Wet process or from strong Sulfuric acid :
Raw materials :
- 1 ton phosphoric acid (100%)
- Phosphate rock → 1635 kg
- Sulfuric acid → 1360 kg
Ca10F2(PO4)6 + 10H2SO4 + 20H2O → 10CaSO4.2H2O + 2HF + 6H3PO4
- The grinded phosphate rock is reacted with dilute phosphoric acid to produce melt, which in a reactor is mixed with concentrated sulfuric acid for 4 to 8 hrs in the temperature range of 75-80ºC.
- Excess air is required to control the temperature.
- Resulting gases includes HF and P2O5 which in the absorption tower is separated and finally treated to give fluorosilicates and dilute phosphoric acid.
- The main product in the liquid form which is phosphoric acid and calcium sulfate is filtered and washed.
- Thus, gypsum and phosphoric acid are separated and after minor purification the phosphoric acid is concentrated into the evaporator.
- Throughout the plant, corrosion resistant materials of construction must be used.
- The most common ones are structural carbon or nickel alloy for evaporator heat exchangers; rubber or carbon-brick for reactor linings; polyester-fiber glass in pipes, ducts, and small vessels.
- Yield of phosphoric acid based on phosphorus content of raw material is 95%