Structural Understanding of MnO–SiO₂–Al₂O₃–Ce₂O₃ Slag via Raman, ²⁷Al NMR and X-ray Photoelectron Spectroscopies

In order to understand sulfur solubility in the MnO–SiO₂–Al₂O₃–Ce₂O₃ slag or inclusion system (mol MnO/mol SiO₂ = M/S = 2.2 or 0.9, Al₂O₃ = 14.2[± 1.8] mol%, Ce₂O₃ = 0–5.6 mol%), the effect of Ce³⁺ ions on the structure of the Mn–aluminosilicate system with different M/S molar ratios has been studied by micro-Raman spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Because constant oxygen partial pressure was maintained at p(O₂) = 2.8 × 10⁻⁷ atm, oxidation state of Mn and Ce would be in Mn²⁺ and Ce³⁺ in the MnO–SiO₂–Al₂O₃–Ce₂O₃ system, which were also confirmed by XPS analysis. Although it was difficult to measure the ²⁷Al solid-state NMR spectra of the quenched MnO–SiO₂–Al₂O₃ system due to the paramagnetic effect, ²⁷Al NMR spectra did show structural changes in aluminate when cerium was added to the Mn–aluminosilicate melts. Addition of Ce₂O₃ in the high M/S(= 2.2) system caused both an explosive enhancement in the intensity of Raman bands at 600 cm⁻¹ and a peak shift in ²⁷Al NMR spectra, indicating the transition from the [AlO₄]^–:0.5Mn²⁺ tetrahedron to a [AlO₆]^3–:Ce³⁺ octahedron complex due to a strong attraction between aluminate and the cerium ion. XPS spectra show that the transition of the aluminate structure upon introduction of Ce³⁺ ions consumes free oxygen in Mn–aluminosilicate melts in high M/S(= 2.2) system. However, the same structural changes were not observed when Ce₂O₃ was added to lower M/S(= 0.9) system, because Ce³⁺ ions primarily interact with the pre-existing [AlO₆]-octahedron in low M/S system.