JP2003089574A - Alumina-chromia-based ceramic and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a fire resistant ceramic sintered compact and a fire resistant binder material, each having corrosion resistance end, which are capable of being used under a corrosive atmosphere caused by a molten salt or molten slug. SOLUTION: A uniform solid solution oxide is obtained by preparing an alumina-chromia ceramic, in which >=0.01 and <=12 wt.% titania is added, by a coprecipitation method. Fire resistant materials having excellent corrosion resistances are provided by producing the sintered compact or the binder material using the solid solution oxide as a raw material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to alumina applied to high-temperature parts that are significantly corroded by molten salts, such as boiler burners, waste incinerator stokers, ash melting furnaces, etc., which are compatible with low-quality fuels containing heavy metals and the like. -Sintered body of chromia ceramics. The present invention also relates to a binder material made of alumina-chromia ceramic powder for refractories having excellent corrosion resistance against molten slag.

[0002]

2. Description of the Related Art Vanadium (V), sodium (N
Alumina-chromia ceramics are materials that are expected to have corrosion resistance against molten salts containing corrosive components such as a), sulfur (S), and oxygen (O). In order to improve the corrosion resistance of this ceramics, high density,
Alumina (Al ₂ O ₃ ) and chromia (Cr
₂ O ₃ ) is preferably in solid solution. Conventionally, alumina powder and chromia powder are used as raw materials, and these are mixed, shaped, and sintered by a solid-phase reaction to obtain an alumina-chromia ceramics sintered body. Here, in order to obtain a sintered body that is homogeneous and has a high density and in which alumina and chromia are completely solid-solved, it is necessary to sinter under conditions of high temperature and high pressure using equipment such as hot press. However,
The method using a hot press has a high manufacturing cost, and
There is a problem that it is not suitable for mass production of sintered bodies.

On the other hand, refractories such as bricks have a particle size of several mm.
The aggregate is composed of coarse ceramic particles and a powder called binder which has a particle size of about 1 mm for binding the aggregates together. Corrosion resistance of refractory
In particular, it is governed by the corrosion resistance of the binder material. In the conventional alumina-chromia refractory material, alumina and chromia powder are mixed as a binder and sintered, so that the solid solution of both is incomplete. Therefore, if the alumina particles are solely present in the binder, they serve as a starting point of the corrosion of the binder, and there is a problem that the entire refractory material is corroded from the binder portion by the high temperature molten salt.

In order to solve the above problem, the inventors have proposed the following technique (Japanese Patent Laid-Open No. 11-31045).
2 and JP-A-2000-319062). That is, by adding about 1% by weight or more and about 4% by weight or less of titania (TiO ₂ ), the sintering rate of alumina-chromia ceramics is increased, and a dense sintered body can be obtained without using hot pressing. Therefore, it is possible to provide a sintered body or a binder having a higher corrosion resistance than before. As for the corrosion mechanism of the sintered body and the binder material obtained by this technique, it is considered that the titania added by the use in the molten salt for a long time segregates to the grain boundary, and this grain boundary becomes the starting point of the corrosion. ing. However, even with this method, since the powder is the raw material, the homogeneity may not be sufficient and a dense sintered body may not be obtained, leaving room for improving the corrosion resistance.

[0005]

SUMMARY OF THE INVENTION The present invention prevents the added titania from segregating at grain boundaries and becoming the starting point of corrosion.
It is an object of the present invention to provide an alumina-chromia ceramics sintered body having high corrosion resistance. Another object of the present invention is to provide a binder material for a refractory material, which is made of alumina-chromia ceramic powder having the same characteristics.

[0006]

The present invention is characterized in that alumina, chromia, and titania are highly dispersed in each other by a coprecipitation method. As a result, the effect of promoting the diffusion of each component can be obtained, so that the amount of titania added can be reduced, and titania is less likely to segregate at the grain boundaries. That is, the present invention has the following configurations. (1) A preparatory step of preparing an aqueous solution containing Al ions, Cr ions, and Ti ions; a coprecipitation step of neutralizing the aqueous solution with an alkali to coprecipitate to obtain a precipitate; and heating the precipitate. And a calcination step of obtaining a mixed oxide powder by molding, a molding step of molding the mixed oxide to obtain a molded article, and a sintering step of sintering the molded article to obtain a sintered body. And a method for producing an alumina-chromia ceramics sintered body. (2) The method for producing an alumina-chromia ceramics sintered body according to (1), wherein the sintering step is performed at a temperature of 1250 ° C. or higher and 1600 ° C. or lower. (3) manufactured by the manufacturing method of (1) or (2) above,
Al ₂ O ₃ and Cr ₂ O _{3 of} 45% by weight or more and 75% by weight or less
And 0.01 wt% or more and 12 wt% or less of TiO ₂ ,
An alumina-chromia ceramics sintered body containing an unavoidable impurity and having a crystal form of a corundum structure. (4) A preparatory step of preparing an aqueous solution containing Al ions, Cr ions, and Ti ions, a coprecipitation step of neutralizing the aqueous solution with an alkali to coprecipitate to obtain a precipitate, and sintering the precipitate. The method for producing a binder for a refractory made of an alumina-chromia ceramics powder, the method comprising: (5) The method for producing a binder for a refractory made of an alumina-chromia ceramic powder according to (4), wherein the sintering step is performed at a temperature of 800 ° C. or higher and 1400 ° C. or lower. (6) manufactured by the manufacturing method of (4) or (5) above,
Al ₂ O ₃ and Cr ₂ O _{3 of} 45% by weight or more and 75% by weight or less
And 0.01 wt% to 1 wt% of TiO ₂ and unavoidable impurities, and a crystal form of a corundum structure, which is a binder for a refractory made of an alumina-chromia ceramic powder. (7) A method for producing an alumina-chromia type sintered body according to (1) or (2) above, and a pulverizing step for pulverizing the alumina-chromia type sintered body to obtain a powder. A method for producing a binder for a refractory made of alumina-chromia ceramic powder.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The alumina according to the present invention will be described below.
A refractory binder material made of a chromia ceramics sintered body and alumina chromia ceramic powder, and a method for producing the same will be described in detail.

The amount of titania added to the alumina-chromia type oxide of the present invention is 0.01% by weight or more and 12% by weight or less based on the weight of the ceramics produced.
Alumina-chromia oxide has a slow diffusion rate of diffusing species, and the vapor pressure of Cr ₂ O ₃ becomes high at high temperature,
It is considered difficult to manufacture a dense sintered body. However, by dissolving a small amount of titania and introducing vacancies into the metal site having a corundum structure, the diffusion of metal ions is accelerated, and a dense sintered body can be manufactured. However, if the amount of titania added is more than 12% by weight, Ti precipitates at the grain boundaries during use at high temperature, and the grain boundaries are easily corroded. Conversely, if the amount of titania added is less than 0.01%, the effect of promoting the diffusion of metal ions is unlikely to appear.

In particular, in the present invention, by adopting the coprecipitation method, Ti ions can be uniformly dispersed in the solid solution oxide of Al ions and Cr ions. When the third component, titania, is highly dispersed by the coprecipitation method of the present invention, unlike the case where a solid solution is prepared by using a powder as a raw material to form a solid solution, even if the titania concentration is 1% by weight or less, the diffusion is accelerated. Further, since the amount of titania added in the method of the present invention is 0.01% by weight or more and 1% by weight or less compared to the powder mixing method,
It was found that the amount of Ti segregated to the grain boundaries during use at high temperature was reduced, which was effective for improving the corrosion resistance.

The composition of the alumina-chromia ceramics of the present invention is such that the Cr ₂ O ₃ concentration is 45 wt% or more and 75 wt% or less and the TiO ₂ concentration is 0.01 wt% or more and 12 wt% or less. In particular, it is possible to provide a binder material composed of ceramic powder having excellent molten salt corrosion resistance.
Chromia is a component with excellent corrosion resistance.If the chromia content is less than the above concentration range, the ceramics produced will be easily attacked by molten salt, and conversely, if the chromia content is more than the above composition range, the vacancy in the sintered body will be high. Since pores are formed and it is difficult to obtain a dense sintered body, corrosion resistance tends to deteriorate, which is not preferable. Further, if there is less titania than the above concentration range, there is no effect of promoting diffusion, and conversely, if there is more titania than the above composition range, titania, which is the third component when the refractory is used at high temperature for a long time, is used. Is segregated at the grain boundaries and is easily corroded, which is not preferable. When the chromia is less than the above composition range, it can be used in applications where corrosion resistance is not required, for example, as an abrasion resistant material.

In the present invention, the precipitate obtained by the coprecipitation method is washed, dried and then heated to 300 ° C. or higher to synthesize the alumina-chromia oxide powder. The coprecipitate is an amorphous hydroxide containing Al ions, Cr ions and Ti ions. When this is heated to 200 ° C. or higher, it dehydrates and becomes an amorphous oxide. Further, when calcined at 300 ° C. or higher, a raw material powder of oxide is obtained.

In the present invention, the oxide raw material powder is molded and then sintered at a temperature of 1250 ° C. to 1600 ° C. to obtain an alumina-chromia ceramics sintered body. 1250
This is because if the temperature is lower than 0 ° C, the sintering rate is slow, and if the temperature is higher than 1600 ° C, the vapor pressure of Cr ₂ O ₃ becomes high and the densification of the sintered body is hindered.

In the alumina-chromia ceramics,
In order to obtain a homogenous sintered body without phase separation without adding titania, a temperature of 1400 ° C or higher and 1600 ° C or lower is required. However, when a small amount of titania is added, the diffusion rate becomes faster, so that a homogeneous and dense sintered body can be obtained even at a temperature of 1250 ° C. or higher and lower than 1400 ° C. In order to obtain a homogeneous and dense sintered body without phase separation, 1300 ° C or higher 1
A preferable sintering temperature is 500 ° C. or lower.

The compactness of the sintered body is an index relating to defects such as cracks in the sintered body. In a non-dense sintered body, many through cracks are often observed, and corrosive molten salt penetrates deep into the cracks to accelerate corrosion. However, since a dense sintered body has few such through cracks, the molten salt corrosion resistance of the sintered body is enhanced.

Sintering can be carried out in the atmosphere. However, when the oxygen partial pressure is high, the vapor pressure of Cr ₂ O ₃ becomes high at a high temperature, and the coarsening tends to be hindered. Therefore, an inert gas atmosphere such as argon or vacuum can be used for the purpose of lowering the oxygen partial pressure. If titania is added in a highly dispersed state by the coprecipitation method, 1400
Since the sintering rate becomes faster even at a temperature of ℃ or less, it is possible to sinter in the air.

In the present invention, the raw material powder obtained by the coprecipitation method is 80
By heating to a temperature of 0 ° C. or higher and 1400 ° C. or lower, a homogeneous alumina-chromia binder oxide powder without phase separation is obtained. As a result of various tests, the following phenomenon was confirmed. The powder obtained by the co-precipitation method is homogeneous, but when the powder is heated to 500 ° C., crystallization occurs and a chromia solid solution precipitates first. When this is heated to 800 ° C., the alumina solid solution is precipitated this time, and as a result, the alumina solid solution and the chromia solid solution are phase-separated into non-uniform powder. The critical temperature of this phase separation is 1330 ° C., and when the above-mentioned phase-separated non-uniform powder is heated to 1330 ° C. or higher, a solid solution powder having a homogeneous corundum structure can be obtained. Therefore, a preferable temperature range is 1330 ° C. or more and 1400 ° C. or less for obtaining a homogeneous powder by heating for a short time. However, when a small amount of titania is added, diffusion becomes faster, so that even if the heating temperature is 1000 ° C., a homogeneous powder without phase separation can be obtained. However, since particles grow at a temperature higher than 1400 ° C., it is not preferable as a powder for a binder. Good calcination temperature is 1000 ℃ or more and 1400 ℃
It is the following. Further, although the powder that is calcined at 800 ° C. or higher and lower than 1000 ° C. has a small amount of phase separation, it becomes homogeneous due to the solid-phase reaction when firing bricks at a high temperature, so it can be used as a binder raw material. it can.
The chromia solid solution is an oxide having a corundum structure in which a small amount of alumina is dissolved in chromia, and the alumina solid solution is an oxide having a corundum structure in which a small amount of chromia is dissolved in alumina. The chromia solid solution and the alumina solid solution can be distinguished by the difference in lattice constant.

In the present invention, a method of obtaining a homogeneous alumina-chromia oxide powder without phase separation by pulverizing a sintered body can also be adopted. As above, 1250 ° C
A homogeneous binder powder can also be produced by crushing a homogeneous sintered body having no phase separation and fired at 1600 ° C. or lower.

[0018]

EXAMPLES Next, examples of the present invention will be described in detail.

(Experimental Example 1) First, the relationship between the firing conditions and the crystal structure in the case where no titania is added will be described. An aqueous solution containing Al (NO ₃ ) ₃ and Cr (NO ₃ ) ₃ each having a predetermined concentration was prepared as a raw material, and the aqueous solution was neutralized by dropwise addition of ammonia water having a concentration of 1 mol / liter. Prepared. The precipitate was washed, then dried, and then calcined in the atmosphere at 300 ° C. for 2 hours to obtain amorphous alumina-
A chromia oxide powder was obtained. 1400 ℃ the above powder
After firing for 5 hours, the crystal structure was examined by X-ray diffraction. As a result, it was confirmed that an oxide solid solution having a corundum structure was obtained in the entire composition region. Then, as shown in Table 1, AlC
As a result of heat-treating the amorphous oxide of rO ₃ composition at various temperatures, when the firing temperature was low, the chromia solid solution was first deposited when the amorphous oxide was crystallized, and the alumina solid solution was further deposited at a higher temperature. . Therefore, at a high temperature of 800 ° C. or higher, it was observed that the chromia solid solution and the alumina solid solution were phase-separated. This phase separation disappeared at 1330 ° C and the critical temperature for phase separation was found to be 1330 ° C. The occurrence of phase separation was identified by X-ray diffraction separation of the reflection of the (300) plane of the corundum structure into two. From this experimental example, it was revealed that a homogeneous corundum structure oxide powder can be obtained by a method of heating to 1330 ° C. or higher in the case of adding no titania.

[Table 1]

(Experimental Example 2) Al (NO ₃ ) ₃ and Cr (N
An aqueous solution containing O ₃ ) ₃ and TiCl _{4 in} a concentration necessary for synthesizing a sintered body having a desired composition was prepared.
A 1 mol / liter aqueous ammonia was added dropwise to the aqueous solution for neutralization to prepare a precipitate. The precipitate was washed, dried, and then calcined in the air at 300 ° C. for 2 hours to give A
A ternary oxide powder was obtained in which TiO ₂ was solid-dissolved in an oxide having a composition of 1CrO ₃ . This ternary oxide powder is calcined,
The results of examining the crystal structure by X-ray diffraction are shown in Table 2. In the case of no addition of titania, phase separation occurs even after heat treatment at 1200 ° C. for 100 hours, resulting in heterogeneity. In contrast,
When titania was added in an amount of 0.01 to 10% by weight, diffusion was promoted and a solid solution oxide having a homogeneous corundum structure with almost no phase separation was obtained. When titania is not added, the diffusion for forming a homogeneous solid solution is slow, and it is considered that the diffusion hardly occurs at 1200 ° C.

[Table 2]

TiO₂-Al₂O₃-Cr₂O₃System coprecipitation method
Uniaxial pressure molding of oxide powder by 9.8MPa
And after isostatic pressing under a pressure of 98 MPa,
Firing at various temperatures from 1200 ° C to 1600 ° C,
The results of producing the sintered body are shown in Table 3. Conventional material is average grain
Al with a diameter of about 1 μm₂O₃Powder and Cr₂O₃Mix with powder,
Molding was performed under the same conditions as the oxide powder obtained by the coprecipitation method
After that, it is a sintered body that is sintered by a solid phase reaction. Comparative material
Is Al having an average particle size of 0.2 μm₂O₃Powder and Cr ₂O₃Powder
And TiO₂Oxide obtained by co-precipitation method by uniformly mixing with powder
After molding under the same conditions as powder, solid phase at high temperature and high pressure
It is a sintered body that is sintered by reaction. Third co-precipitation method
As a result of highly dispersing the titania in a minute amount, the addition amount is relatively small.
Compared with conventional materials and comparative materials, the degree of sintering is faster and more precise even at low temperatures.
A dense sintered body was obtained. In addition, as an index of the compactness of the sintered body
Then, the relative density is used. Relative density is theoretical density
Is the relative density when 100 is defined as (actual density
Degree / theoretical density) × 100.

[Table 3]

An experiment was carried out to compare the corrosion resistance of the sintered body obtained by the coprecipitation method of Experimental Example 3, the comparative material and the conventional material. The sintered body was immersed in a molten salt having a composition of 60 wt% V ₂ O ₅ -40 wt% Na ₂ SO _{4 at} 900 ° C. for 100 hours for corrosion. The corroded sintered body was observed for the corrosion resistance by observing the structure of the cross section with an optical microscope to measure the corrosion depth. The measured corrosion depth is shown in Table 3 above. As a result of highly dispersing the third component, titania, by the coprecipitation method, the amount of titania added was 0.
In the range of 01 wt% or more and 10 wt% or less, a dense sintered body having a high relative density can be obtained, so it is considered that the corrosion resistance of the sintered body is improved as compared with the conventional material and the comparative material. When the amount of titania added exceeds 10% by weight, the sinterability deteriorates, a high relative density cannot be obtained, and the corrosion resistance also deteriorates. On the other hand, if the amount of titania added is less than 0.01% by weight, the effect of addition does not appear, a high relative density cannot be obtained, and the corrosion resistance is poor.

(Experimental Example 5) A test was conducted to examine the correlation between the corrosion resistance of the ceramics for binder in which TiO ₂ is added to the Al ₂ O ₃ --Cr ₂ O ₃ system ceramics and the chromia concentration. The sintered body obtained by the same coprecipitation method as in Experimental Example 3, the comparative material, and the conventional material were subjected to a more corrosive condition than Experimental Example 4, that is, 45 wt% SiO ₂ -33 wt% CaO.
-22 wt% Al ₂ O ₃ composition in molten salt slag 1
It was immersed at 500 ° C. for 1 hour for corrosion. The corroded sintered body was observed for the corrosion resistance by observing the structure of the cross section with an optical microscope to measure the corrosion depth. Table 4 shows the measured corrosion depths. As a result of highly dispersing the third component titania by the coprecipitation method, a sintered body having a high relative density was obtained by adding 0.5% by weight of titania. Especially, the chromia concentration is 50
% Or more, a sintered body excellent in corrosion resistance was obtained. The ceramics sintered body obtained here can be used as a binder by pulverizing it into fine powder. Also, the powder for the binder can be obtained by a step in which the molding is omitted, that is, by heat-treating the oxide powder by a coprecipitation method having a predetermined composition. The composition of the binder can be selected based on the results of this experimental example.

[Table 4]

[0024]

According to the present invention, it is possible to provide a solid solution oxide ceramic having a corundum structure in which alumina and chromia are uniformly solid-dissolved. Further, the solid solution oxide ceramics can be used as a corrosion resistant fire resistant ceramic used in a corrosive atmosphere of a molten salt such as a boiler or a corrosive atmosphere of a molten slag such as an ash melting furnace. Further, it can be used as a binder for a refractory used under the above-mentioned corrosive atmosphere.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yuichiro Murakami             12 Nishikicho, Naka-ku, Yokohama-shi, Kanagawa             Inside Genialing Co., Ltd. F-term (reference) 4G030 AA16 AA22 AA36 BA25 CA01                       GA04 GA08 GA22 GA24 GA25                       GA27                 4G048 AA03 AB01 AB02 AC08 AD03                       AD06 AE05                 4K051 BE03

Claims (7)

[Claims] 1. A preparatory step of preparing an aqueous solution containing Al ions, Cr ions, and Ti ions; a coprecipitation step of neutralizing the aqueous solution with an alkali to coprecipitate to obtain a precipitate; A calcination step of heating to obtain a mixed oxide powder,
Manufacture of an alumina-chromia ceramics sintered body comprising a forming step of forming the mixed oxide to obtain a formed article and a sintering step of sintering the formed article to obtain a sintered body. Method. 2. The sintering step is performed at 1250 ° C. or higher and 1600
The method for producing an alumina-chromia ceramics sintered body according to claim 1, wherein the method is performed at a temperature of ℃ or less. 3. The Al ₂ O ₃ produced by the production method according to claim 1 or 2 and 45% by weight or more and 75% by weight or less of Cr.
₂ O ₃ and 0.01 to 12% by weight of TiO ₂
And an unavoidable impurity, and has a crystal type of a corundum structure, which is an alumina-chromia ceramics sintered body. 4. A preparatory step of preparing an aqueous solution containing Al ions, Cr ions, and Ti ions; a coprecipitation step of neutralizing the aqueous solution with an alkali to coprecipitate to obtain a precipitate; A method for producing a binder for a refractory made of an alumina-chromia ceramic powder, which comprises a sintering step of sintering. 5. The sintering step is performed at 800 ° C. or higher and 1400 ° C.
The method for producing a binder for refractory made of alumina-chromia ceramics powder according to claim 4, wherein the method is performed at the following temperature. 6. The Al ₂ O ₃ produced by the production method according to claim 4 or 5, and 45% by weight or more and 75% by weight or less of Cr.
₂ O ₃ and 0.01% by weight or more and 1% by weight or less of TiO
_A refractory binder made of alumina-chromia ceramic powder, which comprises ₂ and unavoidable impurities and has a crystal form of corundum structure. 7. A method for producing an alumina-chromia type sintered body according to claim 1 or 2, and a pulverizing step of pulverizing the alumina-chromia type sintered body to obtain a powder. A method for producing a binder for a refractory made of alumina-chromia ceramic powder.