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A new technique to determine wettability of powders-imbibition time measurements
Colloids and Surfaces, 21 (1986) 193-203 Elsevier Science Publishers B.V., Amsterdam -
193 Printed in The Netherlands
A New Technique to Determine Wettability of Powders-Imbibition Time Measurements* B.R.MOHAL and S. CHANDER** Mineral Processing Section, Department of Mineral Engineering, University, University Park, PA 16802 (U.S.A.)
The Pennsylvania State
(Received 30 January 1986; accepted in final form 15 May 1986)
ABSTRACT A new technique to determine wettability of powders is presented in this paper. In this technique the times for immersion of individual particles from air into liquids, defined as imbibition times, are measured. Through the measurements of imbibition times for a large number of particles, it is possible to determine homogeneity/heterogeneity of a powder sample with regard to its wetting behavior. Quartz particles give a very narrow distribution of imbibition times showing a homogeneous wetting surface whereas coal particles give a broad distribution indicative of the heterogeneous nature of its surface. The imbibition time measurement technique is considered to have certain advantages over other methods to determine wettability of powders in surfactant solutions. In this study we have also compared the imbibition times with the wetting rate measurements obtained by a modified Walker test.
INTRODUCTION
Several methods are available for determining wetting characteristics of solids. Each of the methods has been used in the past to obtain very valuable information for comparing wetting behavior of various solids in a given liquid or for comparing the wetting behavior of several liquids for a given solid. Each method measures a somewhat different aspect of wetting and therefore quantitative correlations between techniques have been difficult. A general approach is to use a method in which close similarity between the technique and the application (where results of wetting measurements are to be used) can be maintained. The objective of this investigation was to develop a method which could be used to determine the wetting characteristics of single particles in surfactant solutions. Wetting of individual particles is of importance in capture of dust particles by sprays of surfactant solutions. *Dedicated to the memory of Professor G.D. Parfitt. **To whom all correspondence may be addressed.
0166-6622/86/$03.50
0 1986 Elsevier Science Publishers B.V.
194 METHODS FOR DETERMINING WETTING -
A BRIEF REVIEW
The various methods available for measuring wetting characteristics of solids include: contact angle, immersion/sink time, bubble pick-up, induction time, capillary rise, and displacement method measurements. Some of the methods are briefly reviewed in the paragraphs that follow. The contact angle method is perhaps the most widely used. Although there are some difficulties associated with the measurement, and interpretation of contact angles, it is one of the most common methods of assessing wettability. When done properly, contact angle measurements are simple and they provide very valuable information. The main difficulty of the technique, particularly in investigating the wetting behavior of coal, is that the sample must be carefully selected and polished, for measurements to be reliable [ 11. Pellets made from pressing powders have been used by some investigators to estimate contact angles [ 2,3]. An immersion/sink time procedure was initially developed by Draves and Clarkson [4] to determine the effect of surfactants on wettability of cotton fibers. In this procedure, commonly known as Draves test, concentrations of the surfactant required to give a ‘sink time’ of 25 s is measured. The ‘sink time’ is measured as the time taken by a skein of cotton to sink after it has been immersed in the surfactant solution. Walker et al. [5] modified this test to determine the wetting ability of various surfactants for coal dusts. In this technique coal particles are dropped individually onto the surface of surfactant solutions of different concentrations. The most dilute concentration in which coal would sink instantaneously was determined. (The term ‘instantaneously’ was not defined by these authors. Results of the present study show that the wetting process could involve times of the order of milliseconds.) Other investigators modified the Walker technique in various ways. Some investigators report the time for wetting a given amount of powder [3,6,7] whereas others report wetting rate [ 81. Also, there are differences in the manner in which the time of wetting is measured. Some investigators measured the time for the last trace of coal to disappear [3,8] whereas others have determined the times for the vast majority of particles to sink [ 71. Our studies how that there is a gradual decrease in the wetting rate with time. Bartell and Osterhof [9] used a method based upon measurement of the pressure with which one liquid will displace another from a packed bed of particles. Another similar method is the capillary rise method developed by Crow1 and Wooldridge [lo]. In this method a certain amount of powder is placed in a cylindrical glass tube one end of which has a filter paper or porous disc. The filter paper-end of the tube is placed in contact with the surface of the surfactant solution and the height to which the liquid rises is measured as a function of time. Neither of these methods is very suitable for determining wetting of
195
powders in surfactant solutions due to surfactant depletion (through adsorption at the solid) effects at the liquid/gas interface. Some investigators have used indirect methods, such as bubble pick-up [ 111, induction times [ 121, calorimetry [ 13,141, etc., to assess wetting characteristics of powders. In this paper we have used a modified Walker technique to determine the rate of wetting of an assemblage of coal particles. In addition, we have developed a new technique of imbibition time measurements to determine wetting characteristics of individual particles in surfactant solutions. These techniques are discussed in the paragraphs that follow. THE WETTING RATE -
MODIFIED WALKER TECHNIQUE
The difficulties associated with measurement of wetting rate of powders were discussed in the previous section. Therefore, we have introduced yet another modification to the Walker method of assessing wettability in surfactant solutions. The amount of coal imbibed into the liquid, and hence wetted, was measured by placing the pan of a Cahn electrobalance beneath the liquid surface. The wetting rates were determined from measurements of the amount of coal wetted as a function of time. Preliminary experiments showed that the wetting rate decreased gradually with time; a similar observation was made by Zeller [ 151. This is perhaps the reason why some investigators have used somewhat arbitrary definitions of times at which wetting is considered to have occurred. Because of the dependence of wetting rate on time, we have used initial wetting rates to compare the wetting behavior of coals in different surfactant solutions. IMBIBITION TIME
For a highly heterogeneous substance like coal, the use of an averaged parameter to represent wetting characteristics is not very satisfactory and could in fact be misleading as shown later in this paper. Accordingly, a new technique to determine coal wettability was developed. In this technique, the imbibition time (‘t), defined as the time between the instant the particle first impacts at the liquid-gas interface and the instant when the particle detaches from the interface, is measured. Totally hydrophilic particles pass directly into the liquid phase, the only impeding force to their passage being the fluid drag force experienced by the particle as it enters a dense phase. Totally hydrophobic particles make contact with the liquid, lose their kinetic energy and remain at the interface and are not imbibed. Particles with intermediate wetting behavior may remain at the interface until the movement of the three-phase contact completely engulfs the particle and the particle sinks. The downward velocity of a particle as it passes through a gas/liquid interface is schematically shown in Fig. 1 for three kinds of particles. One may
196
TIME
Fig. 1. Velocity of a particle passing through a gas-liquid interface versus time.
consider that the particle impacts the liquid surface with a velocity equal to its terminal settling velocity in air, (Ut)a+ Type I particles are the particles which are completely wetted by the liquid. The time at which such a particle is completely imbibed into the liquid is shown by a vertical arrow corresponding to an imbibition time of Z.. The particle in this case can enter the liquid at a velocity greater than the terminal settling velocity of the paricle in the liquid, (Ut)iiqui& The quantity ?* is expected to depend on the kinetic energy of the particle at the time of impact at the gas/liquid interface. Type II particles are the completely hydrophobic particles which are not wetted by the liquid. These particles lose their kinetic energy after impact and remain at the interface. Type III particles are the particles with intermediate wettability which are imbibed through gradual movement of the three-phase perimeter. The movement of the perimeter depends upon factors such as the adsorption of surfactants at various interfaces and the magnitude of interfacial tensions. Although an induction time, 7, can be defined to represent the time for which the particle remains at the interface after impact occurs and before imbibition starts, as defined by some investigators [12], experimental determination of this parameter is difficult. Since a hydrophilic particle of diameter 200 pm is imbibed in about 0.5 ms, the difference between ? and 7 is expected to be of the order 1 ms or less. The experimental values of ‘i in coals/surfactant systems are in the range of 0.02-200 s. Therefore, imbibition times may be taken as a reasonable measure of induction times. EXPERIMENTAL
METHODS AND MATERIALS
The wetting rates were determined by measuring the amount of coal wetted as a function of time. The weight of coal wetted was measured by placing the pan of a recording electrobalance, Model Cahn 2000, beneath the surface of
197
the liquid. The required amount of coal sample, typically 50 mg, was placed at the interface with a vibro-feeder. The feeder was held over the surface of the solution at a constant height of 1 cm. The initial wetting rate was determined from the plot of the weight of coal immersed versus time. An average of at least four experiments was taken as the initial wetting rate. The coefficient of variation in these experiments was less than 0.08. A Spin Physics Model SP2000 High Speed Motion Analysis System was used to photograph the sequence of events during imbibition of particles into various solutions. The system is capable of recording images up to 12000 frames/s. Therefore, motion taking place at intervals of 0.083 ms can easily be distinguished. Imbibition times, as defined in the previous section, were determined from the recorded tape of events. The liquid was placed in a 1 cm wide spectrophotometer cell. Individual coal particles were dropped onto the liquid surface from a height of about 23 cm through a disposable pipet. The pipet ensured that the particles fall onto the specified viewing area, and also ensured by its length that all particles has the same kinetic energy before impinging on the liquid. The imbibition time was measured from the instant the particle appeared at the interface to the instant it detached from the liquid interface. To obtain the distribution of imbibition times approximately 75-100 particles were observed. A hand-sorted sample of + 2 in quartz was crushed and the + 28 mesh fraction was acid leached and washed with distilled water until there was no change in pH of the wash water. The +28 mesh quartz was then wet ground in a porcelain ball mill. The material was dried at room temperature and stored in a dessicator until use. The three coal samples used in this investigation were: a HVA-bituminous coal from the Upper Freeport seam obtained from the Penn State Data Bank (PSOC 1361p), a sub-bituminous A coal from Wyoming also obtained from the Penn State Data Bank (PSOC 512), and an anthracite sample obtained from Reading, PA. The proximate and ultimate analysis of these sample are given in Table 1. The coal samples were crushed and screened to obtain a 250 x 150 pm fraction which were used for wetting tests. The coal samples were stored in a dessicator at all times. Methanol and NaCl used were obtained as Bakers Analyzed Reagents. The surfactant Triton N-101 (a polyethoxylated nonylphenol with an HLB of 13.4) was obtained from Rohm & Haas. Distilled water from a tin-lined still of specific resistivity greater than 2 Mohm was used in all the experiments conducted at 25°C. RESULTS AND DISCUSSION
Wetting Rates The initial wetting rates for coal as determined by the modified Walker technique are given as a function of surfactant concentration in Fig. 2. For Triton
198 TABLE 1 Proximate and ultimate analysis of coal samples HVA-bituminous
Sub-bituminous
3.2%
Hz0
Proximate analysis (Dry basis%) Ash VM FC Ultimate analysis (DAF basis%) C H N S 0 Cl
Anthracite
10.24%
1.5%
8.47 38.16 53.37
6.64 40.00 53.36
8.2 5.3 86.4
81.94 5.70 1.54 2.12 8.7 -
75.42 5.15 0.84 0.35 18.22 0.02
94.3
-
2.3 0.9 0.6 1.9
N-101, the largest wetting rates are observed for HVA-bituminous coal followed by anthracite and sub-bituminous coals. The results show that below approximately 10e5 M surfactant concentration the wetting rate is zero. The surface tension of Triton N-101 at this limiting concentration was observed to be 40 dyn cm- ‘. The close similarity of this surface tension and the critical surface tension of wetting of coals reported by Parekh and Aplan [ 161 is coincidental, however. We expect that the limiting surfactant concentration for zero wetting rate or the corresponding limiting surface tension will be dependent upon the surfactant type due to adsorption at the coal/surfactant solution interface. The significance of adsorption at the coal/solution interface was P T g u.i + d ks
30
20
E w 2 2
10
2 cl2
0
TRITON
N-101
CONCENTRATION.
mole/liter
Fig. 2. Initial wetting rate for a HVA-bituminous (n), an anthracite (A), and a sub-bituminous (0) coal in aqueous solutions of Triton N-101. The vertical arrow represents the concentration at which imbibition time measurements were made.
199
TIME,
see
Fig. 3. Imbibition times for a wet ground quartz in aqueous 2 x 10m3 M NaCl (0) bituminous coal in a 60 vol. % methanol-water mixture (A).
and a HVA-
discussed by Glanville and Wightman [ 171. Our studies also show that the adsorption at the coal/water interface is a strong function of coal rank and surfactant type [18]. Due to the scatter in the data in Fig. 2, no definite conclusions can be drawn regarding the effect of micelle formation at concentration exceeding the CMC, 3.5 x 10e5 M. Imbibition times The imbibition times for a HVA-bituminous coal in a 60% methanol-water mixture are given in Fig. 3. The methanol-water mixture was used because the wetting rate measurements, presented in Fig. 1, show that coal particles require a surface tension of the liquid to be (approximately) less than 35 dyn cm-’ before they can be wetted. Similar observations were made by other investigators working with different coals [6,8]. For comparison with a hydrophilic material, times for imbibition of quartz in 2 x 10v3 M NaCl are also given in Fig. 3. Quartz being a relatively homogeneous hydrophilic solid gives a very sharp peak on the frequency plot in Fig. 3. The total distance moved by a particle for it to be completely imbibed, after initial contact with the liquid surface, is equal to its diameter which is about 200 pm for the particles investigated. The time required for the particle to move this distance in air at its terminal settling velocity is about 0.00016 s and, similarly, the time required for the particle to move the same distance in water at its terminal settling velocity is about 0.0082 s. The mode of the imbibition time distribution plot for quartz in Fig. 4 is 0.0005 s which lies in between the two limits defined above. These results may be interpreted to mean that the quartz particle experiences an increased drag force immediately after impacting the li uid surface. The mode of the distribution time for quartz is less than &&EK6 which is interpreted to mean that the particle enters the liquid phase at a velocity greater than the terminal settling velocity of the particle in the liquid. Direct
TIME.
set
Fig. 4. Cumulative imbibition times for a wet ground quartz in aqueous 2 x 10m3M NaCl (0) and a HVA-bituminous coal in a 60 vol. % methanol-water mixture (A)
measurements of the velocities of a quartz particle after having been imbibed into the liquid confirm this to be true. The imbibition behavior of coal particles is quite different, however. Within the same coal sample, a large variation in imbibition times is observed. This was the reason for plotting the imbibition time data in Fig. 3 with times on a logarithmic scale. In this figure time intervals on ,/2 scale were used with the geometric mean of the interval as the imbibition time of the interval, The wide distribution in imbibition times is considered to reflect the heterogeneous nature of the coal surface. Several investigators have suggested that coal surfaces consist of a variety of hydrophilic and hydrophobic sites, but no techniques are available to quantitatively determine their relative proportions. We propose that the imbibition time distribution curves provide a means to assess the heterogeneities in wetting properties of coal. The trimodal distribution shown in Fig. 3 suggests that this coal can be considered to consists of three kinds of particles - a fast sinking fraction (least hydrophobic), a slow sinking fraction (most hydrophobic), and an intermediate fraction. The fast sinking fraction is most hydrophilic and its imbibition times in 60% methanol-water mixture are comparable to the imbibition times for quartz in 2 x 10d3 M NaCl. That is, the driving force to wet the fast sinking fraction of coal in 60% methanol-water mixture is comparable to the driving force to wet quartz in 2 x 10e3 M NaCl. The slow sinking fraction represents a very hydrophobic set of particles. Highly hydrophobic particles of certain coal macerals with a water contact angle of about 120"have been measured by Aplan and associates in recent studies at The Pennsylvania State University [191.The particles with intermediate set of imbibition times could either represent locked particles or particles of intermediate wettability. Additional studies are being conducted to further characterize the three coal fractions. The imbibition time data for quartz and coal are also plotted as cumulative percent of particles imbibed as a function of time on the logarithmic scale, Fig. 4. The relative proportions of fast-sinking, slow-sinking and intermediate frac-
201
TIME,
set
Fig. 5. Cumulative imbibition times for a HVA-Bituminous ( q), an anthracite (A), and a subbituminous (0) coal in a 4 x 10d4 M solution of Triton N-101.
tions can be easily determined from such plots. The slope of the curves in Fig. 4 can be taken as a measure of the degree of homogeneity in the system. A large slope implies that the system is homogeneous as was observed for quartz and a small slope is an indication of a heterogeneous system as observed for coal. Both physical and chemical heterogeneities are expected to affect the imbibition time. Since crushed quartz with irregular shaped particles gave imbibition times in a narrow range, we believe that chemical heterogeneities predominantly contribute to the large variation in imbibition times for coal. The cumulative imbibition time plots for wetting of three different coals in 4 x 1O-4 M Triton N-101 are given in Fig. 5. This concentration is marked by a vertical arrow in Fig. 2 from which wetting rates can be obtained. A direct correlation between imbibition times and wetting rates is observed. The imbibition times for the sub-bituminous coal are several orders of magnitude greater than the HVA-bituminous coal indicating a very high sensitivity of these measurements. A unimodal cumulative imbibition time curve can be conveniently described by two parameters: tso and o. The time parameter, t5,,, is defined such that 50% of the particles are imbibed in a time less than tsO(the remaining 50% imbibing at times greater than &). The homogeneity index, a, which is equal to the slope of the cumulative imbibition time curve at tso, is a measure of the homogeneity of the sample; a large slope is indicative of a homogeneous sample whereas a small slope can be considered to represent a heterogeneous sample. As stated before, both chemical and physical heterogeneities may contribute to the homogeneity index. For polymodal distributions, a set of t5,, and TVvalues are needed to describe the wetting behavior of the coal. The data in Figs 4 and 5 were used to calculate the tsO and CTvalues for various coals in different solutions and the calculated values are listed in Table 2. The results clearly show that coals consist of a mixture of particles which differ in their wetting behavior. The large value of B for quartz suggests that the sample is fairly homogeneous in its wetting behavior even though the crushed particles were irregular
202 TABLE 2 The time parameter,
too, and the homogeneity
index, u, for various coals in different
System
t50 (s)
Quartz/water HVA-bit./GO% methanol
0.0005 0.0009 0.0021 0.0600 0.116 35.7 0.71 0.33 98.5
HVA-bit./N-101 Anthracite/N-l01 Sub-bit/N-l01
solutions Type of distribution
6.42 5.5 1.71 1.35 1.02 1.67 0.635 0.81 0.67
Unimodal Trimodal
Bimodal Bimodal Bimodal
in shape. A fraction of the HVA-bituminous coal is readily wetted by 60% methanol-water solution ( &,O= 0.0009 s, o = 5.5) whereas the remainder two fractions have longer tsOand smaller o values. The change in tsOand o in surfactant solutions is due to specific coal-surfactant interactions. SUMMARY
A new technique to determine wettability of powders is presented in this paper. In this technique the times for imbibition of particles from air into liquids are measured. The imbibition times for hydrophilic particles are of the order,of half a millisecond whereas they are several orders of magnitude higher (ranging from 0.02 to 200 s or more) for hydrophobic coal particles. The imbibition times depend upon surfactant concentration and coal type which is not surprising because wetting is a three-phase phenomena. The most convenient way to represent the data is to plot cumulative percent particles imbibed as a function of time plotted on a logarithmic scale. The wetting behavior for a solid-surfactant system can be represented by two parameters. One is a time parmeter, .&, which is defined such that 50% of the particles are imbibed in a time less than t5,,, and the second is a homogeneity index, o, which is equal to the slope of the cumulative imbibition time curve at t5,,. The wetting characteristics of a mixture of particles can be described by a set of such parameters, one set to represent each component in the mixture. ACKNOWLEDGEMENTS
The authors acknowledge the support from the Mineral Institutes Program under Grant No. G1135142 from the Bureau of Mines, U.S. Department of the Interior, as part of the Generic Mineral Technology Center for Respirable Dust,
203
The Pennsylvania State University. The support from the National Science Foundation, under equipment Grant No. CPE-8406276, is also acknowledged.
REFERENCES
6 7 8 9 10 11 12 13 14 15 16 17 18 19
J.A. Gutierrez-Rodriguez, R.J. Purcell Jr, and F.F. ApIan, Colloids Surfaces, 12 (1984) l-25. N.W.F. Kossen and P.M. Heertjes, Chem. Eng. Sci, 20 (1965) 593-599. N. Feldstein, BuMines OFR 127-81 (1981) 47 pp., NTIS PB 82-104654. C.Z. Draves and R.G.Clarkson, Am. Dyest. Rep., 20 (March 1931) 109-116. P.L. WaIker, E.E. Peterson and C.C. Wright, Ind. Eng. Chem., 44 (10) (October 1952) 2389-2392. M.M. Papic and A.D. McIntyre, Coal Age, 78 (June 1973) 85-87. J.A. Kost, G.A. Shirey and C.T. Ford, BuMines OFR 30-82 (1980) 188 pp., NTIS PB 82183344. FJ.0. GlanviIle and J.P. Wightman, Fuel, 58 (1979) 819-822. F.E. Bartell and H.J. Osterhof, Ind. Eng. Chem., 19(11) (1927) 1277-1280. V.T. Crow1 and W.D.S. Wooldridge, SC1 Monograph, Vol. 25, Society of Chemical Industry, London, 1967, pp. 200-212. J.A. Finch and G.W. Smith, Can. Metall. Q., 14(l) (1975) 47-51. V.A. Glembotsky, Izv. Akad. Nauk SSSR OM. Tekh. Nauk, (1953) 1524-1531 (in Russian). A.C. Zettlemoyer, J. CoIIoid Sci., 28 d( 1968) 343. R.W. HeaIy and D.W. Fuerstenau, J. Colloid Sci., 20 (1965) 376. W.H. Zeller, BuMines RI 8815 (1983) 21 pp. B.K. Parekh and F.F. Aplan, Rec. Dev. Sep. Sci., 4 (1978) 107. J.O. Glanvihe and J.P. Wightman, Fuel, 59 (1980) 557-562. B.R. Mohal, S. Chander and F.F. Aplan, unpublished work. B.J. Arnold and F.F. ApIan, unpublished work.
193 Printed in The Netherlands
A New Technique to Determine Wettability of Powders-Imbibition Time Measurements* B.R.MOHAL and S. CHANDER** Mineral Processing Section, Department of Mineral Engineering, University, University Park, PA 16802 (U.S.A.)
The Pennsylvania State
(Received 30 January 1986; accepted in final form 15 May 1986)
ABSTRACT A new technique to determine wettability of powders is presented in this paper. In this technique the times for immersion of individual particles from air into liquids, defined as imbibition times, are measured. Through the measurements of imbibition times for a large number of particles, it is possible to determine homogeneity/heterogeneity of a powder sample with regard to its wetting behavior. Quartz particles give a very narrow distribution of imbibition times showing a homogeneous wetting surface whereas coal particles give a broad distribution indicative of the heterogeneous nature of its surface. The imbibition time measurement technique is considered to have certain advantages over other methods to determine wettability of powders in surfactant solutions. In this study we have also compared the imbibition times with the wetting rate measurements obtained by a modified Walker test.
INTRODUCTION
Several methods are available for determining wetting characteristics of solids. Each of the methods has been used in the past to obtain very valuable information for comparing wetting behavior of various solids in a given liquid or for comparing the wetting behavior of several liquids for a given solid. Each method measures a somewhat different aspect of wetting and therefore quantitative correlations between techniques have been difficult. A general approach is to use a method in which close similarity between the technique and the application (where results of wetting measurements are to be used) can be maintained. The objective of this investigation was to develop a method which could be used to determine the wetting characteristics of single particles in surfactant solutions. Wetting of individual particles is of importance in capture of dust particles by sprays of surfactant solutions. *Dedicated to the memory of Professor G.D. Parfitt. **To whom all correspondence may be addressed.
0166-6622/86/$03.50
0 1986 Elsevier Science Publishers B.V.
194 METHODS FOR DETERMINING WETTING -
A BRIEF REVIEW
The various methods available for measuring wetting characteristics of solids include: contact angle, immersion/sink time, bubble pick-up, induction time, capillary rise, and displacement method measurements. Some of the methods are briefly reviewed in the paragraphs that follow. The contact angle method is perhaps the most widely used. Although there are some difficulties associated with the measurement, and interpretation of contact angles, it is one of the most common methods of assessing wettability. When done properly, contact angle measurements are simple and they provide very valuable information. The main difficulty of the technique, particularly in investigating the wetting behavior of coal, is that the sample must be carefully selected and polished, for measurements to be reliable [ 11. Pellets made from pressing powders have been used by some investigators to estimate contact angles [ 2,3]. An immersion/sink time procedure was initially developed by Draves and Clarkson [4] to determine the effect of surfactants on wettability of cotton fibers. In this procedure, commonly known as Draves test, concentrations of the surfactant required to give a ‘sink time’ of 25 s is measured. The ‘sink time’ is measured as the time taken by a skein of cotton to sink after it has been immersed in the surfactant solution. Walker et al. [5] modified this test to determine the wetting ability of various surfactants for coal dusts. In this technique coal particles are dropped individually onto the surface of surfactant solutions of different concentrations. The most dilute concentration in which coal would sink instantaneously was determined. (The term ‘instantaneously’ was not defined by these authors. Results of the present study show that the wetting process could involve times of the order of milliseconds.) Other investigators modified the Walker technique in various ways. Some investigators report the time for wetting a given amount of powder [3,6,7] whereas others report wetting rate [ 81. Also, there are differences in the manner in which the time of wetting is measured. Some investigators measured the time for the last trace of coal to disappear [3,8] whereas others have determined the times for the vast majority of particles to sink [ 71. Our studies how that there is a gradual decrease in the wetting rate with time. Bartell and Osterhof [9] used a method based upon measurement of the pressure with which one liquid will displace another from a packed bed of particles. Another similar method is the capillary rise method developed by Crow1 and Wooldridge [lo]. In this method a certain amount of powder is placed in a cylindrical glass tube one end of which has a filter paper or porous disc. The filter paper-end of the tube is placed in contact with the surface of the surfactant solution and the height to which the liquid rises is measured as a function of time. Neither of these methods is very suitable for determining wetting of
195
powders in surfactant solutions due to surfactant depletion (through adsorption at the solid) effects at the liquid/gas interface. Some investigators have used indirect methods, such as bubble pick-up [ 111, induction times [ 121, calorimetry [ 13,141, etc., to assess wetting characteristics of powders. In this paper we have used a modified Walker technique to determine the rate of wetting of an assemblage of coal particles. In addition, we have developed a new technique of imbibition time measurements to determine wetting characteristics of individual particles in surfactant solutions. These techniques are discussed in the paragraphs that follow. THE WETTING RATE -
MODIFIED WALKER TECHNIQUE
The difficulties associated with measurement of wetting rate of powders were discussed in the previous section. Therefore, we have introduced yet another modification to the Walker method of assessing wettability in surfactant solutions. The amount of coal imbibed into the liquid, and hence wetted, was measured by placing the pan of a Cahn electrobalance beneath the liquid surface. The wetting rates were determined from measurements of the amount of coal wetted as a function of time. Preliminary experiments showed that the wetting rate decreased gradually with time; a similar observation was made by Zeller [ 151. This is perhaps the reason why some investigators have used somewhat arbitrary definitions of times at which wetting is considered to have occurred. Because of the dependence of wetting rate on time, we have used initial wetting rates to compare the wetting behavior of coals in different surfactant solutions. IMBIBITION TIME
For a highly heterogeneous substance like coal, the use of an averaged parameter to represent wetting characteristics is not very satisfactory and could in fact be misleading as shown later in this paper. Accordingly, a new technique to determine coal wettability was developed. In this technique, the imbibition time (‘t), defined as the time between the instant the particle first impacts at the liquid-gas interface and the instant when the particle detaches from the interface, is measured. Totally hydrophilic particles pass directly into the liquid phase, the only impeding force to their passage being the fluid drag force experienced by the particle as it enters a dense phase. Totally hydrophobic particles make contact with the liquid, lose their kinetic energy and remain at the interface and are not imbibed. Particles with intermediate wetting behavior may remain at the interface until the movement of the three-phase contact completely engulfs the particle and the particle sinks. The downward velocity of a particle as it passes through a gas/liquid interface is schematically shown in Fig. 1 for three kinds of particles. One may
196
TIME
Fig. 1. Velocity of a particle passing through a gas-liquid interface versus time.
consider that the particle impacts the liquid surface with a velocity equal to its terminal settling velocity in air, (Ut)a+ Type I particles are the particles which are completely wetted by the liquid. The time at which such a particle is completely imbibed into the liquid is shown by a vertical arrow corresponding to an imbibition time of Z.. The particle in this case can enter the liquid at a velocity greater than the terminal settling velocity of the paricle in the liquid, (Ut)iiqui& The quantity ?* is expected to depend on the kinetic energy of the particle at the time of impact at the gas/liquid interface. Type II particles are the completely hydrophobic particles which are not wetted by the liquid. These particles lose their kinetic energy after impact and remain at the interface. Type III particles are the particles with intermediate wettability which are imbibed through gradual movement of the three-phase perimeter. The movement of the perimeter depends upon factors such as the adsorption of surfactants at various interfaces and the magnitude of interfacial tensions. Although an induction time, 7, can be defined to represent the time for which the particle remains at the interface after impact occurs and before imbibition starts, as defined by some investigators [12], experimental determination of this parameter is difficult. Since a hydrophilic particle of diameter 200 pm is imbibed in about 0.5 ms, the difference between ? and 7 is expected to be of the order 1 ms or less. The experimental values of ‘i in coals/surfactant systems are in the range of 0.02-200 s. Therefore, imbibition times may be taken as a reasonable measure of induction times. EXPERIMENTAL
METHODS AND MATERIALS
The wetting rates were determined by measuring the amount of coal wetted as a function of time. The weight of coal wetted was measured by placing the pan of a recording electrobalance, Model Cahn 2000, beneath the surface of
197
the liquid. The required amount of coal sample, typically 50 mg, was placed at the interface with a vibro-feeder. The feeder was held over the surface of the solution at a constant height of 1 cm. The initial wetting rate was determined from the plot of the weight of coal immersed versus time. An average of at least four experiments was taken as the initial wetting rate. The coefficient of variation in these experiments was less than 0.08. A Spin Physics Model SP2000 High Speed Motion Analysis System was used to photograph the sequence of events during imbibition of particles into various solutions. The system is capable of recording images up to 12000 frames/s. Therefore, motion taking place at intervals of 0.083 ms can easily be distinguished. Imbibition times, as defined in the previous section, were determined from the recorded tape of events. The liquid was placed in a 1 cm wide spectrophotometer cell. Individual coal particles were dropped onto the liquid surface from a height of about 23 cm through a disposable pipet. The pipet ensured that the particles fall onto the specified viewing area, and also ensured by its length that all particles has the same kinetic energy before impinging on the liquid. The imbibition time was measured from the instant the particle appeared at the interface to the instant it detached from the liquid interface. To obtain the distribution of imbibition times approximately 75-100 particles were observed. A hand-sorted sample of + 2 in quartz was crushed and the + 28 mesh fraction was acid leached and washed with distilled water until there was no change in pH of the wash water. The +28 mesh quartz was then wet ground in a porcelain ball mill. The material was dried at room temperature and stored in a dessicator until use. The three coal samples used in this investigation were: a HVA-bituminous coal from the Upper Freeport seam obtained from the Penn State Data Bank (PSOC 1361p), a sub-bituminous A coal from Wyoming also obtained from the Penn State Data Bank (PSOC 512), and an anthracite sample obtained from Reading, PA. The proximate and ultimate analysis of these sample are given in Table 1. The coal samples were crushed and screened to obtain a 250 x 150 pm fraction which were used for wetting tests. The coal samples were stored in a dessicator at all times. Methanol and NaCl used were obtained as Bakers Analyzed Reagents. The surfactant Triton N-101 (a polyethoxylated nonylphenol with an HLB of 13.4) was obtained from Rohm & Haas. Distilled water from a tin-lined still of specific resistivity greater than 2 Mohm was used in all the experiments conducted at 25°C. RESULTS AND DISCUSSION
Wetting Rates The initial wetting rates for coal as determined by the modified Walker technique are given as a function of surfactant concentration in Fig. 2. For Triton
198 TABLE 1 Proximate and ultimate analysis of coal samples HVA-bituminous
Sub-bituminous
3.2%
Hz0
Proximate analysis (Dry basis%) Ash VM FC Ultimate analysis (DAF basis%) C H N S 0 Cl
Anthracite
10.24%
1.5%
8.47 38.16 53.37
6.64 40.00 53.36
8.2 5.3 86.4
81.94 5.70 1.54 2.12 8.7 -
75.42 5.15 0.84 0.35 18.22 0.02
94.3
-
2.3 0.9 0.6 1.9
N-101, the largest wetting rates are observed for HVA-bituminous coal followed by anthracite and sub-bituminous coals. The results show that below approximately 10e5 M surfactant concentration the wetting rate is zero. The surface tension of Triton N-101 at this limiting concentration was observed to be 40 dyn cm- ‘. The close similarity of this surface tension and the critical surface tension of wetting of coals reported by Parekh and Aplan [ 161 is coincidental, however. We expect that the limiting surfactant concentration for zero wetting rate or the corresponding limiting surface tension will be dependent upon the surfactant type due to adsorption at the coal/surfactant solution interface. The significance of adsorption at the coal/solution interface was P T g u.i + d ks
30
20
E w 2 2
10
2 cl2
0
TRITON
N-101
CONCENTRATION.
mole/liter
Fig. 2. Initial wetting rate for a HVA-bituminous (n), an anthracite (A), and a sub-bituminous (0) coal in aqueous solutions of Triton N-101. The vertical arrow represents the concentration at which imbibition time measurements were made.
199
TIME,
see
Fig. 3. Imbibition times for a wet ground quartz in aqueous 2 x 10m3 M NaCl (0) bituminous coal in a 60 vol. % methanol-water mixture (A).
and a HVA-
discussed by Glanville and Wightman [ 171. Our studies also show that the adsorption at the coal/water interface is a strong function of coal rank and surfactant type [18]. Due to the scatter in the data in Fig. 2, no definite conclusions can be drawn regarding the effect of micelle formation at concentration exceeding the CMC, 3.5 x 10e5 M. Imbibition times The imbibition times for a HVA-bituminous coal in a 60% methanol-water mixture are given in Fig. 3. The methanol-water mixture was used because the wetting rate measurements, presented in Fig. 1, show that coal particles require a surface tension of the liquid to be (approximately) less than 35 dyn cm-’ before they can be wetted. Similar observations were made by other investigators working with different coals [6,8]. For comparison with a hydrophilic material, times for imbibition of quartz in 2 x 10v3 M NaCl are also given in Fig. 3. Quartz being a relatively homogeneous hydrophilic solid gives a very sharp peak on the frequency plot in Fig. 3. The total distance moved by a particle for it to be completely imbibed, after initial contact with the liquid surface, is equal to its diameter which is about 200 pm for the particles investigated. The time required for the particle to move this distance in air at its terminal settling velocity is about 0.00016 s and, similarly, the time required for the particle to move the same distance in water at its terminal settling velocity is about 0.0082 s. The mode of the imbibition time distribution plot for quartz in Fig. 4 is 0.0005 s which lies in between the two limits defined above. These results may be interpreted to mean that the quartz particle experiences an increased drag force immediately after impacting the li uid surface. The mode of the distribution time for quartz is less than &&EK6 which is interpreted to mean that the particle enters the liquid phase at a velocity greater than the terminal settling velocity of the particle in the liquid. Direct
TIME.
set
Fig. 4. Cumulative imbibition times for a wet ground quartz in aqueous 2 x 10m3M NaCl (0) and a HVA-bituminous coal in a 60 vol. % methanol-water mixture (A)
measurements of the velocities of a quartz particle after having been imbibed into the liquid confirm this to be true. The imbibition behavior of coal particles is quite different, however. Within the same coal sample, a large variation in imbibition times is observed. This was the reason for plotting the imbibition time data in Fig. 3 with times on a logarithmic scale. In this figure time intervals on ,/2 scale were used with the geometric mean of the interval as the imbibition time of the interval, The wide distribution in imbibition times is considered to reflect the heterogeneous nature of the coal surface. Several investigators have suggested that coal surfaces consist of a variety of hydrophilic and hydrophobic sites, but no techniques are available to quantitatively determine their relative proportions. We propose that the imbibition time distribution curves provide a means to assess the heterogeneities in wetting properties of coal. The trimodal distribution shown in Fig. 3 suggests that this coal can be considered to consists of three kinds of particles - a fast sinking fraction (least hydrophobic), a slow sinking fraction (most hydrophobic), and an intermediate fraction. The fast sinking fraction is most hydrophilic and its imbibition times in 60% methanol-water mixture are comparable to the imbibition times for quartz in 2 x 10d3 M NaCl. That is, the driving force to wet the fast sinking fraction of coal in 60% methanol-water mixture is comparable to the driving force to wet quartz in 2 x 10e3 M NaCl. The slow sinking fraction represents a very hydrophobic set of particles. Highly hydrophobic particles of certain coal macerals with a water contact angle of about 120"have been measured by Aplan and associates in recent studies at The Pennsylvania State University [191.The particles with intermediate set of imbibition times could either represent locked particles or particles of intermediate wettability. Additional studies are being conducted to further characterize the three coal fractions. The imbibition time data for quartz and coal are also plotted as cumulative percent of particles imbibed as a function of time on the logarithmic scale, Fig. 4. The relative proportions of fast-sinking, slow-sinking and intermediate frac-
201
TIME,
set
Fig. 5. Cumulative imbibition times for a HVA-Bituminous ( q), an anthracite (A), and a subbituminous (0) coal in a 4 x 10d4 M solution of Triton N-101.
tions can be easily determined from such plots. The slope of the curves in Fig. 4 can be taken as a measure of the degree of homogeneity in the system. A large slope implies that the system is homogeneous as was observed for quartz and a small slope is an indication of a heterogeneous system as observed for coal. Both physical and chemical heterogeneities are expected to affect the imbibition time. Since crushed quartz with irregular shaped particles gave imbibition times in a narrow range, we believe that chemical heterogeneities predominantly contribute to the large variation in imbibition times for coal. The cumulative imbibition time plots for wetting of three different coals in 4 x 1O-4 M Triton N-101 are given in Fig. 5. This concentration is marked by a vertical arrow in Fig. 2 from which wetting rates can be obtained. A direct correlation between imbibition times and wetting rates is observed. The imbibition times for the sub-bituminous coal are several orders of magnitude greater than the HVA-bituminous coal indicating a very high sensitivity of these measurements. A unimodal cumulative imbibition time curve can be conveniently described by two parameters: tso and o. The time parameter, t5,,, is defined such that 50% of the particles are imbibed in a time less than tsO(the remaining 50% imbibing at times greater than &). The homogeneity index, a, which is equal to the slope of the cumulative imbibition time curve at tso, is a measure of the homogeneity of the sample; a large slope is indicative of a homogeneous sample whereas a small slope can be considered to represent a heterogeneous sample. As stated before, both chemical and physical heterogeneities may contribute to the homogeneity index. For polymodal distributions, a set of t5,, and TVvalues are needed to describe the wetting behavior of the coal. The data in Figs 4 and 5 were used to calculate the tsO and CTvalues for various coals in different solutions and the calculated values are listed in Table 2. The results clearly show that coals consist of a mixture of particles which differ in their wetting behavior. The large value of B for quartz suggests that the sample is fairly homogeneous in its wetting behavior even though the crushed particles were irregular
202 TABLE 2 The time parameter,
too, and the homogeneity
index, u, for various coals in different
System
t50 (s)
Quartz/water HVA-bit./GO% methanol
0.0005 0.0009 0.0021 0.0600 0.116 35.7 0.71 0.33 98.5
HVA-bit./N-101 Anthracite/N-l01 Sub-bit/N-l01
solutions Type of distribution
6.42 5.5 1.71 1.35 1.02 1.67 0.635 0.81 0.67
Unimodal Trimodal
Bimodal Bimodal Bimodal
in shape. A fraction of the HVA-bituminous coal is readily wetted by 60% methanol-water solution ( &,O= 0.0009 s, o = 5.5) whereas the remainder two fractions have longer tsOand smaller o values. The change in tsOand o in surfactant solutions is due to specific coal-surfactant interactions. SUMMARY
A new technique to determine wettability of powders is presented in this paper. In this technique the times for imbibition of particles from air into liquids are measured. The imbibition times for hydrophilic particles are of the order,of half a millisecond whereas they are several orders of magnitude higher (ranging from 0.02 to 200 s or more) for hydrophobic coal particles. The imbibition times depend upon surfactant concentration and coal type which is not surprising because wetting is a three-phase phenomena. The most convenient way to represent the data is to plot cumulative percent particles imbibed as a function of time plotted on a logarithmic scale. The wetting behavior for a solid-surfactant system can be represented by two parameters. One is a time parmeter, .&, which is defined such that 50% of the particles are imbibed in a time less than t5,,, and the second is a homogeneity index, o, which is equal to the slope of the cumulative imbibition time curve at t5,,. The wetting characteristics of a mixture of particles can be described by a set of such parameters, one set to represent each component in the mixture. ACKNOWLEDGEMENTS
The authors acknowledge the support from the Mineral Institutes Program under Grant No. G1135142 from the Bureau of Mines, U.S. Department of the Interior, as part of the Generic Mineral Technology Center for Respirable Dust,
203
The Pennsylvania State University. The support from the National Science Foundation, under equipment Grant No. CPE-8406276, is also acknowledged.
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