Crown Ethers in the Group and Individual Separation of Rare-Earth Elements through Solvent Extraction: Interactions, Thermodynamics, and Applications

Rare-earth elements (REEs) have a pivotal role in modern technology due to their distinctive physicochemical properties. Solvent extraction is a widely used industrial method for separating REEs from pregnant leach solutions. Crown ethers (CEs) have shown notable selectivity and affinity for REEs and alkali-metal ions. Notable CEs include 15-crown-5, 12-crown-4, 18-crown-6, and dibenzo-18-crown-6 ligands and are employed in both organic and inorganic systems. However, challenges such as solubility in both phases and extractant loss during separation limit their use. Solution such as adding organophilic groups to CEs enhances their solubility and distribution, helping reduce loss. Anion type and concentration also impact the CE retention. This Review explores the application of CEs in REE extraction, their interaction with cations, thermodynamics, and complex formation. It also examines synergistic extraction using mixed CEs and operational parameters and addresses challenges and solutions for industrial use.