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O espinélio MgAl2O4 é utilizado prioritariamente na indústria de refratários. Atualmente há várias técnicas de síntese, visto que não é encontrado na natureza. A técnica de sais fundidos apresenta grande potencial para otimização e redução do custo energético na produção deste óxido. Este trabalho teve como objetivo o desenvolvimento da técnica de síntese via sais fundidos para obtenção do espinélio MgAl2O4, e a caracterização do pó produzido via diferentes técnicas. A dissertação explora as variáveis da produção do espinélio a partir de Al2O3 e MgO em KCl, e também alternando o sal para MgCl2 e o eutético KCl-MgCl2. As seguintes alterações foram testadas para verificar o resultado no pó produzido: redução da temperatura para 1150°C, aumento do MgO nos precursores em 10% e 30%, redução da granulometria da Al2O3 precursores para 0,60 μm, variação da razão sal/precursor (S/P) para 1:1 e 9:1. Foi testado o sal MgCl2 nas proporções de precursores estequiométrica, -50% de MgO e -100% de MgO. Por fim, o sal KCl-MgCl2 na razão eutética foi testado. Para caracterização do pó foram utilizadas as técnica de análise cristalográfica por difração de raios X (DRX) e pela técnica de RAMAN, granulometria por dispersão a laser (GDL), microestrutura por microscopia eletrônica por varredura (MEV) e análise de cristalitos por mircroscopia eletrônica de transmissão (MET). Os pós produzidos apresentaram as fases MgAl2O4, Al2O3 e MgO. A redução de temperatura reduziu a quantidade de espinélio em pequena proporção, a alteração de estequiometria dos precursores no sal KCl reduziu a quantidade de Al2O3 em um primeiro momento e também sobrou MgO na reação para um aumento maior de MgO. A redução de granulometria da Al2O3 propiciou aumento da proporção de espinélio na reação. A proporção S/P 1:1 não mostrou alteração, contudo para 9:1 houve sedimentação e um efeito de diluição/precipitação. Para o sal MgCl2, houve precipitação de MgO a partir do sal, alterando o perfil granulométrico dos pós e as fases obtidas: MgO e MgAl2O4. A redução de MgO aumentou a conversão para MgAl2O4, reduzindo também a granulometria. O sal na proporção eutética apresentou boa conversão de espinélio e pouco aumento de granulometria. / The MgAl2O4 spinel is mainly applied at the refractory industry. There are many techniques to produce that oxide, since it is not found in the nature. The molten salts technique shows potencial energy savings and cost reduction to produce spinel. This work aimed at the development of the molten salts synthesis technique to obtain the MgAl2O4 spinel, and characterization of the powder produced via different techniques. The dissertation explores the variables of production of spinel from Al2O3 and MgO in KCl, and also alternating salt to MgCl2 and KCl-MgCl2 eutectic. The following changes were tested to verify the result of the powder produced: reducing the temperature to 1150 ° C, an increase in the MgO precursor at 10% and 30% reduction in particle size of the Al2O3 precursor to 0.60 micrometres variation ratio of salt / precursor (S / P) to 1: 1 and 9: 1. MgCl2 salt was tested in the stoichiometric proportion of precursors, -50% of MgO and -100% MgO. Finally, the KCl-MgCl2 salt eutectic ratio was tested. For characterization of the powder was used the crystallographic analysis technique of X-ray diffraction (XRD) and RAMAN technique, laser scattering for particle size analisys, scanning electron microscopy (SEM) and crystallite analysis by transmission electron mircroscopy (TEM). The powders produced presented the MgAl2O4 phase, Al2O3 and MgO. The reduction in temperature reduced the amount of spinel in small proportion, the stoichiometry change of the precursors in the KCl salt reduced the amount of Al2O3 at first and also left MgO in reaction to a greater increase of MgO. The reduction of the Al2O3 particle size resulted in an increase in the proportion of the spinel reaction. The ratio S/P 1: 1 showed no change, however to 9: 1 was sedimentation and dilution / precipitation effect. To MgCl2 salt, there was precipitation of MgO from the salt, changing the physical profile of the powder and the obtained phases: MgO and MgAl2O4. The reduction of MgO increased the conversion MgAl2O4, by reducing the grain size. The salt in the proportion eutectic showed good conversion spinel and little increase in particle size.

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Le Réacteur Nucléaire à sel fondu à spectre rapide (Molten Salt Fast Reactor, MSFR) est un concept innovant de quatrième génération développé par le CNRS depuis 2004 et actuellement étudié dans le cadre du projet européen SAMOFAR de H2020. Le MSFR fonctionne avec un combustible nucléaire liquide constitué d’un mélange de sels fluorures LiF-ThF₄-(UF₄/UF₃) (77,5-20-2,5) mol% fondus à haute température (700-900°C). Ce réacteur est particulièrement intéressant pour le cycle de combustible du thorium (²³²Th-²³³U). Ce concept propose un retraitement intégré du combustible nucléaire basé sur des méthodes pyrochimiques afin d’extraire la matière fissile et de séparer les actinides des produits de fission.Un schéma de traitement du sel combustible, proposé lors d’un précédent projet européen (EVOL, FP7), est basé sur les propriétés redox et acido-basiques des éléments produits par les réactions de fission et de capture ayant lieu dans le cœur du réacteur. La base d’évaluation de ce schéma a été dans un premier temps thermodynamique. Une validation expérimentale est actuellement en cours qui consiste à étudier le comportement chimique et électrochimique du sel fondu et des éléments qui y sont solubilisés. Les études précédentes sur les réacteurs sels fondus ne peuvent être utilisées que partiellement pour ce concept car la composition du sel du MSFR définie par le projet européen EVOL est différente en nature et composition des sels proposés jusqu’à présent pour ce type de réacteurs. Or, les coefficients de diffusion et d’activité dépendent des propriétés physico-chimiques du sel fondu (en particulier de la solvatation) et nous avons, lors d’études précédentes, montré que les propriétés de solvatation des sels fondus dépendent fortement de leur nature et de leur composition.Les objectifs de ce travail de thèse sont l’étude du mélange fondu LiF-ThF₄ et du comportement électrochimique de l’uranium et de l’iode dans ce mélange.L’étude électrochimique du comportement de l’uranium a montré la stabilité de deux espèces solubles (UF₄ et UF₃) de cet élément dans le milieu fondu et la possibilité de le réduire à l’état métallique. Ce point est d’importance car la co-existence de ces deux composés permettra de contrôler le potentiel du sel combustible dans le cœur du réacteur et de limiter les réactions de corrosion avec les matériaux de structure. Les coefficients d’activité de U(IV) et de U(III) ont été déterminés. Les valeurs obtenues montrent que la solvatation de l’uranium au degré d’oxydation (IV) par les ions fluorure est beaucoup plus importante que celle de l’uranium au degré (III), ce qui est en accord avec des observations ultérieures dans d’autres sels fluorures. Notre choix pour l’étude des produits de fission dans le sel combustible s’est porté sur l’iode. Dans le cœur du réacteur, la forme stable de l’iode est la forme halogénure soluble I- et dans le schéma général de traitement du sel combustible, il est prévu d’extraire l’iode par une étape de fluoration qui permet de produire le gaz I₂. Cette étude a montré la contribution d’une réaction chimique à l’oxydation des ions iodures en iode gazeux. Cette réaction chimique d’oxydo-réduction correspond à l’oxydation des ions iodures par l’oxygène. Cette réaction n’est expliquée que par l’existence d’un oxyfluorure de thorium soluble ThOF₂. Une efficacité d’extraction de I₂ (g) supérieure à 95 % a été obtenue par électrolyse à potentiel contrôlé. Ces électrolyses, qui simulent la fluoration, permettent de valider l’étape d’extraction de l’iode dans le schéma de traitement.Ce travail de recherche a permis d’acquérir une meilleure connaissance de la stabilité du sel et du comportement chimique et électrochimique de différents composés (U et I) dans le sel. / The Molten Salt Fast Reactor (MSFR) is an innovative concept of GEN IV developed by the CNRS since 2004. It is currently studied in the framework of the European project SAMOFAR of H2020. The MSFR operates with a liquid nuclear fuel consisting of a mixture of fluoride salts LiF-ThF₄- (UF₄ / UF₃) (77.5-20-2.5) mol% melted at high temperature (700-900° C). This reactor is particularly advantageous for the thorium fuel cycle (²³²Th-²³³U). This concept proposes an integrated reprocessing of the nuclear fuel based on pyrochemical methods in order to extract the fissile material and to separate the actinides from the fission products.A scheme for the treatment of the fuel salt, proposed in a previous European project (EVOL, FP7), is based on the redox and acido-basic properties of the elements produced by the fission and capture reactions occurring in the reactor core. The baseline for this scheme was initially thermodynamic. Experimental validation is currently under way to study the chemical and electrochemical behavior of the molten salt and the elements solubilized therein. Previous studies on molten salt reactors can only be partially used for this concept because the composition of the MSFR salt defined by the European EVOL project is different from the composition of the salts proposed up to now for this type of reactor. However, the diffusion and activity coefficients depend on the physicochemical properties of the molten salt (in particular solvation) and in previous studies we have shown that the solvation properties of molten salts are strongly dependent on their nature and their composition.The objectives of this thesis are the electrochemical characterization of the molten mixture LiF-ThF₄ and the study of the electrochemical behavior of uranium and iodine.The electrochemical study of the behavior of uranium shows the stability of two soluble species (UF₄ and UF₃) of this element in the molten medium and the possibility of its reduction to the metallic state. This point is important because the co-existence of these two compounds will make possible to control the potential of the fuel salt in the core of the reactor in order to limit the corrosion reactions with the structural materials. The activity coefficients of U (IV) and U (III) were determined. The values obtained show that the solvation of uranium to the degree of oxidation (IV) by fluoride ions is much greater than that of uranium to degree (III), which is in agreement with subsequent observations in other fluoride salts.Our choice for the study of fission products in the fuel salt has focused on iodine. In the core of the reactor, the stable form of the iodine is the soluble halide form I- and in the general scheme of treatment of the fuel salt, it is planned to extract iodine by a fluorination step in order to produce the gaseous compound I₂. The electrochemical study shows the contribution of a chemical reaction to the electrochemical oxidation of iodide ions in gaseous iodine. This redox chemical reaction corresponds to the oxidation of the iodide ions by oxygen. This reaction is explained for the existence of a soluble thorium oxifluoride ThOF₂. Extraction efficiencies of I₂ (g) greater than 95% were obtained by electrolysis at controlled potential. These electrolysis, which simulate fluorination process, make it possible to validate the method for the extraction of the iodine in the reprocessing scheme.This research has led to a better understanding of salt stability and of the chemical and electrochemical behavior of several compounds (U and I) in the molten salt.

Conducting research experiments on plutonium electrorefining is difficult due to the significant hazards and regulations associated with nuclear materials. Finding a surrogate for plutonium electrorefining studies would enable more fundamental research to be conducted. Potential surrogates were identified by determining the physical properties required to conduct electrorefining using a molten metal and molten salt in CaCl2 at 1123 K. More potential surrogates were identified by changing the matrix salt to be a LiCl-KCl-CaCl2 eutectic salt with electrorefining conducted at 673-773 K. Ce-CeCl3, In-InCl3, Zn-ZnCl2, Sn-SnCl2, and Bi-BiCl¬3 were investigated as potential plutonium electrorefining surrogates. Ce electrorefining in molten CaCl2 resulted in a difficult to separate colloid mixture of Ce, Ca and Cl. Electrorefining rates for In were too slow due to InCl3 volatilizing out of the molten salt. Zn was successfully electrorefined, but the metal obtained did not coalesce into one piece. Sn and Bi were successfully electrorefined and coalesced into solid product rings with high yields and coulombic efficiencies. While a surrogate could not be identified using the same conditions as plutonium electrorefining, two possible surrogates, Sn-SnCl2 and Bi-BiCl3,¬ were found that could imitate the physical configuration (i.e., molten salt on top of molten metal) of plutonium electrorefining at a reduced temperature using a eutectic LiCl-KCl-CaCl2 salt in place of CaCl2. Using this surrogate enables fundamental studies of aspects of plutonium electrorefining. One aspect of plutonium electrorefining research is to improve its efficiency and yield. Plutonium electrorefining is a time-intensive process which generates radioactive waste. Improvements in efficiency and yield can reduce process time and waste. One possible way of improving the efficiency of plutonium electrorefining is to study the impact of using an AC superimposed DC waveform. Four AC superimposed DC and two DC electrorefining runs were performed using bismuth as a plutonium surrogate. All six runs showed a high level of yield and coulombic efficiency. All six cathode rings were confirmed to be high-purity bismuth using scanning electron microscopy with energy dispersive x-ray analysis (SEM-EDS). While the results were inconclusive about the ability of AC superimposed DC waveforms to increase the efficiency of bismuth electrorefining, applying an AC superimposed DC waveform did not appear to decrease the efficiency or yield of the process. The change in waveform also did not result in impurities being present in the product cathode ring. Bismuth, in addition to being identified as a viable plutonium surrogate, has been investigated as a potential liquid electrode for molten salt electrorefining. Because of this, its electrochemical properties are of interest. However, bismuth's electrochemical behavior has received scant attention in eutectic LiCl-KCl melts and no studies were found in the ternary LiCl-KCl-CaCl2 melts. LiCl-KCl-CaCl2 melts offer some advantages over eutectic LiCl-KCl, such as lower melting point and higher oxide solubility. Cyclic voltammetry, square wave voltammetry, chronoamperometry, chronopotentiometry and open-circuit chronopotentiometry were used to measure electrochemical parameters, such as diffusivity and standard redox potential of bismuth electrodeposition in LiCl-KCl and LiCl-KCl-CaCl2 eutectics.

Le néodyme appartient à la série des lanthanides se trouvant dans le tableau périodique. Il est le composant clé des aimants permanents Nd2Fe14B implantés dans différents appareils électroniques mais aussi dans appareils dis « écologiques » comme les éoliennes, les voitures ou vélos électriques. De nos jours seulement 1 % du néodyme présent dans les déchets électroniques est recyclé. De fortes tensions géopolitiques ainsi que la croissance rapide de la demande pour ce type d’aimants conduisent à d’importantes spéculations et fluctuations sur le prix du néodyme aggravées par l’appauvrissement annoncé de la ressource.Depuis quelques décennies, plusieurs procédés de recyclage du néodyme ont été développés au niveau des laboratoires sans avoir été utilisés à l’échelle industrielle. Les procédés existants ont plusieurs désavantages comme un grand nombre d’étapes avant d’obtenir le produit final ou encore l’utilisation de grandes quantités de produits chimiques.Le but de ces recherches est le développement d’un procédé pyrochimique pouvant être une alternative à ceux déjà existants. Ce procédé permettrait de récupérer le néodyme sous sa forme métallique en seulement une étape. Pour atteindre cet objet, il a fallu élaborer un réacteur pouvant résister à de hautes températures ainsi qu’aux sels fondus pouvant être corrosifs selon leur composition. Par la suite, les propriétés électrochimiques du néodyme ont été étudiées dans différents sels fondus à base de chlorures et de fluorures afin trouver la configuration optimale pour l’extraction du néodyme contenu dans les aimants permanents.Nous avons ainsi prouvé qu’il était possible d’extraire le néodyme sous sa forme métallique contenu dans de vieux aimants permanents en une seule étape durant une électrolyse. / Neodymium belongs to the lanthanide’s serie of the period system and is the key component of the permanent magnets Nd2Fe14B which are implemented in electronic devices and “green” technologies like wind turbines or electric cars and bicycles. Nowadays, only one percent of the neodymium in electronic scraps is recycled. Due to the geopolitical considerations and a strong increase of the use of permanent magnets, there is an impoverishment of the raw material resources leading to an instable market.Since some decades, few recycling processes have been developed on a lab scale without any upscaling to the industrial scale. The existing processes have several drawbacks like multiple steps to obtain the final desired product. This means that these methods have a long process time or use a large amount of chemical productsThe aim of this research was the development of a pyrochemical process, which could be an alternative to the existing recycling processes in order to extract neodymium from electronic scraps with less steps, a smaller amount of chemical products and a higher recovery rate of the rare neodymium. For this purpose a special reaction chamber has been created which resists to the severe experimental conditions induced by the use of high temperatures and corrosive molten salts. Moreover the electrochemical behaviour of neodymium in different chloride- and fluoride-based molten salts was studied in order to find the most appropriated setup.We could prove that the pyrochemical method could be the solution to recover neodymium from the old permanent magnets under its metallic form in just one single reaction step during an electrolysis.