3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item A fundamental investigation of the hydrothermal dissolution and oxidation of manganese metal.(2001) Glück, ThomasThe oxides of manganese display a number of allotropic structures, a number of which have significant industrial importance. In particular, mangano-manganic oxide (Mn30 4), is used as a raw material for the manufacture of soft ferrites. The preparation of Mn30 4 by hydrothermal dissolution and oxidation of manganese metal at elevated temperature and pressure yields a raw material that has unique chemical, morphological and structural characteristics. In this work experimental and theoretical investigations were conducted to determine the mechanisms of the dissolution and oxidation reactions as well as the influence that processing conditions have on the morphological and structural properties of the oxidation products. Simplified Pourbaix diagrams were generated to map the stability domains of relevant manganese oxides for hydrothermal processing conditions. These show that the domain of stability of manganous hydroxide, Mn(OHh(aq) may be larger than currently reported in literature. Hydrothermal dissolution experiments were conducted to investigate the influence of hydrogen partial pressure on reaction kinetics. An experimental and mathematical methodology was developed to facilitate the analysis of the non-isothermal dissolution kinetics with polydisperse shrinking particles of Mn metal. The rate of the dissolution reaction was found to be controlled by a heterogeneous chemical reaction occurring on the surface of the manganese metal particles but was not influenced by hydrogen partial pressure. The rate of oxidation of solid Mn(OHh formed after the dissolution of manganese metal by oxygen in a mechanically agitated, gas-sparged reactor was found to follow linear kinetics. Dissolution and reprecipitation processes were found to occur during the oxidation of Mn(OHh particles that results in the formation of a daughter population of particles. The hydrothermal oxidation of Mn(OHh occurs via a homogeneous oxidation mechanism. Mass transfer of oxygen from the gas phase to the liquid was found to be the rate-controlling step of the oxidation process. The changes in morphology and crystal structure of the hydroxide/oxide intermediates and products during hydrothermal oxidation were investigated using a number of different characterisation techniques. Mn30 4 made under hydrothermal conditions can be oxidised by heating in air in contrast to naturally occurring minerals or synthetic Mn304 made via high temperature processes. In addition, this material shows larger deviations from stoichiometry than materials prepared by high temperature synthesis. The crystal defect structures of oxidation intermediates and products were analysed using Rietveld structural refinement techniques and compared to a number of theoretical structural models. The reactivity of hydrothermal Mn30 4 is attributed to the cationic vacancies in tetrahedral and octahedral lattice sites.Item Hydrocarbon reduction of manganese ores(2018) Bhalla, AmitReduction behavior of South African Mamatwan manganese ore using methane-argon- hydrogen gas mixture was investigated experimentally in the temperature range of 1050ºC to 1250ºC. The effect of changing gas mixture composition, time and temperature was studied using a vertical tube furnace. After each test, three representative samples were prepared; one was analyzed by chemical analysis to obtain metallization results as a function of each reducing condition for each time interval over the total reduction period of two hours. Second sample was analyzed by X-ray diffraction to determine the progress of phase changes; the third sample was mounted, polished and submitted for SEM-EDAX in order to examine the morphology of the ore and its changes in the course of reduction. It was seen that CH4 was an effective reductant as it cracked, supplying the reaction site with hydrogen gas and very fine solid carbon. The excess carbon from cracking of methane ensures regeneration of reductants CO and H2 from reaction product gases of CO2 and H2O ensuring low partial pressure of oxygen at the reaction site. Hydrogen gas may also be involved in the reduction of iron oxide components of the ore. Moreover, depending upon temperature and CH4/H2 ratio in the gas phase the activity of carbon in the system reaches values much higher than unity, shifting the reduction reaction by carbon to lower temperatures. It was observed that bulk of the metallization occurred in the first thirty to forty minutes and the metallization reached some kind of a reduction maximum at 73% metallization. The Mn/Fe ratios in the resulting alloy were higher than those in ordinary carbothermic solid-state reduction, indicating the simultaneous reduction of Fe and Mn at these low reducing temperatures due to a low oxygen potential set up by the methane bearing gas mixtures. It was seen that metallization of Mamatwan ore proceed in two stages. First, reduction of the higher oxides to MnO and metallic iron. Second, reduction of any remaining oxides and MnO to mixed carbide of iron and manganese. During first stage values of effective CO-CO2 diffusivities generated by the model were found to lie in the range from 1.45 *10-6 cm2sec-1 to 8.43*10-6 cm2sec-1 at 1100ºC. Apparent activation energy for first stage calculated in the temperature range of 1050ºC to 1250ºC varied from 1.47 kJ/mol to 24.72 kJ/mol indicating possibility of diffusional control. For the second stage the experimental curves could be duplicated with the mathematical model reasonably well with a maximum difference between the experimental and predicted values being about 5 percent. Rate of metallization values during the second stage (Ms) changed between 1.83*10-8 mol.sec-1.cm-2 and 8.55*10-8 mol.sec-1.cm-2. Specific rate constant values (ks) for the second stage, varied from 5.53*10-6 cm/sec to 3.16*10-5 cm/sec which are much smaller than specific rate constant for the first stage of reduction (kf), which varied from 1.64*10-4 cm/sec to 1.15*10-4 cm/sec, as the rate of second stage of the reduction is much slower than the rate of the first stage. X ray analysis revealed that manganese ore was reduced primarily to carbide Mn7C3 at lower temperature range of the experiments, but at 1200ºC the dominant reaction product was Mn5C2 in both mixtures of methane-argon and methane-hydrogen. The S.E.M images revealed that the product metallic phase occurred all throughout the surface, with globular formation in case of reduction where hydrogen was the carrier gas.Item Thermodynamic activity of MnO in manganese slags and slag-metal equilibria(2015-02-09) Cengizler, Hakan