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Structure and chemistry of coals: Transport and nucleation processes in mineral metathesis
Structure and Chemistry of Coals: Transport and Nucleation Processes in Mineral Metathesis E. L. FULLER, JR. Plant Laboratory Department, Y-12 Product Certification Division, Oak Ridge Y-12 Plant, P.O. Box Y, Oak Ridge, Tennessee 37830 Received April 14, 1981; accepted J a n u a r y 19, 1982 Microscopic and X-ray analyses were used to study the metathesis of minerals inherent within the organic matrix of two coals of different rank. Sulfides are oxidized within the hydrated matrix, transported to the surface of the coal particle, further oxidized, and precipitated on the coal surface. The effects of water activity (relative humidity) and oxygen activity are delineated. The rate of transport and morphology of the precipitates depends markedly upon the coal lithotype and morphology. Structural and chemical inferences are related to previous work on the same materials.
INTRODUCTION
Mineral matter in coals can be generally classified into three categories: 1. Admixed minerals are generally of a larger size (> 1 ~zm) and are incorporated by a physical process such as mining practices which acquire some of the mineral strata above and/or below the coal seam. Such mixing also occurs due to cataclysmic geological phenomena. Generally these are either free or rather loosely bound in the coal matrix (i.e., cleat minerals). 2. Incorporated minerals range is size from 0.01 to ca. 1 ~m and result from metamorphic processes to form products of coalification. These include pyritic, silicious, and carbonaceous mineral inclusions within the coal matrix. These are noted as rather large crystals surrounded symmetrically by the organic phase. 3. Inherent minerals are the finely divided inorganic constituents ranging from 1 to 10 nm in size and are relatively uniformly distributed within the organic matrix ( 1). These entities were either inherent in the precursor plant material and/or were absorbed by ion
exchange processes very early in the coalification process. Admixed minerals are readily removed in coal preparation plants by physical processes such as sedimentation, magnetic separation, etc (2). Efficient removal of incorporated minerals is much more difficult since physical separation requires extensive comminution of the coal. The inherent minerals can only be removed by chemical processes involving leaching or solvent attack. In this study, performed by the Oak Ridge Y-12 Plant, ~ we have purposely chosen our samples to be free of admixed and incorporated minerals for several reasons: 1. Inherent minerals are the most difficult to remove and the production of ultraclean hydrocarbon energy sources depend upon its removal. 2. The very structure and chemistry of the "coal" itself depends markedly on the amount and nature of this mineral (3). Operated for the Department of Energy by Union Carbide Corporation, Nuclear Division, under Contract W-7405-eng-26.
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Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
0021-9797/82/100309-19502.00/0 Copyright© 1982 by AcademicPress, Inc. All rights of reproductionin any form reserved.
310
E.L. FULLER, JR. TABLE I Analyses of Coals Mine Rank
Wyodak Subbituminous
Sahara C-Bituminous
Proximate analysesa Ash Volatile matter Fixed carbon Moisture
6.8 46.4 46.8 28.9
12.2 38.8 49.1 5.5
WT%(DAF) WT%(DAF) WT%(DAF) WT%
Ultimate Analysesa Carbon Hydrogen Oxygen Nitrogen Sulfur
72.5 5.45 20.5 1.01 0.53
77.7 5.83 10.6 1.43 4.50
WT%(DAF) WT%(DAF) WT%(DAF) WT%(DAF) WT%(DAF)
Colloid and surface properties b (BET, N2) I~ (0.6, N2) P (S, N2)
2.6 0.7 1.5
2.8 m2/g 1.3 mg/g 2.9 mg/g
(BET, CO2) P (0.6, CO2) P (S, CO2)
200 120 155
128 102 138
m2/g mg/g mg/g
(BET, H20) F (0.6, H20) I~ (S, H20)
270 98 255
68 28 72
m2/g mg/g mg/g
-120
-30
h~ (H20, 25)
J/g
° Determined by the Analytical Chemistry Division, Oak Ridge National Laboratory, b See Ref. (8) for details and other references.
3. Catalytic processing of the coals is dependent to varying degrees (both beneficial and detrimental) by these mineral constituents (4). This finely divided material is quite active and ubiquitous in virtually all coals of rank below that of anthracites. This report is part of an extensive study of the structure and chemistry of coals of varying rank and lithotypes. Two samples of coal were used in this study characteristic of the large western reserve of subbituminous coal and the central reserve bituminous coals. Some pertinent properties of these materials are given in Table I. EXPERIMENTAL
The coal samples used for this study were obtained from freshly opened mine faces and Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
hermetically sealed in stainless-steel containers to minimize reactions with the atmosphere. A subbituminous coal was acquired at the Wyodak mine near Gillete, Wyoming, and the sample of C-bituminous coal was obtained at the Sahara mine in eastern Kentucky. In each case, aliquots of the sample (fragmented pieces one to 100 millimeters) were removed from the sealed stainless-steel containers and stored in loosely covered vessels with care to avoid dust contamination. Within a month the Wyodak sample had a distinct white "bloom" easily detected by the unaided eye. Some details of the morphology of this material are noted in lowpower optical microscopic examination as shown in Fig. 1. The distinctly clear crystalline nature is readily distinguished from the black opaque coal substrate. Detailed examination of the crystals is most easily accomplished by initially locating the crystalline material with optical microscopy and subsequently carrying out analyses with a scanning electron microscope. This technique has been used quite productively to delineate color contrasts for subsequent analyses of the inherent chemical variation (5). Figure I shows that the background coal generally has a structure resuiting from the collapse of the original woody fibrous material which was the precursor to coalification (6). Distinctly different morphology is noted for the crystalline material at higher magnification. The crystals are generally seen to be in the form of bundles grown from nucleation sites within fissures in the coal structure. Some single rods are noted standing alone and/or emanating from the bundles as shown in Fig. 2. Two other types of crystal growth are prevalent as shown in Figs. 3 and 4. There are quite a number of white web-like growths which generally occur in conjunction with a more disordered structure of the substrate coal (in contrast to the striated coal structure of Fig. 1). The nature of the nucleation sites is less distinct but generally seem to occur
STRUCTURE AND CHEMISTRY OF COALS
3 11
FIG. 1. Optical photomicrograph of mineral growth on Wyodak coal surface. The white mineral is readily contrasted to the black coal structure when viewed with reflected light. Magnification: 100-/~m full-scale horizontally.
within the stacked laminae of the coal. This mineral structure is random and the diameter seems to have an undulatory character forming a connected series of nodules. Branching and directional changes occur at
these nodules suggesting that periodic variations of temperature a n d / o r relative humidity may alter the growth processes. This behavior and the existence of somewhat bulbous truncations lead one to suspect that Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
312
E L . FULLER, JR.
FIG. 2a. Scanning electron micrograph of mineral growth on W y o d a k coal surface. Low contrast is noted between mineral and coal structure [ 180-#m full-scale width]. FIG. 2b. Magnified view of Fig. 2a. T h e details of the fibrous nature of the coal substrate are quite evident [90-gm full-scale width]. Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
STRUCTURE AND CHEMISTRY OF COALS
313
FIG. 2c. Magnified view of Fig. 2b. The crystallites are seen to emanate from a nucleation site within the fissure in the coal surface [45-/~m full-scale width], FIG. 2d. Magnified view of Fig. 2c. Crystallites are discerned to be composed of bundles of rods. Some free fibers are noted emanating from the fissure and intertwined in the bundle [ 18-/~m full-scale width]. Journal of Colloid and Interface Science, ~Vol.89, No. 2, October 1982
FIG. 2e. Magnified view of Fig. 2d. The fibrous structure is seen to be composed of needles of about 0.1 m diameter. Some platelet material is attached to the bundle [9-#m full-scale width].
FIG. 3a. Scanning electron micrograph of tendrilic mineral structure on W y o d a k coal. The web-like mineral structure is noted in tfiis zone of fracture when disordered organic phase prevails [9-/~m fullscale width]. Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
314
FIG. 3b. Magnified view of Fig. 3a. The coal matrix appears quite a heterogeneous agglomeration [4.5-urn full-scale width]. FIG. 3e. Magnified view of Fig. 3b. T h e coal structure is seen to be composed of laminae (leaflets) and the tendrils appear to e m a n a t e from this structure. The nodular nature of the tendrils is quite evident and branching occurs at these nodular sites [ 1.8-#m full-scale width]. Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
315
FIG. 3d. Magnified view of Fig. 3c. The bulbous nature of the ends of the tendrils suggests that they are tubular and that growth occurs by a cyclical process involving material transport through the core of the tendrils. Oxidative concretion leads to a rigid shell around a liquid-filled core [0.9-#m full-scale width].
FIG. 4a. Scanning electron photomicrograph of the slab-like mineral growth on Wyodak coal surfaces. The plate structure is quite characteristic of the mineral structure associated with quite homogeneous substrate structures [18-urn full-scale width]. Journal of Colloid and Interface Science,
Vol.89, No. 2, October1982 316
FIG. 4b. Magnified view of Fig. 4a. T h e coal matrix structure is shown to be somewhat homogeneous with conchoidal and smooth fracture [9-~m full-scale width]. FIo. 4c. Magnified view of Fig. 4b. Nucleation is seen to occur at a linear fracture zone. A twinning plan is shown e m a n a t i n g from the slab surface. The smooth nature of the coal surface contains fossilized evidence of plant fibers [4.5-/~m full-scale width]. Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
317
FIG. 4d. Magnified view of Fig. 4c. The rare simultaneous occurrence of both tendrillic and slab mineralization is shown here. The tendril seems to e m a n a t e from a nucleation site on the edge of the slab [ 1.8-~zm full-scale width].
FIG. 5. Typical slab of mineral of Wyodak coal shown e m a n a t i n g from a rather smooth coal surface adjacent to a framboidal coal structure [ 1.8-tzm full-scale width]. Iournal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
318
STRUCTURE AND CHEMISTRY OF COALS
II Mg AI
~ ~ Si
S
3 19
X Co
FIG. 6. Typical elemental analysis of coal A rather uniform distribution of the mineral matter is noted by traces of this type noted for freshly cleaned and/or crystallite free coal surfaces.
these concretions are formed by the transport of material through the center of a tubular structure. The third type of crystal growth we have observed is of a more slab-like or platelet morphology as shown in Figs. 4 and 5. These structures predominate on a much more homogeneous coal substrate where brittle fracture occurs as witnessed by the conchoidal a n d / o r smooth coal structure. All of these platelets are nearly perpendicular to the coal surface and seem to emanate from a linear nucleation zone. Only rarely does one find the coexistence of two mineral forms ema-
nating from a given zone of the coal. The tendrilic structure of Fig. 4 actually seems to emanate from a nucleation site at the edge of the platelet. The free platelet structure noted in Fig. 5 is far more common. In each case we have operated the scanning electron microscope in its X-ray energy dispersive mode to measure the elemental composition of both the coal and the mineral growths to obtain results virtually identical to those shown in Figs. 6 and 7. The elemental abundance within the coal does not vary appreciably for the lithotypes described above since the mineral matter is very finely Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
320
E.L. FULLER, JR.
I
II
S
Ca
FIG. 7. Typical elemental analysis of metathesized minerals on coal. Only calcium and sulfur are detected in each of the metathesized mineral types. This technique cannot detect elements lighter than sodium (i.e., the sulfate oxygen is not detected in this case).
and uniformly dispersed within the coal matrix. In contrast, this technique detects only calcium and sulfur in rods, tendrils, and plates of the mineral growths. These friable appendages can be easily brushed from the coal structure. X - R a y diffraction studies of this white powder showed only a pattern characteristic of pure selenite ( g y p s u m , C a S O 4 ° 2 H 2 0 ) with quite narrow linewidths commensurate with a high degree of order (rather large crystallites). Similar studies with the Illinois N u m b e r Six bituminous coals required much longer periods of air exposure to develop the visibly Journal of Colloid and Interface Science, Vol, 89, No. 2, October 1982
white patches on the surface. This coal structure is much more dense and homogeneous as noted by the smoother fracture surfaces displayed in Fig. 8. The mineral specie does appear to have migrated to preferential patches on this substrate, but no distinct nucleation sites are apparent. The framboidal structure is the result of homogeneous recrystallization to form highly porous agglomerates of calcium sulfate. It is worthy of note that the mineral growth occurred only on the lower side of the bituminous coals. Illumination (sun a n d / or fluorescent) was incident to the upper side
S T R U C T U R E A N D C H E M I S T R Y OF C O A L S
321
FIG. 8a. Scanning electron micrograph of mineral growth on the surface of Illinois coal. Disordered mineral is deposited rather randomly on the smooth coal surface [ 18-ttm full-scale width].
of the 1- to 10-centimeter coal particles. The migration in this case seems to occur due to the gradient (thermal and/or aqueous potential) established by the incident radiant energy. In contrast, the mineral migration led to growth more or less uniformly on all
sides of the Wyodak sample regardless of the incident light. Additional studies with various environmental conditions were performed to better understand the process. Pertinent observaJournal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
322
E.L. FULLER, JR.
FIG. 8b. Magnified view of Fig. 6a. The homogeneousnature of the coal is noted in the conchoidal fracture zones [9-#m full-scale width].
tions are: (1) N o crystallite growth is detectable in low relative humidities (<20%) with or without air; (2) no crystallite growth is noted in high relative humidities (60 to 90%) in the absence of air, and (3)prolonged exposure of air-dried Wyodak coal (100°C Journal of Colloid and Interface Science, VoI. 89, No. 2, October 1982
in air for 24 hours) showed no visible mineral metathesis when placed into a humid (75%) air chamber (18 months at this writing). These results point out the synergetic contributions of the oxygen and water to the metathesis process.
STRUCTURE AND CHEMISTRY OF COALS
323
FIG. 8c. Magnified view of Fig. 6b. Two general mineral morphologies are noted as the smooth and sponge-like deposits on the smooth and conchoidal coal [4.5-1zm full-scale width].
DISCUSSION
Our results lead us to conclude that the sulfide materials within the coal are oxidized consistent with previous work (7): MSx + (3X/2)O2 ~ MOx + SO2,
[1]
where M is either a metal ion or an organic structural entity within the coal. At high relative humidity there is ample free water to establish the equilibria, 502 + H20 ~- H2803 H+(aq) + HSO3(aq).
[2]
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
324
E.L. FULLER, JR.
FIG. 8d. Magnified view of Fig. 6c. The framboidal mineral material is seen to be composed of smaller nodules rather loosely agglomerated [ 1.8-~m full-scale width].
Simultaneously there is an acid attack on the dispersed calcium carbonate CaCO3(s) + 2H+(aq) Ca2+(aq) + H2CO3(aq)
[3]
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
and H/CO3(aq) --~ H:O + CO2(g).
[4]
The net result is the formation of a solution of calcium dihydrogen sulfite within the im-
STRUCTURE AND CHEMISTRY OF COALS
325
FIG. 8e. Magnified view of Fig. 6d. The framboidal particles are formed of small nodules which appear to be truncated growths much akin to the larger smooth particles. Apparently the mineral morphology is controlled by the amount material transported to the growth site [0.9-~m full-scale width].
bibed aqueous phase. The migration is facilitated by the high solubility of this material and the high mobility of the hydrated ionic entities, Ca2+(aq) and HSO3(aq). In the presence of excess oxygen the bisulfate entity is unstable and, after migration to the
surface, oxidation to form the insoluble sulfate occurs by a one- or two-step process Ca2+(aq) + 2HSO~- + O2(g) Ca(HSO4)2(1), [5] where the calcium bisulfate is unstable with Journal of Colloid and Interface Science, Vol.89, No. 2, October1982
E.L. FULLER,JR.
326 respect to calcium sulfate Ca(HSO4)2(1)
CaSO4(s) + H2SO4(aq)
[6]
which is only slightly soluble even in acidic media. Finally at higher humidities calcium sulfate dihydrate (gypsum or selenite) is the stable form
CaSO4(s) + 2H20--~ CaSO4.2H/O(s).
[7]
The acidic entities formed in the process will react readily with the sparingly soluble (rather mobile) calcium carbonate
H20 + CaCO3(aq) + H2SOa(aq) ~CaSO4 °2H20(s) + CO2(g).
[8]
The above processes are commensurate with our observations for a three-step process: 1. Initiation by the oxidation of a sulfide entity, Eq. [ 1], to form a sulfite entity in an acidic medium formed by the imbibed water at relatively high humidity (60-90%). 2. Migration of soluble entities (bicarbonate and bisulfite) to the surface of the coal matrix. 3. Precipitation, either randomly or on surface nucleation sites, by further oxidation (Eq. [5]) coupled with acidic decomposition of carbonates (Eq. [8]). The imbibed water within the coal matrix serves as the media for transport. There is a gradient (8) established at the surface of the coal particles. Sorbed water which is retained below ca. 50% relative humidity is strongly associated with sites on the coal. Above ca. 60% relative humidity the additional water content is somewhat more like liquid water and is available to hydrate ionic species as well as to swell the coal structure (9) and generate channels large enough to accept and pass the rather large hydrated ions. These phenomena are considerably more enhanced in the lower-ranked coals Journal of Colloid and Interface Science, Vol. 89, No, 2, October 1982
(10). Caution must be exercised in assuming the existence of various sizes and amounts of "pores" in the coals of various ranks. More relevant, though, is to consider the degree of coalification as decreasing the absorption capacity by greater consolidation of the impervious organic entities (or macerals). The lower-ranked coals actually absorb water much as wood, cellulose, sponges, etc., and the ionic solubility arises due to the synergetic interaction of the water and the coal matrix much akin to an ion exchange resin. The crystal growth noted here does not occur within the coal bed since there is virtually no oxygen present to initiate the reaction. Furthermore, the undisturbed coal bed is at essentially 100% relative humidity throughout with no water gradient. The gradient exists only when a coal surface is exposed to a lower relative humidity. If the humidity is below ca. 50%, the coal matrix is depleted in water to a point where no liquid-like water is present and no ionic transport occurs. In this state the initiating oxidation occurs and, without an adequate amount of imbibed water, no ionic transport occurs. Subsequent oxidation (Eq. [5]) occurs at the internal site forming the immobile sulfate in a highly dispersed state within the coal matrix. Subsequent exposure to high humidity and/or liquid water has little effect due to the low solubility (mobility) of calcium sulfate. This study is a rather special case since coal mined and stored in Wyoming is exposed to low humidity (10-25%) and very little mineral metathesis is noted. On the other hand, this same coal transported to a more humid area can and does exhibit considerable metathesis. Commercial desulfurization of coals can be implemented with an oxidative leaching process (7), expecially for the low-ranked coals, if and when the market and legislative demands arise. A negative aspect of this phenomena gives rise to the acid mine drainage problem in areas of high rainfall (11) and leaching problems associated with storage of coal processing wastes.
STRUCTURE AND CHEMISTRY OF COALS
Very similar oxidation-transport-oxidation mineral metathesis has been observed to form friable sulfate minerals in refuse dumps (12). More academic, however, are the questions related to the structure and chemistry of coals. First of all, do we define coal as solely the organic matrix, as the hydrated entity (organic plus inorganic plus water) as found in the mine, or as the dehydrated product (organic and inorganic)? More generally, we can note that the "coal" structure and chemistry varies markedly depending on its environment and time history. Most microscopic analyses will give markedly different results at various states; i.e., the sorption properties of the inherent minerals are grossly different than those of the metathesis products in accord with the X-ray analyses reported above. This work also verifies and delineates the difference in character of sorbed water above and below a relative humidity of ca. 50% (13). The former is strongly associated with the coal matrix and is not free to dissolve (via ion hydration) the mineral species. The additional sorbed water (>50% relative humidity) can and does dissolve and serve as a transport medium for soluble minerals within the coal matrix. CONCLUSION
The organic coal matrix serves as a reaction and transport medium if and only if the structure has imbibed the equilibrium amount of water corresponding to a relative humidity in excess of 60 percent. Oxidation of sulfide entities is initiated by the oxygen of the air, and the sulfites thus formed are transported through the hydrated matrix to the surface of the coal particles due to the aqueous gradient. Further oxidation at the coal surface forms gypsum (selenite), which precipitates either by a heterogeneous dispersed mechanism or homogeneously on nucleation sites, depending on the rank of the coal. The detrimental and/or beneficial
327
aspects of these phenomena are readily apparent. Coals should be recognized as complex colloidal (14) systems that exist in a hydrous-reducing environment. Definitive evaluations of the structure and chemistry of a given coal must involve cognizance of changes wrought in exposure to air by virture of mining, transport, and storage operations (15). These changes are intimately related, both in scope and in type, to the nature of the coal as noted in the different allotropic forms of calcium sulfate which are stable on various coal lithotypes. REFERENCES 1. Strehlow, R. A., Harris, L. A., and Yust, C. S., Fuel 57, 185 (1978). 2. Merrit, P. C., "Handbook of Coal Preparation." McGraw-Hill, New York, 1978. 3. Yu, P. P., and Makarewicz, M. A., "Surface Properties of Magnetically Separated Coal Components." ORNL/MIT-289, March 1979. 4. Senkan, S. M., and Fuller, E. L., Jr., Fuel 58, 729 (1979). 5. Dale, J. M., Hulett, L. D., Fuller, E. L., Jr., Richards, H. L., and Sherman, R. L., J. Catal. 61, 66 (1980). 6. Greer, R. T., Energy Sources 4, 23 (1978). 7. Elliot, R. C., "Coal Desulfurization Prior to Combustion." Noyes Data Corporation, 1978. 8. Fuller, E. L., Jr., J. Colloid Interface Sci. 75, 577 (1980). 9. Fuller, E. L., Jr., in "Coal Structure" (M. Gorbaty, Ed.), Advances in Chemistry Series, 1981. 10. Evans, D. G., and Allerdice, D. J., in "Analytical Methods for Coal and Coal Products" (C. Karr, Jr., Ed.), Vol. I, pp. 84, 247. Academic Press, New York, 1978. 11. Hawley, M. E., "Coal, Part I." Dowden, Hutchinson & Ross, Stroudsburg, Penn., 1976. 12. "Trace Element Characterization of Coal Wastes." LA-6835-PR, EPA 600/7-78-028, March 1978. 13. Allerdice, D. J., and Evans, D. G., in "Analytical Methods for Coal and Coal Products" (C. Karr, Jr., Ed.), Part II, Vol. I, Chap. 7. Academic Press, New York, 1978. 14. vanKrevelen, D. W., "Coal." Elsevier, Amsterdam/ New York, 1961. 15. Attar, A., in "Analytical Methods for Coal and Coal Products" (C. Karr, Jr., Ed.), Vol. 3, Chap. 56, p. 585. Academic Press, New York, 1978.
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
INTRODUCTION
Mineral matter in coals can be generally classified into three categories: 1. Admixed minerals are generally of a larger size (> 1 ~zm) and are incorporated by a physical process such as mining practices which acquire some of the mineral strata above and/or below the coal seam. Such mixing also occurs due to cataclysmic geological phenomena. Generally these are either free or rather loosely bound in the coal matrix (i.e., cleat minerals). 2. Incorporated minerals range is size from 0.01 to ca. 1 ~m and result from metamorphic processes to form products of coalification. These include pyritic, silicious, and carbonaceous mineral inclusions within the coal matrix. These are noted as rather large crystals surrounded symmetrically by the organic phase. 3. Inherent minerals are the finely divided inorganic constituents ranging from 1 to 10 nm in size and are relatively uniformly distributed within the organic matrix ( 1). These entities were either inherent in the precursor plant material and/or were absorbed by ion
exchange processes very early in the coalification process. Admixed minerals are readily removed in coal preparation plants by physical processes such as sedimentation, magnetic separation, etc (2). Efficient removal of incorporated minerals is much more difficult since physical separation requires extensive comminution of the coal. The inherent minerals can only be removed by chemical processes involving leaching or solvent attack. In this study, performed by the Oak Ridge Y-12 Plant, ~ we have purposely chosen our samples to be free of admixed and incorporated minerals for several reasons: 1. Inherent minerals are the most difficult to remove and the production of ultraclean hydrocarbon energy sources depend upon its removal. 2. The very structure and chemistry of the "coal" itself depends markedly on the amount and nature of this mineral (3). Operated for the Department of Energy by Union Carbide Corporation, Nuclear Division, under Contract W-7405-eng-26.
309
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
0021-9797/82/100309-19502.00/0 Copyright© 1982 by AcademicPress, Inc. All rights of reproductionin any form reserved.
310
E.L. FULLER, JR. TABLE I Analyses of Coals Mine Rank
Wyodak Subbituminous
Sahara C-Bituminous
Proximate analysesa Ash Volatile matter Fixed carbon Moisture
6.8 46.4 46.8 28.9
12.2 38.8 49.1 5.5
WT%(DAF) WT%(DAF) WT%(DAF) WT%
Ultimate Analysesa Carbon Hydrogen Oxygen Nitrogen Sulfur
72.5 5.45 20.5 1.01 0.53
77.7 5.83 10.6 1.43 4.50
WT%(DAF) WT%(DAF) WT%(DAF) WT%(DAF) WT%(DAF)
Colloid and surface properties b (BET, N2) I~ (0.6, N2) P (S, N2)
2.6 0.7 1.5
2.8 m2/g 1.3 mg/g 2.9 mg/g
(BET, CO2) P (0.6, CO2) P (S, CO2)
200 120 155
128 102 138
m2/g mg/g mg/g
(BET, H20) F (0.6, H20) I~ (S, H20)
270 98 255
68 28 72
m2/g mg/g mg/g
-120
-30
h~ (H20, 25)
J/g
° Determined by the Analytical Chemistry Division, Oak Ridge National Laboratory, b See Ref. (8) for details and other references.
3. Catalytic processing of the coals is dependent to varying degrees (both beneficial and detrimental) by these mineral constituents (4). This finely divided material is quite active and ubiquitous in virtually all coals of rank below that of anthracites. This report is part of an extensive study of the structure and chemistry of coals of varying rank and lithotypes. Two samples of coal were used in this study characteristic of the large western reserve of subbituminous coal and the central reserve bituminous coals. Some pertinent properties of these materials are given in Table I. EXPERIMENTAL
The coal samples used for this study were obtained from freshly opened mine faces and Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
hermetically sealed in stainless-steel containers to minimize reactions with the atmosphere. A subbituminous coal was acquired at the Wyodak mine near Gillete, Wyoming, and the sample of C-bituminous coal was obtained at the Sahara mine in eastern Kentucky. In each case, aliquots of the sample (fragmented pieces one to 100 millimeters) were removed from the sealed stainless-steel containers and stored in loosely covered vessels with care to avoid dust contamination. Within a month the Wyodak sample had a distinct white "bloom" easily detected by the unaided eye. Some details of the morphology of this material are noted in lowpower optical microscopic examination as shown in Fig. 1. The distinctly clear crystalline nature is readily distinguished from the black opaque coal substrate. Detailed examination of the crystals is most easily accomplished by initially locating the crystalline material with optical microscopy and subsequently carrying out analyses with a scanning electron microscope. This technique has been used quite productively to delineate color contrasts for subsequent analyses of the inherent chemical variation (5). Figure I shows that the background coal generally has a structure resuiting from the collapse of the original woody fibrous material which was the precursor to coalification (6). Distinctly different morphology is noted for the crystalline material at higher magnification. The crystals are generally seen to be in the form of bundles grown from nucleation sites within fissures in the coal structure. Some single rods are noted standing alone and/or emanating from the bundles as shown in Fig. 2. Two other types of crystal growth are prevalent as shown in Figs. 3 and 4. There are quite a number of white web-like growths which generally occur in conjunction with a more disordered structure of the substrate coal (in contrast to the striated coal structure of Fig. 1). The nature of the nucleation sites is less distinct but generally seem to occur
STRUCTURE AND CHEMISTRY OF COALS
3 11
FIG. 1. Optical photomicrograph of mineral growth on Wyodak coal surface. The white mineral is readily contrasted to the black coal structure when viewed with reflected light. Magnification: 100-/~m full-scale horizontally.
within the stacked laminae of the coal. This mineral structure is random and the diameter seems to have an undulatory character forming a connected series of nodules. Branching and directional changes occur at
these nodules suggesting that periodic variations of temperature a n d / o r relative humidity may alter the growth processes. This behavior and the existence of somewhat bulbous truncations lead one to suspect that Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
312
E L . FULLER, JR.
FIG. 2a. Scanning electron micrograph of mineral growth on W y o d a k coal surface. Low contrast is noted between mineral and coal structure [ 180-#m full-scale width]. FIG. 2b. Magnified view of Fig. 2a. T h e details of the fibrous nature of the coal substrate are quite evident [90-gm full-scale width]. Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
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313
FIG. 2c. Magnified view of Fig. 2b. The crystallites are seen to emanate from a nucleation site within the fissure in the coal surface [45-/~m full-scale width], FIG. 2d. Magnified view of Fig. 2c. Crystallites are discerned to be composed of bundles of rods. Some free fibers are noted emanating from the fissure and intertwined in the bundle [ 18-/~m full-scale width]. Journal of Colloid and Interface Science, ~Vol.89, No. 2, October 1982
FIG. 2e. Magnified view of Fig. 2d. The fibrous structure is seen to be composed of needles of about 0.1 m diameter. Some platelet material is attached to the bundle [9-#m full-scale width].
FIG. 3a. Scanning electron micrograph of tendrilic mineral structure on W y o d a k coal. The web-like mineral structure is noted in tfiis zone of fracture when disordered organic phase prevails [9-/~m fullscale width]. Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
314
FIG. 3b. Magnified view of Fig. 3a. The coal matrix appears quite a heterogeneous agglomeration [4.5-urn full-scale width]. FIG. 3e. Magnified view of Fig. 3b. T h e coal structure is seen to be composed of laminae (leaflets) and the tendrils appear to e m a n a t e from this structure. The nodular nature of the tendrils is quite evident and branching occurs at these nodular sites [ 1.8-#m full-scale width]. Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
315
FIG. 3d. Magnified view of Fig. 3c. The bulbous nature of the ends of the tendrils suggests that they are tubular and that growth occurs by a cyclical process involving material transport through the core of the tendrils. Oxidative concretion leads to a rigid shell around a liquid-filled core [0.9-#m full-scale width].
FIG. 4a. Scanning electron photomicrograph of the slab-like mineral growth on Wyodak coal surfaces. The plate structure is quite characteristic of the mineral structure associated with quite homogeneous substrate structures [18-urn full-scale width]. Journal of Colloid and Interface Science,
Vol.89, No. 2, October1982 316
FIG. 4b. Magnified view of Fig. 4a. T h e coal matrix structure is shown to be somewhat homogeneous with conchoidal and smooth fracture [9-~m full-scale width]. FIo. 4c. Magnified view of Fig. 4b. Nucleation is seen to occur at a linear fracture zone. A twinning plan is shown e m a n a t i n g from the slab surface. The smooth nature of the coal surface contains fossilized evidence of plant fibers [4.5-/~m full-scale width]. Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
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FIG. 4d. Magnified view of Fig. 4c. The rare simultaneous occurrence of both tendrillic and slab mineralization is shown here. The tendril seems to e m a n a t e from a nucleation site on the edge of the slab [ 1.8-~zm full-scale width].
FIG. 5. Typical slab of mineral of Wyodak coal shown e m a n a t i n g from a rather smooth coal surface adjacent to a framboidal coal structure [ 1.8-tzm full-scale width]. Iournal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
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STRUCTURE AND CHEMISTRY OF COALS
II Mg AI
~ ~ Si
S
3 19
X Co
FIG. 6. Typical elemental analysis of coal A rather uniform distribution of the mineral matter is noted by traces of this type noted for freshly cleaned and/or crystallite free coal surfaces.
these concretions are formed by the transport of material through the center of a tubular structure. The third type of crystal growth we have observed is of a more slab-like or platelet morphology as shown in Figs. 4 and 5. These structures predominate on a much more homogeneous coal substrate where brittle fracture occurs as witnessed by the conchoidal a n d / o r smooth coal structure. All of these platelets are nearly perpendicular to the coal surface and seem to emanate from a linear nucleation zone. Only rarely does one find the coexistence of two mineral forms ema-
nating from a given zone of the coal. The tendrilic structure of Fig. 4 actually seems to emanate from a nucleation site at the edge of the platelet. The free platelet structure noted in Fig. 5 is far more common. In each case we have operated the scanning electron microscope in its X-ray energy dispersive mode to measure the elemental composition of both the coal and the mineral growths to obtain results virtually identical to those shown in Figs. 6 and 7. The elemental abundance within the coal does not vary appreciably for the lithotypes described above since the mineral matter is very finely Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
320
E.L. FULLER, JR.
I
II
S
Ca
FIG. 7. Typical elemental analysis of metathesized minerals on coal. Only calcium and sulfur are detected in each of the metathesized mineral types. This technique cannot detect elements lighter than sodium (i.e., the sulfate oxygen is not detected in this case).
and uniformly dispersed within the coal matrix. In contrast, this technique detects only calcium and sulfur in rods, tendrils, and plates of the mineral growths. These friable appendages can be easily brushed from the coal structure. X - R a y diffraction studies of this white powder showed only a pattern characteristic of pure selenite ( g y p s u m , C a S O 4 ° 2 H 2 0 ) with quite narrow linewidths commensurate with a high degree of order (rather large crystallites). Similar studies with the Illinois N u m b e r Six bituminous coals required much longer periods of air exposure to develop the visibly Journal of Colloid and Interface Science, Vol, 89, No. 2, October 1982
white patches on the surface. This coal structure is much more dense and homogeneous as noted by the smoother fracture surfaces displayed in Fig. 8. The mineral specie does appear to have migrated to preferential patches on this substrate, but no distinct nucleation sites are apparent. The framboidal structure is the result of homogeneous recrystallization to form highly porous agglomerates of calcium sulfate. It is worthy of note that the mineral growth occurred only on the lower side of the bituminous coals. Illumination (sun a n d / or fluorescent) was incident to the upper side
S T R U C T U R E A N D C H E M I S T R Y OF C O A L S
321
FIG. 8a. Scanning electron micrograph of mineral growth on the surface of Illinois coal. Disordered mineral is deposited rather randomly on the smooth coal surface [ 18-ttm full-scale width].
of the 1- to 10-centimeter coal particles. The migration in this case seems to occur due to the gradient (thermal and/or aqueous potential) established by the incident radiant energy. In contrast, the mineral migration led to growth more or less uniformly on all
sides of the Wyodak sample regardless of the incident light. Additional studies with various environmental conditions were performed to better understand the process. Pertinent observaJournal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
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E.L. FULLER, JR.
FIG. 8b. Magnified view of Fig. 6a. The homogeneousnature of the coal is noted in the conchoidal fracture zones [9-#m full-scale width].
tions are: (1) N o crystallite growth is detectable in low relative humidities (<20%) with or without air; (2) no crystallite growth is noted in high relative humidities (60 to 90%) in the absence of air, and (3)prolonged exposure of air-dried Wyodak coal (100°C Journal of Colloid and Interface Science, VoI. 89, No. 2, October 1982
in air for 24 hours) showed no visible mineral metathesis when placed into a humid (75%) air chamber (18 months at this writing). These results point out the synergetic contributions of the oxygen and water to the metathesis process.
STRUCTURE AND CHEMISTRY OF COALS
323
FIG. 8c. Magnified view of Fig. 6b. Two general mineral morphologies are noted as the smooth and sponge-like deposits on the smooth and conchoidal coal [4.5-1zm full-scale width].
DISCUSSION
Our results lead us to conclude that the sulfide materials within the coal are oxidized consistent with previous work (7): MSx + (3X/2)O2 ~ MOx + SO2,
[1]
where M is either a metal ion or an organic structural entity within the coal. At high relative humidity there is ample free water to establish the equilibria, 502 + H20 ~- H2803 H+(aq) + HSO3(aq).
[2]
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
324
E.L. FULLER, JR.
FIG. 8d. Magnified view of Fig. 6c. The framboidal mineral material is seen to be composed of smaller nodules rather loosely agglomerated [ 1.8-~m full-scale width].
Simultaneously there is an acid attack on the dispersed calcium carbonate CaCO3(s) + 2H+(aq) Ca2+(aq) + H2CO3(aq)
[3]
Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982
and H/CO3(aq) --~ H:O + CO2(g).
[4]
The net result is the formation of a solution of calcium dihydrogen sulfite within the im-
STRUCTURE AND CHEMISTRY OF COALS
325
FIG. 8e. Magnified view of Fig. 6d. The framboidal particles are formed of small nodules which appear to be truncated growths much akin to the larger smooth particles. Apparently the mineral morphology is controlled by the amount material transported to the growth site [0.9-~m full-scale width].
bibed aqueous phase. The migration is facilitated by the high solubility of this material and the high mobility of the hydrated ionic entities, Ca2+(aq) and HSO3(aq). In the presence of excess oxygen the bisulfate entity is unstable and, after migration to the
surface, oxidation to form the insoluble sulfate occurs by a one- or two-step process Ca2+(aq) + 2HSO~- + O2(g) Ca(HSO4)2(1), [5] where the calcium bisulfate is unstable with Journal of Colloid and Interface Science, Vol.89, No. 2, October1982
E.L. FULLER,JR.
326 respect to calcium sulfate Ca(HSO4)2(1)
CaSO4(s) + H2SO4(aq)
[6]
which is only slightly soluble even in acidic media. Finally at higher humidities calcium sulfate dihydrate (gypsum or selenite) is the stable form
CaSO4(s) + 2H20--~ CaSO4.2H/O(s).
[7]
The acidic entities formed in the process will react readily with the sparingly soluble (rather mobile) calcium carbonate
H20 + CaCO3(aq) + H2SOa(aq) ~CaSO4 °2H20(s) + CO2(g).
[8]
The above processes are commensurate with our observations for a three-step process: 1. Initiation by the oxidation of a sulfide entity, Eq. [ 1], to form a sulfite entity in an acidic medium formed by the imbibed water at relatively high humidity (60-90%). 2. Migration of soluble entities (bicarbonate and bisulfite) to the surface of the coal matrix. 3. Precipitation, either randomly or on surface nucleation sites, by further oxidation (Eq. [5]) coupled with acidic decomposition of carbonates (Eq. [8]). The imbibed water within the coal matrix serves as the media for transport. There is a gradient (8) established at the surface of the coal particles. Sorbed water which is retained below ca. 50% relative humidity is strongly associated with sites on the coal. Above ca. 60% relative humidity the additional water content is somewhat more like liquid water and is available to hydrate ionic species as well as to swell the coal structure (9) and generate channels large enough to accept and pass the rather large hydrated ions. These phenomena are considerably more enhanced in the lower-ranked coals Journal of Colloid and Interface Science, Vol. 89, No, 2, October 1982
(10). Caution must be exercised in assuming the existence of various sizes and amounts of "pores" in the coals of various ranks. More relevant, though, is to consider the degree of coalification as decreasing the absorption capacity by greater consolidation of the impervious organic entities (or macerals). The lower-ranked coals actually absorb water much as wood, cellulose, sponges, etc., and the ionic solubility arises due to the synergetic interaction of the water and the coal matrix much akin to an ion exchange resin. The crystal growth noted here does not occur within the coal bed since there is virtually no oxygen present to initiate the reaction. Furthermore, the undisturbed coal bed is at essentially 100% relative humidity throughout with no water gradient. The gradient exists only when a coal surface is exposed to a lower relative humidity. If the humidity is below ca. 50%, the coal matrix is depleted in water to a point where no liquid-like water is present and no ionic transport occurs. In this state the initiating oxidation occurs and, without an adequate amount of imbibed water, no ionic transport occurs. Subsequent oxidation (Eq. [5]) occurs at the internal site forming the immobile sulfate in a highly dispersed state within the coal matrix. Subsequent exposure to high humidity and/or liquid water has little effect due to the low solubility (mobility) of calcium sulfate. This study is a rather special case since coal mined and stored in Wyoming is exposed to low humidity (10-25%) and very little mineral metathesis is noted. On the other hand, this same coal transported to a more humid area can and does exhibit considerable metathesis. Commercial desulfurization of coals can be implemented with an oxidative leaching process (7), expecially for the low-ranked coals, if and when the market and legislative demands arise. A negative aspect of this phenomena gives rise to the acid mine drainage problem in areas of high rainfall (11) and leaching problems associated with storage of coal processing wastes.
STRUCTURE AND CHEMISTRY OF COALS
Very similar oxidation-transport-oxidation mineral metathesis has been observed to form friable sulfate minerals in refuse dumps (12). More academic, however, are the questions related to the structure and chemistry of coals. First of all, do we define coal as solely the organic matrix, as the hydrated entity (organic plus inorganic plus water) as found in the mine, or as the dehydrated product (organic and inorganic)? More generally, we can note that the "coal" structure and chemistry varies markedly depending on its environment and time history. Most microscopic analyses will give markedly different results at various states; i.e., the sorption properties of the inherent minerals are grossly different than those of the metathesis products in accord with the X-ray analyses reported above. This work also verifies and delineates the difference in character of sorbed water above and below a relative humidity of ca. 50% (13). The former is strongly associated with the coal matrix and is not free to dissolve (via ion hydration) the mineral species. The additional sorbed water (>50% relative humidity) can and does dissolve and serve as a transport medium for soluble minerals within the coal matrix. CONCLUSION
The organic coal matrix serves as a reaction and transport medium if and only if the structure has imbibed the equilibrium amount of water corresponding to a relative humidity in excess of 60 percent. Oxidation of sulfide entities is initiated by the oxygen of the air, and the sulfites thus formed are transported through the hydrated matrix to the surface of the coal particles due to the aqueous gradient. Further oxidation at the coal surface forms gypsum (selenite), which precipitates either by a heterogeneous dispersed mechanism or homogeneously on nucleation sites, depending on the rank of the coal. The detrimental and/or beneficial
327
aspects of these phenomena are readily apparent. Coals should be recognized as complex colloidal (14) systems that exist in a hydrous-reducing environment. Definitive evaluations of the structure and chemistry of a given coal must involve cognizance of changes wrought in exposure to air by virture of mining, transport, and storage operations (15). These changes are intimately related, both in scope and in type, to the nature of the coal as noted in the different allotropic forms of calcium sulfate which are stable on various coal lithotypes. REFERENCES 1. Strehlow, R. A., Harris, L. A., and Yust, C. S., Fuel 57, 185 (1978). 2. Merrit, P. C., "Handbook of Coal Preparation." McGraw-Hill, New York, 1978. 3. Yu, P. P., and Makarewicz, M. A., "Surface Properties of Magnetically Separated Coal Components." ORNL/MIT-289, March 1979. 4. Senkan, S. M., and Fuller, E. L., Jr., Fuel 58, 729 (1979). 5. Dale, J. M., Hulett, L. D., Fuller, E. L., Jr., Richards, H. L., and Sherman, R. L., J. Catal. 61, 66 (1980). 6. Greer, R. T., Energy Sources 4, 23 (1978). 7. Elliot, R. C., "Coal Desulfurization Prior to Combustion." Noyes Data Corporation, 1978. 8. Fuller, E. L., Jr., J. Colloid Interface Sci. 75, 577 (1980). 9. Fuller, E. L., Jr., in "Coal Structure" (M. Gorbaty, Ed.), Advances in Chemistry Series, 1981. 10. Evans, D. G., and Allerdice, D. J., in "Analytical Methods for Coal and Coal Products" (C. Karr, Jr., Ed.), Vol. I, pp. 84, 247. Academic Press, New York, 1978. 11. Hawley, M. E., "Coal, Part I." Dowden, Hutchinson & Ross, Stroudsburg, Penn., 1976. 12. "Trace Element Characterization of Coal Wastes." LA-6835-PR, EPA 600/7-78-028, March 1978. 13. Allerdice, D. J., and Evans, D. G., in "Analytical Methods for Coal and Coal Products" (C. Karr, Jr., Ed.), Part II, Vol. I, Chap. 7. Academic Press, New York, 1978. 14. vanKrevelen, D. W., "Coal." Elsevier, Amsterdam/ New York, 1961. 15. Attar, A., in "Analytical Methods for Coal and Coal Products" (C. Karr, Jr., Ed.), Vol. 3, Chap. 56, p. 585. Academic Press, New York, 1978.
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