US3464008A - Device for continuously measuring the oxygen content of a molten metal including an electrolytic cell having a solid electrolyte - Google Patents

Description

Aug. 26. 1969 N. MEYSSON ET AL 3,464,008

DEVICE FOR CONTINUOUSLY MEASURING THE' OXYGEN CONTENT OF A MOLTEN METAL INCLUDING AN ELECTROLYTIC CELL HAVING A SOLID ELECTROLYTE 2 Sheets-Sheet 1 Filed 001'.- 18, 1967 .II 5 3 NI l|||ll Q ,M

Aug. 26,1969 N. MEYSSON ET AL 3,464,008

DEVICE FOR CONTINUOUSLY MEASURING THE OXYG CONTENT OF A MOLTEN M AL INCLUDING ELECTROLY CELL' ING A SOLID L CTROLYTE Filed Oct. 18, 19.67 2 Sheets-Sheet z 7 :j 62 y/////// -j56 United States Patent 12 Claims ABSTRACT OF THE DISCLOSURE A device for measuring the oxygen content of molten metal in a container and including an electrolytic cell having a solid electrolyte and constituted by a block of refractory solid electrolyte material mounted in a wall of the container with one face of the block in contact with the metal, and formed with a cavity having an end face adjacent to but spaced from the one face of the block and through which an oxygen-containing reference gas is circulated to maintain a constant partial oxygen pressure in the cavity, an electrode carried by the block in the cavity and surrounded by the reference gas circulated through the latter and having one end in contact with the end face of the cavity, and a conductor carried by the block insulated from the electrolyte and having an end adjacent the face of the block which is in contact with the molten metal and being likewise in contact with the latter; and means in circuit with the electrode and the conductor for measuring the potential difference between said faces of the block.

Background of the invention The present invention relates to a device for continuously measuring the oxygen content of a molten metal of high temperature, especially of molten steel.

It is an object of the present invention to provide for a device for continuously and very exactly measuring the oxygen content of a molten metal.

It is an additional object of the present invention to provide for a device of the aforementioned kind which has a very high resistance to the corrosive action of the liquid metal bath.

It is a further object of the present invention to provide a device of the aforementioned kind which has also an excellent resistance against mechanical and thermal shocks.

It is also an object of the present invention to provide for such a device which can be easily installed in a container for molten metal.

Summary of the invention With these objects in view, the device according to the present invention for measuring the oxygen content of a molten metal in a container mainly comprises an elecice trolytic cell having a solid electrolyte which is conductive for oxygen ions and including a block of refractory material mounted in the wall of the container with one face of the block in contact with the molten metal and another face insulated from the molten metal and being formed with a cavity extending from the other face into the block and having an end face spaced from and adjacent the one face of the block, an electrode disposed in the cavity, means for circulating an oxygen-containing gas through the cavity and about the electrode for maintaining a constant partial oxygen pressure in the cavity, a conductor insulated from the electrolyte and having an end in direct contact with the molten metal, means in circuit with the electrode and the conductor for measuring the voltage difference between said one face of said block and said end face of said cavity, the electrode and the conductor being carried by the block and the wall portion of the block between the face in contact with the molten metal and the end face of the cavity constituting the solid electrolyte of the cell.

The device may also have one or the other of the following characteristics in combination with those mentioned above.

(a) The device may include a passage for preheating the gas circulated through the cavity;

(b) The aforementioned preheating passage may be formed in the refractory block;

(c) The aforementioned preheating passage may be formed in the refractory block and be constituted at least in part by a passage communicating with the cavity in which the electrode is located and through which the gas is circulated;

(d) The aforementioned preheating passage may be filled with granules of solid electrolyte;

(e) The block of refractory material may have an outer surface of the form and dimensions of a pouring nozzle formed in the wall of the container;

(f) The device may also include a pair of thermocouples respectively measuring the temperatures of the electrode and of the molten metal and connected in circuit with the aforementioned measuring device in order to correct the value obtained by the latter for the tem-. perature difference between the electrode and the molten metal.

It is to be understood that the term containerfus ed in the specification and the claims may comprise not only a ladle or similar container in which a bath of molten metal is maintained, but may also comprise a pasage or channel through which the molten metal'continuously flows.

The present invention basically consists ina device for measuring the oxygen content of a liquid metal, especially of molten steel, by means of an electrolytic cell realized in a monolytic block of refractory material.

The electrolyte, constituted by a material conductive of oxygen ions, is accordingly a block of refractory solid electrolyte material in which at least one cavity is formed and one outer face of the block is in contact with the molten metal. The cavity contains an electrode connected with a measuring device for measuring the potential difference between the electrode and a conductor insulated from the electrolyte and in direct contact with the molten metal. An oxygen-containing gas is circulated through the cavity and the partial oxygen pressure of the gas is maintained constant so that the gas may serve as a reference gas to measure the oxygen content of the molten metal.

In a preferred embodiment according to the present invention, the refractory block is also formed with a preheating passage to preheat the reference gas. Finally, the device may also include known means, such as thermocouples, for measuring the temperature difference between the two faces of the block constituting the solid electrolyte, which means are connected in circuit with the measuring device to correct the measuring result obtained.

The monolytic block of refractory material will have a considerable resistance against the corrosive influence of the metal bath, especially molten steel and any slag forming on the bath and, in addition, the block will also have an excellent resistance to mechanical and thermal shocks.

In a preferred embodiment, the outer surface of the refractory block is formed in such a manner that it can be easily mounted in a wall of the container without requiring any essential reshaping of the latter. For this purpose, the block may have the form and exterior dimensions of a casting nozzle so that the opening already provided in the wall of the container may be used for mounting the block.

Since the electrolytic cell according to the present invention is incorporated in the wall of a container in which a bath of molten metal of high temperature is maintained, it is subjected to a high temperature gradient. Therefore, the electromotive force indicated by the cell is the algebraic sum of the electromotive forces, due on the one hand to the difierence in the partial oxygen pressure between the two faces of the wall of the block forming the solid electrolyte and on the other hand due to the thermoelectric or Peltier effect produced by the difference of the temperature of the face of the electrolyte which is in contact with the liquid metal and the end face of the cavity. The measurement of this temperature difference by known means permits an exact correction of the electromotive force indicated by the measuring .device to be effected to thus obtain an exact measuring result indicating the oxygen content of the molten metal.

The block constituting the solid electrolyte may in its entirety be formed from material which will conduct oxygen ions, for example, stabilized zirconia. In a preferred form of realization, the block is of frusto-conical shape to facilitate its mounting and its removal from the wall of a container when such a removal will be necessary due to repair of the lining of the container.

' It is well known that the electromotive force produced by such a cell is indicative of the oxygen content of the molten metal only when the cavity of the block in which the electrode is located contains a reference substance which maintains the partial oxygen pressure in the cavity constant. It, is for this reason that a reference gas is circulated through the cavity in which the electrode is located and the electrode is formed by a metal of good chemical stability such as platinum or platinum-rhodium. In a preferred modification, the cell comprises also a preheating passagefor the reference gas in order to bring the temperature of the latter to the operating temperature of the cell and the electrode; this preheating of the reference gas has the advantage of avoiding inopportune variations of the temperature of the electrode due to unforeseen variations in the delivery of the gas. Since the electric potential is a function of the temperature, the creation of erroneous potentials is thus avoided.

The conductor which. is in direct contact with the molten metal of high temperature, which forms the second electrode of the cell, may be constituted for example from soft iron or a mixture of ceramic and metal, especially CrCr O Themeans for measuring the temperature difference between the two faces of the solid electrolyte may, for instance, be constituted by a pair of thermocouples, one of which is located in the cavity and the other extends into the liquid metal and is protected in this case by an appropriate sheath.

The aforementioned temperature difference may then be obtained by a simple subtraction. When the electrode is formed from platinum, it may also constitute part of the one thermocouple for measuring the temperature in the cavity.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following descriptions of specific embodiments when read in connection with the accompanying drawing.

Brief description of the drawing FIG. 1 is a schematic sectional view of one modification of the device according to the present invention in which the essential measuring elements of the device are supported in a removable block; and

FIG. 2 is a schematic sectional view of another modification in which the block is formed with a passage for preheating the gas circulated through the cavity.

Description of the preferred embodiments Referring now to the drawings, and more specifically to FIG. 1 of the same in which one embodiment of the device according to the present invention is illustrated, it will be seen that the device includes a removably mounted block of zirconia 24 serving as a solid electrolyte of an electrolytic cell and also to carry the essential elements for measuring the oxygen content of the molten metal, for instance steel 23. The molten steel is maintained in a ladle or container of which only a portion of its wall 25 is illustrated in FIG. 1.

The monolytic block 24 is preferably of frustoconical shape to facilitate mounting of the block in a removable manner in an opening of the wall 25 so that the inner face 24' of the block is in contact with the molten metal 23 maintained in the container. The block 24 is formed with a cavity 26 extending from the outer face of the block into the latter, and having an inner end face 32 adjacent but spaced from the face 24' of the block which is in contact with the molten metal. A tube 27 of alumina extends into the cavity and the tube 27 is connected at its outer end by a conduit 28 to a supply of oxygen under pressure, not shown in the drawing. The remainder of the cavity 26, that is the space between the outer surface of the tube 27 and the inner surface defining the cavity, is filled with a powder of porous zirconia 29 which permits evacuation of the oxygen which serves as a reference gas and fed into the cavity through the tube 27. A platinum electrode 30 extending through the tube 27 is continued in the cavity by a wire 31 of the same material applied against the end face 32 of the cavity, and the outer end of the electrode 30 is connected to one end 33 of a resistance 34 of a value r. The inner end of the electrode is soldered at 35 to a wire 36 of a platinum-rhodium alloy containing 10% rhodium. The electrode 30 and the wire 36 thus form a thermocouple which permits measurement of the temperature in the interior of the cavity 26 or more exactly the temperature at the inner face of the solid electrolyte constituted by the block 24.

The upper portion of the block 24 is formed with an opening into which a tube 37 of alumina is cemented, which electrically insulates the block from a sheath of ceramic metal 38 (Cr O -Cr) which functions as conductor. A second sheathv 39 of alumina is placed in the interior of the sheath 38 insulating the conductive sheath 38 from a thermocouple located in the interior of the sheath 39 and composed of a platinum wire 40 and a wire 41 of a platinum-rhodium alloy containing rhodium and soldered at 42 to the wire 40. This thermocouple permits the measurement of the temperature of the molten metal 23, that is the temperature of the face 24' of the electrolyte 24.

The conductor 38 is connected at its outer end to the end 43 of a resistor 44 having a resistance R. The platinum wire 40 is directly connected to a voltmeter 45, whereas the platinum-rhodium wire 41 is connected to the platinum wire 36 located in the tube 27 and forming a thermocouple with the platinum electrode 30. The end 46 of the resistance 34 opposite the end 33 thereof is connected to the end of the resistance 44 opposite to the end 43 and further connected to the voltmeter 45. The ratio of the values of the resistances R/r is in the described embodiment, in which the solid electrolyte is formed by zirconia and the thermocouples are formed by a platinum wire connected to a platinum-rhodium wire with 10% rhodium, equal to 38.45, and the sum of the resistances R+r is in the order of megohms so that the cell will deliver only a small current.

In the arrangement as shown in the wiring diagram of FIG. 1, it results from the arrangement and from the value given to the resistances R and r that the total electromotive force of the cell, that is the electromotive force due to the Peltier eflect and to the potential difference due to the difference of the partial oxygen pressure at the two faces of the electrolyte, will be divided by 39.45, from which value the difference in millivolts of the signals obtained by the thermocouples is subtracted so that the electromotive force at the terminals of the voltmeter will be the potential difference due only to the difference of the partial oxygen pressure between the two faces of the solid electrolyte. In other words, by dividing the total electromotive force by the factor 39.45, millivolts coming from the thermocouple are obtained, and by subtracting the electromotive force arising directly from the temperature difference between the two faces of the electrolyte and measured by the two thermocouples, an electromotive force in millivolts is obtained which is directly proportional to the potential difference solely due to the difference in the partial oxygen pressure between the two faces of the solid electrolyte. Since this potential ditference is proportional to the Naperian logarithms of the concentration of oxygen in the molten metal, the voltmeters 45 can be graduated in such a manner that this value can be directly read on the scale of the voltmeter.

The following will explain further how the above result is obtained:

It is known (see Archiv fiir das Eisenhiitternwesen 1965, vol. 36, No. 9, p. 643) that the thermoelectric effect or Peltier effect of a union of stabilized zirconiaplatinum is equal to 0.47 mv./ C. (millivolts per degree centigrade). Therefore, when the dilference between the temperature T of the molten metal and the temperature T of the block is AT =T T the Peltier effect will be equal to V=0.47AT.

The described cell comprises an electrolyte of zirconia, a first electrode about which the reference gas, i.e., air, circulates and which has a temperature of less than 1,500 C., and a second electrode constituted by the molten metal which has a temperature in the order of 1,600 C. and a partial oxygen pressure which is a function of the activity of the oxygen contained in the molten metal.

In this case, the greater oxygen pressure and the lower temperature will occur in the cavity. The potential difference E between the electrode 30 and the metal 23 is equal to the electromotive force E due to the dilference of the partial oxygen pressure between the two faces of the electrolyte plus the electromotive force V due to the Peltier eifect: E =E+V. In order to obtain the value B, it is necessary to measure the potential difference E between the electrode aed the metal and to subtract therefrom the value V obtained by measuring the temperatures T and T One obains therefore:

The value E permits calculation of the partial oxygen pressure of the molten metal and deduction therefrom, according to the well known law of Nernst, the oxygen content of the molten metal.

In the described example an arrangement is used which permits us to directly obtain the electromotive force E by correcting the output value E in accordance with the temperature difference between the two faces of the electrolyte.

The two thermocouples 30, 36 and 40, 41 connected in circuit as shown in FIG. 1 permit us to obtain a signal Ae which is proportional to the temperature difference AT. In thermocouples formed by a platinum wire and a platinum-rhodium wire containing 10% rhodium the relation between AT and Ae is as follows:

By replacing the above value for AT in the Equation I the following equation is obtained:

0.047 EE OTjAE -39.45A6

which equation may also be written:

E E 39.45 39.45 A6 The division of the output value E by 39.45 is obtained by means of the two resistances 44 of the value R and 34 of the value r in which the ratio In FIG. 1 the potential difference between the point 33 and the right terminal of the voltmeter 45 is Ae (the thermocouples are connected in opposition) and the potential ditference between the two terminals is Since E is proportional to the Napierian logarithms of the concentration of the oxygen in the molten metal, the oxygen content of the molten metal can be directly read on an appropriate scale provided on the voltmeter.

In the embodiment shown in FIG. 2, the element 50 is constituted by a block of solid electrolyte 51, for example, zirconia stabilized with lime, in which two cavities 52 and 53 are formed which are connected with each other by a passage 54. The depth of the cavities 52 and 53 and the position of the passage 54 in the block determine the thickness of the active wall 55 of solid electrolyte which is in contact with the metal bath 56. In the present embodiment, the cavities and the passage are arranged to obtain a uniform thickness of the aforementioned wall of about 15 mm.

The cavity 52 is prolonged towards its rear by a tube 57 permitting the introduction of a reference gas, that is a ga containing oxygen, into the cavity and the cavity 53 is likewise prolonged towards its rear by a tube 58 which receives an electrode 59 the inner end of which is connected to a wire 59' in contact with the active wall portion 55 of the block. The electrode 59 and the wire 59' may for instance be formed from platinum and the inner end of the electrode 59 is again soldered to a wire 66 for instance formed from a platinum-rhodium alloy and forming with the electrode 59 a thermocouple.

The cavities 52 and 53, as well as passage 54 extending between the inner ends of these cavities, are filled with solid electrolyte granules 60 having a diameter in the order of 1 mm.

Air used as a reference gas is introduced under pressure into the tube 57 and passes through the cavity 52 and the passage 54 into the cavity 53 in which it flows about the electrode 59 and the air escapes then to the outer atmosphere through the tube 58.

In this way, the cold air supplied is heated due to contact with the walls of the cavities formed in the block and with the granules 60 filling the cavities so that the heated air will arrive at a constant temperature at the electrode 59.

The block 50 has preferably a frusto-conical shape and the dimensions thereof preferably correspond to those of a seat of alumina 61 fixed in the wall 62 of the container containing molten metal 56 and serving to usually receive a casting nozzle. The block 50 is maintained in place by a seal 63 of alumina cement.

In view of the corresponding shapes of the block 50 and the seat 61, the replacement of the block can be carried out in a simple and expedient manner, for instance during two casting periods, since the block can be placed in its seat and sealed therein from the outside of the container.

The electrode 59, preferably made of platinum, preferably forms one branch of a thermocouple introduced into the cavity 53, as described in connection with the electrode 30 shown in FIG. 1, that is, the inner end of the electrode 59 is soldered to a wire 59 preferably of the same material as the electrode and abutting against the inner end face of the cavity 53 as well as to a wire 66, preferably of an alloy of platinum and rhodium containing of rhodium and forming with the electrode 59 a thermocouple for measuring the temperature at the inner end face of the cavity.

The second pole 64 of the cell may be formed by an iron rod or a rod of Cermet, a mixture of chrome oxide and chrome, extending through a bore in the block 50 so that its inner extremity is in contact with the molten metal 56 in the container. The rod 64 is sealed in the bore of the block and electrically insulated therefrom by a tube 67 of insulating material, for instance alumina. The block carries further a second thermocouple 65 located properly sealed in another bore through the block and extending with its inner extremity into the molten metal 56 adjacent to the inner face of the block so as to sense the temperature of the molten metal adjacent to this face. The two wires 65 and 65" of the thermocouple 65 are insulated from the block by a thin sheath of insulating material, for instance alumina, as described in connection with FIG. 1. The wire 66 of the first thermocouple is connected in series with the wire 65" of the second thermocouple whereas the wire 65 of the second thermocouple is connected to one terminal of a voltmeter, and the electrode 59 as well as the conductor 64 are again connected over resistances 34 and 44 to the other terminal of the voltmeter as shown in the wiring diagram of FIG. 1.

It is to be understood that the various elements as described with reference to FIG. 1 may also be adapted to be used in a cell having a preheating passage for the reference gas.

The types of cells described have various advantages.

First of all, the assembly of elements permitting the measurement of oxygen content of a molten metal is carried by a single member which is a monolytic block.

A further advantage is that this block is stationarily mounted in an opening of the container wall and can be removed very easily without requiring relining of the container containing the molten metal.

A further advantage is that by means of the disclosed device, it is possible to continuously measure the temperature difference between the two faces of the electrolyte.

An additional advantage resides in the fact that the connection of the various elements carried by the block to the measuring device is extremely simple, and that the arrangement permits the direct measurement of the electromotive force produced by the difference of the temperature between the two faces of the electrolyte and its direct substraction from the electromotive force produced by the potential difference at the two faces of the electrolyte.

The device according to the present invention has been successfully used in installations for the continued refinement of molten metal. As all the measuring elements are mounted in only one removable block, the operations for setting and discarding the device are considerably simplified.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of devices for measuring the oxygen content of molten metals differing from the types described above.

While the invention has been illustrated and described as embodied in a device for measuring the oxygen content of a molten metal including a cell of solid electrolyte formed by a monolytic block of refractory material one face of which is in contact with the molten metal, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

1. A device for measuring the oxygen content of molten metal in a container comprising, in combination, a block of refractory solid electrolyte material removably mounted in a wall of the container with one face of said block in contact with the metal in the container and being formed with a cavity having an end face adjacent but spaced from said one face of the block and out of contact with the metal; means communicating with the cavity for circulating an oxygen-containing reference gas therethrough for maintaining in the cavity a constant partial oxygen pressure; an electrode carried by said block in said cavity surrounded by the reference gas circulated through the cavity and having an end in contact with said end face; a conductor carried by said block insulated from said electrolyte and having an end adjacent said one face in direct contact with said molten metal, said block together with said electrode around which the reference gas is circulated and said conductor forming an electrolytic cell with a solid electrolyte formed by the wall portion of the block between said faces, one of which is in contact with the molten metal and the other with the reference gas; and means in circuit with said electrode and with said conductor for measuring the potential difference between said faces of said block.

2. A device as defined in Claim 1, and including means for preheating the reference gas.

3. A device as defined in claim 2, wherein said means for preheating the reference gas is in the form of a passage in said block communicating with said cavity.

4. A device as defined in claim 3, wherein said preheating passage is filled with granules of solid electrolyte.

5. A device as defined in claim 3, wherein said preheating passage and said cavity are filled with granules of solid electrolyte.

6. A device as defined in claim 1, wherein said conductor extends through a bore in said block and including insulating means surrounding the portion of said conductor in said bore for electrically insulating said conductor from said block and for sealing said conductor in said bore.

7. A device as defined in claim 1, wherein said block is of frusto-conical shape and mounted in an opening of the wall of the container of corresponding configuration and adapted to serve as a casting nozzle.

8. A device as defined in claim 1, wherein caid block is formed from zirconia.

9. A device as defined in claim 8, wherein said elec trode is formed from platinum.

10. A device as defined in claim 9, wherein said conductor is formed from Cermet.

11. A device as defined in claim 9, wherein said conductor is formed from iron.

12. A device as defined in claim 1, and including a pair of thermocouples carried by said block, one arranged to measure the temperature at said end face of said cavity 15 and the other arranged to measure the temperature of the molten metal in the region of the face of the block in contact with the metal, said pair of thermocouples being connected in circuit with said electrode and said conductor to said measuring means.

References Cited UNITED STATES PATENTS 3,078,412 2/1963 Blake 32464 3,147,433 9/1964 Claussen 324-71 10 3,317,822 5/1967 Watson 324 32 EDWARD E. KUBASIEWICZ, Primary Examiner US. Cl. X.R.

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