Equipment , Process and Crushing
Equipment , Process and Crushing
Crushing Plants
Stationary crushing plants
In the cement industry, which uses chiefly
medium-hard to hard limestone as its principal raw materials, single-stage
crushing plants equipped with hammer crushers are the commonly preferred type.
Feed Hoppers
The feed hopper, feeding equipment, crusher and
product removal conveyors are the main component units of the plant. The feed
hopper should have a capacity equal to at least twice that of the largest
vehicles supplying rock to the crusher in batches of 20 to 50 tons or more .
Caking of
moist and sticky feed material can be minimized by using a well designed
hopper, with rounded transitions from the end walls to the side walls. If the
hopper is of concrete, it should be lined with steel plates or, preferably, with
steel rails, which give much better protection against wear.
Robustly
constructed apron conveyors have proved most suitable for feeding. They fulfill
all requirements applicable to a feeding system in order to obtain optimum
utilization of the crusher: control of the handling rate within a certain
range, controllability in response to the loading condition of the crusher,
feed over the full working width of the crusher, ability to start under load,
feed can be stopped instantly.
The hopper can be equipped with a hydraulically operated arm with tools for stone splitting, a grab or a scraping arrangement as appropriate, to match the materials it has to handle.
Feeders
The feeder
supplies the crusher with an even flow of material from the feed hopper for optimum
utilization of the crusher, irrespective of irregularities in the supply of
material to the feed hopper.
In some cases, cone crusher and jaw crushers can work without feeders if they are placed directly under the feed hopper. Most other types of crushers need a feeder for securing a regular rate of feed.
Apron Feeders
The apron
feeder is widely used for coarse materials, i.e. primary crushing of rocky
materials, an inclined apron feeder with 20° inclination is normally used, as this will automatically
assure a suitable, stable layer on the apron.
The apron
feeder is furnished with driving station with variable speed.
Gyratory
crushers, which are used as first-stage machines when very hard and coarse feed
material has to be reduced, can receive the material direct from the truck,
without the interposition of a feed hopper. As the preliminary crusher delivers
a product in the 300 - 500 min size range, the second-stage crusher can
function under less severe operating conditions than if the size reduction had
to be performed all in one stage.
While gyratory crushers can often be employed also as second-stage machines, high-speed machines such as impact crushers or hammer crushers are more advantageous for obtaining a finer product.
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Preliminary Screening
Separation
of the finer particles from the raw stone before it is fed to the crusher is
not standard practice in the cement industry. In exceptional cases, however,
the material may first be passed over a grizzly or a reciprocating grid screen.
Preliminary separation of the coarser from the finer material can serve to
relieve the crusher or to improve the quality of the raw material by raising
the concentration of certain desirable constituents. As a rule, it makes for
better performance of the crusher, too.
Protection against foreign bodies
In the
quarrying and loading of raw materials it inevitably occurs that metallic
foreign bodies - excavator bucket teeth, broken drill rods, drill bits, pieces
of rail, chains, etc. - turn up in the feed material supplied to the primary
crusher. If the crusher is fed direct by excavators or dump trucks.
As a rule,
foreign bodies can be more effectively removed from the material after it has
been pre-crushed (first-stage crushing). Magnetic separators and metal
detectors are used for the purpose and help more particularly to protect the
high-speed second-stage crushers.
For dealing
with material containing a substantial amount of tramp iron, belt-type
suspended magnetic separators may be used. A device of this kind is equipped
with a continuous rubber belt which carries the pieces of iron out of the
magnetic field, so that the magnet pole face itself remains clear.
Favourable mounting positions are the points of feed onto, or discharge from, the conveyor, because at these points the material is loosened up and the extraction of tramp iron thus made easier.
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