Rare earth element resources in coal mine drainage and treatment precipitates in the Appalachian Basin, USA

Abstract

Drainages from abandoned coal mines in the Appalachian Basin represent a potential source of rare earth elements and yttrium (REY), many of which are considered to be critical resources due to their use in electronics and batteries. We report REY data from coal mine drainage (CMD) sites and precipitates from CMD treatment systems throughout Pennsylvania. Most CMD discharges are enriched in total rare earth elements relative to concentrations expected in unaffected surface or ground waters, generally by 1–4 orders of magnitude. Relative to the North American Shale Composite (NASC), which approximates the rock with which mine waters interact, CMD waters generally show a pattern of enrichment in the middle REE, including several identified as critical resources (Nd, Eu, Dy, Tb). Low-pH discharges tend to have higher REY concentrations than circumneutral discharges, and a precipitous drop in concentration around pH6.6 suggests adsorption of REY in the mine water flow path at near-neutral pH values. Treatment precipitates, including Fe oxy-hydroxides, CaMg lime slurries, and Si- and Al-rich precipitates, are enriched in REY relative to the average CMD discharge value by factors of 103–104. While the discharges and precipitates analyzed in this study did not overlap in space and time, the overall REY pattern of the precipitates approximate those of the CMD, but with a slight depletion in light rare earth elements. The higher REY concentrations in precipitates from systems treating low-pH discharges most likely reflects the generally higher REY concentrations in these discharges, but this needs to be established through direct comparisons over multiple seasonal cycles. We estimate that coal mine drainage outflows in the Appalachian Basin generate a total of about 538 metric tons of REY per year in a dissolved state, with ~83% of that from low pH (<5) discharges. A portion of these discharges could be targeted for resource recovery by mine drainage treatment systems engineered to optimize high-REY precipitates.