Chemical Process Industries

1. PHOSPHORIC ACID BY SULPHURIC ACID
Prepared by- Mr. Vishal B. Thakare
(M.Tech, Chemical SVNIT Surat)
Assistant Professor PARUL UNIVERSITY
Vadodara
vishalbt88@gmail.com

2. PHOSPHORIC ACID
Wet Process Using Strong Sulfuric Acid
 The wet process is according to the acids (sulfuric acid, nitric acid
or hydrochloric acid) used to decompose the phosphate rock.
 The process using sulfuric acid is the most common among all
particularly for producing fertilizer grade phosphoric acid.
Raw materials
Basis: 1000kg phosphoric acid (100%)
Phosphate rock (32% P2O5) = 1635kg
Sulfuric acid(93-98%) = 1360kg

3. Reaction
Ca3(PO4)2 + 3H2SO4 + 6H2O 2H3PO4 + 3(CaSO4.2H2O)
Phosphoric acid
Side Reaction
CaF2+H2SO4 + 2H2O 2HF+CaSO4.2H2O
6HF + SiO2 H2SiF6 + 2H2O

5. Phosphate rock is ground to 65 % —200 mesh and fed to a chute
where a recycle stream of weak H3PO4 washes it into a reaction tank.
Strong H2SO4, is metered with automatic control which keeps the
acid and rock feed ratio at the desired setting.
A single reactor can be designed by proper baffling and residence
time capacitance to permit a 98 % conversion in 4-6 hours.
In other designs, a series of 4-5 continuous mixing tanks can be used
with slightly better efficiency of extraction by minimizing back
mixing.
Heat of reaction is controlled by circulating cooling air across.

6. The gypsum-phosphoric acid slurry goes to a travelling pan vacuum
filter where the 40 % acid is removed and the cake washed with
water. Filtrate from the latter is returned to the reactor to increase
conversion.
The gypsum can either be dried for use in plaster, paints and
cements, or reacted with ammonium carbonates to give ammonium
sulfate.
Water used for gypsum wash is passes into lagoon for the treatment
of water.
The dilute acid is concentrated in a single effect evaporator to any
grades > 50% acid.