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

A thorough review and critical evaluation of phase equilibria and thermodynamic data for phases in the iron-nickel-sulfur ternary system at I bar pressure has been made over the entire composition range for temperatures from 25 degreesC to above the liquidus. The Gibbs energies of system phases have been modeled, and optimized model parameters have been obtained which reproduce all data simultaneously within experimental error limits. The modeling is based on the recent evaluations by the authors of the Fe-S and Ni-S binary subsystems. For the liquid phase, the recently extended modified quasichemical model for short-range ordering is applied for the first time to a liquid metal-sulfur phase. Although the binary Fe-S and Ni-S liquid solutions were modeled with the extended modified quasichemical model, while the binary Fe-Ni liquid solution was modeled with a simple random-mixing model, it is shown, for the first time, that these can be combined into a consistent model of the ternary liquid phase. Two-sublattice models, within the framework of the compound-energy formalism, are used for the solid solutions: pyrrhotite, pentlandite, pyrite, vaesite, and high-temperature heazlewoodite. The ternary-solution model for iron-nickel pyrrhotite permits very good prediction of thermodynamic properties solely on the basis of binary-model parameters.