Properties of sodium lignosulfonate as dispersant of coal water slurry

Properties of sodium lignosulfonate as dispersant of coal water slurry

Energy Conversion and Management 48 (2007) 2433–2438 www.elsevier.com/locate/enconman Properties of sodium lignosulfonate as dispersant of coal water...

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Energy Conversion and Management 48 (2007) 2433–2438 www.elsevier.com/locate/enconman

Properties of sodium lignosulfonate as dispersant of coal water slurry Dongjie Yang *, Xueqing Qiu, Mingsong Zhou, Hongming Lou School of Chemical and Energy Engineering, Guangdong Provincial Laboratory of Green Chemical Technology, South China University of Technology, Guangzhou 510640, PR China Received 3 September 2006; accepted 12 April 2007 Available online 6 June 2007

Abstract In order to use lignosulfonates (a by-product of pulp and paper processes) as an effective dispersant of coal water slurry five purified sodium lignosulfonate (SL) samples with different molecular weights were prepared by fractionation using ultrafiltration and dialysis. The effect of SL on the apparent viscosity of coal water slurry (CWS) was investigated. The adsorption behavior of the SL on the coal water interface has much greater effect on the viscosity of coal water slurry. The higher adsorption amount and compact adsorption film of SL on the coal surface help reduce the viscosity of CWS, and the zeta potential is also an important factor, which is influenced by the sulfonic and carboxyl group contents of the lignosulfonate molecule. Furthermore, the SL with its molecular weight ranging from 10,000 to 30,000 has both a higher adsorbed amount and zeta potential on the coal surface and the best effect on reducing the viscosity of the coal water slurry. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Sodium lignosulfonate; Molecular weight; Coal water slurry; Dispersant; Apparent viscosity

1. Introduction Lignin is derived from an abundant and renewable resource. Lignosulfonates are a by-product of the pulping industry and are rather cheap and widespread chemicals. Lignosulfonates can be used as concrete admixtures. Apart from that, lignosulfonates have also been used in other fields including applications as oil well dispersants, dyestuff, coal water slurry (CWS) dispersants, agricultural chemicals and other industrial binders [1–3]. Of the 50 million tones of technical lignin produced annually, only 3 million tones of lignosulfonates are used for other purposes. There have been attempts for several decades to increase the utilization of lignin as a raw material, but there is still a wide gap between theory and practice, in other words, between what is technically possible and what is economically achievable. Until now, apart from a few

*

Corresponding author. Tel./fax: +86 20 8711 4722. E-mail address: [email protected] (D. Yang).

0196-8904/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.04.007

exceptions, lignin based products could not compete with products derived from petrochemicals [4]. Recently, continuously increasing demands for energy have led scientists to seek ways of finding new energy sources. Thus, researchers have directed their attention towards various methods of burning coal water slurries for energy generation. A typical coal water slurry (CWS) consists of 60–75% coal, 25–40% water and about 1% chemical dispersants. It is desirable that the coal water slurry has high coal solids content and a low viscosity. In order to control the desired viscosity of the CWS, a dispersant needs to be added [5]. Because of the relatively complex chemical composition and the wide range of molecular weight distribution, lignosulfonates are not an effective dispersant for CWS [6]. In our earlier research [7], it was found that the surface activity and foaming property of lignosulfonate with high molecular weight were more excellent than that of low molecular weight. Furthermore, the effect of molecular weight of the lignosulfonate on the adsorption property in a cement–water dispersed system was investigated, and

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the results showed that the saturation adsorption amount of lignosulfonate with lower molecular weight (less than 30,000) on the surface of cement particles increased with the increase of molecular weight, while the same properties of high molecular weight fractions were independent of their molecular weight. In addition, other studies showed that the lignosulfonate with molecular weight ranging from 5000 to 10,000 had the best effect on the dispersion of titanium dioxide particles in water. In coal water suspension, the molecular weight of the dispersant is also an important factor that affects the viscosity and stability of the CWS. For example, for the common CWS dispersant, humate, it was found that the higher molecular weight had the better effect on reducing the viscosity of CWS [8]. Another common dispersant, NDF (methylene naphthalene sulfonate – styrene sulfonate – maleate copolymer), has the best effect on reducing the viscosity of CWS when the molecular weight is 20,000 [9]. The aim of this investigation is to determine the influence of SL with different molecular weights on the viscosity of CWS and the adsorption behavior of SL on the coal surface.

apparatus (Hangzhou Water Treatment Factory, China). The effective filtration area of each membrane was 0.008 m2. The cut off molecular weights of the membranes used in the experiments is, respectively, 5000, 10,000, 30,000 and 50,000. The operation was under a pressure of 2–4 MPa and a temperature of less than 45 °C. The nominal cut off ratio was more than 90%. The molecular weight distribution of the SL sample was determined by using aqueous gel permeation chromatography (GPC) with Ultrahydragel 120 and Ultrahydragel 250 columns. The GPC analyses were performed using a Waters 1515 Isocratic HPLC pump with a Waters 2487 UV Absorbance Detector (Waters Corp., USA). The influences of the mobile phases on the elution behaviors of the samples are determined. It is confirmed through a series of experiments that the eluent of a neutral aqueous solution containing a low concentration of electrolyte could separate the components of SL. The best results were obtained by using 0.10 mol/L NaNO3 solution with pH 8 as the eluent with the velocity of 0.50 mL/min. The polystyrene sulfonate was used as the standard substance [11–13].

2. Experimental

2.3. Measurements of functional group content of fractions

2.1. Experimental materials

The sulfonic group in the SL sample was determined with the conductometric titration method [14]. The carboxyl and phenolic hydroxyl groups in the SL were also determined by means of the non-aqueous conductometric titration method for weak acid. The Pyridine–acetone mixture of Pyridine or acetone was used individually as solvent. A KOH–benzalcohol standard solution of 0.05– 0.10 mol/L was used as the titrant. The titration temperature was 20–30 °C [15,16]. The above titration experiments were conducted using an automatic potentiometric titrator (809Titrando, Metrohm Corp., Switzerland).

The beneficiated clean Panjiang coal was selected for study. The coal was dried under vacuum at 105 °C for 24 h. The crushed coal was comminuted in the ball mill to obtain products of different particle size distributions by controlling the grinding time. The elemental and proximate analyses of the coal are given in Table 1. The commercial sodium lignosulfonate (SL) was part of a by-product of sulfite pulping from the Guangzhou Paper Making Co. Ltd., China; it was composed of 70 wt% sodium lignosulfonate, about 10 wt% reductive substances and 20 wt% low molecular weight organics such as sugar acid and inorganic salts as well as ash. This commercially available SL was a water soluble, light yellow powder, turning to brown when dissolved in water. 2.2. Ultrafiltration of SL The SL sample in the study was refined through the anion exchange resin and cation exchange resin to remove the low molecular weight organic acid, inorganic salt and other impurities. The SL sample was separated into five fractions with molecular weight ranges [10]: less than 5000, 5000–10,000, 10,000–30,000, 30,000–50,000 and more than 50,000, using a hollow fiber membrane ultrafiltration

2.4. Preparation of CWS and viscosity measurement The coal powder was mixed slowly in a pot containing a quantity of dispersant and deionized water. The contents were continuously stirred by means of a mixer during the addition of coal, and then the stirring of the slurry was continued for another 10 min at 1200 rpm to ensure homogenization of the CWS. The slurry so prepared was left for study of its characteristics. The viscosity measurement was performed employing the Brookfield viscometer. Before measurement, the slurries were allowed to stand for 5 min. The measurements were taken within the first 15 s at a speed of 100 rpm. The temperature was kept at 25 °C.

Table 1 Elemental and proximate analyses of the Panjiang coal Sample

Inherent moisture (wt%)

Ash (wt%)

Volatile matter (wt%)

Carbon (wt%)

Hydrogen (wt%)

Oxygen (wt%)

Sulfur (wt%)

Nitrogen (wt%)

Panjiang

1.08

9.21

30.98

88.03

5.04

2.1

0.17

1.72

D. Yang et al. / Energy Conversion and Management 48 (2007) 2433–2438

2.5. Determination of adsorption amounts on the surface of coal particles Dispersant adsorption experiments were conducted as follows [17]. A slurry with a concentration of 10 wt% coal powder by weight was used in the experiments. Dispersant at different concentrations, on the basis of the coal sample, was mixed thoroughly with a given amount of distilled water. Then, a certain amount of sampling coal was added to the solution. This slurry was stirred at a speed of 500 rpm for a given time at a constant temperature. The slurry was then centrifuged, and the supernatant was analyzed to determine the dispersant concentration using a UV spectrophotometer (UV-1601PC, Shimadzu Corp., Japan). The concentration of the dispersant in the solution was determined from the absorbance at 280 nm using predetermined calibration curves. Adsorption was calculated using the initial and final dispersant concentrations in the solution. 2.6. Determination of zeta potential The zeta potential of coal particles in the CWS was measured by electrophoresis using a microelectrophoretic meter (JS94G, Shanghai Zhongchen Corp., China). As this system was intended for a dilute state, the CWS was prepared by dispersing 0.1 g coal powder into 50 mL of distilled water and dispersed again for 5 min by a stirrer. 3. Results and discussion 3.1. Measurements of molecular weight distribution The molecular weights of the different fractions and the commercial SL were measured using gel chromatography, and the mass average of molecular weight (Mw), the numerical average of molecular weight (Mn) and the polydispersity (Mw/Mn) are given in Table 2. The molecular weight distribution curve of the commercial SL sample is shown in Fig. 1. The SL sample has a relatively wide molecular weight distribution. The fraction with molecular

Table 2 Molecular weight and polydispersity of SL Sample

Cut off range of molecular weights

Mw

Mn

Polydispersity Mw/Mn

Commercial SL Fraction 1 Fraction 2 Fraction 3 Fraction 4 Fraction 5

– Less than 5000 5000–10,000 10,000–30,000 30,000–50,000 More than 50,000

11048 1800 7600 9000 17036 26556

1776 340 1505 3132 5456 9290

6.22 5.29 5.05 2.87 3.12 2.86

weight less than 1000 only accounts for 14 wt% of the total amount. Also, the fraction with molecular weight more than 10,000 only accounts for 36 wt%, while the molecular weight of 80 wt% of the fractions is 1000–50,000 [18]. The Mw and Mn of the commercial SL were 11,048 and 1776, respectively, and the polydispersity of the SL was 6.22. The results of the GPC in Table 2 show that the Mw and Mn increase upon an increase of the cut off molecular weights of the ultrafiltration membranes, and the polydispersity (Mw/Mn) decreases, except Fraction 4. The polydispersities of the SL fractions with the various molecular weight ranges are lower than that of commercial SL. 3.2. Functional group analyses SL is a complex polymer containing different functional groups, such as sulfonic, methoxyl, phenolic hydroxyl, carboxyl, ketonic and carbinol structural groups. These active groups affect, in different ways, the physiochemical, electrochemical processes involved in the adsorption of SL on the coal surface and affect the zeta potentials of the coal particles. Here, three important functional groups, sulfonic, carboxyl and phenolic hydroxyl groups, were measured by means of potentiometric tetrameter, and the results are shown in Table 3. From Table 3, it is concluded that the lower is the molecular weight, the more sulfonic group the fraction contained. Fraction 1 has the maximum sulfonic content of 1.46 mol/kg, while Fraction 5 only has 0.95 mol/kg.

100.00

Mw=11048 MP=9285

0.70

2435

0.60

80.00

40.00

0.20 0.10

20.00

0.00

0.00 5.00

4.80

4.60

4.40

4.20

4.00

3.80

3.60

3.40

3.20

3.00

2.80

2.60

2.40

2.20

Slice Log MW

Fig. 1. The molecular weight distribution curve of commercial SL.

2.00

Cumulative %

0.30

60.00

Mn=1776

Mz=27896

0.40

Mz+1=45398

dwt/d(logM)

0.50

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Table 3 The functional group content of SL Sample

Sulfonic (mol/kg)

Carboxyl (mol/kg)

Phenolic hydroxyl (mol/kg)

Commercial SL Fraction 1 Fraction 2 Fraction 3 Fraction 4 Fraction 5

1.38 1.46 1.23 1.12 1.09 0.95

1.48 1.62 1.51 1.45 1.01 0.85

0.65 0.30 0.55 0.58 0.60 0.57

On the contrary, the lower is the molecular weight, the less is the content of the phenolic hydroxyl. Fraction 1 has the least phenolic hydroxyl content of 0.30 mol/kg, while Fractions 2–4 have the approximate content of 0.60 mol/kg. As to the carboxyl group content, Fraction 1 has the maximum content of 1.62 mol/kg, and Fraction 5 has the least content of 0.85 mol/kg. For this reason, the lignosulfonates have a spherical molecular structure [19], and the greater molecular mass has the lesser average charge density of the molecule. With the increase of molecular weight, the content of the sulfonic and carboxyl groups tends to decrease, and the phenolic hydroxyl group content increases. 3.3. Effect of the SL concentration on the viscosity of CWS In order to evaluate the performance of SL in the CWS, different fractions were used for preparing the slurries, and the effect of dispersant concentration on the apparent viscosity of the CWS is given in Fig. 2. As Fraction 1 cannot be used as the dispersant of the CWS, the effect of Fraction 1 on the viscosity of the CWS has not been measured. The dispersant concentration varied from 0.2 to 1.0 wt% (on dry coal basis). The solid concentration was held constant at 67 wt%. The result in Fig. 2 shows that the apparent viscosity decreases sharply upon an increase of the dispersant concentration and then increases slightly above a certain SL

concentration. This tendency may be attributed to a multi-layer adsorption of the dispersants on the coal particles, and many anionic dispersants of CWS have this characteristic [20]. It is revealed from Fig. 2 that at a concentration of 0.4 wt% of Fraction 3, the minimum viscosity of 600 mPa s at a shear rate of 100 s 1 was obtained, and at 0.6 wt% of Fraction 4, 0.7 wt% of Fraction 5, 0.8 wt% of Fraction 2, the minimum viscosity of 650 mPa s, 700 mPa s and 1050 mPa s, respectively, were obtained at a shear rate of 100 s 1. It was concluded from the results that Fraction 3 has the best effect on reducing the viscosity of the CWS of all the fractions, and Fraction 3 at the concentration of 0.4 wt% has a similar effect to that of commercial SL at 0.8 wt%. The higher is the molecular weight of the SL, the better is the effect on reducing the viscosity of the CWS. Fraction 4 has a similar effect to that of Fraction 5 for the CWS. 3.4. Adsorption investigation The dispersants adsorb on the particle surface, thus modifying their surface properties, which is essential for the dispersion of the coal particles in water to prevent flocculation and agglomeration. The adsorption isotherms of SL on the coal surface are illustrated in Fig. 3. The result of Fig. 3 shows that the amount of adsorption of SL on the coal particles increases along with an increase of their equilibrium mass concentration in the solution. Fraction 1, Fraction 2, and the commercial SL generally have two plateaus where constant amounts of dispersant are adsorbed. These plateaus are caused by double layer adsorption of the dispersant on the coal surface. Fraction 1 and commercial SL have higher adsorbed amounts with the increase of the equilibrium mass concentration, and the gaps between the plateaus are great. The reason is that the molecular weight of Fraction 1 is lowest, and the commercial SL has a wide range of molecular weight distribution and much lower molecular weight impurities

1800 1600 1400

-1

Adsorbed amount (mg.g )

Commercial SL Fraction 2 Fraction 3 Fraction 4 Fraction 5

2000

Apparent viscosity/mPa.s

Commercial SL Fraction 1 Fraction 2 Fraction 3 Fraction 4 Fraction 5

6

2200

1200 1000 800

5 4 3 2 1

600 0

400 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

SL concentration/wt% on dry coal Fig. 2. Effect of SL concentration on the viscosity of CWS.

0

200

400

600

800

Equilibrium mass concentration in solution (mg.L-1) Fig. 3. Adsorption isotherm of SL on coal surface.

1000

D. Yang et al. / Energy Conversion and Management 48 (2007) 2433–2438

like sugar, which is easy to be adsorbed in the large quantities of pores on the coal surface [21,22]. However, the adsorption isotherms of Fractions 3, 4 and 5 for which the Mw are higher than 9000 approximate an L shape, and the influence of molecular weight on the adsorption amount is much smaller. The literature [23] stated that the dispersant molecule exhibited a lying state on the coal surface if the adsorption isotherm approximated an S shape and that the dispersant molecule exhibited a standing state on the coal surface if the adsorption isotherm approximated an L shape. Fig. 3 shows that the adsorption isotherms of Fractions 1 and 2 approximate an S shape. Hence, it is deduced that the molecule lies on the solid–liquid interface, and the adsorption film is loose and thin. The adsorption isotherms of Fractions 3, 4 and 5 approximate an L shape. So, it is considered that the molecule stands on the solid–liquid interface, and the adsorption film is compact and thick. Therefore, Fractions 3, 4 and 5 have a better effect on reducing the viscosity than Fraction 2. Fractions 4 and 5 have more adsorbed amounts than Fraction 3. So, theoretically, Fractions 4 and 5 should have a better effect on reducing the viscosity of the CWS than Fraction 3. However, the viscosity experiment shows the reverse result from Fig. 3. This shows that other factors affect the properties of SL as dispersant besides adsorption, so it is necessary to make further investigation of the zeta potential on the surface of the coal. 3.5. Zeta potential investigation The zeta potential measurement of the coal particles in the coal–water suspension is important to the disclosure of the action mechanism between the dispersant and the coal particles. The zeta potential in the CWS is mainly determined by the coal nature, the dispersant type and the cationic ions in the solution [24]. In the experiment, distilled water was used as the dispersion medium and the same coal was used, therefore the influence of the dispersant on the zeta potential was given in Fig. 4.

50

Zeta potential/-mV

45 40 35

2437

It is found from Fig. 4 that the zeta potential of coal in the SL solution is negative. The zeta potential absolute value increases sharply upon an increase of the dispersant concentration until 400–600 mg/L. The zeta potential of the coal in Fractions 3, 4 and 5 remains generally constant above its concentration of 800 mg/L. However, the zeta potentials in Fractions 1 and 2 decrease slightly above its concentration of 1000 mg/L. The adsorption amount of the dispersant influences the zeta potential absolute value. The adsorbed amount increases with the increase of the dispersant concentration before reaching adsorption saturation, which results in the sharp increase of the zeta potential absolute value. When the adsorbed amount exceeds that of the saturation point, the excessive cationic ionizing from the dispersant is scattered in the diffusion layer, and the electronegativity of the coal surface is counteracted, which causes the zeta potential to drop. Fig. 4 also shows that Fraction 3 had the best ability to increase the zeta potential of the coal particles, especially at the concentration of 400 mg/L; the zeta potential absolute value charge is approximately 52 mV. The adsorption investigation shows that Fraction 5 has more adsorbed amount than Fraction 3 but relatively inferior ability to increase the zeta potential due to its lower molecular charging density. Therefore, Fraction 3 has the better effect on reducing the viscosity of the CWS than the other fractions. Based on the above experimental investigation, the viscosity experiment shows that Fraction 3 has a better effect on reducing the viscosity of the CWS than that of the other fractions. The adsorption investigation proves that Fraction 3, 4 and 5 have more adsorbed amount than Fraction 2. Therefore, it is considered that adsorption is the key factor affecting the dispensability of SL. Compared with the other fractions, Fraction 3 has higher molecular weight and lower polydispersity, so it is deduced that the molecular weight and polydispersity of SL have a close relationship to the adsorption property of SL and further influence its ability to reduce the viscosity of the CWS. Compared with Fractions 4 and 5, Fraction 3 has a relatively stronger ability to increase the zeta potential absolute value of the coal particles due to its higher molecular charging density derived from the sulfonic and carboxyl groups, so Fraction 3 has the smaller adsorption amount but better effect on reducing the viscosity of the CWS than Fractions 4 and 5. Therefore, the contents of the sulfonic and carboxyl groups are also important factors affecting the dispersibility of the SL for the CWS.

Fraction 1

4. Conclusions

Fraction 2

30

Fraction 3 Fraction 4

25

Fraction 5 20 0

200

400

600

800

1000

Dispersant concentration in solution/mg.L

1200 -1

Fig. 4. Effect of dispersant concentration on zeta potential.

The effects of the molecular weight of the SL on its ability to reduce the viscosity of the CWS, the adsorption behavior and the zeta potential in the CWS were investigated by using the China Panjiang coal. The results show that SL with the molecular weight of 10,000–30,000 (Fraction 3) has higher Mw (9000) and a better effect on reducing

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the viscosity of the CWS. SL which molecular weight is greater than 10,000 (Fraction 3, 4 and 5) has a Langmuir adsorption isotherm and more adsorbed amount than that of Fraction 2 on the coal surface. However, Fraction 3 has higher contents of the sulfonic and carboxyl groups compared with those of Fraction 4, which makes the highest zeta potential absolute value of about 52 mV on the coal particles. Therefore, the SL with the molecular weight of 10,000–30,000 has the best effect on reducing the viscosity of the CWS. Acknowledgements The author acknowledges the financial supports of National Natural Science Foundation of China (20676040) and the National Natural Science Major Foundation of Guangdong Province (05103536). References [1] Orlando R, Jean-Louis S. Surface activity of lignin derivatives found in the spent liquor of soda pulping plants. Tappi J 1994;77:123–6. [2] Amel K, Ahmed J, Moncef C. Evaluation of the performance of sulfonated esparto grass lignin as a plasticizer–water reducer for cement. Cem Concr Res 2003;33:995–1003. [3] Nadif A, Hunkeler D, Ka¨uper P. Sulfur-free lignins from alkaline pulping tested in mortar for use as mortar dispersants. Bioresour Technol 2002;84:49–55. [4] Nour-Eddine El M, Joan S. Structural characterization of technical lignins for the production of adhesives: application to lignosulfonate, kraft, soda anthraquinone, organosolv and ethanol process lignins. Ind Crops Prod 2006;24:8–16. [5] Laskowski JS. Does it matter how coals are cleaned for CWS. Coal Prep 1999;21:105–23. [6] Yasuyuki M, Seiichi Y. Preparation and evaluation of lignosulfonates as a dispersant for gypsum paste from acid hydrolysis lignin. Bioresour Technol 2005;96:465–70. [7] Pan X, Zeng F, Fu X. Research on the relationship between the humic substances grades and their ability as CWM dispersants. Coal Conver 1999;22:38–42. [8] Qiu XQ, Yang DJ. Adsorption performances of calcium lignosulfonates on surface of solid particle. J Chem Ind Eng 2003;54(8):1155–9.

[9] Ran N, Dai Y, Hu B, et al. Dispersive effect of methylene naphthalene sulfonate–styrene sulfonate maleate on coal water mixture. J Nanjing Univ Nat Sci 1999;35:643–7. [10] Ola W, Ann-Sofi J, Roland W. Fractionation and concentration of Kraft black liquor lignin with ultrafiltration. Desalination 2003;154: 187–99. [11] Liane ECL, Geraldo L, Sant’Anna J, Ronaldo N. Molecular weight distribution of chlorolignin in bleached Kraft effluent by GPC and ultrafiltration. Bioresour Technol 1999;68:63–70. [12] Chen FG, Li J. Determination of molecular weight and molecular weight distribution of lignin by gel permeation chromatography. J Cell Sci Technol 1999;7:47–51. [13] Li J, Zuo XJ. Determination of relative molecular mass distribution of lignosulfonate and sulfonated soda lignin by aqueous GPC. J Instr Anal 1995;14:47–50. [14] Chen M, Chen JX. Discussion of several problems about determining sulphonic groups in pulp with conductometric titration method. China Pulp Paper 1991;10:35–8. [15] Liu HW, Yu JL, Chen JX. Determination of phenolic hydroxyl and carboxyl groups in lignin with conductometric titration in organic solvent. China Pulp Paper 1990;10:55–8. [16] Staccioli G, Stasiu LD, Mcmillan NJ. Assessment of carboxyl groups of some Canadian Arctic fossil woods to evaluate their degradation. Org Geochem 1997;27:561–5. [17] Yavuz R, Ku¨c¸u¨kbayrak S. An investigation of some factors affecting the dispersant adsorption of lignite. Powder Technol 2001;119: 89–94. [18] Pang YX, Qiu XQ, Yang DJ, et al. Properties of calcium lignosulfonate water reducers with different relative molecular mass. J South China Univ Technol 2002;30:53–8. [19] Micic M, Benitez I, Ruano M, et al. Probing the lignin nanomechanical properties and lignin–lignin interactions using the atomic force microscopy. Chem Phys Lett 2001;347:41–5. [20] Kaushal K, Tiwari SK, Basu KC. High concentration coal–water slurry from Indian coals using newly developed dispersants. Fuel Proc Technol 2003;85:31–42. [21] Sun CG, Li BQ, Yu CW. Characterization of pore size distribution and slurryability of coal. J Fuel Chem Technol 1996;24:434–9. [22] Yavuz R, Kucukbayrak S. Adsorption of an anionic dispersant on lignite. Energy Conver Manage 2001;42:2129–37. [23] Li FQ, Zhu SHQ. Study on construction and performance of modified sodium lignosulfonate. J China Coal Soc 2000;25: 439–42. [24] Ogura T, Tanoura M, Hiraki A. Behavior of surfactants in a highly loaded coal–water slurry. I. Effects of surfactant concentration on its properties. Chem Soc Jpn 1993;66:1343–9.