Method for concentrating aqueous caustic alkali solution

In a continuous process for producing an aqueous alkali metal hydroxide solution by electrolysis of an aqueous alkali metal halide solution using a cation exchange membrane to divide the cell into cathode and anode compartments and concentrating, in an evaporator, the alkali metal hydroxide solution obtained as a catholyte the improvement which comprises utilizing the heat generated by electrolysis as a heat source for the heating side of the evaporator by the steps of:

• 1. circulating a first portion of the catholyte through a heat exchanger where it gives up heat values;
• 2. diverting a second portion of the catholyte to an evaporator;
• 3. adding sufficient water to the circulating catholyte to replace said second portion of catholyte;
• 4. evaporating and thereby cooling said second portion of the catholyte in said evaporator to a selected concentration and temperature;
• 5. collecting a first fraction of said second portion of the catholyte as a final product;
• 6. circulating a second fraction of said second portion of the catholyte through said heat exchanger where it is heated by the heat values of said first portion of the catholyte and thereby reheated;
• and
• 7. returning the reheated second fraction to said evaporator;
• the liquid flow in Steps 1 through 7 being maintained at a rate such that:
• a. the temperature of the catholyte in the cathode compartment is from 50.degree. C to 120.degree. C;
• b. the concentration of alkali metal hydroxide of the catholyte is from 15% to 35%;
• c. the volume of liquid to be concentrated in the evaporator is substantially constant.