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

New hydrophobic eutectic solvents for extraction of a series of trivalent rare earth elements from nitrate solutions, bis(2,4,4-trimethylpentyl)dithiophosphinic acid (DTPA)/menthol and bis(2,4,4-trimethylpentyl)dithiophosphinic acid/trioctylphosphine oxide (TOPO)/menthol, were suggested and studied. The solvents were completely characterized by 31P NMR, IR spectroscopy, and differential scanning calorimetry. The physical properties of the eutectic solvents, including density, viscosity and refractive index, were determined. The dependence of viscosity on temperature was described with high accuracy using the Arrhenius and Vogel–Fulcher–Tammann equations. The extraction of rare earth elements (Pr, Nd, Eu) by the suggested eutectic solvents was studied as a function of the medium acidity, concentration of the salting-out agent, composition of the eutectic solvent, initial metal concentration, volume ratio of phases, etc. To detect the synergistic effect, a comparative analysis of the extraction of the rare earth elements by the following three eutectic solvents was carried out: DTPA/menthol, TOPO/menthol, and DTPA/TOPO/menthol. The addition of TOPO to the DTPA/menthol mixture increased the extraction efficiency of the metal ions from nitrate solutions. It has been shown that the application of the suggested extraction systems can be promising in the hydrometallurgical treatment of wastes containing rare-earth metals.