3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
2 results
Search Results
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 The reducibilty of chromite ore and reactivity of carbonaceous reductants.(2013-02-11) Bhalla, AmitThe purpose of this dissertation is to study the reducibility of chromite ore and reactivity of carbonaceous reductants. The effect of temperature, particle size, composition, reducing agent and reducing atmosphere on the kinetics of the reduction of given chromite (obtained from Xstrata) was studied.using tga,and to test reactivity of the reductant, present experimental data was applied to Arrhenius model. The ore is reduced by reductants namely coke, coal, charcoal and graphite at temperatures between 1000 ºC and 1300 ºC under argon atmosphere. Particle size range of as received, (+50μm-100μm), (+100μm-150 μm), (+150μm–200 μm) were used. For each experiment a calculated mixture of chromite and reductant was mixed with acetone and the mixture was reduced using a TGA furnace.The results indicated that the reduction rate was a function of temperature and particle size.The reduction at 1000 ºC under argon atmosphere is minimal.As temperature is increased to 1100 ºC, 1200 ºC and 1300 ºC it was observed that reduction rate of this chromite increased and sample having finer particle size fraction showed higher reduction rate at all temperature. The effect of the type of reducing agent namely coal, coke, charcoal and graphite was variable.At lower temperatures: 1000ºC and 1100ºC coke, coal, charcoal and graphite was order of reduction from highest reduction to lowest whereas at higher temperatures: 1200 ºC and 1300ºC order was coal, coke, charcoal and graphite.The phases formed at the end of each reduction stage were studied using SEM and optical microscope which helped to confirm the experimental data. To test reactivity of the reductant, present experimental data was applied to various models.A model which was found to be suitable was the Arrhenius model.The value of the activation energies obtained from fitting the data into the Arrhenius equation was used to determine the relative reactivity of the reductants, the values of the chemical reaction rate constants and effective diffusion constants were used to determine the relative speed at which the reductants can reduce the chromite ore. The TGA test suggest that coke has the highest reactivity since its activation energies are lowest and require less energy to induce the reductants reaction The tests also suggest that the rate controlling mechanism is diffusion of species to the reactive site since the effective diffusion coefficients were of the order 10-11, which were far less then the chemical rate constant of the order 10-6 to 10-5.Test also suggest that coal is fastest to react since its De values are high