Effect of Al 2 O 3 on the dissolution behaviour of lime in CaO–SiO 2 –FeO–P 2 O 5 5%–MgO 3%–Al 2 O 3 slag

Lime is one of the primary slag-forming materials in steelmaking, and its rapid, complete dissolution is critical for accelerating metallurgical reactions and reducing the content of free CaO in the slag. In industrial practice, the presence of trace amounts of Al₂O₃ in the slag may influence the dissolution behaviour of lime. In this work, a static lime dissolution experiment was employed, coupled with scanning electron microscopy– energy-dispersive spectroscopy, X-ray diffraction, and FactSage thermodynamic calculations, to investigate the effects of Al₂O₃ on lime dissolution in a CaO–SiO₂–FeO–P₂O₅ slag system containing 3 mass% MgO with varying Al₂O₃ concentrations. The results show that lime dissolution in this slag system leads to four distinct structural zones progressing outward from the lime surface: (1) a reacted lime core, (2) a CaO–FeO(MgO)-enriched layer containing Ca, Fe, Mg, and trace Al, (3) a solid-phase layer comprising dicalcium silicate (Ca₂SiO₄) and Ca₂SiO₄–Ca₃P₂O₈ solid solutions, and (4) the original slag matrix. The formation of solid compounds and solid solution layers was found to impede further lime dissolution. As the Al₂O₃ content increased from 0% to 5%, the average lime dissolution rate increased by approximately 40%. However, a further increase to 7% resulted in a negligible additional improvement of <5%. Mechanistic analysis indicated that Al₂O₃ reacts with CaO and MgO to form MgAl₂O₄ spinel and Ca₃Al₂O₆ phases, which subsequently modulate the dissolution behaviour through phase evolution effects. This study provided a systematic investigation into the non-monotonic effect and underlying mechanism of Al₂O₃ on lime dissolution in a basic oxygen furnace-type slag containing 5% P₂O₅ and 3% MgO.