Process for preparing phosphoric acid

A process for producing wet process phosphoric acid, which comprises the steps of (a) contacting phosphate rock with aqueous phosphoric acid and a slurry comprising gypsum, phosphoric acid and sulphuric acid recycled from step (f) of the process, to produce a first reaction mixture, (b) contacting said first reaction mixture with a hemihydrate containing slurry obtained from step (e) of the process to give a second reaction mixture, (c) allowing the acids in said second mixture to digest the rock to substantial completion to produce a slurry comprising gypsum, phosphoric acid and sulphuric acid, (d) withdrawing a first fraction of said slurry as product which is processed as set forth in step (g) and a second fraction which is processed as set forth in step (e) and a third fraction comprising the remainder of said slurry, the weight ratio of the first fraction to the second fraction being in the range 1 :

• 0.75 to 1 :
• 3, (e) adding sulphuric acid to said second fraction of slurry and mixing under conditions of acid concentration, temperature and time such as to convert at least a part of the gypsum in said slurry into calcium sulphate hemihydrate crystals in the slurry, and recycling said slurry containing hemihydrate to step (b) of the process, (f) recycling said third fraction of the slurry from step (d) to step (a), and (g) filtering the product first fraction of slurry of step (d) to yield an aqueous solution of phosphoric acid as filtrate, the total amount of sulphuric acid present in step (c) being sufficient to digest the rock.