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The aluminum electrolytic capacitor is used extensively in the electric utility industry. A factor limiting the storage of spare capacitors is the integrity of the aluminum oxide dielectric, which over time breaks down contributing to a shelf life currently estimated at one nuclear power electric generating station to be approximately five years. This project examined the electrical characteristics of naturally aged capacitors of several different styles to determine if design parameters were still within limits. Additionally, the effectiveness of a technique known as “Reforming” was examined to determine its impact on those characteristics.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Roger Dean Farr. / M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Includes bibliographical references. / by Michael Stoukides. / Ph.D.

Silica is detrimental to the recovery of zinc from zinc bearing siliceous materials. The difficulties involved in the hydrometallurgical processing of silica are well documented, the major problem being the formation of gels. These are extremely difficult to separate from solution, creating serious problems. This work investigates the mechanism of the formation of granular silica precipitates and how this differs from the mechanism of formation of the gels.Batch precipitation studies of silica from sulfuric acid solutions have revealed valuable information about the processes occurring. A solution of monosilicic acid (Si(OH)4(subscript)) was produced by leaching synthetic zinc orthosilicate (Zn2SiO4) in sulfuric acid. Precipitation was monitored by measuring the turbidity and silica concentration in solution at regular intervals. Electron microscopy was used to examine the precipitate morphology. Variables known to influence silica precipitation are supersaturation, temperature, seeding and the concentrations of sulfuric and hydrofluoric acids and zinc, iron (III) and aluminium sulfates. These were investigated by comparing their effects on precipitation with those of a control experiment. The evaluation criteria used were induction time, precipitation rate, precipitation time, product morphology and filterability. The results of this work define conditions that yield a granular silica precipitate in preference to a gel, low supersaturation being the most important condition. The product morphology is insensitive to precipitation rate over the range of conditions studied.A laboratory scale continuous crystalliser was designed and constructed and used to carry out a series of experiments to determine the operating conditions for suitable processing of natural zinc orthosilicate ore to remove the silica impurity in an easily separable form. The variables examined were ++ / supersaturation as well as mixing and product removal conditions. The relationship between supersaturation and morphology discovered during the batch experiments was confirmed to also occur when employing a continuous basis. This finding has not been previously reported in the literature. The use of classified stirring and classified product removal led to the formation of the highest quality silica product with a reasonable residence time. The most important quality criterion being the filtration rate.Insight into the mechanism of silica precipitation has been gained by analysing information from both batch and continuous experiments. Under the strongly acidic conditions studied, colloidal silica particles are initially formed. At low supersaturation (S </= 2.9) the colloidal particles aggregate and cement together to form solid particles, while at higher supersaturation (S>/= 4.3) the colloidal particles aggregate to form a gel.

Thesis (Ph. D.)--OGI School of Science & Engineering at OHSU, 2004. / Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (leaves 183-191).

Thesis (M. Phil.)--University of Hong Kong, 1984.

This research investigated the mechanism, kinetics, and feasibility of chlorinated aliphatic compounds inactivation by electrochemical reduction using nickel and iron electrodes. Reactions of trichloroethylene (TCE) and tetrachloroethylene (PCE) with zerovalent iron were investigated to determine the role of atomic hydrogen in their reductive dechlorination using Tafel analysis and electrochemical impedance spectroscopy (EIS). Comparison of iron corrosion rates with those for TCE reaction showed that TCE reduction occurred almost exclusively via atomic hydrogen at low pH values and via atomic hydrogen and direct electron transfer at neutral pH values. In contrast, reduction of PCE occurred primarily via direct electron transfer at both low and neutral pH values. The EIS data showed that all the rate limitations for TCE and PCE dechlorination occurred during the transfer of the first two electrons. Carbon tetrachloride (CT) reductive dechlorination was studied at a nickel rotating disk electrode using chronoampermetry (CA) and EIS. Only trace levels of methylene chloride and chloromethane were produced, indicating that sequential hydrogenolysis was not the predominant pathway for methane production. EIS showed that the ratelimiting step for CT reduction was the transfer of the first electron to a physically adsorbed CT molecule. The feasibility of an electrochemical reductive dechlorination method for removing CT from potable water was carried out in a flow-through reactor using bare and polymer coated porous nickel electrodes. Destruction of half-life values for CT with the bare nickel electrode ranged from 3.5 to 5.8 minutes for electrode potentials ranging from -652 to -852 mV with respect to the standard hydrogen electrode (SHE). Faradic current efficiencies could be increased by 100 to 360% by coating the electrode with a silicone polymer. This research also investigated electrochemical oxidation of triclosan, a biocidal agent, using Ebonex® and boron-doped diamond (BDD) film anodes. Product analysis showed that breaking the ether linkage was easier than opening the aromatic ring. Microtox® testing indicated that residual triclosan accounted for nearly all the toxicity in the treated water, despite the fact that chlorinated byproduct concentrations were significantly higher than those of triclosan itself.

published_or_final_version / Chemistry / Master / Master of Philosophy