Grinding ore is usually carried out in a mill. There are many types of mills, but vertical roller mills and rod mills are generally used in metal mines. Gravel mills and autogenous mills have been gradually applied in China in recent years.

The vertical roller mill hydraulic system 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 vertical roller mill cylinder rotates at a specified speed, the steel balls and steel rods are together with the ore. Under the action of centrifugal force and frictional force, they rise to a certain height with the cylinder, and then they are thrown away from the cylinder wall and slide down.

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.