Effects of CaO Addition and Cooling History on Phase Transformation and Iron Recovery from a Steel–Copper–Magnesium Slag Mixture

Steel, copper, and magnesium slags are typical metallurgical solid wastes containing potentially recoverable Fe, Ca, Mg, and Si. Copper slag is relatively rich in iron, but much of this iron is bound in fayalite or Fe–Mg olivine, which limits direct recovery by magnetic separation. Steel slag contains substantial CaO, MgO, and iron oxides, whereas magnesium slag is rich in CaO and also contains MgO; together, they can supply the alkaline and Mg-bearing components required for the phase reconstruction of iron-bearing silicates. The compositional complementarity of the three slags therefore provides a basis for iron recovery and the controlled conversion of non-magnetic constituents. In this study, steel, copper, and magnesium slags were mixed at a mass ratio of 37:37:26. CaO was added to regulate the basicity and promote the redistribution of Fe from olivine-type silicates into magnetically recoverable spinel-group phases. The effects of CaO addition and the combined water-quenching onset temperature/slow-cooling duration on phase transformation, microstructure, and iron recovery were investigated. The samples were held at 1400 °C for 30 min, cooled at 1.5 °C min−1, and water quenching was initiated at 1300, 1200, 1100, or 1000 °C. The results showed that an appropriate CaO dosage promoted the conversion of Fe–Mg olivine toward Ca2SiO4 and Fe- or Mg-bearing oxide phases, whereas excessive CaO favored calcium-ferrite formation. Among the S1–S4 samples, S3, with a basicity of 2.0, showed the highest mean concentrate yield and iron recovery, yielding a concentrate with a T. Fe grade of 19.02% and an iron recovery of 90.89%. In the T1–T4 experiments conducted using S4, lowering the water-quenching onset temperature necessarily extended the slow-cooling duration and was associated with changes in spinel morphology and magnetic-separation performance. The particle-area distribution changed non-monotonically from T1 to T3, whereas T4 exhibited pronounced aggregation into larger blocky spinel domains and showed the highest mean T. Fe grade and iron recovery in this series. When water quenching was initiated at 1000 °C, the concentrate T. Fe grade and iron recovery reached 20.75% and 94.86%, respectively. Within the fixed three-slag ratio and investigated parameter ranges, CaO addition and cooling history affected phase redistribution and magnetic separation, although the resulting magnetic products showed only limited iron enrichment.