THE IMPROVEMENT OF GRINDING AND CLASSIFYING PERFORMANCEUSING CLOSED CIRCUIT SYSTEM
KATSUNORI ISHITO1 , SATOSHI AKIYAMA2 and ZENNOSUKE TANAKA3
1
Plant 1st Department, Nisshin Engineering Inc.,
14-1, Koami-cho, Nihonbashi, Chuo-ku, Tokyo, 103-8544, Japan
2
Powder Technology Laboratory, Technical Research Center, Nisshin Flour Milling Co.,
Ltd.,
5-3-1, Tsurugaoka, Oimachi, Iruma-gun, Saitama, 356-8511, Japan
3
Professor, Material Chemical Engineering Department, Okayama University,
3-1-1, Tsushimanaka, Okayama-city, Okayama, 700-8530, Japan
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 order to save energy cost or running cost, it is recommended to optimize theclassification and grinding condition. Using new classification and grinding technology, we
discuss the improvement of the classification 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.
NOMENCLATURE
P: perimeter of a projected particle
A: projected surface area

1. INTRODUCTION
As interest becomes sharply focused upon fine particles, such as in the fine ceramics, plastic, mineral andmetal powder industry, particle sizes should be in the order of micron or sub-micron range.
classifiers are indispensable for the powder process as well as grinding mills.

In this case,This means that coarse

particles should be sharply discarded from fine particles to determine top size of the products and collect
good fine products before grinder to improve the grinding performance without introducing fine powder.
This paper illustrates that classification and grinding are both principal technology in sharply adjusting
particle sizes and economically grinding particles and shows several up-to-date examples.
12. REQUIREMENTS OF FINE GRINDING AND CLASSIFICATION
There are two typical methods of adjusting the particle size of fine particles. One is
grinding and the other is classification. In manufacturing sub-micron particles, thefollowing technique is required for each method. It is important for grinding mills to
produce as many “less than 1μm particles” as possible, in a short period of time and at anenergy efficient cost. On the other hand, it is important for classifiers to remove the coarse
particles which remain, even after grinding, at a greater yield and accuracy. Closed circuit
system of grinding and classification will be the primary focus of this paper.
3. A MECHANISM OF CLASSIFIERS AND GRINDERS< CENTRIFUGAL FORCE CLASSIFICATION >The forced vortex type classifier has a wide range cut point. For instance, micron order to
100 μm. Precise classification can be achieved with forced particle flows and air flows.
A classification chamber rotating type in the forced vortex type can classify particles down to
the 0.5 μm cut point. This is because of the rectification of air flows and the precisenessin the classification mechanism. Typical examples are: Turbo-Classifier, Acucut, . . . etc.
Figure 1 illustrates the cross-sectional view of the Turbo-Classifier1),2) (from Nisshin
Engineering).
Powder Feed
(1)

(2)

(3)

(4)

(5)

Air
(6)

(7)

(8)

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

Figure 1 Cross-sectional view of Turbo Classifier< MECHANICAL MILL >2

Jet mills need compressed air and have no moving part. On the other hand, mechanicalmills have rotor inside. Figure 2 illustrates the cross-sectional view of the SUPERROTOR3) ( from Nisshin Engineering).
OutletLiner
Rotor
Grinding
Zone

Inlet

Figure 2

Cross-sectional view of Super Rotor

4. CLOSED CIRCUIT SYETEM WITH A GRINDING MILL & CLASSIFIER
An example of a toner manufacturing process is shown in Figure 3. A closed circuit systemof a classifier and a mechanical mill is employed in this process. An effective classificationis possible with this system because dispersed particles can be fed to the classifier by themechanical mill.
Classifier
Bag House
Bag House

CycloneRaw Material
FeederBlowerCycloneMechanical
Mill

Classifier
Blower

Air Cooler
Product
Fine grinding process

Figure 3

Fine classifying process

Flowsheet of toner grinding and classifying process

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 the3

product may contain too many out-of-spec fine particles. To avoid this situation, classifiers
should be incorporated following a grinding mill. A closed circuit system can decrease theload for the grinding mill causing less energy consumption than in a grinding mill-onlysystem.
Today’s copying machines and printers demand improved image quality. This demand hasbrought with it severe specifications on particle size distribution of toner. Most tonercurrently available in the market is manufactured by grinding method. Many efforts havebeen made to increase grinding efficiency in toner manufacture. Higher efficiency isneeded because energy increases in grinding and classifying processes as particle size
becomes smaller and particle 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 mechanicalmill. Turbo Classifier TC-25III removes coarse particles in the system.
4.1 COMPARISON BETWEEN JET MILL AND MECHANICAL MILL
Magnetic toner composed of styrene-acrylic binder resin was ground by two kinds of closed
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 aircooled at 5 degree C to prevent from melting the toner during grinding.
100

Closed Circuit System

24

Mono-Component Toner

Mechanical Mill90

Jet Mill

20
16

Mono-Component

12

Toner

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

28

8

80
Mechanical Mill70

Jet Mill

4

60

0
6

7

8

9

10

11

12 13

6

7

8

9

10

11

12

13

Median Diameter [µm]Median Diameter [µm]Figure 4
Relation between the
median diameter of product and thevolume percentage of fine particles

Figure 5 Relation between the
median diameter and product yieldin grinding and classifying processless than 5µm

4Figure 4 compares the mechanical grinding system with the jet grinding system in terms of
the 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 the specificenergy 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 theSpecific Energy Consumption [kW h/kg]

mechanical grinding 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.
7
Closed Circuit

6

Mechanical Mill
Jet Mill

System5
4

Mono-Component Toner

3
2
1
0
6

7

8

9

10 11

12 13

Median Diameter [µm]Figure 6

Relationship between the median diameter of product and specificenergy consumption of closed circuit grinding system

4.2 TEMPERATURE CONTROL FOR MECHANICAL MILL
Color copiers and printers require color toner that is non-magnetic and made from polyester
resin (in most cases). In general, the polyester resin has a low melting point compared withstyrene-acrylic resin. Non-magnetic toner with polyester resin is more difficult to be groundthan magnetic toner with styrene-acrylic resin. Therefore, an efficient method for grindingcolor toner is needed.
5

Magnetic toner with styrene-acrylic resin and non-magnetic toner with polyester resin wereground by the mechanical mill without a classifier. Figure 7 shows the relationship between
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
toner with styrene-acrylic resin. The manufacture of fine toner requires a rotor running athigh rotational speed. This causes the outlet temperature of the mill to increase, whichmeans the feed rate of raw material must be decreased.
14

Median Diameter [µm]Styrene-Acrylic Toner
Polyester Toner

12

10

8

Mechanical Mill
without Classifier

6
11,00012,00013,000

Rotational Speed [min-1]

Figure 7 Relation between the rotational speed of mechanical milland the median diameter of product ground with mechanical mill

Maximum Throughput [kg/h]

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 atvarious temperatures. Operating conditions were adjusted for an air temperature of 50
degree C at the mill outlet. Figure 8 shows the effect of the temperature of the cooling air
on maximum throughput of the mechanical grinding system. By lowering the temperatureof the cooling air from 5 to -40 degree C, we were able to increase grinding capability by 2.5
times.
20

Closed Circuit Systemwith Mechanical Mill

15

10

5

Polyester Toner

0-50 -40 -30 -20 -10
010Temperature of Cooling Air [degree C]

Figure 8 Relation between the temperature of cooling air and the
maximum throughput of the mechanical grinding system
6

4.3 ENERGY CONSUMPTION

Specific Energy Consumption [kW h/kg]

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 systemincreased as the temperature of the cooling air decreased. However, since processing
capability was improved, the specific energy consumption required for grinding the toner
became rather small.
10
8

Closed Circuit Systemwith Mechanical Mill

6
4
2

Polyester Toner

0
-50

-40

-30

-20

-10

0

10

Temperature of Cooling Air [degree C]

Figure 9 Relation between the temperature of cooling air and the specificenergy consumption of closed mechanical grinding system
4.4 PARTICLE SHAPE OF TONERS
Flowability and electrical charge characteristics of toner change with differences in particle
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
mill system, a pancake jet mill system and the mechanical mill system.

10µm

10µm

(a) Target jet mill

10µm(b) Pancake jet mill(c) Mechanical mill
Figure 10 SEM photographs of the toner ground with three kinds of systems
7

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 shapefactor was calculated by an image analyzer. Compared with other systems, the mechanicalgrinding system, with this particle surface shape factor of close to 1, produces the smoothest
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

5. DEVELOPMENT TREND
5.1 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. A jet
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
Vertical Roller Mill with high performance, and is the result of directly coupling an IHI
Roller Mill with a Nisshin Air Classifier.
In order to increase the classification performance, it is very important that the powder fed tothe 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 is8

used just 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 inmind. Figure 11 shows the SH Mill system flow. In order to classify milled powder within
1 second after milling, the air classifier is mounted on the top of the mill part. This mill canclassify the powder after the milling process and before re-agglomeration. As a result, theefficiency goes up and a much finer classification point can be attained.
Bag House

BlowerReturned

Classifer

Feed

Product
Grind ing

Screw
FeederHydraulic Unit

Control Panel

Figure 11 Flow Diagram of Super Hybrid Mill system

Surface Area [cm2/g]

30,000
CaCO 328,00026,000

Direct Coupling System(Super Hybrid Mill)

24,00022,00020,000Classifier Only

100

500

1,000

Classification Rotor Diameter

2,000[ mm ]

Figure 12 Classification performance improvement by direct coupling
Figure 12 shows the difference in performance between the coupling system and that of theclassifier 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. The surface
area data of the products (SH Mill) and the fine fraction (classifier only) are shown in thisfigure. The fine yield was controlled 10% for both systems. The following advantages
9were obtained with the direct coupling system:
Small Machine
23,000 cm2/g(1.2 μm D50)
29,000 cm2/g(1 μm D50)
+6,000 cm2/g

Classifier Only
Direct coupling
Advantage

Large Machine
21,500 cm2/g
25,500 cm2/g+4,000 cm2/g

The SH Mill was developed mainly for the purpose of producing a large amount of
ultra-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
the SH-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
Electric Power [ kWh/ ton ]1,000
Surface Area [cm2/g]

40,000Talc(SH800,TC-130)
30,00020,000CaCO3

10,0003(SH800,TC-120)500

Conventional Mill

2 00
100
50

SilicaSH-15020
100Super Hybrid Mill

1,00 02,000
Feed Rate [kg/h]

10,0002 0,000

3 0,000
2

Surface Area [cm /g]

Figure 13 SH-800 Grinding Performance Example

Figure 14

Comparison of electric
power usage

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 tonof product 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.
5.2 IN-LINE PARTICLE SIZE MEASUREMENT DEVICE
Although the particle size of products has been measured with off-line devices, in-line
devices are introduced by some manufacturers. Classification condition can be monitored
by measuring the particle size of the products (or classified coarse powder yield)
continuously. This data can automatically adjust the classification conditions throughmicrocomputers. A Laser Method and a classifier method8) have been reported.
10

5.3 MUCH FINER

Average particle size [micron]

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

Nickel powder Grinding using SH-150
6
4

Average
Particle Size(micron)

2
0
1

2

3

Feed Rate [kg/h]

Figure 15 Nickel powder grinding results using SH‑150 (TC‑25)

mechanical mill. Grinding metal powder, such as Nickel powder, is not easy, because it isgenerally difficult to be broken . Figure 15 shows the grinding results using SH-150. The
combination of grinding (roller mill) and air classification made the products finer and finer.
Grinding only can produce 3 to 5 microns average products (Nickel powder). On the other
hand, SH-150 can grind nickel powder down to 1.2 microns.
From now on, this closed circuit grinding and classifying system will become popular in thefine grinding field, especially within the equipment.
6. 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 in toners andmetal powder field.

11

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