013).
Matsui D., Current issues in pediatric medication adherence,
Paediatr Drugs, 9 (2007) 283–288.
Mehta K., Tu Y.H., Perumal A., Orally effective methylphenidate extended release powder and aqueous suspension
product, Google Patents, https://www.google.com/patents/
US8287903 (2012).
Milne C.P., Bruss J.B., The economics of pediatric formulation
development for off-patent drugs, Clin Ther, 30 (2008)
2133–2145.
Osterberg L., Blaschke T., Adherence to medication, New Engl
J Med, 353 (2005).
Pack D.W., Berkland C., Cox A., Kim K., Three-month, zero-­order piroxicam release from monodispersed double-walledmicrospheres of controlled shell thickness, Journal of Biomedical Materials Research Part A, 70A (2004a) 576–584.
Pack D.W., Berkland C., Kim K., PLG microsphere size controls
drug release rate through several competing factors, Pharmaceutical Research, 20 (2003) 1055–1062.
Pack D.W., Berkland C., Kim K.K., Fabrication of PLG microspheres with precisely controlled and monodisperse size
distributions, Journal of Controlled Release, 73 (2001)
59–74.
Pack D.W., Berkland C., King M., Cox A., Kim K., Precise control of PLG microsphere size provides enhanced control ofdrug release rate, Journal of Controlled Release, 82 (2002)
137–147.
Pack D.W., Berkland C., Kipper M.J., Narasimhan B., Kim
K.K., Microsphere size, precipitation kinetics and drug distribution control drug release from biodegradable polyanhydride microspheres, Journal of Controlled Release, 94(2004b) 129–141.
Pande S.V., Kshirsagar M.D., Chandewar A.V., Ion exchange
resins pharmaceutical applications and recent advancement,
2011, 2 (2011).
Passerini N., Albertini B., Perissutti B., Rodriguez L., Evalua-Martin G. Teresk et al. / KONA Powder and Particle Journal No. 34 (2017) 91–105

tion of melt granulation and ultrasonic spray congealing astechniques to enhance the dissolution of praziquantel,
International Journal of Pharmaceutics, 318 (2006) 92–102.
Pediatric research equity act, Public Law 108–155, Washington
DC, 2003.
Pharmacircle, (2016), www.pharmacircle.com, from www.pharmacircle.com.
Powell T.C., Controlled release potassium chloride tablet, Google
Patents, https://www.google.com/patents/US5422122 (1995).
Regulation no. Ec 1901/2006, Ec E.P.a.T.C., Brussels, (2006).
Rocca J.G., Park K., Oral drug delivery: Prospects and challenges, Drug Development and Delivery, 4 (2004) No.4.
http://www.drug-dev.com/Main/Back-Issues/187.aspx.
Sansom L., Oral extended-release products, Aust Prescr, 22(1999) 88–90.
Sastry S.V., Nyshadham J.R., Fix J.A., Recent technologicaladvances in oral drug delivery – a review, Pharmaceutical
Science & Technology Today, 3 (2000) 138–145.
Schier J., Howland M., Hoffman R., Nelson, Fatality fromadministration of labetalol and crushed extended-release

nifedipine, Annals of Pharmacotherapy, 37 (2003) 1420–
1423.
Sugao H., Taste masking of bitter drug powder without loss ofbioavailability by heat treatment of wax-coated microparticles, Journal of Pharmaceutical Sciences, 87 (1997).
Tyle P., Effect of size, shape and hardness of particles in suspension on oral texture and palatability, Acta Psychologica,84 (1993) 111–118.
Vehring R., Pharmaceutical particle engineering via spray drying, Pharmaceutical Research, 25 (2007) 999–1022.
Vummaneni V., Nagpal D., Taste masking technologies: An
overview and recent updates, International Journal of
Research in Pharmaceutical and Biomedical Sciences, 3(2012) 510–525.
Wright D., Medication administration in nursing homes, Nurse
Std, 16 (2002) 33–38.
Yurteri C.U., Hartman R.P.A., Marijnissen J.C.M., Producing
pharmaceutical particles via electrospraying with anemphasis on nano and nano structured particles—a review,
KONA Powder and Particle Journal, 28 (2010) 91–115.

Author’s short biography
Martin G. Teresk
Martin Teresk, PhD, joined Orbis after completing five years at Gilead Sciences,where he served as a process development chemist. Martin’s expertise resides in designing, developing, and validating robust and commercially viable processes for the manufacturing of active pharmaceutical ingredients. He holds a PhD in Organic Chemistryfrom University of Texas at Austin and a BS in Chemistry from Purdue.

Cory J. Berkland
Cory Berkland, PhD, has been developing microencapsulation and drug delivery capabilities for more than a decade. Cory has a PhD in Chemical and Biomolecular Engineering from the University of Illinois, Urbana-Champaign, where he co-invented anddeveloped the Orbis technology. Cory is a Professor of Pharmaceutical Chemistry andChemical and Petroleum Engineering at The University of Kansas.

Nathan H. Dormer
Nathan Dormer, PhD, leads the formulation team at Orbis Biosciences. He has morethan 8 years of experience developing a variety of controlled-release solutions using microsphere techniques. He received his BS in Chemical Engineering from The Universityof Kansas before completing his PhD in Bioengineering from The University of Kansas
with NIH-sponsored training in drug delivery and protein stability. He has authored anumber of publications and book chapters relating to microspheres.

105

KONA Powder and Particle Journal No. 34 (2017) 106–124/Doi:10.14356/kona.2017015

Review Paper

A Review of Advanced Ball Mill Modelling †Luís Marcelo Tavares
1

Department of Metallurgical and Materials Engineering, COPPE, Universidade Federal do Rio de Janeiro, Brazil

Abstract
In the early 1990s the discrete element method (DEM) was used for the first time to simulate media motion intumbling mills. Although it has been over a quarter of a century since this tool was first used to predict media
motion it has not yet reached maturity to be used in predicting product size distribution and throughput of
tumbling mills. However, there has not been shortage of attempts to do so. The literature is relatively vast in the
topic and researchers who embark in this area of research will find it difficult to understand the current status ofdevelopment and also the similarities and fundamental differences that exist amongst the various approaches thathave been proposed and pursued over the years. The paper reviews the literature on the application of models
based on distributed collision energy information to predict size reduction in tumbling mills, in particular ballmills, analyzing critically various approaches proposed, their limitations and achievements, identifying areas thatstill require development until the technology becomes ready for being used for optimizing and designing ballmills. Finally, the advances recently accomplished on the approach proposed by the author and his co-workers arethen reviewed in greater detail.
Keywords: modeling, discrete element method, ball mills, mechanistic

1. Introduction
In spite of the development and increase in popularity
of several other technologies, such as vertical roller mills,
stirred media mills and high-pressure grinding rolls, ballmills still retain a significant part of their importance ingrinding, being widely used from minerals and cement, tochemicals and ceramics. The operation of a ball mill is acapital- and energy-intensive process, so that even marginal improvements in their efficiency result in immense
benefits to the industry. These mills are more often of the
tumbling type, but also include planetary and vibratory
mills, which have all in common the analogous type of
motion of the loose grinding media. It has been long recognized that in order to reduce their size, materials require
the application of energy in some form. This recognition
led to attempts to relate energy and size reduction fromthe latter part of the 19th century (von Rittinger, 1867;
Kick, 1885) throughout the 20th century (Bond, 1952;
Charles, 1957), and even more recently (Morrell, 2004).
These different approaches have one thing in common:
they use a single number to describe energy in grinding.

†1

Received 23 May 2016; Accepted 13 June 2016
J-STAGE Advance Publication online 30 August 2016
Cx. Postal 68505, CEP 21945-970, Rio de Janeiro, RJ, Brazil
E-mail: tavares@ufrj.br
TEL: +55-21-2290-1544 Ext. 246

106

This number, the specific energy, is the ratio between the
power and the throughput in the case of continuous systems, or the power multiplied by the grinding time perunit mass of powder or ore in batch mills. Given typicallyin kWh/t, it has long been recognized as the most important descriptor of the contribution of the process to theoutcome of comminution. The importance of specific energy and the knowledge on how mill power can be controlled by varying mill design and operating variables,
has allowed engineers design mills and improve a large
number of operations throughout the 20th century (Herbst
and Fuerstenau, 1980). These so-called energy-size reduction relationships were originally only used as one-size
descriptors of comminution (Bond, 1952), but with theadvent of the size-mass balance, or population balance
model (Herbst and Fuerstenau, 1980), improvements fromthis type of relationships also became useful to describe
the change of the entire size distribution from the feed tothe product.
A more in-depth look at ball mill operation clearly suggests the richness of the information hidden behind thevalue of specific energy in an operation, which should be
regarded as merely an average number. The emphasis on
this number suggests, in principle, that it would not matter how the material receives energy, just the amount.
Single-particle breakage has allowed us to gain insights
into this by analyzing the different outcomes if a particle
receives energy in the form of a single high-energy im-

Copyright © 2017 The Author. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

pact from the grinding media, or from a multitude of very
low-energy impacts, which have very different outcomes(Pauw and Maré, 1988; Tavares, 2004). Evidence of thisalso appears in the form of the empirical equations thathave been used to account for several variables in grinding in tumbling mills that are part of the design method of
Bond (Rowland and Kjos, 1980), as well as to the need forthe user to conduct additional batch grinding tests to fit
some of the model parameters in the case of the scale-up
method proposed by Herbst and Fuerstenau (1980).
As such, great value can be gained by using distributed
information on specific comminution energies, rather
than averages. Such information has already been available from careful experimentation or, more commonly,
simulations using the discrete element method (DEM),
which conveniently allows predicting the distribution ofcollision energies in a mill (Weerasekara et al., 2013), creating an alternative to the lumped specific energy information used to date to describe size reduction in mills.
DEM makes it possible to predict how collisions vary inmagnitude and frequency as the geometry, the ball charge
and the mill speed are varied. Therefore, instead of correlating each individual parameter such as mill diameter,
filling, speed and ball size, for instance, to the productsize, one can directly take into account the record of thecollisions and their influence on the particle size distribution in the mill.
There has been no shortage in the last 25 years or so of
approaches to use such distributed collision energy information in modelling ball mills. However, it is not straightforward to identify how they differ, their benefits andchallenges in their goal of replacing the traditionalenergy-­size reduction methods such as Bond’s (Bond,
1952) or population balance model (PBM) formulations(Herbst and Fuerstenau, 1980; Austin et al., 1984). The
present work focuses primarily on the research dedicatedto predict particle breakage in ball mills, analyzing inparticular DEM-based approaches to predict size reduction in mills, called here “advanced models”. As such, thisreview does not directly cover other aspects that are also
necessary to predict grinding in continuous systems, suchas material transport and internal classification, which
also were the object of much research throughout the lastquarter of century.

2. Advanced mill modelling prior to DEM
The first attempts in coupling impact energy information to particle breakage data to predict grinding havebeen made in the late 1980s and early 1990s. These wereused to predict the functions that make up the traditional
population balance model. The greatest difficulty at thetime was associated to the collection of information on

the collision energies in the mill. Yashima et al. (1988)
and Nomura et al. (1991) estimated the size-dependent
rates of breakage of particles contained in ball mills using
single-particle slow compression data and experimental
measurements of impact energy spectra using load sensors positioned in the shell of a lab mill, showing somequalitative agreement. Cho (1987) and Höfler and Herbst(1990) demonstrated how both the breakage rate and thebreakage distribution functions varied with collision energy, using data from particle bed breakage tests and impact energy spectra from tests in a batch mill. Their work
proposed that the breakage function Bij commonly used in
the traditional population balance model could be interpreted as the weighed-average of the energy-dependent
breakage distribution function values Bij(Em,k), giving(Cho, 1987)n

Bij   p Em,k  Bij (Em,k )
k 1

(1)

where p(Em,k) is the fraction of collisions with specific energy level Em,k. This equation could be used, associated tothe breakage rate or selection function (Si) to predict
grinding in a batch mill using the equation (Austin et al.,
1984)
i 1
dwi

S i wi  S jbij w j
dt
j 1

(2)

where wi is the fraction of particles in size class i at time t
and bij is the breakage function in density form, so thatbij = Bij – Bi–1,j.
This is certainly a more logical and intuitive approachthan assuming a constant breakage distribution function,
as used in the traditional PBM formulation of ball milling.
These approaches, however, had the serious limitation ofrequiring impact energy data, which was very difficult toestimate experimentally. In the case of Cho (1987), thedistribution of collision energies was back-calculated
from batch grinding data, so that it should be understoodas only a proof of concept of the modeling approach.

3. Role of DEM in advanced mill modelling
A major advance in the modeling of ball mills became
possible through the development of the discrete element
method. DEM provides a numerical procedure for solving
Newton’s second law with an appropriate contact relationship that can be applied to describe the motion of eachgrinding medium contained in the mill charge to predictelement position, velocity and forces of interaction overtime. Very good reviews of the method when applied tocomminution can be found elsewhere (Cleary, 1998;
Weerasekara et al., 2013). The relevance of the method
when applied to milling lies in its ability to account for107

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

the effects of a number of variables explicitly, including
mill speed, mill filling, ball size distribution and liner
configuration. It was first applied to simulate ball millingby Mishra and Rajamani (1992), who originally described
media motion in two dimensions. The technique is now
packaged in a number of commercial as well as open
source software, being able to describe media motion inthree dimensions, which is critical to provide the quantitative information required for mill simulation (Powell
and McBride, 2006; Morrison and Cleary, 2008).
In DEM, particles are either considered undeformable(hard) or deformable (soft) (O’Sullivan, 2011). The latter,
in which particles are allowed to overlap, is the most commonly used. It is supported by proper constitutive laws
describing the contacts, with Hertz-Mindlin being themost commonly used (Weerasekara et al., 2013).
Algorithms that allow describing particle breakage inside the DEM environment are more recent developments,
and include the discrete grain breakage (Herbst andPotapov, 2004), the bonded particle model (Potyondy andCundal, 2004) and the particle replacement (Cleary,2001). As such, DEM with the proper description of particle breakage appeared as a complete and empiricism-freesolution to the current approaches used in modeling ballmills, since it does not only allow describing media motion, but also breakage of particles inside the charge(Morrison and Cleary, 2008). Indeed, as a proof of concept, some researchers have demonstrated that it is possible to include particles in mill simulations and follow
them as they tumble and are progressively broken by theballs. Indeed, Herbst and Potapov (2004) included polyhedral particles that broke inside the charge, Metzger
and Glasser (2013) described breakage of particles insidea batch mill using the bonded particle model, while
Delaney et al. (2013) described breakage of particles in apilot mill using the particle replacement method. Unfortunately, this intuitive approach is not yet feasible to be usedin any realistic way to simulate industrial ball mills, since
breakage algorithms add to the already high computational demand for simulating these mills and also can leadto an “explosion” on the number of particles in the simulations, making computation unfeasible at this time. However, such techniques are quickly reaching maturity for
application in simulation of a number of crusher types(Cleary and Sinnott, 2015; Quist and Evertsson, 2016;
Barrios and Tavares, 2016). With increasing computation
power, these techniques will become, however, graduallymore applicable to tumbling mills, in particular of the autogenous and semi-autogenous type, given that the autogenous media not only also transfers energy but also
undergoes breakage.
DEM simulation of industrial tumbling mills can represent a challenge even in the absence of breakage in theDEM environment. This is because the powder or ore

108particles are much finer in size than the grinding media,
leading to difficulty in keeping track of the immense
number of particles contained in full-scale mills, besides
the need to reduce the time step to avoid overlapping themedia with the smallest powder particles (Cleary andMorrison, 2011). The first limitation can be illustrated
with a simple example: the most common tumbling millused in the lab is the Bond ball mill, used to measure thewidely used Bond ball mill work index (Bond, 1952). The
mill dimensions, 30 cm in diameter and 30 cm in length,
and the test conditions are standardized, with a maximum
ball size of 40 mm and an ore top size of 3.35 mm. Withless than 300 balls, it poses no challenge in running thesimulations, if only grinding media are included (Fig. 1).
For such a small mill and operating at both low mill filling (20 %) and powder filling (50 %), one could include
also the powder charge. Indeed, for a typical powder size
distribution, it would be possible to include particles asfine as about 0.5 mm before running out of memory using
today’s desktop computers, since it could take as much as
10 GB of computer memory to run them, for instance, in
a commercial software such as EDEM® (DEM Solutions,UK) which is substantial (Fig. 1). However, if the case ofa large-diameter industrial mill is considered, which
could have, for instance, 8 m of internal diameter, the user
would already run out of memory if he included particles
with the top size in the feed, if the same ball and particle
size distributions simulated in the laboratory mill areused. Moreover, this is only considering a 30 cm slice ofthe mill, which could have more than two billion elementswith sizes above 0.5 mm (Fig. 1).

Fig. 1

Number of particles used in simulating 30 cm slices ofmills with different diameters (balls in size range of
40–15 mm) and powder following the cumulative size
distribution given by Passing = (x/3.35)0.5, where particle size x is given in mm (20 % ball filling, 50 % interstitial powder filling). Powder finer than designatedsize is accumulated in the informed size.

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

Nevertheless, it is worth mentioning that the increasein computing power in recent years has enticed researchers to include both grinding media and powder in somesimulations of small ball mills, which has been particularly useful to understand the collision frequencies andenergies involved in media-particle contacts (Cleary andMorrison, 2011; Wang et al., 2012; Capece et al., 2014).
This type of simulations, however, represents a challenge
in gathering collision information, since the event of aparticle being nipped by two colliding media is logged as
two separate collisions of lower magnitude.
One pragmatic solution to this in the case of ball mills
is to simplify the problem by not including the ore or
powder particles in DEM, only the grinding media in thesimulations, calculating the product size distribution andthe mill capacity in a post-processing stage using a PBM
scheme on the basis of the frequencies and magnitudes ofcollisions, that, is, the collision energy spectrum, experienced by particles in the mill. The key advantage of thisapproach is that the milling environment can be decoupled from the ore or powder in size reduction, while themain challenge is related to the fact that the mass of material and sizes of particles involved in each of the collisions
are not known. Also, this approach is only valid wheneverthe contribution of the ore or powder particles in transferring energy and inducing breakage in other particles isnegligible, which is a valid assumption in the case of ballmills. As such, DEM would be given the role of only providing information on the mechanical environment in themill, since the ore particles would not appear explicitly inthe simulations. Powell et al. (2008) called these ore particles “sub-DEM particles”, since they would be the background material that is subjected to the collisions throughbeing entrapped between the colliding media, but would
not appear in the DEM simulations explicitly.
In order to ensure that predictions of the collision energy spectra under these circumstances are realistic, it isimportant to ensure that DEM simulations can predictmill media motion accurately. This problem is notstraightforward since the absence of powder (ore) in the
DEM simulations would need to be compensated by modifying some—if not all—of the contact parameters thatgovern the contacts between balls and between them andthe mill liners. This can only be accomplished by proper
contact parameter estimation, which is reviewed elsewhere (Mishra and Murty, 2001). As such, the user should
be aware that apparent contact parameters and not standard parameters for steel-steel contacts (whenever steel
media are used), for instance, should be used in thesesimulations, given the dissipative effect of the backgroundpowder. Typically, the presence of powder increases therolling and sliding friction of the ball charge, while it reduces the coefficient of restitution in the Hertz-Mindlin
model. Obvious challenges are associated to the variationof the motion of the charge as particles become finer. Thisevidently further complicates matters when using datafrom batch milling to calibrate DEM contact parameters,
which is common.
It is common to select contact parameters in DEM simulations so as to match the power predicted to the one
measured either in a batch or an industrial mill. However,caution should be exercised in not using it as the only target when selecting values of contact parameters in ballmill simulations. It is known that several sets of plausible
contact parameters can approximately match the measured mill power, but with reasonably distinct patterns ofmedia motion (Martins et al., 2012; Chagas et al., 2015).
Positions of toe and shoulder of the charge, monitored byvideo recordings of batch mills fitted with a transparent
end covers (Moys et al., 2000; Venugopal and Rajamani,
2001), the position and velocity of an instrumented ball(Martins et al., 2012) or of an irradiated ball using positron emission particle tracking (PEPT) (Govender et al.,2013; Chagas et al., 2015), are excellent alternatives to be
used in conjunction with net mill power to calibrate contact parameters for effective mill simulations.
Once the mill geometry, velocity and the charge size
distribution have been defined, and the various materialand contact parameters selected, the mill can then be simulated. It is important to ensure, though, that a steady motion of the media has been established, which takes
several revolutions if the charge is initially in repose. After
this is established, the mill can then be simulated for a
sufficient period of time to give a consistent output, which
is about two revolutions (Powell et al., 2008), during
which detailed information on the collisions are recorded.
At this point the simulation is interrupted and the interactions between grinding media and between them and themill liners are binned for analysis at a post-processing
stage. This analysis provides the probability distributions,which in effect are the rates at which grinding media interact. The question then arises about the collision energy
information that will be harnessed from DEM. Three options have been considered by researchers:• the dissipated energy• the maximum impact energy• the kinetic energy
The dissipated energy, also called energy loss, resultsfrom the inelastic contact between grinding media or
grinding media and an ore particle, that mimics dissipative processes such as plastic deformation or breakage. It
is calculated as the integral of the damping forces with respect to the displacements over a whole contact period,
accounting for the contributions of both the normal (n)and the shear (s) components of the contact:
E  E n  Es  t contact0

Fn dξ n  t contact0

Fsdξ s(3)

109

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

where Fn and Fs are the components of the contact force
in the normal and shear directions, respectively, andtcontact is the total contact time during a collision.
The maximum impact energy, characterizing the maximum stress experienced in the collision, is the integral of
the contact force and displacement in the normal direction
when the overlap between two particles reaches the maximum (Wang et al., 2012).
The kinetic energy in the collision between two grinding media is calculated by 1/ 2mbvij2 , where νij (= νi – νj) is
the relative normal velocity of the two grinding media at
the collision and mb is average ball mass (= (mb,i + mb,j)/2)
for media–media collision or the mass of one ball for ballliner collision, whereas the reduced mass, given by(= 2mb,imb,j/(mb,i + mb,j)) is also often used (Kwan et al.,2005).
It is convenient, for comparison purposes, to represent
distributions of collision energies in the form of cumulative distribution functions P(E) and total frequency ofcollisions ω. Cumulative distributions are illustrated inFig. 2 for different liner configurations in a batch mill.
Whenever necessary the distribution of collision energies
in density form in the mill p(E) may be calculated fromthe cumulative distribution, with p(E) = dP(E)/dE.
Finally, when grinding is carried out wet, the presenceof water would also influence the contact parameters. As
such, coupling of DEM and techniques such as computational fluid dynamics (CFD) or smoothed particle hydrodynamics (SPH) would allow realistic description of theinfluence of the fluid medium (Cleary et al., 2006). The fidelity of the simulation models able to predict breakage
will then determine whether or not it is feasible to still use

predictions from ball-only DEM simulations or coupled
DEM-CFD or DEM-SPH simulations to predict media
motion. In this case it is important to acknowledge the effect of the presence of water on dampening the collisions(Mayank et al., 2015).

4. DEM-based models of mills
4.1 Models that use lumped information from
DEM
Resorting to empiricism, some researchers have attempted to couple either the total or an average collisionenergy or power obtained using DEM to breakage rates orother measures of breakage intensity from batch grinding
experiments (Kano and Saito, 1998; Mori et al., 2004;
Kwan et al., 2005; Lee et al., 2010).
One interesting approach has been used by Herbst(2004) to predict mill power using DEM and then use it toscale-up the energy-specific breakage rates proposed by
Herbst and Fuerstenau (1980) to solve the equation
i 1
dwi

 S iE wi  S Ej bij w j
dE m
j 1

(4)

where bij is the breakage function back-calculated frombatch grinding tests, and E m = Pθ/H is the specific energy,
where H is the mill hold-up, θ is the residence time and P
is the mill power, estimated using DEM.
The obvious advantage of this approach lies in the factthat DEM captures the influence of liner configuration inmill power, which is not taken into account using powermodels such as those from Rowland and Kjos (1980).
However, in analogy to the traditional population balance
models, it does not use distributed collision energy information in predicting mill performance. Further, since thecontributions of material and machine are not properly
decoupled, this approach becomes significantly more limited when compared to other advanced models of ballmills discussed as follows.

4.2 PBM formulations based on particle bed
breakage
One of the first attempts to apply DEM to ball millmodeling was by Rajamani and co-workers at the University of Utah (Rajamani et al., 1993), who described batch
grinding using the expressionni 1 ndM iω p Ek  mi,k  ω p E k  m j,k bij E k dtj 1 k 1
k 1

Fig. 2110

Collision energy distributions for a 0.6 m diameter millwith different liner profiles balls at 67 % of critical
speed, 30 % mill filling and 25 mm steel balls.

(5)

where N is the number of collision energy classes, ω isthe frequency of collisions, p(Ek) is the fraction of collisions of magnitude Ek, mi,k is the mass of particles in size

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

interval i broken in a single collision of energy Ek, Mi is
the mass of particles in the ith size interval and bij(Ek) isthe breakage function for jth size interval particles generated from collisions of magnitude Ek.
Rajamani et al. (1993) were able to predict reasonably
well the size distribution from grinding limestone in a
25 cm-diameter mill, but only at short grinding times, on
the basis of 2D DEM simulations. The limitation of shortgrinding times was due to the fact that only breakage ofthe top size fraction was predicted.
This equation, however, does not take into account the
instantaneous mass fraction of size i in the mill. This wascorrected with the introduction of the term Mi/H as themass fraction of material contained in size class i, or wi,
in an updated version of the model by Datta and Rajamani(2002), given by
i 1


dwi
ω n 

 p Ek   mi,k wi  m j,k bij E k  w j H k 1
dt
j 1





(6)

The model uses information on the broken mass of particles mi,k at a given collision energy and the energy-­
specific breakage function, both obtained from drop
weight tests with 4-layer particle beds, following the procedure originally proposed by Höfler and Herbst (1990).
The broken mass was also described as logarithmic function of the specific collision energy.
The model has been validated by comparisons between
measured and calculated size distributions in batch grinding of limestone in mills with diameters from 25 to
90 mm, with reasonable agreement (Datta and Rajamani,
2002). However, a correction factor of 0.8 to both appearance and disappearance terms in Eqn. 6 was needed to fit
experimental data. The authors attributed this to the inefficiency of the mill.
Datta and Rajamani (2002) recognized that, in a mill, aparticle bed between two colliding balls will be composedof something between a monolayer and several layers ofmaterial, and not the 4-layer bed considered in their experiments.
One key limitation of this approach is associated to thefact that it relies heavily on data from breakage of particle
beds. This mode of breakage varies as a function of particle size distribution in the bed, impact energy, ball size
and number of layers, but covering all these by experiments would be too time-consuming. Indeed, this approach also relies on estimates of broken mass, which areboth function of material (type, particle size) and environment (ball size and collision energy), which is not desiredfor modeling purposes.
The approach by Datta and Rajamani (2002) has beenused more recently by Wang et al. (2012) to predict resultsfrom batch grinding tests. In this study, the authors compared the type of energy information used in the simulations and also included not only the grinding media in the

DEM simulations, but also medium-sized particles tomimic the powder.
The approach reviewed herein assumed that impacts ofall energies contribute to breakage, which is not a reasonable assumption since single particle impact tests haveshown that a minimum impact energy must be overcome
to cause body breakage (Tavares, 2007). Further, particles
may sustain damage and become weaker due to impacts
that are insufficient to cause outright breakage but may
break after a succession of impacts. Finally, this modelingapproach does not decouple properly surface from bodybreakage of ore particles.

4.3 Microscale PBM formulations based on
breakage probability
Recognizing the limitations of earlier approaches, someresearchers have chosen to use single-particle breakage as
the basis of their modeling. The first comprehensive approach along this line has been proposed by King and
Bourgeois (1993) and later presented in a modified form
by King (2001). It was based on measurements of fractureenergies of individual particles (Fig. 3), determined using
the impact or ultrafast load cell (Tavares and King, 1998;
Tavares, 2007), and on the equivalence between bodybreakage probability and the fracture energy distribution.
King and Bourgeois (1993) accounted for the fact thatthe following outcomes could occur when a particle is hit
by grinding media:• If the energy is insufficient to cause fracture, theparticle will remain intact inside the charge.
• If the energy is sufficient to cause fracture, the particle will undergo primary breakage and rebreakage (secondary breakage), until all energy is dissipated.
The basis of their approach is the postulate that the
breakage functions actually effective in the multiparticle

Fig. 3Distribution of particle fracture energies of a gneissrock over a range of sizes, with a lognormal fit (Eqn. 8).
Reprinted with permission from Tavares and Neves(2008). Copyright: (2008) Elsevier Ltd.

111

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

environment of a ball mill can be synthesized from theuniversal single-particle breakage functions once the patterns of energy distribution and stress application are
known in a bed of particles that is subject to impact (Kingand Bourgeois, 1993). The power of this approach is thatit avoids the limitations of the particle-bed based approaches, reviewed in section 4.2, that cannot properly
cover through experimentation the different bed configurations encountered in practice. On the other hand, one ofits limitations is associated to the need for testing single
particles at fine sizes, which is both time consuming andinconvenient. Besides that, small sample size statistics
can also be considered a limitation of this method (Herbst,
2004).
King (2001) proposed that the breakage rate function ofparticles contained in size class i could be expressed by


1

0

0

S i   p( E )mi*( E ) Fi eE p e dedE(7)

where p(E) is the distribution of collision energies, estimated using DEM, p(e) is the energy partition amongst
particles within the bed, which accounts for the fact thatwhen identical particles are stressed within a bed, they do
not necessarily receive the same amount of energy. Fi(eE)is the distribution of fracture energies of particles in sizeclass i, and mi*( E) is the mass of particles captured in an
impact of magnitude E.
King and Bourgeois (1993) demonstrated that themodel was able to describe qualitatively the results ofbatch grinding experiments, but fell short to provide accurate enough descriptions of particle capture and energy
split in the bed. They based their predictions on 2D DEM
simulations. Nevertheless, this work pioneered the microscale approaches to modeling ball mills.
More recently, Crespo (2011) pursued some aspects ofthis approach by considering the distribution of particle
fracture energies and a perceived distribution of collisionenergies in a ball mill. While the work demonstrated thecapability of his approach of predicting non-first order
breakage rates for coarse particles, it suffered from important limitations. Further, it used very simple descriptions of the breakage function, with rather severe
simplifications, besides oversimplified descriptions ofparticle capture probability, which were back-calculated
from batch grinding data.
Both of these approaches considered that the distributionof particle fracture energies could be described using thelognormal distribution, which has been demonstrated elsewhere to be able to describe such data properly (King and
Bourgeois, 1993; Tavares and King, 1998). It is given by
Fi E
m

lnE m  lnE m50,i 11  erf 


2


2σ 2

(8)

where Em50,i is the median particle fracture energy and σ 2

112

is the variance. The variation of the median fracture energy of the material with particle size could be well described using the expression (Tavares and King, 1998)  d o  EE 1    m50,i  xi  

(9)where xi is the representative size of particles contained in
the ith class, E∞, do and ϕ are model parameters thatshould be fitted to experimental data. In some cases thevariance σ 2 in Eqn. 8 is also found to vary with particle
size and a relationship to describe it has been proposed
elsewhere (Carvalho and Tavares, 2013).
A related approach was used by Bwalya et al. (2001) topredict breakage rates in autogenous grinding. The authors
considered the distribution of fracture energies of the particles, described with a modified version of the Weichert(1992) breakage probability function derived from single
particle breakage in a drop weight tester and collision energies for short grinding times using 2D DEM. Breakage
was generally overpredicted compared to an experimental
mill, although the model correctly predicted the reductionin breakage rates as a function of particle size. With thisapproach they were also able to predict the non-first order
rates of breakage of coarse particles in a mill.
Another modification of the Weichert (1992) breakage
probability function that became very popular was proposed by Vogel and Peukert (2004):
Fi E m  
1  exp  f mat xi E m  E m,min (10)

where fmat and xiEm,min are constants representing material
parameters which should be fit to data, in which Em,min is
the threshold energy, below which particles will not suffer
body breakage.
Recently, Tuzcu and Rajamani (2011) proposed anotherexpression for predicting the energy-specific breakage
rates for powder in a batch mill on the basis of Eqn. 10,
givingn

S iE  k´ω p( E m,k ) Fi ( E m,k )
k 1

Ek
n

k 1Ek(11)

which would be used to solve the batch grinding equation
i 1
Mj
dM i
M

S iE i  S Ej bijE
H
H
dt
j 1

(12)

in which k' is a constant that includes the captured mass
and all other unmodeled effects (Tuzcu and Rajamani,2011), Fi(Em,k) is the probability of fracture of particles insize i at the applied specific collision energy Em,k, described using Eqn. 10. They were able to predict wellbreakage of coarse particles (top size of 31.8 mm) in a
0.9 m diameter batch mill, using Eqns. 11 and 12 and collision energy spectra from 2D DEM simulations.
Capece et al. (2014) proposed yet another method toLuís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

ticle does not vary if it was produced as a progeny of alow-energy or a high-energy stressing event.

calculate the breakage rates, also using Eqn. 10:Sinf mat xiω p( Em,k )(Em,k  Em,min )
k 1

(13)

where Em,k is the specific energy in the kth collision. The authors proposed to conduct batch grinding experiments withvarious narrow-size feeds, then 3D DEM simulations withgrinding media and powder particles of these sizes andthen fit the material parameters ( fmat, xEm,min) on the basisof Eqn. 13. The breakage function was back-calculated
from data of these batch grinding tests and Eqn. 2. The
authors were able to fit one set of material parameters toseveral experiments with alumina, which demonstrates
the relevance of their approach, which circumvents thechallenge of estimating the mass of material captured byincluding the powder particles in the simulations. It has,
however, limited predictive capability for finer powder
sizes, which are known to be broken in beds and not as
individual particles, and for which simulations would require significant computer effort. This approach has similarities to the one proposed by Concas et al. (2006).
Some of the assumptions in these formulations are:i. The approach of King and Bourgeois (1993) andCrespo (2011) assumed that particles are involved in eachcollision that happens in the mill. This means that eachcollision nips some material regardless of the location of
the collision. This is generally considered to be a valid assumption if the fractional mill filling of the media interstices is equal to 1 or higher. The approaches of Tuzcu
and Rajamani (2011) and Capece et al. (2014) do not require this assumption by including powder particles in thesimulations.
ii. Approaches of King and Bourgeois (1993), Crespo(2011) and Tuzcu and Rajamani (2011) did not account for
the fact that particles that do not break in the impacts become progressively weaker, which significantly affects the
chances of particle breakage (Bwalya et al., 2001; Tavares,
2009). As such, the model assumes that the fracture energy distribution of particles contained in a given sizeclass remains unchanged as grinding progresses.
iii. Essentially all approaches assume that breakage ofparticles due to abrasion (surface breakage) is negligible
and no contribution of shearing of particles during ballto-ball or ball-to-wall collisions exists.
iv. The contents of the mill are assumed to be perfectly
mixed, that is, the size distribution of the particle bed
caught between two colliding balls is the same as the prevailing distribution in the entire mill. This assumption isperceived to be valid for dry grinding where internal classification is not as prevalent as in wet grinding. Bwalya etal. (2001), on the other hand, assumed that the probabilityof a particle being involved in a collision in a tumblingmill is a function of particle surface area.
v. The fracture energy or breakage probability of a par-By using the breakage probability, these approaches
actually included an additional variable to particle size inthe population balance model formulation, which is theenergy required for (body) breakage, called particle fracture energy.

4.4 Microscale PBM formulations describing
different breakage mechanisms
A number of approaches have been proposed thattackle some of the limitations of these earlier models. The
commonality amongst them is the fact that they attempt
to describe in greater detail the response of the ore or
powder to the different fragmentation mechanisms, which
could be listed as:
1. Body breakage: the integrity of the original particle iscompletely lost;
2. Surface breakage, further described as:a. Attrition or abrasion: mass loss at the surface ofrounded rocks as other particles slide over them orthey slide over the liner, or as a result of predominantly normal low-energy collisions;b. Chipping: loss of corners, edges and larger asperities from small scale breakage for irregular shaped
particles;c. Rounding: preferential and higher abrasive massloss from sliding at the corners and edges ofblocky particles.
The occurrence of either depends on the intensity andmode of stressing. As such, the approaches reviewed inthis section also have in common the vision that, since thecontributions of the milling environment and of the ore
are appropriately decoupled, the same modeling framework can be used to describe different types of comminution machines, and not only tumbling ball mills.

4.4.1 Unified comminution modelThis model framework, proposed by Powell et al.
(2008) from the University of Cape Town and called unified comminution model (UCM), proposes modeling any
size reduction machine by coupling the mechanical environment in the size reduction equipment, obtained using
DEM, to data that describes the different fundamental
modes of breakage, and tools that characterize the influence of slurry in wet milling, namely CFD and SPH. It
employs the population balance framework to mathematically combine the particles and their associated breakage
products into a mass balance per time step. For a single
component (or material class) and batch operation, it canbe expressed as

113

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

4.4.2 Virtual comminution machine

N
S
dM i

λi (Ed )M i  λi ( E r )M i dt
d 1 r 1i 1

N λ j (Ed )M j (Ed )bij (Ed ) j 1 
d 1Sλ j (Er )M j (Er )bij (Er )

(14)

r 1

where λi(E) is the net breakage frequency at energy E for(E) / λexp
the size class i, given by λi (E)  λDEMii ( E ) , inwhich λDEM
( E ) is the DEM collision frequency at energyi
E for size class i and λexp
i (E) is the mean number of collisions required to break particles in size class i with collision energy E. N and S are, respectively, the number ofnormal and shear energy bins and Mi is the mass of material contained in size i in the mill.
The model recognizes that depending on the magnitudeof the collision energy on a particle three different outcomes could occur (Powell et al., 2008):
i.	Single hit (body) breakage—for a collision with sufficient energy to break the particle, so that the lower
limit for this is defined as Ecrit.
ii.	Multiple impact (body) breakage—the energy of asingle impact is too low to break the rock, however,as it is hit more times at this energy it has an increasing probability of breakage. As a result of this, themodel requires to track the histories of individual
particles (Powell and McBride, 2006).
iii.	No bulk damage—the impact energy is too low tocause any bulk damage to the rock. The upper limit
for this is defined as Eo.
The strength of the approach is its generality and thefact that it recognizes several of the important “pieces ofthe puzzle” that were missing in earlier approaches. However, the authors admitted that several voids existed that
limited the feasibility of applying the model at the time,
including the lack of information on apportioning the energy between colliding media, the left-over energy-input
power not accounted for in particle damage and the description of breakage of the sub-DEM material. Several
features of the model make it more aligned with requirements of modeling autogenous and semi-autogenous mills
than ball mills, since the three different outcomes listed
above may only be applicable to ore particles simulated in
DEM. It is worth noting that the original approach proposed to describe breakage of sub-DEM particles was acknowledged by the authors to be fairly crude, based onrelating energy dissipated in the collisions coupled to thestandard Bond ball mill method (Powell et al., 2008).
A first demonstration of a simplified form of the model
to calculate breakage rates of a full-scale ball mill has
been presented more recently (Powell et al., 2011), although it has not yet been validated for this type of mill.

114

Another approach recently proposed in this category is
the virtual comminution machine (VCM), by Morrison
and Cleary (2008) from the University of Queensland andthe CSIRO, respectively. Also meant to be a framework todeal with different size reduction processes, the VCM isthe one that has evolved the most in coupling DEM to a
description of the slurry flow and discharge classification,
given by smoothed particle hydrodynamics (SPH) (Cleary
et al., 2006). This approach also had the additional featureof providing a model to describe rounding by means of parameterised super-quadric particles (Cleary and Morrison,2016) and fines generation by attrition. Further, it also describes weakening by repeated impacts (called incremental breakage by the authors), using the model proposed byMorrison et al. (2007), which is another modification of
Eqn. 10 by Vogel and Peukert (2004), given bynFi (Em ) 
1  exp b¨(Em  fEo )
 k 1
(15)

If Em is lower than Eo, the contribution from (Em – Eo)is zero. b¨ is a material parameter which may contain theparticle size i. f is a parameter, characterizing the fractionof Eo at which cracks will stop propagating, which was
included in a recent revision of the model to improve fitting to data (Cleary and Morrison, 2016).
It is understood that the model predicts breakage of the
sub-DEM material using a population balance approach,although the equations used have not been published by
the authors. As such, it is unclear how particle capture of
sub-DEM particles is addressed, although recent work inwhich not only grinding media but also fine particles areincluded in the simulations sheds light into this (Clearyand Morrison, 2011). Indeed, the researchers recognizedthat developments that remained to be done included
dealing with the presence of fine material in ball mills(Weerasekara et al., 2013).
Many of the capabilities of the model are relatively recent and significant testing and validation is still required
to demonstrate the predictive capabilities of the VCM.
The developers recognize that good quality characterization of breakage behaviour of the specific rocks being
used by all the breakage mechanisms is required and that
the VCM is only as good as both its numerical model andore characterization components (Weerasekara et al., 2013).
Amongst the modelling approaches in the literature oftumbling mills, the VCM is the one that already describes
the greatest variety of outcomes from breakage listed in
the beginning of this section. Evidently, many of the features of the model, in particular the rounding capability,
are particularly suited for predicting size reduction in autogenous and semi-autogenous mills, but would also be
useful to predict ball milling with a coarse feed.

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

4.4.3 Other approaches

Another interesting approach proposed has been to predict breakage by coupling DEM to a tool, called discretegrain breakage (DGB) (Herbst and Potapov, 2004), andcomputational fluid dynamics (CFD) to describe theslurry flow in the mill. This approach, called High Fidelity Simulation (HFS), attempts to connect single-particle
breakage response to mill performance. It has also been
proposed as a general approach for predicting different
types of crushers and mills, but has been restricted in application to full-scale mills due to the very significant
computational effort associated to its use.
More recently, a refinement of the single-particle based
approach by King and Bourgeois (1993) was proposed by
the author and his research team at the Universidade Federal do Rio de Janeiro (UFRJ). First, the weakening by
repeated impacts and the breakage of particles at much
lower impact energies is incorporated in the model. In
analogy to the other models presented in this section, it
also describes surface and body breakage, using (body)breakage probability to distinguish between the two. Thismodel is presented in greater detail in the following section.
Finally, it is worth noting that the approach proposed
by Tuzcu and Rajamani (2011) also distinguished bodyand surface breakage mechanisms, although it described
the later in a fairly simplistic manner, assuming a constant proportion of fines produced by this mechanism as
grinding time progressed in a batch mill.

4.5 UFRJ mechanistic model
Following the footsteps of the work originally carriedout at the University of Utah during the early 1990s (Kingand Bourgeois, 1993), the research group at UFRJ proposed yet another DEM-based approach to model comminution machines. The model is based on the combinationof DEM that is used to describe the mechanical environment in the comminution machine, to a myriad of empirical and semi-empirical models that describe the different
breakage modes and that are fitted to particle breakage
data, and also to models that describe how sub-DEM particles are captured and how the collision energy is split
among particles when they are assembled in beds. These
pieces of the puzzle are then put together using two models: the microscale population balance model, whichkeeps track of the mass balance of particles in each sizeand ore composition class, and an expression that describes the variation of the distribution of particle fractureenergies inside the machine as comminution progresses,
called fracture energy convolution. This latter is based on
the continuum damage model of particle breakage (Tavares
and King, 2002) and the distribution of particle fractureenergies (Fig. 3). Both equations will vary according to

the mode of operation of the comminution device, if batchor continuous, and also according to the mixing conditions that prevail.
The model considers that when particles suffer an impact, some of them undergo catastrophic (body) breakage
and some do not. If stresses are insufficient to cause bodybreakage, they will undergo surface breakage (abrasion/
chipping) and will also become progressively weaker.
The batch grinding process equation can be derived
from a more general formulation of the traditional population balance model applied to the microscale descriptionof size reduction processes (Carvalho and Tavares, 2009).
The equation that describes the rates of changes in massof material contained in size class i is
dwi
dt

f
1
Ȧ ­°
®wi mi* E p E ³0 >1  bij eE @ Fi eE , t p e dedE
H °̄ ³0
i 1

f

1

0

0

 ¦w j ³ m *j E p E ³ bij eE F j eE, t p e dedE
j 1½°
 Di,s t  Ai,s t ¾
°¿

(16)

where H is the powder or ore load, also called mill
hold-up, wi is the mass fraction of particles contained insize class i in the mill at time t, ω is the frequency ofstressing events, p(E) is the distribution of stressing energies E in the mill, mi*( E) is the mass of particles containedin size class i captured in each collision event of magnitude E and p(e) is the function that represents the energy
split among particles nipped in a bed. bij(eE) is the bodybreakage function in density form, dependent on stressing
energy (Tavares and Carvalho, 2009) and Ai,s and Di,s arefunctions that characterize the appearance and disappearance due to surface breakage, omitted for brevity, andpresented elsewhere (Carvalho and Tavares, 2013). The
product eE is the fraction of the collision energy that isabsorbed by each particle captured in an impact event.
The body breakage function is described on the basisof expressions that account for the effect of particle sizeand stressing energy on the size distribution of the progeny fragments. The reduction in size of the progeny withincreasing applied energy has been described very successfully using the t10 -procedure. The size distribution
from impacts at a specific energy Em on particles contained in size class j is calculated by first estimating thefraction of material passing 1/10th of the original particle
size, called t10, using the expression (Tavares, 2009)E m 
t10, j  A 1  exp  b'E
m50b, j 



(17)

where A and b' are model parameters and Em50b,j is themedian mass-specific particle fracture energy of the par-115

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

ticles that are broken as a result of an impact of magnitude Em. When the collision energy is higher than thefracture energy of the toughest particle contained in sizeclass j, then Em50b,j = Em50,j, otherwise it should be calculated numerically (Tavares and Carvalho, 2009). The validity of Equation (17) has been demonstrated elsewhere(Tavares, 2009).
From the t10 value, calculated using Eqn. 17, it is thenpossible to estimate the proportions passing (tn values) ofdifferent fractions of the original parent size for each collision energy and particle size investigated. This is donewith the aid of a model that is based on the incomplete
beta function (Carvalho and Tavares, 2013). The cumulative breakage function is then calculated from Bij(Em) =interp(t10, tn).
In the case of batch operation, Eqn. 16 should be solved
simultaneously with the equation that describes how thefracture probability distribution of particles contained ineach size i varies with time, given by
Fi E,t  Δt 
Gi Fi* E, t  Δt  H i Fi E,0  I i Fi E, t
Gi  H i  I i(18)

where Gi is the fraction of material in the size class thathas been damaged but remained in the original size range,
Hi is the fraction that appeared due to body and surface
breakage, and Ii is the fraction that was not captured in thetime interval, with expressions given elsewhere (Carvalho
and Tavares, 2013).
The fracture probability distribution of the original
material Fi(E, 0) can be, generally, described well using
the lognormal distribution, given by Eqn. 8. In somecases, an upper truncation of the distribution is also necessary (Carvalho and Tavares, 2013). The median particle
fracture energy varies according to size through Eqn. 9,
and E50,i  Em50,i mp,i . The mean weight of a particle contained in size class i may be estimated by mp,i  ρβxi3,
where ρ is the material specific gravity and β the volume
shape factor.
Also, the standard deviation of the lognormal distribution of particle fracture energies (Eqn. 8) can vary withsize and the variation can be described using an expression analogous to Eqn. 9 (Carvalho and Tavares, 2013).
In Eqn. 18 Fi(E, t) is the distribution of fracture energies of the material contained in size class i that did not
suffer any impact event during the time interval, whereas
Fi*(E, t  Δt ) is the distribution of fracture energies of theparticles in size class i that were captured in a collision
event, but did not fracture, being given by the convolution
equationFi* E, t  Δt

116

Ei*









1  Fi E / (1  D), t  Fi eE' , t0 p(E' )0 

1  Fi (eE' , t)Ei*

0 p(E' )dE'

 p e dedE'

(19)

and
2γ

 2γ(1  D) eE'  5D (2γ  5D  5) E (20)

where Ei* is the maximum fracture energy of particles
contained in class i, which is equal to the value that satisfies Fi Ei*  1 as comminution progresses (Tavares andCarvalho, 2009) and γ is a parameter that characterizes
the amenability of the material to breakage by repeatedimpacts, which is typically independent of particle size
(Tavares, 2009). The model is able to account for the factthat particles that are stressed but do not fracture in a collision event will become progressively weaker. The model
does not have an explicit damage threshold, since theamount of damage sustained by particles becomes progressively smaller as the ratio between the collision energy (eE') and their fracture energy (E) approaches zero.
Illustration of application of Eqns. 18 to 20 to material
ground in a batch mill (Fig. 4) demonstrates the progression of the distribution of fracture energies of the remaining material in the top size range. It shows the
predominant effect of survival of the toughest particles,
along with the weakening of a fraction of the populationof the particles.
The mass of material captured (mi*) between two grind-

Fig. 4

Predicted variation of particle fracture energies of
9.5 × 6.3 mm granulite particles with grinding time ina 0.3 m diameter batch mill with 25 mm steel balls,
67 % of critical speed and 30 % mill filling. Reprinted
with permission from Tavares and Carvalho (2009).
Copyright: (2009) Elsevier Ltd.

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

ing media elements may be estimated from the product of
the number of particles caught between the two grinding
media (Ncap,i) and the average weight of the particles (mp,i),mi*  mp,i N cap,i.
Given the evidence from earlier work (Höfler, 1990),
that particles in an unconfined bed normally only breakwhen they are typically squeezed down to one or two layers between grinding media, particles are modeled as apacked monolayer bed. If particles had spherical shapes,
and if they were arranged according to a dense hexagonal
packing, then the number of particles captured as a function of radius from the center of impact could be estimated by (Barrios et al., 2011a)
N cap,i

1 3  2rc 

4 4  xi 1

2

for rc  xi / 2
for rc  xi / 2(21)

where rc is the radius of the bed, also called radius of capture, and xi is the mean size of the particles caught by thecolliding steel ball, estimated from the geometric mean ofsieves sizes containing the fraction.
A model has been proposed by Barrios et al. (2011a) to
calculate this radius of capture on the basis of diametersof colliding grinding media, size of particles, theirstrength and impact energies. An illustration of the effectof impact energy and particle size on the radius of capture
is presented in Fig. 5. This model has also been used byother researchers (Powell et al., 2011) to estimate masscaptured of sub-DEM material in mill simulations. The
validity of the model in describing the mass of material
captured and the energy distribution in the bed inside themilling environment, however, has not yet been demon-

Fig. 5

Radius of capture in the contact between balls of different sizes and a monolayer bed of particles contained in
the size range sitting on a flat anvil for different particle
sizes and impact energies. Symbols present experiments
and lines the model fit. Reprinted with permission from
Weerasekara et al. (2013). Copyright: (2013) ElsevierLtd.

strated, but is the subject of ongoing work in the author’slaboratory.
In the description of the energy split function in the bedp(e), it is considered that the stressing energy is split
equally among particles positioned in the bed, so thatp(e) = δ(e – 1/Ncap,i), where δ is the Dirac delta function.
However, another more complex expression has also been
proposed to describe the energy split in a more realisticway (Barrios et al., 2011a).
The size-discretized Equations (16) and (18) are solved
simultaneously at each time step using a finite difference
scheme. Time steps are typically in the order of 0.01 s, butshould be chosen to be as small as necessary to guaranteethat particles take part in no more than one collision eventin the interval.
The sensitivity of the model to the geometry of the impact makes it worthwhile to log separately collisions between each two elements in contact, forming a matrix ofprobability distributions of collision energies.
The model has demonstrated to be capable of predicting
non-first order breakage rates of coarse particles (Tavares
and Carvalho, 2009) and also of describing the effects ofseveral design and operating variables in milling in

Fig. 6

Simulated breakage rates for a copper ore as a functionof particle and steel ball size for a ball mill (top) andDEM median impact energy and collision frequency as
a function of ball size (bottom), where Mb is the massof the ball charge (30 % mill filling and 67 % of critical
speed in a 0.3 m diameter mill). Reprinted with permission from Carvalho and Tavares (2013). Copyright:(2013) Elsevier Ltd.

117

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

breakage rates (Carvalho and Tavares, 2013). The effectof ball size on specific breakage rates predicted using themodel is illustrated in Fig. 6, and shows that the model iscapable of accounting for the shift in the size corresponding to the maximum breakage rate with a change in ballsize, which has been widely documented in the literature(Austin et al., 1984; Katubilwa and Moys, 2009). Smallerball sizes produce less energetic impacts and each impact
captures fewer particles, being also less efficient at nipping coarser particles. Offsetting these effects that tend to
decrease the specific rate of breakage as ball size decreases
is the increased frequency of impacts that results from thelarger number of smaller balls in the mill, also shown in
the figure.
Additional work has demonstrated that grinding ofmulti-component mixtures in continuous mills can also be
simulated using the model (Carvalho and Tavares, 2009),
although the validity of the predictions has not yet been
demonstrated. Detailed model validation is underway inthe authors laboratory (Rodriguez, 2016), and Fig. 7 illustrates results that demonstrate that the model can predictbatch grinding well even at prolonged times.
Unlike the UCM and the VCM, the mechanistic millmodel describes single hit breakage and incremental
breakage with no distinction, since it accounts for the distribution of fracture energies of the material and uses
Eqn. 17 to characterize intensity of breakage in bothcases. As such, it is truly a two-variable model, since not
only sizes of particles change as grinding progresses, butalso their fracture energies.
Besides considering the assumptions i, iv, and v of theearlier single-particle based models (4.3), the UFRJ mechanistic mill model does not yet account for the interactionof particles of different sizes in the bed, in spite of evidences that this has been found relevant in the past
(Schönert, 1979).

Fig. 7118

UFRJ mechanistic model validation in dry batch grinding of granulite. Symbols are experimental data andlines model predictions (0.3 m mill, 40 mm steel balls,
30 % filling, 100 % interstitial filling, 67 % of critical
speed).

5. Discussion
5.1 Fracture probability distributions
Several approaches reviewed require information on
fracture probability distributions and their variation withparticle size. Different models (Tavares and King, 1998;
Vogel and Peukert, 2004; Rozenblat et al., 2012) havebeen proposed to describe this variation. Several authors(King and Bourgeois, 1993; Tavares and Carvalho, 2009;
Crespo, 2011) used the lognormal distribution (Eqn. 8)
and the relationship describing the variation of the medianfracture energy with size given by Eqn. 9, which havedemonstrated to be capable of describing material responseover a range of sizes (Tavares and King, 1998; Tavares
and Neves, 2008).
The approach proposed by Vogel and Peukert (2004)
and the Weibull distribution has attracted significant attention, being used, either directly or modified, in describing breakage probability in several approaches(Concas et al., 2006; Morrison and Cleary, 2008; Tuzcu
and Rajamani, 2011; Capece et al., 2014), in particularowing of its simplicity, with only two fitting parameters.
This model (Eqn. 10) identifies a threshold collision energy below which no body breakage would occur. Although the model has explicitly the number of impacts, sothat it can describe weakening by repeated impacts, amore appropriate modification has been proposed byMorrison et al. (2007) that more appropriately describes
this effect.
As such, given its popularity, it is important to analyze
the approach of Vogel and Peukert (2004) in greater detail. Solving Eqn. 10 considering the case in which particles are impacted at exactly their median fracture energy,
so that Fi(Em50) = 0.5 in Eqn. 10, gives  k'' E m50,i xi E m,min 1   
  xi (24)

where k'' = ln(2)/fmat xiEm,min.
The similarity between Equations (24) and (9) is evident, with the key difference that the median fracture energy is inversely proportional to particle size in Eqn. 24(–1 power), whereas it was found to vary, according tomaterial, from –0.8 to about –2.5 for geological materials(Tavares and King, 1998; Tavares and Neves, 2008). As
such, the model given by Eqns. 9 and 10 provides not only
a more realistic description of the variation of breakage
probability with size but also allows decoupling the variability in the breakage probability distributions from theeffect of size in the median fracture energy. It has, however, more parameters (four in total, in contrast to only 2
in Eqn. 10) that need to be fitted from experimental dataon breakage probability or fracture energy distributions
for a range of sizes. This data can be collected with reason-

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

able convenience at intermediate and coarse sizes (Danand Schubert, 1990; Tavares and King, 1998; Rozenblat etal., 2012) but no robust solution appeared yet to deal withbreakage of fine particles, although some alternatives
have been proposed (Marktscheffel and Schönert, 1986;
Barrios et al., 2011b; Ribas et al., 2014). Nevertheless,
Eqn. 10 can be a good compromise between complexity
and capability to describe reality for using in earlier
stages of development of advanced models of milling, inparticular when the model developer has little access todata on particle breakage probability.

5.2 Collision energies
As discussed earlier, at least three options exist in regard to the collision energies from DEM for advanced
mill simulations. It is, however, important to match the information on the collision that is obtained in DEM with theinformation from single-particle breakage (Powell et al.,2008). For instance, not all energy involved in a collision
is absorbed by individual particles and/or a bed, leading
to a rebound that could lead to additional breakage.
Several researchers (Datta and Rajamani, 2002; Tavares
and Carvalho, 2009; Capece et al., 2014) use the dissipated energies as the basis of their simulations. Using themodel approach and data by Datta and Rajamani (2002),
Wang et al. (2012) concluded that the kinetic energies in
the collision were the type of information that could mostreliably predict the batch mill performance. The use of thedissipated energy is considered valid in the UFRJ Mechanistic Model since the energies that are used to model
fracture probabilities (Eqn. 8) and energy-specific breakage function (Eqn. 18) already account only the absorbedby the particles (Tavares, 1999). However, the authors
recognize a mismatch can occur between the sum of dissipated energy and the total energy computed by the center of gravity of the charge, in particular as mill diameters
increase (Carvalho and Tavares, 2013).
Normal and shear components of the collisions contribute to breakage and should be considered in the model
predictions. Indeed, these appear explicitly in the UCM
equations (Powell et al., 2008) and are used actively todescribe the different outcomes of breakage as part of theVCM (Morrison and Cleary, 2016). Other authors (Datta
and Rajamani, 2002; Tuzcu and Rajamani, 2012) simply
group the two components and deal with the combined
spectrum. However, the actual roles of normal and shear
components in the different modes of breakage are not yet
entirely clear (Powell and McBride, 2006), in particularwhen dealing with surface breakage. Originally the UFRJ
model only used the normal components of the collision(Tavares and Carvalho, 2009), but the latest version of themodel, with an improved description of surface breakage,also includes the shear component (Rodriguez, 2016).

5.3 Other
Simulations using the discrete element method havebeen used to investigate mixing between grinding media
and ore charge. Cleary and Morrison (2011) included
powder at different proportions to investigate its effect on
the motion of charge and collision spectra in a laboratoryball mill. They showed that particles were not uniformly
distributed, but tended to concentrate at the shoulder region at very low powder fillings. At higher powder fillings, namely above 100 %, mixing between grinding
media and powder was good, thus justifying the approximation used in nearly all the advanced models. The only
model that does not explicitly consider this assumption is
the VCM (Morrison and Cleary, 2008), although outcomes from relaxing this assumption have not yet beenpresented.
One of the first observations from DEM simulationsof mills is that the frequency of high-energy impacts,
such as those reproduced in standard drop weight tests(Napier-Munn et al., 1996) is very small, shedding lightinto the importance of low-energy collisions and, therefore, to weakening by repeated impacts and to surface
breakage. Whereas the former has attracted significant attention (Tavares and King, 2002; Morrison et al., 2007),
descriptions of surface breakage are still in their infancy,
in particular if compared to volume breakage. In spite ofsome recent worthwhile attempts (Morrison and Cleary,
2008; Yahyei et al., 2015), a need exists to study this modeof breakage in greater detail in the future to properly inform advanced mill models. Indeed, it is important to recognize that surface breakage can be understood as either
the result of normal collisions that are of insufficient magnitude to cause body breakage (Tavares and Carvalho,2009), or to collisions that are predominantly of the shear
type, although this distinction is seldom made by researchers. The lack of proper descriptions and understanding of breakage by either pure or predominant shear
have so far prevented its implementation in the UFRJ
mechanistic model (Carvalho and Tavares, 2013).
Nearly all advanced approaches use some form of population balance framework to mathematically combine theparticles and their associated breakage products into a
mass balance per time step. Several of the approaches(King, 2001; Datta and Rajamani, 2002; Capece et al.,2014) use the linear population balance model equation,e.g., Eqn. 2 in the case of the batch mill, with the only
difference that breakage and selection functions are calculated on the basis of data from DEM simulations. Other
approaches (King and Bourgeois, 1993; Tavares andCarvalho, 2009) use a different formulation of the population balance model equations, called microscale population balance model, since breakage mechanisms andcollision energy spectra are convoluted in the equations.

119

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

Amongst the approaches that consider the breakage probability (fracture energies), only the UFRJ Mechanistic
Model (Tavares and Carvalho, 2009) addresses its change
as the result of both breakage of particles and weakening of
the unbroken particles. Indeed, this model can be, potentially, used to predict the fracture energies of particles thatleave the mill, which can impact downstream processes.
Models differ even at the most fundamental level ofdefinition of body breakage. Whereas several follow thedefinition used in all traditional population balance model
formulations of breakage as departure from the original
size range that contains the particle (Datta and Rajamani,
2002; Capece et al., 2014), other formulations (King and
Bourgeois, 1993; Tavares and Carvalho, 2009) consider
breakage as the loss of a substantial part of the particle
weight, taken arbitrarily as 10 % (Dan and Schubert,
1990). As such, it accounts for the fact that when a particle suffers body breakage, fragments from it can remainin the original size range.
It is evident from this review that very important advances have been made, in particular in the last 10 years,
in advanced ball mill modeling. The approaches that areprogressing further are those that account for breakage
probability and also that distinguish the different breakage mechanisms. However, important challenges remain,in particular related to connecting single particle breakage response at fine sizes to milling response, both batchand continuous, and to modeling surface breakage. It is
the opinion of the author that it will reach maturity in thenext several years, following on the footsteps of other already mature applications of DEM, such as in transfer
chute design and liner design for preventing ultraprojection of grinding media.

6. Conclusions• Several approaches have been proposed since the late1980s to describe beyond mean breakage rates, breakage functions and specific comminution energy, size
reduction in tumbling mills.
• Since the 1990s these approaches evolved significantly,
given the contribution of the discrete element method(DEM) in describing the mechanical environment inmills.
• In spite of the increase in computing power, modelingball mills is still better described considering only thegrinding charge and not the powder in the DEM simulations, relaying on post-processing the data using a proper
population balance formulation to predict grinding.
• Several of the initial approaches in advanced modeling
relied exclusively on particle bed data to describe
breakage. In spite of their simplicity, they became lesspopular given the lack of description of breakage prob-

120

ability and also the difficulty of representing all combinations that would be found in practice.
• Models that rely on single particle breakage and (body)breakage probability have gained popularity in recent
years and were pioneered by the work of King and
Bourgeois (1993). The approaches differ by the expressions used to describe breakage probability. Several ofthese models have adopted the formulation of Vogel
and Peukert (2004), although more flexible alternatives
exist.
• A limited number of approaches decouple body andsurface breakage, and only one approach (VCM) describes explicitly rounding of coarse particles due tosurface breakage, which could be relevant in ball milling of oversized feeds.
• A very limited number of approaches account for
weakening by repeated unsuccessful collisions.
• The approach proposed in the author’s laboratory describes how breakage probability varies due to breakage and weakening of the particles. This approach also
includes sub-models that successfully describe the different mechanisms (body and surface breakage, but notrounding), as well as the mass of powder captured ineach collision, besides efficient solution algorithms thathave been used to predict grinding over a range of conditions. As such, this is the only two-variable model, in
which the material can change not only its size class,but also its fracture energy as grinding progresses.
• The important progress in recent years suggests advanced models of ball mills will reach some level ofmaturity in upcoming years.

NomenclatureA

model parameter of Eqn. 17

Ai,sfunction describing birth by surface breakageb'model parameter of Eqn. 17b''parameter of Eqn. 15

Bijcumulative body breakage function

bij

body breakage function in density form

Bij(Em,k) cumulative energy-specific breakage function for jth
size intervalbij(Em,k) energy-specific breakage function in density form forjth size intervalDdamage sustained by a particle

Di,sdisappearance function for surface breakage

doparameter in Eqn. 9evariable representing the fraction of the collisionenergy absorbed by a particle

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

E

collision energyk''constant in Eqn. 24

E'collision energy (integration variable)Mbmass of the ball chargeEcritlowest magnitude of collision energy required to breaka particle

mbmass of one grinding medium or reduced mass in acollisionEi*maximum fracture energy of particles contained inclass imb.imass of ith ball in a collisionEk

impact energy in kth collision or mean collisionenergy in kth classMi

mass of particles in the ith size interval

mi(E)

Em

specific collision energy

broken mass of particles in size class i resulting froman impact of magnitude E

Emmean specific comminution energy

mi*( E)

mass of particles contained in size class i captured ineach stressing event of magnitude EEm,kspecific energy in the kth collision or mean collisionenergy in kth classmp,imean weight of a particle contained in size class i

Em,minparameter in Eqn. 10, which represents the thresholdspecific energy for body breakage

Nnumber of normal collision energy binsnnumber of collision energy bins

Em50,imedian mass-specific particle fracture energy of particles in size class i

Ncap,inumber of particles contained in size class i captured
in a collision event

Em50b,imedian fracture energy of particles in size class i thatare broken as a result of impact of magnitude Em

Pmill power predicted by DEM

P(E)cumulative distribution of collision energies

En

normal portion of the collision energyp(E)distribution of collision energies in density formEoparameter of Eqn. 15, representing the energy below
which particles do not fracturep(e)energy split among particles in each collisionerf

error functionp(Em,k) distribution of specific collision energies in density
form

Es

shear portion of the collision energyrcradius of capture of the bed

E50,imedian particle fracture energy of particles in sizeclass iS

number of shear collision energy bins

E∞parameter in Eqn. 9, representing the median fractureenergy of infinitely coarse particles

Si

selection or breakage rate of particles in size i

S iEenergy-specific selection functionfparameter of Eqn. 15, characterizing the fraction of Eo
in which cracks will stop propagating

t

time

tcontact

total contact time during a collision

Fi(E,0) fracture probability distribution of the original material in size class itn

fraction of material body broken to 1/nth of the parentsize

Fi(Em)

fracture probability distribution of particles containedin size i at a specific collision energy Em

t10

fraction of material body broken to 1/10th of the parentsize

Fi* E, t distribution of fracture energies of the particles in sizeclass i that were captured in an impact event, but did
not fracturevivelocity of grinding medium i in a collisionvij

relative velocity of grinding media i and j

fmat

parameter in Eqn. 10

wi(t)

Fn

normal component of the contact force

mass fraction of particles contained in size class i in
the mill at time t

Fs

shear component of the contact forcexparticle sizeGifraction of material in size class i that has been damaged but remained in the original size range

ximean size of particles in size class iH

powder or ore load, also called mill hold-up

Greek symbols

Hi

fraction of material that appeared in size class i due tobody and surface breakage

βvolume shape factorIifraction of material that in size class i was not captured in the time intervalγdamage accumulation parameter (Eqn. 20)

δ

Dirac delta function

k'constant in Eqn. 11θmean residence time121

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

λi(E)net breakage frequency at energy E for the size class i

λDEM( E) DEM collision frequency at energy E for size class ii
λexpi (E )

mean number of collisions required to break the particle with collision energy Eξdeformation of media during a collisionρ

specific gravity

σstandard deviation of the log-normal distribution ofparticle fracture energy (Eqn. 8)

ϕparameter in Eqn. 9

ωfrequency of collisions

References
Austin L.G., Klimpel R.R., Luckie P.T., Process engineering ofsize reduction, AIME, New York, 1984.
Barrios G.K.P., Carvalho R.M. de Tavares L.M., Modelingbreakage of monodispersed particles in unconfined beds,
Minerals Engineering, 24 (2011a) 308–318.
Barrios G.K.P., Carvalho R.M. de Tavares L.M., Extending
breakage characterisation to fine sizes by impact on particle
beds, Transactions of the Institution of Mining and Metallurgy C, 120 (2011b) 37–44.
Barrios G.K.P., Tavares L.M., A preliminary model of highpressure roll grinding using the discrete element method
and multi-body dynamics coupling, International Journal of
Mineral Processing (2016), doi: 10.1016/j.minpro.2016.06.
009.
Bond F.C., The third theory of comminution, Transactions of
AIME, 193 (1952) 484–494.
Bwalya M.M., Moys M.H., Hinde A.L., The use of discrete element method and fracture mechanics to improve grinding
rate prediction, Minerals Engineering, 14 (2001) 565–573.
Capece M., Bilgili E., Davé R.N., Formulation of a physicallymotivated specific breakage rate parameter for ball milling
via the discrete element method, AiChe Journal, 60 (2014)
2404–2415.
Carvalho R.M., Tavares L.M., Dynamic modeling of comminution using a general microscale breakage model. Computer-­Aided Chemical Engineering, 27 (2009) 519–524.
Carvalho R.M., Tavares L.M., Predicting the effect of operating
and design variables on breakage rates using the mechanistic ball mill model, Minerals Engineering, 43–44 (2013)
91–101.
Chagas A.S., Carvalho R.M., Bbosa L., Mainza A., Tavares
L.M., Calibration of DEM contact parameters for tumblingmills using PEPT. Proceedings, 14th European Symposiumon Comminution and Classification, Gothenburg, 2015, pp.
1–6.
Charles R.J., Energy-size reduction relationships in comminution, Transactions of the AIME, 208 (1957) 80–88.
Cho H., Breakage mechanisms in size reduction, Ph.D. thesis,
University of Utah, 1987.
Cleary P.W., Predicting charge motion, power draw, segregation

122

wear and particle breakage in ball mills using discrete element methods, Minerals Engineering, 11 (1998) 1061–1080.
Cleary P.W., Recent advances in DEM modeling of tumblingmills, Minerals Engineering, 14 (2001) 1295–1319.
Cleary P.W., Sinnott M., Morrison R.D., Prediction of slurrytransport in SAG mills using SPH fluid flow in a dynamic
DEM based porous media, Minerals Engineering, 19 (2006)
1517–1527.
Cleary P.W., Morrison R.D., Understanding fine ore breakage ina laboratory scale ball mill using DEM, Minerals Engineering, 24 (2011) 352–366.
Cleary P.W., Morrison R.D., Comminution mechanisms, particle
shape evolution and collision energy partitioning in tumbling mills, Minerals Engineering, 86 (2016) 75–95.
Cleary P.W., Sinnott M.D., Simulation of particle flows andbreakage in crushers using DEM: Part 1—Compression
crushers, Minerals Engineering, 74 (2015) 178–197.
Concas A., Lai N., Psiu M., Cao G., Modelling of comminution
processes in Spex mixer/mil, Chemical Engineering Science, 61 (2006) 3746–3760.
Crespo E.F., Application of particle fracture energy distributions to ball milling kinetics, Powder Technology, 210(2011) 281–287.
Dan C.C., Schubert H., Breakage probability, progeny size distribution and energy utilization of comminution by impact,
Aufbereitungs-Technik, 31 (1990) 241–247.
Data A., Rajamani R.K., A direct approach of modeling batch
grinding in ball mills using population balance principles
and impact energy distribution, International Journal of
Mineral Processing, 64 (2002) 181–200.
Delaney G.W., Cleary P.W., Morrison R., Loveday B., Predicting
breakage and the evolution of rock size and shape distributions in Ag and SAG mills using DEM, Minerals Engineering, 50–51 (2013) 132–139.
Govender I., Cleary P.W., Mainza A.N., Comparisons of PEPT
derived charge features in wet milling environments with afriction-adjusted DEM model, Chemical Engineering Science, 97 (2013) 162–175.
Herbst J.A., A microscale look at tumbling mill scale-up using
high fidelity simulation, International Journal of Mineral
Processing, 74S (2004) S299–S306.
Herbst J.A., Fuerstenau D.W., Scale-up procedure for continuous grinding mill design using population balance models,International Journal of Mineral Processing, 7 (1980) 1–31.
Herbst J.A., Potapov A.V., Making a discrete grain breakage
model practical for comminution equipment performance
simulation, Powder Technology, 143–144 (2004) 145–150.
Höfler A., Fundamental breakage studies of mineral particles
with an ultrafast load cell device, Ph.D. thesis, Universityof Utah, 1990.
Höfler A., Herbst J.A., Ball mill modeling through microscale
fragmentation studies: fully monitored particle bed comminution versus single impact tests, Proceedings, 7th European Symposium on Comminution, Ljubluana, 1990, v. 1,
pp. 381–397.
Kano J., Saito F., Correlation of powder characteristics of talc
during planetary ball milling with the impact energy of theballs simulated by the particle element method, Powder

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124

Technology, 98 (1998) 166–170.
Katubilwa F.M, Moys M.H., Effect of ball size distribution on
milling rate, Minerals Engineering, 22 (2009) 1283–1288.
Kick F., Des Gesetz der Proportionalem wider-stand und Seine
Anwendung, Felix, Leipzig, 1885.
King R.P., Modeling and simulation of mineral processing systems, Butterworth-Heinemann, 2001.
King R.P., Bourgeois F.S., A new conceptual model for ball
milling. Proceedings, 18th International Mineral Processing
Congress, Sydney, 1993, pp. 81–86.
Kwan C., Mio H., Chen Y., Ding Y., Saito F., Papadopoulos D.,
Bentham A., Ghadiri M., Analysis of the milling rate ofpharmaceutical powders using the distinct element method(DEM), Chemical Engineering Science, 60 (2005) 1441–
1448.
Lee L., Cho H., Kwon J., Using the discrete element method toanalyze the breakage rate in a centrifugal/vibration mill,Powder Technology, 198 (2010) 364–372.
Marktscheffel M., Schönert K., Liberation of composite
particles by single particle compression, shear and impact
loading, Proceedings, 6th European Symposium on Comminution, Nuremberg, 1986.
Martins S., Li W., Radziszewski P., Faucher A., Makni S.,
Experimental and simulated ball in a tumbling mill—acomparison, Minerals Engineering, 43–44 (2012) 79–94.
Mayank K., Narasimha M., Govender I., Two way coupled
CFD-DEM model to predict tumbling mil dynanics, Proceedings, International Semi-Autogenous Grinding andHigh-Pressure Roll Grinding Technology, Vancouver, 2015,
paper #28.
Metzger M.J., Glasser B.J., Simulation of the breakage ofbonded agglomerates in a ball mill, Powder Technology,
237 (2013) 286–302.
Mishra B.K., Murty C.V.R., On the determination of contact
parameters for realistic DEM simulations of ball mills,
Powder Technology, 115 (2001) 290–297.
Mishra B.K., Rajamani R.K. The discrete element method forthe simulation of ball mills, Applied Mathematical Modeling, 16 (1992) 598–604.
Morrell S., An alternative energy–size relationship to that proposed by Bond for the design and optimisation of grinding
circuits, International Journal of Mineral Processing, 74(2004) 133–141.
Mori H., Mio H., Kano K., Saito F., Ball mill simulation in wet
grinding using a tumbling mill and its correlation to grinding rate, Powder Technology, 143–144 (2004) 230–239.
Morison R.D., Cleary P.W., Towards a virtual comminution
machine, Minerals Engineering, 21 (2008) 770–781.
Morrison R.D., Shi F., Whyte R., Modelling of incremental rockbreakage by impact—For use in DEM models, Minerals
Engineering, 20 (2007) 303–309.
Moys M.H., van Nierop M.A., van Tonder J.C., Glover G., Validation of the discrete element method (DEM) by comparing
predicted load behavior to a grinding mill with measured
data, Developments in Mineral Processing, Elsevier, 2000,
v. 13, pg. C3-39–C3-44.
Napier-Munn T.J., Morrell S., Morrison R.D., Kojovic T., Mineral Comminution Circuits: Their Operation and Optimiza-tion. JKMRC Monograph Series in Mining and Mineral
Processing., Brisbane, 1996.
Nomura S., Hosada K., Tanada T., An analysis of selection
function for mills using balls as grinding media. PowerTechnology, 68 (1991) 1–12.
O’Sullivan C., Particulate discrete element modelling: a geomechanics perspective. Spon Press, v. 4, 2011.
Pauw O.G., Maré M.S., The determination of optimum
impact-breakage routes for an ore, Powder Technology, 54(1988) 3–13.
Potyondy D.O, Cundall P.A., A bonded-particle model for rock,
International Journal of Rock Mechanics and Mining Sciences, 41 (2004) 1329–1364.
Powell M.S., Govender I., McBride A.T., Applying DEM outputs to the unified comminution model, Minerals Engineering, 21 (2008) 744–750.
Powell M.S., McBride A.T., What is required for DEM simulations to model breakage in mills? Minerals Engineering, 19(2006) 1013–1021.
Powell M.S., Weerasekara N.S., Cole S., LaRoche R.D., FavierJ., DEM modeling of liner evolution and its influence on
grinding rate in ball mills, Minerals Engineering, 24 (2011)341–351.
Quist J., Evertsson C.M., Cone crusher modelling and simulation using DEM, Minerals Engineering, 85 (2016) 92–105.
Rajamani R.K, Agrawala S., Mishra B.K., Mill scaleup: ballcollision frequency and collision energy density in laboratory and plant-scale mills, Proceedings, 18th International
Mineral Processing Congress, Sydney, 1993, pp. 103–107.
Ribas L., Cordeiro G.C., Toledo Filho R.D., Tavares L.M.
Measuring the strength of irregularly-shaped fine particles
in a microcompression tester, Minerals Engineering, 65(2014) 149–155.
Rodriguez V.A., Validation of the UFRJ mechanistic ball millmodel, M.S. Thesis, Universidade Federal do Rio deJaneiro, 2016 (in Portuguese).
Rowland C.A., Kjos D.M., Rod and ball mills, in: Mular A.L.,
Bhappu R.B. (Eds.) Mineral Processing Plant Design,
AIME, 1980, pp. 239–278.
Rozenblat Y., Grant E., Levy, Kalman H., Tomas J., Selection
and breakage functions of particles under impact loads,
Chemical Engineering Science, 71 (2012) 56–66.
Schönert K., Aspects of physics of breakage relevant to comminution, Proceedings of the 4th Tewksbury Symposium, Melbourne, 1979, pp. 3.1–3.9.
Tavares L.M., Energy absorbed in breakage of single particles
in drop weight testing, Minerals Engineering, 12 (1999)
43–50.
Tavares L.M., Optimum routes for particle breakage by impact,
Powder Technology, 142 (2004) 81–91.
Tavares L.M., Breakage of single particles: quasi-static, in:Salman A.D., Ghadiri M., Hounslow M.J. (Eds.), Handbookof Particle Breakage, Elsevier, 2007, v. 12, pp. 3–68.
Tavares L.M., Analysis of particle fracture by repeated loading
as damage accumulation, Powder Technology, 190 (2009)
327–339.
Tavares L.M., Carvalho R.M., Modeling breakage rates ofcoarse particles in ball mills, Minerals Engineering, 22123

Luís Marcelo Tavares / KONA Powder and Particle Journal No. 34 (2017) 106–124(2009) 650–659.
Tavares L.M., King R.P., Single-particle fracture under impact
loading, International Journal of Mineral Processing, 54(1998) 1–28.
Tavares L.M., King R.P., Modeling particle breakage by
repeated impacts using continuum damage mechanics,
Powder Technology, 123 (2002) 138–146.
Tavares L.M., Neves P.B., Microstructure of quarry rocks andrelationships to particle breakage and crushing, International Journal of Mineral Processing, 87 (2008) 28–41.
Tuzcu E.T., Rajamani R.K., Modeling breakage rates in mills
with impact energy spectra and ultra fast load cell data,
Minerals Engineering, 24 (2011) 252–260.
Venugopal R., Rajamani R.K, 3D simulation of media motion intumbling mills by the discrete element method, Powder
Technology (2001) 157–166.
Vogel L., Peukert W., Determination of material properties relevant to grinding by practicable labscale milling tests, International Journal of Mineral Processing, 74S (2004) S329–
338.

Von Rittinger P.R., Lehrbuch der Aufbereitungs Kunde, Ernst
and Korn, Berlin, 1867.
Wang M.H., Yang R.Y., Yu A.B., DEM investigation of energy
distribution and particle breakage in tumbling ball mills,
Powder Technology, 223 (2012) 83–91.
Weerasekara N.S., Powell M.S., Cleary P.S., Tavares L.M.,
Evertsson M., Morrison R.D., Quist J., Carvalho R.M., The
contribution of DEM to the science of comminution, Powder Technology, 248 (2013) 3–24.
Weichert R., Application of defect statistics and fracture
mechanics for describing comminution processes, ZementKalk-Gips 45 (1992), 51–57.
Yahyei M., Weerasekara N.S., Powell M.S., Characterization ofsuperficial breakage using multi-size pilot mills, Minerals
Engineering, 81 (2015) 71–78.
Yashima S., Hashimoto H., Kanda Y., Sano S., Measurement ofkinetic energy of grinding media in a tumbling ball mill.
Proceedings of the 16th International Mineral Processing
Congress, Stockholm, 1988, pp. 299–309.

Author’s short biography
Luís Marcelo Tavares
Luís Marcelo Tavares is a Professor at the Department of Metallurgical and Materials
Engineering of Universidade Federal do Rio de Janeiro. He received his bachelor’s degree in Mining Engineering (honours) and his master’s degree from Universidade Federal do Rio Grande do Sul. In 1997 he was awarded a Ph.D. degree in Extractive
Metallurgy at the University of Utah under the supervision of the late Professor R.P.
King. He has been a member of the faculty of the Universidade Federal do Rio de Janeiro(Brazil) since 1998, where he is head of the Laboratório de Tecnologia Mineral and has
been Department chairman. His research interests include advanced models of comminution, particle breakage, physical concentration, classification, iron ore processing anddevelopment of pozzolanic materials. He is a founding member of the Global Comminution Collaborative (GCC) and has received a number of awards from the Brazilian Association of Metallurgy, Materials and Mining (ABM).

124

KONA Powder and Particle Journal No. 34 (2017) 125–140/Doi:10.14356/kona.2017017

Review Paper

A Review of the Modeling of High Pressure Grinding Rolls †Samira Rashidi 1, Raj K. Rajamani 1* and Douglas W. Fuerstenau 2
1
2

Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, USA
Hearst Memorial Mining Building,University of California, USA

Abstract
High Pressure Grinding Rolls (HPGR) technology is accepted as an energy-efficient and cost-effective alternative
for treating specific mineral ore types. HPGR technology has been advancing within manufacturing facilities andresearch centers since its first installation in 1985. Over the last three decades much of the literature on HPGR
have focused on the industrial applications and trade-off studies in comparison with semi-autogenous and ball
milling circuits. Literature on fundamental studies of HPGR technology has been very limited. This paper aims
to provide a review of the modeling of high pressure grinding rolls.
Keywords: HPGR, modeling, interparticle breakage, particles compression, comminution

1. IntroductionMore than thirty years have passed since the first industrial installation of high pressure grinding rolls(HPGR). Through this time, HPGR technology has advanced on many technical fronts. Nowadays, it is accepted as a standard technology for certain types of ore
materials (Battersby et al., 1992; Patzelt, 1992).
The development of HPGR technology originated fromthe fundamental studies that the late Prof. Schönert,
the inventor of HPGR, and his colleagues performed oninterparticle breakage (Schönert, 1985, 1988). From his
extensive research on single-particle breakage under compression and impact loading, Schönert (1988) concluded
that the highest efficiency for particle breakage is only
possible through slow compression loading of single particles. Schönert then extended his research to the breakage of beds of particles in a piston-die press. He observedthat confined particle-bed breakage was less energy efficient than single-particle breakage yet was far more efficient than tumbling mill grinding (See Fig. 1). The
dominant factors affecting the energy efficiency of theparticle-bed stressing in a confined environment include
the form of energy distribution to particles and the particle/particle interaction (Abouzeid and Fuerstenau, 2009).
Following this finding, the same results obtained inparticle-bed loading could be repeated in a continuous
†
1
2

*

Received 25 September 2015; Accepted 9 June 2016
J-STAGE Advance Publication online 30 August 2016
Salt Lake City, UT 84112, USA
Berkeley, CA 94720, USA
Corresponding author: Raj K. Rajamani;
E-mail: raj.rajamani@utah.edu
TEL: +1-801-581-3107 FAX: +1-801-581-4937

mode with a small-scale HPGR. This led to a patent being
issued to Schönert in 1977 (German Patent Number DE2708053). In an oral history interview, Schönert statedthat his patent was for a process and not for a machine1).
Schönert’s patent was for a process that compressed a bedof particles to a certain pressure that may require de-­agglomeration of the product. He immediately began
negotiations for licensing his patent to two German manufacturers, Polysius (currently known as thyssenkrupp
Industrial Solutions) and KHD, to produce the required
roller mills for industrial applications. Manufacturing an
industrial-scale machine that would operate under the
specific conditions of dry grinding circuits, found particularly in cement plants, proved to require extensive designand development. One of the challenges that led bothcompanies to expend a substantial amount of effort was
resolving how to handle wear rate on the rolls. Later,
Köppern, another German manufacturer that had developed the technology of roller presses for the briquetting
process, further contributed in promoting the HPGR technology. Briquetting machines are used for the compactionof granular materials that may share a few similarities
with HPGR machines.

Fig. 1The order of energy efficiency for different modes ofbreakage.

1)

Schönert’s interview is published in the appendix on pages 599 to
629 of Fuerstenau’s oral history (Fuerstenau, 2011).

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

125Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

At this point it might do well to comment on the terminology that has been used to designate high compression
particle-bed comminution. When publishing in English,
Schönert tended to call the technology either “high compression roller mills” or “high-pressure roller mills”,
HPRM. Other designations were also used for the highpressure grinding technology with respect to a wide range
of applications from coarse crushing to very fine grinding, such as ‘high pressure roller crusher’, HPRC. The generic term that has been widely settled upon is HPGR,
high pressure grinding rolls, and is used in this paper aswell. It should be noted that each manufacturer has its
own registered trade name for their particular HPGR.
Quite rapidly, HPGRs were widely installed in cement
plants throughout Germany and Europe. Schönert statedthat he believed the rapid installations of energy-efficient
HPGRs were due to the high cost of electrical power inGermany (Fuerstenau, 2011). The first major applicationof HGPRs by the mining industry was at the diamondmines of DeBeers in South Africa. The motivation in thiscase was the enhanced liberation of diamonds with HPGRs (Fuerstenau, 2011). The strong development and design efforts by the manufacturers led to an increasing
number of installations in a wider range of industries.
The rapid utilization of HPGR technology in industrial
applications led to publications that mainly presented
general information about new installations, benefits
achieved, as well as challenges. However, there were very
few publications by manufacturers at the early stage ofdevelopment that discussed the scale-up of HPGRs. Thatsaid, Schönert and his students at the Technical University of Clausthal were actively involved in the fundamental research that helped delineate interparticle breakage
phenomena in a HPGR through systematic studies with apiston-die apparatus and a 200 mm diameter by 100 mm
length lab-scale HPGR (Otte, 1988; Schönert, 1988, 1991;
Schönert and Lubjuhn, 1990; Norgate and Weller, 1991;
Lubjuhn and Schönert, 1993; Reichardt and Schönert,
2003). Schönert also developed fundamental relationships
for defining the scale-up factors for the design of HPGR
units. Additionally, by capturing high-speed videos,
Schönert investigated the behavior of particle beds insidea HPGR (Schönert, 1991; Lubjuhn and Schönert, 1993).
By the early 1990s, Fuerstenau and his research teamfocused on this energy-efficient grinding technology byutilizing a single-particle roll mill as well as a 200 mm by
100 mm high pressure grinding rolls at the University ofCalifornia, Berkeley, USA. They studied the effect of
HPGR operating parameters and developed fundamental
models to simulate high pressure breakage phenomena.
(Fuerstenau et al., 1991; Fuerstenau et al., 1993; Fuerstenau
et al., 1996; Fuerstenau and Abouzeid, 2007). As accessto laboratory-scale HPGRs became prevalent, more research was dedicated to studying various aspects of high126

pressure grinding performance and attendant models(Austin et al., 1993; Lim et al., 1996b; Rashidi, 2014).
Although the majority of research about HPGR technology is limited to lab-scale units due to the large capacity of these machines, Klein and his research team at the
University of British Columbia have been able to demonstrate interesting pilot-scale results through running tests
in a 750 mm by 220 mm HPGR unit manufactured by
Köppern (Drozdiak et al., 2011; Nadolski et al., 2011;
Rosario et al., 2011). Daniel (2002) also obtained valuable
information about HPGR scale-up through extensive tests
using different sizes of HPGR machines.
As HPGR became accepted as a standard grinding
technology, much of the research was generally directed
toward establishing dimensionless numbers for characterizing grinding operations (Klymowsky and Liu, 1997;
Daniel, 2003; Mcken and Williams, 2006; Morrell, 2006).
Some research was also focused on applying discrete element modeling (DEM) to high pressure grinding techniques (Djordjevic and Morrison, 2006; Barrios et al.,2014). Even though DEM models are computationallydemanding, they provide visual insight into the nature ofparticle breakage in the gap between the rolls.
Overall, HPGR is not yet considered mature in terms ofmodeling. In fact, modeling publications tend to be limited and scattered without proper verification. That being
said however, there has not been a high demand within
the industry for additional investigation and research on
HPGR modeling. Rather, industrial publications showthat the current focus is on developing new applicationsfor HPGR technology as well as alternatives for improving the well-accepted layouts of this technology (Burchardt
et al., 2011; Van der Meer, 2011).
This paper aims to review the evolution of the modeling since the introduction of this technology. We emphasize only those models that have the potential for further
development in the future.

2. Principle of high pressure grinding
The energy efficiency of comminution methods is determined by the mobility or confinement of the particles
being comminuted (Fuerstenau and Kapur, 1995).
Schönert (1996) clearly showed that the most energy-­
efficient method of breakage is the direct application offorce to a single particle or at least a monolayer of particles. The breakage of particles in a confined bed is thesecond most energy-efficient method of comminution, atleast at low reduction ratios (Gutsche, 1993). As exemplified by ball milling, loose-bed comminution has the lowest efficiency, which is due to the hit-and-miss mode ofenergy transfer during particle/ball impacts. Schönert
quickly realized the opportunity of increasing the energy

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

efficiency of comminution by employing particle-bed
breakage.
The efficiency of this method comes from the directtransfer of energy to particles and the highly localized interparticle stresses that are generated at the contact pointsof each particle in the bed (De, 1995). In other words,
particles are stressed by each other provided that the average size of the largest particle is smaller than the widthand height of the bed and the applied force is higher thanthe particle strength (Schönert, 1988, 1991). During compression of the particle bed, smaller particles fill the void
space and thereby participate in the transfer of energy andthe breaking and re-breaking of larger particles to a certain fineness (Klymowsky, 2009). This mode of breakage
requires fewer particles to be in contact with the grinding
medium, that is, the roll surface (Klymowsky, 2009).
There is, however, a negative aspect of particle-bed
grinding, namely a retardation effect. Grinding virtuallycomes to a halt in highly compressed beds at sufficiently
high pressure. This was shown by Fuerstenau et al. (1996)
in a detailed study of confined particle-bed breakage ofbrittle materials whilst using a piston-die apparatus. It
seems that the presence of a sufficient amount of fines inthe bed and the buildup of an isostatic-like pressure field
can lead to a near cessation of particle breakage, as illustrated in Fig. 2. Schönert and Mueller (1990) developed
an expression that gives the fraction of narrowly-sized
feed particles that are broken as a function of compactionenergy, including a maximum fraction that can be broken.
For efficient HPGR comminution, this points to operating
HPGRs at low reduction ratios and to the removal of fines
in recycle systems if the HPGR is to produce a finishedproduct.
A system consisting of rolls that operates at high pressure was conceived by Schönert to translate the breakage
of a confined particle-bed into a continuous form. The
three key criteria for this system include direct transfer ofparticles inside the machine, direct discharge of the product, and the capability of adjusting the level of the applied
force and thereby the energy input (Schönert, 1991). The
basic principle is similar to briquetting machines that
operate with pressures low enough to compact the particles without necessarily increasing the portion of fines(Schewechten and Mulburn, 1990). When it comes to particle bed comminution however, a high grinding pressure
is required in order to compress the bed of particles to abulk specific gravity of 70 to 85 percent of the ore’s specific gravity, which will cause the particles to break andthereby generate fines in the product (Schönert, 1988).
An HPGR machine consists of a pair of counter-­rotating rolls, which are mounted in a very sturdy frame.
While the bearing block of one roll is fixed, the other one
can float linearly to adjust the gap between the two rolls
as the material properties, such as particle size distribu-

Fig. 2 The isostatic-like pressure field in confined particle-bed
breakage shown in piston-die tests (Adapted from
Gutsche, 1993).

tion, moisture, and hardness, change. A hydro-pneumatic
spring system applies the grinding force directly on theparticle bed via the floating roll. Larger gap openings result in higher grinding pressures. The extent of the gap

127

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

opening is controlled by the preset grinding force and initial gap. Each roll unit is normally equipped with a single
drive motor that powers the roll through a gearbox. Feedmaterial is introduced from the top, passes through the
gap where it experiences the grinding force, and thenleaves the machine from the bottom in the form of either
free-flowing particulates (as with quartz) or as a compacted cake (as with limestone, dolomite, coal, etc.).
In a confined mode of particle-bed breakage, energy
absorption is a main factor for the size-reduction process(Schönert, 1996). The specific grinding force has a direct
effect on the amount of energy absorbed to a certain level,
beyond which the bed is saturated and little-to-no breakage occurs (Fuerstenau et al., 1996; Schönert, 1996).
Fig. 3 shows a schematic illustration of the multiple
zones that particles pass through in between the twocounter-rotating rolls. Feed material first enters the acceleration zone (also known as the de-aeration, the pre-­crushing, or the pre-grinding zone) (Schwechten andMulburn, 1990; Austin et al., 1993; Lubjuhn and Schönert,1993) and transfers through the feed hopper into the nextzone, where particle-bed breakage under high pressure
occurs. This transfer is accompanied with compaction
during which, re-arrangement of the particles takes placeand smaller particles fill the void space between the larger
ones. Air that is trapped in the void space, leaves the particle bed from the top. Primarily governed by gravity atthe beginning, the downward movement of the feed canbe resisted by the rate of the upward airflow. For this reason, efficient ventilation is critical to optimal operation of
the HPGR, especially in bigger units. Particles get compacted as they move through and, hence, friction forces
between particle/particle as well as particle/roll surface
increases, which assists with the acceleration of the particle bed. Bulk density also increases as material goes
down the HPGR, where it may reach up to 85 percent of

the ore’s specific gravity.
The zone that starts with the nip angle is called thegrinding zone (also known as the compression zone). True
interparticle breakage takes place in the grinding zone,
where the particles bed is exposed to the highest grindngpressure. The grinding zone is limited to the shortest distance between the two rolls namely the working gap and,on the other hand, the critical gap, which is defined by the
nip angle. The nip angle is influenced by the material
properties, the roll surface pattern, and the roll speed.
In both research and industrial operation of HPGRs,
feed material may either be mono-sized or have a distribution of particles. In research, when comparing energy-­
efficiency with single-particle breakage, the feed must be
narrowly sized. Extensive industrial application of HPGRs
has been the comminution of granulated blast-­furnace
slag in the production of cement. In this case, the particle
bed forms as the feed particles are broken in the grinding
zone. On the other hand, when it comes to feeds that havea distributed particle size, voids in the particle bed are filledwith finer particles as they enter the acceleration zone.
This should lead to a somewhat more energy-efficientcomminution. Among several factors that affect the performance of HPGR grinding, moisture in the feed has beenfound to have a significant effect on HPGR operation
because it influences friction and adhesion between particles as they are being squeezed in the gap (Fuerstenau
and Abouzeid, 1998). Depending on the feed characteristics, optimum moisture content would help with efficient
grinding. Moreover, it also helps, up to a certain extent,with the build-up of an autogenous protection layer on thesurface of the profiled or studded rolls when treating
highly abrasive ores.
The third zone below the working gap is called the relaxation zone. Compacted particles face relatively nopressure as soon as they leave the grinding zone, andFig. 3 Schematic of the gap between rolls showing Acceleration, Grinding and Relaxation zones.

128

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

therefore, the thickness of the flakes in the discharge tend
αi:
to expand slightly right after exiting the grinding zone. If δ  xg necessary, the thickness of the flakes provide a good
αi cos 1 1   g  1  (1)  δF   1000D 
estimate for the working gap. If, of course, the fine particles do not agglomerate however (as seen in the case of
The nip angle theme is central to the models proposed
grinding quartz), then the particles exiting the grinding
in the literature. Austin et al. (1993, 1995) showed that thezone will be free flowing.
specific grinding pressure has an inverse effect on the
Material moving between the two rolls is exposed to a
specific throughput. Depending on the specific grinding
gradient of pressure intensity along the gap. Schönert
pressure and the compressibility of the ore bed, the oper(1991) showed that the grinding pressure increases steeply
ating gap takes on a value of its own. Austin et al. (1993,
in the compression zone, with the peak pressure being at
1995) also showed that a higher pressure reduces thethe working gap or slightly above it. On the other hand,working gap and thereby diminishes the volumetric flow
not all particles along the roll width experience the same
through the rolls. The relationship that Austin et al. (1993,
pressure intensity. Because of this, multiple breakage1995) derived between the relative gap (xg /D) and the spemodes take place depending on where the particles arecific throughput (see Eqn. 7 for definition) is as follows:
passing through the gap and what is their relative size.
xg
Those particles in the bed that are larger than the critical m c1  θg  cos αi  1  θg  cos αi 1  cos αi  (2)1000D
gap, will break when they come in contact with the rollsurface. This is because they are passing from the accelAustin et al. (1993, 1995) relationship allows one to deeration zone to the grinding zone. Depending on the contermine the nip angle and the critical gap (the distance
between the two rolls at the nip angle) by fitting Eqn. 2.
struction of the housing of the rolls, there may also be an
Several grinding tests with similar conditions under atedge effect, where some of the feed particles experience
least three substantially different levels of operating prespartial compression in the grinding zone. In other words,
sure are needed to find the linear relationship between theparticles passing through the center of the roll width are,specific throughput and the relative gap. For details, refer
due to the pressure gradient across the width, more comto Dhawan et al. (2012). The nip angle, depending entirely
pacted and therefore leave the rolls with a higher portion
on the feed characteristics and roll surface pattern, rangesof fines than those passing through either end of the rolls.
from 4 degrees up to 10 or 11 degrees (Schewechten andMulburn, 1990; Schönert, 1991; Lim and Weller, 1999).
The HPGR mass flowrate is proportional to the dimen3. Scale-up and sizing of HPGR
sions of the rolls because the geometry of the gap is diSoon after the introduction of HPGR, Schönert (1985)
rectly related to the capacity of the machine. Schönert
was the first to publish the key parameters, in the form of(1985) and Schönert and Lubjuhn (1990) quantified thisequations, for the scale-up of HPGR. Those equations
relationship as follows:
have been widely quoted by others. Throughput, power,Mρ 1  θ g  xgu m L 3.6 δg xgu m L 3.6
and the specific grinding force are the three main scale-up(3)factors required for sizing a HPGR machine. To begin, aseries of tests need to be carried out in order to obtain
Eqn. 3 is well quoted in the literature and is called thethe necessary data for the quantification of the scale-up
continuity equation. The mass flow between the rolls is ilfactors.
lustrated in Fig. 4. As can be seen, Eqn. 3 can hold true
The energy efficiency of HPGR is achieved via true inas long as the cake (product) density remains constantterparticle breakage. Measuring the nip angle allows one
along the roll width and the phenomenon of the edge
to suitably define the top size of the feed that would minieffect is neglected (Lubjuhn and Schönert, 1993). Only
mize single-particle contact breakage as much as possible
small HPGR machines, with the roll diameter less thanand, instead, expose most of the feed to interparticle
300 mm, can provide uniform compaction through the
breakage mode. Any particle larger than the top size isroll width. As a result, a more practical approach to define
normally broken in the pre-crushing zone repeatedly untilmass flowrate is to conduct actual measurement during
being small enough to enter the compression zone. Nip
the operation. The volumetric flowrate can then be deangle is mostly a function of the material properties andfined as follows (Schönert, 1988):
both the roll dimensions and surface pattern. If material
characteristics inside the grinding zone are known, thenMV
the nip angle can also allow one to measure the volumet(4)ρ
ric flowrate of particles. Klymowsky et al. (2002) used thegeometry of the grinding zone to calculate the nip angle,
With replacing Eq. 3, Schönert (1988) showed that the129

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

volumetric flowrate is a function of the rolls geometry
and material characteristic:uVk  m Vp
1  θg  xgu m L 3.6 
 u 

(5)

With k = (1 – θg)(xg/1000 D) being a material constant,knowing that the working gap is linearly related to the rolldiameter of a given material, and assuming material velocity is equal to the roll velocity, volumetric flowrate canbe measured based on the roll dimensions and the roll
speed within a good approximation. Schönert (1988)
called Vp the potential volumetric flow rate that replaces
Eqn. 5, given as:

Vp  uLD(6)

In order to minimize the influence of the machine sizeon the measured throughput, the specific throughput parameter was introduced. The specific throughput parameter, also widely known as “m-dot” in the industry,
provides the expected throughput for a HPGR, with 1 m
diameter and 1 m length rolls running at 1m/s peripheral
speed (Klymowsky et al., 2002):m c xg
 δg  3.6
D(7)

The necessary data to calculate ṁc can be measured
from the flake density, δg, in the HPGR discharge. At thesame time however, this formula can only be used for labscale machines as long as the edge effect is negligible. For
this reason, the following formula is more practical forreal applications:m f M
uLD(8)Fig. 4

Schematic of gap geometry in HPGR.

HPGR specific grinding force can be defined via HPGR
grinding tests. It is broadly discussed across the HPGR
publications that the compressive milling force is themain controlling parameter for the fineness of the HPGR
product. The specific grinding pressure is considered to
be one of the scale-up factors as the effect from the rolldimensions is taken off through dividing the milling force
by the roll width and length:

Fsp 

F
DL10 6(9)

The specific grinding force is the average force appliedon the projected area of the rolls. At the same time however, there is a pressure gradient along the width. Further,
the pressure across the gap rises to a very high value atthe center of the working gap and then drops very quickly.
De and Fuerstenau (2012) presented a model for calculating the pressure profile across the gap at different angular
positions in the vicinity of the nip angle. This study, verified with a piston-die apparatus, showed that the maximum pressure inside the bed can reach as high as 200 to
250 MPa (De, 1995). Schönert and Lubjuhn (1990) estimated it to be 40 to 60 times the applied specific grinding
force:The value obtained from Eqn. 8 provides information 1 
about the response of the material to high pressure grindFmax  
(10)
 Fsp
 αi c F ing. It should be noted that certain parameters, such as
Extensive results from lab-scale to industrial-size mafeed properties and specifics of the HPGR machine, influchines show that the required power for driving rolls inences the value. For more details refer to Rashidi (2014).
creases with the specific grinding force. HPGRs are
Practice has shown that the specific throughput rate is indesigned in such a way that the grinding force is applied
fluenced by the size of the machine, often caused by theon the floating roll, perpendicular to the projected area ofsize distribution of feed particles and the internal friction
the roll. The applied force is split into two force compobetween particles (Burchardt et al., 2011). Evidently, larger
nents: tangential and radial. The tangential component
feed sizes in the plant-scale HPGRs could behave differcreates a torque on the roll, which determines the required
ently under compression than finer feeds in pilot-scale
power to drive the roll. One of the rolls is normally exHPGRs. This is the reason that most manufacturers prefer
posed to less force compared to the other one, however
to run scale-up tests in large units because it minimizes
Schönert (1988) simplifies it by assuming that equal force
the effect of machine size as much as possible.
is applied to both rolls. Therefore, the total torque needed
In the interparticle mode of breakage, grinding occurs
to drive both rolls is given by:when the compressive force is higher than the particle
strength. A higher specific grinding force increases the D  F sin β DFβ
T 2rate of fines generated in the product at a diminishing rate
 
(11)
2
(Schönert, 1996), as discussed in Section 2. The optimum

130

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

The point at which the grinding force is applied on theroll is determined by the force-acting angle β. The
force-acting angle should be less than the nip angle in order to efficiently apply the grinding force on the particle
bed. The power required for driving the roll is determinedby the force and the force-acting angle. With sin β = β forsmall angles, the power is calculated as follows:uP ωT  2π r  DFβ
 2 βVp Fsp  2βuF
 60 

(12)

There are several necessary measurements to define thescale-up factors, which include the specific throughput,
the optimum specific grinding force in relation to the required fitness, and the specific power consumption, thatcan be determined through a series of tests. Fig. 5 summarizes the interrelationship between the scale-up parameters. HPGR design and, to a great extent, material
properties majorly affect these parameters. Overall, thescale-up of HPGRs has not faced any significant changes
since the introduction of the technology.

4. Particle breakage modelingthe feed experience a fixed-time of residence (Fuerstenau
et al., 1991). Even then, Gutsche and Fuerstenau (2004)
showed that similar to ball mills, the self-similar behavior
holds true for the product size distribution of the HPGR.
In other words, different product cumulative size distributions follow one master curve (Fuerstenau and Kapur,
1995) when expressed as a function of relative size (sizeof particle divided by the product median size) which is
given by: x F  x, E  Z 
 X 50 (14)

Utilizing the self-similarity concept, some researchers
proposed that by fitting an empirical master curve to theself-similar distributions, the product size distribution canbe easily predicted at any specific energy input as long as
the median size is determined (Lim et al., 1996a; Daniel,
2002). Based on these observations, Fuerstenau et al.
(1991) modified the ball mill grinding kinetics equation interms of the cumulative energy input instead of the grinding time to predict the product size distributions in
HPGR:
i 1
dM i E
1,,i  1 (15) S iE M i E bi, j S Ej Mj E ; j dEj 1Developing a realistic mathematical model for the size
reduction in HPGRs has really not been possible due to a
Assuming the breakage function can be normalized,
variety of reasons. The fact that HPGR was put into comthe specific selection function can then be based on themercial use so quickly and given the restricted access toactual energy that goes into grinding by incorporating an
scale-up as well as the industrial datasets are just a few ofenergy dissipation exponent, y, (Fuerstenau et al., 1991):
the reasons why the progress in this area has been slow.
P
In this section, those models that have shown potential
forS i S iE y ;0  y 1
(16)
E
sc
further development are discussed.
As was pointed out, grinding in interparticle bed
For this full batch-grinding model, both selection funcbreakage is directly controlled by the specific energy intion and breakage function had to be back-calculated fromput. With respect to HPGR grinding, Fuerstenau et al.
experimental data, resulting in a total of eight exponents(1993) showed that the reduction ratio for a single pass ofand constants to be simultaneously fitted (Fuerstenau etmaterial varies linearly with the specific energy input,al., 1991; Fuerstenau et al., 1993). An important finding
which holds true for a limited range:
was that the specific selection function is nearly constant,independent of particle size, in contrast to ball milling.
Xf jEsc  eThis should be expected since the same amount of energy
(13)
X 50
is transmitted to all particle sizes in confined particle-bed
Unlike in ball milling, the residence time for individualcomminution. This model was successfully verified usparticles in the bed is the same in HPGR. All particles ining the experimental data obtained in a lab-scale HPGR
that was developed in collaboration between Prof.
Schönert at the Technical University of Clausthal andProf. Fuerstenau at the University of California, Berkeley(Gutsche, 1993). This lab unit with 200 mm diameter by
100 mm width rolls can process sample as small as 10 kg.
This machine was later handed over to the University of
Utah under the supervision of Prof. Rajamani.
The assumption of using the energy-normalized selection function for grinding in HPGRs could only hold true
for low specific energy input where there is a linear function between the energy input and the fineness of theFig. 5 Interrelationship between HPGR scale-up parameters.

131

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

product. Unlike ball milling, where energy is a directwithfunction of time, higher specific energy in particle-bed
breakage does not necessarily relate to a higher reduction
ai* ai  ai (1  ai )(19)
ratio (Austin et al., 1993). Moreover, the application ofthis model needs to be further investigated for larger maSimilarly, two sets of normalizable breakage functions
chines where, for a given specific energy input, the prodwere considered based on the two modes of breakage,uct size distribution varies along the rolls width.
namely contact (nipping) and compression. To simplify
In a separate study, Austin and Trubelja (1994) modithe model however, they assumed that the values for bothfied the roll crusher model to predict the action of grindbreakage functions are the same. The experimental data
ing in HPGR. The roll crusher model was previously
used for fitting the model were obtained in a 100 mm bydeveloped by Austin et al. (1980). It should be noted that
30 mm lab-scale HPGR manufactured by Polysius (Austin
grinding in roll crushers is completely different fromet al., 1993). To verify this model, Schneider et al. (2009)
HPGR grinding. In roll crushers, the gap is fixed and noused six sets of experimental data that were obtained in acompressive pressure is applied to the rolls. Hence, break250 mm in diameter by 100 mm in length lab-scale HPGR
age occurs through contact with the rolls. Individual parequipped with studded rolls, also manufactured by Polyticles larger than the gap repeatedly break into smaller
sius.
sizes until they pass through the rolls.
The multi-component model developed by Austin andAustin et al. (1993) defined two sets of selection funcTrubelja (1994) is primarily empirical and employs a large
tions based on the geometry of the gap in a HPGR. The
number of model parameters. It is difficult to regenerate
first selection function was defined for large particles brothe results by back-calculations unless some constraints
ken by contact, which normally happens above the criticalare imposed on some of the model parameters. Also, thegap in the acceleration zone. This selection function wasedge effect that is a characteristic of larger machines isnormalizable by the working gap as a function of specific
not addressed in this model.
grinding force and therefore, the energy input. The secSince the dominant form of breakage in a HPGR unit isond selection function was defined for particles withparticle-bed comminution, feed properties, especiallyequal or smaller size compared to the critical gap, which
feed size distribution profoundly influences the final
are nipped into the compression zone and broken by interproduct size distribution. Various arrangements of partiparticle stresses. It is important to note that the largest
cles in the bed lead to distinctive stressing patterns underparticle present in the feed is defined by the working gap.
the force loading, which accordingly result in different
Austin and Trubelja (1994) at the time did not suggest any
product size distributions. This led Liu and Schönert
particular compression selection function and, instead,(1996) to pursue the application of the well-developed
used an equation based on the calculated particle size dispopulation balance models for predicting the product size
tribution to determine this selection function. Later,
distribution in a confined particle bed as a function of enSchneider et al. (2009) incorporated the specific grinding
ergy input. They introduced an energy split function inpressure into Austin’s model and defined the compression
order to determine the energy absorption for each size
selection function.
fraction via an iterative algorithm:
Austin and Trubelja (1994) proposed a sequential
n
Ej
kj 

, k j M i E 1
discrete-size model to simulate the product size distribu(20)Ej 1tion. It is based on two sets of particle-size distributions:particles that are left unbroken and are denoted as Pi andAssuming that breakage and selection functions are
particles that enter from larger size fractions after breakcharacteristic features of material, these two parameters
age and are denoted as Pi*. With (1 – ai) and (1 – ai’) to bewere quantified via controlled tests with mono-sized parthe mass of the fraction of particles left unbroken after
ticles in a piston-die apparatus. Liu and Schönert (1996)
nipping (pre-crushing) and compression, respectively, thetests were focused on brittle material such as quartz. They
product size distribution is then calculated as follows:
showed that the influence of the energy input increases as
the distribution of particle sizes widens. However, due toi1 (1  ai )(1  ai );Pi  
the complex iterative nature of the method, Liu and(17)
1 i  n(1  ai )(1  ai )Pi*;
Schönert (1996) did not recommend it for a wide feed size
where,
distribution since the errors between the measured andthe fitted values would inherently increase.
0;
i 1

Klymowsky and Liu (1997) further developed Liu andi 1Schönert’s
model to simulate the entire spectrum of
Pi*   *(18)
* *1 i  nai bi,1  bi, j a j Pj ;product size distribution for an industrial HPGR unit. Asj 1explained earlier, the final HPGR product is a combined132

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

result of three grinding actions that take place as material
passes through the two rolls. Those include pre-crushing,compression, and edge effect. Correspondingly, Klymowsky
and Liu (1997) multi-component HPGR model consistedof three steps for quantifying the product size distribution: (1) predicting the resulting particle size distribution
in the pre-crushing zone through sequential breakage
similar to Austin and Trubelja (1994) method; (2) predicting the resulting particle size distribution in the center using the population balance model similar to Liu and
Schönert (1996) model; (3) defining the theoretical particle size distribution for the edge product and solving theenergy consupmtion balance between all three breakage
zones based on the fact that a pressure gradient exists between the two rolls.
Klymowsky and Liu (1997) created a large experimental dataset based on a 900 mm diameter by 250 mm
pilot-scale HPGR. They set the selection and breakage
functions and the energy split function according to theresults that Liu and Schönert (1996) had obtained in theirideal particle-bed tests on quartz. Nonetheless, around 10
remaining model parameters still needed fitting via
back-calculation. Therefore, to avoid large variations,Klymowsky and Liu (1997) proposed splitting the model
parameters into secondary and primary sets. They suggested that secondary parameters should be determinedthrough a few fundamental tests such as running controlled experiments with narrow-sized feed. Then thevalue for each secondary parameter would be fixed by averaging it over the range obtained in each experimental
dataset. Following this, primary parameters could be further refined through a new set of data and an averaging
technique would be used to fix a value for each primarymodel parameter. They showed that simulation results
based on this technique were promising. This model was
further evaluated by Van der Meer (2010).
Similarly, Morrell et al. (1997b) adopted a modular
structure to predict throughtput, power, and product size
distribution. The product size distributions for three defined breakage zones within a HPGR are independently
calculated and comibined at the end to produce the final
results. To do so, Morrell et al. (1997b) individually applied a modifed version of the Whiten-Awachi-Anderson
crusher model (Anderson, 1988) for each grinding zone.
The Whiten-Awachi-Anderson crusher model features aclassification function that defines the probablity that particls would break in a crusher based on the preset gap
openings given as: K x C  x  1   2
 K 2  K1 

K3

(21)

Crushers operate in a stepwise action and, therefore,
particles bigger than the gap openings (K2 and K1 in Eqn.
21) would break repeatedly until become small enough topass through the gap without further size reduction. Thisis similar to the breakage action in the pre-crushing zone,
where large particles breaks sequentially in order to besmaller than the critical gap to enter the grinding zone
(Morrell et al., 1997b; Daniel and Morrell, 2004). Morrell
et al. (1997a) defined the critical gap based on the geometry of the grinding zones as follows:
0.5
4δ Dx  2

xc 0.5 D  xg   D  xg   g g  (22)δF  
Inside the compression zone, Morrell et al. (1997b)
considered two sub breakage mechanism namely edge-­
effect and compression. While in reality all particles inside the compression zone go under compression to someextent, with the highest force exerted on particles in thecenter of the gap, in their model, Morrell et al. (1997b) assumed that resulting particles from edge-effect go undersingle-particle breakage similar to what was assumed forthe pre-crushing zone. In order to determine the fractionof particles that undergo the edge effect, Morrell et al.
(1997a) defined a split function as follows:



f γ

xg
L

(23)

The calssification function used in Whiten-Awachi-­Anderson crusher model employs three parameters andwith the addition of the fourth parameter, t10, a total of 12
parameters need to be fitted to experimental data to produce the final product size distribution. As shown by
Fuerstenau and Kapur (1995), the HPGR product size distribution follows a master curve, which makes it logical touse t10 as a breakage characteristic parameter. The valueof t10 varies with the energy input. However, as mentioned
earlier, the energy input and grinding force as the dominant factor for controlling the product fineness, forms alinear relationship for only a limited range.
A large number of the parameters are given fixed values
to simplify the approach. However, the remaining criticalparameters are determined via conducting breakage characteristics tests such as drop-weigh test in addition to
HPGR tests. The application of breakage characteristics
tests could be referred as an important feature for theMorrell-Tondo-Shi model. Among the parameters, K2
takes the value of critical gap for the pre-crushing zone
and the value of working gap for the portion of material
going through the edge-effect zone. In other words, K2 isthe controlling factor for the top size of the product.
Meanwhile, the value of K3, which is an ore characteristic,
is only determined for the compression zone and alongwith t10 has to be fitted to experimental data obtained
from HPGR tests. The iterative back-calculation continues until the difference between the simulated and measured energy input reaches a minimum.
Morrell-Tondo-Shi model has been further reviewed by133

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

Daniel (2002) and Benzer et al. (2001) by vast data fromvarious sizes of HPGR machines, putting it in a uniqueposition compared to the rest of the work done so far.
Nevertheless, it does not fully include the fundamental
breakage mechanism that takes place in the compression
zone with respect to all parameters contributing to the final product size distribution.
Torres and Casali (2009) developed a modular structureon the basis of Morrell-Tondo-Shi model to simulate
throughput, power, and product size distribution frompilot-scale HPGR tests. Their model for predicting product size distribution shares a similar structure with Morrell-­Tondo-Shi model, but with a different approach as itfeatures a discretized system for the particle bed. With
this approach, the gap between the two rolls is divided
into N blocks. These blocks start from the critical gap andend at the working gap.
To predict the product size distribution, Torres andCasali (2009) benefited from the population balance modeling technique. They assumed that all particles larger
than the critical gap would instantaneously break and enter the compression zone along with the rest of the particles that are already small enough to form the feed for thecompression zone. The final product size distribution isthe combined particle size distributions from each block.
The resultant particle size distribution is not only a function of the feed particle size, but also depends upon theposition of the particles inside each block in addition to
the position of the corresponding block along the gap.
Therefore, the population balance model was modified
accordingly (Torres and Casali, 2009):uzi 1
dM i, k  z  S i,k M i,k  z bi, j S j,k M j,k  z
dz
j 1

(24)

Eqn. 24 results in N × n matrix of ordinary differential
equations with N and n being the total number of blocks
and particle size classes, respectively. Torres and Casali(2009) suggested an analytical solution for Eqn. 24 as follows:
i

 S


pi,k  Aij,k exp   j,k z  uz 
j 1

(25)

where


0

i 1

bi,l S l,kAij,k
Aij,k  l  j S i,k  S j,ki 1
 f IP  A
i
il ,k


l 1


i j
i j

(26)

i
j

It was assumed that the breakage function is the samefor each particular ore independent of the position inside
the compression zone. The breakage function needed tobe calibrated with the experimental data. Using the func-

134

tional format of selection function, three additional parameters including the specific breakage rate, SiE, required
fitting to experimental data.
Unlike Morrell-Tondo-Shi model, Torres and Casali(2009) considered that compression takes place along theroll width even though the intensity follows a parabolapatter resulting in coarser product towards the ends. However, their model does not offer a way to quantify thefraction of edge product that does not undergo true interparticle breakage as opposed to those particles passing
through the center of the compression zone.
In a simplified approach to apply the population balance model to HPGR grinding, Dundar et al. (2013) assumed that the dominant grinding mechanism in HPGR
is true compression bed. Dundar et al. (2013) determined
the breakage function through running tests in a piston­die setup using a narrow-size feed. The breakage function
parameters were estimated using the Narayanan (1986)
single-particle impact breakage model. With the breakage
function being considered as the characteristics of materials and constant for each ore type, the selection function
was estimated for the total HPGR discharge product via
back-calculation. Therefore, the selection function was
fitted to experimental HPGR data using Austin et al.
(1976) power equation. In contrast to previous models,Dundar et al. (2013) model simplifies the grinding actionin HPGRs to only true particle-bed breakage. Although
this model does not consider individual grinding actionsin industrial HPGRs, the published results are quite
promising for further evaluation.
In another effort to simplify the population balance
model for compression grinding, Esnault et al. (2015) defined the breakage mechanism of the particle-bed compression as a function of porosity. Porosity in the particle-bed
changes with the applied energy. Esnault et al. (2015) assumed that breakage via compression can be defined by
the generation of new surface area referring to Rittinger’slaw:
SS Fn1  x  SS Fn  x 

dSS F  x dE(27)(θn1  θ n )dEdθEsnault et al. (2015) emphasized that the tests for determining the breakage function values need to be performed under low pressure to minimise the occurrence of
re-breakage. Esnault et al. (2015) results were based on
the fine brittle material, which fits with the applicablerange of particle sizes that is suggested for the Rittinger’s
law. Esnault et al. (2015) noted the increased error for
those tests under very high pressure and referred that tothe effect of particle cushioning. Even though, grinding in
HPGR units is more complicated than piston-die apparatus, Esnault et al. (2015) model provided a great under-Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

standing for measuring the breakage in particles bed
related with porosity.
Looking at overall particle size simulation modelingworks (Summarized in Table 1); the focus has been
mainly on adapting the population balance model that has
been well-developed for ball mills to the breakage behavior in HPGR. This approach has had limitations with thefact that particle-bed breakage rate does not necessarily
increases with the energy input beyond the saturation
point. Results published so far indicate that the ideal controlled way to determine the selection and breakage function for particle-bed breakage is through the use ofpiston-die apparatus. The major difficulty in such modeling is to include edge effect.

5. HPGR characteristics tests

also demonstrated that the normalized particle size distributions obtained from a lab-scale HPGR unit and piston-­die setup are matching if the specific energy input is kept
constant.
Daniel (2003) also presented interesting findings to relate the piston-die operating parameters to lab-scale
HPGR operation including the critical gap, working gap,
and the grinding force. He showed that the product size
distribution in both machines for the same feed size distribution and the same material were matching under the
specific energy input range of 2.5–3.5 kWh/t. Similar results have been reported by Kalala et al. (2011). In a novel
approach, Rashidi (2014) defined a new series of controlled tests to estimate the selection and breakage functions via running tests in a lab-scale HPGR.
The most practical meaning for a characteristic test
would be the formulation of a work index. A value thatcan show the performance of the HPGR unit to a specificore regardless of any parameter involved such as feed size
distribution and so on. Bond work index is the world-wide
known factor that with all the limitations serves as awell-established technique to evaluate the response oftumbling mills to grinding a specific ore. However, thisindex itself is not directly applicable to HPGR grinding
technology. Morrell (2004) formulated a general work index formula that could be applied to numerous grinding
equipment including HPGR:

As discussed earlier, the scale-up studies are mostly
based on HPGR testing in pilot-scale machines with atleast a minimum of 500 to 700 mm roll diameter. The
particle-bed breakage is very sensitive to the feed properties and a real test in a HPGR machine would provide themost realistic results about the response of high compression on any specific material. However, HPGRs are
known for their high capacity making it difficult to conduct feasibility studies, requiring large mass of ore material. Even the smallest lab-scale HPGR requires at least
20 to 25 kg material to maintain a long enough feed
Wh KM ih 4  x2f ( x2 )  x1f ( x1)(29)
stream to log data under steady-state operation.
A considerable amount of research has been put into
Where Mih is a breakage related index (HPGR work ininvestigating the possibility of running small-scale batchdex) that is obtained through a specialized test known astests as an indication for the amenability of HPGR.
SMC Test®. However, it is not clear how this parameter is
Piston-die apparatus represents the batch mode of interdefined and how it is related to the strength of the partiparticle breakage. Besides, it requires a small mass ofcles. K is a constant that takes two different values desample, allows controlled compression, and applies compending on the mode of operation (K = 1 for a HPGR inpression at a desired rate. These advantages made the piston-­closed circuit with a screen and K = 1.19 for an open cirdie a standard tool for measuring the breakage rates andcuit HPGR). Morrell (2006) showed that the exponent f(x)the breakage distribution functions in many of the modelin Eqn. 29 takes different values depending on the partiing works.
cle size as follows:
De and Fuerstenau (2012) showed that the rotation anxi
gle is the key parameter to define the grinding zone,f  xi   0.295 (30)
1000000
where the compression happens. They derived an equation
to determine this angle based on the load-displacement
Data that Morrell (2006) used for the verification of
profile in a piston-die test provided that the nip angle isthis model were obtained through a lab-scale HPGR unknown (De, 1995):
der the limited range of 2.5–3.5 kWh/t specific energy input. Later, Morrell (2010) introduced a new parameter, Sh,h  hfαcos 1 1 
1  cos αi 
the material strength parameter, in Eqn. 29, in case feed(28)
 h0  hfparticle size was greater than 25 mm relating to the fact
Knowing the angle of rotation, De and Fuerstenau
that the particle strength changes with the size:(2012) related the pressure profile in a HPGR to the
Wh S h KM ih 4  x2f ( x2 )  x1f ( x1) piston-die test data, and subsequently predicted the(31)
scale-up factors of HPGRs including force, torque, working gap, throughput, and the specific energy input. They
with

135

136

Austin et al., 1993;
Austin and Trubelja,
1994
Liu and Schönert,
1996*Empirical only.
*Depends on the feed material properties.
*Does not account for the edge-effect(partial compression).
*Not suitable for wide feed size distributions.
*Does not consider the pre-crusher and theedge-effect zones.
*Incorporating large number of model
parameters.

*Model accounts for the fact that not all particles
break as a result of the bed compression.
*It adopts roll crusher model that represents thecrushing mode in the acceleration zone.
*First application of the PBM model for HPGR
grinding.
*Using controlled confined particle-bed breakage
tests to quantify breakage characteristics.
*Accounting for three grinding actions that takeplace in an industrial HPGR.
*Defining the grinding parameters through basic
grinding tests.
*Reconciling results from drop-weight test (breakage
characteristic test) with the HPGR model.
*Widely evaluated in industrial applications basedon published material.

*Modifying roll crusher model individually for two dominant
breakage mechanisms in HPGRs: Contact and particle-bed.
*The final product is a combination of broken and unbroken
particles under two separate breakage mechanisms.

*Providing two mass and energy balance equation to accountfor the effect of energy adsorption in different size classes.
*Selection and function are defined by controlled tests in apiston-die apparatus using mono-sized particles.

*A modular model that introduces an energy distributionfunction.
*HPGR tests are combined with piston-die tests to simulate theproduct size distributions.

*A modular model considering three grinding zones in an
HPGR.
*Adopting cone crusher model individually for each grindingzone.
*Applying drop-weight test along with HPGR tests to fit themodel.

*Depends on the feed material properties.
*Does not fully cover the multiple breakage
actions in industrial and semi-industrial
HPGRs.

*A modular discretized PBM model based on Morrell-Tondo-Shi *Embodied in the model the pressure gradient thatmodel.
exists in the compression zone.
*Quantifying selection function in accordance withthe operating conditions and material properties.

*Application of Rittinger’s law to define the particle-­ *Applicable to low energy input due to the
bed breakage under the compressive load.
particle cushioning under higher pressure.

*Application of population balance model.
*Measuring the breakage function via the piston-die test.
*Smoothing out selection function based on material andoperating conditions.

*Modifying population balance model for the compression-bed
breakage based on the creation of new surface area.

*Large number of model parameters.
*Does not consider the partial compression
in the edge effect zone.
*Limited to low energy input levels.

Fuerstenau et al.,
1991; Fuerstenau etal., 1993*Depends on the feed material properties.
*Does not address the variation of particle
size in different HPGR sizes.
*Applicable to low energy input levels.

*Important observation that selection function isconstant for all particle sizes.
*Fully representing the similarities between ball milland HPGR products.

*Modifying ball mill grinding kinetics equation in terms ofenergy input.
*Introducing an energy dissipation component to normalize thespecific selection function.

Ensault et al., 2015

Dundar et al., 2013

Torres and Casali,
2009

Morrell et al.,1997a; Daniel andMorrell, 2004

Klymowsky andLiu, 1997

Reference

Areas for improvement

Salient features

Model structure

Table 1 Summary of HPGR particle size simulation modeling works.

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

S h  35  x1x2

0.2

(32)

Fmax

Maximum specific grinding pressure [N/mm 2]F(xi)Exponent for Morrell work index formulahParticle-bed height at an applied force [mm]h0Initial particle-bed height [mm]hfFinal particle-bed height [mm]6. SummaryjParticle-bed grindability [t/kWh]kMaterial constant

The modeling of HPGR via the principles of selection
and breakage function modeling concept has been partly
successful. Many published works show internal consistency with the data generated in their works. However,the difficulty arises when scaling up the lab-model parameters to large machines. The principal difficulty has
been the postulation of two different grinding zones in
the radial direction and two different zones in the axial
direction of the roll. Moreover, an experimental procedurefor determining breakage functions independently is lacking. Hence, estimation of numerous parameters befuddles
the modeling effort. Next, the models based on a work-­
index like structure, will have to depend on a single size
to represent the entire size distribution. Therefore, more
empirical coefficients are introduced in such models.
Nevertheless, all the models have explained the internal
dynamics of the high compression bed loading and the effect of machine parameters on loading. As of this date,the modeling effort continues to advance.

kj

Energy split function (Schönert model)

K1

Whiten-Awachi-Anderson crusher model parameter

K2

Whiten-Awachi-Anderson crusher model parameter

K3

Whiten-Awachi-Anderson crusher model parameterL

Roll length [m]M

Mass flowrate [t/h]ṁcSpecific throughput based on cake characteristic[ts/m3h]ṁfSpecific throughput based on flowrate [ts/m3h]

MihMorrell HPGR work index

Mi(E)Mass fraction of particles in size class i per unit ofEnergy input

Mi,k (z)Mass fraction of particles in size class i based on its
vertical position (z) in block k

PPower [kW]Pi

Mass fraction of particles left unbroken at size iPi*

Mass fraction of particles that enter size ipi,kMass fraction of particles entering size i in block k

SS(x)Specific surface function

Morrell work index is limited to the higher end of specific energy input range, under which there is a linear relationship between energy input and size reduction. Thismodel being empirical keeps evolving.

Nomenclatureai

Nipping selection function (Austin model)

Si

a'i

Compression selection function (Austin model)

Si

Selection function at size ibi,jBreakage function

T

Torque [Nm]cF

Material constantuRoll peripheral speed [m/s]C(x)Classification functionumMaterial peripheral velocity in the gap [m/s]DRoll diameter [m]urRoll rotational speed [rpm]E

Energy input [kWh]uzMaterial peripheral velocity in the z direction [m/s]

Ej

Energy absorption in size class j [kWh/t]

V

Volumetric flowrate [m3/h]

Esc

Specific energy input [kWh/t]

Vp

Potential volumetric flowrate [m3/h]fSplit function (Morrell-Tondo-Shi model)

Wh

Morrell work index

Feed fraction factor going under single-particle
breakage (Torres-Casali model)x

Particle size

xc

Critical gap at the nip angle [mm]xgWorking gap [mm]XfMedian size in the feed [μm]X50Median size in the product [μm]x2

80 percent passing size for product [μm]

fi

IP

F

Grinding force [N]F(x)Particle size distribution

F(x, E) Particle size distribution function as a function of specific energy input
Fsp

Specific grinding pressure [N/mm 2]

E

The energy-normalized selection function at size i

137

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140x1

80 percent passing size for feed [μm]yEnergy dissipation exponentz

Particle position inside gap (Torres-Casali model)Z

Self-similar distribution function

αAngle of rotation

αi

Nip angle

βForce-acting angleγ

Split factor (Morrell-Tondo-Shi model)

δg

Gap (cake) density [t/m3]

δF

Feed bulk density [t/m3]θPorosityθg

Gap (cake) porosityρMaterial density [t/m3]ωRoll radial velocity [1/s]References
Abouzeid A.-Z.M., Fuerstenau D.W., Grinding of Mineral Mixtures in High-Pressure Grinding Rolls, International Journal of Mineral Processing, 93 (2009) 59–65.
Anderson J.S., Napier-Munn, T.J., Power Prediction for Cone
Crushers, in: Third Mill Operators Conference, AusIMM,
Cobar, 1988.
Austin L.G., Shoji K., Bhatia V.K., Savae K., Klimperl, R.,
Some Results on the Description of Size Reduction as aRate Process in Various Mills, Industrial & Engineering
Chemistry Process Design and Development, 15 (1976)
187–196.
Austin L.G., Van Ordern D.R., Perez J.W., A Preliminary Analysis of Smooth Roll Crushers, International Journal of
Mineral Processing, 6 (1980) 321–336.
Austin L.G., Weller R., Lim I.L., Phenomenological Modellingof the High Pressure Grinding Rolls, in: XVIII International Mineral Processing Congress, Sydney, 1993, pp.
87–96.
Austin L.G., Trubelja M.P., The Capacity and Product Size Distribution of High Pressure Grinding Rolls, in: IV Meetingof the Southern Hemisphere on Mineral Technology, Concepcion, Chile, 1994, pp. 4–67.
Austin L.G., Trubeljal M.P., Seebach H.M. Von, Capacity of
High-Pressure Grinding Rolls, Minerals and Metallurgical
Processing, 1995, pp. 65–73.
Barrios G.K.P., Pérez-prim J., Tavares L.M., A DEM Model of
HPGR Operation Using Bed Compression Models Calibrated with a Piston Die Test, in: 9th International Comminution Symposium, 2014.
Battersby M.J.G., Kellerwessel H., Oberheuser G., Important
Advances in the Development of High Pressure Rolls Comminution for the Minerals Industry, in: Extractive Metallurgy of Gold and Base Metals, Kalgoorlie, 1992, pp.
26–28.

138

Benzer H., Ergun L., Lynch A.J., Oner M., Gunlu A., Celik I.B.,
Aydogan N., Modelling Cement Grinding Circuits, Minerals Engineering, 14 (2001) 1469–1482.
Burchardt E., Patzelt N., Knecht J., Klymowsky R., HPGR’s in
Minerals: What do Existing Operations Tell us for the
Future?, in: Autogenous and Semi-autogenous Grinding
Technology Conference, Vancouver, 2011.
Daniel M.J., HPGR Model Verification and Scale-up, Universityof Queensland, 2002.
Daniel M.J., Particle Bed Compression Comminution using a
Piston-die to Predict the Performance of a HPGR, in:
JKMRC Conference, Brisbane, 2003, pp. 67–83.
Daniel M.J., Morrell S., HPGR Model Verification and Scale-up,
Minerals Engineering, 17 (2004) 1149–1161.
De A.K., Modeling and Optimization of Fine Grinding of Minerals in High-Pressure Roll Mill-Ball Mill Hybrid Comminution Circtuis, Univesity of California, Berkeley, 1995.
De A., Fuerstenau D.W., Simulation of Macroscopic Variables
Related to Comminution in High-Pressure Grinding Rolls,
in: XXVI International Mineral Processing Congress(IMPC), New Delhi, 2012, pp. 1075–1090.
Dhawan N., Rashidi S., Latchireddi R., Rajamani R.K., Evaluation of Scale up Factors in a Lab Scale High Pressure
Grinding Roll, in: XXVI International Mineral Processing
Congress (IMPC), New Delhi, 2012, pp. 1171–1188.
Djordjevic N., Morrison R., Exploratory Modelling of Grinding
Pressure within a Compressed Particle Bed, Minerals Engineering, 19 (2006) 995–1004.
Drozdiak J.A., Klein B., Nadolski S., Bamber A., A Pilot-Scale
Examination of a High Pressure Grinding Roll/Stirred Mill
Comminution Circuit, in: Autogenous and Semi-autogenous
Grinding Technology Conference, Vancouver, 2011.
Dundar H., Benzer H., Aydogan N., Application of Population
Balance Model to HPGR Crushing, Minerals Engineering,
50–51 (2013) 114–120.
Esnault V.P.B., Zhou H., Heitzmann D., New Population Balance Model for Predicting Particle Size Evolution in Compression Grinding, Minerals Engineering, 73 (2015) 7–15.
Fuerstenau D.W., Shukla A., Kapur P.C., Energy Consumption
and Product Size Distributions in Choke-Fed High-­
Compression Roll Mills, International Journal of Mineral
Processing, 32 (1991) 59–79.
Fuerstenau D.W., Kapur P.C., Gutsche O., Comminution of
Minerals in a Laboratory-size, Choke-Fed High-pressure
Roll Mill, in: XVIII International Mineral Processing Congress (IMPC), Sydney, 1993, pp. 175–180.
Fuerstenau D.W., Kapur P.C., Newer Energy-Efficient Approachto Particle Production by Comminution, Powder Technology, 82 (1995) 51–57.
Fuerstenau D.W., Gutsche O., Kapur P.C., Confined Particle Bed
Comminution under Compressive Loads, International
Journal of Mineral Processing, 44–45 (1996) 521–537.
Fuerstenau D.W., Abouzeid A.-Z.M., The Performance of theHigh Pressure Roll Mill: Effect of Feed Moisture, Fizykochemiczne Problemy Mineralurgii, 32 (1998) 227–241.
Fuerstenau D.W., Abouzeid A.-Z.M., Role of Feed Moisture inHigh-Pressure Roll Mill Comminution, International Journal of Mineral Processing, 82 (2007) 203–210.

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

Fuerstenau D.W., Mineral Processing Engineer and Scientist: in
Education, Research, Industry, and International Cooperation, in: an Oral History Conducted by Eleanor Swent in2001, Regional Oral History Office, The Bancroft Library,
University of California, Berkeley, 2011, pp. 599–629.
Gutsche O., Comminution in Roll Mills, University of California, Berkeley, 1993.
Gutsche O., Fuerstenau D.W., Influence of Particle Size andShape on the Comminution of Single Particles in a Rigidly
Mounted Roll Mill, Powder Technology, 143–144 (2004)
186–195.
Kalala J.T., Dong H., Hinde A.L., Using Piston-Die Press to
Predict the Breakage Behaviour of HPGR, in: Autogenous
and Semi-autogenous Grinding Technology Conference,
Vancouver, 2011.
Klymowsky I.B., Liu J., Modelling of the Comminution in a
Roller Press, in: XX International Mineral Processing Congress (IMPC), Aachen, 1997a, pp. 141–154.
Klymowsky I.B., Liu J., Towards the Development of a Work
Index for the Roller Press, in: Kawatra (Ed.), SME Comminution Practices Symposium, 1997b, pp. 99–105.
Klymowsky R., Patzelt N., Knechtz J., Burchardt E., Selectionand Sizing of High Pressure Grinding Rolls, in: A.L. Mular,
D.N. Halbe, and D.J. Barratt (Eds.), Mineral Processing
Plant Design, Practice, and Control Proceedings, 2002, pp.
636–668.
Klymowsky R., HPGR-Accepted Technology in Mining, in:
Mining Magazine Congress, Niagara-on-the-Lake, Canada,
2009.
Lim W.I.L., Morrell S., Tondo L., David D., Modelling the High
Pressure Grinding Rolls, in: Mining Technology Conference, Fremantle, (1996a) 169–176.
Lim W.I.L., Voigt W., Weller K.R., Product Size Distribution
and Energy Expenditure in Grinding Minerals and Ores inHigh Pressure Rolls, International Journal of Mineral Processing, 95 (1996b) 539–559.
Lim W.I.L., Weller K.R., Some Benefits of Using Studded Surfaces in High Pressure Grinding Rolls, Minerals Engineering, 12 (1999) 187–203.
Liu J., Schönert K., Modelling of Interparticle Breakage, International Journal of Mineral Processing, 44–45 (1996) 101–
115.
Lubjuhn U., Schönert K., Material Flow in the Acceleration
Zone and Throughput of High Pressure Roller Mills, in:
XVIII International Mineral Processing Congress (IMPC),
Sydney, 1993, pp. 161–168.
Mcken A., Williams S., An Overview of the Small-Scale Tests
Available to Characterise Ore Grindability, in: Autogenous
and Semi-autogenous Grinding Technology Conference,
Vancouver, 2006.
Morrell S., Lim W., Shi F., Tondo L., Modelling of the HPGR
Crusher, in: Kawatra (Ed.), Comminution Practices, 1997a,
pp. 117–126.
Morrell S., Shi F., Tondo L., Modelling and Scale-up of High
Pressure Grinding Rolls, in: XX International Mineral Processing Congress (IMPC), Aahen, 1997b, pp. 129–140.
Morrell S., An Alternative Energy–Size Relationship to that
Proposed by Bond for the Design and Optimisation of

Grinding Circuits, International Journal of Mineral Processing, 74 (2004) 133–141.
Morrell S., Rock Characterisation for High Pressure Grinding
Rolls Circuit Desing, in: Autogenous and Semi-autogenous
Grinding Technology Conference, Vancouver, 2006.
Morrell S., Predicting the Specific Energy Required for Size
Reduction of Relatively Coarse Feeds in Conventional
Crushers and High Pressure Grinding Rolls, Minerals
Engineering, 23 (2010) 151–153.
Nadolski S., Bamber A.S., Klein S., Drozdiak J., Investigation
into Laboratory Scale Tests for the Sizing of High Pressure
Grinding Roll, in: Autogenous and Semi-autogenous
Grinding Technology Conference. Vancouver, 2011.
Narayanan S., Single Particle Breakage Tests: A Review ofPrinciples and Applications to Comminution Modelling,
Bull. Proc. Austalas, Inst. Min. Metall., 291 (1986) 49–58.
Norgate T.E., Weller K.R., Options for Incorporating
High-Pressure Grinding Rolls into Comminution Circuits,
in: Fourth Mill Operators’ Conference, Burnie, 1991, pp.
19–24.
Otte O., Polycom High Pressure Grinding Principles and Industrial Application, in: Third Mill Operators’ Conference,
Cobar, 1988, pp. 131–135.
Patzelt N., High-Pressure Grinding Rolls, A Survey of Experience, in: IEEE Conference, Dallas, 1992, pp. 149–181.
Rashidi S., A Study of Energy-Size Relationship and Wear Ratein a Lab-Scale High Pressure Grinding Rolls Unit, The
University of Utah, 2014.
Reichardt B.Y., Schönert K., Interparticle Breakage of Fine
Hard Materials by Single and Multiple Compression,
Chemical Engineering & Technology, 26 (2003) 191–197.
Rosario P.P., Hall R.A., Grundy M., Klein B., A Novel
AG-crusher-HPGR Circuit for Hard, Weathered Ores Containing Clays, in: Autogenous and Semi-autogenous Grinding Technology Conference, Vancouver, 2011.
Schewechten D., Mulburn G.H., Experiences in Dry Grinding
with High Compression Roller Mills for End ProductQuality below 20 Microns, Minerals Engineering, 3 (1990)
23–34.
Schneider C.L., Alves V.K., Austin L.G., Modeling the Contribution of Specific Grinding Pressure for the Calculation of
HPGR Product Size Distribution, Minerals Engineering, 22(2009) 642–649.
Schönert K., Sizing of High Pressure Twin Roll Mills, Zement
Kalk Gips, 12 (1985) 728–730.
Schönert K., A First Survey of Grinding with High-Compression
Roller Mills, International Journal of Mineral Processing,22 (1988) 401–412.
Schönert K., Lubjuhn U., Throughput of High Compression
Roller Mills with Plane and Corrugated Rollers, in: 7th
European Syposium on Comminution, ljubljana, 1990, pp.
149–158.
Schönert K., Mueller F., Representation of Breakage Fraction and Breakage Function of Inter-particle Stressing,Aufbereitungs-Tehnik, 31 (1990) 248–256.
Schönert K., The Characteristics of Comminution with High
Pressure Roller Mills, KONA Powder and Particle Journal,
9 (1991) 149–158.

139

Samira Rashidi et al. / KONA Powder and Particle Journal No. 34 (2017) 125–140

Schönert K., The Influence of Particle Bed Configurations andConfinements on Particle Breakage, International Journalof Mineral Processing, 44–45 (1996) 1–16.
Torres M., Casali A., A Novel Approach for the Modelling ofHigh-Pressure Grinding Rolls, Minerals Engineering, 22(2009) 1137–1146.

van der Meer F., High Pressure Grinding Rolls Scale-up andExperiences, in: XXV International Mineral Processing
Congress (IMPC), Brisbane, 2010, pp. 1319–1331.
van der Meer F.P., Feasibility of Dry High Pressure Grinding
and Classification, in: Autogenous and Semi-autogenous
Grinding Technology Conference. Vancouver, 2011.

Author’s short biography
Samira Rashidi
Samira Rashidi received her Ph.D. from the University of Utah in 2014. The principal
emphasis of Samira’s research has been about improving efficiency of the grinding
equipment and circuits via mathematical modeling and simulation. Her Ph.D. research
was focused primarily on energy-size relationship and wear-rate measurement in HPGRs
under the supervision of Prof. Raj Rajamani. Samira currently is a Process Engineer atthyssenkrupp Industrial Solutions (USA). She is involved in all processing aspects of
new and existing grinding systems in the mining, minerals, and cement applications.

Raj K. Rajamani
Raj Rajamani has been on the faculty of the Metallurgical Engineering department of
the University of Utah, Salt Lake City, Utah, USA since 1980. Currently he holds the position of professor. His research interest include population balance modeling of tumbling mills, computational fluid dynamics of hydrocyclones, discrete element modelingof semi-autogenous grinding mills, eddy current sorting of metallic particles and modeling of high pressure grinding rolls. He received the Antoine M. Gaudin Award, presented
by the Society of Mining, Metallurgy and Exploration Engineers Inc. in the year 2009.

Douglas W. Fuerstenau
After receiving his Sc.D. degree at MIT, Dr. Fuerstenau spent a six-year period teaching at MIT and working in industry, after which time he joined the faculty of the University of California. At Berkeley he established an extensive program of teaching andresearch in minerals processing, applied surface chemistry and particle technology. With
his graduate students and postdoctoral researchers, he has published a wide range ofseminal papers in this field. He currently is P. Malozemoff Professor Emeritus of Mineral
Engineering in the University of California.

140

KONA Powder and Particle Journal No. 34 (2017) 141–154/Doi:10.14356/kona.2017020

Review Paper

Liquid Flame Spray—A Hydrogen-Oxygen Flame
Based Method for Nanoparticle Synthesis
and Functional Nanocoatings †Jyrki M. Mäkelä *, Janne Haapanen, Juha Harra, Paxton Juuti and Sonja Kujanpää
1

Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, Finland

Abstract
In this review article, a specific flame spray pyrolysis method, Liquid Flame Spray (LFS), is introduced to produce
nanoparticles using a coflow type hydrogen-oxygen flame utilizing pneumatically sprayed liquid precursor. Thismethod has been widely used in several applications due to its characteristic features, from producing
nanopowders and nanostructured functional coatings to colouring of art glass and generating test aerosols. These
special characteristics will be described via the example applications where the LFS has been applied in the past
20 years.
Keywords: nanopowder, nanoparticle, functional coatings, flame spray pyrolysis, aerosol

1. Introduction
Nanoparticle powder materials are widely used in themodern industry, they have a variety of technological applications, they are commercially available but frequently
custom made for each application using various fabrication methods (Kruis et al., 1998). Thermal gas phase synthesis and particularly aerosol techniques, offer ways togenerate nanoparticles via dry industrial processes (Ulrich,
1971; Kruis et al., 1998). Especially flame methods are
considered to be optimal for up-scaling (Kammler et al.,2001a; Stark and Pratsinis, 2002; Mueller et al., 2003).
Thorough reviews on the flame spray pyrolysis (FSP)
techniques for producing nanopowder have been madepreviously (Pratsinis, 1998; Mädler, 2004; Teoh et al.,2010). The FSP method generates well-defined nanoparticulate material which can be collected as a powder fromthe exhaust aerosol of the flame (Stepuk et al., 2013; Parkand Park, 2015) or sprayed on a surface (Mishra et al.,2014). Apart from nanopowder production, recently the
FSP and related techniques have also been used fairly efficiently in various types of coating applications (Mädler
et al., 2006a; Tricoli et al., 2008; Pimenoff et al., 2009).
With moderately high production rate (ca. g/min), themethod is also applicable for covering large surface areas†
1*

Received 23 May 2016; Accepted 18 August 2016
P.O.Box 692, FI-33101 Tampere, Finland
Corresponding author: Jyrki M. Mäkelä;
E-mail: jyrki.makela@tut.fi
TEL: +358-40-198-1018

with thin nanoparticle coating. One of the most recent applications is coating of a heat-sensitive flexible roll-to-rollmaterial, such as paper or paper board, by a direct deposition of flame-generated nanoparticles in order to e.g. adjust the wetting properties of the substrate (Mäkelä et al.,2011; Haapanen et al., 2015).
In this review article, we introduce a distinct FSP technique, Liquid Flame Spray (LFS), which is based on a coflow type hydrogen-oxygen flame. It utilizes pneumatic
spraying of a liquid precursor in the burner. Most oftenhydrogen is used as the atomizing gas, which having ahigh speed leads to a highly turbulent flame. The turbulent flame in the LFS is steady and robust, it does not require additional side flamelets and, due to the hydrogen,the precursor liquid itself does not need to be a fuel. The
LFS has been widely used in several applications due toits characteristic features. Additionally to powder generation and coating applications, the LFS is also feasible as atest aerosol generator, since the utilization of hydrogen as
the fuel, the exhaust gas does not contain excessive
amounts of carbon containing components. In this review
article, these special characteristics will be described via
the example applications in which the LFS has been applied in the past 20 years.

2. Method description
Flame spray pyrolysis (FSP), or liquid feed flame spraypyrolysis, is usually defined as a method consisting of aspray of micron-sized liquid precursor droplets and a

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

141

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

flame reactor where the liquid precursor is assumed toevaporate, react and nucleate into nanoparticulate form,
as described by e.g. Ulrich (1984), Pratsinis (1998) and
Teoh et al. (2010). The history of the FSP has recently
been described on a more general level by e.g. Purwanto
et al. (2011) and Trommer and Bergmann (2015). The several alternative reaction schemes and particle formation
mechanisms occurring in the FSP reactor when fabricating, e.g., metal oxide powder, has been discussed on a
general level by Mädler (2004).
We define Liquid Flame Spray (LFS) as one certain
type of flame spray pyrolysis, where the combustion gas
and oxidizing gas are hydrogen and oxygen, respectively.
It is specific, that the combustion gas is also acting as an
atomizing gas and no other gaseous components are necessarily needed in the process. It is notable that hydrogen
driven flames have also been widely used and studied
elsewhere, and utilized by e.g. Sokolowski et al. (1977),
Chung and Katz (1985), Cho and Choi (2000), Lee andChoi (2000), and Katusic and Krauss (2011).
The term ‘Liquid Flame Spray’ was originally used by
Tikkanen et al. (1994), Tikkanen et al. (1997) and Grosset al. (1999) in connection with an application where art
glass was coloured by nanoparticles deposited from theflame onto the surface of a molten glass item. In this article, we have maintained this definition that LFS consistsof a liquid precursor atomization by one of the combustion gases, favourably H2. This is not an unambiguous
definition but it clarifies the division within the family ofseveral flame spray pyrolysis modifications in literature,also described by e.g. Trommer and Bergmann (2015).
With this in mind, we have collected Table 1 to list up
papers dealing with LFS, or a very similar technique.
The basic characteristics of the LFS process have beenpresented by Tikkanen et al. (1997) and Aromaa et al.
(2007), and the process fundamentals have been further
discussed by Pitkänen et al. (2005), Keskinen et al.
(2007a) and Keskinen et al. (2008). A schematic of the
LFS-burner is presented in Fig. 1.
In the burner nozzle, the speed of the hydrogen flow, as
an atomizing gas, is close to the speed of sound. This isadvantageous for the atomizing process, as it assists inmaximizing the kinetic energy input into the atomizer,
where generally small precursor droplets are assumed to
be favourable to facilitate evaporation from the droplets.
Keskinen et al. (2008) have measured the droplet size distribution of the LFS atomizer with a conclusion that bynumber, the majority of the generated droplets are in thesubmicron size range, mass median diameter being ataround 3–5 μm.
According to the data by Tikkanen et al. (1997),
Pitkänen et al. (2005), Mäkelä et al. (2006) and Keskinen
et al. (2007a), the maximum temperature of the LFS flame
exceeds 2600 K. A temperature profile obtained with typ-142

Fig. 1Liquid Flame Spray burner. Note, that only two of theavailable coaxial rings for gas flow are necessary forthe basic performance of the LFS-method. The rest of
the rings may be used to modify the flame or to introduce additional components into the process.

Fig. 2Temperature in the center axis of a LFS flame with gas
volume flow rates of H2/O2 = 20/10 l/min. According todata from Pitkänen et al. (2005).

ical gas volume flow rates, (H2/O2 = 20/10 l/min) is presented in Fig. 2 (Pitkänen et al., 2005). This relatively
high temperature level is, again, advantageous to facilitate precursor evaporation and decomposition. Eventually,
it is apparent that the variety of feasible precursor liquids
that are accessible to the LFS is fairly large.
In most of the studies on the LFS, the ratio of hydrogenand oxygen flow rates has been typically stoichiometric,
2/1, but it can also be easily varied. In the studies by both
Cho and Choi (2000) and Lee and Choi (2000) on relatively similar oxy-hydrogen flames, the amount of hydrogen was decreased well below the stoichiometric value.
When hydrogen flow rate was varied in the order of 10 %of O2, both the flame temperature, increasing with thefraction of H2, and the crystal structure of the generated
particles were changed. Similar approach with varying
H2/O2 ratio have been described also by e.g. Kammler etal. (2001b), Hannebauer and Mezel (2003), Dosev et al.
(2006), Goldys et al., (2006), Jang et al. (2006), Pikhitsa
et al., (2007). Sheen et al. (2009) used an oxy-hydrogenJyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

Table 1 Collection of articles utilizing liquid feed-flame spray pyrolysis combined with H2/O2-flame
Reference

Particle compound

Application

Tikkanen et al., 1997, Surface and Coatings Technol.
Pitkänen et al., 2005, IFRF Combustion Journal
Aromaa et al., 2007, Biomolecular Engineering
Aromaa et al., 2013, A Nanotechnol Approach, Wiley
Cho and Choi, 2000, Journal of Aerosol Science.
Chung and Katz, 1985, Combustion and Flame
Harra et al., 2015, Aerosol Science and Technol.
Harra et al., 2016, AIChE Journal
Keskinen et al., 2008, Atomization and Sprays
Lee and Choi, 2000, Journal of Aerosol Science
Katusic and Kress 2011, Patent CA2803211 A1.
Keskinen et al., 2004, Journal of Materials Res.
Sokolowski et al., 1977, Journal of Aerosol Science
Harra et al., 2013, Powder Technology
Hartmann et al., 1985, US Patent
Keskinen et al., 2009, Journal of Applied Physics
Mäkelä et al., 2004, Journal of Materials Science
Nikkanen et al., 2008, Res. Lett. Nanotechnol
Nikkanen et a., 2014, Powder Technology
Dosev et al., 2006, J Aerosol Sci.
Goldys et al., 2006, J Am Chem Soc.
Keskinen et al., 2007b, Catalysis Letters
Mäkelä et al., 2006, Materials Letters
Sainiemi et al., 2007, Nanotechnology
Thybo et al., 2004, J Catalysis
Keskinen et al., 2007a, J. of Nanoparticle Research
Tikkanen et al., 1994, J. of Non-Crystalline Solids
Beneq Oy, 2016, Glass Coloration
Gross et al., 1999, J of Thermal Spray Technol.
Hotoleanu et al., 2002, NOC 2002
Rajala et al., 2003, Rev. Advanced Materials Sci.
Tammela et al., 2006, Proceedings of SPIE 6116
Keskinen et al., 2006, Catalysis Letters
Ejenstam et al., 2015b, Applied Surface Science
Piispanen et al., 2011, Applied Surface Science
Pimenoff et al., 2009, Thin Solid Films
Stepien et al., 2013b, Wear
Stepien et al., 2013c, Nanoscale Research Letters
Valtakari et al., 2016, Nordic Pulp and Paper Res J.
Songok et al., 2014, ACS Appl. Mater. and Interfaces
Keskinen et al., 2005, The Electrochemical Society
Valtakari et al., 2013, J Print Media and Technol. Res.
Aromaa et al., 2012, J Aerosol Science
Mäkelä et al., 2011, Aerosol Science and Technology
Moghaddam et al., 2015, HolzforschungStepien et al., 2011, Applied Surface Science
Stepien et al., 2013a, Langmuir
Teisala et al., 2010, Surface and Coatings Technology
Teisala et al. 2013a, Cellulose
Teisala et al., 2013b, Colloid and Polymer Science
Teisala et al., 2014a, Nordic Pulp and Paper Res J.
Teisala et al., 2014b, Cellulose
Tuominen et al., 2014, J. Adhesion Sci. Technol.
Haapanen et al., 2015, Mater. Chem. Physics
Teisala et al., 2012, Langmuir
Stepien et al., 2012a, Surface and Coatings Technol.
Stepien et al., 2012b, Applied Surface Science

Al2O3
SiO2
TiO2
Ag, TiO2
SiO2
SiO2
SiO2, TiO2
Al2O3
ZrO2
SiO2
NixOy, Mn xOy CoxOy
Ag, PdAl2O3
FexOy
TiO2 SiO2, Al2O3,
SnO2
Ag, Pd, FexOy
Al2O3, ZrO2, FexOy
Al2O3, ZrO2, FexOy
Eu:Y2O3
Er:Gd 2O3
Pd, Al2O3, La2O3
TiO2
SiO2
Au/TiO2
Ag, TiO2
CuO
CoO, Co3O4, CuO, Ag
CoO, Co3O4, CuO, Ag
SiO2, Al2O3, Er2O3
La2O3, SiO2, Er2O3, Al2O3
SiO2, Al2O3, Er2O3
Ag, TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
SiO2, TiO2
TiO2
SiO2
SiO2, TiO2
TiO2
TiO2
SiO2, TiO2

Process fundamentals
Process fundamentals
Process fundamentals
Process fundamentals
Process fundamentals
Process fundamentals
Process fundamentals
Process fundamentals
Process fundamentals
Process fundamentals
Powder generation
Powder generation
Powder and aerosol generation
Powder generation
Powder generation
Powder generation
Powder generation
Powder generation
Powder generation
Powder generation
Powder generation
Nanoparticle suspension
DepositDepositDepositDeposit, Aerosol
Class colouring
Glass colouring
Glass colouring
Optical fiber
Optical fiber
Optical fiber
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating
Functional coating

Specific

SiCl4 -precursor
H 2/O2/Ar counterflow flame
Powder form LFS
Residual particles
Spray droplets
SiCl4 -precursor

Photocatalysis
Gas sensors

Photoluminescence
Fluorescence
Catalysis
Etching mask
Catalysis
Binary particles
Coloured glass powder in liquid

Glass
Anti-bacterial
Corrosion protection
Glass
Glass, Wetting
Mechanical
MechanicalMicrofluidistics on board
Microfluidistics on paper
Photocatalysis
Printing
Wetting
Wetting
Wetting
Wetting
Wetting
Wetting
Wetting
Wetting
Wetting
Wetting
Wetting
Wetting
Wetting fundamentals
Wetting; Surface chemistry
Wetting; Surface chemistry143

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

flame to produce coated particles of SnO2, SiO2 and TiO2,
in binary combinations. Alternatively, if an excess amountof hydrogen will be injected, then the flame will create reducing conditions. In the study by Aromaa et al. (2012), it
was found out that an excess amount of hydrogen in the
flame may have an effect on the surface chemistry of thegenerated particles, and the amount of carbon/hydrocarbons on the surface. But in their case the product Ti-compound remained still in the form of TiO2. Provided thatthe hydrogen flow is kept constant, size and temperatureof the flame can be affected also by varying the solvent
feed rate (Mäkelä et al., 2011), especially for the alcohol
based precursors.
The atomization of the liquid precursor usually brings
along another issue: the fate of the residue of each precursor droplet sprayed into the flame (Strobel and Pratsinis,
2011; Rosebrock et al., 2013; 2016). If the precursor liquid
is sufficiently volatile, the original droplet will evaporate
and a reaction in the gas phase will take place producing
nanoparticles, as expected. If the precursor liquid is notvolatile enough, then a liquid-to-solid reaction will takeplace in the droplet resulting in the formation of residual
particles, as described in Fig. 3.
Depending on the properties of the liquid precursor, e.g.
volatility, concentration and temperature of the decomposition, the remaining residual particles may be moderately

Fig. 3

144

Schematic of the particle formation mechanisms in a
Liquid Flame Spray process. Both agglomerated
nanoparticles and larger residual particles will beformed, resulting in a bimodal product particle size
distribution. Updated from Aromaa et al. (2013).

large. In the worst case, the residual particle diameter
may remain in the order of 100 nm–1 μm. This is not by
any means advantageous for the nanopowder production,
since the specific surface area of the powder will inevitably be decreased. The importunate problem has recently
been solved for the case of conventional methane/oxygen
FSP-flame by introducing specific chemical components
in the precursor to induce droplet explosion (Strobel andPratsinis, 2011; Rosebock et al., 2013; Rosebock et al.,2016). In their case, DEGBE (diethylene glycol monobutyl
ether) or EHA (2-ethylhexanoic acid) were introduced
into the precursor liquid, using concentration levels of
50 % for these chemicals. In the case of our hydrogen
driven LFS, it was recently found out that already lessthan 5 % of EHA in the precursor solution (containing
aluminum nitrate) may be sufficient to induce the desired
explosion of the precursor droplets, and thereby avoiding
residual particle formation (Harra et al., 2016).

3. Applications
3.1 Nanopowder generation
The most widely used application of the FSP is the generation of nanopowders (Ulrich, 1971; Ulrich, 1984). The
product material is most often collected by bag filters
consisting of e.g. glass fiber material (Kammler et al.,2001a; Stark and Pratsinis, 2002). A commercial process
by Degussa/Evonik utilizes a hydrogen-oxygen flame,
similarly to the LFS, but typically gaseous precursors,
such as SiCl4 and TiCl4 (Hartmann et al., 1995). Still,
Evonik also uses a liquid fed FSP system, with a preferenceof hydrogen gas as a fuel, e.g. to produce nanopowders
containing lithium compounds (Katusic and Kress, 2011).
The LFS has been used in nanopowder production fore.g. single component (Tikkanen et al., 1997; Mäkelä et al.,2004), binary component (Keskinen et al., 2004; Keskinen
et al., 2005) and multicomponent nanopowders (Nikkanen
et al., 2008; Nikkanen et al,. 2014). In these studies, theparticulate matter was collected from the exhaust flow of
the flame using an electrostatic precipitator.
Nikkanen et al. (2014) used collected and compacted
composite nanopowder made by the LFS to fabricate carbon nanotubes within the porous powder sample. The
LFS generated nanopowder has also been used for fabricating magnetic composite nanoparticles in a tube furnace(Harra et al., 2013), in order to produce photocatalytic
powder for water purification, and for a study on coatingof ceramic nanoparticles with silver in a tube furnace(Harra et al., 2015).
One consequence of the fuel being hydrogen is that theend product will be mostly water vapour. This fact can beutilized in the powder collection, as described by KeskinenJyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

Fig. 4

Collection of the LFS generated nanoparticles into aliquid suspension utilizing combined particle thermophoresis and water vapour condensation due to the cold
reactor walls. Adapted from Keskinen et al. (2007b).

et al. (2007b). The principle is illustrated in Fig. 4. By introducing a cold walled chamber around the flame, boththe nanoparticles and the water vapour can be collected
into a suspension via combined nanoparticle thermophoresis and vapour condensation, both of which tend totransport the material towards the cooled walls.

3.2 Test aerosol generation
Aerosol nanoparticle sources with controlled particle
size and output concentration are useful for generating
test aerosols for e.g. instrument calibration or filter testing(Kulkarni et al., 2011). Also, with combined aerosol measurement and modeling of flow field and aerosol particle
dynamics, the data describing the evolution of the aerosol
emitted from a localized emission, can be used to assess
the exposure of e.g. engineered nanoparticles in a workplace in different release scenarios (Brouwer et al., 2009).
Among the many methods to generate aerosol nanoparticles (Kulkarni et al., 2011), Liquid Flame Spray can beconsidered an openly operating particle source in an atmospheric pressure. In this respect, the flame can be considered as an exemplary case of a nanoparticle process or
potentially also an aerosol source emulating airborne engineered nanoparticle (ENP) release in the workplace. Ina typical scenario of occupational health, the particles areusually emitted from the process due to leaks in the ducts
or processing chambers or other unintentional releasefrom the process line into the laboratory. Flame technique
is one of the most up-scalable techniques for generating
nanoparticles to fill large containers for high-volume useof ENPs, or e.g. for arranging a flow split into several ex-

posure chambers. The in situ production of nanoparticles
in large quantities with a flame, with natural suspension
into the air, is a much simpler and effective way to generate a well-defined nanoaerosol than to start with a commercial nanopowder and then de-agglomerate anddisperse it into an airborne suspension (Mülhopt et al.,2007).
Monsé et al. (2013; 2014) have characterized the LFS
flame and its gaseous emissions in order to test and use it
for exposure assessment. The CO2-emission of the LFS
flame is rather small, since the only carbon source is the
precursor itself (including solvent). This is advantageouscompared to the conventional flames utilizing e.g. methane or acetylene. However, a slight disadvantage in thisapplication arises from nitrogen oxides. If the LFS flame
operates normally in the free atmospheric conditions,having immediate access to ambient oxygen in the surroundings of the flame, then, due to the thermal reactions,
the NOx level in the exhaust gas can exceed 1.5 ppm(Monsé et al., 2014). Whereas, the available excess oxygen is typically considered to be advantageous for thenanopowder generation to ensure complete oxidation of
the product (Katusic and Kress, 2011), here the excess oxygen is not necessarily crucial. Monsé et al. (2014) succeeded in decreasing the NOx concentration level below
the required 500 ppb by arranging a sheath flow of inert
gas around the flame. In their case, the argon flow introduced from the outmost ring in the annular coflow burner
kept the flame apart from the oxygen of the ambient air,
avoiding excess NOx formation.
Flame generator can also be used as a test aerosol
source in several other purposes such as instrument testing (Mäkelä et al., 2009; Asbach et al., 2009) or filter
loading experiments. Once the nanoparticle aerosol has
been generated, both large scale industrial filters andsmaller scale personal gas mask filters could be considered. In two recent studies (Koivisto et al., 2012; Koivisto
et al., 2015), a potential inhalation dose and performanceof a loose fitting respiratory filter were evaluated using
particle size distribution and concentration data obtained
from a LFS generated aerosol in a test room.

3.3 Accumulation of particle deposit
In some applications, as in catalysis, photocatalysis, orgas measurement, granular highly porous material isneeded, in order to have large surface area available forreaction to occur on. However, these materials often require that certain degree of solidity needs to be maintained. The porous material for, e.g., gas sensing can beprepared by creating a viscous paste which contains thenanopowder and then performing an appropriate heat
treatment to form the final material (Keskinen et al.,2009). The material could also be accumulated by direct145

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

deposition of the nanoparticles from the gas phase(Mädler et al., 2006a; Castillo et al., 2014; Liu et al.,2016).
In our case, the nanoparticle deposition process fromthe flame onto the substrate is mainly a combination ofthermophoresis and diffusion through the boundary layer
above the surface:dN 0.55Nγgas 1 dT
J
D

dx
ρgas
T dx

(1)

where N denotes the particle number concentration, x distance towards the substrate, γ and ρ the dynamic viscosityand density of the gas, respectively, and D denotes the
diffusion coefficient of the particles.
It is convenient to categorize the arrangements of thedeposited nanoparticles on the substrate, depending on themorphology of the generated porous material. Fig. 5 illustrates schematics of some main alternatives. The particles
are arranged on the substrate as a sub-monolayer either
clearly separate (Fig. 5a) or somewhat closer and having
contact with each other (Fig. 5b). Sainiemi et al. (2007)
utilized these two options to create SiO2-nanoparticle
mask for deep reactive ion etching of silicon wafer in order to fabricate nanopillars on the wafer surface. The particles may also form a porous layer either without
sintering (Fig. 5c) or with sintering (Fig. 5d), or evenform a thicker layer (Fig. 5e). Finally, the particles may
also be embedded in the substrate (Fig. 5f), as may occur
with softer substrates, such as e.g. polymer or molten glass.
The heat of the flame assists the process keeping the substrate soft and thereby enhancing particle penetration into
the substrate material. Using polymers as a deposition
substrate for nanoparticles to fabricate polymer nanocom-

Fig. 5

146

Schematic illustration of different alternatives for morphology of the nanoparticle deposit.

posites has a high potential for many applications. Recently
e.g. plasmonic and phosphorescent-superparamagnetic
actuators have been prepared by depositing nanoparticles
from the flame into a polymer film (e.g. PMMA) and covering the nanoparticle film with spin coated layer of thesame polymer as the substrate, resulting in a nanoparticle
layer fully embedded in the polymer (Sotiriou et al.,2013). Also conductive polymer nanocomposites havebeen fabricated with similar process (Blattmann et al.,2015).
Of the alternatives described in Fig. 5a–f, the thin particle layers, such as shown in Fig. 5a–d, are possible to bedeposited practically onto any material, since the interaction time with the flame and substrate can be very short.
Two practical examples have been presented in Fig. 6aand Fig. 6b. If thicker nanoparticle layers are required,
then high nanoparticle concentrations, intensive particle
fluxes or longer deposition times are needed. For longer
deposition time, it is often also required that the substrate
stands heat of the flame relatively well.
The direct deposition process of the LFS generated
nanoparticles, as described above, has been studied by
Mäkelä et al. (2006), Keskinen et al. (2006) and Keskinen
et al. (2007a). It is apparent that along with the accumulation of the nanoparticles, the porous material will be
gradually deformed due to the heat treatment. In case of
Mäkelä et al. (2006), it was observed that the collected
TiO2, which originally consisted mostly of anatase, had atendency to be converted into rutile due to the heat treatment during the deposition process. In Fig. 6, a layer ofdeposited SiO2-Al2O3 (90 %/10 %) is shown in different

Fig. 6a–b) Sub-monolayer types of silver nanoparticle deposits, c–f) Accumulation of a thicker porous SiO2-Al2O3
nanoparticle layer on a silicon wafer (Mäkelä, 2005).

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

scales. For the largest magnifications, it is seen that a certain degree of sintering has occurred.
The sintering of the nanoparticles in a similar process,while fabricating nanostructured porous photocatalytic
TiO2, has been further studied by Thimsen and Biswas(2007) and Thimsen et al. (2008), with a conclusion thatthe eventual sintering of the porous material under theflame is governed by both the sample temperature and theprimary particle size, sintering being enhanced with increasing temperature and with decreasing primary particle size.
It is notable that this deposit of our LFS flame generated particles, shown in Fig. 6e–f, has a strikingly similar
structure to the one presented by Mädler et al. (2006a),
where also thermophoresis is the dominating deposition
mechanism, but clearly differs from the more tree-likestructure presented by Castillo et al. (2014), where diffusion clearly is the dominating deposition mechanism.
The assembling of the porous nanoparticle layer ishighly dependent on the deposited particle morphology
and the Peclet number of the flow field. (Mädler et al
2006; Nasiri et al., 2015) The layer thickness and porosity
can be individually controlled as shown by e.g. Mädler etal. (2006b). Ballistic deposition of individual particles
produces denser layers than those produced by letting agglomerates with low fractal dimension diffuse to the surface.
Comparison of the cross-sectional SEM image of LFSmade TiO2 layer by Stepien et al. (2013c) and simulations
made by Mädler et al. (2006b) indicates that the LFSmade porous TiO2 -nanoparticle layer consists of deposits
of multiple agglomerates produced by flame process. Deposited agglomerates consist of several primary nanoparticles, forming thick layer of nanoparticles with highporosity.

3.4 Glass colouring and processing
In the early history of the LFS, properties of glass material were affected by the deposition of flame generated
nanoparticles onto its surface. Here, both art glass colouring and amplifying optical fiber are discussed.
In Fig. 7a and Fig. 7b the principle of the art glass colouring has been presented. During the high temperatureprocess in molten glass, it is assumed that the nanoparticle will first deposit and plunge into the glass (according
to Fig. 5f), and then the metal compound will be dissolved into the glass matrix, and finally the ions will be
diffused inside the glass material. A typical colouring
agent is CoO3, which creates a strong blue colour within
the soda-lime-silica glass due to dissolved Co2+ ions(Fig. 7b, Gross et al., 1999). The wavelength from the incoming light, which corresponds to the electronic transition of the metal ion, is absorbed in the glass and thus theFig. 7

Glass processing. a) Deposition of nanoparticles onto arotating substrate, an art glass item or an optical fiber
preform, b) glass colouring in practice, blue colour from
CoO3 nanoparticles, c–d) optical micropraphs of a crosssection of coloured yellow glass (Ag-nanoparticles), e)
SEM backscattering graph of a cross section of coloured glass (Ag), f) TEM graph of in-flame sampled
nanoparticles (Au), used for red colour in glass item
shown (in insert). Glasswear design by Markus Eerola.

imparting colour will be made up of the remaining wavelengths in the visible spectrum (Bamford, 1977). However, compounds such as silver (Fig. 7c–e) and gold
(Fig. 7f) are known to dissolve and re-nucleate into reduced nanoparticles. There the colour effect will arise
from light scattering from the nanoparticles in the glass
matrix, being eventually dependent on the final size of thenoble metal nanoparticles. In this application, where the
LFS flame is in close contact with the glass substrate, theheat from the flame is an essential enabling feature of theprocess. The heat prevents the molten glass from cooling
down temporarily and it also intensifies both the particle
diffusion and dissolving inside the glass material. The
LFS is also commercially applied in float glass colouring
under the trademark nHALO© (Beneq, 2016).
There are also several other LFS applications related to
optics, such as doped optical fiber fabrication (Rajala etal., 2003; Tammela et al., 2006), where one needs to beable to control the refractive index of the material (Becker
et al., 1999). Here, aluminum, phosphorus and germanium
are most often used as additives to silica. Furthermore,
the dopant, such as erbium, which is crucial for the amplifying property, also needs to be injected to the fiber preform material. This is most conveniently arranged by aflame-driven nanoparticle deposition process according to147

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

Fig. 7a, resulting in a deposit structure given in Fig. 5e. It
is also important that when the generated deposit, the actual fiber preform, will be further processed into an optical fiber, having the presence of carbonaceous compounds
minimized with the use of hydrogen flame is an advantage (Hotoleanu et al., 2002).
Other optical applications of LFS can be found in fabricating surface plasmon resonance materials (Saarinen etal., 2014). Finally, also functional coatings can be fabricated on glass surfaces, practically by the same mechanisms, using the LFS in a direct deposition process(Pimenoff et al., 2009; Piispanen et al., 2011).

3.5 Functional coatings
In the past 10 years, the Liquid Flame Spray method
has been applied to prepare functional coatings by utilizing direct deposition of the nanoparticles from the flame
onto the substrate. One of the conventional uses ofnanoparticles is catalysis, as described by e.g. Johannessen
et al. (2004), where methane was used as fuel to fabricate
noble metal based nanoparticles. The LFS has also beenused in a somewhat similar application (Keskinen et al.,2007b; Aromaa et al. 2013). Photocatalytic coatings havebeen fabricated by Keskinen et al. (2005) and Keskinen etal. (2006).
Recently, a slightly less conventional application of the
LFS method as a functional coating process has been introduced, where TiO2 or SiO2 nanoparticles are deposited
onto heat sensitive materials, such as paper or paperboard,in a roll-to-roll scheme, in order to control the wetting of
the surface (Teisala et al., 2010; Mäkelä et al., 2011;
Stepien et al., 2011; Teisala et al., 2014a; Haapanen et al.,2015). Here, a layer of nanoparticles, with structure andmorphology described in Fig. 5e has been created. The
feasible line speeds for the process are between 50–
500 m/min. In the process, the LFS flame is directed
towards the moving paperboard. From the model calculations (Aromaa et al., 2012) and from the paper surface
temperature measurements using a pyrometer, the surface
temperature of the paperboard has been estimated to remain below 300 °C. The nanoparticle flux from the flame(T = 600–800 °C) onto the paperboard is estimated to becaused mostly by thermophoresis. A SEM-graph of theporous layer, consisting of ca. 20 nm TiO2 nanoparticles,
is shown in Fig. 8. According to the analysis of Stepien etal. (2013a), the thickness of the porous coating in the
as-prepared form is approximately 600 nm, but the layer
can be solidified by a heavy mechanical compression.
Both superhydrophilic (Tuominen et al., 2014) and superhydrophobic (Teisala et al., 2012; Stepien et al., 2012a;
Stepien et al., 2012b; Stepien et al., 2013a), coatings havebeen fabricated using SiO2 and TiO2, respectively. Additionally, their mechanical properties have been tested

148

Fig. 8The porous structure of superhydrophobic TiO2
nanoparticle deposit on a paperboard (see e.g. Teisalaet al. (2014a) and references from thereon). Insert: water droplet on the superhydrophobic paperboard.

(Stepien et al. 2013b; Stepien et al. 2013c), and surface
chemistry have been characterized (Stepien et al., 2012a;
Stepien et al., 2013a). The results indicate that to achieve,e.g., a full superhydrophobicity on the coating, both surface chemistry and a hierarchical micro-nanostructure isrequired (Teisala et al., 2012; 2013b). Most interestingly, it
is possible to convert the superhydrophobic TiO2 surface
reversibly into superhydrophilic with UV illumination,and back to superhydrophobic with a heat treatment(Teisala et al., 2013a; Teisala et al., 2014b). Surface analysis has shown (Stepien et al., 2013a) that when the coating
has been deposited on a paper or a paperboard, there
seems to be a layer of carbonaceous compounds on the
TiO2 nanoparticles, which will be chemically converted
due to UV. The result has been interpreted so that theflame vaporises some carbonaceous material from thesubstrate, which will re-condense on the particles, andthereby the surface of the nanoparticles is chemically altered. This indicates that the coating process itself may
play a central role in affecting the functionality of thecoating.
This control of the switchable surface wetting of heatsensitive substrates, between superhydrophilic and superhydrophobic states, can be further applied in e.g. printability (Valtakari, et al., 2013) and microfluidistic design
on roll-to-roll materials (Songok, et al., 2014; Valtakari etal., 2016). Also, hydrophobisation of the surface of woodmaterials is an important field (Moghaddam et al., 2013;
Moghaddam et al., 2014), where direct nanoparticle deposition from the LFS has also been applied successfully(Moghaddam et al., 2015; Moghaddam et al., 2016). Super-­hydrophobic coatings can also be used to affect the corrosion properties of the surface (Ejenstam et al., 2013;
Ejenstam et al., 2015a; Ejenstam et al., 2016), with a potential application field also for nanoparticle coatings(Ejenstam et al., 2015b).

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

4. Conclusion
We have presented results obtained during the past
years on the several applications of the Liquid Flame
Spray. As one type of a flame spray pyrolysis, it may beconsidered to be a conventional method for generating
nanopowders from liquid precursors, even though it canutilize a wider range of available precursor compounds
due to the help from the hydrogen combustion. Furthermore, due to its characteristics it can easily be applied indirect nanoparticle coating process of even a larger area,and moreover, the combustion product being water vapour
instead of carbonaceous compounds gives a clear advantage in specific applications.

Acknowledgements
The authors wish to acknowledge funding from Academy of Finland, project nLABS 2014–2018, and from
Finnish Funding Agency for Innovation (Tekes), project
NANORATA2 (2011–2014) grant no. 40094/11. The research leading to these results has also received funding
from European Union’s Seventh Framework Programme
FP7/2007–2013/under REA grant agreements n° [CP-IP
211464-2] (NANODEVICE 2009–2013) and n° [280765](BUONAPART-E 2012–2016). The article reflects only
the authors’ view and that the Union is not liable for any
use that may be made of the information contained
therein.

References
Aromaa M., Keskinen H., Mäkelä J.M., The effect of processparameters on the Liquid Flame Spray generated titania
nanoparticles, Biomolecular Engineering, 24 (2007) 543–
548.
Aromaa M., Arffman A., Suhonen H., Haapanen J., Keskinen J.,
Honkanen M., Nikkanen J.-P., Levänen E., Messing M.E.,
Deppert K., Teisala H., Tuominen M., Kuusipalo J., Stepien
M., Saarinen J.J., Toivakka M., Mäkelä J.M., Atmosphericsynthesis of superhydrophobic TiO2 nanoparticle deposits
in a single step using Liquid Flame Spray, Journal of Aerosol Science, 52 (2012) 57–68.
Aromaa M., Pimenoff J.A., Mäkelä, J.M., Liquid Flame Sprayas a means to achieve nanoscale coatings with easy-toclean properties, in: Daoud W.A. (Ed.), Self-Cleaning
Materials and Surfaces: A Nanotechnology Approach, John
Wiley & Sons Ltd, Chichester, 2013, pp. 229–252.
Asbach C. Kaminski H., Fissan H., Monz C., Dahmann D.,
Mülhopt S., Paur H.-R., Kiesling H.J., Herrmann F., Voetz
M., Kuhlbusch T.A.J., Comparison of four mobility particle
sizers with different time resolution for stationary exposure
measurements, Journal of Nanoparticle Research, 11 (2009)

1593–1609.
Bamford C.R., Color Generation and Control in Glass, Elsevier,
Amsterdam, 1977.
Becker P., Olsson N.A., Simpson J.R., Erbium-Doped Fiber
Amplifiers: Fundamentals and Technology, Academic
Press, San Diego, 1999.
Beneq Oy, 2016, Glass Coloration <http://beneq.com/
glass-coloration.html> accessed 15.05.2016.
Blattmann C.O., Sotiriou G.A., Pratsinis S.E., Rapid synthesisof flexible conductive polymer nanocomposite films, Nanotechnology, 26 (2015) 125601.
Brouwer D., van Duuren-Stuurman B., Berges M., Jankowska
E., Bard D., Mark D., From workplace air measurement
results toward estimates of exposure? Development of astrategy to assess exposure to manufactured nano-objects,
Journal of Nanoparticle Research, 11 (2009) 1867–1881.
Castillo J.L., Martin S., Rodriguez-Perez D., Perea A., Garcia-­
Ybarra P.-L., Morphology and nanostructure of granularmaterials built from nanoparticles, KONA Powder and Particle Journal, 31 (2014) 214–233.
Cho J., Choi M., Determination of number density, size andmorphology of aggregates in coflow diffusion flames using
light scattering and local sampling, Journal of Aerosol Science, 31 (2000) 1077–1095.
Chung S.-L., Katz J.L., The counterflow diffusion flame burner:
A new tool for the study of the nucleation of refractorycompounds, Combustion and Flame, 61 (1985) 271–284.
Dosev D. Guo B., Kennedy I.M., Photoluminescence of Eu3+:
Y2O3 as an indication of crystal structure and particle sizein nanoparticles synthesized by flame spray pyrolysis, J.
Aerosol Sci., 37 (2006) 402–411.
Ejenstam L., Ovaskainen L., Rodriguez-Meizoso I., Wågberg
L., Pan J., Swerin A., Claesson P.M., The effect of superhydrophobic wetting state on corrosion protection – The AKD
example, Journal of Colloid and Interface Science, 412(2013) 56–64.
Ejenstam L., Swerin A., Pan J., Claesson P.M., Corrosion protection by hydrophobic silica particle-polydimethylsiloxane
composite coatings, Corrosion Science, 99 (2015a) 89–97.
Ejenstam L., Tuominen M., Haapanen J., Mäkelä J.M., Pan J.,
Swerin A., Claesson, P.M., Long-term corrosion protection
by a thin nano-composite coating, Applied Surface Science,
357 (2015b) 2333–2342.
Ejenstam L., Swerin A., Claesson P.M., Toward superhydrophobic polydimethylsiloxane-silica particle coatings, Journal of
Dispersion Science and Technology, 37 (2016) 1375–1383.
Goldys E.M., Drozdowicz-Tomsia K., Jinjun S., Dosev D.,
Kennedy I.M., Yatsunenko S., Godlewski M., Optical
Characterization of Eu-Doped and Undoped Gd 2O3
Nanoparticles Synthesized by the Hydrogen Flame Pyrolysis Method, J. Am. Chem. Soc., 128 (2006) 14498–14505.
Gross K.A., Tikkanen J. Keskinen J., Pitkänen V., Eerola M.,
Siikamäki R., Rajala M., Liquid flame spraying for glasscoloring, Journal of Thermal Spray Technology, 8 (1999)
583–589.
Haapanen J., Aromaa M., Teisala H, Tuominen M., Stepien M.,
Saarinen J.J., Heikkilä M.J., Toivakka M., Kuusipalo J.,
Mäkelä J.M., Binary TiO2/SiO2 nanoparticle coating for149

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

controlling the wetting properties of paperboard, Materials
Chemistry and Physics, 149–150 (2015) 230–237.
Hannebauer B., Menzel F., The Combustion of SiCl4 in Hot O2/
H2 flames, Z. Anorg. Chem, 629 (2003) 1485–1490.
Harra J., Nikkanen J.-P., Aromaa M. Suhonen H., Honkanen M.,
Salminen T., Heinonen S., Levänen E., Mäkelä J.M., Gas
phase synthesis of encapsulated iron oxide–titanium dioxide composite nanoparticles by spray pyrolysis, Powder
Technology, 243 (2013) 46–52.
Harra J., Juuti P., Haapanen J., Sorvali M., Roumeli E., Honkanen
M., Vippola M., Yli-Ojanperä J., Mäkelä J.M., Coating ofsilica and titania aerosol nanoparticles by silver vapor condensation, Aerosol Science and Technology, 49 (2015) 767–
776.
Harra J., Kujanpää S., Haapanen J., Juuti P., Hyvärinen L.,
Honkanen M., Mäkelä J.M., Aerosol Analysis of Residual
and Nanoparticle Fractions from Spray Pyrolysis of Poorly
Volatile Precursors. Accepted manuscript online: 11 AUG
2016 11:40PM EST | DOI: 10.1002/aic.15449.
Hartmann W., Mangold H., Kerner D., Flame-hydrolytically
produced titanium dioxide mixed oxide, method of its production and its use, US Patent, (1995) 5451390 A.
Hotoleanu M., Kiiveri P., Tammela S., Särkilahti S., Valkonen
H., Rajala M., Kurki J., Janka K., Characteristics of highly
doped Er-fibers manufactured by the new direct nanoparticle deposition process, in: 7th European Conference on
Networks & Optical Communications (NOC2002), Darmstadt, 2002, pp. 200–204.
Jang H.D., Chang H., Suh Y., Okuyama K., Synthesis of SiO2
nanoparticles from sprayed droplets of tetraethylorthosilicate by the flame spray pyrolysis, Current Applied Physics,
6, Supplement 1 (2006) e110–e113.
Johannessen T., Jensen J.R., Mosleh M., Johansen J., Quaade U.,
Livbjerg H., Flame synthesis of nanoparticles: Applications
in catalysis and product/process engineering. Chemical
Engineering Research and Design, 82 (2004) 1444–1452.
Kammler H.K., Mädler L., Pratsinis S.E., Flame synthesis of
nanoparticles, Chemical Engineering & Technology, 24(2001a) 583–596.
Kammler, H.K., Mueller, R., Senn, O., Pratsinis, S.E., Synthesisof silica–carbon particles in a turbulent H2-air flame aerosol reactor. AIChE Journal, 47 (2001b) 1533–1543.
Katusic S., Kress P., Method for producing mixed oxides comprising lithium, Patent, (2011) CA2803211 A1.
Keskinen H., Mäkelä J.M., Vippola M., Nurminen M.,
Liimatainen J., Lepistö T., Keskinen, J., Generation of silver/
palladium nanoparticles by liquid flame spray. Journal of
Materials Research, 19 (2004) 1544–1550.
Keskinen H., Mäkelä J.M., Hellsten S., Aromaa M., Levänen E.,
Mäntylä T., 2005. Generation of titania nanoparticles byliquid flame spray for photocatalytic applications, in: Devi
A., Fischer R., Parala H., Allendorf M., Hitchman M.
(Eds.), EUROCVD-15: Fifteenth European Conference on
Chemical Vapor Deposition, The Electrochemical Society,
Inc., Pennington, 2005, pp. 491–498.
Keskinen H., Mäkelä J.M., Aromaa M., Keskinen J., Areva S.,
Teixeira C.V., Rosenholm J.B., Pore V., Ritala M., Leskelä
M., Raulio M., Salkinoja-Salonen M.S., Levänen E.,

150

Mäntylä T., Titania and titania-silver nanoparticle deposits
by Liquid Flame Spray and their functionality as photocatalyst for organic- and biofilm removal, Catalysis Letters,
111 (2006) 127–132.
Keskinen H., Mäkelä J.M, Aromaa M., Ristimäki J., Kanerva T.,
Levänen E., Mäntylä T., Keskinen J., Effect of silver addition on the formation and deposition of titania nanoparticles
produced by Liquid Flame Spray, Journal of Nanoparticle
Research, 9 (2007a) 569–588.
Keskinen H., Mäkelä J.M., Heikkinen R., Suopanki A., KeskinenJ., Synthesis of Pd-alumina and Pd-lanthana suspension for
catalytic applications by one-step Liquid Flame Spray,
Catalysis Letters, 119 (2007b) 172–178.
Keskinen H., Aromaa M., Heine M.-C., Mäkelä, J.M., Size andvelocity measurements in sprays and particle producing
flame sprays, Atomization and Sprays, 18 (2008) 1–26.
Keskinen H., Tricoli A., Marjamäki M., Mäkelä J.M., Pratsinis,
S.E., Size-selected agglomerates of SnO2-nanoparticles as
gas sensors, Journal of Applied Physics, 106 (2009) 084316.
Koivisto A.J., Aromaa M., Mäkelä J.M., Pasanen P., Hussein T.,
Hämeri K., Concept to estimate regional inhalation dose ofindustrially synthesized nanoparticles, ACS Nano, 6 (2012)
1195–1203.
Koivisto A.J., Aromaa M., Koponen I.K., Fransman W., Jensen
K.A., Mäkelä J.M., Hämeri K.J., Workplace performance ofa loose-fitting powered air purifying respirator during
nanoparticle synthesis, Journal of Nanoparticle Research,
17 (2015) 177–186.
Kruis F.E., Fissan H., Peled A., Synthesis of nanoparticles in thegas phase for electronic, optical and magnetic applications—a review, Journal of Aerosol Science, 29 (1998) 511–
535.
Kulkarni P., Baron P.A., Willeke K., Aerosol Measurement:
Principles, Techniques, and Applications, John Wiley &Sons, Inc., Hoboken, 2011.
Lee D., Choi M., Control of size and morphology of nano particles using CO2 laser during flame synthesis, Journal of
Aerosol Science, 31 (2000) 1145–1163.
Liu G., Wong W.S.Y, Nasiri N, Tricoli A., Ultraporous superhydrophobic gas-permeable nano-layers by scalable solvent-­
free one-step self-assembly Nanoscale, 8 (2016) 6085.
Mädler L., Liquid-fed aerosol reactors for one-step synthesis of
nano-structured particles, KONA Powder and Particle
Journal, 22 (2004) 107–120.
Mädler L., Roessler A., Pratsinis S.E., Sahm T., Gurlo A., BarsanN., Weimar U., Direct formation of highly porous gas-­sensing films by in situ thermophoretic deposition of flamemade Pt/SnO2 nanoparticles, Sensors and Actuators B, 114(2006a) 283–295.
Mädler L., Lall A.A., Friedlander S.K., One-step aerosol synthesis of nanoparticle agglomerate films: Simulation of film
porosity and thickness, Nanotechnology, 17 (2006b) 4783–
4795.
Mäkelä J.M., Keskinen H., Forsblom T., Keskinen, J., Generation of metal and metal oxide nanoparticles by liquid flame
spray process, Journal of Materials Science, 39 (2004)
2783–2788.
Mäkelä J.M., Unpublished results, Department of Physics, Tam-Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

pere University of Technology, (2005).
Mäkelä J.M., Hellsten S., Silvonen J., Vippola M., Levänen E.,
Mäntylä T., Collection of liquid flame spray generated TiO2
nanoparticles on stainless steel surface, Materials Letters,
60 (2006) 530–534.
Mäkelä J.M., Aromaa M., Rostedt A., Krinke T.J., Janka K.,
Marjamäki M., Keskinen J., Liquid flame spray for generating metal and metal oxide nanoparticle test aerosol, Humanand Experimental Toxicology, 28 (2009) 421–431.
Mäkelä J.M., Aromaa M., Teisala H., Tuominen M., Stepien M.,
Saarinen J.J., Toivakka M., Kuusipalo J., Nanoparticle
deposition from Liquid Flame Spray onto moving roll-toroll paperboard material, Aerosol Science and Technology,
45 (2011) 827–837.
Mishra Y.K., Kaps S., Schuchardt A., Paulowicz I., Jin X.,
Gedamu D., Wille S., Pulan O., Adelung R., Versatile fabrication of complex shaped metal oxide nano-microstructures
and their interconnected networks for multifunctional
applications, KONA Powder and Particle Journal, 31 (2014)
92–110.
Moghaddam M.S., Wålinder M.E.P., Claesson P.M., Swerin A.,
Multicycle Wilhelmy plate method for wetting properties,swelling and liquid sorption of wood, Langmuir, 29 (2013)
12145–12153.
Moghaddam M.S., Claesson P.M., Wålinder M.E.P., Swerin A.,
Wettability and liquid sorption of wood investigated by
Wilhelmy plate method, Wood Science and Technology, 48(2014) 161–176.
Moghaddam M.S., Heydari G., Tuominen M., Fielden M.,
Haapanen J., Mäkelä J.M., Wålinder M.E.P., Claesson P.M.,
Swerin A., Hydrophobisation of wood surfaces by combining liquid flame spray (LFS) and plasma treatment:
dynamic wetting properties, Holzforschung, 70 (2015) 527–
537. DOI: 10.1515/hf-2015-0148.
Moghaddam M.S., Wålinder M.E.P., Claesson P.M., Swerin A.,
Wettability and swelling of acetylated and furfurylated
wood analyzed by multicycle Wilhelmy plate method,
Holzforschung, 70 (2016) 69–77.
Monsé C., Monz C., Dahmann D., Asbach C., Stahlmecke B.,
Lichtenstein N., Buchwald K.E., Merget R., Bunger J.,
Bruning T., Developments for investigations into the healtheffects of zinc oxide particles, Gefahrstoffe Reinhaltung
der Luft, 73 (2013) 144–148.
Monsé C., Monz C., Dahmann D., Asbach C., Stahlmacke B.,
Lichtenstein N., Buckwald K.-E., Megret R., Bünger J.,
Brüning T., Development and evaluation of a nanoparticle
generator for human inhalation studies with airborne zincoxide, Aerosol Science and Technology, 48 (2014) 418–426.
Mueller R., Mädler L., Pratsinis S.E., Nanoparticle synthesis athigh production rates by flame spray pyrolysis, Chemical
Engineering Science, 58 (2003) 1969–1976.
Mülhopt S, Kochek Alavi A., Diabaté S, Paur, H.-R., Aerosol
generation for exposure experiments, in: European Aerosol
Conference, Association for Aerosol Research, Salzburg,
2007, Abstract T09A019.
Nasiri N., Elmoe T.D., Liu Y., Qin Q.H., Tricoli A., Self-assembly
dynamics and accumulation mechanisms of ultra-fine
nanoparticles, Nanoscale, 7 (2015) 9859–9867.

Nikkanen J.-P., Keskinen H., Aromaa M., Järn M., Kanerva T.,
Levänen E., Mäkelä J.M., Mäntylä T., Iron oxide doped
alumina-zirconia nanoparticle synthesis by liquid flame
spray from metal organic precursors, Research Letters in
Nanotechnology, 2008 (2008) 516478.
Nikkanen J.-P., Harju M., Järn M., Linden J., Rintala J., Messing
M.E., Huttunen-Saarivirta E., Saarinen T., Kanerva T.,
Honkanen M., Aromaa M., Levänen E., Pettersson M.,
Mäkelä J.M., Deppert K., Mäntylä T., Synthesis of carbon
nanotubes on FexOy doped Al2O3–ZrO2 nanopowder, Powder Technology, 266 (2014) 106–112.
Park H.K., Park K.Y., Control of particle morphology and sizein vapor-phase synthesis of titania, silica and alumina
nanoparticles, KONA Powder and Particle Journal, 32(2015) 85–101.
Pimenoff J.A., Hovinen A.K., Rajala M.J., Nanostructured coatings by liquid flame spraying, Thin Solid Films, 517 (2009)
3057–3060.
Piispanen M., Hupa L., Comparison of self-cleaning propertiesof three titania coatings on float glass, Applied Surface Science, 258 (2011) 1126–1131.
Pikhitsa P.V., Choi M., Yang S., Kim J.-Y., Jang H., Park J.-H.,
Fast fragmentation of metal oxide nanoparticles via reduction in oxyhydrogen flame, Applied Physics Letters, 90(2007) 163106.
Pitkänen A., Mäkelä J.M., Nurminen M., Oksanen A., Janka K.,
Keskinen J., Keskinen H., Liimatainen J.K., Hellstén S.,
Määttä T., Numerical study of silica particle formation in
turbulent H2/O2 flame, IFRF Combustion Journal, (2005)
200509.
Pratsinis S.E., Flame aerosol synthesis of ceramic powders,
Progress in Energy and Combustion Science, 24 (1998)
197–219.
Purwanto A., Wang W.-N., Okuyama K., Flame Spray Pyolysis
in: Ashgriz. N. (Ed.), Handbook of Atomization and Sprays,
Springer, New York, 2011, pp. 869–879.
Rajala M., Janka K., Kykkänen P., An industrial method fornanoparticle synthesis with a wide range of compositions,
Reviews on Advanced Materials Science, 5 (2003) 493–497.
Rosebrock C.D., Riefler N., Wriedt T., Mädler L., Tse S.D., Disruptive burning of precursor/solvent droplets in flame-spray
synthesis of nanoparticles, AIChE Journal, 59 (2013) 4553–
4566.
Rosebrock C.D., Wriedt T., Mädler L., Wegner K., The role ofmicroexplosions in flame spray synthesis for homogeneous
nanopowders from low-cost metal precursors, AIChE Journal, 62 (2016) 381–391.
Saarinen J.J., Valtakari D., Haapanen J., Salminen T., Mäkelä
J.M., Uozumi J., Surface-enhanced Raman scattering active
substrates by liquid flame spray deposited and inkjet
printed silver nanoparticles, Optical Review, 21 (2014)
339–344.
Sainiemi L., Keskinen H., Aromaa M., Luosujärvi L., Grigoras
K., Kotiaho T., Mäkelä J.M., Franssila S., Rapid fabricationof high aspect ratio silicon nanopillars for chemical analysis, Nanotechnology, 18 (2007) 505303. DOI:10.1088/0957–
4484/18/50/505303.
Sheen S., Yang S., Jun K., Choi M., One-step flame method for151

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

the synthesis of coated composite nanoparticles, Journal of
Nanoparticle Research, 11 (2009) 1767–1775.
Sokolowski M., Sokolowska A., Michalski A., Gokieli B., The“in-flame-reaction” method for Al2O3 aerosol formation,
Journal of Aerosol Science, 8 (1977) 219–230.
Songok J., Tuominen M., Teisala H., Haapanen J., Mäkelä J.M.,
Kuusipalo J., Toivakka M., Paper-based microfluidistics:
Fabrication technique and dynamics of capillary-driven
surface flow, ACS Applied Materials & Interfaces, 6 (2014)
20060–20066.
Sotiriou G.A., Blattmann C.O., Pratsinis S.E., Flexible, Multifunctional, Magnetically Actuated Nanocomposite Films,
Advanced Functional Materials, 23 (2013) 34–41.
Stark W.J., Pratsinis S.E., Aerosol flame reactors for manufacture of nanoparticles, Powder Technology, 126 (2002) 103–
108.
Stepien M., Saarinen J.J., Teisala H., Tuominen M., Aromaa M.,
Kuusipalo J., Mäkelä J.M., Toivakka M., Adjustable wettability of paperboard by liquid flame spray process, Applied
Surface Science, 257 (2011) 1911–1917.
Stepien M., Saarinen J.J., Teisala H., Tuominen M., Aromaa M.,
Kuusipalo J., Mäkelä J.M., Toivakka M., Surface chemical
analysis of photocatalytic wettability conversion of TiO2
nanoparticle coating, Surface and Coatings Technology,
208 (2012a), 73–79.
Stepien M., Saarinen J.J., Teisala H., Tuominen M., Aromaa M.,
Kuusipalo J., Mäkelä J.M., Toivakka M., Surface chemical
characterization of nanoparticle coated paperboard,
Applied Surface Science, 258 (2012b) 3119–3125.
Stepien M., Saarinen J. J., Teisala H., Tuominen M., Aromaa M.,
Haapanen J., Kuusipalo J., Mäkelä J.M., Toivakka M., ToFSIMS analysis of UV-switchable TiO2-nanoparticle-coatedpapersurface, Langmuir, 29 (2013a) 3780–3790.
Stepien M., Chinga-Carrasco G., Saarinen J.J., Teisala H.,
Tuominen M., Aromaa M., Haapanen J., Kuusipalo J.,
Mäkelä J.M., Toivakka, M., Wear resistance of nanoparticle
coatings on paperboard, Wear, 307 (2013b) 112–118.
Stepien M., Saarinen J.J., Teisala H., Tuominen M., Haapanen J.
Mäkelä J.M., Kuusipalo J, Toivakka M., Compressibility ofporous TiO2 nanoparticle coating on paperboard, Nanoscale
Research Letters, 8 (2013c) 444–449.
Stepuk A., Krämer K., Stark, W.J., Flame synthesis of complex
fluoride-based nanoparticles as upconversion phosphors,
KONA Powder and Particle Journal, 30 (2013) 267–275.
Strobel R., Pratsinis S.E., Effect of solvent composition onoxide morphology during flame spray pyrolysis of metal
nitrates, Physical Chemistry Chemical Physics, 13 (2011)
9246–9252.
Tammela S., Söderlund M, Koponen J., Philippov V., Stenius, P.,
The potential of direct nanoparticle deposition for the next
generation of optical fibers, in: Digonnet M.J., Jiang S.
(Eds.), Proceedings of SPIE 6116, Optical Components andMaterials III, Society of Photo-Optical Instrumentation
Engineers, San Jose, 2006, 61160G.
Teisala H., Tuominen M., Aromaa M., Mäkelä J.M., Stepien M.,
Saarinen J.J., Toivakka M., Kuusipalo, J., Development ofsuperhydrophobic coating on paperboard surface using the
Liquid Flame Spray, Surface and Coatings Technology, 205

152(2010) 436–445.
Teisala H., Tuominen M., Aromaa M., Stepien M., Mäkelä J.M.,
Saarinen J.J., Toivakka M., Kuusipalo J., Nanostructures
increase water droplet adhesion on hierarchically roughsuperhydrophobic surfaces, Langmuir, 28 (2012) 3138–
3145.
Teisala H., Tuominen M., Stepien M., Haapanen J., Mäkelä J.M.,
Saarinen J.J., Toivakka M., Kuusipalo J., Wettability conversion on the liquid flame spray generated superhydro­
phobic TiO2 nanoparticle coating on paper and board by
photocatalytic decomposition of spontaneously accumulated
carbonaceous overlayer, Cellulose, 20 (2013a) 391–408.
Teisala H., Tuominen M., Aromaa M., Stepien M., Mäkelä J.M.,
Saarinen J.J., Toivakka M., Kuusipalo J., High- and low-­adhesive superhydrophobicity on the liquid flame spraycoated board and paper: structural effects on surface
wetting and transition between the low- and high-­adhesive states, Colloid and Polymer Science, 291 (2013b)
447–455.
Teisala H., Tuominen M., Haapanen J., Aromaa M., Stepien M.,
Mäkelä J.M., Saarinen J.J., Toivakka M., Kuusipalo J.,
Review on Liquid Flame Spray in paper converting: Multifunctional superhydrophobic nanoparticle coatings, Nordic
Pulp and Paper Research Journal, 29 (2014a) 747–759.
Teisala H., Tuominen M., Haapanen J., Aromaa M., Stepien M.,
Mäkelä J.M., Saarinen J.J., Toivakka M, Kuusipalo J.,
Switchable water absorption of paper via liquid flame spray
nanoparticle coating, Cellulose, 21 (2014b) 2033–2043.
Teoh W., Amal R., Mädler, L., Flame spray pyrolysis: An
enabling technology for nanoparticles design and fabrication, Nanoscale, 2 (2010) 1324–1347.
Thimsen E., Biswas P. Nanostructured photoactive films synthesized by a flame aerosol reactor, AIChE Journal, 53(2007) 1727–1735.
Thimsen E., Rastgar N., Biswas P., Nanostructured TiO2 films
with controlled morphology synthesized in a single step
process: Performance of dye-sensitized solar cells andphoto water splitting, Journal of Physical Chemistry C, 112(2008) 4134–4140.
Tikkanen J., Eerola M., Rajala M., Coating glass by flame
spraying, Journal of Non-Crystalline Solids, 178 (1994)
220–226.
Tikkanen J., Gross K.A., Berndt C.C., Pitkänen V., Keskinen J.,
Raghu S., Rajala M. Karthikeyan J., Characteristics of theliquid flame spray process. Surface and Coatings Technology, 90 (1997) 210–216.
Tricoli A., Graf M., Mayer F., Kühne S., Hierlemann A.,
Pratsinis S.E., Micropatterning layers by flame aerosol
deposition-annealing, Advanced Materials, 20 (2008) 3005–
3010.
Trommer R.M., Bergmann C.P., Flame Spray Technology:
Method for Production of Nanopowders, Springer, Berlin
2015.
Tuominen M., Teisala H., Aromaa M., Stepien M., Mäkelä J.M.,
Saarinen J.J., Toivakka M., Kuusipalo J., Creation of superhydrophilic surfaces of paper and board, Journal of Adhesion Science and Technology, 28 (2014) 864–879.
Ulrich D.G., Theory of particle formation and growth in oxideJyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

synthesis flames, Combustion Science and Technology, 4(1971) 47–57.
Ulrich G.D., Flame synthesis of fine particles, Chemical &Engineering News, 62 (1984) 22–29.
Valtakari D., Bollström R., Tuominen M., Teisala H., Aromaa
M., Toivakka M., Kuusipalo J., Mäkelä J.M., Uozumi J.,
Saarinen, J.J., Flexographic printing of PEDOT: PSS on

coated papers for printed functionality, Journal of Print
Media and Technology Research, 2 (2013) 7–13.
Valtakari D., Stepien M., Haapanen J., Teisala H., Tuominen M.,
Kuusipalo J., Mäkelä J.M., Toivakka M., Saarinen J.J., Planar fluidic channels on TiO2 nanoparticle coated paperboard, Nordic Pulp and Paper Research Journal, 31 (2016)
232–238.

Author’s short biography
Jyrki M. Mäkelä
Dr. Jyrki M. Mäkelä is Professor at the Department of Physics, Tampere University ofTechnology, Finland. He graduated from University of Helsinki, Department of Physics
in 1987 and took the degree of PhD at the same department in 1992. During 1992–2000
he worked in the Aerosol Physics Group at University of Helsinki on nanoparticle measurement, mainly in chamber studies and for atmospheric nanoparticle formation. In
2000 he moved to Tampere University of Technology, TUT, as a Lecturer and received afull Professorship there in 2010. Since 2000 he has been leading the Aerosol Synthesis
Group at TUT, Aerosol Physics Laboratory, focusing on nanoparticle generation and fabrication of functional surfaces via aerosol synthesis. During 2014–2016 he acted as the
Head of the Department of Physics at TUT.

Janne Haapanen
Janne Haapanen is currently working as a researcher and a doctoral student at Tampere University of Technology, Department of Physics, Aerosol Physics Laboratory. He
has been working in Prof. Mäkelä’s Aerosol Synthesis group since 2009. He received his
MSc (Tech.) degree in Technical Physics in 2010 and currently working a researcher anda doctoral student in the same research group. During 2015–2016, he worked as a visiting
researcher at Danish Technological Insititute, Aarhus, Denmark and at Technical University of Denmark, Kgs. Lyngby, Denmark for 7 months. Main topic of research is functional nanocoatings fabricated by Liquid Flame Spray (LFS) method. He has done
research in antibacterial, photocatalytic, superhydrophobic, superhydrophilic, ice-phobic
and superamphiphobic nanocoatings by combining several methods with LFS, such asplasma treatment, ALD and silanization.

Juha Harra
Dr. Juha Harra is currently a postdoctoral researcher at the Department of Physics in
Tampere University of Technology. He received his Master of Science (Technology) andDoctor of Science (Technology) degrees in Technical Physics from Tampere Universityof Technology in 2010 and 2015, respectively. During 2016, he worked six months as avisiting scientist in SP Technical Research Institute of Sweden at the Department ofChemistry, Materials and Surfaces in Stockholm. His main research interest is in aerosol
physics, including nanoparticle synthesis using gas-phase techniques, real-time measurement of aerosol particles, and particle deposition for functional nanocoatings.

153

Jyrki M. Mäkelä et al. / KONA Powder and Particle Journal No. 34 (2017) 141–154

Author’s short biography
Paxton Juuti
Paxton Juuti is presently studying Technical Physics as a doctoral student at the Department of Physics, Tampere University of Technology, Finland. He received his BSc(Tech.) degree in 2013 on the topic of nanoparticle sintering and MSc (Tech.) degree in2015 on real-time monitoring of aerosol synthesis. His research interests include material
functionalization via aerosol routes and monitoring of aerosol processes.

Sonja Kujanpää
MSc (Teck.) Sonja Kujanpää graduated from Tampere University of Technology majoring in Advanced Engineering Physics in 2016. The topic of her thesis was “Determination of the mass size distribution of the industrial nanoparticle application produced
aerosol”. Today she works as a teacher of mathematical subjects in comprehensive school.

154

KONA Powder and Particle Journal No. 34 (2017) 155–167/Doi:10.14356/kona.2017021

Review Paper

Small-Scale Particle Interactions Are Having Significant
Effects on Global Fluidized Bed Behavior †Ray Cocco 1*, Allan Issangya 1, S.B. Reddy Karri 1, Tim Freeman 2,
Heinrich M. Jaeger 3 and Ted M. Knowlton 1
1Particulate Solid Research, Inc., USA
Freeman Technology, UK
3
The James Franck Institute, University of Chicago, USA
2

Abstract
Fluidized bed design and scale-up depends strongly on particle characteristics such as size, shape, and for Geldart
Group A particles, the level of fines (particles smaller than 44 microns). However, recent research has shown thatparticle clustering has a significant effect on fluidized bed hydrodynamics which impacts how these units should
be designed and scaled up. This is especially true with the estimation of the solids entrainment rate and thecyclone collection efficiency. The amount of fines, particle shape and surface morphology play a role on the levelof particle clustering in a fluidized bed. The fine particles are an excellent conduit for moving charge as electrons
or ions which appear to be the dominant mechanism of electrostatics for Geldart Group A material in a bubbling
fluidized bed. This electrostatic force trades off with particle momentum relaxation and rotational to translation
momentum transfer with regard to forming a particle cluster. The issue is the quantification of this effect so more
precise calculations can be made with particle entrainment rates and cyclone collection efficiency. Preliminary
work on particle shear in a packed and fluidized beds, suggest that particle clustering can be measured and may
provide a quantifiable metric for the level of particle clustering.
Keywords: powder, particle, fluidization, entrainment, clustering, cohesive forces

1. Introduction
In the design and operation of fluidized beds, particles
are assumed to behave independently of each other which
results in a fluid-like state once all the particles have beensuspended by the fluid. In other words, the factors responsible for dense granular flow such as particle shape, surface roughness, and stress chains nearly disappear oncethe particles are fluidized. It’s a common convention thathas limited our understanding of fluidized bed hydrodynamics.
Through the years, this common convention has beenchallenged. Particle clustering has been postulated as early
as in the 1890’s where fine-grained granular materials
were observed to break up into “droplets” much like a liquid stream (Khamontoff, 1890). Wilhelm and Kwauk(1948) proposed that Geldart Group A particles clustered
together, which resulted in the smooth fluidization ob†
1
2
3

*

Received 1 June 2016; Accepted 1 September 2016
IL60632, USA
GL20 8DN, UK
IL 60637, USA
Corresponding author: Ray Cocco;
E-mail: ray.cocco@psri.org
TEL: +01-773-523-7227 FAX: +01-773 243-0054

served just after the minimum fluidization velocity wasachieved. This homogeneous or smooth fluidization is
only observed for Geldart Group A particles below theminimum bubbling velocity. Larger particles, such as
Geldart Group B particles, exhibit bubbling fluidization at
the onset of fluidization. The smaller Geldart Group A
particles were viewed as being responsible for particle
clustering, which is not evident with larger particles.
Geldart and Wong (1987) expanded on this by adding
even smaller Geldart Group C particles to a fluidized bedof Geldart Group A particles. The result was a marked
decrease in the measured entrainment rate. They suspected that the smaller particles or fines were adhering to
the larger particle which affect the drag.
A study by Baeyens et al. (1992) was in agreement with
Geldart and Wong’s work. Baeyens et al. noted that particles had to be smaller than 40 microns to exhibit this effect on entrainment. This maximum particle size wascalled the critical particle size and denotes the point
where interparticle forces exceed hydrodynamic forces.
Subbarao (2010) provided an expression for determining
the cluster size which was inline with the findings of
Baeyens et al. Jayaweera et al. (1964) found these clusters
were stable over a range of two to six particles in size.
Clusters larger than six particles were found to split and

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

155

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

form sub groups of clusters. Fortes et al. (1987) observed
similar-sized clusters which were stable up to a Reynolds
number of 1800.
Cocco et al. (2010) used a high-speed camera connected
to a customized boroscope to reveal these particle clusters
in the freeboard region of a fluidized bed containing fluidized catalytic cracking (FCC) equilibrium catalyst witha median particle size of 72 microns and 12 % fines (defined as particle with a size smaller than 44 microns). In
fact, 30 % of the particle behavior in the freeboard wasfound to resemble particle cluster formation with an average particle cluster size of 11 particles. These clusters
tended to be small particles or fines adhering to one or
two larger particles. This finding was also consistent withsimilar observations in a fluidized bed of polyethylene
powder where 75 % of the particle flow in the freeboard
was in the form of particle clusters.
However, Kaye and Boardman (1962) noted that theconcentration of particles in a freeboard tends to be too
low for the formation of a significant amount of particle
clusters. Solids concentrations needed to exceed 0.05 %which is unlikely in a freeboard of a bubbling fluidized
bed. Matsen (1982) also concurred that cluster formationand the size of the cluster formation are strongly dependent on the solids concentration.
The same study by Cocco et al. (2010) used the highspeed camera with a boroscope to reveal that particle
clusters existed in the fluidized bed as well as above thefluidized bed. The region surrounding a passing bubble
showed that most of the particles were behaving as
weakly-­bound clusters. Cocco et al. suggested that perhaps
clusters are formed in the bed and, in part, are subsequently entrained into the freeboard. Particle concentrations in the fluidized bed certainly exceed the criteria ofKaye and Boardman.
Hays et al. (2008) observed that entrainment rates weredependent on fluidized bed height. The taller the bed, thelower the entrainment rate. It was suggested that the reduction in entrainment rates with increasing bed heights
was due to a longer growth time with the deeper bed forparticle clustering. Hays et al. also added baffles into thefluidized bed and noticed that the entrainment rate increased in a system where small particles (less than 44
microns) were present. The baffles were believed to be responsible for breaking up these weakly-bound clusters.
For beds with larger particles, there were not enough particle clusters to exhibit this effect. Both studies further
support that particle clusters are mostly formed in the fluidized bed, and that the resulting entrainment rates and particle clustering in the freeboard are the result of particle
clusters in the bed becoming ejected into the freeboard.
However, without understanding the underlying natureof the formation of particle clustering, it would be difficult to capture the hydrodynamics of a fluidized bed. This156

is more important now than in the past as more fluidized
bed challenges are being addressed with computational
fluid dynamic (CFD) models. In the past, fluidized bed
design and operation was accomplished using empiricalcorrelations which inherently contained the role of particle clustering in the experimental data. With CFD, themodel is more fundamentally based, but it is based on theparticles as being independent entities which is an incorrect assumption for many systems.

2. Impact of particle clustering on fluidized
bed design
The lack of understanding of particle clustering in fluidized beds has compounded the scale-up of these unit
operations. One of the key operating costs with fluidized
beds are the solids loss rates. In catalytic systems, catalyst
loss rates can amount to millions of dollars per year if a
fluidized bed is not optimized. Fluidized bed design needs
to consider particle attrition and particle collection to
manage the loss of solids, which requires knowing the entrainment rate in the freeboard and the grade efficiency ofthe cyclones.
The entrainment rate is needed for cyclone and dipleg
sizing. Under-prediction of the entrainment rates could
lead to undersized cyclones prone to dipleg flooding.
Over-prediction of the entrainment rate leads to an oversized-dipleg which can result in plugging because of too
low of a solids flux moving down the dipleg.
Yet, entrainment rate predictions can be orders of magnitudes different, especially for Geldart Group A particles. Many empirical correlations, which were developedwith experimental data already capture the clustering effect, inherently. However, such correlations are system
specific. This certainly seems to be the case with the disparity of the entrainment rate from published correlations(Chew et al., 2015). Fig. 1 illustrates this discrepancy forthe entrainment rate prediction using six entrainment rate
correlations for FCC powder with 9 % fines in a fluidized
bed at a superficial gas velocity of 1 m/s. For the small
particles, entrainment fluxes were more than two ordersof magnitude different, as shown in Fig. 1. Even thetrends differ with some correlations showing entrainment
significantly increasing with decreasing particle size
while other correlations level off. It is likely that the lackof consideration or enough consideration for particle clustering could be a key issue with the discrepancies in these
entrainment rate correlations (Chew et al., 2015). Furthermore, such differences can also be found in the calculations of the transport disengagement heights (Cahyadi A.
et al., 2015).
Particle clustering can also lead to higher than expected
cyclone collection efficiencies. It is hard to see the down

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

Fig. 1

Fig. 2

Entrainment rate calculations based on FCC catalyst powder with 11 % fines in a 3-meter ID × 12-meter tall
fluidized bed with a bed height of 6 meters and superficial gas velocity of 3 m/s at room temperature.

Cyclone collection efficiency for the collection of titanium dioxide particles in cyclone with a 0.43-m diameter barrel at a solids loading of 0.32 kg solids per m3 ofgas.

side of this scenario, but consider what happens when
new cyclones are added to this fluidized bed with higher
inlet velocities. Higher collection efficiency would be expected but that is not always the case. As shown in Fig. 2
for a 0.43-m diameter cyclone used in the collection oftitania particles, higher inlet velocities resulted in a lower
collection efficiency, not higher as expected. The higher
inlet velocities were postulated as breaking up particle
clusters with the additional trauma to the clusters. Theseclusters were preserved in the cyclone at the lower velocities. As a result, cyclone collection was influenced bysmaller particles at the higher velocity. At the lower inlet
velocities, a particle cluster was retained resulting in thecyclone “seeing” a larger “particle” and providing ahigher collection efficiency.
Until there is a better understanding of the mechanismof cluster formation and how these clusters behave in fluidized operations, such discrepancies as those shown in
Figs. 2 and 3 will continue. Fig. 3 illustrates this concept
for the simulation of a 0.9-meter diameter fluidized bed of

Fig. 3Log linear plot for the entrainment fluxes from experimental data and Barracuda v16 CFD simulations with
0.8 and 1.0 multipliers on the Wen-Yu drag model for a
0.9-m diameter fluidized bed with a 1.2-meter bed
height (at rest).

FCC particles with 9 % fines. As shown in Fig. 3, Barracuda CPFD simulations resulted in 20X higher entrainment fluxes than those observed experimentally for a
0.9-meter diameter fluidized bed of FCC catalyst with a
1.2-meter static bed height. Most commercial CFD codes
do not account for clustering unless invoked with a dragcorrection (Yang et al., 2004). Hence, the entrainment
rates can be significantly over predicted when CFD isused. Reasonable changes in the drag model may not help
either. Fig. 3 illustrates this using a Wen and Yu dragmodel with a 0.8 or 1.0 multiplier. The entrainment rates
from these simulations were relatively unchanged with respect to the differences with the experimental data. The
experimental data are based on the experimental procedures outlined in Issangya et al. (Issangya et al., 2013).

3. Impact of fines levels on particle clustering
As noted by Baeyens et al. (1992), particle clustering in

157

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

a fluidized bed was the result of particles with a size
smaller than 40 microns. Hays et al. (2008) observed thiswith a 0.16-m diameter fluidized bed of FCC catalyst(Geldart Group A, dp50 = 72 microns, ρp = 1500 kg/m3).
As shown in Fig. 4 where grating-like baffles were addedin the bed at 1.6 and 2.5 feet (0.5 and 0.75 meters) above
the distributor plate, the entrainment rate was found to increase when superficial gas velocities exceeded 1 m/s.
Hays et al. proposed that the higher velocities provided
enough shear to break up particle clusters that existed in
the bed. Yet, when larger coke particles (Geldart Group B,
dp50 = 150 microns, ρp = 15600 kg/m3) were used in thisunit, little differences in entrainment rates were observed
over the whole range of superficial gas velocities, as
shown in Fig. 5.
Direct evidence of particle clusters was observed andreported by Cocco et al. (2010). Using a modified borescope connected to a high-speed Phantom V7.2 camera,

Fig. 4

Entrainment rate versus bed height from a bed of
FCC catalyst fines (dp50 = 27 microns, particle density =1500 kg/m3) in a 0.15-meter diameter fluidized bed at asuperficial gas velocity of 0.56 m/s. Reprinted from
Cocco et al., Journal of Powder Technology 203 (2010)
pp. 3–11 with permission from Elsevier.

Fig. 5 Entrainment flux from a 0.3-meter diameter bed of fluidcoke (dp50 = 150 microns, particle density = 1600 kg/m3)
at a static bed height of 0.7 meters with and without
baffles at various superficial gas velocities. Reprinted
from Cocco et al., Journal of Powder Technology 203(2010) pp. 3–11 with permission from Elsevier.

158

Cocco et al. were able to resolve particle clusters well
above the bed in the freeboard. The borescope could capture particle and cluster flows beyond the influence of thewall where electrostatics could be more significant. Experiments were performed in a 0.15-m diameter fluidized
bed at a superficial gas velocity of 0.6 m/s. As shown in
Fig. 6, most of the clusters consisted of small particles orfines attached to larger particles. Clusters of only large
particles were not typically observed.
Yet, Kaye and Boardman (1962) proposed that particle
clustering becomes significant in many systems only
where solids concentrations exceed 0.05 %. Such particle
concentrations are higher than typically measured in the
freeboard region of bubbling fluidized beds at superficial
gas velocities of 0.6 m/s. The particle clusters may not be
forming in the freeboard to any significant degree, but inthe fluidized bed itself. This was confirmed in Cocco etal. (2010) with high-speed video measurements in the fluidized bed. As shown in Fig. 7, particle clustering is prolific in a fluidized bed of FCC catalyst particles.
Fig. 7 shows a selected frame from high-speed video
captured in a fluidized bed of FCC particles using a modified borescope. The image in Fig. 7, is after a bubble
passed by the probe where solids volume fractions concentrations are less than that of the bed emulsion. Nearly
all the particles were behaving as clusters and small particles appear to have an integral role in the cluster formation. This level of particle clustering has a significant
impact on the entrainment rate which is a product of the
bubble bursting on top of the bed (Wen & Hashinger
1960).
It is unknown whether the clusters exist in the denser
emulsion phase and get ejected into the less dense bubble
phase, or if the clusters are solely a product of the bubble
phase in the fluidized bed (Cocco et al., 2010). In the first

Fig. 6

Selective frame of FCC catalyst in the freeboard regionof a 0.15-meter diameter fluidized bed at a superficial
gas velocity of 0.61 m/s. Images were collected at 4000
frames per second with a 20 μs exposure time.

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

resulted in lower entrainment rates than shorter beds.
Cocco et al. (2010) suggested that a taller bed provides alonger residence time in the bed for the clusters to getlarger. What is unknown is how the concentration of fines
in a fluidized bed changes the slope of this relationship.
Fig. 7 illustrates direct evidence of particle clusteringin a fluidized bed. Figs. 4 and 5 show that particle fines(particles smaller than 44 microns) promote the formationof particle clusters which is not unexpected. Fines mobility can be significant in a fluidized bed and serves as aconduit for promotion of particle clustering. However,
how do fines promote this particle clustering?

4. Clusters and cohesive forces
Fig. 7

Selective frame of FCC catalyst in the bed region of a0.15-meter diameter fluidized bed at a superficial gas
velocity of 0.61 m/s. Images were collected at 4000
frames per second with a 20 μs exposure time.

Fig. 8

Entrainment rate versus bed height from a bed of
FCC catalyst fines (dp50 = 27 microns, particle density =1500 kg/m3) in a 0.15-meter diameter fluidized bed at asuperficial gas velocity of 0.56 m/s. Reprinted from
Cocco et al., Journal of Powder Technology 203 (2010)
pp. 3–11 with permission from Elsevier.

case, clustering is prolific and bed hydrodynamics may bestrongly coupled to the level of particle clustering. In thesecond case, clustering may only affect entrainment rates
but not bed hydrodynamics.
Hays et al. (2008) also reported that the entrainment
rate was strongly dependent on the fluidized bed height.
Fig. 8 shows the measured entrainment rate from a 0.15-m
diameter fluidized bed for FCC catalyst powder with 5 %fines at varying bed heights (Cocco et al., 2010; Hays etal., 2008). With the cyclone located well above the transport disengagement height for all cases, the entrainment
rate was found to decrease significantly. A doubling of the
bed height resulted in a 3.5 times reduction in the entrainment rates. Taller fluidized beds of FCC catalyst particles

The questions to address are how do these clusters affect key design parameters such the entrainment rate andcyclone efficiency Specifically, what are the underlying
forces that promote clustering in a fluidized bed? Possible
mechanics for the formation of particle clusters includehydrodynamic interactions (drag minimization) (Geldart,
1987; Zelenko et al., 1996) , inelastic grain-grain collision(collisional cooling (Lu et al, 2005; Wang et al., 2009) andviscous dissipation(Subbarao, 1986; Shuyan et al., 2008)),
electrostatic forces, capillary bridging or van der Waals
forces (Israelachvili, 1992; Podczeck, 1998; Visser, 1989).
Royer et al. (2009) investigated some of these effects in
granular streams of particles freely falling from a small
opening at the bottom of a hopper. For particles in the
50–150 micron range they tracked cluster formation witha high-speed video camera falling alongside the stream.
They found that the glass beads developed clusters during
a 2.5 meter free fall. These cluster formations were observed in an atmosphere or vacuum suggesting that hydrodynamic interactions may not be a cause. When asimilar experiment was performed with 100-micron copper particles, clustering was not observed. If inelasticity
was a driving force for clustering, clustering should havebeen more pronounced as copper has a coefficient of restitution of 0.9 compared to soda glass with a coefficient of
0.97. This together with the fact that the glass particle
clusters did not slowly disintegrate during freefall suggested the presence of small cohesive forces in the nano-­
Newton range.
Clustering in this scenario results when impacting particles cannot escape from the energy well created by attractive forces. In the freefall experiments, the relevant
region is right underneath the hopper opening, where thegranular kinetic temperature has dropped to a level thatclusters are no longer sheared apart, but the collision frequency is still high enough to lead to aggregation. Here,the lighter glass particles did not have enough impact energy to escape after a collision, while the heavier copper

159

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

particles did.
Royer et al. confirmed this using Atomic Force Microscopy (AFM) to measure the contact forces directly. They
found that the adhesion force was short-ranged (a few nm,
i.e. clearly less than 1/1000 of a bead diameter) and a few
10’s of nano-Newtons in magnitude. They proposed thatthe cohesion was due to van der Waals forces or capillary-­
type interactions from molecularly thin layers of water
adsorbed on the particle surfaces. This hypothesis was
further confirmed when Royer et al. added Aerosil silica
nanoparticles to the surface of the glass beads. The addition of the Aerosil nanoparticles artificially roughened thesurface of the glass beads. This resulted in the treatedglass beads exhibiting a much reduced adhesion force in
the AFM measurements and no longer showing particle
clustering in the freefall experiments.
For the glass and metal particles in the experiments byRoyer et al., short-range cohesion dominated over longer-­ranged charging effects. Electrostatic charging can, ofcourse, become important with other types of insulating
particle material. Waitukaitis et al. (2013) performed experiments with freely falling streams of 200–300 micron
zirconium dioxide silicate particles, in which they incorporated a Faraday cup at the bottom and had parallel electrodes along the particle free fall. By applying ahorizontal electric field, the charge on individual particles
could be extracted. Remarkably, while the particle stream
as a whole was found to be essentially uncharged, the distribution of charge on individual particles showed broad
tails around zero, indicating the existence of particles
with large net positive or negative charges, equivalent inmagnitude to the charge of several million electrons. It
was noted that in these experiments the particle density inthe freefall region was adjusted to be low (by using asmaller opening at the bottom of the hopper than Royer et
al. used), such that particle-particle collisions were rare.
Thus, the particle tracking during freefall was essentiallya diagnostic of the triboelectric or contact charging that
had occurred during earlier particle-particle contacts,
most likely inside the hopper during the outflow process.
Waitukaitis et al. (2013; 2014) were able to show thatsuch contact charging among particles made from thesame material involves the transfer of negative charge andthat the charging magnitude can be linked directly to thedispersion in particle size, as proposed earlier by Lacks etal. (2007; 2008). Particles smaller than the mean size tend
to become charged negatively, while larger particles tend
to acquire positive charge. However, at least for zirconium
dioxide silicate particles Waitukaitis et al. found that the
charge carrier could not have been electrons. The densityof electrons in high energy trapped states at the particle
surfaces was shown from thermoluminescence measurements to be at least four orders of magnitude too small
than needed to account for the observed charging of these160

particles.
This suggested that the charge carrier responsible fortriboelectric contact charging might be ions, such as OHions from the presence of water on the surface of theparticles. Pence et al. (1994) and McCarty et al. (2008)
have noted the importance of water, and with it OH- ions,on the surface of particles for contact charging. Unless
one prepares surfaces under ultra-high vacuum conditions, thin water layers are generally unavoidable. Indeed,even in a vacuum at 10 −6 Torr, a molecularly thin layer ofwater adsorbs in one second, which is the definition of aLangmuir. However, we caution that to date there is not
yet consensus about the precise tribocharging mechanism
applicable to granular materials. For example, Baytekin etal. (2011) showed that contact charging of insulating materials can also occur when tiny, nano-sized chunks ofsolid surface material are transferred during contact.
The surface morphology of particles therefore appears
to play a role on several levels as far as clustering is concerned: Asperities may increase the distance two particles
can come into proximity, which controls the strength ofshort-ranged van der Waals and/or capillary forces. In addition, the surface morphology is likely to affect the efficacy of charge transfer during contact, which in turnaffects long-ranged electrostatic interactions.
Lee et al. (2015) used high-speed video of 300 micron,free-falling zirconium dioxide silicate particles to explore
how electrostatic charging increases the efficiency of particle capture and aggregation processes. They were able
to track in detail how particles attract each other over distances of 100’s of microns, undergo multiple collisions,and finally stick to each other. Lee et al. were able tomodel the trajectories between successive collision events
as Kepler-like orbits, taking into account the mutual polarization of the particles.
Lee’s et al. findings also showed that particles lost sometranslational kinetic energy upon collision, while particle
rotation can have a significant effect. They were able tocapture two particles having an electrostatic attraction foreach other but clustering failed resulting from the rotationof one of those particles having a less than sphericalshape. Asperities from a rough surface could have a similar effect (Wilhelm and Kwauk, 1948).
Lee et al. (2015) also noted that cluster formation often
requires multiple bounces to dissipate the kinetic energy
such that cohesion can dominate. Clustering was at its
lowest probability for the collision of two particles. Collisions of a particle with a pre-existing cluster appeared tobe more likely to lead to aggregation, owing to the abilityof a cluster to deform internally. In other words, clusters
have a lower coefficient of restitution than individual particles.
Thus, in the presence of electrostatic forces, particle
clustering appears to be a two-step process. Coulombic

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

forces between oppositely charge particles or forces arising from polarization of a less-charged particle by ahighly charged particle, provide a long-ranged attraction.
We note that polarization forces are always attractive, irrespective of the charge polarity on the participating particles. For single-peaked particle size distributions, where
the peak of the associated charge distribution from collisions will be near zero charge (Waitukaitis et al. (2014),
this means that clusters will tend to be comprised of (near)
neutral particles that have aggregated around highly
charged ones.
The second step in clustering involves short-ranged cohesive forces. Surface roughness and asperities can leadto more rotational momentum upon collision [Geldart andWong, 1987; Cocco et al., 2010). Fines may be less prone
to this, as these particles have both less translational androtational momentum which may also explain why clustering is more evident for many fluidized bed systems
with particles smaller than 44 microns.
Longer-ranged electrostatic interactions make it possible to capture and aggregate particles even if they are notundergoing collisions that transfer rotational momentum
to translational or by some other similar process. By preventing particle escape during multiple bounces, these interactions also lead to efficient capture when the impact
energy would be too large to be sufficiently dissipatedduring a single collision. This greatly increases the clustering propensity. On the other hand, as clusters growthey eventually reach a size, where, depending on the cohesive strength and the background pressure, they will
become large enough that other factors become controlling such as particle shear and drag.

5. Clusters and bulk shear forces
To discern the extent of particle clustering, flow measurements were taken in a 2.5-cm diameter fluidized bed
using Freeman Technology’s FT4 Powder Rheometer inaccordance to ASTM D7891 (2009). If particle clustering
is pervasive throughout the emulsion, it should be detectable with the energy required to shear the fluidized bed. If
particle clustering is restricted to the region around or below the bubbles, such detection should be limited to thenumber of bubbles. At low superficial gas velocities near
the minimum bubbling velocity, this number should be
low and the impact to shear the bed minimal.
The FT4 Powder Rheometer measures the resistancethat a powder exerts on a rotating blade as it moves axially through a powder sample which is packed, fluidizedor compacted. This resistance, measured as torque andaxial force, quantifies the energy needed to rotate theblade. In short, the FT4 provides a measure of the stresses
involved when the powder shears at different axial posi-tions for a packed, compacted or fluidized bed. If clustering is prolific throughout the emulsion of a fluidized bed,
then more energy would be needed to shear the solids
than for a case where clustering is not significant. Similarly, if clustering is restricted to the region around or below the bubbles, the additional shear should be diluted by
the rest of the non-clustering emulsion.
For this study, three samples were examined which
consisted of FCC catalyst powder with varying levels offines concentration (< 44 microns) at 3 % fines (low fines),
12 % fines (medium fines) and 100 % fines (high fines).
Analysis consisted of filling a 5-cm diameter cylinder
with 25 ml of material. Testing was done at room conditions and replicated for all data points. Testing consistedof measuring the energy needed for rotation in a packed
and fluidized bed for up to five times the minimum fluidization velocity.
Fig. 9 shows the results from the permeability testing.
As expected for flow through a packed bed, the pressure
drop owed to the permeability was five times higher forthe high fines case compared to the low or medium fines
cases. This level of pressure drop can be typical of cohesive materials which is in agreement with Figs. 4 and 5with the comparison of entrainment for Geldart Group A
and B materials.
It is interesting that the low and medium fines case had
similar permeabilities, with the low fines case showing
only a slightly lower pressure drop relative to the medium
fines case. Fig. 10 better illustrates this subtle difference.
At low normal stresses (little compaction) this difference
is more significant, but as the normal stresses are increased (higher compaction) this difference becomes
small. The medium fines case appeared to be insensitive
to increases in normal stresses from 2 to 14 kPa, suggesting fines provided a close packed bed upon filling.
This difference is more evident in Fig. 11 where 15 kPa
of normal force on top of a packed bed of particles resulted in a certain level of compaction. The medium fines

Fig. 9Permeability testing of low, medium and high fines
FCC catalyst with air at a superficial gas velocity of
0.002 m/s at room conditions.

161

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

Fig. 10 Permeability testing of low and medium fines FCC
catalyst with air at a superficial gas velocity of
0.002 m/s at room conditions with varying normal applied stress (compaction). conditions.

Fig. 11 Percent change in bed height due to compaction at
15 kPa.

case resulted in the least amount of compaction which isin agreement with Fig. 10. The less availability of fines
results in less of a close pack situation. The high finescase showed the most compaction which is likely the result of weak cohesive forces preventing the bed from approaching a close packed condition with no normal force
on the bed. Once force is applied, the cohesive forces arequickly overwhelmed with the applied force.
Consolidation from vibration at a constant amplitudeand frequency provided a different trend. The low andmedium fines case resulted in comparable consolidation
indices (the degree of compaction with mechanical agitation) whereas the level of compaction for the high finescase was three times higher, as shown in Fig. 12. Thisdifference suggests that even mild perturbations to the
bed can break up the cohesive forces. Thus, if clustering
is significant in the bulk, it appears to provide a lower bed
density in the non-consolidated state. However, even low
levels of energy in the environment seem to break up orreconfigure these clusters.
The most discernible trend with regard to the level offines was observed with the Basic Flowability Energy(BFE) test. BFE is the energy required to establish a par-162

Fig. 12 Measure of the impact that tapping (vibration at constant frequency and amplitude) has on the flow properties of a powder.

Fig. 13 Basic Flowability Energy for varying level of fines in
FCC powder in a packed bed state.

ticular flow pattern in a conditioned, precise volume of
powder. This flow pattern is a downward anti-clockwise
motion of the blade, generating a compressive, relatively
high stress flow mode in a packed bed of powder. The
BFE is calculated from the work done in moving the blade
through the powder from the top of the vessel to the bottom (i.e. during the downward traverse).
Fig. 13 shows the BFE for the three FCC samples in apacked bed state. The low fines case required the mostenergy needed for displacing powder. That level of energy
decreased as the level of fines concentration increased in
the samples suggesting the flowability of the bed becomeseasier with increasing fines level. These results are consistent with the increasing formation of clusters withhigher fines and resulting in more consolidation during
vibration as shown Figs. 11 and 12. The granular stresses
are less apparent with increasing fines levels. Presumably
the higher fines levels provide larger or more clusters
which owing to their weak cohesive forces act as a lubricant when under stress.
Indeed, these results suggest that the BFE might provide a trend that can be calibrated to the level of finesconcentration which may be an indication of the level ofparticle clustering in terms of number and/or size. How-

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

Fig. 14 Energy for varying level of fines in FCC powder withincreasing levels of superficial gas velocity throughthe bed.

ever, the work of Royer et al. (2009) and Waitukaitis et al.
(2013) clearly shows that material type and surface morphology have a significant role in the level of particle
clustering. Though, the BFE might be able to trend with acluster level, that trend is most likely to be different foreach material with respect to size, surface chemistry,morphology, etc.
Fig. 14 shows the Aerated Energy (AE, the energy
measured on the insert when air is being passed throughthe powder at a specified velocity) needed for rotating theblade with increasing levels of superficial gas velocities.
This allows the measurement of the AE from a packed
bed to fluidized bed state. The minimum fluidization velocity is apparent with this test and is represented by theleveling off of the normalized total energy needed to rotate the blade (integrated with respect to axial penetration). For the low fines case, the minimum fluidization
velocity appeared to be at 0.002 m/s, whereas the high andmedium fines case were at 0.004 m/s which is consistent
with experimental measurements based on ASTM 7743
(2008).
Above minimum fluidization velocities, the high finescase consistently resulted in more energy needed for therotating blade with significant cohesive forces prohibiting
uniform fluidization. The medium fines case resulted inlower energy requirements compared to the low fines casefor superficial gas velocities below 0.01 m/s, as a result of
lower permeability and moderate cohesion. At velocities
greater than or equal to 0.01 m/s, the trend reversed andmore energy was needed for the medium fines case compared to the less cohesive low fines case. However, thismay be the result of bed becoming fully fluidized in the
low fines case with the onset of bubbling. With higherfines levels, the minimum bubbling velocity increases(Geldart, 1986) and beyond the minimum bubbling velocity, no further improvements in flow properties are observed.
The results from the FT4 tests between the minimum

fluidization and minimum bubbling velocities, suggestthat clustering might impede the flow of particles in a homogenous or smoothed fluidized bed. Because bed densities at fluidization are much less than that of a packed
bed, the shear forces may be less than the cohesive forces.
Hence, the energy needed for rotating the blade increased
as it would increase for increasing particle sizes.
From the results presented in Figs. 9 through 14, theshear stresses and compaction of a packed bed are influenced by the level of fines concentration. The BFE andAE tests show the best trend with respect to increasingfines concentration, supporting the assumption that clusters could be acting as a lubricant in granular flow ofpacked beds, yet limiting the extend of homogeneous dispersion in fluidized beds.
The AE test quantifies the energy requirements needed
for rotating the blade in a fluidized bed. For measurements between the minimum fluidization and minimum
bubbling velocities, higher fines levels appear to coincide
as a trend with higher AE requirements. Since the bed is
fluidized, the shear stresses associated with particle shapeand surface roughness have less of an influence as in the
BFE test where results predominantly depend on cohesivestrength. Other factors such as particle shapes and structural integrity (i.e., attrition) should also be a consideration.
However, it is still circumstantial that the AE test
trends with the level of clusters either by number or size.
Additional data are needed to provide that trend either bydirectionally measuring cluster number and size in a bed
under similar conditions and using it for correction to entrainment calculations, or comparing it directly to entrainment data.
The results of the FT4 Powder Rheometer testing suggest that cohesive forces presumably resulting in particle
clustering are prolific in the bulk solids and are not confined in the region around or below a bubble. This is evident with the non-monotonic changes in permeability andcompaction with increasing fines level where bubbles were
not present. The AE testing also suggests particle clusters
are not restricted to bubbles as more energy was neededwith increasing fines levels below the minimum bubbling
velocity under homogenous or smooth fluidization.

6. Conclusions
With particle clustering being linked to the morphological and electrostatic nature of the particle surface, entrainment rate calculations are highly material specific.
Coupled with the reflux of particles or clusters in the freeboard because of internals such as cyclones and expanded
heads, it appears unlikely that a fundamentally-based
CFD model would be effective in determining the en-163

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

trainment rate.
Direct evidence of particle clustering in a fluidized bed
has been observed. Particle clusters form in the bed andtheir size may be dependent on the residence time in thebed (i.e., bed height and gas velocity). The mechanism forclustering appears to be resulting from a combination ofshort-ranged cohesion due to van der Waals or capillary
forces and longer-ranged electrostatic interactions, withparticles becoming charged by transfer of electrons or,more likely, ions during contact. The small particles orfines provide an effective conduit in moving the charge
throughout the fluidized bed. Fines have a higher mobility
in fluidized beds. However, rotational movement of a particle could overpower the electrostatic forces, especiallyfor the larger particles. Thus, particles with low sphericity
or high surface roughness may be less likely to form particle clusters.
Despite the mechanism, microscopic particle clustering
affects the fluidization behavior and has a significant effect on entrainment, a design parameter critical for fluidized bed scale-up. To achieve better accuracy withentrainment rate calculations and with the CFD modelingof entrainment, this clustering needs to be accounted for.
Preliminary testing using the FT4 Powder Rheometerfor measuring the energy needed to shear FCC catalyst
samples at varying levels of fines concentration in apacked and fluidized bed suggest fines play a role here as
well. However, such testing under fluidized conditions relaxes the effects of other factors such as particle shapeand surface roughness on the shear stresses. Furthermore,
the results of this aeration test found a trend only with thelevel of fines concentration and not directly with the levelof particle clusters which were unknown.
Nonetheless, particle clustering in a fluidized bed appears to be significant throughout the bed and not confined to the bubble, even though visual observations wererestricted to the region around the bubble. Permeability,
compaction, and unconsolidated shear stress measurements
showed a good trend with the level of fines, a promoter ofparticle clustering, in a packed bed. Furthermore, the AE
testing further confirmed this by showing a monotonic
trend in energy requirements with increasing fines levels
below the minimum bubbling velocity.

Acknowledgements
The authors would like to thank Frank Shaffer and Rupendranath Panday of the US National Energy Technology Laboratory and Alexander Mychkovsky and Steve
Ceccio of the University of Michigan for their help with
consultation with the development of the boroscope. The
authors also thank the US Department of Energy for theirfunding under DE-FC26-07NT43098 and the US National

164

Science Foundation for their funding under DMRCMP-1309611.

References
ASTM D7743-12, Standard Test Method for Measuring the
Minimum Fluidization Velocities of Free Flowing Powders,2008 ed., ASTM International, West Conshohocken, PA,
1900. doi: 10.1520/D7743.
ASTM D7891-15, Standard Test Method for Shear Testing ofPowders Using the Freeman Technology FT4 Powder Rheometer Shear Cell, 2009, ed., ASTM International, WestConshohocken, PA, 1900. doi: 10.1520/D7891.
Baeyens J., Geldart D., Wu S.Y., Elutriation of fines from gas
fluidized beds of Geldart A-type powders—effect of adding
superfines, Powder technology, 71 (1992) 71–80.
Baytekin H.T., Patashinski A.Z., Branicki M., Baytekin B., Soh
S., Grzybowski B.A., The mosaic of surface charge in contact electrification, Science, 333 (2011) 308–312.
Cahyadi A., Neumayer A.H., Hrenya C.M., Cocco C.M., Chew
J.W., Comparative study of Transport Disengaging Height(TDH) correlations in gas–solid fluidization, Powder Technology, 275 (2015) 220–238. doi: 10.1016/j.powtec.2015.02.010.
Chew J.W., Cahyadi A., Hrenya C.M., Karri S.B.R., Cocco R.A.,
Review of entrainment correlations in gas-solid fluidization, Chemical Engineering Journal, 260 (2015) 152–171.
doi: 10.1016/j.cej.2014.08.086.
Cocco R.A, Shaffer F., Hays Karri S.B.R., Knowlton T.M., Particle clusters in and above fluidized beds, Powder Technology, 203 (2010) 3–11.
Colakyan M., Catipovic N., Jovanovic G., Fitzgerald T.J., Elutriation from a Large Particle Fluidized Bed with and without Immersed Heat Transfer Tubes, AIChE Symp. Ser., 77(1981) 66–75.
Colakyan M., Levenspiel O., Elutriation from fluidized beds,
Powder Technology, 38 (1984) 223–232.
Fortes A.F., Joseph D.D., Lundgren T.S., Nonlinear mechanicsof fluidization of beds of spherical particles, Journal of
Fluid Mechanics, 177 (1987) 467–483.
Geldart D., Cullinan J., Georghiades S., Gilvray D., Pope D.J.,
The effect of fines on entrainment from gas fluidized beds,
Transactions of the Institution of Chemical Engineers, 57(1979) 269–275.
Geldart D., Gas Fluidization Technology, Wiley Interscience,
Chichester, (1986), p. 25.
Geldart D., Wong A.C.Y., Entrainment of particles from fluidized beds of bed of fine powder, AIChE Symposium Series,
255 (1987) 1.
Han Y.L., Chyang C.S., Hsiao W.M., Lo K.C., Effect of fines
hold-up in the freeboard on elutriation from a fluidized bed,
J. Taiwan Inst. Chem. Eng., 42 (2011) 120–123.
Hays R., Karri S.B.R., Cocco R.A., Knowton T.M., Small Particles Cluster Formation in Fluidized Beds and its Effect on
Entrainment, Circulating Fluidized Bed IX, 9 (2008) 1–5.
Israelachvili J.N., Intermolecular and Surface Forces, 2nd Ed.,
Academic Press (1992).

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

Issangya A., Karri S.B.R., Knowlton T.M., Cocco R.A., Effectsof bed diameter, baffles, fines content and operation conditions on pressure fluctuations in fluidized beds of FCC catalyst particles, Fluidization XIV. XIV (2013) 1–9.
Jayaweera K.O.L.F., Mason B.J., Slack G.W., The behavior ofclusters of spheres falling in a viscous fluid, Journal of
Fluid Mechanics, 20 (1964) 121–128.
Kaye B.M, Boardman R.P., Cluster formation in dilute suspensions, in Proc. Symp. on the Interaction between Fluids andParticles, Institution of Chemical Engineers London (1962)
17–21.
Khamontoff N., Application of photography to the study ofstructures of trickles of fluid and dry materials, J. Russ.
Phys-Chem. Soc., 22 (1890) 281–284.
Lacks D. J., Levandovsky A., Effect of particle size distribution
on the polarity of triboelectric charging in granular insulator systems, Journal of Electrostatics, 65 (2007) 107–112.
doi: Doi 10.1016/J.Elstat.2006.07.010
Lacks D.J., Duff N., Kumar S.K., Nonequilibrium accumulationof surface species and triboelectric charging in single component particulate systems, Physical Review Letters, 100(2008) 188305. doi: 10.1103/Physrevlett.100.188305
Lee V., Waitukaitis S.R., Miskin M.Z., Jaeger H,M., Direct
observations of particle interactions and clustering incharged granular streams, Nature Physics, 11 (2015) 733–
737.
Lu H.L., Sun Q.Q., He Y.R., Sun Y.L., Ding H.M., Xiang L.,
Numerical study of particle cluster flow in risers with cluster-­based approach, Chemical Engineering Science, 60 (2005)
6757–6767.
Matsen J., Mechanisms of choking and entrainment, Powder
Technology, 32 (1982) 21–33.
McCarty L.S., Whitesides G.M., Electrostatic charging due toseparation of ions at interfaces: contact electrification ofionic electrets, Angewandte Chemie International Edition,
47 (2008) 2188. doi: 10.1002/anie.200701812.
Pence S., Novotny V., Diaz A., Effect of Surface Moisture on
Contact Charge of Polymers Containing Ions, Langmuir, 10(1994) 592. doi: 10.1021/la00014a042.
Podczeck F., Particle-particle adhesion in pharmaceutical powder handling, Imperial College Press (1998).
Rodriguez J.M., Sanchez J.R., Alvaro A., Florea D.F., Estevez
A.M., Fluidization and elutriation of iron oxide particles. A
study of attrition and agglomeration processes in fluidized
beds, Powder Technology, 111 (2000) 218–230.
Royer J.R., Evans D.J., Oyarte L., Guo Q., Kapit E., Möbius
M.E., Waltukaitis S.R., Heinrich J.M., High-speed trackingof rupture and clustering in freely falling granular streams,
Nature, 459 (2009) 1110–1113. doi: 10.1038/nature08115.
Sciazko M., Bandrowski J., Raczek J., On the entrainment ofsolid particles from a fluidized-bed, Powder Technology, 66(1991) 33–39.
Shuyan W., Zhiheng S., Huilin L., Long Y., Wentie L., Yonlong
D., Numerical predictions of flow behavior and cluster sizeof particles in riser with particle rotation model and cluster-­based approach, Chemical Engineering Science, 63 (2008)
4116–4125.
Subbarao D., Clusters and lean-phase behavior, Powder Technology, 46 (1986) 101–107.
Subbarao D., A model for cluster size in risers, Powder Technology, 199 (2010) 48–54. doi: 10.1016/j.powtec.2009.04.015.
Tanaka I., Shinohara H., Hirosue H., Tanaka Y., Elutriation offines from fluidized bed, Journal of Chemical Engineeringof Japan, 5 (1972) 51–57.
Tasirin S.M., Geldart D., The elutriation of fine and cohesive
particles from gas fluidized beds, Chemical Engineering
Communications, 173 (1999) 175–195.
Waitukaitis S.R., Jaeger H.M., In situ granular charge measurement by free-fall videography, Review of Scientific Instruments, 84 (2013) 025104. doi: 10.1063/1.4789496
Waitukaitis S.R., Lee V., Pierson J.M., Forman S.L., Jaeger
H.M., Size -Dependent Same-Material Tribocharging in
Insulating Grains, Physical Review Letters, 112 (2014)
218001–5. doi: 10.1103/PhysRevLett.112.218001.
Wang S.Y., Li X., Lu H.L., Yu L., Ding J.M., Yang Z.M., DSMC
prediction of granular temperatures of clusters and dispersed particles in a riser, Powder Technology 192 (2009)
225–233.
Wen C.Y., Hashinger R.F., Elutriation of solid particles from adense-phase fluidized bed, AIChE J. 6 (1960) 220–226.
Wilhelm R.H., Kwauk M., Fluidization of solid particles,
Chemical Engineering Progress 44 (1948) 201–218.
Visser J., van der Waals and other cohesive forces affecting
powder fluidization, Powder Technology 58 (1989) 1–10.
Yagi S., Aochi T., Elutriation of particles from a batch fluidized
bed, Society of Chemical Engineering (Japan)—Spring
Meeting, 1955.
Yang N., Wang W., Ge W., Wang L., Li J, Simulation of heterogeneous structure in a circulating fluidized bed-riser by
combining the two fluid model with EMMS approach,Industrial and Engineering Chemistry Research, 43 (2004)
5548–5561.
Zelenko V.L., Sergeev Y.A., Tjin T., Iske P.L., Clusters and particle segregation in gas-solid flow through a vertical tube,
Physica A: Statistical Mechanics and its Applications,, 226(1996) 274–294.
Zenz P.A., Weil N.A., A theoretical-empirical approach to themechanism of particle entrainment from fluidized beds,
AIChE J., 4 (1958) 472–479.

165

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

Author’s short biography
Ray Cocco
Ray Cocco holds a Bachelor Degree in Chemical Engineering from University of
Florida and a Doctoral Degree in Chemical Engineering from Auburn University. He isa member of ASTM and the AIChE. He worked in reaction engineering and powder
technology for 17 years at Dow Chemical. He is currently President and CEO of Particulate Solid Research, Inc. (PSRI) which is a research consortium of 35 companies which
focuses on multiphase flows. He serves on several editorial boards including Powder andBulk Engineering, Powder Technology and Advance Powder Technology. He is also the
Chair of the World Congress in Powder Technology in Orlando, Florida in 2018.

Allan Issangya
Allan Issangya holds a Bachelors Degree from Dar es Salaam, Master Degree from
University of Leeds and Doctoral Degree from University of British Columbia in Chemical Engineering. He is a member of the AIChE. He has 20 years of experience in fluidization at Particulate Solid Research Inc., with a current position of Technical Leader. He
has interest in granular-fluid hydrodynamics in fluidized beds, gas bypassing and bed internals.

S.B. Reddy Karri
Reddy Karri holds a Master and Doctoral Degrees in Chemical Engineering from
New Hampshire. He is a member of the AIChE and Chair of the Particle Technology Forum. He extensively consults and trouble shoots the industrial problems in the energy
field. He is currently the Technical Director at Particulate Solid Research, Inc. where heas been for 28 years. He is a resent recipient of the AIChE PTF Fluidization Processing
Award in 2014.

Tim Freeman
Tim has a degree in Mechatronics from the University of Sussex in the UK. He is past
Chair of the American Association of Pharmaceutical Scientists (AAPS) Process Analytical Technology Focus Group and is a member of the Editorial Advisory Board of
Pharmaceutical Technology. He has recently been features on the Industry Expert Panelin European Pharmaceutical Review magazine. Since 1990, Tim Freeman is Managing
Director of powder characterization company Freeman Technology. He was instrumental
in the original design and continuing development of the FT4 Powder Rheometer® andthrough his work with various professional bodies, and involvement in industry initiatives, is an established contributor to wider developments in powder processing.

166

Ray Cocco et al. / KONA Powder and Particle Journal No. 34 (2017) 155–167

Author’s short biography
Heinrich M. Jaeger
Heinrich Jaeger is the William J. Friedman and Alicia Townsend Professor of Physics
at the University of Chicago. He received his Ph.D. in physics in 1987 from the University of Minnesota and has been on the faculty at U Chicago since 1991, directing the
Chicago Materials Research Center from 2001–2006, and the James Franck Institutefrom 2007–2010. He is a fellow of the American Physical Society. Jaeger’s current research focuses on self-assembled nanoparticle-based structures, on the rheology of concentrated particle suspensions, and on studies of the packing and flow properties of dry
granular materials.

Ted M. Knowlton
Ted Knowlton holds a Bachelor, Master and Doctoral Degrees from Iowa State University in Chemical Engineering. Dr. Knowlton managed IGT’s fluidization and solids
transport programs after receiving his Ph.D. at Iowa State University in 1971 until 1996.
He was the Technical Director of Particulate Solid Research, Inc, from 1996 to 2010. Dr.
Knowlton received the Distinguished Service Award from the Fluidization and Fluid-­
Particle Systems Committee of the AIChE in 1990, the Fluor Daniel Lectureship Awardfor his contributions to the area of fluidization in November, 1996, and the Thiele Awardfor outstanding lifelong contributions to the field of Chemical Engineering in 2004.

167

KONA Powder and Particle Journal No. 34 (2017) 168–182/Doi:10.14356/kona.2017022

Review Paper

Advances in Nanoparticle Sizing in Suspensions:Dynamic Light Scattering andUltrasonic Attenuation Spectroscopy †Wu Zhou 1,2, Mingxu Su 1* and Xiaoshu Cai 1
1

Institute of Particle and Two-phase Flow Measurement, Shanghai Key Laboratory of Multiphase Flow andHeat Transfer in Power Engineering, University of Shanghai for Science & Technology, China
2
Key Laboratory of Energy Thermal Conversion and Control of the Ministry of Education, Southeast University, China

Abstract
Size characterization of nanoparticles has gained wide concerns in the past decades, but it remains a challenge formeasurement in suspensions up to now. The extremely small scales of particle size result in great difficulty fortraditional static light scattering method and optical imaging. In addition to the electron microscopy techniques,
the dynamic light scattering (DLS) method is another widely used technique for laboratory analysis of samples.
Moreover, the ultrasonic attenuation spectroscopy (UAS) technique is also being developed rapidly to provide analternative method for nanoparticle sizing. This paper focuses on the latest development in the above twotechnologies for nanoparticle size characterization. As for the former, advances about the image-based DLS
technology in recent years are reviewed, including three different kinds of data processing methods andcorresponding measuring experiments using standard polystyrene particles. Methodology principles, models andexperimental setup were also reviewed for the latter UAS technology. Samples of the same nanoscale silver
particles were tested by the above two methods, as well as by transmission electron microscopy. A sample ofAntimony Tin Oxide (ATO) nanoparticles has also been adopted for measurements and comparisons. Relatively
consistent results can be found by comparing the particle sizes or distributions with various methods. The
dramatically reduced measurement time in image-based DLS indicates the potential for real-time and in-situ
nanoparticle sizing. UAS also provides a suitable way for nanoparticle size characterization at high concentrations.
Keywords: nanoparticle, size characterization, image-based dynamic light scattering, ultrasonic attenuation
spectroscopy, inversion

1. Introductions
Nanoparticles are generally referred to as particles withdiameter less than 100 nm, which are also called as ultrafine particles. Ultrafine particles often exhibit some uniqueeffects such as quantum size effect, surface effect, etc.,
which bring particles many special properties. However,there is no reason to think that here is much difference
between a 90 nm and a 110 nm particle in ability andthere is little evidence for the cutoff diameter for the ultrafine effect (Donaldson K. et al., 2001). There are broad
application prospects for them in catalysis, light filter,
light absorption, medicine, magnetic media, new material
development, and so on. Detailed studies are essential on
†
1
2

*

Received 25 July 2016; Accepted 17 October 2016
516 Jungong Road, Shanghai, 200093, China
Sipailou 2#, Nanjing, 210096, China
Corresponding author: Mingxu Su;
E-mail: sumx@usst.edu.cn
TEL: +86-021-5527-7764 FAX: +86-021-5527-6854

168

methods of nanoparticle characterization. At present,
popular methods used for ultrafine particle sizing include
electron microscopy, laser particle sizing (combined withlarge-angle scattered light reception technology), highspeed centrifugal sedimentation, dynamic light scattering,ultrasonic method, etc. In many cases, ultrafine particles
are suspended in a liquid medium to form a suspension,
which is the basic state for measurement using dynamic
light scattering and ultrasonic method.
Dynamic light scattering (DLS) has also been called as
photon correlation spectroscopy, quasi-elastic light scattering, or intensity fluctuation spectroscopy in the past. It
was first used for the particle sizing in the early 1970s(Pecora R., 1972; Silbey R. and Deutch J.M., 1972) andthen got a booming development in the applications.
Tscharnuter W. (2006) has also made a remarkable review
on the developments of this technology before the 21st
century for particle sizing, including histories, principles,
sampling, instrument and measurement.
In conventional DLS method, a photomultiplier tube

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

is used to receive the scattered light from nanoparticles,
and a digital correlator is utilized to process the one-­dimensional time-series signal to obtain the autocorrelation function of the intensity fluctuation. Features of the
autocorrelation function (ACF) curve reflect the Brownian
motion characteristics of nanoparticles and thus the information of particle size. Commercial instrument based on
DLS has become popular in off-line laboratory analysis.
However, in order to obtain accurate ACF results, a large
number of data is required. The measuring time usually
lasts tens of seconds to hundreds of seconds. It is difficult
to realize a real-time and online nanoparticle size measurement in unstable or rapidly changing processes.
Study of DLS method using a CCD (Charge CoupledDevice) or CMOS (Complementary Metal Oxide Semiconductor) camera as an area detector was first proposedin the nineties of the last century. Wong A.P.Y. and
Wiltzius P. (1993) utilized the scattered light at the same
scattering angles on the same ring in a CCD sensor.
The space-averaged information was used instead of time
averaging, which greatly reduced the measurement time.
Kirsch S. et al. (1996) collected the scattered light by a
CCD camera at nearly identical scattering vectors. The
CCD sensor was equivalent to an array of conventional
photon sensors. The measurement accuracy was improved
since sequential results of different pixels were averaged
based on the conventional time average. However, limitedby the frame rate of cameras, the above experiments(Kirsch S. et al., 1996; Wong A.P.Y. and Wiltzius P., 1993)
were carried out in pure glycerol as dispersion medium,in which Brownian motion of the nanoparticles was veryslow.
In addition, Dynamic Light Scattering Microscopy was
proposed by Dzakpasu R. and Axelrod D. (2004). A progressive scan CCD was used and exposure through a column of the CCD array was adopted. The rapid access totime- and space- sequence signal of dynamic light scattering was realized. That caused an equivalent effect asincreasing of frame rate. In this work, the method wasused for nanoparticle sizing in water. Zakharov P. et al.
(2006) carried out theoretical and experimental researches
in order to suppress complex scattering problems in DLS
measurements with the CCD camera.
As the rapid development of computer technology andimaging technology in recent years, the high-speed camera with tens of thousands of frames per second (fps) isavailable. On the other hand, the developed AOI (area ofinteresting) function in general industrial camera cangreatly improve the frame rate by reducing image size. It
provides the possibility of nanoparticle sizing in water. In
this paper, we are mainly focused on the development of
DLS method using image sensors. Three different dataprocessing algorithms were reviewed here for image-­
based DLS (IDLS) method. The first one is based on the

traditional time-based autocorrelation function. The second one is also based on autocorrelation function but with
two-dimensional image cross-correlation. The third one
is based on one cross-correlation coefficient between two
images in a certain time interval, without the autocorrelation functions. A high-speed camera was used in experiments and the scattered light signal in two-­dimensional
space, as well as in time series were acquired. Images
were manipulated in comparison of the three different
data processing methods.
In addition to DLS, the ultrasonic attenuation spectroscopy (UAS) method is also considered to be very promising as a rapid on-line measurement technology fornanoparticle suspension system, which has advantages ofsimple structure, non-invasive and on-line measurement
and being available to relatively dense suspensions.
In the field of theoretical modelling and experimental
validation involved, Challis R.E. et al. (2005), Povey
M.J.W. (2013) and Mcclements D.J. and Coupland J.N.
(1996) conducted intensive studies from numerical calculation problems of the ECAH model based on scattering
theory to the simplification for long wave limit, as well as
experimental validations. Hipp A.K. et al. (2002) derived
the Core-shell model which was suitable for high concentration conditions. Starting from the elastic ultrasoundscattering of particles and by introducing Lambert-Beerprinciple based on an analogy to the Mie theory in lightscattering, Riebel U. (1992) developed a model for largesolid particles to depict the particle-wave interactions, including entrainment, scattering, and resonances in the
short-wavelength regime. He also concluded theoreticallythe proper order of importance for high concentration
systems as (i) steric, (ii) dependent scattering, and (iii)
multiple scattering. Dukhin A.S. and Goetz P.J. (1996)
and Dukhin A.S. et al. (1996) presented the coupled-phasemodel, in which, the Happel cell model was exploited to
calculate the drag coefficient and a friction coefficient
proportional to the volume fraction was introduced to describe the relationship between the gradient of pressure
and the relative speed between the particle and the medium. The resultant DT Series commercial instruments
were then released by combining acoustic and electroacoustic spectroscopy, whose greatest feature is being
suitable for measuring highly concentrated colloidal
particles which have been able to extend the volume
fraction to 30 % or even more. Applications to a wide
variety of materials were introduced too. Evans J.M. andAttenborough K. (2002) extended the coupled-phasemodel by way of including irreversible heat transfer andvisco-inertial forces, based on the volume-averaged balance equations. They compared the predictions of thecore-shell model and the extended coupled-phase modelwith measurements of ultrasonic velocity and attenuationin two different emulsions for volume fractions up to169

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

70 %, which showed both models were in good agreements with the experimental results. Recently, Challis
R.E. and Pinfield V.J. (2014) proposed a novel way to describe the ultrasonic wave propagation in concentrated
slurries, in which a simple effective medium modification
to the ECAH/LB was put forward to reflect the hydrodynamic effects involved, while it did not yet cause expected
coincident results with those of the core-shell models.
In the characterization of ultrafine particles with UAS,
Mougin P. et al. (2001) conducted an in-situ examinationof solution phase crystallization processes on two organic
compounds: urea and (L)-glutamic acid, in which ultrasonic attenuation spectra were measured by a commercial
ultrasonic spectrometer (Ultrasizer by Malvern Instruments Ltd.) with two pairs of transducers operating over afrequency range from 1 MHz to 150 MHz. The experiment was carried out during a cooling process with temperatures from 50 °C to 15 °C (–0.4 °C/min), in which theparticle sizes (median diameter 0.023–130.8 μm) and concentrations (0.1–0.81 vol %) were monitored continuously.
Hou H. et al. (2010) measured an ultrafine ZnO-H2O suspension with a mean size of approximately 200 nm, inwhich a variable-path method and the short-time Fouriertransform have been adopted to obtain and extract ultrasonic attenuation spectrum and phase velocity synchronously. Wang X.Z. et al. (2009) carried out the on-line
characterization of nanoparticle suspensions using dynamic light scattering, ultrasound spectroscopy and process tomography simultaneously. A silicon nanoparticle(70–100 μm in diameter) suspension with initial volumeconcentration of 24 % was diluted to prepare a series ofsamples with eight different concentrations as low as
0.8 vol %, thus the measurement of particle size distribution was implemented at different concentrations, andanalysis was highlighted on nonlinear effects of the concentration on the final results. Furthermore, a modification (Liu L., 2009) was presented that in ultrasoundparticle sizing, factor analysis as a method of dealing withmultivariate data can be applied to ultrasound spectra to
extract the information of linear dependence of attenuation on concentration and the contribution of frequency toattenuation. On the basis of this work a closed-loop control system was further built to measure and control process parameters (Boonkhao B. and Wang X.Z., 2012).
In this paper, researches of UAS method for nanoparticle sizing are particularly focused on, including the following issues: theories and models for ultrasonicattenuation prediction; inversion method for solving particle size; ultrasonic signal transmission and detection
technology. In order to clearly distinguish the ultrafine
particles, high-frequency broad-band ultrasonic signals
are critical to be utilized, which causes higher requirements from the theoretical model calculation to ultrasound generating and receiving technology.

170

The rest of this paper is arranged as follows. First, detailed reviews of IDLS and UAS for particle sizing are executed, respectively, including measurement principles,data processing, measuring apparatus, and validations experiments using polystyrene particles with various sizes.
Then we focus on the characterizations of nanoscale silverand Antimony Tin Oxide (ATO) particles using IDLS
and UAS, comparisons and analyses are then executed
to come to the final conclusions.

2. Dynamic light scattering
2.1 Measurement principles and data processing
The principle of particle size measurement by DLS
technique is based on the Brownian motion of nanoparticles (King T.A., 1989; Thomas A.J.C., 1991; TscharnuterW., 2006). For spherical nanoparticles dispersed in solutions, the relationship between diffusion coefficient DT of
the Brownian motion and particle diameter D can begiven by Stokes-Einstein equation:

DT 

K BT
3πμD

(1)

where, KB is the Boltzmann constant, T is the absolute temperature and μ is the dynamic viscosity of the dispersion
medium. DT is determined using DLS technology by analysing the scattering signal from nanoparticles, which isilluminated using coherent light beam. The scattering lightfluctuate randomly in relation with Brownian motion.
Smaller particles move faster and the fluctuation of scattered light intensity is also faster. For monodisperse system, the following autocorrelation function (TscharnuterW., 2006) can characterize such a steady fluctuation.
g (2) τ  
1  βexp(2 Γτ )(2)where, τ is the time delay between signals, β is an instrumental constant between 0 and 1. The decay constant Γ isrelated to the diffusion coefficient of the particles DT byΓ  DT q 2(3)

where, q is the norm of scattering vector, which indicatesthe difference between wave vector of the scattered lightand the incident light. It is related to the refractive index
of the dispersion medium n, the scattering angle θ and thewavelength λ of light by
q

4πn
θ
sin  
λ2

2.1.1. IDLS with conventional autocorrelation
function(4)

To exploit the information of decay constant Γ and dif-

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

fusion coefficient DT from one-dimensional fluctuation
information of scattered light I(t), the intensity autocorrelation function will be fitted according to Eqn. 5 as conventional DLS data processing method.
Gτ Ι t Ι t τ1 NΙ ti Ι ti  τ
N i 1(5)

In order to obtain reliable and accurate result, averaging was adopted on a large number of time-series data,
since the fluctuation goes in a random way. It needs a longtime for both data acquisition and data processing. The
typical measurement time for the current conventional
DLS particle sizer is generally 1–2 minutes, which is difficult to meet the requirements of real-time and online
measurement for rapidly changing process. Since theBrownian motion of particles is sensitive to the temperature, a temperature control device is necessary in conventional DLS. It can be generally applicable to off-line
analysis in laboratory.
IDLS is proposed to capture the information of Brownian motion using an image sensor (Wang Z.Y. et al., 2014;
Xu C.Z. et al., 2015). One pixel or several pixels of thesensor is equivalent to one photomultiplier tube. There are
generally hundreds of thousands to millions of pixels in
one image sensor. As shown in Fig. 1, the image is divided into i × j grids, which is equivalent to i × j photomultiplier tubes. i × j time-series of light intensity signals
will be obtained simultaneously. Eqn. 5 will be used to
calculate the averaged autocorrelation functions and particle size D, or averaging can also be taken on the results
Di,j from each autocorrelation function. This takes theplace of conventional averaging on autocorrelation functions from a long time-series signal, so the data acquisition time can be reduced dramatically.
But from the perspective of data processing, the present
IDLS still needs a sufficient amount of data and a longprocessing time to ensure a high accuracy of statisticalresults. Moreover, as for solutions with low viscosity or
small particles, higher requirements are expected on thefrequency of image acquisition, which is in other words

Fig. 1

Images division for data processing (Xu C.Z. et al.,2015).

the frame rate of the camera.

2.1.2. IDLS with ACF from image cross-correlation

Autocorrelation function of discrete-time signals characterizes the similarity between interval signals. It is obtained by correlation processing between one-dimensionalsignals in a certain time interval. In IDLS method, a camera can capture two-dimensional spatial distribution oflight intensity as images, whose similarity can be obtained by image cross-correlation as Eqn. 6.
G τ   R  A m, n , B m, nmn  Am,n  A  Bm,n  B mn  Am,n  A 2 mn Bm,n  B 2

(6)Where, τ is the time delay between two images A andB, both of which have m × n pixels. That is to say, for atime-series of images, the 2D ACF values at different
time delays can be determined by cross-correlation processing between images at the corresponding time delays,
such as between the 1st image and the 2nd one, the 1st andthe 3rd ... and the 1st and the last one (Liu L.L. et al., 2015;
Zhou W. et al., 2015).
Considering three different sizes (79/482/948 nm) ofstandard polystyrene particles, the theoretical time-­
dependent autocorrelation functions with β = 1 can becalculated from Eqn. 1–4, as illustrated in Fig. 2. It showsthat the autocorrelation function values decrease from 1
to nearly 0 in approximately 2 ms, 20 ms and 40 ms, respectively, for the three sizes of particles. Values close to
0 after the respective times have small effects on the
curve fitting. As long as the frame rate of the camera ishigh enough to characterize the light intensity fluctuation,
the data acquisition time will be shortened to be about
2 ms, 20 ms and 40 ms, respectively. If the images arecaptured in the frame rate of 10000 fps, the number ofimages required to cover the typical time-scales in auto-

Fig. 2

Theoretical ACF for nanoparticles of different sizes inwater (Liu L.L. et al., 2015).

171

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

Fig. 3

Variation of theoretical ACF value as particle size at
different delayed times (Zhou W. et al., 2015).

correlation function curves should be about 20, 200 and
400 frames. The time intervals between the 1st image andany other image are the corresponding time delays in the
2D ACF. Conventional DLS acquires one value at a time,requiring a longer time-series data to get an averaged result. IDLS utilizes the information on two-dimensional
space and the cross-correlation coefficients between images are space-averaged values.

2.1.3. IDLS by one cross-correlation coefficient
without ACF

As shown in Fig. 2, under the same time delay, the 2D
ACF is a monotonically decreasing function of particle
size. Lager particles lead to greater correlation coefficient.
So the particle size may be determined by one correlation
coefficient, which can be obtained by cross-correlation of
two-dimensional light intensity signals in a time interval.
Fig. 3 illustrates the variation of correlation coefficient, inanother word theoretical ACF value, as particle size under
different time delays (from 1 μs to 10 ms), with the same
conditions as Fig. 2. It indicates that the data acquisition
time can be shortened to the acquisition time of two images, and the data processing time is shortened to the timeneeded for cross-correlation of these two images.
It should be noticed that the measuring error can be diminished by selecting a proper time interval to maintain amoderate correlation coefficient, since higher accuracy canbe achieved at larger slopes of the curves. It shows from
Fig. 3 that more accurate results can be acquired from correlation coefficient in the range 0.15–0.85. Take 200 nm
particles for example, the time interval should be in therange from 100 μs to 2 ms. There are also other parameters,such as scattering angle and laser wave length, which canbe adjusted to achieve moderate correlation coefficient.

2.2 Experimental system
Experimental setup was developed to validate and

172

Fig. 4

Schematic drawing of IDLS experimental setup.

compare the above mentioned methods. Schematic drawing of the experimental setup is shown in Fig. 4. The
measurement system included light source, lenses, sample
cell, diaphragm iris, high-speed camera and computer. A
green laser diode at 532 nm with 50 mW continuous wave
output was used as the light source. The laser beam was
focused in the sample cell, just as other commercial DSL
nanoparticle sizer has done. The scattered light from thenanoparticles was collected by a lens with focal length of
60 mm. An adjustable diaphragm was applied to define
the measured region, with an opening diameter of 0.8 mm
in the experiments. Limited scattered light was recorded
by a high-speed camera (MotionProX3), instead of aphotomultiplier tube in conventional DLS. The camera
has an 8-bit CMOS sensor with total effective area of
15.4 × 12.3 mm2 and one pixel area of 12 × 12 μm2. The
camera was fixed at a 45° angle with the incident lightduring the experiments.
To avoid the influence of multiple scattering, the solutions were diluted sufficiently to the particle concentration of about 0.1 vol %. The tests were all carried out atroom temperature, which was measured during experiments. To meet the requirement of repeatability and measurement accuracy, all of the measurements were repeatedfive times.

2.3 Validation of IDLS using standard
nanoparticles
Standard polystyrene particles (manufactured by Beijing
Haianhongmeng Reference Material Technology Co.,
Ltd.) with three different sizes (79/482/948 nm) were usedfor validation of the proposed data processing methods.
Accuracies were analysed respectively and the measurement times were compared.

2.3.1. Results using conventional ACF

It was noticed from the captured images that the scattering speckle size was about equal to the size of 4 × 4 pixels
in our experiments. The speckle size is also called as one

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

Table 1 Measured results of standard nanoparticles in different dispersion mediums at different frame rate (Wang Z.Y. et al., 2014)
Dispersant
Frame rate
D N (nm)Distilled water

55 % glycerol water solution

8290 fps

8290 fps

4000 fps

2000 fps

DM (nm)Error (%)

DM (nm)Error (%)

DM (nm)Error (%)

DM (nm)Error (%)

79

74.3

5.9

80.3

1.6

82.3

4.2

84.6

7.1

482

441.7

8.4

485.6

0.8

465.8

3.4

462.5

4.1

948

989.8

4.4

—

—

957.1

0.1

926.5

2.3

“coherence area” or one spot. So the images were divided
into grids of the same size of 4 × 4 pixels. One grid is equal
to one photomultiplier tube and one mean size value can be
determined from the corresponding time-series intensity
fluctuation. Averaged value on all the grids was taken as the
final result. First the samples were tested in distilled waterusing a frame rate of 8290 fps at 200 × 128 resolution. Thatis the maximum frame rate of the camera employed here atthat resolution. 2000 images were captured continuously in
0.24 s at a time. As shown in Fig. 2, 0.24 s is long enoughto cover the typical time scale of the ACF for these threesamples. The final results are illustrated in Table 1. It
shows that even the minimum measurement error can reach
4.4 % at 8290 fps in distilled water. The reason is that notenough efficient signals were captured in the typical timescale, especially for small particles. Camera with higher
frame rate should be used to improve the accuracy.
However, according to Eqn. 1, the diffusion coefficientof nanoparticles was dependent with the viscosity of dispersion medium. Larger viscosity leads to smaller diffusion coefficient, which can increase the timescale of the
autocorrelation function and improve measurement accuracy. Glycerin is a Newtonian fluid with high viscosity, and
it is easy to be mixed with water at any ratio. In this work,
the glycerol water solution of 55 % mass concentration was
adopted as another dispersion medium. The viscosity increased from 0.87 × 10 –3 Pa·s in water to 6.15 × 10 –3 Pa·s
in glycerol-water solution. The measurement results werelargely improved at the same frame rate as shown in
Table 1. No result was obtained for 948 nm particles inglycerol since the Brownian motion was too slow to be
detected. In order to investigate the influence of frame
rate, tests were also carried out at 4000 fps and 2000 fps
in 55 % glycerol-water solution. It shows that the accuracy decreases as the frame rate reduces. In conclusion,
the measurement accuracy may be improved by increasing the viscosity of dispersion medium but IDLS using
conventional ACF method is greatly affected by the frame
rate of the camera.

2.3.2. Results using ACF from image cross-correlation
According to the method described in section 2.1.2, thecross-correlation coefficients between images in a time

Fig. 5

Cross-correlation coefficient and corresponding fitted
ACF curves. (a) by 400 images each; (b) by 20, 160 and
300 images, respectively, for 79, 482, 948 nm particles.

series were calculated to compose the ACF curves, as
shown in Fig. 5. Particles were tested in distilled water at
8290 fps. Fig. 5(a) shows fitted ACF curves using 400
images for all the samples. Results in Table 2 show ahigher accuracy as compared with the results in section
2.3.1 at the same conditions, but using less image data. It
was caused by grid division in the previous method. The
divided grid size was an estimated value, which should be
actually different in different locations of the images. The
latter method does not require grid division.
Fig. 5(b) shows ACF results using 20, 160, and 300 images, respectively, for 79, 482, 948 nm particles. There is
no change in the shape of ACF curves. It indicates that a173

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

Table 2 Measured mean sizes from ACF by image cross-­correlation (Liu L.L. et al., 2015)
D N (nm)

DM (nm)Error (%)

79

77.3

2.15

482

452.6

6.10

948

982.4

3.63

certain result can be determined with images captured in
the typical time scale duration, which dramatically reduced the acquisition time. In our experimental conditions, the acquisition time were shortened to about 2, 20
and 40 ms, respectively.

2.3.3. Results using one cross-correlation coefficient
without ACF

Using the same series of images captured in section
2.3.2, nanoparticle size can be determined from one
cross-correlation coefficient between two images in a certain time interval. Two time intervals, 0.25 ms and 0.5 ms,
were investigated in our experiments for three different
samples, 79/482/948 nm, respectively. The mean particle
size can be calculated directly from corresponding
cross-correlation coefficient with Eqn. 1–4. Theoretical
ACF curves at the above two time intervals and corresponding coefficients are all illustrated in Fig. 6.
The measured results are shown in Table 3. It showsthat the measured mean particle size DM agrees well withcorresponding nominal size D N. The respective relative
errors are generally small. It can also be noticed that, withother conditions remaining the same, coefficients at larger
slope on the ACF curves lead to higher sensitivity. Take
the time interval of 0.5 ms for example, it costs 1 ms toacquire a couple of images and less than 10 ms to process
them using a notebook with i7-3612QM quad-core processor. It shows that the acquisition and processing time areboth significantly reduced, compared with hundreds seconds in traditional DLS.

Fig. 6174

Mean particle size determination from one cross-­correlation coefficient.

Table 3 Measured results using one cross-correlation coefficientτ = 0.25 ms

τ = 0.5 ms

DN
(nm)

DM
(nm)

Error
(%)

DN
(nm)

DM
(nm)

Error
(%)

79

81.5

3

79

80.8

2.3

482

495.6

2.8

482

490.3

1.7

948

970.2

2.3

948

965.7

1.9

3. Ultrasonic attenuation spectroscopy
3.1 Measurement principles
The interaction between ultrasonic beams and particulate two-phase flow is a quite complicated process, inwhich the sound scattering phenomena must be taken into
account in the first place. By assuming small particles aselastic objects, it can be analogous to the light scattering
to reach some similar deduction, and certainly the shapeof particles, the types of ultrasonic beams and incident
angles, as well as the ratio between sound wavelength andparticle size could be significant for the description ofsound scattering. The analytic solution can usually be obtained for those regular-shaped particles like a sphere,
while for more irregular particles the analytic solution israrely available up to now. Ye Z. et al. (1997) has performed quite a few numerical investigations on acoustic
scattering for non-spherical objects from gas, to weak
fluid and rigid spheroids, as well as the multiple scattering
problems from an ensemble of scatterers (Alvarez A. etal., 1999). However for more cases, a numerical solving
method must be relied on. For example, the boundary element method (BEM) can be employed for some special
cases like the scattering sound pressure calculation for
one or several irregular particles, which could bring us ahelpful estimation for the shape effects on sound scattering directive properties and scattering cross section.
Except scattering, there are still several important interaction behaviors, relevant with the viscous and thermal
dissipations. A further observation of the aforementioned
phenomena can help us find obvious intrinsic differencesin that the sound scattering merely causes the redirectionof acoustic energy away from the incident beam, while
the dissipations essentially give rise to the conversion
from acoustic energy to heat. Dukhin A.S. and Goetz P.J.
(2001) has already pointed out that in fact the scatteringand dissipations can be divided into two separated components and analyzed independently, which actually facilitated the understanding of acoustic loss and simplified
the mathematical modelling. Typically, a complete mathematical model is still quite necessary in describing allthese physical phenomena and establishing the rigorous

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

relationship between the particle size and the ultrasonicattenuation spectra. Fortunately, much work has been
done by quite a few researchers as mentioned in the previous section, among which, the thinking of Epstein P.S.
and Carhart R.R. (1953) will be cited in particular todemonstrate a typical procedure of particle sizing.

3.1.1. Ultrasonic attenuation model and calculation

matrix solving will fail frequently. The existence ofabove-­mentioned problems seriously confines the applicable range of the ECAH model in numerical implementation in aspects of frequencies and particle size. Pinfield
V.J. et al. (2005) once proposed a recurrence relation ofBessel functions involved to express the incident wave ina simulation of acoustic propagation in dispersions. A
modified algorithm has also been proposed in detail tofully overcome such a numerical instability problem (SuM. and Cai X., 2002). Avoiding the direct use of Hankel
function and constructing new related variables, recurrence relations of both Bessel and Hankel functions are as
following:

When modelling the interaction between particles andthe surrounding sound waves, Epstein P.S. and Carhart
R.R. (1953) focused on acoustic scattering and attenuation
by droplets suspended in the air. Allegra J.R. and Hawley
S.A. (1972) further extended EC’s model to solid particles
and unified the formulization of ultrasound scattering for2n  1
particle sizing by fulfilling the ECAH model. As a typical (8)Fn ( z)Fn1( z)  Fn2 ( z)zlinear scattering theory, their theoretical model fully accounted for the influence on acoustic waves of mixturenFn ( z) Fn1( z)  Fn ( z)stickiness and heat dissipation, which coupled to the(9)zscattering in the form of viscous waves and heat waves,respectively. By solving the complex wavenumber, thewhere z is the argument for Hankel function and n is themacroscopic phenomena like ultrasonic attenuation andorder. Several related variables and their derived recurphase shift are judiciously connected with the microrence formula are introduced.
scopic process like the interaction between a single partiFn ( z) Fn1( z) n
1n
cle and surrounding waves. Some numerical validations
Ln (
z)



(10)
F
z
F
z
z
M
z
z
(
)
(
)
(
)
n
n
n
and predictions were introduced in the previous papers(Su M. and Cai X., 2002; Tebbutt J.S. and Challis R.E.,
1996), and an example was stressed to demonstrate theF ( z) n  1 1
n 1
n 1
Qn ( z)  n
Ln ( z)  ( 2  1)application of the theoretical model for the submicron
Fn ( z)z M n ( z)zzdroplet characterization by using ultrasonic attenuationand phase velocity spectra simultaneously (Su M. et al.,(11)2009).
where variable Mn(z) is
For a poly-dispersed particle system, the ECAH derived expression for the attenuation coefficient αs can be
2n  1
1
given as:
M


(12)
n ( z)z
M n1( z)
N

qj
3φ
αs
And

 (2n  1)Re( An (D j , f ))
(7)
2
16
k j 1
D 3j n 0
where φ denotes the volume fraction of particles. k is thewavenumber of incident compression wave. D is the diameter of the particle. q j is the frequency distribution ofparticles with radii between Dj and Dj+1. The summationof series of the compression wave-scattering coefficientsAn (a function of particle diameter D and ultrasonic frequency f ) plays the key role for the calculation of thefrequency-dependent attenuation coefficients. The solutionof An finally reduces to a six-order complex linear equations, whose coefficient matrix elements is composed ofspherical Bessel and Hankel functions with complex variables as arguments. With the complex argument, a linear
increase of the imaginary part of the input parameter
leads an exponential variation of these functions. The matrix condition number increases accordingly and the solution of linear equations has numerical instabilities. Whenfunction overflow appears for large argument, numericalM 0 ( z)  TAN( z) for Bessel functionfor Hankel function
M 0 ( z)  i

(13)

The dependence of these variables on the increase of
the variable z is demonstrated in Fig. 7, which shows thatas complex variables increase, the spherical Hankel function and its derivative rapidly decrease toward zero, while
the newly constructed variables Ln(z) and Qn(z) tend to 1
in the end. That is to say, they can be used safely to replace the Hankel function to reduce the condition number
and avoid the potential numerical overflow, which is quite
crucial for the increase of dimensionless parameter (i.e.,a very high frequency). Fig. 8 illustrates the numericalresults using the improved method. It manifests that
calculation range extends to the ultrasonic frequency
1~100 MHz with the particle size 1 nm~1000 μm, which
indeed covers the major particle size range under poten-

175

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

Fig. 7

Variation of different variants with arguments (n = 1,z = w(1 + i)).

rameters to be determined by optimization. So far as weknow, the BFGS (Broyden-Fletcher-Goldfarb-Shanno)
and DFP (Davidon-Fletcher-Powell) are two significant
algorithms to perform the optimization process determining the particle size distribution, although they both possess the drawback that optimization might be trapped into
the local optimized solutions instead that the global one.
Unlike the competitor, the independent mode algorithm
does not assume a priori particle size distribution. The
particle size solution is obtained directly by solving theFredholm integral equations discretized into matrix form.
By rewriting Eqn. 7 according to its discrete form, thelinear equations can be generated in the following form:
3φ
1 q(ΔD ) 
αs ( fi ) (2n  1)Re( An (D j , f i ))  d D  i 
16 ΔD k c2D j 3 n0pi

qi
A i,j

(14)

Fig. 8Predicted ultrasonic attenuation coefficients against
particle diameter (0.5 vol % silver aqueous suspension).

tial investigations, although particles outside this range
have also been successfully measured. At the same time,
it can be observed that the attenuations grow graduallywith the increasing frequency. For a fairly small frequency like 1 MHz, a sensitive peak can be found forparticles in the submicron range (i.e. 100 nm~1000 nm) due
to dissipation mechanism, which indicates the ultrasonicsignal would vary with the particle size seriously. With
the increasing of frequency, the peak moves towards thesmaller particles direction. Thus a broader frequency
range can undoubtedly contain more information about
the particle size, accordingly in favor of the improvementof resolution of particle sizing.

3.1.2. Data inversion for ultrasonic attenuation

Generally, the inversion algorithms to determine particle size distribution have been classified into two types:
dependent and independent modes. The dependent mode
algorithm assumes that the particle system to be measured
conforms to one or two given size distribution functions,e.g. lognormal or Rosin-Rammler (Allen T., 2003), which
are usually characterized by two (unimodal distribution
assumption) or five (bimodal distribution assumption) pa-

176

where A is the coefficient matrix and q is the discrete particle size frequency distribution, the solution vector. The
array p is the experimental spectrum. It must be notedthat the matrix is invariably ill-posed. In addition, solution to such an equation group must obey the non-­negative
constraint. It requires a regularization method (also known
as Twomey algorithm) in order to give stable and real
solution q to ill-posed problems (Tikhonov A.N. and
Arsenin V.Y., 1977; Twomey S., 1977).
q ( A T A  γH ) A T p(15)

where, H is the smooth matrix. Furthermore, an Optimum
Regularization Technique (ORT) (Brandolin A. et al.,1991) has been put forward by selecting an optimum regularization factor γopt and appropriately introducing a setof basic cubic splines to reconstruct the distribution ineach size bin, which can contribute to reducing the condition number of the coefficient matrix and be beneficial to
the smoothness of the solution vector, without altering thelinearity characteristic of equations. In fact, the ORT
method can be further developed by choosing of an optimized regularization factor γopt which serves as a balance
between the smoothness of the PSD solution and thefitting of the reconstructed spectrum. This has been confirmed in particle sizing using light extinction measurement (Su M. et al., 2007) by introducing the L-curve
method (Hanson P.C., 1992), and particle sizing in dense
two-phase droplet systems using ultrasonic attenuationand velocity spectra by introducing the Generalized
Cross-Validation (GCV) criterion (Su M. et al., 2009).

3.2 Experimental Measurement System
For the purpose of measuring the ultrasonic spectrum
based on the insert-substitution method with variable-path

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

ln( A1 / A2 )mode (ISO20998-1:2006), an experimental set-up has
αl

 αw
(16)
been developed for acquisition of broad-band ultrasonicl2  l1
signals. As shown in Fig. 9, signals were generated andwhere A1 and A2 are the amplitude spectra of ultrasonicdetected by a PR-5800 ultrasonic pulse-echo transmitter/signals at the sound path length l1 and l2, respectively. αw
receiver (Parametric, Inc) working with two pairs of broadis the sound absorption of water.
band transducers, whose center frequencies were 10 MHz(V312-SU) and 25 MHz (V324-SU), respectively. According to the insert substitution method, the samples un4. Measurement of polydisperse nanoscale
der study were injected into two cubic sample cells withparticles
thickness of l1 = 10 mm and l2 = 20 mm respectively. Afterward, they encountered successively ultrasonic waves
4.1 Nanoscale silver particles
generated from the transducer and the attenuated ultrasonic waves were received by the other transducer on the
Practical nanoscale silver particles were measured byother side. Temperature of the samples was stabilized at
IDLS using one correlation coefficient and UAS comroom temperature during the testing to eliminate possible
bined with the ORT algorithm, respectively. The nominalinterference on ultrasonic waves.
mean size was claimed as 20 ± 2 nm by the manufacturer
Two typical ultrasonic time-domain signal are showed(Shanghai HuZheng Nanotechnology Co., Ltd.).
in Fig. 10, which indicates evidently the differences inDuring IDLS measurement, samples were dispersed inamplitude and phase between pure water (for calibration)
the 75 % glycerol water solution, since the frame rate ofand the sample suspension. An amplitude spectrum wasthe camera used in our work cannot meet the requirement
obtained after processing the raw signal through fast Fouwhen using water. The viscosity of 75 % glycerol-water atrier transform (FFT). Correspondingly, the attenuation
298 K is 0.03162 Pa·s, which is two orders of magnitude
could be calculated by path-variation measurement:
higher than that of water. The refractive index of 75 %glycerol-water is 1.42. Other experimental parameters
were set the same as section 2.2. The glycerol-water solution makes it feasible to measure small particles using alower frame rate. Take the time interval of 1 ms in themeasurement, the calculated cross-correlation coefficientis 0.821. Theoretical ACF curve at the delay time of 1 ms
is displayed in Fig. 11. The particle size corresponding tocoefficient of 0.821 is 23.15 nm, which is the measured
mean size for the sample of nanoscale silver particles.
The experimental setup and measurement conditions in
UAS were introduced in section 3.2, where high frequency ultrasound was adopted. Before testing, the aqueous suspension with 0.5 vol % silver particles experienced
a 2-minutes sonication process (FS-300, Shanghai Sonxi
Ultrasonic Instrument Company). The PSD measured by
Fig. 9 Sketch map of measurement system.
UAS is illustrated in Fig. 12. Furthermore, the volume-­
based PSD of the same nanoscale silver particles is also
represented in Fig. 12, which was extracted by analysing
their TEM image indicated in Fig. 13. It is obvious that
the UAS yields a relatively broader size distribution than
TEM. This is probably caused by the smoothness operation during the inversion of ultrasonic attenuation spectra
to overcome the ill-posed problems inside. Looking backto the size of IDLS, it appears a slightly bigger value compared with the 19.2 nm (Dv50) of UAS and 18.0 nm of
TEM analysis. The authors infer that this is due to theirrough surfaces slowing the velocity of Brownian motion,
which leads to an overestimated result here.
The recovered spectrum and the measured spectrum
for
UAS are plotted in Fig. 14. A good consistence in tenFig. 10 Ultrasonic pulse signals of water and the sample susdency of variation can be found, while obvious deviations
pension.

177

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

Fig. 11 Measured mean size of nanoscale silver particles by
IDLS using one correlation coefficient.

Fig. 13 TEM image of nanoscale silver particles.

Fig. 12 PSD of the same nanoscale silver particles by UAS
and TEM.

occur when the frequency exceeds 25 MHz, which canα  αR/ N ) ) of 18.2 % beyield a relative deviation ( M
αM
tween the measured and recovered spectra. Authors inferred that it was likely due to the non-linear loss effectcaused by the particle-particle interaction. Thus we calculated the mean distance h between two adjacent particles
according to the Woodcock equation, which yielded avalue of 71.0 nm (corresponding to the median size of
19.2 nm). Meanwhile the thermal and viscous skin depths(Mcclements D.J. and Coupland J.N., 1996) at 25 MHz
were also provided as δT = 42.7 nm and δV = 107.4 nm forwater. The comparison (δV > h) shows that the viscous
waves are very likely to exert an effect on the surrounding particles before vanishing, which makes the attenuation spectrum deviate from the prediction of the linear
theory (ECAH model), especially over the range of higher
frequency. Since the overall trend of recovered spectrum
should satisfy the assumption of theoretical model whichis merely sensitive to the overall trend, it does not causesignificant effect on the inversed results even if the two
attenuation values were lost. Thus to some extent such a
deviation is acceptable.

178

Fig. 14 Recovered and measured ultrasonic spectrum for suspensions of nanoscale silver particles.

4.2 Nanoscale ATO particles
Antimony Tin Oxide (ATO) nanoparticles, as one kindof the advanced materials, has been employed in many industry areas for its attractive properties, such as admirable thermal insulation, unparalleled dispersion, the active
resistance, and high performance in wear resistance andsafety.
The nanoparticle ATO sample in an aqueous suspension (2 vol %) adopted in our work is also provided by
Shanghai HuZheng Nanotechnology Co., Ltd. The particle size distribution of ATO samples measured by ultrasonic attenuation spectrum is ranging from 14.3 nm to40.6 nm (Dv01–Dv99), as shown in Fig. 15. And the volume
median size Dv50 is around 24.8 nm.
Using the same conditions described in section 4.1,
ATO samples are also prepared and measured by IDLS
method. To reveal the reliabilities and stabilities of themeasurement, 500 couples of images were captured andprocessed to get 500 data in a very short time. The results

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

to that on ultrasonic attenuation. Consequently, the resultspresented here also reveal that the size information of nanoscale particles being adequately retrieved by different
principles.

5. Final remarks

Fig. 15

PSD of nanoscale ATO particles by UAS.

Fig. 16 Mean particle size of nanoscale ATO particles by
IDLS.

are shown in Fig. 16, which has the averaged value of
23.9 nm and the standard error of 3.2 %.
According to the values of particle size, the measuring
results of ATO samples by IDLS and UAS methods canbe considered as in good agreement. However, it can benoticed that the small deviation exists with the results ofnanoscale silver particles. The authors would like to clarify that in addition to errors caused by indirect solving
algorithms like the mathematic inversion, various definitions and principles among methods could not be ignored
when considering the aforementioned deviations. For example, the equivalent projection area diameter (Heywood
diameter) is usually used for TEM. The UAS relies on theassumption of spherical sphere to fully interpret the soundscattering behaviour theoretically, which is similar with
the IDLS, whereas the latter actually gets the diameter ofa ball with the same characteristics of Brownian motion.
Strictly speaking, all the principles are aimed at sphericalparticles, so the measuring results are unavoidably influenced by the sphericity, the surface roughness and otherfactors, which often exert different influences for different particle characteristics. For example the influences ofsphericity on Brownian motion may be crucial compared

Two methods (DLS and UAS) with potential for development for nanoscale particles characterization were reviewed. The former is mainly focused on the researches
using CCD cameras, which we called as image-based
IDS. Three different image processing algorithms werereviewed in detail. At the same time, UAS was addressedto the problems of theoretical modelling (including numerical calculations involved), inversion and experimental
signal acquiring and analysis for the broad-band spectrum. Then a sample of nanoscale silver particles withnominal mean size 20 ± 2 nm and ATO was measured byusing IDLS, UAS and TEM. The results were reasonable
and agreed well with each other. The IDLS method basedon one correlation coefficient can avoid acquiring andprocessing a large number of data in conventional DLS
and represent an advantage of ultrafast and stable testing.
At the same time, the broad-band ultrasound contains
available information on particle size, which can ensure areasonable and stable result by combining the ORT algorithm. Thus, a potential ultrasound-based technique will
be available to permit a fast and invasive measurement forconcentrated nanoparticle suspensions without any dilution, where the non-linear effect must be considered carefully.

Acknowledgements
The authors gratefully acknowledge supports from the
National Natural Science Foundation of China (51176128,
51576130, 51573093), the Shanghai Science and Technology Commission of China (13DZ2260900) and the Shanghai Municipal Education Commission (14CG46).

Nomenclature
ACF

Autocorrelation Function

DLS

Dynamic Light Scattering

IDLS

Imaged-based Dynamic Light Scattering

TEM

Transmission Electronic Microscopy

UAS

Ultrasonic Attenuation SpectroscopyAncompression wave-scattering coefficientsDparticle size (nm)179

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

DN

nominal particle size (nm)DM

measured particle size (nm)
2

–1

DT

diffusion coefficient (m s )F

Bessel/Hankel functionfultrasonic frequency

gnormalized autocorrelation functionG

autocorrelation functionhmean distance between two adjacent particles (nm)Ilight intensitykincident compression wave wavenumber (m-1)

KB

Boltzmann constant (J K-1)

lsound path length (m)Lncomplex variables related with the ration of the Bessel/
Hankel functions and their first derivatives

Mncomplex variables related with the ration of the Bessel/
Hankel functions and their second derivativeN

number of frequency in a discrete ultrasonic attenuation
spectrumnrefractive index (–)
column number of image pixels (–)
order in Hankel functionpexperimental spectrum, with element αMqnorm of the scattering vector (m –1)
frequency distribution

Qncomplex variables in the recurrence formula of Bessel/
Hankel functions

Rcross-correlation operatorttime (s)Tabsolute temperature (K)zargument for Bessel/Hankel function

αs

ultrasonic attenuation coefficient(Np m–1)αM

measured ultrasonic attenuation (Np m–1)

αR

recovered ultrasonic attenuation (Np m–1)

β

instrumental constant (–)Γ

decay constant (s –1)

δT

thermal skin depth (nm)δVviscous skin depth (nm)

θscattering angle (°)λ

wavelength (nm)μdynamic viscosity (Pa s)τtime delay (s)φvolume fraction of particles (–)

180

References
Measurement and characterization of particles by acoustic
methods, part 1: concepts and procedures in ultrasonicattenuation spectroscopy, Switzerland, ( ISO20998-1:2006).
Allegra J.R. and Hawley S.A., Attenuation of sound in suspensions and emulsions: Theory and experiments, Journal of
the Acoustical Society of America, 51 (1972) 1545–1564.
Allen T., Powder sampling and particle size determination,
Elsevier Science, Amsterdam, 2003.
Alvarez A., Wang C.C. and Ye Z., A numerical algorithm, of themultiple scattering from an ensemble of arbitrary scatterers, Journal Of Computational Physics, 154 (1999) 231–236.
Boonkhao B. and Wang X.Z., Ultrasonic attenuation spectroscopy for multivariate statistical process control in nanomaterial processing, Particuology, 10 (2012) 196–202.
Brandolin A., García-Rubio L.H., Provedr T., Koehler M.E. and
Kuo C., Latex particle size distribution from turbidimetry
using inversion techniques, experimental validation, ACS
symposium series, 472 (1991) 20–33.
Challis R.E. and Pinfield V.J., Ultrasonic wave propagation inconcentrated slurries–the modelling problem, Ultrasonics,
54 (2014) 1737–1744.
Challis R.E., Povey M.J.W., Mather M.L. and Holmes A.K.,
Ultrasound techniques for characterizing colloidal dispersions, Reports on Progress in Physics, 68 (2005) 1541–1637.
Donaldson K., Stone V., Clouter A., Renwick L. and Macnee W.,
Ultrafine particles, Occupational and Environmental Medicine, (2001) 211–216.
Dukhin A.S. and Goetz P.J., Acoustic spectroscopy for concentrated polydisperse colloids with high density contrast,
Langmuir, 12 (1996) 4987–4997.
Dukhin A.S. and Goetz P.J., Acoustic and electroacoustic spectroscopy characterizing concentrated dispersions emulsions, Advances in Colloid and Interface Science, 92 (2001)
73–132.
Dukhin A.S., Goetz P.J. and Hamlet C.W., Acoustic spectroscopy for concentrated polydisperse colloids with low density contrast, Langmuir, 12 (1996) 4998–5003.
Dzakpasu R. and Axelrod D., Dynamic light scattering microscopy. A novel optical technique to image submicroscopic
motions. I: Theory, Biophysical Journal, 87 (2004) 1279–
1287.
Epstein P.S. and Carhart R.R., The absorption of sound in suspensions and emulsions: I. Water fog in air, The Journal of
the Acoustical Society of America, 25 (1953) 553–565.
Evans J.M. and Attenborough K., Sound propagation in concentrated emulsions: Comparison of coupled phase model andcore-shell model, The Journal of the Acoustical Society ofAmerica, 112 (2002) 1911–1917.
Hanson P.C., Analysis of discrete ill-posed problems by meansof the l-curve, SIAM Rev, 34 (1992) 561–580.
Hipp A.K., Storti G. and Morbidelli I., Acoustic characterization of concentrated suspensions and emulsions. 1. Model
analysis, Langmuir, 18 (2002) 391–404.
Hou H., Su M. and Cai X., Study on measurement of ultrasonicattenuation and velocity spectra in superfine particle suspensions, Chinese Journal of Science Instrument, 31 (2010)

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

241–247.
King T.A., 7 - photon correlation spectroscopy: Technique andscope, in: Bevington G.A.C. (Eds.), Comprehensive polymer science and supplements, Pergamon, Amsterdam,
1989, pp. 911–935.
Kirsch S., Frenz V., Schärtl W., Bartsch E. and Sillescu H.,
Multispeckle autocorrelation spectroscopy and its application to the investigation of ultraslow dynamical processes,
The Journal of Chemical Physics, 104 (1996) 1758–1761.
Liu L., Application of ultrasound spectroscopy for nanoparticle
sizing in high concentration suspensions: A factor analysis
on the effects of concentration and frequency, Chemical
Engineering Science, (2009) 5036–5042.
Liu L.L., Cai X.S., Zhang J. and Xu C.Z., Research on a noval
fast imaging dynamic light scattering method for nanoparticle size measurement, Acta Optica Sinica, 35 (2015) 369–
375 (In Chinese).
Mcclements D.J. and Coupland J.N., Theory of droplet size distribution measurements in emulsions using ultrasonic spectroscopy, Colloids and Surfaces, 117 (1996) 161–170.
Mougin P., Wilkinson D., Roberts K.J. and Tweedie R., Characterization of particle size and its distribution during the
crystallization of organic fine chemical products as measured in situ using ultrasonic attenuation spectroscopy,
Journal of the Acoustical Society of America, 109 (2001)
274–282.
Pecora R., Quasi-elastic light-scattering from macromolecules,
Annual Review Of Biophysics And Bioengineering, 1(1972) 257–276.
Pinfield V.J., Harlen O.G., Povey M.J.W. and Sleeman B.D.,
Acoustic propagation in dispersions in the long wavelength
limit, Siam Journal on Applied Mathematics, 66 (2005)
489–509.
Povey M.J.W., Ultrasound particle sizing: A review, Particuology, 11 (2013) 135–147.
Riebel U., Characterization of agglomerates and porous particles by ultrasonic spectrometry, The Fifth European Symposium Particle Characterization, Nurnberg, (1992).
Silbey R. and Deutch J.M., Quasi-elastic light-scattering fromlarge macromolecules, Journal Of Chemical Physics, 57(1972) 5010–5011.
Su M. and Cai X., The numerical study of acoustical attenuationand velocity in the suspension of superfine particles, Acta
Acustica, 27 (2002) 218–222.
Su M., Cai X., Xue M., Dong L. and Xu F., Particle sizing indense two-phase droplet systems by ultrasonic attenuationand velocity spectra, Science in China Series E-Technological
Sciences, 52 (2009) 1502–1510.
Su M., Xu F., Cai X., Ren K. and Shen J., Optimization of regularization parameter of inversion in particle sizing using
light extinction method, China Particuology, 5 (2007) 295–
299.
Tebbutt J.S. and Challis R.E., Ultrasonic wave propagation in
colloidal suspensions and emulsions: A comparison of fourmodels, Ultrasonics, 34 (1996) 363–368.
Thomas A.J.C., Photon correlation spectroscopy: Technique andinstrumentation, Proc Spie, (1991) 2–18.
Tikhonov A.N. and Arsenin V.Y., Solution of ill-posed problems. , John Wiley and Sons, New York, 1977.
Tscharnuter W., Photon correlation spectroscopy in particle sizing, (Eds.), Encyclopedia of analytical chemistry, John
Wiley & Sons, Ltd, 2006, pp. 5469–5485.
Twomey S., Introduction to the mathematics of inversion inremote sensing and indirect measurement, Elsevier,
Amsterdam, 1977.
Wang X.Z., Liu L., Li R.F., Tweedie R.J., Primrose K., Corbett
J. and Mcneil-Watson F.K., Online characterisation of
nanoparticle suspensions using dynamic light scattering,ultrasound spectroscopy and process tomography, Chemical Engineering Research & Design, 87 (2009) 874–884.
Wang Z.Y., Cai X.S., Xu C.Z. and Liu L.L., Nanoparticle sizing
by image processing with dynamic light scattering, Acta
Optica Sinica, 34 (2014) 282–287 (In Chinese).
Wong A.P.Y. and Wiltzius P., Dynamic light scattering with a
ccd camera, Review of Scientific Instruments, 64 (1993)
2547–2549.
Xu C.Z., Cai X.S., Zhang J. and Liu L.L., Fast nanoparticle sizing by image dynamic light scattering, Particuology, 19(2015) 82–85.
Ye Z., Hoskinson E., Dewey R.K., Ding L. and Farmer D.M., A
method for acoustic scattering by slender bodies .1. Theory
and verification, Journal Of the Acoustical Society Of
America, 102 (1997) 1964–1976.
Zakharov P., Bhat S., Schurtenberger P. and Scheffold F.,
Multiple-­scattering suppression in dynamic light scattering
based on a digital camera detection scheme, Applied Optics
45 (2006) 1756–1764.
Zhou W., Zhang J., Liu L.L. and Cai X.S., Ultrafast imagebased dynamic light scattering for nanoparticle sizing,
Review Of Scientific Instruments, 86 (2015) 115107–
115106.

181

Wu Zhou et al. / KONA Powder and Particle Journal No. 34 (2017) 168–182

Author’s short biography
Wu Zhou
Wu Zhou received her degree from Nanjing University of Science and Technology in2006 and a Ph.D. in Engineering from Southeast University in 2011. She is now an associate professor in the School of Energy and Power Engineering at the University of
Shanghai for Science and Technology. Her research interests include particles and twophase flow measurement, combustion measurement, mainly by image-based methods.
She also interests in mathematical modelling of the above processes. She is a member ofYouth Council of the Chinese Society of Particuology.

Mingxu Su
Prof. Mingxu Su received his degree in Power Engineering from Nanjing Universityof Aeronautics and Astronautics in 1996 and his PhD in Thermal Engineering from the
University of Shanghai for Science and Technology, USST in 2002. In the same year, he
got a postdoctoral position at UMR 6614 - CORIA in Rouen, France. After that, he went
back to the USST being a teacher till now. His main research interests are methods ofparticle size characterization based on ultrasonic spectroscopy and light scattering theories, as well as the industrial applications on particulate two-phase flow measurement.

Xiaoshu Cai
Prof. Xiaoshu Cai graduated from the Department of Engineering Thermalphysics,
Zhejiang University in 1982, and obtained PhD in Department of Power Engineering,
Shanghai Institute of Mechanical Engineering in 1991. He became a professor in University of Shanghai for Science and Technology in 1996. Since 1988 he has been engaged inparticle sizing and two-phase flow measurement techniques. His current research interests include particles and two-phase flow measurement, combustion measurement, emission and environmental monitoring, as well as power engineering and turbo machinery.
He is vice president of Chinese Society of Particuology and president of Shanghai Society of Particuology.

182KONA Powder and Particle Journal No. 34 (2017) 183–196/Doi:10.14356/kona.2017002

Original Research Paper

Microstructural Evaluation of ZrB2/ZrO2 Ceramic Powders
Prepared by Milling-Assisted Magnesiothermic
Reduction of Oxide Raw Materials †Duygu Ağaoğulları *, Özge Balcı, M. Lütfi Öveçoğlu and İsmail Duman
1

Istanbul Technical University, Faculty of Chemical and Metallurgical Engineering, Department of Metallurgicaland Materials Engineering, Particulate Materials Laboratories (PML), Turkey

Abstract
This study reports on the synthesis and microstructural evaluation of ZrB2/ZrO2 ceramic powders prepared bymilling-assisted magnesiothermic reduction of oxide raw materials. Powder blends containing ZrO2, B2O3 and Mg
reactants were milled in different type of mills at different durations and were subsequently annealed in a tube
furnace under Ar atmosphere. An additional purification step (HCl leaching) was conducted on the milled andannealed samples to obtain only Zr-based products. FactSageTM thermochemical software was used in order toshow a preliminary route for the experiments. The effects of milling duration (up to 100 h), milling type (aSPEXTM 8000D Mixer/Mill and a planetary ball mill), excess Mg amount (5–20 wt.%) and annealing duration (6and 12 h) on the formation and microstructure of the products were examined. The milled, annealed and leachedproducts were characterized using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron
microscope (SEM) and differential scanning calorimeter (DSC). Pure ZrB2/ZrO2 ceramic powders having
particles in size range of 200 nm–1 μm and containing two different crystal structures of ZrO2 phase (monoclinic
and tetragonal) were obtained after milling in the SPEXTM 8000D Mixer/Mill for 30 h, annealing at 1200 °C for
12 h and leaching with 5 M HCl.
Keywords: zirconium diboride, zirconium oxide, boron oxide, ceramic powders, high-energy ball milling,magnesiothermic reduction

1. Introduction
There has been recently a growing interest in ZrB2based ceramics due to their outstanding properties suchas high melting point, high electrical and thermal conductivity, as well as excellent chemical inertness, highstrength and high thermal shock resistance (Fahrenholtz
and Hilmas, 2007; Monteverde et al., 2003). Based onthese properties, ZrB2-based ceramics are attracting more
and more attention in ultrahigh temperature applications
where resistance to corrosion, wear and oxidation is demanded (Fahrenholtz and Hilmas, 2007; Zhu et al., 2009a,
2009b). They have been found to be suitable materials invarious usage areas such as electrodes, thermowells, molten metal crucibles, armors, thermal protection systems
for hypersonic flight, atmospheric re-entry vehicles,
rocket propulsion and nose caps (Melendez-Martinez et†
1*Received 1 July 2015; Accepted 3 September 2015
J-STAGE Advance published online 30 January 2016
Ayazağa Campus, 34469 Maslak, Istanbul, Turkey
Corresponding author: Duygu Ağaoğulları;E-mail: bozkurtdu@itu.edu.tr
TEL: +90-212-285-3381 FAX: +90-212-285-3427

al., 2002; Chamberlain et al., 2004; Monteverde et al.,2008; Brouchu et al., 2009). Furthermore, they have beenused as wear parts, nozzles, coating on cutting tools andcathodes for electrochemical processing of aluminium(Upadhya et al., 1997; Opeka et al., 1999). Many of thecomposite materials have been developed to improve thesinterability and mechanical properties of the ZrB2-basedmaterials (Monteverde, 2005; Guo, 2009). Amongst them,
ZrB2-ZrO2 ceramics have raised significant concern due
to the high contribution of ZrO2 phase on the densification, oxidation resistance and fracture toughness of theZrB2-based composites (Zhang et al., 2008; Li W. et al.,2009; Zhu et al., 2009a, 2009b; Li et al., 2010).
Many processes are available for the preparation of
ZrB2 or ZrB2-based ceramic powders: They have been
traditionally produced by high temperature methods using
solid-state reactions, borothermal and carbothermal reductions (Zhang et al., 2009; Qiu et al., 2012). Powders
and coatings of ZrB2 have been also produced by electrometallurgy processes including molten salt electrolysis(Frazer et al., 1975; Malyshev and Gab, 2009). Chemical
vapor deposition (CVD) technique has been applied toobtain ZrB2 coatings by using ZrCl4, BCl3 and H2 gas

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

183

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

mixtures (Gebhardt and Cree, 1965; Greenwood et al.,
1966; Deng et al., 2012). In recent years, low-temperaturesynthesis in an autoclave (Chen et al., 2004; Chen et al.,2012) and self-propagating high-temperature synthesis(SHS) (Radev and Klissurski, 2001; Khanra et al., 2008)
methods have been developed to achieve ZrB2 or ZrB2based ceramic powders. Synthesis of ZrB2 powders byhigh-energy ball milling derived mechanochemical reaction has been examined by a group of researchers using
the starting materials of ZrO2, B2O3 and Mg (Setoudeh
and Welham, 2006; Guo et al., 2011; Akgün et al., 2011;
Jalaly et al., 2013, 2014). The preparation of ZrB2/ZrO2
powders was also reported by using high-energy ball
milling/annealing processes and/or boro/carbothermal
reduction methods (Balcı et al., 2012; Qiu et al., 2012).
Review in the archival literature showed that mechanochemical route or milling-assisted synthesis processes
have been more interesting due to its low-cost, simplicity,
high throughput and product quality.
On the other hand, ZrB2-based dense bodies have been
fabricated by spark plasma sintering (SPS), hot pressing(HP) and pressureless sintering (PS) from the starting
powders containing ZrB2 and some required additives
such as SiC, ZrC, ZrO2, MoSi2, AlN, B4C, Al2O3, etc(Guo et al., 2009; Monteverde et al., 2008; Chen et al.,2009; Li B. et al., 2009). Preparation of ZrB2/ZrO2 ceramics with these methods have also been investigated using
ZrB2 and ZrO2 as raw materials (Basu et al., 2002; Zhang
et al., 2008; Hong et al., 2008; Zhu et al., 2009b; Li W. etal., 2009). Li W. et al (2009) has reported the fabricationof ZrB2/ZrO2 ceramics with various ZrO2 content by using hot pressing and provided a considerable increase in
their fracture toughness values. Zhang et al (2008) has reported the fabrication of ZrB2/ZrO2 ceramics by utilizing
spark plasma sintering of ZrB2 and ZrO2 powders and theincrease in their toughness from ZrB2-rich layer to ZrO2rich layer due to the phase transformation of ZrO2. Thus,
ZrB2/ZrO2 ceramic powders with fine and homogeneous
microstructure can be a potential precursor for the development of ZrB2-based dense bodies.
An efficient method should be developed for the synthesis of submicron-sized ZrB2/ZrO2 precursors with highpurity. Already, it came out as a need for the preparationof their dense bodies since consolidation of ZrB2/ZrO2
precursors obtained from oxide raw materials is cheaper
than that of their commercial ones. High-energy ball milling which is the main driving force of the displacement orredox/reduction reactions in the mechanical alloying andmechanochemical synthesis processes, has the advantageous of enabling rapid preparation for oxide dispersion
strengthened alloys, amorphous materials, solid solution
alloys, non-equilibrium alloys, intermetallics, compounds, nanocomposites, ceramics and advanced materials that are difficult or impossible to be obtained by184

conventional production techniques (Suryanarayana, 2001;
El-Eskandarany, 2001). It can be controlled by several parameters such as type of mill, milling speed, milling duration, ball-to-powder weight ratio, milling container,
milling atmosphere, milling environment, type, size andsize distribution of milling media, etc (Suryanarayana,
2001; El-Eskandarany, 2001). It offers a wide range ofoptions by changing the desired parameters for leading
the reactants towards to the size reduction and reaction.
So, in the present study, milling-assisted magnesiothermic reduction process was used for the synthesis of theZrB2/ZrO2 ceramic powders from oxide raw materials.
Two different types of mills present in the laboratory facilities were employed for the milling process in order tomonitor the reaction products of the ZrO2-B2O3-Mg powder
system. The differences of these mills are speed of rotation, material/diameter of milling media and material/sizeof milling container, meaning that the efficiency of milling is different for this non-equilibrium processing technique. Subsequently, milled powders were annealed andleached to achieve pure and fine-grained ZrB2/ZrO2 ceramic powders.

2. Experimental procedure
Oxide raw materials such as ZrO2 powders (Alfa AesarTM,
in purity of 99.7 %, with an average particle size of
44 μm) and B2O3 powders (ETI Mine, in purity of 98 %,with an average particle size of 467 μm) were used in theexperiments, respectively as zirconium source and as native boron source for the synthesis of ZrB2/ZrO2 ceramic
powders. Mg powders (Alfa AesarTM, in purity of 99.7 %,with an average particle size of 145 μm) were utilized as
reducing agent in order to yield magnesiothermic reduction of the oxide reactants.
Powder blends containing stoichiometric amounts ofreactants were prepared according to the theoretical reduction reaction given in Eq. (1). However, it is alreadyknown that the complete reduction of ZrO2 to ZrB2 can
not be possible considering its high chemical stability(Balcı et al., 2012). Thus, the rationale of preferring stoichiometric amount in the preparation of the powder
blends is impossibility in the complete reduction and conversion of ZrO2 to ZrB2.
ZrO2 + B2O3 + 5Mg → ZrB2 + 5MgO

(1)For each run, powder batches were weighed in a PrecisaTM XB320M sensitive balance. The prepared ZrO2B2O3-Mg powder batches were homogenized in a WABTM
T2C Turbula blender for 1 h and they are hereafter referred to as-blended (ab) ZrO2-B2O3-Mg powders.
As-blended powders were separately milled both in a
SPEXTM 8000D Mixer/Mill (6 g powder batch) and in aDuygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

FritschTM Pulverisette 5 classic line planetary ball mill
(12 g powder batch), with a rotation speed of 1200 and
400 rpm, respectively. Commonly, SPEX TM 8000D
Mixer/Mill is employed for exploratory purposes andplanetary ball mill is utilized to produce larger quantities
of milled powders. In the milling experiments, ball-topowder weight ratio (BPR) was chosen as 10:1. Milling
containers changed according to the type of mills: a hardened steel vial with a capacity of 50 ml was used for the
SPEXTM 8000D Mixer/Mill and a zirconia vial with a capacity of 500 ml was used for the planetary ball mill. Hardened steel balls with diameters of 6 mm and zirconia ballswith diameters of 10 mm were employed as milling media,
respectively. Extent of filling the vial is about 45 % for theutilized mills, knowing that adequate space should begiven to the balls and to the particles to move around freely
in the vial and to collide each other for getting enoughimpact energy (Suryanarayana, 2001; El-Eskandarany,2001). Ar gas (LindeTM, in purity of 99.999 %) was preferred as milling atmosphere. Different milling durations
up to 100 h were conducted during the experiments. The
milling vials were sealed by evacuating to about 10 –2 Paand by back filling with Ar gas in a PlaslabsTM glove box.
The milled powders were unloaded again under Ar atmosphere in the same glove-box.
Annealing experiments of the milled ZrO2-B2O3-Mg
powders were carried out in quartz boats which were inserted in a ProthermTM tube furnace, at 1200 °C for 6 and12 h with a heating and cooling rate of 10 °C/min under
Ar gas flow.
Milled and annealed ZrO2-B2O3-Mg powders wereleached with HCl (MerckTM, in concentration of 37 %)under the effect of ultrasonic stirring using a Bandelin
SonorexTM RK-100 H ultrasonic bath in order to remove
unwanted MgO phase. HCl solutions in concentrations of
3.6 and 5 M were used for the leaching treatments. Leaching parameters such as solid-to-liquid ratio of the solution
and duration were selected respectively as 1 g/10 cm3 and30 min. The solutions containing insoluble solids were
subjected to repeated centrifuging in a HettichTM Rotofix
32A centrifuge with a rotation speed of 3500 rpm for
30 min, to repeated decanting and to repeated washingwith distilled water. Finally, remained solids were dried
under air in a FN 500 stove at 120 °C for 24 h.
The phase compositions of the milled, leached and annealed powders were identified using a BrukerTM D8 Advanced Series X-ray diffractometer (XRD) with CuKα
(1.54060 Å) radiation in the 2θ range of 10–80° incremented at a step size of 0.02° at a rate of 2°/min. International Center for Diffraction Data® (ICDD) powder
diffraction files were utilized for the identification of
crystalline phases. Thermal property of the milled powders was examined in a TATM Instruments SDT Q600
differential scanning calorimeter (DSC)/thermogravimet-

ric analyser (TGA). DSC/TGA experiments were conducted in alumina crucibles up to a heating temperatureof 1200 °C at a rate of 10 °C/min under Ar atmosphere. In
order to identify the phases at the exothermic and endothermic peak points, milled powders were heated to 500,
600, 700, 800 and 900 °C in a ThermoscientificTM tube
furnace under Ar atmosphere and they were subjected toadditional XRD analyses at the same conditions. Microstructural characterizations of the milled, annealed andleached powders were carried out using a ZeissTM Discovery.V12 stereomicroscope (SM) coupled with a ZeissTM
Axiocam ERc5s high resolution digital camera and a
HitachiTM TM-1000 scanning electron microscope (SEM)
operated at 15 kV. For SEM analysis, the specimens wereprepared by following the procedure of dissolving thepowders in C2H5OH (MerckTM, in purity of 99.9 %), syringing them onto a base plate, drying them in air andcoating their surfaces with a thin layer of gold using a
PolaronTM SC7620 Sputter Coater to enhance their conductivities. Several different regions of the samples weremonitored during the SEM analyses: homogeneous distributions of the particles were observed thankful to themilling process conducted for different durations. Thus,
representative SEM images of the samples were shown inorder to determine the size range of the particles insteadof their average size. Lastly, Zr element and also probable
Fe impurity, which could be formed as a result of the wear
and tear from the hardened steel milling vial and balls,
were analyzed in the supernatant liquids by using a Perkin
ElmerTM 1100B atomic absorption spectrometer (AAS).

3. Results and discussion
3.1 Thermodynamic calculations
FactSageTM 6.2 thermochemical software was used forthe determination of reaction possibility in terms of standard Gibbs free energy change versus temperature relation and for the interpretation of the probable reaction
products, in ZrO2-B2O3-Mg system. The standard Gibbs
free energy change versus temperature curve of the ZrO2B2O3-Mg system was plotted up to 2000 °C and illustrated in Fig. 1(a).
The reaction has negative free energy change between–930 and –200 kJ in the temperature range of 0–2000 °C,
which means that the reduction reaction in Eq. (1) is possible and takes place spontaneously at room temperature
and above. However, there is a change in the slope of thecurve in Fig. 1(a). The standard Gibbs free energy changeof the reaction is between –930 and –775 kJ for the temperatures up to 1095 °C but the slope of the curve increases after this temperature at which Mg boils. Thus,boiling of Mg causes a significant change in the free en-

185

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

Fig. 2 XRD patterns of the as-blended powders and thosemilled in the SPEXTM 8000D Mixer/Mill for different
durations: (a) ab, (b) 3 h, (c) 5 h, (d) 7 h, (e) 9 h, (f)15 h, (g) 20 h and (h) 30 h.
Table 1 The details of the crystal structural parameters of thedifferent phases encountered in this study.

Fig. 1

Thermodynamic calculations of the ZrO2-B2O3-Mg
system obtained from FactSageTM 6.2 thermochemical
software for the reaction given in Eq.(1): (a) The standard Gibbs free energy change versus temperature
curve, and (b) Molar amounts of reactants/products
varying according to the molar amount of Mg at
1200 °C.

ergy value and hence slows the reaction proceeding.
Fig. 1(b) shows the FactSageTM plot for the molar amountsof reactants/products varying according to the molar
amount of Mg at 1200 °C. Under the stoichiometricamount of Mg (< 5 moles), the molar amounts of ZrB2 andMgO increase with decreasing amount of unreacted B2O3.
When stoichiometric amount of Mg (5 moles) is used in
the reaction, B2O3 reactant vanishes and 1 mole of ZrB2
and 5 moles of MgO are obtained as the products. Increasing the molar amount of Mg up to 20 moles does notcontribute to the further formation of ZrB2 and MgOphases because excess amounts of Mg convert into gas
phase and leave the system without participating in the
reduction reaction. Thus, maximum amounts of ZrB2 andMgO reaction products can be achieved in the case of using stoichiometric amount of Mg, according to the thermodynamic calculations in Fig. 1(b).

3.2 Milling of the as-blended powders
Fig. 2(a)–(h) displays the XRD patterns of the asblended powders and those milled in the SPEXTM 8000D
Mixer/Mill for different durations (3, 5, 7, 9, 15, 20 and30 h). As clearly seen from Fig. 2, there are no reactions

186

Phase

ICDD
Card #Bravais Lattice

LatticeParameters (nm)ZrO289-9066

Primitive
Monoclinic

a = 0.531
b = 0.520
c = 0.514

B2O3

—

—

—

ZrO2

50-1089

Primitive
Tetragonala = b = 0.359
c = 0.515

Mg

35-0821

Primitive
Hexagonal

a = b = 0.329
c = 0.521

ZrB2

34-0423

Primitive
Hexagonal

a = b = 0.317
c = 0.353

MgO

45-0946

Face-centeredCubic

a = b = c = 0.421

between ZrO2, B2O3 and Mg even after milling for 30 h
because the ZrO2 and Mg phases are still present in the
XRD patterns of the powders. No peaks belonging to the
B2O3 phase are observed in the XRD pattern of the asblended ZrO2-B2O3-Mg powders, probably due to its
amorphous nature. Furthermore, any Fe impurity which
could be released from the hardened steel vial/balls
during milling did not detected in the XRD patterns in
Fig. 2(b)–(h) since the diffractometer is not capable ofanalyzing phase amounts under 2 wt.% of the overall
sample. Table 1 presents the details of the crystal structural parameters of the different phases encountered inthis study.
Similarly, in a related study reported on the carbothermal reduction of ZrO2 and B2O3 powder blends, there exists no reaction even after milling for 6 h in the SPEXTM

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

Fig. 3 SEM images of the powders milled in the SPEXTM
8000D Mixer/Mill for different durations: (a) 15 h and(b) 30 h.

8000D Mixer/Mill with a BPR of 10:1 (Balcı et al., 2012).
Moreover, ZrO2 and Mg peaks broadened and their intensities gradually decreased upon increasing the milling
time from 0 to 30 h, indicating a gradual decrease in theircrystallite sizes and an increase in their lattice strains dueto the continuous deformation during milling. There is a
visible decrease in the intensities of the ZrO2 and Mg
peaks from 0 to 3 h. However, this decrease is seen lessfrom 3 to 30 h of milling duration. Thus, no change occurred in the phase composition of the powders after
milling for 30 h, suggesting that prolonged milling did notcontribute to the chemical reaction, but contributed only
to the particle size reduction.
Representative SEM images of the powders milled in
the SPEXTM 8000D Mixer/Mill for 15 and 30 h are given
in Fig. 3(a) and (b). At first view, it is clearly seen fromthe SEM images that powders milled for 15 and 30 h do
not consist of particles with definite and perfect morphologies throughout the structure. Fig. 3(a) reveals both theround-shaped particles having sizes smaller than 2 μm
and the irregular agglomerates in sizes not larger than
5 μm. It is also easily observed from the microstructure in
Fig. 3(a) that some smaller round-shaped particles areembedded in larger agglomerates. Extended milling to

30 h resulted in the particle size reduction (from 2 μm to
700 nm) and provided the breaking down of the larger agglomerates (from 5 to 2.5 μm). Moreover, it was reported
in previous studies that repeated fracturing and welding
mechanism took place in the vial during milling creates
agglomeration and inhomogeneity in the shapes of theparticles due to the contact of active and small fresh surfaces at each collision between vial walls/balls and particles (Suryanarayana, 2001; Ağaoğulları et al., 2012a,
2016; Ağaoğulları, 2014).
On the basis of XRD patterns in Fig. 2 and SEM images in Fig. 3, it was understood that milling of stoichiometric amount of ZrO2, B2O3 and Mg powders in the
SPEXTM 8000D Mixer/Mill for 30 h yielded only smaller
particles of the reactants without any formation of reduction reaction. Although FactSageTM plot in Fig. 1(b)showed that using 20 moles of Mg as starting material
rather than 5 moles (stoichiometric amount) did not increase the molar amounts of ZrB2 and MgO products,
milling with the addition of excess amounts of Mg (5, 10,
15 and 20 wt.%) was conducted considering that the thermodynamic calculations exhibit only the equilibriumphase compositions at room and elevated temperatures
without taking into account synthesis history and reaction
kinetics. As previously known, the impact energy accumulated in the system during milling converts into othertypes of energies (potential, kinetic, chemical, etc.): theseenergies are effective in the formation of new structuralarrangements which can be deviated from thermodynamic equilibrium states (Suryanarayana, 2001; El-­
Eskandarany, 2001). In this case, the molar amount of Mg
was increased to observe an indication of the reduction
reaction. Taking into account that the peak intensities of
the ZrO2 and MgO phases are similar after milling for 7 hwith those after milling for 30 h (Fig. 2(d) and (h)), experiments with excess Mg amounts were carried out using
7 h of milling duration. Fig. 4(a)–(d) illustrates the XRD
patterns of the powders milled in the SPEXTM 8000D
Mixer/Mill for 7 h with the use of excess amounts of Mg(5, 10, 15 and 20 wt.%), respectively. As seen from Fig.
4(a)–(d), there are still ZrO2 and Mg phases in the microstructure of the milled powders, definitely in the presenceof amorphous B2O3 which could not be observed with adistinctive peak by XRD. Also, there are indications of avery small combined peak of ZrB2 and MgO phases at the2θ range of 40–45° and very small ZrB2 peaks at the 2θ
value of about 34, 66 and 73°. Amongst the XRD patternsin Fig. 4(a)–(d), the highest intensity of ZrB2 and MgOphases were obtained when 20 wt.% excess Mg was usedin the powder blend. It is expected to detect ZrB2 andMgO phases in higher intensities than those in Fig. 4
since the addition of excess Mg could compensate thesurface oxidation of the reactants and could trigger thereactants for the desired reaction. However, the tempera-

187

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

Fig. 4

XRD patterns of the powders milled in the SPEXTM
8000D Mixer/Mill for 7 h with the use of excess
amount of Mg: (a) 5 wt.%, (b) 10 wt.%, (c) 15 wt.% and(d) 20 wt.%.

ture increase in the vial arose from the impact energy
during milling is not sufficient for activating the ZrO2material exhibiting a ceramic character and high chemical
stability. If sufficient temperature increase was obtained
and hence activation energy barrier was exceeded in thesystem under the effect of impact energy, there would bea sudden exothermic reaction following to the achievement of adequate particle size reduction. The driving
force for this exothermic reaction is ignition occurred at
the solid-solid interface of the reduced particles by thehelp of the high oxygen affinity of Mg. Thus, further ignition of the milled powders should be provided by applying
external heat such as annealing.
After obtaining the experimental outputs from millingin the SPEXTM 8000D Mixer/Mill, the type of high-energy
ball mill was changed to monitor its effects on the reaction time and product type, considering that the kineticsof the milling process depend on the stress conditions in
the milling devices. Fig. 5(a)–(f) demonstrates the XRD
patterns of the powders milled in the planetary ball millfor 20, 25, 40, 50, 70 and 100 h. According to the XRD
patterns in Fig. 5(a) and (b), milled powders for 20 and
25 h contain ZrO2 and Mg crystalline phases certainlywith the amorphous B2O3, revealing that any reaction did
not take place till to the end of these durations.
As milling time was increased to 40 h, either a significant decrease in the intensities of the ZrO2 and Mg phases
or small incubations of ZrB2 and MgO phases were observed (Fig. 5(c)). Moreover, the intensity decreases ofZrO2 and Mg phases are seen sharper for 25 and 40 h
milling durations (Fig. 5(b) and (c)). Prolonging milling
time from 40 h to 50, 70 and 100 h enabled the further
size reduction of ZrO2 and Mg particles but did not enable
the formation of complete reduction: ZrB2 and MgOphases have still very small peak intensities in the pres-188

Fig. 5

XRD patterns of the powders milled in the planetaryball mill for different durations: (a) 20 h, (b) 25 h, (c)40 h, (d) 50 h, (e) 70 h and (f) 100 h.

ence of major ZrO2 phase. However, Mg peaks almost
disappeared due to the continuous deformation. As the
XRD analysis results in Fig. 2(b)–(h) and 5(a)–(f) arecompared with each other, it is understood that the only
difference emerged in the phase type as the incubationsof ZrB2 and MgO. Roughly, these incubations could be
expected to occur after milling in the SPEXTM 8000D
Mixer/Mill because the rotation speed of this mill is threetimes higher than that of planetary one which corresponds
to a higher impact energy and shorter reaction time. However, there is no indication of ZrB2 and MgO even after
milling for 30 h in the SPEXTM 8000D Mixer/Mill (Fig.
2(b)–(h)). The emergence of ZrB2 and MgO after 40 h
milling in the planetary ball mill may be explained by thestrong impact forces occurred between ZrO2 reactant andvial/balls which were made from the same material. It
should be also noted that processing parameters of thetwo mills such as material of milling vial/balls, diameterof the milling balls and milling energy did not match eachother and they could not be evaluated in a comprehensive
way. Already, the main aim of using two different mills
was not to compare their corresponding products in thecase of changing one milling parameter by fixing the others, but was to observe the reaction products of the asblended ZrO2-B2O3-Mg powders in two different systems
in the within the laboratory facilities.
Representative SEM images of the powders milled in
the planetary ball mill for 20, 25, 50 and 100 h are given
in Fig. 6(a) to (d), respectively. As seen from Fig. 6(a)and (b), milled powders for 20 and 25 h have irregular-­shaped particles in sizes below 20 μm. There is not a verysignificant difference in the particle sizes of the powders
milled for 20 and 25 h (Fig. 6(a) and (b)), as compatible
with the similar intensities of ZrO2 and Mg phases in their
XRD patterns (Fig. 5(a) and (b)).

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

In other words, further milling for 5 h in planetary ballmill was not adequate to create a remarkable change inthe particle size. Besides, the gradual decrease in the particle sizes of the milled powders can be obviously seenfrom Fig. 6(b) through (d). Fig. 6(c) shows agglomerates
in sizes not larger than 5 μm and also irregular-shaped
particles having sizes below 2 μm. As milling time wasincreased to 100 h, particle sizes decreased to 2 μm andbelow (Fig. 6(d)). Similar to the microstructures obtained
after milling process in the SPEXTM 8000D Mixer/Mill(Fig. 3(a) and (b)), agglomeration of powder particles was
encountered after milling in the planetary ball mill (Fig.
6(c) and (d)). This phenomenon generally prevents theobservation of smaller particles throughout the microstructure. Furthermore, approximate particle sizes wereobtained after milling for 30 h in the SPEXTM 8000D
Mixer/Mill and for 100 h in the planetary ball mill even if
milling parameters for two different equipment could not
be comparable in regard of the material of milling vial/
balls, the diameter of the milling balls and the rotation
speed.

3.3 Annealing of the milled powders

Fig. 6 	SEM images of the powders milled in the planetaryball mill for different durations: (a) 20 h, (b) 25 h, (c)50 h and (d) 100 h.

After milling processes carried out using different typeof mills for different durations, application of an external
heat came out as a need for the reduction reaction and forthe partial/complete conversion of ZrO2 to ZrB2 sincemilling did not yield the characteristic peaks of ZrB2
phase even after very long milling durations. Besides,
milling the stoichiometric powder blends or those addedwith 20 wt.% excess Mg in the SPEXTM 8000D Mixer/
Mill is considered as more advantageous than milling in
the planetary ball mill because similar particle sizes could
be achieved at shorter durations by taking into accounttheir independent process parameters. Thus, powders
milled for 30 h in the SPEXTM 8000D Mixer/Mill with/without excess Mg amounts were subjected to the subsequent annealing processes. Prior to the annealing, thermal
behavior of the sample milled for 30 h was determined by
DSC-TGA experiments in order to suggest an annealing
temperature.
Fig. 7 shows the DSC-TG scans of the powders milledin the SPEXTM 8000D Mixer/Mill for 30 h. According to
the DSC scan, milled powders have an exothermic peak at
about 600 °C and a broad endothermic peak at about
900 °C. Although powders milled for 30 h have ZrO2,
B2O3 and Mg phases (Fig. 2(h)), there are not any endotherms corresponding to the melting of B2O3 at 450 °C
and melting of Mg at 650 °C in the DSC scan in Fig. 7.
Instead of these endotherms, there is an exotherm with asharp peaking point at 600 °C and with an onset temperature of about 400 °C. Homogeneous distribution of active
ZrO2, B2O3 and Mg particles in small sizes enabled them189

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

Fig. 7 DSC-TG scans of the powders milled in the SPEXTM
8000D Mixer/Mill for 30 h.

to react with each other rather than their individual melting. Furthermore, the initial weight of the sample increases from 21.45 to 24.50 mg, as seen in the TG scan. In
order to reveal the reasons of the exothermic and endothermic peaks, milled powders were just heated to 500,
600, 700, 800 and 900 °C without a holding time and they
were cooled instantly down to the room temperature. The
XRD patterns of these milled and heated powders are
given in Fig. 8(a) to (e). After milled powders were
heated to the temperatures of 500, 600, 700 and 800 °C,
the microstructure contained ZrO2 and MgO phases. Thismeans that the exotherm peaking at 600 °C belongs to theoxidation of Mg. This is in good agreement with the TG
scan because it has sharp increase (from 21.45 to
23.62 mg) until 850 °C.
Heating of the milled powders to 900 °C resulted in the
incubation of ZrB2 phase (Fig. 8(e)), indicating that theendothermic peak is a sign of ZrB2 formation. As alreadyknown, monoclinic ZrO2 transforms into tetragonal structure at about 1170 °C (Nettleship and Stevens, 1987).
However, there is no indication of this transformation in
the DSC scan (Fig. 7). This phenomenon can be attributed
to the non-isothermal fast DSC heating conditions withouta holding time, carried out in an alumina crucible inwhich particles of the reactants placed in a very small
volume. Considering the results in Fig. 7 and Fig. 8(a)–(e), the melting temperature of ZrO2 (about 2715 °C) andTamman temperature (0.4–0.5 × Tmelting), the annealing
temperature of the milled powders was selected as 1200 °C.
Fig. 9(a) and (b) represent the XRD patterns of thepowders milled in the SPEXTM 8000D Mixer/Mill for
30 h and annealed at 1200 °C for 6 and 12 h, respectively.
After annealing for 6 h (Fig. 9(a)), ZrB2 and MgO wereobtained with their characteristic peaks in the presence of
unreacted ZrO2 phase. However, an amount of monoclinic
ZrO2 phase transformed into tetragonal (ZrO2-t) structure.
This arose by the help of the holding time for 6 h at
1200 °C because annealing temperature is just a little

190

Fig. 8XRD patterns of the powders milled in the SPEXTM
8000D Mixer/Mill for 30 h and subsequently heated to(a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C and (e)900 °C.

Fig. 9XRD patterns of the powders milled in the SPEXTM
8000D Mixer/Mill for 30 h and annealed at 1200 °C for
different durations: (a) 6 h and (b) 12 h.

higher than the transformation temperature of ZrO2
(1170 °C). When the annealing time was increased from 6
to 12 h by fixing the temperature, the peak intensities of
the ZrB2, MgO and ZrO2-t remarkably increased (Fig.
9(b) in the presence of monoclinic ZrO2 phase which isalmost in the approximate amount with that of in Fig.
9(a). Moreover, Fe impurity which could be incorporate
into the powders from the hardened steel vial/balls during
the milling process did not detected in the XRD patterns
even after annealing, meaning that its probable amount isunder the detection limit of XRD (< 2 wt.%).
Additionally, any emergence of Mg borate compounds
such as MgB4O7, Mg2B2O5 and Mg3B2O6 did not observedin Fig. 9(a) and (b) at the end of the annealing processes.
It was thought as a probability because Mg/MgO andB2O3 can react and form Mg borates in different compositions between the temperatures of 700 and 1000 °C(Ağaoğulları et al., 2012b). Thus, 1200 °C and 12 h can beDuygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

Fig. 10 XRD patterns of the powders: (a) milled in the
SPEXTM 8000D Mixer/Mill for 30 h with the use of
20 wt.% excess amount of Mg, and (b) those of annealed at 1200 °C for 12 h.

respectively evaluated as an optimum annealing temperature and annealing time for yielding ZrB2/ZrO2 ceramic
powders with an amount of MgO which will be easilyleached out using HCl as well as probable Fe impurity.
Due to the fact that the higher annealing temperature witha longer annealing duration may cause excess grain
growth in the resultant powder particles, annealing parameters were not altered for the following experiments.
Fig. 10(a) and (b) exhibit the XRD patterns of the
20 wt.% excess amount of Mg added powders after milling in the SPEXTM 8000D Mixer/Mill for 30 h and thoseafter annealing at 1200 °C for 12 h, respectively. In comparison with the XRD pattern of the powders milled for
7 h with the same amount of excess Mg (Fig. 4(d)), it can
be clearly seen that the peak intensities of the ZrO2 andMg phases decreased and the incubations of the ZrB2 andMgO phases increased in very small amounts after milling for 30 h (Fig. 10(a)). Thus, there is still no reaction
between reactants even after milling for 30 h with the useof 20 wt.% excess amount of Mg.
After annealing of the powders milled for 30 h with theuse of 20 wt.% excess amount of Mg at 1200 °C for 12 h(Fig. 10(b)), ZrB2 phase in high peak intensities and MgO
occurred in the microstructure in addition to the unreacted monoclinic ZrO2 and its transformed form (ZrO2-t).
The utilization of 20 wt.% excess amount of Mg seems toincrease the ZrB2 peak intensities as compared with Fig.
9(b). However, the intensities of the ZrO2 peaks also increased and hence the ZrB2/ZrO2 intensity ratio decreased
with the use of excess Mg. Besides, MgO phase formed inlower amount than that of in Fig. 9(b). The peak intensities of ZrO2-t in Fig. 10(b) are significantly lower thanthose of in Fig. 9(b) because excess amount of Mg(20 wt.%) up to a limited level provides ZrO2 to contribute in the formation of ZrB2 instead of its transformation

to ZrO2-t during annealing at 1200 °C. However, remained excess amount of Mg evaporated from the system
without further participating into the reduction reaction,
as compatible with the FactSageTM plot in Fig. 1(b). Thus,
annealing of the milled powders containing stoichiometric amounts of reactants can give the ideal route for the
production of ZrB2/ZrO2 ceramic powders.
It should be also stated that the thermochemical calculations in Fig. 1(a) and (b) performed using FactSageTM
6.2 software for the theoretical reduction reaction given in
Eq. (1) conflicted with the experimental outputs of the annealing process, when the obtained products were taken
into account. Already, the starting stoichiometric powder
blends were prepared in regard of Eq. (1) considering thehigh stability of ZrO2 and hence its complexity in the reduction. According to the thermochemical calculations,the reaction products obtained from the stoichiometricamounts of raw materials are only ZrB2 and MgO.
Though, ZrB2 and MgO in the presence of both monoclinic and tetragonal ZrO2 were obtained as the reaction
products after milling and annealing of the stoichiometricpowder blends. So, the reduction reaction of ZrO2B2O3-Mg system at 1200 °C should be conformed to theexperimental results. The actual reduction reaction rewritten in regard of achieved outputs is given in Eq. (2).
2ZrO2 + B2O3 + 5Mg → ZrB2 + ZrO2 + 5MgO

(2)

3.4 Leaching of the annealed powders
Following to milling and annealing processes, the reaction products with undesired MgO phase should be purified to achieve fine-grained ceramic powders without any
contamination. Thus, leaching treatment was performed
using different HCl concentrations (3.6 and 5 M) on the
powders milled in the SPEXTM 8000D Mixer/Mill for
30 h and annealed at 1200 °C for 12 h, with the intentionof removing the whole MgO present in the microstructure
and also probable Fe impurity which was not previously
detected by XRD. Fig. 11(a) and (b) illustrate the XRD
patterns of the leached powders. According to Fig. 11(a),3.6 M HCl provided the dissolution of MgO but it is still
not strong enough for the complete elimination of the undesired phase. As HCl concentration was raised to 5 M,
the peak intensities of both monoclinic and tetragonal
ZrO2 and ZrB2 phases increased without any appearanceof MgO (Fig. 11(b)).
Subsequent AAS analyses were conducted on the leachsolutions decanted from the residual solids in order to detect the dissolved Zr and Fe elements. Since 5 M HCl ishighly concentrated, 22 ppm Zr arose from the slight dissolution of ZrO2 phase was detected in the solution. However, trace amount of Zr was found in the leach solution
in the case of using 3.6 M HCl. Furthermore, increasing

191

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

Fig. 11 XRD patterns of the powders milled in the SPEXTM
8000D Mixer/Mill for 30 h, annealed at 1200 °C for
12 h and leached in HCl solution at different concentrations: (a) 3.6 M and (b) 5 M.

Fig. 12 SM images of the (a) as-blended powders, (b) powders
milled in the SPEXTM 8000D Mixer/Mill for 30 h, (c)those annealed at 1200 °C for 12 h and (d) thoseleached in 5 M HCl.

the HCl concentration from 3.6 to 5 M increased the dissolution of Fe but not in a significant amount: the detectedamounts were about 117 ppm for 3.6 M HCl and 125 ppm
for 5 M HCl. In our previous studies about the synthesisof LaB6 and SmB6 from La2O3-B2O3-Mg and Sm 2O3B2O3-Mg powder blends by mechanochemical reactionsin the SPEXTM 8000D Mixer/Mill for 5 h (using the same
conditions such as BPR, milling container, media and atmosphere), the supernatant liquids were respectively foundto have about 11 and 7.81 ppm Fe after leaching with 3.6
and 4 M HCl (Ağaoğulları et al., 2012a, 2015). In thiscase, the amount of Fe impurity released from the leached
ZrB2/ZrO2 ceramic powders is at least ten times higher
than those released from the purified LaB6 and SmB6
powders due to the higher hardness of ZrO2 and ZrB2
compounds. Besides, leaching treatment using 3.6 M wasrepeated for the obtained ZrB2/ZrO2 powders but Fe element was determined as trace amount which was under
the detection limit of AAS. Thus, HCl concentrations
above 5 M were not applied for leaching of the annealed
powders and 5 M HCl was found strong enough to obtainpure ZrB2/ZrO2 ceramic powders.
Fig. 12(a)–(d) shows the SM images of the as-blended,
milled (in the SPEXTM 8000D Mixer/Mill for 30 h), annealed (at 1200 °C for 12 h) and leached (in 5 M HCl)powders, respectively. Monitoring SM images are beneficial to see the general appearances and colors of the powders and to follow the process in a simple way. As the SM
image of the as-blended powders in Fig. 12(a) is compared with those in Fig. 12(b)–(d), the change in the colorsof the starting materials can be obviously seen and the effect of different treatments can be observed step-by-step.
After milling for 30 h in the SPEXTM 8000D Mixer/
Mill, the particles of the reactants were homogeneously
distributed throughout the microstructure, Mg particles

covered the surfaces of the other reactants and it resulted
in a color change in the overall volume (Fig. 12(b)). The
morphology of the annealed powders (Fig. 12(c)) is different than that of milled one (Fig. 12(b)). The microstructure turned to a more incorporated and continuous
form by the influence of heating with a long holding time.
However, the color differences of the Zr-based (ZrO2,
ZrO2-t and ZrB2) and MgO particles are clear in Fig. 12(c)because Zr-based particles are seen as embedded in the
MgO phase. After leaching treatment, the microstructure
became a homogeneous form by eliminating the color
difference via removing the MgO contamination(Fig. 12(d)).
Fig. 13(a) and (b) are the representative SEM imagesof the powders milled in the SPEXTM 8000D Mixer/Millfor 30 h, annealed at 1200 °C for 12 h and leached in 5 M
HCl, at magnifications of 2000 and 8000X, respectively.
The general view of the resultant powders containing bothmonoclinic and tetragonal ZrO2 and ZrB2 phases inFig. 13(a) exhibits irregular agglomerates which preventthe observation of smaller particles in the microstructure.
Fig. 13(b) in higher magnification than Fig. 13(a) clearlyshows the rounded-shaped particles in sizes ranging between 200 nm and 1 μm. On the basis of Fig. 13, it can besaid that the final product is in the form of a ceramic powder in which individual particles of the components canbe observed, rather than that of a composite powder inwhich each particle has the similar composition of resultant phases.
Consequently, submicron-sized and pure ZrB2/ZrO2 ceramic powders containing two different crystal structuresof ZrO2 phase (monoclinic and tetragonal) were preparedby milling, annealing and leaching processes from ZrO2B2O3 -Mg powder blends. The synthesized precursor
material can be utilized as intermediate or final products

192

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

with the minor amounts of ZrB2 and MgO incubations.
•	DSC curve of the milled powders exhibited an exothermic peak at 600 °C and an endothermic peak at
900 °C, respectively corresponding to the oxidationof Mg and incubation of ZrB2. The annealing temperature of the milled powders was selected as
1200 °C in regard of DSC-TGA experiments.
•	Annealing at 1200 °C for 12 h enabled the milled
powders to form ZrB2, MgO and both monoclinic
and tetragonal ZrO2 phases.
•	Pure ZrB2/ZrO2 ceramic powders having particles insize range of 200 nm–1 μm and containing two different crystal structures of ZrO2 phase (monoclinic
and tetragonal) were obtained after milling in the
SPEXTM 8000D Mixer/Mill for 30 h, annealing at
1200 °C for 12 h and leaching with 5 M HCl.

Acknowledgements

Fig. 13 SEM images of the powders milled in the SPEXTM
8000D Mixer/Mill for 30 h, annealed at 1200 °C for
12 h and leached in 5 M HCl, at different magnifications: (a) 2000X and (b) 8000X.

in different application areas and can lead the way for thefurther advanced processing techniques to obtain special
designated bulk forms of composites.

This study was supported by “Istanbul Technical University Scientific Research Projects” with the project title
of “The Production of IVB Group and Lanthanide Group
Metal Borides in Mechanochemical Reaction Medium”
and with the project number of 32743. It was also fundedby “The Scientific and Technological Research Council ofTurkey (TÜBİTAK)” with the project title of “Synthesisof Refractory Metal Borides via Three Different Production Methods from Solid, Liquid and Gas Raw Materials
for Various Application Areas; Sintering, Characterization, Comparison of Process and Final Products” and withthe project number of 112M470.

References
4. Conclusions
In this study, submicron-sized ZrB2/ZrO2 ceramic powders were successfully synthesized via magnesiothermic
reduction of oxide raw materials using a combined process of milling and annealing. An additional HCl leaching
step was conducted on the annealed powders for their purification. Based on the results reported in the present
study, the following necessary conclusions can be drawn:•	Milling of stoichiometric ZrO2-B2O3-Mg powder
blends in the SPEXTM 8000D Mixer/Mill for 3–30 h
did not trigger a reduction reaction since there werestill ZrO2 and Mg phases in the microstructure of thepowders. However, it resulted in a gradual reduction
in particle sizes. On the other hand, milling of thesame blends in the planetary ball mill for 40, 50, 70
and 100 h resulted in the major ZrO2 and Mg phasesAğaoğulları D., Synthesis, Development and Characterizationof Some Rare-Earth Hexaboride Powders and Their Sintered Products, Ph.D. Thesis, Advisor: Prof. Dr. İsmail
Duman, Graduate School of Science, Engineering andTechnology, Istanbul Technical University, (2014).
Ağaoğulları D., Duman İ., Öveçoğlu M.L., Synthesis of LaB6
powders from La2O3, B2O3 and Mg blends via a mechanochemical route, Ceramics International, 38 (2012a) 6203–
6214.
Ağaoğulları D., Balcı Ö., Gökçe H., Duman İ., Öveçoğlu M.L.,
Synthesis of magnesium borates by mechanically activatedannealing, Metallurgical and Materials Transactions A,
43A (2012b) 2520–2533.
Ağaoğulları D., Balcı Ö., Öveçoğlu M.L., Suryanarayana C.,
Duman İ., Synthesis of bulk nanocrystalline samarium
hexaboride, Journal of the European Ceramic Society, 35(2015) 4121-4136.
Ağaoğulları D., Balcı Ö., Öveçoğlu M.L., Duman İ., Preparation

193

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

of LaB6 Powders via Calciothermic Reduction using Mechanochemistry and Acid Leaching, KONA Powder and Particle Journal, 33 (2016) 203–218.
Akgün B., Çamurlu H.E., Topkaya Y., Sevinç N., Mechanochemical and volume combustion synthesis of ZrB2, International Journal of Refractory Metals and Hard Materials,
29 (2011) 601–607.
Balcı Ö., Ağaoğulları D., Duman İ., Öveçoğlu M.L., Carbothermal production of ZrB2-ZrO2 ceramic powders from ZrO2B2O3/B system by high-energy ball milling and annealing
assisted process, Ceramics International, 38 (2012) 2201–
2207.
Basu B., Vleugels J., Biest O., Development of ZrO2-ZrB2 composites, Journal of Alloys and Compounds, 334 (2002)
200–204.
Brouchu M., Gauntt B.D., Boyer L., Loehman R.E., Pressureless reactive sintering of ZrB2 ceramic, Journal of the European Ceramic Society, 29 (2009) 1493–1499.
Chamberlain A.L., Fahrenholtz W.G., Hilmas G.E., Highstrength zirconium diboride-based ceramics, Journal of theAmerican Ceramic Society, 87 (2004) 1170–1172.
Chen L., Gu Y., Yang Z., Shi L., Ma J., Qian Y. Preparation andsome properties of nanocrystalline ZrB2 powders, Scripta
Materialia, 50 (2004) 959–961.
Chen D., Zhang W., Li X., Hu P., Han J., Hong C., Han W.,
Microstructural feature and thermal shock behaviour of
hot-pressed ZrB2-SiC-ZrO2 composite, Materials Chemistry and Physics, 116 (2009) 348–352.
Chen B., Yang L.S., Heng H., Chen J.Z., Zhang L.F., Xu L.Q.,
Qian Y.T., Yang J., Additive-assisted synthesis of boride,
carbide, and nitride micro/nanocrystals, Journal of Solid
State Chemistry, 194 (2012) 219–224.
Deng J., Cheng L., Hong Z., Su K., Zhang L., Thermodynamics
of the production of condensed phases in the chemical
vapor deposition process of zirconium diboride with ZrCl4 –BCl3–H2 precursors, Thin Solid Films, 520 (2012) 2331–
2335.
El-Eskandarany MS. Mechanical alloying for fabrication ofadvanced engineering materials. Norwich, NY: Noyes Publications, 2001.
Fahrenholtz W.G., Hilmas G.E., Refractory diborides of zirconium and hafnium, Journal of the American Ceramic Society, 90 (2007) 1347–1364.
Frazer E.J., Anthony K.E., Welch B.J., Electrodeposition of zirconium diboride from oxides dissolved in molten cryolite,
Electrodeposition and Surface Treatment, 3 (1975) 169–177.
Gebhardt J.J., Cree R.F., Vapor-Deposited Borides of GroupIVA Metals, Journal of the American Ceramic Society, 48(1965) 262–267.
Greenwood N.N., Parish R.V., Thornton P., Metal Borides,
Quarterly Reviews, Chemical Society, 20 (1966) 441–464.
Guo S. Densification of ZrB2-based composites and theirmechanical and physical properties: A review, Journal of
the European Ceramic Society, 29 (2009) 995–1011.
Guo S., Hu C., Kagawa Y., Mechanochemical processing of
nanocrystalline zirconium diboride powder, Journal of theAmerican Chemical Society, 94 (2011) 3643–3647.
Hong C., Zhang X., Li W., Han J., Meng S., A novel function-

194

ally graded material in the ZrB2-SiC and ZrO2 system byspark plasma sintering, Materials Science and Engineering:
A 498 (2008) 437–441.
Jalaly M., Bafghi M.S., Tamizifar M., Gotor F.J., Mechanosynthesis of nanocrystalline ZrB2-based powders by mechanically induced self-sustaining reaction method, Advances inApplied Ceramics, 112 (2013) 383–388.
Jalaly M., Bafghi M.S., Tamizifar M., Gotor F.J. An investigation on the formation mechanism of nano ZrB2 powder by amagnesiothermic reaction, Journal of Alloys and Compounds, 588 (2014) 36–41.
Khanra A.K., Pathak L.C., Godkhindi M.M. Double SHS of
ZrB2 powder, Journal of Materials Processing Technology,
202 (2008) 386–390.
Li B., Deng J., Li Y., Oxidation behavior and mechanical properties degradation of hot-pressed Al2O3/ZrB2/ZrO2 ceramiccomposites, International Journal of Refractory Metals andHard Materials 27 (2009) 747–753.
Li B., Deng J., Wu Z., Effect of cutting atmosphere on dry
machining performance with Al2O3/ZrB2/ZrO2 ceramic
tool, International Journal of Advanced Manufacturing
Technology, 49 (2010) 459–467.
Li W., Zhang X., Hong C., Han W., Han J., Preparation, microstructure and mechanical properties of ZrB2–ZrO2 ceramics, Journal of the European Ceramic Society, 29 (2009)
779–786.
Malyshev V.V., Gab A.I., Theoretical fundamentals and practical implementation of electrometallurgical processes of
producing disperse zirconium diboride powders and coatings, Russian Journal of Non-Ferrous Metals, 50 (2009)
485–491.
Melendez-Martinez J.J., Dominguez-Rodriguez A., Monteverde
F., Melandri C., Portu G., Characterisation and high temperature mechanical properties of zirconium boride-basedmaterials, Journal of the European Ceramic Society 22(2002) 2543–2549.
Monteverde F., Guicciardi S., Bellosi A., Advances in microstructure and mechanical properties of zirconium diboride
based ceramics, Material Science and Engineering: A, 346(2003) 310–319.
Monteverde F., Progress in the fabrication of ultra-high-­temperature ceramics: in-situ synthesis, microstructure andproperties of a reactive hot-pressed HfB2-SiC composite,
Composite Science and Technology, 65 (2005) 1869–1879.
Monteverde F., Bellosi A., Scatteia L., Processing and properties of ultra-high temperature ceramics for space applications, Material Science and Engineering: A, 485 (2008)
415–421.
Nettleship I., Stevens R., Tetragonal zirconia polycrystal(TZP)-A review, International Journal of High Technology
Ceramics, 2 (1987) 1–32.
Opeka M.M., Talmy I.G., Zaykoski J.A., Mechanical, thermal,
and oxidation properties of refractory hafnium and zirconium compounds, Journal of the European Ceramic Society, 19 (1999) 2405–2414.
Radev D.D., Klissurski D., Mechanochemical synthesis and
SHS of diborides of titanium and zirconium, Journal of
Material Synthesis Processing, 9 (2001) 131–136.

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

Setoudeh N., Welham N.J., Formation of zirconium diboride(ZrB2) by room temperature mechanochemical reaction
between ZrO2, B2O3 and Mg, Journal of Alloys and Compounds, 420 (2006) 225–228.
Suryanarayana C., Mechanical alloying and milling, Progressin Materials Science, 46 (2001) 1–184.
Upadhya K., Yang J.M., Hoffmann W.P., Materials for ultrahightemperature structural applications, American CeramicSociety Bulletin, 76 (1997) 51–56.
Qiu H.Y., Guo W.M., Zou J., Zhang G.J., ZrB2 powders preparedby boro/carbothermal reduction of ZrO2: The effects of carbon source and reaction atmosphere, Powder Technology,
217 (2012) 462–466.
Zhang X., Li W., Hong C., Han W., Han J., A novel developmentof ZrB2/ZrO2 functionally graded ceramics for ultrahigh-temperature application, Scripta Materialia, 59 (2008)
1214–1217.
Zhang G., Guo W., Ni D., Kan Y., Ultrahigh temperature ceramics (UHTCs) based on ZrB2 and HfB2 systems: powder synthesis, densification and mechanical properties, Journal of
Physics: Conference Series, 176 (2009) 012041.
Zhu T., Li W., Zhang X., Hu P., Hong C., Weng L., Oxidation
behavior of ZrB2-SiC-ZrO2 ceramic composites in the temperature range of 800–1200 °C, Materials Chemistry andPhysics, 116 (2009a) 593–598.
Zhu T., Li W., Zhang X., Hu P., Hong C., Weng L., Damage tolerance and R-curve behavior of ZrB2-ZrO2 composites,
Material Science and Engineering: A, 516 (2009b) 297–301.

195

Duygu Ağaoğulları et al. / KONA Powder and Particle Journal No. 34 (2017) 183–196

Author’s short biography
Duygu Ağaoğulları
Duygu Ağaoğulları received her M.Sc. degree in 2007 and Ph.D. degree in 2014 inMaterials Science and Engineering Department from Istanbul Technical University. She
worked as a research assistant between 2005 and 2013, and has been affiliated since 2013
in I.T.U. She has been assigned as researcher/scholar in 10 scientific projects. Now, she isworking as a postdoctoral scholar in Materials Science and Engineering Department in
Stanford University. Her main research activities are powder metallurgy, high-tech boron
products and composite materials. She is author/co-author of 21 papers, 1 Patent cited in
Web of Science and 75 international conference proceedings.

Özge BalcıÖzge Balcı received her M.Sc. degree in Metallurgical and Materials Engineeringfrom Istanbul Technical University in 2010 and continues her Ph.D. in the same department. She has been assigned as researcher in 4 scientific projects and gained professional
fellowships in 3 research projects. She has been as a visiting researcher at IFW Dresdenfor 6 months during her Ph.D studies. Her main research activities are powder metallurgy, high-tech boron products and composite materials. She is author/co-author of 11
papers and 1 Patent cited in Web of Science and 42 proceedings presented in international conferences.

M. Lütfi Öveçoğlu
Prof. Dr. M. Lütfi Öveçoğlu received his M.Sc. and Ph.D. degrees in Materials Scienceand Engineering from Stanford University in 1984 and 1987. He has been affiliated with
the Department of Metallurgical and Materials Engineering at Istanbul Technical University since 1990. He is the founder and technical director of Particulate Materials Laboratories (PML), a cluster of 9 laboratories. His main research activities are mechanicalalloying and mechanochemical synthesis of W-based, Al-based and boride-based materials. He is author/co-author of 145 papers and 1 patent cited in Web of Science, 2 bookchapters and 3 edited conference proceedings having about 905 citations.

İsmail Duman
Prof.Dr. İsmail Duman received his M.Sc. degree from Istanbul Technical University
in 1978 and Ph.D. degree from Berlin Technical University in 1985 in Metallurgical Engineering. He has been affiliated with Department of Metallurgical and Materials Engineering at Istanbul Technical University since 1980. His main research activities are
extractive metallurgy, powder metallurgy, development of high-tech boron products and
CVD technologies. He has been the supervisor of 17 M.Sc. and 8 Ph.D. dissertations andhe has been assigned as project director or researcher in 23 industrial/scientific research
projects. He is author/co-author of 49 papers and 7 Patents cited in Web of Science.

196

KONA Powder and Particle Journal No. 34 (2017) 197–206/Doi:10.14356/kona.2017003

Original Research Paper

Comparison of Compression and Material Properties ofDifferently Shaped and Sized Paracetamols †Michal Šimek 1,2*, Veronika Grünwaldová 2 and Bohumil Kratochvíl 1
1
2

Department of Solid State Chemistry, University of chemistry and technology Prague, Czech Republic
Zentiva k.s., Czech Republic

Abstract
Paracetamol is well-known API (active pharmaceutical ingredient) for its bad flow and compression abilities. To
improve the compressibility, a high amount of excipient is commonly mixed with paracetamol to create better
compressible material. Conversely, we used modified crystallization procedures to prepare plate, irregular andspherical particles from original raw paracetamol to prepare directly compressible API. To expand our screening,several sizes of each shape were prepared and material properties analyzed, mainly the flow and compression
abilities, and significant variation of the properties presented; from very poor properties of raw paracetamol to
excellent properties of spherical crystals which exhibited ability to be directly compressed without excipient. The
analysis also showed very small effect of the size modification on tablet compression and material behavior as themain contribution had the shape alteration and compression force.
Keywords: API, shape modification, particle, properties, compression

1. Introduction
Crystal habit, polymorphism, size and other properties
can be modified by the variation of crystallization conditions such as the presence of impurities, ultrasonic frequency, type of solvent and cooling rate (Jordens et al.,2014; Lacmann, 1998). Different crystal habit of a particular drug possesses different in the specific surface andfree surface energy. Therefore, they may exhibit different
physico-mechanical properties, powder flow and compressibility, which are of pharmaceutical interest (El-Zhry
El-Yafi and El-Zein, 2015; Huettenrauch and Moeller,
1983; Marshall and York, 1991; York, 1983).
Attempts to change the crystal habit and the workability of drugs using alternative crystallization procedures ofdrugs has been published; for example ibuprofen (Gordon
and Amin, 1984), hexamethylmelamine (Gonda, 1985),
nitrofurantoin (Marshall and York, 1989) and paracetamol(Ó’Ciardhá et al., 2011). Although, more general influenceof several different shapes and sizes of a single API on
flowability and compressibility has not yet been presented.
†
1
2

*

Received 28 July 2015; Accepted 9 November 2015
J-STAGE Advance published online 30 January 2016
Technická 5, 166 28 Prague, Czech Republic
U Kabelovny 130, 102 37 Prague, Czech Republic
Corresponding author: Michal Šimek;
E-mail: simekm@vscht.cz
TEL: +420-721-728-186

Several shapes and sizes of paracetamol crystals (FormI) were prepared; and their material properties and compressibility compared. We varied the already publisheddifferent methods of paracetamol crystallization to prepare plates (Garekani et al., 1999), irregular (Kaialy et al.,2014) and spherical (Garekani et al., 2000) paracetamol.
Some particle sizes of the described procedures could nothave been reproduced; thus, some of the methods wereredesigned as described.

2. Materials and methods
Different shapes were prepared from raw paracetamol(Anqiu Lu’an Pharmaceutical Co. Ltd, Anqui, Shandong
province, China), which was consisted of very small needle-­
shaped and mainly of irregular particles (Fig. 3). This active pharmaceutical ingredient (API) is a widely used
over-the-counter analgesic and antipyretic (Aghababian,
2010; Ahmad, 2010). Other used solutions and solids of thiswork were obtained from Sigma-Aldrich (Sigma-Aldrich
Co., Prague, Czech Republic).

2.1 Crystallization procedures
The addition of a second substance, usually a liquid
diluent, which reduces the solubility of the solute in thesolvent, is one of the common methods of crystallizationand it is known as salting-out. The diluent must be misci-

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

197

Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206

ble with the crystallization solvent and the solute should
be relatively insoluble in it. The preparation of different
shapes of paracetamol was driven by the change of thesolvent, temperature or usage of additives. The additive
inhibited the crystal growth in some directions and produced unusual shapes of the crystal habit. The change ofthe stirring speed provided different particle size distribution of prepared paracetamols. In every case, the precipitated crystals were collected by filtration using a Büchner
funnel under vacuum with no agitation. They were driedfor 24 h and stored in tightly closed jars. One size of theoriginal supplied raw paracetamol, two sizes of spheres,
three sizes of plates and three sizes of irregulars wereprepared (in amount of 30 g) and used for next experiments. Polymorphic purity of prepared samples wastested.

2.1.1 Plates

Paracetamol (50 g) was dissolved in ethanol p.a.
(120 ml) at 65 °C. The solution was poured into cooled
distilled water (500 ml, 3 °C) with stirring speed 100 rpm
to obtain big plates, or 200 rpm to obtain medium platesof paracetamol. After the cooling of the mixture to 25 °C,
products were filtered and dried in an oven (80 °C, 24 h).
Higher stirring did not produce smaller plates, but irregular shaped particles. To produce smaller plates, the procedure was modified. If methanol was used as a solvent, theprocedure led to small plates. Paracetamol (50 g) was dissolved in methanol p.a. (170 ml) at 40 °C. The solution
was poured into cooled distilled water (500 ml, 3 °C) withstirring speed 600 rpm. The product was filtered anddried as described.

2.1.2 Irregulars

Paracetamol (50 g) was dissolved in the solution of ethanol p.a. (120 ml) and distilled water (500 ml) at 65 °C.
The solution was placed into ice bath and slowly cooled to
25 °C with stirring speed 100 rpm to obtain big irregulars, or 300 rpm to obtain medium irregulars, or 1200 rpm
for small irregulars of paracetamol. Products were filteredand dried in an oven (80 °C, 24 h).

2.1.3 Spheres

Paracetamol (50 g) was dissolved in ethanol p.a.
(120 ml) at 65 °C. The solution was poured into cooled
distilled water (500 ml, 3 °C). The water contained PVP
40000. The stirring speed was set to 200 rpm to obtainmedium spheres, or 300 rpm to obtain small spheres ofparacetamol. After the cooling of the mixture to 25 °C,
products were filtered and dried in an oven (55 °C, 24 h).
The big sized particles were not prepared. Although reduction of the stirring speed led to bigger particles, they
were contaminated by different shape of particles, mainly
irregular and columnar particles.

198

Table 1 Flow properties and angle of repose
Flow Property

Angle of Repose (degrees)

Excellent

25–30

Good

31–35

Fair—aid not needed

36–40

Passable—may hang up

41–45

Poor—must agitate, vibrate

46–55

Very poor

56–65

Very, very poor

> 66

2.1.4 Bulk volume and tapped volume of powders

Paracetamol (20 g) was placed into a measuring cylinder (250 ml) which was mounted into Sotax TD2 Tap
density tester (Sotax Pharmaceutical Testing s.r.o.,Prague, Czech Republic) and the apparent volume of thematerial noted. The machine did 1250 taps and the volume was noted again after tapping. The experiments werein the keeping with European Pharmacopoeia (European
Medicines Agency, 2006a).

2.1.5 Angle of repose

Paracetamol (20 g) was placed into a funnel of Pharmatest PTG S-3XY(Pharma test AG, Hainburg, Germany)and depleted, with the secondary stirring of 20 rpm,
through a release flap (15 mm) into a measurement plate
and the angle of repose from the created powder cone wasmeasured by a detector. The classification system of thepowder flowability from European Pharmacopoeia isshown in Table 1 (European Medicines Agency, 2006b).

2.2 Analysis of particle size and shape
A small amount of paracetamol was fixed on aluminum
stubs using the double sided conductive adhesive carbon
tapes and sputtered with platinum for 240 s (Sputter
SC7640; Quorum Technologies Ltd, Ashford, United
Kingdom) and observed with a scanning electron microscope Mira/Tescan LM (Tescan a.s., Brno-Kohoutovice,
Czech Republic). Particle size distribution (PSD) was analyzed with image analyse software NIS Elements andd-values [lower decil d(0.1), median d(0.5) and upper decild(0.9)] of particle size distributions were obtained (Šimek
et al., 2015).

2.3 DSC analysis
All prepared products was tested of polymorphic purityof paracetamol with differential scanning calorimeter(DSC Pyris 1, Perkin Elmer, Waltham, Massachusetts,USA) was used to determine the solid-state nature of dif-Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206

Table 2 Melting points of paracetamol forms
Form

Tm /°CI

168,6 ± 0,2

II

156,4 ± 0,2ferent paracetamols. An accurate weight of each paracetamol powder (4–8 mg) was placed in 40 μL aluminium
DSC pan, sealed non-hermetically and heated (range 30–
200 °C) at a heating ramp rate of 10 °C/min under a nitrogen gas (50 L/min). Before each measurement, the sample
was allowed to equilibrate for 5 min at 30 °C. Melting
points of known paracetamol forms are shown in Table 2(Di Martino et al., 1996).

2.4 Tablet compression
Each of prepared materials was formulated into tablets.
Tablets contained only paracetamol (200 mg); no excipient was used. The die wall was cleaned with acetone andprelubricated with 4 % w/w magnesium stearate in acetone before each compression. The compression was carried out using Style one Classic compactor (Medel Pharm
Beynost, France) with 7 mm flat faced punches. Ten tablets were produces at compression forces of 5, 10, 15 and
25 kN. Three speeds of compression were used: 25 percent of max speed (87.5 mm/s), 50 percent of max speed(175 mm/s) and 75 percent of max speed (262.5 mm/s).
During the compression, punch separation and appliedmean force were monitored with Analis software v.2(Medel Pharm Beynost, France).

2.5 Analyses of compression data
Both force and displacement data from the upper andlower punches were collected from Analis v.2 software
during a compression cycle and data were processed to
the Heckel equation (Eq. (1)) (Heckel, 1961a, b).
1 
ln 

 kP  A (1)
 1 D 

A typical Heckel plot of Eq. (1) is illustrated in Fig. 1.
Parameter D is the relative density of tablet (the ratio oftablet density to true density of powder) at applied pressure P. k is the slope of the straight line portion of the
Heckel plot and the reciprocal of k is the mean yield pressure (Garekani et al., 1999). Since the tablet dimensions
were measured in the die, it is referred to as apparent
mean yield pressure. The total densification of the powder
bed due to die filling and particle rearrangement, Da, wasobtained from the intercept of the linear portion of thisplot, A, using Eq. (2).

Fig. 1

Heckel plot for paracetamol (Irregular, small) obtained
at a compression speed 25 percent and compression
force 25 kN.

Da  1  e  A (2)

From B, the place where the Heckel plot intercepts theln (1/1-D) axis (Fig. 1), the density of powder at zero pressure, D0, is obtained (Eq. (3)). D0 can be defined as thedensification due to die filling or to initial powder packing.

D0  1  e  B (3)2.6 Measurement of elastic and plastic energy,
elastic recovery
For our system in which both punches were mobile,
the punch separation was plotted against the mean compression force (Garekani et al., 1999; Gibson, 2009). The
area under this curve is the work done or energy. The
work done of compression (plastic energy) and expansion
work of compression (elastic energy) were calculated using the data collected from Analis v.2 software.
Fig. 2 illustrates a typical force-punch separation. Point
A represents the minimal force and maximal punch separation at the beginning of compression. Point B represents
the peak force at minimal punch separation, C shows theminimum punch separation and D represents the punch
separation after decompression when the force is zero,giving the tablet thickness. The total compression energy
is represented by area ABC, and the decompression energy or elastic energy is equal to area under curve CBD.
The difference between area ABC and CBD determines
the plastic energy (the area under curve ABD).
To calculate the elastic recovery of each tablet in thedie we used Eq. (4) (Armstrong and Haines-Nutt, 1972).

199

Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206% Elastic recovery H  H C / H C  100,(4)where Hc and H are the thickness of tablet under maximum pressure and after the compression force was removed, respectively (Garekani et al., 1999).

3. Results and discussion

Fig. 2Force-punch separation plot for paracetamol (Irregular,
small) obtained at a compression speed 25 percent andcompression force 25 kN.

Fig. 3200

Under the conditions, described in the methodology
section, we prepared several samples of paracetamol withdifferent shape (Fig. 3). At the first sign, the properties ofthe materials were mainly relevant to the shape of thecrystals and less appropriate to the crystal size.
Particles size distributions of prepared samples, described with d-values, are presented in Table 3. Samples
of different sizes of each shape were labelled big, medium, or small according to the d-values. The particle size
distribution of raw paracetamol was in good keeping withthe medium class; and two prepared samples of sphericalparacetamols were in keeping with medium and smallclass.

SEM micrographs of prepared paracetamols and raw paracetamol.

Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206

Table 3 Overview of d-values of all prepared paracetamolsd(0.1)[μm]d(0.5)[μm]d(0.9)[μm]

Raw (medium)

17

40

100

Plates (big)

51

113

203

Plates (medium)

37

67

105

Plates (small)

19

40

61

Irregular (big)

51

102

222

Irregular (medium)

44

91

159

Irregular (small)

10

25

57

Spheres (medium)

26

56

103

Spheres (small)

23

39

62

Shape

DSC scans of the paracetamols showed only the melting endotherm of paracetamol Form I. No events such asphase transformation or hydration were seen during the
crystallization process.

3.1 Bulk volume and tapped volume of powders
The difference between untapped and tapped volume isin the rearrangement of the particles in the bulk of material. The rearrangement mainly consisted of two main
events. The first is the movement of the small and verysmall particles into the space between big particles. It
means that samples with a wider PSD, e.g. raw paracetamol, more tended to change the bulk volume according
to tapping than paracetamol with a narrow PSD. The second event relates to a crystal shape and orientation of theparticles due to the crystal shape. For example, plates canbe settled in the bulk in two preferred position—perpen-

Fig. 4

dicularly or parallel. Perpendicular position tend to a big
inter-particle space, therefore, the volume of material bulk
significantly increases. Tapping of plates caused the reorganization of the particles to the parallel position and asignificant reduction of the inter-particle space and theoverall volume of the bulk were significantly reduced
even after several taps. The product was not so dense incomparison to other crystal shapes because the plate particles were not locked together. Conversely, irregular particles and spheres exhibited small change after tapping as
the starting position of the particles was very tight due tothe very good flow of the particles and very good natural
rearrangement, more significant for irregular particles.
The effect of the different sizes of particles was slightly
observed for all types of shape. In all cases, smaller particles tended to smaller bulk volume and smaller tapped
volume. But overall, the effect of size was very small andnegligible.

3.2 Angle of repose
Wide PSD and irregularly-needled shape of raw paracetamol created big contact areas between the particles,
thus, the spill of the raw paracetamol was minimal andmaterial was practically only stacked. Even the flow musthave been supported by secondary stirring to deplete thematerial from the funnel to the measurement plate. Similar to the measurement of the bulk and tapped volumes,
size variation minimally affected the angle of repose andthe main difference was based on the different shape ofparacetamol crystals. Plates created big contact areas and,
when they run from the funnel to the measurements
plates, adhesion forces held particles together, the spreadof the particles on the measurement plate was low and thecreated angle of repose was high. Irregular and spherical

Bulk volume before the tapping started (V0) and tapped volume after 1250 taps (V1250) of different
shapes of paracetamol crystals with different PSD.

201

Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206

Fig. 5Angle of repose of different shapes of paracetamol crystals with different PSD.

particles exhibited a free flow into the measurement plate
and the angle of repose was very small. The smallest angles of repose were measured for spherical particles andthey were even smaller than the values stated in the classification system of the powder flowability (Table 1) of
European Pharmacopoeia (European Medicines Agency,
2006b).

3.3 Compression properties of paracetamols
Compression of raw, irregular or plate-like crystals ofparacetamol produced very weak compacts which had nomeasureable strength, but they had a high tendency to cap
and laminate at all compression forces used. Conversely,
spherical crystals produced very strong compacts under
the same conditions with no tendency to cap. The strengthof these tablets oscillated from 42 N (at compression force
5 kN) to 90 N (at compression force 25 kN).

Crystal shape was the main distributive parameter andsize was the minor distributive parameter of the Heckel
plots of paracetamols (Fig. 6). Irregular and raw paracetamols (irregular particles with a wide PSD) exhibited
higher change of densities for given applied pressure thanplate-like and spherical paracetamols (Table 4). Therefore, the greatest degree of densification during compression was seen for irregular particles, including also raw
paracetamol.
Increased frictional and cohesive forces between thin platelike crystals, due to their large and flat surface, restricted
particle sliding and thus reduced densification rate. Additional fragmentation, easier packing or rearrangement occurred at lower pressure. The differentials of Da-D 0(Table 4) also indicated that densification due to die filling and particle rearrangement for the irregular crystals
was higher than for plate-like or spherical crystals. Spherical crystals exhibited very good flowability (Fig. 5), thus,
the die was filled very tight according to the good natural
particle rearrangement.
The particle size effect was identical for all paracetamols. Bigger particles tended more to rearrangement at
the beginning of compression than smaller particles of thesame shape, as illustrated in higher values of the differential Da-D 0 for bigger particles (Table 4). It was the mostprobably caused by higher amount of inter-particle space
between bigger than smaller particles. The effect of different particle sizes and shapes was reduced when compression force and speed were increased.

3.4 Elastic recovery and tablet capping analysis

Fig. 6202

Heckel plots of different shapes and sizes of paracetamols obtained at compression speed 25 percent.

Generally, elastic recovery of tablets most depended on
both the shape of particles and compression force; which
influenced the total area of permanent bonds created between crystals when compressed. The effect of the force
and shape on the elastic recovery of in-die tablets indi-

Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206

Table 4 The values Da-D 0 derived from the Heckel plots of compressed paracetamols
Irregular

PlatesRaw

Spheres(big)(med.)(small)(big)(med.)(small)(med.)(med.)(small)

5 kN

0.50

0.47

0.41

0.43

0.42

0.36

0.48

0.36

0.35

10 kN

0.44

0.42

0.40

0.42

0.40

0.36

0.44

0.35

0.33

15 kN

0.44

0.41

0.39

0.40

0.39

0.35

0.39

0.34

0.30

20 kN

0.41

0.40

0.36

0.38

0.38

0.34

0.39

0.32

0.30

25 kN

0.38

0.38

0.34

0.37

0.36

0.32

0.39

0.31

0.30

5 kN

0.42

0.38

0.37

0.41

0.37

0.35

0.40

0.36

0.34

10 kN

0.41

0.38

0.36

0.37

0.36

0.34

0.39

0.32

0.31

15 kN

0.40

0.37

0.36

0.37

0.36

0.32

0.38

0.31

0.30

20 kN

0.39

0.36

0.36

0.36

0.35

0.32

0.37

0.30

0.30

25 kN

0.37

0.31

0.33

0.35

0.34

0.29

0.37

0.30

0.29

5 kN

0.42

0.38

0.37

0.38

0.36

0.32

0.39

0.34

0.33

10 kN

0.41

0.36

0.35

0.37

0.36

0.32

0.38

0.32

0.30

15 kN

0.41

0.36

0.34

0.36

0.36

0.31

0.37

0.31

0.29

20 kN

0.40

0.32

0.34

0.35

0.33

0.31

0.37

0.30

0.28

25 kN

0.36

0.29

0.33

0.34

0.31

0.27

0.37

0.29

0.28

25 % speed

50 % speed

75 % speedcated that tablets made from spherical crystals exhibitedthe lowest elastic recoveries (Table 5). Spheres (spherulites) probably fragmented and they created very strong
bonds between particles, which were originally very close
to each other before compression; due to small bulk volume (Fig. 4) and very good powder flow (Fig. 5). Irregular particles, which are the most similar in shape tospheres, exhibited different compression behaviour, as irregular particles were single crystals, not made frommany small crystals like spherulites of paracetamol, thus
the attraction of fragmentation was reduced. Raw paracetamol highly tended to rearrangement of the small particles into the inter-particle space between the big particles
when compressed due to high heterogeneity of particle
sizes (see in Fig. 3); therefore, a lot of the permanent
bonds were created. But the heterogeneous distribution of
the bonds created in the tablet led to high tendency of
capping.
Capping was observed in case of raw, irregular andplates. The degree of capping corresponded with the trendof elastic recovery. Apart from the spheres, the lowest
elastic recovery (5.9 %—irregular, small, 5 kN, 25 %speed) led to capping chance up to 86 percent; and thehighest elastic recovery (22.4 %—plates, small, 25 kN,
75 % speed) led to 100 percent chance of capping. Spheres

exhibited no sign of capping. Besides the theory of higher
fragmentation of the polycrystalline spherules, the most
reasonable explanation of this significantly different behaviour may be attributed to the residuals of PVP remaining on the face of the crystals after crystallization. The
amount of residual PVP was under detectable limit of
DSC analysis; and functioned as particle glue.
The size effect was also observed. Conversely to theeffect of particle rearrangement, the effect was not identical for all the shapes. Except the plate-like particles, bigger particles led to higher elastic recovery of tablets.
Abnormal behaviour of plates may be assigned to highcontent of air in the powder bulk (Fig. 4).
Lower compression speed probably allowed better reorganization of the crystals at the beginning of compression; therefore, the elastic recovery was higher whenhigher compression speed applied. Insufficient time to rearrangement the compressed particles caused weakerbonds between particles, thus, tablets more expanded after compression. Moreover, combination of highest compression speeds and highest compression forces caused
insufficient time for particle rearrangement which resulted in abnormalities from the trend observed at lower
force and speed.

203

Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206

Table 5 Effect of compression force and speed on the elastic recovery of in-die tablets made from all paracetamols
Irregular

PlatesRaw

Spheres(big)[%](med.)[%](small)
[%]

(big)
[%]

(med.)[%](small)[%](med.)[%](med.)[%](small)
[%]

5 kN

8.1

7.8

5.9

9.5

9.6

10.2

8.1

4.2

3.5

10 kN

10.5

10.3

7.8

11.6

11.7

13.7

10.4

4.6

4.3

15 kN

14.1

11.7

10.1

11.9

14.0

14.1

10.9

6.5

5.3

20 kN

15.5

12.8

10.9

13.4

14.1

15.2

11.1

6.9

6.1

25 kN

15.1

13.7

11.9

17.1

17.4

19.8

11.7

7.4

6.2

5 kN

9.5

8.4

7.9

13.0

14.1

14.6

10.9

4.3

4.2

10 kN

11.9

11.8

9.4

15.5

16.3

17.7

11.9

5.2

5.0

15 kN

14.7

13.2

11.4

18.0

18.1

18.4

12.7

6.9

6.1

20 kN

13.4

12.9

12.8

19.2

19.4

19.5

12.9

8.3

7.8

25 kN

11.5

11.6

11.7

18.9

19.5

19.9

13.2

9.4

8.1

5 kN

10.5

9.4

9.1

14.2

15.1

15.7

11.8

5.5

4.7

10 kN

11.3

11.2

10.1

16.2

16.4

16.9

12.6

6.2

5.7

15 kN

10.7

11.9

10.8

18.5

19.4

20.6

13.1

8.6

6.3

20 kN

10.7

10.9

10.8

19.8

20.8

21.3

13.2

9.2

8.1

25 kN

10.1

10.1

10.0

20.1

22.2

22.4

13.4

9.8

8.3

25 % speed

50 % speed

75 % speed

4. Conclusion
Paracetamol is an active pharmaceutical ingredient
known for its bad compression properties. Modified crystallization of paracetamol by a combination of solvents,
stirring speed, cooling rate and additives caused a marked
modification of the crystal habit and material properties;
however, no polymorphic transformation was induced.
Raw paracetamol had a wide PSD of irregular particles
which caused poor flowability and a high difference ofbulk and tapped densities. These properties predetermined this material to be badly compressible, whereas
modified crystallization led to the preparation of sphericalparacetamol which exhibited precisely opposite behavior.
The comparison of material properties shows that theparticles shape, compression force and speed are the main
parameters which influenced the compression and qualityof prepared tables as well as the material properties. Conversely, different particle size had relatively small influence, which was more reduced with increased compression
speed and force.
The most plastic material was the spherical paracetamol. The spherulites, the polycrystalline particles ofparacetamol, probably tended to a significant fragmenta-

204

tion of particles, thus, strong bonds and significantly
larger contact area were made between fragmented particles; and probably a non-detectable residuals of PVP after
crystallization remained on the surface of particles andfunctioned as a particle glue. The elastic recovery of tablet, which were made from spherical paracetamol, wasvery low (from 3.5 to 9.8 percent) and we concluded it
was responsible for no capping of these tablets. Other
used shapes exhibited high elastic recovery and highprobability of the visible capping of produced tablets (atleast 86 percent) at all used conditions. The main difference was caused by the shape of the particles and less by
the compression speed and force. The particles size had
the smaller influence on this property.
Based on the experimental data, crystal manipulation
via particle engineering is an efficient tool in producing
crystals with optimal physico-mechanical properties toaffect the compression behavior of API crystals. This approach might be of interest to those persons who are developing and optimizing the formulation process of poorly
compressible APIs. Our further step is to assess the exact
fragmentation of different shapes and size of paracetamol
by our hot-stage methodology and compare the resultswith the presented evaluation.

Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206

Acknowledgments
The authors thank for financial support from Ministryof Education, Youth and Sports - specific university research (MSMT No 20/2015) for financial support. They
also thank L. Krejčík for DSC analyzes and M. Beneš forthe help with the Style one compactor.

References
Aghababian R.V., Essentials of emergency medicine, Jones andBartlett Publishers, 2010.
Ahmad J., Hepatology and Transplant Hepatology: A Case
Based Approach, Springer, 2010.
Armstrong N.A., Haines-Nutt R.F., Elastic recovery and surface
area in compacted powder systems, J. Pharm. Pharmacol.,24 (1972) 135–136.
Di Martino P., Guyot-Hermann A.-M., Conflant P., Drache M.,
Guyot J.-C., A new pure paracetamol for direct compression: the orthorhombic form, Int. J. Pharm., 128 (1996) 1–8.
El-Zhry El-Yafi A.K., El-Zein H., Technical crystallization for
application in pharmaceutical material engineering: Review
article, Asian Journal of Pharmaceutical Sciences, 10 (2015)
283–291.
European Medicines Agency, European Pharmacopoeia—Part
EP 2.9.34. Bulk density and tapped density of powders,
2006a.
European Medicines Agency, European Pharmacopoeia—Part
EP 2.9.36. Angle of repose, 2006b.
Garekani H.A., Ford J.L., Rubinstein M.H., Rajabi-Siahboomi
A.R., Formation and compression characteristics of prismatic polyhedral and thin plate-like crystals of paracetamol, Int. J. Pharm., 187 (1999) 77–89.
Garekani H.A., Ford J.L., Rubinstein M.H., Rajabi-Siahboomi
A.R., Highly compressible paracetamol: I: crystallizationand characterization, Int. J. Pharm., 208 (2000) 87–99.
Gibson M., Pharmaceutical Preformulation and Formulation, 2,
Informa Healthcare USA, Inc., New York, 2009.

Gonda I., Development of a systematic theory of suspension
inhalation aerosols. I. A framework to study the effects ofaggregation on the aerodynamic behaviour of drug particles, International Journal of Pharmaceutics, 27 (1985)
99–116.
Gordon R.E., Amin S.I., Crystallization of ibuprofen, Google
Patents, 1984.
Huettenrauch R., Moeller U., Molecular galenics. Part 73. Correlation between the degree of order and the wettability ofsolids, Pharmazie, 38 (1983) 198.
Jordens J., Gielen B., Braeken L., Van Gerven T., Determinationof the effect of the ultrasonic frequency on the coolingcrystallization of paracetamol, Chemical Engineering andProcessing: Process Intensification, 84 (2014) 38–44.
Kaialy W., Larhrib H., Chikwanha B., Shojaee S., Nokhodchi
A., An approach to engineer paracetamol crystals by antisolvent crystallization technique in presence of various
additives for direct compression, Int J Pharm, 464 (2014)
53–64.
Lacmann R., Crystallization, Third Edition. J.W. Mullin,
Butterworth-Heinemann, Oxford 1997, 527 Seiten, zahlr.
Abb. und ISBN 0-7506-3759-5, Chemie Ingenieur Technik,
70 (1998) 1468–1468.
Marshall P.V., York P., Crystallisation solvent induced solidstate and particulate modifications of nitrofurantoin, International Journal of Pharmaceutics, 55 (1989) 257–263.
Marshall P.V., York P., The compaction properties of nitrofurantoin samples crystallised from different solvents, International Journal of Pharmaceutics, 67 (1991) 59–65.
Ó’Ciardhá C.T., Frawley P.J., Mitchell N.A., Estimation of thenucleation kinetics for the anti-solvent crystallisation ofparacetamol in methanol/water solutions, Journal of Crystal
Growth, 328 (2011) 50–57.
Šimek M., Grünwaldová V., Kratochvíl B., Hot-stage microscopy for determination of API fragmentation: comparison
with other methods, Pharm. Dev. Technol., in press (2015)
York P., Solid-state properties of powders in the formulationand processing of solid dosage forms, International Journalof Pharmaceutics, 14 (1983) 1–28.

205

Michal Šimek et al. / KONA Powder and Particle Journal No. 34 (2017) 197–206

Author’s short biography
Michal Šimek
In 2012 graduated in Drug synthesis and manufacturing at the Faculty of Chemical
Technology, University of Chemistry and Technology in Prague. From 2012 researcher
at the University of Chemistry and Technology Prague and researcher at the Solid state
department at Zentiva Company in Czech Republic. From 2014, a reviewer for Science
Domain international. Research: particle size analysis and science, formulation of solid
drugs, crystallization and material science.

Veronika Grünwaldová
In 2002 graduated in chemistry at Prague Institute of Technology. In 2010 Ph.D. graduated in analytical chemistry at the Faculty of Science, Charles University in Prague.
Positions: 2002–2006 Institute of Inorganic Chemistry AS CR, researcher; 2003–2006
Academy of Fine Arts, expert consultant; 2007–2012 Zentiva a.s., Analytical scientist;
2012—present Formulation Scientist. Specialization: particles size and morphology,
chemical and phase’s composition of solid pharmaceutical, forensic and art samples, recently development of generics solid formulation. Methodology: SEM, light microscope,
X-ray diffractometer, laser diffractometer and PCCS. Other experimental skills include
the use of EDX, X-ray fluorescence, microtome and macro objectives.

Bohumil Kratochvíl
In 1972 graduated in chemistry at the Faculty of Science, Charles University in Prague.
In 1985 the transition from the Faculty of Science to the University of Chemistry andTechnology, Prague (UCT). Positions : Head of the Department of Solid State Chemistryof the UCT , Vice-dean of the Faculty of Chemical Technology of the UCT and
Vice-Rector for Science and Research of the UCT. Editor in Chief of the Chemicke Listy
journal (1986–2012). Research : Crystallography of coordination, inorganic and smallorganic compounds , recently application of X-ray diffraction methods in pharmaceuticalresearch , development, production and control. Study of solid pharmaceutical phases:
polymorphs, hydrates, cocrystals. Solid dosage forms.

206

KONA Powder and Particle Journal No. 34 (2017) 207–212/Doi:10.14356/kona.2017006

Original Research Paper

Nanocrystallization of Al Powder by Cryomilling Process †Mohammad Hossein Enayati
1

Department of Materials Engineering, Isfahan University of Technology, Iran

Abstract
Al5083 powder was subjected to ball milling in liquid nitrogen for 8 h using an attrition ball milling. The
evolution of morphology of powder particles and the refinement of grain size were studied by scanning andtransmission electron microscopies. The results showed that during cryomilling the morphology of powder
particles changed from spherical to equiaxed shape. Additionally, the size of powder particles increased from
~10 μm to ~20 μm with narrower distribution. Simultaneously the cryomilling was associated with a significant
reduction of grain size so that the final product after 8 h of cryomilling had a nanocrystalline microstructure(~25 nm) with well-developed high angle grain boundaries. These features were discussed in terms of severeplastic deformation, cold-welding and fracturing of powder particles which occur simultaneously duringcryomilling process.
Keywords: cryomilling, aluminium, Al5083, nanocrystalline powder

1. Introduction
Nanocrystalline materials are generally known as thosematerials that have a crystallite size less than 100 nm.
This upper limit of 100 nm is based on the fact that mostproperties (mechanical/physical/chemical) of materials
start to change significantly at this point. Although there
are lots of experimental results showing that the change
in properties can occur in crystallite size much smaller orin some cases larger than 100 nm.
In conventional coarse-grained polycrystals, with atypical grain size of 10 μm, the fraction of boundaries isvery small. In contrast if the grain size reduces to nanocrystalline state (grain size < 10 nm) a high fraction(1018–1021 cm–3) of material is boundaries. As a result,
grain boundaries have significant contribution in physicaland mechanical properties of nanocrystalline materials.
Nanocrystallization leads to the hardness and strength
enhancement and improvement of diffusivity, solid solubility, sintering ability and magnetism.
Vapor deposition, plasma processing, gas-condensation,
chemical precipitation and crystallisation from the amorphous phase are well established processing routes for
obtaining nanocrystalline materials. However, it is wellknown that mechanical milling (MM) and mechanical1

Received 20 January 2016; Accepted 3 March 2016
J-STAGE Advance Publication online 30 April 2016
Isfahan 84156-83111, IranE-mail: ena78@cc.iut.ac.ir
TEL: +98-311-3915730 FAX: +98-311-3912752

alloying (MA) as solid synthesis routes can also be applied for production of a nanocrystalline as well as nanocomposite powders with high uniformity (Fogagnolo J.B.
et al., 2004). Comparison of different preparation methods
in terms of cost and productivity demonstrates that MM/
MA is the most cost effective route capable of producing
nanocrystalline materials in large quantity. It is worth
mentioning that powder particles prepared by MA/MM
are rarely monocrystals. Instead, MA/MM powders havemicrometer dimensions with interior nanometer microstructure. The structure of each particle can be described
as a poly-nanocrystalline. The final powder produced by
MA/MM can be subsequently either consolidated by standard powder metallurgy techniques into bulk materials
with desirable properties or deposited on surfaces of engineering parts using various thermal spraying methods.
It has been proved that grain refinement of powders in
MM/MA to the nanometre size is governed by the plastic
deformation induced during milling. In general, the grain
size decreases continuously with milling time until a minimum (saturation) size is approached (Enayati M.H., 2015;
Hellstern E. et al., 1989). Further refinement seems to bedifficult to achieve for a fixed set of experimental conditions. Transmission electron microscopy (TEM) observations have revealed that nanocrystallization starts with
development of shear bands and dislocations pile-ups at
the early stages of milling (Eckert J. et al., 1992; Hellstern
E. et al., 1989). With increasing milling time more dislocations are created in the grains leading to a dislocation
cell structure and then low-angle boundaries. This stage
is finally followed by development of a fully nanocrystal-

Copyright © 2017 The Author. Published by Hosokawa Powder Technology Foundation. This is an open access article
under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

207

Mohammad Hossein Enayati / KONA Powder and Particle Journal No. 34 (2017) 207–212

line structure with high-angle grain boundaries (Eckert J.
et al., 1992; Cocco G. et al., 1990).
Experimental findings have revealed that the minimum
grain size for a given material is a constant value independent of milling conditions. In fact, the minimum grain
size is determined by intrinsic properties of materials
such as melting temperature, crystal structure and deformation behavior. The milling conditions can only affectthe rate at which the grains refine and approach the minimum size. Eckert et al. (1992) observed that the minimum
grain size, dmin, in a series of FCC metals including Al,
Cu, Ni, Pd, Rh and Ir scales inversely with the melting
point, Tm, or the bulk modulus, B, of the respective metals.
The higher the Tm or B, the smaller the dmin, is obtained.
However, no clear inverse correlation between the minimum grain size and melting temperature was obtained for
BCC metals Cr, Fe, Nb, W as well as for HCP metals Hf,
Zr, Co, Ru . Koch C.C. (1989) observed that only thelower melting temperature FCC metals (≤ Tm for Pd) exhibit an inverse dependence of dmin on Tm.
The dependence of minimum grain size versus melting
temperature has been discussed with respect to the competing rates of creating dislocations due to work hardening and recovery phenomena which scale with the melting
point (Eckert J. et al., 1992; Benghalem A. and Morris
D.G., 1994). Thereby, the minimum grain size achieved
by ball milling is determined by the balance between creation and annihilation of dislocations during processing.
On basis of this model dmin scales inversely with hardness, proportionally with the stacking fault energy andexponentially with the activation energy for recovery.
Hence, the grain size in final material can be controlled
by changing the equilibrium rate of dislocation creation
and recovery. It has been shown that in ball milling experiments not only the milling time but also further parameters, such as type of the mill machine, the milling
tools material, the intensity of milling, the ball-to-powderweight ratio, the milling temperature and the milling atmosphere, may have significant influence on nanocrystallization process.
Experiments show that lowering milling temperatureleads to a finer grain size (Koch C.C., 1997). The dependence of dmin on milling temperature has directed researchers to another version of ball milling process
known as “cryomilling”. This process benefits from ball
milling at cryogenic temperatures. Two different methods
have been applied for cryomilling. In most cases, a cryogenic liquid (e.g. liquid N2) flows continuously inside themill vessel. The raw materials are ball milled with thecryogenic liquid forming a slurry. In some set-ups thecryogenic temperature during ball milling has beenachieved by circulation of cryogenic liquid around theoutside surfaces of milling vessel. Basically, cryomilling
experiments can be carried out in any type of milling ap-

208

Table 1 Some physical and mechanical properties of Al5083
alloy
Physical Properties

Value

Density

2.65 g/cm3

Melting Point

843 K

Electrical Resistivity

0.058 × 10 –6 Ω·m

Thermal Conductivity

121 W/m·K

Thermal Expansion

25 × 10 –6/K

Mechanical Properties

CW↑

CW↓

Modulus of Elasticity (GPa)—72

Proof Stress 0.2 % (MPa)

240

145

Tensile Strength (MPa)

345

300

Shear Strength (MPa)

190

175

Elongation A5 (%)

13

17

Hardness Vickers (HV)95

70

CW↑ Severely cold worked
CW↓ Slightly cold worked

paratus although the attrition ball mill is often preferred.
Cryomilling process includes several advantages overroom temperature ball milling. A benefit of cryomilling isthat the rate of diffusional process like annihilation of
dislocation is extremely limited at cryogenic temperature.
As discussed earlier, under this condition the grain refineswith a much higher rate. Hence the milling time required
to attain a certain grain size is significantly reduced.
Shorter milling time in case of cryomilling implies that alarge quantity of powder can be processed in large-scale
low-energy ball mills within a reasonable milling time.
The present research was aimed to study the process ofnanocrystallization in Al5083 alloy during cryomilling.
Synthesis of nanocrystalline structure in different aluminum alloys have been widely investigated but the nanocrystallization in cryomilling condition have been rarely
reported. Al5083 alloy is a non-heat treatable aluminumalloy. As a results the strengthening of Al5083 is essentially restricted to cold working/work hardening. However, the strength of Al5083 may be further enhanced byscaling the grains size down to the nano range. Becauseof excellent combination of engineering properties (seeTable 1) such as strength, weld ability and corrosion resistance, Al5083 have different applications specificallyin chemical industries and marine environments.

2. Materials and methods
Gas atomized Al5083 powder was used as starting materials. Table 2 lists the chemical composition of as-received

Mohammad Hossein Enayati / KONA Powder and Particle Journal No. 34 (2017) 207–212

Table 2 Chemical composition of as-received Al5083 powder

Table 3 Cryomilling conditions

Element

Weight percent

Parameter

Value

Si

0.15 %

Weight of Powder

1 kg

Fe

0.18 %

Weight of Balls

30 kg

Cu

0.1 %

Ball-to-Powder Weight Ratio

30:1

Mn

0.7 %

Diameter of Balls

6 mm

Mg

4.5 %

Speed of Rotating Shaft

180 rpm

Zn0.25 %

Temperature of Milling Vessel

90 K

Ti

0.05 %

Stearic Acid (Surfactant)

0.2 wt%Cr

0.15 %

Milling Time

8h

Al

BalanceFig. 1 Schematic representation of cryomilling apparatus.

Al5083. The compositions of samples were determinedusing an energy dispersive X-ray spectrometer (EDX).
The main alloying elements in this alloy was found to be
Mg with traces of Cr, Zn, Ti, Fe, Cu, Si and Mn. To prevent oxidation or hydration, the powder was stored in
vacuum chamber before cryomilling.
Cryomilling was performed in an attrition ball mill.
Fig. 1 shows a schematic configuration of milling apparatus.
This type of mill comprises a static cylindrical vessel
with a rotating impeller with which a rather large quantityof powders (several kg) can be processed making it potentially suitable for commercial exploitation. The powder tobe milled was placed in a stationary tank with balls andliquid nitrogen. This mixture was then stirred by the shaft
with arms, rotating at a high speed of about 180 rpm.
Table 3 shows the cryomilling parameters used in thisstudy.
The milling media including milling tank, shaft, impeller and balls were martensitic stainless steel 440C (Fe18Cr-1.2C-1Mn-1Si) with a hardness of 60 HRC. A
problem with ball milling process is the adhesion of pow-

der particles on milling surfaces reducing the powder
yield. The sticking of powders on milling surfaces iscaused by severe cold welding of powder. Extensive cold
welding also results agglomeration of powder particles
which leads to the coarse powder particles. These phenomena are more serious in the processing of ductile materials like aluminum. The adhesion of powders can often
be overcome by adding a surface-active agent to thepowder mixture in order to inhibit the cold welding
process. In this study 0.2 weight percent stearic acid-as asurfactant- was added to the powder mixture. Stearic acidis a waxy solid with chemical formula of C17H35CO2H.
The morphology of powder particles was analyzed in a
Philips XL30 FEG scanning electron microscope (SEM)
operating at 10 kV. The powder particles size was determined on SEM micrographs using ImageJ software. Powder samples were fixed in small quantities on a sample
holder by Ag conductive paint. At least 50 separate particles were chosen for the measurement of powder particle
size.
A CM20 transmission electron microscope (TEM) operating at 200 kV was used for observation and characterization of the internal structure of cryomilled Al5083
powders. A small amount of powder sample was dispersed in methanol using ultrasonication for ten minutes.
The obtained suspension of particles was lifted on acarbon-coated Cu grid (400 mesh) by dipping the grid in
the suspension gently and removing horizontally. The
grid was then dried in air for several hours. The resulting specimens showed some thin areas suitable for TEM
observation.

3. Results and discussions
Fig. 2 shows SEM images of as-received Al5083 powder particles with different magnifications.
As seen Al5083 particles were spherical in shape witha wide size distribution. The spherical morphology of209Mohammad Hossein Enayati / KONA Powder and Particle Journal No. 34 (2017) 207–212

Fig. 2 SEM micrographs of as-received Al5083 powder particles with different magnifications.

Fig. 3 SEM micrographs of Al5083 powder particles after
cryomilling for 8 h.

as-received powder indicates that the powder was prepared by gas atomization process in which a stream ofmolten metal is mechanically disintegrated by a jet ofhigh-pressure gas. Under this situation the cooling rate of
the molten droplets is sufficiently slow for surface tension
forces to spheroidize them before solidification occurs.
The surface image of a particle is presented in Fig. 2c indicating a typical solidification morphology.
The powder particles size was analyzed on SEM using
ImageJ software. The result showed that as-received
Al5083 powder particles had a size distribution of
3–15 μm with a mean size of ~10 μm.
Fig. 3 shows SEM images of Al5083 powder particles
after 8 h cryomilling. The initial spherical morphology
has been destroyed by cryomilling process. Instead a nearly
equiaxed morphology was developed. The final productafter 8 h of milling time had a narrow size distribution
with a mean of ~20 μm indicating that cryomilling has resulted in coarser and more uniform particles.

Changes in particles morphology and microstructureduring ball milling of ductile metal powders are producedby two simultaneous processes; cold welding and fracturing. During ball milling clusters of particles are trapped
between colliding balls and undergo a high level of impact. If the impact stresses are sufficient, the powder particles plastically deform and flatten (see Fig. 3b). As thepowder particles are pressed together their surface area
increases and the surface oxide films rupture, consequently exposing clean underlying metal. When these
fresh surfaces of particles come in contact a metal bond isformed. After a period of milling, particles deform to theextent that cracks initiate, propagate and ultimately fracture the particles. In first stage the cold welding process
dominates and as a result the powder particle size continuously increases. This stage can be also termed the agglomeration stage.
The progressive increase in hardness value during thefirst stage due to the work hardening leads to a decrease210

Mohammad Hossein Enayati / KONA Powder and Particle Journal No. 34 (2017) 207–212

Fig. 4 Bright field (BF) images and selected area diffraction patterns (SADP) of several Al5083 powder particles after
cryomilling for 8 h showing a well-developed nanocrystalline structure.

in the ductility of powder particles and therefore, an increased tendency for particle fracture. Thereby, the firststage is followed by the fragmentation stage. During thefragmentation stage the fracturing of particles occurs
more readily than cold welding and as a result the powder
particle size decreases. The extent of these two events isdetermined by the mechanical properties of the elemental
powders, such as ductility, yield stress and hardness, aswell as the magnitude of the impact provided by colliding
balls. The last stage is a steady-state stage in which there
is a balance between the frequencies of cold welding andfracturing processes so that the average particle size
remains unchanged. As illustrated in the following paragraph these three stages are associated with microstructural changes.
The TEM images presented in Fig. 4 shows bright field(BF) images and selected area diffraction patterns (SADP)
of several Al5083 powder particles after cryomilling for
8 h. A well-defined nanocrystalline structure was observed in all particles. The corresponding selected area
diffraction patterns exhibited the Debye-Scherrer rings
characteristic of a fine grain structure. The BF images
suggested that the powder particles had an average grain
size of ~25 nm. These small grains, in fact, contribute towards the continuous Deby-Scherrer rings in the SADP.
Further the SADP included only the Debye-Scherrer ringsof fcc Al structure without any indication of a third phase(e.g. oxide or nitride compounds).
Similar to conventional room-temperature ball milling
the transformation of coarse-grained structure to a nanosized scale during cryomilling includes several successive
stages; gradual increasing of dislocation density due toplastic deformation of powder particles, formation of
sub-boundaries by annihilation and recovery processes
and eventually development of high-angle boundaries by
the absorption of more dislocations into the boundaries.
For those cryomilling processes in which the powders
are mixed with liquid N2, the reaction of constituents with
N2 must be considered. This reaction seems to proceed toa great extent during ball milling as plastic deformation;
flattening and fracturing of powder particles repeatedly
produce fresh, clean and highly reactive surfaces. The
powder-N2 reaction can be more serious in case of reactive elements such as Al and Ti. However, TEM results in
Fig. 4 do not support this prediction. During cryomillingthe nitrogen picked up was found to be 0.11 wt%. Nitrogendissolved interstitially in Al lattice forming a supersaturated solid solution. It should be noted that because of low
temperature, formation of nitrogen-containing dispersoids
such as AlN are kinetically improbable, although there isa large driving force for the formation of such compounds(e.g. ΔGAlN = –287 kJ mole –1).
However, subsequent heating upon powder consolidation of cryomilled Al5083 can lead to the formation of211

Mohammad Hossein Enayati / KONA Powder and Particle Journal No. 34 (2017) 207–212

various secondary phases including AlN and Al2O3. The
presence of these precipitates was confirmed in previous
studies by atom probe field-ion microscopy (FIM) andhigh-resolution transmission electron microscopy (HRTEM) observations but the volume fraction of precipitates
was not determined (Zhou F. et al., 2001). Furthermore,
the X-ray diffractometry of cryomilled powder after heat
treatment showed no traces of secondary phases suggesting that the amount of the dispersoids are virtually verysmall < 1 vol% (Zhou F. et al., 2001). It is worth notingthat the dispersion particles formed during cryomilling
acquired nanoscale size ranging from 1–30 nm, with majority of 3–10 nm. The average spacing between the dispersoids was measured to be less than 10 nm. These
dispersions were found to be incoherent in nature, highly
stable at high temperature and insoluble in matrix. The
presence of these finely dispersed particles can play a
critical role in retarding grain growth in the nanocrystalline materials through the so-called Zener mechanism
even though their volume fraction is low. Furthermore,they could effectively obstruct dislocation motion andtherefore reduce the minimum grain size.

4. Conclusions
Similar to conventional room temperature ball milling
process cryomilling of aluminum powder led to the nanocrystallization. The ultimate grain size appeared to be
similar to that reported for conventional ball milling(25 nm) although the rate of nanocrystallization was
found to be much faster. Furthermore, in cryomilling thecold-welding process increased the powder particle sizewith more uniform morphology compared to as-received
powder.

References
Benghalem A., Morris D.G., Milling and mechanical alloying ofcopper and some solution alloys seen as a thermomechanical process, Acta Metallurgica et Materialia, 42 (1994)
4071–4081, Doi: 10.1016/0956-7151(94)90184-8.
Cocco G., Soletta I., Battezzati L., Baricco M., Enzo S.,
Mechanical alloying of the Al–Ti system, Philosophical
Magazine B, 61 (1990) 473–486, DOI: 10.1080/
13642819008219288.
Eckert J., Holzer J.C., Kril C.E., Johnson W.L., Structural andthermodynamic properties of nanocrystalline fcc metals
prepared by mechanical attrition, Journal of Materials
Research, 7 (1992) 1751–1761, DOI: 10.1557/JMR.1992.1751.
Enayati M.H., Formation of nanoscale layered structures andsubsequent transformations during mechanical alloying of
Ni60Nb40 powder mixture in a low energy ball mill, Kona
Powder and Particle Journal, 32 (2015), 196–206, DOI:
10.14356/kona.2015010.
Fogagnolo J.B., Robert M.H., Velasco F., Torralba J.M.,
Aluminium matrix composites reinforced with Si3N4, AlN
and ZrB2, produced by conventional powder metallurgy
and mechanical alloying, Kona Powder and Particle Journal, 22 (2004) 143–150, DOI: 10.14356/kona.2004017.
Hellstern E., Fechet H.J., Fu Z., Johnson W.L., Structuraland thermodynamic properties of heavily mechanically
deformed Ru and AlRu, Journal of Applied Physics, 65(1989) 305–310, DOI: 10.1063/1.342541.
Koch C.C., Materials synthesis by mechanical alloying, Annual
Review of Materials Science, 19 (1989)121–143, DOI: 10.
1146/annurev.ms.19.080189.001005.
Koch C.C., Synthesis of nanostructured materials by mechanical milling: problems and opportunities, Nanostructured
Materials, 9 (1997) 13–22, Doi: 10.1016/S0965-9773(97)00014-7.
Zhou F., Lee J., Dallek S., Lavernia E.J., High grain size stability of nanocrystalline Al prepared by mechanical attrition,
Journal of Materials Research, 16 (2001) 3451–3458, DOI:
10.1557/JMR.2001.0474.

Author’s short biography
Mohammad Hossein Enayati
M. H. Enayati (PhD, CEng) is a full Professor of materials science at Isfahan University of Technology and Fellow of Institute of Materials, Minerals and Mining (FIMMM),
UK. Prof. Enayati’s research focuses on the nanostructured and amorphous materials,
mechanical alloying and synthesis of advanced materials for thermal spray coating. He
has authored several books and numerous articles in well recognized international journals in his field, and has received research funding from a wide variety of agencies. His
research has been recognized with numerous awards, most recently from the Iran Nanotechnology Initiative Council and Iranian Nanotechnology Society.

212KONA Powder and Particle Journal No. 34 (2017) 213–223/Doi:10.14356/kona.2017007

Original Research Paper

Identification of Optimal Mill Operating Parameters
during Grinding of Quartz with the Use of
Population Balance Modeling †Evangelos Petrakis *, Elias Stamboliadis and Konstantinos Komnitsas
1

Technical University of Crete, School of Mineral Resources Engineering, Greece

Abstract
It is known that ball milling is an energy intensive process and great efforts have been made over the years to
improve energy efficiency. The use of population balance models (PBMs) can assist in the design of mineralprocessing circuits and the scale-up of laboratory mill results to full-scale. However, since each model has its own
capabilities and limitations, it is believed that a combined use will provide more accurate information for the
reliable description of the process.
In this study, the simulation of grinding of quartz is investigated in order to identify the optimal mill operating
parameters. With the use of population balance modeling the specific rate of breakage and the cumulative
breakage parameters can be determined from mono-size, short grinding time batch tests. The determined
breakage parameters were back calculated by minimizing the residual error between experimental and reproduced
size distributions. By combining two software packages the back calculated breakage parameters were used forthe prediction of the optimum ball filling volume. The proposed procedure can be also applied for theidentification of optimal mill operating parameters for other minerals.
Keywords: comminution, breakage rate, ball filling, kinetic models, simulation

1. Introduction
Size reduction is one of the most fundamental operations in the mineral processing industry. The primary
purpose of size reduction is to obtain an appropriate product size with the least possible energy consumption. However, it is known that a large share of the energy used incomminution is absorbed by the machine and only a smallpart is consumed for size reduction. This implies that theexisting grinding machines are inefficient and engineers
have focused their efforts not only on the design of new
and more efficient machines, but also on the developmentof new approaches for data processing (Yahyaei M. et al.,2015; Umucu Y. et al., 2016).
The main theories that initially described, evaluatedand improved the relationship between specific energy
and size reduction in comminution are those of Rittinger
P.R. (1867), Kick F. (1885) and Bond F.C. (1952). These
theories are based on a specific size modulus of the mate†1*

Received 9 January 2016; Accepted 9 March 2016
J-STAGE Advance Publication online 30 April 2016
73100 Chania, Greece
Corresponding author: Evangelos Petrakis;
E-mail: vpetraki@mred.tuc.gr
TEL: +30-28210-37608 FAX: +30-28210-06901

rial but they do not take into consideration the distribution
modulus. Later, Charles R.J. (1957) and Stamboliadis E.
(2002) used different breakage models by assuming thatboth the initial feed and the final product size distributions are expressed by the Gates-Gaudin-Schuhmann(GGS) equation.
During the last decades, considerable work has been
done on the optimization of energy consumption in
grinding mills using phenomenological grinding kinetics
models based on population balance considerations. Population balance modeling is based on first order kinetics
and uses two functions, namely the specific rate of breakage Si and the breakage function bi,j (Herbst J.A. andFuerstenau D.W., 1980; Austin L.G. et al., 1984). These
functions provide the fundamental size-mass balance
equation for fully mixed batch grinding operations. Many
researchers have underlined the advantages of these functions (Ipek H. and Göktepe F., 2011; Wang X. et al., 2011;
Gupta V.K. and Sharma S., 2014), while the scale-up fromlaboratory data to full-scale mills has also been discussed
in a number of recent studies (Fuerstenau D.W. et al.,
2003; Deniz V., 2013).
Population balance modeling is the most widely usedapproach and assumes that the specific rate of breakage isconstant and is not affected by the grinding time (Austin

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open access article
under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

213

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

al. (1994) also confirmed this relationship.
L.G. and Bhatia V.K., 1972; Austin L.G. and Bagga P.,
Two important factors should be determined before
1981; Nomura S. et al., 1991). Austin L.G. and Bagga P.
carrying out experiments in a mill, namely the ball filling(1981) also have shown that the breakage function paramJ and the powder filling fc. J is the ball filling volume,eters can be estimated from the size analysis of the products obtained after a short grinding time, using as initialwhich is the fraction of the mill filled by the grinding
feed a narrow particle size fraction, namely the one size
media bed (Eqn. (1)), while fc is the powder filling volume
fraction BII method.
in the mill (Eqn. (2))
The primary concept of simulation balance modeling,
Volume of solid balls  1which suggests that the breakage function
parameters
J 
(1)

Mill volume 1 
must be estimated before any of the particles are reselected for further breakage, may result in calculation
Volume of solid powder  1
errors (Austin L.G. et al., 1984; Celik M.S.,
1988; Bozkurt(2)
f c 
Mill volume 1 
V. and Özgür I., 2007). The deviation from first-orderkinetics is also well known in several cases. Celik M.S.
(1988) stated that the breakage rate of anthracite in ballmills exhibited an acceleration, particularly noticeable forlong grinding times, due to extensive agglomeration offines. The acceleration or deceleration of breakage ratesof all individual size classes can be determined during
wet grinding using the G-H method (Rajamani R.K. and
Guo D., 1992). Bilgili E. and Scarlett B. (2005) proposeda phenomenological theory which explains non-first ordereffects as a result of multi-particle interactions. Instead of
the forward experimental estimation of breakage model
parameters, the back-calculation method which calculates
the model parameters by minimizing the error between
experimental and predicted size distributions, is also
widely employed. This method uses all available data at
the same time and enables reduction of the calculation
error. It can be applied in cases where limited data isavailable and thus the need for carrying out several experiments is minimized. In this regard, Austin L.G. et al.
(2007) stated that it is always necessary to confirm thatthe experimental breakage model parameters reproduce
the size distributions and the only way to accomplish thisis to use the back-calculation method.
For the design of grinding circuits in mineral processing plants, many equations describing mill power andgrinding energy are available. These equations can be
categorized into two major groups. The first group describes the relationship between specific energy requirements and size reduction, while the second one relates
power draft to mill size. The first group can be represented by the Bond F.C. (1960) test method. This method
is widely used both for evaluating the grindability of amaterial and for determining the power required to drive
the mill. However, it uses data from laboratory tests while
many empirical correction factors were proposed for the
scale-up to larger mills or other operating conditions. With
respect to the second group, Herbst J.A. and Fuerstenau
D.W. (1973) found out experimentally that a proportional
relationship exists between the specific rate of breakage
and the specific net mill power for a tumbling mill, which
is not affected by the operating parameters. Nomura S. et214

where ε is the bed porosity of balls and powder which isassumed to be 40 % for both (Austin L.G. et al., 1984).
The fraction of space between the balls at rest that is filledwith powder is called interstitial filling U and is calculated by Eqn. (3).

U

fc
 J

(3)

It is mentioned that when U approaches 1 the impacts
are increasingly cushioned by excess particles between
grinding media. Considering that a proportional relationship exists between the specific rate of breakage and thespecific net mill power, the following Eqns. (4)–(8) can beused to determine the specific rate of breakage Si (1/min),for each size class i obtained after grinding, as a functionof J.
Bond F.C. (1960) proposed the following relationshipfor wet grinding in overflow ball mills to scale-up resultsfrom laboratory tests to larger mills
S i  (1  0.937  J ), D  2.4 m(4)Beeck R. (1970) proposed that for dry grinding of
cement the following relationships, based on data from
German and U.S plants respectively, could be used.
S i  (0.294  0.26  J )(5)

S i  (0.374  0.47  J )(6)

Shoji K. et al. (1982) studied the specific rate of breakage in relation to the ball filling volume for quartz grinding using two tumbling ball mills with different capacity.
They proposed the following relationship for mill diameter 0.6 m

1  e 1.20U
S i  
 1  6.6  J 2.3 (7)Austin L.G. et al. (1984), taking into consideration the
differences between various relationships, proposed thatfor batch dry grinding the following relationship should
be used (mentioned here as ‘Bond Modified’)Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

S i  (1  0.937  J ) (1  5.95  J 5 )(8)Later, many other research efforts were made to define
the relationship between Si and J under different operating
conditions. These studies focused on the determination of
Si using the grinding kinetic model. More specifically,
Deniz V. and Onur T. (2002) investigated the breakage
kinetics of pumice samples in relation to the powder filling volume in a ball mill, at J = 20 %, and found that the
maximum normal breakage rate occurred at an interstitial
filling U of approximately 0.4. A more recent study by
Fortsch D.S. (2006) showed that the reduction of J leads
to an increase in capital and installation costs of the milling equipment. He proposed J = 35 % as optimal ball filling volume for cement plants. On the other hand, Metzger
M.J. et al. (2009) using silica sand as a test material
showed that the lower the ball filling J the higher is theamount of the produced product. They also found that
more efficient breakage occurs at ball filling volume
J = 1.5 %.
The present study through batch kinetic experimentswith the use of quartz as test material aims to assess theeffect of ball filling volume J on the specific rate of breakage Si and identify the cumulative breakage function Bi,jby keeping the powder filling volume fc constant. A new
methodology is proposed, involving the use of population
balance modeling, for the identification of the optimum
ball filling during quartz grinding and the scale-up of laboratory mill results to full-scale.

2. Theoretical backgroundLet’s consider a mass of material M in a ball mill thatafter breakage needs to be divided, by using xi screens,
into i + 1 narrow size classes. Normally, for a size class ibounded between two successive screens xi and xi+1 containing a mass fraction mi(t) at time t, it is assumed thatbreakage follows a first-order law. That is, the rate ofbreakage of size i is proportional to the mass of size i in
the mill hold up. According to this, the specific rate ofbreakage Si is independent of time and since the mill holdup M is constant the following Eqns. (9), (10) are obtained(Austin L.G. and Luckie P.T., 1972; Klimpel R.R. andAustin L.G., 1977):
d[mi (t )]S i  mi (t )dt(9) m (t )  S i  tlog  i  
2.3
 mi (0) (10)determined from the slope of this line.
The specific rate of breakage depends on the particle
size of the mill hold up. Austin L.G. et al. (1984) used thefollowing Eqn. (11) to describe this relationship:


x 
Si 
 T   i   Qi
 x0 

(11)

where xi (mm) is the upper size of size class i, x0 is thestandard size (1 mm), αT is a parameter depending on
milling conditions and α is a characteristic parameter depending on the material properties. More specifically αT
is the specific rate of breakage at size xi = 1 mm and has
the same units as Si (1/min). Qi is a correction factor,
which is 1 for small particles (normal breakage) and lessthan 1 for too large particles to be nipped and fracturedby the grinding media (abnormal breakage). In the abnormal breakage region each material size behaves as it iscomposed of a weak and a strong fraction (Ipek H. andGöktepe F., 2011). Qi is calculated by Eqn. (12):1Qi  
Λ 


x
1
(
/
)


i

(12)

where μ is a parameter depending on milling conditions
and denotes the particle size when the correction factor is0.5. Λ is a positive number that depends on the material
type and shows how rapidly the rates of breakage decrease as size increases (Λ ≥ 0). The evolution of the specific rate of breakage vs. size is shown in Fig. 1.
When a material of size j breaks once, the mass fraction of the broken products with size i can be represented
by the breakage function bi,j, i > j. Thus, b2,1 is the massfraction that is reduced from size 1 into size 2, and so on.
The breakage function is usually represented in cumulative form Bi,j as,iBi, j   bk , j
k n

(13)

where bi,j = Bi,j – Bi+1,j.

orwhere mi(t) and mi(0), are the mass fractions for size classi, at times t and 0 respectively. Thus, a log-linear plot ofmi(t) versus t should result in a straight line and Si can be

Fig. 1

Evolution of specific rate of breakage versus size.

215

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

Table 1 Ball mill characteristics and test conditions

Mill

Fig. 2

Balls

Cumulative breakage function versus size.

Austin L.G. and Bagga P. (1981) have shown that for
short grinding times the values of Bi,j can be estimatedfrom the size analysis of the products, using as initial feeda narrow particle size fraction j (the one size fraction BII
method):log[(1  Pi (0)) / (1  Pi (t ))]Bi, j log[(1  Pj 1(0)) / (1  Pj 1(t ))](14)

where Pi(t) is the mass fraction less than size xi at time t.
The above Eqn. (14) presupposes that a small massfraction of particles with smaller sizes will re-break. Experience suggests that good results are obtained when theselected time of grinding results in a 20 % to 30 % material that is broken out from size j (Austin L.G. et al.,1984).
The cumulative breakage function Bi,j can also be represented by the following empirical Eqn. (15) which is thesum of the two straight lines on log-log plot (Austin L.G.
and Luckie P.T., 1972):


x 
x 
Bi, j   j   i 1   (1   j )   i 1  xj 
 xj 



(15)

where xj is the top size, Bi,j is the cumulative breakage
function, γ is the slope of the lower straight line part ofcurve, β is the slope of the upper part and Φj is the intercept, as shown in Fig. 2. γ also, characterizes the relative
mass of fines produced after breakage and therefore isrelated to the grinding efficiency. Generally, the breakage
parameters Φj, γ and β depend on the properties of thematerial. Thus, the parameters of the cumulative breakage
function, which are determined by laboratory experiments, can be used directly for larger scale mills.

3. Material and methods
The material used in the present experimental study isquartz obtained from the quarry of Assiros, in the vicinityof Thessaloniki, N. Greece, with SiO2 content 99.1 % anddensity 2.65 g/cm3. 100 kg of the material was sampled

216

Diameter, D (cm)20.4

Length, L (cm)16.6
3

Volume, V (cm )5,423

Operational speed, Ν (rpm)66

Critical speed, Νc (rpm)

93.7

Diameter, d (mm)25.4

Number77

Weight (g)265,149 2,540 1,738
3Specific gravity (g/cm )7.85

Porosity %40

Ball filling volume, J (%)
3

Material

38

20

Specific gravity (g/cm )2.65

Powder filling volume, fc (%)

4Interstitial filling, U

0.5

10

6.7

1

1.5

from several locations to obtain representative samples.
The microscopic examination using polished and thinsections showed that the material is compact, homogeneous, and does not show the presence of open cracks,
discontinuities or alteration phenomena.
The experiments were performed in a laboratory ballmill that operated at a constant speed of Ν = 66 rpm(1.1 Hz) corresponding to 70 % of its critical speed (Table1). The mill charge consisted of 25.4 mm (1 inch) stainlesssteel balls with density ρb = 7.85 g/cm3.
Three series of tests were performed at J 6.7 %, 10 %and 20 %, while fc that remains constant at 4 %, corresponds to 345 g of quartz.
The sample was homogenized by the cone and quarter
method and 6 kg of the material was used for the tests.
This quantity was crushed at a size of –4 mm using a jaw
crusher. The product of the crusher was wet screened
at 150 μm, while the +150 μm fraction was dried andscreened using a series of screens with an aperture ratio
2 to obtain the feed fractions. As a result, five mono-­
sized fractions of quartz (–3.35+2.36 mm, –1.7+1.18 mm,–0.850+0.600 mm, –0.425+0.300 mm and –0.212+0.150 mm)
were prepared. The grinding tests were performed at various grinding times t (0.5, 1, 2 and 4 or 8 min). The products obtained from each test were wet sieved using aseries of screens with a ratio of 2 for the determinationof particle size distribution. All tests were carried out in
duplicate and average values are given in the results section.
The mill power is calculated by the following formula(Stamboliadis E. et al., 2011):
P  9.9  M  N  D(16)

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

where Μ is the total mass (kg) of the feed material andballs, Ν is the rotational speed (Hz) and D is the internal
diameter of the mill (m).
It should be also noted that the following issues weretaken into consideration:
i. A fresh feed charge was used in each test.
ii.	Following the recommendation of Gupta V.K. andSharma S. (2014), the crushed material was preground in the mill for 2 min in order to avoid abnormal breakage behaviour.
iii.	The feed charge was sampled and screen analyzed
in the same way as the mill product.
iv.	T he mill was layer loaded with balls and feedcharge, following the standard practice (Gupta V.K.
et al., 1985).
The energy consumed by the mill is proportional to thetime t and is given by Eqn. (17):E P  t(17)

where P is the power of the mill (W), t is the grindingtime (s) and Ε is the net energy consumed (J). If m is themass of the feed material (kg) then the specific energy for
grinding ε’ (J/kg) is given by Eqn. (18):




E

m

P t
m

(18)

The specific energy in kWh/t is calculated by dividing
Eqn. (18) by 3,600.
Two software packages were used, namely Moly-Cop
ToolsTM version 1.0 and MODSIMTM academic version
3.6.25 (King, 2001), to identify breakage parameters andmodel the process.
Moly-Cop ToolsTM includes a set of easy-to-use excel
spreadsheets, that can be used to characterize and evaluate the operating efficiency of any given mineral ore
grinding circuit, following standardized methodologies
and widely accepted evaluation criteria. More specifically,
the spreadsheet (BALLPARAM_BACH) uses all the differential equations of a population balance model for the
calculation of Si and Bi,j parameters. The software identifies the best combination of parameters that minimize theresidual error between experimental and reproduced size
distributions, while the objective function SSE is expressed by the following formula:
SSE

2


R
w R
 Wi  i(exp)R i(mod)  100%i(mod)
t 1 i 1


4

n

(19)

where Ri(exp) represents the particle size distribution of themill product (as % retained on screen xi), Ri(mod) represents
the response of each corresponding Ri(exp) for a given setof model parameters, n is the number of screen intervals
used, wi is the weight factor for each screen that quantifies
the relative quality and reliability of each particular

screen size value with respect to the other values (highvalues indicate more reliable measurements) and W is thetotal sum of the weight factors. The software input requires a minimum of four size distributions obtained atfour grinding times t1, t2, t3 and t4.
For the back calculation of the breakage parameters,
the experimentally calculated parameters (α T, α, μ, Λ,Φj, γ and β) are used as initial input parameters in the
Moly-Cop ToolsTM software. As a rule of thumb, the parameters which are generally regarded as characteristicsof the material should be kept constant and only those associated with mill conditions should be changed. In thisstudy, the input parameters are α, μ, Λ, Φj, γ and β and theinitial value of αΤ is the one calculated to give the best fit
between experimental and reproduced size distributions.
After this, the fit is improved by adjusting the parametersμ, Λ, Φj, and γ, while the parameters that depend on thematerial, i.e., α and β are kept constant.
MODSIMTM is a software that uses simulation models
for the design of mineral processing circuits. The main
unit operations include size reduction, crushing andgrinding, classification and concentration based on particle mineralogical composition and solid-liquid separations. Furthermore, MODSIM TM which can up-scalelaboratory mill results to full-scale mill, provides standard grinding mill units, e.g. MILL and GMSU based on
population balance modeling, and thus the calculated experimental breakage parameters can be used as input.
The back calculated breakage parameters from the
Moly-Cop ToolsTM software are used as input in the unit
model MILL of the MODSIMTM software and the distributions of the products are compared. MILL is the ball
milling model that is based on standard Austin’s models(Eqns. (14) and (15)) and takes into consideration the
breakage parameters as does the Moly-Cop. It is mentioned that the MILL model does not take into consideration the operating conditions (e.g. mill diameter, ballfilling volume and mill speed) and a different unit model
should be used to elucidate the ball filling effect on thegrinding process. For this purpose, the Austin’s scaled-up
unit model GMSU of MODSIMTM was used in this study.
The back calculated breakage and other parameters, e.g.
D = 0.204 m, N = 66 rpm, d = 25.4 mm were used as input
in the GMSU model. Size distributions varied in relation
with J. A total of six J values were examined and each of
the obtained distributions was used as input in the Moly-­
Cop ToolsTM software. Then, the optimum value of αΤ (1/min) that minimizes the least-squares objective function(Eqn. (19)) was determined by keeping all other parameters constant.

217

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

4. Results and discussion
4.1 Determination of the specific rate of breakage
Fig. 3 shows the relationship between the remaining
mass fraction of feed size and grinding time for J = 20 %using a normal-log plot. The feed size is the upper size ofthe tested size class. The results indicate that breakage
follows a first order law (i.e. the specific rate of breakage
Si is independent of time) and the specific rate of breakage
can be determined from the slope of the straight lines.
Each line corresponds to a different feed size tested. This
figure also shows an increase of Si values when the feed
size fraction is increased, except for the largest tested size(–3.35+2.36 mm), which exhibits lower breakage ratecompared to the smaller fraction (–1.7+1.18 mm). This reveals that there is an increase in the rate of breakage up toa certain particle size and that grinding media cannot
break coarser material efficiently. The same trend is obtained for J = 10 % and J = 6.7 %.
Regarding the effect of J on breakage rate Si, Fig. 4
shows that the value of Si increases with increasing J suggesting that breakage is more effective at higher J values.
Variations of specific rates of breakage Si for different
feed size fractions are shown in Fig. 5 on a log-log scale.

Fig. 3Fig. 4218

It is shown that Si increases up to 1.7 mm (upper sizeclass), but above this size breakage rates decrease sharply
for all J values, since the particles are too large to benipped and fractured by the grinding media used. It appears that the increase of Si is almost linear up to the optimum size of 1.7 mm and the slope of this linear part
remains constant. In addition, it is also seen that the optimum feed size to obtain a maximum Si is independent of
the ball filling volume J. It is believed that this size depends on the material type, the size of the balls and thetype of the mill used.
The determined Si values for each feed size were usedin Eqns. (11) and (12) and the parameters αT, α, μ, and Λ
were calculated by using a non-linear regression technique. The calculated values are shown in Table 2.

4.2 Determination of the cumulative breakage
function parameters
Fig. 6 shows the experimental values of Bi,j versus size
on log-log scale. The specific graphical representation
corresponds to the (–0.850+0.600 mm) feed size for threedifferent ball filling volumes. The feed sizes presented are

Fig. 5

Variation of specific rate of breakage versus feed size,for various J volumes.

Fig. 6

Cumulative breakage function versus relative feed sizefor various J volumes.

First order plots for J = 20 %.

First order plots for feed size (–3.35+2.36) mm.

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

Table 2 Breakage parameter values for different ball mill filling.
Si
J%

Si

αT

α

1/min

Bi,j
μ

Λ

Φj

γ

β

mm

20

0.55

0.71

1.13

2.11

2.90

0.76

1.02

5.80

10

0.28

0.35

1.15

2.34

3.15

0.77

1.00

5.80

6.7

0.18

0.23

1.17

2.38

3.33

0.77

1.11

5.80

finer than the optimum size (1.7 mm), while as described
previously breakage takes place at the normal region. By
definition, the values of Bi,j are determined from the size
distributions obtained at short grinding times, by fitting
the experimental data to Eqn. (14). The Φj, γ and β parameters were determined using Eqn. (15) and are shown in
Table 2. Since β is generally accepted as characteristic
constant of the material, it was decided to keep it constantat the value of 5.80, as indicated by Austin L.G. et al.
(1984). This was considered necessary in order to reduce
the number of parameters used and thus to investigate thesensitivity of only two parameters (Φj, and γ). From theresults obtained it is observed that changes in ball fillingvolume do not entail any significant changes in the valuesof parameters Φj, and γ. This observation is in accordancewith earlier studies (Fuerstenau D.W. et al., 2004; Ozkan
A. et al., 2009; Chimwani N. et al., 2013), which also statethat the Bi,j parameters are independent of the mill operating conditions.

4.3 Back-calculation of the breakage parameters
From the calculated breakage parameters Si and Bi,j
(Table 2) the size distribution of the grinding products
can be reproduced using the spreadsheet BALLPARAM_
BACH of Moly-Cop ToolsTM.
Fig. 7 presents the reproduced values of cumulative
mass passing for the (–0.850+0.600 mm) feed size after
0.5 min grinding time. The reproduction provides acoarser size distribution compared to the experimental
values and this is also observed at longer grinding times.
It is believed that the deviation between experimental andreproduced values in Fig. 7 is due to the value of α, which
is a characteristic parameter of the material. In order toovercome this shortcoming, a readjustment of the grinding parameters is required. For quartz, the value of α proposed in literature is 0.80 (e.g. Austin L.G. et al., 1984),
which is quite different from the calculated values (about
1.15) shown in Table 2. On the contrary, αT is a parameter
depending on mill operating conditions and characterizes
the grinding process. Thus, the variability of this parameter and the magnitude of its effect on the balling fillingFig. 7Comparison between experimental and reproduced size
distributions at 0.5 min grinding time.

Fig. 8Comparison between experimental and reproduced size
distributions at different grinding times, using the backcalculated breakage parameters.

Table 3 Back calculated breakage parameters for J = 20 %.
α

μ

Λ

Φj

γ

β

2.90

0.73

1.05

5.80

mm
0.80

2.11

should be investigated.
In this study, the input parameters are α = 0.80,
μ = 2.11, Λ = 2.90, Φj = 0.76, γ = 1.02 and β = 5.80 and theinitial value of αΤ is the one calculated to enable the best
fit between the experimental and the reproduced size distributions. The fit can be improved by readjusting the values of the parameters μ, Λ, Φj, and γ, while the parametersthat depend on the material, i.e., α and β are kept constant.
Table 3 summarizes the best combination of the back calculated parameters. These values are closer to those established in literature by other researchers (Yekeler M. et
al. 2001) who obtained an average value of γ = 1.20 during
wet grinding of quartz.
Fig. 8 compares experimental and reproduced size distributions obtained at different grinding times for the(–0.850+0.600 mm) feed size. In this figure a very good219

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

agreement between experimental and predicted values,
within the limits of experimental error, can be observed,
when the back calculated breakage parameters derived by
the Moly-Cop ToolsTM (Table 3) were used.

4.4 Simulation of the grinding process andvalidation of results
It was shown earlier that the value of Si increases withincreasing J and this suggests that breakage is more effective as the ball mill filling volume increases. This conclusion is supported by three series of tests carried out withdifferent J values 6.7 %, 10 % and 20 %.
In order to predict the effect of ball filling volume both
the MODSIM TM and the Moly-Cop ToolsTM software
packages were used. According to this procedure, theback calculated breakage parameters given in Table 3 andother parameters shown in Table 1, i.e., D = 0.204 m,
N = 66 rpm, d = 25.4 mm were used as initial input in theunit model GMSU of MODSIMTM. Fig. 9 shows the particle size distributions obtained from the model GMSU.
The results indicate an agreement between the experimental and the simulated particle size distributions for the

Fig. 9Comparison between experimental and simulated by
MODSIMTM size distributions using the back calculated parameters.

Fig. 10 Comparison between experimental and simulated αΤ
values220

tested ball filling volume J = 20 %, since both software
packages use the standard Austin’s kinetic models.
In order to simulate the grinding process, six J values(5 %, 10 %, 20 %, 30 %, 40 % and 50 %) were used andeach of the obtained, from the unit model GMSU, size
distributions was used as input in Moly-Cop ToolsTM.
Then, the optimum value of αΤ (1/min) that minimizes theleast-squares objective function (Eqn. (19)) was obtained,by keeping all other parameters constant.
Fig. 10 shows very good agreement between the experimental and the simulated αΤ values in relation to the ballfilling volume J. However, the simulation indicated that
there is an optimum ball filling volume (J = 30 %), forwhich the specific breakage rate obtains a maximum
value. The predicted αΤ values were validated by additional experiments for J = 40 % and the results are given
in Fig. 11. It can be seen from this figure that even above
the optimum ball filling volume there is perfect match between experimental and simulated results, indicating thatthe approach used was able to predict accurately the variation between the αΤ values and the ball filling J.
Having successfully validated the simulated results, we
attempted to compare them with those obtained fromother earlier studies (Eqns. (4)–(8)). Fig. 12 presents theFig. 11 Validation of predicted αΤ values.

Fig. 12 Normalized breakage rates predicted by different
studies.

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

predicted specific rates of breakage in relation to the ballfilling volume J. In order to effectively compare the specific rates of breakage, as predicted from different models, the results are expressed as the ratio between Si value
and a reference value Sref, taken as 50 % of the ball fillingvolume J. The approach proposed in the present study isindicated with the dashed line. It is clearly shown that
Eqns. (4), (5), (6) and (8) are inversely proportional to theball filling volume J and this is valid also for J values
higher than the optimum value of about J = 30 %, as predicted in the present study. Eqn. (7) has also an optimum
value J = 40 %, although the specific values of predictedbreakage rates are lower than the ones proposed in thisstudy. However, it can be said that Eqns. (4), (5), (6) and(8) are applicable for higher values of J (in the range of
30 % to 40 %), while the results derived by Shoji K. et al.
(1982) (Eqn. (7)) and the present study are applicable overa wider range of ball filling volume J. The minor difference between the latter two studies may be due to different milling conditions and the material type.

a good match was observed between the experimental andthe simulated αΤ values, in relation to J. The simulated results indicated that efficient grinding occurs at J = 30 %,for which the specific rate Si is maximized. Validation of
the results was also done for a higher J value, 40 %.
The approach proposed provides more accurate information for the reliable description of the grinding process
and enables the scale-up of laboratory results to larger
scale mills. Future work will focus on the evaluation andexploration of other important mill operating parameters,e.g. ball size and mill speed.

Nomenclaturebi,jbreakage function (–)

Bi,jcumulative breakage function (–)

Dinternal diameter of the mill (m)dball diameter (mm)E

grinding consumption energy (J)5. Conclusions

εbed porosity (%)ε’

specific grinding consumption energy (J/kg)

In this study the grinding of quartz was simulated withthe use of population balance modeling. The specific rateof breakage and the cumulative breakage parameters were
determined from mono-size, short grinding time batch
tests using a non-linear regression approach. The experimentally determined parameters (αT, α, μ, Λ, Φj, γ and β)
were compared for three J values (6.7 %, 10 % and 20 %).
It is observed that the breakage rate increases with increasing J, suggesting that grinding in this case becomes
more efficient. Grinding of different feed sizes shows thatthe specific rate of breakage increases up to the size of
1.7 mm, while for coarser sizes it decreases sharply. In
addition, the change of J has minor effect on the parameters of the breakage function Bi,j. The Moly-Cop ToolsTM
software was used to identify the best combination of the
breakage parameters (αT, α, μ, Λ, Φj, γ and β), for which
the reproduced size distributions compare well with theexperimentally determined ones. It was revealed that theexperimental breakage parameters did not predict very
well the size distributions and in order to improve the results a readjustment of the value of α, which is a characteristic parameter of the material, was required. The
results of this study propose that for the more accurate
prediction of the size distributions during grinding, theparameters which characterize the material (i.e. α and β)should be kept constant and only those associated withmill conditions should be modified accordingly.
The simulation of grinding of quartz was enabled by
combining MODSIMTM and Moly-Cop ToolsTM software
packages. Using the back-calculated breakage parameters,

fc

powder filling volume (%)

Jball filling volume (%)

isize class index

M

mass of the feed material and balls (kg)mmass of the feed material (kg)mi(0)

mass fraction of size class i at time 0mi(t)mass fraction of size class i at time tNrotational speed (rpm), (Hz)P

mill power (W)Pi(0)

mass fraction less than size xi at time 0Pi(t)mass fraction less than size xi at time t

Qicorrection factor (–)

Ri(exp)mass retained on screen xi (%)

Ri(mod) predicted mass retained on screen xi (%)
Sref

specific rate of breakage at J = 50 % (1/min)Sispecific rate of breakage (1/min)tgrinding time (min), (s)Uinterstitial filling (–)

wiweight factor (–)

W

total sum of the weight factors (–)

x0

standard size 1 mmxi

the upper size of size class i (mm)αparameter depending on the material properties (–)

221

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

αTparameter depending on milling conditions (–)β

breakage parameter (–)γ

breakage parameter (–)Λ

parameter depending on the material properties (–)μ

parameter depending on milling conditions (–)ρbspecific gravity of the balls (g/cm3)

Φjbreakage parameter (–)

References
Austin L.G., Bhatia V.K., Experimental methods for grinding
studies in laboratory mills, Powder Technol., 5 (1972) 261–
266.
Austin L.G., Luckie P.T., Methods for determination of breakage
distribution parameters, Powder Technol., 5 (1972) 215–222.
Austin L.G., Bagga P., An analysis of fine dry grinding in ballmills, Powder Technol., 28 (1981) 83–90.
Austin L.G., Klimpel R.R., Luckie P.T., Process Engineering ofSize Reduction: Ball Milling, SME–AIME, New York, 1984.
Austin L.G., Julianelli K., Sampaio de Souza A., Schneider
C.L., Simulation wet ball milling of iron ore at Carajas,
Brazil, Int. J. Miner. Process., 84 (2007) 157–171.
Beeck R., Bestimmung der Antriebsleistung von Rohrmühlen
auf Grund von Betriebswerten.”, Zement-Kalk-Gips, 23(1970) 413–416.
Bilgili E., Scarlett B., Population balance modeling of non-linear
effects in milling processes, Powder Technol., 153 (2005)
59–71.
Bond F.C., The third theory of comminution, Trans. AIME, 193(1952) 484–494.
Bond F.C., Crushing and grinding calculations, British Chem.
Eng., 6 (1960) 378–391.
Bozkurt V., Özgür I., Dry grinding kinetics of colemanite,
Powder Technol., 176 (2007) 88–92.
Celik M.S., Acceleration of breakage rates of anthracite during
grinding in a ball mill, Powder Technol., 54 (1988) 227–233.
Charles R.J., Energy–size reduction relationships in comminution, Trans. AIME, 208 (1957) 80–88.
Chimwani N., Glasser D., Hildebrandt D., Metzger M.J.,
Mulenga F.K., Determination of the milling parameters of aplatinum group minerals ore to optimize product size distribution for flotation purposes, Miner. Eng., 43–44 (2013)
67–78.
Deniz V., Onur T., Investigation of the breakage kinetics ofpumice samples as dependent on powder filling in a ballmill, Int. J. Miner. Process., 67 (2002) 71–78.
Deniz V., Comparisons of dry grinding kinetics of lignite, bituminous coal and petroleum coke, Energ. Source, Part A, 35(2013) 913–920.
Fortsch D.S., Ball charge loading—Impact of specific power
consumption and capacity, in: Cement Industry TechnicalConference, Phoenix, Arizona, USA, Conference Record.
IEEE, April 9–14, 2006, pp. 62–72.
Fuerstenau D.W., Kapur P.C., De A., Modelling Breakage

222

Kinetics in Various Dry Comminution Systems, KONA
Powder and Particle Journal, 21 (2003) 121–132.
Fuerstenau D.W., De A., Kapur P.C., Linear and nonlinear particle breakage processes in comminution systems, Int. J.
Miner. Process., 74S (2004) 317–327.
Gupta V.K., Zouit H., Hodouin D., The effect of ball and milldiameters on grinding rate parameters in dry grinding
operation, Powder Technol., 42 (1985) 199–208.
Gupta V.K. and Sharma S., Analysis of ball mill grinding operation using mill power specific kinetic parameters, Adv.
Powder Technol., 25 (2014) 625–634.
Herbst J.A., Fuerstenau D.W., Mathematical simulation of dryball milling using specific power information, Trans.
AIME, 254 (1973) 343–348.
Herbst J.A., Fuerstenau D.W., Scale-up procedure for continuous grinding mill design using population balance models,Int. J. Miner. Process., 7 (1980) 1–31.
Ipek H., Göktepe F., Determination of grindability characteristics of zeolite, Physicochem. Probl. Miner. Process., 47(2011) 183–192.
Kick F., Das Gesetz der proportionalen Widerstände und seine
Anwendungen, Felix, Leipzig (1885).
King RP., Modeling and simulation of mineral processing systems, Oxford: Butterworth-Heinemann, p. 147, 2001.
Klimpel R.R., Austin L.G., The back-calculation of specificrates of breakage and non-normalized breakage distribution
parameters from batch grinding data, Int. J. Miner. Process., 4 (1977) 7–32.
Metzger M.J., Glasser D., Hausberger B., Hildebrandt D.,
Glasser B.J., Use of the attainable region analysis to optimize particle breakage in a ball mill, Chem. Eng. Sci., 64(2009) 3766–3777.
Nomura S., Hosoda K., Tanaka T., An analysis of the selection
function for mills using balls as grinding media, Powder
Technol., 68 (1991) 1–12.
Nomura, S., Tanaka T., Callcott T.G., The effect of mill poweron the selection function for tumbling and vibration ballmills, Powder Technol., 81 (1994) 101–109.
Ozkan A., Yekeler M., Calkaya M., Kinetics of fine wet grinding of zeolite in a steel ball mill in comparison to dry
grinding, Int. J. Miner. Process., 90 (2009) 67–73.
Rajamani R.K., Guo D., Acceleration and deceleration of breakage rates in wet ball mills, Int. J. Miner. Process., 34 (1992)
103–118.
Rittinger P.R., Lehrbuch der Aufbereitungskunde, Ernst and
Korn, Berlin (1867).
Shoji K., Austin L.G., Smalia F., Brame K., Luckie P.T., Further
studies of ball and powder filling effects in ball mills, Powder Technol., 31 (1982) 121–126.
Stamboliadis E., A contribution to the relationship of energy
and particle size in the comminution of brittle particulatematerials, Miner. Eng., 15 (2002) 707–713.
Stamboliadis E., Emmanouilidis S., Petrakis E., A New
Approach to the Calculation of Work Index and the Potential Energy of a Particulate Material, Geomater., 1 (2011)28–32.
Umucu Y., Deniz V., Bozkurt V., Çağlar M. F., The evaluationof grinding process using artificial neural network, Int. J.

Evangelos Petrakis et al. / KONA Powder and Particle Journal No. 34 (2017) 213–223

Miner. Process., 146 (2016) 46–53.
Wang X., Gui W., Yang C., Wang Y., Wet grindability of anindustrial ore and its breakage parameters estimation using
population balances, Inter. J. Miner. Process., 98 (2011)
113–117.
Yahyaei M., Powell M.S., Toor P., Tuxford A., Limpus A.,

Relining efficiency and liner design for improved plant performance, Miner. Eng., 83 (2015) 64–77.
Yekeler M., Ozkan A., Austin L.G., Kinetics of fine wet grinding in a laboratory ball mill, Powder Technol., 114 (2001)
224–228.

Author’s short biography
Evangelos Petrakis
Evangelos Petrakis is a member of the Laboratory teaching staff at the Laboratory of
Mineral Processing of the Mineral Resources Engineering Department of Technical
University of Crete (TUC). He received his diploma degree in 2001 and his M.Sc. in thefield of Geotechnology and the Environment in 2004 from the same Department. He is
now a third-year Ph.D. candidate and his research interests are in the field of simulationand optimization of grinding systems.

Elias Stamboliadis
Elias Stamboliadis graduated from the Department of Mining and Metallurgical Engineering of National Technical University of Athens (NTUA) in 1971. His Master Degree
was in Mineral Process Design from the Royal School of Mines, Imperial College, London(1973) and he received his PhD from McGill University in Montreal (1977). He was aprofessor at the Department of Mineral Resources Engineering of Technical Universityof Crete, Greece, until 2014. Now he is Professor Emeritus in the same Department. His
research interests are in mineral processing and include grinding, liberation, physicalseparation of minerals, surface chemistry of minerals and environmental applications.

Konstantinos Komnitsas
Prof. Konstantinos Komnitsas, 1961, holds a PhD degree in Hydrometallurgy and is
the director of the Laboratories (i) Waste Management and Soil Decontamination, (ii)
Ceramics and Glass Technology and (iii) Ore Beneficiation of the School of Mineral
Resources Engineering of Technical University Crete. He is expert in the fields of metallurgy, hydro- and bio-hydrometallurgy, waste management and valorization, newmaterials, soil decontamination, environmental risk assessment and LCA studies.

223

KONA Powder and Particle Journal No. 34 (2017) 224–233/Doi:10.14356/kona.2017010

Original Research Paper

Changes in Pore Structures of Porous Beds
When Subjected to Vertical Vibration †Rong Yue and Qiang Zhang *1

Department of Biosystems Engineering, University of Manitoba, Canada

Abstract
The objective of this study was to investigate the effect of vibration on critical pore structure parameters related
to flow in porous beds. The discrete element method was used to simulate particle packing in porous beds ofsoybean subjected to vibration. The porous bed was simulated as an assembly of spherical particles withdiameters randomly distributed between 5.5 mm and 7.5 mm. The simulated porous bed was subjected to verticalvibration at a fixed frequency of 15 Hz and multiple amplitudes from 0.5 to 4.0 mm, resulting in vibration
intensities from 0.45g to 3.62g (g = gravitational acceleration). The location (coordinates) of each particle wastracked during vibration. Based on the simulated spatial arrangement of particles, critical flow-related parametersof the porous bed, including porosity, tortuosity, and pore throat width were calculated. It was found that vibration
intensity of 1.81g resulted in the lowest porosity, whereas lower vibration intensity did not have enough energy todensify the bed and higher intensity produced less dense parking due to over-excitement. Local porosity
fluctuated markedly during vibration, with a general trend of decrease as vibration progressed. Vibration
noticeably affected the shape (tortuousness) of flow path. Tortuosity of the porous bed before vibration was higher(2 % to 9 %) than that after vibration. Vibration reduced the pore throat width by 18 % on average (from 3.3 mm
before vibration to an average of 2.7 mm after vibration).
Keywords: porous media, porosity, tortuosity, pore throat, vibration, discrete element method

1. Introduction
The knowledge of pore structure of porous beds (media) is critical in understanding fluid flow through porous
beds. A number of 2D (Lao H.W. et al., 2004; Ljung A.L.
et al., 2012; Xiao Z.F. et al., 2008; Yiotis A.G. et al., 2010)
and 3D models (Balhoff M.T. et al., 2007; Sobieski W. etal., 2012; Thompson K.E., 2002; Thompson K.E. and
Fogler H.S., 1997) have been developed to describe pore
structures of porous media. Those models generally deal
with porous beds with fixed pore structures. However,porous beds are often subject to such mechanical disturbances as vibration during operation. When a porous bed
consisting of randomly packed particles is vibrated, thepore structure is altered due to re-arrangement of particles. The mechanisms by which the pore structure is affected by vibration are extremely complex. An X.Z. et al.
(2005) used the discrete element method to study the densification of porous beds (sphere packing) when subjected†
1*

Received 19 February 2016; Accepted 8 April 2016
J-STAGE Advance Publication online 20 May 2016
Winnipeg, MB R3T 5V6, Canada
Corresponding author: Qiang Zhang;E-mail: Qiang.Zhang@umanitoba.ca
TEL: +1-204-474-9819 FAX: +1-204-474-7512

224

to vertical vibration. They showed that the degree of densiﬁcation was signiﬁcantly affected by both vibration
frequency and amplitude, and identified two densiﬁcation
mechanisms: pushing ﬁlling and jumping filling. Pushingfilling generally occurred at low vibration intensity inwhich the contact between spheres was maintained, while
jumping ﬁlling at high intensity in which the contact
between particles was periodically broken. An X.Z. et al.
(2009) conducted experiments to study the effect of vibration on packing density and observed that there existed
optimum values of vibration frequency and amplitude toachieve the maximum packing density. Frequency andamplitude should be considered separately in terms of
their effect on packing density. Liu C. et al. (2010) used ahigh speed camera to study the behavior of particles in abin subjected to vertical vibration. They reported that all
particles in a porous bed vibrated as a whole (a continuum) when the bed was subjected to low intensity vibration, whereas individual particles vibrated in their own
modes (as a discrete system) when subjected to high intensity vibration.
The effect of vibration on pore structures is complex.
Most studies on vibration of porous beds have been focused on particle packing, and few have explored howchanges in particle packing caused by vibration would

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Rong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

impact such properties as porosity and tortuosity which
are related to flow through the porous bed. Traditionally,
the studies of particle packing are pursued in the realm ofparticulate mechanics, while flow through porous media
has established itself as a sub-discipline in fluid mechanics. This study attempted to establish a connection between flow in porous media and particulate mechanics. A
powerful tool used in particulate mechanics—the discreteelement method, was employed to study pore structuresfrom the angle of flow in porous media. The objective of
this paper was to investigate the effect of vibration onpore structures of porous beds with an emphasis on parameters that are related to flow through the porous beds.
Specifically, a discrete element model (DEM) was used tosimulate the formation of porous beds and changes inpore structure during vibration; and an algorithm was developed to use the DEM simulation data to quantify theflow-related characteristics of the porous bed, including
porosity, flow path (length and shape), tortuosity, and porethroat diameter.

al., 2014). Based on the measurements of 24 randomly
selected kernels, the average long, intermediate, and shortaxis dimensions for soybean used in this study were 7.6,
6.7, and 5.7 mm, respectively. The corresponding geometric mean diameter was 6.6 mm, and sphericity 0.87. Considering variations in measured soybean kernel size, arage of particle sizes from 5.5 and 7.5 mm recommended
by Liu C. et al. (2010) for DEM simulations was adopted
in this study. A coordinate system was set up with x and yrepresenting the horizontal directions, z the vertical direction, and the origin at the bottom-center (Fig. 1).
A simple harmonic excitation was used to vibrate thebin. Vibration excitation was applied to the bin floor (bottom) in the vertical direction with prescribed frequencies
and amplitudes as follows:
Dz(t) = Asin(ωt)(1)

2. Materials and methodology
2.1 Discrete element simulations of porous bed
subjected to vibration
The discrete element method was originally proposed
by Cundall P.A. and Strack O.D.L. (1979) to solve problems in rock mechanics. The method has been widelyused to analyze granular and discontinuous materials. A
commercial discrete element software package PFC3D(Version 4.0 EV, Itasca Consulting Group Inc., Minneapolis,
USA) was used to build a simulation model in this study.
The PFC3D model first simulated the formation of a porous bed, during which the trajectory and rotation of eachparticle were tracked to evaluate its position and orientation in a time stepping simulation. Once the porous bed
was formed, a vibration excitation was applied. Simulations were conducted for a model bin 0.28 m in height and0.15 m in diameter, filled with soybean to a depth of
0.25 m (Fig. 1). This model bin was used by Liu C. et al.
(2010) to study the vibration of soybean in storage. The
same mechanical property parameters for the PFC3D
model (including the stored soybean) used by Liu C. et al.
(2010) were adopted in this study (Table 1). Based on themeasured sizes of bin and soybean kernels, the model bin
was simulated as a cylindrical container filled with spherical particles. It should be noted that soybean kernels are
not spherical, but they resemble a sphere with high average sphericity values (above 0.8), and therefore, most researchers have used spheres to approximate soybean
kernels when simulating bulk soybean characteristics in
DEM models to reduce computation times (Boac J.M. etFig. 1

A Schematic of simulated bin showing the locations(B1-B5) where local porosity was calculated.

Table 1 PFC simulation parameters adopted from Liu C. et al.
(2010)
Parameters

Value

Wall normal stiffness (kn), N/m

1.8 × 107

Wall shear stiffness (ks), N/m

0.9 × 107

Particle normal stiffness (kpn), N/m

4.0 × 106

Particle shear stiffness (kps), N/m

2.0 × 106

Friction coefficient between particles (μp)

0.47Friction coefficient between particle and wall
(Plexiglas) (μw)

0.3Particle density (ρp ), kg/m3

1285

Particle diameter (dp), mm

5.5–7.5

225

Rong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

A frequency of 15 Hz and amplitudes of 0.5, 1.0, 2.0,
3.0, and 4.0 mm were used in simulations. These frequency and amplitude combinations were selected toachieve the maximum densification. According to the experimental results reported by An X.Z. et al. (2009), thedense packing was achieved with vibration intensity
between 1.5g and 4.0g (the peak density occurred at
2.0g–2.5g). The vibration intensity levels, determined as
A(2πf )2, were 0.45g, 0.91g, 1.81g, 2.72g, and 3.62g for amplitudes of 0.5, 1.0, 2.0, 3.0, and 4.0 mm at 15 Hz, respectively.
Vibration was simulated in time steps of 3.3 μs. This
time step (Δt) was determined from the mass (m) and thestiffness (kpn) of particles (Δt = m/k pn ) (Itasca, 2005).
Preliminary simulations were conducted to determine thetotal length of vibration time required to reach the maximum density and it was found the 16 s was sufficient for
all vibration intensities. At each time step, the position (x,y and z coordinates) of each particle was saved in a data
file for late use in calculating flow parameters.
One of the most important parameters affecting flow
through a porous bed is porosity. In this study, both global(overall) and local porosity values were calculated basedon DEM simulation data. The global porosity was calculated as the ratio of the total volume occupied by the particles to the total volume of porous bed as follows:Φ  1  Vp / V


(2)

4 3
πr
3

(3)

Vp  

In a porous bed consisting of randomly packed particles, the pore distribution would not be uniform. Therefore, local porosity was also determined using the built-infunction in PFC3D at five locations, which were denoted as
B1(0.06, 0, 0.119), B2(0.06, 0, 0.129), B3(0.06, 0, 0.130),
B4(0.06, 0, 0.136), and B5(0.06, 0, 0.146) (three numbers
in parenthesis are x-, y- and z- coordinates, respectively)(Fig. 1). Because the bin was relatively small, these fivelocations were selected near the bin center to avoid theboundary effects. The instantaneous local porosity values
at these locations were calculated at every time step
(3.3 μs) during vibration simulation.

centroid of the base triangle to the centroids of the three
exit triangles represented three local airflow paths. The
length of each local path was calculated and the shortest
and longest paths were identified. To compare the widthof each airflow path, the open area (pore throat) between
the three particles that form a triangular face in the tetrahedron unit was calculated for all four face triangles (Fig.
2b). The open area was calculated as the area of facetriangle less the projected area of three particles on thetriangle face (Dudda W. and Sobieski W., 2014). The
equivalent pore throat width was considered as the diameter of a circle which had the same area as the open projected area.
df  2Sfπ(4)

Based on the calculated pore throat width, the widest
and the narrowest local flow paths were identified. To
construct a specific type of global flow path (runningfrom the bottom to the top of the porous bed), say thewidest path, one of the three exit triangles that had thewidest pore throat was selected as the base (inlet) triangle
for the next tetrahedron unit. This process was repeatedfrom the bottom to the top surface of the porous bed anda global flow path was obtained by connecting the local
paths in all associated tetrahedron units (Fig. 3). It should
be noted that simply connecting a series of straight lines(local paths) to form a global path resulted in sharp turnangles between local paths (Fig. 3). However, flowing air
cannot make sharp turns. Therefore, a procedure proposed by Sobieski W. et al. (2012) was used to replaceeach sharp turn angle with an arc (Fig. 3).

2.3 Tortuosity
Different definitions of tortuosity can be found in theliterature (Bear J., 2013). The geometric tortuosity wasused in this study, which was defined as the ratio of theactual flow path length to the depth of porous bed:



Le
L0

(5)

2.2 Airflow path
The airflow path was calculated using the methodology
proposed by Yue R. and Zhang Q. (2013). The porous bed
was treated as a collection of tetrahedron units, and eachtetrahedron contained a pore surrounded by four particles(Fig. 2a). Air (fluid) was assumed to enter the tetrahedron
unit from the base (inlet) triangle (ΔABC) and exit fromthe other three (exit) triangles. The lines connecting the226

Fig. 2A tetrahedron as the basic building block in a porous
bed. (a) four particles associated with a tetrahedron unit,(b) open space (a pore throat) in a tetrahedron unit.

Rong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

Fig. 3Replacement of sharp turn angle between two local
flow paths with arc.

The actual length of a (global) flow path was determined as the sum of all local path lengths. The depth ofporous bed was determined from the positions of particles
in the top layer of bed simulated by the DEM model asfollows (Sobieski W. et al., 2012):
L0  Z h  Z l  d ave(6)

2.4 Experiment
A cylindrical model bin of 0.28 m height and 0.15 m diameter was used to conduct tests to verify the DEM simulations (Fig. 4). The bin was made of Plexiglas and placedon a vibration table (Model 307-97056, Cougar Industries
Inc., Peru, IL, USA). The table was supported by threepiston-spring assemblies to permit only vertical displacement. The vibration table was driven by a pneumatic pistonwith compressed air and controlled by a pressure regulator to achieve different vibration intensities. A portablevibration analyzer (Model 654 CS, Vitec Inc., Cleveland,
OH, USA) was used to measure the vibration amplitudeand frequency and the signal was displayed on an oscilloscope (Model 190 B/C ScopeMeter, Fluke Industrial B. V.,
the Netherlands).
For each test, measured amount (mass) of soybean wasplaced into the bin through a funnel located 350 mmabove the bin, and the top surface was levelled manually
after filling. The total volume of the porous bed was calculated from the filling depth and the bin diameter, andthe packing density (ρb) was then calculated from the total volume and the mass of soybean in the bin. All linear
dimensions were measured by using a ruler with a resolution of 1 mm, and mass measured by an electronic scalewith a resolution of 0.1g.
The liquid displacement method was used to measure
the particle density (ρp) of soybean kernels. The specificFig. 4Model bin test system (the bin was 0.28 m in height andfilled to a depth of 0.25 m).

steps were as follows: (1) 100 kernels were weighed andplaced into a 50-mL flask filled with toluene, (2) the volume of 100 kernels was determined from the displaced
volume of toluene, and (3) the particle density was calculated from the measured kernel mass and volume. A steel
calibration ball of 8580 mm3 (25.4 mm in diameter) wasused to calibrate the toluene volume displacement before
running the test. The (global) porosity was calculated
from the measured bulk and particle densities as follows:Φ  1

bp(7)

3. Results and discussion
3.1 Properties of simulated porous bed andcomparison with experimental data
The DEM model generated a porous bed consisting of
18,188 particles with sizes randomly distributed between
5.5 mm and 7.5 mm (Fig. 5). Based on the measured mass
and volume of the porous bed, as well as the particle density and size, the total number of particles in the test bin
was estimated 18,498. In other words, the total number ofparticles in the simulation was 1.7 % less than the measured value. It should be noted that estimation of the total
number of particles in the bin was based on the assumption that all kernels were spherical and had an average diameter of 6.5 mm (the average of 5.5 and 7.5 mm used insimulations). However, the measured geometric mean diameter of particles was 6.6 mm (as discussed in section
2.1). Using the measured diameter of 6.6 mm, the total
number of particles in the bin was estimated to be 17,670,
which was 2.9 % less than the simulated value.
The simulated global (overall) porosity before vibration
was 0.401, while the measured value was 0.398 ± 0.006

227

Rong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

Fig. 6

Fig. 5

Simulated porous bed with 18,188 particles.

(± standard deviation, n = 6). The simulated porosity waswithin the 95 % CI (Confidence Interval) of the measured
porosity (0.391 to 0.404). The simulated minimum porosity was 0.379, whereas the measured value was
0.366 ± 0.001 after vibration (at 1.0g intensity). The relative change (decrease) in porosity after vibration was
5.5 % in simulations, which was lower than that in measurements (8.0 %). Liu C. (2008) reported relative changes
between 0.7 % to 6.1 % in porosity of soybean in a model
bin after vibration.
The simulated local porosity, calculated from the PFC3D
built-in function in a small region within the porous bed,
varied with the location. Specifically, the local porosity
before vibration was 0.402, 0.395, 0.395, 0.393, and 0.389
at the five selected locations (B1–B5), respectively. The
average of the five locations was close to the measured
value (0.395 vs. 0.398). Local porosity also varied withboth vibration duration and intensity, which is discussed
in the next sections.

3.2 Variation of porosity during vibration
Fig. 6 shows typical curves of simulated local porosity
during vibration. The minimum porosity was reached atabout 9 s for all locations. The local ﬂuctuations in porosity were apparent, and they were attributed to the cyclic
nature of vibration which compressed and relaxed the porous material during each vibration cycle (An X.Z. et al.,2005). The magnitudes of these local fluctuations could
be as high as 0.018 (e.g., from 0.378 to 0.396 in porosity).
The overall trend was that local porosity decreased as vibration progressed, but not monotonically (large global
fluctuations were apparent). Taking location B2 as an example, the porosity increased quickly from the initiallevel of 0.395 to 0.430 in 0.04 s, and then dropped to 0.370
at about 0.3 s (Fig. 6). This observed pattern was similar

228

Simulated local porosity during vibration at locations
B1 and B2, and the average porosity of all five locations for vibration intensity of 1.8g.

to the experimental observations by Nowak E.R. et al.
(1998), but did not fully agree with the DEM simulation
result reported by An X.Z. et al. (2005). They reported
that packing fraction (opposite to porosity) increased
monotonically with time of vibration (local fluctuations
also existed). It should be noted that it was not clear if theporosity reported by An X.Z. et al. (2005) was the local or
global porosity. Given that the pore structure in a porous
bed varies from location to location, one would expectthat local porosity varies with location. An X.Z. et al.
(2005) observed that macro-pores (pores large enough toaccommodate a particle) existed in loosely packed sphereassemblies. Therefore, high porosity could exist locallywhere a macro-pore exists. During vibration, particles
re-arrange themselves and macro-pores would be destructed (An X.Z. et al., 2005). However, it was also possible that some macro-pores were created at an instantthrough particle re-arrangement during vibration, resulting in high local porosity. These instant macro-pores
might not be stable and would be destructed quickly byvibration. During vibration, destruction and creation ofmacro-pores might occur repeatedly, resulting in fluctuations in local porosity; however, more macro-pores would
be destructed than created, leading to a general trend of
decrease in local porosity, as well as gradual decrease in
the global porosity. A similar pattern of fluctuations in local porosity were observed by Liu C. (2008) in his experiments to investigate the vibration effect on porosity ofsoybean. He used a high speed camera to record movements of soybean kernels in a transparent bin made of
Plexiglas. Based on the recorded video images, he tracked
three particles that formed a triangle and calculated thearea of this triangle at different times during vibration.
He considered that changes in the triangle area reflectedchanges in local porosity (i.e., increasing triangle area reflected increasing in porosity, and vice versa). The relativechange in local porosity was then estimated as S/S0, whereS0 was the initial area of triangle and S the triangle area at
a given time. It was apparent that the relative change inlocal porosity fluctuated during vibration, with an overall

Rong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

Fig. 7

Fig. 8Relative change in area of triangle formed by three particles during vibration at 25 Hz frequency and 0.45 mm
amplitude (based on data reported by (Liu C., 2008).

Simulated average local porosity during vibration at intensities of 0.45g, 1.81g, and 3.62g.

trend of decrease, similar to what was observed in his
study (Fig. 7).

3.3 Effects of vibration intensity on porosity
Vibration intensity had noticeable effect on porosity
during vibration (Fig. 8). The higher the intensity, thegreater the fluctuations. Among the five intensity levels
simulated (0.45g, 0.91g, 1.81g, 2.72g, and 3.62g), 1.81g resulted in the lowest porosity (Fig. 9). In other words, there
was a “critical” vibration intensity (1.81g) that resulted ina maximum reduction in porosity, and lower or higher vibration intensities generated less dense porous structure.
A similar observation was reported by An X.Z. et al.
(2009). They measured changes in packing density of anassembly of spheres with diameter of 5.02 mm in a229.70-mm diameter container when subjected to vibration between 0.25g and 5.0g, and observed that the highest packing density occurred in the mid-range vibration
intensities between 2.0g to 2.5g. Theoretically speaking,if the acceleration of particles in a porous bed is greaterthan 1g in the vertical direction, the particles could become “afloat” once in every vibration cycle, which could
result in a high probability of creating a large void into
which another particle can slide, thus providing the best
opportunity to form a dense structure. Therefore, lower
vibration intensity (< 1g) would produce less reduction in

Fig. 9Effect of vibration intensity on porosity.

porosity. However, it should be noted that a porous bed isa discrete system and not all particles vibrate at the same
frequency and amplitude as the excitation applied on thecontaining structures (the bin bottom in this study). Energy transfer in the porous bed when subjected to vibration is extremely complicated, however it is a reasonableassumption that not all vibration energy from the vibration source applied at the bin bottom would be transferredto the particles in the porous bed because of energy dissipation by collision and friction among particles and between particle and the containing structures. Energy
dissipation in granular materials during vibration has
been shown to be significant in many studies in the areaof particle impact damping (Friend R.D. and Kinra V.K.,
2000). Energy dissipation offers a plausible conjecture forthe reason why the maximum reduction in porosity occurred at 1.8g instead of 1g. This also means that the critical vibration intensity that produces the maximum
reduction in porosity would vary with many factors that
influence energy transfer in porous beds, including particle size and shape, particle density, friction between particles, friction between par ticles and containing
structures, restitution coefficient, and even porosity itself,
which dictates the distances (clearances) between particles and between particles and containing structure.
An X.Z. et al. (2009) explained that too high energy input would over-excite particles in porous beds, which
would lead to less dense structures. While low amplitude
vibration induces subtle changes in the microstructure ofa granular material, resulting in higher bulk density,strong vibration leads to fluid-like behavior of the granular material, resulting in lower bulk density (Rosato A.D.
et al., 2002). Hunt M.L. et al. (1994) conducted experiments to study the behavior of a granular material subjected to vertical vibration at acceleration levels from 1g
to 5.5g. They observed that the volume of the granularmaterial began to expand at acceleration of 2g.
At high vibration intensity (3.62g), porosity fluctuatedmarkedly both locally and globally (details are discussed in
Section 3.2). In contrast, at low vibration intensity of 0.45g,
porosity only fluctuated slightly around its average (Fig. 8).

229

Rong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

Fig. 10 Typical simulated airflow paths (as a series of associated tetrahedron units) for different vibration intensities.

This meant that vibration energy at low intensity was
not sufficient to cause significant particle re-arrangement,
resulting in little change in porosity. Liu C. et al. (2010)
used a high speed camera to study vibration of individual
particles in a bin subjected to vertical vibration and observed that all particles in the bin vibrated as a continuumwith the same frequency and amplitude as the excitation
at low intensities (< 1g), whereas individual particles vibrated “irregularly” (not following the harmonic excitation) when the vibration intensities were high (> 1g).

Fig. 11 Variation of simulated tortuosity with vibration intensities (each data point represents the average of threesimulated paths and the error bar indicates the minimum and maximum values).

3.4 Effect of vibration on airflow pathVibration changed the shape and length of airflow path
(Fig. 10). Qualitatively speaking, the deviation of flow
path (the exit point) from the entry point was less before
vibration than after vibration. This was probably because
higher flow resistance in more dense porous beds after vibration made it more difficult for flow to go upwards directly. Nwaizu C. and Zhang Q. (2015) used imaging
techniques (with smokes) to study airflow through grainbulks subjected to vibration and observed that reductionin porosity after vibration caused more lateral “branching” of flow when air entered the grain bulks vertically. In
other words, more air was forced to flow in non-verticaldirections after vibration. It was interesting to note thatwhile there was less degree of deviation of flow path fromthe entry point before vibration, but more sharp local
turns was observed. This became apparent when comparing the flow paths between 0g and 1.81g vibration intensities. Specifically, many small sharp local turns could beseen in the 0g path, whereas long sections of relatively
smooth line existed in the 1.81g path (Fig. 10).
Both the deviation of flow path from the entry point
and the local turns would result in longer flow paths(higher tortuosity). To quantify the differences in airflow
path among different vibration intensities, the simulated
tortuosity values were compared (Fig. 11). The tortuosity
decreased after vibration generally, but no clear pattern
could be identified. A maximum reduction of 9 % was observed for vibration intensity of 1.81g and a minimum

230

Fig. 12 Pore throat width of local flow paths from the bin bottom (tetrahedron no. 1) to the top under different vibration conditions.

2 % for 2.72g. Many studies have shown that tortuousnessof flow through porous media decreases with porosity(Pisani L., 2011). As discussed earlier, the lowest porosity
occurred at 1.81g. Therefore, it was expected that flow
path for 1.81g vibration would be less tortuous. This alsomeant that sharp local turns (dominant before vibration)
had more effect on the tortuousness of flow path than did
the deviation (dominant after vibration) from the entry
point. Although the maximum reduction in tortuosity corresponded to the minimum porosity at 1.81g, the patternof variation in tortuosity with vibration intensity did notquite follow that for porosity shown in Fig. 9.
The pore throat width (the equivalent diameter) of thewidest local flow path was calculated for each tetrahedron
unit associated with a global flow path (Fig. 12). It wasobserved that the throat width varied randomly between
1.3 and 4.6 mm, with one exception before vibration—ahigh value of 6.8 mm was observed. A pore throat of
6.8 mm was apparently greater than the average particle
diameter. This meant that a macro-pore, defined as a porelarge enough to accommodate a particle (An X.Z. et al.,2005), existed in the porous bed before vibration, but nomacro-pores were observed after vibration at any of theRong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

tion was higher (2 %–9 %) than that after vibration.
(5) Pore throats larger than the average particle diameter(due to macro-pores) existed in the porous bed before
vibration, but were not observed after vibration.
(6) Vibration reduced the pore throat width. The highest
percentage of pore throats was in the range of
3–4 mm before vibration, and reduced to 2–3 mm
after vibration. The reduction in average pore throat
width was 22 % after vibration.
Fig. 13 Distribution of pore throat width under different vibration intensities.

five vibration intensities studied.
To further examine the effect of vibration intensity on
pore throat, the distributions of throat width were plotted(Fig. 13). The highest percentage of pore throats was in
the 3–4 mm range before vibration, while the highest percentage after vibration was in the 2–3 mm range. The
percentage of pore throats in the rages of 1–2 mm and
2–3 mm before vibration was much lower than that after
vibration, whereas the percentage in 4–5 mm before vibration was much higher than that after vibration. Theseobservations indicated that vibration reduced the porethroat width. On average, the pore throat width was 3.3,
2.6, 2.8, 2.7, 2.7, and 2.7 mm for 0g (before vibration),
0.45g, 0.91g, 1.81g, 2.72g, and 3.62g, respectively. While
the average pore throat before vibration was greater(22 %) than those after vibration, the differences in porethroat width among the five different vibration intensities
were negligible (< 0.2 mm).

4. Conclusions(1) The discrete element model was capable of predicting some critical pore structure parameters for flow
through porous beds, including porosity, tortuosity,
pore throat width, and flow path geometry (lengthand shape).
(2) The influence of vibration on porosity was clearly
demonstrated. A vibration intensity of 1.81g resulted
in the lowest porosity (6 % lower than that before vibration). Lower vibration intensity did not haveenough energy to cause much change in porosity,
while higher intensity over-excited particles, producing less dense packing than did 1.81g.
(3) Local porosity fluctuated markedly during vibration,
probably due to repeated formation and destructionof instant (unstable) macro-pores (defined as pores
large enough to accommodate a particle).
(4) Tortuousness of flow path was noticeably affected byvibration. Tortuosity of the porous bed before vibra-

NomenclatureA

vibration amplitude (m)dave average particle diameter (m)dfequivalent pore throat width of local path (m)dpparticle diameter (mm)Dzdisplacement in the vertical direction (m)fvibration frequency (Hz)gacceleration due to gravity (m/s2)

kn

wall normal stiffness (N/m)

kpn

particle normal stiffness (N/m)

kps

particle shear stiffness (N/m)

kswall shear stiffness (N/m)

L0depth of porous bed (m)Leactual length of flow path (m)mparticle mass (kg)rparticle radius (m)Sthe triangle area formed by three particles at a given time
(m 2)

S0

the initial area of triangle formed by three particles (m 2)

Sfopen projected area between three particles in a triangularface of tetrahedron (m 2)ttime (s)Vtotal volume of porous bed (m3)VPtotal volume occupied by particles (m3)xx-coordinate (m)yy-coordinate (m)zz-coordinate (m)Zhz-coordinate of the highest sphere centroid in the porous
bed (m)Zlz-coordinate of the lowest sphere centroid in the porous
bed (m)Σ

summationΔtsimulation time step (s)τtortuosity

231

Rong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

ωangular velocity (rad/s)Φsimulated average global porosity (decimal fraction)

ρbpacking density (kg/m3)

ρpparticle density (kg/m3)μpfriction coefficient between particles

μw

friction coefficient between particle and wall

References
An X.Z., Li C.X., Yang R.Y., Zou R.P., Yu A.B., Experimental
study of the packing of mono-sized spheres subjected to
one-dimensional vibration, Powder Technology, 196 (2009)50–55.
An X.Z., Yang R.Y., Dong K.J., Zou R.P., Yu A.B., Micromechanical simulation and analysis of one-dimensional vibratory sphere packing, Physical Review Letters, 95 (2005)
205502.
Balhoff M.T., Thompson K.E., Hjortsø M., Coupling pore-scale
networks to continuum-scale models of porous media,
Computers & Geosciences, 33 (2007) 393–410.
Bear J., Dynamics of fluids in porous media, Dover Publications, 2013.
Boac J.M., Ambrose R.P.K., Casada M.E., Maghirang R.G.,
Maier D.E., Applications of discrete element method inmodeling of grain postharvest operations, Food Engineering Reviews, 6 (2014) 128–149.
Cundall P.A., Strack O.D.L., A discrete numerical model for
granular assemblies, Géotechnique, 29 (1979) 47–65.
Dudda W., Sobieski W., Modification of the pathfinder algorithm for calculating granular beds with various particle
size distributions, Technical Sciences, 17 (2014) 135–148.
Friend R.D., Kinra V.K., Particle impact damping, Journal ofSound and Vibration, 233 (2000) 93–118.
Hunt M.L., Hsiau S.S., Hong K.T., Particle mixing and volumetric expansion in a vibrated granular bed, Journal of Fluids
Engineering, 116 (1994) 785–791.
Itasca, User’s guide pfc3d particle flow code in 3 dimensions,
Itasca Consulting Group, Inc., Minneapolis, Minnesota,
USA, (2005).
Lao H.W., Neeman H.J., Papavassiliou D.V., A pore network232

model for the calculation of non-darcy flow coefficients in
fluid flow through porous media, Chemical Engineering
Communications, 191 (2004) 1285–1322.
Liu C., Study on bulk solid movement and forces in bins, Ph.D.,
China Agricultural University, 2008.
Liu C., Zhang Q., Chen Y., Discrete element simulations ofvibration characteristics of bulk grain in storage bins,
Transactions of the ASABE, 53 (2010) 1653–1659.
Ljung A.L., Frishfelds V., Lundstrom T.S., Marjavaara B.D.,
Discrete and continuous modeling of heat and mass transport in drying of a bed of iron ore pellets, Drying Technology, 30 (2012) 760–773.
Nowak E.R., Knight J.B., Ben-Naim E., Jaeger H.M., Nagel
S.R., Density fluctuations in vibrated granular materials,
Physical Review E, 57 (1998) 1971–1982.
Nwaizu C., Zhang Q., Characterizing tortuous airflow paths ina grain bulk by using smoke visualization, Canadian Biosystems Engineering, 57 (2015) 3.13–13.22.
Pisani L., Simple expression for the tortuosity of porous media,
Transport in Porous Media, 88 (2011) 193–203.
Rosato A.D., Blackmore D.L., Zhang N., Lan Y., A perspective
on vibration-induced size segregation of granular materials,
Chemical Engineering Science, 57 (2002) 265–275.
Sobieski W., Zhang Q., Liu C., Predicting tortuosity for airflow
through porous beds consisting of randomly packed spherical particles, Transport in Porous Media, 93 (2012) 431–451.
Thompson K.E., Pore-scale modeling of fluid transport in disordered fibrous materials, AIChE Journal, 48 (2002) 1369–
1389.
Thompson K.E., Fogler H.S., Modeling flow in disorderedpacked beds from pore-scale fluid mechanics, AIChE Journal, 43 (1997) 1377–1389.
Xiao Z.F., Yang D.Y., Yuan Y.J., Yang B.B., Liu X.D., Fractal
pore network simulation on the drying of porous media,
Drying Technology, 26 (2008) 651–665.
Yiotis A.G., Tsimpanogiannis I.N., Stubos A.K., Fractal characteristics and scaling of the drying front in porous media: A
pore network study, Drying Technology, 28 (2010) 981–990.
Yue R., Zhang Q., Modeling airflow path through grain bulks
using the discrete element method, Annual Meeting of
Canadian Society for Bioengineering (CSBE), CanadianSociety for Bioengineering, Saskatoon, Saskatchewan,
Canada, Paper No. CSBE13-031 (2013).

Rong Yue et al. / KONA Powder and Particle Journal No. 34 (2017) 224–233

Author’s short biography
Rong Yue
Rong Yue is a PhD student in the Department of Biosystems Engineering, Universityof Manitoba, Canada. She received her M.S. and B.S. degrees from Northwest A&F University in China. Her current research focuses on modeling airflow through porous beds
using the discrete element method and pore-scale models. She presented several conference papers on airflow paths through grain bulks and resistance to airflow in grain bulks.

Qiang Zhang
Dr. Qiang Zhang is a Professor in the Department of Biosystems Engineering, University of Manitoba, Canada, and a Registered Professional Engineer (P.Eng.) in the Province of Manitoba. He received his Ph.D. and M.S. degrees in Agricultural Engineeringfrom Pennsylvania State University and B.S. degree in Mechanical Engineering from
Hefei University of Technology, China. Dr. Zhang’s research interests include constitutive behavior of granular materials, loads in bulk solids storage structures, and finite anddiscrete element modeling of bulk solids systems.

233

KONA Powder and Particle Journal No. 34 (2017) 234–240/Doi:10.14356/kona.2017011

Original Research Paper

Visible Light Photocatalytic Bacterial Inactivationon Titanium Dioxide Coatings †Vignesh Nandakumar 1, Zhao Han 1, Zachary Fritz 2,
Vijay Krishna 4, Ben Koopman 3 and Brij Moudgil 1*
1

Department of Materials Science and Engineering, University of Florida, USA
Department of Chemical Engineering, University of Florida, USA
3
Department of Environmental Engineering, University of Florida, USA
4
Department of Biomedical Engineering, Cleveland Clinic, USA
2

Abstract
The increasing number of touch surface mediated infections has steered research to look for alternative strategies
that can prevent infection transmission via pathogen inactivation on surfaces. Anatase, a crystalline form oftitanium dioxide, shows strong UV induced photocatalytic properties. However, nanoparticles of anatase havebeen found to inactivate organic contaminants in the visible spectrum. Degradation of mordant orange andinactivation of S. aureus was evaluated on anatase surfaces under visible light band pass filters across the visible
light region. Inactivation shows a strong co-relation to the absorption spectrum of the dye/microbe on the surfaceof the anatase coating. The phenomenon is similar to dye sensitized solar cells and was not found to exist in
coatings with a higher bandgap such as amorphous silica. Photocatalytic nano-crystalline anatase coatings holdpotential as a visible light active disinfectant to inactivate microbes on touch surfaces over long periods of time.
Keywords: TiO2, visible light photocatalysis, antibacterial coatings

1. Introduction
Decontamination of organic and biological pollutants
has gained significant attention owing to the increase in
the number of infectious diseases. The increase in thenumber of hospital acquired infections in the last decade
has led to significant research in developing alternate
strategies for microbial inactivation and prevention of thetransfer of such pathogens.
Hospital acquired infections or nosocomial infections
are defined as infections that emanate from a stay in medical facilities and are caused by strains of microbes thathave gained resistance to several bactericidal agents(Lobdell et al., 2012). These infections are extremely difficult to treat and have dramatically increased direct costs
ranging from $28 billion to $45 billion in the UnitedStates of America (Scott II, 2009). The number of deaths
caused by Methicillin Resistant Staphylococcus aureus(MRSA) account to 99,000 annually in the Unites States†

Received 22 July 2015; Accepted 12 May 2016
J-STAGE Advance Publication online 9 July 2016
1,2,3
Gainesville, Florida 32611, USA
4
Cleveland, Ohio 44195, USA* Corresponding author: Brij Moudgil;
E-mail: bmoudgil@perc.ufl.edu
TEL: +1-253-846-1194 FAX: +1-352-846-1196234

alone (Klein et al., 2007). The transfer of such pathogens
causing nosocomial infections is primarily touch surface
mediated (Otter et al., 2013; Weber et al., 2013). Thus
there is a clear need for inactivation of microbes on thesesurfaces to reduce the number of nosocomial infections.
Photocatalytic coatings have shown tremendous potential for inactivating organic contaminants and bacteria(Erkan et al., 2006; Lilja et al., 2012; Wei et al., 2014).
The principle of photocatalysis for microbial inactivation
has been discussed in many reviews and primarily involves the generation of reactive oxygen species (ROS)
including hydroxyl free radicals and superoxide free radicals (Fujishima et al., 2008; Henderson, 2011; Linsebigler
et al., 1995). Titanium dioxide is one such heterogeneous
photocatalyst that is an extremely efficient generator of
ROS in the presence of UV light (Ishibashi et al., 2000;
Konaka et al., 1999). Inactivation of various bacteria,
fungi and oxidative stresses on mammalian cells has beenshown due to the ROS generated on UV photo-excited
TiO2 surfaces (Chen et al., 2009; Sunada et al., 2003;
Tucci et al., 2013). In addition, complete degradation oforganics on TiO2 surfaces ensures that there is no residue
on the surface thereby reducing ‘masking effects’ which
often reduce the efficacy of active coatings. Although
TiO2 surfaces show very high potential for photocatalysis,
they have only been used for surfaces that can receive a

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Vignesh Nandakumar et al. / KONA Powder and Particle Journal No. 34 (2017) 234–240

significant amount of UV light. Anatase, which is a crystalline form of TiO2, has very good photocatalytic properties but absorbs only in the near UV and UV region andhas been largely ignored for use in indoor applications.
This has led to extensive research into the area of visible
light activated photocatalysis on modified titanium dioxide surfaces (Mitoraj et al., 2007; Yu et al., 2003). Dopingof TiO2 with nitrogen and carbon has been shown to reduce the bandgap and thus enhance visible light photocatalysis. Asahi and co-workers have modified titanium
dioxide with nitrogen doping to show significant increasein the photocatalytic activity under visible light (Asahi etal., 2001). Shen and group have also shown visible light
photocatalysis on anatase surfaces when doped with carbon (Shen et al., 2006). The evaluation of the antibacterial
performance of unmodified titanium dioxide in the presence of visible light however has largely been ignored primarily owing to a very small contribution of visible lightin anatase induced photocatalysis.
However TiO2-based dye sensitized solar cell applications were based on the premise that light-absorbing organic molecules can insert electrons into the TiO2
conduction band, generate free electrons on TiO2 and activate photocatalysis (Bach et al., 1998; Krasovec et al.,2009; Oregan and Gratzel, 1991; Yu et al., 2003) We extended this concept using contaminant dyes and microbes
to sensitize an otherwise inactive anatase photocatalyst to
become active in the visible spectrum(Krishna et al.,2015). This property of anatase can be exploited for coating indoor surfaces which over a long period of time canlead to significant inactivation of microbes and thus reduction in the number of touch surface mediated infections.

2. Experimental section
2.1 Chemical reagents
Most Chemicals were purchased from Fisher Scientific.
Mordant Orange was acquired from Sigma-Aldrich Inc.
(St. Louis, MO) and anatase and silica were obtained from
MKnano (M K Impex Corp. Mississauga, TO).

2.2 Microbial culture and enumeration
Staphylococcus aureus (ATCC 25923) was cultured in
Tryptic Soy Agar (Becton, Dickinson and Company,
Sparks, MD) and grown for 48 hours in the incubator at
37 °C. An individual colony from the streak plate was recovered and inoculated in Tryptic Soy broth (TSB) andgrown to an OD600 of 0.6 at 37 °C in a shaker incubator at
150 RPM. The culture was washed thrice and harvested
by centrifugation at 4800 RCF and 4 °C. The final concentration for use on the tiles was set to 2 × 106 cells per

ml. Enumeration of bacteria were performed using thepour plate method. 0.1 ml of bacterial suspension wastransferred to a petri dish and covered with melted TSA
and mixed well. After the agar solidifies at room temperature the plates were incubated at 37 °C for 24 hours.

2.3 Coating preparation
Coatings were made of anatase (7 nm crystallite size)and silica (15 nm crystallize size). The coatings were prepared in accordance with a protocol suggested by Bai etal (Bai et al., 2012). 10 mg of anatase was added to 10 mlof water to have a final concentration of 0.1 wt% and sonicated (Misonix Sonicator 3000, Farmingdale, NY) for 30
minutes at 90 W with NaOH as the electrostatic stabilizer
at a pH of around 9.5. The dispersions were then applied
as uniform coatings on ceramic tiles of dimensions
2.5 cm × 2.5 cm, with a total volume of 0.4 ml of coating
spread over the surface of the tile. A second coat of anatase was applied after the tiles were dried overnight to ensure complete coverage on the surface of the tile. Coatingsof silica were prepared the same way and all coatings
were stored in a dark cabinet until use.

2.4 Estimation of bacterial inactivation
Tiles coated with anatase and silica were inoculatedwith 0.1 ml of bacteria at a starting concentration of
1–2 × 106 cells per ml. The tiles were then exposed to alight intensity of 1.8 W/m2 under fluorescent lamps (General Electric model T8 Ultramax F28T8-SPX41) for 18 h
at 23 °C and 40 % RH. A dark control with anatase double coat was also included for the experiments. Microbial
recovery from tiles was performed by the sonication of
the inoculated tiles at 90 W in 20 ml DI water for 1 minute. 0.1 ml of the bacteria suspension after sonication wasenumerated as mentioned before. A zero hour reading
taken immediately after initial bacteria inoculation on selected tiles was acquired to estimate the number of bacterial cells that can be recovered from the tile. The tiles
were then exposed to visible light under various filters toassess the performance of the coatings at the limits of theabsorption spectrum of the contaminant. The recovered
microbes were then cultivated in TSA and enumeratedusing a colony counter. Effect of sonication on bacterial
viability was also assessed to eliminate possible inactivation due to sonication% Bacterial Inactivation =C 0 – Ct
*100
Ct

(1)

Where C0 is the concentration of bacteria in CFU at time
t = 0 and Ct is the concentration of the bacteria at an exposure of time‘t’.

235

Vignesh Nandakumar et al. / KONA Powder and Particle Journal No. 34 (2017) 234–240

2.5 Estimation of dye degradation
Tiles coated with anatase were inoculated with 0.002 mlof mordant orange at a concentration of 100 ppm. The
tiles were then exposed to a light intensity of about~1.8 W/m2 for 24, 48, 72 and 96 h at 23 °C and 40 % RH.
Reflectance of coated and uncoated tiles were measured
with a spectrometer. Absorbance was calculated as thenegative log10 of reflectance expressed as a fraction. Dye
degradation was calculated by comparing the reflectancemeasurements of coated tiles at 0 hour exposure to thegroups exposed to light.
% dye degradation =A0 – At*100
At

Fig. 1

Absorption spectrum of anatase (green), S. aureus (yellow) and Mordant Orange (Orange) across UV and visible spectrum obtained from reflectance measurementson ceramic tiles.

Fig. 2

Spectral distribution of the fluorescent lamp used forbacterial inactivation on tile surfaces. Image courtesyof General Electric Company (General Electric, 2016).
Copyright: (2016) G.E.

Fig. 3Inactivation of S. aureus on anatase, silica and in thedark over 18 hours at 23 °C and 40 % RH.

(2)

Where A0 is the absorption of dye at time t = 0 and At isthe absorption of the dye at an exposure of time‘t’.

2.6 Effect of optical cut-off filters on theinactivation experiments
Optical filters with cut-off wavelengths were chosen inaccordance with the absorption spectrum of the dye andthe bacteria. Anatase tiles inoculated with bacteria or dye
and were placed under 400, 495 and 550 nm filters. Bacterial and dye degradation was calculated for an exposureof 18 and 24 hours respectively at 23 °C and 40 % RH.
The results were then plotted comparing the photocatalytic performance under different cut off filters.

3. Results and discussion
3.1 Absorbance spectrum of mordant orange,
anatase and S. aureus
The absorbance spectrum of mordant orange, S. aureus
and anatase on a ceramic tile substrate was acquired using the spectrometer. Fig. 1 illustrates an absorption cut
off for anatase at 380 nm. The absorption of staphylococcus aureus and mordant orange extends well into the visible spectrum with a cut off at around 550 nm.
The spectrum indicates that any absorption in the visible region is accounted for by the dye or the bacterium on
the surface of anatase.

3.2 Photocatalytic inactivation of Staphylococcus
aureusPhotocatalytic inactivation of S. aureus was estimatedon coatings of anatase and silica. Anatase coating, kept in
the dark, was included as a negative control. Test coatings
were exposed to a light intensity of about 2 W/m 2, 32 °C,
40 % RH and an exposure time of 18 h. The spectral dis-236

tribution of the fluorescent lamp used for the experimentis shown in Fig. 2 (GeneralElectric). A 400 nm filter wasused for all visible light photocatalysis to eliminate thesmall amount of UV present in the luminescent bulb
spectra. Fig. 3 shows about 45 % photocatalytic inactivation of S. aureus on unmodified anatase surfaces. The in-Vignesh Nandakumar et al. / KONA Powder and Particle Journal No. 34 (2017) 234–240

activation on silica and dark control are significantly
lower and are attributed to desiccation of microbes overthe period of 18 h. The experiment indicates a synergy
between the bacteria and the anatase surface introducing
photocatalysis on anatase surfaces.
The lower inactivation on the silica and dark surfaces
also eliminates the contribution of photolysis or oxidative
degradation of dye under experimental conditions.

3.3 Contribution of bacteria and dye(contaminant) to the process of visible light
photocatalysis
The contribution of contaminants to the process of visible light induced photocatalysis was studied by measuring
the inactivation of S. aureus as the model bacterium and5-(4-Nitrophenylazo) salicylic acid, (mordant orange) asa model dye. The kinetics of degradation for mordantorange on unmodified anatase surface were studied as amodel for the bacteria. The dye was placed under longpass filters with exposure for 24, 48, 72 and 96 hours. The
absorption spectrum of mordant orange exhibits a peak at
410 nm and gradually decreases with a cut off at 550 nm
as shown in Fig. 1. Long pass filters cut off wavelengths
at 400 nm, 495 nm and 550 nm filters were chosen to ensure non-interference of UV light (400 nm filter), reduced
absorbance of dye (495 nm) and cut off of dye absorption(550 nm).
Dye degradation data was analyzed by comparing thereflectance data of dye at the start of the experiment to thetiles at different time points of the experiment. Results in
Fig. 4 illustrate a reduction in photocatalytic activity ofanatase surfaces with increasing wavelength cut off filters. A sharp decrease in the photocatalytic activity at the
495 filter is seen due to significantly decreased participation of the dye in the process. The photocatalytic degradation under a 550 nm is comparable to the dark control

Fig. 4Degradation of dye on anatase coatings over a period of
24 h, 48 h, 72 h and 96 h under various optical filters.
The absorption of the dye clearly sensitizes the visible
light photocatalysis of anatase.

indicating no participation of dye in the process. The reduction in light intensities under various filters were recorded and show a maximum reduction of 5 % at the
495 nm cut off and 16 % at the 550 nm cutoff wavelengths. Subsequently, inactivation of S. aureus on anatase surfaces was carried out under similar spectral
conditions to validate the mechanism of photocatalytic
inactivation with biological contaminants. The resultsplotted in Fig. 5 show that S. aureus inactivation undervisible light gradually decreased with increasing cut off
limits up to 550 nm in accordance with dye degradation
results. It is also noted from the absorption spectrum in
Fig. 1 that the only component absorbing in the visible
spectrum is the bacteria. Additionally, the trend of bacterial inactivation on anatase in visible light follows thetrend in absorption intensity of bacteria. This indicatesthe antibacterial potential of unmodified anatase with visible light.
From the data obtained we hypothesize that the mechanism of microbial inactivation on unmodified anatase surfaces is similar to that of TiO2 dye sensitized solar cells(DSSC). In TiO2 based DSSC’s irradiation with visible
light leads to (i) excitation of the dye molecule, (ii) subsequent oxidation of dye molecule, and (iii) transfer of electrons from the dye molecule to the TiO2 conduction band.
The electrons then diffuse through the circuit to thecounter electrode. The electrolyte is reduced from I3- to Iwhich diffuses and combines with the oxidized dye molecule to complete the circuit. Similarly in a contaminant
activated photocatalyst, the dye or microbe acts as theelectron source when irradiated with visible light. Fig. 6
is a schematic of the hypothesized mechanism of actionon unmodified anatase surfaces when in contact with or-

Fig. 5Inactivation of S. aureus on anatase under different optical filters over 18 hours at 23 °C and 40 % RH. The
inactivation of bacteria decreases with the increase in
cut off wavelengths of the long pass filters from 400 to
550 nm.

237

Vignesh Nandakumar et al. / KONA Powder and Particle Journal No. 34 (2017) 234–240

and contribution to this project. Experimental work was
carried out the University of Florida Water Reclamation
and Reuse laboratory and at the Particle Science andTechnology Laboratory. We would like to thank Dr. Wei
Bai for the SEM micrographs.

Nomenclatures

Fig. 6

Schematic for possible mechanism of inactivation on
unmodified TiO2 surfaces (Krishna et al., 2016).

ganic contaminants such as microbes(Krishna et al.,2015). Electrons inserted in the conduction band of TiO2
catalyze the small molecules like oxygen and water into
super oxide free radicals. These reactive radicals in turnoxidize most organic molecules to CO2 and water. The
generation of free radicals in the photocatalyst with availability of microbial contaminants leads to their continued
inactivation.

4. Conclusions
The present study illustrates the potential of unmodified nano-crystalline anatase as a photocatalytic disinfectant in visible light when in contact with microbes. While
the kinetics of inactivation of bacteria on such unmodified
surfaces is not as fast as in the case of conventional disinfectants, photocatalytic coatings can last much longer andhave a very high cumulative contribution towards surface
disinfection. Masking by inactivated microbes often hampers the performance of active coatings by forming abarrier between the active species and the microbes.
However, the complete degradation of microbes (organic
contaminants) on anatase surfaces ensures no ‘masking
effect’ unlike on other potent antimicrobial surfaces likecopper and silver. Thus, anatase surfaces in comparison
can function for long inactivation periods. We would also
like to further indicate that these coatings can be applied
to surfaces and complemented with additional disinfection treatments to have increased efficacies of surface disinfection. A combination of disinfection approaches canbe used for surfaces that do not receive large amounts of
UV light and kill pathogens that are transferred throughtouch mediation.

Acknowledgements
The authors would like to acknowledge the National
Science Foundation Grant No. 1127830 for their financial
support and the industry members of the Centre for Particulate and Surfactant Systems (CPaSS) for their help

238

CFU/mlColony forming units per ml

A0

Absorbance at time 0At

Absorbance at time t

C0

Concentration at time 0 (CFU/ml)

Ct

Concentration at time t (CFU/ml)

RCF

Relative Centrifugal Force

RH

Relative Humidity (%)

RPM
W/m

2

Rotations per minuteWatt/ Square meter

References
Asahi R., Morikawa T., Ohwaki T., Aoki K., Taga Y., Visible-­light photocatalysis in nitrogen-doped titanium oxides,
Science, 293 (2001) 269–271.
Bach U., Lupo D., Comte P., Moser J.E., Weissortel F., SalbeckJ., Spreitzer H., Gratzel M., Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies, Nature, 395 (1998) 583–585.
Bai W., Krishna V., Wang J., Moudgil B., Koopman B.,
Enhancement of nano titanium dioxide photocatalysis intransparent coatings by polyhydroxy fullerene, Appl. Catal.
B-Environ., 125 (2012) 128–135.
Chen F.N., Yang X.D., Wu Q., Antifungal capability of TiO2
coated film on moist wood, Build. Environ., 44 (2009)
1088–1093.
Erkan A., Bakir U., Karakas G., Photocatalytic microbial inactivation over Pd doped SnO2 and TiO2 thin films, J. Photoch.
Photobio. A, 184 (2006) 313–321.
Fujishima A., Zhang X.T., Tryk D.A., TiO2 photocatalysis andrelated surface phenomena, Surf. Sci. Rep., 63 (2008) 515–
582.
GeneralElectric, Spectral Power Distribution Curves: Tri-­
Phosphor Fluorescent Colors, SPX41. Commercial Products
and Solutions. http://www.gelighting.com/LightingWeb/na/
resources/tools/lamp-and-ballast/pop _cur ves.jsp?15,
accessed 06.06.2016.
Henderson M.A., A surface science perspective on TiO2 photocatalysis, Surf. Sci. Rep., 66 (2011) 185–297.
Ishibashi K., Fujishima A., Watanabe T., Hashimoto K., Quantum
yields of active oxidative species formed on TiO2 photocatalyst, J. Photoch. Photobio. A, 134 (2000) 139–142.
Klein E., Smith D.L., Laxminarayan R., Hospitalizations anddeaths caused by methicillin-resistant Staphylococcus
aureus, United States, 1999–2005, Emerg. Infect. Dis., 13(2007) 1840–1846.
Konaka R., Kasahara E., Dunlap W.C., Yamamoto Y., Chien

Vignesh Nandakumar et al. / KONA Powder and Particle Journal No. 34 (2017) 234–240

K.C., Inoue M., Irradiation of titanium dioxide generates
both singlet oxygen and superoxide anion, Free Radical
Bio. Med., 27 (1999) 294–300.
Krasovec U.O., Berginc M., Hocevar M., Topic M., Unique TiO2
paste for high efficiency dye-sensitized solar cells, Sol.
Energ. Mat. Sol. C, 93 (2009) 379–381.
Krishna V., Bai W., Han Z., Thakur A., Koopman B., Moudgil
B., Contaminant-Activated Visible Light Photocatalysis for
Reducing Healthcare-Acquired Infections, In review,
Nature (2016).
Lilja M., Forsgren J., Welch K., Astrand M., Engqvist H.,
Stromme M., Photocatalytic and antimicrobial properties ofsurgical implant coatings of titanium dioxide deposited
though cathodic arc evaporation, Biotechnol. Lett., 34(2012) 2299–2305.
Linsebigler A.L., Lu G.Q., Yates J.T., Photocatalysis on TiO2
Surfaces—Principles, Mechanisms, and Selected Results,
Chem. Rev., 95 (1995) 735–758.
Lobdell K.W., Stamou S., Sanchez J.A., Hospital-Acquired
Infections, Surg. Clin. N. Am., 92 (2012) 65–77.
Mitoraj D., Janczyk A., Strus M., Kisch H., Stochel G., Heczko
P.B., Macyk W., Visible light inactivation of bacteria andfungi by modified titanium dioxide, Photoch. Photobio.
Sci., 6 (2007) 642–648.
Oregan B., Gratzel M., A Low-Cost, High-Efficiency Solar-Cell
Based on Dye-Sensitized Colloidal TiO2 Films, Nature, 353(1991) 737–740.
Otter J.A., Yezli S., Perl T.M., Barbut F., French G.L., The roleof ‘no-touch’ automated room disinfection systems in

infection prevention and control, J. Hosp. Infect., 83 (2013)
1–13.
Scott II R.D., The Direct medical costs of healthcare-associatedinfections in U.S. hospitals and the benefits of prevention,
Centers for Disease Control and Prevention (2009). http://
stacks.cdc.gov/view/cdc/11550/, accessed 06.06.2016.
Shen M., Wu Z.Y., Huang H., Du Y.K., Zou Z.G., Yang P.,
Carbon-­doped anatase TiO2 obtained from TiC for photocatalysis under visible light irradiation, Mater. Lett., 60(2006) 693–697.
Sunada K., Watanabe T., Hashimoto K., Studies on photokillingof bacteria on TiO2 thin film, J. Photoch. Photobio. A, 156(2003) 227–233.
Tucci P., Porta G., Agostini M., Dinsdale D., Iavicoli I., Cain K.,
Finazzi-Agro A., Melino G., Willis A., Metabolic effects ofTiO2 nanoparticles, a common component of sunscreensand cosmetics, on human keratinocytes, Cell Death Dis., 4(2013).
Weber D.J., Anderson D., Rutala W.A., The role of the surface
environment in healthcare-associated infections, Curr. Opin.
Infect. Dis., 26 (2013) 338–344.
Wei X.J., Yang Z.D., Tay S.L., Gao W., Photocatalytic TiO2
nanoparticles enhanced polymer antimicrobial coating,Appl. Surf. Sci., 290 (2014) 274–279.
Yu J.C., Xie Y.D., Tang H.Y., Zhang L.Z., Chan H.C., Zhao J.C., Visible light-assisted bactericidal effect of metalphthalocyanine-­sensitized titanium dioxide films, J. Photoch. Photobio. A,
156 (2003) 235–241.

Author’s short biography
Vignesh Nandakumar
Vignesh Nandakumar is a third year graduate student in the Materials Science andEngineering at the University of Florida. His Scientific activities and research interests
focus on understanding cell materials interactions, and designing systems that can manipulate surface properties of cells to remove, kill or enhance adhesion onto surfaces.
Vignesh received his Bachelors from SASTRA University, Tanjore, India and his M.S. in
Material Science and Engineering from the University of Florida, Gainesville, Florida.

Zhao Han
Zhao Han is an Analytical Chemist/Material Scientist of Western Digital (WD) located in San Jose, California. Zhao is a graduate of University of Florida, where he received a M.S. in Materials Science and Engineering (MSE) 2013. He also received his
B.S. in MSE from Beijing University of Technology 2011 with expertise in Metal Processing. During his graduate study, Zhao researched on visible light activated TiO2 antimicrobial coating and developed the mechanism of contaminant activation by using
optical cut-off filters and organic dye. After graduation, Zhao started his professional career at WD and focuses on hard disk drive development.

239

Vignesh Nandakumar et al. / KONA Powder and Particle Journal No. 34 (2017) 234–240

Author’s short biography
Zachary Fritz
Zachary Fritz is a research scientist at a food-testing biotech company and will be pursuing graduate studies in Biomedical Engineering at Rutgers University (New Brunswick,
New Jersey). His research interests include immunological engineering and tissue engineering. He received his B.S.’s in Chemical Engineering and Microbiology from the
University of Florida (Gainesville, Florida).

Vijay Krishna
Vijay Krishna, Ph.D., was a Postdoctoral Fellow with the Particles Engineering Research Center at the University of Florida. Dr. Krishna has been awarded 6 patents and isprimary inventor on 12 current patent applications. He has authored 16 journal publications and presented at more than 30 technical conferences. Dr. Krishna earned a Ph.D. in
Materials Science and Engineering at the University of Florida. He is currently a Project
Scientist at the Lerner Research Institute, Cleveland Clinic. His research interests include carbon nanostructures, bio-nano interactions, surface and colloid science.

Ben Koopman
Ben L. Koopman, Ph.D., is a Professor and Undergraduate Coordinator in the Department of Environmental Engineering Sciences at the University of Florida. He earned a
B.S. in Civil Engineering from Oregon State University before pursuing a Ph.D. in thesame field at the University of California, Berkeley. His research interests include removal of microorganisms by surface-modified filter media and bio films, photocatalytic
antimicrobial coatings, water treatment, one-dimensional modelling of activated sludge
secondary settling tanks, and biological nutrient removal.

Brij M. Moudgil
Dr. Brij M. Moudgil is a Distinguished Professor of Materials Science and Engineering at the University of Florida. His current research interests are in particulate materials
based systems for enhanced performance in bioimaging, nanomedicine, photocatalytic
degradation of hazardous microbes, polymer and surfactant adsorption, dispersion andaggregation of fine particles and nanotoxicity. Dr. Moudgil received his B.E from theIndian Institute of Science, Bangalore, India and his M.S and Eng.Sc.D degrees from
Columbia University, New York. He has published more than 400 technical papers and has
been awarded 14 patents. He is a member of the U.S National Academy of Engineering.

240

KONA Powder and Particle Journal No. 34 (2017) 241–247/Doi:10.14356/kona.2017012

Original Research Paper

Eddy Current Separation of Nonferrous Metals
Using a Variable-Frequency Electromagnet †Nakul Dholu, James R. Nagel, Dave Cohrs and Raj K. Rajamani *1

Department of Metallurgical Engineering, University of Utah, USA

Abstract
We present a novel technique for sorting nonferrous metal scrap by using eddy current separation. However,rather than vary the magnetic field with a spinning rotary drum, our system utilizes a fixed electromagnet excitedby an alternating electric current. The technique requires no moving parts other than a feeding mechanism andhas the capacity to operate at excitation frequencies up to 50 kHz and beyond. Sorting results are demonstratedusing various combinations of metal spheres, which resulted in nearly perfect performance in terms of grade andrecovery. We also demonstrate sorting of aluminum alloys from other aluminum alloys, with consistent grade andrecovery between 85–95 %.
Keywords: eddy-current separator, electromagnet, nonferrous, metals, alloys

1. Introduction
Metal recycling is a multibillion dollar industry withtremendous economic opportunities and positive environmental impact. In 2015 alone, global aluminum production from mined ore reached well over 58 million metrictons, selling at an average price of $0.88 per pound(USGS, 2016). Global copper production likewise reachedover 18 million tons with an average price of $2.77 perpound. Nearly all of this material eventually ends up in
municipal waste streams wherein local scrap recyclers
generate significant wealth by collecting and selling thevaluable metals. In 2013, for example, 55 % of all aluminum produced in the United States came from recycled
scrap (USGS, 2015). Copper, in comparison, was produced from 33 % recycled material. In principle, thesevalues could reach much higher if not for the great difficulty in sorting various materials into more pure streams.
Consequently, there is a strong economic pressure to find
new ways to efficiently sort municipal waste streams into
their basic, elemental components.
To meet this challenge, many industrial machines andprocesses have been engineered specifically to sort valuable materials from a mixed stream of waste (Gaustad etal., 2012). For example, separation of ferrous metals, suchas iron and steel, is readily accomplished through large,†
1*

Received 3 May 2016; Accepted 14 June 2016
J-STAGE Advance Publication online 9 July 2016
Salt Lake City, UT 84112, USA
Corresponding author: Raj k. Rajamani
E-mail: raj.rajamani@utah.edu
TEL: +1-801-581-3107

permanent magnets mounted into specialized conveyors
and collectors (Weiss, 1985). This typically leaves behinda stream of nonferrous metals like copper, aluminum andbrass, mixed with other nonmetallic fluff like rubber, textiles, and plastic. Recovery of nonferrous metals then typically occurs through the use of standard eddy current
separators, which consist of a spinning drum of permanent magnets placed under a moving conveyor belt
(Schloemann, 1975; Lungu and Rem, 2003; Lungu, 2005).
As metallic particles pass over the magnets, electricaleddy currents are induced by the alternating magnetic
field, which in turn produces a net magnetic force that alters their trajectories.
Once the nonferrous metals have been separated fromthe fluff, it is desirable to further separate the base metals
into purified streams. For example, aluminum and copper
are two common metals that might exist in relative massconcentrations of 90 % Al to 10 % Cu. Since copper is adistinctly red metal that contrasts against the gray of aluminum, one common sorting method might utilize computerized optical sorting via basic color recognition(Kutila et al., 2005). Alternatively, one could exploit thefact that copper is nearly three times more dense than aluminum, thereby utilizing the popular technique of dense
medium separation (Weiss, 1985). In many developing
countries, the cost of human labor is so low that metals
are simply sorted directly by hand (Spencer, 2005). All ofthese methods have various costs, capabilities, andthroughputs associated with them, and individual recycling firms must generally weigh the pros and cons within
their own economic niche.
In this paper, we will introduce a new method for sepa-

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

241

Nakul Dholu et al. / KONA Powder and Particle Journal No. 34 (2017) 241–247

rating nonferrous metals from other nonferrous metals,
including nonferrous alloys. Dubbed electrodynamic sorting (EDX), our system utilizes a time-varying magnetic
field generated by an alternating electric current within afixed electromagnet. The principle was originally pioneered by Saveliev to extract gold particles from rockyore (Saveliev, 1998) but has since remained relatively unnoticed and unrefined. The process is very similar to traditional eddy current separators mentioned earlier, but
without the need for mechanically intense spinning of any
heavy, rotary magnets. Another distinct advantage to ourapproach is the ability to tune the frequency of excitation
to as high as 50 kHz and beyond. This allows for much
more active recovery of smaller particles in the realm of
1.0 cm and below, which can be especially difficult for
rotary-based separators and human separators to workwith. Our system also has the distinct advantage of being
completely dry, thus avoiding the wet slurry contamination commonly used in many flotation-based separators.
Beginning in Section 2, we discuss the basic physicalmechanism of eddy current separation as a derived principle of Maxwell’s equations. Section 3 then follows witha high-level description of the EDX system, including amechanical diagram and electrical circuit model. Sorting
data is then presented in Section 4, wherein metal spheresof various sizes and conductivities are separated by oursystem. Section 5 finally concludes with a discussion offurther applications and research goals for EDX.

2. Theory of eddy current separation
Eddy current separation is a highly mature technology
with many practical applications in the recycling industry.
The basic principle is derived from the fact that when apermanent magnet passes over a conductive metal object,electric charges within the metal tend to experience a net
magnetic force (Rem et al., 1997). This causes the charges
to flow around in distinct swirling patterns, commonlyreferred to as eddy currents or Foucault currents. If themagnetic field is strong enough and the relative motion
quick enough, this force can significantly accelerate theentire metal particle. Thus, a common design theme withmodern eddy current separators is a large, rotary drum
implanted with a series of permanent magnets. When the
drum is mechanically rotated at a very high velocity,
nearby metal particles tend to deflect along distinct kinematic trajectories away from other nonmetallic fluff. A
physical barrier is then placed somewhere between the
two trajectories, thus separating material into two distinct
streams.
For the case of electrodynamic sorting, we completely
do away with the spinning permanent magnets, replacing
them instead with a single fixed electromagnet excited by242

an alternating electric current. This significantly changes
the mathematical nature of the physics involved as now
the relative velocity between the magnet and particle is nolonger a significant factor. We may therefore begin with
Faraday’s law of electromagnetic induction, which statesthat a time-varying magnetic field B will give rise to anelectric field E in accordance with (Jackson, 1999)

 E 

B
.
t

(1)

If we next assume sinusoidal steady-state operation at angular frequency ω, all time derivatives can be replacedwith ∂/∂t = –jω. The phasor form of Faraday’s law istherefore written asE j B .

(2)

Now let us consider a metal particle placed somewhere in
the magnetic field B. Because of the induced electric fieldE, electric charges within the metal are accelerated accordingly, thus giving rise to an eddy current density J.
The relation between E and J is then given by the point
form of Ohm’s law, written as
J = σE ,(3)

where σ is the electrical conductivity of the metal. Changing magnetic fields therefore give rise to electric fields,
which in turn give rise to eddy currents in metallic objects.
Next, we note that electrical currents also give rise tomagnetic fields of their own in accordance with Ampere’slaw,  Be 
µ0J ,

(4)

where Be indicates the magnetic fields produced just by
the eddy current density J. This implies more changing
magnetic fields that must be accounted for in Eqn. (1),which in turn cause changes in the resultant eddy current
J. Thus, a complete solution for J, E, and B requires us tosolve Eqns. (1)–(4) simultaneously. While such a processextends well beyond the scope of this article, we can atleast note that once a solution for J is finally obtained, it
becomes possible to calculate the net Lorentz force F acting on the metal particle. This is given by the magnetic
force law,F J  B dV ,(5)

where the integral is carried out over the volume definedby the space within the particle. Thus, if the excitation
field B is strong enough and the frequency ω great
enough, the net force F will be great enough to significantly deflect arbitrarily large particles of metal.

Nakul Dholu et al. / KONA Powder and Particle Journal No. 34 (2017) 241–247

3. Description of electrodynamic sorting
The basic configuration of our EDX system is depicted
in Fig. 1. The core of the system is a wire-wound ferrite
toroid, excited by an alternating electric current. A specialized gap is then cut out from the toroid and serves as
the concentration point of time-varying magnetic fields.
As particles enter the gap, eddy currents are induced inaccordance with various factors, like conductivity, geometry, B-field variation, and frequency of excitation. In
particular, particles with very high conductivity will tend
to experience very strong forces, thus pushing them awayfrom their natural kinematic trajectories. Dense particles,
however, have greater inertia and will therefore resist being accelerated. Where exactly the particles land will
therefore depend on a combination of both factors, which
are fortunately varied enough to produce highly distinct
trajectories among most materials of interest. Thus, asimple mechanical splitter is all that is required to finallyseparate materials into their corresponding bins.
Fig. 2 shows an electrical circuit diagram for the EDX
drive electronics. A small-signal voltage Vs is produced bya standard signal generator, which determines the frequency and amplitude of the excitation signal. This signal
is then fed to the high-power amplifier U1, which isneeded in order to drive the core with enough current tosaturate the magnet. For this particular system, we used
the AE Techron model 7796, which is rated to deliver upwards of 5 kW of continuous power. In practice, however,Fig. 1

Fig. 2

Schematic diagram of the electrodynamic sorting system.

Equivalent circuit model for EDX drive electronics.

our system usually only requires about 350 W to operate.
At the output of the power amplifier, we employ a series RLC circuit to drive the core. Because the primaryelectromagnet is comprised of a wire-wound ferrite, it
naturally manifests as a lumped inductance L with values
typically ranging between 1–10 mH. In series with the inductor is an equivalent resistance R, representing the internal thermal loading of the magnet that occurs during
operation. Typical values may reach as high as 20 Ω, but
can vary strongly with the size of the magnet, the magnetic material, and the frequency of operation. However,this value is usually dwarfed by the reactive impedance of
the inductor, which can easily reach many hundreds ofOhms when excited to several kilohertz. We therefore
placed a lumped capacitance C in series with L, thus creating a resonant RLC circuit. When excited at the resonant frequency f0 = 1/( 2π LC ) the capacitive reactance
perfectly negates the inductive reactance, thus leaving
only the series resistance R to impede current.
Fig. 3 shows an isometric view of the magnetic core itself, which has been shaped into a square toroid. The dimensions are given by an outer radius of 440 mm, inner
radius of 240 mm, and height of 30 mm. The gap is a specialized double-cut, which is used to funnel flux into atight volume of space and serves as the feeding point forsmall metal particles. At the point of inner radius, the gap
separation starts at 10 mm with a flare angle of 10°. The
flare then extends radially outward for 30 mm before
opening up to a widened flare of 40°.
Considering that the core must channel strong magnetic
fields at high frequency, the specific choice of material isvery important. In particular, electrical conductivity must
be very low, or else eddy currents induced within thecore, itself tend to result in heavy thermal dissipation. It isalso important that the material be magnetically soft,
meaning that hysteresis effects must be small as well.
NiZn ferrite is well-known for satisfying both of theseproperties (Leary et al., 2012) and was therefore used toconstruct our EDX magnet.
Finally, the core must be wound with electrical wiringand excited by an alternating current. The exact numberof turns in the windings is somewhat arbitrary, thoughmany practical issues must be carefully weighed. In principle, if the drive current I is held constant, then a doubling of the number of turns N will subsequently double

Fig. 3Ferrite core geometry.

243Nakul Dholu et al. / KONA Powder and Particle Journal No. 34 (2017) 241–247

the magnetic field intensity B within the gap. However,this also increases the inductance of the coil by a factor offour, thus quadrupling the corresponding voltage at agiven frequency. If this voltage gets too large, then the insulation around the wiring itself can potentially break
down and halt the system. Likewise, we can reduce thevalue of N to maintain a reasonable voltage, but only by
reducing the current density around the core. This causes
B to fall accordingly, which can only be compensated forby increasing I. After careful trial-and-error, we eventually settled on a value of N = 300 turns, which wasenough to saturate the magnet at a peak current ofroughly I = 4.5 A.

4. Sort demo and results
In order to quantify our sorting data, it is first necessary to define the quality metrics for a given sorting process. We begin by considering two initial masses, m0 and
M0, of differing materials mixed together at the input of
the sorter. At the output are two bins, labeled bin A andbin B. The goal of the sort process is for all of mass m0 tocollect in bin A and for all of mass M0 to collect in bin B,
with no errors either way. We therefore define mA as thetotal mass from material m0 that falls into bin A, with mB
as the total mass falling into bin B. Likewise, we can define MA and MB as the total masses from M0 that fall into
their respective bins.
Let us now define the recovery of bin A as the fractionof initial material that collects in its proper bin:

RA 

mA
.
m0

(6)

Likewise, the recovery of bin B is defined as

RB 

MB
.
M0

(7)

Next, we define the grade of bin A as the fraction of total
mass in bin A comprised of its target material. This is
written as

GA 

mA
,
mA  M A(8)with the grade of bin B likewise following

GB 

mB
.
mB  M B(9)Thus, in a perfectly ideal world, both R and G would approach 100 % for each bin.
Fig. 4 shows a photograph of our experimental test bed
with the feeder, magnet, and sort bins all visible. The divider is fixed at a distance of 60 cm below the magnet but
could be translated horizontally to an optimal position for

244

Fig. 4 Primary EDX components (power amplifier not shown).
Table 1 Electrical conductivity and mass density for various
metals and alloys under consideration
Material

Conductivity [MS/m]

Density [g/cm3]

Copper

58.5

9.0

Brass

15.9

8.5

Al-1100

34.4

2.7

Al-6061

24.6

2.7

Al-2024

17.3

2.8

each sort. To demonstrate the EDX system, we mixed together various batches of metal spheres with contrastingmaterial properties and then sorted them accordingly. In
all cases, the number of spheres was at least 100 for eachmaterial being sorted.
Table 1 lists the materials under consideration for thisstudy as well as their relevant physical properties. In particular, we were interested in two distinct groups of trials.
The first case focused solely on the dissimilar metals Al/
Cu, Cu/Brass, and Al/Brass, using uniform spheres with
6.0 mm diameter. In this case, the magnet was excited toa frequency of 6.5 kHz and driven to a peak current of
4.5 A. The second case then focused on sorting metal alloys, including Al-1100, Al-6061, and Al-2024. In thiscase, the sphere diameters were slightly larger at 12.5 mm
and the frequency was lowered down to 1.9 kHz.
Table 2 shows a summary of our sorting trials. For thecase of dissimilar metals, the disparity in trajectories was
somewhat dramatic and highly consistent. For example,
upon reaching the separator, the disparity between aluminum spheres and copper spheres was well over 6 cm (10diameters), with random variations of only 1–2 cm. As a

Nakul Dholu et al. / KONA Powder and Particle Journal No. 34 (2017) 241–247

consequence, both recovery and grade were consistently
100 % over multiple runs. Similar results were also manifest with Cu/Brass and Al/Brass, likewise giving perfectoutcomes. Figs. 5a and 5b show samples of typical sorting results both before and after the sort.
Only when we attempted to sort aluminum alloys fromother alloys did separation errors begin to emerge. In thiscase, the mass density of the particles is virtually identical, leaving only small variations in conductivity bywhich to sort. Thus, the separation distances were typically in the range of only 3–4 cm (3 diameters), with almost as much variance causing errors. Even so, the results
were still consistently very good, leading to both grade
and recovery of well over 85 %. A typical sorting outTable 2 Grade and recovery results under various sorting
conditions
Materials (A/B)

RA [%]

RB [%]

GA [%]

GB [%]

Al/Cu

100

100

100

100

Al/Brass

100

100

100

100

Cu/Brass

100

100

100

100

Al-1100/Al-2024

100

100

100

100

Al-1100/Al-6061

99

91

92

99

Al-6061/Al-2024

91

86

87

91

Fig. 5come is further depicted in Fig. 5c, which shows the alloy
mixtures before and after the sort.

5. Discussion and conclusion
As the results in Table 2 show, the principle of electrodynamic sorting exhibits a powerful capacity for sorting
nonferrous metals from other nonferrous metals. This capacity was also extended to metal alloy separation, being
demonstrated with various combinations of aluminum alloys. While we only limited ourselves to particles on theorder of 0.5–1.0 cm, the basic principle could easily be redesigned for both larger and smaller geometries. The system utilizes no moving parts other than those needed tophysically feed material into the magnet and requires no
special chemical treatment or slurry in order to processmaterial.
While such results are initially very impressive, it isimportant to remember that our particles were perfectly
uniform spheres. This provided a tremendous level ofconsistency that would likely not exist under real-world
conditions. It also has the added advantage of being mathematically symmetric, thus lending itself to closed-form
analytic solutions to the resulting force equations (Lohofer,
1989). This is invaluable in identifying the primary fac-(a) Sorting results between 6.0 mm copper and brass spheres. (b) 6.0 mm aluminum and copper spheres.
(c) 12.5 mm aluminium alloys. Al-6061 spheres are painted black with Al-2014 spheres painted white.

245

Nakul Dholu et al. / KONA Powder and Particle Journal No. 34 (2017) 241–247

tors which contribute to particle forces as well as theirvarying significance when compared to each other. In
practice, however, real-world scrap is highly variable insize, geometry, and even composition, and will naturally
exhibit much greater variance in trajectories during thesorting process. We are therefore currently in the processof collecting real-world scrap material and testing oursystemʼs performance accordingly. The results of theseinvestigations, as well as the challenges we overcome,will be the subject of future publications.
One particular feature that requires further research isthe problem of throughput. In practice, it will be necessary to process many hundreds of kilograms per hour inorder to gain any interest from industrial recyclers. However, due to the bottleneck inherent to the magnetic gap,
our current system can only process perhaps 50–100 kg
per hour when pushed to its limits. Increasing throughput
also tends to add greater chaotic variation to the feed process, thereby reducing grade and recovery in the outcome.
Further design iterations must therefore increase throughput by at least a factor of 10 while also maintaining reasonable values in both grade and recovery. For example,
the use of a conveyor belt will greatly improve consistency when feeding material to the magnet while simultaneously increasing throughput. Such challenges are also
currently being investigated by our research team, and theresults will likewise be presented in future publications.

Acknowledgements
This work was funded by the United States Advanced
Research Project Agency-Energy (ARPA-E) METALS Pro­
gram under cooperative agreement grant DE-AR0000411.

NomenclatureBmagnetic field intensity (T)Ccapacitance (F)Eelectric field intensity (V/m)

EDXelectrodynamic sortingF

force (N)f0resonant frequency (Hz)GA

grade of sort bin A (%)

GB

grade of sort bin B (%)

I

electric current (A)jimaginary unitJelectric current density (A/m)

L

inductance (H)m0initial mass of material 1 (kg)

246

M0

initial mass of material 2 (kg)mAmass of material 1 in bin A (kg)MAmass of material 2 in bin A (kg)mBmass of material 1 in bin B (kg)MBmass of material 2 in bin B (kg)Nwire turn numberRelectrical resistance (Ω)

RArecovery of sort bin A (%)

RB

recovery of sort bin B (%)

RLC

resistor, inductor, capacitor

Vvolume (m3)σelectrical conductivity (S/m)µ0permeability of free space (H/m)ω

angular frequency (rad/s)

References
Gaustad G., Olivetti E., Kirchain R., Improving aluminum
recycling: A survey of sorting and impurity removal technologies, Resources, Conservation, and Recycling, 50(2012) 79–87.
Jackson J.D., Classical Electrodynamics, Wiley, New York(1999).
Kutila M., Viitanen J., Vattulainen A., Scrap metal sorting withcolour vision and inductive sensor array, International Conference on Computational Intelligence for Modeling, Control and Automation, and International Conference onIntelligent Agents, Web Technologies and Internet Commerce (CIMCA-IAWTIC’06) 2 (2005) 725–729.
Leary A.M., Ohodnicki P.R., McHenry M.E., Soft magnetic
materials in high-frequency, high-power conversion applications, Journal of Minerals, Metals & Materials Society,64 (2012) 772–781.
Lohofer G., Theory of an electromagnetically levitated metal
sphere I: absorbed power, SIAM Journal of Applied Mathematics, 49 (1989) 567–581.
Lungu M., Rem P., Eddy-current separation of small nonferrous
particles by a single-disk separator with permanent magnets, IEEE Transactions on Magnetics, 39 (2003) 2062–
2067.
Lungu M., Separation of small nonferrous particles using anangular rotary drum eddy-current separator with permanent magnets, International Journal of Minerals Processing,78 (2005) 22–30.
Rem P.C., Leest P.A., van den Akker A.J., A model for eddy
current separation, International Journal of Minerals Processing, 49 (1997) 193–200.
Saveliev V., System and method for separating electrically conductive particles, US Patent, (1998) 5772043A.
Schloemann E., Separation of nonmagnetic metals from solid
waste by permanent magnets. I. Theory, Journal of Applied
Physics, 46 (1975) 5012–5021.

Nakul Dholu et al. / KONA Powder and Particle Journal No. 34 (2017) 241–247

Schloemann E., Separation of nonmagnetic metals from solid
waste by permanent magnets. II. Experiments on circulardisks, Journal of Applied Physics, 46 (1975) 5022–5029.
Spencer D.B., The high-speed identification and sorting of nonferrous scrap, Journal of Materials, 57 (2005) 46–51.
USGS (United States Geological Survey), 2015, minerals
yearbook <http://minerals.usgs.gov/minerals/pubs/commodity/

aluminum/myb1-2013-alumi.pdf> accessed 16.06.2016.
USGS (United States Geological Survey), 2016, Mineral Commodity summaries 2016 <http://minerals.usgs.gov/minerarals/
pubs/mcs/2016/mcs2016.pdf> accessed 16.06.2016.
Weiss N.L., SE Mineral Processing Handbook, Volume 2, Society of Mining Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineers (1985).

Author’s short biography
Nakul Dholu
Nakul Dholu completed his undergraduate degree in Metallurgical Engineering from
Gujarat Technological University, Gandhinagar, India, in 2013. He graduated with MS
degree in Metallurgical Engineering as well from the University of Utah, USA, in 2016
under the supervision of his Professor Dr. Raj Rajamani. Currently he is working as a
Metallurgist-I at the Freeport-McMoRan Inc. in Arizona, USA. His research interests arein the field of light metal scrap recycling techniques and developing energy efficientmineral processing operations.

James Nagel
Dr. James Nagel completed his undergraduate degree in 2004 and MS degree in 2006,
both in Electrical Engineering, from Brigham Young University in Provo, Utah. He thencompleted his PhD in 2011, also in Electrical Engineering, from the University of Utah
in Salt Lake City, Utah and was awarded the Stockham Medal of Excellence for Conspicuously Effective Teaching. He now works as a research associate for the Universityof Utah, where his research focuses on applied electromagnetics with a specialty in numerical methods.

Dave Cohrs
Dave studied Environmental Science with emphasis on Analytical Chemistry at University of Maryland, Baltimore County in Maryland. He managed water quality laboratories at public aquariums for 16 years and has consulted for aquariums, zoos, andacademia on water chemistry and life support topics. He now works for the University of
Utah as a research associate.

Raj K. Rajamani
Raj Rajamani has been on the faculty of the Metallurgical Engineering Department of
the University of Utah, Salt Lake City, Utah, USA since 1980. Currently he holds the position of professor. His research interests include population balance modeling of tumbling mills, computational fluid dynamics of hydrocyclones, discrete element modelingof semi-autogenous grinding mills, eddy current sorting of metallic particles and modeling of high pressure grinding rolls. He received the Antoine M. Gaudin Award, presented
by the Society of Mining, Metallurgy and Exploration Engineers Inc. in the year 2009.

247

KONA Powder and Particle Journal No. 34 (2017) 248–257/Doi:10.14356/kona.2017014

Original Research Paper

Impact Dispersion Using 2D and 3D
Composite Granular Packing †Surajit Sen 1*, T. R. Krishna Mohan 2 and Mukesh Tiwari 3
1

Department of Physics, State University of New York, USA
CSIR Centre for Mathematical Modelling and Computer Simulation (C-MMACS), India
3
Group in Computational Science and High Performance Computing, DA-IICT, India
2

Abstract
We present a study of efficient dispersion of an impact onto structured and potentially scalable granular beds. We
use discrete element method based dynamical simulations of shock wave propagation and dispersion in 2D and
3D arrangements of granular spheres. The spheres are geometrically packed in a nested columnar structure,
which leads to the severe attenuation and spreading of the incident energy within the structure. We further showthat by incorporating inhomogeneity in material properties, or by introducing layers of a dissimilar material in
the middle of the arrangement, impact mitigation can be enhanced significantly. Such an arrangement cantherefore be useful in the design of effective impact decimation systems. Using a 2D arrangement we first show
the basic idea behind impact dispersion in such an arrangement. With this understanding the system is scaled to
3D. The influence of the system size and material properties on the wave propagation within the packing is alsopresented.
Keywords: elastic grains, mass mismatch, impact dispersion, granular chain, solitary waves

Introduction
Impact dispersion is an important area of research withwide ranging combat related applications, applications inthe study of earthquake resistant structures and so on.
Impulse propagation in granular systems has been an important area of research for more than 30 years due to theunique properties observed as a result of the highly nonlinear contact law between the grains (Nesterenko V.F.,
2001; Jaeger H.M. et al., 1996; Katsuragi H., 2016).
Starting with the pioneering work of Nesterenko(Nesterenko V.F., 1983), who showed the existence of solitary waves in a 1-dimensional arrangement of equal radius spheres, extensive work based on experiments (Coste
C. et al., 1997), numerical simulations (Sinkovits R.S. and
Sen S., 1995; Sen S. and Manciu M., 2001) and analyticalmethods (Mackay R.S., 1999; Lindenberg K. et al., 2011)
have been performed for one dimensional chains (see e.g.,
Sen S. et al., 2008 for an exhaustive review). These stud†
1
2
3

*

Received 18 April 2016; Accepted 18 June 2016
J-STAGE Advance Publication online 30 July 2016
Buffalo, New York 14260-1500, USA
Bangalore, Karnataka 560037, India
Gandhinagar, Gujarat 382007, India
Corresponding author: Surajit Sen;
E-mail: sen@buffalo.edu
TEL: +1-716-907-4961 FAX: +1-716-645-2507

248

ies have been instrumental in the design of novel granularmaterials with widespread applications. It is now well understood that introducing a mass mismatch in these one
dimensional granular systems, either by changing the sizeof the spheres such as in tapered (Doney R. and Sen S.,
2005; Nakagawa M. et al., 2003; Melo F. et al., 2006) anddecorated chains (see e.g., Doney R. and Sen S., 2006;,
Machado L.P. et al., 2014), or by changing of the material
properties by introducing spheres of different materials(Hong J. and Xu A., 2002; Hong J., 2005; Nesterenko V.F.
et al., 2005; Daraio C. et al., 2006; Wang P.J. et al., 2007;
Fraternali F. et al., 2010) leads to the rapid disintegrationof the moving solitary wave, which would be of importance in designing materials for impact decimation.
Impact response of 2-dimensional granular systems has
been studied extensively through experiments and numerical simulations of photoelastic disks by Shukla et al.
(Rossmanith H.P. and Shukla A., 1982; Zhu Y. et al.,
1996; Sadd M.H. et al., 1993). More recently, Nishida etal. have studied projectile impact and wave propagation in
two dimensional granular arrangement, including the effect of dissimilar materials (Nishida M. et al., 2001;
Tanaka K. et al., 2002; Nishida M. and Tanaka Y., 2010).
Leonard et al. through a series of works have systematically studied the role of the intruders and disorder on two
dimensional square and hexagonal arrangements of granular systems (Leonard A. and Daraio C., 2012; Leonard A.

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

et al., 2012, 2013, 2014). The role of the intruders and thedifferent modes of propagation have been studied numerically by Awasthi et al. (Awasthi A.P. et al., 2012; Awasthi
A.P. et al., 2015). The force propagation speed and effects
of dissipation has also been analyzed (Abd-Elhady M.S.
et al., 2010; Pal R.K. et al., 2013; Burgoyne H.A. andDaraio C., 2015). The 3-dimensional studies on monodispersed sphere ar rangements have explored wave
propagation (Manjunath M. et al., 2014), effect of impact
conditions on the rebound velocity of the incident projectile (Nishida M. et al., 2004) and the impact response ofheterogeneous granular systems (Burgoyne H.A. et al.,2015).

2. Model and numerical method
2.1 Quasi-2D bed
Fig. 1 shows the schematic of a recently proposed two
dimensional granular arrangement (Tiwari M. et al., 2016)
for impact decimation, wherein, spherical granular particles are assembled in a block-type arrangement. The
spheres in a block are of equal masses and have the samematerial properties. The radius of the spheres in any blockis, however, twice that of spheres in the block immediately below it. It should be emphasized that the geometries of the systems discussed here all require a factor of
two reduction in sphere radius at each interface. Withineach block we have rectangular arrangement of monodispersed spheres, and the blocks are placed in such a waythat at the interface the sphere with larger radius is placed
symmetrically above the two smaller spheres in the block
below. This is also shown in the enlarged view in Fig. 1.

Fig. 1

Schematic of a part of the two dimensional granular arrangement with two blocks. The side walls are to be regarded as infinitely far away. The lower block is madeup of spheres of radius r and the top block has spheresof radius 2r.

The spheres are initially assumed to be in contact with noprecompression, and interact elastically upon compression. The normal and tangential components of the force
depend on the overlap and can be written as,

Fn  k nδn ,(1)

Ft  k tδt ,(2)

δn and δt are the normal and tangential overlap, respectively. For two spheres with separation r and radii R1 andR2 the normal overlap δn = R1 + R2 – r and the contact
force is nonzero only when δn > 0. The tangential overlap
δt is the relative tangential displacement between the two
spheres for the entire contact duration and the tangential
force is limited by the coulomb criterion Ft = min(Ft,μFn). We use the nonlinear Hertz contact interaction for
describing the normal force in our simulations (Hertz H.,1881). The spring constants kn and kt therefore depend on
the normal overlap in addition to the material parameters,
4k n  Y * R *δn and k t  8G * R *δn . For spheres with3
the same Young’s modulus Y and Poisson’s ratio ν the efY
fective Young’s modulus Y * and the effective
2 1  ν 2Y
. R* is the effective
shear modulus G * 
4 1  2ν1  νR1R2
radius and equals. For numerical simulation
R1  R2of the system we use the open source DEM package
LIGGGHTS (Cundall P.A. and Strack O.D.L., 1979; Kloss
C. et al., 2012; Plimpton S.J., 1995). We do not include
gravity in our numerical simulations. The effect due togravity would become significant for a large number ofparticles and weak impulse. For strong impulse the disturbance would still travel as solitary waves (Hong J. and Xu
A., 2001). No dissipation of any form is taken into account, however, we do consider the effect of static friction
on the distribution of energy. Incorporating dissipation
would undoubtedly improve the impact decimation capabilities of the systems we study. However, ignoring dissipation in the following calculations help us set an upperbound on how much energy can be dispersed by the systems we examine.
Since the spheres interact only if they are in contact,
the energy due to the impact by the striker travels along

Table 1 Densities, Young’s moduli and Poisson’s ratio ofdifferent materials used in numerical simulations.
ρ (kg/m3)

Y (Pa)

ν
9

Steel

7833

193 × 10

Teflon2170

1.46 × 109

0.3
0.46

249

Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

the chains of spheres in line with the contact point of thestriker. We assume that the striker impact is in between
two of the spheres in the upper most layer. For simplicity,
we also assume that the striker is of the same mass andmaterial as the spheres in the top block. The energy in the
top block therefore travels sideways in the top layers, anddownwards along two vertical chains. For a single block
system with a square arrangement of purely elastic
spheres it has been observed that around 71.5 % of the energy propagates downwards whereas the sideward propagation is around 24.5 % (Leonard A. et al., 2013). Our
numerical simulations were consistent with these observations. The sideward propagation was observed to besignificantly suppressed if static friction was included in
our numerical simulations. We therefore, focus only on
the downward propagating part. We will briefly commentin the following on what happens when the mass of thestriker is increased or decreased.

2.2 Impact decimation by a quasi 2D bed

the smaller radii of the spheres across the interface.
In Fig. 2 we show the propagation of kinetic energy ina two block system at different time instants for particles
made from steel in both the blocks. The energy as expected propagates in a localized manner in the top block(Fig. 2(a)) along two chains. As this energy crosses theinterface and moves to the block with lighter spheres it
tends to spread out in its direction of propagation (seeFig. 2(b)), and after this transient phase it is seen to moveas well separated discrete packets or solitary wave trains
with much smaller energies in each pulse. The energy ineach pulse of the wave train depends on the difference ofthe masses of the two spheres (Job S. et al., 2007). It is
worth noting that impact of a striker which is more massive than the masses of the grains in the bed leads to theformation of solitary wave trains in the first block itself.
No such trains form when a striker of lighter mass impacts on to the bed.
Presence of more blocks would lead to further frag-The system in the absence of multiple blocks is only
one dimensional. Presence of multiple blocks makes it
quasi two dimensional, since due to the proposed arrangement of spheres at the interface of two blocks, the energy
from any excited larger spheres gets transmitted to two
smaller spheres that are in contact with it in the block below. The spatial extent, however, remains the same due toFig. 2250

Surface plots of the kinetic energy propagation in a two
block system for three different time instants: (a) the
energy is in the upper block (b) at the interface and (c)the energy is in the lower block. The kinetic energy isnormalized to its maximum value for that time instant.
The spheres in upper and lower blocks are shown in
red and yellow respectively.

Fig. 3

Normal force experienced by the larger sphere at theinterface of a 3 block system made of (a) steel grainsand (b) block made from Teflon spheres sandwiched
between two blocks made of steel spheres. The solidline is for a sphere at the top interface while dashedline is for the bottom interface.

Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

In the previous section we observed that a multi-block
system results in significant decimation of the impact energy. Using a hard sphere approximation we provide anestimate of the output kinetic energy of the system. We
consider a simplified picture in which we first estimate
the transmitted energy in a general three sphere collision

Fig. 4

Schematic for the sphere arrangement at the interface.
The vertical downward direction is the x-direction andhorizontal direction is the y-direction.

piy,1  piy,2,xp
pix1  f   2 pi,1cos θ,i 1 02
p
pi21  f   2 pi2,1
i 1 0

(3)

where pix, y represents the x, y component of the momenm
tum of the spheres in the ith block,   i 1 is the ratio ofmithe masses of the sphere in the two blocks, and for the ar2 2
rangement considered at the interface, cos θ . Due
3to the symmetry in the collision, the two smaller spheres
move with equal momentum ( pix,1  pix,2 ) after impact. The
final momentum of the spheres from Eq. (3) ispix,1
2cos 2θ,pi 1 0   2cos 2θ

(4)

pi 1
  2cos 2θ
.
pi 1 0   2cos 2θ

(5)

If – 2 cos2 θ > 0 ( > 16/9) from Eq. (5), the top spherewould continue to move in its original direction. This
leads to multiple collisions at the interface in the proposedsystem. Each collision leads to the generation of a solitary
wave. From Eq. (4), the momentum transfer is inversely
related to the mass ratio. Therefore larger mass mismatch
would result in a smaller momentum or energy transmission at each collision. It has been shown that the total energy carried by the solitary wave is E = P2/2meff where
the effective mass of the solitary wave is meff ≈ Ωm. The
value of Ω is approximately 1.4 (Job S. et al., 2007;
Tichler A.M., et al., 2013). We further assume that the energy and momentum transfer between subsequent blocks
is through the collision between solitary waves, and thatthe individual pulses in the solitary wave train are separated significantly. To calculate the output energy of thesystem we therefore only need to consider the leading solitary wave pulse in each block. Under these assumptions
the complete energy transfer can be thought to occur as
discrete events. At the initial time, the impact by the
striker results in the generation of a solitary wave in the
top block. Therefore, the collision process here is between
a particle and solitary wave treated as a quasiparticle. At
the second interface, the collision is between the twoleading solitary waves on either side of the interface. Finally, in the lowest block the solitary wave transfers its
϶

2.3 Hard sphere collision approximation (energy
transmission in a multi-block system)with the arrangement shown in Fig. 4. This is similar to
the arrangement of the spheres at the interface of i – 1th
and ith block. We ignore any horizontal momentum transfer to these spheres from the adjacent spheres. Using conservation of energy and momentum we can write for the
momentum of the spheres϶

mentation of these pulses at each interface. An effective
impact decimation system can be constructed by having a
multiple block arrangement. We consider two such arrangements each consisting of three blocks. In the firstarrangement all the blocks are made from steel spheres,
whereas, in the second arrangement a block made fromlighter material such as teflon is sandwiched between two
blocks made from steel spheres. The masses differ by afactor of 8 at each interface in the first arrangement
whereas in the second arrangement they differ by a factorof 29.5 and 2.2 at the steel-teflon and teflon-steel interface
respectively. Even though the numbers of interfaces are
the same the difference in mass mismatch is seen to affectthe impact decimation ability significantly. The normal
force experienced by the larger sphere at the interface forthe two arrangements are shown in Figs. 3(a) and (b) respectively. At the first interface (solid line in Fig. 3) sustained small amplitude oscillations are observed in thesecond arrangement. For larger mass mismatch values thesolitary wave in the top block splits into more number ofpulses with smaller amplitude when it moves to the second block. Each of these pulses split further at the second
interface since they still move from a heavier side to alighter side. This results in the further decimation of eachpulse. This is shown through the dashed line in Fig. 3.

251

Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

complete energy to the last particle which is taken to be
the output energy of the system. The ratio of the momentum of the last particle in an n block system and thestriker can then be written as: pn,L 
pn,L



pst 0  psw,n  psw,n  psw,n1   p2,1   psw,1  .... .
 psw,n1  psw,n2   psw,1   pst 0 (6)pn,L is the momentum of the last particle in the nth block,
psw,i is the momentum of the leading solitary wave pulsein the ith block and pst(0) is the initial momentum of the
pn, L
p
2
2Ωcos 2θ1striker,, and, sw,1 . θ1 ispsw,n Ω  1
pst 0 1  2Ωcos 2θ1
the angle that the striker makes with the two spheres(cos θ1  1 / 3 ). If the spheres are made from the samematerial Eq. (6) can be simplified and written as: p   2cos 2θ  n1  psw,1 p n,L  n,L  .
pst 0  psw,n     2cos 2θ   pst 0 (7)

For alternating blocks of hard and soft material, such as
steel-teflon-steel-teflon we can obtain a similar expression,pn,Lpst 0p pn,L  2cos 2θ 



2 
 psw,n   st,tf  2cos θ qFig. 5

Average Kinetic energy for 1–4 block system with (a)each block made of steel spheres and (b) alternate
blocks with steel and Teflon spheres. The circles are
obtained through numerical simulation whereas (×) is
obtained from Eqs. (7) and (8), and, (▲) is obtained
from Eqs. (9) and (10).

 2cos 2θ   psw,1 

,
(8)
2  
  tf ,st  2cos θ   pst 0 mwhere,  st,tf  st . The powers p and q equals, respecmtf
 pn,L   2  n1pn,Ln 1 if n is even, while, if n is odd they tively, n/2 andpst 0  psw,n   Ω  1 2pq
are both equal to (n – 1)/2. A better approximation to the 2Ωcos 2θ   2Ωcos 2θ   psw,1  . (10)energy transmission is obtained if we assume that within
2  
2  
  st,tf  2Ωcos θ    tf ,st  2Ωcos θ   pst 0 each block the solitary wave transfers its momentum andenergy to the last particle of the block. Each collision of mst   p n2,L 
The average kinetic energy transmitted this last particle with the top particle in the next block mn   pst2 0 
is shown in Fig. 5 for multiple blocks (1–4 block system),leads to the generation of a solitary wave. Therefore at
mn is the mass of the sphere in the nth block. The averageeach interface the collision is between the particle on one
here is over the energies of all the excited spheres in theside and the leading edge of the solitary wave treated as abottom layer of the last block. For comparison, in Fig. 5
quasiparticle on the other side. Under this assumption we
we
only show the kinetic energy when the leading pulse
obtain for spheres of the same material
reaches the last layer. In both the cases, Eqs. (9) and (10)are able to provide a better estimate, however, for dissimn 1
 p n,L   2 
p n,L


ilar materials the fit is only qualitative. In Fig. 5, the en
pst 0  psw,n   Ω  1 ergy shown is the leading energy of the pulse reaching then 12last sphere. The energy with which the last particle ejects
 2Ωcos θ   psw,1 
(9)

 
,
is slightly larger than this value. Nevertheless, this differ   2Ωcos 2θ   pst 0 
ence is insignificant. More importantly, even with threeblocks the average output energy is only around 0.4 % ofand, for alternate blocks of steel and teflon

252

Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

Fig. 6(Top) Schematic of the 3-dimensional system with two blocks. (Bottom) Propagation of energy in a single block system (Left) vertically downward and (Right) sideward.

the impact energy. This energy does not show any significant dependence on the size of the system.

3. Impact decimation by a 3D bed
The advantage of the proposed system in two dimensions is that it can be easily extended to three dimensions.
In Fig. 6 (top) we show a typical three dimensional system with two blocks. As is evident from the enlarged
view of the sphere arrangement at the interface, the larger
sphere is in contact with four spheres in the layer below.
The centers of the spheres form a pyramid like structure,with the base made from the plane connecting the centersof the four small spheres and the larger sphere is centeredat the top of the pyramid at a vertical distance of 7 fromthe base. This pyramid structure forms the basis of energy partitioning at each interface. Each excited spheretransmits its energy to four spheres in the downward direction. The energy propagation for a single excitedsphere is therefore through four chains vertically downward and eight chains in the lateral direction (see Fig. 6(bottom)). The process gets repeated at each interface andthe energy is transferred to four times the number of ex-

cited spheres in the upper block. We show this energy
partitioning for a completely elastic 1-block system inFig. 7. What is notable is that the net energy transmission
in the horizontal and vertical directions is almost thesame. The rebound energy of the striker is higher (approx
12.5 %) than that of the two dimensional system (approx4 %). Before energy exits from the system the distribution
between potential and kinetic energy follows the virial
theorem. Since the energy partitioning along the two directions are the same we also look at the effect of static
friction on the distribution of energy. As shown in thebottom plot in Fig. 7(b) static friction does have the effectof reducing the sideward energy propagation. For higher
values of static friction and when the boundaries are sufficiently far away or the side walls are made to be energy
absorptive, these multi-block systems may be useful inmany impact dispersion applications. The system properties would not change if the grain sizes were smaller aslong as the elastic properties of the spheres were unaffected. Realistically this means the systems envisioned
here could well be realized possibly even in the micronscale. In a regularly arranged closed packing we do not
expect much rotational motion to occur, and therefore it ispossible that energy propagation in the sideward direction

253Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

Fig. 7(a) Partitioning of the normalized energy in a 1 block
system, between the striker (solid line), four particles
in the bottom layer (dashed-dotted line), and eight particles in the top layer (dashed-line). The dashed line
shows the overall kinetic energy of the entire system(bed and striker). (b) Variation of normal (○) and sideward (×) transmission of total kinetic energy propagation with static friction μ.

would be significant. At this moment we concentrate only
on the downward propagation of energy.
In Fig. 8 we show the vertical force propagation in athree block system. Two different systems are considered.
In the first, all the blocks are made of steel spheres, whilein the second, a block made from lighter mass Teflon
spheres is sandwiched between blocks made from heavier
steel spheres. In both the systems within the top block theforce propagates as a single hump. At the top interface thetransmission of force is significantly different. In the firstcase when the mass mismatch is comparatively smaller theforce gets transmitted almost entirely and over a shortertime interval. As mass mismatch increases, the force
propagates slowly inside the Teflon block and is more uniformly distributed over the overall length of the block.
There is also a sharp reduction of the maximum force on
the sphere at the interface in the second arrangement.

254

Fig. 8Force at an interface sphere in a 3-block system for (a)all blocks made from steel sphere and (b) block withteflon spheres sandwiched between blocks made fromsteel sphere. The solid line shows the force on interfacial sphere in the top block while the dashed line shows
the force on the interfacial sphere in the second block.
The inset shows the vertical cross sectional view of theoverall force distribution at different time instants. The
color bar shows the absolute value of the normal force
acting on a sphere.

4. Conclusions
The wave propagation in the presence of mass mismatch was investigated numerically for an impact decimation system. Our numerical simulations show that sucha system is capable of suppressing incident impact significantly. The setup presented here takes advantage of thefact that when a solitary wave crosses from a denser to alighter medium, a series of solitary waves with smaller
energies are generated. This splitting is also borne out of
the approximate hard sphere collision theory for the system under consideration. A larger mass difference at theinterface results in more splitting and thereby lesser energy in each pulse. A block with smaller mass spheres
sandwiched between blocks with heavier mass spheres onboth sides shows better impact decimation capability. The

Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

arrangement of spheres at the interface (triangular in 2D
and pyramid in 3D) leads to energy being partitioned between more spheres at the interface. The proposed three
dimensional system is therefore seen to demonstrate superior impact dispersion capabilities. The sideward propagation is however signif icantly larger in three
dimensions. While we have not considered this situationin the current work, in actual design of systems thiswould be quite important. Since the impact dispersion
and decimation is due to the geometry of the packing at
the interface, the qualitative features should not be affected by varying the size or the angle of impact of thestriker.

NomenclatureFnnormal force (N)Fttangential force (N)knnormal spring constant (N/m)

kttangential spring constant (N/m)δnnormal overlap (m)δttangential overlap (m)μstatic friction

Y

Young’s modulus (Pa)νPoisson’s ratioRradius of the sphere (m)ρdensity of the sphere (kg/m3)

R* effective radius of two particles in contact
Y* effective Young’s modulus
G

Shear modulus (Pa)

References
Abd-Elhady M.S., Abd-Elhady S., Rindt C.C.M., Van Steenhoven
A.A., Force propagation speed in a bed of particles due toan incident particle impact, Adv. Powder Technol., 21 (2010)
150–164.
Awasthi A.P., Smith K.J., Geubelle P.H., Lambros J., Propagation of solitary waves in 2D granular media, Mechanics ofMaterials, 54 (2012) 100–112.
Awasthi A.P., Wang Z., Broadhurst N., Geubelle P., Impact
response of granular layers, Granular Matter, 17 (2015)
27–31.
Burgoyne H.A., Daraio C., Elastic–plastic wave Propagation in
uniform and periodic granular chains, Journal of Applied
Mechanics, 82(8) (2015) 081002.
Burgoyne H.A., Newman J.A., Jackson W.C., Daraio C., Guided
impact mitigation in 2D and 3D granular crystals, Procedia
Engineering, 103 (2015) 52–59.

Coste C., Falcon E., Fauve S., Solitary waves in a chain of beads
under Hertz contact, Phys. Rev. E, 56 (1997) 6104–6117.
Cundall P.A., Strack O.D.L., A discrete numerical model for
granular assemblies, Géotechnique, 29(1) (1979) 47–65.
Daraio C., Nesterenko V.F., Herbold E.B., Jin S., Energy trapping and shock disintegration in composite granularmedium, Phys. Rev. Lett., 96 (2006) 058002.
Doney R., Sen S., Impulse absorption by tapered horizontalalignment of elastic spheres, Phys. Rev. E, 72 (2005)
041304.
Doney R., Sen S., Decorated, tapered and highly nonlinear
granular chain, Phys. Rev. Lett., 97 (2006) 155502.
Fraternali F., Porter M.A., Daraio C., Optimal design of composite granular protectors, Mech. Adv. Mat. Struct., 17(2010) 1–19.
Hertz H., Über die Berührung fester elastischer Körper, Journal
für die reine und angewandte, Mathematik, 92 (1881) 156–
171.
Hong J., Xu A., Effects of gravity and nonlinearity on the waves
in the granular chain, Phys. Rev. E, 63 (2001) 061301.
Hong J., Xu A., Nondestructive identification of impurities in
granular medium, Appl. Phys. Lett., 81 (2002) 4868–4870.
Hong J., Universal power-law decay of the impulse energy in
granular protectors, Phys. Rev. Lett., 97 (2005) 108001.
Jaeger H.M., Nagel S.R., Behringer R.P., Granular solids, liquids, and gases, Rev. Mod. Phys., 68 (1996) 1259.
Job S., Melo F., Sokolow A., Sen S., Solitary wave trains in
granular chains: experiments, theory and simulations,
Granular Matter, 10 (2007) 13–20.
Katsuragi H., Physics of soft impact and cratering, Springer,
Tokyo (2016). ISBN 978-4-431-55648-0.
Kloss C., Goniva C., Hager A., Amberger S., Pirker S., Models,algorithms and validation of opensource DEM and CFDDEM, Progr.Computat. Fluid Dyn., 12 (2012) 140–152.
Leonard A., Daraio C., Stress wave anisotropy in centeredsquare highly nonlinear granular systems, Phys. Rev. Lett.,
108 (2012) 21430.
Leonard A., Fraternali F., Daraio C., Directional wave propagation in a highly nonlinear square packing of spheres, Exp.
Mech., 57 (2013) 327–337.
Leonard A., Daraio C., Awasthi A., Geubelle P., Effects of weakdisorder on stress-wave anisotropy in centered square nonlinear granular crystals, Phys. Rev. E, 86(3) (2012) 031305.
Leonard A., Chong C., Kevrekidis P.G., Daraio C., Travelingwaves in 2D hexagonal granular crystal lattices, Granular
Matter, 16 (2014) 531–542.
Lindenberg K., Harbola U., Romero H., Rosas A., Pulse propagation in granular chains, Am. Inst. Phys. Conf. Ser., 1339
(2011) 97–110.
Machado L.P., Rosas A., Lindenberg K., A quasi-unidimensional
granular chain to attenuate impact, Eur. Phys. J. E, 37(11)(2014) 1–7.
Mackay R.S., Solitary waves in a chain of beads under Hertz
contact, Phys. Lett. A, 251 (1999) 191–192.
Manjunath M., Awasthi A.P., Geubelle P.H., Plane wave propagation in 2D and 3D monodisperse periodic granular
media, Granular Matter, 16 (2014) 141–150.
Melo F., Job S., Santibanez F., Tapia F., Experimental evidence

255

Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

of shock mitigation in a Hertzian tapered chain, Phys. Rev.
E, 73 (2006) 041305.
Nakagawa M., Agui J.H., Wu D.T., Extramiana D., Impulse dispersion in a tapered granular chain, Granular Matter, 4(2003) 167–174.
Nesterenko V.F., Propagation of nonlinear compression pulses
in granular media, J. Appl. Mech. Tech. Phys., 24 (1983)
733–743.
Nesterenko V.F., Dynamics of heterogeneous materials, Springer,
New York, 2001.
Nesterenko V.F., Daraio C., Herbold E.B., Jin S., Anomalous
wave reflection at the interface of two strongly nonlinear
granular media, Phys. Rev. Lett., 95 (2005) 158702.
Nishida M., Tanaka K., Kunimochi T., Takagi T., Discrete element simulation in projectile impacts on granular materials,
Proc. 23rd Int. Symp. Shock Waves, (2001) 655–661.
Nishida M., Tanaka K., Matsumoto Y., Discrete element method
simulation of the restitutive characteristics of a steel spherical projectile from a particulate aggregation, JSME international journal, Series A, Solid mechanics and material
engineering, 47(3) (2004) 438–447.
Nishida M., Tanaka Y., DEM simulations and experiments for
projectile impacting two-dimensional particle packings
including dissimilar material layers, Granular Matter, 12(2010) 357–368.
Pal R.K., Awasthi A.P., Geubelle P.H., Wave propagation inelasto-plastic granular systems, Granular Matter, 15 (2013)
747–758.
Plimpton S.J., Fast parallel algorithms for short-range molecular
dynamics, J. Comput. Phys., 117(1) (1995) 1–19.
Rossmanith H.P., Shukla A., Photoelastic investigation of

256

dynamic load transfer in granular media, Acta Mech., 42(1982) 211–225.
Sadd M.H., Tai Q., Shukla A., Contact law effects on wave
propagation in particulate materials using distinct element
modelling, Int. J. Non-linear Mech., 28 (1993) 251–265.
Sen S., Manciu M., Solitary wave dynamics in Generalized
Hertz chains: An improved solution to the equation of
motion, Phys. Rev. E, 64 (2001) 056605.
Sen S., Hong J., Bang J., Avalos E., Doney R., Solitary waves in
the granular chain, Phys. Rept., 462 (2008) 21–66.
Sinkovits R.S., Sen S., Nonlinear dynamics in granular columns, Phys. Rev. Lett., 74 (1995), 2686.
Tanaka K., Nishida M., Kunimochi T., Takagi T., Discrete element simulation and experiment for dynamic response of
two-dimensional granular matter to impact of a sphericalprojectile, Powder Technol., 124 (2002) 160–173.
Tichler A.M., Go’mez L.R., Upadhyaya N., Campman X.,
Nesterenko V.F., Vitelli V., Transmission and reflection ofstrongly nonlinear solitary waves at granular interfaces,
Phys. Rev. Lett., 111 (2013) 048001.
Tiwari M., Krishna Mohan T.R., Sen S., Decorated granularlayers for impact decimation, Granular Matter, 18 (2016)
1–5.
Wang P.J., Xia J.H., Li Y.D., Liu C.S., Crossover in the power-­law behavior of confined energy in a composite granular
chain, Phys. Rev. E, 74 (2007) 041305.
Zhu Y., Shukla A., Sadd M., The effect of microstructural fabricon dynamic load transfer in two dimensional assemblies of
elliptical particles, J. Mech. Phys. Solids, 44 (1996) 1283–
1303.

Surajit Sen et al. / KONA Powder and Particle Journal No. 34 (2017) 248–257

Author’s short biography
Surajit Sen
Surajit Sen is a Professor of Physics at the State University of New York at Buffalo.
His research interests are focused on understanding the dynamics of strongly nonlinear
systems. He has worked on impulse propagation in granular systems since 1995. He received his PhD in Physics from the University of Georgia in 1990 and is a Fellow of theAmerican Physical Society and the American Association for the Advancement of Science and serves as an editor of International Journal of Modern Physics B and Modern
Physics Letters B.

T. R. Krishna Mohan
Krishna Mohan, T. R. is currently working as a Scientist at CSIR Fourth Paradigm Institute in Bangalore, India where he has been working since 1991. In between, he worked
at State University of New York at Buffalo for around four years at the Department of
Physics and Department of Pharmaceutics. He received his PhD. from Jawaharlal Nehru
University, New Delhi, India in 1988 working on nonlinear dynamics. His current interests include, apart from impulse absorption in granular systems, modelling of CNS,
earthquake dynamics and nonlinear dynamics of FPU-like systems.

Mukesh Tiwari
Mukesh Tiwari is currently an Associate Professor at the Dhirbubhai Ambani Instituteof Information and Communication Technology (DA-IICT) where he has been working
since 2009. He received his M.Sc degree in Physics from Indian Institute of Technology,
Delhi in 2003 and PhD in Optical Science and Engineering from the University of New
Mexico, USA in 2008. His research interests are mainly focused on statistical physics
and nonlinear systems.

257

KONA Powder and Particle Journal No. 34 (2017) 258–274/Doi:10.14356/kona.2017016

Original Research Paper

Flow of Bi-modal Slurry through Horizontal Bend †Deo Raj Kaushal 1*, Arvind Kumar 2, Yuji Tomita 3,
Shigeru Kuchii 4 and Hiroshi Tsukamoto 41

Department of Civil Engineering, IIT Delhi, India
Department of Mechanical Engineering, YMCA UST, India
3
Kyushu Institute of Technology, Japan
4
Kitakyushu National College of Technology, Japan
2

Abstract
Extensive experimental investigations were carried out for pressure drop and concentration profile in the flow of
bi-modal slurry comprising silica sand and fly ash with mean diameter of 450 and 75 μm, respectively, at six silica
sand:flyash ratios (namely, 100:0, 90:10, 80:20, 70:30, 60:40 and 0:100) in a 53 mm diameter horizontal bend. Flowvelocity was varied up to 3.56 m/s (namely, 1.78, 2.67 and 3.56 m/s) at two efflux concentrations of 8.82 % and16.28 % for each silica sand:flyash ratio. The experimental data were compared with the CFD modelling results
using an Eulerian two-phase model available in the FLUENT software. Eulerian model predicted almost all theexperimental data collected in the present study for pressure drop and concentration profile with fair accuracy.
The bend loss coefficient kt was found to decrease with increase in percentage of flyash for silica sand:flyash ratio
up to 70:30. Further increase in flyash did not show any significant change in the value of kt.
Keywords: Eulerian model, CFD modelling, Bi-modal slurry, pipe bend, pressure drop, concentration profile

1. Introduction
Transportation of granular solids in slurry form
through pipeline is widely applied in industries due to its
several inherent advantages, such as, continuous delivery,
flexible routing, ease in automation and long distancetransport capability. Slurry pipelines are used for transportation of mineral ore, coal, flyash and tailings materials. In dredging operations, sand dredged from rivers isoften pumped with water through a pipeline to a construction site. The flow mechanism of slurry transport
through pipeline is of vital importance to the designers ofslurry transport system. Pipeline bends normally incur
relatively higher pressure drop and are prone to excessive
erosion compared to the straight equivalent length of pipe.
Flow in bends is quite complex and complexity is further
increased for two-phase mixtures. Studies available in literature have shown that the flow in bends for solid-liquid
mixture system is much more complicated than in gas-

†
1
2
3
4

*

Received 18 January 2016; Accepted 20 July 2016
J-STAGE Advance Publication online 30 August 2016
Hauz Khas, New Delhi-110016, India
Faridabad, Haryana, India
1-1 Sensui cho, Tobata, Kitakyushu 804-8550, Japan
5-20-1 Shii, Kokura-minami, Kitakyushu 802-0985, Japan
Corresponding author: Deo Raj Kaushal;
E-mail: kaushal@civil.iitd.ac.in
TEL: +91-98-1828-0867 FAX: +91-11-2658-1117

258

solid system (Kumar, 2010). Reported literature for solid-­
liquid flow in bends is limited which may be due tocomplexities involved. It is already well established thatbends in general are prone to excessive erosion wear andhence need frequent replacements resulting in shut-downsof plants leading to loss of man hours. In order to increase
the longevity of the pipe bends and to minimise the pressure loss through bends, a detailed knowledge of the flow
properties in the pipe bends and downstream of the bend
is essential. Most of the experimental and computational
work related to the effect of bends in slurry pipeline system concern the erosion, concentration profiles and pressure drops for the flow of mono-dispersed particles(Kaushal et al., 2013). However, there is only few studies
available on flow of bi-modal and multi-sized particles
through pipe bends.
Nasr-El-Din and Shook (1987) have measured the concentration profile and velocity distribution for flow of bimodal slurry in a vertical pipe with short and long radiusbends. They observed axi-symmetric flow for short radiusbend whereas the coarser and finer particles migrate towards the outer and inner wall, respectively, for the longradius bend. Ahmed et al. (1994) have measured the pressure drop in a 90° horizontal bend using two multi-sizedparticulate slurries namely iron ore and zinc tailings slurries. From their studies they concluded that the presenceof the finer particles reduces the additional pressure dropin the flow of multi-sized particulate slurries across the

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

bends. They found from bi-modal viewpoint that particles
re-distribute more in the bend as compared to the straightpipe but could not identify the plane of movement. The
extent of re-distribution reduces as the flow velocity andconcentration increases. Gupta et al. (1997) confirmed thefindings of Ahmed et al. (1994) for multi-sized particulateslurries. They observed the maximum concentration to beat the bottom of the bend along the vertical plane even at
higher concentration also. They however observed thatlargest size particles moved outward and its concentration
was higher in outer bottom periphery. The finer particles
were observed to be uniformly distributed. Literature on
studies of pressure drop by Kazanskij and Hinsch (1974),
Boothroyde et al. (1979), Hayashi et al. (1980) and Verkerk(1982) reveals that mixing of fine particles in small percentage up to 20–30 % in coarse slurry helps to reduce thepressure drop and makes the distribution of particles homogeneous.
Advent of highly sophisticated computers with advanced numerical techniques involved in computational
fluid dynamics (CFD) analysis made it possible to analyse
the operation of slurry transport systems using numericalsimulations but the literature review clearly reveals thatthe application of CFD for such systems is few (Thinglas
and Kaushal, 2008a, 2008b). Kaushal et al. (2012) numerically simulated using mixture and granular-Eulerian twophase models for mono-dispersed high concentrate slurryflow in Pipeline. Standard k-ε model per phase and thestandard wall function with no slip were used. They concluded that mixture model failed to predict but granular-­
Eulerian model per phase gave better predictions forpressure drop at all efflux concentrations and flow velocities. However, this model failed to predict solid concentration distribution, more dominantly at higher concentrations
and flow velocities, for few experimental data near thepipe bottom. Kaushal et al. (2013) made use of Eulerianmodel in FLUENT software to calculate the pressure dropand concentration profile in mono-­dispersed silica sandslurry flow through pipe bend. Eulerian model was foundreasonably effective with the slurry pipe bend on the basisof good agreement between their experimental and computational results. Messa and Malavasi (2014) simulated
the fully-suspended slurry flow in horizontal pipe basedon the standard k-ε model. Phase diffusion fluxes are introduced in all conservation equations to reproduce theeffect of the turbulent dispersion of particles. The mixture
viscosity approach is employed to model the effect of thepresence of multiple particles on interfacial momentum
transfer. Equilibrium wall function for smooth walls is
applied to both phases and no slip is assumed between thephases at the inlet section. In comparison with similar
models, the authors revealed that the used model showed
better performance in terms of numerical efficiency, quality of the solution and range of applicability. Gopaliya andKaushal (2015) investigated the effects of particle size on
slurry flow parameters using CFD by implementing granular RNG k-ε turbulence model with standard wall function
and no-slip at the wall. It is reported that high deviationsin the values between simulated and experimental data of
local solid concentration values are observed in the lower
half of the cross-­section just near the wall for coarser
grain sizes. Gopaliya and Kaushal (2016) analysed the effects of particle size on various slurry flow parameterssuch as pressure drop, solid phase velocity distribution,
friction factor, granular pressure, turbulent viscosity, turbulent kinetic energy and its dissipation. However, there
is no study available in literature for modelling the flow of
bi-modal or multi-sized particulate slurries through pipeline using three-dimensional CFD based Eulerian twophase model. The present research work delves deep into
the transport mechanism of bi-modal slurries throughpipe bend by conducting experiments and numerical simulations.
In the present study, measurements of concentrationand particle size distribution are made at downstream ofthe bend at different locations to understand the phenomena of re-distribution of particles belonging to two size
fractions of the bi-modal slurry. Concentration profiles
and pressure drops are modelled using Eulerian model ina pipe bend for the flow of bi-modal silica sand and fly ash
slurries at six silica sand:flyash ratios. The CFD basedmodelling results are compared with the experimental
data.

2. Experimental setup
In the present study, pilot plant test loop having inside
diameter of 53 mm and length of 30 m is used. Pipe bendis laid horizontally in the loop. Fig. 1 shows the schematiclayout of the pilot plant test loop. The loop is described indetail by Kumar (2010) Kaushal et al. (2013).
We measured concentration profiles by sampling tube
in the mid-vertical plane in the downstream of bend exit
as shown in Fig. 2 Sampling tube was having a 4 mm x
6mm rectangular slot 2 mm above the end to collect representative samples in the pipe line. Samples are collectedfrom different heights from bottom of the pipe in the vertical plane of the cross-section to measure the concentration proﬁle under near isokinetic conditions. As the flow
is under pressure, slurry comes out from the samplingtube with the same velocity and volumetric concentration
as it is flowing in the pipeline at the location of measurement. During the collection of samples it is ensured thatthe flow of the slurry through the sampling tube outlet isnonstop and uniform. At the end of the pipe loop a sampling point is provided in the vertical portions to collectan average efflux sample. The size of rectangular opening259

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

Fig. 1

Fig. 2

Schematic diagram of pilot plant test loop.

Locations of pressure tappings and concentration samplers

was optimized after extensive trials to ensure smooth
flow of slurry. The accuracy of the sampling tube ischecked by integrating the measured concentration profile
to obtain overall concentration and comparing it with themeasured efflux concentration. An error included in themeasurement was found to be ±1 %. This error may be attributed to the slip velocity between primary water phaseand secondary silica sand or fly ash phase. The bend geometry used in the simulations was identical to that ofbend used in experiments. The bend was horizontallylaid in (x, y) plane. In the present study, concentration
samples are collected from six different locations foreach experimental run by traversing the sampling probe
in the mid-vertical plane of pipe cross-section located atz′ = 0.094, 0.189, 0.377, 0.566, 0.755 and 0.943, where260z′ = z/D. Three sampling probes located at 5D, 25D and
50D from bend exit are used for collecting such concentration samples (Fig. 2). Overall volumetric concentrations αs are experimentally measured for each run, wheresusbscript “s” represents the slurry. The slurry samples
are dried in oven and sieved to determine the concentrations of silica sand (α ss) and flyash (αf ) considering
αs = αss + αf. A sieve of aperture diameter of 150 micron
is used to separate out silica sand and flyash particles
from each dried sample.
Two methods were applied for the flow rate measurement (measuring tank and electro-magnetic flow meter)in this study as shown in Fig. 1. Magnetic flow meter was
also used in adjusting the flow velocity to the desired
value. The accuracy of flow rate measurement by electro-

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

separately. Other steps in computations are similar to that
described in earlier work by Kaushal et al. (2013).

Table 1 Details of pipe bend used in the present study.
(a) R/r and length of bend
Radius ofcurvature (R)Radius ratio(R/r)

Length ofpipe bend

148.4 mm

5.6

4.40 D

4. Modeling results
4.1 Pressure drop(b) Inside radius of pipe bend: r = 26.5 mm(c) D = Pipe diameter = 53 mm(d) ks = 0.15 mm and ks /D = 0.0003

magnetic flow meters is of the order of ±0.5 %. The calibration of the meters was periodically checked for slurryflows at various concentrations using measuring tank.
Efflux sampler is used to monitor the efflux concentration as shown in Fig. 1. If we consider the concentration
profile in vertical section of pipeline as uniform acrossthe pipe cross-section and Vss = Vf = Vw = Vsl = Vm, the efflux concentration (Cvf ) of slurry may be defined as
Cvf = Css + Cf, where Css and Cf is the efflux concentrationof silica sand and flyash, respectively. Css and Cf may bemathematically defined as follows:

Css 

1
αssdA
A

(1)

Cf 

1
αf dA
A

(2)

The normalized pressure drops at different locations[Δh/(Vm 2/2g), where, Δh = Δp/(ρ mg)] are presented inFig. 3 and 4 for Cvf = 8.82 and 16.28 % for 70:30 silica
sand:flyash ratio at Vm = 1.78, 2.67 and 3.56 m/s.
From Fig. 3 and 4, it is clear that the flow is slightly
disturbed before and after the bend. The permanent pressure loss reduces with increase in Vm. This reduction isinterpreted from the slope of the pressure drop line, upstream and downstream of the bend.
It is observed from Fig. 3 and 4 that the pressure drop
reduces as the amount of flyash increases. However, thedecrease in pressure drop is negligible for amount ofExperiments are carried out on rough bend of whichgeometric details are tabulated in Table 1. Specific gravity of silica sand and flyash is measured experimentally as
2.65 and 2.1, respectively. Silica sand and fly ash particles
are having median diameter of 450 μm and 75 μm withgeometric standard deviation of 1.15 and 1.10, criticaldeposition velocity of 1.5 m/s and 1.1 m/s, respectively.
Mixtures having different silica sand:flyash ratios at aparticular efflux concentration are prepared by mixing thesilica sand and flyash in that ratio by volume.

3. Mathematical model
The ranges of Stokes number of particles and Dean
number for the pipeline slurry flows carried out in thepresent study are 0.02 to 2.0 and 33385 to 59152, respectively. The mathematical model implemented is describedin Kaushal et al. (2013) and given in Appendix A. Water,
silica sand and fly ash are considered as three phases having different properties in the calculations. Water is considered as fluid and primary phase. Silica sand and fly ash
are considered as granular and secondary phases withdifferent concentration, particle size and mass densities.
Momentum equations have been solved for each phaseFig. 3

Normalised Pressure drop [Δp/(ρmVm2/2)] distributions
at different flow velocities at Cvf = 8.82 % (sand:flyash = 70:30).

261

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

flyash more than 30 %.
Ito (1960) has shown that to establish permanent pressure loss across a bend, considerable lengths of the pipeon both sides of the bend have to be included and this isshown schematically in Fig. 5 as described below:
i. A-B-C-D-E shows the actual hydraulic gradient.
ii. A-B′-C′-D′-E′ shows hydraulic gradient in a corresponding straight pipe.

Fig. 4

Normalised Pressure drop [Δp/(ρmVm2/2)] distributions
at different flow velocities at Cvf = 16.28 % (sand:flyash = 70:30).

Fig. 5

Typical hydraulic gradient in pipe flow in the presenceof bend.

262

iii. A-B′-C′′-D′′-E′′ shows hydraulic gradient without
pipe bend or when the pipe bend is removed. It isclear from these hydraulic grade lines that totalhead loss across the pipe bend is Δht.
iv. The total bend loss coefficient (kt) is defined asΔht(3) Vm 2  2g 
The measured and simulated bend loss coefficients (kt)are evaluated for each run as and are shown graphicallyin Fig. 6 and 7 for Cvf = 8.82 % and 16.28 % as a function
kt 

Fig. 6

Measured and predicted bend loss coefficient (kt) at
Cvf = 8.82 % for various sand:flyash ratios.

Fig. 7

Bend loss coefficient (kt) at Cvf = 16.28 % for various
sand:flyash ratios.

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

of Vm at different mix proportions. It is observed thatvalue of kt increases as Cvf increases. Value of kt reduces
as flow velocity increases for all sand:flyash ratios and efflux concentrations. However, the rate of this reduction
decreases with increase in amount of flyash. A reduction
in total bend loss coefficient is seen with increase in theflyash mixed in the bi-modal slurry up to 70:30. However,further increase in flyash do not show any significant
change in the value of kt. The uniform distribution of fineFig. 8

particles, that is flyash, across the pipe for the bi-modal
slurry provides a uniform suspension of increased viscosity for the coarse particles. This uniform suspension offiner particles provides better suspension to coarse particles resulting into the reduced pressure drop. At the lowest velocity, appreciably high value of kt is obtained for all
the mix proportions when compared to that obtained atlower concentration. Fig. 6 and 7 also show that the CFD
modeling gives good predictions with error of ±10 %.

Cross-sectional concentration distributions of (a) αss, (b) αf and (c) αs in fraction at different locations for
sand:flyash = 70:30 at Vm = 3.56 m/s and Cvf = 16.28 %.

263

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

In the present study, the turbulent viscosity at bend outlet is 0.151, 0.268 and 0.305 kg/m-s for Cvf = 16.28 % at
Vm = 1.78, 2.67 and 3.56 m/s, respectively. Turbulent viscosity at bend outlet is 0.056, 0.230 and 0.307 kg/m-s for
Cvf = 8.82 % at Vm = 1.78, 2.67 and 3.56 m/s, respectively.

4.2. Concentration distribution
Cross-sectional concentration distributions of silica
sand (α ss), flyash (αf ) and mixture (α s) for different silica sand:flyash ratios are calculated using CFD based
FLUENT (2006) software. One of such concentration distributions αss, αf and αs for silica sand:flyash ratio of 70:30
at Cvf = 16.28 % and Vm = 3.56 m/s is shown in Fig. 8(a),(b) and (c) for silica sand, fly ash and mixture, respectively. The particles are forced outwards in the vicinity ofbend due to the interaction of centrifugal force and pressure driven secondary flows. In a curved flow, there appears a centrifugal force and in order to balance this forcepressure increases outward. However, near the wall, fluid
velocity is negligibly small and there appears pressure
driven flow near the wall inward, thus, secondary flow
appears.
Flyash particles are forced outwards along with the silica sand particles (Fig. 8). However, Nasr-El-Din andShook (1987) observed experimentally that coarser particles (0.45 mm diameter) moving towards the outer andfiner particles moving towards the inner edge for long radius bend.
The concentration profiles at bend inlet, X = 25D and
50D shows maximum concentration away from bottom of

Fig. 9264

the pipeline at lower velocities for lower concentration
and at all velocities for higher concentration. In Fig. 9,
distributions of υfz, υsz and υssz in m/s at Cvf = 16.28 % and
Vm = 3.56 m/s are shown. The effect of bend on velocity
profiles is more prominent at X = 5D. The observations
made previously in Fig. 8 are reaffirmed in Fig. 10 showing the z-component of velocity for sand, flyash and water. In this figure, very low values of υfz(x, y), υsz(x, y) andυssz(x, y) at X = 50D indicate vanishing of the secondary
flows.
Fig. 11 depicts the direction and magnitude of velocity
component in the perpendicular plane for Cvf = 16.28 %and Vm = 3.56 m/s at X = 5D, 25D and 50D. This figure reaffirms the observations in earlier figures.
Figs. 12 to 17 present concentration profiles at X = 5D,
25D and 50D from bend outlet at Vm = 1.78, 2.67 and3.56 m/s and Cvf = 8.82 % and 16.28 %. Normalized conα  z
α  z
centration is defined as ss
for silica sand, f
for
Css
Cf
α  z
flyash and sfor overall slurry. Figs. 12 to 17 shows
C vf
good resemblance in experimental and CFD values.
Concentration profiles of sand in bimodal (sand:flyash = 70:30) slurry at Cvf = 8.82 % and 16.28 % are shownin Fig. 18 to 21. It is observed that spread in the solids
concentration profiles is the least at X = 5D and the particles are occupied in the entire cross-section of the pipeline. However, the spread in solids concentration profile
increases with increasing X to the extent that solids concentration becomes zero at topmost zone in the pipeline at
X = 25D and 50D. Furthermore, this area of occupancy byDistributions of υfz, υsz and υssz in m/s at Cvf = 16.28 % and Vm = 3.56 m/s.

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

particles increases with increase in velocity of flow.
Fig. 19 shows predicted mid-vertical concentration
profiles of sand, in monodispersed (sand:flyash = 100:0)
slurry measured by Kaushal et al. (2013) and in bimodal
slurry (sand:flyash = 70:30) at different X and Vm at
Cvf = 8.82 %. Asymmetry in the concentration profiles for
bi-modal slurry is less. Such a reduction in asymmetry
may be attributed to the uniform distribution of flyash
across the pipe in the bi-modal slurry, which provides a
uniform suspension of increased viscosity and less asymmetric mid-vertical solids concentration profile for thecoarse particles. Figs. 20 and 21 reaffirms the fact thatre-distribution of solid particles takes place downstreamof the bend. This effect decays with increase in distancefrom bend.

5. Conclusions
Following conclusions are drawn the present study:a. Pressure drop for bi-modal mixture of silica sand andflyash having particle diameter 450 μm and 75 μm, respectively, is found to be less than that of mono-­
dispersed (silica sand) slurry flow. In bi-modal slurry
flow, the bend loss coefficient (kt) decreases with increase in percentage of flyash up to silica sand:flyashratio of 70:30. Further increase in flyash does not show
any significant change in the value of kt.
b. CFD based Eulerian two-phase model predicts pressure
drops with fair accuracy (within percentage error of±10 %) for the flow of bi-modal slurry in the efflux concentration upto 16.28 % at flow velocities ranging from1.78 to 3.56 m/s.

Fig. 10 Vertical velocity υfz(x, y), υsz(x, y) and υssz(x, y) at different locations for sand:flyash = 70:30 in m/s for
Cvf = 16.28 % and Vm = 3.56 m/s.

265Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

c. Concentration profiles become uniform in the downstream of bend near the bend exit.
d. The uniform distribution of fine particles across thepipe for the bi-modal slurry provides a uniform suspension of increased viscosity for the coarse particles. This
uniform suspension of finer particles provides bettersuspension to coarse particles, which in turn make there-distribution of particles after the bend exit at a much
faster rate.
e. Redistribution of particles take place in the downstream of the bend. The effect of bend is seen in thedownstream of bend near the exit, and this effect de-cays with increase in distance.
f. Asymmetry in the mid-vertical solids concentration
profiles for bi-modal (sand:flyash = 70:30) is less thanthe mono-dispersed (sand:flyash = 100:0) particulateslurry.
g. More work needs to be performed to expand the CFD
based Eulerian two-phase model for more complex
slurries containing particles with broad or multimodal
size distributions. Also work should be done to determine how best to deal with slurries containing different
species of particles with multiple densities.

Fig. 11 Perpendicular velocity component at different locations for sand:flyash = 70:30 in m/s for Cvf = 16.28 %and Vm = 3.56 m/s.

266

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

Fig. 12 Concentration distributions at Cvf = 8.82 % (sand:flyash = 70:30) and Vm = 1.78 m/s (Normalised Concentration
for sand = αs(z′)/Css) and Normalised Concentration for fly ash = αf(z′)/Cf ).

Fig. 13 Concentration distributions at Cvf = 8.82 % (sand:flyash = 70:30) and Vm = 2.67 m/s (Normalised Concentration
for sand = αs(z′)/Css) and Normalised Concentration for fly ash = αf(z′)/Cf ).

267

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

Fig. 14 Concentration distributions at Cvf = 8.82 % (sand:flyash = 70:30) and Vm = 3.56 m/s (Normalised Concentration
for sand = αs(z′)/Css) and Normalised Concentration for fly ash = αf(z′)/Cf ).

Fig. 15 Concentration distributions at Cvf = 16.28 % (sand:flyash = 70:30) and Vm = 1.78 m/s (Normalised Concentration
for sand = αs(z′)/Css) and Normalised Concentration for fly ash = αf(z′)/Cf ).

268

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

Fig. 16 Concentration distributions at Cvf = 16.28 % (sand:flyash = 70:30) and Vm = 2.67 m/s (Normalised Concentration
for sand = αs(z′)/Css) and Normalised Concentration for fly ash = αf(z′)/Cf ).

Fig. 17 Concentration distributions at Cvf = 16.28 % (sand:flyash = 70:30) and Vm = 3.56 m/s (Normalised Concentration
for sand = αs(z′)/Css) and Normalised Concentration for fly ash = αf(z′)/Cf ).

269

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

Fig. 18 Predicted mid-vertical concentration profiles of sand at different X and Vm for sand:flyash = 70:30 at
Cvf = 8.82 % and 16.28 % (Normalised Concentration = αs(z′)/Css).

Fig. 19 Predicted mid-vertical concentration profiles of sand in monodispersed and bimodal (sand:flyash = 70:30) slurry at different X and Vm at Cvf = 8.82 % (Normalised Concentration = αs(z′)/Css).

270

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

Fig. 20 Concentration profile of sand for sand:flyash = 70:30 at Cvf = 8.82 % (Normalised Concentration = αs(z′)/Css).

Fig. 21 Concentration profiles of sand for sand:flyash = 70:30 at Cvf = 16.28 % (Normalised Concentration = αs(z′)/Css).

Nomenclaturekt

bend loss coefficients (–)

kturbulent kinetic energy (m 2/s2)

εturbulent dissipation rate (m 2/s3)

Ksf

υs

υf

inter-phase drag coefficient (–)

CLlift coefficient taken as 0.5 in the present study (–)

Pssolid pressure gradient (Pa)P

static pressure gradient (Pa)Cvm

coefficient of virtual mass force taken as 0.5 in thepresent study (–)τfstress tensor for fluid (Pa)τsstress tensors for solid (Pa)λsbulk viscosity of the solids (Pa.s)

dsparticle diameter put as 440 μm for silica sand and75 μm for flyash (μm)

go,ssradial distribution function (–)αs,maxstatic settled concentration measured as 0.52 and 0.50
for sand and flyash particles, respectively (–)

Dinternal diameter of pipeline (mm)dmeanmean diameter of particles (μm)

zdistance from bottom of the pipe (mm)αsoverall concentration of slurry (–)αss

concentration of silica sand (–)αfconcentration of flyash (–)

Vssvelocity of silica sand (m/s)

Vfvelocity of flyash (m/s)

Vwvelocity of water (m/s)

Vslvelocity of slurry (m/s)

Vmmean flow velocity (m/s)

Cvf

efflux concentration of slurry (–)

Cssefflux concentration of silica sand (–)

Cfefflux concentration of flyash (–)

Apipe cross-sectional area (m 2)Δppressure drop with reference to the velocity inlet (Pa)ρmdensity of the flowing fluid (Kg/m3)

gacceleration due to gravity (m/s2)

Θsgranular temperature (Pa)Δht

total head loss across the pipe bend (m)essrestitution coefficient (–)

velocity of solid phase (m/s)
velocity of fluid phase (m/s)

271

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

μf

shear viscosity of fluid (Pa.s)

μs

shear viscosity of solids (Pa.s)

Rradius of curvature of pipe bend (mm)rinternal radius of pipeline (mm)kspipe roughness (mm)References
Ahmed M., Singh S.N., Seshadri V., Pressure drop in a longradius 90° horizontal bend for the flow of multi-sized heterogeneous slurries, International Journal of Multiphase
Flow, 1994 (21) 329–334.
Boothroyde J., Jacobs B.E.A., Jenkins P., Coarse particle
hydraulic transport, Proceedings Hydrotransport, BHRA
Fluid Engineering, Cranfield, Bedford, England, 6 (1979),
405–428.
FLUENT 6.3.26 User’s Guide, Chapter 23: Modelling Multiphase Flows, Fluent Incorporation, USA, 2006.
Gidaspow D., Bezburuah R., Ding J., Hydrodynamics of circulating fluidized beds, kinetic theory approach in fluidization
VII, Proc. 7th Engineering Foundation Conference on Fluidization, 1992.
Gopaliya M.K., Kaushal D.R., Analysis of Effect of Grain Sizeon Various Parameters of Slurry Flow through Pipeline
Using CFD, Particulate Science and Technology: An International Journal, 33 (2015) 369–384.
Gopaliya M.K., Kaushal D.R., Modeling of sand-water slurryflow through horizontal pipe using CFD, J. Hydrol. Hydromech., 64 (2016) 261–272.
Gupta R., Singh S.N., Seshadri V., Migration of solid particles
in the heterogeneous slurry flow through a 90° bend, Indian
Journal of Engineering and Material Science, 4 (1997)
10–20.
Hayashi H., Sampei T., Oda S., Ohtomo S., Some Experimental
Studies on Iron Concentrate Slurry Transportation in Pilot
Plant, Proceedings of Hydrotransport, BHRA Fluid Engineering, Cranfield, Bedford, England, 7 (1980) D2.
Ito H., Pressure losses in smooth pipe bends, Journal of BasicEngineering, ASME, 82 (1960) 131-140.
Kaushal D.R., Thinglas T., Tomita Y., Kuchii S., Tsukamoto H.,
CFD Modeling for Pipeline Flow of Fine Particles at High
Concentration, Int. J. of Multiphase Flow, 43 (2012) 85–100.
Kaushal D.R., Kumar A., Tomita Y., Kuchii S., Tsukamoto H.,
Flow of mono-dispersed particles through horizontal bend,
Int. J. of Multiphase Flow, 52 (2013) 71–91.
Kazanskij B.H., Hinsch J., Influence of added fine particles on
the flow structure and the pressure losses in sand-water
mixture, Proceedings of Hydrotransport, BHRA Fluid
Engineering, Colardo, USA, 3 (1974) 11–21.
Kumar A., Kaushal D.R., Kumar U., Bend Pressure Drop
Experiments compared with FLUENT, Institution of Civil
Engineers journal of Engineering and Computational
Mechanics, Thomas Telford Publication London, 161 (2008)
35-42.
Kumar A., Experimental and CFD modeling of hydraulic andpneumatic conveying through pipeline, Ph.D. Thesis,
Indian Institute of Technology Delhi, 2010.
Lun C.K.K., Savage S.B., Jeffrey D.J., Chepurniy N., Kinetic

272

theories for granular flow: inelastic particles in couette flow
and slightly inelastic particles in a general flow field, Journal of Fluid Mechanics, 140 (1984) 223–256.
Messa G.V., Malavasi S., Numerical Prediction of Particle Distribution of Solid-Liquid Slurries in Straight Pipes andBends, Engineering Applications of Computational Fluid
Mechanics, 8 (2014) 356–372.
Nasr-El-Din H., Shook C.A., Effects of 90° bend on slurry
velocity and concentration distribution, Journal of Pipelines, 6 (1987) 239–252.
Thinglas T., Kaushal D.R., Comparison of two dimensional andthree Dimensional CFD Modeling of Invert Trap Configuration to be used in Sewer Solid Management, Particuology,
6 (2008a) 176–184.
Thinglas T., Kaushal D.R., Three Dimensional CFD Modelingfor Optimization of Invert Trap Configuration to be used in
Sewer Solid Management, Particulate Science and Technology, 26 (2008b) 507–519.
Verkerk C.G., Transport of Fly Ash Slurries. Proceedings ofHydrotransport, BHRA Fluid Engineering, Johannesburg,
South Africa, 8 (1982) 307–316.

Appendix A: Mathematical modelTwo-phase Eulerian model adopted in present researchassumes that the slurry comprises solid “so” and fluid “fl”.
These phases are considered as separated but continuous
and interpenetrating in such a way that αso + αfl = 100 %.
These phases are made to satisfy continuity and momentum equations separately. Equations of fluid and solid
phases were coupled using inter-phasial exchange coefficients and pressure. Different forces existing in the slurryflow are listed below.

1. Body forces, ρg 
2. Drag force, K sf υs  υf  

3. Lift force, CLαs ρf υf  υs     υf 
4. Pressure gradient for solid phase, Ps
5. Static pressure gradient, P 
6. Virtual mass force: C vmαs ρf υf υf  υs υs 
7. Viscous forces,   τ f
The continuity equation (equation A1) and momentum
equations (equations A2 and A3) need to be satisfied byeach phase separately. These equations are coupled using
exchange coefficients and pressure (equations A4 andA5).
Continuity equation:  α t ρ tυt   0(A1)
Momentum equation for fluid phase:   αf ρf υf υf   αf P    τ f   
αf ρf g  K sf υs  υf  



 

C vmαf ρf υs υs  υf υf  CLαs ρf υf  υs    υf 

(A2)

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

Wall function:

Momentum equation for solid phase:   αs ρs υs υs   αsP  Ps    τ s   
αs ρf g  K fs υf  υs  



 

C vmαs ρf υf υf  υs υs  CLαs ρf υs  υf     υf 

(A3)

The region very near to the walls needs special attention. In this region, the flow parameters have very highgradients. This region is dealt with a very fine boundary
layer meshing. Standard wall functions along with RNG
k-ε model is used. Wall functions are semi-empirical formulas used for reducing the difficulty in calculation andbetter results close to the wall.

tr 

2
 

τ s αs μs  υs   υs   αs  λs  μs    υs I
3 




(A4)

Boundary Conditions

and
tr 
 

τ f α f μ f   υf   υf  ,



(A5)

Bulk viscosity of solids λs is proposed by Lun et al.
(1984) as:
1

4
Θ 2

λs
αs ρsd s g o,ss 1  ess   s 
3
 π 

(A6)

Radial distribution function go,ss is proposed by Gidaspow et al. (1992) as:
1 1

3


 α
(A7)
g o,ss 1   s    αs,max  Granular temperature Θs is taken into account in themathematical model by using transport equations as described by Kaushal et al. (2013).

Turbulence closure for the fluid and solid phase:

Turbulent quantities for the fluid and solid phase arecalculated using RNG k-ε model and Tchen’s theory, respectively, supplemented by additional terms accounting
interfacial turbulent momentum transfer as described by
Kaushal et al. (2013).

There are three boundaries available in the given flow
domain namely the inlet boundary, the wall boundary andthe outlet boundary. Constant concentration and flow velocity were considered at the inlet boundary. A 0.15 mmof pipe wall roughness is adopted during simulation. Wallboundary conditions are adopted phase-wise: no slip for
liquid phase and specularity co-efficient of 0.5 for solid
phase. Pressure outlet boundary condition is adopted at
the outlet boundary where mixture pressure is mentioned.

Solution Process and Convergence Criteria

The convergence criteria is decided on the basis ofproper residual values for different parameters at the endof computation for better results. In the present study aresidual value of 10 –3 is set as convergence criteria. The
solution was converged and computations were stopped in
1475 iterations for slurry flow with sand:flyash = 70:30 at
Vm = 3.56 m/s and Cvf = 16.28 % through pipe bend. The
actual values of the residuals at convergence for mass, u,v and w components of water, sand and flyash in x, y and z
directions, k and ε for water and volume fractions of sandand flyash were 2.9024×10 –4, 9.1087×10 –5, 5.5728×10 –6,
3.0933×10 – 6 , 1.1206×10 –5, 3.3552×10 – 6 , 2.4197×10 – 6 ,1.2064×10 –5, 4.6818×10 –6 , 3.1317×10 –6 , 2.9304×10 – 4 ,9.9960×10 –4, 3.5810×10 –4 and 2.6986×10 –4, respectively.

Author’s short biography
Deo Raj Kaushal
Dr. D.R. Kaushal was trained to be a slurry pipeline designer during his Ph.D. at IIT
Delhi and Post Doctoral Research at KIT Japan. He has been an Associate Professor of
Hydraulics in Water Resources Engineering Section of Civil Engineering Department at
IIT Delhi since 2004. He has visited KIT Japan and KCT Japan as visiting Professor.
During his teaching and postdoctoral research, he has carried out several research andconsultancy projects. He has worked with several mining, mineral-processing and production companies, either through direct consultation or through collaboration withother companies or research organizations.

273

Deo Raj Kaushal et al. / KONA Powder and Particle Journal No. 34 (2017) 258–274

Author’s short biography
Arvind Kumar
Dr. A. Kumar is currently Professor in Mechanical Engineering Department at YMCA
University of Science and Technology , Faridabad (Haryana) India. He obtained his
B.Tech. (Mechanical Engg.) from AMU, Aligarh and M.Tech. and PhD From Indian Institute of Technology Delhi, India. He is having 25 years Teaching, Research and Industrial experience. His areas of specialization include Computational Fluid Dynamics,
Multiphase flow, Slurry flow in pipeline system. Energy efficiency and energy management in thermal system, heat transfer, Refrigeration and Air-conditioning.

Yuji Tomita
Prof. Yuji Tomita was born in Dalian, China in 1942. From 1970 to 2006, he has beenworking at Kyushu Institute of Technology in the mechanical engineering department.
He taught fluid mechanics and powder technology. His research topics include fluid dragreduction, pipeline conveying of solids by pneumatic, slurry and capsule method, performance characteristics of blow-tank solids conveyor, rotary and injection feeders, flushingphenomena of fine powder by compression and vibration, and air entrainment by freefalling powder jet.

Shigeru Kuchii
Prof. Shigeru Kuchii is a Professor from Department of Creative Engineering,
National Institute of Technology (NIT), Kitakyushu College. He received the B.S, M.S
and Ph D. degrees all in Mechanical Engineering from Kyushu Institute of Technology,
Japan. He is working on research topic in Powder Technology, Mechanical Engineering,
Mechatronics and Information Technology.

Hiroshi Tsukamoto
Prof. Hiroshi Tsukamoto is the President of National Institute of Technology, Kitakyushu College. After receiving the Ph.D. degree from the University of Tokyo, Tokyo,
Japan, he was a Lecturer in the Department of Mechanical Engineering, Universityof Tokyo, and an Associate Professor, a Professor in the Department of MechanicalEngineering, and a Professor of biological functions and engineering, Kyushu Instituteof Technology, Kitakyushu, Japan, where he has been a Professor Emeritus since 2009.
Dr. Tsukamoto is a fellow of The Japan Society of Mechanical Engineers, and a memberof The Engineering Academy of Japan.

274

KONA Powder and Particle Journal No. 34 (2017) 275–281/Doi:10.14356/kona.2017018

Original Research Paper

A Study of the Mixing Index in Solid Particles †Shih-Hao Chou, Yue-Lou Song and Shu-San Hsiau *1

Department of Mechanical Engineering, National Central University, Taiwan

Abstract
This study focuses on investigating the mixing state by different mixing indices for different pre-organizedparticles. We also try to propose a new mixing index and discuss the advantages and disadvantages of different
cases.
The results show that the Avg. distance mixing index indicates the degree of mixing more accurately than othermixing indices for a case of complete segregation. In some cases the degree of mixing would be higher with theLacey mixing index compared with the other indices, with the size of the cells greatly affecting this value. In
other words, the use of any single specific mixing index on its own cannot accurately reflect the correct degree of
mixing for all different kinds of systems. The only difference between mixing indices is which one can provide
more accurate and stable results.
Keywords: mixing index, degree of mixing, granular material, pre-organized condition

1. Background
The mixing of powders is very important in industry as
well as in our daily life. Many products or semi-products
are made from different ingredients in the form of powders which need to be well mixed to make ensure a highquality end product. For example, for manufacturing
pharmaceuticals, avoiding segregation and achieving awell-mixed powder is essential. Thus, the problem of
mixing/segregation in powdered or granular materials has
been studied by many industrial engineers and academics.
Powders can be mixed by mechanical excitation, gasflow agitation, gravity-driven free-falling, and many other
different complex methods (Masuda H. et al., 2006). Different types of blenders/mixers have been developed forindustry. The mechanisms of powder mixing include convective mixing, shear mixing and diffusive mixing (Lacey
P.M.C., 1954; Kaye B.H., 1997; Ottino J.M. and Khakhar
D.V., 2000). The rate of mixing by diffusive mixing is low
compared with convective mixing mechanisms. However,diffusive mixing is essential for microscopic homogenization (Gotoh K. et al., 1997).
What is a so-called “well-mixed” granular material?
The mixing status is usually determined based upon one
or several samples taken from a batch. The way the sam†1*

Received 23 June 2016; Accepted 1 August 2016
J-STAGE Advance Publication online 30 August 2016
No. 300, Jhongda Rd. Jhongli City, Taoyuan County 32001, Taiwan
Corresponding author: Shu-San Hsiau;
E-mail: sshsiau@cc.ncu.edu.tw
TEL: +886-3-426-7341 FAX: +886-3-425-4501

ple is taken, the number of samples, the locations the
samples are taken from, the size of the samples, and soon, all influence the evaluation of the mixing status(Weidenbaum S.S., 1958). The statistical method for investigating the samples is also critical. This paper studies
determination of the mixing index for particle mixing by
sampling.
As mentioned above, the mixing index is an importantparameter to indicative of the status of mixing, with avalue of 1 indicating that the material is fully mixed; 0represents fully segregated. However there are many different mixing indices with different ways of determining
the status some of which are investigated in this study todetermine which one is the best and the most user
friendly?

2. Introduction to some mixing indices
Currently, there are a number of suitable mixing indices that can be used to analyze granular mixing behavior.
Gayle J.B. et al. (1958) proposed a mathematical method
to solve the mixing problem based on the chi squarevalue. Their method can not only be used for a binary
mixture system but also to for systems with any finite
number of components. Leggatt C.W. (1951) studied themixing process for weeds and crops, and determined thedispersion of homogeneity in a unit weight. Poux M. et al.
(1991) also made a careful review of many different mixing indices.
Most investigations of mixing indices are based on the

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

275

Shih-Hao Chou et al. / KONA Powder and Particle Journal No. 34 (2017) 275–281

statistical method but photometric and kinetic approaches
are also employed. However, the conditions for power
mixing are defined differently in different mixing indicesindicating the uncertainty and complicated nature of themixing process. In this study, some of the mixing indicescommonly used in academic research or in industry arereviewed; only binary mixtures are considered.
(1) Lacey Mixing Index
Lacey P.M.C. (1943) first introduced what is now
known as the Lacey mixing index in 1943, by dividing thearea of interest into N cells. Assume that ϕi is the concentration of the reference component in the “ith” cell and ϕmis the overall concentration of the reference component.
The variance of σ 2 for the concentration of a referencecomponent in each cell can be expressed by

σ2 

N


i 1

i  m 2 ,
N 1

(1)

where N is the total number of cells occupied by the total
particles; ϕi is the local concentration of tracer particles ineach cell; ϕm is the average concentration of tracer particles
in the whole system. Theoretically, for a binary-mixture
solid system, the maximum value of mixture variance, σ0,
can be expressed byσ 02 m 1  m  .

by

(2)

The possible minimum variance in a cell, σr, is defined

σ r2 

m 1  m
n

,

(3)

where n denotes the possible number of particles in a cell.
Then the Lacey mixing index M can be defined byMσ 02  σ 2.
σ 02  σ r2

(4)

Given the concentrations of the reference component ineach cell, the mixing index can be defined and calculatedby the statistical method. Lacey pointed out that this index is related to the number of the cells. Although it is noteasy to represent the real mixing condition by one single
index, the Lacey mixing index is still a good parameterfor comparing the mixing status at different times or fordifferent apparatus (Lacey, 1943). However this mixing
index may be greater than 1 in some cases which causes
some problems. This issue will be discussed further below.
(2) Kramer Mixing Index
Kramer proposed this mixing index (Kramer H.A.,
1968) based on the same statistical variances as Lacey asin Eqs. (1–3):M276

σ0  σσ0  σr .

(5)(3) Lacey, Weidenbaum and Bonilla Mixing Index (L.
W.B. Mixing Index) (Weidenbaum S.S. and Bonilla C.F.,
1953)
This index was first proposed for a horizontal cylindrical mixer. The mixing index M can be defined by

M

σr
,
σ

(6)

where σ and σr are the same as defined in Eqs. (1) and (3).
(4) Mixing index by distance between the volume centersof two species (Avg. distance mixing index)
Two differently colored particles (with the same volumes) are often used to perform mixing/segregation experiments/simulations in academic studies. The distance(width or height, depending on the mixing/segregation direction) between the volume centers of each species isused to denote the mixing status. For example, Fig. 1(a)shows pre-organized particles (black on the left and whiteon the right) in a container. They are in a completely segregated state. The distance between the two volume centers is denoted by Δseg. Fig. 1(b) shows a similar case but
the particles are organized so that black is on the bottom
and white above. The distance between the volume centers of the two species in the horizontal direction (or in
the vertical direction) is denoted as Δx. Thus the mixing
index can be defined as follows:MΔ seg  Δ x
Δ seg  Δ mix ,(7)

where Δmix is the value of Δx in a fully-mixed state which
is usually 0 as shown in Fig. 2. This index is really basedon the idea of the geometric positions of the particles as
determined by identifying the volume (weight) center ofparticles. Calculation for this method would appear to beeasy, but failure may occur because Δx could be 0 evenwhen the particles are segregated, for example, if all theblack particles were located in the upper-right and lower-­left quadrants.

Fig. 1Pre-organized particles with (a) black on the left andwhite on the right; (b) black on the bottom and white
above in a container.

Shih-Hao Chou et al. / KONA Powder and Particle Journal No. 34 (2017) 275–281

Fig. 4

Fig. 2

Schematic representation of the fully-mixed particle
state.

M

Schematic representation of the method of calculationof the coordinate mixing index.

ΩΩ ran .

(8)

If a cuboid box contains a, b, c particles in three directions (total abc particles), the coordinate number of thewell-mixed arrangement would beΩ ran  a  1 bc  b  1 ca  c  1 ab .

Fig. 3

The relation between the mixing degree and the lengthof the boundary between two different species.

3. Coordinate mixing index
As can be seen in Fig. 3, the mixing status (mixing degree or mixing index) has a strong relation with the lengthof the boundary between the two different species. The
right configuration in Fig. 3 has a better mixing condition
than the left configuration, with the length of the boundary between the black-white areas of the right one being
four times that of the left one. Based on the above idea,
we try to develop a new mixing index, the coordinate
mixing index, in this study.
An examination of Fig. 4 is useful to explain how thecoordinate mixing index is defined. Since the lengths of thecontact interface between the black and white particles
should be the same, whether counting either the white or
the black particles, here we only use the white particles as
the target. We can use the (coordinate) number of different-­
color contacts as the length, therefore the numbers insideeach white circle (particle) denote the number of black
particle(s) located in the neighboring position of the whiteparticles. For example, in the lower left corner, the whiteparticle has two black particles around it, so the coordinate number is 2. After counting the coordinate number
for each white particle, the sum of all white particles is
the total coordinate number for this system. The coordinate number for the granular system in Fig. 4 is 17. A
higher coordinate number indicates a better mixing status.
Let Ω denote the coordinate number of the system, andΩran denote the coordinate number of the well-mixed(most random) system. Then the mixing index can be defined as(9)

It is simpler to test and explain Eq. (9) using a 2D system with a and b particles distributed in the two directions. The right-hand image in Fig. 3 represents a 3 × 3
particle system (a = 3, b = 3) in a well-mixed condition.
For each row there are a – 1 = 2 coordinate numbers.
With b = 3 rows, there would be (a – 1) × b = 6 coordinate
numbers. Similarly there are (b – 1) × a = 6 coordinate
numbers in the column direction. Therefore Ωran = (a – 1)
b + (b – 1)a = 12 coordinate numbers.

4. Results and discussions
In this study, we analyze the performance of different
mixing indices with different examples of pre-organization,
as in Fig. 5. Three different types of examples are examined: series A includes three different completely segregated initial system configurations; series B is devoted toinvestigating cases with different percentages of mixing
particles; and in series C we investigate mixing startingwith different special pre-organized patterns. The Lacey
mixing index has been widely used in previous studies.
However, the extent of the Lacey mixing index could be
more than 1 in some cases. Therefore, we compare theLacey mixing index and coordinate mixing index in a simulation of the granular mixing process in a vibrating bed.
Fig. 6 shows the degree of mixing obtained using different kinds of mixing indices for three different completely segregated initial configurations. In case I andcase II, the degree of mixing with each mixing index is 0
except for the coordinate mixing index. This is because of
the existence of a contact interface between the black andwhite particles in each case, which has to be counted
when calculating the coordinate mixing index. In addition, although the Avg. distance mixing index is still indicates zero, the degree of mixing is calculated to be greater277

Shih-Hao Chou et al. / KONA Powder and Particle Journal No. 34 (2017) 275–281

Fig. 5

Fig. 6

Schematic representation of the configurations of black-white particles for different pre-arranged cases.

The relation between the degree of mixing for different
kind of mixing indices and different initial configurations of black-white particle systems. Total particle
number of system is 1728; cell number N is 216; andnumber of particles (cell size) is n = 8.

than 0 in case III. Overall, the degree of mixing obtainedwith the L.W.B. mixing index is the highest while theLacey Mixing Index is slightly larger than the Kramer
Mixing Index.
Therefore, if a contact interface still exists between theblack and white particles even in a well segregated situation, the degree of mixing obtained with the coordinate
mixing index would not be zero. Differences in the length(number) of the interface also cause little deviation; thedegree of mixing is similar. Meanwhile, the degree of
mixing for the Avg. distance mixing index for series A is
calculated to be zero. This is because the variances in theinitial completely segregated state and the symmetricityof the system both affect the calculated value.

278

Fig. 7The degree of mixing with the Lacey mixing index
with different sizes of cells for series B. The cell number N = 216 for n = 8, N = 64 for n = 27, N = 27 forn = 64, and N = 8 for n = 216. Symbol fill: black, exact
value.

The number and size of the cells should be chosen before using the Lacey mixing index to calculate the degreeof mixing. If the size of the system is the same, the number of cells is dependent on the number of particles in acell. For example, observation of series B shows 4 different configurations having 0 %, 33 %, 66 % and 100 % ofparticle mixing, respectively. The mixing states of theparticle system are defined by four different volume ratioswith fully mixed particles. For example, a mixing degreeof 33 % means that 1/3 of the volume of particles is in acompletely mixed condition in the system. Fig. 7 shows
the degree of mixing obtained with the Lacey mixing index for different sizes of cells for cases IV to VII. In caseV, the calculated degree of mixing is close to the definedShih-Hao Chou et al. / KONA Powder and Particle Journal No. 34 (2017) 275–281

Fig. 8The degree of mixing for different mixing indices forseries B. The size of cell size is n = 8.

degree of mixing with n = 64. The error becomes larger
while the cell size increases. However, this tendency does
not occur in case VI. In case VII, the degree of mixing
will be close to 1 when the cell size becomes larger. The
degree of mixing even exceeds 1 with n = 8. In summary,accuracy is higher if we use a suitable cell size. The accuracy is highest for case V where all the white and black
particles are well-mixed in each cell. This situation could
not be achieved in case VI. Therefore, reducing the sizeof the cells could lead to variation in the system if wecannot find a suitable size of cell.
According to the results discussed above, n = 8 is chosen for all mixing indices, while the cell size varies in theformulation and the ensuing discussion. Fig. 8 shows thedegree of mixing obtained using different mixing indicesfor series B with a cell size of 8. We found the degree of
mixing obtained with the Avg. distance mixing index to
be closest to the pre-mixed value for this system. The error for the degree of mixing for the coordinate mixing index was almost 10 %, and for the Lacey mixing index it
was 15 % in all cases. The degree of mixing obtainedwith the Kramer mixing index was either higher or lower
than the pre-mixed value for different cases, but the value
was abnormally high for case VII. We also found the degree of mixing to be larger than one for mixing indicesbased on the statistical method.
The degree of mixing obtained with different mixing
indices for series C is shown in Fig. 9. In this figure, we
can see that in addition to the coordinate mixing index,
calculation with the other mixing indices shows a relatively high degree of mixing. From case VIII to case X,
the degree of mixing calculated from the Lacey Mixing
Index and Kramer Mixing Index is the same as for thewell-mixed case, but this result is unreasonable. This isbecause if the number of different species of particle are
the same in a cell, the mixing degree is indicated as wellmixed regardless of whether the different species of particle are mixed or not. The Avg. distance mixing index fails

Fig. 9The degree of mixing for different mixing indices forseries C. The cell size is n = 8.

for case X and case XI, because the systems are symmetrical. In addition, since σ = 0 in each case for series C, thedegree of mixing obtained with the L.W.B. mixing index
is close to infinity. In these cases, this unreasonable situation does not happen when the coordinate mixing index isused. This also indicates that the coordinate mixing index
has better accuracy and wider applicability.
From the above discussion, it can be seen that the degree of mixing obtained with the Lacey mixing index will
be larger than one in some cases. This situation is contrary to expectations and is well known. The mixing behavior results in a situation between complete segregation
and being well-mixed (the degree of mixing is between 0
and 1). Therefore, below we will discuss the mixing behavior of particles under different operating conditions.
The dynamic behaviors of the particles were simulated by
the discrete element method (DEM) in a vibrated granular
bed, and the degrees of mixing were calculated with theLacey mixing index and coordinate mixing index. The
only difference between these two cases are the electrostatic force exist or not. The granular beds are energized
by vertical sinusoidal oscillations at different vibrated acceleration and amplitude. The relevant operating conditions and parameters could be referred to the previous
study (Lu L.S. and Hsiau S.S., 2005).
The evolution of the degree of mixing over time under
different operating conditions is shown in Figs. 10(a) and(b). We can see that both the trend and the final mixing
degree for the different mixing indices are almost thatsame, as shown in Fig. 10(a). In Fig. 10(b), we consider
the influence of the electrostatic force between the particles. To simplify the electrostatic effect, the two groups ofglass beads are given opposite charges and the charge
strength is assumed to be constant. In this case, the degree of mixing is greatly increased (Lu L.S. and Hsiau
S.S., 2005). We found that the degree of mixing foundwith the Lacey mixing index would not exceed one, but
this is unreasonable. At the same time, the final degree of279

Shih-Hao Chou et al. / KONA Powder and Particle Journal No. 34 (2017) 275–281

5. Conclusions

Fig. 10 Comparison of time evolution of mixing degree forthe case where (a) Γ = 3, f = 20 Hz and the electrostatic force does not exist; and (b) Γ = 5, f = 20 Hz, but
electrostatic force does exist. The dimensionless acceleration amplitude Γ was defined as Γ = a/g, wherea is the acceleration; g was the acceleration due togravity. The vibrational frequency is f.

mixing for the coordinate mixing index is quite stable.
The final average mixing degree is about 0.95. If the overall concentration is about 0.5 in the system, the Lacey
mixing index could be rewritten as

n 
σ2 

M
1 
,
n  1  0.25 (10)and if the M > 1, the root mean square of the deviation σ
is given byσ 2 i  m  

1
.
4n

(11)

From the above equation, it can be seen that the size ofthe cell will affect the degree of mixing. Meanwhile, themixing degree for the Lacey mixing index will be larger
than one, if the variance σ 2 for the concentration of a reference component in each cell and overall concentrationis smaller than a critical value.

280

In the granular mixing process, there are many factorsthat can affect the state of mixing in the system. The degree of mixing results also differ for different mixing indices. In this study, we discuss the performance obtainedwith different indices using different pre-organized particle configurations.
The results show that the cell size has a significant effect on the degree of mixing calculation given the criterion of the mixing index about a standard deviation. In
some cases the degree of mixing for the Lacey mixing index, L.W.B. mixing index and Kramer mixing index will
be larger than 1. Moreover, if the number of different species of particle in a cell is the same, the degree of mixing
will be higher than the correct value regardless of how
these different species of particle are mixed. In somecases, the Avg. distance mixing index will fail because
the variation in the initial state and the symmetry of thesystem both affect this index.
In this study, we propose a new mixing index, called
the coordinate mixing index. This index is relatively simple and widely applicable. According to the results obtained with different examples of pre-organized particles,
we found that the degree of mixing obtained with the coordinate mixing index to always be between 0 and 1.
Therefore, for now, the coordinate mixing index may bealso a suitable index.

Acknowledgements
The authors would like to acknowledge the support
from the National Science Council of the R.O.C. for thiswork through Grants MOST 103-2221-E-008-042-MY3.

Nomenclaturef

The vibrational frequency

N

Total number of cells occupied by the total particlesnThe possible number of particles in a cellΔx Distance between the volume centers of the two species in
the horizontal direction (or in the vertical direction)
ΓDimensionless acceleration amplitude was defined as Γ = a/g,
where a is the acceleration; g was the acceleration due togravityΩ

The coordinate number of the system

ϕiConcentration of the reference component in the “ith” cellϕm Overall concentration of the reference component

Shih-Hao Chou et al. / KONA Powder and Particle Journal No. 34 (2017) 275–281

References
Gayle J.B., Lacey O.L., Gary J.H., Mixing of Solids. Chi Square
as a Criterion, Journal of Industrial and Engineering Chemistry, 50 (1958) 1279–1282.
Gotoh K., Masuda H., Higashitani K., Powder Technology
Handbook, 2nd ed., Marcel Dekker, New York, 1997.
Kaye B.H., Powder Mixing, Chapman & Hall, UK, 1997.
Kramer H.A., Effect of grain velocity and flow rate upon theperformance of a diverter type sampler, U.S. Department ofAgriculture, Agricultural Research Service, (1968) ARS
No.51–25.
Lacey P.M.C., The mixing of solid particles, Transactions of theInstitution of Chemical Engineers, 21 (1943) 53–59.
Lacey P.M.C., Developments in the theory of particle mixing,
Journal of Applied Chemistry, 4 (1954) 257–268.
Leggatt C.W., Method of making the homogeneity test, Association of Official Seed Analysts News Letter., 25 (1951) 3–8.

Lu L.S. and Hsiau S.S., Mixing in vibrated granular beds withthe effect of electrostatic force, Powder Technology, 160(2005) 170–179.
Masuda H., Higashitani K., Yoshida H., Powder Technology
Handbook, 3rd ed., CRC Press, Boca Raton, 2006.
Ottino J.M., Khakhar D.V., Mixing and segregation of granularmaterials, Annual Review of Fluid Mechanics, 32 (2000)
55–91.
Poux M., Fayolle P., Bertrand J., Bridoux D., Bousquet J., Powder mixing some practical rules applied to agitated systems,
Powder Technology, 68 (1991) 213–234.
Weidenbaum S.S., Bonilla C.F., A Fundamental Study of the
Mixing of Particulate Solids, Ph.D. Thesis, Columbia University (1953).
Weidenbaum S.S., Mixing of Solids, in: Advances in Chemical
Engineering II, Academic Press, New York, 1958, pp. 211–
321.

Author’s short biography
Shih-Hao Chou
Dr. Shih-Hao Chou received his bachelor and PhD degrees in Department of Mechanical Engineering, National Central University, Taiwan in 2007 and 2013. He is currently
a post-doctoral researcher at National Central University, Taiwan. His research interests
include powder technology, mixing and segregation, DEM modeling of granular flow.

Yue-Lou SongMr. Yue-Lou Song received his bachelor in Department of Mechanical Engineering,
National Central University, Taiwan in 2012. He served as an assistant of Department
office (2008-2009). His research interests include mixing behavior in a vibrated granularbed.

Shu-San Hsiau
Shu-San Hsiau is an Associate Vice President for Research & Development of National Central University (NCU). He received his MS and PhD degrees from California
Institute of Technology. He start his professorship life in NCU and majored in Mechanical Engineering in 1993. He also is a Distinguished Professor of Mechanical Engineering
and Institute of Energy Engineering of NCU. His researches including Powder Technology, Clean Coal Technology, Modelling and Design of MOCVD, Energy Technology,
Debris Flow and Avalanche, Hot-Gas Cleanup and Thermo-Fluids. He served as thechairman of Mechanical Engineering (2010–2013) and also as the director of Institute ofEnergy Engineering (2008–2013).

281

KONA Powder and Particle Journal No. 34 (2017) 282–289/Doi:10.14356/kona.2017019

Original Research Paper

Characterisation of the Wetting Behaviour of Poor
Wetting Food Powders and the Influence ofTemperature and Film Formation †John J. Fitzpatrick 1*, Justine Salmon 1, Junfu Ji 2 and Song Miao 212

Process & Chemical Engineering, School of Engineering, University College Cork, Ireland
Teagasc Food Research Centre, Moorepark, Ireland

Abstract
Characterisation of the wettability of five poor wetting food powders was performed using static immersion andcontact angle measurements. The effect of temperature (20, 50 and 70 °C) on wettability showed varying effects
on the powders. Higher temperatures majorly improved the wettability of chocolate and high fat powders but
worsened the wettability of sodium caseinate and milk protein isolate. Rate-limiting regime testing was
performed by pouring a fixed mass of powder on to the surface of water in an agitated beaker and visuallyassessing what was rate-limiting rehydration after 1 minute. The rate limiting regime tended to be floating at
lower agitation speeds and dispersed clumps of varying sizes at higher speeds. However, there were majordifferences observed between the powders. Some of the powders formed strong films at powder/water interfaces,that could act as a barrier to water penetration and wettability. Consequently, force displacement testing was
performed on a layer of powder on the water surface to assess the strength of any powder film formed. Some ofthe powders formed strong films that may in-part explain their poor wetting behaviour and their propensity toform strong clumps that were difficult to disrupt.
Keywords: food powders, rehydration, wettability, clumping, film/skin strength

1. Introduction
Ease of rehydration is a very important functional
property of food powders (Schubert, 1993), as many foodpowders eventually end up being rehydrated. Powders
that rehydrate easily and are not problematic are essential
for industrial users and home consumers. Some foodpowders rehydrate easily while others are more difficult(Barbosa-Canovas et al., 2005). Consequently, a good understanding of rehydration behaviour is necessary alongwith a knowledge of the factors that influence poor rehydration behaviour.
Rehydration has a number of steps, but it can bebroadly broken down into wetting, where the powder is
initially contacted with water to wet the powder particles,
followed by solubilisation where the powder particles
disperse and dissolve in water. It is desirable that boththese steps occur rapidly and completely, however somepowders may prove difficult to wet and/or solubilise.
†
1
2

*

Received 24 June 2016; Accepted 19 August 2016
Western Road, Cork, Ireland
Teagasc, Fermoy, Co. Cork P61 C996, Ireland
Corresponding author: John J. Fitzpatrick;E-mail: j.fitzpatrick@ucc.ie
TEL: +353-21-4903089

282

There are many factors that can influence the wettabilityof a powder. Poor wetting powders tend to float on thesurface of still water and sink very slowly into water.
Powder particle surface composition will influence powder hydrophobicity, and this will influence wetting behaviour with water. Powder formation, for example byspray drying, may influence powder properties that influence rehydration (Gaiani et al., 2010). Porowska et al.
(2015) showed that spray drying parameters and feed
composition can be manipulated to influence the surface
composition which can influence wetting behaviour.
Powder physical properties, such as particle size, densities, porosity and morphology, can influence wetting behaviour. Powders with higher densities tend to sink more
rapidly. Powders with larger particle size tend to have better wetting behaviour because water can penetrate moreeasily into the larger void spaces between the particles.
Consequently, granulation is commonly used to improve
the wettability of powders, however slimy skin or film
formation at the powder/water interface may impede water penetration into the bulk powder (Ji et al., 2016).
The wetting process can be made more difficult by theformation of powder clumps by some poor wetting powders, as these clumps need to be broken down (Barbosa-­
Canovas et al., 2005). Clumps may form where skins or

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

John J. Fitzpatrick et al. / KONA Powder and Particle Journal No. 34 (2017) 282–289

films occur at the water/powder interface which can inhibit water penetration into the powder clump. Agitation
helps improve sinking, clump disruption and dispersionof powder particles. Many food powders are rehydrated intanks that are mechanically agitated. The tank configuration, agitator type used and mechanical power intensity
coupled with water temperature and its conductivity may
all impact on rehydration behaviour (Richard et al.,2013; Jeantet et al., 2010; Schober and Fitzpatrick, 2005;
Fitzpatrick and Cuthbert, 2004).
The wettability of powders has been assessed using avariety of techniques, including the International Dairy
Federation standard method (IDF, 1979), the Washburn
method (Washburn, 1921), dynamic contact angle measurement (Dupas et al., 2013) and the turbidity method(Gaiani et al., 2007, 2009). Food powder rehydration may
be complex in nature, and quantitative techniques may
not fully capture this complexity. Powders may float on
the surface for prolonged periods of time; they may settle
and form sediments at the bottom of the vessel; they may
disperse as clumps that prove difficult to disrupt. Consequently, Mitchell et al. (2015) described a qualitative approach to help provide further insight into describing andunderstanding powder rehydration. They describe arate-limiting regime mapping approach that qualitatively
describes the particular aspect or regime that limits theoverall rehydration in a stirred vessel at a specific condition of agitation.
The focus of this paper is on poor wetting powders. In
particular, it examines factors that influence poor wetting
behaviour, investigates the influence of temperature andagitation on the wetting, clumping and dispersion behaviour of poor wetting powders, and investigates clumping and the strength of skins formed at powder waterinterfaces.

2. Materials and methods
2.1 Powders and their composition
Five poor wetting powders and one easy wetting powder were tested and compared and these are presented inTable 1. Skim milk, 73 % high fat milk, milk protein isolate (MPI) and sodium caseinate powders were obtained
from dairy powder manufacturers in Ireland, and chocolate and wheat flour powders were purchased in a local
supermarket. The composition data provided in Table 1
were obtained from product packaging or dairy powder
manufacturer data, except for the water content which
was measured by an oven drying test (102 °C for 24hours).

Table 1 Powders and their % composition
proteinfat

carbohydrate

Skim milk

35

1

52

4.8

7

Chocolate

6.3

6

75

2.9

0.9

High fat

9

73

13

2

3

Caseinate

85

1.3

1.3

8.8

4

MPI

86

1.5

0.5

7.4

6

Flour

10

1.3

76

11.8

0.8

powderwater

ash

Table 2 Powder particle size and densities
particlesize(microns)

apparent
densitypoured
BD1(kg m –3)

tappedBD1Skim milk

132

1220

490

548

Chocolate

109

1530

638

722

High fat

76

934

280

433

Caseinate

90

1310

500

620

MPI

50

809

291

387

Flour

85

1490

590

729

powder1

bulk density

2.2 Powder particle size and densities
The median (d50) of a volume particle size distribution,
particle apparent density, poured and tapped bulk densities were measured, as described by Fitzpatrick et al.
(2016), and these data are provided in Table 2.

2.3 Wettability
The method used was an extension of the International
Dairy Federation (IDF) method (IDF, 1979), and is described by Fitzpatrick et al (2016). The method essentiallymeasures the time for a given mass of powder to sink beneath the water surface, and this time is referred to as thewetting time. Powders having a wetting time less than 1
minute can be considered as easy wetting powders andpowders having wetting times greater than 5 minutes canbe considered as poor wetting powders. The temperatureof the water was maintained at either 20, 50 or 70 °C.
Poor wetting powders may take very long time to wet or
may not totally wet. To overcome this time limitation, the% wettability was evaluated as defined in equation 1. Thisconsisted of measuring the amount of powder that disappeared from the water surface after 60 minutes, as described by Fitzpatrick et al. (2016).

% Wettability 100 mass of powder disappeared(1)
mass of initial powder

283

John J. Fitzpatrick et al. / KONA Powder and Particle Journal No. 34 (2017) 282–289

This test was applied to evaluate the wetting time of apowder if less than 60 minutes, and the % wettability at
60 minutes for those powders whose wetting time wasgreater than 60 minutes. This represents an improved approach for assessing and comparing the wettability ofpoor wetting powders.

2.4 Rate-limiting regime mappingThis is a similar method to that described by Fitzpatrick
et al. (2016) and is based on that reported by Mitchell etal. (2015). A 2 L glass beaker with a diameter of 125 mm
was filled with 1600 mL of water, as illustrated in Fig. 1a.
An agitator with a four-bladed pitch-blade impeller wasinserted axially into the water at a fixed location and was
operated at speeds between 100 to 750 rpm. Little or novortex formation occurred at speeds up to 300 rpm. At
400 rpm and higher, a vortex formed and deepened progressively with higher speed, as illustrated in Fig. 1b, engaging the impeller at around 750 rpm.
The water was heated to the required temperatures (20,50 and 70 °C) using a hotplate. 16 grams of powder (1 % w/v)
was weighed and emptied into the agitated water. The
rehydration was visually observed and a qualitative evaluation of any rate-limitation(s) to achieving complete dispersion or dissolution was made at the end of the first minute.
Key words were used to describe any rate-limitations,such as sedimentation (if powder remained on the bottomof the beaker); floating (if powder remained floating on
the surface of the water); and clumps in dispersion (ifpowder formed clumps that were visible in the dispersion). The procedure was carried out in duplicate for eachpowder.

Fig. 1Rate-limiting regime mapping: (a) beaker and agitator;(b) agitation at 600 rpm, deep vortex.

Fig. 2

Plunger moving axially through chocolate powder(1 min, 20 °C).

Fig. 3Force displacement: (a) water; (b) SMP (1 min, 20 °C).

2.5 Film strength at powder water interface
A force-displacement test was used to measure thestrength of films developed at the powder/water interface.
110 g of water was filled into a small glass beaker with internal diameter and height of 65 mm and 40 mm, respectively. Powder was sieved onto the water surface until itfilled the beaker giving a powder thickness of 7 mm. Excess powder was removed to produce a smooth surface. A
35 mm plunger was then centered and moved axiallythrough the powder at a constant speed of 0.08 mm/s,
as illustrated in Fig. 2, and force vs displacement wasmeasured using a TA HD Plus Texture Analyser (Stable
Microsystems, UK). With no powder present the force-­
displacement through water is illustrated in Fig. 3a. Powders displaying measurable film strength showed behavior
similar to that illustrated in Fig. 3b with a noticeable
peak. This is the peak force required for the plunger topenetrate through the film and was used as an index offilm strength. Powders displaying negligible film strength284

John J. Fitzpatrick et al. / KONA Powder and Particle Journal No. 34 (2017) 282–289

displayed a similar linear behavior to that illustrated inFig. 3a. Measurements were conducted at water temperatures of 20, 50 and 70 °C and powder water contact timesof 1 and 20 minutes.

3. Results and discussion
3.1 Wettability and effect of temperature
Results from wettability testing are presented in Tables
3 and 4, along with contact angle data presented in Table
5. The poor wetting powders, except for the wheat flour,
had contact angles greater than 90 degrees. Hydrophobic
surfaces tend to be the major determinant of high contact
angles, and surface chemistry and composition determinehydrophobicity. Consequently, bulk composition, which
influences surface composition, is having a major impact
on the wettability of the powders. The wettability results
were closely related to the contact angle data, except for
the SMP and wheat flour where the flour had a lower contact angle than the SMP even though it had a longer wetting time.
At 20 °C, the high fat powder had very poor wettability
at only 5 % after 60 minutes, where it essentially floatedon the water surface. This is most likely due to the hydrophobic nature of fat and its low apparent density,
which is lower than the density of water. The chocolate
powder is poor wetting, displaying 50 % wettability after
60 minutes. Its poor wettability is most likely due to its
composition, in particular a significant fat content of
5.9 %, giving rise to a hydrophobic surface, as evidenced
by its contact angle data. Its particle size and densities
were not unusually low and should not be contributing to
its poor wettability. The high milk protein powders, sodium caseinate and MPI, were both poor wetting powders, having poorer wettability than the chocolate powder.
Both powders displayed high contact angles, and this ismost likely due to their composition. Consequently, theirhigh protein contents, being derived mainly from caseins,were mainly responsible for the poor wettability of these
powders. Crowley et al. (2015) showed how composition
influenced the contact angle of milk protein concentrate
powders, with higher protein (lower lactose) content resulting in higher contact angles. The low apparent density
and relatively small particle size of the MPI powder may
also contribute to its poor wettability.
The effect of temperature on wettability is presented inTables 3 and 4. For the easy wetting SMP with a wetting
time less than 1 minute, wettability improved with increasing temperature. However, temperature displayed
differing impacts on the poor wetting powders. Increasing
temperature had a very beneficial impact on the chocolate
powder. Likewise, for the high fat powder, where at 50

Table 3 Powder wetting times
powder

20 °C

50 °C

70 °C

Skim milk

51 s

16 s

10 s

Chocolate

>1h

2 min

1.2 minHigh fat

>1h

3 min

1 min

Caseinate

>1h

>1h

>1h

MPI

>1h

>1h

>1h

Flour

31 min13 min>1h

Table 4

% Wettability of poor wetting powders after 60
minutes

powder

20 °C

50 °C

70 °C

Chocolate

50

100

100

High fat

5

100

100

Caseinate

27

15

15

MPI

22

16

18

Flour

100

100

73

and 70 °C the powder sank just below the surface, wetted
and formed a layer due to the lower density of fat. The
greatly improved wetting behaviour may be related to
the fact that milk fats melt in the range of –40 to 37 °C(McCarthy, 2006).
Increasing temperature did not improve the wettabilityof the caseinate and MPI powders; in fact the wettability
disimproved as shown in Table 4. Both of these powders
are high protein powders containing caseins which arethermally resistant in the experimental range and the caseins appear to be maintaining their surface hydrophobicity and poor wettability. The flour powder displayed adifferent behavior with an optimum wetting temperaturesomewhere between 20 and 70 °C. Increasing temperaturefrom 20 to 50 °C showed improved wettability but increasing further to 70 °C showed a wettability that wasworse than at 20 °C. The reduced wettability at 70 °C is
most likely due to the effect of temperature on the gelatinisation of starch which occurs typically in the range of
53 to 64 °C for wheat flours (Fennema, 1985) and the interaction between starch and wheat proteins which canform films when heated (Potter, 1986).

3.2 Rate-limiting regime mapping
Rate-limiting regime mapping tests were undertaken tovisually investigate the effect of agitation and temperatureon the wetting, sinking, clumping and dispersion of thepowders. For SMP, the rate limiting regime at low agitation (200 rpm and less) was sedimentation at 20 °C.
Dispersed clumps were observed at 300 and 400 rpm agi-

285

John J. Fitzpatrick et al. / KONA Powder and Particle Journal No. 34 (2017) 282–289

tation speeds, however the powder was well dispersed at
500 rpm and higher. Sedimentation was less rate-limitingat higher temperature, but dispersed clumps were still observed up to 400 rpm. Clumping is most likely due to thepresence on the particle surface of fat and some types ofproteins that make the surface less hydrophilic. Furthermore, fat and proteins often concentrate to the surface inspray dried powders. Kim et al. (2002) showed the surface
fat content was much higher than the bulk value in spraydried dairy powders.
The rate-limiting regime map of the chocolate powder
is presented in Table 6. The dominant rate-limitation at
20 °C was floating, most likely due to the significant fat
content. Increasing temperature had a major effect on the
chocolate powder with it being well dispersed by 300 rpm
at both 50 and 70 °C. The high fat powder essentiallyfloated on the surface of the water up to 300 rpm at 20 °C,
as illustrated Fig. 4a. This could be expected due to its
inherently hydrophobic nature and its low apparent density. At 400 rpm and greater, a stronger and stronger vortex formed and more powder was drawn into dispersion
as fine clumps. However, Fig. 4b shows many powder
clumps still circulating in the vortex, although these became less and less at higher speeds, but some were stillpresent at 750 rpm along with fine clumps in dispersion.
Increased temperature had a major effect by reducing thefloating limitation and increasing dispersion, as illustrated
by comparing Fig. 4a & c.
Floating was the dominant rate-limiting regime forboth the MPI and sodium caseinate powders at 20 °C,
most likely due to their high casein protein content, asboth powders have low fat contents. The MPI powder behaved somewhat similar to the high fat and chocolate
powders in the sense that increased dispersion occurred at
higher speeds and complete dispersion was only achievedat the highest speed of 750 rpm. Temperature did not appear to significantly influence the regime map of MPI,
and the rehydration behaviour appeared similar at all
three temperatures.
The rate-limiting regime map of the sodium caseinate
powder is presented in Table 7. Fig. 5a shows that floating was the dominant rate-limiting regime up to 400 rpm
at 20 °C. A stronger vortex developed at 500 rpm andmore powder submerged into the water but formed many
dispersed clumps, thus clumping became the rate-limiting
regime. Even at the highest speed of 750 rpm, powder

Table 5 Contact angle formed between deionized water(20 °C) and powder surface over a 10 second period
powder

1s

2s

5s

10 s

Skim milk

82

60

58

58

Chocolate

104

94

91

86

High fat

104

103

102

98

Caseinate

130

130

130

129

MPI

150

145

142

137

Flour

38

30

25

21

Table 6 Rate-limiting regime map for chocolate powder
Speed
(rpm)

20 °C

100
200
300
400

some floating +sedimentationsome floating +sedimentation

light
sedimentation

light
sedimentation

Speed
(rpm)
100
200
300

500
well-dispersed

well-dispersed

20 °C

50 °C

floating(decreased withagitation)

floating

some floating+ clumps indispersion

some floating+ smaller clumps
in dispersion

600
700

700

Pictures from rate limiting regime mapping for high fat
powder.

Table 7 Rate-limiting regime map for sodium caseinate powder

400

well-dispersed

600

286

70 °C

floating(decreased
with agitation)

500

750

50 °C

Fig. 4750

big clumps indispersion

70 °C

floating

some floating+ big clumps indispersion

floating +clumping
big clumps on
surface + indispersion

John J. Fitzpatrick et al. / KONA Powder and Particle Journal No. 34 (2017) 282–289

The strength of these films could make the clumps moreresistant to disruption by the agitation system, and makeit more difficult to disperse the powder residing within
the clumps. Consequently, the next section of this work
examined the film strength developed by the powders
tested and how it is influenced by temperature.

3.3 Film strength at powder/water interface

Fig. 5

Pictures from rate limiting regime mapping for sodium
caseinate powder at 60 seconds.

clumps remained in dispersion. Many of the powder
clumps were large in size (Fig. 5b) and the surface had astrong skin or film with dry powder inside that greatly
impeded waters ability to penetrate into the bulk. Furthermore, the surface skin of the sodium caseinate wasnoticeably much stronger than skins developed on otherpoor wetting powders. Consequently, sodium caseinate
powder clumps proved difficult to disrupt by the agitation.
At higher temperatures, especially at 70 °C, floating wasmore persistent and the powder dispersed more slowly, as
illustrated by comparing Fig. 5a & c. Clumping appeared
to become more severe at higher temperatures with larger
and stronger clumps being formed.
The most complex behavior was displayed by the flour
powder. Both sedimentation and floating were observed
up to 300 rpm at all temperatures. Floating was more
dominant at lower speed, but even at 100 rpm, somelumps of powder would sink to the bottom and this would
increase with higher speed. As the vortex became stronger at 400 and 500 rpm, floating disappeared and therate-limiting regime was due to clumps either becoming
dispersed in the water or sinking to the bottom. Sedimentation ceased at 600 and 700 rpm, and rate-limitation was
due to clumps in suspension. The major impact of temperature was on clumping. There was the least amount ofclumping at 50 °C while there were more clumping andlarger clumps formed at 20 and 70 °C.
Overall, poor wetting powders tend to display floating
and clumping behaviours. Many of the reasons for clumping are the same as those for floating, that is, hydrophobic
surfaces and slimy skin or film formation. In stirred systems, wettability may be a rate-limiting step whereby
powder clumping may occur on the surface and whensubmerged, and the agitator may have difficulty and takesignificant amount of time to sink and break up theclumps. Some of the poor wetting powders, most noticeably, sodium caseinate, flour and chocolate, developed
noticeable slimy skins/films at the powder/water interface
at 20 °C, which would further inhibit water penetration.

The SMP is an easy wetting powder but did show ameasurable film strength of 70 mN at 20 °C at 1 min, as
illustrated in Fig. 3b. There were no other measurements
as SMP had wetted at all the other conditions. The chocolate powder and high fat powders were poor wetting powders at 20 °C, however their film strengths were low with
80 mN at 1 and 20 minutes for chocolate and negligible
strength for the high fat at all contact times. Consequently, strong film formation was not a major impediment to the wetting of these powders.
The sodium caseinate, MPI and flour powders all displayed measurable film strengths. These are presented inTable 8 as a function of time and temperature. They all
showed similar film strengths of around 150 to 200 mN

Table 8 Film strength (mN) of sodium caseinate, MPI andflour
powder

20 °C

50 °C

70 °C

Sodium caseinate
1 min

180

150

170

20 min

200

860

900

MPI
1 min

0

170

160

20 min

160

380

520

Flour
1 min

200

150

170

20 min

220

410

490

Fig. 6

Effect of temperature on film strength of sodium caseinate at 20 minutes contact time.

287

John J. Fitzpatrick et al. / KONA Powder and Particle Journal No. 34 (2017) 282–289

after one minute contact time, irrespective of temperature, except for the MPI which showed negligible strength
at 20 °C although this did increase to 160 mN after 20
minutes. However, increasing the temperature to 50 and
70 °C had a major impact on film strength at the longer
exposure time of 20 minutes, especially for the caseinate
as illustrated in Fig. 6. The caseinate displayed the strongest film strength, which concurs with that visually observed in the rate-limiting regime mapping. Consequently,
these results suggest that the caseinate has a propensity todevelop very strong films at these higher temperatures,
especially when the powder film is exposed for longer
time. The MPI and flour showed similar behaviour to caseinate at higher temperatures but the film strengths were
not as strong.

4. Conclusions
In this study, composition was a major cause of poor
wettability due to either higher fat or high milk protein
contents, making the powder surfaces hydrophobic. Furthermore, the low apparent densities of the high fat and
MPI powders may contribute to their poor wettability andtheir tendency to float. The formation of a slimy skin orfilm at the powder/water interface may contribute to poor
wettability where water penetration into the bulk isgreatly impeded. This enhances both floating and clump
formation.
Vortexing in a stirred tank greatly improved wetting ofeven very poor wetting powders by rapidly submerging
powder into the water and disruption of clumps by the action of the impeller. However, the formation of strong
films by some powders, such as sodium caseinate, may
result in the formation of clumps with strong skins thatare difficult to disrupt and thus impede powder dispersion. Force-displacement testing can be applied to assess
the susceptibility of powders to clumping and strong skinformation by measuring the strength of films formed at
powder/water interfaces.
Temperature had a varied impact on the wettability ofthe poor wetting powders. Increasing temperature greatly
improved the wetting and rehydration abilities of the
chocolate and high fat powders. On the other hand, it reduced the wettability of the caseinate and MPI powders,which in-part is due to higher temperatures producing
stronger films that impede the penetration of water into
the bulk.

References
Barbosa-Canovas G.V., Ortega-Rivas E., Juliano P., Yan H.,
Bulk properties, in: Food Powders: Physical Properties,

288

Processing, and Functionality, Kluwer Academic/Plenum
Publishers, New York, 2005.
Crowley S.V., Desautel B., Gazi I., Kelly A.L., Huppertz T.,
O’Mahony J.A., Rehydration characteristics of milk protein
concentrate powders, J. Food Engineering, 149 (2015) 105–113.
Dupas J., Verneuil E., Ramaioli M., Forny L., Talini L.,
Lequeux F., Dynamic wetting on a thin film of soluble
polymer: Effects of nonlinearities in the sorption isotherm,
Langmuir, 29 (2013) 12572–12578.
Fennema O., Ed, Food Chemistry, Marcel Dekker, New York,
1985.
Fitzpatrick J.J., Cuthbert R., Effect of temperature on the reconstitution of milk powder to high solids content in a stirredtank, Milchwissenschaft, 59 (2004) 55–58.
Fitzpatrick J.J., van Lauwe A., Coursol M., O’Brien A.,
Fitzpatrick K.L., Ji J., Miao S., Investigation of the rehydration behaviour of food powders by comparing the behaviourof twelve powders with different properties, Powder Technology, 297 (2016) 340–348.
Gaiani C., Schuck P., Scher S., Desobry S., Banon S., Dairy
powder rehydration: Influence of protein state, incorporation mode, and agglomeration, J. Dairy Science, 90 (2007)
570–581.
Gaiani C., Schuck P., Scher S., Desobry S., Banon S., Use of aturbidity sensor to determine dairy powder rehydration
properties, Powder Technology, 190 (2009) 2–5.
Gaiani C., Morand M., Sanchez C., Arab Tehrany E., Jacquot
M., Schuck P., Jeantet R., Scher S., How surface composition of high milk protein powders is influenced by spraydrying temperature, Colloids and Surfaces B: Biointerfaces,
75 (2010) 377–384.
IDF, International IDF Standard 87: 1979. International Dairy
Federation, Brussels, Belgium, 1979.
Jeantet R., Schuck P., Six T., Andre C., Delaplace G., The influence of stirring speed, temperature and solid concentrationon the rehydration time of micellar casein powder, Dairy
Science and Technology, 90 (2010) 225–236.
Ji J., Fitzpatrick J., Cronin K., Maguire P., Zhang H., Miao S.,
Rehydration behaviours of high protein dairy powders: The
influence of agglomeration on wettability, dispersibility andsolubility, Food Hydrocolloids, 58 (2016) 194–203.
Kim E.H.J., Chen X.D., Pearce D., Surface characterization offour industrial spray-dried powders in relation to chemicalcomposition, structure and wetting property, Colloids andSurfaces B: Biointerfaces, 26 (2002) 197–212.
McCarthy O.J., Physical characteristics of milk fat and milk fat
based products, in: Fox P.F., McSweeney P.L.H. (Eds),
Advanced Dairy Chemistry, vol. 2 Lipids. Springer, New
York, 2006, pp. 725–778.
Mitchell W.R., Forny L., Althaus T.O., Niederreiter G., Palzer
S., Hounslow M.J., Salman A.D., Mapping the rate-limiting
regimes of food powder reconstitution in a standard mixing
vessel, Powder Technology, 270 (2015) 520–527.
Porowska A., Gianfrancesco A., Fries L., Dosta M., Palzer S.,
Heinrich S., Influence of feed composition and drying
parameters on surface composition of a spray dried multicomponent particle, Drying Technology, 33 (2015) 1911–

John J. Fitzpatrick et al. / KONA Powder and Particle Journal No. 34 (2017) 282–289

1919.
Potter N.N., Food Science, AVI publishing Company, Connecticut,
1986.
Richard B., Le Page J.F., Schuck P., Andre C., Jeantet R.,
Delaplace G., Towards a better control of dairy powder
rehydration processes. International Dairy Journal, 31(2013) 18–28.
Schober C., Fitzpatrick J.J., Effect of vortex formation on pow-

der sinkability for reconstituting milk powders in water tohigh solids content in a stirred-tank, J. Food Engineering,71 (2005) 1–8.
Schubert H., Instantization of powdered food products, International Chemical Engineering, 33 (1993) 28–44.
Washburn E.W., The dynamics of capillary flow, PhysicalReview, 17 (1921) 273–283.

Author’s short biography
John J. Fitzpatrick
Dr. John Fitzpatrick is a senior lecturer in Process & Chemical Engineering. His research interests are in food particle & powder technology and sustainability & environmental protection. He has been involved in research projects in powder flowability andcaking; protein precipitation and sugar crystallisation; dry powder mixing and rehydration of food powders.

Justine Salmon
Justine Salmon is a student in food science at the “Agrocampus Ouest” food engineering school in Rennes (France). She undertakes her studies through an apprenticeship andworks at Entremont in the production of dairy powders. As part of her studies, she carried out a work placement in University College Cork and participated in a research project on the rehydration behaviour of poor wetting powders.

Junfu Ji
Junfu Ji is currently a Walsh fellow in Teagasc Food Research Centre Moorepark andalso a PhD candidate in Process & Chemical Engineering, University College Cork. His
research focuses on dairy powder agglomeration processes (fluidised bed and high shear
mixer granulation) and the rehydration behaviour, physical & structural properties andflowability of dairy powders and their agglomerates.

Song Miao
Dr. Song Miao is Senior Research Officer in Food Technology at Teagasc Food Research Centre Moorepark. His current research programme focuses on development ofnovel food structures for functional delivery, water structuring and powder technologies.
His research interests cover dehydration and granulation, foods structural and texturaldesign, powder technology, state transition and phase transition in foods, encapsulationof functional food ingredients, stabilization of probiotics and dairy ingredients.

289

KONA Powder and Particle Journal No. 34 (2017) 290–291/Doi:10.14356/kona.2017024

Infomation Article

The 50th Symposium on Powder Technology
The 50th Symposium on Powder Technology was held at Tokyo Marriott Hotel, Japan on Tuesday, September 13,
2016. It was organized by Hosokawa Powder Technology Foundation (HPTF) with the sponsorship of Hosokawa
Micron Corporation. About 160 people from the industries and universities attended this symposium. The theme ofthe symposium this year was “Powder Technology to Create a Prosperous Future”. There were six lectures withquestions and answers followed by a get-acquainted party for further free discussions.
The first lecture was given by Dr. Makino, who was presented the KONA Award last year, on his lifework research in the field of coal utilization power engineering including the recent trends of CO2 capture and storage. The
lectures were carried out by the professors with the diversity of the fields including the cancer treatment with the
inorganic/bio-complex nanoparticle, the new food flours produced by glassy state foods materials, the continuous
production of nanoparticles by supercritical hydrothermal synthesis. The other one lecture was from the industrial
field introducing the Internet-of-Things (IoT), which has been applied to powder processing systems for several
manufacturing industries.
The last lecture was Special Lecture for the 25th Anniversary of the founding of the HPTF, which was given by
Prof. Akira Fujishima, Special University Professor Emeritus of the University of Tokyo and President of Tokyo
University of Science, who discovered the titanium dioxide photocatalysis. Prof. Fujishima looked back the historyof the discovery of the titanium oxide photocatalysis and explained the important applications across the various
fields, as well as the preparation method and characteristics of the diamond electrode.
The contents of the symposium are shown in the followings.

The 50th Symposium on Powder Technology
Theme: Powder Technology to Create a Prosperous Future
Opening address Mr. Yoshio Hosokawa (President of Hosokawa Powder Technology Foundation)

Session 1 Chaired by Prof. emeritus Ko Higashitani (Kyoto University)• Lecture 1“Coal Utilization Power Engineering and Powder Technology for the Stable Power Supply in
Low-Carbon Society”
Dr. Hisao Makino (Central Research Institute of Electric Power Industry, JAPAN)• Lecture 2“The Cancer Treatment Strategy with the Inorganic/Bio-Complex Nanoparticle”
Prof. Chiaki Ogino (Kobe University)
Session 2 Chaired by Prof. emeritus Kikuo Okuyama (Hiroshima University)• Lecture 3“Glassy State Foods Materials Produce New Food Flour”
Prof. emeritus Rikuo Takai (Tokyo University of Marine Science and Technology)290• Lecture 4“Continuous Production of Nanoparticles by Supercritical Hydrothermal Synthesis-Nanoparticles Synthesis, Nanohybrid Materials, and Surface Controlled Nanocatalysts-”
Prof. Tadafumi Adschiri (Tohoku University)• Lecture 5“The Internet-of-Things, Opportunities to Improve Manufacturing Processes”
Dr. Iain Crosley (Hosokawa Micron Ltd. (UK))
Session 3

Chaired by Prof. emeritus Minoru Takahashi (Nagoya Institute of Technology)

Special Lecture for the 25th Anniversary of the founding of HPTF
• Lecture 6“Titanium Dioxide Photocatalysis and Diamond Electrode”
Special University Professor Emeritus of the University of Tokyo, Akira Fujishima (Presidentof Tokyo University of Science)

Symposium on Powder Technology

Special Lecture of 25th Founding Anniversary

Get-acquainted Party

291

KONA Powder and Particle Journal No. 34 (2017) 292/Doi:10.14356/kona.2017025

Infomation Article

The 23rd KONA Award
Dr. Hisao Makino, Executive Research Scientist of CRIEPI (Central Research Institute of Electric Power Industry), was selected as the winner of the 23rd KONA Award. The KONA Award is sponsored by Hosokawa Powder
Technology Foundation and presented to scientists or groups who have achieved distinguished research work in thefield of particle science and technology.
After finishing all the required curricula of Kyoto University for B.E. in 1977 and M.E. in 1979 respectively witheach degree, Dr. Hisao Makino obtained his Doctoral degree from the same university in 1995. He joined CRIEPI
in 1979 and became a Director of Chemical Energy Engineering Department of CRIEPI in 1997. Through thecarrier as an Acting Vice President nominated in 2008, he has been serving as an Executive Research Scientist of
CRIEPI since 2012. In parallel with his steady works in CRIEPI, he performed his academic contributions as a
Visiting Research Fellow of Delft University of Technology in the Netherlands from 1989 to 1990. He has also
carried out his educational activities by serving as a Visiting Professor of Gunma University since 2000 as well as
Kyushu University since 2009.
His major research subject is the development of clean coal technologies for stable electric power supply. One ofhis distinguishing accomplishments is the development of new type high performance burners such as an advanced
low NOx burner and an advanced high turn down burner. He also investigated the estimation methods of theperformance of electrostatic precipitator in pulverized coal combustion power stations and the control methods ofdegradation of the denitrification catalyst. Recently, he is developing a new type power generation system using
oxygen and carbon dioxide blown gasification technology for the system of high efficiency power generation andhigh performance removal of carbon dioxide.
He has published over 100 research papers and more than 100 reviewed articles in journals and academic magazines. He has a great deal of experiences invited as an lecturer in about 100 conferences including about 20 international conferences. His research field covers the major subjects of powder technology, they are, (1) high temperature
reaction of pulverized hydrocarbon resources, (2) dust collection technology in flue gas, (3) analysis of flow
behavior in gas-solid phase flow, (4) measurement technology of fine particles, and (5) analysis of particle
characteristics of coal ash.
He served Vice President of the Society of Powder Technology, Japan from 2003 to 2015 and President of thisSociety 2011–2015. And he has been serving the Vice president of Association of Powder Process Industry andEngineering, Japan since 2014 and the Vice President of The Japan Institute of Energy since 2008.
In summary, his contributions to academics and societies are outstanding as we can realize that the products of
his research works are promoting the construction of high efficiency and clean power generation system for fossil
fuel utilization.
On March 22, 2016, Mr. Yoshio Hosokawa, President of Hosokawa Foundation, presented the 23rd KONA Award
to Dr. Hisao Makino at the presentation ceremony held at Hosokawa Micron Corporation in Hirakata.

292

KONA Powder and Particle Journal No. 34 (2017) 293/Doi:10.14356/kona.2017026

Infomation Article

General Information
History of the Journal
In the history of KONA Powder and Particle Journal, twoorganizations have been playing an important role. The one is theCouncil of Powder Technology, Japan (CPT) and the other is the
Hosokawa Powder Technology Foundation (Hosokawa Foundation).
The CPT was established in 1969 by Hosokawa Micron Corporation
as a nonprofit organization to enhance the activities of research anddevelopment on powder science and technology. The Hosokawa
Foundation was established in 1991 as a public-service corporation
approved by the Ministry of Education, Culture, Sport, Science andTechnology of Japan. The issues from No.1 (1983) to No.12 (1994)
of KONA were published by CPT and the issues from No.13 (1995)
by the Hosokawa Foundation.
The aim of KONA in the early days was to introduce excellent
Japanese papers to the world and thus KONA consisted of papers
recommended by some Japanese academic societies and translated
from Japanese to English. From the issue of No.8, the CPT changed
its editorial policy to internationalize the KONA and to incorporate
papers by authors throughout the world in addition to translated
papers. In response to this change, three editorial blocks have beenorganized in the world; Asian-Oceanian, American and European.
The policy and system have not changed even after the Hosokawa
Foundation has taken over from the CPT. From the issue of No.27(2009), publication of translated papers has been terminated andonly original papers have been published. The CPT is active still
today and collaborates with the Hosokawa Foundation.
Aims and Scope
KONA publishes papers in a broad field of powder science andtechnology, ranging from fundamental principles to practical applications. The papers describing technological experiences and critical reviews of existing knowledge in special areas are also welcome.
The submitted papers are published only when they are judged
by the Editor to contribute to the progress of powder science andtechnology, and approved by any of the three Editorial Committees.
The paper submitted to the Editorial Secretariat should not havebeen previously published.
Category of Papers•	Invited articles
	
Review papers and special articles invited by the KONA
Editorial Committees.
• Contributed papers
	Original research and review papers submitted to the KONA
Editorial Committees, and refereed by the reviewers and editors.
Submission of Contributed Papers“Inquiry Letter” for submission on the KONA website (http://www.
kona.or.jp) needs to be sent to the Editor-in-Chief (mail to:contact_zasubmit@hmc.hosokawa.com) prior to the submission of themanuscript. After then, papers will be guided to each KONA
Editorial Secretariat as follows.
• Asian/Oceanian Editorial Secretariat
Dr. T. Yokoyama or Dr. L. Cui
Hosokawa Powder Technology Foundation
1-9, Shodaitajika, Hirakata-shi, Osaka, 573-1132 Japan• European / African Editorial Secretariat
Dr. S. Sander or Ms. L. Kneisl
Hosokawa Alpine AG
Peter-Dörfler-Straße 13-25, D - 86199 Augsburg, Germany• American Editorial Secretariat
Dr. C. C. Huang
Hosokawa Micron Powder Systems
10 Chatham Road, Summit NJ 07901 USA
Publication in KONA is free of charge.
Please visit KONA’s website for the most current Submission
Information at http://www.kona.or.jp
Publication Schedule
KONA is published annually. The publication date is around
January 10th.
Subscription
KONA is distributed free of charge to senior researchers atuniversities and laboratories as well as to institutions and libraries
in the field throughout the world. The publisher is always glad toconsider the addition of names of those, who want to obtain thisjournal regularly, to the mailing list.

Instructions to Authors(1) Manuscript format•	Electric files should be submitted to the Editorial Secretariat byonline. Authors’ short biography with less than 100 words perperson and color photographs of all the authors should be
attached to the final version.
•	The structure of manuscripts should follow the following order;
title, authors, affiliations, abstract, graphical abstract, keywords,
main text, (acknowledgement), (appendix), (nomenclature), references. The items with parentheses are not mandatory.
•	Full postal addresses must be given for all the authors. Indicate
the corresponding author by the mark“*” after the name. Telephone and fax numbers and e-mail address should be provided
for the corresponding author.
•	Abstract should not exceed 200 words.
•	G raphical abstract should be a concise, visual summary of thearticle which will be displayed in the contents list both online
and print.
•	The appropriate number of keywords is 5 or 6.
•	The maximum pages printed in KONA are supposed to be:
15 for an original paper and 25 for a review paper.
•	Symbols and units should be listed in alphabetical order with
their definition and dimensions in SI units.
•	The color figures will appear in color both on the KONA
Website (http://www.kona.or.jp) and also in the paper version.
•	Concerning references, the alphabetical system should be
adopted. Please use reference management software such as
Endnote to manage references as far as possible.
List: References should be arranged first alphabetically andthen further sorted chronologically if necessary. More than one
reference from the same author(s) in the same year must be
identified by the letters “a”, “b”, “c”, etc., placed after the yearof publication.
Examples:- Reference to a book:Strunk Jr. W., White E.B., The Elements of Style, fourth ed.,Longman, New York, 2000.
- Reference to a chapter in an edited book:
Mettam G.R., Adams L.B., How to prepare an electronicversion of your article, in: Jones B.S., Smith R.Z. (Eds.),
Introduction to the Electronic Age, E-Publishing Inc., New
York, 2009, pp.281–304.
- Reference to a journal publication:
Tsuji Y., Tanaka T., Ishida T., Lagrangian numerical simulationof plug flow of cohesionless particles in a horizontal pipe,
Powder Technology, 71 (1992) 239–250.
Text: All citations in the text should refer to:1. Single author: the author’s name (without initials, unlessthere is ambiguity) and the year of publication;
2. Two authors: both authors’ names and the year of publication;
3. Three or more authors: first author’s name followed by “etal.” and the year of publication.
Citations may be made directly (or parenthetically). Groups ofreferences should be listed first alphabetically, then chron­ologically.
Examples: “as demonstrated (Hidaka J. et al., 1995; Tsuji Y.,
1992a, 1992b, 1993). Mori Y. and Fukumoto Y. (2002) haverecently shown ....”(2) Copyright and permission•	The original paper to be submitted to KONA has not been
published before in any language or in any journal or media; it
is not submitted and not under consideration for publication inwhole or in part elsewhere.
•	Authors are responsible for obtaining permission from the
copyright holders to reproduce any figures, tables and photos
for which copyright exists.
•	The KONA Journal applies the Creative Commons Attribution
License to all works published by the Journal. Copyright stays
with the agreed copyright owner, and the Hosokawa Powder
Technology Foundation is granted the exclusive right to publish
and distribute the work, and to provide the work in all forms
and media.
•	Users of the journal will be able to reuse the contents in any
way they like, provided they are accurately attributed. No
permission is required from either the authors or the publisher.

Free electronic publication of KONA is available at http://www.kona.or.jp2935+66 406'45646*+404$1%36.66#/)./)#/.66.//66-456.

Data Driven Manufacturing

Addressing your manufacturing challenges
Gen4 converts data mining and machine learning into measurable manufacturing improvements.
With modular solutions we work at a pacethat meets your specific business needs.
,*3 4056,5%2*3$6400

"526'523
 *"126 5!135 analysis and interrogation((

 online advisory systems
 intelligent monitoring solutions((,5%256&01%

 remote data collection

 closed loop control systems

Understand • Monitor • Control-04+3

'524312)&1+32106,$"35!

-&6-&6 4053
126,!423' 1+5

T: +44 (0)1928 755100 • info@hosokawagen4.com • hosokawagen4.com