Phase Distribution During Slag Formation in Mn Ferroalloy Production

Tichaona Mukono,Maria Wallin,Merete Tangstad

doi:10.1007/s11663-022-02441-5

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.

Highlights

THE primary use of manganese metal is in the steel industry, where it is an alloying constituent mainly added in the form of ferroalloy.[1,2,3,4] Industrially, production of ferromanganese alloys is done in submerged arc furnaces (SAF) or blast furnaces
The accuracy of the electron probe microanalyzer (EPMA) measurements was within ±1 wt pct; as such, phases containing maximum concentrations < 1 wt pct of a particular component do not exhibit solid solution with that particular component since these concentrations are below the limits of uncertainty of the measurements and are negligible
The phase development and reduction of slag from Comilog and Assmang ores as a function of temperature and in relation to position within the coke bed are discussed based on metallographic observations in EPMA, phase compositions measured by wavelength-dispersive spectrometry (WDS) and FactSage 7.3-based thermodynamic equilibrium calculations

Summary

Introduction

THE primary use of manganese metal is in the steel industry, where it is an alloying constituent mainly added in the form of ferroalloy.[1,2,3,4] Industrially, production of ferromanganese alloys is done in submerged arc furnaces (SAF) or blast furnaces. Production of manganese ferroalloys with silicon requires higher temperatures; blast furnace cannot be used to produce this product, and, as such, the major part of global ferroalloy production is METALLURGICAL AND MATERIALS TRANSACTIONS B through submerged arc furnaces (SAFs).[2,5,6,7,8] In this article, the production of Mn ferroalloys containing < 1 pct Si will be discussed. The feed materials for the high carbon ferromanganese (HCFeMn) submerged arc furnace process contain oxides that undergo reduction as well as non-reduceable oxides. In HCFeMn production, the main oxides of manganese ores include pyrolusite (MnO2), bixbyite (Mn2O3) and hausmanite (Mn3O4) as well as other natural minerals that could be present including braunite (3Mn2O3ÁMnSiO3), calcite (CaCO3), manganite (c-MnOOH), jacobsite (Fe2MnO4) and other associated minerals.[1,9]

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