Caustic and chlorine production process

Claim 1

In the process of producing sodium hydroxide and chlorine by the stage-wise electrolysis of sodium chloride-containing brine, wherein in the first stage an electrolytic cell, consisting of anode and cathode compartments separated by at least one interposed buffer zone, is provided and wherein membranes separate the anode and cathode compartments from the buffer zone and brine is charged to the anode compartment and water is charged to both the buffer zone and to the cathode compartment and as a result of the electrolysis a concentrated sodium hydroxide solution of more than 250 g/l NaOH content is recovered from the cathode compartment and a dilute sodium hydroxide solution of 50-200 g/l NaOH content is recovered from the buffer zone; and the dilute sodium hydroxide solution is then charged to the cathode compartment of at least one second stage electrolysis cell consisting of membrane-separated anode and cathode compartments wherein charging of brine to the anode compartment and electrolysis of the same produces a concentrated sodium hydroxide solution of at least 250 g/l NaOH content in the cathode compartment of this second-stage cell; the improvement which comprises providing at least one first-stage electrolytic cell consisting of membrane-separated anode and cathode compartments without an intermediate buffer zone and wherein brine is charged to the anode compartment and electrolysis of the brine is conducted in a manner so as to produce in the cathode compartment a dilute sodium hydroxide solution containing about 10-25% by weight sodium hydroxide; charging the dilute sodium hydroxide solution to the cathode compartment of at least one second-stage electrolytic cell in lieu of water, introducing brine in the anode compartment of the second-stage cell, electrolyzing the brine and recovering from the cathode compartment of the second-stage cell a sodium hydroxide solution containing 25-50% by weight sodium hydroxide.