Iron redox equilibria in CaO-Al2O3-SiO2 and MgO-CaO-Al2O3-SiO2 slags

Measurements have been made of the ratio of ferric to ferrous iron in CaO-Al2O3-SiO2 and MgO-CaO-Al2O3-SiO2 slags at oxygen activities ranging from equilibrium with pCO(2)/pCO approximate to 0.01 to as high as air at temperatures of 1573 to 1773 K. At 1773 K, values are given by log (Fe3+/Fe2+) = 0.3(+/-0.02) Y + 0.45(+.-0.01) log (pCO(2)/pCO) - 1.24(+/-0.01) where Y = (CaO + MgO)/SiO2, for melts with the molar ratio of CaO/SiO2 = 0.45 to 1.52, 10 to 15 mol pct Al2O3, up to 12 mol pct MgO (at CaO/SiO2 approximate to 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 pCO(2)/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 pet MgO (at CaO/SiO2 approximate to 1.5), and 3.6 to 4.7 wt pct Fe. These show a strongly nonideal behavior for the iron redox equilibrium, with Fe3+/Fe2+ proportional to (pCO(2)/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 Mossbauer and NMR (Nuclear Magnetic Resonance) spectroscopy of quenched melts.