An improved experimental technique has been developed to measure, concurrently, the oxygen partial pressures and temperatures within a pilot scale iron ore sinter pot as a function of time. The measurements and thermodynamic calculations have demonstrated that the oxygen partial pressure at peak bed temperature and during cooling can be oxidising or reducing relative to hematite. Examples of typical microstructures and phase assemblages observed in product sinters are presented. Potential mechanisms of hematite and magnetite formation at sub-liquidus and sub-solidus conditions are demonstrated. The relative impacts of changes to coke rate and draft pressure drop on the process conditions and proportions of the phases formed in the sinter have been measured. Increasing coke rate was shown to result in a faster sinter heating rates, higher peak bed temperatures and times at peak temperature. Higher draft pressures across the sinter bed resulted in faster sinter heating rates and shorter times at peak temperature.

The cooling rate of the liquid oxide can be controlled in industrial sintering processes through the draft pressure and has the potential to influence microstructure formation. The solidification of a liquid within the hematite primary phase field in the ternary “Fe₂O₃”–CaO–SiO₂ system in air was undertaken at different cooling rates to determine the impact of cooling rate on the formation of product microstructures. Samples with a bulk composition of 72.7 wt% Fe₂O₃ and a CaO/SiO₂ ratio of 3.46, were cooled from 1623 K (1350°C) at 2 K/s, 0.5 K/s, 0.1 K/s and 0.01 K/s and quenched at 5 K temperature intervals from 1533 K (1260°C) to 1453 K (1180°C). During cooling, four stages of phase assemblage formation were consistently observed at all cooling rates; in order of formation these are, Liquid+hematite (I), Liquid+hematite+dicalcium silicate(C₂S)(II), Liquid+C₂S+calcium diferrite (CF₂)(III) and C₂S+CF₂+calcium ferrite (CF)(IV). An intergrowth of silico-ferrite of calcium and aluminium-I (SFCA-I) and Ca_7.2Fe²⁺_0.8Fe³⁺₃₀O₅₃ was observed to form in some conditions in regions free of hematite, present in liquids solidifying at 0.5 K/s and 0.1 K/s. The sizes and shapes of microstructures were observed to systematically change with cooling rate, with a slower cooling rate typically resulting in coarser coupled microstructures and larger individual crystals. A larger proportion of coupled microstructures are observed at slower cooling rates, this appears to be related to the degree of undercooling prior to the nucleation of new phases. The equilibrium silico-ferrite of calcium (SFC) phase was not observed at any of the cooling rates investigated.

The solidification of liquids having bulk compositions in the high iron region of the ‘Fe ₂ O ₃ ’-CaO-SiO ₂ system having selected CaO/SiO ₂ ratios has been investigated. The compositions investigated were selected such that the bulk compositions were within the hematite primary phase field, fully liquid at temperature (1350°C) and pass through the SFC primary phase field on cooling. Specifically, three CaO/SiO ₂ ratios (wt/wt) were investigated, 2.78, 3.46 (Nicol S, Jak E, Hayes P. 2019b. Microstructure evolution during controlled solidification of “Fe ₂ O ₃ ”-CaO-SiO ₂ liquids in air. Met. Trans. B. 50:2706–2722) and 4.75 with bulk compositions comprising 69.23, 72.74 and 74.94 wt% Fe ₂ O ₃ respectively. The phases present and their composition was determined with electron probe microanalysis (EPMA) and X-ray diffraction (XRD). During cooling, four stages of solidification were observed at all three CaO/SiO ₂ ratios, and an additional two stages at the highest CaO/SiO ₂ ratio. Non-equilibrium phase and liquidus compositions were observed. Significantly, the silico ferrite of calcium phase (SFC) was not observed in any of the samples, although the formation of this phase was anticipated from phase equilibrium considerations.

The solidification of “Fe2O3”–CaO–SiO2 liquids in air at a controlled cooling rate of 2.0 K/s for a range of Fe2O3 concentrations in the bulk has been investigated. The compositions investigated were selected such that the bulk compositions were within the hematite primary phase field and had a CaO/SiO2 ratio of 3.46 wt/wt.Non-equilibrium phase assemblages were formed for all bulk compositions investigated. Specifically, the silico ferrite of calcium (SFC) phase was not formed on cooling. The microstructures and proportions of the phase assemblages formed were found to vary with the Fe2O3 concentration in the bulk.