Effect of CaF₂ Addition on the Silicothermic Reduction of MnO in Ferromanganese Slag

The effect of temperature and CaF₂ (fluorspar) addition on the silicothermic reduction behavior of MnO in the CaO-40 pct MnO-SiO₂ (C/S = 1.0) system, which is used to simulate high carbon ferromanganese (HCFeMn) slag, was investigated at 1773 K (1500 °C). The production of SiO₂ was stoichiometrically balanced with the consumption of MnO in the slag phase in the CaF₂-containing systems based on the reaction “[Si] + 2(MnO) = (SiO₂) + 2[Mn],” whereas the SiO₂ production was lower than the MnO consumption in the CaO-MnO-SiO₂ ternary system, which may have originated from the production of SiO gas in the latter. From the temperature dependence of the mass transfer coefficient of SiO₂, the activation energy of the silicothermic reduction process was determined to be about 217.9 kJ/mol, which was very close to the activation energy reported in the literature for mass transfer in the slag phase. The mass transfer coefficient of SiO₂ exhibited a maximum value at 5 mass pct CaF₂, which originated from an increase in the apparent viscosity of the slag due to the precipitation of solid cuspidine at CaF₂ content greater than 5 pct. Consequently, the addition of fluxing additive CaF₂ should be carefully determined, because an excess of CaF₂ results in the formation of cuspidine during silicothermic reactions.