Effect of sulfides on gold dissolution in ammoniacal thiosulfate medium
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I. INTRODUCTION
THIOSULFATE as an alternative lixiviant for gold has received much attention in recent years due to the growing environmental and public concerns over the use of cyanide. Acceptable gold leaching rates using thiosulfate were achieved in the presence of ammonia with cupric ion acting as the oxidant.[1–19] Most work was carried out on complex ores containing high copper, carbonaceous ores, or ores containing high concentrations of lead, zinc, or manganese. The gold dissolution rates and percentage recovery varied depending on the deportment of gold in the ores. A wide range of conditions appeared to have been used, including relatively severe conditions, compared with those used in the cyanidation process. The gold concentration in the ore varied from 1 to 62 g/ton. The addition of copper in the cupric form varied from 0.001 to 0.1 mol/L, whereas the ammonia concentration varied from 0.1 to 6 mol/L and the thiosulfate concentration from 0.1 to 2 mol/L. The pH conditions were all alkaline with most leaches carried out at pH 9 to 10. The gold leaching from ores appears to be case specific, depending on the ore mineralogy. The successful application of thiosulfate leaching depends not only on the dissolution behavior of gold, but also critically on the behavior of the associated minerals. The leaching of gold in thiosulfate solutions is an electrochemical reaction, with the constituent half reactions being the oxidation of gold to gold thiosulfate, and the reduction of Cu(II) amine to Cu(I) thiosulfate. These half-cell reactions are shown in Eqs. [1] and [2], respectively.[20] Au ⫹ 2S2O32⫺ → Au(S2O3)3⫺ ⫹ e⫺
[1]
Cu(NH3)42⫹ ⫹ 3S2O32⫺ ⫹ e⫺ → Cu(S2O3)35⫺ ⫹ 4NH3 [2]
D. FENG, Research Fellow, and J.S.J. VAN DEVENTER, Professor, are with the Department of Chemical Engineering, The University of Melbourne, Victoria, 3010, Australia. Contact e-mail: [email protected]. au Manuscript submitted November 26, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
The thiosulfate leaching system is complicated by the homogeneous reduction of Cu(II) by thiosulfate according to the simplified overall reaction shown in Eq. [3]. 2Cu(NH3)42⫹ ⫹ 8S2O32⫺ → 2Cu(S2O3)35⫺ ⫹ 8NH3 ⫹ S4O62⫺
[3]
Gong et al.[3] suggested that the kinetics of leaching gold from an auriferous pyrite concentrate could be controlled by a corrosion reaction of the ammonia-thiosulfate-copper system on pyrite. From X-ray powder diffraction studies, Qian et al.[21] observed less chalcopyrite present in thiosulfate leached residues than in the original ore. They concluded that an ammonia-thiosulfate solution containing copper leached chalcopyrite by a corrosive process. Scanning electron microscopy showed clear scarring on leached chalcopyrite grains. These authors also observed some dissolution of pyrite in the residues. Copper sulfide minerals, other than chalcopyrite, also dissolved readily in an aerated thiosulfate leach solution, particularly when ammonia was in the solution. Based on our recent studies,[22] other sulfide minerals also showed different
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