Ion retardation method of separating chloride and chlorate ions from concentrated alkali metal hydroxide solutions

摘要

Alkali metal hydroxide solution is freed of chloride and/or chlorate by treatment with a composite anion and cation exchange resin which comprises two or more intimately associated and apparently molecularly entangled resins, at least one of which is an insoluble cross-linked resin which may or may not possess ionisable groups and at least two of which are ionisable resins containing anion and cation exchange groups respectively. The spent resin, which contains hydroxyl and chloride and/or chlorate ions, may be regenerated by washing with water, by which means the hydroxide ions are removed first and the chloride and/or chlorate ions are removed second (Figs. 1 and 2, not shown). The operations of adsorption and desorption may be repeated alternately. The alkali metal hydroxide solution treated may contain 30-50 per cent sodium hydroxide. 10-15 per cent lithium hydroxide, or up to 75 per cent of francium hydroxide. The composite resin may contain 0.3-3, preferably about 1, chemical equivalents of cation exchange groups per chemical equivalent of anion exchange groups. The alkali metal hydroxide solution and the wash water may be passed successively through a bed of the composite resin in granular or powdered form. Alternatively, a stream of the composite resin in granular form may be circulated between absorption and desorption zones. One or more of the ionisable resin constituents may contain both anion and cation exchange groups. All the resin constituents may be insoluble and cross-linked. However, one insoluble cross-linked resin constituent may be intimately associated with one or more normally soluble resin constituents so as to render them insoluble. Resins which may be constituents of the composite resin are sulphonated polystyrene, sulphonated polyvinyl toluene, sulphonated styrene-vinyl toluene copolymer, polyacrylic acid, polymethacrylic acid, styrene-maleic anhydride copolymer, styrene-ethyl-vinylbenzene-divinylbenzene copolymer, styrene - alphamethylstyreneethylvinylbenzenedivinylbenzene copolymer, vinylxylene-divinylbenzene copolymer, vinyltoluene-divinylbenzene copolymer, sulphonated styrene-divinylbenzene copolymer, sulphonated styrene-ethylvinylbenzene - divinylbenzene copolymer, sulphonated vinyltoluene - ethylvinylbenzenedivinylbenzene copolymer, acrylic acid - ethylvinylbenzene - divinylbenzene copolymer, methacrylic acid - ethylvinylbenzene - divinylbenzene, styrene - ethylvinylbenzene - divinylbenzene copolymer containing quaternary ammonium groups, styrene-ethylvinylbenzene-divinylbenzene copolymer containing -CH2 NHCH2CH2NH2, -CH2(CH3)3N+, or -CH2 (CH3)2(CH2CH2OH)N+ groups, divinylbenzene- or divinyltoluene-styrene copolymer, divinylbenzene - vinylbenzene sulphonic acid, polyvinyl benzene sulphonic acid, polyvinyl benzenetrimethylammonium hydroxide. The composite resin may be prepared by absorbing a liquid polymerizable material in an insoluble cross-linked resin which may be dry or swollen by water or an organic liquid, and may be in the form of granules or powder. In the examples beads of 50-100 Tyler mesh size are used. The liquid polymerizable material may be in undiluted form or may contain an inert liquid solvent. It may contain a plasticizer, lubricant or catalyst, e.g. sodium or potassium persulphate, tertiary-butyl hydroperoxide, di(tertiary-butyl) peroxide, or hydrogen peroxide. The temperature may be 40-100 DEG C., and the insoluble cross-linked resin containing absorbed liquid polymerizable material may be immersed in an inert liquid, during polymerization. Examples of composite resins are styrene-ethylvinylbenzene - divinylbenzene copolymer containing -CH2(CH3)3N+ or -CH2(CH3)2 (-CH2CH2OH)N+ groups associated with polyacrylic acid or polyvinylbenzene sulphonic acid; vinyltoluene - ethylvinylbenzene - divinylbenzene copolymer associated with polyvinylbenzene trimethylammonium hydroxide; and the reaction product of trimethylamine and nuclearly chloromethylated styrene-divinyl-benzene copolymer associated with polyacrylic acid.