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–FeS–SiO₂ liquid phase at SiO₂ saturation under reducing condition were experimentally investigated in the temperature range from 1200°C to 1400°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 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 thermo-dynamic 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.