Process for reducing formation of gypsum scale durng the washing of calcium sulphate hemihydrate crystals

In a process for producing phosphoric acid wherein phosphate rock is acidulated with a mineral acid and crystals of calcium sulphate hemihydrate are formed, separated off from the phosphoric acid and thereafter washed, the improvement which comprises washing the calcium sulphate hemihydrate crystals with an aqueous wash liquor which prior to contact with the crystals, contains P.sub.2 O.sub.5 in amounts less than 30% and H.sub.2 SO.sub.4 in amounts of less than 10 parts by weight per 100 parts by weight of H.sub.3 PO.sub.4 present in the aqueous wash liquor and contains from 0.0002 to 1.0% by weight of an additive selected from the group consisting of compounds of the empirical general formula:

• R.sup.1 R.sup.2 S(O).sub.n I
• wherein n has a value of 3 or 4 and when n is 3 R.sup.2 is hydrogen and R.sup.1 is an alkyl or aralkyl group and when n is 4 R.sup.1 is an alkyl or aralkyl group and R.sup.2 is hydrogen or an R.sup.1 group;
• alkylene oxide condensates of an alcohol, phenol or an alkyl phenol and mixtures and salts, esters and amides of said compounds whereby the deposition of calcium sulphate dihydrate scale due to the recrystallisation of the calcium sulphate hemihydrate to calcium sulfate dihydrate is reduced;
• the total contact time between the calcium sulphate hemihydrate crystals and the wash liquor during washing being less than 10 minutes.