| Coal beneficiation is the basis of clean and efficient utilization of coal,and also the guarantee of green and high-quality development of coal.Flotation is one of the important coal beneficiation technologies and the most efficient means to deal with fine-grained coal sludge at present.Usually,coal particles collide with air bubbles and adhere into the flotation concentrate,and the concentrate is theoretically pure coal material.However,in actual flotation,hydrophilic particles such as clay minerals are also found in the flotation concentrate.This is due to the formation of a tail vortex at the end of the bubble during the flotation process,and some particles are entrained into the flotation concentrate by the tail vortex,so probing the movement pattern of particles in the bubble trailing vortex region is the key to optimize the flotation effect of coal slurry.Based on the analysis of interface chemistry,fluid mechanics,probability theory and mathematical statistics and vortex dynamics,combined with 3D particle image velocimetry system,high-speed motion acquisition system and flotation verification test,the paper firstly analyzes the morphology of bubble tail vortex zone in flotation by using 3D particle image velocimetry system,and initially grasps the effect of turbulence intensity on bubble tail vortex morphology.Further,the energy dissipation mode of the tail vortex area was elaborated and an energy dissipation model of the bubble tail vortex area was constructed.On the basis of this,a high-speed camera system was used to explore the influencing factors of particle motion in the wake vortex area,to investigate the mechanism of particle trajectory motion on the spatial and temporal scales from a macroscopic point of view and to establish a model of particle wind-up in the wake vortex area of flotation bubbles.The flotation tests were conducted to verify the suitability of the flotation bubble tail vortex zone wicking model for flotation recovery.The following main conclusions were obtained from the study.In terms of bubble tail vortex morphology,there is a region at the end of the cylinder that is lower than the incoming flow velocity,i.e.,the low velocity region also known as the tail vortex region.With the increase of the flow velocity,the height of the relative tail vortex area increases first with Re and then gradually decreases.The separation angle and the vortex shedding in the wake area are the factors that lead to the change of the height of the wake area,with the increase of fluid velocity,the separation angle first decreases rapidly,and then almost constant at 94.2°.The vortex shedding in the cylindrical wake area also leads to the asymmetric distribution of the cylindrical wake,and the frequency of cylindrical vortex shedding increases with the increase of the incoming flow velocity and decreases with the increase of the cylindrical size.In terms of wake vortex energy dissipation,pressure,bulk force and energy density are distributed hierarchically at low flow velocities,but the hierarchy continuously decreases with increasing flow velocity.The pressure,viscous shear stress,bulk force and energy density are roughly symmetrically distributed along the vertical line x/d=0.The distribution of these forces is mainly concentrated in the rectangular region where two horizontal lines(y/d=2,y/d=4)and two vertical lines(x/d=2,x/d=2)intersect.According to the characteristics of the total energy change of the fluid the whole flow phase can be divided into three phases: near phase,around phase and away phase.Under the same conditions,the order of the magnitude of energy loss caused by the three forces is: volume force > viscous shear stress >pressure,and volume force work caused by energy loss is the main part of the three energy losses.The vortex energy dissipation,total energy loss and the percentage of vortex dissipation to total energy loss all increased with the increase of flow velocity,and when the flotation bubble velocity reached the maximum,the percentage of vortex dissipation to total energy loss in the tail vortex region was 28.2%.In terms of particle dynamics,there are two main types of particle trajectories in the cylindrical tail vortex region: coiled and uncoiled trajectories.The difference between the two types of trajectories lies in the different directions of the combined forces of the particles in the vertical direction of point I inside the tail vortex.The coiled-suction particles move on the cylindrical surface and then enter the tail vortex zone,where centrifugal force and dynamic pressure provide sufficient energy.After a similar circular motion,the particles leave the tail vortex region under dynamic pressure.And the uncoiled particles move on the cylindrical surface and then settle freely.By analyzing the critical motion of the particles,the particles affected by the cylindrical boundary layer and the particles closer to the vortex in the tail vortex region are more likely to enter the tail vortex region,while in the tail vortex region,small-sized particles are more likely to be swirled by the vortex than large-sized particles.In terms of the swirl probability model,the swirl probability of the tail vortex gradually increases with the decrease of coal particle size,increases with the decrease of particle density,and increases with the increase of fluid flow velocity.The particle mobility depends on the particle size,density and fluid flow rate are the key factors affecting the tail vortex suction probability,and the tail vortex suction rate increases with the increase of particle mobility(K),while at the same time,the increase of the suction probability is mainly concentrated in K<12.5.The flotation recovery is consistent with the change trend of the tail vortex suction probability through the flotation test,which verifies the suitability of the flotation bubble tail vortex zone suction model for the flotation recovery.The model’s suitability for flotation recovery was verified.Figure [52] Table [5] Reference [92]... |
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