The effect of alumina in slag on manganese and silicon distributions in silicomanganese smelting
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I.
INTRODUCTION
SILICOMANGANESE is produced from slag formed in ferromanganese production, which contains about 25 to 35 pct Mn, by reducing it in a submerged arc furnace with coke. Standard silicomanganese contains about 67 pct Mn, 16 pct Fe, 16 pct Si, and 2 pct C. t~,21 The economics of silicomanganese smelting is enhanced by maximizing the proportions of MnO and SiO2 reduced from the slag to the alloy, within the constraints imposed by the relevant chemical equilibrium. The equilibrium that controls the distribution of silicon and manganese between the slag and alloy is, according to Rankin, t31 the same whether the alloy is ferromanganese or silicomanganese: 2(MnO) + [Si] = (SiO2) + 2[Mn]
[1]
where the parentheses indicate a species present in slag and the brackets indicate a species present in the alloy. The equilibrium constant of Reaction [1], in expanded form, is K =
N2nNsio2~Mn~/SiO2
N~noNsiTsiT2MnO
[2]
where Ni is mole fraction and Ti is the activity coefficient of the species i in the slag or alloy. Distribution ratios for manganese and silicon between slag and alloy are a practical way of expressing equilibrium data and can be written as DMn = Dsi =
NMnO/NMn Nsio2/Nsi
[3] [4]
Equation [2] can thus be written to express the equilibrium constant as a function of the distribution ratios and the activity coefficients of all species as follows: Dsi ~Mn~/SiO2
K - --
- -
D ,o r is
[5]
D.R. SWINBOURNE, Senior Lecturer, is with the Department of Chemical and Metallurgical Engineering, Royal Melbourne Institute of Technology, Melbourne, 3001 Australia. W.J. RANKIN, Professor, is Director of the G.K. Williams Cooperative Research Centre for Extractive Metallurgy, University of Melbourne, Parkville, Australia. R.H. ERIC, Professor, is Head of the Department of Metallurgy and Materials Engineering, University of the Witwatersrand, Witwatersrand 2050, South Africa. Manuscript submitted November 19, 1993. METALLURGICALAND MATERIALSTRANSACTIONSB
The group containing the distribution ratios can be termed the "apparent equilibrium constant," K', and the activity coefficients group can be termed the "system activity coefficient," Ysys, for convenience; thus, K = K'Ysys
[6]
The apparent equilibrium constant will be constant only if %ys remains constant. In practical silicomanganese smelting, the composition of the alloy varies very little, and so, the term 32Mn/3,Siwill be substantially constant. However, the composition of the slag, for example the basicity or the alumina content, can vary, and so, 7sio2/~,o would not be constant. Thus, the apparent equilibrium constant would be expected to be a function principally of slag composition. Some work has been done on the distribution of manganese and silicon between slag and various ferrous alloys, but very little has been done on silicomanganese alloys containing low iron contents. Turkdogan and Hancock t41 studied the silicon-manganese equilibrium in the production of high-carbon ferromanganese and found that (1) an increase in slag basicity increases th
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