Effect of silica on crystallization process of calcium ferrite: thermodynamic and kinetic analysis

SiO₂ is the main component of gangue in sinters and a crucial constituent in the formation of the SiO₂–Fe₂O₃–CaO (SFC) system. The non-isothermal crystallization kinetics of the SFC system were investigated using differential scanning calorimetry. The crystallization process of SFC was studied under different cooling rates (5, 10, 15, and 20 K/min), and the crystalline phases and microstructures of the SFC crystals were verified through X-ray diffraction and scanning electron microscopy. The results indicate that when the SiO₂ content is 2 wt.%, increasing the cooling rate promotes the precipitation of CaFe₂O₄ (CF) in the SFC system, thereby inhibiting the precipitation of Ca₂Fe₂O₅ (C₂F). In contrast to the CaO–Fe₂O₃ (C–F) system, the addition of SiO₂ does not alter the precipitation mechanisms of C₂F and CF. By further adding SiO₂, the precipitation of Ca₂SiO₄ (C₂S) significantly increases. Simultaneously, the CaO content in the liquid phase decreases. This leads to the crystallization process of the CF₄S (4 wt.% SiO₂) system bypassing the precipitation of C₂F and directly forming CF and CaFe₄O₇ (CF₂). In the case of the CF₈S (8 wt.% SiO₂) system, the crystallization process skips the precipitation of C₂F and CF, directly yielding CF₂. The crystallization process of both CF₂S (2 wt.% SiO₂) and CF is similar, comprising two reaction stages. The Ozawa method was used to calculate the activation energy for the crystallization of C₂F and CF as − 329 and − 419 kJ/mol, respectively. Analysis using the Malek method reveals model functions for both stages.