Raman spectroscopy and quantum theory calculations on complexes in the KF-AlF3-Al2O3 system

Abstract

The ionic structure of the KF-AlF3-Al2O3 system was studied using in situ high-temperature Raman spectroscopy and quantum mechanics calculations. The complexes in the KF-AlF3-Al2O3 system with different molar ratios of KF to AlF3 were AlF63−, AlF52−, AlF4−, Al2OF4, Al2OF84−, Al2OF62−, and Al2O2F42−. The structures of Al2OF4, Al2OF84−, Al2OF62−, and Al2O2F42− were optimized considering that the counterion K+, Al2OF4 exhibits D2d point group symmetry, K4Al2OF8 and K2Al2OF6 exhibit C2v point group symmetry, and K2Al2O2F4 exhibits C2h point group symmetry. The experimental main Raman bands of Al2OF84−, Al2OF62−, and Al2O2F42− were at 495 cm−1, 465 cm−1, and 410 cm−1, respectively, after comparing their theoretical Raman spectra. The forms and contents of the oxygen-containing complex ions in the system were related to the molar ratio of KF to AlF3 and to the concentration of alumina. AlF63−, AlF52−, AlF4−, and Al2OF4 were found in the molten KF-AlF3-Al2O3 system with a KF-to-AlF3 molar ratio of 1.22 at a temperature of 1003 K. In the KF-AlF3-Al2O3 system with a KF-to-AlF3 molar ratio of 2 at a temperature of 1273 K, when the alumina concentration was less than or equal to 10 wt%, it contained AlF63−, AlF52−, AlF4−, Al2OF4, and Al2OF84−, and when the alumina concentration reached 12 wt%, Al2OF62− and Al2O2F42− appeared. In the KF-AlF3-Al2O3 system with a KF-to-AlF3 molar ratio of 3 at a temperature of 1273 K, when the alumina concentration was less than or equal to 10 wt%, the system contained AlF63−, AlF52−, AlF4−, Al2OF4, and Al2OF84−. When the alumina concentration increased to 14 wt%, Al2OF62− and Al2O2F42− began to appear but AlF4− disappeared. As the concentration of alumina increased, the contents of Al2OF62− and Al2O2F42− increased. The ionic reactions that formed the various oxygen-containing complexes were obtained. The occurrence of these chemical reactions was related to the molar ratio of KF to AlF3 in the system and the alumina concentration. When the KF-AlF3-Al2O3 molten system with KF-to-AlF3 molar ratios of 1.22, 2 and 3 were used as the aluminum electrolytes, the ion discharged at the anode was Al2OF84−(except for the system with a KF-to-AlF3 molar ratio of 1.22 in which the Al2OF4 was discharged at the anode), and the ion discharged at the cathode was Al2OF4 according to wave function analyses.