Optimization of cobalt removal from an aqueous sulfate zinc leach solution for zinc electrowinning

• Cementation of cobalt by zinc with arsenic activation occurs in two distinct stages.
• First stage controlled by reaction with an activation energy of 43 kJ/mol
• Second stage controlled by mass transfer with a low activation energy
• Rate of cobalt removal increased by recirculation of Co–Cu–As–Zn precipitate.
• Rate of cobalt removal increased by using finer zinc dust.

The removal of cobalt from zinc sulfate leach solutions by copper and arsenic activated cementation on zinc powder prior to zinc electrowinning was studied by considering the thermodynamics and kinetics of the cementation process. Arsenic increases the thermodynamic driving force for cobalt cementation in a zinc sulfate solution with the cementing of the cobalt ions as cobalt arsenide at more positive potentials compared to cobalt metal plating. The influence of temperature, the size and quantity of the zinc powder used, and the recirculation of cobalt precipitate on the kinetics of cobalt removal were investigated using batch cementation experiments. It was shown that the rate of cobalt cementation can be enhanced significantly by using finer zinc powder and by recirculating some of the cobalt precipitate such that similar or improved cobalt removal may be achieved at lower operating temperatures or with less zinc added. It was found that the cobalt is cemented by the zinc at an initially high rate with an activation energy of 43 kJ/mol, followed by a slower temperature insensitive stage.