The metal saturation line and tie-lines in the nickel-cobalt-sulfur ternary system between 1273 and 1573 K
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I.
INTRODUCTION
Some nickel ores contain cobalt, which is recovered as a by-product 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 thermodynamics of the Ni-Co-S system is required. As a first step toward the understanding of the thermodynamics of this system, the phase diagram, the metal saturation line, and the position of the tie-lines at different temperatures must be known. There are very few experimental results available for the Ni-Co-S ternary system. An isotherm of the Ni-Co-S phase diagram at 873 K has been determined by the application of differential thermal analysis (DTA) and X-ray diffraction.[1] The only experimental data available at high temperature in the literature are those reported by Jacob[2] at 1273 K. The metal saturation line and tie-lines of the NiCo-S ternary system have been determined by Jacob. Kongoli and Pelton[3] have applied a quasi-chemical model to predict the Ni-Co-S ternary phase diagram at 1273 K. Activities of nickel and cobalt in the Ni-Co alloys have been determined by a number of researchers: Maruyama and Ban-Ya[4] used a transportation technique at 1873 K; Tomiska et al.[5] used a Knudsen cell mass spectrometry technique between 1480 and 1875 K; Vrestal and Kucera[6] used an effusion method at 1500 K; and Oishi et al.[7] used a gas equilibration technique at 1273 K. The heat of mixing of nickel and cobalt has also been measured by Predel and Mohs[8] and Iguchi et al.[9] using a calorimetric technique at 1780 and 1813 K, respectively. The thermodynamic properties of Ni-Co were assessed by Fernandez-Guillermet.[10] II.
EXPERIMENTAL
A. Sample Preparation The sulfides were prepared from electrolytic grade cobalt and nickel together with sulfur. The impurities in the elec-
MANSOUR SOLTANIEH, Postdoctoral Fellow, J.M. TOGURI, INCO/NSERC Professor, and R. SRIDHAR, Adjunct Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON Canada M5S 3E4. Manuscript submitted February 9, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
trolytic cobalt were (mass pct) 0.0008C, 0.020Ni, and 0.0008P, and the electrolytic nickel contained (mass pct) 0.0002Co, 0.0003Fe, 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. Thirty-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 backfilled with argon gas. This procedure was repeated ten times. At the end of the cycle, the quartz tube containing the starting materials was sealed under vacuum. The sample was then heated very slowly between 473 to 723 K, over a period of six days, to effect sulfidization. After this treatment, the temperature of the sample was raised to 1073 K for
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