Electrode processes in the KF-AlF3-Al2O3 melt

Abstract

The kinetics of anodic and cathodic processes in KF-AlF3 and KF-AlF3-Al2O3 melts obtained by means of stationary polarization and cyclic voltammetry have been studied. Studies were conducted at different cryolite ratios [CRs, (moles of KF)/(moles of AlF3)] and different alumina contents at 700-800 °C for the Cu-Al anode and at 800 °C for the tungsten cathode. The limiting current density of oxygen evolution on the Cu-Al anode was 0.31 A cm-2 at 700 °C and CR = 1.4. It drastically increases with CR and temperature. The limiting current density of aluminium reduction on the W cathode was in the range 0.45-0.50 A cm-2. The cathodic process is diffusion-controlled while the anodic one shows mixed kinetics. The diffusion and mass transfer coefficients for the cathodic process (W electrode) and the mass transfer coefficient for the anodic process (Cu-Al electrode) were determined. Based on the obtained results, the electrolysis of 1.4KF-AlF3-Al2O3(sat) with the Cu-Al anode and W cathode was performed. The aluminium purity was determined using an optical emission spectrometer. The XRD and SEM-EDX methods were used to study the phases formed on the anode surface. © 2020 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.