Crushing and Grinding
Crushing is the
first mechanical stage in the process of comminution in which the main
objective is the liberation of the valuable minerals from the gangue.
In the nature minerals are usually
intergrowth with other mineral species. In order to separate them, they must be
unlocked through comminution (crushing and grinding). The particle size of the
ore after its mining is progressively reduced until the free liberated
particles of mineral can be separated. Some comminution machines accomplish
their size reduction objective through slow compression, some through
high-velocity impacts, and others primarily through shear forces.
Classification
The following
classification of basic size-reduction steps can be used:
|
Size reduction step
|
Upper size
|
Lower size
|
|
Primary
crushing
|
1 m
|
100 mm
|
|
Secondary
crushing
|
100 mm
|
10 mm
|
|
Coarse grinding
|
10 mm
|
1 mm
|
|
Fine grinding
|
1 mm
|
100 μm
|
|
Very fine
grinding
|
100 μm
|
10 μm
|
|
Superfine
grinding
|
10 μm
|
1 μm
|
Circuits of Crushing
Crushing may be
in open or closed circuits, single stage or multi-stage, single pass or
multi-pass depending on product size.
Open-circuit
Crushing
In open-circuit
crushing, undersize material from the screen is combined with the crusher
product and is then routed to the next operation (undersize is a part of
finished product and finished product consists of a range of particle sizes)
Closed-circuit
Crushing
In
closed-circuit crushing, only undersize material from the screen is the
finished product. The oversize material will be re-circulated.
Choice of Crushing Equipment
The choice of
crushing equipment is influenced by
- Feed and product size (as mentioned
in above table)
- Nature of Material, e.g
- Hardness - very hard materials are better in low speed or low
contact machines
- Structure - fibrous materials need tearing or cutting action
- Moisture content - materials with 5 - 50% moisture do not flow
easily and can be difficult to process
- Stickiness - sticky materials need easily cleaned machines
- Soapiness - if coefficient of friction is low crushing may be difficult
- Explosives - need inert atmosphere
- Hazardous to health - need good confinement
- Closeness of distribution
Reduction Ratio
It is the ratio
of maximum particle size entering to maximum particle size leaving the crusher.
It is an indicator of extent of size reduction caused by particular equipment.
Mathematically,
Reduction Ratio
= Size of the largest lump in the
feed
Size of the
largest lump in the product
Particle Size and Shape
Exact size of an
irregular particle cannot be measured. For irregular particles it is desirable
to quote the size of a particle in terms of a single quantity and the
expression most often used is “equivalent diameter”.
This refers to
the diameter of a sphere that would behave in the same manner as the particle
when submitted to some specified operation.