Crystal Phases Formed in a CaO-Fe₂O₃ System Under a High Cooling Rate in Air

A CaO-Fe₂O₃ system is a fundamental binary system for the iron ore sintering process. Although the basic reactions have been investigated since the 1960s, melting and solidification caused by the combustion of coke results in an unstable state owing to extreme temperature variations. In this study, using a hot thermocouple method, samples of 10 pct CaO-90 pct Fe₂O₃ and 20 pct CaO-80 pct Fe₂O₃ were melted on a thermocouple and quenched with several techniques. The obtained samples were precisely examined by XRD. It was found that the sample containing 10 pct CaO-90 pct Fe₂O₃ changed to 10 pct CaO-13 pct FeO-77 pct Fe₂O₃ under an oxygen partial pressure (\( P_{{{\text{O}}_{2} }} \)) of 0.21 during melting. For the 10 pct CaO sample, the crystal phases found at a low cooling rate (509 K/s) were WF_ss, C₄WF₈ (C: CaO, W: FeO, F: Fe₂O₃), and C₂W₄F₉. When the sample composition was 20 pct CaO, the precipitated crystal phases were C₄WF₄, C₄F₇, and C₄WF₈. On the other hand, the crystal phases for high cooling rates (1590 and 7900 K/s) with 10 pct CaO were WF_ss (solid solution of WF and F), F, and C₂W₄F₉. The formation of the equilibrium phases WF_ss, F, C₄WF₄, and C₄WF₈ can be understood by examining the isothermal section of the phase diagrams, while the unstable phases C₂W₄F₉ and C₄F₇ are discussed on the basis of the reactions under an equilibrium state.