Effect of MgO on the liquid/spinel/matte/gas equilibria in the Si–Fe–Mg–O–Cu–S system at controlled P(SO2) 0.3 and 0.6 atm

In this study, the effect of MgO on the phase equilibria of iron silicate slags in equilibrium with Fe3O4 spinel and matte of fixed 72 wt % Cu were identified at controlled P(SO2) 0.3 and 0.6 atm. The experimental process includes equilibration, quenching and Electron Probe X-ray Micro Analysis (EPMA). Spinel (Fe3O4) substrates were applied to confirm the equilibrium was achieved in the primary phase field of spinel. It was found that the presence of MgO increases the liquidus temperature of slag under a constant SiO2 content, i.e., more SiO2 is required to be fluxed to keep a stable smelting temperature with MgO. The effects of CaO and MgO on the liquidus of the system were further compared and found that the influence of MgO is stronger than CaO at 1250 °C under the same weight content while opposite situation occurs at 1200 °C, which is different from the FactSage predictions. Thus, the present study provides important information not only for the industrial smelting process but also for the thermodynamic modelling.