The effect of CaF2 on the viscosities and structures of CaO-SiO2(-MgO)-CaF2 slags

The viscosities of CaO-SiO2(-MgO)-CaF2 slags were measured to clarify the effect of CaF2 on the viscous flow of molten slags at high temperatures and the solidification behavior of slags. Furthermore, the infrared (IR) spectra of the quenched slags were analyzed to understand the structural role of CaF2 in the modification of slag structure. The CaF2 affects the critical temperature (TCR) of the slags; that is, the higher the content of CaF2, the lower the TCR of the slags. It is suggested that some extent of undercooling as a driving force is needed for the precipitation of solid particles in the melt. In the composition of B (≡(mass pct CaO)/(mass pct SiO2)) = 1.0, the TCR was decreased about 150 to 200 K by addition of 10 mass pct MgO, while the TCR was increased about 100 K by MgO addition at B = 1.3. The effect of CaF2 on the viscous flow of molten slags can be understood based on a decrease in the degree of polymerization by F− as well as by O2− ions and this was confirmed by the IR spectra of the quenched slags. The relative intensity of the IR bands for [SiO4]-tetrahedra with low NBO/Si decreased, while that of the IR bands for [SiO4]-tetrahedra with high NBO/Si increased with increasing CaF2 content. The decrease in viscosity of the CaO-SiO2-MgO-CaF2 (B = 1.0) system by CaF2 addition was negligible, while the effect of CaF2 on the viscosity was significant in the more basic system (B = 1.3).