Experimental and molecular dynamics simulation study on the enhancement of low rank coal flotation by mixed collector

Fuel 266 (2020) 117046

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Experimental and molecular dynamics simulation study on the enhancement of low rank coal flotation by mixed collector Lei Zhanga, Xiaole Suna, Bao Lia, Zhixuan Xiea, Jianying Guoa, Shengyu Liua,b, a b

T

⁎

College of Mining Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China Key Laboratory of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Low rank coal Mixed collector Adsorption Molecular dynamics simulation

Flotation collectors have a significant effect on the flotation performance and efficiency of fine low rank coal (LRC) particles. In this study, a mixed collector consisting of a nonpolar collector (dodecane) and a polar collector (C12EO4, tetraethylene glycol monododecyl ether) was used to improve the flotation performance of LRC. The results show that flotation behavior can be enhanced with the addition of C12EO4 in dodecane. The yield, combustible matter recovery, and flotation efficiency index increase with a larger volume ratio of C12EO4 in the mixed collector. A molecular dynamics simulation (MD) approach was applied to explore the microscopic strengthening mechanism of the mixed collector. The simulation demonstrated that the interaction strength and spreading ability of the mixed collector on the LRC surface increased due to electrostatic attraction and hydrogen bonding between the mixed collector and the LRC surface (as indicated by interaction energy and contact surface area), thereby enhancing flotation performance of the LRC. The effect of C12EO4 on the emulsification and dispersibility of dodecane is also discussed.

1. Introduction Low rank coal (LRC) is an important fossil energy source with large reserves. However, LRC must be first upgraded to use it efficiently. The properties of LRC, such as low density, fragility, and high concentration of fine particles, limit the application of gravity separation in upgrading LRC. Froth flotation is an effective method for fine coal separation. ⁎

Flotation reagents, especially the flotation collector, play a significant role in the cleaning efficiency of coal slimes in the coal preparation and utilization industry [1]. Nonpolar oils, such as kerosene and diesel oil, are the most common collectors used in coal flotation, which are effective for medium or high rank coals. However, due to the abundance of oxygen-containing groups, satisfactory LRC flotation performance cannot be achieved using common oily collector alone [2–5].

Corresponding author at: Taiyuan University of Technology, Taiyuan 030024, China. E-mail address: [email protected] (S. Liu).

https://doi.org/10.1016/j.fuel.2020.117046 Received 5 July 2019; Received in revised form 1 December 2019; Accepted 7 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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It has been proved that a mixed collector containing nonpolar and polar collectors (oxygenated surfactants) is effective in LRC flotation [2,6–9]. For instance, Xia et al. [2] applied a mixture of 4-dodecylphenol (DDP) and dodecane in the flotation of lignite. They found that the dodecane primarily covers the hydrophobic sites on the lignite surface, while the DDP primarily covers the hydrophilic sites. S. Chander et al. [6] dispersed the surfactant BCR1 (PO-EO-PO type, PO: propylene oxide; EO: ethylene oxide) in dodecane as the LRC collector. Using this collector, the yield of coal, flotation selectivity, and rate markedly increased. Liu et al. [7] used a mixture of dodecane and nvaleraldehyde in LRC coal flotation and obtained a higher clean coal yield compared with dodecane alone. These studies indicate that a mixed collector consisting of polar and nonpolar collectors could be applied for improved LRC flotation. However, the micro-mechanism of LRC flotation enhancement by mixed collector has not been reported. Currently, the application of molecular dynamics (MD) simulations is becoming more popular for studying phenomena that cannot be observed by experimental methods, tracking the dynamic evolution process of a system, and explaining the micro-mechanisms. In the area of mineral processing, molecular dynamics simulations are often used to study the interaction of minerals and collectors [10–16]. For example, Wang et al. [10] used molecular dynamics simulation to describe the co-adsorption of mixed surfactants (dodecylamine hydrochloride and sodium oleate) on the muscovite surface in an aqueous solution. They found a micelle structure forms, which creates a hydrophobic state on the muscovite surface. Shen et al. [11] studied the performance of a dodecylamine chloride/fatty acid collector on kaolinite flotation using flotation tests and computational methods. The simulation results show that the octanoic acid molecules were interleaved among dodecylamine chloride ions and co-adsorbed at the kaolinite (0 0 1) surface, thus increasing the hydrophobicity of the kaolinite (0 0 1) surface. Based on molecular dynamics calculations, Zhang et al. [12] indicated the adsorption mode of octanol on mineral surfaces, when combined with sodium oleate for flotation, was through hydrogen bonds. Liu et al. [13] investigated the adsorption behavior and mechanism of an isopropanol amine collector on magnesite ore and found from simulation results that adsorption was achieved due to the electrostatic and hydrogen bond affinities. Therefore, it is possible to use MD simulation to study the adsorption behavior of a mixed collector on the LRC surface and explore the micro-mechanism involved. In the current investigation, a mixed collector consisting of dodecane (C12) and tetraethylene glycol monododecyl ether (C12EO4) was applied to the flotation of LRC. In addition, the adsorption behavior of a mixed collector on an LRC surface model and oxygen-containing group models were studied by MD simulation. Based on the simulation results, the enhancement mechanism in the flotation performance of LRC using a mixed collector is fully discussed.

Fig. 1. The molecular model of dodecane (a), C12EO4 (b) and Hatcher lignite coal (c), and the LRC surface model (d) The fixed atoms in LRC surface model were shown in red. Colored balls represent O in red, C in gray, H in white. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2.2. Flotation tests Flotation tests were completed in an XFG flotation machine with a 500 mL cell with pulp density at 10%. The impeller speed of the flotation machine was 2000 rpm, and the airflow rate was 0.17 L/min. The dosage of the collector was 1600 mL/t, and octanol was used as the frother, with a dosage of 200 mL/t. Dodecane (C12) was used as the nonpolar collector and non-ionic surfactant tetraethylene glycol monododecyl ether (C12EO4) consisting of polyethylene oxide and hydrocarbon chains was used as the polar collector. C12EO4 is an oil-soluble surfactant and that is miscible with dodecane. The molecular structures of dodecane and C12EO4 are displayed in Fig. 1(a) and Fig. 1(b), respectively. The mixture of C12 and C12EO4 at different volume ratios was used as a mixed collector. The temperature during the experiment was 298 K, and tap water was used in all flotation tests. First, the coal sample was added to the flotation cell and pre-wetted for 2 min. Next, the collector was added and the pulp was conditioned for 1 min. Then, the frother was added and the pulp was conditioned for another 10 s. Finally, the air flow was started and the flotation concentrate was collected within 3 min. The concentrate and tailing samples were filtered, dried, weighed and combusted for ash determination. The flotation performance was evaluated by combustible matter recovery and flotation efficiency index, which were calculated using Eqs. (1) and (2), respectively. Combustible matter recovery and flotation efficiency index are the common parameters to evaluate flotation performance in coal flotation [1–2,17–18]. The aim of coal flotation is to recover the combustible matter. Combustible matter recovery is the ratio of combustible matter in the flotation concentration to the combustible matter in the feed. A higher combustible matter recovery suggests a higher flotation efficiency. The flotation efficiency index, taking both concentrate yield and ash content into account, is a comprehensive parameter to evaluate whether the flotation has been improved. Further, a higher numerical value indicates better flotation performance [17].

2. Experiment and simulation 2.1. Coal sample An LRC sample from Inner Mongolia, China was used in this study. The LRC was crushed and screened to smaller than 0.5 mm. The proximate and ultimate analyses of the coal are given in Table 1.

Table 1 The proximate and ultimate analysis of coal sample. Proximate analysis/%

MC (100 − AC ) × 100 MF (100 − AF )

(1)

MC A − AC × F × 100 100 − AF AF MF

(2)

Combustible matter recovery (%) =

Ultimate analysis/%

Mad

Aad

Vdaf

Cdaf

Hdaf

Oadaf

Ndaf

Sdaf

4.25

29.33

36.45

82.57

5.37

10.15

1.29

0.62

Flotation efficiency index (%) =

where MC is the weight of the concentrate, MF is the weight of the feed,

ad: air-dry basis; d: dry basis; daf: dry ash-free basis; a: by difference. 2

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AC is the ash content of the concentrate (%), and AF is the ash content of the feed (%). 2.3. Simulation details The Hatcher lignite coal model was selected to represent LRC, as shown in Fig. 1(c). Before constructing an LRC surface model, the Hatcher lignite model was optimized by DMol3 module implemented in Materials Studio 6.0. The exchange correlation energy was calculated within the generalized gradient approximation using the form of the functional proposed by Perdew and Wang (PW91) [19] and a double numerical plus polarization basis set [20,21]. The self-consistent field convergence was set to 10−5, and the convergence criteria fixed for the energy, maximum force, and maximum displacement were 2 × 10−5 Ha, 4 × 10−3 Ha/Å, and 5 × 10−3 Å, respectively. Twenty-two optimized Hatcher lignite molecules were constructed as an LRC surface model, followed by an annealing algorithm [22,23], with the aim of structural relaxation of the LRC surface model. At the top of the LRC surface model, an 80 Å thick vacuum slab was added to prevent any influence of the period boundary conditions. Taking into account that coal is a water-insoluble substance, to save time, the bottom two-thirds of the model was constrained, and the top one-third was free, as shown in Fig. 1(d). Similarly, the C12 and C12EO4 molecules were optimized using the same parameters as the Hatcher lignite coal model. A collector box (the same length and width as the LRC surface model) containing 24 C12 molecules or 20 C12 and 4 C12EO4 molecules was put into the LRC surface model to investigate the adsorption behavior. To explore the distribution of a mixed collector on oxygen-containing groups of LRC, a graphene model grafting different oxygen functional groups (carboxyl, hydroxyl, carbonyl, and ether) was constructed, as shown in Fig. 2. Molecular dynamics simulations were performed using the Nosé thermostat and canonical ensemble with a 1 fs time step. For full interaction, 500 ps simulations were conducted. The Ewald method with a precision of 0.00 1 kcal/mol was used for electrostatic interactions, and the Van der Waals interaction was calculated under the atom-based option with a cutoff of 12.5 Å. The output files were analyzed and discussed. The MD simulations were performed using the Forcite

Fig. 3. The flotation results of LRC coal (a) Effect of the volume ratio of C12 and C12EO4 on concentrate yield and ash content of low rank coal; (b) Effect of the volume ratio of C12 and C12EO4 on combustible matter recovery and flotation efficiency index of low rank coal.

module in Materials Studio 6.0. A polymer consistent force-field was employed for all simulations. 3. Results and discussion 3.1. Flotation results The flotation results are displayed in Fig. 3. It can be observed in Fig. 3(a) that the concentration yield of LRC using only dodecane or C12EO4 is not satisfied. Dodecane is a non-polar collector and C12EO4 is a polar collector, but they obtain almost the same yield. Moreover, the ash content of clean coal using a C12EO4 collector is higher than with a dodecane collector, indicating that the selectivity of C12EO4 is poor. This is because LRC is a heterogeneous substance containing polar and non-polar parts. Dodecane interacts with the non-polar parts of LRC, and C12EO4, a polar collector, interacts with the polar parts of LRC. When used alone, there is a limited coal surface to interact with the collector. Another reason for the low yield of LRC using C12EO4 may be the viscosity. It has been reported that the viscosity of C12EO4 is 35 cP [24], while the viscosity of dodecane is 1.434 cP [25]. The high viscosity of C12EO4 limits its dispersibility in pulp. Flotation efficiency is closely related to the dispersibility of an oily collector. An increase in collector dispersibility results in an increase in flotation recovery [26,27]. Hence the concentration yield using dodecane or C12EO4 alone is very low and nearly equal. However, the polar collector C12EO4 can also interact with the ash-forming hydrophilic minerals in coal. Thus, the surface hydrophobicity of both organic matter and minerals is improved by C12EO4, resulting in higher ash content using C12EO4

Fig. 2. The top view (a) and side view (b) of graphite; and the constructed model of oxygen-containing groups (c). 3

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compared to dodecane. It can be concluded that the use of dodecane or C12EO4 alone does not yield efficient flotation. The yield increases significantly using the mixed collector. Within the research scope of this work, as the ratio of C12EO4 increases, the yield and ash content of clean coal increase. The mixed collector contains both polar and non-polar parts. The polar ethoxy bonds with the hydrophilic oxygen-containing functional groups of the coal surface through hydrogen bonds [28]. The non-polar alkane chain interacts with the hydrophobic parts of the LRC surface by hydrophobic bonding [28]. Thus, the interaction between collector and LRC strengthens, helping to increase the yield of concentrate. The combustible matter recovery and flotation efficiency index results in Fig. 3(b) show the same trends as the yield of concentrate. LRC flotation can be enhanced by adding an oil-soluble nonionic surfactant to dodecane. The dosage of C12EO4 plays an important role in enhancing the hydrophobicity of the LRC surface.

Table 2 The interaction energies and energy components of different collector-LRC models. Model

C12 collector – Lignite Mixed collector – Lignite

Eint

Evdw

Eelec

−280.25 −373.13

−278.97 −323.21

−1.28 −49.92

where Ecomplex is the energy of the LRC-collector complex. ELRC and Ecollector are the energies of the LRC surface model and the collector model, respectively. The energy components, Van der Waals (Evdw) interaction, and electrostatic interaction (Eelec) were also calculated using a similar method. Negative values of interaction energy correspond to the attractive force between two components, whereas positive values represent repulsive forces. The more negative the value of Eint, the stronger the interaction. Table 2 summarizes the interaction energy results. The Eint in the mixed collector system is more negative than in the dodecane system, indicating that the mixed collector has more affinity to the LRC surface model, which is in accordance with the flotation results. To gain deeper insight into the adsorption mechanism of the collector, the components of interaction energy were analyzed [29,30]. Evdw and Eint are nearly equal and Eelec is small in the dodecane system because electrostatic force usually exists between polar molecules. Dodecane is a non-polar collector and it only interacts with the nonpolar parts of the LRC surface model through hydrophobic bonding. Although there are polar parts on the LRC surface model, the interaction of polar parts and dodecane is very weak. The value of Eelec is negligible, which accounts for the difficulty in LRC flotation using a dodecane collector. However, with the addition of C12EO4 in dodecane, Eint and Eelec can be greatly increased, as shown in Table 2. The increase of Eelec suggests that there is interaction between polar molecules in the mixed collector simulation model. In other words, the ethoxy groups of C12EO4 interact with the polar parts of the LRC surface model, effectively covering the oxygen-containing groups. As a result, the increased hydrophobicity of the LRC surface greatly improves LRC flotation performance.

3.2. Molecular dynamics simulation results The microscopic enhancement mechanism of a mixed collector on LRC flotation was investigated by MD simulation. In this section, we use 20 dodecane and 4 C12EO4 molecules as the mixed collector, which corresponds to a volume ratio of 3:1. 3.2.1. Interaction energy Fig. 4 depicts the initial and equilibrium adsorption configurations of single and mixed collectors on the LRC surface model. It can be observed that both collectors interacted with the LRC surface model. However, it is difficult to determine the adsorption strength from configurations alone. Thus, the interaction energy (Eint) is calculated using Eq. (3) to estimate the adsorption intensity of a collector on the LRC surface model.

Eint = Ecomplex − (ELRC + Ecollector )

Interaction energy (kcal/mol)

(3)

3.2.2. Contact surface area In addition to interaction energy, we introduce the contact surface area (CSA) to evaluate the intensity of adsorption, which can be calculated using Eq. (4):

CSA = (SASALRC + SASAcollector − SASA complex ) / 2

(4)

where SASALRC, SASAcollector, and SASAcomplex are the solvent-accessible surface areas (SASA) of the LRC surface model, collector, and LRCcollector complex, respectively. A larger contact surface area leads to stronger adsorption intensity. In this study, we use a probe with a radius of 1.4 Å to calculate the solvent-accessible surface area [22,23]. The calculated results indicate that the contact surface area between collector and the LRC is greatly increased, as shown in Table 3. The increase in contact surface area allows increased interaction between the LRC surface and the collector, and increased collector spreading on the LRC surface. The polar ethoxy of C12EO4 in the mixed collector bonds with the hydrophilic sites on the LRC surface, thus increasing the coverage by the mixed collector. Increased coverage enhances the Table 3 The contact surface areas of different collector-LRC models.

Fig. 4. The initial and equilibrium configurations of single (a), (b) and mixed collectors (c), (d). 4

Model

Contact surface area (Å2)

C12 collector – Lignite Mixed collector – Lignite

2752.28 2959.93

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3.2.4. Adsorption behavior of the mixed collector on oxygen-containing groups of LRC The adsorption behaviors of mixed collectors on the oxygen-containing groups of LRC were investigated by MD simulation to obtain the distribution characteristics of the mixed collector. In this section, two C12EO4 and ten C12 molecules are used, corresponding to a volume ratio of 3:1. Fig. 6 illustrates the initial and equilibrium configurations of the mixed collector on different oxygen-containing groups. As shown in Fig. 6, almost all collector molecules lie flat on the oxygen-containing groups. To elucidate the adsorption conformations further, relative concentration profiles of representative atoms in C12 (C) and C12EO4 (O) along the Z-axis (perpendicular to the surface of the oxygen-containing group models) were calculated. The concentration profiles quantify the distribution of collectors on different oxygen-containing groups, as shown in Fig. 7. It can be clearly observed that the distribution of the O atom in C12EO4 is closer to oxygen-containing groups than the C atom in C12, indicating that C12EO4 has a great affinity for oxygen-containing groups than C12. This distribution of mixed collectors could form an effective coverage layer on the hydrophilic parts of LRC, increasing the hydrophobicity and floatability of LRC. The interaction energies of the mixed collector and oxygen-containing groups were calculated by the same method described in Section 3.2.1. Table 4 indicates that for all oxygen-containing groups, the interaction energies become more negative when using the mixed collector, due to the introduction of C12EO4. It was found that the mixed collector shows the most affinity for the carboxyl group, followed by the hydroxyl group, which is consistent with the previous result suggesting that C-O/OH and COO/COOH are responsible for the adsorption of EO-based surfactants [31]. It is acknowledged that hydrogen bonding of the polar parts of the LRC surface with the oxygenated functional groups of the reagent greatly enhances the LRC flotation response [23,28,32,33]. Here, we use geometrical criteria to define the existence of a hydrogen bond: intermolecular hydrogen-acceptor distance is less than 2.5 Å, and the donor-hydrogen-acceptor angle is greater than 135° [34]. It was found that there are two kinds of hydrogen bonds between ethoxy and carboxyl or hydroxyl groups, as shown in Fig. 8. One is the bond between the O in ethoxy and the H in COOH or OH (HB1), the other is the bond

Fig. 5. The MSDs and diffusion coefficients of different collector-LRC models.

hydrophobicity of the LRC surface. The results of contact surface area match well with the interaction energy results. 3.2.3. MSD To describe the dynamics properties of the collector, the mean square displacement (MSD) was calculated. Based on the MSD, the diffusion coefficient (D) was calculated according to Eq. (5) [3]:

D=

1 × KMSD 6

(5)

where KMSD is the slope of the MSD curve. A larger diffusion coefficient value suggests lower dynamics properties. Fig. 5 shows the MSD of collectors in both simulation systems under a kinetic equilibrium state. It is observed that the diffusion coefficients of the single and mixed collectors are 1.17 × 10−8 m2/s and 0.70 × 10−8 m2/s, respectively, indicating that the diffusion of the mixed collector was restricted with the addition of C12EO4. This is attributed to the stronger adsorption intensity of the mixed collector on the LRC surface model, which limits its diffusion ability. The MSD is consistent with the interaction energy and contact surface area results.

Fig. 6. The initial and equilibrium configurations of composite collector on different oxygen-containing groups. 5

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Fig. 7. The relative concentration profile of reprehensive atoms along Z-axis: (a) carboxyl; (b) carbonyl; (c) hydroxyl; (d) ether group.

investigated experimentally and theoretically [36]. It has been demonstrated that the emulsion droplet, with smaller size, better stability, and uniform distribution can be formed in a dodecane-water system by adding C12EO4. In other words, oil droplets can be easily dispersed in C12EO4 solution. The increase in collector dispersibility results in increased flotation recovery [26,27]. Therefore, the concentration yield of LRC increases significantly with a mixed collector.

Table 4 The interaction energies of different collector-oxygen-containing group models. Type

Model

Energy (kcal/mol)

Carboxyl group

C12 Mixed C12 Mixed C12 Mixed C12 Mixed

−234.31 −315.60 −115.59 −169.14 −231.02 −267.34 −113.46 −150.50

Carbonyl group Hydroxyl group Ether group

4. Conclusions The enhancement of LRC flotation using a mixed collector consisting of dodecane and C12EO4 was investigated through experimental and MD simulation methods. The results show that flotation recovery is improved using a mixed collector, and the ratio of dodecane and C12EO4 is an important factor. Based on the MD simulation results, it was found that the adsorption of C12EO4 on the LRC surface is mainly driven by hydrogen bonding and electrostatic attraction, resulting in increased interaction and spreading ability of the collector on the LRC surface model, which is beneficial to LRC flotation.

between the terminal hydroxyl H of EO chain and the O in COOH or OH (HB2). Thus, combined with the interaction energy results in Section 3.2.1, it is known that the adsorption of C12EO4 on the LRC surface occurs mainly through electrostatic attraction and hydrogen bonding. Similarly, the contact surface area was also computed as a measurement of adsorption intensity of collectors on oxygen-containing groups. In comparison with the single collector, the contact surface areas between the mixed collector and oxygen-containing groups are enlarged, indicating that the mixed collector coverage of oxygen-containing groups is increased, as shown in Table 5. The simulation results of oxygen-containing groups are consistent with the LRC surface model.

CRediT authorship contribution statement

3.3. Emulsification

Lei Zhang: Conceptualization, Methodology, Formal analysis, Writing - original draft. Xiaole Sun: Investigation. Bao Li: Formal analysis, Writing - original draft. Zhixuan Xie: Investigation. Jianying Guo: Writing - review & editing. Shengyu Liu: Writing - review & editing.

Aside from the adsorption of a polar collector on oxygen-containing groups to increase the hydrophobicity of the LRC surface, the emulsification of dodecane with surfactants is often used to explain the synergistic mechanism of a mixed collector for LRC flotation [6,35]. In a previous study, the emulsification effect of C12EO4 on dodecane was 6

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Fig. 8. The hydrogen bonds between C12EO4 and carboxyl group (a) and hydroxyl group (b). Table 5 The contact surface areas of different collector-oxygen-containing group models. Type

Model

Surface area (Å2)

Carboxyl group

C12 Mixed C12 Mixed C12 Mixed C12 Mixed

1467.36 1678.15 1442.84 1628.66 1575.70 1634.46 1422.57 1577.22

Carbonyl group Hydroxyl group Ether group

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (21878211 and 21376161). References [1] He JF, Liu CG, Yao YK. Flotation intensification of the coal slime using a new compound collector and the interaction mechanism between the reagent and coal surface. Powder Technol 2018;325:333–9. [2] Xia WC, Ni C, Xie GY. Effective flotation of lignite using a mixture of dodecane and 4-dodecylphenol (DDP) as collector. Int J Coal Preparation Utilization 2016;36(5):262–71. [3] Xia YC, Zhang R, Xing YW, Gui XH. Improving the adsorption of oily collector on the surface of low-rank coal during flotation using a cationic surfactant: an experimental and molecular dynamics simulation study. Fuel 2019;235:687–95. [4] Xia YC, Yang ZL, Zhang R, Xing YW, Gui XH. Performance of used lubricating oil as flotation collector for the recovery of clean low-rank coal. Fuel 2019;239:717–25.

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