Notes on the Potential for the Concentration of Rare Earth Elements and Yttrium in Coal Combustion Fly Ash

TLDR

Central Appalachian coals, especially Fire Clay with REY‑enriched volcanic ash tonstein, are known to contain high levels of rare earth elements, and many variables from mine to power plant can affect the concentration of REY in combustion products. During mining, roof, floor rock, and partings may be included in the product; beneficiation removes some REY to meet utility specifications; and at the power plant, pulverization and removal of harder rock further alter the REY content before combustion. The Fire Clay tonstein contributes 20–35 % of the REY in the coal‑parting sequence, and even when tonstein is absent, some Fire Clay coals are still enriched in REY, implying that shipped run‑of‑mine products carry virtually the same REY content as the mined coal.

Abstract

Certain Central Appalachian coals, most notably the Fire Clay coal with a REY-enriched volcanic ash fall tonstein, are known to be enriched in rare earth elements. The Fire Clay tonstein has a greater contribution to the total coal + parting REY than would be inferred from its thickness, accounting for about 20%–35% of the REY in the coal + parting sequence. Underground mining, in particular, might include roof and floor rock and the within-seam partings in the mined product. Beneficiation, necessary to meet utility specifications, will remove some of the REY from the delivered product. In at least one previously published example, even though the tonstein was not present in the Fire Clay coal, the coal was enriched in REY. In this case, as well as mines that ship run-of-mine products to the utility, the shipped REY content should be virtually the same as for the mined coal. At the power plant, however, the delivered coal will be pulverized, generally accompanied by the elimination of some of the harder rock, before it is fired into the boiler. Overall, there are a wide range of variables between the geologic sample at the mine and the power plant, any or all of which could impact the concentration of REY or other critical materials in the coal combustion products.