1997 TMS Annual Meeting: Wednesday Session

APPLICATIONS OF SENSORS AND MODELING TO MATERIALS PROCESSING: Session V

Session Chairs: J.M. Toguri, Dept of Metallurgy and Materials Science, Univ. of Toronto, 184 College Street, Toronto, Ontario, Canada, M5S 1A4; U. Pal, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Room 4-134, Cambridge, MA 02139

8:30 am

MEASUREMENT OF GAS COMPOSITIONS USING SOLID ELECTROLYTES: R.V. Kumar, D.J. Fray, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom

Sensors are required to measure gaseous species such as SOX, NOX, HCl, and water vapour at elevated temperatures. There are very few solid electrolytes which respond to these gases, and furthermore, the species are usually present with other gases to which the electrolyte may respond. In addition, for long periods of operation, a stable reference is required. In order to meet these challenges, sensors consisting of two electrolytes in intimate contact have been developed. To measure HCl, a proton-conducting electrolyte is interfaced with strontium chloride. The reaction for the cell: (-) Pt, Air + HCl(g))/SrCl2//SrCeO3 + 10 mole % Nd2O3// Air + HCl(g), Pt(+). At the various interfaces, the reactions are: anode: 2Cl- + SrHCl = SrCl2 + HCl(g) + 2e-interface: 2H+ + 2Cl- = 2HCl(interface) cathode: HCl(interface) + SrCl2 + 2e- = SrHCl + 2Cl- overall cell reaction: HCl(interface) = HCl(g). The net result is that the potential is independent of the oxygen potential of the gas and the reference is given by the interaction of the two electrolytes. There is no need for an external reference which overcomes the problems of forming gas thigh seals at elevated temperatures. Other examples of this approach for the measurement of SOX, NOX, CO2, and H2O will be given.

8:55 am

DEVELOPMENT AND APPLICATION OF OXYGEN SENSORS IN COPPER METALLURGY: S. Seetharaman, Du Sichen, A. Jakobsson, Division of Theoretical Metallurgy, Royal Institute of Technology, S-100 44 Stockholm, Sweden

Oxygen concentration cells with stabilized zirconia electrolytes are widely used today to measure the activity of oxygen in liquid metals. In the case of liquid copper, the activity of oxygen in the molten metal as well as the effect of a third element on the same have been extensively studied using galvanic cells of the type (-) Pt, W or cermet / Cu-O-M // ZrO2 (stabilized) // AO, A / Pt (+, where M is the third element in molten copper and AO / A represents a suitable oxide/metal reference electrode. The effect of a number of "third elements" like Mn, Zn, As, Se, and Te on the oxygen activity in liquid copper have been studied by this method. The Mn and Zn interact very strongly with oxygen in Cu ( ), while, in the case of arsenic, the interaction parameter is nearly zero. The behavior of Se and Te in liquid copper with respect to the effect on the activity of oxygen is unique. At oxygen levels of the order of 10-4, these elements lower the activity coefficient of oxygen in the melt, while, at lower oxygen levels, the interaction parameter values are positive. These results are discussed in the light of some of the theoretical models for solute interactions in liquid metals. The application of these sensors in flash smelting, converter, and casting sections of copper production are discussed along with their implications on process optimization.

9:20 am

APPLICATION OF RAMAN SPECTROSCOPY TO HIGH­TEMPERATURE ANALYTICAL MEASUREMENTS. J.P. Young, S, Dai, Y. Lee, H. Xiao, Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831­6142

There are numerous analytical applications of scatter­emission and absorption spectroscopy to liquids and solids in the temperature range of 0° to 350°C; process control is made impossible by using fiberoptic probes and Raman spectroscopy, for example, in these measurements. We have developed a unique all­silica fiberoptic probe which can have analytical applications in the temperature range where silica is a solid, 0° to 1600°C, and in chemical situations where it is inert. We have also developed techniques to use the ratio of the Stokes/anti­Stokes Raman emission of diamond to measure temperature in the range of 400 to 1000°C. We will discuss our results of such measurements in molten chloride and fluoride salts, other liquids, and solids. We will also describe our plans for temperature measurements by Raman spectroscopy through optical fibers in the range of 1000 to 2500°C, a range that can include molten metals such as steel.

9:45 am

SOLID STATE CURRENT-POTENTIAL SWEEP SENSOR FOR THE IN-SITU MONITORING OF COMPOSITION AND TRANSPORT PROPERTIES IN HIGH TEMPERATURE METALLURGICAL SLAGS: S.C. Britten, V. Stancovski, U. Pal, Dept of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 4-134, Cambridge, MA 02139

An in-situ solid-state electrochemical technique for measuring the concentrations of easily dissociable oxides in slags at temperatures between 1200 to 1600°C is being developed. The technique consists of using a stabilized zirconia solid electrolyte, which conducts oxygen ions, to separate a reference gas compartment from the slag of interest. Using a potentiostat, a direct current potential sweep is applied between the inner and outer compartments of the electrolyte, driving oxygen ions from the slag into a reference gas. With the use of open circuit reference electrodes, the resulting magnitude of the current potential profile reveals the concentration of dissociable oxides such as those of iron, manganese, and chromium. The open circuit potential recovery indicates the type of oxide present and its thermodynamic activity within the slag. The technique should therefore determine multiple properties of several different oxides with only one measurement. The limits imposed by the electronic short-circuit property of the zirconia electrolyte on the sensitivity of the technique are also under investigation.

10:10 am BREAK

10:20 am

APPLICATIONS OF SENSORS IN MOLTEN SALT TECHNOLOGY FOR METAL PROCESSING: P.T. Velu, R.G. Reddy, Dept of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487

The use of emf measurements to derive classical thermochemical data has been in use for many years. The reaction associated with electrochemical process is harnessed in a galvanic cell and energy involved measured in terms of electromotive force. The property requirements of sensor electrodes used differ for various aqueous, molten salt, and solid electrolyte systems. The electrodes for molten salt electrolytes need special care for construction. This paper describes the studies carried using an Ag/AgCl reference electrode. This electrode was used to measure the solubility of compounds in molten salt electrolytes. The results obtained using this electrode are in excellent agreement with the values from chemical analysis method.

10:45 am

NOVEL SOLID STATE SENSOR FOR MEASURING ARSENIC IN MOLTEN METALS: G.M. Kale, Dept of Mining and Mineral Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom; D.J. Fray, Dept. of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3 QZ, United Kingdom

A novel solid state sensor for measuring arsenic dissolved in molten metals has been developed based on a novel solid electrolyte conducting K+ ions. The solid state sensor can be schematically represented as: (-)Mo, Zn-As/ /K-323 SE/ /K-Fe2O3+Fe2O3+02, Fe-Cr(+). The open circuit emf of the above sensor was measured in molten zinc containing less than 0.1 wt % As at a temperature of 823K. The emf of the sensor was found to vary linearly as a function of 1nXAs, where XAs is the mole fraction of As in An(1). The ionic conductivity of the novel solid electrolyte (K-323) was measured over a range of temperature (370<T<770K) using a vector impedance analyzer. The K+ ion conductivity of K-323 at 823K was found to be 1 X 10-3-1cm-1. The performance of these arsenic sensors is currently being evaluated in plants in the United Kingdom and Germany.

11:10 am

MEASUREMENT OF NITROGEN AND SULPHUR IN MOLTEN METALS USING SOLID ELECTROLYTES: Y.C. Avniel, T.E. Warner, D.J. Fray, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom

One of the major problems of the iron and steel industry is the presence of phosphorus, sulphur, nitrogen, and silicon. Careful control and removal of these elements is required, and an on-line measurement of the concentration during removal would greatly improve the efficiency of the process. The oxygen content of molten metals is usually measured by using a sensor based upon stabilized zirconia, in which the oxygen ion is mobile. However, for nitrogen and sulphur, stable ionically conducting solid electrolytes for nitrogen and sulphur do not exist. The approach adopted in this work is to use electrolytes which conduct alkali and alkaline earth elements. For example, when a sodium ion conducting electrolyte is placed in a situation where sodium is not present, the sodium comes to equilibrium with the species which forms the most stable compound with sodium. Examples are given where sodium, strontium, and calcium beta alumina respond to sulphur and oxygen. Regions of response to a given species depend upon the concentration of other species in the melt and the standard free energy of formation of the various compounds. In the case of nitrogen measurement, it proved necessary to develop a new electrolyte, based upon rare earth oxides, in which nitrogen is soluble in the anion lattice. Results are presented for the measurement of sulphur and nitrogen partial pressures and for these species in molten iron and steel.