Energy & Fuels 1991,5,561-568 flow sintering to occur. Previous work on ash fouling has indicated that the formation of melilites and sulfates contributed to severe fouling.u Plagioclase that was observed in the sintered ash at 950 OC is a normal constituent of superheater deposits.G Formation of the melilites and the sodalites is the result of complex reactions between alkalies, alkaline earth metal sulfates, and aluminosilicates.u Sintering ashes in the laboratory replicates the compounds that are formed in ash deposits on boiler tubes and appears to be a realistic representation of the processes providing greater particle-to-particle contact in ash deposits as a result of viscous sintering. Conclusions
In sintering testa of three different particle size fractions of coal ashes, which were prepared by ASTM ashing procedures (750 "C), the sinter point measured by the electrical resistance method and the thermophysical and thermochemical changes of the ashes were correlated. The sinter point decreased with decreasing particle size. The (44) Rindt, D. K.; Jones, M. L.; Schobert, H. H. In Fouling and Slagging Resulting from Impurities in Combustion Gases; Bryers, R. W., Ed.;Hemisphere: Washington, DC, 1981;pp 17ff. (45) Nicholls, P.; Selvig, W. A. US. Bur. Mines Bull. 1932, 364.
561
results of the sinter point determination using the electrical resistance method indicated the onset of sintering due to the formation of a liquid phase from the lowest melting point components, which themselves may form from the interaction of alkali and alkaline earth metal sulfates, particularly anhydrite, with aluminosilicates or silica, The onset of sintering and the change in electrical resistance are related to the same chemical change in coal ashes, this interaction of alkalies and alkaline earths with the other minerals present. In the compressive strength test, there was an inverse relationship between the sinter strength and the amount of anhydrite in the sintered ash, and a direct relationship between strength and the amount of hauyne. This suggested that the growth of strength is due, at least in part, to a reaction of anhydrite with quartz or clays to form calcium aluminosilicate "glue" to bond unmelted ash particles together. The compressive strength test results also substantiated qualitatively the Frenkel sintering equation. Acknowledgment. Financial support for thisstudy was provided by the Commonwealth of Pennsylvania under the Coal Water Fuel Project. We are pleased to thank John Hurley and Sharon Miller for providing the coal samples used in this work, and Carl Martin for technical assistance.
Demineralization Effects on the EPR Properties of Argonne Premium Coals B. G. Silbernagel* and L. A. Gebhard Corporate Research, Exxon Research & Engineering Co., Annandale, New Jersey 08801
R. A. Flowers I1 and J. W. Larsen Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received December 15, 1990. Revised Manuscript Received March 20, 1991 Because of the nature of their collection and handling, Argonne Premium Coals provide a unique opportunity to study the chemistry of the hydrocarbon and mineral species encountered in coal in its native state. Electron paramagnetic resonance (EPR) observations are reported for Argonne Premium Coals before and after demineralization by either a citric acid wash or HCl/HF treatments. Increases in radical density of up to a factor of 3 are observed after citric acid washing (for Beulah-Zap lignite) and up to a factor of 3.3 after HCl/HF demineralization (Upper Freeport). Changes in g values and line widths are also observed. These data suggest that the carbon radicals in the organic material of the coal are interacting with inorganic paramagnetic species which occur in both mineral and ionic forms. This interaction renders some of the carbon radicals unobservable in the EPR experiment and affects the EPR properties (g values, line widths, microwave saturation behavior) of those that are observable. Demineralization is recommended before quantitative EPR studies of coal organic matter. Introduction
Coals contain unpaired electrons. An 85 w t 9% carbon coal containing 1Ol8 spins/g in its organic matter has one unpaired electron for about every 43000 carbon atoms. The presence of these electrons raises two issues: (1)Are they responsible for any chemical reactions in the coal? (2) Can they be examined by electron paramagnetic resonance (EPR) spectroscopy to gain information about coal structure or reactivity? This paper is concerned with the second issue, We will demonstrate that some of the unpaired electrons on molecules (carbon radicals) couple
magnetically with inorganic matter in the coals. These interactions will render some of the carbon radicals "invisible" in the EPR experiment and will change the EPR properties of others. We are concerned here with the analytical use of coal radicals and therefore will not (reluctantly) discuss their role in chemical reactions. Early work is summarized in van Krevelen's book' and an excellent summary of EPR studies of coals appears in the book by Petrakis and (1) van Krevelen, D. W. Coal; Elsevier: New York, 1981;p 394-99.
0887-0624/91/2505-0561$02.50/00 1991 American Chemical Society
562 Energy & Fuels, Vol. 5, No. 4, 1991
coal Pocahontas No. 3 Upper Freeport Pittsburgh No. 8 Lewiston-Stockton Blind Canyon Illinois No. 6 Wyodak- Anderson Beulah-Zap Beulah Illinois No. 6 (SIU)
rank lvb mvb hvb hvb hvb hvb sub B lig lig hvb
Silbernagel et al.
Table I. Properties of Argonne Premium Coals" hydrocarbon propertiesb selected mineral propertiesc %C H/C 01C % ash % FezO9in ash % Fe total % Fe as pyrite 91.0 0.59 0.021 4.77 15.8 0.53 0.05 85.5 0.66 0.066 13.18 17.3 1.60 1.60 83.2 0.77 0.079 9.25 19.5 1.26 1.13 19.84 82.6 0.76 0.089 0.14 80.7 0.86 0.108 4.71 10.0 0.33 0.24 77.7 0.77 0.130 15.48 18.0 1.95 2.59 8.77 75.0 0.86 0.180 10.2 0.626 0.05 9.72 72.9 0.80 0.209 0.14 74.1 0.66 0.258 80.7 0.66 0.125 15.48 18.0 1.95 2.56
OFrom ref 8. lvb, mvb, and hvb = low-, medium-, and high-volatile bituminous; sub B = subbituminous; lig = lignite. *Hydrocarbon analysis are on a dry, ash-free basis. Iron analyses are for the dry coal.
Grandy.* Radicals persist in coals either because lack of molecular mobility in the solid preventa their combination to form bonds or because the unpaired electrons are sufficiently delocalized on the host molecule to be p table.^ Since the radical population does not decrease when coals undergo extensive molecular rearrangements, their persistence is more likely due to delocali~ation.~The EPR g values of low-rank coals suggest significant localization of the electron on heteroatoms which are part of the molecule, which decreases with increasing rank as the unpaired electrons, are increasingly found on hydrocarbons without associated heteroatoms. The g values increase again at very high rank with the formation of graphitic structures.2 The intensity of the radical signals follows that Curie law as the temperature is varied, demonstrating that the origin of the radicals is not charge t r a n ~ f e r . ~ All of these observations were made on whole coals. We will show that, due to the presence of inorganic species, not all of the existing radicals were observed and that the line shapes and g values are affected by the inorganics. Not all of the published observations (for example, radical densities) are correct. While we have no reason to question the gross structural conclusions, much of the past EPR work on coals will have to be revisited using carefully demineralized coals. Our study of demineralized coals was prompted by large differences in the EPR results obtained when comparing whole coal results with those of demineralized macerals. Several years ago, an extensive series of EPR observations were performed on an series of coal macerals that had been separated by the density gradient centrifugation technique6 In this study, the density of carbon radicals and their associated EPR properties (g values, line widths, and microwave saturation behavior) were examined as a function of rank (Le., extent of coalification) of the coal for materials ranging from subbituminous to low volatile bituminous (lvb) coals. For each of the generic maceral types examined (vitrinites, inertinites, and exinites) distinct trends were observed with increasing rank. The density of carbon radicals generally increases with rank, (2)Petrakis, L.;Grandy, D. W. Free Radicals in Coals and Synthetic Fuels; Elsevier: New York, 1983. (3)Binder, C. R. Ph.D. Thesis, The Pennsylvania State University, 1965; cited in ref 2,p 35. (4)Flowers, R. A., 11; Gebhard, L. A.; Larsen, J. W.; Silbernagel, B.
G. Unpublished observations. (5)Retcofsky, H.L.;Thompson, G. P.; Hough, M.; Friedel, R. A. In Organic Chemistry of Coal; h e n , J. W., Ed.; ACS Symposium Series No. 71; American Chemical Society: Washington, DC, 1978; p 142. Retcofsky, H. L.;Hough, M. R.; Maguire, M. M.Clarkson, R. B. In Coal Structure;Gorbaty, M. L., Ouchi, K., E&.; Advances in Chemistry Series No. 192;American Chemical Society: Washington, DC, 1981;pp 37-58. (6) Silbernagel, B. G.; Gebhard, L. A.; Dyrkacz, G. R.; Bloomquist, C. A. A. Fuel 1986,65,558.
reflecting the chemistry associated with the transformation of the organic matter. The g values fall with increasing rank due to the progressive loss of oxygen from the organic matter. Line widths increased as a consequence of the increasing carbon radical density, as did the ability of the radicals to absorb microwave radiation. In subsequent applications of electron spin echo (ESE) techniques,' this microwave saturation response was demonstrated to arise from both the increase in carbon radical density and more efficient molecular order which occurs in the higher coal ranks. This (EPR) analysis has recently been extended to samples of Argonne Premium Coals. The Argonne Premium Coals are chosen to reflect a broad range of organic matter types and coal ranks, from lignites to low volatile bituminous coals, and the samples have been handled with great care from the time of their collection at the mine faces8 Data are being accumulated by researchers throughout the world and the results are being compiled by researchers at the Argonne National Laboratories. A preliminary survey of these coalse revealed EPR results that were considerably at variance with the isolated maceral studies. Radical densities were unexpectedly low in the higher rank coals, saturation responses were higher in the low rank coals than in the high rank ones, and anomalous g values were observed in a number of cases. These data collectively suggest that the EPR properties are influenced by the presence of magnetic species in the coals, which render some of the radicals unobservable and alter the properties of the ones that can be observed. To test this hypothesis, we have studied a series of Argonne Premium Coal samples: the "as received" parent coals (parent), samples that have been citric acid washed (CAW),IOand samples that have been demineralized with HCl/HF (demineralized).l' We have examined the EPR spectra of the paramagnetic transition-metal species at each stage of the process as well as the properties of the carbon radicals. We find that the mild acidity and the ion-exchange capabilities of the CAW process are sufficient to remove several classes of paramagnetic species (carbonates and iron ionic pairs) and that the details of the process depend upon the rank of the coal. After demineralization, the magnitude of the radical densities and the (7)Thomann, H.; Silbemagel, B. G.; Jin, H.; Gebhard, L. A,; Tindall, P.; Dyrkacz, G. R. Energy Fuels 1988,2,333. (8)Vorres, K. S. Users Handbook for the Argonne Coal Sample Program; Argonne National Laboratories, October 1, 1989. (9)Silbemagel, B. G.; Gebhard, L. A.; Bemardo, M.; Thomann, H. In Magnetic Resonance of Solid Carbonaceow Fuels; Botto, R. E., Ed.; ACS
Advances in Chemistry Series No. 2 2 9 American Chemical Society: Washington, DC, in press. (10) Radenacher, W.; Mohrhauer, P. Brenmt. Chem. 1955,36,236. Bishop, M.; Ward, D. L. Fuel 1958,37,191. (11)Shawver, S.Ph.D. Thesis, University of Tennessee, 1987.
Demineralization Effects on the EPR Properties of Coals
Energy & Fuels, Vol. 5, No. 4, 1991 563
EPR properties are very similar to those observed in the
transferred to a tared bottle and dried to a constant weight a t 110 o c . C. EPR Samples and Studies. The Argonne Premium Coal samples used for the EPR measurements were handled in a nitrogen drybox. A measured amount of coal was transferred into a 4-mm EPR tube and a vacuum pumpout valve was placed on top of the tube. The sample was then removed from the drybox, placed on a vacuum line, evacuated a number of times, and back-filled with helium gas. The sample was then sealed. For normal samples, this evacuation process is necessary to remove molecular oxygen from the pores of the coal, since this paramagnetic molecule can influence the EPR properties.I2 Although we have no reason to believe that oxygen is present in these samples, the usual procedure was employed to ensure that the present samples had the same treatment history as previous ones. Back-filing the tube with helium gas ensures the same thermal response for observations at low temperatures. Evacuation had the added benefit of reducing the amount of water to low levels. The experiments were performed in a Bruker ESP 300 spectrometer, operating at a frequency of 9.8 GHz. Most of the observations reported here were made at ambient temperature. Two kinds of EPR scans have been used: a broad (7 kG)scan a t 1 mW microwave power and 5-G modulation amplitude to examine the transition-metal resonances, and a narrow (45 G) scan at 0.1 mW microwave power and 0.05-G modulation amplitude to examine the carbon radical resonance signals. The data in the following figures will be presented as the derivative of the microwave absorption. Bruker computer software was used to eatabliab baselines and integrate the resulting signals. The g values for the vitrinite component of the coal signals were determined from the zero crossing of the derivative curve and are found to be consistent with the mean value of the integral of the same component. A Varian pitch sample is used as the standard for the g value measurements (taking = 2.00300) and as the secondary standard for the intensity measurements (calibrated with respect to an NBS ruby standard). The accuracies of the measurements are carbon radical density determinations f 5 % g values f0.0001, line widths f10%.
isolated maceral studies, both in systematics and in the absolute magnitude of the effects. We suggest that demineralization is an important first step in the systematic and quantitative study of carbon radicals in coal. Experimental Procedures A. Argonne Premium Coals. Because of the important role that the samples play in this work, it is important to begin with a brief description of their pedigree and properties. These materials are discussed a t length in the Users Handbood for the Argonne Premium Coal Sample Program, edited by Karl S. Vorres of Argonne National Laboratories (ref 8). One ton samples of each of the eight coals were collected as large chunks at the mine face and sealed in an argon atmosphere. They were transferred to Argonne National Laboratories at a controlled temperature and transferred to a nitrogen-filled chamber where the materials were processed to powders of standard mesh size. Small amounts of sample (5- and 10-g lots of -100- and -20-mesh coals, respectively) were prepared in sealed ampules to be sent to the individual research teams. These materials have been examined by many groups and over 150 publications detailing various properties have already appeared. A selected set of elemental data for the eight samples is given in Table I. The wide variations in weight percent of carbon, (H/C),, and (O/C),, show the loss in oxygen and hydrogen with increasing coalification and the consequent increase in the total percentage of carbon in the coal. Except for the LewistonStockton coal, all of the coals contain 285% vitrinite macerals, with 8-10% associated inertinite. Clays are the principal minerals, but significant levels of calcite, quartz, and pyrite are also present. Among inorganic elements, the aluminum and silicon associated with the minerals are the largest components, but there is also a significant amount of iron, occurring a t levels from 0.3 to 3% of the dried coal, a fraction of which is likely to occur in magnetic forms (pyrite, FeSz, is an exception, since iron is diamagnetic in that phase). A comparison of the anticipated levels of total and pyritic iron is given in Table I, and the minor discrepancies which are observed (notably the iron as pyrite sometimes exceeding the total iron) are most likely the consequence of different researchers and techniques employed in the determinations. The as-received levels of water are also indicated in ref 8 and can be as much as 30% for lower rank coals like Wyodak subbituminous and Beulah-Zap lignite. Because many of the EPR analyses will be reported on a per gram basis, it is important to know how much water remains in the coal after the preparation of the sealed EPR samples. We have performed wide-line proton NMR measurements which indicate that there is no more than 1-2 w t % of free water in the present samples.
B. Sample Handling and Preparation. Citric Acid Wash Procedure. Approximately 10 g of coal was weighed into a 500-mL round-bottom flask in a Vacuum Atmospheres drybox. Under a dry nitrogen blanket, 250 mL of 1M aqueous citric acid solution was added. The solution was refluxed for 24 h under dry nitrogen. The mixture was then cooled and filtered through a weighed and dried Soxhlet thimble. The thimble and coal were placed in a Soxhlet extractor and charged with 250 mL of distilled water. The extraction was carried out until the siphon water was determined to be neutral using litmus paper. The thimble containing the coal was dried to constant weight in a vacuum oven at 110 OC. HCl/HF Demineralization Procedure. Approximately 5 g of coal was weighed into a Nalgene beaker. Approximately 40 mL of 49% aqueous HF solution was added to the coal and the mixture was stirred and heated to 60 OC for approximately 1 h under a nitrogen blanket. The coal/HF solution was diluted to 600 mL with distilled water, cooled, and filtered through a medium-porosity sintered glass funnel. The coal was washed with an additional 150 mL of distilled water. The coal was transferred to a glass beaker and 50 mL of 5 N HC1 was added. The solution was stirred and heated to 60 O C for 1 h under nitrogen. The coal/HCl solution was diluted to 250 mL, filtered through a medium-porosity sintered glass funnel and washed with 600-mL of warm distilled water in 25mL portions. Litmus paper was used to determine the acidity of the wash water. The coal was
Experimental Results A. Transition-Metal EPR Spectra. We begin with an analysis of the transition-metal EPR spectra because it provides a measure of the efficacy of the demineralization procedures. The intensities of the various species observed are listed in Table I1 and selected examples of the data are presented in Figures 1-3. Examples of the paramagnetic species in the parent coals are shown in Figures l a and 2a, and these can be identified by reference to previously known spectra of various transition-metal ions.13 In addition to a narrow, intense signal near g = 2 (at a magnetic field of Ho-3400 G ) associated with the carbon radicals, there is a family of six absorptions, also centered near g = 2, which is associated with Mn2+ impurities which occur in calcite. Even though the levels of manganese in the samples are small (- 100 ppm) this very strong resonance signal is observed. In the Beulah lignite parent coal sample (Figure 2a) there is also a strong signal with a g value near 4.3 (Ho = 1800 G), which has previously been identified as coming from iron impurities in quartz (Fe/Si02).13 It occurs in a large number of the samples in this suite and is likely associated with the quartzite in the samples. The most prominent signal, which occurs in practically all of the parent coal samples, is a broad (AHpp N 1700 G)absorption centered around g = 2.44. As we will discuss in section IV, the shape and EPR properties (12) Thomann, H.;Goldberg, 1. B.; Chiu, C.; Dalton,L. R. In Magnetie Resonance, Introduction, Advanced Topics and Applications to Fossil Energy; Petrakie, L., Fraiward, J. P. Eds.; NATO AS1 Series C124; D. Reidel: Dordrecht, The Netherlands, 1984. (13) Wertz, J. E.;Bolton, J. R.Electron Spin Resonance: Elementary Theory and Practical Applications; McGraw-Hill: New York, 1972; Chapter 11.
564 Energy & Fuels, Vol. 5, No. 4, 1991
Silbernagel et al. 300 N -
2
I
z
200
Y
m C
2
100
u
,
n
m
& Lo
I
ffl
c
-200
-0
-100
-
-400 I
-200
-300
d-. 1000
2000
3000
4000
5000 Field
6000
7000
3000
4000
5000
Field
Y
-
2000
:OOO
(gauss!
z
I00
t
6000
700(
(gauss)
n
400-
ci u
C
200
d
C m
m
o I
-200 -400 -600
-
-100
lj
-800 r 1--_L-i-1000 2000 3000
4000
5000
Field
t
400
6000
c
-150
1
I
1
2000
3000
I
I
1;
7000
(gauss!
I'
I
mc
I
-0
Lo
-100
-200 f
-400
-200
-
-
li
I
1
L
~
I
I
-300
-
I
I
I
1
I
1
I
1000
4000
5000 6000 F i e l d (gauss!
700(
Figure 2. Transition-metal EPR properties for Beulah lignite. (a, top) The parent coal shows a very large, dipolar broadened iron pair signal. A strong Fe/Si02signal is also present and there is a small Mn2+/carbonatesignal as well. (b, middle) Citric acid wash completely removes the iron pair and Mn2+/carbonate signals. There is a signal in the vicinity of g = 2 and the remaining Fe/Si02absorption. (c, bottom) Complete demineralization reduces the magnitude of the g = 2 signal.
signal in the starting sample, it is not possible to determine whether some or all of this signal was present in the starting coal. The more rigorous HCl/HF demineralization has little additional effect on the Beulah lignite sample (Figure 2c), but large new signals are introduced in the Pocahontas No. 3 lvb material. A similar effect is observed for the other high-rank material, Upper Freeport mvb coal, and may be an artifact of the demineralization process.
Demineralization Effects on the EPR Properties of Coals
Energy & Fuels, Vol. 5, No. 4, 1991 565
Table 11. Transition-Metal EPR Intensities sample Pocahontas No. 3 parent CAW demineral. Upper Freeport parent CAW demineral. Pittsburgh No. 8 parent CAW demineral. Lewiston-Stockton parent CAW Blind Canyon parent CAW Illinois No. 6 parent CAW Wyodak-Anderson parent CAW Beulah-Zap parent CAW Beulah parent CAW demineral. Illinois No. 6 (SIU) parent demineral. Big Brown Lignite parent CAW demineral.
derivative intensities Fe/Si02 @ = 4.2) Mn'+/carbonate Fe & = 2) 3.6 X lo-' 0.14X lo-' 0.31 X lo-' 0.22 x 10-4
4.22X lo-'
0.40 X lo-' 0.16 X lo-'
1.30 X lo-'
0.79X lo-'
0.59 X lo-' 0.62X lo-'
8.47X lo-'
4.95x 104 0.89 X lo-'
0.15X lo-' 0.48X lo-' 0.28 X lo-'
2.36 X lo4 0.59X lo-' 3.09 X lo-' 0.57X lo-'
0.52X lo-'
0.11x 104 0.45X lo-' 0.27X lo-' 0.36 X lo-'
3000
4000
1.30 X lp
1.60X lo-'
0.71 X lop0
3.26 X lo-'
1.25 X l p
3.37 x lo-'
1.02 x lozo
7.08X lo-'
1.98X l p
5000 6000 F i e l d (gauss)
0.03 X lo-' 0.06x 104
13.9 X lo-'
4.23 X l p
10.8 X lo-'
3.51 X 1020
0.23X lo-' 0.07 X lo-' 0.03X lo-'
6.30X l p 0.17X l p 0.19x lozo
0.04X lo-'
0.36X l@ 0.21 x lolo
0.24X lo-'
0.23X lo-' 0.40X lo-' 0.37X lo-'
0.05 X lo-' 0.09x 10-4 1
2000
3.82X lo-'
0.63X lo-'
400 k
1000
0.67X loa0
0.31 X lo-'
0.72 X lo-' 0.62X lo-'
0.02x 104 0.02x lo-'
2.25 X lo-'
0.40X lo-' 0.91 x lo-'
0.68x lo-' 0.45X lo-'
3.05 X lo-'
0.45 X lo-' 1.31 X lo-' 2.61 X lo-'
Fe ion pair
integrated Fe ion pair, apinsfg of sample
700(
Figure 3. Presence of a new paramagnetic signal after demineralization of the SIU sample of Illinois No. 6 coal.
The most anomalous of the materials is the sample of Illinois No. 6 coal obtained from Southern Illinois University (SIU). This is the only case where the iron pair signal is absent in the parent coal (see Table 11), and the demineralization leads to the appearance of a strong signal centered around g = 2.4 (Figure 3). We suggest below that thisdeparture from the normal behavior may be associated with weathering of the sample prior to the demineralization. B. Carbon Radical EPR. As shown in Figure 4, the carbon radical spectra are smooth, featureless absorption
2.19 X lom 0.24X loa0 0.17X 1@
signals with widths on the order of 5 G. For five of the coals (PocahontasNo. 3, Upper Freeport, Pittsburgh No. 8, Lewiston-Stockton, and Illinois No. 61, narrow and broad components of the signals are observed, as seen in Figure 4a for Pocahontas No. 3 lvb coals. Earlier isolated maceral studies6 suggest that the narrow component is associated with inertinite coal macerals, while the broader component is principally associated with vitrinites. For the lower rank coals (Wyodak, Beulah, Beulah-Zap, Big Brown) only the broad component is observed. The EPR properties are presented in Table 111. Carbon radical density increases dramatically in the higher rank coals after demineralization, with gains of a factor of 2.8 for Pocahontas No. 3,3.35 for Upper Freeport, and 1.85 for Pittsburgh No. 8. Intermediate radical density gains are observed after citric acid washes. Much smaller changes are observed for the lower rank coals. The 2-fold drop in radical density after demineralization of the Illinois No. 6 sample from SIU is likely related to the appearance of the large number of paramagnetic species seen in Figure 3. After this demineralization, the radical densities increase nearly monotonically with increasing coal rank. Furthermore, the observed magnitudes of the radical densities agree closely with those observed previously for isolated vitrinite coal macerals. Demineralization leads to a reduction in the line widths and the g values for most of the samples. The resulting values are consistent with previous experience in the isolated maceral studies6 and with the known chemistry of coalification. For example, most mineral-free low-rank coals have line widths of AHm N 5 G andg 2.0035, while
Silbernagel et al.
566 Energy & Fuels, Vol. 5, No.4,1991
2
m -
Y" Y)
$
loot ao
1
c I 40
60
I
II
I
t
c
-
Table 111. Carbon Radical EPR Data
Y
20
:
o
ffl
-20
-40 -60 -80 -100
3470
500
-200
3480
3490
3500 Field
3510 (gaUSS)
I-
t
I
i \\/
i
F i e l d (gauss)
Figure 4. Typical carbon radical specieg in parent coals: (a, top) Pocahontas No.3 shows a mixture of narrow and broad components; (b, bottom) only a broad component is observed in Beulah lignite.
the corresponding high-rank coals have AHppH 6 G and g 2.0027. These differences in g value and line width have previously been attributed to the relatively smaller numbers of phenoxy radical species in the higher rank coals. It is interesting to note that citric acid washing has the opposite effect in the higher rank coals (Pocahontas No. 3, Upper Freeport, Pittsburgh No. 8, LewistonStockton, and Illinois No. 6), with line widths increasing after the wash. We observe that this is correlated with the appearance of the g = 2 transition-metal signal in the product after the citric acid wash (see,e.g., Figure lb). The ratio of the narrow to broad signals also changes, particularly after the CAW treatments (Table N).The strong broadening and intensity falloff observed for the narrow signals suggests that the inertinites may be particularly strongly affected by migration of the paramagnetic ions during the CAW process. Significant changes in the saturation behavior are also seen. Data for typical samples are shown in Figure 5, where the ratio of the integrated intensity of the carbon radical signal to the square root of the applied microwave power is plotted as a function of the log of the applied power (Le. I / P 1 / 2vs log P ) . We define P as the magnitude of microwave power for which I/P1IJfalls to 0.5 of its low-power value and have listed the PIl2values in Table V. For the vitrinites, we observe an increase in Pl12values with increasing coal rank and the magnitudes are consistent with samples of isolated macerals of comparable rank. An inversion of the order is seen for the inertinites.
sample Pocahontas No. 3 parent CAW demineral. Upper Freeport parent CAW demineral. Pittsburgh No. 8 parent CAW demineral. Lewiston-Stockton parent CAW Blind Canyon parent CAW Illinois No. 6 SIU parent parent CAW SIU demineral. Wyodak-Anderson parent CAW Beulah-Zap parent A parent B CAW Beulah parent CAW demineral. Big Brown lignite parent CAW demineral.
vitrinite G value
radical density, 1019 spins/g of line width, G carbon
2.002 65 2.002 83 2.002 65
0.3115.19 -15.83 0.38j5.02
1.16 1.79 3.27
2.006 4 2.002 7 2.002 65
0.3914.37 0.7316.65 0.515.74
0.78 1.43 2.61
2.002 87 2.003 31 2.002 8
4.74 5.1 1.411533
0.72 1.09 1.33
2.002 79 2.002 77
0.9414.83 1.04j5.11
1.37 1.95
2.003 43 2.003 24
7.57 6.75
1.13 1.16
2.002 94 2.002 74 2.003 97 2.003 83
03914.92 1.1516.02 1.68j7.11 1.0415.65
1.02 0.87 0.99 0.41
2.004 04 2.003 49
7.93 5.29
1.56 0.76
2.002 97 2.002 83 2.003 33
6.84 8.02 5.66
0.35 1.52 1.03
2.003 66 2.003 43 2.003 46
5.83 5.01 4.92
0.18 0.39 0.32
2.004 09 2.003 47 2.003 57
6.65 5.11 5.01
0.17 0.21 0.18
Table IV. Variation of InertinitdVItrinite Ratios with Treatment" sample inertinitet vitriniteb LU€-",G Pocahontas No. 3 6.6719.43 0.3110.28 parent CAW c c demineral. 5.97 0.38 Upper Freeport parent 0.3910.36 5.3416.69 CAW 1.61 0.73 4.04 0.50 demineral. Pittsburgh No. 8 parent cj2.0 ~10.50 CAW C c demineral. 0.75 1.41 Lewiston-Stockton parent 0.69/1.21 0.9410.91 CAW 0.32 1.04 Illinois No. 6 parent 0.8011.98 1.1510.55 CAW 0.86 1.68 For parent coala we include data from two APC batches: current samples/samples from ref 9. bIntansity ratios from derivative data. Trace inertinite component observed.
This may be associated with the large paramagnetic signals seen in the demineralized Pocahontas No. 3 and Upper Freeport samples.
Discussion The present observations demonstrate that ion exchange and demineralization procedures remove or alter the
Energy & Fuels, Vol. 5, No. 4, 1991 567
Demineralization Effects on the EPR Properties of Coals 600
w-
t
in
4 I
>
Y
400
C Lo
: 200 H
F rc40
z8
c
2c
Upper Freeport demin Beulah Zap CAW Pittsburgh 8 demin Pocahontas demin
-200
-400
1000
2000
3000
4000
5000
Field
6000 (gauss1
700(
Figure 6. Integrated intensity of the iron pair signal in as-received Illinois No. 6 coal. The intensity shows the characteristic shape of a Pake doublet,13which comes from the dipolar coupling of
two magnetic spins.
P s 0 40
z8
3 2c.
-
w-wa Pittsbur h 8 demin
Pocahontas demin Upper Freeport demin
Figure 5. (a, top) Saturation response after demineralization. Saturation response is similar to that of isolated macerals of comparable rank. (b, bottom) Saturation response of Pocahontas and Upper Freeport inertinites. Saturation response remains somewhat low after demineralization, suggesting that some magnetic impurities may still be sequestered in the inertinite macerals. Table V. Selected Saturation Results Pll2, m W parent coal
demineralized coal
3.0 19.0
22.0 19.0
4.0 21.0
15.0 32.0
vitrinite inertinite
1.2 71.0
9.8 100.0
Beulah-Zap vitrinite
19.0
11.00
Pocahontas No. 3 vitrinite inertinite Upper Freeport vitrinite inertinite Pittsburgh No. 8
I, CAW only.
transition-metal ions that occur in the coal. The removal of the minerals or organically complexed ions has a substantial effect on the EPR properties of the carbon radicals in the coal. After the demineralization process the remaining radicals have the EPR parameters and microwave saturation behavior previously encountered in isolated coal macerals. One major concern is whether carbon radicals are being created during the course of the citric acid washing or demineralization processes. In the earlier isolated coal maceral studies this issue was examined at length, with carbon radical behavior measured at each stage of the process: starting coals, grinding, demineralization, and centrifugati0n.B The effects were found to be minor in that case. Chemically, it is difficult to see how exposure to aqueous citric acid at room temperature could cause the bond cleavage or the redox process necessary to increase
the radical population. The only reasonable explanation is that existing radicals are being made visible by the removal of ion-exchangeableimpurities. In the present case, the systematic changes in the g value, line width, and saturation properties are consistent with the removal of paramagnetic impurities rather than the creation of substantial numbers of new radicals. In the higher rank coals, the increase in radical density which is observed is associated with "unmasking" of radicals which were rendered unobservable by the presence of the paramagnetic impurities. It is also clear that there are a number of types of paramagnetic impurities, some of which are ion exchangeable and others which are associated with the minerals. The ions removed by the CAW procedure strongly influenced the number of carbon radicals and their EPR properties. This would be expected if these exchangeable species were located on the surface of coal pores, since the paramagnetic spcies would then be in contact with the organic matter at the atomic level. It is also interesting to speculate on the source of the iron pair signal seen in all of these samples. The line shape observed, Figure 6, known as a Pake doublet14arises from two magnetic dipoles in close proximity to one another. At higher density, adjacent pairs can also contribute to the width of the line, as can be seen by comparing the signals from Pocahontas No. 3 and Beulah-Zap. The width of the Pake doublet spectrum is related to the magnitude of the dipoles and their spacing by the expression AH = gpB/1'9, from which one deduces a spacing between the iron ions of -2.75 A,in good agreement with the distance of -3 A expected on the basis of ionic radii for the ions. The g value of 2.44 is also consistent for a pair of Fe3+spins." There is considerable evidence for the presence of such paired iron ions.15 At low pH the preferred form of iron in aqueous solution is in the form of dimers and a common iron form resulting from such solutions is FeOOH, which is known to occur naturally as the minerals Lepidocrocite and Goethite. One question which should be addressed is the stability of the paramagnetic species observed here. The Argonne coals have been subjected to a minimum of exposure to the atmosphere and it may be that some of the species seen (14) Pake, G. E.J. Chem. Phys. 1948,16, 327. (15) Cotton, F. A.; Wilkinson, G. Adoanced Organic Chemistry: A Comprehensive Text; Interscience-Wiley: New York, 1972; pp 858-84.
Energy & Fuels 1991,5, 568-573
568
here may be lost after extended exposure to the atmosphere, either by oxidation or loss of humidity. For example, fairly mild heating of FeOOH (at 200 "C)produces cu-Fe20, (hematite). Three of the samples examined here have not had the same rigorous protection from the atmosphere: the SIU sample of Illinois No. 6 coal and the Beulah and Big Brown Lignites. The SIU Illinois No. 6 coal is conspicuous in that it is the only material in which the iron pair signal is not observed; prominent signals are present in both of the lignites. The present observations have important implications for the study of carbon radicals in coals. The presence of mineral matter can significantly alter the EPR properties of carbon radicals, particularly in the higher rank coals. These results suggest that a significant fraction of the
radicals may not be observable in coal. Furthermore, the radicals that are observable may not be representative of the coal, but may be a subset which are less influenced by the magnetic impurities. We suggest that such factors be carefully considered when analyzing the EPR properties of coals and their products. Acknowledgment. We are particularly grateful to A. R. Garcia for his assistance in preparation of the samples and for the use of wideline NMR techniques to determine the level of water in the EPR samples. We have benefited from a number of stimulating discussions with G. N. George, M. L. Gorbaty, and J. C. Scanlon. Ragistry No. Citric acid, 11-92-9;hydrofluoric acid, 1664-39-3; hydrochloric acid, 7647-01-0.
Speciation of Uranium in a South Texas Lignite: Additional Evidence for a Mixed Mode of Occurrence Mysore S. Mohan,* J. Drew Ilger, and Ralph A. Zingaro Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 Received December 21, 1990. Revised Manuscript Received March 20, 1991
New experimental evidence indicates that uranium in a Jackson lignite (Karnes County, South Texas) occurs principally (70-90%) in the form of uranyl humates and the rest in the form of poorly crystallized mineral(s). The low-temperature ash (LTA) of the lignite was separated by density gradient fractionation. Examination of the fractions by scanning electron microscopy-energy-dispersive X-ray spectrometry showed the presence of grains (10-30 pm) of uranium minerals only in the highest density (>2.90 g/mL) fraction. The X-ray spectral data indicate that the uraniferous grains are composites of coffinite (U(Si04)l-JOH)~Jand possibly its alteration product uraninite (UO,). Minor amounts of autunite (Ca(U02)2(P04)2-10-12Hz0) also appear to be present. Much of the uranium (-90%) which is extremely fine-grained is recovered in the lower density fractions and probably originates from the destruction of uranyl humates during the ashing. Extractions of the ground lignite with HC1 solutions of varying pH (0.7-3.0) have been carried out. The experimentally obtained uranium extraction efficiencies were compared with corresponding values calculated assuming a predominantly humic mode of occurrence for uranium. If suitable simplifications are made, the agreement is quite good. Introduction
In our earlier publications'J we reported the results of some detailed investigations on the modes of occurrence of uranium in a South Texas (eocene) lignite. A variety of experimental strategies which included float-ink separations, solvent extractions, and characterization of lignite-derived humic fractions strongly indicated a mixed mode of occurrence of uranium. Approximately 60-80% of the total uranium appears to be bound to the humic fraction, probably in the form of uranyl humates. The remainder appears to be present as fine-grained minerals. As has been noted in the earlier a precise differentiation between uranium which is truly organically associated, e.g., as uranyl humates, and fine-grained uranium minerals intimately associated with the organic matrix is difficult to establish unequivocally. Even the comparatively mild techniques used to isolate humic ma(1) Mohan, M. s.; Zingaro,R. A.; Macfarlane, R. D.; Irgolic, K. J. Fuel 1982,61,853-858. (2) Ilger, J. D.; Ilger, W. A.; Zingaro, R. A,; M o b , M. S.Chem. Geol. 1987,63, 197-216.
terial, such as extraction with dilute alkali followed by precipitation, inevitably causes significant disruption of the original matrix and the information obtained may not be certain with respect to elucidating the original mode of occurrence. Also, the identification and characterization of the accessory minerals (such as the U-containing minerals) in the unmodified coal or even in its low-temperature ash (LTA) will be difficult because of their (comparatively) low concentrations. In the investigation reported in this paper we have attempted to further characterize the uranium by methods which should result in a minimal disruption of the sample matrix under study. A U-containing accessory mineral has been characterized by scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDS) after preconcentration by density-gradient separation of the low-temperature ash (LTA). The uranium-humic acid association has been investigated by extracting the ground coal with dilute hydrochloric acid solutions at different pH values. The experimentally obtained uranium extraction efficiencies have been compared with those that would be expected if the
0881-0624/91/2505-0568$02.50/00 1991 American Chemical Society
