Chemical leaching of an Indian bituminous coal and characterization of the products by vibrational s

International Journal of Minerals, Metallurgy and MaterialsVolume 19, Number 4, Apr 2012, Page 279Do!:10.1007/s12613012055140Chemical leaching of an Indian bituminous coal and characterization of theproducts by vibrational spectroscopic techniquesB. Manoj2)and A.G. Kunjomana"1)Department of Physics, Christ University, Hosur Road, Bangalore-29 Kamataka 560029, India2)Research and Development Centre, Bharathiar University, Coimbatore, Tamil Nadu, lodia(Received: 29 April 2011: revised: 28 November 2011; accepted: 29 November 201l)Abstract: High volatile bituminous coal was demineralized by a chemical method. The vibrations of the"aromatics"structure of graphite,crystalline or non-crystalline, were observed in the spectra at the 1600 cm" region. The band at 1477 cm is assigned as Va band, the bandat 1392 cm as Vi band and the band at 1540 cm as Gr band. Graphite structure remains after chemical leaching liberates oxygenatedfunctional groups and mineral groups. The silicate bands between 1010 and 1100 cm" are active in the infrared (IR)spectrum but inactive inthe Raman spectrum. Absorption arising from C-H stretching in alkenes occurs in the region of 3000 to 2840 cm". Raman bands because ofsymmetric stretch of water molecules were also observed in the spectrum at 3250 cm"and 3450 cm". Scanning electron microscopy analysis revealed the presence of a graphite layer on the surface. Leaching of the sample with hydrofluoric acid decreases the mineral phase andincreases the carbon content. The ash content is reduced by 845wt%with leaching from its initial value by mainly removing aluminum andsilicate containing mineralsKeywords: bituminous coal; characterization; graphite; leaching,mal spectroscopy;ash content of the coal. It will in turn affect the quality of1. Introductioncoal as fuel. Concerted efforts are there to demineralize theRaman spectroscopy had been used extensively to char-coal samples to get mineral free coal [4-5]. In the presentacterize the structural features of carbonaceous matter sincestudy, the change of functional groups and mineral groupTuinstra and Koenig [1] first correlated Raman bands towith hydrofluoric acid (HF) leaching on high volatile Indianstructural parameters measured from X-ray diffraction coal was investigated by IR and Raman spectroscopy. Su(XRD)of polycrystalline graphite. Raman spectroscopy had face characteristics of very low ash bituminous coal ob-also been applied to study the structural features of tained after chemical demineralization were carried out withcoal-derived products [2-3]. In these studies, the Raman scanning electron microscopy(SEM) and energy dispersivespectral characteristics, mainly those of graphite(G)and spectroscopy (EDS)defect bands(D), were used to investigate the coal structuresand their correlation to other characteristics, e.g. coal rank2. Experimentaland the 'graphite crystalline size parameters. In nearly allspectro"copic studies of organic and inorganic materials it is dried in a dessicator to remove the absorbed water. Samples30%10%for24halong with Fourier transform infrared(FTIR) spectra. Thewashed with enormous amounts of distilled water. Thetwo methods are completely complementary and could pro- slurry was filtered and dried at about 80 C for removing thevide considerable structural information when used together.absorbed water and allowed to cool slowly in a dessicator.The minerals present in coal play a significant role in the The Ftir spectrum was recorded by a Shimadzu FTIR-8400orresponding author: B Manoj E-mail: manoj b@christuniversity in中国煤化工SpringerO University of Scicnce and Technology Beijing and Springer-Verlag Berlin Heidelberg 2012CNMHGInt J. Miner. MetalL Mater oL19, No4, Apr 2012pectrometer in the region of 4000-400 cm". The Raman band, the band at 1392 cm"is assigned as vl band and thespectra were obtained using a Bruker RFS 100/S spec- band at 1540 cm"as gr band. These bands representtrometer equipped with a Raman accessory. This comprised mainly the aromatic semi-quadrant ring breathing for aro-a Spectron laser system SL301 Nd-YAG laser operating at a matic ring systems having more than two fused benzenewavelength of 1064 nm, and a raman sampling compart- rings.ment incorporating 1800 optics. The Raman detector was aThe bands observed at 1460 and 1375 cm in the ramanhighly sensitive standard Ge detector and was operated at22C. Under these conditions raman shifts would be ob.spectra of coal are attributed to Ch band and absorption at1250 cm is attributed to phenolic structures. The absorp-served in the spectral range of 3500-1200 cm. A laserpower of 200 mw was used.tion due to phenoxy group at-1250 cm, presumably phe-ols, is probably very weak, and the majorA SEM micrographs of the virgin and residual samples likely due to the graphite structure. This makes more sensewere obtained by scanning electron micrograph(sEM) to assign a small amount of this jointly to phenolic and ChJEOL model JSM-6390 LV. Each of the samples was structure and the major portion of the absorption to themounted on disc and coated with gold. The energy disper- graphite structure. Many of these bands with HF leachingsive spectrometer(EDS) was obtained by JEOL model decrease their intensities. In the Raman spectra of coal,JED-2300. Elemental analysis was carried out using weak absorption peak at 1080 cm is observed. This isVaroEL III CHNS analyzerprobably due to Si-O stretching or C-H band on aromaticrings [5]. This singlet with HF leaching was split up into3. Results and discussionthree peaks with less intensity.3. 1. Vibrational spectroscopic analysis of bituminousFT Raman spectra of bituminous coal and their chemicalleached products with wavenumber 2600-3600 cm"areThe Raman spectra of the bituminous coal sample andshown in Fig. 2. The vibrations of the CH2 group, thethe HF leached products within 1200-1650 cm"region areasymmetric stretch, symmetric stretch and scissoring andshown in Fig. 1. On studying the structures of the bitumiwagging vibrations appear in the regions of 3000-550, 2965+30, 1455+55 and 1350-85 cm respectively. Absorptionnous coal samples using Raman spectroscopy, the g band at1598 cm and the d band at 1345 cm are commonly as-arising from C-H stretching in alkenes occurs in the regionsigned as the graphite and defect bands respectively [4-7].of 3000-2840 cm". For the two CH2 groups, the asymmetricCH, stretching bands are observed at 2950 and 2920 cm inThe bands in the region between G band and d band are as-signed as Ve band(standing for valley right)at 1480 cm-lthe IR spectrum and only a weak band at 2940 cm in theRaman spectrum. With HF leaching, the intensity of thisVl band(standing for valley left)at 1388 cm and gr band(standing for G right)at 1542 cm, prior to the literatureband is decreased.information [6]. The band at 1477 cm is assigned as VRThe C-H stretching in CH, group occurred at lower fre-00101605002650cm0.002700cm50 cm0.0073250cm00042920cm1000600031430cm11300cm10.0021365cm1480cm000100010.00012001250130013501400145015001550160016502800Wavenumber动y300032003600Wavenumber/cm-HF leached products(KH)(1200-1650 cm regia(kx and its Flg. 2 FT RaFig 1. FT Raman spectrum of bituminous c中国煤化工coal( KX) andleached producCNMHGquencies than that of the aromatic ring (3000-3100 cm). In gion. The presence of graphite type, non-crystalinrarneB. Manojet aL, Chemical leaching of an Indian bituminous coal and characterization of the products by vibrationalour present study, two weak bands appearing at 3020 andture, is the most important contribution to the study of3000 cm" in the FT-Raman are assigned to asymmetric structure [6]. Graphite structure remains after chemicalstretching vibration. These bands are not visible in the IR leaching liberates oxygenated functional groups and minerSpectrum.groups. It is obvious that the graphite structure, crystallineThe peaks in the 3000-3600 cm" region contain promi-or amorphous, would not be volatile or extractable. Thepresent authors have the opinion that the changes in inten-nent peaks positioned at -3250 and-3450 cm". The band at sity of bands in the iR spectrum of coal are due to the libera-3250 cmin the spectra is attributed to the strong interno- tion of extractable organic components of coal from thelecular coupling of water molecule symmetric stretch vibra-molecular sieve, which has been reported by many authorstions within a symmetric hydrogen bond. This band also earlier[U]arises because of OH stretching of AHOH groups. The peakat-3450 cm 'in the spectra is assigned to the weaker cou-The intense bands at 602 and 570 cm" are doubly as-pling of the water molecule stretching modes, which is as- signed to halloysite and aromatic ring breathing. The band issociated with a more disordered and asymmetric 4-coorinfrared active but weak. The characteristic ir bands in thedinate(tetrahedral) hydrogen bonding network. This peak680-600 cm region are attributed to the C-h bendingshows the decrease of intensity with HF leaching. The in. modes. This band might be due to ring breathing vibrationtensity loss could be correlated to the displacement of thepresent in the coalwater molecules in the symmetric hydrogen-bonding net-Characteristic doublets for a-quartz are observed at 797work by the halide(fluoride)ions. The fluoride ion, which is and 778 cm"in the IR spectrum[8]. These absorption bandsminimally polarizable, has no effect on the 3450 cm 'peak in bituminous coal with chemical leaching are eliminated.in the Raman spectrum.The intense infrared bands at 1014. 1036. and 1108 cm" aredue to the silicate bands in the sample [7-13]. These peaksIn the IR spectra of the coal sample( Fig. 3), in addition decrease their intensities in bituminous coal when treatedthe absorption band observed at 1605 cm", absorption with he. This result is in confimation with that of the ear-bands at 1435, 1375, 1165, 1105 cm and a spike at 1030 lier study on bituminous coal carried out by Wu and Steelcm"are observed. These bands indicate that the coal sam- [9]. The C-C stretching bands(1161-758 cm)are weak inples constitute complex polymeric materials, which were IR and not simple characteristic frequencies in Raman. Theearlier reported by the same author in subbituminous coal identification of these vibrational bands is more difficult4-5]. The C-H bending frequencies have a higher intensity because other types of c-C stretching vibrations also occurthan the C-H stretching band. This is due to the intense in the same region(around 1300 cm-and could interactbroad absorption produced in this region by"graphite" with each othercomponents. Aromatics are also present in small amountsThere are many weak Raman absorptions in the regionWith HF leaching, these bands remain more or less constant 1200-1800 cm-'other than graphite bands. The weak ab-in bituminous coal. In Raman spectrum, bituminous coalshows more intense absorption with HF leaching in this re-sorption band at 1540 cm in the spectrum is assignable tothe amorphous carbon structures with aromatics 3-5 ringsThe bands at 1465 and 1380 cm are due to methylene or501090 cm-lmethyl group, which indicate the presence of amorphous11030 cm; Kxcarbon structure in the sample. In the IR spectrum all these835KH309bands show a decrease in absorption with increase in HF2920cm1605cm1g30↑1435concentration during leachingKH109In addition to the above band, the in-plane deformation of15CHe bands is observed at 1490 and 1460 cm in the IRpectrum and at 1520 and 1460 cm" in the Raman spectrumof bituminous coal. The rocking modes of CH2 are expectedin the range of 895+85 cm. This band is not prominent in002500200015001000500the raman spectrum of bituminous coal3.2. Minerals中国煤化工Fig. 3. FTIR spectrum of bituminous coal (KX and itsCNMleached products(KH)(HF 10%-30%)The SEM-Elwu ovule are shown in282Int J. Miner. Metall Mater., Vol 19, No, 4, Apr 2012ig. 4. It is seen that the samples are not very uniform, some of the coal shows a layer-like structure, which is characterispoints are rich in carbon, and at some points the minerals tic of a graphite layer [9-13]. This further suggests that theco-exist with the carbon materials. The EDS analysis of the mineral particles are not trapped in the coal layer, whichsurface reveals that, except for carbon, hydrogen, oxygen, may be easily separated using chemical leaching. Thenitrogen and traces of sulphur, silicon and aluminium mechanism of fluoride attack is the reaction of hydrogen(0.9wt% Al, 1. 2wt% Si)are the major elements on the stud- fluoride(HF)with silicon dioxide(SiO2) to form siliconied surface of Korba coal(Fig. 4). A bulk structure can be tetrafluoride(SiF4)seen,which in turn is composed of a homogeneously dis- 4HF+SiO,,SiF+2H Otributed network comprised of small fistulous and filamen-Silicon tetrafluoride is soluble in water and will reacttous crystallites showing minerals. Luminous and non lumi-nous features can be seen in the matrix. This indicates thatwith additional hydrogen fluoride to form fluorosilicic acidare distributed in the(hexafluorosilicic acid)as followsCracks and veins are also seen. The bright luminosity is due 2HF+SiF4+H2SiF6 or SiO2+6HF-H2SiF6+2H20to the presence of lithophiles like aluminium and silicatesThis is removed during the leachingChalcophile elements like sulphur are present only as traceelements上 lement wt4 2 Element wt%.C73.7020kVx50005m0048日0.001.002.00rg0.001002.00Energy /ke VFig. 5. SEM/EDS analysis of the HF leached sample(KH).Fig 4. SEM/EDS analysis of the virgin sample(KX)Table 1 shows the contents of C.h.n. s and o in thevirgin coal(KX) and demineralized coal(KH). When bitu3.3. Effect of HF leaching on demineralizationminous coal is treated with HF(30%), the sulphur contentSEM-EDS analysis of the demineralized coal(Fig. 5) decreased from 0.33 ]wt% to 0. 148wt% because of the reactreated with 30% HF solution reveals that large particles of tion with bassanite. Other elements such as C, H and N inkaolinite and bassanite are extensively removed, and creased during leaching whereas o content decreased frombrightness of the surface is decreased. It can be seen that 22. 54wt% to 15.207wt%. From the proximate analysis, it isleaching caused morphological changes in the particle and found that the ash content decreased from 82wt% todid enormous modification to the surface by leaching of the 1. 27wt% with HF (30%)leaching. Wu and Steel [9]earlierinorganic minerals. Rubiera et al. reported a change in sur- reported a reduction of ash content from 530wt%toface morphology in the case of bituminous coal when 1.37wt% in UK bituminous coal by one-stage HF leachingleached with 25% HF and nitric acid [10]. Estimation of the The present result is a better result than the previous re-elemental composition by EDS shows that the surface has ported ones. Leaching is performed with various concentra81wt% carbon and 18.85wt% oxygen where as minerals like tions of HF. As the concentration of HF in leaching in-Si and Al are absent on the studied surface. The morphology creases, the ash content decreases by 84.5%(Fig. 6.Table 1 Proximate elemental analysis of virgin and HF treated sampleCoalProximate analysis /wt%%Elemental analysis by ultimate analysis wt%Fixed carbon Volatile matter AshMoisture28.988.205.406990中国煤化工332259KH65.051.275,10CNMHGI515.20B. Manojef aL, Chemical leaching of an Indian bituminous coal and characterization of the products by vibrational..with leaching whereas oxygen content decreased from2259w%to15.207w%References[ F. Tuinstra and J.L. Koenig, Raman spectrum of graphite, J.Chem. Phys,53(1970),p.1126[2]RA. Friedel and G L. Carlson, Difficult carbonaceous mate-KHI0 KH20rials and their infra-red and Raman spectra: reassignment forcoal spectra, Fuel, 51(1972), p. 194Fig. 6. Effect of HF leaching on demineralization.3] Y. Wang, D.C. Alsmeyer, and RL. McCreery, Raman spec-troscopy of carbon materials: structural basis of observed4. Conclusionsspectra, Chem. Mater, 2(1990), P557[4]B. Manoj and A.G. Kunjomana, FT-Raman spectroscopicThe vibrations of the"aromatics"structure of graphitestudy of Indian bituminous and sub-bituminous coal, Asian J.crystalline or non-crystalline, were responsible for the obMater. Sci., 2(2010), No 4, p. 204served spectra in the 1600 cm-'region. The band at 1477 [] B Manoj and A.G. Kunjomana, Chemical solubilization ofcm is assigned as VR band, the band at 1392 cm"as Vucoal using HF and characterization of products by FTIR, FTRaman, SEM and elemental analysis, J. Miner. Mater. Charband and the band at 1540 cm as gr band. graphite struc-act. Eng,9(2010,No.10,p919ture remains after chemical leaching liberates oxygenated [6]X. Li, J I. Hayashi, and C.Z. Li, FT-Raman spectroscopicfunctional groups and mineral groups in the sample. In thestudy of the evolution of char structure during the pyrolysis ofIR spectrum all these bands show a decrease in absorptiona victorian brown coal, Fuel. 85(2006), p 1700in HF concentration with leaching Charac- [7] J. Robertson, Diamond-like amorphous carbon, Mater. Sci.EngR,37(2002),p.l29teristic doublet for a-quartz at 797 and 778 cm in the IR [8] A Georgakopoulos, A. lordanidis, and V. Kapina, Study ofspectrum and this band is removed with leaching. The in-low rank Greek coals using FTIR spectroscopy, Energyense infrared bands at 1014 cm 1036 cm and 1108 cmSources,25(2003),p.995are due to the silicate bands in the sample. These peaks de- [9] Z.H. Wu and K M. Steel, Demineralization of a UK bitumi-creased the intensity in bituminous coal when treated withnous coal using HF and ferric ions, Fuel, 86(2007), P 2194HF. The peak at-3450 cm ' in the spectra is assigned to the [o] F. Rubiera, A. Arenillas, B. Arias, JJ. Pis, I. Suarez-Ruiz,eaker coupling of the water molecule stretching modesK M. Steel, and J W. Patrick, Combustion behaviour of ultraThis band showed a decrease in intensity with HF leachingclean coal obtained by chemical demineralization, Fuel82(2003),p2145The intensity loss could be correlated to the displacement of [11] D.N. Shooto and E.D. Dikio, Morphological characterizationwater molecules in the symmetric hydrogen-bonding netof soot from the combustion of candle wax, Int J. electro-work by halide( fluoride)ions. The morphology of the coalchem. sci.,6(2011),p.l269.shows a layer-like structure, which is characteristic of a [12] 0.0O. Sonibare, T Haeger, and S.F. Foley, Structural charac.graphite layer. Leaching using hf decreases remarkably theterization of Nigerian coals by X-ray diffraction, Raman andIR spectroscopy, Energ, 35(2010), P 5347ash content in high volatile bituminous coal from 10.23wt%to 1. 27wt%. From the ultimate analysis it is found that theo [13]W.Geng, T. Nakajima, H. Takanashi, and A Ohki. Analysisof carboxyl group in coal aromaticity by Fourier transformcarbon content is increased from 6990wt% to 74.59wt%infrared(FT-IR)spectrometry, Fuel, 88(2009), p. 139中国煤化工CNMHG