Thermodynamic Calculation of Liquidus Surface of FeOx–CaO–SiO2 System

Abstract

As world's reserve of high grade iron ores declines, the gangue minerals, such as Al2O3, of iron ore have been increasing. To counter the adverse impact of alumina on sinter quality, MgO fluxes are often added to increase sinter MgO. The liquidus surface of Al2O3 and MgO containing FeOx–CaO–SiO2 system is therefore of fundamental importance to understanding of the melting mechanism in the sintering process. In this study, the literature data on the liquidus surface of the FeOx–CaO–SiO2 system containing Al2O3 and MgO under various temperatures and oxygen partial pressures, which are relevant to the iron ore sintering, were carefully reviewed, and the differences between various data sources were analysed. A modified version of the thermodynamic package: Multi-Phase Equilibrium (MPE) software developed by CSIRO was compiled, including refinement of its slag database, to cover the compositional and atmospheric conditions encountered in the sintering process. The effects of oxygen partial pressure and additions of Al2O3 and MgO on the liquidus of the FeOx–CaO–SiO2 system were modelled. The model predictions represent experimental results well, and show that the liquid field at 1573 K shrinks with the oxygen partial pressure decreasing from 10−3 to 10−5 atm and separates into two distinct liquid fields. As the oxygen partial pressure decreases further from 10−5 to 10−8 atm, both liquid fields expands and merges to form a continuum. Addition of Al2O3 and MgO is found to increase the stability of the magnetite (spinel) phase, shift the liquidus away from the FeOx corner, and therefore suppress the formation of the melt during sintering.