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AbstractAbstract
[en] The method used by Comurhex for the treatment of fluorine-rich effluents relies on the addition of solid Ca(OH)2 (portlandite) to precipitate fluorite (CaF2) in strongly alkaline solutions. This method is effective in a settler, and the development of a new fluidized bed process has been proposed to increase its efficiency. Successful assays were conducted on a pilot scale and they demonstrated that precipitation in a fluidized bed reactor actually ensures a satisfactory defluorination. However, the phenomena involved remained poorly known, and the main goal of the present work was to elucidate the mechanisms, their kinetics, and to control the crystallization. Our first contribution is a thermodynamic reappraisal of the KOH-portlandite-calcite system that extends existing solubility data to higher KOH concentrations in solution and incorporates new solubility data for fluorite. The outcome is an aqueous speciation model (Pitzer type) accounting for the solids solubility as a function of the KOH concentration and the temperature, and allowing portlandite under-saturation and fluorine supersaturation to be evaluated in process solutions. Our second contribution focuses on the kinetics and the mechanisms involved during the formation of fluorite, using various mixing designs in batch reactors. Reaction rates are fast, except in experiments involving dilute reactants. Microprobe and granulometric data both indicate that much of the fluorine results from the conversion of the portlandite grains. A shrinking core model controlled by diffusive transfer of fluoride in the porous fluorite corona successfully fits the kinetics of fluorine uptake and the particle grain size. The last part of the experimental work was carried out on a small fluidized bed pilot. These experiments were poorly conclusive. The very small particle size and the large flow rates lead to particle blow off, but not to fluidization. As a consequence, the capacity of the fluidized bed to promote an increase of the particle grain size remains largely unknown. (author)
[fr]
Le procede adopte par Comurhex pour le traitement des effluents tres charges en fluor est base sur l'addition d'un compose alcalin Ca(OH)2 (portlandite) pour faire precipiter de la fluorine (CaF2). Mis en oeuvre par le passe dans un decanteur le developpement d'un nouveau procede en lit fluidise a ete envisage pour augmenter l'efficacite du traitement des effluents. Des essais concluants ont ete realises sur une installation pilote qui ont demontre l'efficacite de la precipitation en lit fluidise a assurer une reduction satisfaisante de la concentration du fluor. Cependant, les phenomenes mis en jeu dans le reacteur sont mal connus. Notre objectif principal est la maitrise de la precipitation de la fluorine dans le reacteur a lit fluidise ainsi qu'une meilleure comprehension des mecanismes mis en jeu. Pour ce faire, il a ete necessaire de reprendre l'etude thermodynamique du systeme KOH-portlandite-calcite-fluorine, de maniere a etendre les donnees existantes aux fortes concentrations de KOH en solution. Un modele thermodynamique capable de predire la solubilite des solides en presence de variations de la concentration de la potasse et de la temperature du systeme a ete etabli. La deuxieme partie du travail a ete focalisee sur la comprehension des mecanismes mis en jeu pendant la formation de la fluorine. Les experiences realisees au laboratoire ont conduit a l'obtention d'une fluorine issue d'une conversion de la portlandite. Ce mecanisme a ete modelise en considerant que le remplacement de la portlandite par la fluorine s'explique par un modele a coeur retrecissant controle par la limitation diffusionnelle du fluorure dans la couche de fluorine formee. La derniere partie experimentale de ce travail a ete realisee sur le pilote a lit fluidise. Les experiences effectuees n'ont pas donne de resultats satisfaisants. Il n'a pas ete possible de faire faire fonctionner le pilote de facon coherente avec les objectifs de son dimensionnement. En particulier, compte-tenu de la tres petite taille des particules, les debits de fonctionnement conduisent a l'envolement de celles-ci, mais pas a l'etablissement de l'etat fluidise. (auteur)Original Title
Etude de la precipitation de la fluorine en milieu alcalin et application a un procede en reacteur a lit fluidise
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Source
14 Oct 2013; 253 p; 64 refs.; Available from the INIS Liaison Officer for France, see the INIS website for current contact and E-mail addresses; These Docteur de l'Ecole Nationale Superieure des Mines de Saint-Etienne Specialite: Genie des Procedes
Record Type
Miscellaneous
Literature Type
Thesis/Dissertation
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Country of publication
ALKALI METAL COMPOUNDS, ALKALINE EARTH METAL COMPOUNDS, CALCIUM COMPOUNDS, CALCIUM HALIDES, ELEMENTS, FLUORIDES, FLUORINE COMPOUNDS, FUEL DISPERSION REACTORS, FUNCTIONAL MODELS, HALIDE MINERALS, HALIDES, HALOGEN COMPOUNDS, HALOGENS, HOMOGENEOUS REACTORS, HYDROGEN COMPOUNDS, HYDROXIDES, MATHEMATICAL MODELS, MINERALS, NONMETALS, OXYGEN COMPOUNDS, PARTICLE MODELS, POTASSIUM COMPOUNDS, REACTORS, SEPARATION PROCESSES, STATISTICAL MODELS
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