Synthesis and Evaluation of Sulphonated Acetone–Formaldehyde Resin Applied As Dispersant of Coal–Water Slurry

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Energy Conversion and Management 48 (2007) 204–209 www.elsevier.com/locate/enconman

Synthesis and evaluation of sulphonated acetone–formaldehyde resin applied as dispersant of coal–water slurry

Mingsong Zhou, Xueqing Qiu *, Dongjie Yang, Weixing Wang

School of Chemical and Energy Engineering, Guangdong Provincial Laboratory of Green Chemical Technology, South China University of Technology, Wushan, 510640 Guangzhou, Guangdong, PR China

Received 23 October 2005; accepted 28 April 2006 Available online 22 June 2006

Abstract

The water-soluble sulphonated acetone–formaldehyde (SAF) resin was synthesized by a reaction among acetone, formaldehyde and sodium sulfite. It is applied as a dispersant of a coal–water slurry, and the influence of the synthesis processing parameter on the inherent viscosity and the dispersing ability of the SAF resin was investigated. The composite SAF resin was regarded as an aliphatic high polymer containing block hydrophilic and hydrophobic structures in the molecular chain, which show little surface activity. The dispersing ability of the SAF resin dispersant in a coal–water slurry was evaluated, and the effect of the SAF mass concentration on the rheological property of the slurries was determined. The results showed that the SAF resin has potential to be developed as a new type of dispersant of a coal–water slurry. Ó 2006 Elsevier Ltd. All rights reserved.

Keywords: Sulphonated acetone–formaldehyde; Dispersant; Inherent viscosity; Rheological property; Coal–water slurry

1. Introduction of dispersants, such as sodium naphthalene sulfonate formaldehyde condensate, sodium polystyrene sulfonate and non- Many studies have been performed on the coal–water ionic surfactants, as well as lignosulphonate and humate, slurry (CWS), which has been regarded as a promising fuel have been known as useful dispersants for the CWS [6–9]. instead of petroleum oil due to the rapid depletion of the However, the sodium naphthalene sulfonate formaldehyde latter. The most important rheological characteristic of a condensate, the sodium polystyrene sulfonate and the coal–water slurry is its viscosity. It is desirable that a par- non-ionic dispersants have the deﬁciency of their increasing ticular coal–water slurry has a high pulp solids percentage raw material cost, and the lignosulphonate, humate disper- and a low viscosity value. To achieve the demands that the sants have the disadvantage of low performance. Therefore, coal content in the CWS should be attained as high as pos- a relatively low cost and high performance resin, water- sible and the amount of dispersant utilized to prepare the soluble sulphonated acetone–formaldehyde (SAF) resin, CWS should be as low as possible, many investigations was synthesized as the dispersant for the CWS in the present have been published [1–4]. work. The water soluble sulphonated acetone–formalde- Surface active chemicals, i.e., dispersants, were presented hyde resin, which was developed in the 1990s [10,11], was to modify the interparticle forces in the coal–water interface. known as the superplasticizer in the construction industry. The viscosity and rheological behavior were dependent on To our best knowledge, the application of SAF as a disper- the dispersant type and its concentration [5]. Many series sant for the CWS has not been reported. The objective of the present study was to determine * Corresponding author. Tel./fax: +86 20 8711 4722. experimentally the eﬀect of the synthesis processing param- E-mail addresses: [email protected] (M. Zhou), cexqqiu@ eter on the inherent viscosity of the SAF resins and the vis- scut.edu.cn (X. Qiu). cosity of the slurries. Also, the application property of the

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SAF as a dispersant for the CWS, the dispersing ability and Sodium sulfite was dissolved in water and the rheological behavior of the slurries, were also the temperature of the solution was investigated. maintained at 45 °C 2. Experimental Acetone was added and reaction was carried 2.1. Experimental materials out at 45 ˚C for 20 min

The commercial products of acetone, formaldehyde Formaldehyde aqueous solution of 37% solution (37%) and sodium sulfite were used as crude mate- concentration was fed dropwise and the rials of the chemical synthesis. temperature should be maintained below 65 Beneficiated clean Datong coal was selected for study. The coal was first crushed in a jaw crusher to obtain a below 10 mm product. Then, the coal was dried under vac- The temperature was raised to 80 ~ 85 °C, uum at 105 °C for 24 h. The crushed coal was comminuted and the reaction continued for 3 h in the ball mill to obtain products of different particle size distributions by controlling the grinding time. The elemen- The SAF resin of approximately 35% solid content tal and proximate analyses of the coal are given in Table 1.

2.2. Synthesis of SAF resin Fig. 1. Synthesis process of SAF resin.

Sodium sulfite was dissolved in water in a reactor flask with a control sample, and then, the experimental sample equipped with a temperature control electric heating scanning was conducted. device, a motor stirrer, a thermometer, a dropping funnel and a reflux condenser. The temperature of the solution 2.4. Measurements of the surface tension of the resin was maintained at 45 °C in the dissolution course. When the solution became clear, the acetone was added, and then A surface tension meter (JK99B, Shanghai Zhongchen the reaction was performed at 45 °C for 20 min. Then, the Corp.) was used to measure the surface tension of the formaldehyde aqueous solution of 37% concentration was SAF resin solution, which was performed in distilled water. fed into the reactor by means of a dropping funnel. During the feeding course, the temperature of the solution will 2.5. Measurements of the inherent viscosity of the resin increase automatically and should be controlled below 65 °C. After feeding of the formaldehyde solution, the tem- The Ubbelohde viscometer was used to measure the perature was raised to 80–85 °C and the reaction continued inherent viscosity of the SAF resin using the dilution and for 3 h. After cooling, the obtained SAF resin, according to extrapolation method [12]. the above procedure, has a solid content of approximately 35% and the final pH of the product is above 12. The syn- 2.6. CWS preparation and viscosity measurements thesis process is shown in Fig. 1. The coal powder was mixed slowly in a pot containing 2.3. Infrared spectrum analysis known quantities of additive and deionized water. The con- tents were continuously stirred by means of a mixer during The Fourier transform infrared (FT-IR) spectrometry of addition of the coal, and then, stirring of the slurry was Auto system XL/ i-series /Spectrum 2000 PE was used in continued for a further 10 min at 1200 rpm to ensure an infrared spectrum analysis. A certain amount of sample homogenization of the CWS. The slurry, so prepared, was mixed with potassium bromide in a mortar box. After was left for study of its characteristics. each grinding, the mortar box was thoroughly cleaned with The viscosity measurement was performed employing acetone. The grinding powder of the sample and potassium the Brookfield viscometer. Before measurement, the slur- bromide was pressed using the tablet machine for subse- ries were allowed to stand for 5 min. The measurements quent infrared spectrum analyses. The Spectra were col- were taken within the first 15 s employing a rotation speed lected within a scanning range of 400–4000 cm1. The of 100 rpm. The temperature was kept at 25 ± 1 °C. The FT-IR was first calibrated for background signal scanning measured viscosity value is the apparent viscosity.

Table 1 Elemental and proximate analyses of Datong coal sample Coal Moisture (wt%) Ash (wt%) Volatilematter (wt%) Carbon (wt%) Hydrogen (wt%) Oxygen (wt%) Sulfur (wt%) Nitrogen (wt%) Datong 2.94 6.53 27.40 74.56 4.54 11.05 0.85 0.22 中国科技论文在线 http://www.paper.edu.cn

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2.7. Rheological behavior measurements

A rheometer (RV–I , Haake corp.) was used to measure the rheological property of the CWS. The Z41 rotor was Fig. 3. Chemical structure of the SAF resin. used. The shearing rate range is: up run 0–200 S1, down run 200–0 S1. The time of the up run and down run is 3 min, and the temperature is kept at 25 °C. The measured rheological curve was ﬁtted and used to determine the rhe- 70

ological behavior. ] -1

3. Results and discussion 60

3.1. Infrared spectrum analysis 50 The infrared spectrum was used to analyse the chemical

structure of the resins. Fig. 2 shows the infrared spectrum Surface tension [mN.m of the SAF resin. The adsorption peak in 3435 cm1 is 40 the stretching vibration peak of the hydroxyl group; those in 1644 cm1 and 1705 cm1 are the characteristic adsorp- 1E-3 0.01 0.1 1 10 100 Concentration, g/L tion peaks of the carbonyl group; and those in 1182 cm1 and 1042 cm1 are the characteristic adsorption peak of Fig. 4. Surface tension of the SAF resin. the sulfonic group. The analysis proved that the SAF resin is an aliphatic high polymer containing hydroxyl, carbonyl 3.3. Effect of the dosage of the crude materials on inherent and sulfonic groups. The chemical structure of the SAF viscosity and dispersing ability of SAF resin is shown in Fig. 3. In the industry, as a norm, the polymer molecular 3.2. Surface tension of SAF weight is reported by inherent viscosity, denoted by [g]. It is related to the molecular weight, M, through the Mark– The SAF resin is a high polymer surfactant with many Houwink equation [13]: hydrophilic groups and hydrophobic groups in the molec- KM a ular chain. Fig. 4 shows that the surface tension decreases ½g¼ very little with the increase in concentration of the SAF K and a are constants specific to the solvent and temper- resin. There is no evident critical micelle concentration like ature used in the measurements. The inherent viscosity with other surfactants. Therefore, it is proved that the SAF determines the polymer molecular weight and, further, resin has little surface activity, and there are many block influences the dispersing property of the SAF resin. In hydrophilic and hydrophobic structures in the molecular the experiment, it is proved that the inherent viscosity of chain. SAF resins is affected greatly by the mixture ratio of the

Fig. 2. Infrared spectrum of the SAF resin. 中国科技论文在线 http://www.paper.edu.cn

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crude materials. Therefore, the effect of the mole ratios of low molecular weight of the SAF resins cause poor dispers- sodium sulfite to acetone and formaldehyde to acetone ing performance in the CWS, the suitable sodium sulfite on the inherent viscosity and dispersing property of the addition is necessary. SAF resins were studied, and the results were given in Figs. It is shown in Fig. 6 that with the increase of the mole 5 and 6, respectively. ratio of formaldehyde to acetone, the inherent viscosity It is seen from Fig. 5 that the inherent viscosity of the of the SAF resins increases abruptly at first, then decreases SAF resin decreases abruptly as the mole ratio of sodium abruptly and then increases again, and the apparent viscos- sulfite to acetone is increased, reaches its minimum and ity of the CWS decreases abruptly at first and then then levels off, while the apparent viscosity of the CWS increases slightly. Around the mole ratio of 2.0, the CWS decreases abruptly before the mole ratio of 0.40 and then reaches the minimum viscosity of about 580 mPa s and increases after that. In the synthesis reaction, the sodium the SAF resin has the inherent viscosity of about 8 ml/g. sulfite adds to the water solubility of the resins, however, In the synthesis reaction, the addition of formaldehyde excessive sodium sulfite leads to lower molecular weight is to polymerize the monomers, so excessive formaldehyde so there is a suitable addition of the sodium sulfite. In leads to high molecular weight of the SAF resins, and the the experiment, it is found that the suitable mole ratio of lack of formaldehyde results in low molecular weight. sodium sulfite to acetone is 0.4. At this mole ratio, the However, the synthesis reaction is complex. Less dosage SAF resin has the best dispersing performance in the of the formaldehyde also causes poor water solubility and CWS. As to the reason, less sodium sulfite causes poor high inherent viscosity of the SAF resins, so only a suitable water solubility, but excess sodium sulfite causes lower mole ratio of formaldehyde to acetone (about 2.0) can pro- molecular weight. Because both poor water solubility and duce resins with the suitable molecular weight and excellent dispersing ability in the CWS, as shown in Fig. 6.

1800 35 3.4. Eﬀect of amount of the dispersant on apparent Apparent viscosity viscosity of CWS 1600 Inherent viscosity 30

Inherent viscosity [ml.g The SAF resin synthesized in this experiment was used 1400 25 as dispersant for CWS preparation. The viscosity values

1200 20 of the CWS obtained using solid ratios of 65% at neutral pH in the presence of diﬀerent concentrations of SAF resin 1000 15 are given in Fig. 7. Fig. 7 shows the inﬂuence of the amount of SAF resin 10 800 on the apparent viscosity measured at a shear rate of -1

] 1 5 Apparent viscosity [mPa.s] 600 100 s . It is seen that the apparent viscosity decreases abruptly in proportion to increasing the amount of the dis- 0 400 persant, reaches a minimum and then increases a little. The 0.30 0.35 0.40 0.45 0.50 0.55 minimum viscosity is about 700 mPa s at the mass concen- Moll ratio of sodium bisulfite to acetone tration of 0.75%, so the amount of 0.70–0.80% is suitable Fig. 5. Eﬀect of the mole ratio of sodium sulﬁte to acetone on inherent for the CWS preparation of 65% solid concentration in viscosity and dispersing ability of SAF. the present experiment. At higher dispersant concentrations, the viscosity is likely to increase again slightly.

24 2700 Apparent viscosity 22 2400 Inherent viscosity 1800 20 Inherent viscosity [ml.g 2100 18 1600

1800 16 1400 14 1500 12 1200 1200 10 900 1000 8 -1 ] Apparent viscosity [mPa.s] Apparent viscosity [mPa.s] 600 6 800

300 4 600 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 n(formaldehyde):n(acetone) SAF concentration [wt% on Dry Coal]

Fig. 6. Eﬀect of the mole ratio of formaldehyde to acetone on inherent Fig. 7. Eﬀect of dispersant mass concentration on apparent viscosity of viscosity and dispersing ability of SAF. CWS. 中国科技论文在线 http://www.paper.edu.cn

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3.5. Effect of the dispersant mass concentration on decrease in the magnitude of sy, from 12.19354 to rheological behavior of CWS 0.39892. The flow characteristic exponentials n with the SAF mass concentrations of 0.4%, 0.6% and 0.8% are One of the major requirements to be met in preparing a 0.54194, 0.97435 and 0.92538, respectively. Because n <1, CWS is that it must have as high a coal concentration as these slurries belong to the pseudoplastic fluid class with possible but a minimum viscosity to allow ease of handling the property of dilution characteristics under shearing . during preparation, storage, transfer and atomization The flow characteristic exponential n with the SAF concen- [14,15]. The characterization of the rheological behavior tration of 1.0% is 1.05420, or n > 1, so the slurry belongs to of the CWS is the most important influence factor. It is well the dilatant plastic fluid class with the property of thicken- known that the CWS is a nonlinear non-Newtonian fluid. ing characteristic under shearing. Probably most non-Newtoian fluids exhibit shear thinning To evaluate further the effect of added mass concentra- behavior, but as shown in Fig. 8, the CWSs with the SAF tion of SAF resin on the rheological property of the CWS, mass concentrations of 0.4%, 0.6% and 0.8% show pseudo- the relation between the SAF concentration and the yield plastic character, while that of 1.0% shows a dilatant plas- stress sy, the consistency coefficient K and the flow charac- tic character. In the present study, the shear stress – shear teristic exponential n were studied. It was found that a rate curves were fitted to the Herschel–Bulkley model lower SAF mass concentration resulted in higher yield [16,17]. stress, higher consistency coefficient value and lower flow s ¼ s þ Kcn characteristic exponential value. However, it is seen from y Fig. 7, less dispersant addition resulted in high viscosity where sy, K and n are rheological constants referred to as of the slurry, which is undesirable in industry. The added the yield stress, the fluid consistency coefficient and the mass concentrations of 0.6% and 0.8% are relatively suit- flow characteristic exponential, respectively. able because of the lower viscosity and dilution character- Using the Herschel–Bulkley model, the experimental istics under shearing of the slurry. On the other hand, results from Fig. 8 were correlated and the estimated values excessive added mass concentration leads to higher flow of sy and the model parameters were listed in Table 2. characteristic exponential value, and the slurry trends to From Fig. 8 and Table 2, it is clear that increasing the the dilatant plastic fluid class. Though the slurry showed SAF resin concentration from 0.4% to 0.8% resulted in a relatively lower apparent viscosity, the viscosity would be incremented under high shear rate in industrial application due to its dilation characteristic. Therefore, it is concluded that the suitable added mass concentration of SAF 160 0.4 wt% resin for the CWS should be more than 0.6% and lees 0.6 wt% 140 0.8 wt% than 1.0%. 120 1.0 wt%

100 4. Conclusions 80

60 The characterization results showed that the SAF resin

40 is an aliphatic high polymer containing hydroxyl, carbonyl

Shear stress [Pa] and sulfonic groups, and the molecule chain is considered 20 to have block hydrophilic and hydrophobic structures. 0 The inherent viscosity of SAF resins decreases in pro-

-20 portion to increasing amounts of sodium sulfite, while it 0 50 100 150 200 is complicated for the formaldehyde addition amount in -1 Shear rate [S ] the synthesis reaction. The inherent viscosity of SAF resin Fig. 8. Effect of the mass concentration of the SAF resin on rheological is influenced greatly by the addition amount of the sodium behavior of CWS. sulfite and formaldehyde, and the SAF resin with the inherent viscosity of 8–10 ml/g is most suited to be a dispersant of the CWS. Table 2 The amount of 0.70–0.80% of SAF resin is suitable for Rheological parameters derived from Herschel–Bulkley model fits to the the CWS preparation of 65% solid concentration. The rhe- data of Fig. 8 ological investigation showed that the lesser amount of the n Addition mass concentration sy (Pa) K (Pa s ) n SAF resin favors the flow pattern of the CWS but makes of SAF (wt% on dry coal) higher viscosity, while excessive amount of the SAF resin 0.4 12.19354 5.30205 0.54194 makes the viscosity increase slightly and the slurry to be 0.6 6.38759 0.81754 0.97435 dilation characteristic. Under the suitable addition 0.8 0.39892 1.09882 0.92538 amount, the SAF resin is proved to be a high performance 1.0 0.78613 0.51403 1.05420 dispersant for the CWS. 中国科技论文在线 http://www.paper.edu.cn

M. Zhou et al. / Energy Conversion and Management 48 (2007) 204–209 209

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