Distribution of bismuth between copper and FeO x –CaO–SiO 2 slags under copper converting conditions

Abstract

Ferrous calcium silicate (FCS) slags (FeO_x–CaO–SiO₂) have been proposed for use in continuous copper converting, because they attack refractories to a much lesser extent than calcium ferrite slag. However, their ability to absorb unwanted minor elements, such as bismuth, from the molten copper is less well known. Bismuth in copper anodes causes problems in the electrorefinery, while in finished copper products it adversely affects their mechanical properties. In this work, the distribution ratio of bismuth between copper and copper saturated FCS slags at 1300°C and an oxygen partial pressure of 10⁻⁷ MPa (10⁻⁶ atm) was determined by three-phase equilibration. Copper entrainment in slag caused difficulties during data analysis. It was found that the slag/metal bismuth distribution ratio decreased as the CaO content of the slag increased but was much less affected by the Fe/SiO₂ (w/w) ratio. The inferred distribution ratios were projected onto the FeO–CaO–SiO₂ phase diagram, as was the calculated value of the limiting activity coefficient of BiO_1·5 in the slags. The strong similarity between the way in which both bismuth and copper distributions are affected by the slag composition, as well as theoretical considerations, show that BiO_1·5 behaves in slags as a neutral metal oxide. As a result, bismuth removal from blister copper is maximized with FCS slags of low CaO content, the Fe/SiO₂ (w/w) ratio being relatively unimportant.