A thermodynamic analysis on thiosulfate leaching of gold under the catalysis of Fe3+/Fe2+ complexes

Abstract

• Thermodynamic analysis of Fe-catalyzed thiosulfate leaching of gold is performed. • Catalysis of EDTA 4– , ox 2– , CN – and cit 3– complexes of Fe 3+ /Fe 2+ are involved. • Possible mechanisms for Fe-catalyzed thiosulfate leaching of gold are elucidated. • Reasons for low reactivity of Fe complexes towards thiosulfate are proposed. Thiosulfate is an eco-friendly gold lixiviant that is promising to replace the highly toxic cyanide. Copper is a conventional catalyst in accelerating the rate of thiosulfate leaching of gold, but it causes high thiosulfate consumption and complex gold recovery process. Attempts on the replacement of copper by other transitional metals such as nickel, cobalt and iron have been made to address the two issues. Among the catalysts, iron is cheap with great potential of development and industrial application. Few research has focused on the thermodynamics for iron-based thiosulfate systems. In this paper, a systematic thermodynamic analysis on the thiosulfate leaching of gold catalyzed by Fe 3+ /Fe 2+ and the ligand of EDTA 4– , oxalate (ox 2– ), cyanide (CN – ) or citrate (cit 3– ) is conducted by constructing a series of predominance area and speciation diagrams. Information on the stability, speciation, and redox behavior of iron species under various solution conditions are provided to better understand the complex solution chemistry and possible mechanisms for the iron-catalyzed gold thiosulfate leaching. The likely reasons for the high stability and low consumption of thiosulfate under the catalysis of Fe 3+ /Fe 2+ complexes are also proposed.