Adsorption performance of nonionic surfactant on the lignite particles

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Adsorption performance of nonionic surfactant on the lignite particles with different density Jie Wang, Yaqun He, Xiangyang Ling, Juan Hao, and Weining Xie Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 30 Apr 2017 Downloaded from http://pubs.acs.org on April 30, 2017

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Adsorption performance of nonionic surfactant on the lignite particles with different density Jie Wang1, Yaqun He1, 2*, Xiangyang Ling1, Juan Hao1, Weining Xie1 1

School of Chemical Engineering and Technology, China University of Mining and Technology,

Xuzhou, Jiangsu 221116, China 2

Advanced Analysis and Computation Center, China University of Mining and Technology,

Xuzhou, Jiangsu, 221116, China Abstract: The floatability of lignite is usually known very poor. Generally, surfactants are applied to improve its hydrophobicity in the flotation process. This research focuses on the adsorption performance of nonionic surfactant, sorbitan monooleate, on the lignite particles with different density, including -1.45 g/cm3, -1.80+1.45 g/cm3 and +1.80 g/cm3. From Fourier transform infrared spectroscopy (FTIR) results, it was obvious that organic oxygen functional groups determined the surface properties of lower density lignite, while inorganic Si-O determined the surface properties of higher density sample. Qualitative similarities are found in the curves of the surface electrokinetic potentials versus pH values. Points of zero charge (PZC) of the samples were measured to be at pH values of 2.3, 3.0 and 3.0, respectively. Hence, organic oxygen functional groups are similar with Si-O on coal surface in adsorbing OH- and H+ in electrolyte solution. The X-ray photoelectron spectroscopy (XPS) results show that the lower density lignite particle has stronger adsorptions of sorbitan monooleate. In other words, organic oxygen functional groups have the priority over inorganic Si-O in adsorbing sorbitan monooleate. In addition, polar groups of the surfactant have different adsorbing ability with organic oxygen functional groups. Interestingly, the oxygen content of +1.8 g/cm3 lignite nearly does not change

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after the pretreatment by sorbitan monooleate solutions. Therefore the sorbitan monooleate molecule cannot almost adsorb on the mineral surface. Keywords: lignite coal flotation; zeta potential; non-ionic surfactant; adsorption performance; XPS Introduction Froth flotation has a long history in the fine coal preparation.1 Fine coal particles are separated selectively from associated minerals in flotation process. Since coal is not a chemically unique substance, its natural hydrophobicity varies with its rank, petrographic composition and degree of oxidation.2 Low-rank coals, especially lignite, are generally known as the most difficult to float coals.3 Their poor floatability has been mainly attributed to the high oxygen content and the abundance of hydrophilic functional groups at their surface.4-7 Many researchers have focused on improving the hydrophobicity of difficult-to-float lignite through heat pretreatment8 and introducing appropriate chemical agents9. These ionic and non-ionic surfactants may adsorb on the surface of lignite or minerals selectively.

10, 11

It has been suggested that this adsorption is due to

hydrogen bonding and electrostatic attraction. The adsorption of surfactants on particles has relationship with the electric double layer of the particles.12 The zeta potential is to determine the potential at the boundary of a slipping plane, which can reflect the polarity of the coal sample.13, 14 As coal is combustible rock consisting mainly organic macerals and inorganic minerals, it is necessary to confirm that the electrokinetic potentials of coal-rich and mineral-rich particles. The preferential adsorption of the surfactant can improve the hydrophobicity of the specific particles, which will be beneficial for the separation. Three coal samples of low ash lignitic, oxidized and unoxidized coal have been investigated to illustrate the relationship between the pH and the


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surface electrical properties.15 The oxygen functional groups contribute to the more negative surface of the lignitic and oxidized coal. In theory, the polar groups of the surfactant adsorb on the charged sites of the oxygenated coal surface, with non-polar group towards the water.16, 17 But in practice, the interaction between the polar groups and the charged sites has not been investigated. At present, XPS and FTIR have been widely applied to analyze the elements and functional groups on the surface of materials.18-22 From the variation of the oxygen functional groups on the coal surface before and after the pretreatment, it is possible to analyze the interaction between different hydrophilic functional groups and the polar groups of the surfactant molecule. Based pn the interaction analysis, it is beneficial for optimizing the surfactant structure. Huge reserves of lignite have been detected in Inner Mongolia, China. Investigations on improving the floatability of lignite have a great significance for the clean coal utilization. Non-ionic surfactant, sorbitan monooleate has been applied to improve the floatability of the lignite effectively.23, 24 In this study, the non-ionic surfactant, sorbitan monooleate was applied to investigate the different adsorption performance on the coal-rich and mineral-rich particles. Firstly the lignite particles were separated into three parts of -1.45 g/cm3, -1.80+1.45 g/cm3 and +1.80 g/cm3 in float and sink test. XRD and FTIR were employed to characterize the phase compositions and surface functional groups of the three coal samples. In addition, zeta potential tests were conducted to measure the differences in surface charged properties versus pH values. Combined the surface functional groups, the effects of organic and mineral groups on the electric double layer of the different density particles were discussed. The three density lignite particles were pretreated by the nonionic surfactant, sorbitan monooleate solutions (0.1% mass ratio). XPS was used to detect the element contents on the three samples before and after the pretreatment. Based


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on the XPS analysis, the peak fitting of O1s binding energy was discussed for the compositions of oxygen functional groups. Furthermore, the interaction between hydrophilic oxygen functional groups (lignite coal) and the polar groups (sorbitan monooleate) was investigated qualitatively. 2 Materials and methods 2.1 Materials The lignite sample was obtained from Inner Mongolia Province, China. Fine fraction (-0.5 mm) was separated in organic liquids with densities of 1.45 and 1.8 g/cm3 in sink and float tests. After the separation, each product was filtrated, washed and dried at 105 ℃ for 6 h. The three density products were weighed and then ground to -74 µm separately for ash content, FTIR and XRD analysis. 2.2 Methods 2.2.1 X-ray diffraction analysis The three density lignite samples were analyzed by XRD to detect the phase compositions. It was obtained using Bruker D8 Advance XRD with a copper anode at 40kV. The X-ray intensities were measured in the range of 3°≤2θ≤70° with a step size of 0.019° 2θ. The resulting diffractograms were further smoothened and analyzed using phase method. 2.2.2 Fourier transforms infrared spectroscopy Characteristics of the coal surface functional groups were recorded on a Bruker Vertex 80v with -1

wave number from 4000 cm -400 cm-1 and resolution of 4cm-1. Pure KBr was used to obtain a reference spectrum. The raw coal sample was dried in a far infrared oven 2 hours. Approximately 100 mg potassium bromide (KBr) powder and a bit of dried coal sample, with the mass ratio of coal to KBr about 1:100 were placed in an agate mortar to be thoroughly mixed and then put in a


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diffuse reflectance test apparatus. 2.2.3 Zeta potential distribution analysis The three coal samples were further ground to -2 µm. A dilute suspension of the ground coal particles was used to measure the Zeta-potential versus pH value. Zeta potential of the fine coal particles was measured by Zetaplus Zeta analyzer, Brookhaven Company, USA. The pH of the suspension was adjusted by the addition of hydrochloric acid/ sodium hydroxide. All of the pH determinations were made using a digital pH-meter. Each sample was tested for 5 times to achieve an average value. 2.2.4 Pretreatment by sorbitan monooleate solutions The three raw coal samples (-0.5 mm) were pretreated by the sorbitan monooleate solutions (0.1% mass ratio) in neutral condition. According to the liquid-to-solid ratio of 25: 1, 25 cm3 sorbitan monooleate solutions and 1 g raw coal samples were added in a 100 cm3 glass beaker. Then, the slurry was stirred by a magnetic stirring apparatus for 5 minutes. After the setting time of 10 minutes, the supernatant liquor was removed. In order to remove the covered but leave the adsorbed sorbitan monooleate from the residual coal surface, plenty of deionized water was taken to flush the coals. Finally, coal samples were dried in a vacuum oven at 105 ℃ for 6 hours. 2.2.5 X-ray photoelectron spectroscopy The surface properties of raw and pretreated coals were measured by XPS. The XPS experiments were carried out at room temperature in an ultrahigh vacuum (UHV) system with the surface analysis system (ESCALAB250 Xi, America). The data processing (peak fitting) was performed with the XPS peak fit software. 3 Results and discussions


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3.1 Physical phase composition Table 1 Ash contents of the coal samples from slow-ash test Coal samples

-1.45 g/cm3

-1.80+1.45 g/cm3

+1.80 g/cm3

Ash contents /%

8.22

27.32

63.51

The ash contents of the three coal samples are shown in Table 1. It is obvious that higher density sample has more ash contents. The ash content of -1.45 g/cm3 coal sample is 8.22%, which meets the requirement of market specification. Figure 1 shows the XRD spectra of the fractions of -1.45 g/cm3, -1.80+1.45 g/cm3 and +1.80 g/cm3. Broad amorphous lignite coal peaks are vividly shown in the spectrum of -1.45 g/cm3 (Figure 1a). Two crystalline peaks of quartz at 2θ = 20° and nearly 27° are exhibited in Figure 1a. However, the intensity and amounts of these crystalline peaks of quartz and gypsum become stronger in the higher density fractions, which indicates that more mineral contents are maintained. Therefore, it indicates that the main components of the minerals are quartz and gypsum. Analyses of phase compositions indicate that the amount of the crystalline peaks increases with the increase of density, while the content of amorphous peaks decreases with this trend. In addition, the slow-ash test gave the ash contents of 27.32% and 63.51% for lignite fractions of -1.80+1.45 g/cm3 and +1.80 g/cm3, which can partly represent the amounts of minerals, respectively. In fact, different phase and compositions would result in the different surface properties, which is the basis of froth flotation.


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(a)

(b)

(c) Figure 1 X-ray diffraction pattern of different density fractions of lignite coal: a. -1.45 g/cm3 component; b. +1.45 g/cm3-1.80 g/cm3 component; c. +1.80 g/cm3 component, Q-quartz, G-gypsum. 3.2 Surface functional groups and electrokinetic potentials Table 2 Band assignments for the FTIR spectra of coals. Wavenumber (cm-1)

Functional groups

3400-3700

-OH(minerals)

3100-3400

-OH

3000-3100

Aromatic CHx stretching

2800-3000

Aliphatic CHx stretching


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1650-1800

C=O

1550-1650

Aromatic C=C ring stretching

1300-1550

Aliphatic CHx bending

1100-1300

C-O-C stretching

900-1100

Si-O stretching

650-900

Aromatics CHx Out-of-plane deformation

-650

Al-O formation, Si-O

(a)

(b)

(c) Figure 2 Surface functional groups of the three density lignite coal sample, (a) -1.45 g/cm3 lignite coal, (b) -1.80 g/cm3+1.45 g/cm3 lignite coal, and (c) +1.80 g/cm3. The FTIR signals deriving from chemical functional groups are listed in Table 2.25-27 Figure 2


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presented the surface functional groups of the three density lignite samples. For the -1.45 g/cm3 sample, the wide organic -OH peak, at 3400 cm-1 occupies a quite high proportion of the coal surface. The absorbance of C=C peak, at 1600 cm-1 is presented the strongest peak in Figure 2a. Besides, the characteristic peaks of aliphatic CHX are found at the ranges of 2800 cm-1-3000 cm-1 and 1300 cm-1-1550 cm-1. Thus, it is obvious that most of the peaks are organic functional groups on the surface of -1.45 g/cm3 lignite sample. However, for -1.80+1.45 g/cm3 lignite, the organic peaks of -OH and C=C are decreased significantly in comparison with FTIR spectrum of -1.45 g/cm3. On the contrary, the mineral peaks of Si-O are increased remarkably at the ranges of 900 cm-1-1100 cm-1 and -650 cm-1. In addition, mineral -OH peaks at 3400 cm-1-3700 cm-1 are presented alongside the organic -OH. Therefore more mineral peaks are exhibited on the surface of -1.80+1.45 g/cm3 lignite. From Figure 2c, the organic peaks are further decreased, while the inorganic peaks are increased remarkably. The absorbance of the inorganic peaks are much higher than the organic peaks in Figure 2c. Consequently, more organic functional groups are exposed on the surface of the lower density lignite, while more mineral functional groups are exhibited on the surface of the higher density lignite. Zeta-potential versus pH curves for the three density lignite samples are given in Figure 3. Generally, variation of the zeta-potential with pH of the suspensions illustrates the potential determining behavior of hydronium and hydroxyl ions. It is shown that the zeta potential is decreased with the increase of the pH value for all the three samples in Figure 3. Qualitative similarities between the three zeta-potential versus pH curves are quite apparent. Especially in the range of 5.0-8.0, it can be noted that the change in the zeta-potential is not pronounced among the three coal samples. Besides, the pH of coal flotation slurry is usually in this range. However, there


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exist two aspect differences, in which one is the ±5 mV fluctuations in the zeta-potential at each pH value for the three coal samples. On the other hand, PZC of -1.45 g/cm3 component was located at about 2.3, which is similar to the research results of low ash lignite.15 But the PZCs of the -1.80+1.45 g/cm3 and +1.80 g/cm3 components were both at 3.0. The component of higher density sample is mainly quartz from XRD results. The PZC of the higher density sample is similar with that in Liu et al.’s result.28

Figure 3 Zeta-potential variations as a function of pH at lignite coal water interface

−COH ( surf ) + H + ( aq ) ↔ −COH 2+ ( surf )

(1)

−COOH ( surf ) + H + ( aq ) ↔ −COOH 2+ ( surf )

(2)

C = O + H + → C − OH + + H + → C − OH 22+

(3)

−COH ( surf ) + OH − ( aq ) ↔ −CO − ( surf ) + H 2O( aq )

(4)

−COOH ( surf ) + OH − ( aq ) ↔ −COO − + H 2O( aq )

(5)

SiOH 2+ ( surf ) ↔ SiOH ( surf ) + H + ( aq )

SiOH ( surf ) ↔ SiO − ( surf ) + H + ( aq )

(6)

(7)

The surface charge on lower and higher density lignite is induced by ionization or protonation of the surface functional groups in the presence of electrolytes. Combined the FTIR results, the


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zeta-potential of the lighter coals is more related to the organic functional groups, while that of the heavier coals is more relative with the mineral group of Si-O. Surface oxygen functional groups affect not only the charged properties in aqueous media but also the electronic properties, which play an important role in CO2 adsorption. Liu et al. applied molecular simulation to investigate the typical relationship between CO2 adsorption and surface oxygen functional groups.29, 30 Here the average surface charged properties of the three density lignite were considered. On the lignite surface, ionisable groups are mainly hydroxy and carboxyl groups.31, 32 As the pH decreases, the more protons (H+) interact with the surface functional groups and protonate them, e.g. C=O reacts to C-OH+ or C-OH+ reacts to C-OH22+ in Equation (1) to (3). Therefore the surface of the coal becomes more positive. With the increase of pH, the more OH- interacts with hydroxy and carboxyl groups in the forms of Equation (4) and (5). However, quartz forms hydroxylated surfaces when in contact with water molecules. Adsorption/dissociation of H+ from the surface hydroxyls can account for the surface charge on quartz in Equation (6) and (7).33 Therefore the ability of ionization or protonation of organic functional groups is similar with that of the Si-O functional groups in the same concentrations of H+/OH- from Figure 3. It is difficult to adjust the pH of the slurry to make the lower and higher density lignite charged oppositely between the narrow gap between 2.3 and 3.0. Hence, it brings the difficulty for anionic and cationic surfactant in the adsorption difference on the three density coal samples. 3.3 Adsorption performance of sorbitan monooleate The basic physical properties of sorbitan monooleate were illustrated in Table 3. From its structural formula, the molecule consists of nonpolar hydrocarbon chain and polar groups. Moreover, the polar groups contain an ester, epoxy ether bond and three hydroxyls. Table 4


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exhibited the element contents of carbon and oxygen on the particle surface before and after pretreated by sorbitan monooleate solutions (0.1% mass ratio). The contents of carbon were increased by 23.71%, 13.44% and 10.22% in the samples of -1.45 g/cm3, -1.80+1.45 g/cm3 and +1.80 g/cm3, respectively. However, the amounts of oxygen were decreased by 13.74%, 7.16% and 4.96% in the three coal samples, respectively. Thus the lower density lignite has the stronger ability of adsorbing sorbitan monooleate molecules. More carbon and less oxygen atoms will benefit for improving the hydrophobicity of the lighter lignite. Table 3 Basic physical properties of sorbitan monooleate Parameters

Values

molecular weight

428

fusion point

10-20 ℃

solubility

insoluble in water

density

1.0±0.05 g/cm3 OH

structural formula OH OH

O O O C7H14 C8H17

Table 4 Contents of C1s and O1s from wide sweep results of XPS Coal samples

C1s

O1s

Raw

Pretreated

Raw

Pretreated

-1.45 g/cm3

43.51

67.22

38.87

25.13

-1.80 g/cm3+1.45 g/cm3

26.33

39.77

49.71

42.55


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+1.80 g/cm3

17.66

27.88

(a)

55.19

50.23

(b)

Figure 4 Curve fitting the O1s peaks to surface functional groups of -1.45 g/cm3 coal samples (a) O1s of raw coal, (b) O1s of pretreated coal. Figure 4a and b showed the binding energy of O1s in the raw and processed coal sample of -1.45 g/cm3. In general, the content of oxygen atom is decreased by 13.74% on the coal surface after pretreated by sorbitan monooleate solutions. The peak of O1s can be fitted in three type peaks of O-H, O-C and O=C in raw lignite, with the relative contents of 13.06%, 13.62% and 12.19% respectively. It is well known that the hydrophilicity of coal sample is determined by the oxygen functional groups on the coal surface. It indicates that the three organic oxygen functional groups contribute to the hydrophilic property of the raw coal sample of -1.45 g/cm3. On the other hand, the O1s is fitted to two types of peaks of O-C and O-H in Figure 4b. Particularly, the O-C is shown as the main oxygen functional group with the content of is 22.76% on the pretreated lignite surface. However, the O-H is much comparatively weaker with the content of 2.37%. It is decreased significantly on the surface of pretreated coals. But the oxygen functional group O=C peak is excluded from the pretreated lignite in Figure 4b. Besides, the polar group -COOR of the surfactant is not presented on the surface of the pretreated lignite. Therefore, it could be deduced


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that the oxygen functional groups of O=C and O-H could form associated hydrogen-bond with the polar groups of -OH and -COOR of sorbitan monooleate molecule. Nevertheless, the O-C functional group is increased from 13.62% to 22.76% after the pretreatment. In consequence, the epoxy ether bond can hardly form hydrogen bond with the oxygen functional groups on lignite surface, which increases the O-C content on the pretreated lignite surface. Therefore, the speculation can be verified that the sorbitan monooleate molecule is adsorbed on coal surface with the hydrocarbon chain towards outside and the polar groups towards the coal surface.

(a)

(b)

Figure 5 Curve fitting the O1s peaks to surface functional groups of -1.80 g/cm3+1.45 g/cm3 coal samples (a) O1s of raw coal, (b) O1s of pretreated coal. Figure 5 presented the binding energy of O1s on -1.80+1.45 g/cm3 lignite surface before and after pretreated by sorbitan monooleate solutions (0.1% mass ratio). Generally, the content of oxygen atom decreases by 7.16% after the pretreatment process. The binding energy of O1s in raw -1.80+1.45 g/cm3 lignite can be fitted to Si-O, C=O and -COOR in Figure 5a. Their contents are 26.45%, 19.31% and 3.96%, respectively. It is exhibited that the Si-O occupies more than half of the oxygen functional groups on the surface of this density raw lignite. Thus, it is demonstrated that the reason that lead to the hydrophilicity of raw coal sample of -1.80+1.45 g/cm3 is mineral


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Si-O, O=C and COOR. On the other hand, the pretreatment changed the composition of oxygen functional groups significantly. The O1s of the pretreated coal is fitted to three kind peaks of O-H, Si-O and C=O in Figure 5b. In detail, their contents are 12.21%, 22.07% and 8.27%, respectively. The O-H functional group is derived from sorbitan monooleate molecule due to its absence in raw coal. In addition, the -COOR peak is eliminated from the pretreated coals. Therefore, the -COOR has priority than -OH of sorbitan monooleate molecule to form hydrogen-bond with -COOR on the coal surface. Moreover, the content of O=C is largely decreased from 19.31% to 8.27%. It is indicated that -COOR has stronger affinity to the polar groups of sorbitan monooleate molecules than that of O=C on the coal surface. The contents of Si-O changed from 26.45% to 22.07% after the pretreatment. Hence it can be concluded that the sorbitan monooleate molecules cannot almost adsorb on the mineral Si-O, which results in the less adsorption of sorbitan monooleate on the coal sample of -1.80 +1.45 g/cm3 than that on the lighter lignite.

(a)

(b)

Figure 6 Curve fitting the O1s peaks to surface functional groups of +1.80 g/cm3 coal samples (a) O1s of raw coal, (b) O1s of pretreated coal. Figure 6 exhibited the binding energy of O1s in +1.80 g/cm3 lignite before and after pretreatment by sorbitan monooleate solutions (0.1% mass ratio). The peak of O1s are only fitted


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in two kind peaks of Si-O and O=C (Figure 6a). Their contents are 34.31% and 20.88% for Si-O and O=C, respectively. Thus, it is indicated that the mineral oxygen atom play an important role in the hydrophilicity of +1.80 g/cm3 lignite. After the pretreatment by sorbitan monooleate solutions, the oxygen content is decreased by 4.96% through the wide sweep analysis of the coal samples. At the same time, the compositions of oxygen functional groups are changed after the pretreatment. In detail, three kind peaks of O-H, Si-O and O=C is fitted to the O1s peak in the pretreated sample with the contents of 8.77%, 34.07% and 7.40%, respectively. Besides, the O=C peak was declined from 20.88% to 7.40%. Hence, it should be the circumstance that the O=C functional group form hydrogen bonds with O-H or COOR of the sorbitan monooleate molecule. However, this interaction force is comparatively weak, which result in the existence of O=C functional group. Since the O-H peak is not presented in raw coal, it is derived from sorbitan monooleate molecules on the pretreated sample due to the excess O-H in the surfactant molecule. For Si-O peak, its content is changed from 34.31% to 34.07% before and after the pretreatment. Its invariability is more outstanding than that in -1.80+1.45 g/cm3 lignite sample. The fewest sorbitan monooleate molecules are adsorbed on +1.80 g/cm3 lignite surface. Figure 7 showed the schematic diagram of interfacial interaction between sorbitan monooleate molecule and lignite particle in water media. In conclusion, the lower density lignite can form hydrogen bonds with the polar groups of the sorbitan monooleate molecule, while the mineral Si-O can hardly form hydrogen bonds with the reagent. Therefore the hydrophobicity of lower density lignite will be improved by the adsorption of non-ionic surfactant.


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(a)

(b)

Figure 7 Schematic diagram of interfacial interaction between sorbitan monooleate molecule and particle in water medium, sorbitan monooleate in red round, water in blue round, (a) lower density lignite (b) higher density lignite. 4 Conclusions This research focuses on the surface charged properties and adsorption performance of nonionic surfactant, sorbitan monooleate, on the lignite coal surface of different density, including -1.45 g/cm3, -1.80+1.45 g/cm3 and +1.80 g/cm3. FTIR results indicated that the surface of lower density lignite is covered with more organic functional groups, while the higher density coal is covered with more inorganic oxygen groups. Therefore the zeta-potential of the lighter coals is more related to the organic functional groups, while that of the heavier coals is more relative with the mineral Si-O groups. However, it is quite apparent that three zeta-potential versus pH curves are similar to each other qualitatively. Hence the organic oxygen functional groups have the similar ionization/ protonation ability with inorganic Si-O functional groups. Since the PZC of the three coal samples are located at pH values of 2.3, 3.0 and 3.0, it is difficult to adjust the pH of the slurry to make the different density lignite to charge oppositely. But on the other hand, there are great diversities in the adsorption performance of the nonionic surfactant on the surface of the lignite particles. The increments of C1s are 23.71%, 13.44% and 10.22% for the lignite samples


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from low to high density. The decrements of O1s are 13.74%, 7.16% and /4.96% for the three samples, respectively. It is concluded that the lower density lignite adsorbs more sorbitan monooleate molecules on the surface. In addition, polar groups of the surfactant have different adsorption performance with organic functional groups on the lower density lignite surface. The polar groups, including -OH and -COOR of the sorbitan monooleate molecule easily form associated hydrogen-bond with the oxygen functional groups, containing C=O and O-H on lignite coal surface, while epoxy ether bond almost cannot. Furthermore, the interaction force between the C=O on the coal surface and polar groups of sorbitan monooleate molecule is comparatively weak. It is verified that the polar groups of the sorbitan monooleate molecules adsorb on the oxygen functional groups on the lignite surface. Besides, due to the excess -OH in sorbitan monooleate molecule, it is presented on the surfaces of all the three samples after the pretreatment, which will be beneficial for the structure optimization of the surfactant. On the other hand, Si-O plays a dominant role in the hydrophilicity of the raw higher density sample. Interestingly, its content nearly does not change after the pretreatment by sorbitan monooleate solutions. Hence it can be concluded that the sorbitan monooleate molecule cannot almost adsorb on the mineral Si-O. That is the main reason that causes the less adsorption of sorbitan monooleate on the higher density lignite particle, which will be helpful for the lignite beneficiation in flotation process. Acknowledgements This work was supported by the National Natural Science Foundation of China (No.51574234). We also want to thank the Fundamental Research Funds for the Central Universities (2014QNB10). We would like to thank Advanced Analysis and Computation Center of China University of Mining and Technology for their technical support.


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Adsorption performance of nonionic surfactant on the lignite particles

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