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Trace elements in minerals of German bituminous coals
International Journal of Coal Geology, 14 ( 1989 ) 137-153 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
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T r a c e e l e m e n t s in m i n e r a l s of G e r m a n b i t u m i n o u s coals WILHELM PICKHARDT
Neckarstrafle 16, 4300 Essen 18, Fed. Rep. of Germany (Received and accepted December 6, 1988)
ABSTRACT Pickhardt, W., 1989. Trace elements in minerals of German bituminous coals. In: W. Pickhardt {Editor), Erich Stach Memorial Issue. Int. J. Coal. Geol., 14: 137-153. From coal seams of the Ruhr District, samples of the main mineral groups: sulphides, carbonates, clay minerals and coal tonsteins of different origin and composition were isolated. In these samples the contents of following trace elements with environmental relevance were determined: arsenic, beryllium, cadmium, chrome, cobalt, copper, lead, manganese, mercury, molybdenum, nickel, strontium, uranium, vanadium and zinc. In addition, a sample of superclean coal before and after demineralization was examined, to elucidate which trace elements are bound either to the organic substance of the coal or predominantly to the different mineral groups.
INTRODUCTION
The growing impact on the environment by the activities of various industries, and also by the great increase in power production with more and more emissions of sulphur dioxide, oxides of nitrogen, carbon dioxide and also of heavy-metal oxides has become a great problem. The use of hard coal in power plants for electricity generation, too, is of public interest in this respect. During the last 20 years the German coal mining industry has carried out in its research institute, Bergbau-Forschung GmbH in Essen, a number of research projects with regard to environmental protection in order to investigate the possibilities to reduce environmental impact when using hard coal and to offer suitable process engineering solutions of this problem. As to the preparation of hard coal, our effects were first concentrated on the reduction of the sulphur content of steam coals by means of coal preparation measures. In a further comprehensive research, program investigations have been made into "trace elements in the German hard coal, their distribution during combustion in power plants and the biological effects of particulates emitted from power plants". Further investigations in superclean coal samples 0166-5162/89/$03.50
© 1989 Elsevier Science Publishers B.V.
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which are used for the production of activated coal and activated coke, have confirmed that most of the trace elements in the coal are located in the mineral groups intergrown with the coal and not in the organic substance, that means: trace elements can be reduced together with the ash content during the coal preparation process. Special investigations into the trace-element contents of the different mineral groups intergrown with the coal of our German coal districts have started only recently; this paper deals with the first results. S A M P L E S FOR T H I S I N V E S T I G A T I O N S
For the investigations the following samples have been chosen: - 1 sample of s u p e r c l e a n coal which was analyzed after depyrization and demineralization; - 1 k a o l i n i t e sample which was separated from a coal seam and which consisted of pure kaolinite; 1 illite sample which was separated from a coal seam, too, consisting of pure illite; - 5 "coal t o n s t e i n " samples of different types from 5 different seams which have been made available by Dr. K. Burger and which have been investigated petrographically; - 1 e p i g e n e t i c p y r i t e sample resulting from a special preparation by means of the Deister table; the pyrite concentrate was separated by a float- and sinktest with bromoform at a density of > 2.83 kg/dm3; - 1 s y n g e n e t i c p y r i t e sample from the roof bank of a drilling core of the Katharina seam, the border seam between Westfal A/Westfal B; - 2 e p i g e n e t i c calcite samples from the drill core of a seam in the exploration zone of the Ruhr District; therefrom, 1 sample was handpicked, that is chosen by means of a stereo magnifying glass, one sample separated by a floatand sink-test at a density of 2.6 kg/dm3; - 1 e p i g e n e t i c calcite sample from a calcite vein, about 3 cm wide, in the hanging side of a seam in the eastern part of the Ruhr District, and - 1 s y n g e n e t i c s i d e r i t e sample from a drill core of a seam of the Ibbenbiiren District, the northern continuation of the Ruhr District. -
INVESTIGATION METHODS
After sample preparation polished sections and in some cases thin sections were microscopically tested as to their "purity" and afterwards investigated by X-ray. In view of the trace-element analysis all samples were decarbonized at low temperatures (LTA). Some individual samples (epigenetic calcite and coaltonsteins) were analyzed by microprobe in the scanning-electron-microscope in addition to reflected-light and fluorescence microscopical investigation. For the trace-element analyses atomic absorption spectrometry (AAS) and
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atomic absorption spectrometry with inductively coupled plasma and mass spectrometry ( I C P / M S ) were used. REMARKS Coal as organogenic sedimentary rock has formed under heavily changing conditions of sedimentation. The chemo-physical reactions during coalification depending on the water submergence are very complicated and have changed very much as well. The syngenetic mineral inclusions and the mineral turn overs depend on many factors. The epigenetic minerals in the joints and cavities of the coal ground mass depend on the changing concentrations of the elements in the mineral solutions. The individual crystal lattices of the different minerals are, moreover, accessible in a different way for other ions and other elements. The conclusion to be drawn from these conditions, all of which, could by no means be mentioned here, is, that it is to be expected that the trace-element concentrations in the individual minerals and mineral groups in dependence of the forming conditions must be very different. The trace-element concentrations are definitely not comparable with the results of investigations of samples from ore deposits which have formed under different conditions. RESULTS Table 1 shows the mean trace-element contents of different sedimentary rocks - shales, limestones, sandstones - together with the average content of German coal originating from all districts with an ash content of < 10%. On the whole, 102 coal samples were investigated. The values for the other sedimentary rocks have been taken from geochemical tables. As can be seen from this table, the shales contain the highest concentrations of nearly all trace elements while hard coal has concentrations which are approximately between the shales and the limestones or sandstones respectively. Some trace elements obviously show extreme significant maxima in the sedimentary rocks; these are: - chromium, cobalt, copper, nickel and vanadium in the shales, strontium is enriched by a factor of 10 to 20 in shales and - as can be expected in limestones compared to sandstones and hard coal and the elements beryllium and zinc are much more abundant in shales than in the other sedimentary rocks and hard coal. -
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Assessing the analytic data from this table as well as from the following tables which are the average values of numerous analyses it has to be taken into account that individual values may scatter more or less around these averages. This is due to both samples as well as analyses.
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TABLE 1 Average contents of trace elements (mg/kg) in sedimentary rocks and German hard coals ( < 10% ash)
arsenic (As) beryllium (Be) cadmium {Cd ) chrome (Cr) cobalt (Co) copper (Cu) lead (Pb) mercury (Hg) molybdenum (Mo) nickel (Ni) strontium (Sr) uranium (U) vanadium (V) zinc (Zn)
shale
limestone
sandstone
hard coal
6.6 3 0.3 100 20 57 20 0.4 2 95 450 3.2 130 80
1 <1 0.04 11 0.1 4 9 0.04 0.4 20 610 2.2 20 20
1 <1 < 0.1 35 0.3 < 10 7 0.03 0.2 2 20 0.45 20 15
4 <2 < 0.5 10 10 20 14 <0.5 2 25 35 <1 30 35
One can seen, however, a clear differentiation between individual trace elements during the formation of sedimentary rocks. The trace-element contents of German hard coals represented in this first table show the average values of prepared or high-grade coals for coke making and for power generation. Table 2 contains the trace-element contents of a superclean coal and its residues after depyritization and in addition after demineralization. This analysis serves to show the trace-element contents in the superclean coal and in the completely depyritized and subsequently demineralized coal, that means in the pure organic coal substance. Although this is, indeed, only one isolated measurement, it gives, nevertheless, information on the amount of trace-element contents in the coal ground mass. This provides information which trace elements exist in measurable concentration in the organic coal matrix. This table shows that taking a superclean coal sample with an ash content of 1.10%, after depyritization and demineralization there remains a coal ground mass showing only low contents of: less than 1 m g / k g of cadmium, lead, manganese, mercury and uranium, -however, measurable contents of chromium, cobalt, copper, molybdenum, nickel, vanadium and, to a lesser degree - strontium and zinc. One can start from these values of the depyritized or demineralized coal in order to make the following statement: The elements cobalt, molybdenum and uranium are predominantly linked together with the organic substance of coal. -
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TABLE 2 Average c o n t e n t s of trace elements (mg/kg) in superclean coal and after depyritization and
demineralization
water % ash (dry) % sulphur (dry) % arsenic ( A s ) beryllium ( B e ) cadmium ( C d ) chrome (Cr) cobalt {Co ) copper (Cu) lead ( P b ) manganese (Mn) mercury (Hg) molybdenum {Mo) nickel ( N i ) strontium ( S r ) uranium ( U ) vanadium (V) zinc {Zn)
superclean coal
after depyritization
after demineralization
1.16 1.0 0.92 3.5 1 < 1 4 4 10 < 1 30 < 1 3 8 20 < 1 16 10
---1 1 < 1 4 4 7 < 1 < 1 < 1 2 7 5 < 1 10 < 10
---n.d. < 1 < 1 5 3 6 < 1 < 1 < 1 2 6 2 < 1 10 < 10
n.d. = not determined.
- The elements beryllium, nickel, vanadium and to some extent also chromium and mercury are linked to both the mineral groups as well as to the organic substance of coal. - The other elements studied are mainly contained in the minerals. In the first table with the average contents of trace elements in different sedimentary rocks the values stated for shales were taken from geochemical tables. Table 3 shows values of trace-element analyses which have been obtained from pure clay minerals separated from thin layers in coal seams. Both clay minerals, kaolinite and illite, show significant differences in some elements: - The concentrations of chromium, cobalt, copper, nickel, strontium, vanadium and zinc are higher in the illite sample than in the kaolinite sample. With some reservation, the values for uranium with 16 mg/kg are undoubtedly too high and for vanadium with 3 mg/kg too low. The value for vanadium should be assessed with special discretion. - Beryllium, lead and manganese are found in both samples in about the same concentration. This holds also for the elements cadmium, mercury and molybdenum although at a lower concentration level. Compared with the values for shales in Table 1 the values for the illite sample
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TABLE 3 Average contents of trace elements (mg/kg) in isolated clay minerals
arsenic (As) beryllium (Be) cadmium (Cd) chrome (Cr) cobalt (Co) copper (Cu) lead (Pb) manganese (Mn) mercury (Hg) molybdenum (Mo) nickel {Ni) strontium (Sr) uranium (U) vanadium (V) zinc (Zn)
Kaolinite isolated
Illite isolated
2 5 < 1 20 1 30 20 10 < 1 <1 20 55 16 3 ( .9) 10
4 5 <1 220 6 50 20 80 <1 <1 50 160 1 250 30
are about of the same order of magnitude. Only the elements nickel, strontium and zinc show higher concentrations in shales than in the illite sample. The samples of 5 coal tonsteins from Dr. Burger which were investigated are chosen from the following seams of the Ruhr District: Hagen 4 - coal tonstein, a mixed-layer tonstein of the middle Westfalian C, Zollverein 3 - coal tonstein, a granular tonstein of the lower Westfalian B, Zollverein 8 - coal tonstein, a "f~Tbergangs/pseudomorphous" tonstein (according to Dr. Burger) of the lower Westfalian B, - Karl 2 - coal tonstein, a typical pseudomorphous tonstein of the middle Westfalian A and Wilhelm - coal tonstein, a crystal tonstein of the middle Westfalian A. Table 4 contains a list of the results. At first glance it strikes that the contents of trace elements are, apart from three exceptions, very different. As to the exceptions, it can be stated that: the elements cadmium and mercury are contained in all tonstein samples to less than 1 mg/kg; the contents of the elements beryllium, nickel, uranium and cobalt scatter with factors of about 2 to 10; all other contents vary within the tonstein samples with the factors of about 20 to 100. The greatest scatter show the following elements: chromium with 2 mg/kg {Hagen 4-tonstein) to up to 210 mg/kg in Karl 2tonstein, that is a factor of about 100; -
-
-
-
-
-
-
-
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TABLE 4 Average contents of trace elements {mg/kg) in different coal-tonsteins
arsenic (As) beryllium (Be) cadmium (Cd) chrome (Cr) cobalt (Co) copper (Cu) lead ( P b ) manganese ( M n ) mercury (Hg) molybdenum (Mo) nickel (Ni) strontium (Sr) uranium {U) vanadium (V) zinc (Zn)
Hagen 4
Zollv. 3
Zollv. 8
Karl 2
Wilhelm
0.3 3 < 1 2 8 20 10 10 < 1 < 1 90 25 8 10 10
17 3 < 1 50 5 400 50 400 <1 20 150 590 3 50 20
4 4 < 1 110 15 120 180 70 < 1 10 140 230 9 90 50
5 6 < 1 210 50 30 60 100 < 1 1 220 100 8 200 400
3 7 < 1 60 6 20 80 9 < 1 4 190 860 1 10 10
a r s e n i c with 0.3 m g / k g (Hagen 4-tonstein) to up to 17 mg/kg (Zollverein 3tonstein), that is a factor of 50, - m a n g a n e s e with 10 m g / k g (Hagen 4-tonstein) to up to 400 m g / k g (Zollverein 3-tonstein) and z i n c with 10 m g / k g (Hagen 4-tonstein) to up to 400 m g / kg (Karl 2-tonstein), that is a factor of 40. all other elements have scatter ranges around factors of 10 to 35. Particularly low are the trace-element contents in Hagen 4-tonstein. Only the elements uranium, zinc, vanadium and cobalt are present in the same order of magnitude as in some of the other tonstein samples. Also in comparison with the concentrations in the pure clay mineral samples (kaolinite and illite Table 3 ) the Hagen 4-tonstein is relatively "poor" in trace elements. Maximum contents of arsenic, copper, manganese and strontium can be observed in the Zollverein 3-tonstein, whereas the Karl 2-tonstein shows higher contents of chromium, nickel, vanadium and zinc. Maximum values for lead (200 mg/kg) and strontium (860 mg/kg) are contained in the tonsteins Zollverein 8 and Wilhelm, respectively. The formation conditions and the original material are certainly of decisive importance for the great differences in the trace element concentrations of the various tonstein samples. The accessory minerals as for instance apatite, zirconia, diaspore and partly also sulfides as well as carbonates which can be observed in the tonsteins in different quantities and different association are of great importance. Whether the trace-element combinations with the stated maximum values -
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can contribute to the identification of a tonstein has to be proven in future by regional investigations in order to obtain a basis for statistics. The figures on Plate 1 give an idea of the samples which have been used for the trace-element analysis in sulphides and carbonates. Both, polished-sections micrographs with reflected light as well as micrographs from the scanning-electron-microscope have been taken as well as trace analyses by means of energy-dispersive X-ray (EDAX) of mineral impurities on the mineral surfaces have been made. It shows how much the different modern investigation techniques can successfully supplement the conventional coal petrographic investigation methods - primarily the reflected-light microscope. Figure 1 on Plate 1 shows the section of an epigenetic pyrite concentrate from fine-coal preparation on the Deister table with mainly idiomorphous pyrite crystals. Small coal residues, as for instance on the grain in the upper right part of the figure, were removed before trace analysis by means of plasma-low-temperature decarbonization. During preparation of the samples of epigenetic pyrite the two FeS2-modifications (markasite and pyrite) can, naturally, not be separated; however, it can, indeed, be assumed that their trace-element contents are not extremely different although their crystal lattice and also their formation temperatures are different. Figure 2 on Plate 1 shows a picture which is well known to all coal petrographers: A coal sample which is completely peppered and intimately intergrown with syngenetic silt-grade pyrite crystals and framboids. We know such figures especially from seams which are superposed by marine sediments; this sample originates of the roof part of the Katharina seam in the Ruhrcoal District, the boundary seam between Westphal A and B with the famous goniatite-horizon in the roof. For this sample we had to take a drill core from the exploration zone as this seam is no longer exploited due to this pyrite-intergrowth since in this case there is no preparation technique available to obtain an acceptable sulphur value in the coking and steam coals. As typical representative for a syngenetic pyrite sample the famous Katharina seam once again came back into favour. The material for one of the two epigenetic calcite samples in Figure 3 on Plate 1 has also been taken from a drill core, namely from the seam K, middle Westphal B. As mineral intergrowth the seam coal contains almost exclusively epigenetic calcite and only impurities of pyrite. On the basis of this seam sample two individual preparations have been produced and investigated, one subsample (a) by means of dense-medium separation, the other sample (b) hand-picked under the stereomicroscope. As will be seen in Table 5, considerable differences in the trace-element contents are nevertheless noticeable.
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TRACEELEMENTSIN MINERALSOF GERMANBITUMINOUSCOALS
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As one example for minerals of different genesis in the German hard coal the Figure 4 on the Plate 1 shows a rare occurrence of syngenetic siderite in a seam of the northern continuation of the Ruhr District intersected by drilling. The seam has a thickness of about 1 m; about 70 cm of this seam show this intimate siderate-coal intergrowth. The macroscopic picture of this split of seam is identical to a sapropelic coal although this seam coal has the rank of anthracite. The appearance is similar as for instance the pyrite facies in the Katharina seam shown before; only the siderite crystals are more densely interstratified in the coal. To some extent the individual crystal are also agglomerated to greater lenses. As an additional sample of an epigenetic calcite we took sampling material from a more than 20-mm-thick calcite vein directly from the roof of a seam. The roof consists of a fine-grained clayey sandstone with this thicker calcite joint. The sampling material has carefully been broken out of this joint. Although this sample (c) does not originate directly from the coal itself, it delivers, nevertheless, analytical values for epigenetic calcite. Figures 1 until 4 on Plate 2 shows the surface of a cleavage face of the crystals and can clearly and impressively observe the complete cleavability of calcite down into the/~m-range on this micrograph from the scanning-electron-microscope taken from individual excavations out of the crystal structure and texture. In the lower part of the individual micrographs the respective scale for the magnification is shown. Figures 5 until 8 on Plate 2 shows once again the surface of a cleavage face of this epigenetic calcite sample; here again, the complete cleavability is clearly visible, and on the surface there are some impurities from other minerals in the size of some ~um.Even such minute particles can still be analyzed and identified today by the modern investigation methods and techniques. First of all we made an EDAX analysis of such a particle. Such a particle is to be seen in Figure 1 on the Plate 3 in the middle of this micrograph of the scanning-electron-microscope. Its size is 2-3 #m. The EDAX analysis clearly shows by means of the pronounced peaks for nickel and sulphur that the mineral traces consist of millerite, the nickel sulphide (NiS). As shown by Figures 2 until 5 on Plate 3 the same particle has been investigated by means of an X-ray microprobe in order to confirm the preceding PLATE2 1-4. Perfect cleavage of epigenetic calcite in scanning-electron-micrograph (different magnification ). 5-8. Perfect cleavage of epigenetic calcite in the same sample in scanning-electron-micrograph with traces of other minerals on the surface {different magnification).
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TRACEELEMENTSIN MINERALSOFGERMANBITUMINOUSCOALS
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TABLE 5 Average contents of trace elements (mg/kg) in sulphides and carbonates of different origin pyrite
calcite (a) epigenetic
calcite (b) epigenetic
calcite siderite epigenetic syngenetic
12 8 26 70 75 180 200 2800 6 20 150 750 6 250 6000
5 3 18 10 70 390 100 8600 <1 4 60 120 1 50 2000
2 1 1 3 40 320 2 1800 <1 1 235 n.d. <1 6 9 (? )
epigenetic syngenetic arsenic (As) beryllium (Be) cadmium (Cd) chrome (Cr) cobalt (Co) copper (Cu) lead (Pb) manganese (Mn) mercury (Hg) molybdenum (Mo) nickel (Ni) strontium (Sr) uranium (U) vanadium (V) zinc (Zn)
98 <1 24 n.d. 45 620 3650 n.d. 6 4 180 100 n.d. 212 3840
100 18 <1 200 85 410 1300 500 6 70 360 330 8 400 300
100 4 3 130 45 100 20 n.d. <1 10 100 n.d. <1 90 350
result. T h e l i g h t - p o i n t d i s t r i b u t i o n m a k e s clear t h a t t h e c r y s t a l c o n t a i n s o n l y s u l p h u r a n d nickel a n d t h a t t h e iron as well as t h e calcium show no emission. In T a b l e 5 one c a n see t h e results o f t h e t r a c e - e l e m e n t analyses in t h e seq u e n c e in w h i c h was p r e s e n t e d t h e individual samples in f o r m of t h e microg r a p h or t h e s c a n n i n g - e l e c t r o n - m i c r o s c o p e m i c r o g r a p h respectively. As w i t h t h e samples of t h e coal t o n s t e i n s , h e r e again, we find with t h e sulphides a n d c a r b o n a t e s v e r y strongly v a r y i n g c o n t e n t s of all trace e l e m e n t s e x c e p t for cobalt. C o m p a r i n g t h e t r a c e - e l e m e n t c o n t e n t s of epigenetic a n d syngenetic pyrite h i g h e r c o n t e n t s for c a d m i u m , lead a n d zinc in epigenetic p y r i t e are to be observed, w h e r e a s t h e e l e m e n t s b e r y l l i u m , cobalt, m o l y b d e n u m , nickel, s t r o n t i u m a n d v a n a d i u m in s y n g e n e t i c p y r i t e are r e p r e s e n t e d in h i g h e r c o n c e n t r a tion. T h e values for c a d m i u m (24 m g / k g ) in t h e epigenetic p y r i t e a n d for b e r y l l i u m in t h e syngenetic p y r i t e (18 m g / k g ) seem to be relatively high. Since, however, c a d m i u m is a c c o m p a n y i n g zinc, t h e c a d m i u m value m i g h t be realistic, p a r t i c u l a r l y due to t h e values in one o f t h e epigenetic calcium samples w h i c h are r e l a t i v e l y high as well. O n t h e whole, 9 e l e m e n t s show differences w i t h factors r a n g i n g f r o m 2 t o 20. T h e two epigenetic calcite samples a a n d b in t h e middle of t h e table have
PLATE 3 1. Impurity of particle from Plate 2 (Figs. 5-8) with result of EDAX analysis. 2-5. Results of Xray-micro-probe of this particle.
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been taken from a seam sample; I mentioned already that the subsample in the left column was prepared by dense-medium separation and the other subsample, by handpicking. The majority of the elements in these two samples shows - not quite comprehensible - considerable differences, too. The differences for the contents of arsenic, manganese, molybdenum, strontium, uranium, vanadium and zinc are particularly striking and cannot be explained exactly. The sample of the calcite vein in the direct roof of the seam from which we saw the scanning-electron-microscope micrograph (right column) shows on the other hand, by far lower trace-element levels compared with the other calcite samples. In this sample the relatively high nickel content of 235 mg/kg is striking; this is most probably to be attributed to the millerite crystals (NiS) which have been identified. The value for zinc is undoubtedly to be explained by analysis uncertainties. The syngenetic siderite sample shows very low contents of trace elements. Only the value for arsenic about 100 m g / k g is for carbonates very high and is of the same order of magnitude as the arsenic contents in the pyrite samples. Unfortunately, the determination of manganese interfered with the iron content so that the results could not be assessed. No doubt, the siderite sample contains also a significant manganese content. If a comparison is made between the trace-element contents of these two mineral groups independent of the formation conditions, there are only two elements which are represented in significantly different amounts, namely: - lead, which is, as can be expected, represented in the sulphides with higher concentrations, and - manganese, which shows expectedly in the carbonates higher contents than in the sulphides. For the other elements the different contents compensate in the sum more or less for the two mineral groups in dependence on the formation conditions. This is an astonishing result. However what shows the picture comparing all mineral groups of this investigation. CONCLUSION
Table 6 is compiled to compare the ranges of the trace-element contents of the different mineral groups: - The content of arsenic is relatively high in pyrite and also in syngenetic siderite. In the clay minerals and also partly in the coal tonsteins it is relatively low. - B e r y l l i u m can reach maximum values in syngenetic pyrite. In the mineral groups contents are generally of about the same order of magnitude. - C a d m i u m is present in the clay minerals and in the coal tonsteins only in ppb-amounts. In the epigenetic sulphides and carbonates, however, higher contents (up to 26 ppm), parallel to higher zinc contents, are observed.
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TABLE 6 Range of average contents of trace elements (mg/kg) in the different mineral groups of German hard coals and superclean coal
arsenic (As) beryllium (Be) cadmium (Cd) chrome (Cr) cobalt (Co) copper (Cu) lead (Pb) manganese ( M n ) mercury (Hg) molybdenum (Mo) nickel (Ni) strontium (Sr) uranium (U) vanadium (V) zinc (Zn)
kaolinite/illite
coaltonsteins
sulphides
carbonates
superclean coal
2-4 -5 < 1 20-220 1-6 30-50 -20 10-80 < 1 < 1 20-50 50-160 1-16 3-250 10-30
0.3-17 3-7 < 1 2-210 5-50 20-400 10-200 10-400 < 1 < 1-20 90-220 25-860 1-9 10-200 10-400
~ 100
2-100 1-8 1-26 3-130 40-75 100-390 2-200 1800-8600 -6 1-20 60-235 120-750 -6 -250 350-6000
3.5 1 < 1 4 4 10 < 1 30 < 1 3 8 20 < 1 16 10
-18 -24 -200 45-85 410-620 1300-3650 -500 ~6 4-70 180-360 100-330 -8 210-400 300-3840
occurs in all mineral groups in strongly varying quantities with maximum values of about the same order of magnitude. In general, the trace element c o b a l t is contained in the sulphides and carbonates in higher concentrations than in the clay minerals and the coal tonsteins. - C o p p e r shows higher concentrations in the sulphides than in the carbonates and the clay minerals. - L e a d is a very environmentally-relevant trace element. Nevertheless, it was used for our drinking water pipes for many centuries. It is to be observed significantly with higher contents in the sulphides; in the other mineral groups it is less abundant. M a n g a n e s e is actually an unimportant trace element and cannot be classified as environmentally-relevant. Manganese has partly been determined as supplement only. It is evident that it is present in the carbonates in higher concentrations than in the other mineral groups. - In general, the environmentally-relevant element m e r c u r y can be traced in concentrations in the ppb-range in the minerals; only in the sulphides and some epigenetic carbonates it can be present in concentrations of up to 6 mg/ kg. - Molybdenum shows in the syngenetic sulphides partly higher contents than in the other mineral groups. - The trace element n i c k e l is predominantly bound to the sulphide and car- Chromium
-
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bonate mineral groups but also to the coal tonsteins whereas the pure clay minerals show low contents. Strontium is present in all mineral groups. It is an environmentally non relevant element in the proper sense and has not always been analyzed. In general it can be identified, as expected, in the carbonates in higher concentrations. - The trace element uranium shows no enrichment typical for minerals even though in general some lower quantities can be identified in the clay minerals. - The trace element zinc is clearly enriched in the sulphides but surprisingly also in the carbonate minerals. In this respect an "impurity" of the epigenetic calcite with pyrite crystals may be of some importance. The clay minerals and coal tonsteins, except the Karl 2-tonstein, show considerable lower contents of zinc. The values of the superclean coal on the right-hand side of Table 6 serve for information.
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ACKNOWLEDGEMENTS
The investigations have been carried out at Bergbau-Forschung GmbH, the research institute of the German coal mining industry and were sponsored by the "Ministerium fiir Wirtschaft, Mittelstand und Technologie" des Bundeslandes Nordrhein-Westfalen. I wish to thank the colleagues at Bergbau-Forschung for the analyses, especially Mrs. S. Griitzner for the investigations with the scanning-electronmicroscope and also Dr. K. Burger for providing of the coal-tonstein samples.
REFERENCES Babu, S.P., 1975. Trace elements in fuel. Advances in Chemistry Series 141, American Chemical Society, Washington. Bergbau-Betriebsblatt 22 022, 1980. Rohstoffuntersuchungen im Steinkohlenbergbau; Bestimmung der Gehalte an Spurenelementen. FABERG, Essen. Fiedler, H.I. and RSsler, H.J., 1988. Spurenelemente in der Umwelt. Ferdinand Enke Verlag, Stuttgart. Gerhard, L., Kautz, K., Pickhardt, W., Scholz, A. and Zimmermeyer, G., 1985. Untersuchung der Spurenelementverteilung bei der Verbrennung von Steinkohle in drei Kraftwerken. VGB Kraftwerkstechnik, 65: 753-763. Gluskoter, H.J. et al., 1977. Trace elements in coal: Occurrence and distribution. Ill. State Geol. Survey, Circ. 499. Hatch, J.R., Gluskoter, H.J. and Lindahl, P.C., 1976. Sphalerite in coals from the Illinois Basin. Econ. Geol., 71: 613-624. Kautz, K., Kirsch, H. and Laufhiitte, D.W., 1975. Ober Spurenelemente in Steinkohlen und den daraus entstehenden Reingasst~iuben. VGB Kraftwerkstechnik, 55: 672-676. Kautz, K., Pickhardt, W., Riepe, W., Schaaf, R., Scholz, A. and Zimmermeyer, G., 1984. Spure-
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nelemente in der Steinkohle, ihre Verteilung bei der Verbrennung und ihre biologische Wirkung. Gltickauf-Forschungsh., 45: 228-237. Kirsch, H., Schirmer, V. and Schwarz, G., 1980. Die Herkunft der Spurenelemente Zn, Cd und V in Steinkohlen und ihr Verhalten bei der Verbrennung. VGB Kraftwerkstechnik, 60: 814-824. Kirsch, H., Rehme, H.J., Reichel, H.H. and Schwarz, G., 1981. Zum Verhalten der anorganischen Substanzen bei der Verbrennung von Steinkohlen in Wirbelschichtfeuerungen. VGB Kraftwerkstechnik, 61: 482-492. Love, L.G., Coleman, M.L. and Curtis, G., 1983. Diagenetic pyrite formation and sulphur isotope fractionation associated with a Westphalian marine incursion, northern England. Trans. R. Soc. Edinburgh, 74: 165-182. Matsumoto, R. and Iijima, A., 1981. Origin and diagenetic evolution of Ca-Mg-Fe carbonates in some coalfields of Japan. Sedimentology, 28: 239-259. Miller, R.N., Zarzab, R.F. and Given, P.H., 1979. Determination of mineral matter contents of coals by low temperature ashing. Fuel, 58: 4-10. Nicholls, G.D., 1968. The geochemistry of coal-bearing strata. In: D.G. Murchison and T.S. Westoll (Editors), Coal and Coal-bearing Strata. Oliver and Boyd, Edinburgh, pp. 269-307. RSsler, H.J. and Lange, H., 1976. Geochemische Tabellen. Ferdinand Enke Verlag, Stuttgart. Scholz, A., 1987. Das Vorkommen von Spurenelementen in Steinkohlen. Gltickauf-Forschungsh., 48: 99-103. Spears, D.A. and Caswell, S.A., 1986. Mineral matter in coals: cleat minerals and their origin in some coals from the English Midlands. Int. J. Coal. Geol., 6: 107-125. Stach, E., Mackowsky, M.Th., Teichmtiller, M., Taylor, G.H., Chandra, D. and Teichmtiller, R., 1982. Coal Petrology. Gebrtider Borntr~iger, Stuttgart, Berlin, 535 pp.
137
T r a c e e l e m e n t s in m i n e r a l s of G e r m a n b i t u m i n o u s coals WILHELM PICKHARDT
Neckarstrafle 16, 4300 Essen 18, Fed. Rep. of Germany (Received and accepted December 6, 1988)
ABSTRACT Pickhardt, W., 1989. Trace elements in minerals of German bituminous coals. In: W. Pickhardt {Editor), Erich Stach Memorial Issue. Int. J. Coal. Geol., 14: 137-153. From coal seams of the Ruhr District, samples of the main mineral groups: sulphides, carbonates, clay minerals and coal tonsteins of different origin and composition were isolated. In these samples the contents of following trace elements with environmental relevance were determined: arsenic, beryllium, cadmium, chrome, cobalt, copper, lead, manganese, mercury, molybdenum, nickel, strontium, uranium, vanadium and zinc. In addition, a sample of superclean coal before and after demineralization was examined, to elucidate which trace elements are bound either to the organic substance of the coal or predominantly to the different mineral groups.
INTRODUCTION
The growing impact on the environment by the activities of various industries, and also by the great increase in power production with more and more emissions of sulphur dioxide, oxides of nitrogen, carbon dioxide and also of heavy-metal oxides has become a great problem. The use of hard coal in power plants for electricity generation, too, is of public interest in this respect. During the last 20 years the German coal mining industry has carried out in its research institute, Bergbau-Forschung GmbH in Essen, a number of research projects with regard to environmental protection in order to investigate the possibilities to reduce environmental impact when using hard coal and to offer suitable process engineering solutions of this problem. As to the preparation of hard coal, our effects were first concentrated on the reduction of the sulphur content of steam coals by means of coal preparation measures. In a further comprehensive research, program investigations have been made into "trace elements in the German hard coal, their distribution during combustion in power plants and the biological effects of particulates emitted from power plants". Further investigations in superclean coal samples 0166-5162/89/$03.50
© 1989 Elsevier Science Publishers B.V.
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which are used for the production of activated coal and activated coke, have confirmed that most of the trace elements in the coal are located in the mineral groups intergrown with the coal and not in the organic substance, that means: trace elements can be reduced together with the ash content during the coal preparation process. Special investigations into the trace-element contents of the different mineral groups intergrown with the coal of our German coal districts have started only recently; this paper deals with the first results. S A M P L E S FOR T H I S I N V E S T I G A T I O N S
For the investigations the following samples have been chosen: - 1 sample of s u p e r c l e a n coal which was analyzed after depyrization and demineralization; - 1 k a o l i n i t e sample which was separated from a coal seam and which consisted of pure kaolinite; 1 illite sample which was separated from a coal seam, too, consisting of pure illite; - 5 "coal t o n s t e i n " samples of different types from 5 different seams which have been made available by Dr. K. Burger and which have been investigated petrographically; - 1 e p i g e n e t i c p y r i t e sample resulting from a special preparation by means of the Deister table; the pyrite concentrate was separated by a float- and sinktest with bromoform at a density of > 2.83 kg/dm3; - 1 s y n g e n e t i c p y r i t e sample from the roof bank of a drilling core of the Katharina seam, the border seam between Westfal A/Westfal B; - 2 e p i g e n e t i c calcite samples from the drill core of a seam in the exploration zone of the Ruhr District; therefrom, 1 sample was handpicked, that is chosen by means of a stereo magnifying glass, one sample separated by a floatand sink-test at a density of 2.6 kg/dm3; - 1 e p i g e n e t i c calcite sample from a calcite vein, about 3 cm wide, in the hanging side of a seam in the eastern part of the Ruhr District, and - 1 s y n g e n e t i c s i d e r i t e sample from a drill core of a seam of the Ibbenbiiren District, the northern continuation of the Ruhr District. -
INVESTIGATION METHODS
After sample preparation polished sections and in some cases thin sections were microscopically tested as to their "purity" and afterwards investigated by X-ray. In view of the trace-element analysis all samples were decarbonized at low temperatures (LTA). Some individual samples (epigenetic calcite and coaltonsteins) were analyzed by microprobe in the scanning-electron-microscope in addition to reflected-light and fluorescence microscopical investigation. For the trace-element analyses atomic absorption spectrometry (AAS) and
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atomic absorption spectrometry with inductively coupled plasma and mass spectrometry ( I C P / M S ) were used. REMARKS Coal as organogenic sedimentary rock has formed under heavily changing conditions of sedimentation. The chemo-physical reactions during coalification depending on the water submergence are very complicated and have changed very much as well. The syngenetic mineral inclusions and the mineral turn overs depend on many factors. The epigenetic minerals in the joints and cavities of the coal ground mass depend on the changing concentrations of the elements in the mineral solutions. The individual crystal lattices of the different minerals are, moreover, accessible in a different way for other ions and other elements. The conclusion to be drawn from these conditions, all of which, could by no means be mentioned here, is, that it is to be expected that the trace-element concentrations in the individual minerals and mineral groups in dependence of the forming conditions must be very different. The trace-element concentrations are definitely not comparable with the results of investigations of samples from ore deposits which have formed under different conditions. RESULTS Table 1 shows the mean trace-element contents of different sedimentary rocks - shales, limestones, sandstones - together with the average content of German coal originating from all districts with an ash content of < 10%. On the whole, 102 coal samples were investigated. The values for the other sedimentary rocks have been taken from geochemical tables. As can be seen from this table, the shales contain the highest concentrations of nearly all trace elements while hard coal has concentrations which are approximately between the shales and the limestones or sandstones respectively. Some trace elements obviously show extreme significant maxima in the sedimentary rocks; these are: - chromium, cobalt, copper, nickel and vanadium in the shales, strontium is enriched by a factor of 10 to 20 in shales and - as can be expected in limestones compared to sandstones and hard coal and the elements beryllium and zinc are much more abundant in shales than in the other sedimentary rocks and hard coal. -
-
-
Assessing the analytic data from this table as well as from the following tables which are the average values of numerous analyses it has to be taken into account that individual values may scatter more or less around these averages. This is due to both samples as well as analyses.
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TABLE 1 Average contents of trace elements (mg/kg) in sedimentary rocks and German hard coals ( < 10% ash)
arsenic (As) beryllium (Be) cadmium {Cd ) chrome (Cr) cobalt (Co) copper (Cu) lead (Pb) mercury (Hg) molybdenum (Mo) nickel (Ni) strontium (Sr) uranium (U) vanadium (V) zinc (Zn)
shale
limestone
sandstone
hard coal
6.6 3 0.3 100 20 57 20 0.4 2 95 450 3.2 130 80
1 <1 0.04 11 0.1 4 9 0.04 0.4 20 610 2.2 20 20
1 <1 < 0.1 35 0.3 < 10 7 0.03 0.2 2 20 0.45 20 15
4 <2 < 0.5 10 10 20 14 <0.5 2 25 35 <1 30 35
One can seen, however, a clear differentiation between individual trace elements during the formation of sedimentary rocks. The trace-element contents of German hard coals represented in this first table show the average values of prepared or high-grade coals for coke making and for power generation. Table 2 contains the trace-element contents of a superclean coal and its residues after depyritization and in addition after demineralization. This analysis serves to show the trace-element contents in the superclean coal and in the completely depyritized and subsequently demineralized coal, that means in the pure organic coal substance. Although this is, indeed, only one isolated measurement, it gives, nevertheless, information on the amount of trace-element contents in the coal ground mass. This provides information which trace elements exist in measurable concentration in the organic coal matrix. This table shows that taking a superclean coal sample with an ash content of 1.10%, after depyritization and demineralization there remains a coal ground mass showing only low contents of: less than 1 m g / k g of cadmium, lead, manganese, mercury and uranium, -however, measurable contents of chromium, cobalt, copper, molybdenum, nickel, vanadium and, to a lesser degree - strontium and zinc. One can start from these values of the depyritized or demineralized coal in order to make the following statement: The elements cobalt, molybdenum and uranium are predominantly linked together with the organic substance of coal. -
-
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TABLE 2 Average c o n t e n t s of trace elements (mg/kg) in superclean coal and after depyritization and
demineralization
water % ash (dry) % sulphur (dry) % arsenic ( A s ) beryllium ( B e ) cadmium ( C d ) chrome (Cr) cobalt {Co ) copper (Cu) lead ( P b ) manganese (Mn) mercury (Hg) molybdenum {Mo) nickel ( N i ) strontium ( S r ) uranium ( U ) vanadium (V) zinc {Zn)
superclean coal
after depyritization
after demineralization
1.16 1.0 0.92 3.5 1 < 1 4 4 10 < 1 30 < 1 3 8 20 < 1 16 10
---1 1 < 1 4 4 7 < 1 < 1 < 1 2 7 5 < 1 10 < 10
---n.d. < 1 < 1 5 3 6 < 1 < 1 < 1 2 6 2 < 1 10 < 10
n.d. = not determined.
- The elements beryllium, nickel, vanadium and to some extent also chromium and mercury are linked to both the mineral groups as well as to the organic substance of coal. - The other elements studied are mainly contained in the minerals. In the first table with the average contents of trace elements in different sedimentary rocks the values stated for shales were taken from geochemical tables. Table 3 shows values of trace-element analyses which have been obtained from pure clay minerals separated from thin layers in coal seams. Both clay minerals, kaolinite and illite, show significant differences in some elements: - The concentrations of chromium, cobalt, copper, nickel, strontium, vanadium and zinc are higher in the illite sample than in the kaolinite sample. With some reservation, the values for uranium with 16 mg/kg are undoubtedly too high and for vanadium with 3 mg/kg too low. The value for vanadium should be assessed with special discretion. - Beryllium, lead and manganese are found in both samples in about the same concentration. This holds also for the elements cadmium, mercury and molybdenum although at a lower concentration level. Compared with the values for shales in Table 1 the values for the illite sample
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TABLE 3 Average contents of trace elements (mg/kg) in isolated clay minerals
arsenic (As) beryllium (Be) cadmium (Cd) chrome (Cr) cobalt (Co) copper (Cu) lead (Pb) manganese (Mn) mercury (Hg) molybdenum (Mo) nickel {Ni) strontium (Sr) uranium (U) vanadium (V) zinc (Zn)
Kaolinite isolated
Illite isolated
2 5 < 1 20 1 30 20 10 < 1 <1 20 55 16 3 ( .9) 10
4 5 <1 220 6 50 20 80 <1 <1 50 160 1 250 30
are about of the same order of magnitude. Only the elements nickel, strontium and zinc show higher concentrations in shales than in the illite sample. The samples of 5 coal tonsteins from Dr. Burger which were investigated are chosen from the following seams of the Ruhr District: Hagen 4 - coal tonstein, a mixed-layer tonstein of the middle Westfalian C, Zollverein 3 - coal tonstein, a granular tonstein of the lower Westfalian B, Zollverein 8 - coal tonstein, a "f~Tbergangs/pseudomorphous" tonstein (according to Dr. Burger) of the lower Westfalian B, - Karl 2 - coal tonstein, a typical pseudomorphous tonstein of the middle Westfalian A and Wilhelm - coal tonstein, a crystal tonstein of the middle Westfalian A. Table 4 contains a list of the results. At first glance it strikes that the contents of trace elements are, apart from three exceptions, very different. As to the exceptions, it can be stated that: the elements cadmium and mercury are contained in all tonstein samples to less than 1 mg/kg; the contents of the elements beryllium, nickel, uranium and cobalt scatter with factors of about 2 to 10; all other contents vary within the tonstein samples with the factors of about 20 to 100. The greatest scatter show the following elements: chromium with 2 mg/kg {Hagen 4-tonstein) to up to 210 mg/kg in Karl 2tonstein, that is a factor of about 100; -
-
-
-
-
-
-
-
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TABLE 4 Average contents of trace elements {mg/kg) in different coal-tonsteins
arsenic (As) beryllium (Be) cadmium (Cd) chrome (Cr) cobalt (Co) copper (Cu) lead ( P b ) manganese ( M n ) mercury (Hg) molybdenum (Mo) nickel (Ni) strontium (Sr) uranium {U) vanadium (V) zinc (Zn)
Hagen 4
Zollv. 3
Zollv. 8
Karl 2
Wilhelm
0.3 3 < 1 2 8 20 10 10 < 1 < 1 90 25 8 10 10
17 3 < 1 50 5 400 50 400 <1 20 150 590 3 50 20
4 4 < 1 110 15 120 180 70 < 1 10 140 230 9 90 50
5 6 < 1 210 50 30 60 100 < 1 1 220 100 8 200 400
3 7 < 1 60 6 20 80 9 < 1 4 190 860 1 10 10
a r s e n i c with 0.3 m g / k g (Hagen 4-tonstein) to up to 17 mg/kg (Zollverein 3tonstein), that is a factor of 50, - m a n g a n e s e with 10 m g / k g (Hagen 4-tonstein) to up to 400 m g / k g (Zollverein 3-tonstein) and z i n c with 10 m g / k g (Hagen 4-tonstein) to up to 400 m g / kg (Karl 2-tonstein), that is a factor of 40. all other elements have scatter ranges around factors of 10 to 35. Particularly low are the trace-element contents in Hagen 4-tonstein. Only the elements uranium, zinc, vanadium and cobalt are present in the same order of magnitude as in some of the other tonstein samples. Also in comparison with the concentrations in the pure clay mineral samples (kaolinite and illite Table 3 ) the Hagen 4-tonstein is relatively "poor" in trace elements. Maximum contents of arsenic, copper, manganese and strontium can be observed in the Zollverein 3-tonstein, whereas the Karl 2-tonstein shows higher contents of chromium, nickel, vanadium and zinc. Maximum values for lead (200 mg/kg) and strontium (860 mg/kg) are contained in the tonsteins Zollverein 8 and Wilhelm, respectively. The formation conditions and the original material are certainly of decisive importance for the great differences in the trace element concentrations of the various tonstein samples. The accessory minerals as for instance apatite, zirconia, diaspore and partly also sulfides as well as carbonates which can be observed in the tonsteins in different quantities and different association are of great importance. Whether the trace-element combinations with the stated maximum values -
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can contribute to the identification of a tonstein has to be proven in future by regional investigations in order to obtain a basis for statistics. The figures on Plate 1 give an idea of the samples which have been used for the trace-element analysis in sulphides and carbonates. Both, polished-sections micrographs with reflected light as well as micrographs from the scanning-electron-microscope have been taken as well as trace analyses by means of energy-dispersive X-ray (EDAX) of mineral impurities on the mineral surfaces have been made. It shows how much the different modern investigation techniques can successfully supplement the conventional coal petrographic investigation methods - primarily the reflected-light microscope. Figure 1 on Plate 1 shows the section of an epigenetic pyrite concentrate from fine-coal preparation on the Deister table with mainly idiomorphous pyrite crystals. Small coal residues, as for instance on the grain in the upper right part of the figure, were removed before trace analysis by means of plasma-low-temperature decarbonization. During preparation of the samples of epigenetic pyrite the two FeS2-modifications (markasite and pyrite) can, naturally, not be separated; however, it can, indeed, be assumed that their trace-element contents are not extremely different although their crystal lattice and also their formation temperatures are different. Figure 2 on Plate 1 shows a picture which is well known to all coal petrographers: A coal sample which is completely peppered and intimately intergrown with syngenetic silt-grade pyrite crystals and framboids. We know such figures especially from seams which are superposed by marine sediments; this sample originates of the roof part of the Katharina seam in the Ruhrcoal District, the boundary seam between Westphal A and B with the famous goniatite-horizon in the roof. For this sample we had to take a drill core from the exploration zone as this seam is no longer exploited due to this pyrite-intergrowth since in this case there is no preparation technique available to obtain an acceptable sulphur value in the coking and steam coals. As typical representative for a syngenetic pyrite sample the famous Katharina seam once again came back into favour. The material for one of the two epigenetic calcite samples in Figure 3 on Plate 1 has also been taken from a drill core, namely from the seam K, middle Westphal B. As mineral intergrowth the seam coal contains almost exclusively epigenetic calcite and only impurities of pyrite. On the basis of this seam sample two individual preparations have been produced and investigated, one subsample (a) by means of dense-medium separation, the other sample (b) hand-picked under the stereomicroscope. As will be seen in Table 5, considerable differences in the trace-element contents are nevertheless noticeable.
146
W. PICKHARDT
TRACEELEMENTSIN MINERALSOF GERMANBITUMINOUSCOALS
147
As one example for minerals of different genesis in the German hard coal the Figure 4 on the Plate 1 shows a rare occurrence of syngenetic siderite in a seam of the northern continuation of the Ruhr District intersected by drilling. The seam has a thickness of about 1 m; about 70 cm of this seam show this intimate siderate-coal intergrowth. The macroscopic picture of this split of seam is identical to a sapropelic coal although this seam coal has the rank of anthracite. The appearance is similar as for instance the pyrite facies in the Katharina seam shown before; only the siderite crystals are more densely interstratified in the coal. To some extent the individual crystal are also agglomerated to greater lenses. As an additional sample of an epigenetic calcite we took sampling material from a more than 20-mm-thick calcite vein directly from the roof of a seam. The roof consists of a fine-grained clayey sandstone with this thicker calcite joint. The sampling material has carefully been broken out of this joint. Although this sample (c) does not originate directly from the coal itself, it delivers, nevertheless, analytical values for epigenetic calcite. Figures 1 until 4 on Plate 2 shows the surface of a cleavage face of the crystals and can clearly and impressively observe the complete cleavability of calcite down into the/~m-range on this micrograph from the scanning-electron-microscope taken from individual excavations out of the crystal structure and texture. In the lower part of the individual micrographs the respective scale for the magnification is shown. Figures 5 until 8 on Plate 2 shows once again the surface of a cleavage face of this epigenetic calcite sample; here again, the complete cleavability is clearly visible, and on the surface there are some impurities from other minerals in the size of some ~um.Even such minute particles can still be analyzed and identified today by the modern investigation methods and techniques. First of all we made an EDAX analysis of such a particle. Such a particle is to be seen in Figure 1 on the Plate 3 in the middle of this micrograph of the scanning-electron-microscope. Its size is 2-3 #m. The EDAX analysis clearly shows by means of the pronounced peaks for nickel and sulphur that the mineral traces consist of millerite, the nickel sulphide (NiS). As shown by Figures 2 until 5 on Plate 3 the same particle has been investigated by means of an X-ray microprobe in order to confirm the preceding PLATE2 1-4. Perfect cleavage of epigenetic calcite in scanning-electron-micrograph (different magnification ). 5-8. Perfect cleavage of epigenetic calcite in the same sample in scanning-electron-micrograph with traces of other minerals on the surface {different magnification).
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W. PICKHARDT
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TRACEELEMENTSIN MINERALSOFGERMANBITUMINOUSCOALS
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TABLE 5 Average contents of trace elements (mg/kg) in sulphides and carbonates of different origin pyrite
calcite (a) epigenetic
calcite (b) epigenetic
calcite siderite epigenetic syngenetic
12 8 26 70 75 180 200 2800 6 20 150 750 6 250 6000
5 3 18 10 70 390 100 8600 <1 4 60 120 1 50 2000
2 1 1 3 40 320 2 1800 <1 1 235 n.d. <1 6 9 (? )
epigenetic syngenetic arsenic (As) beryllium (Be) cadmium (Cd) chrome (Cr) cobalt (Co) copper (Cu) lead (Pb) manganese (Mn) mercury (Hg) molybdenum (Mo) nickel (Ni) strontium (Sr) uranium (U) vanadium (V) zinc (Zn)
98 <1 24 n.d. 45 620 3650 n.d. 6 4 180 100 n.d. 212 3840
100 18 <1 200 85 410 1300 500 6 70 360 330 8 400 300
100 4 3 130 45 100 20 n.d. <1 10 100 n.d. <1 90 350
result. T h e l i g h t - p o i n t d i s t r i b u t i o n m a k e s clear t h a t t h e c r y s t a l c o n t a i n s o n l y s u l p h u r a n d nickel a n d t h a t t h e iron as well as t h e calcium show no emission. In T a b l e 5 one c a n see t h e results o f t h e t r a c e - e l e m e n t analyses in t h e seq u e n c e in w h i c h was p r e s e n t e d t h e individual samples in f o r m of t h e microg r a p h or t h e s c a n n i n g - e l e c t r o n - m i c r o s c o p e m i c r o g r a p h respectively. As w i t h t h e samples of t h e coal t o n s t e i n s , h e r e again, we find with t h e sulphides a n d c a r b o n a t e s v e r y strongly v a r y i n g c o n t e n t s of all trace e l e m e n t s e x c e p t for cobalt. C o m p a r i n g t h e t r a c e - e l e m e n t c o n t e n t s of epigenetic a n d syngenetic pyrite h i g h e r c o n t e n t s for c a d m i u m , lead a n d zinc in epigenetic p y r i t e are to be observed, w h e r e a s t h e e l e m e n t s b e r y l l i u m , cobalt, m o l y b d e n u m , nickel, s t r o n t i u m a n d v a n a d i u m in s y n g e n e t i c p y r i t e are r e p r e s e n t e d in h i g h e r c o n c e n t r a tion. T h e values for c a d m i u m (24 m g / k g ) in t h e epigenetic p y r i t e a n d for b e r y l l i u m in t h e syngenetic p y r i t e (18 m g / k g ) seem to be relatively high. Since, however, c a d m i u m is a c c o m p a n y i n g zinc, t h e c a d m i u m value m i g h t be realistic, p a r t i c u l a r l y due to t h e values in one o f t h e epigenetic calcium samples w h i c h are r e l a t i v e l y high as well. O n t h e whole, 9 e l e m e n t s show differences w i t h factors r a n g i n g f r o m 2 t o 20. T h e two epigenetic calcite samples a a n d b in t h e middle of t h e table have
PLATE 3 1. Impurity of particle from Plate 2 (Figs. 5-8) with result of EDAX analysis. 2-5. Results of Xray-micro-probe of this particle.
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W. PICKHARDT
been taken from a seam sample; I mentioned already that the subsample in the left column was prepared by dense-medium separation and the other subsample, by handpicking. The majority of the elements in these two samples shows - not quite comprehensible - considerable differences, too. The differences for the contents of arsenic, manganese, molybdenum, strontium, uranium, vanadium and zinc are particularly striking and cannot be explained exactly. The sample of the calcite vein in the direct roof of the seam from which we saw the scanning-electron-microscope micrograph (right column) shows on the other hand, by far lower trace-element levels compared with the other calcite samples. In this sample the relatively high nickel content of 235 mg/kg is striking; this is most probably to be attributed to the millerite crystals (NiS) which have been identified. The value for zinc is undoubtedly to be explained by analysis uncertainties. The syngenetic siderite sample shows very low contents of trace elements. Only the value for arsenic about 100 m g / k g is for carbonates very high and is of the same order of magnitude as the arsenic contents in the pyrite samples. Unfortunately, the determination of manganese interfered with the iron content so that the results could not be assessed. No doubt, the siderite sample contains also a significant manganese content. If a comparison is made between the trace-element contents of these two mineral groups independent of the formation conditions, there are only two elements which are represented in significantly different amounts, namely: - lead, which is, as can be expected, represented in the sulphides with higher concentrations, and - manganese, which shows expectedly in the carbonates higher contents than in the sulphides. For the other elements the different contents compensate in the sum more or less for the two mineral groups in dependence on the formation conditions. This is an astonishing result. However what shows the picture comparing all mineral groups of this investigation. CONCLUSION
Table 6 is compiled to compare the ranges of the trace-element contents of the different mineral groups: - The content of arsenic is relatively high in pyrite and also in syngenetic siderite. In the clay minerals and also partly in the coal tonsteins it is relatively low. - B e r y l l i u m can reach maximum values in syngenetic pyrite. In the mineral groups contents are generally of about the same order of magnitude. - C a d m i u m is present in the clay minerals and in the coal tonsteins only in ppb-amounts. In the epigenetic sulphides and carbonates, however, higher contents (up to 26 ppm), parallel to higher zinc contents, are observed.
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TABLE 6 Range of average contents of trace elements (mg/kg) in the different mineral groups of German hard coals and superclean coal
arsenic (As) beryllium (Be) cadmium (Cd) chrome (Cr) cobalt (Co) copper (Cu) lead (Pb) manganese ( M n ) mercury (Hg) molybdenum (Mo) nickel (Ni) strontium (Sr) uranium (U) vanadium (V) zinc (Zn)
kaolinite/illite
coaltonsteins
sulphides
carbonates
superclean coal
2-4 -5 < 1 20-220 1-6 30-50 -20 10-80 < 1 < 1 20-50 50-160 1-16 3-250 10-30
0.3-17 3-7 < 1 2-210 5-50 20-400 10-200 10-400 < 1 < 1-20 90-220 25-860 1-9 10-200 10-400
~ 100
2-100 1-8 1-26 3-130 40-75 100-390 2-200 1800-8600 -6 1-20 60-235 120-750 -6 -250 350-6000
3.5 1 < 1 4 4 10 < 1 30 < 1 3 8 20 < 1 16 10
-18 -24 -200 45-85 410-620 1300-3650 -500 ~6 4-70 180-360 100-330 -8 210-400 300-3840
occurs in all mineral groups in strongly varying quantities with maximum values of about the same order of magnitude. In general, the trace element c o b a l t is contained in the sulphides and carbonates in higher concentrations than in the clay minerals and the coal tonsteins. - C o p p e r shows higher concentrations in the sulphides than in the carbonates and the clay minerals. - L e a d is a very environmentally-relevant trace element. Nevertheless, it was used for our drinking water pipes for many centuries. It is to be observed significantly with higher contents in the sulphides; in the other mineral groups it is less abundant. M a n g a n e s e is actually an unimportant trace element and cannot be classified as environmentally-relevant. Manganese has partly been determined as supplement only. It is evident that it is present in the carbonates in higher concentrations than in the other mineral groups. - In general, the environmentally-relevant element m e r c u r y can be traced in concentrations in the ppb-range in the minerals; only in the sulphides and some epigenetic carbonates it can be present in concentrations of up to 6 mg/ kg. - Molybdenum shows in the syngenetic sulphides partly higher contents than in the other mineral groups. - The trace element n i c k e l is predominantly bound to the sulphide and car- Chromium
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bonate mineral groups but also to the coal tonsteins whereas the pure clay minerals show low contents. Strontium is present in all mineral groups. It is an environmentally non relevant element in the proper sense and has not always been analyzed. In general it can be identified, as expected, in the carbonates in higher concentrations. - The trace element uranium shows no enrichment typical for minerals even though in general some lower quantities can be identified in the clay minerals. - The trace element zinc is clearly enriched in the sulphides but surprisingly also in the carbonate minerals. In this respect an "impurity" of the epigenetic calcite with pyrite crystals may be of some importance. The clay minerals and coal tonsteins, except the Karl 2-tonstein, show considerable lower contents of zinc. The values of the superclean coal on the right-hand side of Table 6 serve for information.
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ACKNOWLEDGEMENTS
The investigations have been carried out at Bergbau-Forschung GmbH, the research institute of the German coal mining industry and were sponsored by the "Ministerium fiir Wirtschaft, Mittelstand und Technologie" des Bundeslandes Nordrhein-Westfalen. I wish to thank the colleagues at Bergbau-Forschung for the analyses, especially Mrs. S. Griitzner for the investigations with the scanning-electronmicroscope and also Dr. K. Burger for providing of the coal-tonstein samples.
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