JPH05254956A - Production of cordierite-based porous body - Google Patents

Abstract

PURPOSE:To obtain a cordierite-based porous body having a large pore diameter while using starting materials having a small grain size. CONSTITUTION:Starting materials for cordierite ceramics such as magnesia, silica and aluminum silicate are blended with 35-80% mullite particles and the resulting compsn. is fired at 1,350-1,440 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cordierite porous body.

[0002]

BACKGROUND OF THE INVENTION Cordierite (2MgO.2Al ₂ O ₃ .5SiO ₂ ) has excellent thermal shock resistance properties, and in recent years, its porous body has become a catalyst carrier for exhaust gas purification. Is used as.

Further, the use of its porosity has been drawing attention in recent years, for example, it is applied as a high temperature gas filter for collecting dust in high temperature gas.

In this case, the properties of the cordierite porous material are influenced by the pore diameter and the porosity, and therefore it is important to control the pore diameter and the porosity to the predetermined pore diameter and porosity.

The cordierite porous material is extruded from a magnesia raw material such as kaolin and talc, which are plate-like particles, an alumina (Al ₂ O ₃ ) raw material, and a silica (SiO ₂ ) raw material, and fired. By doing so, it is generally produced (Japanese Patent Laid-Open No. 50-75611).

By extruding the raw material containing the plate-like particles in this way, the cordierite crystals can be oriented in one direction and the coefficient of thermal expansion in the same direction can be lowered.

That is, kaolin, which is a plate-like particle, plays an important role in orienting cordierite crystals in the extrusion direction.

A method for controlling the pore size of this cordierite porous material is disclosed in JP-A-53-82822,
JP-A-50-75612, JP-A-58-13381
No. 0, those described in JP-A-63-143421 are known.

The method described in Japanese Patent Application Laid-Open No. 53-82822 is to control the particle size of the magnesia raw material such as talc to control the pore size of the cordierite porous material. , Average particle size 5 to 150
It is stated that a cordierite-based porous material having an average pore diameter of 5 to 50 μm can be obtained by using a magnesia raw material of μm.

However, in this method, it is necessary to use a raw material having a large particle size, and for example, in order to obtain a cordierite porous body having a pore size of 50 μm,
There is a problem that it is necessary to use talc having a large particle size and an average particle size of 50 μm. Such coarse particles are troublesome to handle, and cause wear of equipment such as an extruder and a press.

On the other hand, in the method described in JP-A-50-75612, a part or the whole amount of talc or clay in the raw material is used as a silica or silica-alumina raw material such as pyroferrite, kyanite, quartz or fused silica. The content of the replacement is to provide a cordierite porous body having at least 20% of pores having a diameter larger than 10 μm. By this method, those having an average pore diameter of 20 μm or more have not been obtained.

On the other hand, the method described in JP-A-58-133810 is a metal component of Group 1 and 2 of the periodic table which is eutectic with magnesia, alumina, silica and the like at a firing temperature or lower, such as calcium. It is stated that a metal component such as potassium, lithium, or the like is added to a cordierite raw material, and that a cordierite porous body having an average pore diameter of 50 to 800 μm can be obtained by this method.

In the case of this method, calcium, potassium,
Although an oxide such as lithium is added, the addition of these oxides inevitably results in a high coefficient of thermal expansion and deterioration in thermal shock resistance.

The method of Japanese Patent Laid-Open No. 63-143421 discloses that a composition contains a pore-forming agent (pore-forming agent) and the pore-forming agent is volatilized during firing to form pores. Ratio is 30 to 50%, average pore diameter is 45
A cordierite porous material having a particle size of up to 150 μm is obtained.

However, in the case of this method, 180 to 3
Since the pore size of 00 μm is used, the size of the pore size becomes irregular, the rate of formation of closed pores is high, and the decrease in strength is unavoidable.

In addition to the above, as a general method for controlling the pore diameter of a cordierite porous body, a small amount of glass flux or cordierite raw material (talc, kaolin, etc.) is added to aggregate cordierite particles and fired. And controlling the particle size of the aggregate cordierite particles to control the pore diameter and the like.

However, in the case of this method, for example, when an attempt is made to obtain a porous body having a pore size of 50 μm and a porosity of about 35%, the following equation derived from a model in the middle stage of firing d = 0.46D√ε (d: pore size , D: aggregate particle diameter, ε: porosity), it is necessary to use large aggregate particles having a particle diameter of about 180 μm.

When such large aggregate particles are used,
As described above, handling of the raw material powder is troublesome, and problems such as wear of the apparatus are caused, and the strength of the obtained fired body becomes insufficient, and the porosity also becomes small.

[0019]

The present invention has been made in order to solve such problems, and its gist is the cordierite ceramics such as a magnesia raw material, a silica raw material, and an aluminum-silicate raw material as a starting raw material. The mullite particles are used in an amount range of 35 to 80% by weight together with the raw materials for the composition, and the composition comprising the raw materials is used in the range of 1350 to
It consists in firing at a temperature of 40 ° C.

[0020]

As described above, according to the present invention, mullite is used as a raw material for cordierite together with other raw materials such as a magnesia raw material and a silica raw material in a predetermined ratio, and this is used at a predetermined temperature, that is, cordierite is melted. The mullite is converted to cordierite by firing at an appropriate temperature so that it does not end up.

Examples of the magnesia raw material include talc, calcined talc, magnesite, calcined magnesite, magnesium carbonate, magnesium oxide, magnesium hydroxide and the like, and silica raw materials include quartz and fused quartz. it can.

It is a known fact that mullite reacts with magnesia raw material, silica raw material and the like to form cordierite.

FIG. 2 shows a conventional kaolin-talc-
4 shows a change in crystal phase (by X-ray diffraction) when an alumina-based raw material was fired at 1200 to 1400 ° C. (“Ceram. Bulletin”).
in, 60, p203-205 (1981)).

From this figure, the mullite produced by heating the starting kaolin is reduced by reacting with cristobalite similarly produced from kaolin, protoenstatite produced by heating the starting talc, and alumina (corundum) added as the starting material. At the same time, it can be seen that cordierite is generated at around 1250 ° C. and almost converted to cordierite crystals at around 1400 ° C.

However, conventionally, such mullite has not been used for controlling the pores of a cordierite porous body because the thermal expansion coefficient of mullite is higher than that of cordierite.

The characteristics of the cordierite ceramics are that they have a low coefficient of thermal expansion and that they have excellent thermal shock resistance properties. If mullite having a large coefficient of thermal expansion is added to such cordierite, it will become This is because it is generally considered that the thermal expansion coefficient rises and the thermal shock resistance characteristic deteriorates accordingly.

FIG. 4 shows a general relationship between these thermal expansion coefficients and thermal shock characteristics. As shown in the figure, the larger the thermal expansion coefficient, the lower the thermal shock characteristics.

That is, it is considered that the characteristics of cordierite are diminished by the addition of mullite.

Therefore, it is the actual situation that the pores are controlled by adjusting the particle size of raw materials other than kaolin, such as talc and silica. The reason why kaolin is not used for controlling pores is that it is difficult to obtain a coarse particle size capable of controlling pores.

However, when the present inventor actually used mullite particles which had been crystallized in advance as a raw material, if a predetermined amount of mullite was left in the obtained cordierite porous body, its thermal shock resistance property was obtained. It has been found that the value does not decrease at all, but rather improves. The reason for this is not clearly known, but the following can be considered as a guess.

As described above, the thermal expansion coefficients of the mullite crystal and the cordierite crystal are different (while the thermal expansion coefficient of mullite is approximately [Equation 1], the thermal expansion coefficient of cordierite is approximately [Equation 2]) Therefore, when both crystals coexist in the fired body, a large number of microcracks are generated between both crystal phases due to the difference in thermal expansion coefficient between them in the cooling process after firing. It is considered that the thermal shock resistance is improved by absorbing the strain caused by the thermal shock by the microcracks.

[0032]

[Equation 1]

[0033]

[Equation 2]

This mullite has hitherto been industrially widely used as a raw material for various refractory materials, and it is easily available in various particle sizes, and by changing the particle size, fine cordierite porous materials can be obtained. Pore size can be controlled.

Moreover, when such mullite particles are used, it is possible to obtain a porous body having a relatively small particle size and a large pore size. For example, in the above-mentioned conventional method, when the pore size of the cordierite porous body is controlled by controlling the particle size of talc, as shown in FIG. It is necessary to use talc.

However, according to the present invention, it is possible to obtain a porous body having a pore size of the same size by using mullite having a particle size smaller than this.

In the present invention, it is necessary that the content of mullite particles in the raw material composition is 35 to 80% by weight.

This is because when the amount of mullite particles is less than 35% by weight, the amount of mullite aggregate that contributes to pore formation is too small to obtain a porous body having a sufficient pore diameter and porosity, and conversely, when it exceeds 80% by weight. This is because the amount of cordierite produced is too small and sufficient thermal shock characteristics cannot be obtained.

Regarding the size of the mullite particles, it is desirable to use those having an average particle diameter of 3 to 50 μm.

By using mullite particles having a particle size of 3 μm or more, it is possible to prevent the mullite particles from completely reacting during firing, and to leave a predetermined amount in the fired body, and vice versa. When it is larger than 50 μm, the reactivity of the mullite particles is remarkably lowered, the amount of the mullite particles remaining in the fired body is excessively increased, and the thermal shock property is deteriorated.

In the present invention, mullite particles contained as a raw material are reacted to generate cordierite, and at the same time pores are formed, and it is desirable to use such mullite particles having excellent reactivity. ..

Mullite can be roughly classified into electro-melting mullite and synthetic mullite. Therefore, in the present invention, it is desirable to use synthetic mullite having excellent reactivity. However, depending on the case, it is also possible to use an electrofusion mullite.

The purpose of the present invention is to obtain a cordierite porous material, but as described above, it is desirable to leave mullite as a sub-phase in the fired body,
The residual amount of such mullite is preferably 1 to 50% by volume.

On the other hand, the amount of cordierite as the main crystal phase is preferably 50% by volume or more.

Regarding the reaction rate of the raw material mullite, at least 30% or more, preferably 60, of the raw material mullite is used.
% Or more, and more preferably 80% or more is preferably converted to cordierite crystals by the reaction.

The present invention is highly effective when applied to a method for producing a cordierite porous body having a porosity (apparent porosity) of 30% or more and an average pore diameter of 5 to 70 μm.
O ₂ : 34 to 64% by weight (desirably 40 to 55% by weight), MgO: 6.8 to 21% by weight (desirably 10 to
18% by weight) and Al ₂ O ₃ : 29 to 60% by weight (preferably 34 to 50% by weight), which is highly effective when applied to the production of cordierite ceramics.

[0047]

EXAMPLES Next, examples of the present invention will be specifically described. Using various raw materials shown in Table 1, a raw material composition prepared by mixing these at a predetermined ratio was mixed with 80 parts by weight of water and a dispersant of several%, and the mixture was dehydrated by a filter press. After drying and crushing, PVA (polyvinyl alcohol) was added as a binder and press-molded to 15 mm × 40 mm × 5 mm.

The molded body was fired at a predetermined temperature in an electric furnace, and the average pore diameter, porosity and various physical properties of the obtained fired body were measured. The results are shown in Table 2 and FIG.

[0049]

[Table 1]

[0050]

[Table 2]

For comparison, as shown in Table 3, the same test was carried out on the kaolin-talc-alumina raw material used in the production of the conventional cordierite porous body, which was press molded and extruded. went. Table 3 shows the results
Are also shown.

[0052]

[Table 3]

The average pore diameter and other physical properties were measured under the following conditions. Porosity: Measured by Archimedes method Average pore diameter: 50 of total pore volume measured by mercury porosimetry
Pore diameter equivalent to% Total pore volume: Mercury porosimetry Thermal expansion coefficient: Measured value in the range of room temperature to 800 ° C Thermal shock resistance: The obtained 15 mm × 40 mm × 5 mm sample was placed in an electric furnace for 1 A cycle in which the sample was held for a time, taken out of the furnace, and rapidly cooled was repeated 10 times, and the sample was checked for cracks or damages at that time. If no abnormality was found, the temperature was further raised to carry out the test.

From Table 2, it can be seen that the average pore size of the cordierite porous material of the present invention is controlled by the particle size of the raw mullite particles regardless of the raw material composition.

Further, as can be seen from the comparison with FIG. 3, in the case of the conventional method, talc with a particle size of 50 μm must be used to obtain a porous body with an average pore size of 50 μm, whereas in the example of the present invention the average particle size is By using mullite particles having a diameter of 28 μm, those having a pore diameter of 50 μm have been obtained.

That is, according to the present invention, a porous body having a large pore size can be obtained from a raw material having a relatively small particle size as compared with the conventional one.

On the other hand, regarding the thermal shock resistance property, the cordierite-based porous body produced in the example of the present invention has a large thermal expansion coefficient due to the addition of mullite.
No cracks or cracks were observed even at a thermal shock temperature difference of 1000 ° C. or more, and the thermal shock resistance characteristics were comparable to or superior to those of the conventional sample having a low coefficient of thermal expansion composed of the talc-kaolin raw material shown in Table 3. Has been obtained.

Regarding the thermal shock resistance, it has been conventionally said that the maximum factor is the thermal expansion coefficient, but in the case of the present invention example, despite the fact that the mullite raw material has a large thermal expansion coefficient,
The thermal shock resistance of the obtained fired product is good.

The embodiment of the present invention has been described in detail above, but this is merely an example, and the present invention can be implemented in a mode in which various modifications are made based on the knowledge of those skilled in the art without departing from the spirit of the invention. is there.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the average particle size of raw material mullite and the average pore size of a porous body obtained in an example of the present invention.

FIG. 2 is a diagram showing how mullite reacts to generate cordierite.

FIG. 3 is a diagram showing a relationship between a particle size of a raw material used in a conventional method and an average pore diameter of a porous body obtained.

FIG. 4 is a diagram showing a relationship between a thermal expansion coefficient and thermal shock resistance, which are generally established.

Claims (4)

[Claims] 1. A mullite particle is used as a starting material together with a material for cordierite ceramics such as a magnesia material, a silica material and an aluminum-silicate material in an amount of 35 to 35.
A method for producing a cordierite porous body, comprising using the composition in the range of 80% by weight and firing the composition composed of the raw materials at a temperature of 1350 to 1440 ° C. 2. The mullite particles have an average particle size of 3
The method for producing a cordierite-based porous body according to claim 1, wherein the porous body has a diameter of about 50 μm. 3. The cordierite-based porous body contains 50% by volume or more of cordierite as a main crystal phase,
The method for producing a cordierite porous body according to claim 1 or 2, wherein mullite is contained as a subphase in the balance in the range of 1 to 50% by volume. 4. The cordierite porous body according to claim 1, wherein the cordierite porous body has a porosity of 30% or more and an average pore diameter of 5 to 70 μm. Manufacturing method.