Theoretical Contributions to Ionic Liquid Design for Selective Extraction of Rare Earth Elements from Nigerian Mineral Processing Streams

Nigeria possesses rare earth mineral deposits in Nasarawa, Zamfara, and Cross River States that represent underdeveloped critical mineral resources, yet selective hydrometallurgical separation of individual rare earth elements from complex matrices remains technically challenging. This dissertation makes original theoretical and experimental contributions to the rational design of task-specific ionic liquids for selective extraction of individual rare earth elements from simulated and authentic Nigerian mineral processing streams. Quantum chemical computations at the DFT-B3LYP/TZVP level were performed on a library of 80 ionic liquid cation-anion combinations to predict extraction free energies, complexation geometries, and selectivity between adjacent lanthanides. Based on computational screening, twelve candidate ionic liquids incorporating phosphonate, carboxylate, and phosphoramide functional groups were synthesised. Solvent extraction experiments were conducted using synthetic rare earth chloride solutions and authenticated leachate from a Nasarawa State monazite concentrate. Distribution ratios, separation factors, and stripping efficiency were measured as a function of aqueous phase pH, ionic liquid concentration, diluent polarity, and temperature. Extended X-ray absorption fine structure spectroscopy was employed to characterise rare earth coordination environments in ionic liquid phases. Molecular dynamics simulations were performed to investigate interfacial organisation at the ionic liquid-aqueous interface. Results show that the designed phosphonate ionic liquid achieves separation factors for neodymium over lanthanum of 12.4 at optimised conditions, superior to commercially available organophosphorus extractants. Original structure-selectivity relationships derived from combined computational and experimental data constitute the primary scientific contribution of this work. Keywords: ionic liquids, rare earth elements, solvent extraction, DFT calculations, Nasarawa State