Process and apparatus for electrodialysis of an alkali sulfate containing aqueous solution

A multistage process for the electrodialysis of a spent spin bath liquor incurred in the production of regenerated cellulose by way of the xanthogenate process, said liquor containing alkali sulfate and Ca.sup.++, comprising the steps of

• providing a plurality of electrodialytic three-compartment cell means operating in series, each cell means comprising acid and base compartments and two exclusively cationic exchange membranes in the cell interior, and being delimited by bipolar ion exchange membranes,
• conducting said liquid exclusively through first acid compartments of said cell means in series, each of said first acid compartments being delimited by one of the bipolar ion exchange membranes and one of the exclusively cationic exchange membranes in its respective cell means,
• simultaneously conducting concentrated aqueous Na.sub.2 SO.sub.4 solution through second acid compartments of said cell means in series, each of said second acid compartments being delimited by two exclusively cationic exchange membranes in its respective cell means, thereby acidifying said Na.sub.2 SO.sub.4 solution and thereby migrating Ca.sup.++ from said first acid compartment through one of said exclusively cationic exchange membranes into said concentrated aqueous Na.sub.2 SO.sub.4 solution in said second acid compartment to a concentration of 3 ppm or less, and
• feeding said acidified Na.sub.2 SO.sub.4 solution containing Ca.sup.++ in a concentration of 3 ppm or less into said first acid compartment of the first of said series operating cell means.