Interaction of B 2 O 3 with molten KF-AlF 3 and KF-NaF-AlF 3

Abstract

Abstract The B 2 O 3 interaction with the KF-AlF 3 and KF-NaF(15.7–15.9 mol%)-AlF 3 melts, with cryolite ratio CR = ( n KF + n NaF )/ n AlF3 varied within 1.3–1.5, was studied by thermal analysis and Raman spectroscopy. The vibration bands at 790 and 773 cm − 1 obtained in the KF-AlF 3 -B 2 O 3 solidified sample were attributed to the fluorooxoborate and tetrafluoroborate ions, respectively. The BF 4 − was not identified in the KF-NaF-AlF 3 -B 2 O 3 spectrum. The B 2 O 3 reaction with the KF-AlF 3 melt was proposed to proceed, originally, with formation of the KBF 4 and K 2 Al 2 OF 6 . Then, when the melt reaches a saturation with alumina, the B 2 O 3 interacts with the KBF 4 producing the potassium fluorooxoborates K 2 B 2 OF 6 and K 3 B 3 O 3 F 6 . The ratio between the various boron ions 3BF 4 − + B 3 O 3 F 6 3 − = 3B 2 OF 6 2 − in the molten mixture depends on the B 2 O 3 concentration and temperature. In the NaF containing cryolite mixtures, defined by the liquidus temperature about 70° higher than in potassium system, the Na 2 B 4 O 7 compound is also formed. The Al 2 O 3 , being added to the KF-AlF 3 -B 2 O 3 melt, dissolves with formation of the K 2 B 2 OF 6 , K 3 B 3 O 3 F 6 and K 2 Al 2 OF 6 . However , the poorly soluble complex compound Na 2 Al 2 B 2 O 7 is formed in the KF-NaF-AlF 3 -B 2 O 3 -Al 2 O 3 system. The suggested mechanism of the B 2 O 3 interaction with the potassium-cryolite-based melts in the temperature range of 580–860 °C correlates to the liquidus temperature data obtained in the KF-AlF 3 and KF-NaF-AlF 3 systems with additions of B 2 O 3 and Al 2 O 3 .