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An experimental study of heat of immersion of coal
Colloids and Surfaces, 28 (1987) 109-121 Elsevier Science Publishers B.V., Amsterdam
An Experimental Coal T.G. MELKUS
109 -
Printed
in The Netherlands
Study of Heat of Immersion
of
and S.H. CHIANG
University of Pittsburgh, Pittsburgh, PA (U.S.A.) W.W. WEN U.S. Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, PA (U.S.A.) (Received
5 January
1987; accepted in final form 22 June 1987)
ABSTRACT The heats of immersion of six coals were measured at 303 K in water with a Setaram C-80 microcalorimeter. The effects of coal rank, surface oxidation state, moisture content, and petrographic constituents on the wetting characteristics and surface properties were investigated. The six coals and four other solids (pyrite, kaolin, slate, and graphite) were characterized in terms of relative hydrophobicity or hydrophilicity via the comparison of heat of immersion measurements in water and in methanol. Heat of immersion measurements were also made of coal in surfactant solutions (Aerosol OT, Triton X-l 14, and dodecylpyridinium chloride) of varying concentrations. The results of the various experiments were found to correlate well with independent flotation and vacuum filtration tests.
INTRODUCTION
Surface calorimetry assists in the assessment of the chemistry of the surface and its interactions with any molecules that can contact it [ 11. More specifically, immersion calorimetry has been shown to be a valuable means of determining solid surface properties [ 21, such as wetting tendencies, surface polarity, and site energy distribution. Heat of immersion (AH,,,) is defined as the heat that is released (or absorbed) when a clean solid is immersed in a liquid. A variety of solids, including coal, have been used in heat of immersion studies. Of interest to the work presented here, the effect of coal rank [ 31, moisture content [ 4,5], and oxidation [ 61 on the dHi,, of coal have been previously reported. In addition, the heat of immersion of coals in various liquids have been used to investigate its pore structure [ 7-131. The work presented* here differs from previous studies in that the calorim*Reference in this report to any specific commercial product, understanding and does not necessarily imply its endorsement Department of Energy.
process or service is to facilitate or favoring by the Unites States
0166-6622/87/$03.50
B.V.
0 1987 Elsevier Science Publishers
110
eter was used to measure the heat of immersion of coal in its natural state (whose physical characteristics had not been altered by evacuation). Great care was taken to keep the coal (after it had been crushed) in an inert atmosphere at all times prior to the dHimm measurement. Thus, the effects of oxidation and moisture content of the natural state of various coals could be determined in this study. The purpose of making these dHi,m measurements was to study the wetting characteristics of coal particles. EXPERIMENTAL
Test equipment A Setaram C-80 heat flux calorimeter was used to measure the heat of immersion. This calorimeter works on the Tian-Calvet heat-flow principle and is manufactured by Setaram Instruments, Lyon, France. Two cells (one experimental, one reference) are placed in the calorimetric block, which functions as a heat sink. The temperature of this block is controlled precisely ( + 0.01 K over a range of ambient temperature to 573 K) using a temperature programmer/controller. Two identical and independent heat-flux detectors, consisting of conductive thermocouples, connect the vessels thermally to the block so that the vessel temperature is always as close as possible to that of the block. The detectors are differentially connected so that thermal perturbations to the block are eliminated and only the signal owing to the heat exchange with the sample is generated. The heat-flux detectors emit a signal that is proportional to the heat per unit time transferred. This signal is then processed by an Apple II computer that is programmed to (1) integrate the signal (i.e., calculate the total quantity of heat emitted or absorbed) and (2) analyze the signal over time (i.e., determine the kinetics and equilibrium). To perform the heat of immersion measurements, mixing cells (Fig. 1) were used to mix the solid and the liquid. In a typical experiment, a layer of mercury ( 600-800 ~1) is placed in the small internal sample vessel; then the coal sample ( 30-50 mg) is placed on top of the mercury and covered with a cap. This vessel is then inserted into the mixing-cell body, and another layer of mercury (800-1200 ~1) is added to act as a barrier to prevent any mixing prior to activation of the reversing mechanism. The mixing cell is then filled nearly to the top with 4-5 ml of the immersion fluid (usually water) in order to minimize air space. The reference cell is filled in the same manner, but the coal sample is omitted. Both cells are then loaded into the calorimeter and allowed to come to complete thermal equilibrium (usually 3-4 h) . Once equilibrium is attained, the experiment is initiated by activating the reversing mechanism, which inverts the entire calorimetric assembly every 10 s. The cap and mercury act as a mixer to homogenize the solution. The number of inversions depends upon the ease with which the materials mix; 20-30 min was usually required for the
111
Teflon plug Retaining ring Bushing
Stopper Threaded
Immersional
cover
fluid
Mercury
Fig. 1. Cross section of the mixing cell of the Setaram C-80 calorimeter.
experiments reported here. All experiments were carried out in the isothermal mode of operation at slightly above room temperature (303 K ) . Coal samples Coal is a heterogeneous mixture of organic and inorganic materials, and the most common way to classify coal is by rank. Coal rank is determined by carbon content, volatile matter, and calorific value. Six different coals were used in this study. Their rank, seam, origin, and proximate and ultimate analyses are presented in Table 1. The coal was mined at the coal face and shipped in chunks (of the order of inches) under argon. These chunks were kept under argon prior to the crushing process, which took place in an oxygen environment. The coal samples were crushed using a jaw crusher followed by a hammerhill to obtain 28 mesh x 0 coal particles. The samples were then dry screened on a rotap to obtain the desired size range; particles of 100 x 200 mesh (0.074-0.147 mm) were used in all experiments. The coal was then microriffled to obtain a representative sample (2-3 grams ) for use in the calorimeter experiments. It was determined that the heat of immersion is highly dependent on the moisture content of the coal (especially for the low-rank coals). Therefore, all of the samples (except those
North Dakota Montana Illinois Pennsylvania Pennsylvania Pennsylvania
State
the original moisture content
Beulah Zap Rosebud Illinois No. 6 Pittsburgh Upper Freeport Mammoth
Lignite Subbituminous Bituminous, HVC Bituminous, HVA Bituminous, MV Anthracite
“These numbers represent
Seam
and ultimate analyses of Coal
Coal rank
Proximate
TABLE 1
(29.70)’ (21.40)” (8.04)” (1.99)” (0.74)a (2.91)’ 36.36 36.34 35.35 36.10 21.99 2.88 36.91 41.35 46.39 59.18 53.04 87.70
Fixed carbon
(as received)
Volatile matter
analysis
4.77 5.63 5.54 5.84 5.49 2.14
Hydrogen
Ultimate
71.65 75.94 77.58 84.02 86.25 95.27
Carbon
analysis
0.87 0.99 1.08 1.73 1.56 0.78
Nitrogen
1.31 0.97 6.03 1.10 2.01 0.73
Sulfur
(moisture, ash free )
analyses were performed.
8.91 9.12 13.23 3.98 24.18 6.44
Ash
of each of the coals before laboratory
17.82 13.19 5.03 0.74 0.79 2.98
Moisture
Proximate
21.40 16.47 9.82 7.31 4.69 1.08
Oxygen
113
used for the moisture study) were placed in an argon-circulating oven at 313 K for four days (to drive off moisture until a constant weight was achieved). The samples were then placed in sample bottles and purged with argon so that the coal surface was kept as fresh as possible and was not subjected to oxidation. Each time a bottle was opened to remove a sample, it was again purged with argon before it was resealed. Moisture and ash analyses were performed using a LECO MAC-400 Proximate Analyzer on each coal sample to check the uniformity of the samples. In addition to the six different coals, lithotypes of Upper Freeport seam coal and four solids (grahite, kaolin, pyrite, and slate) were used in the dHi,m measurements. Specimens of the Upper Freeport seam lithotypes were separated from the hand-picked samples of banded coals, ground via mortar and pestle to 100 x 200 mesh particles, and then placed in an argon-circulating oven at 313 K for four days (to drive off moisture until a constant weight was achieved). The graphite and kaolin were both obtained from Fischer Scientific; the graphite was > 98% pure and ~325 mesh ( < 0.044 mm), and the kaolin was dry screened to 100 x 200 mesh (0.074-0.147 mm) particles. The pyrite was hand-picked from coarse refuse of a coal preparation plant in Western Pennsylvania, and the slate was hand-picked from the Bureau of Mines experimental mine near Pittsburgh, PA. Both the pyrite and slate were prepared in the same manner as the six coal samples. For each of the systems considered, the heat of immersion measurement was performed at least three different times to ensure reproducibility of results. The liquids that were used in the experiments were deionized distilled water and commercially available organic liquids. RESULTS AND DISCUSSION
The calorimeter was calibrated by measuring the heat of dissolution of potassium chloride (NBS 1655) in water at 299 K. ThedH,i, value was 4.23 2 0.04 kcal mol-‘, which agrees well with the literature value of 4.1998 kcal mol-l
[141. Coal rank
The results for the six different coals tested (Fig. 2) show that dHi,m varies significantly with coal rank. A similar trend exists with the floatability of coal [ 151. Floatability is inversely related to the heat of immersion. Thus, the least floatable form of coal, lignite, has the highest heat of immersion [ 31. Similarly, anthracite is slightly more difficult to float than high-volatile bituminous coal and correspondingly has a higher dHi,m than Pittsburgh seam coal. Fuller [ 91 also considered the variation of heat of immersion with coal rank and observed
114 14
1’1’1’
l
0 12 -
0 A
o
5 : 9
n
io-
A
I
‘I’
’
Liqnite /Beulah Zap Subbituminous /Rosebud Bituminous HVC/lllinais No.6 Bituminous HVC/Pittsburqh Bituminous W/Upper Freepart Anthracite /Mammoth
z g
6
g j
6-
10
74
76 02 06 90 COAL RANK IN PERCENT CARBON
Fig. 2. Heat of immersion in water: Effect of coal rank (coal rank expressed in percent carbon on a moisture- and ash-free basis).
a somewhat similar trend, even though his calorimetric treatment of the coal were quite different.
technique
and pre-
Surface oxidation state Oxidation of the coal surface has an adverse effect on its floatability; even a highly floatable coal will become difficult to float if highly oxidized [ 151. In order to study the effect of surface oxidation on the heat of immersion, the following experiments were performed. Eighteen coal samples (three each of the six coals that were used in the coal rank study) were dried in an argon atmosphere at 313 K for four days. Their moisture contents after drying are given in Table 2. Heat of immersion measurements were made in water for six TABLE 2 Moisture content of coal after drying in argon oven for four days Coal rank/seam
Percent moisture
Lignite/Beulah Zap Subbituminous/Rosebud Bituminous, HVC/Illinois No. 6 Bituminous, HVA/Pitteburgh Bituminous, MV/Upper Freeport Anthracite/Mammoth
1.96 1.80 1.30 0.78 0.54 0.60
115 I
I
I
I
I
I
OBituminous HVA/Pittsburgl D LigniWBeulah Zap 3 Subbltuminous I Rosebud l bituminous MV/UpperFmpc A Anthracite / Mammoth L Bituminousj HVC/lllinols No.6
1 h
.o -
&I~XIDATGN
TIMi
IN 0~;s
-
Fig. 3. Effect of oxidation at 369 K on the heat of immenion in water for six different coals. samples, one each of the six coals. The other twelve samples were then oxidized in an air-circulating oven at 369 K. Six samples were oxidized for one day, and six samples were oxidized for four days before the dHimm measurements were made. As shown in Fig. 3, oxidation had the same qualitative effect on all of the coals considered, regardless of rank. An oxidized coal tends to be more hydrophilic, which corresponds to a higher dHi,m in water. The heat of immersion values reported in this study are generally lower than those reported in the literature. Most of the di!&,., data reported by others are similar to the results reported here for oxidized coals. This consistent difference is a result of the kind of calorimeters used by earlier researchers [ 3,9]. They required that the coal sample be outgassed at a very high temperature, usually for a period of 24 h. This procedure altered the physical characteristics of the coal surface in such a way which appears to be similar to an oxidized coal surface in this investigation.
Moisture content Heat of immersion ting liquid contained
is highly dependent on the amount of preadsorbed wetby the solid. This was readily observed in measuring the
116
MOISTURE IN PERCENT
Fig. 4. Effect of moisture content on the heat of immersion in water of lignite coal/Beulah Zap.
dHi,m of lignite coal in water (Fig. 4). The dHi,, measurements were made after the coal was dried in an inert atmosphere for varying periods of time (2 h to 4 days). The corresponding moisture content was also determined. Heat of immersion curves were also obtained for the other five coals as a function of moisture content, and they were similarly shaped. This same decrease in the heat of immersion with increasing moisture content has been reported by Nordon and Bainbridge [ 41 for a bituminous coal and by Glanville et al. [ 51 for a subbituminous coal. Those coals that show an exponential decrease in the heat of immersion with increasing amounts of adsorbed water on their surface have a heterogeneous surface structure, according to Chessick and Zettlemoyer [ 21. Petrographic constituents Higher ranked coals, such as bituminous and anthracite, can be further categorized by their petrographic constituents. Lithotypes, appearing as bright and dull bands of varying thickness, are the layers of the maceral-mineral mixtures that make up the strata of the coals seam. Each lithotype - vitrain, clarain, fusain, or durain - has its own particular physical properties that are based on hardness, friability, and specific gravity. Separation of these macrostopically recognizable bands is possible by physical means. Coal seams contain varying amounts of each lithotype. This was evident in the hand-picked sample of Upper Freeport seam coal that was used in this study. The sample contained vitrain, clarain, fusain, and pyrite. The results of the heat of immersion measurements are presented in Table 3. As expected, the standard deviation of error was less for the lithotypes than for the channel sample of coal, since the lithotypes are less heterogeneous. Furthermore, microscopic studies of the various lithotypes have revealed that
117 TABLE 3 Heats of immersion
in water for Upper Freeport
seam coal, its lithotypes
-AH,,, Upper Freeport Vitrain Clarain Fusain Pyrite
coal (channel
sample)
0.26 0.23 0.35 0.42 0.12
+ + k f +
and’its pyrite
(cal g-l) 0.03 0.01 0.02 0.02 0.06
vitrain and fusain are more homogeneous than clarain and durain [ 151. This was suggested by a smaller standard deviation of error for the vitrain and fusain samples. Of the petrographic constituents, vitrain is the most floatable and has the lowest AHi,,. Methanol as the immersional fluid
Heat of immersion measurements have also been used to identify solids in terms of hydrophobicity and hydrophilicity [ 21. In general, hydrophobic solids are characterized by higher heat values for immersion in organic liquids than in water, whereas the reverse is true for hydrophilic solids. In order to investigate this occurrence, the same six coals that were previously described were used, as well as samples of graphite, kaolin, pyrite, and slate. All ten samples were initially placed in an argon-circulating oven at 313 K for four days (to drive off moisture until a constant weight was achieved). The heat of immersion measurements were subsequently made in water and methanol, and the results are given in Table 4. In these experiments, methanol is used as an organic wetting agent to characterize a solid in terms of hydrophobicity and hydrophilicity. As previously reported [ 21 graphite, a nonpolar hydrophobic solid, has a higher dHi,m in an organic liquid than in water. The same result was obtained in this study. Although coal is a complex heterogeneous solid, the results indicate that it could be classified as a predominantly hydrophobic solid regardless of rank. In addition, a relatively hydrophobic characteristic can be seen by comparing the magnitude of change in the dHi,m for each of the solids. As expected, MV bituminous Upper Freeport seam coal would be considered to be the most hydrophobic of the coals, followed by Pittsburgh seam, Illinois No. 6 seam, etc. In addition, pyrite would also be classified as a slightly hydrophobic solid, whereas slate and kaolin would be considered predominantly hydrophilic solids, since their heats of immersion are less in an organic liquid than in water.
118 TABLE 4 Heats of immersion of various solids in water and methanol Solid
Graphite Upper Freeport seam coal Pittsburgh seam coal Illinois No. 6 seam coal Mammoth seam coal Rosebud seam coal Beulah Zap seam coal Pyrite Slate Kaolin
-AH,,,
(Cd
g-‘)
Water
Methanol
0.04 f 0.01 0.26 f 0.03 0.49 f 0.04 3.60 + 0.33 0.59 f 0.04 12.42 k 0.53 13.71+ 0.47 0.76 k 0.06 1.00 k 0.03 1.27kO.10
2.37 + 0.04 1.35kO.15 2.2OkO.14 8.71 k 0.52 1.08f0.10 13.05 f 0.53 15.20 + 0.43 0.92 + 0.05 0.53 + 0.02 0.41+ 0.03
Surfactant solutions as immersional fluids Three different surfactants were used as immersional fluids to determine the effect on the wetting properties of Pittsburgh seam coal (Table 5). In addition, the surfactant/water concentration was varied to determine its effect, and the results were compared to vacuum filtration dewatering results obtained by Gala [ 161 and Venkatadri [ 171. The Pittsburgh seam coal was prepared as previously described. The results of these heat of immersion measurements are given in Figs 5,6 and 7, where they are plotted in conjunction with the moisture content of the dewatered filter cakes versus surfactant concentration (given in mole fraction - moles of surfactant per total moles of solution). In all cases, the dHi,, measurements correlated well with the dewatering results. In general, whenever surfactant adsorption takes place on the surface of the coal particle in such a fashion that the surface becomes more hydrophobic, the heat of immersion decreases, and dewatering by vacuum filtration improves. This is the case TABLE 5 Surfactants used as immersional fluids Name/type
Manufacturer
Chemical name
Aerosol-OT/anionic
American Cyanamid
Triton X-lll/nonionic DPC/cationic
Rohm and Haas MCB Manufacturing Chemists
Sodium di (2-ethyl-hexyl) sulfosuccinate Polyoxyethylene Dodecylpyridinium chloride
-
119 0.22.
I
I
i
I __&_
z 0.20 %
I
I
I
I
I
Moisture contmlt -
-A R imm
11__~_~///O~~
. .
if g 0.12-
. .
/ .\
.\
/ -0.3
1’ -0’
?
E ;
-
O.!O-
2
I 10-e 2
I
I
I
I
I
I
__*__
F 0.20B
concentration
on the moisture
0.2
’
I 10-5
10-6 2 5 10-T 2 5 5 AEROSOL-01 SURFACTANT CONCENTRATION IN MOLE FRACflON
Fig. 5. Effect of Aerosol-OT surfactant cakes and the heat of immersion.
& +
content
of vacuum filter
Moisture Content
- 0.7
-AHtmm
z
8 \ O.l6-
- 0.6 : s
B t 3 O.l6::
__
z - 0.5 g
I % 0.14 -
w e - 0.4 %
P 8 O.l2-
- 0.3
k! 5 O.lO-
- 0.2
s I
10-6
I
I
I
I
1
I
I
1
5 5 10-T 2 10-6 2 5 2 TRITON X-114 SURFACTANT CONCENTRATION IN MOLE FRACTION
Fig. 6. Effect of Triton X-114 surfactant cakes and the heat of immersion.
concentration
on the moisture
content
8 t Y
’
, 10-5
of vacuum filter
for very low surfactant concentrations (less than 1.85 x lo-” mole fraction for Aerosol-OT, 1.0~10-~ mole fraction for Triton X-114, and 2.0~10-~ mole fraction for DPC). If the surfactant concentration is increased above these
120
-
Moisture Content
_-*--_
P
I
10-6
I
-A
H imm
I
2 5 10-S DPC SURFACTANT CONCENTRATION
I
I
2
5 IN MOLE FRACTION
Fig. 7. Effect of dodecylpyridinium chloride surfactant vacuum filter cakes and the heat of immersion.
concentration
10-e
on the moisture content
of
values, the coal particles become more hydrophilic because of the geometric orientation of adsorbed surfactant molecules on the coal surface [ 161. Therefore, the heat of immersion of the coal particles, as well as the moisture content of the dewatered coal filter cake, increases. CONCLUSIONS
Coal is a heterogeneous mixture of organic and inorganic materials and is therefore quite difficult to assess both structurally and chemically. In the experiments reported here, however, immersional calorimetry has proved to be quite helpful in measuring surface and interfacial properties of coal and other solids in various liquid media, and the following conclusions can be drawn: The heat of immersion measurements were capable of distinguishing differences in surface properties owing to coal rank, surface oxidation state, moisture content, and petrographic constituent content. The trends that were shown to exist in the heat of immersion measurements (for varying coal rank, lithotypes, and oxidation) coincide with trends reported for flotation. The ten solids that were investigated were characterized as being either hydrophobic or hydrophilic, and their relative wetting tendencies were also established. Alteration of the surface properties of a hydrophobic solid was shown to occur by the preferential adsorption of surfactant in a water solution.
121
The results obtained by the heat of immersion measurements correlated well with vacuum filtration experiments in which the same surfactants were used. ACKNOWLEDGEMENTS
The financial support from the U.S. Department of Energy’s Postgraduate Research Training Program administered by Oak Ridge Associated Universities is gratefully acknowledged.
REFERENCES 1 2 3 4 5
6 7 8 9 10 11 12
13 14 15
16 17
G. Steinberg, What You Can Do With Surface Calorimetry, Chemtech, (1981) 730-737. J.J. Chessick and A.C. Zettlemoyer, Immersional Heats and the Nature of Solid Surfaces, Adv. Catal., 11 (1959) 263-299. V.N. Petukhov and L.A. Popova, Evaluation of the Flotation Activity of Coals by the Heat of Wetting, Khim. Tverd. Topl., 9 (1975) 160-162. P. Nordon and N.W. Bainbridge, Heat of Wetting of a Bituminous Coal, Fuel, 62 (1983) 619-622. J.O. Glanville, S.T. Hall, D.L. Messick, K.L. Newcomb, K.M. Phillips, F. Webster and J.P. Wightman, Heat of Immersion in Water of Wyodak No. 3 Coal as a Function of Moisture Content, Fuel, 65 (1986) 647-649. K.M. Phillips, J.O. Glanville and J.P. Wightman, Heat of Immersion of Virginia-C Coal in Water as a Function of Surface Oxidation, Colloids Surfaces, 21 (1986) 1-8. J.W. Larsen and E.W. Kuemmerle, Heat of Wetting of Coal by Tetralin: Evidence for Structural Disruption at 25”C, Fuel, 57 (1978) 59. J.D. Glanville and J.P. Wightman, Wetting of Powdered Coals by Alkanol-Water Solutions and Other Liquids, Fuel, 59 (1980) 557-562. E.L. Fuller, Jr, Structure and Chemistry of Coals: Calorimetric Analysis, J. Colloid Interface Sci., 75 (1980) 577-583. E. Widyani and J.P. Wightman, Thermodynamics and Kinetics of Immersion of Coal by nAlcohols, Colloids Surfaces, 4 (1982) 209-212. B. Chawla and E.M. Arnett, Thermochemistry of Acid-Base Interactions of Three Coal Samples of Different Rank at Elevated Temperatures, J. Org. Chem., 49 (1984) 3054-3059. W. Rudzinski, J. Zajac and C.C. Hsu, Excess Isotherms and Heats of Immersion in Monolayer Adsorption from Binary Liquid Mixtures on Strongly Heterogeneous Solid Surfaces, J. Colloid Interface Sci., 103 (1985) 528-541. J.O. Glanville, K.L. Newcomb and J.P. Wightman, Factors Affecting the Rate and Extent of Heat Release during the Immersion of Powdered Coals in Alkanols, Fuel, 65 (1986) 485-488. S.R. Gunn, Comparison Standards for Solution Calorimetry, J. Phys. Chem., 69 (1965) 2902-2913. R.E. Zimmerman, Flotation in Practice, in J.W. Leonard (Ed.), Coal Preparation, 4th edn, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., New York, 1979, p. 10-87. H.B. Gala, Use of Surfactants in Fine Coal Dewatering, Ph.D. Dissertation, School of Engineering, University of Pittsburgh, 1982. R.A. Venkatadri, Effect of Surface-Active Agents on Filtration and Post-Filtration Characteristics of Fine Coal, Ph.D. Dissertation, School of Engineering, University of Pittsburgh, 1984.
An Experimental Coal T.G. MELKUS
109 -
Printed
in The Netherlands
Study of Heat of Immersion
of
and S.H. CHIANG
University of Pittsburgh, Pittsburgh, PA (U.S.A.) W.W. WEN U.S. Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, PA (U.S.A.) (Received
5 January
1987; accepted in final form 22 June 1987)
ABSTRACT The heats of immersion of six coals were measured at 303 K in water with a Setaram C-80 microcalorimeter. The effects of coal rank, surface oxidation state, moisture content, and petrographic constituents on the wetting characteristics and surface properties were investigated. The six coals and four other solids (pyrite, kaolin, slate, and graphite) were characterized in terms of relative hydrophobicity or hydrophilicity via the comparison of heat of immersion measurements in water and in methanol. Heat of immersion measurements were also made of coal in surfactant solutions (Aerosol OT, Triton X-l 14, and dodecylpyridinium chloride) of varying concentrations. The results of the various experiments were found to correlate well with independent flotation and vacuum filtration tests.
INTRODUCTION
Surface calorimetry assists in the assessment of the chemistry of the surface and its interactions with any molecules that can contact it [ 11. More specifically, immersion calorimetry has been shown to be a valuable means of determining solid surface properties [ 21, such as wetting tendencies, surface polarity, and site energy distribution. Heat of immersion (AH,,,) is defined as the heat that is released (or absorbed) when a clean solid is immersed in a liquid. A variety of solids, including coal, have been used in heat of immersion studies. Of interest to the work presented here, the effect of coal rank [ 31, moisture content [ 4,5], and oxidation [ 61 on the dHi,, of coal have been previously reported. In addition, the heat of immersion of coals in various liquids have been used to investigate its pore structure [ 7-131. The work presented* here differs from previous studies in that the calorim*Reference in this report to any specific commercial product, understanding and does not necessarily imply its endorsement Department of Energy.
process or service is to facilitate or favoring by the Unites States
0166-6622/87/$03.50
B.V.
0 1987 Elsevier Science Publishers
110
eter was used to measure the heat of immersion of coal in its natural state (whose physical characteristics had not been altered by evacuation). Great care was taken to keep the coal (after it had been crushed) in an inert atmosphere at all times prior to the dHimm measurement. Thus, the effects of oxidation and moisture content of the natural state of various coals could be determined in this study. The purpose of making these dHi,m measurements was to study the wetting characteristics of coal particles. EXPERIMENTAL
Test equipment A Setaram C-80 heat flux calorimeter was used to measure the heat of immersion. This calorimeter works on the Tian-Calvet heat-flow principle and is manufactured by Setaram Instruments, Lyon, France. Two cells (one experimental, one reference) are placed in the calorimetric block, which functions as a heat sink. The temperature of this block is controlled precisely ( + 0.01 K over a range of ambient temperature to 573 K) using a temperature programmer/controller. Two identical and independent heat-flux detectors, consisting of conductive thermocouples, connect the vessels thermally to the block so that the vessel temperature is always as close as possible to that of the block. The detectors are differentially connected so that thermal perturbations to the block are eliminated and only the signal owing to the heat exchange with the sample is generated. The heat-flux detectors emit a signal that is proportional to the heat per unit time transferred. This signal is then processed by an Apple II computer that is programmed to (1) integrate the signal (i.e., calculate the total quantity of heat emitted or absorbed) and (2) analyze the signal over time (i.e., determine the kinetics and equilibrium). To perform the heat of immersion measurements, mixing cells (Fig. 1) were used to mix the solid and the liquid. In a typical experiment, a layer of mercury ( 600-800 ~1) is placed in the small internal sample vessel; then the coal sample ( 30-50 mg) is placed on top of the mercury and covered with a cap. This vessel is then inserted into the mixing-cell body, and another layer of mercury (800-1200 ~1) is added to act as a barrier to prevent any mixing prior to activation of the reversing mechanism. The mixing cell is then filled nearly to the top with 4-5 ml of the immersion fluid (usually water) in order to minimize air space. The reference cell is filled in the same manner, but the coal sample is omitted. Both cells are then loaded into the calorimeter and allowed to come to complete thermal equilibrium (usually 3-4 h) . Once equilibrium is attained, the experiment is initiated by activating the reversing mechanism, which inverts the entire calorimetric assembly every 10 s. The cap and mercury act as a mixer to homogenize the solution. The number of inversions depends upon the ease with which the materials mix; 20-30 min was usually required for the
111
Teflon plug Retaining ring Bushing
Stopper Threaded
Immersional
cover
fluid
Mercury
Fig. 1. Cross section of the mixing cell of the Setaram C-80 calorimeter.
experiments reported here. All experiments were carried out in the isothermal mode of operation at slightly above room temperature (303 K ) . Coal samples Coal is a heterogeneous mixture of organic and inorganic materials, and the most common way to classify coal is by rank. Coal rank is determined by carbon content, volatile matter, and calorific value. Six different coals were used in this study. Their rank, seam, origin, and proximate and ultimate analyses are presented in Table 1. The coal was mined at the coal face and shipped in chunks (of the order of inches) under argon. These chunks were kept under argon prior to the crushing process, which took place in an oxygen environment. The coal samples were crushed using a jaw crusher followed by a hammerhill to obtain 28 mesh x 0 coal particles. The samples were then dry screened on a rotap to obtain the desired size range; particles of 100 x 200 mesh (0.074-0.147 mm) were used in all experiments. The coal was then microriffled to obtain a representative sample (2-3 grams ) for use in the calorimeter experiments. It was determined that the heat of immersion is highly dependent on the moisture content of the coal (especially for the low-rank coals). Therefore, all of the samples (except those
North Dakota Montana Illinois Pennsylvania Pennsylvania Pennsylvania
State
the original moisture content
Beulah Zap Rosebud Illinois No. 6 Pittsburgh Upper Freeport Mammoth
Lignite Subbituminous Bituminous, HVC Bituminous, HVA Bituminous, MV Anthracite
“These numbers represent
Seam
and ultimate analyses of Coal
Coal rank
Proximate
TABLE 1
(29.70)’ (21.40)” (8.04)” (1.99)” (0.74)a (2.91)’ 36.36 36.34 35.35 36.10 21.99 2.88 36.91 41.35 46.39 59.18 53.04 87.70
Fixed carbon
(as received)
Volatile matter
analysis
4.77 5.63 5.54 5.84 5.49 2.14
Hydrogen
Ultimate
71.65 75.94 77.58 84.02 86.25 95.27
Carbon
analysis
0.87 0.99 1.08 1.73 1.56 0.78
Nitrogen
1.31 0.97 6.03 1.10 2.01 0.73
Sulfur
(moisture, ash free )
analyses were performed.
8.91 9.12 13.23 3.98 24.18 6.44
Ash
of each of the coals before laboratory
17.82 13.19 5.03 0.74 0.79 2.98
Moisture
Proximate
21.40 16.47 9.82 7.31 4.69 1.08
Oxygen
113
used for the moisture study) were placed in an argon-circulating oven at 313 K for four days (to drive off moisture until a constant weight was achieved). The samples were then placed in sample bottles and purged with argon so that the coal surface was kept as fresh as possible and was not subjected to oxidation. Each time a bottle was opened to remove a sample, it was again purged with argon before it was resealed. Moisture and ash analyses were performed using a LECO MAC-400 Proximate Analyzer on each coal sample to check the uniformity of the samples. In addition to the six different coals, lithotypes of Upper Freeport seam coal and four solids (grahite, kaolin, pyrite, and slate) were used in the dHi,m measurements. Specimens of the Upper Freeport seam lithotypes were separated from the hand-picked samples of banded coals, ground via mortar and pestle to 100 x 200 mesh particles, and then placed in an argon-circulating oven at 313 K for four days (to drive off moisture until a constant weight was achieved). The graphite and kaolin were both obtained from Fischer Scientific; the graphite was > 98% pure and ~325 mesh ( < 0.044 mm), and the kaolin was dry screened to 100 x 200 mesh (0.074-0.147 mm) particles. The pyrite was hand-picked from coarse refuse of a coal preparation plant in Western Pennsylvania, and the slate was hand-picked from the Bureau of Mines experimental mine near Pittsburgh, PA. Both the pyrite and slate were prepared in the same manner as the six coal samples. For each of the systems considered, the heat of immersion measurement was performed at least three different times to ensure reproducibility of results. The liquids that were used in the experiments were deionized distilled water and commercially available organic liquids. RESULTS AND DISCUSSION
The calorimeter was calibrated by measuring the heat of dissolution of potassium chloride (NBS 1655) in water at 299 K. ThedH,i, value was 4.23 2 0.04 kcal mol-‘, which agrees well with the literature value of 4.1998 kcal mol-l
[141. Coal rank
The results for the six different coals tested (Fig. 2) show that dHi,m varies significantly with coal rank. A similar trend exists with the floatability of coal [ 151. Floatability is inversely related to the heat of immersion. Thus, the least floatable form of coal, lignite, has the highest heat of immersion [ 31. Similarly, anthracite is slightly more difficult to float than high-volatile bituminous coal and correspondingly has a higher dHi,m than Pittsburgh seam coal. Fuller [ 91 also considered the variation of heat of immersion with coal rank and observed
114 14
1’1’1’
l
0 12 -
0 A
o
5 : 9
n
io-
A
I
‘I’
’
Liqnite /Beulah Zap Subbituminous /Rosebud Bituminous HVC/lllinais No.6 Bituminous HVC/Pittsburqh Bituminous W/Upper Freepart Anthracite /Mammoth
z g
6
g j
6-
10
74
76 02 06 90 COAL RANK IN PERCENT CARBON
Fig. 2. Heat of immersion in water: Effect of coal rank (coal rank expressed in percent carbon on a moisture- and ash-free basis).
a somewhat similar trend, even though his calorimetric treatment of the coal were quite different.
technique
and pre-
Surface oxidation state Oxidation of the coal surface has an adverse effect on its floatability; even a highly floatable coal will become difficult to float if highly oxidized [ 151. In order to study the effect of surface oxidation on the heat of immersion, the following experiments were performed. Eighteen coal samples (three each of the six coals that were used in the coal rank study) were dried in an argon atmosphere at 313 K for four days. Their moisture contents after drying are given in Table 2. Heat of immersion measurements were made in water for six TABLE 2 Moisture content of coal after drying in argon oven for four days Coal rank/seam
Percent moisture
Lignite/Beulah Zap Subbituminous/Rosebud Bituminous, HVC/Illinois No. 6 Bituminous, HVA/Pitteburgh Bituminous, MV/Upper Freeport Anthracite/Mammoth
1.96 1.80 1.30 0.78 0.54 0.60
115 I
I
I
I
I
I
OBituminous HVA/Pittsburgl D LigniWBeulah Zap 3 Subbltuminous I Rosebud l bituminous MV/UpperFmpc A Anthracite / Mammoth L Bituminousj HVC/lllinols No.6
1 h
.o -
&I~XIDATGN
TIMi
IN 0~;s
-
Fig. 3. Effect of oxidation at 369 K on the heat of immenion in water for six different coals. samples, one each of the six coals. The other twelve samples were then oxidized in an air-circulating oven at 369 K. Six samples were oxidized for one day, and six samples were oxidized for four days before the dHimm measurements were made. As shown in Fig. 3, oxidation had the same qualitative effect on all of the coals considered, regardless of rank. An oxidized coal tends to be more hydrophilic, which corresponds to a higher dHi,m in water. The heat of immersion values reported in this study are generally lower than those reported in the literature. Most of the di!&,., data reported by others are similar to the results reported here for oxidized coals. This consistent difference is a result of the kind of calorimeters used by earlier researchers [ 3,9]. They required that the coal sample be outgassed at a very high temperature, usually for a period of 24 h. This procedure altered the physical characteristics of the coal surface in such a way which appears to be similar to an oxidized coal surface in this investigation.
Moisture content Heat of immersion ting liquid contained
is highly dependent on the amount of preadsorbed wetby the solid. This was readily observed in measuring the
116
MOISTURE IN PERCENT
Fig. 4. Effect of moisture content on the heat of immersion in water of lignite coal/Beulah Zap.
dHi,m of lignite coal in water (Fig. 4). The dHi,, measurements were made after the coal was dried in an inert atmosphere for varying periods of time (2 h to 4 days). The corresponding moisture content was also determined. Heat of immersion curves were also obtained for the other five coals as a function of moisture content, and they were similarly shaped. This same decrease in the heat of immersion with increasing moisture content has been reported by Nordon and Bainbridge [ 41 for a bituminous coal and by Glanville et al. [ 51 for a subbituminous coal. Those coals that show an exponential decrease in the heat of immersion with increasing amounts of adsorbed water on their surface have a heterogeneous surface structure, according to Chessick and Zettlemoyer [ 21. Petrographic constituents Higher ranked coals, such as bituminous and anthracite, can be further categorized by their petrographic constituents. Lithotypes, appearing as bright and dull bands of varying thickness, are the layers of the maceral-mineral mixtures that make up the strata of the coals seam. Each lithotype - vitrain, clarain, fusain, or durain - has its own particular physical properties that are based on hardness, friability, and specific gravity. Separation of these macrostopically recognizable bands is possible by physical means. Coal seams contain varying amounts of each lithotype. This was evident in the hand-picked sample of Upper Freeport seam coal that was used in this study. The sample contained vitrain, clarain, fusain, and pyrite. The results of the heat of immersion measurements are presented in Table 3. As expected, the standard deviation of error was less for the lithotypes than for the channel sample of coal, since the lithotypes are less heterogeneous. Furthermore, microscopic studies of the various lithotypes have revealed that
117 TABLE 3 Heats of immersion
in water for Upper Freeport
seam coal, its lithotypes
-AH,,, Upper Freeport Vitrain Clarain Fusain Pyrite
coal (channel
sample)
0.26 0.23 0.35 0.42 0.12
+ + k f +
and’its pyrite
(cal g-l) 0.03 0.01 0.02 0.02 0.06
vitrain and fusain are more homogeneous than clarain and durain [ 151. This was suggested by a smaller standard deviation of error for the vitrain and fusain samples. Of the petrographic constituents, vitrain is the most floatable and has the lowest AHi,,. Methanol as the immersional fluid
Heat of immersion measurements have also been used to identify solids in terms of hydrophobicity and hydrophilicity [ 21. In general, hydrophobic solids are characterized by higher heat values for immersion in organic liquids than in water, whereas the reverse is true for hydrophilic solids. In order to investigate this occurrence, the same six coals that were previously described were used, as well as samples of graphite, kaolin, pyrite, and slate. All ten samples were initially placed in an argon-circulating oven at 313 K for four days (to drive off moisture until a constant weight was achieved). The heat of immersion measurements were subsequently made in water and methanol, and the results are given in Table 4. In these experiments, methanol is used as an organic wetting agent to characterize a solid in terms of hydrophobicity and hydrophilicity. As previously reported [ 21 graphite, a nonpolar hydrophobic solid, has a higher dHi,m in an organic liquid than in water. The same result was obtained in this study. Although coal is a complex heterogeneous solid, the results indicate that it could be classified as a predominantly hydrophobic solid regardless of rank. In addition, a relatively hydrophobic characteristic can be seen by comparing the magnitude of change in the dHi,m for each of the solids. As expected, MV bituminous Upper Freeport seam coal would be considered to be the most hydrophobic of the coals, followed by Pittsburgh seam, Illinois No. 6 seam, etc. In addition, pyrite would also be classified as a slightly hydrophobic solid, whereas slate and kaolin would be considered predominantly hydrophilic solids, since their heats of immersion are less in an organic liquid than in water.
118 TABLE 4 Heats of immersion of various solids in water and methanol Solid
Graphite Upper Freeport seam coal Pittsburgh seam coal Illinois No. 6 seam coal Mammoth seam coal Rosebud seam coal Beulah Zap seam coal Pyrite Slate Kaolin
-AH,,,
(Cd
g-‘)
Water
Methanol
0.04 f 0.01 0.26 f 0.03 0.49 f 0.04 3.60 + 0.33 0.59 f 0.04 12.42 k 0.53 13.71+ 0.47 0.76 k 0.06 1.00 k 0.03 1.27kO.10
2.37 + 0.04 1.35kO.15 2.2OkO.14 8.71 k 0.52 1.08f0.10 13.05 f 0.53 15.20 + 0.43 0.92 + 0.05 0.53 + 0.02 0.41+ 0.03
Surfactant solutions as immersional fluids Three different surfactants were used as immersional fluids to determine the effect on the wetting properties of Pittsburgh seam coal (Table 5). In addition, the surfactant/water concentration was varied to determine its effect, and the results were compared to vacuum filtration dewatering results obtained by Gala [ 161 and Venkatadri [ 171. The Pittsburgh seam coal was prepared as previously described. The results of these heat of immersion measurements are given in Figs 5,6 and 7, where they are plotted in conjunction with the moisture content of the dewatered filter cakes versus surfactant concentration (given in mole fraction - moles of surfactant per total moles of solution). In all cases, the dHi,, measurements correlated well with the dewatering results. In general, whenever surfactant adsorption takes place on the surface of the coal particle in such a fashion that the surface becomes more hydrophobic, the heat of immersion decreases, and dewatering by vacuum filtration improves. This is the case TABLE 5 Surfactants used as immersional fluids Name/type
Manufacturer
Chemical name
Aerosol-OT/anionic
American Cyanamid
Triton X-lll/nonionic DPC/cationic
Rohm and Haas MCB Manufacturing Chemists
Sodium di (2-ethyl-hexyl) sulfosuccinate Polyoxyethylene Dodecylpyridinium chloride
-
119 0.22.
I
I
i
I __&_
z 0.20 %
I
I
I
I
I
Moisture contmlt -
-A R imm
11__~_~///O~~
. .
if g 0.12-
. .
/ .\
.\
/ -0.3
1’ -0’
?
E ;
-
O.!O-
2
I 10-e 2
I
I
I
I
I
I
__*__
F 0.20B
concentration
on the moisture
0.2
’
I 10-5
10-6 2 5 10-T 2 5 5 AEROSOL-01 SURFACTANT CONCENTRATION IN MOLE FRACflON
Fig. 5. Effect of Aerosol-OT surfactant cakes and the heat of immersion.
& +
content
of vacuum filter
Moisture Content
- 0.7
-AHtmm
z
8 \ O.l6-
- 0.6 : s
B t 3 O.l6::
__
z - 0.5 g
I % 0.14 -
w e - 0.4 %
P 8 O.l2-
- 0.3
k! 5 O.lO-
- 0.2
s I
10-6
I
I
I
I
1
I
I
1
5 5 10-T 2 10-6 2 5 2 TRITON X-114 SURFACTANT CONCENTRATION IN MOLE FRACTION
Fig. 6. Effect of Triton X-114 surfactant cakes and the heat of immersion.
concentration
on the moisture
content
8 t Y
’
, 10-5
of vacuum filter
for very low surfactant concentrations (less than 1.85 x lo-” mole fraction for Aerosol-OT, 1.0~10-~ mole fraction for Triton X-114, and 2.0~10-~ mole fraction for DPC). If the surfactant concentration is increased above these
120
-
Moisture Content
_-*--_
P
I
10-6
I
-A
H imm
I
2 5 10-S DPC SURFACTANT CONCENTRATION
I
I
2
5 IN MOLE FRACTION
Fig. 7. Effect of dodecylpyridinium chloride surfactant vacuum filter cakes and the heat of immersion.
concentration
10-e
on the moisture content
of
values, the coal particles become more hydrophilic because of the geometric orientation of adsorbed surfactant molecules on the coal surface [ 161. Therefore, the heat of immersion of the coal particles, as well as the moisture content of the dewatered coal filter cake, increases. CONCLUSIONS
Coal is a heterogeneous mixture of organic and inorganic materials and is therefore quite difficult to assess both structurally and chemically. In the experiments reported here, however, immersional calorimetry has proved to be quite helpful in measuring surface and interfacial properties of coal and other solids in various liquid media, and the following conclusions can be drawn: The heat of immersion measurements were capable of distinguishing differences in surface properties owing to coal rank, surface oxidation state, moisture content, and petrographic constituent content. The trends that were shown to exist in the heat of immersion measurements (for varying coal rank, lithotypes, and oxidation) coincide with trends reported for flotation. The ten solids that were investigated were characterized as being either hydrophobic or hydrophilic, and their relative wetting tendencies were also established. Alteration of the surface properties of a hydrophobic solid was shown to occur by the preferential adsorption of surfactant in a water solution.
121
The results obtained by the heat of immersion measurements correlated well with vacuum filtration experiments in which the same surfactants were used. ACKNOWLEDGEMENTS
The financial support from the U.S. Department of Energy’s Postgraduate Research Training Program administered by Oak Ridge Associated Universities is gratefully acknowledged.
REFERENCES 1 2 3 4 5
6 7 8 9 10 11 12
13 14 15
16 17
G. Steinberg, What You Can Do With Surface Calorimetry, Chemtech, (1981) 730-737. J.J. Chessick and A.C. Zettlemoyer, Immersional Heats and the Nature of Solid Surfaces, Adv. Catal., 11 (1959) 263-299. V.N. Petukhov and L.A. Popova, Evaluation of the Flotation Activity of Coals by the Heat of Wetting, Khim. Tverd. Topl., 9 (1975) 160-162. P. Nordon and N.W. Bainbridge, Heat of Wetting of a Bituminous Coal, Fuel, 62 (1983) 619-622. J.O. Glanville, S.T. Hall, D.L. Messick, K.L. Newcomb, K.M. Phillips, F. Webster and J.P. Wightman, Heat of Immersion in Water of Wyodak No. 3 Coal as a Function of Moisture Content, Fuel, 65 (1986) 647-649. K.M. Phillips, J.O. Glanville and J.P. Wightman, Heat of Immersion of Virginia-C Coal in Water as a Function of Surface Oxidation, Colloids Surfaces, 21 (1986) 1-8. J.W. Larsen and E.W. Kuemmerle, Heat of Wetting of Coal by Tetralin: Evidence for Structural Disruption at 25”C, Fuel, 57 (1978) 59. J.D. Glanville and J.P. Wightman, Wetting of Powdered Coals by Alkanol-Water Solutions and Other Liquids, Fuel, 59 (1980) 557-562. E.L. Fuller, Jr, Structure and Chemistry of Coals: Calorimetric Analysis, J. Colloid Interface Sci., 75 (1980) 577-583. E. Widyani and J.P. Wightman, Thermodynamics and Kinetics of Immersion of Coal by nAlcohols, Colloids Surfaces, 4 (1982) 209-212. B. Chawla and E.M. Arnett, Thermochemistry of Acid-Base Interactions of Three Coal Samples of Different Rank at Elevated Temperatures, J. Org. Chem., 49 (1984) 3054-3059. W. Rudzinski, J. Zajac and C.C. Hsu, Excess Isotherms and Heats of Immersion in Monolayer Adsorption from Binary Liquid Mixtures on Strongly Heterogeneous Solid Surfaces, J. Colloid Interface Sci., 103 (1985) 528-541. J.O. Glanville, K.L. Newcomb and J.P. Wightman, Factors Affecting the Rate and Extent of Heat Release during the Immersion of Powdered Coals in Alkanols, Fuel, 65 (1986) 485-488. S.R. Gunn, Comparison Standards for Solution Calorimetry, J. Phys. Chem., 69 (1965) 2902-2913. R.E. Zimmerman, Flotation in Practice, in J.W. Leonard (Ed.), Coal Preparation, 4th edn, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., New York, 1979, p. 10-87. H.B. Gala, Use of Surfactants in Fine Coal Dewatering, Ph.D. Dissertation, School of Engineering, University of Pittsburgh, 1982. R.A. Venkatadri, Effect of Surface-Active Agents on Filtration and Post-Filtration Characteristics of Fine Coal, Ph.D. Dissertation, School of Engineering, University of Pittsburgh, 1984.