160
Energy & Fuels 1988,2, 150-157
< Illinois No. 6 coal B < oxidized Upper Freeport coal < Illinois No. 6 coal A < unoxidized Upper Freeport coal < graphite. This order seems indicative of the relative hydrophobicity of the materials. Adding traces of sodium oleate to the agglomeration system markedly increased the recovery of pyrite, oxidized Upper Freeport coal, and 11linois No. 6 coal. The results suggest that deposition of oleic acid onto the coal or pyrite surface or possibly adsorption of various oleate species accounted for an increase in hydrophobicity and recovery. An attempt to separate mixtures of carbonaceous materials and pyrite produced divergent results that require further explanation. On the one hand, a good separation of graphite and pyrite was
achieved by selective agglomeration with heptane; on the other hand, a similar separation was not achieved with coal and pyrite.
Acknowledgment. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This work was supported by the Assistant Secretary for Fossil Energy, through the Pittsburgh Energy Technology Center. The experimental work of L. Kotlarz and D. Clark is gratefully acknowledged. Registry No. Graphite, 7782-42-5;pyrite, 1309-36-0; heptane, 142-82-5;sodium oleate, 143-19-1.
Low-Temperature Coal Weathering: Its Chemical Nature and Effects on Coal Properties? M. M. Wu,* G . A. Robbins, R. A. Winschel, and F. P. Burke Research & Development, Consolidation Coal Company, 4000 Brownsville Road, Library, Pennsylvania 15129 Received August 10, 1987. Revised Manuscript Received November 30, 1987 A systematic study was performed in which both hvAb and mvb coals were weathered for up to 500 days at 25-80 "C under closely controlled conditions in the laboratory. Periodic samples were analyzed by a wide range of elemental, spectroscopic, physical, and empirical methods. These systematic and closely controlled experiments confirmed many conclusions reached by others using less rigorous experimental designs. Gieseler plastometry and Audibert-Amu dilatometry were found to be the most responsive to the early stage of weathering, thus demonstrating that moderate weathering will destroy coal thermoplasticity. The higher rank coal weathered significantly more slowly. The weathering rate is highly dependent on temperature. This study also allowed several new conclusions regarding coal weathering. Rates of change with time of log Gieseler fluidity, heating value, and carbon and oxygen contents show similar dependencies on temperature. The principal chemical changes from organic matrix weathering were the low of aliphatic groups and the production of carbonyl groups. These reactions occurred rapidly at 80 "C, moderately fast at 50 "C and slowly, if at all, at 25 "C. Froth flotation tests, performed on the hvAb coal, showed that losses in flotation recovery from weathering are strongly affected by pyrite oxidation at 25 "C and by organic matrix oxidation a t 80 "C. The different temperature dependencies of pyrite and organic matrix oxidation underscore the importance of using realistic temperatures to study natural weathering.
Introduction Weathering can affect the behavior of coal in many production and end-use processes.' Extensive laboratory research has been performed to determine the chemical nature of coal weathering or oxidation. However, as has been rep~rted,~" published studies have come to significantly different conclusions concerning the chemical nature of coal weathering. Most reported coal weathering simulation experiments have been conducted at temperatures greater than 100 "C to accelerate the oxidation rate. However, the mechanism and chemical nature of coal weathering are reportedly different at temperatures above and below about 70-80 0C.576 The chemical structural changes induced by coal weathering are also reportedly dependent on coal rank and humidity.1g6.7i8 As noted by Gethner? ill-defined or poorly controlled coal weathering
experiments can result in inaccurate conclusions. The work reported here is a systematic laboratory study of coal weathering at realistic temperatures and times using different ranks of coal under well-controlled experimental conditions. The objectives of this study are (1)to determine the effects of weathering on coal properties with emphasis on froth flotation and thermoplastic properties, (2) to compare the abilities of various techniques to measure the degree of weathering, and (3) to better define the chemical nature of low-temperature coal weathering. (1)Cox, J. L.;Nelson, C. R. B e p r . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1984,29(1),102-107. (2)Gethner, J. S.Appl. Spectrosc. 1987,4,50-63. (3)Rhoads, C. A.;Senftle, J. T.; Coleman, M. M.; David, A.; Painter, P. C. Fuel 1983,62,1387-1392. (4)Liotta, R.; Brons, G.; Isaac, J. J. Fuel 1983,62,781-791. (5)Larsen, J. W.;Lee, D.;Schmidt, T.; Grint, A. Fuel 1986, 65, 595-596.
'Presented at the Symposium on the Surface Chemistry of Coals, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10, 1987.
(6) Yohe, G. Rep. 1nuest.-Ill., State Geol. Suru. 1958,207, 5-51. (7)Isaac, J. J.; Liotta, R. Energy Fuels 1987,1 , 349-351. (8)Schmidt, L. D.; Elder, J. L.; David, J. D. Znd. Eng. Chem. 1940,32, 548-555.
0887-0624/88/2502-Ol50$01.50/00 1988 American Chemical Society
Energy & Fuels, Vol. 2, No. 2, 1988 151
Low- Temperature Coal Weathering
4
Exhaust Air
0.6 m Coal and 1 cm Ceramic Saddles
\ 9.6 Standard L/hr
,Water
Jacket
7 TCs
11
' 1
--------*--
1
I
--
Circulating Themstated U a t e r Bath 25, 50 o r WOC
, I
Dry Air, 48.1 Standard L/hr
T h e m s t a t e d U a t e r Bath
Figure 1. Coal weathering unit.
In this paper, weathering is defined as the progressive changes in coal properties that occur as coal is exposed to humid air at temperatures of 80 OC or less. Medium- and high-volatile bituminous coals were weathered at temperatures of 25,50, and 80 OC in flowing humid air for as long as 500 days. Absolute humidity, which was constant for all experiments, was equal to 80% relative humidity at 20 "C.Weathered coals were sampled periodically and characterized by a variety of relevant techniques, including ultimate and proximate analyses, forms of sulfur, Gieseler plastometry, free-swelling index (FSI), Audibert-Arnu dilatometry, heat of combustion, slurry pH, alkaline extraction, and petrography. The froth flotation performance of the fresh and weathered coals was measured as a function of weathering time at several collector dosages. The weathered coals were also characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) to study the chemical nature of weathering.
Experimental Section Weathering Unit. Experiments were performed simultaneously in three weathering units. A schematic diagram is shown in Figure 1. The coal (1.2 kg; -0.6 mm particle size) used in the study was dispersed in a fixed-bed reactor (7.62 cm i.d.) with 0.92 cm Intalox ceramic saddles (ca. 1.2 kg) used to prevent air channeling. The temperature of the coal bed was controlled by circulating water from a thermostated bath through the reactor's outer jacket. The water circulator maintained the coal bed at temperatures up to 80 OC to within 1 O C . Two thermocouples were located a t one-third and two-thirds of the coal bed height to monitor bed temperature. Air was introduced to the bottom plenum of the reactor at 802 cm3/min (STP). A third thermocouple was inserted in the reactor bottom to monitor the temperature of the incoming air. Air humidity (80% relative humidity a t 20 "C) was controlled by dividing the air into two controlled-flow parallel streams with one stream passing through two water saturators in series. Feed air and reactor off-gas, sampled several times for gas chromatographic analysis, showed no sig-
nificant difference in oxygen composition, thus indicating that these reactors are being operated at differential conditions with respect to oxygen and that the air flow is sufficient for uniform coal weathering. Periodic samples were taken from the weathering units as follows. The entire reactor contents were emptied over a screen to remove the saddles. The entire coal sample was riffled 10 times and sampled for analyses. The remaining coal was remixed with the saddles and then repacked into the reactor. The reactor was tapped while being packed to settle the bed. Bed back-pressure, typically 3.45 kPa, was monitored to ascertain the absence of channeling. Coals. The high volatile bituminous coal used in this study was fresh run of mine (ROM) Pittsburgh seam coal from a deep mine in Monongalia County, WV. Aliquots of the whole ROM Pittsburgh seam coal were ground to pass a 0.6-mm (28-mesh) screen. The medium-volatilebituminous coal used will be called Horsepen seam coal. It is the natural -0.6 mm portion (obtained by screening but no grinding) of fresh ROM coal from a strip mine in McDowell County, WV. At the time of sampling, the Lower Horsepen, Pocahontas 10, and Pocahontas 11seams were being actively extracted. Analyses and properties of both feed coals are listed in Table I. Since subsequent grinding of weathered coal could create fresh, unweathered surfaces, the fresh coals used in this study had particle sizes of less than or equal to 0.6 mm so that flotation tests and spectroscopic analyses could be performed without further grinding. Analyses. Gieseler plastometer and free-swelling index (FSI) measurements followed ASTM procedures. Audibert-Arnu dilatometer measurements followed the IS0 procedure. The alkaline extraction test was performed by the published procedure? Ultimate and proximate analyses, forms of sulfur, and heat of combustion were measured by standard procedures. Fourier transform infrared (FTIR) spectra were collected on the neat samples (not ground further after removal from weathering unit) by using diffuse reflectance (Harrick accessory) with a Nicolet Model 7199 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) was performed with a Perkin-Elmer PHI Model 560 ESCA/SAM spectrophotometer with a magnesium X-ray source. (9)Lowenhaupt, D.E.;Gray, R.J. Int. J. Coal Geol. 1983,1,63-73.
Wu et al.
152 Energy & Fuels, Vol. 2, No. 2, 1988 100
Table I. Analyses and Properties of Horsepen and Pittsburgh Seam Coals Horsepen Pittsburgh seam seam Proximate Analysis (wt %, As Determined) 0.61 2.29 moisture 24.59 36.57 volatile matter 5.82 10.41 ash 68.98 50.73 fixed carbon Ultimate Analysis (wt %, Dry Basis) carbon 84.06 hydrogen 4.69 nitrogen 1.33 sulfur total 1.11 pyritic 0.41 sulfatic 0.02 organic (by difference) 0.68 oxygen (by difference) 2.96 ash 5.86
Gieseler Plastometer max fluidity, DDPM 1836 softening temp, "C 387 resolidification temp, O C 511 max fluidity temp, "C 472 plastic range, "C 124
2.54 1.37
0.06 1.11 6.48 10.66 30 996 30 600 348 481 434 133
Free Swelling Index B1/2
7li2
Audibert-Arnu Dilatometer contraction, % dilatation, % Ti, O C
Tz,"C Ts,O C Tz - T3,O C ,
28 191 377 418 496 78
27 103 334 404 454
50
Alkaline Extraction (% Transmittance) 88.8
Flotation Recovery (% maf) without collector 94.1 with collector (0.25g of fuel oilikg of coal)
90.0
94.5
Wet Screen Analysis (wt %) for a Tyler Mesh 28 X 48 mesh (600 X 300 pm) 48 X 100 mesh (300 X 150 pm) 100 X 200 mesh (150 X 75 pm) -200 mesh (-75 pm)
39.4 26.4 14.3 19.9
36.3 23.7 17.7 22.3
Binding energies reported herein are referenced to the CISXPS peak at 284.6 eV for charge calibration.'O Froth Flotation Tests. Froth flotation testa were made with a Denver Model D1 flotation cell. Coal slurry (5 w t % coal) was first charged to the cell and conditioned at 1500 rpm for 3 min to ensure thorough wetting of the coal. Collector (No. 2 fuel oil), when used, was added and the slurry was conditioned for 15 s. Frother (methylisobutylcarbinol or MIBC) was then added, and the slurry was conditioned for another 15 s. The air valve was opened, and the froth was manually removed with a spatula for 2 min. The froth concentrate and tailings were filtered, dried, and then analyzed.
Results and Discussion Weathering Effects on Coal Properties. Only small changes in elemental compositions were observed upon weathering, as reported by others." For example, after 144 days of weathering at 80 "C, Horsepen seam coal ~~
W 70
W
3 -
$ @ 5
% 20
74.04 4.93 1.39
Heating Value (J/g, Dry) 34 423
w
~
(10)Wagner, C. D., Riggs, W.M., David, L. E., Moulder, J. E., Muilenborg, G. E., Eds. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1979;p 56. (11)Rose, H. J.; Sebastian, J. J. S. Fuel 1932, 11, 284-297.
10 0
-10
--
I
0
20
40
w
80
YE*THERINC llYE (MTS) AT M 'C
Figure 2. Thermoplastic and flotation properties of Horsepen seam coal as functions of weathering time a t 80 "C.
showed small but significant decreases in hydrogen content (4.2% vs 4.7% mf (mf = moisture free)) and carbon content (81.8% vs 84.1% mf), an increase in oxygen content (5.8% vs 3.0% mf, by difference) and a slight increase in sulfate sulfur content (0.05% vs 0.02% mf). A slight decrease in the heating value (32 928 vs 34 423 J/g, mf) was also observed. In contrast, thermoplastic properties such as Gieseler maximum fluidity and Audibert-Arnu dilatation exhibited rapid decreases with increasing weathering time. These two thermoplastic properties, in addition to FSI and froth flotation coal recovery (% maf (maf = moisture and ash free)), are plotted in Figure 2 as percentages of their initial values (Table I) as functions of weathering time up to 84 days at 80 "C. Gieseler maximum fluidity is plotted as a logarithm in Figure 2. After 5C-60 days, log Gieseler fluidity became negative, indicating that Gieseler maximum fluidity was below 1 DDPM (dial division per minute) (essentially no fluidity). After 30-40 days, dilatation became negative, indicating that there was no dilatation, only contraction. These results demonstrate that even moderate weathering will destroy coal thermoplasticity and therefore make it unsuitable for metallurgical coke making. This confirms earlier observations.12J3 In contrast, flotation recovery using frother (9.6 mg of MIBC/L of slurry; 0.19 g of MIBC/kg of coal) but no collector showed a more gradual decrease with weathering time. This result indicates that weathering has a more profound qualitative effect on the thermoplastic properties than on flotation recovery, as observed by others.14 Gieseler fluidity and Audibert-Arnu dilatation were essentially lost at a point at which about half of the coal still responded to froth flotation. Similarly to flotation recovery, FSI decreased much less rapidly in early weathering than maximum fluidity and Audibert-Arnu dilatation. This indicates that FSI is not very sensitive to slight-to-moderate weathering, as also noted by Gray et al.14 However, FSI measurements may be more meaningful for severely weathered coal because, with extensive weathering, the plastometer and dilatometer results decrease to such low values that sensitivity is lost. The rates of change with weathering time of the thermoplastic properties and flotation recovery of Horsepen seam coal increased with temperature. Gieseler maximum fluidity decreased from 1836 to 446 DDPM (81% of the (12)Huffman, G. P.;Huggins, F. E.; Dunmyre, G. P.; Pignoco, A. J.; Lin, M.-C. Fuel 1985,64, 847-856. (13)Cagigas, A.; Escuder, J. B.; Low,M. J. D.; Pis, J. J.; Tascon, J. M. D. Fuel Process. Technol. 1987,15, 245-256. (14)Gray, R. J.; Roades, A. H.; King, D. T. Trans. Am. Inst. Min.,
Metall., Pet. Eng., SOC.Min. Eng. AIME 1976, 260, 334-342.
Energy & Fuels, Vol. 2, No. 2, 1988 153
Low-Temperature Coal Weathering
'loz/\ I
A 0
L
f 10 WAVENUMBERS
1 ;
0
4
80
'
I
'
1
160 240
-
I
320
S
I
I
I
'
400 480
Figure 5. FTIR spectra of Horsepen seam coal.
Weathering Time (Days)
-
Figure 3. Gieseler maximum fluidity of Pittsburgh seam coal as functions of weathering time and temperature. 8 7
I
1 7
' 5 1
i
i
I
z 0
i
0
20
40
60
80
1W
120
+
PITISBURGH
CI
HORSEPEN
140
WEATHERING TIME (day.) at BO'C
Figure 6. Oxidation index as a function of weathering time for 80 "C weathered coals.
(15) Larsen, J. W.; Lee, D.; Shawver, S. E.; Schmidt, T. E. Annual report, GRI Contract No. 6081-361-0532,February 1984.
slight changes in heating value and oxygen and carbon contents were observed, the order of the reaction with respect to the parameter being described is poorly determined. A zero-order model was chosen for simplicity. Since fluidity is modeled by using its logarithm, the expression is actually first order with respect to Gieseler maximum fluidity. From the Arrhenius expression and the Pittsburgh seam coal data, temperature dependencies of log Gieseler maximum fluidity, heating value, oxygen content, and carbon content, as expressed by the activation energy, were calculated to be 14 f 4, 12 f 3, 13 f 1 and 11.7 f 0.3 kcal/mol, respectively, for these four measurements. Though the similar values do not prove a direct or indirect causal relationship, they do indicate that these properties, which all reflect oxidation of the coal organic matrix, respond similarly to weathering temperature. In addition, such relationships may provide the beginnings of a more unified understanding of coal weathering phenomena. Chemical Nature of Low-TemperatureWeathering. The diffuse-reflectance FTIR spectra (neat) of the fresh Horsepen seam coal and the same coal weathered for 51 days at 80 "C are shown in Figure 5. The difference spectrum clearly indicates a decrease in the C-H stretch intensity at about 2900 cm-' and an increase in the carbonyl peak intensity at about 1700 cm-l. The former is attributed to the loss of C-H groups by oxidation, and the latter is attributed to the production of the carbonyl groups involving, perhaps, ketones, carboxylic acids, esters, and aldehydes.2 Both are associated with coal weathering. Since these two absorption bands are related to the organic structure of coal, the ratio of the integrated areas under these two bands can provide a good indication of organic
154 Energy & Fuels, Vol. 2, No. 2, 1988
Wu et al.
Oxldatlon Index 'I
~
0 50% A 25'C
-P
Irm.0
j
n
0 ' 0
I
I
100
I
I
200
I
I
300
I
I
400
0.1
i
0.01
+ ,
Weathering Time (Days)
Figure 7. Oxidation index of Pittsburgh seam coal as functions
of weathering time and temperature.
matrix oxidation. An oxidation index developed by US. Steel using this concept12 is plotted as a function of weathering time in Figure 6. The oxidation index is defined as the ratio of the integrated intensity of the carbonyl band (1635-1850 cm-l) to that of the C-H stretching band (2746-3194 cm-') in the diffuse-reflectanceFTIR spectrum. Figure 6 showed fairly consistent increases in oxidation index with time for both Horsepen and Pittsburgh seam coals. Such changes can be attributed to the continuing oxidation of C-H groups to carbonyl groups with increasing weathering time, as concluded by others.12J6J7 The oxidation index of the Pittsburgh seam coal (hvAb) not only increases more rapidly with time, but also has a higher initial value than the Horsepen seam coal (mvb). This indicates that the initial value and the rate of change with time of the oxidation index are rank dependent. In addition to rank dependence, the rate of change with time of the oxidation index also depends on weathering temperature. In Figure 7, the oxidation index of Pittsburgh seam coal is plotted as functions of weathering time and temperature. Changes are rapid at 80 "C, slow at 50 "C and small or nonexistent at 25 "C, showing that the production of carbonyl groups occurred rapidly at 80 "C, moderately fast at 50 "C, and slow, if at all, at 25 "C. The increase of carbonyl group production with increasing weathering temperature was also reported by others3J2 using hvA bituminous coals. In comparison, Liotta et al.4 reported that the oxidation of an Illinois No. 6 coal (hvCb) at ambient temperature resulted in the formation of ether groups (evident at ca. 1100 cm-' in the IR spectra) and that this was the dominant mechanism of oxidation. To the extent that ether formation results in the loss of IR intensity in the C-H band,4 the oxidation index values obtained in this study should be increased with ether formation. The lack of variation in the oxidation index of the 25 "C weathered coals appears inconsistent with significant formation of ethers at the conditions of this study. However, it is recognized that ethers may be formed at concentrations too low to affect the oxidation index value. As discussed earlier, the loss of Gieseler maximum fluidity is also dependent on weathering temperature (Figure 3). Figure 8 shows log Gieseler maximum fluidity plotted as a function of the oxidation index of the 80 "C weathered Horsepen and Pittsburgh seam coals. This figure shows that for both coals, the initial rapid loss of Gieseler fluidity is correlated to an increase in the oxidation index. The oxidation index continues to increase even after Gieseler fluidity is essentially destroyed, i.e., less than (16)Huggins, F.E.;Huffman, G.P.; Dunmyre, G. P.; Lin, M.-C. Fuel Process. Technol. 1987, 15, 233-244. (17)Khan, M. R.Energy Fuels 1987,1, 366-376.
0.4
0.6
0.8
,
,
1.0
,
,
1.2
,
,
,
1.4
,
,
1.6
-
1.8
2.0
Figure 8. Gieseler maximum fluidity of coals weathered at 80 "C for various times as a function of oxidation index. N(E)/E 3 2 1
0
-172
-170
-168
-166
-164
-162
Binding Energy, eV
Figure 9. XPS SZpspectra of Pittsburgh seam coal. Table 11. Surface Analyses of Weathered Coals by XPS
coal Horsepen seam (mvb)
Pittsburgh seam (hvAb)
weathering time, days at 80 "C 0 31 51 84 0 45
surface organic
surface sulfur
O/C,
(oxidized/
atom % 5.6 7.2 9.0 10.5 7.7 15.6
unoxidized) 0.6
1.7 0.6 3.0
5 DDPM. This result demonstrates that weathering reactions continue to proceed even after the coal thermoplasticity is destroyed. The surface (ca. 4 nm deep) elemental compositions (C, total and organic 0, S, N, Si, Al) of selected samples were obtained by XPS. As an example of the data generated, X P S gave the following atomic percentages18for the fresh Horsepen seam coal: C = 84.7%, total 0 = 11.1%, Si = 2.3%, Al = 1.1%, S = 0.4%, and N = 0.4%. After 84 days weathering at 80 "C, XPS gave the following atomic percentages for the same coal: C = 81.3%, total 0 = 14.3%, Si = 2.0%, A1 = 1.2%, S = 0.4%, and N = 0.8%. Organic oxygen was calculated by Perry and Grint's methodIg by subtracting inorganic oxygen from total oxygen, assuming that inorganic oxygen is associated with Si and Al as SiOz and A120,. The surface organic oxygen content increased from 4.8% (fresh coal) to 8.5% after 84 days weathering at 80 "C for Horsepen seam coal, and it increased from 6.0% (fresh coal) to 12.0% after 45 days weathering at 80 "C for Pittsburgh seam coal.18 The atomic ratio of organic oxygen to carbon (organic O/C ratio) obtained by XPS has (18)Wu, M. M.; Winschel, R. A. Proceedings-Third Annual Pittsburgh Coal Conference;University of Pittsburgh Pittsburgh, PA, 1986; pp 591-607. (19)Perry, D. L.; Grint, A. Fuel 1983, 62, 1024-1032.
Energy & Fuels, Vol. 2, No. 2, 1988 155
Low-Temperature Coal Weathering
'I 5
n
A
0
-294
-292
-290
-288
-286
-284
-282
-280
B i n d i n g Lnergy, eV
Figure 10. XPS CISspectra of Pittsburgh seam coal: (A) coal weathered at 80 "C for 45 days; (B) fresh coal.
been used as an indication of coal surface 0xidati0n.l~The surface organic O/C ratios of both coals, shown in Table 11, increases with weathering time. The surface organic O/C ratio is higher for the Pittsburgh seam coal (hvAb) a t all weathering times, and it also increases more rapidly than for the Horsepen seam coal (mvb), demonstrating again the rank dependence of coal weathering. The ratio of oxidized to unoxidized surface sulfur in Table I1 was calculated from the XPS SZpspectra, an example of which is shown in Figure 9. Two distinct SZp peaks were resolved, at binding energies of 164 and 169 eV. Binding energies were determined with reference to the C1, pe-ak at 284.6 eV.'O The peak at 164 eV is assigned to u n o x i d d sulfur species (pyritic and organic sulfur).lg@ The peak at 169 eV is assigned to oxidized sulfur species.1° We believe that the oxidized sulfur is almost entirely sulfate sulfur. Sulfur in model coal organic sulfur compounds is relatively inert to oxidation at fairly severe oxidizing conditions whereas pyritic sulfur is relatively easy to oxidize to sulfate even under mild oxidizing conditions.21 Of course, the forms of sulfur analysis, which will be discussed later, also showed the production of sulfate upon weathering. The relative intensity (atomic percentage) of the SZppeak at 169 eV to that of the peak at 164 eV increased upon weathering (Figure 9 and Table 11), indicating that surface pyritic sulfur was oxidized to sulfate. An asymmetric C1, peak with a small shoulder at high binding energy (286-291 eV) was observed by XPS in the 50 and 80 "C weathered coals (but not the 25 OC weathered coals), indicating that carbon-oxygen functional groups were generated upon weathering, in agreement with the FTIR data and results of other w ~ r k e r s . ~ ~ ~ ~ (20) Frost, D.C.; M e r , W. R.; Tapping, R. L. Fuel 1974,53,206-211. (21) Morrison, G. F. Chemical Desulfurization of Coal; IEA Coal Research London, 1980. (22) Clark, D. T.; Cromarty, B. J.; Dilks, A. J. Polym. Sci., Polym. Chem. Ed. 1976, 16,3173-3184.
The XPS C1, spectra of the weathered and fresh Pittsburgh seam coal are compared in Figure 10. The curve fitting routine of the Perkin-Elmer instrument found the best fit to be three peaks with binding energies of 288.9, 287.3, and 284.6 eV assigned to carboxylic, carbonyl, and aromatic and aliphatic carbons, respectively. The assignments of carboxylic and carbonyl C1, peaks are consistent with previous XPS studies of coalslg and polymer^,^^,^^ when a slight difference in the binding energy of the aromatic and aliphatic C1, peak is taken into consideration (284.6 eV in this study and 285.0 eV in several other s t ~ d i e s ' ~ * For ~ ~ ,Pittsburgh ~~). seam coal weathered for 45 days at 80 OC, the ratio of oxidized carbon to total carbon was 8.5 atom %. For Horsepen seam coal, this ratio was 4.1 atom % after 84 days of weathering at 80 "C. Interestingly, these values (obtained from the C1, peak) are nearly the same as the increase in the surface organic O/C ratio from weathering (Table 11). This is consistent with the concept that on average only one oxygen atom is added with each carbon atom that oxidizes. Of course, this only addresses the carbon that remains with the coal and not the carbon that leaves as CO,. The fact that small amounts of carboxylic groups were identified in the C1, spectra (Figure 10) indicates that either this average value of one oxygen added per carbon oxidized is offset by minor ether formation or that the carboxylic groups were formed from other oxygenated groups, such as carbonyl, present in the fresh coal. These data also show that both the organic O / C ratio and the oxidized carbon to total carbon ratio are rank dependent. Since oxygen is most readily accessed by the coal surface, one might expect that more of the surface carbon would be oxidized than the 4.1% and 8.5% that were. However, the relatively low values observed are consistent with work by Huffman et al.12J6 It may be that mass transfer of oxygen into the organic interior is rapid relative to the kinetics of the oxidation reactions. It may also be that only certain active sites on the coal surface will oxidize readily at these conditions. Since the XPS CISpeak is the direct measurement of the fraction of oxidized carbon relative to total carbon on the coal surface, the combination of X P S and other spectroscopic techniques may provide information on which organic moieties are most susceptible to oxidation in low-temperature weathering. On the basis of transmittance FTIR, Painter3 reported that air oxidation was indeed selective and that aliphatic carbon, especially in benzylic sites, is highly susceptible to the oxidation. As discussed below, our results are consistent with Painter's. Oxidized aliphatic carbon as a percentage of total carbon was calculated by multiplying the fraction of aliphatic carbon in the fresh coal (based on I3C nuclear magnetic resonance spectroscopy, or 13C NMR) by the loss in the aliphatic C-H stretch intensity from FTIR. 13C NMR results2*show that the carbon in hvAb and mvb coals is typically 20%-30% aliphatic. The loss of aliphatic C-H stretch band intensity was calculated by using the integrated area of the absorbance spectrum over the range 2750-2980 cm-' for both fresh and weathered coals. For this work, transmission spectra were obtained by using KJ3r pellets. The calculation method is illustrated in Table 111. The amount of aliphatic carbon lost from weathering was determined by this method to be 7.0%-10.4% of the total for the Pittsburgh seam coal weathered at 80 ~ ~ ~carbon "C~for 45 days and as 2.4%-3.8% of the total carbon for (23) Dilks, A.; Clark, D. T. J.Polym. Sci., Polym. Chem. Ed.1981,19, 2847-2860. (24) Cerstein, B.C.; Murphy, P. D.; Ryan, L. M. In Coal Structure; Meyers, R.A., Ed, Academic: New York, 1982; pp 87--126.
Wu et al.
156 Energy & Fuels, Vol. 2, No. 2, 1988
Table 111. Carbon Oxidation in Weathered Pittsburgh and Horsepen Seam Coals % loss of aliphatic C-H fraction of aliphatic oxidized aliphatic band intensity carbon in coal from carbon as % of total weathered coal (%) from IR NMR carbon -8.7 f 1.7" 0.25 f 0.05 Pittsburgh seam coal (45 days at 80 "C) 34.8 X -3.1 f 0.7* 0.25 f 0.05 Horsepen seam coal (84days at 80 " C ) 12.7 X "Oxidized carbon to total carbon by XPS equals 8.5%, and the increase in organic O/C (atomic) by XPS equals 7.9%. *Oxidized carbon to total carbon by XPS equals 4.1%, and the increase in organic O/C (atomic) by XPS equals 4.9%. 100.0
,
90.0
I
12.5
.2"00,
.--_
2 5
% -,
700
6 60.0 0 L
4 300 -
b 20 0
0 Flotation Recovery 0 XPS Szp Peak R a t i o
0
1
+ 0 LL
O
/
0
~
100
~
200
~
~
300
I
400
~
0.0'
'
500
Weathering Time, Days
Figure 12. Flotation recovery and XPS SZppeak ratio vs weathering time for Pittsburgh seam coal weathered a t 25 "C.
(25)
67-75.
Sun, S. C. Trans. A m . Inst. Min. Metall. Pet. Eng. 1954, 199,
(26) Sun, S. C. Trans. Am. Inst. Min. Metall. Pet. Eng. 1954, 199, 396-401. (27) Celik, M. S.; Somasundaran, P. Sep. Sci. Technol. 1986, 21(4), 393-402. (28) Firth, B. A.; Nichol, S. K. Coal Prep. (Gordon & Breach) 1984,
I, 53-70.
tained by using 0.11 g of MIBC/kg of coal and 0.25 g of collector/kg of coal. As shown in Figure 11, both parameters decrease together at any one temperature; however, it is apparent that there is no one-to-one correspondence for the entire data set. Flotation recovery and oxidation index respond differently to temperature; therefore, the cause of the loss in flotation recovery must be different from the cause of the increase in the oxidation index. We believe that, in addition to organic matrix oxidation, pyrite oxidation has a strong effect on flotation recovery, particularly at the low weathering temperatures. Figure 12 shows the flotation recovery and XPS sulfur peak ratio as functions of weathering time for the 25 OC weathered Pittsburgh seam coal. Sulfatic sulfur contents (by forms of sulfur analysis) increased gradually from 0.05 to 0.31 wt % maf for these same samples. The XPS sulfur peak ratio is the relative intensity (atomic percentage) of the S2ppeak at 169 eV (oxidized) to the S2ppeak at 164 eV (unoxidized)as noted previously. The ratio is an indication of pyrite surface oxidation. The loss of flotation recovery correlates with the increase in the XPS sulfur peak ratio. XPS and ultimate analysis data, as well as the data shown in Figures 3,4, and 7, show that these samples, which were weathered at 25 OC,have only slight organic matrix oxidation even after 489 days. Therefore, the loss in flotation recovery does not appear to be caused by organic matrix oxidation at 25 "C. Other workmwith naturally weathered bituminous coals showed losses in flotation recovery with pyrite oxidation, but in the absence of observable organic matrix oxidation. A t higher temperatures, e.g., 80 "C, flotation recovery is apparently more influenced by organic matrix oxidation, A t 80 "C,organic matrix oxidation is greatly accelerated as discussed previously; however, pyrite oxidation shows only a modest increase in rate. For example, the Pitts(29) Robbins, G. A.; Oblad, H. B.; Meenan, G. F. Proceedings-Third Annual Pittsburgh Coal Conference; University of Pittsburgh Pittsburgh, PA, 1986; pp 33-46.
~
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Energy & Fuels 1988,2, 157-163 burgh seam coal weathered for 45 days at 80 OC and for 268 days at 50 "C contained only 0.10 and 0.19 wt % of maf sulfatic sulfur, respectively. This indicates that the loss in flotation recovery a t the higher temperatures is strongly affected by organic matrix oxidation. The different behavior with temperature of pyrite and organic matrix oxidation underscores the importance of
157
using realistic temperatures to study natural. weathering. Acknowledgment. We gratefully acknowledge the contributions of €3. B. Oblad and D. E. Lowenhaupt of Consolidation Coal Co. and D. H. Ralston of Conoco, Inc. This work was supported by Consolidation Coal Co. Registry No. Pyrite, 1309-36-0.
Correlation between Electrophoretic Mass Transport and Bulk Properties of Concentrated Coal Suspensionst E. Z. Casassa and E. W. Toor* Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received July 13, 1987. Revised Manuscript Received December 28, 1987
The purpose of this study of six bituminous coals, some of which were available in both ROM and beneficiated forms, was to relate the surface chemistry of a coal to its beneficiation and oxidation history and to test the hypothesis that coal-water slurry properties depend upon slurry t potential. The particle size distribution, degree of oxidation, and density were determined for freshly ground samples of each of the dry coal powders. Slurry rheology and sedimentation rate and settled volume were examined as functions of pH and of coal concentration. The pH dependences of the electrophoretic mobility of both dilute suspensions and 50 w t % slurries of each coal were observed. Proximate, ultimate, and mineral ash analyses, surface analyses by XPS, and the concentrations of nine inorganic ions in slurry liquor were also obtained for each coal. Slurry rheology and stability toward sedimentation were in general related to the electrophoretic mobility measured in concentrated suspensions by the mass transport method, rather than to that determined in dilute suspensions by microelectrophoresis. Electrokinetic sonic amplitudes measured in concentrated slurries agreed with mass transport results. High soluble ash content of a coal powder decreased slurry electrophoretic mobility across the pH range, whereas oxidation of the carbonaceous surface produced slurry particles with high negative electrophoretic mobility at pH above 6. Low slurry electrophoretic mobility correlated with good stability toward sedimentation and high viscosity, while high electrophoretic mobility produced low viscosities and very poor stability toward sedimentation. These effects on slurry properties were more marked for powders of low median particle size.
Introduction The behavior of dilute dispersions of charged colloidal particles can be interpreted according to the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory of colloidal Stability.' Particles experience a van der Waals attraction toward each other that can be expressed as a negative potential energy diminishing with distance between the particles. Electrostatic charge on particle surfaces produces a positive repulsive potential energy that also decays to zero as interparticle separation increases. The stability of a dispersion depends upon the relative magnitudes of these opposing potential energies. High interparticle repulsion, arising from large electrostatic charges on particle surfaces, leads to stable dispersion of individual colloidal particles and to low viscosity. When the surface charge is lower, the balance between the repulsive potential and the attractive potential may allow either flocculation in a secondary minimum to form loosely bound aggregates or coagulation in a primary minimum to form tightly Presented at the Symposium on the Surface Chemistry of Coals, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10, 1987.
0887-0624/88/2502-Ol57$01.50/0
bound aggregates. Aggregation increases viscosity and decreases stability toward sedimentation. Increasing the ionic strength of the fluid compresses the electric double layer around the particles and changes the balance between attractive and repulsive potentials. The DLVO theory is strictly applicable only to dilute colloidal dispersions, but can be applied qualitatively to concentrated slurries, where many-body interactions occur. A second complication is that typical colloidal behavior is limited to dispersions for which the particle size and density relative to the fluid permit Brownian motion, which prevents settling of the unflocculated individual particles in the dilute dispersion. For coal-water suspensions this limit is on the order of 1-2 vm, considerably lower than the median particle size of most coal-water slurries. Thus high interparticle repulsion does not promote stability toward sedimentation in coal-water suspensions although it favors dispersion of the powder as single particles and promotes the most efficient packing, in which interparticle distances are maximized. (1) Verwey, E. J. W.; Overbeek, J. T. G . Theory of the Stability of Lyophobic Colloids, Elsevier: New York, 1948.
0 1988 American Chemical Society
