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EXTRACTION OF GOLD AND SILVER FROM

TURKISH GOLD ORE BY AMMONIACAL

THIOSULPHATE LEACHING

FATMA ARSLAN1 AND BARIS SAYINER2

1Mining Faculty, Mining Engineering Department,Mineral and Coal Processing Section, IstanbulTechnical University, Maslak, Istanbul, Turkey2Koza Altın Isletmeleri A.S., Ovacık Gold Mine,Bergama, Izmir, Turkey

Although cyanide is a widespread leaching reagent for the recovery

of precious metals, there still are continuing investigations on

alternative processes due to the related environmental problems con-

cerned. Thiosulphate leaching of precious metals is one of the pro-

cesses that were developed as an alternative and nontoxic technique

to conventional cyanidation. The aim of this experimental study was

to investigate the possibilities of leaching gold and silver in ammo-

niacal thiosulphate solutions on a laboratory scale. Samples used

in the experiments were taken from Ovacık Gold Mine, located in

the Bergama region of Turkey. The influence of temperature, copper

sulphate concentration, ammonia concentration, thiosulphate con-

centration, sodium sulfite concentration, and solid–liquid ratio on

gold and silver leaching recoveries was investigated and optimum

leaching conditions were determined. At these conditions, 99.57%Au and 95.87% Ag leaching recoveries were achieved.

Keywords: gold, silver, thio-sulphate, ammonia, gold leaching

Address correspondence to Fatma Arslan, Mining Faculty, Mining Engineering

Department, Mineral and Coal Processing Section, Istanbul Technical University, 34469

Maslak, Istanbul, Turkey. E-mail: [email protected]

Mineral Processing & Extractive Metall. Rev., 29: 68–82, 2008

Copyright Q Taylor & Francis Group, LLC

ISSN: 0882-7508 print=1547-7401 online

DOI: 10.1080/08827500601141784

Downloaded By: [Islamic Azad Consortium] At: 15:32 15 December 2008

1. INTRODUCTION

Cyanide is the widespread leaching reagent used for the recovery of

precious metals, since it is cheap, well established, and efficient for gold

and silver extraction. There has been a growing interest in developing

leaching processes that are alternatives to the cyanide leaching of pre-

cious metal ores containing gold and silver. The main factors responsible

for this interest are the concerns regarding the toxicity of cyanide, the

inability of cyanide solutions to effectively leach some types of ores such

as manganiferrous silver and gold ores, and the fear of cyanide, especially

in recent metallurgical operations starting in new precious metals mining

districts. Despite the noteworthy safety regulations currently applied in

cyanidation plants all around the world, real environmental risks and

human toxicity hazards still remain (Zipperian et al. 1988).

Another reason for the renewed interest in noncyanide leaching

reagents is the increased dissolution rate of gold and silver. Rapid leaching

rates involve smaller leach tanks, requiring lower capital costs and energy

consumption (Zipperian et al. 1988). However, until now, noncyanide

reagents have not been widely employed for gold and silver recovery, but

they may find application in future treatment operations when environ-

mental constraints do not allow the customary practice of cyanidation.

2. AMMONIACAL THIOSULPHATE LEACHING

The thiosulphate process may be considered as a nontoxic alternative to

conventional cyanidation; there are continuing studies under investi-

gation (Abbruzzese et al. 1995; Ayata and Yildiran 2001; Aylmore and

Muir 2001; Feng and Van Deventer 2001; Jeffrey et al. 2001; Langhans

et al. 1992; Umetsu and Tosawa 1972; Zipperian et al. 1988;). Leaching

by thiosulphate permits a decreasing interference from foreign cations

and results in a smaller environmental impact (Zipperian et al. 1988).

Ammoniacal thiosulphate solution dissolves gold in the form of an anio-

nic aurocomplex, which is stable over a wide range of pH and Eh values.

The presence of ammonia hinders the dissolution of iron oxides, silica,

silicates, and carbonates, the most common gangue minerals found in

gold- and silver-bearing ores. Finally, in the thiosulphate system, the

oxidant necessary to oxidize metallic gold to gold(I) is present in the

solution as copper(II) ions. Several oxidants such as ozone, hydrogen

peroxide, oxygen, ferric ion, and formamidine disulfide have been used

in the studies (Hiskey 1981).

GOLD AND SILVER EXTRACTION FROM TURKISH GOLD ORE 69


The chemistry of the ammonia–thiosulphate system is very compli-

cated due to the simultaneous presence of complexing ligands such as

ammonia and thiosulphate, the Cu(II)–Cu(I) redox couple, and the

possibility of oxidative decomposition reactions of thiosulphate involving

the formation of additional sulphur compounds such as tetra-thionate

(Garrels and Christ 1965; Kerley 1981). The redox equilibrium between

the cuprous–cupric couple in ammoniacal solution is represented by the

following reaction (Abbruzzese et al. 1995; Ayata and Yildiran 2001;

Feng and Van Deventer 2001; Jeffrey et al. 2001; Langhans et al. 1992;

Umetsu and Tosawa 1972; Zipperian et al. 1988):

CuðNH3Þ2þ4 þ 3S2O2�3 þ e� ! CuðS2O3Þ5�3 þ 4NH3 ð1Þ

Therefore, during the thiosulphate leaching of precious metals, some

chemical reactions involving dissolution, oxidation, and complexation

occur. Only a few oxidizing agents, such as copper ions, are adequate in

ammoniacal solutions.

The following reaction shows the role of copper(II) ions, present in

the form of the tetramine complex, in the oxidation of gold from the

metallic state to aurous Auþ ion (Abbruzzese et al. 1995; Langhans

et al. 1992; Smith and Martel 1974; Zipperian et al. 1988):

Auþ 5S2O2�3 þ CuðNH3Þ2þ4 !AuðS2O3Þ3�2 þ 4NH3 þ CuðS2O3Þ5�3 ð2Þ

or without copper ions

2Auþ 4S2O2�3 þ 2Hþ þ 1=2O2 ! 2AuðS2O3Þ3�2 þH2O: ð3Þ

The dissolution reaction for silver can be written as (Abbruzzese et al. 1995)

2Agþ 4S2O2�3 þ 2Hþ þ 1=2O2 ! 2AgðS2O3Þ3�2 þH2O: ð4Þ

3. MATERIAL AND METHOD

The sample used in this experimental study was taken from the Bergama–

Ovacık gold ore deposit in Turkey, where the preproduction activities

started. Also, a cyanidation plant (Cyanide=Carbon-In-Pulp=INCO SO2=

Air Process for cyanide destruction) was built and is currently operational.

Cyanide and thiourea leaching of this ore were studied earlier (Tukel et al.

1996). Approximately 92% of Au and 80% of Ag was extracted at the end

of 24-h leaching and cyanide consumption was about 1.28 kg=t of ore. On

the other hand, 94.3% Au and 28.3% Ag recoveries were achieved by

thiourea leaching and thiourea consumption was 16 kg=t ore at the end

70 F. ARSLAN AND B. SAYINER


of 2.5-h leaching. As a result of this study, although thiourea leaching

resulted in slightly higher extractions in terms of Au, cyanide leaching of

this ore is preferred when other factors are considered. The aim of this

experimental study was to investigate the possibility of extracting Au and

Ag from Ovacık gold ore by using thiosulphate leaching as an alternative

to cyanide and thiourea leaching, which were studied earlier.

A sample subjected to the experiments was taken from the Ovacık gold

ore deposit, (Bergama, Turkey) and ground below 38mm before thiosulphate

leaching experiments. Chemical analysis of the ore sample is given in Table 1.

Agitation leaching with a constant stirring speed was utilized in the

experiments. At the end of the experiments, solid–liquid separation was

done by filtration. Gold and silver analyses were made from the leach cakes

by using fire assay, atomic adsorption, and inductively coupled plasma

(ICP) techniques. Thiosulphate analysis of the leach liquors was done by

iodimetric titration in order to find thiosulphate consumption. Each point

in the graphs represents a single experiment and no sample solution was

taken during the experiments.

4. RESULTS AND DISCUSSION

In the experiments, the effects of CuSO4, ammonia, thiosulphate, and

sulfite concentrations, solid–liquid ratio, and temperature on Au and

Ag extractions were investigated.

Table 1. Chemical analysis of the ore sample

used in the experiments

Element Content

Au (g=t) 6.4

Ag (g=t) 10.4

Cu (g=t) 43.7

Fe (%) 0.025

Zn (g=t) 173.5

Pb (g=t) 22.7

Co (g=t) 3.1

Ni (g=t) 36.8

Mg (%) 0.34

Cd (g=t) 3.0

Mn (g=t) 159.3

S (%) 0.02

SiO2 (%) 86.7



4.1. Effect of CuSO4 Concentration

Constant experimental conditions were: 0.5 M thiosulphate, 0.1 M

ammonia, 1=10 solid–liquid ratio, and 20�C temperature. Ag and Au leach-

ing efficiencies versus time curves for different copper concentrations are

illustrated in Figures 1 and 2, respectively. As seen in Figure 1, increasing

the CuSO4 concentration up to 0.01 M shifted Ag leaching recovery-time

curves to higher values. However, after that concentration Ag leaching

recoveries were sharply dropped to lower values. The main reason for this

is probably that increasing the Cu2þ ion concentration reduces the stability

region of the Cu(NH3)42þ complex while widening the stability region of

the solid copper compounds such as CuO, Cu2O, CuS, and Cu2S. Silver

may also precipitate in the form of Ag2S. Thus, increasing the copper

ion concentration causes the formation of solid copper compounds and

the precipitation of silver by having higher consumption of thiosulphate

as a result of changing Eh-pH equilibrium of the system. The precipitation

reaction takes place according to the following reaction (Kerley 1981):

Cu2Sþ 2AgðS2O3Þ3�2 þ 2S2O2�3 ! Ag2Sþ 2CuðS2O3Þ5�2 ; ð5Þ

A similar effect was also observed in Au leaching recoveries (Figure 2) and

the highest leaching recoveries were observed by using 0.01 M CuSO4

concentration.

Figure 1. Effect of CuSO4 concentration on the Ag leaching recovery.



4.2. Effect of Ammonia Concentration

Constant experimental conditions were 0.5 M thiosulphate, 0.01 M

CuSO4, 1=10 solid–liquid ratio, and 20�C temperature. Ag and Au leach-

ing efficiencies versus time curves for different ammonia concentrations

are illustrated in Figures 3 and 4, respectively. A curve, produced when

Figure 2. Effect of CuSO4 concentration on the Au leaching recovery.

Figure 3. Effect of ammonia concentration on Ag leaching recoveries.



no ammonia and copper sulfate were added, is also included in the graph

for comparison.

Increasing ammonia concentration up to 1 M shifted Ag leaching

recovery-time curves to higher values (85%) and had a decreasing

effect after that concentration. High ammonia concentrations increase

solution pH and reduce the area of thermodynamic stability region

of Cu(S2O3)25� and Cu(NH3)4

2þ while widening the thermodynamic

stability regions of solid copper species such as CuO and Cu2O, result-

ing in lower Ag leaching recoveries (Abbruzzese et al. 1995). Addition-

ally, it was shown that solid (NH4)5Cu(S2O3)3 forms. This solid reduces

the oxidant activity of the cuprous-tetra-ammine complex and covers

the mineral surface, hindering the thiosulphate attack. Similar results

were observed in Au leaching recoveries (Figure 4) for the same

reasons.

4.3. Effect of Thiosulphate Concentration

Thiosulphate concentration varied between 0.3–1.2 M and constant

experimental conditions were 0.01 M CuSO4, 1 M ammonia, 1=10

solid–liquid ratio, and 20�C temperature. Increasing the thiosulphate

concentration increased Ag and Au leaching recoveries up to 93%

with the addition of 1.2 M thiosulphate, as seen in Figures 5

and 6.

Figure 4. Effect of ammonia concentration on Au leaching recoveries.



4.4. Effect of Solid/Liquid Ratio

The solid–liquid ratio was taken as 1=1, 1=3, 1=10 and constant experi-

mental conditions were 1.2 M thiosulphate, 0.01 M CuSO4, 1 M ammonia,

and 20�C temperature. Results for Ag and Au are demonstrated in

Figures 7 and 8, respectively. As seen from these figures, increasing the

Figure 5. Effect of thiosulphate concentration on Ag leaching recoveries.

Figure 6. Effect of thiosulphate concentration on Au leaching recoveries.



solid–liquid ratio had a decreasing effect in Ag leaching recoveries and did

not have much affect on Au leaching recoveries.

4.5. Effect of Temperature

Experiments were performed at three different temperatures in order to

see temperature’s effect on Ag and Au leaching recoveries. Constant

Figure 7. Effect of the solid–liquid ratio on Ag leaching recoveries.

Figure 8. Effect of the solid–liquid ratio on Au leaching recoveries.



experimental conditions were 1.2 M thiosulphate, 0.01 M CuSO4, 1 M

ammonia, and 1=10 solid–liquid ratio. Results are shown in Figures 9

and 10.

As seen from Figure 9, increasing temperature over 40�C had a

decreasing effect on Ag leaching recovery and some precipitation is

observed in leach liquors. The temperature influence on Au leaching

efficiency was similar to that of Ag and increasing the temperature to 60�C

decreased Au leaching recoveries very sharply, as seen in Figure 10. At

temperatures over 40�C, the decreased Au and Ag recoveries may be

ascribed to passivation due to cupric sulphide, formed by thermal reaction

between Cu(II) ions and thiosulphate (Abbruzzese et al. 1995):

Cu2þ þ S2O2�3 þH2O! CuSþ SO2�

4 þ 2OH�: ð6Þ

At 60�C, the kinetics of cupric sulphide film formation is very fast, hinder-

ing Au and Ag dissolution. An increase in temperature from 25 to 60�C

facilitates the loss of thiosulphate by decomposition of sulfur compounds.

Under these conditions, only a small fraction of the added thiosulphate

remains available for Au and Ag complexation (Abbruzzese et al. 1995;

Lee 1974).

2S2O2�3 þH2Oþ 1=2O2 ! S4O2�

6 þ 2OH� ð7Þ

Figure 9. Effect of temperature on Ag leaching recoveries.



2S2O2�3 þ 3H2O! 4SO2�

3 þ 2S2� þ 2Hþ: ð8Þ

As a consequence, the gold and silver recoveries fall.

Thiosulphate consumption at different temperatures is shown in

Table 2. Although increasing temperature decreased Ag and Au leaching

recoveries, thiosulphate consumption increased with increasing tempera-

ture at the end of 4-h leaching. Thiourea consumption is also found to be

high, as given in the literature, due the formation of complexes with

impurity metals and by oxidation of products that are ineffective in

complexing gold and silver (Hiskey 1981).

4.6. Effect of Sulfite Addition

Sodium sulfite concentration varied between 0 and 0.2 M and constant

experimental conditions were 1.2 M thiosulphate, 0.01 M CuSO4, 1 M

Figure 10. Effect of temperature on Au leaching recoveries.

Table 2. Thiosulphate consumption (kg=t ore) for the experiments run at

different temperatures

Time (h) 20�C 40�C 60�C

0.5 75.6 62.2 55.4

2 82.4 95.6 82.6

4 95.6 108.8 153.8



ammonia, 20�C, and 1=10 solid–liquid ratio. Results in terms of Ag

leaching recovery versus time at different sulfite concentrations and

Au recovery versus sulfite concentration at constant time of 4-h are

shown in Figures 11 and 12, respectively. Thiosulphate consumptions

for the experiments are given in Table 3.

Figure 11. Effect of sodium sulfite concentration on Ag leaching recoveries.

Figure 12. Effect of sodium sulfite concentration on Au leaching recoveries.



Silver leaching recoveries slightly increased with increasing sulfite

concentration, as seen in Figure 11. Almost 100% Ag leaching recov-

ery was observed with the addition of 0.2 M sodium sulfite at the end

of 4-h leaching. As shown in Figure 12, approximately 100% of gold

was leached with the small addition of sulfite (0.0 M). Increasing sulfite

concentration reduced thiosulphate consumption, as given in Table 3.

As a result of these experiments, it was shown that almost all Au and

Ag present in Ovacik gold ore can be recovered by applying thiosulphate

leaching. In earlier studies, 90% of Au and 80% of Ag were leached by

cyanidation while higher gold efficiencies were observed in thiourealeach-

ing experiments (Tukel et al. 1996). However, silver efficiencies in thiourea

leaching remained below 30%. Therefore, thiosulphate leaching experi-

ments represent better results in terms of Au and Ag leaching efficiencies.

5. CONCLUSIONS

The following conclusions are made as a result of the thiosulphate leaching

of Bergama–Ovacık gold ore containing 6.4 g=t Au and 10.4 g=t Ag.

. Increasing copper sulphate concentration up to 0.01 M had an increasing

effect on Au and Ag leaching recoveries. After that point, leaching

recoveries started to decline.

. Increasing ammonia concentration up to 1 M caused Au and Ag leaching

recoveries to increase and had a decreasing effect after that point.

. Increasing thiosulphate concentration increased Ag leaching recoveries

up to 93% with the addition of 1.2 M thiosulphate, while similar Au

leaching recovery was observed at 0.5 M thiosulphate concentration.

. Increasing solid–liquid ratio resulted in decreasing Au and Ag leaching

recoveries.

. Increasing temperature over 40�C had a decreasing effect on Au and

Ag leaching recoveries.

Table 3. Thiosulphate consumption (kg=t ore) for the experiments carried out at different

sulfite concentrations

Time (h) 0.0 M 0.01 M 0.05 M 0.1 M 0.2 M

0.5 75.6 57.5 49.7 10.7 12.0

2 82.4 57.3 50.7 28.6 18.7

4 95.6 57.3 59.6 37.5 22.2



. Almost 100% Ag and Au leaching efficiencies were observed with

the addition of sodium sulfite during leaching. Increasing sulfite

concentration had a reducing effect on thiosulphate consumption from

95.6 to 22.2 kg=t in 4-h leaching time.

. As a result of this study, it was shown that Au and Ag recoveries

from Ovacık gold ore could be increased by applying thiosulphate

leaching.

In conclusion, it can be said that thiosulphate leaching experiments

represents better results than cyanidation and thioureation in terms of

Au and Ag leaching efficiencies.

REFERENCES

Abbruzzese, C., Fornari, P., Massidda, R., Veglio, F., and Ubaldini S., 1995,

‘‘Thiosulphate leaching for gold hydrometallurgy.’’ Hydrometallurgy, 39,

pp. 265–276.

Ayata, S. and Yildiran, H., 2001, ‘‘A novel technique for silver extraction from

silver sulfide ore.’’ Turkish Journal of Chemistry, 25(2), pp. 187–191.

Aylmore, M. G. and Muir, D. M., 2001, ‘‘Thiosulfate leaching of gold—A

review.’’ Minerals Engineering, 14(2), pp. 135–174.

Feng, D. and Van Deventer, J. S. J., 2001, ‘‘Preg-robbing phenomena in the thio-

sulphate leaching of gold ore.’’ Minerals Engineering, 14(11), pp. 1387–1402.

Garrels, R. M. and Christ, L. C., 1965, Solutions, Minerals and Equilibria,

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