Full definition
A cylindrical rotating vessel partially filled with steel or ceramic grinding media (balls) that reduces ore and other materials to fine particle sizes (typically 0.01-1 mm) through the combined action of impact and attrition as the tumbling balls cascade and catapult within the rotating shell. Dimensions: 1-8 m diameter × 1-14 m length (industrial). Speed: 60-80% of critical speed (critical speed = the RPM at which centrifugal force equals gravity at the shell). Mill capacity: 1-200+ tonnes/hour depending on size, feed material, and target grind. Ball media: forged steel (high-chrome, 40-65 HRC), cast steel, or ceramic (alumina, zirconia — for contamination-free grinding). Ball sizes: 20-125 mm diameter, selected by feed and product size. Liners: rubber (NR, 35-55 Shore A — for secondary and regrind mills, reduces noise 10-20 dB, lighter, longer life in many applications), steel (for primary mills with large media), or composite (rubber body with steel inserts). Power consumption: 5-20 kWh/tonne (the largest energy consumer in most mining operations). Brands: Metso Outotec (Vertimill, horizontal mills), FLSmidth (Fuller-Traylor), and Thyssenkrupp. Per Bond Work Index methodology for sizing. Rubber liners by Metso (Megaliner), Weir Minerals, and Polycorp.