Thermodynamics of the MnO-FeO-MnS-FeS-SiO2 System at SiO2 Saturation under Reducing Condition: Immiscibility in the Liquid Phase

Abstract

Phase Equilibria in the MnO-FeO-MnS-Fe-SiO2 liquid phase at SiO2 saturation under reducing condition were experimentally investigated in the temperature range from 1200 degrees C to 1400 degrees C in order to provide a fundamental knowledge on new FerroManganese (FeMn) alloy process. High temperature equilibration, quenching and Electron Probe Micro-Analysis (EPMA) were employed to obtain equilibrium compositions of liquid phase which was separated into oxide-rich liquid and sulfide-rich liquid. Concentration of Mn defined as R-Mn = n(Mn)/n(Fe)+n(Mn) in the sulfide-rich liquid was always lower than R-Mn in the oxide-rich liquid. In order to understand the liquid separation and the distribution of Mn in the two liquid phases, a thermodynamic modeling of this liquid oxysulfide was performed by taking into account strong chemical Short-Range Ordering (SRO) in the framework of the Modified Quasichemical Model in the Quadruplet Approximation. Contrary to the general understanding that Mn attracts S stronger than Fe does and it would have resulted higher Mn content in the sulfide-rich liquid, the present experimental results show that Fe is enriched in the sulfide-rich liquid. This implies that Fe attracts S stronger than Mn does in the liquid phase concerned in the present study. Such a behavior is attributed to the fact that Mn is bound by SiO2 through a formation of (Mn-O-Si) Second-Nearest-Neighbor (SNN) pair, thus oxide-rich liquid attracts more Mn while Fe is distributed more to sulfide-rich liquid. A number of points to be considered for the production of low phosphorus FeMn alloy through the two-phase liquid separation are discussed.