In-line removal of impurities by ion exchange during a recrystallization step in the phosphoric acid production process

During the production of phosphoric acid by the HemiDiHydrate process, calcium sulfate dihydrate (DH, gypsum) is formed as a byproduct, which is mainly disposed. Some impurities from the feedstock, like lanthanides and some heavy metals, mainly end up in the gypsum. During the solvent mediated recrystallization of calcium sulfate hemihydrate (HH) to gypsum, an existing step within the HemiDiHydrate production process, the impurities might be removed from the slurry by ion exchange. However, conventional set-ups, like packed beds, cannot be used for the recrystallizing slurry with 200-400 g solids/kg slurry. Lanthanides can be recovered during the recrystallization of calcium sulfate hemihydrate to calcium sulfate dihydrate in industrial recrystallization acid by ion exchange in stirred vessels as well as in the developed transverse flow ion exchange module. However, due to the relatively small amounts of ion exchange resin per kg of slurry (50 g resin/(kg slurry) for the stirred vessels, and 20 g resin/(kg slurry) for the module), the extraction efficiencies are low (15% for the stirred vessel, and 4% for the module). It should be possible to increase the extraction efficiency by increasing the amount of ion exchange resin per kg of slurry.