Hydrometallurgy 106 (2011) 58–63

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Thiourea–thiocyanate leaching system for gold

Xiyun Yang a,⁎, Michael S. Moats b, Jan D. Miller b, Xuming Wang b, Xichang Shi a, Hui Xu a

a School of Metallurgical Science and Engineering, Central South University, Hunan 410083, Chinab Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, Salt Lake City, UT 84112, USA

⁎ Corresponding author. Tel.: +86 731 88877352; faE-mail address: xiyun.yang@yahoo.com (X. Yang).

0304-386X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.hydromet.2010.11.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 September 2010Received in revised form 30 November 2010Accepted 30 November 2010Available online 13 December 2010

Keywords:Gold leachingThioureaThiocyanateSynergistic effectRotating discKinetics

The leaching of gold in thiourea–thiocyanate solutions has been studied by the rotating-disc technique usingferric sulfate as oxidant. The effects of initial concentrations of ferric, thiourea (Tu) and thiocyanate as well astemperature and pH on gold leaching rates were studied. An initial gold leaching rate in the order of 10−9 molcm−2 s−1 was obtained at 25 °C, which was higher than rates obtained when either ferric–thiocyanate orferric–thiourea solutions were used separately. The synergistic effect was attributed to the formation of amixed ligand complex Au(Tu)2SCN. Determinations of apparent activation energy indicate that the processwas controlled by a combination of chemical reaction and diffusion in the mixed lixiviant system. Open circuitpotentials show that thiocyanate stability is increased in the mixture.

x: +86 731 88710171.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

A possible alternative reagent to cyanide for gold leaching isthiocyanate, as first reported by White (1905). However, it was notuntil 1986 that research on the subject was resumed by Fleming(1986), followed by a thermodynamics study of the thiocyanatesystem for leaching of gold and silver ores published by Barbosa-Filhoand Monhemius (1989). Thiocyanate is a technically viable andinteresting lixiviant for gold (Broadhurst and Du Preez, 1993).Leaching is performed between pH 1 and 2, which allows the use offerric sulfate as an oxidizing agent (Barbosa-Filho and Monhemius,1994a).

Themechanism of dissolution of gold by ferric–thiocyanate solutionsis directly linked to the auto-reduction process, in which ferric isspontaneously reduced to ferrous while oxidizing SCN−. This oxidationproceeds through the formation of several intermediate species,particularly (SCN)3− and (SCN)2, which act both as oxidants and ascomplexants for gold upon reduction (Barbosa-Filho and Monhemius,1994b). The formation of (SCN)3− and (SCN)2 must be continuous due totheir relatively fast decomposition by hydrolysis into more stableoxidation products. The instability of (SCN)3− and (SCN)2 towardshydrolysis is amajor drawback of the ferric–thiocyanate leaching system.Experimental results with this system at 25 °C yielded an initial goldleaching rate in the order of 10−10 mol cm−2 s−1 (Barbosa-Filho andMonhemius, 1994c). Rates as high as 10−9 or even 10−8 mol cm−2 s−1,depending on reagent concentrations, could be obtained by raising the

temperature to 85 °C, but this was seen as a limitation to the commercialuse of thiocyanate as a gold lixiviant as it would add cost to conventionalagitated leaching (Barbosa-Filho and Monhemius, 1994d).

Occasionally, the combination of lixiviants can produce a syner-gistic effect. Copper sulfate is a good coordinator for gold dissolutionin thiosulfate solution compared to other oxidants (Breuer and Jeffrey,2002; Li et al., 1995). The presence of thiourea (Tu) can improve thegold oxidation half reaction in thiosulfate solutions (Chandra andJeffrey, 2004). The addition of a small amount of thiourea into athiocyanate solution appears to catalyze the gold dissolution leadingto significant benefits such as a faster leaching rate (Yang et al.,2010a). The aim of this paper is to determine the effect of majorparameters on the dissolution rate associated with a mixedthiocyanate/thiourea leaching system using ferric sulfate as anoxidant.

2. Experimental

The rotating disc technique was employed for leaching experi-ments. A gold disc (99.9% purity, 14.0 mm diameter) was purchasedfrom Pine Instrument. An analytical rotator (Model: ASR, PineInstrument) was used, and the rotational speed was controlled bythe ASR Speed Controller (Pine Instrument). Before each experiment,the disc surface was carefully ground and polished with successivelyfiner grades of alumina powder of size 1.0, 0.3 and 0.05 μm. Afterrinsing with deionized water, dipping in 0.5 M HCl for more than10 min, and rinsing with water again, the disc was ready for theleaching experiments.

The gold leaching experiments were carried out in a cylindricalreactor immersed in a thermostatically controlledwater bath. A 1.00 L

59X. Yang et al. / Hydrometallurgy 106 (2011) 58–63

solution was freshly prepared for each experiment and it was leftopen to the atmosphere during the experiment. The pH was adjustedby the addition of NaOH or H2SO4. Ferric sulfate was used as theoxidant. Solution samples, about 5 mL, were taken at selected timeintervals and analyzed for gold with a model IRIS intrepid II XSP ICP(Thermo Electro Corporation).

The standard conditions employed were a thiourea 5 mM; ferricsulfate 0.055 M; thiocyanate 0.02 M; pH 1.5; temperature 25 °C and adisc rotational speed of 200 rpm. Variations from these conditions arenoted in the text and in figure captions. All the chemicals wereanalytical grade and used as received.

Potential measurements between the gold disc and referenceelectrode (SCE) were made through a Luggin capillary using a GamryInstrumentPCI4G750Potentiostat. The solution volumewas200 mLanddeaerated with nitrogen before open circuit potential measurements.

The FTIR Transmission spectra were recorded in the range of 4000–400 cm−1 on a Bio-Rad 6000 FTIR spectrometer with a broadbandMCTdetector. A cell with two CaF2 windows and a 0.05 cm Teflon spacer wasused in this study. All the spectra were obtained at room temperaturefrom the average of 500 scanswith a resolution of 4 cm−1. The solutionsexamined included0.4 MTu, 0.4 MTu+0.1 MAuCl3 andNaSCN+0.4 MTu+0.1 M AuCl3, the concentration of NaSCN was controlled at 0.4 M,0.8 M and 1.8 M, respectively. To diminish the interference of water andto avoid as much as possible that from the acid, deuterated oxide wasused as solvent and the spectrum of 0.03 M H2SO4 in deuterated oxidewas used as a baseline.

3. Results and discussion

3.1. Effect of ferric sulfate concentration

The effect of ferric sulfate concentration on the gold leaching rate inthe presence of thiourea and thiocyanate is shown in Fig. 1. As can beobserved, the gold leaching rate slightly increases with increasing ferricsulfate concentration from 0.0055 M to 0.022 M. Further increasing theferric sulfate concentration to 0.055 Mhas only aminor effect on the rateof gold dissolution. In sulfuric acid solutions with ferric and thioureapresent (e.g. thiourea only solutions), gold dissolves according toreaction (1). The gold leaching rate is independent of ferric concentra-tion andkeeps constantwith timewhen thiourea concentration is below12 g L−1 and ferric is above 0.1 g L−1 (Li and Miller, 2007).

2Au + 2Fe3+ + 4Tu + SO2−4 → ½AuðTuÞ2�2SO4 + 2Fe2+ ð1Þ

In sulfuric acid solutions with ferric and thiocyanate present (e. g.thiocyanate only solutions), gold dissolves according to reaction (2)

0 20 40 60 800.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5 0.0055M 0.011M 0.022M 0.055M

Gol

d co

ncen

teat

ion/

ppm

Time/min

Fig. 1. Effect of ferric sulfate concentration on the leaching rate of gold. Conditions: Tu0.005 M, thiocyanate 0.02 M, pH 1.5, temperature 25 °C, 200 rpm.

(Li et al., 2008). The gold leaching rate decreases with time and ferricconcentration has only a minor effect on the rate of gold dissolution.

Au + Fe3+ + 2SCN− → AuðSCNÞ−2 + Fe2+ ð2Þ

In thiourea–thiocyanate solutions, the gold leaching rate remainsconstant with time, which means the mixed ligand exhibits behaviorsimilar to thiourea solutions. Increasing ferric concentration leads to asmall increase in the gold leaching ratewhen ferric sulfate concentrationincreases from0.0055 M to 0.022 Mwith no increase above 0.022 M. It issuggested that this response to ferric ions is related to reactionmechanism. When ferric is in the concentration range of 0.0055 M to0.022 M, increasing ferric concentration increases the oxidation powerof the oxidant and leads to higher dissolution rate. When ferric is above0.022 M, the gold dissolution is controlled by a surface reaction and thegolddissolution reaction is independent on ferric concentration,which issimilar to thiourea only solution (Li andMiller, 2007). It is reported thathigh ferric concentration is favorable to thiocyanate stability (BroadhurstandDu Preez, 1993) and thus ferric was fixed at 0.055 M for subsequentexperiments.

3.2. FTIR measurements

Previously, SERS results suggested a possible formation of a mixedligand complex [Au(Tu)2]SCN in the thiourea–thiocyanate leachingsystem (Yang et al., 2010a). To further support the formation of themixed ligand complex, FTIR measurements were performed in Tu, Tu–AuCl3 and SCN−–Tu–AuCl3 solutions. The results are shown in Fig. 2. Thespectrum of deuterated thiourea shows bands at 1378 and 1466 cm−1

related to the stretching of C–S and C–N bonds. The band at 1515 cm−1

has been assigned to the bending mode of the ND2 groups. The band at1625 cm−1 that becomes stronger upon the addition of auric chloride isrelated to formamidine disulfide (FDS). The gold is predominantly stableas an aurous ion when complexed with thiourea. Auric is reduced toaurousby the oxidationof thiourea to FDS. Thebandat 3407 cm−1 canbeassigned to the stretchingmode of ND2 groups.WhenAuCl3was added, anew band at 1561 cm−1 can be seen, which indicates the formation ofthe gold–thiourea complex in the presence of sulfuric acid (Bolzan et al.,2003).

When thiocyanate was added into the above solution, a band relatedto the stretching mode of sulfur in SCN− appears at 2063 cm−1 (Bronand Holze, 1999). With increasing thiocyanate concentration, the bandof ND2 stretching mode at 3047 cm−1 shifts toward higher wavenum-bers likely due to concentration increase. The C–N stretching mode at1466 cm−1 shifts toward lower wavenumbers. The band at 1561 cm−1

associatedwith the gold–thiourea complex disappears into the shoulderof the increasing C–N stretching peak at 1466 cm−1.

1000 1500 2000 3500

e

d

cba

1466

2063

3407

3412

3417

3424

34071561

1625

1515

14461378

1216

Wavenumber/cm-1

Inte

nsity

Fig. 2. FTIR spectra in (a) 0.4 M Tu; (b) 0.4 M Tu+0.1 MAuCl3; (c) 0.4 M SCN−+0.4 MTu+0.1 MAuCl3; (d) 0.8 M SCN−+0.4 M Tu+0.1 MAuCl3 and (e)1.8 M SCN−+0.4 MTu+0.1 MAuCl3 solutions.

0 20 40 60 800

1

2

3

4

5

6 1mM Tu

5mM Tu

10mM Tu

Gol

d co

ncen

tratio

n/pp

m

Time/min

Fig. 4. Effect of Tu concentration on the leaching rate of gold. Conditions: ferric 0.055 M,thiocyanate 0.01 M, pH 1.5, temperature 25 °C, 200 rpm.

5

-

60 X. Yang et al. / Hydrometallurgy 106 (2011) 58–63

The red shift of the C–N stretchingmode band upon the addition andincrease of thiocyanate indicates the increased character of the bond ofC–N. This is interpreted as being caused by forming a newmixed ligandcomplex in the solution. The stability constants for the soluble species ofAu(Tu)2+, AuSCN and Au(SCN)2− are 21.5, 15.3 and 19.2, respectively(Groenewald, 1975; Barbosa-Filho and Monhemius, 1994c). Accordingto the theory of mixed ligand complex (Farkas et al., 2000), the mostprobable complex is Au(Tu)2SCN involving the mixed solutions. It isreported that Ag(Tu)2+ and Zn(Tu)22+ interact with SCN− to form Ag(Tu)2SCN and Zn(Tu)2(SCN)2 (Marco et al., 1992; Alia et al., 1999). It issuggested that gold dissolves according to reaction (3).

Au + Fe3+ + SCN− + 2Tu → AuðTuÞ2SCN + Fe2+ ð3Þ

3.3. Effect of thiocyanate concentration

The effect of thiocyanate concentration on the gold leaching rate isshown in Fig. 3. It is apparent from Fig. 3 that increasing thethiocyanate concentration from 0.005 M to 0.05 M decreases the goldleaching rate. When thiocyanate concentration is below 0.02 M, goldconcentration increases linearly with time indicating that leachingrate is not time dependent over the range studied. When thethiocyanate concentration is 0.05 M the gold leaching rate decreaseswith time, which is similar to the behavior observed in thiocyanateonly solutions.

Due to the low thiourea concentration used (0.005 M), the requiredthiocyanate concentration for reaction (3) to proceed is also low. Thus,the added thiocyanate appears to decrease the rate of reaction andeventually lead to passivation at high thiocyanate concentration.

3.4. Effect of thiourea concentration

The results at various concentrations of thiourea at a fixedthiocyanate concentration of 0.01 M are shown in Fig. 4. The goldleaching rate increases with increasing thiourea concentration, whichis similar to previous reports in thiourea only solutions (Li and Miller,2007).

To better understand the leaching process, the gold leaching rate forthemixed thiourea/thiocyanate solution comparedwith that in thioureaor thiocyanate only solutions is shown in Fig. 5. The sum of goldconcentration with two individual lixiviants is also presented. It can beseen that the mixed solution leads to a higher gold leaching rate thaneither lixiviant alone or the sum of the two individual lixiviants.

It is easy to get the initial gold leaching rate from the resultspresented previously. These results are summarized in Table 1 andindicate that the dissolution rate is strongly dependent on theconcentration of thiourea and the ratio of thiourea to thiocyanate. A

0 20 40 60 800

1

2

3

4

5

6

0.005M 0.01M 0.02M 0.05M

Gol

d co

ncen

trat

ion/

ppm

Time/min

Fig. 3. Effect of thiocyanate concentration on the leaching rate of gold. Conditions: Tu0.005 M, ferric 0.055 M, pH 1.5, temperature 25 °C, 200 rpm.

higher ratio and thiourea concentration are favorable to enhancedleaching kinetics.

3.5. Electrochemical measurements

Previous electrochemical results indicate the lowest polarizationresistance for gold dissolution at potentials of 0.3 and 0.4 V vs. SCEwas observed at SCN−to Tu ratios between 5:1 and 10:1 (Yang et al.,2010b). The leaching results in the presence of ferric indicate 1:1 isbetter than 5:1 to 10:1. One possible explanation for this apparentdiscrepancy is that the leaching was performed in the presence of theferric ion and the electrochemical studies were not.

Ferric ion forms complexes with SCN− (Barbosa-Filho andMonhemius, 1994a) and thiourea (Li and Miller, 2007). Due todifferences in complex stability constants, it is likely that the actualfree SCN− to Tu ratio is different from the SCN− to Tu ratio based onthe initial addition. While a rigorous calculation of stability constantshasnot beenperformed, a quick examinationof the relativemagnitudeofstability constants indicates that ferric ion is likely to complexmorewiththiourea than with thiocyanate. This would result in the actual SCN− toTu ratio in the ferric solution being higher thanwhat appears to be addedin the form of the chemicals. Thus, the optimal free SCN− to Tu ratio forthe ferric–thiourea–thiocyanate leaching solution is probably similar tothose measured electrochemically.

The rates observed are the samemagnitude as reported for cyanideleaching of gold (typically, 10−9 mol cm−2 s−1) or leaching with0.1 M thiourea+0.01 M formamidine disulphide; and are one order

0 20 40 60 800

1

2

3

4

0.01M SCN 5mM Tu

5mM Tu,0.01M SCN-

Tu+SCN-

Gol

d co

ncen

tratio

n/pp

m

Time/min

Fig. 5. Effect of ligand on gold dissolution and the sum of gold concentration in 5 mM Tuand 0.01 M NaSCN individually. Conditions: ferric 0.055 M, pH 1.5, temperature 25 °C,200 rpm.

Table 1Gold leaching rate at various concentrations of thiourea and thiocyanate.

Tu, M Thiocyanate, M Tu/thiocyanate Rate, J, mol cm−2 s−1 (J×109)

0.005 0.005 1/1 4.300.005 0.02 1/4 2.790.005 0.05 1/10 2.220.001 0.01 1/10 0.930.005 0.01 1/2 3.440.01 0.01 1/1 4.210.005 0 ∝ 2.500 0.01 0 0.22

0 20 40 60 80 100 120 140 160 180 2000.41

0.42

0.43

0.44

0.45

0.46

0.47

0.48

0.49

0.50

0.05M 0.02M 0.01M 0.005M 0 M

Pot

entia

l/(V

vs.

SC

E)

Time/min

Fig. 7. Effect of thiocyanate concentration on open circuit potential reading for Tu/thiocyanate mixed solution on gold disc. Conditions: ferric 0.055 M, Tu 0.005 M, pH 1.5,temperature 25 °C, 200 rpm.

61X. Yang et al. / Hydrometallurgy 106 (2011) 58–63

of magnitude higher than those obtained with thiocyanate (Barbosa-Filho andMonhemius, 1994c; Li andMiller, 2002; Groenewald, 1976).

The dissolution in thiocyanate only solution is very slow, whichmeans that thiocyanate does not participate significantly in the directdissolution of gold, rather thiocyanate must be assisting thiourea toincrease the rate of dissolution, which again suggests that thioureaand thiocyanate have some synergistic effects and gold dissolvesaccording to reaction (3).

To understand the synergistic effect further, the open circuit ormixedpotential of gold was measured over time. Fig. 6 shows the open circuitpotentials of gold in ferric–thiocyanate solutions with various thiocya-nate concentrations. It is interesting to note that there is a decrease in thepotential readings in the ferric–thiocyanate solution due to the oxidationof thiocyanate, resulting in the formation of the intermediate thiocyanatespecies, such as (SCN)2 and (SCN)3− (Barbosa-Filho and Monhemius,1994c), and the reduction of ferric ion.

The reverse occurs in the thiourea system as shown in Fig. 7 (0 MNaSCN). Themixedpotentials are lowat zero time and then increase andtend to plateau due to the production of the species formamidinedisulphide, generatedby theoxidationof thioureaby ferric ion. Theweakadsorption of formamidine disulphide and thiourea on the gold surfaceslightly increases the potential initially (Yang et al., 2010a,b,c; Zhanget al., 2001). Formamidinedisulphide acts as oxidant for gold dissolutionand is reduced to thiourea again (Li andMiller, 2002). After some period,the system reaches a stable state and the potential tends to plateau.

In the mixed system with thiocyanate concentration from 0.005 to0.02 M, the potentials show the same tendency with thiourea while0.05 M thiocyanate appears more similar to the response in thiocyanateonly solutions.

Themixed potentialmeasured at 20 min (from Fig. 7) is compared tothe calculated current density for gold dissolution in Fig. 8. The currentdensity was calculated from the leaching rate given in Table 1 usingFaraday's law and a one electron transfer reaction (Au→Au++e). It

Pot

entia

l(V v

s. S

CE

)

0 20 40 60 80 100 120 140 160 180

0.50

0.51

0.52

0.53

0.54

0.55

0.56

0.57

0.58

0.05M

0.02M

0.01M

0.005M

Time/min

Fig. 6. Effect of thiocyanate concentration on open circuit potential reading inthiocyanate only solution on gold disc. Conditions: ferric 0.055 M, pH 1.5, temperature25 °C, 200 rpm.

appears that several competing reactions are occurring simultaneouslymaking a mechanistic determination difficult at this time. The authorsbelieve the addition of small amounts thiocyanate is activating the goldsurface by alleviating some surface passivation possibly caused by [Au(Tu)2]2SO4 (Zhang et al., 2001). Further addition of thiocyanate can leadto surface blocking (e.g. deactivation) by the proposed formation of Au(SCN) (Li et al., 2008). Additionally, thiocyanate can also complex ferricion and thus reduce the oxidation power of the leaching solution. Howthese three mechanisms interact to generate the response observed inthis study needs further investigation.

By comparing the potential change with time in Figs. 6 and 7, it isconcluded that thiocyanate stability is increased in this mixed system.

3.6. Effect of pH value

The effect of pH on gold dissolution in a mixed thiourea/thiocyanate solution is shown in Fig. 9. Increasing the pH value doesnot affect the gold leaching rate significantly for the mixed lixiviantsystem. What is observed is the gold leaching rate for pH 1.5 is a littlehigher than that for pH 1.0 and 1.9. In the thiourea only solution, ahigh pH value is favorable for gold dissolution (Li andMiller, 2007). Inthe thiocyanate only solution, a low pH value is favorable forthiocyanate stability (Barbosa-Filho and Monhemius, 1994a). Fromreactions (1) to (3), it is known that hydrogen ions do not take part inthe reaction. The small effect of pH value appears to be related tothiocyanate stability and the reaction kinetics of thiourea.

0.20 0.25 0.30 0.35 0.400.450

0.455

0.460

0.465

0.470

0.475

0.480

0.485

0.490

0.495

0.500

0.05

0

0.02

0.01

0.005

Pot

entia

l/( V

vs.

SC

E)

Calculated current density/(mAcm-2 )

Fig. 8. Mixed potential at 20 min vs. calculated cureent density from gold dissolutionsrate as a function of thiocyanate concentration (M as listed in plot). Conditions: ferric0.055 M, Tu 0.005 M, pH 1.5, temperature 25 °C, 200 rpm.

0 20 40 60 800.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

pH:1.0 pH:1.5 pH:1.9

Gol

d co

ncen

tratio

n/pp

m

Time/min

Fig. 9. Effect of pH value on the leaching rate of gold. Conditions: ferric 0.055 M,thiocyanate 0.01 M, thiourea 0.005 M, temperature 25 °C, 200 rpm.

Table 2Gold leaching rates observed at several disc rotating speeds.

Rotation speed, ω, rpm ω1/2 Rate, J, mol cm−2 s1(J×109)

100 10 2.43200 14.1 3.44400 20 3.51

62 X. Yang et al. / Hydrometallurgy 106 (2011) 58–63

3.7. Effect of temperature

The effect of temperature on gold dissolution is presented inFig. 10. It is shown that the gold leaching rate increases withincreasing temperature in the mixed thiourea/thiocyanate solution.To better understand the leaching mechanisms, the activation energywas determined. The activation energy is simply calculated from theslope of the logarithm of the rate deduced from experimental resultsagainst 1/T (e.g. Arrhenius plot) as shown in Fig. 11. From the slope,the apparent activation energy is calculated as 34.40 kJ mol−1

according to the Arrhenius equation and suggests that the process is

0 20 40 60 800

1

2

3

4

5

6

7 20OC

25OC

30OC

35OC

Gol

d co

ncen

tratio

n/pp

m

Time/min

Fig. 10. Effect of temperature on the leaching rate of gold. Conditions: ferric 0.055 M,thiocyanate 0.01 M, thiourea 0.005 M, pH 1.5, 200 rpm.

0.00324 0.00328 0.00332 0.00336 0.00340-3.2

-3.1

-3.0

-2.9

-2.8

-2.7

-2.6

-2.5

-2.4

slope=-4137

In(d

[Au]

/dt)

1/T

Fig. 11. Arrhenius plot of In(d[Au]/dt) vs. 1/T. Data taken from Fig. 10.

either controlled by a surface reaction or a combination of surfacereaction and mass transfer.

To confirm the rate-controlling step, the effect of rotating speeds ongold dissolution was studied and the results are shown in Table 2. It canbe seen that increasing the rotating speed does increase the goldleaching rate. According to Levich (1962), for reactions at a rotating discthat are controlled by the diffusion of reactants or products, there is alinear dependence between theflux of a given species to or from thediscand the square root of the disc's angular velocity. The data in Table 2indicates that there is no linear relationship between the gold leachingrate and the square root of the disc's angular velocity. This analysis is inagreement with the calculated activation energy, which indicates thatgold dissolution is a mixed-control process where mass transfer controlis present. In thiocyanate solutions, the process is controlled by surfacereaction (Barbosa-Filho and Monhemius, 1994c). It seems that theaddition of small amounts of thiourea into thiocyanate solutions leads tothe control step changing from surface reaction to a mixed process. Infact such a transition in rate control was suggested by Li and Miller forthe acid thiourea system (Li and Miller, 2007). These results showagreement with previous analysis of the ferric concentration.

4. Conclusions

The addition of small amounts of thiourea to thiocyanate–ferricsolutions reveals a synergistic effect on the dissolution of gold. Thedissolution rate is higher than those obtained when either ferric–thiocyanate or ferric–thiourea solutions are used separately at theconcentrations used in the mixture. The synergistic effect is ascribedto the formation of a mixed ligand complex Au(Tu)2SCN. Determina-tions of apparent activation energies indicate that gold dissolution is amixed-control process unlike the surface controlled reactions ob-served in thiocyanate solutions. An initial gold leaching rate in theorder of 10−9 mol cm−2 s−1 was obtained and was dependent on thethiourea to thiocyanate ratio. These initial dissolution rates comparefavorably to cyanide leaching rate.

Acknowledgements

The authors are grateful to Newmont Mining Corporation for theirfinancial support of this project. This international collaboration wasmade possible through financial support of Dr. Xiyun Yang from theChina Scholarship Council. Discussion with Jinshan Li is greatlyappreciated. The assistance with the FTIR measurements by Xihui Yinis gratefully acknowledged.

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