Structural behavior of F− in mould flux melt of CaO-SiO2-Al2 O3-Na2 O-CaF2 system

The influence of fluorine on the structure of CaO-SiO₂-Al₂O₃-Na₂O-CaF₂ continuous-casting-type slag was measured by Raman spectroscopy, and the degree of polymerization of mould flux and the structural behavior of F⁻ in the melt were investigated by classifying and quantifying the structural species of F⁻ ions. The results exhibit that the main structural units of Si-O tetrahedra are Q⁰, Q¹ and Q² and the actual measured number of non-bridging oxygen ions in the [SiO₄]-tetrahedra (denoted by NBO/T) increases from 2. 73 to 3. 44 with increasing the molar ratio of F to (F+O) (denoted by X_F/X_(F+O)) from 0. 06 to 0. 19. It means that the degree of polymerization of melt structure decreases with an increase in X_F/X_(F+O). In addition, most of F⁻ ions were distributed in Si-O tetrahedra and Al-O tetrahedra. With increasing X_F/X_(F+O), the complex structural units Al-O tetrahedra are gradually replaced by discrete structural units AlF^1– because of the breakage of Al-O bonds in Al-O tetrahedra by F⁻ ions, and the Si-O (bridging oxygen) bonds of Si-O tetrahedra are broken to form [SiO_nF_1–n]-tetrahedra by F⁻ ions coordinating with Si⁴⁺.