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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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Surface roughness plays a significant role in floatability of coal. In the present paper, coking coal surface was polished by three different sandpapers and the surface properties were characterized ...by contact angle and roughness measurements. The effect of surface roughness on floatability was investigated by adhesion force measurement system for measuring interaction forces between droplets/bubbles and coking coal surfaces with different roughness. The results showed that the contact angle decreased with increasing roughness yet the adhesion force between the water droplet and coal surface increased owing to the increased contact line and the appearance of line pinning. Maximum adhesion forces between water and surfaces were 111.70, 125.48, and 136.42 μN when the roughness was 0.23, 0.98, and 2.79 μm, respectively. In contrast, under a liquid environment, the adhesion forces between air bubble/oil droplet and coal surfaces were decreased with increasing roughness because of the restriction by water. Maximum adhesion forces of increasing roughness were 97.14, 42.76, and 17.86 μN measured at interfaces between air bubble and coal surfaces and 169.48, 145.84, and 121.02 μN between oil droplet and surfaces, respectively. Decreasing roughness could be beneficial to the spreading of oil droplets and the adhesion of bubbles which is conducive to flotation separation.
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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.
•Evolutionary of contact angle of bubble from macroscale to nanoscale is researched.•GML coexists with bubble only when internal gas density is high.•Contact angle is large regardless of surface ...hydrophobicity after formation of GML.•GML attracts gas molecules from bulk and assists the stability of interfacial nanobubble.
Since solid-liquid interfacial nanobubbles (INBs) were first imaged, their long-term stability and large contact angle have been perplexing scientists. This study aimed to investigate the influence of internal gas density and external gas monolayers on the contact angle and stability of INB using molecular dynamics simulations. First, the contact angle of a water droplet was simulated at different nitrogen densities. The results showed that the contact angle increased sharply with an increase in nitrogen density, which was mainly caused by the decrease in solid-gas interfacial tension. However, when the nitrogen density reached 2.57 nm−3, an intervening gas monolayer (GML) was formed between the solid and water. After the formation of GML, the contact angle slightly increased with increasing gas density. The contact angle increased to 180° when the nitrogen density reached 11.38 nm−3, indicating that INBs transformed into a gas layer when they were too small. For substrates with different hydrophobicities, the contact angle after the formation of GML was always larger than 140° and it was weakly correlated with substrate hydrophobicity. The increase in contact angle with gas density represents the evolution of contact angle from macro- to nano-bubble, while the formation of GML may correspond to stable INBs. The potential of mean force curves demonstrated that the substrate with GML could attract gas molecules from a longer distance without the existence of a potential barrier compared with the bare substrate, indicating the potential of GML to act as a gas-collecting panel. Further research indicated that GML can function as a channel to transport gas molecules to INBs, which favors stability of INBs. This research may shed new light on the mechanisms underlying abnormal contact angle and long-term stability of INBs.
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The relationship between shear force and selective liberation of middling coking coal was investigated by crushing the coal at different shear angles (15°, 30°, 45°, 60°, and 75°). Subsequent ...analysis of the middling coking coal revealed that as the ash content of coal increased, the shear strength and cohesion increased, while the internal friction angle gradually decreased. Increasing the shear angle caused a gradual increase in the quantity of coarse particles and a decrease in the quantity of fine particles, ultimately leading to a dominant particle size of >10mm at a shear angle of 75°. The cumulative ash and cumulative yield curves showed that the liberation of middling coking coal through crushing was optimum when the shear angle was smaller than the internal friction angle. Lamellar tear fractures observed with low-shear-angle crushing could contribute to the liberation of coal, but the step-like morphology characteristics of medium and high shear angles was not conducive to liberation. Because the liberation of middling coking coal through crushing was optimum when the shear angle was smaller than the internal friction angle, a high ash content or high shear angle may make the release of clean coal more difficult.
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•An evaluation index of selective crushing was proposed.•Smaller shear angle is good for middling coal crushing.•High ash content and high shear angle make the release of clean coal more difficult.
The objective of this study is to explain the different flotation responses observed in oxidized coal flotation when using a nonpolar flotation collector, dodecane, versus a polar flotation ...collector, oleic acid. Particularly, the effect of each flotation collector on bubble-coal particle attachment was investigated. Colloidal probe atomic force microscopy (AFM) was used to directly measure the force between a model coal surface and a model bubble in the presence of either dodecane or oleic acid. Pyrolytic graphite (PG) treated with oxygen plasma and a polymethylmethacrylate (PMMA) particle were selected to represent the model oxidized coal surface and model bubble. High speed visualization for bubble-oxidized coal attachment was used to monitor the attachment behavior between bubble and oxidized coal in presence of dodecane and oleic acid, respectively. It was found that the force between the oxidized PG and the PMMA particle in Milli-Q water was monotonically repulsive, illustrating that oxidized coal particles attach onto bubble surface with difficulty. The flotation recovery using a traditional hydrocarbon oil, dodecane, was always lower than when oleic acid was used at a low dosage (300-1100 g/t). The force measurements showed that an attractive hydrophobic force was introduced when a 0.01 mM oleic acid solution was used, while the force was still repulsive in presence of a 0.01 mM dodecane solution. The minimum contact time for successful attachment between oxidized coal surface and bubbles in the presence of 0.01 mM oleic acid is much shorter than that in 0.01 mM dodecane. However, a high flotation recovery of 85.81% was obtained when dodecane concentration was further increased to 1700 g/t. The significant jump into contact effect observed in the AFM force curves and the short induction time in the presence of 1 mM dodecane solution was responsible for this high flotation recovery.
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.
A theoretical analysis was conducted to study the role of collectors and depressants in flotation, based on the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, where the hydrophobic force is ...considered. The collector-coated hydrophilic particle and the depressant-coated hydrophobic particle are simplified to a sphere uniformly covered with respectively hydrophobic and hydrophilic nanometer-sized hemispherical asperities of identical radius. Results show that the role of a collector in bubble-particle attachment is to create an attractive hydrophobic force and thus overcome the repulsive van der Waals and electrostatic forces. Moreover, increasing the length of the hydrophobic part of the collector molecule is a more effective way to enhance flotation recovery, compared to increasing the collector concentration. For a depressant, however, its function mechanism is to create a strong electrostatic double-layer force, while the suppression of the hydrophobic force plays a secondary role in decreasing the bubble-particle attachment barrier. The depressant molecule length is also a dominant parameter in designing a powerful depressant.
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