Effect of CaF₂ and CaO/Al₂O₃ on viscosity and structure of TiO₂-bearing slag for electroslag remelting

The relationship between the viscosity and structure of CaF₂–CaO–Al₂O₃–MgO–TiO₂ slag with different CaF₂ contents and CaO/Al₂O₃ ratios was studied using the rotating cylinder method, Fourier transform infrared spectroscopy, and Raman spectrometry. The activity coefficients of CaF₂ and the CaO/Al₂O₃ ratio were determined to understand the correlation between viscosity and structure of the slag. The results suggest that the slag viscosity reduces gradually with an increase in CaF₂ content from 14.1 to 28.1 wt.% or CaO/Al₂O₃ ratio from 0.9 to 1.5, and correspondingly apparent activation energy for viscous flow reductions. The addition of CaF₂ does not change the structure of the molten slag; however, the relaxation effect of the anionic species and the hindrance effect of the cationic species are promoted by substituting part of the non-bridging oxygens (NBO) with F⁻ ions from CaF₂, which is attributed to the formation of NBO–Ca²⁺–F⁻ and NBO–Ca²⁺–NBO, respectively. However, as the CaO/Al₂O₃ ratio increases, some of the Q⁴ units in the aluminate structure are depolymerized into Q² units, so that the relative strength of the Al–O–Al linkage decreases, and the relative fraction of Ti₂O₆⁴⁻ chains increases, whereas that of O–Ti–O chains decreases slightly, resulting in depolymerization on the slag structure. Additionally, the effect of the CaO/Al₂O₃ ratio on the structure was greater than that of CaF₂ because of the greater depolymerization effect. The variation in the activity can indirectly explain the relationship between the viscosity and structure of the aluminate structural units based on thermodynamic analysis.