Acid–base properties of molten oxides and metallurgical slags

Abstract

The application of the Lux–Flood concept to acid–base reactions in oxide systems is discussed in the light of spectroscopic information concerning the “state” of oxide ions. In slag systems (e.g. molten silicates) it is possible for the basicity to be expressed in terms of electron donor power as an optical basicity (Λ), and not in terms of the activity of O^2– ions. CaO is chosen as the reference oxide of unit basicity; Λ-values are either measured spectroscopically or calculated from the equation Λ=ΣX_iΛ_i, where X_i and Λ_i are equivalent fractions and (assigned) basicity values for slag components. The optical basicity scale provides a means for comparing acid–base indicators in oxide melts, rather analogous to the use of the Hammett acidity function (H_O) in strong protonic acids and is applicable to multicomponent systems. Possible applications of optical basicity to metallurgy are exemplified by reference to the sulphide capacities (C_s) of iron and steelmaking slags. Data for ≈ 130 blast furnace slags show a good correlation between log C_s and Λ, even for slags which do not contain SiO₂. Evidence is supplied that optical basicities can also be derived from ESCA measurements; Λ-values obtained in this way might be useful in assessing the properties of steel making slags.