Method for removal of monovalent ions from ZnSO.sub.4 electrolyte by electrodialysis

A method for the purification of zinc sulfate electrolyte containing concentrations of monovalent cations including at least one cation chosen from the group consisting of thallium, sodium and potassium and monovalent anions including at least one anion chosen from the group consisting of chloride and fluoride by electrodialysis, which method comprises the steps of feeding zinc sulfate electrolyte at rates in the range of about 2 to 40 L/h.m.sup.2 per membrane pair to diluate cells of an electrodialysis unit comprising a multiplicity of alternating monovalent cation permselective exchange membranes and monovalent anion permselective exchange membranes, said membranes having a membrane matrix of a styrene di-vinyl benzene copolymer and having active groups in an amount in the range of about 3 to 4 milli-equivalents per gram of dry resin, the active groups of the monovalent cation permselective exchange membranes being sulfonic acid radicals and the active groups of the monovalent anion permselective membranes being a derivative of trimethylamine, said membranes defining alternating diluate and concentrate cells, an anode compartment and a cathode compartment, said membranes being arranged in the electrodialysis unit such that monovalent anionic permselective exchange membranes form the end membranes next to the electrode compartments, an anode positioned in the anode compartment and a cathode made of platinum coated titanium positioned in the cathode compartment;

• rinsing said anode compartment and said cathode compartment with a circulating rinse solution having a pH in the range of about 2 to 3 and containing sodium sulfate in a concentration in the range of about 0.1 to 1.0M, a soluble form of antimony in an amount in the range of about 2 to 10 mg Sb/L of rinse solution and a soluble form of cobalt in an amount in the range of about 2 to 5 mg Co/L of rinse solution;
• maintaining a concentration of zinc in said rinse solution of less than about 150 mg/L;
• applying an electrical current between the anode and the cathode at a value such that the value of the corresponding current density is in the range of about 50 to 300 A/m.sup.2 ;
• maintaining the temperature in the unit in the range of about 20.degree. to 50.degree. C.;
• feeding electrolyte at a pH having a value in the range of about 2 to 5.5;
• passing flows of solutions through the diluate and concentrate cells at a linear velocity sufficient to maintain turbulent flow in said cells;
• passing said flows of solutions through the diluate and concentrate cells and said rinse solution circulating through said anode compartment and said cathode compartment at flow rates such that the differential pressure across the membranes is less than about 50 kPa;
• withdrawing a diluate from said diluate cells;
• and recovering purified zinc sulfate electrolyte with reduced concentrations of monovalent cations and monovalent anions.