Surface processes in silver and gold cyanidation

Int. J. Miner. Process. 58 Ž2000. 351–368 www.elsevier.nlrlocaterijminpro

Surface processes in silver and gold cyanidation Milton E. Wadsworth

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Department of Metallurgical Engineering, UniÕersity of Utah, 209 Browning Building, Salt Lake City, UT 84112-1183, USA Received 15 May 1999; accepted 21 May 1999

Abstract Rate processes for silver and gold cyanidation are controlled by a series of coupled electrochemical reactions. These include anodic metal dissolution and oxygen reduction. This study demonstrates basic differences between silver and gold dissolution both in reference to the metal and the accompanying oxygen discharge. The kinetics of oxygen reduction are compared for silver and gold. At a fixed voltage, the cathodic current for gold is less than that of silver except at large negative potentials where the kinetics are oxygen diffusion-limited. A mathematical model is presented for oxygen reduction based upon two paths: one path with peroxide formation followed by alternate catalytic decomposition and desorption, and a second path with continued peroxide reduction to hydroxyl ions. The model permits separate calculations of the first and second waves of oxygen reduction. Based upon mixed potential models, silver dissolution is shown to be controlled by both the first and second waves of oxygen reduction with the number of electrons approaching four for high cyanide concentrations. Gold dissolution is controlled by surface reactions with mixed potentials associated mainly with the first wave of oxygen control and limited to two-electron transfer. The model permits an evaluation of the number of electrons which may be transferred for a given potential and shows this number is voltage-dependent. Silver dissolution is shown to include both diffusion or mixed diffusion plus charge transfer kinetics, influenced by the degree of agitation and both oxygen and cyanide concentration-dependent. Gold dissolution kinetics are controlled by crystal-dissolution overpotentials and are independent of agitation. Models of oxygen discharge and metal dissolution are used to explain observed characteristics and kinetics of silver and gold leaching. q 2000 Elsevier Science B.V. All rights reserved. Keywords: surface processes; silver; gold

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Tel.: q1-801-581-8767; Fax: q1-801-581-4937; E-mail: [email protected]

0301-7516r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 Ž 9 9 . 0 0 0 2 0 - 4

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1. Introduction Wagner and Traud Ž1938. first outlined the concept of mixed potential as being that potential for which anodic and cathodic currents within an electron-conducting phase are equal and opposite in sign; i.e., i a s yi c . Kudryk and Kellogg Ž1954. first used the concept to explain gold dissolution kinetics and compared rates based on mixed potentials with measured rates of dissolution. Wadsworth Ž1985; 1989; 1991. modeled anodic and cathodic branches based on the Kudryk–Kellogg data and calculated the mixed potential and corrosion Žleaching. currents as functions of oxygen and cyanide concentrations. Hiskey and Sanchez Ž1990. and Li and Wadsworth Ž1992a; b; c; 1993. used similar equations to model silver cyanidation reactions and developed mathematical rate expressions for the anodic Žmetal dissolution. and cathodic Žoxygen reduction. branches. Hiskey and Sanchez Ž1990. presented a detailed review of various models proposed for oxygen reduction on metals. Quantitative modeling of the oxygen reduction branches has not been satisfactory because of the complexity of the process and the influence of the metal substrate. In this presentation, oxygen reduction is considered in greater detail and is shown to contain separable, additive first and second wave reduction branches. While these branches differ for silver and gold, they differ only in the magnitude of the rate constants. There are striking differences between gold and silver anodic dissolution in cyanide solutions. Both metals react by processes consistent with mixed potential models ŽKudryk and Kellogg, 1954; Wadsworth, 1985; Hiskey and Sanchez, 1990; McCarthy et al., 1998. but the kinetics differ in many ways. Silver dissolves more rapidly than gold under the same conditions, but silver is sensitive to the cyanide concentration ŽLi, 1991., while gold becomes independent of cyanide concentration at higher levels of concentration ŽZheng et al., 1995; Jeffrey, 1998.. Silver is sensitive to the degree of agitation ŽLi, 1991. while gold is essentially independent of agitation ŽJeffrey, 1998.. These factors will be considered in terms of surface electrochemical processes for both oxygen reduction and metal dissolution.

2. Experimental Experimental procedures have been described in prior publications of Li Ž1991., Li Ž1992., Jeffrey Ž1998. and Wadsworth et al. Ž1998.. Steady-state conditions are assumed to occur at slow scan rates; in this case 1 mV sy1 . Jandel SigmaPlot and TableCurve programs were used for data analysis.

3. Mixed potentials and leaching kinetics Fig. 1 presents an anodic Žpositive current. silver dissolution curve for a rotating disk electrode Ž300 rpm. in a de-aerated solution containing 0.01 M CNy. Also shown is a typical cathodic Žnegative current. curve for oxygen reduction on a silver rotating disk electrode in an air-saturated solution. Both curves were obtained at 248C. The corrosion

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Fig. 1. Silver dissolution and oxygen reduction showing mixed potential, Em : rotating electrode, 300 rpm, pH 11.0.

Žor dissolution. current is the current corresponding to the mixed potential, Em , where anodic and cathodic currents within the electron-conducting phase Žin this case, the silver electrode. are equal, but opposite in sign; i a s yi c . Usually, absolute values of current densities i Žwhere i s i a s yi c . vs. potential are plotted as shown in Fig. 2. Two

Fig. 2. Rotating silver electrode showing Ag dissolution and oxygen reduction: 248C, pH 11.0, 300 rpm.

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anodic curves at cyanide concentrations 0.01 M and 0.005 M are shown and the dashed curve represents oxygen reduction. The cross-over points define the mixed potentials and mixed currents for both curves. Other important information may be derived from these plots. The mixed potentials for the two cyanide concentrations occur at potentials near the oxygen diffusion-limiting rate for oxygen reduction. At higher cyanide concentrations, where the anodic curves cross at the O 2 limiting rate, the rate of dissolution is totally oxygen diffusion-limited. For the two curves shown, the dissolution current is below Žalthough near. the limiting value for oxygen; consequently, the kinetics will be strongly oxygen diffusion-dependent but include both oxygen diffusion and charge transfer. The anodic Žsilver. curves increase to plateau values where cyanide diffusion is rate-controlling. The anodic currents for the cyanide concentrations shown are predominantly charge transfer-controlled with cyanide diffusion a very small component. As the cyanide concentration is decreased, cyanide diffusion becomes more dominant and, finally, totally controlling when the cyanide horizontal plateau crosses the oxygen curve. It must be noted that the mixed potential and corresponding mixed currents do not represent actual condition present during cyanide leaching. The anodic curves were obtained in the absence of oxygen and the oxygen reduction curves in the absence of cyanide. The influence of competitive adsorption or chemical reactions are not present as may be true under actual leaching conditions. The mixed current prediction of the rate and the measured rate, while often close, cannot be expected to be exactly the same. Mixed potential concepts are useful for modeling the leaching system. Fig. 3 illustrates similar data for gold. The differences between silver and gold are striking and, as will be shown, explain differences experienced under actual leaching conditions. The oxygen curve for gold has a different shape than that for silver, and the

Fig. 3. Four anodic curves for gold showing mixed potentials with oxygen reduction in air-saturated solutions.

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mixed potentials fall in the lower region of the curve and most importantly, the mixed potentials and currents are essentially independent of cyanide concentration. The crossover points fall on the threshold of two or more passivation peaks. Passivation with increasing voltage includes formation of AuCN films followed by AuŽIII. oxide films at higher potentials. Discussions of the complex nature of gold electrochemistry is beyond this presentation. Nicol Ž1980. has reviewed the electrochemistry of gold for both acidic and alkaline systems and has presented important observations of the major influence heavy metal impurities have on the rate of Au dissolution. Zheng et al. Ž1995. and Jeffrey Ž1998. have presented data to illustrate the quantitative influence of selected metal impurities ŽCu, Pb and Ag. on the kinetics of gold dissolution. DuPont researchers Kristjansdottir and Thompson Ž1996., Thompson Ž1996. and Hinds and Thompson Ž1998. have described classes of organic activators which increase the rate of gold dissolution in cyanide solutions. The independence of gold dissolution kinetics on cyanide concentration and degree of agitation as well as the positive effect of heavy metals and organic activators all indicate surface processes are controlling. These conclusions are based on laboratory results under well-controlled conditions and must be extended to actual leaching systems. It seems logical that free gold particles in an agitation tank would be influenced by the factors enumerated above. Also, it may be shown, for the particle size range commonly used in pulp leaching, pore diffusion is much more rapid than the rate of gold particle dissolution. Therefore, parameters noted in the laboratory are expected to be operative in pulp leaching of ‘‘free milling’’ ores. In the case of heap leaching of much larger ore fragments, pore diffusion kinetics become controlling as the diffusion paths become longer ŽBartlett, 1992.. Fig. 4 expands some of the data shown in Figs. 2 and 3 and illustrates more directly the differences between silver and gold. The oxygen curve for gold falls well below that

Fig. 4. Comparison of silver and gold dissolution and relative positions on individual oxygen reduction curves.

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Fig. 5. Comparison of measured rates of silver and gold dissolution: silver 500 rpm, gold independent of rpm.

for silver and the mixed current for gold falls far below that for silver. Gold dissolution appears predominantly charge transfer-limited, while silver appears predominantly oxygen diffusion-limited for these conditions. Fig. 5 illustrates actual leaching rates measured in the laboratory using rotating disk electrodes for silver ŽLi, 1991. and gold ŽJeffrey, 1998; Wadsworth et al., 1998.. The silver results are for 500 rpm, and the gold results are for 300 rpm. However, it has been shown by the above-investigators that gold dissolution is independent of rotation speed. The rate for silver increases steadily approaching a maximum corresponding to four-electron transfer for oxygen. Gold approaches a maximum at a relatively low rate of dissolution and independent of cyanide concentration. The maximum leaching rate for silver for these conditions is more than seven times that for gold. None of these data indicate the influence of the separate steps of oxygen reduction or oxygen efficiency vs. voltage for silver and gold dissolution. A more detailed modeling of oxygen reduction on silver and gold surfaces is required.

4. Oxygen reduction on silver and gold Oxygen reduction on metal electrodes has been studied extensively and several earlier reviews ŽVetter, 1967; Erdey-Gruz, 1970. cover an extensive literature on the subject. Hiskey and Sanchez Ž1990. provided a detailed review of oxygen reduction on metal surfaces for their work on silver cyanidation. The majority of research has been on platinum but broad similarities exist for many metals. The reduction process is complex, dependent upon the metal substrate and solution pH. Two general models include: Ž1.

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reduction with peroxide as an intermediate; and, Ž2. direct reduction to hydroxyl ions. Two stages or waves are apparent in the voltage–current curves for oxygen reduction on many metals including silver and gold. The two stages or waves overlap, explaining the apparent range of n values Žnumber of electrons. measured for oxygen reduction. Delahay Ž1950. measured the amount of oxygen consumed at a given potential for several metals and found n to vary between 2 and 4. He considered the intermediate n-values to be the sum of simultaneous two- and four-electron processes. The following steps are consistent with the peroxide path. Oxygen diffuses to the surface and, at a given potential, establishes a steady-state surface concentration wO 2 xs . The two-electron reaction sequence is: O 2 Ž s . q Hqq 2e HOy 2 Ž s.

° HO

y 2

™ 12 O q OH .

Ž s.

y

Ž 1.

2

The four-electron reaction sequence is:

° HO Ž s. Ž s . q H q 2e ™ 2OH .

O 2 Ž s . q Hqq 2e HOy 2

y 2

q

y

Ž 2.

The catalytic decomposition of peroxide, Eq. Ž1., is an electrochemical process ŽGerischer, 1956; Winkelmann, 1956., is voltage-dependent, and is the sum of the electron donor and acceptor reactions:

™ O q H q 2e HO q H q 2e ™ 2OH the sum being: 2HO ™ O q 2OH . HOy 2 y 2

y 2

q

2

q

y

Ž 3.

y

Ž 4.

2

The model depicted in Scheme 1 consists of two and four electron processes and corresponds to the ‘end-on’ paths as described by Hiskey and Sanchez Ž1990. following concepts advanced by Yeager Ž1976.. In Scheme 1, k 0 –k 2 and k 0 –k 3 sequences represent two-electron processes and the k 0 –k 4 sequence represents a four-electron

Scheme 1.

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358

process. Oxygen at the metal surface in the liquid phase is designated O 2 Žsl., and O 2 Žs., Ž . and HOy 2 s refer to adsorbed oxygen and peroxide. At a sufficiently low scan rate, it is assumed that steady-state concentrations are established for any given voltage. Accordingly: d n O 2 Žsl.

s

DO 2Ž w O 2 x y w O 2 x sl .

d

dt d n O 2 Ž s. dt

y k 0 w O 2 x sl q kX0 w O 2 x s s 0

s w O 2 x sl k 0 y w O 2 x s kX0 y w O 2 x s k 1ey

Ž1y a c1 . F E RT

Ž 5. a c1 F E

X q HOy 2 s k 1e

RT

s0

Ž 6. and d n HO y2 Žs. dt

s w O 2 x s k 1ey

Ž1y a c1 . F E RT

a c1 F E

y

HOy 2 s

ž

kX1e

q kX2 q k 4 ey

RT

Ž 1y a c 4 . FE

RT

/

s0

Ž 7.

where kX2 s k 2 q k 3 , DO 2 is the coefficient of diffusion for oxygen, d is the thickness of the diffusion boundary for a fixed rotation speed, and a is the transfer coefficient. At steady-state, the cathodic current may be expressed as: ic s y

nFDO 2Ž w O 2 x y w O 2 x sl .

sy

nFDO 2 w O 2 x

d

ž

d

1y

w O 2 x sl . wO2 x

/

Ž 8.

The combined Eqs. Ž5. – Ž7. provide an expression for wO 2 xsl which may be substituted in Eq. Ž8.. The transfer coefficient for oxygen may be taken as ; 0.5 ŽVetter, 1967.; i.e., a c1 s a c4 s 1r2 and the solution of Eqs. Ž5. – Ž8. is: FE

ic s y

nFDO 2 w O 2 x

d

ž

FE

k 0 k 1 kX2 ey 2 RT q k 0 k 1 k 4 ey RT X

/

Ž 9.

where: Xs

DO 2

d q

kX0 kX2 q

DO 2

d

ž

DO 2

d

/

FE

kX0 kX1e 2 RT

FE

Ž kX0 k 4 q k 1 kX2 . q k 0 k 1 kX2 ey 2 RT

q

ž

DO 2

d

/

FE

k 1 k 4 q k 0 k 1 k 4 ey RT .

Ž 10 .

Eq. Ž9. is the sum of two steps for oxygen reduction: i c s i c1 q i c 2 s y

nFDO 2

d

ž

1

1 q

X1

X2

/

Ž 11 .

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359

where: DO 2 Ž kX0 k 4 q k 1 kX2 .

X1 s 1 q

q

d

FE

DO 2 kX0 k 0 kX1

d

q

k 0 k 1 kX2 e 2 RT q

ž

DO 2 k 4 k 0 kX2

d

DO 2 kX0 kX1

k4 kX2

/

FE

ey 2 RT

FE

k 0 k 1 kX2

d

q

ey RT

Ž 12 .

and: DO 2

X2 s 1 q

q

q

d k0

ž

DO 2 Ž kX0 k 4 q k 1 kX2 .

d FE

DO 2 kX0 kX2

d k 0 k1 k 4

e

RT

q

q

kX2

k 0 k1 k 4

k4

DO 2 kX0 kX1

3FE

d k 0 k1 k 4

FE

/

e

2 RT

e 2 RT .

Ž 13 .

TableCurve analysis of the data for oxygen reduction on both silver and gold indicated several of the terms in Eqs. Ž12. and Ž13. are negligibly small. These are the second and fifth terms of Eq. Ž12. and the second, fourth and fifth terms of Eq. Ž13.. Also, the third terms of Eqs. Ž12. and Ž13. are essentially independent of the angular velocity, or: DO 2 k 4

d

k 0 kX2

<

k4

Ž 14 .

kX2

and: DO 2 Ž kX0 k 4 q k 1 kX2 .

d

k 0 k1 k 4

<

kX2

Ž 15 .

k4

Eq. Ž9. now becomes:

yi c s



nFDO 2 w O 2 x

nFDO 2 w O 2 x

d 1q

k4 kX2

FE

ey 2 RT q

DO 2

kX0

d k 0 kX1

FE

e 2 RT

0 0 q

1q

d kX2 k4

FE

s yi c1 y i c 2

e 2 RT

Ž 16 . with the right-hand terms representing the two reduction steps for oxygen. Also, from the electron balance: ic n

s

i c1 n1

q

ic2 n2

Ž 17 .

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360

where n1 and n 2 are the number of electrons associated with each step. The value of n 2 is expected to be 4, the maximum value of n for oxygen reduction. Letting i s yi c , Eq. Ž9. becomes: a1 a2 is q Ž 18 . FE FE FE

y

ž 1 q be

2 RT

q ce 2 RT

2 RT

/ ž 1 q de /

where: a1 s

n1 FDO 2 w O 2 x

d

; a2 s

n 2 FDO 2 w O 2 x

d

; bs

k4 kX2

; cs

DO 2 kX0

d

k 0 kX1

; ds

1 b

.

Ž 19 . The thickness of the diffusion boundary Ž d . may be calculated using the Levich equation: d s 1.61 vy1 r2 Õ 1r6 DO1r3 Ž 20 . 2 where v and Õ are the angular velocity and kinematic viscosity. and the terms for a1 , a 2 , and c may be given in terms of v 1r2 :

w O 2 x Õy1 r6 . v 1r2 ; a2 s n 2 Ž 0.62 FDO2r3 w O 2 x Õy1 r6 . v 1r2 ; a1 s n1 Ž 0.62 FDO2r3 2 2 kX0 a1 n1 c s X Ž 0.62 DO2r3 Õy1r6 . v 1r2 ; s . Ž 21 . 2 k 0 k1 a2 n2 Solid lines in Fig. 6 are oxygen reduction curves on rotating silver electrodes ŽLi, 1991. for five rotational speeds and a scan rate of 1 MV sy1 . Data were transferred to TableCurve for evaluation. The dashed lines are calculated curves using Eq. Ž18. with b s 1.88 = 10y6 . Fig. 7 illustrates values for a1 , a2 , and c plotted vs. v 1r2 . The lines

Fig. 6. Oxygen reduction on a rotating silver electrode showing experimental and calculated curves.

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361

Fig. 7. Linear regression lines for calculated values of a1 , a2 , and c vs. square root of the angular velocity.

shown are linear regression lines. Results correspond well with the Levich equation. Also shown are the calculate a1ra2 values. For ideal two- and four-electron reduction steps, a1ra2 is expected to be 0.5. The calculated values are slightly greater than the expected 0.5 value but approach 0.5 for higher rotation speeds. Values greater than 0.5

Fig. 8. Oxygen reduction on a rotating silver electrode showing measured and calculated curves.

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M.E. Wadsworthr Int. J. Miner. Process. 58 (2000) 351–368

Fig. 9. Oxygen reduction on a rotating silver electrode showing first and second waves.

may result from continued reduction of some of the oxygen produced by the catalytic decomposition of the surface peroxide ion. Higher rotation speeds are expected to increase the rate of desorption of the peroxide ion from the metal surface, limiting the extent of catalytic decomposition.

Fig. 10. Oxygen reduction on a rotating gold electrode showing experimental and calculated points.

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363

Fig. 11. Linear regression lines for calculated values of a1 , a 2 , and c vs. square root of the angular velocity.

Having separate equations for the first and second partial currents makes it possible to plot the nominal two-electron Žyi c . and four-electron Žyi c . curves. Fig. 8 illustrates the data for 300 rpm, showing the experimental data, the separate first and second partial

Fig. 12. Calculated first and second waves for oxygen reduction on a rotating gold electrode.

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M.E. Wadsworthr Int. J. Miner. Process. 58 (2000) 351–368

currents or waves, and the sum according to Eq. Ž18.. Fig. 9 illustrates the calculated first and second partial currents for the five rotational speeds. Fig. 10 shows results for gold electrodes at five rotational speeds. The average b value was 8.31 = 10y7 . Experimental results are shown as points and curves calculated with Eq. Ž18. as dashed lines. Fig. 11 presents a1 , a 2 , and c plotted vs. the square root of the angular velocity. Again, results correspond well with the Levich equation. Fig. 12 presents the first and second partial currents for oxygen reduction on gold for five rotational speeds.

5. Coupled reduction and oxidation during cyanidation In Fig. 2, it is clear that the mixed potential, and therefore the leaching rate, may vary considerably with cyanide concentration. At low concentrations, the rate becomes increasingly limited by cyanide diffusion and at high cyanide concentrations, it approaches a plateau value corresponding to oxygen diffusion rate control. Similarly, as is evident in Fig. 3, the mixed potential for gold increases with cyanide concentration approaching a plateau for concentrations greater than ; 0.005 M NaCN. The plateau value for gold falls far below that for silver ŽFig. 4. and therefore cannot be related to oxygen diffusion control. These same results are evident for actual rates of leaching as shown in Fig. 5. The plateau value for silver approaches a limit corresponding to oxygen diffusion for a four-electron process. For gold, the low rate corresponding to the plateau strongly suggests gold leaching kinetics are limited by surface processes. A model which includes surface imperfections Žkinks and edges. has been proposed by Wadsworth et al. Ž1998. and is illustrated in Fig. 13. Cyanide adsorbed on planar surfaces diffuses to kink

Fig. 13. Gold surface showing kink Žd., edge Žc. and planar Žb. sites; Ža. refers to cyanide and its gold complex in solution.

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Fig. 14. Rotating silver electrode Ž248C, 300 rpm, pH 11.0. showing silver dissolution and oxygen reduction curves and current densities a – a, b – b, and c – c.

sites where charge transfer takes place. The population of these surface sites is far smaller for gold than for silver and is rate-limiting for gold dissolution. It is interesting to examine these relationships with the first and second waves of oxygen reduction for both silver and gold. Fig. 14 illustrates results for a silver electrode rotating 300 rpm. Above ; 200 p.m. NaCN, the mixed potential is mainly associated

Fig. 15. Calculated values of n vs. overpotential for gold and silver electrodes at 300 rpm.

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M.E. Wadsworthr Int. J. Miner. Process. 58 (2000) 351–368

Fig. 16. Four anodic curves for gold showing relationship with first and second waves of oxygen reduction.

with the second Žfour-electron. wave of oxygen reduction. The percent of current associated with the first Žtwo-electron. wave is shown for each curve. At 25 p.m. NaCN, silver dissolution is controlled 99% by the first wave while at 490 p.m., silver dissolution current is made up of 42% first wave and 58% second wave. This result illustrates the important fact that the oxygen efficiency and apparent variation of n with potential results from mixed four- and two-electron processes as proposed and demonstrated by Delahay Ž1950. where n Žbased on oxygen up-take. diminished as the potential was increased for several metal electrodes. An apparent value of n may be calculated by Eq. Ž17.. Fig. 15 is a plot of n vs. potential for 300 rpm silver and gold electrodes with n 2 s 4 and n1 s 2 from Figs. 9 and 12. Also shown are the ranges of potential expected for each metal during cyanide processing. Silver falls in the upper range of n values Ž n 4. while gold is entirely in the two-electron range. Fig. 16 shows the first and second waves for gold, for the 300 rpm data, and gold dissolution Žanodic. curves for four cyanide concentrations. The four anodic curves have mixed potentials associated totally with the first wave of oxygen reduction and the rate Žmixed current. approaches a constant value independent of the cyanide concentration. These results suggest gold is confined to the first oxygen reduction nominal two-electron process.

™

6. Summary and conclusions Cathodic curves for oxygen reduction are consistent with a model in which oxygen diffusion, surface adsorption, charge transfer and chemical reactions occur on the metal

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surface. The kinetics at a fixed potential establish steady-state concentrations of reaction intermediates and align reaction along two general paths, one characterized as a nominal two-electron process and a second as a four-electron process. An initial first charge transfer process forms peroxide on the surface. Peroxide may leave the surface by decomposition or desorption, both processes occurring without a net transfer of charge. These reactions constitute the two-electron path and dominate the process for the more positive potentials. As the potential becomes more negative, continued reduction of peroxide occurs, accounting for the four-electron path. This path dominates at sufficiently negative overpotentials. The model provides a way to calculate the partial currents contributing to the nominal two-electron and four-electron cathodic partial currents. For the high range of cyanide concentration, oxygen reduction is more efficient on the silver surface Ž n 4., while oxygen reduction on gold is confined to n s 2. This higher oxygen efficiency results in rapid surface reactions with solution diffusion becoming the dominant rate-controlling process. As the cyanide concentration decreases, the oxygen efficiency diminishes and surface reactions become increasingly important components of the rate limiting process. The partial currents for oxygen reduction on gold demonstrate that gold dissolution kinetics are confined to the two-electron process. The rate of gold dissolution is controlled by crystal dissolution overpotentials and the number of reactive surface sites. Dissolution takes place at rates far below those expected for solution diffusion control. The mixed current approaches a cyanide-independent plateau value consistent with observed leaching measurements.

™

Acknowledgements Special thanks are given here to Dr. Ximeng Zhu, Associate Research Professor of Metallurgical Engineering for running the many electrochemical curves for gold.

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