The main features of scandium and zirconium extraction from their oxides to aluminum during the aluminothermic and electrolytic preparation of Al–Sc and Al–Zr alloys and master alloys in the KF–AlF3,...

A review of the existing methods for producing aluminum master alloys with silicon, zirconium, scandium and boron are given. Basic parameters, advantages and disadvantages of the existing methods are analyzed, indicating the need to develop new more energy-efficient technologies. The prospects of obtaining aluminum master alloys in the electrolysis of melts based on the KF-NaF-AlF ₃ -Al ₂ O ₃ -MeO (MeO = SiO _2, ZrO ₂ , Sc ₂ O ₃ , and B ₂ O ₃ ) system are considered. For this purpose, the results of physical and chemical measurements in these melts are presented, including data on the solubility of Al ₂ O ₃ , SiO _2, ZrO ₂ , Sc ₂ O ₃ , and B ₂ O ₃ oxides in KF-NaF-AlF ₃ melts, data on the effect of oxide additives on the liquidus temperature of studied melts, as well as data on the kinetics of electrowinning of aluminum and alloying element from KF-AlF ₃ -Al ₂ O ₃ -MeO melts. Based on the measurements, the parameters were selected and electrolysis tests were carried out for obtaining aluminum master alloys with silicon, zirconium, scandium, and boron from its oxides. The composition and structure of the obtained master alloys were studied.

Effects of the KF-NaF-AlF₃ melt composition, ZrO₂ concentration and aluminothermic synthesis parameters on the conversion rate of zirconium to aluminum were investigated. The kinetics of zirconium electroreduction on glassy-carbon obtained from KF-AlF₃-ZrO₂ melts at a temperature of 750°C was studied via the cyclic voltammetry method. Parameters of the electrolysis of KF-NaF-AlF₃-ZrO₂ melts were derived on the basis of obtained results, and the theoretical possibility for the electrolytic production of Al-Zr master alloys (having up to 15 wt% zirconium content) was demonstrated.

Kinetics of the cathode process in the KF-AlF₃ and KF-AlF₃-Al₂O₃ melts has been studied by means of the steady-state polarization and voltammetry techniques. The kinetic parameters of the aluminum electrodeposition in dependence on such factors as cathode material, process temperature, alumina concentration in the melt, etc. were determined. Based on the experiment data analysis, a mechanism of the cathode process in the KF-AlF₃-Al₂O₃ melts was proposed. The operating parameters were defined and electrolysis was carried out.

The kinetics of the cathode process on tungsten in KF-AlF₃-Al₂O₃ melts and suspensions was studied by means of voltammetry and galvanostatic polarization. The effects of the temperature, [KF]/[AlF₃] ratio, Al₂O₃ content on the kinetic parameters of electrowinning of aluminum from the KF-AlF₃-Al₂O₃ melts and suspensions were determined. Introducing Al₂O₃ slurry into the suspension and increasing its proportion make the diffusion of the electroactive ions to the cathode more difficult. The parameters for the suspension electrolysis were chosen on the basis of electrochemical measurements. The possibility of aluminum production during the electrolysis of KF-AlF₃-Al₂O₃ suspension with 45 wt% Al₂O₃ in the solid phase was demonstrated.

The main kinetic features of the aluminum and scandium electrowinning during the electrolysis of the KF-AlF₃-Al₂O₃-Sc₂O₃ melts were studied by means of voltammetry, stationary polarization, as well as electrolysis. Aluminum electrowinning from the KF-AlF₃-Al₂O₃ melt on tungsten cathode occurs in the potential range from −0.1 to −1.3 V relative to the potential of the aluminum electrode. Replacing Al₂O₃ by Sc₂O₃ in KF-AlF₃ melts leads to a co-electrowinning of aluminum and scandium with the same kinetic parameters. Based on the electrochemical measurements, the parameters of stable galvanostatic electrolysis of the melts under study were selected and electrolysis tests were carried out to obtain Al-Sc master alloys. The cathode current efficiencies during co-electrowinning of aluminum and scandium on tungsten were evaluated, and the microstructure of the obtained Al-Sc master alloys was studied. A novel approach for the synthesis of the Al-Sc master alloys proposed that includes the electrolysis of the KF-AlF₃-based melts with the addition of Al₂O₃ and Sc₂O₃, electrowinning of aluminum and scandium on a solid cathode, and dissolution of formed cathode deposit in liquid aluminum.