High-Alumina Type Calcium Ferrite: A new mineral phase for low-carbon ironmaking in the future

As high-quality iron ore resources gradually diminish, the global steel industry faces long-term challenges due to declining ore quality. Notably, the continuous increase in Al₂O₃ content in iron ore has led to a decline in the metallurgical performance of sinter and fluctuations in slag properties. Calcium ferrite plays a crucial role as a binding phase in high-alkalinity sinter, exhibiting excellent physical strength and metallurgical performance. Therefore, we propose incorporating excess Al₂O₃ into composite calcium ferrite to enable sinter with Al₂O₃ content exceeding 3%, thus entering a previously restricted compositional zone. In the synthesis of high-alumina composite calcium ferrite, two high-Al₂O₃ phases were identified: Type A (Al_1.2 Ca_2.8 Fe_8.7 O₂₀ Si_0.8) and Type B (Ca₄ Al_4.18 Fe_1.82 Si₆ O₂₆). Results show that Type A has a higher cell density (4.13 g/cm³) and longer Fe-O bond length (2.2193 Å) compared to Type B (3.46 g/cm³ and 1.9415 Å), with a significantly greater lattice oxygen concentration (7.86% vs. 1.85%), demonstrating advantages in strength and reducibility. Type A contains a lower proportion of silicates, is predominantly composed of calcium ferrite, and exhibits minimal porosity. Melting point and viscosity tests indicate that Type A SFCA begins to form a liquid phase at 880°C, with a viscosity range of 0-0.35 Pa·s, significantly lower than that of Type B (1220°C and 0-20 Pa·s). This suggests that Type A SFCA has a lower initial melting temperature and viscosity, facilitating increased liquid phase generation and improved flow properties, thus enhancing adhesion to surrounding ore particles. Compressive strength tests reveal that Type A (36.83-42.48 MPa) significantly outperforms Type B (5.98-12.79 MPa) and traditional sinter (5.02-13.68 MPa). Additionally, at 900°C, Type A achieves a final reducibility of 0.89, surpassing Type B's 0.83. In summary, Type A SFCA demonstrates superior structural, thermophysical, and metallurgical properties, highlighting its promising potential for industrial applications.