Market Survey on Copper - Indian Bureau of Mines

(a) Rod Mills: For many years the standard comminution circuits
comprised two-three stage crushing followed by open circuit rod mills and then
ball mills in closed circuit with classifiers. Now, many concentrators have
dispensed with rod mills and use fine size feed (10 mm) for ball milling only.
(b) Ball Mills: Crusher product from ball milling must be finer for those
concentrators who do not employ rod mills. A much greater amount of power is
used in ball mills than in crusher or rod mills because the reduction is much
greater. Power consumption is usually in 4-6 kW/h per tonne range, higher for
those mills that use ball mills for primary grinding and lower for those that have
rod mills.
(c) Autogenous Mills: This method of grinding is especially applicable
when the ore is wet and sticky or contains a lot of fines. Paradoxically, it
should also be considered when ore is tough & hard and blocky because the ore
serves well as the grinding media. When the ore varies greatly in hardness,
semi-autogenous grinding has been beneficial with capital and operating costs
both being 10-20% lower than that could be obtained with conventional
crushing and grinding. The ball charge in semi-autogenous mills varies from 6-
8% of mill volume. In recent years, secondary and tertiary crusher or secondary
crushers and rod mills have been substituted by autogenous mills which utilise
coarse and hard pieces of ore itself as grinding media, so that rod and ball
wear, a costly affair in conventional grinding is eliminated.
(iii) Sizing/Classification
An important step in grinding is to ensure that final discharge from the
grinding circuit is fine enough to respond to flotation. Coarser material must be
separated and returned to the grinding mill. Classification is usually done in
hydrocyclones or mechanical classifiers. Cyclones utilise centrifugal force to
accelerate setting rate of particles.
Presently, hydrocyclones have replaced mechanical classifiers as they
are more efficient especially in fine size separation. They require less space and
consume less power. Due to short residence time of particles in cyclones,
oxidation of copper sulphides is minimised. They are almost universally used
for classification between 150 and 5 microns, although coarse separation is also
possible. The pulp (ground material) from grinding mills is pumped at high
speed (5-10 m/second) into hydrocyclone through an opening tangential to the
cylindrical wall so that pulp is given a rotational motion inside the cyclone.
This results in a centrifugal force which accelerates the settling rate of ore
particles. Faster settling particles move towards the wall of the cyclone where
the velocity is lowest and migrate to the apex opening. Pressure drop across the
cyclone and feed flow rate are the two most important operating variables in
cyclone operation.
32