A method of recovering metals and producing a secondary slag from base metal smelter slag produced by a copper or nickel smelter includes mixing the smelter slag with at least one reducing agent selected from the group consisting of carbon, calcium carbide, ferrosilicon and aluminum, said carbon and calcium carbide (if present) being from about 1% to about 40% by weight of the slag and said aluminum (if present) being from about 2% to about 35% by weight of the slag and with from zero to about 70% calcium oxide by weight of the slag, heating the mixture aluminothermically if Al is present as reducing agent above the melting point to reduce the smelter slag to a metal alloy containing iron and possibly silicon and aluminum, depending on the quantity of aluminum added to the smelter slag, and heavy metals such as copper, nickel and cobalt which were in the smelter slag and thereby also producing a secondary slag containing at least one compound selected from the group consisting of calcium silicate, calcium aluminate, fused alumina and calcium iron aluminum silicate, and separating the metal alloy from the secondary slag.
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