GB632081A - Improved process for the preparation of solutions containing sodium hydroxide - Google Patents

Abstract

Solutions containing caustic soda are prepared by reacting hydrated lime with sodium sulphate in a medium of water under such conditions that a double salt, Na2SO4.CaSO4 (Glauberite) is formed, and separating Glauberite together with other solid substances from the resulting solution of caustic soda. The concentration of sodium sulphate may exceed the minimum concentration at which Glauberite can be stable in the same medium and at the same temperature by at least 10 per cent; and the reaction may be carried out at a temperature much higher than the lowest temperature at which the Glauberite can be stable, and if desired at a temperature at which increased pressure is necessary. Hydrated sodium sulphate may be crystallized from the resulting solution by cooling and further sodium sulphate may be precipitated by the addition of a substance which reduces its solubility in the solution, e.g. ammonia or alcohol. Sodium sulphate may be recovered from the precipitate formed during the reaction by leaching. The medium in which the reaction takes place may consist partly in the mother lye from an evaporation plant for salt making. The solutions obtained, preferably after crystallization and removal of hydrated sodium sulphate, may be used directly for the opening of straw, watersoftening or purifying solutions such as brine. The solutions may also be evaporated to produce concentrated and even solid caustic soda.ALSO:Solutions containing caustic soda, prepared by reacting hydrated lime with sodium sulphate (see Group III), are used for water-softening. Such a solution, according to one example, contains 25 grams of NaOH and 22 grams of Na2SO4 per litre.