Methods for Characterization of Inorganic and Mineral Matter in Coal

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Energy & Fuels 2003, 17, 271-281

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Methods for Characterization of Inorganic and Mineral Matter in Coal: A Critical Overview Stanislav V. Vassilev† and Juan M. D. Tasco´n*,‡ Central Laboratory of Mineralogy and Crystallography, Acad. G. Bonchev Str., Bl. 107, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, and Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain Received May 22, 2002

The present state of the methods commonly used for inorganic and mineral matter characterization in coal is described and summarized. The application of various separation procedures, macroscopic observations, reflected and transmitted optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, differential thermal and thermogravimetric analyses, Mo¨ssbauer spectroscopy, infrared spectroscopy, and chemical analyses are briefly discussed. A short critical overview on the advantages and limitations of the above listed methods as well as some recommendations during their utilization are also presented.

Introduction Coal, as a sedimentary rock, is a complex heterogeneous mixture of organic and inorganic constituents containing intimately mixed solid, liquid, and gaseous phases with forming (>10%), major (1-10%), minor (0.1-1%), or accessory ( IM > MM. However, lower values of ash yields than IM and even MM contents in coal are commonly detected in the high-temperature ash (HTA) produced at 500-815 °C as a result of advanced decomposition of original coal minerals (clay minerals, sulfides, and certain carbonates and sulfates), loss of adsorbed and combined water, and partial volatilization of elements such as Br, C, Cl, Hg, N, Se, S, and probably Ag, As, Be, Cd, Co, F, Ge, Mo, Ni, Pb, Sb, Sn, Te, Tl, V, (5) Given, P. H.; 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; Karr, C., Jr., Ed.; Academic Press: New York, 1978; Vol. 2, Chapter 20, pp 3-41. (6) Kiss, L. T.; King, T. N. Reporting of low-rank coal analysissthe distinction between minerals and inorganics. Fuel 1979, 58, 547-549. (7) Huggins, F. E.; Srikantapura, S.; Parekh, B. K.; Blanchard, L.; Robertson, J. D. XANES spectroscopic characterization of selected elements in deep-cleaned fractions of Kentucky No. 9 coal. Energy Fuels 1997, 11, 691-701.

10.1021/ef020113z CCC: $25.00 © 2003 American Chemical Society Published on Web 01/11/2003

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Zn, and others at these incineration temperatures. The ash yields of coal can be measured routinely, while the determination of IM and MM contents in coal as well as the isolation of IM and MM from coal is a complex procedure. Thus, the statement3 that “a reliable estimation of the IM and MM contents in coal can be quickly and routinely achieved” is an illusion (see below). The detailed knowledge on IM and MM in coal is important in the following: (1) determination of content, concentration/depletion trends, distribution, migration, and modes of occurrence of elements, minerals, and phases in coal; (2) elucidation of genesis, environment of deposition, and source material for coal deposits; (3) identification and correlation of coal seams; (4) prospection to locate economic mineral resources in coal deposits and source areas; (5) characterization and evaluation of economically valuable components occurring in coal products; (6) elucidation of formation and behavior of coal processing products; (7) prediction, elucidation, reduction, or elimination of various problems resulting from mining, transportation, preparation, beneficiation, storage, pyrolysis, gasification, liquefaction, and combustion of coal, namely: • technological problems such as processing of inert material, oxidation, autogenous heating (spontaneous coal ignition), swelling, grinding, agglomeration, abrasion-erosion, corrosion, slagging, and fouling; • global and local environmental impacts such as pollution of the air, water, soil, and plants by gas, liquid, and solid emissions containing, e.g., dust, fly ash, acidic or alkaline solutions, and toxic and potentially toxic elements and compounds. Hence, the methods for characterization of IM and MM in coal have a great importance to elucidate most of the above-listed directions. Detailed or summarized data on some methods commonly used for characterization of MM in coal have been reported earlier.8-17 The (8) Jenkins, R. G.; Walker, P. L., Jr. Analysis of mineral matter in coal. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1978; Vol. 2, Chapter 26, pp 265292. (9) Finkelman, R.; Gluskoter, H. Characterization of minerals in coal: problems and promises. In Proceedings of the 1981 Engineering Foundation Conference “Fouling and Slagging Resulting from Impurities in Combustion Gases”; Bryers, R., Ed.; Henniker: New Hampshire, 1981; pp 299-318. (10) Mraw, S. C.; De Neufville, J. P.; Freund, H.; Baset, Z.; Gorbaty, M. L.; Wright, F. J. The science of mineral matter in coal. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1983; Vol. 2, pp 1-63. (11) Raask, E. Mineral Impurities in Coal Combustion; Hemisphere: Washington, DC, 1985; 484 pages. (12) Ward, C. R. Review of mineral matter in coal. Australian Coal Geol. 1986, 6, 87-110. (13) Nayak, R.; Bauer, F. W.; Tonden, T. P. Mineral matter in coals origin, identification, high-temperature transformation, and boiler erosion. J. Coal Qual. 1987, 6, 37-43. (14) Korobetskii, I. A.; Shpirt, M. Ya. Genesis and Properties of the Coal Mineral Components (in Russian); Nauka: Novosibirsk, 1988; 227 pages. (15) Martı´nez-Alonso, A.; Martı´nez-Tarazona, M. R.; Tasco´n, J. M. D. Mineral matter in Spanish bituminous and brown coals. Part. 1. Development of methodology and identification of inorganic constituents. Erd. Kohle Erdgas Petrochem. 1992, 45, 121-128. (16) Martı´nez-Tarazona, M. R.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Mineral matter in Spanish bituminous and brown coals. Part. 2. Mineral matter quantification. Erd. Kohle Erdgas Petrochem. 1993, 46, 202-209. (17) van Krevelen, D. W. Coal. Typology-Physics-Chemistry-Constitution, 3rd ed.; Elsevier: Amsterdam, 1993; pp 137-142.

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purpose of the present report is to extend this knowledge and to provide additional information on the recent state of the methods commonly used. A short critical overview on the advantages and limitations as well as some recommendations for the application of different methods for IM-MM characterization of coal are also done. Macroscopic Observations The macroscopic observations of the samples are the first step during the IM-MM investigation of coal. This type of observation describes some characteristics such as color, hardness, visible texture, massive inclusions and partings, friability, cracking, porosity, weathering, and type of mineralization (inherent or extraneous) of the specimens and gives some preliminary information that can be helpful for the subsequent studies. For example, some coarse-grained detrital (quartz, feldspars), syngenetic (Fe carbonate and sulfide concretions), and epigenetic (gangue carbonates, sulfates and sulfides) minerals as well as silicified wood and volcanic ash material are occasionally present as visible crystals, grains, aggregates, layers, lenticles, bands, nodules, coatings, or cleat infillings and can be recognized, described, and even isolated for future investigations by different methods. Separation Procedures The ashing procedures, physical separations, and chemical leaching of coal are very important initial tools because they isolate relatively “bulk” IM, MM, or fractions enriched in different minerals and inorganic phases. So, the subsequent identification and characterization of the minerals and phases by various methods is more effective, complete, and reliable. However, the separation procedures alter more or less irreversibly the IM and MM of coal samples and the results should be interpreted with caution. Ashing Procedures. Low-Temperature Ashing. The most widely used method for removing OM from coal and concentrating IM and MM in a low-temperature ash (LTA) is the oxygen plasma ashing at 150200 °C.8,12,14-16,18-56 Subsequently, the investigation of this IM residue by various chemical and mineralogical methods is conducted more easily than for the bulk coal samples. However, it should be stated that the LTA yield is not the actual IM or MM of coal because the LTA is derived from both IM and OM in coal, and inorganic compounds not originally present in coal (in particular for lignites) may be formed. For instance, portions of some organically bound elements in coal (Al, Ba, C, Ca, Fe, K, Mg, N, Na, S, Si, Ti) produce inorganic phases in the LTA residue (see below). It is an illusion that this ashing supplies a relatively unaltered MM. Unfortunately, various unwanted transformations and reactions occur during this low-temperature treatment.1 They may include some oxidation of Fe sulfides (pyrite, marcasite, pyrrhotite) and Fe sulfates (Fe2+-bearing coquimbite, jarosite, and roemerite to Fe3+-bearing szomolnokite, rozenite, and melanterite), total or partial dehydration of water-containing minerals (gypsum, hexahydrite, Fe hydrosulfates, montmorillonite), formation of various artificial sulfates (bassanite, anhydrite, Fe hydrosulfates, barite), nitrates of Na and Ca, carbon-

Characterization of Inorganic and Mineral Matter in Coal

ates (calcite, magnesite, dolomite), silicates (clay minerals, zeolites), Ca-, Mg-, Fe-, Ti-oxyhydroxides, and amorphous matter, as well as incomplete oxidation of OM and some loss of highly volatile elements (Br, C, Cl, Hg, N, Se, S, others). For example, up to 92% of Cl,20 80% of Hg, 30% of Se, and at least 90% of Br are lost, while the X-ray amorphous material can be as much as 20% in some LTA samples.41 Thus, a number of original minerals in coal experience varying degrees of induced alteration (to some extent similar to natural weathering) during this type of ashing. The new inorganic and mineral formations are due to the crystallization of (18) Gleit, C. E.; Holland, W. D. Use of electrically excited oxygen for the low-temperature decomposition of organic substances. Anal. Chem. 1962, 34, 1454-1457. (19) Gleit, C. E. Electronic apparatus for ashing biological specimens. Am. J. Med. Electron. 1963, 2, 112-118. (20) Gluskoter, H. J. Electronic low-temperature ashing of bituminous coal. Fuel 1965, 44, 285-291. (21) O’Gorman, J. V.; Walker, P. L., Jr. Mineral matter characteristics of some American coals. Fuel 1971, 50, 135-151. (22) Frazer, F. W.; Belcher, C. B. Quantitative determination of the mineral-matter content of coal by radio frequency-oxidation technique. Fuel 1973, 52, 41-46. (23) O’Gorman, J. V.; Walker, P. L., Jr. Thermal behaviour of mineral fractions separated from selected American coals. Fuel 1973, 52, 71-79. (24) Augenstein, D. A.; Sun, S. C. Characterization of coal refuse by low-temperature ashing. Trans. Soc. Mining Eng. AIME 1974, 256, 161-166. (25) Ward, C. R. Mineral matter in Australian bituminous coals. Proc. Aust. Inst. Min. Metall. 1978, 267, 7-25. (26) Miller, R. N.; Yarzab, R. F.; Given, P. H. Determination of the mineral-matter contents of coals by low-temperature ashing. Fuel 1979, 58, 4-10. (27) Sanyal, A.; Williamson, J. Slagging in boiler furnaces: An assessment technique based on thermal behaviour of coal minerals. J. Inst. Energy 1981, 54, 158-162. (28) Guilianelli, J. L.; Williamson, D. L. Comparison of microwave and radio frequency low-temperature ashing of coal using 57Fe Mo¨ssbauer spectroscopy of sulphur forms. Fuel 1982, 61, 1267-1272. (29) Liu, K. H. D.; Johannes, A. H.; Hamrin, C. E. Characterization and catalytic activity of coal mineral matter. 1. Effect of hydrogen pretreatment and exposure to sulphur and nitrogen compounds. Fuel 1984, 63, 18-23. (30) Palmer, C. A.; Filby, R. H. Distribution of trace elements in coal from the Powhatan No. 6 mine, Ohio. Fuel 1984, 63, 318-328. (31) Adolphi, P.; Sto¨rr, M. Glow discharge excited low-temperature ashing. A new technique for separating mineral matter of coals. Fuel 1985, 64, 151-155. (32) Goodarzi, F.; Foscolos, A. E.; Cameron, A. R. Mineral matter and elemental concentrations in selected Western Canadian coals. Fuel 1985, 64, 1599-1605. (33) Allen, R. M.; Carling, R. W.; VanderSande, J. B. Microstructural changes in coal during low-temperature ashing. Fuel 1986, 65, 321326. (34) Korobetskii, I. A.; Podolskii, A. P.; Zaostrovskii, A. N.; Balabanova, N. V. Isolation of the mineral fraction from coals and carbonaceous rocks (in Russian). Khim Tverd. Topl. 1986, 20, 117121. (35) Unuma, H.; Takeda, S.; Tsurue, T.; Ito, S.; Sayama, S. Studies of the fusibility of coal ash. Fuel 1986, 65, 1505-1510. (36) Huggins, F. E.; Huffman, G. P.; Dunmyre, G. R.; Nardozzi, M. J.; Lin, M. C. Low-temperature oxidation of bituminous coal: its detection and effect on coal conversion. Fuel Process. Technol. 1987, 15, 233-244. (37) Beaton, A. P.; Goodarzi, F. The geochemistry and petrography of lignites from Southern Saskatchewan, Canada. J. Coal Qual. 1989, 8, 110-117. (38) Pike, S.; Dewison, M. G.; Spears, D. A. Sources of error in lowtemperature plasma ashing procedures for quantitative mineral analysis of coal ash. Fuel 1989, 68, 664-668. (39) Ward, C. R. Minerals in bituminous coals of the Sydney basin (Australia) and the Illinois basin (U.S.A.). Int. J. Coal Geol. 1989, 13, 455-479. (40) Burchill, P.; Richards, D. S.; Warrington, S. B. A study of the reactions of coals and coal minerals under combustion-related conditions by thermal analysis-mass spectrometry and other techniques. Fuel 1990, 69, 950-956. (41) Finkelman, R. B.; Palmer, C. A.; Krasnow, M. R.; Aruscavage, P. J.; Sellers, G. A.; Dulong, F. T. Combustion and leaching behavior of elements in the Argonne Premium Coal samples. Energy Fuels 1990, 4, 755-766.

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liberated components from OM, inorganic amorphous constituents, and moisture, as well as the recrystallization or amorphization of preexisting minerals and phases in coal.1 Hence, the LTA yield can be closer to the actual ash yield than to the real IM and MM contents of coal. The resultant LTA residue only partly represents the original minerals in coal, and the data from investigations of LTA should always be accompanied by parallel direct investigations of the coal samples in situ. Another obvious limitation is that the actual microtextural and microstructural relationships between the OM and IM are absent from this residue and they cannot be observed. Additionally, the LTA production is a time-consuming process (up to several days) and yields a limited amount of ash (normally about several hundred milligrams). This ashing also requires special equipment, sample preparation procedures, and experimental conditions. Despite the above listed limitations, the LTA treatment is still a useful and subsidiary technique for the concentration of IM from coal for subsequent chemical analyses, and to a lesser extent for investigations of more inert minerals (certain silicates, carbonates, oxides, phosphates) at these ashing conditions. High-Temperature Ashing. The high-temperature ashing, performed commonly at 500-815 °C, is a routine procedure for the combustion of OM and determination of ash yield during each coal examination. This inorganic residue is also used for chemical analyses of the coal ash and occasionally for some phase and mineral identifications (amorphous material and inert silicates, oxides, and phosphates). However, the chemical and mineralogical characterization of the IM-MM in HTA samples should be always interpreted with much caution. This is due to the above-described limitations, namely volatilization of elements and alteration, decomposition, transformation, crystallization, recrystallization, and amorphization of the minerals and phases originally present in coal, but at a higher level of change than in LTA.1,8,11,12,14,16,21,23,25 Hence, the HTA and LTA residues are complex inorganic mixtures including the following: (1) primary minerals (preexisting minerals in coal that have not undergone phase transformations), mainly inert silicates, oxides, and phosphates; (2) secondary phases (new phases formed during ashing), namely some silicates, oxides, carbonates, sulfates, and amorphous phases; and (3) tertiary minerals (new minerals formed during HTA and LTA storage), i.e., certain sulfates, carbonates, and hydroxides.1 Physical Separations. The density fractionation of crushed coal in heavy liquids (chemically inert to coal constituents) is a very important procedure for the concentration and subsequent characterization of IM and MM in coal.1,7,15,16,30,40,45,52,57-72 For example, the coal fractions separated by heavy liquids with density (42) Martı´nez-Tarazona, M. R.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Interactions between carboxyl groups and inorganic elements in Spanish brown coals. Fuel 1990, 69, 362-367. (43) Beaton, A. P.; Goodarzi, F.; Potter, J. The petrography, mineralogy and geochemistry of a Paleocene lignite from Southern Saskatchewan, Canada. Int. J. Coal Geol. 1991, 17, 117-148. (44) Garcı´a, A. B.; Moinelo, S. R.; Martı´nez-Tarazona, M. R.; Tasco´n, J. M. D. Influence of weathering process on the flotation response of coal. Fuel 1991, 70, 1391-1397. (45) Mohan, M. S.; Ilger, J. D.; Zingaro, R. A. Speciation of uranium in a South Texas lignite: Additional evidence for a mixed mode of occurrence. Energy Fuels 1991, 5, 568-573.

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2.9 g/cm3 are normally the most useful because they are related, respectively, to the enrichment of OM, some rock-forming minerals and phases (quartz, feldspars, clay and mica minerals, calcite, magnesite, dolomite, ankerite, gypsum, volcanic glass), and heavy accessory minerals (As, Cu, Pb and Zn sulfides, Al, Cr, Fe and Ti oxides, zircon, garnet, Fe sulfates, barite, celestine, anglesite, witherite, strontianite, smithsonite, apatite, monazite, xenotime, schee(46) Ward, C. R. Mineral matter in low-rank coals and associated strata of the Mae Moh basin, Northern Thailand. Int. J. Coal Geol. 1991, 17, 69-93. (47) Martı´nez-Tarazona, M. R.; Spears, D. A.; Palacios, J. M.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Mineral matter in coals of different rank from the Asturian Central basin. Fuel 1992, 71, 367372. (48) Douchanov, D.; Minkova, V.; Martı´nez-Alonso, A.; Palacios, J. M.; Tasco´n, J. M. D. Low-temperature ashing of Bulgarian lignites. Erd. Kohle Erdgas Petrochem. 1993, 46, 461-466. (49) Irdi, G. A.; Booher, H. B.; Martello, D. V.; Frommell, E. A.; Gray, R. J. The petrography and mineralogy of two Indian coals. Fuel 1993, 72, 1093-1098. (50) Ogunsola, O.; Lam, W. Mineralogical composition of Nigerian coals. Fuel Sci. Technol. Int. 1993, 11, 1319-1329. (51) Shirazi, A. R.; Lindqvist, O. An improved method of preserving and extracting mineral matter from coal by very low-temperature ashing (VLTA). Fuel 1993, 72, 125-131. (52) Spears, D. A.; Martı´nez-Tarazona, M. R. Geochemical and mineralogical characteristics of a power station feed-coal, Eggborough, England. Int. J. Coal Geol. 1993, 22, 1-20. (53) Williamson, J.; West, S. S.; Laughlin, M. K. The behaviour of bed material during fluidized bed gasification: The effects of mineral matter interactions. Fuel 1994, 73, 1039-1045. (54) Martı´nez-Tarazona, M. R.; Spears, D. A. The fate of trace elements and bulk minerals in pulverized coal combustion in a power station. Fuel Process. Technol. 1996, 47, 79-92. (55) Sentorun, C.; Schobert, H. H. Determination of the mineral matter contents of some Turkish lignites by low-temperature ashing. In Proc. 9th Intern. Conf. Coal Science; Ziegler, A., van Heek, K. H., Klein, J., Wanzl, W., Eds.; P&W Druck und Verlag GmbH: Essen, Germany, 1997; Vol. 1, pp 401-404. (56) Helle, S.; Alfaro, G.; Kelm, U.; Tasco´n, J. M. D. Mineralogical and chemical characterisation of coals from Southern Chile. Int. J. Coal Geol. 2000, 44, 85-94. (57) Rekus, A. F.; Haberkorn, A. R. Identification of minerals in single particles of coal by the X-ray powder method. J. Inst. Fuel 1966, 39, 474-477. (58) Dixon, K.; Skipsey, E.; Watts, J. T. The distribution and composition of inorganic matter in British coals. Part 3: The composition of carbonate minerals in the coal seams of the East Midlands Coalfields. J. Inst. Fuel 1970, 43, 229-233. (59) Paulson L. E.; Beckering, W.; Fowkes, W. W. Separation and identification of minerals from Northern Great Plains Province lignite. Fuel 1972, 51, 224-227. (60) Gluskoter, H. J.; Ruch, R. R.; Miller, W. G.; Cahill, R. A.; Dreher, G. B.; Kuhn, J. K. Trace elements in coal: Occurrence and distribution. Circular 499; Illinois State Geological Survey: Urbana, IL, 1977; 154 pages. (61) Fowkes, W. W. Separation and identification of minerals from lignites. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1978; Vol. 2, Chapter 27, pp 293314. (62) Shpirt, M. Ya. The mineral components of coals (in Russian). Khim. Tverd. Topl. 1982, 16, 35-43. (63) Harvey, R. D.; DeMaris, P. J. Size and maceral association of pyrite in Illinois coals and their float-sink fractions. Org. Geochem. 1987, 11, 343-349. (64) Martı´nez-Tarazona, M. R.; Palacios, J. M.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. The characterization of organomineral components of low-rank coals. Fuel Process. Technol. 1990, 25, 81-87. (65) Martı´nez-Tarazona, M. R.; Spears, D. A.; Tasco´n, J. M. D. Organic affinity of trace elements in Asturian Bituminous coals. Fuel 1992, 71, 909-917. (66) Bettelheim, J.; Dillon, A. N. The response of British powerstation coals to density separation. J. Inst. Energy 1992, 65, 160-165. (67) Ruppert, L. F.; Minkin, J. A.; McGee, J. J.; Cecil, C. B. An unusual occurrence of arsenic-bearing pyrite in the Upper Freeport coal bed, West-Central Pennsylvania. Energy Fuels 1992, 6, 120-125. (68) Querol, X.; Ferna´ndez-Turiel, J. L.; Lo´pez-Soler, A. The behaviour of mineral matter during combustion of Spanish subbituminous and brown coals. Mineralogical Magazine 1994, 58, 119-133. (69) Vassilev, S. V.; Yossifova, M. G.; Vassileva, C. G. Mineralogy and geochemistry of Bobov Dol coals, Bulgaria. Int. J. Coal Geol. 1994, 26, 185-213.

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lite) plus certain heavy Fe sulfides and carbonates (pyrite, marcasite, pyrrhotite, siderite). Naturally, in most cases these minerals are finely dispersed in coal and intimately associated with OM in particles and they are difficult to separate in heavier concentrates and commonly pass to lighter fractions (clay minerals, sulfates, carbonates, and occasionally sulfides). On the other hand, the application of gravity methods for some lignites shows that the separation of minerals is relatively ineffective.61 Various accessory minerals of As, Au, Ba, Ce, Cl, Cr, Cu, F, Hf, La, Mn, Mo, N, P, Pb, Sb, Sn, Sr, Ti, U, V, W, Y, Zn, and Zr were identified as discrete and finely dispersed species (0.5-1 µm can be observed and analyzed and the distribution of elements (“elemental map”) in phases may also be conducted and photographed. The grains of minerals containing heavy elements are easily distinguished from the OM and silicates in backscattered electron image. Thus, this method is an irreplaceable tool for identification, characterization, and microtextural and/or microstructural

Vassilev and Tasco´ n

observations in situ of the minerals and phases present in coal. The relationships between the minerals and macerals are easily recognized, and genetic interpretations are common. The sample preparation technique is relatively simple and rapid, including the use of polished or unpolished blocks, pellets, sections, powder, and grains, depending on the samples. A disadvantage of SEM-EDX is that elements with atomic weights below that of Na cannot be detected. This includes problems with the identification of important coal minerals such as hydroxides, hydrates, borates, carbonates, nitrates, and fluorides, and use of SEM-WDX is required for some of them. Another limitation of SEM is that many of the accessory minerals in coal have a grain size less than 0.5-1 µm and their observation and analysis is impossible by this method. On the other hand, some waterrich samples (lower-rank coals and minerals with water molecules) also contribute to problems connected with their observation and analysis due to dehydration. Additionally, the detailed characterization of samples is a time-consuming process. Quantitative micromorphometric and mineral measurements in the samples by SEM are possible. For example, in computer-controlled SEM (CC-SEM) or automatic image analysis (AIA), the mineral particles are distinguished from the coal matrix by their brightness (average atomic number) in the backscattered electron image using ground coal samples mounted and polished in an epoxy resin. Such investigations of MM conducted by CC-SEM have been reported.33,36,72,74,75,133,137,142-155 In this system, the min(104) Thiessen, G. Composition and origin of the mineral matter in coal. In Chemistry of Coal Utilization; Lowry, H., Chairman; John Wiley & Sons Inc.: New York, 1947; Vol. I, pp 485-495. (105) Francis, W. Coal. Its Formation and Composition; Edward Arnold: London, 1961; 806 pages. (106) Dixon, K.; Skipsey, E.; Watts, J. T. The distribution and composition of inorganic matter in British coals: Part 1-Initial study of seams from the East Midlands Division of the National Coal Board. J. Inst. Fuel 1964, 37, 485-493. (107) Kemezys, M.; Taylor, G. H. Occurrence and distribution of minerals in some Australian coals. J. Inst. Fuel 1964, 37, 389-397. (108) Mackowsky, M. Th. Mineral matter in coal. In Coal and Coalbearing Strata; Murchison, D., Westall, T., Eds.; American Elsevier: New York, 1968; pp 309-321. (109) Harris, L. A.; Rose, T.; DeRoos, L.; Greene, J. Quantitative analyses of pyrite in coal by optical image techniques. Econ. Geol. 1977, 72, 695-697. (110) Ting, F. T. C. Petrographic techniques in coal analysis. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1978; Vol. 1, Chapter 1, pp 3-26. (111) Berkowitz, N. An Introduction to Coal Technology; Academic Press: London, 1979; 345 pages. (112) Stach, E.; Mackowsky. M.-Th.; Teichmu¨ller, M.; Taylor, G. H.; Chandra, D.; Teichmu¨ller, R. Stach’s Textbook of Coal Petrology; Gebru¨der Borntraeger: Berlin, 1982; 535 pages. (113) Wiese, R. G.; Fyfe, W. S. Occurrences of iron sulfides in Ohio coals. Int. J. Coal Geol. 1986, 6, 251-276. (114) Van der Flier-Keller, E.; Fyfe, W. S. Mineralogy of Lower Cretaceous coals from the Moose River basin, Ontario, and Monkman, British Columbia. Can. Mineral. 1988, 26, 343-353. (115) Van der Flier-Keller, E.; Fyfe, W. S. Relationships between inorganic constituents and organic matter in a Northern Ontario lignite. Fuel 1988, 67, 1048-1052. (116) Querol, X.; Chincho´n, S.; Lo´pez-Soler, A. Iron sulfide precipitation sequence in Albian coals from the Maestrazgo basin, Southeastern Iberian Range, Northeastern Spain. Int. J. Coal Geol. 1989, 11, 171189. (117) Hower, J. C.; Wild, G. D.; Pollock, J. D.; Trinkle, E. J.; Bland, A. E.; Fiene, F. L. Petrography, geochemistry, and mineralogy of the Springfield (Western Kentucky No. 9) coal bed. J. Coal Qual. 1990, 9, 90-100. (118) Kortenski, J. Carbonate minerals in Bulgarian coals with different degrees of coalification. Int. J. Coal Geol. 1992, 20, 225-242. (119) Kortenski, J.; Kostova, I. Occurrence and morphology of pyrite in Bulgarian coals. Int. J. Coal Geol. 1996, 29, 273-290.

Characterization of Inorganic and Mineral Matter in Coal

eral grains are located and sized, their major elemental compositions are determined by using the X-ray channels for these elements, and the elemental combinations characterize most of the common minerals in coal. This automatic system is capable of analyzing thousands of particles for hours, and the computer summarizes the (120) Kostova, I.; Petrov, O.; Kortenski, J. Mineralogy, geochemistry and pyrite content of Bulgarian subbituminous coals, Pernik Basin. In Coalbed Methane and Coal Geology; Gayer, R., Harris, I., Eds.; Geol. Soc., Special Publ. No. 109, 1996; pp 301-314. (121) Mukhopadhyay, P. K.; Lajeunesse, G.; Crandlemire, A. L. Mineralogical speciation of elements in an Eastern Canadian feed coal and their combustion residues from a Canadian power plant. Int. J. Coal Geol. 1996, 32, 279-312. (122) Gluskoter, H. J. Mineral matter and trace elements in coal. ACS Symposium Series, Vol. 141, Trace Elements in Fuel; Babu, S, Ed.; American Chemical Society: Washington, DC, 1975; Chapter 1, pp 1-22. (123) Finkelman, R. B. Determination of trace element sites in the Waynesburg coal by SEM analysis of accessory minerals. Scanning Electron Microsc. 1978, 1, 143-148. (124) Finkelman, R. B.; Stanton, R. W. Identification and significance of accessory minerals from a bituminous coal. Fuel 1978, 57, 763-768. (125) Raymond, R., Jr.; Gooley, R. Electron probe microanalyzer in coal research. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1979; Vol. 3, Chapter 48, pp 337-356. (126) Russell, S. J.; Rimmer, S. M. Analysis of mineral matter in coal, coal gasification ash, and coal liquefaction residues by scanning electron microscopy and X-ray diffraction. In Analytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1979; Vol. 3, Chapter 42, pp 133-162. (127) Renton, J. J. Mineral matter in coal. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; Chapter 7, pp 283-326. (128) Herman, R. G.; Simmons, G. W.; Cole, D. A.; Kuzmicz, V.; Klier, K. Catalytic action of minerals in the low-temperature oxidation of coal. Fuel 1984, 63, 673-678. (129) Cloke, M., Hamilton, S., Wright, J. P. Evidence for mineral matter forms in coal liquefaction extracts. Fuel 1987, 66, 1685-1690. (130) Cressey, B. A.; Cressey, G. Preliminary mineralogical investigation of Leicestershire low-rank coal. Int. J. Coal Geol. 1988, 10, 177-191. (131) Finkelman, R. B. The inorganic geochemistry of coal: a scanning electron microscopy view. Scanning Electron Microsc. 1988, 2, 97-105. (132) Steinmetz, G. L.; Mohan M. S.; Zingaro R. A. Characterization of titanium in United States Coals. Energy Fuels 1988, 2, 684-692. (133) Birk, D. Coal minerals: Quantitative and descriptive SEMEDX analysis. J. Coal Qual. 1989, 8, 55-62. (134) Martı´nez-Tarazona, M. R.; Palacios, J. M.; Tasco´n, J. M. D. SEM-EDX characterisation of inorganic constituents of brown coals. Inst. Phys. Conf. Ser. 1990, 98, 327-330. (135) Tasco´n, J. M. D.; Martı´nez-Alonso, A.; Palacios, J. M.; Jaramillo, A.; Quintero, G.; Mondrago´n, F. Characterization of mineral constituents of Amaga´ and Cerrejo´n coals (in Spanish). In Proc. 1st Natl. Conf. Coal Sci. Technol.; Imp. Caribe: Medellı´n, Colombia, 1991; pp 70-77. (136) Garcı´a-Valle`s, M.; Pradell, T.; Martı´nez, S.; Vendrell, M. Mineralogical characterization of the Garumnian subbituminous lignite from the Central Pyrenees by SEM-EDX, X-ray-diffraction and Mo¨ssbauer-spectroscopy. Fuel 1993, 72, 971-975. (137) Sykes, R.; Lindqvist, J. K. Diagenetic quartz and amorphous silica in New Zealand coals. Org. Geochem. 1993, 20, 855-866. (138) Lyons, P. C.; Spears, D. A.; Outerbridge, W. F.; Congdon, R. D.; Evans, H. T., Jr. Euramerican tonsteins: overview, magmatic origin, and depositional-tectonic implications. Palaeogeogr. Palaeoclimatol. Palaeoecol. 1994, 106, 113-134. (139) Patterson, J. H.; Corcoran, J. F.; Kinealy, K. M. Chemistry and mineralogy of carbonates in Australian bituminous and subbituminous coals. Fuel 1994, 73, 1735-1745. (140) Seredin, V. Elemental metals in metalliferous coal-bearing strata. In Proceedings of the 9th International Conference on Coal Science; Ziegler, A., van Heek, K. H., Klein, J., Wanzl, W., Eds.; P&W Druck und Verlag GmbH: Essen, Germany, 1997; Vol. 1, pp 405408. (141) Pollock, S. M.; Goodarzi, F.; Riediger, C. L. Mineralogical and elemental variation of coal from Alberta, Canada: An example from the No. 2 seam, Genesee Mine. Int. J. Coal Geol. 2000, 43, 259-286. (142) Huggins, F. E.; Kosmack, D. A.; Huffman, G. P.; Lee, R. J. Coal mineralogies by SEM automatic image analysis. Scanning Electron Microsc. 1980, 1, 531-540. (143) Allen, R. M.; VanderSande, J. B. Analysis of submicron mineral matter in coal via scanning transmission electron microscopy. Fuel 1984, 63, 24-29.

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results (size, distribution, shape, and volume percentages of minerals). However, the limitations of this technique are the surface analysis and significant percentage (e10%) of undefined or unclassified miscellaneous silicates, sulfides, hydrosulfates, carbonatesoxyhydroxides, and fine-grained (3-4%8,223 for minerals in LTA samples, and mineral quantification is possible therein. Different methods (curve-fitting programs, spectral subtraction, clustering theory) have been applied to quantification of certain minerals (clay minerals, carbonates, quartz, and pyrite).8,222-227 The application of XRD for qualitative mineral identifications with subsequent FTIRS for quantification of minerals could be a more favorable approach. Chemical Analyses

Characterization of Inorganic and Mineral Matter in Coal

halite, arcanite), chlorides (sylvite, halite), amorphous inorganic matter, pore water, and organically bound K in different proportions.71 Variable modes of element occurrences were more or less detected for the other major and minor elements included in these normative calculations.71,237-239 Therefore, an identification of mineral species prior to these calculations should be always carried out, and the calculation method should be adapted to each coal deposit. Conclusions The coal ash is the inorganic residue resulting from the incineration of coal and is composed of original and new-formed inorganic phases generated from both IM and OM of coal. The IM of coal comprises the solid crystalline (mineral), solid noncrystalline (amorphous), and fluid inorganic constituents in coal. The MM of coal, as a part of IM, consists of various mineral species (mostly crystalline) that belong to mineral classes strictly divided and defined in the mineralogical sense. A number of techniques are available to characterize the IM-MM present in coal; however, there is no one universal method for a complete qualitative and quantitative characterization due to the complex character of these constituents. Each method has different advantages and limitations and the best approach is to use a combination of procedures and methods depending on the purpose of study, degree of detail desired, and available techniques for IM-MM characterization. Unlike ash yield, the determination and isolation of IM and MM is a complex procedure and cannot be conducted easily and measured routinely, at present. The determination of total MM or IM contents in coal on the basis of the available chemical treatments, ashing procedures, chemical analyses, and simplified correction formulas should always be interpreted with caution. Methods such as XRD, DTA-TGA, MS, IRS, and chemical analyses should be always accompanied by direct methods of observations in situ (with preserved textural and structural mineral-maceral relationships), namely optical, SEM, and TEM microscopy. On the other hand, the mineralogical study of coal should be always combined with chemical analyses conducted on the bulk coal samples, LTA, HTA, separated fractions, and/or individual minerals and phases. The best results are normally achieved by the selection of an analytical scheme including macroscopic and preliminary microscopic observations, then, sequential combination of suitable separation procedures and chemical analyses, followed by detailed XRD and optical microscopy, and later by SEM, TEM, and other techniques. This scheme should be selected after some initial observations on the coal samples. As an illustration, a detailed and idealized characterization of IM and MM in coal may include several steps. The specimens should first be examined macroscopically and microscopically (by ordinary stereomicroscope) to obtain some informative data for the dominant texture, size, distribution, association, assemblage, type, abundance, and morphol-

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ogy of minerals and phases in coal. Some minerals can be even isolated by handpicking under the microscope, and this is a big advantage for their subsequent studies by different methods. The second step includes ashing procedures (LTA and HTA), physical separations (size, magnetic, froth flotation, electrostatic, and in particular density fractionations), and chemical leaching (acid demineralization, treatment by hydrogen peroxide, and sequential fractionation). Then, the bulk coal and ash samples, and separated fractions are studied by chemical analyses (wet methods, XRF, AAS, ICP-AES, ICPMS, NAA). After that, the above-mentioned samples are investigated qualitatively and quantitatively by XRD, optical microscopy (under reflected and particularly under transmitted light), SEM, and CC-SEM, and the minerals and phases are characterized (properties, size, crystallinity, structure, composition, morphology, microstructural and microtexture relationships, mineral assemblages and generations, stages and substages of mineralization). Later, the most important bulk samples and fractions can be additionally investigated by SEMEDX and SEM-WDX for more precise identification and characterization (in particular, composition, morphology, mineral-maceral relationships, and genetic interpretations), while the concentrates abundant in accessory minerals can be examined by TEM for their more accurate identification. Other subsidiary techniques such as IRS, DTA-TGA, and MS may be also utilized for some fractions. The above-described scheme can also be applied for IM-MM characterization of solid products from preparation (upgraded coals, slimes, host rocks), pyrolysis (char), and combustion (fly ashes, bottom ashes, lagooned ashes) of coals. Note Added in Proof. Two recent review papers of particular relevance to the subject reviewed here have come to the authors’ knowledge only after this paper was accepted in definitive form, and therefore they are not discussed here. In one of them, Ward [Ward, C. R. Intern. J. Coal Geol. 2002, 50, 135-168] addresses the analysis and significance of mineral matter in coal seams, including a section devoted to methods for mineral matter analysis. In the other paper, Huggins [Huggins, F. E. Intern. J. Coal Geol. 2002, 50, 169-214] provides an overview of analytical methods for inorganic constituents of coal, with a specific section on the determination of coal mineralogies. Acknowledgment. Financial support from the European Coal and Steel Community (Project 7220-PR/ 099) is gratefully acknowledged. The authors thank two anonymous reviewers for their useful comments and suggestions for improving the present manuscript. Note Added after ASAP Posting. This article was released ASAP on 01/11/03 with an incomplete Acknowledgment at the end of the paper. The correct version was posted on 02/24/2003. EF020113Z