The Tilting Pan Filter
Description
The Tilting Pan
Filter belongs to the top feed group and has found its main
application in the phosphoric acid industry and, to some extent, in
the washing of phosphate rock. As already mentioned in the
Horizontal Belt Filters
section there was a traditional rivalry over the years between the
Pan and Belt Filter which now, owing to the substantial progress in
rubber technology, swings in the favor of the later.
|
 |
The
operation of Tilting Pan Filters is based on a series of
horizontal independent trapezoidal pans mounted on a
circular supporting structure that rotates under vacuum
during the filtration cycle and invert under blow to
discharge the cake. During filtration the cake may be
washed counter-currently, generally in three stages,
while the mother and wash filtrates are flowing to
separate vacuum receivers. After the final drying
portion of the cycle the cake is discharged dry for
piling or sluiced as a slurry for disposal to ponds.
Following cake discharge the pans pass over radial
manifolds with high impact jets that clean the cloth and
dislodge any cake leftovers that remained on the pan's
surface.
|
|
At the last
stage before relevelling, suction is applied to evacuate the
wash water that accumulates inside the pan, and at the same
time it dries the cloth to avoid dilution of the mother
filtrate. Since during this stage air passes freely through
the exposed cloth, applying suction with the main vacuum
pump will cause a loss of vacuum to the entire system,
hence, an auxiliary small vacuum pump is incorporated to
separate between the vacuum zones. |
 |
|
|
The main valve,
that is also called distributor, controls the cycle segments
with bridges that open and close the port of each pan as it
passes from high vacuum through cake blow-off to low vacuum.
Compartments within the valve separate the various zones to
ensure that the separation is sharp and no mixing of mother,
strong, middle and weak filtrates occurs during the cycle.
|
To speed up the
evacuation of the various filtrates there are valves with two bridge
circles, one for the main stream and the another that opens to
atmosphere and purges the pan below the deck just before entering a
new zone.
The speed of rotation
determines the cycle time and this, being an important design
parameter, is limited mechanically by the mass inertia of the
swinging pan and its wet cake load at the point of discharge.
Typical cycle times for Pan Filters are 2½ to 3 minutes depending on
the size of the filter and the design of the peripheral cam that
controls the tilting velocity.
Pan Filters are
available for dry or wet cake discharge depending on the method of
disposal. The discharge zone for dry discharge consists of two
chutes, one for the dry cake and the second for cloth washing. The
zone for wet discharge has one chute only in which the cake is
sluiced and then the cloth is washed. Dry cake discharge consumes
about 3-4% more effective filtration area than the wet discharge so
this should be taken in account while calculating the required area.
The Tilting Pan Filter
is constructed from the following subassemblies:
-
Trapezoidal pans
with sloping bottoms for fast evacuation of filtrates and each
fitted with a roller arm assembly that tilts and relevels the
pan as it passes trough the inverting track. Special
circumferential fasteners enable the quick replacement of filter
cloths.
-
A central valve with
bridge setting to control the various zones throughout the
filtration cycle, and in some instances, two bridge circles with
the second serving to purge the pan. The valve has internal
compartments for the mother and wash filtrates.
-
A feed and wash
boxes that may be set in a slight slope so that the distribution
over the trapezoidal surface will be proportional to the pan's
area. This will ensure that the formed cake is even in thickness
and that the applied wash liquids are distributed evenly.
-
A structure that
rotates over rollers, supports the pans and is fitted with a
toothed rim that drives the filter.
Selection Criteria
As mentioned in an
earlier discussion the Pan Filters have been set aside once the
rubber technology enabled the manufacture
Horizontal Belt Filters
with large filtration areas. Presently there are no outstanding
features to prefer a Tilting Pan Filter over a Belt Filter although
many of them are in extensive use.
Operational Sequence
The cycle of a Tilting
Pan Filter that includes three counter-current washing stages
consists of the following zones:
Maintenance
The major parts
requiring inspection for preventive maintenance are:
-
The pans for
corrosion even if fabricated from UB6, Sanicaro 28 or 914L for
the harsh phosphoric acid conditions.
-
The wear plate of
the main valve for erosion since its large diameter makes it
vulnerable to loss of vacuum.
-
The twisting rubber
hoses or "O" ringed elbows which connect the rotating pan necks
with its pipe and wear plate to the face of the stationary
valve.
-
The condition of the
high impact nozzles mounted on the cloth wash manifold and, for
wet cake discharge, the sluicing manifold.
-
The support rollers
which take the vertical load of of the entire machine and the
horizontal thrust rolls that keep the rotating frame concentric.
-
The toothed rim and
sprocket which drives the Pan Filter.
Back |
