Modulating lanthanide affinity through DFT-guided ligand design for rare earth element extraction from coal ash of a gasification unit in a DRI plant

A study of coal ash from a gasification unit in a direct reduced iron (DRI) plant as a secondary resource for the extraction of rare earth elements (REEs) is presented, with a focus on ligand-assisted separation strategies. Experimental characterisation using wavelength dispersive X-ray fluorescence (WD-XRF) and inductively coupled plasma mass spectrometry (ICP-MS) revealed that coal ash is predominantly composed of SiO ₂ and Al ₂ O ₃ with a total REE concentration of 650 mg/kg, indicating its potential as a viable secondary resource. The study specifically examines the coordination behaviour of a modified Cyanex 301 (bis(phenyl)dithiophosphinic acid (Ph ₂ DTPA)). In this ligand, phenyl groups replace the branched alkyl chains of Cyanex 301, and the phosphorus-centred framework is retained. The above change is intended to adjust the electronic environment and steric properties of the ligand. Density functional theory (DFT) calculations have been carried out at the B3LYP/SDD level to provide precise insight into the interactions. A comprehensive analysis of the lanthanide complexes by considering structural parameters, thermochemical properties, frontier molecular orbitals and nuclear magnetic resonance (NMR) spectroscopy has been carried out. The important parameters, including interaction energies, enthalpy, entropy and Gibbs free energy changes, the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the electrophilicity index and NMR chemical shifts, have been determined to evaluate the stability and feasibility of the complex formation. The theoretical findings highlight the potential of the Ph ₂ DTPA ligand as a promising extractant for separating and recovering REEs from coal ash leachates, providing a basis for future synthesis and experimental solvent extraction investigations.