Turbulent Motion Of Fine Mineral Particles And Mechanism Of Flotation Process

It is difficult for fine minerals to be captured by bubbles,and the low collision probability directly affects the flotation efficiency.The slip velocity and contact collision of mineral particles and bubbles are decisive factors and processes that affect flotation efficiency.With the further understanding of the flotation process,the flotation of fine particles has developed from“viscous flotation”under the action of surface force of large bubbles to microbubble flotation,and then to turbulence enhanced flotation.Through fluid movement,achieving momentum and energy transfer of particles,increasing particle kinetic energy,changing particle motion and contact collision behavior with bubbles has become an important direction for improving mineral flotation capacity and efficiency.However,due to the complexity of turbulence and the particularity of flotation systems,the mechanism of turbulence on particle motion and contact collision is not clear.Although predecessors have established a series of theoretical collision models,the particle motion and contact collision mechanism under complex flow conditions such as turbulence are not clear,and there is a lack of quantitative collision probability models that accurately consider the impact of turbulence.It is of important theoretical and practical significance to further reveal fine particle-bubble turbulent collision mechanism,explore the particle momentum transfer and flotation mechanism in turbulent flotation.The purpose of this study is to clarify the mechanism of fine particle-bubble turbulent collision,and establish a turbulent collision model,which is carried out according to the route of turbulent motion-particle motion-particle-bubble collision process.Through high time resolution PIV measurements of liquid-solid two phase flows in isotropic turbulence,the correlation between particle and turbulent eddy motion is intuitively discovered;Using Eulerian-Lagrange numerical calculation and wavelet analysis,the motion mechanism of particles in turbulent eddy field was analyzed and evaluated,and a calculation model for particle slip velocity under the action of turbulent eddy was established;Using high-speed dynamic measurement and particle image velocimetry,the dynamic process and behavior of fine particle-bubble turbulent collision were measured,and the mechanism of turbulent eddy action in the process of fine particle-bubble collision was revealed;A measurement platform for the turbulent collision probability of fine particles and bubbles was established independently.Based on the established particle slip velocity model,turbulent collision probability model was modified,which improving the prediction accuracy of the model.The main research work and conclusions are as follows:A synchronous measurement and numerical calculation method for two-phase flow in a turbulent multiphase system was studied and established.Continuous PIV measurements of particle motion characteristics in turbulent flow fields were carried out using continuous lasers and high-speed cameras with high temporal resolution,and their turbulent motion characteristics were captured.Based on the differences in particle size and scattered light intensity between particles and flow field tracers,phase separation measurements of particles and flow field were achieved.The instantaneous velocities of the two phases were obtained through cross correlation calculations and particle tracking,respectively.A CFD-DEM numerical calculation method for liquid-solid two-phase coupling was established.The Eulerian Lagrange method was used to calculate the turbulent flow of particles with different physical properties,the LES turbulence model and Smagorinsky-Lilly model were used to solve the turbulent flow field,the Free stream drag model was used to calculate the drag force on particles,and the accuracy of the coupled numerical calculation was verified.The study found that the fluid generates macroscopic eddies with opposite rotation directions behind the grid spoiler column,and the eddies continuously fall off from the spoiler column and mix with each other,forming an isotropic turbulent flow field behind the grid;Particle motion is related to eddy motion,and particles always follow a certain scale of eddy motion.Turbulent flow achieves momentum transfer to particles through eddy motion.The correlation between particles and turbulent eddy motion was studied and quantitatively analyzed.Based on the multi-frequency characteristics of turbulence,the velocity signals of different scale eddies in the turbulent flow field were extracted using7 scale Wavelet transform;Based on Taylor’s"freezing hypothesis",spatiotemporal coordinates are transformed to quantify the motion characteristics of turbulent eddies.The results show that turbulence has an obvious coherent structure;The eddy scale increases as the frequency of the velocity signal decreases,and gradually decreases with the attenuation of turbulent energy at the same frequency.The tracking measurement of single particle turbulent motion shows that particles follow the turbulent eddy to move along the flow direction,and rotate with the eddy;The calculated and measured values of slip velocity of particles with a diameter of 74μm in different scales of turbulent eddies are compared,and the effective scale range of turbulent eddy is 35-119μm.The results of turbulent energy spectrum show that the turbulent eddy located in the inertial sub-region affect particles motion effectively,and the relationship between the velocity and the size of the eddy conforms to W_λ=C（ελ）^1/3.The effect of turbulence on the slip velocity of particles with different particle sizes and densities was studied and clarified.The greater the particle slip velocity,the higher the probability of particles leaving the streamline.Under the same turbulent dissipation rate,the particle slip velocity increases with the increase of particle size,while the fine particle slip velocity is small,and it is not easy to diffuse radially with the streamline.When turbulence is weak,density has a significant impact on particle slip velocity;When the turbulence is strong,the particle slip velocity is less affected by the density.When the turbulent dissipation rates are 0.057 m²/s³ and 0.014 m²/s³,the particle density increases from 2520 kg/m³ to 4500 kg/m³,and the slip velocities of 45μm particles increased by 3%and 50.4%,respectively.The calculation model of particle slip velocity in turbulent flow field was studied and modified.Based on the mechanical equilibrium relationship of particle motion in turbulent flow,the particle slip velocities in turbulent eddies with different scales were calculated.The results show that:1)in the spanwise direction perpendicular to gravity,turbulent centrifugal force and drag force dominate particle motion;When the particle size is smaller than the Kolmogorov minimum eddy scale,the motion correlation between the particle and the minimum eddy is strong,and turbulent eddies with an average scale of 67μm and 60μm act on the movement of particle with d_p=45μm and74μm respectively;When the particle size is greater than the Kolmogorov minimum eddy scale,the turbulent eddies with average scale 60μm-188μm dominant the movement of particle with d_p=125μm,150μm and 200μm.As the turbulent dissipation rate decreases,the scale of the turbulent eddy acting on particle motion increases,with a scale range of 318μm-663μm.The research shows that there is no constant proportional relationship between particle size and the scale of the turbulent eddy that dominates its motion;By introducing the relative scale coefficient between particles and turbulent eddies,the calculation model of particle slip velocity under turbulence was modified.2)In the flow direction opposite to gravity,turbulent centrifugal force,drag force,and gravity jointly dominate particle motion.By introducing a dimensionless parameter F_t（F_t=g/a）,a calculation model for particle flow slip velocity dominated is established through binary fitting.The mechanism of turbulent eddy interaction in fine particle-bubble collisions was studied and revealed.Combining single-bubble flotation system with oscillating grid turbulence generation system,the oscillating grid turbulence collision experimental platform was constructed.The characteristics of turbulent flow fields in oscillating grids were measured using particle image velocimetry,and the turbulent dissipation rates of flow fields with different oscillation frequencies were calculated.The study showed that the turbulent dissipation rate increases with the increase of grid oscillation frequency.By controlling the oscillation frequency,stable regulation of the intensity of turbulent flow fields was implemented.Through high time resolution particle image velocimetry and high-speed dynamic measurements,the process of fine particle-bubble turbulent collision was found as follows:bubbles pass through multiple turbulent eddies,collide with passing particles;when bubbles passing through a high vorticity region,they rotate under the influence of the turbulent eddies,and briefly stay at the eddy center;during the rotation process,they collide with particles.Therefore,the strengthening mechanism of turbulent eddies on particle-bubble collisions is not only manifested in enhancing the kinetic energy of fine particles,promoting fine particles to break away from the streamline and collide with bubbles,but also causing bubbles to rotate during the passage between eddies,resulting in multi-angle shear collisions with particles.The fine particle-bubble turbulent collision probability model was studied and modified.With established oscillating grid turbulence collision experiment system,the fine particle-bubble turbulence collision probability experiments were conducted.Model comparison and analysis indicate that the existing model has a narrow application range and overestimates the impact of bubble slip velocity on collision probability.Finally,based on the comprehensive study of particle slip velocity and collision mechanism,the probability of particle bubble turbulent collision was quantitatively described,and existing collision probability calculation models were modified to improve the accuracy of prediction.