Microstructure Evolution During Controlled Solidification of “Fe₂O₃”-CaO-SiO₂ Liquids in Air

The principal chemical components in iron ore sintering are Fe₂O₃, CaO, and SiO₂. This sintering process consists of three key steps: heating, holding at peak temperature, and cooling. During the cooling stage, a liquid oxide solidifies to form the final sinter microstructures. To investigate the fundamental processes taking place during the cooling of sinters, a new experimental technique has been developed that allows the stages of solidification to be determined in isolation, rather than inferred from the final microstructures. Fe₂O₃-CaO-SiO₂ oxide samples of a bulk composition having a CaO/SiO₂ mass ratio of 3.46 and 73.2 wt pct Fe₂O₃ were cooled in air from 1623 K (1350 °C) at 2 K/s, quenched at 5 K temperature intervals from 1533 K to 1453 K (1260 °C to 1180 °C), and analyzed using Electron Probe Micro X-Ray Analysis (EPMA). During cooling, four distinct stages were observed, consisting of the phase assemblages Liquid + Hematite (I), Liquid + Hematite + C₂S(II), Liquid + C₂S + CF₂(III), and C₂S + CF₂ + CF (IV). This solidification sequence differs from that predicted under equilibrium and Scheil–Gulliver Cooling. Importantly, no Silico-Ferrite of Calcium (SFC) phase was observed to form on solidification of the liquid. Based on the microstructures formed and liquid compositions, measured by EPMA, it was demonstrated that kinetic factors play a major role in determining the phases and microstructures formed under the conditions investigated.