Phase Equilibria of the Iron-rich Corner of the CaO–Fe₂O₃–Al₂O₃ System at 1240°C in Air

Abstract

The compositional ranges of SFCA-I and other SFCA series and the phase equilibrium relationships were investigated in the iron-rich corner of the CaO–Fe₂O₃–Al₂O₃ system at 1240°C in air using powder and single crystal XRD as well as EPMA. To obtain the desired composition, reagent-grade CaCO₃, Fe₂O₃, and Al₂O₃ powders were weighed, mixed, and equilibrated at 1240°C in air. Each of the obtained samples was divided into two parts: one was pulverized into a powder and analyzed by XRD, and the other was subjected to microstructural observation and compositional analysis using EPMA. The crystalline structures of the SFCA series for some samples were analyzed by a single XRD. The results revealed that not only SFCA-I but also SFCA-II and SFCA have been found in the compositional area of this study; the Al/(Al+Ca+Fe) concentration ranges are 7.89–20.33 mol% for SFCA-I, 17.62–24.55 mol% for SFCA-II and 20.93–27.60 mol% or even higher for SFCA. The compositions of SFCA are on the line satisfying M₁₄O₂₀, while those of SFCA-I and SFCA-II deviate from the lines satisfying M₂₀O₂₈ and M₁₇O₂₄, respectively, and are closer to the line satisfying M₁₄O₂₀. Since the compositions were derived based on the assumption that all Fe irons are Fe³⁺, the apparent compositions of SFCA-I and SFCA-II deviate from M₂₀O₂₈ and M₁₇O₂₄, respectively, owing to the presence of trace amounts of Fe²⁺.