Research On Motion Law Of Media In Planetary Ball Mill Based On Discrete Element Method

The advantages like high energy density and stree intensity in grinding process make planetary ball mill widely used in ultra-fine grinding, mechanical alloying and mechanochemistry of hard and brittle materials. How to greatly improve the milling efficiency, solve the instability problem of product quality, as well as reduce the wear of media and liner and overcome the overheating problem of product is meaningful for the process. Given the milling ball is the energy transfer medium between the machaine and powder, two subject, milling ball and powder, were discussed separately based on the existing mechanical structure of lab planetary ball mill. Firstly the motion law of media was investigated. The variation related to motion, force and position were analyzed by combining discrete element simulation and mathematic analysis of media. Then the breakage of agglomerates, the areas where the breakage occurs and its relation with media movement were explored through the observation of milling process simulation and the analysis of simulation and cited experimental data. Finally, the improvement of machine to fit the requirements of efficiency miling was proposed in terms of conclutions drawed from last Chapters.The theories, concepts and simulation calculating process of discrete element method and the software EDEM based on the method were brifely introduced to lay a foundation for the investigation of the motion law of media and the breakage mechanism.Based on three assumptions, the movement of media in the dynamics framework was presented. In order to deepen the knowledge of the motion law of media in planetary ball mill, the variation related to motion, force and position were analyzed by combining discrete element simulation and mathematical analysis of media. It is suggested that the motion of ball is generated by the contact with vial wall and buttom, and adherent movement does not exist. The ball bounce movement is produced and maintain by the angular velocity component cox, coy through the entire milling process. The single ball sustains a steady state such as the static of relative position and the periodic change in ball translational and angular velocity. The futher study on the steady state shows that: Translational velocity of media plays a leading role during the process. The vial wall contact makes the medial circle the revolution center at the same speed roughtly and swing versus vial. The vial button contact reduces the swing amplitude and extends its period. The equilibrium position of media can be changed by tangential force acting on media, the finiteness of which makes the deflection of media equilibrium position has corresponding limitation. The media jump relative to vial during steady state.A numerical simulation using the discrete element method was performed to investigate the area where the breakage occurs and the breakage mechanism during the process by studying the breakage of agglomerates with different bond strengths or various speed ratios k. The information generated by simulation was used for ploting and analyzing the breakage event countour map. The simulations show that the areas close to vial wall where the majority of breakage occurs suffer the most density grinding effect of media. Speed ratio k controls the exitence of impact breakage and has an influence on the intensity of abration breakage, but considered bond strength doesn’t present its effect on the impact breakage. The simulation parameter of volume ratio of agglomerate to media influences the material broken process. The analysis of experimental data shows initial breakage and deformation of product building basis for further finely grinding benefits form strong impact. For further grinding of product and even the control of breakage-agglomerate dynamic balance shifting to the direction of fine product, abration breakage plays a major role and harvests an evident result.To practice the derived conclusions above, a speed control system based on the existing mill electrical system was designed. The equivalent dynamic model of mill mechanical transmission parts and the control system mathematical model of mill were established. The simulation model in Matlab/Simulink environment was builded to tune PID controller parameters and verify its effectiveness. Lab mill electrical system was improved to meet the requirement of speed control, and the corresponding parameters of inverter were also set.