T.I. Pfeiffer, D.A. Bagdonas, M.S. Behrens
University of Wyoming,
United States
Keywords: coal, coal mine waste, coal-derived by-products, cost-effective feedstock, solvent extraction, flash pyrolysis, organic tar products removal, enriched REE stream, REE selectivity, weak acid extraction
Summary:
Coal mine waste and coal-derived by-products from industrial processes can serve as an alternative, cost-effective feedstock for rare earth element (REE) recovery. Although coal-based materials contain lower total REE concentrations (500-2000 ppm on ash basis) than conventional materials (40000 ppm+), their abundant, low-cost availability and suitability for simple extraction make them an attractive source for domestic REE production. Screening efforts have identified coal deposits in Wyoming’s Powder River Basin (PRB) with exceptionally high REE concentrations with substantial total reserves. Proprietary coal utilization technologies developed by the University of Wyoming School of Energy Resources (SER)—solvent extraction (SE) and flash pyrolysis (FP)—can utilize normal and high ash-PRB coals to produce an enriched REE stream whilst producing other valuable organic products, that can subsidize downstream REE separation. This approach consists of: 1) Targeting REE-enriched coal deposits with significant reserves; 2) Utilizing coal and mine waste coal that are widely available at low-cost to produce organic products; and 3) Applying a simple, economic extraction process to selectively dissolve and separate REEs from most non-REE material. To support commercialization, Wyoming has begun construction of a $31M demonstration plant near Gillette, WY. The plant will produce asphalt binders via SE and coal tar and char via FP, with the tar being sold for making mesophase pitch or to refineries for processing. The char is sold as soil amendments, building material components (bricks, pavers), or polymer fillers. Plant capacity for char will be 45t/month. A larger, industrial-scale plant is anticipated. SER’s processes convert carbon to products, simultaneously improving REE extractability and enriching REE concentration in SE residues and FP chars by up to 5.8 times relative to the original coal. REE are easily extracted with weak acids from the intermediate coal products (residue, char). Flash pyrolysis char exhibited excellent REE recovery rates near 100% for light REEs, while solvent extraction residues, with their lower treatment temperature, showed superior leaching behavior for heavy REEs, thereby improving overall selectivity and separation efficiency. Stepwise extraction removed most non-REE to reach total REE contents up to 26,000 ppm based on the combined content of REE and Al, Ca, Fe, Mg, Na, Si, and Ti. Additionally, preliminary testing demonstrates environmental and safety benefits as compared to other types of raw materials. Using PRB feedstocks, the SE process residues and FP char contained only 1-2% instead of 5+% radioactive elements (Th, U) per unit of REE. Together, these findings highlight a synergistic and economically sustainable pathway to extract and concentrate rare earths from Wyoming’s abundant coal resources—transforming what was considered waste streams into a strategically valuable domestic REE source.