PURPOSE: To prevent the formation of scale by dispersing hard components by keeping chlorine ion concentration high in dust collection treatment water to be used again for treatment.

CONSTITUTION: When using again treated water of pH 9 or more, obtained from the coagulating sedimentation of the coverter waste gas dust collection water, sodium chloride, etc., is added to the treated water in order to prevent the formation of scale. In this case, the preferred concentration of chlorine ion necessary for preventing the scaling in treated water is 2000ppm or more, corresponding to more than 4 times as much as calcium hardness of treated water. The presence of 10000W 40000ppm of chlorine ion in treated water, corresponding to more than 8 times as much as the calcium hardness, will bring on more preferable results. Furthermore, it is better to add concurrently scale dispersants selected from acrylic acid homopolymers, the copolymers of acrylic acid and any other monomer, phosphonic acids, and phosphonocarboxylic acids.