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INTRODUCTION
The increasing demand for high alloy and stainless steels world wide, has put great emphasis on the optimization of the applicable production processes to reduce production costs. The price of nickel has increased dramatically over the past few years<n resulting in an increased price of nickel-chromium stainless steels. This is just one of the factors that has aroused interest in the increased production of manganese containing stainless steels, for instance the SAE 200 series, in which nickel is partly replaced by relatively inexpensive manganese. Manganese ore is freely available on the local market and, with the exciting prospect of the availability of a direct steelmaking route to produce manganese-containing stainless steel, a fundamental knowledge of the activity of manganese oxide in the liquid slag during the smelting stages of the process is required. It is important to quantify the thermodynamic behaviour of the manganese oxide in the slag because this knowledge is required to minimize manganese losses to the slag and to optimize the slag composition. However, there is very little information available in the literature on the thermodynamic properties of this system, and there is consequently much incentive to determine the activity of manganese oxide in the applicable slag systems.
Conventionally, the gas-slag equilibrium technique has been used successfully for the determination of metal-oxide activities in several slag systems in the temperature range 1673 to 1923 K<2><3><4 H5)(6). For instance, Pretorius<2> determined the CrO-activity in a (Cr0-Si02-Ca0)-slag at 1773 K. A platinum crucible contained the slag which was equilibrated with a mixture of (C02 + H2) gas of known oxygen potential. Equilibrium between the slag, gas and metal is established through the reaction: x<Qr>Pt + 1/2[02] = (Cr 0) x slag· [1.1]
At equilibrium, the chromium content of the platinum crucible is fixed by the oxygen potential of the gas and the chromium oxide in the slag. The chromium oxide activity was determined by Pretorius<2> as a function of slag composition by the subsequent chemical analysis of the chromium content of the platinum crucible and the chromic oxide content of the slag.
Equilibrium between the gas and slag phases is typically attained after periods varying from 12 to 48 hours when the gas-slag equilibrium technique is used<2><3><4><S><6). Consequently, it is imperative that the flow rates of the different gases are accurately controlled to ensure that the oxygen potential of the gas is constant and known throughout the equilibration period. However, slags containing volatile components can not be studied by this technique, since the evolution of gases from the slag will change the oxygen potential of the system. Furthermore, the composition of the slag will continuously change as a function of time due to the evaporation of the volatile components.
CHA.P’fER 1 INTRODUCTION
CHAPTER 2 BACKGROUND
2.1 Fundamental Aspects of the Electrochemical Measuring Technique .
2.1.1 Limitations of the Electrochemical Technique
2.1.2 Analysis of the EMF Recording .
2. 2 Previous Work
CHAPTER 3 ACTIVITY MEASUREMENTS IN THE Fe-Cr-0 SYSTEM AT 1873 K
3.1 Introduction
3.2 Design of the Plug-type Oxygen Probe
3.3 Experimental Aspect
3.3.1 Al20 3 crucibles Coated with Cr20 3
3.3.2 Cr20 3-crucible (no slag
3.3.3 Cr20 3 crucible ((Cr20rsaturated-SiOrCaO)-slag)
3.4 Results and Discussion
CHAPTER 4 .. ACTIVITY MEASUREMENTS IN (Mn0-Si02-Mg0)-SLAGS IN EQUILIBRIUM WITH (MnO-MgO) SOLID SOLUTIONS AND Mn METAL AT 1873 K
4.1 Introduction .
4.2 Thermodynamic Relations
4.3 Experimental Aspects
4.3.1 Analysis of the Phase Composition of the Slag
4.3.2 Oxygen Activity Measurements
4.4 Results and Discussion
4.4.1 Analysis of the Slag Phase Compositions and the Determination of the MnO Activity in the Liquid Slag
4.4.2 Oxygen Activity Measurements
CHAPTER 5 SUMMARY AND CONCLUSIONS
APPENDIX 1
REFERENCES
