<i>In-Situ</i> X-ray Diffraction Analysis Reveals Complex Calcium Ferrite Phase Formation during Heating and Cooling of Silico-Ferrite of Calcium (SFC) Compositions

The phase evolution during heating of two Al2O3-free mixtures designed to form the SFC iron ore sinter bonding phase was investigated using in-situ synchrotron X-ray diffraction over the temperature range 293-1623 K. Improved fundamental understanding of the complex Ca-rich ferrite phases which form under sintering conditions has the potential to improve the sintering process. Results reported herein confirmed that during heating at an oxygen partial pressure of 5×10-3 atm, the γ-CFF phase (nominal composition Fe14.82Ca3.0O25, crystallographic space group P321), and not the β-CFF phase (Fe14.85Ca2.95O25, P3–c1), formed as a precursor phase to SFC (M14O20, where M = Fe, Ca and Si, space group P1–). Results also confirmed that during heating in air an Al2O3-free analogue of triclinic SFCA-I, designated SFC-I, formed as a precursor phase to SFC, and there was no evidence for the formation of the α-CFF phase that has been reported previously to form under equilibrium conditions. The question of whether an Al2O3-free analogue of SFCA-III, SFC-III, could also be formed during crystallisation from the melt during cooling was raised and in separate laboratory in-situ XRD experiments there was no evidence for the formation of SFC-III during cooling; rather, the β-CFF phase formed along with dicalcium silicate (CaO.SiO2, C2S) as the first phases to crystallize from the melt at high temperature, and with magnetite converting to hematite as cooling continued. Any presence of β-CFF in industrial sinter would likely indicate, and be a useful marker for, localized regions of Al2O3 deficiency in the sinter mixture, where that sinter mixture has experienced melting.