Effect of Chloride Ions in Electrowinning Solutions on Zinc Deposition Behavior and Crystal Texture

Abstract

To clarify the effect of chloride ions on Zn electrowinning in terms of deposition behavior and crystal texture, the partial polarization curve, current efficiency, and AC impedance for 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 degree of depolarization increased when increasing the concentration of chloride ions from 300 to 6,000 mg/L. The exchange current density i₀ for Zn deposition also increased with increasing chloride ion concentration, which indicates that the chloride ions promote the charge-transfer process during Zn deposition. The polarization resistance for Zn deposition measured by AC impedance decreased with increasing chloride ion concentration. Conversely, the capacity of the electric double layer increased with increasing chloride ion concentration from 300 to 1,200 mg/L; however, from 3,000 to 6,000 mg/L, the capacity decreased and approached that obtained from a chloride ion-free solution. On the other hand, the partial polarization curve for hydrogen evolution was almost unchanged by the presence of chloride ions. As a result, the current efficiency of Zn deposition increased by 5–7% and 1.7% in the initial stage of deposition (up until the thickness of Zn reached approximately 2.1 μm) and at 500 A/m² for 6 hours (482 μm), respectively. With the addition of 300 to 1,200 mg/L of chloride ions, the deposited Zn exhibited a preferred orientation of {0001} plane, and it increased with increasing chloride ion concentration, which is attributed to a decrease in the overpotential for Zn deposition; however, no preferred orientation of a specific plane was shown in the deposits when increasing the concentration of chloride ions above 3,000 mg/L. Some spherical concavities resulting from traces of evolved hydrogen gas were observed on the surface of deposited Zn, and the size of the concavities increased with increasing chloride ion concentration. The size of platelet crystals of the deposited Zn decreased with increasing chloride ion concentration, which indicates that the chloride ions possibly promote the nucleation rate of Zn crystals due to specific adsorption onto the cathode.