Semi-Autogenous Grinding (SAG) is a method of grinding rock into fine powder whereby the grinding media consist of larger chunks of rocks and steel balls. A SAG mill is generally used as a primary or first stage grinding solution. SAG mill is a tumbling mill with a large rotating drum, is filled with ore, water, and steel grinding balls. The result is a fine slurry. The mill is so named because the rock itself does some of the grinding.