The consumption of stainless steel is growing rapidly, at the highest rate of all metals. During the last decade, the melting and rolling capacity of stainless steel has increased significantly, especially because of huge investments in China. The production and supply of the necessary raw materials, especially nickel, has not increased to meet rising demand, which has resulted in a substantial increase in the price of the most expensive raw material used in austenitic stainless steel. The end-users of stainless steel are not happy about these price increases and are constantly looking for cheaper substitutes, such as plastic, aluminium, galvanized and/or painted carbon steels, and also stainless steel grades with low nickel contents. The profit margin per unit produced decreases with growing production and competition, and all means are necessary to defend profit margin in such a fierce competitive environment. The European producers have been forced to start making nickel free ferritic stainless steels (e.g., 1.4509)and manganese alloyed austenitic steels (e.g., 200-series/1.4432). They are also developing special grades, which combine high corrosion resistance with improved strength, with corresponding weight and cost savings, such as duplex stainless steels. All of these alternatives may substitute for the traditional high nickel containing austenitic steel grades in specific applications at a lower cost. The most important factor in the ability of a particular stainless steel producer to obtain a competitive edge is the production efficiency. New integrated production lines are run with only a few operators, such as Outokumpu Tornio Works cold rolling mill 2 (also called RAP5), which is a fully integrated rolling-annealing pickling line. All failures in production are identified, and actions are taken to improve processes and product quality.
One of the key issues is to achieve better raw material yield throughout the entire process. Recovery of chromium is of special importance because chromium is the major alloying element in stainless steel. Chromium is alloyed into the stainless steel as ferrochromium, which is produced in a submerged arc-furnace (SAF). SAF is a combination of blast furnace and electric arc furnace, which provides enough energy to reduce the stable chromite pellets and lumpy ore into metallic ferrochromium. The reduction of chromite occurs in two distinct stages. The first occurs between solid chromite, coke and carbon monoxide, and the second occurs in the Al2O3-CaO-MgO-SiO2 based slag. The liquid slag dissolves the already partially reduced solid chromite as the ionic species Cr2+, Cr3+, Fe2+ Fe3+ and O2-, along with some additional impurity elements. Carbon reacts with oxygen anions in the slag and forms carbon monoxide gas (CO). As a consequence, the slag supersaturates with respect to cations. The iron and chromium cations, which comprise the less stable oxides in the slag, are reduced and preferentially precipitated out of the slag, forming metal droplets. The metal droplets coalesce and disperse out of the slag as a heavier phase and collect onto the bottom of the furnace as ferrochromium melt. The chromium yield depends on the thermodynamic reaction equilibrium between the slag and ferrochromium, but also on the reaction kinetics. The thermodynamics of Cr-containing slags were studied by Yanping Xiao in her thesis, and chromite reduction kinetics in the solid and liquid states have been studied by Marko Kekkonen. In particular, low viscosity slag speeds up the reactions and helps the metal droplets to segregate out of the slag, which consequently improves the yield of the ferrochromium process, but also the recovery of chromium and other metals in stainless steel melting in an electric-arc-furnace.
There is little previously published data regarding the viscosity of chromium containing slags. Some research was conducted in the late Soviet Union on the viscosities and electrical conductivities of ferrochrome process slags mainly by Zhilo et al. Unfortunately, the compositional analysis in these studies did not separate the different oxidation stages of chromium. A comparative analysis with the present study also raises doubt about the viscometer, which was not sensitive enough to detect below 0.2 cPa. The main reasons for the lack of experimental viscosity data are: 1) the high melting temperature of chromium containing slags, which often exceeds the heating range of the experimental furnaces or limits of the available construction materials, and 2) the multiple oxidation stages of chromium (Cr2+, Cr3+, Cr4+, Cr6+), which add a degree of difficulty due to demanding atmospheric control. In reducing conditions, such as in the ferrochromium process, the chromium appears simultaneously as Cr2+ (CrO) and Cr3+ (CrO1.5). The distribution of the total chromium content (CrOx) into CrO and CrO1.5 is dependent on the oxygen partial pressure, the temperature, and the total amount of chromium and other oxide species in the slag. The low PO2 increases the portion of CrO and lowers the liquidus temperature of the slag. The viscosity measurements are very expensive to perform because of the high temperature refractory materials which often can only be used for a limited time. These measurements are also very time-consuming due to the long heating and cooling times of furnaces, along with sample preparation and analysis. Furthermore, the experimental runs often fail, and the results are subject to fairly large errors.
Mathematical viscosity models may be used to interpolate the viscosity values at compositions where the measured data does not exist, or extrapolate the viscosity data to the composition ranges where the measurement could not be made in practice due to the high melting temperatures. A successful viscosity model can also decrease the errors of independent measurements, and may provide more reliable data than a separate measurement alone. The viscosity model may be incorporated with thermodynamic and kinetic computer aided models, and used to model and optimise real production processes.
CONTENTS
1 INTRODUCTION
2 STRUCTURE OF SLAG
3 DEFINITION OF VISCOSITY
4 MEASUREMENT OF VISCOSITY
5 VISCOSITY MODELS
5.1 Urbain model and the modified Urbain model
5.2 KTH model
5.3 Reddy model
5.4 CSIRO model
5.5 Iida model and the modified Iida model
5.6 Models based on optical basicity (NPL)
5.7 Pyrosearch quasi-chemical viscosity model
5.8 Nakamoto - Tanaka model
5.9 Modelling viscosity of a heterogeneous liquid
6 EXPERIMENTAL TECHNIQUES AND METHODS
6.1 Viscosity measurement arrangement
- 6.1.1 Furnace
6.1.2 Viscometer
6.1.3 Spindle and crucible
6.1.4 Furnace atmosphere
6.1.5 Arrangement
7 EXPERIMENTAL PROCEDURES
7.1 The measurable CrOx oxide systems
7.2 Sample preparation
7.3 Viscosity measurement procedure
7.4 Sample analysis
8 SUMMARY OF THE PUBLICATIONS
8.1 Experimental study of the viscosities of selected CaO-MgO-Al2O3-SiO2 slags and application of the Iida model
8.2 Viscosity of CaO-CrOx-SiO2 slags in relatively high oxygen partial pressure atmosphere
8.3 Viscosity of SiO2-CaO-CrOx slags in contact with metallic chromium and application of the Iida model
8.4 Experimental study and modelling of viscosity of chromium containing slags
8.5 Assessment of viscosity of slags in ferrochromium process
9 RESULTS AND ERROR ANALYSIS
10 DISCUSSION
10.1 Effect of chromium oxide addition on viscosity
10.2 The applied viscosity models
11 CONCLUSIONS
12 REFERENCES
13 APPENDICES
13.1 Article I
13.2 Article II
13.3 Article III
13.4 Article IV
13.5 Article V
13.6 Derivation of the Eyring equation for viscosity
13.6.1 Determination of the Maxwell-Boltzmann equation, i.e. the classical law for the distribution of energy
13.6.2 The theory of absolute reaction rates
13.6.3 Reaction rate theory for viscosity
13.7 Derivation of the Bockris equation for viscosity
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