High-alumina type calcium ferrite: A new mineral phase for low-carbon ironmaking in the future

With the gradual reduction in high-quality iron ore resources, the global steel industry faces long-term challenges. For example, the continuous increase in the Al₂O₃ content of iron ore has led to a decrease in the metallurgical performance of sinter and fluctuations in slag properties. Considering calcium ferrite (CF) and composite CF (silico-ferrite of calcium and aluminum, SFCA) play a crucial role as a binding phase in high-alkalinity sinter and exhibit excellent physical strength and metallurgical performance, we propose incorporating excess Al₂O₃ into SFCA to form a new binding phase with excellent properties for high-quality sinter preparation. In the synthesis of high-Al₂O₃ SFCA, two high-Al₂O₃ phases were identified as types A (Al_1.2Ca_2.8Fe_8.7O₂₀Si_0.8) and B (Ca₄Al_4.18Fe_1.82Si₆O₂₆). Results show that type A SFCA sample had a higher cell density (4.13 g/cm³) and longer Fe–O bond length (2.2193 Å) than type B (3.46 g/cm³ and 1.9415 Å), with a significantly greater lattice oxygen concentration (7.86% vs. 1.85%), which demonstrate advantages in strength and reducibility. Type A SFCA sample contained a lower proportion of silicates, was predominantly composed of SFCA, and exhibited minimal porosity. Melting point and viscosity simulation tests indicate that type A SFCA sample formed a liquid phase at 880°C with a viscosity range of 0–0.35 Pa·s, which is notably lower than that of type B SFCA sample (1220°C and 0–20 Pa·s). This finding suggests that type A SFCA sample has a low initial melting temperature and viscosity, which facilitates increasing liquid-phase generation and improving flow properties. Such a condition enhances the adhesion to surrounding ore particles. Compressive strength tests reveal that type A SFCA sample (36.83–42.48 MPa) considerably outperformed type B SFCA sample (5.98–12.79 MPa) and traditional sinter (5.02–13.68 MPa). In addition, at 900°C, type A SFCA sample achieved a final reducibility of 0.89, which surpassed that of type B SFCA sample (0.83). In summary, type A SFCA sample demonstrate superior structural, thermophysical, and metallurgical properties, which highlight their promising potential for industrial applications.