GB662314A - Improved method of producing alkali metal hydroxides - Google Patents

Abstract

An alkali metal hydroxide solution of at least 5 per cent by weight concentration is prepared by treating a water-insoluble strongly basic anion exchange resin containing quaternary ammonium hydroxide groups with a stream of an aqueous solution of an alkali metal halide of at least 10 per cent by weight concentration and withdrawing separately the more concentrated mid-portion of the resultant alkali metal hydroxide solution. The preferred resin is the reaction product of a chloromethylated benzene-insoluble copolymer containing a polyvinyl-aromatic compound and a monovinyl-aromatic compound with a tertiary mono-alkyl or di-alkyl N-substituted alkanolamine. The preparation of such resins is described in Specification 679,850, [Group IV(a)]. The invention may be conducted cyclically. According to one example aqueous calcium hydroxide is passed through the bed of granular resin until the solution entering and leaving the bed has the same composition, the resin thus being converted to the quaternary ammonium hydroxide form. After washing the resin with water, a saturated aqueous sodium chloride solution is passed through the bed, the effluent sodium hydroxide solution being collected as a series of fractions. The first and last fractions, being of low concentration, are discarded and mid-fractions containing various concentrations of hydroxide and chloride are retained, the later of these retained fractions being saturated with sodium chloride. In a second cycle, calcium hydroxide solution is again passed through the resin bed. After washing the resin, the retained mid-fractions from the first cycle are fed through the bed in the order in which they were collected. The effluent sodium hydroxide solution is collected in a series of fractions and treated as in the first cycle. This cyclic treatment is continued until a condition of balance is obtained whereupon the cycles are carried out as before except that the feeding of the retained mid-fractions is followed by the feeding of sufficient saturated sodium chloride solution to complete the reaction and the particular mid-fraction of the solution from the sodium chloride treatment in each cycle having a maximum sodium hydroxide concentration (about 16.6 per cent) is reserved for evaporation or like recovery. Calcium hydroxide may be replaced by strontium or barium hydroxide and potassium hydroxide may also be prepared by the process.