Residues From Coal Conversion and Utilization: Advanced Mineralogical Characterization and Disposed Byproduct Diagenesis

Abstract

Prior to the initiation of this study, understanding of the long-term behavior of environmentallyexposed Coal Combustion By-Products (CCBs) was lacking in (among others) two primary areas addressed in this work. First, no method had been successfully applied to achieve full quantitative analysis of the partitioning of chemical constituents into reactive or passive crystalline or noncrystalline compounds. Rather, only semi-quantitative methods were available, with large associated errors. Second, our understanding of the long-term behavior of various CCBs in contact with the natural environment was based on a relatively limited set of study materials. This study addressed these areas with two objectives, producing 1) a set of protocols for fully quantitative phase analysis using the Rietveld Quantitative X-ray Diffraction (RQXRD) method and 2) greater understanding of the hydrologic and geochemical nature of the long-term behavior of disposed and utilized CCBs. The RQXRD technique was initially tested using 1) mixtures of National Institute of Standards and Technology (NIST) crystalline standards, and 2) mixtures of synthetic reagents simulating various CCBs, to determine accuracy and precision of the method, and to determine the most favorable protocols to follow in order to efficiently quantify multi-phase mixtures. Four sets of borehole samples of disposed or utilized CCBs were retrieved and analyzed by RQXRD according to the protocols developed under the first objective. The first set of samples, from a Class F ash settling pond in Kentucky disposed for up to 20 years, showed little mineralogical alteration, as expected. The second set of samples, from an enbankment in Indiana containing a mixture of chain-grate (stoker) furnace ash and fluidized bed combustion (FBC) residues, showed formation of the mineral thaumasite, as observed in previously studied exposed FBC materials. Two high-calcium CCBs studied, including a dryprocess flue gas desulfurization (FGD) by-product disposed in the Midwest, and a mixture of Class C fly ash and wet process FGD by-product codisposed in North Dakota, appeared relatively unchanged mineralogically over the up to 5 and 17 years of emplacement, respectively. Each of these two materials contained mineralogies consistent with short-term hydration products of their respective starting (dry) materials. The hydration product ettringite persisted throughout the duration of emplacement at each site, and the diagenetic ash alteration product thaumasite did not form at either site. Explanations for the absence of thaumasite in these two sites include a lack of significant carbonate, sulfate, and alkalinity sources in the case of the North Dakota site, and a lack of sulfate, alkalinity, and sufficient moisture in the Midwest site. Potential for future thaumasite formation in these materials may exist if placed in contact with cold, wet materials containing the missing components listed above. In the presence of the sulfite scrubber mineral hannebachite, the ettringites formed had ...