(633b) Transmission Electron Microscopy Observations of Rare Earth Minerals in the Fire Clay Coal, Kentucky, and in the Derived Combustion Ashes

The Fire Clay coal, eastern Kentucky, is arguably the premier coal-based rare earth element resource in the eastern United States. While the high concentration is, in part, a function of the presence of a volcanic ash-fall deposit within the coal, the Dean (Fire Clay) coal in Knox County, Kentucky, does not contain the megascopically-visible ash-fall tonstein present in most other sections of the coal bed. Like the Fire Clay tonstein, a low-ash portion of the coal is enriched in rare earth elements (>2400 ppm, on ash basis). In addition to kaolinite produced in the diagenesis of volcanic glass, transmission electron microscopy studies indicate the coal contains primary kaolinite, La-Ce-Nd-Th monazite, barium niobate, native gold, and Fe-Ni-Cr spinels. The mineral assemblages, particularly the kaolinite-monazite association and its similarity to the tonsteins in the coal to the east, demonstrate the coal was subject to the REE-enriched volcanic ash fall, apparently just at a more dilute level than at locations where the tonstein is present.

Fly ash was collected from individual mechanical and electrostatic precipitator hoppers at a 130-MW pulverized-coal utility unit burning the Fire Clay coal. REE-bearing phases occur within fine minerals and, possibly, as part of the fly ash glass phase. Transmission electron microscopy was used to identify Y-Ce-Nd-La-bearing regions within a glassy matrix. Y- and U-bearing zircons were also found in the fly ash. Total REY occupies a narrow range from 719 to 775 ppm, showing an overall lack of partitioning of the REY elements with respect to particle size and temperature in ash collection systems. The heavy REE increase and the light REE/heavy REE ratio decreases from the mechanical hoppers through to the ESP rows.