Critical thermodynamic assessment and modeling of the Fe-Ni-S system

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. INTRODUCTION

METAL-SULFUR systems play an essential scientific and industrial role in the fields of metallurgy, materials science, geochemistry, and cosmochemistry. Thermodynamic databases of multicomponent metal-sulfur systems can support improvement and developmental work in these fields on several levels. The thermodynamic modeling of the iron-nickel-sulfur system is an essential contribution to the development of an extended database for multicomponent metal-sulfur systems. The system Fe-Ni-S is dominated by a ternary solid solution, Fe-Ni pyrrhotite, connecting the monosulfide phases of the binary systems Fe-S and Ni-S over a wide homogeneity and temperature range up to liquidus temperatures.[1] The solid solutions with highest sulfur contents are formed between pyrite (FeS2) and vaesite (NiS2), which exhibit limited mutual solubility. At lower sulfur contents, the high-temperature heazlewoodite phase covers a large ternary stability field due to substantial solubility of up to 30 mol pct iron. Below 600 °C, solid-state equilibria in the central composition region are characterized by the occurrence of the ternary compound pentlandite ((Fe,Ni)9S8). In all phases of the alloy subsystem Fe-Ni, very little solubility of sulfur occurs. The ternary monosulfide pyrrhotite phase of the Fe-Ni-S system has been the subject of thermodynamic modeling by Hsieh et al.[2] A statistical thermodynamic model for ternary phases with the nickel arsenide structure was formulated, which assumes the presence of metal vacancies and metal interstitials. The model was used in the framework of a thermodynamic analysis of phase equilibria of the Fe-Ni-S system P. WALDNER, Assistant Professor, is with the Department of Physical Chemistry, University of Leoben, A-8700 Leoben, Austria. A.D. PELTON, Professor and Director, is with the Center for Research in Computational Thermochemistry (CRCT), École Polytechnique de Montréal, Montréal, PZ, Canada H3C 3A7. Contact e-mail: [email protected] Manuscript submitted December 9, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B

between 900 °C and 1350 °C by Hsieh et al.[3] An associated solution model was used for the ternary metal-sulfur liquid phase, with 12 empirical parameters to account for ternary interactions. The ternary disulfide phase was taken to be a pseudobinary solution of FeS2 and NiS2. Kongoli and Pelton[4] used an earlier version of the modified quasichemical model for short-range ordering to predict the thermodynamic properties of liquid Fe-Ni-S solutions based only upon parameters obtained from critical optimization of data for the binary subsystems. Equilibrium sulfur pressures over the ternary liquid solutions and phase equilibria with solid alloy phases between 1200 °C and 1400 °C were calculated. However, equilibria between solid and liquid sulfide phases were not modeled. The aim of the present work is to provide a comprehensive critical thermodynamic evaluation and optimization of the FeNi-S system over the entire composition range from room temperature up to liquidus te