Abstract
The transformation of separate crystals of CaSO4·2H2O (DH) into CaSO4·O.5H2O (HH) in an aqueous solution of H3PO4 and H2SO4 at 90–97°C was studied using a JSM-35CF SEM. The source DH and acids were reagent or commercial (taken from the production of H3PO4 by a wet process). DH was allowed to convert into HH and periodic samples of the suspension were drawn, filtered, and investigated in the solid phase by SEM. It was shown that the dehydration of reagent DH specimens occurred through the recrystallization mechanism, whereas the transformation of commercial DH into HH was basically topochemical and developed by a relay race mechanism without any phase transformation front. The observed difference between commercial and reagent DH specimens may be explained by differences in their crystalline structure, initial particle size and by the presence of impurities in DH and acids. It is established that chemical impurities can change the mechanism of the phase transformation of the crystals DH into HH.
Published Version
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