Phase Distribution During Slag Formation in Mn Ferroalloy Production

Abstract

Phase development during slag formation and reduction of Comilog and Assmang ores in a setup that simulates the conditions in an industrial SAF process under different coke bed temperatures, 1473 K (1200 °C), 1673 K (1400 °C) and 1773 K, (1500 °C) was investigated. Graphite crucibles were utilized to contain the charge and as a heating element in an induction furnace. A temperature profile was established, cross-sectional excavations were made, and samples were core drilled at selected positions. The resultant phases, i.e., slag and metal, were examined in an electron probe microanalyzer (EPMA) coupled with wavelength-dispersive spectrometry (WDS). The equilibrium phase relations in the MnO–SiO2–CaO–MgO–Al2O3 oxide system were calculated using FactSage 7.3 thermochemical software. Differences in ore compositions revealed significant differences in phase development during slag formation and reduction behavior of these manganese sources. The phase development is highly influenced by temperature and position in relation to the coke bed, with a two-phase region, i.e., manganosite + liquid, dominant on top of the coke bed and a dominant liquid slag inside the coke bed. The solid monoxide phase in Comilog has been found to be purely MnO. In contrast to Comilog, the solid monoxide solution phase in Assmang has significantly high FeO content and forms a (Mn,Mg,Fe)O solid solution, which becomes more enriched in MgO with increases in temperature and reduction extent. Due to lower concentration of CO gas for prereduction in this experimental setup, iron oxides are reduced to FeO instead of Fe and subsequently FeO is stabilized in the monoxide solid solution.