Dissertations / Theses on the topic 'Chromium plating'

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.

A consideration of pertinent aspects of general electro- chemistry and electrodeposition was followed by a review of the relevant literature relating to hard chromium plating technique and understanding (and problems and proposed improvements therein). Following the establishment and evaluation (using a Standard Solution at 55 0 C) of a reproducible technique for the measurement of (low current density) partial cathodic polarization data during plating, the Tafel regions of the resulting curves were analysed using the relationship - dn Cp -2.303 kT to provide an indication as to the nature of the rate-determining step in the plating and hydrogen evolution (cathodic) reactions. Information thus generated was used with a modified Arrhenius- type expression and further (experimentally determined) data to provide values of the thermodynamic constants of activation for the plating reaction. A series of experiments were undertaken to determine the effects upon various (physical) plate and plating parameters of three new inorganic additions (at selected concentrations) to a Standard Solution at 55 0 C. The results of these tests were commented upon with regard to potential industrial applications. The electrochemical investigative techniques referred to earlier were then employed with each new solution tested to determine the effect (if any) of the new additions upon those aspects of the deposition mechanism yielded, and to relate these effects to the observed (corresponding) changes in plate and plating parameters. The new additions were found to have profound effects upon a number of (physical) plate and plating parameters, and upon experimentally determined theoretical aspects of the deposition mechanism (the relationship between the two is discussed). Whilst no definite conclusion has been reached with regard to the precise mechanism of chromium plating, the techniques employed and the data generated have been used to provide significant pointers with regard to the direction future investigations might take.

The electroplating industries use soluble nickel salts in numerous applications. Over the past few years this has become problematic due to the reclassification of these salts from a harmful substance to now a toxic substance. The introduction of the legislation REACH (Registration, Evaluation, Authorisation & restriction of CHemicals) has meant the electroplating industry has had to use less harmful chemicals where possible, thus meaning companies are investing in research to find an alternative to the nickel deposit. In this study, alternative deposits under investigation as a potential replacement to the nickel deposit under the decorative chromium deposit has been characterised in terms of the appearance, surface topography and corrosion resistance by using spectrophotometry, scanning electron microscopy (SEM), atomic force microscopy (AFM), linear polarisation, electrochemical impedance spectroscopy (EIS) and accelerated corrosion salt spray methods. Spectrophotometry identified that a white Cu-Sn alloy gave a bluer appearance in comparison to the nickel deposit, when the chromium deposit was plated on top this modified the colour slightly giving the white Cu-Sn alloy with chromium a similar appearance to the nickel and chromium deposit. The yellow Cu-Sn alloy was yellow in colour with a visibly dull appearance, but when chromium was plated on top of the yellow Cu-Sn alloy the colour was improved but still remained dull. The SEM and AFM results identified that the white Cu-Sn alloy deposit had similar nodulated topography to the nickel deposit and when the chromium was plated on top the topography changed only slightly. While the yellow Cu-Sn alloy deposits showed a more crystalline structure and increased roughness in comparison to the nickel deposit, the chromium deposit plated on top did not change the structure of the underlying deposit but it did reduce the roughness slightly. Electrochemical corrosion tests showed the white Cu-Sn alloy to have a higher polarisation resistance compared to the nickel deposit, thus suggesting it would provide similar corrosion protection to the nickel deposit. The yellow Cu-Sn alloy proved to have a faster corrosion rate in comparison to the nickel deposit. Accelerated corrosion tests proved the white Cu-Sn alloy to be more corrosive than the nickel despite the electrochemical test results, it was concluded that the white Cu-Sn alloy deposit was porous and therefore provided less corrosion protection to the substrate in comparison to the nickel deposit. The yellow Cu-Sn alloy had a lower corrosion protection than the nickel deposit and when combined as a duplex Cu-Sn alloy with the white Cu-Sn alloy deposit there was no improvement in corrosion performance. The plated chromium deposit did improve the corrosion protection for most deposits but none of the alternatives could match the corrosion protection offered by the standard nickel with chromium deposit. This study concluded that the white Cu-Sn alloy with chromium deposit was found to be a potential alternative to the nickel with chromium deposit for applications where appearance is primary, no alternative could be found to match the corrosion protection provided by the standard nickel and chromium process.

Before the persistent and toxicological properties of perfluoroalkyl and polyfluoroalkyl substances (PFASs) were known they were used worldwide. The thermal and chemical stability from the carbon-fluorine bond makes these substances desirable for industrial applications. One particular PFAS that have received much attention is perfluorooctanesulfonic acid (PFOS) which is included in the Stockholm convention. Analyses have detected PFASs, especially the ones with longer carbon chain, in the environment, wildlife and humans so replacements are being searched for. This study investigates groundwater around a former hard chromium plating site, which is one of the registered exceptions where PFOS still is used. Five different located sampling points were collected and analyzed from this site in Iggesund, which is in the northern parts of Sweden. The substances analyzed for were thirteen perfluorinated carboxylic acids (PFCAs: C4-C14, C16 and C18), five perfluorinated sulfonic acids (PFSAs: C4, C6, C8 and C10) and 6:2 fluorotelomer sulfonate (6:2 FTS). The 6:2 telomer substances is being used as a replacement for similar compounds with longer carbon chains, but are believed to undergo biotransformation to persistent alkyl acids like perfluorohexanoic acid (PFHxA) in the environment. Solid phase extraction was performed on 500 mL filtered groundwater with and the concentrated samples were analyzed on an Ultra Performance Liquid Chromatography (UPLC), tandem mass spectrometer. The glass microfiber filters used for filtrating the groundwater were analyzed for particle bound PFASs. Results from the water extraction analysis showed eight detected PFAS, with PFOS as the major contributor (72 - 9600 ng/L). The PFOS concentrations differs substantially between the five groundwater samples where the two sampling points located south of the facility, and closest to the actual hard chromium plating, were clearly higher than the other three. Two other substances detected in high concentration were perfluorobutanesulfonic acid (PFBuS), 8-1550 ng/L, and perfluorohexanesulfonic acid (PFHxS), 18-140 ng/L which can be produced as an impurity during the production of PFOS. PFCAs in all samples were detected in lower concentrations (1-25 ng/L). Additional to the PFAS analysis, the groundwater was also measured for chromium since the carcinogenic hexavalent form is used in the chromic acid bath during hard chromium plating. In the same two samples that had the highest PFAS concentration, chromium was found in high concentration, 34900-44800 μg/L. These result indicates that the chromic acid baths is the source for the elevated concentrations found in the groundwater. If the facilities are left to deteriorate it will lead to continuously spreading of chromium and the highly water soluble PFASs downstream to Iggesundså which is of concern for the environment and water living organisms.

Baths containing sulphuric acid as catalyst and others with selected secondary catalysts (methane sulphonic acid - MSA, SeO2, a KBrO3/KIO3 mixture, indium, uranium and commercial high speed catalysts (HEEF-25 and HEEF-405)) were studied. The secondary catalysts influenced CCE, brightness and cracking. Chromium deposition mechanisms were studied in Part II using potentiostatic and potentiodynamic electroanalytical techniques under stationary and hydrodynamic conditions. Sulphuric acid as a primary catalyst and MSA, HEEF-25, HEEF-405 and sulphosalycilic acid as co-catalysts were explored for different rotation, speeds and scan rates. Maximum current was resolved into diffusion and kinetically limited components, and a contribution towards understanding the electrochemical mechanism is proposed. Reaction kinetics were further studied for H2SO4, MSA and methane disulphonic acid catalysed systems and their influence on reaction mechanisms elaborated. Charge transfer coefficient and electrochemical reaction rate orders for the first stage of the electrodeposition process were determined. A contribution was made toward understanding of H2SO4 and MSA influence on the evolution rate of hydrogen. Anodic dissolution of chromium in the chromic acid solution was studied with a number of techniques. An electrochemical dissolution mechanism is proposed, based on the results of rotating gold ring disc experiments and scanning electron microscopy. Finally, significant increases in chromium electrodeposition rates under non-stationary conditions (PRC mode) were studied and a deposition mechanisms is elaborated based on experimental data and theoretical considerations.

Технологический процесс изготовления штока шаровых кранов предусматривает ряд отделочных операций, которые применяются перед и после операции хромирования его поверхности и должны обеспечить требуемые показатели её качества – точность формы и шероховатость поверхности. Однако более 42% поверхности штоков имеют дефекты поверхности в виде отдельных царапин, окалины, надиров, что отрицательно влияет на надёжность и долговечность изделия. Например, операция шлифования приводит к локальному пригоранию поверхности и изменению структуры поверхностного слоя из-за высокой температуры в зоне резания. Шлифованная поверхность не является износостойкой и не обеспечивает требуемого качества слоя, покрытого хромом.

Polymer electrolyte membrane fuel cells (PEMFCs) are envisioned to replace internal combustion engines (ICEs) as vehicle power sources and to compete with conventional technologies in backup power systems, residential combined heat and power (CHP) devices and consumer electronics. However, the commercial viability is still hindered due to the cost and durability that are significantly related to the precious metal catalysts used in these cells. In the current work, two different methods towards reducing the precious metal content by the use of transition metal (Cr) with Pt as electrocatalysts for PEMFCs are reported. The commercial Pt/C catalysts layer consists of three components i.e. Pt, carbon support and Nafion ionomer. The Nafion ionomer binder does not penetrate through the microporous carbon support and thus limits the triple phase boundary region (catalytic reaction takes place at triple phase boundary), which in turn leaves a significant amount of Pt being inactive or not utilised. In the first part of this work, Nafion- stabilised Pt-Cr alloys are synthesised using a novel wet chemical synthesis. The aim of this work is to improve the triple phase boundary region with the introduction of Nafion during the synthesis and alloying with Cr that can enhance the catalytic activity. Thus, in turn improving the utilisation of the catalyst and also reducing the amount of Pt loading.

Chapter 1 provides a concise review of concepts and previous studies in liquid crystal chemistry as they relate to metallomesogens, highlighting their advantages over purely organic liquid crystal systems. Chapter 2 reports on a mild synthetic route starting from [Co(DH)₂(py){(CH₂)ₘOH}] (DH = the monoanion of dimethylglyoxime; py = pyridine; m = 3 or 8) to give the ester-functionalized cobaloximes [Co(DH)₂(py){(CH₂)ₘOC(O)(CH₂)ₙC₆H₄R}] (n = 0, 2 or 5; R = H, o-, m-, p-OCH₃, p-OC₇H₁₅ or p-OC₉H₁₉), 1- 9. The esterification occurred chemoselectively to extend the hydroxy alkyl group, without affecting the oxime entities on the DH ligands. None of these complexes displayed mesogenic properties. This chapter also details the synthesis and characterization of the new complexes [Co(DH)₂(C₆H₄₀CₘH₂ₘ₊₁)(n-OST)] (m = 8, 10 or 12; n-OST = trans-4-n-alkoxy-4'stilbazole; n = 8, 10 or 12), 11 - 17, which possess greater structural anisotropy and consequently displayed an enantiotropic smectic C mesophase. The extended rod-like core and the central position of the bulky metal system proved useful in promoting the formation of the mesophase. Variation of the above cobaloxime-stilbazole system afforded the complexes [Co(DH)iC₆H₄₀C₁₂H₂₅)(R-OST)] 18, [Co(DH)₂(C₁₄H₂₉)(12- OST)] 19, and [Co(DH)₂(C₁₄H₂₉)(R-OST)] 20 (where R = C(O)C₆H₄₀C₉H₁₉). These complexes were non-mesomorphic, despite possessing favourable rod-like structures. The synthesis and characterization of the novel octahedral stilbazole complexes [PtMe₂X₂(n-OST)₂] (X= I or Br; n = 3 - 10 or 12), 21- 30, is described in Chapter 3. The shorter-chain homologues melted cleanly to the isotropic state while the complexes with n = 8 - 10 or 12 developed an unusual pseudo-isotropic phase around 80 °C and on further heating, the samples displayed a smectic C phase around 150 °C. The alignment of the molecules to give the mesophase occurred despite the fact that the complexes possessed an angular structure resulting from the cis geometry of the n-OST ligands. The complexes [PtMe₂RX(n-Diim)], [PtMe₂X₂(8-Diim)] and [ {PtMe₂Br(9- Diim)}₂(μ-p-(CH₂)₂C₆H₄)] (R =Me, Et, n-Pr; X= I or Br; n-Diim = N,N'-bis-4'-(4-nalkoxybenzoyloxyphenyl)- 1,4-diaza-1,3-butadiene), 31- 42, were synthesized in a one- pot procedure from the dimeric complex [Pt₂Me₄(μ-SMe₂)₂] and the respective n-Diim ligand to give the square-planar intermediate [PtMe₂(n-Diim)], which underwent subsequent oxidative addition in the presence of an alkyl halide RX (R = Me, Et, n-Pr) or a dihalide X₂ (X = Br or I). An aromatic-like metallocyclic ring was proposed to explain the anomaly in the NMR resonances of the protons close to the coordination sphere in the square-planar complexes. The longer-chain complexes (i.e. n = 6 - 10) exhibited wide temperature-range enantiotropic nematic phases. Further, modification of the axial ligands to increase the bulkiness of the central metal core (R = Me to n-Pr, X = Br or I) generally led to lower-melting materials. However, many of the complexes were thermally unstable, decomposing at their clearing temperatures. Chapter 4 describes the synthesis and characterization of the novel ionic complexes Y[PtC₁₃(olefin)] (where Y = K, H₂₁C₁₀NH₃, NMe₄, NEt₄ and olefin = CH₂=CHC₆H₄- C(O)OC₆H₄₀R, R = C₇H₁₅ , C(O)C₆H₄₀CH₃ or C(O)C₆H₄₀C₈H₁₇), 43 - 50. By careful choice of the reaction conditions and work-up solvents, most of the individual reaction steps produced high yields of the desired products. The meso genic behaviour was found to be dependent on the length of the rod-like core, the length of the flanking alkoxy chain, and more importantly, the nature of the cation used. Only the long-chain alkylammonium complexes displayed stable nematic phases. Variation of the now established mesogemc platinum(II)-olefin system afforded the complexes [PtC₁₂(olefin)(N-donor)] (N-donor = 4,4'-bipy, H₂NC₄H₉, H₂NC₁₀H₂₁ or 4-0ST), 51- 54. Except for the 4,4'-bipy complex 51 which was non-mesomorphic, the remaining complexes displayed well-formed nematic phases. The greater structural anisotropy of the stilbazole complex 54 produced a mesophase with a wider temperature range than that observed for the alkylamine complexes. Finally, Chapter 5 reports on a simple one-pot synthesis leading to the new complexes [ {η⁶-C₆H₄R₂-1,4}Cr(CO)₃] (where R = OC(O)C₆H₄₀CnH₂n+l (n = 1, 6 - 10) or OC(O)C₆H₄C₅H₁₁ ) and [ {η⁶-C₆H₄(R')(C₆H₄R')-1,4 }Cr(CO)₃] (R' = OC(O)C₆H₄₀C₁₀H₂₁), 55 - 62. The procedure involved thermal reaction of Cr(CO)₆ with hydroquinone and subsequent esterification of the para hydroxyl functionalities with various mesogenic carboxylic acid compounds. Infrared monitoring of the reaction mixture provided useful data in identifying the reaction intermediates. Despite the bulky nature of the central chromium system, the rigid unit comprising three aromatic rings proved sufficient to stabilize enantioptropic nematic phases similar to those observed for the metal-free analogues. Modification of the organic backbone in these complexes to incorporate cyclohexylene rings and the biphenylene unit had pronounced effects on the temperature range of the observed mesophase.

The growing demand in the industry towards sustainability and the globalization of manufacturing lead to an industrial climate of constant development and improvements, and automotive interior design is not excluded. In 2017 the European Union declared that the material used to generate the bright chrome surfaces of car interiors (CR VI) is toxic and carcinogenic. This implies that Volvo will ban the use of Chrome VI for decorative parts from September 2024.  Finding a more eco-friendly alternative to replace CR VI not only functionally but also in terms of perceived quality and user experience will be an urgent and decisive action.  In this project, the parameters of gloss, haze, color temperature, and metallic depth are investigated in order to figure out which elements ensure the perceived quality of chrome surfaces.  For this purpose, a user study based on different sensory tests and soft metrology was carried out with 48 people, as well as seven samples, five of them more eco-friendly alternatives. This project aims to introduce a set of tools to assess and guarantee the perception of quality by supporting the development of "chrome-look" surfaces in the automotive interior with new sustainable materials. Through this study, correlating soft and hard metrology, it is observed what makes a surface perceived as "high-quality" and which of the more eco-friendly alternatives could be the most optimal to replace Chrome VI in Volvo's car interiors.

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This work presents oxygem reduction reaction (ORR) studies on platinum (Pt/C) and platinum-chromium alloys containing 50% (PtCr/C) or 70% in chromium (Pt3Cr) dispersed on high surface area, in acid aqueous solutions with and without metanol, with cyclic voltammetry, chronoamperometry and rotating disk electrode techniques. Electrochemical measurements have been also performed in a direct methanol unit fuel cell. The electrocatalysts were characterized by energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses. The ORR studies in absence of methanol in solution showed that chromium adittion does not change the reaction mechanism observed on the Pt/C and the electrocatalytic activity increases in the order PtCr/C < Pt/C < Pt3Cr/C. Such increase was attributed to a favourable interatomic distance for O2 dissociative adsorption. The alloys showed higher tolerance to methanol presence in solution than Pt, mainly the PtCr electrode due to smaller amount of active sites to methanol adsorption with respect to Pt. In solutions containing low concentration of methanol, the ORR mechanism was not affected by methanol presence. The polarization curves recorded with the unit cell showed that na increase in temperature results in an increase in the electrocatalitic activity of the anode, as well as na increase in methanol diffusion through the Nafion® membrane with consequent cathode depolarization. Furthermore, the system containing Pt3Cr/C as cathodic electrode material presented the best electrochemical performance, with power densities of ~ 55 mW cm-2, in contrast to ~ 40 mW cm-2 for the unit cell with Pt/C cathode and ~ 20 mW cm-2 with PtCr/C.
This work presents oxygem reduction reaction (ORR) studies on platinum (Pt/C) and platinum-chromium alloys containing 50% (PtCr/C) or 70% in chromium (Pt3Cr) dispersed on high surface area, in acid aqueous solutions with and without metanol, with cyclic voltammetry, chronoamperometry and rotating disk electrode techniques. Electrochemical measurements have been also performed in a direct methanol unit fuel cell. The electrocatalysts were characterized by energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses. The ORR studies in absence of methanol in solution showed that chromium adittion does not change the reaction mechanism observed on the Pt/C and the electrocatalytic activity increases in the order PtCr/C < Pt/C < Pt3Cr/C. Such increase was attributed to a favourable interatomic distance for O2 dissociative adsorption. The alloys showed higher tolerance to methanol presence in solution than Pt, mainly the PtCr electrode due to smaller amount of active sites to methanol adsorption with respect to Pt. In solutions containing low concentration of methanol, the ORR mechanism was not affected by methanol presence. The polarization curves recorded with the unit cell showed that na increase in temperature results in an increase in the electrocatalitic activity of the anode, as well as na increase in methanol diffusion through the Nafion® membrane with consequent cathode depolarization. Furthermore, the system containing Pt3Cr/C as cathodic electrode material presented the best electrochemical performance, with power densities of ~ 55 mW cm-2, in contrast to ~ 40 mW cm-2 for the unit cell with Pt/C cathode and ~ 20 mW cm-2 with PtCr/C.

Thesis (MScIng)--University of Stellenbosch, 2004.
ENGLISH ABSTRACT: This thesis details results obtained in an experimental study conducted to determine the effects of operating temperature, oxygen partial pressure, bulk chromium oxide content and bulk FeOx/MgO ratio on the solubility of chromium oxide in melter type slags in the platinum industry. Two PGM-containing layers in the Bushveld Complex in South Africa, the Merensky and UG2 reefs, are currently being mined for the extraction of base metals and platinum group metals (PGM). While the Merensky reef is a pyroxenitic layer, the UG2 reef is a platiniferous chromitite seam. Due to a gradual depletion in Merensky ore reserves, platinum producers have been moving towards the processing of more UG2 concentrates, which are higher in chromium oxide content. The technical difficulties associated with the smelting of concentrates with high chromium oxide contents is a matter of concern. The formation of chromite spinels in melts increases liquidus temperatures and viscosities and subsequently hampers tapping of slags and mattes from furnaces. Bottom build-up from the smelting of high chromium oxide containing concentrates could reduce effective furnace volume. From the literature reviewed it was found that very few published investigations covered melt compositions and oxygen partial pressures similar to those encountered in the platinum industry. Relevant studies were found to deal with significantly lower bulk chromium oxide and iron oxide contents. It became clear that a need exists for information on the behaviour of chromium oxide and its effects on phase chemistry and stability in melter slags. It was decided to study the phase equilibria through drop-quench experiments using six synthetic slags with bulk FeOx/MgO ratios between 0.6 and 1.9 and bulk chromium oxide contents between 1.2 and 7 wt%. Temperatures investigated were 1400, 1500 and 1600°C. The oxygen partial pressure was varied between 6.8x10-10 atm at 1400°C to8.3x10-5 atm at 1600°C. Experiments were conducted in a sealed vertical tube furnace and the required oxygen partial pressure in the furnace tube was maintained by controlling the flow rates of purified CO and CO2 gas mixtures through the tube. Reaction products were quenched after a reaction time of between 20 and 24 hours, depending on temperature, and the phase compositions were analysed by microprobe. The experimental study revealed that chromium oxide partitions very strongly into the spinel phase relative to the liquid phase, especially at lower temperatures, and higher oxygen partial pressures and bulk chromium oxide contents. The solubility of chromium oxide in the liquid phase was found to increase with increasing temperature and decreasing oxygen partial pressure. An increase in bulk chromium oxide contents of 1 wt%, under otherwise constant conditions, resulted in an increase in slag liquidus temperature of approximately 100°C over the range of temperatures investigated. At 1500°C and bulk chromium oxide contents of 3.7 and 6.4 wt% a reduction in oxygen partial pressure from 1.1x10-5 to 1.1x10-7 atm resulted in increases in soluble chromium oxide of 0.9 and 2.0 wt%, respectively. A further decrease in oxygen partial pressure to 6.7x10-9 atm resulted in increases in soluble chromium oxide of 2.8 and 4.7 wt%, respectively. Experimental results were compared to values predicted by the multi-phase equilibrium (MPE) model developed by CSIRO, and found to agree well. Slag basicity was not varied experimentally and therefore the model was used to predict its effect on the solubility of chromium oxide in the liquid phase and the stability of crystalline phases. At constant temperature, an increase in basicity resulted in a decrease in the solubility of chromium oxide in the liquid phase as well as stabilisation of the spinel phase. It was concluded that practicable combinations of one or more of four main factors, namely increased operating temperature and decreased bulk chromium oxide content, slag basicity and oxygen partial pressure, should be applied and evaluated in a plantenvironment to optimise furnace operation. The MPE model would be a valuable tool in predicting the outcomes of such investigations.
AFRIKAANSE OPSOMMING: Hierdie tesis detaileer die resultate wat verkry is uit ‘n eksperimentele studie uitgevoer om die effek van bedryfstemperatuur, die parsiële druk van suurstof, die algehele chroomoksied inhoud en die algehele FeOx/MgO verhouding op die gedrag van chroomoksied in smelter slakke in die platinum industrie te bestudeer. Twee PGM-bevattende ertslae in die Bosveldkompleks in Suid Afrika, die Merensky en UG2 riwwe, word huidiglik gemyn vir die ekstraksie van basismetale en platinumgroep metale (PGM). Die Merensky rif is ‘n piroksenitiese laag terwyl die UG2 rif ‘n platinumbevattende chromitiet laag is. As gevolg van ‘n geleidelike afname in reserwes van Merensky erts beweeg platinumprodusente al meer na die verwerking van groter hoeveelhede UG2 erts. Die tegniese probleme wat gepaard gaan met die smelting van konsentrate met hoë chroomoksied inhoud kan ‘n rede tot kommer wees. Die vorming van chromiet spinelle in die slak- en matfases verhoog likuidus temperature en viskositeite en bemoeilik die tap van hierdie fases uit oonde. Die opbou van soliede fases verlaag ook die effektiewe oondvolume. Uit die literatuurstudie is gevind dat gepubliseerde studies waarin slak samestellings en parsiële suurstofdrukke wat betrekking het op die platinumindustrie bespreek is, baie beperk is. Dit is gevind dat relevante navorsing gedoen is met aansienlik laer algehele chroom- en ysteroksied konsentrasies. Gevolglik het dit duidelik geword dat ‘n behoefte bestaan vir inligting oor die gedrag van chroomoksied in oonde en die effekte daarvan op fasechemie en –stabiliteit in smelter slakke. Daar is besluit om eksperimente uit te voer deur die gebruik van ses sintetiese slakke met algehele FeOx/MgO verhoudings tussen 0.6 en 1.9 en algehele chroomoksied konsentrasies tussen 1.2 en 7.0 % (op ‘n massabasis). Temperature van 1400, 1500 en 1600°C en suurstof parsiële drukke tussen 6.8x10-10 atm by 1400°C en 8.3x10-5 atm by 1600°C is ondersoek. Eksperimente is uitgevoer in ‘n geseëlde vertikale buisoond en dievereiste suurstofdruk in die oond is gehandhaaf deur beheer van die vloeitempos van gesuiwerde CO en CO2 gas deur die oond. Reaksieprodukte is in water geblus na ‘n reaksietyd van tussen 20 en 24 ure, afhangende van die reaksietemperatuur. Fasesamestellings is bepaal deur mikrosonde analises. Die eksperimentele studie het bewys dat chroomoksied baie sterk in die spinelfase konsentreer relatief tot die vloeistoffase, veral by laer temperature, suurstofdrukke en algehele chroomoksied konsentrasies. Dit is gevind dat die oplosbaarheid van chroomoksied in die vloeistoffase toeneem met toenemende temperatuur en afnemende suurstofdruk. ‘n Toename in die algehele chroomoksied konsentrasie van 1 massa%, onder andersins onveranderde toestande, het ‘n toename van ongeveer 100°C in likuidus temperature veroorsaak tussen 1400 en 1600°C. By 1500°C en algehele chroomoksied konsentrasies van 3.7 en 6.4 massa%, het ‘n verlaging in suurstofdruk vanaf 1.1x10-5 tot 1.1x10-7 atm respektiewelike toenames in die chroomoksied oplosbaarheid van 0.9 en 2.0 massa% veroorsaak. ‘n Verdere verlaging in suurstofdruk tot 6.7x10-9 atm het respektiewelike toenames in chroomoksied oplosbaarheid van 2.8 en 4.7 massa% veroorsaak. Eksperimentele resultate is vergelyk met waardes wat voorspel is deur die multifase ewewigsmodel (MPE), ontwikkel deur CSIRO, en goeie ooreenstemming is gevind. Verskillende slak basisiteite is nie eksperimenteel ondersoek nie en daarom is die model gebruik om die effek daarvan op die oplosbaarheid van chroomoksied in die vloeistoffase asook die stabiliteit van kristallyne fases te bepaal. By konstante temperatuur het ‘n toename in slak basisiteit ‘n afname in chroomoksied oplosbaarheid veroorsaak en die spinelfase gestabiliseer. Die aanbeveling is gemaak dat ‘n kombinasie van een of meer van vier hooffaktore, naamlik hoër bedryfstemperature en laer algehele chroomoksied konsentrasies, slak basisiteit en suurstofdruk, in die praktyk toegepas en geëvalueer moet word om sodoendeoptimum bedryfkondisies te bepaal. Die multifase ewewigsmodel is ‘n nuttige instrument wat gebruik kan word om die resultate van sulke ondersoeke te voorspel.

The finding of a suite of unusual minerals associated to ophiolitic chromitites questions traditional model for the formation of these rocks. These findings include minerals typically considered indicators of ultra-high pressure and super-reducing conditions (i.e., diamond). Several authors proposed new genetic models for ophiolitic chromitites which involved their formation and/or evolution in the deep mantle (>410 km), opening an intense debate that is still active. This Ph.D. thesis contributes to this debate by studying four examples of ophiolitic chromitites from different geodynamic settings (fore-arc, back-arc, and supra-subduction mantle interacted with mantle plume) in order to evaluate the origin of these “exotic” minerals in ophiolitic chromitites and their possible meaning into the evolutionary history of these enigmatic rocks. The studied examples of chromitites formed in fore-arc settings include chromitite veins from Havana-Matanzas ophiolite (Cuba) and chromitite pods from Loma Las Cabirmas hill in Loma Caribe peridotite (Dominican Republic). The chromitites from Havana-Matanzas are characterized by abundant platinum-group mineral (PGM) inclusions and by being associated with orthopyroxenite in ophiolitic mantle section. Chromite studies reveal that the chromitites and orthopyroxenites formed coetaneously from Si- and Mg-rich andesitic melts with boninitic affinity originated in the fore-arc mantle of an intra-oceanic arc. The melts infiltrated the pre-existing harzburgite, dissolving olivine while precipitating orthopyroxene and chromite. The abundance of PGM is interpreted to result from the “collector” effect of chromite during the chromitite formation, which was enhanced due to the small volume of chromitites formed. The Loma Las Cabirmas chromitites display unusually high platinum-group elements (PGE) contents and hydrothermal PGE mineralizations. The study of these chromitites suggests that hydrothermal fluids infiltrated the chromitites during serpentinization of the rocks and gave place to the in situ alteration of magmatic PGM in the chromitites and the precipitation of hydrothermal Pt-rich mineralization in uvarovite- and chromian clinochlore-filled fractures, within the thermal range of 350–150 ºC. Therefore, the PGE bulk geochemistry and mineralogy of these chromitites was largely impacted by post-magmatic processes. Microdiamonds were found in situ in ophiolitic chromitites from Tehuitzingo serpentinite (Mexico), interpreted as an ophiolitic mantle section formed in a back-arc geodynamic setting. Micrometric diamonds were located within healed fractures crosscutting the chromite grains. These diamonds were associated with amorphous C bearing O-species and with a low-pressure and low-temperature mineral assemblage. Thermodynamic calculations show that the chromite from the healed fractures recrystallized at 670–510 ºC. Consequently, the diamonds are inferred to be metastable and form from C undersaturated CO2–H2O–CH4 fluids infiltrating the chromitite during serpentinization processes. The diamond formed due to the super-reducing conditions achieved within the sealed inclusions. The formation of microdiamonds in (ultra)reduced low-pressure and low-temperature environments during serpentinization challenges the idea that diamond in ophiolitic rocks constitutes an unequivocal indicator of ultra-high pressure conditions. Chromitites from the northern and central part of the Loma Caribe peridotitic belt (Dominican Republic) were studied as an example of chromitites hosted in an ophiolite with geochemical fingerprints of deep-seated plume activity. These chromitites display unusual compositions, different from other ophiolitic chromitites. Instead, the analyzed chromite grains exhibit major and trace element composition that overlaps the field of komatiitic chromite. Moreover, the composition of parental melts of these chromitites is similar to plume derived products of the Dominican Republic. Therefore, I inferred that these chromitites formed in the lithospheric mantle after parental melts derived from an enriched peridotite source influenced by the Caribbean mantle plume. Altogether, my results show the importance of differentiating features resulting from magmatic primary processes from those resulting from alteration processes. Only then it will be possible to reconstruct the genetic story and evolution of the chromitites and propose accurate models.
En els darrers anys s’han trobat una sèrie de minerals considerats “exòtics” en cromitites ofiolítiques que han fet replantejar a la comunitat científica internacional els models de formació d’aquestes roques. Aquests minerals inusuals són sovint considerats indicadors de condicions d’ultra-alta pressió i súper reductores (i.e. diamant), de manera que els models proposats a partir d’aquestes troballes inclouen un origen i/o una evolució de les cromitites a nivells profunds mantèl·lics (>410 km), contraposant-se a la formació en el mantell litosfèric proposada pels models tradicionals. Aquesta tesi doctoral estudia quatre exemples de cromitites ofiolítiques que pertanyen a diferents contexts geodinàmics amb l’objectiu d’identificar minerals o trets inusuals en les cromitites i/o en les seves roques associades per tal d’avaluar-ne el seu origen i el seu possible significar en el marge de la historia evolutiva d’aquestes roques. Els casos estudiats són: (1) les venes de cromitita de l’ofiolita d’Habana-Matanzas (Cuba), pertanyents a un context de avant-arc; (2) els pods de cromitites de Loma Las Cabrimas (peridotita de Loma Caribe, República Dominicana), també pertanyents a un context d’avant-arc i caracteritzades per les seves mineralitzacions hidrotermals de baixa temperatura de Pt; (3) les cromitites de la serpentina de Tehuitzingo (Mèxic), interpretada com una secció mantèl·lica ofiolítica formada en un context de rere-arc; per últim, (4) les cromitites del nord i del centre del cinturó peridotític de Loma Caribe (República Dominicana), que constitueixen un exemple de cromitites encaixades en una ofiolita amb evidències geoquímiques d’activitat de ploma mantèl·lica. En conjunt, els resultats demostren la importància de diferenciar els efectes de processos magmàtics primaris d’aquells derivats de processos d’alteració per tal de fer una interpretació correcta de la història evolutiva d’aquestes roques.

Bibliography: pages [144]-152.
The methodology was developed to characterise the complex MF2 circuit at Impala Platinum Ltd. The circuit was divided into individual units and each unit was evaluated over a period of time. The performance of each unit in terms of platinum, palladium, rhodium and chromite recoveries was determined by sampling around the unit. The changes in ore floatability characteristics were monitored by carrying out batch flotation tests on samples taken from the feed to each unit. The mixing in the units was characterised by performing residence time distribution studies on the units. The batch flotation results were modelled using a distributed rate approach. The model yielded parameters which were associated with three floatability classes, viz. a fast floating, a medium floating and, a slow floating class and a class for valuable minerals which did not float. The model was not able to discriminate between mineralogical and particle size effects on flotation rates. This would be an important prospect of a future investigation. The rate constants associated with each floatability fraction obtained from the batch flotation together with residence time distribution results were used in association with the actual recoveries from each unit. These factors were used to simulate the performance of the circuit. The same model was used for all the units and the differences in cell design as indicated by hydrodynamics, aeration rate, etc. were accounted for in the· mixing parameters and flotation rates. This procedure to simulate plant performance was successfully applied to three different circuits thus confirming its validity. The difference in the three simulated circuits was in the cleaning stages. Among other things, the simulation shows that the best ·platinum, palladium and rhodium recoveries could be obtained if the greatest cleaning capacity was situated in the secondary stage of the circuit. Although the changes in the circuit configuration could be simulated to establish the effect on the platinum, palladium and rhodium recoveries, the chromite recovery could not be simulated. This is due to the fact that chromite is mainly floated by entrainment which is not incorporated into the model. Since the chromite recovery is crucial for the design of optimum cleaning circuits in the flotation of UG-2 ore, research needs to be done to quantify the entrainment phenomenon.

Le massif de Tropoja est le plus septentrional des massifs de la ceinture orientale des ophiolites d'Albanie dont les caractères pétrologiques et métallogéniques s'apparentent aux produits magmatiques des arcs insulaires immatures. Quatre niveaux enrichis en chromite ont été définis d'après leur stratigraphie, leur structure et leur composition : trois dans le manteau et un dans les cumulats. D'importantes variations de la fugacité en oxygène ont accompagné la formation des chromitites et des minéralisations en élement du groupe du platine qui leur sont associées. Au Sud du massif, une forte fugacité en oxygène couplée à une faible fugacité en soufre seraient responsables de la précipitation, directement à partir du magma, d'alliages de fer et de platine, qui forment la minéralisation platinifère caractéristiques des cumulas de Tropoja. Cette minéralisation, rare dans les autres massifs ophiolitiques, est située dans un niveau bréchique, à l'interface de dunites basales et de pyroxénites. Les teneurs de platine, qui peuvent atteindre 11 grammes par tonne, sont corrélées positivement aux concentrations de chrome. Trois stades minéralisants y ont été définis, le dernier étant lié à des interactions entre minéraux et fluides tardifs. D'autres caractéristiques minéralogiques de cette minéralisation, importantes pour le traitement des minerais, ont été obtenues grâce à une étude statistique. Au Nord du massif, les minéralisations platinifères, localisées dans le manteau, sont associées à des sulfures de métaux de base.

Exploration for chromitite-associated Pt deposits is hampered by a poor understanding of the distribution and behaviour of Pt in the surficial environment. This study investigates Pt content, residence sites and PGE mineralogy of soils developed on till and colluvium above the Tulameen ultramafic complex in southern British Columbia. Seventy-six soil profiles, as well as sediments, bogs and waters were sampled above the dunite core of the Tulameen complex, within which Pt occcurrences consist of massive-to-discontinuous segregations of platinic chromitite. Pt content of the -212 um fraction of soils and sediments was determined by fire assay-inductively coupled plasma spectroscopy. Samples from fourteen selected profiles were then examined in detail to determine Pt mineralogy and its distribution between different size, density and magnetic fractions. Pt concentrations in the -212 um fraction of C horizon soils range from 2 to 885 ppb and are closely related to soil dunite content, as estimated from MgO content and verified by XRD mineralogy. Dunite colluvium (mean: 24.2% MgO), locally-derived dunitic till (mean: 16.5% MgO) and exotic non-dunitic till (mean: 5.7% MgO) have median Pt concentrations of 88 ppb, 36 ppb and 8 ppb, respectively. This trend is evident in all size and density fractions. Pt content of heavy mineral (SG > 3.3) fractions is 10-2Ox greater than in light mineral fractions. Pt is most abundant in the heavy magnetic fraction from non-dunitic tills and dunitic tills remote from known mineralization, but the proportion of Pt in the heavy non-magnetic fraction increases with increasing proximity to mineralization. SEM and microprobe studies of heavy fractions from C horizons identified Pt-Fe-Cu alloys as free grains, and as inclusions in Mg-silicates and chromites. Chromite occurs as Mg-Cr-rich anhedral fragments and as Fe-rich euhedral to subhedral crystals. The latter, relatively more important in the magnetic fraction, are interpreted as Pt-poor grains disseminated throughout the dunite whereas fragments are relatively more important in the non-magnetic fraction and are interpreted as remnants of Pt-bearing massive chromitite segregations. The abundance of chromite fragments in soils near chromitite segregations accounts for the high Pt content of the non-magnetic heavy fractions of these soils. The -270 mesh fraction or the magnetic heavy fraction of C horizon soils would be the most suitable sample media for reconnaissance geochemical sampling. However, the greater contrast, more limited dispersion and Mg-Cr-rich chromite association of the non-magnetic heavy fraction make it a more suitable media for detailed geochemical sampling.
Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate

La transformation exogène qui affecte les roches émergées de la ceinture intertropicale, a édifié aux dépends des péridotites de Nouvelle-Calédonie, d'épais manteaux latéritiques enrichis en divers métaux. L'objectif de cette étude a été de caractériser les évolutions chimique et minéralogique des éléments et minéraux du groupe platine dans cet environnement latéritique. L'approche pétrographique et pétrologique a permis de mettre en évidence une modification précoce de la minéralisation initiale pendant la serpentinisation par un processus de désulfuration des sulfures précurseurs, et une précipitation de nouveaux minéraux du groupe platine (PGM). Dans l'environnement exogène, les PGM évoluent selon leur composition chimique initiale. L'isoferroplatine (Pt3Fe) parait stable, alors que le tétraferoplatine (PtFe), la tulameenite(Pt2FeCu) et les oxydes de Pt-Fe s'altèrent, avec une lixiviation sélective de leurs composants chimiques : S > Cu > Pd > Fe > Pt.

The UG3 at Modikwa Platinum Mine occurs as a platiniferous, planar chromitite seam. It is stratigraphically located in the Upper Critical Zone of the Eastern Bushveld Complex. Field work study comprise of underground mapping, sampling, surface mapping, borehole core logging, microprobing and microscopic investigations carried out at the Rhodes University. The UG3 at the Modikwa Platinum Mine is about 22cm thick chromitite seam underlain by a white fine grained anorthosite and overlain by a brown medium grained feldspathic pyroxenite. It is an incomplete cyclic unit consisting of chromite and feldspathic pyroxenite. The UG3 reef at the Modikwa Platinum Mine lease area represents a Platinum Group Metal resource of 300 million tons of ore at an in situ grade of 2.5g/t. Under the current market conditions the UG3 reef remains unprofitable to mine in an underground operation due to the operational cost involved. However, it has been illustrated that the UG3 chromitite seam can increase profit margins in an open pit operation provided it is mined together with the economic UG2 chromitite seam. The extraction of the UG3 as ore in the four Modikwa UG2 open pits would result in a combined operating cash profit of R330 million. The UG3 chromitite seam is platiniferous. The platinum-group minerals (PGM) range in size from less than 10μm to about 70μm. The PGMs are associated with sulphides and are both located in the interstitial silicates and are concentrated in the chromitite seam. The PGMs show a strong preference to contact boundaries of the silicate grains, the chromite grains and the sulphide phases. In some instances, they are enclosed within the chromite grains in association with sulphides. The general sulphide assemblage comprises pentlandite and chalcopyrite whereas, the PGMs assemblage comprises cooperite, ferroplatinum, laurite, FeRhS and PtRhS.

Conception d'un pot catalytique et etude de l'efficacite de la catalyse. Mise au point d'un support metallique. Preparation du support par passivation de l'alliage fer-chrome-aluminium. Stabilisation de l'alumine par ajout de terres rares. Utilisation de platine et de palladium comme materiaux catalytiques

La séquence plutonique du massif de Fizh (partie nord de l'ophiolite d'Oman) offre une complexité crustale, liée à une diversité des corps magmatiques et du système filonien. La cartographie détaillée montre une succession d'événements magmatiques dont les séries principales sont formées de gabbros lités, des wehrlites et pyroxénites, des gabbronorites. Le magmatisme gabbroïque est généré dans une grande chambre magmatique axiale, alors que le magmatisme wehrlitique est dit hors-axe ; ceci est conditionné par la segmentation axiale de la paléodorsale. L'asymétrie ainsi observée dans la séquence crustale est marquée par la distribution et l'abondance des corps ultrabasiques. Les variations longitudinales dans les séquences stratigraphiques montrent des discontinuités au sein de la paléodorsale dans ce massif. Les granitoïdes sous forme de filons ou de stocks, attestent une succession d'événements magmatiques et les modalités de l'accrétion. En effet, les plagiogranites sont liés à la cristallisation fractionnée des gabbros, au cours de l'épisode d'accrétion normale. Les granodiorites et les diorites quartziques sont rattachées spatialement aux ultrabasites et formées par cristallisation fractionnée pendant la phase d'écaillage intra-océanique. Les granites et les leucogranites sont tardifs et à cachet collisionnel. Ils sont issus de la fusion partielle de la semelle métamorphique et liés à l'épisode obductif chaud. Les gisements de chromite sont variés et sont localisés dans le manteau et à la limite manteau-croûte. Les concentrations de chromite dans les ultrabasites sont importantes et présentent un effet sur le comportement des éléments du groupe du platine. Les sulfures magmatiques (pentlandite et chalcopyrite) précipitent dans les wehrlites et les pyroxénites en même temps que le plagioclase. Le comportement des PGE dans les chromitites montre qu'elles sont moins différenciées que celles des autres massifs ophiolitiques. Les PPGE et particulièrement le Pd précipitent dans les ultrabasites en même temps que les sulfures d'origine magmatique

Sélectionner des traitements de surface pour l’industrie nécessite de prendre en compte :les propriétés à conférer au substrat, la nature et la géométrie de celui-ci et les caractéristiques du milieu extérieur. Certaines combinaisons de ces paramètres rendent difficile la sélection d’un traitement unique, d’où le recours à des multitraitements de surface. Dès lors, se posent les questions suivantes :

- Utiliser des multitraitements de surface peut se faire en scindant les différentes requêtes en sous-ensembles, de manière à ce que chaque traitement réponde à l’un d’eux. Dans quel ordre ces requêtes doivent-elles être introduites par rapport au substrat ?

- Comment sélectionner les traitements de surface répondant à chaque requête individuelle ?

- Comment classer des multitraitements en termes d’adéquation au problème posé ?

Dans ce travail, les première et troisième questions sont abordées, en explorant les requêtes concernant habituellement les dispositifs de moulage de l’aluminium :

- Résistance aux contraintes d’origine thermique.

- Résistance à la corrosion par les métaux fondus.

- Résistance au frottement.

L’analyse de la bibliographie relative aux traitements de surface utilisés dans ces systèmes a été analysée et des « architectures »-types ont été identifiées (chapitre 3). On prévoit, par exemple, un traitement conférant la résistance à la fatigue superficielle, ainsi qu’un revêtement étanche et résistant à l’aluminium fondu. Une barrière thermique est parfois préconisée.

Pour chacune des architectures, des traitements de surface individuels peuvent être sélectionnés. Un « facteur de performance » permettant de classer les solutions par rapport au problème de la fatigue thermique a été construit (chapitre 4) et discuté dans deux situations :

- Lorsqu’un revêtement est présent, et que les contraintes d’origine thermique (différence de dilatation thermique couche-substrat) menacent de le rompre lors de l’immersion dans un milieu corrosif à haute température. Des essais de corrosion dans de l’aluminium fondu ont été réalisés sur un acier revêtu par du nitrure de chrome dopé à l’aluminium, synthétisé par déposition physique en phase vapeur (chapitre 5 – collaboration :Inasmet).

- Lorsque des variations thermiques rapides menacent de rompre le substrat et la (les) couches. Des essais de fatigue thermique ont été réalisés sur de l’acier à outils pour travail à chaud non traité, boruré ou recouvert d’un multitraitements (zircone yttriée / NiCrAlY / boruration / acier). Le revêtement en zircone yttriée a été obtenu par projection par plasma. L’essai de fatigue thermique a été modélisé et le facteur de performance, discuté (chapitre 6).

Au chapitre 7, les architectures-types ont été introduites dans une méthodologie de sélection des multi-traitements de surface, qui a été appliquée dans deux cas :

- Celui des moules de fonderie, devant résister à la fatigue thermique et à la corrosion par l’aluminium fondu. Le facteur de performance a été extrapolé à d’autres situations qu’aux chapitres 5 et 6. Les solutions habituellement proposées pour résoudre ce problème sont retrouvées.

- Celui de deux pièces en acier frottant l’une contre l’autre en présence d’aluminium fondu.

To select surface treatments, one must account for the required functional properties, the substrate features and the solicitations the substrate must endure. Certain combinations of these parameters make it difficult to select a single surface treatment, a reason why several successive treatments are preferred. To select them, one needs to determine:

- How to divide the several requests into groups and how to stack up these groups from the substrate to the outer surface, so that each treatment deals with one specific group of requests/properties.

- How to select individual layers for each group of properties.

- How to rank the multi-treatments in terms of relevance for a given application.

In this work, one tries to answer the first and the third questions, by studying the case of aluminium foundry, in which the industrial devices frequently face the following solicitations:

- Thermal stress (thermal fatigue, thermal expansion mismatch).

- Presence of corrosive molten metal.

- Sliding wear.

In the literature, several “standard” architectures are proposed (chapter 3), like a diffusion layer reducing superficial fatigue plus a corrosion barrier layer. A thermal barrier coating is also sometimes proposed.

For each of these architectures, one can select individual treatments. To rank them, one devised a “performance index” for thermal stress (chap.4), which is discussed for two cases:

- For large differences between layer and substrate thermal expansion coefficients, when both are put into contact with a high temperature corrosive medium, the layer may be damaged. One discusses this case by examining the corrosion caused by molten aluminium for a steel substrate coated by anticorrosive chromium nitride doped with aluminium. The layer is produced by physical vapour deposition (chap. 5 – cooperation: Inasmet).

- Repeated fast surface temperature transients can also damage the substrate and/or the layer by thermal fatigue. One conducted thermal fatigue tests with samples of hot work tool steel, respectively untreated, simply borided and protected by a multilayer. In the last case, top coat is yttria stabilised zirconia, followed by a nickel superalloy and then a borided layer (undercoat). One synthesized the zirconia coating by plasma spray and one modelled the thermal fatigue (chap. 6).

In chap. 7, architectures from chap. 2 are introduced in a multi-treatment selection routine, which is applied in two cases:

- Foundry moulds for molten aluminium, withstanding both thermal fatigue and corrosion. The devised performance index is extrapolated beyond the tests of chap. 5 and 6 to treatments for this industrial application, thereby quantifying their respective merits.

- A foundry device exposed to molten metal and sliding wear.

Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

Engineered components can gain desirable properties when coated with surface materials. Wear-resistant coatings can improve the performance of contacting surfaces and allow for an extended life of the parts. Hard chromium has been the plating material of choice for certain wear and corrosion- resistant coatings because of its desirable combination of chemical resistance, adhesion, and mechanical properties. However, hexavalent chromium, a component of the process for applying hard chromium coatings, has been recognized by the EPA as having hazardous health and environmental impacts. Existing and planned environmental regulations restricts the use of process chemicals containing hexavalent chromium ions. This substantiates a need to develop an environmental friendly process for alternative coatings. Praxair has reported that Cr-Ni-C particles have a better corrosion resistance than current chromium carbide and nickel chromium powders. Today, Cr-Ni-C provides great qualities for flame spray and does not contain the toxic compounds used to deposit hard chromium, but is not compatible with application by cold spray. The purpose of this thesis project is to compare two processes for plating metal powder, chromium nickel carbide (Cr-Ni-C, CRC-410-1 from Praxair), with nickel. The particles were encapsulated using three different methods: one electroplating method previously used on particles, and two electroless plating processes using different solutions. The Cr-Ni-C particles were successfully encapsulated with Ni by one of the electroless deposition methods. The electrolytic deposition experiments did not yield the uniformity of coating without agglomeration that is being attained in industrial practice today. Further research on this method is recommended, due to the material operational cost in an industrial setting that is projected to be over 200 times cheaper than electroless deposition method. In the meantime, it should be possible to produce enough coated powder by electroless deposition to validate the utility of this coated powder in depositing wear- and corrosion-resistant coatings of Cr-Ni-C by cold spray.

A hard chromium electroplating process is normally used for preventing mechanical and electrical parts such as roller, piston and mold from the harmful environments and giving the good physical properties on the surface such as increased wear resistance, increased hardness, low frictional coefficient as well as good aesthetic look on the surface of workpieces. The problem that often found in this process is the deflected workpieces after plating process due to the low plating performance during a plating period. These deflected workpieces are needed to replating it again. However, the replating method causes a large amount of resource consumptions and increases the production time. To handle this problem, the plating solution temperature is needed to maintain the plating solution temperature at a set point about 50 °C in order to improve the plating performance during the plating period and decrease the occurrence of the deflected workpieces. In this work, the mathematical models are developed to explain the dynamic behavior of the plating solution temperature during the plating time and validated with the real data from a plant. The conventional (PID) controller is applied to this process for the purpose of keeping the plating solution temperature at the set point throughout the plating time. The result demonstrates that the developed mathematical models can be used to explain the dynamic behavior of the plating solution temperature because it gives the good simulation of the plating solution temperature with a slightly different from the real data. Furthermore, the PID controller shows the high control performance for maintaining the plating solution temperature at the set point throughout the plating period with small overshoot at the beginning of every batch.

碩士
長庚大學
機械工程學系
98
Mg alloy, AZ91D, which has a two-phase structure, was successfully electroplated in an alkaline Cu-plating bath. The Cu-coated Mg alloy specimen was further electroplated in eco-friendly acidic Cu and then trivalent Cr baths to obtain an anti-wear and anti-corrosion Cr/Cu coating. Experimental results show that the wear and corrosion resistance of the Mg alloy specimen was considerably improved by trivalent Cr electrodeposition. The hardness of the as-plated Cr deposit was drastically increased by using reduction-flame heating for 0.5 s. A superior wear and corrosion resistance was obtained when a Cu-coated Mg alloy specimen was electroplated with a trivalent Cr deposit, followed by heating with reduction-flame heating for 0.5 s. The corrosion behavior of Cu-coated and Cr/Cu-coated AZ91D specimens was studied electrochemically in 0.1 M H2SO4 with different NaCl concentrations. The corrosion resistance of Cr/Cu-coated AZ91D decreased with increasing NaCl concentration in 0.1 M H2SO4 solution. After immersion in a 0.1 M H2SO4 with a NaCl-content above 3.5 wt.%, the surface of Cr/Cu-coated AZ91D suffered a few blisters. Cracks through the Cr deposit provided active pathways for corrosion of the Cu and the AZ91D substrate. Formation of blisters on the Cr/Cu-coated AZ91D surface was confirmed based on the results of an open-circuit potential test, which detected an obvious potential drop from noble to active potentials. The above-mentioned results were measured via bonding strength, hardness, wear and corrosion tests.

Pt, Rh and chromite solubility in basalt-rhyolite mixtures at 1400°C, 0.1 MPa, FMQ +3.4 and FMQ (fayalite-magnetite-quartz-buffer) is presented. Melt samples were equilibrated on Pt90Rh10 loops and in chromite crucibles for up to 100 hours. Pt/Rh solubility decreased 20 and 7-fold respectively, progressing to rhyolitic compositions. Cr solubility decreased 3-fold (FMQ+3.4) and 4-fold (FMQ) over the same compositional interval. At FMQ+3.4, reduced Cr solubility, compared to FMQ, reflects Cr3+ and Fe3+ exchange, stabilizing Fe3+-rich spinel. As SiO2 increases, decreasing Pt/Rh and Cr solubility is interpreted to result from low abundances of non-bridging oxygens and lack of octahedral sites for Cr3+.Numerically simulated diffusive exchange between basalt and rhyolite predicts rapid alloy saturation at high fO2, but undersaturated mixtures at low fO2. Magma mixing produced Fe3+-rich spinel at high fO2; chromite undersaturation occurs near FMQ. Combined chromite and Pt-rich alloy saturation does not appear to result from magma mixing near FMQ.

Extraction of platinum group elements (PGE) is a major source of revenue in South Africa and the reserves represent about 75 per cent of world reserves. Most of the remaining Platinum Group Mineral (PGM) reserves are located in the UG-2 chromitite layer of the Bushveld Igneous Complex. Platinum concentrators experience significant losses of PGE in their secondary milling circuits due to insufficient liberation of platinum-bearing particles. The chromium oxide (Cr2O3) content in UG-2 concentrates is typically 3%, which results in operational problems in the downstream smelting process. Ways of improving the design of the secondary milling circuit were investigated, with the purpose of improving PGE recovery and reducing Cr2O3 entrainment in the subsequent flotation stage. Batch-scale laboratory and pilot plant tests were carried out to investigate the optimal design of a secondary milling circuit configuration. The optimal design consisted of a conventional hydrocyclone to de-slime the feed, followed by gravity separation with a spiral concentrator circuit to separate the ore into lights (silicates-rich) and heavies (chromite-rich) fractions. Separate milling of the light and heavy fractions made it possible to grind the silicate-rich fraction finer and to avoid over-grinding of the chromite. The total milling energy was redistributed between the silicates and chromite ball mills with 88% of the energy input to the silicates mill and 12% to the chromite mill thus reducing chromite over-grinding. The effects on the recovery of PGE, and the entrainment of Cr2O3 were measured in combined batch rougher flotation tests. The results indicated a 2% improvement in the secondary rougher flotation PGE recovery for the densifier underflow sample as compared to the standard MF-2 circuit, and most significantly the Cr2O3 entrainment was reduced by over 30% overall. Attritioning of the chromite-rich heavies fraction and ball milling of the silicates-rich lights fraction resulted in a 52% reduction of Cr2O3 in the rougher flotation concentrate and a 0.4% increase in PGE recovery (0.4%) as compared to the standard circuit. The improved reduction in chromite entrainment may be attributed to the lower fines generation with attritioning (52.8%- 106μm) as compared to ball milling with a 12% energy input (83.6% -106μm). Over 50% of the chromite minerals remained in the +106μm of the attritioned heavies product as compared with 21% for the ball milled spiral heavies stream. This accounted for a significant proportion of the overall chromite reduction in the flotation concentrate and supported the motivation for the inclusion of a separate grinding circuit for the chromite and silicate particles. Pilot plant testwork on a VHG (very high grade) spiral concentrator circuit followed by laboratory milling and rougher flotation tests confirmed the above conclusions. A 3.7% improvement in PGE recovery was noted with a 32% Cr2O3 reduction in the secondary rougher flotation concentrate as compared to the standard circuit. The statistical reliability of the laboratory and pilot plant data were quantified at various stages of the testwork due to the heterogeneous nature of the feed material and representative sampling. The repeat analyses on selected flotation tests for the high grade ore revealed that the variances were below 0.5%, 4%, and 7% for the head grades, PGE and Cr2O3 recoveries respectively. The flotation results for the standard and significantly improved milling circuits had variances in the 4E recoveries for the low grade ore and pilot plant ore of below 5.5% and 1% respectively. Low variances (<1%) in the Cr2O3 recoveries were noted for the low grade and pilot plant ores. A preliminary cost estimate was undertaken based on the pilot plant data to determine what value the proposed circuit could add for an additional 3.7% PGE recovery. An additional revenue of approximately R50 000 per day could result based upon the platinum mineral recovery only. The other precious metals, i.e. palladium and rhodium were neglected and would further increase the overall revenue. The minimum payback period for the estimated capital investment would be approximately 4 years. This confirmed the benefit of this improved secondary milling circuit design as a viable option. A closed-circuit operation of the silicates mill should offer more significant benefits compared to the open circuit option; however, this was not considered in the current testwork. This project has confirmed the benefit of separate ball milling and the use of a spiral concentrator as an effective gravity separation device in the secondary milling circuit for a chromite-rich (>50%) UG-2 platinum ore.
Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2011.

The magnetization reversal mechanism in Co/Pt and CoCrTa/Pt thin-film multilayers was investigated in situ using a Magneto-Optical Kerr Effect (MOKE) apparatus. Multilayers Nx(Co/Pt) and Nx(CoCrTa/Pt) with different amounts of doped $\rm Cr\sb{17}Ta\sb5$ were sputtered in high vacuum. The growth was interrupted at intervals of two bilayers to perform perpendicular MOKE loops in the range from N = 2 to N = 20. The Draaisma and De Jonge model was adapted to analyze the hysteresis loops with the domain wall energy density as a fitting parameter. The broad consistency of the experimental data with theory shows that the incorporation of Cr and Ta in the magnetic layer lowers the domain wall energy density. The model successfully predicts the thickness dependence of the loop squareness, which decreases with increasing N. According to the model, the domain repetition length d is in the range 750A to 2100A, and the domain wall energy density $\rm\sigma\sb{w}$ is in the range of 1.68 $\rm erg/cm\sp2$ to 11.12 $\rm erg/cm\sp2.$

M.Sc.
This study focuses on the Merensky Reef (MR) occurring within the Two Rivers Platinum mine property in the farm Dwarsriver 372 KT, on the Southern sector of the Eastern Limb of the Bushveld Complex. Five MR exploratory drill core intersections were obtained. Petrographic and mineral chemical characteristics of these drill core samples focused on the characterization of minerals like clinopyroxene, orthopyroxene, plagioclase, chromite and olivine. Data of the cryptic variation of orthopyroxene, plagioclase and chromite, from a 10 m interval (approximate thickness of the section studied); from footwall through the MR to the hangingwall lithologies at Dwarsriver are described in this study. Locally the vertical cryptic variation of these minerals is broadly consistent with regional trends of the RLS. The lateral variation (i.e. along strike) is less pronounced; however, locally these minerals appear to be chemically evolving moving to the south of the property. Footwall orthopyroxene compositions vary from a minimum of En66 and reach a maximum of En84. Those of the MR range from En71 to En85. Hangingwall orthopyroxene compositions range from En60 and reach a maximum of En80. Plagioclase compositions in the footwall units range from a minimum of An69 and reach a maximum of An85. Those of the MR range from a minimum of An35 to a maximum of An84. This wide range in plagioclase compositions is believed to be as a result of the increased presence of fluids within the MR interval. The hangingwall plagioclase compositions range from An64 to An84. By analogy of the Western Limb, where the lithologies of the Northwestern sector are believed to be proximal to the feeder of this limb; the local lateral variation in the present study suggests that the lithologies of either the Central or Western sectors are most probably proximal to the feeder for the Eastern Limb. PGM assemblages associated with and adjacent to the MR chromitite stringers were evaluated using an MLA. Data obtained from this technique is in broad agreement with regional studies of the MR. With the aid of wholerock PGE assays the MLA technique has proven to be a powerful tool in evaluating PGM assemblages relatively quickly, from a few carefully selected samples. The mineralogical associations of the PGM with the gangue and host minerals have shown three main associations. These are the associations of chromite, BMS and silicates with the PGM, of which the base metal sulfide (BMS) association is remarkable given that these have a relatively low modal abundance. The relatively high mineralogical association of the BMS with PGM has been explained by a model involving a base metal sulfide liquid which possibly scavenged chalcophile and siderophile elements. Chromite chemistry and modal analyses of MR secondary silicate phases, which peak adjacent to the chromitite stringers, suggests elevated fluid overprinting within and adjacent to the chromitite stringers. The upper chromitite stringers, however, have higher abundances of PGM phases that are believed to be secondary in origin relative to the basal chromitite stringers. Generally the PGM associated with the upper chromitite stringers are also bigger in size averaging 70 μm as opposed to 27 μm for those associated with the basal chromitite stringers. The increase in grain size of the PGM along with the higher modal abundance of secondary PGM phases associated with the upper stringers is believed to be as a result of fluids. These fluids although affecting both the upper and basal chromitite stringers, appear to have had a relatively higher influence on the upper chromitite stringers. The most common PGMs encountered in this study are isoferroplatinum, sperrylite, michenerite, maslovite, cooperite, laurite and braggite.

Στα πλαίσια της παρούσας διατριβής μελετήθηκαν οι διεργασίες κοιτασματογένεσης των χρωμιτιτικών εμφανίσεων των οφιολιθικών συμπλεγμάτων του Βούρινου και της Πίνδου, σε συνδυασμό με τη συνδεόμενη με αυτές μεταλλοφορία των στοιχείων της ομάδας του λευκόχρυσου (PGE). Ιδιαίτερη έμφαση δόθηκε επίσης στην πετρογενετική εξέλιξη των μανδυακών ενοτήτων των δύο οφιολιθικών ακολουθιών, προκειμένου να εξαχθούν γενικότερα συμπεράσματα σχετικά με την προέλευση και τη διαμόρφωση των οφιόλιθων του ΒΑ Ελλαδικού χώρου. Γεωτεκτονικά το οφιολιθικό σύμπλεγμα του Βούρινου τοποθετείται στην Υποπελαγονική ζώνη, ενώ εκείνο της Πίνδου βρίσκεται τεκτονικά επωθημένο επί του Ηωκαινικής ηλικίας φλύσχη της ομόνυμης ισοπικής ζώνης. Αμφότερα τα συμπλέγματα θεωρείται ότι αντιπροσωπεύουν υπολείμματα του ενός ενιαίου ωκεανού, τα οποία σήμερα βρίσκονται επωθημένα επί του δυτικού περιθωρίου της Πελαγονικής μικροπλάκας. Αν και εμφανίζουν έντονα χαρακτηριστικά τεκτονικής καταπόνησης και οι δύο ακολουθίες διατηρούν όλο τους τύπους πετρολογικών ενοτήτων που συνθέτουν το φάσμα ενός πλήρως ανεπτυγμένου οφιολιθικού συμπλέγματος. Όλοι οι σχηματισμοί που μελετήθηκαν στα πλαίσια της παρούσας διατριβής προέρχονται από τις μανδυακές σειρές των δύο οφιολιθικών συμπλεγμάτων. Οι μανδυακοί τεκτονίτες αποτελούν τον κύριο όγκο του συμπλέγματος του Βούρινου, ενώ οι μανδυακής προέλευσης σχηματισμοί στο οφιολιθικό σύμπλεγμα της Πίνδου συνθέτουν την επονομαζόμενη ενότητα της Δραμάλας. Στον Βούρινο η δειγματοληπτική έρευνα κάλυψε τις σημαντικότερες από κοιτασματολογικής άποψης περιοχές του βόρειου και νότιου τμήματός του, ενώ στην Πίνδο δείγματα συλλέχθηκαν από τα κεντρικά και νότια τμήματα της όλης ενότητας της Δραμάλας, καθώς και από μανδυακής προέλευσης, ευμεγέθη, εξωτικά τεμάχη εντός της ενότητας της Αβδέλλας mélange. Η υπαίθρια μελέτη των μανδυακών ενοτήτων και στα δύο συμπλέγματα οδήγησε στις ακόλουθες διαπιστώσεις: α) το βόρειο τμήμα του οφιολιθικού συμπλέγματος του Βούρινου εμφανίζει σαφώς εντονότερα χαρακτηριστικά πλαστικής παραμόρφωσης σε σχέση με το νότιο, στο οποίο επικρατούν χαρακτήρες εύθραυστης-ημιπλαστικής παραμόρφωσης. Μια ανάλογη διάκριση, ασθενέστερα ωστόσο εκπεφρασμένη, θα μπορούσε να ειπωθεί πως ισχύει και όσον αφορά στην διάρθρωση των περιοχών Μηλιάς-Πεύκης, αντίστοιχα, στην Πίνδο. β) Η στατιστική ανάλυση των κινηματικών δεικτών συμφωνεί με μια προς τα ΒΑ διεύθυνση επώθησης για τον βόρειο Βούρινο και τη Μηλιά και με μια ΝΑ διεύθυνση τοποθέτησης για τον νότιο Βούρινο και το υπερβασικό τέμαχος της Πεύκης. γ) Οι χρωμιτιτικές εμφανίσεις φιλοξενούνται από δουνίτες και συγκεκριμένα στην πλειονότητα των θέσεων που παρατηρείται η ανάπτυξη χρωμιτικής μεταλλοφορίας το συνοδό είδος περιδοτίτη είναι ο λεπτοκρυσταλλικός δουνίτης (υγιής ή σερπεντινιωμένος). Πέρα από την σε όρους κοκκομετρικού μεγέθους διαφορά του σε σχέση με τον αντίστοιχο αδροκρυσταλλικό λιθότυπο, μια επιπλέον διαφοροποίησή του από αυτόν είναι η έλλειψη φολιωμένης δομής στον πρώτο. δ) Οι χρωμιτίτες του βόρειου Βούρινου και της Μηλιάς είναι συνήθως θυλακοειδούς μορφής, ασύμφωνα-παρασύμφωνα ανεπτυγμένοι σε σχέση με τους πιο άμεσους προς αυτούς αδροκρυσταλλικούς δουνιτικούς-χαρτσβουργιτικούς περιδοτίτες, ενώ τα χρωμιτικά κοιτάσματα του νότιου Βούρινου και της Πεύκης ακολουθούν στρωματοειδή, σύμφωνη-παρασύμφωνη δομική ανάπτυξη. ε) Οι χρωμιτίτες και των δύο συμπλεγμάτων εμφανίζουν μεγάλη ποικιλία ιστολογικής ανάπτυξης, ωστόσο διαπιστώθηκε μια συστηματική διαφοροποίηση όσον αφορά στο κύριο τύπο ιστού της κάθε χρωμιτικής μεταλλοφορίας. Έτσι στους χρωμιτίτες του βόρειου Βούρινου και της Μηλιάς επικρατεί ο συμπαγής και ο ιστός τύπου λεοπαρδάλεως, ενώ στους χρωμιτίτες του νότιου Βούρινου και της Πεύκης το επικρατέστερο είδος είναι ο ιστός κατά πλάκες. στ) Στο νότιο Βούρινο και στην Πεύκη παρατηρούνται συχνά εναλλασσόμενοι χαρτσβουργιτικοί-δουνιτικοί "πάγκοι", ενδεικτικοί μιας στρωματομένης κατάστασης του υπολειμματικού τμήματος του μανδύα που εκτίθεται επί των συγκεκριμένων περιοχών. Επίσης, ο αριθμός των πυροξενιτικών (ροδινγκιτιωμένων και μη) φλεβών είναι υψηλότερος στις περιοχές αυτές σε σχέση με εκείνων του βόρειου Βούρινου και της Μηλιάς. Στην πλειονότητα των περιπτώσεων οι φλέβες διατέμνουν τα κοιτάσματα χρωμίτη, στοιχείο που υποδεικνύει ότι δεν συνδέονται γενετικά μαζί τους, με μόνη εξαίρεση μια χρωμιτική εμφάνιση που φιλοξενείται σε πυροξενιτική φλέβα στην περιοχή της Πεύκης. ζ) Πάντα κοντά στις περιοχές εμφάνισης των κοιτασμάτων η τυπική λιθολογική διαδοχή προς αυτά είναι η: ±κλινοπυροξενομιγής χαρτσβουργίτης-κανονικός χαρτσβουργίτης-μεταβατικός χαρτσβουργίτης-αδροκρυσταλλικός δουνίτης-λεπτοκρυσταλλικός δουνίτης-χρωμιτίτης. Η οποιαδήποτε απόκλιση από την γενική αυτή διαδοχή είναι σπάνια, ακόμη και σε περιπτώσεις μικρών χρωμιτικών φακών. η) Η συχνή παγίδευση χαρτσβουργιτικών τεμαχών εντός αδροκρυσταλλικών δουνιτών υποδηλώνει την προέλευση των τελευταίων από τους πρώτους, ωστόσο περιπτώσεις αντικατάστασης φολιωμένων χαρτσβουργιτών από λεπτοκρυσταλλικούς δουνίτες υποδεικνύουν ότι κάποιοι λιθότυποι δεν έχουν, όπως αναμένεται, υπολειμματική προέλευση. θ) Η παρουσία κλινοπυροξενομιγών χαρτσβουργιτών είναι πιο συστηματική στην Πίνδο, ενώ στον Βούρινο απαντώνται πιο συχνά στείροι λεπτοκρυσταλλικοί δουνίτες κυρίως υπό τη μορφή φλεβών, πάγκων ή φακών. ι) Το χρωμιτικό κοίτασμα του Κορυδαλλού φιλοξενείται μέσα σε ένα εξωτικό, δουνιτικό τέμαχος, το οποίο περιέχεται εντός της οφιολιθικής mélange της ενότητας της Αβδέλλας που αναπτύσσεται στη συγκεκριμένη περιοχή. Τέλος, η συνολική αξιολόγηση όλων των διαθέσιμων γεωλογικών, ορυκτοχημικών, πετρολογικών και κοιτασματολογικών στοιχείων, υποδηλώνει ότι οι μανδυακές ενότητες και των δύο ακολουθιών γεννήθηκαν σε ένα κέντρο ώριμης διάνοιξης (MOR) και στη συνέχεια αναδιαμορφώθηκαν ως τμήματα ενός περιθωρίου πάνω από μια ζώνη καταβύθισης (SSZ). Η μετάβαση από τον Βούρινο στη Μηλιά και την Πεύκη και εν τέλει στον Κορυδαλλό πιθανά ισοδυναμεί με τη μετάβαση από το περιβάλλον μιας λεκάνης μπροστά από νησιωτικό τόξο στο (χώρο κάτω από το) ίδιο το τόξο και τέλος σε μια μικρών διαστάσεων περιθωρειακή λεκάνη πίσω από αυτό.
In the present thesis the processes of the genesis of the chomitite occurrences of the Vourinos and Pindos ophiolite complexes combined with their content in platinum group elements are studied. Special attention is given to the petrogenetic evolution of the mantle units of both ophiolite sequences in an attempt to understand better the origin and the formation of the ophiolites of the NW Greek area. In geotectonic terms the ophiolite complex of Vourinos belongs to the Ypopelagonic isopic zone, while the Pindos ophiolites are tectonically thrusted onto the Eocenic flysch of the Pindos zone. Both complexes are considered as remnants of a certain ocean, which today are found tectonically thrusted over the west margin of the Pelagonic micro-plate. Beyond the fact that both complexes show characteristics of intense tectonic deformation, they do preserve all the types of the petrologic sequences that compose a complete ophiolite complex. All the formations that have been studied in the present thesis are coming from the mantle units of both complexes. The main part of the Vourinos ophiolite complex is composed of mantle tectonites, while the mantle formations in the Pindos ophiolite complex compose the so-called Dramala unit. In the Vourinos complex the sampling covered the most important ore hosting areas of its northern and southern part, while in the Pindos complex several samples were taken from the central-southern parts of the Dramala unit and from exotic blocks of mantle origin of the Avdella mélange unit. The field investigation in both complexes has led to the following conlusions: a) The northern part of the Vourinos ophiolite complex shows clearly more characteristics of plastic deformation compared to its southern part in which characteristics of brittle to semi-plastic deformation prevail. A similar, but not so well-expressed discrimination can be made as far as concerns the structure of the areas of Milia and Pefki in the Pindos complex. b) The statistical analysis of several motional indicators shows a NE thrust direction for the northern sector of the Vourinos complex and the area of Milia and a SE direction of motion for southern Vourinos and Pefki. c) The chromitites are hosted in dunites and in the majority of the chromitite occurrences the accompanying rock-type is thin-grained dunite (serpentinised or not). In contrast to coarse-grained dunite the thin-grained variety of dunite is not foliated. d) The chromitites of northern Vourinos and Milia are usually of podiform type, discordantly to sub-concordantly developed to their neighbouring thick-grained dunitic to harzburgitic peridotites. The chromite ores of southern Vourinos and Pefki are stratiform, having a concordant to sub-concordant structure. e) Chromitites of both complexes show a great variety of chromite textures, although in each deposit a certain type of texture predominates. In northern Vourinos and Milia the dominant textural type is massive and leopard chromitite, while in southern Vourinos and Pefki the most common texture in chromitite is schlieren. f) In southern Vourinos and Pefki are commonly observed interlayered bodies of harzburgite and dunite, which are indicative of a stratified structure of the residual mantle superimposed on these areas. The number of the pyroxenite dykes (rodingitised or not) is higher in these areas compared to those in northern Vourinos and Milia. In their vast majority these dykes cut chromitite bodies, which indicates that they are not genetically related to the ores, except for one case of a chromitite that is hosted in a pyroxenite dyke in the northern part of the area of Pefki. g) Always in the vicinity of the chromitites the lithology follows the sequence: ±Clinopyroxene-bearing harzburgite, normal harzburgite, transitional harzburgite, coarse-grained dunite, thin-grained dunite, chromitite. A different lithological transition is quite unusual. h) The common entrapment of harzburgite blocks in dunites indicates that the latter comes from the former, although replacements of foliated harzburgites from thin-grained dunite that show no mark of foliation supports the fact that some rocks may not be simply of residual origin. I) The presence of clinopyroxene-bearing harzburgites is more common in the Pindos ophiolite complex, while in the Vourinos ophiolite complex bodies of thin-grained dunite in the form of dykes, layers or lenses are more commonly observed. Finally, taking into account all the available geological, mineral chemistry and petrological data, it can be said that the mantle units of both complexes have been initially formed in a mid-ocean ridge and then have been furtherly affected by the processes that take place in the mantle wedge above a SSZ. Thus the transition from Vourinos to Milia and Pefki and then to Korydallos is obviously equivalent to the "passage" from a fore arc basin to the arc and finally to a small back arc basin.

La minéralisation platinifère étudiée dans ce travail se situe dans le grand massif ultrabasique du sud de la Nouvelle-Calédonie, à l'embouchure de la Rivière des Pirogues. La transformation exogène qui affecte les roches émergées de la ceinture intertropicale, a édifié aux dépends des péridotites de Nouvelle-Calédonie, d'épais manteaux latéritiques enrichis en métaux. L'objectif de cette étude a été de caractériser les évolutions chimiques et minéralogiques qui affectent les phases porteuses d'Eléments du Groupe du Platine (EGP) dans cet environnement latéritique. L'étude pétrographique et pétrologique de trois profils d'altération dans la zone anomale en platine-palladium, a permis de mettre en évidence le rôle majeur de la serpentinisation hydrothermale dans l'évolution de la minéralisation platinifère. Cette altération pré-météorique a néoformé et formé des Minéraux du Groupe du Platine (MGP) par désulfuration de phases préexistantes et par précipitation de nouveaux minéraux. Au contact de l'atmosphère et de l'hydrosphère, les MGP évoluent différemment selon leur composition chimique. Les phases très riches en platine de type isoferroplatine (Pt3Fe) sont stables dans le manteau d'altération, alors que les phases plus riche en fer, cuivre et palladium, comme le tétraferroplatine (PtFe), la tulameenite (Pt2FeCu) ou les oxydes de Pt-Fe subissent une dissolution incongruente, avec une lixiviation sélective de leurs composants chimiques. La caractérisation géochimique à la microsonde électronique des cortex d'altération, révèle le gradient de mobilité suivant lors de la transformation latéritique des MGP : S > Cu > Pd > Fe > Pt. Quel que soit le minéral parental, la phase néoformée est relativement enrichie en Pt, et tend à acquérir la configuration chimique de l'isoferroplatine (Pt3Fe).