Structure of the organic crystallite unit in coal as determined by X-ray diffraction

lining Science and Technology( China)21(2011)667-671Contents lists available at SciVerse Science Direct氵容设Mining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcStructure of the organic crystallite unit in coal as determined by X-ray diffractionSong Dangyu. Yang Cunbeia. Zhang Xiaokui, Su Xianbo Zhang XiaodongSchool of Resources and Environment, Henan Polytechnic University, Jiaozuo 454000, ChinaSchool of Energy Science and Engineering, Henan Polytechnic University, iaozuo 454000, ChinaARTICLE INFOA BSTRACTarticleX-ray diffraction(XRD) was used to study the structure of the organic crystallite unit( La. Le, doo?)in coalsReceived 21 January 2011ceived in revised form 15 Fehruary 2011collected from Henan and Shanxi Provinces, XRD patterns of coal were collected in a step-scan modeAccepted 5 March 2011(O 1/step)over an angular range of 2-90(20), allowing 8 s at each step. the structure of the crystalliteAvailable online 6 November 2011unit was determined from the Scherrer equation and peak parameters deduced from whole pattern fitting. The results show that the structure of the crystallite unit in coal is mainly controlled by the coalKeywords:rank. As the coal rank increases the average diameter of a coal crystallite unit(L,)increases, the interlayer spacing(doo2)decreases slightly, and the average height of a coal crystallite unit( L)increases atfirst but then decreases. a new diffraction peak from the crystallite unit in coal was found at a low scattering angle in the XRD pattern(2-10 ) This suggests a structure with an inter layer spacing from 1.9 toWhole pattern fitting2.8 nm exists in coal crystallites.o 2011 Published by Elsevier B V. on behalf of China University of Mining Technology1. Introductioncalculation, and analytical methods, have led to obvious differences in the various conclusions[11-13).Coal is a complex material composed of organic matter and inor-After removing minerals from the raw coal samples and apply-ganic minerals. Because of their large specific surface area and high ing a large scan angle with a wide step distance and slow scarabsorption capacity, the crystallites greatly affect methane absorp- speed a series of XRD patterns has been collected and is reportedtion and desorption from coal [ 1. Many domestic and international here. the morphology of the crystallite unit in ten coals that werescholars have studied the macromolecular structure of coal over the collected from Henan and Shanxi provinces and have different coalyears. The tools they used include chemical analysis, energy spec- ranks is studied by fitting the whole pattern. a new diffractiontrum analysis, NMR spectroscopy, and transmission/scanning elec- peak from crystallite units was discovered in the low angle scatter-ron microscopy. However, the coal macromolecular structure is ing region with a diffraction angle of about 6. This has not beenstill not quite clear due to its complexity and the variability of the or- reported in earlier publicationsganic structure in coal [2-6].Although it is difficult to establish an accurate macromolecularstructure, it is possible to describe the crystallite structure in coalsby applying X-ray diffraction(XRD). The XRD peaks of the organic 2 Sample collection and preparationmatter in coal show broad humps typical of amorphous matterMany studies demonstrate that the shape of the crystallites andTen raw coal samples having different ranks were collected fol-he amount of crystalline material in coal can be deduced from lowing the distribution of coal resources in Henan and Shanxithe broad XRD peaks of coals [7-10]provinces. The samples were ground to pass a 74 um sieve andResearch on using XRD to determine the structure of organic stored in brown, glass stoppered bottles. The proy' * te analysismatter in coals has been developing for decades. Research mainly and the vitrinite maximum reflectance of the ye givenfocuses on the morphology of the crystallite unit in coals of differ- in Table 1. these data were obtained accordinent rank. However, limitations in accuracy and power of early dif- dards GB/T 212-2008 and GB/T 6948-1998fractometer, and different raw coal sample preparationAll the raw coals contain small amountAlthough the content is relatively low yXRD pattern compared to the broads Corresponding author. tel:13986265Because of this it is very difficE-mail address: dangyusongahpu. edu.cn(D Song).parameters of the中国煤化工1674-5264/s- see front matter b 2011 Published by Elsevier B V on behalf of China University of Mining Techdoi:10.016/mc20111000YHCNMHGD Song et aL/ Mining Science and Technology(China) 21(2011)667-671Proximate analysis and vitrinite maximum reflectance of raw coals and organic matter.Sample no.mpling siteCoal typeMad(%)Vaar(%) Romsh yield of organic part(e)Quasi-natural coke340Jiaocheng, Shanxi26.036.5672.33Xiqu, TaiyuaCoking coal5.6820034567895972.25904Dongqu, Taiyuanean coa8.1518.231935043Yongdingzhuang Datong28.500.999Anthracite9.27Qianqiu, YimaLong flame coalNo 6 coal mine, Pingdingshan 1/3 coking co.14811.8032.10037removed by an acid dissolution method to alleviate this problem part is a broad and low intensity peak special test conditions were[14]. The dissolution proceeds as followsworked out: copper Ka radiation(40 kV, 40 mA); graphite mono-chromator: step-scan mode; scanning range from 2 to 90: step(1)6.*0.0001 g of air dried coal was weighed and trans- distance of 0. 1 and, scanning speed of 8 s/step [13. Comparedferred to a Teflon beaker. To this 1-2 mL of absolute alcohol to conditions used for inorganic minerals this step distance iswere added followed by the slow addition of 40 mL HCl much longer and the scanning time at each step is increased(5 mol/L). This makes the coal wet enough for stirring with significantlya plastic rod. the beakers were then warmed for 4050 min in water at 55-60C, stirring at 5-10 min intervals. 4. Results and discussionThen the beakers were removed from the water bathallowed to stand for a moment, and filtered through a slowThe organic matter in coal is composed of crystalline and amor-qualitative grade filter paper.phous regions( Fig. 1). The amorphous fraction does not contribute(2)The solid on the filter is transferred back into the original to peak intensity and is contributes only to the background of theTeflon beaker along with 5-10 mL of hot water. Then XRD pattern. The structure of the crystallite fraction is similar to40 mL of HF (5 mol/L) was slowly added to the Teflon bea- graphite and the positions of the broad peak are close to theker, the beaker was heated, and the residue was filtered as 002, 100, and 110 peaks of graphite. Hence, the XRD peaks ofdescribed in Step(1).the crystallite unit in coal are named 002, 100, and 1 10.(3)Again, the residue was removed from the filter paper andreturned to the original Teflon beaker with 5-10 mL of hot 4.1. Influence of mineral removal on crystallite structurewater. Then 50 mL of concentrated HCl (1. 19 g/cm)wasslowly added to the teflon beaker, which was heated thesuspension was filtered as described in Step(1). The TeflonTo test whether demineralization changed the crystallite strucbeaker and rod were scrubbed with the filter paper and the responding demineralized organic matter, were scanned using thefilter paper plus the collected solid was placed into an Erlen- same instrument parameters(Fig. 2). Comparing the XRD patternneyer flask.of raw coal to the corresponding demineralized organic matter it(4)50 mL HNO,(2 mol/L)was added to the Erlenmeyer flaskcan be seen that: (1)there is no obvious diffraction peak of the min-containing the residual coal and boiled for 30 min with a erals in the XRD pattern of the organic matter, which indicates thatfunnel over the mouth of the Erlenmeyer flask. Rinse the residual inorganic matter is not present to affect the diffractionfunnel with distilled water into the Erlenmeyer flask. Filter peaks of the organic crystallite unit: ( 2)the position of the broadthrough a slow qualitative grade filter paper and rinse withot water until no iron ions are present in the filtrate(test intensity changestaks is not changed after demineralization but the diffractionwith potassium thiocyanate solution).e diffraction intensity of the weakly caking coal after demineral(5)Wash the filter paper and the residual coal with hot water ization strengthens, to some degree. in the low angle region(20<25)and place them into a weighing bottle. Dry in an oven at The diffraction intensity of the anthracite also strengthens obviously50C for 5-6 h, with the air circulation system on Remove in the region between 15 and 50. By comparing the background va-the weighing bottles and put them into a desiccator to cool. lue of the XRd pattems one may see that the increase in absolute dif-are allowed to cool for 1 h. The samples on the filter paper background value: The relative intensity remains mostly unchangedare transferred with a spoon into lidded weighing bottlesand weighedThese treated coal samples are stored in sealed brown glass bottles for later use. the ash contents of the demineralized coals weredetermined again(Table 1). Except for the anthracite coal from Jia-acheng in Shanxi Province the ash yield of the treated coals is lessthan 1.0%. The ash yield of the lean coal from Datong is only 0.07%(a)A single crystallite in coalThe obvious change in the ash yield between the raw coals and thetreated coals indicates that the removal method is effective3. Experimental○Crystaldiffractometer from the german firm Bruker wasated coals. because the Xrd peak of the organicYH中国煤化工ro crystallitesCNMHGFlz 1. SchemaD Song et aL/ Mining Science and Technology(China )21(2011)667-671aw coal, Yong3600Datong"Raw coal, Yema. Jizozuo-Organic par YongdingzhangDatong3000200028(°)20Flg. 2. Comparison of XRD patterns of raw coals and the demineralized organic matter from the coals.e Long flame coal....-..--..Weakly caking coal- Yongding2. PinggDatong…1/3 co4000at coal, Malan, Taiyuanoking coal, Xiqu TaiyuanL9,Pgg工y300Anthracite. Yema, JiaozuoAnthracite, Jiaocheng, Shanxi200026(FI. 3. xRD patterns of coals of different rankBecause the peak shape parameters related to the crystallite structure crystallite units can be calculated by the Scherrer formula, anare mainly the position and the full width at half maximum( FWHM) empirical equation giving the crystal grain size by calculation fromof the diffraction peak the differences inthe XRD patterms after demin- the position, intensity, and shape of the diffraction peaks (8 .Beeralization have little influence on the deduced crystallite structure of cause the diffraction peaks of the organic matter are characterizedthe organic matter.by low intensity, broad shape, and a high and variable backgroundvalue it is very hard to obtain accurate FWHM values or the posi4.2. Characteristics of the original XRD patternstion of the diffraction peaks directly. Slight differences in theFWHM or the position of the diffraction peaks can cause obviouslI samples after demineralization were scanned using the d8 changes in the calculated results. In this research the Xrd back-Advance diffractometer the results are shown in Fig 3. Three main ground is removed by a cubic spline combined with manual inter-diffraction peaks are located around 6, 25, and 43 in all the sam- vention at key points. the position and FWHM are deduced by aoles. In contrast to the patterns of the mineral matter the diffrac- Pseudo-Voigt whole pattern fitting [15, 16 Table 2 lists the posi-tion pethe organic crystallite units are low and broad. a tion, inter-layer spacing, and the FWHM of diffraction peaks anddiffraction peak can be observed at about 83 for high coalification the crystallite morphology parameters of each sample calculatedcoals such as anthracite and quasi-natural coke. Looking at the ori- by the Scherrer formula. The physical meaning of the relativeginal XRD patterns of different rank coals the diffraction intensity parameters is shown in Fig. 1in the scattering angle region where 20 exceeds 40 gradually be-The relationships between the vitrinite maximum reflectancecomes stronger as the coal rank increases. Through in the interme- and the average diameter( L,), and the average diameter of the coaldiate region(10<20<40) the diffraction intensity does not crystallite units(Lc), are provided in Fig 4. As shown there the mor-strictly increase with increasing coal rank At the low angle scatter- phology of the crystallite units is controlled by the degree of coal-g angle region the diffraction intensity decreases with an in- ification. with the rise in coal rank L first increases and thencrease in the coal rank for coal ranks higher than anthracitedecreases. The inflexion point is located where the vitrinite maximum reflectance is about 3.5%. Also note that the increase per unit4.3. Morphology of the crystallite unitchange in reflectance is much higher than for the decrease. L, in-creases with increa中国煤化工 ctance. whichIt is impossible to deduce the structure of the crystallite units illustrates that the laeases continudirectly from the XRD patterns. However, the structure of the ously with rising coalCNMHGdoo?)decreasesD Song et aL/ Mining Science and Technology( China)21(2011)667-671Parameters of the XRD peak and crystallite unit structure of the coals.Sample no. 2%ew(@) 2000() 2010() FWHMoo( FWHMno( doew(A) doo(A) dno(A) k(nm) L(nm)72953.977190122.184.3921234567702325.7254.81019.7483.4602.1723.6355.13825.3443.1255.39626.993.5122.18023235.28125.1885.5613.5331.6003.14129097415279693.5062.35525.38843.3786.324284393.5051.6216.2564.62743.3046.534557325.71143.3685.0276.2631.6042.7928.6533.5782.1152.02924.90843.455.9966.551218783.5722.16526704.58.01002.0(a)L(b)L4. Relationship between vitrinite maximum reflectance and La, Le0360.3621.0l800.344.0208.0Fit 5. Relationship between vitrinite maximum reflectance and the inter-layerspacing. doo.yer Fig. 6 Relationship between the vitrinite maximum reflectance and the inter-layespacing of the new diffraction peak.with an increase in coal rank until reaching anthracite ll Fig. 5)although the reduction is slight: The difference between the max- Because this diffraction peak has not been reported previously furimum and minimum is only 0. 17A, far less than the change in L ofher work should done to obtain more information about the crys4.1 A. This indicates that the average height of the crystallite units allite unit structure.increases from long flame coal to anthracite I as the coal rankincreases Because the reduction in dooz is very small the increased 5. Conclusionscrystallite unit height mainly arises from an increase in the num-per of aromatic layers. Coal ranked higher than anthracite I shows(1)The morphology and structure of the organic crystallitea slight decrease in L although the decrease is much smaller thanunits in coal can be obtained by XRD based on the Scherrerthe increase this illustrates that the crystallite unit height de-formula, XRD whole pattern fitting, and demineralization ofcreases to a constant level slightly after anthracite L.the raw coalIn this study a new diffraction peak from the crystallite units(2)The crystallite unit structure of coal is controlled by thewas found in the low angle scattering range that has not beencoal rank. an increase of coal rank is correlated with a grad-reported in previous publications. It is located at 20 between 4. 9oual increase in the diameter of crystallite unit. The interand 7.3. The corresponding inter-layer spacing is 1.9-2.8 nmlayer spacing decreases slightly at first and then tends toThe relationship between inter-layer spacing and vitrinite maxistabilize. Thcit increases firstmum reflectance is shown in Fig. 6. As the coal rank increasesand then de中国煤化工 that the volumethe inter- layer spacing increases at first but then decreases. Theof the crystCNMHGaromatic layersinflexion is located at the vitrinite maximum reflection of 2.5%.IncreasesD Song et aL/ Mining Science and Technology(China)21(2011)667-671671(3)A new diffraction peak is found in the XRd pattern of coal 14]Fletcher TH, Kerstein AR, Pugmire R). Solum MS. Grant DM. Chemicalat a diffraction angle (20)of about 6, which has never beenpercolation model for devolatilisation-3use of 13carbon NMR data toreported previously. Further work should be done to obtain(5SH以BhxC理rAy心ction analysis on graphene layersmore structural information about the crystallite unit.of Assam coal. ] Che[6 Li XM, Cao DY, Liu DMrent types of coal metamorphism byHTEM, Min Sci Technol 2010: 20(6): 835-8.Acknowledgments[7] Warren BE. X-ray diffraction in random layers lattices. Phys Rev941:599):693-8[8] Biscoe ]. Warren BE. AnX-raystudy of carbon black. J Appl Phys 1942: 13: 364-71This research was supported in part by Program for New Cen- (i0)VanKrevelen Dw.Coal-Topology-Physics-Chemistry-Constitution.Elseviertury Excellent Talents in University of China(No. NCET-10-0133)Science Publisher: 1993. pp 225-240and in part by Innovation Scientists and Technicians Troop Con- [11] Jiang B, Qin Y Song DY, Wang C XRD structure of high rank tectonic coals andstruction Projects of Henan Province(No. 114100510004)Thankswere given to Dr. Liu Yinghui for reading an earlier version of the [121 Ju Yw. Jiang B, Hou QL Wamg GL Relationship between nanoscalemanuscript and providing thoughtful comments and constructiveture and metamorphic deformed environments.suggestions. The authors wish to acknowledge Guo Hongyu, for (131 LL Sahajwalla v Kong C. Hafrisb D Quantitative x-ray diffraction analysis(14) Moh as Pad s iomt ia ar Ms msm d bo. 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