The large percentage of the phosphoric acid in fertilizer
phosphates is produced from sulfuric acid and phosphate rock.
This phosphoric acid, known as "wet-process" acid, is
then used in the manufacture of triple superphosphate, ammonium
phosphates and other high analysis fertilizer products.
The primary chemical reaction in the production of
wet process phosphoric acid is between the calcium phosphate constituent
of the phosphate rock and sulfuric
acid to form phosphoric acid and calcium sulfate. The principal
difference in the processes used throughout the world is the degree
of hydration of the calcium sulfate, which can be varied by changing
the temperature and P2O5
concentration in the reaction. The most popular process which
is used by the Florida producers is known as the dihydrate process
in which gypsum is precipitated.
Phosphoric acid production by the wet process requires
three main operations: reaction between the phosphate rock and
sulfuric acid, separation of the gypsum from the phosphoric acid
and the concentration of the phosphoric acid to the desired P2O5 level. The production
units include large reaction vessels where finely ground phosphate
rock (dry or wet slurried) is first treated with weak phosphoric
acid and then reacted with approximately 55% sulfuric acid. Additional
process efficiencies and proper acid concentrations. The calcium
sulfate is then separated by vacuum filtration and washed for
optimum recoveries of phosphoric acid. Various concentrations
of phosphoric acid are produced from the filtration operation
with the weak portions being recirculated for proper process controls
and the end product containing usually 28 - 32% P2O5. In the final step, the filtration product
is concentrated under vacuum and with heat to the desired P2O5 level, depending
upon the specific end use, with the highest concentration normally
being 52-54% P2O5.
This concentration is also used for acid shipped for fertilizer