Ponded and Landfilled Fly Ash as a Source of Rare Earth Elements from a Kentucky Power Plant

2 Hower et al. / Coal Combustion and Gasification Products 9 (2017) Table 1 Broader ash source table Power plant Capacity (MW) Location Combustor type Coal origin Plant I (unit 1) 123 Kentucky Pulverized coal Fire Clay coal, eastern KY Plant W 633 South Carolina Pulverized coal Fire Clay coal, eastern KY Plant H (unit 3) 180 Kentucky Pulverized coal Various coals, Illinois Basin NH 112 Pennsylvania Circulating fluidized bed Anthracite coal and culm, northeast Pennsylvania Plant D 216 Kentucky Pulverized coal Various coals, eastern Kentucky KSU Kentucky Stoker Various coals, eastern Kentucky This work is part of a larger U.S. Department of Energy National Energy Technology Laboratory–funded study of the recovery of REY from beneficiated ponded or landfilled fly ash, or both. Overall, we initially considered/screened landfilled ash at several other power plants (Table 1); some of that preliminary work will be discussed below. Circulating fluidized bed combustion (CFBC) ash also was considered; it was eliminated from consideration since beneficiation of CFBC ash would require dry processing because contact with water will initiate pozzolanic reactions with this type of material. Dry separation technologies are essentially limited to classification and electrostatic separation. Because of the fine nature of the particles present in CFBC ash, neither of these technologies has been shown to be appropriate for this type of substrate. A stoker boiler under consideration, while producing a relatively high REE ash, was not chosen because the volume of ash production could not support sustained production of REE. In reality, these decisions are pragmatic, not rigorous. We had to decide on an ash with a reasonable amount of REY, characteristics amenable to beneficiation, a supply sufficient to support a pilot plant, and access as determined by the company and state regulations. Of the utility-scale pulverized fuel combustion plants, our preferred choice was eliminated from consideration when the utility covered the ca. 20-year-old landfill, effectively eliminating it from consideration. As an alternative, in this study, we investigated the REE, yttrium (Y), and scandium (Sc) concentrations (REYSc) in coal ashes from a pond at a closed Kentucky power plant. The fuel supply typically was a blend of central eastern Kentucky low-S, high volatile A bituminous coals, similar to the typical coal feed at the originally planned study site. The pond had been used for about 20 years before the mid-2010 closure of the power plant. In light of current concerns about the long-term environmental viability of ash disposal ponds, the ash is currently (started in 2015, planned to continue through 2017) being excavated and moved from the current site on the river floodplain to a landfill at another companyowned facility. As a rough estimate based on the age of the pond and the amount of ash produced per year, as extrapolated from ash production amounts backing the state-wide numbers published by Hower et al. (1996, 1999a, 2005, 2009, 2014), we estimate that there is about 2 Mt of ash in the pond/landfill. For an ash feed rate of 100 t/day in a commercial extraction plant, this supply would last decades. 2. Methods A summary of the advantages and limitations of the various analytical techniques is shown in Table 2. 2.1. Sampling Samples were collected at a central Kentucky power plant (plant D in the code used in our previous studies of Kentucky power plants) on 13 April 2016 by University of Kentucky Center for Applied Energy Research (CAER) staff with the assistance of utility personnel. The excavation of the ash provided some level, albeit unknown, of mixing of the ash. Individual samples were collected at 20 sites (five sites along four rows, samples 93977–93996; Figure 1, Table 3) along with an additional split at each site for the composite sample (sample 93997). The composite sample was homogenized and split at CAER. Each of the individual samples was also split, and all were submitted for chemical, X-ray mineralogy, and petrographic analyses at the CAER. Additional splits of the composite sample were reserved for additional experimental work at the CAER and at Physical Sciences Inc. (PSI). 2.2. Chemistry Moisture, ash, and carbon analyses (the latter from the ultimate analysis) were conducted at CAER following the appropriTable 2 Techniques summary Technique Scale Advantages Limitations Optical microscopy Lower limit of a few microns Micron-scale and larger descriptions Not chemically based XRD Single to few percent Mineral determination Very low % minerals can be lost in background ICP techniques Whole sample Chemical analysis of whole samples Bulk analysis SEM-EDS Submicron Chemical analysis of specific areas Limited use for low-concentration trace elements HRTEM Few nanometers Chemical analysis of nanoscale areas Surface or thin sample technique SEM/FIB Nanometer High-precision chemical analysis; milling allows measurements in three dimensions Very small area may limit precision TEM Nanometer High-precision chemical analysis; milling allows measurements in three dimensions; XRD analysis Very small area may limit precision Note: XRD = X-ray diffraction; ICP = inductively couple plasma; SEM-EDS = scanning electron microscopy–energy dispersive X-ray spectroscopy; HRTEM = high-resolution transmission electron microscopy; SEM/FIB = scanning electron microscopy–focused ion beam; TEM = transmission electron microscopy. Hower et al. / Coal Combustion and Gasification Products 9 (2017) 3 Fig. 1. (A) View of April 2016 ash pond sampling sites. (B) Google Earth view of sampling site. ate American Society for Testing and Materials (ASTM) standards. Major oxide and minor element concentrations were quantified by X-ray fluorescence at the CAER following procedures outlined by Hower and Bland (1989). The REYSc elements, Hf, and Tl were extracted from the fly ash samples by heated digestion with a 1:1 HF:HNO3 acid mixture followed by analysis with inductively coupled plasma atomic absorption spectroscopy at the CAER. 4 Hower et al. / Coal Combustion and Gasification Products 9 (2017) Table 3 Sample locations, reported as north latitude and west longitude, for the 20 individual samples Sample MA no. Row Location N lat W long 93977 75645 1 1 37.87778 84.26228 93978 75646 1 2 37.87807 84.26238 93979 75647 1 3 37.87818 84.26252 93980 75648 1 4 37.87833 84.26260 93981 75649 1 5 37.87840 84.26280 93982 75650 2 1 37.87805 84.26267 93983 75651 2 2 37.87815 84.26275 93984 75652 2 3 37.87822 84.26282 93985 75653 2 4 37.87830 84.26285 93986 75654 2 5 37.87850 84.26287 93987 75655 3 1 37.87832 84.26207 93988 75656 3 2 37.87842 84.26208 93989 75657 3 3 37.87850 84.26220 93990 75658 3 4 37.87857 84.26220 93991 75659 3 5 37.87867 84.26223 93992 75660 4 1 37.87827 84.26218 93993 75661 4 2 37.87838 84.26228 93994 75662 4 3 37.87843 84.26233 93995 75663 4 4 37.87855 84.26235 93996 75664 4 5 37.87860 84.26237 Note: MA no. = materials analysis number; lat = latitude; long = longitude. 2.3. Petrology Fly ash petrology was performed on epoxy-bound pellets prepared to a final 0.05-μm alumina polish using 50×, reflected-light, oil-immersion optics on Leitz Orthoplan microscopes at the CAER following procedures described by Hower (2012). 2.4. Particle size analysis Particle size analysis was conducted on a Cilas 1090 laser particle size analyzer at the CAER using the liquid dispersion mode. The instrument has a measurement range of 0.04–500 μm. 2.5. X-ray diffraction mineralogy Selected samples were examined by powder X-ray diffraction (XRD) at the CAER. If necessary, the samples were ground by hand in a ceramic mortar and pestle just before XRD analysis. The powdered samples were then dry mounted in aluminum holders. The samples were scanned at 8–60◦ 2θ with copper K-α radiation on a Philips X’Pert diffractometer (model PW3040-PRO) operating at 45 kV and 40 mA. Crystalline substances or “minerals” were identified in the diffractograms with an International Centre for Diffraction Data (Newtown Square, PA) powder diffraction database. Further details on the XRD methodology are in the leaching report in Appendix A. 2.6. Transmission electron microscopy High-resolution transmission electron microscopy (HRTEM) was carried out at the National Institute for Occupational Safety and Health laboratory in Cincinnati, OH. The fly ash sample was deposited onto a carbon support film on a Cu TEM grid. TEM observations were made using an FEI Tecnai transmission electron microscope TF20 at 200 kV. Energy-dispersive X-ray spectroscopy analysis was carried out using an EDAX Genesis spectrometer. TEM was conducted on a JEOL 2100 field thermionic emission analytical electron microscope equipped with a silicon drift detector– based energy dispersive spectroscopy system for chemical mapping at the Virginia Tech National Center for Earth and Environmental Nanotechnology Infrastructure (NanoEarth), Blacksburg, VA. Initial isolation and separation of the specimen to be analyzed was made with an FEI Helios 600 NanoLab DualBeam scanning electron microscopy/focused ion beam (SEM/FIB). The specimen was milled to <100-nm thickness. 2.7. Beneficiation A representative portion of the composite sample collected from the ash pond was obtained for beneficiation into separate products. The first beneficiation step was to remove extraneous oversize (+60 mesh or 0.25 mm) material such as bottom ash by wet screening. The −0.25-mm material was diluted to 10 wt% solids and subjected to froth flotation to remove unburned carbon. Appropriate dosages of collector and frother were added to generate both hydrophobicity and air bubble surface area necessary for effective flotation; froth products were removed by hand scraping, and then were filtered and dried. Rougher and scavenger flotation stages were followed by a cleaner flotation step on the combined rougher/scavenger froth products. The cleaner tailings were c