Development of electrochemical sensing techniques for the determination of activity-composition relations in liquid alloys and slags at 1873 K

[摘要] A plug-type oxygen probe containing a magnesia stabilized solid zirconia electrolyterod, was developed. This oxygen sensor was evaluated in the Fe-Cr-0 system at1873 K by determining the Raoultian chromium and Henrian oxygen activities of anFe-Cr alloy which was in equilibrium with pure Cr20 3 and a Cr20rsaturated liquidslag respectively. However, the slag reacted with the oxygen probe and therefore,a crucible assembly was developed to prevent direct contact between the slag and theoxygen sensor. The results obtained with this crucible assembly proved that nointeraction between the oxygen probe and slag occurred and that the smallerslag/metal interface did not effect equilibrium between the slag and the metal.The Raoultian chromium activities of the Fe-Cr alloy containing 5 to 25% chromiumdetermined in this study, deviate negatively from ideal Raoultian behaviour. Theoxygen and chromium activities were used to determine the standard free energychange for the formation of solid Cr20 3 from the elements at 1873 K, and the valueobtained is in good agreement with existing data. Evidently, the plug-type oxygenprobes and the redesigned crucible assembly can be used to determine the oxygenactivity of an Fe-Cr alloy, in equilibrium with a liquid slag at 1873 K, accuratelyand reliably. Therefore, these measurements were extended to the determination ofthe activity of MnO in (MgO-Si~-MnO)-slags.The chemical composition of the equilibrium phases pertaining in (Mg0-Si0Si02-MnO)slagssaturated with (MnO-MgO) solid solutions and in contact with manganesemetal, were determined by microprobe analyses. These results together with theknown activity-composition relations of (MnO-MgO) solid solutions at 1873 K, wereused to determine the activity of MnO in the slag. This method proved to be reliableand the MnO activities so obtained, could serve as a check for activities determinedwith the electrochemical technique. Furthermore, the same method could be used for the determination of the high temperature phase constitution of the slag when theoxygen potential of the slag/metal system is determined by electrochemical means.In the course of the determination of the oxygen potential of (MgO-SiOz-MnO)-slagsat 1873 K by means of electrochemical oxygen probes, the magnesia stabilizedzirconia solid electrolytes reacted with manganese metal and vapour which had adetrimental influence on the accuracy and reliability of the EMF (electromotiveforce) measurements. Accordingly, it was illustrated that zirconia solid electrolytesare not suitable for application in slag/metal systems containing manganese metal at1873 K. Previous workers have shown that thoria solid electrolytes are not attackedby manganese metal at these high temperatures and should consequently be used withthe crucible assembly developed in this study, for the determination of activities in(Mg0-Si02-Mn0)-slags at 1873 K.

[发布机构] University of Pretoria