The thermodynamics of the Ni-Co-S ternary system
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INTRODUCTION NICKEL ores generally contain cobalt, which is recovered as a byproduct of nickel extraction. During the pyrometallurgical production of nickel from sulfide concentrates containing small amounts of cobalt, approximately half of the cobalt is lost to the slag phase. To minimize this loss, knowledge of the thermodynamic properties of the Ni-Co-S system is required. Information available in the literature regarding the thermodynamics of cobalt at high temperatures in systems of metallurgical interest is limited. There are a number of studies in which the distribution of cobalt has been determined between an alloy and slag phase[1,2,3] and between a matte and slag.[4–7]. The activities of cobalt in cobalt-gold alloys in equilibrium with cobalt oxide have also been reported.[8,9] There are some experimental measurements on the activity coefficients of sulfur in dilute solutions of sulfur in metallic alloys. Alcock and Cheng[10] measured the activity coefficients of a very dilute solution of sulfur in the Ni-Co and Fe-Co binaries using H2S/H2 gas mixtures at a temperature of 1540 8C. Baren and Gokcen[11] performed similar studies to those of Alcock and Cheng,[10] but over a temperature range from 1470 8C to 1650 8C. They also determined the activity coefficients of sulfur in a dilute solution of sulfur in Fe-Ni-Co alloys at 1552 8C.[12] There are no data available for either sulfur or metal activities in the Ni-Co-S system in the temperature and composition range of interest to matte smelting. Therefore, in this article, the activities of sulfur in the Ni-Co-S ternary system were experimentally determined over the temperature range from 1373 to 1573 K. A H2S/H2 gas mixture was used to fix the partial pressure of sulfur over the matte sample. Then, the activities of nickel and cobalt were calculated by application of the Gibbs–Duhem equation. II. EXPERIMENTAL A. Sample Preparation The sulfides were prepared from electrolytic-grade cobalt and nickel together with sulfur. The impurities present in M. SOLTANIEH, Postdoctoral Fellow, J.M. TOGURI, Professor, and R. SRIDHAR, Adjunct Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 3E4. T. TAKASU, Assistant Professor, is with the Department of Materials Science and Engineering, Kyushu Institute of Technology, Kitakyushu 8048550, Japan. Manuscript submitted May 21, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
the electrolytic cobalt were (in mass pct) 0.0008 C, 0.020 Ni, and 0.0008 P, while the electrolytic nickel contained (in mass pct) 0.0002 Co, 0.0003 Fe, and 0.0015 C. Prior to use, any residual oxide on the surface of the nickel and cobalt was removed by hydrogen treatment at 873 K for 2 hours. Two-gram samples of the required matte composition were prepared by mixing the appropriate amount of nickel, cobalt, and sulfur in a quartz tube. The quartz tube was evacuated by a mechanical pump and back-filled with argon gas. This procedure was repeated ten times. At the end of the cycle, the qua
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