Display omitted
•The counterintuitive stability of nanobubbles is still an open question.•The interfacial nanobubbles stability theories and models in recent ten years are reviewed and analyzed.•Some ...suggestions for the future research on nanobubbles stability are proposed.
As nanobubbles are of far-reaching application in interface science, mining science, medical treatment, sewage treatment and other fields, researchers have carried out considerable research on the properties and influencing factors of nanobubbles. However, the abnormal lifetime of nanobubbles—super stability is still an open question. According to the classical thermodynamic theories, bubbles at the nanoscale in water should be dissolved quickly, but a great number of research results in recent years indicate that nanobubbles can exist at the solid–liquid interface stably. The thorough understanding and mastery of the underlying stability mechanism of nanobubbles is the premise of the research and applications of nanobubbles. In this paper, the interfacial nanobubbles stability theories and models in recent ten years are reviewed, including contamination (impurity) theory, dynamic equilibrium theory and its extension theory, contact line pinning effect and nanobubbles internal pressure theory, etc. Furthermore, some suggestions for the future research on stability theories are proposed and the research focuses and development directions of nanobubbles in the future are also prospected.
The filtration and dewatering of fine clean coal not only ensure industrial water recycle in coal washing plant, but also reduce the moisture of coal product in order to meet the requirements of ...combustion or coking industry. Fine clean coal is mainly composed by organic matter, and the property difference of different organic matter determines the filtration and dewatering behavior. In this investigation, vitrinite and inertinite were separated from a clean bituminous coal, and the comparative filtration and dewatering behavior of vitrinite and inertinite were conducted. The results showed that inertinite has lower dewatering rate and higher filter cake moisture than vitrinite. The analysis of filter cake structure showed that inertinite particle is easier to be broken into small particles due to the difference of mechanical properties, thus forming more compact filter cake than vitrinite. The analysis of particle surface properties showed that vitrinite is more hydrophobic than inertinite, which makes water easier drained from filter cake. The simulation study showed that the structure of inertinite is more porous than that of vitrinite, and the interaction between inertinite and water is stronger than that between vitrinite and water. This study provides a theoretical basis for improving coal dewatering by selectively improving coal maceral hydrophobicity.
The present work aimed to study the effect of slurry conditioning on flocculant-aided filtration of coal tailings by the analysis of filtration kinetics and filter cake structure. Laboratory ...filtration tests of the coal tailings showed that both the shear rate and agitation time have significant effects on filtration rate and cake moisture. Moderate agitation at the shear rate of 92 s−1 was favorable for fast filtration, but high cake moisture was encountered. The low-field 1H nuclear magnetic resonance (NMR) measurements of the filter cake showed that the slurry conditioning has a significant effect on the residual water in large pores and a negligible effect on the residual water in small pores. The X-ray micro-tomography (XRM) measurements indicated that the filter cake formed at the shear rate of 92 s−1 has more macro-pores and higher porosity than that formed at the shear rate of 53 s−1, hence more residual water was entrapped in filter cake. The slurry conditioning in the presence of flocculant will change the structure of filter cake and affect the filtration performance. There was a paradox between fast filtration rate and low filter cake moisture. The findings enable better understanding of the effect of slurry conditioning on flocculant-aided filtration of coal tailings.
The dynamic pore wetting plays an important role in the flotation of porous particle, such as coal, fly ash, gasification slag, etc. Water phase get into the pores and crevices on the surface and ...inside of the particles, creating a dense water layer that covers their surface and decreases their floatability. It is important to reduce the adverse impact of dynamic pore wetting on the flotation performance of porous particles. Thus, this review is to highlight the dynamic pore wetting behavior, and its role in the porous particle flotation as well as the proposed regulation methods to weaken the pore wetting. The characterization approaches of the dynamic pore wetting are discussed including the optical method for the capillary and the nuclear magnetic resonance (NMR) method for porous particles. The surface tension and viscosity of liquid significantly affect the pore wetting degree of porous particles during the 60 min wetting process. Pre-wetting time adjustment, pore compression, pore filling, and hydrophobic modifications are introduced to decrease the wetting percentage of pores in porous particles and further improve their flotation performance. This critical review will provide some guidance for studying the dynamic wetting behavior of pores and enhancing the floatability of porous particles.
•Bulk micro-nanobubble solutions were prepared using a fine-bubble generator under different preparation times and aeration rates.•The dissolved oxygen contents of bulk micro-nanobubble water under ...different conditions were evaluated.•The bubble concentrations and sizes in bulk micro-nanobubble water under different conditions were evaluated.•The relationship between the dissolved oxygen concentration and the cavitation behaviors of bubbles was discussed.
Fundamental research on bulk micro-nanobubbles (BMNBs) has grown rapidly due to the demand for their industrial applications and potential role in interfacial sciences. This work focuses on examining properties of such bubbles, including the number, concentration, zeta potential, and surface tension in water. For this purpose, BMNBs were generated by the hydrodynamic cavitation (HC) mechanism. Distilled water and air in the experiments were the liquid and gas phases, respectively. The characterization of bulk microbubbles (BMBs) and bulk nanobubbles (BNBs) were performed through focused beam reflectance measurement (FBRM) and nanoparticle tracking analysis (NTA) techniques, respectively. Zeta potential and surface tension of aqueous solutions were measured at different time and aeration rates. The results showed that aeration rate and preparation time had an important role in the properties of BNBs (concentration, bubble size, and surface charge) and BMBs (number, and bubble size). The instability of BMBs led to the rapid changes in the dissolved oxygen (DO) content in the water. The number of BMBs decreased when preparation time and aeration rate increased, but their size remained constant. By enhancing the preparation time and aeration rate, the concentration of BNBs improved first and then reduced. Additionally, the surface tension of an aqueous solution containing BNBs was significantly lower than that of pure water.
Coal-series kaolin (CSK) in Xuzhou, Jiangsu Province of Eastern China was characterized by chemical analyses, X-ray diffraction (XRD) and zeta potential measurement as well as by scanning electron ...microscope equipped with an energy dispersive spectrometer (SEM-EDS). The authigenic ultrafine quartz grains (d90=15.8μm) closely associated with this CSK sample was the major impurity mineral, which cannot be separated from kaolin by screening and size classification process. This work investigated the separation of fine quartz from kaolin by flotation. Single mineral flotation tests indicated that effective separation of quartz from kaolin was possible with acidic pH value (pH=3), depressant of starch and collector of dodecyl amine (DDA). Rougher flotation tests of coal-series kaolin revealed that the SiO2/Al2O3 weight ratio of the concentrate was qualified for industrial applications in the presence of 160g/t starch and 100g/t DDA, at acidic pH pulp. A specially designed two-stage cleaner flotation flowsheet increased the concentrate recovery from 37.26% to 46.55% compared to a rougher flotation process.
Display omitted
•The micron quartz grains closely are associated with kaolin in this ore.•Starch can selectively depress quartz in acidic solution.•A qualified product having a proper recovery was obtained in multi-stage flotation.
Graphite ore collected from Hunan province, south China was characterized by chemical analysis, X-ray diffraction, and optical microscopy. Rougher and multi-stage flotation tests using a mechanical ...flotation cell and a flotation column containing an additional centrifugal force field were carried out to promote its grade and economic value. In rougher flotation, both the mechanical flotation cell and flotation column reduced the ash content of the graphite ore from 15.43% to 10.8%, while the yield of the flotation column (91.41%) was much higher than that of the mechanical flotation cell (50%). In the presence of hydrophobic graphite, the seriously entrained gangue restricted further improvement in the quality and economic value of the graphite ore. Therefore, multi-stage flotation circuits were employed to diminish this entrainment. Multi-stage flotation circuits using the two flotation devices further decreased the ash content of the graphite ore to ~8%, while the yield when using the flotation column was much higher than that obtained from the mechanical flotation cell employed. On the other hand, the ash removal efficiency of the flotation column was 3.82-fold higher than that observed for the mechanical flotation cell. The Cleaner 3 flotation circuit using the flotation column decreased the ash content in graphite from 15.43% to 7.97% with a yield of 77.53%.
Micro-Fourier transform infrared (micro-FTIR) spectroscopy was used to correlate the surface chemistry of low rank coal with hydrophobicity. Six square areas without mineral impurities on low rank ...coal surfaces were selected as testing areas. A specially-designed methodology was applied to conduct micro-FTIR measurements and contact angle tests on the same testing area. A series of semi-quantitative functional group ratios derived from micro-FTIR spectra were correlated with contact angles, and the determination coefficients of linear regression were calculated and compared in order to identify the structure of the functional group ratios. Finally, two semi-quantitative ratios composed of aliphatic carbon hydrogen, aromatic carbon hydrogen and two different types of carbonyl groups were proposed as indicators of low rank coal hydrophobicity. This work provided a rapid way to predict low rank coal hydrophobicity through its functional group composition and helped us understand the hydrophobicity heterogeneity of low rank coal from the perspective of its surface chemistry.
Conventional hydrocarbon oil cannot adhere effectively to oxidized coal, resulting in a low yield of clean coal. In this study, a high-speed homogenizer was used to emulsify LDD (laurylamine ...dipropylene diamine) and kerosene, which enhanced the flotation efficiency of oxidized coal. The flotation results showed an increase from 4.12% (only kerosene) to 23.33% (emulsified oil). An increase in contact angle indicated that the mixture reagent can increase the hydrophobicity of coal particles, which is attributed to the adsorption of LDD onto the coal particle surface and the decrease of the oil droplet A lower surface tension of LDD allows it to produce a stable layer of froth than the layer generated by kerosene alone.
Kaolinite, as a mineral in fine coal, has an important influence on the flotation of coal particles. In this study, the effects of ultrafine kaolinite particles on the flotation recovery of coal ...particles were investigated. Flotation tests were carried out using a mixture of coal particles and different amounts of ultrafine kaolinite particles. Combined with the Stefan–Reynold theory, the effect of liquid film drainage rate between coal bubbles in a kaolinite suspension was calculated. The yield of flotation clean coal increases quickly with the increasing content of ultrafine kaolinite particles. The ultrafine kaolinite particles can reduce the surface tension of the suspension, weaken the bubble coalescence, and stabilize the structure of the froth layer. In addition, the ultrafine kaolinite particles increase the apparent viscosity of the flotation pulp slightly. It is concluded that the role of ultrafine kaolinite particles on the positive effect of froth properties conceals the negative effect on the liquid film drainage rate between coal particles and bubbles caused by the kaolinite particles, which ultimately leads to an increasing yield of clean coal with an increasing content of kaolinite particles. This study is important for understanding the influence of ultrafine kaolinite on coal particle flotation.
PDF
