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.