MINERAL ANALYSIS OF COALS FROM FIVE COAL FORMING BASINS IN THE U.S. USING A DIRECT X-RAY DIFFRACTION (XRD) ANALYSIS OF WHOLE COAL

The traditional way to determine the bulk mineralogy of inorganic constituents in coal by X-ray diffraction (XRD) analysis was to first low temperature ash (LTA) the coal in a radio frequency activated oxygen plasma asher and analyze the ash by XRD. During ashing, the temperature is maintained at less than 66°C/150°F while organic components are oxidized and lost as CO₂. Removal of the organic component is necessary to concentrate mineral phases. This process can be slow (1 to 2 weeks) especially for low rank coals. In addition, LTA almost always generates secondary by-product and dehydrated minerals such as bassanite from gypsum, dehydrated clays, and partial destruction of pyrite. Electron microscopy or petrographic studies were sometimes necessary to confirm the presence of minerals in the coal and identify alteration minerals. Advances in XRD technology with solid-state strip detectors have made direct analysis of the whole coal and determination of major/minor mineral phases routinely possible.

To determine the effectiveness of direct XRD analysis, coals that represent five major coal forming basins in the United States were selected and results of whole coal and LTA XRD analysis were compared. Coal from each basin has a unique mineralogy that reflects the depositional setting and later authigenic alteration. The relative concentrations of identified minerals were determined on these largely amorphous samples (65 – 95%) by running Rietveld analysis. Acquisition parameters for the presented data were collected using 6-hour scan time (4 to 65 degrees 2θ with a 0.0167 degree step size, 750 sec counting time per step and a strip detector using an active area of 127 points in 2.12 degrees 2θ,). Our results demonstrate that a 3 hour scan, half the counting used above, is adequate to identify most mineral phases in the whole coal samples. This study shows that air-drying and grinding of whole coal generates less secondary dehydrated minerals when compared to LTA. Direct analysis of whole coal reduces preparation time and more accurately reflects the true mineralogy of the coal.