Interaction between SiO₂ and a KF–KCl–K₂SiF₆ Melt

Abstract

The solubility mechanism of silica in a fluoride-chloride melt has been determined in situ using Raman spectroscopy. The spectroscopy data revealed that the silica solubility process involved Si-O bond breakage and Si-F bond formation. The process results in the formation of silicate complexes, fluorine-bearing silicate complexes, and silicon tetrafluoride in the melt. Mass spectrometry of the vapor phase over the KF-KCl-K2SiF6 and KF-KCl-K2SiF6-SiO2 melts and differential scanning calorimetry coupled with thermal gravimetric analysis of these melts were performed to verify the silica solubility mechanism.

DFT-based calculations of silicon complex structures in the KF-KCl-K2SiF6 and KF-KCl-K2SiF6-SiO2 melts

“…The results are presented in Table II. groups, which was reflected in the spectra at 988 and 1043 K. Moreover, the presence of K 3 SiF 7 was determined only in the solid sample by means of X-ray phase analysis. 9 The data proves that as the temperature increases, the bond length in the [SiF 6 ] 2complex rises, while the bond energy experiences vibrations. Checking the temperature stability did not lead to a change in bond angles.…”

“…In practice, in the presented systems, the different types of the electrode deposits and various types of complexes were found in an experimental investigation. 9 When studying the bond energies in the complexes, the principle expressed by Eq. (1) was applied.…”

“…The experimental study of the structure of complex compounds is difficult and a not always solvable task in attempts to explain the processes occurring in molten salts. [7][8][9] There are a number of approaches providing an opportunity to consider the structure of complex groups and to explore the interaction processes of the substances in molten salts. The study of the melts structure can be performed on quenched melt samples.…”

“…7 Moreover, at room temperature, Raman data of solid samples may not take into account the destruction of individual components during the melting of the sample, as well as instability of the particles at high temperature. The study of the structure of melts, 8,10 including alkali halides, 9 can be realized in situ at high temperatures. A Raman spectroscopy study of molten salts gives a sufficiently clear idea regarding the structure of complexes.…”

“…They are well studied by the direct method (in situ) of the Raman spectroscopy. The high-temperature Raman spectra of KF−KCl−K 2 SiF 6 and KF−KCl−K 2 SiF 6 −SiO 2 melts were obtained in literature, 9 where the process of the silicon oxide dissolution was considered. It was established that complexes of the [SiF 6 ] 2type are common for KF−KCl−K 2 SiF 6 melts.…”