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INTRODUCTION

THE liquid phase of the copper-sulfur system is characterized by the existence of two extended regions of miscibility gaps between them the solid solution digenite melts congruently at a stoichiometry around Cu2S. According to the review of Chakrabarti and Laughlin[1] the liquidus maximizes at 1403 ± 2 K (1130 ± 2 C) in relation with the congruent melting point of digenite while the liquid solution demixes within two miscibility gaps above 1378 K (1105 C) at copper-rich compositions on one side of digenite and above 1086 K (813 C) at higher sulfur compositions on the other side. Including the melt, nine condensed Cu-S phases are reported for 1 bar total pressure, which are the solids anilite Cu1.75S, covellite CuS, high- and low-temperature chalcocite, djurleite, digenite and the terminal phases based on Cu and S. Thermodynamic modeling studies on the copper-sulfur melts were the subject of several studies such as those of Kellogg[2,3] and Larrain et al.[4] using an associate solution description. Sharma and Chang[5] took into account the same approach for the liquid solution and additionally considered covellite, high-temperature chalcocite and digenite for a thermodynamic analysis of the

PETER WALDNER is with the Department of General, Analytical and Physical Chemistry, University of Leoben, Franz-Josef-StraOˆe 18, 8700 Leoben, Austria. Contact e-mail: [email protected] Manuscript submitted November 6, 2019. Article published online February 20, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS B

Cu-S system. Dinsdale et al.[6] published a partial assessment of the copper–sulfur system regarding a two-sublattice model for the properties of the liquid phase. The modified quasichemical model was chosen by Kongoli et al.[7] and Degterov and Pelton[8] to model the liquid solution with two different Gibbs energies for the copper-rich phase and a sulfur-rich molten phase. The extended modified quasichemical model was applied to the molten copper-sulfur solution phase for the first time by Waldner and Pelton.[9] Lee et al.[10] performed Gibbs energy modeling for various Cu-S solid phases where an associate solution model specifically for the liquid phase was considered to compute the phase diagram of the Cu-S system. Within a critical assessment and thermodynamic modeling of the Cu-O and Cu-O-S systems, Shishin and Decterov[11] described the Gibbs energy of the Cu-S liquid solution phase on the basis of the optimization by Waldner and Pelton.[9] Jantzen et al.[12] took copper sulfide Cu2S as the liquid solution constituent into account for their calculation of the Cu-S phase diagram. The purpose of this study is a comprehensive thermodynamic description of the copper-sulfur liquid solution over the whole composition range considering all experimental data available in the literature. Based on recent studies on the solid system phases by Waldner,[13,14] the obtained model should provide the final contribution for the completion of the thermodynamic modeling of all known copper-sulfur phases to present