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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 ϭ 22°C to 1215°C (alumina-free compositions) and T ϭ 22°C to 1260°C (compositions containing 1 and 5 wt pct Al 2 O 3 ) using synthetic mixtures of hematite (Fe 2 O 3 ), calcite (CaCO 3 ), quartz (SiO 2 ), and gibbsite (Al(OH) 3 ). The formation of SFCA and SFCA-I is dominated by solid-state reactions, mainly in the system CaO-Fe 2 O 3 . Initially, hematite reacts with lime (CaO) at low temperatures (T ϳ 750°C to 780°C) to form the calcium ferrite phase 2CaOиFe 2 O 3 (C 2 F). The C 2 F phase then reacts with hematite to produce CaOиFe 2 O 3 (CF). The breakdown temperature of C 2 F to produce the higherFe 2 O 3 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 ϭ 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 ϭ 1200°C and is also faster in the higher-alumina-containing systems.

Quenching experiments were used to investigate the solid solution range, thermal stability, and selected phase relationships of silico-ferrite of calcium and aluminum (SFCA) within the Fe 2 O 3 -CaO-Al 2 O 3 -SiO 2 (FCAS) system. SFCA was found to be stable within a plane that connects the end members CF 3 (CaO и 3Fe 2 O 3 ), CA 3 (CaO и 3Al 2 O 3 ), and C 4 S 3 (4CaO и 3SiO 2 ). Chemical substitution in the four component system follows the coupled substitution mechanism 2(Fe 3+ , Al 3+ ) } (Ca 2+ , Fe 2+ ) ϩ Si 4+ with the greatest range in chemical substitution occurring in the direction of the Al 3+ } Fe 3+ exchange (ranging from 0 wt pct Al 2 O 3 to ϳ31.5 wt pct Al 2 O 3 ). The extent of Al 3+ } Fe 3+ substitution decreases with increasing temperature, and it was estimated that SFCA completely decomposes by ϳ1480 ЊC. Coupled substitution involving Ca 2+ and Si 4+ for 2M 3+ is not as extensive as the Al 3+ } Fe 3+ exchange, having a maximum range between 3 and 11 wt pct C 4 S 3 component. Additional phases encountered in the experimental program included hematite; magnetite; quench liquid; dicalcium silicate; Fe-bearing gehlenite; calcium alumino-ferrite solid solutions, C(A, F) 6 and C(A, F) 2 , plus an unidentified phase, possibly representing a solid substitution between SFCA-I and C(A, F) 3 . Schematic phase diagrams have been constructed to show the relationship of SFCA with these surrounding phases.

The formation mechanisms of the complex Ca-rich ferrite iron ore sinter bonding phases SFCA and SFCA-I, during heating of a synthetic sinter mixture in the range 298-1 623 K and at pO2 = 0.21, 5 × 10 -3 and 1 × 10 -4 atm, were determined using in situ X-ray diffraction. SFCA and, in particular, SFCA-I are desirable bonding phases in iron ore sinter, and improved understanding of the effect of parameters such as pO2 on their formation may lead to improved ability to maximise their formation in industrial sintering processes. SFCA-I and SFCA were both observed to form at pO2 = 0.21 and 5 × 10 -3 atm, with the formation of SFCA-I preceding SFCA formation in each case, but via distinctly different mechanisms at each pO2. No SFCA-I was observed at pO2 = 1 × 10 -4 atm; instead, a Ca-rich phase designated CFAlSi, formed at 1 420 K. By 1 456 K, CFAlSi had decomposed to form melt and a small amount of SFCA. Such a low pO2 during heating of industrial sinter mixtures is, therefore, undesirable, since it would not result in the formation of an abundance of SFCA and SFCA-I bonding phases. In addition, CFA phase, which was determined by Webster et al. (Metall. Mater. Trans. B, 43(2012), 1344) to be a key precursor phase in the formation of SFCA at pO2 = 5 × 10 -3 atm, was also observed to form at pO2 = 0.21 and 1 × 10 -4 atm, with the amount decreasing with increasing pO2.KEY WORDS: iron ore sinter; complex Ca-rich ferrite sinter bonding phases; SFCA and SFCA-I; in situ X-ray diffraction; oxygen partial pressure; phase formation mechanisms; synchrotron X-ray diffraction.

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Owing to the depletion of world lump iron ore stocks, pre-treated agglomerates of ®ne ores are making up a growing proportion of blast-furnace feedstock ($80%). These agglomerations, or`sinters', are generally composed of iron oxides, ferrites (most of which are silicoferrites of calcium and aluminium, SFCAs), glasses and dicalcium silicates (C2S). SFCA is the most important bonding phase in iron ore sinter, and its composition, structural type and texture greatly affect its physical properties. Despite its prevalence and importance, the mechanism of SFCA formation is not fully understood. In situ powder X-ray diffraction investigations have been conducted into the formation of SFCA, allowing the study of the mechanism of its formation and the observation of intermediate phases with respect to time and temperature. Studies have been carried out to investigate the effects of changing the substitution levels of aluminium for iron. The use of the Rietveld method for phase quanti®cation gives an indication of the order and comparative rates of phase formation throughout the experiments.

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