A mössbauer study of the behavior of iron cations in iron oxide-containing melts at 1400 °C

Abstract

A Mössbauer spectroscopy technique has been developed to study the distribution of iron cations in slag melts as a function of distance from a reaction surface. A wide range of slag compositions of interest to the nonferrous smelting industry was reduced and oxidized using mixtures of CO, CO₂, and Ar. Mössbauer spectra were collected on quenched slag samples for varying conditions between iron saturation and magnetite saturation, l(10^-13) to 4(10^-6) atm P_O2 at 1400 °C. Slag sections were analyzed with a spatial resolution of approximately 500 /am, and details of the ferric and ferrous distribution at the surface and in subsurface regions were studied. In Fe_xO-SiO₂-Al₂O₃ melts exposed to a reducing gas mixture, there is a gradient in ferric iron concen-tration decreasing toward the surface. In the Fe_xO-Al₂O₃ melt, the reduction of ferric anions at the surface appears to release O_2- ions, which increases the coordination of Fe²⁺. However, in the presence of silica, which is surface active and acidic, reduction of ferric iron does not lead to an excess of O_2- at the surface. Lime has a dramatically different effect. It reverses the ferric gradient so that, even during reduction, the ferric concentration at the surface is enhanced. Ferric oxide is surface active in these melts, and under certain conditions, liquid-phase mass transfer is rapid enough to replenish the surface. The presence of a polymer-forming ion, Fe³⁺, at the surface results in a scarcity of O^2- ions and a reduced coordination of Fe²⁺. In the presence of both SiO₂ and CaO, the behavior of ferric iron during reduction depends, to an extent, on the CaO/SiO₂ ratio. The behavior of iron oxide melts during reduction is very complex, and the surface-active nature of SiO₂ and ferric oxide has a significant influence on the reaction. A quantitative analysis of the overall reaction would have to take these factors into account.