Electrochemical co-reduction between aluminium and lanthanide ions in fluoride salts: application to nuclear wastes pyrochemical treatment

Nuclear reaction produces fission products such as lanthanides; these neutro-phage elements represent a poison for the nuclear chain reaction and have to be extracted. This thesis concerns lanthanides extraction (neodymium, cerium and samarium) by electrodeposition in the LiF-CaF2 eutectic mixture at 840 C. The process used is an alloy formation with aluminium by electrochemical co-reduction on an inert tungsten electrode. This technique is based on the simultaneous reduction of Al(III) and Ln(III) ions to obtain an alloy at the cathode. A decrease of the lanthanide activity in the alloy is obtained due to Al-Ln formation and a shift of lanthanide reduction potential to more positive values is observed: the theoretical extraction efficiency is then approaching 100 %. When both Ln(III) and Al(III) ions are present in the molten salt, new reduction waves appear between Al and Ln pure metal depositions and are attributed to Al-Ln alloy formation by co-reduction. The alloy compositions have been characterised in function of electrolysis potential: the more the electrolysis potential is negative, higher is the Ln content in the alloy. Finally, extractions by potentiostatic electrolyses have been carried out and the extraction efficiencies for the different lanthanides are higher than 99.4 %. (author)