Ion exchange extraction during continuous recrystallization of CaSO4 in the phosphoric acid production process: lanthanide extraction efficiency and CaSO4 particle shape

In the HemiDiHydrate (HDH) phosphoric acid production processes, calcium sulfate hemihydrate (HH) is formed from the reaction of phosphate ore with sulfuric acid. In a second step, the HH crystals are recrystallized to calcium sulfate dihydrate (DH, gypsum). With the phosphate ore, impurities enter the production processes. Some of these impurities, such as the lanthanides, mainly end up in the DH crystals. Some impurities, such as aluminum fluoride and lanthanides, not only cause contamination of the DH crystals, but also retard the crystallization and influence the HH and DH particle shapes. By adding ion exchange resin during the continuously operated recrystallization, lanthanides are removed resulting in cleaner DH and phosphoric acid. The simultaneous ion exchange extraction accelerates the recrystallization and can lead to differences in DH particle shape.