| Flotation occurs in gas-liquid-solid three-phase flow system,and the particles with strong hydrophobicity are easy to collide and adhere to bubbles,and then float to achieve separation from gangue minerals.Due to the small mass and low inertia of fine particles,it is difficult for fine particles and bubbles to collide,and it is very difficult to recover flotation.High intensity turbulence can improve the collision probability between fine particles and bubbles,so it is widely used in fine particle flotation equipment.Our research group designed vortex generators（VG）with different structures to be added to the pipe flow section to induce micro-scale vortices with different structures and sizes,so as to enhance the turbulent environment,increase the collision probability between fine-grained mineral particles and bubbles,and effectively improve the separation efficiency of fine-grained minerals,especially fine-grained minerals,without increasing power consumption.Previous studies focused on the movement and collision of ore particles under turbulence,but the size and distribution of bubbles have a great impact on the mineralization process,which can not be ignored.The purpose of this thesis is to investigate the influence of flow field inside vortex mineralized tubes on the bubble size,and mineralized tubes with different vortex induced structures are taken as the research object.Using high-speed dynamic technology,CFD（Computational Fluid Dynamics）technology,PIV（Particle Image Velocity）technology,the bubble size and distribution and the main mechanism of bubble breakage were studied.The main research contents and achievements include the following aspects:An experimental measurement platform was built to study the bubble population at the output of the vortex mineralization tube,and the accuracy of the experimental method was verified by pre-experiments.The measurement of the bubble size and distribution in the tube of five VG structures:light tube,triangle,trapezoid,ellipse,and rectangle was achieved by using high-speed camera technology,and the circulation conditions were 0.21m3/h,0.28m3/h,0.35m3/h,0.42 m3/h,0.49m3/h,and0.56m3/h,respectively.The influence law of different structures and flow rates on the bubble size and distribution in the vortex mineralization tube was investigated.At the same flow rate,especially at low flow rate,there are differences in the bubble size inside the mineralization tube with different structures,among which the bubble size in the rectangular VG tube is the smallest compared with other tube sections at each flow rate,and is lower than that of the light tube,triangle,trapezoid and ellipse by 64.37%,58.35%,56.69%and 31.07%,respectively,at 0.21 m3/h circulation volume;at the same structure,with flow rate rises,the bubble size decreases,and the difference of bubble size in mineralized tubes of different structures has a tendency to decrease.The rectangular and elliptical VG tubes had advantages in increasing the proportion of small bubbles with sizes less than 500μm,which increased by 9.31%and 8.47%,respectively,compared with the light tube.The performance of reducing the bubble size was in the order of rectangular,oval,trapezoidal,triangular and light tube in order of high and low,and the performance was consistent with the available flotation data.Based on the CFD-PBM（Population Balance Module）coupling model,the numerical study of bubbly flow in vortex mineralized tubes with triangular,trapezoidal,oval,rectangular and light structures was carried out under the circulating flow of 0.21 m3/h and gas intake of 1 L/min;the numerical study of bubbly flow in vortex mineralized tubes with trapezoidal VG structure was carried out,when the circulating flow was 0.21 m3/h,0.28 m3/h,0.35 m3/h,0.42 m3/h,0.49 m3/h and 0.56m3/h and the gas intake was 1 L/min.The characteristics of the two-phase turbulent flow field considering the bubble coalescence and fragmentation process are obtained.Combined with the experimental data,it is shown that the increase of turbulent kinetic energy,turbulent dissipation rate,shear rate,vorticity and the decrease of the minimum vortex scale all promote the bubble fragmentation.Taking each parameter as the index,the influence law of the flow field on the bubble size can be predicted:（1）Under different structures,the average turbulent kinetic energy of rectangular structure was 0.013 m2/s2,which was186.22%,110.19%,72.46%and 18.34%higher than that of light tube,triangle,trapezoid and oval（the same below）;The average turbulent dissipation rate was 2.84m2/s3,which was 437.23%,118.72%,64.41%and 44.94%higher than that of the other four structures,respectively.The average vorticity of the structure was the largest,which was 3.22×103 s-1,which was 106.66%,21.46%,and 23.96%higher than that of triangle,trapezoid,and oval,respectively.And shearing effect inside the tube was the strongest,with an average shear rate of 414.70,which was 156.39%,18.99%,18.16%and 3.63%higher than the other four structures,respectively.Compared with other structures,the Kolmogorov scale in the flow field of this structure was 24.45μm,which indicated that the small-scale vortex had high kinetic energy and strong effect,the probability of bubble-vortex collision was high,and the probability of bubble breakage was increased.The bubble size reduction performance of the five structures was ranked as light tube,triangle,trapezoid,oval and rectangle,which was consistent with the experimental data.（2）Under different flow rates,the maximum flow of 0.56 m3/h had obvious advantages.In this condition,the average turbulent kinetic energy,turbulent dissipation rate,vorticity and shear rate were the highest,which were 0.05 m2/s2,40.66 m2/s3,1.58×103 s-1 and 739.26,respectively.Taking 0.49m3/h as an example,the relative growth rates of each parameter were29.94%,83.96%,58.70%and 18.20%,respectively.Compared with other flows,the Kolmogorov scale was the smallest,which was 12.57μm.The performance ranking of bubble size reduction was the ranking of flow rate from low to high,which was consistent with experimental data.At the mechanism level,a visual experimental platform for studying the bubble breaking process under the action of turbulent eddies is built,and the numerical simulation of single bubble breaking process under the same conditions is carried out based on LES（Large Eddy Simulation）-VOF（Volume of Fluid）model.The results show that the bubble breaking mechanism in the vortex mineralized tube is the joint action of three factors:turbulence pulsation,vortex body collision,viscous shear and the sharp edge of the vortex generator.The research on the bubble size and distribution of the flow field in vortex mineralization tubes provides a new reference for the structural optimization and operation parameter setting,and is of great help to improve the flotation recovery rate;The mechanism of bubble breakage in mineralized tubes is revealed,which provides a new reference for bubble dynamics research in flotation field.There are 49 figures,15 tables and 105 references in this thesis. |
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