Solid-state amperometric sensor for the in-situ monitoring of slag composition and transport properties

Open-circuit potential (OCP) measurements with zirconia-based sensors have been used for several decades in the metals industries to determine the oxidation state of high-temperature melts. Although amperometric techniques are known to provide additional kinetic and compositional analysis of the system, they have not been utilized by the metals industries to the same extent. In this work, three different amperometric techniques were examined to monitor high-temperature slags by utilizing zirconia-based sensors. Applied direct current (DC) potential steps and impedance frequency sweeps at fixed applied DC potentials were shown to clearly distinguish between slags having the same oxidation state but different quantities of FeO(x) in low concentrations. The techniques were also shown to reveal information on the transport properties, slag structures, dissociable oxide type, and buffering capacity of the slag to a given oxidation state. The third technique, coulometric titration, was shown to easily differentiate between slags containing different quantities of FeO(x) in larger concentrations. The ion-selective membrane properties of the zirconia allowed the coulometric titration to proceed at current efficiencies near 100 pet. This may be useful in developing technology for the recovery of metals from oxide waste streams.