No description available.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The crystallization kinetics of hydronium jarosite have been studied in the area of the Fe₂O₃-SO₃-H₂O phase diagram where hematite is the stable phase. Hydronium jarosite has been shown to be a kinetically favored intermediate to hematite over a wide range of chemical and thermal conditions. A model useful for predicting the crystal size distribution as a function of temperature, free acid and iron concentrations, and residence time has been developed. Hydronium, sodium, and potassium jarosite have been shown to convert, via a solid phase reaction, to hematite. A model useful for predicting the conversion rate as a function of temperature, free acid concentration, and particle size has been developed. A predictive model, the growing core model, has been developed. It is useful for predicting the crystal size distribution and the product split between hydronium jarosite and hematite when both crystallization and conversion are occurring simultaneously. The cardinal assumption in this model is that crystal growth and conversion occur at separate cites on the crystal surface simultaneously. The model, with only one adjustable constant, has been verified with experimental results. The effect of double draw off (DDO) operation in this system has been demonstrated. It has been shown, via the growing core model and experimental results, that DDO operation can produce a high iron, low sulfur product. Without DDO operation, this high product quality can only be achieved through higher operating temperature, high neutralization rates, or very large vessel size.

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Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references. Also available in print.

Thesis (Ph. D.)--University of Hong Kong, 1998. / Includes bibliographical references (leaves 26-31).

The study of pulsed n. m. r. in single crystal metallic samples, initiated by McLachlan, has been extended to liquid helium temperatures with special emphasis on Sn¹¹⁹ . Contrary to McLachlan's belief it was found that cooling to 4. 2°K and lower afforded significant improvements to the signal to noise ratio and in many cases the n. m. r. signals (including spin echoes in Sn¹¹⁹ ) could readily be seen on an oscilloscope without the use of a signal averager. The theory of magnetic resonance in metallic samples was studied in some detail with particular emphasis on the experimental situation where matters are complicated by the high conductivity which modulates both the amplitude and the phase of the exciting r.f. magnetic field as it penetrates into the sample. It is shown theoretically that several assumptions must be made to show that the conventional methods of pulsed n. m. r. used to measure T₂ (by either spin echo or free induction decay) and T₁ yield true meaningful results. In particular it is found that the spin lattice relaxation time T₁ is obtained by the conventional two pulse sequence only when the magnetic field is exactly on resonance; this was observed to be the case experimentally as well. In sharp distinction to McLachlan's findings for Sn¹¹⁹, the spin-spin relaxation time T₂ obtained by FID methods (175 ± 18μsec) was much smaller than that obtained by spin echo techniques (390 ± 48μsec) in the present research on Sn¹¹⁹. The spin-lattice relaxation time was also measured at liquid helium temperatures and yielded a value of 56 ± 4 millisec deg for T₁, in excellent agreement with Dickson although twice as large as McLachlan’s value. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

No description available.