Filtration section of phosphoric acid plant

Filtration section:

Reactor slurry overflowing from the central compartment of reactor goes to filter feed tank, which has been provided with an agitator to keep the slurry in uniform suspension. This tank has a capacity equivalent to half an hour production to facilitate running of the filter when the reactor is down for small shutdowns. A filter feed pump takes suction from filter feed tank and pumps the slurry to belt filter.

The filter provided is of belt type having a belt and filter cloth, which moves in parallel. The belt and filter cloth make an endless loop. When at the top they receive slurry and facilitate filtration and washing of the cake. At one end finally the cake is discharged the belt and filter cloth get separated from where they move separately but in parallel and finally return to receive slurry again. During this return period the belt and filter cloth are washed thoroughly to remove clogged gypsum. The belt provides support and facilitates drainage of collected acid and wash liquors. The filter cloth retains the gypsum and allows the acid and wash liquors. The filter cloth retains the gypsum and allows the acid and wash liquors to pass through.

The filter is provided with a mother liquor distribution box and three wash liquor boxes to facilitate counter current washing. The filter is provided with variable speed drive to increase or decrease the speed of the filter to optimize load and efficiency as required. Filtration is carried out under vacuum for which a liquid ring type vacuum pump is provided to create necessary vacuum. Separated acid and various wash liquors by special arrangement are kept separately and are taken out through seal legs and collected in filtrate seal tanks from where they are pumped top different destinations. The vacuum zone of the filter is divided into different sections viz., cloudy section, mother liquor section first wash second and third wash section. After the third wash, once the cake is dewatered the vacuum is broken and cake is discharged.

The slurry from the tank is fed into the filter through slurry feed box. This distributes the slurry uniformly on the filter. The cake deposits on the filter and acid is pulled down by application of vacuum. The very first fraction of the acid because the cloth is highly porous initially as the cake has not formed completely will contain more solids and is also relatively dilute. Accordingly, this is kept separate and through cloudy ports in the filter box goes to filtrate seal tank. Most of the remaining acid goes to the mother liquor section from where it is drained to filtrate seal tank. As the acid is received, it is continuously pumped to the storage tank by the vertical submersible pump.

By the time the cake passes the mother liquor section on filter most of the acid is drained. However the cake retains enough acid that needs to be removed completely for obvious economic reasons. This is achieved by employing washing in counter current fashion in three stages. Immediately after the mother liquor section the washed filtrate, this contains about 8 to 12% P2O5. As the liquor passes through the cake it picks up some more acid retained in the cake and gets concentrated to 15 to 18% P2O5 acid. This is collected in the seal tank which is then mixed with acid coming from cloudy port. This is then pumped by pump and goes to reactor acid. This return acid supplies the necessary water and product acid required to maintain the desired concentration of acid and solids ratio in the reactor slurry.

After the first wash, the cake is further washed, which contains about 5 to 8% P2O5 and is collected in the filtrate seal tank. The cake is given one more wash with P2O5 free water coming from the wash water tank to remove the acid in the cake to the minimum possible. As the liquor passes through the cake the same gets concentrated from 5 to 8% P2O5 and is collected as filtrate, which is used for a second wash. By the time the cake passes third wash zone, the P2O5 content in cake is less than 1% with free moisture of about 25%.

At the end of the filter, the cake is discharged onto a moving belt conveyor. This conveyor picks up the gypsum and transports the same to gypsum yard where it is handled separately. The belt and filter cloth get separated, they are washed with water to remove gypsum crystals embedded within the fabrics of the filter cloth. The belt is also cleaned off gypsum. Both during return run dewater and will be ready to receive the slurry again.

A tank is provided to receive various waters from the plant. These include bleed offs from fumes scrubber filter condenser sump, seal waters from filter as well as evaporator vacuum pumps. This water is conserved and no water is wasted. Any additional water requirement is met through utility header. A level based controller opens / closes the valve on utility water header, and maintains the required wash temperature, which is required for effective washing of the cake. A temperature based controller opens or closes the valve on the low pressure steam header.

In the application of vacuum for required filtration not only the acid / wash liquors, but also air is pulled through the cake from various places. As this vapor liquid mixture comes down through barometric leg the liquid portion goes down where as vapor is pulled sideward. However, since vapor can contain some acid / water droplets the mixture passes through the vapor liquid separator. This is a kind of centrifugal separator where the mixture is entered tangentially. The spinning action caused by tangential entry and high velocity coalesce the droplets and separate the liquid portion. The vapors containing mainly air and some fluorine bearing compounds like silicon tetra fluoride combine form separates and passes through an acid trap and then to a filter condenser. Any droplets and mist that has escaped in primary vapor liquid separates are further separated in acid trap before clean vapors go to condenser. The collected liquid containing acid is routed to filtrate from where it is taken into the process. Water is circulated in the filter condenser to absorb fluorine compounds. Gases are also simultaneously cooled as water cascades down through disc and doughnut place assembly. Clean gases then go to vacuum pump, which maintain the necessary vacuum in the entire circuit. In the vacuum pumps the pressure is boosted and air is let out which finally goes to fumes scrubber for further cleaning. A bleed off is taken from circulating water of filter condenser and sent to wash water tank and fresh water is made up to ensure that vapors are condensed and scrubbed effectively. The acid fromĀ  filtrate pump directly goes to any of the two weak acid storage tanks where it is used for subsequent concentration.