Effect of SiO₂ on Crystallization of Calcium Ferrites in Fe₂O₃–CaO–SiO₂–Al₂O₃ System in Cooling Process

SiO₂ plays an important role in the formation of calcium ferrite as a main binding phase in iron ore sinter. But the mechanism of SiO₂ influence on the crystallization of calcium ferrite has been still unclear. The crystallization behavior of Fe₂O₃–CaO–SiO₂–Al₂O₃ melt in a non-equilibrium state was investigated using X-ray diffraction, optical microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The results show that SiO₂ was an essential impact on the amount formation and morphology of calcium ferrite crystallization during the sintering process. With the increase of SiO₂ content, the crystal morphology changed in the order of blocky, columnar, acicular to grains. The crystallization size increased first and then decreased. The crystallization sequence of samples with 4.0 mass pct SiO₂ was observed as (Fe₂O₃, C₄F₁₄, SFCA-I) → CF→ SFCA → β-C₂S. Increase of the cooling rate promoted the formation of Fe₂O₃, C₄F₁₄, and glass phase and the transformation of SFCA-I to SFCA. However, it also inhibited the crystallization of CF and SFCA, simultaneously the transformation of β-C₂S to γ-C₂S. The thermodynamic calculation shows that the crystallization temperatures and the primary crystal phase varied with the increase of SiO₂ content, which agreed with the experimental results. In addition, the increase of SiO₂ significantly reduced the liquid phase amount, simultaneously increased its viscosity at high-temperature range, which was contributing to decrease of the crystallization size.