International Journal of Mineral Processing, 22 (1988) 313-343 313 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Factors Affect ing Wear in Tumbling Mills: Influence of Composit ion and Microstructure

J.J.MOORE *1, R. PEREZ, A. GANGOPADHYAY and J.F. EGGERT

Mineral Resources Research Center, University of Minnesota Minneapolis, MN 55455 (U.S.A.) Department of Metallurgy, University of Antioquia, Medellin (Colombia) Department of Material Science and Engineering, Northwestern University, Evanston, Ill. (U.S.A.) Whirlpool Inc., Kentucky (U.S.A.)

(Received December 24, 1985; accepted after revision August 11, 1986)

ABSTRACT

Moore, J.J., Perez, R., Gangopadhyay, A. and Eggert, J.F., 1988. Factors affecting wear in tum- bling mills: influence of composition and microstructure. Int. J. Miner. Process., 22: 313-343.

A large range of iron-carbon alloys were investigated under milling conditions. The alloy sys- tems used included AISI 1020 mild steel, high-carbon low-alloy forged steel, forged martensitic stainless steel, forged austenitic stainless steel and NiHard, 20% chromium, 27% chromium, 30% chromium white cast irons. A range of mineral slurries was also used which included a corrosive copper-nickel sulfide ore, abrasive quartzite, taconite, molybdenite, and abrasive and corrosive quartzite-pyrrhotite combinations. These ranges of alloy systems and minerals provided suitable comparisons with respect to corrosive and/or abrasive conditions of the mineral slurry together with a suitable variation in microstructural constituents of the ball.

Marked-ball wear tests were conducted under laboratory conditions in the main, but a small investigation aimed at identifying the importance of impact was also conducted using large in- dustrial tumbling mills. Seven wear mechanisms were identified from examination of ball surfaces, using scanning electron microscopy. It was determined that wear rate was dependent upon a com- plex interaction of parameters involving microstructure, hardness ratio of the ball to the mineral, mineral slurry chemistry and pH and oxygen potential, ball mix and size. Adiabatic shear bands were also found in balls which were used in the large industrial mill. A galvanic series of the minerals and grinding media materials was determined from electrochemical tests, indicating the tendency of each grinding material to corrosive wear. The effect of media composition and micro- structure was also determined using a strain index method developed in this investigation. In all the tests conducted the harder grinding media produced the most efficient grinding.

Wet grinding efficiency was found to be dependent on media hardness, while wear under wet grinding was much higher, in general, than that under dry grinding. With respect to mill size it was found that the smaller laboratory mills tended to overemphasise corrosive wear compared with the larger laboratory mills.

m

*Present address: Department of Chemical and Materials Engineering, University of Auckland, Private Bag, Auckland, New Zealand.

0301-7516/88/$03.50 © 1988 Elsevier Science Publishers B.V.

314

I N T R O D U C T I O N

In the U.S. alone approximately 0.5 billion pounds of steel grinding media are consumed each year in wet grinding and in excess of 1 billion pounds world- wide [ 1 ]. This amounts to between $100 to $200 million. 77% of the grinding media consumed in the U.S. is in the form of forged high carbon, e.g., 0.85% C, steel, 20% cast high carbon steel, 3% cast white iron [2].

Wet grinding provides a complex aggressive, abrasive and corrosive environ- ment in that continual abrasion may remove any protective film that is pro- duced in the wet slurry environment, and that the surface area of the anodic balls is much smaller than that of the cathodic mineral being ground.

Factors that affect ball wear can be summarized under three headings: (1) the ore - - w h e r e hardness (abrasiveness), mineralogy, e.g., the presence of corrosive species, and particle size are the more important parameters; (2) the mill - - where composition, microstructure and mechanical properties of the balls and liner, quantity and size of balls, size of mill and mill speed are the more important parameters; (3) the mill environment such as the mill water chemistry and pH, oxygen potential in the mill, percent solids, and temperature.

Hardness alone is not a good indicator of the wear resistance of grinding media and considerable differences in media wear behavior have been reported. These include balls of similar chemical composition but different microstruc- ture; balls of similar hardness but of different composition and microstructure; while balls of similar composition, hardness and microstructure behave differ- ently in different ore slurries [ 3,4 ]. The importance of microstructure on wear rate is demonstrated in Fig. 1 for 0.8% carbon steel grinding balls. The large

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Rockwell C Hardness

Fig. 1. Effect of h a r d n e s s ~ n d microstructure on the wear of cast grinding balls.

315

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Fig. 2. Metal wear in dry and wet ball milling for ores of varying abrasion indices.

difference in wear rate between dry and wet grinding (Fig. 2 ) was, until re- cently, thought to be due solely to corrosion, but recent research at the Uni- versity of Minnesota, has indicated the importance of slurry rheology [ 5 ].

This paper reviews the research work conducted to date at the Mineral Re- sources Research Center (MRRC) at the University of Minnesota on the ef- fect of media composition, microstructure and hardness on wear.

RESEARCH AT MRRC

Grinding media

Eight different grinding ball alloys have been used in laboratory marked- ball tests with corrosive Cu-Ni sulfide ores [ 6 ], abrasive quartzite, taconite or molybdenite ores [7], and abrasive and corrosive quartzi te-pyrrhoti te ore combinations [ 8 ]. The alloy systems used include AISI 1020 mild steel (MS) , high-carbon low-alloy (HCLA) forged steel, forged martensit ic stainless steel ( MSS ), forged austenitic stainless steel (ASS) , and NiHard ( NiH ), 20% Cr, 27% Cr, 30% Cr high chromium cast irons (HCCI) . This provides an ex- t remely wide range in composition, microstructure and hardness with consid- erable variation in abrasion and corrosion resistance. The chemistries and microstructures of these ball compositions are given in Table I and Figs. 3 and 4, respectively.

Fig. 3. Typical microstructures of (a) mild steel, (b) high-carbon low-alloy steel, (c) martensitic stainless steel and (d) austenitic stainless steel.

Wear mechanisms

Examination of the worn surface of these balls in the scanning electron mi- croscope (SEM) have allowed seven wear mechanisms to be identified [ 91. These are outlined below and in Figs. 5-11.

(i) Indentation craters. A hard mineral particle, positioned between two balls, extrudes the ball material to the sides. This material is not detached, but is moved upwards and out from the ball surface producing a crater. This wear mechanism is more prominent in softer media materials (Fig. 5a).

(ii) Gouging. A h ar d mineral particle cuts into the ball surface and scoops

318

a portion of the metal in the direction of particle movement. This wear mech- anism is also generally present in the softer media materials (Fig. 6).

(iii) Plowing. A hard mineral particle plows across the surface of the ball pushing out the ball material to the sides, producing a ridge in the ball's sur- face. This mechanism predominates in the harder media materials (Fig. 5).

(iv) Scuffing. Ball-to-ball contact results in a relatively thick abraded re- gion of approximately parallel ridges (Fig. 7).

(v) Strain-induced corrosion. The conjoint action of corrosion and strain, the strained material being the heavily deformed edges of the material after plowing or indentation has taken place (Fig. 8).

(vi) Pitting. The selective dissolution of material at preferential sites due

Fig. 4. Typical microstructures of (a) Ni-Hard cast iron, (b) 20% Cr cast iron, (c) 27% Cr cast iron and (d) 30% Cr cast iron.

319

Fig. 5. SEM photomicrographs of (a) MS ball tested under dry grinding, (b) MS ball tested under N2 flushing, (c) HCLA steel ball tested under dry grinding and (d) HCLA steel ball tested under N2 flushing. Note predominant wear mode is indentation in (a) and (b) and plowing in (c} and (d).

to galvanic coupling between heterogeneities (e.g., inclusions, segregated areas, phases) and the surrounding matrix material (Figs. 9,10).

(vii) Spalling. The subsurface cracking and subsequent removal of material at the surface of the ball (Fig. 11 ).

Several of the above wear mechanisms will operate in any ball-mineral slurry system, but, in general, one will predominate depending on the microstructure, hardness and corrosion resistance of the ball material and the abrasive and corrosive nature of the slurry being ground.

320

Fig. 6. SEM photomicrographs of (a) MS ball tested under nitrogen flushing, (b) HCLA steel ball tested under dry grinding and (c) ASS ball tested under dry grinding. Note the gouges pro- duced on the surface of the balls.

Fig. 7. SEM photomicrographs of (a) Ni-H ball tested under N2 flushing and 10% pyrrhotite addition to quartz slurry and (b) 30% Cr C.I. ball tested under nitrogen flushing. Note the scuffing marks on the surface of the balls.

321

(d)

(e)

Fig. 8. SEM photomicrographs of (a) HCLA steel ball tested under oxygen flushing and 10% pyrrhotite addition, (b) 20% Cr C.I. ball tested under oxygen flushing and 10% pyrrhotite addi- tion, (c) Ni-H ball tested under oxygen flushing and 10% pyrrhotite addition. Note: Preferential electrochemical attack of highly deformed ridges in plow marks. Schematic drawings of develop- ment of strain-induced corrosion of ridges: (d) side elevation of plow or ridge developed by abra- sive wear (quartz slurry in nitrogen flushing), and (e) uneven nature of plow mark or ridge under abrasive and corrosive conditions (quartz+ 10% pyrrhotite slurry and oxygen flushing).

Fig. 9. SEM photomicrographs of ball surfaces of 30% Cr cast iron: (a) under oxygen flushing, (b ) with 10% pyrrhotite and oxygen flushing. Note increase in pitting in (b ) compared with (a) .

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7 8 ~ • ~ - 4 1 p r n

Fig. 11. Subsurface cracking (spalling) produced in (a) HCLA steel ball tested under N2 flushing, (b) 27% Cr C.I. tested under N2 flushing, and (c) MSS ball tested under air flushing.

Effect of impact

In large-diameter mills severe impact conditions may exist which may give rise to increased spalling. In a recent investigation at M R R C [ 10 ] conducted on 5-inch diameter balls used in a 27-ft. diameter SAG mill, the impact was found to be sufficient to produce adiabatic shear bands (Fig. 12a) which are normally associated with ballistic deformation. These extremely hard narrow bands are prone to create fracture and spalling. Adiabatic shear bands with a strain of 16 were also present in the martensit ic steel liner on the SAG mill (Fig. 13 ). These shear bands promoted cracking which was seen to propagate via sulfide inclusions in both liners and balls (Fig. 14). Such severe impact

324

|

Fig. 12. Photomicrographs of 127-mm diameter grinding ball. (a) Adiabatic shear band at the surface, (b) plastic deformation below the surface, and (c) formation of a white phase below the surface.

deformation was also seen to produce a white transformation product at the surface of the ball (Fig. 12c). The microhardness value of this transformation product was similar to that for austenite in the steel composition. However, further detailed investigations are needed to confirm this phase identification. The properties of this transformation phase may play an important role in the overall wear characteristics of the media. Such large-diameter balls were also found to have significant macrosegregation of carbon between the edge and center (Fig. 15), presumably produced during the preceding continuous cast- ing operation. This could give rise to considerable differences in austenite transformation products on subsequent heat treatment and also provide in- creased thermal stress. This combination of factors may be the cause of these large grinding balls exploding, both before and during use in the tumbling mills.

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Fig. 13. Photomicrographs of adiabatic shear bands in mill liners.

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Fig. 14. Photomicrographs of (a) mill liner just below the surface at fracture edge showing long thin cracks, joining the MnS inclusions and (b) HCLA steel ball used in a laboratory ball mill showing a crack generated from the surface through a MnS inclusion.

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A considerable drop in hardness was noticed at the ball surface (Fig. 16) which may be due to the softer t ransformation product and/or the lower carbon con- tent of the ball at its surface.

The fractured surface of the liners also showed signs of fatigue failure (Fig. 17). Most liner failures appeared to be close to the columnar-equiaxed tran- sition in the liner solidification structure. This transit ion structure is known to be usually associated with a high concentrat ion of inclusions. Indeed, these liner cracks were also usually found to be propagated via sulfide inclusions close to the fracture surface (Fig. 14a). The effect of impact of the balls on the liner was seen to produce a hardened surface of between 1 and 3 mm depth which appeared as a white area on the fracture surface (Fig. 18).

Wear test data

The importance of microstructure and hardness on wear rate in a corrosive slurry environment [ 11 ] is well demonstra ted in Fig. 19. Generally, increasing the ball hardness resulted in a decrease in wear. The exception to this is seen in the decarburized layer on the surface of the mild steel ball. Dry grinding provides a good indication of the abrasion resistance of the alloy and the single phase ferrite decarburized mild steel produced a much higher wear rate than the combination of ferrite and pearlite phase structure. However, the softer single phase a ferrite produced a lower wear rate during wet grinding than the harder combination of ferrite and pearlite bulk structure. This indicates that

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there was a galvanic coupling between the c~ and pearlite phases which resulted in increased corrosive wear.

Using nitrogen flushing through the mill has been found to simulate wet grinding under largely non-corrosive conditions so that the difference between dry grinding and wet grinding with nitrogen flushing is largely due to wet abra- sive grinding. The pH of 11.2 was effected by adding CaO to the slurry and, again, essentially simulated wet abrasive grinding conditions. However, the reduced ball wear produced with the addition of lime may be due to the soft lime providing a somewhat protective coating on the the balls' surfaces. On the other hand, oxygen flushing through the mill simulated a highly corrosive slurry environment. Again, the effect of galvanic coupling between different phases of the ball under these highly corrosive, oxygen flushing conditions, resulting in increased wear was demonstrated with the 20% Cr HCCI ball. The complex

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Fig. 22. Wear (g/ball per tonne fore) versus surface area of ball in 36-inch mill. mo -- molybdenite ore; tac = taconite ore.

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Fig. 24. Change in wear rate due to mixing of mild steel balls with 30% chromium cast iron balls in a quartz + 10% pyrrhotite system.

microstructure plus the aggressively corrosive conditions resulted in an in- creased wear rate compared with the HCLA and MSS steel balls, even though the HCCI ball was harder than the HCLA and MSS balls. The importance of the mineralogy on wear rate is demonstra ted in Fig. 20. Addition of a corrosive const i tuent such as 10% pyrrhoti te to a highly abrasive quartzite slurry pro- duced increased wear rates for the HCLA and NiH balls especially at high oxygen potentials in the mill [ 12 ]. The corrosion-resistant HCCI and ASS balls and to a lesser extent the MSS balls were essentially unaffected by such corrosive slurry conditions. However, the slight decrease in wear rate for the mild steel when 10% pyrrhoti te was added to the quartzite slurry under oxygen flushing conditions was totally unexpected. This is the subject of an extensive electrochemical research program at MRRC.

334

T A B L E II

Galvanic series in a slurry of quartzi te + 10% pyrrhot i te

Pyrrhot i te Taconi te Austeni t ic stainless steel 30% Cr HCCI Mar tens i t ic stainless steel 20% Cr HCCI HCLA steel NiH cast i ron Mild steel

CATHODIC

ANODIC

T A B L E III

Electrochemical data for pyr rhot i te -ba l l mater ial couples in grinding mill water

Combina t ion Galvanic potent ia l (mV vs current SCE) (irA)

Mild steel-pyrrhotite Nitrogen - 600 1.723 Air - 484 26.5 Oxygen - 462 30.44 Ni hard C.L-pyrrhotite Nitrogen - 614 1.84 Air - 5 4 4 10.36 Oxygen ( - 4 6 0 ) - ( - 4 9 0 ) 60-110 HCLA-pyrrhotite Nitrogen - 662 2.634 Air - 600 36.36 Oxygen - 520 47-60 20% Cr C.L pyrrhotite Nitrogen ( - 5 9 8 ) - ( - 6 2 0 ) 1.6-4.57 Air - 555 19.84 Oxygen - 4 1 0 20.17 MSS-pyrrhotite Nitrogen - 586 1.6-2.11 Air ( - 392 ) - ( - 440) 4.8-8.0 Oxygen - 369 13.24 30% Cr C.L-pyrrhotite Nitrogen + 90 0.01 Air + 210 0.004 Oxygen + 158 0.006

335

Hardness can be a good indication of wear resistance for media of similar microstructure as shown in Fig. 21. Again, increasing the pH, even for a cor- rosive sulfide ore, decreased the wear rate. In all the wear tests conducted, wear rate was seen to be proportional to the surface area (size) of the grinding balls [ 7 ] (Fig. 22 ) while, for the HCCI media, wear rate decreased with increasing carbide volume [8] (Fig. 23).

Galvanic coupling was also shown to take place between different balls when mixed in the same test [ 8 ] (Fig. 24). This has an important bearing on in- dustrial mill testing of different grinding media in that the mill must be cleared of dissimilar balls before a new ball composition is tested.

a) Bri t t le Non-Conducting Inclusion, e.g. , AI203 SiO 2

Before Milling After (During) Hil l ing

ANODI C RX.

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J 2e-

Low Oxygen (Low En}

High Oxygen (High EO}

CATHODIC RX.

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Fe - Fe 2+ + 2 e-

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Fig. 25. Schematic representation of pitt ing corrosion mechanisms.

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Fig. 26. Wear rate versus media hardness: (a) 18-inch mill; and (b) 36-inch mill.

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Ball to Mineral Hardness Ratio

Fig. 27. Wear rate versus ball to mineral hardness ratio: (a) 18-inch mill; and (b) 36-inch mill.

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Fig. 28. Effect of increasing HB:H M on decreasing media wear.

A galvanic series for the grinding media investigated was determined from electrochemical tests conducted in the mill water from a slurry mixture of quartzite + 10% pyrrhotite [8]. This is given in Table II.

Electrochemical measurements, i.e., combination potentials and galvanic currents, were determined in the quartzite + 10% pyrrhotite slurry mill water. These data are given in Table III where it is evident that increasing the oxygen potential in the mill increased the galvanic current and decreased the combi- nation potential. The corrosion resistance of the HCCI and stainless steels is also evident. However, the electrochemical data for mild steel is not consistent with the wear data given in Fig. 20 and it must be concluded that the abrasion of the soft mild steel ball must play an important role.

The expected electrochemical reactions that take place are outlined below. Cathodic sites: e.g., unabraded or less strained regions in the ball surface.

RT P~)/~ 1/2 O2 + H 2 0 + 2 e = 2 0 H - ; Eo =E°o +-~ In a2 oH

Thus, increasing the oxygen potential in the mill increases the cathodic potential.

339

Anodic sites: e.g., galvanic coupling between phases and/or abraded or strained regions in the balls.

F e = F e 2+ +2e

Sulfides, present as inclusions or mineral particles a t tached or embedded in the ball surface may produce the following reaction:

MS (inclusion or mineral) + 202 ÷ 2H20 = M (OH) 2 ÷ H2 SO4

The generation of H2SO4 will decrease the pH and may result in an increas- ingly aggressive corrosive environment and even hydrogen embri t t lement in certain Fe-C alloys.

Fig. 29. Photomicrographs below the worn surface of (a) MS ball under closed mill wet grinding, (b) HCLA steel ball under dry grinding, (c) 30% Cr C.I. ball under dry grinding, and (d) ASS ball under dry grinding (note extensive slip lines).

340

Possible pitting corrosion mechanisms [ 7,8,11] are outlined in Fig. 25. These may result from either brittle, non-conducting particles in the ball surface, e.g., alumino-silicate inclusions or silicate minerals embedded in the ball surface on grinding. Fragmentation of these minerals will produce a pit in which oxy- gen or hydrogen concentration cells could develop depending on the slurry mineralogy and mill environment. Conducting minerals such as sulfides pres- ent as inclusions or in the ore mineralogy will provide a galvanic coupling with the ball.

Performing marked-ball grinding tests in different size laboratory mills (i.e., 8-inch and 36-inch diameter mills) indicated that the effect of corrosion, as simulated by oxygen-flushing through the mill, is less emphasized in the larger mill [ 9 ] ( Fig. 26).

The ratio of hardness of the ball, HB to that of the mineral, HM, i.e. HB:HM, has been found to be an important parameter in determining media wear (Fig. 27), in that increasing the HB:HM ratio decreases the wear rate and the pre- dominant mode of wear under abrasive ore conditions changes from indenta- tion and gouging to plowing. However, the effect of HB:HM ratio on media wear was most pronounced in smaller 8-inch mills than in 36-inch mills (Fig. 28).

1.7

1.6

~Mild steel 1.5 - Mild Steel

1.4 ~ 1.:3 1.2 "o---o .... \ 30"/, Cr C.l.

- " o - . . o ~

1.1 "-, 30"/, Cr C.I.

1.0 o ~ ' - - - o . . . . . . . .

Depth (prn)

Fig. 30. Change in strain index from the surface towards center during dry grinding of quartz with MS and 30% Cr HCCI.

1.7

341

1.6

I\~Ms 1.S

1.4

'~ 1 .3

1 .2

1.1

1.0

Ni -H

HCLA

Cr C . I .

~ S O ~ ; Cr C . I .

40 fl0 120 180 200 270 280

Depth Belou t h e S u r f a c e , pm

Fig. 31. Variation of strain index with distance below the surface for various grinding media after dry grinding in quartz.

The extent of deformation (Fig. 29 ) and strain induced in the balls' surfaces on grinding is also an important criterion in media material development and selection. This has been measured by determining a strain index from micro- hardness measurements. A microhardness profile is determined in the ball from surface towards the center after a grinding test. Each microhardness value is divided by the microhardness value taken from a similar ball at the same po- sition but which has not been used in a wear test. Thus a strain index of 1.0 indicates no increase in hardness. The strain index profiles for MS and 30% Cr HCCI balls in an 8-inch mill are given in Fig. 30 together with the surface deformation structures. Similar profiles are shown in Fig. 31 for all the grind- ing media alloys investigated. As expected, the softer materials exhibit high strain indices but the total depth of the strain appears to be approximately constant at about 120-160 ~m for the 8-inch mill tests. Indeed, the depth of strain appears to be greater for the 30% Cr HCCI. This strain index could prove to be an important criterion for grinding media selection and development.

342

~ - - - ° l

e - ° - - O e - - - - e

KSS b a l l t e s ted under N 2 f l u s h i n g condi t ions hISS b a l l t e s ted under dry grinding condi t ions XCLA b a l l tes ted under N 2 f l u s h i n g condit ions HCLA b a l l t e s ted under dry grinding condi t ions

1oo 90

80

-~ 7o

"~ 60

~ so

~ 40

~ 30

~ 20

0 20 300

, ' / / /

I I I I I I I I I 40 b0 80 100 200

S i z e , m i c r o n s

Fig. 32. Size distribution of quartz after dry grinding and wet grinding in N2 flushing.

More work needs to be done in this area, e.g., correlation of strain index with transmission electron microscopy and dislocation density.

In all tests conducted to date the harder grinding media produced the most efficient grinding (Fig. 32).

CONCLUSIONS

The wear rate of grinding media is sensitive to microstructure, e.g., mechan- ical and electrochemical properties; hardness ratio of ball to mineral, HB:HM; ore mineralogy, such as presence of sulfides; slurry pH and oxygen potential in the mill; ball mix and size.

Several wear mechanisms have been identified as indentation, gouging, plowing, strain-induced corrosion, pitting and spalling, including the forma- tion of adiabatic shear bands under high impact conditions.

Wet grinding efficiency is dependent on media hardness while wear under wet grinding is much higher, in general, than that under dry grinding.

Smaller laboratory mills e.g., 8 inch, tend to overemphasize corrosive wear compared with larger 36-inch laboratory mills.

343

ACKNOWLEDGMENTS

The au thor s express apprec ia t ion for the suppor t of this research to the U.S. Bureau of Mines unde r the Gener ic Minera l Techno logy Cen te r P r o g r a m in C o m m i n u t i o n , the U.S. D e p a r t m e n t of E n e r g y unde r C o n t r a c t DEFC07831D12441 and the Na t iona l Science F o u n d a t i o n under G r a n t CPE8018395. Th i s work is publ i shed wi thou t pr ior approva l by the Bureau of Mines.

REFERENCES

1 J.F. Remark and O.J. Wick, 1976. Corrosion control in ball and rod mills. Int. Corrosion Forum, Houston, TX, March 1976, Paper 121.

2 D.E. Nass, 1974. Steel Grinding Media Used in the United States and Canada. Material for the Mining Industry, Climax Molybdenum, Ann Arbor, MI, p. 173.

3 T.E. Norman and E.R. Hall, 1968. Abrasive Wear of Ferrous Materials in Climax Operations. Evaluation of Wear Testing, ASTM Spec. Tech. Publ., 446.

4 P. Moroz and J. Lorenzetti, 1981. The effects of matrix hardness and microstructure on the wear of steel grinding balls during wet copper ore grinding. Int. Conference on Wear of Ma- terials, San Francisco, Ca.

5 I. Iwasaki, K.A. Natarajan, K. Adam, J.N. Orlich, 1985. Nature of corrosive and abrasive wear in ball mill grinding. Engineering Foundation Conference on Recent Developments in Comminution, December, Hawaii, 1985.

6 R. Perez, 1982. MS Thesis, University of Minnesota. 7 J. Eggert, 1985. MS Thesis, University of Minnesota. 8 A. Gangopadhyay, 1985. PhD Thesis, University of Minnesota. 9 A.K. Gangopadhyay and J.J. Moore, 1985. An assessment of wear mechanisms in grinding

media. J. Miner. Metall. Process., Minerals and Metallurgical Processing, 1985, pp.145-151. 10 J.J. Moore, to be published. 11 R. Perez and J.J. Moore, 1983. The influence of grinding ball composition and wet grinding

conditions on metal wear. In: K.C. Ludema {Editor), Proc. Int. Conf. Wear of Materials, Reston, VA, American Society of Mechanical Engineers, New York, 1983, p. 67.

12 A. Gangopadhyay and J.J. Moore, 1985. The Role of Abrasion and Corrosion in Grinding Media Wear. Wear, 104: 49-64.