Hower, and six other authors, reported in 2017 that 20 samples, with one composite sample built out of the 20 individual samples, of ponded ash were collected at a Kentucky power plant. The coal feed throughout the history of the power plant was central eastern Kentucky low- to moderate-S high volatile A bituminous coal. The pond was in the process of being excavated, with the ash moved to a landfill on another company property. In spite of the long history of the pond and the expected variation in coal supply, and perhaps because of the mixing due to the excavation, there is little variation in ash-basis rare earth elements plus yttrium among the 20 sites.
Much of the interest in recovering rare earth elements (REEs) has concentrated on current ash production; for example, Mardon and Hower (2004) investigated the REE distribution in an array of ash-collection hoppers at an active Kentucky power plant. However, older ash, whether in ponds or landfills, are additional possible sources of REEs. Stored ash represents many years, sometimes many decades, of a resource that is not subject to the vagaries of planned and unplanned outages, operating schedules determined by a regional dispatcher, and other problems that could be encountered when relying on “live” ash production. For many power plants, coal supplies changed over the years, shifting between coal seams within the same coalfield or even changing coalfields as environmental regulations affected the coal quality needs of the utilities. Additionally, stored ash may be the product of a more suitable (with respect to REE content) coal supply and might not contain non-coal pollution control constituents, such as activated carbon used for Hg control or calcium oxide for SOx control.
The light rare earth elements include La through Sm, and the heavy rare earth elements are Eu to Lu. The REE plus yttrium (REY) are divided into light (La through Sm), medium (Eu through Dy plus Y), and heavy (Ho through Lu) fractions. The authors also noted L-type (LaN/LuN > 1), M-type (LaN/SmN < 1, GdN/LuN > 1), and H-type (LaN/LuN < 1) enrichment patterns. Within the Central Appalachians, the Fire Clay coal and its correlatives is considered to be the premier REY resource, owing to enrichment associated with a REY-rich volcanic ash fall parting (tonstein). Among the rare earth minerals found with the Fire Clay tonstein and coal are REE and Y bearing zircon; Y-bearing Ca phosphates (crandallite); and Y-, La-, Ce-, Nd-, Dy- and Gd-bearing apatite.
The fuel in use over the decades of plant operation was Pennsylvanian, central eastern Kentucky, high volatile A bituminous, low to medium-S coal. The coal combustion products were placed in ponds on either side of the plant. The ponds, on a terrace of the Kentucky River, are being excavated and moved to another utility-owned site in the same county. The power plant, although not yet formally decommissioned, will not be upgraded to comply with current environmental regulations.
The composite fly ash has 24.76% C, high for most power plants but expected based on our experience with fly ash from this power plant. The REE content is 457 ppm with 59 ppm of Y and 33 ppm of Sc (all on the ash basis). TEM analyses of Central Appalachian coal-derived fly ashes indicate that REE and Y can be found in minerals,
such as zircon and monazite, and within glass. The association with either glass or minerals or both is not always clear with the TEM analyses. HRTEM analysis of an Al-Si glass showed that a number of REEs are associated with the glass. The latter observation is suspect, however, because REEs were also found to be in particles
within the amorphous carbon surrounding the glass. The assessment of the association of REE in fly ash is dependent on the scale of the observation, with TEM not capable of distinguishing the REE associations with the amorphous carbon and HRTEM not capable of detailed analysis of the large glass spheres. Taking in all scales, we see a complex array of associations with minerals, glass, and the secondary carbons coating the inorganic fly ash particles, together suggesting that no one method of extraction may be universally sufficient to extract significant amounts of REY.
Overall, although this deposit shows promise, it is the product of the combustion of, arguably, the richest coal-based REY region in the eastern United States; by itself, it would provide a small portion of the annual U.S. REY needs. That being said, with no other U.S. production at the time of this writing, any domestic REY production is important.
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