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 pO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ p_{{{\text{O}}_{2} }} $$\end{document} (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 AlO1.5). 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.