Phosphoric Acid → Calcium Sulfate, Hemihydrate

Participants

Reaction Steps

Also, the present inventors have previously proposed in Japanese Patent Sho 45-10408 (U.S. Pat. No. 3,653,826) a process for the filtration and hydration of the hemihydrate in which calcium sulfate hemihydrate is formed during a digestion step and filtered off, then calcium sulfate dihydrate of good quality is obtained by hydration during a hydration step and, finally, highly concentrated phosphoric acid is recovered. In this process, easily filterable agglomerate crystals of calcium sulfate hemihydrate can be obtained in the following manner: Phosphate rock is digested under specified conditions, i.e., it is first mixed with phosphoric acid, and then sulfuric acid is added into the mixture, whereby 70-80% of the CaO existing in the phosphate rock combines with sulfur to form calcium sulfate hemihydrate and the other 20-30% of the CaO is transformed into calcium dihydrogen phosphate [Ca(H2PO4)2 ]. Then more sulfuric acid is added to the mixture, in order to turn the calcium phosphate into calcium sulfate hemihydrate and phosphoric acid under conditions such that 2-3% of free sulfuric acid will be present in the reaction vessel. The present inventors found that agglomerate crystals of calcium sulfate hemihydrate having good filtrability can be obtained by recycling a part of the slurry formed into a digestion system. Thus, a method using hemihydrate filtration and hydration has become industrially practical.

Provenance & Evidence