FILTRATION MACHINE- Batch and Semicontinuous Filters
In
beneficiation plant, Continuous filters tend to be more widely used in the
mineral and coal processing field, particularly where large tonnages are
involved. This preference reflects the lower capacities of batch or
semicontinuous units and the increased labor requirements, both of which result
in higher operating costs. However, at low-tonnage plants and under special
conditions, these filters can have distinct advantages. Also, where pressure
drops must be used that are higher than those obtained by continuous vacuum
filters (because of the low cake permeability), the pressure filter may be
applicable. For instance, tailings may be filtered to recover water or to
dewater them to a high enough solids concentration to allow land disposal. The
mineral and coal processing industry use batch and semicontinuous filters of
four types: plate and frame filters, recessed plate filters, vertical disk
pressure filters with or without sluice discharge, and automatic discharge
plate and frame-type filters. All four employ pressure filtration.
Plate
and Frame Filter.
The plate
and frame filter uses a plate that has a grooved or other type of drainage
system supporting the filter cloth. Both sides of the plate have grooved
patterns so that filtration occurs on either side. The frames will contain the
feed and filter cake and seal against the plate. Usually there are connecting
ports through the four corners of both plates and frames so that either feed or
filtrates can be accommodated. The feed may enter through one or more of the
corners where the ducts are accommodated into the frame interior but not into
the plate. The plates contain ducts in other corners for conducting the
filtrate. The filter cloths are draped over the plates and holes of the ports
in the corners are matched. A seal is made by the filter cloth between the
plates and the frames, although slurry can leak if the cloth is wrinkled or if
a piece of cake sticks between the plate and the frame. This possibility can
usually be prevented by using a gasketed plate.
Figure 1
shows a typical plate and frame unit and also shows the hydraulic closing
system. One head is stationary and the other can be moved by a double-acting
hydraulic cylinder. Thus, relatively high hydraulic pressures are employed to
close the press so that normal operating pressures up to 125 to 250 psig can be
obtained.
Also
shown in Figure 1 is a shifter mechanism, which discharges cake by mechanical
movement of the plates and frames. An operator should make sure that the cake
discharges and that pieces do not hang up on the sealing surfaces; such pieces
could cause a very leaky joint or even break the plate or frame on closure. The
operator usually has a wooden paddle to take care of these instances, and as
soon as the paddle breaks a photocell path, the mechanical operation stops.
After the operator has
corrected
the situation, he restarts operation by pressing a button that moves with him.
In
smaller filters, a hand crank can be used in place of the hydraulic cylinder at
a lower cost. The plates and frames can also be moved manually, but doing so
usually requires two operators.
The
filter cakes can be washed by using the feed lines for introducing wash fluid.
Because the cake is being washed from a point source, the fluid tends to follow
shorter paths to the filtrate side and the wash is not as effective. The wash
is more efficient if every other plate is a washing plate. The wash fluid
enters this plate behind the filter media on both sides and passes through the
cloth to the opposite filter media and plate. Thus, a more consistent short
flow path is obtained across the cake area.
Many
different filter media can be used, ranging from canvas to synthetic woven
fibers to nonwoven synthetics. To obtain very clear filtrates, special papers
are also employed either as the sole media or over a backing cloth.
Plates
and frames were formerly made of wood, cast iron or other metals, and
rubber-covered steel. Currently, thermoplastics, such as polyethylene and
polypropylene, have largely supplanted the earlier materials. These newer
materials not only reduce costs but also greatly reduce weight. They are used
at the normal operating pressures of 125–250 psig.
Frames
generally are 1–2 in. deep depending on the specific cake permeability. Plates are usually
square and are 12–48 in. on each side. The frame depth should be carefully determined,
because the plate and frame filter work best when the frame is entirely full of
cake at the end of the filtration cycle. If it is not, the frame may contain
too much fluid and produce a high-moisture cake. In addition, a thicker final
cake may result in an appreciably lower filtration rate in terms of pounds of
dry solids per hour from the press.
The feed
pump may be either a centrifugal pump or a positive displacement pump,
depending on the type of solids in the feed and the filtration pressures. If
final filtration pressures permit and the solids or flocculi are not injured by
the pump, single-stage or multistage centrifugal pumps can be used.
At higher
pressures or where the solids or flocculi are fragile, the positive
displacement type pump is used. Controls should be used to prevent excessive
pressures that could injure the filter
press.
A common
auxiliary is an air or gas compressor that blows the cake at the end of the
filtration cycle for further dewatering. Where tailings or refuse are filtered,
the solids must be flocculated because they contain large amounts of colloids.
Flocculation requires a mix tank preceding the feed pump and complementary
equipment to prepare and dilute flocculant. Bench-scale investigations will be
required to determine the type and dosage of flocculant, the mixing power, and
the duration of mixing. Because flocculi generally deteriorate with time, the
length of time that flocculated pulp is stored will be important, as the
filtration rate is not constant at all times with batch equipment.
Recessed
Plate Pressure Filter.
A similar
type of press but one that eliminates the frame is the recessed plate pressure
filter. The plate has a center feed and all feed enters through this port. The
filter cloth must be sewn or a fixture employed to seal the cloth on both sides
of the plate at the feed port. In addition, the plate is recessed to allow for
cake buildup. This recess is usually 1/2–l in. deep, yielding a cake thickness
of 1–2 in. Filtrate is collected at any one or more of the four corners;
filtrate ports are cast in the plates as with the plate and frame filter. Cake
washing, if necessary, is best practiced by using every other plate as a
washing plate with wash fluid entering at the top behind the filter media on
both sides of the plate. The wash fluid passes more evenly through the cake to
the opposite plate, where the filtrate is collected at the bottom corners.
The usual
plate sizes vary from 12-in. square to as much as 6 × 9 ft. As many as 175
plates may be incorporated into one press; the maximum plate size yields a
filtration area of 18,585 ft2. Plates are available
in a wide range of materials but molded plastic dominates, particularly in the
large sizes. Auxiliary equipment is similar to that discussed under plate and
frame filters. However, with the larger units, the feed ports may be doubled to
achieve the proper hydraulics and feed distribution. In addition, the press is
constructed so that the feed ports may be blown out by compressed air through the
follower end (a movable closure head); the compressed air removes the higher
moisture core and also further dewaters the cake. Mechanical plate shifters are
also employed on recessed plate filters. Because of the large size of these
filters, large plate shifters that move as many as 12 plates at a time for cake
discharge can be used. This device reduces the time for cake discharge to a very
few minutes and increases the overall filtration rate.
Figure 2
is a picture of a typical recessed plate with a gasketed construction that
eliminates leakage and reduces filter-media wear.
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