Interaction of B2O3 with molten KF-AlF3 and KF-NaF-AlF3

The B2O3 interaction with the KF-AlF3 and KF-NaF(15.7-15.9 mol%)-AlF3 melts, with cryolite ratio CR = (nKF + nNaF)/nAlF3 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-AlF3-B2O3 solidified sample were attributed to the fluorooxoborate and tetrafluoroborate ions, respectively. The BF4− was not identified in the KF-NaF-AlF3-B2O3 spectrum. The B2O3 reaction with the KF-AlF3 melt was proposed to proceed, originally, with formation of the KBF4 and K2Al2OF6. Then, when the melt reaches a saturation with alumina, the B2O3 interacts with the KBF4 producing the potassium fluorooxoborates K2B2OF6 and K3B3O3F6. The ratio between the various boron ions 3BF4− + B3O3F63− = 3B2OF62 − in the molten mixture depends on the B2O3 concentration and temperature. In the NaF containing cryolite mixtures, defined by the liquidus temperature about 70° higher than in potassium system, the Na2B4O7 compound is also formed. The Al2O3, being added to the KF-AlF3-B2O3 melt, dissolves with formation of the K2B2OF6, K3B3O3F6 and K2Al2OF6. However, the poorly soluble complex compound Na2Al2B2O7 is formed in the KF-NaF-AlF3-B2O3-Al2O3 system. The suggested mechanism of the B2O3 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-AlF3 and KF-NaF-AlF3 systems with additions of B2O3 and Al2O3.