Thermodynamics and phase relationships of transition metal-sulfur systems: Part V. A reevaluation of the Fe-S system usi
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I.
INTRODUCTION
THE thermodynamic
and phase equilibrium data of the subsystem Fe-FeS of the Fe-S binary were evaluated by Hillert and Staffansson ~ taking pyrrhotite to be a stoichiometric compound at equal atomic composition. Subsequently, the data for the entire system were evaluated by Sharma and Chang-' taking into consideration the homogeneity range of pyrrhotite. The equations derived by Libowitz 3 were used to describe the thermodynamics of the pyrrhotite phase as well as the parameter values recommended by Libowitz 3 on the basis of some data then available in the literature. For the liquid phase, Sharma and Chang 2 used an associated solution model in order to account for the rapid change of the component activities in the vicinity of equal atomic composition. A total of 10 parameters was used by Sharma and Chang'- to describe the thermodynamics of the liquid phase. Most recently, Fernandez Guillermet, Hillert, Jansson, and Sundman 4 reevaluated the Fe-S system using a two-sublattice model for the liquid and the pyrrhotite phases. Fernandez Guillermet et al. 4 showed that the activity equations for the liquid phase used by Sharma and Chang 2 when extended to high sulfur concentrations do not give values in agreement with those of Ditman and Vecko. 5 In their optimization, Sharma and Chang 2 did not consider the data of Ditman and Vecko 5 while Fernandez Guillermet et al. 4 did. Moreover, Fernandez Guillermet et al. 4 used more parameters for the liquid phase than did Sharma and Chang. 2 The objective of the present study is to reevaluate the Fe-S system using the same models as Sharma and Chang-" for the liquid and pyrrhotite phases. However, in the present study, more experimental data are used in optimizing the solution parameters for the pyrrhotite and liquid phases while keeping the same number of parameters: some of the data used in the present study were not available then. Y - Y . CHUANG and K.-C. HSIEH, Project Associates, and Y AUSTIN CHANG, Professor and Chairman, Department of Metallurgical and Mineral Engineering, are with The University of Wisconsin-Madison, 1509 University Avenue, Madison, WI 53706 Manuscript submitted September 24, 1984. METALLURGICALTRANSACTIONS B
II.
EVALUATION OF THE THERMODYNAMIC DATA OF THE VARIOUS PHASES
A. Pure Sulfur and Iron
The Gibbs energy differences among the various forms of sulfur and iron are given in Table I. The equation for OG~ _ ~o~g . taken from Sharma and Chang 2 based on the t~s, Js JANAF data. 6 The equation for ~ - ~ is from A,gren v based on the data recommended by Orr and Chipman, s while those for GF~ are obtained by Chuang, Schmid, and Chang, 9 again based on the evaluated data of Orr and Chipman. 8 B. The Liquid Phase
The model used for the liquid phase is given by Sharma and Chang 2 except the equilibrium constant, K, for the assumed species 'Fe', 'S', and 'FeS' is expressed in terms of the formation of 'FeS" as given in Table II and in accordance with the notation of Schmid and Chang.t~ The symbols Yl, Y2, Y3 and f~, f2, f3 are used to
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