| Coal preparation is a key step in ensuring the efficient use of coal resources and promoting the sustainable development of the energy industry.Coal separation is of significant importance to enhance the efficiency of energy use and protect the environment.As one of the key technologies in the coal sorting process,flotation is currently the most effective means of processing fine-grained coal slurry.Appropriate use of flotation chemicals and bubble adjustment are important factors to ensure the effectiveness of coal slurry flotation.In addition to the effect of flotation chemicals on the adsorption of gas-liquid-solid three-phase in the slurry,which has an effect on the flotation effect,the trailing vortex generated by the movement of bubbles in the flotation process is one of the most important reasons for the impact of flotation on the ash content of the coal concentrate.Therefore,it is a precondition to improve the flotation technology to explore the influence mechanism of the frothing agent on the bubble tailing vortex in the flotation slurry and the particle movement in the flow field of the tailing vortex area.The essence is to explain the influence mechanism of the frothing agent on the gas-liquid interface,flow field characteristics and particle dynamics in the flotation process.Based on the multidisciplinary principles of interface chemistry,fluid mechanics,dynamics and mathematical statistics,combined with modern technical means such as high-speed camera motion acquisition and three-dimensional particle image velocimetry,this paper carries out an in-depth investigation on how the concentration of the frothing agent affects the flow field of bubbles and their trailing vortex areas.The morphological characteristics of bubbles and trailing vortices are analysed,and the characteristics of the flow field under different frothing agent concentrations are grasped.The mechanism of the frothing agent on the boundary layer separation phenomenon at the gas-liquid interface is clarified,which provides an important theoretical basis and practical guidance for the entrainment of fine mineral particles in the trailing vortex of gas bubbles.The dynamics of bubbles and bubble trailing vortices are analysed to reveal the correlation between bubbles and trailing vortices,and the dissipation law of trailing vortices is grasped through the analysis of energy evolution.Mineral particles are introduced into the flow field containing bubble trailing vortex under the influence of frothing agent,and the particles are tracked and observed by using high-speed camera technology and professional analysis software,so as to clarify the dynamic characteristics of the particles and the energy evolution law,to analyse the kinetic parameters of the particles in the flow field,and to master the influence mechanism of the characteristics of the flow field on the particle suction and entrainment.Through the mathematical statistics and probability analysis of the trajectory of the mineral particles,the probability distribution model of the particles in the bubble trailing vortex area is constructed under the influence of the concentration of the frothing agent,the particle size and the density of the mineral particles.The main conclusions of the experimental study are as follows:As the concentration of the frothing agent increases,the size of the bubbles gradually decreases,and the aspect ratio gradually increases,and a critical value exists at a concentration of 1.6×10-4 mol/L.The bubble size of the bubbles decreases with the concentration of the frothing agent,and the length-to-diameter ratio gradually increases,and a critical value exists at a concentration of 1.6×10-4 mol/L.The bubble size of the bubbles decreases with the concentration of the frothing agent,and the length-to-diameter ratio gradually increases.As the concentration of the foaming agent increases,the range of the low velocity region of the flow field gradually decreases,the symmetry is gradually significant,and the trailing vortex is mainly concentrated in the region where the flow velocity is less than 0.09 m/s.The flow velocity of the gas-liquid interface is also reduced by the concentration of the frothing agent.Due to the change of the properties of the gas-liquid interface by the frothing agent and the phenomenon of uneven distribution of the frothing agent by the bubbles under the action of the flow field,the separation angle of the boundary layer of the fluid on the surface of the bubbles increases gradually with the increase of the concentration of the frothing agent,and when the concentration of the frothing agent is greater than 1.6×10-4 mol/L,there is a stable separation angle of 196.70°.Characterisation of the movement of bubbles and bubble trailing vortices,there are horizontal oscillations during the movement of bubbles in the flow field,and at the same time there is a trailing vortex that is consistent with the oscillation frequency of the bubbles.The oscillation frequency of the bubble is affected by the energy change during the oscillation motion,and the oscillation frequency decreases significantly with the increase of the frothing agent concentration,and there exists a critical concentration of 1.6×10-4 mol/L.The numerical model of the relationship between the oscillation frequency and the concentration of the frothing agent can be fitted as f=0.37e^（-0.236c）+11.35.As the distance from the bubble increases,the oscillation frequency of the bubble trailing vortex remains essentially constant,while the oscillation amplitude of the bubble trailing vortex gradually increases.The trajectory characteristics of particles passing through the bubble trailing vortex region in the flow field were clarified,including three types of trajectories,namely,"escape,offset and entrainment".The velocity,kinetic energy and energy dissipation of the particles under different trajectory characteristics were analysed.The escape particles are not sucked into the trailing vortex region of the bubble,their velocity is not affected by the trailing vortex,and their energy is not dissipated;the offset particles are attracted to the trailing vortex and their lateral velocity changes significantly,and there is energy dissipation and the kinetic energy decreases;the entrainment particles’velocity in the trailing vortex region decreases sharply,and there are large changes in the direction of the velocity,so that the kinetic energy of the particles in the trailing vortex region of the bubble is obviously smaller than the energy dissipated by the particles.Small size particles are more likely to be entrained.Particles in the bubble trailing vortex are subjected to buoyancy,gravity,drag,additional mass force,Basset force,pressure gradient force,Magnus force and Saffman lift force.A dynamic force analysis of the particles shows that drag is the main cause of particles being entrained into the trailing vortex region and the main force preventing the particles from leaving the trailing vortex region,while the Saffman force is the main force prompting particles to stay in the bubble trailing vortex region.Through the mathematical statistics of a large number of particle trajectories,a model of the probability of particle entrapment by entrainment was constructed under the experimental factors.when the particle size was 0.100 mm and the density was 1.35 g/cm3,the probability of particle escape gradually increased and the probability of particle entrapment gradually decreased with the increase in the concentration of the frothing agent,and there was a critical frothing agent concentration of 1.6×10-4 mol/L.The mathematical relationship model between the concentration of frothing agent and the probability of particle entrapment was P=38.21+37.64e^（-0.188c）,and the correlation coefficient R2=0.99.As the size of the particles increased,the probability of particle escape increased and the probability of particle entrapment decreased,and there was a critical concentration of frothing agent 1.6×10-4 mol/L.The mathematical relationship model between the entrapment probability is P=38.21+37.64e^（-0.188c）,and the correlation coefficient R2=0.99.The probability of particles being entrapped decreases gradually with the increase of particle size,and at the blistering agent concentration lower than 1.6×10-4 mol/L,the relationship model between the entrapment probability and particle size can be expressed as P=A+Be^（-d/C）.The higher the concentration of the blowing agent in the solution,the smaller the effect of particle size on the entrapment probability.The probability of particle entrapment decreases linearly with increasing particle density for the same blistering agent concentration and particle size.The entrapment probability model was validated by further more extensive experiments and analysis of variance,which proved that the constructed probability model has certain accuracy and applicability.The entrainment probability of pulsed minerals can be improved to a certain extent by regulating the concentration of the frothing agent in the flotation process,which in turn reduces the ash content of the flotation concentrate and improves the flotation performance.The research results of the paper can enrich the theory of hydrodynamics and particle dynamics in flotation,and provide theoretical support for the optimisation of bubble regulation technology in coal slurry flotation and improvement of flotation effect.Figure[55]Table[9]Reference[139]... |
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