An Electrochemical Study of the Dissolution of Pure Gold And ...

An Electrochemical Study of the Dissolution of Pure Gold And Gold-Silver Alloys in Thiosulfate Leach Solutions

E. A. Orabya,b and M. I. Jeffreya*

a Parker Centre (CSIRO Minerals Down Under Flagship), P.O Box 7229, Karawara, WA

6152, Australia b Parker Centre (Faculty of Engineering and Science, Curtin University), Australia

A fundamental electrochemical study of gold and gold/silver alloys in thiosulfate solutions showed that although the oxidation is a complex process, there was no selective dissolution of either gold or silver from the alloys during oxidation. The presence of copper and thiourea catalyses the oxidation of the alloys, resulting in good leaching for thiosulfate systems when utilising either of these species. Surprisingly for copper containing solutions, the oxidation rate is largely independent of potential, and is higher for pure gold than for the 4% Ag and 8% Ag alloys. For thiourea containing solutions, the rate of alloy oxidation increases with increases in the silver content of the alloy. In the absence of copper or thiourea, the only alloy which showed appreciable oxidation was 50% Au / 50% Ag, although the rate of oxidation did increase with an increase in silver content from 0 to 50%.

Introduction The cyanidation process has been used for recovery of precious metals for more than

100 years due to its simplicity, high leaching efficiency, and low total cost. However, there are some disadvantages of using cyanide such as its toxicity and low efficiency in extracting gold from refractory and carbonaceous ores or concentrates. Many researchers around the world have been attempting to evaluate and optimise alternatives to cyanides; thiosulfate is considered as one of the less-toxic alternatives to cyanide, and in some cases, is more effective at recovering gold than cyanide (1).

The leaching of gold and silver in thiosulfate solutions is an electrochemical reaction.

The anodic half reactions for gold and silver oxidation are shown in Equations [1] and [2] respectively.

[1]

[2] The standard potential of the gold oxidation to the gold thiosulfate complex as shown

in Eq. (1) is 0.15 V (2). The oxidation potential for silver to the silver thiosulfate complex, 0.02 V (3), is considerably more negative than that of gold.

Over the last two decades, kinetic and electrochemical studies of gold and silver

leaching in thiosulfate solutions have been the subject of many investigations (4-13). Many studies have also focussed on the electrochemistry of the gold oxidation half

−−− +→+ eOSAuOSAu 3232

232 )(2

−−− +→+ eOSAgOSAg 3232

232 )(2

ECS Transactions, 28 (6) 237-248 (2010)10.1149/1.3367917 © The Electrochemical Society

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reaction in thiosulfate solutions, with the effects of the main reagents in the solutions such as thiosulfate, ammonia, and copper concentrations and other parameters, such as temperature, and pH being reported. For instance, it has been shown that ammonia improves the rate of gold oxidation in thiosulfate solutions, with the reaction being very slow in the absence of ammonia due to surface passivation of gold (10, 14-16). The presence of copper(II) ions is also catalytic in increasing the rate of the gold oxidation half reaction in thiosulfate solutions (11, 15), and recently it has been observed that low concentrations of thiourea significantly catalyses the gold oxidation half reaction (16-17).

Despite the abundance of work on the leaching and electrochemistry of gold in

thiosulfate solutions, there has been very little work published on the leaching of gold-silver alloys in thiosulfate solutions. This is potentially significant, as alloying the gold with low levels of silver leads to a very large enhancement in the rate of gold leaching in the cyanidation process (18-21), and naturally gold most often occurs alloyed with some silver. The objective of this study was to evaluate the effect of alloying silver (4, 8, 20, and 50 wt% Ag) on the gold and silver leaching in thiosulfate solutions.

Experimental Methods

All experiments were carried out using solutions prepared from analytical reagents

and distilled water. Electrochemical experiments were carried out using pure gold, and gold containing 4, 8, 20, and 50 wt% silver rotating discs. These were polished with a waterproof silicon carbide paper (FEPA # 2400) and 3 micron aluminium oxide powder prior to the experiment. The pure gold disc was made from pure gold foil (99.99% Au). All discs are 1.7 cm in diameter and have a surface area of 2.27 cm2.

Two different electrochemical techniques were used: coulometric experiments at a

fixed potential, and linear sweep voltammetry, both using a Radiometer PGP201 potentiostat. The reaction cell was a three-electrode system in which a platinum wire was used as the counter electrode. All potentials were measured relative to the saturated calomel electrode (+ 0.242 V vs. SHE) but reported relative to the standard hydrogen electrode (SHE). Linear sweep voltammetry experiments were performed at 30 ºC, with a rotation rate of 300 rpm and a scan rate of 1 mV s-1. All the solutions were de-aerated using pure nitrogen gas for 20 minutes. The current was measured at a potential range of 0 - 300 mV.

Coulometric experiments were conducted for 30 minutes at different potential values

using pure gold or gold-silver discs in an electrochemical cell containing thiosulfate solution. The current was measured with time, and the charge density (mC.cm-2) was obtained by the integration of the current-time plot for each disc in different solutions. All experiments, unless otherwise specified, were performed at a rotation rate of 300 rpm and at a temperature of 30 ºC. All the solutions were de-aerated using pure nitrogen for 20 minutes. After 30 minutes, the samples were collected and analysed for gold and silver by ICP–OES.

ECS Transactions, 28 (6) 237-248 (2010)



Results and Discussions Gold and Gold/silver alloy leaching rates

Two of the candidate thiosulfate leach systems are the copper-ammonia, and iron(III)-

thiourea systems. For the copper-ammonia system, the oxidant is the cupric tetraamine complex, and the complete reaction for the dissolution of gold can be represented by Equation [3]. There are two variations of the iron(III)-thiourea, in which iron(III) is the oxidant (complexed with either EDTA or oxalate), and thiourea is added as a catalyst. The leaching reaction with the ferric-EDTA oxidant is shown in Equation [4].

[3]

EDTAFeOSAuOSEDTAFeAu +−−+ +⇔++ 23232

232

3 )(2 [4] The gold leach rate obtained for both systems is shown in Figure 1 for the range of

gold/silver alloys . The leach behaviour of the two systems is significantly different, with the gold leach rate of the 4 and 8% silver alloys being higher than pure gold for the iron(III)-EDTA-thiourea system, but lower than pure gold for the copper-ammonia system. The total alloy dissolution rate (defined as the sum of the silver and gold leach rates) are also shown in Figure 1 (right), from which similar behaviour is evident. Electrochemical experiments were carried out in an attempt to elucidate the mechanisms that lead to these results.

Figure 1. Gold and total alloy dissolution rates from pure gold, 4, 8, 20, and 50% Ag discs in: a) 200 mM thiosulfate, 400 mM NH3 and 10 mM Cu(II); and b) 50 mM thiosulfate, 5 mM thiourea and 5 mM ferric-EDTA.

Linear sweep voltammetry

The voltammetry of gold and gold/silver alloys was initially carried out in ammonium thiosulfate solutions of different thiosulfate concentrations. Figure 2 shows those obtained for 100 mM ammonium thiosulfate (ATS). It is evident that pure gold only oxidises very slowly over a wide potential range, but as the quantity of silver alloyed with the gold increases, the oxidation of the alloy becomes significantly enhanced, especially

0.0

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Silver, %

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d di

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-2.s-1

thiosulfate-ammonia-copper

thiosulfate-thiourea-ferricEDTA

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Silver, %

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Tot

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thiosulfate-ammonia-copper

thiosulfate-thiourea-ferricEDTA

−−+− ++⇔++ 53323

3232

243

232 )(4)()(5 OSCuNHOSAuNHCuOSAu




for the alloy containing 50% gold and 50% silver. These results are consistent with those previously published for an alloy containing 2% silver (22). For 25 mM thiosulfate, a very similar trend was observed, with rapid oxidation of the 50% silver alloy, but poor oxidation of the other alloys. From a leaching perspective, the voltammetry results suggest that if the gold in an orebody is alloyed with a high percentage of silver (electrum), then a weak oxidant (e.g. oxygen) and low thiosulfate concentrations should be adequate to ensure fast dissolution of the gold.

Figure 2. Linear sweep voltammograms showing the effect of silver content on gold/silver oxidation. Experimental conditions: (left) 100 mM ATS, 300 rpm, 30 ºC; and (right) 25 mM ATS, 300 rpm, 30 ºC.

Figure 3 (left) shows the oxidation of the gold/silver alloys in ammonium thiosulfate

solutions in the presence of 400 mM ammonia. The voltammograms show similar behaviour to those obtained in the absence of ammonia in that the rate of oxidation of the alloy increases with an increase in the percentage of silver in the alloy. The key difference however is that the oxidation of the pure gold and all the gold/silver alloys was more active in the presence of ammonia. Increasing the ammonia concentration also increases the rate of alloy oxidation, as shown in Figure 3 (right) for the 4% silver alloy. Therefore it would be expected that the rate of alloy leaching would increase as the ammonia concentration increased.

Figure 4 shows the effect of thiourea on the rate of alloy oxidation in thiosulfate

solutions. By comparing the voltammograms in the absence of thiourea, it is apparent that for the alloys containing up to 20 % silver, the rate of oxidation in the presence of thiourea is significantly higher, especially at less positive potentials. It is interesting to note though that the percentage of silver in the alloys does not have a significant influence on the rate of oxidation of the alloy in the presence of thiourea.

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4% Ag

8% Ag

20% Ag

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Pure gold

4% Ag

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50% Ag




Figure 3. Linear sweep voltammograms showing the effect of silver content thiosulfate concentration on gold/silver alloy oxidation. Experimental conditions: (left) 50 mM ATS, 400 mM ammonia, 300 rpm, 30 ºC; (right) 4% Ag alloy, 300 rpm, 30 ºC.

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0 50 100 150 200 250 300

Potential, mV

Curr

ent,

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-2

Pure gold

4% Ag

8% Ag

20% Ag

Figure 4. Linear sweep voltammograms showing the effect of silver content on gold/silver oxidation in thiosulfate solutions: 50 mM ATS, 5 mM Tu, 300 rpm, 30 ºC.

Although voltammetry is a rapid and powerful tool for predicting the effect of the various parameters on the leaching of gold/silver alloys, there are a number of phenomena observed during leaching which were not explained by the voltammograms. For example:

• For the copper-ammonia system, the alloy leach rates were lower for 4 and 8% silver

alloys than for pure gold. The voltammetry predicts that the leach rate should increase when increasing the percentage of silver in the alloy

• For the iron(III)-EDTA-thiourea system, the alloy leach rates increased with percentage silver in the alloy, yet the voltammograms were very similar regardless of the silver content in the alloy.

4% Ag

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None

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0246

81012141618

20


Curr

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4% Ag8% Ag

20% Ag

50% Ag




• For pure gold, the leach rate in the copper-ammonia system were higher than the iron(III)-EDTA-thiourea system, despite the voltammetry indicating that the gold should be much more readily oxidized.

To establish which of these factors can be related to the oxidation half reaction, and

which factors can be related to the cathodic half reaction, longer term coulometric experiments were carried out.

Coulometric Experiments at Fixed Potential

Both the gold and total alloy oxidation rates were determined for three thiosulfate solutions representing the potential leach systems proposed: ammoniacal thiosulfate without copper; the copper-ammonia thiosulfate system; and the non ammonia/ammonium systems where thiourea is an alternative catalyst to accelerate the gold oxidation half reaction (17, 22). These experiments were carried out at four different electrode potentials, with the gold leach rate shown in Figure 5, and the total alloy leach rate shown in Figure 6. Since the electrode potential was controlled by a potentiostat, the leach rates obtained are a direct reflection on the anodic oxidation of the alloys.

Figure 5. Gold dissolution rates from pure gold, 4, 8, 20, and 50% Ag discs at different potentials in thiosulfate solutions: a) 50 mM ATS, 400 mM NH3, b) 50 mM ATS, 400 mM NH3, 10 mM Cu(II), c) ATS, 5 mM Tu.

0.22 V

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a) ATS + ammoniab ) ATS + ammonia + copper(II) c) ATS + Tu

0.256 V

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ATS + ammoniaATS + ammonia + copper(II)ATS + Tu

0.292 V

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a) ATS + ammoniab) ATS + ammonia + copper(II)c) ATS + Tu

0.35 V

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a) ATS + ammoniab) ATS + ammonia + copper(II)c) ATS + Tu




It is clear from Figure 5 that at 0.22 V, the gold dissolution rate of the copper-ammonia thiosulfate system is the highest, and significantly higher than the solutions in the presence of thiourea. These results confirm that copper is very effective in catalysing the gold oxidation half reaction, and explains why the copper-ammonia leach system works well, as proposed previously (11, 15). This result is not simply due to the presence of the copper(II) oxidant, as the potential of the disc was fixed using a potentiostat, and hence the alloy was not necessarily at the open circuit potential (mixed potential) of the leach system. The gold leach rates obtained at fixed potential also show a decrease when comparing pure gold to the alloys with a low silver content. This is also consistent with the leaching results.

Figure 5 also shows that for the ammoniacal thiosulfate solution without copper, the

gold leach rate is very slow for the alloys with a low silver content, but increases significantly for the 20 and 50% silver alloys. The effect of thiourea as a catalyst is also confirmed for all the gold/silver alloys studied, with the rate of gold leaching from the alloys also increasing with the percentage of silver in the alloy. This is consistent with the leach results for the iron(III)-EDTA-thiourea system.

As the potential is increased, it is interesting to note that there is very little change in

the gold dissolution rate for the copper-ammonia thiosulfate system. In contrast, the gold leach rate increases significantly for the other two systems studied, which is more typical for electrochemical reactions. However, it should be remembered that the standard potential for the oxidation of thiosulfate is 0.25 V (16), and therefore the adoption of a more negative potential (utilisation of less oxidising oxidants) in the leach system is likely to result in a lower thiosulfate consumption. Hence the data at 0.22 V is the most relevant to the thiosulfate leaching systems in practical use.

It should be remembered that the data shown in Figure 5 only represents the rate of

gold dissolution; for the alloys, especially those with a high silver content, the rate of oxidation of the alloy will be higher. The alloy dissolution rates from each disc at different potential values are shown in Figure 6, from which, the increase in the rate of reaction at higher silver contents in the alloy is much more apparent. At 0.22 V, as the silver increases above 4%, the alloy oxidation rate increases for all three of the systems. It is however interesting to note that for the copper-ammonia thiosulfate system, in a similar manner to the leach data, the oxidation rate of pure gold is higher than the oxidation rate for the 4% silver containing alloy. Similar behaviour is also seen for the thiourea containing system at the three higher potential values. For solutions containing thiosulfate-ammonia without copper, the total leach rate increased as the percentage of silver increased. These results are consistent with the voltammetry in that as the percentage of silver increased, the oxidation current also increased. It is possible that the presence of silver reduces or inhibits the passivation of the gold-silver alloy during leaching.

Another key observation can be made by calculating the ratio of oxidation rate of

gold to silver (obtained from the solution assays), and comparing it to the alloy composition, as shown in Figure 7 (on a mole basis). It is clear that the majority of the data points lie on the dashed line which has a slope of 1, indicating that there is no selective dissolution of either silver or gold from the alloy substrate for the three systems investigated. The deviation for the thiosulfate plus ammonia system at the high alloy




ratios of Au:Ag is considered to be due to experimental error, since the quantity of silver leached into solution was very low due to both the low leach rates and the low proportion of silver in the alloy.

Since the current was also measured during the oxidation of the alloys, the alloy

oxidation rate can also be calculated using Faraday’s law. This was only completed for the non copper systems, as the presence of the copper(I) / copper(II) couple results in additional current flow. The results are shown in Tables I and II respectively for the fixed potential of 0.292 V. The current efficiency was calculated as the ratio between the measured alloy dissolution rate and the calculated rate from the charge. It is clear that at the applied potential of 0.292 V, the only electrochemical reaction occurring at the electrode is the oxidation of the alloy. Similar results were obtained at 0.22 and 0.256 V, although at the higher potential value (0.35 V, Table III), the current efficiency was found to decrease slightly, due to the simultaneous oxidation of the thiosulfate at the surface of the alloy. It is interesting to note that the rate of thiosulfate oxidation on the alloy surface increased as the percentage of silver in the alloy increased.

Figure 6. Total dissolution rates of gold/silver from pure gold, 4, 8, 20, and 50% Ag discs at different potentials in thiosulfate solutions: a) 50 mM ATS, 400 mM NH3, b) 50 mM ATS, 400 mM NH3, 10 mM Cu(II), c) 50 mM ATS, 5 mM Tu.

0.22 V

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b ) ATS + ammonia + copper(II)

c) ATS + Tu

0.256 V

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c) ATS + Tu

0.35 V

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c) ATS + Tu

0.292 V

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Figure 7. Ratio of the gold to silver dissolved in solution at a fixed potential of 0.22 V versus the ratio of gold to silver in the alloy for the systems: a) 50 mM ATS, 400 mM NH3, b) 50 mM ATS, 400 mM NH3, 10 mM Cu(II), c) 50 mM ATS, 5 mM Tu. TABLE I. Total measured and calculated leach rates of the gold/silver alloys. Experimental conditions: 50 mM ATS, 400 mM ammonia, 300 rpm, 30 ºC, 0.292 V.

Disc type Rate from Charge,

105 x mol.m-2.s-1

Dissolution rate, 105 x mol.m-2.s-1

Current efficiency, %

Pure Gold 0.06 0.05 95

4% Ag 0.46 0.48 104

8% Ag 0.79 0.83 105

20% Ag 4.61 4.48 97

50% Ag 13.16 12.61 96 TABLE II. Total measured and calculated leach rates of the gold/silver alloys. Experimental conditions: 50 mM ATS, 1 mM Tu, 300 rpm, 30 ºC, 0.292 V.

Disc type Rate from Charge,

105x mol.m-2.s-1

Dissolution rate, 105 x mol.m-2.s-1

Current efficiency, %

Pure Gold 1.43 1.41 99

4% Ag 0.59 0.61 103

8% Ag 0.92 0.94 102

20% Ag 1.54 1.58 103

50% Ag 3.87 3.79 98




A plot of log (alloy oxidation rate) versus the electrode potential was constructed for the three systems, and Figures 8, 9, and 10 show the Tafel style plots for the different alloys in each solution respectively. For the copper-ammonia system, the Tafel plot is almost vertical, indicating that the applied potential has very little influence on the rate of oxidation of the alloys. Such a phenomenon is usually observed for diffusion controlled reactions, and may indicate the diffusion of a copper species (e.g. a copper complex) is important in controlling the rate of alloy oxidation. However, it is very difficult to estimate the concentration of the various copper complexes to confirm this, as there is a dynamic mix of both copper(I) and copper(II) ammine and thiosulfate complexes in the copper-ammonia leach system. For the ammonia-thiosulfate and thiosulfate-thiourea systems, the Tafel plots are more typical of those expected for metal oxidation reactions, and the Tafel slopes for the active area of these plots were extracted and the results are tabulated in Table IV. For all the alloys, the Tafel slopes were higher than 60 mV, indicating that the alloys were not undergoing active dissolution in the thiosulfate solutions. TABLE III. Current efficiency for the dissolution of the alloys in solutions containing (a) 50 mM ATS, 400 mM ammonia (b) 50 mM ATS, 5 mM Tu. Experimental conditions: 300 rpm, 30 ºC and 0.35 V.

Disc type Current efficiency, %

ATS + NH3 ATS + Tu

Pure Gold 102 103

4% Ag 97 94

8% Ag 87 92

20% Ag 85 91

50% Ag 82 90

0.15

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0.25

0.3

0.35

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0.45

-7.5 -7 -6.5 -6 -5.5 -5 -4.5 -4 -3.5

Log total rate, mol.m-2.s-1

Pote

ntia

l, V

Pure gold4% Ag8% Ag20% Ag50% Ag

Figure 8. Tafel plots for gold/silver oxidation in thiosulfate-ammonia solutions (50 mM ATS, 400 mM NH3).




0.15

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0.25

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-5.5 -5 -4.5 -4 -3.5


Pote

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Figure 9. Tafel plots for the total leach rate in thiosulfate-ammonia-copper solutions at different percentages of silver (50 mM ATS, 400 mM NH3, 10 mM Cu(II).

0.15

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0.25

0.3

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-6 -5.5 -5 -4.5 -4 -3.5


Pote

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l, V


Figure 10. Tafel plots for gold/silver oxidation in thiosulfate-thiourea solutions (50 mM ATS, 5 mM Tu). TABLE IV. Tafel slopes for each alloy in the different thiosulfate systems.

Disc type

Tafel slope, mV

ATS + NH3 ATS + Tu ATS + NH3+Cu(II)

Pure gold 77.7 85.8 1431

4% Ag 79.8 116 513

8% Ag 93.2 125 406

20% Ag 99.2 131 307

50% Ag 204 162 252




Conclusions The leach rate of gold/silver alloys was found to increase with the concentration of

silver in the alloy for both the copper-ammonia iron(III)-EDTA-thiourea based thiosulfate leach systems. Electrochemical studies, including voltammetry and controlled potential experiments confirmed that the rate of oxidation of the alloys increases with the content of silver in the alloy. There is selective dissolution of silver or gold from the alloys during oxidation, and copper and thiourea were found both to be effective catalysts for the oxidation of the alloys, which is similar to the behaviour of pure gold. It was however surprising that for the system containing copper, the rate of alloy oxidation was not very dependent on applied potential, and hence exhibit a high Tafel slope. Another unexpected observation was that both the leaching and oxidation rate of pure gold in the presence of copper was higher than for the alloys containing 4% and 8% silver.

References

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p. 1141, AusIMM, Parkville, Victoria (1993). 15. P. L. Breuer, M. I. Jeffrey, Hydrometallurgy, 65, 145 (2002). 16. I. Chandra and M. I. Jeffrey, Hydrometallurgy, 73, 305 (2004). 17. H. G. Zhang, M. J. Nicol and W. Staunton, in Treatment of Gold Ores, G.

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18. Y. U. Choi, E. C. Lee, K. N. Han, Metall. Mater. Trans. B, 22b, 755 (1991). 19. G. Q. Liu, Cyanidation of gold and silver in oxygen-enriched solution. Master of

Science, Queen`s University, Canada (1993). 20. M. I. Jeffrey and I. M. Ritchie, J. Electrochem. Soc., 147, 3272 (2000). 21. K.Cerovic, H. Hutchison and R. F. Sandenbergh, Miner. Eng., 18, 585 (2005). 22. I. Chandra and M. I. Jeffrey, Hydrometallurgy, 77, 191 (2005).




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