Electrochemical characterization of the liquid aluminium bipolar electrode for extraction of noble metals from spent catalysts

Regularities of the cathodic and the anodic behaviour of aluminium on tungsten in halide melts are investigated. The study of such systems is relevant in connection with the prospects for their use as bipolar electrodes for efficient extraction of noble metals from spent petrochemical Al2O3-based catalysts by the electrometallurgical method with associated production of high-purity aluminium and oxygen. Stationary and non-stationary polarization curves were obtained for the reduction and the oxidation of aluminium in molten NaCl - KCl + 5 wt.%AlF3 and 1.3KF - AlF3 systems at 700-850 degrees C (973-1123 K) in the range of current densities from 0.01 to 1.5 A.cm(-2) and in the range of potential sweep rates from 0.01 to 5 V.s(-1). Electrochemical characterization of the electrode showed that electroactive particles in chloride-based melts have higher diffusion coefficients (in comparison with fluoride melt) due to the high concentration of Na(I) and K(I) cations, low density and viscosity of the melt. Electrochemical dissolution and reduction of aluminium in both chloride-based and fluoride melts are quasi-reversible diffusion-controlled processes complicated by several parallel phenomena. Cathodic reduction of Al(III) in both studied melts occurs at high overvoltages (more than 200 mV at 0.1 A.cm(-2)) with the limiting current densities of 0.3-0.4 A.cm(-2) at 800 degrees C. The cathodic reduction of Na(I) starts in chloride-based melts at the potential of -0.5 - (-0.7) V relative to Al reference electrode; the reduction of K(I) in both studied melts starts at -1.0 -(-1.2) V. In general, the kinetic parameters of the processes show that the extraction in two-sectioned cell can be beneficially performed at 800 o degrees C with high current densities, and the liquid aluminium bipolar electrode seems to be the promising solution. Key words: bipolar electrode, metal refining, high-purity aluminium, platinum group metals, catalyst processing, polarization curves, kinetics of electrode processes, electrolysis of melts