Investigation of rare earth element extraction from coal byproducts using supercritical CO2

Abstract

The increasing demand for rare earth elements (REEs) in modern technologies has led to growing interest in their efficient recovery from alternative sources. Coal ash, a waste product from coal combustion, has been identified as a potential reservoir of valuable REEs, with concentrations ranging from 270 to 1480 mg/kg. This study investigates the recovery of REEs from various ranks of coal ashes using environmentally benign supercritical carbon dioxide (SC-CO₂) with tributyl phosphate (TBP) and nitric acid (HNO₃) as complexing agents. It suggests the optimal extraction conditions for potential industrial application. Experimental results indicate that sub-bituminous coal ash exhibits the highest REE recovery (60 %), followed by bituminous (48 %) and anthracite (38 %). The extraction mechanism involves three key steps: (1) dissolution of metal oxides into metal ions using HNO₃, (2) complexation of metal ions with TBP, and (3) extraction and dissolution of metal complexes in SC-CO₂. The optimum extraction conditions were determined at 60 °C, 2175 psi (15 MPa), a solid-to-chelating-agent ratio of 10:1, 120-min residence time, and TBP-HNO₃ ratio of 1:5. Under these conditions, anthracite ash achieved a recovery of 120 mg/L, bituminous ash 330 mg/L, and sub-bituminous ash 180 mg/L. The five-stage purification process that effectively purified REEs by reducing impurities such as Al, Ca, Fe, K, Mg and Mn with minimal environmental impact due to CO₂ recyclability. This research highlights supercritical fluid extraction (SCFE) as a green, scalable alternative for REEs recovery, supporting circular economy principles and offering an estimated $4.3 billion annual economic potential from U.S. coal ash.