CORRELATIONS OF RARE EARTH ELEMENTS WITH MAJOR MINERAL CONSTITUENTS OF AN ILLINOIS BASIN COAL

Identification of the modes of occurrence of elements in coal has been the focus of many studies. More recently, that focus has been directed toward rare earth elements (REEs) since they are critical commodities. Targeted techniques for the commercial recovery of REEs can be developed most successfully by using fundamental approaches to better understand their occurrence in coal.

REE minerals (e.g. monazite and xenotime) are rarely observed in Illinois Basin coal beds. As such, most of the REE content seems to be associated with more commonly-occurring mineral phases, such as silicates and aluminosilicates. An organic association of REEs in Illinois Basin coals appear to be minimal. The design of this study was to perform a mineralogical sorting on a typical Illinois Basin coal sample, and document the variations of REEs that occur with changes in mineralogy. The sorting was made through consecutive density separations, resulting in seven density fractions from <1.3 g/cm³ to >2.9 g/cm³. The mineralogy of the density fractions was investigated by microscopy, X-ray diffraction, and X-ray fluorescence. From the lightest fraction to the second heaviest fraction a transition from coal with disseminated clays and pyrites to aggregated clays and other silicates was observed. The minerals in the heaviest fraction were pyrite, marcasite, silicates, sphalerite, and trace amounts of other heavy minerals.

The individual REEs show different correlative behaviors with changes in mineralogy. The REEs were classified into multiple correlation categories accordingly, with some REEs falling into more than one category. The correlations signify possible chemical associations.