REGENERATION OF SULFURIC-ACID FROM A SULFURIC-ACID ELECTROLYTE USED FOR ELECTROLYTIC ANODIZING OF ALUMINUM

It is demonstrated that those electrodialyzers which contain either anionic (MA-41) or sulfocationic (MK-40) membranes are unsuitable for the regeneration of sulfuric acid from a sulfuric acid electrolyte used for anodizing of aluminum, due to the high aluminum transfer numbers through cationic membranes and to contamination of the reduced sulfuric acid by aluminum ions. Electrodialyzers are proposed where a bipolar membrane (MB-3) divides the hydrogen and aluminum ions. This permits the production of sulfuric acid at a concentration of 140 g/liter, with an aluminum ion content not greater than 1.2 g/liter, from a sulfuric acid electrolyte almost completely used-up in the anodizing of aluminum. Ion transfer numbers through anionic and cationic membranes were determined under the conditions of electrodialysis treatment of the sulfuric acid electrolyte used for anodizing aluminum, and equations are presented for calculating the current efficiency for the reduced sulfuric acid.