Abstract
The goal in the electrowinning of metals is the production of high quality and purity cathodes. In a nickel/cobalt electrowinning operation, one of the ongoing issues is the occurrence of black deposits on the nickel and cobalt cathodes.An investigation has been made into the effects of dithionate and thiosulfate ions on the reduction and oxidation processes of cobalt and nickel. Dithionates are not responsible for the formation of black metal sulphides. In the presence of small amounts of thiosulfate, metal sulphides are formed in addition to the metals in a reaction involving reduction of thiosulfate in the presence of these metal ions. Chemical reduction of thiosulfate by metallic cobalt and nickel to form metal sulphides has been established. In addition, reduction of cobalt ions by thiosulfate catalysed by the metal/metal sulphide surface accounts for the excess anodic charge observed during open circuit of cobalt metal/sulphide in thiosulfate solutions.