Chromous capacities of ferrochromium and matte smelting slags

Abstract

Purpose: of this paper is to extend the slag capacity concept to sparingly soluble cationic species encountered for example in matte smelting and ferrochromium processing in terms of chromous capacities. Design/methodology/approach: In this context in order to develop the chromous capacity the experimental data on slag-metal equilibrium for ferrochromium smelting and on chromium distribution between slag and matte phases encountered in sulphide smelting of PGM (Platinum Group Metals) and chromium containing copper nickel concentrates were used. Findings: The calculated chromous capacities were in the 10-8 range for ferrochrome smelting slags and 10-4 range for matte smelting slags. These values indicate the very low and moderate chromous ion dissolving ability of ferrochromium smelting slags and matte smelting slags respectively. Research limitations/implications: Due to highly reducing conditions in ferrochromium smelting and the imposed low oxygen partial pressures in sulphide smelting of chromium and PGM containing concentrates chromium in the slag was assumed to be in its divalent state. The slag-metal/matte reaction for dissolution of chromium into the slag as a cation under reducing conditions -in contrast to reactions for dissolution of species such as sulphur and phosphorus into the slag as anions under oxidizing conditions – required the release of electrons. Thus to maintain charge neutrality the dissolution of chromium from the metal or the matte phases into the slag as Cr2+ ions was accompanied by consumption of these electrons by reduction of Fe2+ ions in the slag into metallic Fe which dissolved in the metal/ matte phase. This was supported by the experimental equilibrium data. Hence the slagmetal/ matte reaction defining the chromous capacity involved both O2- and Fe2+ ions of the slag phase. Moreover for matte smelting the thermodynamic activities of species in the matte phase were assumed to conform to ideal associated solution model due to lack of data. For future work it is advisable to determine such activities. Practical implications: The calculated chromous capacities were in the 10-8 range for ferrochrome smelting slags and 10-4 range for matte smelting slags. The results clearly indicate that in matte smelting lower basicity slags would be preferable to render chromium more soluble in the slag in contrast to ferrochromium smelting where higher basicity slags would be suitable to minimize chromium losses to the slag. Originality/value: This research and hence the paper is probably the first on dissolution of chromium as chromous ion in slags, thus it is novel and original.