Rare earth elements and yttrium in Pennsylvanian coals and shales in the eastern part of the Illinois Basin

Abstract

The objective of this study was to evaluate rare earth elements and yttrium (REY) concentrations in Pennsylvanian coal-bearing strata in the eastern part of the Illinois Basin (Indiana), specifically targeting coals and marine shales associated with the coals. Samples were collected from several coal mines and three boreholes. In total, 105 samples were analyzed using inductively coupled plasma - optical emission spectrometry (ICP-OES), including 69 coals, 31 shales, and 5 paleosols. Concentrations of REY in the coals vary from less than 200 ppm to more than 3000 ppm (on ash basis). Indirect evidence, including REY versus ash relationships, REY distribution patterns, and REY content in raw coal compared to that in washed coal, suggests that the organic association may be an important mode of REY occurrence in these coals. The Lower Block and the Upper Block Coal Members of the Brazil Formation and some coals of the Staunton Formation have the highest REY concentrations among those studied. Concentrations of REY in marine shales range from less than 200 to 800 ppm on ash basis. The majority of shale samples studied vary little from the REY values of the continental crust, but some show enrichments. Our results suggest that, while post-combustion coal ash of the high-REY coals are a potential source of these elements, significant efforts and resources are needed to make the studied shales a viable REY resource.