Influence of CaF₂ on the Viscosity and Structure of Manganese Ferroalloys Smelting Slags

Addition of CaF₂ to the CaO-SiO₂-MnO (CaO/SiO₂ = 0.5) system, which corresponds qualitatively to a silicomanganese ferroalloy smelting slag, affected not only the critical (crystallization) temperature (T _CR) but also the viscosity at high temperatures, and its influence on slag properties was strongly dependent on the content of MnO in the slag. The viscosity of CaF₂-free 10 mass pct MnO slag was relatively high, i.e., about 10 dPa s at 1773 K (1500 °C), but decreased continuously upon addition of CaF₂ to the system. In contrast, the viscosity of the 40 pct MnO system was very low, i.e., 1 dPa s at 1773 K (1500 °C), and CaF₂ did not have a large effect. This indicates that Mn²⁺ is a strong network modifier in manganese ferroalloy smelting slags. Nevertheless, CaF₂ addition was very effective at decreasing the viscosity of low MnO slags at low temperatures. The activation energy for the viscous flow of silicate melts decreased linearly in response to CaF₂ addition, but this tendency was less pronounced in the more basic composition of the slag. The effect of CaF₂ on the viscosity and activation energy for viscous flow of melts was analyzed quantitatively using micro-Raman spectra of quenched glass samples and the silicate polymerization index, i.e., Q³/Q² ratio. The polymerization index decreased continuously with increasing CaF₂ content in less basic (10 pct MnO or C/S = 0.5) slags, whereas it was not affected by CaF₂ content in highly basic (40 pct MnO and C/S = 1.0) slags. Bulk thermophysical properties of the CaO-SiO₂-MnO-CaF₂ slags were quantitatively correlated with the structural information of the slags.