Effect of chloride ions on deposition behavior and morphology of zinc in electrowinning

To clarify the effect of chloride ions on the deposition behavior and crystal morphology of Zn at the initial stage of an electrowinning, the partial polarization curve, current efficiency, and AC impedance for a Zn deposition as well as its morphology and crystal orientation were investigated. In solutions containing chloride ions, the partial polarization curve for Zn deposition was evidently depolarized and the polarization resistance was decreased. By increasing the concentration of the chloride ions from 600 to 6,000 mg/L, the degree of depolarization and the decrease in the polarization resistance became significantly large. On the other hand, the partial polarization curve for the hydrogen evolution was almost unchanged by the presence of the chloride ions. As a result, the current efficiency of the Zn deposition at the initial stage at current densities from 100 to 2,000 A/m² increased 3 or 10 % with the chloride ions. The crystal of the deposited Zn from the solutions containing the chloride ions became aggregated and large. This tendency was remarkable as the concentration of the chloride ions was increased to 6,000 mg/L. In the Zn deposition at 500 A/m², the orientation index of the {0001} plane and the crystallite size of Zn increased a bit with the chloride ions. This is attributed to the decrease in overpotential for the Zn deposition with the chloride ions.