Kinetics Study of Solvent and Solid-Phase Extraction of Rare Earth Metals with Di-2-Ethylhexylphosphoric Acid

The kinetic features of solvent and solid-phase extraction of yttrium and iron (III) from simulated and industrial phosphoric acid solutions are revealed. Di-2-ethylhexylphosphoric acid (D2EHPA) was used as a liquid extractant, and D2EHPA-containing Levextrel resin-a co-polymerization product of styrene and divinylbenzene in the presence of D2EHPA-was used as a solid-phase extraction agent. Significant dependence of yttrium extraction rate constant on the stirring rate was revealed using the formal first-order kinetic equation. The data obtained characterizes a diffusion-limited process with an activation energy of 16.2 +/- 1.3 kJ/mol. Temperature increase during the iron (III) extraction process leads to a changeover of a rate-limiting stage from kinetic to diffusion, accompanied by drop of activation energy from 40.0 +/- 1.4 to 11.4 +/- 1.2 kJ/mol. Effective separation of elements at the extraction stage is possible at temperatures of 283-300 K under non-equilibrium conditions of the ferric ions transport from aqueous to organic phase. This condition ensures a high yttrium-iron separation coefficient of 23.2 in 1.5-2 min. Extraction kinetics by Levextrel resin are described by Fick's second law equation, which establishes the laws of diffusion in the solid grain of the organic phase with an activation energy of 18.5 +/- 2.0 kJ/mol.