Role of copper(II), carbonate and sulphite in gold leaching and thiosulphate degradation by oxygenated alkaline non-ammoniacal solutions

Researchers have been unable to incorporate the copper(II)-thiosulphate complexes in Eh-pH or species distribution diagrams due to the unreliability of the reported stability constants. The present work describes a concurrent thermodynamic and kinetic analysis of literature data to show that the reaction between copper(II) and thiosulphate ions takes place via the formation of an unstable Cu(S2O3)(n)(H2O)(p)(-2(n-1)) complex followed by its first or second order decomposition. The oxidation of thiosulphate to tetrathionate, predominantly via the second order disproportionation with a rate constant of 1.2 x 10(5) lmol(-1)s(-1) produces Cu(S2O3)(-), which is stabilized as Cu(S2O3)(n)(-(2n- 1)) (n = 2 or 3). This analysis allows the calculation of two stability constants 10(2.4) and 10(5.2) for Cu(S2O3)(0) and Cu(S2O3)(2)(2-) respectively, which predict the predominant species: Cu(S2O3)(2)(2-) (pH < p), Cu(OH)(2)(0) (9 < pH < 11) and Cu(OH)(3)(-) (pH > 11). The disproportionation of Cu(S2O3)(0) and Cu(S2O3)(-) leads to the precipitation of Cu, CuS and Cu2S. The calculated equilibrium constants predict that copper(II) complexes such as Cu(S2O3)(0), Cu(S2O3)(2)(2-), Cu(OH)(2)(0) and Cu(OH)(3)(-) can leach gold to produce Au(S2O3)(2)(3-), Au(OH)(0), and Au(OH)(2)(-), where the latter two are converted to the more stable Au(S2O3)(2)(3-) complex. The rate of thiosulphate oxidation by oxygen during gold leaching from copper gold ores in alkaline media is of the order approximate to 10(-6) mol l(-1)s(-1), irrespective of the presence or absence of ammonia. This is three orders of magnitude smaller than the rate of oxidation of thiosulphate by copper(II) in acid media, indicating the involvement of mixed complexes of the type Cu(II)(L)(n)(S2O3)(m)(O-2) in ammoniacal (L = NH3) and non-ammoniacal (L = OH-) media, compared to the disproportionation of Cu(II)(S2O3)(1-2) in acid media with no oxygen. The gold leaching in oxygenated alkaline thiosulphate media obeys a shrinking core kinetic model with an apparent rate constant of the order 10(-5) s(-1). In the presence of 0.4 M total carbonate in alkaline media, the predominant copper(II) complex is Cu(CO3)(2)(2-). This minimizes thiosulphate degradation to tetrathionate via the disproportionation of Cu(II)(S2O3)(1-2). However, the presence of carbonate enhances the overall thiosulphate consumption and trithionate and sulphate formation, leading to a lower gold extraction. In a thiosulphate deficient leach liquor, such as that produced in the presence of carbonate, gold precipitation takes place via disproportionation reactions. The addition of sulphite reproduces thiosulphate from tetrathionate, but slowly precipitates gold. (c) 2004 Elsevier Ltd. All rights reserved.