Formation du sulfate de calcium hémihydrate de type α à partir de gypse par un procédé de dissolution-cristallisation : étude cinétique expérimentale et modélisation

Abstract : In mineral or pharmaceutical industry, many transformations of solids in solution (polymorphic or not), take place by the dissolution of the first solid and the crystallization of the second solid. Instead of the traditional drying process, the conversion of gypsum (calcium sulfate dihydrate) to alpha-bassanite (calcium sulphate hemihydrate) can be carried out in aqueous solution by increasing the temperature sufficiently. At this moment, the gypsum becomes more soluble in water than the hemihydrate, which then has the possibility to crystallize. An apparatus and an original procedure have been designed to investigate the sensitivity of the kinetics of transformation and the characteristics of the hemihydrate crystals obtained under its operating conditions. In fact, this dissolution-recrystallization route allows to control the aspect ratio of the hemihydrate crystals, and even their average size and size dispersion, by the choice of its physicochemical conditions such as temperature, pH, use of additives, and seeding the solution with the appropriate hemihydrate particles. Image analysis techniques are developed to identify the aspect ratio of the particles.A kinetic model taking into account the dissolution of gypsum, the nucleation and the growth of calcium sulphate hemihydrate and their equilibrium in aqueous phase and solubility had been developed and the equations of this model are solved with the help of MATLAB software. Coupled with a numerical optimization method, this resolution under MATLAB makes it possible to examine the validity of the assumptions on the selected processes and to identify their kinetic parameters.
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Yi Rong. Formation du sulfate de calcium hémihydrate de type α à partir de gypse par un procédé de dissolution-cristallisation : étude cinétique expérimentale et modélisation. Génie des procédés. PSL Research University; École nationale supérieure des Mines d'Albi-Carmaux, 2018. Français. ⟨NNT : 2018PSLEM070⟩. ⟨tel-02138372⟩

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