Influence of Inherent Coal Mineral Matter on theStructural Characteristics of Graphite Materials

Prepared from Anthracites

David Gonzalez, Miguel A. Montes-Moran, and Ana B. Garcia*

Instituto Nacional del Carbon, 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 mattercompositionsand, therefore, microtextureswere obtained by consecutive immersion in mixturesof 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 themineral matter. Graphite materials were then prepared by heating the anthracites in thetemperature interval of 2400-2600 °C for the purpose of studying the influence of the anthracitemineral 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-raydiffractometry (XRD), as well as the relative intensity of the Raman D-band (ID/It), were used toassess the degree of structural order of the materials. A progressive increase in this degree ofstructural order with increasing mineral matter content of the anthracite was observed. Thecatalytic effect of the mineral matter on the graphitization of the anthracites relies mainly onpromotion of the growth of the crystallites along the basal plane. Reasonably good linearcorrelations 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 ironcarbonates ankerite and siderite were observed to be the main active catalyst compounds duringthe 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, wasobserved to improve the catalytic effect of the mineral matter.

Introduction

Graphite materials with structural characteristicscomparable to those of commercially available syntheticgraphites can be obtained from anthracites that havebeen heated at temperatures of g2400 °C.1-5 In additionto the treatment temperature, some of the characteris-tics of the anthracite also influence the graphitizationprocess. Among them, the mineral matter has beensuggested to act as a graphitization catalyst.1,6-8 More-over, the microtexture of the anthracite was also relatedto its ability to graphitize, specifically when there is apreferential planar orientation of the polyaromatic basicstructural units (BSUs).1,9 Finally, more-ordered graph-

ite materials were prepared from anthracites with ahigher H/C atomic ratio, inferring that the elementalcomposition influences the graphitization process.4 Inprevious work,4,8,10 anthracites with different mineralmatter contents were graphitized. A significant im-provement of the structural order was observed for thegraphite materials prepared from the anthracites withthe highest mineral matter content. Because of thedifferent microtexture and organic matter compositionof the anthracites studied, these results were, however,inconclusive in establishing the specific contribution ofthe mineral matter to this improvement.

In the present work, an anthracite with a low degreeof graphitizability8 was separated into different fractionsby consecutive immersion in mixtures of organic liquidsof increasing density. Using this conventional coalwashability procedure (the float-sink test), anthraciteswith different mineral matter content and compositionwere obtained. The organic matter composition and,therefore, the microtexture of these anthracites (includ-ing the raw one) were similar, thus reducing the numberof possible factors that would affect the graphitizationto the mineral matter. On the basis of this consideration,

* Author to whom correspondence should be addressed. Telephone:+34 98 511 89 54. Fax: +34 98 529 76 62. E-mail: anabgs@incar.csic.es.

(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) Gonzalez, D.; Montes-Moran, 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) Gonzalez, D.; Montes-Moran, 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.

(10) Zeng, S. M.; Rusinko, F.; Schobert, H. H. Producing High-Quality Carbon and/or Graphite Materials from Anthracites byCatalytic Graphitization; Commonweath of Pennsylvania, Pennsyl-vania Energy Development Authority, Harrisburg, PA, Final TechnicalReport (Grant 9303-4019), 1996.

263Energy & Fuels 2005, 19, 263-269

10.1021/ef049893x CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 11/05/2004

the anthracites were heated in the temperature intervalof 2400-2600 °C for the purpose of studying theinfluence of the inherent mineral matter (amount andcomposition) 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, andtitanium) have been used as graphitization catalysts forvarious graphitizable and nongraphitizable carbonmaterials.11-17 To explore the catalytic effect of thedifferent constituents of the anthracite mineral matter,the content of the above-mentioned elements, as wellas potassium, in the anthracites was determined. Spe-cial attention was given to aluminosilicate minerals,which are one of the main constituents of the coalmineral matter and have been observed to be remark-ably effective catalysts for other carbon materials,particularly in the presence of iron.11,12

The interlayer spacing (d002) and crystallite sizesalong the c-axis (Lc) and the a-axis (La), calculated fromX-ray diffractometry (XRD), as well as the relativeintensity of the Raman D-band (ID/It), are used in thisstudy to assess the degree of structural order of thegraphite materials that have been prepared. Both XRDand Raman spectroscopy techniques have been usedextensively in the characterization of carbon materialsthat have been obtained from different precursors.5,18-26

Experimental Section

Anthracites. An Spanish anthracite, denoted ATO, with astatistical mean random reflectance of R0 ) 3.63 (ISO 7404/5)was selected for this research. Anthracites with different ash

contents (ATOD1-ATOD4) were obtained from ATO by con-secutive immersion in mixtures of organic liquids (xylol,perchloroethylene, and bromoform) of increasing density (1.47-1.80 g/cm3). Their ash contents, elemental analyses, and sulfurforms are reported in Table 1. This procedure, which is aconventional coal washability test (the float-sink test), relieson the density differences between the organic matter and themineral matter, as described in ASTM Standard D4371-91.The major inorganic elements (aluminum, calcium, iron,potassium, magnesium, manganese, silicon, and titaniumpresent in the mineral matter of the anthracites were analyzedin 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 lithiumtetraborate and metaborate (6 g) to prepare the glass disks.The concentration of these inorganic elements, expressed asa weight percentage of the ash, are given in Table 2.

Graphitization. The anthracites at a particle size of e212µm were carbonized at 1000 °C in a tube furnace, undernitrogen flow, for 1 h at a heating rate of 2 °C/min, and thengraphitized. The graphitization experiments were performedat 2400, 2500, and 2600 °C in a graphite furnace for 1 h underan argon flow. The heating rates were 20 °C/min from roomtemperature to 2000 °C, and then 10 °C/min from 2000 °C tothe prescribed temperature.

X-ray Diffractometry (XRD). The diffractograms of thesamples were recorded in a Siemens model D5000 powderdiffractometer that was equipped with a monochromatic CuKR X-ray source and an internal standard of silicon powder.Diffraction data were collected by step scanning with a stepsize of 0.02° 2θ and a scan step time of 1 s. For each sample,five diffractograms were obtained, using a different represen-tative batch of sample for each run. The d002 value wasevaluated from the position of the (002) peak, applying Bragg’sequation. The Lc and La values were calculated from the (002)and (110) peaks, respectively, using the Scherrer formula, withK values of 0.9 for Lc and 1.84 for La.27 The broadening ofdiffraction peaks due to instrumental factors was correctedwith the use of a silicon standard.

(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-Hodar, 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.; Martınez-Alonso,

A.; Tascon, 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.; Martınez-Alonso,

A.; Tascon, J. M. D. Carbon 1994, 32, 1523.(26) Montes-Moran, M. A.; Young, R. J. Carbon 2002, 40, 845. (27) Biscoe, J.; Warren, B. J. Appl. Phys. 1942, 13, 364.

Table 1. Ash Contents, Elemental Analyses, and Sulfur Forms of the ATO and ATDO1-ATDO4 Anthracites

parameter ATOD1 ATOD2 ATOD3 ATO ATOD4

ash (wt %, db) 2.07 3.66 6.54 10.12 19.07elemental analysis (wt %, daf)

carbon 94.76 94.45 94.20 93.13 91.54hydrogen 2.35 2.41 2.17 2.03 2.16nitrogen 0.86 0.87 0.85 0.87 0.77organic sulfur 0.87 0.88 0.96 1.01 1.03

sulfur forms (wt %, db)total 0.86 0.85 0.92 1.07 0.91pyrite n.d.a n.d.a 0.02 0.15 0.05sulfate n.d.a n.d.a n.d.a 0.01 0.02organic (diff.) 0.86 0.85 0.90 0.91 0.84

a Not detected.

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 13.35 12.01 15.54 13.17 16.32Ca 0.18 0.33 0.26 0.23 0.12Fe 11.18 7.41 4.49 5.02 1.97K 0.99 1.24 2.22 2.30 3.05Mg 5.67 5.22 2.13 1.36 0.45Mn 0.03 0.03 0.02 0.02 0.01Si 11.68 12.89 20.53 25.36 27.31Ti 0.38 0.62 0.79 0.63 0.93

a Given in terms of weight percentage of ash.

264 Energy & Fuels, Vol. 19, No. 1, 2005 Gonzalez et al.

Raman Spectroscopy. Raman spectra were obtained in aRenishaw 1000 System that used the green line of an argonlaser (λ ) 514.5 nm) as an excitation source and was equippedwith a charge-coupled device (CCD) camera. The 50× objectivelens of an Olympus model BH-2 optical microscope was usedboth to focus the laser beam (at a power of ∼25 mW) and tocollect the scattered radiation. Extended scans from 3000 to1000 cm-1 were performed to obtain the first- and second-orderRaman bands of the samples, with typical exposure times of30 s. To assess differences within samples, at least 21measurements were performed on different particles of eachindividual sample. The intensity (I) of the bands was measuredusing 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 theRaman D-band (ID/It, where It ) IG + ID + ID′) of theATO and ATOD1-ATOD4 anthracites after heat treat-ment, are summarized in Table 3. Typical standarderrors of crystallite sizes are <2% and <9% of thereported values for Lc and La, respectively. The d002values are much more precise, with standard errors of<0.1%. The average intensities of the Raman bandswere calculated with errors of <6%.

Effect of Mineral Matter Content. The degree ofstructural order of the materials that have been pre-pared increases as the mineral matter content (ex-pressed in terms of ash content in Table 1) of theanthracite increases (see Table 3). For example, at 2400°C, the ATOD1, ATO, and ATOD4 anthracites, with ashcontents of 2.07, 10.12, and 19.07 wt %, respectively,lead to materials with d002 values of 0.3448, 0.3430, and0.3398 nm, respectively. The catalytic effect of theinherent mineral matter on the graphitization of theATO and ATOD1-ATOD4 anthracites relies mainly onpromotion of the growth of the crystallites along thebasal plane, as indicated by the higher absolute growthof La (from 12.5 nm to 39.3 nm), relative to that of Lc

(from 5.0 nm to 11.0 nm). To explore a possible correla-tion between the mineral matter content of the anthra-cites and the XRD crystalline parameters of the mate-rials that are prepared, linear regression analyses wereperformed with the values appearing in Tables 1 and3. This type of analysis gave equations with correlationscoefficients of R2 ) 0.940, 0.986, and 0.977 for La valuesof the materials that are obtained at 2400, 2500, and2600 °C, respectively.

In previous work,8 preferential crystallite growth inthe direction of the basal plane occurred during thegraphitization of the ATO anthracite at temperaturesof >2000 °C. The increase of the La value with HTT incarbon materials is due to two processes: (i) vegetativegrowth (in-plane), in which the existing crystallites(BSUs) grow via the incorporation of amorphous carbon,and (ii) coalescence of the crystallites along the a-axis.Although the in-plane growth starts at temperaturesof <1000 °C and can continue as long as low-organizedcarbon remains in the material, the coalescent increaserequires temperatures up to 2400 °C, depending on theability of the carbon to graphitize.28 Low-organizedcarbon structures have been even observed to remainas such in graphite materials that have been preparedfrom anthracites at temperatures up to 2800 °C.8,29

Therefore, it can be assumed that the two crystallitegrowth processes coexist during the graphitization ofthe ATO and ATOD1-ATOD4 anthracites, thus con-tributing to the increase of La with ash content (seeTables 1 and 3). Following a model proposed to explainthe catalytic graphitization of hard carbons,30 the activeconstituents (metals) of the mineral matter of theanthracites studied in this work would preferentiallyreact with the nonorganized carbon in the boundariesof the BSUs (turbostratic domains) to form carbides;their further decomposition would lead to graphiticcarbon, and the size of the already-existing graphite-like layers would be increased. As a consequence of thisvegetative growth, the lateral coalescence of the crys-tallites that are arranged around the walls of theflattening pores during the graphitization process1 isfavored, thus explaining the dependence of growth inthe a-axis direction (increased La) on the ash content ofthe anthracite.

The trend followed by the relative intensity of theRaman D-band (ID/It, where It ) IG + ID + ID′) of thematerials, relative to the ash content of the anthracite,is similar to that of the XRD parameters (see Table 3).The decay of this band ratio indicates an improvementof the crystallite orientation of the materials,20,22-26

confirming the catalytic effect of the inherent mineralmatter on the graphitization of the ATO and ATOD1-ATOD4 anthracites. Unlike La, no linear relationshipwas observed between this Raman parameter and theash content of the anthracite, or between ID/It and(1/La), despite the linear correlation that has beenexperimentally postulated.23

The evolution of the structural order of the materialsprepared from ATO and ATOD1-ATOD4 with thetemperature is dependent on the ash content of theanthracite (see Table 3). Thus, the graphitization of the

(28) Emmerich, F. G. Carbon 1995, 33, 1709.(29) Deubergue, A.; Oberlin, A.; Oh, J. H.; Rouzaud, J. N. Carbon

1987, 8, 375.(30) Oberlin, A.; Rouchy, J. P. Carbon 1971, 9, 39.

Table 3. XRD Crystalline Parameters and Raman Ratio(ID/It) of the Materials Prepared from the ATO and

ATOD1-ATOD4 Anthracites

Crystallite Size (nm)temp,T (°C)

interlayerspacing,d002 (nm)

along c-axis,Lc

along a-axis,La ID/It (%)a

ATOD1 Anthracite2400 0.3448 5.0 12.5 38.32500 0.3439 6.0 12.1 37.32600 0.3421 7.4 14.9 34.3

ATOD2 Anthracite2400 0.3423 7.2 13.8 32.12500 0.3433 6.2 13.5 35.82600 0.3405 9.8 19.0 28.3

ATOD3 Anthracite2400 0.3416 7.6 14.7 24.72500 0.3425 6.7 16.3 32.12600 0.3404 9.6 19.4 21.7

ATO Anthracite2400 0.3430 6.4 19.5 31.72500 0.3410 8.7 25.3 18.22600 0.3401 10.2 27.1 18.3

ATOD4 Anthracite2400 0.3398 11.0 39.3 15.72500 0.3402 11.0 38.1 19.12600 0.3402 10.5 37.6 17.0

a It ) IG + ID + ID′. ID is the Raman D-band intensity.

Structure of Graphite Material from Anthracites Energy & Fuels, Vol. 19, No. 1, 2005 265

anthracites with ash contents of e10.12 wt % (ATDO1-ATOD3 and ATO) progresses as the temperature in-creases from 2400 °C to 2600 °C. However, no improve-ment in the degree of crystallinity is observed in thesame temperature range for materials prepared fromthe ATOD4 anthracite (19.07 wt % ash). In previouswork,8 the graphitization of the ATO anthracite beyond2500-2600 °C was determined to be very limited, withno significant changes in the structural parametersobserved. It was then concluded that the breakage ofthe pores walls as a consequence of their flattening,1should occur in this temperature interval. As mentionedpreviously, the mineral matter preferentially promotesthe side-by-side coalescence of the crystallites (BSUs)during the graphitization of the ATO and ATOD1-ATOD4 anthracites. In addition to increasing La, thiscoalescence should also facilitate the progressive poreflattening, thus decreasing the temperature at whichtheir breakage occurs. On the basis of this consideration,an increase of the ash content of the anthracite from10.12 wt % (ATO) to 19.07 wt % (ATOD4) should reducethis temperature, limiting the structural evolution ofthe materials at temperatures of >2400 °C (see Table3).

An unexpected slight decrease of the Lc and ID/Itvalues can be noticed for the materials that have beenprepared from the ATOD2 and ATOD3 anthraciteswhen increasing the temperature from 2400 °C to 2500°C (see Table 3). Correspondingly, an increase of the d002values is observed. Similar variations of this structuralparameter were previously reported by Gilyazov31 tooccur during the graphitization of anthracites. Accordingto this author, the decomposition of the metal carbidesin this temperature interval disorders the carbon net-work and stresses the internal structure, resulting inan increase of the d002 value. A further temperatureincrease would cause the relaxation of the internalstress, allowing the progress of the graphitization.Silicon carbide is the only carbide that is thermody-namically stable at temperatures of g2400 °C. Siliconconcentrations of 0.48 and 1.43 wt % were determinedin the ATOD2 and ATOD3 anthracites, respectively;these values are higher than that for the ATOD1anthracite (0.24 wt %), which does not exhibit the above-mentioned effect (see Table 3). However, this effectwould be expected in the ATO anthracite, which has asilicon content of 2.85 wt %, which is not the case. Thiscan be explained in terms of the mineral matterdistribution. The ATOD2 and ATOD3 anthracites havemuch higher proportions of particles with different typesof associations of the organic and mineral matters (OM-MM) than the ATOD1 and ATO anthracites, as mea-sured by optical microscopy on the polished surface ofpetrographic pellets through the examination of at least1000 particles (ISO 7404/3). Number percentages valuesof 23%, 48%, 49%, and 23% were determined for theATOD1, ATOD2, ATOD3, and ATO anthracites, respec-tively. These results suggest that the presence in theanthracite of a higher amount of OM-MM particles (i.e.,particles where an intrinsic contact between the twomatters of coal (organic and mineral) exists, particularlywhen the mineral matter is finely distributed in theorganic matter) enhances the internal stress caused by

the silicon carbide decomposition, thus providing anexplanation for the different behavior of ATO anthra-cite, with respect to that of the ATOD2 and ATOD3anthracites with increasing temperature.

Effect of Mineral Matter Composition. The best-ordered materials have been achieved from the ATOD4anthracite, which has the highest proportions of silicon,aluminum, potassium, and titanium in its mineralmatter (see Tables 2 and 3). Specifically, it can beobserved that the degree of structural order of thegraphite materials increases as the amount of siliconand potassium in the ashes increases, suggesting thatthe catalytic effect of the mineral matter on the graphi-tization of the ATO and ATOD1-ATOD4 anthracitesrelies mainly on the presence of these two elements.Although silicon is a forming element of quartz anddifferent clay minerals in coals, potassium is only foundin the aluminosilicate illite. This clay was identified byXRD in the low-temperature ash (LTA) residues of theanthracites under study, as well as in those of theircarbonized products. As an example, the XRD profilesof the LTA residues of the ATO anthracite and thecarbonized sample (ATOC) are given in Figure 1. In thissense, the increment of the potassium concentration canbe only related to the presence of a greater amount ofillite. The concentration of this element in the anthracite(as calculated from the data in Tables 1 and 2) has beenplotted against the XRD parameters of the materialsthat have been prepared at 2500 °C in Figure 2. Thecrystallite sizes La and Lc increase as the weightpercentage of potassium in the anthracite increases,while the interlayer spacing d002 decreases. As in thecase of the bulk mineral matter, reasonably good linear(31) Gilyazov, U. Sh. Khimiya Tverdogo Topliva 1979, 13, 46.

Figure 1. X-ray diffraction (XRD) profiles of the low-temper-ature ash (LTA) residues of the anthracite (ATO) and thecarbonized anthracite (ATOC). Legend for the minerals thathave been identified is as follows: A, anquerite; K, kaolinite;Q, quartz; D, dolomite; I, illite; P, pyrite; and S, siderite.

266 Energy & Fuels, Vol. 19, No. 1, 2005 Gonzalez et al.

correlations were attained between the proportion ofillite in the anthracite and the La values of the materialsthat have been prepared. Therefore, this mineral mattercomponent seems to be one of the main active catalystsof the graphitization of the ATO and ATOD1-ATOD4anthracites. Clay minerals of the illite group have beenpreviously reported to be effective graphitization cata-lysts of carbon materials.11

As just mentioned, an increment of the amount ofsilicon in the mineral matter cannot be exclusivelyrelated to a higher illite content. However, we considerthat the catalytic effect of kaolinite, which is the otherclay mineral that is identified, or quartz during thegraphitization is less significant than that of the illite.Thus, an increase in the degree of structural order ofthe materials is not accompanied by an increase in theamount of kaolinite in the mineral matter of theanthracite, as shown by the aluminum contents (seeTables 2 and 3). Indeed, this clay does not appear afterthe carbonization process (see Figure 1). Kaolinite canundergo a dehydroxylation process under HTT, with astructural collapse that leads to the formation of mulliteand/or amorphous alumina and silica.32 Mullite has notbeen identified in the mineral matter of the carbonizedanthracite (see Figure 1); therefore, it can be inferredthat alumina and silica are the decomposition products.On the other hand, no specific variation of the structuralparameters of the materials with a Si/Al weight ratioin the anthracite range was observed; the increase ofthis ratio is associated with a larger proportion ofquartz. Thus, the ATOD4 anthracite, with a Si/Alweight ratio of 1.7, leads to materials with a higherdegree of structural order than those obtained from theATO anthracite, which has a Si/Al weight ratio of 1.9,suggesting that the catalytic role of this anthracitemineral matter constituent is not very significant. It hasbeen previously reported to occur during the graphitiza-tion of carbon materials.11

Apparently, there is no influence of calcium, magne-sium, and manganese on the structural characteristicsof the materials that have been prepared. Thus, theconcentration of magnesium in the most graphitizableATOD4 anthracite is 0.09 wt %, versus the values of0.14 wt % in the ATOD3 and ATO anthracites. Ankerite

(Ca, Mn, Mg, Fe carbonate), and dolomite (Ca, Mgcarbonate), which decarbonate to their correspondingoxides through heating, were identified in the mineralmatter of the ATO and ATOD1-ATOD4 anthracites viaXRD (see Figure 1). Both magnesium and manganeseoxides have been reported to catalyze the graphitizationof carbon materials, although larger concentrations wererequired to observe a significant effect.11,14,15

Metallic iron and different iron compounds also havebeen reported to catalyze the graphitization pro-cess.11-13,17,30 Moreover, this metal was observed to bean active catalyst for the synthesis of silicon carbidethrough the formation of iron silicide in an intermediatestage.33 The iron compounds ankerite and siderite wereidentified via XRD in the mineral matter of the anthra-cites that have been studied, whereas pyrite was onlyfound in very small proportions in the ATO, ATOD3,and ATOD4 anthracites (see Figure 1 and Table 1). Theweight percentages of iron carbonates in the anthraciteshave been plotted versus the XRD parameters of thematerials that have been prepared at 2500 °C (Figure3). Crystallite sizes increase as the iron carbonatescontent increases, up to a maximum value; correspond-ingly, the d002 value decreases to a minimum. A furtherincrease in the amount of iron leads to materials witha lower degree of crystallinity. Thus, materials with Lavalues of 38 and 25 nm were obtained from anthraciteswith iron concentrations of 0.36 wt % (ATOD4) and 0.44wt % (ATO), respectively. In addition to this differentmetal content, these two anthracites also have a differ-ent distribution of the iron minerals, as observed byscanning electron microscopy, coupled with energy-dispersive X-ray analysis (SEM-EDX) (Figure 4). Thus,although a relatively abundant population of free sid-erite particles were identified in ATO, no iron mineralparticles could be found in the ATOD4 anthracite.Therefore, the iron minerals in the ATOD4 anthracitemust be finely distributed in the organic matter. Theseresults seem to indicate that the catalytic effect of themineral matter is associated with both its amount andits distribution, and the intrinsic association betweenthe organic matter and the mineral matter of theanthracite improves that effect. In this sense, graphi-tized regions that surround the mineral particles were

(32) Martınez-Alonso, A.; Martınez-Tarazona, M. R.; Tascon, J. M.D. Erdoel Kohle 1992, 45, 121.

(33) Narciso-Romero, F. J.; Rodriguez-Reinoso, F. J. Mater. Sci.1996, 31, 779.

Figure 2. Variation of the XRD parameters of the materialsthat have been prepared at 2500 °C, relative to the potassiumcontent in the anthracite.

Figure 3. Variation of the XRD parameters of the materialsthat have been prepared at 2500 °C, relative to the ironcarbonate content in the anthracite.

Structure of Graphite Material from Anthracites Energy & Fuels, Vol. 19, No. 1, 2005 267

previously observed via high-resolution transmissionelectron microscopy in heat-treated anthracites at tem-peratures as low as 1300 °C.6

Conclusions

(1) The degree of structural order of the graphitematerials that have been prepared from anthracites ofcomparable organic matter composition and microtex-ture progressively increases as the mineral matter

content increases, thus inferring the catalytic effect ofthis inorganic matter of coal during the graphitizationprocess.

(2) The mineral matter of the anthracite preferentiallypromotes the growth of the crystallites in the directionof the basal plane. Reasonably good linear correlationswere observed between the anthracite ash content andthe a-axis crystallite size (La) of the materials. As aconsequence of this effect and the flattening of the

Figure 4. Scanning electron microscopy (SEM) photomicrographs of (a) ATO anthracite and (b) ATOD4 anthracite. In panel a,image (1) shows a general view of ATO anthracite and image (2) shows one of the mineral-free particles in ATO anthracite; thebottom image is an energy-dispersive X-ray (EDX) analysis of the mineral-free particle, showing the presence of iron. In panel b,image (1) shows a general view of ATOD4 anthracite and image (2) shows one of the mineral-free particles in ATOD4 anthracite;the bottom image is an EDX analysis of the mineral-free particle, showing no presence of iron.

268 Energy & Fuels, Vol. 19, No. 1, 2005 Gonzalez et al.

anthracite pores during graphitization, no structuralevolution of the materials with increasing temperatureoccurred from a given mineral matter content.

(3) Among the different constituents of the mineralmatter, the clay mineral illite and the iron carbonateswere determined to be the main active catalyst com-pounds during the graphitization of the anthracites.

(4) In addition to the amount and composition of themineral matter, the distribution of the mineral matteralso influences the graphitization process of the anthra-cite. A fine distribution in the organic matter, such as

that in the case of the iron compounds, improved thecatalytic effect of the mineral matter.

Acknowledgment. Financial support from DGICYT(under Project No. MAT2001-1843) and FICYT (underProject No. PB-EXP01-01) is gratefully acknowledged.One of the authors (D.G.) thanks FICYT for a personalgrant. Thanks are also owed to Dr. M. A. Banares(Instituto de Catalisis, CSIC, Madrid, Spain) for provid-ing access to the Raman spectrometer.

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Structure of Graphite Material from Anthracites Energy & Fuels, Vol. 19, No. 1, 2005 269