Tailings from an industrial copper slag cleaning milling and flotation circuit have been pre-reduced at low temperatures, well below its melting point, using CH4-H-2-Ar mixtures with the aim of metallizing its iron oxide component to metallic iron. The treatment carried out at 700-900 degrees C, with a duration of up to 90 min, indicated a complete transformation of magnetite within the first 30 min of the process. At the same time, significant fractions of its harmful impurities, antimony, arsenic, bismuth, lead and zinc, present as oxides in the slag were reduced from magnetite and the glassy inter-granular phase, and removed from the slag as flue dust in the process off-gas. The obtained fine slag powder with low impurity contents can be melted after the pre-reduction to synthetic rock or to fast cooled slag granules for further use.