An Environmentally Extractive Metallurgy Approach: Recovery of Rare Earth Elements from Coal Ash via Selective Extraction with Deep Eutectic Solvents (DES)

Rare earth elements (REE) are strategic commodities for the green energy transition and high-tech industries. Coal fly ash from power plants contains REE concentrations ranging from 230 to 450 mg/kg (ppm). However, conventional extraction of REEs from the stable aluminosilicate matrix requires strong inorganic acids, such as HCl or H₂SO₄, which lead to significant environmental impacts, acid gas emissions, and high reagent consumption. This study evaluates a green hydrometallurgical approach using eco-friendly Deep Eutectic Solvents (DES) composed of choline chloride (Hydrogen Bond Acceptor/HBA) and an organic acid (oxalic or citric acid as Hydrogen Bond Donor/HBD) at molar ratios ranging from 1:4 to 4:1. The results demonstrate that pre-treatment (calcination) at 600 °C for 2 hours significantly alters the aluminosilicate matrix, thereby increasing REE recovery from 58.4% to 82.7% under extraction conditions of 90 °C, a 4-hour duration, and a solid-to-liquid (S/L) ratio of 1:5 g/mL. The DES exhibited high selectivity, resulting in extremely low dissolution rates of matrix impurities (Fe and Al) compared to conventional strong acids. Selective precipitation using oxalic acid successfully isolated REE-oxalate precipitates with a separation efficiency of 82%–94% and an enrichment factor of 5.62. Techno-Economic Analysis (TEA) and sensitivity analysis confirm that DES solvent recycling and energy efficiency at moderate temperatures yield a highly attractive Return on Investment (ROI) and a payback period of less than 3.5 years, even under scenarios involving critical market price fluctuations for REE (specifically neodymium and dysprosium).