Extraction of Rare-Earth Elements with Deep Eutectic Solvent Di(2,4,4-trimethylpentyl)phosphinic Acid/Phenol

A hydrophobic deep eutectic solvent based on di(2,4,4-trimethylpentyl)phosphinic acid and phenol is proposed as an extractant for the separation of a number of rare-earth element ions from nitrate solutions. Experimental data on the interphase distribution of Pr, Nd, Tb, Dy, and Yb ions in the di(2,4,4-trimethylpentyl)phosphinic acid/phenol system are obtained with varying key process conditions: medium acidity, salting-out agent concentration, component ratio in the eutectic solvent, metal concentration, etc. The study establishes the mechanism of rare-earth-metal cation extraction by the proposed eutectic solvent and the composition of the extracted compounds using the tilt angle method. A study of metal ion re-extraction from the organic phase with mineral acid solutions is conducted, and the possibility of repeated use of the proposed eutectic solvent in chemical engineering processes is assessed. The obtained results indicate the potential of using the deep eutectic solvent di(2,4,4-trimethylpentyl)phosphinic acid/phenol for the extraction of rare-earth-metal cations in magnetic waste recycling processes.