Dissertations / Theses on the topic 'Chromide'

Contrairement à la grande majorité des bactéries qui possèdent un chromosome unique, Vibrio cholerae, l'agent pathogène responsable de la maladie du choléra, contient un génome divisé en deux réplicons : un chromosome principal (Chr1) et un chromosome secondaire (Chr2). Les chromosomes secondaires sont apparus indépendamment dans de nombreux phylum bactériens et sont souvent associés à des caractères pathogènes ou symbiotiques (par exemple Leptospira, Agrobacterium, Burkholderia). Les chromosomes secondaires résultent de l'acquisition et de la domestication de grands plasmides. En effet, certains "mégaplasmides" ont cohabité avec leur hôte au cours d'une longue période évolutive et sont devenus un composant essentiel du génome de leur hôte. Au cours de leur domestication, les chromosomes secondaires ont acquis différentes caractéristiques permettant leur maintien à travers les divisions cellulaires assurant leur transfert vertical aux générations suivantes. L'étude des bactéries à génome multipartite permet de s'interroger sur le bénéfice sélectif d'un organisme à posséder plusieurs chromosomes. Ou encore, comment un élément génétique acquis horizontalement peut-il se connecter à un réseau génétique préétabli ? L'une des principales étapes de la domestication des mégaplasmides est leur intégration dans le cycle cellulaire de l'hôte. Chez V.cholerae, la réplication de Chr2 est coordonnée avec celle de Chr1. Chr1, qui est 3 fois plus grand que Chr2, est initié en premier et lorsque les 2/3 se sont répliqués, un signal d'initiation déclenche la réplication de Chr2 de sorte que les deux chromosomes terminent leur réplication de façon concomitante. Ce signal d'initiation est émis lorsque la fourche de réplication passe par un locus non codant de Chr1 nommé crtS (Chr2 Replication Triggering Site)2,3. Cependant, le mécanisme moléculaire à l'origine de ce signal d'initiation est encore très difficile à élucider. Notre travail vise à élucider le mécanisme de ce nouveau point de contrôle bactérien. Afin de s'attaquer à ce problème, nous avons développé plusieurs approches de pointe pour étudier le mécanisme à différents niveaux : -Echelle génomique : ChIPseq à haute résolution -Single-cell : microscopie à fluorescence -Moléculaire : CryoEM
Contrary to the vast majority of bacteria that possesses a unique chromosome, Vibrio cholerae, the pathogenic agent responsible for cholera disease, contains a genome divided into two replicons: a main chromosome (Chr1) and a secondary chromosome (Chr2). Secondary chromosomes appeared independently in many bacterial phylum and are often associated with pathogenic or symbiotic traits (i.e. Leptospira, Agrobacterium, Burkholderia). Secondary chromosomes result from the acquisition and domestication of large plasmids1. Indeed, some “megaplasmids” have cohabited with their host over a long evolutionary period and became an obligatory component of their host’s genome, essential for their normal metabolism. During their domestication, secondary chromosomes acquired different features allowing their maintenance through cell divisions ensuring their vertical transfer to subsequent generations. The study of multipartite genome bacteria allows questioning the selective benefit of an organism to possess several chromosomes. In addition, how can a horizontally acquired genetic element connect to a pre-established genetic network? One of the major steps in megaplasmid domestication is their integration into the host cell cycle. In V.cholerae, Chr2 replication is coordinated with that of Chr1. Chr1, which is 3 times larger than Chr2, is initiated first and when 2/3 has replicated, an initiation signal triggers the replication of Chr2 so that both chromosomes terminate replicating concomitantly. This initiation signal is emitted when the replication fork passes through a non-coding Chr1 locus named crtS (Chr2 Replication Triggering Site)2,3. However, the molecular mechanism behind this initiation signal is still very elusive. Our work aims to elucidate the mechanism of this novel bacterial replicative checkpoint. In order to tackle this problem we developed several cutting-edge approaches to study the mechanism at different levels: -Genomic scale: high resolution ChIPseq -Single cell: fluorescent microscopy -Molecular: CryoEM

De par leurs propriétés mécaniques et anti-corrosion élevées, les revêtements métalliques de chrome sont utilisés dans de nombreux domaines tels que l’aéronautique, l’automobile, le ferroviaire, la défense,… Aujourd’hui le chromage dur industriel, réalisé à partir de chrome hexavalent, est fortement menacé par la directive européenne REACH qui prévoit l’interdiction d’utilisation de sels de chrome VI (classé CMR) à l’horizon 2017. Dans ce contexte le projet HCTC (Hard Chromium by Trivalent Chromium) regroupe 16 partenaires industriels et 2 partenaires académiques sous la coordination de l’Institut de Recherche Technologique Matériaux, Métallurgie, Procédés (IRT M2P) afin de développer une alternative à base de sel de chrome trivalent. Le développement d’un procédé de substitution à partir de sels de chrome trivalent nécessite de répondre aux interrogations liées notamment à la difficulté de réduire les ions Cr3+ formant en solution aqueuse un complexe hexa-aqua [Cr(H2O)6]3+ très stable et difficilement réductible. Afin de faciliter la réduction, l’ensemble des travaux recensés dans la littérature prône l’utilisation d’un agent complexant organique afin d’augmenter l’accessibilité du chrome. Une première partie de ce travail de thèse a été de caractériser les dépôts de chrome métallique réalisés à partir de la formulation de chrome III (EXDBA 1318) afin de mettre en évidence l’influence des paramètres de chromage (température du bain de traitement, densité de courant,…) sur les propriétés physico-chimiques des dépôts (Morphologie, cristallinité, composition chimique). Les résultats ont montré que la diminution de fissures tranversantes est généralement accompagnée d’une augmentation du taux de carbure de chrome qui dégrade l’état cristallin. La compréhension et l’optimisation du procédé a permis la réalisation de dépôts avec des propriétés physico-chimiques permettant de lancer les premiers essaies industriels dont les premiers retours sont très encourageants pour certaines applications (essentiellement automobile).Dans une deuxième partie plus fondamentale, des analyses par HPLC-ICP/AES ont mis en évidence la complexation du chrome par différents agents complexant. Les résultats obtenus ont montré que la complexation étaient d’autant plus importante que le pH est haut (pH=5) et le rapport molaire [Cr]/[Cplx] faible (1/10). Les mécanismes de réduction associés aux taux de complexation ont ensuite été déterminés par des mesures de voltammétrie cyclique en utilisant des électrodes sérigraphiées modifiées par de l’or. Les voltammogrammes obtenus montrent que la complexation du chrome par un agent complexant permet de passer d’un mécanisme de réduction en deux étape (Cr3+  Cr2+  Cr) en une seule (Cr3+  Cr).Une dernière partie du travail a été de définir l’influence de la chimie sur bain (pH et rapport molaire [Cr]/[Cplx]) sur les propriétés physico-chimiques des dépôts. Les résultats obtenus ont montré que pour des solutions faiblement complexées il est préférable de travailler à des pH bas et d’appliquer de faibles densités de courant (<15A/dm²) tandis qu’une augmentation du taux de complexation du chrome par l’agent complexant nécessite d’appliquer des densités de courant plus fortes. Ce travail a aussi permis de montrer l’existence d’une zone de pH optimale pour un rapport molaire [Cr]/[Cplx] donnée. L’existence de cette zone de pH révèle l’existence d’un domaine d’équilibre en solution, favorable à la réduction du chrome. L’ensemble de ces travaux plus fondamentaux ont permis d’améliorer les connaissances sur le procédé de chromage à partir d’un sel de chrome trivalent et ainsi de réfléchir à une nouvelle formulation pour des applications plus critiques où la solution actuelle a pu montrer quelques limites
Due to their high mechanical and anti-corrosion properties, chromium metal coatings are used in many fields such as aeronautics, automotive, railway, defense, etc. Today industrial hard chromium coating, made from of hexavalent chromium, is strongly threatened by the European REACH Directive, which provides for the prohibition of the use of chromium VI salts (classified as CMR) by 2017. In this context, the HCTC project (Hard Chromium by Trivalent Chromium) groups together 16 industrial partners and 2 academic partners under the coordination of the Institute for Materials, Metallurgy and Process Technology Research (IRT M2P) to develop an alternative based on trivalent chromium salt. The development of a substitution process from trivalent chromium salts requires answering the questions related in particular to the difficulty of reducing the Cr3+ ions forming in aqueous solution a very stable hexa-aqua [Cr(H2O)6]3+ complex and hardly reducible. In order to facilitate the reduction, all the works listed in the literature advocate the use of an organic complexing agent in order to increase the accessibility of chromium.A first part of this thesis work was to characterize the chromium metal deposits made from the chromium III formulation (EXDBA 1318) in order to highlight the influence of chromium parameters (bath temperature, density of current, ...) on physico-chemicals properties of the deposits (Morphology, crystallinity, chemical composition). The results showed that the decrease of transverse cracks is generally accompanied by an increase in chromium carbide which degrades the crystalline state. The understanding and the optimization of the process allowed the realization of deposits with physico-chemical properties allowing to launch the first industrial tests whose first returns are very encouraging for certain applications (mainly automobile).In a second, more fundamental part, HPLC-ICP / AES analyzes have demonstrated the complexation of chromium by different complexing agents. The results obtained showed that the complexation was all the more important that the pH is high (pH = 5) and the molar ratio [Cr] / [Cplx] low (1/10). Reduction mechanisms associated with complexation rates were then determined by cyclic voltammetry measurements using gold-modified screen-printed electrodes. The voltammograms obtained show that the complexation of chromium by a complexing agent makes it possible to go from a reduction mechanism in two steps (Cr3+  Cr2+  Cr) to a single one (Cr3+  Cr).A final part of the work was to define the influence of bath chemistry (pH and [Cr]/[Cplx] molar ratio) on the physico-chemical properties of coatings. The results obtained showed that for weakly complexed solutions it is preferable to work at low pH and to apply low current densities (<15A / dm²) while an increase in the chromium complexation rate by the complexing agent requires the application of higher current densities. This work also showed the existence of an optimal pH zone for a given [Cr]/[Cplx] molar ratio. The existence of this zone of pH reveals the existence of a balance domain in solution, favorable to the reduction of chromium. All of these more fundamental works have made it possible to improve the knowledge on the chromium plating process from a trivalent chromium salt and thus to think about a new formulation for more critical applications where the current solution has been able to show some limits

Ce travail de thèse s’inscrit dans un projet de grande envergure piloté par l’IRT M2P, intitulé « Hard Chrome by Trivalent Chromium » qui regroupe 15 partenaires industriels et 2 partenaires académiques. L’objectif principal est de substituer les composés du chrome hexavalent par des électrolytes à base de chrome trivalent dans les procédés de chromage « dur », en réponse à l’application du règlement européen REACH en septembre 2017, même si des demandes d’autorisations temporaires sont en cours. Dans le cadre de cette thèse, des études électrochimiques ont été réalisées sur des bains commercial et synthétique. Celles-ci ont conduit au choix d’un matériau jouant le rôle d’anode, à une meilleure compréhension des mécanismes mis en jeu et de ce fait à une optimisation des conditions d’utilisation du bain commercial. D’autre part, une étude approfondie des courbes transitoires permet de mieux appréhender le comportement des électrolytes de chrome trivalent vis-à-vis des courants pulsés. Ainsi, un plan d’expérience sur les paramètres des pulses avec pour sortie de multiples caractérisations des dépôts de chrome montre que les courants pulsés influent sur plusieurs propriétés telles que : la teneur en carbone, la structure cristalline, la microdureté, la morphologie de surface et la microfissuration. Grâce au traitement des données issues du plan d’expériences, l’optimisation des paramètres de polarisation aboutit à l’élaboration de séquences de pulses optimisées. L’utilisation de celles-ci, employées seules ou en combinaison avec d’autres séquences de pulses issues également du plan d’expériences, aboutit à l’obtention de dépôts de chrome trivalent dont les propriétés sont modulables en fonction des paramètres procédés. Dans l’optique de la mise en place d’une nouvelle stratégie de choix des paramètres de l’impulsion anodique, un dispositif permettant la mesure du pH local en fonction des séquences et à proximité immédiate de la surface a été élaboré. Les premières tendances confirment les modes d’action des courants pulsés, et les choix qui ont été faits pour l’optimisation
This thesis work is part of an ambitious project handled by IRT M2P, named “Hard Chrome by Trivalent Chromium” which gathers 15 industrials partners but also 2 academic partners. The main objective is to substitute the hexavalent chromium compounds in hard chromium electroplating process before there ban by European instances (REACH, ECHA) in September 2017, excepted for those authorized. As part of this thesis, electrochemical studies were realized on commercial and synthetic baths. Thanks to this studies, a material has been chosen to act as an anode for the making of the trivalent chromium coatings ; furthermore, the utilization conditions of the commercial bath has been optimized. Moreover, another study based on transient curves allows a better comprehension of the behaviour of the trivalent chromium electrolytes regarding pulsed current. This work permitted the elaboration of pulse sequences in the form of an experimental design. The realization and characterization of trivalent chromium coatings as a part of the experimental design show that pulsed current have an effect on several properties of the coatings such as carbon content, crystalline structure, microhardness, surface morphologies an microcracking. Thanks to the analyses of the results from the experimental design, an optimization of pulsed current has been made to obtain optimized pulse sequences. The utilization of those pulse sequences, used alone or combine with some pulse sequences of the experimental design lead to the elaboration of trivalent chromium coatings which properties are adjustables depending on the set parameters of the process. To further optimize the properties of the coatings, the basis of an analysis tool based on local pH measurements are developed; this analysis tool must make it possible to select the most interesting pulse sequences for the realization of trivalent chromium coatings

Chromium has been widely used in many industrial applications. As a result of chromite ore processing, large amounts of chromite ore processing waste (COPW) material that can be classified as hazardous have been produced and released into the environment. Therefore, knowledge of migration behavior and leaching rates of chromium through waste materials and soils are of primary concern for environmentally sound management of land-disposal hazardous wastes. Haskö
k (1998) experimentally studied leaching rates of total Cr and Cr(VI) using laboratory columns packed with chromium COPW material produced by a sodium chromite plant. Based on the experimental results of Haskö
k (1998), present study aim, through mathematical modeling, to understand the dissolution kinetics of chromium during leaching of COPW material and to investigate the effectiveness of intermittent leaching involving a sequence of batch (dissolution) and leaching (mass flushing) operational modes. Obtained results show that a coupled system of two first order differential equations was able to capture the essential characteristics of leaching behavior of COPW material. In addition, the kinetics of chromium dissolution from COPW appeared to be controlled by the difference between aqueous phase concentration and a saturation concentration, by the mass fraction of dissolvable chromium remaining in the solid phase, and finally by the contribution of a constant dissolution rate manifested as a steady-state tailing behavior. As a result of performed simulations it was seen that intermittent leaching could be 65%and 35% more effective than continuous leaching for total Cr and Cr(VI), respectively.

Chromium is an important alloying element for stainless steels and other Cr-bearing steels. During the steelmaking process chromium is added to the steels mainly in the form of ferrochrome, which is largely produced by the energy-intensive smelting reduction process of chromite ore in the submerged arc furnace. To reduce the overall energy consumption during the ferrochrome production process and the chromium alloying process, direct chromiumalloying by chromite ore has been proposed. The application of this process will integrate the processes for ferrochrome production and chromium alloying, and thus has the potential to cut the production costs of the Cr-bearing steels by avoiding, or at least partially avoiding, the usage of ferrochrome. Further, this new alloying process has the capacity to improve therecovery of chromium from chromite ore. This thesis presents fundamental studies on the carbothermic reduction of synthetic iron chromite (FeCr2O4) and chromite ore, which aim at designing a direct alloying precursor to be applied in the industrial process. Thermogravimetric Analysis (TGA) experiments have been carried out to investigate the carbothermic reduction processes of FeCr2O4 in the absence/presence of metallic iron, and of chromite ore in the absence/presence of mill scale. In the case of using the mixture ‘FeCr2O4 +iron powder + graphite’, it is found that the presence of metallic iron enhances the reduction of FeCr2O4, and this enhancing effect increases with increasing iron addition. The enhancing effect of iron addition on the reduction of FeCr2O4 is due to the fact that the reduction of component Cr2O3 in FeCr2O4 is enhanced, and this effect is attributed to the presence of solidiron which can decrease the activity of chromium by having chromium in situ dissolved in the iron. In the case of using the mixture ‘chromite ore + petcoke’, it is found that the reduction of iron ions in the chromite ore starts before that of chromium ions in the ore and the reduction of iron ions and chromium ions in the ore overlaps to some degree. (Cr,Fe)7C3 is found to bethe intermediate phase during the reduction and a chromium gradient is found in the spinel phase of the fractional reduced sample at 1673 K. A four-stage reduction process is proposed: one stage involving the reduction of iron ions in the chromite ore and three stages involving the reduction of chromium ions in the ore. The activity aspects of component FeCr2O4 and component MgCr2O4 in the chromite ore have been considered. The difficulty in the reductionof the chromite ore is attributed to the fact that, as the reduction proceeds, the activity of component MgCr2O4 in the fractional reduced ore will decrease to a very low level, which makes the further reduction very difficult. In the case of using the mixture ‘chromite ore + mill scale + petcoke’, it is found that mill scale is reduced to iron before 1573 K. The asreduced iron is disseminated around chromite ore particles and, at the same time, some carbonis dissolved in the iron via diffusion. Reduction of chromite ore is enhanced with the addition of mill scale at temperatures higher than 1623 K, and the enhancing effect increases with increasing mill scale addition. The enhancing effect, in this case, is attributed to the presence of molten Fe-Cr-C phase in the vicinity of chromite ore, which can decrease the activity of chromium by having chromium in situ dissolved into the melt. Induction furnace experiments have been carried out to investigate the effectiveness of some different alloying mixtures. The experimental results have confirmed the necessity ofadjusting the composition of the slag to ensure high chromium yield in the final product and the experimental results show that, by using iron scrap, chromium yield can reach 90%. The present findings have led to the proposal of using ‘chromite ore + mill scale + petcoke’ as alloying precursor for direct chromium alloying. The effectiveness of this alloying precursor needs to be further explored by induction furnace experiments, followed by full scale Electric Arc Furnace experiments.

Godkänd; 2014; 20141027 (andbra); Nedanstående person kommer att hålla licentiatseminarium för avläggande av teknologie licentiatexamen. Namn: Xianfeng Hu Ämne: Processmetallurgi /Process Metallurgy Uppsats: Fundamental Studies on Direct Chromium Alloying by Chromite Ore with Designed Alloying Precursor Examinator: Professor Bo Björkman Institutionen för samhällsbyggnad och naturresurser Avdelning Mineralteknik och metallurgi Luleå tekniska universitet Diskutant: Professor Timo Fabritsius University of Oulu Finland Tid: Fredag den 15 december 2014 kl 10.00 Plats: E246, Luleå tekniska universitet

The thesis is focused on the characteristics of the key features of the operating system Google Chrome OS and on the evaluation of deployment of this operating system in practise. The theoretical part describes the purpose and philosophy of the commercial operating system Google Chrome OS and its relation with Chrome web browser. The connection with open-source projects Chromium and Chromium OS is explained. Details of application development and deployment options, in the form of virtualization and domain administration, are analysed. The practical part applies theoretical knowledge from the development of applications for Chrome OS to create thin client application for remote control of hardware sensor over the internet. This sensor is designed and built on the platform Arduino. The sensor measures the temperature and humidity of the ambient air and remote controls high-voltage switching relays. Communication over TCP/IP uses modern MQTT protocol, for which the server is configured with MQTT Broker Mosquitto. The next section demonstrates the procedure for building a customized build of Chromium OS from the publicly provided Google source codes. Based on the evaluation of the acquired theoretical and practical knowledge, I assess the possibility of deploying the Chrome OS in private and commercial areas.

In this thesis work, a high grade chromite concentrate obtained from PinarbaSi, Kayseri region of Turkey was reacted with sulfuric acid solution to determine the optimum conditions of leaching of chromite ores. Conventional methods for producing chromium are still valid in industry. The main process in production includes soda melting and taking it into solution in the form of chemical compounds such as sodium dichromate. Three and six valence states of chromium have importance in production. Trivalent chromium, chrome ore or chromite have no harmful effects. However, compounds of hexavalent chromium show toxic, irritating and corrosive action to people and environment. In the mentioned conventional method, main products consist of hexavalent chromium compounds. In this study, only trivalent chromium remained in the leach solution and did not change to hexavalent state. Obtained product after leaching was chromium(III) sulfate. The maximum extraction of chromium in the absence of perchloric acid was 94.1 % under the conditions of 175 °
C, 6 hours and 84.6 wt % sulfuric acid. The maximum extraction of chromium in the presence of perchloric acid was 98.7 % under the conditions of 175 °
C, 2 hours, 84.6 wt % sulfuric acid and ½
perchloric acid / chromite ratio. The latter one was also the highest recovery value obtained during the experiments. Moreover, in none of the analyzed samples, appreciable amounts of hexavalent chromium was found during analyses.

The industrial importance of the chromium industry to South Africa is emphasised by the fact that it is considered the largest chromite (chromium ore) and ferrochrome (chrome-iron alloy) producing country in the world. Although South Africa holds three quarters of the world's chromite ore reserves, the chrome-to-iron (Cr-to-Fe) ratio of the local chromite ores is relatively low (1.47 to 1.55), compared to other deposits in the world (2.6 to 3.5). Additionally, iron is more readily reduced than chromium. The combination of these two factors implies that ferrochrome produced from South African chromite ore contains 47-53% chromium. Current pricing practises in the world ferrochrome industry dictate that ferrochrome producers are only paid for the chromium content in the ferrochrome, which implies that South African ferrochrome producers export a large percentage of their product without any financial benefit. Research to improve the Cr-to-Fe ratio is therefore essential to support sustainability of the local ferrochrome industry. Conventional beneficiation methods such as gravity concentration, magnetic separation and floatation are unlikely to increase the Cr-to-Fe ratio, since both iron and chromium are part of the same mineral phase, i.e. the spinel, which requires structural dissociation. It has been proven on laboratory scale that high temperature carbochlorination (CO and Clz atmosphere) can be used to selectively remove iron from chromite. However, such methods are unlikely to be implemented on an industrial scale due to health, environmental and cost considerations. In light of this, an alternative approach to chromite chlorination, avoiding the use of chlorine and other toxic gasses, was investigated during this study. Since it was found that NaCI addition significantly improved the effectiveness of carbochlorination of chromite, the effect of adding only NaCI during high temperature treatment of chromite was investigated. The material utilised during this investigation consisted of local chromite, anthracite (source of carbon) and attapulgite clay (serving as a binder). These materials were mixed in a ratio and subsequently milled to 0 90 = 75fJm to represent materials and specifications similar to those used during pelletisation of the chromite in the pre-reduction ferrochrome production process. This mixture could also be used to generate a partially reducing atmosphere (CO rich) during high temperature treatment, which was similar to the reaction conditions utilised during carbochlorination. The abovementioned milled mixture was pelletised into cylindrical pellets with a die set and a hydraulic press. This experimental investigation was based on a mono-variance procedure, in that the four different variables investigated, i.e. maximum pellet treatment temperature, exposure time, wt% NaCI addition to the pellets and the atmosphere the pellets were exposed to, were varied one at a time during experimentation. After each alteration of the afore-mentioned variables, the Cr-to-Fe ratios, together with other parameters, were measured. Analyses undertaken included Scanning Electron Microscopy, with Energy-Dispersive X-ray Spectroscopy (SEM-EDS), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and cured breaking strength. Although this investigation did not focus on the cured breaking strength of the pellets, it is a very important industrial parameter and was therefore measured. Results indicated that the addition of NaCI had a definite effect. In both oxidising and partially reducing atmospheres the cured breaking strength of the cured pellets increased up to 800°C exposure, whereafter it decreased. This was attributed to melting of NaCI at 801 °C. In the oxidising atmosphere, the cured breaking strength increased again at temperatures higher than 1 000°C, due to the formation of a thin oxldised layer on the outside of the pellets, which could be confirmed by SEM analysis. Fine, metallic-like crystals were noticed inside and on the lids of crucibles in which pellets containing NaCI as an additive were cured at temperatures of 900°C or higher. SEM-EDS analysis and weight-ratio calculations revealed that these crystals were pure FeO. This indicated that some iron might have been liberated from the chromite spinel matrix. However, ICP-OES analyses revealed that Cr-to-Fe ratios did not change significantly under any of the experimental conditions (NaCI addition 5wt% to 15wt%, curing between 500'C and 1200'C, and oxidative/partially reducing atmospheres). The observed FeO crystals did not make any meaningful difference to the Crto- Fe ratio of the chromite, but was of great academic interest as iron was extracted from the chromite spinel. This indicated that it is not only the formation of low melting point species, such as those proposed in previous mechanistic studies of carbochlorination of chromite, but that molten NaCI alone could also initiate the extraction of iron out of chromite. According to the knowledge of the author, this is the first report of its nature in open literature. SEM and SEM-EDS analyses also proved that the addition of NaCI to the chromite/carbon/clay mixtures enhanced the rate of chromite pre-reduction. This finding was in agreement with earlier literature reports. In conclusion, it can be stated that the addition of NaCI alone cannot alter the Cr-to-Fe ratio of chromite during high temperature treatment. NaCI addition did, however, have an effect on other important parameters i.e. initiation of iron removal, cured breaking strength and the rate of chromite pre-reduction. From the results and experience gained in this study, certain recommendations with regard to possible future studies could also be made. This included investigating i) other single component additives to possibly alter the Cr-to-Fe ratio during high temperature treatment, ii) the effect of industrially relevant additives such as CaO/CaC03, Mg03 and SiOz on the rate of chromite pre-reduction and iii) the effect of different clays (e.g. attapulgite, bentonite, etc.) on the rate of chromite pre-reduction.
Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2010.

Neste trabalho estudou-se a evolução da redução da pelota auto-redutora de cromita contendo coque de petróleo, ferro-silício, cal hidratada, sílica e cimento Portland ARI (alta Resistência Inicial), para a obtenção da liga ferro-cromo alto carbono (FeCrAC). As principais variáveis estudadas foram: influência das adições de Fe-75%Si em sinergismo com coque de petróleo, adição de fluxantes, temperatura e tempo de redução. Além disso, foram realizadas experiências para confirmação dos resultados de auto-redução num forno rotativo de laboratório. Inicialmente os materiais (cromita, ferro-silício, coque de petróleo, cal dolomitica, sílica e cimento Portland ARI), foram caracterizados por análise química e análise granulométrica. Após a caracterização, os materiais, foram aglomerados na forma de pelotas (P1, P2, P3, P4 e P5), com adições de 0, 1, 2 e 4% Fe-75%Si, e adições de 2% Fe-75%Si e de fluxantes (3,83% cal dolomitica e 2,88% sílica), respectivamente. A redução das pelotas foi feita num forno de indução podendo atingir temperaturas de até 1973K (1700oC). Os ensaios experimentais foram realizados nas temperaturas de 1773K (1500°C), 1823K (1550oC) e 1873K (1600oC), utilizando-se cadinhos de grafite. Após os ensaios de redução os produtos obtidos (escória e metal) foram analisados por microscopia ótica, por microscopia eletrônica de varredura (MEV) e por análise de espectro de dispersão de energia (EDS). O processo de redução nas pelotas 1, 2, 3 e 4 segue os seguintes fenômenos i) via intermediários gasosos (CO/cromita) formam-se glóbulos metálicos nucleados na superfície das partículas de cromita, inicialmente rico em ferro; ii) estes crescem, pela redução na superfície da cromita deixando óxidos refratários na periferia da partícula de cromita original; iii) uma escoria incipiente se forma com os componentes da pelota (aglomerantes inorgânicos, cinza do redutor e fluxantes) e com a dissolução da ganga das partículas pequenas reduzidas da cromita; iv) a escória incipiente dissolve parte refratária da superfície da cromita, liberando a fase metálica e a escória vai se tornando cada vez mais refratária; v) o nódulo metálico segue crescendo e enriquecendo-se de cromo, reduzindo os óxidos de cromo e eventualmente de ferro dissolvido na escória incipiente; vi) o coalescimento da fase metálica é favorecido pela formação de escória e dissolução da ganga refrataria da cromita. O processo de redução da pelota 5 pela presença de fluxantes forma uma quantidade maior de escória inicial e apresenta os seguintes fenômenos: i) as reações indireta e direta reduzem as partículas finas de cromita, com formação de nódulos metálicos e fase escória nos primeiros instantes de redução; ii) os nódulos metálicos são formados pela redução das partículas finas de cromita. As partículas grandes sofrem pequena redução superficial e são encobertas pela escória, permanecendo dispersas na mesma; iii) a formação de escória encobrindo a cromita prejudica a redução gasosa aumentando o tempo de redução da mesma, porem facilita o coalescimento da fase metálica; iv) o nódulo metálico segue crescendo e enriquecendo-se de cromo, reduzindo aos poucos as partículas grandes de cromita. Existe regeneração do gás redutor (Boudouard) que pode ser diretamente com C do redutor ou com C dissolvido na fase metálica. A auto-redução carbotérmica das pelotas de cromita, na faixa de temperatura 1773K (1500oC) a 1873K (1600°C), sofre grande influência da temperatura, seja com ou sem adição de Fe-75%Si. O aumento da temperatura de 1773K (1500°C) para 1873K (1600°C) diminui o tempo para atingir redução completa conforme segue: i) 8 vezes para pelota sem Fe-75%Si; ii) 4 vezes para pelota com 1% de Fe-75%Si; e iii) 3 vezes para pelota com 2% de Fe-75%Si. Há um efeito significativo de adições de Fe-75%Si em pelotas auto-redutoras de cromita no tempo para atingir redução completa. O teor benéfico destas adições foi de 2%, contribuindo com aproximadamente 9% de calor necessário para redução completa, para as temperaturas ensaiadas de 1873K (1600ºC), 1823K (1550ºC) e 1773K (1500ºC). A evolução da redução é altamente sensível (diminui) com adição de fluxantes formadores de escória com temperatura líquidus abaixo de 1773K (1500ºC). A evolução da redução pela reação indireta (CO/cromita) é notavelmente mais rápida que a redução pela reação direta (C/cromita e C dissolvido na fase metálica/óxido de cromo na escória). A redução gasosa atuante nos primeiros estágios de redução, vai sendo prejudicada à medida que aumenta a quantidade de escória. As pelotas (1, 2, 3 e 4) sem adição de fluxantes (sílica e cal dolomítica), após reduzidas, são altamente porosas e têm pequena formação de fase escória se comparar com aquelas com adição de fluxantes com formação maior de fase escória (pelota 5). A pelota 3 com 2% de Fe-75%Si apresentou melhores resultados em relação ao tempo de redução. A pelota com adição de 4% Fe-75%Si (pelota 4), não apresentou diminuição do tempo de redução, devido a uma maior formação de escória que prejudica a reação indireta (mais rápida). As evidências micrográficas, auxiliadas por análises por EDS, mostraram que as reduções das partículas de cromita, foram praticamente completas quando as frações de reação se aproximam da unidade, confirmando a confiabilidade da metodologia utilizada. A redução da pelota auto-redutora, independente da sua composição, acontece de forma não isotérmica apesar de ser ensaiada numa temperatura isotérmica, apresentando-se um gradiente de temperatura entre a superfície e o centro da pelota, ao longo do tempo, mas esta desaparece conforme a reação progride tornando-se uniforme ao final da reação; evidenciando que a transferência de calor é a etapa lenta do processo devido: às reações de redução serem bastante endotérmicas; ao tamanho das pelotas; às altas temperaturas; e por ser um material poroso e refratário. A resistência a compressão das pelotas (1, 2, 3, 4 e 5) após 28 dias de cura e antes de serem reduzidas foi de ~4 kgf/pelota, porém tornou-se bastante alta após reduzidas (150 a 400 kgf/pelota); tornando-as aptas para carga em reatores de fusão. Estes resultados foram confirmados com ensaios no forno rotativo de laboratório, utilizando-se a pelota 2 (2% de Fe-75%Si), evidenciando: i) que as reduções de Cr e Fe foram praticamente completas (fração média de reação de 0,99) em 30 minutos de ensaio a 1500ºC; ii) a coalescência das partículas metálicas, obtidas por redução depende da capacidade da escória de dissolver os óxidos remanescentes na partícula de cromita reduzida; iii) há formação de fase incipiente de escória não-continua, aos 5 minutos de ensaio, pela parte da ganga do minério de cromita com os componentes de aglomerantes e/ou fluxantes; iv) a recuperação do teor metálico é alto (99%), em 30 minutos de ensaio, a 1500º C. Os resultados mostram um grande potencial do processo de auto-redução na produção de ferro-cromo alto carbono (FeCrAC).
The evolution of reduction of the self-reducing pellets of chromite for obtaining ferro-chromium high carbon (FeCrHC) was analyzed. The influences of Fe-75%Si additions, addition of fluxing agents, temperature and time of reduction were studied. The materials (chromite, ferro-silicon, petroleum coke, dolomite lime, silica and cement Portland), were characterized by chemical and particle size analysis. After characterization, the materials were agglomerated in the form of pellets (P1, P2, P3 and P4), with additions of 0, 1, 2 and 4% Fe-75%Si, respectively, and P5 with additions of 2% Fe-75%Si and fluxing agents (3.83% dolomite lime and 2.88% silica). The reduction of pellets was made using induction furnace with capability to reach temperatures up to 1973K (1700ºC). The experiments were performed at temperatures of 1773K (1500ºC), 1823K (1550ºC) and 1873K (1600ºC), using graphite crucibles. After the reduction the products (slag and metal) were analyzed by optical microscopy, scanning electronic microscopy (MEV) and energy dispersion spectrum analysis (EDS). The reduction process in pellets 1, 2, 3 and 4 followed phenomena as: i) gaseous reduction (CO/chromite) produces metallic globules on the surface of chromite particles, initially rich in iron; ii) these globules grow continuing the reduction at the periphery of chromite particles, leaving refractory oxides at this area of the original chromite particle; iii) an incipient slag is formed with the components of the pellet (inorganic binders, ash of reducer and fluxing agents) and with the dissolution of gangue from small particles of the reduced chromite; iv) the incipient slag dissolves refractory oxides remaining at the periphery of the chromite particles, liberating the metallic phase and the slag becomes more refractory; v) the metallic phase grows and becomes richer in chromium by reducing chromium oxides and eventually of iron dissolved in the incipient slag; vi) the coalescence of the metallic phase is favored by the slag formation and dissolution of refractory gangue of the chromite. The reduction process of pellet 5 follows as: i) indirect and direct reactions reduce fine particles of chromite, with formation of metallic nodules and slag phase at the beginning of reduction; ii) the metallic nodules are formed by the reduction of fine particles of chromite. Large chromite particles are reduced at the peripherical surfaces and are embebeded by the slag and remain dispersed in it; iii) the slag formed is harmful for the gaseous reduction and the time for completing the reduction is increased, but facilitates the coalescence of the metallic phase; iv) the metallic nodule follows growing and becomes richer in chromium. The carbothermic self-reduction pellets of the chromite at the temperature range of 1773K (1500ºC)-1873K (1600ºC), presents great influence of the temperature, either, with or without addition of Fe-75%Si. The increase of the temperature from 1773K (1500ºC) to 1873K (1600ºC) decreases the time for completing the reduction as: i) 8 times for pellet without Fe-75%Si; ii) 4 times for pellet with 1% of Fe-75%Si; and iii) 3 times for pellet with 2% of Fe-75%Si. A significant effect of additions of Fe-75%Si in self-reducing pellets of chromite in the reduction time was observed. The best addition was with 2% and its contribution was approximately 9% of necessary heat for complete the reduction, for the temperatures of 1873K (1600ºC), 1823K (1550ºC) and 1773K (1500ºC). The evolution of reduction is highly sensitive (it decreases) with addition of fluxing agents which form the slag with liquidus temperature below 1500ºC. The evolution of reduction for the indirect reaction (CO/chromites) is remarkably faster than that of the reduction by the direct reaction (C/chromite and C dissolved in the metallic phase/chromium oxide in the slag). At the beginning the gaseous reduction is predominant but it becomes less important with formation of larger amount of slag. The pellets (1, 2, 3 and 4) without addition of fluxing agents (silica and dolomite lime), after reduced, are highly porous and have small formation of slag phase than pellet 5 with addition of fluxing agents. Pellet 3 with 2% of Fe-75%Si presented the best results with relation to time for completing the reduction of chromite. The pellet with addition of 4% Fe-75%Si (pellet 4) did not present advantage with relation to that of 2% addition due to larger volume of slag formation. The micrograph analysis showed that the reductions of chromite particles practically were complete when the reaction fractions approach to the unit, confirming the confidence of the methodology used for determining the reaction fraction. The reduction of the self-reducing pellet, regardless its composition, happens by not isothermal way although it is submitted at isothermal temperature. The temperature gradient between surface and the core of the pellet is larger at the beginning but it disappears as the reaction progresses, becoming uniform with time. The heat transfer showed to be the slowest step of the process due to, the endothermic reactions of reduction, the size of the pellets, the high temperatures and porous nature and refractory material. The compression strength of the pellets (1, 2, 3, 4 and 5), after 28 days of curing, before of the reduction was ~4kgf/pellet but it increased up to 150 - 400 kgf/pellet; which are acceptable for charging the melting furnace for metal/slag separation. These results were confirmed by using laboratory rotating furnace, with pellet 2 (2% of Fe-75%Si), as: i) the reductions of Cr and Fe were practically complete (fraction of reaction 0,99) after 30 minutes of experiment at 1500ºC; ii) the coalescence of metallic particles, depends the capability of the slag to dissolve remaining oxides in the reduced chromite particle; iii) incipient not-continuous slag phase forms, at 5 minutes of experiment, from the gangue of the chromite and from the components of binders and/or fluxing agents; iv) the yield of metallic recovery is high (99%), after 30 minutes of experiment at1500º C. The results show that the self-reduction process presents a great potential for the ferro-chromium high carbon production (FeCrHC).

La durabilité chimique des alliages métalliques résulte notamment de la nature des défauts ponctuels assurant le transport au travers du film d’oxydation formé en surface. L'élaboration de couches d'oxyde modèles par oxydation thermique en pression contrôlée et ALD (Atomic Layer Deposition) et l'étude de leurs propriétés semi conductrices (conditionnées par la nature des défauts ponctuels) devrait permettre une meilleure compréhension des mécanismes de formation de ces couches d'oxyde
The chemical durability of the metal alloy results in particular from the nature of point defects providing transport through the oxidation film formed on the surface. Models oxide layers, grown by thermal oxidation and Alomic Layer Deposition, will be studied by photoelectrochemistry. This will provide us information about the semiconductive properties of the oxide, determined by the point defect in the oxide layer, and should allow us a better understanding of the formation mechanism of these oxide

In my thesis, I focus on the operating system Google Chrome OS, determination of its characteristics, definition of target group of the operating system's users and comparison of available applications with those available for other operating systems. In the theoretical part, historical development of Google is analysed first, followed by the issue of Google Chrome web browser and analyse of the operating system Google Chrome OS. The practical part starts with evaluation of survey on the operating system Google Chrome OS and is followed by multi-criteria analysis of variants comparing various categories of applications available for the operating system Google Chrome OS and other operating systems, in this case represented by the operating system Microsoft Windows 8.1 and distribution of Linux operating system Ubuntu 14.04 LTS.

Thesis deals with the operating system Google Chrome. First, the system is described (how it works, its user interface). Next there is a characteristic of applications that can be downloaded to the system and how they work. Another part is devoted to security, which consists of several elements. The end of the theoretical part is focused on Chromebooks - devices in which the operating system is sold on the market. The practical part is firstly characterized Dart programming language, which is then used for an application. This application shows not only work with the language, but also the benefits of Google Apps. It also shows compatibility with Windows and compares these two systems. In conclusion are described advantages and disadvantages of Chrome system and summarizes all knowledge arising from this work.

L’écotoxicité des éléments traces vis-à-vis du vivant est contrôlée par leur bioaccessibilité, leur biodisponibilité et leurs interactions avec le(s) site(s) d’action à l’intérieur des organismes. Dans le cas des certains éléments, l’existence dans le milieu naturel de plusieurs formes redox ayant des propriétés différentes doit aussi être prise en considération. Dans le cas spécifique du Cr, le consensus général considère le Cr(III) comme moins écotoxique que le Cr(VI). Toutefois, plusieurs études récentes soulignent que ce consensus devrait être réexaminé. Plus particulièrement, une connaissance insuffisante de la spéciation du Cr(III) pendant les tests en laboratoire pourrait avoir conduit à une sous-estimation de sa biodisponibilité et écotoxicité. Ce travail de thèse s’intéresse donc aux relations entre spéciation, biodisponibilité et bioaccumulation du Cr(III) et Cr(VI) dans différents modèles biologiques pour mieux appréhender leur potentiel écotoxique ainsi que leurs mécanismes d’interaction avec le vivant. Des études ont été menées chez les algues Pseudokirchneriella subcapitata et Chlamydomonas reinhardtii, le bivalve Corbicula fluminea et des cultures de cellules d’ovaire d’hamster chinois (CHO). L’ensemble des résultats de ce travail suggère que la validité du paradigme ‘le Cr(VI) est plus écotoxique que le Cr(III)’ serait dépendante des conditions d’exposition et du modèle biologique utilisé. Ce paradigme, accepté tant par la communauté scientifique que par les gestionnaires environnementaux, nécessite donc un réexamen afin d’assurer une meilleure gestion des risques liés au Cr
The ecotoxicity of the trace elements to the living organisms is controlled by their bioaccessibility, their bioavailability and their interactions with the site(s) of action inside the organisms. In the case of certain elements, the existence in the natural environment of several redox forms having different properties must also be taken into consideration. In the specific case of Cr, general consensus considers Cr(III) to be less ecotoxic than Cr(VI). However, several recent studies highlight that this consensus should be re-examined. In particular, insufficient knowledge of Cr(III) speciation during laboratory tests may have led to an underestimation of its bioavailability and ecotoxicity. This work focuses on the relationship between speciation, bioavailability and bioaccumulation of Cr(III) and Cr(VI) in different biological models in order to better understand their ecotoxic potential as well as their mechanisms of interaction with living organisms. Studies have been conducted on the algae Pseudokirchneriella subcapitata and Chlamydomonas reinhardtii, the bivalve Corbicula fluminea and cultures of Chinese hamster ovary cells (CHO). The results of this work suggest that the validity of the paradigm ‘Cr(VI) is more ecotoxic than Cr(III)’ would be dependent on the exposure conditions and the used biological model. This paradigm, accepted by both the scientific community and the environmental managers, requires a reassessment to ensure a better management of Cr risks

Le travail réalisé au cours de cette thèse concerne l'analyse des conséquences de l'oxydation à 950°C des alliages base Ni sur la composition de l'alliage à proximité de l'interface alliage/oxyde. Deux catégories d'alliages ont été analysées : alliages à faible teneur en chrome conduisant à l'oxydation interne et l'alliage chromino-formeur Ni-16Cr-9Fe.Une description complète des mécanismes de l'oxydation interne du chrome est obtenue à partir du développement du modèle analytique de Wagner d'oxydation interne et du développement du modèle numérique de Feulvarch. Ces modèles décrivent l'évolution de l'oxydation interne jusqu'à la transition oxydation interne / oxydation externe à 11 %poids de chrome environ.L'analyse par la spectroscopie d'électrons Auger de l'alliage modèle Ni-16Cr-9Fe à 950°C oxydé pendant 10 heures a permis d'explorer la zone à proximité immédiate de l'interface alliage/oxyde et de déterminer la concentration en chrome à l'interface à 0,5%poids (i.e. dans 20 premiers nanomètres), ce qui est en accord avec le modèle analytique de Wagner d'oxydation en couche compacte. La description des profils de déchromisation et des profils de cavités pour plusieurs temps d'exposition allant de 100h à 5000h a permis de mettre en évidence une corrélation forte entre ces deux phénomènes (même constante parabolique). Pour ces durées d'oxydation les profils de déchromisation présentent un point d'inflexion ce qui se traduit par l'augmentation de la teneur en chrome à l'interface. Les résultats sont interprétés dans le cadre d'un nouveau modèle analytique avec l'hypothèse d'injection des lacunes produites par l'effet Kirkendall au point d'inflexion.

Les oxydes de chrome permettent d’obtenir une large variété de couleurs, conduisant à leur utilisation répandue dans les arts céramiques. À la manufacture de Sèvres, leurs propriétés colorantes ont été explorées dès la découverte du chrome à la fin du 18ème siècle. Les travaux initiés par Brongniart ont conduit à une série de 76 pigments pour porcelaine composés d’oxydes de chrome, régulièrement synthétisés au laboratoire de la manufacture. Nous avons mené une étude exhaustive de ces pigments, permettant une classification basée sur leur analyse minéralogique et mettant en évidence le rôle du Cr3+ dans ces pigments, avec l’absence de Cr6+. Le mélange de ces pigments avec un composant incolore est appliqué sur la porcelaine, et porté à haute température afin d’obtenir un émail, c’est-à-dire une couche décorative partiellement vitreuse à la surface de la porcelaine. Les pigments composés de chrome se retrouvent dans les décors verts, roses, bleu-vert, marron et noirs. Après avoir caractérisé la spéciation et l’environnement du chrome dans l’ensemble de ces pigments, cette thèse s’est concentrée sur un pigment particulier, majoritairement composé de gahnite au chrome ZnAl2O4:Cr3+. Ce type de pigment a été identifié dans une série d’œuvres produites par la manufacture, entre la fin du 19ème et le début du 20ème siècle, correspondant à la période de l’art nouveau. La solution solide complète ZnAl2-xCrxO4 a été étudiée afin de préciser l’environnement structural du chrome, montrant l’importance des seconds voisins Cr dans les variations de couleur. Au cours de la cuisson de l’émail, un mécanisme de dissolution progressive des grains de pigments a été mis en évidence, résultant de l’interaction avec le composant incolore en cours de vitrification. Notre travail montre la faible solubilité du Cr dans le verre et sa spéciation préférentielle dans les cristaux, entrainant parfois des changements de phases ou des modifications de composition le long de solutions solides lors du traitement thermique à haute température. Ces modifications ont permis de mieux comprendre l’origine de la couleur dans les décors de porcelaine ainsi que les mécanismes de formation des émaux
Chromium oxides are widely used in the ceramic industry because they are characterized by a large variety of colors. At the French porcelain manufacture of Sèvres, their coloring properties have been explored since the discovery of chromium at the end of the 18th century. The researches initiated by Brongniart lead to a group of 76 pigments for porcelain composed of chromium oxides, regularly synthesized at the laboratory of the manufacture. An exhaustive study was done on these pigments, which enables to propose a classification based on their mineralogical analysis and highlights the role of Cr3+ in the color of the pigments, without Cr6+. The mixture of one or two pigments and an uncolored frit is applied on the porcelain, and fired at high temperature to obtain a glaze, i.e. a partly glassy decorative layer on the top of the porcelain. Pigments composed of chromium are used to obtain green, pink, greenish-blue, brown and black glazes. This thesis focuses on a particular pigment, mainly composed of gahnite doped with chromium ZnAl2O4:Cr3+. This kind of pigment was identified in a collection of artefacts produced by the manufacture between the end of the 19th century and the beginning of the 20th century. This period corresponds to the artistic movement Art Nouveau. The solid solution ZnAl2-xCrxO4 was studied to get the structural environment of Cr, showing the role of the second neighbor Cr in the color variation. During firing of the glaze, a mechanism of gradual dissolution of the initial grains of pigments was determined, resulting of reactions with the uncolored frit. Our work shows the low solubility of Cr in glass and its preferential speciation in crystals, leading to crystalline phase changes or modifications of composition along solid solutions during the thermal treatment at high temperature. These modifications enable to understand the origin of color in porcelain glazes and the mechanism of glaze formation

As a part of the continuing search for bioactive compounds with the Madagascar International Cooperative Biodiversity Group (ICBG), and in collaboration with the Natural Products Discovery Institute of the Institute for Hepatitis and Virus Research (IHVR), thirteen plant extracts were investigated for antiplasmodial activity, thirteen plant extracts were investigated for antiproliferative activity, and one extract was investigated for inhibitors of the shikimate pathway in Plasmodium falciparum. Bioassay-guided fractionation of the extracts led to the identification of nineteen compounds with both antiplasmodial and antiproliferative activity, and thirteen compounds with only antiproliferative activity. Thirteen of these compounds (2.1 – 2.9, 3.3, 3.4, 4.5, and 5.1) were previously unknown. In addition total synthesis was used to confirm the structure of one new compound (4.5) and two other new natural-product like compounds (4.6 and 4.7) were also synthesized and investigated for antiplasmodial activity.
Ph. D.

Na área de Química, uma das linhas mais tradicionais de pesquisa envolve o estudo de formação de complexos. Estes trabalhos são realizados de modo sistemático em nossos laboratórios, geralmente envolvendo cátions de metais de transição com vários ligantes (haletos, pseudo-haletos e carboxilatos). A intensa coloração desenvolvida pela maioria dessas reações de complexação, entre um metal e um pseudo-haleto, tem motivado o desenvolvimento de novos métodos espectrofotométricos para a determinação de íons em amostras reais. Espera-se que o crômio(III), da mesma forma que outros metais de transição, como o ferro, níquel e cobalto, forme complexos com o pseudo-haleto denominado azida ou azoteto (N3-). Desta forma, dando continuidade a esta linha de pesquisa, este trabalho teve como objetivo otimizar as condições experimentais para o sistema crômio(III)/azoteto. Diversos parâmetros como acidez, concentração de ligante, natureza e concentração de solvente, bem como o efeito da temperatura, foram investigados. Reunidas as melhores condições (otimização), montou-se o método analítico que possibilita a determinação do metal. As condições ideais empregadas foram: concentração analítica de ligante e ácido perclórico de 493 e 12,0 mmol L-1, respectivamente, numa temperatura de 25ºC, em meio aquoso. Para a determinação no ultravioleta, o tempo de espera para cada medida foi de 1 hora, enquanto que na região do visível, foi de apenas 30 minutos. Os máximos de absorção ocorreram em torno de 646, 480 e 287 nm, com coeficientes de absortividade molar médios de 184±1, 157±1 e (1,481±0,008) 104 L mol-1 cm-1, respectivamente. As condições estabelecidas foram promissoras, permitindo uma boa repetitividade nas regiões do visível e ultravioleta. Para o método desenvolvido, os íons mais interferentes na região do visível foram: Fe3+, Co2+ e Cu2+ e, na região do ultravioleta, foram HC8H4O4-, Fe3+, Cu2+ e Hg2+. Por meio de um sistema com duas equações, a determinação simultânea de crômio e ferro mostrou-se possível e com erros dentro dos limites analiticamente aceitáveis. Sem a utilização do sistema de equações, verificou-se ainda a possibilidade de se determinar crômio diretamente, mesmo na presença de ferro, no comprimento de onda de 646,5 nm, com um erro menor que 5%, desde que a concentração de ferro não ultrapasse 4 mg L-1. Aplicações analíticas para o método proposto foram testadas em dois suplementos alimentares contendo crômio. Os resultados foram comparados e mostraram-se concordantes com os da técnica de espectrometria de absorção atômica, para o suplemento contendo crômio quelato. Para o suplemento alimentar contendo picolinato de crômio, houve uma discordância dos resultados, indicando a existência de interferentes da matriz que não puderam ser eliminados. Um teste com uma amostra sintética de crômio mostrou um erro de cerca de 1,0% para o método proposto. Em condições ideais, e com um controle dos interferentes presentes, o método proposto mostra-se preciso, simples e barato. Considerando valores de absorbância entre 0,2 e 0,8 (faixa ideal), pode-se determinar o metal nos intervalos de 1,27 a 5,09 mmol L-1 a 480,5 nm, de 1,09 a 4,35 mmol L-1 a 646,5 nm e, finalmente, entre 0,702 a 2,81 mg L-1 no ultravioleta (287 nm).
In Chemistry, the study of complexes formation is one of the most traditional research lines. These studies are done under systematic manners in our laboratories, usually involving metallic transition cations with several ligands (halides, pseudohalides and carboxylates). The intense coloration developed by the major part of those complexation reactions, between a metal and a pseudohalide, has encouraged the development of new spectrophotometric methods for determination of ions in real samples. Conforming others transition metals, (iron, nickel and cobalt), which form complexes with the pseudohalide azide (N3-), we tried to observe the same behavior to chromium(III). Continuing this research line, this work had as principal objectives optimizes the experimental conditions for the chromium(III)/azide system. Several parameters were investigated as acidity, ligand concentration, nature and solvent concentration, as well as temperature effect for optimization of experimental conditions. Gathering all these conditions, the analytical methods were tested for the metal determination. The ideal conditions employed were: analytical ligand concentration and perchloric acid of 493 and 12.0 mmol L-l respectively, at 25 °C, in aqueous medium. For the studies in the ultraviolet region, the contact time before the measures was one hour, while for studies in the visible band measures were made after 30 minutes. The wavelengths of maximum absorption were encountered at 646, 480 and 287 nm, with molar absorptivity coefficients (mean) of 184±1, 157±1 and (1.481±0,008) 104 L mol-1 cm-1, respectively. The established conditions were promising, allowing a good precision in the visible and ultraviolet region. For the developed method, the more interfering ions on visible band were: Fe3+, Co2+ and Cu2+ and, on the ultraviolet band, were HC8H4O4-, Fe3+, Cu2+ and Hg2+. Through a system of two equations, the simultaneous determination of chromium and iron ions was possible with errors within the tolerance range. Without the simultaneous equations system, it was verified directly the possibility of chromium determination in the presence of iron, at a wavelength of 646.5 nm, with an error smaller than 5%, since the concentration of iron doesn’t exceed 4 mg L-l. In this proposed method, analytical applications were tested for two alimentary supplements containing chromium. The results were compared, being concordant with those of atomic absorption technique for the supplement containing chromium chelate, while for the alimentary supplement containing chromium picolinate, the results were in disagreement with the results, indicating the existence of matrix interferences which could not be eliminated. A test with a synthetic sample of chromium had an error of about 1.0% for the proposed method. Under ideal conditions, and with some control of possible interfering, the proposed method can be precise, accurate, simple and cheap. Considering the absorbance values between 0.2 and 0.8 (ideal range), the metal can be determined from 1.27 to 5.09 mmol L-l in 480 nm; 1.09 to 4.35 mmol L-l in 646 nm and, finally, from 0.702 to 2.81 mg L-l at 287 nm (ultraviolet).

Les oxydes de chrome permettent d’obtenir une large variété de couleurs, conduisant à leur utilisation répandue dans les arts céramiques. À la manufacture de Sèvres, leurs propriétés colorantes ont été explorées dès la découverte du chrome à la fin du 18ème siècle. Les travaux initiés par Brongniart ont conduit à une série de 76 pigments pour porcelaine composés d’oxydes de chrome, régulièrement synthétisés au laboratoire de la manufacture. Nous avons mené une étude exhaustive de ces pigments, permettant une classification basée sur leur analyse minéralogique et mettant en évidence le rôle du Cr3+ dans ces pigments, avec l’absence de Cr6+. Le mélange de ces pigments avec un composant incolore est appliqué sur la porcelaine, et porté à haute température afin d’obtenir un émail, c’est-à-dire une couche décorative partiellement vitreuse à la surface de la porcelaine. Les pigments composés de chrome se retrouvent dans les décors verts, roses, bleu-vert, marron et noirs. Après avoir caractérisé la spéciation et l’environnement du chrome dans l’ensemble de ces pigments, cette thèse s’est concentrée sur un pigment particulier, majoritairement composé de gahnite au chrome ZnAl2O4:Cr3+. Ce type de pigment a été identifié dans une série d’œuvres produites par la manufacture, entre la fin du 19ème et le début du 20ème siècle, correspondant à la période de l’art nouveau. La solution solide complète ZnAl2-xCrxO4 a été étudiée afin de préciser l’environnement structural du chrome, montrant l’importance des seconds voisins Cr dans les variations de couleur. Au cours de la cuisson de l’émail, un mécanisme de dissolution progressive des grains de pigments a été mis en évidence, résultant de l’interaction avec le composant incolore en cours de vitrification. Notre travail montre la faible solubilité du Cr dans le verre et sa spéciation préférentielle dans les cristaux, entrainant parfois des changements de phases ou des modifications de composition le long de solutions solides lors du traitement thermique à haute température. Ces modifications ont permis de mieux comprendre l’origine de la couleur dans les décors de porcelaine ainsi que les mécanismes de formation des émaux
Chromium oxides are widely used in the ceramic industry because they are characterized by a large variety of colors. At the French porcelain manufacture of Sèvres, their coloring properties have been explored since the discovery of chromium at the end of the 18th century. The researches initiated by Brongniart lead to a group of 76 pigments for porcelain composed of chromium oxides, regularly synthesized at the laboratory of the manufacture. An exhaustive study was done on these pigments, which enables to propose a classification based on their mineralogical analysis and highlights the role of Cr3+ in the color of the pigments, without Cr6+. The mixture of one or two pigments and an uncolored frit is applied on the porcelain, and fired at high temperature to obtain a glaze, i.e. a partly glassy decorative layer on the top of the porcelain. Pigments composed of chromium are used to obtain green, pink, greenish-blue, brown and black glazes. This thesis focuses on a particular pigment, mainly composed of gahnite doped with chromium ZnAl2O4:Cr3+. This kind of pigment was identified in a collection of artefacts produced by the manufacture between the end of the 19th century and the beginning of the 20th century. This period corresponds to the artistic movement Art Nouveau. The solid solution ZnAl2-xCrxO4 was studied to get the structural environment of Cr, showing the role of the second neighbor Cr in the color variation. During firing of the glaze, a mechanism of gradual dissolution of the initial grains of pigments was determined, resulting of reactions with the uncolored frit. Our work shows the low solubility of Cr in glass and its preferential speciation in crystals, leading to crystalline phase changes or modifications of composition along solid solutions during the thermal treatment at high temperature. These modifications enable to understand the origin of color in porcelain glazes and the mechanism of glaze formation

Ce travail de thèse a eu pour buts : - d'édifier une échelle de basicité dans les milieux verres fondus. Une cellule électrochimique permettant la détermination in situ des activités en Na2O et appelée méthode des piles de concentrations a été développée. Cette méthode a permis de mesurer la basicité de verres de compositions simplifiées et industrielles pour les comparer entre eux et critiquer les modèles de basicité existants. - de déterminer la solubilité de Cr2O3 dans les fontes de verres et montrer que la teneur en chrome dissous dépend de la température d'expérience, de la basicité du verre, de sa structure et de la pO2 dans l'atmosphère environnante. L'oxyde de chrome est un oxyde amphotère dans ces milieux. - d'étudier l'interaction entre le chrome métal et les fontes des verres Na2O-xSiO2 pour mettre en évidence l'influence de la basicité des verres, de la température et de la pression partielle en oxygène sur les cinétiques et le mécanisme de corrosion du chrome
The aims of this work were : - To build a basicity scale in glass melts. A specific electrochemical cell was designed to measure in situ the Na2O activities. This method allowed the basicity measurement of several silicate melts (of with simplified and industrial compositions) to compare these glasses between each other and to criticise the existing glass basicity models. - To determine the Cr2O3 solubility in glass melts. The chromium content that is dissolved in the glass depends on experimental temperature, glass basicity, melt structure and pO2 in the atmosphere surrounding the glass. Cr2O3 exhibits an amphoteric behaviour in glass melts. -To study the interaction between Cr metal and Na2O-xSiO2 glass melts in order to determine the influence of glass basicity, experimental temperature and partial pO2 on chromium corrosion kinetics and mechanism

Les déplacements chimiques expérimentaux observés à partir des spectres des oxydes simples et d'oxydes de Cr ou de V de structure spinelle à degré d'oxydation connu permettent d'établir une relation entre la valeur de ce déplacement chimique et le degré d'oxydation. Applications: 1) mise en évidence d'une très faible teneur en V (ii) dans les composés du système Fe(1+x)O(4) 2) dosage semiquantitatif du Cr (vi) dans les catalyseurs "chromite de cuivre"

Les couches de conversion actuellement utilisées dans l'industrie aéronautique, pour protéger le métal de la corrosion et favoriser l'adhérence de la peinture, contiennent du chrome hexavalent, composé toxique et cancérigène dont l'utilisation va prochainement être interdite par la réglementation européenne REACh. L'une des pistes envisagée pour remplacer ces couches chromatées est l'utilisation de couches de conversion TCP (Trivalent Chromium Protection). Ces travaux portent sur l'étude des couches de TCP et s'attachent à caractériser chaque étape du traitement de surface industriel, incluant les étapes de prétraitement et de post-traitement. Des techniques d'analyse de surface (XPS, ToF-SIMS, AFM, MEB et PM-IRRAS) ont été utilisées pour analyser la composition chimique et la morphologie de la surface après chaque étape du traitement. Cette étude a été réalisée sur un alliage d'aluminium AA 2024-T3, très utilisé en aéronautique pour ses propriétés mécanique, mais présentant une faible résistance à la corrosion. Les résultats obtenus ont notamment mis en évidence que la couche de TCP se forme sur toute la surface de l'alliage (composés intermétalliques et cavités), et ont permis de comprendre comment le post-traitement permet d'améliorer la résistance à la corrosion de la couche de TCP. Ces travaux s'intéressent également à des couches de conversion sans chrome, à base de zirconium, étudiées ici pour servir de point de départ au développement d'une conversion sans chrome qui respecterait les exigences de résistance à la corrosion
Conversion coatings are used in aerospace industry to protect the metal from corrosion and to promote paint adhesion. Currently, chromate conversion coatings are used, but chromate is toxic and carcinogenic and its use will be forbidden by the European REACh regulation. TCP (Trivalent Chromium Protection) conversion coatings, are considered as a promising alternative to replace chromate conversion coating. This work focuses on the characterisation of the TCP layer and considers each step of the industrial surface treatment, including pre-treatment and post-treatment steps. Surface analytical techniques (XPS, ToF-SIMS, AFM, SEM and PM-IRRAS) were used to analyse the chemical composition and morphology of the surface after each step in the process. This work was done on an aluminium alloy AA 2024-T3, commonly used in the aerospace industry for its good mechanical properties, but poorly resistant to corrosion. The results obtained demonstrate, among other things, that the TCP layer totally covers the surface (intermetallic compounds and cavities) and enable to understand how the post-treatment can improve the corrosion resistance of the TCP coating. This work also focuses on conversion coating based on zirconium, which are studied here to be used as a starting point to develop a new conversion coating without chromium, meeting the corrosion resistance requirement

La corrosion haute température d’alliages par les verres fondus est un problème rencontré dans plusieurs procédés industriels tels que la vitrification des déchets nucléaires ou encore la production de la fibre de verre. Les alliages base nickel contenant une grande quantité de chrome sont particulièrement intéressants pour ces applications, notamment grâce à leur bonne résistance à la corrosion due à leur capacité à développer une couche homogène et couvrante de chromine (Cr2O3) lors d’une oxydation à haute température. De par la faible solubilité de l’oxyde dans les silicates fondus, cette couche protège l’alliage d’une corrosion rapide par le verre. Lors de la mise en contact avec le milieu corrosif à haute température, la couche de chromine est soumise à une compétition entre sa dissolution par le verre fondu et à sa formation simultanée par oxydation de l’alliage. La dissolution totale de la couche est appelée dépassivation et est néfaste car elle conduit à une corrosion accélérée de l’alliage par remise en contact direct avec le liquide. La stabilité de la couche d’oxyde au contact du verre est donc un point clé pour optimiser la durée de vie des matériaux. Ainsi, il convient de comprendre les phénomènes assurant ou non la stabilité de cette couche d’oxyde. In fine, le développement d’outils prédictifs permettrait l’optimisation des matériaux et des conditions opératoires des procédés industriels. Pour répondre à cette problématique, l’alliage modèle Ni-30Cr a été sélectionné de même que des compositions de verres binaires Na2O-SiO2 et borosilicatés Na2O-B2O3-SiO2. L’approche consiste en une étude expérimentale visant à déterminer les conditions de température et de composition assurant la stabilité de la couche d’oxyde développée sur l’alliage immergé. Une modélisation de la corrosion de l’alliage est ensuite proposée afin de définir les paramètres prédisant la stabilité de la couche d’oxyde. L’étude de ce modèle révèle que la dépassivation est liée à un manque d’O2 dans liquide, ce qui limite la cinétique de formation de l’oxyde. La confrontation du modèle aux résultats expérimentaux permet valider cette conclusion
High temperature corrosion of alloys induced by molten glasses is an issue encountered in several industrial processes such as nuclear waste vitrification or glass fiber production. Nickel-based alloys containing a large amount of chromium are especially interesting for these applications, notably due to their good corrosion resistance thanks to their ability to develop an homogeneous and covering chromia (Cr2O3) layer through high temperature oxidation Due to its low solubility in molten silicates, this layer protects the alloy from a fast corrosion induced by the glass. When put in contact with the corrosive medium at high temperature, the chromia layer is submitted to a competition between its dissolution by the molten glass and its simultaneous formation by oxidation of the alloy. The complete dissolution of the scale is called depassivation and is detrimental because it leads to an accelerated corrosion of the alloy due to a direct contact with the melt. The stability of the oxide scale is the key point for optimizing the materials lifetime. Thus, the phenomena ruling the stability or not of the oxide scale must be fully understood. In fine, the development of forecasting tools might allow to optimize the materials and the operating conditions of the industrial processes. To answer to these issues, the Ni-30Cr model alloy was selected so as binary Na2O-SiO2 and ternary Na2O-B2O3-SiO2 glass compositions. The conditions (temperature and composition) ensuring the stability of the oxide scale developed on the immersed alloy are studied through an experimental approach. A modelling of the alloy corrosion is then proposed, which allows to define the parameters forecasting the oxide scale stability. The study of the model reveals that the depassivation is linked to a lack of O2 in the liquid, which limits the formation kinetic of the oxide. The model is validated after comparison with experimental results

Ce travail traite des possibilites de recuperation de l'acide chromique a partir des effluents de traitement de surface. L'utilisation de l'ether couronne dc18-6 dans un procede d'extraction liquide-liquide ou membrane liquide semble interessante. L'extraction de l'acide chromique par le cd18-6 en presence et en absence d'acide sulfurique a montre que l'espece extraite predominante dans la phase organique est (h#3o#+l)hcr#2o#7#. Malgre la presence d'une forte concentration d'acide sulfurique, celui-ci est faiblement coextrait. Les resultats ont montre que l'utilisation de l'ether couronne dc18-6 permet de concentrer l'acide chromique de 2 a 8 fois a partir d'un effluent contenant 0,09 a 0,5 m (5 a 25 g/l) en crvi. D'autres extractants plus usuels qui peuvent solvater h#+ ou h#3o#+ (tbp, butex) extraient tres faiblement l'acide chromique dans des conditions similaires a celles utilisees par le dc18-6. Par contre, le topo extrait fortement l'acide chromique, mais aussi l'acide sulfurique du fait de sa forte basicite; il ne permet donc pas la concentration de l'acide chromique. La presence des ions chlorures modifie fortement les proprietes extractantes du dc18-6 par rapport au cr(vi) du fait de la formation et de l'extraction de l'anion tres stable cro#3cl#

The Voskhod podiform chromitite is one of more than 30 chromitite deposits that collectively form the Main Ore Field (MOF) within the Kempirsai Massif, in Kazakhstan. The MOF is the largest podiform chromitite ore-field in the world. The Voskhod deposit, encased in a serpentinised dunite halo, is situated within harzburgite units that comprise the mantle sequence of the Kempirsai ophiolite. This study arose from a unique opportunity to work on drill core samples through an un-mined podiform chromite deposit and investigate its internal structure, composition and genesis. The 18Mt ore-body has a strike of 600 m, is 170 m to 360 m wide and has an average thickness of 39 m. It has an immediate dunite halo between 1 m and 5 m thick. The ore body is made up of multiple stacked chromitite layers. Mineralised layers are separated by barren dunite or by weakly disseminated dunite lenses ranging from <1 m to 50 m. The style of mineralization varies throughout the ore body; the central region is dominated by thick (>5 – 45 m) units of massive chromite (>80% chromite), with progression towards the south west disseminated chromite (10 – 40% chromite) becomes increasingly abundant. Drill core logging and cross-section profiling of the internal structure of the ore body has identified an intricately connected network of what appear to be chromite-filled channel-ways. Outside of the halo the host rocks are inter-layered harzburgite and dunite. Accessory chromite in harzburgite has an average Cr# of 0.31 compared to Cr# 0.49 in the dunite. The harzburgites are depleted, having formed from intermediate degrees of partial melting (~15 – 18 %) of a fertile mantle source at a mid-ocean ridge (MOR) setting. The dunite units have transitional geochemical fingerprints that imply they formed from the interaction of MOR mantle harzburgite with both mid ocean ridge baslt-melt and an arc derived-melt. They are not the products of extremely high degrees of partial melting. The encasing dunite halo is extensively serpentinised (>80%). Chromite is only present as an accessory phase having an average Cr# of 0.62. The dunite has a geochemical signature indicating that it formed by reaction between residual harzburgite and a boninite melt in supra-subduction zone (SSZ) tectonic setting. A variety of geochemical fingerprints have been identified; residual MOR harzburgite, reacted-MOR dunite, reacted-SSZ dunite and harzburgite, indicating that the mantle section has had a ii polygenetic tectonic evolution, recording both ocean basin opening (MOR setting) and closing (SSZ setting) events. Trace element and REE whole rock geochemistry of the chromitites and associated host rocks provide evidence of depletion and a later-stage LREE-enrichment event. LREE-enrichment is most intense within and immediately adjacent to the chromitite. Chromites from the ore zone are at the Cr-rich extreme for podiform chromites (Cr# ave. 0.80-0.85) and are TiO2 poor (ave. 0.16 wt%), similar to chromite in boninite worldwide and nearby. Al/Ti ratios have been used to calculate the composition of the parent melts from which the Voskhod podiform chromitite crystallised: compositions that are synonymous with a boninite melt composition. Chemical variation in chromite is systematic and on a much smaller scale than was anticipated. Even variations in a single thin section provide key evidence for different magmatic processes. An apparent melt-rock reaction in harzburgite has been examined in freeze-frame. The chromite chemistry has been investigated at 50 cm, 1 cm and 1 mm scales. Compositional differences were identified on the basis of MgO% and FeO(t)% compositions. Diagrams FeO-Fe2O3 and Cr# - Mg# were used to demonstrate the variations and identify relationships. Broad cryptic layering on a 50 cm scale has been found as well as fine-cryptic layering on a 1 – 8 cm scale. The variations are interpreted to reflect differences in the mineral phases crystallised from the melt; periods when on chromite only crystallised are distinguished from periods when both chromite with olivine crystallised. It seems likely that the deposit is made up of thousands of episodes of chromite accumulation that formed in an intermittently replenished open-system. It also seems likely that the conduit was never a single melt-filled cavity; instead melt flow was focused through the mantle over an extended period. The conduit appears to be comprised of multiple branches, as chromite (± olivine) crystallised from the melt the channel-way became blocked and the melt was forced to deviate and make a new pathway through the mantle. As time elapsed the process resulted in the formation of stacked chromitite lenses, creating an orebody that has an internal arrangement of chromitite and dunite unites which resemble a stacked braided 'delta'.

With device scaling, the current optical lithography technique is reaching its technological limit to print small features. Extreme Ultra-Violet (EUV) lithography has shown promise to print extremely thin lines reliably and cost-effectively. Many challenges remain before introducing EUV to large scale manufacturing. The main challenge addressed in this study is particle removal from EUV mask surfaces (CrON1, CrON2, and fused silica) and thermal oxide (SiO₂). Effective pre-clean procedures were developed for each surface. As chemical cleaning methods fail to meet SEMATECH criteria, addition of megasonic energy to EUV mask cleaning baths is seen as a promising cleaning methodology. As the requirement to print fine lines needs to be met, all materials used in EUV mask fabrication either absorb the incident EUV wavelength light or reflect it. Therefore, the masks used in the industry will be reflective instead of the conventional transmissive masks. Also, for the same reason, no protective pellicle can be used leading to all the surfaces unprotected from particle contamination. To avoid the detrimental effect of the particle contamination, a cleaning study for nano-particle removal was performed. A dark field microscope was utilized to study the removal of gold nano-particles from surfaces. The cleaning procedures utilized H₂SO₄ and NH₄OH chemistries with and without megasonic irradiation. The cleaning variables were bath concentration, temperature, and megasonic power. The contamination variables were the gold nanoparticles charge and size, from 40nm to 100nm. For 100 nm negatively charged gold nano-particles deposited on a CrON1 surface, a 1:10 H₂SO₄:DI bath at boiling temperature (101°C) without megasonics gave high particle removal efficiency (PRE) values as did a 1:10 H₂SO₄:DI bath at 35°C with 100W megasonics. Comparison of removal of poly diallyl-dimethyl ammonium chloride (PDAC) coated and uncoated gold nano-particles deposited on a CrON1 surface using dilute H₂SO₄ baths indicated that the coated, positively charged nano-particles were more difficult to remove. PRE trends for different baths indicate surface dissolution (shown to be thermodynamically favorable) as the particle removal mechanism. However, experimental etch rates indicated minimal surface etching in a 10 minute bath. Increased surface roughness indicated possible local galvanic corrosion at particle sites. Low surface etching results meet SEMATECH requirements. During the fused silica surface cleaning study, particle charge (negative) and size (100 nm) of the contamination source and cleaning bath chemistry (NH₄OH) were kept constant. Low PREs were obtained at room temperature for all NH₄OH bath concentrations; however, high PREs were obtained at an elevated temperature (78°C) without megasonics and at room temperature in more dilute chemistries with megasonic power applied. Similar PRE trends were demonstrated for thermal SiO₂ surfaces. The experimental etch rates of the thermal SiO₂ agree with published values.

University of Central Florida College of Engineering Thesis
This research addressess emissions of hexavalent chromium mist from hard chromium electroplating operations. Most of these emissions are typically captured by a ventilation stack and directed to a pollution control device; those which escape capture are called fugitive emissions. Releases of toxic materials such as hexavalent chromium must be reported annually to the Environmental Protection Agency (EPA) under provisions of the Superfund Amendments and Reauthorization Act (SARA) Title III of 1986 via the Toxic Release Inventory (TRI), Form R. The objectives were: (1) to provide estimates of fugitive hexavalent chromium emissions for the completion of Form R; and (2) to develop a predictive model for stack and fugitive emissions versus process and ventilation parameters. The database for stack emissions included published results from EPA studies. Fugitive release data were generated by field characterization at two operating facilities. Supplemental data for stack releases were also obtained during this field activity. The fugitive releases were documented to represent a small portion of the total atmospheric discharge; in most instances, the fugitive releases were less than the detection capability of the smapling/analytical protocols. Stack releases were successfully correlated with a measure of production activity (ampere-hours), production capacity (mass of chromium in the process bath), tank dimensions (plating bath surface area), and ventilation efficiency (ventilation slot area). This effort was supported by the EPA in the form of a cooperative agreement with the American Electropaters and Surface Finishers Society (AESF).
M.S.;
Civil and Environmental Engineering
Engineering;
Environmental Engineering
112 p.
viii, 112 leaves, bound : ill. ; 28 cm.

As a result of increasing cost, efficiency and environmental pressures ferrochrome producers strive towards lower overall energy consumption. Increases in local electricity prices have placed particular pressure on South African ferrochrome producers. Pelletised chromite pre–reduction is likely the currently applied ferrochrome production process option with the lowest specific electricity consumption. In this process fine chromite, together with a carbonaceous reductant and a clay binder is milled, pelletised and pre–reduced. In this dissertation it is demonstrated that the functioning of the clay binder in this process is not as straightforward as in conventional metallurgical pelletisation processes, since the cured pre–reduced pellets are characterised by an oxidised outer layer and a pre–reduced core. Conventional performance characteristics of clay binders (e.g. compressive strength and abrasion resistance) therefore have to be evaluated in both oxidative sintering and reducing environments. Two clay samples, i.e. attapulgite and bentonite, were obtained from a local ferrochrome producer and investigated within the context of this study. Results indicated that the compressive and abrasion resistance strengths of oxidative sintered pellets for both clays were substantially better than that of pre–reduced pellets. Thus, although the objective of the chromite pre–reduced process is to achieve maximum pre–reduction, the strength of pre–reduced chromite pellets is significantly enhanced by the thin oxidised outer layer. The strength of the bentonite–containing pellets was found to be superior in both pre–reducing and oxidative sintering environments. This is significant, since the attapulgite clay is currently the preferred option at both South African ferrochrome smelting plants applying the pelletised chromite pre–reduction process. Although not quantitatively investigated, thermo–mechanical analysis indicated that the hot strength of the attapulgite pellets could be weaker than the bentonite–containing pellets. The possible effects of clay binder selection on the level of pre–reduction were also investigated, since it could have substantial efficiency and economic implications. For both case study clays investigated, higher clay contents resulted in lower pre–reduction levels. This has relevance within the industrial process, since higher clay contents are on occasion utilised to achieve improved green strength. The average pre–reduction of the bentonite–containing pellets were also consistently higher than that of the attapulgite–containing pellets. Again, this is significant, since the attapulgite clay is currently the preferred option. In general the case study results presented in this dissertation indicated that it is unlikely that the performance of a specific clay binder in this relatively complex process can be predicted; based only on the chemical, surface chemical and mineralogical characterisation of the clay.
Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2012.

O processo de transformação da pele em couro envolve uma seqüência complexa de reações químicas e processos mecânicos, no qual o curtimento representa fundamental estágio, por propiciar à pele características como qualidade, estabilidade hidrotérmica e excelentes propriedades para uso. O sulfato básico de cromo trivalente é o agente curtente predominantemente empregado no curtimento de peles em todo o mundo. É produzido a partir do cromato de sódio, industrialmente obtido do minério de cromo. Consideráveis quantidades de resíduos sólidos contendo cromo são geradas pelas indústrias coureira e calçadista. Estes resíduos tem sido motivo de preocupação constante, uma vez que são considerados perigosos devido a presença do cromo. O processo de incineração destes resíduos é uma importante alternativa a ser considerada, em decorrência de suas características de redução de massa, volume e possibilidade de aproveitamento da energia térmica dos gases de combustão. O processo de incineração dos resíduos das indústrias coureira e calçadista dá origem a cinzas contendo cerca de 40% de cromo que pode ser submetida a um processo de recuperação. Este trabalho apresenta os resultados da pesquisa sobre a utilização das cinzas, provenientes da incineração dos resíduos sólidos da indústria coureira e da indústria calçadista, para a produção de cromato de sódio(VI). No processo de planejamento e de condução dos experimentos foram utilizadas as técnicas de Planejamento Fatorial 2k, Metodologia de Superfície de Resposta e Análise de Variância na avaliação da produção de cromato de sódio(VI). Os fatores investigados foram: temperatura, taxa de aquecimento, tempo de reação, vazão de ar e quantidade de dolomita. A partir das variáveis selecionadas identificaram-se como parâmetros importantes a temperatura e a taxa de aquecimento. As superfícies de resposta tridimensionais obtidas a partir dos modelos de segunda ordem ajustados aos dados experimentais, apresentaram o comportamento do efeito conjugado dos fatores temperatura e taxa de aquecimento sobre a variável resposta grau de oxidação, desde a temperatura de inicio da reação química até a temperatura limite utilizada industrialmente. As condições de operação do processo de produção de cromato de sódio(VI) foram otimizadas. Os níveis ótimos dos fatores de controle aplicados as cinzas dos resíduos da indústria calçadista, geradas em uma planta piloto com incinerador de leito fixo, com tecnologia de gaseificação e combustão combinadas, apresentaram um grau de oxidação superior a 96% para as cinzas coletadas no ciclone e de 99,5% para as cinzas coletas no reator de gaseificação. Os resíduos sólidos, as cinzas e o produto de reação foram caracterizados por análises químicas, fluorescência de raio-X, microscopia eletrônica de varredura e difração de raio-X.
The transformation process of hides and skins in leather involves a sequence of complex chemical reactions and mechanical processes. Amongst these, tanning is the fundamental stage, which gives these leather characteristics: top handling quality, high hydrothermal stability and excellent properties of use. Trivalent chromium salts, typically hydrated basic chromium sulfate, are the most widely used tanning agent in the world. It is derived from Sodium Chromate that is industrially derived from ore chromate. Most quantity of solid wastes with chromium comes from leather and leather shoes industries. These leather wastes have been being the reason of constant worry because chromium makes it dangerous. The leather waste incineration process is an important alternative to be considered, because it results in reduction of mass, volume and allow make use of thermal energy. The incineration process waste residues, of leather and leather shoes industries, origins ashes with 40% of chromium that can pass through a process of recuperation. This paper presents the research results about ashes utilization, deriving from waste residues incineration of leather and shoes industry to produce Sodium Chromate (VI). The process of experimental and the process of planning were established according to statistical design of experiments. It was utilized techniques of 2k factorial designs, Response Surface Methodology and Analysis of Variance in the production evaluation of sodium chromate (VI). The factors studied were: temperature, heating rate, time reaction, flow rate of air and quantity of dolomite. Based on selected variables were identified as important parameters: temperature and heating rate. A second order polynomial regression equation was derived to plot 3-D response surface that clearly illustrate the relationship between the temperature and the rate heating of oven on chromium oxidation degree, since the beginning temperature of the chemical reaction until the limit temperature used at industries. The operating condition of the production process of sodium chromate (VI) was optimized. Using the great levels of the controlling factors on waste ashes residues of shoes industries, created at a pilot plant with fixed bed reactor with integrated gasification combined cycle, presented an oxidation degree of over 96% of ashes collected from cyclone, and of 99,5% of ashes collected from gasification chamber. The leather waste, ashes and reaction product were characterized by chemical analysis, fluorescence X-ray, scanning electron microscopy (SEM) and X-ray diffraction (XRD).

A method was developed to synthesize chromium-chromium sulfide (Cr-CrS) cermets (ceramic-metal composites) without the use of a hot isostatic press. This synthesis method allowed for the production of materials that range from almost pure ceramics to highly metalized cermets. These materials were then impacted under ballistic loading to determine if a change in properties occurred as the cermet became more metalized. Six techniques were developed to increase the quality of cermet samples: controlling the rheology of the molten precursor mixture, vacuum degassing the molten precursor mixture, preheating the mould before pouring in the molten green mixture, using the vortex mixer to process mixtures of higher viscosity, reacting the precursor mixture under high pressure of an inert gas, and directional solidification of the cermet after reaction. Ballistic trials revealed that under certain conditions ductile-like behaviour was observed in Cr-CrS cermets. Perhaps even more interestingly, these tests revealed that even the ceramic phase seemed to support this ductile-like behaviour.
Une technique de synthèse a été développée pour fabriquer des cermets (composites en céramique et métal) de chrome et sulfide de chrome (Cr-CrS) sans avoir besoin d'une presse isostatique. Cette technique peut être employé pour fabriquer des matériaux qui sont presque des céramiques purs jusqu'aux cermets avec une concentration de métal élever. Des échantillons ont été soumis à des tests balistiques pour déterminer si les propriétés du cermet changent avec la concentration de métal. Six techniques ont été développées pour améliorer la qualité des échantillons : contrôle de la rhéologie du mélange précurseur fondu, dégazage sous vide du mélange précurseur fondu, préchauffage de la moule, utilisant un agitateur vortex pour les mélanges de viscosité plus élevée, synthèse du cermet sous haute pression de gaz inerte, et la solidification dirigée du cermet après la réaction. Des essais balistiques ont révélé que, sous certaines conditions, les cermets Cr-CrS—y compris la phase céramique—ont démontré une certaine ductilité.

In addition to the main chromosome, many bacterial genomes have a second element, often called a "second chromosome" or "megaplasmid". It has become clear that, while they carry some essential genes, these replicons do not have the properties normally expected of chromosomes. In this thesis a systematic analysis of all publicly available sequenced bacterial genomes demonstrates that these large secondary replicons represent a single class of elements with a distinct and consistent set of properties. The term 'chromid' is proposed to distinguish them from both chromosomes and plasmids. Chapter Two utilises compositional measures to demonstrate that chromids are present in approximately one in ten sequenced genomes, and found in many bacteria of medical, agricultural' and environmental importance. Moreover, the nucleotide composition and codon usage of chromids is very similar to that of the chromosomes they are associated with, despite possessing plasmid replication and partitioning systems. Chapter Three determines that chromids possess a number of orthologous genes held on chromosomes in other species, as well as being particularly rich in genus-specific genes. This, combined with chromids of different genera having unrelated replication and maintenance systems, suggests that the creation of a new chromid may be associated with the origin of a genus. Chapter Four demonstrates that only genes held on the chromosome maintain high levels of phylogenetic consistency, and Chapter Five highlights the complexity of the interaction between gene composition, expression and function, investigating differences between the three classes of replicon. No large-scale study incorporating all published data has previously been conducted. Yet the identification and characterisation of this component provides an important framework for understanding the evolution and organisation of bacterial genomes. By proposing a clear definition, this thesis seeks to encourage consistent annotation and discussion of the evolution and functional biology of this distinct and common type of replicon.

The magnesia-chromite refractory composites are the best candidates for the lining of non-ferrous metal smelting and refining furnaces, due to their high melting temperature, chemical inertness, and excellent thermal shock resistance. However, their high sintering temperatures (>1700°C) increase the processing complexity and costs. In this investigation, the primary goal was to study the sintering of these composites, with the long-term engineering goal to reduce their sintering temperature to <1500°C in air. The materials were synthesized via chemical methods using nitrates to assure chemical homogeneity. Al₂O₃, Fe₂O₃, and SnO₂ affected the kinetics of the MgCr₂O₄ formation and increased the density of MgCr₂O₄ in the order Fe₂O₃>Al₂O₃>SnO₂. The enhanced MgCr₂O₄ densification was attributed to the cation distribution in spinel structure (inversion phenomenon), caused by the inherent affinity of Fe⁺³ and Al⁺³ to tetrahedral sites. Fe₂O₃ and Al₂O₃ showed to form inverted spinel, while SnO₂ resulted in the formation of normal spinel solid solutions. Twelve magnesia-chromite composites were synthesized to study the effects of Al₂O₃, Fe₂O₃ and Cr₂O₃ on their sintering conditions; Cr₂O₃ decreased the density, while Fe₂O₃ and Al₂O₃ enhanced the densification of composites. The microstructural studies revealed that Fe₂O₃ and Al₂O₃ reduced the dihedral angle between MgO and spinel, while Cr₂O₃ increased it. The increased densification by Fe₂O₃ and Al₂O₃ was attributed to the decreased dihedral angle and formation of inverted solid solutions. The optimized composition [MgO6.9Cr₂O₃6.9Al₂O₃2.7Fe₂O₃]mol% (MK) reached nearly full density in air at 1475°C for 70minutes; 1700°C is currently used for magnesia-chromite refractories. In order to study the effects of the particle size on densification, magnesia-chromite composites (NMK) with average particle size of ~20 nm were synthesized via Pechini's method. Reducing the particle size from 1.2 um for MK to 20 nm for NMK reduced the onset sintering temperature by 200°C to 1000°C. The densification results were evaluated using master sintering curve theory for the first time for this kind of composites. The sintering activation energy was 443.7 and 302.6 kJ/mol for MK and NMK respectively. It was hypothesized that the oxygen diffusion through lattice and grain boundaries was rate controlling mechanism for MK and NMK respectively.

A range of natural chrome ores, containing iron in two valence states, have been investigated using the technique of Mossbauer spectroscopy. The instrumentation required to cry out the technique was studied and an additional item of equipment was designed and constructed to extend the temperature range over which the effect could be observed. All the chrome ores yielded a complex spectra which could be attributed to a combination of octahedrally and tetrahedrally co-ordinated Fe2+ and Fe3+ quadrupole doublets. A model for fitting the spectra was devised assuming the presence of octahedrally co-ordinated Fe3+ and tetrahedrally co-ordinated Fe2+. The inhomogeneous nature of the samples caused a broadening of the tetrahedral Fe2+ contribution which was fitted as a distribution of three doublets. The Mossbauer parameters obtained were compared with those of pure compounds of a similar structure. Spectra recorded from 12K to 600K enabled effective Debye temperatures to be calculated. Second order Doppler shift (S.O.D.S.) calculations gave values for theta[D] of 385K and 658K for the Fe2+ and Fe3+ contributions respectively. Absorption area data gave similar values. An examination of the temperature dependence of the quadrupole splitting of the Fe2+ contributions appeared to confirm the validity of the proposed fitting model. Estimations of the Fe2+ / Fe3+ ratio were made using the data obtained and yielded values of 1.6 - 3.6.

Chromite belongs to the spinel group with the general chemical formula XY2O4, where X and Y represent divalent and trivalent metal ions, respectively. Four types of chromite ore deposits occur as either lode or secondary deposits. Lode chromite ore deposits comprise stratiform and podiform deposits, whereas secondary chromite ore deposits comprise laterite and placer deposits. Chromite is an important industrial mineral used in the refractory industry. Moreover it is the only industrial source of chromium for chemical and metallurgical industries and to be marketed as a valuable commodity it requires very demanding quality parameters that differ according to the kind of application. Chromite ore cannot be usually sold as it is and, therefore, some kind of ore beneficiation is required in order to separate chromite from gangue minerals. The most commonly used beneficiation methods for chromite ores are the gravity methods, such as the shaking table, jig, spiral and Reichert cone methods. The present work deals with the geologic studies that, coming after prospection, lead to the evaluation of the chromite ore quality and to planning and/or improvement of beneficiation plants. The quality of chromite ore deposits, in order to rank them for possible exploitation, was studied in five deposits of Madagascar (Andriamena, Antanimbary, North Befandriana, North Belobaka and North Toamasina), in collaboration with UT Group s.r.l., for the development of chromite mining in the country. The basic geological, mineralogical and geochemical study of the deposits led to the reconstruction of genetic models for each of them. This was the starting point for a more detailed study on the quality of the chromite ore. The five most important chromite ore localities, investigated for this work, are all characterized by outcropping chromitite bodies hosted within mafic/ultramafic intrusions of probable Neoproterozoic to Cambrian age. Metamorphism and alteration affected, at different degrees, all chromitites, but never completely obliterate their primary characters. Chromitite host rocks are peridotite, orthopyroxenite or orthoamphibolite, while primary gangue phases are orthopyroxene, olivine, rare plagioclase, ilmenite, rutile, pyrrhotite and pentlandite. Secondary assemblage comprises serpentine, talc, Cr-chlorite, tremolitic to actinolitic amphibole and magnetite. Geologic, textural, mineralogical and mineral chemistry data are compatible with an ophiolite origin for North Befandriana chromitites and a layered intrusion origin for Andriamena, North Toamasina, North Belobaka and Antanimbary chromitites. These latter show differences that can be related to a different position of the chromitite bodies within the stratigraphic sequence of a layered intrusion. North Befandriana is a high quality deposit that could be exploited without any beneficiation of the ore, Andriamena needs beneficiation to reach market standard, Antanimbary and North Belobaka is low quality for metallurgical or chemical use but could be a good prospect for refractory market. Finally North Toamasina chromite ore is not suitable for any market even after beneficiation. An innovative study of geologic processes that can affect chromite ore beneficiation was applied to Vavdos chromite deposit (Greece) hosted in the Vavdos ultramafic massif belonging to the Halkidiki ophiolite of the Circum-Rhodope orogenic belt. Here metamorphic modification of chromite led to redistribution of Cr2O3 from chromite to silicates. The effect of the redistribution is to lower the efficiency of gravity plants as Cr2O3 contained in silicate phases will be preferentially discharged into the tailing during enrichment. The influence of this process that is widespread in chromite ores, on chromite enrichment was evaluated quantitatively. Generally accepted assumption that chromite ores do host Cr only in chromite is misleading as metamorphosed chromite ores host significative amounts of Cr in gangue phases and especially in Cr-chlorite. This study, of a completely metasomatized chromite ore, shows that about 3 wt% of total Cr2O3 in the rock is hosted in Cr-chlorite; while only about 0.2 wt% of total Cr2O3 is hosted in serpentine. As Cr-chlorite can host even more Cr2O3 than at Vavdos, and as the deepest alteration of chromite due to metasomatism occurs for ores containing about 34% chromite, the amount of Cr2O3 redistributed within the gangue can be even higher than at Vavdos, especially in low grade disseminated ores, where probably about 5-6% of Cr2O3 can be hosted in the gangue, a value that could rise to 7-8 wt% for high Cr2O3 Cr-chlorite. The effect of this wrong assumption is a mistake in the calculation of plant efficiency that will be overestimated. Mistakes due to redistribution of Cr2O3 during metamorphism can be easily avoided through mineralogical analysis that can detect the presence of Cr-chlorite in the ore. Planning of beneficiation in Cr-chlorite-bearing chromite ores requires additional investigation, concerning Cr2O3 content in Cr-chlorite and Cr-chlorite amount in the ore. A procedure to evaluate efficiency and results of gravity chromite enrichment plants was tested on Brieville enrichment plant (Madagascar), which belongs to Kraomita Malagasy mining company. Here a detailed study of chromite sand quality parameters at each step within the plant together with the measuring of all sand flow rates led to the reconstruction of separation efficiency at each step of chromite processing. The results of grain size, XRD, XRF, EMP and grain counting analyses together with separation efficiency (SE) and liberation degree (LD) evaluation allow to conclude that Brieville plant does not properly work due to the low sorting of sands feeding shaking tables that negatively affects their separation efficiency. Moreover the low degree of liberation of chromite, especially in the coarsest grain sizes, negatively affects the re-cycling process of mix materials. Plant efficiency and quality of final product can be improved by: moving the Cr2O3-enriched mixes to the concentrate and the Cr2O3-depleted mixes to the waste; grinding to finer grain size the overall feed of shaking tables and the mixes that will be re-cycled and substituting hydrosizers with screens. The first change, that does not involve any additional operational cost, has been effectively applied to the plant just after publication of the present study. The other changes involve additional operational costs and require a detailed economic analysis before being applied to the plant. Finally a completely new technology for high performance beneficiation of chromite sand that leads to the production of chromite refractory sands was tested. For this work chromite concentrate sand from Krasta enrichment plant (Albania) was used. The innovative beneficiation plant of Omega Foundry Machinery LTD. comprises a drum magnet and the new Inclined Fluidised Separator that uses an air cushion as the fluidizing agent during gravity driven grain separation. Refractory chromite sand chemical and technical requirements are the most demanding in chromite market and no chromite ore can attain them by simple crushing and grinding. On the other hand usual enrichment methodologies either cannot meet the required parameters or have a very low refractory sand recovery. The combination of drum magnet and Inclined Fluidised Separator in the Omega Foundry Machinery LTD. pilot plant not only produces a good quality refractory sand, but the result is reached with a high recovery, making of this plant an optimal solution for the production of refractory chromite sand. The Inclined Fluidised Separator is particularly performing as it combines a very high recovery of silica in the waste with an increase of the grain size of concentrate.

Composants principaux des couches de conversion chimique sur le marché, les composés à base de chrome hexavalent vont être interdits par la réglementation européenne REACh à partir de l'année 2024. Depuis plusieurs années, les industriels du secteur aéronautique développent donc des solutions de substitution basées sur le chrome trivalent, moins toxique et moins néfaste pour l'environnement que le chrome hexavalent, tout en conférant des propriétés anticorrosion prometteuses aux alliages d'aluminium. Le déploiement des solutions de conversion chimique à base de CrIII à l'échelle industrielle a mis en évidence une sensibilité forte de ces procédés aux propriétés de surface des alliages d'aluminium. Ce projet de thèse vise à améliorer la robustesse du procédé de conversion au chrome trivalent en identifiant les paramètres de premier ordre contrôlant les performances anticorrosion des couches. Pour cela, des éléments de compréhension concernant les modifications de surface causées par la préparation de surface et les mécanismes de croissance de la couche ont été apportés. Les travaux de thèse ont été réalisés sur l'alliage d'aluminium 2024 largement employé dans le domaine aéronautique. Différents lots d'alliage, états de surface et états métallurgiques ont été considérés afin de déterminer l'impact de la métallurgie de l'alliage sur les propriétés de surface après préparation de surface ainsi que sur les propriétés anticorrosion de la couche formée. L'étude de la réactivité de l'alliage dans le bain de conversion chimique a ensuite permis de mettre en évidence les mécanismes de nucléation et croissance de la couche de conversion. Un modèle de formation de la couche en surface de l'alliage 2024-T3 a ainsi été proposé
Hexavalent chromium, one of the main components of the chemical conversion coatings on the market, will be banned by European REACh regulations from 2024. From several years, manufacturers in the aeronautical sector have been developing substitution solutions based on trivalent chromium. These solutions with promising anti-corrosion properties as compared to hexavalent chromium are less toxic and less harmful to the environment than hexavalent chromium. However, the deployment of the CrIII conversion coatings to the industrial scale highlighted a higher sensitivity of these processes, as compared to the CrVI solutions, to the surface properties of the aluminium alloys. This work aims at improving the robustness of the trivalent chromium conversion process of the aluminium alloys by identifying the first order parameters controlling the anticorrosion performances of the conversion coatings. For that purpose, elements of understanding concerning the surface modifications caused by the surface preparation as well as data on the mechanisms of growth of the coating were brought. The thesis work was carried out on aluminium alloy 2024, which is widely used in the aeronautical field. Various alloy batches, surface states and metallurgical states were considered in order to determine the impact of alloy metallurgy on the surface properties after surface preparation as well as on the anticorrosive properties of the coating. The study of the reactivity of the alloy in the chemical conversion bath gave relevant data on the mechanisms of nucleation and growth of the conversion layer. As a result, a model for the formation of the conversion coating for 2024-T3 alloy was proposed

Progress in catalytic ethylene oligmerization and polymerization processes creates an enormously valuable and active field in both industry and academic perspective. From the naive point of view, gathering ethylene molecules to generate carbon carbon bonds can be assumed as the no electron, no energy and facile chemical process which results from interaction of several π-electrons but this process never can afford macro- and micromolecule without applying catalysts. It is no wonder that major efforts have been committed to designing novel catalysts to perform poly- and oligomerization processes, both recently and in the past which provide the extensive library of catalysts today. Among all the metals applied in preparation of homogeneous catalysts for ethylene oligo- and polymerization, chromium has been ideal in providing versatile systems in terms of selectivity and activity for oligomer and polymer productions. In the first part of the current work we chose two different ligand systems (iminopyridine and bisiminophosphine) to investigate the activity of their chromium complexes for ethylene oligmerization and polymerization at different conditions. In addition we took on the substantial challenge of explaining the activities through isolation of intermediates during the catalytic runs. The major focus in this part was rationalizing catalytic behaviour of the catalysts through elucidation of metal oxidation state. To fulfill this goal, syntheses of chromium-aluminate complexes in different valence states as well as description ligand-metal interactions were presented. In the second part of this thesis, our mainly focus of study was on the development in the well- established field of radical chemistry of alkylaluminum. Preferential cation radical and diradical formation and exploring their stabilities through both experimental and computational methods will be discussed. Also continuation of this work led us to report unprecedented double alkylation on the ligand system.

Actuellement, 80 à 85% des cuirs sont tannés au chrome. Ce procédé permet d'obtenir d'excellentes propriétés physico-mécaniques dans des conditions économiques optimales. Les sels de chrome sous forme trivalente utilisés lors de la fabrication du cuir ne sont pas nocifs; cependant sous certaines conditions et associés à d'autres substances, le chrome trivalent peut se transformer en chrome hexavalent, très allergisant et pouvant s'avérer dangereux pour la santé du consommateur. Depuis le 1er mai 2015, la présence de chrome hexavalent dans le cuir est soumise à la législation européenne; la teneur en chrome hexavalent ne doit pas excéder plus de 3 mg/kg de poids sec total du cuir (REACh ANNEXE 17, §47). A ce jour, de nombreux travaux de recherche ont été réalisés et des premières hypothèses ont été avancées pour expliquer la formation du chrome hexavalent dans le cuir. L'objectif de ce projet de recherche a été de rationaliser et d'approfondir ces différentes études pour essayer de mieux appréhender cette problématique. Ce projet de thèse a permis d'améliorer la spécificité et la fiabilité la méthode d'analyse pour la détermination de la teneur en chrome hexavalent en introduisant la chromatographie ionique. Ensuite, l'influence de la composition du cuir (matière grasse, indice d'iode, pH, chrome total et chrome extractible) sur la formation de chrome hexavalent dans le cuir a été étudiée afin d'expliquer l'origine et d'éventuels mécanismes d'oxydation du chrome trivalent (en chrome hexavalent). Enfin, cette étude s'est intéressée aux facteurs environnementaux (température, humidité relative, lumière et durée d'exposition), aux conditions de fabrication du cuir ou encore au cycle de vie du cuir (transport, transformation en produit fini, utilisation par le consommateur) afin de déterminer l'impact de ces paramètres sur la formation du chrome hexavalent. A l'issue de ce travail, une méthode de vieillissement "artificiel" représentative de l'évolution réelle d'un cuir a été proposée, et ce quelles que soient les caractéristiques et la nature du cuir
Currently, 80 to 85% of leather is tanned with chrome. This process allows to obtain excellent physico-mechanical properties with optimum economic conditions. The chromium salts in trivalent form used in the manufacturing of leather are not harmful; however under some conditions and associated with other substances, trivalent chromium can be transformed into hexavalent chromium, highly allergenic and potentially harmful to the health of the consumer. Since 1st May 2015, the presence of hexavalent chromium in leather is subject to European legislation; the hexavalent chromium content must not exceed 3 mg/kg of the total dry weight of leather (REACh ANNEX 17, § 47). To date, a great amount of research has been done and initial hypothesis have been able to explain the formation of hexavalent chromium in leather. The objective of this research project was to rationalize and to deal with these various studies in order to understand this problem in more detail. In this thesis project, the introduction of ion chromatography has improved the specificity and reliability of the analysis method for the determination of hexavalent chromium. Secondly, the influence of leather composition (fat liquor, iodine value, pH, total chromium and extractible chromium) on the formation of hexavalent chromium in leather was studied. The objective was to explain the potential oxidation mechanisms of trivalent chromium (in hexavalent chromium). The third part of this study deals with environmental factors (temperature, relative humidity, light and duration of exposure), leather manufacturing conditions or the life cycle of leather (transport, transformation into finished product, consumer use). The aim was to determine the impact of these parameters on the formation of hexavalent chromium. To conclude, an "artificial" ageing method representing the natural evolution of leather, and which corresponds to the characteristics and the nature of the leather, has been proposed