Mineralogical characterization of lower rank coals by SEM-based automated image analysis and energy-dispersive X-ray spectrometry

Abstract

Automated image analysis (AIA) based on scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry was used to characterize the mineral matter in two western subbituminous coals from the Adaville No. 11 seam (Kemmerer, WY) and the Dietz No. 1 and 2 seams (Decker, MT). The samples were ground to — 200 mesh and cleaned by float-sink separation at 1.40 and 1.38 specific gravity, respectively. The samples were characterized before and after cleaning for mineral phase and size distributions. The Adaville sample contained primarily quartz and an iron-rich mineral, along with traces of apatite, while the Dietz coal contained primarily quartz and kaolinite. The mineral particles in the Adaville coal were notably coarser. The total mineral matter content determined by AIA agreed reasonably well with that calculated from the ASTM ash content. Studies on the association of the mineral particles with the coal matrix were performed by SEMAIA to classify the composite mineral-coal particles into 10 particle types or classes of increasing density. The AIA density distributions were consistent with distributions expected from the float-sink cleaning. They also indicated significant differences in the association of minerals with coal between the raw coals. Specifically, the Dietz coal contained relatively more liberated mineral matter and less material between 1.4 and 1.8g cm −3 than did the Adaville coal.