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

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.