PURPOSE:To prevent drop of working efficiency of the electrolytic cell, when electrolyzing salt water by ion exchange membrane method, by pretreating the salt water of the raw material with precision filter and chelate resin or the like to keep the total amount of Ca and Mg ions in the salt water entering the electrolytic cell at less than 0.08ppm. CONSTITUTION:Salt water containing alkali chlorine metal salt such as NaCl is electrolyzed by the ion exchange membrane method to produce chlorine and hydroxide of alkali metal. At this time, the aqueous solution containing NaCl of the raw material such as sea water is filtered by a filter using filter medium such as active carbon and alpha-cellulose having a higher precision than conventional sand filter, and the concentration of solid content is kept under 3ppm. Then, passing through a chelate ion exchange resin tower, the ion contents such as Ca and Mg are reduced to less than 0.08ppm. This salt water is supplied into the electrolytic cell by the ion exchange membrane method, and sedimentation of fine Ca and Mg particles in the exchange membrane is prevented, thereby preventing rise of electrolytic voltage, drop of electrolytic current efficiency, and damage of the exchange membrane.
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