Structural behavior of F− in mould flux melt of CaO-SiO2-Al2O3-Na2O-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 XF/X(F+O)) from 0.06 to 0.19. It means that the degree of polymerization of melt structure decreases with an increase in XF/X(F+O). In addition, most of F− ions were distributed in Si-O tetrahedra and Al-O tetrahedra. With increasing XF/X(F+O), the complex structural units Al-O tetrahedra are gradually replaced by discrete structural units AlF4− 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 [SiOnF4−n]-tetrahedra by F− ions coordinating with Si4+.