Agitated Ball Mills

Agitated Ball Mills are a variation of regular Ball Mills and are often used for the grinding of wet media. Most of the agitated media mills are used for slurries, only a minority is in industrial use for the fine grinding of dry powders. The grinding energy is transferred into the media by agitation with a rotating stirrer. Also combined agitation by stirrer and rotating body is known. The grinding chamber arrangement and shaft direction is vertical and there are three basically different mill types. The so called tower-mill has a screw-type rotor which lifts the grinding media and actually uses the gravity as driving force for media motion. This mill cannot be operated with very small media. Second type is an agitated media mill with stationary grinding media filling; material transport is done by gravity or pneumatically. The third type is operated with a circulating media filling; material and powder are flowing through the agitated mill chamber. These two types of dry agitated media mills create high kinetic forces on the media and can be used with very small ball sizes. The grinding mechanism can be influenced by variation of the agitator speed and the ratio of media filling. High rotation speed and low media filling level effect a tendency to impact stress, whereas a high filling ratio of beads shifts the mechanism more to shear stress. In contrast to agitated wet ball mills, dry ball mills are typically low speed mills with predominant shear grinding.