Kinetics of electroreduction of fluorozirconates in fluoride melts

At present, the demand for aluminum alloys, including those with zirconium additives, is growing significantly. One of the methods for producing such alloys is the reduction of alkali and alkaline earth metal fluorozirconates in molten salts; this method is characterized by a high degree of extraction and process intensity. According to scientific and technical literature, the use of electrolysis can contribute to increasing the efficiency of such processes, in connection with which, it is relevant to study the electrochemical behavior of fluorozirconates in molten media. Using the cyclic chronovoltammetry method, some patterns of electroreduction of zirconium and aluminum from a low-melting melt of KF–AlF₃–Al₂O₃–ZrO₂ at a temperature of 750°C were studied, depending on the composition of the additive and the substrate material. A series of polarization curves were obtained, both in a pure melt and with additives of potassium fluorozirconate, at potential sweep rates from 0.01 to 2 V. It was shown that cathode currents of aluminum electrolysis appear at potentials of –1.6…–1.7 V, relative to the potential of the CO/CO₂ electrode, and a further shift of the potential to the cathode region leads to the joint release of aluminum and potassium. When K₂ZrF₆ is introduced into the KF-AlF₃-Al₂O₃ melt, a discharge area of zirconium ions appears on the cathode branch of the voltammograms at potentials of –1.4 and –1.6 V. When comparing the voltammograms obtained with the addition of zirconium oxide and potassium fluorozirconate under otherwise equal conditions, it was found that when the oxide is added, two inflections are observed on the cathode branch of the voltammograms at potentials of –1.4 and –1.7 V, while only one inflection is observed at a potential of –1.3 V upon reduction of fluorozirconate. The presence of two responses when introducing the oxide can be caused by both the electrolysis of elemental zirconium in two electrochemical stages and the discharge of zirconium-containing ions of different composition. Analysis of the diffraction patterns of the melts shows that, regardless of the composition of the additive, both potassium fluorozirconate and zirconium oxide can be present in the melt. Based on the data obtained, it can be concluded that zirconium can either discharge in several stages or form a number of intermediate compounds when interacting with the components of the melt.