Kinetics of Heavy Rare Earth Element Extraction from Phosphoric Acid Solutions

Rare earth elements are indispensable for a wide range of advanced technologies, which underscores their strategic importance. This study investigates the kinetics of extracting heavy rare earth elements—lutetium, thulium, yttrium, erbium, and dysprosium—from industrial phosphoric acid solutions generated during apatite processing. A comparative approach using both solvent and solid-phase extraction with di-(2-ethylhexyl)phosphoric acid (D2EHPA) was applied to elucidate the underlying mechanisms. Optimal solvent extraction parameters (Vaq:Vorg = 2:1, φD2EHPA = 0.2, 298 K, stirring at 350 rpm) achieved efficiencies exceeding 85%. Efficient solid-phase recovery was attained under mild conditions (298 K, m:V = 1:10, shaking at 100 opm). The rate-limiting steps were identified as diffusion-controlled for solvent extraction, governed primarily by agitation intensity, and as a mixed external–internal diffusion regime for solid-phase extraction. Calculated activation energies for each element corroborate these findings.