Thermodynamics and kinetics of alumina and magnesium oxide in calcium ferrite sintering process

Al₂O₃ and MgO serve as the primary gangue components in sintered ores, and they are critical for the formation of CaO–Fe₂O₃–xAl₂O₃ (wt%, C–F–xA) and CaO–Fe₂O₃–xMgO (wt%, C–F–xM) systems. Therefore, in this study, a nonisothermal crystallization thermodynamics behavior of C–F–xA and C–F–xM systems was examined using differential scanning calorimetry, and a phase identification and microstructure analysis for C–F–xA and C–F–xM systems were carried out by X-ray diffraction and scanning electron microscopy. Results showed that in C–F–2A and C–F–2M systems, the increased cooling rates promoted the precipitation of CaFe₂O₄ (CF) but inhibited Ca₂Fe₂O₅ (C₂F) formation. In addition, C–F–2A system exhibited a lower theoretical initial crystallization crystalline phase precipitation temperature (1566 K) compared to the C–F system (1578 K). This temperature further decreases to 1554 K and 1528 K in the C–F–4A and C–F–8A systems, respectively. However, in C–F–xM system, the increased MgO content raised the crystallization temperature. This is because that the enhanced precipitation of MF (a spinel phase mainly comprised Fe₃O₄ and MgFe₂O₄) and C₂F phases suppressed the CF precipitation reaction. In kinetic calculations, the Ozawa method revealed the apparent activation energies of the C–F–2A and C–F–2M systems. Malek’s method revealed that the crystallization process in C–F–2A system initially followed a logarithmic law ( \mathrmln\alpha or \mathrmln\alpha ^2 ), later transitioning to a reaction order law ((1−α)⁻¹ or (1−α)^−1/2, n = 2/3) or the \mathrmln\alpha ^2 function of the exponential law. In C–F–2M system, it consistently followed the sequence ƒ(α) = (1−α)² (α is the crystallization conversion rate; n is the Avrami constant; ƒ(α) is the differential equations for the model function of C₂F and CF crystallization processes).