Deep Copper Recovery from Autogenous Smelting Slags under Strongly Reducing Conditions

Abstract

Deep reduction of copper smelting slags is a promising route to recover entrained non-ferrous metals and to generate slags suitable for further processing. This work investigates the depletion of fayalite–magnetite slags formed during autogenous smelting of copper concentrates in conditions simulating the oxidation and reduction zones of a two-zone Vanyukov furnace. Laboratory charges of 100 g, containing 14.63–16.82 wt.% Cu, 25.6–27.6 wt.% Fe, 30 wt.% S and 15 wt.% SiO₂, were smelted at 1350 °C with controlled oxygen injection to produce slags containing 0.93–1.54 wt.% Cu, 30.05–32.30 wt.% SiO₂ and 7.8–9.8 wt.% Fe₃O₄. Subsequent reduction at 1300 °C was carried out with activated carbon in a fivefold stoichiometric excess relative to magnetite, at an oxygen-containing blast flow rate of 5 L/h and a 1 h holding time. Chemical analysis shows that Fe₃O₄ in slag decreases from 7.8–7.95 wt.% to 2.5–2.6 wt.%, while copper content drops from 0.93–1.033 wt.% to 0.43–0.50 wt.% under oxygen partial pressures of 10⁻¹²–10⁻¹¹ atm. X-ray diffraction and electron microscopic studies reveal a transition from fayalite–magnetite slags with dispersed metallic copper and sulfides to fayalite-dominated matrices containing ferruginous sphalerite, copper minerals of the bornite–chalcopyrite type, iron oxides and glassy phases. Simultaneous thermal analysis demonstrates that all major endothermic and exothermic events are completed by 1300 °C, supporting this as an optimal temperature for deep slag depletion. The results define an operating window—slag composition, temperature, reductant dosage and pO₂—under which copper losses to slag can be reduced to about 0.5 wt.% in industrially relevant fayalite slags.