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A study of trace elements in lithotypes of some selected Indian coals
International Journal of Coal Geology, 8 (1987) 269-278
269
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
A S t u d y of T r a c e E l e m e n t s in L i t h o t y p e s of some S e l e c t e d Indian Coals REKHA GHOSH, TAPAN MAJUMDER and DIPTENDU N. GHOSH
Department of Applied Geology, Indian School of Mines, Dhanbad-826004, India (Received March 19, 1985; revised and accepted October 10, 1986)
ABSTRACT Ghosh, R., Majumder, T. and Ghosh, D.N., 1987. A study of trace elements in lithotypes of some selected Indian coals. Int. J. Coal Geol., 8: 269-278. Ashes of the lithotypes from some Indian coals were analyzed by emission spectroscopy for some selected elements. Based on the combined concentration and differentialfixation,the elements Pb and Co appear to be supplied by the woody portion of the proto-coal material whereas Ga, Nb, Ni, Cr and In can be attributed to the non-woody portions of the proto-coal. On the other hand, Cu, M o and part of the available Cr appear to come from both organic and inorganic sources, whereas V, Mn, Sr, La and Ba have been attributed to an extraneous inorganic source. The differential fixation of the trace elements appears to be mainly dependent on the physicochemical environment of the basin.
INTRODUCTION
Information on the differential distribution of trace elements in coal lithotypes is likely to have potential application in future coal research. The present study is aimed at deciphering the nature and mode of trace-element occurrence in the lithotypes of some selected Indian coals of different age and province. Previously, trace-element distributions in coals of different ages, types and ranks have been studied on whole coal basis by Mukherjee and Dutta (1950), Palmer and Filby (1983, 1984) and others. Studies on the differential distribution of trace elements in coal macerals are few in number; e.g. Chen et al. (1981), Minkin et al. (1982, 1983). All these studies were specifically based on microprobe analysis of microscopic components of the coal. However, only a few works exist on the distribution of trace elements in different coal lithotypes. The present study was undertaken to analyze and interpret the concentration and distribution patterns of trace elements in some typical Indian coals, with the aim to understand the mode of fixation of the trace elements in the lithotypes. 0166-5162/87/$03.50
© 1987 Elsevier Science Publishers B.V.
270 TABLE 1 Indian coal samples analyzed Sample
Age and location
Coal characteristics
Code
1,TL
L.Barakar ( Autunian ), Mahanadi Valley, Orissa
Rich in fusain and durain, poor in vitrain, non-coking coal of low ash content
1 V, 1 C. D,F
2,K
Karharbari (Autunian), Giridih Coalfield, Bihar
Strongly coking bituminous coal of low ash content. Rich in vitrain and poor in clarain and durain. Fusain not visible
2V, 2C, 2D
3,LB
Lower Barakar (Autunian), Jharia Coalfield
Fairly coking bituminous coal of moderate ash and rich in vitrain and durain. Fusain and clarain could not be separated
3V, 3D
4,UB
Up.Barakar ( Autunian ), Jharia Coalfield
Superior coking bituminous coal of low ash (15-20%). Rich in durain and vitrain, fusain rather low
4V, 4C, 4D, 4F
5,R
Raniganj, ( Thuringian ), Raniganj Coalfield
Poor coking bituminous coal of medium ash (10-22%). Rich in vitrain and durain; fusain could not be separated
5V, 5D
6,T
Tertiary, Baragolai Coalfield, Assam
Highly coking, unbanded lustrous bituminous coal of low ash. Rich in vitrain, moderate clarain but devoid of durain and fusain. High sulphur with visible pyrite grains
6 V, 6C
V = vitrain; D = durain; C = clarain; F = fusain. TL = Talchir; K = Karharbari; LB and UB = Lower and Upper Barakar; R = Raniganj; T = Tertiary. SAMPLES AND THEIR GEOLOGY T h e location, e q u i v a l e n t s t r a t i g r a p h i c age along with m e g a s c o p i c description of r e p r e s e n t a t i v e coal s a m p l e s f r o m different coalfields of I n d i a are given in Table 1. T h e coal c h a r a c t e r i s t i c s are f r o m P a r e e k (1971), S h a r m a a n d R a m (1978), B a s u (1983) a n d M a z u m d e r (1984). EXPERIMENTAL F r o m the six samples, f r a g m e n t s of vitrain, clarain, d u r a i n a n d fusain [ t h e macroscopic units of coal as suggested by Stopes (1919) ] were carefully selected, picked a n d s e p a r a t e d m a n u a l l y after d i s i n t e g r a t i o n , w h e r e v e r possible. T h e grains were o b s e r v e d u n d e r a b i n o c u l a r m i c r o s c o p e to e n s u r e the p u r i t y of t h e s e p a r a t e d fractions.
271 Ashing is widely used as a procedure for separating the mineral content of a coal whereby the minerals are retained in a relatively unaltered state (Gluskoter, 1965 ). For the present study, the cleaned chips of separated coal lithotypes were ashed at a temperature of 500 + 25 ° C. The resulting values in Table 2 show that the percentage of ash is generally low in vitrain and fusain, while it is higher in clarain and durain fractions. After being mixed with speakpure graphite the ash was analyzed with emission spectroscopy and the results obtained for some selected trace elements are given in Table 2. GEOCHEMISTRY OF THE LITHOTYPEASH To study the distribution of the various trace elements in the lithotypes, both their content and relative concentration were taken into consideration. Table 3 shows the Pearsons cross product correlation coefficients (Davis, 1973) of some selected trace elements with the ash contents of different lithotypes. The average content of the trace elements in the four lithotypes of the various coals (Fig. la) shows the preferential fixation of the trace elements in the different lithotypes. A combined study of the tables and the figures indicates the possible site of fixation of the major trace elements and their probable distribution in time and space.
Lead. In view of the maximum content of Pb in fusain, and considerable amounts in vitrain and clarain, which are derived mainly from the woody portion of the plants ( Stopes, 1919), Pb seems to be fixed in the wood component. This is also supported by its low content in durain as well as the high correlation (0.83 and 0.79 ) with the ash of vitrain and vitrain-fusain combined (Table 3B and C). Cobalt. Cobalt has a poor correlation with the ash of all lithotypes, so that its status can only be deduced from its relative concentration in the different lithotypes. Amongst the four constituents, Co is showing a relatively high concentration with vitrain and decreasing amounts in clarain, durain and fusain (Fig. la}, which seems to indicate its possible fixation in the woody portions of plants. Niobium, gallium, nickel and indium. These elements exhibit a near similar pattern with respect to fixation in the different phases, as shown in Fig. la. They show maximum contents in clarain and comparatively decreasing amounts in durain, fusain and vitrain. Gallium shows a significant high correlation coefficient with ash of vitrain-fusain (0.91, Table 3C); with durainclarain (0.72, Table 3D); with durain (0.98, Table 3E); and also with ash of all lithotypes (Table 3A). Nickel shows some minor deviation from the above
4.7 9.8 12.2 5.2
13.8 39.8 41.0
8.7 15.1
23.1 35.1 39.2 31.2
13.1 17.1
3.1 48.4
IV 1C 1D 1F
2V 2C 2D
3V 3D
4V 4C 4D 4F
5V 5D
6V 6C
0.30 4.84
7.86 6.84
16.17 17.55 11.76 15.60
10.44 4.53
12.42 7.96 4.10
3.29 5.39 4.88 3.64
Pb
3.9 58.08
28.82 42.75
46.2 42.12 66.64 37.44
13.05 18.12
22.08 39.8 57.4
16.45 26.46 42.7 15.6
Cu
1.2 14.52
13.1 17.1
9.24 21.06 11.76 15.60
20.01 18.12
24.84 27.86 24.6
9.4 12.74 10.98 3.64
Ni
n.d. n.d.
1.97 2.57
2.31 3.51 3.92 3.12
11.31 7.55
9.66 9.95 0.25
2.82 3.43 3.05 1.30
Co
n.d. n.d.
1.31 0.86
2.31 1.76 1.96 1.56
0.87 0.76
1.38 1.99 2.05
0.94 1.96 1.83 0.78
Mo
1.5 14.52
13.1 10.26
69.3 21.06 27.44 18.72
20.01 7.55
13.8 39.8 69.7
10.34 14.7 36.6 13.0
V
0.6 n.d.
91.7 94.05
69.3 70.2 66.64 46.8
5.22 5.29
19.32 51.74 61.5
25.85 49.0 67.1 31.2
Mn
0.3 4.84
2.62 2.57
8.09 8.78 9.8 7.8
2.61 2.27
2.76 7.96 10.25
0.94 1.47 2.44 1.04
Ga
n.d. n.d.
1.31 n.d.
n.d. n.d. n.d. n.d.
0.87 1.51
n.d. n.d. 4.10
2.82 2.45 1.22 0.52
Ge
0.3 4.84
1.31 1.71
2.31 3.51 3.92 3.12
2.18 1.51
1.38 3.98 2.05
0.47 0.98 1.22 0.52
In
1.5 33.8
13.1 8.55
23.1 14.04 16.6 15.6
10.44 15.1
13.8 27.86 45.1
4.7 3.92 2.44 1.04
Nb
2.4 53.24
13.1 18.81
23.1 35.1 23.52 31.2
8.70 7.55
13.8 39.8 8.2
10.34 21.56 24.4 10.4
La
21.0 96.8
117.9 171.0
207.9 315.9 215.6 312.0
104.4 181.2
110.4 278.6 164.0
56.4 88.2 73.2 41.6
Ba
21.0 33.8
26.2 34.2
138.6 59.6 196.0 62.4
69.6 75.5
20.7 55.7 69.7
11.7 29.4 30.5 20.8
Sr
5.4 87.12
15.72 17.1
69.3 59.67 62.72 49.92
17.4 19.63
41.4 51.74 73.8
32.9 53.9 97.6 36.4
Cr
T h e observed trace element values were multiplied by a factor corresponding to the ash % to obtain the tabulated value (e.g. in 1V with a n ash % of 4.7 the observed Cu content was 350 ppm, and 16.45 p p m the tabulated value ).
Ash%
Code
Distribution of trace elements ( p p m ) in coal lithotypes of some selected Indian coal samples
TABLE 2
t~
t~
0.68 17
0.90 6
0.775 8
0.538 9
0.63 5
0.132 17
0.83 6
0.792 8
0.081 9
0.174 5
B.
C.
D.
E.
0.132 5
0.183 9
0.143 8
0.082 6
0.25 17
Ni
-0.24 5
0.003 8
-0.021 7
0.021 5
-0.003 15
Co
0.446 5
0.244 8
0.592 7
0.92 5
0.41 15
Mo
0.404 5
0.156 9
0.303 8
0.75 6
0.213 17
V
0.027 5
0.036 8
0.672 8
0.45 6
0.21 16
Mn
0.98 5
0.72 9
0.914 8
0.90 6
0.537 17
Ga
--
0.743 4
-0.067 4
--0.51 3
0.48 8
Ge
1.0 5
0.99 9
0.799 8
0.60 6
0.954 17
In
0.609 5
0.66 9
0.682 8
0.98 6
0.689 17
Nb
S.~S,.
Covx~
where Covx~ is the covariance of x, y; Sx, S~. are the standard deviations of x and y.
Y -
A = all lithotypes; B = vitrain; C = vitrain + fusain; D = durain + clarain; E = durain. ,n = number of samples. The Pearsons cross product correlation coefficient was determined using the standard formula {Davis, 1973 ) :
n
n
n
n
n
t.
Cu
Pb
Variables
Correlation coefficient of trace elements with ash of coal lithotypes
TABLE 3
0.482 5
0.672 9
0.935 8
0.89 6
0.814 17
La
0.31 5
0.167 9
0.975 8
0.52 6
0.43 17
Ba
0.449 5
0.119 9
0.386 8
0.58 6
0.21 17
Sr
0.068 5
0.133 9
0.488 8
0.66 6
0.36 17
Cr
P,D
274
(a)
(b) 360 2OO
IO0
J
Cu
~g" 200 150
IOO ~25~ ~
S0 150 7O 9 6
3 35~ ~ 55 155 I
E z~
~
~
~
11oo ~OO 700 500
Bo
300 ~
100
13C)9O 50
3g 25 15
96 60 30 30 20
In
I0
Ni
75 55
o.
35 li 20 10
6
G~
3 Vitroin
Cloroin
Ouroin
Fulo;n
code 1"1 SQmpte no: 1
~
LB
UB
R
2
3
4
5
6
Fig. 1. a. Distribution of trace elements ( p p m ) in four coal lithotypes of some selected Indian coals, b. Distribution of trace elements in selected Indian coals in time a n d space.
275 pattern. Thus, it appears that these elements are fixed in the organic phase, preferably in the non-woody portion, in view of their highest concentration in clarain, as shown in Fig. la. Because clarain is considered as a mixture of durain and vitrain (Van Krevelen and Schuyer, 1957 ), mineral matter in clarain may be considered partly detrital. Moreover, as Cr is generally detrital in nature ( Hinst, 1962 }, a part of the Cr present can be attributed to a detrital source.
Lanthanum and barium have relatively higher contents in clarain (Fig. la) and a tendency to be concentrated in fusain. They show high correlation with the ash of vitrain and fusain {0.93 and 0.97, Table 3C). From their high concentration in clarain, they may be regarded as extraneous mineral matter. However, their secondary concentration in fusain could be explained by the presence of mineral matter within the cell cavities of fusain (Francis, 1954 ).
Copper can be attributed to two sources, i.e. fixation in organic phase as well as in external mineral matter. This conclusion is based on its high correlation (0.90) with ash of vitrain ( Table 3B) and vitrain-fusain combined (0.78, Table 3C), which indicates its fixation in the organic part. The inorganic status is indicated both from its higher concentration in clarain and durain ( Van Krevelen and Schuyer, 1957 ), as well as the rare presence of chalcopyrite observed microscopically in some samples, which is supported by the high correlation with ash of all the lithotypes {0.68, Table 3A). In view of a similar distribution pattern (Fig. l a ) , Mo may also be attributed a dual status which is supported by its correlation with ash of vitrain (0.92, Table 3B ) and with ash of vitrainfusain combined ( 0.59, Table 2C ). Vanadium, strontium, manganese and germanium. The first three elements are conspicuous by their low peaks in durain only (Fig. la). Durain consists of a considerable amount of mineral matter (Van Krevelen and Schuyer, 1957; Pareek, 1971 ), which supports an inorganic fixation for these elements. Manganese has a significant correlation with ash of vitrain and fusain (0.67, Table 3C) while V has significant correlation with ash of vitrain (0.75, Table 3B). Germanium exhibits a prominent peak in durain like the previous three elements and also has a secondary concentration in vitrain. Thus, in view of its near similar distribution as observed in Fig. la, Ge may also be assigned to the inorganic phase. Leutwein and Roesler (1956) have subclassified the fixation of some trace elements to the following phases: Ge, Be, Zn and Cr exclusively to coal substance; Cu, Pb, Zn, Ag, Ni, Mo, Sb preferably to coal substance; Ga, Co and V primarily to foreign ash, but Mn, St, Ba exclusively to foreign ash.
276 TRACE-ELEMENT DISTRIBUTION IN TIME AND SPACE
Because the coals of this study come from different stratigraphic ages, the total trace element in them was plotte(~against their ages as shown in Fig. lb. The successive ages with their European equivalents given in brackets are Talchir ( Autunian ), Karharbari (Autunian), Lower Barakar (Autunian), Upper Barakar (Autunian), Raniganj (Thuringian) and Tertiary. From Fig. lb the following observations can be made: (a) All the trace elements except Ge and Co show distinct and prominent peaks in the ash of the Upper Barakar sample and conversely all these elements show depletion in the Lower Barakar sample. (b) The elements Ni, In and Nb have a consistent high content in the ash of Karharbari age in comparison to that of the Upper Barakar age. The elements Ga, V and Ba also exhibit a distinct secondary peak in the Karharbari sample. However, these six elements show a marked depletion in coal ash of Talchir (Autunian) as well as in the Raniganj (Thuringian) sample. It may be mentioned here that Mukherjee and Ghosh (1976) in their study on trace-element distribution in Permian coals of India, indicated fixation of trace elements to either organic phase or inorganic phase. They also had noted a marked difference in the fixation of trace elements between the Upper Barakar and the Lower Barakar. It has been postulated that the Gondwana basin knew a history of oscillation during deposition (Fox, 1939; Krishnan, 1968), in which coal-bearing formations were developed in the major portions. The marked depletion of the abovementioned trace elements in the Talchir, Lower Barakar and Raniganj samples together with the corresponding enrichment of the same elements in the intervening Karharbari and Upper Barakar samples attest to the oscillating nature of the basin during deposition. This marked difference in trace-element fixation can be either due to their supply from different sources or to different depositional conditions. The possibility of different sources appears less probable because the same sets of trace elements are encountered repeatedly in coals of different ages. Therefore, the physicochemical condition of the basin appears to be the main influencing factor. The coal quality is greatly controlled by the Eh-pH conditions of the depositional basin (Van Krevelen and Schuyer, 1957). This view is further supported by the fact that coals with higher traceelement contents have a better coking quality than those with lower contents, e.g. Karharbari and Upper Barakar coals are of superior coking quality ( Sharma and Ram, 1978) while the other three Permian samples studied, Talchir, Lower Barakar and Raniganj, which contain comparatively lower amounts of trace elements exhibit inferior coking properties. From Fig. lb it appears that the majority of the trace elements like Ni, In, Nb, Ga, V, Ba and Pb occur in nearly similar amounts in the Talchir coal
277 (Sample No. 1 ), Lower Barakar, Mahanadi Valley and in the Jharia coal of the same age ( Sample No. 3 ) from the Damodar Valley. Though separated in space it may be possible to postulate, in view of the close similarity of traceelement contents, that the Mahanadi and Damodar valleys form parts of the same Gondwana Basin. The conspicuous presence of Mn in near similar amounts in the coal of Talchir, Karharbari, Upper Barakar and Raniganj can be explained due to the proximity and association of banded iron stone beds which areogenerally rich in Mn. In general the Tertiary coals are markedly poor in trace elements (Sample No. 6) compared to the Gondwana coals. A possible explanation could be that the Gondwana coals were formed in a closed inland basin while the Tertiary coals were formed in an open basin (Krishnan, 1968 ) in which the majority of the trace elements possibly had a chance to escape to sea. CONCLUSION In conclusion it may be summarized that the fixation of trace elements appears to be independent of the age of the coal but is more dependent on the basin condition and the preferential affinity of the trace elements towards either organic (nonwoody or woody portion) or inorganic (inherent or extraneous) phase. The elements Ga, Nb, Cr and In can be attributed to the non-woody portion while Pb and Co appear to be supplied by the woody portions of the plants. Cu, Mo and part of the available Cr appear to be fixed in the organic phase and in the inorganic phase. V, St, Mn, La and Ba can be attributed primarily to the inorganic source. Ge may also be assigned to this source in view of the similarity of distribution, although with some reservation in view of its low content. Based on similar trace-element distributions it may be suggested that the two different areas Mahanadi-Valley and Damodar Valley form part of the same Gondwana basin. ACKNOWLEDGEMENT The authors are thankful to the Director-General, Geological Survey of India for trace element analytical facilities, to Prof. T.C. Rao, Head of the Department of Fuel and Mineral Engineering for ashing facility and to the Director, Indian School of Mines, for all other facilities. The authors appreciate and acknowledge the suggestions of Dr. S.C. Banerjee of Central Mining Research Station, Dhanbad. However, the views expressed are those of the authors.
278 REFERENCES Basu, T.N., 1983. Correlation and classification of the Basal and Lower Permian coal measures of Peninsular India. Trans. Min. Geol. Metall. Inst. India, 80(1 ): 96-125. Chen, J.R. et al., 1981. Trace elemental analysis of bituminous coals using the Heidelberg proton microprobe. Nucl. Instrum. Methods, 181: 151-167. Davis, J.C., 1973. Statistics and Data Analysis in Geology. John Wiley & Sons Inc., London, 550 pp. Fox, C.S., 1931. The natural history of Indian coals. Mem. Geol. Surv. India, Vol. 57. Fox, C.S., 1939. The Lower Gondwana coalfields of India. Mem. Geol. Surv. India, Vol. 59. Francis, W., 1954. Coal, its Formation and Composition. Edward Arnold Ltd., London, 567 pp. Gluskoter, H.J., 1965. Electronic low-temperature ashing of bituminous coal. Fuel, 44: 285-291. Hirst, D.M., 1962. The geochemistry of Modern Sediments from the gulf of Baria II. Geochem. Cosmochim. Acta, 26: 1147-1187. ICCP (International Committee for Coal Petrology), 1963. Handbook of Coal Petrography. C.N.R.S. Paris. Krevelen, D.W. Van and Schuyer, J., 1957. Coal Science. Elsevier Publishing Company, Amsterdam, London, New York, Princeton, 352 pp. Krishnan, M.S., 1968. Geology of India and Burma. Higginbothams (P) Limited, Madras, 536 pp. Leutwein, F. and Roesler, H.J., 1956. Geochemische Untersuchungen an paleozoischen und mesozoischen Kohlen Mittel- und Ostdeutschlands. Freiberg. Forschungsh, C 19. Akademie Verlag, Berlin. Mazumder, B.K., 1984. Genetic development of coal and coal behaviour. Q.J.G.M.M.S.I., 56 (3) : 105-127. Minkin, J.A., Chao, E.C.T., Thompson, C.L., Nobling, R. and Blank, H., 1982. Proton microprobe determination of elemental concentrations in coal macerals. Scanning Electron Microscopy. SEM Inc., AMF O'Hare, Chicago, pp. 175-184. Minkin, J.A., Chao, E.C.T., Thompson, C.L., Wandless, M.V., Dulong, F.T., Larson, R.R. and Neuzil, S.G., 1983. Submicroscopic ( < 1 ~tm) mineral contents of vitrinites in selected bituminous coal beds. Ron Geoley, (Ed.), Microbeam Analysis. San Francisco Press, San Francisco. Mukherjee, B. and Dutta, R., 1950. A note on the constituents of the ashes of Indian Coal determined spectrographically. Fuel., 29 (8) : 190-192. Mukherjee, B. and Ghosh, A., 1976. Distribution and behaviour of trace elements in some Permian coals of India. Indian Mineral., 17: 23-30. Palmer, C.A. and Filby, R.H., 1983. Determination of mode of occurrence of trace elements in the Upper Freeport coal bed using size and density separation procedure. International Conference on Coal Science, Pittsburgh, PA, pp. 365-368. Palmer, C.A. and Filby, R.H., 1984. Distribution of trace elements in coal from the Powhatan No. 6 mine, Ohio. Fuel, 63: 318-328. Pareek, H.S., 1971. Coal Resources of India. Mem. Geol. Surv. India, Vol. 88, 575 pp. Sharma, N.L. and Ram, K.S.V., 1978. Introduction to Geology of coal and Indian coalfields, Dhanbad Publishers, New Sketch Press, Bihar, India, 146 pp. Stopes, M.C., 1919. On the four visible ingredients in banded bituminous coals; studies in the composition of coal No. 1. Proc. R. Soc. London, 90B: 470-487.
269
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
A S t u d y of T r a c e E l e m e n t s in L i t h o t y p e s of some S e l e c t e d Indian Coals REKHA GHOSH, TAPAN MAJUMDER and DIPTENDU N. GHOSH
Department of Applied Geology, Indian School of Mines, Dhanbad-826004, India (Received March 19, 1985; revised and accepted October 10, 1986)
ABSTRACT Ghosh, R., Majumder, T. and Ghosh, D.N., 1987. A study of trace elements in lithotypes of some selected Indian coals. Int. J. Coal Geol., 8: 269-278. Ashes of the lithotypes from some Indian coals were analyzed by emission spectroscopy for some selected elements. Based on the combined concentration and differentialfixation,the elements Pb and Co appear to be supplied by the woody portion of the proto-coal material whereas Ga, Nb, Ni, Cr and In can be attributed to the non-woody portions of the proto-coal. On the other hand, Cu, M o and part of the available Cr appear to come from both organic and inorganic sources, whereas V, Mn, Sr, La and Ba have been attributed to an extraneous inorganic source. The differential fixation of the trace elements appears to be mainly dependent on the physicochemical environment of the basin.
INTRODUCTION
Information on the differential distribution of trace elements in coal lithotypes is likely to have potential application in future coal research. The present study is aimed at deciphering the nature and mode of trace-element occurrence in the lithotypes of some selected Indian coals of different age and province. Previously, trace-element distributions in coals of different ages, types and ranks have been studied on whole coal basis by Mukherjee and Dutta (1950), Palmer and Filby (1983, 1984) and others. Studies on the differential distribution of trace elements in coal macerals are few in number; e.g. Chen et al. (1981), Minkin et al. (1982, 1983). All these studies were specifically based on microprobe analysis of microscopic components of the coal. However, only a few works exist on the distribution of trace elements in different coal lithotypes. The present study was undertaken to analyze and interpret the concentration and distribution patterns of trace elements in some typical Indian coals, with the aim to understand the mode of fixation of the trace elements in the lithotypes. 0166-5162/87/$03.50
© 1987 Elsevier Science Publishers B.V.
270 TABLE 1 Indian coal samples analyzed Sample
Age and location
Coal characteristics
Code
1,TL
L.Barakar ( Autunian ), Mahanadi Valley, Orissa
Rich in fusain and durain, poor in vitrain, non-coking coal of low ash content
1 V, 1 C. D,F
2,K
Karharbari (Autunian), Giridih Coalfield, Bihar
Strongly coking bituminous coal of low ash content. Rich in vitrain and poor in clarain and durain. Fusain not visible
2V, 2C, 2D
3,LB
Lower Barakar (Autunian), Jharia Coalfield
Fairly coking bituminous coal of moderate ash and rich in vitrain and durain. Fusain and clarain could not be separated
3V, 3D
4,UB
Up.Barakar ( Autunian ), Jharia Coalfield
Superior coking bituminous coal of low ash (15-20%). Rich in durain and vitrain, fusain rather low
4V, 4C, 4D, 4F
5,R
Raniganj, ( Thuringian ), Raniganj Coalfield
Poor coking bituminous coal of medium ash (10-22%). Rich in vitrain and durain; fusain could not be separated
5V, 5D
6,T
Tertiary, Baragolai Coalfield, Assam
Highly coking, unbanded lustrous bituminous coal of low ash. Rich in vitrain, moderate clarain but devoid of durain and fusain. High sulphur with visible pyrite grains
6 V, 6C
V = vitrain; D = durain; C = clarain; F = fusain. TL = Talchir; K = Karharbari; LB and UB = Lower and Upper Barakar; R = Raniganj; T = Tertiary. SAMPLES AND THEIR GEOLOGY T h e location, e q u i v a l e n t s t r a t i g r a p h i c age along with m e g a s c o p i c description of r e p r e s e n t a t i v e coal s a m p l e s f r o m different coalfields of I n d i a are given in Table 1. T h e coal c h a r a c t e r i s t i c s are f r o m P a r e e k (1971), S h a r m a a n d R a m (1978), B a s u (1983) a n d M a z u m d e r (1984). EXPERIMENTAL F r o m the six samples, f r a g m e n t s of vitrain, clarain, d u r a i n a n d fusain [ t h e macroscopic units of coal as suggested by Stopes (1919) ] were carefully selected, picked a n d s e p a r a t e d m a n u a l l y after d i s i n t e g r a t i o n , w h e r e v e r possible. T h e grains were o b s e r v e d u n d e r a b i n o c u l a r m i c r o s c o p e to e n s u r e the p u r i t y of t h e s e p a r a t e d fractions.
271 Ashing is widely used as a procedure for separating the mineral content of a coal whereby the minerals are retained in a relatively unaltered state (Gluskoter, 1965 ). For the present study, the cleaned chips of separated coal lithotypes were ashed at a temperature of 500 + 25 ° C. The resulting values in Table 2 show that the percentage of ash is generally low in vitrain and fusain, while it is higher in clarain and durain fractions. After being mixed with speakpure graphite the ash was analyzed with emission spectroscopy and the results obtained for some selected trace elements are given in Table 2. GEOCHEMISTRY OF THE LITHOTYPEASH To study the distribution of the various trace elements in the lithotypes, both their content and relative concentration were taken into consideration. Table 3 shows the Pearsons cross product correlation coefficients (Davis, 1973) of some selected trace elements with the ash contents of different lithotypes. The average content of the trace elements in the four lithotypes of the various coals (Fig. la) shows the preferential fixation of the trace elements in the different lithotypes. A combined study of the tables and the figures indicates the possible site of fixation of the major trace elements and their probable distribution in time and space.
Lead. In view of the maximum content of Pb in fusain, and considerable amounts in vitrain and clarain, which are derived mainly from the woody portion of the plants ( Stopes, 1919), Pb seems to be fixed in the wood component. This is also supported by its low content in durain as well as the high correlation (0.83 and 0.79 ) with the ash of vitrain and vitrain-fusain combined (Table 3B and C). Cobalt. Cobalt has a poor correlation with the ash of all lithotypes, so that its status can only be deduced from its relative concentration in the different lithotypes. Amongst the four constituents, Co is showing a relatively high concentration with vitrain and decreasing amounts in clarain, durain and fusain (Fig. la}, which seems to indicate its possible fixation in the woody portions of plants. Niobium, gallium, nickel and indium. These elements exhibit a near similar pattern with respect to fixation in the different phases, as shown in Fig. la. They show maximum contents in clarain and comparatively decreasing amounts in durain, fusain and vitrain. Gallium shows a significant high correlation coefficient with ash of vitrain-fusain (0.91, Table 3C); with durainclarain (0.72, Table 3D); with durain (0.98, Table 3E); and also with ash of all lithotypes (Table 3A). Nickel shows some minor deviation from the above
4.7 9.8 12.2 5.2
13.8 39.8 41.0
8.7 15.1
23.1 35.1 39.2 31.2
13.1 17.1
3.1 48.4
IV 1C 1D 1F
2V 2C 2D
3V 3D
4V 4C 4D 4F
5V 5D
6V 6C
0.30 4.84
7.86 6.84
16.17 17.55 11.76 15.60
10.44 4.53
12.42 7.96 4.10
3.29 5.39 4.88 3.64
Pb
3.9 58.08
28.82 42.75
46.2 42.12 66.64 37.44
13.05 18.12
22.08 39.8 57.4
16.45 26.46 42.7 15.6
Cu
1.2 14.52
13.1 17.1
9.24 21.06 11.76 15.60
20.01 18.12
24.84 27.86 24.6
9.4 12.74 10.98 3.64
Ni
n.d. n.d.
1.97 2.57
2.31 3.51 3.92 3.12
11.31 7.55
9.66 9.95 0.25
2.82 3.43 3.05 1.30
Co
n.d. n.d.
1.31 0.86
2.31 1.76 1.96 1.56
0.87 0.76
1.38 1.99 2.05
0.94 1.96 1.83 0.78
Mo
1.5 14.52
13.1 10.26
69.3 21.06 27.44 18.72
20.01 7.55
13.8 39.8 69.7
10.34 14.7 36.6 13.0
V
0.6 n.d.
91.7 94.05
69.3 70.2 66.64 46.8
5.22 5.29
19.32 51.74 61.5
25.85 49.0 67.1 31.2
Mn
0.3 4.84
2.62 2.57
8.09 8.78 9.8 7.8
2.61 2.27
2.76 7.96 10.25
0.94 1.47 2.44 1.04
Ga
n.d. n.d.
1.31 n.d.
n.d. n.d. n.d. n.d.
0.87 1.51
n.d. n.d. 4.10
2.82 2.45 1.22 0.52
Ge
0.3 4.84
1.31 1.71
2.31 3.51 3.92 3.12
2.18 1.51
1.38 3.98 2.05
0.47 0.98 1.22 0.52
In
1.5 33.8
13.1 8.55
23.1 14.04 16.6 15.6
10.44 15.1
13.8 27.86 45.1
4.7 3.92 2.44 1.04
Nb
2.4 53.24
13.1 18.81
23.1 35.1 23.52 31.2
8.70 7.55
13.8 39.8 8.2
10.34 21.56 24.4 10.4
La
21.0 96.8
117.9 171.0
207.9 315.9 215.6 312.0
104.4 181.2
110.4 278.6 164.0
56.4 88.2 73.2 41.6
Ba
21.0 33.8
26.2 34.2
138.6 59.6 196.0 62.4
69.6 75.5
20.7 55.7 69.7
11.7 29.4 30.5 20.8
Sr
5.4 87.12
15.72 17.1
69.3 59.67 62.72 49.92
17.4 19.63
41.4 51.74 73.8
32.9 53.9 97.6 36.4
Cr
T h e observed trace element values were multiplied by a factor corresponding to the ash % to obtain the tabulated value (e.g. in 1V with a n ash % of 4.7 the observed Cu content was 350 ppm, and 16.45 p p m the tabulated value ).
Ash%
Code
Distribution of trace elements ( p p m ) in coal lithotypes of some selected Indian coal samples
TABLE 2
t~
t~
0.68 17
0.90 6
0.775 8
0.538 9
0.63 5
0.132 17
0.83 6
0.792 8
0.081 9
0.174 5
B.
C.
D.
E.
0.132 5
0.183 9
0.143 8
0.082 6
0.25 17
Ni
-0.24 5
0.003 8
-0.021 7
0.021 5
-0.003 15
Co
0.446 5
0.244 8
0.592 7
0.92 5
0.41 15
Mo
0.404 5
0.156 9
0.303 8
0.75 6
0.213 17
V
0.027 5
0.036 8
0.672 8
0.45 6
0.21 16
Mn
0.98 5
0.72 9
0.914 8
0.90 6
0.537 17
Ga
--
0.743 4
-0.067 4
--0.51 3
0.48 8
Ge
1.0 5
0.99 9
0.799 8
0.60 6
0.954 17
In
0.609 5
0.66 9
0.682 8
0.98 6
0.689 17
Nb
S.~S,.
Covx~
where Covx~ is the covariance of x, y; Sx, S~. are the standard deviations of x and y.
Y -
A = all lithotypes; B = vitrain; C = vitrain + fusain; D = durain + clarain; E = durain. ,n = number of samples. The Pearsons cross product correlation coefficient was determined using the standard formula {Davis, 1973 ) :
n
n
n
n
n
t.
Cu
Pb
Variables
Correlation coefficient of trace elements with ash of coal lithotypes
TABLE 3
0.482 5
0.672 9
0.935 8
0.89 6
0.814 17
La
0.31 5
0.167 9
0.975 8
0.52 6
0.43 17
Ba
0.449 5
0.119 9
0.386 8
0.58 6
0.21 17
Sr
0.068 5
0.133 9
0.488 8
0.66 6
0.36 17
Cr
P,D
274
(a)
(b) 360 2OO
IO0
J
Cu
~g" 200 150
IOO ~25~ ~
S0 150 7O 9 6
3 35~ ~ 55 155 I
E z~
~
~
~
11oo ~OO 700 500
Bo
300 ~
100
13C)9O 50
3g 25 15
96 60 30 30 20
In
I0
Ni
75 55
o.
35 li 20 10
6
G~
3 Vitroin
Cloroin
Ouroin
Fulo;n
code 1"1 SQmpte no: 1
~
LB
UB
R
2
3
4
5
6
Fig. 1. a. Distribution of trace elements ( p p m ) in four coal lithotypes of some selected Indian coals, b. Distribution of trace elements in selected Indian coals in time a n d space.
275 pattern. Thus, it appears that these elements are fixed in the organic phase, preferably in the non-woody portion, in view of their highest concentration in clarain, as shown in Fig. la. Because clarain is considered as a mixture of durain and vitrain (Van Krevelen and Schuyer, 1957 ), mineral matter in clarain may be considered partly detrital. Moreover, as Cr is generally detrital in nature ( Hinst, 1962 }, a part of the Cr present can be attributed to a detrital source.
Lanthanum and barium have relatively higher contents in clarain (Fig. la) and a tendency to be concentrated in fusain. They show high correlation with the ash of vitrain and fusain {0.93 and 0.97, Table 3C). From their high concentration in clarain, they may be regarded as extraneous mineral matter. However, their secondary concentration in fusain could be explained by the presence of mineral matter within the cell cavities of fusain (Francis, 1954 ).
Copper can be attributed to two sources, i.e. fixation in organic phase as well as in external mineral matter. This conclusion is based on its high correlation (0.90) with ash of vitrain ( Table 3B) and vitrain-fusain combined (0.78, Table 3C), which indicates its fixation in the organic part. The inorganic status is indicated both from its higher concentration in clarain and durain ( Van Krevelen and Schuyer, 1957 ), as well as the rare presence of chalcopyrite observed microscopically in some samples, which is supported by the high correlation with ash of all the lithotypes {0.68, Table 3A). In view of a similar distribution pattern (Fig. l a ) , Mo may also be attributed a dual status which is supported by its correlation with ash of vitrain (0.92, Table 3B ) and with ash of vitrainfusain combined ( 0.59, Table 2C ). Vanadium, strontium, manganese and germanium. The first three elements are conspicuous by their low peaks in durain only (Fig. la). Durain consists of a considerable amount of mineral matter (Van Krevelen and Schuyer, 1957; Pareek, 1971 ), which supports an inorganic fixation for these elements. Manganese has a significant correlation with ash of vitrain and fusain (0.67, Table 3C) while V has significant correlation with ash of vitrain (0.75, Table 3B). Germanium exhibits a prominent peak in durain like the previous three elements and also has a secondary concentration in vitrain. Thus, in view of its near similar distribution as observed in Fig. la, Ge may also be assigned to the inorganic phase. Leutwein and Roesler (1956) have subclassified the fixation of some trace elements to the following phases: Ge, Be, Zn and Cr exclusively to coal substance; Cu, Pb, Zn, Ag, Ni, Mo, Sb preferably to coal substance; Ga, Co and V primarily to foreign ash, but Mn, St, Ba exclusively to foreign ash.
276 TRACE-ELEMENT DISTRIBUTION IN TIME AND SPACE
Because the coals of this study come from different stratigraphic ages, the total trace element in them was plotte(~against their ages as shown in Fig. lb. The successive ages with their European equivalents given in brackets are Talchir ( Autunian ), Karharbari (Autunian), Lower Barakar (Autunian), Upper Barakar (Autunian), Raniganj (Thuringian) and Tertiary. From Fig. lb the following observations can be made: (a) All the trace elements except Ge and Co show distinct and prominent peaks in the ash of the Upper Barakar sample and conversely all these elements show depletion in the Lower Barakar sample. (b) The elements Ni, In and Nb have a consistent high content in the ash of Karharbari age in comparison to that of the Upper Barakar age. The elements Ga, V and Ba also exhibit a distinct secondary peak in the Karharbari sample. However, these six elements show a marked depletion in coal ash of Talchir (Autunian) as well as in the Raniganj (Thuringian) sample. It may be mentioned here that Mukherjee and Ghosh (1976) in their study on trace-element distribution in Permian coals of India, indicated fixation of trace elements to either organic phase or inorganic phase. They also had noted a marked difference in the fixation of trace elements between the Upper Barakar and the Lower Barakar. It has been postulated that the Gondwana basin knew a history of oscillation during deposition (Fox, 1939; Krishnan, 1968), in which coal-bearing formations were developed in the major portions. The marked depletion of the abovementioned trace elements in the Talchir, Lower Barakar and Raniganj samples together with the corresponding enrichment of the same elements in the intervening Karharbari and Upper Barakar samples attest to the oscillating nature of the basin during deposition. This marked difference in trace-element fixation can be either due to their supply from different sources or to different depositional conditions. The possibility of different sources appears less probable because the same sets of trace elements are encountered repeatedly in coals of different ages. Therefore, the physicochemical condition of the basin appears to be the main influencing factor. The coal quality is greatly controlled by the Eh-pH conditions of the depositional basin (Van Krevelen and Schuyer, 1957). This view is further supported by the fact that coals with higher traceelement contents have a better coking quality than those with lower contents, e.g. Karharbari and Upper Barakar coals are of superior coking quality ( Sharma and Ram, 1978) while the other three Permian samples studied, Talchir, Lower Barakar and Raniganj, which contain comparatively lower amounts of trace elements exhibit inferior coking properties. From Fig. lb it appears that the majority of the trace elements like Ni, In, Nb, Ga, V, Ba and Pb occur in nearly similar amounts in the Talchir coal
277 (Sample No. 1 ), Lower Barakar, Mahanadi Valley and in the Jharia coal of the same age ( Sample No. 3 ) from the Damodar Valley. Though separated in space it may be possible to postulate, in view of the close similarity of traceelement contents, that the Mahanadi and Damodar valleys form parts of the same Gondwana Basin. The conspicuous presence of Mn in near similar amounts in the coal of Talchir, Karharbari, Upper Barakar and Raniganj can be explained due to the proximity and association of banded iron stone beds which areogenerally rich in Mn. In general the Tertiary coals are markedly poor in trace elements (Sample No. 6) compared to the Gondwana coals. A possible explanation could be that the Gondwana coals were formed in a closed inland basin while the Tertiary coals were formed in an open basin (Krishnan, 1968 ) in which the majority of the trace elements possibly had a chance to escape to sea. CONCLUSION In conclusion it may be summarized that the fixation of trace elements appears to be independent of the age of the coal but is more dependent on the basin condition and the preferential affinity of the trace elements towards either organic (nonwoody or woody portion) or inorganic (inherent or extraneous) phase. The elements Ga, Nb, Cr and In can be attributed to the non-woody portion while Pb and Co appear to be supplied by the woody portions of the plants. Cu, Mo and part of the available Cr appear to be fixed in the organic phase and in the inorganic phase. V, St, Mn, La and Ba can be attributed primarily to the inorganic source. Ge may also be assigned to this source in view of the similarity of distribution, although with some reservation in view of its low content. Based on similar trace-element distributions it may be suggested that the two different areas Mahanadi-Valley and Damodar Valley form part of the same Gondwana basin. ACKNOWLEDGEMENT The authors are thankful to the Director-General, Geological Survey of India for trace element analytical facilities, to Prof. T.C. Rao, Head of the Department of Fuel and Mineral Engineering for ashing facility and to the Director, Indian School of Mines, for all other facilities. The authors appreciate and acknowledge the suggestions of Dr. S.C. Banerjee of Central Mining Research Station, Dhanbad. However, the views expressed are those of the authors.
278 REFERENCES Basu, T.N., 1983. Correlation and classification of the Basal and Lower Permian coal measures of Peninsular India. Trans. Min. Geol. Metall. Inst. India, 80(1 ): 96-125. Chen, J.R. et al., 1981. Trace elemental analysis of bituminous coals using the Heidelberg proton microprobe. Nucl. Instrum. Methods, 181: 151-167. Davis, J.C., 1973. Statistics and Data Analysis in Geology. John Wiley & Sons Inc., London, 550 pp. Fox, C.S., 1931. The natural history of Indian coals. Mem. Geol. Surv. India, Vol. 57. Fox, C.S., 1939. The Lower Gondwana coalfields of India. Mem. Geol. Surv. India, Vol. 59. Francis, W., 1954. Coal, its Formation and Composition. Edward Arnold Ltd., London, 567 pp. Gluskoter, H.J., 1965. Electronic low-temperature ashing of bituminous coal. Fuel, 44: 285-291. Hirst, D.M., 1962. The geochemistry of Modern Sediments from the gulf of Baria II. Geochem. Cosmochim. Acta, 26: 1147-1187. ICCP (International Committee for Coal Petrology), 1963. Handbook of Coal Petrography. C.N.R.S. Paris. Krevelen, D.W. Van and Schuyer, J., 1957. Coal Science. Elsevier Publishing Company, Amsterdam, London, New York, Princeton, 352 pp. Krishnan, M.S., 1968. Geology of India and Burma. Higginbothams (P) Limited, Madras, 536 pp. Leutwein, F. and Roesler, H.J., 1956. Geochemische Untersuchungen an paleozoischen und mesozoischen Kohlen Mittel- und Ostdeutschlands. Freiberg. Forschungsh, C 19. Akademie Verlag, Berlin. Mazumder, B.K., 1984. Genetic development of coal and coal behaviour. Q.J.G.M.M.S.I., 56 (3) : 105-127. Minkin, J.A., Chao, E.C.T., Thompson, C.L., Nobling, R. and Blank, H., 1982. Proton microprobe determination of elemental concentrations in coal macerals. Scanning Electron Microscopy. SEM Inc., AMF O'Hare, Chicago, pp. 175-184. Minkin, J.A., Chao, E.C.T., Thompson, C.L., Wandless, M.V., Dulong, F.T., Larson, R.R. and Neuzil, S.G., 1983. Submicroscopic ( < 1 ~tm) mineral contents of vitrinites in selected bituminous coal beds. Ron Geoley, (Ed.), Microbeam Analysis. San Francisco Press, San Francisco. Mukherjee, B. and Dutta, R., 1950. A note on the constituents of the ashes of Indian Coal determined spectrographically. Fuel., 29 (8) : 190-192. Mukherjee, B. and Ghosh, A., 1976. Distribution and behaviour of trace elements in some Permian coals of India. Indian Mineral., 17: 23-30. Palmer, C.A. and Filby, R.H., 1983. Determination of mode of occurrence of trace elements in the Upper Freeport coal bed using size and density separation procedure. International Conference on Coal Science, Pittsburgh, PA, pp. 365-368. Palmer, C.A. and Filby, R.H., 1984. Distribution of trace elements in coal from the Powhatan No. 6 mine, Ohio. Fuel, 63: 318-328. Pareek, H.S., 1971. Coal Resources of India. Mem. Geol. Surv. India, Vol. 88, 575 pp. Sharma, N.L. and Ram, K.S.V., 1978. Introduction to Geology of coal and Indian coalfields, Dhanbad Publishers, New Sketch Press, Bihar, India, 146 pp. Stopes, M.C., 1919. On the four visible ingredients in banded bituminous coals; studies in the composition of coal No. 1. Proc. R. Soc. London, 90B: 470-487.