Study On The Working Performances Of Large Tumbling Ball Mill Based On The Discrete Element Method

Tumbling ball mill is widely used in mineral processing, metallurgy, building material and chemical industry because of its merits such as high crushing ratio,well stability,strong adaptability and so on. Its large scale trend is the demand of practical production and the major research development at home and abroad.But investigation on the grinding process applying traditonal theoretical and practical method is more difficult with the increase of the mill scale.As the mill starting with full load is a transient process, it is difficult to obtain experience in practice and there is few conclusions for reference.It was a point of view that the torque and the power during the starting process are especially high.People intended to select motor with larger power and torque, while meeting the working demand,it also increases the grinding cost. Investigation results show that only enough attention be paid on the mill design and parameters such as filling ratio can maximum grinding performance.If grinding parameters are selected unsuitably,the large ball mill can not work normally,which will bring huge economy loss.The research on the grinding parameters mainly focused on the effect on the power which was not able to reflect the comprehensive performances of the mill.Discrete element model of large scale ball mill 5585 containing ores was established using EDEM software based on discrete element method to simulate the grinding process.The dynamic change of charge motion during strating can be viewed directly,and the transient torque was gained.The maximum requirements of the torque and power during starting were determined based on the normal torque and the normal power. Then regarding the specific power,particle breakage rate as well as proportion of the energy consumed by collision between charge and liners to the total energy loss as evalution indexes of the mill working performance, the optimized matching to speed ratio,filling level,the ratio of media to ore and structure parameters of liner (height,number and angle)was made. The main research contents were as follows:(1) Based on the knowledge of the calculation thought of the DEM and relevent software development, the generally used contact models were introduced, the contact parameters defining method and time step control method were described detailly.(2) Charge motion especially the parabolic movement law in the ball mill was investigated,the critical speed and optimum operating speed were calculated and common formulas of effective power draw were presented.(3) Dynamics analysis of tumbling ball mill during starting process was finished.DEM model of a large tumbling ball mill including ores was established,and the changes of charge motion and transient torque were analyzed. Combining with the orthogonal test method,the maximum torque and power needed by mill to start smoothly were determined based on the stable torque and power. Comparison with the theoretical results showed that the simulation results agree well with it when the rotation angle of charge is smaller than the repose angle, DEM can simulate the dynamic motion of discrete particles, so it can predict the transient torque exactly during staring process.The maximum torque and power during starting process of the ball mill is less than twice of the normal torque and power when the starting time is greater than 3 seconds.(4) Applying uniform design method, with filling level,speed ratio,media-to-ore ratio and the liner structure parameters as test factors,with specific power draft,breakage fraction,and the proportion of the energy consumed by collision between charge and liners to the total energy loss as evalution indexes, the simulation scheme was established. Combined DEM simulation with regression analytical technology,the influences of each factors on each index were analyzed quantitatively and optimum grinding parameters were determined. The specific power and the broken fraction of the particle increased apparently while the liner wear of the mill decreased after optimization.The results also indicates that filling level has significance effect on the specific power and liner wear,it is the most important factor to the working performances of the ball mill.(5) Based on Hertz contact theory and media motion analysis,the mathematical model of grinding performance including impact grinding and attrition grinding was established to optimize fill level and rotation speed ratio. The optimization results agreed well with the simulation results, which proved the reliability of DEM simulation for ball mill again.And it also shows the advantage of DEM simulation.Research work of this thesis can be used as reference for motor selecting,and it also has certain derective meaning for improving ball mill efficiency and reducing consumption of steel and energy.