A combination of density-based clustering method and DEM to numerically investigate the breakage of bonded pharmaceutical granules in the ball milling process

Abstract

Ball milling is widely used in industry to mill particulate material. The primary purpose of this process is to attain an appropriate product size with the least possible energy consumption. The process is also extensively utilised in pharmaceuticals for the comminution of the excipients or drugs. Surprisingly, for ball mill, little is known concerning the mechanism of size reduction. Traditional prediction approaches are not deemed useful to provide significant insights into the operation or facilitate radical step changes in performance. Therefore, the discrete element method (DEM) as a computational modelling approach has been used in this paper. In previous research, DEM has been applied to simulate breaking behaviour through the impact energy of all ball collisions as the driving force for fracturing. However, the nature of pharmaceutical material fragmentation during ball milling is more complex. Suitable functional equations which link broken media and applied energy do not consider the collision of particulate media of different shapes or collisions of particulate media (such as granules) with balls and rotating mill drum. This could have a significant impact on fragmentation. Therefore, this paper aimed to investigate the fragmentation of bounded particles into DEM granules of different shape/size during the ball milling process. A systematic study was undertaken to explore the effect of milling speed on breakage behaviour. Also, in this study, a combination of a density-based clustering method and discrete element method was employed to numerically investigate the number and size of the fragments generated during the ball milling process over time. It was discovered that the collisions of the ball increased proportionally with rotation speed until reaching the critical rotation speed. Consequently, results illustrate that with an increase of rotation speed, the mill power increased correspondingly. The caratacting motion of mill material together with balls was identified as the most effective regime regarding the fragmentation, and fewer breakage events occurred for centrifugal motion. Higher quantities of the fines in each batch were produced with increased milling speed with less quantities of grain fragments. Moreover, the relationship between the number of produced fragment and milling speed at the end of the process exhibited a linear tendency.