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

Al₂O₃ and MgO are the primary gangue components in sintered ores, critical for the formation of the CaO-Fe₂O₃-Al₂O₃(CF-A) and CaO-Fe₂O₃-MgO (CF-M) systems. This study examines the non-isothermal crystallization kinetics of CF-A and CF-M systems using Differential Scanning Calorimetry (DSC). Crystallization processes at different cooling rates (5, 10, 15, and 20 K/min) were investigated, with phase identification and microstructural analysis performed via X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Results show that at 2 wt.% Al₂O₃ and MgO, increasing cooling rates promotes the precipitation of CaFe₂O₄ (CF) in both systems, inhibiting Ca₂Fe₂O₅ (C₂F) formation. However, crystallization weakens when cooling rates exceed 20 K/min. The addition of Al₂O₃ and MgO does not alter the precipitation mechanisms of C₂F and CF compared to the Fe₂O₃-CaO (C-F) system. Increased Al₂O₃ content reduces the initial crystallization temperatures of CF and CF₂A to 1578 K and 1566 K, respectively, slowing the C₂F and CF precipitation processes and delaying the peritectic reaction temperature from 1489 K to 1473 K. In CF4A (4 wt.% Al₂O₃) and CF8A (8 wt.% Al₂O₃) systems, the liquid-to-solid transition involving C₂(A, F) precipitation and the peritectic reaction involving C(A, F) phases become more gradual, with peritectic reaction temperatures delayed to 1455 K and 1416 K, respectively.  Increasing MgO content raises the crystallization temperature of the CF4M (4 wt.% MgO) system. Enhanced precipitation of MF (The spinel phase is mainly composed of Fe₃O₄ and MgFe₂O₄) and CF phases suppresses the CF-related peritectic reaction, which ceases in the CF8M system. The crystallization behaviors of CF2A (2 wt.% Al₂O₃), CF2M (2 wt.% MgO), and CF are similar. Using the Ozawa method, the apparent activation energies of CF2A and CF2M systems are higher than those of single-step processes. Malek’s method shows that the CF2A system initially follows a logarithmic law (lnα or lnα²), later transitioning to a reaction order law ((1-α)^-1 or (1-α)^-1/2, n=2/3) or the lnα² function of the exponential law. The CF2M system consistently follows the sequence ƒ(α) = (1-α)².