Bis(2,4,4-trimethylpentyl)phosphinic acid/phenol deep eutectic solvent: Physicochemical properties and application prospects for the extraction of trivalent rare earth elements

A new hydrophobic deep eutectic solvent, bis(2,4,4-trimethylpentyl)phosphinic acid/phenol (BTMPPA/phenol), for extraction of a series of trivalent rare earth elements from nitrate solutions was proposed. The solvent was completely characterized by 31P NMR, FTIR spectroscopy, and differential scanning calorimetry. The physical properties of the deep eutectic solvent, such as density, viscosity, and refractive index, were determined. The temperature influence on viscosity was described with high accuracy using the Arrhenius and VFT equations. The BTMPPA/phenol deep eutectic solvent was proposed for the first time as an extractant for the extraction of rare earth elements. The extraction degrees of La, Eu, Gd and Y were 20, 44, 39 and 67 %, respectively, in a one-step procedure. The extraction of Y, La, Eu, and Gd cations from aqueous solutions by the proposed solvent was studied as a function of the solution acidity, concentration of the desalting agent, composition of the eutectic solvent, initial metal concentration, volume ratio of phases, etc. The stability of the BTMPPA/phenol solvent in extraction/re-extraction cycles was analyzed. A 0.5 M nitric acid solution was used as the re-extractant for the extraction of the metals of interest from the organic phase. It was shown that the proposed extraction system is promising for the practical recycling of spent fluorescent lamps containing rare-earth metals by the hydrometallurgical method.