Thiosulfate leaching of gold catalyzed by hexaamminecobalt(III): Electrochemical behavior and mechanisms

Thiosulfate is a green gold lixiviant that has promise to replace cyanide at the gold mining industry. Hexaamminecobalt(III) (Co(NH3)63+) is effective in catalyzing the thiosulfate leaching of gold from ores, but the electrochemical behavior and catalytic leaching mechanisms remain unclear. This paper reports an in-depth investigation into the electrochemical behavior of gold leaching in ammoniacal thiosulfate solutions catalyzed by Co(NH3)63+. The electrochemical results suggested that Co(NH3)63+ is more efficient than Cu(NH3)42+ in catalyzing the ammoniacal thiosulfate leaching of gold. Compared with Cu(NH3)42+, the coulombic efficiency of gold dissolution (QAu/QT) under the Co(NH3)63+ catalysis is much higher. In the presence of Co(NH3)63+, the QAu/QT was nearly 100% at 150–250 mV and higher than 78.9% at 250–350 mV; at 250 mV, the QAu/QT was approximately 100% in the initial electrolysis of 300 s and remained basically unchanged at around 75% after 1200 s. Leaching results further showed that the catalysis of Co(NH3)63+ not only achieved a continuous leaching of gold with time, but also significantly decreased the consumption of thiosulfate from 36.5% (for Cu(NH3)42+) to 20.9% after leaching 24 h. In the ammoniacal thiosulfate solution, Co(NH3)63+ is likely reduced to mixed ligand Co(II) complexes of Co(NH3)x(S2O3)0 (x = 4, 5), which are readily oxidized back to Co(NH3)63+. Thus, an improved catalytic mechanism of gold leaching is proposed that the redox cycle between Co(NH3)63+ and Co(NH3)x(S2O3)0 (x = 4, 5) catalyzes the leaching of gold with ammoniacal thiosulfate.