Effect of ionic impurities on the crystallization of gypsum in wet-processphosphoric acid

The dihydrate phosphoric acid production process was simulated in a bench-s cale glass reactor. The influences of selected ionic contaminants (Na+, K+, Mg2+, Al3+, F-, and Fe3+) on the crystallization of the gypsum byproduct w ere investigated. Except for fluorine, the presence of the impurities aided gypsum precipitation, with iron and potassium showing the greatest effect. At cation impurity levels of ca. 0.1%, filter cake resistance was lowest, but filtration rates decreased at higher impurity levels. Magnesium and iro n were largely retained in the acid, whereas sodium and fluorine were large ly removed during the precipitation of gypsum. Sodium, magnesium, iron, and aluminum led to increased coprecipitation of phosphate, whereas fluorine a nd potassium had no effect.