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THERE is growing acceptance in the use of calcium ferrite-based slags in the processing of copper concentrates and in the continuous production of copper metal. Commercial processes such as the Mitsubishi process,[1–4] the Kennecott flash converting process,[5–7] as well as other proposed continuous and single-stage converting processes[8–10] advocate the use of calcium ferrite slags. The advantages of calcium ferrite slags over the more conventional iron silicate slags include  A lower dissolved copper content at a given activity of copper in slag,[11,12]  A higher capacity to dissolve ferric iron and, hence, magnetite, and[13]  A higher capacity for some acidic minor elements such as arsenic and antimony.[14,15]

Most of the thermodynamic investigations into calcium ferrite slags have concentrated on liquidus boundaries at copper-making temperatures,[10,16–20] iron redox equilibria,[16–18,21–24] and the solubility of copper in slag.[11,15,19,25,26] Nikolic et al.[27] and Hidayat et al.[28] investigated the thermodynamics of complex slag systems relevant to modern copper smelting and converting

MICHAEL SOMERVILLE, Research Team Leader, and SHOUYI SUN and SHARIF JAHANSHAHI, Senior Principal Research Scientists, are with the CSIRO - Minerals Down Under National Research Flagship, Box 312 Clayton South, VIC 3168, Australia. Contact e-mail: [email protected] Manuscript submitted July 7, 2014. Article published online August 13, 2014. 2072—VOLUME 45B, DECEMBER 2014

processes as part of a thermodynamic modeling study. Jak et al.[5] also investigated calcium ferrite-based slag systems in a study on the control of slag freeze linings which are used in new copper converting processes. In most studies, copper metal is assumed to dissolve into the slag as an univalent species (CuO0.5). The dissolved copper has a linear dependence on the oxygen partial pressure in logarithm, especially at low oxygen partial pressures of about 0.22.[11] At higher oxygen partial pressures, the linear dependence has been observed to break down, and the copper content of slag has been shown to increase with a higher dependency. There is some uncertainty where the change in dependence on oxygen partial pressure occurs. In the work of Eerola et al.,[14] the change occurred at an oxygen partial pressure of about 107 atm, while for Yazawa and Takeda[11] and Takeda et al.,[15] the change occurred at about 105 atm. Eerola et al.[14] attributed this increased dependence to a decrease in the activity coefficient of CuO0.5 (cCuO0.5). The formation of a bivalent copper oxide species (CuO), which would also account for the increase in dependency, was also considered by Eerola et al.[14] and Takeda et al.[15] However, no quantitative understanding was derived from those studies. In this work, the dissolution of copper into calcium ferrite slags was investigated with particular reference to the copper species and the Fe3+/Fe2+ ratio of the slag at high oxygen partial pressures. The bivalent copper oxide species (CuO) was assumed to exist