The purpose of the reaction step is

The purpose of the reaction step is not only to extract the phosphate from the rock but also to ensure slow growth of gypsum crystals to a relatively large size. To attain this goal, reaction systems are designed to prevent direct contact between the two reactants, phosphate rock and sulfuric acid. A high concentration of free sulfuric acid would result in coating the phosphate rock with calcium sulfate reaction product, thus blocking further reaction.
A serious case of “reaction blocking” in a phosphoric acid plant can take several hours or even days to correct. On the other hand, a high concentration of calcium ions (low sulfate)in the slurry will increase the amount of phosphate cocrystallized with the gypsum. Hence, the aim of designers and operators of reaction systems is to maintain a uniform composition of the slurry, avoiding pockets of high sulfate or calcium concentration. The liquid phase usually consists of phosphoric acid (about 30% P2O5) with about 1,5% free sulfuric acid; the optimum concentration of free sulfuric acid varies with rock composition. The solid phase is mainly gypsum. The proportion of solids in the slurry is about 35%-45%. Phosphate rock particles introduced into this slurry dissolve rapidly in the phosphoric acid in the liquid phase, which causes supersaturation with calcium sulface and results in the growth of gypsum crystals.