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

Abstract

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 pCO2/pCO≈0.01 to as high as air at temperatures of 1573 to 1773 K. At 1773 K, values are given by $$\begin{gathered} \log {\text{ }}\left( {\frac{{Fe^{3 + } }}{{Fe^{2 + } }}} \right) = 0.3( \pm {\text{ }}0.02){\text{ }}Y + {\text{ }}0.45( \pm {\text{ }}0.01){\text{ }}\log \hfill \\ \left( {\frac{{pCO_2 }}{{pCO}}} \right) - 1.24( \pm {\text{ }}0.01) \hfill \\ \end{gathered} $$ 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≈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 $$\frac{{Fe^{3 + } }}{{Fe^{2 + } }} \propto \left( {\frac{{pCO_2 }}{{pCO}}} \right)^{0.37} $$ 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.