Features of solid electrode application for cryolite-alumina melt electrolysis

The purpose of the article is to study the processes of boron, titanium and aluminum electrodeposition on carbon graphite cathodes, identify the possible ways to overcome the existing restrictions on the use of solid cathodes for cryolite-alumina melt electrolysis. X-ray diffraction analysis (XRD) was carried out using Shimadzu XRD-6000 automated x-ray diffractometric equipment (CuK_α radiation, graphite monochromator). The microstructure of the sample surface was examined with the use of the JEOL JSM 7001F scanning electron microscope (Japan). X-ray microanalysis and energy dispersive microanalysis (SEM-EDS analysis) as well as EDS-mapping were performed using the Oxford Instruments energy dispersive spectrometer (UK). The results of the electrochemical synthesis of titanium diboride TiB₂ at the temperature of ~ 975°C and current density of 0.82 A/cm² with the subsequent deposition of aluminum on the carbon cathode and surface study by electron microscopy suggested the causes of the unstable electrolysis and poor quality of coatings. These causes can include chemical heterogeneity and physical microdefects of the surface, which initiate the concentration of current on the micro-sites of defects with simultaneous increase in the electrolysis rate. The subsequent shortage of electroactive ions in the cathode space approximates and then reaches the current density above the limiting diffusion one successively for the discharge of boron, titanium and aluminum with a subsequent increase in voltage to background decomposition potentials (cryolite, sodium and aluminum fluoride). These processes result in the interaction of intrinsic and impurity aggressive elements with the surface material, the progressive development of physical micro-defects as well as cathode passivation with refractory sediments and electrolyte components. Ultimately, the normal process of electrolysis is disrupted and followed by the need to stop. One of the reasons for the lack of progress in the development of electrolysis technology with the use of inert electrodes lies in the specific electrochemical processes on the solid surface of polycrystalline cathodes. The established causal-and-effect relationship between the heterogeneity of the electrode surface, the instability of the process of electrolysis and cathode passivation makes it possible to determine the application conditions of solid cathodes. To solve these problems, it is proposed to use the technologies of composite cathode boronation and low-temperature synthesis of titanium diboride developed in laboratory conditions.

electrochemical synthesis, electrodeposition, solid electrode, chemical heterogeneity, physical micro defects, limiting diffusion density of current