Effect of the Nature of Environment on Comminution Processes Ponisseril Somasundaranl and Israel J. Lin2 Henry Krumb School of Mines, Columbia University, New York, N Y 20027
The past reports in the literature on the effects that the physical and chemical nature of the environment has on such comminution processes as grinding and drilling are reviewed. The two major mechanisms put forward to explain certain commercially important beneficial effects of additives during grinding are the Rehbinder's mechanism based on adsorption-induced surface energy changes and Westwood's mechanism based on adsorption-induced mobility of near-surface dislocations. The mechanisms are discussed in the light of the available data in literature and critically reviewed. Literature data for this purpose has been of limited use because of the lack of control of important system variables during past comminution experiments.
S o l i d s when subjected to a certain degree of tension by impact or abrasion, fragment to smaller particles, but as often stated b y researchers in the past, only with a n efficiency of 1% or so with respect to the new surface created during the operation. It has been the aim of workers in numerous fields t o discover methods for obtaining improved performance during comminution as this operation is used widely in indust'ries such as chemicals, cement, pigments and paints, ceramics, minerals, plastics, pharmaceuticals, and cereals. For several years, comminution was treated as a physical process controlled merely b y the mechanical conditions of the comminuting system. Little attention was paid to the physicochemical conditions of the system until recently when researchers began studying the effect of the nature of the environment on comminution. Even t'hough only about 1% of the energy spent for comminution could be accounted for creating new surfaces, the initial approach to the problem has been mostly o n t'he belief t h a t if comminution involved primarily creation of new surfaces, reduction of the surface free energy of the solid being fragmented should increase the effective efficiency of the operation. Those workers interested in wet grinding or wet drilling strived to effect changes in the solid-liquid interfacial properties b y changing the properties of the liquid phase and those int'erested in dry operations strived to effect, changes in the solid-gas interfacial propert'ies b y changing the gaseous phase in which the solid was fragmented. X critical examination of the past results is necessary to obtain any useful general information from them since these results are often found to be in contradiction to each other. An understanding of the effect's of changes in the environment on comminut,ion is essential for properly utilizing these effects to increase the efficiency of such operations as crushing, grinding, pulverizing, drilling, and machining. Effects of environment on fracture processes are also of concern t'o structural engineers particularly because certain ductile niet'als and alloys become brittle aiid fail in a few environments. lllechaiiisms for this stress corrosion cracking are still not clear. It is the purpose of this paper to review reported effects of enviroiiment on comminution processes and to discuss the T o whom correspondence should be addressed. from Technion-Israel Institute of Technology, Haifa, Israel. 1
* On leave
mechanisms t h a t are responsible for these effects. Toward this goal, we shall first summarize the work reported by various workers in the field of comminution and allied topics. I n the last part of the paper, among others the two major mechanisms proposed to explain the effects are discussed, the first b y Rehbinder et al. based on adsorption-induced surface energy reduction of solids and the second by Westwood et al. based on adsorption-induced alterations in the movement of dislocations a t the surface of the solids. -4rguments put forward by various authors in support of either mechanism are critically examined. Certain system variables t h a t need control or characterizat'ion during cornminution so t h a t the results could be more confidently used to understand the basic processes involved are also indicated. Effects of Environment on Comminution Processes
Grinding. Size reduct'ion essent.ially involves rupturing of chemical bonds to create ne!$- surfaces (42). Any phenomenon that can enhance t'he above rupturing can be expected t'o facilitate the creation of the new surface. Similarly any process that can reduce the surface energy might also be expected to enhance t,he size reduction. Furthermore, reduction of surface energy should in general retard rejoining of the ruptured surfaces aiid particle agglomeration counteracting the process of grinding. If fragmeritat'ion can be considered to be a chemical reaction, one can enhance it b y providing the required activation energy by elevating the temperature (23) or b y lowering t'he energy requirement b y some means. Different ways in which the energy requirement can be lowered become evident when we closely examine the effect of various environments on fragmentation. E$ecf of the Presence of Ti-der. The most' common environmental effect on grinding is that due to t'he presence of water during comminution. Wet grinding, usually meaning grinding in the presence of water, is often found to be more efficient than dry grinding (9, 16, 26, 40, 108, 15'9). Figure 1 shows the grindability in tons of ground material/hp-hr of dolomite as a function of the amount of material in the mill for both wet grinding and dry grinding. It can be seen that wet grinding is more efficient than dry grinding under all conditions of mill filling. Furthermore, when the amount of material in t'he mill was either decreased below 50 lb or increased above 125 Ib the dry grinding efficiency decreased, whereas Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
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Courtesy, Trans. A I M E , 220, 397 (1962).
Figure 1. W e t and dry grindabilities of dolomite as a function of the amount of material in the ball mill; data of Coghill and DeVaney (26) 10
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Figure 2. Log-log plots of rate of formation vs. particle size for grinding in water, carbon tetrachloride, methyl cyclohexane, and nitrogen; after Lin and Mitzmager (78)
the wet grinding efficiency remained constant a t least' throughout the 50-200 lb mill feed range. As pointed out by Lin and Mitzmager (78) the effect of water could primarily be due to a reversible react'ion between unsatisfied surface bonds and water molecules. I n the case of silica the following reaction bet'ween the siloxane and water could be expected to effect the splitting of the oxygen bridge bet,ween two silicon atoms and thus to enhance fragmentation (97). -Si-0-Si-
+ H20+ -SiOH.OHSi
Of course, such effects should be obtained even in the presence of sufficient amount of water in the vapor form. The authors are not aware of any grinding n-ork reported in literature as a funct,ion of humidity in tumbling mills. Locher et al. (79) have reported the grinding rate of soda lime glass to be higher in humid air at 2OoC than in vacuum. There are also qualitative reports that water in the form of vapor can cause deleterious effects on the mechanical properties of met'als (15, 22, 31, 41, 49, 55, 66, 87, 94, 134, 162). For esamples, Sichols and Rostoker (87) found in their investigation of the effect of adsorbed species on the fatigue life of high-strength steels that water was one of the most severe embrittling agents they evaluated. Water vapor could affect grinding due to similar reasons. The increased efficiency of wet' grinding could, on the other hand, be mainly due to physical reasons. 322
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For example, during wet grinding the cushioning effect owing to the presence of dust layer on the particles or owing to the burial of the coarse particles in the dust will be minimal as the fine particles tend to remain suspended in the liquid during grinding. Thus coarser particles are protected to a lesser degree from impact in the case of wet grinding and this would result in a n increase in the efficiency of the grinding process. The lower efficiency shown in Figure 1 of dry grinding a t high mill feeds could in fact be the result of this cushioning effect. Effects, if any, owing to changes in the viscosity or specific gravity of the environment could also be responsible for the reported observations. Grinding in Organic Liquids. Grinding is reported, in general, to be more efficient when done in organic liquids such as glycerine, benzene, and methanol than in water. For example, Kiesskalt (64) reports a twelvefold increase in the surface area when his sample was ground in such liquids as isoamyl alcohol instead of in water. Engelhardt (32, 33) observed t h a t while 5.8 X 106 ergs of energy was required for producing 1 em2 of quartz by grinding it in water, it was necessary to spend only about 3.2 X lo6 ergs for generating the same amount of new quartz surface when the grinding vvas done in alcohol. H e also determined the abrasion hardness of quartz in various media, b u t there is no correlation between the reported hardness values and such fundamental properties of the liquids as the dielectric constant. The effect of grinding in organic liquids can be clearly seen if one examines the data of Lin and Mitzmager (78) reproduced in Figure 2. The rate of production of particles in the range (37-370 p ) investigated was higher for grinding in carbon tetrachloride and methyl cyclohexane than for grinding in nitrogen. An interesting observation that they made was that the efficiency of grinding was lower in the two organic liquids than in water, but it became the same as that for grinding in water when small amounts of water were present in the liquids in the dissolved form. Correlation of these results with such properties as the surface free energy of the materials under various conditions is unfortunately not possible since detailed information on these properties are currently unavailable. E$ect of Surfactant (Organic) Additives to the Grinding Environments. -1ddition of surface active agents has been reported by numerous workers in the past to produce significant effects in grinding and other related processes. I n general, grinding is reported to be enhanced by the addition of moderate amounts of surfactants. For example, a flotation reagent such as Flotigam P, when added to the wet ball-milling of quartzite and limestone was found by Szantho (138) to produce as much as 100% increase in the specific surface area when the reagent concentrations were in the range of 00.03% (see Figure 3 ) . Further increase in the concentration of the surfactant, however, causes a decrease in the specific surface area. With certain reagents such as sodium oleate in larger concentration they observed a net decrease in specific surface area possibly owing to agglomeration as discussed later. The effect on the grinding of quartz in a ball mill of the addition of a surfactant such as Armac T which carries a charge opposite that of the mineral is shown in Figure 4 (44). Quartz is negatively charged above p H 2 (126) and Armac T is a cationic surfactant. It is interesting to note that the addition of this reagent even in moderate amounts is detrimental to grinding. Furthermore, the decrease in grinding efficiency becomes higher with increase in the negative characteristics of the mineral as a result of the increase of the solution pH. The zeta potential of the mineral particles is known to de-
crease with adsorption of oppositely charged surfactants, thereby effecting a n increase in their flocculation (131). T h e adsorption and flocculating effects of the long-chain surfactant are increasingly significant at higher particle surface charge. It is not clear at this point whether the reportedly lower grinding efficiency is due to a n experimental artifact introduced b y the flocculation of fine particles, or the direct result of change in interfacial properties brought about by t h e surfactant. -Malati and eo-workers (82) also obtained a finer product when they added flotation reagents to their wet rod-milling circuit t h a n in the absence of the reagent; b u t they found this influence only for ground products in the size range of 0.005-0.3 mm. This selectivity of effect could again be due to the agglomeration of the finer size particles. Unless special care is taken to deflocculate the products before determining their size, the amount of very fine particles could appear to be smaller. Flocs in the mill during grinding also could cause dissipation of some of the energy owing to their deflocculation on impact. A more serious problem due to the adsorption of the surfactants on the particles can be their hydrophobization and t h e resultant attachment to the air bubbles produced in the system during the process and stabilized b y the surfactant. If a sufficient amount of air gets attached to the particles, they can remain levitated to indirectly cause a loss in the grinding efficiency. In fact, from the work of Kapur et al. (61) it is known t h a t the mineral, if it is less dense t h a n the environment, will suffer a reduction in its comminution. Other reports of the effect of surfactant addition include t h a t of Piven and Saloichenko (95) on the grinding of ultraporcelain and talc in the presence of 0.05-0.1% polysilosane, Voznyuk and Danckuk (143) on the grinding of quartzite, Frangiskos and Smith (37) on the drop-weight crushing of limestone and quartz in the presence of silicones, and Khodakov and Rehbinder (63) on the ball milling of quartz in the presence of some surfactants. As indicated earlier, fruitful grinding essentially consists of producing new surfaces. Since the adsorption of a surfactant on the solid will lower the free energy of the solid-liquid interface, the improvement in grinding performiance owing to its addition could be interpreted to be the result of the easiness with which new surface could be produced in its presence. This reasoning is in line with the esplanation offered b y Rehbinder and eo-workers (103) for the observed increase in drilling rate on the addition of these agents. On the other hand, it is also possible t h a t it is a n indirect result of several other phenomena that could occur ill the system, such as the interaction of the surfact'ant molecules adsorbed 011 the surface (38, 125, 128, 129, 132) and the resultant effects on various int'erfacial properties. I n some cases it could also be the result of the ability of the reagents to enhance the dispersion of the particles and thus indirectly to facilitate fragmentation. This is supported b y works which report a n increase in the fineness of the product even during d r y grinding, and interpret it as the result of a decrease in the attractive forces between particles when surfactants such as napht'henic acid are added for the d r y grinding of ilmenite, magnetite, y-alumina, etc. (13, 19>36, 81, 85, 109). Kukolev et al. (70, 71) found t h a t with the use of orgaiiosilicone a t concentrations of 0.005% in the grinding of alumina it is possible to reduce the time required for a given amount of size reduct'ion b y a factor of 4 in the case of ball milling and by a factor of 6 in the case of vibratory milling. Use of additives during dry grinding is very common in the cement industry. Snow (124) in his recent review on size reduction has discussed the work of various people in this
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(441 area. Examples of the additives used in the cement industry are glycols, amines, organo-silicones, organic acetates, carbon blacks, wool grease, and calcium sulfate (7, 8, 29, 48, 50, 90, 111, 114, 115, 118-120, 123, 137, 163, 164). Some of these reagents, particularly triethanol amine, are found to be effective in preventing ball-coating, b u t not agglomeration of particles (89). This effect of additives is very beneficial since it is known t h a t while ball-coating impairs the grinding efficiency, agglomeration impairs the quality of the cement. According to Schneider (112), the throughput is increased by 25-50Yo when glycol is used as grinding aid. The beneficial effect of organosilicones in grinding is illustrated by the fact t h a t the grinding time for a particular amount of grinding can be reduced by as much as 70Y0 merely b y adding 0.010.05y0 of organosilicones (84).Other examples of the use of surface active agents include that of silicones in the ball milling of quartz (44), of various nonpolar liquids such as acetone, nitromethane, benzene, carbon tetrachloride, and hexane in the vibratory milling of ground glass, marble, and quartz (165) and of wool grease in the milling of gypsum, limestone, and quartz (50, 143).I n fact, use is made of this effect in the ordinary machine shop practice where soap solutions are generally used during the cold-working of metals. It is claimed that the soap solutions, in addition to their cooling effect on the cutters, help the cutting process itself. T h e presence of certain reagents such as amines in t h e gaseous phase has been reported by Lin and Mitzmager (77) Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
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Courtesy, Trans. A I M E , 232, 59 (1965),
Figure 6. Cumulative percentage finer than the starting size vs. viscosity for rod mill grinding of 10 X 14-mesh quartzite in corn syrup-water mixtures for various grinding times; after Hockings et al. ( 5 1 )
to enhance the grinding process. Locker et al. (79) have also found increased grinding rate for soda lime glass in ball mill in the presence of vapor of decanol a t 80°C than in vacuum. They did not, however, observe any effect of environment for the ball milling of cement clinker. Effects of the gaseous environments have been observed b y material engineers on the mechanical behavior of metals and have been explained by a variety of mechanisms involving physical as well as chemical interaction of the gaseous species with the surface layers (31, 151). Effect of Inorganic Electrolytes as Additives. Ever since Rehbinder and eo-workers (103) observed about 35 years ago that the efficiency of a drilling operation can be increased significantly b y adding certain electrolytes which they called hardness reducers to the flushing medium, various workers have studied the effect of these electrolytes on the grinding of rocks and minerals in tumbling mills (14, 20, 21, 37, 78, 83,155). Even though some of the results are in contradiction with each other, there is a general tendency for the results to show that the grinding is more efficient in the presence of optimum amounts of electrolytes (see Figure 5 ) . The optimum concentration is determined b y both the valency of the active 324
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
ion of the salt and the manner of grinding. Increasing the concentration of additives above certain levels is reported to cause poor grinding in some cases (20,83, 121). In addition to any effect t h a t these electrolytes might have on the hardness of the materials, their influence on the flocculation or dispersion of particles is also possibly a major reason for their overall observed effect. This reasoning is supported b y the work of Beyer (14) who found t h a t the addition of dispersing agents always improved the comminution of solids. It is interesting to note t h a t Kukolev and Melnishenko (72) found the addition of caustic soda to be beneficial for the grinding of magnesite (MgC03), but not for the grinding of dolomite (LfgC03.CaC03) in ball mills. Soap on the other hand was found to accelerate the grinding of dolomite with no effect on magnesite. These workers report the above behavior of reagents to be due to the difference in adsorption of the electrolytes on the two minerals; but there is no reported work to the author's knowledge on the adsorption of these chemicals on the above minerals or mechanisms of adsorption which can explain such peculiar behavior. There is no reason why sodium hydroxide should not have the same adsorption on MgC03 as on hfgC03.CaC03, especially in view of the fact t h a t Ghosh et al. (43) have observed a positive effect of sodium hydroxide as well as sodium carbonate on the grinding of CaCOa in stamp mill. They obtained a n increase of up to 50y0 in the specific surface area of the product on addition of the above reagents to the mill. Mallikarjunan and coworkers (46, 83) studied the effect of pH, among other things, on the grinding of quartz and calcite. They report maximum increase in the surface area of the product a t around p H 7 for quartz in one of their works. I n another work they report a continuous increase in surface area with increase in p H of a solution containing 1 g/l. sodium chloride in the p H range of 4-8 that was investigated. The effect of p H on the grinding of calcite in 1 g/l. sodium chloride solution is reported to be a n increase in surface area with decrease in p H in the p H range of 4-8. It is known that the points of zero charge of calcite and quartz are 10.5 and -2, respectively (126, 127). It might be noted that the surface area change owing to grinding reported by the above workers increases as the p H of the environment is shifted away from the point of zero charge. Since flocculation of materials will decrease with increase in difference between the point of zero charge and the actual environmental pH, the above effects could be interpreted as owing to either a n improved grinding efficiency when particles flocculate or owing to artifacts produced by flocculation on the size analysis of the ground product. Significant differences in grinding rates owing to the presence of SaC1, FeC13, and NiCll in the environment have been reported, but the effects are not consistent so that it is not possible a t this point to make any inference on the responsible mechanisms. It can be said, however, that the addition of salts t o grinding systems, in general, enhances the operation, particularly since it has been reported that mineral dressing plants usually work on a higher efficiency when seawater is used to make up the pulp instead of the ordinary t a p water (76, 92, 205). Furthermore, in the ceramic industry also the grinding of metallic and refractory type materials is found to be more efficient when multivalent electrolytes are used as additives (28,29,67, 98, 110,117, 118, 120, 140). Influence of the Physical A-ature of the Environment. Physical properties such as the viscosity of the grinding environment can be expected to have a n effect on the hydrodynamic behavior of particles as well as on the grinding mediums such as balls and rods and therefore on the grinding performance.
Schweyer (113) was the first to investigate the effect of the viscosity of the environment on the rate of grinding in pebble mills. H e found the grinding to be normally dependent o n t h e viscosity of the medium up to about 20,000 mill revolutions and then to be independent of it. Similar results were obtained b y Hockings et al. (51) who st'udied the rates of production of fine particles in corn syrup-water mixtures. These results reproduced in Figure 6 show t h a t the rate of production of fines is lower a t higher viscosities and t'hat the effect is most significant in the beginning of grinding. Kapur e t al. (61) studied grinding of quartz and pyrite in air, water, and tetrabromoethane for mill revolutions u p to 2310 Rev and noticed t h a t while the grinding efficiency was better in water than in air t h e performance of the mill with tetrabromoethane was relatively poor. The result obtained for quartz is in line with the fact t h a t quartz is lighter t h a n tetrabromoethane and therefore is likely t,o remain afloat and thus out of the p a t h of the impact'ing rods or balls. The effect of the physical properties of the environment on grinding performance can be due to either a change in the flow conditions of the pulp or a ret,ardation of the impact a t large viscosit'ies or specific gravities of the environment. Further result's in this area are provided by Clarke and Kitchener (24) who examined the effect of pulp viscosity on fine grinding in ball mills and by Oberson and Brown (88) who conducted grinding experiment,s in heavy media. When a material is ground in the presence of anot'her material, there is oft'en significant amount of preferential grinding (33, 130). For example, in the wet ball-milling of 1: 1 mixtures of 4 X 8-mesh limestone and quartz, the former mineral was fouiid to be preferentially ground in the initial st,ages and t'he latter in the later stages (130). I n the case of wet rod-milling, however, there was no preferential grinding in the initial st'ages, b u t significant preferent'ial grinding of quartz, occurred in the later st'ages. For all cases of grinding, Locher et al. (79) have report'ed a reduction in grinding of quartz in the presence of limestone. The authors attributed this to agglomeration of limestone and the adherence of the finer to coarser quartz protecbing it during grinding. Numerous workers (2-4, 18, 58, 60) studied the effect' of temperature on grinding b y conducting t'he experiments a t lon- temperature, for example, in the presence of liquid nitrogen. Low temperatures are often used for the safe grinding of inflammable or volat'ile materials. Both grinding a t subambient temperatures and preheating t'he samples before grinding are found to help the grinding process, possibly because of the large number of cracks produced owing t o thermal shocks received b y the samples. Drilling. Since Rehbinder reported in 1944 that, t h e addition of electrolyt'es during drilling of rocks like quartzite and granite increased the rate ol' penetration of the drill by 20 t'o SO%,, a large number of workers have provided additional evidence confirming this finding (17, 59, 75, 128, 135, 147, 154). The phenomenon of softening of the minerals on'ing to the addition of external reagents referred to in the literature as "Rehbinder effect" has reportedly been used in Russia to increase the efficiency of drilling operations (101, 103). The above effect was interpret'ed by Rehbinder to adsorptioninduced reductions in t'he surface free energy of the materials subjected to drilling. On the basis of this reasoning, several researchers have examined drilling behavior in the presence of organic reagents such as glycol, glycerine, and some detergents, since these reagents could be expected to reduce bhe interfacial energy of the solids that they come in contact with ( 7 2 , 116).Figure i shows the effect' of glycerine, ethylene
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Figure 7. Effect of various additives on the penetration rate in drilling of quartzite; after Strebig et al. ( 7 35)
glycol, and anionic detergents on the penet'rat'ion rate of drills. It can be seen that these reagents increase t'he drilling rate significantly. Westwood and eo-workers have recently conducted a detailed study of this phenomena b y examining the penetration rate of carbide spade drill in freshly cleaved 31gO and CaF2 monocrystals in the presence of water, toluene, the homologues of heptane through hexadecane, and aqueous solut,ions of aluminum chloride. On the basis of a rigorous examination of the results that t'hey obt'ained for penetrat'ion rat'es and the micrographs of the test specimens, they concluded that the observed changes in hardness were due to adsorption-induced mobility of near surface dislocations or of the chemical components of t'he solid rather than due t'o a n y changes in interfacial energies. A negative Rehbinder effect has been reported by some workers, but it is not clear from these reports whet'her these observat'ions are due t'o a reversal of t'he change in surface properties caused by the addition of excess reagents. For example, if the zeta potent.ia1 of t'he solid is a governing factor in determining the hardness of minerals it' is possible t h a t the reversal of charges by organic or cert'ain inorganic electrolytes could be the cause for the observed effects (128, 189). Possibility that the hardness of solid might depend on their zeta potential is strengt'hened b y the works of Englemann and eo-workers (34) and of West'wood (153) which show that the maximum penetration rate is, in general, a t the isoelect,ric point. of the solution. The environment's which help increase the surface charge decrease the hardness and therefore t'he penetration rate; interestingly these environments, however, increase the wear rat'e by abrasion processes (153). The possible role of zeta potent'ial of particles in determining their dispersion and t,heir resultant efficient removal from t'he drilling area is also to be noted in this connection. It is equally important t'o note that the amount of friction would be influenced by additives, particularly t'hose referred to as lubricants. I n fact Strebig and eo-workers (135) report a n increase in bit life with the addition of reagents, but even this observation is not without a n except'ion t'o the fact t h a t most of the reports in this area are conflictive. For example, Long and Agnew (80) have reported that' during t'heir study of the effect of lubricating agents on drilling n-ith diamond bits they have observed Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
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Figure 8. Illustration of the influence of adsorbed water on the influence on hardthe electromechanical effect-i.e., ness of small currents passed axially through single crystal germanium Dry signifier desorbed and in toluene; wet signifies tested after extended exposure to air of 40% r.h.; after Westbrook and Jorgensen ( I 47)
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Figure 9. Influence of various environments on depth of penetration in M g O ; after Westwood and Latanision ( I 58)
increased wear as well as rise in temperature. This is in conflict with most of the ot'her reports on t'he effect of additives on friction aiid wear during drilling. Effect of Additives on the Mechanical Properties of Materials. T h e postulate t h a t t h e effect that' addit,ives have on drilling rat.es is due to their ability to induce a change in hardness is supported by the result's obtained b y direct determination of hardness by several researchers in the presence of various additives. For example, the hardness of metals such as t'ellurium, as determined by the technique of amplitude damping of a n oscillatory pendulum was found to be reduced by the addition of surface active agent's (74, 75, 104, 136). Rehbinder and eo-workers have found the hardness of metals, when tested in the presence of elect'rolytes, to be dependent on the potential difference across the double layer a t the solidliquid interface. The results of Westbrook and Jorgensen (146, 148) on the effect of moisture on the hardness of germanium crystals and their electro-mechanical behavior and those of Restmood and Latanision (158) on the effects of a homologous series of alcohols on the drill penetrat'ion of magnesium oxide are reproduced in Figures 8 and 9, respect'ively. The implications of the effect's of ions m-ith polyvalency on the hardness of materials (142, 149, 150, 156-157) must be noted, since the presence of such species can be expected to reduce t'he zeta potential significantly b y compression of the double 326
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layer as u-ell as by specific adsorption. It must be mentioned t h a t here also some workers have reported that they have failed to produce a n y change in hardness b y additives when the hardness determination was done by indentation techniques (17 , 1 4 8 ) . I n addition to the hardness, several other mechanical properties of solids have been tested for changes by various workers in the presence of different environments (10, 11, 30, 46, 47, 55, 62, 68, 69, 102). Majority of this work has been on the effect of water on the mechanical properties. A classic example of this is the observed increase in strength of salt crystals when exposed to water. The above phenomenon knon-n as Joffe's effect is ascribed to the elimination of surface defects such as cracks b y dissolution (53, 54). On the other hand, as mentioned earlier, water in the adsorbed form has been reported to be a most pori-erful embrittling agent (66, 56, 134). Sichols and Rostoker (87) who studied the effects of adsorbed organic liquids and water found that the latter had the most effect on the mechanical properties of materials. The effect of organic molecules varied systematically with their chain lengths. The fracture strength of brittle materials such as glass, hardened steel, rock salt, and ice have been reported to vary anywhere from a few percent to 300% depending on the nature of the environment surrounding the material, with the presence of polar molecules having the maximum effect in reducing the fracture of glass (11, 12, 47, 53, 64, 57, 73, 86, 91, 93, 96). Like water vapor, the presence of oxygen in the atmosphere has also been reported to influence the fatigue behavior of metals (1, $ 5 ) . lJ7adsivorth and Hutchings (144, 145) reduced the amount of gaseous atmosphere present by reducing the environmental pressure from 1 a t m to 10-5 torr and very interestingly found a n increased fatigue life for aluminum, copper, and gold. The same effect has been found to occur also for lead, steel, and nickel, even though the responsible mechanisms have not been confirmed. Uhlig (141) recently presented a n excellent discussion on the peculiar effects of chemical reagents on the fracture strength of metals. H e noted that while hot sodium hydroxide and sodium nitrate solution reduce the tensile strength of iron, 18-8 stainless steel is unaffected by them but affected by chloride and hydroxyl ions. Iodide ions, however, are reported to have no effect but when added to chloride solutions prevent the cracking which othervise occurs. Other structural alloys stated to be affected by environment include titanium alloys that fail in chlorides, bromides, and iodides; aluminum alloys that fail in chlorides; and copper alloys that fail in air containing ammonia and amines. It is thus abundantly clear that like the grinding characteristics, the mechanical behavior of materials are also significantly affected by the nature and amount of various species in the surrounding environment.
Discussion
On the basis of the results discussed in the preceding sections, one can conclude t h a t the presence of inorganic as u-ell as organic additives, in general, enhance the fracturing of materials during comminution operations such as grinding and drilling as well as during failure of struct'ural materials. It is important to establish the actual mechanisms by which the above occur so that, t,he most efficient' use of the beneficial effects or prevention of the deleterious effects be made in practice. However, because of the large possibilities for experimental artifacts owing to lack of careful control of important vari-
ables such as solution pH, ionic strength, etc., or owing to agglomeration of the samples used for size and surface area analysis, majority of the past results are not of much scientific value. I n fact, some of the results can be misleading. For example, if the finer product had any tendency to agglomerate due to the addition of the reagent, not only could a n y fracture-enhancing ability of the additive be masked but also the final result could superficially and indeed wrongly indicate t h a t the reagent had a retarding effect on fracturing. This could be due to both the wasting of some of the impact energy for the deaggregation of the flocs and the effect of the larger flocs on the hydrodynamic behavior of the system. Furthermore, the results could be misleading if the investigators did not take special care to deflocculate the sample before surface area or size is determined. I t was mentioned earlier t h a t there are conflicts in the reported results on the effects of ionic surfactants and polyvalent electrolytes on comminution. This could also be due to the fact t h a t proper provisions were not made in the experiments to prevent the multieffects of the addition of reagents. ;Idsorption of these ions of the reagents which carry charges opposite to t h a t of the material being comminuted will cause a reduction of the zeta potential of the mineral, but' some of these ions with specific adsorpiion characteristics will a t larger concentrations even reverse the zeta potential of the miiieral and will continue to increase it with further increase in concentration. Figure 10 illustrates the effect of alkylamnioniuni chlorides on the zeta potential of quartz (132). It can be seen that these reagents can cause a reversal of the surface charge when their solution concentration is above a given level. This concentration of zeta reversal (CZR) is dependent on such factors as the chain length of the surfactant, solution p H , ionic strength, and the point of zero charge of the mineral which in t'urn is dependent on its previous physico-chemical and mechanical history (126). If a researcher happened to work a t concentrations near the CZR he could observe the effect of the additive on zeta potential in the form of a n effect on comminution. On the other hand, if the selected concentration is very much higher t'han the CZR, a zeta potential opposite in sign b u t possibly larger in magnitude than the zeta potential in the absence of the additive could result. Any effect of this larger zeta poteiitial on comminution could be expected to be even opposite to that observed a t concentrations below CZR, since a t these potentials the extent of flocculation and such pheiiomena could be lower than in the absence of additives. K i t h o u t a correct understanding of the interfacial properties ten1 under such conditions, the researchers are likely to attribute t'he results directly to the surfactant addition and cause various reports to be considered as conflicting with each other. T h e problem could be even more complex than that out'liiied so far. For example, we know that the electro-chemical properties of the solid-liquid interface can influence such physico-chemical properties as solubility and reactivity of the mineral ( 1 2 7 ) . .Igain, a n y effect t h a t solubility or reactivity of t'he mineral has on comminut,ion could be interpreted by some workers as a direct effect of the additives on comminution. A n additional experimental art'ifact that could exist in the presence of surface active agent,s could arise from the frothing of the pull) during grinding and the subsequent attachment of the particles to the froth. As mentioned earlier, the particles could remain levitated under such circumstances and thus avoid the impacting rods aiid balls with resultant reduction in grinding efficiencj-.
,
-1001
1001
I
I
I
I
I
I
I
6'
Id6 I 0-5 lo-4 IO-' lo-2 C O N C E N T R A T I O N OF E L E C T R O L Y T E , M O L E PER L I T E R
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I 10.'
Courtesy, J . P h y s . Chem., 68, 3562 (1961).
Figure 10. Effect of hydrocarbon chain length on the zeta potential of quartz in solutions of alkylammonium acetates and in solutions of ammonium acetate; after Somasundaran et al. ( I 32) CRACK PROPAGATION
Figure 1 1 . Propagation of crack A is a broken bond and B is not broken yei
Unless the researchers have examined their grinding systems for the presence of the above-mentioned artifacts, the reported results, even t'hough real, could be misleading. Majority of the reported results have been obtained without any regard to the above-mentioned problems and therefore are not of much use for eit'her any meaningful interpretation or deduction of responsible mechanisms. There are mainly two mechanisms that have been put forward in the past to explain the effects of additives on the comminution of materials. The first by Rehbinder and coworkers (100) and applied to the part'icular case of grinding by Rose aiid Sullivan (107) is based on the concept that fruitful comminution involves the production of new surface and t h a t to accomplish it a n amount of energy that' is proportional to the free energy of the surface should be spent,. If the free energy of the solid surface could somehow be reduced, it would be possible to produce more surface wit,h the same amount of energy input and to minimize t'he chalice of bhe rejoining of the surfaces. Addition of surface active agents to reduce the effective surface energy of the solid particles should then enhance t,he grinding process. Even though this reasoning appears to be basically sound, when one examines the fracturing process more closely it becomes evident t h a t it need not necessarily be the sole reason for the observed effects. For fracture to occur, cracks must be present or must originate and propagate. Figure 11 illustrates the microscopic nature of cracks. Bond A has been broken, but bond B has not yet been broken. -it the t,ime of breaking of bond B, the result'iiig new interface is likely to be of the solid-gas type since the Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , NO. 3, 1972
327
Figure 1 2. Schematic representative of silicon-oxygen banding in quartz and the manner in which water could activate fracturing Electrochemical nature of Type 1 broken bonds would b e negative whereas that of Type II would b e positive
liquid probably cannot penetrate in the liquid state into the crack at the same speed as that of the propagation of crack. One might note that the liquid inside cracks can possess sufficient pressure to enable it to penetrate into cracks because of the possibly very low radius of curvature of the advancing miniscus and a very low contact angle. Yet, as mentioned above, the penetration up to the crack tip is not likely to be a t the speed of crack propagation if a t all it is able to penetrate at a n y speed. Indeed, we might postulate that it is not necessary for the liquid to penetrate as liquid into the crack in order to act as a sink for the bond energy created by the fracture; the reagent can esert its influence even in the vapor form, and it is likely that the vapor pressure of the liquid in the cracks is relatively high. Such phenomenon could play a n important role in determining the influence of various liquids on fracturing. It must be noted in this connection that the normal effect of inorganic electrolytes is to reduce the vapor pressure of a solution. However, the presence of inorganic additives, particuarly those containing polyvalent ions, can decrease the contact angle of the liquid with the freshly formed surface and thus can once again decrease the radius of the advancing meniscus, and hence, increase the vapor pressure. Organic additives indeed decrease the surface tension, but they usually also increase the contact angle. T h e recent experiments of Restwood and co-workers which were described earlier briefly have shed considerable light on the mechanism by which additives might be influencing the comminution process. They studied the effect of the presence of liquids such as water, toluene, hydrocarbon homologues from heptane to hexadecane, and aqueous solutions of aluminum chloride on the penetration of carbide spade drill in freshly cleaved magnesium oxide and fluorite monocrystals. It was found during these studies that the drilling efficiency is directly related to the near-surface dislocation mobility. The Rehbinder effect, attributed by Rehbinder to adsorptioninduced reduction in the surface free energy of the solids, is, according to Westwood, more likely due to changes in the electronic states near the surface and point and line defects caused b y the adsorption of the additive on the surface of the solid. Such changes are known to influence the specific interactions between dislocations and point defects which control the dislocation mobility and hence the hardness (58). Both the increase in hardness of calcium fluoride owing to adsorption of certain reagents and the increase in drilled particle size with surface activity are cited by Westwood as good reasons for rejecting Rehbinder’s explanation based o n reductions in the surface free energy of the solid. This reasoning by JVestm-ood is obviously on the assumption that adsorption always reduces surface free energy. B u t we have a multitude of evidence in literature for the fact that adsorption of certain reagents can take place not only to reduce the zeta potential 328
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to zero, b u t also to reverse the sign of the charge and then even t o increase it (5). In fact, adsorption of appropriate potential-determining ions will cause a n increase in the surface potential also. This is because it is the change in energy of the total system containing both particles and solution t h a t is the criterion for the occurrence of adsorption and not the surface free energy of the solid alone. It is then possible t h a t in the cases where a change in hardness was observed as a result of adsorption, a change in surface free energy also took place. Similarly, the production of larger size particle with increased surface activity cannot strictly be used to reject Rehbinder’s mechanism since it is not known whether it is the surface free energy of the solid that is responsible for determining their distribution modulus of ground or drilled products. I n all probability, the phenomena determining the distribution modulus (slope of the straight line portion of the log-log plot of cumulative percentage finer than a given size vs. the size) of the product are much more complex, especially if one considers the various size distributions that can be obtained for the same solid using various grinding methods (27, 65, 66). It must be pointed out t h a t examination of the drilled specimens using etching techniques b y Westwood support his hypothesis rather strongly. It must also be mentioned that the effect of additives on the drilling of glass, which probably has little dislocation in it, is reported by Westwood as owing to the diffusion to the surface of sodium ions under favorable electrical conditions a t the interface and the resultant decrease in the hardness of glass (159). However, the validity of the mechanism is to be carefully examined since the increase in hardness owing to additives is more or less instantaneous whereas the diffusion of the sodium ions to the surface from the bulk can be expected to be relatively slower. Westwood believes that he can justify adequate sodium diffusion to the surface for drilling rates of the order of 10 @-set assuming a n activation energy which is reduced by lattice strains to 13 kcal/mol from about 15-18 kcal/mol (152). It must be mentioned that Westwood’s arguments, in general, are supported by some recent works like that of Robinson (106) who studied the environment-sensitive drilling behavior of limestone and reportedly found that reductions in surface free energy are not of prime importance in the drilling of nonmetallic solids. We must also examine the possibility of formation of a n activated complex of the additive with the chemical constituents of solid as a precursor of the actual fracturing of a bond. Figure 12 illustrates the formation of surface silanol groups in the case of the fracturing of silica in the presence of water. Here the water molecules, whether from vapor phase or liquid phase, corrode the solid a t the crack-tip and thereby facilitate the breaking of bonds. Since the total energy of two bonds between -Si and -OH is higher than the energy of one bond between a Si and a n 0, the silanol groups form easily and with this action facilitate the propagation of the crack. One cannot neglect the possibility of the molecules of the additive diffusing through various crack tips even before the actual beginning of the comminution and thus being available for reactions during the fracture formation. It has been suggested for the case of the effect of surfactant additives t h a t one of the mechanisms to be considered is the penetration of the surface active molecules deep into the roots of cracks nuclei exerting pressure there and tending to separate the walls of the crack (74).The effect of long-chain alkyl or aryl surfactants cannot be easily explained by this mechanism since their dfiusion would be limited owing to their larger size. The observation by West-
wood (79) t h a t the maximum influence of the long-chain surfactants is when the chain has 6 to 7 CH, and C H I groups might indeed be due to the fact t h a t the higher chains cannot diffuse rapidly through the solid structure. Certain reagents while known to enhance some type of fragmentation are known to retard some other types. For example, heptylalcohol enhances drilling of soda lime glass with diamond bit b u t minimizes ball milling at sufficiently low rpm t h a t there is no cascading of the grinding medium (158). This is reportedly owing to t h e fact t h a t while in t h e former a n inipact type of cutting mechanism is predominant, in the latter t h e major mechanism is of a plowing type. Certain additives could help in the formation of new cracks b y local corrosions. Such corrosions could be expected to be most prominent a t the sites of surface irregularities. Any change a t a surface, whether owing t o a chemical reaction or a structural change, could be expected to produce such irregularities ( 3 5 ) .As pointed out b y R e y l (160, 161), surface energy of a solid can be reduced significantly by structural changes such as polarization of the surface ions, increase of t'he number of ions with maximum polarizability or decrease of the number of ions with minimum polarizability and a larger exposure of the most polarizable ions a t the surface. X liquid which can wet a freshly formed surface mill lower the energy of that surface and therefore enhance the creation of the new surface b y comminution. Sudden changes in temperature or pressure of the system would also enhance the formation and propagation of cracks because of the shocks that the particles would be subjected to during such changes. The mechanism responsible for st'ress corrosion cracking is far from being clear particularly because of the previously ment'ioned apparent inconsistency with which various environments affect various metals. While a number of authors believe this type of cracking to be due to adsorpt'ion of ions in defect' surface sites, several others believe it to be due to accelerated elect'rochemical dissolution as well as progressive fracture of brittle oxide films present on metals (141). Owing to the reasons discussed in the beginning of this chapter, reagents can influence the rate of grinding or drilling also on-ing t'o t'heir role in determining the dispersion and hence the flow of particles produced during these processes (99, 107). Furthermore, in the case of drilling, additives will reduce the bit wear owing to their lubricating and cooling actions. However, these actions of the additives cannot, in general, be considered to be the main reason for their influence since they, even t,hough always beneficial as coolants, effect' improved drilling in bhe case of some minerals but, decreased drilling in the case of some others. Also, Selim and co-workers (116) have found t h a t the addition of certain organic compounds to water increases the coefficient of friction of the bit with quartzite even though it facilitates the cutt'ing of quartz. The above discussion shows that use of experimental dat,a for identification of true mechanisms is possible only if relevant properties of the solid and of the solution, if any, that is in contact. with the solid during its fracture are simultaneously determined. The essential properties that could be checked without much difficult'y include the zeta potential and adsorption capacity of the solid, surface t'ension, p H , ionic st'rength, temperature and chemical composition of the solution, part'ial pressure of various gases in the environment during fracturing, and t'he frot,hing and flocculation characteristics of the system during grinding as well as during subsequent sizing. Absolute value of the interfacial energy of t'he solut'ion is indeed hard t80determine but changes in it
could be followed b y standard methods. Previous history of the solid and any pretreatment t h a t it has been subjected to must also be established carefully. Nature of the dislocation density and other physical characteristics such as surface roughness must be followed microscopically. Concentration or deficiency of important elements near the surface must be determined if possible. Finally and most importantly the machine characteristics such as speed and size of it and nature of the comminuting medium must be noted. Summary and Conclusions
The physical as well as the chemical nature of the environment is found to influence significantly the performance of comminution operations such as grinding and drilling. These effects are of commercial importance since higher comminution efficiency can be obtained by adding reagents. In fact, they are used beneficially in some industries, particularly in the cement industry. On the other hand the deleterious effects of environment o n structural materials are often of a significant magnitude. While the effects of changes in physical conditions such as viscosity or density of the environment during grinding are easily explained on the basis of the hydrodynamical behavior of the particles as well as the grinding medium, the effects of the addition of chemical reagents are far more diverse and sometimes conflicting with each other. They are not easily explained to the satisfaction of all past esperimental observations. In general, there are t n o major mechanisms proposed for the observed effects; one by Rehbinder and co-workers based on surface energy reductions owing to the adsorption of additives on solids and the other b y Westwood and co-workers based on the effect of adsorption on the movement of near-surface dislocations. Rehbinder's hypothesis is supported mainly b y correlations between possible reductions in the surface energy of solids owing to the adsorption of reagents, particularly of the organic type, and observed changes in penetration rates during drilling. Kestwood has put forward significant evidence with his etching n ork to support his hypothesis. However, his arguments against Rehbinder's hypothesis are far from being conclusive. Fracturing is composed of two processes-crack initiation and crack propagation. It is likely t h a t one mechanism is responsible for the effect on the first process and the other for that on the second process. Only more careful experimentation could establish the relative importance of the two mechanisms in the overall comminution processes. Results from majority of the past work are not of much use for rigorous scientific interpretations since researchers of both grinding and drilling have not controlled or measured all the important system variables such as ionic strength, surface potential of particles, etc. They most often have not characterized their starting materials in terms of the history, pretreatment, or crystal structure. Furthermore, several workers have not taken into account experimental artifacts introduced by the effect of such phenomenon as flocculation of particles on the determination of the size of the ground product. frothing, etc. Further work with more careful control of all the important variables such as surface potential, surface energy, adqorptioii capacity, and surface dislocation density of the solid; surface tension, p H , ionic strength, temperature, and chemical composition of the solution; partial pressure of various gases in the atmosphere; and the machine characteristics like the size and speed of it, is essential to elucidate the responsible mechaniqms. Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 3, 1972
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References
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I~LCI:IVLD for review J d y 12, 1971
ACCLPTED April 27, 1972
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