Research On Particles Vibrational Segregation Kinetics And Variable-trajectory Screening Process

Coal preparation is the source technology to realize the clean utilization of coal resources.Screening is one of the most important links in the coal washing process.The vibrated segregation and stratification of particles on the screen surface is the key prerequisite for the efficient screening of mixed particles.Most of the traditional research on particle vibration separation has focused on the segregation mechanisms under specific conditions,as well as the predictions of the transition boundary of different separation modes,which lacks in-depth analysis on the process dynamics of particle segregation when subject to a wide range of parameters.Previous research on velocity theories of particle segregation in the separation process and the corresponding mathematical models are merely applicable to spherical particles.However,the bulk materials in practical industrial productions are mostly irregular non-spherical particles,thus previous theories and models need to be further studied and developed.In addition,most of the previous unit-combined vibrating screening technologies,which characterized by sectional-enhancing segregation and screening of particles,are usually exerted with consistent excitation trajectories for different screen panels to regulate the feed distribution on the screen surface and control the passing of small particles.And there is a lack of in-depth theoretical research on variable-trajectory enhancement screening.To solve the above problems,investigations were conducted to study the vibrated segregation dynamics of complex particles,as well as the screening mechanism and parameter control of the variable-trajectory screening process.The segregation kinetics of non-spherical particles and the response of vibration separation of complex particles to various parameters were revealed.Based on the constant-gradient decreased vibration parameters and variable vibration-trajectories,the stratification and screening of particles were strengthened to achieve efficient 6 mm screening technology.The segregation kinetics and velocity model of a single non-spherical particle were studied.Based on periodic boundary conditions,the models for single spherical/non-spherical large particles segregated in a non-convection bed consists of small particles was established;The segregation behaviors of large particles under the synergistic effect of particle size,shape,and vibration amplitude are comparatively analyzed.And a velocity phase diagram(amplitude-frequency phase diagram based on a wide range of vibration parameters)for non-spherical large particles has been established to reveal the macroscopic variations of segregation velocity with vibration conditions;The various non-monotonic critical conditions of the segregation velocity were unified,and the shape factor has been introduced to propose a segregation velocity model for non-spherical particles;Based on the regulation of the particle inertia moment,the orientation distributions of the non-spherical particles in the segregation process and their influence on the segregation behavior were revealed.A binary homogeneous mixed particles segregation model with“Brazil fruit”segregation tendency driven by convection mechanism was established,and the response of the segregation degree and large particles'segregation velocity to the vibration parameters and the packing structures were analyzed.The non-monotonic variation trend of segregation degree in the steady separation process with vibration amplitude and frequency has been revealed,and a predictive formula for the segregation degree was proposed;The distribution characteristics of large particles'segregation velocity were analyzed.Based on the velocity's convergence and its fitting performance to different amplitude-frequency combinations,a velocity model of large particles was established;The influence of the packing ratio of the large and the small particles on the granular temperature and the oscillation frequency spectrum was analyzed,and the response of the segregation behavior to the large particle volume fraction has been revealed.The stratification and screening behavior of complex non-spherical particles have been investigated.Considering the sidewall friction and periodic boundaries,models for segregations triggered by“void filling”and“global convection”were respectively established,based on which the regulation of the sidewall effect on segregation behavior was comparatively revealed;The segregation performance of four groups of particles with different shape combinations in a non-convective environment was analyzed,and the competition mechanism between the sphericity and the granularity of the particles was revealed.A dimensionless phase diagram based on the size-ratio and sphericity was proposed to predict the final segregation mode;Based on the mixing-entropy in Boltzmann's form,the influence of intermediate size and larger size on the vibration segregation of complex non-spherical particles were analyzed;Based on the above segregation models,a 2D-vibrated screen mesh was attached beneath the particle bed to equivalently show the particles'movement on the equal-amplitude screen and passing behavior of small particles.And the batch screening process of complex non-spherical particles was thus analyzed.In addition,the number of collisions between the particles and the screen was further counted,and the influence of feeding rate and vibrations on the segregation-screening process was revealed.Research on the mechanism of variable-trajectory enhanced screening and parameter optimizations was conducted.Considering the response characteristics of screening efficiency and particle moving speed to the product of amplitude and n-order frequency,an efficient regulation method based on(A_d×f_d)for screening behavior was proposed,A_d and f_d are dimensionless amplitude and dimensionless frequency respectively.Based on this method,the particle passing distribution and screening efficiency for a three-section unit-combined screen that subjects to the constant-gradient decreased vibration amplitude/frequency were investigated.Based on different combinations of three typical vibration trajectories:linear vibration(Z),elliptical vibration(T),and circular vibration(Y),the mechanism of variable-trajectory enhanced screening was studied.Single-factor analysis and multi-factor response surface analysis were carried out to reveal the interaction and impact significance of vibration parameters and processing parameters.The result shows that,among the variable-trajectory screening processes,the screening efficiency of the Z-Z-Z unit-combined screen was relatively better;The Quadratic model can be used to establish the correlation model of amplitude gradient(AG),frequency gradient(FG),feed rate(Q)to screening efficiency.The significance order of the three factors on screening efficiency was:FG>AG>Q;The further optimization based on the correlation model showed that when the amplitude gradient was 0.53 mm,the frequency gradient was 4.7Hz,and the feed rate was 6.31 kg/(s?m~2),the screening efficiency was 88.11%,reaching the optimal value.