Numerical Study Of Particle-Bubble Collisions In Flotation Turbulence

Flotation is a separation process in which valuable particles are collected by rising bubbles in liquid phase and this technology has been widely applied to industrial processes,to name but a few:the separation of minerals,coals and plastic particles,the deinking recycling of waste papers,and the processing of wastewater.Taking mineral processing as an example,mineral particles have been grinded to be finer only in order to increase the yield of useful minerals,with the decrease of high-quality,easy-mined and easy-processed minerals.In this situation,the flotation of minerals has been increasingly difficult since flotation is a rather complex multiphase process with its inherent turbulence mechanism unclearly revealed,especially the collision of solid particles and air bubbles.These unsolved problems significantly limit the optimization of flotation equipment,which negatively affects the yield of fine particles.Applying the direct numerical simulation and two kinds of discrete phase models,this thesis is intended to systematically study the turbulence effects on the distribution and collision of solid particles and air bubbles.Firstly,a brief introduction to the multiple relaxation time lattice Boltzmann method and two models-the point-particle model and the particle-resolved model is given.The method is used to solve continuum phase,and the two models are applied to solve the motion of dispersed phases.More specifically,the point-particle model solves motion equation of particles and the particle-resolved model satisfies the non-slip boundary condition directly at particle surfaces by means of a bounce-back scheme.Secondly,the accuracy and reliability of the applied numerical methods are validated carefully.In the simulation of single-phase turbulence,the statistics of a three-dimensional homogenous isotropic turbulence agree well with those found in literature.In the particle-resolved simulation,the terminal velocity of single solid particle settling in quiescent water is almost identical with experimental results.In the point-particle simulation of turbulence laden with many solid particles and air bubbles,the statistics related to collision also reach a high-level agreement with those in literature.Thirdly,turbulence effects on the distribution and collision of solid particles and air bubbles are investigated within the framework of point-particle model.The results show that both the solid particles and air bubbles preferentially concentrate in different areas when their Stokes numbers are about one.As for the collision kernels,the turbulence transport effect dominates compared with the preferential concentration effect.This makes the collision kernels always increase when turbulence intensity goes up.Besides,the effects of bubble diameter,solid particle diameter and density on the collision statistics are studied.At last,particle-resolved simulations are conducted to study the turbulence effects on flotation dynamics based on some assumptions:zero gravity and low density ratio.Then the collision statistics are obtained for nearly the first time.The results show that both the radial distribution function and radial relative velocity are non-physical and the kinematic kernels are invalid.However,the dynamic kernels are still reasonable compared with the predicted value by point-particle simulations.Then the collision efficiency is calculated.