Abstract: For purification of liquids used for cooling and/or lubrication interspersed with metallic and/or other materials contaminations, especially for separation of rolling scale and of substances containing oil or grease from the industrial process water in open or exposed cooling circulation cycles in the smelting industry, especially in rolling mills, it is proposed to separate the coarse rolling scale by gravity sedimentation from the contaminated process water, to subsequently remove the mid-size and fine rolling scale particles from the process water prepurified in this manner by magnetic separation, to divide by flotation the thus finished clarified or purified process water into fractions containing oil concentrate, water and the finest rolling scale particles carrying little oil, the purified process water is to be used as a cooling agent, the entire rolling scale separated out of the process water is smelted in the form of sinter and the concentrate containing oil is either rejected or subjected to a wast