Alternative Methods for in Situ Determination of Iron Oxide in Steelmaking Slags

The most important component of steelmaking slags is the iron oxide since it serves as the oxidizing agent in refining reactions. The measurement of the iron oxide content is performed in the steelworks by taking a sample out of the liquid slag, cooling it and subsequently analysing it. The procedure takes some time. It would be highly desirable, therefore, to develop methods for rapid in situ determination. Iron oxide is present in liquid slags in the form of FeO (ferrous oxide) and Fe2O3 (ferric oxide). The ratio Fe2O3/FeO (or Fe3+/Fe2+) depends on the total iron content, the contents of the other slag components, the temperature, and the oxygen potential. Under reducing conditions mainly FeO is stable, and under oxidizing conditions mainly Fe2O3. If slag and metal are close to equilibrium, the iron oxide content of the slag can be computed from the content of certain elements in the metal and the thermodynamic data for the slag/metal equilibrium. For slag with high iron oxide content, the relevant reaction is [O] + [Fe] = (FeO) which yields aFeO=K(T) aO. Hence, the ferrous oxide activity aFeO can easily be obtained by in situ measurement of the oxygen activity aO of the liquid metal with EMF probes involving stabilized zirconia as solid electrolyte. However, in many stages of conventional steelmaking the slag metal reactions are not so close to equilibrium. Consequently, the indirect determination of iron oxide content via metal analysis is not sufficiently accurate, or may even be rather wrong. The direct in situ measurement of iron-oxide content and of Fe3+/Fe2+ ratio with EMF probes is possible1–6. The oxygen potential has to be fixed, if the oxide content is to be determined, by the presence of metallic iron, which can be performed in various ways. Possible cell designs have been suggested in previous studies5,6. However, there are difficulties. The zirconia tubes are attacked by the slag melts. In the practical application, the signal is found to oscillate and there is a large error in the determined iron oxide content. Thus, due to such problems, the direct determination of iron oxide content with EMF cells is not yet widely carried out in industry. In principle, the iron oxide content can be determined by measurement of such slag properties that are markedly dependent on this component. Besides the thermodynamic activity, certain transport properties can be utilized. They may be easily measurable as such, or it might be possible to measure kinetic phenomena controlled by them. New methods for in situ iron oxide determination can be conceived from application of such alternative principles. In the present paper methods are suggested that are based on transport of electrical charge in the slag melt. The first part deals with the application of electrical conductivity, and in the second part it is proposed to use the measurement of direct current overvoltage and alternating current impedance for in situ analysis of iron oxide.