- 13 December 2006 -

Page 1 2
Next page: efficiency of operation

Tilting pan filters - the industrial work-horses of filtration

The tilting pan filter in phosphoric acid plants is employed to filter the digested phosphoric acid/gypsum slurry and recover the acid content. Dr MP Sukumaran Nair explains how the process works.

This rotary equipment is one of the early developments in the commercial manufacture of phosphoric acid and was developed by the Prayon Technologies SA, Belgium . Even today, this equipment serves as the heart of phosphoric acid plant and the economic and efficient operation depend heavily on it. Such filters have very high filtration yields are generally used to separate slurries which require large filtration surfaces.

The phosphoric acid process

In phosphoric acid manufacturing, mineral rock phosphate (Ca10(PO4}6 F2) containing 30-35% P2O5 is ground to fine powder and is reacted with sulphuric acid in a heavily agitated reactor called an attack tank. Phosphoric acid is liberated from the phosphates and under the design conditions of the plant a 30% acid solution is produced. The digested slurry containing 38-40% solids is pumped to the feed box of the filter which has a number of trays. The feed to the filter under atmospheric condition allow coarse solids to pre-coat the filter media so that the solids content of the filtrate is minimal and also helps to improve the cloth wash efficiency.

A pre-separation distributor valve separates the filtrates from different segments of the filter to maximize the strength of product acid, control the strength of acid recycled back to the reactor, and maximizes efficiency of the counter-current wash system. Filter vacuum is maintained around 200 mm Hg absolute using liquid ring vacuum pumps. The higher pressure drops across the filtration circuit helps to maintain a higher differential pressure across the filter cake, facilitating better acid recovery.

As the slurry in the feed tray moves forward, vacuum is applied from the bottom of the cloth laden filter trays and filtration takes place exactly as in the case of a Buchner filter.

The tilting pan filters has a number of horizontal independent trapezoidal trays mounted on a circular supporting structure. Tray design allows for the flooding of the surface of filter cake to create displacement washing to achieve maximum efficiency with the least quantity of wash liquor. The trays rotate under vacuum during the filtration cycle and invert under blow off to discharge the cake. During filtration, the cake is washed counter-currently with filtrate from successive stages. The filtration process is most often laid out in three stages. The feed and wash liquor quantities are specific to plant load factor. The mother and wash filtrates flow into separate compartments of the filter seal tank through a barometric leg. At the end of the drying portion of the cycle the cake is discharged into the cake hopper either in dry condition to a conveyor or slurried with water for wet disposal and sent to ponds.

After the cake is discharged, the trays pass over radial manifolds with high impact jets that clean the cloth and dislodge any cake leftovers on the tray surface. Before the tray is levelled to receive fresh slurry, any wash water accumulated inside the trays is evacuated by applying suction. This also helps to dry the cloth to avoid any dilution of the mother filtrate. During this stage as 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 vacuum pump is provided between the vacuum zones. A structure rotates over rollers, supports the pans and is fitted with a toothed rim that drives the filter.

Page 1 2
Next page: efficiency of operation