Prereduction of Comilog- and Nchwaning ore

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