The Formation Process of Silico-Ferrite of Calcium (SFC) from Binary Calcium Ferrite

Silico-ferrite of calcium (SFC) is a significant equilibrium crystalline phase in the Fe₂O₃-CaO-SiO₂ (FCS) ternary system and a key bonding phase in the sintering process of fine iron ore. In this work, the formation process of SFC from binary calcium ferrite has been determined by X-ray diffraction and field-emission scanning electron microscopy. Experiments were carried out under air at 1473 K (1200 °C) by adding SiO₂ and Fe₂O₃ into CaO·Fe₂O₃ (CF). It was found that the formation of SFC is dominated by solid-state reactions in the FCS ternary system, in which Fe₂O₃ reacts with CaO·Fe₂O₃ to form the binary calcium ferrite phase. The chemical composition of binary calcium ferrite is Ca_2.5Fe_15.5O₂₅ and approximately Ca₂Fe₁₂O₂₀ (CaO·3Fe₂O₃). Then Si⁴⁺ and Ca²⁺ ions take the place of Fe³⁺ ion in preference located on the octahedral layers which belongs to (0 0 18) plane of binary calcium ferrite. The crystal structure of binary calcium ferrite gradually transforms from orthorhombic to triclinic, and the grain is refined with the addition of silica due to the smaller radius of Si⁴⁺ ion. A solid solution SFC forms completely when the content of SiO₂ reaches approximately 3.37 wt pct at 1473 K (1200 °C).