Studies on comminution mechanism of the dry tower mill KD-3

Proceedings of the XXI International Mineral Processing Congress

C4-44

STUDIES

ON COMMINUTION TOWER

MECHANISM MILL

OF THE DRY

KD-3

A. Shibayama*, S. Mori ~ A. Bissombolo ~ *Department of Recycle Plant Engineering, Kubota Cooperation 2-35, Jinmucho, Yao, 581-8686, Japan ~ of Earth Resources Engineering, Kyushu University 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan

Abstract In order to obtain economically lot of fine particles, a dry tower mill KD-3, composed of a comminuting section, a classifying column and a collecting section, has been developed through this study. Grinding tests were carried out on limestone to clarify the comminution mechanism in the KD3. A particular attention was paid to important factors such as the size of the grinding media (steel balls of q) 10 nma, q) 13 mm, q) 16 mm, q~20 mm), the speed of the stirring screw (from 1.08 s~ to 1.58 s-~) and the role of the net in the classifying column. The study has revealed an optimum efficiency for steel balls of q) 20 mm size and a stirring screw rotating at 1.58 s-1. Besides, a series of nets settled in the classifying column have improved the efficiency of the classifying operation by reducing the buoyancy of the materials and regulating the maximum size of products collected in the cyclone (dmax is about 10 ~tm). The air flow rate in the duct and the pressure of drops in the column were approximately quadratic curves, but the dmaxof products collected in the cyclone was proportional to the pressure of drops in the classifying column. Keywords: dry tower mill, comminution, grinding media, stirring screw, classifying column

Introduction Some industrial mineral producers have extensively used the tower mill through the world as a new type of fine grinding pulverizer. However, few studies have been carried out on the mechanism and conditions to achieve a higher fine grinding performance with this tower mill. Recently, a series of modifications made in the structure of the cyclone and the classifying column have led to some new types of dry tower mill. The latest version of the tower mill (KD-3) gives a product whose m a x i m u m particle diameter is smaller than 10 pm (Mori et al., 1997). The present work deals with the influence of the grinding media (steel balls) size and the speed of the stirring screw on the grinding efficiency, the role of the nets settled in the classifying column of the tower mill KD-3. Further development would permit to achieve a higher recovery of fine particles from the pulverized products.

Experiment and method The dry tower mill KD-3, illustrated in Figure 1, is a modified version of the commonly used laboratory tower mill. Its comminuting cell (A) is separated from the classifying column (B) to regulate the maximum size of particles in the products collected in the section (C). The inside diameter and length of the comminuting cell

Comminution, Classification and Agglomeration

C4-45

are 0.26 m and 0.6 m respectively and the grinding media is made of steel balls (loading mass: 40.0 kg). Tests were carried out on 1.0 kg of limestone ore (density 2.70-103 kg/m3), sizing from 420 pm to 3.36 mm and provided by the Todaka Tsukumi Mine (Japan). The rate of the air circulating inside the tower mill, later referred as the duct air flow rate (Ua), is measured through an orifice | settled inside the duct and the particles size distribution were performed on a sedimentation balance (Hara and Moil, 1995).

| '~,~ I ~ I, 1

B

KD-3

L

_

,

|

i

!

,/

(~ Comminuting cell

| Stirring spiral

| Motor

| Classifying cell

| Baffle plate

| Inlet of feed

(~ Cyclone

| Airflow meter

| Electric blower

A.Comminuting section

B.Classifying section C.Collecting section

Figure 1: Schematic view of the tower mill KD-3.

The limestone is ground twice for 60 min and the fresh-formed particles are transported upwards to the classifying column | and the cyclone Q by a circulating air flow generated inside the mill (by the blower | through the bottom of the comminuting cell (i)). A sudden increase in the diameter of the classifying column reduces the buoyancy of the materials and allows only fine particles to be collected while coarse particles settle at the bottom of the column. The cyclone C2 is provided with a central spiral ribbon (CSR) that is supposed to increase the centrifugal force of fine particles settling down the spirals. The firstly investigated parameters were the size of the steel balls (q) = 10 mm, q0 = 13 ram, q0 = 16 mm, q0 - 20 mm) and the speed of the stirring screw (from 1.08 s-~ to 1.58 s-l). Then, followed the role of the nets (S 1, $2, and $3) and the type of columns (CLA0, CLA1, CLA2, and CLA3) used to reduce the air turbulence and improve the classifying efficiency. Table I gives the characteristics of sieves and Figure 2 illustrates different views of columns.

Proceedings of the XXI InternationalMineral Processing Congress

C4-46

Table I: Kind of sieves using in column of KD-3: S1, S2, $3. Types of n e t s S1 $2 $3

Opening(mm) 5.0 3.5 1.0

Diameterof line (mm) 0.75 1.00 0.20

As seen, the column CLA0 does not have a net, but a baffle plate settled in the column. The CLA1 has two nets: one (S1) is settled downside and the other ($3) upside the baffle plate. In the case of CLA2, $3 is settled above S1 upside the baffle plate. Its diameter is half that of the column. As for CLA3, both S 1 and $3 are now settled downside the baffle plate and $3 is the lower net. Upside the plate, S 1, $2 and $3 are put in this order from the bottom to the top. Besides, the diameter of $2 is two third and that of $3 is one third of the diameter of the classifying column.

:Eli X

-

X-Y Cross section

V

Baffle plate CLA1

CLA0

I

!

d

Sl $2 $3 Sl $3

S1 $3 ~.~":"

I CtA3

CLA2

Figure 2: Schematicview of modified columns of KD-3. Results and analysis

Influence of the airflow rate Previous studies carried out on a laboratory tower mill in which the comminution and classification mechanisms occur in the same column have shown an increase in the dmax value with the airflow rate. At duct airflow of 1.35 m/s for instance, the dmax was high enough (about 70 ~tm). These studies suggested that a better classifying/collecting effect could help producing more fine particles. The following steps have been made with this purpose: - enlarge the diameter of the classifying column; - provide the classifying column with a series of nets to separate the classifying and collecting functions, stabilize the air turbulence and increase the process selectivity;

Comminution, ClassificationandAgglomeration

c4-47

insert into the cyclone a central spiral ribbon that would increase the centrifugal force of collected particles and improve, in the same way, the air-solid separation process. These modifications have improved the efficiency of the Tower Mill KD-3: the dmax value of particles in the cyclone products has dropped to 10 ~tm (Figure 3) and the product fineness or median size of particles (ds0) is up to 1.5 ~tm. -

Influence of the grinding media size Figure 3 gives the weight of cyclone and column products obtained after pulverization of limestone ore with steel balls of different diameters (q0 - 10 mm, q) = 13 mm, q) = 16 mm, q) = 20 mm) at increasing duct airflow rates. At lower rates, the weight of products is not influenced by the size of the grinding media. This dependence increases sharply at higher values. So, the weight of cyclone products obtained with 20 mm diameter balls, for example, is four times, and that of column products - fifteen times, as large as that obtained with 10 mm diameter balls. These results are an evidence of the highly comminuting efficiency of 20 mm diameter balls to produce many fine particles (Mori et al., 1995). Figure 4 represents the maximum diameter of particles collected in the cyclone and the column at increasing duct airflow rates. It is shown that the size of the balls is not a major factor in the determination of the maximum diameter of particles in the products. 0.3

0.7

f

~ q 0 ~q013 + q ~

exl)

l0 m m mm 16mm

----~---q~ ~ 0 + q 0 ~ q ~

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mm mm rnm mm

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,

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0.5 1.0 1.5 Duct air flow rate (m/s)

,

i

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2.0

Figure 3: Weight of cyclone products (left) and column products (right) at different airflow rates.

Influence of the stirring screw speed Figures 5 and 6 give the results of the tests carried out at various values of the stirring screw speed and the duct airflow rate (Shibayama et al., 1997). The plot in Figure 5 between the specific energy (J/kg) and the duct airflow rate reveals that these two parameters are inversely proportional. Although some differences are observed at lower duct airflow rates, the energy consumption of the KD-3 remains almost constant at higher rates. It is likely to obtain a high grinding efficiency by increasing the speed of the stirring screw without increasing the specific energy. That does not influence the shape of the particles size distributions of products in the cyclone and the column (Figure 6). In these plots, both ds0 and dmax increase with the duct airflow rate independently of the stirring screw speed.

Proceedings of the XXI International Mineral Processing Congress

C4-48

15

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0.0

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Duct air flow rate (m/s)

1.0

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Duct air flow rate (m/s)

Figure 4: Maximum diameter (dm~x) of cyclone products (left) and column products (fight) versus duct air flow rates. 250

50 Screwspeed(s-l)

Screwspeed(s-l) 200 ~150

----II--

1.08

--k-+

1.25 1.42

---0-

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0.5 1.0 1.5 Duct air flow rate (m/s)

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Figure 5: Specific energy (kWh/t) for cyclone prducts (left) and column products (fight) at various duct air flow rates. 5

2.0 E

9

=,nn

o2

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1.5

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Figure 6: Median size (ds0) of cyclone products (left) and column products (right) at different duct air flow rate.

C4-49

Comminution, Classification and Agglomeration

Role of the nets in the classifying column Fine particles, freshly produced by the grinding motion of the steel balls and the stirred screw in the comminuting cell of the tower mill KD-3, are directly removed by a circulating air flow generated inside the mill. The classifying operation occurs in the column next to the comminuting cell. To understand the behavior of these particles during the classifying action and the role of the nets in the column, a series of experiments was carried out modifying the structure of the column (Figure 2) and assuming that: the air flow rates in the classifying column (Uc0 and the comminuting cell (Uce) can be converted from the measured duct air flow rate (Ud) in accordance with the calculated cross area of each section of the tower mill KD-3; the relationships between Ucl, Uce and Ua are presented below: Ucl = 0.02 Ud (1) U c e -- 0 . 1 9 Ua (2) - the maximum diameter of particles in the cyclone and column products is determined by the column and the cell airflow rates respectively; - particles in the column and the cell are supposed to be not secondary, but primary particles; besides, the dropping rate (Up) of particles whose diameter is Dp may have the same value as the column airflow rate (Ucl). Therefore, the Stoke's law, applied to the particles inside the column and the cell, gives an estimate of the maximum particles diameter of cyclone and column (dmax-cl)as a function of Ucl and Uce (equations 3 and 4): -

(dmax)

(dmax-cy)

products

dmax-cy= ( I g(pml8q-Pa) xgcl)

(3)

dmax-cl= (I ' 1811

)

(4)

)

(5)

Xgce g(Pm - P a ) The plots of these equations, represented in Figure 7 above, are referred as estimated curve 1 and estimated curve 2 respectively. The introduction of nets in the classifying column modifies slightly the curves representing the maximum particles diameter of cyclone and column products:

dmax-cy'= (I

18q

g(Om-Oa)

daxc /I

x Uct xC~

/ xC2

9 X Uce (6) g(Pm - P a ) Where: C1 = 0.65 -~ 1.5 and C2 - 0.6 The maximum particles diameter of products can be also calculated from the pressure drop (Pd) in the duct, using the relationship: Pd -- 835.31 Ud2 + 44.18 Ud (7) Several observations show a relatively high air current in the center area of the column blowing up particles coarser than those of the estimated curve 1 to the cyclone (cases CLA0 and CLA1). For CLA2 and CLA3, many of finely ground --

(dmax)

C4-50

Proceedings of the XXI International Mineral Processing Congress

particles, blown up from the comminuting cell through the column and the nets are supposed to cohere and form secondary particles (Shibayama and Moil, 1999). 50

50

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t

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0 0.1 0.2 0,3 Comminuting cell air flow rate (m/s)

Comminuting cell air flow rate (m/s)

Figure 7: Maximum size (din.x) of particles collected in the cyclone (left) and column (right) at variuos duct, column and cell air flow rates. Figure 8 gives, for each type o f column, the relationship between the maximum diameter of particles (dn~x) and the fineness of products (ds0) in the cyclone and

column. 4

4

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~'3

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v

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10

. . . . . . .

20

,

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,

30

40

dmax of products (~m)

Figure 8: Relationship between ds0 and dm~ of cyclone (black) and column (white) products.

Comminution, Classification and Agglomeration

C4-51

In the case of CLA0, the overlap of curves in Figure 8, as well as the superposition of the particle-size distributions curves of products in Figure 9 denote a poor efficiency of the classifying structure. As for the columns CLA1, CLA2 and CLA3, the dmax versus ds0 charts indicate a sharp classification of products confirmed by the distinct separation of distribution curves in Figure 9 (case of CLA3). It is obvious that settling a sieve to screen the pulverized products in the column reduces the air turbulence and increases the classifying efficiency of the mill. So, the maximum diameter of particles in the products is influenced by the type of the nets and the way they are settled on the baffle plate. 100,

dJ

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.

.

.

.

.

.

.

.

100

Figure 9: Particle size distribution of cyclone (black) and column (white) products. Conclusions

The Tower Mill is a new type of pulverizer specially conceived for a fine grinding. Its efficiency largely depends on the shape and structure of each of the three constituting sections: comminuting, classifying and collecting. In this present study a high recovery of fine particles (dmax smaller than 10~tm) have been achieved on the latest version of the Tower Mill KD-3. The experimental results of these investigations can be summarized as follows: - the performance of KD-3 increases with the size of the grinding media: a higher efficiency is observed with 20 mm diameter balls rather than with 10 mm diameter balls; - the weights of cyclone and column products, as well as the electric consumption of the stirring motor increase with the speed of the stirring screw while the specific energy (J/kg) tends to be constant; besides, the size distribution curves of particles in the cyclone and column products are not influenced by the ball size and the stirring screw speed; the maximum particle diameters of pulverized products can be estimated applying

C4-52

Proceedings of the XXl International Mineral Processing Congress

the Stoke's law of sedimentation; the maximum and median particle diameters o f products have a proportional relationship; besides, the use of a column provided with a series o f "screening" nets (CLA3) decreases the air turbulence and improves the efficiency o f the classification of products. References

Hara, T. and Mori, S., 1995. Application of Ultrasonic Waves in the Preparation of Suspension for a Particle Size Distribution Analysis of Powder by Sedimentation: Kyushu Daigaku Kougaku Syuuhou (Technology Reports of Kyushu University, Japan), 68. 211-216. Mori, S., Shibayama, A. and Hara, T., 1997. Development of a Tower Mill for Fine Grinding. Proceedings of the XX International Mineral Processing Congress, 2. Aachen, Germany, 61-69. Mori, S., Hara, T., Hamada, K., Okada, Y., Izumi, T. and Shibayama, A., 1995. The Effect of Ball Size on Limestone Pulverization by the Tower Mill KD-3: Kyushu Daigaku Kougaku Syuuhou (Technology Reports of Kyushu University, Japan), 68. 577-582. Shibayama, A. and Mori, S., 1999. Roles of the Net in the Column on Tower Mill KD-3: Shigen-toSozai (Journal of the Mining and Materials Processing Institute of Japan), 115. 83-88. Shibayama, A., Mori, S., Hara, T., Izumi, T. and Nagatome, S., 1997. Influence of Stirring Screw Speed on Grinding Efficiency of the Tower Mill KD-3: Shigen-to-Sozai (Journal of the Mining and Materials Processing Institute of Japan), 113. 842-846.