GB857688A - Improvements relating to electrodialytic processes - Google Patents

Abstract

857,688. Electrodialysis. PERMUTIT CO. Ltd. Feb. 7, 1957 [Feb. 22, 1956], No. 5524/56. Class 41 An electrodialytic cell has at least three compartments separated from each other by barriers A all selectively permeable to ions of the same sign, alternate compartments containing solutions of two different electrolytes. In a three compartment cell, Fig. 1, employing amonic-selective membranes A, electrolyte solutions MX, NY may be converted respectively to MY, NX by passage of the anions Y, X into adjacent compartments under the influence of an applied voltage. If cation-selective membranes are employed, the same result is achieved, but in both cases the products formed in the electrode compartment, (NX in the first case, MY in the second) may be destroyed by electrode reactions, and to prevent this a four compartment or a multi compartment cell is employed, the membranes all being selectively permeable to ions of the same sign. To prevent contamination of the products of the electrodialyis by electrode products, one of the electrodes is separated from the rest of the cell by a membrane of opposite type to that forming the rest of the membranes, and is cationic-selective if adjacent the cathode and anion-selective is adjacent the anode. The production of mixtures due to the passage of ions through more than one compartment is avoided by passing the two solutions continuously in countercurrent through alternate compartments, and one solution may be more concentrated than the other. Instead of membranes, the inter-compartment barriers may comprise a granular ion-exchange material between two sheets of a material, e.g. regenerated cellulose, equally, permeable to cations and anions, the interstices of the granular material teing filled with an aqueous liquid of higher electrical resistance, e.g. one of the electrolyte solutions being employed. Some or all of the compartments may contain ion-exchange material, e.g. if anion-selective membranes are employed, the exchanger is a strongly basic anion-exchanger, if cation-selective, a strongly acid cation-exchanger is employed. In softening water, a sodium chloride solution is passed through alternate compartments of a cell employing all cation-selective membranes, and water to be softened in counter-current through the other compartments which contain a cation-exchanger. Anion-exchangers may be employed instead of cation-exchangers in which case anion-selective membranes are employed. In one example, Fig. 4, sodium hydroxide solution was introduced into compartment 21 and lithium sulphate solution into chamber 2, lithium hydroxide solution being obtained at 20 and a mixture of sodium hydroxide and sulphate solutions at 1. The membranes were all anion-selective, and a voltage of 25 was employed. The invention is stated to be applicable to the removal of bicarbonates from water, alternate compartments having cation-exchange material therein, or to convert sodium chromate and sulphuric acid to chromic acid and sodium sulphate. Specification 767,103 is referred to.