High-temperature phase relations and thermodynamics in the iron-lead-sulfur system
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INTRODUCTION
AT high temperatures when liquid phases are involved, current knowledge of the thermodynamic properties and phase equilibria of many binary and ternary metal-sulfur systems is rather limited. Such information is essential for the understanding, designing, and possible improvement of the extraction and refining processes, particularly those of nonferrous metals. Solidstate regions of metal-sulfur systems have received more attention in the past due to promising electronic and optical properties of some sulfide compounds. Nevertheless, accurate knowledge on liquidus boundaries and liquid solubility of S in such regions are also essential in order to produce, develop, and improve the properties of semiconducting sulfides, especially during their refining to very high purities. The most recent trend in Pb production is direct smelting of PbS in a single step, which will involve a matte phase containing FeS along with PbS. Thus, to a first approximation, FeS-PbS melts resemble commercial Pb matte. Hence, a knowledge concerning the thermodynamics and phase equilibria of the Fe-Pb-S system is of considerable theoretical and practical significance. The FeS-PbS pseudobinary system, which has been investigated several times, l~-SI is a simple eutectic system. The eutectic temperature has been reported in the range 835 ~ t3j and 848 ~ The eutectic composition is in the vicinity of 48 mol pct PbS, and no terminal solid solutions are reported. Nesterov et al.t6] measured the vapor pressure of PbS over FeS-PbS melts in the range 1000 ~ to 1250 ~ The PbS activities were observed to behave almost ideally with very small negative departures from ideality. The data, however, covered only a limited composition range, Xpbs < 0.35, and hence, were insufficient to depict the activitycomposition relationships in FeS-PbS melts. Eric and R. H U R M A N ERIC, Professor and Head, and H A K A N O Z O K , Doctoral Postgraduate Student, are with the Department of Metallurgy and M a t e r i a l s E n g i n e e r i n g , U n i v e r s i t y o f the W i t w a t e r s r a n d , Johannesburg, Private Bag 3, WITS 2050, South Africa. Manuscript submitted March 24, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
Timucin 171 derived activities of PbS in FeS-PbS liquid solutions at 1200 ~ using the dew-point technique and concluded that the system behaved ideally. However, they observed an anomaly in PbS activities calculated from the PbS branch of the liquidus curve and suggested a further investigation of the system. 171 In the Fe-Pb-S ternary system, the binary Fe-FeS is well established, ts-~ll This system is characterized by a congruent melting phase, pyrrhotite (FeS), at approximately 50 at. pct S and a eutectic between Fe and pyrrhotite. The liquidus in equilibrium with Fe at approximately 20 at. pct S is rather fiat, suggesting a tendency toward immiscibility. Pyrrhotite has an ordered NiAs structure and exists over a range of homogeneity, depending upon S pressure and temperature. The general features of the phase relationshi
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