TRANSMISSION ELECTRON MICROSCOPY (TEM) OF THE <2 MICROMETER SIZE FRACTION OF A COAL ASH REVEALS MATERIAL COMPLEXITY

Coal ash is a major waste stream generated from the coal industry and is of great environmental and economic concern. Identifying components in coal that are of health concerns and that may be of economic benefit in recycling these wastes are a research priority. TEM investigations of the fine fraction of coal ash are relatively uncommon and may provide new insight into these materials.

Bright field TEM investigation on grain mounts of the < 2 micrometer fraction of a sample of a coal ash from the Morehead State University heading plant was conducted. Imaging indicates the fine fraction of the sample is dominated by poorly crystalline material with lesser amounts of calcite. The poorly crystalline material is composed of spherical to ovoid particles which are inter-grown and have two size distributions. A texture where spheres are approximately 100 to 300 nm constitutes approximately 80% of sample particle material. A texture where spheres are approximately 5 to 50 nm, with particles approximately 20 nm in diameter being most common comprises 15% of sample material. Approximately 5% of the sample material is calcium carbonate.

Imaging of the poorly crystalline material shows lattice fringes which are curvilinear and commonly have spacings of approximately 0.37 nm. Selected area electron diffraction data of these textures are rings with typical d-spacings of 0.389 nm, 0.220 nm, and 0.123 nm which closely match lattice fringe dimensions. Energy dispersive spectroscopy (EDS) spot analysis from poorly crystalline textures indicates a range of chemical compositions exist along two apparent end members. One compositional end member is consists of approximately 39.8% O, 31.3% Si, 28.4% S, and 0.5% Fe. A more complex end member consists of 39.7% O, 5.1% S, 3.2% P, 23.4% Si, 5.9% Al, 21.9% Na, and 0.8% K. The small particle size and thus high surface area, high sulfur content and high silica content may make the fine fraction of this particular coal ash an inhalation health hazard. There are no particular materials of economic interest in the fine size fraction of this coal ash. Results indicate further TEM investigations on the fine fractions of other coal ash samples are warranted.