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.
Ionen‐ und Elektronenleitung in CaO‐FeO‐Fe2O3‐SiO2‐Schmelzen
Die elektrische Leitfahigkeit von Schmelzen aus CaO, SiO2, FeO und Fe2O3 mit einem molaren Verhaltnis CaO/SiO2 = 0,79 und Eisenoxidgehalten von 0–44 Mol-% wurde bei 1600°C untersucht. Das Fe3+/Fe-Verhaltnis x wurde durch Gleichgewichtseinstellung mit einer Gasphase zwischen 0,25 und 0,77 variiert. Die Leitfahigkeit x als Funktion von x last sich durch x = x0 – ax + bx(1 – x) wiedergeben. Aus x0 werden die Beweglichkeiten der Ca2+- und Fe2+-Ionen bestimmt. Der quadratische Term kann einer Elektronenleitung zugeordnet werden, die durch Ladungsaustausch zwischen Fe2+ und Fe3+ ermoglicht wird. Der Ladungsaustausch wird nach zwei Modellen behandelt, die jeweils als Grenzfalle anzusehen sind. Im einen Fall wird Tunnelung der Elektronen zwischen unbeweglichen Leitungszentren, im anderen eine bimolekulare, stosabhangige Reaktion angenommen. 
 
 
 
The electrical conductivity of melts consisting of CaO, SiO2, FeO and Fe2O3 having a molar ratio CaO/SiO2 = 0.79 and iron oxide contents from 0–44 mole-% have been investigated at 1600°C. The Fe3+/Fe ratio was varied between 0.25 and 0.77 by equilibration with a gas phase. The conductivity x can be expressed as a function of x by the equation x = x0 – ax + bx(1 – x). The mobilities of the Ca2+ and Fe2+ ions have been determined from x0. The quadratic term can be assigned to an electron conduction by charge transfer between Fe2+ and Fe3+. The charge transfer has been treated according to two models, both of which are to be regarded as limiting cases. In one case tunnelling of the electrons between stationary conduction centres is assumed, while in the other a bimolecular reaction dependent upon collision number is assumed.
Development and Application of Oxygen Sensors to Steelmaking Operations
Synopsis: An oxygen sensor with Cr/Cr2O3 reference electrode has been developed and used practically in steelmaking operation. The relation between the dissolved oxygen content at blow off and the conditions of LD refining operation was studied with the help of the data obtained by this cell. Consequently. Al content of killed steels in the ladle, Si content of semi-killed steel, and Mn content of rimmed steel have been precisely adjusted from the dissolved oxygen content measured, because it gives the quantities of the materials to be added at the LD tapping process. The main results obtained are summarized as follows: (1) The oxygen sensor was successfully applied to practical use in steelmaking. (2) Dissolved oxygen contents of the steel in the LD converter are closely related to Mn content and bath temperature as well as to C contents of steel. (3) Dissolved oxygen contents are not affected by the "after blows" and the furnace life. (4) Because the relation between the dissolved oxygen at blow off and Al content of killed steel in ladle has been clarified, more precise adjustment of Al content in ladle became possible, compared to the previous method using carbon-oxygen relations. (5) Because oxygen content of steels are closely related to the Si content of Si-semi-killed steels and Mn content of rimmed steel, precise adjustments of the crow of Si-semi-killed ingot top and the degree of rimming action of steels became possible.