1. Introduction
The efficient and clean utilization of coal resources is of significant interest in recent years. Coal gasification technologies, especially the opposed multi-burner (OMB) gasification technology using coal–water slurry (CWS) as a feedstock, can achieve the efficient and clean utilization of coal resources, which attracted the attention of the world [
1]. The desirable CWS should have properties, such as a high coal content for economic consideration, excellent stability for storage, and a viscosity as low as possible for transportation and combustion. To achieve the demands, the selection of a dispersant plays an extremely important role [
2,
3]. Some dispersants have been commercialized, such as naphthalene sulfonated formaldehyde condensates, sulfonated melamine formaldehyde polymers and the aminosulfonic acid series, etc. They are in the process of synthesis or use may result in undesirable environmental effects, as they can accidentally or intentionally release formaldehyde into the environment [
4,
5,
6,
7,
8]. Therefore, an environmentally friendly and biodegradable additive is urged to be developed as a new high-performance dispersant for coal–water slurries using cheap and abundant natural renewable resources as raw materials.
Cellulose is the main representative of such resources. Cellulose is one of the most widely distributed biopolymers on earth, having many unique advantages, such as low cost, renewability, being environmentally friendly, natural biodegradation, chemical stability and biocompatibility [
9,
10]. Cellulose-based materials have attracted much attention and have already been applied in food, water treatment, membrane materials, drug delivery, 3D printing and other fields [
11,
12,
13,
14,
15]. In the last few years, research and technology of water-soluble cellulose derivatives as water-reducing agents of cement concrete have made remarkable discoveries in the field of bio-renewable materials [
16,
17]. Wang, L. J. have synthesized water-soluble sulfobutylated cellulose ether (SBC); the study results indicate that water-soluble SBCs have the potential to be developed as a set-retarding and water-reducing agent—even a high-range water-reducing agent [
18]. However, to the best of our knowledge, there is no report on the direct synthesis of dispersants for coal–water slurries.
In this investigation, we have developed cellulose derivatives containing sulfonate and carboxylate esters through one-pot synthesis. This method utilizes sulfamic acid to obtain high degrees of sulfonate substitution (DS sulfonate), while sulfamic acid is used as a catalyst for the esterification reactions. The CWS application properties, such as viscosities and stabilities, were investigated in the presence of the dispersants. Based on the investigations, the feasibility of the described method provided a basis for the synthesis, characterization and application of cellulose-based dispersant.
2. Materials and Methods
2.1. Materials
Cotton linter was obtained from the Aksu Cotton Linter Processing Company (the content of cellulose in cotton linter was 94.34%) (Xinjiang, China). Sulfamic acid, urea and other reagents (analytical grade) were obtained from the Tianjin Guangfu Chemical Company (Tianjin, China).
Zhundong coal from Fukang, Xinjiang Province in China, was used in this study.
Table 1 shows the results of the proximate analyses of coal.
The particle size distribution of the coal sample is given in
Table 2.
2.2. Methods
The synthetic route for S-CSHEs included esterification by sulfamic acid and succinic anhydride, as shown in
Scheme 1.
The degrees of degradation, sulfonation and esterification of S-CSHE could be controlled stoichiometrically, and by adjusting the time and temperature of the chemical reaction. Sulfamic acids were used as reactants, while sulfamic acid also proved to be a cost-effective and recyclable catalyst for esterification. The catalytic application of sulfamic acid was recognized due to its easy separation, recyclability, greater economic viability and environmental friendliness [
19,
20,
21,
22,
23,
24].
Preparation of cellulose derivatives containing sulfonate and carboxylate groups were cotton linters cellulose (8.1 g, ~0.05 mol), sulfamic acid (13.55 g, ~0.15 mol), succinic anhydride (6 g, ~0.06 mol), urea (3.0 g, ~0.05 mol), and DMF, and were mixed in a 100 mL three-necked flask equipped with a thermometer. The mixture was warmed to 120 °C and was stirred for 6 h. The products were filtered, washed with ethyl alcohol, and dried at 80 °C under vacuum. Then, it was dissolved in distilled water, and the solution was dialyzed against distilled water. The final samples were obtained after being freeze-dried by using a freezer dryer at −50 °C for 2 days.
2.2.1. Determination of Degree of Substitution (DS)
The extent of ester, as defined by the succiniclic acid content (O-C=O %) and the sulfur content (S%), was measured using an X-ray photoelectron spectroscopy (D/max2500, Bruker AXS, Beijing, China). The DS was calculated according to Equation (1) and Equation (2), as follows [
8,
25]:
where ω
s and ω
c are the mass fraction of the sulfonic and carboxyl groups; 122, 102 is the molar quantity of the substituted group; 162 is the molar quantity of a grape unit; 44 is the molar quantity of the O-C=O; 32 is the molar quantity of the S.
2.2.2. Fourier Transform-Infrared (FT-IR) Spectroscopy
The chemical structures of cellulose and sulfonic-cellulose succinate half ester (S-CSHE) were characterized by FT-IR spectroscopy (FT/IR-430, JASCO, Tokyo, Japan). The samples were prepared by grinding to a fine powder (sample/KBr 1:1000 ratio). FT-IR spectra were recorded over the wavenumber range from 500 cm−1 to 4000 cm−1.
2.2.3. Nuclear Magnetic Resonance Spectroscopy (NMR)
13C-NMR spectra of S-CSHE samples were recorded in D2O (50 mg/mL) containing tetramethylsilane as an internal standard. The spectra were acquired on an Inova-400 m superconducting nuclear magnetic resonance spectrometer(Bruker, Billerica, MI, USA) at room temperature, collecting 16 scans.
2.2.4. Scanning Electron Microscopy (SEM)
The morphologies of the samples were imaged with field emission scanning electron microscopy (FESEM, Hitachi SU800 equipped with an EDS system) (Garl Zeiss AG, Oberkochen, Germany), in which all samples were spread on conductive carbon tape and sputtered with gold at an acceleration voltage of 10 kV.
2.2.5. Molecular Weight Distribution
The samples were dissolved in high-purity water with a concentration of 8.00–10.00 mg/mL at the appropriate temperature; prior to injection, the sample solutions were filtered. Following the principle of GPC, the molecular weight distributions of the samples were estimated in
Table 3.
2.2.6. Viscosity Measurement and Rheological Property Determination
The viscosities of CWS were measured by using a rheometer (AR-2000, TA Instrument Company, New Castle, DE, USA). The shear rate range was 0–100 s−1, and the temperature was maintained at 25 °C.
2.2.7. Adsorption Amount Measurement
The coal–water slurry was prepared with 10 wt.% of coal and the known concentration of a dispersant. The slurry was stirred at 1200×
g rpm for 10 min, then centrifuged, and the supernatant was used for determining dispersant equilibrium concentration, employing a UV–vis spectrophotometer (UV-265FW, Shimadzu Corp., Tokyo, Japan). The concentration of the dispersant in the solution was determined from the characteristic absorption wavelength according to a predetermined calibration curve. The adsorption amount was calculated as follows [
8,
25]:
where
is the adsorption amount per unit mass coal (mg·g
−1),
and
are the initial and final mass concentrations of the dispersant (mg·L
−1), V is the total volume of the solution (L) and m is the mass of the coal sample (g).
4. Conclusions
Based upon this study, S-CSHEs can be developed as a new dispersant for coal–water slurries by means of the chemical modification of cotton linters cellulose using C4H4O3-NH₂SO3H-DMF as a modifying agent by a one-pot synthesis. The results demonstrated that the DS is crucial to control the fluidity, stability and viscosity of CWS paste. It was noted that, as DS increased, the molecular weight was diminished. Furthermore, the flexibility of the S-CSHE molecule and the roughness of the molecular surface became smaller. It could be concluded that S-CSHE (DS = 0.75) had a better viscosity-reducing ability and dispersibility, as well as stabilizing ability for CWS. The improvements in the application performance were mainly attributed to the special cellulose structure. The CWS prepared using S-CSHE (0.38 wt.%, DS = 0.75) for Zhundong coal showed the rheological characteristics of shear-thinning, and were consistent with the Herschel–Bulkley model. Meanwhile, this research is a new way to enhance the utilization value of the renewable cotton linter of Xinjiang and widens the selection range of the dispersant of CWS.