Low-rank Coal Surface Research Articles

Improving Low Rank Coal Flotation Using a Mixture of Oleic Acid and Dodecane as Collector: A New Perspective on Synergetic Effect

The mixed collector can improve low rank coal flotation efficiency, but its synergistic mechanism needs to be further explored. In this paper, oleic acid-dodecane (OA-D), oleic acid (OA), and dodecane (D) were employed to treat the low rank coal for revealing new synergistic mechanism of the mixed collector. First the surface free energy of the coal, the surface free energy of coal-water and coal-water-coal were calculated. Then wetting heat measurement, X-ray Photoelectron Spectroscopy (XPS) and FTIR were used to analyze synergistic mechanism of the mixed collector in depth. The results showed that OA-D obtained a higher combustible recovery than using OA and D, respectively. The essence of synergetic mechanism of OA-D was that they formed a relatively ordered “supramolecular structure” on the low rank coal surface, especially there were hydrophobic and van der Waals forces between the oleic acid chain and the dodecane chain that can promote the formation of a continuous collector film.

Comparison of the adhesion kinetics between air or oily bubble and long flame coal surface in flotation

Flotation is an effective separation technology with a wide range of applications, and oily bubbles have recently been shown to be more effective than air bubbles at separating low-rank coal. However, the differences in the spreading behaviours of oily and air bubbles at the microscale have not yet been clarified. In this study, high-speed camera technology was used to characterise the interaction process of air and oily bubbles with low-rank coal particles. The three-phase contact (TPC) line formation behaviours of air and oily bubbles on the low-rank coal surface were observed in a deionised solution. At 0–20 ms, the oily bubbles spread more quickly on the low-rank coal surface than the air bubbles; the TPC spreading completion percentages of the oily and air bubbles were 97.58% and 70.73%, respectively. At 20–100 ms, the oily bubbles spread more slowly than the air bubbles because they respectively had 2% and 30% of the TPC spreading process to complete. The results indicated that the oily bubble surface was more hydrophobic than the air bubble surface.

Role of sodium dodecyl benzene sulfonate in enhancing coal dewatering: Experiments and theoretical calculations

The surface of low-rank coal has many oxygen functionalities, which leads to poor coal slime dehydration. To improve the effect of coal slime dewatering, sodium dodecyl benzene sulfonate (SDBS) was used as accelerator in the coal slime dehydration process. In addition, the mechanism of acceleration was studied using X-ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations. The results of dehydration experiments showed that adding a certain amount of SDBS into the slurry water was conducive to promoting the removal of water. When the concentration of the SDBS solution was 0.06 g/L, the water content of the filter cake decreased from an initial value of 59.98% to 54.55%. XPS measurements and MD simulations show that the polar groups (sulfonic acid) of SDBS are adsorbed on the oxygen-containing functional groups on the coal surface, thus, exposing the nonpolar groups (alkyl chains) of the molecule to the solution. The calculation of the self-diffusion coefficient (D) of water molecules in the simulated system indicate that the presence of SDBS improves the mobility of water at the coal surface, which is helpful in improving the loss of water from the coal surface, thus, improving its hydrophobicity. Crucially, the results of the MD simulations are consistent with those from experiment. Highlights Addition of a certain amount of SDBS helps to remove moisture from the coal slime. Oxygen-containing functional groups on the coal surface are effectively covered by SDBS. The interaction mechanism of SDBS at coal/water interface was studied by combining experiments and theoretical calculations.

Microscopic spreading characteristics of non-polar oil droplet on low rank coal surface: Effects of surfactant pretreatment and oxygen-containing groups

Numerous oxygen-containing groups present on the low rank coal (LRC) surface makes a poor flotation performance of LRC only using common oily collector. Increasing the spreading strength of oily collector on the LRC surface is of great significance for improving its flotation behavior. In this study, molecular dynamics simulation (MDS) was employed to investigate the microscopic spreading characteristics of non-polar oil droplet on the LRC surface. For this purpose, the configurations of non-polar oil droplet, spreading area, and interaction energy were calculated. It was found that on the raw LRC surface, the non-polar oil droplet remained spherical with a small spreading area and weak interaction strength. Besides, a non-ionic surfactant, dodecyl nonaethoxyl ether (C12EO9) was used to modify the surface wettability of LRC to enhance the non-polar oil droplet spreading. After surfactant pretreatment, a pancake-like oil film was formed on the LRC surface with a larger spreading area and more negative interaction energy compared with the case of the raw LRC surface. Based on these observations, the microscopic insights were provided about the spreading of non-polar oil droplet on the surface of LRC, as well as the role of surfactant enhancing its spreading intensity. In addition, the effect of the oxygen-containing groups on the spreading behaviors of non-polar oil droplet was also explored. The results suggested that carboxyl and hydroxyl are the main groups that hinder the spreading of the non-polar oil droplet on LRC surface. The simulation results are in accordance with the available experimental data.

Effect of Hydration Layer on the Adsorption of Dodecane Collector on Low-Rank Coal: A Molecular Dynamics Simulation Study

The hydration layer has a significant effect on the adsorption behavior of reagents during the flotation process of low-rank coal. Understanding the effect of hydration layer on the adsorption of common collectors on low-rank coal is a prerequisite for proposing a new enhanced coal floatation method. In this study, a smooth low-rank coal surface model with a density of 1.2 g/cm3 was constructed and compared with the XPS results. Three different systems, coal-water, coal-collector, and coal-water-collector, were constructed. Molecular dynamics method was applied to study the adsorption behaviors of water and dodecane molecules. Simulation results revealed that a stable hydration layer with a thickness of about 5 Å was formed due to the strong attraction of coal surface. The negative value of interaction energy (IE) indicated that dodecane molecules could spontaneously adsorb on the coal surface. Dodecane molecules were successfully adsorbed on the coal surface when it was located inside the hydration layer. While the dodecane molecule was outside the hydration layer, it could not pass through the hydration layer on the surface of low-rank coal.

Molecular Dynamics Simulation Study of Bubble Attachment at the Coal Surface with Varying Coalification Degrees.

Establishing the dynamics of wetting film thinning and rupture during the bubbles attached on the coal surface is extremely important for flotation. However, studying the dynamics of bubble attachment from the molecular level using molecular dynamics simulation (MDS) has rarely been reported. In this work, the dynamics of bubble attachment at three different coal [low-rank coal (LRC), bituminous coal (BC), and anthracite coal (AC)] surfaces with varying degrees of coalification were studied using MDS. In the bubble attachment process, the wetting film between the bubble and coal surface gradually become thinner until it ruptures. By comparing the bubble attachment dynamics on three different coal surfaces, the results indicate that the bubble attachment rate on the surface with strong hydrophobicity is faster than that on the surface with weak hydrophobicity. Besides, the number of hydrogen bonds between the molecules of the wetting film is decreased with the attachment of bubbles; however, it is sharply decreased on the BC surface and slowly reduces on the LRC surface before the film rupture. At the same time, the radial distribution functions (RDFs) of hydrogen bonds in the wetting film at the moment of bubble attachment on the coal surface are analyzed, indicating that the peak intensity of the RDF decreases at the time of bubble attachment. The findings in this study may help to better comprehend the dynamics of bubble attachment, which is valuable for future design in practical applications.

Molecular dynamics simulation and experimental characterization of anionic surfactant: Influence on wettability of low-rank coal

The influence of the anionic surfactant sodium fatty alcohol polyoxyethylene ether carboxylate (AEC) on the wettability of coal surfaces was studied by combining molecular dynamics simulations and experimental research. First, a molecular dynamics simulation of the AEC adsorption process on a low-rank coal surface was carried out. The simulation results show that AEC promotes the adsorption of water molecules on the coal surface, moves more water molecules to the coal surface, and improves the mobility of water molecules; AEC molecules were detected at the coal-water interface. After adsorption, the AEC molecules were connected to each other through alkyl chains to form a spherical-like structure, which covered the coal surface laterally. The calculated interaction energy between coal and water is negative, which shows that the adsorption process is spontaneous, and AEC enhances the interaction between coal and water. Second, the abovementioned simulation results are verified by experiments. The adsorption experiments show that AEC adsorbs on the lignite surface by monolayer adsorption, which can be described by the Langmuir isothermal adsorption equation; the contact angle first increases and then decreases, reaching a maximum near the critical micelle concentration (CMC). The XPS analysis shows that the change in the content of the oxygen-containing functional groups is the main reason for the wettability of the lignite surface. Among these oxygen-containing functional groups, the C-O group has the most significant effect on the wettability.

Effect of pores on the flotation of low-rank coal: An experiment and simulation study

Low rank coal contains a large number of pores and cracks which have an adverse influence on its flotation behavior. Effects of pores on the hydration layer and flotation behavior of low-rank coal were studied by molecular dynamics method and experiments in this paper. A specific surface area/pore analyzer was employed to determine the pore structure. Pore distribution analysis indicated that the pores in the low-rank coal are mainly micropores. Pore volume and average pore diameter in low-rank coal can be reduced by pore compression pretreatment. Coal-bubble attachment experiments were conducted to study the effect of pores on the effect of pores on particle bubble adhesion process. Results showed that coal particles with less pores were more likely to adhere to the surface of the bubble. And a probable mechanism for the effect of pores on the adhesion of coal particles to bubbles was proposed. Molecular dynamics simulation results revealed that the pores on the surface of low-rank coal reduce the mobility of water molecules and form a more stable hydration layer. Flotation results indicated that reducing the pore content in low-rank coal could reduce the collector dosage in flotation process and improve the flotation behavior of low-rank coal.

Molecular insight into the mechanism of benzene ring in nonionic surfactants on low-rank coal floatability

In this work, a combined molecular dynamics (MD) simulations and experiments were adopted to study the microscopic mechanism of the benzene ring in nonionic surfactants on low-rank coal (LRC) flotation. Two nonionic surfactants with identical headgroups, nonylphenol ethoxylate (NPEO) and dodecyl ethoxyl ethers (C12EO), were selected as the research objects. Two LRC surface models with varying metamorphism degrees were constructed. The rationality of the constructed surface models was firstly discussed from the chemical composition and structure. The morphology of the surfactants and the adsorption energy obtained by MD suggested that the arrangement of polar oxygen atoms in the surfactant depended on its interaction with the polar groups of LRC, and the presence of the benzene ring in the nonionic surfactant hardly changed its arrangement on the LRC surface. The benzene ring in the nonionic surfactant can enhance its adsorption on the higher rank coal surface via the π-bonds while it cannot promote its adsorption on the lower rank coal surface. The results obtained from the experiments are in accordance with the simulation results.

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

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.

Oily collector pre-dispersion for enhanced surface adsorption during fine low-rank coal flotation

The prior adsorption of water molecules on low-rank coal surface suppresses the adsorption of collectors and decreases the surface hydrophobicity during flotation. To improve the adsorption of oil molecules, this study proposed an innovative flotation flowsheet, i.e., oily collector pre-dispersion. Oily collector pre-dispersion enhances the flotation recovery by 5–10% compared with the traditional flowsheet. The underlying oil/water‒coal interaction mechanisms were investigated by combining experiment and simulation. The results indicated that oil droplets adsorbed quickly on the low-rank coal surface in an air environment and, notably, the contact angle was smaller than that of a water droplet. However, in a water environment, the contact angle of the oil droplet on the low-rank coal surface was large and stable. Molecular dynamics simulations of coal/oil/water adsorption configurations revealed that oil molecules self-aggregated into spheres when water molecules surrounded the low-rank coal surface. In contrast, water molecules were largely repelled when the oil molecules were preadsorbed on the low-rank coal surface. Adsorption capacity measurements directly confirmed that the adsorption capacities of oily collectors improved with collector pre-dispersion. Thus, water/coal adsorption can be controlled by realizing full contact between oil and low-rank coal. This finding suggests a technical modification that can be adopted to enhance low-rank coal flotation.

Improved floatability of low-rank coal through surface modification by hydrothermal pretreatment

Abstract Low-rank coal (LRC) contains a developed pore structure and rich oxygen-containing functional groups (OCFG). Therefore, it cannot easily be upgraded using flotation technology. In this study, simple and effective hydrothermal pretreatment (HP) was performed to reduce the porosity and increase the hydrophobicity of LRC surface. LRC characteristics, such as coal composition, surface morphology, pore structure, and surface functional groups were analyzed before and after HP. The results showed that, after HP, the coals exhibited lower amounts of moisture and volatile matter, but substantially greater amounts of fixed carbon than raw coal. Pore structure analysis indicated the collapse and closure of many pores on the coal surface, indicating in low porosity in the pretreated coal. In addition, X-ray photoelectron spectroscopy revealed more hydrophobic functional groups and less OCFG on the coal surface. It is, therefore, HP resulted in excellent flotation recovery, with increased recovery of 70.7% combustible material.

Effect of microemulsion on low-rank coal flotation by mixing DTAB and diesel oil

It is difficult to recover low-rank coal using traditional oily collectors because a large number of oxygen-containing functional groups and pores exist on its surface. In this study, a microemulsion was prepared by mixing dodecyl trimethylammonium bromide (DTAB) and diesel oil; its effect on low-rank coal flotation was studied via water contact angle and wrap angle measurements. The results show that the clean coal could not be effectively recovered using a single reagent of DTAB or diesel oil. However, the microemulsion can improve the clean coal recovery with a small amount of diesel oil. The recovery of clean coal was greatest when the mass ratio of DTAB and diesel was 1:2. The addition of DTAB could act as a dispersant in the oily collector, and thus increased the contact area between coal and reagent. More importantly, DTAB and diesel oil could be adsorbed on the low-rank coal surface because of their synergistic effect. On the one hand, DTAB can improve the dispersion of diesel oil. On the other hand, the combination of diesel and DTAB promoted the spread of diesel on the particle surface. The microemulsion could improve the hydrophobicity and floatability of coal particles and enhance the stability of particle-bubble adhesion. The relationship between wrap angle and time could be well fitted by the first-order kinetic equation and the results show that the attachment rate gradually increased and reached its maximum when the mass ratio of DTAB and diesel was 1:2.

Synergistic adsorption of polar and nonpolar reagents on oxygen-containing graphite surfaces: Implications for low-rank coal flotation

Conventional oily collectors cannot improve the floatability of low-rank coal because of the presence of containing-oxygen groups on the mineral surface, and the effect of the application of a single polar reagent is also limited. In this study, an innovative approach using molecular dynamics simulations to study the adsorption of mixed collectors on an oxygen-containing graphite surface was carried out. The adsorption structures of the collectors were clearly revealed, which has important implications for flotation recovery. In the simulations, when a nonpolar collector, dodecane, was used alone, it was only adsorbed on the carbon atom sites, and the hydrophilic oxygen-containing groups were not covered and remained exposed. In contrast, when a polar collector, dodecanoic acid, was used alone, the molecules self-aggregated, which reduced the surface coverage and increased the collector consumption. However, the use of a mixture of nonpolar and polar molecules resulted in a good dispersion over the mineral surface and significantly improved the coverage of oxygen-containing groups. The nonpolar molecules could be adsorbed on the oxygen-containing sites, being directed by the polar molecules, resulting in a synergistic effect. In addition, water contact angle tests show that the mixed collector can significantly improve the hydrophobicity of the low-rank coal surface, which is beneficial for improving the floatability of coal particles. Flotation tests verified the theoretical and experimental results: when the mass ratio of dodecanoic acid was 40%, the combustible matter recovery was significantly increased compared to that using a single collector. Consequently, the addition of a polar component reduces the required dosage of the traditional oily collector. This investigation provides a better understanding of the interaction mechanism of mixed collectors in low-rank coal flotation.

Adsorption behavior of mixed dodecane/n-valeric acid collectors on low-rank coal surface: Experimental and molecular dynamics simulation study

In this investigation, a mixed collector (MC) of dodecane and n-valeric acid was used to improve low rank coal flotation. Fourier transformation infrared spectroscopy (FTIR) measurements and contact angle measurements were conducted to explain flotation responses. Furthermore, using a molecular dynamics (MD) simulation, the changes of water/collector (dodecane or MC)/coal systems were studied before and after adsorption to indicate the absorption behavior and mechanism. The results showed that a better flotation performance of low-rank coal was obtained using the MC compared to a conventional collector of dodecane. The hydroxyl (OH) or carbonyl (CO) or carboxyl (COOH) migration of the coal with the MC was stronger than that with dodecane as a solo collector, and the MC can enhance the hydrophobicity and lipophilicity of the coal surface. MD simulation results revealed the adsorption behavior of the MC on low-rank coal surfaces. Dodecane was mainly adsorbed on the hydrophobic surface of coal, and n-valeric acid was more inclined to act on the oxygen-containing functional group of coal, i.e., a hydrophilic surface. The mobility of = dodecane molecules on the coal surface was reduced owing to the presence of n-valeric acid, which increased the lipophilicity and hydrophobicity of the low-rank coal surface, thus attracting dodecane molecules of the MC and restricting their movement, and accelerating the departure of water molecules from the low-rank coal surfaces. There was also a higher interaction energy between the MC and low-rank coal. The simulation results are in good agreement with the measured results; this effectively explains the flotation results.

Investigation of nonylphenol ethoxylate on the surface characteristics of low rank coal

The surface characteristics of sub-bituminous coal have been determined in the presence of nonylphenol ethoxylate with 9 ethylene oxide groups (NP-9). The surface characterization techniques of zeta potential measurement, contact angle measurement, and surfactant adsorption have been used. Addition of NP-9 causes the zeta potential profiles to become less negative resulting from the surfactant molecules shielding the negative charge of the sub-bituminous coal. Changes in the sub-bituminous coal surface hydrophobicity are assessed using contact angle measurements. As the concentration of NP-9 gradually increases, the contact angle first increases and then decreases with the maximum contact angle at about critical micelle concentration (CMC) of NP-9. FTIR and XPS results show that oxygen functional groups on the low-rank coal surface were significantly covered by the NP-9, which indicates more hydrophobicity for the low-rank coal after adsorbing surfactant. The surfactant adsorption has been investigated as a function of concentration and temperature. The adsorption isotherms are well fitted by the Langmuir equation. The adsorption data are utilized to calculate thermodynamic parameters. The thermodynamic functions indicate that the whole adsorption process is exothermic, being driven by enthalpy-entropy synergically.

Improving bubble-particle attachment during the flotation of low rank coal by surface modification

Based on the problems caused by many oxygen-containing functional groups and poor floatability on the surface of low rank coal, the characteristics of low rank coal were analyzed systematically by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS) techniques. The bubble-particle induction time was used to determine the characterization of the bubble-particle attachment, and the bubble-particle attachment of low rank coal modified by soaking the coal samples in an acid or alkaline solution was analyzed. The floatability of the modified coal surface was verified by flotation tests. The results show that the particle size of 0.125–0.074 mm of the coal sample exhibited better bubble-particle attachment characteristics. The small bubble, the larger approach velocity of bubble and the larger bubble deformation were more helpful to enhance the bubble-particle attachment. For an acid solution, the smaller the pH was and the longer the soaking time was, the better the floatability of the coal sample and the higher the combustible material recovery were. The combustible material recovery of low rank coal was increased to 78.79% by soaking the sample in an acid solution of pH = 0 for 180 min. On the contrary there was a best concentration for the alkaline solution.

Effect of oxidized diesel oil on the flotation response of the low-rank coal by plasma oxidation method

The purpose of this investigation was to explore the influence of the oxidized diesel oil on the low-rank coal flotation response and flotation kinetics by plasma oxidation method. FTIR analysis showed that the polar groups of the diesel oil after oxidation, such as OH, CO, COOH and C–O–C, showed an obvious increase; and the polar groups generated could bond with the polar functional groups on the low-rank coal surface. Moreover, compared to the diesel oil before oxidation, a stronger interaction between the low-rank coal and the diesel oil after oxidation was found according to the reaction heat analysis. And, the contact angle of the low-rank coal wetted by the diesel oil before and after oxidation is 34.5° and 23.7°, respectively. Therefore, the diesel oil after oxidation showed better positive effect on enhancing the contact angle of the low-rank coal and restraining the negativity of the wetting heat of the low-rank coal wetted by water compared to the raw diesel oil. The FTIR analysis illustrates that the polar oxygen functional groups of the low-rank coal surface are decreased with the oil addition, especially after the oxidized diesel oil addition, indicating that the oxidized diesel oil can better improve the surface hydrophobicity of coal sample compared with the raw diesel oil. Furthermore, five kinetics models were chosen to assess the effect of the diesel oil after oxidation on the kinetics performance of the low-rank coal flotation, and the five test kinetics flotation models could give the excellent fit for the low-rank coal flotation data obtained by the diesel oil before and after oxidation. And, the kinetics fitted results illustrated that the diesel oil after oxidation strongly influenced the flotation kinetics of the low-rank coal and could give a better kinetics effect for the low-rank coal flotation than diesel oil before oxidation.

Synergistic effect of particle heterogeneity and size on the surface hydrophobicity characterization of low rank coal

In this article, five particle size fractions such as +0.500 mm, 0.500–0.250 mm, 0.250–0.125 mm, 0.125–0.074 mm, and −0.074 mm were used to investigate the synergistic effect of particle heterogeneity and size on the hydrophobicity characterization of low rank coal surfaces using induction time and wetting thermodynamic measurements. The high induction time result of +0.500 mm size fraction with low ash content proved that the physical property, especially particle size, has a significant effect on induction time and wetting thermodynamic measurements. Moreover, LHR value and combustible recovery of 0.250–0.125 mm size fraction were higher than that of 0.125–0.074 mm size fraction, which indicated that the induction time measurement was greatly affected by the chemical property, i.e. heterogeneity. Furthermore, wetting thermodynamic measurements can be used as the supplement of induction time measurements while the ash content of the particle size fractions is close.

Waste colza oil used as renewable collector for low rank coal flotation

The high cost of low rank coal flotation using diesel as the collector significantly restricts the improvement in economic efficiency of the coal plant. Waste colza oil (WCO) from the catering industry was used as a renewable and alternative collector for ultrafine low rank coal flotation in this investigation. Scanning electron microscopy (SEM) test was carried out to check the surface morphologies of the coal sample. Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) tests were performed to examine the coal chemical compositions. The flotation yield using WCO was always higher than that of using diesel, especially at a high dosage. When 6 kg/t dosage was used, 35.68% yield was obtained with WCO, 5.54% higher than that with diesel. The mechanism of WCO used as an alternative collector was discussed based on the induction time, FTIR, and XPS tests. The induction time of the low rank coal after conditioning with 6 kg/t diesel and WCO were 200 ms and 100 ms, respectively, indicating that WCO was a more effective collector for improving the floatability of low rank coal compared with traditional diesel. XPS tests confirmed the adsorption of WCO on coal surface as the WCO has a large amount of CO groups according to FTIR results. The hydrogen bonding between oxygen-containing groups in WCO and low rank coal surface was responsible for the improvement of floatability and flotation recovery.