The separation and concentration processes of heavy rare-earth metals—yttrium, ytterbium, erbium, and dysprosium—during stripping from the organic phase based on di-2-ethylhexylphosphoric acid (D2EHPA, or DEHPA) solutions were investigated in this work. Optimal conditions providing high separation factors of rare-earth metals (REM) and their extraction degree to the aqueous phase were determined. The usage of sulfuric acid solutions with a concentration of 2–6 mol/L, depending on the type of extracted rare-earth element, was proposed as a stripping agent for rare-earth metals (REM), and the usage of oxalic acid solution was proposed as an iron stripping solution from the organic phase. To increase the REM stripping efficiency, the antagonistic effect of tributyl phosphate in the di-2-ethylhexylphosphoric acid-kerosene-tributyl phosphate system was considered. The possibility of increasing the capacity of the organic solvent by cleaning the organic phase from iron ions using oxalic acid solutions was revealed. The influence of temperature, aqueous and organic phase ratio, stirring rate, and re-extractant concentration on the distribution and separation factors of adjacent heavy rare-earth-metal (HREM) pairs during the re-extraction process were determined. A schematic diagram of the laboratory-tested separation process of heavy rare-earth metals into individual components with the obtaining of yttrium and ytterbium concentrates containing more than 99% of the target components was proposed.

The thermodynamic properties of di-2-ethylhexylphosphoric acid (D2EHPA) in organic solvents are critical for optimizing metal extraction processes in hydrometallurgy, necessitating precise determination of activity coefficients in binary systems such as D2EHPA–n-hexane. This study was devoted to the determination of n-hexane’s concentrations in the vapor phase over D2EHPA solutions at 293.0 K using gas chromatography (GC) and isopiestic (IP) methods. Comparison with literature data confirmed the superior reliability of GC measurements at low n-hexane concentrations. The experimentally determined activity coefficients of hexane, obtained via GC, served as the initial input parameters for UNIFAC modeling. The optimized interaction parameters were 1144 ± 25 (CH₂-HPO₄) and 228 ± 50 (HPO₄-CH₂), with the infinite dilution activity coefficient for D2EHPA ${\gamma }_{\infty }=22.1$. These results experimentally clarify the non-ideal behavior of D2EHPA–n-hexane mixtures, establishing a validated thermodynamic modeling framework for organophosphorus extractant systems. This work establishes a fundamental basis for investigating ternary systems, such as D2EHPA–aliphatic solvent–aromatic solvent and D2EHPA–metal complex–solvent systems, paving the way for enhanced liquid–liquid extraction efficiency.

This study examines the activity coefficients of benzene, toluene, and di-(2-ethylhexyl)phosphoric acid (D2EHPA) in binary benzene–D2EHPA and toluene–D2EHPA systems, as well as the ternary n-hexane–toluene–D2EHPA system, using gas chromatography at 293.0 K. The primary objective was to determine UNIFAC model interaction parameters and validate their accuracy for predicting thermodynamic behavior in these systems. Experimental measurements revealed activity coefficient maxima for benzene and toluene at mole fractions of 0.8–0.9, decreasing to 0.46–0.67 in dilute solutions. The UNIFAC interaction parameters were calculated as follows: ACH–HPO₄ (−334, 4605), ACCH₃–HPO₄ (680, 467), and refined CH₂–HPO₄ (54, 1199). The UNIFAC model achieved deviations of less than 2% from experimental data in both binary and ternary systems. A novel methodology incorporating intermediate standards for gas chromatography was developed to overcome challenges in measuring volatile solvent concentrations, enhancing measurement precision. These findings enable accurate prediction of activity coefficients in mixtures of alkanes, cycloalkanes, and monoaromatic hydrocarbons with D2EHPA, offering significant implications for optimizing metal liquid–liquid extraction processes.