GB756205A - Production of alkali metal hydroxides - Google Patents

Abstract

<PICT:0756205/III/1> <PICT:0756205/III/2> Substantially all the alkali metal hydroxide content of an alkali metal amalgam obtained from a mercury cathode electrolytic cell is recovered by causing the amalgam to flow vertically through a first decomposition zone to which water is fed concurrently with the amalgam so as to partly decompose the amalgam and produce an alkali metal hydroxide solution of a concentration of at least 68 per cent and optionally 73-78 per cent, recovering the hydroxide solution and passing the partly decomposed amalgam from the said decomposition zone through a heat-exchanger, in heat-exchange relationship with undecomposed amalgam flowing towards the said zone, to a second decomposition zone wherein the amalgam is caused to flow horizontally in counter-current with fresh water so as to substantially completely decompose the amalgam and produce an alkali metal hydroxide solution of low concentration, e.g. 35-40 per cent, recovering the hydroxide solution and returning the decomposed amalgam to the electrolytic cell. As shown, amalgam to be denuded enters a vertical decomposer (Fig. 1) through inlet 1 and passes through heat-exchanger L and pipe F to distributer 3 from where it is directed on to graphite catalyst bodies contained in a series annular baskets A (see also Fig. 2, not shown) together with decomposing water which has been admitted at inlet S and passed through direct heat-exchanger M. Hydrogen and water vapour generated by the reaction passes through heat-exchanger M wherein the watervapour is condensed, and hydrogen passes out at 6. Concentrated alkali metal hydroxide is removed at 7 while partially denuded amalgam is passed through heat-exchanger L and outlet 4 to a horizontal decomposer (Fig. 3) through an inlet 8 wherein it passes over graphite catalyst strips Q (see also Fig. 3a, not shown) to outlet 9 in countercurrent to fresh water which passes from inlet 10 to outlet 11. Hydrogen produced in the vertical decomposer is passed into the horizontal decomposer at 12, and removed, together with hydrogen produced in the horizontal decomposer, at 13. The electrolytic cell may be arranged above the vertical decomposer (Fig. 4, not shown) or below the horizontal decomposer (Fig. 5, not shown).