Solids and liquids in the (Fe, Mg, Ca)S-system: experimentally determined and thermodynamically modelled phase relations

Abstract

Thermodynamic descriptions and experimentally verified phase relations in the FeS-MgS-CaS system are important both for steelmaking and for natural reduced systems. Experimental and thermodynamic data for such oxygen-poor systems are sparse due to the difficulty of conducting experiments under conditions at which these sulfides are stable. In this study, phase relationships were determined for FeS-MgS at 1170–1550 °C, for FeS-CaS at 1025–1600 °C, for MgS-CaS at 900–1500 °C and for FeS-MgS-CaS at 1050 and 1360 °C. Experiments were performed in evacuated silica glass tubes with excess Fe⁰ to favour troilite (FeS) rather than pyrrhotite (Fe_1–xS) for the FeS-rich phase. Textural interpretations and measured compositions indicate that the FeS-CaS system melts eutectically at 1063 ± 3 °C at 7 ± 1 mol% CaS. The FeS-MgS system is also modelled to be eutectic (at 1180 and 2.5 mol% MgS), yet, experimentally, its eutectic or peritectic character could not be unequivocally determined. This system’s liquidus has a higher dT/dX than previously reported. The MgS-CaS system was found to have a symmetric miscibility gap that closes at 1210 °C. Differences to the outcome of previous experimental studies can be explained by the presence of troilite rather than pyrrhotite in our experiments when Fe-rich solid solution coexists with liquid or solid solution. The experimental data are fit by a thermodynamic model that reproduces the experimentally determined phase relations, and is capable of predicting melting phase relations for the FeS-MgS-CaS ternary.