Hydrodynamics Mechanism Of Particle Motion On The Bubble Surface In Flotation Process And Flow Enhancement

The contradiction between the equilibrium flotation process and the non-linear separation feature of mineral properties is the key problem restricting the efficiency and capability of the mineral flotation process,which is originated from the difficulty of efficient particle-bubble mineralization.The interaction between particles and bubbles is the core of this mineralization process,in which the particle motion on the bubble surface significantly affects the micro-scale behavior of the gas-solid interface,together with the particle-bubble collision process and attachment process.Particle movement is essentially caused by its own gravity and the motion of fluid particles.Variations in the flow environment can directly break the force balance of particles,change particle movement,and affect the micro-scale behavior of particles on the bubble surface.Therefore,it is of scientific and practical significance for enhancing the separation and recovery of complex mineral resources to deeply probe the kinetic mechanism of particles on the bubble surface under the action of fluid,reveal the fluid adaptation mechanism of the mineralization process,and construct multi-scale fluid flow strengthen the mineral flotation process.This thesis focuses on the particle-bubble interaction process from the perspective of fluid intensification and adaptation to study the movement behavior of particles on the bubble surface in the flow condition using the high-speed dynamic microscopy camera technology.Computational fluid dynamics(CFD)numerical simulation was employed to analyze the fluid motion characteristics on the bubble surface and to reveal the force mechanism of particles on the bubble surface.Based on the driven force of particle motion,a sliding contact time model and an attachment dynamics model on the bubble with mobile boundary conditions could be established.Single-bubble mineralization experiments under different flow states were carried out to reveal the fluid adaptation mechanism of the particle-bubble mineralization probability.Thus,a multi-scale fluid flow strengthened mineral flotation process was constructed,and the performance was verified through flow field simulation and coal slime separation tests.The main conclusions are as follows:The mechanism of fluid influencing particle motion behavior on the bubble surface was ascertained.Axial fluid motion became dominant above the bubble in the reverse flow process,whereas in the near-wall region of the bubble,the fluid changed from axial to transverse motion.It was found that the particle-bubble interaction process included four types of behaviors:non-collision,sliding after collision,non-jumping adhesion after collision,and jumping adhesion after collision.The jumping phenomenon represented the rupture of the liquid film and the formation of a three-phase periphery.The countercurrent flow increased the velocity of the particles,and enhances their movement around the bubble,hence reducing the probability of collision.In addition,the particles slid faster on the bubble surface,which cannot provide enough contact time for film drainage and generated a stable thin film to decrease the attachment probability.The increase of particle hydrophobicity and particle size increased the particle motion velocity and speed up its sliding on the bubble surface,thereby decreasing the induction time and inducing film rupture easily.In contrast,fine particles had insufficient kinetic energy to overcome the film thinning resistance and reach the critical film thickness for rupture.This force characteristics and driving mechanism of particle motion on the bubble surface under the action of fluid were revealed.In the process of particle motion,tangential forces were mainly dominated by the gravity that caused particles to move tangentially,while buoyancy and fluid drag forces acted in the opposite direction.The fluid drag coefficient(k_f)of particles was significantly influenced by their properties,with k_fdecreasing as surface hydrophobicity increased and particle size decreased.The bubble with mobile boundary conditions promoted particle motion,and after film rupture,the influence of fluid drag forces was weakened,further enhancing particle tangential motion.In addition,fluid flow,particle hydrophobicity,and particle size had a relatively limited effect on the evolution of tangential forces.As for the radial forces,gravity worked as a driving force on the top surface of the bubble,promoting particle movement towards the bubble,and was converted into resistance on the lower surface,while the buoyancy and fluid resistance acted in the opposite direction.During the particle sliding process on the bubble surface,the"jumping"phenomenon caused an instantaneous increase in radial hydrodynamic resistance,which led to the film rupture and the rapid generation of capillary forces.Due to the large wetting perimeter of coarse particles,capillary forces were stronger.Fluid flow affected particle motion by controlling the fluid resistance.As the reverse flow velocity increased,fluid resistance became larger,gradually dominating the particle approach process and preventing particle-bubble collision.In general,gravity collision dominated the process of countercurrent mineralization collision,and fluid flow enhancement greatly reduced the probability of gravity collision,which was positively correlated with particle hydrophobicity and particle size.Based on the driving mechanism of particle-related forces on the bubble surface,an inertial force model for particles was established.Furthermore,the fluid streamline equation was introduced to establish a sliding contact time model,together with an attachment probability model for particles on the bubble surface with mobile boundary conditions under different flow regimes.The fluid adaptation mechanism of the particle-bubble mineralization process were clarified.Single-bubble mineralization pipes with reverse flow and stirring flow were constructed.In the single-bubble pipe with reverse flow,the fluid movement in the mineralization zone of reverse flow was stable with low turbulent kinetic energy and dissipation rate,inducing the generation of large-scale turbulent eddies.Increasing the feed rate enhanced the reverse fluid movement and reduced the mineralization probability.In the single-bubble pipe with stirring flow that was generated by the rotor rotation,the fluid flow at the pipe bottom was strengthened,and the turbulent kinetic energy and dissipation rate were increased,thus giving rise to the small-scale turbulent micro-eddies.Increasing the stirring speed enhanced the bottom turbulent flow and improved the mineralization probability between fine particles and bubbles,but reduced the mineralization probability of weakly hydrophobic or coarse particles.There existed an adaptive relationship between the mineralization probability and the fluid scale.The mineralization process induced by reverse flow has a stable flow field,which is suitable for the flotation of weakly hydrophobic and coarse particles,while fine particles require small-scale micro-eddies to strengthen their collision process and increase the mineralization probability.A flotation process enhanced by multi-scale fluid flow was constructed,which was compatible with the non-linear characteristics of mineral separation.Based on the mineralization methods using reverse flow and stirring flow,a restricted space stirring flow mineralization process was introduced,and fluid flow with different scales was systematically integrated to establish the flotation process synergistically intensified by it.A laboratory turbulent mineralization-static separation flotation device was designed.The flow field simulation results showed that the fluid scale in this device exhibited a stepwise change.The fluid movement in the reverse mineralization unit of the static separator is smooth,inducing the generation of large-scale eddies,while the swirling mineralization unit formed a centrifugal field,causing the small-scale eddies smaller than 100μm.The pulp in the turbulent mineralization device was violent,bringing about the turbulent micro-eddies smaller than 30μm,and strengthening the mineralization between fine particles and bubbles.Fine coal flotation tests verified the device performance.The optimal operating conditions were found at a feed rate of 0.2m~3/h,a medium circulation rate of 0.6 m~3/h,and an impeller speed of 1600 rpm,which resulted in a yield of 45.92%and an ash content of 8.43%for the cleaned coal.The thesis has 115 figures,12 tables,and 168 references.