How does a Double Roll Crusher works - Mineral Processing
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Description:
Roll crushers, or crushing rolls, are
still used in some mills, although they have been
replaced in most installations by cone crushers.
They still have a useful application in handling
friable, sticky, frozen, and less abrasive feeds, such
as limestone, coal, chalk, gypsum, phosphate, and
soft iron ores. Jaw and gyratory crushers have a
tendency to choke near the discharge when crushing
friable rock with a large proportion of maximum
size pieces in the feed.
The mode of operation of roll crushers
is extremely simple, the standard spring rolls
consisting of two horizontal cylinders
which revolve towards each other. The set is
determined by shims which cause the spring-loaded
roll to be held back from the solidly mounted roll.
Crushing rolls Unlike jaw and gyratory crushers, where reduction
is progressive by repeated pressure as the material
passes down to the discharge point, the crushing
process in rolls is one of single pressure.
Roll crushers are also manufactured with only
one rotating cylinder, which revolves towards a
fixed plate. Other roll crushers use three, four, or
six cylinders. In some crushers the diameters and
speeds of the rolls may differ. The rolls may be
gear driven, but this limits the distance adjustment
between the rolls; and modern rolls are driven by
V-belts from separate motors.
Multi-roll machines may use rolls in pairs or in
sets of three. Machines with more than two rolls
are, however, rare in modern mills. The great disadvantage
of roll crushers is that, in order for reasonable
reduction ratios to be achieved, very large rolls
are required in relation to the size of the feed particles.
They therefore have the highest capital cost
of all crushers.
Peripheral
speeds vary between about 1 ms -1 for small
rolls, up to about 15 ms -1 for the largest sizes of
1800 mm diameter upwards.
Smooth-surfaced rolls are usually used for
fine crushing, whereas coarse crushing is often
performed in rolls having corrugated surfaces, or
with stub teeth arranged to present a chequered
surface pattern. "Sledging" or "slugger" rolls have
a series of intermeshing teeth, or slugs, protruding
from the roll surfaces. These dig into
the rock so that the action is a combination of
compression and ripping, and large pieces in relation
to the roll diameter can be handled. Their
main application is in the coarse crushing of soft or
sticky iron ores, friable limestone, coal, etc., rolls
of 1 rn diameter being used to crush material of top
size 400 mm.
Wear on the roll surfaces is very high and they
often have a manganese steel tyre, which can be
replaced when wom. The feed must be spread
uniformly over the whole width of the rolls in order
to give even wear. One simple method is to use a
flat feed belt of the same width as the rolls.
Since there is no provision for the swelling of
broken ore in the crushing chamber, roll crushers
must be "starvation fed" if they are to be prevented
from choking. Although the floating roll should
only yield to an uncrushable body, choked crushing
causes so much pressure that the springs are continually
"on the work" during crushing, and some
oversize escapes. Rolls should therefore be used in
closed circuit with screens. Choked crushing also
causes interparticle comminution, which leads to
the production of material finer than the set of the
crusher.
For full working theory of roll crusher read the book:
Mineral Processing Technology
An Introduction to the Practical Aspects of Ore Treatment and Mineral
Recovery, by Barry A. Wills, Tim Napier-Munn
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