34
Energy & Fuels 1990, 4, 34-37
Estimation of the Distribution of Surface Sites and Contact Angles on Coal Particles from Film Flotation Data D. W. Fuerstenau,* J. Diao, and J. S. Hanson Department of Materials Science and Mineral Engineering, University of California, Berkeley, California 94720 Received July 5, 1989. Revised Manuscript Received October 30, 1989
Because of their heterogeneity, coal particles may vary widely in composition and, consequently, each particle may have its own unique set of surface properties. The distribution of surface wettabilities of coal particles can be determined by a film flotation technique, using a series of aqueous methanol solutions to vary the surface tension of the liquid. This paper shows how such measurements of the critical wetting surface tension can be used to quantify the contact angle of the particles and the distribution of hydrophobic sites on the surface of the particles. The mean contact angles of particles calculated from film flotation for sulfur, graphite, and a number of different coals are in reasonable agreement with the values reported in the literature for bulk material.
Introduction Characterization of coal particles in terms of their wetting properties is important for understanding the behavior of coal in such surface-based processes as flotation, agglomeration, filtration, and dust abatement. The wetting properties of solids have commonly been studied by measuring contact angles on flat surfaces' and by determining the critical wetting surface tension from the well-known Zisman plot2 from those measurements. Such methods have been very successful for assessing the wettability of homogeneous materials, such as polymers. However, because of the heterogeneity of coal, the results obtained using this approach by different researchers are often i n c o n ~ i s t e n t . ~ ~ ~ Because of this pronounced heterogeneity, the properties of coal particles can range from those of a virtually pure inorganic mineral species to that of a carbonaceous material. Consequently, each particle may have its own unique set of surface characteristics. Standard measurements of liquid penetration rates, heat of immersion, immersion times, and freezing-front points can assess only average properties of particular samples. However, a film flotation technique, developed recently in our laboratories, permits determination of the fraction of particles that sink or float on liquids of different surface tension from which the distribution of wetting surface tensions (surface energies) can be a s s e ~ s e d . ~ ~ ~ In the investigation reported in this paper, the distribution of the wetting characteristics of coal particles (100 X 150 pm in diameter) was determined by film flotation using a series of aqueous methanol solutions of different compositions. By separating the particles into the lyophobic fraction (those remaining on the surface) and the lyophilic fraction (those imbibed into the solution) at each (1) Good, R. J. In Surface and Colloid Science; Good, R. J., Stromberg, R. R., Eds.; Plenum Press: New York, 1979; Vol. 11, pp 1-30. (2) Zisman, W. A. Contact Angle, Wettability and Adhesion; Gould, R. F., Ed.; Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964; pp 1-51. (3) Parekh, B. K.;Aplan, F. F. In Recent Deuelopments in Separation Science; CRC Press: Boca Raton, FL, 1978; Vol. 4,pp 107-113. (4) Yang, G. C. C. Ph.D. Thesis, University of California, Berkeley, 1983. (5)Fuerstenau, D. W.; Williams, M. C.; Diao, J. Presented at the AIME Annual Meeting, New Orlean, March 1986. (6) Fuerstenau, D. W.; Williams, M. C. Colloids Surf. 1987,22, 87-91.
0887-0624/90/2504-0034$02.50/0
surface tension, the distribution of coal particles with relation to their wetting surface tension was determined. This paper shows how such information can be used to delineate additional surface characteristics of the particles, namely, their contact angle and fraction of hydrophilic and hydrophobic sites on the surface.
Theoretical Considerations The critical wetting surface tension of a solid is an important parameter that can be used as an index of the wettability of the solid. As defined by Zisman,2 it is the surface tension of a liquid that just forms a zero contact angle on the solid. In our earlier research,'^^ we verified theoretically and experimentally that, for film flotation, the critical wetting surface tension of hydrophobic particles can be taken to be the surface tension of the liquid a t which the particle just sinks into the liquid. The contact angle of those particles that sink at the given surface tension is essentially zero since the effect of particle size and density is sensibly negligible in the practical size range.'q8 Since the contact angle of a small particle cannot unambiguously be determined directly, it is useful to find a way to assess its value from other kinds of experiments. In the approach developed here, it is necessary to know the value of yc of the individual particles. Since any point along a film flotation plot (a plot of the fraction of particles floating versus the liquid surface tension) represents conditions for zero contact angle at a specific surface tension, in our analysis the distribution of wetting surface tensions obtained from film flotation results will be the starting experimental information. Our analysis begins with the Neumann-Good equation of state:9
(7) Diao, J. M.S.Thesis, University of California, Berkeley, 1987. (8) Fuerstenau, D. W.; Diao, J. L.; Narayanan, K. S.; Urbina, R. H. In
Interfacial Phenomena in Biotechnology and Materials Separations; Attia, Y. A., Ed.; Process Technology Proceedings 7; Elsevier: Amsterdam, New York, 1988, pp 429-441. (9) Neumann, A. W.; Good, R. J.; Hope, C. J.; Sejpal, M. J. Colloid Interface Sci. 1974,49, 291-304.
0 1990 American Chemical Society
Energy & Fuels, Vol. 4, No. 1, 1990 35
Surface Sites and Contact Angles on Coal Particles
where ysv, ysL, and yLv are the interfacial energies at the solid/vapor, solid/liquid and liquid/vapor interfaces, respectively. Substituting this relation into the Young equation (2) Ysv - YSL = YLV cos 6 yields
The contact angle, 6, of a particle for a liquid of a given surface tension can be calculated with eq 3 using the values of ycmeasured in the film flotation experiments. The basis for this is taking yc as being equivalent to ysv, an assumption that has been discussed by Neumann et ala9 Because coal is a complex mixture of various carbonaceous substances and inorganic minerals, the surface of coal particles will appear as a patchwork assembly of carbonaceous material, hydrocarbon, oxygen functional groups, and mineral matter. The carbonaceous and hydrocarbon materials contribute to the hydrophobic character of coal, while the oxygen functional groups and mineral matter contribute to the hydrophilic behavior of coal. Therefore, the coal surface can be taken as a composite material that has different surface energies in different parts of the coal surfaces. One of the objectives of this study was to evaluate the fraction of hydrophobic sites (area with low surface energies) on the surface of coal particles am and the fraction of hydrophilic sites (area with high surface energies) CYHL from the contact angles of the coal particles by using the Cassie equation.l0 Such a calculation makes it possible to visualize the heterogeneous nature of the coal particles in the assemblage. In investigating the wettability of porous surfaces, Cassie and Baxter" derived an expression for predicting the contact angle on this special type of composite surface. Cassie,lo and later Philippoff et a1.,12 extended the expression to include heterogeneous solid surfaces. The Cassie relation has been confirmed theoretically by Johnson and Dettre13 and by Neumann and Good14from more sophisticated free energy analyses and was verified experimentally by Philippoff et a1.12 This equation has been used to estimate the surface coverage of surfactants on particles from contact angle measurements12 and to account for the variation of the contact angle with coal rank by assuming reasonable values for the contact angle of both the hydrophobic and hydrophilic sites and the fraction of these sites on the surfaces of the coal particles.15J6 In order to simplify our calculation, we assume that only two general types of sites occur on coal surfaces: (i) hydrophobic sites, which have a water contact angle OH! = 105" (that is, paraffm wax), and (ii) hydrophilic sites, which have a water contact angle OHL = 0". Since under these conditions (YHB
+ (YHL = 1
(4)
(10) Cassie, A. B.D. Discuss. Faraday SOC.1948,3, 11-16. (11) Cassie, A. B.D.; Baxter, S. Trans. Faraday SOC.1944,40,546-551. (12) Philippoff, W.; Cooke, S. R. B.; Cadwell, D. E. Trans. AZME 1952, 193,283-286. (13) Johnson, R.E.;Dettre, R. H. Contact Angle, Wettability and Adhesion; Gould, R. F., Ed.; Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964; pp 112-135. (14) Newmann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972,38, 341-358. (15) Rosenbaum, J. M.;Fuerstenau, D. W. Znt. J. Miner. Process. 1984, 12, 313-316. (16) Keller, Jr., D. V. Colloids Surf. 1987,22, 21-35.
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SURFACE TENSION OF WETTING SOLUTION,
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Figure 1. Cumulative percentage of lyophobic (hydrophobic) Cambria No. 78 coal particles m a function of their wetting surface tension, as obtained from film flotation with aqueous methanol solutions.
the contact angle of this composite surface can be written as COS 6 = CXHB COS 6HB + CYHLCOS 6HL (5) With these assumptions, the surface composition of the coal particles, that is, am and am, can be calculated using eqs 4 and 5 from their water contact angles calculated from film flotation results using eq 3.
Materials and Methods Materials used in this investigation included a number of U.S. coals, Ceylon graphite (99.0% carbon), and sulfur (Nevada). In each case, 100 X 150 pm particles were obtained by grinding the as-received materials in a small ceramic mill to avoid iron contamination, followed by sizing with sieves. Paraffin-wax-coated coal particles were prepared by a simple vapor deposition pro~ e d u r e . T~ o accomplish this, the vapor generated by heating paraffin wax was passed through a bed of particles (1.5 g) by applying a vacuum to the system. To ensure a uniform coating on all particles, the bed was tumbled every 15 min, with a total coating time of 3 h. Oxidation of the coal wm carried out thermally in air for 19 h a t 200 "C in a mechanical convection oven. The film flotation experiments5" were conducted by sprinkling the 100 x 150 pm particles onto the surface of the liquid, and the fraction of particles remaining at the liquid/vapor interface was determined by separating the sink and float material with each solution. The liquids used for the film flotation experiments were aqueous methanol solutions, with compositions ranging from pure methanol to pure water so that the surface tension could be controlled between 22.4 and 72.8 mN/m. All film flotation experiments were carried out a t 20 "C.
Results and Discussion Because of the heterogeneity of coal, the wetting behavior of coal particles may change continuouslyfrom that of hydrophobic organic materials to those of hydrophilic inorganic matter. Figure 1presents the cumulative distribution of lyophobic (hydrophobic) Cambria No. 78 coal particles as a function of their wetting surface tension, as obtained by film flotation with aqueous methanol solutions. From the results given in Figure 1, the frequency distribution of the critical wetting surface tension of Cambria No. 78 coal particles can be determined since every point along the curve in Figure 1 must represent those particles for which the contact angle is zero. The frequency distribution plot is presented in Figure 2. These two figures clearly show the heterogeneous nature of coal particles. In contrast, ideal homogeneous particles will
36 Energy & Fuels, Vol. 4, No. 1, 1990
Fuerstenau et al.
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Figure 2. Frequency distributionof Cambria No. 78 coal particles as a function of their critical wetting surface tension, as determined from film flotation with aqueous methanol solutions. have the same critical wetting surface t e n ~ i o n . ~ , ~ From such a distribution, four wetting parameters have been defined." The critical wetting surface tension of the most hydrophobic particles in the assembly, ypin, is the surface tension of the liquid at which none of the particles remain (float) at the liquid surface. The critical wetting surface tension of the most hydrophilic particles in the powder, yf", is the surface tension of the liquid at which all the particles remain at the liquid surface. The mean critical wetting surface tension of all partaicles, T ~ can , be calculated from the film flotation frequency distribution by using the equation
where ycis the critical surface tension of the particles and is the frequency distribution function. 7crepresents the mean wettability of the assembly of particles. The standard deviation of the frequency distribution function, by,, reflects the heterogeneity of the surface and is given by
20
40
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C O N T A C T ANGLE O F P A R T I C L E S ,
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Figure 3. Frequency distribution of Cambria No. 78 coal particles as a function of their contact angle in the liquid with surface tension of 60.0 and 72.8 mN/m, respectively.
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f(yJ
(7) High uycvalues correspond to more heterogeneous materials. The contact angles of particles for a given liquid surface tension were calculated from their distributed values of yc (assuming yc = ysv), using eq 3. The weight percent of particles for different intervals of the contact angle was taken from the distribution given in Figure 2. Figure 3 shows the frequency distribution of Cambria No. 78 coal particles as a function of their contact angle in liquids with surface tension of 60.0 and 72.8 mN/m, respectively. The results plotted in this figure indicate that the coal particles in an assembly have a wide range of contact angles and, as expected, that the lower the surface tension of the liquid the smaller the contact angle of the coal particles. This distribution of contact angles serves to illustrate why coal particles reside at the liquid/vapor surface over a range of surface tensions in film flotation experiments. The surface composition of coal particles can be determined from the distribution of their wetting surface tensions by calculating the fraction of lyophobic sites on the surface of the particles, using eq 4 and 5 and the calculated contact angles for water. The frequency distribution of Cambria No. 78 coal particles as a function of the percentage of hydrophobic sites on their surface is given in
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Figure 4. Frequency distribution of Cambria No. 78 coal particles as a function of the fraction of their surface that is comprised of lyophobic sites. Table 1. Wetting Parameters of As-Received, Wax-Coated, and Oxidized Cambria No.78 Coal Obtained from Film Flotation Results treatment T ~mN/m , 8, deg ~ H wax-coated 25.3 101 0.92 as-received 43.0 68 0.49 oxidized at 200 "C for 19 h 67.0 24 0.07
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Figure 4. The weight percent of particles for the different intervals plotted in Figure 4 was taken from Figure 2. Thii figures shows that the surfaces of the coal particles are covered by varying amounts of hydrophobic components and gives some insight into the extreme heterogeneity of the surface of the coal particles. The validity of the film flotation method for characterizing coal particle surfaces was tested by conducting film flotation tests on wax-coated and also on oxidized Cambria No. 78 coal particles. Figure 5 gives the cumulative distribution of as-received, wax-coated, and oxidized Cambria No. 78 coal particles as a function of their critical wetting surface tension, as determined by film flotation response. The wetting parameters calculated from the film flotation results are given in Table I. It can be seen from the results given in Figure 5 and Table I that the cumulative distribution curve of wax-coated coal particles moves to a lower
Energy & Fuels, Vol. 4, No. 1, 1990 37
Surface Sites and Contact Angles on Coal Particles
Table 11. Contact Angles, 8, of Paraffin, Sulfur, Graphite, and Coals in Water Calculated from Film Flotation Results with 100 X 150 pm Particles Measured by Captive-Bubble (CB) or Sessile-Drop (SD)Methods on Flat Surfaces
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flat surface measurements particle film flotation mineral 8, deg method ref 8, deg 105 CB 7 paraffin wax 101 SD 17 86 87 sulfur 71 77 CB 18 graphite SD 19 90 87 Braztah coal SD 19 74 56 Somerset coal 57 CB 18 Cambria No. 33 62 CB 4 85 coal SD 4 91 SD 19 72 Geneva coal 81 SD 19 86 64 CB 18
SURFACE TENSION OF WETTING SOLUTION, mN/m
Figure 5. Cumulative percentage of lyophobic (hydropobic) particles for as-received,wax-coated, and oxidized (200 O C , for 19 h) Cambria No. 78 coal particles as a function of their wetting surface tension, determined from film flotation using aqueous methanol solutions.
wetting surface tension as compared with that of as-received coal particles. Also yc of wax-coated coal particles is less than that of as-received coal particles, and the values of .d and CXHBof wax-coated coal particles are higher than those of the as-received coal particles. This increase in hydrophobicity is due to replacing (covering up) the original hydrophilic and hydrophobic sites by a uniform paraffin-wax surface. On the other hand, the cumulative distribution curve of oxidized coal particles moves to higher wetting surface tensions and the value of 9, is higher than that of the as-received coal particles. The values of '6 and CXHBfor oxidized coal particles are smaller than those of as-received coal particles, indicating, as expected, that coal particles are more hydrophilic when they are oxidized (due to the formation of hydrophilic oxygen functional groups through the process of oxidation). These findings serve to confirm our film flotation approach for assessing wettability and surface site distributions. To further verify our new approach, the mean contact angles of particle assemblies were compared with the values reported for direct measurements on flat surfaces of bulk material. Since a flat surface of a bulk sample can be considered to be a composite of small particles with different contact angles, the contact angles obtained from both methods should agree with each other if our approach is correct and if the polishing procedures used in preparing the flat surfaces do not change their wetting properties significantly. The contact angles of sulfur, graphite, and coals in water were calculated from film flotation data using the foregoing procedures, and the results are presented in Table 11. This table also gives the contact angles measured by captive-bubble and/or sessile-drop methods on flat surfaces of the same samples in our laboratory. In (17) Olsen, D. A.; Moravec, R. W.; Osteraas, A. J. J . Phys. Chem. 1967, 71, 4464-4466. (18) Osoka, A. S. K. Ph.D. Thesis, University of California, Berkeley, 1983. (19) Rosenbaum, J. M. Ph.D. Thesis, University of California, Berkeley, 1981.
addition, the results for paraffin (wax-coatedcoal particles) are included in Table 11. From the results given in Table 11, it can be seen that the two sets of values are in quite good agreement with each other, especially for the more homogeneous minerals. This further supports the efficacy of the model for estimating the distribution of contact angles of particles from their critical wetting surface tension distribution. For coals the agreement is also reasonably good if one recalls that coal is highly heterogeneous, that it is very susceptible to oxidation, and that the polished surface may not be as same as the particle surface. Even when the same direct method was used, the contact angle determined with flat surfaces of the same coal sample by different researchers varies considerably, as shown in Table 11. However, the mean contact angle of coal particles calculated from their ycdistribution determined from film flotation results is very reproducible, due to its statistic mean.
Summary and Conclusions The distribution of the critical wetting surface tension of an assembly of particles was determined by the film flotation method, using a series of aqueous methanol solutions to vary the surface tension of the wetting liquid. The. mean value and the standard deviation of the distribution were shown to be reliable parameters for assessing the wettability and heterogeneity of coal. A model has been developed for estimating the contact angles of coal particles from the critical wetting surface tension distribution. The model can also be used to estimate the fraction of hydrophobic sites on the surface of the particles. Tests with homogeneous materials show that contact angles calculated with the model in conjunction with film flotation results are in good agreement with values obtained by captive-bubble or sessile-drop experiments. This model provides further insight into the heterogeneous nature of the coal surface and of the distribution of particles in an assembly of coal particles. Acknowledgment. We acknowledge the U S . Department of Energy, Pittsburgh Energy Technology Center, Grant DE-FG22-86PC90507, for the support of this research. Registry No. Methanol, 67-56-1; sulfur, 7704-34-9; graphite, 1782-42-5.
