Study on the Rheology and Stabilization of a Concentrated Coal

Energy & Fuels 2009, 23, 3217–3226

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Effect of Organized Assemblies. Part 5: Study on the Rheology and Stabilization of a Concentrated Coal-Water Slurry Using Saponin of the Acacia concinna Plant⊥ Debadutta Das,† Sagarika Panigrahi,† Pradipta Kumar Senapati,‡ and Pramila K. Misra*,† Centre of Studies in Surface Science and Technology, School of Chemistry, Sambalpur UniVersity, Jyoti Vihar-768 019, Orissa, India, and Institute of Minerals and Materials Technology, Council of Scientific and Industrial Research, Bhubaneswar-751 013, Orissa, India ReceiVed December 8, 2008. ReVised Manuscript ReceiVed February 2, 2009

The present work involves the preparation of a highly concentrated coal-water slurry employing three different low-rank coals of Indian origin having variable ash content. The formulation, rheology, and stabilization of the slurry have been investigated using saponin extracted from the seeds and pericarps (mods) of the Acacia concinna plant as a dispersant. The saponins extracted from both the seeds and pericarps of the plant are found to stabilize the slurry. The rheological characteristics of the slurry have been analyzed by varying pH and coal loading in the presence and absence of saponin. The coal-water slurry follows the Bingham plastic model and behaves as a non-Newtonian fluid in the presence of saponin. The stabilization of the coal-water slurry has been attributed primarily to the diminution of the coal particle-particle interaction because of steric hindrance offered by adsorbed saponin. The measurements of ζ potential, yield stress, and viscosity of coal in the presence of saponin further provide evidence to this effect.

Introduction Many countries including India that are scarce in fossil fuels spend a considerable amount of valuable foreign exchange in importing crude oil from abroad. Because of the increasing demand of energy sources as well as the ongoing energy crisis (since 1970), the quest for an alternative for fuel oil has become a dire need for modern society. Very often coal-water slurries have been used as a plenteous source for energy generation in many industrial sectors.1-3 The preparation of a coal-water slurry having a high content of coal with considerable stability has therefore been the subject of intensive research recently. During the attainment of the coal-water slurry with a high coal concentration, the viscosity of the slurry increases because of the increase in the particle-particle interaction, thereby leading to a severe adverse effect on the storage as well as transportation of the slurry. Plenty of additives4-17 has been developed so far to improve the stability of the highly * To whom correspondence should be addressed. Telephone: +916632430983 (Res),+916632430114(office),and09938333244(mobile).Fax:+916632430158. E-mail: [email protected]. † Sambalpur University. ‡ Institute of Minerals and Materials Technology, Council of Scientific and Industrial Research. ⊥ This paper has been dedicated to Prof. Gopabandhu Behera, Retired Professor of Sambalpur University, Jyoti Vihar, India and Prof. Ponisseril Somasumdaran, Columbia University, New York on their seventieth birth day. (1) Bienstock, D.; Foo, O. K. Proceedings of the First European Conference on Coal-Liquid Mixtures (I CHEM Symposium Series Number 83), Pergamon Press, Elmsford, NY, 1983; p 1. (2) Shirato, R. Proceedings of the Seventh International Conference on Coal Slurry Fuel Preparation and Utilization. New Orleans, LA, May 2124, 1985; p 1978. (3) Whaley, H. Proceedings of the Third International Symposium on Coal-Oil Mixture Combustion, April, 1981. (4) Qiu, X.; Zhou, M.; Yang, D.; Lou, H.; Ouyang, X.; Pang, Y. Evaluation of sulphonated acetone-formaldehyde (SAF) used in coalwater slurries prepared from different coals. Fuel 2007, 86, 1439–1445.

concentrated coal-water dispersion. Currently, some new techniques, such as microwave irradiation,18 exposure to ultrasonic radiation,19,20 use of organic solvents in place of (5) Tiwari, K. K.; Basu, K. S.; Bit, C. K.; Banerjee, S.; Mishra, K. K. High-concentration coal-water slurry from Indian Coals using newly developed additives. Fuel Process. Technol. 2003, 85, 31–42. (6) Boylu, F.; Atesok, G.; Dincer, H. The effect of carboxymethyl cellulose (CMC) on the stability of coal-water slurries. Fuel 2005, 84, 315–319. (7) Pawlik, M. Polymeric dispersants for coal-water slurries. Colloids Surf., A 2005, 266, 82–90. (8) Dincer, H.; Boylu, F.; Sirkeci, A. A.; Atesok, G. The effects of chemicals on the viscosity and stability of coal-water slurries. Int. J. Miner. Process. 2003, 70, 41–51. (9) Yavuz, R.; Kucukbayrak, S. An investigation of some factors affecting the dispersant adsorption of lignite. Powder Technol. 2001, 119, 89–94. (10) Tadros, Th. F.; Taylor, P.; Bognolo, G. Influence of addition of a polyelectrolyte, nonionic polymers, and their mixtures on the rheology of coal/water suspensions. Langmuir 1995, 11, 4678–4684. (11) Ukigai, T.; Sugawara, H.; Tobori, N. Effects of polyelectrolytes on coal-water mixtures in dispersed/coagulated states. Chem. Lett. 1995, 24, 371–372. (12) Yoshihara, H. Graft copolymers as dispersants for CWM. Coal Prep. 1999, 21, 93–103. (13) Saeki, T.; Tatsukawa, T.; Usui, H. Coal Prep. 1999, 21, 161. (14) Saeki, T.; Usui, H.; Ogawa, M. Effect of molecular structure of polysaccharide on the stability of coal-water mixtures. J. Chem. Eng. Jpn. 1994, 27, 773–778. (15) Usui, H.; Saeki, T.; Hayashi, K.; Tamura, T. Sedimentation stability and rheology of coal-water slurries. Coal Prep. 1997, 18, 201–214. (16) Zaiden, H. Japanese Patent 58,96,693, SGKS Co. Ltd., Osaka, Japan, June 8, 1983; Chem. Abstr. 1985, 103, 144674. (17) Botsaris, G. D.; Glazman, Y. M. In Interfacial Phenomena in Coal Technology; Botsaris, G. D., Galzman, Y. M., Eds.; Marcel Dekker: New York, 1989; p 199. (18) Meikap, B. C.; Purohit, N. K.; Mahadevan, V. Effect of microwave pre-treatment of coal for improvement of rheological characteristics of coalwater slurries. J. Colloid Interface Sci. 2005, 281, 225–235. (19) Zhaobing, G.; Feng, R.; Zheng, Y.; Fu, X. Improvement in properties of coal-water slurry by combined use of new additive and ultrasonic irradiation. Ultrason. Sonochem. 2007, 14, 583–588.

10.1021/ef800915y CCC: $40.75  2009 American Chemical Society Published on Web 05/12/2009

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water,21 modification of the surface chemistry of coal,22,23 etc., have been found to improve the stabilization of the coal dispersion substantially. Some treatments, such as hydrothermal dewatering24 and thermal watering,25 have also been adopted for achieving suspension from low-rank coal. A high concentration brown coal suspension has been achieved from a low-rank coal by reducing its water content and macroporosity through thermal and chemical means.26 Some synthetic additives, such as sodium dodecyl sulfate (SDS) and cetyltrimethyl ammonium bromide have also been found to stabilize the coal-water slurry to a substantial extent.27,28 The tremendous upsurge in the use of surfactant in coal technology stems largely from the fact that modification of coal through surfactant adsorption leads to either inhibition or minimization of the particle-particle association. Consequently, a fairly stable29-31 coal-water/liquid dispersion is yielded. Many works29-31 have been reported so far using commercially available surfactants. Some naturally biodegradable polysaccharides (i.e., gums) have also been used as efficient additives for stabilizing coal-water slurries.13-16 However, most of the reported additives are synthetic in nature and are neither innocuous to the ecosystem nor to the environment. Development of the eco-friendly and biodegradable additives for a coal-water slurry is therefore, essential in the present context. Plentiful coal reserves of India are mostly used as fuel for combustion processes in different industries. Our long-term objective aims at developing suitable natural-plant-based and environmentally safe, biodegradable additives to stabilize the coal-water slurry using these coal samples. In our earlier study,32 we have seen that saponin extracted from fruits of a Sapindous laurifoilia plant is as effective as some synthetic additives, such as SDS in formulating a concentrated coal-water slurry. Using that saponin, the slurry containing 64% weight fraction of coal could be achieved, which is stable for about a month. The present study is an attempt to use the surface-active (20) Li, Y. X.; Li, B. Q. Study on the ultrasonic irradiation of coalwater slurry. Fuel 2000, 79, 235–241. (21) Shin, Y. J.; Shen, Y. H. Preparation of coal slurry with organic solvents. Chemosphere 2007, 68, 389–393. (22) Woskoboenko, F.; Siemon, S. R.; Creasy, D. E. Rheology of Victorian brown coal slurries 1. Raw coal-water. Fuel 1987, 66, 1299– 1304. (23) Leong, Y. K.; Boger, D. V. Surface chemistry effects on concentrated suspension rheology. J. Colloid Interface Sci. 1990, 136, 249–258. (24) Hauserman, W. D.; Patel, R. C.; Wilson, D. G. Proceedings of the 17th International Symposium on Coal Slurry Fuel Preparation and Utilization, New Orleans, LA, 1985; p 268. (25) Woskoboenko, F.; Stacy, W. O. The solar dried coal slurry process. Proceedings of the 1985 International Conference on Coal Science, Sydney, Australia, 1985; pp 505-509. (26) Leong, Y. K.; Boger, D. V.; Christie, G. B.; Mainwaring, D. E. Rheology of low viscosity, high concentrated brown coal suspensions. Rheol. Acta 1993, 32, 227–285. (27) Aktas, Z.; Woodburn, E. T. Effect of addition of surface active agent on the viscosity of a high concentration slurry of a low rank British coal in water. Fuel Process. Technol. 2000, 62, 1–15. (28) Gu¨rses, A.; Ac¸ıkyıdız, M.; Dogˇer, C¸; Karaca, S.; Bayrak, R. Fuel Process. Technol. 2006, 87, 821–827. (29) Yeboah, Y. D. Surface modified coals for enhanced catalyst dispersion and liquefaction. Final report, Sept 1, 1996-Aug 31, 1999, prepared for the United States Department of Energy under contract number DE-FG22-95PC95229-07 (available on the Web). (30) Mishra, S. K.; Kanungo, S. B.; Rajeev. Adsorption of sodium dodecyl benzene sulfonate onto coal. J. Colloid Interface Sci. 2003, 267, 42–48. (31) Mishra, S. K.; Panda, D. Studies on the adsorption of Brij-35 and CTAB at the coal-water interface. J. Colloid Interface Sci. 2005, 283, 294– 299. (32) Das, D.; Panigrahi, S.; Misra, P. K.; Nayak, A. Effect of organized assemblies, Part 4: Formulation of highly concentrated coal-water slurries using a natural surfactants. Energy Fuels 2008, 22, 1865–1872.

Das et al.

component of a Acacia concinna plant33,34 (commonly named as Shikakai in Hindi) for stabilizing the coal-water slurry. A. concinna is a member of the family Leguminoseae (subfamily Mimoseae). This plant is abundantly available in the eastern as well as southern parts of India. The surface-active component, saponin, extracted from the pericarps and seeds of this plant are different from each other, but both are a complicated mixture of saccharine derivatives and belong to a class of naturally occurring non-ionic surfactants. These saponins are triglycosides of acacia acid,33-35 containing glucose, arabinose, and xylose as sugar moieties (called glycon) linked through oxygen to the acacic acid moiety (called aglycone), with the International Union of Pure and Applied Chemistry (IUPAC) name being 3β,16β,21β-trihydroyolean-12-ene-18R-28-oic acid (molecular structure 1). Unlike saponin from S. laurifolia (which has one

linking position for sugar units in the aglycone part), these saponins have four linking positions: at C3-OH, C16-OH, C21-OH, and C28-COOH of acacic acid. The saponin from seeds is a mixture of acacinin A, acacicnin B, and a free sugar conicinin. Acacinin A has two sugar linkages at C3-OH and C16-OH, whereas acacinin B has only one position for the sugar linkage. However, in acacinin A, normally the sugars are found to be attached at C3-OH alone. The free sugar units are galactose, fructose, and glucose. Saponin from pericarps is a mixture of acacinin C, acacinin D, acacinin E, and free sugars. In acacinin D, the sugars are attached in C3-OH and/or C16-OH (molecular structure 2).

In view of the structural difference of the saponin extracted from A. concinna from that of S. laurifoilia, we are enthusiastic to investigate the efficiency of this saponin for stabilizing coal-water dispersion. The saponin extracted from both seeds and pericarps is allowed to adsorb on the coal surface as such without further chromatographic separation. The rheological characteristics of the coal-water slurry have been investigated as a function of the solid load, additive concentration, and pH (33) Varshney, I. P.; Handa, G.; Pal, R. Structure of glycosides of Acacia concinna DC seeds. J. Ind. Chem. Soc. 1973, 50, 544–545. (34) Varshney, I. P.; Shamuddin, K. M. Absolute structure of acacic acids. Bull. Chem. Soc. Jpn. 1970, 43, 3830–3840. (35) Vraneshy, I. P.; Handa, G.; Pal, R.; Srivastav, H. C. Partial structure of acacininsA new saponin from the seeds of Acacia concinna DC. Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem. 1976, 14, 228–229.

Concentrated Coal-Water Slurry

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Table 1. Particle Size Parameters of Coal Samplesa

Table 2. Proximate Analyses of the Coal Samples

particle size (µm) coal type coal A coal B coal C

d10

d50

d90

3.894 3.222 3.337

27.318 23.949 33.131

95.70 100.789 109.531

a d , d , and d 10 50 90 are the diameter percentage points at 10%, 50%, and 90%, respectively.

moisture (%) ash (%) volatile matter (%) fixed carbon (%) value (kcal/kg)

coal A

coal B

coal C

13.15 8.02 33.44 45.39 6438

13.65 18.14 28.06 39.66 5745

10.02 39.84 29.75 30.39 4937

Table 3. Ultimate Analyses of the Coal Samples coal A (low ash)

coal B (medium ash)

coal C (high ash)

78.70 5.83 2.86 1.85 10.62 7.411

78.81 5.91 3.70 1.89 10.06 7.834

77.21 5.63 3.62 1.94 11.27 6.856

carbon (%) hydrogen (%) sulfur (%) nitrogen (%) oxygen (%) carbon/oxygen ratio

Table 4. Stability of the Coal-Water Slurry with Days at Different Weight Concentrations of Coal Containing 0.8% Weight Fraction of Coal in the Slurry number of days

Figure 1. Particle size distribution of coal samples.

in the presence of natural additive, saponin, as well as synthetic additive, SDS.

weight concentration (%)

coal A

coal B

coal C

58.0 60.0 62.0 64.0. 64.8

16 23 24 24 25

14 18 19 20 23

8 14 15 18 20

Isolation of Saponin.36 The plant was collected from forest zones of Paralakhemundi and Koraput of Orissa situated in the eastern part of India. The defatted powder of the plant materials was exhaustively extracted with absolute ethanol. The extract was concentrated on a water bath, and the residual amount of ethanol was removed under vacuum. The residue thus separated was successively extracted with petroleum ether, diethylether, carbon tetrachloride, chloroform, and acetone. It was finally dissolved in ethanol, filtered, and poured into a large amount of ether/acetone (1:1) mixture, as a result of which saponin precipitated out. The precipitation was repeated 5-6 times to obtain saponin of fairly good quality. At the end, the solution of saponin in ether/acetone was decolorized by passing through a bed of activated charcoal and was then evaporated. The solid obtained was refluxed in isopropyl or butyl alcohol. Upon gentle cooling, colorless powder separated out from the solution. The product was redissolved in a small quantity of ethanol to which a large amount of anhydrous acetone was added to precipitate colorless saponin. The compound was isolated using a multistep dissolution-extraction-evaporation procedure involving organic solvents to put forth the mechanism of slurry stabilization using a pure sample of saponin. The saponin thus extracted however is assumed to retain its natural character because the unit operations, such as dissolution, distillation, and extraction, do not involve any chemical change. Procurement of the Coal Sample.32 Coal samples collected from Talcher Coal Field placed in Orissa (the eastern part of India) were obtained as described in our earlier paper.32 Three types of coal samples designated as coal A, coal B, and coal C with an ash content of 8.02% (low-ash coal), 18.14% (medium-ash coal), 39.06% (high-ash coal), respectively, were selected for preparation of a high concentration slurry in distilled water. The particle size distribution of the coal samples measured by a Malvern particle size analyzer shows the d10, d50, and d90 of the coal samples, as given in Table 1. A representative diagram for the particle size distribution is shown in Figure 1. The proximate and ultimate analyses data measured by air-dried basis are given in Tables 2 and 3. The ultimate analysis shows the maximum percentage of

moisture to be around 13%. The coal was dried in a hot air oven for 24 h to reduce the moisture percentage to 2-3% by weight and was sealed to avoid contamination with moisture. Preparation of the Coal-Water Slurry. The coal-water slurry was prepared by agitating the coal-water mixture containing saponin in a helical ribbon mixer at 50-150 rpm (to avoid particle disintegration) using a variable frequency drive. The suspension has fair stability during the measurement for at least 48 h. Rheological Measurement.32 The rheological studies of the coal-water slurry were carried out using a HAAKE rotational viscometer (model RV 30), consisting of a measuring drive unit, temperature vessel with a circulator, sensor system, and a data logger. A sensor system MV I was chosen for the rheological measurements. The detailed procedure was described in our earlier paper.32 All experiments were conducted at room temperature of 30 °C. The pH of the coal-water slurry with natural and commercial additives measured in a pH meter was in the range of 4.5-5.0. The rheological measurement was controlled by a software rotation, version 3.0. The best-fit model was fitted to the shear stress-shear rate data to obtain the nature of the coal-water slurry. The various parameters, such as shear stress, shear rate, true viscosity, apparent viscosity, and temperature, along with the curve were displayed on the computer screen. ζ probe 24 V (52-60 Hz) T3A equipped with a microprocessor was used for the measurement of the ζ potential of coal particles containing 5% weight fraction of coal in water. This unit automatically calculates the electrophoretic mobility of the particles and converts it into ζ potential. The ζ potential values are at least an average of five different measurements. All measurements were made at the ambient temperature. Static Stability Tests.15,32,37 Coal-water slurries were prepared at weight concentrations of 58-65% and were then poured into glass cylinders of about 100 mL. The top of the cylinders was sealed, and the cylinders were stored at room temperature. The static stability of the coal-water slurry was evaluated by applying a rod penetration method; i.e., a glass rod of fixed weight and diameter was to put into the slurry to observe whether soft sedimentation appeared during storage. The soft sedimentation was identified because it can be easily disintegrated by mild stirring and transferred

(36) Vraneshy, I. P. Leguminaseae; saponins. Indian J. Chem. 1969, 7, 446–449.

(37) Yuchi, W.; Li, B.; Li, W.; Chen, H. Effects of coal characteristics on the properties of coal-water slurry. Coal Prep. 2005, 25, 239–249.

Experimental Section

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Figure 2. Variation of the apparent viscosity of coal A, coal B, and coal C with the variation of the coal weight (saponin ) 0.8% weight fraction in the coal-water slurry, w/v, at a shear rate of 77 s-1).

to other containers without any difficulty. The appearance of soft sediment by days was used as an indicator of static stability. The data are given in Table 4.

Results and Discussion Variation of Coal Loading. Figure 2 shows the variation of apparent viscosity of the slurry by varying the coal in the range of 55-65% weight fraction in the presence of 0.8% saponin. The apparent viscosity is found to increase with the solid weight concentration. Beyond 64.8% weight fraction of coal in the slurry, the apparent viscosity cannot be measured because of the formation of a hard sediment of coal. Variation of the apparent viscosity with the shear rate containing 64.8% of coal A, coal B, and coal C with 0.8% saponin is shown in panels a and b of Figure 3. The apparent viscosity of the slurry containing the same weight fraction of coal is found to increase with an increase in the amount of coal as well as ash content in coal. The presence of the particle invariably increases the suspension viscosity to a greater value than that of the fluid itself because of the increase in the interparticle interaction. Because of the strong interaction of hydrophilic ash, coal with a high content of ash sips more water to form gel. The apparent viscosity, therefore, follows the order coal A < coal B < coal C in accordance with the ash content. Variation of the Additive Concentration. The apparent viscosity of the slurry (with 64.8% weight fraction of coal) is found to decrease with an increase of the additive concentration until a minimum value is obtained at around 0.8% weight fraction of saponin (Figure 4). Beyond 0.8%, there is no appreciable change in the viscosity. Several interactions, such as hydrogen bonding,38-40 hydrophobic interaction,41-45 ion pairing,38,39 ion exchange,38-40 polarization of π electron,40,46 (38) Rupprecht, H.; Liebl, H. Influence of tensides on colloid chemical behaviour of highly disperse silic acids in polar and nonpolar solvents. Kolloid Z. Z. Polym. 1972, 250, 719. (39) Law, J. P., Jr.; Kunze, G. W. Soil Sci. Soc. Am. J. 1966, 30, 321. (40) Snyder, L. R. Interactions responsible for the selective adsorption of nonionic organic compounds on alumina. Comparisons with adsorption on silica. J. Phys. Chem. 1968, 72, 489–494. (41) Wakamatsu, T.; Fuersteanau, D. W. In Adsorption from Aqueous Solution; Weber, W. J., Jr, Matijevic, E., Eds.; American Chemical Society: Washington, D.C., 1968; Vol. 16, pp 1-172. (42) Giles, C. H.; D’Silva, A. P.; Easton, I. A. A general treatment and classification of the solute adsorption isotherm. Part 2. Experimental interpretation. J. Colloid Interface Sci. 1974, 47, 766–778.

Figure 3. (a) Plot of the apparent viscosity of coal A, coal B, and coal C with the variation of the shear rate (64.8 weight fraction of coal in the coal-water slurry). (b) Plot of the apparent viscosity of coal A, coal B, and coal C with the variation of the shear rate (60.0 weight fraction of coal in the coal-water slurry).

dispersion forces,39,47,48 etc., usually determine the interactions between the substrate and a solid surface. At natural pH, the coal surface is mostly hydrophobic in nature, with a low fraction of positively charged active site possibly because of the presence of -OH2 and -COOH2 groups on its surface. Saponin, therefore, partitions preferably from bulk solution because of hydrophobic interaction/dispersion forces with the coal surface. The partitioning continues with an increase in the saponin concentration in bulk solution until the threshold concentration, critical micelle concentration (cmc), of saponin32 is achieved, beyond which saponin aggregates to form molecular clusters called micelle. The micelle provides a thermodynamically favouable situation for saponin to stay preferably in the bulk solution. The onset of saponin aggregation in the bulk solution therefore limits the monomer concentration of saponin. Because (43) Dick, S. G.; Fuersteanau, D. W.; Healy, T. W. Adsorption of alkyl benzene sulphonote (ABS) at the alumina-water interface. J. Colloid Interface Sci. 1971, 37, 595–602. (44) Misra, P. K.; Mishra, B. K.; Somasundaran, P. Organization of amphiphiles. Part 5. In situ fluorescence probing of the adsorbed layer of polyoxyethylated alkyl phenols at silica-water interface. J. Colloids Interface Sci. 2003, 265, 1–8. (45) Misra, P. K.; Mishra, B. K.; Somasundaran, P. Organization of amphiphiles. Part 4. Characterization of the microstructure of the adsorbed layer of decylethoxylene nonyl phenol. Colloids Surf., A 2005, 252, 169– 174.



Figure 4. Variation of the apparent viscosity of coal A, coal B, and coal C with the variation of the saponin concentration (64.8% weight fraction of coal in the coal-water slurry).

surfactants adsorb at interfaces as monomer only,45 the formation of aggregates in the bulk solution stops further partitioning of saponin onto the coal surface and, hence, the decrease in apparent viscosity is labeled up. The adsorbed saponin stability may provide steric hindrance for the association among the coal particles, leading to a decrease in viscosity. Similar to our earlier investigation,32 0.8% weight fraction of saponin (w/v) from A. concinna also renders an optimum effect on stabilizing the slurry, which indicates that the surface activity of saponin from both plants lies in the same concentration range. The saponin isolated from the pericarps was found to be more effective than that extracted from seeds and was therefore, used throughout the study. The maximum coal concentration in the coal-water slurry achieved using seeds is 63.50%. Rheological Behavior Studies of the Coal-Water Slurry. For a Newtonian fluid in a laminar flow, the plot of the shear stress versus shear rate yields a straight line passing through the origin, with the slope of the line being equal to viscosity. The shear stress is measured with variation of the shear rate of the coal-water slurry containing 64.8% weight fraction of coal in the presence of 0.8% of both saponin and synthetic surfactant, SDS. A linear shear stress-shear rate relationship with an initial shear stress threshold is found for both of the surfactants. This clearly indicates non-Newtonian49,50 characteristics of the water suspension of the coal samples in the weight concentration range of 55-64.8%. The linear plot with an initial intercept as shown in a representative plot (panels a-c of Figure 5) also indicates that these fluids belong to Bingham plastic fluids obeying eq 1 τ ) τ0 + γη

(1)

where τ and γ denote shear stress and applied shear rate, respectively. τ0 is the yield stress, and η is defined as the coefficient of rigidity. The relatively high value of yield stress (46) Misra, P. K.; Dash, U.; Somasundaran, P. Effect of organized assemblies, Part VII: Adsorption behavior of polyoxyethylated nonyl phenol at silica-cyclohexane interface and its efficiency in stabilizing the silicacyclohexane dispersion. Ind. Eng. Chem. Res. 2009, 48, 3403–3409. (47) Kolbel, H.; Kuhn, P. Angew. Chem. 1959, 71, 211–215. (48) Kolbel, H.; Horig, K. Angew. Chem. 1959, 71, 691–697. (49) Skarvelakis, C.; Antonini, G. Rheological behaviour of multiphase slurries for combustion applications. Fuel 1996, 75, 1758–1760. (50) Guo, D. H.; Li, X. C.; Yan, J. S.; Jiang, L. Rheological behaviour of oil-based heavy oil, coal and water multiphase slurries. Fuel 1998, 77, 209–210.

Figure 5. Rheological behavior of the (a) coal A, (b) coal B, and (c) coal C samples in the presence of saponin and SDS.

in compared to that of saponin obtained from S. laurifolia32 may likely be due to the increased coal loading in the present case. The yield stress (Table 5) is found to increase with the increase in the ash content of coal. The hydrophilic ash content may increase the solid volume of coal because of the strong interaction with water, which in turn may increase the viscosity. Effect of pH on the ζ Potential of Coal and Viscosity of the Coal-Water Slurry. The rheological behavior of the

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Table 5. Apparent Viscosity and Yield Stress for Coal A, Coal B, and Coal C at a Shear Rate of 77 s-1 for Different Coal-Water Slurry Weight Concentrations apparent viscosity (Pa s)

weight concentration (%)

coal A

coal B

coal C

55 60 62 64.8

0.41 0.52 0.61 0.899

0.45 0.55 0.74 0.995

0.48 0.60 0.78 1.233

yield stress (Pa) coal A coal B coal C 34 48 58 62

41 49 60 70

43 54 65 78

coal-water slurry is largely determined by the surface characteristics of coal. The functional groups present at the surface play a very crucial role in influencing the yield stress and apparent viscosity of coal.51,52 The coal surface contains polar groups, such as -COOH2+ and -OH2+, attached to the hydrocarbon skeleton connected by cross-links.53,54 The predominating mechanism of surface charge generation on the coal is due to the pH-dependent dissociation or protonation of these groups.55-57 At acidic pH, positive ζ potential is due to the protonation of the hydroxyl and carboxyl groups, and at alkaline pH, negative ζ potential is due to the dissociation of these groups. The pH at which the surface charge of a particle is zero is referred to as the point of zero charge (PZC)58,59 or isoelectric point (IEP).60,61 The plot of the ζ potential of coal (5% in water) with the variation of pH is shown in panels a-c of Figure 6. The bare coals exhibit an IEP of approximately 5.4-6.5, which falls within the same range as reported earlier.60,61 However, in the presence of saponin, the ζ potential is decreased in all cases with a concomitant shift of IEP of all coals toward alkaline pH (Table 6). The most preferred orientation of saponin on the coal surface during the adsorption is with the hydrophobic aglycone part lying flat on the coal surface and with the glycol part (sugar residues) protruding vertically outward into the bulk water. This arrangement would minimize the unfavorable free energy because of dissolution of the large nonpolar moiety in bulk water as well as an increase in the favorable mixing of the bulky glycosides of the saponin molecule because of hydrogen bonding between glycoside units and the water molecule. The ζ potential of a particle is a measure of its electrophoretic mobility when it is kept within an electrolytic cell. On (51) Mishra, S. K.; Senapati, P. K.; Panda, D. Rheological behaviour of coal-water slurry. Energy Sources 2002, 24, 159–167. (52) Boger, D. V.; Leong, Y. K.; Christie, G. B., Mainwaring, W. E. Flow behaviour of high studies brown-coal-water suspensions as liquid fuel. Proceedings of the Australasian Institute of Mining and Metallurgy Annual Conference on Coal Power, New Castle, Australia, 1987. (53) Whitehurst, D. D.; Mitchell, T. O.; Farcasiu, M. Coal Liquifaction; Acdemic Press: New York, 1980. (54) Davidson, R. M. In Coal Science; Gorgaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1982; Vol. 1, p 84. (55) Misra, P. K.; Panigrahi, S.; Somasundaran, P. Organization of amphiphiles, Part VIII: Role of polyoxyethylated alkyl phenols in optimizing of the beneficiation process of a hydrophilic mineral. Int. J. Miner. Process. 2006, 80, 229–237. (56) Misra, P. K.; Somasundaran, P. Organization of amphiphiles, Part VI: A comparative study of the orientation of polyoxyethylated alkylphenols at air-water and silica-water interface. J. Surfactants Deterg. 2004, 7, 373–378. (57) Somasundaran, P. Interfacial chemistry of particulate flotation. AIChE Symp. Ser. 1975, 71, 1–15. (58) Kosal, E.; Ramachandran, R.; Somasundaran, P.; Maltesh, C. Flocculation of oxide using polyethylene oxide. Powder Technol. 1990, 62, 253–259. (59) Fuerstenau, D. W.; Pradip. Adsorption of frothers at coal/water interfaces. Colloids Surf. 1982, 4, 213–227. (60) Miller, J. D.; Laskowski, J. S.; Chang, S. S. Dextrin adsorption by oxidized coal. Colloids Surf. 1983, 8, 137–151. (61) Roh, N. S.; Shin, D. H.; Kim, D. C.; Kim, J. D. Rheological behaviour of coal-water mixtures 1, Effect of surfactants and temperature. Fuel 1995, 74, 1313–1318.

Figure 6. Variation of ζ potential of (a) coal A, (b) coal B, and (c) coal C with the change in pH in the presence and absence of saponin.

application of electric current, the particle moves toward the opposite electrode depending upon the magnitude of charge. The decrease in ζ potential of coal on adsorption of saponin may be due to the diminution of the exposed surface charge on the coal particle resulting from the coverage of some fraction



Table 6. PZC of Different Coals in the Absence and Presence of Additive and SDS system

point of zero charge

naked coal A coal A with additive coal A with SDS naked coal B coal B with additive coal B with SDS naked coal C coal C with additive coal C with SDS

5.5 6.7 6.5 6.1 6.6 6.1 6.5 6.9 6.8

of its surface by the nonpolar moiety of saponin.31,50,55 Because saponin is a large molecule (molecular mass of 1782 and rms velocity value of 2.05 at 30 °C), there might be the possibility for the decrease in ζ potential because of mechanical displacement of the original shear plane by the projected sugar chain.31,50,55 Such a decrease in the ζ potential because of the adsorption of the surfactant at the solid-liquid interface has been the common phenomena for surfactant adsorptions55 on solid surfaces. Further, saponin also contains a large number of -OH and -COOH groups, which may also contribute to the surface charge of coal depending upon the pH of the medium. The extent of dissociation of these functional groups with pH may be responsible for the shift of the IEP of coal when saponin adsorbs onto it. The apparent viscosity of all coal-water slurries (60%, w/v) is found to decrease with an increase of pH, as shown in panels a-c of Figure 7 and labeled gradually beyond pH 8.0. The electrostatic repulsion between intense negative charges developed on the coal surface because of the ionization of the polar groups prevents particle-particle association as the pH of the slurry increases, and hence, a well-dispersed suspension is obtained at high pH. The larger value of negative ζ potential of coal at alkaline pH in the presence of synthetic additive, SDS, compared to saponin is due to the negative sulfate group, which adds up to the surface potential of coal. This stabilization of the coal-water slurry may be attributed to the mechanical shock provided by the bulky hydrated glycoside head for the association among coal particles (Figure 8). The yield stress with the variation of pH for different coals has been measured in the presence of saponin (0.8%) and is shown in Figure 9. The maximum yield stress at the IEP is due to the absence of residual charge on the coal particle, which promotes particle-particle association, thereby leading to agglomeration of coal particles. Effect of the Temperature on the Apparent Viscosity of the Coal-Water Slurry. The decrease in the apparent viscosity with temperatures at 298, 303, 308, and 318 K (panels a and b of Figure 10) is attributed to the increase of the kinetic energy of the coal particle and rapid movement of the dangled chains of saponin sugar units at the coal-water interface. The solid volume because of shrinking of the coal content with an increase of the temperature may also be responsible for the decrease of the viscosity. The temperature-dependent apparent viscosity is found to fit the following simple relation called the GuzmanAndrade equation,62 which is widely used for many pure liquids and solutions: η ) A1 exp(A2/T)

(2)

where T is the absolute temperature and A1 and A2 are constants, which upon rearrangement give the following equation: ln η ) ln A + (A2/T)

(3)

(62) Andrade, E. N. da C. Distribution of grain size in annelated metals. Nature 1930, 125, 309.

Figure 7. (a) Effect of pH with viscosity for (a) coal A, (b) coal B, and (c) coal C (coal concentration ) 60%, shear rate ) 77 s-1, and saponin ) 0.8%).

A linear correlation with a correlation coefficient of ∼0.99 is obtained in all cases (Figure 11). Comparison to Commercial Additives. The efficiency of stabilizing the coal-water slurry of saponin has been compared by measuring the apparent viscosity of the coal-water slurry in the presence of commercial additive sodium carboxymethylcellulose (CMC) and a synthetic sodium dodecylbenzene

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Das et al.

Figure 8. Schematic representation of a coal particle with adsorbed saponin.

Figure 10. (a) Effect of the temperature on the apparent viscosity for coal A, coal B, and coal C (coal concentration ) 60%, shear rate ) 77 s-1, and saponin ) 0.8%). (b) Effect of the temperature on the apparent viscosity for coal A, coal B, and coal C (coal concentration ) 60%, shear rate ) 77 s-1, and SDS ) 0.8%). Figure 9. Plot of the yield stress versus pH of the slurry.

sulfonate (SDDBS). The effect of the solid weight concentration on the apparent viscosity is studied in the presence of 0.8% CMC, and the result is given in Figure 12. The decrease in the apparent viscosity of the coal-water slurry also comes out to be within the same range as that of saponin and SDS under investigation. The plot of the apparent viscosity of 64.8% coal-water slurry versus shear rate is measured in the presence of 0.8% of both CMC and SDDBS separately, and the results are shown in Figures 13 and 14. Upon comparison to that of the saponin as well as the synthetic additive SDS, the same trend of the apparent viscosity is found and 0.8% CMC also stabilizes the slurry to the same extent. Because the number of moles of CMC in 0.8% would be less than that of the small molecule, such as SDS, SDDBS, or saponin, the results converge to the same value, even though the former is known to be a high-performance dispersant. The increase of the CMC percentage does not improve the coal load, possibly because of the saturation of the coal site by CMC. General Basis of Slurry Stabilization. The basic principle behind the mechanism of stabilization of the coal-water slurry is to increase the net interparticle repulsion, which is usually induced in the following way:7 (i) increasing the surface charge (electrostatic repulsion) or (ii) introducing groups that provide a mechanical barrier (steric repulsion), or (iii) increasing the steric wettability of the solid surface to eliminate the hydrophobic interaction. Because the coals under study are relatively less charged at the natural pH (ζ potential is around +15 mV), the agglomeration of coal

Figure 11. Guzman-Andrade equation applied to the coal-water slurry in the presence of saponin (coal concentration ) 60%, shear rate ) 77 s-1, and saponin ) 0.8%).

particles is mainly due to the hydrophobic association. With the fact that the ζ potential of the coal-water slurry is decreased in the presence of saponin as well as the synthetic additive SDS and the slurry is stabilized to the same extent irrespective of the nature and size of the additives, the basis underlying the stability would, therefore, be mostly the steric repulsion. For a hydrophobic solid, such as coal, the most likely driving force for the adsorption of the additive is the


Figure 12. Variation of the apparent viscosity of coal A, coal B, and coal C with variation of the coal weight (CMC ) 0.8% weight fraction in the coal-water slurry, w/v, at shear rate ) 77 s-1).

Figure 13. Plot of the apparent viscosity of coal A, coal B, and coal C with the variation of the shear rate (64.8% weight fraction of coal in the coal-water slurry) in the presence of CMC.

Figure 14. Plot of the apparent viscosity of coal A, coal B, and coal C with the variation of the shear rate (64.8% weight fraction of coal in the coal-water slurry) in the presence of SDDBS.

hydrophobic interaction. The additives would therefore adsorb to the coal surface, with their hydrophobic groups adhering to the coal surface and the hydrophilic chain remaining suspended in bulk water. The suspended hydrophilic head groups of the additives form an effective barrier around each particle. The mechanical barrier increases the resistance of the dispersed particles to maintain a mechanical shock and prevents them for coalescing when they collide, inhibiting the close approach of one particle to another. This in turn put a control on coal congregation. The extent of steric


Figure 15. Schematic representation of the steric stabilization of the coal-water slurry by small and large molecules.

stabilization, therefore, mainly depends upon the fraction of the coal surface occupied by the exposed hydrophilic groups of the additive on the surface of coal. During the adsorption process, the solute (adsorbate) continues partitioning to the solid surface until a monolayer coverage occurs, encompassing all of the active sites. For a fixed number of sites, the number of molecules required for complete coverage of the solid site will be more for a small molecule, whereas for a larger molecule, the number of molecules required for the same coverage will be less. Hence, for a larger additive, such as CMC, because the size of the hydrophilic group is large, the number would be less, whereas in the case of a small molecule, such as SDS, because the size of the head group is small, the number of molecules adsorbed will be more. However, the fraction of the solid particle that they cover at the highest coverage (saturated adsorption) is the same in both cases, thus rendering the same effect to the steric stabilization (Figure 15). In fact, while studying the efficiency of beneficiating silica by adsorbing a series of polyoxyethyleted non-ionic surfactants with varying oxyethylene groups (varying from 10 to 40) as well as different hydrophobic chain length (ocyl and nonyl), we44,45,55,56 have seen that that extent of flotation recovery is the same in all cases despite their differences in molecular weight as well as adsorption densities on the silica surface and they also shift the shear plane to the same extent (despite different lengths in hydrophilic chain length and, hence, the size), resulting in the same surfacial charge over silica. During the study of adsorption behavior of a series of ethoxylated non-ionic surfactants on coal, Celik63 has also concluded that the shift of the shear plane leading to the change in ζ potential because of the adsorption of the surfactant is independent of the size of the head group (ethoxylated chain). Further, it is found that, even though saponin is non-ionic in nature, it has the same effect as that of an anionic synthetic additive, such as SDS and SDDBS, and a non-ionic additive, such as CMC. This similarity in behavior is due to the presence of a large number of groups containing oxygen atoms in saponin and CMC, having lone pairs of electrons, which provide a negatively charged atmosphere similar to SDS and SDDBS. To add to the fact, during the measurements of pK of a number of aldimines in the presence of a series of non-ionic surfactants, (63) Celik, M. S. Adsorption of ethoxylated sulfonate and nonionic homologs on coal. J. Colloid Interface Sci. 1989, 129, 428–437.

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Tween series and Triton-X-100,64,65 and anionic surfactant, SDS, we have seen the similarities in the environment provided by them. Fernadez and Fromherz66 have also made similar observations in the measurement of the electrical potential and polarity of SDS and Triton-X-100 micelles using lipoid pH indicators as probes. Mishra et al.31 have observed the similarities in behavior between Brij-35, a non-ionic surfactant, and an anionic surfactant, SDDBS, during their studies on the adsorption characteristics of these two surfactants at the coal-water interface. Comparative Cost Analysis. The saponin that was extracted through the chemical methods stabilized the coal-water slurry and a coal-water slurry containing 64.8% could be achieved. The extract obtained by dissolving the fruit in cold water alone (150 g of fruit in 1 L of water) without subjecting it to any chemical means is also found to stabilize the slurry to the same extent. The later method requires only the cost of the fruit, which is a waste forest product in the locality and, hence, requires only a collection charge. This may be around 3 U.S. dollars of additive per 100 kg of slurry in India, whereas the saponin if extracted through chemical means would cost around 15 U.S. (64) Panigrahi, S.; Chakravorty, M.; Misra, P. K. Effect of organized assemblies, II: Environmental effect on the pK of (o/p) hydroxybenzylidene(4/6)-nitro-2-aminobenzothiazoles. J. Colloid Interface Sci. 2007, 306, 137– 142. (65) Misra, P. K.; Mishra, B. K.; Behera, G. B. Determination of dissociation constants of salylidene-2-aminobenzothaizole and p-hydroxybenzylidene-2-aminobenzothiazole in different surfactant systems. Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem. 1988, 27, 889–892. (66) Fernadez, M. S.; Fromherz, P. Lipoid pH indicators as probes of electrical potential and polarity in micelles. J. Phys. Chem. 1977, 81, 1755– 1761.

Das et al.

dollars. The commercially available additives on the other hand may require around 50 U.S. dollars for the same amount of slurry. Conclusion Aqueous transport of coal fuels to a thermal power station is considered to be the best because of its low cost and acceptable environmental pollution. This work shows that the plant-based additive saponin from A. concinna (both pericarps and seeds) can be suitably substituted for a synthetic additive, such as SDS. A few pilot experiments with other commercially available additives, such as CMC and SDDBS, also yielded the same results. The tentative cost estimation indicates that the cost of natural additives is cheaper than the available synthetic additives. Because the additive contains carbon, hydrogen, and oxygen only, during the combustion of the coal-water slurry, the contribution to the air pollution is negligible. Acknowledgment. The authors express their sincere thanks to the Institute of Minerals and Materials Technology, Bhubaneswar, for providing laboratory facilities and the Department of Science and Technology, Government of India, New Delhi, for sanctioning a project to P.K.M. (Project DST, sanction letter SR/S1/PC-39/2004, dated March 14, 2006). The authors also thank CSIR for a senior research fellowship award to one of the authors (S.P.) and the University Grants Commission and Department of Science and Technology, Government of India, for financial support to the School through sanction of DRS and FIST, respectively. EF800915Y

Study on the Rheology and Stabilization of a Concentrated Coal

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