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. Hex-aamminecobalt(III) (Co(NH3)(6)(3+)) 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 so-lutions catalyzed by Co(NH3)(6)(3+). The electrochemical results suggested that Co(NH3)(6)(3+) is more efficient than Cu(NH3)(4)(2+) in catalyzing the ammoniacal thiosulfate leaching of gold. Compared with Cu(NH3)(4)(2+), the coulombic efficiency of gold dissolution (QAu/QT) under the Co(NH3)(6)(3+) catalysis is much higher. In the presence of Co(NH3)(6)(3+), 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)(6)(3+) not only achieved a continuous leaching of gold with time, but also significantly decreased the consumption of thiosulfate from 36.5% (for Cu(NH3)(4)(2+)) to 20.9% after leaching 24 h. In the ammonia-cal thiosulfate solution, Co(NH3)(6)(3+) 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)(6)(3+). Thus, an improved catalytic mechanism of gold leaching is proposed that the redox cycle between Co(NH3)(6)(3+) and Co(NH3)x(S2O3)(0) (x = 4, 5) catalyzes the leaching of gold with ammoniacal thiosulfate. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigated the electrochemical behavior of gold leaching in ammoniacal thiosulfate solutions catalyzed by Co(NH3)6(3+), and found that Co(NH3)6(3+) is more efficient than Cu(NH3)4(2+). A new catalytic mechanism involving redox cycle between Co(NH3)6(3+) and Co(NH3)x(S2O3)(0) (x = 4, 5) was proposed for the leaching of gold with ammoniacal thiosulfate.