Effect of Alumina on Crystallization Behavior of Calcium Ferrite in Fe₂O₃-CaO-SiO₂-Al₂O₃ System

Al₂O₃ is a gangue component in iron ores, significantly influencing the formation and crystallization of calcium ferrite in the sintering process. But the mechanism of the Al₂O₃ effect on the crystallization of calcium ferrite is rarely reported. In this work, a crystallization device was designed to investigate the crystallization behavior of calcium ferrite in Fe₂O₃-CaO-SiO₂-Al₂O₃ melt under non-isothermal conditions. XRD, SEM-EDS, and optical microscopy were used to identify the crystalline phase and the microstructure of samples. The result shows that the crystal morphology of SFCA changed in the order of strip, column, and needle as the Al₂O₃ content increased. The crystallization sequence of samples containing Al₂O₃ was observed as Ca₄Fe₁₄O₂₅ (C₄F₁₄) → Fe₂O₃ → Ca_3.18Fe_15.48Al_1.34O₃₆ (SFCA-I) → CaFe₂O₄ (CF) → Ca₅Si₂(Fe, Al)₁₈O₃₆ (SFCA) → γ-Ca₂SiO₄ (C₂S). The generation pathway of SFCA-I was found to be C₄F₁₄ + Si⁴⁺ + Al³⁺ → SFCA-I. Increasing the cooling rate can promote the formation of C₄F₁₄, SFCA-I, Fe₂O₃ and the amorphous phase. However, it prevented the crystallization of CF and SFCA while inhibiting the transformation of β-C₂S to γ-C₂S. When the Al₂O₃ content reached or exceeded 2.5 mass pct, the viscosity of Fe₂O₃-CaO-SiO₂-Al₂O₃ melt increased sharply, resulting in the decrease in the crystal size of calcium ferrite.