Abstract
In the HemiDiHydrate (HDH) phosphoric acid production processes, calcium sulfate hemihydrate (HH) is formed from the reaction of fluoroapatite ore with sulfuric acid, after which it is recrystallized into calcium sulfate dihydrate (DH, gypsum). Some impurities from the feedstock are incorporated in the HH during the first reaction and finally end up mainly in the sparingly soluble gypsum. By introducing ion exchange resin during the phosphoric acid-mediated recrystallization, lanthanides that are dissolved from the HH can be removed from the acid before they become incorporated again in the gypsum. With 250 g resin/kg phosphoric acid slurry, lanthanides were removed to up to 53% of the input. The resulting decrease of the lanthanide concentration in the gypsum is lower, up to 34%. The extraction efficiency is influenced by the recrystallization reaction. Lanthanides that are most readily taken up by the gypsum are extracted least. Although the sulfonic acid resin has a preference for the trivalent lanthanides, calcium ions are also extracted.
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