ALTERNATIVE RARE EARTH ELEMENT RESOURCE EVALUATION OF COAL BY-PRODUCTS: A TYPE EXAMPLE FROM THE POWDER RIVER BASIN, WYOMING

Coal by-products have the potential to provide trace metal alternative resources such as Rare Earth Elements (REEs). The suggested economic cut-off grade of Rare Earth Oxides (REO) in these coal by-products is relatively high and thus excludes many U.S. coals. In addition to REO concentrations, major element compositions, the proportion of critical REEs, and overall volumes should play dominant roles in alternative REE resource evaluation. Some coal by-products with traditionally uneconomic REO concentrations have highly favorable chemistry for extraction, and very large quantities of material; surprisingly these by-products may be the best candidates for alternative REE stocks.

One such source of high volume, easily extractable REEs comes from coal-fired power stations in the Powder River Basin (PRB), Wyoming. PRB coal by-products are calcic to calsialic with consistent trace element concentration patterns. REE patterns from PRB coal by-products exhibit middle to heavy REE enriched profiles (normalized to Upper Continental Crust). Critical REEs are consistently above 35% and over 37% in some sample groups. The REO grade, 400-500 ppm, may be robust enough to be a potential resource if extraction rates and overall material volumes are considered.

Using the most efficient methods currently known, fly ash exhibits significantly different extraction rates between Al-Si-enriched and Ca-enriched stocks.¹For example, Appalachian basin coal by-products are Al-Si-enriched and Ca-poor, resulting in mean REE extraction rates less than 35%. In contrast, PRB coal by-products which are Ca-enriched have mean REE extraction rates above 70% resulting in higher per volume mean extraction rates of REEs. Although Appalachian basin coal by-products have some of the highest REE-concentrations in the U.S., lower extraction rates and volume make them less economic than PRB coal by-products; with modest REE-concentrations but excellent extraction rates and high volume. Additionally, USGS coal data suggests PRB coal sources that could yield higher REE-concentrations than what is currently being utilized as fuel.

¹ Taggart, R.S., Hower, J.C., Dwyer, G.S., and Hsu-Kim, H. (2016). Trends in the rare earth element content of U.S.-based coal combustion fly ashes. Environmental Science & Technology, 50(11), 5919-5926.