GB812377A - A regenerative process for spent aqueous alkaline solutions - Google Patents

Abstract

812,377. Electrodialysis. ESSO RESEARCH & ENGINEERING CO. Oct. 10, 1956 [Jan. 18, 1956], No. 1696/56. Class 41. A process for separating the cationic alkaline constituents from a spent aqueous alkaline solution, e.g. containing mercaptides, comprises applying a positive electrical potential to the solution and applying a negative potential to an aqueous solution of an electrolyte, e.g. caustic soda solution, which is separated from the first solution by a cation exchange membrane, and recovering the constituents transferred to the electrolyte. The anode may be separated by an anionic exchange membrane from the spent solution and an aqueous electrolyte solution, e.g. caustic soda solution, placed in the anode chamber so formed. In one example, spent alkaline caustic solution was circulated through the middle chamber and 0.09N NaOH solution through the anode and cathode chambers at 3.7 to 4.7 volts for a current of 0.54 to 0.91 amps. Current densities of 15 to 25 amps. per square foot are mentioned. A cell for carrying out the process may comprise alternate cation and anion exchange membranes, Fig. 3, arranged in a vessel inert to the material to be recovered, e.g. of iron, stainless steel, ceramic, cement or rubber, and the membranes are separated by thin washers, e.g. rubber or ceramic. Inert electrodes are employed, e.g. of iron, carbon, nickel, copper, graphite, stainless steel or nickel-plated iron. Spent solution is delivered to compartment S1 flowing through each compartment S1, S2 ... in turn to Sn and the electrolyte solution, e.g. of the base recovered, at a concentration of 0.1N to 1N, to the compartments Rl ... Rn or Rn to R1. Alternatively, the spent solution may go from S 1 to S n to S 2 to S n-1 . . . &c. through non-conducting passages. The electrode compartments A, C may contain any electrolytic solution, e.g. the spent solution or the recovered base, and the anode compartment may contain sodium chloride to give chlorine on electrolysis. The alkali may be the water-soluble bases of Group I elements, the alkaline earths, or ammonia, or may be organic bases. Thus the hydroxides of sodium, potassium, lithium and calcium, and triethanol amine, may be recovered. Where mercaptides are dissolved in the spent solution, hydrogen sulphide is given off. The ion exchange membranes, e.g. of ionexchange resin, may be strengthened with glass wool or fibre, or with metal gauze, and may incorporate a porous material, e.g. wood, plaster, portland cement, natural fibre or ceramic.