Effects of Cadmium Impurities on the Electrowinning of Zinc

Abstract

A study was carried out on the effect of cadmium, a major impurity in commercial zinc electrolytes, on the zinc plating current efficiency in acid sulfate baths. The synthetic electrolyte (200 g · dm−3 Zn++ and 0–100 mg · dm−3Cd++) was continuously circulated through the electrolysis cell. The electrolysis was carried out with a current density of 480–490 A · m−2 at 37°C. Hydrogen overpotential, potential sweep, atomic absorption, x‐ray diffraction, and electron microscopy were used to determine the effect of cadmium on the behavior of the zinc plate. The results indicate that cadmium may either increase or decrease the zinc plating current efficiency. For Cd++ less than 20 mg · dm−3, increasing cadmium concentrations in the electrolyte reduce the hydrogen overpotential and the zinc plating current efficiency. For Cd++greater than 20 mg · dm−3, increasing cadmium concentrations cause a refinement in the structure of the zinc plate with subsequent increase in the hydrogen overpotential and in the zinc plating current efficiency. The incorporation of cadmium into the zinc plate neither significantly alters the crystal orientation of the deposit nor alters the hydrogen evolution mechanism.