Reaction equilibria in the production of manganese ferroalloys
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INTRODUCTION
M A N G A N E S E ferroalloys are either produced in blast furnaces or in electric furnaces. The alloys are classified as high-carbon ferromanganese, medium- and low-carbon ferromanganese, and silicomanganese. High-carbon ferromanganese and silicomanganese are produced by carbothermic reduction, whereas medium- and low-carbon ferromanganese are produced by oxygen refining of high carbon ferromanganese or by silicothermic reduction of MnO dissolved in slag. Several investigations have been carried out over the years to study equilibrium relations associated with the production of manganese ferroalloys,tt-~~ Even though most industrial slags and alloys have been included in these studies, the results have rarely been used to predict and control the production processes. The difficulty arises from a misty knowledge of the real state of equilibrium and confusion due to complex multicomponent slags. It is well known from the literaturetn-~sl that silicon addition to manganese melts gives rise to a considerably decreased carbon solubility, for example, from 8 wt pct C for Mn-C,,, to 1.8 wt pct C for Mn-Si (18 wt pct)-C~, at 1500 ~ The solubility is somewhat affected by iron, i.e., the higher the Fe/Mn ratio, the lower the carbon content of the melt. The silicon content at the graphite/SiC coexistence point varies with the Fe/Mn ratio and the temperature, being, for example, 22.0 wt pct Si at 1550 ~ for an iron alloy (Fe-Si-C,~,) and 18.77 wt pct Si for a corresponding manganese alloy (Mn-Si-C~,). t~9,2~
WEIZHONG DING, formerly Doctorate Student, Department of Metallurgy, University of Trondheim, Norwegian Institute of Technology, is Associate Professor, Department of Metallurgy, Shanghai University of Technology, 200072 Shanghai, China. SVERRE E. OLSEN, Professor, is with the Department of Metallurgy, University of Trondheirn, Norwegian Institute of Technology, N-7034 Trondheim, Norway. Manuscript submitted December 1, 1993. METALLURGICALAND MATERIALS TRANSACTIONS B
A number of investigations have been undertaken to measure activities in manganese alloys,t2,~2,21-2sl Although the published data are not in satisfactory agreement with each other, a general tendency is obvious. Both carbon and silicon additions lower the Mn activity and give large negative deviations from ideality. Activities have also been measured in the slag system. Some results have been reported for binary MnO-SiO2 slags and for different ternary and multicomponent slag systemsY 9-371 Several investigations have been carried out to determine distribution equilibria between the slag and metal phases. Tuset et aLt3] and Kolq6] studied slag/metal equilibria for low-carbon alloys, whereas carbon-saturated alloys were employed in the works of Turkdogan and Hancock,t21 Rankin and See, tSJ Tanaka, I7] and Gzielo and Pacula: 1~ These authors did not study the equilibrium with the gas phase, however. The equilibrium between a carbon-containing metal and an oxide slag will define a certain CO pressure. Complete equilibrium with the gas phase
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