CHLORATE AND OXYGEN FORMATION IN ALKALI CHLORIDE MEMBRANE ELECTROLYSIS

During the operation of an industrial-scale membrane electrolysis plant over a number of years, a record was kept of the formation of the byproduct oxygen in the anode gas and chlorate in the anolyte parallel to the decline of the current efficiencies of the main products. It was found that the current efficiencies of the byproducts increase linearly with the declining current efficiencies of the main products, chlorine and caustic soda. Of the two types of anode used, one exhibited considerably more oxygen formation than the other. The high-oxygen anode was associated with distinctly lower chlorate formation than the low-oxygen anode. The increasing oxygen contents and chlorate formation rates associated with falling caustic current efficiency are reported for both types of anode. If hydrochloric acid is used to destroy the chlorate, the amount of acid must be increased as the caustic current efficiency falls. The amounts of hydrochloric acid required for the two types of anode are calculated as examples for 96% and 93% caustic current efficiency.