Rare Earth Extraction from Ion-Adsorption Clays in U.S. Coal By-Products

Ion-adsorption clays (IACs) mined in South China are probably the most important rare earth resource in the world, as they account for >80% of the global supply of heavy rare earth elements (HREEs) that are critical for high-tech manufacturing industries. Recent studies conducted by the U.S. Geological Survey (USGS) showed that both the climate and geological condition in the Central Appalachian Mountains were conducive to forming regolith-hosted ion-adsorption clays, which raised the prospect of finding IAC deposits in the region. It happens that ~80% of the mineral matter in coal is clay and/or clay-based sedimentary rocks, while much of the sediments in coal basins are detrital in origin. It is, therefore, the objective of this chapter to discuss the possibility of extracting rare earth elements (REEs) from coal by-products. It appears that IACs are present in coal by-products; however, it is more difficult to extract the REEs from these materials as compared to those in South China. The reasons for the difficulty are reviewed on the basis of the limited experimental and thermodynamic data available in the literature.