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

Abstract

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.

Appendix A

In regard to the reactions with respect to pure Cr₂O₃, three reactions are considered to occur, which would lead to the formation of the vapor species Cr, CrO, and CrO₂, as shown in Eqs. [3] and [4]. Based on several measurements of pure Cr₂O₃, the ratio of the corresponding partial pressures of CrO₂, CrO, and Cr is roughly about 3:6:100[12,13] in a wide temperature range. For the present slags, the partial pressure of CrO is attributed to two species in the condensed phase, viz. CrO_1.5 and CrO, whereas the partial pressure of CrO₂ is generated solely by CrO_1.5 according to reaction [3] and the following:

$$ \left( {\text{CrO}} \right)_{\text{slag}} \Rightarrow {\text{Cr}}_{\left( g \right)} ,{\text{CrO}}_{\left( g \right)} $$

(A1)

$$ \left( {{\text{CrO}}_{ 1. 5} } \right)_{\text{slag}} \Rightarrow {\text{Cr}}_{\left( g \right)} ,{\text{CrO}}_{\left( g \right)} ,{\text{CrO}}_{ 2\left( g \right)} $$

(A2)

In the present analysis, it is assumed that the vaporization capacity of CrO in slag is the same as that of CrO_1.5 in forming the gaseous species of CrO and Cr, which could be considered reasonable under the high temperatures used in the measurements. The contributions of CrO and CrO_1.5 to the partial pressure of CrO(g) reflected the X _CrO/X _CrO1.5 ratio in the condense phase. Moreover, for Cr₂O₃ in our sample, the partial pressure ratio of three chromium species is supposed to obey the behavior of pure Cr₂O₃. According to the ratio of the partial pressure of CrO₂ and CrO in pure Cr₂O₃, the part of the partial pressure of CrO contributed by Cr₂O₃ in condense phase can be obtained from the following reaction as 2pCrO₂:

$$ {\frac{{X\left( {\text{CrO}} \right)_{\text{slag}} }}{{X\left( {{\text{CrO}}_{ 1. 5} } \right)_{\text{slag}} }}} = {\frac{{p_{\text{CrO}} \left( {\text{from CrO}} \right)}}{{p_{\text{CrO}} \left( {{\text{from CrO}}{}_{1.5}} \right)}}} = {\frac{{p_{\text{CrO}} - 2p_{\text{CrO2}} }}{{2p_{\text{CrO2}} }}} $$

(A3)