Crushing and grinding are usually carried out in a sequence of operations by which thelump size is reduced step by step. There are 3 stages of crushing and 2 stages of grinding.
i. Primary Crushing (coarse crushing): In primary crushing, ore or run-of-mine ore (up to 1 m in size) is crushed down to about 10 cm and it is done in a jaw or gyratory crusher.
ii. Secondary Crushing (intermediate crushing): In this case, ore is crushed from 10 cm to less than 1 – 2 cm size; for this purpose jaw, cone or roll crushers are used. These secondary crushers consume more power than primary crushers.
iii. Tertiary Crushing (fine crushing): By tertiary crushers ore is crushed from 1 – 2 cm to less than 0.5 cm. Short head cone crushers, roll crushers, hammer mills can be used for this purpose.
The two stages of grinding are:
i. Coarse Grinding: Rod mills are generally used as coarse grinding machines.They are capable of taking feed as large as 50 mm and making a product as
fine as 300 microns.
ii. Fine Grinding: Fine grinding, which is the final stage of comminution, is performed in ball mills using steel balls as the grinding medium. The ball mill, after feeding 0.5 mm material may give a product that is less than 100 microns.
Grinding is usually done wet.