The effects of thiosulfate ions on the deposition of cobalt and nickel from sulfate solutions

• Metal sulfides are formed in addition to the metals in a cathodic reaction involving reduction of thiosulfate in the presence of these metal ions.
• The presence of thiosulfate ions in the electrolyte is responsible for the formation of black metal sulfides from reduction of nickel and cobalt ions
• The anodic passivity of nickel is prevented in the presence of thiosulfate ions.
• Chemical reduction of thiosulfate by metallic cobalt and nickel to form metal sulfides has been established.
• Reduction of cobalt ions by thiosulfate is catalyzed by the metal/metal sulfide surface.

A detailed electrochemical study of the effects of thiosulfate ions on the reduction of nickel(II) and cobalt(II) ions in sulfate solutions in the pH range 3 to 4 has been undertaken. Even in the presence of small amounts of thiosulfate, metal sulfides are formed in addition to the metals in a reaction involving reduction of thiosulfate in the presence of these metal ions. In addition, chemical reduction of thiosulfate by metallic cobalt and nickel to form metal sulfides has been established. Reduction of cobalt ions by thiosulfate catalyzed by the metal and/or metal sulfide surface accounts for the excess anodic charge observed during open circuit contact of cobalt metal and/or cobalt sulfide with thiosulfate solutions. It has been shown that thiosulfate is responsible for the formation of black cathodic films of the metal sulfides periodically observed during electrowinning of these metals. A simple method for the control of the peroxide addition to the electrolyte in order to oxidize residual thiosulfate has been devised based on the results of this study.