Membrane electrolytic process for producing concentrated caustic

In a process for the manufacture of sodium hydroxide by electrolysis at a temperature of about 70.degree. to 110.degree. C. of an aqueous solution of sodium chloride in an electrolysis cell comprising an anode compartment and a cathode compartment separated from each other by a cation exchange membrane, wherein a more concentrated sodium chloride solution is continuously introduced into the anode compartment;

• a less concentrated sodium chloride solution is continuously withdrawn from the anode compartment;
• and the resulting sodium hydroxide solution is continuously recovered from the cathode compartment,
• wherein the improvement of operating the process in a sustained manner, tolerant of upsets in sodium hydroxide concentration, by using as the cation exchange membrane a membrane consists of at least one layer, in which the material of at least the layer closest to the cathode is a copolymer of tetrafluoroethylene with a monomer Z of the general formula ##STR6## wherein n is 0 or 1;
• and Y=F or CF.sub.3 ;
• and the mole ratio of tetrafluoroethylene to monomer Z in the resin is about 7.6 to 8.2, the membrane material on the side exposed to the catholyte being itself nonporous but having a porous catholyte diffusion layer in direct contact therewith or in close proximity thereof between the membrane and the cathode,
• the sodium chloride concentration in the feed solution being about equal to the concentration of saturated brine at the operating temperature, and the concentration of the sodium chloride solution leaving the anode compartment being about 18-24 weight percent, no water being added to the catholyte during steady state operation, except water passing through the membrane from the anode compartment to the cathode compartment,
• and recovering from the cathode compartment a catholyte in which the sodium hydroxide concentration is about 42-54 weight percent.