The rotating drum-type ball mill is the industrial and large-scale laboratory standard for continuous or batch grinding. While it shares the "rolling" principle with the jar mill, a drum mill is typically a self-contained unit where the drum itself is the grinding chamber, often featuring internal "liners" to assist in lifting the media.

Inside the drum, the grinding media (balls or rods) follow a cycle of being lifted and falling. The internal geometry of the drum is key to its efficiency. Unlike smooth-walled jars, drum mills often feature "lifter bars." These prevent the media from simply sliding along the bottom, ensuring they are carried to the "shoulder" of the drum before falling. 

Three States of Motion: (1) Cascading: Low speed; balls roll down the surface. Best for fine grinding and attrition. (2) Cataracting: Optimum speed; balls are thrown in an arc and impact the "toe" of the charge. Best for breaking down large particles. (3) Centrifuging: Overspeed; balls stick to the wall. No grinding occurs.

References:

1. M. V. C. Machado, et al., Experimental and Numerical Study of Grinding Media Flow in a Ball Mill, Chemical Engineering Technology, 2017, 40, 1835-1843
2. D. Li, et al., Numerical study on the effect of drum on the flow behavior of binary-size particles in rotating drums, Powder Technology, 2021, 386,  108-119