Electrochemical aspects of grinding media-mineral interactions in magnetite ore grinding

International Journal o f Mineral Processing, 13 (1984) 53--71 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

53

ELECTROCHEMICAL ASPECTS OF GRINDING MEDIA--MINERAL I N T E R A C T I O N S IN M A G N E T I T E O R E G R I N D I N G

K.A. NATARAJAN and I. IWASAKI Mineral Resources Research Center, University o f Minnesota, 56 East River Road, Minneapolis, MN 55455 (U.S.A.)

(Received November 20, 1982; revised and accepted September 12, 1983)

ABSTRACT Natarajan, K.A. and Iwasaki, I, 1984. Electrochemical aspects of grinding media--mineral interactions in magnetite ore grinding. Int. J. Miner. Process., 13:53--71. Electrochemical aspects of grinding media--mineral interactions in magnetite ore grinding are illustrated from the viewpoint of corrosive wear of grinding balls and changes in the surface properties of the ground minerals. The corrosive wear of mild steel and HCLA steel was found to be small in contact with magnetite, while the combined presence of pyrrhotite and oxygen accelerated the corrosion of the above bail materials. Austenitic stainless steel exhibited passive behavior under all the above conditions. Galvanic coupling of mild steel with magnetite or pyrrhotite resulted in the formation of a surface reaction product consisting of iron hydroxide species on the mineral.

INTRODUCTION Grinding in steel r o d and ball mills is a c o m m o n practice preceding a conc e n t r a t i o n process ( f l o t a t i o n , for example) in the t r e a t m e n t o f m a n y lowgrade ores. N a s c e n t metals as well as mineral surfaces are c o n t i n u a l l y being e x p o s e d in grinding and the e l e c t r o c h e m i c a l i n t e r a c t i o n s b e t w e e n a b r a d e d and u n a b r a d e d areas o f the surface as well as b e t w e e n the grinding m e d i a and the minerals can o c c u r in g r o u n d pulps. S u c h e l e c t r o c h e m i c a l i n t e r a c t i o n s b e t w e e n t h e grinding m e d i a and the minerals in the slurry in ore grinding n o t o n l y accelerate the corrosive wear o f balls b u t also a f f e c t the surface p r o p e r ties and, c o n s e q u e n t l y , the f l o t a t i o n response o f the g r o u n d minerals. M a n y electroactive sulfide and o x i d e minerals exhibit rest p o t e n t i a l s n o b l e r t h a n t h o s e o f grinding balls in mill w a t e r and thus c o u l d initiate the corrosive wear of grinding m e d i a in galvanic c o n t a c t , especially in the presence o f o x y g e n . The relative i m p o r t a n c e o f corrosive wear in wet grinding has n o t been q u a n t i f i e d , b u t B o n d ( 1 9 6 4 ) s p e c u l a t e d t h a t well over half o f the m e d i a wear results f r o m c o r r o s i o n or dissolution f r o m the active nascent metal surfaces c o n t i n u o u s l y b e i n g e x p o s e d in grinding. H o e y et al. ( 1 9 7 5 ) d e m o n strated the role p l a y e d b y corrosive wear t h r o u g h the use o f c o r r o s i o n inhib-

54 itors and r e p o r t e d a reduction in ball wear of up to 50% through corrosion inhibition in the grinding of a nickel-copper sulfide ore in laboratory ball mills. The floatability of p y r r h o t i t e from magnetite ores was found to be influenced by the t y p e of media used in grinding; ores ground in a stainless steel or porcelain mill exhibiting higher floatability than the ones ground in a carbop steel mill (Cline et al., 1974). It is therefore essential to have a clear understanding of the mechanism by which the wear of grinding balls is accelerated an d /or the surface properties of ground minerals are altered through mineral--grinding media interactions before the adverse effects of such interactions may be mitigated. This work was undertaken in order to investigate the electrochemical aspects o f grinding media--mineral interactions in magnetite ore grinding with a view to elucidate (a) the corrosive wear of grinding balls and (b) the changes in surface properties of the ground minerals due to such interactions. The electrochemical behavior of ball materials such as mild steel, HCLA steel and austenitic stainless steel was studied through rest-potential measurements and by monitoring the combination potentials and galvanic currents in contact with magnetite in plant mill water. Since small amounts of pyrrhotite are often present as impurities in many magnetite deposits (Pavlica and Iwasaki, 1982), the effect of introducing p y r r h o t i t e in the magnetite--ballmaterial co mb in a t i on on the electrochemical behavior of the ball material was also investigated. The nature of any surface change in magnetite and p y r r h o t i t e due to contacts with the ball material was also evaluated. EXPERIMENTAL High-purity magnetite samples obtained from Ward's Natural Science Establishment, Inc., were cut to a size of a p p r o x i m a t e l y 0.5 cm 2 and m o u n t e d o n t o a lucite tubing to serve as an electrode. Pyrrhot i t e electrodes o f the same size were also made from mineral samples obtained from the same supplier. Mild steel, HCLA steel and austenitic stainless steel electrodes representing ball materials and of 0.5 cm diameter were made from steel specimens cut from the respective t y p e of grinding balls and mounting o n t o lucite tubings. Electrical contact with the electrodes was established with a drop of mer cu r y to a copper wire. Although mild steel is not used in commercial grinding operations, t hey were used in this study since t he y exhibit active behavior through a wide range of pH with relatively poor abrasion resistance. HCLA steels having good abrasion resistance are used widely in industrial grinding mills. Austenitic stainless steels on the ot her hand exhibit very high corrosion resistance and thus a comparison of the electrochemical data obtained for the three types of steels could bring out the role of corrosive wear as well as of their influence on the surface properties of magnetite and p y r r h o t i t e , under different experimental conditions.

55 Rest potentials for the various ball material and mineral electrodes as well as the combination potentials and galvanic currents developed in different mineral--ball-material couples were measured in a typical grinding mill water (of magnetic taconite) that analyzed 120 ppm chloride and 40 ppm sulfate with a pH of 8.5 and a specific conductance of 775 micromhos per cm. A Saturated Calomel Electrode (SCE) was used as the reference electrode and all the above measurements were carried out using an EG & G Model 350A Corrosion Measurement Console. Cyclic voltammetric curves for the magnetite and mild steel electrodes were traced in a 0.1 M KNO3 solution maintained at a pH of 8.5 through the addition of sodium hydroxide. A Wenking potentiostat in combination with a Beckman Model 9030 function generator was used for this purpose. A rotating electrode assembly specially constructed for this study and attached to a Sargent cone drive stirring motor, as shown in Fig. 1, was used in electrochemical measurements with continually abraded electrode surfaces. The assembly consisted of a rectangular aluminum fixture (A) in which two centrally placed, cylindrical copper shafts (B and C) (12 cm length, 0.5 cm diameter and placed about 2.5 cm apart from each other) could freely rotate by means of a bearing-supported base. The top portion of each copper shaft (leaving 1 cm at the tip for electrical connection and 6 cm towards the b o t t o m for attaching the lucite m o u n t e d electrodes) was enclosed in an aluminum casing (D and E) (5 cm length with about 2 cm diameter) provided with grooves for inserting drive belts. The copper shafts were connected through belts to a double pulley arrangement(F) that was in turn connected to the central rotating shaft of the stirring motor (G). The internal spacing between the aluminum casing and the copper shaft was insulated. Mineral or ball material electrodes premounted onto a lucite tubing (about 8 to 10 cm in length and 0.5 cm diameter) could be scre~ed to the b o t t o m tip of the copper shafts and the electrical connection between the electrode tip and copper shaft made through a mercury contact. The rotating electrodes could be connected to the desired terminals of the corrosion measurement console through contact wires sliding into the upper tip of the copper shaft. The electrodes could be raised or lowered and also the speed of rotation adjusted. Electrochemical measurements under surface abrasion conditions were carried out while both the electrodes were simultaneously abraded against a quartz slurry (--48 +65 mesh at 60% solids), unless otherwise mentioned, at a rotation speed of 250 rpm. Quartz particles are known to be very abrasive contributing to ball wear in ore grinding and this mineral constitutes a major gangue c o m p o n e n t in many magnetite ores. The electrodes were raised above the slurry to study their electrochemical behavior in the absence of surface abrasion. Measurements were also made in the absence of any ore slurry with the rotating electrodes.

56

Fig. 1. Rotating electrode assembly. RESULTS Electrochemical behavior of the ball materials either alone or in contact with magnetite and/or pyrrhotite and conversely, the nature of surface change on the minerals due to ball--mineral interactions in the mill water axe detailed below.

Rest-potential measurements Variation of the potentials as a function of time for the various mineral and ball material electrodes immersed in plant mill water (air exposed) is shown in Fig. 2. Both magnetite and pyrrhotite exhibit relatively noble potentials when compared to the ball materials. Either magnetite or pyrrhotite

57

in contact with any of the ball materials used in this study would thus act as cathodes accelerating the corrosion of grinding balls. Among the ball materials, austenitic stainless steel is less active while both the HCLA and mild steel exhibit more or less equally active behavior as evident by the gradual

-100

£

- 200

~

E -300 (j, o3 hi

o 0 A

Magnetite Pyrrhotite

x

HCLA Steel

0

Mild Steel

-400

Austenltic Stainless Steel

-500~L J~'~X

~-

~ X ~ x

x--

-600 r - ~ I O

~

20

I OI DI 40 Time, rain

D60

Fig. 2. Variation of potentials with time for different mineral and ball material electrodes.

decrease in their rest potentials with time towards more negative values. Magnetite and pyrrhotite indicate steady-state potential values closer to each other in the mill water, possibly due to the presence of a similar type of oxidized layer on both surfaces. The Eh-pH responses of magnetite, pyrrhotite, mild steel, HCLA steel and austenitic stainless steel are plotted in Fig. 3 and it can be seen that the potentials of magnetite and pyrrhotite at higher pH values and especially in the presence of oxygen, approach the experimental water-oxygen line, indicating that their surfaces were coated with an oxide (or hydroxide) layer and the oxygen reduction reaction was controlling the potential irrespective of the electrode material (Natarajan and Iwasaki, 1972). The measured potentials for mild steel, HCLA steel and austenitic stainless steel also approached the experimental water-oxygen line on prolonged aeration with oxygen at higher pH values (beyond about 10). The Potential drift towards the experimental water-oxygen line signifies the presence of a mixed potential mechanism until the surface becomes fully passivated. In air-exposed solutions in the acid pH range, the potentials of mild steel and HCLA steel lay near the equilibrium line for Fe°--Fe 2÷. At

58

higher pH, the potentials drift towards the stability regions for Fe(OH)2 and Fe(OH)3.

Electrochemical behavior of magnetite--ball-material couples Combination potentials as well as galvanic currents measured after coupling the magnetite electrode with mild steel, HCLA steel and austenitic stainless steel electrodes in plant mill water (exposed to air) are given in Fig. 4 as a function of time. The combination potentials for magnetite in contact with mild and HCLA steels were active and the shift of the potential towards more negative values with time resembles the individual behavior of mild steel and HCLA steel as represented in Fig. 2. Galvanic currents in the magn e t i t e - m i l d steel couple were almost four times higher than those in the magnetite--HCLA-steel couple and the galvanic current in both cases showed

No Aeration

02 • •

Magnetite

Pyrrhotite M i t d steel H C L A steel Stainless steel

• 0 [] [] A

II

x •

N2 0

1.0 -I)2-Fe (OH) 3 4- Fe205

0.8

0.6 0.4

0.2 > -

0

-0.2

-0.4

-0.6

-0.8

-1.00

2

4

6

8

10

12

14

pH

Fig. 3. Eh-pH diagram for the Fe-H~O-O 2 system (activity of dissolved species at 10 -3 M).

59

an increasing trend with time attaiiiing a steady-state value after about 20 minutes. The electrochemical behavior of the magnetite--austenitic-stainlesssteel couple appeared different from the above in that there was a gradual increase in the combination potential with time towards less active values along with a decrease in galvanic current to a value a few orders of magnitude lower than that attained in the presence of mild steel or HCLA steel. The passive behavior of austenitic stainless steel is readily apparent from the above observation and the presence of magnetite appeared to afford added protection to the stainless steel. The corrosion rate of austenitic stainless steels was reported to be drastically reduced in contact with a cathodic protector such as magnetite even in sulfuric acid (Tomashov and Chernova, 1967) and such a cathodic protection due to the presence of magnetite would be more effective only for readily passivating metals and alloys. .--O

. . . . .

-O-

2O p" 2OO i

~

Austenitic Stainless Steel X H C L A Steel

!

/ D

(.J (D v

> F

0

(~

0

i~

Mild

16

Steel

Potential ....

Current

ci

".T-

12

no - 2 0 0

._o

f,

__ 4 0 0

~

_

x _

_ _x . . . . . . .

x

x-600

x

~

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. . . . . .

x -

-x--

_

- 4

e

~'--i ....

20 Time,

40 rain

~----I

0 60

Fig. 4. V a r i a t i o n o f c o m b i n a t i o n p o t e n t i a l s a n d galvanic c u r r e n t s w i t h t i m e for magnet i t e - b a l l - m a t e r i a l couples.

Combination potentials and galvanic currents measured for magnetite in contact with either mild steel or HCLA steel under different aeration conditions in the mill water are given in Table I. Measured potentials for the individual magnetite and mild steel electrodes before galvanic contact and soon after breaking the contact are also given. Oxygen aeration decreased the galvanic current by an order of magnitude for the magnetite--mild-steel couple

Nitrogen Air Oxygen

Type of aeration

--110 --110 --170

Magnetite

--430 --460 --570

Mild steel

Potentials soon after polishing; before c o n t a c t , mV (SCE)

--390 --400 --540

--520 --620 --250

3.8 21 2.4

Galvanic current after 20 rain of c o n t a c t I m m e d i a t e l y After 20 min uA on c o n t a c t of c o n t a c t

C o m b i n a t i o n potential m V (SCE)

Magnetite--mild-steel couple

--380 --320 --160

Magnetite

--540 --650 --270

Mild steel

Potentials soon after breaking 20 min contact, m V (SCE)

3 6 1.5

Galvanic c u r r e n t after 20 m i n of contact uA

Magnetite--HCLA-steel couple

Electrochemical behavior of magnetite in c o n t a c t with mild steel and HCLA steel in mill water (stationary electrodes)

TABLE I

05 O

61 in comparison with an air-exposed system. Though the combination potential for the magnetite--mild-steel couple in the mill water after 20 minutes of contact was more negative under nitrogen aeration than that obtained under oxygen aeration, the galvanic currents under both aeration conditions were lower. Galvanic current was the highest in the presence of air, while the formation of a surface film in the presence of oxygen as well as the absence of oxygen decreased the galvanic current. The galvanic currents in the magnetite--HCLA-steel couple were not significantly affected by different modes of aeration unlike the case with the magnetite--mild-steel couple. Interaction between abraded iron and mineral surfaces During ore grinding abraded metallic iron particles or flakes are also generated mainly due to the erosive wear of grinding balls and such metallic particles could attach themselves onto ground mineral surfaces such as magnetite and pyrrhotite either through magnetic interaction or due to physicochemical forces. A close and intimate contact between the mineral and the grinding ball could thus be established in the grinding mills. It has been indicated above that the electrochemical behavior of a magnetite electrode in contact with the ball material (mild steel, for example), becomes more active as evidenced by a shift in the rest potential of the magnetite towards more negative values soon after breaking the contact (see Table I). Even one hour after breaking the galvanic contact with mild steel, the potential for magnetite did not attain its original value (i.e., potential attained before any galvanic contact), indicating the presence of a surface reaction product on magnetite after coupling with the steel. For pyrrhotite electrodes, also, it was observed that the combination potential after coupling with mild steel shifted in the active direction and even after breaking the contact, the potential of pyrrhotite remained in the active range for at least an hour (Adam et al., 1984). The electrochemical behavior of a magnetite electrode with abraded iron particles (from HCLA steel ball) attached to the surface is shown in Figs. 5 and 6. As can be seen from Fig. 5, a freshly polished magnetite electrode exhibited an increase in potential with time, reaching a steady-state value after about 20 minutes. The potential was seen to decrease sharply immediately upon contacting the magnetite electrode with abraded iron particles of a HCLA steel ball. For example, from a steady-state value of about +5 mV (vs SCE) for the magnetite, the potential dropped to - 4 5 0 mV (vs SCE) on contacting with iron particles in the mill water. The potential was seen to increase on tapping the electrode due to detachment of the iron particles, and on repolishing the electrode surface the original value of the potential attainable in the absence of surface contamination could readily be observed. The electrochemical behavior of the magnetite electrode, therefore, resembled t h a t of steel under conditions where the magnetite surface was covered with abraded iron particles. Depending on the extent of such a surface coverage, a mixed potential would be observed. Current-potential curves

62

for magnetite in the presence and absence of surface-attached iron particles are compared with those obtained for mild steel in Fig. 6. A 0.1 M solution of potassium nitrate maintained at a pH of 8.5 was used as an inert electrolyte in these studies. For freshly polished magnetite, two separate peaks (A •w"---Abraded iron attached to surface

-100

> E

- 200

L5 ts.l

-.300

-400

0

I 20

I 40

,\, 60

80

100 lZO 140 160 180 200

T i m e , min

Fig. 5. Electrochemical behavior of magnetite with surface-attached iron particles.

o.8

0.6 0.4

Scan rate 0 . S V / s e c

~

.

.

.

.

.

~\

.

.

.

-----.

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.,,h / the surface

L

/\,\

~ A--_

r

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-0.6 1.8

1.2

0.6

0

POTENTIAL,

-0.6

-1.2

V

Fig. 6. Current-potential curves for magnetite in the presence and absence of surfaceattached iron particles.

63 and D) were observed in the negative p o t e n t i a l range due to the r e d u c t i o n o f Fe 3÷ and Fe 2÷ species in the lattice with t w o peaks (B and C) in the reverse d i r e c t i o n c o r r e s p o n d i n g to a n o d i c reactions. Curves o b t a i n e d with a mild steel e l e c t r o d e had larger a n o d i c and c a t h o d i c peaks which were n o t as well d e f i n e d as t h o s e f o r the magnetite. T h e behavior o f t h e m a g n e t i t e surface covered with iron particles closely r e s e m b l e d t h a t o f the mild steel electrode.

Influence of pyrrhotite T h e e f f e c t o f i n t r o d u c i n g a third e l e c t r o d e , n a m e l y , p y r r h o t i t e on the m a g n i t u d e o f t h e c o m b i n a t i o n p o t e n t i a l s and galvanic currents generated in d i f f e r e n t magnetite--ball-material couples i m m e r s e d in t h e mill w a t e r is illustrated in Table II. Galvanic currents in the p y r r h o t i t e - - m i l d - s t e e l or HCLA-steel couples were t w o to t h r e e times higher t h a n t h o s e in the magn e t i t e - m i l d - s t e e l or HCLA-steel couples. Galvanic currents in the multimineral c o m b i n a t i o n , m a g n e t i t e - - p y r r h o t i t e - - m i l d steel o r H C L A steel were still higher and the presence o f o x y g e n f u r t h e r e n h a n c e d t h e c u r r e n t . If the p y r r h o t i t e e l e c t r o d e was r e m o v e d f r o m the c o m b i n a t i o n , t h e initial value o f tbe galvanic c u r r e n t for the m a g n e t i t e - - b a l l - m a t e r i a l c o u p l e could be readily observed. T h e c o m b i n a t i o n p o t e n t i a l s were relatively n o b l e with galvanic c u r r e n t s t w o to t h r e e orders o f m a g n i t u d e lower for c o m b i n a t i o n s involving austenitic stainless steel. T h e passive behavior o f austenitic stainless steel is readily evident and the role o f o x y g e n and o f the presence o f a c a t h o d i c prot e c t o r such as m a g n e t i t e in p r o t e c t i n g stainless steel could also be seen as p o i n t e d o u t already.

TABLE II Influence of pyrrhotite on the electrochemical behavior of magnetite--ball-material cornbinations in mill water (stationary electrodes} Type of electrode combinations

Combination potential mV (SCE)

Galvanic current uA

1. Mild steel--magnetite 2. Mild steel--pyrrhotite 3. Mild steel--magnetite--pyrrhotite 4. Oxygen aeration in (3) 5. HCLA steel--magnetite 6. HCLA steel--pyrrhotite 7. HCLA steel--magnetite--pyrrhotite 8. Oxygen aeration in (7) 9. Austenitic stainless steel--magnetite 10. Austenitic stainless steel--magnetite--pyrrhotite 11. Oxygen aeration in (10)

--620 --600 --580 --390 --500 --430 --400 --350

10--11 25--30 28--31 42--60 5--7 15--16 18--20 20--22

--150 --90 --60

0.04 0.09-*0.05 0.02

64

Effect of electrode/solution movement and surface abrasion So far, the results o f electrochemical measurements with stationary electrodes in a stagnant electrolyte were illustrated. The potentials and galvanic currents were seen to be influenced by the relative m o t i o n o f the electrodes with respect to the electrolyte or vice versa and also by surface abrasion o f the electrodes. The effect o f stirring o f the electrolyte o n the c o m b i n a t i o n potential and galvanic current for a magnetite--mild-steel c o u p l e is s h o w n in Fig. 7. The potential shifted in the nobler direction with a corresponding decrease in current on stirring the solution. A similar effect was n o t i c e d o n rotating the electrode. In a grinding mill, ore particles dispersed in a slurry and the steel grinding media are in c o n t i n u o u s m o t i o n . At the same time, fresh mineral surfaces are generated due to size reduction while the ball surfaces also undergo differential abrasion. To simulate practical grinding c o n d i t i o n s , therefore, a few parallel electrochemical measurements were made while the electrode surfaces were being c o n t i n u a l l y renewed by abrasion. 22

//----q~

200

Stirring

/

CD O3 3> E

- -

Potential

....

Current <~

3_

0

14

c o°.

I

-200

I

-

>

I

m

I 23

Eo - 4 0 0

600 NO stirring

• "Stirring 2O

40 Time,

60

min

Fig. 7. Effect of stirring on the electrochemical behavior of a magnetite--mild-steel couple. Corrosion currents f l o w b e t w e e n abraded and unabraded regions on the same metal surface immersed in mill water with abraded areas serving as a n o d e s relative to unabraded cathodic sites. The possibility o f f o r m a t i o n o f differential abrasion cells of the type:

65

(--) abraded region in ball I electrolyte I unabraded ball surface (+) could be visualized. The galvanic current generated due to differential abrasion in mild steel electrodes is illustrated in Fig. 8. In these tests, two rotating mild steel electrodes were contacted in the mill water in the presence of oxygen with one of them continuously being abraded against a quartz slurry while the other electrode was kept above the slurry, immersed in the supernatant. The mild steel electrode, whose electrochemical behavior was monitored, was cathodic to the other mild steel electrode to begin with, in the absence of surface abrasion, as shown in the figure. On abrasion against the quartz slurry, the indicator electrode assumed anodic behavior with a corresponding shift in the galvanic current.

-

-

....

-20C

Potential

Current

o

.................

--~

0

i

o9

<~

E -24C

c (.9

nO -280 g o

L) c

Abrasionagainst

~ J

fX\

-320

Quortzite~ y

E

o (D

-360

-400

L 0

I 20

L

I 40

~

I 60

Time,

J

I 80

L

I 100

i

120

min

Fig. 8. E l e c t r o c h e m i c a l b e h a v i o r o f a d i f f e r e n t i a l a b r a s i o n cell.

The effect of electrode rotation and surface abrasion on the magnitude of combination potentials and galvanic currents measured with different magnetite-ball-material couples in the mill water is shown in Table III. The galvanic currents monitored with rotating electrode couples were smaller than those attained with stationary electrodes. In the absence of surface abrasion, the magnetite--austenitic-stainless-steel couple exhibits relatively noble behavior under all aeration conditions with the galvanic currents almost an order of magnitude lower than those obtained for the mild-steel--magnetite and HCLA-steel--magnetite couples. The combination potentials obtained with mild steel and HCLA steel electrodes in combination with magnetite are more or less the same. However, in the absence of abrasion, the presence of oxygen was found to decrease the galvanic current for the mild-steel--

66 magnetite couple more than that for the HCLA-steel--magnetite couple due to a g r e a t e r surface passivation f o r mild steels. T h e change o f c o m b i n a t i o n p o t e n t i a l s a n d galvanic c u r r e n t s t o w a r d s m o r e active values on a b r a s i o n c o r r e s p o n d s to surface a c t i v a t i o n o f t h e various ball materials. B e y o n d a critical v e l o c i t y o f r o t a t i o n in t h e p r e s e n c e o f c o n s t a n t a b r a s i o n , t h e passive l a y e r will n o t be able t o r e n e w itself and thus t h e m a t e r i a l will be susceptible to c o r r o s i o n c o n t i n u o u s l y w i t h o x y g e n r e d u c t i o n at t h e c a t h o d e surface ( m a g n e t i t e ) a n d iron d i s s o l u t i o n f r o m t h e ball material. T h e p r e s e n c e o f o x y g e n u n d e r a b r a d e d c o n d i t i o n s c o n t r i b u t e s t o a larger c u r r e n t t h a n in deo x y g e n a t e d solutions, a n d such a c o n t r i b u t i o n f r o m o x y g e n t o w a r d s galvanic c o r r o s i o n was m o r e evident in t h e m a g n e t i t e - - m i l d - s t e e l couple. F o r t h e m a g n e t i t e - - H C L A - s t e e l couple, galvanic c u r r e n t values u n d e r d i f f e r e n t m o d e s o f a e r a t i o n while being c o n t i n u a l l y a b r a d e d did n o t vary m u c h . TABLE III Electrochemical data for the magnetite--ball-material couples in mill water in the presence and absence of surface abrasion (rotating electrodes)

Type of electrode combinations and aeration

No abrasion Steady-state value after 1 hr Combination potential, mV vs SCE

A. Magnetite--mild steel Nitrogen --570

Air Oxygen

--450 --330

B. Magnetite--HCLA steel Nitrogen --510

Air Oxygen

--400 --300

Galvanic current, uA

Combination potential, mV vs SCE

Galvanic current, uA

0.5 0.3 0.05

--660 --560 --500

0.8 1.5 0.5--1.5

0.9 0.4 0.25

--600 --480 --420

0.9--1.0 0.9--1.2 0.9--1.2

--700 --380 --350

1.8--2.0 1.5--2.0 1.0--2.0

C. Magnetite-austenitic stainless steel Nitrogen --174 0.09 Air --110 0.07

Oxygen

--92

Abrasion (against - 4 8 +65 mesh quartz slurry at 60% solids) Steady-state value after 15--20 rain

0.04

T h e b e h a v i o r o f t h e p y r r h o t i t e - - b a l l - m a t e r i a l c o u p l e s w i t h e l e c t r o d e rotat i o n a n d a b r a s i o n was similar to t h e a b o v e o b s e r v a t i o n s , e x c e p t for the f a c t t h a t t h e galvanic c u r r e n t s f o r p y r r h o t i t e in c o n t a c t w i t h mild steel and H C L A steel were a l m o s t 1 0 - - 2 0 t i m e s higher, w i t h t h e p r e s e n c e o f o x y g e n f u r t h e r increasing t h e galvanic c u r r e n t . Galvanic c u r r e n t s m e a s u r e d in t h e p y r r h o t i t e - - a u s t e n i t i c - s t a i n l e s s - s t e e l c o u p l e u n d e r similar c o n d i t i o n s were a b o u t t w o orders o f m a g n i t u d e l o w e r t h a n t h o s e o b t a i n e d w i t h mild steel and H C L A steel a n d t h e p r e s e n c e o f o x y g e n d e c r e a s e d t h e c u r r e n t f u r t h e r .

67 DISCUSSIONS

Ball wear data gathered from marked ball grinding tests for a magnetite ore in the presence and absence of pyrrhotite under different aeration conditions using mild steel, HCLA steel and austenitic stainless steel balls are discussed elsewhere (Natarajan et al., 1984) and relevant information from such grinding results will be used in this section to interpret the electrochemical data illustrated above. Corrosion behavior o f ball materials in contact with magnetite Galvanic currents generated in magnetite--ball-material couples immersed in plant mill water were small. In the absence of surface abrasion, oxygen aeration tends to decrease the current due to surface passivation of the steels, with couples involving austenitic stainless steel indicating the maxim u m decrease in current. Even under conditions where the electrodes were continually being abraded against a quartz slurry, the presence of oxygen produced only a small increase in the galvanic current in any of the magnetite-ball-material combinations. Results from laboratory grinding tests on a magnetic taconite ore sample (with magnetite as the only principal electroactive mineral) indicated that corrosive wear of mild steel and HCLA steel balls was relatively small and the presence of oxygen increased the ball wear by about 10--15%. With austenitic stainless steel balls, the ball wear was not at all affected by a change in the mode of aeration. In a ball mill grinding, the abrasive effect may not be such as to maintain continuously the same active surfaces and it would be more appropriate to consider differentially abraded ball surfaces with the possibility that the active and passive regions interchange periodically depending on the rate at which surface films are repaired and destroyed. With austenitic stainless steel balls, the rate of surface passivation inside the grinding mill may have been faster and as such enhanced galvanic currents attainable on continuous surface renewal (as shown in Table III) may not be realized in practice. The role of differential abrasion of the ball surfaces in initiating ball corrosion has been brought out already. Commercial grinding mills are aerated and the corrosion mechanism for the grinding balls involves oxygen reduction at the mineral (magnetite, for example) or ball surface itself {differentially abraded) and dissolution of iron from the anodic sites on the balls. Role o f pyrrhotite and multi-electrode combinations in ball wear The additional presence of pyrrhotite significantly enhanced the galvanic current attained in a magnetite--mild-steel or HCLA-steel couple, especially in the presence of oxygen. However, with austenitic stainless steel, no such increase in galvanic current could be observed. In fact, the additional presence of magnetite and oxygen along with a pyrrhotite--austenitic-stainless-

68 steel couple in the mill water decreased the galvanic current indicating an enhancement of the passivity for the stainless steel with added cathodic protection from magnetite and oxygen. Results from laboratory grinding tests with a magnetite ore containing 10% by weight of pyrrhotite indicated a 46% and 185% increase in the corrosive wear of mild steel and HCLA steel balls, respectively, in the presence of oxygen in comparison with the ball wear obtained for the magnetite ore alone. There was a slight decrease in ball wear (by about 6--7%) for the austenitic stainless steel balls in the grinding of the same ore mix in the presence of oxygen. There appears to be a good correlation between the electrochemical behavior of the mineral--ball-material couples and the corrosive wear data obtained from laboratory grinding tests. The role of pyrrhotite in increasing the corrosive wear of mild steel and HCLA steel balls in magnetite ore grinding needs to be further examined in the light of a multi-electrode system. The following electrochemical aspects of a multi-electrode system are relevant (Tomashov, 1966): (a) An electrode with the most positive initial potential will always be c a t h o d i c ; t h e most negative, anodic. (b) The potential of an intermediate electrode and hence its electrochemical behavior depends on its proximity to the electrodes in an already existing couple. If placed near a principal cathode, it would tend to behave as an anode; on the contrary, the intermediate electrode would behave as a cathode, if placed near a more anodic principal electrode. With reference to the present study, the minerals and ball materials having freshly abraded surfaces could be arranged in the following order in a galvanic series in mill water: Magnetite Cathodic Pyrrhotite I Austenitic stainless steel HCLA steel Mild steel Anodic In a magnetite--pyrrhotite--mild-steel combination, the magnetite would be the cathode with the mild steel as anode; pyrrhotite serving as an intermediate electrode. The pyrrhotite could behave anodically when in close proximity with magnetite. Corrosive wear of mild steel could be expected to increase under the above conditions due to the following reasons: (a) Increase in cathodic surface area, since both pyrrhotite and magnetite are cathodic to mild steel. (b) Dissolution of pyrrhotite in contact with magnetite in the presence of oxygen leading to the generation of dissolved sulfide species in solution. Soluble sulfide ions in the slurry could cause the breakdown of existing passive oxide films on mild steel or HCLA steel balls. Extensive pitting of mild steels in sulfide containing water has been observed even at alkaline pH (Gupta, 1981). Scanning electron micrographs of mild steel and HCLA steel ball surfaces after grinding the magnetite ore containing pyrrhotite revealed extensive pitting (Natarajan et al., 1984}.

69 On the o th er hand, the p y r r h o t i t e in close p r o x i m i t y to the steel ball materials could also behave as a cathGde, then too, accelerating the corrosive wear of the ball. In a grinding mill, establishment and breaking of two- or multi-electrode contacts could occur f r om time to time. Also, the electrochemical behavior o f the minerals as well as of the ball materials with respect to their relative active or cathodic nature would be different before, after, and during mutual contacts. F o r example, rest-potential measurements with mild steel, pyrrhotite and magnetite electrodes soon after breaking a three-electrode cont act in the mill water indicated that the p y r r h o t i t e had become more active than before contact, with a negative potential closer to that of mild steel. As can be seen from Table I, the rest potential of magnetite deviated in the negative direction due to coupling with mild steel. Thus, it is essential to understand the electrochemical behavior of the various minerals and the ball materials, n o t only before, but also after and during grinding-media--mineral contacts inside a grinding mill in order to gain an insight into the electrochemistry of ore grinding.

Effect o f grinding-media--mineral interaction on mineral surfaces Magnetite surfaces after contacting with mild steel in the mill water were f o un d to have a surface film composed of FeOOH when examined through X-ray photo-electron spectroscopy. Similarly, a p y r r h o t i t e surface revealed the presence o f a reaction pr oduct layer that contained sulfate and oxygen, after contacting with mild steel (Adam et al., 1984). The flotation response o f p y r r h o t i t e was also f ound to be adversely affected if previously cont act ed with mild steel and the reason for such a behavior was attributed to the electrochemical interaction between p y r r h o t i t e and the mild steel leading to the f o r m a t i o n o f a surface film on pyrrhotite. In the grinding of a magnetite ore containing p y r r h o t i t e in a carbon steel mill, the following electrochemical reactions could take place under conditions where the p y r r h o t i t e may behave either as an anode or a cathode, depending on its p r o x i m i t y to the magnetite or ball material as brought out above. (a) On ball surfaces Fe = Fe 2+ + 2e (anodic) (1) 02 + 2H20 + 4e = 4OH- (cathodic) (2) (b) On magnetite surfaces reaction (2) as above (c) On pyrrhotite surfaces (1) Anodic behavior with respect to magnetite FeS = Fe 2÷ + S O+ 2e (3) FeS + 4H20 = Fe 2÷ + SO~- + 8H + + 8e (4) (2) Cathodic behavior with respect to ball material Reaction (2) as above (3) Dissociation FeS = Fe 2÷ + S 2(5)

70

Ferrous ion released by pyrrhotite due to anodic oxidation can form iron hydroxide species of the type Fe20(OH)3, FeOOH and/or Fe(OH)SO4 which can coat the mineral surface. A similar surface film on pyrrhotite surfaces serving as cathodes can also be envisioned due to reaction of ferrous ion released by pyrrhotite dissociation with the h y d r o x y l ion generated by the reduction reaction of oxygen. Grinding the same ore in an austenitic stainless steel mill may not similarly affect the nature of the pyrrhotite surface since the electrochemical reactivity between grinding media and the minerals in the slurry can be expected to be only minimal due to the passive nature of stainless steel. SUMMARY

Electrochemical interactions between grinding media and minerals in a grinding mill influence the corrosive wear of balls and the surface properties of the ground mineral. There exists a good correlation between the electrochemical data obtained for various mineral--ball-material combinations and the ball wear data obtained through marked ball grinding tests on a magnetite ore containing pyrrhotite. In the absence of pyrrhotite, the corrosive wear of mild steel and HCLA steel balls in contact with magnetite in the presence of oxygen was small. The addition of pyrrhotite accelerates the corrosive wear of the above ball materials, especially in the presence of oxygen. Austenitic stainless steel was found to be corrosion resistant even in the presence of a sulfide mineral such as pyrrhotite and oxygen. The electrochemical behavior of magnetite and pyrrhotite was also affected due to coupling with ball materials in a mill slurry. A surface reaction product consisting of iron hydroxide species was found to be formed on the mineral surfaces due to such interactions which could affect their surface properties. ACKNOWLEDGEMENT

The financial support provided for this project by the National Science Foundation under Grant CPE 8018395 is gratefully acknowledged.

REFERENCES Adam, K., Natarajan, K.A. and Iwasaki, I., 1984. Grinding media wear and its effect on the flotation of sulfide minerals. Int. J. Miner. Process., 12: 39--54. Bond, F.C., 1964. Metal wear in crushing and grinding. Chem. Eng. Progress, 60: 90. Cline, W.A., Connolly, L.P. and Grandy, G.A., 1974. Marcona pyrrhotite flotation from magnetite. SME/AIME Preprint 74-B-347. Gupta, D.V.S., 1981. Corrosion behavior of 1040 carbon steel. Corrosion, 37: 611. Hoey, G.R., Dingley, W. and Freeman, C., 1975. Corrosion inhibitors reduce ball wear in grinding sulfide ore. C.I.M. Bull., 68: 120. Natarajan, K.A. and Iwasaki, I., 1972. Eh-pH response of noble metal and sulfide mineral electrodes. Trans. SME/AIME, 252: 437.

71 Natarajan, K.A., Riemer, S.C. and Iwasaki, I., 1983. Corrosive and erosive weax in magnetic taconite grinding. Preprint, SME/AIME Annu. Tech. Meeting, Atlanta, Georgia. Natarajan, K.A., Riemer, S.C. and Iwasaki, I., 1984. Influence of pyrrhotite on the corrosive wear of grinding balls in magnetite ore grinding. Int. J. Miner. Process., 13: 73--81. Pavlica, J.J. and Iwasaki, I., 1982. Electrochemical and magnetic interactions in pyrrhotite flotation. SME/AIME Preprint 82-2. Tomashov, N.D., 1966. Theory of Corrosion and Protection of Metals. Macmillan, New York, N.Y., 672 pp. Tomashov, N.D. and Chernova, G.P., 1967. Passivity and Protection of Metals against Corrosion. Plenum Press, New York, N.Y., 208 pp.