Bismuth electrodeposition in molten salts

Abstract

The electrodeposition of bismuth from BiCl 3, Bi 2O 3 and BiOCl dissolved in chloride eutectics and borax melts has been studied. The cathodic deposition of the metal from the fused mixture KCl-LiCl-BiCl 3, at 400–600°C, and low mole fractions of BiCl 3, has been determined by a diffusion process. At higher values of the mole fraction, the discharge of ions at the cathode proceeds without polarization. The cathodic current efficiency increases with the mole fraction of the bismuth salt when temperature and cd are kept constant. In molten CaCl 2-NaCl, Bi 2O 3 appears to be slightly soluble, at 600 to 900°C. The polarization curve of a Mo electrode shows concentration polarization; the limiting cd increases with the temperature. The current efficiency is negligible at 600°C but increases considerably at higher temperature. Concentration polarization is also found in the cathodic deposition of bismuth from a BiOClBaCl 2CaCl 2-NaCl mixture at 900°C. Here, the variation of cathode current efficiency as a function of cd shows a maximum. Polarograms obtained with Mo electrodes for dilute solutions of Bi 2O 3 in molten borax, at 800–1000°C, have wave heights proportional to the concentration. At higher mole fraction the diffusion wave tends to disappear. The dependence of the current efficiency upon cd goes through a maximum at 800°C. Using Pt electrodes, polarograms can be obtained only at 800 and 900°C and 0·01 mole fraction. At larger oxide content and higher temperature there is no concentration polarization. In the same way, polarization curves with Bi electrodes show no controlling diffusion process at any mole fraction and temperature.