Redox Equilibrium of Niobium in Calcium Silicate Base Melts

The oxidation state of niobium has been determined at 1873 K (1600 °C) in CaO-SiO₂-NbO _x melts with CaO/SiO₂ ratios (mass pct) of 0.66, 0.93 and 1.10, and 5.72 to 11.44 pct Nb₂O₅ (initial). The slag samples were equilibrated with gas phases of controlled oxygen pressure, then quenched to room temperature and analyzed chemically. The niobium is mainly pentavalent with small amounts in the tetravalent state. It was found that the Nb⁵⁺/Nb⁴⁺ ratio increases with oxygen pressure at a constant CaO/SiO₂ ratio and constant content of total niobium, closely according to the ideal law of mass action, which is proportional to \( {\text{p}}_{{{\text{O}}_{2} }}^{1/4} . \) The ratio also increases with total niobium content, and it seems to have a maximum at a basicity of about 0.93. The color of the solidified slag samples is described and is explained with the help of transmission spectra.