Comparative thermodynamic analysis of gold dissolution in four organic carboxylic acid-catalyzed thiosulfate systems using Eh–pH and species distribution diagrams

The non-ammonia thiosulfate leaching system is considered to be an attractive and eco-friendly route for gold leaching without the usage of cyanide or ammonia. In this paper, the role of four organic carboxylic acids, named oxalic acid (Ox), malic acid (Mal), citric acid (Cit), and tartaric acid (Tart) in copper-thiosulfate system has been studied comparatively. Based on the available thermodynamic data, a series of E_h–pH and species distribution diagrams for copper-Ox/Mal/Cit/Tart-thiosulfate system under various conditions have been constructed from thermodynamic calculation. The results show that thiosulfate is the only effective complexing agent for aurous ions, while the organic carboxylic acids work as a copper ligand. The copper-Mal/Cit/Tart-thiosulfate system exhibits potential advantages over the traditional copper-ammonia-thiosulfate system, such as wider pH range and lower thiosulfate consumption. The calculation also indicates the redox potentials of Cu(II)/Cu(I) for the leaching systems are in the subsequence of E⁰[Cu(Ox)₂²⁻/Cu(S₂O₃)₃⁵⁻] >E⁰[Cu(NH₃)₄²⁺/Cu(S₂O₃)₃⁵⁻] >E⁰[Cu₂(Mal)₂H₋₂²⁻/Cu(S₂O₃)₃⁵⁻] ≈E⁰[Cu₂(Cit)₂H₋₂⁴⁻/Cu(S₂O₃)₃⁵⁻] > E⁰[Cu(Tart)₂H₋₄⁶⁻/Cu(S₂O₃)₃⁵⁻]. Increasing thiosulfate concentration leads to a sharp decline in the content of copper-Ox/Mal/Cit/Tart complex, and the stability of the formed cupric complex follows the descending order of tartaric acid>citric acid>malic acid>oxalic acid.