Phosphoric acid manufacture reaction unit

From the grinding unit the feed is sent to reaction units

Reaction section:

Phosphate rock is subjected to react with sulfuric acid react in this section resulting out the precipitate of calcium sulfate dihydrate CaSO4. 2H2O and liquid phosphate acid.

Various equipments in this section are

  • A reactor
  • Agitators to maintain homogeneity
  • Cooling air fan, connecting ducts, and air nozzles to supply the necessary air for cooling the slurry.
  • Off gas duct leading to fume scrubber followed by exhaust fans and stack.
  • An overflow founder to transport the slurry to filter feed tank.

The reactor is a single tank reactor with central and annulus compartments and the total volume is divided into five processing zones. The annulus compartment is provided with two agitators and the central compartment is provided with one agitator. The central compartment is connected to the outer compartment through an under the port.

The agitator blades are designed such that in addition to ensure a high level of axial mixing, they also provide large circulation of slurry within the annulus compartment. This ensures constant and proper control of free, sulfate content in the slurry which is to be maintained at an optimum level to get gypsum crystals of desired shape and size. In the annulus, the ground is fed at first agitator. A high powered agitator is provided to ensure complete dissolution of rock phosphate into the slurry.

A surface baffle which is nothing but an SS plate dipping form top into the slurry is provided immediately after first agitator. This prevents rock going directly together processing zones. The reaction zone in the annulus is further extended to give sufficient time for complete reaction of rock and growth of the crystals. About 12 hours residence time is provided in the reactor to ensure large sized crystals.

Sulfate level, which is vital to obtain the large sized crystals are almost constant throughout the annulus because of the large back mixing flow produced by the agitators and the “flywheel” effect obtained from there circulating slurry. Reactor temperature is maintained at about 78oC to promote formation of dehydrate crystals and rapid rock digestion.

To maintain this temperature a cooling air fan supplies the necessary air, which then passes through 4 numbers of nozzles provided. The fan is provided with variable speed drive by which fan speed can be increased or decreased to maintain the reactor slurry temperature at the desired level. The slurry form annulus flows to central compartment through under the port.

The central compartment gives about one hour residence time for the slurry. This helps in desupersaturating the slurry and promotes the growth of the gypsum crystal for better filtration. As the level builds up the equivalent slurry outflow through an overflow launder which starts from central compartment and up to filter feed tank.

At the mouth of the overflow launder a gate is provided to control the level of the slurry in the reactor. By opening, closing this gate the slurry level can be dropped or increased so that an optimum gap is maintained between slurry surface and cooling air nozzle. With this arrangement effective cooling of slurry can be achieved with minimum airflow.

The off gases from reactor which contains small amounts of fluorine vapors are routed to fume scrubber for effective scrubbing of the same before the air can be let out to the atmosphere. These off gases consists of those that released during the reaction (mainly CO and some amount of fluorine vapors in the form of  SiF) and the air supplied with cooling air fan, which picks up water from the slurry by evaporative cooling.

The off gas duct is provided with impingement baffles immediately above the reactor for cleaning particulate matter and for capturing fine droplets carried because of the entrainment.

The off gases from reactor which contains small amounts of fluorine vapors are routed to fume scrubber for effective scrubbing of the same before the air can be let out to the atmosphere. These off gases consists of those that released during the reaction (mainly CO2 and some amount of fluorine vapors in the form of  SiF4) and the air supplied with cooling air fan, which picks up water from the slurry by evaporative cooling.

The off gas duct is provided with impingement baffles immediately above the reactor for cleaning particulate matter and for capturing fine droplets carried because of the entrainment.

Partially cleaned gases then go to fume scrubber where the velocity of the gases is reduced and a large number of spray nozzles are provided. These sprays provided large surface area and effectively scrub the all fluorine bearing compounds.

The fume scrubber also cleans the gases coming from various ventilation units of the plant like filter hood, seal tank area and vacuum pump area. After spray section the gases go to packed section where final scrubbing is effected to ensure the last traces of fluorine compounds are removed.

The water required for scrubbing is supplied by a circulating pump. This water is kept in closed circuit with continuous bleed off and make up water facilities. As an exhaust fan provides necessary driving force for the removal of off gases from reactor and negative pressure in reactor filter areas avoids pollution. The clean gases are finally let out through the stack.