GB664023A - Improved method of dehydrating sodium hydroxide solutions - Google Patents

Abstract

Aqueous sodium hydroxide solution preconcentrated below 150 DEG C and containing about 50-75 per cent solids is further concentrated by adding sucrose and evaporating at progressively increasing temperatures above 150 DEG C in contact with nickel surfaces. The sucrose reduces any sodium chlorate in the sodium hydroxide to sodium chloride and also reacts with any oxygen in the evaporator whereby corrosion of the nickel surfaces is inhibited. A 50 per cent aqueous NaOH solution is concentrated in a vacuum evaporator 2 at 2811 of Hg. to a 70 per cent solution which is fed at 115 DEG C to a storage tank 7 where it <PICT:0664023/III/1> is held at 100 DEG C and fed with aqueous sugar solution from a supply tank 8. The 70 per cent liquor is then fed to an evaporator 11 which is a continuous single-pass vertical tubular heat-exchanger wherein the tubes and other parts in contact with the liquor are of nickel. The evaporator 11 is heated with "Dowtherm" (R.T.M.) circulated via pipes 13 and 14 and operates at 2011 of Hg. at the top. An entrained mixture of molten NaOH and water vapour at about 370 DEG C. is discharged via pipe 15 to a separator 16. In modifications the sugar is fed to the evaporator 2 and a small flow of inert gas, e.g. nitrogen or natural gas, is fed into the bottom of the evaporator 11 to prevent "bumping" of the liquid in the tubes.