Study of media mechanics in tumbling mills by the discrete element method.

摘要

This dissertation presents a new approach to the problem of analyzing the mechanics of grinding media inside tumbling mills. A two-dimensional discrete element method (DEM) has been employed to study the multibody collision events associated in the grinding process. Here, the model is two-dimensional. In other words, the movement of an assembly of discs inside a circular chamber of small width is modeled. In the discrete element method described here, the motion of the charge is modeled by considering forces acting at each contact point and following the motion of individual balls. According to the model, a force displacement law is used at the contact points, and Newton's Second Law of motion is used for particle trajectory calculation. Furthermore, the precise geometry of the inner shell of the mill is imposed in the ball-wall impact calculation. In this manner, the motion of hundreds of balls is monitored accurately.; The validity of the computer code has been checked against experiments; motion analysis studies are done to compare the position of toe and shoulder of the ball charge at varying degree of filling and speed. Movement of two balls, arbitrarily selected from an assembly of balls, is captured by a video camera and compared with model prediction. Finally, by means of actual experiment, the power draft of a 10-inch diameter mill is predicted by means of a special scheme that uses two-dimensional data.; A computer graphics program, facilitates visual interpretation of the numerical results. Different snapshots of the graphics output are compared, by changing the important operating parameters, for the qualitative assessment of the overall motion of the mill charge.; Several numerical experiments were carried out by means of the computer code to obtain diverse information pertaining to charge motion. The changes in power draft with the mill filling and speed are studied. Also, in order to characterize the behavior of the charge, change in the distribution of energy and fall-heights with mill speed and filling is studied. The trends in these distribution results are compared for different size mills.; The results obtained here are quite fundamental in nature. This work clearly brings a different approach to standing comminution problems and demonstrates how qualitative information complements fundamental and applied experimental work.