Reaction sequences in the formation of silico-ferrites of calcium and aluminum in iron ore sinter

Abstract

Complex silico-ferrites of calcium and aluminium (low-Fe form, denoted as SFCA; and high-Fe, low-Si form, denoted as SFCA-I) constitute up to 50 vol pct of the mineral composition of fluxed iron ore sinter. The reaction sequences involved in the formation of these two phases have been determined using an in-situ X-ray diffraction (XRD) technique. Experiments were carried out under partial vacuum over the temperature range of T=22 °C to 1215 °C (alumina-free compositions) and T=22 °C to 1260 °C (compositions containing 1 and 5 wt pct Al₂O₃) using synthetic mixtures of hematite (Fe₂O₃), calcite (CaCO₃), quartz (SiO₂), and gibbsite (Al(OH)₃). The formation of SFCA and SFCA-I is dominated by solid-state reactions, mainly in the system CaO-Fe₂O₃. Initially, hematite reacts with lime (CaO) at low temperatures (T ∼ 750 °C to 780 °C) to form the calcium ferrite phase 2CaO·Fe₂O₃ (C₂F). The C₂F phase then reacts with hematite to produce CaO·Fe₂O₃ (CF). The breakdown temperature of C₂F to produce the higher-Fe₂O₃ CF ferrite increases proportionately with the amount of alumina in the bulk sample. Quartz does not react with CaO and hematite, remaining essentially inert until SFCA and SFCA-I began to form at around T=1050 °C. In contrast to previous studies of SFCA formation, the current results show that both SFCA types form initially via a low-temperature solid-state reaction mechanism. The presence of alumina increases the stability range of both SFCA phase types, lowering the temperature at which they begin to form. Crystallization proceeds more rapidly after the calcium ferrites have melted at temperatures close to T=1200 °C and is also faster in the higher-alumina-containing systems.