| dc.description.abstract | The production of high carbon ferromanganese (HCFeMn) alloys is an energy intensive process where manganese ores are smelted in a submerged arc furnace (SAF) using carbon reductants thereby generating CO2 emissions. In the prereduction zone of the SAF, higher manganese oxides in the ore are reduced to MnO through solid-gas exothermic reactions and at a temperature around 800 ℃, the unwanted endothermic Boudouard reaction is also active. As such, the total coke and energy consumption is highly dependent on if the prereduction occurs by CO gas or solid C. Improvement of existing SAF ferromanganese process in resource and energy efficiency as well as reduction of CO2 emission through ore pretreatment in a separate unit is being explored in this work. A successful pretreatment limits the extent of Boudouard reaction thereby reducing the carbon and energy footprint of the process. Pretreated raw materials are expected to give a more stable and predictable furnace operation and in addition, contribute to lowering CO2 emissions. However, the effect of utilizing pretreated ores on the SAF process is not known and this work seeks to increase and contribute to the knowledge about furnace performance from use of pretreated ores in ferromanganese production.
Experimental investigations were carried out at both laboratory and pilot scale. Firstly, laboratory scale experiments were carried out to investigate the prereduction behavior of manganese ores and to gain knowledge on the effect of extent of prereduction on the high temperature metal producing reaction i.e., MnO+C=Mn+CO. Three commercial ores used in industry, namely Comilog, Nchwaning and UMK were studied. Secondly, pilot-scale experiments have been conducted at SINTEF/NTNU in a 440 kVA AC electric furnace using different feed mixtures of untreated manganese ore, manganese ore preheated in air and manganese ore prereduced with solid carbon. Comilog and Nchwaning ores in both untreated and pretreated forms were used in the experiments. In addition, untreated UMK ore and a blend of untreated UMK and Kudumane referred to as Mintek mix, were used in separate pilot experiments. In total, 8 pilot experiments were carried out. Post experimental investigations were carried out on the pilot experiments, through digouts of the prereduction zone and excavations of the cokebed zone. Focus was on establishing the prereduction behavior of the various ores and establishing the effect of using pretreated ores on the metal producing reactions. Lastly, accounting material and energy balance calculations for the 8 pilot experiments were then calculated in HSC Chemistry software. Through mass and energy balances, carbon and energy consumption were established based on the offgas composition and the degree of degree of prereduction was estimated.
TGA data showed that the extent of ore prereduction will not have any effect on the metal producing reaction i.e., MnO + C = Mn + CO. Any remaining prereduction will be completed in the low temperature range 1200 – 1400 ℃ prior to the metal producing reaction. This was in agreement with findings from pilot experiments, were pretreatment of manganese ores was seen to have no effect on the metal producing reaction. Phase development in untreated and pretreated charge mixtures of Comilog, Nchwaning, UMK and Kudumane ores, which is dependent on temperature and composition of the ore, showed that the initial slag formed is mainly liquid + solid monoxide phase and slag reduction will occur on top of the coke bed with an increased dissolution of monoxide in slag rendering the slag fluid enough to trickle down the coke bed. The coexistence of a solid phase and a liquid slag phase explains the high MnO activity in the slag as well as affect the viscosity of the slag. Therefore, low reduction extent will occur inside the coke bed area due to the lower MnO activity which emanates from complete dissolution of solid monoxide phase. Hence, the final slag for example in untreated, pretreated and prereduced Comilog ores, will be tapped at compositions which are in the same region in the stable period of the furnace operation.
In laboratory experiments, the prereduction of ores mixed with solid carbon i.e., coke in the induction furnace improved the extent of prereduction. The decomposition of carbonates leads to better prereduction due to the CO produced in the Boudouard reaction. Hence, UMK ore was found to have a better prereduction compared to Nchwaning which is low in carbonates and Comilog which is known to have a higher CO reactivity at similar CO contents. However, in TGA experiments were the CO/CO2 gas flow is high and stable, Comilog is seen to have a higher extent of prereduction followed by UMK and lastly Nchwaning. Contrastingly, in pilot furnace digouts, UMK ore was seen to have a lowest O/Mn ratio with increasing furnace depth, followed by Comilog and lastly Nchwaning. Pretreatment of Comilog ore was not seen to affect the degree of prereduction, which correlates well with laboratory scale results and energy calculations. Pretreatment of Nchwaning ore was seen to increase the extent of prereduction compared to untreated Nchwaning ore, when looking at the extent of prereduction adjacent the electrode. On the other hand, no differences were observed for Nchwaning ore adjacent the furnace lining. As such, we see an increased extent of prereduction for preheated Nchwaning in digout of pilot experiments, but we do not see it based on the offgas composition and mass and energy calculations. It is believed that furnace digouts are subject to uncertainties. The furnace is shutdown 40kWh after the last tapping and the chemical reactions in the cokebed area are expected to stop within minutes of shutdown since metal producing reactions in the cokebed are highly endothermic. However, it is possible that exothermic reduction in the prereduction zone will continue as the furnace cools.
Based on furnace offgas composition, mass, and energy calculations, preheated and prereduced Comilog experiments show more stable operation compared to untreated Comilog experiment. As such pretreatment in Comilog series gives a stable operation. However, for Nchwaning ore, ore preheating does not show stable operation. For a fixed charge the lower CO2 to (CO + CO2) ratio in the offgas indicates higher CO2 consumed in the Boudouard reaction and consequently higher carbon and energy consumption. Offgas measurements in the Comilog series show that the use of untreated Comilog ore leads to considerably lower CO/CO2 off-gas composition compared to pretreated Comilog ores, largely related to the high oxygen level of Comilog ore. For untreated and preheated Nchwaning there was no significant differences in the offgas composition. The degree of prereduction is generally high for Comilog series, followed by UMK and Mintek mix and lastly Nchwaning experiments. As such Comilog will have the lowest carbon and energy consumption. The degree of prereduction in UMK was significantly higher compared to Nchwaning series experiments, hence the total carbon consumption was lower for UMK. Carbonates will have a double effect consuming both energy on decomposition and energy by Boudouard reaction and as such Mintek mix had the highest energy and carbon consumption.
Due to the higher amount of exothermic reactions, the untreated Comilog ore will give lower energy consumption compared to pretreated ore if added cold in the SAF. However, charging hot pretreated ore will result in lowering of the energy consumption in SAF by 300 kWh/ton alloy. Prereduced Comilog did not have any significant difference from preheated Comilog with regards to energy and carbon consumption owing to insignificant change in oxygen level when Comilog is prereduced with solid carbon. The degree of prereduction in the Comilog series is in the same area within the uncertainties of the experiment. This gives the same carbon consumption and hence the overall CO2 emissions. | en_US |
