Fundamental Studies on the Recovery of Rare Earth Elements from Coal and Coal Byproducts

Rare earth elements (REEs) are critical for the development of renewable energy resources, national security, and advanced manufacturing. With the recent closure of the Mountain Pass Mine in California, the U.S. relies entirely on foreign imports mainly from China. According to the coal data base reported by the U.S. Geological Survey (USGS), it would be possible to extract the critical materials from coal and coal byproducts. However, the rare earth minerals (REMs) found in the U.S. coals are very small in size ( 5 µm) and low in grades, which make it difficult to extract the critical materials economically. To meet these challenges, it is proposed to conduct fundamental research that can lead to the development of advanced separation technologies for efficient recovery of REMs. In flotation, hydrophobic particles dispersed in water are collected on the surface of air bubbles to be separated from hydrophilic particles. However, air bubbles have difficulties in collecting ultrafine particles due to the repulsive van der Waals forces present in the thin liquid films (TLFs) of water formed between the bubbles and particles during the initial collision step. On the contrary, the van der Waals forces in the TLFs formed between oil drops and mineral particles are attractive, suggesting that oil may be better suited for the recovery of ultrafine REM particles. It is, therefore, proposed to measure the surface forces in TLFs to better understand the roles of both the hydrodynamic and surface forces operating during oil-particle and bubble-particle interactions. The fundamental information derived from the force measurements will be useful for developing kinetic models for the process of recovering ultrafine REMs. Recent studies showed that the heavy REEs present in U.S. coals are preferentially partitioned to clay minerals, suggesting that the clayey byproducts may be a major source of the critical materials. It is, therefore, proposed to study the fundamental mechanisms by which REEs are adsorbed and desorbed on clay minerals in aqueous phase. Based on the improved understanding of the mechanisms, methods of identifying new lixiviants for ion-exchange leaching will be developed. It is also proposed to determine the surface charges of clay minerals from atomic compositions and crystal structures using a first principles model. Such information will be useful for explaining different ion-exchange capacities of the kaolinites and other clay minerals found in different coal seams. Some of the REMs are soluble in different acids. It is necessary, however, to optimize the process by minimizing the acid consumption and hence minimize the environmental impacts. It is, therefore, proposed to conduct leaching experiments on high purity REMs to determine the rate constants, reaction orders, and activation energies for different acids. In addition, species diagrams will be constructed for different REMs using the thermodynamic data available in the literature. The diagrams will be constructed in the presence of different anions to study the efficacy of the acids.