Grinding ore is usually carried out in a grinding mill. There are many types of grinding mills, but ball mills and rod mills are generally used in metal mines. Pellet mills and autogenous mills are also used in China, but in smaller numbers overall. A ball mill and rod mill are rotating cylinders with hollow shafts at both ends, containing a considerable number of steel rods and balls. The working principle of a grinding mill is shown in Figure 3-7.

Ore and water are fed into the cylinder from one hollow shaft and discharged from the other. As the cylinder rotates at a specified speed, the steel balls (or steel rods) are together with the ore, and under the action of centrifugal force and friction, they rise with the cylinder to a certain height, then detach from the cylinder wall and fall and slide down. They then rise to the same height again with the cylinder, and fall down again, repeating this cycle periodically, causing the ore to be crushed by impact and abrasion.

Figure 3-7 Schematic diagram of the grinding process in a ball mill The ground ore and water form a slurry (wet grinding), which is discharged from the hollow shaft at the discharge end, completing the grinding operation.