THE IMPROVEMENT OF GRINDING AND CLASSIFYING PERFORMANCE

BY

Carl K. Ishito
Vice Manager
Nisshin Engineering Co., Ltd.
Satoshi Akiyama
Chief Engineer
Nisshin Flour Milling Co., Ltd.
Dr. Zennosuke Tanaka
Professor
Okayama University

1. ABSTRACT
In the powder handling process, it is important to control the particle size distribution.
Especially for dry process, because of the stickiness or agglomeration of powder, it becomes
more difficult to classify and/or grind powder with the finer particle size distribution. In orderto save energy cost or running cost, it is recommended to optimize the classification and grinding
condition.
Using new classification and grinding technology, we discuss the improvement of theclassification and grinding performance. This discussion includes closed loop grinding &classification systems, classification with improved dispersion mechanism and other topics.
Powder to be discussed in this paper is mainly toner.

1

mills have rotor inside. Figure 2 illustrates the cross-sectional view of the SUPER ROTOR3) (from Nisshin Engineering).
5. CLOSED CIRCUIT SYETEM WITH A GRINDING MILL & CLASSIFIER
An example of a toner manufacturing process is shown in Figure 3. A closed circuit
system of a classifier and a mechanical mill is employed in this process. An effectiveclassification is possible with this system because dispersed particles can be fed to the classifierby the mechanical mill.
Typically, milled particles have a broad size distribution. For a grinding mill to achieve
narrow size distribution products, it would be necessary to operate the mill for an extremely long
period of time. Even then, some coarse particles may still remain in the product or the productmay contain too many out-of-spec fine particles. To avoid this situation, classifiers should beincorporated following a grinding mill. A closed circuit system can decrease the load for thegrinding mill causing less energy consumption than in a grinding mill-only system.
Today’s copying machines and printers demand improved image quality. This demandhas brought with it severe specifications on particle size distribution of toner. Most tonercurrently available in the market is manufactured by grinding method. Many efforts have been
made to increase grinding efficiency in toner manufacture. Higher efficiency is needed because
energy increases in grinding and classifying processes as particle size becomes smaller andparticle distribution narrower. In the past, jet mills were widely used for toner grinding.
However, mechanical mills with higher energy efficiency than jet mills have been developed.
These mills can prevent over-grinding and become widely available.
We have developed a closed circuit grinding system coupling a highly efficient mechanicalmill and a precision air classifier as follows. Super Rotor SR-25 is used for the mechanical mill.
Turbo Classifier TC-25III removes coarse particles in the system.
5.1 COMPARISON BETWEEN JET MILL AND MECHANICAL MILL
Magnetic toner composed of styrene-acrylic binder resin was ground by two kinds ofclosed circuit grinding systems using the mechanical mill and a conventional jet mill4). Particle
diameter of the feed material was 500 µm.

It was ground in the mechanical mill with air2cooled at 5 degree C to prevent from melting the toner during grinding.
Figure 4 compares the mechanical grinding system with the jet grinding system in terms ofthe over-grinding. The percentage of fine particles less than 5 µm was smaller with themechanical grinding system than with the jet grinding system.
The fine particles less than 5 µm of those ground toner were removed by an air classifier.
The Turbo Classifier TC-40II was used for fine classification. Figure 5 shows product yieldwith each system. For the same median particle size, the product yield with the mechanicalgrinding system was higher than that with the jet grinding system, because the mechanicalgrinding system produced fewer particles less than 5 µm.
Figure 6 shows the relation between the median particle diameter of product and thespecific energy consumption of the grinding system. The air compressor in the jet grinding
system consumes 70% or more of the total energy. The energy consumed by the air cooler is
included in the values shown for the mechanical grinding system. When using the mechanicalgrinding system, if particle diameter became finer than 8 µm, although energy consumption
increased, that consumption was still 40% less than with the jet grinding system.
5.2 TEMPERATURE CONTROL FOR MECHANICAL MILLColor copiers and printers require color toner that is non-magnetic and made frompolyester resin (in most cases). In general, the polyester resin has a low melting pointcompared with styrene-acrylic resin. Non-magnetic toner with polyester resin is more difficult
to be ground than magnetic toner with styrene-acrylic resin. Therefore, an efficient method for
grinding color toner is needed.
Magnetic toner with styrene-acrylic resin and non-magnetic toner with polyester resin
were ground by the mechanical mill without a classifier. Figure 7 shows the relationshipbetween the rotational speed of the mill and the median particle diameter of product. The
non-magnetic toner with polyester resin was more difficult to be ground than the magnetic tonerwith styrene-acrylic resin. The manufacture of fine toner requires a rotor running at highrotational speed. This causes the outlet temperature of the mill to increase, which means thefeed rate of raw material must be decreased.

3

Non-magnetic toner with polyester resin was ground to an 8 µm particle diameter by theclosed mechanical grinding system. The raw material was fed with the cooling air at various
temperatures. Operating conditions were adjusted for an air temperature of 50 degree C at themill outlet. Figure 8 shows the effect of the temperature of the cooling air on maximum
throughput of the mechanical grinding system. By lowering the temperature of the cooling air
from 5 to -40 degree C, we were able to increase grinding capability by 2.5 times.
5.3 ENERGY CONSUMPTION
Figure 9 shows the relationship between the temperature of the cooling air and the specificenergy consumption of the grinding system. The energy consumption in the system increased
as the temperature of the cooling air decreased. However, since processing capability wasimproved, the specific energy consumption required for grinding the toner became rather small.
5.4 PARTICLE SHAPE OF TONERS
Flowability and electrical charge characteristics of toner change with differences inparticle shape. Spherically shaped particles hold many advantages. Printing concentration, forexample, is deep. Figure 10 shows SEM photographs of the toner ground with a target jet millsystem, a pancake jet mill system and the mechanical mill system.
The surface shape factor3) of the toner particles was determined by eq.(1).
Surface factor = P2/4πA(1)where P is perimeter of a projected particle and A is projected area. If a particle is spherical, its
surface shape factor is 1.
Table 1 shows the surface factors for each toner ground with these mills. The surface
shape factor was calculated by an image analyzer. Compared with other systems, themechanical grinding system, with this particle surface shape factor of close to 1, produces thesmoothest and rounded surface shape.
Table 1 Surface shape factors for the toner with three kinds of mills
Type of Mill
Target Jet Mill
Pancake Jet Mill
Mechanical Mill

Surface Shape Factor [-]
1.41
1.35
1.26

4

6. DEVELOPMENT TREND
6.1 TO FIND THE OPTIMUM SYSTEM: (A SYSTEM OF GRINDING MILLS AND
CLASSIFIERS)
In advanced closed circuit systems, there are grinding mills with built-in precise classifiers.
Typical examples are the Super Hybrid Mill7) and AFG Counter Jet Mill. These grinding mills
can reportedly produce 0.5 μm (median dia.) particles, although their minimum grinding limit
is generally 1 to 3 μm.
A large mechanical mill is often used for grinding minerals (such as calcium carbonate,talc and silica sand) to micron or sub-micron order particles in a high capacity at less cost. Ajet mill can also be used for fine grinding; however the jet mill has a high grinding energy costand is not practical for large capacity grinding. The Super Hybrid Mill (SH Mill) was
developed by IHI and Nisshin Engineering to meet these demands. The SH Mill is a VerticalRoller Mill with high performance, and is the result of directly coupling an IHI Roller Mill witha Nisshin Air Classifier.
In order to increase the classification performance, it is very important that the powder fed
to the classifier should be dispersed into single particles as completely as possible.
When theclassifier is used together with the mill, the mill’s function is to disperse the agglomerated
particles as well as to grind coarse particles into fine. In this way, ground and dispersed
particles in mills can be effectively classified by classifiers. In general, if the classifier is usedjust after the mill in the system, the fine powder yield is improved by about 5% in the calcium
carbonate process. Nisshin and IHI developed the SH Mill with this concept in mind. Figure
11 shows the SH Mill system flow. In order to classify milled powder within 1 second aftermilling, the air classifier is mounted on the top of the mill part. This mill can classify thepowder after the milling process and before re-agglomeration. As a result, the efficiency goesup and a much finer classification point can be attained.
Figure 12 shows the difference in performance between the coupling system and that of
the classifier by itself. The SH Mill was used for the direct coupling system, and groundmaterial was classified just after grinding.
10,000 cm2/g (Average Diameter 4 to 5 μm)
surface area calcium carbonate was used as both of the Feed to the classifier.

5The surface area

data of the products (SH Mill) and the fine fraction (classifier only) are shown in this figure.
The fine yield was controlled 10% for both systems. The following advantages were obtainedwith the direct coupling system:
Small Machine
Large Machine
2Classifier Only
23,000 cm /g(1.2 μm D50)
21,500 cm2/g
Direct coupling
29,000 cm2/g(1 μm D50)
25,500 cm2/g
Advantage+6,000 cm2/g+4,000 cm2/g
The SH Mill was developed mainly for the purpose of producing a large amount ofultra-fine powder, such as 5 μm to sub-micron order, at the least cost. Figure 13 shows theperformance of the largest model (SH-800) of SH Mill. The TC-120 or TC-130 is used in theSH-800 as the built-in classifier. 3,900 cm2/g talc is produced by the SH-800 with the TC-130
at 800 kg/h feed (product) rate
Figure 14 shows the difference in electric power usage between the SH Mill and theconventional roller mill. To produce 20,000 cm2/g silica, the SH Mill uses 200 kWh per ton ofproduct compared with 500 kWh per ton for the conventional model. This difference isrelatively small in the coarse grinding field, however, it is very big in the fine grinding field.
This is typical of the improvement in performance that can be obtained by the direct couplingsystem over the performance that either machine can deliver separately.
6.2 IN-LINE PARTICLE SIZE MEASUREMENT DEVICE
Although the particle size of products has been checked with off-line devices before.
in-line devices are introduced by some manufacturers. Classification conditions will beautomatically adjusted by microcomputers. A Laser Method and a classifier method8) havebeen reported.
6.3 MUCH FINER
Already, there is a classifier which can classify down to 0.5 μm cut point in the ceramic
field. For toners, 6.5 μm toner could be produced (ground and classified) using a mechanical
mill.

6

7. CONCLUSION
1) In the toner grinding process, mechanical mill produces less fine particles (below 5 μm)
more rounded shape particles than jet mills.
2) In the toner grinding process, mechanical mill can be used down to 6.5 μm (average).
3) In the toner grinding process, closed circuit system of classifier and mechanical mill gives
less energy cost than jet mill above 6.5 μm products (magnetic toner).
4) In the toner grinding process, cool air (-40 degree C) increases the capacity of grinding
process by 2.5 times compared with 5 degree C normal temperature.
5) Grinding mill with built-in classifier increases performance enormously.
< References >1) M. Yasuguchi, Precise Air Classifier TURBO-CLASSIFIER, Industrial Machinery, No. 438,
1987, p80-86
2) Y. Yamada , T. Ibori and I. Shimogawa, Effect of Circulating Mechanism on the Air
Classifier, Rosemont, IL, 1989, p187-192
3) N. Tonoike, S. Akiyama, T. Ichikawa, H. Okabe and K. Ishito: Fine Pulverizing of Heat
Sensitive Plastics using Mechanical Mills, Powder and Bulk Solids Handling and Processing,
1997, p363-370
4) Y. Yamada , O. Kano and K. Yabuta: Coupling System of Air Jet Pulverizer and Air
Classifier, Proceedings of the Technical Program, Powder and Bulk Solids Handling andProcessing, 1992, p205-2125) K. Gotoh, H. Masuda and K. Higashitani: Powder Technology Handbook, Marcel Dekker
Inc., 1997, p456) M. Jimbo, The Newest Ultra-Fine Grinding Process and the Precise ClassificationTechnologies, Epotech, May 31. 1986.
7) M. Kano and K. Ishito: Dry Continous Sub-micron Grinding by Super Hybrid Mill, FC
Report, No. 8, 1989, p333-337
8) Y. Yamada , M. Yasuguchi and K. Iinoya: Development of a Fineness Measuring Air
Classifier, Proceedings of the Technical Program, Powder and Bulk Solids Handling andProcessing, 1985, p4119) Carl K. Ishito: Air Classification Technologies Update, Proceedings of the TechnicalProgram, Powder and Bulk Solids Handling and Processing, 1992, p27-36

7

Powder Feed
(1)

(2)

(3)

(4)

(5)
OutletAirLiner

(6)

Rotor
GrindingZone

(7)

(8)
Inlet

Air Flow(1) classification rotor(3) dispersion disc(5) coarse fraction outlet(7) scroll casing(2) dispersion blades(4) classification blades(6) auxiliary blades(8) balance rotor

Figure 2 Cross-sectional view ofSuper Rotor

Figure 1 Cross-sectional view ofTurbo Classifier

Classifier
CycloneBag House

Bag House

Raw MaterialFeeder
BlowerCycloneMechanical
MillClassifier
Blower

Air Cooler
ProductFine grinding process

Fine classifying process

Figure 3 Flowsheet of toner grinding and classifying process

8Closed Circuit System

24Mechanical Mill

Product Yield [%]Fine Particles less than 5µm [vol.%]

100

28

Jet Mill

2016129080
Mechanical Mill70

8

Jet Mill

460

0678910116712 139810111213

Median Diameter [µm]Median Diameter [µm]Figure 5 Relation between themedian diameter and product yieldin grinding and classifying process

714

Closed Circuit System6Mechanical Mill

5Median Diameter [µm]Specific Energy Consumption [kW h/kg]

Figure 4 Relation between themedian diameter of product and thevolume percentage of fine particles
less than 5µm

Jet Mill

432106789101112

Styrene-Acrylic Toner
Polyester Toner

12108613

Median Diameter [µm]Mechanical Mill
without Classifier

11,00012,00013,000

Rotational Speed [min-1]

Figure 6 Relation between the median
diameter of product and specific energy
consumption of closed circuit grinding
system9

Figure 7 Relation between therotational speed of mechanical milland the median diameter of productground with mechanical millSpecific Energy Consumption [kW h/kg]

Maximum Throughput [kg/h]

20
Closed Circuit Systemwith Mechanical Mill15105

Polyester Toner

0-50-40-30-20-10010

Temperature of Cooling Air [degree C]

Closed Circuit Systemwith Mechanical Mill

8642

Polyester Toner

0-50-40-30-20-10010

Temperature of Cooling Air [degree C]

Figure 8 Relation between the temperatureof cooling air and the maximum throughputof the mechanical grinding system

Figure 9 Relation between thetemperature of cooling air and thespecific energy consumption of closed
mechanical grinding system

10µm(a) Target jet mill

1010µm(b) Pancake jet mill

10µm(c) Mechanical mill

Figure 10 SEM photographs of the toner ground with three kinds of systems

10Bag House

BlowerReturned

Classifer

FeedProduct
Grind ing

Screw
FeederHydraulic Unit

Control Panel

Figure 11 System Flow of the Super Hybrid Mill

40,000
CaCO 328,00026,000Surface Area [cm2/g]

Surface Area [cm2/g]

30,000

Direct Coupling System(Super Hybrid Mill)

24,00022,00020,000Classifier Only

1005001,000

2,000

Talc(SH800,TC-130)
30,00020,000
CaCO 3(SH800,TC-120)

10,000

0

1,000

2,000

Feed Rate [kg/h]

Classification Rotor Diameter [ mm ]Figure 13 SH-800 Grinding
Performance Example

Figure 12 Classification performance
improvement by direct coupling

11Electric Power [ kWh/ ton ]

1,000
500

Conventional Mill200
10050

Super Hybrid MillSilicaSH-150

2010
10,00020,000Surface Area [cm30,0002/g]

Figure 14 Comparison of electric power usage12