Ball mill works on impact principle that is size reduction is done by the impact. It contains a hollow cylinder shell that rotates around its axis and is partially filled with the balls. The ball mill may be operated in a batch or continuous fashion and wet or dry.
In case of continuously operated ball mill, the material to be ground is fed from the left through a 60 degree cone and the product is discharged through a 30 degree cone to the right. As the shell rotates, the balls are lifted up on the rising side of the shell and then they cascade down from near to the top of the shell. In doing so, the solid particles in between the balls are ground and reduced in size by impact.
The ball mill contains the balls of various sizes. As the shell rotates, the large balls segregate near the feed end and small balls segregate near the end product that is going to be discharge. The initial breaking of the feed particles, therefore, is done by the largest balls dropping the largest distance and the small particles are ground by small balls dropping from much smaller distances. If the rate of the feed is increased, coarser product will be obtain and if the speed of the rotation is increased (that should be less than critical speed), the fineness for a given capacity increases.
During grinding the balls themselves wear and are constantly replaced by new ones so that the ball mill contains balls of various ages and of various sizes. In case of batch operated ball mills, a measured quantity of solid to be ground is charged into the ball mill through the opening in the shell. The opening is then closed and the ball mill is rotated for several hours. It is then stopped and the product is discharged.
In case of wet grinding, there is low power consumption, increased capacity, reduction in the formation of fines, facilitates the removal of product and no dust formation.
Advantages of Ball Mill
- The cost of installation is low.
- The cost of power required is low.
- It is suitable for materials of all degree of hardness.
- It is suitable for batch as well as continues operations.
- The grinding medium is cheap.