(2006) 118–125www.elsevier.com/locate/hydromet

Hydrometallurgy 82

The effect of sulfide minerals on the leaching of goldin aerated cyanide solutions

X. Dai 1, M.I. Jeffrey ⁎,1

Department of Chemical Engineering, Monash University, Clayton Vic 3800, Australia

Available online 8 June 2006

Abstract

The effect of galvanic interactions and sulfide mineral dissolution on the gold leaching kinetics in aerated cyanide solutions isinvestigated. Gold leaching kinetics were measured using a rotating electrochemical quartz crystal microbalance (REQCM), whichmeasures the leaching rates in-situ. When the mineral is electrically in contact with gold, the gold dissolution rate increases as aresult of the extra surface area available for oxygen reduction. However the leaching behavior of gold in the presence of solublesulfide minerals is complicated. It is shown that the formation of sulfide ions in solution results in the passivation of the dissolutionof gold. The addition of lead was found to be an effective means of alleviating this problem, as lead promotes the removal ofsulfide by precipitation as PbS.© 2006 Published by Elsevier B.V.

Keywords: Gold leaching; Kinetics; Cyanidation; Mineral sulfides; Electrochemistry; Passivation; Lead; Galena; Chalcocite

1. Introduction

Since a large proportion of gold ores contain sulfideminerals, the effects of these minerals on gold dissolu-tion in cyanide solution have interested many research-ers. Early studies on the dissolution of gold in cyanidesolution in the presence of sulfide minerals have shownthat heavy metal components, such as Cu, Fe and Zn,significantly increase the consumption of both cyanideand oxygen (Habashi, 1967). In addition, the sulfidecomponent has been shown to have a strong impact onthe gold leaching kinetics (Fink and Putnam, 1950;Hedley and Tabachnick, 1968). Liu and Yen (1995)conducted a systematic study of the kinetics of gold

⁎ Corresponding author. Tel.: +61 8 9334 8081; fax: +61 8 93348001.

E-mail address: matthew.jeffrey@csiro.au (M.I. Jeffrey).1 Now at CSIRO Minerals, Bentley, WA, Australia.

0304-386X/$ - see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.hydromet.2006.03.005

dissolution in the presence of various sulfide minerals inboth air-saturated and oxygen-enriched systems. Theirresults demonstrated that the leaching behavior of gold inthe presence of sulfide minerals depended strongly onboth the solubility of the sulfides and the oxygen con-centration in the solution. For instance, in an oxygen-enriched cyanide solution, the presence of chalcopyrite,pyrrhotite, arsenopyrite and pyrite were found toincrease the gold dissolution rate, while others, such asstibnite and chalcocite, caused a reduction in the golddissolution rate. Deschenes et al. (2000, 1998),Deschenes and Wallingford (1995) tried to improve theleaching performance of gold in the presence of somesulfide minerals by using pre-leaching, injecting oxygenand adding lead nitrate. It was reported that pre-leachingsuccessfully overcame the effect of pyrite and pyrrhotiteon cyanide consumption and the gold leaching kinetics,but it had no beneficial effect when chalcopyrite waspresent. It was suggested that lead nitrate prevented

119X. Dai, M.I. Jeffrey / Hydrometallurgy 82 (2006) 118–125

copper and iron dissolution by forming a passive layer atthe surface of sulfide minerals.

Since the sulfide minerals are to some extent solublein cyanide solutions, there will always be some sulfurspecies present in the leaching solution. It is generallybelieved that the presence of such species results in highconsumption of cyanide and oxygen. However, someresults of kinetic studies suggest that sulfur species alsodirectly affect the gold leaching reaction. Weichselbaumet al. (1989) found that the addition of trace amounts ofsodium sulfide to the cyanide solution dramaticallyhindered gold leaching and ascribed it to the formationof a passive layer of Au2S on the gold surface. Similarresults were obtained by Lorenzen and van Deventer(1992), who studied the kinetics of gold leaching in thepresence of reactive sulfide minerals such as pyrrhotite.Jeffrey and Breuer (2000) used gold containing 5%silver rather than high purity gold to investigate theeffect of sulfur species and suggested that solublesulfide hinders the rate of gold leaching by forming aprotective layer of the type Au/Sx. They showed that thiswas a monolayer of sulfur which forms at potentialsnegative of the bulk sulfur deposition potential. Theformation of a monolayer of sulfur on gold in alkalinenon-cyanide solutions has also been noted by others(Briceno and Chandler, 1990; Hamilton and Woods,1983). It was also shown that the sulfur formed ischemically attacked by cyanide, resulting in higherleach rates at higher cyanide concentrations (Jeffrey andBreuer, 2000).

Another interesting aspect of the gold leachingbehavior is the galvanic interaction between gold andthe mineral, as naturally occurring gold is likely to be incontact or in association with various minerals.Lorenzen and van Deventer (1992) tried to differentiatethe effect of galvanic interaction from the formation of apassivating film on the gold surface by using pure goldrotating disc electrically contacted with mineral disc inone vessel and then in two separate vessels. Theysuggested that when gold is in contact with conductingminerals, it will passivate as a result of the enhancedmagnitude of the cathodic current. It was argued that thehigher cathodic current causes a positive shift in themixed potential into the passive region, resulting indecreased gold dissolution rates.

One complication with most previous studies is thathigh purity gold was utilized, for which the leaching ishindered by a film of AuCN (Jeffrey and Ritchie, 2001).However most naturally occurring gold contains silver,for which the cyanide leaching reaction occurs at a highrate (Jeffrey and Ritchie, 2000b). The present study aimsto establish the effects of sulfide minerals such as

chalcocite on the leaching of gold containing 5% silverusing a rotating electrochemical quartz crystal microbal-ance (REQCM) to directly measure the gold leaching rateand the corresponding mixed potential when it is electri-cally connected to various mineral sulfide electrodes.

2. Experimental methods

These experiments were carried out using solutionsprepared from analytical grade reagents and Milliporewater. For the experiments without the employment ofpre-leaching, cyanide was initially added into the bulksolutions containing sulfide minerals. At different timeintervals, sample solutions were taken and filtered, andthe filtrate was used to measure gold leach rates using theREQCM, which is described elsewhere (Jeffrey et al.,2000). The concentration of dissolved metal from thesulfides was measured using a Varian SpectrAA-400Atomic Absorption Spectrometer (AAS). For experi-ments with pre-leaching, a similar procedure was adoptedbut cyanide was added after a pre-leaching period of 4 h.

The leaching and electrochemical experiments wereperformed at a rotation rate of 300 rpm and at atemperature of 20 °C. All potentials were measuredrelative to the saturated calomel electrode (+0.242 V vs.SHE), but are reported relative to the SHE. Prior to eachexperiment, gold was electroplated onto the quartzelectrode at 25 A m−2 from a solution containing0.02 mol L−1 potassium dicyanoaurate, 0.23 mol L−1

potassium cyanide, 0.086 mol L−1 potassium carbonate,and 0.5 mmol L−1 silver nitrate. This resulted in a de-posit containing approximately 5% silver by mass(which for simplicity will be referred to as gold), thusensuring that the experiments more closely mimickedindustrial conditions. The linear sweep voltammetry wascarried out using a PAR273 potentiostat, and a scan rateof 1 mV s−1 was adopted. A platinum wire was used asthe counter electrode.

3. Results and discussion

3.1. Electrochemistry of gold and sulfide minerals incyanide solutions

The oxidation of gold and some sulfide minerals insolutions containing 5 mmol L−1 cyanide and 50 mmolL−1 sodium perchlorate as background electrolyte wasstudied using linear sweep voltammetry, and thepolarization curves are shown in Fig. 1. It can be seenthat gold oxidation commences at −620 mV andapproaches a diffusion limited current of 15 A m−2 at0 mV. The oxidation of various sulfide minerals was also

-600 -400 -200 0 200 4000

5

10

15

20

i / A

m-2

chalcocite

Au

E / mV

gold chalcocite covellite galena pyrite marcasite pyrrhotite chalcopyrite

Fig. 1. Gold and mineral oxidation polarization curves in 5 mmol/Lcyanide and 50 mmol/L sodium perchlorate.

-600 -400 -200 0-10

-8

-6

-4

-2

0

i / A

m-2

E / mV

Chalcocite Galena Pyrite Marcasite Pyrrhotite Chalcopyrite

Fig. 2. Oxygen reduction polarization curves on various sulfideminerals in 5 mmol/L cyanide and 50 mmol/L sodium perchlorate.

120 X. Dai, M.I. Jeffrey / Hydrometallurgy 82 (2006) 118–125

studied, and the polarization curves in Fig. 1 show thatlittle current is observed until the potential exceeds200mV for pyrrhotite, covellite, pyrite, marcasite, galenaand chalcopyrite. The oxidation of chalcocite occursmost readily among the sulfide minerals studied, with thereaction commencing at −150 mV. However, it is stillmuch more noble than gold in cyanide solutions. Theseresults demonstrate that gold cannot be cathodicallyprotected by these minerals when in electrical contact.

In a similar manner, the polarization curves for thereduction of oxygen in the absence of cyanide on themineral surface were obtained (Fig. 2). It is clear thatoxygen reduction occurs on all the minerals, withchalcocite having the most active surface for reduction,and galena the least active surface. Since the oxidation ofgold can occur at potentials more positive than−600mV,it is clear from Fig. 2 that there is the opportunity forgalvanic interaction between the gold and sulfide mine-rals. It can be expected that particularly for chalcocite,oxygen reduction on the mineral surface in contact withgold will give rise to higher dissolution rates.

3.2. Effect of galvanic interactions on gold leaching

In order to study the galvanic interactions betweensulfide minerals and gold in more detail, two electrodes,one gold electrode and one mineral electrode, weresimultaneously leached in 5 mmol L−1 cyanide solution.The REQCMwas used to measure the gold leaching rateand the corresponding mixed potential when it waselectrically connected to a chalcocite electrode. The twoelectrodes were not connected together electrically untilleaching had proceeded for 5 min. As shown in Fig. 3, itis clear that the mass of gold decreases at a steady rateprior to the electrical connection of the mineral. From theslope of mass vs. time, the gold leaching rate is calcula-

ted to be 5.6×10−5 mol m−2 s−1 under these conditions.This leaching rate is typical for the dissolution of gold inair saturated solutions when the rate is oxygen diffusioncontrolled (Jeffrey and Ritchie, 2000b), and hence itrepresents the maximum rate at which gold can beleached in air saturated cyanide solutions.

After the gold and chalcocite electrodes wereelectrically connected together, the gold leaching rateincreased, reaching a value of 13×10−5 mol m−2 s−1. Atthe same time the mixed potential increased rapidly from−350 to −60 mV, before slowly decreasing to −200 mV.As shown in Fig. 2, this is in the potential region whereoxygen reduction can occur on the mineral surface.Therefore, it is believed that the connected mineralelectrode provides extra surface area for oxygen reduc-tion to occur, and hence the increase in the total cathodiccurrent causes a positive shift in the mixed potential andan increase in the gold leaching rate. A similar effect wasobserved for the other sulfide minerals, even galena, forwhich oxygen reduction is more difficult. However theincrease in leaching rate when galena was connected wasless significant than for chalcocite.

It is worth noting that these results are inconsistentwith those of Lorenzen and van Deventer (1992), as theystated that the enhanced cathodic current shifts the mixedpotential to the passivation region for gold, and hencehinders the dissolution process. It is most likely that thisdifference can be ascribed to the fact that Lorenzen andvan Deventer (1992) used a pure gold electrode, forwhich the leaching is hindered (Jeffrey and Ritchie,2001). As shown in Fig. 1, the gold/silver alloy does notexperience passivation, and hence an increase in themixed potential gives rise to higher leaching rates.

The secondmechanism bywhich sulfide minerals canimpact on the gold leaching process is as a result of their

0 200 400 600-200

-150

-100

-50

0

E /

mV

Δm /μ

g

t / s

-400

-300

-200

-100

0

Fig. 3. Effect of the galvanic interaction between gold and chalcociteon the leaching of gold and on the mixed potential in 5 mmol/Lcyanide.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

2

4

6

pyrite pyrrhotite marcasite

[Fe]

/ pp

m

t / hr

0.0

0.5

1.0

1.5

2.0

105

r / m

ol m

-2s-1

Fig. 4. The iron concentration (top) and gold leaching rate (bottom) asa function of the contact time for solutions containing 10 mmol/Lcyanide and 1 g/L sulfide minerals.

121X. Dai, M.I. Jeffrey / Hydrometallurgy 82 (2006) 118–125

solubility. The experiments shown in Fig. 3 were onlyrun for a short period of time, and hence the non-oxidative dissolution of the sulfide minerals was limited.Since most sulfide minerals are to some extent soluble incyanide solution, it has been suggested that the dissolvedspecies may affect the gold leaching reaction. Theremainder of the present study will address this issue.

3.3. Effect of galena (PbS) on gold leaching kinetics

The REQCM was used to study the leaching of goldin air saturated 10 mmol L−1 cyanide solutions in thepresence of galena (PbS). At various time intervals asample of solution was filtered and the gold leaching ratewas determined by measuring the mass as a function oftime using the REQCM. The steady state leach rate wasthen calculated and found to remain constant above6×10−5 mol m−2 s−1 for three hours.

It is worth noting that the concentration of leadmeasured in the solution for the experiments was lessthan 5 ppm. However with time, the colour of the solidschanged from black to a milky white. Such a resultsuggests that in cyanide solutions, the galena is convertedto lead hydroxide via the oxidation of the sulfide.Previous studies have shown that lead catalyses theoxidation of soluble sulfide to thiocyanate through theprecipitation of lead sulfide (Hedley and Tabachnick,1968; Jeffrey and Breuer, 2000). Thus it is believed thatgalena is oxidized to thiocyanate, producing lead ionswhich precipitate to form themilkywhite lead hydroxide.

3.4. Effect of pyrite (FeS2), pyrrhotite (Fe1−xS) andmarcasite (FeS2) on gold leaching rates

Using the same method as described above for gale-na, gold leaching experiments were performed to studythe effect of pyrite, pyrrhotite and marcasite. Fig. 4

shows the gold dissolution rates as well as the ironconcentrations as a function of contact time of thesolution with 1 g/L of mineral. It can be seen that thesolubility of these three minerals in cyanide solution isvery low with less than 2 mg/L iron detected after threehours of leaching. Surprisingly, pyrite appeared to bemore reactive than the other two iron mineral samples.The gold leaching rate was also measured as a functionof time, but due to the low solubility of the minerals, thegold dissolution rate is barely affected by the presenceof these minerals (Fig. 4). A leaching rate of ca.5.5×10−5 mol m−2 s−1 was observed for pyrrhotite andmacarsite. However it is interesting to note that forpyrite, the gold leaching rates, ca. 4.5×10−5 mol m−2

s−1 are slightly lower than that for the other twominerals. Such a result is consistent with the apparenthigher reactivity of pyrite than marcasite and pyrrhotite.The non-oxidative dissolution of the mineral produces alow concentration of dissolved sulfide, which is wellknown to passivate gold leaching in cyanide solutions(Fink and Putnam, 1950; Hedley and Tabachnick, 1968;Weichselbaum et al., 1989; Jeffrey and Breuer, 2000).

3.5. Effect of the copper cyanides on the goldleaching rate

One of the complications with studying theleaching of gold in the presence of copper sulfides isthat there is the possibility of low free cyanide con-centration due to the formation of copper cyanide speciessuch as Cu(CN)3

2−. Thus preliminary experiments werecarried where cuprous oxide was added to the cyanidesolution, and the copper concentration and gold leaching

0 21 3 21 30

1

2

3

4

5

10mM CN-

12mM CN-

13.5mM CN-

cyan

ide:

copp

er r

atio

105

r / m

ol m

-2 s

-1

t / hr0

2.4

2.8

3.2

3.6

4.0

4.4

Fig. 5. The gold leaching rate (left) and cyanide to copper ratio (right)as a function of the contact time for solutions containing 2 mmol/LCu2O and various cyanide concentrations.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

40

80

120

160

200

240

280

without lead 10ppm lead 20ppm lead 100ppm lead

[Cu]

/ pp

m

t / hr

Fig. 6. The effect of lead addition on the copper concentration vs. timeprofile for solutions containing 13.5 mmol/L cyanide and 2.4 g/Lchalcocite.

1

2

3

4

5

without lead 10ppm lead 20ppm lead 100ppm lead

105

r / m

ol m

-2 s

-1

122 X. Dai, M.I. Jeffrey / Hydrometallurgy 82 (2006) 118–125

rate were measured as a function of time. The results inFig. 5 show that Cu2O dissolved rapidly in the cyanidesolution, forming soluble copper cyanide complexes asreflected in the low cyanide to copper ratio. For thesolution initially containing 10 mmol L−1 cyanide, it isclear from Fig. 5 that the cyanide to copper ratio is below3. Hence the dominant species in the solution would beCu(CN)2

−and Cu(CN)32−. In contrast, when the solution

contained 13.5 mmol L−1 cyanide, the cyanide to copperratio is well over 3, and hence the solution would containfree cyanide.

The effect of the copper to cyanide ratio on the goldleaching rate is shown in Fig. 5. It is clear that the goldleaching rate remains at ca. 4.5×10−5 mol m−2 s−1 aslong as there is sufficient free cyanide in solution. At thecyanide to copper ratio of approximately 3, where mostof the copper cyanide is present as Cu(CN)3

2−, reasonablegold leaching rates of ca. 1.5×10−5 mol m−2 s−1 can stillbe achieved. However, when the cyanide to copper ratiodrops to 2.5 and a large proportion of copper is present asCu(CN)2

−, the gold leaching rates are extremely low. Thissuggests that Cu(CN)3

2− has a reasonable capability ofleaching gold according to Eq. (1), while Cu(CN)2

−

cannot. This conclusion is consistent with otherresearchers' work (Jeffrey et al., 2002; Muir et al.,1991; Parsons et al., 1993; Zheng et al., 1995).

4Au þ 8CuðCNÞ2−3 þ O2 þ 2H2O→4AuðCNÞ−2þ 8CuðCNÞ−2 þ 4OH− ð1Þ

0.0 0.5 1.0 1.5 2.0 2.5 3.00

t / hr

Fig. 7. The effect of lead addition on the leaching rate for gold in airsaturated 13.5 mmol/L cyanide solutions after contact with 2.4 g/Lchalcocite for various time intervals.

3.6. Effect of chalcocite (Cu2S) on gold leaching rates

The gold leaching rate and the dissolved copperconcentration were measured for different contact timeof the cyanide solution with chalcocite. The resultsplotted in Fig. 6 show that chalcocite dissolves rapidly in

cyanide solutions; with 140 mg/L copper being observedafter 10 min, and 230 mg/L after 1 h. The high solubilityobviously poses a problem in cyanidation, as coppercyanide complexes consume free cyanide and reduce thegold leaching rate. In addition, the presence of dissolvedsulfide ions is known to be detrimental to the leaching ofgold. Thus experiments were performed to establishwhether the addition of 10, 20 or 100 mg/L lead couldreduce the solubility of the chalcocite. Fig. 6 shows thatthe addition of 10 or 20 mg/L has little effect, whilst100 mg/L lead does reduce the dissolution of chalcocitewith only 200 mg/L copper detected after 3 h. Despitethis, a reasonable quantity of sulfide would be leachedinto the solution.

The gold leaching rate was thenmeasured in solutionsthat had been in contact with chalcocite for various timesup to 3 h (Fig. 7). It can be seen that the presence ofchalcocite in cyanide solution after 0.25 h dramatically

-600 -500 -400 -300 -200 -1000

2

4

6

8

i / A

m-2

E / mV (SHE)

20 ppm lead No lead

Fig. 8. Polarization curves showing the effect of lead addition on thegold oxidation half reaction in 13.5 mmol/L cyanide solutions aftercontact with chalcocite for 3 h.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

1

2

3

4

5

6

t / hr

105

r / m

ol m

-2 s

-1

No galena with galena

Fig. 9. The effect of 200 mg/L galena addition on the leaching rate ofgold in 13.5 mmol/L cyanide solutions after contact with 2.4 g/Lchalcocite for various time intervals.

0 1 2 3 40

10

20

30

40

50 No mineral Chalcocite + 20 ppm lead Covellite + 20 ppm lead Chalcocite Covellite Galena

[Au]

/ pp

m

t / hr

Fig. 10. The effect of lead addition on gold dissolution in a solutioncontaining 10 mmol/L cyanide, about 50 mg/L pure gold powder andvarious sulfide minerals.

123X. Dai, M.I. Jeffrey / Hydrometallurgy 82 (2006) 118–125

retards gold dissolution in the absence of lead, and where10 mg/L lead was added. In the past it has been arguedthat such an effect is due to the consumption of cyanide.However, from the data in Fig. 6, the amount of copperdissolved is only 140 mg/L and one would expectsufficient free cyanide to leach the gold readily. Fig. 7also shows that when 20 mg/L lead was added, the goldleaching rate is substantially faster. These results indicatethat some other factor apart from cyanide consumption isresponsible for the adverse effect of chalcocite on goldleaching, and this effect can be alleviated by leadaddition. It has been previously proposed that a passivefilm of sulfur formed on the surface of gold may beresponsible for the reduced gold leaching rates in sulfidecontaining solutions (Jeffrey and Breuer, 2000). If this isthe case, then the role of the added lead is to prevent thepassive film from forming. This has been observed forsynthetic solutions containing lead nitrate and sodiumsulfide (Jeffrey and Breuer, 2000).

It is worth noting that when 100mg/L lead was added,although the gold leaching rate was high initially, after1 h or more of contact between the solution and mineral,the leaching rate shown in Fig. 7 is severely hindered.Such a result suggests that the addition of too much leadcan be detrimental to the gold leaching process, and it hasbeen suggested in the past that overuse of lead couldresult in the passivation of the gold surface, presumablyby lead hydroxide (Habashi, 1970).

The effect of lead on the gold oxidation half reactionwas studied using standard electrochemical techniques.Fig. 8 shows the comparison of the polarization curves inthe solutions with and without lead after being in contactwith chalcocite for 3 h. It is clear that the oxidation ofgold is active in the solution containing lead, whereas itis passive in the absence of lead. This result is consistentwith the above conclusions. The peak appearing in the

polarization curve of the solution containing lead isrelated to the formation and decomposition of a metalliclead or gold–lead alloy film (Deschenes et al., 2000;Jeffrey and Ritchie, 2000a).

Further experiments were conducted to investigatethe addition of galena as a source of lead for improvingthe dissolution of gold in cyanide solutions in thepresence of chalcocite. This was accomplished byadding 200 mg/L galena into the bulk solution contain-ing chalcocite. The results shown in Fig. 9 withoutgalena addition are the same as that shown in Fig. 7.After 10 min the gold leaching rate in the presence ofgalena and chalcocite is still low due to the slowdissolution of galena. However after 1 h of leaching inthe presence of chalcocite, it is clear that the goldleaching rate is significantly enhanced by the presenceof galena in the bulk solution – cf. Fig. 7 with 20 mg/Llead added into the bulk solution. The polarization curveof gold oxidation in the solution that had been contacted

124 X. Dai, M.I. Jeffrey / Hydrometallurgy 82 (2006) 118–125

with galena and chalcocite for 3 h was also similar tothat when 20 mg/L lead was added as lead nitrate.

3.7. Gold powder leaching in the presence of varioussulfide minerals

The experimental results shown above illustrate theeffect of galvanic interactions and mineral dissolution ongold leaching separately. In order to study galvanicinteractions and mineral dissolution together, goldpowder leaching experiments were carried out in thepresence of ground sulfide minerals. The limitation ofthese experiments is that a pure gold powder was used, ascompared to a gold/silver alloy in the other leachingexperiments. However, as shown in Fig. 10, the dataobtained from this experiment is very informative. In theabsence of minerals, the gold leaching rate was relativelyslow, with a gold concentration of 35 ppm beingobtained in 4 h (equivalent to 70% gold dissolution).In the presence of galena, the leaching of gold was muchmore rapid, consistent with the above discussion. In thecase where either chalcocite or covellite was present, thegold leaches very slowly. After 4 h, only 15% gold wasleached in the presence of chalcocite, and only 7% goldin the presence of covellite. After 24 h, the amount ofgold extracted was 59% and 36% in the presence ofchalcocite and covellite, respectively. These results areconsistent with a passive film formed on the surface ofgold when soluble sulfide is present in the solution.Experiments performed with the addition of 20 mg/Llead as lead nitrate to the cyanide solutions containingchalcocite or covellite show much more rapid goldleaching (Fig. 10) verifying the experiments carried outusing the REQCM.

4. Conclusions

For gold leaching in the presence of sulfide minerals,there are twomajor mechanisms by which the sulfide canimpact on the gold leaching rate (a) through the galvanicinteraction between gold and the conducting mineral incontact with gold and (b) through the dissolution of themineral which impacts on the solution speciation. Anelectrochemical study of the oxidation of gold andsulfide minerals in the cyanide solutions shows that noneof the mineral studied, including chalcocite, covellite,galena, pyrite, pyrrhotite, marcasite and chalcopyrite, areoxidized at more negative potentials than gold, and thusdo not cathodically protect the gold when in directcontact with the mineral. Oxygen reduction occurs atdifferent rates on the sulfide mineral surfaces, and hencewhen the gold is in contact with the mineral, the higher

cathodic current shifts the mixed potential morepositively, resulting in a higher dissolution rate.

The leaching behavior of gold in the presence ofsulfide minerals is quite complicated due to thedissolution of the mineral to form sulfide in the solutionthat then impacts on the gold leaching kinetics. It hasbeen shown that gold leaching in cyanide solutions isseverely hindered by the presence of chalcocite due tothe high solubility of this mineral in cyanide solutions.The gold leaching rate in solutions containing chalcocitecan be improved by the addition of 20 mg/L lead duringthe cyanide process. The role of lead is to promote theprecipitation of PbS and enhance the anodic oxidation ofgold via a gold–lead alloy film. The addition of galena tothe cyanide solution containing chalcocite was shown tohave the same effect as the addition of lead nitrate.

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