Effect of SiO2 on the Formation of Acicular Calcium Ferrite in Sinter

Abstract

SiO2 greatly influences the formation of acicular calcium ferrite (ACF), which is the main binder phase in sinter, but the mechanism of its influence is not yet clear. Experiments were carried out under air at 1200 °C with different amounts of SiO2 mixed with Fe2O3 and Ca(OH)2. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and three-dimensional (3D) reconstruction were used to characterize the phase and morphological changes of the sintered samples. The results show that SiO2 can form silicon-ferrite of calcium (SFC), which is beneficial to the formation of ACF in sinter. The SFC content first increased and then decreased with the increasing SiO2 content. The sintered sample contained the highest contents of SFC and ACF when w(SiO2) was 3 to 4 pct. The morphological changes in calcium ferrite observed with the increasing SiO2 content followed a sequence of massive → platy → acicular; both platy calcium ferrite and ACF were the longest in one direction in their 3D morphology. The inclusion of Si4+ in calcium ferrite crystals changes the crystal growth habit from having no obvious orientation to having a preferential orientation along the (040) or (320) surfaces, as determined by the first-principles calculations and XRD analysis of block samples. Viscosity was the main environmental factor influencing the size of ACF in samples with sufficient liquid phase.