Extraction of rare earths(III) from hydrochloric acid solutions with mixtures of new tridentate carbamoylmethylphosphine oxides Ph₂P(O)CH₂CON(R)CH₂CH₂P(O)Ph₂ (R = Me, Bu, Oct) and 4-benzoyl-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one in organic solvents has been studied. The observed strong synergistic effect has been attributed to the formation of hydrophobic mixed-ligand rare earth complexes in the organic phase. Stoichiometry of the extractable complexes has been determined, and extraction constants have been calculated. The effect of organic solvent and composition of the aqueous phase on the extraction efficiency has been analyzed.

Modified extractants, carbamoylmethylphosphine oxides Ph₂P(O)CH₂CON(R)CH₂P(O)Ph₂ (R = Et, i-Pr, Bu, Oct), containing a phosphoryl group in the amide part of the molecule, have been synthesized. The extraction of microquantities of REE(III), U(VI), and Th(IV) with solutions of the obtained extractants in organic solvents from nitric acid solutions has been studied. The stoichiometry of the extractable complexes was determined, and the influence of the extractant structure, the organic diluent nature, and the aqueous phase composition on the efficiency and selectivity of the U(VI), Th(IV), and REE(III) extraction into the organic phase was considered. It has been shown that the modification of diphenyl(N,N-dialkylcarbamoylmethyl)phosphine oxide by introducing an additional coordinating group CH₂P(O)Ph₂ into the amide part of the molecule leads to an increase in the efficiency of heavy REE(III) extraction and a decrease in the efficiency of light REE(III), and also Th(IV) and U(VI) extraction from nitric acid solutions. The degree of REE(III), U(VI), and Th(IV) extraction into the organic phase by the carbamoylmethylphosphine oxides increases significantly in the presence of the ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide.