Ultraviolet/visible spectroscopy of molten slags and glasses (up to 1600°C)

The aim of this research project is to obtain information about the molecular structure of liquid oxides (silicates and aluminates) by application of spectroscopic methods. The chemical structure is correlated with the properties of the oxide melts (e.g. basicity, redox state, equilibria). Spectroscopic methods are also suitable for the control of melts during smelting. For these investigations, spectroscopic methods have been developed (wavelength range 220 up to 780 nm) for research studies of temperatures up to 1600°C at different oxygen partial pressures. Measurements of slags with and without 3d cation dopings have been investigated. The eigenabsorption (electron transition valence band/conduction band in the silicates) in the ultraviolet-range in undoped liquid silicates is connected with the dissociation of these silicates. Slag systems of samples from CaO–SiO2; CaO–MgO–SiO2; CaO–Al2O3–SiO2 and CaF2–CaO–Al2O3 have been doped with oxides of 3d cations: Fe2O3 [Fe2+/Fe3+]; Cr2O3 [Cr6+-species] and CoO [Co2+] with mass contents 0.80% up to 50% and samples without dopings have been prepared. With an increase of basicity, a red-shift of the charge transfer bands has been measured, which has been quantitatively evaluated. The height and the long-wave decrease (Urbach-range) of the Fe3+-bands depend on the redox state of the slag. In liquid slags, the Cr6+-species is spectroscopically detectable as Cr2O72−-anion, whilst in the glassy slags at room temperature the CrO42−-anion exists. The Cr2O72−-anion is not stable in liquid slags and in a diffusion-controlled thermal dissociation, gaseous CrO3 is evaporated. Within the visible range, the d–d-absorption bands of the tetrahedral Co2+(O2−)4-complex decrease with an increase of temperature. By quantitative evaluation of the spectra, the dissociation degree and the dissociation enthalpy have been determined as a function of temperature up to a temperature above the liquidus temperature.