Physical-Chemical Properties of Potassium Cryolite-Based Melts Containing KBF4

Recent developments in the field of low-temperature aluminum electrolysis utilizing oxygen-evolving inert anodes prove the efficiency of potassium cryolite-based electrolytes. Such electrolytes give an opportunity to perform electrolysis at temperature below 800 °C. The potassium cryolite-based electrolytes can be used as prospective media for other electrochemical and chemical technological processes including aluminum alloys production. However, the introduction to the electrolytic bath the metal-containing additives along with alumina can result in electrolysis violation. It is a consequence of significant change in electrolyte properties with varied composition. In present work the KBF₄impact on the physical-chemical properties of the potassium cryolite-based melts was studied.

Liquidus temperature, electrical conductivity, density, alumina solubility in the KF-AlF₃ and KF-NaF(15.9 mol%)-AlF₃ molten mixtures containing KBF₄ were determined. Cryolite ratio (CR), the ratio between molar concentrations of the alkali metal fluoride and aluminum fluoride (CR=N_MF/N_AlF3), was varied from 1.3 to 1.5. It should be mentioned that the cryolite ratio for the mixed KF-NaF-AlF₃ cryolite melts was calculated as CR=(N_KF+N_NaF)/N_AlF3. Elemental composition of the molten mixtures being studied was controlled by ICP analysis and X-ray fluorescence spectroscopy on XRF-1800 (Shimadzu).

The liquidus temperature of the cryolite melts was measured by the specially developed thermal analysis technique. The data obtained in the KF-AlF₃-KBF₄ and KF-NaF(15.9 mol%)-AlF₃-KBF₄ systems showed that the liquidus temperature rises with the KBF₄ content. It should be noted that the NaF, LiF, and CaF₂, potential additives to the electrolyte for low-temperature electrolysis, being added to the KF-AlF₃ melts with low CR, impact the liquidus temperature the same manner as KBF₄.

The alumina additions to the potassium cryolite-based melts decrease the liquidus temperature as it is shown in Fig.1. The quasi-binary “cryolite mixture”– alumina phase diagrams are simple eutectics. The KBF₄ presence in the KF-NaF-AlF₃-Al₂O₃ electrolytes shifts the eutectic point to the right and increases the alumina solubility in these melts. The eutectic points were found equal to 759 and 761°C at the Al₂O₃ content 4 and 5 mol%, respectively,  in the [KF-NaF(15.9mol%)-AlF₃+KBF₄ (3mol%)]-Al₂O₃ and [KF-NaF(15.9 mol%)-AlF₃+KBF₄(5 mol%)]-Al₂O₃ mixtures with CR=1.3. The Al₂O₃addition of 6 mol% to the both electrolytes results in significant rise of the liquidus. The bend point on the cooling curves corresponding to the temperature of primary crystallization was not detected because the beginning temperature of the cooling process in these tests was below the liquidus. The presence of not dissolved deposit at the bottom of crucible at 850 °C was also confirmed by visual observation.

The original technique to study the electrical conductivity of aggressive fluoride and oxide-fluoride melts was developed. It consists in combination of two types cells: capillary (BN) and with parallel electrodes (Mo).  The resistivity of electrolyte was measured by means of impedancemeter (Zahner electrik IM6E) in a frequency range from 100 Hz to 1 MHz using a signal with amplitude 5 mV/s. The electrical conductivity of the potassium and potassium-sodium cryolites was measured in the capillary-type cell whereas the electrical conductivity of electrolytes with KBF₄ and Al₂O₃additions was studied in the cell with parallel electrodes. The temperature dependence of a cell constant was taken into account that made the electrical conductivity measurements more accurate in a wide temperature range.

The density of cryolite melts was measured using the Archimedean method. A platinum sphere suspended with platinum wire and attached to an electronic balance unit (Mettler AT20) was immersed into the electrolyte located in the glassy carbon crucible. The measurements were performed in the temperature range 700-800 °C. The temperature dependence of the Pt sphere volume was considered and determined in the calibration test with FLiNaK (eutectic).  The density of the KF-AlF₃-KBF₄ melts decreases with increasing the KBF₄ content. So, the addition of the KBF₄ 3 mol% reduces the density of the molten mixture on about 5%.