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talline activity by sulfur was unexpected. It is possible that the sulfur complexes formed inside the pores hehave like 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. Literature Cited Breck, D. W., “Zeolite Molecular Sieves”, Wiley, New York, N.Y., 1974. Chen, N. Y. (to Mobil Oil CorD.). U S . Patent 3373 110 (Mar 12. 1968). Chen, N. Y., Maziuk, J., Schwa&, A. B., Weisz, P. B., Oii Gas J., 66 (47),154
(1968). Chen, N. Y., Lucki, S. J., Mower,
E. B., J. Catal.,
13, 329 (1969).
Chen, N. Y.. Lucki, S. J.. Anal. Chem., 42, 508 (1970). Chen, N. y., b r w o O 4 w. E.. Adv. Chem SW., NO. 121, 575 (1973). 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
(1975). 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 119741 .,. 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 I . _ .
(1962).
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 additive 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 references, and Hartley et al. (1976) have recently prepared an updated synopsis of the grinding additive literature. Many of the studies reported consist of subjecting mal
Physical Research Laboratory. Functional Products Department. 0019-7882/78/1117-0518$01.0010
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 hypotheses 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 Rehbinder 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 phenomena 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 0 1978 American Chemical Society
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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 Kalinkovskaya (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 following 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 ineffective 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 agglomerative 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
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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%
