Grinding ore is usually done in a grinder. There are many types of grinding machines, but in metal mines, ball mills and rod mills are generally used. A ball mill is a rotating cylinder with hollow shafts at both ends, which contains a considerable number of steel bars and balls.

The grinding principle of a ball mill is that when ore and water are fed into a cylinder from one hollow shaft and discharged from the other hollow shaft. When the cylinder of the ball mill rotates at the specified speed, the steel ball and steel rod are together with the ore. Under the action of centrifugal force and friction, they rise to a certain height with the cylinder, and then detach from the cylinder wall for throwing and sliding down. Then they rise to the same height with the cylinder, fall down again, and move periodically, causing the ore to be crushed by impact and grinding. The ground ore and water form a slurry (wet grinding), which is discharged by a hollow shaft at the discharge end to complete the grinding operation.

So the speed of the ball mill directly determines the movement status of the steel balls inside the cylinder and the effectiveness of the grinding operation.

When the speed is low, the ball in the ball mill is lifted to a smaller height and slides down from the top of the ball load due to the gravity of the ball itself, forming a falling state. At this time, the impact force of the ball is small, but the grinding effect is strong. The ore is mainly ground and crushed, and the grinding effect is not high.

When the rotation speed is high, the height at which the ball is lifted is also high. After the ball rises to a certain height and detaches from the cylinder, it falls along a parabolic trajectory and is in a throwing state. At the throwing point, there is a significant impact effect, and the ore is mainly crushed by the impact, resulting in the highest grinding efficiency.

When the speed of the ball mill exceeds a certain limit (high speed), the ball rotates with the cylinder and does not fall, entering a centrifugal operation state. At this point, the ball has no impact effect and the grinding effect is also very small, causing the grinding effect to stop.