Study of sodium lignosulfonate prepare low-rank coal-water slurry:Experiments and simulations

Lin Li,Chuandong Ma,2,Mengyu Lin,2,Mingpu Liu,Hao Yu,Qingbiao Wang,Xiaoqiang Cao,Xiaofang You,

1 School of Chemistry and Biological Engineering,Shandong University of Science and Technology,Qingdao 266590,China

2 College of Safety and Environment Engineering,Shandong University of Science and Technology,Qingdao 266590,China

3 National Engineering Laboratory for Coalmine Backfilling Mining,Shandong University of Science and Technology,Tai'an 271019,China

4 Department of Resource and Civil Engineering,Shandong University of Science and Technology,Tai'an 271019,China

Keywords:LCWS Low-rank coal Sodium lignosulfonate MD simulation

ABSTRACT The effect of sodium lignosulfonate(SL)as additive on the preparation of low-rank coal-water slurry(LCWS)was studied by experiments and molecular dynamics(MD)simulations.The experimental results show that the appropriate amount of additives is beneficial to reduce the viscosity of LCWS and increase the slurry concentration.Adsorption isotherm studies showed that SL conforms to single-layer adsorption on the coal surface,and ΔGa 0ds was negative,proving that the reaction was spontaneous.Zeta potential measurements showed that SL increased the negative charge on coal.FTIR scanning and XPS wide-range scanning were performed on the coal before and after adsorption,and it was found that the content of oxygen functional groups on coal increased after adsorption.Simulation results show that when a large number of SL molecules exist in the solution,some SL molecules will bind to hydrophobic hydrocarbon groups on coal.The rest of the SL molecules,their hydrophobic alkyl tails,come into contact with each other and aggregate in solution.The agglomeration of SL molecules and the surface of coal with static electricity will also produce electrostatic interaction,which is conducive to the even dispersion of coal particles.The results of mean square displacement(MSD)and self-diffusion coefficient(D)show that the addition of SL reduces the diffusion rate of water molecules.Simulation results correspond to experimental results,indicating that MD simulation is accurate and feasible.

1.Introduction

There are abundant reserves of low-rank coal[1]resources in the world,but the direct combustion of low-rank coal has brought serious environmental problems[2–4].Clean use of coal is important for future policy.Low-rank coal-water slurry(LCWS)is a kind of environmental friendly fuel with fluidity[5].It contains of 60%–70%pulverized coal,minimal additives and 30%–40%water.

The addition of a small amount of additives is beneficial to the stability and uniform distribution of coal particles in LCWS.The adsorption effect of additives on coal is closely related to the dispersion performance of coal.Various factors will affect the solid–liquid interface adsorption,such as van der Waals forces,hydrogen bonds and electrostatic repulsion forces[6].Adsorption behavior of additives on coal particles has been extensively studied.Honghong Chang et al.[7]studied the wettability and adsorption mechanism of cationic Gemini additives with different chain lengths(hydrophobic)on coal pitch by using the soliddrop analysis and electrophoresis technology,and concluded that the wettability was the result of electrostatic interaction and van der Waals adsorption.Guanhua Ni et al.[8]found that the surface tension of water will decrease with the increase of additive dosage.Shengyu Liu et al.[9]found that the adsorption of egg-1,2-dimethyl lauryl bromide(12-2-12),a cationic Gemini surfactant,on the surface of lignite would cause the wettability change of lignite and slow the adsorption of dry lignite to water.During the adsorption process,oxygen functional groups on coal are beneficial to the adsorption of surfactants,and electrostatic force plays a leading role in the adsorption process.Jianying Guo et al.[10]studied the effects of two non-ionic surfactants n-dodecyl beta-D-maltoside(C12G2)and dodecyl hepta glycol(C12E7)on the hydrophilicity of lignite.Compared with the original lignite,the strongly polar oxygen-containing groups on the surface of lignite after adsorption of C12E7 were covered by ethers with weak polarity in C12E7,which weakened the interaction between water and lignite.By adsorbing C12G2 and introducing polar hydroxyl group,the surface polarity of lignite was improved and hydrophilicity was enhanced.Although there are a large number of additives on coal adsorption researches,the microscopic mechanism of the interaction of coal/additive/water three-phase system in the adsorption process has not been explained from the molecular level.

MD simulation[11–13]provides us with powerful help in studying the mechanism of microcosmic action in the experimental process.MD simulation can not only study the micro-mechanism of solid surface(minerals)and surfactants from the molecular level,but also provide the energy,power and structure information which cannot be obtained in the experimental process.Smit et al.[14]used molecular simulation to study the energy and position of straight and branched alkanes in the zeolite silicalite.Hao Du et al.[15]conducted a preliminary MD simulation study on the interface phenomenon on the surface of talcum powder,and the simulation results showed that there was no close contact between hydrophobic talcum powder and water molecules due to the absence of hydrogen bond site,leaving a gap of 3(1?=0.1 nm)? on the surface.It was found that the cationic surfactant dodecyltrimethylammonium bromide(DTAB)preferentially adsorbed on the talcum powder base surface through hydrophobic interaction,which was consistent with the previous experimental results.These studies show that MD simulation has become an important tool for studying adsorption and energy calculation.However,due to the complex chemical structure of coal[16,17],there are few studies on the interaction mechanism of coal/surfactant/water three-phase system.And mainly to study the wettability and the mechanism of action of the collector in the flotation process.There is a lack of microscopic mechanism research on the effect of additives on the slurry formation of coal-water slurry(CWS),especially low-grade coal pulping.Xiaofang You et al.[18]studied the filtration and dehydration process of lignite in the presence of TRITON X-100 (TX-100) by experimental method and MD simulation.MD simulation was used to study the single-layer adsorption of TX-100 on the surface of lignite.The calculation results of mean square displacement(MSD)and self-diffusion coefficient(D)showed that TX-100 could improve the mobility of water molecules and facilitate water removal,which was consistent with the experimental results.Yangchao Xia et al.[19]studied the effect of DTAB as collector on flotation of coal by combining experimental study with molecular dynamics simulation.The promotion of DTAB on low-order coal and dodecane adsorption reactions was verified in MD simulations.You et al.[20]studied the adsorption of nonylphenol ethylic acetate on the surface of subbituminous coal by 10 epoxy ethane units with experimental and computational methods,and found that the results of interaction energy calculated by MD simulation are consistent with Langmuir isotherm fitting results.

To further understand the interaction mechanism of LCWS additives in the coal/water interface,the micro mechanism of coal/additives/water three phase system interaction was discussed from the molecular level by combining experiment and simulation.It is helpful to understand the influence of different components on the slurrability of LCWS.

2.Materials and Methods

2.1.Materials

The coal samples(bituminous coal)used in the test were taken from Shendong coal mine group,and elemental analysis were carried out.The results are shown in Table 1.

The particle size of coal sample is analyzed after crushing and grinding(BT-9300Z,Dandong better instrument Co.Ltd.).The distribution of particle size of coal sample is shown in Fig.1.

Table 1 Ultimate analysis of the coal

Fig.1.Particle size distribution.

Sodium lignosulfonate(SL)is produced by Shanghai Aladdin biochemical technology Co.,Ltd.(analytical grade,purity ≥98.0%).It is a kind of additive commonly used to prepare CWS.Its structural formula is shown in Fig.2.

2.2.Experimental methods

2.2.1.Preparation and rheology determination of LCWS

The amount of coal,water and additives in LCWS was calculated according to the predetermined solid concentration.First,the additive was dissolved in water.The pulverized coal was then poured into the additive solution.A high-speed mixer was used to stir the mixture for 10 min at 1000 r·min?1.The additive dosage was set as 0.4%–1% of the dry coal sample.The total mass of slurry is 100 g.Rheological analysis of LCWS samples(25°C)was performed by using a rotating viscometer (NXS-11 (B),Chengdu Instrument Co.Ltd.,China).Shear rate ranges from 0 to 120 s?1.The slurry viscosity measured at shear rate of 100 s?1is apparent viscosity η100.The lower apparent viscosity η100shows that CWS with good slurry forming ability.The Herschel–Bulkley model[21]as Eq.(1).

τ,shearing stress,Pa;τ0,yield stress,Pa;ν,shearing rate,s?1;S,consistency index;n,flow index.

2.2.2.Determination of stability of LCWS

“Glass rod penetration”method[22,23]was used to evaluate the stability of LCWS.After the preparation of LCWS,it was stored in the measuring cylinder at room temperature for 24 h (The diameter of the measuring cylinder was 3 cm.LCWS layer 15 cm in height).A glass rod(5 mm in diameter and 20 g in mass)falls freely from the surface of the slurry and records the falling distance.For calculation of penetration ratio(Eq.(2))[23].

where d distance of rod travel(cm);dtmaximum distance of rod travel(cm).

The Penetration ratio is high,indicating good stability.

2.2.3.Additive adsorption experiment

Fig.2.Molecular structure of sodium lignosulfonate.

Configuring a series of SL solutions with concentrations between 0 and 1000 mg·L?1.Mixing 1 g of coal and 50 ml of additive solution in a 100 ml conical flask.Placing the mixture in a water bath oscillator(25°C,150 r·min?1).The mixture was shaken for 5 h and allowed to stand for 2 h to bring the mixture to equilibrium.The solution was separated from the solid by 45 μm filters.The effect of the addition of SL on the preparation of LCWS was studied.

The wavelength of UV absorbance was 280 nm,SP-756 UV–visible spectrophotometer,Shanghai Spectrum Co.,Ltd.The measured absorbance is brought to the additive concentration standard curve(Fig.S1)to obtain the remaining additive concentration.Blank tests can eliminate interference items.The adsorption amount of additives was calculated by Eq.(3).

A,amount of additive adsorbed by the unit mass of coal(dry basis),mg·g?1;

C0,concentration of the original surfactant solution,mg·L?1;

C1,additive concentration that remained in the solution after adsorption,mg·L?1;

Cb,converted concentration from the absorbance of the blank sample,mg·L?1;

V,volume of the surfactant solution,L(V=0.05 L in the study);

m,mass of coal(dry basis)used to adsorb the surfactant,g(m=1 g in the study).

The experiment was performed three times,and the error of the three results did not exceed 15%.

The adsorption of SL on the coal/water interface is mainly singlelayer adsorption and multi-point adsorption[24].By fitting Langmuir isothermal adsorption model(Eq.(4))[25],the corresponding data of equilibrium adsorption amount were analyzed.

where qmaxis the maximum adsorption amount,and K is the equilibrium constant.qeis the amount of SL adsorbed by a unit mass of coal particle,mg·g?1;ceis the mass concentration of SL solution after adsorption equilibrium,g·L?1.

The calculation of the Gibbs free energy,can be obtained from the following Eq.(5):

where R is the ideal gas constant(8.314 J·mol?1·K?1),and T is the absolute temperature.

2.2.4.Zeta potential measurements

The coal suspension potential was measured by Zeta potential analyzer,Nano Brook Zeta PALS(Brookhaven Instruments Corp,USA).0.1 g pulverized coal was oscillated for 5 h (25 °C,150 rpm) in 50 ml water or dispersant solutions of different concentrations and left for two hours to reach adsorption equilibrium.The supernatant was taken for Zeta potential measurement.Zeta potential was measured three times for each concentration solution,and the average of the three times was taken.

2.2.5.FTIR spectra

Infrared spectra of coal and SL were obtained on a KBr disk in the region of 4000–400 cm?1using a German Brucker EQUI NX55 Fourier transform infrared(FTIR)spectrophotometer.

2.2.6.XPS test

The XPS experiment was conducted at room temperature.The narrow-range scan test of element C was carried out with ESCALAB 250Xi X-ray photoelectron spectrometer (XPS) manufactured by Thermo Fisher,USA.The excitation source during the test was a monochromatic aluminum anode target(AlKα)with a beam spot size of 500 μm.The vacuum of the analysis room is 5×10?8Pa.The scanning pass energy is 20 eV with a resolution of 0.05 eV.Data processing (peak fitting)using XPS peak fitting software with intelligent background subtraction and Gaussian/Lorentz peak shape.

2.3.MD simulation

The Forcite module in Materials Studio 8.0 was used to simulate the adsorption of additives on low-rank coal[27].COMPASS force field is used in the simulation.Subbituminous coal is a matter system with complex structure and diverse chemical composition,mainly composed of organic materials with a content of 85%–95%.There is no uniform chemical structure representing subbituminous coal.Subbituminous coal is a highly crosslinked polymer composed of subbituminous coal macromolecules through bridge bonds,hydrogen bonds,van der Waals forces,etc.That is,the single molecule of coal is the basic structural unit,which is the basis for the construction of the sub-bituminous coal model in this paper.The macromolecules involving multiple single molecule models are suitable models for representing coal[28].According to existing studies,Hatcher coal model[20,29](Fig.3(a))is selected to represent low-rank coal,because it has low order coal structure characteristics[19].In addition,Hatcher coal model is helpful to directly describe the transition from Alder lignin model to subbituminous coal[30].The composition ratio of oxygen-containing functional groups was confirmed by XPS results.

First,based on the Hatcher model,a low-order coal molecular model was established,and geometric optimization was carried out.Then,20 optimized low-rank coal molecules were randomly loaded into a rectangular simulated cell 36.36 ×36.36 ×132.47 ?3(X×Y× Z)with a three-dimensional periodic boundary condition.The structural relaxation of the coal model was achieved in the range of 1098 K–298 K,and the system was reoptimized and balanced for 1 ns at 298 K.The 12.5 ? van der Waals interaction cut-off time was used.Refer to Lyu et al.simulation section for the simulation process[31].The low-rank coal model is presented in Fig.3 (b).Meanwhile,an additive molecular model can be built and geometric Optimization can be carried out by using the Geometry Optimization module Smart method(Fig.3(c)).

Fig.3.(a)Hatcher coal molecular model,(b)Subbituminous coal surface model,(c)SL molecule model,(d)Initial adsorption configuration.Red ball represents O,gray ball represents C,white ball represents H,and yellow ball represents S.

A vacuum layer of 15 ? is set to avoid the influence of periodic boundary conditions.The size of all systems is about 36.36×36.36×132.47 ?3.The Forcite module was used to optimize all the coal-water models by the Smart Minimizer method,so as to reduce the unreasonable contact between atoms.The materials in the simulation are composed of 20 coal molecules,12 SL molecules and 2000 water molecules,as shown in Fig.3(d).

3.Results and Discussion

3.1.Rheological analysis of LCWS

3.1.1.SL dosage experiment

The amount of additive will have a significant impact on the properties of LCWS.In order to evaluate the performance of SL in LCWS,different concentrations of additives were used to prepare slurry.As shown in Fig.4,the relationship between the amount of different additives and the apparent viscosity.(LCWS concentration is fixed at 60%).

It can be seen from Fig.4 that the apparent viscosity η100of LCWS is 747.68 mPa·s when the additive concentration is 0.4%.With the increase of additive concentration,η100is on the decline.η100decreased to 549.77 mPa·s at 0.8%,and remained basically unchanged when the additive concentration increased to 1.0%.This may be because the additive occupies more adsorption sites on coal as the amount of additives increases.A stronger electrostatic repulsion is generated on the coal surface to prevent coal particles from coalescing.Therefore,η100is further reduced.When the additive concentration increases to 0.8%,the adsorption of the additive reaches equilibrium.The adsorption of additives on coal remained unchanged and η100remained basically unchanged.Considering the cost in actual production,0.8%was selected as the additive amount in the test process.

Fig.4.Relationship between additive concentration and apparent viscosity η100 of LCWS.

3.1.2.Rheological studies

The ideal LCWS should have a high solids concentration,low viscosity,good stability and good flow properties,which can be characterized by rheology [32].In Fig.5,LCWS shear rate and shear stress curves under different dry coal consumption.

Fig.5.LCWS shear rate and shear stress curves under different dry coal consumption(pH=7,25°C).

As can be seen from Fig.5,LCWS at different slurry concentrations has shear thinning characteristics.The presence of the y-axis intercept indicates that LCWS has a non-Newtonian fluid performance.The y-intercept of LCWS increases with the increase of LCWS concentration,which indicates that the shear thinning property of LCWS decreases with the increase of LCWS concentration.Herschel-Bulkley model was used to fit shear rate and shear stress of LCWS with different concentrations.Where the larger the value of S,the more viscous the slurry.In addition,when n=1,the fluid is Newtonian fluid,and when n>1,the slurry is expansive plastic fluid,and when n <1,the slurry is pseudoplastic fluid.The resulting parameters are shown in Table 2.

As can be seen from Table 2,R2is greater than 0.98.At the same time,in conjunction with Fig.5,the experimental points have a higher correlation with the fitted curve.This indicates that Herschel-Bulkley model is suitable for describing the rheological properties of LCWS.The n values obtained by fitting are all less than 1,indicating that the LCWS is a pseudoplastic fluid.This is due to the hydrogen bond and polarity on the coal particles after adsorption of additives,which adsorbs a large amount of water and forms hydration film.Therefore,the slurry forms a three-dimensional network structure,so the viscosity of the LCWS is higher when the static or shear rate is low.However,when the slurry is subjected to strong shearing action,the three-dimensional network structure of the slurry is destroyed,and the viscosity of the slurry is lowered,thereby causing the LCWS to be shear-thinned pseudoplastic fluid.In addition,the yield stress of LCWS increases with increasing concentration.When the slurry concentration was 60%and 62%,the yield stress τ0was small,being 3.84 Pa and 4.72 Pa,respectively.At this time,the S values are also correspondingly small,0.63 and 0.56,respectively,indicating that the consistency of the slurry is low.When the concentration is 64%,the yield stress τ0is 5.74 Pa,the S value is 5.95,and the consistency of the slurry is increased.When the concentration increased to 66%,the yield stress τ0and S values increase sharply,being 18.19 Pa and 13.77 respectively.At this time,the slurry consistency is high,corresponding to the poor liquidity performance in the test process.This is mainly due to the higher degree of agglomeration of coal particles,increased friction collision,reduced free water ratio,decreased lubrication and buffering effect,resulting in an increase in yield stress[33].

Table 2 Herschel-Bulkley parameter values of slurry with different concentrations were fitted

3.1.3.Slurry ability

Fig.6 shows the relationship between η100and solid concentration.

Fig.6 reflects that as the solids concentration increases,the apparent viscosity η100also increases.This is mainly because as the solid concentration increases,the amount of solid particles in the slurry also increases,and the collision and friction between the particles increase.Moreover,the proportion of water in the slurry decreases,leading to a decrease in lubrication and buffering between the particles.In the industrial production of LCWS,the upper limit of apparent viscosity η100is 1000 mPa·s.The corresponding concentration of 1000 mPa·s is defined as C1000.In the experiment,C1000was obtained by preparing LCWS with different concentrations,characterizing its slurrability index,and then using interpolation method.The C1000obtained in the experiment was 64.3%.The apparent viscosity η100in Fig.6 does not change much when the concentration of LCWS is 60%–64%,indicating that the particle interaction is small at this time,and the proportion of free water is relatively high,which plays a role of lubrication and buffering.When the concentration is 66%,the η100of the LCWS increases rapidly.It shows that the degree of agglomeration between coal particles is higher,the interaction between particles is increased,and the friction and collision is increased.The proportion of free water is further reduced,and lubrication and cushioning are reduced.The apparent viscosity increases rapidly.

Fig.6.Relation curve between apparent viscosity and mass concentration.

3.1.4.Stability

Fig.7.The relationship between penetration ratio and LCWS concentration.

Adding SL will have a positive effect on slurry stability.As shown in Fig.7,the stability of LCWS at various concentrations during the experiment were evaluated.It can be seen that the penetration ratio of LCWS(24 h)decreased with the increase of concentration.When the LCWS concentration is low,60%and 62%,the stability is good,and the penetration ratio are 97.79% and 90.57%,respectively.When the LCWS concentration are 64%and 66%,the penetration ratio become worse,which are 55.93%and 40.00%,respectively.The additive can provide steric hindrance effect to form a double electric layer on the surface of particles,which can avoid condensation and sedimentation.However,with the increase of slurry concentration,the degree of particle agglomeration increases and the proportion of free water decreases,leading to the decrease of penetration ratio with the increase of slurry concentration.Although particle agglomeration occurs,the uniform dispersion state of the LCWS can be restored after mechanical agitation.

3.2.Adsorption studies

The adsorption capacity of the additive on the coal has an important influence on the wettability of the coal and particle accumulation.Fig.8 shows the relationship between the adsorption capacity and the equilibrium concentration.

It can be seen that the adsorption amount of coal particles in SL solution increases with the increase of equilibrium concentration.When the concentration of the solution was 0–200 mg·L?1,the adsorption increased rapidly and reached a stable level after 400 mg·L?1.The adsorption isotherm was approximately L,indicating that the dispersant molecules were adsorbed in a single layer on coal.Therefore,it is considered that the adsorption film formed by the SL molecule at the solid–liquid interface is dense and thick.It is beneficial to improve the wettability of coal particles.The hydration film formed on particles separates the particles,prevents agglomeration,and reduces the friction between the particles when they are under stress,thus achieving the effect of viscosity reduction.The experimental data were fitted with Langmuir isothermal adsorption model andwas calculated.The results are shown in Table 3.

Fig.8.Effect of additive concentration on adsorption.

Table 3 Langmuir parameters and ΔGa 0ds of adsorption additives for coal

In Table 3,R2is 0.99,which means that the adsorption of SL is mainly single-layer.is negative,indicating that the reaction is spontaneous.

3.3.Zeta potential analysis

LCWS Zeta potential can be used to predict LCWS electrostatic interaction.Zeta potential in LCWS is mainly determined by the nature of coal,the type of dispersant and the cation in the solution[34].Fig.9 shows the change of Zeta potential of coal sample with SL concentration when pH=7.

In the absence of additives,the Zeta potential of the coal sample is?9.90 mV.This is because the oxygen-containing group on the coal,when the pH value is 7,produces a group of anion charge,thus producing a negative Zeta potential.As can be seen from Fig.9,when SL concentration is 0.2%–0.4%,Zeta potential value drops sharply,from?15.24 mV at 0.2% to ?35.54 mV at 0.4%,Zeta potential value decreased by 20.3 mV.At 0.6%to 1.0%,the potential value does not change much,from ?39.15 mV at 0.6%to ?42.98 mV at 1.0%,the zeta potential value only drops by 3.83 mV.It is because the amount of additive adsorption has a large effect on the Zeta potential value.Before the saturated adsorption is reached,the amount of adsorption increases as the concentration of the additive increases,resulting in a rapid decrease in the potential value of the suspension.At the same time,the hydrophilic groups of coal pointing to the aqueous phase increases.Conducive to coal dispersed in water.Thereby the whole system is more stable and the performance of the slurry is improved.When the adsorption approaches saturation,the adsorption amount remains basically unchanged,so the Zeta potential does not change much.The addition of additives significantly reduced the Zeta potential and apparent viscosity(Fig.4),and prevented particle agglomeration.This indicates that the strong negative charge attached to the coal causes a strong electrostatic repulsion,which is conducive to the dispersion of the coal in the solution.

Fig.9.Zeta potential analysis of additive.

3.4.Functional groups of coal and SL

3.4.1.FTIR

Coal contains complex functional groups,such as hydroxyl,methyl,carboxyl and aromatic ring.The types and contents of functional groups in coal directly affect the hydrophilicity of coal and then affect the slurryability.FTIR has been widely used to analyze functional groups in coal and their changes under different treatment conditions[35,36].Fig.10 shows the FTIR spectrum before and after coal adsorption,with similar functional groups but different strengths.

The peak near 3416 cm?1represents the absorption of hydroxyl groups.1618 cm?1is the stretching vibration of benzene ring.The peak near 1400 cm?1is aliphatic bending vibration.1148 cm?1is the symmetric vibration of S=O in the sulfonic acid group.After adsorption,the absorption intensity of benzene ring,hydroxyl functional groups and S=O in sulfonic acid functional groups increased significantly.It indicates that SL successfully adsorbed on the coal.

3.4.2.XPS

To analyze the influence of SL on functional groups in coal before and after adsorption,XPSPEAK 4.1 software was used to fit C1s spectrum,as shown in Fig.11.

The spectra were divided into four peaks of isolated components,with binding energies of 284.75 eV (C--C/C--H),285.60 eV (C--O),286.60 eV(C=O),and 289.10 eV(O=C--O),respectively.Comparing the C1s XPS peak fitting spectrum of coal samples before and after adsorption,The fitting curve in the figure almost coincides with the original curve,indicating that the fitting effect is better.The content of hydrophobic groups(C--C/C--H)reflects the strength of hydrophobicity while the content of oxygen functional groups (C--O,C=O,and O=C--O)reflects the hydrophilicity of the coal.In Table 4,the content of C--C/C--H groups was 80.81%before adsorption and decreased to 71.52%after adsorption,indicating that C--C and C--H were the main forms of functional groups on the surface of coal.Oxygen functional groups C--O,C=O,and O=C--O increased from 11.02%,3.45%,and 4.72%before adsorption to 18.89%,4.59%,and 5.00%after adsorption,respectively.The content of oxygen-containing groups increases,and the content of C--C/C--H groups decreases.It is indicated that after adsorbing SL,the hydrophobic group of SL will bind to the C--C/C--H group on the coal surface,and the oxygen-containing functional group is exposed to the outside.This leads to an increase in the hydrophilicity of the coal,thus increasing the wettability.

Fig.10.FTIR before and after adsorption.

Fig.11.XPS narrow scan spectrogram.

Table 4 Functional group content before and after adsorption

Fig.12.Configuration of SL in simulation system at different time.

3.5.MD simulation results

3.5.1.Adsorption configuration of SL on the coal surface

The adsorption process of SL on coal is shown in Fig.12.

It can be seen that the SL molecules are first uniformly dispersed in water.Over time,some of the SL molecules approach and adsorb on the coal.The rest of SL molecules are agglomerated in water.

As can be seen from the density distribution curve Fig.13,the distribution curve of SL and the coal molecule distribution curve partially coincide,which also indicates that SL can be adsorbed on the coal.The surface roughness and pore structure of the coal result in the coincidence of coal/water density curves[37].There are two peaks in the SL molecular density distribution curve.The first peak is formed at 39 ?,because the hydrophobic end of SL molecule binds to the hydrocarbon group on coal and adsorbs on coal.Mainly by van der Waals forces,hydrogen bonds,etc.Here,the density distribution of water molecules also has a peak,indicating that the hydrophobic tail group of SL is bound to coal,and the hydrophilic head group is exposed to the solution,which is easy to bond with water molecules (contributes to hydration film formation).The second peak (46 ?) of the SL density distribution is that the negatively charged SL molecules are freed in the solution by the electrostatic repulsion of the coal surface(ζ=?9.9 mV)[38]and agglomerate with each other.The negative charge on the surface of the agglomerated SL in the solution and the more negative charge on coal after adsorption of some SL molecules can produce stronger electrostatic repulsion(Fig.S2).It may be more conducive to the uniform dispersion of coal in the solution.

Fig.13.Density distribution along the Z-axis.

3.5.2.Migration and diffusion capacity of water molecules

The adsorption of SL molecules on coal affects the kinetics of water molecules in the water/additive/coal system.These effects can be studied by MSD and D.The MSD was calculated using Eq.(6)[39].

Here,N represents the number of atoms,ri(0)represents the initial position,and ri(t)is the position after time t.The angled brackets denote the population mean.

The MSD curve for water molecules is shown in Fig.14.The presence of SL significantly affected the mobility of the water molecules and reduces their diffusion ability.

According to the relationship developed by Einstein[40],D can be expressed as Eq.(7).

The relationship between the MSD and D is given by Eq.(8).

The value of D for the water molecules in water/SL/coal simulated system was calculated,and the data from the first 500 ps were selected for analysis,as shown in Fig.14.When SL was added,the value of D for water was 0.34×10?8m2·s?1,whereas,in the absence of SL,D was 10.00×10?8m2·s?1.The MSD diagram of water molecules has different gradients,and the gradient of water/coal system is steeper than that of water/additive/coal system.This is because after adsorption,hydrophobic groups on the coal are covered,and more hydrophilic groups are exposed in the water phase.Hydrophilic groups interact with water molecules through van der Waals and electrostatic forces,this makes it easier for water molecules to stick to the surface of the coal.The movement of water molecules in the water/additive/coal system tends to be gentle,which improves the hydrophilicity of coal.Therefore,the addition of additives can improve the hydrophilicity of coal.The results show that the presence of SL reduces the mobility of water molecules on coal.The mobility of water molecules decreases,and the accumulation of water molecules is beneficial to the formation of hydration film,which is helpful to improve the stability of LCWS.Simulation results correspond to experimental results,indicating that MD simulation is accurate and feasible.

Fig.14.Mean square displacement of water molecules in different simulation systems.

3.5.3.Adsorption energy

The adsorption energy Eadsorptioncan be used to calculate the relative strength of the interaction between the surfactant and the coal.The Eadsorptionof the SL and coal can be calculated using Eqs.(9)and(10)[41].

Here,Etotalrepresents the total energy of the system,Ecoal+waterrepresents the total energy of coal and water,ESL+waterrepresents the total energy of SL and water,and Ewaterrepresents the energy of water.

The energy for the adsorption of SL on the coal is shown in Table 5.

As listed in Table 5,the value of Eadsorptionobtained from the simulation is ?92.37 kJ·mol?1,a negative value,which indicates that the systems become more stable after the SL molecules adsorbed on the coal.The large absolute value of the adsorption energy indicates a stable adsorption structure between the additive and the coal particles,and a good hydrophobic effect on the coal particles.Moreover,the large binding energy(Ebinding)value indicates a strong interaction between the SL molecules and the coal surface.

Table 5 Energetics of the adsorption of SL on the coal

4.Conclusions

The effect of sodium lignosulfonate as additive on the preparation of low-rank coal-water slurry was studied by the combination of simulation and experiment.

The experimental results show that the apparent viscosity of LCWS decreases with the increase of additive.When the additive is increased to a certain amount,the apparent viscosity basically remains unchanged with the increase of additive amount.Rheological study shows that LCWS has shear thinning property,but with the increase of slurry concentration,shear thinning property becomes worse.When the slurry viscosity in the experiment is 1000 mPa·s,the corresponding concentration of LCWS is about 64.3%.The stability of LCWS gradually deteriorates with the increase of slurry concentration.

Adsorption isotherm studies showed that SL conforms to singlelayer adsorption on the coal surface,andwas negative,proving that the reaction was spontaneous.Zeta potential measurements showed that SL increased the negative charge on coal and enhanced the electrostatic interaction,which was conducive to the dispersion of coal particles in the solution.FTIR scanning and XPS wide-range scanning were performed on the coal before and after adsorption,and it was found that the content of oxygen containing groups on coal increased after adsorption.It indicates that the hydrophilicity of coal is enhanced after adsorption.

Simulation results show that a large number of SL molecules in the solution will be partly combined with hydrocarbon groups on the coal surface through its hydrophobic tail,and hydrophilic head group will be exposed in the water.Some SL molecules are agglomerated in water,and their hydrophobic alkyl tails are in contact with each other,while the sulfonate groups are evenly distributed in the water due to the electrostatic interaction between them.The SL group,which is dispersed in solution,has a negative charge on its surface and on the coal surface.Therefore,a strong electrostatic repulsion is generated,which is advantageous for the coal particles to be dispersed in the solution.The mean square displacement and migration diffusion coefficient of water molecules indicate that the addition of SL reduces the diffusion rate of water molecules and improves the hydrophilicity of the coal surface.The adsorption energy(?92.37 kJ·mol?1)is negative indicating that the systems become more stable after the SL molecules adsorbed on the coal.The simulation results are consistent with the experimental results.

Declaration of competing interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work,there is no professional or other personal interest of any nature or kind in any product,service and/or company that could be construed as influencing the position presented in,or the review of,the manuscript entitled“Study of sodium lignosulfonate prepare low rank coal water slurry:Experiments and simulations“.

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Acknowledgements

This work was supported by SDUST Research Fund (Grant No.2018TDJH101),Key Research and Development Project of Shandong(Grant No.2019GGX103035),National Natural Science Foundation of China(Grant Nos.51904174,52074175),Young Science and Technology Innovation Program of Shandong Province(Grant No.2020KJD001),and Project of Shandong Province Higher Educational Young Innovative Talent Introduction and Cultivation Team.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.07.064.