Abstract: Method of dissolving out a sodium sulphate component of a residual product obtained upon evaporation and cooling of a sodium hydroxide solution originating from a diaphragm electrolysis, the product occuring in form of a crystallized salt mixture containing sodium chloride and sodium sulphate. First of all residues of the sodium hydroxide are removed from the product. Thereafter the mixture of sodium chloride and sodium sulphate is so slurried in water and treated that a maximum of the sodium chloride and a first part of the sodium sulphate are dissolved up to a solution being saturated of said two components while a second part of the sodium sulphate and a possible rest of the sodium chloride remain suspended. Whereafter the suspended solid substance is then separated from the sodium chloride-sodium sulphate mother brine.

Abstract: Method of concentrating sodium or potassium hydroxide liquids by means of a heated evaporator whereby for a preliminary pre-evaporation of the liquid to be concentrated the liquid is first of all heated by direct contact with the superheated evaporation vapors issuing from the evaporator; a suitable apparatus for this method is disclosed which comprises an evaporator and a pre-evaporator this arranged at the upstream side of the evaporator and heated by direct contact with the evaporation vapors issuing from the evaporator.