Abstract:
A plug-type oxygen probe containing a magnesia stabilized solid zirconia electrolyte
rod, was developed. This oxygen sensor was evaluated in the Fe-Cr-0 system at
1873 K by determining the Raoultian chromium and Henrian oxygen activities of an
Fe-Cr alloy which was in equilibrium with pure Cr20 3 and a Cr20rsaturated liquid
slag respectively. However, the slag reacted with the oxygen probe and therefore,
a crucible assembly was developed to prevent direct contact between the slag and the
oxygen sensor. The results obtained with this crucible assembly proved that no
interaction between the oxygen probe and slag occurred and that the smaller
slag/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% chromium
determined in this study, deviate negatively from ideal Raoultian behaviour. The
oxygen and chromium activities were used to determine the standard free energy
change for the formation of solid Cr20 3 from the elements at 1873 K, and the value
obtained is in good agreement with existing data. Evidently, the plug-type oxygen
probes and the redesigned crucible assembly can be used to determine the oxygen
activity of an Fe-Cr alloy, in equilibrium with a liquid slag at 1873 K, accurately
and reliably. Therefore, these measurements were extended to the determination of
the activity of MnO in (MgO-Si~-MnO)-slags.
The chemical composition of the equilibrium phases pertaining in (Mg0-Si0Si02-MnO)slags
saturated with (MnO-MgO) solid solutions and in contact with manganese
metal, were determined by microprobe analyses. These results together with the
known activity-composition relations of (MnO-MgO) solid solutions at 1873 K, were
used to determine the activity of MnO in the slag. This method proved to be reliable
and the MnO activities so obtained, could serve as a check for activities determined
with the electrochemical technique. Furthermore, the same method could be used for the determination of the high temperature phase constitution of the slag when the
oxygen 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)-slags
at 1873 K by means of electrochemical oxygen probes, the magnesia stabilized
zirconia solid electrolytes reacted with manganese metal and vapour which had a
detrimental influence on the accuracy and reliability of the EMF (electromotive
force) measurements. Accordingly, it was illustrated that zirconia solid electrolytes
are not suitable for application in slag/metal systems containing manganese metal at
1873 K. Previous workers have shown that thoria solid electrolytes are not attacked
by manganese metal at these high temperatures and should consequently be used with
the crucible assembly developed in this study, for the determination of activities in
(Mg0-Si02-Mn0)-slags at 1873 K.
