Effect of Alumina on Crystallization Behavior of Calcium Ferrite in Fe2O3-CaO-SiO2-Al2O3 System

Abstract

Al2O3 is a gangue component in iron ores, significantly influencing the formation and crystallization of calcium ferrite in the sintering process. But the mechanism of the Al2O3 effect on the crystallization of calcium ferrite is rarely reported. In this work, a crystallization device was designed to investigate the crystallization behavior of calcium ferrite in Fe2O3-CaO-SiO2-Al2O3 melt under non-isothermal conditions. XRD, SEM-EDS, and optical microscopy were used to identify the crystalline phase and the microstructure of samples. The result shows that the crystal morphology of SFCA changed in the order of strip, column, and needle as the Al2O3 content increased. The crystallization sequence of samples containing Al2O3 was observed as Ca4Fe14O25 (C4F14) → Fe2O3 → Ca3.18Fe15.48Al1.34O36 (SFCA-I) → CaFe2O4 (CF) → Ca5Si2(Fe, Al)18O36 (SFCA) → γ-Ca2SiO4 (C2S). The generation pathway of SFCA-I was found to be C4F14 + Si4+ + Al3+ → SFCA-I. Increasing the cooling rate can promote the formation of C4F14, SFCA-I, Fe2O3 and the amorphous phase. However, it prevented the crystallization of CF and SFCA while inhibiting the transformation of β-C2S to γ-C2S. When the Al2O3 content reached or exceeded 2.5 mass pct, the viscosity of Fe2O3-CaO-SiO2-Al2O3 melt increased sharply, resulting in the decrease in the crystal size of calcium ferrite.