Investigation of the Thermodynamic Stability of C(A, F)₃ Solid Solution in the FeO-Fe₂O₃-CaO-Al₂O₃ System and SFCA Phase in the FeO-Fe₂O₃-CaO-SiO₂-Al₂O₃ System

Silico-ferrite of calcium and aluminum (SFCA) is the major bonding phase in iron ore sintering process and is critical to enhancing the sinter properties, such as reducibility and mechanical strength in subsequent blast furnace operations. The phase relations foundations of the alumina-free silico-ferrite of calcium (SFC) have been previously experimentally investigated in air by the authors (Chen et al. ISIJ Int 59:795–804, 2019, Cheng et al. Metall Mater Trans B 51:1587–1602, 2020) and in 1 atm CO₂ (Chen et al. ISIJ Int, 59:805–809, 2019). Present investigation using equilibration and quenching followed by electron probe X-ray microanalysis (EPMA) technique, follows those previous works on the SFC, with the focus on the effects of: (i) Al₂O₃ (in the “Fe₂O₃”-CaO-Al₂O₃ and the “Fe₂O₃”-CaO-SiO₂-Al₂O₃ system in air), and (ii) the effect of \( p_{{{\text{O}}_{2} }} \) (the “Fe₂O₃”-CaO-Al₂O₃ in 1 atm CO₂ atmosphere), to investigate the thermodynamic stability of the C(A, F)₃ [Ca(Al, Fe)₆O₁₀] solid solution in the “Fe₂O₃”-CaO-Al₂O₃ system in both air and pure CO₂ atmospheres between 1150 °C and 1250 °C; and the silico-ferrite of calcium and aluminum (SFCA) solid solution with 1, 2 and 4 wt pct of Al₂O₃ in bulk compositions in the “Fe₂O₃”-CaO-SiO₂-Al₂O₃ system at temperatures in the range between 1255 °C and 1340 °C. Present study shows that C(A, F)₃ is stable over a wide range of Al₂O₃ concentration (8.8 to 26.7 wt pct Al₂O₃, or 12.5 to 34.8 mol pct AlO_1.5). It also becomes less stable in terms of both the temperature and the compositional stability range as the oxygen partial pressure is reduced. The SFCA phase in the “Fe₂O₃”-CaO-SiO₂-Al₂O₃ system is found to be present in the range of 1 to 4 wt pct Al₂O₃ bulk compositions selected in air. The relative stability of this phase increases with increased Al₂O₃ in the bulk material. Tie-lines joining the SFCA and the corresponding liquid and hematite phases are constructed over the range of composition investigated at sub-liquidus temperatures. The new experimental measurements show that the CaO/SiO₂ ratio in the SFCA phase is almost identical to that in the liquid. The distribution ratio of Al₂O₃ between SFCA and liquid is in the range 2/1 to 3/1.