Energy & Fuels 2005, 19, 263-269
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Influence of Inherent Coal Mineral Matter on the Structural Characteristics of Graphite Materials Prepared from Anthracites David Gonza´lez, Miguel A. Montes-Mora´n, and Ana B. Garcia* Instituto Nacional del Carbo´ n, CSIC, Francisco Pintado Fe 26, 33011 Oviedo, Spain Received April 27, 2004. Revised Manuscript Received September 20, 2004
Anthracites with different mineral matter content and composition but similar organic matter compositionsand, therefore, microtextureswere obtained by consecutive immersion in mixtures of organic liquids of increasing density from an anthracite with a low degree of graphitizability, thus reducing the characteristics of the anthracite that affect the graphitization process to the mineral matter. Graphite materials were then prepared by heating the anthracites in the temperature interval of 2400-2600 °C for the purpose of studying the influence of the anthracite mineral matter (amount and composition) on their ability to graphitize. The interlayer spacing (d002) and crystallite sizes (along the c-axis (Lc) and along the a-axis (La)), calculated from X-ray diffractometry (XRD), as well as the relative intensity of the Raman D-band (ID/It), were used to assess the degree of structural order of the materials. A progressive increase in this degree of structural order with increasing mineral matter content of the anthracite was observed. The catalytic effect of the mineral matter on the graphitization of the anthracites relies mainly on promotion of the growth of the crystallites along the basal plane. Reasonably good linear correlations between the mineral matter content and the La value of the material were attained. Among the different constituents of the mineral matter, the clay mineral illite and the iron carbonates ankerite and siderite were observed to be the main active catalyst compounds during the graphitization of anthracites. In addition to the amount and composition of the mineral matter, the distribution of the mineral matter also influences the graphitization process of the anthracite. A fine distribution in the organic matter, such as that in the case of the iron compounds, was observed to improve the catalytic effect of the mineral matter.
Introduction Graphite materials with structural characteristics comparable to those of commercially available synthetic graphites can be obtained from anthracites that have been heated at temperatures of g2400 °C.1-5 In addition to the treatment temperature, some of the characteristics of the anthracite also influence the graphitization process. Among them, the mineral matter has been suggested to act as a graphitization catalyst.1,6-8 Moreover, the microtexture of the anthracite was also related to its ability to graphitize, specifically when there is a preferential planar orientation of the polyaromatic basic structural units (BSUs).1,9 Finally, more-ordered graph* Author to whom correspondence should be addressed. Telephone: +34 98 511 89 54. Fax: +34 98 529 76 62. E-mail:
[email protected]. (1) Oberlin, A.; Terriere, G. Carbon 1975, 13, 367. (2) Bustin, R. M.; Rouzaud, J. N.; Ross, J. V. Carbon 1995, 33, 679. (3) Bustin, R. M.; Ross, J. V.; Rouzaud, J. N. Int. J. Coal Geol. 1995, 28, 1. (4) Atria, J. V.; Rusinko, F., Jr.; Schobert, H. H. Energy Fuels 2002, 16, 1343. (5) Gonza´lez, D.; Montes-Mora´n, M. A.; Garcia, A. B. Energy Fuels 2003, 17, 1324. (6) Evans, E. L.; Jenkins, J. L.; Thomas, J. M. Carbon 1972, 10, 637. (7) Oya, A.; Fukatsu, T.; Otani, S.; Marsh, H. Fuel 1983, 62, 502. (8) Gonza´lez, D.; Montes-Mora´n, M. A.; Suarez-Ruiz, I.; Garcia, A. B. Energy Fuels 2004, 18, 365. (9) Blanche, C.; Rouzaud, J. N.; Dumas, D. Extended Abstracts, 22nd Biennal Carbon Conference 1995, 152.
ite materials were prepared from anthracites with a higher H/C atomic ratio, inferring that the elemental composition influences the graphitization process.4 In previous work,4,8,10 anthracites with different mineral matter contents were graphitized. A significant improvement of the structural order was observed for the graphite materials prepared from the anthracites with the highest mineral matter content. Because of the different microtexture and organic matter composition of the anthracites studied, these results were, however, inconclusive in establishing the specific contribution of the mineral matter to this improvement. In the present work, an anthracite with a low degree of graphitizability8 was separated into different fractions by consecutive immersion in mixtures of organic liquids of increasing density. Using this conventional coal washability procedure (the float-sink test), anthracites with different mineral matter content and composition were obtained. The organic matter composition and, therefore, the microtexture of these anthracites (including the raw one) were similar, thus reducing the number of possible factors that would affect the graphitization to the mineral matter. On the basis of this consideration, (10) Zeng, S. M.; Rusinko, F.; Schobert, H. H. Producing HighQuality Carbon and/or Graphite Materials from Anthracites by Catalytic Graphitization; Commonweath of Pennsylvania, Pennsylvania Energy Development Authority, Harrisburg, PA, Final Technical Report (Grant 9303-4019), 1996.
10.1021/ef049893x CCC: $30.25 © 2005 American Chemical Society Published on Web 11/05/2004
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Gonza´ lez et al.
Table 1. Ash Contents, Elemental Analyses, and Sulfur Forms of the ATO and ATDO1-ATDO4 Anthracites
a
parameter
ATOD1
ATOD2
ATOD3
ATO
ATOD4
ash (wt %, db) elemental analysis (wt %, daf) carbon hydrogen nitrogen organic sulfur sulfur forms (wt %, db) total pyrite sulfate organic (diff.)
2.07
3.66
6.54
10.12
19.07
94.76 2.35 0.86 0.87
94.45 2.41 0.87 0.88
94.20 2.17 0.85 0.96
93.13 2.03 0.87 1.01
91.54 2.16 0.77 1.03
0.86 n.d.a n.d.a 0.86
0.85 n.d.a n.d.a 0.85
0.92 0.02 n.d.a 0.90
1.07 0.15 0.01 0.91
0.91 0.05 0.02 0.84
Not detected.
the anthracites were heated in the temperature interval of 2400-2600 °C for the purpose of studying the influence of the inherent mineral matter (amount and composition) on their ability to graphitize. Regarding the composition of the coal mineral matter, most of its major inorganic elements (such as aluminum, calcium, iron, magnesium, manganese, silicon, and titanium) have been used as graphitization catalysts for various graphitizable and nongraphitizable carbon materials.11-17 To explore the catalytic effect of the different constituents of the anthracite mineral matter, the content of the above-mentioned elements, as well as potassium, in the anthracites was determined. Special attention was given to aluminosilicate minerals, which are one of the main constituents of the coal mineral matter and have been observed to be remarkably effective catalysts for other carbon materials, particularly in the presence of iron.11,12 The interlayer spacing (d002) and crystallite sizes along the c-axis (Lc) and the a-axis (La), calculated from X-ray diffractometry (XRD), as well as the relative intensity of the Raman D-band (ID/It), are used in this study to assess the degree of structural order of the graphite materials that have been prepared. Both XRD and Raman spectroscopy techniques have been used extensively in the characterization of carbon materials that have been obtained from different precursors.5,18-26 Experimental Section Anthracites. An Spanish anthracite, denoted ATO, with a statistical mean random reflectance of R0 ) 3.63 (ISO 7404/5) was selected for this research. Anthracites with different ash (11) Marsh, H.; Warburton, A. F. J. Appl. Chem. 1970, 20, 133. (12) Oya, A.; Marsh, H. J. Mater. Sci. 1982, 17, 309. (13) Dhakate, S. R.; Mathur, R. B.; Bahl, O. P. Carbon 1997, 35, 1753. (14) Mochida, I.; Ohtsubo, R.; Takeshita, K.; Marsh, H. Carbon 1980, 18, 117. (15) Mochida, I.; Ohtsubo, R.; Takeshita, K.; Marsh, H. Carbon 1980, 18, 25. (16) Yu, J. K.; Ueno, S.; Li, H. X.; Hiragushi, K. J. Eur. Ceram. Soc. 1999, 19, 2843. (17) Maldonado-Ho¨dar, F. J.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hanzawa, Y.; Yamada, Y. Langmuir 2000, 16, 4367. (18) Kajiura, K.; Tanabe, Y.; Yasuda, E. Carbon 1997, 34, 1169. (19) Oberlin, A. Carbon 1984, 22, 521. (20) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. J. Mater. Chem. 1998, 8, 2875. (21) Franklin, R. E. Acta Crystallogr. 1951, 4, 253. (22) Waldek Zerda, T.; Gruber, T. Rubber Chem. Technol. 2000, 73, 284. (23) Tunistra, F.; Koening, J. L. J. Chem. Phys. 1970, 53, 1126. (24) Lespade, P.; Marchand, A.; Couzi, M.; Cruege, F. Carbon 1984, 22, 375. (25) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Carbon 1994, 32, 1523. (26) Montes-Mora´n, M. A.; Young, R. J. Carbon 2002, 40, 845.
Table 2. Elemental Concentrations of Aluminum, Calcium, Iron, Potassium, Magnesium, Manganese, Silicon, and Titanium in the Ashes of the ATO and ATOD1-ATOD4 Anthracitesa Concentration (wt %) element
ATOD1
ATOD2
ATOD3
ATO
ATOD4
Al Ca Fe K Mg Mn Si Ti
13.35 0.18 11.18 0.99 5.67 0.03 11.68 0.38
12.01 0.33 7.41 1.24 5.22 0.03 12.89 0.62
15.54 0.26 4.49 2.22 2.13 0.02 20.53 0.79
13.17 0.23 5.02 2.30 1.36 0.02 25.36 0.63
16.32 0.12 1.97 3.05 0.45 0.01 27.31 0.93
a
Given in terms of weight percentage of ash.
contents (ATOD1-ATOD4) were obtained from ATO by consecutive immersion in mixtures of organic liquids (xylol, perchloroethylene, and bromoform) of increasing density (1.471.80 g/cm3). Their ash contents, elemental analyses, and sulfur forms are reported in Table 1. This procedure, which is a conventional coal washability test (the float-sink test), relies on the density differences between the organic matter and the mineral matter, as described in ASTM Standard D4371-91. The major inorganic elements (aluminum, calcium, iron, potassium, magnesium, manganese, silicon, and titanium present in the mineral matter of the anthracites were analyzed in the ashes using X-ray fluorescence (XRF) spectrometry (Siemens, model 3000 spectrometer), using fused glass disks. The ash sample (0.6 g) was fused with a mixture of lithium tetraborate and metaborate (6 g) to prepare the glass disks. The concentration of these inorganic elements, expressed as a weight percentage of the ash, are given in Table 2. Graphitization. The anthracites at a particle size of e212 µm were carbonized at 1000 °C in a tube furnace, under nitrogen flow, for 1 h at a heating rate of 2 °C/min, and then graphitized. The graphitization experiments were performed at 2400, 2500, and 2600 °C in a graphite furnace for 1 h under an argon flow. The heating rates were 20 °C/min from room temperature to 2000 °C, and then 10 °C/min from 2000 °C to the prescribed temperature. X-ray Diffractometry (XRD). The diffractograms of the samples were recorded in a Siemens model D5000 powder diffractometer that was equipped with a monochromatic Cu KR X-ray source and an internal standard of silicon powder. Diffraction data were collected by step scanning with a step size of 0.02° 2θ and a scan step time of 1 s. For each sample, five diffractograms were obtained, using a different representative batch of sample for each run. The d002 value was evaluated from the position of the (002) peak, applying Bragg’s equation. The Lc and La values were calculated from the (002) and (110) peaks, respectively, using the Scherrer formula, with K values of 0.9 for Lc and 1.84 for La.27 The broadening of diffraction peaks due to instrumental factors was corrected with the use of a silicon standard. (27) Biscoe, J.; Warren, B. J. Appl. Phys. 1942, 13, 364.
Structure of Graphite Material from Anthracites
Energy & Fuels, Vol. 19, No. 1, 2005 265
Table 3. XRD Crystalline Parameters and Raman Ratio (ID/It) of the Materials Prepared from the ATO and ATOD1-ATOD4 Anthracites Crystallite Size (nm) along a-axis, along c-axis, La Lc
temp, T (°C)
interlayer spacing, d002 (nm)
2400 2500 2600
0.3448 0.3439 0.3421
ATOD1 Anthracite 5.0 12.5 6.0 12.1 7.4 14.9
38.3 37.3 34.3
2400 2500 2600
0.3423 0.3433 0.3405
ATOD2 Anthracite 7.2 13.8 6.2 13.5 9.8 19.0
32.1 35.8 28.3
2400 2500 2600
0.3416 0.3425 0.3404
ATOD3 Anthracite 7.6 14.7 6.7 16.3 9.6 19.4
24.7 32.1 21.7
2400 2500 2600
0.3430 0.3410 0.3401
2400 2500 2600
0.3398 0.3402 0.3402
a
ATO Anthracite 6.4 8.7 10.2
ID/It (%)a
19.5 25.3 27.1
31.7 18.2 18.3
ATOD4 Anthracite 11.0 39.3 11.0 38.1 10.5 37.6
15.7 19.1 17.0
It ) IG + ID + ID′. ID is the Raman D-band intensity.
Raman Spectroscopy. Raman spectra were obtained in a Renishaw 1000 System that used the green line of an argon laser (λ ) 514.5 nm) as an excitation source and was equipped with a charge-coupled device (CCD) camera. The 50× objective lens of an Olympus model BH-2 optical microscope was used both to focus the laser beam (at a power of ∼25 mW) and to collect the scattered radiation. Extended scans from 3000 to 1000 cm-1 were performed to obtain the first- and second-order Raman bands of the samples, with typical exposure times of 30 s. To assess differences within samples, at least 21 measurements were performed on different particles of each individual sample. The intensity (I) of the bands was measured using a mixed Gaussian-Lorentzian curve-fitting procedure.
Results and Discussion The interlayer spacing (d002) and crystallite sizes (Lc and La), as well as the relative intensity of the Raman D-band (ID/It, where It ) IG + ID + ID′) of the ATO and ATOD1-ATOD4 anthracites after heat treatment, are summarized in Table 3. Typical standard errors of crystallite sizes are
