JP2009068856A - Carbon activity measuring probe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a practical carbon activity measuring probe having a mixed sub-electrode, easy to manufacture, good in measuring precision and having excellent durability causing no quality deterioration even if the probe is preserved in air. <P>SOLUTION: The sub-electrode 23 comprising magnesium carbonate (MgCO<SB>3</SB>) or a mixture of magnesium carbonate and magnesium oxide (MgO) is provided on the outer surface of a solid electrolyte pipe 13 having oxygen ion conductivity to constitute a standard electrode element (sensor element 3) becoming a pair along with a counter electrode (acting electrode 25). Local equilibrium is established between carbon and oxygen in a molten metal and magnesium carbonate constituting the sub-electrode or the mixture and the activity of oxygen in a local equilibrium layer is measured to measure the activity of carbon in the molten metal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a carbon activity measurement probe for quickly and accurately measuring the carbon concentration in molten metal in a metal refining process, particularly the carbon activity in molten iron in a steel refining process. The present invention relates to a practical carbon activity measuring probe that is excellent in durability without deterioration in quality even after long-term storage.

  In the metal refining process, the carbon content in the molten metal can be measured by pumping the molten sample, cooling and solidifying it to make a solid sample for instrument analysis, or until the molten sample is pumped to become solid. There is a CD method that determines the carbon concentration by performing thermal analysis with a thermocouple using the gap between the two, but instrumental analysis has the disadvantage that it takes a lot of time from sampling to obtaining the analysis result, and the CD method is However, when elements other than carbon coexist, they are affected, and when the carbon concentration is low, the accuracy is deteriorated. Another disadvantage is that it takes time to pump the sample and wait for coagulation.

  As a method for overcoming these drawbacks, a method for estimating a carbon concentration by directly using an oxygen sensor has been proposed (for example, see Non-Patent Document 1). Since this method does not solidify the molten iron, the carbon concentration in the molten iron can be measured quickly. However, since each device is affected by the individuality of the refining device, different calibrations are required for each device. In order to overcome the above disadvantages, a probe for measuring carbon activity in molten iron having a mixed sub-electrode has been proposed (see, for example, Patent Document 1). In this patent document 1, metal carbide and oxide are mixed to form a mixed sub-electrode. However, the carbide is generally hard, and an appropriate particle size for obtaining a mixed sub-electrode is obtained. However, there are drawbacks in that it is difficult to produce a probe and there is a slight inaccuracy due to the difficulty of production.

  In order to solve the disadvantages of the probe having a mixed subelectrode using carbide, it is possible to use a mixture of a metal carbonate and a metal oxide made of the same metal as the metal constituting the carbonate as a subelectrode constituent material. It has been proposed (see, for example, Patent Document 2). However, the metal carbonates and metal oxides mentioned in Patent Document 2, especially sodium carbonate and sodium oxide, have the property of causing deliquescence or defoliation when stored in the air, and long-term storage of the probes on which they are prepared. It is a cause that makes it difficult to use and practically difficult to use.

US Pat. No. 5,393,403 JP-A-2005-43297 Edited by Steelmaking Sensor Subcommittee, Steelmaking 19th Committee, “New development of sensors for steelmaking”, Japan Society for the Promotion of Science, 1989, p. 4.88-p. 4.93

  Therefore, in view of the above-mentioned situation, the present invention intends to solve the carbon activity measuring probe having the mixed sub-electrode, which is easy to manufacture and has a high measurement accuracy. The object is to provide a practical carbon activity measuring probe having excellent durability without deterioration.

  In order to solve the above-mentioned problems, the present invention provides a carbon activity measuring probe for measuring carbon activity in a molten metal, wherein a sub-electrode made of magnesium carbonate or a mixture of magnesium carbonate and magnesium oxide is provided with oxygen ion conductive material. By providing it on the outer surface of the solid electrolyte having the property, it constitutes a standard electrode element that is paired with the counter electrode, and between the carbon in the molten metal, oxygen, and the magnesium carbonate or mixture constituting the sub electrode. A carbon activity measuring probe characterized in that the carbon activity in the molten metal is measured by establishing local equilibrium and measuring the oxygen activity in the local equilibrium layer.

  Here, the constituent mass ratio of magnesium oxide in the mixture (magnesium oxide mass / (magnesium carbonate mass + magnesium oxide mass)) is preferably 0.9 or less.

  According to the present invention as described above, since the sub-electrode is composed of magnesium carbonate or a mixture of magnesium carbonate and magnesium oxide, the sub-electrode can be easily and inexpensively manufactured, and it has excellent measurement accuracy and is manufactured. Even if it is stored in the air for a long time from start to use, a practical carbon activity measurement probe with excellent durability and no deterioration in quality is obtained without deliquescence or wind deflation of the secondary electrode. It is done.

  Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  1 and 2 show typical embodiments of a probe for measuring carbon concentration according to the present invention, in which reference numeral 1 denotes a probe, 2 denotes a carbon sensor, and 3 denotes a sensor element.

As shown in FIG. 1, the probe for measuring carbon concentration 1 of the present invention has a sub-electrode 23 made of a mixture of magnesium carbonate (MgCO ₃ ) and magnesium oxide (MgO) as a solid electrolyte tube 13 having oxygen ion conductivity. By providing on the outer surface, a standard electrode element (sensor element 3) that is paired with a counter electrode (working electrode 25) is formed, and between the carbon and oxygen in the molten metal and the mixture constituting the sub electrode It is characterized in that the carbon activity in the molten metal is measured by establishing local equilibrium and measuring the oxygen activity in the local equilibrium layer. In the following description of the embodiment, an example in which the sub-electrode 23 is formed of the above mixture will be described. However, the sub-electrode 23 can be formed of only magnesium carbonate (MgCO ₃ ). Magnesium carbonate has a decomposition temperature of 600 ° C. (see Iwanami Rikagaku Dictionary 4th edition, p761), and the molten metal temperature when using the carbon concentration measurement probe 1 of this case is usually 1200 to 1700 ° C. Even if it is composed only of magnesium carbonate, it decomposes during measurement to produce magnesium oxide, which is the same as that composed of the above mixture. As is well known, the probe 1 is configured by attaching a sensor element 3 and the like, which will be described later, to a paper tube or the like. However, since the basic configuration of the probe 1 is described here, the paper tube is not shown. .

  More specifically, the sensor element 3 includes a cap or cover 5 made of a hollow quartz tube having one end (upper side in the figure) opened and the other end (lower side) closed. The quartz cap 5 has two circular openings 7 and 9 formed in a state facing the side wall at a predetermined position in the axial direction, and one circular opening formed in the lower closed end portion. 11. The solid electrolyte tube 13 is a one-end-closed tube having oxygen ion conductivity such as zirconia that constitutes a known oxygen sensor. The solid electrolyte tube 13 includes a metal serving as the standard electrode 15 and its metal. A predetermined amount of a mixture made of the oxide is filled, and one end of a standard electrode lead wire 17 is inserted into and connected to the standard electrode 15.

  The solid electrolyte tubes 13 are arranged concentrically in the quartz cap 5 and are fixed to the housing 19 by high-temperature adhesives 21 respectively. The lower end of the solid electrolyte tube 13 extends below the lower edge of the openings 7 and 9 of the quartz cap 5, and the upper surface of the mixture constituting the standard electrode 15 is above the upper edge of the openings 7 and 9. The amount is enough to be positioned. The lower surface of the high temperature adhesive 21 does not reach the lower edge of the openings 7 and 9. The quartz cap 5 is filled with a sub-electrode material made of a mixture of magnesium carbonate and magnesium oxide constituting the sub-electrode 23 up to the position of the lower edge of the openings 7 and 9, and the lower portion of the solid electrolyte tube 13. Is embedded in the quartz cap 5, and a void 24 is defined above the mixed sub-electrode 23 in the quartz cap 5.

  The working electrode 25 is connected to a working electrode lead wire 27, and the working electrode 25, the working electrode lead wire, the solid electrolyte tube 13, the standard electrode 15, and the standard electrode lead wire 17 constitute a conventional oxygen sensor 12. Further, the carbon sensor 2 including the sensor element 3 and the working electrode 25 is configured by further including the quartz cap 5 and the mixed sub-electrode 23. As shown in FIG. 2, the carbon sensor 2 and the thermocouple 31 are combined to constitute the carbon concentration measurement probe 1. The carbon activity in the molten steel can be known by inserting the carbon concentration measuring probe 1 into the molten metal and knowing the electromotive force of the oxygen sensor and the electromotive force of the thermocouple.

That is, when the carbon probe is immersed in the molten iron, the molten iron enters the space 24 from the openings 7 and 9 of the quartz cap 5. A local equilibrium is formed between the carbon in the molten iron that has entered the void 24, the oxygen in the molten iron, and the sub-electrode 23. The local equilibrium reaction between magnesium carbonate (MgCO ₃ ), magnesium oxide (MgO), carbon (C) in molten iron, and oxygen (O) in molten iron constituting the sub-electrode is expressed by equation (1). The equilibrium constant K ₁ is expressed by equation (2).
MgCO ₃ = MgO + C +2 O (1)
_{K 1 = (a MgO × a} C × a O 2) / a MgO3 ··· (2)

Here, C is the carbon in the molten iron, O means each oxygen in molten iron shows _{a MgO, a C, a O} , a MgO3 respectively MgO, C, O, the activity of MgCO _3. When pure substances are used for oxides and carbonates, the respective activities are 1. In this case, equation (2) is
a _C × a _O = 1 / K ₁ (3)
It becomes. Since K ₁ is a constant when the temperature is determined, the carbon activity and the oxygen activity have a one-to-one correspondence when the temperature is constant. Therefore, if the activity of oxygen can be known, the activity of carbon can be known.

The following equilibrium relationship exists between oxygen in molten iron and oxygen measured by an oxygen sensor.
2 O = O ₂ (4)
Here, O is oxygen in the molten iron, and O ₂ is oxygen that can be measured by an oxygen sensor. The equilibrium constant K ₄ of the reaction of the formula (4) is expressed by the formula (5).
K ₄ = P _o2 / a _O ² (5)
Here, P _o2 is the oxygen partial pressure.

The relationship of the equation (6) among the oxygen partial pressure P _o2 (W) in the molten iron, the oxygen partial pressure P _o2 (R) at the reference electrode, the temperature T, and the electromotive force EMF of the oxygen sensor There is.
_{EMF = RTln (P o2 (W} ) / P o2 (R)) ··· (6)
In equation (6), R is a gas constant (R = 8.3144J / (mol · K), F is a Faraday constant (F = 96500J / (V · mol)), T is the Kelvin temperature, and EMF is in volts. measured using a. Thus, knowing the EMF in the oxygen sensor, if it is possible to know the temperature with a thermocouple, (6) can know P _o2 (W) is from equation the P _o2 (W) is (5 ) can know a _O by substituting the formula if. in other words it is possible to know the a _O a (3) the activity of the carbon by substituting the formula a _C, carbon from EMF and temperature carbon probe The activity a _C of can be known.

  Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and can of course be implemented in various forms without departing from the gist of the present invention.

  In the following, the results of testing the carbon concentration measurement accuracy and the hygroscopicity that causes deliquescence or defrosting of the sub-electrode material will be described for the examples of the carbon activity measurement probe according to the present invention.

  The probe for measuring carbon activity of the examples has the following configuration. That is, FIG. 2 shows a longitudinal section showing a state in which the carbon probe 1 is introduced into molten iron and the carbon concentration is measured. As described above, the sensor element 3 and the counter electrode 25 constitute the carbon sensor 2, and this and the thermocouple 31 are attached to a paper tube or the like (not shown) to constitute the carbon probe 1. The standard electrode lead wire 17 and the counter electrode lead wire 27 are connected to a potentiometer 35, and the thermocouple 31 is connected to a temperature measuring device 33 through a lead wire 32.

  The quartz cap 5 was manufactured as follows (see FIG. 1). That is, one end having an outer diameter of 11 mm, an inner diameter of 9 mm, and a length of 35 mm was sealed. Then, an opening 11 having a diameter of 2 mm was opened in the center of the sealed lower end. Furthermore, openings 7 and 9 having a diameter of 5 mm were provided on the side wall at a position 25 mm above the lower end so as to face each other in the radial direction.

After fixing the solid electrolyte tube 13 and the quartz cap 5 receiving the standard electrode 15 to the ceramic housing 19 with the high-temperature adhesive 21 respectively, the reagent MgCO ₃ and the reagent MgO constituting the sub-electrode 23 are added. The mixed powder was filled into the quartz cap 5 up to the height of the lower edge of the openings 7 and 9 in the side wall. A space 24 having a height of 5 mm was defined above the sub-electrode 23 in the quartz cap 5. In the void 24, the solid electrolyte is exposed. Molten iron flows into the quartz cap 5 from the openings 7 and 9 on the side walls and mixes with the sub electrode 23 to form the above-mentioned local equilibrium. Rather than filling the quartz cap 5 with the sub-electrode 23, the space 24 is defined above the sub-electrode 23, and the opening 11 is formed at the lower end of the cap 5. There is an advantage that the molten iron can easily flow in, and the molten iron that has flowed in easily comes into contact with the sub-electrode 23, so that the time until local equilibrium is formed is shortened. That is, measurement can be performed in a short time.

The main members constituting this probe are shown below.
Solid electrolyte tube 5 constituting the oxygen sensor 12: ZrO2 end-closed tube stabilized with 8 mol% MgO Standard electrode 15 of the oxygen sensor: Mixed powder of Cr and Cr ₂ O ₃ Counter electrode (working electrode 25): diameter 3 mm Mo rod of standard electrode and counter electrode lead wire: Mo wire with a diameter of 0.29 mm Sub electrode 23: Powder in which powder reagent MgCO ₃ and powder reagent MgO are mixed in a mass ratio 1: x, and the value of x ranges from 0 to 9 Tried between.
Thermocouple 31: Type-Rh

(Measurement accuracy of carbon concentration)
A carbon probe was put into the molten iron, and immediately after the EMF of the probe was stabilized, the molten iron was collected with a sampler, and this was chemically analyzed to determine the carbon concentration. FIG. 3 shows the relationship between the logarithm of the carbon concentration in the molten iron at 1550 ° C. and the EMF measured by the carbon probe. The conditions for obtaining FIG. 3 are as follows.
Mass mixing ratio of sub-electrode constituent materials: MgCO ₃ : MgO = 1: 1
Temperature: 1550 ° C
Carbon concentration: between 0.001 and 1% Crucible containing molten iron: Made of porous alumina Atmosphere: 100% N ₂
From FIG. 3, it can be seen that the relationship between the logarithm of the carbon concentration obtained from the analysis value and the EMF is a good linear relationship and has sufficient measurement accuracy with no practical problem.

(Hygroscopic comparison test of secondary electrode substance)
The comparative substances are Na ₂ O ₂ , Na ₂ CO ₃ , MgO, and MgCO ₃ reagents. Na ₂ O ₂ and Na ₂ CO ₃ are combined in a conventional probe to constitute a set of sub-electrode materials, and MgO and MgCO ₃ are combined in a probe of the present invention to form a set of sub-electrode materials. It constitutes. About 1 g of a powder sample of each of these reagents was placed on a watch glass so as to be flat, and was left in an atmosphere at a temperature of 26 ° C. and a humidity of 70%, and the mass was measured every 15 minutes.

The results are shown in FIG. The horizontal axis represents the test time expressed in minutes, and the vertical axis represents the mass in grams that 1 g of sample absorbed. Although only the results of Na ₂ O ₂ , Na ₂ CO ₃ and MgCO ₃ are shown in the graph, MgO did not change in mass at the test time of 120 minutes, so the entry in the graph was omitted. Compared with the other samples, Na ₂ O _2, which has a particularly high moisture absorption, using the result of 120 minutes, it has a hygroscopic capacity of about 50 times for Na ₂ CO _{3 and} about 92 times for MgCO ₃ . I understand. It can also be said that MgO is so large that it cannot be numerically displayed. It can also be seen that Na ₂ CO ₃ has a hygroscopicity about 1.8 times that of MgCO ₃ . Also from this result, it can be seen that the sub-electrode of the probe of the present invention made of MgCO ₃ or MgO having low hygroscopicity hardly causes deliquescence or defoliation, and there is almost no deterioration in quality even when stored in the air.

Sectional drawing which shows the structure of the carbon probe of this invention. Sectional drawing which shows the state which measures carbon activity using a carbon probe. The graph which shows the relationship between the electromotive force of a carbon probe, and the carbon concentration in molten iron. The gram which shows the result of a hygroscopic comparison test.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Carbon activity measurement probe 2 Carbon sensor 3 Sensor element 5 Cover 7, 9, 11 Opening 12 Oxygen sensor 13 Solid electrolyte 15 Standard electrode 17 Lead wire 19 Housing 23 Mixed subelectrode 24 Space 25 Counter electrode 27 Lead wire 31 Thermocouple 32 Lead wire 33 Temperature measuring device 35 Potentiometer

Claims (2)

In the carbon activity measurement probe for measuring the carbon activity in the molten metal,
By providing a sub-electrode made of magnesium carbonate or a mixture of magnesium carbonate and magnesium oxide on the outer surface of a solid electrolyte having oxygen ion conductivity, a standard electrode element that is paired with a counter electrode is formed. The carbon activity in the molten metal is measured by establishing a local equilibrium between the carbon, oxygen, and the magnesium carbonate or mixture constituting the sub-electrode and measuring the oxygen activity in the local equilibrium layer. A probe for measuring carbon activity.   The probe for measuring carbon activity according to claim 1, wherein the constituent mass ratio of magnesium oxide in the mixture is 0.9 or less.

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