Coal mineralogy

Fourier transform infrared (FTIR) spectroscopy of coal low-temperature ashes was applied to the determination of coal mineralogy and the prediction of ash properties during coal combustion. Analytical methods commonly applied to the mineralogy of coal are critically surveyed. Conventional least-squares analysis of spectra was used to determine coal mineralogy on the basis of forty-two reference mineral spectra. The method described showed several limitations. However, partial least-squares and principal component regression calibrations with the FTIR data permitted prediction of all eight ASTM ash fusion temperatures to within 50 to 78 F and four major elemental oxide concentrations to within 0.74 to 1.79 wt % of the ASTM ash (standard errors of prediction). Factor analysis based methods offer considerable potential in mineral-ogical and ash property applications. 

Despite considerable effort to understand the chemical nature of the organic and inorganic portions of coal, and substantial advances in instrumentation and methodologies, much of coal s chemical nature remains intractable. This paper reviews methods and their limitations for the determination of specific minerals in coal, and presents results of efforts at Consolidation Coal Company to develop FTIR methods for routine coal mineralogy. 

This survey of five major methods for coal mineralogy and their limitations includes only methods which can provide a "complete" mineral analysis. Methods of limited applicability, such as Mossbauer, are omitted. 

X-rav Diffraction. XRD is the most common method used for coal mineralogy (6.7.8). Its major advantage is the ability to unequivocally identify many minerals. The main disadvantages are 1) reliance on reference minerals, 2) requires careful attention to sample preparation, and 3) low sensitivity to certain minerals (especially many clays) due to poor crystallinity and to particle orientation effects. Many laboratories analyze a separate concentrated clay... 

Infrared Spectroscopy. The use of IR (9.10.11.12) and FTIR (3.4) for coal mineralogy has been reported. Painter and coworkers (3) demonstrated that FTIR can provide a virtually complete analysis. Painter, Brown and Elliott (4), and others (9.10.11) discuss sample preparation, reference minerals, and data analysis. The advantages of IR are 1) high sensitivity to molecular structure, 2) unequivocal identification of a number of minerals, 3) small sample size (a few milligrams), and 4) rapid analysis time (once LTA is available). Disadvantages include 1) reliance on reference minerals, 2) requires careful attention to sample preparation, and 3) limited selectivity (discrimination among similar minerals). 

The next section of this paper describes the use of classical least-squares analysis of FTIR data to determine coal mineralogy. This is followed by promising preliminary results obtained using factor analysis techniques. 

Experience in this laboratory has shown that even with careful attention to detail, determination of coal mineralogy by classical least-squares analysis of FTIR data may have several limitations. Factor analysis and related techniques have the potential to remove or lessen some of these limitations. Calibration models based on partial least-squares or principal component regression may allow prediction of useful properties or empirical behavior directly from FTIR spectra of low-temperature ashes. Wider application of these techniques to coal mineralogical studies is recommended. 

Hubbard, F. H., McGill, R. J., Dhir, R. K. Ellis, M. S. 1984. Clay and pyrite transformations during ignition of pulverized coal. Mineralogical Magazine, 48, 251-256. 

Information on coal mineralogy has been used primarily to assess the technological performance... 

Birk D. (1989) Quantitative coal mineralogy of the Sydney Coalfield, Nova Scotia, Canada, by scanning electron microscopy, computerized image analysis, and energy-dispersive X-ray spectrometry. Can. J. Earth Sci. 27, 163-179. 

Although particulate methods are potentially more informative than bulk methods because of the extra information obtained on particle size, their application in coal mineralogy has been relatively limited The main reason for this has been the lack of automation, as such microscopic measurements are time-consuming and tedious if sufficient data have to be generated manually Recently, however, the application of microcomputers and image-analysis systems to microscopy has progressed to the extent that most of the operation, data collection, and analysis can now be done automatically A number of reports of such automated microscope techniques used in coal research have appeared recently in the literature (3-8) ... 

In this paper we describe the use of a scanning electron microscope (SEM) equipped with automatic-image-analysis (AIA) capability for the determination of coal mineralogies Because one of the main limitations of the SEM-AIA technique is its inability to distinguish the various iron-bearing minerals, Fe Mdssbauer spectroscopy is used to supplement data from SEM-AIA with respect to iron-bearing minerals The combination of these two techniques usually provides a detailed, quantitative characterization of the minerals in coal ... 

Coal in the gronnd does not generally pose an environmental threat although coal mineralogy (Chapter 7) can influence groundwater properties and in fires in the coal seams can lead to subsidence, contamination of groundwater, and health and safety issues related to heat and emissions from the fires. 

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