Electrolytic method for the simultaneous manufacture of concentrated and

Abstract

Hydroxides, such as alkali metal hydroxides, e.g., sodium hydroxide, are made in concentrated and dilute aqueous solutions simultaneously by electrolyzing brine in a cell having a plurality of compartments or zones therein, preferably three, wherein anode and cathode compartments are separated by a buffer compartment formed by permselective membranes of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether or of a sulfostyrenated perfluorinated ethylene propylene polymer, while adding water to the buffer zone at such a rate as to produce a dilute hydroxide solution therein at the same time that a relatively concentrated hydroxide solution is being made in the cathode compartment, while maintaining a high caustic current efficiency. For example, sodium hydroxide drawn off from the cathode compartment can have a concentration of from 280 to 340 grams per liter while the more dilute caustic obtainable from the buffer compartment may be at 60 to 150 g./l., with the caustic current efficiency being 80% or more. The more concentrated caustic can be further evaporated to 50% caustic and the dilute caustic solution may be used in nearby industrial applications, e.g., pulping operations, production of hypochlorite, manufacture of chlorates and neutralizations of acids, or may be evaporated to increase its concentration.