Simultaneous removal of Co, Cu, Cd and Ni from zinc sulfate solution by zinc dust cementation

Abstract

Instead of conventional two or three-stage zinc dust cementation purification, one-stage purification was proposed to remove copper, cadmium and nickel as well as cobalt by CuSb activation simultaneously from solution for an electrolytic zinc production process under development by MMG Limited. Copper was rapidly removed, forming a porous copper shell around a zinc particle. The shell acted as a cathode for the reduction of cobalt, cadmium, and nickel while the zinc particle dissolved anodically. The measurement of hydrogen evolution as well as slurry potential explains how the reduction and re-dissolution of cobalt, cadmium and nickel as a function of time is influenced by temperature, pH, and antimony and zinc dust dosages, providing a deeper understanding of impurity removal. Cobalt, cadmium and nickel were initially removed without back reaction. When zinc dissolved to a certain degree, the shell potential exceeded their redox potentials and the impurities started to re-dissolve. The shell potential was determined by the electrical contact between a zinc particle and the shell, the portion of the remaining zinc and the surface product on the zinc particle. The zinc dissolution weakened the electrical contact and therefore increased the shell potential. Temperature, pH, and antimony and zinc dust dosages not only affect the impurity reduction but also zinc dissolution, the shell potential and finally the impurity re-dissolution in different degrees. Copper, cadmium, cobalt and nickel can be completely removed in one step. The optimal conditions for impurity removal are recommended.