Supercritical Extraction of Lanthanide Tributyl Phosphate Complexes: Current Status and Future Directions

Many researchers have studied the extraction of lanthanides with tributyl phosphate in supercritical carbon dioxide. Potential applications include the enhanced extraction or separation of lanthanides from ores and recycled materials by making use of the unique solvation properties of supercritical CO2. In some cases, tributyl phosphate has been used to extract lanthanides from their solid nitrate salt form or from nitrate solutions. In other cases, tributyl phosphate/nitric acid adducts have been used to extract lanthanides from oxides, hydroxides, ores, phosphors, magnets, and waste batteries. Flow-through-type experiments have been useful for measuring extraction kinetics for various lanthanide-containing materials. Equilibrium-type experiments have helped to show the effect of different parameters on phase equilibria, often making use of spectroscopy to measure supercritical lanthanide concentrations in situ. Several studies have noted that extraction decreases when more water is added to the system; this is likely due to the condensation of aqueous droplets, which segregate lanthanides and thus inhibit extraction. It is proposed that varying degrees of water dissolution account for the inconsistent effect of pressure or temperature on extraction across various studies.