AG (Autogenous) / SAG (Semi-autogenous) Grinding Mills

Autogenous (AG) and Semi-Autogenous (SAG) grinding mills have seen increased application in large mineral processing operations in recent years. They are rotating/tumbling mills in which comminution (size reduction) takes place without grinding aids. Consisting of a large diameter cylinder relative to their length (typically 2 or 2.5 to 1) these mills use large lumps of ore as the grinding media. Autogenous mills use ore exclusively as their grinding media but if hardness and abrasiveness of the ore does not lend itself well to full AG milling then a small charge of steel balls (6 to 15%) is added to assist in size reduction. This is known as a Semi-Autogenous (SAG) mill. AG/SAG mills bring about size reduction by a combination of impact, attrition and abrasion forces. Ore is swept up one side of the mill and at a certain point it falls back to impact the toe of the charge (bottom of mill). Ore particles in the body of the charge slide when moving to different heights and are subjected to attrition and abrasion forces. The inside liner of AG/SAG mills consists of steel or rubber liners and are fitted with lifter bars.

AG/SAG mills can accomplish the same size reduction work that normally takes multiple stages of crushing, screening and grinding methods which accounts for its popularity. Often the product can be finished size or ready for final grinding in a ball mill or pebble mill. They can grind run-of-mine rock or primary crusher products with their feed size limited to what can practically be conveyed. In addition they can be less costly to operate with the reduction or elimation of expensive balls or rods.

- FLSmidth Minerals, Metso Minerals, Outotec Oyj, Polysius Corporation images.