Synthesis of rare earth phosphates in molten LiCl–KCl eutectic: Application to preliminary treatment of chlorinated waste streams containing fission products

Abstract

The precipitation of rare earth phosphates (RE = La, Ce, Pr, Nd, and Lu), from RECl 3 was investigated in molten LiCl–KCl eutectic at 500 °C in air. Ammonium dihydrogenphosphate (NH 4H 2PO 4) was used as the phosphorus precursor. X-ray diffraction analysis indicated the formation of compounds with monazite (La, Ce, Pr, and Nd) or xenotime (Lu) structures. 31P NMR spectroscopy measurements confirmed that lanthanum formed pure monazite, which indicates a LaCl 3 → LaPO 4 conversion factor near 100%. These results demonstrate that the stoichiometric addition of NH 4H 2PO 4 is sufficient to obtain quantitative precipitation at 500 °C of anhydrous rare earth phosphates in molten LiCl–KCl. The use of this type of precursor, which has the advantage of not modifying the chemical composition of the medium after recovery of the rare earth phosphates, could be considered during the first step of purification of chlorinated baths containing fission products arising from spent fuel reprocessing by a pyrochemical process.