Abstract: The aim of this invention is to resolve problems in sludge treatment methods, and to render sludge containing harmful substances produced in large amounts in chromium plating works harmless with reproducibility by effective use of certain substances. The invention resolves these problems by separating metal ions such as iron, copper, zinc and chromium contained in chromium plating effluent or a mixed solution of chromium plating solution, chromium plating effluent and chromic acid wash water in an impurity recovery electrolysis tank A in the form of a sludge, extracting the sludge, washing the sludge with water, separating the wash water containing chromic acid, drying the sludge which sedimented in the water washing step by natural or assisted drying, adding a natural organic substance to the sludge and reduction calcinating the sludge.

Abstract: Closed recycle system for chromium plating solution, solution wastes from chromium plating and chromic acid cleaning water in chromium plating composed of a chromium plating chamber including a chromic acid mist recovery device in communication with a recovery vessel for recovering and liquefying chromic acid mists formed upon chromium plating, and a chromic acid mist cleaning tower in communication with the recovery vessel for cleaning chromic acid mists flowing from the chromic acid mist recovery device. The recovery vessel is preferably positioned underground, and also preferably positioned in the recovery vessel. Liquid wastes containing noxious substances formed in chromium plating factories are utilized, and generation of materials leading to public pollution is prevented.