October_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 0 2 6 REE CONCENTRATIONS When reporting REE concentra- tions, it is important to specify which el- ements are included as part of the re- ported total REE (TREE) content. In this article, REE refers to the lanthanides and yttrium. REE concentrations that con- tain scandium are referred to as REE+Sc . Using this nomenclature, the average REE concentration in U.S. coal is be- tween 62 and 65.5 ppm on a dry whole sample or drymass basis [6,10] . This trans- lates to more than 46,000 short tons of REEs being mined in U.S. coals in 2018, using the EIA 2018 U.S. coal produc- tion of 756 million short tons (MMst) [11] . This is more than four times the esti- mated apparent consumption of ap- proximately 11,000 tons of REEs in 2015, and shows the potential opportunity for coal to meet U.S. REE demand [1] . More- over, production and use of coal pro- duces other large-volume materials, such as power generation ash, coal re- fuse, and AMD sludge, which can also be considered as potential REE sources. Using NETL’s EDX data, the aver- age REE concentration on a dry ash ba- sis for each U.S. coal basin is shown in Table 1. This table, which characterizes more than 3000 datasets, shows that AMD sludge exhibits the highest av- erage REE concentration compared to coal and coal by-product samples ana- lyzed to date. The higher REE concentration in AMD sludge has renewed inter- est in these materials as a potential REE source. The Central Appalachia ba- sin features the highest average REE concentra- tion. This supports publi- cations that have shown enriched REE concentra- tions in the bituminous fire clay coalbed in east- ern Kentucky. [12,13] The relatively high REE concentrations found in AMD sludge potentially open new opportunities for extracting coal-based REEs that could not have been fully appreciated by examining large datasets in the pub- lished literature, such as the COALQUAL database and REE concentrations in coal mine drainage water samples [6,14] . Additionally, AMD sludge contains a higher heavy REE (HREE) concentration (Fig. 4) in comparison to other coal- based materials. TABLE 1 — REE DRY ASH BASIS CONCENTRATIONS FOR COAL AND COAL BY-PRODUCTS BY DOMESTIC U.S. BASIN Basin N REE (ppm) Northern Appalachia AMD Sludge 573 728 Central Appalachia AMD Sludge 70 521 Northern Appalachia 400 354 Central Appalachia 279 509 Southern Appalachia 35 343 Illinois 769 332 Rocky Mountain 120 113 West/Northwest 2 334 Gulf Lignite 30 242 Powder River Basin 93 364 Lignite 416 319 Other* 264 381 Total 3051 Note: N is number of datasets; REE [ppm] is average concentration. *Samples include materials from unidentified plants, blended ash samples, roadcuts, and other coal-based samples. No attempt was made to infer a basin if the EDX website data did not identify a basin. Fig. 4 — Percent HREE (Tb through Lu, Y) compared to total REE concentration.