Experimental Studies on the Oxidation States of Chromium Oxides in Slag Systems

In view of the importance of the thermodynamic behavior of chromium in the slag phase as well as the serious discrepancies in the earlier reports on the valence state of chromium in slag melts, the oxidation state of chromium oxides in CaO-SiO₂-CrO_x and CaO-MgO-(FeO-) Al₂O₃-SiO₂-CrO_x were investigated experimentally in the present study using two different experimental techniques. The gas–slag equilibrium technique was adopted to study the CaO-SiO₂-CrO_x system between 1823 K (1550 °C) and 1923 K (1650 °C) and equilibrated with mixtures of CO-CO₂-Ar gases corresponding to three different oxygen partial pressures (between 10⁻⁴ and 10⁻⁵ Pa). After equilibrating, the samples were quenched and subjected to analysis using the X-ray absorption near edge structure method to determine the distribution ratio of Cr²⁺/Cr³⁺ in the slags. The second technique examined the applicability of the high-temperature mass spectrometric method combined with the Knuden effusion cell for quantifying the valence states of Cr in the multicomponent system CaO-MgO-(FeO-) Al₂O₃-SiO₂-CrO_x up to a maximum temperature of 2000 K (1727 °C). The results showed that the Knudsen cell-mass spectrometric method could be used successfully to estimate the valence ratio for Cr in silicate melts. According to the present study, the Cr²⁺/Cr³⁺ ratio increased with increasing temperature and a decreasing slag basicity as well as the oxygen partial pressure prevailing in the system. A mathematical correlation of X _CrO/X _CrO1.5 as a function of temperature, oxygen partial pressure, and basicity was developed in the present work based on the present results as well as on those assessed from earlier data.