Modélisation des équilibres thermodynamiques impliquant le fer dans la cryolithe lors de l'électrolyse de l'aluminium à l'aide d'anodes inertes

RÉSUMÉ Depuis la fin du 20e siècle, l’industrie de l’aluminium s’intéresse sérieusement au développement d’anodes dites inertes pour remplacer les anodes de carbone dans le procédé Hall-Héroult. La ferrite de nickel (NiFe2O4) est une candidate prometteuse pour le remplacement des anodes traditionnelles en carbone. L’usage de telles anodes peut toutefois engendrer la présence de fer et de nickel dans le bain cryolithique et donc dans l’aluminium produit. La vitesse de dissolution dans l’électrolyte de telles anodes doit être minimisée. De plus, la présence de fer dans le métal à ce stade de la production (avant la mise en alliage) n’est pas souhaitable. Il devient alors intéressant de modéliser le comportement des anodes inertes dans la cryolithe afin de comprendre les phénomènes de dissolution en cause. Le mandat du présent projet, qui s’inscrit dans le projet de «laboratoire virtuel pour l’industrie de l’aluminium» (CRSNG, Rio Tinto Alcan, Alcoa, Hydro Aluminium) est d’ajouter le fer à une base de données déjà existante sur le bain cryolithique. Le système chimique global à l’étude est NaF-AlF3-Al2O3-CaF2-FeO-Fe2O3. L’étude thermodynamique doit donc porter sur les ions suivants : Na+, Ca2+, Al3+, Fe2+, Fe3+ // F-, O2-, Va- (lacune anionique). Les cations Fe2+ et Fe3+ ont été ajoutés au modèle Na+, Ca2+, Al3+ // F-, O2-, Va- de Chartrand et Pelton [1] . Les propriétés thermodynamiques des composés purs du fer ont été évaluées. Le modèle quasichimique avec l’approximation des quadruplets a été utilisé afin de décrire le comportement du bain cryolithique liquide. Les solutions solides ont été modélisées selon le «Compound Energy Formalism» (CEF). Les paramètres du modèle pour le liquide et des modèles de solutions solides ont été optimisés à partir des données expérimentales disponibles. Les mesures expérimentales ont été critiquées et analysées et seules celles jugées fiables ont été sélectionnées. Les paramètres d’interaction du modèle du liquide ont été évalués dans les systèmes binaires Fe-F (FeF2-FeF3), NaF-FeF2, NaFFeF3, CaF2-FeF3. Les systèmes binaires FeF2-AlF3 et CaF2-FeF2 sont des estimations basées sur des analogies avec des systèmes chimiquement proches et FeF3-AlF3 est supposé idéal. Les paramètres d’interaction du modèle du liquide ont aussi été évalués pour les systèmes ternaires NaF-FeF2-AlF3 et NaF-FeF3-AlF3 (selon les mesures disponibles pour les sections Na3AlF6-FeF2 et Na3AlF6-FeF3). Les solutions solides modélisées sont : la cryolithe cubique de haute température ((Na+,Va)8(Na+)4(AlF6 3-,FeF6 3-)1(AlF6 3-(AlF4 -),FeF6 3-(FeF4 -))3), la cryolithe de basse température (Na3(Al,Fe3+)F6), la chiolite (Na5(Fe3+,Al)3F14), AlF3-FeF3 ((Al,Fe3+)F3), la weberite (Na2(Al,Fe3+)(Fe,Mg2+)F7) et CaAlF5-CaFeF5 (Ca(Al,Fe3+)F5).----------ABSTRACT Since the end of the 20th century, the aluminium industry is seriously interested in replacing consumable carbon anodes in the Hall-Heroult process by the so-called inert anodes. Nickel ferrite (NiFe2O4) is a promising material for the replacement of the traditional carbon anodes. However, the use of such anodes can cause iron and nickel to be dissolved in the cryolitic bath and therefore in the final product, which is not desirable. The dissolution rate of inert anodes in the electrolyte has to be minimized. Modeling the thermodynamic and phase equilibria behavior of inert anodes in contact with cryolite is useful to understand the dissolution processes. The mandate of this project, included in the project of «a virtual laboratory for the aluminium industry» (CRSNG, Rio Tinto Alcan, Alcoa, Hydro Aluminium) is to add the iron to an already existing database for the cryolitic bath. The global system under consideration is NaF-AlF3-Al2O3-CaF2-FeO-Fe2O3. The thermodynamical study has to take in account the following ions Na+, Ca2+, Al3+, Fe2+, Fe3+ // F-, O2-, Va- (anionic vacancy). The two cations Fe2+ and Fe3+ have been added to the existing model for the Na+, Ca2+, Al3+ // F-, O2-, Va- system optimized by Chartrand and Pelton [1] . Thermodynamic properties of new pure compounds have been evaluated. The quasichemical model in the pair approximation has been used in order to describe the cryolitic bath (liquid) behavior. New solid solutions have been modeled using the Compound Energy Formalism (CEF). Parameters for the liquid phase and solid solutions models have been optimized using available experimental data. Experimental techniques have been examined and only reliable data have been selected. Parameters for the liquid phase have been evaluated from experimental data for the following binary fluoride systems: Fe-F (FeF2-FeF3), NaF-FeF2, NaF-FeF3, CaF2-FeF3. The binary systems FeF2-AlF3 and CaF2-FeF2 are estimations based on chemically similar systems and the FeF3-AlF3 liquid solution is assumed to be ideal. Parameters for the liquid phase have also been evaluated in the two ternary fluorides systems NaF-FeF2-AlF3 and NaF-FeF3-AlF3 (based on available data for the isoplete sections Na3AlF6-FeF2 and Na3AlF6-FeF3). The new evaluated solid solutions for the fluoride systems are : high-temperature cubic cryolite ((Na+,Va)8(Na+)4(AlF6 3-,FeF6 3-)1(AlF6 3-(AlF4 -),FeF6 3-(FeF4 -))3), low-temperature cryolite (Na3(Al,Fe3+)F6), chiolite (Na5(Fe3+,Al)3F14), AlF3-FeF3 ((Al,Fe3+)F3), weberite (Na2(Al,Fe3+)(Fe2+,Mg)F7) and CaAlF5-CaFeF5 (Ca(Al,Fe3+)F5)

Hydrothermal Synthesis of Frustrated Lanthanide Pyrochlores and Transition Metal Double Perovskites and Germanates

Magnetically frustrated materials hold promise of unique behavior allowing for the novel study of quantum phenomena. Such materials are poised to become an integral foundation for technological advancement in the post-Silicon Age. Crystalline materials are given special focus where the rigid lattice allows more detailed study of these quantized effects and frustration behavior. As opposed to polycrystalline powders, large single crystals can be preferentially aligned enabling the study of anisotropic behavior. Two cubic structure types have garnered significant interest due to their 3-D tetrahedral arrangement of symmetry-related metal centers with the potential for magnetic frustration: pyrochlores and perovskites. The supercritical hydrothermal crystal growth technique has been applied to a host of refractory oxides to enhance phase purity and minimize crystalline defects often introduced by conventional flux or melt-based based techniques. The supercritical growth in aqueous alkali at temperatures much lower than those of floating zone melts, while maintaining a sealed environment, avoids the possibility for reactant sublimation. In comparison to melt-based techniques, the modest thermal conditions also aid in minimizing atomic site displacements by insertion into different sites than those expected (“stuffing”). Through supercritical hydrothermal conditions, it was possible to obtain the entire series of lanthanide stannate pyrochlores. Stannous oxide, originally applied for in situ ceria reduction, was found beneficial for all lanthanide oxide reactants regardless of oxidation state allowing facile growth of faceted, mm-scale stannate octahedra in only days. This has allowed for the first single-crystal neutron scattering study of the quantum spin liquid candidate Ce2Sn2O7 with similar experiments to follow across the lanthanide series. The sealed, pressurized supercritical hydrothermal technique was also applied in the growth of stoichiometric orthoscandates and site-ordered cubic barium double perovskites. Both systems had structures occur in crystallographic settings previously only possible through thin-film epitaxial growth or application of high pressures and temperatures. Attempts to expand the palette of lanthanide-containing germanium perovskites yielded novel P212121 LnTM(GeO4)(OH) (TM = 3d transition metal) adelite-type structures possessing chiral [TMO4]∞ a-axis chains. These adelites demonstrated the first known inclusion of lanthanides and germanium in the adelite-descloizite supergroup. The synthetic possibilities of the refractory lanthanide oxides and often-overlooked germanium offer great synthetic possibilities poised to further expand the adelite family while also providing a few magnetically frustrated surprises themselves or through the high-symmetry cubic garnet and spinel products encountered during exploratory adelite synthesis under hydrothermal conditions

NUCLEAR CHEMISTRY ANNUAL REPORT 1970

Papers are presented for the following topics: (1) Nuclear Structure and Nuclear Properties - (a) Nuclear Spectroscopy and Radioactivity; (b) Nuclear Reactions and Scattering; (c) Nuclear Theory; and (d) Fission. (2) Chemical and Atomic Physics - (a) Atomic and Molecular Spectroscopy; and (b) Hyperfine Interactions. (3) Physical, Inorganic, and Analytical Chemistry - (a) X-Ray Crystallography; (b) Physical and Inorganic Chemistry; (c) Radiation Chemistry; and (d) Chemical Engineering. (4) Instrumentation and Systems Development