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The role of surfactant adsorption in the improved dewatering of fine coal
Fuel 78 (1999) 501–506
The role of surfactant adsorption in the improved dewatering of fine coal Bimal P. Singh* Mineral Beneficiation Division, Regional Research Laboratory, (Council of Scientific and Industrial Research), Bhubaneswar 751, Orissa 751013, India Received 29 August 1997; received in revised form 5 August 1998; accepted 16 September 1998
Abstract This paper describes the adsorption kinetics and isotherms for cationic and anionic surfactants on coal surfaces. Both types of surfactants adsorb significantly rapidly; although the cationic surfactant achieved much higher levels of adsorption than the anionic one. The adsorption isotherms are of typical Langmuir type and employed for calculation of free energies. The results indicate that adsorption occurs through hydrophobic interaction in addition to electrostatic attraction between the surfactant molecules and the coal surface. Surfactant adsorption is shown to be a significant factor in the improved dewatering of coal particles. These improvements have been attributed to the changes which surfactant adsorption causes in the wetting characteristics of the coal surfaces. The results obtained are evaluated in terms of surface effects and by reference Laplace–Young equation. Furthermore, this equation has been used to quantify dewatering phenomena. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Adsorption; Surfactant; Dewatering; Kinetics; Wetting; Surface tension
1. Introduction The dewatering of aqueous mineral slurries such as iron ore [1–3], clay [4,5], gypsum [6] and coal [7–9] has received considerable attention as the problems involved in both bulk formation and the dewatering become more difficult with decreasing particle size. The use of chemical additives as dewatering aid to the mining and mineral processing has become increasing important in recent years. The advent of sophisticated equipment and increased awareness of energy conservation has accelerated this process. Such reagents provide lower filter cake moisture resulting in reduced thermal drying costs and transportation, improved product quality, increased calorific value and minimised freezing during winter storage. This communication is a supplement to previous works [9,10] which showed that the addition of a small amount of appropriate surfactant to the filtration dewatering of fine clean coal improved dewatering significantly. A direct correlation between the point of zero charge (PZC), surface tension reduction in the coal system and residual moisture reduction in the filter cake have been reported. The mechanism by which a surfactant brings about improved dewatering is complex and little understood, but appears to involve the reduction of the filtrate surface tension and an increase in * Fax: 0674-581635; e-mail: [email protected]
the solid to liquid contact angle [9,11]. As a result, the adsorption characteristics of the chemical additives on the solid should play a crucial role. This paper present the results of experimental work carried out to study the role of surfactant adsorption in the dewatering of fine coal. The adsorption kinetics and isotherms for cationic and anionic surfactants on fine coal are described and discussed. Filtration dewatering results are also presented and interpreted in the light of surfactant adsorption. 2. Theory The mechanism thought to be solely responsible for surfactant dewatering of aqueous mineral slurries was one of surface tension reduction as given by the Laplace–Young equation for capillary rise, h
2glg cos usl grr
1
where g lg is the liquid/air surface tension, u sl is the solid/ liquid contact angle, r is the capillary radius, g denotes acceleration due to gravity (vacuum in the case of coal filtration), and r is the liquid density. Thus, the lowering of the surface tension leads to lower capillary forces and improved cake dewatering. Dewatering results do not always correlate with surface tension reductions of the filtrates, suggesting that surfactant adsorption at the solid/
0016-2361/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(98)00169-0
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B.P. Singh / Fuel 78 (1999) 501–506
⬍ 0.02 mm. Detailed descriptions of the criteria for the selection of these materials, their purity and general chemistry have been given elsewhere [9]. Sodium dodecyl sulphate (SDS) and dodecyl ammonium bromide (DAB) were used as anionic and cationic surfactants, respectively. These two surface active agents was kindly supplied by Merck, Germany. The pH of the surfactant was adjusted to 5.2, which was the approximate pH of the washery water for filtration.
Table 1 Characteristics of coal sample Size (mm) ⫹1000 ⫺1000 ⫹ 500 ⫺500 ⫹ 300 ⫺300 ⫹ 150 ⫺150 ⫹ 75 ⫺75 ⫹ 45 ⫺45 Head
Weight (%)
Ash (%)
1.9 10.0 13.7 15.8 8.5 21.9 28.2
27.7 25.0 21.0 23.0 24.3 12.7 17.8
100.0
19.4
3.2. Methods
Proximate analysis Moisture % Volatile % Ash % Fixed carbon %
0.08 19.55 19.40 60.97
Elemental analysis Oxygen % Hydrogen % Nitrogen % Chlorine
11.63 6.22 1.75 0.03
liquid interface also plays an important role. An increase in the solid/liquid contact angle serves to lower the cos u sl and therefore lower the capillary forces which control cake drainage. A surface active agent may hydrophobize the surface by adsorbing in such a way that its hydrophobic part is oriented away from the surface. The Young equation provides the relationship between the solid/liquid contact angle and the work of adhesion: Wsl glg 1 ⫹ cos usl
2
The work of adhesion is a measure of how strongly water is bound by the surface and offers an additional mechanism for filtration dewatering of a mineral cake. For a completely hydrophilic surface, u sl 0⬚, cos u sl 1 and Wsl is at a maximum. On the contrary, for a completely hydrophobic surface, u sl 180⬚, cos u sl 0 and Wsl is minimized. Therefore, surfactant adsorption can result in disruption of hydration layers at the interface with the effect of either hydrophobizing or hydrophilizing the surface of the solids.
3.2.1. Adsorption studies Adsorption tests were conducted by the addition of 25 ml of surfactant solution of known strength to 1 g of coal sample in a 100-ml conical flask which was shaken in a constant temperature (25⬚C) water bath. Abstraction of the surfactant was calculated from the difference between the initial and final values. The equilibrium surfactant concentration at the end of an experiment was determined by the method developed by Epton [12]. For kinetic studies a number of glass stoppered test tubes containing the same quantity of coal (1 g) and surfactants at the same concentration were agitated in a constant temperature (25⬚C) water bath and the solution was withdrawn periodically to measure the concentration. 3.2.2. Electrokinetic measurement The electrophoretic mobility (charge on the particles) of the coal sample was measured with the Rank Brother Mark II microelectrophoresis apparatus (Make, UK) utilizing the two electrode modes and a flat quartz cell. Details of this apparatus and the procedures for preparing test suspension have appeared elsewhere [13]. 3.2.3. Dewatering experiment The filtration apparatus consisted of a 0.06-cm diameter Buchner funnel fitted with Whateman 41 filter paper, connected to a vacuum pump. Tests were performed using a 30% w/w coal slurry at pH 5.2 and at a constant vacuum 93 kPa. Dewatering was continued for a period of 4 min after disappearance of water from the top of the cake. Residual cake moisture content was determined by drying overnight at 105⬚C, with moisture content being expressed as
3. Experimental 3.1. Materials The fine size coal, froth flotation concentrate from a coal washery in Bihar, India, was available as a dry cake. The sieve size, ash distribution and proximate analysis are given in Table 1. The sample contained 19.4% ash with an average particle size of 30 mm. The BET surface area with adsorbate N2 was 3.8 (m 2/g). The average pore size was specified to be
Moisture wt%
wet weight ⫺ dry weight wet weight
3
3.2.4. Surface tension and contact angle measurement The surface tensions of the filtrate were recorded at 25⬚C on a Fischer surface tension meter using the ring method, and the contact angle was measured by the captive bubble technique [9].
B.P. Singh / Fuel 78 (1999) 501–506
Fig. 1. Zeta potential of coal sample as a function of pH. W, distilled water; K, SDS (9.2 × 10 ⫺6 mole dm ⫺3); A, DAB (6.9 × 10 ⫺6 mole dm ⫺3).
4. Results and discussion 4.1. Particle surface charge Fig. 1 shows that coal particles are positively charged at lower pH. The reversal of charge from positive to negative occurs at a pH of about 5.2. Above pH 5.2, coal particles again exhibit negative surface charge, as shown in the curves. Hence, the sample under investigation shows a point of zero charge (PZC) at pH 5.3. It has been observed that there was a slight downwards shift in electrophoretic mobility with surfactants SDS and DAB. Of course, with further DAB at a higher pH there was an upward swing. In this study the surfactants concentration was kept constant, i.e., 6.9 × 10 ⫺6 and 9.2 × 10 ⫺6 mole dm ⫺3 for DAB and SDS, respectively. Adsorption of anionic and cationic surfactants should provoke shifts in mobility respectively downwards and upwards and similar shifts in PZC. One of the explanations could be adsorption of the surfactants in the pores of the coal. The pore size is ⬍ 0.02 mm, which is very narrow; such pores do not participate in the electrophoretic mobility. The porous surface area calculated was found to
503
be 3.6 m 2/g. One can thus assume that condensation inside the mesopores takes place at such low concentrations, and that the negative shift in electrophoretic mobility may be attributed to humic acid present in the system, desorbed from the pores and readsorbed on the external surface. The higher particle charge on the coal sample can be attributed to the presence of surface adsorbed humic material. Above pH 5, the humic material becomes negatively charged and confers a high zeta potential. Firth and Nicol [14] have made similar observations on the effect of adsorbed humics on coal slurries. The fundamental mechanism that contributes to the charge on the surface of coal particles in aqueous media is due to the dissociation of ionogenic groups on the coal surfaces, such as those having carboxylic, phenolic and hydroxylic functional groups. For such groups, the degree and sign of the charge developed depend on the pH of the liquid phase. Since H ⫹ and OH ⫺ are potential determining ions, the surface charge is governed by the ionization of these groups [15,16]. 4.2. Adsorption kinetics The adsorption kinetics of DAB and SDS on fine clean coal at their natural pH (5.2) are plotted as amount of adsorbent (mg/g) at 25 and 60⬚C, respectively (Fig. 2). Both surfactants adsorbed rapidly and equilibrium adsorption was reached within a contact time of 150 s, which is less than the typical 4 min dewatering time used on the coal sample. The initial concentration of the surfactants in the solution was 1.15 mole dm ⫺3 (20 mg/g). The equilibrium adsorption at 25⬚C for DAB was 18 mg/g, which represents 90% of the total available DAB in the solution, whereas the equilibrium adsorption for SDS was 7 mg/g, which represents only 35% of the available surface active agent. Thus, the adsorption of the cationic surfactant (DAB) was an order of magnitude greater than that of the anionic surfactant (SDS). This difference is attributed to the highly electronegative surface of coal at the system pH. Increasing the temperature from 25 to 60⬚C decreased the amount of DAB adsorption from 18 to 17 mg/g and SDS adsorption from 7 to 5 mg/g. Therefore, the temperature dependence of DAB adsorption was very low compared with SDS adsorption. This difference in adsorption pattern at different temperatures suggests that the two surfactants may be adsorbed by different mechanisms. The low temperature dependence for DAB is possibly due to chemisorption, whereas the higher temperature dependence of SDS is typical of physisorption. 4.3. Adsorption isotherms
Fig. 2. Kinetics of adsorption of cationic (DAB) and anionic (SDS) surfactants on coal sample at 25⬚C. W, DAB 25⬚C; A, DAB 60⬚C; K, SDS 25⬚C; X, SDS 60⬚C.
Fig. 3 shows the adsorption isotherms of DAB and SDS on the coal sample as a function of equilibrium concentration in solution. Isotherms were obtained at 25⬚C using an adsorption time of 20 min and at their natural pH (5.2) plotted as amount of adsorbent (mg/g). The isotherm for DAB appears to reach a plateau at about 6.9 ×
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for DAB and a poorer result in dewatering compared to SDS. The adsorption of these ionic molecules on a hydrophobic surface like coal may occur by vander Waal’s interactions between the hydrophobic part of the reagent and the surface of coal. Furthermore, if there are some active sites consisting of various functional groups which can interact with the polar part of the surfactant, these interactions may also contribute to the adsorption process. 4.4. Dewatering studies
Fig. 3. Equilibrium adsorption isotherms of DAB and SDS from aqueous phase on coal at 25⬚C. W, DAB; K, SDS.
10 ⫺6 mole dm ⫺3 concentration in solution, whereas with SDS the plateau is reached at 9.2 × 10 ⫺6 mole dm ⫺3 concentration. In both cases, the maximum adsorption densities were achieved at an equilibrium concentration well below the bulk solution cmc value. Isotherms typically are of Langmuir type with adsorption at the plateau corresponding to surface saturation [17]. The adsorbed quantity was converted into surface coverage using the specific surface area of the coal and the cross-sectional area of the adsorbed surfactant (35 A 2 above the Kraft temperature). The coverage is close to 1.7 and 5 layers for SDS and DAB, respectively. Another significant observation was that the level of adsorption of the anionic surfactant (SDS) was much lower than the cationic surfactant (DAB). This is because of the affinity of DAB for greater adsorption at the negatively charged coal surface than the negatively charged SDS (electrostatic repulsion). Therefore, in dewatering phenomena it is expected that most of the DAB will be adsorbed on the coal hence resulting in a smaller reduction in surface tension
The results for dewatering of the coal sample, with and without any chemical additives, at their optimum dosages as a function of pH are shown in Fig. 4. The optimum cake moisture content was 21% without any chemical additives, whereas with SDS and DAB it was found to be 11% and 18%, respectively. The effect of cationic surfactant (DAB) on the cake moisture reduction was marginal (18%), whereas the anionic surfactant showed a considerable reduction in final cake moisture to about 11%. The surface tension of the filtrate was also noted at different concentrations of surfactants. The effects of reducing the filtrate surface tension by the addition of either an anionic or cationic surfactant were quite different. The results reveal a substantial fall in retained moisture with anionic surfactant addition of the order of 9.2 × 10 ⫺6 mole dm ⫺3; but the corresponding changes in the slurry surface tension with SDS and DAB were of the order of 30 and 46.8 mN/m, respectively. Detailed surface tension measurements with surfactant concentrations are shown in Table 2. There was no significant reduction in surface tension above a concentration of 11.5 × 10 ⫺6 mole dm ⫺3. Contact angle measurement results in the presence and absence of surfactants SDS and DAB are shown in Table 3. The highest angle (54, advancing/deg) most hydrophobic using water were measured for SDS and DAB (36, advancing/deg) at optimum surfactant concentrations (SDS, 9.2 × 10 ⫺6 and DAB, 6.9 × 10 ⫺6 mole dm ⫺3). It was observed that the contact angles on coal are difficult to measure, because the advancing angle may change by as much as 30⬚ as the drop moves from the hydrophobic to a more hydrophilic region of the surface. In general, the contact angle measurement results are in good agreement with dewatering results. Fig. 5 shows the moisture content of the filter cake as a function of the surfactant concentration used in slurry pretreatment. The residual moisture content was minimal with SDS (11%) and 18% with DAB and the corresponding optimum surfactant concentrations were 9.2 × 10 ⫺6 and 6.9 × 10 ⫺6 mole dm ⫺3 for SDS and DAB, respectively. 5. Discussion
Fig. 4. Moisture content of filter cakes as a function of the pH of the slurry. W, As recieved coal sample; A, DAB (6.9 × 10 ⫺6 mole dm ⫺3); K, SDS (9.2 × 10 ⫺6 mole dm ⫺3).
The point of zero charge (PZC) for the coal sample under investigation was pH 5.3. The cake moisture was found to be minimum at the PZC (Fig. 4). This is attributed to minimization of electrostatic repulsive forces near the PZC
B.P. Singh / Fuel 78 (1999) 501–506
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Table 2 Surface tension of surfactants DAB (mole dm ⫺3) × 10 ⫺6
Surface tension (mN/m)
SDS (mole dm ⫺3) × 10 ⫺6
Surface tension (mN/m)
2.5 4.6 6.9 9.2 11.5
59.0 50.0 47.0 46.8 46.0
2.5 4.6 6.9 9.2 11.5
40.2 36.0 31.4 30.0 29.5
which induces agglomeration/flocculation. The zeta potential of particles comprising industrial coal slurries explains the dependence of dewatering on pH. With increasing pH, particle charge and particle repulsion increases, resulting in a more open cake and hence larger r (Eq. 1). As a consequence, cake permeability and dewatering improved. The interpretation of dewatering results using the Laplace–Young relationship in the form of Eq. (1) can be typified in the presence of anionic SDS and cationic DAB. In the case of SDS, the negative charge present at the coal surface prevented significant adsorption of SDS, thereby a large amount of SDS was adsorbed at the gas/liquid interface, causing large reductions in the surface tension, to which the enhancements in filtration dewatering were attributed. The role of SDS is therefore, primarily to alter beneficially g ga in Eq. (1). In the case of DAB, however, a significant amount of DAB is adsorbed at the solid/water interface (via electrostatic attraction) and very little surfactant remains available for adsorption at the gas/liquid interface. As a result only a small reduction in the surface tension was attained and hence the poorer results. The adsorption isotherms for such a system can be adequately described by a simple equation that is readily derived from exchange adsorption between solute and solvent molecules in dilute solution [18]
u= 1 ⫺ u G= Gm ⫺ G cK=55:6
4
where u is the surface fractional coverage, G is the adsorption density (moles/unit area) at equilibrium concentration of the adsorbate in solution, c is the concentration in Table 3 Measured contact angle of coal sample Surfactant concentration (mole dm ⫺3)
Contact angle (advancing/deg)
0 4.6 (SDS) 6.9 (SDS) 9.2 (SDS) 18.4 (SDS)
9 52 53 54 50
4.6 (DAB) 6.9 (DAB) 9.2 (DAB) 18.4 (DAB)
32 36 32 30
solution (moles per litre), and Gm is the adsorption at saturation. The equilibrium constant K is related to the adsorption free energy 7G0ads as follows:
7G0ads ⫺RT ln K
5
The data from the experiments can be plotted in accordance with the following rearrangement of Eq. (4): c=55:6 1=G 1=Gm K ⫹ c=55:6Gm
6
and K can be calculated from the slope and intercept of the straight line. Table 4 summarises the saturation adsorption density and the standard free energy of adsorption estimated from the exchange adsorption equation. The maximum free energy of adsorption was obtained for DAB ( ⫺ 55.9 kJ/m) The nature of the coal surface can affect the adsorption in two ways. If adsorption takes place due to displacement of water molecules by the non-polar part of the adsorbing molecules interacting with the non-polar surface sites, then the adsorbed layer will be non-homogeneous and the maximum uptake will depend on both the fraction of the surface that is hydrophobic and the structure of the adsorbate. Secondly, since coal exhibits a great deal of microporosity, the extent of gas and solution adsorption will depend on whether the adsorbate can enter the pores [19]. The contact angle measurements data clearly show that in the case of SDS, the contact angle was higher in order of magnitude than that of the DAB at a particular narrow range of concentrations. After that there was again a decrease in the contact angle with increasing surfactant concentration. The role of the surfactant is therefore to alter the surface properties such that the contact angle (u ls) is changed. The SDS addition is most effective at low levels, corresponding to a monolayer coverage or less. From this study it would appear that the formation of a filter cake with low residual moisture content can be assisted by adjusting process filtration dewatering parameters in accordance with Eq. (1). Table 4 Free energy of surfactant adsorption Surfactants
Adsorption at plateau (mg/g solid)
Free energy of adsorption (kJ/m)
DAB SDS
18.0 7.0
⫺55.9 ⫺37.2
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B.P. Singh / Fuel 78 (1999) 501–506
on coal surfaces were ⫺ 55.9 and 37.2 kJ/mol, respectively. The experimental results revealed that adsorption occurs through hydrophobic interaction, as well as the electrostatic one. All results are in good agreement with the proposed Laplace–Young model.
References
Fig. 5. Moisture content of filter cake as a function of surfactant concentration used in slurry pretreatment. W, SDS (9.2 × 10 ⫺6 mole dm ⫺3); K, DAB (6.9 × 10 ⫺6 mole dm ⫺3).
These results are in agreement with Young’s equation for capillary drainage. 6. Conclusions Both cationic (DAB) and anionic (SDS) surfactants adsorb rapidly onto coal particles. DAB achieved much higher levels of adsorption than the anionic surfactants because of electrostatic attraction. These effects are explained by a reduction in the filtrate surface tension and an increase in the solid–liquid contact angle. SDS is capable of achieving these with a much lower level of surface adsorption. The adsorption isotherms exhibit typical Langmuir-type behavior in the concentration range investigated. The free energy of adsorption estimated for DAB and SDS
[1] Singh BP, Besra L. Sep Sci and Technol 1997;32(13):2181. [2] Sastry KVS, Kawulok-Englund DJ, Hosten C. J Ann Meet Minn Sect AIME (Proc) 1981;18(1):54. [3] Kobler RW, Dahlstrom DA. Trans, AIME 1980;266:2015. [4] Wright HJL, Kitchener JA. J Colloid Interface Sci 1976;56(1):57. [5] Oxford TP, Bromwell LG. In: Proc Int Symp Fine Particles Process, Las Vegas, 1980:1661. [6] McCall MT, Tadros ME. Colloid and Surfaces 1980;1:161. [7] Silverblatt CE, Dahlstrom DA. Ind Eng Chem 1954;46:1201. [8] Nicol SK, Day JC, Swanson AR. In: Proc Int Sym Fine Particles Process, Las Vegas, 1980:1661. [9] Singh BP. Filtration and Separation 1997;34(2):159. [10] Singh BP, Besra L, Reddy PSR, Sengupta DK. Fuel 1998;77(1y2):1349. [11] Mwaba CC. Mineral Engineering 1991;4(1):49. [12] Milwidsky BM. Practical detergent analysis. New York: MacMairDonald Co, 1970:44. [13] Staff operator’s mannual for Mark II microelectrophoresis apparatus. Cambridge, UK: Rank Brothers Ltd. [14] Firth BA, Nicol SK. Int J Min Proc 1981;8:239. [15] Healy TW, White LR. Adv Colloid Interface Sci 1978;103:303. [16] Singh BP, Singh R. Fuel Sci and Technol 1994;13(1):11. [17] Ottewill RH. In: Schick MC, editor. Nonionic surfactants. New York: Dekker, 1967, ch. 19. [18] Kipling JJ. Adsorption from solution of non-electrolyte. New York: Academic Press, 1965:248. [19] Fuerstenau DW, Pradip. Colloids and Surfces 1982;4:229.
The role of surfactant adsorption in the improved dewatering of fine coal Bimal P. Singh* Mineral Beneficiation Division, Regional Research Laboratory, (Council of Scientific and Industrial Research), Bhubaneswar 751, Orissa 751013, India Received 29 August 1997; received in revised form 5 August 1998; accepted 16 September 1998
Abstract This paper describes the adsorption kinetics and isotherms for cationic and anionic surfactants on coal surfaces. Both types of surfactants adsorb significantly rapidly; although the cationic surfactant achieved much higher levels of adsorption than the anionic one. The adsorption isotherms are of typical Langmuir type and employed for calculation of free energies. The results indicate that adsorption occurs through hydrophobic interaction in addition to electrostatic attraction between the surfactant molecules and the coal surface. Surfactant adsorption is shown to be a significant factor in the improved dewatering of coal particles. These improvements have been attributed to the changes which surfactant adsorption causes in the wetting characteristics of the coal surfaces. The results obtained are evaluated in terms of surface effects and by reference Laplace–Young equation. Furthermore, this equation has been used to quantify dewatering phenomena. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Adsorption; Surfactant; Dewatering; Kinetics; Wetting; Surface tension
1. Introduction The dewatering of aqueous mineral slurries such as iron ore [1–3], clay [4,5], gypsum [6] and coal [7–9] has received considerable attention as the problems involved in both bulk formation and the dewatering become more difficult with decreasing particle size. The use of chemical additives as dewatering aid to the mining and mineral processing has become increasing important in recent years. The advent of sophisticated equipment and increased awareness of energy conservation has accelerated this process. Such reagents provide lower filter cake moisture resulting in reduced thermal drying costs and transportation, improved product quality, increased calorific value and minimised freezing during winter storage. This communication is a supplement to previous works [9,10] which showed that the addition of a small amount of appropriate surfactant to the filtration dewatering of fine clean coal improved dewatering significantly. A direct correlation between the point of zero charge (PZC), surface tension reduction in the coal system and residual moisture reduction in the filter cake have been reported. The mechanism by which a surfactant brings about improved dewatering is complex and little understood, but appears to involve the reduction of the filtrate surface tension and an increase in * Fax: 0674-581635; e-mail: [email protected]
the solid to liquid contact angle [9,11]. As a result, the adsorption characteristics of the chemical additives on the solid should play a crucial role. This paper present the results of experimental work carried out to study the role of surfactant adsorption in the dewatering of fine coal. The adsorption kinetics and isotherms for cationic and anionic surfactants on fine coal are described and discussed. Filtration dewatering results are also presented and interpreted in the light of surfactant adsorption. 2. Theory The mechanism thought to be solely responsible for surfactant dewatering of aqueous mineral slurries was one of surface tension reduction as given by the Laplace–Young equation for capillary rise, h
2glg cos usl grr
1
where g lg is the liquid/air surface tension, u sl is the solid/ liquid contact angle, r is the capillary radius, g denotes acceleration due to gravity (vacuum in the case of coal filtration), and r is the liquid density. Thus, the lowering of the surface tension leads to lower capillary forces and improved cake dewatering. Dewatering results do not always correlate with surface tension reductions of the filtrates, suggesting that surfactant adsorption at the solid/
0016-2361/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(98)00169-0
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B.P. Singh / Fuel 78 (1999) 501–506
⬍ 0.02 mm. Detailed descriptions of the criteria for the selection of these materials, their purity and general chemistry have been given elsewhere [9]. Sodium dodecyl sulphate (SDS) and dodecyl ammonium bromide (DAB) were used as anionic and cationic surfactants, respectively. These two surface active agents was kindly supplied by Merck, Germany. The pH of the surfactant was adjusted to 5.2, which was the approximate pH of the washery water for filtration.
Table 1 Characteristics of coal sample Size (mm) ⫹1000 ⫺1000 ⫹ 500 ⫺500 ⫹ 300 ⫺300 ⫹ 150 ⫺150 ⫹ 75 ⫺75 ⫹ 45 ⫺45 Head
Weight (%)
Ash (%)
1.9 10.0 13.7 15.8 8.5 21.9 28.2
27.7 25.0 21.0 23.0 24.3 12.7 17.8
100.0
19.4
3.2. Methods
Proximate analysis Moisture % Volatile % Ash % Fixed carbon %
0.08 19.55 19.40 60.97
Elemental analysis Oxygen % Hydrogen % Nitrogen % Chlorine
11.63 6.22 1.75 0.03
liquid interface also plays an important role. An increase in the solid/liquid contact angle serves to lower the cos u sl and therefore lower the capillary forces which control cake drainage. A surface active agent may hydrophobize the surface by adsorbing in such a way that its hydrophobic part is oriented away from the surface. The Young equation provides the relationship between the solid/liquid contact angle and the work of adhesion: Wsl glg 1 ⫹ cos usl
2
The work of adhesion is a measure of how strongly water is bound by the surface and offers an additional mechanism for filtration dewatering of a mineral cake. For a completely hydrophilic surface, u sl 0⬚, cos u sl 1 and Wsl is at a maximum. On the contrary, for a completely hydrophobic surface, u sl 180⬚, cos u sl 0 and Wsl is minimized. Therefore, surfactant adsorption can result in disruption of hydration layers at the interface with the effect of either hydrophobizing or hydrophilizing the surface of the solids.
3.2.1. Adsorption studies Adsorption tests were conducted by the addition of 25 ml of surfactant solution of known strength to 1 g of coal sample in a 100-ml conical flask which was shaken in a constant temperature (25⬚C) water bath. Abstraction of the surfactant was calculated from the difference between the initial and final values. The equilibrium surfactant concentration at the end of an experiment was determined by the method developed by Epton [12]. For kinetic studies a number of glass stoppered test tubes containing the same quantity of coal (1 g) and surfactants at the same concentration were agitated in a constant temperature (25⬚C) water bath and the solution was withdrawn periodically to measure the concentration. 3.2.2. Electrokinetic measurement The electrophoretic mobility (charge on the particles) of the coal sample was measured with the Rank Brother Mark II microelectrophoresis apparatus (Make, UK) utilizing the two electrode modes and a flat quartz cell. Details of this apparatus and the procedures for preparing test suspension have appeared elsewhere [13]. 3.2.3. Dewatering experiment The filtration apparatus consisted of a 0.06-cm diameter Buchner funnel fitted with Whateman 41 filter paper, connected to a vacuum pump. Tests were performed using a 30% w/w coal slurry at pH 5.2 and at a constant vacuum 93 kPa. Dewatering was continued for a period of 4 min after disappearance of water from the top of the cake. Residual cake moisture content was determined by drying overnight at 105⬚C, with moisture content being expressed as
3. Experimental 3.1. Materials The fine size coal, froth flotation concentrate from a coal washery in Bihar, India, was available as a dry cake. The sieve size, ash distribution and proximate analysis are given in Table 1. The sample contained 19.4% ash with an average particle size of 30 mm. The BET surface area with adsorbate N2 was 3.8 (m 2/g). The average pore size was specified to be
Moisture wt%
wet weight ⫺ dry weight wet weight
3
3.2.4. Surface tension and contact angle measurement The surface tensions of the filtrate were recorded at 25⬚C on a Fischer surface tension meter using the ring method, and the contact angle was measured by the captive bubble technique [9].
B.P. Singh / Fuel 78 (1999) 501–506
Fig. 1. Zeta potential of coal sample as a function of pH. W, distilled water; K, SDS (9.2 × 10 ⫺6 mole dm ⫺3); A, DAB (6.9 × 10 ⫺6 mole dm ⫺3).
4. Results and discussion 4.1. Particle surface charge Fig. 1 shows that coal particles are positively charged at lower pH. The reversal of charge from positive to negative occurs at a pH of about 5.2. Above pH 5.2, coal particles again exhibit negative surface charge, as shown in the curves. Hence, the sample under investigation shows a point of zero charge (PZC) at pH 5.3. It has been observed that there was a slight downwards shift in electrophoretic mobility with surfactants SDS and DAB. Of course, with further DAB at a higher pH there was an upward swing. In this study the surfactants concentration was kept constant, i.e., 6.9 × 10 ⫺6 and 9.2 × 10 ⫺6 mole dm ⫺3 for DAB and SDS, respectively. Adsorption of anionic and cationic surfactants should provoke shifts in mobility respectively downwards and upwards and similar shifts in PZC. One of the explanations could be adsorption of the surfactants in the pores of the coal. The pore size is ⬍ 0.02 mm, which is very narrow; such pores do not participate in the electrophoretic mobility. The porous surface area calculated was found to
503
be 3.6 m 2/g. One can thus assume that condensation inside the mesopores takes place at such low concentrations, and that the negative shift in electrophoretic mobility may be attributed to humic acid present in the system, desorbed from the pores and readsorbed on the external surface. The higher particle charge on the coal sample can be attributed to the presence of surface adsorbed humic material. Above pH 5, the humic material becomes negatively charged and confers a high zeta potential. Firth and Nicol [14] have made similar observations on the effect of adsorbed humics on coal slurries. The fundamental mechanism that contributes to the charge on the surface of coal particles in aqueous media is due to the dissociation of ionogenic groups on the coal surfaces, such as those having carboxylic, phenolic and hydroxylic functional groups. For such groups, the degree and sign of the charge developed depend on the pH of the liquid phase. Since H ⫹ and OH ⫺ are potential determining ions, the surface charge is governed by the ionization of these groups [15,16]. 4.2. Adsorption kinetics The adsorption kinetics of DAB and SDS on fine clean coal at their natural pH (5.2) are plotted as amount of adsorbent (mg/g) at 25 and 60⬚C, respectively (Fig. 2). Both surfactants adsorbed rapidly and equilibrium adsorption was reached within a contact time of 150 s, which is less than the typical 4 min dewatering time used on the coal sample. The initial concentration of the surfactants in the solution was 1.15 mole dm ⫺3 (20 mg/g). The equilibrium adsorption at 25⬚C for DAB was 18 mg/g, which represents 90% of the total available DAB in the solution, whereas the equilibrium adsorption for SDS was 7 mg/g, which represents only 35% of the available surface active agent. Thus, the adsorption of the cationic surfactant (DAB) was an order of magnitude greater than that of the anionic surfactant (SDS). This difference is attributed to the highly electronegative surface of coal at the system pH. Increasing the temperature from 25 to 60⬚C decreased the amount of DAB adsorption from 18 to 17 mg/g and SDS adsorption from 7 to 5 mg/g. Therefore, the temperature dependence of DAB adsorption was very low compared with SDS adsorption. This difference in adsorption pattern at different temperatures suggests that the two surfactants may be adsorbed by different mechanisms. The low temperature dependence for DAB is possibly due to chemisorption, whereas the higher temperature dependence of SDS is typical of physisorption. 4.3. Adsorption isotherms
Fig. 2. Kinetics of adsorption of cationic (DAB) and anionic (SDS) surfactants on coal sample at 25⬚C. W, DAB 25⬚C; A, DAB 60⬚C; K, SDS 25⬚C; X, SDS 60⬚C.
Fig. 3 shows the adsorption isotherms of DAB and SDS on the coal sample as a function of equilibrium concentration in solution. Isotherms were obtained at 25⬚C using an adsorption time of 20 min and at their natural pH (5.2) plotted as amount of adsorbent (mg/g). The isotherm for DAB appears to reach a plateau at about 6.9 ×
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for DAB and a poorer result in dewatering compared to SDS. The adsorption of these ionic molecules on a hydrophobic surface like coal may occur by vander Waal’s interactions between the hydrophobic part of the reagent and the surface of coal. Furthermore, if there are some active sites consisting of various functional groups which can interact with the polar part of the surfactant, these interactions may also contribute to the adsorption process. 4.4. Dewatering studies
Fig. 3. Equilibrium adsorption isotherms of DAB and SDS from aqueous phase on coal at 25⬚C. W, DAB; K, SDS.
10 ⫺6 mole dm ⫺3 concentration in solution, whereas with SDS the plateau is reached at 9.2 × 10 ⫺6 mole dm ⫺3 concentration. In both cases, the maximum adsorption densities were achieved at an equilibrium concentration well below the bulk solution cmc value. Isotherms typically are of Langmuir type with adsorption at the plateau corresponding to surface saturation [17]. The adsorbed quantity was converted into surface coverage using the specific surface area of the coal and the cross-sectional area of the adsorbed surfactant (35 A 2 above the Kraft temperature). The coverage is close to 1.7 and 5 layers for SDS and DAB, respectively. Another significant observation was that the level of adsorption of the anionic surfactant (SDS) was much lower than the cationic surfactant (DAB). This is because of the affinity of DAB for greater adsorption at the negatively charged coal surface than the negatively charged SDS (electrostatic repulsion). Therefore, in dewatering phenomena it is expected that most of the DAB will be adsorbed on the coal hence resulting in a smaller reduction in surface tension
The results for dewatering of the coal sample, with and without any chemical additives, at their optimum dosages as a function of pH are shown in Fig. 4. The optimum cake moisture content was 21% without any chemical additives, whereas with SDS and DAB it was found to be 11% and 18%, respectively. The effect of cationic surfactant (DAB) on the cake moisture reduction was marginal (18%), whereas the anionic surfactant showed a considerable reduction in final cake moisture to about 11%. The surface tension of the filtrate was also noted at different concentrations of surfactants. The effects of reducing the filtrate surface tension by the addition of either an anionic or cationic surfactant were quite different. The results reveal a substantial fall in retained moisture with anionic surfactant addition of the order of 9.2 × 10 ⫺6 mole dm ⫺3; but the corresponding changes in the slurry surface tension with SDS and DAB were of the order of 30 and 46.8 mN/m, respectively. Detailed surface tension measurements with surfactant concentrations are shown in Table 2. There was no significant reduction in surface tension above a concentration of 11.5 × 10 ⫺6 mole dm ⫺3. Contact angle measurement results in the presence and absence of surfactants SDS and DAB are shown in Table 3. The highest angle (54, advancing/deg) most hydrophobic using water were measured for SDS and DAB (36, advancing/deg) at optimum surfactant concentrations (SDS, 9.2 × 10 ⫺6 and DAB, 6.9 × 10 ⫺6 mole dm ⫺3). It was observed that the contact angles on coal are difficult to measure, because the advancing angle may change by as much as 30⬚ as the drop moves from the hydrophobic to a more hydrophilic region of the surface. In general, the contact angle measurement results are in good agreement with dewatering results. Fig. 5 shows the moisture content of the filter cake as a function of the surfactant concentration used in slurry pretreatment. The residual moisture content was minimal with SDS (11%) and 18% with DAB and the corresponding optimum surfactant concentrations were 9.2 × 10 ⫺6 and 6.9 × 10 ⫺6 mole dm ⫺3 for SDS and DAB, respectively. 5. Discussion
Fig. 4. Moisture content of filter cakes as a function of the pH of the slurry. W, As recieved coal sample; A, DAB (6.9 × 10 ⫺6 mole dm ⫺3); K, SDS (9.2 × 10 ⫺6 mole dm ⫺3).
The point of zero charge (PZC) for the coal sample under investigation was pH 5.3. The cake moisture was found to be minimum at the PZC (Fig. 4). This is attributed to minimization of electrostatic repulsive forces near the PZC
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Table 2 Surface tension of surfactants DAB (mole dm ⫺3) × 10 ⫺6
Surface tension (mN/m)
SDS (mole dm ⫺3) × 10 ⫺6
Surface tension (mN/m)
2.5 4.6 6.9 9.2 11.5
59.0 50.0 47.0 46.8 46.0
2.5 4.6 6.9 9.2 11.5
40.2 36.0 31.4 30.0 29.5
which induces agglomeration/flocculation. The zeta potential of particles comprising industrial coal slurries explains the dependence of dewatering on pH. With increasing pH, particle charge and particle repulsion increases, resulting in a more open cake and hence larger r (Eq. 1). As a consequence, cake permeability and dewatering improved. The interpretation of dewatering results using the Laplace–Young relationship in the form of Eq. (1) can be typified in the presence of anionic SDS and cationic DAB. In the case of SDS, the negative charge present at the coal surface prevented significant adsorption of SDS, thereby a large amount of SDS was adsorbed at the gas/liquid interface, causing large reductions in the surface tension, to which the enhancements in filtration dewatering were attributed. The role of SDS is therefore, primarily to alter beneficially g ga in Eq. (1). In the case of DAB, however, a significant amount of DAB is adsorbed at the solid/water interface (via electrostatic attraction) and very little surfactant remains available for adsorption at the gas/liquid interface. As a result only a small reduction in the surface tension was attained and hence the poorer results. The adsorption isotherms for such a system can be adequately described by a simple equation that is readily derived from exchange adsorption between solute and solvent molecules in dilute solution [18]
u= 1 ⫺ u G= Gm ⫺ G cK=55:6
4
where u is the surface fractional coverage, G is the adsorption density (moles/unit area) at equilibrium concentration of the adsorbate in solution, c is the concentration in Table 3 Measured contact angle of coal sample Surfactant concentration (mole dm ⫺3)
Contact angle (advancing/deg)
0 4.6 (SDS) 6.9 (SDS) 9.2 (SDS) 18.4 (SDS)
9 52 53 54 50
4.6 (DAB) 6.9 (DAB) 9.2 (DAB) 18.4 (DAB)
32 36 32 30
solution (moles per litre), and Gm is the adsorption at saturation. The equilibrium constant K is related to the adsorption free energy 7G0ads as follows:
7G0ads ⫺RT ln K
5
The data from the experiments can be plotted in accordance with the following rearrangement of Eq. (4): c=55:6 1=G 1=Gm K ⫹ c=55:6Gm
6
and K can be calculated from the slope and intercept of the straight line. Table 4 summarises the saturation adsorption density and the standard free energy of adsorption estimated from the exchange adsorption equation. The maximum free energy of adsorption was obtained for DAB ( ⫺ 55.9 kJ/m) The nature of the coal surface can affect the adsorption in two ways. If adsorption takes place due to displacement of water molecules by the non-polar part of the adsorbing molecules interacting with the non-polar surface sites, then the adsorbed layer will be non-homogeneous and the maximum uptake will depend on both the fraction of the surface that is hydrophobic and the structure of the adsorbate. Secondly, since coal exhibits a great deal of microporosity, the extent of gas and solution adsorption will depend on whether the adsorbate can enter the pores [19]. The contact angle measurements data clearly show that in the case of SDS, the contact angle was higher in order of magnitude than that of the DAB at a particular narrow range of concentrations. After that there was again a decrease in the contact angle with increasing surfactant concentration. The role of the surfactant is therefore to alter the surface properties such that the contact angle (u ls) is changed. The SDS addition is most effective at low levels, corresponding to a monolayer coverage or less. From this study it would appear that the formation of a filter cake with low residual moisture content can be assisted by adjusting process filtration dewatering parameters in accordance with Eq. (1). Table 4 Free energy of surfactant adsorption Surfactants
Adsorption at plateau (mg/g solid)
Free energy of adsorption (kJ/m)
DAB SDS
18.0 7.0
⫺55.9 ⫺37.2
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on coal surfaces were ⫺ 55.9 and 37.2 kJ/mol, respectively. The experimental results revealed that adsorption occurs through hydrophobic interaction, as well as the electrostatic one. All results are in good agreement with the proposed Laplace–Young model.
References
Fig. 5. Moisture content of filter cake as a function of surfactant concentration used in slurry pretreatment. W, SDS (9.2 × 10 ⫺6 mole dm ⫺3); K, DAB (6.9 × 10 ⫺6 mole dm ⫺3).
These results are in agreement with Young’s equation for capillary drainage. 6. Conclusions Both cationic (DAB) and anionic (SDS) surfactants adsorb rapidly onto coal particles. DAB achieved much higher levels of adsorption than the anionic surfactants because of electrostatic attraction. These effects are explained by a reduction in the filtrate surface tension and an increase in the solid–liquid contact angle. SDS is capable of achieving these with a much lower level of surface adsorption. The adsorption isotherms exhibit typical Langmuir-type behavior in the concentration range investigated. The free energy of adsorption estimated for DAB and SDS
[1] Singh BP, Besra L. Sep Sci and Technol 1997;32(13):2181. [2] Sastry KVS, Kawulok-Englund DJ, Hosten C. J Ann Meet Minn Sect AIME (Proc) 1981;18(1):54. [3] Kobler RW, Dahlstrom DA. Trans, AIME 1980;266:2015. [4] Wright HJL, Kitchener JA. J Colloid Interface Sci 1976;56(1):57. [5] Oxford TP, Bromwell LG. In: Proc Int Symp Fine Particles Process, Las Vegas, 1980:1661. [6] McCall MT, Tadros ME. Colloid and Surfaces 1980;1:161. [7] Silverblatt CE, Dahlstrom DA. Ind Eng Chem 1954;46:1201. [8] Nicol SK, Day JC, Swanson AR. In: Proc Int Sym Fine Particles Process, Las Vegas, 1980:1661. [9] Singh BP. Filtration and Separation 1997;34(2):159. [10] Singh BP, Besra L, Reddy PSR, Sengupta DK. Fuel 1998;77(1y2):1349. [11] Mwaba CC. Mineral Engineering 1991;4(1):49. [12] Milwidsky BM. Practical detergent analysis. New York: MacMairDonald Co, 1970:44. [13] Staff operator’s mannual for Mark II microelectrophoresis apparatus. Cambridge, UK: Rank Brothers Ltd. [14] Firth BA, Nicol SK. Int J Min Proc 1981;8:239. [15] Healy TW, White LR. Adv Colloid Interface Sci 1978;103:303. [16] Singh BP, Singh R. Fuel Sci and Technol 1994;13(1):11. [17] Ottewill RH. In: Schick MC, editor. Nonionic surfactants. New York: Dekker, 1967, ch. 19. [18] Kipling JJ. Adsorption from solution of non-electrolyte. New York: Academic Press, 1965:248. [19] Fuerstenau DW, Pradip. Colloids and Surfces 1982;4:229.