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)

Size Reduction 1

•It’s the process that reduces large solid particles to smaller particles by mechanical means.

Factors affecting size reduction
1.Hardness .
2.Toughness.
3.Stickiness.
4.Abrasiveness.
5.Temperature.
6.Material Structure .
7.Moisture Content .
8.Physiological Effect .
9.Purity Required .
10.Ratio of Feed Size to Product Size

SIZE REDUCTION METHODS

There are many different types of size reduction method employed.
1.Cutting methods
•The material is cut by means of a sharp blade or blades.
2. Compression methods:
•The material is crushed by application of pressure.
3. Impact methods:
•Occurs when the material is stationary and is hit by an object moving at high speed or when the moving particles strikes a stationary surface. The materials shatter to smaller pieces.
4.Attrition methods: The material is subjected to pressure as in compression but the surfaces are moving relative to each other, resulting in shear forces which break the particles. 5. Combined impact and attrition methods
•A ball mill , fluid energy mills and pin mills are examples of a comminuting method which produces size reduction by both impact and attrition of particles.

Equipments
•All milling or Comminution equipment have three basic components:
1. A structure for feeding material to the mill.
2. The milling chamber and its working parts.
3. A receiver or collector in which the milled product is deposited.