High quality steels have a low sulfur content. Steel is purified from sulfur by treating it with highly basic slags. Scarce fluorspar is used to provide slags with the required fluidity, which decomposes during smelting with the liberation of toxic fluorine into a workshop atmosphere. Alumina is also a thinner for highly basic slags, which is present in sufficient concentrations in aluminothermic production slags that are sent to landfill and create pressure on the environment. Slag dumps are “technogenic deposits” that can be a raw material base. In particular, industrial ferroalloy slags with a high alumina content may be used to prepare a thinning slag-forming mixture that can be used in steelmaking instead of scarce fluorspar. Preparation of a slag-forming mixture from waste slag will reduce the environmental pressure on slag dumps and their use as a slag thinner during steel refining in the working area of a steelmaking workshop by eliminating toxic fluorine emissions released during fluorspar decomposition. Replacement of scarce fluorspar with a slag-forming mixture in steelmaking will reduce steel cost by lowering aluminum consumption for slag deoxidation.