The Tilting Pan Filter

The Tilting Pan Filter is predominantly employed in the phosphoric acid industry and, to some extent, in the washing of phosphate rock. 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 countercurrently, 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, which is called the 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 establishes 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 2.5 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 machine 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.

The maintenance of these machines can be labor intensive. Major components that require frequent inspection for preventive maintenance include:

• Evacuation of leftover cloth during relevelling

• The pans for corrosion

• The wear plate of the main valve for erosion since its large diameter makes it vulnerable to a loss of vacuum

• The twisting rubber hoses or "o" ringed elbows which connect the rotating pan necks with pipe and the 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 the entire machine and the horizontal thrust rolls that maintain the rotating frame concentric

• The toothed rim and sprocket which drives the pan filter

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