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•Surface hydrophobicity of low-rank coal was enhanced by the adsorption of DTAB.•The head group of DTAB adsorbed on the low-rank coal surface and alkyl chains was exposed.•The ...adsorption of DTAB decreased the interaction energy and number of hydrogen bonds.•The mobility of water molecules was improved due to the surface repulsive effect.
This study investigated the mechanism of improving the surface hydrophobicity of low-rank coal by adsorbing dodecyltrimethylammonium bromide (DTAB). Experimental tests were conducted to analyze the adsorption characteristic of DTAB on the coal surface including the adsorption amount, Zeta potential, wetting heat, and X-ray photoelectron spectroscopy measurement. Furthermore, the adsorption configuration, spatial location of the simulation systems, and the coal–water interaction were investigated by molecular dynamics (MD) simulation at the atomic scale. Experimental results indicate that the hydrophobicity of the low-rank coal first increased, then decreased as the DTAB concentration increased. The electrostatic force plays a dominant role in the adsorption of DTAB on low-rank coal surface. The adsorption of DTAB reduced the wetting heat between water and low-rank coal because the oxygen-containing groups were covered. The decrease in hydrophobicity at high DTAB concentration was due to the formation of a bilayer or micelle adsorption. MD simulation results show that the adsorption of DTAB reduced the thickness of the water adsorption layer. The nitrogen atoms of DTAB were oriented toward the coal surface, such that the alkyl chains were oriented to the water phase, thereby producing an additional repulsive effect and subsequently inhibiting the adsorption of water molecules. As a result, the number of hydrogen bonds and the interaction energy between water molecules and low-rank coal decreased, indicating that the hydrophobicity of low-rank coal was enhanced.
Coal fly ash, the main industrial waste in coal-fired power plants generated during the coal combustion, usually re-utilized as raw materials in construction industry. However, a lot of unburned ...carbons are contained in raw fly ash, which not only decreasing the strength of fly ash concrete but also causing a huge waste of the resources. Separation of unburned carbon from fly ash is an efficient way to achieve a higher efficiency in the utilization of waste fly ash and greater economic and environmental benefits. This review highlights current methods for separating unburned carbon from fly ash, such as sieving, gravity separation, electrostatic separation, froth flotation, and oil agglomeration. The mineralogy features of unburned carbon affecting the separation efficiency are also presented. This review is closed with a brief discussion on the future research directions addressing different problems related to the recovery of unburned carbon.
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Fatty acids, which are enriched in vegetable oil, have attracted much attention in low-rank coal flotation because of their unique chemical structure. In this study, density functional theory ...calculations, molecular dynamics simulations, and atomic force microscopy were employed to investigate the adsorption structure and forces between collectors and hydrophilic surfaces. The results show that fatty acids can be easily adsorbed onto surfaces through hydrogen bonds, and can cover the oxygen sites. The existence of hydration film on hydrophilic surfaces prevented nonpolar molecules from being able to adsorb, while polar fatty acids could adsorb and expel water molecules. The adhesion force between the RCOOH-terminated probe and the surface appeared in the retraction process, which differed significantly from that of the RCH3-terminated probe, indicating that polar fatty acids are more suitable as flotation collectors for low-rank coal than nonpolar hydrocarbon oil. The simulation and AFM test revealed the mechanisms of polar fatty acids, and can provide guidance for low-rank coal flotation applications.
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•A novel low-rank coal flotation flowsheet (collector pre-dispersion) was proposed.•Collector pre-dispersion enhanced the low-rank coal flotation recovery by 5–10%.•The oil–water–coal ...contact angle was significantly larger than that of oil–coal.•Oil molecules self-aggregated into spheres when water molecules were adsorbed first.•Water molecule adsorption was inhibited when oil molecules were adsorbed first.
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.
Wetting film thinning measurement was introduced to clarify the wettability and floatability of solid surfaces with varying roughness. The wettability was quantified using the contact angle ...measurement combined with the dynamic force microbalance test between solid surfaces and water droplets, while the floatability was investigated by the bubble-solid surface dynamic attachment observation and the induction time measurement. The results show that the water contact angles reduce (14.53°, 12.74°, and 6.71°) with the increase of glass surface roughness, while the water droplet-glass adhesion forces intensify (11.1, 19.1 and 19.2 μN) owing to the stable wetting film. The distortion of the contact surface and the Wenzel state are the causes. In contrast, the hydrophobized surfaces have the growing apparent contact angles (38.08°, 69.81°, and 81.01°), declining adhesion strength and shortening induction time (863, 352 and 12 ms) along with the increasing surface roughness. The weak wettability and fine floatability on the rough hydrophobized surface is reflected in the fast wetting film drainage dynamics and three-phase contact formation, which may be attributed to the wetting film with short diameter on tiny rough nubs and the entrapped air in the grooves as a bridge between the bulk bubble and the solid surface.
Mixed surfactants have a prominent synergistic effect and show advantages in many aspects. In this work, the effects of a mixture of dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate ...(SDS) on the flotation of low-rank coal were studied from the wetting rate, contact angle, surface tension, and zeta potential. Furthermore, the adsorption configuration of the mixed surfactant on the surface of oxygen-containing graphite was simulated at the molecular level by molecular dynamics simulation. The experimental results show that the combustible matter recovery of low-rank coal flotation is improved using the mixed surfactant, and the contact angle test and wetting rate test confirmed the synergistic effect of the mixed surfactant. In the mixed surfactant system, the addition of SDS with an opposite charge to DTAB can reduce the mutual repulsion between DTAB molecules and enhance the degree of DTAB alignment in solution, which was analyzed by surface tension and zeta potential tests. Meanwhile, the simulation results reveal the adsorption behavior of anionic and cationic surfactants on the surface of oxygen-containing graphite from the molecular level and also verify the experimental results. This investigation provides a good understanding of the interaction mechanism of mixed surfactants in low-rank coal flotation.
Coal gangue (CG), an industrial solid waste with high contents of Li and Ga, has attracted the attention of researchers. However, the utilization of CG remains an economic challenge. Pre-enrichment ...of Li and Ga by flotation was carried out with a view to improving the comprehensive utilization of CG. Mineral composition, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and elemental composition were used to investigate the embeddedness of each mineral and the mode of elemental occurrence in the CG. The results showed that the main mineral compositions of the CG were kaolinite, quartz, and pyrite. Li and Ga were mainly associated with kaolinite and other clay minerals. Li and Ga had a high correlation with Al2O3 and SiO2, while Li and Ga were highly correlated with SiO2/Al2O3, indicating that Li and Ga may be associated with one or more high-alumina minerals. In addition, flotation tests proved that synergistic sorting of ash impurities and valuable components from the CG was a cost-effective method. The ash content of the final product was increased by 3% under the process of prediscarding concentrate–dissociation–secondary flotation, and the contents of Li and Ga in the final product were also slightly enriched, and the recovery rate of the carrier minerals of Li and Ga can reach 66.1%.
The coal structure is extensively used for studying the properties of coal, and the construction of accurate and reliable coal structure models can promote these researches. In this study, Fourier ...transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) were used to analyze the changes in the coal structure as a function of the coalification degree, and a semiquantitative model construction method based on FTIR, XRD, and X-ray photoelectron spectroscopy (XPS) analyses was proposed. With an increase in the coalification degree, the size of the aromatic cores in the coal increased. During the conversion from lignite to bituminous coal, the content of aliphatic structures increased, while the content of oxygen-containing functional groups (OFGs) significantly decreased. Conversely, during the conversion from bituminous coal to anthracite, the content of aliphatic structures decreased while the content of OFGs slightly increased. The calculated FTIR spectra of the coal structure models were consistent with the experimental FTIR spectra, which confirmed the accuracy of the models. Furthermore, the models successfully explained the microscopic mechanism underlying the differences in the wettability of the coal samples with varying coalification degrees. The construction method and coal structure models in this study will be more widely applied in future research.
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•Suitable pretreatment concentration of DTAB increased clean coal yield of low-rank coal.•Dodecane could form spherical shape on low-rank coal surface under water environment.•Exposed ...hydrophobic structure of coal-DTAB complex enhanced surface lipophilicity.•Adsorption of DTAB increased attraction and interaction energy between low-rank coal and dodecane.
The effect of a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), on low-rank coal flotation using an oily collector (dodecane) was investigated by a combination of experimental tests and molecular dynamics simulations. Flotation results showed that the addition of DTAB during pulp conditioning increased the clean coal yield, while a high concentration of DTAB exerted a negative influence. To explain the flotation behavior and analyze the interaction between low-rank coal, DTAB and dodecane, a series of experimental tests were conducted and the results indicated that a relatively lower concentration of DTAB is beneficial for the adsorption of dodecane on low-rank coal surface. In addition, by measuring the induction time, it was found that low-rank coal particles were easily adhered to bubbles in the presence of suitable concentration of DTAB. However, a high concentration of DTAB brings about an adverse effect. Later, molecular dynamics (MD) simulations were carried out to illustrate the positive effect of DTAB on the adsorption reaction between low-rank coal and dodecane. The results showed that the abundant number of oxygen-containing groups in low-rank coal are responsible for the limited adsorption of nonpolar dodecane molecules on its surface, while the pre-adsorption of DTAB on the surface of low-rank coal enhances dodecane adsorption. This can be ascribed to the exposed hydrophobic structure of the coal-DTAB complex. As a result, dodecane molecules were more easily adsorbed on the surface of low-rank coal and the attraction of low-rank coal to dodecane in the presence of DTAB resulted in a lower mobility of dodecane molecules, and a larger interaction energy between dodecane and low-rank coal. The simulation results are in good agreement with experimental results.
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