Improving the flotation performance of coking coal using the reverse-direct flotation process

In order to improve the flotation performance of the coking coal particles, the flotation tests of the coking coal particles were conducted in the direct flotation, reserve flotation, and reverse-direct flotation processes. It was found that the concentrate ash content of coking coal particles was higher than 20%, which cannot be effectively reduced using the direct and reserve flotation processes. However, the flotation concentrate with the ash content of 12.53% can be obtained from the reverse-direct flotation process. In the reverse-direct flotation process, the surface hydrophobicity was reduced with the dextrin and 1-dodecylamine (DDA) addition at the reverse flotation process stage. For the addition of diesel collectors at the direct flotation process stage, the surface hydrophobicity of the coking coal samples was improved.     

A new process based on a combination of gravity and flotation for the recovery of clean coal from flotation tailings

Beneficiation of clean coal from flotation tailings with a high intergrown ash content is gaining popularity in China. It not only improves the utilization rate of coal resources but also reduces environmental pollution. In this paper, a novel gravity–flotation process is proposed to concentrate low-ash fine coal and includes the disposal of tailings via gravity, grinding liberation, and froth flotation. The experimental results show that the clean coal had an ash content of 12.33% and the yield from the flotation tailings was 32.24% with an ash content of 46.39%, which has economic value in the current market.     

Experimental research of surface roughness effects on highly-loaded compressor cascade aerodynamics

Aircraft engines deteriorate during continuous operation under the action of external factors including fouling, corrosion, and abrasion. The increased surface roughness of compressor passage walls limits airflow and leads to flow loss. However, the partial increase of roughness may also restrain flow separation and reduce flow loss. It is necessary to explore methods that will lower compressor deterioration, thereby improving the overall performance. The experimental research on the effects of surface roughness on highly loaded compressor cascade aerodynamics has been conducted in a low-speed linear cascade wind tunnel. The different levels of roughness are arranged on the suction surface and pressure surface, respectively. Ink-trace flow visualization has been used to measure the flow field on the walls of cascades, and a five-hole probe has been traversed across one pitch at the outlet. By comparing the total pressure loss coefficient, the distributions of the secondary-flow speed vector, and flow fields of various cases, the effects of surface roughness on the aerodynamics of a highly loaded compressor cascade are analyzed and discussed. The results show that adding surface roughness on the suction surface and pressure surface make the loss decrease in most cases. Increasing the surface roughness on the suction surface causes reduced flow speed near the blade, which helps to decrease mixing loss at the cascades outlet. Meanwhile, adding surface roughness on the suction surface restrains flow separation, leading to less flow loss. Various levels of surface roughness mostly weaken the flow turning capacity to various degrees, except in specific cases.     

Spin-up and Spin-Down Times of Rotational Air Flow Depended on Mean Air Velocity and Surface Roughness on Inner Surface in Cylindrical Vortex Chamber

The spin-up time TA in the spin-up process and spin-down time TD in the spin-down process of rotational air flow in the cylindrical vortex chamber with three kinds of the artificial surface roughness were investigated. One of the most different characteristics between the smooth and rough surface conditions was the spin-down time TD. Also the transient response of the tangential velocity on the spin-up process was analyzed numerically and this result was compared with the experimental results. The above stated results are reported here in detail.     

Effects of ultrasonic pretreatment on the flotation performance and surface properties of coking middlings

The effects of ultrasonic pretreatment on the flotation performance of coking middlings were investigated through a flotation cell. The surface properties of coking middlings before and after ultrasonic pretreatment were indicated using X-ray fluorescence analyzer (XRF), scanning electron microscope (SEM) and DSA100 contact angle analyzer. The flotation results demonstrated that the combustible recovery increased by 7.18%, while the ash value of flotation tailings decreased by 2.74% at most after ultrasonic pretreatment. Flotation results were supported by XRF analysis, SEM analysis, and contact angle measurements results. These results show that the ultrasonic pretreatment has a beneficial influence on the flotation of coking middlings.     

Effects of surface roughness of capillary wall on the profile of thin liquid film and evaporation heat transfer

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On the role of surface roughness in the aerodynamic performance and energy conversion of horizontal wind turbine blades: a review

Renewable energy is one of the main pillars of sustainable development, especially in developing economies. Increasing energy demand and the limitation of fossil fuel reserves make the use of renewable energy essential for sustainable development. Wind energy is considered to be one of the most important resources of renewable energy. In North African countries, such as Egypt, wind energy has an enormous potential; however, it faces quite a number of technical challenges related to the performance of wind turbines in the Saharan environment. Seasonal sand storms affect the performance of wind turbines in many ways, one of which is increasing the wind turbine aerodynamic resistance through the increase of blade surface roughness. The power loss because of blade surface deterioration is significant in wind turbines. The surface roughness of wind turbine blades deteriorates because of several environmental conditions such as ice or sand. This paper is the first review on the topic of surface roughness effects on the performance of horizontal‐axis wind turbines. The review covers the numerical simulation and experimental studies as well as discussing the present research trends to develop a roadmap for better understanding and improvement of wind turbine performance in deleterious environments. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of NaCl on the flotation response of low-rank coal

In this study, the effect of adding NaCl on the flotation enhancement mechanism of low-rank coal was investigated. A gradual reduction in the negativity of the zeta potential of low-rank coal with NaCl addition was conducive to reducing the bubble–particle attachment time and enhancing low-rank coal recovery. In addition, the reduced bubble size and improved foam stability with NaCl addition were moderately beneficial to coal recovery. The flotation rate constant, k, and the maximum combustible recovery, ε_∞, increased remarkably as the NaCl concentration increased, indicating that the flotation kinetic of low-rank coal was enhanced by the addition of NaCl.     

Effect of ultrasound on the true flotation of lignite and its entrainment behavior

In this study, the ultrasound was fixed in the pulp zone of flotation cell and its effect on the true flotation of lignite was analyzed. Flotation results indicated that the simultaneous ultrasound treatment increased the concentrate yield and decreased the concentrate ash content. Screening analysis of flotation products revealed that the ultrasound could crush coarse coal to fine coal and scanning electron microscopy (SEM) tests indicated that the ultrasound could reduce the coverage of high-ash coal fines on the coarse particle surface. Thus, the flotation recovery of coarse lignite particle was increased. In addition, the true flotation and entrainment of ?0.074 mm fine particles were studied by the sink-float test and the method of Trahar. It was found the ultrasound significantly enhanced the true flotation of fine particles and improved the overall water recovery in lignite flotation.     

Effects of flotation on the release behavior of sulfur and nitrogen during coal pyrolysis

Two Chinese coals were selected to investigate the effects of flotation on the release behavior of sulfur and nitrogen during pyrolysis. The results show that the removal rate of minerals and sulfur-containing compounds from raw coal by flotation are closely related to coal properties. The significant alterations of sulfur and nitrogen forms on coal surface are mainly presented in the decrease of sulfidic-S, thiophenic-S, and pyridinic-N, the increase of sulfones-S and quaternary-N after flotation. The release of sulfur- and nitrogen-containing gases during pyrolysis of raw and clean coals has evident differences, which are mainly caused by the change of the relative proportions of different sulfur and nitrogen forms in the process of flotation.     

Research on the reverse flotation of kaolinite from fine coal on basis of adsorption behavior

The strong hydrophilic properties of both sub-bituminous coal and kaolinite make it difficult to separate by direct flotation. In this paper, the removal of kaolinite from fine sub-bituminous coal was investigated by reverse flotation tests using N, N-dimethyl dodecyl amine (DRN₁₂) as a kaolinite collector. In addition, the adsorption behavior of DRN₁₂ on coal and kaolinite surfaces was also studied to explore its interaction mechanism The experimental results showed that the beneficiation of kaolinite from the raw coal was effective only in the acid pulp with DRN₁₂ less than 1.5kg/t. Moreover, in acid solutions, DRN₁₂ preferentially adsorbs on kaolinite surface by electrostatic force, and the adsorption capacity of DRN₁₂ on kaolinite surface was much higher than on coal, which caused an increase of kaolinite hydrophobicity and floatability.     

Influence of machining parameters on surface roughness and susceptibility to hydrogen embrittlement of austenitic stainless steels

In the present work, an investigation on the susceptibility to hydrogen embrittlement of AISI 304 and 310 austenitic stainless steels was performed. The hydrogen embrittlement process leads to degradation of mechanical properties and can be accelerated by the presence of surface defects combined with elevated surface hardness. Tensile test specimens of the selected materials were machined by turning with different cutting parameters in order to create variations in surface finish conditions. The samples thus prepared were submitted to tensile tests before and after hydrogen permeation by cathodic charging. Regarding the AISI 304 steel, it was possible to notice that the presence of strain-induced martensite on the material surface led to severe hydrogen embrittlement. In the case of the AISI 310 steel, due to its higher nickel amount, no martensite formation could be detected, and this steel was found to be less susceptible to embrittlement in the tested conditions.     

Effect of surface roughness of the metallic interconnects on the bonding strength in solid oxide fuel cells

In this study, the joint strengths of glass-ceramic sealant placed between two metallic interconnectors (Crofer® 22 APU) are experimentally investigated depending on the surface conditions of the metallic interconnector and electrolyte/electrode materials (YSZ and NiO). The surfaces of the interconnectors are sanded with sandpaper having five different grits (60, 120, 240, 320 and 2000 grits). Thus, roughened surfaces are obtained and the adhesion is examined for each case. Profilometer is used to inspect the surface roughness of the samples. The fracture strengths of 24 samples prepared for each case are determined via tensile tests. Similarly, different electrolyte/electrode materials with modified surfaces are sandwiched between two glass ceramic layers and their mechanical performances are also measured. The results reveal that the joining strength tends to increase with the amount of surface roughness. It is also found that NiO adheres better to glass-ceramic material than YSZ. The microstructures of the adhesion interface of some cases are also investigated by a scanning electron microscopy. The images showed that good adhesion is achieved without any delamination or cracks at the interfaces. Chemical formation between the glass-ceramic sealants, interconnects and SOFC components is further investigated by XRD analyses.     

A model of the combustion of a single small coal particle using kinetic parameters based on thermogravimetric analysis

A mathematical model for the combustion in air of a single entrained spherical coal particle, 30 μm in diameter, has been developed incorporating thermogravimetric analysis data of Whitwick coal. The model is based on a set of ordinary differential equations, describing the reaction rates and the mass and heat transport processes. The system of equations was solved numerically. The combustion mechanism of the particle was described by locating the reaction zone at the solid surface, where gas-phase combustion of volatiles and heterogeneous reaction between gaseous oxygen and the carbon and hydrogen in the solid occurred in parallel. The combustion process was chemical-reaction-rate-controlled, with the oxygen partial pressure at the surface almost that of the surrounding bulk gas. The simulation results using this model, with the kinetic parameters for devolatilization and combustion derived from the experimental thermogravimetric data, are consistent with previously reported combustion lifetimes of approximately 1 s, for particles of this size and rank. They are also consistent with the anticipation that higher ambient gas temperatures should result in shorter burn-out times. The use of thermogravimetric data in the modelling of the combustion of small particles of these low-rank coals is a potentially valuable method for characterization of feedstocks for pulverized coal-fired boilers. © 1998 John Wiley & Sons, Ltd.     

Effect of surface roughness of composite bipolar plates on the contact resistance of a proton exchange membrane fuel cell

Polymer electrolyte membrane fuel cell performance strongly depends on properties of the fuel cell stack bipolar plates. Composite bipolar plates, though low cost and convenient in manufacturing, raise a major concern due to their high interfacial contact resistance caused by the mechanical treatment used to remove the polymer-rich layer on the surface. It is observed that most of this contact resistance is governed by electrical properties of the interface layer between the contacting surfaces. Measurements of contact resistance of mechanically polished composite bipolar plate/gas diffusion layer interface reveal a substantial influence of surface topography on the contact resistance, which varies significantly depending on the substrate surface treatment and roughness of composite bipolar plates.     

Tuning thermal expansion behavior and surface roughness of tubular Al2O3 substrates for fabricating high-performance carbon molecular sieving membranes for H2 separation

Tubular alumina substrates have been widely used as supporting membranes in gas separation. Owing to the demand for supporting membranes with a dense or ultra-micropore texture, the quality/quantity control of substrates is required to prevent the formation of defects due to rough surfaces, high curvature, and high difference in thermal expansion between the polymer precursor and the alumina substrate. This study proposes a new strategy to modify the pore texture, surface properties, and thermal expansion coefficient of the substrate by filling it with TiO₂ nanoparticles and using the grinding/polishing method. The effect of CMS preparation conditions, including coating cycles and pyrolysis temperature on the microstructure of the carbon matrix is also discussed. A tubular CMS membrane with excellent permselectivity toward H₂/CO, H₂/N₂, and O₂/N₂ (77.52, 162.94, and 13.87, respectively), and permeabilities of 55.9 barrer and 6.49 barrer are obtained for H₂ and O₂, respectively.     

Influence of liquid hydrogen and nitrogen on MHD triple diffusive mixed convection nanoliquid flow in presence of surface roughness

An innovative study of influence of surface roughness and nanoparticles on mixed convection flow is considered in presence of liquid hydrogen and liquid nitrogen. In fact, in order to understand the effects of surface roughness and nanoparticles on the flow characteristics of MHD triple diffusive mixed convection nanoliquid flow along an exponentially stretching rough surface, the flow problem is modelled in terms of highly nonlinear partial differential equations subject to the appropriate boundary conditions. Then, those equations are made non-dimensional with the application of non-similar transformations. The resultant nonlinear dimensionless coupled partial differential equations with boundary constraints are solved by using the Quasilinearization technique in combination with the implicit finite difference scheme. The liquid hydrogen and liquid nitrogen are considered as species concentration components. The surface roughness is modelled by a sine wave representation and hence the sinusoidal variations have been observed in gradients such as skin-friction coefficient, heat and mass transfer rates. It is observed that the effects of surface roughness on the skin-friction coefficient are more prominent near the origin than that in downstream. The addition of nanoparticles into the ambient ordinary fluid enhances the skin-friction coefficient and reduces the magnitude of wall heat transfer rate for both cases of smooth and rough surfaces. The rapid variations have been observed in the wall mass transfer rate due to the surface roughness in comparison to that of skin-friction coefficient and wall heat transfer rate. Further, the magnitude of wall mass transfer rate of liquid nitrogen is higher than that of liquid hydrogen.     

Effect of particle size on sodium distribution and sodium removal by water washing of Zhundong coal

The utilization of Zhundong (ZD) coal has been limited by its high sodium content. In the paper, the effect of particle size on the distribution of the sodium contained in the coal and the sodium removal from the coal was addressed. The results indicate that the water-soluble sodium content and insoluble sodium content increased and the acid-soluble sodium decreased with increasing particle size. The occurrence modes of the sodium contained correlate closely to the properties of ZD coal. The water-soluble sodium and water-soluble chlorine contained in ZD coal may exist in the form of sodium chloride. Due to the competitive effects between adsorbing capability of the leachate and the diffusing capability of sodium ion, the removal sodium rate increases first and then decreases with increasing particle size.     

Combined experimental and numerical investigations on the roughness effects on the aerodynamic performances of LPT blades

The aerodynamic performance of a high-load low-pressure turbine blade cascade has been analyzed for three different distributed surface roughness levels (Ra) for steady and unsteady inflows. Results from CFD simulations and experiments are presented for two different Reynolds numbers (300000 and 70000 representative of take-off and cruise conditions, respectively) in order to evaluate the roughness effects for two typical operating conditions.     

Effect of different grinding conditions on the dissociation and flotation of difficult-to-float coal

The recovery of clean coal can be improved through grinding. In this study, grinding and flotation experiments were conducted on difficult-to-float coal. Results show that the particle size obtained from ball milling is finer than that obtained from rod milling. However, the particle size distribution obtained from rod milling is more evenly distributed than that obtained from ball milling. The contact styles of steel rod and steel ball are “line contact” and “point contact,” respectively. Results from the flotation experiments are consistent with those from the orthogonal experiments.