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Call number: LD2668 .T4 1986 C443 / Master of Science / Chemistry

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, 1994 / A quantitative description of galvanic interactions between sulphide minerals based on thermodynamic and kinetic parameters has been developed. The basis for quantitative description involves conducting a voltage balance over the galvanic couple. The contributions to the voltage balance include the galvanic couple cell emf, kinetic descriptions of the anodic and cathodic half reactions, the voltage characteristics 'of mineral-mineral contacts and solution voltage losses. The rates of the anodic and cathodic half' reactions were modelled by the Butler-Volmer equation and ti1ediffusion equation. A potentiostat was used to vary the voltages losses across mineral-mineral contacts. TIle galvanic couples were constructed. as rotating ring disc electrodes and hence electrolyte voltage losses were negligible. Three galvanic couples, copper-platinum, copper-pyrite and galena-pyrite, were electrochemically characterised under different conditions of ferric concentration, electrode rotation rate and temperature. The effect of illumination on the anodic dissolution of galena was investigated. The electrochemical model is in good agreement with experimentally measured galvanic currents. Galvanic interaction is a dynamic function and various models are developed which account for dynamic behaviour in galvanic cells. / MT2017

Digitized by Kansas Correctional Industries

In this study, an electrolytic micro-machining tester is employed to investigate the effects of the supply voltage, the immerse depth of tungsten rod, and the machining time on the current waveform, the material removal rate, and the geometry of the tungsten needle. The tungsten rod to be electrolyzed is dipped in an aqueous electrolyte of 10 wt% sodium hydroxide as the anode, and the stainless steel ring as the cathode. The spindle rotating speed and the stirring rotating speed are set to be 100 rpm and 200rpm, respectively. According to analyze the topography of the tungsten needle, four machined regimes have been identified as:¡]1¡^non-machined regime,¡]2¡^incomplete machined regime,¡]3¡^complete machined regime,¡]4¡^over machined regime. In order to obtain the perfect tungsten needle, the experiments are conducted in the complete machined regime. Results show that the tungsten rod becomes a short cone for the immerse depth of 5 mm, and a long cone for the depth of 10mm. When the immerse depth of 10 mm and the supply voltage of 3V, the surface of tungsten needle becomes rough slightly and the tip radius of tungsten needle is about 2£gm. With increasing the supply voltage to 4.5 V, the surface of tungsten needle is uniform with a downward trend in material removal rate, and the tip radius can achieve a submicron. For the supply voltage of 6V, because the material removal rate varies violently, it becomes very difficult to control the diameter of tungsten needle. During the machining time between 0 to 10 min for the supply voltage of 4.5V, the diameter of tungsten rod is decreased from 1000 to 200£gm, but during the machining time between 10 to 12.5 min, the tungsten rod gradually transforms into the needle due to a downward trend in current, and the tip radius is decreased from 200£gm to submicron. Hence, the machining time must be controlled accurately to manufacture the needle in a submicron radius.

Process variables of electrolytic technology to reduce the energy consumption of harvesting Nonnocloropsis salina were investigated including electro-coagulation, electro-floatation, and electro-flocculation. Electro-coagulation and electro-flocculation showed significant cost savings, however electro-floatation did not. The objectives were to determine the effects of electrode material, pH adjustment and electro-polymer addition for electro-coagulation and determine the performance characteristics for electro-coagulation and electro-flocculation. Both treatments proved to be competitive with the energy consumption of a centrifuge. The best electrolytic treatments were electro-coagulation with aluminum and nickel electrodes. Energy requirements at optimum conditions were 239 and 344 kWh/ton. The best treatment combination using electro-flocculation was 432 kWh/ton with no electrode consumption, which could lead to potential cost savings.

This investigation applied the techniques of polarography and controlled potential electrolytic reduction to the study of a series of unsymmetrical benzils. Eleven unsymmetrical benzils were synthesized and studied in 50% ethanol-water (by volume) solvent systems buffered at pH 1, 5, and 13. The results showed that, with unsymmetrical benzils substituted with electron-withdrawing or electrondonating groups, the carbonyl closest to the ring with the least electron-donating power was the preferred reaction site. However, the dicarbonyl system was nevertheless reduced as a unit, and Hammett sigma values of disubstituted symmetrical benzils were approximately additive. This was ascertained by comparing the ease of reduction of eight unsymmetrical benzils to the ease of reduction of the corresponding disubstituted symmetrical benzils. It was proposed that the electrons were added to the oxygen of one carbonyl, and that the second carbonyl was involved in the reduction chiefly through complete polarization of the dicarbonyl system at the electrode. In general, the reduction of unsymmetrical benzils appeared to follow the same path previously reported for symmetrical benzils.