Thermodynamics of Selenium and Tellurium in Calcium Ferrite Slags
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INTRODUCTION
THE presence of trace amounts of minor impurity elements in metal products can have a significant adverse effect on their electrical and mechanical properties. Selenium (Se) and tellurium (Te) are common minor components of sulfide ore, and as such must be managed during the extraction of metals such as copper and nickel. Selenium, for example, has a high affinity for copper, and even at concentrations < 1 ppm can affect the softening properties of Cu metal. Although Se and Te have value in that they have uses in electronic components and as a colorant in glass, they are not in great demand. While it is preferable to remove certain impurities at the final refining step, these and other minor elements will become increasingly problematic as high grade ore reserves are depleted and low grade deposits containing larger amounts of impurities are relied upon. Knowledge of the thermodynamic behavior of these elements is therefore necessary to successfully eliminate them from the final metal product, and to assess whether waste materials could potentially pose an environmental hazard if not handled appropriately. The pyrometallurgical extraction of copper from sulfide ore has traditionally been carried out with an M.D. JOHNSTON, former Postgraduate Student, University of Western Australia, Crawley, Western Australia 6009, Australia and the Centre for Sustainable Resource Processing, CSIRO Minerals, Clayton, Victoria, 3168, Australia. S. JAHANSHAHI, Research Program Leader, Centre for Sustainable Resource Processing, and Research Program Manager–High Temperature Processing, is with CSIRO Minerals Contact e-mail: [email protected] F.J. LINCOLN, Honorary Research Fellow, Chemistry, is with the School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Crawley, Western Australia, 6009, Australia. Manuscript submitted: January 4, 2007. Article published online June 8, 2007.
METALLURGICAL AND MATERIALS TRANSACTIONS B
iron silicate (fayalite, Fe2SiO4) based slag. However, this slag type has inherent deficiencies in that it has high viscosity, high solubility of copper, and low solubility of ferric iron oxides. A recent alternative has been the use of calcium ferrite (CaO-FeOx) slag in the copper converting step. Studies of the thermodynamics and physico-chemical properties of the CaO-FeO-Fe2O3 system[1,2,3] have been used to establish the applicability of this type of slag to certain processes. The thermodynamics of calcium ferrite slag in regard to its direct application in the Mitsubishi process for copper converting is discussed by Tanaka et al.[4] This type of slag allows for easier operation over fayalite in terms of improvement in the factors mentioned previously. It is also expected to have different holding capacity for impurity elements such as Se and Te due to its basic, rather than acidic, nature. If this is the case, there arises the possibility that these elements will exit the process stream early and become dispersed with the waste slag. The behavior of Se and Te has
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