The Table Filter
Description
The Table Filters belong
to the top feed group, introduced in the early 40's and were rather
small and of a simple design. Their main limitation was at the
discharge zone since the cake was contained in a fixed rim and
special sealing arrangements had to be provided in order to avoid
the spillage of brine at the table's circumference. Another problem
was that the thin heel left between the scroll and the surface of
the table was dislodged by applying a back blow but not removed from
the surface of the passing cell. So, as it reached the feed zone it
was mixed with the incoming slurry without the cloth being washed.
This has caused progressive media blinding which effected filtration
rate and required frequent stoppage of the operation for cloth
washing.
As the demand for
more area and higher throughputs has gone up in the 60's the Table
Filters and Tilting
Pan Filters were redesigned and
upgraded since the rubber technology was unable to catch-up with the
filtration area requirements of
Horizontal Belt Filters.
A typical flowscheme for
a counter-current washing may be seen on the left. The flowscheme
consists of three washing stages and is a conventional design for
phosphoric acid plants. The filter in the flowscheme is a table
filter however the same diagram applies also to Tilting Pan Filters
and Horizontal Belt Filters.
In the old design the
rim that contained the cake was fixed to the main structure and had
an opening just at the discharge point to allow cake removal. In
this design a thin heel was left continuously on the filtering
surface, could not be removed and caused loss of filtration rate due
to blinding. Likewise owing to the friction between the moving cake
and the stationary rim the drive was subjected to high power
consumption.
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To solve the
problems encountered with fixed rim a radically new
engineering concept was developed in the late 60's. The new
table filter features a rubber belt rim that rotates
together with the cells but is removed from the table just
after leaving the final wash and drying zone and before
reaching the cake discharge hopper. The rim then moves away
from the table, the cake discharges and the heel is removed
by high impact jets to a cloth wash water to a separate
chute. It then continuous to a cloth drying zone for such
applications that the dilution of mother liquor is undesired
and returns to the slurry feed section. |
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Special
rollers are diverting the rim at the cake discharge zone
and a tension take-up roll ensures that the belt stays
tight against the table and seals the circumference
against leakages.
The
filtration area of large Table Filters is more than 200
m2 and since there are few moving parts, contrary to Pan
Filters, the table can rotate at a cycle time of 1½
minutes. |
These filters can handle
thick cakes and may be operated at high vacuum levels. The major
subassemblies of the Table Filter are:
-
A series of fixed
trapezoidal cells that form a rotating table and each connected
to a stationary valve in the center of the filter. The cell is
designed with steep sloped bottom for fast evacuation of the
filtrate.
-
A valve that may be
raised from the top and has a bridge setting and compartments to
control the various zones.
-
An internal rim
fixed to the table at the inner circumference and a continuous
rubber belt that surrounds the table at the periphery and
confine the slurry, wash liquids and the cake during the
filtration cycle.
-
Rollers that support
the vertical loads, centering thrust and others that move the
rim away from the table in the discharge zone and maintain it
under tension.
-
Radial rubber dams
that separate between the feed, wash stages, cake discharge and
cloth wash, and cloth drying zones to prevent the mixing of
filtrates.
-
A variable pitch
screw the transports the cake radially towards the point of
discharge.
Selection Criteria
The criteria for
selecting a Table Filter are:
-
When the process
downstream requires a de-lumped cake since the screw
disintegrates the solid lumps while conveying them to the
periphery.
-
When the
solids are fast settling and cannot be kept as a homogenous
slurry in bottom or side feed filters such as
Drum
or Disc Filters.
-
When very short
cycle times are required for fast dewatering cakes such as
phosphate slurry.
-
When a clear
filtrate is required right from the start it is good practice to
form a thin heel that serves as a filter medium over the exposed
cloth. This is done by either a "cloudy port outlet" that is
recirculated or, if solids are settling fast, by allocating a
portion of the table after the cloth drying dam and prior to
entering the vacuum zone to act as a "sedimentation pool".
-
When intensive cake
washing is required.
-
When a large
filtration area is required but a
Horizontal Belt Filter
does not fit into the layout.
-
When cakes tend to
crack under vacuum measures such as a flapper or pressure roll
may assist in sealing the cracks thus avoiding loss of vacuum.
Operational Sequence
The cycle of a Table
Filter that includes three counter-current washing stages consists
of the following zones:
Maintenance
When compared to
Tilting Pan Filters
the Table Filter has less moving parts so the major components to be
checked as part of the preventive maintenance schedule are:
-
The clearance
between the cake discharge screw and the cloth surface. If, for
some reason, this clearance becomes smaller the friction and
shear forces during the radial transport of the cake towards the
discharge hopper may cause cloth wear.
-
The condition of the
rubber belt that surrounds the table at its periphery and serves
to contain the feed slurry, wash liquid and cake.
-
The table support
rollers, centering thrust rollers and the rollers in the
discharge zone.
-
The valve and wear
plate faces for tight sealing against vacuum leaks.
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