The kinetics of cobalt removal by cementation from an industrial zinc electrolyte in the presence of Cu, Cd, Pb, Sb and Sn additives

The effect of the addition of various additives on the kinetics of cobalt removal and zinc-dust dissolution in an industrial zinc electrolyte solution during cementation was investigated. Optimum cobalt removal with minimal zinc-dust dissolution occurs at 85°C with the addition of 15 mg/L copper, 10 mg/L cadmium and 2 mg/L antimony, using 3.5 g/L zinc dust. It was determined that tin could substitute antimony, but a substantially greater concentration of tin is required for adequate cobalt removal. At temperatures 85–90°C, the amount of dissolved zinc dust was always less than 10% of the initially-added zinc dust, but increased up to 80% at 95°C. For different additive combinations, there was no effect on zinc-dust dissolution. The initial rate of cobalt removal was satisfactorily fitted to a first-order rate equation. The apparent rate constant was determined to be 1.6±0.2×10 −3 s −1 at 85°C and initial conditions of [Cu]=[Cd]=30 mg/L, [Sb]=2 mg/L, [Co]=8–30 mg/L and Zn dust=3.5 g/L. Under similar initial conditions, it was found that the initial rate constant was dependent on the surface area of zinc dust available, up to approximately 6 m 2/L (3.5 g/L zinc dust), after which a plateau was observed. The apparent activation energy for the process was estimated equal to 51 kJ/mol. This high value suggests that the cobalt removal process is controlled by a surface chemical reaction.