RARE EARTH ELEMENTS AND YTTRIUM IN AMD-ASSOCIATED SEDIMENTS IN THE ILLINOIS BASIN, USA

Acid mine drainage (AMD) associated with abandoned coal mines could represent a potential source of critical and strategic metals, including lanthanide rare earth elements plus yttrium (REY). Extensive and horizontally stratified AMD sediments found in association with AMD-impacted natural and engineered systems may also contain elevated concentrations of REY and, as such, become promising sources of critical metals. In the Illinois Basin, these AMD sediments are usually a mixture of nanocrystalline Fe(III) precipitates, Fe(III)_NP (i.e., goethite, schwertmannite, and jarosite), as well as detrital minerals, predominantly clays, originating from the partial weathering of coal mining waste.

Geochemical analyses of the stratified AMD sediments, collected at Tab-Simco, an abandoned coal-mine site in Illinois, showed a wide range of REY concentrations ranging from 10 ppm in the Fe(III)_NP-rich layers up to 250 ppm in the clay-rich layers. REY concentrations normalized to the North American Shale Composite (NASC) displayed no prevailing patterns regardless of the dominant mineralogy of the AMD sediments. In all samples, a systematic direct correlation occurs between REY and Al contents indicating that clay minerals were important reservoirs for residual REY.

The combined use of chemical extractions and synchrotron-based X-ray fluorescence (XRF), and X-ray diffraction (XRD) was further employed to characterize the distribution of REY-phases at the microscale. The microXRD patterns of the AMD sediments showed that the REY-bearing phases were dominated by detrital clay minerals in addition to minor amounts of REY-rich minerals including zircon, xenotime, monazite, and apatite. The microXRF maps collected for yttrium revealed that it was consistently spatially collocated with the clay minerals suggesting that clays immobilized yttrium, and possible other REY, as surface-bound complexes or incorporated yttrium into their structure. These results are interpreted to indicate that, contrary to previous reports, the REY in AMD sediments are primarily associated with detrital clays minerals. Even though the clay-rich AMD sediments are substantially lower grade than other types of REY mineral resources, their abundance could make them an economically‐important source of REY.