The activity coefficient of cobalt oxide in silica-saturated iron silicate slags
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INTRODUCTION
THE behavior of cobalt is of economic importance during nickel and copper smelting because cobalt enters the smelting circuit and represents a potentially valuable by-product. [IJ However, the recovery of cobalt can be as low as 45 pet and rarely exceeds 60 pet. The recovery depends on the distribution of cobalt between matte and discard slag in the smelting stage, and an important factor affecting this distribution is the activity coefficient of cobalt oxide in slag. The cobalt distribution can be used also as a practical indicator of the extent of oxidation within a smelting furnace.[2l The distribution of cobalt between slag and alloy has been previously studied, t3-1~ Smith and Masson 13j equilibrated cobalt silicate melts with Pt-Rh-Co alloys at controlled oxygen pressures and reported an average cobalt oxide activity coefficient of 1.2 relative to the solid, for melts containing around 0.6 mole fraction of CoO at both 1723 and 1773 K. Wang et al. 14,51 equilibrated silica-saturated iron silicate slags with liquid Co-Au and Co-Cu alloys, respectively, and reported the cobalt content of contiguous alloys and slags for temperatures of 1523, 1573, and 1623 K and oxygen pressures of between 10 -6 and 10 -~~ arm. Activity coefficients for cobalt oxide were calculated from these data by Grimsey and Toguri 161and summarized by the following relationship: 3'CoO = 1.94 + 0.123 (wt pct Co) [1] where 3'coo is the activity coefficient of cobalt oxide relative to the pure solid and wt pet Co is the weight percent of cobalt in slag. ERIC. J. GRIMSEY, formerly Senior Lecturer in Pyrometallurgy, Murdoch University, Murdoch, Western Australia, is Professor and Head, Department of Minerals Engineering and Extractive Metallurgy, WA School of Mines, Egan St. Kalgoorlie, Western Australia, 6430. XULIANG LIU, formerly Postgraduate Student, Murdoch University, Murdoch, Western Australia, is Research Fellow, G.K. Williams CRC for Extractive Metallurgy, Department of Chemical Engineering, University of Melbourne, Victoria, 3052, Australia. Manuscript submitted February 21, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
Reddy t71 equilibrated alumina-saturated iron silicate slags (Fe/SiO2 of 1.34) with liquid Co-Cu alloys at temperatures of 1473 to 1573 K and reported the infinite dilution activity coefficient of cobalt oxide (relative to the pure liquid) as In Y~oo = 19.63 - 1.298 • 10-2T
[2]
This equation implies a relatively large heat of solution for cobalt oxide since when the data predicted by this relationship are refitted to the usual temperature-dependent form over the range 1473 to 1623 K, the resulting expression is In 3'~ = 3.006 • 104/T - 19.89. This gives a heat of solution for cobalt oxide in slag of 249 kJ/mol, which is 7.5 times higher than that measured for nickel oxide in iron silicate slags, ll~l Reddy and Healy ISl equilibrated liquid Cu-Co alloys with iron-free cobalt silicate slag at 1523 K while Fontana et al.[9] reported the solubilities of cobalt in iron silicate slags containing lime that w
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