A flexible electric furnace operation has widely been used in production of manganese ferroalloys such as high carbon ferromanganese (HC FeMn) and silicomanganese (SiMn), according to requirement. There are two operational strategies referred to as the discard slag practice (or basic slag operation) and the high MnO practice (or fluxless operation). In the latter, the MnO-rich slag is reprocessed for production of SiMn in duplex processing. In this way, the MnO content of the discard slag can be reduced to 5~10% MnO. There has been a shift in demand in many countries away from the use of HC FeMn towards SiMn and refined alloys of both.
Low carbon SiMn with 20~30% Si is produced by upgrading standard SiMn alloy by the addition of Si wastes from ferrosilicon industry. The Mn(-Fe)-Si-C system is an important metallic system in production of SiMn. Graphite is the stable phase in the lower Si range. When the Si content is raised to a certain value at a fixed temperature, the coexistence point is reached where the liquid alloy will be in equilibrium with both graphite and silicon carbide (SiC). At Si-contents above this point, SiC will become the stable phase. Therefore, the addition of Si decreases the solubility limit of C in SiMn alloy melt. Notwithstanding this well-known thermodynamic information, there are few experimental data for understanding the refining mechanism of carbon (or SiC) from SiMn melt.
Based on the above backgrounds, in the present study, the dissolution of SiC particle at 1600oC in the CaO-SiO2-MnO slag was observed in situ by means of confocal scanning laser microscopy in order to make the determination of dissolution mechanism. The SiC particle is initially wetted by molten slag from the outer surface and the wetting between SiC and slag phase is more dominant in the composition of higher CaO/SiO2 ratio. When the SiC particle is wetted by molten slag, the gas bubbles that are mainly CO gas is generated by the reaction between SiC and MnO in slag phase and are continuously evolved at the wetted area, which is pronounced as the CaO/SiO2 ratio increases. The dissolution of SiC particle in the slag through the reaction with MnO is enhanced in the composition of higher CaO/SiO2 ratio not only due to greater thermodynamic driving force but also due to accelerated mass transport kinetics.
Duplex production of HC FeMn and SiMn, with refining processes
(reproduced from 'Production of Manganese Ferroalloys', Olsen et al., SINTEF and Tapir academic press, 2007)