VIBRATING SCREENS
This article is meant to be used with and as a backup to the
information found in the SDM Screen spreadsheet found at (../software/default.html)
and Sizing Vibratory screens at (
Sizing Vibratory Screens)
Vibrating screens are primarily classification devices, used
for separating a feed stream into two or more products of different
size ranges. They are
also used for dewatering clean coal and refuse products from
cleaning processes, and for media recover following dense medium
separation. If they are
used to remove excess of water and fines from a feed stream prior to
cleaning they are often referred to as desliming screens.
The general nomenclature and definition of screens whether
they are used for classification or dewatering is essentially the
same.
Clean coal and refuse from a wet cleaning device is usually
accompanied by large volumes of water which must be removed from the
coal. At the same time,
the coal is often separated into various size ranges for subsequent
crushing or further dewatering action.
Coarse coal may be sized and dewatered on the same screen.
Coarse coal to be dewatered can either be presized before
cleaning or can contain all the fines.
If it is presized, with the fine coal removed it can be
dewatered at openings ranging from 0.5 mm to 6 mm, depending upon
the market requirements of the coal, unless it is necessary to
remove the fine coal caused by degradation in the process.
Fine coal is not usually sized since the main purpose of the
screen is to retain the coal solids while removing water.
Dewatering screen selection is based on handling a bed depth
thin enough to be free-draining.
The depth of coal is a function of the size of coal being
dewatered. The smaller the
average particle size, the more difficult it is to drain and the
thinner the bed must be.
The presence of fine coal particles tends to fill up the
voids and hold the water.
Dewatering screens are usually 3.5 M (12 ft.) or longer
depending upon the dewatering efficiency required and the size
distribution of the coal being dewatered.
Bed depth is normally considered to be no more than three
times the largest particle size as a maximum.
When dewatering fine coal (12 mm or 6 mm x 0), sizing is not
important. The purpose
of the screen is to retain the coal solids while removing the water.
The openings in the screen surface are usually small (0.25 mm
to 0.5 mm) and it is necessary to provide sufficient screen area to
pass the water. In
order to obtain satisfactory screen surface life, practice is to use
profile wire screen surface.
Because of the tendency of fine coal to pack, stratify, or
form a blanket or cake, better dewatering can be obtained if the bed
is periodically disturbed.
In order to mix up the bed of coal, cross-dams are usually
used, which forces the coal to climb over the dam and makes the bed
more porous permitting the free drainage of water.
Many operators use spray water to break-up the bed of coal or
a combination of dams and sprays may be used.
If there is a large amount of free water with the feed coal,
a stationary sieve or sieve bend ahead of the screen may be used to
reduce the water. The
capacity of fine coal dewatering screens is influenced by the amount
of water with the feed.
If the amount of water is too great, the high entrance velocity will
cause the coal to flush-down the deck reducing the available area
for the watering. Under
these conditions, the surface moisture of the dewatered product
will be high and under extreme conditions, free water will be
discharged with the coal.
In order to prevent this, the amount of water admitted to the
feed must be limited.
The use of dense medium in cleaning coal is based on the difference
in specific gravity between the coal and its impurities.
A suspension of finely ground media (usually magnetite) and
water with the specific gravity maintained at a point between the
specific gravity of the coal and the refuse is used.
Media is expensive and it is desirable to recover the maximum
amount possible and to recirculate it inside the circuit.
Following the dense medium process, the clean coal and refuse
products are handled separately to remove the water and recover the
media.
A media recovery screen has two sections, a drain and a rinse
section. The first
portion of the screen drains the media from the stream and returns
it back to the system.
The second portion washes and then dewaters the coal, or refuse.
Drain and rinse screens commonly have two compartments
underneath them; the first portion the drain, and the second portion
the rinse product. The
drain portion returns directly to the dense medium circuitry, often
times through a control system to help maintain the gravity.
The rinse portion goes to dilute media for media recovery.
In order to perform these operations, long screens, often 5 M
(16 ft.) or longer, are usually selected.
The drain section is usually the first 1 to 1.5 M (3 to 4
ft.) at the feed end of the screen.
Following the drain section, material is washed using spray
water and the media recovered.
Media recovery screens are selected on the same basis as
others, using the depth of bed that can be successfully drained and
rinsed. In general the
finer the coal, the more difficult to drain and the thinner the bed
must be.
To reduce the moisture in a fine refuse product, recovery
screens are often used to partially dewater the material before
disposal. This water is
then reused or discarded, as necessary.
Equipment Sizing
Raw Coal/Desliming Screens
Raw and desliming screens are sized on the basis of product
through the screen deck or the overflow discharged over the screen
deck. Spray or rinse
water is added for maximum efficiency in desliming. The underflow
should at least be 10% solids.
Dense medium Drain and Rinse Screens
Screen capacity for drain and rinse screens is specified in
regard to feed capacity, based on average grain size.
Rinse water is added in two locations, one immediately
following the drain section, and the second half way along the rinse
section. Rinse water
normally comes from two sources; partly fresh and partly as
recirculated water from the dilute media recovery circuit.
At finer average grain sizes the rinse water should be
increased. For minimum
magnetite loss higher flow rates of fresh water are preferred.
A general excepted factor of 2% degradation in particle size
in a dense medium vessel circuit is excepted.
Clean Coal Dewatering Screens
Clean coal dewatering screens are sized in a similar manner
to raw coal and dense medium drain and rinse screens.
Sufficient spray water should be added to bring the underflow
concentration up to 10% by weight solids.
Refuse Dewatering
Refuse dewatering screens will be sized similar to dense
medium cyclone drain and rinse screens.
Calculations
for other vibrating screen sizing can be done using the SDM Screen
spreadsheet found at (http://www.smartdogmining.com/tools/Software/default.html)
Performance
Screen performance is very intimately related to sizing and has been
covered in the previous section.
Screen efficiency for raw coal is commonly taken at 80 to 85%
of the near size material.
Following desliming stage overall efficiency should be on the
order of 95%. The
moisture content of the deslimed coal will be in the range of 16 to
20% by weight. A value
of 15% could be used to be optimistic with a 18% being a good plant
design factor.
A general accepted factor of 2% degradation in particle size in a
dense medium vessel circuit is used.
Media carry-over from drain to rinse section can be 2-5% of
media fed to screen.
Experience has shown that coal can have a degradation of 6%
and the refuse can have up to 2% degradation.
Media carry-over can reach 5-10%.
In as much as the refuse and fine clays are removed the
product surface, moisture will be lower than from raw coal
screening. It is
commonly expected that + 38 mm (1-1/2") dewatered clean coal will
have a moisture content of 3 to 5% by weight.
38 mm by 12 mm
(1 1/2" by 1/2") coal will have a surface moisture of 5 to 7%, and
12 mm x 6 mm (1/2" by 1/4") coal with surface moisture of 15%.
Calculations
for other screen performance for alternative configurations can be
done using the SDM Screen spreadsheet found at (http://www.smartdogmining.com/tools/Software/default.html)
