In the summer of 2019, ten projects were initiated through funding from the State Energy Research Center (SERC) at the Energy & Environmental Research Center (EERC). Recently, one project was completed focused on in situ extraction of rare-earth elements (REEs) from North Dakota coal.
REEs consist of the lanthanide series of elements with atomic numbers from 57 to 71, plus yttrium (Y) and scandium (Sc). Because of their unique properties, REEs are crucial materials used in an incredible array of consumer goods, energy system components, and military defense applications. However, the United States is currently 100% reliant on Chinese imports of these critical materials, and the REE market is considered an issue of national security. Identifying and developing alternative domestic sources of REEs and methods developed to produce them are of high interest. Recently, coal and coal by-products have been identified as promising alternative resources.
To date, the explored methods of extraction require coal to be mined from the ground and processed further through crushing, grinding, and coal cleaning prior to leaching to extract the REEs. However, coal seams that contain the highest levels of REEs identified in North Dakota are in areas not currently being mined in the extreme southwestern portion of the state. The possibility also exists that deeper coal seams not economical for surface mining may contain high concentrations of REEs and critical minerals. One alternative REE extraction technique that may address both limitations is in situ leaching (ISL).
ISL, also known as in situ recovery (ISR) or solution mining, is a process by which a solution is injected into an ore body that leaches out the target mineral(s) from the ore. The mineral-laden solution is then brought to the surface from a production well. Next, the solution is processed to remove the targeted minerals. ISL causes minimal disturbance to the environmental structures.
Testing showed that recovery of REEs from coal ranged from 5% to 9% of available REEs in the coal, while other valuable metals such as Manganese, Nickel, Cobalt, and Germanium provided greater variation in extraction percentage. Variations in pH, strength, and type of acid used for extraction from the coal were identified as crucial aspects in designing an extraction process. Economically, metals other than REEs provided the bulk of the value of the extracted material. Of the REEs, Scandium was recovered at the lowest percentage but has the greatest value. Future is being pursued to maximize the recovery rate of REEs and high-value metals to enhance the economic potential and should also include site-specific investigation into potential coal seams for ISL operation.