JPS63270351A - Corrosion-resistant protection tube for thermocouple - Google Patents

Abstract

PURPOSE:To obtain a thermocouple-protection tube having low apparent porosity, high strength, etc., and excellent corrosion resistance to molten glass, slag, etc., and various high-temperature gases, by mixing chromium oxide with magnesium oxide at a specific ratio and forming and calcining the mixture. CONSTITUTION:75-85wt.% of chromium oxide powder is mixed with 25-15wt.% of magnesium oxide powder. The mixture is mixed with proper amounts of water and a vehicle such as polyvinyl alcohol and carboxymethylcellulose, formed in the form of a thermocouple-protection tube and calcined at a high temperature in a neutral or reducing atmosphere to obtain the objective corrosion-resistant protection tube for thermocouple having an apparent porosity of <=5%. Chromium oxide reacts with magnesium oxide at a high temperature to form picrochromite. Accordingly, a dense protection tube having excellent corrosion resistance, etc., and containing crystalline phase composed of single picrochromite phase or a phase resembling thereto can be produced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to continuous temperature measurement over a long period of time by directly immersing it in a molten material such as high-temperature glass or slag, or by placing it in a high-temperature gas atmosphere of various types. The present invention relates to a thermocouple protection tube that can be used suitably for temperature measurement and temperature control in coal gasification, liquefaction plants, glass melting furnaces, etc.

[Prior art] [Problems to be solved by the invention] Temperature detection of the atmosphere at high temperatures, such as temperature detection of molten materials such as glass, molten materials in metal casting and refining, or plants such as gasification and liquefaction of coal. For control, thermocouples such as platinum-platinum-rhodium thermocouples and chromel-alumel thermocouples are generally used.

When temperature is measured using such a thermocouple, a thermocouple protection tube is used to prevent the thermocouple from reacting with the melt or gas to be measured and being damaged.

Thermocouple protection tubes commonly used in the past have been made from porcelain materials such as mullite, alumina, and zirconia. However, since these protective tubes have insufficient corrosion resistance against molten materials such as glass and slag, and various gas atmospheres, various improvements have been attempted.

Japanese Patent Publication No. 57-49512 is made of ceramic with a composition of 1 to 35% by weight of chromium oxide and the balance is aluminum oxide, and Japanese Patent Publication No. 58-46692 is made of a magnetic protective tube body made of alumina, mullite, etc. A protection tube made of a silicate heat-insulating refractory is provided around the periphery, and JP-B No. 62-4354 contains 70 to 90% by weight of silicon nitride; the balance is 0% each of magnesium oxide and lanthanum group oxide.
．． The appearance is a silicon nitride protection tube with a porosity of 1% or less, which is obtained by sintering a powder mixture of 5% by weight or more. It is not a protective tube with improved corrosion resistance that can be used for temperature measurement of molten materials such as slag or slag, or various gas atmospheres.

However, as a corrosion-resistant protective tube that can be used in gasifiers for coal and petroleum residual oil, JP-A-59-8627
No. 4 only proposes a composite protection tube consisting of an outer tube made of quartz glass and an inner tube made of boron nitride.

On the other hand, although it is not a detection tube, in the field of refractories, JP-A-52
No. 80315, a refractory molded product made by press-molding a mixture of magnesia periclase, chromium ore, and dichromium oxide powder and sintering it at 1700°C, JP-A-53-104
No. 613 is a thermal shock-resistant magkuro ribbon brick made by blending and firing refractory raw materials consisting of electrofused magnesia clinker, electrofused magnesia clinker, chromium ore, and chromium oxide, and Japanese Patent Publication No. 57-37552 is made of magnesia raw material fine powder and chromium. Basic refractories manufactured from clinker obtained by heating and rapidly cooling a mixture with mineral raw material fine powder have been proposed, but all of them are used for applications such as refractories and bricks, and the apparent porosity is 14 to 14. It is extremely porous (30%) and cannot be used in thermocouple protection tubes.

Moreover, in JP-A-57-36804, a porous sintered body consisting of chromium oxide and a metal oxide selected from magnesium oxide, nickel oxide, cobalt oxide, and manganese oxide,
Moisture sensing elements carrying phosphorus and sulfur have been disclosed, but
Since this moisture-sensitive element is required to have a porous structure, that is, to be a perforated tube, its corrosion resistance and mechanical strength are insufficient, and the phosphorus and sulfur carried thereon corrode the base material at high temperatures. This became clear as a result of studies conducted by the present inventors, and it was confirmed that it could not be used for corrosion-resistant protective tubes. Furthermore, JP-A-59-18156 discloses a method for producing a high-purity magnesia-chromia-based oxide talincar by melting and pulverizing a mixture of a magnesium compound and a chromium compound. The manufacturing technology of semi-molten clinker itself is used to make porous refractories such as clinker, and it is dense and has excellent mechanical strength, which can be used for thermocouple protection tubes, and is highly resistant to corrosive atmospheres. There is no disclosure of producing one that is corrosion resistant.

[Means for solving problems]

The present invention has been made as a result of intensive studies in view of the above-mentioned circumstances, and the outline thereof is as follows.

Chromium oxide 75-85% by weight, magnesium oxide 25-85% by weight
A corrosion-resistant thermocouple protection tube consisting of 15% by weight and having an apparent porosity of 5% or less, preferably 1% or less.

The protection tube mentioned above is made of picrochromite (MgCrzO4
It is desirable to form a single phase or a phase close to this, and for this purpose, the most preferable composition is 77.5 to 82.5% by weight of chromium oxide and 22.5 to 17.5% by weight of magnesium oxide. This is preferable. The total amount of chromium oxide and magnesium oxide in the material of this protective tube is 98% by weight or more, and the content of iron oxide is 0.5% by weight.
Those containing the following are preferred from the viewpoint of corrosion resistance.

[For production]

In the present invention, as mentioned above, we focused on chromium oxide, which has good corrosion resistance against molten glass, and magnesium oxide, which has excellent corrosion resistance against basic slag, and added them to the above-mentioned compounding ratio, that is, 75 to 85% by weight of chromium oxide. A protective tube containing 25 to 15% by weight of magnesium oxide has sufficient corrosion resistance against melts such as glass and slag, as well as against high-temperature oxidizing or reducing gases. It is something that

In the above compounding ratio, if chromium oxide is less than 75 layers and magnesium oxide is greater than 25 layers, corrosion resistance will be poor when it comes into contact with molten materials such as glass and slag.
Also, if the chromium oxide content is more than 85% by weight, and the magnesium oxide content is less than 15% by weight, the product will be more easily attacked by reducing gases such as NZ and 82, and its gas resistance will decrease.

In other words, the above formulation can exhibit corrosion resistance against melts such as glass and slag, and corrosion resistance against gases at the same time.

Furthermore, the present invention requires a protection tube having the above composition, and an apparent porosity of 5% or less, preferably 1% or less.

If the apparent porosity is greater than 5%, not only will the corrosion resistance against melts and gas resistance be insufficient, but also the mechanical strength as a protective tube will be too low. Especially when the apparent porosity is 1
% or less, a protective tube with excellent corrosion resistance, gas resistance, and mechanical strength can be obtained.

In the protective tube of the present invention, chromium oxide and magnesium oxide react at high temperatures to form picrochromite (MgCrzL).
Therefore, it is recognized that the most corrosion resistant material is obtained when the material is composed of a single phase (such as when free chromium oxide or magnesium oxide is not present). In particular, picrochromite has a melting point of 2350°C and is stable up to this high temperature. However, considering the permissible range due to its characteristics, 77.5 to 82.5% by weight of chromium oxide,
A product containing 22.5 to 17.5% by weight of magnesium oxide has a crystalline phase consisting of a single picrochromite (MgCrzOn) phase or a phase close to this, and a protective tube containing almost no free chromium oxide or magnesium oxide can be obtained. Therefore, it was confirmed that a dense material (apparent porosity of 1% or less) with extremely excellent corrosion resistance and mechanical strength could be obtained.

However, in any case, as mentioned above, the density of the protective tube is not dense, and if the porosity of conventional refractories is apparently extremely high, the mechanical strength is also low, so it cannot be used as a thin-walled protective tube. It is clear that you will not get it.

Furthermore, in a preferred embodiment of the present invention, the total amount of chromium oxide and magnesium oxide in the material is 98% by weight or more,
The content of iron oxide is 0.5% by weight or less.

The reason for this is as a result of a detailed study of the corrosion resistance of ceramics against molten materials such as glass and slag at high temperatures.
We have found that even in dense ceramics with no pores, erosion progresses mainly through the grain boundaries, especially the glass phase that exists there. It is assumed that impurities are mixed in in each process leading to the formation of the protective tube, and in order for the resulting protective tube to exhibit excellent corrosion resistance, the ratio of chromium oxide and magnesium oxide should be 98% by weight or more, more preferably 99% by weight, as described above.
The content of iron oxide, which is necessary and has a particularly negative effect on corrosion resistance, must be 0.5% by weight or less.

In the present invention, in manufacturing the protective tube, chromium oxide or other chromium oxide source and magnesium oxide or other magnesium oxide source, or picrochromite powder obtained by reaction sintering may be used.

The magnesium oxide source and chromium oxide source are mixed in a predetermined weight ratio after firing, but usually powder or granules are uniformly mixed, formed into a protective tube shape, and then neutralized. or in a reducing atmosphere at a temperature of 1600 to 20
By firing at 00°C, a protective tube with an apparent porosity of 5% or less can be obtained.

In the above-mentioned case, the protective tube may be formed by dry pressing such as mold pressing or rubber pressing, or other ceramic forming techniques such as slurry casting or extrusion may be used.

〔Example〕

Examples will be described below.

Example 1: 3% by weight of polyvinyl alcohol (PVA) was added to a predetermined amount of chromium oxide powder (average particle size: 0.5 μm) and magnesium oxide powder (average particle size: 0.02 μm), mixed wet in a ball mill, and then dried. - Granulation was performed, and the product was filled into a rubber mold having the required shape of a protective tube and molded using a hydrostatic press method. This molded body was heated to 1600°C in a weakly reducing atmosphere.
Heated at a temperature of ~2000℃ for 2 hours to create an outer diameter of 15φ1
We made a protective tube with an inner diameter of 11φml and a length of 300 mm. The bulk density, bending strength, and coal slag of the obtained protective tube were evaluated at 1600 °C under two types of atmospheres: oxidizing and reducing.
The amount of erosion after immersion at ℃ for 10 hours was measured. The results obtained are shown in Table 1. The chemical composition of the coal slag is shown in Table 2.

According to this, protective tubes 1 to 6 corresponding to the embodiment of the present invention
It can be seen that the material has an apparent porosity of 5% or less and is excellent in bending strength and corrosion resistance.

Table 2 Example 2 Chromium oxide powder (average particle size 0.5 μm) 64% by weight, magnesium carbonate powder (average particle size 0.1 μm) 36% by weight, and carboxymethylcellulose (CMC) 5% by weight
After adding an appropriate amount of water and kneading, the mixture was molded into the desired shape using an extruder.

This was heated at 1900° C. for 2 hours in a slightly reducing atmosphere to obtain a protective tube with an outer diameter of 15φlIn, an inner diameter of 11φ and a length of 500mm. This protective tube had a bulk density of 4.16 g/- and an apparent porosity of 0.5%.

Example 3 Chromium oxide powder (average particle size 0.5 μm) 72×, magnesium hydroxide powder (average particle size 0.1 μm) 28i
A slurry was prepared by adding 0.5% by weight of ammonium polyacrylate and polyvinyl alcohol (PVA) and 0.5% by weight of water, and the slurry was cast into a plaster mold of a predetermined shape. This was heated at 1900° C. for 2 hours in a weakly reducing atmosphere to obtain a protective tube with an outer diameter of 15φ, an inner diameter of 11φ, and a length of 300 fins.

This protective tube was dense with a bulk density of 4.20 g/cffl and an apparent porosity of 0.1%. When this sintered body was subjected to X-ray diffraction, it was found to be a single phase of picrochromite (MgCr 204).

Example 4 Electrofused picrochromite lumps were wet-pulverized using a ball mill, 3% by weight of polyvinyl alcohol (PVA) was added thereto, mixed, dried, granulated, and filled into a rubber mold of a predetermined shape. It was molded using a hydrostatic press method.

This molded body was heated at 1900° C. for 2 hours in a slightly reducing atmosphere to obtain a protective tube having an outer diameter of 15 mm, an inner diameter of 11 mm, and a length of 300 cm.

This protective tube had a bulk density of 4.15 g/crK and an apparent porosity of 2.7%.

Next, the protective tubes of Examples 3 and 4 were subjected to chemical analysis, and the results are shown in Table 3.

Table 3 Weight % In all cases, the proportions of chromium oxide and magnesium oxide in the materials were 98% or more, and FezO= was 0.5% by weight or less.

A protection tube of the example construction (protection tube Hiroshi 3) and an alumina protection tube of the same size as this as a comparative example were prepared, a platinum-platinum rhodium (PR13%) thermocouple was inserted, and they were placed in molten slag at 1500℃. Continuous temperature measurements were performed by immersion.

The results showed that with the alumina protection tube, it became impossible to measure the temperature after 14 hours due to corrosion, whereas with the protection tube of Example 1, temperature measurement became impossible after 14 hours.
Even after 00 hours, there was no corrosion and temperature measurement was possible.

〔Effect of the invention〕

As is clear from the above comparative tests, the present invention provides a protective tube that is less likely to be corroded even when it comes in contact with molten materials such as glass and slag, or various high-temperature gases, and has excellent mechanical strength. be able to.

Claims (1)

[Claims] 1) Chromium oxide 75-85% by weight, magnesium oxide 2
A corrosion-resistant thermocouple protection tube comprising 5 to 15% by weight and having an apparent porosity of 5% or less. 2) The corrosion-resistant thermocouple protection tube according to claim 1, which has an apparent porosity of 1% or less. 3) Claims consisting of 77.5 to 82.5% by weight of chromium oxide and 22.5 to 17.5% by weight of magnesium oxide, with a crystalline phase consisting of a single picrochromite (MgCr_2O_4) phase or a phase close to this. The corrosion-resistant thermocouple protection tube according to item 1. 4) The total amount of chromium oxide and magnesium oxide in the material is 9
The corrosion-resistant thermocouple protection tube according to claim 1, wherein the iron oxide content is 8% by weight or more and 0.5% by weight or less.