An analytical electron microscopy investigation of municipal solid waste incineration bottom ash
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An analytical electron microscopy investigation of municipal solid waste incineration bottom ash James E. Krzanowski Mechanical Engineering Department, University of New Hampshire, Durham, New Hampshire
T. Taylor Eighmy and Bradley S. Crannell Environmental Research Group, University of New Hampshire, Durham, New Hampshire
J. Dykstra Eusden, Jr. Geology Department, Carnegie Hall, Bates College, Lewiston, Maine (Received 12 June 1996; accepted 7 July 1997)
Incinerator bottom ash samples have been characterized using analytical electron microscopy (AEM) techniques, including electron diffraction, energy dispersive spectroscopy, and electron energy loss spectroscopy. The samples were first separated by magnetic properties and density. Three resulting fractions were examined: the magnetic, high-density (MHD) fraction, the nonmagnetic/high-density (NMHD) fraction, and the nonmagnetic, low-density (NMLD) fraction. Examination of these samples revealed a variety of submicron microstructural features. For the MHD fraction, metal oxides, iron silicates, aluminum silicates, and calcium phosphate compounds were found in addition to amorphous material. The NMHD fraction contained elements similar to the MHD fraction but had more amorphous material; crystalline silicates were less common. Compounds such as MgO and chloroapatite were also found. The NMLD fraction contained SiO2 and numerous metal oxides. The results of some of these analyses were used to model leaching behavior of the ash. Based on the AEM results, three mineral phases were chosen as candidates for aqueous geochemical thermodynamic equilibrium modeling of pH-dependent leaching: chromite, chloroapatite, and zincite. In two of these three cases (chromite, chloroapatite), the selected mineral phase provided excellent agreement with the experimentally observed leaching behavior. AEM was shown to be a useful tool for elucidating mineralogy of complex environmental samples.
I. INTRODUCTION
The incineration of municipal solid waste (MSW) is one means of dealing with nonrecyclable solid waste. The benefits of MSW incineration are the reduction of solid waste volume and the opportunity to recover energy and use it for power generation in a waste-to-energy (WTE) facility. The by-products of the incineration process are bottom ash residues (material that resides on the grate of the incinerator after the combustion process is completed) and residues captured in air pollution control devices. Bottom ash is typically disposed of in specially engineered landfills, or used in construction applications.1 Leaching of constituents from bottom ash by infiltration of rainwater can release these constituents, some of which can be an environmental concern (e.g., salts and metals). In order to understand the leaching behavior, the chemical speciation of the ash must be understood. This involves not only knowledge of the elements present but also the nature of the compounds formed and the microstructure of the ash particles. Once speciat
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