Computer simulation of equilibrium relations in managanese ferroalloy production
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INTRODUCTION
IN manganese ferroalloy production, the distribution of Mn and Si between carbon-saturated Mn-Fe-Si alloys and slags containing various amounts of MnO, SiO2, Al2O3, CaO, and MgO is a result of simultaneous reactions taking place inside the furnace. An understanding of these reactions will help the metallurgists to predict and control the element distributions among the different phases. The basic reactions among the slag, metal, and gas phases are described by the following equations: ðMnOÞ 1 C 5 Mn 1 COðgÞ
[1]
ðSiO2 Þ 1 2C 5 Si 1 2COðgÞ
[2]
ðSiO2 Þ 1 Si 5 2SiOðgÞ
[3]
Mn 5 MnðgÞ
[4]
where the parentheses denote the slag phase and underscores the alloy phase. Manganese and silicon will redistribute between liquid metal and slag phases according to the reaction 2ðMnOÞ 1 Si 5 2Mn 1 ðSiO2 Þ
[5]
The two-phase metal/slag Reaction [5] is actually a combination of Reactions [1] and [2]. Extensive laboratory investigations[1–7] have been carried out over years to study equilibrium relations of importance for the production of manganese ferroalloys. The influence of different operating parameters was established semiquantitatively from the experimental results.[1–4] For example, it was found that the effect of temperature on the metal/slag equilibrium (Reaction [5]) is relatively small, whereas the temperature plays an important role for the metal/slag/gas equilibria (Reac-
KAI TANG, Research Scientist, is with SINTEF Materials and Chemistry, N-7465 Trondheim, Norway. Contact e-mail: [email protected] SVERRE E. OLSEN, Professor Emeritus, is with the Department of Materials Technology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. Manuscript submitted May 18, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
tions [1] and [2]). Increasing temperature shifts the equilibrium to lower contents of MnO in the slag and thus benefits the recovery of manganese. Increasing the slag ‘‘basicity’’ ratio, (CaO 1 MgO)/SiO2, and decreasing the CO partial pressure will also reduce the MnO content considerably. Although the experimental results may provide a basic image of equilibrium compositions of metal and slag for a given system, it is still difficult to give exact quantities and compositions of reaction products. The reason is that the final state of a given system is dependent not only on chemical equilibrium, but also on stoichiometric restrictions, i.e., the materials balance. Thermodynamic modeling is a powerful tool for quantitative characterization of important equilibrium relations. Chemical equilibrium and mass balance are automatically satisfied through both mathematical and thermodynamic solutions. Based on existing experimental data, the model is capable of using thermodynamic interpolations and extrapolations to predict equilibrium relations in regions where there is lack of experimental knowledge. The objective of the present investigation is to simulate thermodynamically the complex heterogeneous equilibria involved in the process of ferromanganese production by use of assesse
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