High-temperature phase relations and thermodynamics in the silver-tin-sulfur system
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I. 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. Solid-state regions of metal-sulfur systems have received more attention in the past, due to the promising electronic and optical properties of some sulfide components. Nevertheless, accurate knowledge of the liquidus boundaries and liquid solubility of S in such regions is also essential in order to produce, develop, and improve the properties of semiconducting sulfides, especially during their refining to very high purities. Hence, a knowledge concerning the thermodynamics and phase equilibria of the Ag-Sn-S systems containing the Ag2S-SnS pseudobinary melts is of considerable theoretical and practical significance. Furthermore, separation of tin (and silver) from various scrap dental, brazing, solder, and electrical-contact alloys may be possible by forming a liquid matte from which SnS may be fumed off. On the Ag2S-SnS pseudobinary system, there are only two investigations in literature,[1,2] which are not in good agreement. Nekrasov et al.[1] reports a eutectic temperature of 519 8C at 47.5 mol pct SnS, whereas Moh and Klein[2] show five invariant reactions between 500 8C and 600 8C, derived from their Ag-Sn-S ternary. It is expected that Nekrasov’s pseudabinary Ag2S-SnS phase diagram is more representative, which also reports on solid-solution ranges. It appears that below 170 8C (eutectoid temperature, with composition at approximately 5 mol pct SnS), g-Ag2S (acanthite) and b-SnS (gerzenbergite) are stable. At 178 8C, acanthite ¨ R. HURMAN ERIC¸, formerly Visiting Professor, Faculty of Applied Earth Sciences, Delft University of Technology, 2628 RX Delft, The Netherlands, is Professor and Head, School of Process and Materials Engineering, University of Witwatersand, Johannesburg, Private Bag 3, WITS 2050, South Africa. GOKHAN AKBAY, formerly Postgraduate Student, School of Process and Materials Engineering, University of Witwatersand, is Lieutenant, Turkish Army, Ankara, Turkey. Manuscript submitted April 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
transforms into b-Ag2S, which has a fairly wide solid-solution range, and, up to a eutectic temperature of 519 8C, bAg2S and b-SnS remain stable. Nekrasov’s diagram indicates a maximum solid solubility of about 25 mol pct SnS in bAg2S. The b-Ag2S phase transforms to a-Ag2S at 580 8C, which, according to Nekrasov, has no solid-solubility region. In order to handle the b-Ag2S to a-Ag2S transformation, Nekrasov has inserted a binary peritectic reaction into the system at 16 mol pct SnS and 602 8C. Here, the liquid solution reacts with a-Ag2S to form a b-Ag2S solid solution (on cooling direction). Unless firmly established by experimental work, the peritectic reaction reported by Nekras
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