grinding aids klimpel

518

talline activity by sulfur was unexpected. It is possible that

the “inhibitors” described earlier and impede the passage Of the reactants through the pores. On the other hand, the inhibition effect could be the indirect result of sulfur poisoning the hydrogenation sites leading to the buildup of inhibitor concentration.

Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978

Chen, N. Y.. Lucki, S. J.. Anal. Chem., 42, 508 (1970).

Dudzik, Z., Preston, K. F., J. Colloid Interface Sci., 26, 374 (1968). Dudzik, 2.. us. Patent 3516947 (June 23, 1970). Giannetti, J. P.. Perrotta, A. J., Ind. Eng. Chem. Process Des. Dev., 14, 86

Gorring, R, L,, J , catal,, 31, 13 (1973), Miale, J. N., Weisz, P. B., J . catal., 20, 288 (1971). Robson, H. E., Hamner, G. P., Arey, W. F., Jr., Adv. Chem. Ser., No. 102, 417

the sulfur complexes formed inside the pores hehave like Chen, N. y., brwoO4 w. E.. Adv. Chem SW., NO. 121, 575 (1973).

(1975).

119741 I . _ . .,. Weisz, P. B., Frilette, V. J., J. Phys. Chem., 64, 382 (1960). Weisz, P. B., Frilette, V. J., Maatman, R. W., Mower, E. B., J. Catal., 1, 307 Literature Cited

Breck, D. W., “Zeolite Molecular Sieves”, Wiley, New York, N.Y., 1974. Chen, N. Y. (to Mobil Oil CorD.). US. Patent 3373 110 (Mar 12. 1968).

(1962). Chen, N. Y., Maziuk, J., Schwa&, A. B., Weisz, P. B., Oii Gas J., 66 (47), 154

Chen, N. Y., Lucki, S. J., Mower, E. B., J. Catal., 13, 329 (1969). (1968). Received f o r review December 7, 1977

Accepted April 17, 1978

Chemical Grinding Aids for Increasing Throughput in the Wet Grinding of Ores

Richard R. Kllmpel*’ and Wllly Manfroy2

Physical Research Laboratory and Functional Products Department, The Dow Chemical Company, Midland, Michigan 48640

The effects of chemical grinding aids for wet ore grinding have been analyzed in both batch laboratory ball mills and continuous industrial scale ball and rod mills. Two important industrial results have been achieved: first, increased feed rate at constant product size; and second, the production of a finer product at constant feed rate. All data have been characterized using the concepts of specific rate of breakage, S, and breakage product distribution, B. The engineering mechanism involved with the use of selective chemical additives is one of allowing more dense slurries to interact with the tumbling media while still maintaining the slurry fluidity and mass transport characteristics of less dense slurries.

Introduction Laboratory and industrial grinding tests have shown that

the process of size reduction can be significantly influenced by chemicals added to the powder or slurry being ground. The terms grinding aid or grinding addi t ive refer to a substance which when mixed into the mill contents causes an increase in the rate of size reduction. The increased rate can be used to grind a higher feed rate to the desired product size or it can be used to produce a finer product size a t fixed feed rate. Whether the use of a grinding aid is justified in any given situation depends on the cost of the substance vs. the improvement of output or product quality obtained with its use. Obviously, an expensive chemical must be effective in very small concentrations if it is to be economically justifiable; the cost criteria is calculated on the basis of the cost of the grinding additive per ton of material ground.

Although there is direct experimental verification of the advantageous effect of grinding additives, no sound engineering explanation has yet been offered which explains or predicts the general behavior of additives for general mineral processing usage. Rose and Sullivan (1958) have listed most of the work undertaken prior to 1950, Snow (1973) has summarized the implications of selected ref- erences, and Hartley et al. (1976) have recently prepared an updated synopsis of the grinding additive literature. Many of the studies reported consist of subjecting ma-

l Physical Research Laboratory. Functional Products Department.

terials with simple geometric shapes to some type of hardness or controlled single fracture test. On the other hand, a number of the studies were carried out on operating industrial scale mills, with little control or precise monitoring of the effect of the additives.

Out of this work has come a bewildering array of hy- potheses to explain the action of grinding aids. The prevention of particle agglomeration and grinding media coating as well as altering the strength of macro porous rocks by liquid presoaking have been suggested. Another mechanism which is often quoted is attributed to Reh- binder and Kalinkovskaya (1932), who suggested that the adsorption of additive on the surface of solid particles lowers the cohesive force which bonds the material of the particles together. In particular, adsorption on the surfaces of a flaw in the surface of a solid could affect the bonding forces and surface energy at the point where fracture initiates as discussed by Griffith (1920) and Austin and Klimpel (1964). Westwood (1966) has demonstrated in a series of articles the effect of adsorbed molecules on various surface mechanical properties and he refers to the phe- nomena in general as chemomechanical effects. This phenomenon suggests that the adsorbed molecules may “pin” dislocations near the surface thus preventing easy movement of dislocations under stress gradients. Since plasticity is due to the movement of the dislocations, the region near the surface of the solid is thus rendered more brittle.

The surrounding molecular environment can certainly affect the critical stress-strain required to produce fracture under conditions where the fracture initiates from a flaw

0019-7882/78/1117-0518$01.0010 0 1978 American Chemical Society

Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978 519

in the surface. Examples are certain slow compressive, tensile, or bending tests on certain materials. For example, the tensile strength of glass fibers is strongly dependent on damage to the surface and is affected by immersion in different chemical environments. However, the normal action in a grinding mill is more comparable to striking a small piece of solid with a large hammer, and fracture will occur whether the surface is flawed or not. It is likely that the stress waves produced by massive high speed impact will activate flaws throughout the solid and a maze of fractures will propagate a t high speed (near the speed of sound in the material). Simple calculation shows that molecules of an additive cannot diffuse down the cracks at anything like the speed of sound, so they cannot affect the crack energy at the propagating tip of the crack. Thus it is highly improbable that the Rehbinder and Kalin- kovskaya (1932) chemomechanical effect could explain the mode of operation of grinding aids in ball mills, etc. Similarly, chemomechanical effects due to the movement of dislocations can only occur in the time scale of such movements, which are far slower than the speeds of crack propagation, and chemomechanical effects have only been demonstrated in processes where local plastic flow is important. Locher and von Seebach (1972) have given strong experimental evidence against chemomechanical effects in dry grinding of cement clinker using grinding aids.

In summary, one must look for explanations of the effect of grinding aids other than those involving effects on the fracture energy or degree of surface plasticity. The fol- lowing sections will explore the effects of additives on fluidity in slurries and flowability in dry powders.

The classic work on the action of vaporous grinding additives in dry grinding is the work of Locher and von Seebach (1972). In studies of the dry ball milling of cement clinker using organic vapors as grinding aids they demonstrated by adsorption studies that vapors were inef- fective unless they were adsorbed in substantial amounts and at high rates (incidentally, requiring adsorption at 120 O C for ethylene glycol and 70 “C for butylamine). The correlation between the amounts adsorbed and the ag- glomerative forces between fine particles exposed to the adsorbents was very pronounced. The effect of grinding aids was observed only under conditions where agglomeration of fine particles could affect the grinding action, and there was no effect of the grinding additives on the rates of fracture of coarse particles. They concluded that the fracture process in tumbling ball mills was not affected by the adsorption but that agglomeration and flow properties were affected for fine sizes.

Wet Grinding of Ores The most important “grinding additive” in the strict

sense of the definition of a grinding aid is, of course, water. As is well known in practice, grinding in water is advantageous over dry grinding, in tumbling ball mills. One needs hardly look for any chemomechanical action here, however, since the effect must surely be one of bringing and keeping the particles in advantageous positions to receive a breakage action. Mass transport of the slurry within continuous tumbling ball mills is complicated and difficult to predict using known scientific principles. Yet engineers and industrial operators have been able to characterize mass transport effects in several ways, by residence time distributions, or the “feel” or “sound” of the mill, for example. It is also known that slurry transport in tumbling mills can be influenced by the type of mill, the nature of the material, the temperature, the density of the material and balls, particle and ball size, degree of

800 0 I

700

0 10 20 30 40 50 Grinding Time, Minutes

Figure 1. Zero-order production plot for copper ore ( p = 2.60 g/cm3) ground in 8-in. batch ball mill with 106 1-in. steel balls. Feed: 100% <2000 pm, 50% <710 pm, 25% <250 pm, 12.5% <44 pm.

BW

750

700

650

600

60 62 64 66 68 70 72 74

Slurry Density. Weight %

Figure 2. Weight of solid, in grams, less than 44 pm produced for copper ore ground for 40 min in 8-in. batch ball mill.

mill filling, and slurry density. Of all these factors, industrial experience has shown that the modification of slurry density by water addition (or removal) is the easiest and most rapid technique for altering slurry transport in grinding mills. Despite this experience, the quantification of the effect of water has only been recently studied.

As reported in a series of papers, Klimpel and Manfroy (1977a,b,c) carried out a number of experiments in a laboratory batch ball milling, giving results which were then scaled-up and verified on several overflow discharge ball and rod mills of industrial size. They studied the effect of the water-to-solids ratio under well controlled experimental conditions in which a constant slurry volume was maintained in the test mill, to correspond with a continuous overflow mill. The volume of solid plus volume of water was held at a value sufficient to occupy 120% of the void volume of the ball charge. Figure 1 shows a typical grinding result, giving the amount of material produced less than a specified size when the percent solids of a constant volume of copper ore slurry is varied over a range corresponding to industrial practice. The rate of production of small sizes (<44 pm, in this case) is a constant over much of the grinding time but this rate varies with the slurry density, as shown in Figure 2.

To assist in analyzing the results, the well-known concepts of specific rate of breakage for size i material, Si, and cumulative daughter fragment distributions of breakage of size j material to less than size i material, Bij were employed (e.g., Austin, 1971). The collection of the

520

Table I. Comparison of S W Values for Wet Grinding of Copper Ore in a Ball Mill (Fraction Ball Filling = 0.33, 1-in. Steel Balls, Mill Diameter = 200 mm, Speed of Mill = 67% Critical)


0.30

‘a 0.20 z c 5 m-

0.10 5 e m 0.07 6

slurry density specific gravity of ore mill vol, cm3 wt % solids vol % solids S, , min-’

2.60 5790 60 36.1 0.433 formal bulk 6 2 38.1 0.417

density = 64 40.1 0.398 (2.60) (0 .6) = 68 44.5 0.351 1.56 g/cm3 72 49.2 0.297

74 51.8 0.272

-

-

- -

0.50

g 0.05 ’ 4 0.03

0.02 t 0.01 ~

30 50 100 200 500 lMxl 2000

Size (pmj

Figure 3. Back-calculated S curves for copper ore slurries ground in 8-in. batch ball mill for various slurry densities.

Si and Bij values for the size ranges in question (called S and B curves) can then be used to establish appropriate mass balance differential equations for the particular grinding situation at hand (e.g., Austin et al. (1975), Klimpel and Austin (1972)).

A series of batch grinds, a t 10, 20, and 40 min, were performed on the same feed for various slurry densities. Using the S and B back-calculation technique developed by Klimpel and Austin (1977) the appropriate shifts in S and B can be quantified. It is difficult to get the correct values of the primary breakage distributions by back- calculation on a distributed feed, so that the statistical confidence levels on B tend to be rather wide. It was originally thought in the work of Klimpel and Manfroy (1977a) that the B values were changing with various slurry densities, but independent experimental measurements did not confirm this. Therefore, the back-calculated B values were examined and the best mean set was chosen. The back-calculation program was then rerun with the char- acteristic parameters of this mean set as inputs, to generate S values for the different slurry densities. Figure 3 shows typical results. Within significance levels, the slope CY of the lines is constant for different slurry densities showing that the fundamental breakage mechanisms are not being affected.

Since the normalized B values, B, = Bi.j for all j , and a are the same in all tests, the S value for any chosen size interval is representative of the relative time-scale factor of breakage between the various tests. Thus one can compare Si W values to indicate the comparison between the absolute rates of breakage where W is the weight of the charge in the mill. Table I shows the results where i = 1 represents the largest particle sieve interval in the charge. The values of S W are almost constant over most of the range, 60 wt % to 68 wt 70, but a t the higher slurry

S, W, g/min w, g 1000 433 1054 44 0 1110 44 2 1231 432 1362 405 1434 390

densities, where it is known that the effective viscosity increases rapidly with higher solids loading, there is a significant decrease in S W. Presumably this is due to poor distribution of the solid between the grinding surfaces due to hindered movement of the slurry in the tumbling action. There must, of course, be a transition between the higher grinding rates in wet grinding and the lower rates of dry grinding.

Combining the information of Figure 3 and Table I gives an interesting insight into the use of water as a grinding additive. The actual amount of material being broken over a certain time span is a function of the product of S and the charge of the mill W. When the percents are varied, W is varied, and when one considers the absolute breakage out of a given size interval i over a unit time period, the product S W is almost a constant over a sizeable range of percent solids. Thus, for the case of a constant slurry volume, a reservoir or critical grinding volume concept, as orginally suggested by Austin and Klimpel(1964), appears applicable. Of course, if W gets too small or too large, this result will break down. In a continuous grinding machine, the operating region over which S W is almost constant is where most industrial ball and rod mills are deliberately operated.

With regard to the use of this information on continuous overflow discharge mills, although the residence time of the solid is changed by water addition, the fact that S W remains constant maintains a constant rate of production of fines, because T = W / F and hence Sr is also constant for a constant mill feed. On the other hand, for grate discharge mills variations of slurry density also have large effects on the hold-up and breakage rates in the mill. If the slurry density becomes too high, the holdup in the mill increases to force the slurry out, and it can increase to such an extent that the mill overfills and passes beyond the peak in power draw, so that S W decreases. If the slurry density is too low, the mill becomes underfilled and again S W can decrease, although this rarely happens in practice because the fluidity only changes slowly with slurry density for dilute slurries. The essential difference between wet overflow and wet grate discharge systems is that the grate discharge is more sensitive to slurry fluidity than the overflow discharge.

Fluidity Control Using Chemical Additives in the Wet Grinding of Ores

The use of chemicals such as silicates, phosphates, sulfonates, etc., as grinding aids in wet grinding circuits is well known (Rose and Sullivan (1958), Perry and Chilton (1973), Hartley et al. (1976)). Their use, however, has been limited by several factors such as the lack of a consistent engineering characterization of their behavior, the rela- tively large amounts of additive sometimes required to “see” an effect, and the downstream processing problems in flotation, filtration, thickening, etc., that often ac- company the use of grinding additives. As a result, the industrial use of additives has been most successful in selected grinding applications involving high grinding costs


1050

1000 ED - I 3

950

In :: e 5

2

900

4 850

e 0

0 m -

800 5 s

750

b

T T

T

V No Additive

0.2 mg/g XF-4272 Q b 0.6 mgig XF-4272 @ @ Experimental Grinding Aid.Dow Chemical

I I I I I 78 80 a2 a4 86

Slurry Density, Weight %

Figure 4. Weight of solid, in grams, less than 44 pm, produced for taconite ore ground for 45 min in 8-in. batch ball mill.

G I / I

4

g 22 Slurry Density At 84 Weight Yo

0.5 1 0

Concentration Of Aid, kgIMetric Ton

Figure 5. Variation of rate of grinding with concentration of Dow Chemical grinding aid XF-4272 on a taconite ore.

due to excessive ball or liner wear, long grind times for materials of low grindability or very fine product size, or higher value products such as pigments, speciality chemicals, etc.

In the second part of the work by Klimpel and Manfroy (1977b), detailed study was undertaken to find efficient chemical grinding additives that avoided the limitations mentioned above, especially the minimization of downstream processing problems (Klimpel and Manfroy, 1977~). The basic assumption of their work was that the effect of the additive is to control the fluidity and hence, the mass transport of the slurry in the milling environment. They screened many viscosity control agents, surfactants, and general purpose dispersants and found that fluidity control effects in the presence of high shear forces were of significance. Figure 4 shows a typical result, which gives the amount of material produced less than a specified size when the slurry density is varied, for batch grinding of a constant volume of taconite slurry. The tests with no additive show the same trends as discussed in the previous section. However, the production of fine material was increased by an efficient fluidity modifier, holding slurry

0.50

All Slurry Densities A t 84 Weight % 0.30 -

5 0.20 - 1

m-

m 0.07 - 0.05

5 -

0.02 - @ Experimental Grinding A id - Dow Chemical

30 50 100 200 500 1000 2000

Figure 6. Back-calculated S curves for taconite ore slurries ground in 8-in. batch ball mill showing effect of grinding additive.

Table 11. of Breakage SI W for Taconite Ore

Size h i

Effect of Fluidity Modifier on Absolute Rates

concn of

slurry density specific tive, gravity g/kg of wt % vol % SI, SI W, of ore solid solids solids min-' g/min - 3.30 0 80 54.9 0.327 616

0 82 58.1 0.305 608 0 84 61.5 0.270 570

0.2 84 61.5 0.290 612 0.6 84 61.5 0.332 700

density constant. This is also shown in Figure 5, which demonstrates that the rate of production of fines increases with the concentration of the fluidity modifier. The right-hand part of the curve in Figure 4 is in the region where S W is decreasing similarly to the values given in Table I a t higher slurry density. The effect of the fluidity modifier is clearly to keep the slurry in the efficient grinding region.

This conclusion is supported by the more exact S and B analysis. Figure 6 shows the S values for batch grinding of this ore with and without the grinding aid. Again, it was assumed that B values did not significantly change and the figure shows that the CY values were constant. Table I1 shows the values of S W for data on taconite grinding under similar conditions; again the maximum value of S W is increased by the fluidity modifier.

The importance of using efficient chemicals for the purpose of increasing S W is illustrated in Table 111. It is obvious that the polymeric fluidity modifier caused far greater increases in S W than the sodium silicate in the grinding of a copper ore.

A number of industrial grinding circuit trials involving rod and ball mills with overflow discharges have been performed using these efficient chemical additives as reported by Klimpel and Manfroy (1977b,c). These results were very encouraging, with several tests demonstrating an ability to increase the rates of production between 9 and 15% in both open and closed circuit operations. This has been done while maintaining a similar product size distribution, and keeping the total mill power draw the same as for no additive. Figure 7 shows a typical result from one of these trials in the copper industry involving a ball mill in closed circuit with a classifier. The rate of

522

Table 111. Concentrations of Additive with a Copper Ore Slurry of Fixed Volume % Solids = 58.3%


Comparison of the Values of Absolute Breakage Rate SW for Grinding in a Ball Mill, with Various

concn of aid,

specific gravity glkg of

2.34 0 0 0 0 0.25 0.50

of ore solid

concn of sodium silicate, S,W, slurry density,

g/kg of solid S,, min-' w, g glmin wt % solids

0.5 0.171 1500 257 7 1 0.5 0.167 1500 251 73 0.5 0.164 1500 246 75 0.5 0.156 1500 234 77 0.5 0.174 1500 261 77 0.5 0.201 1500 302 77

Table IV. Performance in Terms of Increase in Throughput with Grindine Additive with a Taconite Ore

Prediction of Industrial Scale Rod Mill

constant size constant size within given for all

slurry slurry density, slurry densities den- chemical feed rate sitv. addition feed rate

w i % rate, % % solids Iblton tonslh increase tons/h increase

84 0 410 0 410 base 0.4 410 0 410 0 0.6 410 0 410 0 0.85 410 0 410 0 1.7 410 0 410 0

0.4 428 4.4 422 2.9 0.6 437 6.6 431 5.1 0.85 448 9.3 442 7.8 1.7 478 16.6 471 14.9

88 0 410 0 392 -4.4 0.4 450 9.8 430 4.9 0.6 470 14.6 449 9.5 0.85 494 20.5 472 15.1 1.7 526 28.3 503 22.7

86 0 410 0 404 -1.5

make-up feed and the product size distributions were monitored continuously. All other factors such as percent solids, classifier control, etc., were kept constant over the duration of the test.

Several important conclusions can be drawn from in- spection of Figure 7. The first is the ability of the additive to cause an increase in fineness of grind with approximately constant feed rate; the period from 2 and 3 produced an average size distribution which was 4% finer than the desired control size as compared to the period of 1 to 2. When the additive is shut off, the % greater than the

+6 .B ,z L lJ7 +4

+ Steady State (No Aid) 3 Polymer Added $Polymer Shut Off b Polymer Added

U C

.10 7 8 9 1 0 1 1 N o o n 1 2 3 4 5 6 7 + + $ ea

Time

Figure 7. Typical response curve for industrial scale mill.

control size goes back t,o its original level. At time 4, the additive was again used with-a subsequent increase in fineness of grind. After time 5, the feed rate was slowly increased. After 3 h with the aid, the feed rate was 10% higher than earlier, but with the product size being the same as the base case without additive. When the additive was shut off and the feed rate reduced, the mill output returned to approximately its earlier operating condition. Also, during the period of increased feed rate with the grinding additive, the mill power draw was identical with the base case without chemical. This has been shown to be true for all the overflow ball and rod mills investigated.

Several methods by which the chemical usage can be scaled up from laboratory tests to industrial scale have been developed. The simplest approach consists of crudely estimating the approximate S and B curve for the industrial circuit in question. This was done using the

Table V. Results of Industrial Rod Mill Operated with Additions of Grinding Aid polymer slurry addition, density, product size

time, h comment rate, tonslh lblton wt % solids % <35 U.S. mesh

Day 1 0 normal operation 405 0 84 38.4 2 aid added 405 0.6 85 38.3 3.5 higher feed 425 0.6 86 42.7 4.7 higher feed 465 0.6 87 37.8 6.8 higher feed 485 0.6 89 34.2 7.1 aid off 485 0 a

feed off 000 0 87 22.5 40.6

7.5 8.7 normal operation 440 0

Day 2 0 normal operation 440 0 87 41.1

aid added 440 0.4 87 40.2 1.7

steady state 490 0.4 89 39.2 3.7 aid off 440 0 4.5

_ _ 2.5 higher feed 490 0.4 88

6.5 normal operation 440 0 _ _ 45.3 - _ _ -

a Pebble-sized particles appeared in mill product.


problems in flotation, pelletizing, thickening, etc. Pre- liminary economic evaluation shows that the use of efficient chemical grinding additives is an economically viable concept.

Determining S and B grinding parameters for laboratory batch ball mill testing has clearly helped in explaining and quantifying the engineering usage of grinding aids. The ability of mill circuit models to use these grinding parameters to predict and explain operational plant data is good. The additions of either water or grinding aid to a mill having a fixed slurry volume showed similar operating responses. The power draw of overflow discharge mills is constant regardless of the feed rate if the slurry properties are modified by the addition of a grinding aid. The engineering mechanism involved with the use of the selective chemical additives appears to be one of allowing more dense slurries to interact with the tumbling media while still maintaining sufficient slurry fluidity (control of viscosity) in the mill. Acknowledgment

The authors wish to acknowledge the work of their co-workers on this product, M. Katzer, S. Sewell, and J. Shackelford, Dow Chemical USA. They also wish to thank The Dow Chemical Company for permission to publish this paper. Literature Cited Austin, L., Powder Techno/., 5, 261-266 (1971). Austin, L. G., Klimpel, R. R., Ind. Eng. Chem., 56 (11). 16-29 (1964). Austin, L. G., Luckie, P. T., Wightman, D., Int. J. Min. Roc., 2, 127-150 (1975). Griffith, A. A,, Phi/. Trans. Roy. SOC. (London), 221A, 163 (1920). Hartky, J. N., Prisbey, K. A., Wick, 0. J., "Chemical Enhancement of Ore Gsirding

Efficiency," Annual Intermountain Minerals Conference of the AIME, Vail, Colo., 1976.

Herbst, J. H., Fuerstenau, D. W., Trans. SMEIAIME, 241, 538-543 (1968). Klimpel, R. R., Austin, L. G., DECHEMA Monogr., 69, 449-473 (1972). Kiimpel, R. R. , Austin, L. G., Int. J. Min. Proc., 4, 7-32 (1977). Klimpel. R. R., Manfroy, W., "Computer Analysis of Viscosity Effects on Selection

for Breakage and Breakage Distribution Parameters in the Wet Grinding of Ores," " 14th International Symposium on Applications of Computers in the Mineral Industry (1966)", S. Ramani, Ed., Chapter 16, pp 197-206, published by SMEIAIME, 1977.

Klimpel, R. R., Manfroy, W.. "Grinding Aids for Increased Throughput," Ottawa, Canada, January 1977, "Proceedings of Ninth Annual Meeting of Canadian Mineral Processors", R. Bruce, Ed., pp 206-218, published by Can. Min. Proc., 1977.

Klimpel, R. R., Manfroy, W., "Development of Chemical Grinding Aids and Their Effect on Selections-for-Breakage and Breakage Distribution Parameters in the Wet-Grinding of Ores," Proc. 12th Int. Min. Proc. Congress Sao Paulo, Brazil, Aug-Sept, 1977.

Locher, F. W., von Seebach, H. M., Ind. Eng. Chem. Process Des. Dev., 11, 190-197 (1972).

Rehbinder, P. A,, Kalinkovskaya. N. A,, J. Tech. Phys. (USSR), 2, 726-755 (1932).

Rose, H. E., Sullivan, R. M., "The Role of Additives in Grinding," "Ball, Tube, and Rod Mills", Chemical Publishing Co., pp 236-251, New York, N.Y., 1958.

Snow, R. H., "Chemical Engineer's Handbook", 5th ed, Chapters 8, 12, H. R. Perry and C. H. Chilton, Ed., McGraw-Hill, New York, N.Y., 1973.

Westwood, A. R., Metall. SOC. Conf., 35, 1-65 (1966). Westwood, A. R., J. Mat. Sci., 9, 1871-1695 (1974).

Received for review December 23, 1977 Accepted June 12, 1978

approximate analytical solution for the continuous plug-flow case suggested by Herbst and Fuerstenau (1968)

1 - P(x,T) = [I - P(x,O)] exp(-SiT)

where it is assumed that SjBi, is a function of i only and size i is less than size j . The feed and output size distributions, P(x,O) and P(x,T) , were used to obtain the set of values of S , using this equation. Then the proportional influence of grinding additive measured in a laboratory test was used to scale these Si values, enabling an estimate to be made on the increase in feed rate (reduction in WIF) a t a constant product specification or of the increase in fineness a t the same T . Table IV gives a typical set of full-scale open circuit mill predictions for various slurry densities and chemical addition rates in the grinding of taconite ore. It is important to note that because of the large changes in particle size distributions that can ac- company batch grinding tests, the chemical reagent use required to identify a significant S change in a batch grind test is often two or three times that of a similar large-scale contiiiuous grind test.

The necessity of having a higher mill slurry density in order to get a significant grinding aid effect is quite pronounced. Table IV is essentially providing a "break-even'' chart of potential production at similar product particle size. Therefore, for a given slurry density and chemical addition rate, if the feed rate is less than the figure stated the product will be finer than the base case. If the feed rate is higher, the product quality will be coarser. Table V gives a set of open circuit rod mill results run on the same taconite ore, to check the applicability of this simple approach for predicting changes in circuit output. It is seen that the predictions of Table IV are qualitatively correct.

It is not possible to maintain a grate discharge mill a t its best filling if the slurry density, fluid properties, and feed rate are not closely controlled, because the discharge area cannot be opened and closed like a valve. Wet overflow ball or rod mills are made more flexible and are preferred in practice. I t is for this type of mill that the fluidity modifiers will have a very important economic impact in coming years. Conclusion

The effects of chemical grinding aids for wet ore grinding have been analyzed in both laboratory ball mills and industrial scale ball and rod mills. Two important grinding results have been achieved with the use of chemical grinding aids: first, increased feed rate a t constant quality of product size; and second, the production of a finer product a t constant feed rate. Some ability has also been demonstrated to interchange the two results on the same industrial equipment. The use of appropriate additives does not cause noticeable downstream operational

grinding aids klimpel

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