Method of operating chlor-alkali electrolytic cells

A method of operating an electrolytic cell comprising:

• (a) providing an electrolytic cell having a catholyte compartment containing a cathode, an anolyte compartment containing an anode, and a liquid-permeable asbestos-free diaphragm separating said catholyte and anolyte compartments, said diaphragm comprising a base mat of asbestos-free material comprising fibrous synthetic polymeric material resistant to the environment of said electrolytic cell, and a topcoat comprising water-insoluble inorganic particulate material comprising,
• (i) at least one oxide or silicate of a valve metal,
• (ii) optionally clay mineral, and
• (iii) optionally hydrous oxide of at least one of the metals zirconium and magnesium;
• (b) introducing alkali metal chloride brine into said anolyte compartment;
• (c) applying an electrical potential across said cathode and anode;
• and
• (d) withdrawing hydrogen gas and an aqueous solution comprising alkali metal hydroxide from said catholyte compartment, and chlorine gas from said anolyte compartment;
• wherein the improvement comprises adding clay mineral and alkali metal polyphosphate to said anolyte compartment while said electrolytic cell is operating.