Energy Fuels 2010, 24, 5479–5482 Published on Web 10/01/2010
: DOI:10.1021/ef100444p
Direct Determination of Pyrite Content in Argonne Premium Coals by the Use of Sulfur X-ray Near Edge Absorption Spectroscopy (S-XANES) Trudy B. Bolin* Argonne National Laboratory, Argonne, Illinois 60439 Received April 9, 2010. Revised Manuscript Received September 1, 2010
Argonne premium coal samples are used by researchers worldwide as standards in coal research. The set consists of a suite of eight samples of varying rank from the United States. The sulfur X-ray near edge absorption spectroscopy (S-XANES) third-derivative analysis method uses a well-defined library of model compounds to curve fit each sample spectrum and enables sulfur speciation to within about 10 mol % for materials such as coals and kerogens. This direct, non-destructive characterization technique, used in conjunction with others, such as X-ray photoelectron spectroscopy, can provide valuable information about chemical and thermal sulfur transformations. The S-XANES third-derivative analysis method provides quantitative results for organic sulfur species in coal but has not been used to quantify inorganic sulfur forms to date. In general, the direct determination of the pyrite content, a metal sulfide, has been problematic. Through wet chemical methods, several of the Argonne premium coal samples are known to exceed 50 mol % pyrite but only show a weak pyrite feature in the S-XANES absorbance and thirdderivative spectrum. We show that particle-size effects are responsible for attenuating the pyrite signal for high-pyrite-containing Argonne premium coals. Grinding techniques are discussed that decrease the particle size and produce spectra and results that are in better agreement with those from established wet chemical methods for pyrite determination. of the high atomic concentration of pyrite in the coal samples.3 A combination of the S-XANES absorbance least-squares fitting technique and Mossbauer spectroscopy was used to calibrate the pyrite content in Argonne premium coals.3,10 Recently, it was noted that the least-squares method was not accurate when used to estimate pyrritic sulfur, and Mossbauer spectroscopy was used to examine the pyrite.6 Iron XAS has also been performed on the Argonne premium coal samples.7 It has been determined that not all of the iron exists as pyrite. The assumption that all iron exists as pyrite will lead to errors using the American Society for Testing and Materials (ASTM) method for organic sulfur determination. The S-XANES analysis offers the prospect of directly quantifying the relative amounts of sulfur as pyrite, organic sulfur, and sulfate if the above-mentioned uncertainties can be addressed. In this paper, we present evidence that particle-size effects are responsible for attenuating the pyrite signal, and this effect can be mitigated by finely grinding the coal sample prior to S-XANES analysis.
1. Introduction All sedimentary organic matter and fossil fuels contain sulfur in varying quantities and diverse forms. The Argonne premium coal samples were assembled to aid researchers in comparing their work to one another and enhance the quality of coal research.2 A basic understanding of sulfur chemistry in these materials is extremely important for predicting their reactivity and physical properties. Organic sulfur forms in Argonne premium coals,4 kerogens,1 and petroleum asphaltenes5 have been previously quantified by the sulfur X-ray near edge absorption spectroscopy (S-XANES) third-derivative analysis method. However, previous investigators have not reported the pyrite content for Argonne premium coals even though pyrite was used to fit the spectra.4 In an X-ray absorption spectroscopy (XAS) investigation, it was hypothesized that there may be oxide overlayers on the pyrite particles attenuating the pyrite signal.4 In the analysis of S-XANES absorbance spectra, researchers hypothesized that particle-size effects did indeed affect the attenuation of the signal and, thus, they further ground the Argonne premium coal samples down to a particle size of less than 35 μm by the use of a wig-l-bug.3 They concluded that there were self-absorption effects because
2. Experimental Section The bulk S-XANES measurements were obtained at the Advanced Photon Source (APS) beamline 9-BM at Argonne National Laboratory. Quantification of organic sulfur forms was achieved by curve-resolving the third derivative of the absorbance spectrum using spectra of model compounds containing aliphatic disulfide, aliphatic sulfide, aryl sulfide, and thiophenic. The experimental arrangement at 9-BM consists of a Si(111) monochromator, and focusing is achieved with a rhodium-coated
*To whom correspondence should be addressed. E-mail: bolitru@ aps.anl.gov. (1) Kelemen, S. R.; Afeworki, M.; Gorbaty, M. L.; Sansone, M.; Kwiatek, P. J.; Walters, C. C.; Freund, H.; Siskin, M.; Bence, A. E.; Curry, D. J.; Solum, M.; Pugmire, R. J.; Vandenbroucke, M.; Leblond, M.; Behar, F. Energy Fuels 2007, 21 (3), 1548–1461. (2) Vorres, K. S. Energy Fuels 1990, 4 (5), 420–426. (3) Huffman, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy Fuels 1991, 5 (4), 574–581. (4) George, G. N.; Gorbaty, M. L.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991, 5 (1), 93–97. (5) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111 (9), 3182–3186. r 2010 American Chemical Society
(6) Huggins, F. E.; Seidu, L. B. A.; Shah, N.; Huffman, G. P.; Honaker, R. Q.; Kyger, J. R.; Higgins, B. L.; Robinson, J. D.; Pal, S.; Seehra, M. S. Int. J. Coal Geol. 2009, 78 (1), 65–76. (7) Wasserman, S. R.; Winans, R.; McBeth, R. Energy Fuels 1996, 10 (2), 392–400.
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Energy Fuels 2010, 24, 5479–5482
: DOI:10.1021/ef100444p
Bolin
Figure 1. Parity plot for the aromatic (blue [) and aliphatic (red 9) organic sulfur content for data collected at the APS and the Stanford Synchrotron Radiation Laboratory (SSRL).
toriodal mirror. Harmonics are rejected by the use of a Rh-coated flat mirror. The energy resolution is approximately 0.1 eV at 2.5 keV. To provide for positional stability, a feedback system using a position-sensitive ion chamber with two sets of plates sensitive to horizontal and vertical beam motion is used. Air absorption is eliminated by the use of helium in the flight path and the sample chamber. The sample chamber is directly attached to a Lytle detector. The detector and sample chamber are separated by a 2.5 μm thick sheet of aluminized mylar. The S-XANES absorbance spectra were analyzed using the third-derivative method previously described.1,4,5 Figure 1 shows a comparison of aromatic/aliphatic sulfur (mol % organic basis) for Argonne premium coals from analysis of data currently collected at APS to analysis of data previously reported.4 The results for the organic sulfur forms agree. Figure 2 shows the S-XANES absorbance and third-derivative spectra of Argonne premium coals samples that were not additionally ground after removal from the bottle. The appearance of both the absorbance and third-derivative spectrum are almost identical to those previously reported.4 This figure highlights the effects of large particles on the pyrite XANES and third derivative. The feature at 2468.5 eV is due to pyrite and should be much more prominent based on the pyrite content determined by standard methods.4 Table 1 shows the pyrite content on a total sulfur mole percent basis obtained by the ASTM bulk method8 and values determined by S-XANES third-derivative analysis of the samples that were not additionally ground. The table clearly shows that there is poor agreement between the two methods for sulfur because of pyrite. The effects of grinding on the S-XANES spectra were investigated to see if the relative pyrite signal strength is changed. Grinding was chosen because of the large variety of natural pyrite particle sizes that may be present in any given sample. Natural pyrite particle sizes range from micrometers to hundreds of micrometers depending upon the formation process.9,10 Argonne premium coal samples with the highest pyrite content were selected for grinding: Illinois No. 6 (4.8 wt % S2, 97% iron
Figure 2. Third-derivative and XANES spectra for not additionally ground samples. The feature indicated by the arrow is due to pyrite.
Table 1. Predicted Pyrite Content from Wet Chemical Methods as Compared to S-XANES
Beulah Zap Wyodak Illinois No. 6 Blind Canyon Pittsburgh No. 8 Lewis Stockton Upper Freeport Pocahontas
bulk (mol % pyrite)
XANES (mol % pyrite)
18 28 58 41 63 24 77 24
0 0 9 6 0 0 8 0
content because of pyrite7), Upper Freeport (2.3 wt % S, 81% pyrite), and Pittsburgh No. 8 (2.2 wt % S, 94% pyrite). For each coal sample, different subtypes were made: (1) particles sieved to a size range of 149-58 μm (from -100 to þ250 mesh), not additionally ground, (2) finely ground sieved particles (149-58 μm), and (3) finely ground (out-of-the-bottle, unsieved). Subtype 2 samples were created from the samples of subtype 1 by reclaiming the subtype 2 samples after acquiring their spectra and subsequently grinding them to produce a finely ground sample. Grinding was performed with an agate mortar and pestle. For both subtypes 2 and 3, the fine-grinding process involved taking a small amount of sample (a few milligrams) and grinding
(8) American Society for Testing and Minerals (ASTM). ASTM D2492. 1977 Annual Book of ASTM Standards; ASTM: West Conshohocken, PA, 1977; Part 26: Gaseous Fuels; Coal and Coke Atmospheric Analysis. (9) Ryan, B.; Ledda, A. A review of sulphur in coal: With specific reference to the Telkwa deposit, north-western British Columbia. Geological Fieldwork 1997; British Columbia Geological Survey Branch (BCGS), Ministry of Energy, Mines and Petroleum Resources: Victoria, British Columbia, Canada, 1997; Paper 1998-1. (10) Southam, G.; Donald, R.; Rostad, A.; Brock, C. Geology 2001, 29 (1), 47–50.
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Energy Fuels 2010, 24, 5479–5482
: DOI:10.1021/ef100444p
Bolin
Figure 4. Third-derivative spectra for Illinois No. 6 (top) and Pittsburgh No. 8 (bottom) samples. The spectra for each set correspond to (top) non-sieved material straight out the bottle and ground, (middle) from -100 to þ250 mesh and ground, and (bottom) from -100 to þ250 mesh and not additionally ground.
Figure 3. Pennsylvania Upper Freeport derivative spectrum (top) and absorption spectrum (bottom). In both plots, the spectra correspond to (top) out-of-the-bottle, unsieved, and ground, (middle) from -100 to þ250 mesh and ground, and (bottom) from -100 to þ250 mesh and not additionally ground.
for a period of 20 min, being careful to spread the sample out in the mortar as much as possible while grinding to obtain as even a particle distribution as possible. The actual final size was not precisely measured. It was noticed after grinding that the sample became slightly shiny in appearance. After grinding, the samples were immediately dusted onto sulfur-free tape, which was mounted onto a Teflon holder, transported to the sample chamber, and purged with helium to minimize oxidation. Figure 5. Soft X-ray energy attenuation length for pyrite in a typical fluorescence geometry.
3. Results and Discussion The impact of grinding was significant. Figure 3 shows the S-XANES absorbance and third-derivative spectra for the Upper Freeport coal sample: (1) unground (149-58 μm), (2) finely ground (149-58 μm), and (3) finely ground (out-of-thebottle) samples. Clearly, the pyrite-related feature has been significantly enhanced by finely grinding the 149-58 μm and out-of-the-bottle samples. Likewise, Figure 4 shows the S-XANES third-derivative spectra for the Illinois No. 6 and Pittsburgh No. 8 samples. Again, the pyrite-related feature has been significantly enhanced by finely grinding the 149-58-μm-sized or out-of-the-bottle samples. Table 2 shows the values from before and after grinding of Illinois No. 6, Pittsburgh No. 8, and Upper Freeport coal. Finely ground samples give results for pyrite that are in much better agreement with the bulk values. We can understand the effects of grinding by considering the physics involved in the S-XANES measurement. Figure 5 shows a plot of attenuation length for pyrite at 2.5 keV. The
Table 2. Old and New Pyrite Mole Percents for Three Selected Argonne Premium Coals
Upper Freeport Illinois No. 6 Pittsburgh No. 8
old (mol %)
new (mol %)
bulk (mol %)
8 9 0
57 41 35
77 58 63
penetration depth is approximately 1.5 μm, indicating that, even though a large particle of pyrite, which can be as large as a few millimeters in diameter,11 contains a large amount of pyritic sulfur, a photon of that energy can only “see” a very small fraction, hence, the reduction in signal. The fact that the finely ground samples give results close to bulk values indicates that the average particle size is on the order of the penetration depth (∼1.5 μm). (11) Huffman, G. P.; Huggins, F. E. Fuel 1978, 57 (10), 592–604.
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Energy Fuels 2010, 24, 5479–5482
: DOI:10.1021/ef100444p
Bolin
The particle-size effects for large, relatively dense materials, such as iron-containing pyrite at low X-ray energies, can be so severe that they nearly extinguish their S-XANES signal relative to the sulfur in the organic matter. The penetration depth of organic matter is on the order of hundreds of micrometers. This effect is clearly shown in Table 1 and Figure 2 for not additionally ground Argonne premium coals.
This effect can be minimized by finely grinding the coal sample prior to S-XANES analysis. The finely ground samples give results for pyrite that are in better agreement with the bulk values. These results have significant implications for XANES analysis of other systems and elements where particle-size effects will exist for mixtures of relatively dense materials, such as metal sulfides.
4. Summary
Acknowledgment. Use of the APS was supported by the Office of Science, Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC02-06CH11357. The author thanks S. R. Kelemen and M. Sansone for helpful discussions.
Particle-size effects are responsible for attenuating the pyrite signal relative to organic sulfur in Argonne premium coals.
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