Wetting of powdered coals by alkanol-water solutions and other liquids

Wetting of powdered coals by alkanolwater solutions and other liquids

James 0. Glanville” and James P. Wightman Department of Chemistry, Virginia Polytechnic lnstitu te and State University, Blacksburg, Virginia 24061, USA (Received 5 December 1979)

The wetting rates of powdered coals by alkanol-water solutions have been measured. A critical concentration of each alkanol is needed before any wetting occurs and this critical wetting concentration is lower for alcohols with longer carbon chains. A critical surface tension of 0.033 J m-* for Pocahontas No. 3 coal is estimated from the measured wetting concentrations. Studies with a Calvet microcalorimeter show that the heats of immersion of powdered coal in water-methanol mixtures vary smoothly with changing concentration, reaching a maximum value of 16 J g-’ at 30 mol % methanol. Both for alkanols and other liquids, the heat of immersion of coal dust is released over long periods of up to nine hours.

An understanding of the surface properties of coal has practical application to such processes as coal flotation’, coal dust suppression’, and the chemical treatment of coa13. In a previous paper4, we reported the results of wetting experiments on coal dust by aqueous solutions of surface active agents following the procedure of Walke?. This test method, which measures the rate at which coal dust placed on a liquid surface is wetted, has also found use in the evaluation of the wettability of pharmaceutical powder@. There has been a recent discussion’ of the physical chemistry involved in the wetting processes of the test. A related experiment has been used to estimate the critical surface tensions (CST) of libres*. Historically, the study of surface activity, and of the adsorption of solute molecules into the surface layer of aqueous solutions, has been much concerned with investigations of simple alkanol solutionsg; many papers being devoted to the study of alkanol-water mixtures. Furthermore, there has been considerable interest in the nature of the preferential adsorption from alkanollwater mixtures onto the surfaces of graphitic materials”. As we are interested in the wetting characteristics of coal dust, we aimed to investigate how coal interacted with alkanollwater mixtures. These mixtures can be considered simpler than solutions of detergents, for which they can serve as a model as well as being of interest in their own right. There is at least one practical application of alkanols or alkanol-water mixtures for wetting coal dust”: their use as the collection fluid for coal dust in the midget impinger utilized for airborne dust sampling and measurement. In our previous experiments4 with surface active agents we found in certain cases that a minimum, or threshold, concentration of the agent was necessary to produce any discernible wetting of coal. We now find * Present address: Wen-Don Roanoke, VA 24038, USA

0016-2361/80/080557TO6.$02.00 @ 1980 IPC Business Press

Corporation,

P.O. Box 13905,

that as the concentration of an alkanol in water is altered, a similar radical change in wetting properties takes place; phenomena possibly indicative of a critical surface tension of wetting12. The critical surface tensions of polished coal samples of various ranks have been reported recently’ 3. Having studied the wetting rates, a kinetic phenomenon, we studied the energetics of the wetting process using a microcalorimeter to determine heats of immersion of coal dust in various liquids. This showed that the heat released by immersed coal dust was not liberated at equal rates by different alkanols. This finding led us to study the rates of heat release on immersion of coal dust in a variety of organic liquids.

EXPERIMENTAL Wetting rate determinations

Wetting rates were measured by timing the disappearance of coal dust samples deposited on the surface of the solutions according to the procedure of Walker’. All experiments were conducted at 25 f 2°C and the average of at least three separate trials was used to establish each datum point. Two coals were used: a ~45 pm (325 US mesh) size sample obtained by crushing coal from the Pocahontas No. 3 seam, which we have previously characterized4, and a coal dust obtained from the US National Bureau of Standards as Standard Reference Material 1632a. This latter coal is reportedI to have been prepared from a bituminous Pennsylvania coal seam, crushed and sieved to pass through a 250 pm (60 US mesh) screen. Wetting rates were determined on 0.500 g samples of the Pocahontas No. 3 dust and on 0.250 g-samples of the standard dust. As previously, we estimate a relative error of k 15% in the measurement of wetting rates.

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Wetting of powdered

coals: J. 0. Glanville

and J. P. Wightman

.

a 00

0

..

0

LO

Methanol

I

I

I

1

50

60

70

80

.

/I -.-

10

20

90

(mOl%)

0

-1

c

I

I 0 .’

30

0’

LO

Ethanol

.0’

50

0’

60

I

80

90

I mol%)

Figure 1 Wetting rates of Pocahontas No. 3 (0) and NBS-SRM 1632a (0) coal dusts by (a) methanol/water and (b) ethanol/water mixtures

Reagents

Alkanols, and other liquids, were obtained as reagent grade materials and were used without further purification. Solutions were prepared by weighing appropriate amounts of alkanols and distilled water. Heat I$ immersion

experiments

The heats of immersion of coal dust from the Pocahontas No. 3 seam in various solutions and pure liquids were determined using a Calvet ms70 microcalorimeter (Setaram Instruments, Lyon, France), which has been used infrequently in heats of immersion of the calorimeter was done studies” - ’ 8. Calibration by passing a known current for a given time through a precision resistor. Coal dust samples (15Q-200 mg) were weighed to the nearest tenth milligram and placed in small, cylindrical glass tubes (ca. 3 cm x 0.5 cm) with break-off tips. The tubes were evacuated at a pressure of approximately 1 mPa for 15 min and then sealed under vacuum. The evacuated, sealed, coal-containing tubes were placed inside stainless-steel vessels containing about 3 cm3 of the wetting liquid, and the whole assembly was lowered into the microcalorimeter maintained at a constant temperature of 359°C and allowed to attain thermal equilibrium. Experience revealed that an equilibration time of at least 12 h was desirable before sample bulb breakage and that for maximum instrument stability an equilibration time of up to 40 h was preferable.

558

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RESULTS

AND DISCUSSION

I

I 70

After steady-state had been established, the break seal of each sample was broken by remote mechanical action, the liberated heat detected by an array of thermocouples, and the thermocouple output recorded by electronic integration of the detector signal. The signal was displayed sequentially in digital form on a paper tape. The ms70 is a very stable calorimeter, and our experiments were well above the detection threshold (* 50 pJ). Once thermal equilibrium was established with blank samples the calorimeter remained at steadystate indefinitely. Corrections were not made either for the heat of bulb breaking (estimated to be 0.3 J g-i for a 150 mg sample) or for the heat absorbed by the evaporation of the immersing liquid as the vacuum seal is broken (estimated to be of the same order of magnitude as the heat of bulb breaking but of opposite sign). A detailed description of the Calvet microcalorimeter, and an analysis of sources of error in its use, has been published recently”. The reproducibility of the measurement of the heat of immersion of Pocahontas No. 3 coal dust in methanol was measured by repeating the experiment seven times, which gave a value of 16.4& 1.2 J g-i, corresponding to a reproducibility of *7x.

Wetting

of coals by alkanol

solutions

It has been recognized for many years that ethanol and other simple alkanols are excellent wetting agents for textiles and other materials2’. However, to achieve equivalent wetting to that of surface active agents of the detergent type, the alkanols must be used at very high concentrations, which is not economically feasible for commercial practice. This does not suggest that alkanols in dilute aqueous solutions are not surface active, but that their superficial adsorption2i is very weak compared with that of soaps and detergents. Figures 1 and 2 show the rates of wetting of the two coals by four alkanol-water mixtures of varying composition. It is clear that at a sufficient alkanol concentration both coals are rapidly wet, but at an insufficient concentration neither coal is wet and remains floating on the surface of the solution. Thus, there is a critical concentration necessary to wet coal for each alkanol. Figures 1 and 2 show that this critical wetting concentration shifts progressively to lower values as the carbon chain of the alkanol increases in length, which is not surprising as the increase in surface activity of alkanols with increasing chain length has been established9,22. To obtain an estimate of the critical wetting concentration of each alkanol-water solution, we extrapolated through the region of maximum slope to the concentration axis. This procedure is not entirely satisfactory, and we are aware that the critical wetting concentrations we have established are only approximate. Estimates of the critical wetting concentrations of the various alkanol solutions for the two coal samples are shown in Table 1 in both mol % and wt % . In our earlier study4 of the wetting of coal dust by aqueous solutions of the surface active agent sodium bis(2-ethylhexyl) sulphosuccinate, we also observed a

Wetting of powdered

critical concentration below which this material would not cause wetting. Several of the alkanol solutions show an increased wetting rate passing through a maximum at a concentration only slightly in excess of the critical concentration. We have not observed this type of behaviour with surface active agents, and a study of the wetting rates of a pharmaceutical powder’ showed a minimum in the wetting rate at concentrations of surface active agent slightly in excess of the critical concentration. We believe that the explanation for this behaviour is that several mechanisms of wetting are likely to be occurring, and they shift in their relative efficiencies at different concentrations’.

a

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I/ 00

g&o

. .

l

’

0

0

OoO

.

0

.

.

10

20

0 30

LO

Straight-chain

0

0

50

60

Propanol

0 70

I 90

lmol%’

1

b

.

120 -

60

. .

:

In

.

.

.

z -

._F z g

. .

60-

al ‘;L

cO 0

0

0

0

10-

0

0

ooo

0

0

20

30

50 60 LO is0 - Propanol lmol%’

70

80

90

Wetting rates of Pocahontas No. 3 (0) and NEKYSRM Figure 2 1632a (0) coal dusts by (a) straightchain propanol/water and (b) isopropanol/water mixtures

Tab/e

1

Critical concentrations

of straight-chain

alkanols for the wetting

Pocahontas Mol %

Alcohol

of coal NBS coal (SRM

No. 3 coal Wt %

Mol %

Y

1632al

WtX

Propanol Butanol

42.5 15.0 3.2 6.1 1.2

56.8 31.1 9.9 17.8 4.8

0.031 0.032 0.035 0.034 0.033

Ref.

Y (J m-*l

(J rnm2) Methanol Ethanol Straight-chain iso-Propanol Straight-chain

No. 3 coal in

The study of the heats of immersion (wetting) of various coals in water has been used for many years to investigate the surface areas and other surface properties of the coals. The review by van Krevelen3’ and by Marsh31 include the work of Dryden”‘, Gallcott33, Lahiri34 and Maggs 35. There have also been extensive studies of the heats of immersion of graphitized carbon in various liquids; the older work in this area has been

0

‘0

rates and surface tension

Heats of immersion of Pocahontas methanol-water mixtures

0

OO’

and J. P. Wightman

The approximate surface tensions (y) of the alkanolwater mixtures at the critical wetting concentrations are shown in Table 1. The surface tension results were obtained from the references cited in the last column of Table 1. The critical wetting concentrations measured for Pocahontas No. 3 coal correspond to live alkalinewater solutions having an average surface tension of 0.033+0.002 J rne2. This value is taken to be the critical surface tension of Pocahontas No.3 coal. Similarly, the critical surface tension of NBS coal is estimated to be 0.040+0.002 J rnw2. The difference between the two values could be attributed to differences in the particle size distribution between the coals, and differences in surface chemistry (degree of oxidation, etc) and structure between the coals. We intend to pursue this further with a variety of coals of various size fractions. Recently, a value of approximately 0.045 J me2 for the critical surface tension of coal irrespective of rank has been obtained13. This value was obtained from polished coal specimens using the Zisman technique in which the contact angles of a series of liquids of varying surface tension were determined. It is difficult to relate our results to this value directly. In one case the measurement is in a static system with a polished coal surface using pure liquids, and in the other in a dynamic system with a powdered coal and binary liquid mixtures. It is not clear whether the polishing of the coal does not in fact alter the coal surface. However, we are wary of ascribing too great a significance to the present correlation. There are many reports in the literature of the lack of correlation between wetting kinetics in practical systems and simple, static surface tension measurements24-26, and our present theoretical knowledge of the dynamics of wetting of porous, powdered solids is severely limited27*28. Furthermore, a detailed knowledge of the nature of porosity in coals is only now beginning to emerge29.

.

.

.

Wetting

coals: J. 0. Glanville

20.3 6.5 2.6 3.9 1

0.040 0.042

31.2 15.1 8.2 11.9 4

0.038 0.038 0.040

(23) (23) (22) ( 6) (22)

Y Avg 6.946

‘YAvg 6.633

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Wetting of powdered coals: J. 0. G/anvil/e and J. P. Wightman

-I

z

(i:

I”

0

I 25

0

I 75

I 50 Methanol

(mot

100”

%I

Figure 3 Heats of immersion (ml of Pocahontas No. 3 coal dust in methanol/water mixtures compared with surface tension data (0)

reviewed by Dryden36. Zettlemoyer37 and his coworkers have investigated the heats of immersion of graphitized carbon in surfactant solutions and various aqueous systems. More recently, studies of the heat of immersion of coal have been used to probe such phenomena as the effects of alkali treatment on the surface of coa13*, and the effects or organic colloids on the flotation characteristics of coal’. Related to these are recent gas chromatographic measurements of the heats of adsorption of organic molecules on coal surfaces3,39*40 undertaken to study the accessibility of coal surfaces to the reagents used in coal-conversion processes. The heat of immersion of Pocahontas No. 3 coal dust in methanol-water mixtures is shown in Figure 3. It is seen that the smooth change in the heat of immersion as a function of concentration is in contrast to the wetting rate curve for methanol-water mixtures in Figure 1. There is an absence in the immersional heat data of the threshold demonstrated in the wetting rate curve. Furthermore, the maximum heat of immersion, about 16 J g-i, is obtained with methanol concentrations above about 30 mol %, suggesting that methanol is the principal adsorbate in these solutions. It may be significant that methanol-water mixtures containing less than about 20 mol Y0methanol are incapable of wetting the coal dust. Additional experimental support for this view can be seen in the shape of the surface tension curve for methanol-water mixtures also shown in Figure 3. The similarity of the two curves suggest a relation between surface tension and heat of wetting; the two being rectilinearly related as shown by the results in Figure

solutions of a range of surface tensions; however, the approach may be negated by the preferential adsorption phenomenon. There is also a significant experimental difference between the methods of rate and heat determination that complicates direct comparison of the results. Wetting experiments were conducted at atmospheric pressure, whereas the heat of immersion experiments (of necessity with the experimental set-up used) were conducted using evacuated coal samples. We are presently modifying the microcalorimeter to evaluate the heats of immersion at atmospheric pressure. Heats of immersion of Pocahontas No. 3 coal in various liquids

The heats of immersion of Pocahontas No.3 coal dust in a variety of liquids are in Table 2. Significant differences between the immersional heats of this coal in various organic liquids are observed with methylene chloride yielding the greatest heat value and hexane the least. The alkanols are intermediate in the amount of heat released and within experimental uncertainty the four simplest alkanols (methanol, ethanol, straightchain propanol and straight-chain butanol) liberate the same amount of heat. The origin of the heat liberated during immersion is undoubtedly complex. Marsh3’, in his review of the pre-1965 literature, notes that such mechanisms as chemisorption, imbibition, mechanical trapping, swelling of the coal, and interactions of the immersing liquid with specific surface groups on the coal have all been invoked as possible sources of the experimentally

I

I

I

4.

Thus we speculate that the adsorbate on the coal surface is preferentially methanol over water, and predominately so at higher solution concentrations of methanol. The preferential adsorption of one component of a binary mixture onto a solid surface is an established phenomenon, as demonstrated41 for butanol onto a graphite surface from a butanol-water mixture. This same phenomenon is recognized to invalidate the use of the Zisman method of measuring the surface energies of many solids by mixtures of liquids4’. Here, the determination of critical surface tension of wetting would be conveniently easy if binary liquid mixtures could be used to produce wetting test

560

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1980,

Vol 59, August

I 0.07

I 0.03

I

0.05 Surface

tension

1J

m-*)

Heats of immersion of Pocahontas No. 3 coal dust in Figure 4 methanol/water mixtures as a function of surface tension

Wetting of powdered coals: J. 0. Glanville and J. P. Wightman Tab/e 2 Heats of immersion of a West Virginia liquids and their approximate release times

coal dust in various

Approximate

Heat of immersion

time to reach

Liquid

J g-’

equilibrium

Methanol Ethanol Straight-chain Propanol Straight-chain Butanol Methylene chloride Pyridine Tetrahydrofuran

16.2 15 14 15 21 15

0.4 4

11 7.6 5.4 5.0 2.1 3.7

1.5 0.6 0.8 3 2.5 2.7

Tetralin Water Ethylene Decalin Hexane

Glycol

Ih)

7 9 2 2

somewhat accessible to ethanol and not accessible to large alcohols.’ Our results from the study of heats of immersion seem to lend considerable support to this view. In a half an hour only methanol seems able to sufficiently penetrate the coal structure to release all of the potential immersional heat in the system, the other alkanols eventually release a similar amount of heat but the process of penetration is increasingly longer with increasing molecular size. This finding, that the kinetics of the interaction of coal dust with various liquids can be studied at moderate temperatures, promises to be helpful in elucidating the question of the accessibility of coal structures to solvents and reagents. ACKNOWLEDGEMENTS

observed heat. We observed, in the single case of methylene chloride, that this liquid was markedly darkened in colour after immersion of the coal, suggesting that in this case partial dissolution of the coal may also make a contribution to the immersional heat. Especially noteworthy among the liquids of Table 2 is the result that the immersional heat of tetralin is almost four times as great as that of decalin.*

The authors wish to express their thanks to E. Glanville and L. Haley for assistance with the determination of wetting rates, and to Yoonok Kang for advice and assistance with the microcalorimeter. REFERENCES

Rate of heat release after immersion

Immersional calorimetry is not usually concerned with the kinetics of the wetting process. Our study of the immersional heats of coal shows that the heat release can occur over periods of hours. Our microcalorimeter is designed to follow the kinetics of long term (>looO s, = 16 min) thermal events, and as the liberation of heat after the breaking of the seal takes several hours for most of the liquids studied as shown in Table 2, this can be easily followed. Coal immersed in methanol yields a heat release over about a half hour, a time which may be inaccurate owing to the response of the microcalorimeter. Each of the higher alkanols releases heat at a successively slower rate, up to a maximum of nine hours for straight-chain butanol, but the amount of heat ultimately released by the alkanols is the same. Precisely what is the cause of this slowed heat release is not clear, although there are many possible mechanisms from which to select. Of particular interest is the rapid rate of heat release on immersion in tetralin compared to decalin. Tetralin releases more heat of immersion than decalin, and this heat is liberated much more quickly. This infers that the decalin interacts much more slowly and weakly with the coal than does tetralin, which may explain why tetralin is a much better solvent for coal liquefaction processes than decalin despite the fact that decalin should be a richer source of hydrogen. Larsen, Kennard and Kummerle3, have reported that although the equilibrium between Bruceton coal and most organic vapours is rapid, complete equilibrium with the total coal surface may not be rapid. In the specific case of the alkanols their gas chromatographic studies led them to speculate as to existence of an area on the coal surface ‘accessible to methanol, *

We are indebted to Professor Larry Taylor include these liquids as part of our study

for suggesting

that we

9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 21 28 29

Huang, H. H., Calara, J. V., Bauer, D. L. and Miller, J. D Rec. Deu. Sep. Sci. 1978, 4, 115 Kruglitskii, M. M., Tretinnik, V. V. and Kiselova, I. D. Akad. Nauk. URSRN Visnvk. Kiev 1917. 4. 41 Larsen, J. W., Ken&d, L. and Kummerle. E. W. Fuel 1978, 57, 309 Glanville, J. 0. and Wightman, J. P. Fuel 1979, 58, 819 Walker, Jr., P. L., Petersen, E. E. and Wright, C. C. fnd. Eng. Chem. 1952, 44, 2389 Stevens, P., Gypen, L. and Jennen-Bartholomeussen, R., Farm. Tijdschr. Be/g. 1974, 51, 150 Carino, L. and Mallet, J. Powder Technol. 1975, 11, 189 Mutchler, J. P., Menkart, J. and Schwartz, A. M., in ‘Pesticidal Formulations Research’, Advances in Chemistry Series No. 86, (Ed. R. F. Gould) Am. Chem. Sot., Washington, 1969, p 7 Clint, J. H., Corkill, J. M., Goodman, J. F. and Tate, J. R. in ‘Hydrophobic Surfaces’ (Ed. F. M. Fowkes) Academic Press, 1969, p 180 Hansen, R. S. and Craig, R. P. J. Phys. Chem. 1954, 58, 211 Jacobsen, M., Terry, S. L. and Ambrosia, D. A. Ind. Hyg. Assoc. J. 1970, 31, 442 Adamson, A. W. ‘Physical Chemistry of Surfaces’, Third Edition, John Wiley, 1976. Parekh, B. K. and Aplan, F. F. Rec. Dev. Sep. Sci. 1978, 4, 107 J. Paul Cali, US National Bureau of Standards Information Bulletin. Robert, L. Bull. Sot. Chim. Fr. 1967, 2309 Thorne, P, Ph.D. Dissertation, Univ. Bristol (UK), 1974 Bailey, R. R. and Wightman, J. P. J. Colloid Interfuce Sci. 1979, 70, 112 Kang, Y., Skiles, J. A. and Wightman, J. P. J. Phys. Chem. 1980, 1448 Partyka, S., Rouquerol, F. and Rouquerol, J. R. J. Colloid Interface Sco. 1979,68, 21 Sluhan, C. A. Pap Trade J. 1940, 86 Guggenheim, E. A. and Adam, N. K. Proc. Roy. Sot. (London) 1933, A139,218 Addison, C. C. J. Chem. Sot. 1945, 98 International Critical Tables 1928 Cooper, W. F. and Nuttall, W. H. J. Agri. Sci. 1915, 7, 219 Wilkes, B. G. and Wickert, J. N. Ind. Eng. Chem. 1937, 29, 1234 Thomas, W. D. E. and Potter, L. Annual Report of the Long Ashton Research Station 1961, 124 Padday, J. F. in ‘Wetting, Spreading and Adhesion’ 1978 Academic Press, p 465 Schwartz, A. M. Adv. Colloid Interface Sri. 1975, 4, 365 Mahajan, 0. P. and Walker, Jr., P. L. US Department of Energy, Technical Report FE-2030-TR7, 1978

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Wetting of powdered coals: J. 0. Glanville and J. P. Wightman 30 31 32 33 34 35 36 37

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Van Krevelen, D. W. ‘Coal’ Elsevier, 1961 Marsh, H. Fuel 1965,44, 253 Dryden, 1. G. C. Proc. Symp. Nature Coal. Jealyora, India 1959, 112 Gallcott, T. G. Fuel 1964, 43, 203 Sharma, S. K., Mitna, D. C., Dar Gupta, N. N. and Lahiri, A. Coke Gas 1960, 22, 5 Maggs, F. A. P. Nature 1952, 169, 193 Dryden, I. G. C. in ‘The Chemistry of Coal Utilization’, 1963 Supplementary Volume, (Ed. H. H. Lowry) John Wiley, p 233 Zettlemoyer, A. C., Schneider, C. H. and Skewis, J. D.

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Proceedings of the Second International Congress of Surface Activity 1957, p 472 Senkan, S. M. and Fuller, E. L. Jr. Fuel 1979, 58, 2606 Larsen, J. W. Coal Liquefaction bv Alkylation, EPRI Report AF-23, Electric Power Research xtitute, Palo Alto, California, December 1976 Larsen, J. W., Choudhury, P., Greene, T. and Kummerle, E. W. Preprints, Am. Chem. Sot. Div. Fuel Chem. 1979, 24, 197 Kipling, J. J. ‘Adsorption from Solutions of Non-Electrolytes’ Academic Press, 1965 Good, R. J. Surface Colloid Sci. 1979, 11, 1