Thermodynamics of Selenium and Tellurium in Calcium Ferrite Slags M.D. JOHNSTON, S. JAHANSHAHI, and F.J. LINCOLN New measurements have been made on the equilibrium distribution of selenium and tellurium between molten slag and metal phases under a controlled CO-CO 2 atmosphere. Distribution coefficients of Se and Te between magnesia-saturated calcium ferrite slag and copper or silver alloy were measured at 1473 to 1673 K and oxygen patrial pressures of 10 )11 to 10 )0.68 atm. For the calcium ferrite slag, the highest distribution coefficients were recorded at the highest lime content, under strongly reducing conditions and at higher temperature. The selenide and tel- luride capacities were also calculated for each slag and were found to increase with increasing temperature and increasing lime activity. Activity coefficients of Se in the slag phase were found to be much lower than those of Te, and values for both minor elements decreased considerably with increasing activities of lime and ferrous oxide in the slag. Selenium showed strong negative deviation from the ideal behavior, while Te showed positive deviation up to 1573 K. DOI: 10.1007/s11663-007-9048-z Ó The Minerals, Metals & Materials Society and ASM International 2007 I. INTRODUCTION THE presence of trace amounts of minor impurity elements in metal products can have a significant adverse effect on their electrical and mechanical prop- erties. 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 prod- uct, 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 iron silicate (fayalite, Fe 2 SiO 4 ) 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-FeO x ) slag in the copper converting step. Studies of the thermodynamics and physico-chemical properties of the CaO-FeO-Fe 2 O 3 system [1,2,3] have been used to establish the applicability of this type of slag to certain processes. The thermody- namics of calcium ferrite slag in regard to its direct application in the Mitsubishi process for copper con- verting 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 been determined in iron silicate [5] and sodium carbonate [6] slag-copper systems, and while some minor elements have been studied in calcium ferrite slag, gaps in the knowledge remain. The aim of this work is to characterize the behavior of Se and Te in terms of key operating variables and slag composition for representative cop- per production systems making use of calcium ferrite– based slag. Both the solubility and oxidation state of the minor elements in the slag are to be determined, with additional focus on the generation of thermody- namic activity data. It is hoped that the successful prediction of the occurrence of Se and Te in the calcium ferrite slag and metal phases can lead to the application of procedures for their removal from copper metal at the desired step, enabling sustainable, economical copper production from previously unus- able ore. M.D. JOHNSTON, former Postgraduate Student, University of Western Australia, Crawley, Western Australia 6009, Australia and the Centre for Sustainable Resource Processing, CSIRO Minerals, Clay- ton, 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: sharif.jahanshahi@csiro.au 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 VOLUME 38B, JUNE 2007—433