Effect of ZnO on Copper Loss and Fe3O4 Reduction During the Copper Slag Cleaning

A mass action concentration model of FeO-Fe2O3-SiO2-CaO-MgO-Al2O3-ZnO multi-component slag was established to address the problems of high viscosity and high copper content in oxygen-enriched top-blown copper smelting slag. Theoretical calculations, combined with experimental investigations, were performed to evaluate the effects of ZnO additions (0–25 wt.%) on phase transformation, Fe3O4 reduction and copper content in the slag at temperatures ranging from 1200 to 1400 °C. The results indicate that ZnO addition decreases the mass concentrations of Fe2O3, Fe3O4, Fe2SiO4 and SiO2, while increasing the proportions of zinc-bearing structural units and enhancing the reducing capability of the slag. In the copper slag system, Zn2+ substitutes for Fe in Fe3O4 and Fe2SiO4 through isomorphous substitution, forming Fe-Zn spinel and Fe-Zn olivine. As the addition of ZnO gradually increases to 25 wt.%, the liquid slag fraction increases and the slag viscosity decreases, resulting in a reduction in copper content from 5.64 wt.% to 2.04 wt.%. Furthermore, ZnO promotes the transformation of Fe3O4 into zinc–iron spinel, which is more readily reducible by carbothermic reaction than Fe3O4 itself, thereby facilitating the overall reduction of iron oxides in the copper slag. These findings provide a theoretical basis for slag-type regulation and the efficient separation of copper from slag via high-temperature gravity settling of matte.