Abstract: Disclosed is an improved process for purifying phosphoric acid where a calcium sulfate hemihydrate slurry is first filtered to produce a first filtrate and a filter cake which comprises calcium sulfate hemihydrate, the calcium sulfate hemihydrate filter cake is slurried to produce a slurry containing less than 2% by weight free sulfate ion, which is hydrated in a hydration tank to produce a calcium sulfate dihydrate slurry, and the calcium sulfate dihydrate slurry is filtered on a second filter to produce a second filtrate and a gypsum filter cake. The improvement comprises recycling the second filtrate to the second filter in an amount (1) sufficient to reduce the flow rate of the calcium dihydrate slurry from the hydration tank to the second filter so that the residence time of the slurry in the hydration tank is at least two hours and (2) at least sufficient to maintain liquid on the second filter. Also disclosed is an apparatus for performing this process.