A method for preparing calcium sulphate comprising a production of DCP by the attack of a source of phosphate by an acid, a digestion of the isolated DCP by the sulphuric acid under conditions giving rise to the formation of a first slurry of gypsum suspended in an acidic aqueous phase having a content of free SO.sub.3 equal to or less than 1.5% and a content of free P.sub.2O.sub.5, a conversion of at least part of said first slurry by heating to a temperature greater than 80 C. and potentially by adding sulphuric acid, with solubilisation of the gypsum crystals and recrystallisation of the solubilised calcium sulphate in a second slurry of -calcium sulphate hemihydrate crystals suspended in an aqueous phase based-on phosphoric acid, wherein the content of free SO.sub.3 is less than 10% by weight, and a separation between said aqueous phase and a filter cake based on particularly pure -calcium sulphate hemihydrate.

A source of phosphate for agriculture and food industry comprises a phosphate salt in solid form of formula M.sub.n(HPO4)y.zH2O in which M is Na, K, NH4, n=2, and y=1; or M is Ca, n=1, y=1; or M is Al or Fe, n=2, y=3; and in which z is 0, 1 or 2, wherein said phosphate salt has a phosphate content expressed as a P2O5 content of between 30 and 50% by weight of the phosphate salt, and it has a cadmium content lower than 0.40 ppm.

A method for preparing calcium sulphate comprising a production of DCP by the attack of a source of phosphate by an acid, a digestion of the isolated DCP by the sulphuric acid under conditions giving rise to the formation of a first slurry of gypsum suspended in an acidic aqueous phase having a content of free SO.sub.3 equal to or less than 1.5% and a content of free P.sub.2O.sub.5, a conversion of at least part of said first slurry by heating to a temperature greater than 80 C. and potentially by adding sulphuric acid, with solubilisation of the gypsum crystals and recrystallisation of the solubilised calcium sulphate in a second slurry of -calcium sulphate hemihydrate crystals suspended in an aqueous phase based-on phosphoric acid, wherein the content of free SO.sub.3 is less than 10% by weight, and a separation between said aqueous phase and a filter cake based on particularly pure -calcium sulphate hemihydrate.

The present invention relates to a composition containing an anionic surfactant based on an alkali salt of sulfosuccinic acid dialkylester to promote the obtention of an inverse microemulsion which is used as foam control composition in a phosphoric acid production medium. The present invention also discloses a process to produce said composition by stirring the components of such a composition to obtain a microemulsion. The invention also aims at a method for controlling foam during the production of phosphoric acid by wet process, avoiding its formation or decreasing its amount.