Organic and inorganic associations of rare earth elements in central Appalachian coal

Abstract Coal contains trace amounts of rare earth elements (REE). They are either in the form of discrete minerals or chemically bound to organic matter. Methods of distinguishing the organic and inorganic associations of REE in coal have been mostly based on the correlations between the REE concentrations and the ash yields. Quantitative characterization of organically associated REE is still very challenging. In this work, we confirm the existence of organically-associated REE in coal and present an approach to quantifying its mass fraction. The coal sample used in this study was a cleaned low-ash coal collected from a coal preparation plant in Kentucky, USA and originally from the central Appalachian basin coal region. The results from particle size and density separations of the coal suggested that the REE are associated with both organic and inorganic matters. It was found that as the ash yield increases, both the total REE concentration on the whole coal basis and the LREE/HREE ratio increase, while the total REE concentration on the ash basis reduces dramatically. The results also suggested that the organic matter is relatively enriched in HREE. Based on these observation and findings, a method was developed to quantitatively determine the distribution of REE in the organic and inorganic phases of the coal. It was found that the total REE concentrations in the organic and inorganic phases are 31 ppm and 1141 ppm, respectively. The results also showed that in the low-ash Appalachian coal sample used in this work, 25% of the REE is associated with the organic matter. The method developed in this work for quantitative evaluation of the organic/inorganic mode of occurrence may be applied to other coal samples and for many other trace elements in coals.