Calcium sulphate hydrates formation in aqueous sulpho-phosphoric media containing rare earth impurity beyond 80 °C

•Gypsum transforms into hemihydrate at 99 °C in aqueous sulpho-phosphoric solutions.•Transformation depends on the concentration of the solution.•It is partial when CH3PO4 is lower than 0.03 M, and total beyond 1 M.•Gadolinium in sulphuric solutions favours the transformation but inhibits it in sulpho-phosphoric media.•The bassanite-like phase formed at 99 °C in sulpho–phosphoric media with CH3PO4 ≥ 0.1 M contains syncristallized HPO42−.

Powder X-ray diffraction and infrared spectroscopy monitoring, show that the calcium sulphate dihydrate transforms into hemihydrate in aqueous solution at about 99 °C. The transformation yield depends on the composition of the aqueous solution. It increases with the concentration of phosphoric acid in solution. The transformation becomes almost total with a large excess of phosphoric acid.Gadolinium at concentration (10−4 M), in aqueous solutions containing calcium nitrate (0.08 M) and sulphuric acid (CH2SO4 = 0.09 M) favours the formation of the hemihydrates. This process is accentuated as the lanthanide concentration increases. However this impurity seems to inhibit the under hydrated calcium sulphate formation in aqueous solution containing sulphuric and phosphoric acids (CH2SO4 = 0.09 M – CH3PO4 = 0.01 M).In the sulpho−phosphoric media (CH2SO4 = 0.09 M, 0.1 M ≤ CH3PO4 ≤ 0.3 M), the infrared spectra of the solid phase exhibit the band 845 cm−1 attributed to ions HPO42−. With more concentrated phosphoric acid (CH3PO4 = 1 M), IR spectroscopy reveals all bands characterizing the hemihydrate, as well as the band situated at 875 cm−1 relative to phosphate ions syncrystallized.The dehydration of dihydrate into hemihydrate would proceed mainly by dissolution – re crystallization mechanism.