Method of treating salt bath liquid

In a method of treating salt bath liquid used for surface treatment of steel, said surface treatment of steel comprising the steps of:

• providing a salt bath furnace containing said salt bath liquid;
• immersing said steel in said salt bath liquid for surface treatment;
• withdrawing said steel from said salt bath liquid;
• immersing said steel in cooling water of a cooling bath to cool said steel;
• withdrawing said steel from said cooling bath;
• immersing said steel in a neutralizing liquid of a neutralizing bath for neutralizing alkalis sticking to the surface of said cooled steel;
• withdrawing said steel from said neutralizing liquid bath;
• immersing said steel in a pickling liquid of a pickling bath for pickling said steel;
• the improvement comprising:
• washing foreign matter, sedimented from a bottom portion of said salt bath furnace and withdrawn from said furnace, by means of washings;
• providing a storage tank;
• storing said cooling water flowing from said cooling bath, said neutralizing liquid flowing from said neutralizing bath, and said washings used for said washing, in said storage tank;
• taking out a portion of said pickling liquid from said pickling bath;
• taking from said storage tank said cooling water, neutralizing liquid, and washings, and combining them with said portion of pickling liquid, thereby reducing hexavalent salts contained in said combined liquid to trivalent salts whereby said combined liquid is reduced;
• adding sodium hydroxide to said reduced liquid, thereby converting metal ions remaining in said reduced liquid into insoluble metal hydroxides and isolating said insoluble metal hydroxides from said reduced liquid;
• and
• feeding said liquid from which said insoluble metal hydroxides are isolated to an electrolyzer for recovering sodium hydroxide.