Mineral Liberation

In order to separate the minerals from gangue (the waste minerals), it is necessary to crush and grind the rock to unlock, or liberate, valuable minerals so that they are partially or fully exposed. This process of size reduction is called comminution. The crushing and grinding process will produce a range of particles with varying degrees of liberation. Any particles that exceed a target size required for physical separation or chemical extraction are returned to the crushing or the grinding circuit.

Crushing and grinding

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.

What is mineral dressing

The first process most ores undergo after they leave the mine is mineral dressing(processing), also called ore preparation, milling, and ore dressing or ore beneficiation. Ore dressing is a process of mechanically separating the grains of ore minerals from the gangueminerals, to produce a concentrate (enriched portion) containing most of the ore minerals anda tailing (discard) containing the bulk of the gangue minerals.

Since most ore minerals are usually finely disseminated and intimately associated withgangue minerals, the various minerals must be broken apart (freed) or “liberated” before theycan be collected in separate products. Therefore, the first part in any ore dressing process willinvolve the crushing and grinding (which is also known by a common name called“comminution”) of the ore to a point where each mineral grain is practically free.