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Density gradient centrifugation of − 100 mesh coal: An alternative to using micronized samples for maceral separation
Org. Geochem. Vol. 14, No. 3, pp. 307-313, 1989
0146-6380/89 $3.00 + 0.00 Copyright ~; 1989 Maxwell Pergamon Macmillan plc
Printed in Great Britain. All rights reserved
Density gradient centrifugation of --100 mesh coal: An alternative to using micronized samples for maceral separation STEVEN H. POE*, DARRELLN. TAULBEEand ROBERT A. KEOGH Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, KY 40511-8433, U.S.A. (Received 13 May 1988; accepted 7 November 1988)
A~traet--Using micronized coal for density gradient centrifugation (DGC) may increase maceral separation but the small size of the coal particles also leads to uncertain petrographic and chemical analyses as well as a product size that may not be useful for additional comparative testing. By contrast DGC maceral separations performed on - 100 mesh (150 p m) coal gave excellent, repeatable results with concentrations usually over 95% and a product size which could be more easily utilized for further tests. Key words--coal, maceral, separation, density gradient centrifugation
INTRODUCTION AND REVIEW
Coal is a complex and heterogeneous material composed of varied organic components and mineral assemblages. The microscopically differentiated organic constituents are called macerals. The three main maceral groups are vitrinite, inertinite, and liptinite (exinite). Coal is composed of varied proportions of these maceral groups. Since each maceral type has distinctly different physical and chemical characteristics, the proportion of each maceral type has a direct bearing on the behavior of a coal. For this reason it is advantageous to isolate and study each maceral individually. This laboratory wanted reasonably clean ( > 9 0 % purity) separations of the main maceral groups to study for liquefaction potential. For this project, it was not necessary to separate the main maceral groups down into individual maceral types although it was deemed important to reliably determine the amount and type of the individual macerals present in the concentration. One method commonly used (Dyrkacz et aL, 1984; Crelling, 1987) for maceral separation is density gradient centrifugation (DGC) which separates macerals on the basis of density. This method yields high maceral concentrates, but the extremely small particle size used is frequently a disadvantage. When pioneering this technique, Dyrkacz and Horwitz (1982) ground coal to an average particle size of 3 microns in an effort to clearly separate all maceral types. By decreasing the particle size to a minimum, there would obviously be less likelihood of having particles composed of more than one maceral. Using
*Present address: Indiana Department of Environmental Management, P.O. Box 6015, Indianapolis, IN 462066015, U.S.A.
an ultra-fine or micronized coal may enhance the separation efficiency, but unfortunately creates additional problems. In coal crushed to a 3/~m average size there are many particles smaller than two microns, which was the minimum size Dyrkacz and Horwitz (1982) listed as practical for distinguishing between vitrinite and inertinite. Approximately 40% of these particles were too small to be identified by Dyrkacz. They assumed that this unidentifiable portion had the same composition as the identified fraction. This assumption is highly questionable due to differential grinding, density overlap, density changes, and incomplete separation. McCabe (1942), Hsieh (1976), and Koshina and Maglicheva (1980), reported that coal macerals were subject to differential grinding; that is, some (usually inertinites) were easily fractured to fine particle sizes while others were much more resistant to grinding. Particular problems were reported with maceral combinations and borders. Thus, ultra-fine grinding of coal cannot be expected to yield uniform results in all size fractions and values should not be assumed if a significant portion of the sample is too small to be identified. Dyrkacz et aL (1984) reported that vitrinite density distributions overlapped significantly with the density distributions of inertinites. This was not due to incomplete separation or grinding but to a portion of each maceral group having the same density. The authors said, "This behavior places inherent limits on the ability to completely separate macerals using density measurements alone." More significantly, in the case of micronized coal, is the possibility of impurities in the unanalyzed portion, particularly as it is this portion which can be expected to be high in inertinite. Again, no purities should be assumed if much of the sample cannot be identified.
307
308
STEVENH. POE et al.
Diessel (1985) has reported that a portion of the inertinites from Australian coals fluoresce weakly. It is doubtful that this would be readily observable in micronized Pennsylvanian coals, but may possibly help in distinguishing between the vitrinites and inertinites. Lloyd and others (1988) have reported a definite density increase in the macerals of micronized coal. He suspects that this may be due to the use of a roller mill to ultra-fine grind the coal which could compact the maceral pores. These density changes could have an effect on density gradient separations, with an expanded density distribution causing macerals to separate out at densities other than normal. In spite of the ultra-fine particle size, all of the macerals in micronized coal are not liberated (Dyrkacz and Horwitz, 1982). For instance, most of the micrinite, a fine (2 pm average) granular inertinite maceral, remains bound within other maceral types and usually does not separate easily, no matter what grinding size is used. Further problems arise in using a micronized DGC separation after it is made. Micronized coal may not be very suitable for further testing or comparisons. Lloyd and others (1986) report different chemical data from analyses of - 6 0 mesh and micronized coal. Table 1 shows typical results of multiple analyses of the same sample at - 6 0 mesh and then, after grinding, on the same sample micronized. It is apparent that the chemical tests, standardized at - 6 0 mesh, do not yield the same results for the micronized coal. This rather compromises the usefulness of micronized maceral separations, if comparisons cannot be made with normal data. The Kentucky Center for Applied Energy Research was involved in a coal liquefaction project for which maceral separations were performed. Initially, samples were ground ultra-fine resulting in many of the difficulties mentioned above. This approach also introduced a new variable into the liquefaction runs that normally utilized - 1 0 0 mesh coal. Even if a reliable micronized separation could be obtained, liquefaction analyses on micronized samples could not easily be compared with standard runs. These problems prompted a decision to attempt DGC maceral separations utilizing the laboratory's standard liquefaction particle size of - 1 0 0 mesh (150/~m).
EXPERIMENTAL
Most of the samples utilized in this study were high vitrinite coals from western Kentucky. Other samples, including a high exinite bench of the Lower Elkhorn seam in eastern Kentucky, were separated as well. DEMINERALIZATION
After being stage-ground to - 1 0 0 mesh, forty (1.0) g aliquots of raw coal were demineralized at room temperatures with two successive 400ml aliquots of 24% HF. Neoformed fluoride minerals were removed with a saturated boric acid solution followed by six water rinses. Pyrite reduction was accomplished by centrifugation instead of the more commonly used chemical dissolution techniques (HNO3 or LiAIH4) which can induce measurable chemical alteration of the organic matrix of the coal (Durand and Nicaise, 1980). DENSITY-GRADIENT CENTRIFUGATION
The density gradient was formed by pumping aliquots of aqueous CaCI solutions stepwise to the outer wall of the centrifuge rotor (spinning at 2000 rpm) until the rotor was full (1.91 capacity ). This was followed by 8 g of - 100 mesh, demineralized coal sonically dispersed in 200 ml of aqueous surfactant solution (Brij-35/coal ratio of 1:4) added to the middle of the spinning rotor. The rotor speed was then increased to 16,000rpm for one hour, resulting in a force of 7000--25,000 g across the rotor. The rotor speed was slowly reduced to 2000 rpm and the CsCI gradient was forced from the rotor by pumping an immiscible high density fluorocarbon (Fluorinert) to the outer wall of the rotor. The solution exiting from the center of the rotor was routed through a portable density meter to determine collection cut points. Macerals were recovered by filtration on to 0.45/~m pore size teflon filters at whilch time they were thoroughly rinsed with water to reduce residual surfactant. The remaining surfactant was removed by overnight Soxhlet extraction with benzene/methanol (4:1). Several 8 g aliquots of demineralized coal were processed in this manner with the appropriate density fractions from each run being combined to provide sufficient material for analyses.
Table 1. Dupficative Chemical Analysis of Micronized and -60 Mesh Coal, Dry Basis Carbon Hydrogen Nitrogen Oxygen
Btu
Ash
Volatile Matter
Sulfur
#60 Mesh Coal
13831
18.78
34.37
5.94
57.25
3.90
1.55
16.33
5 Micron Coal
13512
16.99
33.22
4.99
64.58
4.19
1.47
12.22
Data courtesy of Dr. K. W. Kuehn, Western Kentucky University. Published in Lloyd, et. al., 1986.
Density gradient centrifugation of -100 mesh coal
309
MACERAL SEPARATION NO. 31
100
Reslnlte Exlnlte ~]] Mlcdnite SemI-Fusinite Fusinite [ ~ Vitrinite
-!! i !
o Density Cut Points
Fig. I. Initial separation• Same coal shown in Fig. 2. For each coal, an initial separation consisting of two runs divided into 15-20 density divisions was made. Micropellets were made of each of the density divisions and petrographic analyses made to determine the maceral separation efficiency. The results were used to select 7-9 density divisions, to be used for a subsequent large scale separation of the targeted
maceral(s). Sufficient DGC runs were then made to provide about 30 g of the targeted maceral concentrate for both chemical and petrographic analyses as well as liquefaction studies. The objective of the initial separation was to determine the density maximum for the maceral groups of each particular coal prior to starting large
MACERAL SEPARATION NO. 32 Weight %
O
0
0
'¢
~
901
•
", .....
8oi 701 6o I 5o
0 Reslnlte Exlnlte ~ ] Micrinite Semi-Fusinite Fuslnits I"1 Vltdnite
40. •
301 20. 10. O'
Density Cut Points
Fig. 2. Large scale separation. Same coal as Fig. 1,
STEVENH. POE et
310
al.
STAGE GRINDING - I00 Mesh
Table 2. Results of Vitrinite Concentrations
1
DEMINERAMZATION HF -- H3RO4
1 l
SPLIT
l
INITIAL DGC Aqueous CsCl/Srij-35
LARGE-SCALEDGC Aqueous CsCl/Brij-35
1 1 Filtration/Sonication/Rinse l SOXHLET EXTRACTION
1 1 Filtration/Sonication/Rinse 1 SOXHLET EXTRACTION
15-20 Density Fractions
Benzene/MeOH 4:1
7-9 Density Fractions
~
Vitrinite Concentrate %
98,2
71071
W, Ky. #9
90.9
71077
W, Ky. #11
92.5
99,0
71094
W. Ky. #9
87.3
99,0
71095
W. Ky. #9
89.5
99,0
71148
W. Ky. #9
86.3
96,6
71151
W. Ky. #9
88.3
96,0
Lower Elkhorn
24.5
84.8
3761
Benzene/MeOH 4:1
PETROGRAPHICANALYSES 9 mm Micropellets
Coal
Sample No.
Initial Vitrinite %
CHEMICAL LIQUEFACTION ANALYSES
Fig, 3. Density gradient centrifugation (DGC) procedure flow chart, scale separation. This optimizes the DGC runs to provide the maximum amount of a targeted maceral. Figure 1 shows the petrographic results of a typical initial separation and Fig. 2 shows the large scale separation of the same coal. In these and subsequent graphs, the raw parent coal is shown in the left column for comparison. A flow chart of the separation scheme is shown in Fig. 3. RESULTS
The objective of this work was to obtain vitrinite concentrations of greater than 90% purity and this was easily achieved with - 100 mesh coal (Table 2). Most separations reached purities over 95%. The
major contaminants left in the vitrinite concentrates were the smaller particles of inertinite and liptinite which would not readily separate from the vitrinite even when ground to a finer size [Fig. 4(a)]. At - 100 mesh the coal particles are la~'ge enough to be confidently identified in a routine maceral analysis using standard 500 x magnification. Figures 4(a) and 4(b), showing - 1 0 0 mesh and micronized coals at the same magnification, demonstrate the superiority of - 1 0 0 mesh sample over micronized samples for maceral analyses. Thus all of the samples were analyzed, not just a certain size fraction. The quantity, type, and form of the impurities are also reliably identified. Maceral separation No. 19 (Fig. 5) is a high exinite bench of an eastern Kentucky coal. This sample is composed of a choppy and poorly sorted mixture of
MACERAL SEPARATION NO. 19 H I G H - E X I N I T E COAL Weight % o °
m
to
0o ¢~ o
to
~
to
~" r~
• oo ~t
~, ¢~
lOO
Resinite m~ ExJnite [ ] Macrinite [ ] Micrinlte Semi-Fusinite Fusinite [ ~ Vitrinite
90 80 70
! 60 a. iii
50
i
40
-
3O 2O 10 0
Density Cut Points
Fig. 5. Initial separation of high exinite coal.
Fig. 4.(a) - I00 mesh D G C separation, high vitrinite concentration, W. KY No. 9 coal. (b) Micronized coal, W. KY No, 9. Photo courtesy of Dr K. W. Kuehn. Western Kentucky University. (c) - 100 mesh D G C separation, liptinite concentration. L. Elkhorn seam. 311
Density gradient centrifugation of - 100 mesh coal MACERAL SEPARATION NO. 34
,OOl
'":::::i
90
o__
I
Exlnlte
!
Macrinite
[ ~ Mior|nite
] 80 i
~
Semi-Fu,init.
~ Fusintte r ~ Vitdnite
0. 6O
5o
313
coals, to check the degree of maceral segregation before subjecting it to large particle size DGC. Normal coals have given excellent results but atypical samples may prove difficult no matter what size fraction is used. To test our procedures and analyses, duplicate DGC separations were made on one coal sample. The results showed less than 2% mean variation between the two different runs (Fig. 6), providing evidence for the reproducibility of this procedure.
i
001--'1° MACERAL SEPARATION NO. lS
~ 80
oo}j:
I
LI. DENSITYCUT POINTS
Fig. 6. Different separation of the same coal sample, to test repeatability. 40% liptinites, 25% vitrinites, and 35% inertinites. An attempt was made to obtain greater than 90% concentrations of each maceral group, but was not completely successful due to the poor segregation of the raw coal. The initial separation, shown in Fig. 5, had vitrinite concentrations of only 61% while liptinites and inertinites reached 99 and 91% respectively. The impurities in the vitrinite concentrate were small fragments of inertinite and liptinite enclosed within the vitrinites [Fig. 4(c)]. In an attempt to get a better concentration, selected fractions from the large scale separation were crushed to - 2 0 0 mesh and re-centrifuged. The vitrinite concentration after crushing reached 85%, with 5% liptinites and 10% inerts. The liptinite and inertinite fractions reached concentrations as high as 98% and 96%, respectively. Since there was an increase in bi-maceral border fragments, further grinding was judged impractical. This particular sample provides an example of a coal that was not well suited for - 1 0 0 mesh separation due to its atypical nature and low vitrinite concentration. It may be necessary, when working with unusual
CONCLUSIONS
DGC separations at - 100 mesh gave good, repeatable results for most coals, with maceral group concentrations generally over 95% purity. The final separation product at this size was also more amenable to subsequent comparative tests and analyses. For a project requiring this type of separation, using a large particle size may be preferable to using micronized coal. Acknowledgements--This work was funded by the Kentucky Energy Cabinet and the U.S. Department of Energy, Grant No. DE-FC22-86PC90017. REFERENCES
Crelling J. C. (1986) The occurrence and properties of pseudovitrinite. The Society for Organic Petrology Abstracts and Programs, Vol. 3, pp. 28-29. Diessel C. F. K. (1985) Fluorometric analysis of intertinite. Fuel 64, 1542-1546. Durand B. and Nicaise G. (1980) Procedures for kerogen isolation. In Kerogen (Edited by Durand B.), pp. 35-53. Editions Technip, Paris. Dyrkacz G. R., Bloomquist C. A. and Rustic L. (1984) High resolution density variations of coal macerals. Fuel 63, 1367-1373. Dyrkacz G. R. and Horwitz E. P. (1982) Separation of coal macerals. Fuel 61, 3-12. Hsieh S. (1976) Effects of bulk-components on the grindability of coals. Ph.D. thesis, Penn State University. Koshina M. and Maglicheva A. (1980) Changes in the microcomponent composition during the grinding of a hard coals. Khimiya Tverdogo Topliva 14 (No. 4), 12-18. Lloyd W. G., Riley J. T., Kuehn K. W. and Kuehn D. W. (1988) Chemistry and reactivity of micronized coals. DOE No. DE-FG22-85PC80514, Final Report. Feb. 15, 1988. Lloyd W. G., Riley J. T., Kuehn K. W. and Kuehn D. W. (1986) Chemistry and reactivity of micronized coals. DOE No. DE-FG22-85PC80514, Progress Report No. 4, Aug. 15, 1986. McCabe L. C, (1942) Practical significance of the physical constitution of coal. J. Geol. 50 (No. 4), 406-410.
0146-6380/89 $3.00 + 0.00 Copyright ~; 1989 Maxwell Pergamon Macmillan plc
Printed in Great Britain. All rights reserved
Density gradient centrifugation of --100 mesh coal: An alternative to using micronized samples for maceral separation STEVEN H. POE*, DARRELLN. TAULBEEand ROBERT A. KEOGH Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, KY 40511-8433, U.S.A. (Received 13 May 1988; accepted 7 November 1988)
A~traet--Using micronized coal for density gradient centrifugation (DGC) may increase maceral separation but the small size of the coal particles also leads to uncertain petrographic and chemical analyses as well as a product size that may not be useful for additional comparative testing. By contrast DGC maceral separations performed on - 100 mesh (150 p m) coal gave excellent, repeatable results with concentrations usually over 95% and a product size which could be more easily utilized for further tests. Key words--coal, maceral, separation, density gradient centrifugation
INTRODUCTION AND REVIEW
Coal is a complex and heterogeneous material composed of varied organic components and mineral assemblages. The microscopically differentiated organic constituents are called macerals. The three main maceral groups are vitrinite, inertinite, and liptinite (exinite). Coal is composed of varied proportions of these maceral groups. Since each maceral type has distinctly different physical and chemical characteristics, the proportion of each maceral type has a direct bearing on the behavior of a coal. For this reason it is advantageous to isolate and study each maceral individually. This laboratory wanted reasonably clean ( > 9 0 % purity) separations of the main maceral groups to study for liquefaction potential. For this project, it was not necessary to separate the main maceral groups down into individual maceral types although it was deemed important to reliably determine the amount and type of the individual macerals present in the concentration. One method commonly used (Dyrkacz et aL, 1984; Crelling, 1987) for maceral separation is density gradient centrifugation (DGC) which separates macerals on the basis of density. This method yields high maceral concentrates, but the extremely small particle size used is frequently a disadvantage. When pioneering this technique, Dyrkacz and Horwitz (1982) ground coal to an average particle size of 3 microns in an effort to clearly separate all maceral types. By decreasing the particle size to a minimum, there would obviously be less likelihood of having particles composed of more than one maceral. Using
*Present address: Indiana Department of Environmental Management, P.O. Box 6015, Indianapolis, IN 462066015, U.S.A.
an ultra-fine or micronized coal may enhance the separation efficiency, but unfortunately creates additional problems. In coal crushed to a 3/~m average size there are many particles smaller than two microns, which was the minimum size Dyrkacz and Horwitz (1982) listed as practical for distinguishing between vitrinite and inertinite. Approximately 40% of these particles were too small to be identified by Dyrkacz. They assumed that this unidentifiable portion had the same composition as the identified fraction. This assumption is highly questionable due to differential grinding, density overlap, density changes, and incomplete separation. McCabe (1942), Hsieh (1976), and Koshina and Maglicheva (1980), reported that coal macerals were subject to differential grinding; that is, some (usually inertinites) were easily fractured to fine particle sizes while others were much more resistant to grinding. Particular problems were reported with maceral combinations and borders. Thus, ultra-fine grinding of coal cannot be expected to yield uniform results in all size fractions and values should not be assumed if a significant portion of the sample is too small to be identified. Dyrkacz et aL (1984) reported that vitrinite density distributions overlapped significantly with the density distributions of inertinites. This was not due to incomplete separation or grinding but to a portion of each maceral group having the same density. The authors said, "This behavior places inherent limits on the ability to completely separate macerals using density measurements alone." More significantly, in the case of micronized coal, is the possibility of impurities in the unanalyzed portion, particularly as it is this portion which can be expected to be high in inertinite. Again, no purities should be assumed if much of the sample cannot be identified.
307
308
STEVENH. POE et al.
Diessel (1985) has reported that a portion of the inertinites from Australian coals fluoresce weakly. It is doubtful that this would be readily observable in micronized Pennsylvanian coals, but may possibly help in distinguishing between the vitrinites and inertinites. Lloyd and others (1988) have reported a definite density increase in the macerals of micronized coal. He suspects that this may be due to the use of a roller mill to ultra-fine grind the coal which could compact the maceral pores. These density changes could have an effect on density gradient separations, with an expanded density distribution causing macerals to separate out at densities other than normal. In spite of the ultra-fine particle size, all of the macerals in micronized coal are not liberated (Dyrkacz and Horwitz, 1982). For instance, most of the micrinite, a fine (2 pm average) granular inertinite maceral, remains bound within other maceral types and usually does not separate easily, no matter what grinding size is used. Further problems arise in using a micronized DGC separation after it is made. Micronized coal may not be very suitable for further testing or comparisons. Lloyd and others (1986) report different chemical data from analyses of - 6 0 mesh and micronized coal. Table 1 shows typical results of multiple analyses of the same sample at - 6 0 mesh and then, after grinding, on the same sample micronized. It is apparent that the chemical tests, standardized at - 6 0 mesh, do not yield the same results for the micronized coal. This rather compromises the usefulness of micronized maceral separations, if comparisons cannot be made with normal data. The Kentucky Center for Applied Energy Research was involved in a coal liquefaction project for which maceral separations were performed. Initially, samples were ground ultra-fine resulting in many of the difficulties mentioned above. This approach also introduced a new variable into the liquefaction runs that normally utilized - 1 0 0 mesh coal. Even if a reliable micronized separation could be obtained, liquefaction analyses on micronized samples could not easily be compared with standard runs. These problems prompted a decision to attempt DGC maceral separations utilizing the laboratory's standard liquefaction particle size of - 1 0 0 mesh (150/~m).
EXPERIMENTAL
Most of the samples utilized in this study were high vitrinite coals from western Kentucky. Other samples, including a high exinite bench of the Lower Elkhorn seam in eastern Kentucky, were separated as well. DEMINERALIZATION
After being stage-ground to - 1 0 0 mesh, forty (1.0) g aliquots of raw coal were demineralized at room temperatures with two successive 400ml aliquots of 24% HF. Neoformed fluoride minerals were removed with a saturated boric acid solution followed by six water rinses. Pyrite reduction was accomplished by centrifugation instead of the more commonly used chemical dissolution techniques (HNO3 or LiAIH4) which can induce measurable chemical alteration of the organic matrix of the coal (Durand and Nicaise, 1980). DENSITY-GRADIENT CENTRIFUGATION
The density gradient was formed by pumping aliquots of aqueous CaCI solutions stepwise to the outer wall of the centrifuge rotor (spinning at 2000 rpm) until the rotor was full (1.91 capacity ). This was followed by 8 g of - 100 mesh, demineralized coal sonically dispersed in 200 ml of aqueous surfactant solution (Brij-35/coal ratio of 1:4) added to the middle of the spinning rotor. The rotor speed was then increased to 16,000rpm for one hour, resulting in a force of 7000--25,000 g across the rotor. The rotor speed was slowly reduced to 2000 rpm and the CsCI gradient was forced from the rotor by pumping an immiscible high density fluorocarbon (Fluorinert) to the outer wall of the rotor. The solution exiting from the center of the rotor was routed through a portable density meter to determine collection cut points. Macerals were recovered by filtration on to 0.45/~m pore size teflon filters at whilch time they were thoroughly rinsed with water to reduce residual surfactant. The remaining surfactant was removed by overnight Soxhlet extraction with benzene/methanol (4:1). Several 8 g aliquots of demineralized coal were processed in this manner with the appropriate density fractions from each run being combined to provide sufficient material for analyses.
Table 1. Dupficative Chemical Analysis of Micronized and -60 Mesh Coal, Dry Basis Carbon Hydrogen Nitrogen Oxygen
Btu
Ash
Volatile Matter
Sulfur
#60 Mesh Coal
13831
18.78
34.37
5.94
57.25
3.90
1.55
16.33
5 Micron Coal
13512
16.99
33.22
4.99
64.58
4.19
1.47
12.22
Data courtesy of Dr. K. W. Kuehn, Western Kentucky University. Published in Lloyd, et. al., 1986.
Density gradient centrifugation of -100 mesh coal
309
MACERAL SEPARATION NO. 31
100
Reslnlte Exlnlte ~]] Mlcdnite SemI-Fusinite Fusinite [ ~ Vitrinite
-!! i !
o Density Cut Points
Fig. I. Initial separation• Same coal shown in Fig. 2. For each coal, an initial separation consisting of two runs divided into 15-20 density divisions was made. Micropellets were made of each of the density divisions and petrographic analyses made to determine the maceral separation efficiency. The results were used to select 7-9 density divisions, to be used for a subsequent large scale separation of the targeted
maceral(s). Sufficient DGC runs were then made to provide about 30 g of the targeted maceral concentrate for both chemical and petrographic analyses as well as liquefaction studies. The objective of the initial separation was to determine the density maximum for the maceral groups of each particular coal prior to starting large
MACERAL SEPARATION NO. 32 Weight %
O
0
0
'¢
~
901
•
", .....
8oi 701 6o I 5o
0 Reslnlte Exlnlte ~ ] Micrinite Semi-Fusinite Fuslnits I"1 Vltdnite
40. •
301 20. 10. O'
Density Cut Points
Fig. 2. Large scale separation. Same coal as Fig. 1,
STEVENH. POE et
310
al.
STAGE GRINDING - I00 Mesh
Table 2. Results of Vitrinite Concentrations
1
DEMINERAMZATION HF -- H3RO4
1 l
SPLIT
l
INITIAL DGC Aqueous CsCl/Srij-35
LARGE-SCALEDGC Aqueous CsCl/Brij-35
1 1 Filtration/Sonication/Rinse l SOXHLET EXTRACTION
1 1 Filtration/Sonication/Rinse 1 SOXHLET EXTRACTION
15-20 Density Fractions
Benzene/MeOH 4:1
7-9 Density Fractions
~
Vitrinite Concentrate %
98,2
71071
W, Ky. #9
90.9
71077
W, Ky. #11
92.5
99,0
71094
W. Ky. #9
87.3
99,0
71095
W. Ky. #9
89.5
99,0
71148
W. Ky. #9
86.3
96,6
71151
W. Ky. #9
88.3
96,0
Lower Elkhorn
24.5
84.8
3761
Benzene/MeOH 4:1
PETROGRAPHICANALYSES 9 mm Micropellets
Coal
Sample No.
Initial Vitrinite %
CHEMICAL LIQUEFACTION ANALYSES
Fig, 3. Density gradient centrifugation (DGC) procedure flow chart, scale separation. This optimizes the DGC runs to provide the maximum amount of a targeted maceral. Figure 1 shows the petrographic results of a typical initial separation and Fig. 2 shows the large scale separation of the same coal. In these and subsequent graphs, the raw parent coal is shown in the left column for comparison. A flow chart of the separation scheme is shown in Fig. 3. RESULTS
The objective of this work was to obtain vitrinite concentrations of greater than 90% purity and this was easily achieved with - 100 mesh coal (Table 2). Most separations reached purities over 95%. The
major contaminants left in the vitrinite concentrates were the smaller particles of inertinite and liptinite which would not readily separate from the vitrinite even when ground to a finer size [Fig. 4(a)]. At - 100 mesh the coal particles are la~'ge enough to be confidently identified in a routine maceral analysis using standard 500 x magnification. Figures 4(a) and 4(b), showing - 1 0 0 mesh and micronized coals at the same magnification, demonstrate the superiority of - 1 0 0 mesh sample over micronized samples for maceral analyses. Thus all of the samples were analyzed, not just a certain size fraction. The quantity, type, and form of the impurities are also reliably identified. Maceral separation No. 19 (Fig. 5) is a high exinite bench of an eastern Kentucky coal. This sample is composed of a choppy and poorly sorted mixture of
MACERAL SEPARATION NO. 19 H I G H - E X I N I T E COAL Weight % o °
m
to
0o ¢~ o
to
~
to
~" r~
• oo ~t
~, ¢~
lOO
Resinite m~ ExJnite [ ] Macrinite [ ] Micrinlte Semi-Fusinite Fusinite [ ~ Vitrinite
90 80 70
! 60 a. iii
50
i
40
-
3O 2O 10 0
Density Cut Points
Fig. 5. Initial separation of high exinite coal.
Fig. 4.(a) - I00 mesh D G C separation, high vitrinite concentration, W. KY No. 9 coal. (b) Micronized coal, W. KY No, 9. Photo courtesy of Dr K. W. Kuehn. Western Kentucky University. (c) - 100 mesh D G C separation, liptinite concentration. L. Elkhorn seam. 311
Density gradient centrifugation of - 100 mesh coal MACERAL SEPARATION NO. 34
,OOl
'":::::i
90
o__
I
Exlnlte
!
Macrinite
[ ~ Mior|nite
] 80 i
~
Semi-Fu,init.
~ Fusintte r ~ Vitdnite
0. 6O
5o
313
coals, to check the degree of maceral segregation before subjecting it to large particle size DGC. Normal coals have given excellent results but atypical samples may prove difficult no matter what size fraction is used. To test our procedures and analyses, duplicate DGC separations were made on one coal sample. The results showed less than 2% mean variation between the two different runs (Fig. 6), providing evidence for the reproducibility of this procedure.
i
001--'1° MACERAL SEPARATION NO. lS
~ 80
oo}j:
I
LI. DENSITYCUT POINTS
Fig. 6. Different separation of the same coal sample, to test repeatability. 40% liptinites, 25% vitrinites, and 35% inertinites. An attempt was made to obtain greater than 90% concentrations of each maceral group, but was not completely successful due to the poor segregation of the raw coal. The initial separation, shown in Fig. 5, had vitrinite concentrations of only 61% while liptinites and inertinites reached 99 and 91% respectively. The impurities in the vitrinite concentrate were small fragments of inertinite and liptinite enclosed within the vitrinites [Fig. 4(c)]. In an attempt to get a better concentration, selected fractions from the large scale separation were crushed to - 2 0 0 mesh and re-centrifuged. The vitrinite concentration after crushing reached 85%, with 5% liptinites and 10% inerts. The liptinite and inertinite fractions reached concentrations as high as 98% and 96%, respectively. Since there was an increase in bi-maceral border fragments, further grinding was judged impractical. This particular sample provides an example of a coal that was not well suited for - 1 0 0 mesh separation due to its atypical nature and low vitrinite concentration. It may be necessary, when working with unusual
CONCLUSIONS
DGC separations at - 100 mesh gave good, repeatable results for most coals, with maceral group concentrations generally over 95% purity. The final separation product at this size was also more amenable to subsequent comparative tests and analyses. For a project requiring this type of separation, using a large particle size may be preferable to using micronized coal. Acknowledgements--This work was funded by the Kentucky Energy Cabinet and the U.S. Department of Energy, Grant No. DE-FC22-86PC90017. REFERENCES
Crelling J. C. (1986) The occurrence and properties of pseudovitrinite. The Society for Organic Petrology Abstracts and Programs, Vol. 3, pp. 28-29. Diessel C. F. K. (1985) Fluorometric analysis of intertinite. Fuel 64, 1542-1546. Durand B. and Nicaise G. (1980) Procedures for kerogen isolation. In Kerogen (Edited by Durand B.), pp. 35-53. Editions Technip, Paris. Dyrkacz G. R., Bloomquist C. A. and Rustic L. (1984) High resolution density variations of coal macerals. Fuel 63, 1367-1373. Dyrkacz G. R. and Horwitz E. P. (1982) Separation of coal macerals. Fuel 61, 3-12. Hsieh S. (1976) Effects of bulk-components on the grindability of coals. Ph.D. thesis, Penn State University. Koshina M. and Maglicheva A. (1980) Changes in the microcomponent composition during the grinding of a hard coals. Khimiya Tverdogo Topliva 14 (No. 4), 12-18. Lloyd W. G., Riley J. T., Kuehn K. W. and Kuehn D. W. (1988) Chemistry and reactivity of micronized coals. DOE No. DE-FG22-85PC80514, Final Report. Feb. 15, 1988. Lloyd W. G., Riley J. T., Kuehn K. W. and Kuehn D. W. (1986) Chemistry and reactivity of micronized coals. DOE No. DE-FG22-85PC80514, Progress Report No. 4, Aug. 15, 1986. McCabe L. C, (1942) Practical significance of the physical constitution of coal. J. Geol. 50 (No. 4), 406-410.