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Accurate Phase Quantification of Mineral Matter in Bulk Lignite Samples from Western Peloponnese (Greece) Stefanos Papazisimou,† Stavros Kalaitzidis,† Kimon Christanis,*,† Gordon Cressey,‡ and Eva Valsami-Jones‡ Section of Earth Materials, Department of Geology, University of Patras, GR-26500 Rio-Patras, Greece, and Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom Received October 18, 2003. Revised Manuscript Received January 9, 2004
Lignite samples from western Peloponnese, Greece, are examined using an X-ray diffractometer that was equipped with a position-sensitive detector, to quantify the mineral phases. The contents of the determined minerals correlate well with the results of chemical analyses and examinations via scanning electron microscopy, coupled with energy-dispersive X-ray analysis. The proposed quantification method can be applied rapidly and accurately to organic-rich sediments.
1. Introduction The identification and quantification of minerals contained in lignites and high-rank coals are very important in understanding the geochemical conditions established during peat formation in the palaeomires and, thus, in the elucidation of coal genesis. The mineralogical composition of coal is essential in determining the mode of occurrence of a large number of elements. Moreover, it provides information about the behavior of coals during combustion for power generation. The latter concerns bottom and fly ash formation, boiler corrosion, and slagging.1,2 X-ray diffractometry (XRD) on samples of coal and coal ashes has been applied, among other methods, in many studies, to explain the occurrence, abundance, and origin of minerals present, as well as the behavior of coals during various utilizations.3-9 Quantification of mineral matter on the basis of XRD patterns is not reliable, because of effects such as preferred orientation of the mineral grains in the sample holder and variation in the absorption of X-rays by the minerals and the organic matter. Nevertheless, various methods, mainly on a semiquantitative basis, have been applied for the study of minerals in coals. These are * Author to whom correspondence should be addressed. E-mail:
[email protected]. † University of Patras. ‡ The Natural History Museum. (1) Ward, C. R. Coal Geology and Coal Technology; Blackwell: Melbourne, Australia, 1984; 345 pp. (2) Gupta, R.; Wall, T. F.; Baxter, L. A. The Impact of Mineral Impurities in Solid Fuel Combustion; Plenum: New York, 1999; 768 pp. (3) O’Gorman, J. V.; Walker, P. L., Jr. Fuel 1973, 52, 71-79. (4) Ward, C. R. Fuel 1974, 53, 220-221. (5) Mitchell, R. S.; Gluskoter, H. J. Fuel 1976, 55, 90-96. (6) Nankervis, J. C.; Furlong, R. B. Fuel 1980, 59, 425-430. (7) Vassilev, S. V.; Kitano, K.; Takeda, S.; Tsurue, T. Fuel Process. Technol. 1995, 45, 27-51. (8) Vassilev, S. V.; Vassileva, C. G. Fuel Process. Technol. 1996, 48, 85-106. (9) Vassilev, S. V.; Tasco´n, J. M. D. Energy Fuels 2003, 17, 271281.
based on integrated intensities of the main diffractogram peaks.10,11 The use of the full profile of an XRD pattern provides considerably more information about mineral quantification than the use of only integrated intensities. One of the most-recent methods for the quantification of mineral facies in coal samples and ashes using the full profiles of the X-ray patterns is a computer software system that incorporates Rietveld techniques and full-pattern fitting.12,13 The advent of curved position-sensitive detectors (PSDs) has enabled the rapid acquisition of diffraction patterns with angular ranges up to 120° 2θ in various multiphase samples (including clay-bearing samples), whereas the use of computer software has permitted the rapid whole-pattern profile stripping and accurate quantification of the mineral phases contained.14,15 In the present study, an attempt is made to apply the rapid quantification method of minerals, proposed by Cressey and Schofield14 and Batchelder and Cressey,15 on dry lignite samples from the Magoula deposit and lignite seams outcropping near Zacharo and Kyparissia (Figure 1). All the study areas are located in Neogene basins in western Peloponnese, Greece. 2. Regional Setting The margins of the Neogene basins of Pyrgos, Zacharo, and Kyparissia in western Peloponnese consist (10) Rao, C. P.; Gluskoter, H. J. Occurrence and Distribution of Minerals in Illinois Coals. Illinois State Geological Survey, Circular 476, 1973; 56 pp. (11) Renton, J. J. Semiquantitative Determination of Coal Minerals by X-ray Diffractometry. In Mineral Matter and Ash in Coal; Vorres, K. S., Ed.; ACS Symposium Series 301; American Chemical Society: Washington, DC, 1986; pp 53-60. (12) Ward, C. R.; Bocking, M.; Ruan, C. Int. J. Coal Geol. 2001, 47, 31-49. (13) Ward, C. R.; Taylor, J. C.; Matulis, C. E.; Dale, L. S. Int. J. Coal Geol. 2001, 46, 67-82. (14) Cressey, G.; Schofield, P. F. Powder Diffr. 1995, 11 (1), 35-39. (15) Batchelder, M.; Cressey, G. Clays Clay Miner. 1998, 46 (2), 183-194.
10.1021/ef034072y CCC: $27.50 © 2004 American Chemical Society Published on Web 02/06/2004
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Figure 1. Schematic geological map of western Peloponnese, Greece.
mainly of Upper Triassic to Oligocene limestones, cherts, and flysch of the Pindos, Tripolitsa, and Ionia zones.16-21 The 12.4 Mt lignite deposit of Magoula22 is located in the eastern portion of the Pyrgos basin, 12 km northeast of the city of Pyrgos (refer to Figure 1). The sediments (16) Aubouin, J. Ann. Geol. Pays Hell. 1959, 10, 1-403. (17) Aubuin, J.; Dercourt, J. Bull. Soc. Geol. France 1962, 7 (3), 785794. (18) Richter, D. Z. Dtsch. Geol. Ges. 1976, 127, 467-483. (19) Fleury, J. J.; De Wever, P.; Izart, A.; Dercourt, J. Geological Map of Greece, Scale 1:50.000; Sheet Goumero; IGME: Athens, Greece, 1981. (20) Jacobshagen, V. Geologie von Griechenland; Borntraeger: Berlin, 1986; p 363.
that fill the basin are from the Upper Pliocene to Pleistocene Ages and consist mainly of sands, silts, clays, lignites, and sandstones deposited in a variety of limnic, brackish, and marine environments.23-26 In the Zacharo basin, two lignite seams outcrop in two (21) Degnan, P. J.; Robertson, A. H. F. Sediment. Geol. 1998, 117, 33-70. (22) Vagias, D. Results of Lignite Exploration in Elias Prefecture, Greece. Institute of Geological and Mineral Exploration (in Greek); Internal Report; IGME; Athens, Greece, 1994; 40 pp. (23) Hageman, J. Ann. Geol. Pays Hell. 1977, 28, 299-333. (24) Frydas, D. Kalkiges Nannoplankton aus dem Neogen von NWPeloponnes. Neues Jahrb. Geol. Palaeontol. Monatsh. 1987, 5, 274286. (25) Kamberis, E.; Ioakim, Ch.; Tsaila-Monopolis, St.; Tsapralis, V. Palaeontol. I Evolucio 1992, 24/25, 363-376.
Bulk Lignite from Western Peloponnese (Greece)
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Figure 2. Sequential procedure of whole-pattern profile matching and stripping, from the initial lignite pattern of sample S7 (pattern a).
small open-pit mines at the proximity of the villages of Trypes (0.5 km to the northeast) and Koumouthekras (1.8 km to the north), with thicknesses of 1 and 2 m, respectively (see Figure 1). According to Streif et al.,27 the Neogene sediments of this basin consist of components ranging from sand to clay, deposited under shallow marine conditions during the Upper Pliocene Age. In the Kyparissia basin, a 1-m-thick lignite seam outcrops 2.5 km northeast of Kyparissia at the margins of a former open-pit mine. The sediments that fill the basin are from the Pliocene/Pleistocene Age and consist of conglomerates, sandstones, marls, sands, and red clays.28 (26) Tsaila-Monopolis, S.; Ioakim, Ch.; Kamberis, E. Biostratigraphical Correlation in the Neogene and Quaternary Sediments in the Katakolon Oil Wells (NW Peloponnese). In Memorial Volume for Prof. Panagos; National Technical University: Athens, Greece, 1993; pp 813-828. (27) Streif, H.; Perissoratis, C.; Metropoulos, D.; Bornovas, J. Geological Map of Greece, Scale 1:50.000; Sheet Olympia; IGME: Athens, Greece, 1982.
Lignite seams from Zacharo and Kyparissia are mined in small open pits and used for power generation. The Magoula lignite deposit may also be used in the future for the same purpose. 3. Field and Laboratory ProceduressExperimental Section Twenty-two lignite samples from the Magoula deposit were selected from cores provided by the Institute of Geological and Mineral Exploration (IGME). Each sample represents an entire bench, with thicknesses up to 2.5 m. Eight samples from the lignites outcropping at Trypes, Koumouthekras, and Kyparissia were channel-sampled, according to the method described by Thomas.29 The thickness of each sample does not exceed 20 cm. The lithotypes in each sample were described (28) Metropoulos, D.; Perissoratis, C.; Angelopoulos, J.; Bornovas, J. Geological Map of Greece, Scale 1:50.000; Sheet Kyparissia; IGME: Athens, Greece, 1979. (29) Thomas, L. Handbook of Practical Coal Geology; Wiley: Chichester, England, 1992; 325 pp.
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Table 1. Ash and Carbon, Hydrogen, Nitrogen, Oxygen, and Sulfur Contents of the Studied Lignites
Table 2. Content of Major Elements in the Studied Lignites, Determined via ICP-MS
Content (%)a sample
ash
C
N
S
4.3 4.2 3.5 5.6 2.8 3.2 3.5 3.7 3.9 4.6 4.1 2.8 3.0 4.4 3.6 3.5 4.4 3.6 3.8 4.7 4.7 4.2
Magoula 2.9 2.8 1.3 4.0 1.0 3.7 9.2 3.0 0.9 3.9 0.7 5.3 1.4 3.9 1.1 4.0 5.5 2.7 7.3 3.1 1.6 3.0 0.8 3.3 1.0 4.0 1.9 3.7 1.3 4.5 2.2 2.9 7.7 2.5 1.0 3.5 2.0 4.0 6.5 3.0 3.0 3.6 3.5 3.5
O
O/C ratio H/C ratio
S7 S9 S11 S15 S17 S19 S24 S25 S34 S36 S38 S42 S51 S55 S56 S57 S58 S61 S65 S67 S71 S75
17.9 22.5 25.3 18.8 33.0 21.9 21.5 16.7 30.5 25.0 11.2 28.5 34.1 17.7 27.2 33.9 36.0 22.8 24.0 31.1 17.8 28.3
24.0 21.7 25.1 8.3 28.1 30.8 27.7 29.4 22.9 12.2 28.4 31.2 27.5 25.6 24.9 22.5 14.1 27.6 25.6 11.9 20.3 17.4
0.37 0.35 0.45 0.11 0.68 0.61 0.49 0.49 0.50 0.19 0.41 0.70 0.68 0.41 0.48 0.48 0.30 0.50 0.47 0.21 0.30 0.30
1.08 1.09 1.01 1.21 1.06 1.01 0.99 0.98 1.34 1.17 0.95 1.02 1.18 1.12 1.12 1.19 1.49 1.04 1.11 1.30 1.12 1.18
S77 S78
Trypes 25.7 53.2 3.7 1.5 1.0 14.9 14.5 48.5 4.7 8.3 0.7 23.3
0.21 0.36
0.84 1.17
S81 S82 S84
Koumouthekras 37.8 39.1 3.3 1.5 1.1 17.1 39.3 44.9 4.6 8.1 0.8 2.5 37.3 32.3 2.5 1.0 1.9 25.0
S90 S93 S95
Kyparissia 27.2 47.8 3.1 1.4 2.4 18.2 11.5 59.1 3.6 1.3 3.1 21.4 16.0 54.0 3.5 1.5 3.5 21.5
a
48.1 46.3 41.4 55.2 31.2 38.1 42.0 45.1 34.6 47.7 51.7 33.4 30.4 46.8 38.5 35.1 35.3 41.5 40.7 42.9 50.6 43.1
Elemental Content (wt %, dry basis)
H
0.33 0.04 0.58 0.29 0.27 0.30
1.02 1.23 0.93 0.78 0.73 0.79
Dry basis.
according to the nomenclature used by the International Committee for Coal Petrology (ICCP).30 All samples were oven-dried at 105 °C to constant weight. Ash contents were determined at 750 °C for 4 h in a Raypa model HM9 muffle furnace. The elemental contents of carbon, hydrogen, nitrogen, and sulfur on dry samples were determined with a Carlo Erba analyzer. The oxygen content was calculated by difference, by subtracting the sum of the ash, carbon, hydrogen, nitrogen, and sulfur contents from 100. The contents of the major elements (such as aluminum, silicon, iron, calcium, magnesium, and potassium) were determined at The Natural History Museum in London, using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on solutions obtained after dry lignite samples were digested in closed polytetrafluoroethylene (PTF) vessels, using a microwave oven (Mars 5). Polished blocks were examined via scanning electron microscopy (SEM), using two SEM microscopes (JEOL model LSM 6300, at the University of Patras, Greece, and Cambridge Instruments model Stereoscan S250 MK3 at the Department of Earth Sciences, University of Bristol, UK); both microscopes were equipped with energy-dispersive X-ray (EDX) analyzers. 3.1. XRD Experiments. The quantification method utilizes a curved PSD with an output array that consists of 4096 channels, representing an arc of 120° 2θ (channel width of 0.03°). The detector enables diffraction patterns to be acquired rapidly by measuring the diffracted intensity at all angles simultaneously around the 120° arc. All the data were collected (30) I.C.C.P. (International Committee for Coal Petrology) International Handbook of Coal Petrography, Supplement, Commision I; Centre National de la Recherche Scientifique: Paris, 1993; p 19.
sample
Al
Si
Fe
Ca
Mg
K
S
0.39 0.70 0.83 0.88 1.10 0.51 0.47 0.66 1.11 0.51 0.61 1.02 1.13 0.50 0.87 1.03 1.09 0.39 0.83 0.88 0.49 0.54 0.75 0.39 1.13
0.18 0.28 0.44 0.35 0.87 0.35 0.44 0.14 0.68 0.31 0.37 0.69 0.85 0.29 0.47 0.62 0.68 0.30 0.38 0.44 0.19 0.39 0.44 0.14 0.87
4.12 4.87 4.83 3.10 4.62 6.27 4.80 4.89 3.27 4.26 3.61 4.64 4.11 4.46 5.57 3.54 4.13 3.99 5.09 4.22 4.33 4.51 4.42 3.10 6.27
S7 S9 S11 S15 S17 S19 S24 S25 S34 S36 S38 S42 S51 S55 S56 S57 S58 S61 S65 S67 S71 S75 mean min max
0.85 1.47 1.77 1.57 2.72 1.80 1.73 0.96 3.57 2.00 1.61 2.94 4.13 1.57 2.38 3.39 4.09 1.60 2.08 2.68 1.17 2.32 2.20 0.85 4.13
0.48 2.35 3.36 3.09 7.39 3.73 3.80 1.93 6.33 3.63 3.29 4.80 6.63 2.71 4.26 5.59 6.71 3.27 3.96 4.42 1.94 4.99 4.03 0.48 7.39
Magoula 1.39 4.66 1.89 3.12 2.18 1.98 1.53 1.4 3.70 1.00 3.40 1.96 1.54 2.41 0.90 2.98 2.84 2.22 2.48 3.20 1.52 1.63 2.75 2.09 2.97 2.28 2.20 2.34 2.98 3.39 2.65 3.58 3.54 2.38 2.13 2.65 1.77 2.76 2.20 4.03 1.50 2.85 1.36 3.3 2.25 2.65 0.90 1.00 3.7 4.66
S77 S78
1.96 0.82
2.32 1.02
Trypes 1.47 6.85 0.48 5.64
0.17 0.08
0.18 0.09
1.25 1.56
S81 S82 S84 mean min max
3.48 4.98 3.39 2.93 0.82 4.98
8.05 8.35 7.82 5.51 1.02 8.35
Koumouthekras 1.59 1.50 1.65 1.49 2.45 1.53 1.53 3.40 0.48 1.49 2.45 6.85
0.70 0.94 0.68 0.51 0.08 0.94
0.46 0.58 0.59 0.38 0.09 0.59
1.34 1.19 2.45 1.56 1.19 2.45
S90 S93 S95 mean min max
2.65 1.05 1.28 1.66 1.05 2.65
2.39 1.45 2.83 2.22 1.45 2.83
Kyparissia 1.43 4.57 1.32 0.62 1.71 0.96 1.49 2.05 1.32 0.62 1.71 4.57
0.15 0.005 0.02 0.06 0.005 0.15
0.13 0.02 0.06 0.07 0.02 0.13
2.72 3.47 4.46 3.55 2.72 4.46
using a powder diffraction system (ENRAF-NONIUS PDS 120) with a curved PSD and a fixed-beam sample detector geometry. A Ge(111) monochromator was used to select only Cu KR1 radiation from the primary beam, and the tubeoperating conditions were 45 kV and 45 mA. Horizontal and vertical slits between the monochromator and the sample were used to restrict the beam to an area of 0.24 mm × 5.0 mm; thus, the irradiated area was constant. Measurements were made in reflection geometry with the powder sample surface at a fixed angle of ∼5° to the incident beam. The sample was rotated continuously around an axis vertical to its surface. This statistically increases the number of the crystallites orientated in diffracting positions. The tilt of the sample surface to the beam was held constant during data acquisition. Data acquisition times of only 5 min were used for all the analyses. Unground NIST silicon powder (SRM 640) was used as an external standard for the 2θ calibration. The 2θ linearization was performed with the ENRAF-GUFI software, using a least-squares cubic spline function. All mineral standards used in this study are obtained from the mineral collection of the Natural History Museum in London. 3.2. Sample Preparation. The samples and the mineral standards were prepared according to the method of Batchelder and Cressey.15 They were only gently ground until a smooth powder was prepared. Dry samples were top-loaded into a circular well mount that was 15 mm in diameter and 1
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mm deep. To avoid inducing a high degree of preferred orientation of platy crystals parallel to the top surface, each sample was packed and leveled using only the narrow (knife) edge of a small steel spatula until a smooth flat surface was obtained that was level with the rim of the circular well holder. The packing procedure required
