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.
•The wetting ability of modified diesel oil for coal sample did not entirely increase with the decrease of surface tension, and liquid viscosity was also a relevant factor.•Great viscosity is good ...for adhesion, but excessive viscosity was bad for its wettability.•The induction time between coal particles and reactive oily bubbles modified by 2-ethylhexanol and DDAB showed a significant decrease.•It was crucial to the flotation of low rank coal that proper surfactant was used to generate reactive oily bubbles.
It is critical for flotation to accomplish the bubble-mineral particle attachment as well as mineralization process. The surface free energy of mineral particle and liquid is a key parameter which affects the wettability and wetting process of mineral in mineralization. In this paper, the surface free energy components of coal sample were calculated by Washburn equation and Oss Chaudhury-Good-Van theory. Additionally, the wetting ability of modified diesel oil for coal sample was studied by Washburn dynamic capillary method while Lipophilic Hydrophilic Ratio and relative contact angle were calculated. Furthermore, the effect of both surfactants, 2-ethylhexanol and didodecyldimethylammonium bromide (DDAB), on the induction time between oily bubbles and coal particles was investigated. Results indicated that wetting ability of modified diesel oil for coal sample did not entirely increase with the decrease of surface tension, and liquid viscosity was also a relevant factor. The adhesion strength enhanced whereas the detachment probability reduced with the increase of collector viscosity. But too large viscosity was bad for fluidity of liquid, which therefore decreased its wettability for coal sample. The induction time between coal particles and reactive oily bubbles modified by 2-ethylhexanol and DDAB decreased by 74.35% and 86.45% respectively. As a result, the maximum combustible matter recovery of 72.20% (increasing by 22.23%) and highest flotation efficiency index of 56.50% were obtained when the weight ratio of 2-ethylhexanol to diesel oil was 0.04, while that of using reactive oily bubbles modified by DDAB showed an apparent decrease. It indicated that it was crucial to the flotation of low rank coal that proper surfactant was used to generate reactive oily bubbles.
In this paper, a relationship between the roundness, roughness and induction time of low-rank coal particles was investigated. With the increase of grind time from 1 to 5 minutes, the roundness of ...low-rank coal particles slightly increased. The results of induction time measurements indicated that the particle surface roughness played a significant role in the surface hydrophobicity of low-rank coal particles. Moreover, it was also found that the roundness and roughness of low-rank coal particle surfaces presented a certain degree of change after both grinding and abrasion processes. Contrary to roundness change, during the grinding process, the roughness of low-rank coal particles significantly decreased. It was also observed that the roundnesses of low-rank coal samples after the grinding process significantly increased with the increase of abrading time. However, unlike the roundness increase with the abrading time increase, the roughness of low-rank coal samples significantly decreased. Therefore, the induction times of low-rank coal samples showed an increased trend after the grinding and abrading processes, which may be attributed to the difficult rupture of wetting film on the smooth solid surface. The grinding and abrading processes may result in a further decrease in surface hydrophobicity and adversely impact on the flotation performance of low-rank coal.
This paper focuses on enhancing the flotation of coking coal with ultrasonic pretreatment and the corresponding improving mechanism. The effects of ultrasound on the coal surface properties and the ...dispersion of oil collector in flotation slurry were fully studied. The SEM, FTIR and XRF analyses indicated that, after the ultrasonic pretreatment, the fine particles covered on coarse coal particles were removed and the elements forming gangue minerals were decreased, which improved the surface hydrophobicity of the coal sample. What's more, the optical microscope measurements for collector dispersion showed that the size of diesel oil droplets was greatly reduced after the ultrasonic treatment, suggesting the enhanced dispersion of collector in flotation pulp. As a result of synergistic effects of ultrasonic treatment on the coal surface cleaning and promotion of collector dispersion in pulp, a maximum combustible matter recovery of 79.79% was attained with the flotation process 2 (i.e., the ultrasonic treatment was performed after the addition of collector to the pulp). Furthermore, the enhanced collector dispersion in pulp caused by the ultrasonic treatment may be conducive to reduce the collector consumption in actual flotation practice.
This study focused on the mechanisms of NaCl and CaCl
2
enhancing the oily-bubble flotation of lignite. The degree of surface hydration of coal particles before and after conditioning with NaCl/CaCl
...2
was evaluated with the Einstein theory of dispersion viscosity, while the effect of these two electrolytes on lignite floatability was evaluated via attachment time and flotation tests. The results indicated the adsorption amount of Ca
2+
on the coal sample was more than that of Na
+
in the same electrolyte concentration. Owing to the cationic adsorption, the zeta potential of lignite particles increased with increasing NaCl and CaCl
2
concentrations, which thus decreased the electrostatic repulsion between coal particles and oily bubbles. Moreover, the hydration of the coal surface decreased when NaCl/CaCl
2
was added to flotation pulp, meaning the surface hydrophilicity of the coal sample was reduced. As a result, the attachment time between coal particles and oily bubbles decreased significantly, and the combustible-matter recovery correspondingly increased up to a maximum of 50.97% and 90.06% with increasing NaCl and CaCl
2
solutions concentration, respectively. However, it is necessary to keep the electrolyte solution concentration within a reasonable range for a good selectivity during the oily-bubble flotation.
The upgrading of low-rank coal is important for its utilization. The lignite used in this study had extremely low surface hydrophobicity and responded poorly to the conventional flotation process. ...Therefore, to enhance its flotation, a laboratory device known as oily-bubble flotation was used. With this device, the combustible matter recovery at a collector dosage of 6.4 kg/t reached 32.94% which was comparable to that of the conventional flotation at the collector dosage of 50 kg/t. For improving recovery, utilization of electrochemical conditions for pulp, i.e., pH, and NaCl and CaCl
2
solutions, was used to further enhance the oily-bubble flotation performance. Based on the response-surface methodology, a functional model for combustible matter recovery was established, and the regulator scheme in oily-bubble flotation was optimized to achieve a combustible recovery of 59.19% with pH and NaCl and CaCl
2
concentrations of 6.03, 61.26 mmol/L, and 80 mmol/L, respectively. Results showed that an increased combustible matter recovery of 78.69% could be obtained using this optimization scheme.
The main purpose of coal separation is to reduce ash, sulfur, mercury and other mineral contaminants in the coal to increase the calorific value and benefit the environment. Dry coal beneficiation ...has obvious advantages over the wet process although the latter is currently the predominant method in use throughout the world. A vibrated fluidized bed was constructed for separating dry fine coal particles from unwanted gangue particles. An experimental investigation of vibrational energy transmission, and the interaction between vibration and gas flow, was performed. The motivation for these experiments was a theoretical development of the principles involved in forming a dense-media vibrated fluidized bed (DMVFB). The mechanism of bubble breaking by vibration is discussed. A formula for calculating the critical vibration frequency at which bubbles can be efficiently broken and bubble formation restrained is proposed. The experimental results demonstrate that the density of a dense-media vibrated fluidized bed is uniform, with a maximum relative error of 1.68% under optimal technological and operating conditions. The <
6 mm fine coal was efficiently separated with a probable error
E value of 0.07 t/m
3. A lower limit of separation of 0.5 mm was achieved. The DMVFB separation efficiency is higher than wet jig with
E value of 0.11 t/m
3.
A dense-media vibrated fluidized bed (DMVFB) was constructed for separating dry fine coal particles from unwanted gangue particles. The experimental results demonstrate that the density of a DMVFB is uniform, with a maximum relative error of 1.68%. The <
6 mm fine coal was efficiently separated by DMVFB with a probable error E value of 0.07, better than by wet jig.
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With the advantages of its fast speed, effective and moderate controllable conditions, desulfurization of coal by microwave has become research focus in the field of clean coal technology. Coal is a ...homogeneous mixture which consists of various components with different dielectric properties, so their abilities to absorb microwaves are different, and the sulfur-containing components are better absorbers of microwave, which makes them can be selectively heated and reacted under microwave irradiation. There still remain controversies on the principle of microwave desulfurization at present, thermal effects or non-thermal effects. The point of thermal effects of microwave is mainly base on its characters of rapidly and selectly heating. While, in view of non-thermal effect, direct interactions between the microwave electromagnetic field and sulfur containing components are proposed. It is a fundamental problem to determine the dielectric properties of coal and the sulfur-containing components to reveal the interaction of microwave and sulfur-containing compounds. However, the test of dielectric property of coal is affected by many factors, which makes it difficult to measure dielectric properties accurately. In order to achieve better desulfurization effect, the researchers employ methods of adding chemical additives such as acid, alkali, oxidant, reductant, or changing the reaction atmosphere, or combining with other methods such as magnetic separation, ultrasonic and microorganism. Researchers in this field have also put forward several processes, and have obtained a number of patents. Obscurity of microwave desulfurization mechanism, uncertainties in qualitative and quantitative analysis of sulfur-containing functional groups in coal, and the lack of special microwave equipment have limited further development of microwave desulfurization technology.
In this paper, the back-calculated induction times of low-rank coal particles on the rising bubble with mobile surfaces were back-calculated from the micro-flotation rate constants. The ...back-calculated induction times slightly increased with the flotation recovery increase or the surfactant concentration decrease. It is because the drainage time accounting for most of the induction time is affected by the force exerted on the wetting film. Moreover, the force exerted on the wetting film is characterized by the Reynolds number. Furthermore, the Reynolds number increased with increasing bubble rising velocity due to an increase in bubble size as a result of decreasing surfactant concentration. Therefore, the back-calculated induction times could reflect the difference in the flotation recoveries at the same surfactant concentration. Meanwhile, it indicated that the hydrodynamic condition in the flotation process had a significant effect on the back-calculated results of induction times. From this investigation, it can be speculated that the back-calculated induction time of particles sliding on the rising bubble with mobile bubble surfaces is greatly influenced by the Reynolds number.
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