- Email: [email protected]
Natural weathering of coal
Natural Gayle
weathering
R. Ingram
and J. Donald
of coal Rimstidt
Department of Geological Sciences, Virginia Polytechnic Blacksburg, VA 24067, USA (Received 76 October 1987; revised 13 October 7983)
Institute
and State University,
The effects of weathering on the mean maximum vitrinite reflectance, moisture content, and lowtemperature ash mineralogy of the low volatile bituminous, Lower Mississippian Price Formation coals of southwest Virginia were determined on samples from depth profiles of three weathered coal beds. The results show that the outermost ~50 cm of each seam is intensely weathered showing humic acid formation, higher moisture content and physical disintegration. Beyond this depth (which corresponds to the average frost depth of this area), the amount of weathering decreases sharply. Freezing and thawing appear to be important in weathering the outermost zone, and, together with wetting and drying, disintegrate the coal creating reactive surfaces that oxidize readily. Reflectance values from weathered coal were found to be reliable indicators of rank if measurements were taken away from identifiable weathering features. The low-temperature ash mineralogy (silicates) showed no change with weathering. (Keywords:
coal; weathering;
oxidation)
Geologists, who need to evaluate samples of weathered coal from outcrops or abandoned surface mines, often need to know to what extent weathered surface samples relate to the general nature of the unweathered, buried coal. This requires an understanding of the mechanisms and agents of weathering, the rate and depth at which it takes place, and the physical and chemical changes it produces. The aim of this study was to determine the relation between the degree of weathering and the vitrinite reflectance of the Lower Mississippian coals in Montgomery and Pulaski Counties, Virginia. Chandra’ -3 and Benedict and Berry4 have studied the effect of low-temperature oxidation on the reflectance of coal. Chandra examined coals that were oxidized both naturally and under controlled laboratory conditions, and concluded that there was no significant change in reflectance from the unoxidized to oxidized coals. However, Benedict and Berry, using samples oxidized in the laboratory, reported that reflectance decreases, reaches a minimum, then increases steadily with continuing oxidation. The work on naturally weathered coals reported here indicates that the vitrinite reflectance of samples from a weathered outcrop can be a reliable indicator of rank. The Price Formation coals are low sulphur, high ash, and nonagglomerating5. Previously published proximate analyses show that these coals contain 84.4% fixed carbon and yield 15.6% volatiles, on a dry, mineral matter-free basis. Thus, according to the ASTM classification6, the Price coals are low volatile bituminous coals. EXPERIMENTAL This study determined the effects of weathering on the petrography, moisture content and mineralogy of the Price Formation coal. The degree of weathering of samples from depth profiles at three weathered outcrops 0016-2361/84/030292~5$3.00 @ 1984 Butterworth & Co. (Publishers)
292
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Vol 63, March
determined by an alkali-extraction test. This test relates the per cent of weathered material in the coal to the amount of extractable humic acids. Humic acids are operationally defined here as natural organic substances that are soluble in strong alkali solutions. The moisture content was determined by weight loss on heating. The mean maximum vitrinite reflectance was determined from 100 measurements taken on each sample, and the lowtemperature ash mineralogy of the coal was evaluated by powder X-ray diffraction. The Price Formation coals were sampled from a roadcut along Virginia State Route 100 on Cloyds Mountain (4 114 500 mN, 525000 mE) and along a Norfolk and Western railroad cut on the western end of Price Mountain (4 115 500 mN, 544 500 mE). The roadcut on Cloyds Mountain was opened in 1955’. This cut exposes both the Langhorne and Merrimac beds which show an apparent dip of 45” to the southeast (Figure I). Each seam is surrounded by claystones. The Langhome seam is highly deformed making bedding difficult to ascertain. The Merrimac is less deformed and has numerous partings. The exposure on Price Mountain was formed in 19128. The seam sampled here is in the hinge of a fold and shows severe deformation. The position of this seam within the Upper Member of the Price Mountain is unknown. This seam is also surrounded by claystones. Samples were taken by hand augering with a 3 in (7.5 cm) diameter auger from the surface into the outcrop as nearly parallel to bedding as possible. Entire samples, from lo-15 cm in length, were immediately sealed in airtight plastic bags and labelled. On Cloyds Mountain, samples were taken down the dip of the bedding. This depth was later corrected to a horizontal depth from the surface. A horizontal depth of 228 cm was reached in the Longhorne seam and a horizontal depth of 122 cm in the Merrimac. On Price Mountain, the samples were taken was
Natural weathering of coal: G. R. Ingram and J. 0. Rimstidt
Ro\ute 100 seams
_____-_ ____---_ -___-----------
-------------
____-----------_--_ _ _ Claystone _ _ _ _ _ -___---------------___-----____---I-ID Borehole
-
----
7
?
_.-
CWI _____-------___--------___---------___---------__----------
-----
7
-__---------____--------__---------__ -----------------____------------_ Price
mountain
seam
1 Schematic diagrams showing the sampling geometries for the three coal seams studied. Both seams in the Route 100 (Cloyds Mountain) outcrop had the same geometry (shown at top). The face of the cut strikes NE and the beds strike N 55” E and dip 45” SE. The bore hole, oriented down dip, was begun 3 m below the top of the outcrop. The location of each sample is expressed as a horizontal depth (Langhorne HD=228 cm, Merrimac HD = 122 cm) from the outcrop surface. The Price Mountain (shown at bottom), located in the hinge of a fold, is too deformed to allow the bedding to be ascertained. The horizontal borehole was begun -20 m below the top of the outcrop and penetrated to a horizontal depth of 208 cm Figure
horizontally to a depth of 208 cm. Sampling was stopped when it became physically impractical to proceed. The amount of weathering (natural oxidation) of the coals was determined by the alkali-extraction test described by Lowenhaupt and Gray’. This procedure involves boiling ground coal in a 1 M NaOH solution to
extract the humic acids, produced by weathering, from the coal as soluble sodium salts. This test is not valid for lignites which contain humic acids not produced by weathering. The optical transmittance of the filtered solution at 520 nm was measured using a spectrophotometer. Lowenhaupt and Gray found a linear relation between the percent transmittance of the extract from weathered coal samples and the percent of petrographically identifiable weathered material in each sample. Although results for both raw and washed coal were reported, only the raw coal data were used for this study. These data have correlation coefficient of 0.92 (slope = - 0.6, y-intercept = 59) for samples showing between 3% and 60% oxidation. The intervals between O-3% and 60-100% oxidation coal have transmittance values either too high or too low to be measured accurately. The results of the alkali-extraction test were used to choose samples for the other parameters examined in this study. Representative samples were chosen from the more weathered to the less weathered coal, so that if different samples showed similar amounts of weathering, only one of them was chosen for further study. The moisture content of the coal was determined to test the assertion that more weathered coal contains more moisture. The method used to determine per cent moisture was modified from ‘Oven method for coal crushed to pass a No. 60 sieve” O. The maximum reflectance of 100 particles was measured on pellets made from representative samples taken from each weathering profile. The standard deviation of such measurements taken on a seam is usually f 0.1 y0 R1 ’ . The standard deviation for each seam in this study was ~0.2% R. The higher standard deviation of these measurements probably reflects the difficulty of taking measurements on very weathered particles. In this study, submacerals of the vitrinite group were not distinguished. Measurements were not taken on obvious fractures, pits and darkened rims which are characteristic of weathered coal grains, so the results are averages of measurements taken on apparently unweathered material. The low-temperature ash mineralogy was determined by X-ray diffraction. RESULTS
AND
DISCUSSION
The weathering of coal involves complex physical and chemical processes. Freezing and thawing, and wetting and drying are the dominant factors involved in the physical disintegration of coal. During freezing, fractures in the coal are extended and enlarged when the increased volume of ice forces unfrozen water farther into the coal”. Physical breakdown from wetting and drying is caused by internal stresses that are set up from unequal volume changes that take place as water is absorbed and desorbed faster on the outside of a coal particle than on the inside’ 3. Thus, wetting and drying create fractures that can be extended and enlarged during freezing and thawing; freezing and thawing force water into new areas where wetting and drying can take place. The main process in chemical weathering is the formation of humic acids by the oxidation of organic substances in the coal. During the first stages, peroxide complexes form on the coal surface”. Later, these peroxide complexes decompose to water and carbon dioxide, leaving oxygen-bearing functional groups (such as hydroxyl, carboxyl, carbonyl. ethers, phenols and
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anhydrides). With further oxidation, humic acids form, and ultimately water-soluble coal acids are createdr3. Hydrolysis reactions probably also produce acid groups. The rate of these reactions increases with increasing temperature. Oxidation rates can be accelerated by heat from oxidation, 6.74 kJ g-r for C + 0, = CO,, or the heat of wetting, 0.09 kJ g - ‘. In low rank coals these autocatalytic processes often lead to spontaneous combustion of the outcrop. Results from the alkali-extraction test show the outer 46-52 cm of each bed in this study was intensely weathered14 (Figure 2). This phenomenon appears to be related to the common freeze-thaw (frost) depth of this area (4560 cm) l5 . Physical p r oc esses disintegrate the coal to increase the reactive surface area of the grains and make oxygen penetration into the bed easier. The processes that produce physical deterioration of the coal are most active in the outer 50 cm of the outcrop, and thus, this zone also shows the most intense chemical weathering (Figure 2). Length of exposure of a bed appears to be a less important factor determining the depth of severe weathering because the thickness of the intensely weathered zone is approximately the same on the different age outcrops (26 years for the Cloyds Mountain outcrop and 69 years for the Price Mountain outcrop). At depths > ~50 cm, the severity of the coal weathering declines rapidly. Humic acids from the coal below the freeze-thaw zone were probably produced by oxygenated ground water slowly filtering down from the original surface before the present outcrop was made. The
260 50 40 30 20 10 0 50 2
40
; aI
30
5
20
s
10 0
C 50 40
Depth (cm)
Figure 2 Weathering profiles for the Langhorne (A), Merrimac (B), and Price Mountain (C) Beds. Note that the outer x50 cm (stippled) show the greatest weathering
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5-
01 0
’
’
10
1
’ 20
1
’
30 Weathering
’
1 40
’
1 50
1
6
6
(%I
Figure 3 Per cent moisture versus per cent weathering for the Langhorne (@). Merrimac (A), and Price Mountain (U) beds. Note that the most weathered coal has the highest moisture content
bed on Price Mountain is less weathered than the coals from the Cloyds Mountain because the Cloyds Mountain beds are exposed at the top of the outcrop while the Price Mountain bed was buried x20 m below the surface before being exhumed (Figure 1). Although vegetation may play a part in the weathering of coal, the vegetation on both outcrops was sparse enough so that it was not a major weathering factor in this study. In the outer, more severely weathered zone of the beds, moisture content of the coal was the highest (Figure 3). Beyond this zone per cent moisture appears to be unaffected by weathering and is relatively constant. Four types of water in coal have been defined: bulk, capillary, multilayer and monolayer’6. Bulk water is the liquid water found in the larger voids and interstices of the coal; it has the properties of free water while the capillary, multilayer and monolayer water properties are influenced by surface forces. Water that is condensed in the capillaries is capillary water. Multilayer water occurs in pores of a few molecular diameters and is weakly hydrogen bonded to the monolayer water. The monolayer water is strongly hydrogen bonded (chemisorbed) to oxygen-containing functional groups on the coal surface. Carboxylic acid groups are the most significant hydrophilic sites for water chemisorption in the monolayer. To a lesser extent phenolic hydroxyl groups are important hydrophilic sites for monolayer water. Both of these are common functional groups in humic acids”. The technique used to measure the moisture content in this study lo does not measure free liquid water, but rather it measures the amount of capillary and multilayer water, and to a lesser extent monolayer water. Water that is strongly chemisorbed or water from decomposition of functional groups is not usually released by this technique16. The first step in the procedure involves drying the coal samples under ambient laboratory conditions to remove any bulk water. Thus, the remaining water molecules are bound by surface forces as capillary, multilayer and monolayer water and represent an equilibrium amount that is controlled by the relative humidity of the laboratory air. The more weathered coal in the outer zones of the outcrop shows a higher moisture content than the lessweathered coal because both the surface area is increased by physical weathering and the number of hydrophilic functional groups that chemisorb water are increased by chemical weathering. The increased surface area produces more area for water adsorption and more capillary spaces
Natural weathering of coal: G. R. Ingram and J. D. Rimstidt
that can retain water. The increased number of carboxyl and phenol groups formed during the oxidation of coal substance to humic acids provide more sites for hydrogen bonding of water in the monolayer. Weathering of coal produces numerous petrographic effects. Many of the samples examined show micropores, microcracks and darkened weathering rims which are characteristic of weathered coal. While weathering features are normally confined to vitrinite, inertinite particles show occasional microfractures and pores. Weathering rims are the result of humic acid formation on the edges of coal particle during weathering. Micropores and microcracks form as these humic acid gels dry’*. Humic acid gels have a lower reflectivity than unweathered vitrinite; thus, the more weathered areas around the edges of coal particles and fractures appear darker in reflected light examination. However, even in the most weathered samples, a grain was rarely totally affected by weathering features. Thus, vitrinite reflectance measurements could still be taken on the apparently unweathered portions within the grain. The reflectance measurements of this study showed that it is valid to use weathered outcrop coal to determine the reflectance of a bed if measurements are taken away from weathering features (rims, cracks, pores) of a coal particle14 (Figure 4). The fact that a weathered coal sample can give a reliable value of a coal’s mean reflectance is in agreement with Chandra’. He found no change in the reflectance from naturally weathered to unweathered coal. He also found no change in reflectance in coals oxidized at low temperatures in the laboratory when compared with unoxidized coals1*3. However, Benedict and Berry4 measured the reflectance of coal oxidized at a low temperature in the laboratory and reported that reflectance decreases, reaches a minimum, than steadily increases with continuing oxidation. However, because of the narrow range of the differences (between 0.06 and 0.08%) in the extreme reflectances and because the position of these extremes is inconsistent, the scatter of their data may well be a result of the precision of their measurements rather than changes,caused by oxidation. The low-temperature ash within each bed does not show major changes connected with weathering. Minerals identified by X-ray diffraction in the ash were quartz, kaolinite, mixed layer illite-montmorillonite, gypsum, hemihydrate, anhydrite and feldspar. In each seam, the relative proportions of these minerals were unchanged from the more-weathered to less-weathered samples14.
I
I
T
22 tT
1.21 0
’
’ 10
’
’ 20
’
’ ’ ’ 30 40 Weathering (7~)
’
’ 50
’
Figure 4 Per cent reflectance, with one standard deviation, versus per cent weathering for the Langhorne (a), Merrimac (A), and Price Mountain (H) beds
1 >60
CONCLUSIONS During weathering many physical and chemical changes take place as the coal is influenced by surface conditions and attempts to adjust to them. The physical processes of weathering which produce disaggregation are freezing and thawing, working in concert with wetting and drying. Wetting and drying create fractures that are extended and enlarged by freezing and thawing, and freezing and thawing forces water into new areas where wetting and drying can take place. During chemical weathering of coal, humic acids are produced by the oxidation and hydrolysis of the organic substances. Under conditions of very intense weathering, humic acids break down to soluble coal acids and ultimately to carbon dioxide and water. Areas of more intense physical weathering are also areas of more intense chemical weathering. Physical breakdown of the coal accelerates the chemical breakdown by increasing the amount of reactive surface area and by providing heat from wetting and oxidation that accelerates the rates of chemical reactions. It also makes oxygen penetration into the outcrop easier. This study demonstrated a direct correlation between the amount of physical and chemical weathering. The intensely weathered outer zones of the outcrops ( z 50 cm) showed the most humic acid formation and the highest moisture content. Beyond this, the amount of weathering dropped sharply. The frost depth for the area of study is z 50 cm. Thus, intense chemical weathering is directly tied to the surface zone that showed intense physical weathering as a result of freezing and thawing. The fact that the outcrops were of different ages did not seem to influence the depth of weathering. Therefore, it is likely that freezing and thawing is the dominant factor in determining the depth of intensely weathered coal. Coal weathering produces several noticeable petrographic changes. Microfractures, micropores, and weathering rims are usually obvious. These rims, which are lower in reflectivity, are the result of humic acid formation around the edges of the coal grains. Microfractures and micropores are formed as these humic acids shrink on drying. The petrographic weathering effects usually do not affect the entire grain. Any chemical analyses of intensely weathered coal will probably be distorted. However, samples from below the intensely weathered zone show only minor changes produced by oxidation of the coal by groundwater. Samples taken at depths > ~50 cm should give a much better indication of the unweathered nature of the coal. However, even in the most weathered zones, silicate minerals in the ash appeared to be unaffected so that surface samples should be adequate to study silicate mineralogy of the coal. However, it is likely that the sulphide and sulphate minerals would be severely affected perhaps to a considerable depth. Sulphides and sulphates were not sufficiently abundant in the coals used to verify this. Although the bulk chemical properties of the coal may be altered to the point that proximate analyses can not be used to determine the rank of the coal, the rank can still be determined by carefully measuring the reflectance of the apparently unaltered portions of the vitrinite grains. This is confirmed by the fact that per cent reflectance of all samples used in this study was between 1.5 and 2.0%, the range for low volatile bituminous coals. This agrees well
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Natural weathering of coal: G. R. Ingram and J. D. Rimstidt
with the rank determination proximate analyses.
made from the previous 7 8
ACKNOWLEDGEMENTS The field work for this project was supported by the Virginia Division of Mineral Resources. The research was supported in part by the Virginia Mining and Mineral Resources Research Institute Allotment Grant 1980-S 1.
9 10 II 12 13
REFERENCES 1 2 3 4
5
6
296
Chandra, D. Econ. Geol. 1958,53, 102 Chandra, D. Fuel 1962,41, 185 Chandra, D. Econ. Geol. 1966,61, 754 Benedict, L. G. and Berry, W. F. ‘Recognition and Measurement of Coal Oxidation’, Bituminous Coal Research, Inc., Monroeville, Pennsylvania, 1964 Stevens, D. W. ‘A survey of the factors which affect mining of the Lower Mississippian coals in Montgomery Co., Virginia’, MS. thesis, Virginia Polytechnic Institute and State University, 1959 Crelling, J. C. and Dutcher, R. R. ‘Principles and Applications of
FUEL, 1984, Vol 63, March
14
15 16 17 18
Coal Petrology’, SEPM Short Course Notes No. 8, 1980 Collins, D. R. Virginia Highway Department, personal communication, 198 1 Harman. W. M. Norfolk and Western Railway, personal communication, 1981 Lowenhaupt, D. E. and Gray, R. J. Inter-nut. J. Coal Geol. 1980,1, 63 US Bureau of Mines, Bull. 1967, 638, 3 Stach, E. ‘Stach’s Textbook of Coal Petrology’, 2nd edition, Gebruder Bomtraeger, 1975 Bloom, A. L. ‘Geomorphology, A Systematic Analysis of Late Cenozoic Landforms’, Prentice-Hall, Inc. 1978, ch. 6 Fryer, J. F. and Szladow, A. J. ‘Storage of Coal Samples’, Research Council of Alberta Information Series, No. 88. 1971 Ingram, G. R. ‘The effect of weathering on the vitrinite reflectance of the Lower Mississippian coals in Montgomery and Pulaski Counties, Virginia’, MS. thesis, Virginia Polytechnic and Institute and State University, 1981 Walker, R. D. Personal communication, 1981 Allardice, D. J. and Evans, D. G. ‘Analytical Methods for Coal and Coal Products’, Academic Press, 1978, Vol. 1, ch. 7 Wershaw, R. L., Pinckney, D. J. and Booker,S. E. J. Res. US Geol. Survey 1977,5, 565 Teichmuller, M. and Teichmuller, R. ‘Coal and Coal Bearing Strata’, American Elsevier Publishing Co., Inc. 1968, ch. 15
weathering
R. Ingram
and J. Donald
of coal Rimstidt
Department of Geological Sciences, Virginia Polytechnic Blacksburg, VA 24067, USA (Received 76 October 1987; revised 13 October 7983)
Institute
and State University,
The effects of weathering on the mean maximum vitrinite reflectance, moisture content, and lowtemperature ash mineralogy of the low volatile bituminous, Lower Mississippian Price Formation coals of southwest Virginia were determined on samples from depth profiles of three weathered coal beds. The results show that the outermost ~50 cm of each seam is intensely weathered showing humic acid formation, higher moisture content and physical disintegration. Beyond this depth (which corresponds to the average frost depth of this area), the amount of weathering decreases sharply. Freezing and thawing appear to be important in weathering the outermost zone, and, together with wetting and drying, disintegrate the coal creating reactive surfaces that oxidize readily. Reflectance values from weathered coal were found to be reliable indicators of rank if measurements were taken away from identifiable weathering features. The low-temperature ash mineralogy (silicates) showed no change with weathering. (Keywords:
coal; weathering;
oxidation)
Geologists, who need to evaluate samples of weathered coal from outcrops or abandoned surface mines, often need to know to what extent weathered surface samples relate to the general nature of the unweathered, buried coal. This requires an understanding of the mechanisms and agents of weathering, the rate and depth at which it takes place, and the physical and chemical changes it produces. The aim of this study was to determine the relation between the degree of weathering and the vitrinite reflectance of the Lower Mississippian coals in Montgomery and Pulaski Counties, Virginia. Chandra’ -3 and Benedict and Berry4 have studied the effect of low-temperature oxidation on the reflectance of coal. Chandra examined coals that were oxidized both naturally and under controlled laboratory conditions, and concluded that there was no significant change in reflectance from the unoxidized to oxidized coals. However, Benedict and Berry, using samples oxidized in the laboratory, reported that reflectance decreases, reaches a minimum, then increases steadily with continuing oxidation. The work on naturally weathered coals reported here indicates that the vitrinite reflectance of samples from a weathered outcrop can be a reliable indicator of rank. The Price Formation coals are low sulphur, high ash, and nonagglomerating5. Previously published proximate analyses show that these coals contain 84.4% fixed carbon and yield 15.6% volatiles, on a dry, mineral matter-free basis. Thus, according to the ASTM classification6, the Price coals are low volatile bituminous coals. EXPERIMENTAL This study determined the effects of weathering on the petrography, moisture content and mineralogy of the Price Formation coal. The degree of weathering of samples from depth profiles at three weathered outcrops 0016-2361/84/030292~5$3.00 @ 1984 Butterworth & Co. (Publishers)
292
FUEL,
1984,
Ltd
Vol 63, March
determined by an alkali-extraction test. This test relates the per cent of weathered material in the coal to the amount of extractable humic acids. Humic acids are operationally defined here as natural organic substances that are soluble in strong alkali solutions. The moisture content was determined by weight loss on heating. The mean maximum vitrinite reflectance was determined from 100 measurements taken on each sample, and the lowtemperature ash mineralogy of the coal was evaluated by powder X-ray diffraction. The Price Formation coals were sampled from a roadcut along Virginia State Route 100 on Cloyds Mountain (4 114 500 mN, 525000 mE) and along a Norfolk and Western railroad cut on the western end of Price Mountain (4 115 500 mN, 544 500 mE). The roadcut on Cloyds Mountain was opened in 1955’. This cut exposes both the Langhorne and Merrimac beds which show an apparent dip of 45” to the southeast (Figure I). Each seam is surrounded by claystones. The Langhome seam is highly deformed making bedding difficult to ascertain. The Merrimac is less deformed and has numerous partings. The exposure on Price Mountain was formed in 19128. The seam sampled here is in the hinge of a fold and shows severe deformation. The position of this seam within the Upper Member of the Price Mountain is unknown. This seam is also surrounded by claystones. Samples were taken by hand augering with a 3 in (7.5 cm) diameter auger from the surface into the outcrop as nearly parallel to bedding as possible. Entire samples, from lo-15 cm in length, were immediately sealed in airtight plastic bags and labelled. On Cloyds Mountain, samples were taken down the dip of the bedding. This depth was later corrected to a horizontal depth from the surface. A horizontal depth of 228 cm was reached in the Longhorne seam and a horizontal depth of 122 cm in the Merrimac. On Price Mountain, the samples were taken was
Natural weathering of coal: G. R. Ingram and J. 0. Rimstidt
Ro\ute 100 seams
_____-_ ____---_ -___-----------
-------------
____-----------_--_ _ _ Claystone _ _ _ _ _ -___---------------___-----____---I-ID Borehole
-
----
7
?
_.-
CWI _____-------___--------___---------___---------__----------
-----
7
-__---------____--------__---------__ -----------------____------------_ Price
mountain
seam
1 Schematic diagrams showing the sampling geometries for the three coal seams studied. Both seams in the Route 100 (Cloyds Mountain) outcrop had the same geometry (shown at top). The face of the cut strikes NE and the beds strike N 55” E and dip 45” SE. The bore hole, oriented down dip, was begun 3 m below the top of the outcrop. The location of each sample is expressed as a horizontal depth (Langhorne HD=228 cm, Merrimac HD = 122 cm) from the outcrop surface. The Price Mountain (shown at bottom), located in the hinge of a fold, is too deformed to allow the bedding to be ascertained. The horizontal borehole was begun -20 m below the top of the outcrop and penetrated to a horizontal depth of 208 cm Figure
horizontally to a depth of 208 cm. Sampling was stopped when it became physically impractical to proceed. The amount of weathering (natural oxidation) of the coals was determined by the alkali-extraction test described by Lowenhaupt and Gray’. This procedure involves boiling ground coal in a 1 M NaOH solution to
extract the humic acids, produced by weathering, from the coal as soluble sodium salts. This test is not valid for lignites which contain humic acids not produced by weathering. The optical transmittance of the filtered solution at 520 nm was measured using a spectrophotometer. Lowenhaupt and Gray found a linear relation between the percent transmittance of the extract from weathered coal samples and the percent of petrographically identifiable weathered material in each sample. Although results for both raw and washed coal were reported, only the raw coal data were used for this study. These data have correlation coefficient of 0.92 (slope = - 0.6, y-intercept = 59) for samples showing between 3% and 60% oxidation. The intervals between O-3% and 60-100% oxidation coal have transmittance values either too high or too low to be measured accurately. The results of the alkali-extraction test were used to choose samples for the other parameters examined in this study. Representative samples were chosen from the more weathered to the less weathered coal, so that if different samples showed similar amounts of weathering, only one of them was chosen for further study. The moisture content of the coal was determined to test the assertion that more weathered coal contains more moisture. The method used to determine per cent moisture was modified from ‘Oven method for coal crushed to pass a No. 60 sieve” O. The maximum reflectance of 100 particles was measured on pellets made from representative samples taken from each weathering profile. The standard deviation of such measurements taken on a seam is usually f 0.1 y0 R1 ’ . The standard deviation for each seam in this study was ~0.2% R. The higher standard deviation of these measurements probably reflects the difficulty of taking measurements on very weathered particles. In this study, submacerals of the vitrinite group were not distinguished. Measurements were not taken on obvious fractures, pits and darkened rims which are characteristic of weathered coal grains, so the results are averages of measurements taken on apparently unweathered material. The low-temperature ash mineralogy was determined by X-ray diffraction. RESULTS
AND
DISCUSSION
The weathering of coal involves complex physical and chemical processes. Freezing and thawing, and wetting and drying are the dominant factors involved in the physical disintegration of coal. During freezing, fractures in the coal are extended and enlarged when the increased volume of ice forces unfrozen water farther into the coal”. Physical breakdown from wetting and drying is caused by internal stresses that are set up from unequal volume changes that take place as water is absorbed and desorbed faster on the outside of a coal particle than on the inside’ 3. Thus, wetting and drying create fractures that can be extended and enlarged during freezing and thawing; freezing and thawing force water into new areas where wetting and drying can take place. The main process in chemical weathering is the formation of humic acids by the oxidation of organic substances in the coal. During the first stages, peroxide complexes form on the coal surface”. Later, these peroxide complexes decompose to water and carbon dioxide, leaving oxygen-bearing functional groups (such as hydroxyl, carboxyl, carbonyl. ethers, phenols and
FUEL, 1984, Vol 63, March
293
Natural weathering
of coal: G. R. Ingram and J. D. Rimstidt
anhydrides). With further oxidation, humic acids form, and ultimately water-soluble coal acids are createdr3. Hydrolysis reactions probably also produce acid groups. The rate of these reactions increases with increasing temperature. Oxidation rates can be accelerated by heat from oxidation, 6.74 kJ g-r for C + 0, = CO,, or the heat of wetting, 0.09 kJ g - ‘. In low rank coals these autocatalytic processes often lead to spontaneous combustion of the outcrop. Results from the alkali-extraction test show the outer 46-52 cm of each bed in this study was intensely weathered14 (Figure 2). This phenomenon appears to be related to the common freeze-thaw (frost) depth of this area (4560 cm) l5 . Physical p r oc esses disintegrate the coal to increase the reactive surface area of the grains and make oxygen penetration into the bed easier. The processes that produce physical deterioration of the coal are most active in the outer 50 cm of the outcrop, and thus, this zone also shows the most intense chemical weathering (Figure 2). Length of exposure of a bed appears to be a less important factor determining the depth of severe weathering because the thickness of the intensely weathered zone is approximately the same on the different age outcrops (26 years for the Cloyds Mountain outcrop and 69 years for the Price Mountain outcrop). At depths > ~50 cm, the severity of the coal weathering declines rapidly. Humic acids from the coal below the freeze-thaw zone were probably produced by oxygenated ground water slowly filtering down from the original surface before the present outcrop was made. The
260 50 40 30 20 10 0 50 2
40
; aI
30
5
20
s
10 0
C 50 40
Depth (cm)
Figure 2 Weathering profiles for the Langhorne (A), Merrimac (B), and Price Mountain (C) Beds. Note that the outer x50 cm (stippled) show the greatest weathering
294
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5-
01 0
’
’
10
1
’ 20
1
’
30 Weathering
’
1 40
’
1 50
1
6
6
(%I
Figure 3 Per cent moisture versus per cent weathering for the Langhorne (@). Merrimac (A), and Price Mountain (U) beds. Note that the most weathered coal has the highest moisture content
bed on Price Mountain is less weathered than the coals from the Cloyds Mountain because the Cloyds Mountain beds are exposed at the top of the outcrop while the Price Mountain bed was buried x20 m below the surface before being exhumed (Figure 1). Although vegetation may play a part in the weathering of coal, the vegetation on both outcrops was sparse enough so that it was not a major weathering factor in this study. In the outer, more severely weathered zone of the beds, moisture content of the coal was the highest (Figure 3). Beyond this zone per cent moisture appears to be unaffected by weathering and is relatively constant. Four types of water in coal have been defined: bulk, capillary, multilayer and monolayer’6. Bulk water is the liquid water found in the larger voids and interstices of the coal; it has the properties of free water while the capillary, multilayer and monolayer water properties are influenced by surface forces. Water that is condensed in the capillaries is capillary water. Multilayer water occurs in pores of a few molecular diameters and is weakly hydrogen bonded to the monolayer water. The monolayer water is strongly hydrogen bonded (chemisorbed) to oxygen-containing functional groups on the coal surface. Carboxylic acid groups are the most significant hydrophilic sites for water chemisorption in the monolayer. To a lesser extent phenolic hydroxyl groups are important hydrophilic sites for monolayer water. Both of these are common functional groups in humic acids”. The technique used to measure the moisture content in this study lo does not measure free liquid water, but rather it measures the amount of capillary and multilayer water, and to a lesser extent monolayer water. Water that is strongly chemisorbed or water from decomposition of functional groups is not usually released by this technique16. The first step in the procedure involves drying the coal samples under ambient laboratory conditions to remove any bulk water. Thus, the remaining water molecules are bound by surface forces as capillary, multilayer and monolayer water and represent an equilibrium amount that is controlled by the relative humidity of the laboratory air. The more weathered coal in the outer zones of the outcrop shows a higher moisture content than the lessweathered coal because both the surface area is increased by physical weathering and the number of hydrophilic functional groups that chemisorb water are increased by chemical weathering. The increased surface area produces more area for water adsorption and more capillary spaces
Natural weathering of coal: G. R. Ingram and J. D. Rimstidt
that can retain water. The increased number of carboxyl and phenol groups formed during the oxidation of coal substance to humic acids provide more sites for hydrogen bonding of water in the monolayer. Weathering of coal produces numerous petrographic effects. Many of the samples examined show micropores, microcracks and darkened weathering rims which are characteristic of weathered coal. While weathering features are normally confined to vitrinite, inertinite particles show occasional microfractures and pores. Weathering rims are the result of humic acid formation on the edges of coal particle during weathering. Micropores and microcracks form as these humic acid gels dry’*. Humic acid gels have a lower reflectivity than unweathered vitrinite; thus, the more weathered areas around the edges of coal particles and fractures appear darker in reflected light examination. However, even in the most weathered samples, a grain was rarely totally affected by weathering features. Thus, vitrinite reflectance measurements could still be taken on the apparently unweathered portions within the grain. The reflectance measurements of this study showed that it is valid to use weathered outcrop coal to determine the reflectance of a bed if measurements are taken away from weathering features (rims, cracks, pores) of a coal particle14 (Figure 4). The fact that a weathered coal sample can give a reliable value of a coal’s mean reflectance is in agreement with Chandra’. He found no change in the reflectance from naturally weathered to unweathered coal. He also found no change in reflectance in coals oxidized at low temperatures in the laboratory when compared with unoxidized coals1*3. However, Benedict and Berry4 measured the reflectance of coal oxidized at a low temperature in the laboratory and reported that reflectance decreases, reaches a minimum, than steadily increases with continuing oxidation. However, because of the narrow range of the differences (between 0.06 and 0.08%) in the extreme reflectances and because the position of these extremes is inconsistent, the scatter of their data may well be a result of the precision of their measurements rather than changes,caused by oxidation. The low-temperature ash within each bed does not show major changes connected with weathering. Minerals identified by X-ray diffraction in the ash were quartz, kaolinite, mixed layer illite-montmorillonite, gypsum, hemihydrate, anhydrite and feldspar. In each seam, the relative proportions of these minerals were unchanged from the more-weathered to less-weathered samples14.
I
I
T
22 tT
1.21 0
’
’ 10
’
’ 20
’
’ ’ ’ 30 40 Weathering (7~)
’
’ 50
’
Figure 4 Per cent reflectance, with one standard deviation, versus per cent weathering for the Langhorne (a), Merrimac (A), and Price Mountain (H) beds
1 >60
CONCLUSIONS During weathering many physical and chemical changes take place as the coal is influenced by surface conditions and attempts to adjust to them. The physical processes of weathering which produce disaggregation are freezing and thawing, working in concert with wetting and drying. Wetting and drying create fractures that are extended and enlarged by freezing and thawing, and freezing and thawing forces water into new areas where wetting and drying can take place. During chemical weathering of coal, humic acids are produced by the oxidation and hydrolysis of the organic substances. Under conditions of very intense weathering, humic acids break down to soluble coal acids and ultimately to carbon dioxide and water. Areas of more intense physical weathering are also areas of more intense chemical weathering. Physical breakdown of the coal accelerates the chemical breakdown by increasing the amount of reactive surface area and by providing heat from wetting and oxidation that accelerates the rates of chemical reactions. It also makes oxygen penetration into the outcrop easier. This study demonstrated a direct correlation between the amount of physical and chemical weathering. The intensely weathered outer zones of the outcrops ( z 50 cm) showed the most humic acid formation and the highest moisture content. Beyond this, the amount of weathering dropped sharply. The frost depth for the area of study is z 50 cm. Thus, intense chemical weathering is directly tied to the surface zone that showed intense physical weathering as a result of freezing and thawing. The fact that the outcrops were of different ages did not seem to influence the depth of weathering. Therefore, it is likely that freezing and thawing is the dominant factor in determining the depth of intensely weathered coal. Coal weathering produces several noticeable petrographic changes. Microfractures, micropores, and weathering rims are usually obvious. These rims, which are lower in reflectivity, are the result of humic acid formation around the edges of the coal grains. Microfractures and micropores are formed as these humic acids shrink on drying. The petrographic weathering effects usually do not affect the entire grain. Any chemical analyses of intensely weathered coal will probably be distorted. However, samples from below the intensely weathered zone show only minor changes produced by oxidation of the coal by groundwater. Samples taken at depths > ~50 cm should give a much better indication of the unweathered nature of the coal. However, even in the most weathered zones, silicate minerals in the ash appeared to be unaffected so that surface samples should be adequate to study silicate mineralogy of the coal. However, it is likely that the sulphide and sulphate minerals would be severely affected perhaps to a considerable depth. Sulphides and sulphates were not sufficiently abundant in the coals used to verify this. Although the bulk chemical properties of the coal may be altered to the point that proximate analyses can not be used to determine the rank of the coal, the rank can still be determined by carefully measuring the reflectance of the apparently unaltered portions of the vitrinite grains. This is confirmed by the fact that per cent reflectance of all samples used in this study was between 1.5 and 2.0%, the range for low volatile bituminous coals. This agrees well
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Natural weathering of coal: G. R. Ingram and J. D. Rimstidt
with the rank determination proximate analyses.
made from the previous 7 8
ACKNOWLEDGEMENTS The field work for this project was supported by the Virginia Division of Mineral Resources. The research was supported in part by the Virginia Mining and Mineral Resources Research Institute Allotment Grant 1980-S 1.
9 10 II 12 13
REFERENCES 1 2 3 4
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Chandra, D. Econ. Geol. 1958,53, 102 Chandra, D. Fuel 1962,41, 185 Chandra, D. Econ. Geol. 1966,61, 754 Benedict, L. G. and Berry, W. F. ‘Recognition and Measurement of Coal Oxidation’, Bituminous Coal Research, Inc., Monroeville, Pennsylvania, 1964 Stevens, D. W. ‘A survey of the factors which affect mining of the Lower Mississippian coals in Montgomery Co., Virginia’, MS. thesis, Virginia Polytechnic Institute and State University, 1959 Crelling, J. C. and Dutcher, R. R. ‘Principles and Applications of
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Coal Petrology’, SEPM Short Course Notes No. 8, 1980 Collins, D. R. Virginia Highway Department, personal communication, 198 1 Harman. W. M. Norfolk and Western Railway, personal communication, 1981 Lowenhaupt, D. E. and Gray, R. J. Inter-nut. J. Coal Geol. 1980,1, 63 US Bureau of Mines, Bull. 1967, 638, 3 Stach, E. ‘Stach’s Textbook of Coal Petrology’, 2nd edition, Gebruder Bomtraeger, 1975 Bloom, A. L. ‘Geomorphology, A Systematic Analysis of Late Cenozoic Landforms’, Prentice-Hall, Inc. 1978, ch. 6 Fryer, J. F. and Szladow, A. J. ‘Storage of Coal Samples’, Research Council of Alberta Information Series, No. 88. 1971 Ingram, G. R. ‘The effect of weathering on the vitrinite reflectance of the Lower Mississippian coals in Montgomery and Pulaski Counties, Virginia’, MS. thesis, Virginia Polytechnic and Institute and State University, 1981 Walker, R. D. Personal communication, 1981 Allardice, D. J. and Evans, D. G. ‘Analytical Methods for Coal and Coal Products’, Academic Press, 1978, Vol. 1, ch. 7 Wershaw, R. L., Pinckney, D. J. and Booker,S. E. J. Res. US Geol. Survey 1977,5, 565 Teichmuller, M. and Teichmuller, R. ‘Coal and Coal Bearing Strata’, American Elsevier Publishing Co., Inc. 1968, ch. 15