A pilot plant scale SAG mill suitable for use in design of a commercial scale SAG mill. The ball mill has a cylindrical outer chamber having flanges at opposed ends. The cylindrical outer chamber contains a removable grinding chamber in the form of a sleeve. The removable grinding chamber has a diameter of 2-5 feet and a ratio of diameter:length in the range of 3:1 to 1:1. The removable grinding chamber contains a plurality of steel balls. The removable grinding chamber extends partly down the length of the cylindrical chamber and has longitudinal lifters attached to the internal surface of the sleeve. The lifters are capable of lifting steel balls located in the removable grinding chamber during rotation of the cylindrical outer chamber. The removable grinding chamber has means at one end for receiving particulate ore from a feed hopper and a removable diaphragm at the opposed end, the removable diaphragm having outlet ports therein for discharge of ground particulate ore into the cylindrical outer chamber. The cylindrical outer chamber has discharge ports for discharge of ground particulate from the SAG mill, and means to rotate the cylindrical chamber about a longitudinal axis.