Understanding size segregation in tumbling mills

Abstract

Several parameters affect the grinding performance in tumbling mills. Among them, the energy spectra has been considered as a key qualitative parameter. Another parameter that critically affects the milling performance is size segregation in the mill. The energy spectra and size distribution of the impacting media are interrelated. These must correctly evolve and optimally set in response to milling parameters. The present work focuses on the radial size segregation in a mill. It is observed that at lower mill speed bigger balls move near the periphery of mill and smaller balls clutter around the kidney of the charge. In contrast, at higher speed reversal in the positioning of balls take place i.e. smaller balls move closer to the periphery and the larger ones occupy the positions relatively closer to the center of mill. At an intermediate speed, charge dynamics attain a perfect transition state representing uniform distribution of contacts—a condition favorable for grinding. Discrete element simulations have been carried out to understand the rationale for size segregation and its reversing pattern. The segregation pattern at lower milling speed can be explained by analyzing the rapid flow layer. As mill speed increases, the height from which balls fall start getting differentiated according to their size. At sufficiently high mill speed, centrifugal and frictional forces cause a situation where smaller balls are closer to the periphery. The improved understanding may help avoid the segregation and achieve the uniform energy spectra.