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A comparative study on structural differences of xylite and matrix lignite lithotypes by means of FT-IR, XRD, SEM and TGA analyses: An example from the Neogene Greek lignite deposits Ioannis K. Oikonomopoulos, Maria Perraki, Nikolaos Tougiannidis, Theodora Perraki, Manfred J. Frey, Prodromos Antoniadis, Werner Ricken PII: DOI: Reference:
S0166-5162(13)00109-2 doi: 10.1016/j.coal.2013.04.002 COGEL 2146
To appear in:
International Journal of Coal Geology
Received date: Revised date: Accepted date:
12 November 2012 10 April 2013 11 April 2013
Please cite this article as: Oikonomopoulos, Ioannis K., Perraki, Maria, Tougiannidis, Nikolaos, Perraki, Theodora, Frey, Manfred J., Antoniadis, Prodromos, Ricken, Werner, A comparative study on structural differences of xylite and matrix lignite lithotypes by means of FT-IR, XRD, SEM and TGA analyses: An example from the Neogene Greek lignite deposits, International Journal of Coal Geology (2013), doi: 10.1016/j.coal.2013.04.002
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ACCEPTED MANUSCRIPT A comparative study on structural differences of xylite and matrix lignite lithotypes by means of FT-IR, XRD, SEM and TGA analyses: An example from the Neogene Greek lignite deposits
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Ioannis K. Oikonomopoulos a, Maria Perraki b, Nikolaos Tougiannidis c, Theodora Perraki b, Manfred J. Frey c, Prodromos Antoniadis b, Werner Ricken c a
c
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Core Laboratories LP., Petroleum Services Division, 6316 Windfern, Houston, Texas 77040, USA.
[email protected] b National Technical University of Athens, School of Mining and Metallurgical Engineering, Division of Geological Sciences, 9 Heroon Politechniou Str. 15773 Zografou, Athens, Greece.
[email protected],
[email protected],
[email protected],
[email protected] University of Cologne, Faculty of Mathematics and Natural Sciences, Institute of Geology and Mineralogy, 49a Zuelpicher Str. 50674 Cologne, Germany.
[email protected],
[email protected],
[email protected]
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Corresponding author: Dr. Ioannis K. Oikonomopoulos Core Laboratories, Petroleum Services Division 6316 Windfern Road Houston, TX 77040, USA Tel: +1 713-328-2553 Fax: +1 713-328-2170 E-Mail:
[email protected]
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Abstract
The FT-IR spectra for both Neogene xylite and matrix lignite samples from six different Greek lignite deposits (NW Greece) show significant differences. In particular in the aliphatic stretching region (3000-2800 cm-1) the intensities of the vibrations are more prominent in the xylite lithotype as in the matrix lignite lithotype.
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The intense bands in the region 3402-3416 cm-1 are attributed to -OH stretching of H2O and phenol groups for both xylite and matrix lignite lithotypes. The bands at ~3697 cm-1 and ~3623 cm-1 as well as at ~538 cm-1 and 470 cm-1, which are more evident in the FT-IR spectra of the matrix lignite, are attributed to a higher clay content in the samples of this lithotype. Data resulting from X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and thermoanalytical methods (TG/DTG and DTA) indicate main differences of the xylite and matrix lignite lithotype as well. Typical peaks in X-ray diagrams confirm the high content of clay minerals in matrix lithotype, and their minor contribution in the xylite lithotype. Scanning electron microscopy (SEM), combined with the FT-IR and XRD results, reveals prevalence of gypsum, pyrite/marcasite, quartz, and clays in the matrix lignite lithotype as compared to the xylite lithotype and also textural differences such as the
ACCEPTED MANUSCRIPT heterogeneity in the mass of matrix lignite and the homogeneity of the xylite. Compositional differences of the examined lignite materials, as identified by FT-IR, were also confirmed by TG/DTG/DTA. While cellulose decomposition was observed
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by an endothermic peak of DTA curves, at around 360 °C, in the xylite samples, lignin degradation became a prominent process observed in the temperature range
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from 200oC to 400oC in the matrix lignite samples. This was also observed by an exothermic peak on DTA curves. Each of the heat effects was accompanied with a
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partial mass loss registered on TG curves. As a whole, the data resulting from the combined research by means of FT-IR, XRD, SEM, and TG/DTG/DTA of both xylite and matrix lignite lithotypes confirm the significant differences between these two lignite lithotypes reflecting their different structure, texture, and maturity stage.
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Keywords: Greece, Neogene, lignite, xylite, FT-IR, DTA, SEM 1. Introduction
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The organic structure of coal can be regarded as consisting of heterogeneous aromatic compounds, with increasing aromaticity from low rank (lignite, brown coal)
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to high rank coals (semianthracite, anthracite) (Killops and Killops, 1993). The organic deposits in Greece are described as low rank brown coals or lignites and based on their macroscopic characteristics they are classified in a number of lithotypes such as matrix lignite, mixed xylite-rich/matrix lignite, and xylite (Kaouras
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et al., 1991; Riegel et al., 1995). The term lignite refers to the maturity stage of coals, whereas the terms xylite and matrix relate to the lithotype of lignite. Fourier Transform Infra Red (FT-IR) spectroscopy is a sensitive technique in determining coal rank and also differentiates coals from areas which are geographically close in proximity (Gentzis et al., 1992). FT-IR spectroscopy is also a widely used analytical technique for determining the different functional groups of coal structures and has been extensively employed in the characterization of the mineral and organic matter of coals (e.g. Cloke et al., 1997; D’ Alessio et al., 2000; Geng et al., 2009; Georgakopoulos et al., 2003; Guiliano et al., 1990). In a detailed study carried out by Drobniak and Mastalerz (2006), the FT-IR spectra indicate that in a series of Miocene conifer wood samples from the Belchatow brown coal deposit in Poland, which have different maturation stage, cellulose is abundant in the less mature woody samples and is almost absent in the highest mature samples. Demethylation and oxidation
ACCEPTED MANUSCRIPT appear to be important processes of lignin modification, whereas demethoxylation is less prominent (Drobniak and Mastalerz, 2006). In the same study, it is concluded that the progressive elimination of cellulose and modification of lignin are the dominant
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processes of wood transformation. Up until recently, only a limited number of FT-IR studies of Greek lignites has
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been carried out. The presence of phenolic and alcoholic C-O bonds and C-O-C bonds with aliphatic carbons in the initial xylite sample named BEX from the Vevi area
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were reported by Georgakopoulos et al. (2003). The FT-IR spectra of lignite and humic clay samples, from the Apofysis-Amynteo lignite deposits, NW Greece, revealed the high abundance of C=O and C-O-R structures as well as clay and silicate minerals, respectively.
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In our case, the studied material (xylite and matrix lignite) belongs to different maturity stage, since matrix lignite lithotype has been defined as a fine detrital humic groundmass, whereas xylite lithotype represent the woody tissue preservation
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suggesting less humification as compared to the matrix lignite lithotype (Taylor et al., 1998). Due to the different maturity stage of the examined samples and also the
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heterogeneous and complicated organic structure of lignite material, a series of analytical methods such as Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, thermo-gravimetric (TG/DTG) and differential thermal analysis (DTA) were employed to provide information on the structural and compositional features of the examined lignite
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lithotypes.
The aim of the present study is to highlight the main compositional and
chemical differences of xylite and matrix lignite lithotypes, from the Neogene Greek lignite deposits, confirming in the micro-scale their totally different macroscopical features.
2. Geological setting The sampling areas (Fig.1) are located in the NNW-SSE trending Florina, Ptolemaida-Amynteo, and Kozani-Servia (FPS) graben. After the end of the Alpine orogenesis and during the Early Miocene, a period of intense tectonic faulting begins in NW Macedonia. Fault activity of major and profound faults of NW-SE direction resulted in the formation of the Florina, Ptolemaida-Amynteo, and Kozani-Servia graben (Fig.1). This graben extends to the North of the Greek borders with
ACCEPTED MANUSCRIPT F.Y.R.O.M. (Monastiri area) and is more than 150 km long (Metaxas et al., 2007). Subsequent tectonic episodes of NE–SW extension resulted in the fragmentation of the FPS graben and the creation of many independent sedimentary basins (Florina,
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Amynteo, Ptolemaida, Kozani and Servia) (Pavlides and Mountrakis, 1987).
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FIGURE 1
Formations belonging to the Pelagonian Zone mainly occupy the east and
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south-southwest margins of the graben. The west and northeast margins consist of Palaeozoic metamorphic rocks and Mesozoic crystalline limestones of the Pelagonian mass (Metaxas et al., 2007). The
Neogene-Quaternary
sediments
which
fill
the
basin
overlie
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unconformably on the basement rocks and can be divided into three members (Fig.1): The Lower Member (Upper Miocene to Lower Pliocene) includes the Komnina Formation and consists mainly of sands, sandy clays, clay marls,
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marls, and lignite seams.
The Middle Member (Pliocene) is comprised of lignite seams intercalated with
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marl and clay layers and includes the Ptolemais Formation. The Upper Member (Quaternary) includes the Proastion and Perdika Formations and consists of limnic and terrestrial sediments such as sandy clays, clay marls, sandy marls, and lignites. Terrestrial and fluvioterrestrial conglomerates are
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also present.
Quaternary sedimentation is represebted by lateral fans and alluvial deposits
(Pavlides and Mountrakis, 1987). The major Achlada-Scopos-Papadia (A-S-P) fault, which is NE-SW directed,
was formed during the Pre-Neogene neotectonic activity (Pavlides and Mountrakis, 1987), whereas syn-sedimentary normal faults were active during the entire ligniteforming period in the Ptolemais Basin (Doutsos and Koukouvelas, 1998).
3. Materials and methods Lithological features of each of the samples studied here were macroscopically described and the lignite lithotype was determined according to the guidelines established by Taylor et al. (1998). Samples with less than 10% by volume woody
ACCEPTED MANUSCRIPT tissues were classified as matrix lignite, whereas those with pure woody tissues were classified as xylite. All samples were crushed into coarse fragments and were air-dried in room
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temperature for the purposes of the present study. Thirteen xylite and thirteen matrix lignite samples from five lignite deposits (Achlada, Vevi, Kleidi, South-Field and
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Lava) of western Macedonia, Greece, were examined by means of FT-IR spectroscopy. The FT-IR measurements were carried out by a Fourier Transform Infra
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Red (FT-IR) spectrophotometer (Perkin Elmer GX-1). The FT-IR spectra, ranging in wavenumber from 4000 cm–1 to 400 cm–1, were obtained using the KBr pellet technique. The pellets were prepared by pressing a mixture of sample and dried KBr (sample: KBr, approximately 1:200), at 8 tons/cm2. All spectra were normalized to the
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weight of the sample in order the band absorbance to be compared among the samples. The Spectrum 3.02 software was used for spectra smoothing and normalizing. Bands were identified by comparison to published data (Cloke et al.,
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1997; Das, 2001; Drobniak and Mastalerz, 2006; Ibarra et al., 1996; Koch et al., 1998; Lide, 1991; Mastalerz and Bustin, 1995, 1996; Sobkowiak and Painter, 1992; Van
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Krevelen, 1993; Wang and Griffith, 1985). Band assignments used in this paper are listed in Table 1.
The samples were also examined by means of X-ray Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), Thermo-Gravimetric (TG/DTG) and Differential Thermal (DTA) analysis. X-ray powder diffraction patterns were obtained
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using a Bruker D-8 Focus diffractometer, with Ni-filtered CuKα1 radiation (λ=1.5405 Å), operating at 40 kV, 30mA. Where necessary, samples of pulverized and wet free lignite were heated up to 550 oC for approximately 1 hour in a Barnstead/Thermolyne 30400-60 oven. Afterwards, the samples were cooled to room temperature and examined by means of XRD. The morphology and the textural relations of the multiphase inclusions were studied by a Jeol 6380LV scanning electron microscopy (SEM). Experimental testing was carried out at a 20 kV accelerating voltage condition. TG/DTG/DTA were obtained simultaneously using a thermal analyzer (Mettler, Toledo 851) at a heating rate of 10 οC/min, atmospheric air and temperature range 25 οC - 1000 οC and 1200 οC. All analyses were carried out at the School of Mining and Metallurgical Engineering of the National Technical University of Athens (NTUA), Greece.
ACCEPTED MANUSCRIPT 4. Results and Discussion 4.1. FT-IR study of xylite and matrix lignite samples Representative FT-IR spectra of xylite and matrix lignite samples from the
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Achlada, Vevi, Kleidi, Amynteo, Southfield, and Lava lignite deposits are shown in Figure 2 and Figure 3. Although the examined samples were obtained from variable
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lignite deposits and also variable basins, strong similarities among the FT-IR spectra of each lithotype were observed (Fig.2 and Fig.3). Whereas the FT-IR spectra of the
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xylite samples (Fig. 2) are very similar to one another; minor differences among the matrix lignite FT-IR spectra (Fig. 3) are attributed to the high inhomogeneity of this material. On the other hand, the spectra of figures 2 and 3 differ significantly in the bands assigned to phenolic (C-O) and aliphatic carbon (C-H) groups and to the
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accompanying mineral phases. Representative spectra show typical infrared characteristics of the organic matter of lignite (low-rank coals), including aliphatic CH stretching bands at 2924 cm-1 and 2856 cm-1, C=C or C=O aromatic ring stretching
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vibrations at ~1610 cm-1 and at ~1506 cm-1, as well as aliphatic C-H stretching bands,
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at 1455 cm-1, 1370 cm-1 and 822 cm-1.
Figure 2
The main FT-IR absorption bands of both xylite and matrix lignite samples are summarized in Table 1.
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Wavenumber (cm-1)
Assignment
O-H stretching vibrations of hydrogen-bonded hydroxyl groups in polymeric association 2930 Asymmetric aliphatic C-H stretch vibrations–methylene (CH2) 2850 Symmetric aliphatic C-H stretch vibrations–methylene (CH2) 1610 Aromatic ring (C=C in plane) stretching symmetric 1510 C=O stretching vibrations 1458 Asymmetric aliphatic C-H deformation of methylene and methoxyl 1430-1420 aromatic C=C stretching vibrations 1370 Symmetric aliphatic C-H bending of methyl groups CH3 1266 C-O stretch vibration (in lignin-gualacyl ring with C-O stretch) 1224 C-O stretch vibration (in lignin-gualacyl ring and C-O stretch) 1031 C-O-H deformation in cellulose 822 Aromatic out-of-plane-rings with 2 neighbouring C-H groups ~534 Si-O-AlVI vibrations (Al in octahedral co-ordination) of clay minerals ~468 Si-O-Si bending vibrations of clay minerals Table 1: Characteristic FT-IR bands and bands assignments used in this paper and based on published 3400
studies (Das, 2001; Drobniak and Mastalerz, 2006; Ibarra et al., 1996; Mastalerz and Bustin, 1995, 1996; Van Krevelen, 1993).
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Figure 3
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Major differences are observed by comparing the FT-IR spectra of xylite and matrix lignite lithotypes in figure 4 as it can also be seen in figures 2 and 3. The
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intense and broad hydroxyl band at ~3392 cm-1of the xylite samples is attributed to both the -OH stretching vibrations of phenol groups, and to the -OH stretching
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vibrations of hydrogen-bonded hydroxyl groups of absorbed water of clay minerals, which are identified in traces. This band is downshifted compared to the matrix lignite samples at a band of ~3406 cm-1. The latter band is accompanied by two additional weaker bands at around 3698 cm-1 and 3620 cm-1. These are attributed to the crystal
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water which occurs in clay minerals of the matrix lignite samples (Geng et al., 2009). Bands at ~3698 cm-1 arise from the in-phase symmetric stretching vibration of the OH groups, either “outer” or “inner” surface OH of the octahedral sheets, which form
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weak hydrogen bonds with the oxygen of the next tetrahedral layer (Balan et al., 2001). The peak at ~3620 cm-1 corresponds to the stretching vibrations of the “inner
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OH groups” lying between the tetrahedral and octahedral sheets (Geng et al., 2009; Madejova, 2002). At the nearby wavenumbers’ area, the predominant FT-IR feature for xylite samples, in contrast to matrix lignite ones, is the strong band at ~2931 cm-1 that is attributed to asymmetric and symmetric aliphatic C-H stretching vibrations of methylene (-CH2) (Ibarra et al., 1996; Michaelian and Friesen, 1990; Wang and
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Griffiths, 1985). This band is observed at slightly lower wavenumbers (~2925 and ~2856 cm-1) in matrix lignite samples.
Figure 4
Significant differences for functional groups that contain oxygen in their molecules are also observed in the 1700-1100 cm-1 zone that includes also the region of cellulose and lignin bands. The strong band at ~1618 cm-1 of the matrix lignite samples is attributed either to C=O or C=C aromatic ring stretching vibrations, as well as to OH bending vibrations of adsorbed water, and it is slightly shifted at lower wavenumbers (1606 cm-1) for the samples belong to the xylite lithotype. The stretching vibrations at ~1506 cm-1 due to C=O structures along with the vibrations due to symmetric aliphatic C-H vibration of methylene (-CH2) and methoxyl (OCH3)
ACCEPTED MANUSCRIPT at ~1455 cm-1, the band at ~1370 cm-1 attributed to symmetric aliphatic C-H bending vibration of methoxyl groups (OCH3), and the band at ~1265 cm-1 due to C-O stretching vibrations (Ibarra et al., 1996; Mastalerz and Bustin, 1995, 1996; Van
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Krevelen, 1993) tend to decrease in matrix lignite. Since the bands attributed to C=O and C-O-R structures at the 1800-1100 cm-1 region tend to decrease during the
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process of natural maturation of coal (Ibarra et al., 1996); the elimination of stretching vibrations at the 1800-1100 cm-1 region indicates increasing coalification from xylite
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to matrix lignite lithotype. Thus, it could be stated as a transformation factor of organic matter to be considered in determining the coalification stage. The out-ofplane vibration due to the C-H bonds at ~823 cm-1 also decreases in matrix lignite, suggesting increasing aromaticity. The last is a typical characteristic of lignite in a
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higher maturation stage (Ibarra et al., 1996). Our results are in accordance with previous studies on vitrinite reflectance of xylite-rich and xylite-free lignites from NW Macedonia (Greece), which point out the different maturity stage of these
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materials. Georgakopoulos and Valceva (2000) reported random reflectance on euulminite at a range 0.15-0.18% for xylite-rich lignites and 0.26-0.34% for lignites
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without xylite fragments. The band at ~1033 cm-1 is typical for the xylite lithotype samples and is attributed to C-O-H bonds in cellulose as well as to C-O stretching vibrations of aliphatic ethers (R-O-Ŕ) and alcohols (R-OH) (Ibarra et al., 1996); whereas the ~1104 cm-1 and ~1030 cm-1 bands are typical for the matrix lignite samples and arise from the Si-Oapical and Si-O-Si stretching vibrations of the enclosing
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mineral matter respectively (Çetinkaya and Yürüm, 2000; Georgakopoulos et al., 2003; Velde, 1978). Drobniak and Mastalerz (2006) concluded that the FT-IR spectra of a series of woody samples show that cellulose is abundant in the least-transformed woody samples and barely detectable in the highest-transformed woody samples. The results of the present study are in accordance with this assumption, since the bands represented by cellulose and lignin region are prominent at the samples that belong to the xylite lithotype and more restricted in the samples of matrix lignite lithotype which represent (by definition) a more transformed organic material. Furthermore, as described above, the aromatic structures are more evident in the samples of matrix lignite lithotype which also implies more transformed material comparing as to xylite. The prominent band at ~680 cm-1 in matrix lignite samples could be attributed to aromatic out-of-plane C-H vibrations rather than to mineral matter (Georgakopoulos et al., 2003). The ~914 cm-1 band at the matrix lignite lithotype arises from the
ACCEPTED MANUSCRIPT bending vibrations of “inner” OH groups of Al-OH-Al of the kaolinite structure. The band at ~531 cm-1 originates from Si-O-AlVI vibrations (Al in octahedral coordination), whereas the band at ~469 cm-1 is attributed to the Si-O-Si bending
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vibrations (Madejova, 2003; Van Jaarsveld et al., 2002). In general, concerning the contribution of mineral mater in both xylite and
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matrix lignite lithotype (Fig.4); the strong vibrations corresponding to the occurrence of clay and silicate minerals (bands at ~3698 cm-1, 3620 cm-1, 1031 cm-1, 915 cm-1,
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531 cm-1, and 469 cm-1) are prominent in the FT-IR spectra of the matrix lignite samples, whereas in the xylite samples, a limited number of barely detectable vibrations are observed. This could be assumed as a result of the less cohesive structure of matrix lignite that allows the water movement through the lignite mass
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and thus the precipitation of clay and silicate minerals. The abundance of silicate minerals in the matrix samples from the Achlada, Vevi, and Kleidi lignite deposits compared to the Southfield lignite deposits, suggests the fluvial paleoenvironment in
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which Achlada, Vevi, and Kleidi were formed; and the limnic conditions in which the Southfield deposits were formed, indicating slow transportation rate of inorganic
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material into the peatland (Antoniadis et al., 2005; Oikonomopoulos et al., 2008).
4.2. XRD analysis of xylite and matrix lignite samples The X-ray diffraction (XRD) analysis of the samples revealed that the mineral assemblages present in matrix lignite are predominantly illite-muscovite, gypsum, and
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quartz, as it can be seen in Figure 5b of representative xylite and matrix lignite samples. Illite-muscovite was identified by the sharp diffraction patterns at d001=~9.95 Å and d003=~3.34 Å, gypsum was identified by its typical peak at d020=~7.6 Å and quartz by its typical peaks at d101=~3.34 Å and d100=~4.28 Å. Traces of other minerals such as calcite at d104=~3.06 Å and pyrite at d311=~1.64 Å were also detected. Since the peak observed at ~7.1 Å might be attributed to both kaolinite and chlorite, in order to identify these two minerals, the thermal behaviour of the samples was examined. The samples were heated up to 550 oC for 1 hour, then cooled at room temperature and examined by x-ray power diffraction. The lack of the characteristic diffraction pattern at d=~7.1 Å due to the collapse of kaolinite as shown in figure 6b, clearly indicates the presence of kaolinite.
FIGURE 5
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Upon heating matrix lignite samples to 550 oC, a decrease in intensity of the typical diffraction pattern at d020=~7.6 Å was present due to the collapse of gypsum. d210=~2.85 Å (Fig.6b), indicates the presence of anhydrite.
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The last in combination with the occurrence of typical peaks at d020=~3.50 Å and
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Regarding the X-ray diagram of the xylite bulk sample (Fig.5a) the background overlaps the typical diffraction pattern of the gypsum. However, the
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formation of anhydrite (peaks at d020=~3.50 Å and d210=~2.85 Å) after heating up to 550 oC (Fig.6a) indicates the presence of gypsum at xylite bulk samples. The presence of pyrite is more evident at the heated matrix lignite samples.
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FIGURE 6
Comparing the X-ray diagrams of figure 5 and figure 6, it is observed that clay
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minerals are almost exclusively present in the matrix lignite lithotype since in xylite samples only traces of illite-muscovite was observed (Fig.6a). These findings confirm
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the results of the FT-IR-studies, in which the typical bands of clay minerals are barely detectable (almost absent) on the xylite spectrum. This may be attributed to the cohesive structure of woody material that prevents the water movement through the xylite mass.
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4.3. Scanning Electron Microscopy (SEM) The examination of matrix lignite samples by scanning electron microscopy
(Fig.7) reveals that gypsum (calcium sulphate) and quartz (silica dioxide) represent the bulk of mineral constituents. Small amounts of pyrite/marcasite (iron sulphide) and clays are also present. The calcium sulphate mineralization associated with matrix lithotype of lignite occurs mainly as individual crystals of gypsum in an elongated form (Fig. 7a) within the detritus plant mass. Angular fragments of quartz were also detected (Fig. 7b). Pyrite (Fig.7d,e) displays in "framboidal" textures; the framboids take the form of spheroidal aggregates, ranging from 1 to 100 μm across, which are built up of discrete, equidimensional euhedral pyrite microcrystals. They are usually intimately associated with the macerals of the coal deposit (Ward, 2002), occurring for example within ulminite mass of Greek lignites (Oikonomopoulos et al., 2008). Among others,
ACCEPTED MANUSCRIPT Ramdohr (1975), Kortenski and Kostova (1996), and Butler et al. (2000) interpreted the formation of framboidal pyrite controversially: initially, framboids were widely accepted to represent fossilized microorganisms ("vererzte Bakterien" sensu
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Ramdohr, 1975). However, the experimental synthesis of framboids under laboratory conditions suggests their abiotic precipitation from Fe - S - solutions (Butler et al.,
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2000; Sweeney and Kaplan, 1973).
Furthermore, the matrix lithotype of lignite presents heterogeneity (Fig.7f) due
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to different plant remains which compose the matrix mass. In contrast, the pure woody material (xylite) occurs as a homogenous mass with cracks (Figs.7e,f) which were formed during shrinkage and/or pressure from the overlying sediments and are
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therefore post-coalification in age.
FIGURE 7
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Xylite shows a totally different texture than the matrix lignite. Minor amounts of minerals are also present on the surface of xylite or they appear only to fill cracks
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in the woody material.
4.4. TG/DTG and DTA study of xylite and matrix lignite samples For the xylite samples there is a weight loss up to 100 oC (Fig.8a) which is due to absorbed water. At temperatures above 100ºC, chemical bonds begin to break and
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the rate at which the bonds are broken increases as the temperature increases. By the slight slope of xylite TG curve between 100°C and 200ºC, it could be assumed that pyrogenic water (Iordanidis et al., 2001) is the major product that is released, whereas noncombustible products, such as carbon dioxide and traces of organic compounds are also produced (Brebu and Vasile, 2010). The steep slope of the xylite TG curve, at the temperature range from 250 oC up to 400 oC (Fig.8a), suggests rapid weight loss which is mainly due to oxidative decomposition of cellulose and its residues (Kaur et al., 1986). During decomposition of cellulose volatile compounds are produced (Brebu and Vasile, 2010) and breakdown of lignin is present in minor contribution as it is also confirmed from the xylite DTA curve. As temperature increases at around 450°C, the production of volatile compounds is complete and the continuing weight loss is due to degradation of the remaining organic compounds and produced char. (Brebu and Vasile, 2010).
ACCEPTED MANUSCRIPT In contrast, the slight slope of the matrix TG curve (Fig.8a) suggests continuous weight loss during heating up to ~1000 oC. The progressive weight loss is attributed to the gradual decomposition of the lignin content, the organic compounds
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that contain aromatic rings in their molecules (more stable compounds in low temperatures) and the presence of inorganic material. Heating of lignin yields phenols
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from cleavage of ether and carbon–carbon linkages and produces more residual char than does heating of cellulose. Lignin generally decomposes at a slower rate than
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cellulose, although the degradation period begins somewhat earlier than for the cellulose. Heating up by 10 °C/min, lignin decomposes very slowly and the degradation rate increases slightly above 750 °C (Brebu and Vasile, 2010) as it can be seen by the higher weight loss of xylite samples up to 1000 οC (82.72 wt-%), than the
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matrix lignite ones (22.75 wt-%). The above results are in accordance with the FT-IR observations, since the bands due to the cellulose region are prominent in the xylite samples and less detectable in the matrix lignite samples; whereas the bands that are
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attributed to aromatic rings are more evident in the matrix lignite samples than in the
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xylite ones.
FIGURE 8
The intense and sharp peak of the xylite DTG curve observed at the temperature range from ~200 oC up to ~400 oC indicates the higher devolatisation rate
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of the xylite lithotype compared to the matrix one. The sharp peak at ~340 οC (Fig.8b) can be attributed to cellulose content of the xylite sample (Charland et al., 2003). Since the peak height of the DTG curve provides a relative measure of reactivity (Vamvuka et al., 2004), the xylite samples seem to be more reactive than the matrix lignite samples. This reactivity difference could be regarded as the result of the compositional differences between the two lignite lithotypes since, as previously mentioned, lignin generally decomposes at a slower rate than cellulose and therefore a prevalence of lignin in the matrix samples makes them less reactive than the xylite samples. In general, the DTG curves of lignin-rich material decomposition show wide and flat peaks that are different for the sharper DTG peaks of cellulose-rich materials and, inducing a flat tailing section at higher temperatures for wood decomposition (Brebu and Vasile, 2010). The bulk of the burning process for matrix lignite occurred
ACCEPTED MANUSCRIPT mainly between 200 oC and 550 oC, whereas the peak of the DTG curve at ~100 οC is associated with drying phase of sample (Fig.8b). Observing the DTA curves the first endothermic peak occurs in the range 40-
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110°C for each sample and is obviously connected with gradual moisture evaporation. An exothermic peak at ~360 οC in the xylite DTA curve is associated with the
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decomposition of cellulose, whereas the decomposition of lignin is characterized by an exothermic peak in the temperature range from 200 οC - 400 οC (Fig.8c). The
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exothermic peak, in the same temperature range, of the matrix DTA curve (Fig.8c), is also typical for lignin and thus it is attributed to the destruction of aliphatic groups, CH groups, carbohydrate components and (to some extent) of oxygeneous (alcoholic, phenolic) and amino groups (Kucerik et al., 2004). The thermal degradation of lignin
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is a complex process because the materials have many components with different decomposition pathways (Brebu and Vasile, 2010). Lignin thermally decomposes over a broad temperature range, because various
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oxygen functional groups from its structure have different thermal stability and their scission occurs at different temperatures. Due to its complex composition and
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structure, the thermal degradation of lignin is strongly influenced by its nature, moisture content, and reaction temperature (Brebu and Vasile, 2010). Drobniak and Mastalerz (2006) indicated that in a series of woody specimens that include groups of samples showing variable maturity stage, from the least-transformed group of samples showing low gelification up to the highest-transformed group showing high
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gelification, there is an overall increase in aromaticity, an increase in lignin / cellulose ratio, and an increase in oxygen functionalities. In the same study it is concluded that demethylation and oxidation appear to be important processes of lignin modification, whereas demethoxylation is less prominent. While cellulose decomposition was a continuing process in the least-transformed samples which are less gelified, lignin modification, particularly demethoxylation and demethylation, became a prominent process in the group of more mature samples showing higher gelification and stated as highest-transformed. (Drobniak and Mastalerz, 2006). Cellulose decomposition and transformation of lignin are most likely processes that proceed simultaneously and were suggested in numerous studies (Hatcher, 1988; Mastalerz and Bustin, 1994; Senftle et al., 1986; Stout et al., 1988). The results of the present study are generally in accordance with the above mentioned suggestions indicating, however, that while cellulose decomposition was more
ACCEPTED MANUSCRIPT obvious in the samples that were classified under the xylite lithotype, as evidenced by the steep slope of TG curve and the typical for cellulose exothermic peak of the DTA curve, lignin degradation, became a prevalent process for the samples that were
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classified under the matrix lignite lithotype. The endothermic peak at ~500 οC of the matrix DTA curve (Fig.8c) is attributed to the dehydroxylation of the kaolinite, due to
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the loss of OH groups surrounding the AlVI atoms (Van Jaarsveld et al., 2002). The process of the above dehydroxylation leads to the progressive transformation from the
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octahedral co-ordinated Al, in kaolinite, to a tetrahedral co-ordinated form, in metakaolinite, caused by the breaking of OH bonds (Van Jaarsveld et al., 2002). Illitemuscovite provides endothermic peaks at higher temperatures. Xylite samples present higher weight loss up to 1000 οC (82.72 wt-%), than
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the matrix lignite ones (22.75% wt-%). The water which is evolved during pyrolysis arises from the -OH of constituent water and from the condensation of phenols
5. Conclusions
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(MacPhee et al., 2004).
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The comparative study of the xylite and matrix lignite lithotypes from the organic seams of six lignite deposits in Florina and Ptolemais sub-basins, NW Greece, by means of FT-IR spectroscopy, in combination with scanning electron microscopy (SEM), X-ray diffraction analysis (XRD) and thermoanalytical methods (TG/DTG and DTA), resulted in the followings: The FT-IR spectra of all samples confirm major differences between the xylite
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and the matrix lignite lithotype. Bands attributed to vibrations of C-O and C=O structures and bands due to symmetric and asymmetric aliphatic vibrations (1700-1100 cm-1) are prominent in the xylite lithotype of lignite and less detectable in the matrix lignite lithotype. Since the bands attributed to C=O and C-O-R structures at the 1800-1100 cm-1 region tend to decrease during the process of natural maturation of coal (Ibarra et al., 1996); the elimination of stretching vibrations at the same region indicates increasing coalification from xylite to matrix lignite lithotype. The out-of-plane vibration due to the C-H bonds decreases in matrix lignite, suggesting increasing aromaticity in the latter.
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The vibrations corresponding to the occurrence of clay minerals (~3697 cm -1, 3620 cm-1, 1034 cm-1, 915 cm-1, 531 cm-1, 469 cm-1 and 435 cm-1) are prominent in the FT-IR spectra of all the matrix lignite samples and barely
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detectable in xylite samples. This observation is also confirmed by the X-ray diffraction study which indicates that clay minerals, mainly illite/muscovite,
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are present in the matrix lignite lithotype and almost absent in the xylite one. These observations could be assumed as the result of the less cohesive
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structure of matrix lignite that allow the water movement through the lignite mass and thus results in the precipitation of clay minerals. The formation of anhydrite in the heated samples indicates the presence of gypsum in both raw materials.
Scanning electron microscopy (SEM) revealed that gypsum (calcium
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sulphate), pyrite/marcasite (iron sulphide), quartz (silica dioxide) and clays represent the bulk of mineral constituents for the samples classified as matrix
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lignite. Texture differences such as the heterogeneity of the samples classified as matrix lignite lithotype and the homogeneity of the samples classified as
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xylite lithotype, were also observed. The matrix lignite shows heterogeneity due to different plant remains which compose the matrix mass; whereas the pure woody material (xylite lithotype) occurs as a homogenous mass having cracks that were formed during shrinkage and/or pressure from the overlying
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sediments and are therefore post-coalification in age. Minor amounts of mineral matter appear only to fill cracks of the xylite lithotype.
The TG/DTG/DTA curves of the xylite lithotype present higher weight loss
when comparing to the matrix lignite lithotype, as well as a sharp DTG peak at ~320 °C, accompanied with an endothermic peak of DTA curve, which is typical for cellulose decomposition. In contrast, the lignin decomposition is characterized by an exothermic peak in the temperature range 200oC – 400oC. While cellulose decomposition was more obvious in the samples of xylite lithotype, as evidenced by the steep slope of TG curve and the “typical” cellulose exothermic peak of the DTA curve, lignin degradation, became a prevalent process for the samples of matrix lignite lithotype. TG/DTG/DTA curves also confirm the minor contribution of mineral matter in the xylite lithotype and its important participation in the matrix lignite lithotype.
ACCEPTED MANUSCRIPT 6. Acknowledgments The authors would like to thank Prof. R. Littke for the editorial handling of the present study. The careful and constructive reviews of the two anonymous reviewers
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are also highly appreciated. Many thanks are given to the mining engineers Th. Balis and O. Grammenopoulos of the Achlada lignite mine for their support during
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sampling as well as the technician D. Sutor of Core Laboratories LP for the
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proofreading of this paper.
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FIGURES CAPTIONS
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Fig.1: Location map of the Florina-Ptolemais-Kozani-Servia graben (after Tougiannidis, 2009). AM= Amynteon – Mine, ApM = Apophysis – Mine, MF =Main Field – Mine, KFM = Komanos – Mine, KF = Kardia – Mine, WF = West Field – Mine, SF = South Field – Mine. Fig.2: FT-IR spectra of representative xylite samples obtained from the studied lignite
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deposits. Significant similarities can be observed among the xylite FT-IR spectra, although the examined samples belong to variable basins and lignite deposits. Ach: Achlada, Ve: Vevi, Kl: Kleidi, La: Lava. Fig.3: FT-IR spectra of representative matrix samples obtained from the studied lignite deposits. Noteworthy similarities can be seen among the FT-IR spectra of matrix lignite lithotype. Ach: Achlada, Ve: Vevi, Kl and KL: Kleidi, Sf: Southfield. Fig. 4: Comparative FT-IR spectra of representative xylite (a) and matrix lignite (b) samples from Vevi lignite deposits (Ve). Fig.5: X-ray diffraction diagrams of representative xylite (a) and matrix (b) lignite samples, from Achlada lignite deposits. Qz: Quartz, Cc: Calcite, Gy: Gypsum, Ill-Mu: Illite-Muscovite, Ka: Kaolinite, Py: Pyrite.
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Fig. 7: SEM images of mineral matter associated with matrix lignite and xylite lithotypes of the Greek lignite deposits (a): calcium sulphate in the form of gypsum
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(Gy) within matrix lithotype, (b): silica dioxide, in the form of quartz (Qz) within matrix lithotype, (c) and (d): iron sulphide, in the form of pyrite (Py) within matrix
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lithotype, (e) and (f): homogeneous xylite mass with cracks, which are partially filled with mineral matter.
Fig. 8: TG/DTG/DTA diagrams of representative xylite and matrix lignite samples from the Vevi and Achlada lignite mines respectively. (a): TG curves both for xylite
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and matrix lignite lithotype; (b): DTG curves both for xylite and matrix lignite
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lithotype; (c): DTA curves both for xylite and matrix lignite lithotype.
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Study of xylite vs matrix lignite by means of FT-IR, SEM, XRD, and TG/DTG/DTA FTIR confirms progressive elimination of aliphatic vibrations at different lithotypes Vibrations due to clay minerals are almost absent in xylite FTIR spectra SEM confirms major differences (texture, minerals) between xylite and matrix lignite TGA curves of the studied lithotypes show different content in cellulose and lignin
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