The coal-based direct reduction followed by magnetic separation (CDRMS) is an efficient iron extraction and dephosphorization process, which requires adding additives to improve the phosphorus removal rate. Compared with other additives, sodium carbonate has the advantages of good iron index, high phosphorus removal rate and less environmental pollution. Its role in phosphorus-rich oolitic iron ore (PROIO) where phosphorus exists in the form of apatite has been proved. However, the influence on the phosphorus transformation process in the lattice of iron minerals is not clear. In this paper, the effect of sodium carbonate on phosphorus removal in iron minerals and iron recovery during CDRMS was studied. Compared with not adding chemicals, the addition of sodium carbonate significantly reduced the phosphorus content of direct reduced iron (DRI) from 0.69% to 0.09%. The iron grade increased from 93.28% to 95.08%, and the iron recovery rate rose from 90.61% to 96.48%. The mechanism of sodium carbonate was revealed by using a synchronous thermal analyzer (TG–DSC), X-ray diffractometer (XRD), X-ray photoelectron spectrometer (XPS), scanning electron microscope and energy dispersive spectrometer (SEM–EDS), and vibrating sample magnetometer (VSM). The results show that sodium carbonate reacted with silicon and aluminum components to form nepheline, and the lattice substitution of phosphorus in iron minerals and silicon in nepheline prevents the reduction of phosphorus. In addition, sodium carbonate promotes the reduction of iron minerals, resulting in an increase in the magnetic properties of the reduction products.

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.