Experimental Phase Equilibria Studies in the FeO-Fe2O3-CaO-Al2O3 System in Air

The CaO·3(Al,Fe)2O3, (C(A,F)3), phase forms a solid solution in the FeO-Fe2O3-CaO-Al2O3 system and is an end member of the SFCA (Silico-Ferrite of Calcium and Aluminum) suite of solid solutions observed in industrial iron ore sinters. SFCA acts as a bonding phase in these sinter materials so an accurate description of the phase equilibria and thermodynamic properties of this phase can be used to assist in the design and operation of Fe-sintering and Fe-making processes. New experimental data are reported on the liquidus and sub-liquidus phase equilibria in the iron-rich region of the FeO-Fe2O3-CaO-Al2O3 system in air. The study was undertaken using equilibration/quenching and microanalysis techniques enabling the compositions of the liquid and solid phases in equilibrium at high temperature to be directly and accurately measured. The compositional limits of the C(A,F)3 phase and associations with the primary phase fields of CA [CaO·(Al,Fe)2O3], CA2 [CaO·2(Al,Fe)2O3], CA6 [CaO·6(Al,Fe)2O3], Hematite [(Fe,Al)2O3], Spinel [(Fe,Ca)O·(Fe,Al)2O3)], C2F [2CaO·(Fe,Al)2O3], C2F3A (2CaO·3Fe2O3·Al2O3), CF [CaO·(Fe,Al)2O3], and CF2 [CaO·2(Fe,Al)2O3] have been determined in the high-iron, low-melting temperature region of the system. The C(A,F)3 is found to be stable over a wide range of temperatures (1213 °C to 1400 °C) and alumina compositions (3.3 to 33.6 mol pct AlO1.5). The solid-phase C2F3A (2CaO·3Fe2O3·Al2O3), which is isostructural to SFCA-I indicated by previous researchers, has been found to be stable up to 1225 °C in the FeO-Fe2O3-CaO-Al2O3 system in air.