Production of calcium sulphate alpha-hemihydrate from citrogypsum in unheated sulphuric acid solution

Alpha-hemihydrate of calcium sulphate can be obtained by suspending gypsum in boiling or almost boiling aqueous solution of an inorganic acid salts or sulphuric acid at atmospheric pressure. Our task was to try to obtain it from citrogypsum in unheated solution of this acid. The experiment proved it possible if 10, 15, 20 30, or 40 wt.% solution was used, and that the resulting alpha-hemihydrate developed at various ratios and had different properties, depending on the acid solution concentration and the gypsum/acid solution mixing ratio. An a[pha-hemihydrate composed of smooth, compact, acicular monocrystals, which according to reference data promises the best utilitarian aspect. was obtained using 20 wt.% acid solution at 0.25 g cm(-3) mixing ratio.