Conversion of zinc chloride to zinc sulphate by electrodialysis—a new concept for solving the chloride ion problem in zinc hydrometallurgy

Abstract

The results presented in this paper describe the course and conditions of electrodialytic conversion of zinc chloride to zinc sulphate using mono- and bipolar ion exchange membranes. The object of this work was to find if such a process can be applied to produce zinc sulphate solutions with low concentrations of chloride ions from solutions of zinc sulphate–zinc chloride obtained from raw materials containing high concentration of chloride. As a result of this work, the influence of the technological parameters on the effectiveness of the process has been determined. The parameters studied include the chemical composition of the working solutions over a wide range of concentrations of individual constituents, the current load of the electrodialytic apparatus, the linear flow velocity of the solution feed, as well as the character of membranes applied in the apparatus. Empirical equations describing the current efficiency of the process, the increase in concentration of chloride ions in the sulphate solution and the selectivity coefficient of the process have been devised, as a function of four independent variables: the current load of the electrodialytic apparatus, the concentration of zinc as zinc sulphate and zinc chloride in the inlet sulphate–chloride solution and the initial concentration of zinc in the sulphate solution, based on the results of complete factorial experimental technique. Analysing the empirical equations has shown that a one-stage electrodialysis process may be applied to produce zinc sulphate solution, with a low enough concentration of chloride ion to meet the requirements of the electrolytic zinc production process, from solutions obtained by leaching zinc-bearing raw materials containing up to 1% concentration of chloride with sulphuric acid.