Oxygen reduction on sulphide minerals

Electroanalytical Chemistry and Intet][b.cial Electrochemistry, 60 (1975) 151 162

151

i~:~ Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

OXYGEN REDUCTION ON SULPHIDE MINERALS PART I. KINETICS AND MECHANISM AT ROTATED PYRITE ELECTRODES

T. B1EGLER, D. A. J. R A N D and R. W O O D S

CSIRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne, Victoria 3207 (Australia) Received 12th December 1974)

The electronic conductivity of sulphide minerals causes them to participate m coupled charge-transfer processes analogous to those involved in the corrosion of metals in contact with electrolyte solutions. Such electrochemical processes are important in the weathering of sulphides in nature 1-3 and at several stages in the winning of metals from sulphide ores, e.g. oxidation during mining, storage and transport, concentration by froth flotation4 7 and metal extraction by a variety of leaching techniques s. Electrochemical studies of these systems have generally placed emphasis on understanding the mechanisms of the anodic processes involved, such as phase changes of sulphides and metal dissolution in leaching 8 and collector adsorption and oxidation reactions in flotation9. The reduction of dissolved molecular oxygen is the most important cathodic process in these systems. Its kinetics, as is usual with corrosion systems, can be examined by electrochemical techniques, but this type of study has received only passing interest to date. In their investigation of the galena-xanthate-oxygen flotation system by voltammetric techniques, Tolun and Kitchener 5 concluded that galena was "comparatively inefficient as an oxygen-reduction catalyst". By comparing the cathodic behaviour when helium, air or oxygen was bubbled past a stationary pyrite electrode, Peters and Majima ~° derived polarization curves for oxygen reduction over a range of temperatures which showed a mass-transfer controlled limiting current. These authors were mainly concerned with the oxidation and reduction reactions of pyrite itself and their reported data do not allow proper analysis of the oxygen reduction kinetics. In order to obtain detailed information on the characteristics of oxygen reduction on sulphide minerals, we have studied this reaction using the rotating electrode technique. In this communication, we describe the preparation of suitable electrodes and report kinetic and mechanistic data for the oxygen reduction reaction on pyrite surfaces. EXPERIMENTAL

TWQ types of rotating electrode systems were used in this work, a Beckman variable speed assembly and a laboratory-constructed system. The latter will be described in detail in a later paper.

152

1. BIEGLER. D. A. J. RAND. R. WOODS

Electrodes were cut from massive natural pyrite specimens selected to be free. so far as possible, from inclusions, cracks and voids. The cut samples were approximately cubic with 3 5 mm edges. The next steps in electrode manufacture depended slightly on the rotator used. but in both cases the first step was to attach a tinned copper wire to the pyrite by means of conducting epoxy cement (Epo-Tek 410E. Epoxy Technology Inc.. Watertown. Mass., U.S.A.). For the Beckman system, one end of a length of brass rod ( - 3 cm long) was drilled and tapped to fit the threaded rotating spindle and the electrode contact wire was soldered to the other end. Brass rod and pyrite were embedded in epoxy resin (Araldite D) and, after curing, the cast was machined to form a cylinder of about 1 cm diameter with the working face of the electrode exposed. Electrode assemblies for use with the laboratory-constructed rotator were machined to fit a Teflon holder which was attached to the rotating shaft. Electrode surfaces were prepared on successively finer grades of silicon carbide paper down to 600 grade and. where required, polished with 0.1 /an Linde B alumina (slurried with water) using a Syntron vibratory polisher. Final polishing and electrode surface renewal between runs were carried out by hand using an aqueous shirry of 0.05 Hm alumina on Buehler "Microcloth". Most of the experiments were carried out with an electrode prepared from a pyrite specimen having n-type metallic conductivity. Its exposed area was 0.17 cm 2 and its resistance 1.1 ohm. A second specimen used (area 0.25 cm 2. resistance 15 ohm) had p-type semiconductivity. The three-compartment glass cell was designed to allow the rotating electrode to be positioned close to a centrally located Luggin probe. Reference and counter electrodes were isolated by glass sinters. A saturated calomel reference electrode (SCE) was generally used and. when the cell contained perchlorate, an intermediate compartment containing 1 M sodium chloride was added in order to avoid liquid junction potential instability due to precipitation of potassium perchlorate. Gold wire was selected for the counter electrode material rather than platinum so that any uncertainty arising from the possibility of platinum dissolution and redeposition on the cathode l~ was eliminated. After a set of measurements with each solution. the reversible hydrogen potential in that solution was determined using a platinum hydrogen electrode. Solutions were prepared from reagent grade chemicals and doubly distilled water. The observations that (a) sweep and steady-state measurements were in good agreement, (b) currents at constant potential did not change significantly over the time scale of an experiment, and (c) results were reproducible with different batches of reagents, all suggested that the purity of these solutions was sufficient to provide reliable results. It appears that this system is less demanding of solution purity than is, for example, oxygen reduction on noble metal electrocatalysts. Potentiostatic experiments were performed with a Wenking 68TS3 potentiostat programmed with a laboratory-constructed linear sweep generator. Voltammograms were recorded on a Hewlett Packard 7035A XY recorder. Potentials are quoted with respect to the SCE unless otherwise stated. Nitrogen was used to purge oxygen fi-om sohltions. Experiments were carried out at room temperature (20_+ 2 C).

153

OXYGEN REDUCTION ON SULPHIDE MINERALS. I RESULTS AND DISCUSSION

The following data were obtained with the pyrite specimen having n-type metallic conductivity. The p-type pyrite gave essentially the same behaviour. The influence of electrical properties on the electrochemistry of the pyrite~oxygen system will be considered in detail in a later paper. General behaviour in acid solutions

Figures 1 and 2 show cyclic voltammograms at various rotation speeds (W) A

I0

~

IOOHz B

~ G

e4

0 i

I

i

__I

+0?

i

I

i

I

-02

0

i

-04

.OTENT,AL/ VOLT ( . 5 CE) Fig. 1. Cyclic voltammograms for (A) rough and (B) polished pyrite in Oz-saturated 1 M HCIO 4 at the rotation speeds indicated. Sweep speed 20 mV s -1. Curves for N 2 are at 20 Hz. Where hysteresis is appreciable, sweep direction indicated by arrow. A 125 B

/

'E I00 ~075 ~a

o

0

~

I

I

+0'2

i

l

0

i

1

-OZ

i

I

I

-04

POTENTIAL/VOLT(VS 3 CE) Fig. 2. Cyclic voltammograms at low rotation speed (0.5 Hz) on (A) rough and (B) polished pyrite in Oz-saturated (upper curves) and Oz-free (lower curves) 1 M HCIO4. Sweep speed 20 mV s ~.

T. BIEGI.ER. D. A. J. RAND, R. ~OODS

154

for a pyrite electrode (roughly ground and polished) in oxygen-free and oxygensaturated (1 atm) 1 M perchloric acid. In the absence of oxygen, the current in the potential range +0.4 to - 0 . 4 V (vs. SCE) is small and this region can be considered the working range of the electrode. At the anodic extreme of the range, pyrite oxidizes to ferric and sulphate ions 1°, while at the cathodic end both the evolution of hydrogen and the reduction of pyrite to ferrous ion and hydrogen sulphide occur t°. Because of the higher surface area. these processes are apparent earlier with the rough surface and limit the working range of the electrode. On curves recorded at high sensitivity, especially for the rougher surface (Fig. 2, curves A), several small peaks become apparent. These indicate that surface processes are taking place in the working potential range, for example, a series of surface compositional changes corresponding to the various possible phases indicated by the Fe S phase diagram 12. The general features of the oxygen reduction voltammogram depend on the surface condition and the rotation speed. Though a single voltammetric wave is usually found, it never has a simple S-shape: in the case of low rotation speeds there is a suggestion of two separate steps (Fig. 2). The oxygen wave starts at more positive potentials and limiting currents are better defined on the rough than on the polished surface. On both types of surface, lowering the rotation speed favours the attainment of a limiting current plateau. 12 t

A

[ i 10[-

o

<

// /

///

2

4

G

8

/

I

+

10

Fig. 3. Dependenceof current for oxygen reduction on rotation speed (14") in (1,2) O2-saturated and (3) air-saturated l M HCIO~. (1) Rough pyrite, current at -0.4 V: (2. 3) polished pyrite, current at -0.5 V. Broken lines give extrapolation of linear section at lob 14'. The maximum currents iu reached before background discharge, corrected for base currents in absence of oxygen, are plotted as a function of W ~' in Fig. 3, curves 1, 2. At low W the plots coincide and are linear, indicating pure mass transfer control, and the slope is 1.21 mA cm-2 s ~. As W increases, the deviations from linearity are greater for the polished than for the rough surface. The theory for currents at a rotating disc electrode should be applicable to this system despite the rectangular geometry of the pyrite surface. According to the form of the Levich equation used by Gregory and Riddiford ~s the limiting diffusion

O X Y G E N R E D U C T I O N O N S U L P H I D E MINERALS. 1

155

current it> is given by 0.554 n F [ O ~ ] D ~ -~(27tW) * mA cm -2 iD = 0.8934 + 0.316(D/v) °" 36

(1)

where [03] is the bulk oxygen concentration, D is the oxygen diffusion coefficient and v the kinematic viscosity of the solution. Taking for the various parameters the values* [ o b ] = 1.21 × 10 -3 moll 1 v=0.941 x 10 - 2 c m 2 s - 1 and O = 1.93 × 10 -5 cm 2 s 1, we obtain i~:

0.298 n W ~ mA cm -2

--

Thus, the experimental slope gives a value of n very close to 4, showing that in the limiting current region the reaction is O 2 + 4 H + +4e-~2 H20

(2)

Figure 1 indicates the absence of mass transfer control at the foot of the oxygen wave, the current being independent of rotation speed. In this region, slow sweep (2-20 mV s-1) and point-by-point measurements of current are in reasonable agreement (Fig. 4), showing that electrode deactivation (e.9. by impurities) is not a serious problem during the time scale of the experiments. Hyste.resis is negligible for cyclic voltammograms in this potential region. Potential vs. log i plots are linear over about two orders of magnitude of current, and the value of c~E/01og i (referred to henceforth as the Tafel slope) is -(127_+ 3) mV. Extrapolation to the reversible oxygen potential (1.229 V vs. the experimentally determined reversible hydrogen potential) yields an exchange current of 9 × 10 -12 A cm -2. \ \ \ \

OZ

\~ \

,~0"1

0"001

0"01

CURRENT/mA

01

Fig. 4. Tafel plots for polished pyrite in O2-saturated l M HCIO4 at W = 5 0 Hz. (1) Steady-state; slope - [25 mV; (2) sweep, 10 mV s-~; slope - 1 3 0 mV.

* Most of the relevant figures for oxygen in 1 M perchloric acid at 20" are not directly available in the literature, and these values are interpolated or estimated from viscosity, density and solubility data in the International Critical Tables. The diffusion coefficient taken is that in 0.025 M sulphuric acid 14.

156

T. BIEGLER. D, A. J. R4ND. R. W O O I ) S

Activation controlled currents for the rough surface are about eight times greater than for smooth pyrite, a result consistent with the idea that the increased activity of the unpolished surface is due to its higher real area rather than to the presence of intrinsically more active sites. Some hysteresis is evident on certain of the curves in Fig. 1 and appeared on other voltammograms during the course of this work. Factors such as nature of the electrolyte, solution pH. cathodic limit of the sweep, electrode rotation speed and type of pyrite specimen used had some effect on both the nature (higher or lower activity on positive-going sweeps, presence of a crossover point in a cycle) and the extent of the hysteresis, but no particular pattern emerged. It is likely that this type of behaviour is at least partly due to slow reactions of the pyrite surface causing subtle changes in surface stoichiometry and activity. The behaviour in 1 M sulphuric acid is similar to that described above for 1 M perchloric acid. The Tafel slope is - ( 1 2 9 + 3 ) mV and exchange current 1.0× 10 ~ A c m - 2 both values very close to those found in 1 M perchloric acid. In 1 M hydrochloric acid the ~aves are more drawn out and limiting currents less well-defined than in the other acids. E-tog i plots are curved, with a slope tending towards - 1 3 0 mV at the low current limit. This section of the plot gives an exchange current of 4 × 10 ~= A c m -2

E{]~ects of pH Voltammograms in oxygen-saturated solutions of various pH values were determined for smooth pyrite, and the results are summarized in Table 1. The main observations are as follows. T h e wave shape changes noticeably with pH (Fig. 5). At pH 2. splitting into two waves is discernible and is still clearer at pH 4.65 where the two waves are about equal in height. At higher pH values the split becomes less evident. TABLE I KINETICS O F R E D U C T I O N O F O X Y G E N (SATD.) IN VARIOUS ELECTROLYTES

Solution

pH

ii, W "-" 2

,,,.4 ,',n

i (E

.¢

)

~~ li,q ~

E at 10 /L4

V r,., seE)

-Era, ?

V (,.~ see

ml

1 1 1 1

M H2SO 4 M HC104 M HCI M NaClO4 +0.01 M HCIO 4 1 M acetate buffer 0.1 M phosphate buffer 0.05 M Na2B40. 0.l M NaCIO., +0.011 M NaOH

0.96 1.21 1.05

130 127 130

0.10 0.09 0.04

0.251 0.266 0.250

2.0 4.65

0.97 0.97

114 85

0.08 0.04

0.359 0.516

6.61 9.06

1.05 1.18

85 70

0.01 -0.03

0.632 0.774

1.30

67

--0.13

0.954

-

12.1

" Slope of linear portion of iv t's. W ~ plot at 1o~ rotation speeds W. b Reversible hydrogen potential.

157

OXYGEN REDUCTION ON S U L P H I D E MINERALS. 1

Increasing the pH extends the working range of pyrite to more cathodic potentials because of the pH-dependence of the hydrogen evolution and pyrite reduction reactions; the shift in cathodic limit parallels the shift in the reversible hydrogen potential. On the other hand, the foot of the oxygen wave, as indicated in Table 1 by the potential at which the current reached 10/xA, moves much less with pH, though there is some indication of greater pH-dependence in alkaline solution. The dependence of maximum current i,i on rotation speed is similar at all pH values to that shown in Fig. 3. The slope of the linear segment at low rotation speed (Table 1) shows no systematic dependence on electrolyte composition and is probably affected only by changes in diffusion coefficient, kinematic viscosity and oxygen solubility; no change in the number of electrons involved is indicated. As the pH increases, the increased working range of the electrode allows better development of the limiting current plateau at high W. but the i,T vs. W ~ plots still deviate significantly from linearity at high rotation speed. Current/voltage curves all show an activation-controlled region at the foot of the wave, where the current is independent of rotation speed, and the Tafel slopes decrease with pH, from around - 1 3 0 mV in molar acid solutions to - 6 7 mV at pH 12 (Table 1).

~0 z

0Z

0

-0 Z

04

06

02

0

02 -04 -O'G -08

p H 9I

F-. 4

E

0

-OZ -04

-06 -0

0

-OZ -0-4 -0 G -08

-OZ -04 -O'G -08 POTENrtAL/VOLT(re SCE~

-I'0

-tO

Fig. 5. Cyclic voltammograms on polished pyrite in O2-saturated solutions of pH values indicated. Solution compositions as in Table 1. W = 20 Hz.

158

T. BIEGLER. D. A. J. RAND. R. W O O D S

Order of reaction Values of ixt in air-saturated 1 M perchloric acid are included in Fig. 3 (curve 3). The linear portion at low W has a slope of 0.228 mA cm z s~ compared with 1.21 mA c m - 2 s ~ in oxygen-saturated solution. The ratio of these slopes is 0.19, a value close to the partial pressure of oxygen in air. This behaviour is consistent with the interpretation of the slope as corresponding to pure mass transfer control. In the activation controlled region, the Tafel slope is - 1 1 8 mV and the currents are between 0.20 and 0.28 of the current at the same potential in oxygensaturated solution. These values indicate that the reduction rate is first order in oxygen concentration. Detection off soluble intermediate Formation of a stable intermediate, presumably hydrogen peroxide, was suggested by the appearance, under certain conditions, of two waves (see above). The involvement of peroxide as an intermediate in acid solution was determined by the following experiments carried out with polished pyrite in oxygen-saturated 1 M sulphuric acid. Two sets of conditions were selected, a potential on the rising portion of the wave at high rotation speed and another near the limiting current plateau at low rotation speed (Table 2). The electrode was held potentiostatically, the current integrated, and the two runs terminated at approximately equal charge values. The solution was subsequently tested for hydrogen peroxide by addition of titanyl sulphate solution (0.05 M in 2 M sulphuric acid), the yellow colour developed being used to give a colorimetric estimation of the peroxide concentration. TABLE 2 H Y D R O G E N P E R O X I D E F O R M A T I O N IN O2-SATURATED 1 M H2SO 4

H202

Potential /V (vs. SCE)

W /Hz

Current /pA

,'glass tran.sler limiting current it,,/Ft,4

Charge C

tool

-0.2 -0.35

50 1.5

482 183

1140 197

2.9 2.9

7 × 10 6 2 × 10 6

The results in Table 2 show significantly more hydrogen peroxide formed in the high rotation gpeed case. This is consistent with the expectation that when the current is close to the four-electron limiting value little intermediate can be escaping from the electrode surface. If we suppose that the reaction mechanism is 02

2.H. +2e) H iI 2° 2 4,

2H- +2el 2 H 2 0 i

(3)

Solution and that under the conditions chosen for the low rotation speed case the first step is under diffusion control, then the fraction of peroxide lost to the solution can be calculated from the measured current. Thus, il will be half of the mass transfer limiting current if> for the overall reaction which can be calculated from

O X Y G E N R E D U C T I O N O N S U L P H I D E MINERALS. I

159

the slope of the linear portion of the iu vs. W ~ plot (see Table 1). This gives i~=98.5 #A from which i2= 183-98.5=84.5 /~A. Hence, the fraction of peroxide lost to the solution is 14/98.5 (14~o) and the quantity corresponding to the measured charge is 2.1 x 10 - 6 tool, in excellent agreement with the amount of peroxide detected. Note that the 7 x 10-6 mol found in the high rotation speed experiment corresponds to a loss of 64~o of the total quantity of peroxide formed during the electrolysis, based on the charge passed and reaction via scheme (3). Mechanism o f oxygen reduction on pyrite

The reduction of oxygen on pyrite shows two overlapping waves, the extent of resolution of which varies in an irregular manner with pH and solution composition. The presence of two waves is associated with formation of a soluble intermediate which has been identified in acid solutions as hydrogen peroxide. In these aspects of its behaviour, pyrite is therefore similar to a number of metals on which oxygen reduction has been studied is. On mercury, reduction to peroxide is fully resolved as a separate wave, while on silver, for example, only a single wave for complete reduction to water is found, though hydrogen peroxide is detected as an intermediate with the rotating ring-disc electrode 16. Pyrite appears to lie between mercury and silver in its activity for reduction, or catalytic decomposition, of peroxide. The Tafel slope of -130 mV in acid solutions and the lack of pHdependence of the position of the wave up to at least pH 7 are consistent with a rate-determining first step 02+e--+O

2

(4)

as has been postulated for a number of electrodes including platinum1 v, m e r c u r f 8, silver ~8 and gold is in acid solutions. The TaM behaviour at higher pH requires consideration of the next stage in the formation of hydrogen peroxide. As the pH increases, the overpotential of oxygen reduction on pyrite decreases by virtue of the cathodic shift of the reversible potential of the O2/H20 2 couple with respect to the SCE. Thus, at pH 12 the reversible potential for equal concentrations of oxygen and hydrogen peroxide is -0.18 V, which is within the range of the Tafel region on pyrite at this pH. The reverse of (4), and subsequent reaction steps, must therefore become important in determining the kinetics. . The mechanism O 2 + e ~ Oz O 2 + 2 H + + e -~ H20 2

(5a) (5b)

in which the first step is in quasi-equilibrium and the addition of the second electron is rate-determining, would give a Tafel slope of - 2 . 3 R T / ( I + ~ ) F , i.e. - 4 0 mV for c~=0.5. If step (5b) is also reversible the value of OE/O log i for i~ io would be - 2 . 3 R T / 2 F or - 3 0 mV, and the wave would shift by - 5 9 mV pet pH unit. These mechanisms have been used to account for behaviour of oxygen reduction on several metals in alkaline solution1518; they are supported by the observations that on mercury, silver and gold, the O2/H20 2 couple can actually set up the reversible potential~ s. 19.

160

T. BII!GLI R D. A. J. RAN[). R. WOODS

In the case of pyrite, the fitll in Tafel slope with increasing pH and the onset of pH dependence of the position of the wave above pH 9 indicate a similar influence of reverse steps, but fafel slopes as small as those predicted by the mechanisms considered above are not observed in the pH range studied. The actual Tafel slopes of around - 7 0 mV suggest that the quasi-equilibrium assumption cannot be applied to (5a) and that the rates of all three steps in (5) need to be taken into account to explain the kinetics. Reaction scheme (3) proposed above allows quantitative analysis of the kinetics of reduction of intermediate formed in the reduction of oxygen. We rewrite (3) in the form O~ k 02 L H 2 0 ~ k H20

(6)

3

H20~ where the superscripts refer to surface and bulk species. In the steady state, the rate of formation of hydrogen peroxide is equal to the rate of its removal by further reaction (step 2) and by diffusion into the bulk (step 3). i.e. B , ( [ O b] -- [O~]) W" = k2[H20~] + B3( [H_,O~] - [H 20~] ) W ~

(7)

where the constants of the Levich flux equation are included in the B terms. Under conditions where the rate of formation of peroxide is controlled purely by mass transfer of oxygen, i.e. [O2]=0. and there is negligible peroxide in the bulk solution, eqn. (7) gives [ H 2 0 2 ] -- BI tO b] W ½ k2 + B3 W½

(8)

The total current is given by i = 2 F O 1 [O b] W ~ -at-2 F k 2 [H202] = 2 F B 1 [O~]

W ~ 1 + /~2-+~B~-~i~

(9)

Since the mass transfer limiting current for the overall process is given by i,, = 4 F B 1 [O~]

W ~'

(10)

we can write "

'l+kz/(kz+B~

W~)~

(11)

or, rearranging, ½il) i-½i,~-

B3 W ~

1+

k2

(12)

Plots of the left-hand side of eqn. (12) rs. W ~ are shown in Fig. 6 for the voltammograms of Fig. 1. Considering the assumptions made in the above derivation, the fit of the results to eqn. (12) is fair. Serious deviations occur at higher potentials and rotation speeds on smooth pyrite where it seems clear that

161

OXYGEN REDUCTION ON SULPHIDE MINERALS. I

3

1

Pohshed Ptj rtte

Rouqh PLjrJte

!

04-

o

~

~

'-I

'

;

'

;

,o

W~/Hz~

1

s

lo

so

kz B3'/Hz~/2

Fig. 6. Analysis of oxygen reduction currents in l M HCIO 4 according to eqn. (12). ( + ) -0.5 V,

(©) -0.4 V, (V) -0.3 V, ([Z]) -0.2 V t:s. SCE. Fig. 7. Potential dependence of the reciprocal slopes of plots in Fig. 6 for ([Z]) polished and (V) rough pyrite. the condition that peroxide formation be under pure diffusion control is not fulfilled. Other possible factors likely to disturb the fit are a catalytic decomposition pathway for hydrogen peroxide and an alternative parallel oxygen reduction mechanism which bypasses peroxide formation. The reciprocal slopes of the lines in Fig. 6 are plotted against potential in Fig. 7. These plots should give the potential dependence of k2 (see eqn. 12). They yield a Tafel slope of - 2 4 5 mV, which is the same as that reported for the second stage of the oxygen reduction reaction on mercury 18. Furthermore, the results in Fig. 7 lead to a ratio of 7.5 for the k2/B 3 values for rough and smooth pyrite at constant potential, and since B 3 is expected to be independent of surface micro-roughness this value should represent the ratio of the real electrode surface areas. It is in good agreement with the ratio determined earlier from currents in the Tafel region at the foot of the oxygen reduction wave. The consistency of the Tafel slope and area ratio with other results gives added support to the validity of the above analysis. The qualitative features of oxygen reduction voltammograms on polished and rough pyrite are now understandable in terms of scheme (6). Increase of rotation speed increases the loss of peroxide through step (3) and therefore the 4-electron limiting current is reached only at more negative potentials. Because of the small potential dependence of the rate constant for peroxide reduction, the accessible potential range may not be adequate to reach this limiting current. The loss of intermediate by diffusion depends on the geometric electrode area while the rate of its further reduction depends on the real area. Hence, roughening the surface layouts attainment of the limiting current, as shown in Figs. 1 and 2. Though these considerations satisfactorily account for the observations on wave shape and the amounts of peroxide found in solution, it is apparent that ring~tisc studies would be useful in allowing a more detailed analysis of the kinetics.

162

T. B1EGLER. D. A. J. RAND. R. W O O D S

ACKNOWLEDGEMENT

The authors wish to thank Dr. H. Linge for helpful discussion. SUMMARY

The electro-reduction of oxygen was studied at rotated electrodes of the sulphide mineral pyrite (FeSz).Kinetic parameters were obtained from currentpotential measurements at the foot of the oxygen reduction wave. In oxygensaturated 1 M acid solutions, the Tafel slopes and exchange currents were of the order of - 1 3 0 mV and 10 -11 A cm --2 respectively. The results at low pH indicated that the first electron transfer step to form O2 is rate-determining, while in alkaline solution the rates of subsequent steps become important. At low rotation speeds, linear sweep voltammograms reached a limiting current corresponding to 4-electron reduction of oxygen to water. The limiting current plateau was not reached at higher speeds because of the loss of a soluble intermediate, identified as hydrogen peroxide. The dependence of current on rotation speed, potential and surface roughness was analysed in terms of a mechanism involving kinetic control of peroxide reduction and diffusion control of its escape into the solution bulk. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

M. Sato and H. M. Mooney, Geophysics, 25 (1960) 226. M. Sato, Econ. Geol., 55 (1960) 1202. E. H. Nickel. J. R. Ross and M. R. Thornber, Econ. Geol.. 69 (1974) 93. S. G. Salamy and J. C. Nixon, Aust. J. Chem.. 7 (1954) 146. R. Tolun and J. A. Kitchener, Inst. Mininy Met. Trans.. 73 (1964) 313. R. Woods, Australas. Inst. Mining Met. Proc.. 241 (1972) 53. A. Granville. N. P. Finkelstein and S. A. Allison. Inst. Mininct Met. Tra~s.. Sect. C. 81 (19721 1. H. Majima and E. Peters, Miner. Process. Colzgr. 8th. Lemn,qrad. 1968. l~?~l. II. Instituta Mekhanobr, Leningrad, 1969, p. 5. R. Woods, J. Phys. Chem., 75 (1971) 354. E. Peters and H. Majima, Can. Met. Quart.. 7 (1968) l11. D. A. J. Rand. Proc. Roy. Aust. Chem. Inst.. 41 (1974) 8. L. A. Taylor, Carne(jie Inst. WashingtoJl. Yearb.. 68 (1970) 259. D. P. Gregory and A. C. Riddiford, J. Chem. Soc.. (1956) 3756. V. G. I