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Selected elements in major minerals from bituminous coal as determined by INAA: implications for removing environmentally sensitive elements from coal
ELSJSWIER
International
Journal of Coal Geology 32 (1996) 151- 166
Selected elements in major minerals from bituminous coal as determined by INAA: implications for removing environmentally sensitive elements from coal C.A. Palmer, P.C. Lyons U.S.Geological
Survey, MS.
956 National Center, Reston, VA 20192, USA
Received 15 June 1995; accepted 8 September 1995
Abstract The four most abundant minerals generally found in Euramerican bituminous coals are quartz, kaolinite, illite and pyrite. These four minerals were isolated by density separation and handpicking from bituminous coal samples collected in the Ruhr Basin, Germany and the Appalachian basin, U.S.A. Trace-element concentrations of relatively pure (- 99+%) separates of major minerals from these coals were determined directly by using instrumental neutron activation analysis (INAA). As expected, quartz contributes little to the trace-element mass balance. Illite generally has higher trace-element concentrations than kaolinite, but, for the concentrates analyzed in this study, Hf, Ta, W, Th and U are in lower concentrations in illite than in kaolinite. Pyrite has higher concentrations of chalcophile elements (e.g., As and Se) and is considerably lower in lithophile elements as compared to kaolinite and illite. Our study provides a direct and sensitive method of determining trace-element relationships with minerals in coal. Mass-balance calculations suggest that the trace-element content of coal can be explained mainly by three major minerals: pyrite, kaolinite and illite. This conclusion indicates that the size and textural relationships of these major coal minerals may be a more important consideration as to whether coal cleaning can effectively remove the most environmentally sensitive trace elements in coal than what trace minerals are present. Keywords;
bituminous
coal; mass balance;
trace elements;
mineralogy;
Carboniferous;
environment
1. Introduction There elements
has long been an interest in trace elements in coal. Large data bases of trace in coals have been established by the U.S. Geological Survey (Zubovic et al.,
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Journal of Coal Geology 32 (1996) 151-166
1961; Swanson and Huffman, 1976; Bragg et al., 1994). Early work concentrated on organic-inorganic relations of trace elements. Goldschmidt (1935) postulated the occurrence of metal-organic complexes in coal and attributed the concentration of V, Co and Ni to the presence of these complexes. Zubovic (1966, 1976) determined organic affinities for 15 elements and suggested that metals with high organic affinity are present in chelates. Gluskoter et al. (1977) used float-sink methods to determine organic-inorganic associations of trace elements in coal. Lyons et al. (1989) calculated ratios of elemental concentrations in vitrinite as compared to whole coal to determine organic-inorganic associations. From these studies, it is clear that most trace elements are associated with the mineral matter in coal. Exceptions are Br (Lyons et al., 19891, Cl (Palmer and Filby, 1983) and Se (Palmer et al., 19931, all of which are known to be organically associated or have a tendency toward organic association. Chou (1991) suggests that chlorine ions are adsorbed on inner surfaces of micropores in macerals. Hower et al. (1994b) found that Cl and Br were concentrated in the very low ash, high vitrinite fraction. Several other elements are occasionally organically associated, such as Ge, B, and Sb (Gluskoter et al., 1977; Lyons et al., 1989). Recently, many studies have been conducted to identify the mode of occurrence of trace elements in coal. These works have become increasingly important because the chemical form of an element will affect its behavior during coal cleaning and combustion. Finkelman (1981) conducted studies by using scanning electron microscopy combined with an energy dispersive X-ray analyzer (SEM-EDXA) to identify trace elements in the coal matrix. This method was excellent in identifying heavy trace elements in minerals such as lead selinide (Pb and Se> and rare earth phosphates (rare earth elements). Also, SEM-EDXA has been successful in identifying minor elements in some major and minor minerals in coal. However, many trace elements in major minerals in coal are not detectable by this technique; detection limits are highly variable and mass dependent. This may lead to a somewhat distorted view of the importance of some “minerals” in the trace-element mass balance. Another microprobe technique, laser micro-mass spectrometry (LAMMA) was used by Lyons et al. (1987) and Morelli et al. (1988) to analyze vitrinites and other macerals. Although this technique is more sensitive for some trace elements (e.g. Li, Na and K), the results for elements of atomic weight greater than 100 are harder to interpret because of interferences by organic peaks in the mass spectra. Other microprobe techniques such as proton induced X-ray emission analysis (PIXE) (Chen et al., 1981; Minkin et al., 1982) also have been used. Several indirect methods have been used to obtain additional information on the mode of occurrence of inorganic constituents in coal. Nicholls (1968) related several trace elements to ash content. Palmer and Filby (1984) correlated mineralogy and trace-element content of size and density separates of low-temperature ash (LTA) from coal. Finkelman et al. (1990) and Palmer et al. (1993) used leaching studies to approximate the mode of occurrence of trace elements in coal. X-ray absorption fine structure (XAFS) has been used to determine the chemical form of several elements such as Cl, Ti, V, Cr, Mn, Zn, As, Se and Br (Huggins and Huffman, this volume). Over 100 minerals have been identified in coal (Finkelman, 198 1; Swaine, 1990). However, only a few of these minerals are common in bituminous coals from the United
CA. Palmer, P.C. Lyons/International
Journal of Coal Geology 32 (1996) 151-166
I53
States and Europe. The four most abundant minerals in Euramerican coals are quartz, pyrite, kaolinite and illite. Minor minerals that may be locally important include calcite (and other carbonates), mixed-layer clays (and other clays) and sphalerite. In this papel we will examine the trace-element content of selected mineral separates of these four major minerals. Because mixed-layer clays and illite are similar, especially in terms of X-ray diffraction, they will be considered together in this study. We will also consider together the two iron sulfides, pyrite and marcasite, for purposes of this study.
2. Samples
and procedures
The samples for this study came from a variety of sources. Each sample was chosen because of its high purity. The kaolinite and illite, from the Ruhr Basin of Germany, were splits of powdered samples described by Pickhardt (1989). The quartz came from two Appalachian tonstein beds: the Upper Banner coal bed of Wise County, Virginia, and the Fire Clay coal bed of Magoffin County, Kentucky (see Lyons et al., 1992). The pyrite was separated from the Pittsburgh (No. 8) coal bed of Belmont County, Ohio (Palmer and Filby, 1984) and the Upper Freeport coal bed of Indiana County, Pennsylvania (Palmer and Wandless, 1985). The pyrite, kaolinite and illite samples were all examined by X-ray diffraction (XRD) analysis; all peaks were attributed to the minerals in question. The quartz samples were handpicked under a low-power stereomicroscope, and the purest grains were selected. The samples were analyzed by instrumental neutron activation analysis (INAA). Each sample was irradiated in a nuclear reactor for 8 h at a flux of 3 X 10” neutrons/cm* s- ’ Using techniques described by Palmer and Baedecker (1989) to determine 29 elements, the samples were then counted on a Ge detector for about 4000 s, three or four days after irradiation, and for 10,000 or more seconds, six to eight weeks after irradiation.
3. Results Table 1 shows the results of the INAA analysis of the samples used in this study. The concentrations and the relative errors (one standard deviation) are also shown. For samples where an element was below the detection limit, “upper limits” were calculated based on the assumption that the minimum detectable peak was less than three standard deviations above background (Palmer and Baedecker, 1989). The upper limits were higher in cases where the sample size is very small, especially the handpicked quartz grains. The chemical data for the seven quartz samples, the two pyrite samples, the one kaolinite and the one illite samples are given in Table 1. As expected, quartz contained much lower concentrations of most trace elements than other minerals. Only Na, SC, Hf and Th were detected in all quartz samples. The remaining 25 elements in quartz, as determined by INAA, had one or more values below the detections limit; twelve elements (Cr, Co, Ni, Zn, As, Se, Nd, Tb, Yb, Ta, W and U) were below the detection limit in all seven quartz samples. The concentration of trace elements in kaolinite were generally lower than in illite except for Br, Sm, the heavy rare earth elements (Tb, Yb and Lu) and the lithophile
CA. Palmer, P.C. Lyons/International
Journal of Coal Geology 32 (1996) 151-166
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CA. Palmer, P.C. Lyons/International
Journal of Coal Geology 32 (1996) 151-166
elements (Hf, Ta, W, Th and U), which are characterized by large atomic radii. Uranium is enriched by nearly a factor of six in kaolinite over illite and by more than ten times over the other minerals studied. The reason for this is unclear but one possibility is micron-size uranium-containing components in the kaolinite. The concentrations of Na, K, SC, Cr, Rb, Cs, La, Ce and Nd in illite were greater than all of the other minerals included in this study. Illite has many sites available for substitution because of its layered structure and is known to be a “garbage pail” for trace elements. For example, LAMMA results showed more elements in illite than kaolinite (Lyons and Hosterman, 1989). The concentration of Ba was higher in only one quartz sample than in illite. The concentration of Sr (where detected) was greater for illite than for the other samples, but the upper limits reported in six samples were greater than the concentration reported for illite. The concentrations of K, Rb and Cs were about ten times greater in the illite sample than in any other sample, which is understandable because of the K-rich nature of this mineral. The concentration of Fe, Co, As, Se and Sb were higher in the pyrite samples than in any of the other samples. Pyrite is the only mineral in this study where the purity of the mineral can be assessed from the chemical data as well as the XRD data. For both of the pyrite samples in this study, the Fe concentration was 46.6% which is the stoichiometric value for pure pyrite. As, Se and Sb were enriched in both pyrite samples by four to nearly fifty times the highest value determined in other minerals. These elements are chalcophile elements that would be expected to substitute more readily into the pyrite (Fe&) structure than in the structures of any of the other minerals in this study. The association of As with pyrite is well documented (Fleischer, 1955; Finkelman, 1981). Minkin et al. (1979) found Se in pyrite. Finkelman (1981) also postulated that Sb could be concentrated in pyrite, and that much of the Sb must also be in other minerals or associated with the organic matrix because of the mass-balance problems. Cobalt, which can be a chalcophile element is generally considered a lithophile element in the Earth’s crust (Mason, 1966). Cobalt is clearly associated with illite as well as pyrite. Cobalt can easily substitute for Fe in the pyrite structure (Deer et al., 1962). Finkelman (1981) stated that most of the Co is associated with sulfides, but some Co also occurs in the clays, as shown in Table 2. The relatively high Co value in the illite, as shown in Table 2, agrees with the data of Finkelman (1981). Table 2 Comparison
of data from this study with the data of Pickhardt
Element
Cr co Ni Zn As Sr U
Kaolinite
(1989). Values in
Illite
ppm
Pyrite
Calcite
This study
Pickhardt
This study
Pickhardt
This study
Pickhardt
Pickhardt
8.8 0.943 < 26 23 2 < 110 25.3
20 1 20 10 0.52 55 16
161 12.4 36 36 4 189 4.25
220 6 50 30 2.35 160 1
21.6 18.5 nd 27.5 718 < 300 1
200 65 270 2070 9 215 4
28 62 155 2700 6 290 2.3
nd = not determined.
CA. Palmer, P.C. Lyons/International
Journal
qf Coal Geology 32 (1996) 151-166
157
4. Discussion Table 2 shows a comparison of the data determined in this study with data in Pickhardt (1989). In general, the data in our study agrees with Pickardt’s data. The comparison is not nearly as good as one would expect based on the fact that our samples are splits of his powdered samples and that the methods Pickardt used to analyze trace elements, i.e. atomic absorption and ICP-MS, generally compare well with INAA results techniques (Palmer and Klizas, in press). Differences in some elements of up to a factor of four are reported in the two data sets (see Table 2). Whether these differences are due to sampling or due to analytical problems can not be determined from our study. Differences reported for pyrite are much greater than differences in kaolinite and illite. This is not surprising because the pyrite samples were not taken from the same geographic locations and stratigraphic positions. The mineral content of coals differ due to various factors. A few coals will have significant concentrations of minerals that are absent or in trace quantities in most coals. Some minerals, especially epigenetic minerals, may be altered by substitution particularly in locations of hydrothermal activity or in areas of high concentration of trace metals. However, assuming that the trace-element content measured in the four minerals of our study are representative of coal in general, one can do mass-balance calculations to test whether or not one can account for the inorganic element content of a whole coal based on its known content of the four minerals in question. Information on both the mineral content and the trace-element content of coal are required in order to do these mass-balance calculations. Ideally the minerals should come from the same coal but pure clay minerals from the same coal are difficult to obtain. Relatively few papers report both trace-element content and mineral content of coal. As a result only a few samples have been characterized for both minerals and trace elements. The sample most intensively studied for mineral content and trace elements is from the Herrin (No. 6) coal bed of Illinois. Data were available from many laboratories for both mineral content and trace-element content (Finkelman et al., 1984). In addition to quartz (20%), pyrite (23%), kaolinite (14%) and illite and mixed-layer clays (31%) the Herrin sample also contained calcite (9%). With the exception of Sr and possibly Zn and Ni, we do not believe that the 9% calcite will significantly affect the material balance for the other elements determined in this study. Table 2 shows data for calcite from coal determined by Pickhardt (1989); the highest concentrations of trace elements are for Zn, Sr and Ni, in order of importance. Palmer (1983) presented data for two coal beds for both mineral and trace element content. For the Upper Freeport coal bed, samples were taken from three different benches. These benches have very different mineralological and trace-element contents. In addition, Hower et al. (1990, 1994a) presented mineralogical and limited trace-element data. The Springfield and Fire Clay data reported in these two papers are averages and do not represent individual coals. Table 3 shows the low temperature ash (LTA) content for each of these coals as well as the concentrations of each mineral on a LT,4 basis as determined by the various studies. All these coals are high volatile bituminous coal. Because there are large differences in mineralogy between these coals (quartz ranges from 9-20%; pyrite and marcasite ranges from O-67%; kaolinite ranges from
158
C.A. Palmer, P.C. Lyons/International
of Coal Geology 32 (1996) 151-166
Journal
Table 3 Low temperature ash (LTA) content and mineral content in percent for coals discussed in this paper. Numbers in parenthesis reference papers from which the data was taken
Appalachian Basin Coals Pittsburgh (No. 8) (1) Upper Freeport (I ) Bench I Bench 2 Bench 3 Fire Clay (2) Letcher County Floyd County Pike County Perry County Knott County
LTA
Quartz
Pyrite + Marcasite
Kaolinite
Illite + Mixed Layered Clays
Calcite
14
16
30
15
25
0
8.1 12.3 29.7
9 21 24
43 0 67
15 30 5
27 38 2
0 8 0
11.6 15.4 14.4 14.3 30.9
11 11 10 17 14
2 4 3 0 6
25 27 24 18 20
62 5 61 65 60
0 0 0 0 0
15.5 16.8
11 20
17 23
9 14
54 31
7 9
Illinois Basin Coals Springfield (3) Herrin No. 6 (4) (1) (2) (3) (4)
Data Data Data Data
taken taken taken taken
from from from from
Our Study I
Palmer (1983). Hower et al. (1994a,b). Hower et al. (1990). Finkelman et al. (1984)
Pickhardt n
Herrln = Herrln No.6
(1989)
,..-_ ~~_q~“rnh ‘In ’ nnw Freeport Bench 1 UF 2 = &per F reeport Bench 2 UF 3 = Upper F reeport Bench 3
!:;I
~19
._
”
Spring gfleld = SprIngfIeld No 9 FC Let = FIreClay.Letcher County FC Knott = Fire Clay,Knott County FC Pike = Fire Clay,Pike County FC Pery = FIreClay Perry County
Fig. 1. Ratios of Zn, As, Sr and U concentrations calculated by using the mass balance of the major minerals to the concentration of each trace element in the low-temperature ash (Table 4; this study) as compared with similar ratios using data from Pickhardt (1989) (Table 2).
C.A. Palmer, P.C. Lyons/International
Journal of Coal Geology 32 (1996) 1.5-166
159
S-30% and illite and mixed-layed clays ranges from 2-65%) these data are good to test the mass-balance calculations. Table 4 shows the average elemental concentrations for each mineral analyzed in this study. These concentrations were used to calculate the contribution of trace elements found in major minerals in coal to the total trace-element content of the LTA using the mineral contents given in Table 3. A ratio of this calculated concentration to the measured concentration in the LTA of less than one suggests that not all of the given element could be accounted for by the major minerals. As shown in Table 4, a ratio of greater than one suggests that the concentrations for the major minerals given do not fully represent the values in the sample of interest. At least 70% of the elements have ratios less than one. Although less than 30% of the elements in any coal had values within *25% of the expected value, most (nearly 60%) agreed within a factor of 2. Nearly all of those elements that did not agree within a factor of two had ratios less than one. Nearly 80% of the values for the samples reported by Hower et al. (1990, 1994a,b) have ratios of less than 0.5. This is due to the fact that Hower et al. (1994a,b) reported only 5 elements and two of those elements (Zn and Ni) have ratios of less than 0.5 for ~11 samples reported and Cr has ratios less than 0.7 for all samples reported. Zn may be associated with the calcite as indicated by Pickhardt (1989). who found an average of 2700 ppm Zn in his calcite. This concentration would raise the calculated
Cl Lt
Co NI
Cr
ierrln
Our study I
Co
PlttS
Pickhardt (1989) II
Herrm = Hernn (No 6)
CO
JF 1
Cr Co
I
UF2
Cr
!
co UF3
jcf
CO
pprlngfl
PC = Pl~sbwh 0% 8) UF I= Upper Freepott (Bench 1) UF 2 = Upper Freeport (Bench 2) UF 3 = Upper Freepott (Bench 3)
Sprmgfleld = Sprmgfleld (No 9) FC Let = Fire Clay.Letcher County FC Knott = Fire Clay, Knott CoiiQ FC Pike = Fire Clay,Plke County FC Perry Fire Clay, Perry Count\i ??
Fig. 2. Ratios of Cr, Co and Ni concentrations calculated by using the mass balance of the major minerals to the concentration of each trace element in the low-temperature ash (Table 4; this study) as compared with similar ratios using data of Pickhardt (1989) (Table 2).
Tb(ppm) Yb(ppm) Lu(ppm) Hf(ppm) Ta(ppm) W(ppm) Th(ppm) U(ppm)
Sm (ppm) Eu @pm)
As(ppm) Se(ppm) Br(ppm) Rb(ppm) Sr(ppm) Sb @pm) Cs (ppm) Ba(ppm) La (ppm) Ce @pm) Nd (ppm)
Co @pm) Ni (ppm) Zn @pm)
Fe (%)
Cr(ppm)
K (%I SC (ppm)
Na (%)
Pyrite average
0.026 0.02 0.55 22 47 18 nd 28 719 39 nd 6.00 <300 2.49 0.10 < 300 1.48 1.60 < 40 0.18 0.08 < 0.6
0.07 0.3 0.2 < 27 0.05 <4 i 290 < 80 <3 < 13 1.71 14 64 0.08 0.5 600 4.51 10 < 30 0.57 0.09 < 0.2 <1 0.03 0.78 <3 <2 1.97
0.425 0.146 11.2 8.8 0.287 0.943 < 26 23 0.52 <3 14.5 1.5 < 110 0.28 1.99 159 36.4 73 28.4 11.03 0.157 2.05 1.3 0.81 8.88 9.9 8.8 59.6 25.3
Kaolinite
0.665 5.64 24.5 167 1.46 12.4 36 36 2.35
0.28 1.87 9.32 58 11 8.2 11 21 166 8.88 3.80 90 71 0.88 8.86 344 33.8 56.8 17 4.86 0.55 0.54 2.48 0.31 3.04 2.02 2.86 15.3 5.09 5.49 305 30.5 57.9 nd 6.10 1.34 0.79 3.66 0.61 2.74 0.79 1.83 11.6 6.10
1.46
0.45 1.48 18 91 16 24 98 213 12 12 nd 79 183
Measured
-
*
Herrin (No. 6)
trace element data
Calculated
with measured
Illite
trace element content based on mineral compared
Quartz average
Table 4 Data for calculated
1.32 0.83
1.56
0.63 1.26 0.5 1 0.63 0.68 0.34 0.11 0.10 13.6 0.73 nd 1.14 0.39 0.60 1.61 1.13 1.11 0.98 nd 0.80 0.41 0.68 0.68 0.51 1.11 2.55
Ratio 0.25 1.5 8.0 50 14 8.8 9.0 21 216 12 3.6 74 57 1.0 7.2 283 29 49 15 4.4 0.46 0.5 1 2.3 0.29 2.8 2.0 2.6 15 5.2
-
Calculated 0.22 0.79 17 109 16 22 0 71 39 12 0 50 321 1.0 4.2 279 41 69 24 5.9 1.5 1.1 3.7 0.46 3.1 1.1 0 11 5.0
Measured
Pittsburgh **
(No. 8)
1.35 1.03
1.47 0.18 1.01 1.71 1.01 0.71 0.70 0.61 0.73 0.32 0.45 0.61 0.63 0.91 1.87 _
0.29 5.61 0.94 _
1.11 1.91 0.47 0.46 0.90 0.39
Ratio
Calculated 0.26 1.6 8.6 57 23 13 9.7 27 353 19 3.6 79 57 1.5 7.8 278 31 51 16 4.5 0.50 0.53 2.3 0.32 2.9 2.0 2.7 15 5.43
5.7 0.80 2.3 0.74 nd 11 nd
1.9
Measured 0.21 0.72 16.9 81 20 53 nd 111 685 17 nd 49 321 4.0 2.7 185 37 75 37 7.3 2.2
nd = not determined ~~ = value not calculated because measured ’ Data taken from Finkelman et al. (1984). ’’ Data taken from Palmer (1983).
Co (ppm) Ni (ppm) Zn (ppm) As @pm) Se (ppm) Br (ppm) Rb (ppm) Sr (ppm) Sb (ppm) Cs (ppm) Ba @pm) La (ppm) Ce (ppm) Nd @pm) Sm (ppm) Eu (ppm) Tb (ppm) Yb (ppm) Lu (ppm) Hf (ppm) Ta (ppm) W (ppm) Th (ppm) U (ppm)
Fe (%)
Cr (ppm)
K (%) SC (ppm)
Na (%)
1.41
1.60 0.18 0.38 2.86 1.50 0.82 0.68 0.42 0.62 0.23 0.29 0.42 0.40 1.22 2.76 _
0.24 0.51 1.11 _
Ratio 1.26 2.23 0.51 0.70 1.18 0.24 _
1) Measured 0.21 2.1 24 157 1.7 16 nd 268 3.3 8.2 nd 195 374 2.0 14 374 69 130 < 20 9.7 2.9 2.0 5.9 0.73 5.7 1.9 nd 21 7.3
**
1.27 1.26
0.76 0.23 0.46 0.68 0.62 0.84 2.01 _
0.56 0.23 0.20 0.80 1.15 0.66 0.60 _
0.08 0.3 1 0.00
Ratio 1.84 1.08 0.52 0.42 0.39 0.31
Upper Freeport (Bench 2)
or was below the detection limit
Calculated 0.39 2.3 13 66 0.67 5.0 14 21 1.0 0 6.46 109 85 0.41 11 431 46 78 24 7.3 0.67 0.93 4.0 0.46 4.8 3.8 4.6 26 9.2
value was not determined
**
Upper Freeport (Bench Calculated 0.43 2.6 14 77 2.6 6.5 16 24 30 1.5 6.7 125 94 0.56 13 469 51 86 27 7.9 0.77 0.98 4.3 0.49 5.1 3.9 5.0 21 9.5
Measured 0.22 2.2 25 123 4.3 18 nd 218 24 9.4 nd 125 455 2.5 10 404 74 132 47 8.7 2.5 2.0 5.5 0.74 5.0 1.3 nd 20 6.7 _____
**
1.35 1.41
1.Ol 0.21 0.23 1.23 1.16 0.69 0.65 0.57 0.91 0.30 0.48 0.78 0.67 1.02 2.95 _
0.11 1.27 0.16 _
Ratio 1.99 1.20 0.58 0.63 0.61 0.35 _
Upper Freeport (Bench 3)
71
_ _ _
22 28
-
105 2.4
Calculated _ _
Calculated 0.41 3.1 95 8.7 9.9 19 26 123 4.0 151 109 50
***
Measured nd nd 143 3.7 nd 116 85 nd nd nd 1332
* ’* *
Fire Clay Pike County
Measured 0.25 1.5 153 23.7 31.1 105 455 54 37 90 161 306
Springfield
* * * Data taken from Hower et al. (1990). * * * * Data taken from Hower et al. (1994a.b)
Co (ppm) Ni (ppm) Zn (ppm) As (ppm) Rb (ppm) Sr (ppm) Ba (ppm)
K (%) Cr (ppm) Fe (o/o)
Na (%o)
Co (ppm) Ni (ppm) Zn (ppm) As (ppm) Br (ppm) Rb (ppm) Sr (ppm) Ba (ppm)
K (%I Cr (ppm) Fe (%)
Na (o/n)
Table 4 (continued)
110 1.0 _ 23 28 _ _ _ 83
0.73 0.63 0.19 0.33 _
0.05
_
Calculated _ _
76
_ _
22 29 _
106 1.9
Calculated _
Ratio _
Ratio 1.63 2.11 0.62 0.37 0.32 0.18 0.06 2.28 0.11 1.68 0.68 0.16
***
Measured nd nd 329 4.2 nd 97 84 nd nd nd 1874
****
Fire Clay Perry County
Measured nd nd 293 5.2 nd 131 127 nd nd nd nd 1932
Fire Clay Letcher County
103 3.7 _ 21.6 27.9 _ _ _ 77.2
0.36 0.37 0.17 0.23 _ _ _ _ 0.04
0.04
0.24 0.33 _ _ _
0.33 0.24 _
_
Ratio
Calculated _ _
Ratio _
Measured nd nd 244 3.8 nd 78.4 60.4 nd nd nd nd 1711.8
**
Fire Clay Knott County
0.05
0.28 0.46 _ _ _ _
0.42 0.99 _
_
Ratio
C.A. Palmer, P.C. Lyons/International
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content to 243 ppm Zn in the Herrin No. 6 coal and would over account for the observed Zn content. If data of Pickhardt (1989) for Zn in minerals was used instead of the data in this study, 5 coals would have ratios greater than 1 (2 with ratios of 9 or more) instead of ratios generally less than 0.3; see Fig. 1. Therefore, the calculated Zn would be greater than the reported Zn. Ratios greater than 1 suggest that the elemental concentrations calculated for the minerals are not representative of the coals in this study. Ratios less than 1 suggest that minerals other than the major minerals may be present. In the case of Zn, sphalerite is often present, especially in Illinois basin coals (Cobb and Russell, 1976) such as the Herrin No. 6 and the Springfield No. 9. Other coals such as the Pittsburgh No. 8 and the Fire Clay coal are Appalachian coals. These coals are also low in Zn and are not as likely to contain sphalerite. We were also not able to account for the observed Ni content based on our mineral data. This may be because Ni was not determined in pyrite because a Ni standard was not included in that particular irradiation. Pickhardt (1989) found 270 ppm Ni in pyrite which, if taken together with nickel in other minerals, would represent about 75% of the expected Ni in the LTA of the Herrin No. 6. coal. If one uses Pickhardt’s data for Ni in the other minerals, the ratio is even higher (see Fig. 2). Most of the Cr appears to be associated with illite but less than 75% of the Cr is accounted for by the major minerals using the concentrations in this study. Use of values for Cr reported by Pickhardt (I 989) yields much greater ratios up to nearly 2 (see Fig. 2). Finkelman (1981) stated that the association of chromium in coals remains a mystery and concluded that much of the Cr may be associated with the clays. Less than 16% of the Ba was accounted for in any of the coals reported by Hower et al. (1990, 1994a) even though Ba was slightly over accounted for in the other coals. This suggests that Ba is in barite or Ba phosphates below the X-ray diffraction detection limit in the generally high-Ba Kentucky coals. Generally less than one third of the Co and Sr is accounted for in the minerals analyzed. Use of the values of Pickhardt (1989) for these elements gives ratios closer to one (see Figs. 1 and 2). Arsenic has a ratio of over 5 in 2 coals and ratios of over 1 in 5 coals. This is because one of the pyrite samples used in our study has over 0.1% As. This seems quite high, although Fleischer (1955) reported that 10% of the pyrites in his study had over 1% As and Hower et al. (in press) reported variable As in pyrite as high as 3.5 wt.% in the Fire Clay coal. On the other hand, Pickhardt’s value of 99 ppm As may also be too high an average for pyrite as it would yield a ratio of slightly over 2 in one coal. Arsenic can readily enter the pyrite structure and, therefore, is highly variable in most coals. K and Cs each had a ratio of greater than 2 in only one sample. The only other elements having a ratio greater than 2 are Ta in four of the five coals measured; the ratio for the fifth coal was 1.87. Ta was only found in kaolinite and illite. Clearly, the clay of the Pickhardt (1989) samples are not representative for Ta in the coals of our study.
5. Conclusions Surprisingly, the agreement between the reported values of the trace elements in the LTA of the Illinois and Appalachian coals of our study and the calculated values using a
164
C.A. Palmer, P. C. Lyons /International
Journal of Coal Geology 32 (I 996) 151- I66
mass-balance approach is within a factor of 2 for most elements. Exceptions include Zn, Ba and Ta. Most of the trace-element contents in the coals considered in this study are accounted for by kaolinite, illite and pyrite. Quartz, which is a major mineral in most Euramerican coals, does not contain major quantities of any element except possibly for Ba (Table 1). Calcite, where present, is a major contributor of Sr, Zn and Ni. Coal-cleaning methods designed to remove four of these major minerals (illite, kaolinite, pyrite and calcite) prior to burning of the coal should remove most of the environmentally sensitive trace elements. Exceptions are Br and other organically fixed trace elements. Also, in cases where these minerals are dispersed as ultrafine ( < 2 pm> grains in the macerals of coals, they may remain in the coal and contribute to airborne contamination.
Acknowledgements We thank Dr. W. Pickhardt for providing the clay samples for this study. We also thank Drs. W.H. Orem and P.A. Baedecker of the U.S. Geological Survey, Dr. J.C. Hower of the Center for Applied Energy Research, and Dr. C.-L. Chou of the Illinois State Geological Survey for reviewing this paper.
References Bragg, L.J., Oman, J.K., Tewalt, S.J., Oman, C.L., Rega, N.H., Washington, P.M. and Finkelman, R.B., 1994. U.S. Geological Survey Coal Quality (COALQUAL) Database: Version 1.3. U.S. Geol. Surv. Open-File Rep, 94-205 (CD-ROM). Chen, J.R., Kneis, H., Martin, B., Nobiling, R.,. Traxel, K., Chou, E.C.T. and Minkin, J.A., 1981. Trace elemental analysis of bituminous coals using the Heidelberg proton microprobe. Nucl. Instrum. Methods, 181: 151-157. Chou, C-L., 1991. Distribution and forms of chlorine in Illinois Basin coals. In: J Stringer and D.D. Banerjee (Editors), Coal Science and Technology 17: Chlorine in Coals. Elsevier, New York. pp. 11-13. Cobb, J.C. and Russell, S.L., 1976. Sphalerite mineralization in coal seams of the Illinois Basin. Geol. Sot. Am., Abst. Programs, 8(6): 816. Deer, W.A., Howie, R.A. and Zussman, J., 1962. Rock Forming Minerals, Volume 1, Ortho and Ring Silicates. Wiley, New York, 371 pp. Finkelman, R.B., 1981. Modes of Occurrence of Trace Elements in Coal. U.S. Geol. Surv. Open-File Rep., 81-99, 301 pp. Finkelman, R.B., Fiene, F.L., Miller, R.L. and Simon, F.O., 1984. Interlaboratory Comparison of Mineral Constituents in a Sample from the Herrin (No. 6) Coal Bed from Illinois, U.S. Geol. Surv. Circ., 932, 42 PP. Finkelman, R.B., Palmer, C.A., Krasnow, M.R., Amscavage, P.J., Sellers, G.A. and Dulong, F.T., 1990. Combustion and leaching behavior of elements in the Argonne Premium Coal samples. Energy Fuels, 4: 755-766. Fleischer, M., 1955. Minor elements in some sulfide minerals, Econ. Geol., 50th Anniversary Volume, Part II. pp. 970-1024. Gluskoter, H.J., Ruth, R.R., Miller, W.G., Cahill, R.A., Dreher, G.B. and Kuhn, J.K., 1977. Trace Elements in Coal: Occurrence and Distribution. Ill. State Geol. Surv. Circ., 499, 154 pp. Goldschmidt, V.M., 1935. Rare element in coal ashes. Ind. Eng. Chem., 27(9J: 1100-I 102. Huggins, F.E. and Huffman, G.P., this volume. Modes of occurrence of trace elements in coal from XAFS spectrometry.
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Hower, J.C., Wild, G.D., Pollock, J.D., Trinkle, E.J., Bland, A.E. and Fiene. F.L., 1990. Petrography, geochemistry, and mineralogy of the Springfield (Western Kentucky No. 91 coal bed. J. Coal Qual.. 9(31: 90-100. Hower, J.C., Andrews, W.M., Jr., Wild, G.D., Eble, C.F., Dulong, F.T. and Salter, T.L.,1994a. Quality of the Fire Clay coal bed, southeastern Kentucky. J. Coal Qual., 13(l): 13-26. Hower, J.C., Taulbee, D.N., Rimmer, S.N. and Morrell, L.G., 1994b. Petrographic and geochemical anatomy of lithotypes from the Blue Gem coal bed, southeastern Kentucky. Energy Fuels, g(3): 7 19-728. Hower, J.C., Robertson, J.D., Wong, AS., Eble, C.F. and Rupert, L.F., in press. Arsenic and lead concentrations in the Pond Creek and Fire Clay coal bed, Eastern Kentucky. J Coal Qual. Lyons, PC. and Hosterman, J.W., 1989. Laser microprobe (LAMMA 10001 Chemical analysis of clay minerals: potential application to in situ analysis of clay minerals in coal. In: C.R. XI Congr. lnt Stratigraphie et de Geologic du Carbonifere, Beijing 1987, 5, pp. 264-272. Lyons, P.C., Hercules. D.M., Morrelli, J.J., Sellers, G.A., Mattem. D., Thompson-Rizer, C.L., Brown. F.W. and Millay, M.A., 1987. Application of laser microprobe (LAMMA 10001 to “fingerprinting” of coal constituents in bituminous coal. Int. J. Coal Geol., 7: 185-194. l,yona, P.C.. Palmer, C.A., Bostick, N.H., Fletcher, J.D., Dulong, F.T., Brown, F.W., Brown, Z.-A., Krasnow. M.R. and Romankiw, L.A., 1989. Chemistry and origin of minor and trace elements in vitrinite concentrates from a rank series from the eastern United States, England and Australia. In: P.C. Lyons and B. Alpem (Editors), Coal: Classification, Mineralogy, Trace-element Chemistry, and Oil and Gas Potential. Int. J. Coal Geol., 13: 481-527. Lyons, P.C., Outerbridge, W.F., Triplehom, D.M., Evans, H.T., Jr., Congdon, R.D., Capiro, M., Hess. J.C. and Nash, W.P.. 1992. An Appalachian isochron: A kaolinized Carboniferous air-fall volcanic ash deposit (tonstein). Geol. Sot. Am. Bull., 104: 1515-1527. Mason, B., 1966. Principles of Geochemistry. Wiley, New York, 329 pp. Minkin, J.A., Chao. E.C.T. and Thompson, C.L., 1979. Distribution of elements in coal macerals and minerals: determination by electron microprobe. Am. Chem. Sot. Div. Fuel Chem. Preprint, 24(l): 242-249. Minkin, J.A., Chao, E.C.T., Thompson, CL., Nobiling, R. and Blank, H., 1982. Proton microprobe determination of elemental concentrations in coal macerals. Scanning Electron Microsc.. I : 175- 181. Morelli, J.J., Hercules, D.M., Lyons, P.C., Palmer, C.A. and Fletcher, J.D., 1988. Using Laser micro mass specrometry with the LAMMAinstrument for monitoring relative elemental concentrations in vitrinite. Mikrochim. Acta, III: 105-l 18. Nicholls, G.D., 1968. The geochemistry of coal-bearing strata. In: D. Murchison and T.S. Westoll (Editors). Coal and Coal-bearing Strata. Elsevier, New York, pp. 269-307. Palmer, C.A.. 1983. The Distribution of Trace Elements in Minerals found in Coal. PhD. Dissertation. Washington State Univ., Pullman, Washington, 213 pp. Palmer. C.A. and Baedecker, P.A., 1989. The determination of 41 elements in whole coal by instrumental neutron activation analysis. In: D.W. Golightly and F.O. Simon (Editors), Methods for Sampling and Inorganic Analysis of Coals. U.S. Geol. Surv. Bull., 1823: 27-34. 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 procedures. In: Proc. 1983 Int. Conf. Coal Science, International Energy Agency, pp. 365-368. Palmer, C.A. and Filby, R.H., 1984. Distribution of trace elements in coal from the Powhatan #6 mine, Ohio. Fuel, 63: 31X-328. Palmer, C.A. and Klizas, S.A., in press. Compilation of multi-technique analysis of eight argonne premium coal samples for 51 elements. In: C.A. Palmer (Editor), The Chemical Analysis of Argonne Premium Coal Samples. US. Geol. Surv. Bull., B2144. Palmer, C.A. and Wandless, M.-V., 1985. Distribution of trace elements in coal minerals of selected eastern United States coals. In: Proc. 1985 Int. Conf. Coal Science. Pergamon, New York, pp. 792-795. Palmer, CA., Krasnow, M.R., Finkelman, R.B. and d’Angelo, W.M., 1993. An evaluation of leaching to determine modes of occurrence of selected toxic elements in coal. J. Coal Qual., 12(4): 135-141. Pickhardt, W., 1989. Trace elements in minerals of German bituminous coals. Int. J. Coal Geol., 14: 137 - 153. Swaine. D.J., 1990. Trace Elements in Coal. Butterworths, London, 278 pp. Swanson, V.E. and Huffman, C., Jr., 1976. Guidelines for Sample Collecting and Analytical Methods used in
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Survey for determining
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Chemical
Composition
of Coal. US. Geol. Surv. Circ., 735,
11 PP. Zubovic, P., 1966. Minor element distribution in coal samples of the Interior Coal Province. In: R.F. Gould (Editor), Coal Science. Am. Chem. Sot. Publ. Advances in Chemistry Series No 55., Washington D.C., pp. 232-247. Zubovic, P., 1976. Geochemistry of trace elements in coal. In: F.A. Ayers (Compiler), Symposium Proceedings: Environmental Aspects of Fuel Conversion Technology II. Environmental Protection Agency, EPA-600/2-76-149, pp. 47-63. Zubovic, P., Stadnichenko, T. and Sheffey, N.B., 1961. The association of minor element associations in coal and other carbonaceous sediments. U.S. Geol. Surv. Prof. Pap. 4OOB, Article 411, pp. D345-D348.
International
Journal of Coal Geology 32 (1996) 151- 166
Selected elements in major minerals from bituminous coal as determined by INAA: implications for removing environmentally sensitive elements from coal C.A. Palmer, P.C. Lyons U.S.Geological
Survey, MS.
956 National Center, Reston, VA 20192, USA
Received 15 June 1995; accepted 8 September 1995
Abstract The four most abundant minerals generally found in Euramerican bituminous coals are quartz, kaolinite, illite and pyrite. These four minerals were isolated by density separation and handpicking from bituminous coal samples collected in the Ruhr Basin, Germany and the Appalachian basin, U.S.A. Trace-element concentrations of relatively pure (- 99+%) separates of major minerals from these coals were determined directly by using instrumental neutron activation analysis (INAA). As expected, quartz contributes little to the trace-element mass balance. Illite generally has higher trace-element concentrations than kaolinite, but, for the concentrates analyzed in this study, Hf, Ta, W, Th and U are in lower concentrations in illite than in kaolinite. Pyrite has higher concentrations of chalcophile elements (e.g., As and Se) and is considerably lower in lithophile elements as compared to kaolinite and illite. Our study provides a direct and sensitive method of determining trace-element relationships with minerals in coal. Mass-balance calculations suggest that the trace-element content of coal can be explained mainly by three major minerals: pyrite, kaolinite and illite. This conclusion indicates that the size and textural relationships of these major coal minerals may be a more important consideration as to whether coal cleaning can effectively remove the most environmentally sensitive trace elements in coal than what trace minerals are present. Keywords;
bituminous
coal; mass balance;
trace elements;
mineralogy;
Carboniferous;
environment
1. Introduction There elements
has long been an interest in trace elements in coal. Large data bases of trace in coals have been established by the U.S. Geological Survey (Zubovic et al.,
0166-5162/96/$15.00 Copyright PIZ SO166-5162(96)00035-3
0 1996 Elsevier Science B.V. All rights reserved
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Journal of Coal Geology 32 (1996) 151-166
1961; Swanson and Huffman, 1976; Bragg et al., 1994). Early work concentrated on organic-inorganic relations of trace elements. Goldschmidt (1935) postulated the occurrence of metal-organic complexes in coal and attributed the concentration of V, Co and Ni to the presence of these complexes. Zubovic (1966, 1976) determined organic affinities for 15 elements and suggested that metals with high organic affinity are present in chelates. Gluskoter et al. (1977) used float-sink methods to determine organic-inorganic associations of trace elements in coal. Lyons et al. (1989) calculated ratios of elemental concentrations in vitrinite as compared to whole coal to determine organic-inorganic associations. From these studies, it is clear that most trace elements are associated with the mineral matter in coal. Exceptions are Br (Lyons et al., 19891, Cl (Palmer and Filby, 1983) and Se (Palmer et al., 19931, all of which are known to be organically associated or have a tendency toward organic association. Chou (1991) suggests that chlorine ions are adsorbed on inner surfaces of micropores in macerals. Hower et al. (1994b) found that Cl and Br were concentrated in the very low ash, high vitrinite fraction. Several other elements are occasionally organically associated, such as Ge, B, and Sb (Gluskoter et al., 1977; Lyons et al., 1989). Recently, many studies have been conducted to identify the mode of occurrence of trace elements in coal. These works have become increasingly important because the chemical form of an element will affect its behavior during coal cleaning and combustion. Finkelman (1981) conducted studies by using scanning electron microscopy combined with an energy dispersive X-ray analyzer (SEM-EDXA) to identify trace elements in the coal matrix. This method was excellent in identifying heavy trace elements in minerals such as lead selinide (Pb and Se> and rare earth phosphates (rare earth elements). Also, SEM-EDXA has been successful in identifying minor elements in some major and minor minerals in coal. However, many trace elements in major minerals in coal are not detectable by this technique; detection limits are highly variable and mass dependent. This may lead to a somewhat distorted view of the importance of some “minerals” in the trace-element mass balance. Another microprobe technique, laser micro-mass spectrometry (LAMMA) was used by Lyons et al. (1987) and Morelli et al. (1988) to analyze vitrinites and other macerals. Although this technique is more sensitive for some trace elements (e.g. Li, Na and K), the results for elements of atomic weight greater than 100 are harder to interpret because of interferences by organic peaks in the mass spectra. Other microprobe techniques such as proton induced X-ray emission analysis (PIXE) (Chen et al., 1981; Minkin et al., 1982) also have been used. Several indirect methods have been used to obtain additional information on the mode of occurrence of inorganic constituents in coal. Nicholls (1968) related several trace elements to ash content. Palmer and Filby (1984) correlated mineralogy and trace-element content of size and density separates of low-temperature ash (LTA) from coal. Finkelman et al. (1990) and Palmer et al. (1993) used leaching studies to approximate the mode of occurrence of trace elements in coal. X-ray absorption fine structure (XAFS) has been used to determine the chemical form of several elements such as Cl, Ti, V, Cr, Mn, Zn, As, Se and Br (Huggins and Huffman, this volume). Over 100 minerals have been identified in coal (Finkelman, 198 1; Swaine, 1990). However, only a few of these minerals are common in bituminous coals from the United
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Journal of Coal Geology 32 (1996) 151-166
I53
States and Europe. The four most abundant minerals in Euramerican coals are quartz, pyrite, kaolinite and illite. Minor minerals that may be locally important include calcite (and other carbonates), mixed-layer clays (and other clays) and sphalerite. In this papel we will examine the trace-element content of selected mineral separates of these four major minerals. Because mixed-layer clays and illite are similar, especially in terms of X-ray diffraction, they will be considered together in this study. We will also consider together the two iron sulfides, pyrite and marcasite, for purposes of this study.
2. Samples
and procedures
The samples for this study came from a variety of sources. Each sample was chosen because of its high purity. The kaolinite and illite, from the Ruhr Basin of Germany, were splits of powdered samples described by Pickhardt (1989). The quartz came from two Appalachian tonstein beds: the Upper Banner coal bed of Wise County, Virginia, and the Fire Clay coal bed of Magoffin County, Kentucky (see Lyons et al., 1992). The pyrite was separated from the Pittsburgh (No. 8) coal bed of Belmont County, Ohio (Palmer and Filby, 1984) and the Upper Freeport coal bed of Indiana County, Pennsylvania (Palmer and Wandless, 1985). The pyrite, kaolinite and illite samples were all examined by X-ray diffraction (XRD) analysis; all peaks were attributed to the minerals in question. The quartz samples were handpicked under a low-power stereomicroscope, and the purest grains were selected. The samples were analyzed by instrumental neutron activation analysis (INAA). Each sample was irradiated in a nuclear reactor for 8 h at a flux of 3 X 10” neutrons/cm* s- ’ Using techniques described by Palmer and Baedecker (1989) to determine 29 elements, the samples were then counted on a Ge detector for about 4000 s, three or four days after irradiation, and for 10,000 or more seconds, six to eight weeks after irradiation.
3. Results Table 1 shows the results of the INAA analysis of the samples used in this study. The concentrations and the relative errors (one standard deviation) are also shown. For samples where an element was below the detection limit, “upper limits” were calculated based on the assumption that the minimum detectable peak was less than three standard deviations above background (Palmer and Baedecker, 1989). The upper limits were higher in cases where the sample size is very small, especially the handpicked quartz grains. The chemical data for the seven quartz samples, the two pyrite samples, the one kaolinite and the one illite samples are given in Table 1. As expected, quartz contained much lower concentrations of most trace elements than other minerals. Only Na, SC, Hf and Th were detected in all quartz samples. The remaining 25 elements in quartz, as determined by INAA, had one or more values below the detections limit; twelve elements (Cr, Co, Ni, Zn, As, Se, Nd, Tb, Yb, Ta, W and U) were below the detection limit in all seven quartz samples. The concentration of trace elements in kaolinite were generally lower than in illite except for Br, Sm, the heavy rare earth elements (Tb, Yb and Lu) and the lithophile
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Journal of Coal Geology 32 (1996) 151-166
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elements (Hf, Ta, W, Th and U), which are characterized by large atomic radii. Uranium is enriched by nearly a factor of six in kaolinite over illite and by more than ten times over the other minerals studied. The reason for this is unclear but one possibility is micron-size uranium-containing components in the kaolinite. The concentrations of Na, K, SC, Cr, Rb, Cs, La, Ce and Nd in illite were greater than all of the other minerals included in this study. Illite has many sites available for substitution because of its layered structure and is known to be a “garbage pail” for trace elements. For example, LAMMA results showed more elements in illite than kaolinite (Lyons and Hosterman, 1989). The concentration of Ba was higher in only one quartz sample than in illite. The concentration of Sr (where detected) was greater for illite than for the other samples, but the upper limits reported in six samples were greater than the concentration reported for illite. The concentrations of K, Rb and Cs were about ten times greater in the illite sample than in any other sample, which is understandable because of the K-rich nature of this mineral. The concentration of Fe, Co, As, Se and Sb were higher in the pyrite samples than in any of the other samples. Pyrite is the only mineral in this study where the purity of the mineral can be assessed from the chemical data as well as the XRD data. For both of the pyrite samples in this study, the Fe concentration was 46.6% which is the stoichiometric value for pure pyrite. As, Se and Sb were enriched in both pyrite samples by four to nearly fifty times the highest value determined in other minerals. These elements are chalcophile elements that would be expected to substitute more readily into the pyrite (Fe&) structure than in the structures of any of the other minerals in this study. The association of As with pyrite is well documented (Fleischer, 1955; Finkelman, 1981). Minkin et al. (1979) found Se in pyrite. Finkelman (1981) also postulated that Sb could be concentrated in pyrite, and that much of the Sb must also be in other minerals or associated with the organic matrix because of the mass-balance problems. Cobalt, which can be a chalcophile element is generally considered a lithophile element in the Earth’s crust (Mason, 1966). Cobalt is clearly associated with illite as well as pyrite. Cobalt can easily substitute for Fe in the pyrite structure (Deer et al., 1962). Finkelman (1981) stated that most of the Co is associated with sulfides, but some Co also occurs in the clays, as shown in Table 2. The relatively high Co value in the illite, as shown in Table 2, agrees with the data of Finkelman (1981). Table 2 Comparison
of data from this study with the data of Pickhardt
Element
Cr co Ni Zn As Sr U
Kaolinite
(1989). Values in
Illite
ppm
Pyrite
Calcite
This study
Pickhardt
This study
Pickhardt
This study
Pickhardt
Pickhardt
8.8 0.943 < 26 23 2 < 110 25.3
20 1 20 10 0.52 55 16
161 12.4 36 36 4 189 4.25
220 6 50 30 2.35 160 1
21.6 18.5 nd 27.5 718 < 300 1
200 65 270 2070 9 215 4
28 62 155 2700 6 290 2.3
nd = not determined.
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qf Coal Geology 32 (1996) 151-166
157
4. Discussion Table 2 shows a comparison of the data determined in this study with data in Pickhardt (1989). In general, the data in our study agrees with Pickardt’s data. The comparison is not nearly as good as one would expect based on the fact that our samples are splits of his powdered samples and that the methods Pickardt used to analyze trace elements, i.e. atomic absorption and ICP-MS, generally compare well with INAA results techniques (Palmer and Klizas, in press). Differences in some elements of up to a factor of four are reported in the two data sets (see Table 2). Whether these differences are due to sampling or due to analytical problems can not be determined from our study. Differences reported for pyrite are much greater than differences in kaolinite and illite. This is not surprising because the pyrite samples were not taken from the same geographic locations and stratigraphic positions. The mineral content of coals differ due to various factors. A few coals will have significant concentrations of minerals that are absent or in trace quantities in most coals. Some minerals, especially epigenetic minerals, may be altered by substitution particularly in locations of hydrothermal activity or in areas of high concentration of trace metals. However, assuming that the trace-element content measured in the four minerals of our study are representative of coal in general, one can do mass-balance calculations to test whether or not one can account for the inorganic element content of a whole coal based on its known content of the four minerals in question. Information on both the mineral content and the trace-element content of coal are required in order to do these mass-balance calculations. Ideally the minerals should come from the same coal but pure clay minerals from the same coal are difficult to obtain. Relatively few papers report both trace-element content and mineral content of coal. As a result only a few samples have been characterized for both minerals and trace elements. The sample most intensively studied for mineral content and trace elements is from the Herrin (No. 6) coal bed of Illinois. Data were available from many laboratories for both mineral content and trace-element content (Finkelman et al., 1984). In addition to quartz (20%), pyrite (23%), kaolinite (14%) and illite and mixed-layer clays (31%) the Herrin sample also contained calcite (9%). With the exception of Sr and possibly Zn and Ni, we do not believe that the 9% calcite will significantly affect the material balance for the other elements determined in this study. Table 2 shows data for calcite from coal determined by Pickhardt (1989); the highest concentrations of trace elements are for Zn, Sr and Ni, in order of importance. Palmer (1983) presented data for two coal beds for both mineral and trace element content. For the Upper Freeport coal bed, samples were taken from three different benches. These benches have very different mineralological and trace-element contents. In addition, Hower et al. (1990, 1994a) presented mineralogical and limited trace-element data. The Springfield and Fire Clay data reported in these two papers are averages and do not represent individual coals. Table 3 shows the low temperature ash (LTA) content for each of these coals as well as the concentrations of each mineral on a LT,4 basis as determined by the various studies. All these coals are high volatile bituminous coal. Because there are large differences in mineralogy between these coals (quartz ranges from 9-20%; pyrite and marcasite ranges from O-67%; kaolinite ranges from
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of Coal Geology 32 (1996) 151-166
Journal
Table 3 Low temperature ash (LTA) content and mineral content in percent for coals discussed in this paper. Numbers in parenthesis reference papers from which the data was taken
Appalachian Basin Coals Pittsburgh (No. 8) (1) Upper Freeport (I ) Bench I Bench 2 Bench 3 Fire Clay (2) Letcher County Floyd County Pike County Perry County Knott County
LTA
Quartz
Pyrite + Marcasite
Kaolinite
Illite + Mixed Layered Clays
Calcite
14
16
30
15
25
0
8.1 12.3 29.7
9 21 24
43 0 67
15 30 5
27 38 2
0 8 0
11.6 15.4 14.4 14.3 30.9
11 11 10 17 14
2 4 3 0 6
25 27 24 18 20
62 5 61 65 60
0 0 0 0 0
15.5 16.8
11 20
17 23
9 14
54 31
7 9
Illinois Basin Coals Springfield (3) Herrin No. 6 (4) (1) (2) (3) (4)
Data Data Data Data
taken taken taken taken
from from from from
Our Study I
Palmer (1983). Hower et al. (1994a,b). Hower et al. (1990). Finkelman et al. (1984)
Pickhardt n
Herrln = Herrln No.6
(1989)
,..-_ ~~_q~“rnh ‘In ’ nnw Freeport Bench 1 UF 2 = &per F reeport Bench 2 UF 3 = Upper F reeport Bench 3
!:;I
~19
._
”
Spring gfleld = SprIngfIeld No 9 FC Let = FIreClay.Letcher County FC Knott = Fire Clay,Knott County FC Pike = Fire Clay,Pike County FC Pery = FIreClay Perry County
Fig. 1. Ratios of Zn, As, Sr and U concentrations calculated by using the mass balance of the major minerals to the concentration of each trace element in the low-temperature ash (Table 4; this study) as compared with similar ratios using data from Pickhardt (1989) (Table 2).
C.A. Palmer, P.C. Lyons/International
Journal of Coal Geology 32 (1996) 1.5-166
159
S-30% and illite and mixed-layed clays ranges from 2-65%) these data are good to test the mass-balance calculations. Table 4 shows the average elemental concentrations for each mineral analyzed in this study. These concentrations were used to calculate the contribution of trace elements found in major minerals in coal to the total trace-element content of the LTA using the mineral contents given in Table 3. A ratio of this calculated concentration to the measured concentration in the LTA of less than one suggests that not all of the given element could be accounted for by the major minerals. As shown in Table 4, a ratio of greater than one suggests that the concentrations for the major minerals given do not fully represent the values in the sample of interest. At least 70% of the elements have ratios less than one. Although less than 30% of the elements in any coal had values within *25% of the expected value, most (nearly 60%) agreed within a factor of 2. Nearly all of those elements that did not agree within a factor of two had ratios less than one. Nearly 80% of the values for the samples reported by Hower et al. (1990, 1994a,b) have ratios of less than 0.5. This is due to the fact that Hower et al. (1994a,b) reported only 5 elements and two of those elements (Zn and Ni) have ratios of less than 0.5 for ~11 samples reported and Cr has ratios less than 0.7 for all samples reported. Zn may be associated with the calcite as indicated by Pickhardt (1989). who found an average of 2700 ppm Zn in his calcite. This concentration would raise the calculated
Cl Lt
Co NI
Cr
ierrln
Our study I
Co
PlttS
Pickhardt (1989) II
Herrm = Hernn (No 6)
CO
JF 1
Cr Co
I
UF2
Cr
!
co UF3
jcf
CO
pprlngfl
PC = Pl~sbwh 0% 8) UF I= Upper Freepott (Bench 1) UF 2 = Upper Freeport (Bench 2) UF 3 = Upper Freepott (Bench 3)
Sprmgfleld = Sprmgfleld (No 9) FC Let = Fire Clay.Letcher County FC Knott = Fire Clay, Knott CoiiQ FC Pike = Fire Clay,Plke County FC Perry Fire Clay, Perry Count\i ??
Fig. 2. Ratios of Cr, Co and Ni concentrations calculated by using the mass balance of the major minerals to the concentration of each trace element in the low-temperature ash (Table 4; this study) as compared with similar ratios using data of Pickhardt (1989) (Table 2).
Tb(ppm) Yb(ppm) Lu(ppm) Hf(ppm) Ta(ppm) W(ppm) Th(ppm) U(ppm)
Sm (ppm) Eu @pm)
As(ppm) Se(ppm) Br(ppm) Rb(ppm) Sr(ppm) Sb @pm) Cs (ppm) Ba(ppm) La (ppm) Ce @pm) Nd (ppm)
Co @pm) Ni (ppm) Zn @pm)
Fe (%)
Cr(ppm)
K (%I SC (ppm)
Na (%)
Pyrite average
0.026 0.02 0.55 22 47 18 nd 28 719 39 nd 6.00 <300 2.49 0.10 < 300 1.48 1.60 < 40 0.18 0.08 < 0.6
0.07 0.3 0.2 < 27 0.05 <4 i 290 < 80 <3 < 13 1.71 14 64 0.08 0.5 600 4.51 10 < 30 0.57 0.09 < 0.2 <1 0.03 0.78 <3 <2 1.97
0.425 0.146 11.2 8.8 0.287 0.943 < 26 23 0.52 <3 14.5 1.5 < 110 0.28 1.99 159 36.4 73 28.4 11.03 0.157 2.05 1.3 0.81 8.88 9.9 8.8 59.6 25.3
Kaolinite
0.665 5.64 24.5 167 1.46 12.4 36 36 2.35
0.28 1.87 9.32 58 11 8.2 11 21 166 8.88 3.80 90 71 0.88 8.86 344 33.8 56.8 17 4.86 0.55 0.54 2.48 0.31 3.04 2.02 2.86 15.3 5.09 5.49 305 30.5 57.9 nd 6.10 1.34 0.79 3.66 0.61 2.74 0.79 1.83 11.6 6.10
1.46
0.45 1.48 18 91 16 24 98 213 12 12 nd 79 183
Measured
-
*
Herrin (No. 6)
trace element data
Calculated
with measured
Illite
trace element content based on mineral compared
Quartz average
Table 4 Data for calculated
1.32 0.83
1.56
0.63 1.26 0.5 1 0.63 0.68 0.34 0.11 0.10 13.6 0.73 nd 1.14 0.39 0.60 1.61 1.13 1.11 0.98 nd 0.80 0.41 0.68 0.68 0.51 1.11 2.55
Ratio 0.25 1.5 8.0 50 14 8.8 9.0 21 216 12 3.6 74 57 1.0 7.2 283 29 49 15 4.4 0.46 0.5 1 2.3 0.29 2.8 2.0 2.6 15 5.2
-
Calculated 0.22 0.79 17 109 16 22 0 71 39 12 0 50 321 1.0 4.2 279 41 69 24 5.9 1.5 1.1 3.7 0.46 3.1 1.1 0 11 5.0
Measured
Pittsburgh **
(No. 8)
1.35 1.03
1.47 0.18 1.01 1.71 1.01 0.71 0.70 0.61 0.73 0.32 0.45 0.61 0.63 0.91 1.87 _
0.29 5.61 0.94 _
1.11 1.91 0.47 0.46 0.90 0.39
Ratio
Calculated 0.26 1.6 8.6 57 23 13 9.7 27 353 19 3.6 79 57 1.5 7.8 278 31 51 16 4.5 0.50 0.53 2.3 0.32 2.9 2.0 2.7 15 5.43
5.7 0.80 2.3 0.74 nd 11 nd
1.9
Measured 0.21 0.72 16.9 81 20 53 nd 111 685 17 nd 49 321 4.0 2.7 185 37 75 37 7.3 2.2
nd = not determined ~~ = value not calculated because measured ’ Data taken from Finkelman et al. (1984). ’’ Data taken from Palmer (1983).
Co (ppm) Ni (ppm) Zn (ppm) As @pm) Se (ppm) Br (ppm) Rb (ppm) Sr (ppm) Sb (ppm) Cs (ppm) Ba @pm) La (ppm) Ce (ppm) Nd @pm) Sm (ppm) Eu (ppm) Tb (ppm) Yb (ppm) Lu (ppm) Hf (ppm) Ta (ppm) W (ppm) Th (ppm) U (ppm)
Fe (%)
Cr (ppm)
K (%) SC (ppm)
Na (%)
1.41
1.60 0.18 0.38 2.86 1.50 0.82 0.68 0.42 0.62 0.23 0.29 0.42 0.40 1.22 2.76 _
0.24 0.51 1.11 _
Ratio 1.26 2.23 0.51 0.70 1.18 0.24 _
1) Measured 0.21 2.1 24 157 1.7 16 nd 268 3.3 8.2 nd 195 374 2.0 14 374 69 130 < 20 9.7 2.9 2.0 5.9 0.73 5.7 1.9 nd 21 7.3
**
1.27 1.26
0.76 0.23 0.46 0.68 0.62 0.84 2.01 _
0.56 0.23 0.20 0.80 1.15 0.66 0.60 _
0.08 0.3 1 0.00
Ratio 1.84 1.08 0.52 0.42 0.39 0.31
Upper Freeport (Bench 2)
or was below the detection limit
Calculated 0.39 2.3 13 66 0.67 5.0 14 21 1.0 0 6.46 109 85 0.41 11 431 46 78 24 7.3 0.67 0.93 4.0 0.46 4.8 3.8 4.6 26 9.2
value was not determined
**
Upper Freeport (Bench Calculated 0.43 2.6 14 77 2.6 6.5 16 24 30 1.5 6.7 125 94 0.56 13 469 51 86 27 7.9 0.77 0.98 4.3 0.49 5.1 3.9 5.0 21 9.5
Measured 0.22 2.2 25 123 4.3 18 nd 218 24 9.4 nd 125 455 2.5 10 404 74 132 47 8.7 2.5 2.0 5.5 0.74 5.0 1.3 nd 20 6.7 _____
**
1.35 1.41
1.Ol 0.21 0.23 1.23 1.16 0.69 0.65 0.57 0.91 0.30 0.48 0.78 0.67 1.02 2.95 _
0.11 1.27 0.16 _
Ratio 1.99 1.20 0.58 0.63 0.61 0.35 _
Upper Freeport (Bench 3)
71
_ _ _
22 28
-
105 2.4
Calculated _ _
Calculated 0.41 3.1 95 8.7 9.9 19 26 123 4.0 151 109 50
***
Measured nd nd 143 3.7 nd 116 85 nd nd nd 1332
* ’* *
Fire Clay Pike County
Measured 0.25 1.5 153 23.7 31.1 105 455 54 37 90 161 306
Springfield
* * * Data taken from Hower et al. (1990). * * * * Data taken from Hower et al. (1994a.b)
Co (ppm) Ni (ppm) Zn (ppm) As (ppm) Rb (ppm) Sr (ppm) Ba (ppm)
K (%) Cr (ppm) Fe (o/o)
Na (%o)
Co (ppm) Ni (ppm) Zn (ppm) As (ppm) Br (ppm) Rb (ppm) Sr (ppm) Ba (ppm)
K (%I Cr (ppm) Fe (%)
Na (o/n)
Table 4 (continued)
110 1.0 _ 23 28 _ _ _ 83
0.73 0.63 0.19 0.33 _
0.05
_
Calculated _ _
76
_ _
22 29 _
106 1.9
Calculated _
Ratio _
Ratio 1.63 2.11 0.62 0.37 0.32 0.18 0.06 2.28 0.11 1.68 0.68 0.16
***
Measured nd nd 329 4.2 nd 97 84 nd nd nd 1874
****
Fire Clay Perry County
Measured nd nd 293 5.2 nd 131 127 nd nd nd nd 1932
Fire Clay Letcher County
103 3.7 _ 21.6 27.9 _ _ _ 77.2
0.36 0.37 0.17 0.23 _ _ _ _ 0.04
0.04
0.24 0.33 _ _ _
0.33 0.24 _
_
Ratio
Calculated _ _
Ratio _
Measured nd nd 244 3.8 nd 78.4 60.4 nd nd nd nd 1711.8
**
Fire Clay Knott County
0.05
0.28 0.46 _ _ _ _
0.42 0.99 _
_
Ratio
C.A. Palmer, P.C. Lyons/International
Journal of Coal Geolog? 32 (1996) 151-166
163
content to 243 ppm Zn in the Herrin No. 6 coal and would over account for the observed Zn content. If data of Pickhardt (1989) for Zn in minerals was used instead of the data in this study, 5 coals would have ratios greater than 1 (2 with ratios of 9 or more) instead of ratios generally less than 0.3; see Fig. 1. Therefore, the calculated Zn would be greater than the reported Zn. Ratios greater than 1 suggest that the elemental concentrations calculated for the minerals are not representative of the coals in this study. Ratios less than 1 suggest that minerals other than the major minerals may be present. In the case of Zn, sphalerite is often present, especially in Illinois basin coals (Cobb and Russell, 1976) such as the Herrin No. 6 and the Springfield No. 9. Other coals such as the Pittsburgh No. 8 and the Fire Clay coal are Appalachian coals. These coals are also low in Zn and are not as likely to contain sphalerite. We were also not able to account for the observed Ni content based on our mineral data. This may be because Ni was not determined in pyrite because a Ni standard was not included in that particular irradiation. Pickhardt (1989) found 270 ppm Ni in pyrite which, if taken together with nickel in other minerals, would represent about 75% of the expected Ni in the LTA of the Herrin No. 6. coal. If one uses Pickhardt’s data for Ni in the other minerals, the ratio is even higher (see Fig. 2). Most of the Cr appears to be associated with illite but less than 75% of the Cr is accounted for by the major minerals using the concentrations in this study. Use of values for Cr reported by Pickhardt (I 989) yields much greater ratios up to nearly 2 (see Fig. 2). Finkelman (1981) stated that the association of chromium in coals remains a mystery and concluded that much of the Cr may be associated with the clays. Less than 16% of the Ba was accounted for in any of the coals reported by Hower et al. (1990, 1994a) even though Ba was slightly over accounted for in the other coals. This suggests that Ba is in barite or Ba phosphates below the X-ray diffraction detection limit in the generally high-Ba Kentucky coals. Generally less than one third of the Co and Sr is accounted for in the minerals analyzed. Use of the values of Pickhardt (1989) for these elements gives ratios closer to one (see Figs. 1 and 2). Arsenic has a ratio of over 5 in 2 coals and ratios of over 1 in 5 coals. This is because one of the pyrite samples used in our study has over 0.1% As. This seems quite high, although Fleischer (1955) reported that 10% of the pyrites in his study had over 1% As and Hower et al. (in press) reported variable As in pyrite as high as 3.5 wt.% in the Fire Clay coal. On the other hand, Pickhardt’s value of 99 ppm As may also be too high an average for pyrite as it would yield a ratio of slightly over 2 in one coal. Arsenic can readily enter the pyrite structure and, therefore, is highly variable in most coals. K and Cs each had a ratio of greater than 2 in only one sample. The only other elements having a ratio greater than 2 are Ta in four of the five coals measured; the ratio for the fifth coal was 1.87. Ta was only found in kaolinite and illite. Clearly, the clay of the Pickhardt (1989) samples are not representative for Ta in the coals of our study.
5. Conclusions Surprisingly, the agreement between the reported values of the trace elements in the LTA of the Illinois and Appalachian coals of our study and the calculated values using a
164
C.A. Palmer, P. C. Lyons /International
Journal of Coal Geology 32 (I 996) 151- I66
mass-balance approach is within a factor of 2 for most elements. Exceptions include Zn, Ba and Ta. Most of the trace-element contents in the coals considered in this study are accounted for by kaolinite, illite and pyrite. Quartz, which is a major mineral in most Euramerican coals, does not contain major quantities of any element except possibly for Ba (Table 1). Calcite, where present, is a major contributor of Sr, Zn and Ni. Coal-cleaning methods designed to remove four of these major minerals (illite, kaolinite, pyrite and calcite) prior to burning of the coal should remove most of the environmentally sensitive trace elements. Exceptions are Br and other organically fixed trace elements. Also, in cases where these minerals are dispersed as ultrafine ( < 2 pm> grains in the macerals of coals, they may remain in the coal and contribute to airborne contamination.
Acknowledgements We thank Dr. W. Pickhardt for providing the clay samples for this study. We also thank Drs. W.H. Orem and P.A. Baedecker of the U.S. Geological Survey, Dr. J.C. Hower of the Center for Applied Energy Research, and Dr. C.-L. Chou of the Illinois State Geological Survey for reviewing this paper.
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