Scanning Microscopy


Research at the University of North Dakota Energy and Environmental Research Center (EERC) has focused on methods to characterize the inorganic components in coals. Because the scanning electron microscope and electron probe microanalysis system (SEM/EPMA) provide both morphologic and chemical information, the SEM/EPMA system is well-suited to the characterization of discrete minerals in coal. Computer-controlled scanning electron microscopy (CCSEM), along with simultaneous automated digital image collection, is one means of gaining more detailed insight into coal mineralogy. Computer-stored images of coal surfaces already analyzed for minerals using CCSEM can be reanalyzed to discern mineral morphologies and coal-to-mineral associations. Limitations may exist when using just CCSEM to characterize chemically and physically complex clay minerals without complimentary data on the association of the minerals to the coal organic matrix. Mineralogic investigations of San Miguel and Beulah lignites and Upper Freeport bituminous coal using CCSEM and automated digital image collection are given with a particular reference to the clay minerals present. Total mineral quantities generated for the three coals were in good agreement with total ash content, provided that organically bound constituents were taken into account for the lignites. Classification of the more complex aluminosilicate minerals was aided by the use of distribution plots of Si/Al ratios and concentrations of ion exchangeable cations derived from the CCSEM analysis. Morphologic analysis of stored SEM images proved to be helpful in characterizing kaolinite group minerals.

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