Little over a decade ago, Professor Derek Fray's group at the University of Cambridge, UK, discovered that a solid metal oxide can be directly reduced to metal by cathodically polarising it in a molten salt electrolytic cell. The simple electrochemical process attracted worldwide attention and significant efforts have been made since then to study and develop the process for production of many metals/alloys from their oxides. The process, with necessary modifications, is being studied and adapted as a head-end step in the pyrochemical reprocessing of spent oxide nuclear fuels. This review discusses the solid-state electro-reduction process, its development and the present status in both public and nuclear domains.

The reduction of zirconium dioxide pellets by electro-deoxidation in molten calcium chloridecalcium oxide (900°C) has been studied. In this technique, the solid oxide is cathodically polarized against a graphite counter electrode under a constant applied potential. Unlike other metal oxides that have been reduced by this technique, only a small area around the cathodic current-collector wire was reduced to zirconium metal with zirconia pellets sintered at~1100°C; the rest of the sample was largely calcium zirconate. Pellets sintered above 1200°C showed better reduction near the cathode wire and the reduction extended to the entire surface of the pellet with the passage of time. However, reduction of the inner core was found to be increasingly difficult, because the surface metal layer thickened on continuous electrodeoxidation. An analysis of the experimental results showed that the poor electrical conductivity of the intermediate compound, CaZrO 3 and its blocky morphology inhibited the electrodeoxidation process. The increase in the sintering temperature of the pellet made it better conducting. However, the pores formed in the thick zirconium metal layer in such samples were too small for an ideal contact between the inner core and the molten electrolyte and hence the reduction of the inner core remained incomplete. Within the scope of this study, it is concluded that preforms with good grain growth and porosity are necessary for the electro-deoxidation of solid zirconium oxide.

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A solid metal oxide cathode undergoes significant chemical changes during the molten salt electro-deoxidation process. The changes in the chemical composition lead to changes in the electrical resistivity and potential of the electrode. Two novel electrochemical techniques, based on these two parameters, have been employed to study the electro-deoxidation of solid TiO 2 and ZrO 2 in molten calcium chloride at 900°C. The in situ resistance measurements carried out by the IR drop method conclusively proved that TiO 2 electrode remains highly conducting throughout the electro-deoxidation process and hence is amenable for reduction. The ZrO 2 electrode, on the other hand, developed very high resistance midway in the electrodeoxidation, and could not be reduced completely. The resistance measurements give strong indication that the electron-transfer reactions taking place at the cathode determine the rate and efficiency of the electro-deoxidation process to a great extent. The low-current galvanostatic electro-deoxidation of TiO 2 electrodes, in conjunction with a graphite pseudo reference electrode to monitor the half cell potentials, showed that the metal oxide passes through two stages during the electrolysis; a high current, low resistant stage 1, where Ca 2? ions are inserted to the metal oxide cathode to produce different intermediate compounds and stage 2 where electro-deoxidation of the cathode take place continuously. Removal of oxygen, from the cathode, in stage 1 of the electro-deoxidation is considered to be insignificant. The anodic and cathodic voltages in this stage remained more or less stable at *1.4 V and *-1 V, respectively. When the oxygen ions in the melt were depleted at the end of this stage, both the anode and cathode potentials were increased in the anodic direction and this behaviour suggested that the graphite pseudo reference electrode was changed from a C/CO electrode in stage 1 to a Ca 2? /Ca electrode in stage 2.