Made available in DSpace on 2014-06-11T19:34:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2004Bitstream added on 2014-06-13T19:47:49Z : No. of bitstreams: 1 padilha_rq_dr_guara.pdf: 4651598 bytes, checksum: 77a7edcf3c2fe16e30e62ec60549c1f8 (MD5) / Universidade Estadual Paulista (UNESP) / Este estudo objetiva analisar a influência da espessura de uma camada intermediária de níquel químico sob cromo duro eletrodepositado no aço de alta resistência, ABNT 4340 (utilizado em trens de pouso de aeronaves), com dureza de 39 HRc e 52 HRc. A análise é realizada por meio de dados obtidos nos ensaios de fadiga por flexão rotativa, das amostras revestidas com cromo duro, com e sem camada intermediária de níquel químico. As espessuras com melhor desempenho foram submetidas a ensaio de fadiga axial, em corpos de prova com e sem entalhe. Os resultados demonstram que a aplicação da camada de níquel químico é benéfica, em todas as espessuras analisadas, porém, há espessura que seu desempenho é ainda melhor. O tema proposto surgiu do interesse da ELEB - Embraer Liebherr Equipamentos do Brasil, em buscar por novas alternativas ou melhorias ao atual revestimento de cromo duro convencional. / The aim of this study is to analyze the effect of the thickness of an intermediate layer of chemical nickel under chromium electroplating on a high resistance steel ABNT 4340 (used in aircrafts landing gears), with hardness of 39 and 52 HRc. The analysis is done through data obtained from rotating bending fatigue tests of samples coated with hard chromium, with and without intermediate layer of chemical nickel. The thickness with better performance are submitted to axial fatigue tests, with and without notch. The results demonstrate that the application of a layer of chemical nickel is beneficial, in all of the analyzed thickness although some thicknesses present even better performance. The proposed work is due to the interest of the Brazilian Company of Aeronautics, Division of Equipments - EMBRAER/ELEB - in searching for new alternatives or improvements to the current coating with conventional hard chromium.

Orientadores: Bernardino Ribeiro de Figueiredo, Romulo Simões Angelica / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Geociencias / Made available in DSpace on 2018-08-11T02:16:06Z (GMT). No. of bitstreams: 1 DalBosco_SandraMaria_M.pdf: 475525 bytes, checksum: d539864c9285d28b195d0358b87cc60e (MD5) Previous issue date: 2007 / Resumo: Além do enriquecimento de metais pesados em águas subterrâneas devido aos processos biogeoquímicos que ocorrem na natureza, as atividades industriais, agrícolas e outras têm contribuído para um aumento significativo nas concentrações de íons metálicos em águas, representando importantes fontes de contaminação dos corpos aquáticos e provocando preocupações principalmente quando se considera que tais íons podem ser disseminados via cadeia alimentar. O cromo, objeto deste estudo, é um dos metais potencialmente tóxicos encontrados em água subterrânea. Em águas naturais, o cromo pode ocorrer nas formas químicas Cr(III) e Cr(VI), estáveis em meio aeróbio. A ingestão de águas contaminadas com Cr(VI) pode causar vários danos à saúde como dermatite alérgica, ulcerações na pele, perfurações do septo nasal e câncer. Embora o Cr(III) seja reconhecido como menos móvel e menos tóxico que o Cr(VI), vários processos podem induzir o intercâmbio entre as espécies Cr(VI) e Cr(III) revelando a importância de prevenir concentrações excessivas de cromo em água. Neste trabalho, foi avaliado o comportamento de um arenito contendo zeólita, proveniente da Formação Corda, Bacia do Parnaíba, como trocador catiônico, visando à remoção de cromo de soluções aquosas. A pesquisa restringiu-se ao Cr(III) que, em solução aquosa, ocorre como um cátion, ao contrário do Cr(VI), que ocorre como ânion em solução, necessitando, por isso, de um trocador aniônico ou de uma pré-redução a Cr(III) para ser removido de soluções aquosas. Para os testes de troca iônica, foram utilizadas duas frações granulométricas do material, <250 µm (Zeo60) e <177 µm (Zeo80). A presença de estilbita como a espécie de zeólita predominante nas amostras foi indicada por difratometria de raios X. Os testes foram realizados com 1,0 g da amostra em 60 mL de solução contendo Cr(III) em concentrações conhecidas, submetidos à agitação constante até que a reação atingisse o equilíbrio. O equilíbrio nas reações de troca iônica foi alcançado rapidamente, em cerca de 1 hora para Zeo80 e de 4 horas para Zeo60. A concentração de cromo presente na solução após as reações foi determinada por espectrometria de absorção atômica. Foi demonstrada uma eficiência de 99 % na remoção de cromo de uma solução contendo 10 mg L-1 do metal em ambas as amostras. Nos testes realizados a 25, 40 e 60 ºC, a temperatura não demonstrou influência significativa na remoção do metal de solução. A influência do pH da solução foi avaliada no intervalo de 3,0 a 5,0 e um aumento significativo na remoção de cromo foi registrado em pH = 5 atingindo-se o mesmo total de cromo removido da solução para ambas as amostras. Os testes de dessorção revelaram uma elevada capacidade de regeneração de ambas as amostras: cerca de 90 % do cromo adsorvido na amostra foi liberado para a solução no caso da Zeo60, e 93 % no caso da Zeo80. Os resultados obtidos indicam a possibilidade do uso do material na remoção do cromo de efluentes e água contaminada. Além disso, o fato de se tratarem de amostras naturais, não modificadas e, portanto de baixo custo, encontradas em depósitos sedimentares, ou seja, de fácil obtenção, pode também ser um estímulo para contínuos estudos visando viabilizar a sua aplicação em larga escala / Abstract: Heavy metal enrichment in groundwater can be due to natural biogeochemistry processes as well as to industrial, agricultural and other activities that contribute significantly to groundwater contamination and, hence, giving rise to great concerns when considering the extension of ionic dispersion via food chain. Chromium, object of the present study, is one of these potential toxic metals found in groundwater. In natural water, chromium may be found in the Cr(III) and Cr(VI) chemical forms, stable in aerobic environment. Ingestion of contaminated water for Cr(VI) can lead to health problems such as allergic dermatitis, ulcerate of skin, nasal septum perforation and cancer. Despite of Cr(III) being recognized as less mobile and less toxic than Cr(VI), several natural processes can induce interchange between Cr(VI) and Cr(III) species, revealing the importance of preventing excessive chromium concentration in water. In the present study, zeolite-bearing sandstone, originated from the Parnaiba Paleozoic basin, northeastern Brazil, was tested as a cationic exchange material for chromium removal from aqueous solution. Two grain-size fractions <250 µm (Zeo60) and <177µm (Zeo80) were used. Essays were restricted to removal of Cr(III) species which occur cationic species in aqueous solution whereas the negative charged Cr(VI) species were not considered in the present study. Tests were performed using 1.0 g of each sample in 60 mL of solution with known initial Cr(III) concentrations kept agitated until equilibrium was attained (1 hour for Zeo80 and ca. 4 hours for Zeo60). After reactions, the final Cr concentrations were determined by Atomic Absorption Spectrometry. The material response to ionic exchange showed to be very efficient for Cr(III) removal from solution at around 99 % when the cation initial concentration was 10 mg L-1. Tests were carried out at 25, 40 and 60 ºC but the influence of temperature was not significant. The influence of solution pH was observed in the interval from 3.0 to 5.0 and an increase of Cr removal occurred at pH = 5 of equal amounts of chromium for both samples. The material easily regenerates after the desorption tests: ca. 90 % of adsorbed Cr was released from the Zeo60 sample and ca. 93 % from the Zeo80 sample. All the results support the recommendation of using this material for chromium removal from contaminated water and effluents. In addition, this low-cost material is natural, easily accessed from sedimentary deposits and does not need any previous treatment which can be a stimulus for continuing studies oriented to consider its application in large scale / Mestrado / Geologia e Recursos Naturais / Mestre em Geociências

M.Sc. / When a pure chromium single crystal is cooled through its Néel temperature, it undergoes a paramagnetic to a multi-wave vector incommensurate spin-density-wave magnetic transition. Should a chromium single crystal be cooled through its Neel temperature in the presence of a strong enough magnetic field, 4-5 T, then all the spin-density-wave vectors that occur as it undergoes the paramagnetic to incommensurate spin-density-wave magnetic transition, are forced to be aligned parallel to the direction of the applied magnetic field, producing what is called a single spin-density-wave wave vector state in the crystal. The single spin-density-wave wave vector state will remain in the crystal when the magnetic field is removed providing the crystal is not heated above its Neel temperature. If the crystal when in the single spin-density-wave wave vector state is orientated so that the single spin-density-wave wave vector is aligned perpendicular to the direction of an applied magnetic field, then the spin polarisation vectors of the magnetic moments will align themselves so that they are perpendicular to the applied magnetic field and perpendicular to the single wave vector, producing what is called a single spin-densitywave wave vector and single spin polarisation vector state in the crystal. There is a difference in value between the elastic constants measured when a chromium single crystal is in the single spin-density-wave wave vector state to the elastic constants measured when the crystal is in the multi-wave vector spin-density-wave state.When pure chromium is alloyed with other materials the topology of the Fermi surface is changed resulting in some of the alloys being able to undergo paramagnetic to commensurate spin-densitywave magnetic transitions, as well as being able to undergo commensurate spin-density-wave to incommensurate spin-density wave transitions. This study makes use of a magnetic field of strength 4.5 Tin an attempt to produce a single spin-density-wave wave vector state in a Cr + 0.3 at.% Ru and a Cr + 1.6 at.% Si single crystals. Both single crystals undergo paramagnetic to commensurate spin-density-wave transitions when cooled through their, respective Neel temperatures, as well as the Cr + 0.3 at.% Ru single crystal being able to undergo a commensurate spin-density-wave to an incommensurate spin-density-wave transition if cooled further, down to 77 K, well below it's Neel temperature. The effects of the applied magnetic field on the Cr + 0.3 at.% Ru and Cr + 1.6 at.% Si single crystals was determined by measuring the elastic constants of the respective crystals.

M.Sc. (Chemistry) / Please refer to full text to view abstract

A lack of methods for the separation and determination of air-sensitive Cr(II) and stable Cr(III) complexes in aqueous and organic phase motivated this study. The formation of kinetically stable Cr(III) complexes with common anions was studied using ion chromatography coupled to inductively-coupled plasma emission spectrometry (ICP-OES). Distribution diagrams for the complex species were determined from the results. Estimations of the stability constants were made from these diagrams and compared with stability constants published in the literature. Separation and determination of Cr(II) and Cr(III) and their complexes with PDCA and EDTA in aqueous phase was investigated using ion chromatography. ICP-OES was used for detection purposes. The separation and determination of Cr(II) and Cr(III) complexes with a ligand, bis-(2-decylsulphanyl-ethyl)amine, developed by Sasol Technology (Pty) Ltd., was attempted. The study was undertaken as the Cr(III) complex is used as a catalyst in the trimerisation of ethylene to 1-hexene. The mechanism is still unknown and determining which species are present in the catalytic cycle could clarify which oxidation-reduction couple is involved during the cycle. Reversed phase HPLC was investigated as a possible method for the separation of these complexes. No separation of the species was, however, achieved. / Prof. P.P. Coetzee

A mapping project of the surface geology of the Lavino chrome mine and its surroundings was initiated in order to establish the surface geological relationships in the area. In so doing the chromitite layer presently being mined has been identified and potential exploration targets in the area have been outlined. The Lavino Chrome mine field area is situated within the eastern lobe of the Bushveld Igneous Complex. The area is bounded by in the north by the Steelpoort Lineament, in the west by the Dwars River fault and in the east by the contact with the Transvaal Sequence floor rocks. Layered igneous rocks (pyroxenites, norites and anorthosites) of the Rustenburg Layered Suite dominate the geological landscape at the Lavino mine. The fact that outcropping igneous rocks of the Critical Zone abut directly against the quartzite floor rocks on the mine property makes this area unique in the Bushveld Complex. The hills in the field area are capped by mafic/ultramafic iron-rich sheet - like bodies. Extensive strike-slip faulting is seen in outcrop in the area to the north/northwest of present mining operations. On the basis of field relationships, the main chromitite layer presently being mined at Lavino is identified as the Middle Group chromitite layer MG 1. Three other prominent chromitite layers stratigraphically associated with MG 1 are identified as the Middle Group chromitites MG 2, MG 3 and MG 4. Several other less prominent outcropping chromitite layers are tentatively identified as those belonging to the Lower and Upper group of chromitites. The disconformable nature of the contact between the layered igneous rocks and the Transvaal Sequence floor rocks has resulted in the development of a wedge of undifferentiated pyroxenites in the north of the field area. The economically important LG 6 chromitite layer may be developed in subcrop within this wedge.

Studies have been made of the reactions which take place when zirconium compounds and vegetable tannins react with chromium tanned leather, in order to elucidate the mechanisms of the reactions which occur on retannage. Statistical procedures have been used in all investigations because of the variable nature of the substrate, and computer techniques have been applied to the repetitive statistical computations. Although chromium and vegetable tannages are well understood, further information on the reaction of zirconium with collagen was necessary before attempting to interpret the results of the studies of combination tannages with chromium, and this has been obtained by a comparative study of the reactions of chromium and zirconium with modified collagen. It is concluded that the mechanism of the reaction of basic zirconium sulphate with collagen is multipoint attachment of the tanning material by residual valency forces, although charge effects with basic groups may be supplementary. Zirconyl chloride reacts with carboxyl groups but does not form satisfactory, stable cross-links with collagen. Further evidence for this theory was obtained from the investigation of the reaction of zirconium compounds with chromium tanned collagen. Zirconyl sulphate does not interfere with effective chromium tannage and therefore can have little affinity for the carboxyl groups on the protein, but it displaces chromium complexes loosely held by auxiliary valencies without reducing the shrinkage temperature of the chromium leather Zirconyl chloride, although only fixed to a limited extent, apparently forms co-ordination compounds with the carboxyl groups, disrupting the chromium tannage because there is an over-all loss of hydrothermal stability. There is no evidence that zirconium co-ordinates with, or releases acid from chromium-collagen complexes, since combination chromium/zirconium tanned leathers are stable on storage. Retannage of chromium tanned leather with vegetable tanning materials generally results in loss of strength and a product which tends to deteriorate on ageing. Lower initial strength is probably due to the increased avidity of chromium tanned pelt for vegetable tannins, resulting from the liberation of internally neutralised reactive sites which are not normally available in vegetable tannage, and from the co- ordination of vegetable tannins and non-tannins to the chromium complex with the displacement of sulphate radicals. From a study of the retannage of chromium tanned modified collagen, it appears that basic groups probably play an important part in the rapid absorption of vegetable tannin. These reactions result in overloading of the fibre and an increased number of cross-links, both of which tend to produce weak leather. Deterioration with age is primarily a hydrolytic degradation of the protein which is catalysed by acid liberated from the chromium complexes by the entry of vegetable tannins, those factors which favour the formation of acid causing greater and more rapid deterioration.

Some properties of soluble chromium complexions containing coordinated aliphatic acids have been studied. The work falls naturally into two sections. In the first, the coordination of a series of ⊄, β and⊁amino acids by chromium chloride has been studied by physical methods. The tanning action of chromium chloride in the presence of these amino acids has also been studied. The absorption spectra of the complexes were similar to those reported previously for trivalent chromium solutions, having two pronounced maxima in the visible region. From the variations in these absorption maxima, it is suggested that the absorption maximum in the 580 m u region is influenced by coordination of the chromium with the ligand, while the maximum in the 420 m u region is also affected by the olation of the basic chromium salts. The spectrophotometric evidence indicates that raising the pH or the concentration of the ligand in the solution increases the amount of coordination, and further, that the tendency for coordination increases as the hydrocarbon chain separating the carboxyl and amino groups becomes longer. This suggests that tho stability of the complex is not dependent on chelate ring formation, but is influenced by the pK₁ value of the carboxyl group of the . ligand. Potentiometric titrations support the hypothesis that only the carboxyl group is coordinated, to an extent depending on its pK₁ value, since the curves have shown that the amino group is still free to titrate. Paper electrophoresis has shown that all the complexes prepared were cationic, indicating that the amino acids were coordinated as dipolar ions. The tanning action of the masked chromium solutions has confirmed the deductions made from the physical measurements. Increasing the amount of amino acid added to the solution lowered the chromium fixation and the hydrothermal stability of the leather, and further, that for solutions at the same pH containing the same amount of masking agent, tanning action was least for the ⊁ amino acid and greatest for the ⊄ amino acids. Comparison of the present data with the corresponding results obtained with chrome alum solutions showed that coordination of the amino acids was greater in the case of the chromium chloride solutions. The second section of the experimental work was an investigation of the coordination of substituted acetic and propionic acids by chromium chloride and chromium sulphate. Spectrophotometric and potentiometric methods were applied and the various solutions were also used in miniature tanning experiments. Certain difficulties were encountered in the preparation of some of the complexes, and it was not possible to carry the work to a point where conclusive results could be obtained. Nevertheless, the work reported suggests that chelate ring formation occurs in the coordination of hydroxy-carboxylic acids, resulting in exceptionally high stability of the complex. In the case of the other ligands, containing amino, chloro and bromo groups, as well as with acetic and propionic acids, the results suggest that coordination involves the carboxyl group only, and that the pY value of this group is an important factor determining the stability of the complexes.

It was been reported that untreated wastes from ferrochrome plants release heavy metals including lead, chromium, copper, cadmium, zinc and nickel in the environment. Some of these metals such as hexavalent chromium are potentially toxic and carcinogenic, and can cause a serious threat to human health. The contamination of terrestrial and aquatic ecosystems by hexavalent chromium is worldwide of major environmental concern, especially in South Africa which is the largest producer of chromite and ferrochrome. Therefore, the pre-treatment of these wastes before landfill is of great importance to prevent the contamination of the ecosystems. In the present study, vermiculite, a natural occurring mineral, has been tested for its adsorption effectiveness in removing Cr(VI) from ferrochrome dust leachate. Batch adsorption studies have been carried out to determine the effect of pH, contact time and adsorbent dose on the removal of Cr(VI). The process was found to be highly pH dependent. The optimum conditions for the removal were found to be at pH 1.5, contact time 2 hours and adsorbent dose 10 g.L-1. According to kinetic and isotherm studies, the process is best fitted by the pseudo-second order kinetic model, and to both Langmuir and Freundlich isotherms. The maximum adsorption capacity was found to be 23.25 mg.g-1. Thermodynamic parameters show the spontaneous and endothermic nature of Cr(VI) adsorption onto vermiculite. / Dissertation (MSc)--University of Pretoria, 2011. / Materials Science and Metallurgical Engineering / unrestricted

Over the past decade, advances in ethylene oligomerization have witnessed explosive growth of interest from both commercial and academic standpoint, with chromium metal invariably being the metal of preference. A common feature in this literature was the extended long debate regarding the mechanism, metal oxidation states responsible for selectivity and the role of the ligand. This thesis work embarked on the isolation and characterization of new active intermediates called “single component catalysts” (or self activating) to address two important questions: (1) how the catalyst precursors re-arrange upon activation and (2) the real oxidation state of the activated species. Four different ligands systems have been examined for this purpose. The first part is a study on the NPIIIN ligand which can be described as a dynamic and non-spectator ligand. Upon aluminum alkyl activation, a series of single component chromium catalysts for selective ethylene oligomerization and polymerization have been isolated, fully characterized and tested. New selective single component chromium(I) catalysts have also been isolated and tested positively for ethylene trimerization. The second part includes a new series of chromium complexes based on the NPVN ligand. This ligands enabled to obtain the first polymer-free extremely active catalytic system. In both NPN ligand systems, a new activation pathway was discovered by using vinyl Grignard reagent [(CH2=CH)MgCl] as activator and/or reducing agent. The third part explores new modified pyrrole-chromium complexes which were found to be highly active and selective ethylene trimerization catalysts. This part was a continuation of previous work from our lab to complete the mechanistic picture of this highly successful pyrrole-chromium catalyst independently commercialized by Phillips-Chevron and Mitsubishi. Interestingly upon aluminum alkyl treatment, the first example of a Schrock-type chromium ethylidene complex has been isolated and characterized and found to be a potent catalyst for selective ethylene trimerization. Finally, the other ligands introduced in this thesis are new systems called pyridine-SNS and Si-SNS that introduce some modification to the known commercial SNS catalyst (Sasol technology). The introduction of a pyridine ring or a silyl unit in the ligand scaffold has allowed to understand the mechanism of action of this remarkable system.