An Electrochemical Study of Gold Dissolution in Thiosulfate Solution with Cobalt-Ammonia Catalysis

Alkali agents could be used to enhance the extraction of zinc from solution, but excess alkali can lead to emulsification of the solution. In this study, the redox couples of Co(III)/Co(II) could be formed based on the thermodynamic analysis, and catalyze the dissolution of gold in a cobalt-ammonia-thiosulfate system. The catalytic mechanism of cobalt-ammonia for gold dissolution in the thiosulfate solution was further proposed by the analysis of polarization curves to be that NH3 catalyzes the dissolution of gold in the anodic area, and Co(NH3)(6)(3+) catalyzes the reduction of dissolved oxygen in the cathodic area. In the dissolution of gold, the concentration of NH3, Co3+ and S2O32- increased, and the dissolution of gold increased. The apparent activation energy for gold dissolution is 41.15 kJ/mol in a temperature range of 25-40 degrees C, and this demonstrates that gold dissolution is chemically controlled. The SEM-EDS and XPS analyses indicate that the precipitate coating on gold surfaces is mainly S-2(-) in CoS and S-8. The decomposition of thiosulfate was also carried out in different thiosulfate solutions. Results have shown that the increase of NH3 concentration can reduce thiosulfate decomposition, while the increase of Co3+ and S2O32- concentration can accelerate thiosulfate decomposition. The apparent activation energy for thiosulfate decomposition with the temperature range of 26.6-41 degrees C is determined to be 22.12 kJ/mol, which shows that thiosulfate decomposition is mixed chemical-diffusion-controlled.