Size reduction 2

Equipments of size reduction :

1-Ball mill

It is an example of a comminuting equipment which produces size reduction by both impact and attrition methods.

Types of ball mill

A. batch

B. continuous

C. continuous closed – circuit milling.

A. Batch type
The ball mill is a cylindrical shell usually filled to about half its volume with grinding media (ball) in different sizes. The balls may be of metal or porcelain. The large balls crush the feed and smaller ones form the fine product. The shell is mounted in such a way that it can be rotated on its horizontal longitudinal axis by means of motor-driven rollers.

B. continuous ball milling
Harding Mill It is a conical shell with grinding media of different sizes Feed enter from the hallow end of the mill and milled material exits through coarse screen. The outlet is covered with a coarse screen to prevent the loss of the balls.

C. continuous closed-circuit ball milling.
It is arranged using the continuous mill with a gas or liquid classifying system.
Air and feed enters through the opening at end of the mill.
The classifier removes the smallest particles while the larger particles are recycled to the mill inlet for further size reduction.

Speed of Rotation
1. At low speeds
The mass of balls will slide or roll over each other and only a negligible amount of size reduction will occur.
2. At high speed (centrifugation )
At and the above the critical speed the mill is said to be centrifuging and grinding of material not occur.
3. At about two-thirds of the critical speed (Optimum speed)