A useful and simple equation describing the grinding momentum is M x V (mass x velocity), which shows how the attritor fits into the family of mills. For example, ball mills use large media, normally 1/2” or larger, and run at a low (10–50) rpm. The other mills, such as sand, bead and horizontal, use smaller media from 0.3 mm to 2 mm, but run at a very high rpm (roughly 800–1,200). High-speed dispersers with no media run even faster rpm (1,200–1,800). Various types of attritors fall in between these (see Table).
For efficient fine grinding, both impact action and shearing force must be present (see Figure 1). When wet grinding in the attritor, impact action is created by the constant impinging of the grinding media due to its irregular movement. Shearing action is present in the attritor as the balls (media) in their random movement spin in different rotation and, therefore, exert shearing forces on the adjacent slurry. As a result, both liquid shearing force and media impact force are present. Such combined shearing and impact results in size reduction and good dispersion.