How Impurity Effects in Zinc Electrowinning Process

How Impurity Effects in Zinc Electrowinning Process

The presence of impurities in the electrolyte is a major problem for the zinc electrowinning industry. Low levels of impurities greatly influence the cathodic deposition of zinc, leading to a decrease in zinc current efficiency and to changes in deposit morphology. Metallic impurities nobler than zinc may affect the purity of zinc deposits by co-deposition and some of them (Co and Ni) induce the redissolution of deposited zinc. However, not only the absolute magnitude of the various impurities but also the synergistic interactions among them determine the quality of deposits from the solution.

The metallic impurities affect the zinc electrodeposition process. Cd favors zinc deposition by diminishing the nucleation overpotential and it is co-deposited with zinc on an aluminum cathode. The grain size of the deposit is larger in the absence of cadmium. Fe increases the nucleation overpotential, inhibiting zinc deposition, but has no significant influence on the morphology of the deposit when the glue is present. Cu has a harmful effect on zinc electrowinning. The cathodic deposit is non-adherent, consisting of porous microspheres and a parallel discharge of Cu ions takes place. Hydrogen evolution and zinc deposition are enhanced.

The zinc deposition CE (current efficiency) decreased with an increasing atomic number of the elements in each period of the periodic table. The addition of glue to the electrolyte counteracted the detrimental effects of Sb and Ge on the CE of zinc deposition. The glue-Sn interaction was anomalous in that the CE decreased, reached a minimum, and then increased with increasing glue concentration.

The impurities also affected the zinc deposition cyclic voltammograms. Certain impurities such as Sb, Pb, and Tl affected the nucleation overpotential whereas others such as As (V), Sn, Ge, and Ga affected the plating overpotential. Trivalent arsenic resulted in a characteristic pre-wave prior to zinc deposition whereas Ni and Co produced a significant cathodic current in the reverse scan following zinc stripping.