Copper(i) speciation in mixed thiosulfate-chloride and ammonia-chloride solutions: XAS and UV-Visible spectroscopic studies

Thiosulfate and ammonia mixtures may be more environmentally benign alternatives to cyanide for leaching Au from ores. In this method, the Cu(i)/Cu(ii) couple acts as a redox mediator aiding in the oxidative dissolution of metallic Au. Information about the speciation of Cu(i) and Cu(ii) in these lixiviant solutions is paramount to the optimization of gold ore processing conditions. With this in mind, we have carried out XANES, EXAFS and UV-Vis spectroscopic studies of the speciation of Cu(i) in mixed thiosulfate-chloride and ammonia-chloride solutions. In thiosulfate-chloride solutions, the EXAFS studies indicate that the geometry of the predominant Cu(i) complex is distorted trigonal (triangular planar), with an average of 2 sulfur atoms + 1 oxygen atom occupying the coordination sphere. This indicates that the stability of the [Cu(S₂O₃)₃]^5- complex is lower than previously proposed. Formation constants for Cu(i) thiosulfate complexes have been derived on the basis of systematic UV-Vis measurements of solutions with varying [S₂O₃]/[Cl] ratios. Only one mixed chloride-thiosulfate complex, [Cu(H₂O)(S₂O ₃)Cl]^2-, was found to predominate over the range of conditions investigated. For Cu(i) in ammonia-chloride solutions, our results confirm the broad stability of [Cu(NH₃)₂]⁺ and we have also identified a stable mixed amminechlorocopper(i) complex, [CuCl(NH₃)]⁺. XAS reveals that these two complexes share a linear geometry. This study demonstrates that combinations of methods are required to decipher the geometry and thermodynamic properties of transition metal complexes in mixed ligand chemical systems where many species may coexist. Our results allow more comprehensive predictions of solution speciation and contribute to efforts to design improved methods to process gold ore with thiosulfate and ammonia lixiviants.