Prereduction of Comilog- and Nchwaning ore
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Manganese ferroalloys are mainly used as an alloying element in steel production and are normally produced in submerged arc furnaces. Raw materials such as manganese ore, metallurgical coke and fluxes (and quartz for SiMn) are premixed in proper ratios and fed to the charge top. As the manganese ore descends in the furnace, it will experience increasing temperatures and an ascending furnace gas, which is largely composed of CO(g) and CO2(g). In addition, the gas contains smaller amounts of water vapor and/or hydrogen, which may be introduced through the moisture in the raw materials. These gas components are thus mainly present in the upper parts of the industrial furnace. While in solid state, higher manganese and iron oxides in the ore will be reduced to MnO and Fe. The gas-solid reaction between the ore and the furnace gas is largely decisive of the energy consumption and the characteristics of the off-gas, including amount, composition and temperature. This is due to two main factors. Firstly, manganese ores have varying oxygen levels, where a high oxidation level correlates to a larger extent of the exothermic reduction of higher manganese oxides to MnO. This further implies that high oxygen ores will have a larger influence on the CO/CO2 ratio in the gas phase. Secondly, the reaction between the ore and the furnace gas is governed by kinetics, which is affected by the characteristics of the given ore. The ore-gas kinetics is important as it decides the extent of reduction that occurs in the active region of the Boudouard reaction, which is said to be at temperatures exceeding 800°C in an industrial ferromanganese furnace. The Boudouard reaction is highly endothermic, carbon consuming, and gas producing. It further leads to an increased energy in the off-gas. As such, a higher furnace efficiency is obtained when the Boudouard reaction is minimized, i.e. the prereduction of the manganese ore is completed at temperatures below 800°C. The prereduction behavior of manganese ores has been studied previously, however there is still a lack of knowledge on the topic. This is partly credited to the high number of manganese ores, as the prereduction behavior is dependent on the ore characteristics. Furthermore, the focus of previous investigations have largely been the final step of reduction, i.e. Mn3O4 to MnO, where the ores were precalcined and investigated at high temperatures. As such, the information on how the ores behave at low temperatures as they are fed to the furnace is scarce. An increased understanding of this part of the furnace could potentially reveal measures leading to increased energy efficiency and lower CO2 emissions. This work focused on the prereduction behavior of two commercial manganese ores, i.e. Comilog and Nchwaning ore. Comilog is a high oxygen ore, where the majority of the ore is composed of MnO2-minerals. Nchwaning is a semi-oxidized ore, containing various Mn2O3-oxides, hematite and calcite. Furthermore, Comilog and Nchwaning differ in physical properties, such as porosity. Due to these differences, the ores show different reduction behaviors, and will have different effects on energy efficiency and gas characteristics. The goal was to elucidate the reaction behavior and quantify the effect of parameters that may vary in an industrial furnace, e.g. particle size, gas composition and temperature. The results were further used to estimate the effect on the off-gas characteristics through simulated mass and energy balances. When heated in reducing atmosphere, the tetravalent oxides in Comilog reduce to manganosite(MnO) in an overall single step. When the temperature reached approximately 580-600°C, any present MnO2 was rapidly decomposed to Mn2O3, which further reduced to MnO. The reduction rate in the initial stages, i.e. prior to rapid reduction step, was found to be proportional to the inverse particle size (1/rp) and the partial pressure of CO to the power of 0.7. The reaction rate of the rapid decomposition step was merely dependent on the amount of MnO2 that was present when threshold temperature was reached. The reduction was observed to proceed to similar reaction steps in both isothermal(400-600°C) and non-isothermal heating(25-1000°C). It is suggested that a more dense structure is formed when the ore is subjected to isothermal heating, as a topochemical reaction front was formed. This was not observed in non-isothermal experiments. This may either be due to the rapid initial temperature increase, or due to the low reaction temperatures. The manganese and iron oxides in Nchwaning ore were found to reduce at highly similar temperature ranges, however the manganese oxide reduction was initiated prior to the iron oxides in small particle sizes(< 4 mm). Trivalent oxides reduced to MnO in a single step, and the reduction of hematite subsided with the formation of wüstite(FeO). It was found that the reduction rate was proportional to the inverse particle size (1/rp) and the partial pressure of CO to the power of 1.5. Carbonates were found to decompose at temperatures 800-1000°C regardless of particle size and CO-concentration. The reduction behavior of Nchwaning ore was less reproducible compared to Comilog ore, due to a more heterogeneous nature. It was found that representative investigations may not be performed using small sample sizes. Nonetheless, chemical analysis showed that the ore reduced through similar reaction steps when reduced isothermally(600-900°C) and non-isothermally(25-1000°C). Nchwaning ore particles did not appear to follow a shrinking core behavior, as a reaction front was not observed at any evaluated conditions. Comilog ore is reduced at a lower temperature range compared to Nchwaning ore. Furthermore, the temperatures of Comilog ore is largely affected by the heat production accompanying the reduction, whereas Nchwaning ore largely follows the furnace temperature. This is both due to the higher oxidation level of Comilog ore, and the faster reduction rate of MnO2 to MnO in Comilog compared to Mn2O3 to MnO in Nchwaning. A simple model was constructed for the reduction of Nchwaning and Comilog ore, which was able to describe the reduction extent with reasonable agreement. The model included the quantified dependency on the particle size and partial pressure of CO. Further, the reduction rate was described according to a first order reaction. Activation energies were determined to be 17 kJ/mol for Comilog ore and 63 kJ/mol for Nchwaning ore. Due to the different reduction behavior, it was found that the use of Nchwaning ore generally leads to an increased energy and carbon consumption, as a larger extent of the reduction occurred in the active region of the Boudouard reaction. Furthermore, this leads to a higher content of CO(g) in the off-gas. A certain amount of moisture is present in the raw materials, which gives rise to water vapor and potentially hydrogen, where the latter depends on the kinetics of the water-gas shift reaction. Comilog- and Nchwaning- ore responded differently to the presence of hydrogen and water vapor in the gas mixture. For Comilog ore, it was found that hydrogen promoted the reduction rate, both when added as H2(g) and H2O(g) in a CO-CO2 mixture for a fixed oxygen pressure, where the effect was larger for the former. It was found that the water-gas-shift reaction proceeded to some extent, but was not at equilibrium at temperatures between 25-750°C. The reduction of Nchwaning ore appeared to be insignificantly affected by the presence of hydrogen. As such, it is believed that the water-gas shift reaction was close to or at equilibrium. Surface moisture will evaporate at 100°C, and it was further found that the chemically bound moisture in Comilog was expelled at temperatures 200-400°C. Hence, the presence of water vapor and/or hydrogen will have a low effect on the reaction rates in an industrial furnace. For a given charge mixture containing 10% of moisture, the energy in the off-gas will increase with increasing extent of the water-gas shift reaction. Compared to no reaction between water vapor and CO(g), the energy in the off-gas is increased 1.5% relative to the total energy consumption of the furnace if the water-gas shift reaction is at equilibrium. It was found that both ores are subjected to disintegration during heating in reducing atmosphere, where Nchwaning showed a lower extent compared to Comilog. A close correlation between the reduction extent and decrepitation was observed. The decrepitation of Comilog ore was not related to the thermal stresses resulting from the rapid decomposition step. It was found that a decreasing heating rate correlated to an increased disintegration. It is suggested that when the reduction proceeds at lower temperatures, a more dense structure is formed, which in turn makes the ore more susceptible towards decrepitation.