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The effects of grinding media shape on several milling variablesAndrs Felipe Escrraga

Escuela de Ingeniera de Materiales, Universidad del Valle. Cali, Colombia____________________________________________________________________________________

Abstract

The objective of this paper is to compare Cylpebs, ball, worn and concave grinding media in terms of

particle distribution, mill power, particles breakage rate and milling kinetics. For every experiment were

used different sizes of grinding media and milling machines. The results of experiments indicated that the

ground product using the Cylpebs of the same mass and the same size distribution as the balls contained

slightly less oversize. At lower speeds, Cylpebs media draw more power followed by worn balls and lastly

spherical balls. Cylpebs produced faster breakage rates than ball charges under the same conditions. The

differences between the breakage rates are more significant for the coarse fractions than for the fine

fractions. The expected increase on Concaves grinding efficiency related to their larger specific surface

area does not occur.______________________________________________________________________________

1. INTRODUCTION

Grinding media exert a significant influence on milling performance in terms of product size, energy

consumption and grinding costs due to media wear. Grinding balls and rods are traditionally used as

grinding media in the mineral industry. In recent years, grinding charges with unconventional shapes have

appeared on the market. One example is the cylindrically shaped media called Cylpebs (Shi, 2004).

Cylpebs have greater surface area and higher bulk density than balls of similar mass and size, due to their

shape. Cylpebs of the same diameter and length have 50% greater surface area, and 45% greater weight,

than balls of the same material. In addition, they have 9% higher bulk density than steel balls, and 12%

higher than cast balls. Studying the comminution processes and understanding different parameters that

affect it has increasingly became of interest to many researchers in the field of mineral processing. Milling

kinetics (Austin et al., 1984 andHerbst and Lo, 1989), load behaviour (Liddell and Moys, 1988,Powell and

Nurick, 1996a,Powell and Nurick, 1996b,Powell and Nurick, 1996c,van Nierop and Moys, 1997 andDong

and Moys, 2003)and mill power (Yildirim et al., 1998 andMorrell, 1993)have been studied as functions of

media size, feed particle size distribution, fraction of mill filled with balls and powder, mill diameter, mill

speed and other variables affecting them.

Spherical balls are the dominant media shape used in the tumbling mills. However spherical balls wear into

non-spherical fragments as a result of breakage due to impact and different wearing mechanisms inside

the mill. Thus at any one time a mill charge is a mixture ranging from larger spherical balls to worn irregular

fragments (Banisi et al., 2000 andVermeulen and Howat, 1989). To date, not much has been done to

assess the impact of worn balls on mill performance. The objective of this paper is to compare shape

grinding media in terms of particle distribution, mill power, particles breakage rate and milling kinetics. The

effect of using concave surfaces is also analyzed.
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2. EXPERIMENTAL DESIGN AND PROCEDURE

For this paper were realized five different experiments within independence in variables and test conditions.

The following designs and procedures have not relationship but only the results can be correlational by

theory assumptions and concepts.

2.1. Effects of grinding media shapes on particle distribution

As the objective of the first experiment is to predict the grinding performance of Cylpebs in a full-scale ball

mill, a model-based procedure for scale-up of ball mill was adopted. The experiment was conducted in a

standard Bond ball mill loaded with various grinding media to treat the same feed ore. Ore samples used

for the laboratory tests were collected from the ball mill feed conveyor in the Phosphate Hill Beneficiation

Plant during a grinding circuit survey. The ore is locally known as soft ore, with a Bond ball mill Work

Index of 14.3 kWh/t, and the breakage characteristic parameter A = 63.9 and b = 0.93 determined with a

drop weight tester (Napier-Munn et al., 1996). The ore sample was prepared using a jaw crusher and a

rotary divider to obtain the identical sub-samples for each test. [1]

The test conditions for both the locked-cycle and the single-stage open circuit tests are summarised

inTable 1. In the batch grinding tests 338 mill revolutions was selected in order to be consistent with the

locked-cycle Cylpebs test. This grinding time was kept constant across all the single-stage open circuit

tests.

Test type Chargetype

Test conditions

Single-stagetest

Charge 1 Standard Bond balls charge; batch grinding for 338 revolutions; open circuit

Single-stagetest

Charge 2 Cylpebs of the same mass and similar size as balls, greater surface area;batch grinding for 338 revolutions; open circuit

Single-stagetest Charge 3 Cylpebs of the same surface area and similar size as balls, smaller mass;batch grinding for 338 revolutions; open circuitSingle-stage test

Charge 4 Cylpebs of the same mass and surface area; fine media omitted; batchgrinding for 338 revolutions; open circuit

Table 1. Comparative tests conditions

2.2. Effects of grinding media shapes on mill power

The second experiment was conducted in 0.54 m-internal diameter mill with 0.4 m length. The internal mill

length was adjusted to 100 mm using a movable diaphragm. The mill is driven by a 2.5 kW variable speed

motor mounted in a mill rig, which has been described previously (Moys et al., 1996). Three types of media

were available to study the effect of media shape on mill power; spherical media, cylpebs and worn balls.

The worn media were obtained from an industrial dry ball mill used by ESKOM (South Africa power utility)to produce pulverized fuel following an experiment done to characterise ball size and shape distribution

(Lameck, 2005). [2]

Material properties such as coefficient of friction, size and shape (Zhou et al., 2002)affect media packing.

With this fact in consideration Eq.(1) was used to calculate the spherical ball charge mass at 15%, 20%

and 25% filling levels. The voidage of 0.4 was used in calculating spherical balls mass. The charge mass

involving cylpebs and worn balls were obtained as respective equivalent weight to that of spherical balls.
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The respective percent filling levels for cylpebs were 14.6, 19.5 and 24.3 at the voidage of 0.36 (0.02) and

that of worn balls at 0.37 (0.01) voidage were 14.2, 18.9 and 23.6. The brackets show the standard

deviations of the measured media voidage. This implies that media shapes were compared based on the

same mass criterion. Measurements were performed with speeds ranging from 20% to 90% of critical

speed. Balls between 22.4 and 26.5 mm size class for spheres and worn balls and 24 22 mm (l d) for

cylpebs were used. [2]

Equation (1)

where J is the load volume as a fraction of the mill volume, and L is the internal mill length.

2.3. Effects of grinding media shape on breakage rate

For the third experiment the grinding media used were 20 20 mm diameter Cylpebs and 20 mm diameter

balls, made from cast iron. Specific gravities of Cylpebs and balls were 7.35 and 7.69 g/cm3and their

surface areas 18.84 and 12.56 cm2, respectively. Seven different mono sized feed fractions of quartz with a

specific gravity of 2.68 g/cm3

, containing 99.62% SiO2, were used for all the tests. Grinding experimentswere carried out in a stainless steel laboratory mill of 30.5 cm length and 30.5 cm diameter, with a smooth

lining, rounded corners and operating at 70 rpm. During all the tests, mill feeds were constant, 3000 g

being used. 19794.2 g of Cylpebs and 19776.9 g of balls were used separately in the grinding charge. [3]

In order to determine the specific rate of breakage, feed samples were prepared in seven different mono

sizes and ground batch wise using Cylpebs and ball grinding charges for selected periods (0.5, 1, 2, 4,

8 min). After each grinding period, the product was discharged and a 375 g of sample was representatively

taken by riffling, followed by dry sieving for 15 min on a Rotap. [3]

2.4. Effects of grinding media shape on milling kinetics

The fourth experiment was conducted using a 0.54 m internal diameter laboratory mill. The mill is 0.4 m

internal length fitted with twelve 20 mm high trapezoidal lifters with 45 face angle and 50 mm base width.The mill is driven by a 2.5 kW variable speed motor mounted on a mill rig and has been described

previously (Moys et al., 1996). The mill filled with balls to 20% by volume was operated at 75% critical

speed. Single sized feed quartz (300 425, 600 860 and 1180 1700 m) was used at 20%, 50% and

80% fractional filling, U. [4]

Perfectly spherical balls in various size ranges were used to conduct experiments. The worn media were

obtained from an industrial dry ball mill used by ESKOM (South Africa power utility) to produce pulverized

fuel following an experiment done to characterise ball size and shape distribution (Lameck, 2005). The

typical characteristic features of industrial ball shape distribution are shown inFig. 1. It was clearly

observed that balls below 31 mm, which constituted about 1540% of the sample, are no longer spherical.

Thirty millimeter top size balls were used-all perfectly spherical or all non-spherical in the two ball loads

used. The balls size distribution used in experiments was decided based on this experiment as well. [4]
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Figure 1. Characteristic feature of balls inside ball mill.

Material properties such as coefficient of friction, size and shape affect media packing (Zhou et al., 2002).

With this fact in consideration, spherical balls charge mass was calculated by assuming the voidage of 0.4

at the desired mill filling level. The charge mass involving worn balls was equivalent to that of spherical

balls. This implies that media shapes were compared based on the same mass criterion.

2.5. The effect of using concave surfaces

The fifth experiment was consisted of batches of the same feed that was first ground with conventionalgrinding balls. Then, part of the grinding medium balls was replaced by concave-convex balls, increasingthe specific surface area of the grinding charge. The ore used was an iron ore concentrate from acommercial plant. Two types of concave balls made of cast iron were used, as sketched in Figure 2. Themain characteristics of these bodies are presented in Table 2. [5]

Figure 2: Concave grinding bodies.

Crushing bodies Diameter (mm) Weight (g) Overall area (cm2) Concave area (cm2)

Type I 38.8 181 47.3 9.8Type II 38.8 148 47.3 13.4

Table 2. Concave crushing bodies. The radius of the concavities is the same as the sphere radius

3. DISCUSSION OF RESULTS

3.1. Effects of grinding media shapes on particle distribution results

Four batch dry grinding tests were undertaken using the Bond ball mill with various media charge

conditions. In the four tests the mill run at the identical 338 revolutions and treated the same mass of the

same ore. Results of the single-stage batch tests are given in Table 3 in terms of percent passing two size

fractions (1.18 mm representing the coarse end and 0.038 mm the fine end). The full size distributions of

feed and product are given inFig. 3,Fig. 4 andFig. 5.
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contact between the end faces on the ore particles. The line contact and area contact increase the

tendency for grinding to take place preferentially on the larger particles. Once the large particles are caught

on the line or between the face areas, this prevents the smaller particles being broken further, which is

similar to the rod mill practice. As a result, the advantage of a greater surface area of Cylpebs is cancelled

out by the grinding action of line contact and area contact, and the product is almost identical at the fine

end to that ground by balls at the same specific energy and same media size distribution. The benefit of the

greater surface area of Cylpebs can only be realised at the coarse end and the difference is marginal.

Fig. 4 shows the effect of energy input. Although having the same surface area and similar media size, the

smaller charge mass of Cylpebs means a smaller energy input. The difference in product sizes above

1.7 mm is not significant between Cylpebs and balls, showing that the effect of surface area may, again,

dominate the grinding action in the coarse size fractions of particles. The energy input effect appears in

sizes smaller than 1.7 mm, which results in almost parallel lines, with a smaller input energy producing a

consistently coarser product. This is as expected.

Figure 4. Ball charge compared with Cylpebs of the same media size and the same surface area, smaller

mass.

An interesting observation inFig. 5 is that when the Cylpebs smaller than 28 mm were omitted from the

charge, the size distribution curve of the ground product became steeper, i.e. producing less fines

(0.038mm) and less oversize, compared with the same charge mass and the same surface area. This

phenomenon may indicate that large grinding media offer a larger probability for impact breakage, but less

abrasion breakage. Such a breakage mechanism would lead to rapid disappearance of the coarser

particles in the feed and avoid excessive fines generation. A similar observation was reported previously in

a separate research project at the JKTech using a laboratory ball mill at various media size distributions to

grind silicon metal.
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Figure 5. Ball charge compared with Cylpebs of the same mass and the same surface area, fine Cylpebs

omitted.

3.2. Effects of grinding media shapes on mill power results

Sensitivity of power draw to media shape and operating parameters in terms of speed and charge filling

level was analysed and is shown inFig. 6.The power drawn was found to be sensitive to media shape atall charge filling levels studied. At a mill speed of about 72%, which is about the speed most mills are

operated; power drawn by the cylpebs and spherical media is similar at all charge levels studied, however

beyond this speed, power drawn by cylpebs starts to decrease but that drawn by spheres is still increasing.

Different media shapes reached the maximum power draw at different mill speeds. The maximum power

for spheres occurs at a speed greater than 90% of critical (i.e., higher than the highest speed tested).

Figure 6. Variation of mill power draw with mill speed (J15 & J25).

3.3. Effects of grinding media shape on breakage rate resultsThe first order plots for different feed sizes of quartz ground by balls and Cylpebs are illustrated in Fig.

7 andFig. 8, respectively. The results indicate that grinding of all size fractions can be described by first

order grinding kinetics with 0.91970.9991 correlation coefficients.
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Figure 7. First order plots for dry grinding with ball charge.

Figure 8. First order plots for dry grinding with Cylpebs charge.

Variations in specific rates of breakage at different feed particle sizes for ball and Cylpebs grinding charges

are shown inFig. 9.The specific rate of breakage increases up to 1180 + 850 m feed size, but abovethis size fraction breakage rates decrease sharply for both grinding charges, since the particles are too

large and strong to be properly nipped and fractured by the Cylpebs and the balls, and have a slow specific

rate of breakage.

Fig. 9. Variation of the specific rate of breakage with size.

Parameters of specific rates of breakage aT, , , were obtained by non-linear regression and are 0.73,

1.57, 1.27, 3.65 for ball charge and 0.82, 1.50, 1.28, 3.29 for Cylpebs charge, respectively. As can be seen
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fromFig. 3,Cylpebs give faster rates of breakage than do balls. This can be attributed to the linear and

point contacts of Cylpebs on each other.

3.4. Effects of media shape on milling kinetics results

The results presented inFig. 10 compare the first order plot of quartz with feed size of 1180 1700 m and

600 850 m for spherical and worn grinding media shapes. The results suggested that, for the two mediashapes studied, breakage followed first order.

Figure 10. First order plot of batch ball milling of quartz.

The variation of specific rate of breakage with feed sizes shown in Fig. 11 (bottom axis), indicated that

spherical media have higher breakage rate but the difference narrows as the feed becomes finer.

Figure 11. Variation of specific rate of breakage with particle size and fractional filling.

It is understood that larger particles undergo impact breakage when impacted by balls with sufficient kineticenergy. As particles get smaller, they become difficult to break by impact hence breakage rates decrease

with decreasing particle size. Considering the two media charges, worn balls have surface, linear and point

contacts while spheres have only point contact. This might lead one to expect increased breakage by

abrasion for worn balls. On the other hand, the use of test screens in sizing the media might be the factor

accounting for higher breakage rate for spherical media with coarse feed. For the same screen size class,

the non-spherical (worn) media constituted heavier balls as relatively bigger balls could easily reorient and

pass through the screen aperture. To maintain an equal total mass and the same screens mass fractions,

1489 worn balls were used compared to 1758 spherical balls. The increased spherical balls number results
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in more ball-to-ball and ball to liner collisions for spherical balls than worn balls, which results in higher

catastrophic breakage by impact for larger particle sizes. While the trend observed shown faster breakage

rate with spherical media,Ipek (2006) comparison of spherical media and cylpebs suggested that cylpebs

have higher rate of breakage than spheres.

The variations in specific rates of breakage, S with fractional fillings U were studied using 600 850 m

material feed size (Fig. 11, top axis). It was observed that the specific rate of breakage decreased with

increasing fractional filling, U.

Austin et al. (1984) equation relating specific rate of breakage, S with particle size xi when xi d (d is ball

diameter) was extended to include the effect of fractional filling U and Eq.(2) was obtained as

representative of the effect of feed size and interstitial filling have on specific rate of breakage.

Equation (2)

where a is proportionality constant resulting from Austin et al. (1984) equation and the effect of mill filling.

3.5. Effect of using concave surfaces as grinding results

Assumptions

Grinding efficiency increases with an increase in the available surface area of grinding medium. A grinding

ball with a concavity has a smaller volume than a sphere of same radius. Therefore, the convex-concave

ball has a smaller mass with the same surface area, providing a larger specific surface area. A concave

surface allows a larger contact area with convex surfaces than the contact area between two convex

surfaces; therefore, a concavity may simultaneously exert stress on a larger number of particles than a

convex surface. The particles that are caught between the convex portion of a grinding ball, and the

concave portion of another, would be trapped under the crushing action of the grinding medium for a longer

time than in the case of two convex surfaces, where the particle would be under crushing action for a

shorter contact time. In the case of contact between convex and concave surfaces, the probability of

crushing action over larger particles is greater than for smaller particles, causing preferential grinding of

larger particles.

Figure 12. Action of the concave grinding bodies.

Experiment

The following parameters were adopted for the first round of experiments: mill dimensions, 20x 30 cm 2, 67

rpm (72.0% of critical speed), 38% filling, 17.5 kg ball charge, 2.975 kg ore charge, 697 ml water (81%

solids), 20 min, 40 min and 60 min grinding time. In this round of experiments, only the Type I concave was

used, and just one size of balls, with the same weight as the Type I concave.
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Figure 13. Blaine surface area (BSA) for the first round of tests, as a function of the percentage of type I

concave crushing bodies in the charge.

The Blaine surface area (BSA) data for the ground products, as seen in Figure 13, have a clear tendency to

be reduced with an increase in the concave bodies of the charge.

Figure 14: Size distribution of the first round of tests. The curves for the other tests are not plotted.

In most cases, the larger the concave area in the charge, the coarser is the ground product.


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4. CONCLUSIONS

1. Laboratory tests were conducted using a standard Bond ball mill to compare the milling performance of

Cylpebs against balls. Effects of the three charge conditionsmass, size distribution and surface

areawere investigated. Single-stage batch grinding tests indicated that the ground product using the

Cylpebs of the same mass and the same size distribution as the balls contained slightly less oversize.

This may be due to the greater surface area of the Cylpebs. This advantage, however, may be

balanced by the line contact and area contact grinding action of the Cylpebs. As a result, Cylpebs

produce a similar product at the fine end compared with the balls at identical charge mass, and hence

at the identical specific energy input level. When compared at the same surface area and a similar size

distribution, the Cylpebs produced a much coarser product, because their specific energy level was

smaller than for the balls. When compared at the same mass and the same surface area, the fine-

Cylpebs truncated media produced a product with significantly less fines (0.038 mm) and slightly less

oversize.

2. It was also observed that mill power was sensitive to grinding media shape used. Power increases tothe maximum with increasing mill speed for all media shapes studied. The maximum power draw for

cylpebs media occurs at a relatively lower speed in comparison to the other media shapes studied. At

lower speeds, cylpebs media draw more power followed by worn balls and lastly spherical balls.

3. Cylpebs produced faster breakage rates than ball charges under the same conditions. The differences

between the breakage rates are more significant for the coarse fractions than for the fine fractions.

4. Spherical balls have slightly higher rates of breakage than worn balls. For normal operating material

fractional filling (0.8 < U 1.0), the specific rates of breakage for single size feed of 600 850 m

were similar for the two shapes, but for lower material fractional filling, higher rates were observed with

spherical media. The difference is more significant with coarse feed. However considering that worn

balls in industrial mill charge constitute about 1540% and that only marginal differences in breakage

rate was observed, the effect of the presence of worn balls inside the mill charge is negligibly small.Thus, the benefit that can be obtained does not justify the cost of their removal from mills.

5. The assumption of increasing grinding efficiency with an increase in specific charge area was not

proven. In all tests, there was a reduction of the product BSA (Blaine surface area) for increasing

concave surface area and total surface area. For up to about 10% of the concave area of the charge,

there was a trend toward a finer ground product. Apparently, the increase in the proportion of concave

bodies with a less regular shape causes the charge to have less smooth movement, causing a

negative impact on the grinding efficiency. The expected increase on bodies grinding efficiency related

to their larger specific surface area does not occur, perhaps for the same reason.


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REFERENCES

All references are availables online atwww.sciencedirect.com.

[1] F. Shi. Comparison of grinding mediaCylpebs versus balls. Elsevier March 2004.

[2] N.S. Lameck, K.K. Kiangi, M.H. Moys. Effects of grinding media shapes on load behaviour and mill

power in a dry ball mill. Elsevier March 2006.

[3] H. Ipek. The effects of grinding media shape on breakage rate. Elsevier July 2005.

[4] N.S. Lameck *, M.H. Moys Effects of media shape on milling kinetics. Elsevier February 2006.

[5] F.L. von Kfiiger, J.D. Donda, M.A.R. Drummond, A.E.C. Peres. The effect of using concave surfaces as

grinding media.Proceedings of the XXI International Mineral Processing Congress 2002.
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