MINERAL CHARACTERIZATION OF COAL COMBUSTION PRODUCTS USED AS STRUCTURAL FILL AND CAPPING MATERIAL AT AN ABANDONED COAL MINE IN SOUTHWESTERN INDIANA

Coal combustion products are used in abandoned mine land (AML) reclamation because of availability and acid-neutralization capacity. The potential for these materials to leach additional elements into groundwater has raised concerns regarding their use in these applications. In 1996, the Midwestern AML site in southwestern Indiana was reclaimed using a fixated scrubber sludge (FSS) cap composed of flue gas desulphurization sludge, fly ash, and lime. The cap was placed over a pyritic refuse deposit and ponded ash, a mixture of bottom ash and fly ash, which was used as structural fill in adjacent highwall ponds. X-ray diffractometry (XRD) was conducted on core samples containing FSS and/or ponded ash collected from the site in 2011 to characterize potential mineralogical changes since emplacement. The ponded ash is composed of glass with smaller amounts of feldspar, mullite, hematite, magnetite, and wollastonite. The FSS is composed of mostly glass and hannebachite with lesser amounts of gypsum, quartz, ettringite, hematite, magnetite, and calcite. Cores of FSS show a decrease in gypsum and increases in hannebachite and amorphous material with depth. An XRD analysis of FSS collected from the production facility in 1997 and preserved had a similar composition to FSS from the bottom of the cores collected from the field in 2011, suggesting that FSS higher in the core has undergone mineral transformations during the 15 years since reclamation. Mineralogy of ponded ash indicates little change with depth, possibly from leaching that occurred at the production facility prior to emplacement. Although FSS is relatively impermeable, a change in mineralogy with depth over time indicates FSS can potentially leach elements into groundwater when exposed to ambient conditions. To fully assess the net benefit of FSS in AML reclamation additional work will focus on elemental analyses of core samples and laboratory leaching experiments.