Grinding ore is usually done in a grinding machine. There are many types of grinding machines, but ball mills and rod mills are generally used in metal mines. Pebble mills and autogenous mills have been gradually used in China in recent years. A ball mill is a rotating 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 a specified speed, the steel balls and steel rods rise to a certain height with the cylinder under the action of centrifugal force and friction, and then fall off the cylinder wall to be thrown and slid down. Then they rise to the same height with the cylinder again, and then fall down, cyclically, so that the ore is impacted and ground. The ground ore and water form a slurry (wet grinding), which is discharged from the hollow shaft at the discharge end to complete the grinding operation.