Ultrasound-assisted dicyclohexano-18-crown-6 solvent extraction of rare earth elements from monazite leachate

This work investigates the ultrasound-assisted solvent extraction (USX) of rare earth elements (REEs) from a real monazite ore leachate using dicyclohexano-18-crown-6 (DCH18C6). The research addresses the challenges of slow mass transfer and low selectivity in conventional systems by combining the intrinsic size-selective molecular recognition of DCH18C6 with ultrasound-intensified mass transfer, which accelerates the approach to equilibrium without altering the ligand's underlying complexation thermodynamics. Kinetic results demonstrate that ultrasound (100 W/L) achieves extraction equilibrium in just 4 min, a 5-fold reduction compared to the 20 min required for mechanical stirring. A fundamental divergence in phase affinity was identified: light REEs (LREEs) showed preferential transfer to the organic phase (D_La = 0.30 ± 0.01), while heavy REEs (HREEs) exhibited a strong inherent hydrophilicity, remaining in the aqueous phase (D_Lu = 0.05). Notably, at pH 5.5, USX achieved a D_Lu of 2.75, representing a 4.3-fold enhancement over the silent system. Under integrated conditions, the system effectively partitioned LREEs from HREEs with a maximum grouping separation factor (β_La/Lu ) of 6.3. This trend was further intensified during the 4-min stripping stage, where HREEs were almost completely recovered into the acidic phase (98.5% for Lu), resulting in a β(La/Lu) of 372 ± 350, though this value and its mechanical-stirring counterpart (244 ± 240) both carry very large uncertainty due to the low absolute D values for Lu under both conditions. These results confirm that the synergy of acoustic cavitation and size-selective ligands offers a rapid pathway for high-purity REE fractionation.