The Tilting Pan Filter

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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.

Pan Filter

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.

Filter cycle
 
Feed Box
Pan-Filtering
Pan-Discharging
Cake Hopper
Main Valve
Vacuum Hoses
Walkway
Pan Bearings
Outer Support Rim
Inner Support Rim
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Cross section

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 2 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:


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: