METHOD OF AND DEVICE FOR MEASURING OXYGEN ACTIVITY IN SLAG AND CONSUMABLE TYPE CRUCIBLE USED FOR SAID DEVICE

Abstract: This invention relates to a method and device permitting measurement of oxygen activity in the molten pig iron slag and molten steel slag with high precision. According to the conventional method of measurement depending on the oxygen-concentration cell composed of a one-end-closed tube type solid electrolyte fixed to the tip of the probe, having a contrasting platinum electrode wound around the outer surface thereof, and located directly in the molten slag layer, positioning of the cell in the flowing slag is difficult and insufficient contact of the contrasting electrode with the slag layer is caused on account of thermal contraction of the electroyte resulting from heat of the slag, whereby effort possibly occur in measurement. Unlike the direct measurement of oxygen activity in the slag, this invention provides a method in which a specific metal whose oxygen activity equilibrates with that of the slag when brought into contact with the slag is contained in the same vessel together with the slag and oxygen activity in the specific metal is measured for indirect measurement of oxygen activity in the slag, whereby the problem resulting from insufficient contact of the solid electrolyte with the contrasting electrode can be solved and measurement of oxygen activity in the slag with high precision is ensured.

Description: Technical Field The present invention relates to a method of measuring the oxygen activity of the slag in molten iron or molten steel with high accuracy which are produced in the steel making process, a measuring apparatus used for the method and a disposable crucible used for the apparatus. Background Art A method is conventionally known of measuring the oxygen activity of the slag in molten iron or molten steel by winding a platinum contact electrode around the outer surface of a solid electrolyte of one end closed tube type in a close contact state or coating the outer surface thereof with a platinum contact electrode to prepare an oxygen concentration cell, embedding the oxygen concentration cell in the end of the probe, and disposing the probe directly in the molten slag layer which is existent in the upper layer of the molten steel in a converter or a ladle during operation. Since the surface of the slug vehemently flows, however, it is difficult to exactly locate the oxygen concentration cell, which serves as a detecting portion, in the slag, so that the ratio at which measurement succeeds is low. Since the detecting portion is exposed to a high temperature for a long time, a firm heat-resistant material is necessary, so that the probe becomes thick and heavy, and difficult to handle. In addition, it is necessary to bring platinum into contact with the surface of the solid electrolyte in order to produce the oxygen concentration cell in the above-described form. Since the solid electrolyte is densified by the heat of the slag during operation, it is difficult to constantly maintain the contact state and the defective contact between the solid electrolyte and the contact electrode is produced, which results in production of an error in measurement. Furthermore, although it is necessary to stop the progress of the steelmaking process for measurement, it is actually impossible to stop the steelmaking process for a long time. It is therefore impossible to use a probe-type oxygen measuring apparatus in an industrial furnace. Accordingly, it is an object of the present invention to solve these problems in the prior art and to provide a method of measuring the oxygen activity of slag which is capable of measuring the oxygen activity of a slag with high accuracy and an oxygen activity measuring apparatus which is used for the method and which is very easy to handle. Disclosure of the Invention The present inventor thought of measuring the oxygen activity of a slag not directly but indirectly by selecting a specific metal between which and a slag an oxygen equilibrium is established when it comes into contact with the slag, accommodating the metal and the slag in the same container and measuring the oxygen activity of the metal. By materializing this idea, the arrangement of the contact electrode in close contact with the surface of the solid electrolyte is obviated and the problem caused by the defective contact between the solid electrolyte and the contact electrode is solved. In a first aspect of the present invention, there is provided a method of measuring the oxygen activity in a slag comprising the steps of: dissolving a specific metal which is hardly alloyed with iron and hardly produces an oxide in an inert atmosphere and which has a larger specific gravity than the slag in a steel crucible; immersing a solid electrolyte of one end closed tube type with a reference electrode provided therewithin in the specific metal and bringing a contact electrode into contact with the specific metal so as to form an electric closed circuit between the reference electrode and the contact electrode through the specific metal; and measuring the oxygen activity of the specific metal. In a second aspect of the present invention, the above-described measuring method is materialized and there is provided an apparatus for measuring the oxygen activity in a slag comprising: a steel crucible disposed in a smelting furnace which is filled with an inert gas and accommodation the specific metal and the slag therein; and a solid electrolyte provided with a reference electrode therewithin and immersed in the slag; wherein the electromotive force between the steel crucible and the reference electrode is measured with the steel crucible as the contact electrode. As the smelting furnace for dissolving the slag and the specific metal in the steel crucible, it is possible to use a resistance furnace or a high frequency induction furnace, but a condensing radiation furnace is preferred from the point of view of high-speed heating. As a method of holding the steel crucible, it is possible to place the steel crucible on a pedestal or to vertically slidably hang the steel crucible. In the former case, it is also possible to use a conductive pedestal so that the pedestal also serves as a lead wire from the steel crucible. In the latter case, it is also possible to use a removable metal hanger so that the metal hanger also serves as a lead wire from the steel crucible. It is possible to dissolve and solidify the specific metal on the inner surface of the steel crucible in advance and replace the steel crucible for each measurement in place of inserting the specific metal in the steel crucible at the time of measurement. The oxygen activity in a slag is measured in the following manner by using the apparatus of the present invention. A metal which is not alloyed with iron and hardly produces an oxide in an inert atmosphere, namely, the specific metal is accommodated in the steel crucible in a solidified state and a slag being measured is charged in the steel crucible. The steel crucible is then accommodated in a furnace which is filled with an inert gas and heated so as to melt the slag and the specific metal. A solid electrolyte of one end closed tube type with a reference electrode provided therewithin is then passed through the slag layer in the molten state so as to be immersed in the specific metal. By inferring the oxygen activity in the specific metal, the oxygen activity in the slag is measured. Since the specific metal is not alloyed with iron and does not produce an oxide, an oxygen equilibrium is established between the slag and the specific metal and the oxygen activity in the specific metal has a correlation with the oxygen activity in the slag. It is therefore possible to measure the oxygen activity in the slag from the oxygen activity in the specific metal. In addition, since the special metal has a conductivity unlike the slag, which is an insulant, an electric closed circuit is formed between the reference electrode and the contact electrode, so that it is possible to obtain the oxygen activity in the specific metal by measuring the electromotive force between the reference electrode and the steel crucible. In the case of using a condensing radiation furnace as the heat source for the smelting furnace, high-speed melting is possible, thereby greatly shortening the time required for the measurement of the oxygen activity. In the case of vertically slidably hanging the steel crucible as a method of holding the steel crucible, it is possible to set and take out the steel crucible together with the solid electrolyte from above. In the case of using a conductive metal hanger or a pedestal which is provided separately from the steel crucible, since these members can also function as a lead wire from the steel crucible, it is unnecessary to provide a connecting terminal for leading the electromotive force on the steel crucible. If a disposable crucible is provided by dissolving and solidifying the specific metal on the inner surface of the steel crucible in advance, the trouble of inserting the specific metal in the steel crucible for each measurement is saved, and since the steel crucible once used is discarded, there is no slag remaining in the steel crucible as in the case of repetitive use, thereby enabling measurement with high accuracy. Brief Description of the Drawings Fig. 1 is a schematic explanatory sectional view of an embodiment of an apparatus for measuring the oxygen activity in a slag according to the present invention; Fig. 2 is an explanatory view of the structure of the embodiment shown in Fig. 1 and peripheral devices; Figs. 3(I) and 3(II) are explanatory views of other examples of the steel crucible and the pedestal; and Fig. 4 is another embodiment of an apparatus according to the present invention which adopts a steel crucible hanging system. Best Mode for Carrying out the Invention The details of the present invention will now be explained with reference to the illustrated embodiments. Fig. 1 is a schematic explanatory sectional view of the structure of an oxygen activity measuring apparatus according to the present invention. In Fig. 1, the reference numeral 1 represents a reaction tube composed of a heat-resistant material such as alumina, mullite and quartz. The reaction tube 1 is situated at the center of a furnace body 2, which is the heat source in a smelting furnace. As the smelting furnace, it is possible to use a condensing radiation furnace, a high frequency induction furnace or a resistance furnace. A condensing radiation furnace is especially preferred. A condensing radiation furnace is advantageous in that since the temperature control is easy, the time required for raising the temperature is short and strong heating is possible, it is possible to melt a slag in a short time. The reaction tube 1 is hermetically sealed in the vertical direction by a bottom plate 3 and a cover 4, and the bottom plate 3 is provided with a gas inlet 5 for introducing an inert gas such as nitrogen gas and argon gas to the interior of the reaction tube 1, while the cover 4 is provided with a gas outlet 6 for discharging the inert gas from the interior of the reaction tube 1. The reference numeral 7 represents a pedestal for placing the steel crucible thereon, and a temperature element 8 is disposed in the pedestal 7 with a temperature sensing portion 9 in contact with the upper end surface of the inner surface of the pedestal 7. The lower end of the temperature element 8 is led out of the reaction tube 1 through the bottom plate 3, and lead wires 10 led out of the temperature element 8 are connected to a temperature measuring instrument 11. The reference numeral 12 represents a steel crucible placed on the pedestal 7 in contact therewith. The steel crucible 12 accommodates a slag S, which is the object of measurement, and a specific metal M. As the specific metal M, any metal may be adopted that is not alloyed with iron and hardly produces an oxide and that has a larger specific gravity than the slag S. For example, silver and copper are usable. In this embodiment, silver is used. The slag S and the specific metal M are melted by the heat of a smelting furnace, and a one end closed tube type solid electrolyte 13 with a reference electrode (not shown) provided therein is immersed in the molten specific metal M. A lead 14 on the reference electrode side which is led from the solid electrolyte 13 is introduced to the outside of the reaction tube 1 through the cover 4 of the reaction tube 1. On the other hand, a lead wire 15 is led out of the pedestal 7 which is electrically conductive with the steel crucible 12 through the bottom plate 3, thereby constituting a lead on the contact electrode side. Thus, an oxygen concentration cell is formed between the lead on the reference electrode side and the lead on the contact electrode side. A measuring instrument 16 for measuring the electromotive force produced by the oxygen concentration cell is provided between the lead on the reference electrode side and the lead on the contact electrode side. The oxygen activity measuring apparatus having the above-described structure is used after peripheral devices are arranged therearound so as to constitute a complete measuring apparatus, as shown in Fig. 2. The symbol A represents a smelting furnace which accommodates the reaction tube 1 and the furnace body 2, and an elevating equipment B for elevating the solid electrolyte 13 is disposed above the smelting furnace A. The symbol C represents an elevating equipment control panel for controlling the elevating equipment B. An inert gas is supplied to the reaction tube 1 disposed in the smelting furnace A by using a inert gas cylinder D disposed outside of the smelting furnace. and the environment in the furnace including the furnace temperature is controlled by a furnace control panel E. The temperature obtained from the temperature element 8 and the electromotive force produced by the oxygen concentration cell is input to a processor unit F having the measuring instruments therein and processed. The oxygen activity measuring apparatus having the above-described structure is used in the following manner. The slag S and the specific metal M which are collected from a converter and a ladle are first charged into the steel crucible 12. At this time, although the specific metal M is in a solidified state, the slag S may be either in the solidified state or in the molten state. It goes without saying, however, that use of the slag S in the molten state is preferred from the point of view of shortening the measuring time. The steel crucible 12 is then accommodated in the reaction tube 1 by removing the bottom plate 3 to take out the pedestal 7, placing the steel crucible 12 on the pedestal 7 in contact therewith, and inserting the pedestal 7 with the steel crucible 12 placed thereon into the reaction tube 1 from below. When the steel crucible 12 is disposed in the reaction tube 1 in this way, an inert gas is poured from the gas inlet 5 to fill the reaction tube 1 with the inert gas, and the smelting furnace A is then operated to rapidly heat the interior of the reaction tube for the purpose of melting the slag S and the specific metal M. The specific metal is preferably rapidly heated. Especially, by using a condensing radiation furnace as the smelting furnace A, it is possible to rapidly raise the temperature and, in addition, a fear of producing noise which influences the instruments is precluded unlike a high frequency induction furnace. After the slag S and the specific metal M have been molten, the solid electrolyte 13 is lowered from above the steel crucible 12 and passed through the slag layer, and the end portion of the solid electrolyte 13 is immersed in the specific metal M. In this state, an oxygen concentration cell is formed between the reference electrode and the steel crucible, which is the contact electrode, with the solid electrolyte 13 and the special metal M therebetween, and the electromotive force produced by the oxygen concentration cell is measured by the instrument 16 which is connected between the lead on the reference electrode side and the lead on the contact electrode side. The temperatures of the slag S and the specific metal M are constantly measured by the temperature element 8 provided in the pedestal 7 and indicated to the temperature measuring instrument 11. By processing these values, the oxygen activity of the specific metal are calculated. Since a metal which is not alloyed with iron and hardly produces an oxide is used as the special metal M, an oxygen equilibrium is established between the specific metal M and the slag S and a correlation holds between both oxygen activities. It is thus possible to obtain the oxygen activity in the slag S on the basis of the oxygen activity in the specific metal M. When the measurement of the oxygen activity is completed, the inert gas in the reaction tube 1 is discharged from the gas outlet 6. Thereafter, the steel crucible 12 is taken out from below the reaction tube 1, thereby completing the measurement process. When a condensing radiation furnace is used as the smelting furnace A, since it is possible to raise the temperature in a short time, the time required until the steel crucible 12 is taken out since it is accommodated in the reaction tube 1 is about 1 to 15 minutes. In this way, measurement of the oxygen activity is enabled very quickly. In order to maintain the measuring accuracy of this apparatus, it is necessary to regularly calibrate the apparatus by inserting a standard in place of the slag. As the standard, a pure ferrous oxide or a mixture of ferrous oxide with a halide of alkaline earth metals and/or a halide of alkaline metals is used in this embodiment. It is naturally possible to use another standard so long as it has a constant oxygen activity. The steel crucible 12 and the pedestal 7 shown in Fig. 1 have a structure in which the steel crucible 12 comes into contact with a flat mounting surface of the pedestal 7, but the steel crucible 12 and the pedestal 7 may have different configurations. For example, it is also preferable that the bottom surface of a steel crucible 12a and the mounting surface of a pedestal 7a are undulate, as shown in Fig. 3(I), thereby ensuring the holding of the steel crucible 12 and electric contact therebetween. It is also possible to combine a steel crucible 12b having a tapered under surface and a cylindrical pedestal 7b having a tapered upper edge, as shown in Fig. 3(II). Fig. 4 shows another embodiment in which a steel crucible 12c is hung and the pedestal is dispensed with. A retaining flange 17 is formed on the outer periphery of the opening of the steel crucible 12c, and the steel crucible 12c is hung by a plurality of metal hangers 18 which are hung down from a cover 4c in the form of arms. The steel crucible 12c and the metal hangers 18 have a different engaging structure and the configurations of the steel crucible 12c and the metal hangers 18 are appropriately selected in accordance with the engaging structure. If such a steel crucible hanging system is adopted, since the steel crucible 12c and the solid electrolyte 13 can be taken out of the reaction tube 1 merely by elevating the cover 4c, the trouble of taking out the steel crucible from below the reaction tube 1 and taking out the solid electrolyte from above the reaction tube 1 is omitted, thereby greatly simplifying the take-out operation and setting operation. As another example of a steel crucible, it is possible to produce a disposable type steel crucible by melting and solidifying a specific metal such as silver on the inner surface of the steel crucible, and use a new disposable type steel crucible for each measurement. In this way, not only is the time for charging the specific metal in the steel crucible for each measurement saved but also since no slag other than the slag which is the object of measurement, for example, the slag subjected to the precedent measurement, remains in the steel crucible, measurement of oxygen activity with higher accuracy is enabled. Industrial Applicability As described above, according to the method and apparatus for measuring the oxygen activity in a slag of the present invention, a specific metal between which and a slag, which is the object of measurement, an oxygen equilibrium is established is selected and the specific metal is charged in a steel crucible together with the slag so as to measure the oxygen activity in the slag by measuring the oxygen activity in the specific metal. It is thus possible to measure the oxygen activity in the slag by measuring the electromotive force produced by the oxygen concentration cell formed between the lead on the reference electrode side and the lead on the contact electrode side through the specific metal. Since the solid electrolyte is situated in the specific metal in an immersed state, the contact between the specific metal and the solid electrolyte is complete. In addition, since the steel crucible is used as the contact electrode, the contact between the specific metal and the contact electrode can be made complete, thereby enabling measurement with high accuracy. In the case of using a condensing radiation furnace as the smelting furnace, since high-speed heating is easy and temperature control is also easy, it is possible to greatly shorten the measuring time and, in addition, noise which influences the instruments is not produced unlike a high frequency induction furnace. In the case of adopting a steel crucible hanging system, it is possible to take out both the steel crucible and solid electrolyte from above the reaction tube, thereby greatly facilitating the setting operation and take-up operation. Furthermore, in the case of adopting a steel crucible hanging system, the hanging metal can serve as the lead on the contact electrode side, and in the case of adopting a system of placing the steel crucible on the pedestal, the pedestal can serve as the lead on the contact electrode side. In either case, the structure of the apparatus can be simplified without the need for providing a lead wire separately. In the case of producing a disposable type steel crucible by melting and solidifying the specific metal in the steel crucible in advance, it is unnecessary to charge the specific metal for each measurement. It is possible to save the trouble of inserting the specific metal or to prevent the specific metal from being left in the steel crucible after measurement. In addition, since a new steel crucible is used for each measurement, it is possible to prevent a slag other than the slag which is the object of measurement for mixing therewith.

Claims: A method of measuring the oxygen activity in a slag comprising the steps of: melting a specific metal which is not alloyed with iron and hardly produces an oxide in an inert atmosphere and which has a larger specific gravity than a slag, which is the object of measurement, together with said slag in a steel crucible; immersing a solid electrolyte of one end closed tube type with a reference electrode provided therewithin in said specific metal and bringing a contact electrode into contact with said specific metal so as to form an electric closed circuit between said reference electrode and said contact electrode through said specific metal; and measuring the oxygen activity of said specific metal. An apparatus for measuring the oxygen activity in a slag according to the method of claim 1 comprising: a steel crucible accommodated in a smelting furnace filled with an inert gas; a specific metal which is not alloyed with iron and hardly produces an oxide in an inert atmosphere, which has a larger specific gravity than a slag, which is the object of measurement, and which is charged in said steel crucible in a molten state together with said slag; and a solid electrolyte of one end closed tube type with a reference electrode provided therewithin which is immersed in said specific metal; means for measuring the electromotive force between said steel crucible which also serves as a contact electrode and said reference electrode. An apparatus for measuring the oxygen activity in a slag according to Claim 2, wherein said smelting furnace is a condensing radiation furnace. An apparatus for measuring the oxygen activity in a slag according to Claim 2, wherein said steel crucible and said solid electrolyte are vertically slidably hung down in said smelting furnace. An apparatus for measuring the oxygen activity in a slag according to Claim 2, further comprising a conductive pedestal provided below said steel crucible,said pedestal also serving as a lead wire from said steel crucible which also serves as said contact electrode. An apparatus for measuring the oxygen activity in a slag according to Claim 2, further comprising a metal hanging for removably hanging said steel crucible, said metal hanging also serving as a lead wire from said steel crucible which also serves as said contact electrode. An apparatus for measuring the oxygen activity in a slag according to Claim 2, wherein said specific metal which is charged in said steel crucible together with said slag is silver. An apparatus for measuring the oxygen activity in a slag according to Claim 2, wherein a pure ferrous oxide or a mixture of ferrous oxide with a halide of alkaline earth metals and/or a halide of alkaline metals is used as a material for calibrating the measuring accuracy of said oxygen activity. A disposable crucible used for an apparatus according to claim 2 for measuring the oxygen activity in a slag comprising: a steel crucible; and a specific metal which is not alloyed with iron and hardly produces an oxide, which has a larger specific gravity than said slag, and which is melted and solidified on the inner surface of said steel crucible in advance.

Inventors: FUJIWARA RYUJI, IWASE M -GO KYOTO DAIGAKU KUMA, MOCHIZUKI RYO, FUJIWARA, RYUJI, IWASE, M., MOCHIZUKI, RYO, IWASE, M. 216-GO, KYOTO DAIGAKU KUMANO SHOKUIN
Applicants: HERAEUS ELECTRONITE CO LTD, HERAEUS ELECTRONITE CO., LTD.
Application year: 1990
Main classification: G01N27
IPC codes: G01N27, G01N33