GB723344A - Improvements in or relating to process for concentration of aqueous solutions of fusible caustic alkali - Google Patents

Abstract

<PICT:0723344/III/1> <PICT:0723344/III/2> Water is evaporated from solutions and melts of fusible caustic alkalis (NaOH or KOH) by continuously injecting the aqueous alkali into the lower portion of a vertical elongated dehydration zone which contains a hot concentrated solution of the alkali or a melt of the anhydrous alkali, the solution or melt being at such a temperature above the boiling point of the feed-alkali that flash-vaporization of water in the feed occurs and superheated steam is formed. The mixture of steam and liquid ascends the dehydration zone and discharges into a separating zone from which steam and liquid product are continuously withdrawn while some liquid is returned via a heat transfer zone to the lower portion of the dehydration zone. The process may be used for the concentration of caustic soda solution (e.g. from 50 to 70 per cent) or for making substantially anhydrous caustic soda. In the latter case, the feed is introduced into hot molten anhydrous caustic soda maintained at a temperature with the range 377 DEG -400 DEG C. The parts of the apparatus which come in contact with the alkali are fabricated from a nickel alloy stainless steel. In the apparatus shown, the heat transfer zone comprises ten conduits 4 (length 6 metres, inside diameter 3.8 c.m.) which are mounted in furnace 10 heated by gas burner 11. Heat insulated chamber 1 is the separating zone. The dehydration zone is conduit 3 (length 7.5 metres, inside diameter 15.2 cm.) which communicates at the base with conduits 4 via header 5, and which discharges at the upper end into chamber 1. The apparatus is charged with anhydrous NaOH, at a temperature of 385-400 DEG C., up to the level 7 which is maintained by out-flow 9. Caustic soda solution (70 per cent) at 121 DEG C. is introduced into conduit 3 via feed line 17 and injection nozzle 6 (jet diameter 0.178 cm.) at 277 litres per hour. Superheated steam flashes off from the feed; and a mixture of this steam and concentrated caustic soda rises rapidly up conduit 3, and is discharged into chamber 1. The mixture strikes disentraining baffle 14, and is separated into steam (which leaves the apparatus via outlet 15) and anhydrous caustic soda which falls into the pool thereof maintained in the lower part of chamber 1. Anhydrous NaOH is thereby established. The supply of air and gas to burner 11 is automatically controlled to maintain the temperature of the caustic soda in the lower part of chamber 1 at 390 DEG C. The temperature of the gases leaving the burner average 815 DEG C., while the waste gases leaving stack 13 average 630 DEG C. After 24 hours continuous operation, 405 kilograms of anhydrous NaOH (0.55 per cent water) are withdrawn as product via out-flow 9. Instead of using combustion gases as heat transfer medium, mercury vapour, or a molten mixture of sodium nitrate and nitrite, or a silicone polymer, may be employed. The process may be carried out at super- or sub-atmospheric pressures, as well as at normal pressure.