PHOSPHORIC ACID

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 usages.