Analysis of Mineral Matter in Coal

Understanding the type of original mineral matter in the coal can sometimes be the key to finding the ultimate cause for fouling and depositions in the process. There are several methods to identify the minerals in the parent coal  

By the temperature of appearance, the mineral decomposition reaction can be identified. Heating and cooling a sample in a DTA may even reveal much more information because transformation reactions of the certain compounds also can be detected (e.g., quartz). 

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We will then consider how such routines can be applied to the quantitative determination of specific functional groups in coal. Published work concerning the analysis of mineral matter in coal... 

FTIR has been used successfully in the qualitative and quantitative analysis of mineral matter in coal (1,5,13) The... 

Straszheim, W.E. Markuszewski, R. Application of Scanning Electron Microscopy and Automated Image Analysis for Characterization of Mineral Matter in Coal, Am. Chem. Soc. Div. Fuel Chem. Preprints, 1985, 30(1), 47-55. 

Ode, W. H., Coal Analysis and Mineral Matter in Chemistry of Coal Utlization, Supplementary Volume (H. H. Lowry, Ed.), p. 202-231, John... 

The results of the carbon and hydrogen analysis may be reported on any number of bases, differing from each other in the manner by which moisture values are treated. Inclusion of the hydrogen of moisture and water of hydration of mineral matter in the hydrogen value is common practice for the as-determined and as-received bases. Hydrogen values on a dry coal basis, however, are commonly corrected for the hydrogen of moisture. No corrections are normally made to the determined hydrogen value for the water of hydration of mineral matter, due to the uncertainty of the estimate of its value. 

Determination of a good value for the percent of mineral matter content (% MM) is a very important component of coal analysis. If this quantity cannot be determined directly by the acid demineralization or low-temperature ashing procedure discussed previously, or by other suitable methods, it is possible to calculate a reasonable value for the mineral matter in coal, provided that the necessary data are available. 

The evaluation of coal mineral matter by the ashing technique can be taken further insofar as attempts can then be made to determine the individual metal constituents of the ash. On the occasion when the mineral matter has been separated from the coal successfully, it is then possible to apply any one of several techniques (such as x-ray diffraction, x-ray fluorescence, scanning electron microscopy and electron probe microanalysis) not only to investigate the major metallic elements in coal but also to investigate directly the nature (and amount) of the trace elements in the coal (Jenkins and Walker, 1978 Prather et al., 1979 Raymond and Gooley, 1979 Russell and Rimmer, 1979 Jones et al., 1992). Generally, no single method yields a complete analysis of the mineral matter in coal and it is often necessary to employ a combination of methods. 

Given, P. H., and Yarzab, R. F., Analysis of the Organic Substance of Coals Problems Posed by the Presence of Mineral Matter, in Analytical Methods for Coal and Coal Products, Vol. 11, C. Karr, Jr. (Ed.), pp. 3-41, Academic Press, Inc., New York, 1978. 

A few years ago, an ad hoc group of workers interested in coal minerals. The Mineral Matter in Coal Group, prepared and distributed a round-robin low temperature ash to ten laboratories. Each laboratory was to prepare, mount and quantify the mineral components in the ash by their respective XRD techniques. The data were then compared. Even though a wide variety of techniques was used for each phase of the analysis, with the exception of the clay mineral estimates made by one laboratory (significantly lower than the others) and the pyrite estimate made by another (too high), the data compared reasonably well. The averages of all the submitted estimates are summarized in Table III. 

Silicate Minerals in Coal. The silicate minerals, kaolinite and potassium aluminosilicate species together with quartz constitute the bulk of mineral matter in most coals. The approximate amounts of different silicate species of the bituminous coal mineral matter can be estimated from ash analysis. 

Mineral matter in coal is often determined indirectly with the ash analysis (determined by direct combustion of the sample) (Chapter 8) forming the basis of the calculation. However, determination and chemical analysis of the ash content of coal gives the average content of the inorganic elements in a particular coal but is not an indication of the nature or distribution of the mineral matter in coal. Nevertheless, ash analysis can provide valuable data which, when used with data from other sources, may give a representation of the mineral content of coal. 

Analysis of coal-mineral association in samples of Upper Freeport and Indiana No. 3 coals showed significant differences in the association of minerals with the coal matrix. In terms of bulk properties, the mineral matter was generally more liberated in the Upper Freeport coal than in the Indiana No. 3 coal. Also, based on bulk properties, pyrite was found to be preferentially liberated in the Upper Freeport coal, while no such preferential liberation was observed for minerals in the Indiana No. 3 coal. For both coals, the... 

Air drying removes most of the surface moisture of coal, while a temperature of approximately 107°C (225°F) is needed to remove inherent moisture. At temperatures of approximately 200 to 300°C (392 to 572°F), moisture from the decomposition of organic materials is driven off, but water of hydration requires a considerable amount of energy for expulsion. For example, the water of hydration in clay minerals may require a temperature in excess of 500°C (932°F). However, the issues of decomposition moisture and water of hydration of mineral matter are not usually dealt with in conventional analysis because the temperatures specified in the test methods for moisture determination are well below those needed to remove such moisture. 

It is therefore impossible to determine accurately the composition of the pure coal substance from the usual ultimate analysis simply by making allowance for the quantity of ash left behind as a residue when the coal is burned. Results obtained in this fashion are, as a consequence, quoted as being on a dry, ash-free basis, and no claim is therefore made that these results do in fact represent the composition of the pure coal substance. If, however, it were possible to calculate accurately the quantity of mineral matter originally present in the coal sample, then by making due allowance for this material, the composition of the pure coal material could be deduced with reasonable precision and certainly with a greater accuracy than could be obtained by adopting the analytical figures calculated to a dry, ash-free basis. 

An ultimate analysis that can claim to represent the composition of the organic substance of a coal is said to be on a dry, mineral-matter-free (dmmf) basis. The dmmf basis is a hypothetical condition corresponding to the concept of a pure coal substance. Since the dry, ash-free basis for coal neglects changes in mineral matter when coal is burned, the dmmf basis is preferred whenever the mineral matter can be determined or calculated. 

ASTM method D3175 (H), and forms of sulfur by ASTM method D2492 (21). Elemental analysis of the ash was performed using ASTM method D3682 (22). Carbon aromaticity was determined using 13c NMR CP-MAS procedures described elsewhere (13). X-ray powder diffraction analysis of the mineral matter in the whole coal was performed using a Rigaku powder dif-... 

When the product coal shown in Figure 3 was subjected to low temperature ashing as described above and that ash product subjected to particle size analysis, a curve as is illustrated in Figure 4 results. Clearly the enveloped mineral matter in the product coal particles is considerably smaller in diameter than the coal particles from which they came and as such are not available for separation by the T-Process. The T-Process separation rejects all particles of pure mineral matter and agglomerates as product coal all particles that have any fraction of coal exposed to the liquid system. 

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