Grinding ore is usually carried out in a mill. There are many types of mills, but ball mills and rod mills are generally used in metal mines. The ball mill is a rotary cylinder with hollow shafts at both ends, and a considerable number of steel rods and steel balls are installed in the cylinder.
When ore and water are fed into the cylinder from the hollow shaft at one end, and discharged from the hollow shaft at the other end. When the cylinder of the ball mill rotates at the specified speed, the steel balls and steel rods together with the ore, under the action of centrifugal force and friction force, rise to a certain height with the cylinder, and then drop and slide down from the cylinder wall. Then they rise to the same height with the cylinder, and then fall down, periodically, so that the ore is crushed by impact and grinding. The ground ore and water form ore slurry (wet grinding), which is discharged from the hollow shaft at the discharge end to complete the grinding operation.