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On the identification and origin of pseudovitrinite
International Journal o f Coal Geology, 4 (1985) 309--319 Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands
309
ON THE IDENTIFICATION AND ORIGIN OF PSEUDOVITRINITE
DENNIS D. KAEGI Raw Materials and Primary Research, Inland Steel Research Laboratories, East Chicago, I N 46312, U.S.A.
(Received June 6, 1984; revised and accepted January 21, 1985)
ABSTRACT Kaegi, D.D., 1985. On theidentiflcation and origin ofpseudovitrinite. Int. ~ Coal Geol., 4:309--319. Representative subsamples of Pocahontas No. 3 coal obtained at a fresh mining cut in a coal mine in West Virginia, U.S.A., were oxidized in air at 50°C or stored at ambient temperature in argon. Periodically, subsamples were removed from their locations and analyzed both petrographically and through Gieseler plastometry. With increasing oxidation, as indicated by plastometer results, the percentage of vitrinite decreased while the percentage of oxyvitrinite increased. Also, the percentage of slitted pseudovitrinite initially increased, but, subsequently decreased. The same trends occurred for both the 50°C and ambient samples, although at a lower rate for the ambient samples. This has been interpreted as indicating that slitted pseudovitrinite is an oxidation product of vitrinite that is intermediate, in terms of degree of oxidation, between vitrinite and oxyvitrinite. Vitrinite and slitted pseudovitrinite reflectances were found to be approximately the same, however, a shallow trend of decreasing reflectance with degree of oxidation may be indicated. No explanation is submitted for an observed decrease (with oxidation) in percentage of micrinite; however, an explanation of the origin of slitted structures in vitrinite material as it relates to cell structure and geological conditions is offered.
~TRODUCTION
A p p r o x i m a t e l y 15 years ago, B e n e d i c t et al., ( 1 9 6 6 ) p r o p o s e d t h a t s o m e vitrinite behaves d i s t i n c t l y d i f f e r e n t l y d u r i n g c a r b o n i z a t i o n , f r o m t h e bulk o f t h e vitrinite in c o k i n g coal. T h e y f u r t h e r p r o p o s e d criteria f o r distinguishing this material a n d n a m e d it " p s e u d o v i t r i n i t e " . T h e y f o u n d it to o c c u r in m o s t , if n o t all, coals a n d classified it as a semi-inert maceral. T h e degree o f inertness was assigned b y c a l c u l a t i o n based on the d i f f e r e n c e in reflect a n c e b e t w e e n p s e u d o v i t r i n i t e and vitrinite {Benedict et al., 1 9 6 8 ) . With this innovative t r e a t m e n t , t h e y were able to achieve satisfactorily a c c u r a t e p e t r o g r a p h i c p r e d i c t i o n s o f c o k e stability in cases w h e r e less t h a n satisfact o r y a c c u r a c y had been achieved w i t h o u t this t r e a t m e n t . Also, based o n available i n f o r m a t i o n , it was suggested t h a t p s e u d o v i t r i n i t e was given its
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©1985 Elsevier Science Publishers B.V.
310 characteristics (as they differ f~om vitrinite) b y moderate oxidation of the parent vegetal material at an early stage in the coalification process. In support of the hypothesis that pseudovitrinite was formed by oxidation during the earliest stages of coalification, Benedict et al. (1966) listed the following points: (1) Some features of pseudovitrinite were produced in oxidation studies of Benedict and Berry (1965). (2) Coals with pseudovitrinite exhibit chemical properties similar to those produced by oxidation. (3) Pseudovitrinite is poorly reacting in the coking process and in its rheological properties, as are oxidized coals. (4) Pseudovitrinite is devoid of pyrite, thus suggesting a non-reducing environment. (5) Pseudovitrinite is most abundant in coal seams containing large percentages of inert macerals, which probably originated by an oxidative process. A number of the supporting points for the hypothesized origin of pseudovitrinite concern observations that low-temperature (92 ° C) oxidation of coal produced features and effects similar to those in and of pseudovitrinite (Benedict and Berry, 1965). The pseudovitrinite concept was further evaluated to determine its merit for inclusion as a separate entity for identification during petrographic analysis (Kaegi, 1981}. Part of this evaluation concerned a study of the origin of slitted pseudovitrinite as related to coal oxidation. In light of the observation that pseudovitrinite-like features could be produced by oxidation (Benedict et al., 1966), it was hypothesized that some or all pseudovitrinite is produced during the oxidation of coal rather than through precoalification oxidation. The current paper presents some of the results of experimentation conducted to study this hypothesized origin of pseudovitrinite. IDENTIFICATION OF PSEUDOVITRINITE Prior to extensive experimentation, it was necessary to consider the criteria for identification o f pseudovitrinite in polished particulate sections. The identification criteria of Benedict et al. (1966) are: (1) Greater reflectance from polished surfaces in oil. (2) Slitted structures. (3) R e m n a n t cellular structures. (4) U n c o m m o n fracture patterns. (5) Higher relief. (6) Paucity or absence of pyrite inclusions. (7) Occurrence usually as comparatively large particles. The first three of these criteria were stated as being sufficient for identifying pseudovitrinite in high-volatile and medium-volatile coals, but the other criteria were needed for tow-volatile coals because the reflectance difference
311
and occurrence of remnant cellular structure diminished at this rank (Benedict et al., 1966). In all cases where comparison to another maceral was implied, that comparison was to vitrinite. Such comparison is, in fact, necessary for all criteria except those of slitted structure and remnant cellular structure. In attempting to utilize the above criteria, the need for comparison to vitrinite was found to severely restrict the application of the pseudovitrinite concept. This is because multi-seam, multi-mine, or blend samples usually contain coals o f different rank and different optical properties. Many of the criteria required comparison of the suspected pseudovitrinite to vitrinite from the same coal. In multi-rank samples, however, it is often impossible to determine from which coal a suspected pseudovitrinite particle is derived and, thus, to which vitrinite it should be compared. As one example, while using the original pseudovitrinite identification criteria, periodic analyses of preparation plant product coal showed significant fluctuations in pseudovitrinite content depending on the locations of the numerous mining units that contributed to the preparation plant feed at any specific time. It was found, however, that this was due to a geographic trend in rank, such that some mining units produced slightly higher reflectance coal than did others. When these units were operating, the total preparation plant feed appeared to contain vitrinite and slightly higher reflecting pseudovitrinite, while petrographic analysis of the coal mined by the individual units showed none of the coal to contain appreciable amounts o f pseudovitrinite. Because o f the problems associated with the criteria that required comparison to other particles, it was decided b y the author that these criteria were inappropriate since they were not generally applicable. Furthermore, it was thought that, since the remnant cellular structure criteria had already been incorporated into maceral identification as a means of distinguishing telinite from coUinite, it should not also be used as a criteria for distinguishing other vitrinite material. In addition, the pseudovitrinite characterized by Benedict et al. (1966) is considered to be semi-inert, yet Hacquebard (1958) concluded that cellular vitrinite (e.g., telinite)possessed more coking ability than associated noncellular vitrinite (e.g., collinite). This apparent disagreement a b o u t the behavior o f cellular vitrinite has yet to be resolved, b u t it may be caused by the grouping of materials (that behave differently from each other) in the original definition o f pseudovitrinite. Regardless of the reason for this discrepancy, it was additional cause not to use the occurrence of cellular structure as an identification criterion for pseudovitrinite. From the foregoing discussion it is apparent that the only original criterion for pseudovitrinite identification that was not rejected by the author was that of slitted structure. This feature has been found to be a relatively simple one to incorporate into an analytical procedure, and one that is n o t restricted to use on individual coal " m o n o - r a n k " samples. The use o f this criterion can become complex or useless for very small particles, as can any morphological feature. However, it has been found that for maximum parti-
312 cle diameters between approximately 150 tzm and 2000 pm, identification of the slitted structure is n o t significantly affected by particle size. Therefore, the only feature currently used by the author to distinguish vitrinite and pseudovitrinite is the occurrence of slits. Specifically, any vitrinitic material t h a t contains one or more slits, n o t apparently due to sample preparation or inorganic inclusions, is called slitted pseudovitrinite. Examples of this pseudovitrinite are shown in Fig.1.
Fig.1. Photomicrographs of slitted pseudovitrinite particles. Reflected light, oil immersion, 625x.
Since m a n y of the pseudovitrinite identification criteria proposed by Benedict et al. (1966) are not utilized by us, the propriety of the use of the name "pseudovitrinite" in describing only vitrinite material that contains slits may be questioned. However, it appears that this material would be identified as pseudovitrinite by use of the original criteria, and, therefore, use of only the slitted structure criterion results in identifying a subset of the total pseudovitrinite assemblage as originally defined. Also, the slitted structures criterion is the only criterion which Benedict et al. (1966) stated for use for high-, medium-, and low-volatile coals. EXPERIMENTAL The coal chosen for experimentation was one known to contain varying amounts of slitted pseudovitrinite. Specifically, a 4.5-kg channel sample of Pocahontas No. 3 seam coal was taken from a working, momentarily
313
exposed, mining face. Immediately upon sampling, the coal was placed in an air-tight glass container and purged with nitrogen. The sample was thus maintained until the following day; at which time it was further processed. The nitrogen-protected sample was divided into eleven representative subsamples. One of these subsamples was prepared and analyzed immediately, and was designated as " f r e s h " or as having zero oxidation time. Onehalf of each of the remaining ten subsamples was placed in an enameled steel pan, each sample forming a 15-mm-thick layer, and exposed to the atmosphere in an oven maintained at 50°C. These subsamples will hereafter be referred to as the 50°C samples. The remaining one-half of each subsample was placed in a steel can, which was then purged with argon prior to sealing. These subsamples were maintained at room temperature (23°C), and will hereafter be referred to as the ambient samples. After 7, 30, 60, and 120 days, a 50°C and an ambient sample were removed from their containers and characterized by several techniques that have been reported to be good indicators of coal oxidation. As an indication of the extent of oxidation, Gieseler plastometer analyses will be discussed here. The plastometer tests were conducted in accordance with accepted standards (ASTM, 1978), and petrographic analyses were conducted as described earlier (Kaegi, 1981). RESULTS
Table 1 and Figs. 2 and 3 present the plastometer and petrographic data for the samples. Gieseler plastometry indicated relatively severe oxidation of the 50°C samples and less severe oxidation of the ambient samples. For example, m a x i m u m fluidity decreased from about 1400 ddpm to about 80 ddpm for the sample exposed at 50°C for 120 days, and to about 690 ddpm for the samples stored at ambient temperature for 120 days (Fig. 2). Similarly, softening temperature and temperature of maximum fluidity
1600
1200
800
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400
x
0
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I
~------ambient h
I
!
40 80 120 Time (days) F i g . 2 . M a x i m u m fluidity as a f u n c t i o n o f length o f o x i d a t i o n t i m e . 0
314 60
I~llL'~"~"',~lambient 50
v ¢n c
vitrinite
50° C ~
40
30 20
.o_ Q. o} o
pseudovitrinite
10
~L E o
0
u_ 30
E _= o
2010 ~ S_.
0
~
oxyvitrinite
,e°,
40 80 120 Tm i e(days)
Fig.3. Petrographic
composition
as a function
of length of oxidation
time.
increased rapidly with time for the 500(7 samples and more slowly for the ambient samples (Table 1). Petrographic analyses showed a rapid increase in the oxyvitrinite content for the 50°C samples and a less rapid increase for the ambient samples. Oxyvitrinite, an unofficial maceral category, represents the material classically recognized as originating from low-temperature oxidation (weathering) or vitrinite material. Examples of oxyvitrinite are shown in Fig.4. Additionally, the slitted pseudovitrinite content appeared to peak at an intermediate oxidation time for the 50°C samples and increased with a possible final decrease at the longest oxidation time for the ambient samples. Decreases in vitrinite content occurred in both the 50°C and ambient sample series. Micrinite c o n t e n t and vitrinite reflectance appeared to decrease for the 50°C samples, while similar trends in the ambient samples were not apparent (Table 1). Also, vitrinite and slitted pseudovitrinite reflectances appear to be about the same at a given condition and time (Table 1).
* M e a n m a x i m u m in oil
Vitrinit e reflectance* Pseudovitrinite reflectance*
Vitrinite S litted p s e u d o v i t r i n i t e Oxyvitrinite Fusinite S emifusinite Macrinite S e m i m acrinite Micrinite Exinite
Petrography (%)
1.31 1.33
55.1 8.4 9.9 4.3 19.6 0.1 1.3 11.0 0.2
M a x i m u m f l u i d i t y Qddpm) 1296 S o f t . t e m p e r a t u r e (~C) 407 Maximum fluidity temp. (°C) 462 Solid. t e m p e r a t u r e ( ° C ) 503
Gieseler plastometry
60
120
1.31 1.31
51.0 12.7 1.7 5.2 18.5 0.0 2.6 19.0 0.0
1007 409 462 499
1.30 1.32
47.1 14.1 5.5 4.5 24.4 0.2 1.6 8.0 0.2
420 414 468 504
1.30 1.27
39.0 10.5 12.0 3.0 26.1 0.2 2.3 7.0 0.0
172 428 471 501
1.27 1.28
28.9 4.5 29.0 4.0 25.0 0.2 2.3 6.0 O.1
79 427 474 502
1.32 1.30
55.9 5.4 1.1 5.0 20.1 0.0 1.6 11.5 0.4
1404 404 463 500
1.32 1.33
56.1 5.9 1.0 4.6 19.1 0.0 0.8 12.1 0.2
1343 407 461 500
1
1.32 1.33
56.0 6.0 1.0 5.2 19.1 0.0 0.7 12.0 0.1
1233 407 465 498
7
1.28 1.34
55.2 6.0 2.0 4.1 18.7 0.2 1.6 12.2 0.4
1088 410 468 508
30
0
30
1
7
Stored at a m b i e n t t e m p e r a t u r e (days)
Exposed at 50°C (days)
Gieseler p l a s t o m e t e r a n d coal p e t r o g r a p h i c a n a l y s i s r e s u l t s
TABLE 1
1.31 1.34
50.8 7.2 5.0 3.9 18.6 0.6 2.1 11.8 O.O
723 419 469 504
60
1.32 1.33
48.7 4.9 10.4 4.4 19.2 0.1 1.3 11.0 0.0
691 417 469 504
120
c~ h.a
316
Fig.4. Photomicrographs of slitted pseudovitrinite particles. Reflected light, oil immersion, 625x. DISCUSSION The noted changes in plastometer results confirmed the expected oxidation of the coal with less severe effects for the ambient than for the 50°C samples. This relatively milder oxidation for the ambient samples was expected considering the lower temperature and reduced availability o f oxygen. However, it appears that oxygen trapped with the ambient coal was sufficient to effect oxidation. Thus, the data represent two different rates of oxidation on the same coal, the lower rate being for the ambient samples. This is shown graphically in Fig.2. While the plastometer data may possibly be of significance in regard to the storage of reference coal samples for long periods of time, of immediate interest relative to the pseudovitrinite issue are the petrographic indications o f oxidation. Figure 3 shows plots of the percentage of vitrinite, oxyvitrinite, and slitted pseudovitrinite for both the 50°C and ambient samples as a function of time. Considering the 50°C samples first, the vitrinite content steadily decreased while the oxyvitrinite content steadily increased with time. The slitted pseudovitrinite content initially increased, b u t after 30 days of exposure, thereafter decreased. As would be expected, the decrease in vitrinite content is largely accounted for by the increase in oxyvitrinite. This is because, as vitrinite becomes increasingly oxidized, it would more frequently be identified as oxyvitrinite. However, to fully account for the vitrinite
317
decrease, the slitted pseudovitrinite content must also be considered. This is more easily seen in Fig.5, which is a plot o f the sum of the oxyvitrinite and slitted pseudovitrinite c o n t e n t in the 50°C samples versus oxidation time. Also shown is the vitrinite percentage versus time. The two curves are essentiaUy mirror images o f each other. This is interpreted as indicating that oxyvitrinite and slitted pseudovitrinite can both be formed by oxidation o f vitrinite. Also, since the later decrease in slitted pseudovitrinite is accompanied by an acceleration in oxyvitrinite formation, it appears that increased oxidation converts slitted pseudovitrinite to oxyvitrinite.
°
60-
\ -.o
O~R 4 0
" I L vitrinite
O...N E~
oca ~-,, , ~ : 20 E o )
0
~l=~..-II="~se ~
1
0
ud~ v Jtr,nit e oxyvitrinite
I
I
40 80 Time ( d a y s )
I
120 "
Fig. 5. C o n c u r r e n t decrease in v i t r i n i t e c o n t e n t a n d increase in t h e s u m o f p s e u d o v i t r i n i t e and oxyvitrinite content.
The conversion of slitted pseudovitrinite to oxyvitrinite is, perhaps, not t o o surprising if the identification criteria for slitted pseudovitrinite and oxyvitrinite are considered. It appears that the slitted structure of pseudovitrinite (shown in Fig.l) can increase in intensity as the particle becomes further oxidized. Eventually, as the slits grow in size and begin to intersect, the particle takes on the appearance of brecciated oxyvitrinite (shown in Fig.4). Therefore, in this context, slitted structures may be considered to be preliminary indications o f oxidation, and pseudovitrinite identified on the basis o f slitted structure may be considered an oxidation product intermediate between vitrinite and oxyvitrinite. With this scenario, one can readily rationalize the designation of pseudovitrinite as a semi-inert. Also, these observations support earlier findings of Roga and Pampuch (1956) who reported that the oxidation of coal in either air or water generated fissures in the oxidized particles, thus, documenting the well known decrepitation of coal as it weathers. The fact that the slitted structures generally form in perpendicular preferential directions not necessarily related to bedding plane may indicate that slits form at the sites of remnant closed cell lumens. If this is correct, the degree to which the cell walls are sealed to one another would influence the rate at which these cells open (to form slits) during oxidation. Therefore, the vitrinite that proceeds through slitted pseudovitrinite as an oxidation intermediate may be that with more poorly sealed or fused remnant
318
cells. If this is the case, then it would be expected that original source vegetation and subsequent geological conditions would influence the amount of slitted pseudovitrinite that would form as a result of oxidation. One might extrapolate this theory to hypothesize that some coals might n o t produce any slitted pseudovitrinite when oxidized because of the coal's origin and/or coalification history. In regard to the ambient samples, the same trends in vitrinite, slitted pseudovitrinite, and oxyvitrinite contents were found, although the degree of these changes was less severe. It is n o t certain that the slitted pseudovitrinite content of the ambient samples increased and subsequently decreased as did that of the 50 ° C samples, since only one data point was obtained for a period of time longer than that time at which the apparent highest slitted pseudovitrinite c o n t e n t was observed. However, if it is assumed that these data (for 120 days of storage) do indicate that the peak in slitted pseudovitrinite content was reached after 60 days of storage, then the slitted pseudovitrinite peak for the ambient samples was lower than the slitted pseudovitrinite peak for the 50°C samples. Since the data are n o t conclusive, the point will n o t be belabored. In the context of the proposed role of remnant cellular structure in slitted pseudovitrinite formation, however, such an effect would be expected, since the driving force (heat) for opening the partially sealed cells was lower for the ambient samples than for the 50°C samples. The slitted pseudovitrinite present in the "fresh" coal may also have been produced b y post-coalification oxidation. Coal seams are permeable enough to allow the flow of oxygen-containing gas or liquid (air or water) through them. This permeability is utilized in underground gasification (Elder, 1963), and is demonstrated by the existence and movement of methane in coal seams. Additionally, Benedict et al. (1966) n o t e d that many of the microscopic features o f their pseudovitrinite were produced in the coal oxidation study of Benedict and Berry (1965). With this thought, perhaps the use of the term "fresh" for recently mined or sampled coal should be reconsidered. The presence o f pseudovitrinite in certain coals may simply indicate in situ oxidation of the coal. The decrease in micrinite content for the 50°C samples has not been explained. Initially, it was thought that brecciation of vitrinite as it oxidized might be interfering with the identification o f micrinite contained in the vitrinite. However, the lack o f a similar trend in the ambient samples (even though over 10% oxyvitrinite was produced) refutes this theory. CONCLUSIONS
Oxidation of a medium-volatile coal showed that the proportion of vitrinite in the coal decreased, while the oxyvitrinite content increased, and the slitted pseudovitrinite content went through a maximum. From these results it is concluded that: (1) At least some pseudovitrinite is produced by low-temperature oxidation.
319
(2) Upon continued oxidation, slitted pseudovitrinite assumes the morphology of brecciated vitrinite (oxyvitrinite). (3) Slitted pseudovitrinite is a product of oxidation intermediate between vitrinite and oxyvitrinite.
REFERENCES American Society for Testing and Materials, 1978. ASTM part 26. Benedict, L.G. and Berry, W.F., 1965. Recognition and measurement of coal oxidation. Presented at GSA Coal Group, Miami. Benedict, L.G., Thompson, R.R., Shigo, J.J., III, and Aikman, R.P., 1966. Pseudovitrinite in Appalachian coking coals. Presented at ICCP Nomenclature, Madrid. Benedict, L.G., Thompson, R.R. and Wenger, R.O., 1968. Relationship between coal petrographic composition and coke stability. Blast Furn. Steel Plant, 56: 217--224. Elder, J.L., 1963. The underground gasification of coal. In: H.E. Lowry (Editor), Chemistry of Coal Utilization. Wiley and Sons, New York, N.Y., pp. 1023--1040. Hacquebard, P.A., 1958. The value of a quantitative separation of the maceral vitrinite into its constituents telinite and collinite for the petrography of coking coals. Proc. Third Int. Congr. Coal Petrology, Heerlen, pp. 131--138. Kaegi, D.D., 1981. Predicting coke stability form coal petrographic analysis. ISS-AIME Ironmaking Proc., 40: 381--392. Roga, B. and Pampuch, R., 1956. Weathering of Coal. Prace Glownege Inst. Gornictwa, Ser. B, 189 pp.
309
ON THE IDENTIFICATION AND ORIGIN OF PSEUDOVITRINITE
DENNIS D. KAEGI Raw Materials and Primary Research, Inland Steel Research Laboratories, East Chicago, I N 46312, U.S.A.
(Received June 6, 1984; revised and accepted January 21, 1985)
ABSTRACT Kaegi, D.D., 1985. On theidentiflcation and origin ofpseudovitrinite. Int. ~ Coal Geol., 4:309--319. Representative subsamples of Pocahontas No. 3 coal obtained at a fresh mining cut in a coal mine in West Virginia, U.S.A., were oxidized in air at 50°C or stored at ambient temperature in argon. Periodically, subsamples were removed from their locations and analyzed both petrographically and through Gieseler plastometry. With increasing oxidation, as indicated by plastometer results, the percentage of vitrinite decreased while the percentage of oxyvitrinite increased. Also, the percentage of slitted pseudovitrinite initially increased, but, subsequently decreased. The same trends occurred for both the 50°C and ambient samples, although at a lower rate for the ambient samples. This has been interpreted as indicating that slitted pseudovitrinite is an oxidation product of vitrinite that is intermediate, in terms of degree of oxidation, between vitrinite and oxyvitrinite. Vitrinite and slitted pseudovitrinite reflectances were found to be approximately the same, however, a shallow trend of decreasing reflectance with degree of oxidation may be indicated. No explanation is submitted for an observed decrease (with oxidation) in percentage of micrinite; however, an explanation of the origin of slitted structures in vitrinite material as it relates to cell structure and geological conditions is offered.
~TRODUCTION
A p p r o x i m a t e l y 15 years ago, B e n e d i c t et al., ( 1 9 6 6 ) p r o p o s e d t h a t s o m e vitrinite behaves d i s t i n c t l y d i f f e r e n t l y d u r i n g c a r b o n i z a t i o n , f r o m t h e bulk o f t h e vitrinite in c o k i n g coal. T h e y f u r t h e r p r o p o s e d criteria f o r distinguishing this material a n d n a m e d it " p s e u d o v i t r i n i t e " . T h e y f o u n d it to o c c u r in m o s t , if n o t all, coals a n d classified it as a semi-inert maceral. T h e degree o f inertness was assigned b y c a l c u l a t i o n based on the d i f f e r e n c e in reflect a n c e b e t w e e n p s e u d o v i t r i n i t e and vitrinite {Benedict et al., 1 9 6 8 ) . With this innovative t r e a t m e n t , t h e y were able to achieve satisfactorily a c c u r a t e p e t r o g r a p h i c p r e d i c t i o n s o f c o k e stability in cases w h e r e less t h a n satisfact o r y a c c u r a c y had been achieved w i t h o u t this t r e a t m e n t . Also, based o n available i n f o r m a t i o n , it was suggested t h a t p s e u d o v i t r i n i t e was given its
0166-5162/85/$03.30
©1985 Elsevier Science Publishers B.V.
310 characteristics (as they differ f~om vitrinite) b y moderate oxidation of the parent vegetal material at an early stage in the coalification process. In support of the hypothesis that pseudovitrinite was formed by oxidation during the earliest stages of coalification, Benedict et al. (1966) listed the following points: (1) Some features of pseudovitrinite were produced in oxidation studies of Benedict and Berry (1965). (2) Coals with pseudovitrinite exhibit chemical properties similar to those produced by oxidation. (3) Pseudovitrinite is poorly reacting in the coking process and in its rheological properties, as are oxidized coals. (4) Pseudovitrinite is devoid of pyrite, thus suggesting a non-reducing environment. (5) Pseudovitrinite is most abundant in coal seams containing large percentages of inert macerals, which probably originated by an oxidative process. A number of the supporting points for the hypothesized origin of pseudovitrinite concern observations that low-temperature (92 ° C) oxidation of coal produced features and effects similar to those in and of pseudovitrinite (Benedict and Berry, 1965). The pseudovitrinite concept was further evaluated to determine its merit for inclusion as a separate entity for identification during petrographic analysis (Kaegi, 1981}. Part of this evaluation concerned a study of the origin of slitted pseudovitrinite as related to coal oxidation. In light of the observation that pseudovitrinite-like features could be produced by oxidation (Benedict et al., 1966), it was hypothesized that some or all pseudovitrinite is produced during the oxidation of coal rather than through precoalification oxidation. The current paper presents some of the results of experimentation conducted to study this hypothesized origin of pseudovitrinite. IDENTIFICATION OF PSEUDOVITRINITE Prior to extensive experimentation, it was necessary to consider the criteria for identification o f pseudovitrinite in polished particulate sections. The identification criteria of Benedict et al. (1966) are: (1) Greater reflectance from polished surfaces in oil. (2) Slitted structures. (3) R e m n a n t cellular structures. (4) U n c o m m o n fracture patterns. (5) Higher relief. (6) Paucity or absence of pyrite inclusions. (7) Occurrence usually as comparatively large particles. The first three of these criteria were stated as being sufficient for identifying pseudovitrinite in high-volatile and medium-volatile coals, but the other criteria were needed for tow-volatile coals because the reflectance difference
311
and occurrence of remnant cellular structure diminished at this rank (Benedict et al., 1966). In all cases where comparison to another maceral was implied, that comparison was to vitrinite. Such comparison is, in fact, necessary for all criteria except those of slitted structure and remnant cellular structure. In attempting to utilize the above criteria, the need for comparison to vitrinite was found to severely restrict the application of the pseudovitrinite concept. This is because multi-seam, multi-mine, or blend samples usually contain coals o f different rank and different optical properties. Many of the criteria required comparison of the suspected pseudovitrinite to vitrinite from the same coal. In multi-rank samples, however, it is often impossible to determine from which coal a suspected pseudovitrinite particle is derived and, thus, to which vitrinite it should be compared. As one example, while using the original pseudovitrinite identification criteria, periodic analyses of preparation plant product coal showed significant fluctuations in pseudovitrinite content depending on the locations of the numerous mining units that contributed to the preparation plant feed at any specific time. It was found, however, that this was due to a geographic trend in rank, such that some mining units produced slightly higher reflectance coal than did others. When these units were operating, the total preparation plant feed appeared to contain vitrinite and slightly higher reflecting pseudovitrinite, while petrographic analysis of the coal mined by the individual units showed none of the coal to contain appreciable amounts o f pseudovitrinite. Because o f the problems associated with the criteria that required comparison to other particles, it was decided b y the author that these criteria were inappropriate since they were not generally applicable. Furthermore, it was thought that, since the remnant cellular structure criteria had already been incorporated into maceral identification as a means of distinguishing telinite from coUinite, it should not also be used as a criteria for distinguishing other vitrinite material. In addition, the pseudovitrinite characterized by Benedict et al. (1966) is considered to be semi-inert, yet Hacquebard (1958) concluded that cellular vitrinite (e.g., telinite)possessed more coking ability than associated noncellular vitrinite (e.g., collinite). This apparent disagreement a b o u t the behavior o f cellular vitrinite has yet to be resolved, b u t it may be caused by the grouping of materials (that behave differently from each other) in the original definition o f pseudovitrinite. Regardless of the reason for this discrepancy, it was additional cause not to use the occurrence of cellular structure as an identification criterion for pseudovitrinite. From the foregoing discussion it is apparent that the only original criterion for pseudovitrinite identification that was not rejected by the author was that of slitted structure. This feature has been found to be a relatively simple one to incorporate into an analytical procedure, and one that is n o t restricted to use on individual coal " m o n o - r a n k " samples. The use o f this criterion can become complex or useless for very small particles, as can any morphological feature. However, it has been found that for maximum parti-
312 cle diameters between approximately 150 tzm and 2000 pm, identification of the slitted structure is n o t significantly affected by particle size. Therefore, the only feature currently used by the author to distinguish vitrinite and pseudovitrinite is the occurrence of slits. Specifically, any vitrinitic material t h a t contains one or more slits, n o t apparently due to sample preparation or inorganic inclusions, is called slitted pseudovitrinite. Examples of this pseudovitrinite are shown in Fig.1.
Fig.1. Photomicrographs of slitted pseudovitrinite particles. Reflected light, oil immersion, 625x.
Since m a n y of the pseudovitrinite identification criteria proposed by Benedict et al. (1966) are not utilized by us, the propriety of the use of the name "pseudovitrinite" in describing only vitrinite material that contains slits may be questioned. However, it appears that this material would be identified as pseudovitrinite by use of the original criteria, and, therefore, use of only the slitted structure criterion results in identifying a subset of the total pseudovitrinite assemblage as originally defined. Also, the slitted structures criterion is the only criterion which Benedict et al. (1966) stated for use for high-, medium-, and low-volatile coals. EXPERIMENTAL The coal chosen for experimentation was one known to contain varying amounts of slitted pseudovitrinite. Specifically, a 4.5-kg channel sample of Pocahontas No. 3 seam coal was taken from a working, momentarily
313
exposed, mining face. Immediately upon sampling, the coal was placed in an air-tight glass container and purged with nitrogen. The sample was thus maintained until the following day; at which time it was further processed. The nitrogen-protected sample was divided into eleven representative subsamples. One of these subsamples was prepared and analyzed immediately, and was designated as " f r e s h " or as having zero oxidation time. Onehalf of each of the remaining ten subsamples was placed in an enameled steel pan, each sample forming a 15-mm-thick layer, and exposed to the atmosphere in an oven maintained at 50°C. These subsamples will hereafter be referred to as the 50°C samples. The remaining one-half of each subsample was placed in a steel can, which was then purged with argon prior to sealing. These subsamples were maintained at room temperature (23°C), and will hereafter be referred to as the ambient samples. After 7, 30, 60, and 120 days, a 50°C and an ambient sample were removed from their containers and characterized by several techniques that have been reported to be good indicators of coal oxidation. As an indication of the extent of oxidation, Gieseler plastometer analyses will be discussed here. The plastometer tests were conducted in accordance with accepted standards (ASTM, 1978), and petrographic analyses were conducted as described earlier (Kaegi, 1981). RESULTS
Table 1 and Figs. 2 and 3 present the plastometer and petrographic data for the samples. Gieseler plastometry indicated relatively severe oxidation of the 50°C samples and less severe oxidation of the ambient samples. For example, m a x i m u m fluidity decreased from about 1400 ddpm to about 80 ddpm for the sample exposed at 50°C for 120 days, and to about 690 ddpm for the samples stored at ambient temperature for 120 days (Fig. 2). Similarly, softening temperature and temperature of maximum fluidity
1600
1200
800
E
"2""~
400
x
0
I
I
~------ambient h
I
!
40 80 120 Time (days) F i g . 2 . M a x i m u m fluidity as a f u n c t i o n o f length o f o x i d a t i o n t i m e . 0
314 60
I~llL'~"~"',~lambient 50
v ¢n c
vitrinite
50° C ~
40
30 20
.o_ Q. o} o
pseudovitrinite
10
~L E o
0
u_ 30
E _= o
2010 ~ S_.
0
~
oxyvitrinite
,e°,
40 80 120 Tm i e(days)
Fig.3. Petrographic
composition
as a function
of length of oxidation
time.
increased rapidly with time for the 500(7 samples and more slowly for the ambient samples (Table 1). Petrographic analyses showed a rapid increase in the oxyvitrinite content for the 50°C samples and a less rapid increase for the ambient samples. Oxyvitrinite, an unofficial maceral category, represents the material classically recognized as originating from low-temperature oxidation (weathering) or vitrinite material. Examples of oxyvitrinite are shown in Fig.4. Additionally, the slitted pseudovitrinite content appeared to peak at an intermediate oxidation time for the 50°C samples and increased with a possible final decrease at the longest oxidation time for the ambient samples. Decreases in vitrinite content occurred in both the 50°C and ambient sample series. Micrinite c o n t e n t and vitrinite reflectance appeared to decrease for the 50°C samples, while similar trends in the ambient samples were not apparent (Table 1). Also, vitrinite and slitted pseudovitrinite reflectances appear to be about the same at a given condition and time (Table 1).
* M e a n m a x i m u m in oil
Vitrinit e reflectance* Pseudovitrinite reflectance*
Vitrinite S litted p s e u d o v i t r i n i t e Oxyvitrinite Fusinite S emifusinite Macrinite S e m i m acrinite Micrinite Exinite
Petrography (%)
1.31 1.33
55.1 8.4 9.9 4.3 19.6 0.1 1.3 11.0 0.2
M a x i m u m f l u i d i t y Qddpm) 1296 S o f t . t e m p e r a t u r e (~C) 407 Maximum fluidity temp. (°C) 462 Solid. t e m p e r a t u r e ( ° C ) 503
Gieseler plastometry
60
120
1.31 1.31
51.0 12.7 1.7 5.2 18.5 0.0 2.6 19.0 0.0
1007 409 462 499
1.30 1.32
47.1 14.1 5.5 4.5 24.4 0.2 1.6 8.0 0.2
420 414 468 504
1.30 1.27
39.0 10.5 12.0 3.0 26.1 0.2 2.3 7.0 0.0
172 428 471 501
1.27 1.28
28.9 4.5 29.0 4.0 25.0 0.2 2.3 6.0 O.1
79 427 474 502
1.32 1.30
55.9 5.4 1.1 5.0 20.1 0.0 1.6 11.5 0.4
1404 404 463 500
1.32 1.33
56.1 5.9 1.0 4.6 19.1 0.0 0.8 12.1 0.2
1343 407 461 500
1
1.32 1.33
56.0 6.0 1.0 5.2 19.1 0.0 0.7 12.0 0.1
1233 407 465 498
7
1.28 1.34
55.2 6.0 2.0 4.1 18.7 0.2 1.6 12.2 0.4
1088 410 468 508
30
0
30
1
7
Stored at a m b i e n t t e m p e r a t u r e (days)
Exposed at 50°C (days)
Gieseler p l a s t o m e t e r a n d coal p e t r o g r a p h i c a n a l y s i s r e s u l t s
TABLE 1
1.31 1.34
50.8 7.2 5.0 3.9 18.6 0.6 2.1 11.8 O.O
723 419 469 504
60
1.32 1.33
48.7 4.9 10.4 4.4 19.2 0.1 1.3 11.0 0.0
691 417 469 504
120
c~ h.a
316
Fig.4. Photomicrographs of slitted pseudovitrinite particles. Reflected light, oil immersion, 625x. DISCUSSION The noted changes in plastometer results confirmed the expected oxidation of the coal with less severe effects for the ambient than for the 50°C samples. This relatively milder oxidation for the ambient samples was expected considering the lower temperature and reduced availability o f oxygen. However, it appears that oxygen trapped with the ambient coal was sufficient to effect oxidation. Thus, the data represent two different rates of oxidation on the same coal, the lower rate being for the ambient samples. This is shown graphically in Fig.2. While the plastometer data may possibly be of significance in regard to the storage of reference coal samples for long periods of time, of immediate interest relative to the pseudovitrinite issue are the petrographic indications o f oxidation. Figure 3 shows plots of the percentage of vitrinite, oxyvitrinite, and slitted pseudovitrinite for both the 50°C and ambient samples as a function of time. Considering the 50°C samples first, the vitrinite content steadily decreased while the oxyvitrinite content steadily increased with time. The slitted pseudovitrinite content initially increased, b u t after 30 days of exposure, thereafter decreased. As would be expected, the decrease in vitrinite content is largely accounted for by the increase in oxyvitrinite. This is because, as vitrinite becomes increasingly oxidized, it would more frequently be identified as oxyvitrinite. However, to fully account for the vitrinite
317
decrease, the slitted pseudovitrinite content must also be considered. This is more easily seen in Fig.5, which is a plot o f the sum of the oxyvitrinite and slitted pseudovitrinite c o n t e n t in the 50°C samples versus oxidation time. Also shown is the vitrinite percentage versus time. The two curves are essentiaUy mirror images o f each other. This is interpreted as indicating that oxyvitrinite and slitted pseudovitrinite can both be formed by oxidation o f vitrinite. Also, since the later decrease in slitted pseudovitrinite is accompanied by an acceleration in oxyvitrinite formation, it appears that increased oxidation converts slitted pseudovitrinite to oxyvitrinite.
°
60-
\ -.o
O~R 4 0
" I L vitrinite
O...N E~
oca ~-,, , ~ : 20 E o )
0
~l=~..-II="~se ~
1
0
ud~ v Jtr,nit e oxyvitrinite
I
I
40 80 Time ( d a y s )
I
120 "
Fig. 5. C o n c u r r e n t decrease in v i t r i n i t e c o n t e n t a n d increase in t h e s u m o f p s e u d o v i t r i n i t e and oxyvitrinite content.
The conversion of slitted pseudovitrinite to oxyvitrinite is, perhaps, not t o o surprising if the identification criteria for slitted pseudovitrinite and oxyvitrinite are considered. It appears that the slitted structure of pseudovitrinite (shown in Fig.l) can increase in intensity as the particle becomes further oxidized. Eventually, as the slits grow in size and begin to intersect, the particle takes on the appearance of brecciated oxyvitrinite (shown in Fig.4). Therefore, in this context, slitted structures may be considered to be preliminary indications o f oxidation, and pseudovitrinite identified on the basis o f slitted structure may be considered an oxidation product intermediate between vitrinite and oxyvitrinite. With this scenario, one can readily rationalize the designation of pseudovitrinite as a semi-inert. Also, these observations support earlier findings of Roga and Pampuch (1956) who reported that the oxidation of coal in either air or water generated fissures in the oxidized particles, thus, documenting the well known decrepitation of coal as it weathers. The fact that the slitted structures generally form in perpendicular preferential directions not necessarily related to bedding plane may indicate that slits form at the sites of remnant closed cell lumens. If this is correct, the degree to which the cell walls are sealed to one another would influence the rate at which these cells open (to form slits) during oxidation. Therefore, the vitrinite that proceeds through slitted pseudovitrinite as an oxidation intermediate may be that with more poorly sealed or fused remnant
318
cells. If this is the case, then it would be expected that original source vegetation and subsequent geological conditions would influence the amount of slitted pseudovitrinite that would form as a result of oxidation. One might extrapolate this theory to hypothesize that some coals might n o t produce any slitted pseudovitrinite when oxidized because of the coal's origin and/or coalification history. In regard to the ambient samples, the same trends in vitrinite, slitted pseudovitrinite, and oxyvitrinite contents were found, although the degree of these changes was less severe. It is n o t certain that the slitted pseudovitrinite content of the ambient samples increased and subsequently decreased as did that of the 50 ° C samples, since only one data point was obtained for a period of time longer than that time at which the apparent highest slitted pseudovitrinite c o n t e n t was observed. However, if it is assumed that these data (for 120 days of storage) do indicate that the peak in slitted pseudovitrinite content was reached after 60 days of storage, then the slitted pseudovitrinite peak for the ambient samples was lower than the slitted pseudovitrinite peak for the 50°C samples. Since the data are n o t conclusive, the point will n o t be belabored. In the context of the proposed role of remnant cellular structure in slitted pseudovitrinite formation, however, such an effect would be expected, since the driving force (heat) for opening the partially sealed cells was lower for the ambient samples than for the 50°C samples. The slitted pseudovitrinite present in the "fresh" coal may also have been produced b y post-coalification oxidation. Coal seams are permeable enough to allow the flow of oxygen-containing gas or liquid (air or water) through them. This permeability is utilized in underground gasification (Elder, 1963), and is demonstrated by the existence and movement of methane in coal seams. Additionally, Benedict et al. (1966) n o t e d that many of the microscopic features o f their pseudovitrinite were produced in the coal oxidation study of Benedict and Berry (1965). With this thought, perhaps the use of the term "fresh" for recently mined or sampled coal should be reconsidered. The presence o f pseudovitrinite in certain coals may simply indicate in situ oxidation of the coal. The decrease in micrinite content for the 50°C samples has not been explained. Initially, it was thought that brecciation of vitrinite as it oxidized might be interfering with the identification o f micrinite contained in the vitrinite. However, the lack o f a similar trend in the ambient samples (even though over 10% oxyvitrinite was produced) refutes this theory. CONCLUSIONS
Oxidation of a medium-volatile coal showed that the proportion of vitrinite in the coal decreased, while the oxyvitrinite content increased, and the slitted pseudovitrinite content went through a maximum. From these results it is concluded that: (1) At least some pseudovitrinite is produced by low-temperature oxidation.
319
(2) Upon continued oxidation, slitted pseudovitrinite assumes the morphology of brecciated vitrinite (oxyvitrinite). (3) Slitted pseudovitrinite is a product of oxidation intermediate between vitrinite and oxyvitrinite.
REFERENCES American Society for Testing and Materials, 1978. ASTM part 26. Benedict, L.G. and Berry, W.F., 1965. Recognition and measurement of coal oxidation. Presented at GSA Coal Group, Miami. Benedict, L.G., Thompson, R.R., Shigo, J.J., III, and Aikman, R.P., 1966. Pseudovitrinite in Appalachian coking coals. Presented at ICCP Nomenclature, Madrid. Benedict, L.G., Thompson, R.R. and Wenger, R.O., 1968. Relationship between coal petrographic composition and coke stability. Blast Furn. Steel Plant, 56: 217--224. Elder, J.L., 1963. The underground gasification of coal. In: H.E. Lowry (Editor), Chemistry of Coal Utilization. Wiley and Sons, New York, N.Y., pp. 1023--1040. Hacquebard, P.A., 1958. The value of a quantitative separation of the maceral vitrinite into its constituents telinite and collinite for the petrography of coking coals. Proc. Third Int. Congr. Coal Petrology, Heerlen, pp. 131--138. Kaegi, D.D., 1981. Predicting coke stability form coal petrographic analysis. ISS-AIME Ironmaking Proc., 40: 381--392. Roga, B. and Pampuch, R., 1956. Weathering of Coal. Prace Glownege Inst. Gornictwa, Ser. B, 189 pp.