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Doppleritization of xylitic coal in the light of petrographic and chemical investigations
International Journal of Coal Geology, 2 (1982) 181--194 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
181
DOPPLERITIZATION OF XYLITIC COAL IN THE LIGHT OF PETROGRAPHIC AND CHEMICAL INVESTIGATIONS
MARIAN WAGNER Institute of Fuel Materials, University of Mining and Metallurgy in Cracow, al. Mickiewicza 30, 30-059 KrakOw (Poland) (Received July 17, 1981; revised and accepted February 26, 1982)
ABSTRACT Wagner, M., 1982. Doppleritization of xylitic coal in the light of petrographic and chemical investigations. Int. J. Coal Geol., 2:181--194. The paper presents the results of petrographic and chemical investigations of xylitic coal, xylite and their ash. As is known, a xylitic coal has the function of a lithotype whereas xylites are inclusions brown coals only. All these xylites were arranged according to the increasing degree of doppleritization of xylem in the sequence: common xylite, poorly, moderately and intensely doppleritized varieties, dopplerite coal. The distinctive criteria were their differentiated physical properties, which are also reflected in the variable petrographic and chemical compositions. The petrographic differences result from the replacement of textinite by ulminite and gelinite, while the differentiation of the chemical constitution is due to the increasing carbon content and the increasing number of functional groups that determine the aromatic nature of the internal structure of the coal. Infrared absortion spectroscopy, Raman spectroscopy and chemical analysis of ash have shown that the aromatization of coal and the resulting increase in the degree of order of the structural unit are due to the formation of organomineral compounds in the course of doppleritization. It appears from the investigations that the doppleritization of vegetable matter gives rise to gels that are a mixture of organomineral humic compounds. The process of humification leads to the formation of gel made up of humins. It follows then that there are two ways of initial coalification of plant material, both referred to as biochemical gelification.
INTRODUCTION X y l i t i c c o a l is t h e f o s s i l w o o d o f T e r t i a r y c o n i f e r s . I t o c c u r s in b r o w n c o a l s e a m s in t h e f o r m o f p r e s s e d s t e m s l y i n g h o r i z o n t a l l y , u p r i g h t t r u n k s , s t u b s a n d o t h e r t r e e f r a g m e n t s o f d i f f e r e n t sizes. X y l i t i c c o a l is c h a r a c t e r i z e d b y d i v e r s e p h y s i c a l f e a t u r e s in t h a t i t m a y r e s e m b l e t h e p r e s e n t day wood, show longitudinal or both longitudinal and transversal splitting, o r i t m a y o c c u r in t h e f o r m o f gel t h a t r e s e m b l e s a w o o d f r a g m e n t o n l y in s h a p e . The transformation of soft brown coal to hard brown coal varieties
0166-5162[82/0000--0000[$02.75 © 1982 Elsevier Scientific Publishing Company
182 involves intense gelification of fossil wood. At the bright sub-bituminous coal stage it is n o t possible to distinguish the xylite varieties. The gelification of fossil w o o d can be considered as physical, chemical and biochemical changes of lignin and hydrocarbons. It is generally held that it is the first stage of transformation (decomposition) of plant matter, which results in the formation of humic or organomineral gel. At the subsequent stages of coalification this gel is converted into vitrain. The transformation of plant material at the peat and brown coal stages is referred to as humification or biochemical gelification (Stach et al., 1978, pp. 247--250). It is the first stage of enrichment of the decomposing plant matter in carbon, involving moderate oxidation and the formation of humic acids which in the subsequent stages of coalification are converted into humins, i.e. c o m p o u n d s of low chemical activity. In an aqueous medium, humic acids can react with inorganic bases or salts. The products of these reactions are organomineral c o m p o u n d s called dopplerite. The complex of physical and chemical p h e n o m e n a leading to the formation of such c o m p o u n d s will be referred to in this paper as doppleritization. There are very few papers published to-date that deal with the process of doppleritization. More attention has been given to dopplerite originating from peat and brown coal deposits. Dopplerite has been defined as a Ca humic c o m p o u n d or complex humic c o m p o u n d of Ca, Fe, A1, K and Na (Potoni~ and Stockfish, 1932; Rammler and Jacob, 1951). It appears that the biochemical gelification of plant matter in a coal sedimentation environment proceeds in two different ways. One leads to the formation of humins, i.e. purely organic compounds, whilst the other gives rise to dopplerite. Therefore the term "humification" is not s y n o n y m o u s with "biochemical gelification", as it defines only one of the possible ways of transformation of humic material. This paper aims to determine the geochemical conditions of doppleritization of xylitic coal. Petrographic and chemical investigations were carried o u t on xylitic coal samples showing varying degrees of doppleritization, derived from the same coal seam. MATERIAL AND METHODS The macroscopic features alone do n o t allow to distinguish xylites that have been subject to humification from those that have undergone doppleritization. It is, however, relatively easy to distinguish one variety from the other if the c o n t e n t of internal ash, i.e. that combined with the organic matter of coal, is determined, b u t such a procedure requires a strict control of the purity of the investigated material in thin sections. As appears from the author's experiments the intensely doppleritized xylites have an ash c o n t e n t of more than 10 wt.%, while the xylites that have undergone humification contain only a b o u t 6 wt.% of ash.
183
The present investigations were carried out on doppleritized xylites derived from the so-called Glogbw bed (Upper Oligocene) in the area of GlogSw (Frankiewicz, 1975). The experimental material was represented by samples of c o m m o n cylite (L-O), poorly (L-l), moderately (L-2, L-3) and intensely doppleritized (L-4) xylites, and doppleritic coal (L-5). The internal ash content in this series of xylites varied from 1.2 to 11.1 wt.% (Table I). The detailed investigations comprised: (a) Macro- and microscopic petrographic observations in polarized transmitted light, carried out with a Zeiss "Laboval 2 pol" microscope (b) Chemical analysis of C and H contents (by Sheffield's method), the O content in the carboxyl groups of coal, and the determination of CaO, MgO, A1203, Fe203, Na20 and K20 contents in the xylite ash obtained at 450°C
TABLE I Q u a n t i t a t i v e analysis of I R a b s o r p t i o n spectra of x y l i t i c coal
Sample
L-0
L-1
L-2
L-3
L4
L-5
Aa
1.3
1.1
4.1
10.8
10.9
11.1
Ca
47.2
49.0
53.7
53.7
53.4
55.1
Ha
5.9
5.8
5.5
5.0
4.7
4.6
C/H
8.0
8.4
9.7
10.8
11.7
11.9
HCH3
2.48
2.05
1.74
2.46
2.45
3.27
HC~
3.19
3.48
3.58
2.14
1.83
0.93
Har
CCH3
rain CC~
rain Car
fmin
max2 CCH
C max
fmax
40.52
--7.94
--0.02
81.04
32.58
0.38
42.63
--9.15
--0.02
85.26
33.49
0.41
40.00
2.15
0.07
80.00
42.15
0.47
0.23 21.01
nat
0.07
0.29 16.72
0.06
0.22 12.96
0.21 23.83
25.14
0.34
47.73
49.000
0.58
21.79
26.25
0.37
43.58
48.04
0.59
10.41
39.74
0.55
20.82
50.15
0.65
0.39 18.32
0.07
0.45 18.34
0.67
nar
0.06
23.74
0.11
A a = ash content calculated in the analytical state; C a and H a ffiC and H contents calculated in the analytical state; C / H = atomic ratio of C to HCH~ ; C c ~ = percentage of C and H in CH~-type groups relative to their content in coal; Car ffipercentage o f C in aromatic bonds relativeto their total content in coal; fmax, fmln = m a x i m u m and m i n i m u m coefficient of aromatization; nar/nal ffiquantitative ratio of vibrating aromatic to aliphatic bonds systems.
184 (c) Infrared absorption spectroscopy in the basic infrared region, using a UNICAM SP 1200 grid spectrometer (d} Raman spectroscopy carried o u t with a Zeiss GDM-1000 spectrometer equipped with a m o n o c h r o m a t o r and an ILA-120(2W)Ar laser. The laser beam p o w e r was 40 and 70 mW, depending on the kind of xylite, at a wavelength of 514.5 nm. The resolution of the Raman spectrometer with m o n o c h r o m a t o r was a b o u t 10 cm -1 . Coal samples for spectroscopic investigations were prepared with the u t m o s t care. Each sample was dried at 105°C in a nitrogen atmosphere for 2 hours. Then the samples were ground in a Fritsch planetary mill for 16 hours in the damp-free atmosphere, and prepared in KBr discs in proportions by weight of 1:300 coal to potassium bromide by pressing at a pressure of 10 MPa. PETROGRAPHIC FEATURES OF DOPPLERITIZED XYLITES The change from c o m m o n xylite through doppleritized xylites to doppleritic coal is progressive. It manifests itself in the gradual disappearance of the macroscopic features of wood, the change of colour from brown to black, and the change in lustre from dull to vitreous. In contrast to c o m m o n xylites, the doppleritized varieties do n o t show fibrous parting or elasticity. The latter have a conchoidal fracture and exhibit great friability due to the dense n e t w o r k of endogenous fissures. These features are particularly conspicuous in partly dried coal, being less pronounced in freshly exposed seains. The degree of doppleritization of xylites is also clearly visible under the microscope. On the basis of microscopic features, three degrees of doppleritization can be distinguished, corresponding to the macroscopic division (Jacob, 1958; Suss 1959; Brzyski and Majewski, 1974). C o m m o n and poorly doppleritized xylites are made up of textinite. It has been noticed that in the doppleritized varieties the zones of spring growth of xylem are generally disturbed, showing locally the features peculiar to texto-ulminite. The textinite is partly impregnated with corpohuminite. Xylites showing a medium degree of doppleritization are made up of textinite and texto-ulminite. The structure of spring growth zones of xylem is frequently obliterated due to the crushing, tearing and random displacem e n t of the cell walls. The cell walls of the summer growth of xylem are well preserved. The cell lumina are filled with gelinite. Intensely doppleritized xylites are made up almost entirely of eu-ulminite. The tracheids are poorly marked off, being e m b e d d e d in the granular gel mass. Doppleritic coal has a similar structure. In the gel ground mass, made up of uliminite, fragments of corroded tracheids are visible only locally. From the above observations it is evident that intense doppleritization
185
of xylitic coal involves not only the impregnation of xylem with gel but also the chemical destruction of the cell walls (Roselt, 1969, 1976). These processes result in the progressive replacement of textinite by ulminite and eventually by gelinite (levi-gelinite). The humic material owing its origin to the destruction of wood tissue partly or completely fills the fissures. From the above considerations it follows that the doppleritization of xylitic coal is a process involving several stages. INFRARED ABSORPTION SPECTRA
IR spectra of the investigated xylite varieties are differentiated, the differences consisting in the disappearance or development of absorption bands at the specified frequencies (Fig. 1). The IR spectrum of c o m m o n xylite has been found to be similar to that of lignin, while the absorption displayed by doppleritic coal has several features in c o m m o n with the spectrum of sodium acetate (CH3COONa), an organomineral c o m p o u n d used for spectroscopic analysis.
L-O L4 L~2
L-3
L-4
[_-5
CH,COONa 40
35
30
25
20
15
12
10
B
4X100 cn~1
Fig. 1. I n f r a r e d absorption spectra of xylites. L-O = c o m m o n xylites; L-1 = p o o r l y d o p p leritized xylite; L-2, L - 3 = m o d e r a t e l y d o p p l e r i t i z e d xylites; L - 4 = i n t e n s e l y d o p p l e r i t i z e d xylite; L - 5 = d o p p l e r i t i c coal.
186 At present much attention is given to the absorption displayed by coal at 1500--1650 cm -1 . The bands produced by xylitic coal varieties in this range are distinct, although they are characterized by the variable morphology of lines. The spectrum of c o m m o n xylite shows an intense and sharp peak at 1510 cm -1 , and a broad but less intense band at 1610 cm -1 . Its left part shows a weak inflexion at 1700 cm-' (Fig. 1). In xylites showing a higher degree of doppleritization the 1500 cm-' band disappears, but simultaneously the 1600 cm-' band increases in intensity and becomes markedly broadened. If we assume that the 1600 cm -' absorption band reflects solely the aromatic structure of coal, then the dependence between the coefficient of aromatization f and the optical density of this band should be linear. The appropriate calculations have shown (Table I), however, that this assumption is incorrect (Fig. 2). It appears therefore that the growth of the 1600 cm -1 band is associated n o t only with aromatic --C=C - vibrations, but also with vibrations of functional groups containing oxygen. The relationship between the optical densities of the 1600 and 1700 cm -1 bands (Fig. 2) indicates that the intensity of their vibrations depends on the bonds containing oxygen.
A D17 0 i
~
0,4~ ~ . o~- . . . ~ "
0,7 o,o1
0,2.
0,5 ,
i
0,1 }
0,0
0,4 +
0,5
B
frnax ar I
~
1,0
D160~
i i I
I --------:~.
~ . I .
/
Q2
0,4
{3,6
O,B
1,0
D1600
Fig. 2. A. Relationship between the optical densities of the 1700 and 1600 cm -1 absorption bands. B. Relationship between the m a x i m u m coefficient of aromatization and the optical density o f the 1600 cm -1 band.
The 1700 cm -1 band in the spectrum of lignin is to be attributed to the C=O groups of ketone radical. In c o m m o n xylite it is displaced to a frequency characteristic of quinones. In the spectra of doppleritized xylites this band is caused by C=O stretching vibrations of aromatic acids. This is evidenced by sharp peaks at 1220 (vc_o) and 880 cm -1 (VoH) that disappear, however, in doppleritic coal. The absorption close to 1700 cm-' displayed by this coal may be due to the carbonyl group of ketones. The 1600 cm-' band produced by c o m m o n xylite and poorly or moderately doppleritized varieties is accompanied by absorption with peaks at 1500 and 1260 cm -1 , which markedly decrease in intensity, and have
187 been attributed to skeleton vibrations of heteroaromatic rings (Czuchajowski and Sliwiok, 1974, p. 68). The intensity of these bands indicates that in the process of doppleritization of xylites, the aromatic bonds of lignin are substituted by heteroaromatic linkages. Xylites showing a high degree of doppleritization display an absorption centered close to 1630 cm -1 , which in doppleritic coal exceeds the vibrations of heteroaromatic systems in intensity. Its growth is associated with vibrations of chelate bonds between the carboxyl groups of ketoadipic acids, or, more precisely, with vibrations of the =C--C=O . . . . . . HO--type bonds. The inflexions on the right part of this band are presumably due to the incorporation of a metal ion in the carboxyl group and to the formation of a linkage intermediate between single and double bonds in C O 0 - , which also points to the rise of an organomineral compound. The bands appearing near to 470 and 2360 cm -1 lend additional support to the hypothesis of incorporation of metal in the organic structure of coal. The 1600 cm -1 band is certainly also caused by aromatic --C=C - vibrations, y e t the effect of such vibrations on its intensity is insignificant. This is implied by the relationship between the optical densities of the 1600 and 1700 cm -1 bands, as well as by the comparison of this band for brown coal and bituminous coal in which aromatic skeleton vibrations alone are responsible for its development (Friedel, 1970, p. 146; Hacura et al., 1977, pp. 1--17). The aromatic skeleton vibration of the structural unit of coal lies in the range from 650 to 900 cm -~ . As doppleritization progresses, the nature of aromatic substitution changes, which testifies to the aromatic condensation of this unit. At the initial stages of doppleritization, single benzenetype rings are substituted in the positions 1, 3, 4. Intensely doppleritized coal varieties display absorption between 730 and 790 cm -1 , arising from rings substituted in the positions 1 and 2. Simultaneously a peak indicative of substitution disappears in the position 3. The inference that aromatic condensation takes place in the structural unit is also borne out by the 2830 cm -~ band caused by aromatic C--H vibrations. In the spectrum of c o m m o n xylite this band is barely distinguishable whereas in doppleritized coal it is pronounced. The sharpness of the 2830 cm -1 band is also due to the elimination of CH2 vibrations that occur close to 2870 cm -~ . The band at 1450 cm -1 has the same character. It is attributed to CH2 groups and disappears with the progressing doppleritization of xylites. The infrared spectrum of xylite coal contains a band that is absent in the spectrum of bituminous coal. It is a strong absorption centered close to 1440 cm -~ , arising from m e t h o x y l (OCH3) groups. In intensely doppleritized xylites this band disappears. The absorption appearing near to 2040 cm -~ and caused by hydroaromatic skeleton vibrations has a different character (Friedel, 1970, p. 146). It is absent in the spectrum of c o m m o n xylite and distinct in doppleritic coal. Hydroaromatic compounds are also responsible for the band
188 at 1000 cm -1 . These two bands testify to the presence of five-membered rings in the structural unit of coal, which may form by condensation of ketoadipic acids (van Krevelen and Schuyer, 1959, pp. 98--103). A strong and diffuse absorption band appearing close to 3400 cm -I is difficult to interpret. It generally testifies to the presence of hydroxyl (OH) groups, presumably of phenolic nature. QUANTITATIVE DETERMINATION OF SOME FUNCTIONAL GROUPS A qualitative description of IR spectra fails to provide all the information necessary to account for the structure of the c o m p o u n d s studied. Therefore, various approximate methods are used to calculate the c o n t e n t of certain groups in their structure. In this paper Oelert's m e t h o d (1970) was used to determine the percentage of hydrogen atoms in CH3, CH2 and aromatic CH compounds, and the content of carbon in CH2 and aromatic CH groups; Brown's m e t h o d (1955) to determine the quantitative ratio of vibrating aromatic to aliphatic bonds; and Bloom's m e t h o d (fide Roga and Wnekowska, 1966, pp. 206--207) to estimate the percentage of oxygen in COOH groups. Most hydrogen atoms occur in aliphatic compounds. Their amount is greatest in the CH3-type compounds (Table I) because in doppleritic coal hydrogen makes up a b o u t 67.1% of the total content of this element. The lowest content of hydrogen in CH3 groups has been noted in moderately doppleritized xylite, being only about half of that in doppleritic coal. The H c o n t e n t on CH2 groups is greatest in moderately doppleritized xylite. As the degree of doppleritization increases, hydrogen in these groups decreases from 64.6% down to 19% of the total H c o n t e n t in these coal varieties. From the data listed in Table I it appears that in the process of doppleritization of xylites, the number of CH3 groups decreases and then substantially increases while the reverse relation applies to CH2 groups. The percentage of hydrogen in aromatic groupings is n o t high, varying from 4 to 14% of the total content. At the initial stage of doppleritization it decreases, causing the opening of a part of the aromatic rings of lignin, and then increases nearly three-fold. This behaviour may be due to the aromatic condensation of the structural unit of coal. The total hydrogen content in the doppleritization series decreases by a b o u t 20% (Table I). The carbon content in CH2 and CH3 groups is subject to changes similar to those observed for hydrogen. Moderately doppleritized xylite has the lowest C c o n t e n t in CH3 groups (about 13%) while in doppleritic coal it is nearly twice as high. On the other hand, the carbon c o n t e n t in CH2 is highest in c o m m o n xylite and poorly doppleritized varieties, showing a nearly fourfold decrease in the direction of doppleritic coal. The carbon c o n t e n t in aromatic c o m p o u n d s is greatest in doppleritic
189 coal, running up to 40% at the least and 50% at the most (Table I). In xylites showing a lower degree of doppleritization it is considerably lower. The increasing degree of aromatization of coal is best reflected by the coefficient of aromatization f, which in c o m m o n xylite is 0 at the least and 0.38 at most, amounting respectively 0.55 and 0.65 in doppleritic coal. The ratio of vibrating aromatic to aliphatic bonds is highest in moderately doppleritized cylite. It is also high in doppleritic coal but low in the other xylites. This suggests that the medium degree of doppleritization involves the condensation of aromatic rings into one system which breaks up again and is subject to recondensation at the subsequent stages of this process. Such systems would be likely to form if one of the condensed rings were a quinone ring that would then be subject to opening due to the formation of heteroaromatic bonds and then ketoadipic acids and organomineral c o m p o u n d s (Fig. 4, stages II--IV). The total carbon c o n t e n t in the doppleritization series, recalculated to the analytical state, increases by a b o u t 6%. The c o n t e n t of oxygen in COOH is a measure of the amount of these groups in the xylite varieties under study. Their a m o u n t is greatest in intensely doppleritized xylite and less in doppleritic coal. The simultaneous increase in the content of metallic elements in ash {Table II) shows explicitly that doppleritization involves the replacement of carboxyl by an organomineral system, possibly according to the reaction: I
I
I
I
=C--C=O ' ' ' H O - ~ =C--C=O . . . M e - - O - . Infrared absorption spectra yielded by xylites from which bitumens have been extracted (with the Soxhlet method) are nearly identical with the spectra presented above. A slight difference has only been noted in the intensity of bands corresponding to certain aliphatic groupings. RAMAN ROTARY SPECTRA The xylites under study show marked differences in the shape of Raman spectra (Fig. 3). Poorly doppleritized xylite does not display any vibration. In xylites showing a medium degree of doppleritization (L-2, L-3) the 1600 cm I band increases in intensity and a band begins to develop close to 1380 cm -1 . In doppleritic coal these bands are pronounced, and the shape of spectral lines is similar to that of low-rank bituminous coal (type 31 according to Polish standards or type 800 according to the international classification). The occurrence of bands in Raman spectra of intensely doppleritized xylites testifies to the formation of locally ordered graphite-like domains, because the 1600 and 1380 cm -' bands are associated with vibrations of E2g and A~ classes of the crystal symmetry group D~h of graphite (Tsu et al., 1977). The average size L of these areas is probably a b o u t 5 nm, similar to that in sub-bituminous coal and somewhat less than in bitumi-
190
L-3
20
1B
16
14
12
10
B
F,xlO0 cm1
Fig. 3. Raman rotary spectra of xylites.
L-1
=
poorly doppleritized xylite;
L-2,
L-3
=
moderately doppleritized xylites; L - 4 -- intensely doppleritized xylite; L - 5 = doppleritic coal; B C = low-rank bituminous coal (type 31 according to Polish standards or type 800 according to international classification). nous coal (Zerda et al., 1981}. The structurally ordered areas are to be located in the core of the statistical structural unit of coal. From the above considerations it seems feasible that such a unit, presumably made up of one or two aromatic rings, is formed in the process of doppleritization of xylites. CHEMICAL COMPOSITION OF XYLITE ASH The ash c o n t e n t recalculated to the analytical state increases from 1.3 wt.% in c o m m o n xylite up to 11.2 wt.% in doppleritic coal (Table II). It is internal ash, structurally connected with the organic matter of coal. The contents of oxides and sulphur determined in the ash are differentiated. The c o m m o n xylite ash has the highest c o n t e n t of CaO, A1203 and K20. Most oxides, except MgO and Fe203, decrease in the ash of moderately doppleritized xylite. The ash of intensely doppleritized xylite has the lowest c o n t e n t of such oxides as MgO, Fe203, Na20 and K:O, whereas in the doppleritic coal ash nearly all oxides and sulphur increase.
191 TABLE II Chemical analysis of xylite ash and oxygen content in COOH grouping (in percentage) Sample
Aa
CaO
MgO
A1203 Fe203 N a 2 0
L-0 L-2 L-4 L-4
1.38 4.11 10.89 11.14
9.15 3.19 3.84 4.86
2 . 5 2 10.99 3.02 6.54 0.76 6.86 1.26 5.72
10.06 11.13 5.65 5.66
2.77 2.40 2.19 3.38
K:O
OcOOH
Sash
0.20 0.13 0.06 0.08
3.35 4.96 5.29 4.52
0.62 0.97 2.14 3.11
The increased contents of oxides in c o m m o n xylite and doppleritic coal are associated with the chemical mechanism of transformation of lignin, and with the final stage of the process referred to as doppleritiza+ tion. The transformation of lignin to soluble compounds involves its reaction with alkalis in a weak basic medium, and thence the fairly high c o n t e n t of CaO, Na~O, K:O, as well as A1203 and Fe203, in c o m m o n xylite ash. The resulting salts, called alkalilignin, are n o t stable and readily change into acids and other products (Prosiflski, 1969, p. 334). The second stage of formation or organomineral compounds is associated with the last stage of doppleritization. The reaction of humic acids with inorganic bases or dissociated salts gives rise to dopplerite, i.e. CaO, MgO and Na20 humic compounds. The increased contents of oxides of these metals compared with their a m o u n t in the ash of intensely doppleritized xylite lend support to this statement. Taking into consideration Dragunov's formula for humic acid, given by Roga and T o m k o w (1971), it can be stated t h a t the replacement of carboxyl groups of this acid by C O M e " groups (Me = 1/2 Ca) results in a 4.5% Ca c o n t e n t in the molecular mass of a humic compound. In the case of Na, it will be considerably lower. The occurrence of Ca, Mg and Na humic compounds in brown coal has been reported by Kuhl (1960), Kruszewski (1968), and other authors. GENESIS OF DOPPLERITIC COAL The doppleritization of fossil wood is a chemical process that occurs in the environment of a peat-bog or a brown coal deposit (Welte, 1952). This process operates in wood that has n o t y e t undergone p r o f o u n d de-struction during its decay, which is evidenced by the substantial c o n t e n t of lignin. The environment in question shows a weak alkaline reaction. Under continental conditions, an environment of this kind is provided by a low peat-bog under a thin layer of hard water. The effect of OH- ion on lignin involves the disruption of etheric bonds between its radicals and d e m e t h o x y l a t i o n , while the formation of free phenol groups may be responsible for the recondensation of radicals. Furthermore, the alkaline medium causes changes in the side chains
192
of lignin, and particularly the enolization of ketone systems (Fig. 4) as a result of the formation of soluble salts of alkaline metals. The further evolution of these salts involves the formation of quinones and reactive ketoadipic acids due to the opening of a part of rings. It seems feasible that the mixture of the decomposition products of cellulose and lignin in a weakly oxidizing environment gives rise to ketoadipic acids, c o m m o n l y referred to as humic acids. The presence of these c o m p o u n d s is responsible for the strong acidity of the environment, typical of peat-bogs and brown coal deposits.
CH~
I
~H2 C-O i~
OH
HO',CHz /
H-CI - - 0 H-C-OH
~'OCH~ 0I
CH~ H-C
CH~
H-C
_OCHs
II
.,~C-ON°
* 2 NI:OH I,, ~
--CH:. H#
~
II
Jr"
V'nr~v~" OH
&-ONo
A
IIIQ
lllb
ICH~ CH II OH
H,CO~ x . ~
xl~,o~o,
~
014
H-C
R~
CH
OH
R'
HBCO.~
IL~O ~ l 0
OH
H ~ C O ~ f ~-H
l` II j''HZ-'7~'U'C
CZOH
R
R
CH~
0H
V
IV
""°2'2"°'2:
OH
Ro
temp ,c*neH(O3 H2CO
/C,.~
CIH
%1,-c
~OH
O-,Me.O
Fig. 4. The hypothetical course of doppleritization of lignin radicals. I = the stage of dissolution of lignin (the formation of alkalilignin); H = the stage of formation of quinone; I I I = the stage of formation of ketoadipic acid; I V = the stage of formation of dopplerite (organomineral compound). V = the stage of formation of vitrain in subbituminous coal.
The formation of dopplerite is p r o m o t e d by brackish sea waters or alkaline fresh waters. This process involves the bonding of cations of some metals to humic acids (~-ketoadipic acids). In reactions of this type a water molecule is presumably expelled. This causes the condensation of organomineral sol, which may then be subject to in situ diagenesis, or may be displaced to a different geochemical environment and undergo coagulation (Breger, 1955, p. 132). During diagenesis the organomineral, presumably lyophobic, sol coagulates. The coagulation m a y be caused by dehydration due to an increasing pressure of the overburden layer and the resulting increase in temperature, or by a change in the electric charge in response to structural changes, e.g. of clay minerals, in the changing environmental conditions (Wagner, 1980). The coagulation is accompanied by the process of ageing of col-
193
loids, as well as by recrystallization and chemical changes. One of the changes induced by elevated temperature is the decomposition of organomineral c o m p o u n d s (Fig. 4). Hence the vitrain of sub-bituminous and bituminous coal does n o t contain the same a m o u n t of ash combined organically with coal as dopplerite, although there are exceptions to this rule. In conclusion, it can be stated that the doppleritization and humification of w o o d in a coal-forming environment ultimately give rise to chemically different products, even though the physical nature of these processes is similar as they involve colloidal transformation. When the coagulation of organomineral sol is n o t induced by the increasing temperature, the dopplerite retains its chemical character, which fact has been noted in some peat beds and brown coal deposits. ACKNOWLEDGEMENTS
The author wishes to express his thanks to Dr W. Carius from the Pedagogical University of Erfurt /GDR / for taking Raman spectra, to Dr T. Zerda from the Silesian University for taking IR spectra, and to Dr K. Matl from the University of Mining and Metallurgy Cracow for the critical reading of the manuscript and valuable discussion.
REFERENCES Brown, J.K., 1955. The infrared spectra of coals. J. Chem. Soc., 744. Breger, I.A., 1955. Association of uranium with a naturally occurring coal extract. Bull. Geol. Soc. Am., 66 (12): 130--133. Brzyski, B. and Majewski, S., 1974. Les xylites doppl~ritis~s du gisement de charbon brun de la carri~re de Patn6w du Bassin de Konin. Zesz. Nauk. Akad. Gorn.-Hutn., Cracow Geol., 19: 7--26. Czuchajowski, L and Sliwiok, J., 1974. Spektroskopowe metody badafi zwiazkSw organicznych NMR, IR, UV. Uniw. Slaski 59, Katowice, 138 pp. Frankiewicz, J., 1975. Structural-geological characteristic of the "Glogowski" browncoal bed in the region Lubin-GlogSw. Zesz. Nauk. Akad. Gorn.-Hutn., Cracow Geol., 24: 35--44. Friedel, R.A., 1970. Spectrometry of Fuels. Plenum Press, New York, N.Y., 250 pp. Hacura, A., Frackowiak, M. and Zerda, T., 1977. Spektroskopia absorpcyjna w podczerwieni wegli humusowych i sapropelowych. Arch. Uniw. Slask., GIG, Katowice, 21 pp. Jacob, H., 1958. Dopplerit. Seine Petrographie und Genesis. Geologie, 7 (1): 61--75. Kruszewski, T., 1968. Warunki geologiczne i budowa wegla brunatnego z rejonu Konina. Prace GIG, Ser Dodatkowa, 180 pp. Kuhl, J., 1960. Chemiczno-mineralna budowa nieorganicznych substancji mineralnych w weglu b r u n a t n y m Konina. Kwart. Geol., 4 (II): 32--70. Oelert, H.H., 1970. Chemical composition of mierinites and semifusinites. Fuel, 49: 119 -125. Potoni~, R. and Stockfisch, K., 1932. Uber Oxyhumodile-Kohlenvariet~ten der Oxydationszone yon Wiechbraunkohlenfl6zen. Mitt. Labor. Preuss. Geol. Landesanstalt, H. 16, pp. 75--78.
194 Prosi~ski, S., 1969. Chemia drewna. PWRiL, Warszawa, 488 pp. Ramler, E. and Jacob, H., 1951. Uber die Brikettierreigensehaft von Dopplerit. Freiberg. Forschungsh. A-3. Roga B. and Tomkow, K., 1971. Chemiczna teehnologia wegla. WNT, Warszawa, 586 pp. Roga, B. and Wnekowska, L., 1966. Analiza wegla i koksu. PWNT, Warszawa, 250 pp. Roselt, G., 1969. Zum Problem der Vergelung der Kohlne. Freiberger Forschungsh. C, 242: 13--28. Roselt, G., 1976. Methode und Anleiting zur weitgehend objektiven Ermittlung der Variet~ten humifizierter Xylite unserer Weichbraunkohlen. Wiss. Geol. yon ErdS1, Erdgas und Kohlen, Freiberg, p. 34. Stach, E. (Editor), 1978. Petrologia uglej. Izd. "Mir", Moscow, 554 pp. Suss, M., 1959. Zur Petrographie des Xylits. Freiberg. Forsehungh. A, 148: 14--33. Tsu, R., Gonzales, J.H., Hernandez, I.C. and Luengo, C.A., 1977. Raman spectra of coal. Solid. State Commun., 24: 809. Van Krevelen, D.W. and Schuyer, J., 1959. Wegiel. Chemia wegla i jego struktura. PWN, Warszawa, 310 pp. Wagner, M., 1980. Coal matter in the flysch of the Magura nappe between JordanSw and Nowy Sacz (Carpathians, Poland). Annal. Soc. Geol. Pol., 50: 99--117. Welte, E., 1952. Uber die entstehung von Humis~iuren und Wege ihrer Reindarstellung. Z. Pflannzenernahn., Dfing., Bodenkd., 56: 105--139. Zerda, T., John, A. and Chmura, K., 1981. Raman studies of coals. Fuel, 60: 375--379.
181
DOPPLERITIZATION OF XYLITIC COAL IN THE LIGHT OF PETROGRAPHIC AND CHEMICAL INVESTIGATIONS
MARIAN WAGNER Institute of Fuel Materials, University of Mining and Metallurgy in Cracow, al. Mickiewicza 30, 30-059 KrakOw (Poland) (Received July 17, 1981; revised and accepted February 26, 1982)
ABSTRACT Wagner, M., 1982. Doppleritization of xylitic coal in the light of petrographic and chemical investigations. Int. J. Coal Geol., 2:181--194. The paper presents the results of petrographic and chemical investigations of xylitic coal, xylite and their ash. As is known, a xylitic coal has the function of a lithotype whereas xylites are inclusions brown coals only. All these xylites were arranged according to the increasing degree of doppleritization of xylem in the sequence: common xylite, poorly, moderately and intensely doppleritized varieties, dopplerite coal. The distinctive criteria were their differentiated physical properties, which are also reflected in the variable petrographic and chemical compositions. The petrographic differences result from the replacement of textinite by ulminite and gelinite, while the differentiation of the chemical constitution is due to the increasing carbon content and the increasing number of functional groups that determine the aromatic nature of the internal structure of the coal. Infrared absortion spectroscopy, Raman spectroscopy and chemical analysis of ash have shown that the aromatization of coal and the resulting increase in the degree of order of the structural unit are due to the formation of organomineral compounds in the course of doppleritization. It appears from the investigations that the doppleritization of vegetable matter gives rise to gels that are a mixture of organomineral humic compounds. The process of humification leads to the formation of gel made up of humins. It follows then that there are two ways of initial coalification of plant material, both referred to as biochemical gelification.
INTRODUCTION X y l i t i c c o a l is t h e f o s s i l w o o d o f T e r t i a r y c o n i f e r s . I t o c c u r s in b r o w n c o a l s e a m s in t h e f o r m o f p r e s s e d s t e m s l y i n g h o r i z o n t a l l y , u p r i g h t t r u n k s , s t u b s a n d o t h e r t r e e f r a g m e n t s o f d i f f e r e n t sizes. X y l i t i c c o a l is c h a r a c t e r i z e d b y d i v e r s e p h y s i c a l f e a t u r e s in t h a t i t m a y r e s e m b l e t h e p r e s e n t day wood, show longitudinal or both longitudinal and transversal splitting, o r i t m a y o c c u r in t h e f o r m o f gel t h a t r e s e m b l e s a w o o d f r a g m e n t o n l y in s h a p e . The transformation of soft brown coal to hard brown coal varieties
0166-5162[82/0000--0000[$02.75 © 1982 Elsevier Scientific Publishing Company
182 involves intense gelification of fossil wood. At the bright sub-bituminous coal stage it is n o t possible to distinguish the xylite varieties. The gelification of fossil w o o d can be considered as physical, chemical and biochemical changes of lignin and hydrocarbons. It is generally held that it is the first stage of transformation (decomposition) of plant matter, which results in the formation of humic or organomineral gel. At the subsequent stages of coalification this gel is converted into vitrain. The transformation of plant material at the peat and brown coal stages is referred to as humification or biochemical gelification (Stach et al., 1978, pp. 247--250). It is the first stage of enrichment of the decomposing plant matter in carbon, involving moderate oxidation and the formation of humic acids which in the subsequent stages of coalification are converted into humins, i.e. c o m p o u n d s of low chemical activity. In an aqueous medium, humic acids can react with inorganic bases or salts. The products of these reactions are organomineral c o m p o u n d s called dopplerite. The complex of physical and chemical p h e n o m e n a leading to the formation of such c o m p o u n d s will be referred to in this paper as doppleritization. There are very few papers published to-date that deal with the process of doppleritization. More attention has been given to dopplerite originating from peat and brown coal deposits. Dopplerite has been defined as a Ca humic c o m p o u n d or complex humic c o m p o u n d of Ca, Fe, A1, K and Na (Potoni~ and Stockfish, 1932; Rammler and Jacob, 1951). It appears that the biochemical gelification of plant matter in a coal sedimentation environment proceeds in two different ways. One leads to the formation of humins, i.e. purely organic compounds, whilst the other gives rise to dopplerite. Therefore the term "humification" is not s y n o n y m o u s with "biochemical gelification", as it defines only one of the possible ways of transformation of humic material. This paper aims to determine the geochemical conditions of doppleritization of xylitic coal. Petrographic and chemical investigations were carried o u t on xylitic coal samples showing varying degrees of doppleritization, derived from the same coal seam. MATERIAL AND METHODS The macroscopic features alone do n o t allow to distinguish xylites that have been subject to humification from those that have undergone doppleritization. It is, however, relatively easy to distinguish one variety from the other if the c o n t e n t of internal ash, i.e. that combined with the organic matter of coal, is determined, b u t such a procedure requires a strict control of the purity of the investigated material in thin sections. As appears from the author's experiments the intensely doppleritized xylites have an ash c o n t e n t of more than 10 wt.%, while the xylites that have undergone humification contain only a b o u t 6 wt.% of ash.
183
The present investigations were carried out on doppleritized xylites derived from the so-called Glogbw bed (Upper Oligocene) in the area of GlogSw (Frankiewicz, 1975). The experimental material was represented by samples of c o m m o n cylite (L-O), poorly (L-l), moderately (L-2, L-3) and intensely doppleritized (L-4) xylites, and doppleritic coal (L-5). The internal ash content in this series of xylites varied from 1.2 to 11.1 wt.% (Table I). The detailed investigations comprised: (a) Macro- and microscopic petrographic observations in polarized transmitted light, carried out with a Zeiss "Laboval 2 pol" microscope (b) Chemical analysis of C and H contents (by Sheffield's method), the O content in the carboxyl groups of coal, and the determination of CaO, MgO, A1203, Fe203, Na20 and K20 contents in the xylite ash obtained at 450°C
TABLE I Q u a n t i t a t i v e analysis of I R a b s o r p t i o n spectra of x y l i t i c coal
Sample
L-0
L-1
L-2
L-3
L4
L-5
Aa
1.3
1.1
4.1
10.8
10.9
11.1
Ca
47.2
49.0
53.7
53.7
53.4
55.1
Ha
5.9
5.8
5.5
5.0
4.7
4.6
C/H
8.0
8.4
9.7
10.8
11.7
11.9
HCH3
2.48
2.05
1.74
2.46
2.45
3.27
HC~
3.19
3.48
3.58
2.14
1.83
0.93
Har
CCH3
rain CC~
rain Car
fmin
max2 CCH
C max
fmax
40.52
--7.94
--0.02
81.04
32.58
0.38
42.63
--9.15
--0.02
85.26
33.49
0.41
40.00
2.15
0.07
80.00
42.15
0.47
0.23 21.01
nat
0.07
0.29 16.72
0.06
0.22 12.96
0.21 23.83
25.14
0.34
47.73
49.000
0.58
21.79
26.25
0.37
43.58
48.04
0.59
10.41
39.74
0.55
20.82
50.15
0.65
0.39 18.32
0.07
0.45 18.34
0.67
nar
0.06
23.74
0.11
A a = ash content calculated in the analytical state; C a and H a ffiC and H contents calculated in the analytical state; C / H = atomic ratio of C to HCH~ ; C c ~ = percentage of C and H in CH~-type groups relative to their content in coal; Car ffipercentage o f C in aromatic bonds relativeto their total content in coal; fmax, fmln = m a x i m u m and m i n i m u m coefficient of aromatization; nar/nal ffiquantitative ratio of vibrating aromatic to aliphatic bonds systems.
184 (c) Infrared absorption spectroscopy in the basic infrared region, using a UNICAM SP 1200 grid spectrometer (d} Raman spectroscopy carried o u t with a Zeiss GDM-1000 spectrometer equipped with a m o n o c h r o m a t o r and an ILA-120(2W)Ar laser. The laser beam p o w e r was 40 and 70 mW, depending on the kind of xylite, at a wavelength of 514.5 nm. The resolution of the Raman spectrometer with m o n o c h r o m a t o r was a b o u t 10 cm -1 . Coal samples for spectroscopic investigations were prepared with the u t m o s t care. Each sample was dried at 105°C in a nitrogen atmosphere for 2 hours. Then the samples were ground in a Fritsch planetary mill for 16 hours in the damp-free atmosphere, and prepared in KBr discs in proportions by weight of 1:300 coal to potassium bromide by pressing at a pressure of 10 MPa. PETROGRAPHIC FEATURES OF DOPPLERITIZED XYLITES The change from c o m m o n xylite through doppleritized xylites to doppleritic coal is progressive. It manifests itself in the gradual disappearance of the macroscopic features of wood, the change of colour from brown to black, and the change in lustre from dull to vitreous. In contrast to c o m m o n xylites, the doppleritized varieties do n o t show fibrous parting or elasticity. The latter have a conchoidal fracture and exhibit great friability due to the dense n e t w o r k of endogenous fissures. These features are particularly conspicuous in partly dried coal, being less pronounced in freshly exposed seains. The degree of doppleritization of xylites is also clearly visible under the microscope. On the basis of microscopic features, three degrees of doppleritization can be distinguished, corresponding to the macroscopic division (Jacob, 1958; Suss 1959; Brzyski and Majewski, 1974). C o m m o n and poorly doppleritized xylites are made up of textinite. It has been noticed that in the doppleritized varieties the zones of spring growth of xylem are generally disturbed, showing locally the features peculiar to texto-ulminite. The textinite is partly impregnated with corpohuminite. Xylites showing a medium degree of doppleritization are made up of textinite and texto-ulminite. The structure of spring growth zones of xylem is frequently obliterated due to the crushing, tearing and random displacem e n t of the cell walls. The cell walls of the summer growth of xylem are well preserved. The cell lumina are filled with gelinite. Intensely doppleritized xylites are made up almost entirely of eu-ulminite. The tracheids are poorly marked off, being e m b e d d e d in the granular gel mass. Doppleritic coal has a similar structure. In the gel ground mass, made up of uliminite, fragments of corroded tracheids are visible only locally. From the above observations it is evident that intense doppleritization
185
of xylitic coal involves not only the impregnation of xylem with gel but also the chemical destruction of the cell walls (Roselt, 1969, 1976). These processes result in the progressive replacement of textinite by ulminite and eventually by gelinite (levi-gelinite). The humic material owing its origin to the destruction of wood tissue partly or completely fills the fissures. From the above considerations it follows that the doppleritization of xylitic coal is a process involving several stages. INFRARED ABSORPTION SPECTRA
IR spectra of the investigated xylite varieties are differentiated, the differences consisting in the disappearance or development of absorption bands at the specified frequencies (Fig. 1). The IR spectrum of c o m m o n xylite has been found to be similar to that of lignin, while the absorption displayed by doppleritic coal has several features in c o m m o n with the spectrum of sodium acetate (CH3COONa), an organomineral c o m p o u n d used for spectroscopic analysis.
L-O L4 L~2
L-3
L-4
[_-5
CH,COONa 40
35
30
25
20
15
12
10
B
4X100 cn~1
Fig. 1. I n f r a r e d absorption spectra of xylites. L-O = c o m m o n xylites; L-1 = p o o r l y d o p p leritized xylite; L-2, L - 3 = m o d e r a t e l y d o p p l e r i t i z e d xylites; L - 4 = i n t e n s e l y d o p p l e r i t i z e d xylite; L - 5 = d o p p l e r i t i c coal.
186 At present much attention is given to the absorption displayed by coal at 1500--1650 cm -1 . The bands produced by xylitic coal varieties in this range are distinct, although they are characterized by the variable morphology of lines. The spectrum of c o m m o n xylite shows an intense and sharp peak at 1510 cm -1 , and a broad but less intense band at 1610 cm -1 . Its left part shows a weak inflexion at 1700 cm-' (Fig. 1). In xylites showing a higher degree of doppleritization the 1500 cm-' band disappears, but simultaneously the 1600 cm-' band increases in intensity and becomes markedly broadened. If we assume that the 1600 cm -' absorption band reflects solely the aromatic structure of coal, then the dependence between the coefficient of aromatization f and the optical density of this band should be linear. The appropriate calculations have shown (Table I), however, that this assumption is incorrect (Fig. 2). It appears therefore that the growth of the 1600 cm -1 band is associated n o t only with aromatic --C=C - vibrations, but also with vibrations of functional groups containing oxygen. The relationship between the optical densities of the 1600 and 1700 cm -1 bands (Fig. 2) indicates that the intensity of their vibrations depends on the bonds containing oxygen.
A D17 0 i
~
0,4~ ~ . o~- . . . ~ "
0,7 o,o1
0,2.
0,5 ,
i
0,1 }
0,0
0,4 +
0,5
B
frnax ar I
~
1,0
D160~
i i I
I --------:~.
~ . I .
/
Q2
0,4
{3,6
O,B
1,0
D1600
Fig. 2. A. Relationship between the optical densities of the 1700 and 1600 cm -1 absorption bands. B. Relationship between the m a x i m u m coefficient of aromatization and the optical density o f the 1600 cm -1 band.
The 1700 cm -1 band in the spectrum of lignin is to be attributed to the C=O groups of ketone radical. In c o m m o n xylite it is displaced to a frequency characteristic of quinones. In the spectra of doppleritized xylites this band is caused by C=O stretching vibrations of aromatic acids. This is evidenced by sharp peaks at 1220 (vc_o) and 880 cm -1 (VoH) that disappear, however, in doppleritic coal. The absorption close to 1700 cm-' displayed by this coal may be due to the carbonyl group of ketones. The 1600 cm-' band produced by c o m m o n xylite and poorly or moderately doppleritized varieties is accompanied by absorption with peaks at 1500 and 1260 cm -1 , which markedly decrease in intensity, and have
187 been attributed to skeleton vibrations of heteroaromatic rings (Czuchajowski and Sliwiok, 1974, p. 68). The intensity of these bands indicates that in the process of doppleritization of xylites, the aromatic bonds of lignin are substituted by heteroaromatic linkages. Xylites showing a high degree of doppleritization display an absorption centered close to 1630 cm -1 , which in doppleritic coal exceeds the vibrations of heteroaromatic systems in intensity. Its growth is associated with vibrations of chelate bonds between the carboxyl groups of ketoadipic acids, or, more precisely, with vibrations of the =C--C=O . . . . . . HO--type bonds. The inflexions on the right part of this band are presumably due to the incorporation of a metal ion in the carboxyl group and to the formation of a linkage intermediate between single and double bonds in C O 0 - , which also points to the rise of an organomineral compound. The bands appearing near to 470 and 2360 cm -1 lend additional support to the hypothesis of incorporation of metal in the organic structure of coal. The 1600 cm -1 band is certainly also caused by aromatic --C=C - vibrations, y e t the effect of such vibrations on its intensity is insignificant. This is implied by the relationship between the optical densities of the 1600 and 1700 cm -1 bands, as well as by the comparison of this band for brown coal and bituminous coal in which aromatic skeleton vibrations alone are responsible for its development (Friedel, 1970, p. 146; Hacura et al., 1977, pp. 1--17). The aromatic skeleton vibration of the structural unit of coal lies in the range from 650 to 900 cm -~ . As doppleritization progresses, the nature of aromatic substitution changes, which testifies to the aromatic condensation of this unit. At the initial stages of doppleritization, single benzenetype rings are substituted in the positions 1, 3, 4. Intensely doppleritized coal varieties display absorption between 730 and 790 cm -1 , arising from rings substituted in the positions 1 and 2. Simultaneously a peak indicative of substitution disappears in the position 3. The inference that aromatic condensation takes place in the structural unit is also borne out by the 2830 cm -~ band caused by aromatic C--H vibrations. In the spectrum of c o m m o n xylite this band is barely distinguishable whereas in doppleritized coal it is pronounced. The sharpness of the 2830 cm -1 band is also due to the elimination of CH2 vibrations that occur close to 2870 cm -~ . The band at 1450 cm -1 has the same character. It is attributed to CH2 groups and disappears with the progressing doppleritization of xylites. The infrared spectrum of xylite coal contains a band that is absent in the spectrum of bituminous coal. It is a strong absorption centered close to 1440 cm -~ , arising from m e t h o x y l (OCH3) groups. In intensely doppleritized xylites this band disappears. The absorption appearing near to 2040 cm -~ and caused by hydroaromatic skeleton vibrations has a different character (Friedel, 1970, p. 146). It is absent in the spectrum of c o m m o n xylite and distinct in doppleritic coal. Hydroaromatic compounds are also responsible for the band
188 at 1000 cm -1 . These two bands testify to the presence of five-membered rings in the structural unit of coal, which may form by condensation of ketoadipic acids (van Krevelen and Schuyer, 1959, pp. 98--103). A strong and diffuse absorption band appearing close to 3400 cm -I is difficult to interpret. It generally testifies to the presence of hydroxyl (OH) groups, presumably of phenolic nature. QUANTITATIVE DETERMINATION OF SOME FUNCTIONAL GROUPS A qualitative description of IR spectra fails to provide all the information necessary to account for the structure of the c o m p o u n d s studied. Therefore, various approximate methods are used to calculate the c o n t e n t of certain groups in their structure. In this paper Oelert's m e t h o d (1970) was used to determine the percentage of hydrogen atoms in CH3, CH2 and aromatic CH compounds, and the content of carbon in CH2 and aromatic CH groups; Brown's m e t h o d (1955) to determine the quantitative ratio of vibrating aromatic to aliphatic bonds; and Bloom's m e t h o d (fide Roga and Wnekowska, 1966, pp. 206--207) to estimate the percentage of oxygen in COOH groups. Most hydrogen atoms occur in aliphatic compounds. Their amount is greatest in the CH3-type compounds (Table I) because in doppleritic coal hydrogen makes up a b o u t 67.1% of the total content of this element. The lowest content of hydrogen in CH3 groups has been noted in moderately doppleritized xylite, being only about half of that in doppleritic coal. The H c o n t e n t on CH2 groups is greatest in moderately doppleritized xylite. As the degree of doppleritization increases, hydrogen in these groups decreases from 64.6% down to 19% of the total H c o n t e n t in these coal varieties. From the data listed in Table I it appears that in the process of doppleritization of xylites, the number of CH3 groups decreases and then substantially increases while the reverse relation applies to CH2 groups. The percentage of hydrogen in aromatic groupings is n o t high, varying from 4 to 14% of the total content. At the initial stage of doppleritization it decreases, causing the opening of a part of the aromatic rings of lignin, and then increases nearly three-fold. This behaviour may be due to the aromatic condensation of the structural unit of coal. The total hydrogen content in the doppleritization series decreases by a b o u t 20% (Table I). The carbon content in CH2 and CH3 groups is subject to changes similar to those observed for hydrogen. Moderately doppleritized xylite has the lowest C c o n t e n t in CH3 groups (about 13%) while in doppleritic coal it is nearly twice as high. On the other hand, the carbon c o n t e n t in CH2 is highest in c o m m o n xylite and poorly doppleritized varieties, showing a nearly fourfold decrease in the direction of doppleritic coal. The carbon c o n t e n t in aromatic c o m p o u n d s is greatest in doppleritic
189 coal, running up to 40% at the least and 50% at the most (Table I). In xylites showing a lower degree of doppleritization it is considerably lower. The increasing degree of aromatization of coal is best reflected by the coefficient of aromatization f, which in c o m m o n xylite is 0 at the least and 0.38 at most, amounting respectively 0.55 and 0.65 in doppleritic coal. The ratio of vibrating aromatic to aliphatic bonds is highest in moderately doppleritized cylite. It is also high in doppleritic coal but low in the other xylites. This suggests that the medium degree of doppleritization involves the condensation of aromatic rings into one system which breaks up again and is subject to recondensation at the subsequent stages of this process. Such systems would be likely to form if one of the condensed rings were a quinone ring that would then be subject to opening due to the formation of heteroaromatic bonds and then ketoadipic acids and organomineral c o m p o u n d s (Fig. 4, stages II--IV). The total carbon c o n t e n t in the doppleritization series, recalculated to the analytical state, increases by a b o u t 6%. The c o n t e n t of oxygen in COOH is a measure of the amount of these groups in the xylite varieties under study. Their a m o u n t is greatest in intensely doppleritized xylite and less in doppleritic coal. The simultaneous increase in the content of metallic elements in ash {Table II) shows explicitly that doppleritization involves the replacement of carboxyl by an organomineral system, possibly according to the reaction: I
I
I
I
=C--C=O ' ' ' H O - ~ =C--C=O . . . M e - - O - . Infrared absorption spectra yielded by xylites from which bitumens have been extracted (with the Soxhlet method) are nearly identical with the spectra presented above. A slight difference has only been noted in the intensity of bands corresponding to certain aliphatic groupings. RAMAN ROTARY SPECTRA The xylites under study show marked differences in the shape of Raman spectra (Fig. 3). Poorly doppleritized xylite does not display any vibration. In xylites showing a medium degree of doppleritization (L-2, L-3) the 1600 cm I band increases in intensity and a band begins to develop close to 1380 cm -1 . In doppleritic coal these bands are pronounced, and the shape of spectral lines is similar to that of low-rank bituminous coal (type 31 according to Polish standards or type 800 according to the international classification). The occurrence of bands in Raman spectra of intensely doppleritized xylites testifies to the formation of locally ordered graphite-like domains, because the 1600 and 1380 cm -' bands are associated with vibrations of E2g and A~ classes of the crystal symmetry group D~h of graphite (Tsu et al., 1977). The average size L of these areas is probably a b o u t 5 nm, similar to that in sub-bituminous coal and somewhat less than in bitumi-
190
L-3
20
1B
16
14
12
10
B
F,xlO0 cm1
Fig. 3. Raman rotary spectra of xylites.
L-1
=
poorly doppleritized xylite;
L-2,
L-3
=
moderately doppleritized xylites; L - 4 -- intensely doppleritized xylite; L - 5 = doppleritic coal; B C = low-rank bituminous coal (type 31 according to Polish standards or type 800 according to international classification). nous coal (Zerda et al., 1981}. The structurally ordered areas are to be located in the core of the statistical structural unit of coal. From the above considerations it seems feasible that such a unit, presumably made up of one or two aromatic rings, is formed in the process of doppleritization of xylites. CHEMICAL COMPOSITION OF XYLITE ASH The ash c o n t e n t recalculated to the analytical state increases from 1.3 wt.% in c o m m o n xylite up to 11.2 wt.% in doppleritic coal (Table II). It is internal ash, structurally connected with the organic matter of coal. The contents of oxides and sulphur determined in the ash are differentiated. The c o m m o n xylite ash has the highest c o n t e n t of CaO, A1203 and K20. Most oxides, except MgO and Fe203, decrease in the ash of moderately doppleritized xylite. The ash of intensely doppleritized xylite has the lowest c o n t e n t of such oxides as MgO, Fe203, Na20 and K:O, whereas in the doppleritic coal ash nearly all oxides and sulphur increase.
191 TABLE II Chemical analysis of xylite ash and oxygen content in COOH grouping (in percentage) Sample
Aa
CaO
MgO
A1203 Fe203 N a 2 0
L-0 L-2 L-4 L-4
1.38 4.11 10.89 11.14
9.15 3.19 3.84 4.86
2 . 5 2 10.99 3.02 6.54 0.76 6.86 1.26 5.72
10.06 11.13 5.65 5.66
2.77 2.40 2.19 3.38
K:O
OcOOH
Sash
0.20 0.13 0.06 0.08
3.35 4.96 5.29 4.52
0.62 0.97 2.14 3.11
The increased contents of oxides in c o m m o n xylite and doppleritic coal are associated with the chemical mechanism of transformation of lignin, and with the final stage of the process referred to as doppleritiza+ tion. The transformation of lignin to soluble compounds involves its reaction with alkalis in a weak basic medium, and thence the fairly high c o n t e n t of CaO, Na~O, K:O, as well as A1203 and Fe203, in c o m m o n xylite ash. The resulting salts, called alkalilignin, are n o t stable and readily change into acids and other products (Prosiflski, 1969, p. 334). The second stage of formation or organomineral compounds is associated with the last stage of doppleritization. The reaction of humic acids with inorganic bases or dissociated salts gives rise to dopplerite, i.e. CaO, MgO and Na20 humic compounds. The increased contents of oxides of these metals compared with their a m o u n t in the ash of intensely doppleritized xylite lend support to this statement. Taking into consideration Dragunov's formula for humic acid, given by Roga and T o m k o w (1971), it can be stated t h a t the replacement of carboxyl groups of this acid by C O M e " groups (Me = 1/2 Ca) results in a 4.5% Ca c o n t e n t in the molecular mass of a humic compound. In the case of Na, it will be considerably lower. The occurrence of Ca, Mg and Na humic compounds in brown coal has been reported by Kuhl (1960), Kruszewski (1968), and other authors. GENESIS OF DOPPLERITIC COAL The doppleritization of fossil wood is a chemical process that occurs in the environment of a peat-bog or a brown coal deposit (Welte, 1952). This process operates in wood that has n o t y e t undergone p r o f o u n d de-struction during its decay, which is evidenced by the substantial c o n t e n t of lignin. The environment in question shows a weak alkaline reaction. Under continental conditions, an environment of this kind is provided by a low peat-bog under a thin layer of hard water. The effect of OH- ion on lignin involves the disruption of etheric bonds between its radicals and d e m e t h o x y l a t i o n , while the formation of free phenol groups may be responsible for the recondensation of radicals. Furthermore, the alkaline medium causes changes in the side chains
192
of lignin, and particularly the enolization of ketone systems (Fig. 4) as a result of the formation of soluble salts of alkaline metals. The further evolution of these salts involves the formation of quinones and reactive ketoadipic acids due to the opening of a part of rings. It seems feasible that the mixture of the decomposition products of cellulose and lignin in a weakly oxidizing environment gives rise to ketoadipic acids, c o m m o n l y referred to as humic acids. The presence of these c o m p o u n d s is responsible for the strong acidity of the environment, typical of peat-bogs and brown coal deposits.
CH~
I
~H2 C-O i~
OH
HO',CHz /
H-CI - - 0 H-C-OH
~'OCH~ 0I
CH~ H-C
CH~
H-C
_OCHs
II
.,~C-ON°
* 2 NI:OH I,, ~
--CH:. H#
~
II
Jr"
V'nr~v~" OH
&-ONo
A
IIIQ
lllb
ICH~ CH II OH
H,CO~ x . ~
xl~,o~o,
~
014
H-C
R~
CH
OH
R'
HBCO.~
IL~O ~ l 0
OH
H ~ C O ~ f ~-H
l` II j''HZ-'7~'U'C
CZOH
R
R
CH~
0H
V
IV
""°2'2"°'2:
OH
Ro
temp ,c*neH(O3 H2CO
/C,.~
CIH
%1,-c
~OH
O-,Me.O
Fig. 4. The hypothetical course of doppleritization of lignin radicals. I = the stage of dissolution of lignin (the formation of alkalilignin); H = the stage of formation of quinone; I I I = the stage of formation of ketoadipic acid; I V = the stage of formation of dopplerite (organomineral compound). V = the stage of formation of vitrain in subbituminous coal.
The formation of dopplerite is p r o m o t e d by brackish sea waters or alkaline fresh waters. This process involves the bonding of cations of some metals to humic acids (~-ketoadipic acids). In reactions of this type a water molecule is presumably expelled. This causes the condensation of organomineral sol, which may then be subject to in situ diagenesis, or may be displaced to a different geochemical environment and undergo coagulation (Breger, 1955, p. 132). During diagenesis the organomineral, presumably lyophobic, sol coagulates. The coagulation m a y be caused by dehydration due to an increasing pressure of the overburden layer and the resulting increase in temperature, or by a change in the electric charge in response to structural changes, e.g. of clay minerals, in the changing environmental conditions (Wagner, 1980). The coagulation is accompanied by the process of ageing of col-
193
loids, as well as by recrystallization and chemical changes. One of the changes induced by elevated temperature is the decomposition of organomineral c o m p o u n d s (Fig. 4). Hence the vitrain of sub-bituminous and bituminous coal does n o t contain the same a m o u n t of ash combined organically with coal as dopplerite, although there are exceptions to this rule. In conclusion, it can be stated that the doppleritization and humification of w o o d in a coal-forming environment ultimately give rise to chemically different products, even though the physical nature of these processes is similar as they involve colloidal transformation. When the coagulation of organomineral sol is n o t induced by the increasing temperature, the dopplerite retains its chemical character, which fact has been noted in some peat beds and brown coal deposits. ACKNOWLEDGEMENTS
The author wishes to express his thanks to Dr W. Carius from the Pedagogical University of Erfurt /GDR / for taking Raman spectra, to Dr T. Zerda from the Silesian University for taking IR spectra, and to Dr K. Matl from the University of Mining and Metallurgy Cracow for the critical reading of the manuscript and valuable discussion.
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194 Prosi~ski, S., 1969. Chemia drewna. PWRiL, Warszawa, 488 pp. Ramler, E. and Jacob, H., 1951. Uber die Brikettierreigensehaft von Dopplerit. Freiberg. Forschungsh. A-3. Roga B. and Tomkow, K., 1971. Chemiczna teehnologia wegla. WNT, Warszawa, 586 pp. Roga, B. and Wnekowska, L., 1966. Analiza wegla i koksu. PWNT, Warszawa, 250 pp. Roselt, G., 1969. Zum Problem der Vergelung der Kohlne. Freiberger Forschungsh. C, 242: 13--28. Roselt, G., 1976. Methode und Anleiting zur weitgehend objektiven Ermittlung der Variet~ten humifizierter Xylite unserer Weichbraunkohlen. Wiss. Geol. yon ErdS1, Erdgas und Kohlen, Freiberg, p. 34. Stach, E. (Editor), 1978. Petrologia uglej. Izd. "Mir", Moscow, 554 pp. Suss, M., 1959. Zur Petrographie des Xylits. Freiberg. Forsehungh. A, 148: 14--33. Tsu, R., Gonzales, J.H., Hernandez, I.C. and Luengo, C.A., 1977. Raman spectra of coal. Solid. State Commun., 24: 809. Van Krevelen, D.W. and Schuyer, J., 1959. Wegiel. Chemia wegla i jego struktura. PWN, Warszawa, 310 pp. Wagner, M., 1980. Coal matter in the flysch of the Magura nappe between JordanSw and Nowy Sacz (Carpathians, Poland). Annal. Soc. Geol. Pol., 50: 99--117. Welte, E., 1952. Uber die entstehung von Humis~iuren und Wege ihrer Reindarstellung. Z. Pflannzenernahn., Dfing., Bodenkd., 56: 105--139. Zerda, T., John, A. and Chmura, K., 1981. Raman studies of coals. Fuel, 60: 375--379.