Process for preparation of purified calcium sulfate in an aqueous medium

Abstract

A process for the preparation in an aqueous medium of purified calcium sulfate, with high whiteness, characterized in that it comprises: a) the formation of an aqueous solution by dissolution of impure hemihydrated calcium sulfate, at a concentration at most of 13.0 grams per liter (expressed in dissolved CaSO.sub.4) and at a pH of at least 5.5, b) the separation of the aqueous solution containing the dissolved calcium sulfate from the insoluble solid phase formed by the impurities to be removed, c) the recrystallization of the purified calcium sulfate in dihydrated form from the aqueous solution resulting from step (b), possibly in the presence of a seeding primer placed in the reaction medium, d) the separation after recrystallization of the aqueous phase poor in dissolved calcium sulfate from the solid phase composed of the recrystallized purified calcium sulfate, e) the recycling of the aqueous phase poor in dissolved calcium sulfate back to step (a) for dissolving impure calcium sulfate, yields calcium sulfate having at will the appearance of polymorphous or longilinear monomorphous crystalline structures. The purified crystalline structures obtained in accordance with the process are intended more particularly for fields of use as varied as cements and plasters and, more especially, for those of paper, paint and plastics.

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