The Role of Thiosulfate and Sulfite in Gold Thiosulfate Electrowinning Process: An Electrochemical View

In this study, electrochemical and analytical techniques are employed to explore the effect of thiosulfate, sulfite, and a mixture of thiosulfate-sulfite in the electrolyte on gold electrowinning process. The open circuit potential (OCP) analysis showed negative potential shifts from -0.02 V (in the electrolyte containing Au-TS (E1)) to -0.21 V, -0.33 V and -0.42 V in the presence of thiosulfate oversupply (electrolyte E2), sulfite (electrolyte E3), and thiosulfate-sulfite mixture (electrolyte E4). The linear sweep voltammetry (LSV) results exhibited -0.4 V potential shift by the addition of sulfite to the gold-thiosulfate electrolyte, attributing to the formation of Au(S2O3)(SO3)25- as the most stable gold complex. It turned out that sulfite can control the kinetic and thermodynamics of gold reduction reaction. Based on the chronoamperometry (CA) results, presence of thiosulfate accelerated the kinetics of gold reduction reaction. However, adding sulfite to the electrolyte stabilized the kinetic about 10 times faster. Analytical and electrochemical investigations revealed that sulfite addition could control undesired cathodic side reactions such as thiosulfate decomposition or water reduction. Finally, SEM and EDS analysis depicted the morphological changes in the deposited gold layer from different electrolytes during CA, showing that thiosulfate oversupply results in formation of greater number of smaller grains through accelerating the nucleation process, while sulfite addition led to the formation of a smoother purer gold layer on the surface of cathode.