Extraction of Rare Earth Elements from Nitrate Solutions\nby Hydrophobic Eutectic Solvents Based on Phosphorus-Containing Compounds

New\nhydrophobic eutectic solvents for extraction of a series of\ntrivalent rare earth elements from nitrate solutions, bis(2,4,4-trimethylpentyl)dithiophosphinic\nacid (DTPA)/menthol and bis(2,4,4-trimethylpentyl)dithiophosphinic\nacid/trioctylphosphine oxide (TOPO)/menthol, were suggested and studied.\nThe solvents were completely characterized by <sup>31</sup>P NMR,\nIR spectroscopy, and differential scanning calorimetry. The physical\nproperties of the eutectic solvents, including density, viscosity\nand refractive index, were determined. The dependence of viscosity\non temperature was described with high accuracy using the Arrhenius\nand Vogel–Fulcher–Tammann equations. The extraction\nof rare earth elements (Pr, Nd, Eu) by the suggested eutectic solvents\nwas studied as a function of the medium acidity, concentration of\nthe salting-out agent, composition of the eutectic solvent, initial\nmetal concentration, volume ratio of phases, <i>etc.</i> To detect the synergistic effect, a comparative analysis of the\nextraction of the rare earth elements by the following three eutectic\nsolvents was carried out: DTPA/menthol, TOPO/menthol, and DTPA/TOPO/menthol.\nThe addition of TOPO to the DTPA/menthol mixture increased the extraction\nefficiency of the metal ions from nitrate solutions. It has been shown\nthat the application of the suggested extraction systems can be promising\nin the hydrometallurgical treatment of wastes containing rare-earth\nmetals.