Iron redox equilibria in CaO-Al 2 O 3 -SiO 2 and MgO-CaO-Al 2 O 3 -SiO 2 slags
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~FeFe ! 5 0.3(50.02) Y 1 0.45(50.01) log 2 1.24(50.01) ~pCO pCO ! 3+
log
2+
2
where Y 5 (CaO 1 MgO)/SiO2, for melts with the molar ratio of CaO/SiO2 5 0.45 to 1.52, 10 to 15 mol pct Al2O3, up to 12 mol pct MgO (at CaO/SiO2 ' 1.5), and with 3 to 10 wt pct total Fe. Available evidence suggests that, to a good approximation, these redox equilibria are independent of temperature when expressed with respect to pCO2/pCO, probably from about 1573 to 1873 K. Limited studies have also been carried out on melts containing about 40 mol pct Al2O3, up to 12 mol pct MgO (at CaO/SiO2 ' 1.5), and 3.6 to 4.7 wt pct Fe. These show a strongly nonideal behavior for the iron redox equilibrium, with
~
!
Fe 3+ pCO2 } 2+ Fe pCO
0.37
The nonideal behavior and the effects of basicity and Al2O3 concentration on the redox equilibria are discussed in terms of the charge balance model of alumino-silicates and the published structural information from Mo¨ssbauer and NMR (Nuclear Magnetic Resonance) spectroscopy of quenched melts.
I.
INTRODUCTION
THE major component compositions of the slags in the several iron bath smelting processes which are currently under development[1–4] are broadly similar to those in the iron blast furnace process, except for the iron oxide concentration. Average ‘‘FeO’’ concentrations appear to be about 3 to 8 wt pct, compared with ≤0.2 wt pct in tapped blast furnace slags.[5] The local state of oxidation within the slag phase of a bath smelting process is likely to vary over a wide range. Low activities of oxygen must exist where metallic iron is being produced, yet, depending on the particular process, higher oxygen activities must occur in the local environment near to where iron oxide–containing materials are being absorbed or near to where postcombustion processes occur. The present study was undertaken to establish the equilibrium relationships between the Fe3+/Fe2+ ratio and the activity of oxygen for CaO-SiO2-Al2O3-MgO-‘‘FeO’’ slags, at compositions and temperatures relevant to the bath smelting processes, for the subsequent interpretation of experiments on the rates of gas-slag and metal-slag reactions. However, the results should prove useful as a diagnostic tool in studies of the practical processes through chemical LIXIANG YANG, Research Engineer, and G.R. BELTON, Scientific Director–Metallurgy, are with BHP Research Newcastle Laboratories, Wallsend NSW 2287, Australia. Manuscript submitted September 4, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
analysis of quenched slag samples. It seems unlikely that interactions among redox pairs[6,7] involving minor metal oxide constituents, such as manganese and titanium oxides, would significantly change the relationship between oxygen activity and the Fe3+/Fe2+ ratio in the slag. High concentrations of sulfur may affect the ratio through the complexing of the Fe3+ ion by the S22 ion. Such complexes are known to occur as the amber chromophore in commercial glasses,[8,9] but there is no experimental information available on whether similar effects occu
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