Ball mills are giant drums that are filled to between 30 and 40% of their volume with freely moving grinding media - in this case balls. The diameter of the balls is between one and several centimetres to suit the application - smaller balls for fine milling results and larger ones for coarser milling tasks.
The drive sets the milling drum into slow rotation and the bed of milling media is thus activated, i.e. the balls are lifted and then impact back against the bed. Ball mills are usually operated at 75% of the critical speed. The critical speed is the speed where, mathematically calculated, the grinding media would centrifuge out. Mills of 2 m in diameter therefore rotate in operation at about 23 revolutions per minute, which corresponds to a peripheral speed of 2.4 m/s.
A mill of this size equipped with steel grinding media converts approx. 50 kW per m³ of feedstock. The feed product is fed continuously through the trunnion and is discharged through slots on the opposite side. The discharge rate can be adjusted as a function of the number of open slots. The ground product is transported - often pneumatically - and is charged to a downstream classifier. The coarse material is returned to the mill together with the feed product.