US 3723279 A
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l/V VEN TORS' RICHARD J. F RUEHA/V E' THE M 7T TURKDUGA/V lay R. J. FRUEHAN ET AL AND APPARATUS FOR OXYGEN DETERMINATION TEMPERATURE OXYGEN IND/CA TOI? March 27, 1973 Filed May 21, 1970 ,mii/ZW( w75.
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R. J. FRUEHAN ET Al- APPARATUS FOR OXYGEN DETERMINATION Sheets-Sheet 2 RICHARD J. FRUEHAN THEM T. TURKDOGAN Homey United States Patent O 3,723,279 APPARATUS FOR OXYGEN DETERMINATION Richard J. Fruehan, Franklin Township, Westmoreland County, and Ethem T. Turkdogan, Pittsburgh, Pa., assignors to United States Steel Corporation Continuation-impart of application Ser. No. 727,314, May 7, 1968. This application May 21, 1970, Ser.
Int. cl. Goin 27/46 U.S. Cl. 204-195 S 10 Claims ABSTRACT OF THE DISCLOSURE This application which is a continuation-in-part of our co-pending application, Ser. No. 727,314, iiled May 7, 1968, now abandoned, relates to apparatus for measuring the oxygen content and/or activity of fluids at elevated temperatures, and more particularly, to apparatus that rapidly determines the oxygen content of liquid steel in a ladle or furnace without sampling. Since our invention, at present, is most useful and most needed for this purpose, this use Will be stressed hereinafter. However, our invention is also applicable for determining oxygen in other fluids at temperatures above approximately 700 C., such as liquid copper and hot furnace gases.
The steel-making process can be considerably advanced if the amount of oxygen dissolved in molten steel is continuously and immediately known during the various stages of the process. Such knowledge would permit previously impossible manipulations of a melt of steel by providing better control of the oxidation and deoxidation reactions.
The oxygen content of liquid steel has heretofore been determined by the analysis of samples of the liquid steel. The various analytical methods now used require elaborate equipment and do not provide results rapidly enough for best control of the steel-making process. A solid oxide electrolyte galvanic cell with a gaseous reference electrode has been used in laboratory experiments to provide a rapid determination of oxygen in liquid steels, but these cells are not practical for use in steel production for various reasons. They are relatively expensive, their accuracy is not satisfactory, and their quality is not uniform nor satisfactory.
According to our invention, we provide an oxygen cell having a solid oxide electrolyte fused in a refractory tube. A thermocouple is embedded into a condensed phase reference electrode which is placed inside the tube. The thermocouple, in addition to providing a temperature determination, serves as one connection to an oxygen indieating apparatus. A dise of molybdenum cermet which contacts the liquid steel is also connected to the oxygen indicating apparatus by a -wire of the same material that connects the thermocouple to the oxygen indicating apparatus. A metallic cap, which dissolves in the liquid steel, protects the cell or probe from the slag when plunged into the melt. Within a few seconds after immersion in the liquid steel, a steady and reliable EMF is achieved which is measured by the oxygen indicating apparatus.
It is therefore an object of our invention to provide apparatus for rapid or continuons determination of the oxygen content in high temperature uid over a wide range of temperatures.
Patented Mar. 27, 1973 Another object is to provide a disposable probe for determining oxygen content in molten metal `without sampling the melt.
A further object is to provide such a probe which iS relatively inexpensive, reliable and accurate.
A still further object is to provide such a probe that requires no thermo-electromotive force corrections.
Still another object is to provide such a probe which also determines the temperature of the liquid metal.
These and other objects will be more apparent after referring to the following specification and attached draw ings in which:
FIG. 1 is a sectional view of the oxygen probe of our invention and includes its connections to be oxygen and temperature indicating apparatus;
FIG. 2 is a graph showing the relationship between the measured EMF and oxygen content in parts per million; an
FIG. 3 is a graph comparing the measured oxygen content of liquid steel with the oxygen content determined by the neutron activation method of analysis.
Referring more particularly to the drawings, reference numeral 2 indicates a calcia stabilized zirconia,
Zr02(CaO) disc `which is placed in one end of a refractory container 4. Disc 2 may be fused to the bottom of a silica tube 4, or inserted into the bottom of an alumina tube 4. Disc 2 is preferably about 3 mm. in diameter and 3 mm. high. Contained in tube 4 above disc 2 is a mixture 6 of Cr and Cr2O3, about 90 to 98 weight percent Cr, the balance Cr203. Partially embedded in the mixture 6 is a conventional thermocouple 8 having a double bore silica tube 10 and a pair of wires 12, preferably Pt-Pt/ 10% Rh in contact with the mixture 6. The upper ends of wires 12 are each connected to Contact prongs 14. Tube 4, thermocouple 8 and part of prongs 14 are enclosed in an alumina container tube 16. A disc 18 of molybdenum cermet, approximately to 85 weight percent Mo and the balance A1203, is fitted tightly into the bottom of a silica tube 20 to prevent the penetration of liquid metal into the inside of tube 20. A Pt Wire 22 is attached to disc 18 and to a contact prong 24 through a silica protective tube 26. Tube 16 is filled with a refractory cement 28, such as Alundum, which is an electrical and thermal insulator. Tube 20 is filled with alumina powder 29. 'The top of tube 16 is covered with a cap 30 made of a plastic, such as Teflon. Prongs 14 and 24 project through cap 30 to form an electrical plug. An alumina support ring 32 is attached to the bottom of tube 16. The bottom of tube 16 is sealed by a metal cap 34. A long protective cardboard tube 36, only a portion of which is shown, rests on support rings 32. A receptacle 3S fits over prongs 14 and 24 with lead wires 40 connecting prongs 14 and 24 to a temperature and oxygen indicator 42. Prongs 14 and their respective lead wires 40 are compensating alloys conventional for a Pt-Pt/ 10% Rh thermocouple. Prong 24 and its lead Wire 40 is the same compensating alloy as that connected to the Pt thermocouple wire. Indicator 42 is preferably a potentiometric recorder or similar device with t-wo indicating arms. One indicating arm, for indicating temperature, is connected to wires 12. A second indieating arm, indicating in the range of minus 0.2 volt to plus 0.5 volt, is for oxygen content, and is connected to Wire 22 and Pt wire 12.
In operation, the probe is plugged into a permanent receptacle provided at the furnace or ladle above the liquid metal L and then plunged into the melt to a suitable depth. The cardboard tube 36, which extends above the top of the melt, protects the wires and upper elements of the probe from damage by the metal. lf the protective cardboard tube causes significant local carbon deoxidation, this may be overcome by replacing the last few inches of the cardboard tube with a refractory tube, such as silica. Cap 34 protects the probe from chemical attack by the slag as the probe is lowered through the slag into the molten metal. The cap 34 is made of material, such as copper or brass, which will not affect the oxygen content of the steel and which readily dissolves in the steel so as to expose the electrolyte and the molybdenum cermet disc to the molten steel. The cap 34 may also be made of steel. When the stabilized zirconium oxide electrolyte contacts the electrode of liquid steel, a galvanic cell results which may be represented as follows:
Pt(s), Cr(s)+Cr203(s)[Zr (CaO) (s)|Fe-O (1),
(Mo cermet), Pt(s) The voltage across the electrolyte is given by the free energy equation as follows:
ercntO glp e l RT E=2.303 -10 KP02,
nF (l) where The temperature dependence of the equilibrium constant K and 1102i for Cr-Cr203 equilibrium from known thermodynamic data is 19,700 10g @021/2 (atl'ILl/z) By combining these equations and substituting values of the constants, the following equation is obtained for weight percent oxygen in liquid steel in terms of EMF, (E) in milivolts and T in degrees Kelvin for the Cr-Cr203 reference electrode It should be noted that E is actually a measure of the oxygen activity in liquid steel. The relationship of the activity of the oxygen in the liquid steel to the oxygen content of the liquid steel is well known and is shown in FIG. 2 where the EMF indicates the oxygen activity.
By using lead wires to the oxygen indicator of the same material, preferably platinum, no thermo-electromotive force corrections are required since all connections are at the liquid metal temperature and the lead wires will be at the same temperature. Fusing a small disc of electrolyte into the silica tube greatly increases its resistance to thermal shock.
While the EMF of Equation 5 neglects the effect of alloying elements in steel, the equation is essentially correct for low alloy steels. An approximate correction for alloying elements can be made by the following expression:
Actual log (wt. percent O) :Apparent log (percent O) (e, percent -l-ej percent j -I-ek percent k-llog (percent) 0=4.62
where apparent log percent 0=the value from Equation 5 using measured E percent i, percent j, percent k, etc.==concentrations of various alloying elements in liquid steel, such as Mn, Si, C, etc.
ei, ej, ek, etc.=interaction coefficients for various alloying elements.
For large tonnage, low carbon steel-making the concentrations of Mn, Si, C are smal lenough that the above correction becomes insignificant for all practical purposes.
A steady EMF is achieved in a few seconds after plunging the probe into the melt and provides continuous oxygen content readings up to 40 minutes. The cell has sufficient reliability and accuracy for use in the steelmaking process as shown in FIG. 3 where the oxygen content of samples determined by the neutron activation method is compared with the oxygen content as measured by the EMF of the cell.
While the specific components set forth above are preferred since they have proven very successful in use in determining the oxygen content of liquid steel, they may be varied as long as they meet the following requirements. The electrolyte 2 must be a solid oxide and an oxygen ion conductor with insignificant electronic conductivity under the conditions of its use, particularly at the ternperature and oxygen partial pressure to which it is subjected. It must not react with the materials it contacts. Zr02 with 3 to 10% by weight of Ca0 and Th02 with 3 to 20% by weight of Y203 are preferred for use with steel. MgO, A1203, and Zr02 with 3 to 10% by weight of MgO or Y2O3 and Th02 with La203 may also be used, but electrolytes are not all suitable for use with all types of fluids.
The reference electrode y6 must be a condensed phase mixture of chromium and its oxide which does not readily melt at the temperature to which it is subjected nor react significantly with the materials it contacts. It is preferred to use a mixture of between about 90% to 98% by weight of pure chromium and the remainder Cr203. However, a chrome alloy may be substituted for the chromium. The alloying metal must be one having an oxide less stable than Cr203 so that Cr203 will be the equilibrium oxide phase. The melting point of the alloy must be high enough so that the alloy is solid at temperature of use. The dissociation pressure in Equation l would be corrected accordingly. Suitable alloys include up to 10% by weight of manganese or iron, or up to 5% nickel.
While it is preferred to use a Mo cermet Mo, 20% A1203) for member '118, it is only necessary that the material be an electronic conductor, will not melt at its temperature of use, will not readily dissolve in the fluid so that a steady and reliable voltage reading can be taken, and when dissolved in the liquid will not affect the voltage reading. Such materials are Cr cermet (80% Cr, 20% Al203), Cr203, Mo wire or rod, Fe wire or rod and graphite rod. Not all materials can be used for all types of liquids.
A probe suitable for determining the oxygen content of liquid copper may have a reference electrode of Cr-Cr203, and a liquid metal contact of Cr cermet or Cr203.
When used for determining the oxygen contents of the gases in soaking pits, heat treatment furnaces, hot exhaust gases or the like, the probe is altered slightly. Wire 22 must contact the electrolyte 2 where it is in contact with the gas and the cermet contact may be omitted. The balance of the probe is, of course, not in contact with the gas.
The specific probe described has been used in the oxygen content range from about 10 p.p.m. to about 1000 p.p.m. Below that concentration it is preferred to use ThO2(Y2O3), which will provide predominantly ionic con duction at oxygen contents down to 1 p.p.m.
While silica has been used extensively in the probe, other refractories, such as alumina, may be used which would better resist the effects of liquid steel. The refractory must, of course, not react with other components of the probe.
Since the probe is made of inexpensive materials and is relatively easy to manufacture, the probe can be used once and thrown away in the same manner that immersion thermocouples are used. Because a conventional thermocouple is incorporated into the probe, temperatures may be determined at the same time as oxygen content, and auxiliary equipment requirements are minimal.
While the preferred embodiment of our invention is described as an instrument to use in a vessel, ladle or furnace as a conventional immersion thermocouple is used, other adaptations are possible. As an example, a zirconia thimble, small enough to avoid thermal shock, could be located in the lining of a tundish to contact the liquid steel with the other essential parts of the probe arranged in a manner already described.
While several embodiments of our invention have been shown and described, it will be apparent that other adaptations and modiiications may be made 'without departing from the scope of the following claims.
1. Apparatus for determining the oxygen content of a fluid at a temperature of at least 700 C. which comprises a galvanic cell including a refractory container closed at one end thereof by a solid oxide electrolyte adapted to contact said fluid, and a reference electrode inside said refractory containers and in electrical contact with said electrolyte, said electrolyte being an oxygen ion conductor having insignificant electronic conductivity under conditions of use, said reference electrode being a mixture of Cr203 and a material of the class consisting of chromium and an alloy of chromium with a metal having an oxide less stable than Cr203, said material having a melting point such that it will be solid at the temperature of said fluid, and means for measuring the EMF of the cell when the electrolyte contacts the uid.
2. Apparatus according to claim 1 in which said electrolyte is of the group consisting of MgO, A1203, Zr02 with 3 to 10% by weight of CaO, ThO2 with 3 to 20% by weight of Y2O3, ZrOZ with 3 to 10% by weight of MgO, Zr02 with 3 to 10% by weight of Y2O3, and 'I'h02 with La203.
3. Apparatus according to claim 1 which includes a refractory electronic conductor adapted for electrical contact with said electrolyte, said conductor being made of material which is relatively insoluble in said uid, and means connecting said conductor and said reference electrode to said EMF measuring means.
4. Apparatus according to claim 3 in which said conductor is of the group consisting of a molybdenum cermet composed of about 80% by weight of Mo and about 20% by weight of A1203, a chromium cermet composed of about 80% by weight Cr and about 20% by weight of A1203, molybdenum, graphite and Cr203.
5. Apparatus according to claim 4 in which said EMF measuring means is a two position potentiometric recorder; and which apparatus includes a thermocouple, said thermocouple having a junction embedded in said reference electrode, a first thermocouple wire connected to both potentiometric positions, and a second thermocouple wire connected to one potentiometric position to indicate temperature; and in which said means connecting said conductor to said EMF measuring means includes a junction of said conductor and a wire of the same material as vsaid first thermocouple wire, said last named wire being connected to the other potentiometric position to indicate oxygen content, and said junctions being located to operate at the same temperatures under conditions of use.
6. Apparatus according to claim 1 in which said electrode is a mixture of between about to 98% by weight of chromium and the remainder Cr203.
7. Apparatus according to claim 6 for use in a melt of steel having a layer of slag thereon, which apparatus includes a steel-soluble cap shielding said apparatus from slag upon immersion with said melt, said cap being of the group consisting of copper, brass, and steel.
8. Apparatus according to claim 7 which includes a refractory electronic conductor adapted to contact said melt, said conductor being made of material which is relatively insoluble in said melt, and means connecting said conductor and said reference electrode to said EMF measuring means.
9. Apparatus according to claim 1 in which said material of said reference electrode is an alloy of chromium with another metal.
10. Apparatus according to claim 9 in which said alloying metal is of the group consisting of up to 10% by weight manganese, up to 10% by weight iron and up to 5% by weight nickel.
References Cited UNITED STATES PATENTS 3,464,008 8/1969 Meysson et al 204-195 3,468,780 9/ 1969 Fischer 204-195 3,481,855 12/1969 Kolodney et al 204-195 3,619,381 11/1971 Fitterer 204--195 S OTHER REFERENCES Fitterer: Reprint from Journal of Metals, August 1966, pp. 1-6.
Tretjakow et al.: Berichte der Bunsengesellschaft, vol. 69, No. 5, 1965, pp. 396-402.
Horsley: A ERE Report R3427, pp. l8 and FIG. 2, 1961.
TA-HSUNG TUNG, Primary Examiner U.S. C1. X.R. 204-1 T Patent No. 3v723v279 Dated MaC' 27 1973 Inventms) RICHARD J., FRUE E'ua AL.
It is certified that` error appears in the above-dentfied patent and that said Letters Patent are hereby correclted as shown below:
Col 5p line 28 mwont-,aimersW should fread M- container msigned andv sealed this 20th day of Noven'ber 1973.,
Attesting Officer Acting Commissioner of Patents FORM PO-105O (1C-59) USCOMMDC 50376-P69 U.S. GOVERNMENT PRINTING OFFICE |969 0-'366-3342;