While comprising just 17 elements of the periodic table, the group known as rare earth elements (REEs) provides significant value to our national security, energy independence, environmental future, and economic growth. REEs are found throughout the earth’s crust but most often occur in low concentrations. They do not occur naturally in elemental form but in a variety of minerals and coal. The rare earth elements can be divided into light rare earth elements (LREEs) and heavy rare earth elements (HREEs), with LREEs typically being more abundant in REE-bearing mineral deposits.
The REEs include the lanthanides: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and transition elements: scandium (Sc) and yttrium (Y).
The critical REEs (CREEs) are those deemed as supply risk and highly important to the United States' clean energy technologies going forward. REEs are important elements used in high technology products, such as catalysts, cell phones, hard drives, hybrid engines, lasers, magnets, medical devices, televisions, and other applications (see infographic).
Both coal processing refuse and coal utilization ash residues have received attention for the economic value of the REEs they might contain, thus the potential opportunities for recovering REEs from coal and coal by-products are twofold. REEs are present in materials that have already been extracted and the potential savings from co-production of coal and REEs versus production from a mine dedicated solely to recovering REEs could offset the penalties resulting from utilization of the coal-based sources with lower REE concentrations. Certain coal and coal by-products have elevated concentrations of HREEs—which are lowest in supply, rank high in criticality and price, and are projected to increase in demand—making them potentially attractive targets for REE recovery despite their overall lower concentration. Basket price can be used as a rough tool to compare the compositional distribution of REE-bearing ores based on the current relative value of each element or oxide as a pure—nearly 100 percent pure—commodity. The REE distributions found in coal and coal by-products can exhibit a compositional distribution that would command a higher basket price than some more conventional and well-recognized REE ores.
Assessments of the amounts of REEs that are likely to be found in coal deposits and the mineral matter associated with these deposits was performed. The initial NETL estimates indicate that 6 million MTs of REEs could potentially be recovered from the known coal reserves in select western state coal basins in Montana, Wyoming, Colorado, Utah, New Mexico, and Arizona. Similar estimates indicate that nearly 5 million MT are potentially available from among the coal deposits found in Pennsylvania, West Virginia, Kentucky, and Virginia. In addition, separate estimates were made for the total tonnage of REEs that might be available in current coal ash reserves and coal mine refuse/reject and the calculations suggest that several additional million MTs of REEs could be found in these sources as well. The actual amounts that could be recovered would be a function of mining practices and the economics of recovery from the operations actively producing, processing, and utilizing newly mined coal.
Long-term opportunities to economically recover REEs from coal and coal by-products depend on successful R&D focused on near-term opportunities and on leveraging the knowledge gained to develop novel recovery approaches applicable to low-concentration sources. These new processing systems must incorporate steps to recover, separate, and/or refine REEs without generating additional unwanted products. These advances may facilitate recovery of REEs from large volumes of fly and bottom ash as well as other spent and low-concentration sources. Moreover, the co-production of REEs with other useful materials available in coal and coal by-product sources offers the potential to further enhance the economic viability of REE recovery from coal and coal by-products. The figure ("Opportunities to recover REEs from the coal value chain") provides a conceptual schematic of the coal value chain as it relates to opportunities to recover REEs.
The REE Program is thus focused on developing a viable pathway to improve the economics and reduce the environmental impact through innovative applications of existing and novel separation technologies for the recovery of REEs from the Coal and Coal By-Product value chain.