Structural Characterization of Graphite MaterialsPrepared from Anthracites of Different Characteristics:

A Comparative Analysis

David Gonzalez, Miguel A. Montes-Moran, Isabel Suarez-Ruiz, andAna B. Garcia1

Instituto Nacional del Carbon, CSIC, Francisco Pintado Fe 26, 33011-Oviedo, Spain

Received July 18, 2003

Graphite materials were prepared from two Spanish anthracites, AF and ATO, by heating atdifferent temperatures within the range 2000-2800 °C. XRD and Raman spectroscopy wereemployed to characterize the degrees of crystallinity and crystal orientation of the materials. Inaddition to studying the evolution of typical crystal parameters such as interlayer spacing, d002,and crystallite sizes, La and Lc, with temperature, this work aimed to evaluate the influence ofelemental composition, texture (as measured by optical microscopy), and mineral matter of theraw anthracites on their ability to graphitize. Two temperature segments were discerned duringthe development of crystallinity. The first segment exhibited major improvements in crystalparameters, which afterward reached a plateau value. Raman parameters indicated that furtherimprovement in crystal orientation could be obtained after heating at the highest temperature(2800 °C). The limiting temperature at which the materials showed their highest degree ofstructural order, i.e., the temperature at which the plateau was reached, was lower for the mostgraphitizable anthracite (AF). This anthracite was found to have higher hydrogen and mineralmatter (specifically Al, Fe, K, and Si) contents. However, the textural anisotropy of this mostgraphitizable anthracite was lower than that of the other anthracite under study (ATO). Opticalmicroscopy characterization of the carbonized materials showed that this trend changed afterheating the anthracites at 1000 °C, i.e., the anisotropy of the texture in the carbonized AF washigher than that of the corresponding carbonized material prepared from ATO. It was concludedthat the structural and textural changes of the anthracites during carbonization, which are relatedwith both their microtexture and hydrogen content, influence the graphitization process.

Introduction

Synthetic graphite is a highly valuable material withmany applications,1,2 its manufacturing process involv-ing the selection of carbon materials (precursors) thatgraphitize readily.1 Currently, petroleum coke of variousgrades is used as the main filler material in themanufacturing of synthetic graphite. Coal, with knownworld reserves more than five times higher than thatof oil,3 appears as a possible precursor of differentcarbon materials. Among the different classes of coals,anthracites were found to graphitize when heated attemperatures above 2000 °C.4,5 Anthracites carboncontent is over 90%, which is arranged in a macromo-lecular structure of condensed aromatics rings forminglarge units (graphene layers) bridged or “cross-linked”by aliphatic and/or ether groups,6 conferring on them a

certain structural order. The removal of the “cross-links”by heating the anthracite at high temperatures shouldfacilitate the reorganization of the aromatic units intographite-like structures. Conversion of anthracites intographite materials offers a potential alternative topetroleum coke. Moreover, graphite materials with anacceptable degree of structural order were obtained fromanthracites at temperatures g2700 °C.5,7,8

The most significant changes in the structure of thegraphite materials prepared from anthracites werefound to occur in the temperature range 2000-2700 °C.No significant improvements of the interlayer spacingwere observed at higher graphitization temperatures,thus suggesting that a plateau may be reached at sometemperature below 2700 °C.7 For potential use ofanthracites as precursors of graphite materials, theknowledge of this limiting temperature at which thematerial prepared reaches its highest degree of crystal-linity is more than interesting. To determine thistemperature, a step-by-step study of the developmentof the structural order of the graphite materials in the

* Corresponding author. Tel: +34 98 511 89 54. Fax: +34 98 52976 62. E-mail: anabgs@incar.csic.es.

(1) Pierson H. O. Handbook of carbon, graphite, diamonds andfullerenes; Noyes: Park Ridge, NJ, 1993; pp 87-121.

(2) Kalyoncu, R. S. Graphite; U.S. Geological Survey MineralsYearbook, U. S. Department of Interior: Reston, VA, 2001; p 314. InU.S. Geological Survey Minerals Yearbook, U.S. Department ofInterior, Reston, VA, 2001: pp 341-343.

(3) Schobert, H. H.; Song, C. Fuel 2002, 81, 15-32.(4) Franklin, R. E. Proc. R. Soc. 1951, 209, 196-218.(5) Oberlin, A.; Terriere, G. Carbon 1975, 13, 367-376.

(6) van Krevelen, D. W. Coal: Typology, Physics, Chemistry andConstitution; Elsevier: Amsterdam, 1993; pp 777-810.

(7) Atria, J. V.; Rusinko, F., Jr.; Schobert, H. H. Energy Fuels 2002,16, 1343-1347.

(8) Bustin, R. M.; Rouzaud, J. N.; Ross, J. V. Carbon 1995, 33, 679-691.

365Energy & Fuels 2004, 18, 365-370

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above-mentioned temperature range should be done. Inthis research, Spanish anthracites of different charac-teristics were heated in the temperature interval 2000-2800 °C with the aim of studying the evolution with thetemperature of the crystallinity and orientation of thematerials obtained. While there are some data on thegraphitization behavior of the anthracites at 2000 °Cand the cited g2700 °C,5,7,8 to the authors’ knowledge,the information available at temperatures betweenthose temperatures is very scarce and practically limitedto experiments carried out at 2500 °C in which TEMwas the main technique used to characterize the struc-tural order of the materials.5,9,10

In addition to the treatment temperature, somecharacteristics of the anthracite influence the graphi-tization process. Among them, the coal mineral matterhas been suggested to act as a graphitization catalyst.5,11

Thus, graphitized regions surrounding mineral particleswere observed by HRTEM in heat-treated anthracitesat temperatures as low as 1300 °C.11 Also a significantimprovement of the structural order of the graphitematerials by using anthracites with high mineral mattercontents was found.12 These findings seem to indicatethat the catalytic effect of the mineral matter is associ-ated with both its amount and its distribution, a closeproximity between the organic and the mineral mattersof the anthracite improving that effect. The texture ofthe anthracites was also related to their ability tographitize, specifically when there is preferential planarorientation of the polyaromatic basic structural unitsBSUs.5,13 Finally, more ordered graphite materials wereprepared from anthracites with a higher H/C atomicratio,7 inferring the influence of the elemental composi-tion on the graphitization. Therefore, all these influenc-ing factors should be taken into account to study thegraphitization of the anthracites. On the basis of thatconsideration, the second objective of this research isto evaluate globally the influence of the elementalcomposition, texture as measured by optical microscopy,and mineral matter of the anthracite on its ability tographitize.

The interlayer spacing, d002, and crystallite sizes alongthe c axis, Lc, and the a axis, La, calculated from XRD,as well as the relative intensity of the D Raman band,ID/It, are used in this study to assess the degree of thestructural order of the graphite materials prepared.Both X-ray diffraction and Raman spectroscopy tech-niques have been employed extensively for the charac-terization of carbon materials.14-22 Given that the

presence of imperfections in the graphite materials leadsto lower densities, helium densities were also measuredto estimate the structural characteristics of thesematerials.23

Experimental Section

Anthracites: Selection and Characterization. Twoanthracites, denoted AF and ATO, from Villablino in north-west Spain were selected for this research. Their proximateand elemental analyses are reported in Table 1. Majorinorganic elements Al, Fe, K, and Si present in the mineralmatter of the anthracites were analyzed in the ashes by X-rayfluorescence spectrometry in a Siemens 3000 using fused glassdisks. The ash sample (0.6 g) was fused with a mixture oflithium tetraborate and metaborate (6 g) to prepare the glassdisks. The concentration of these inorganic elements, expressedas wt % of coal organic matter (100 - wt % ash) to bettercompare the results among the two anthracites, are given inTable 2.

Graphitization. The anthracites at a particle size of e 212µm were carbonized at 1000 °C in a tube furnace, undernitrogen flow, for 1 h with a heating rate of 2 °C min-1, andthen graphitized. The graphitization experiments were carriedout at 2000, 2200, 2300, 2400, 2500, 2600, 2700, and 2800 °Cin a graphite furnace for 1 h under an argon flow. The heatingrates were 20 °C min-1 from room temperature to 2000 °C,and 10 °C min-1 from 2000 °C to the prescribed temperature.

Reflectance Measurements. The measurements wereperformed on particulate pellets with randomly orientedparticles using a MPV-Combi (Leitz) microscope, reflectedlight, and oil immersion objectives (50×). The pellets wereprepared by a modified procedure of the ISO 7404/02 standard.The statistical mean random reflectance (Ro) was obtainedfollowing the analytical method described in ISO 7404/5. Themaximum and minimum apparent reflectances (R′max andR′min) were measured in polarized light and during a rotationof the microscope stage through 360°. For each sample, aminimum of 200 particles were measured. Based on thesemeasured values, the principal axes of the reflectance indicat-

(9) Duber, S.; Rouzaud, J. N.; Beny, C.; Dumas, D. ExtendedAbstracts, 21st Biennal Carbon Conf. 1993, 316.

(10) Deubergue, A.; Oberlin, A.; Oh, J. H.; Rouzaud, J. N. Carbon1987, 8, 375.

(11) Evans, E. L.; Jenkins, J. L.; Thomas, J. M. Carbon 1972, 10,637.

(12) Zeng, S. M.; Rusinko, F.; Schobert, H. H. In Producing high-quality carbon and/or graphite materials from anthracites by catalyticgraphitization; Commonwealth of Pennsylvania, Pennsylvania EnergyDevelopment Authority, Harrisburg, PA, Final Technical Report (Grant9303-4019), 1996.

(13) Blanche, C.; Rouzaud, J. N.; Dumas, D. Extended Abstr. 22ndBiennal Carbon Conf. 1995, 694.

(14) Kajiura, K.; Tanabe, Y.; Yasuda, E. Carbon 1997, 34, 169.(15) Oberlin, A. Carbon 1984, 22, 521.(16) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martınez-Alonso,

A.; Tascon, J. M. D. J. Mater. Chem. 1998, 8, 2875.(17) Franklin, R. E. Acta Crystallogr. 1951, 4, 253.(18) Waldek Zerda, T.; Gruber, T. Rubber Chem. Technol. 2000, 73,

284.

(19) Tunistra, F.; Koening, J. L. J. Chem. Phys. 1970, 53, 1126.(20) Lespade, P.; Marchand, A.; Couzi, M.; Cruege, F. Carbon 1984,

22, 375.(21) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martınez-Alonso,

A.; Tascon, J. M. D. Carbon 1994, 32, 1523.(22) Montes-Moran, M. A.; Young, R. J. Carbon 2002, 40, 845.(23) Pierson, H. O. Handbook of carbon, graphite, diamonds and

fullerenes; Noyes: Park Ridge, NJ, 1993; pp 43-69.

Table 1. Proximate and Elemental Analyses and SulfurForms of AF and ATO Anthracites

ATO AF

proximate analyses (wt % db)ash 10.12 19.74volatile matter 4.12 8.72

elemental analyses (wt % daf)carbon 93.13 91.00hydrogen 2.03 3.01nitrogen 0.87 1.40organic sulfur 1.01 0.92oxygen (diff.) 2.96 3.67

sulfur forms (wt % db)total 1.07 1.08pyrite 0.15 0.33sulfate 0.01 0.02organic (diff.) 0.91 0.74

Table 2. Concentrations of Al, Fe, K, and Si in AF andATO Anthracites (wt % of coal organic matter)

anthracite Al Fe K Si

AF 2.70 2.32 0.50 5.03ATO 1.48 0.57 0.26 2.85

366 Energy & Fuels, Vol. 18, No. 2, 2004 Gonzalez et al.

ing surface RIS: RMAX, RMIN, and RINT, and the RIS param-eters: Rev ) reflectance of equivalent volumen isotropic RIS,Rst ) RIS style, and Ram ) RIS anisotropy magnitude werecalculated following the method described by Kilby 24 andDuber et al.25 In addition, BW (RMAX - RMIN) as a measure ofthe bireflectance was also calculated. The values of theseoptical parameters for the raw anthracites and their carbon-ized samples are shown in Table 3.

X-ray Diffraction. The diffractograms of the samples wererecorded in a Siemens D5000 powder diffractometer equippedwith monochromatic Cu KR X-ray source and an internalstandard of silicon powder. Diffraction data were collected bystep scanning with a step size of 0.02° 2θ and a scan step timeof 1 s. For each sample, five diffractograms were obtained,using a different representative batch of sample for each run.The interlayer spacing, d002, was evaluated from the positionof the (002) peak applying Bragg’s equation. The crystallitesizes, Lc and La, were calculated from the (002) and (110)peaks, respectively, using the Scherrer formula, with valuesof K ) 0.9 for Lc and 1.84 for La.26 The broadening of diffractionpeaks due to instrumental factors was corrected with the useof a silicon standard.

Raman Spectroscopy. Raman spectra were obtained in aRenishaw 1000 System using the green line of an argon laser(λ ) 514.5 nm) as excitation source and equipped with a CCDcamera. The 50× objective lens of an Olympus BH-2 opticalmicroscope was used both to focus the laser beam (at a powerof approx 25 mW) and to collect the scattered radiation.Extended scans from 3000 to 1000 cm-1 were performed toobtain the first- and second-order Raman bands of the samples,with typical exposure times of 30 s. To assess differenceswithin samples, at least 21 measurements were performed ondifferent particles of each individual sample. The intensity I,width W, and frequency ν of the bands were measured usinga mixed Gaussian-Lorentzian curve-fitting procedure.

Transmission Electron Microscopy (TEM). Transmis-sion electron microscopy was performed using a Philips CM200microscope working at 200 kV giving a resolution of 0.144 nmin lattice fringes. The 002 lattice fringes technique (002 LF)was used to image directly the profile of aromatic layers.

Helium Density. Density measurements of the sampleswere made with a Helium Pycnometer Accupyc 1330 fromMicromeritics.

Results and Discussion

Effect of the Graphitization Temperature. Theinterlayer spacing, d002, and crystallite sizes, Lc and La,of AF and ATO anthracites after heat treatments areplotted versus temperature in Figures 1 and 2, respec-tively. Typical standard errors of crystallite sizes areless than 2% and 9% of the reported values for Lc andLa, respectively. d002 values are much more precise, withstandard errors lower than 0.1%.

Two temperature segments may be distinguished inthe evolution of the crystallinity of the graphite materi-als prepared from AF and ATO anthracites. Thus, asthe graphitization temperature increases from 2000 °Cto 2400 °C in AF and to 2500-2600 °C in ATO, theinterlayer spacing, d002, decreases to reach plateauvalues of approximately 0.337 and 0.340 nm, respec-tively (Figure 1). In a parallel way, a growth of thecrystallite sizes, particularly at temperatures above2200 °C, is observed (Figure 2). A preferential lateralcoalescence of the crystallites occurs in this temperatureinterval, as confirmed by the higher absolute growth ofthe La with regard to that of Lc. Basically, no significantimprovements in the degree of crystallinity of thegraphite materials prepared from AF and ATO anthra-cites were attained at temperatures above 2400 °C and2500-2600 °C, respectively. As an example, crystallitesizes in the basal planes direction, La, of 25.3, 27.1, and28.6 nm were calculated for the graphitized samples ofATO at 2500, 2600, and 2700 °C; the correspondinginterlayer spacing, d002, being 0.3410, 0.3401, and0.3404 nm.

The variation with the temperature of the relativeintensity of the D Raman band ID/It (It ) ID + IG + ID′)follows a similar tendency (Figure 3). Standard errorsof this ratio are less than 6%. The drop of the D bandrelative intensity with the temperature indicates animprovement of the crystallite orientation of the graph-ite material.16,18-22 Similar to crystalline parameters,the ID/It Raman ratio remains almost unchanged duringthe second temperature segment. However, there aresome slight differences between the information pro-vided by XRD and Raman spectroscopy about thegraphitization process of the anthracites. On the basisof Raman data, the second graphitization temperaturezone of ATO anthracite clearly starts at 2500 °C with a

(24) Kilby, W. E. Int. J. Coal Geology 1988, 9, 267.(25) Duber, S.; Pusz, S.; Kwiecinska, B. K.; Rouzaud, J. N. Int. J.

Coal Geol. 2000, 44, 227.(26) Biscoe, J.; Warren, B. J. Appl. Phys. 1942, 13, 364.

Table 3. Optical Parameters, Measured (%) and Calculated, of AF and ATO Anthracites and Their Carbonized (1000 °C)Samples AFC and ATOC

measured values RIS axes RIS parameters

sample Ro R′max R′min RMAX RINT RMIN Bwa Rev Rst Ram

AF 2.37 2.65 2.10 2.82 2.55 1.62 1.21 2.26 -17.59 0.900AFC 7.17 7.31 5.47 7.67 7.05 3.70 3.97 5.84 -22.19 0.115ATO 3.63 4.22 3.19 4.70 3.93 2.40 2.30 3.53 -11.96 0.105ATOC 7.01 7.65 6.10 7.90 7.43 4.94 2.97 6.61 -21.40 0.078

a Bw ) RMAX - RMIN.

Figure 1. Values of the interlayer spacing, d002, of thegraphite materials prepared from ATO and AF anthracites.

Characterization of Graphite Materials from Anthracites Energy & Fuels, Vol. 18, No. 2, 2004 367

plateau value of 18-19% for the ID/It ratio. Moreover,the graphitized sample from AF anthracite at thehighest temperature (2800 °C) shows a further decreaseof the ID/It ratio to 10.5% (Figure 3). Since the plateauvalue is 13-14% (Figure 3), the difference between bothvalues is statistically significant, thus suggesting animprovement of the crystallite orientation of the mate-rial.

Bands in the second order of the Raman spectrum areobservable only in highly orientated carbons, the most

important feature being the G′ band occurring atapproximately 2700 cm-1 that, in the case of graphite-like materials, tends to split into a doublet.27 Therefore,the percentage of measured particles exhibiting thedoublet of the G′ band should constitute an indicationof the degree of crystallite order for a given material.Those percentages are plotted in Figure 4 for thegraphitized samples prepared from both anthracites.Practically, no particles with doublet of the G′ band weredetected until AF and ATO anthracites were heated at2300 and 2500 °C, respectively. Results of Figure 4 seemto confirm that, regardless of the crystallographicmeasurements, (i) the increase of graphitization tem-perature to 2800 °C improves slightly the structuralorder of the materials prepared from AF anthracite, and(ii) 2500 °C is the temperature at which ATO anthracitereaches its highest degree of structural order.

Helium densities of the graphitized samples from AFand ATO anthracites tend to increase with the temper-ature of treatment (Figure 5), giving an indication thatmore graphite-like materials are being obtained. How-ever, as in the evolution of the crystalline and Ramanparameters (Figures 1-4), no significant improvementsin the structural order of the materials produced from

(27) Angoni, K. J. Mater. Sci. 1998, 33, 3693.

Figure 2. Crystallite sizes, Lc and La, of the graphitematerials prepared from (a) AF and (b) ATO anthracites.

Figure 3. Values of the relative intensity of the D Ramanband, ID/It, calculated for the graphite materials prepared fromATO and AF anthracites.

Figure 4. Percentages of particles exhibiting the G′1, G′2doublet in the second-order Raman spectrum, N*, of thegraphite materials prepared from ATO and AF anthracites.

Figure 5. Helium densities of the graphite materials preparedfrom ATO and AF anthracites.

368 Energy & Fuels, Vol. 18, No. 2, 2004 Gonzalez et al.

AF and ATO anthracites, as indicated by the value ofthis physical property, were found at temperaturesabove 2400 °C and 2500 °C, respectively.

According to TEM observations of graphitized an-thracites5,10 and other hard carbon precursors such aspolyimide films,28 polyaromatic basic structural unitsBSUs arrange around the walls of pores that wouldflatten progressively as the temperature increases.During this progressive flattening, the BSUs coalesce,leading to the appearance of imperfect flat lamellarareas which still retain pore walls normal to the basalplane. Therefore, the crystallinity degree of the materialimproves significantly as shown by the evolution of theXRD parameters during the graphitization of the men-tioned polyimide films.28 The flattened pores collapseat a given temperature, allowing the stacking of thecarbon layers in a graphite-like structure. From thistemperature at which the interlayer spacing, d002, of thegraphitized polyimide was close to the minimum oneachieved, the value of this crystalline parameter wasfound to decrease gradually toward a plateau value.28,29

On the basis of the evolution of the crystallinity ofthe graphitized polyimide films with temperature, theflattened pores of AF and ATO anthracites wouldcollapse at temperatures close to 2400 °C and 2500-2600 °C, respectively, at which the d002 plateau valuesof 0.337 and 0.340 nm were reached (Figure 1). Theanalysis of the XRD profiles showed that of the (10)band splits into (100) and (101) well-defined peaks, andan additional (112) peak appears in AF and ATO treatedat temperatures g2400 °C, and g2500-2600 °C, re-spectively. These facts are associated with the develop-ment of a three-dimensional crystalline structure,17 thusconfirming that the breakage of the pore walls occursat these temperatures. Once the pores have collapsedand the carbon layers are stacked in a more or lessgraphitic structure, the progress of the graphitizationof these anthracites is very limited, specifically in thecase of AF (Figures 1-4). From this point on, theevolution of the crystallinity of the materials with thetemperature depends on the activation energy neededto remove the crystal defects and to promote thecoalescence of different groups of graphite-like crystal-lites along the c and a axis. According to the results ofthe present work, an increase in the temperature oftreatment from 2400 to 2800 °C, such in the case of AFanthracite, is not enough to improve the crystallinitydegree of the graphite material obtained (Figures 1-4).A plausible explanation for this behavior could be thepresence of amorphous and/or low-organized carbonmaterial between the crystallites. This type of carbonhas been reported to prevent the coalescence of thecrystallites by not favoring their complete orientation.30

In this sense, lamellar and amorphous carbon structureswere observed by TEM to coexist even after heating AFanthracite at the highest temperature of 2800 °C(Figure 6). Nevertheless temperatures as high as2800 °C seem to promote a better orientation of the

crystallites, as indicated by Raman measurements ofID/It ratio (Figure 3).

Effect of Anthracite Characteristics. A compara-tive analysis of the graphitization results of the anthra-cites, shown in Figures 1 to 4, leads to the conclusionthat materials with higher degree of crystallinity andorientation can be obtained from AF. Thus, d002 valuesof 0.3371 and 0.3404 nm were measured for samples ofAF and ATO anthracites graphitized at 2700 °C, thecorresponding Raman ID/It ratio being 13% and 19%,respectively. This significant difference in the degreeof graphitizability can be explained attending to theirdifferent characteristics.

Considering the elemental composition of the anthra-cites in Table 1, AF exhibits higher hydrogen contentthan ATO, the H/C atomic ratio being also higher. Asthe heat treatment of the anthracite proceeds, theheteroatomic (oxygen, nitrogen, and sulfur) cross-linkersare removed as their corresponding hydrogen com-pounds, “unlocking” the aromatic layers and, conse-quently, eliminating a significant obstacle to theirstructural reorganization. The residual hydrogen wouldbe located in light molecules, probably of aromaticnature, that can act as a suspensive medium for theBSUs, facilitating their rearrangement, in a way similarto that occurring during the pyrolysis of other carbon-aceous materials.31 This remaining hydrogen can bequantified approximately as the amount of hydrogen perhundred grams of carbon after the stoichiometric cor-rection for the hydrogen that would be removed withthe heteroatoms (net hydrogen).32 Anthracites havingthe highest net hydrogen content were found to be themost graphitizable ones and vice versa.7 It seems to bethe case of AF and ATO anthracites with calculatedhydrogen net contents of 2.38 wt % and 1.50 wt %,respectively. In addition to a higher amount of nethydrogen, AF also shows higher ash content than ATO(19.74 wt % against 10.12 wt % in Table 1). Asmentioned in the introduction of this work, the coalmineral matter may behave as a graphitization cata-lyst.5,11 Improvements of the XRD crystalline param-eters of the graphitized materials with increasing coalash content have been previously found,12 thus agreeingwell with our results. Among the major constituents of(28) Inagaki, M.; Takeichi, T.; Hishiyama, Y.; Oberlin A. In Chem-

istry and Physics of Carbons; Thrower, P. A, Radovic, L. R., Eds.;Marcel Dekker: New York, 1999; Vol. 26, pp 245-333.

(29) Hishiyama, Y.; Igarashi, K.; Kanaoka, I.; Fuji, H.; Kaneda, T.;Koidesawa, T.; Shimazawa, Y.; Yoshida, A. Carbon 1997, 35, 657.

(30) Emmerich, F. G. Carbon 1995, 33, 1709.

(31) Chermin, H. A. G.; Van Krevelen, D. W. Fuel 1957, 36, 85.(32) Donath, E. E. In Chemistry of Coal Utilization; Lowry, H. H.,

Ed.; Marcel Dekker: New York, 1963; pp 1041-1080.

Figure 6. TEM lattice fringe imaging of AF anthracitegraphitized at 2800 °C.

Characterization of Graphite Materials from Anthracites Energy & Fuels, Vol. 18, No. 2, 2004 369

coal mineral matter, clay minerals are known to be oneof the most effective graphitization catalysts of severalcarbon materials, particularly in the presence of iron.33

Different types of aluminum silicates such as illite andkaolinite, have been identified in the LTA residues ofAF and ATO anthracites,34 the contents of Al, Si as wellas K which is present in the illite, and Fe being higherin the most graphitizable AF (Table 2).

The microtexture of the anthracites was related totheir ability to graphitize, 9, 13 specifically when thereis a preferential planar orientation of the polyaromaticbasic structural units (BSUs). Apart from other methodssuch as TEM, 5,9,13,15 the microtexture can be studiedon the basis of anthracite optical properties which aredefined by the reflectance indicating surface (RIS).Principal axes (RMAX, RMIN and RINT) and RIS param-eters (Rev, Rst, Ram) characterize the textural anisotropyand the structure of the BSUs (microtexture) of theanthracites as well as their changes during heat treat-ment.24,25,35 Assuming that the anisotropy of the texture(spatial arrangements of the BSUs), as estimated frombireflectance values (Bw), increases with the degree ofplanar orientation, anthracites with larger BW valuesshould be expected to show a better degree of graphi-tizability. However, according to the BW values in Table3, the most graphitizable AF anthracite exhibits a lowerdegree of textural anisotropy than ATO anthracite. Onthe other hand, due to the transformations caused bythermal treatment which include devolatilization, aro-matization, and rearrangement of the BSUs,36 valuesof RMAX and RMIN of the AF and ATO anthracitesincrease during carbonization. The magnitude of thisincrease is higher for RMAX, leading in turn to carbon-ized samples with larger anisotropy of the texture asshown by the values of the BW parameter. Unlike rawanthracites, the carbonized AFC exhibits a higherdegree of textural anisotropy than ATOC (Table 3).

The RIS parameter Rev has been suggested to char-acterize the structure of the BSUs, 36 its improvementduring heating being a consequence of their chemicalstructural ordering. As shown in Table 3, a higher valueof this parameter is obtained for ATO, suggesting amore ordered structure of the BSUs in this anthracitewhich also shows a larger anisotropy of the texturewhen compared to that of AF, as measured by the BWparameter. At temperatures up to 1000 °C, changes inthe structure of the BSUs are mainly of a chemicalnature as a consequence of the removal of heteroatomsand other light compounds. The significance of thesechanges depends on the amount of hydrogen availableand the microtexture of the initial anthracite. Therefore,

AF anthracite shows a greater growth of Rev duringcarbonization (Table 3). However, on the basis of thevalues of Rev, the structure of the BSUs is still moreordered in the carbonized sample of ATO anthracite(ATOC) than in AFC sample. Nevertheless, one shouldkeep in mind that, although higher values of RISparameter Rev were calculated for anthracites with amore anisotropic texture of the BSUs as determined bythe bireflectance BW, no direct relation was foundbetween these two optical parameters.36 In conclusion,the analysis of the optical properties of the samples inTable 3 suggests that the ability of the anthracites tographitize depends, among other factors, on the anisot-ropy of the texture of their carbonized form, the presenceof a higher amount of hydrogen in the raw anthracitefacilitating both the structural ordering and the rear-rangements of the BSUs in the carbonization process.

Conclusions

Two temperature segments, the second one being aplateau, were found to happen in the evolution of thecrystallinity and degree of orientation of the graphitematerials prepared from the anthracites. Once thecarbon layers are stacked in a more or less graphiticstructure after the pores collapse, the presence ofamorphous and/or low-organized carbon between thecrystallites prevents their coalescence, thus limiting theprogress of the graphitization.

More graphite-like materials were achieved from theanthracite with higher hydrogen and mineral mattercontents. The degree of preferential planar orientationof the BSUs in the carbonized form of this anthracite,as measured by optical microscopy, was also higher.

The structural and textural changes of the anthracitesduring carbonization, which are related with both theirmicrotexture and hydrogen content, influence the graphi-tization process. The ability of the anthracites tographitize was found to depend, among other factors,on the anisotropy of the texture of their carbonized formrather than on the anisotropy of the raw anthracite asreported previously.

The limiting temperature at which the graphitematerials prepared from the anthracites reach theirhighest degree of structural order is lower for theanthracite exhibiting higher anisotropy of the textureand/or degree of preferential planar orientation of theBSUs after the carbonization process.

Acknowledgment. Financial support from DGICYT(project MAT2001-1843) and FICYT (project PB-EXP01-01) is gratefully acknowledged. One of the authors (D.Gonzalez) thanks FICYT for a personal grant. Thanksare also due Dr. M. A. Banares of the Instituto deCatalisis, CSIC, Madrid (Spain) for providing access tothe Raman spectrometer.

EF030144+

(33) Marsh, H.; Warburton, A. F. J. Appl. Chem. 1970, 20, 133.(34) Gonzalez, D. Doctoral Thesis, 2003.(35) Kilby, W. E. Int. J. Coal Geol. 1991, 19, 201.(36) Pusz, S.; Duber, S.; Kwiecinska, B. K. Fuel Process. Technol.

2002, 77-78, 173.

370 Energy & Fuels, Vol. 18, No. 2, 2004 Gonzalez et al.