Controlled Solidification of Liquids Within the SFC Primary Phase Field of the “Fe₂O₃”-CaO-SiO₂ System in Air

To provide fundamental information on the phases and microstructures formed during sintering, a liquid with a bulk composition within the silico ferrite of calcium (SFC) primary phase field in the ternary “Fe₂O₃”-CaO-SiO₂ system in air was solidified at a controlled rate. Samples of a bulk composition with a CaO/SiO₂ ratio of 4.00 and 69.24 wt pct Fe₂O₃, were cooled from 1623 K (1350 °C) at 2 K/s, with samples quenched at temperatures between 1513 K (1240 °C) to 1453 K (1180 °C). The silico ferrite of calcium and aluminium I (SFCA-I) and Ca_7.2Fe _0.8 ²⁺ Fe₃₀³⁺O₅₃ phases were observed to form an intergrowth (‘SFC-I’) rather than the anticipated SFC phase. Solidification was found to occur in three stages, Liquid + ‘SFC-I’, Liquid + ‘SFC-I’ + C₂S + CF₂, and C₂S + CF₂ + CF, where C₂S denotes dicalcium silicate, CF denotes calcium ferrite and CF₂ denotes calcium diferrite. The phases formed and the solidification sequence differ from those predicted under equilibrium and Scheil–Gulliver Cooling. Although not directly applicable to industrial operations, this research clearly shows that the formation of both the SFCA and SFCA-I phase in iron ore sinters is controlled by kinetic processes rather than equilibrium conditions.