JPH05115721A - Exhaust bas filter and production thereof - Google Patents

Abstract

PURPOSE:To extend the cycle life of an exhaust gas filter for a diesel engine vehicle by enhancing the heat resistance and crystallization temp. thereof. CONSTITUTION:A ceramics fiber based on aluminum oxide and silicon oxide is bonded by an inorg. binder based on aluminum oxide and silicon oxide and having an m.p. lower than that of the ceramics fiber to form a honeycomb structure. The sheet consisting of the ceramic fiber and the inorg. binder is preliminarily heat-treated to form fine crystals of mullite in the ceramics fiber and finally baked to form corrugated cardboard like cells 1a, 1b and the baked sheet is wound to constitute an exhaust gas filter. By this method, the generation of a white powder from the exhaust gas filter can be suppressed and the heat resistance of the fiber is enhanced to a large extent. As a result, the exhaust gas filter can be baked at high temp. and the mechanical strength and heat shock resistance of the exhaust gas filter are enhanced and the life thereof can be extended.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas filter for removing fine particles (particulates) contained in exhaust gas such as a diesel engine.

[0002]

2. Description of the Related Art In recent years, air pollution due to NOx and suspended particulate matter (hereinafter abbreviated as fine particles) in metropolitan areas has advanced. Of these, the fine particles floating in the atmosphere
Black smoke emitted from diesel vehicles is 2
It is said to occupy 0 to 30%. Moreover, it contains mutagenic and carcinogenic components such as polycyclic aromatic hydrocarbons. Therefore, as the most effective measure against the black smoke, there is an exhaust gas purifying apparatus that traps particles contained in the exhaust gas in the exhaust system and then performs self-regeneration. This exhaust gas purification device is provided in the exhaust system, and mainly comprises a filter for capturing fine particles in exhaust gas (hereinafter, abbreviated as exhaust gas filter) and a regenerating device for burning the captured fine particles. The regeneration method includes, for example, an electric heater method and a burner method, but each regeneration method has advantages and disadvantages, and a decisive method is not yet available.

The mainstream of conventional exhaust gas filters is a ceramic monolithic wall flow type particulate matter filter (hereinafter referred to as a ceramic monolithic type filter) as disclosed in US Pat. No. 4,364,761. there were.

As shown in FIG. 1, this type of ceramic monolithic filter has a honeycomb structure having cells 1a and 1b arranged in parallel to the length direction, and among two adjacent cells 1a and 1b. The cell 1b has a structure in which the inlet side is filled with the inflow side plug 2 and the cell 1a is filled with the outflow side plug 3 in the outlet side. Therefore, the exhaust gas containing fine particles flows into the cell 1a whose outlet side is clogged, passes through the porous wall, and is pushed to the adjacent cell 1b whose inlet side is clogged. At this time, since the fine particles cannot pass through the porous wall, the exhaust gas is filtered when passing through the porous wall, and the fine particles are trapped in the filter.

As the amount of trapped particulates increases, the porous walls become clogged with particulates and the back pressure of the diesel engine exhaust system increases. Therefore, it is necessary to suppress the increase in the load on the engine due to the increase in back pressure by removing the fine particles when the amount of the captured fine particles exceeds a certain amount.

The fine particles consist of a fixed carbon component and a soluble organic component (SOF) which can be dissolved in an organic solvent (dichloromethane is often used), both of which are flammable and may be slightly heated depending on the type of engine and load conditions. Although there is a difference, it will burn if heated to a temperature of about 600 ° C or higher. Therefore, conventionally, a method of regenerating the filter by reburning these fine particles using a heating means such as an electric heater or a burner has been attempted.

A conventional exhaust gas filter uses cordierite, which is a typical low thermal expansion ceramic, so as to withstand the temperature gradient generated during regeneration. However, cordierite ceramic has a drawback that the low back porosity of the diesel engine exhaust system causes a heavy load on the engine due to its low porosity. In addition, there is also a drawback that if the temperature rises too much during heating and regeneration, it will melt. Therefore, Japanese Patent Laid-Open No. 2-4
In the 3022 publication, an attempt was made to prevent an increase in back pressure and melting of the filter by using a plate-like body obtained by binding and sintering silica-alumina fiber with sericite clay as a filter element. ..

[0008]

However, in such a conventional structure, in order to prevent the melting of the filter, silica-alumina clay having a high melting point must be used as the filter material. In Japanese Unexamined Patent Publication (Kokai) No. 2-43022, sericite clay is used and fired and sintered at a temperature of 1250 ° C.

However, 100 silica-alumina fibers are used.
Mullite (3Al ₂ O ₃
・ 2SiO ₂ ) crystals are deposited. As a result, crystal growth of mullite occurs in the fiber, the surface of the fiber becomes brittle, mullite powder is separated from the surface layer by a slight external force such as wind pressure and vibration, and the mechanical strength of the filter decreases. there were. Further, there is a risk that an operator of a vehicle equipped with a filter inhales this powder into the body by inhalation and accumulates it in the body, which may impair health. Further, when the exhaust gas filter is mounted on a diesel turbo engine vehicle, there is a risk that the mullite powder is sucked into the engine by the turbocharger and wears the engine.

The surface layer of the conventional silica-alumina fiber fired at 1000 ° C. or higher is composed of mullite crystals and aluminosilicate glass, and if fired at 1260 ° C. or higher, the aluminosilicate glass is softened, so that it is 1250 ° C. It was necessary to bake below. Therefore, quartz in the sericite clay does not sufficiently dissolve into the liquid phase, and sufficient mechanical strength cannot be obtained after firing, so that there is a problem that the filter is destroyed by thermal stress during heating and regeneration.

The present invention is intended to solve such a problem, and is a crystal of mullite crystals when a plate-like body obtained by binding and sintering silica-alumina fibers with silica-alumina clay is used as a filter element. By suppressing growth,
The object of the present invention is to suppress the exfoliation of mullite powder, to enable the exhaust gas filter to be fired at high temperature, and to increase the mechanical strength of the exhaust gas filter.

[0012]

In order to solve this problem, the present invention is formed by binding ceramic fibers mainly composed of aluminum oxide and silicon oxide with an inorganic binder mainly composed of aluminum oxide and silicon oxide. A honeycomb structure, in which mullite crystals having an average length of 0.2 μm as an upper limit are contained in the ceramic fibers.

Aluminosilicate or aluminoborosilicate fibers are used as the ceramic fibers.

Amorphous ceramic fibers composed mainly of aluminum oxide and silicon oxide are preliminarily heat-treated in a crystal nucleation temperature region of mullite crystals, and then a binder composed mainly of an inorganic binder and an organic sizing material is used. The honeycomb structure is manufactured by binding the crystalline ceramic fibers to each other and then heating and firing at a temperature equal to or higher than the crystal nucleation temperature.

Further, after a honeycomb structure formed by binding an amorphous ceramic fiber composed mainly of aluminum oxide and silicon oxide and an inorganic binder with an organic paste material is heat-treated in a crystal nucleation temperature region of mullite crystals. The heating and firing is performed at a temperature equal to or higher than the crystal nucleation temperature.

Further, the ceramic fiber is manufactured by using an aluminosilicate type or an aluminoborosilicate type.

Further, clay containing sericite as a main crystal component is used as an inorganic binder, and heating and firing is performed at a temperature of 1350 ° C. as a lower limit.

[0018]

According to this method, the silica-alumina fiber is preheated in the crystal nucleation temperature range to deposit mullite crystals, which are a stable phase of the silica-alumina system, in this temperature range. Since no crystal growth of mullite crystals occurs, a large number of mullite microcrystals are precipitated in the fiber. Since a large number of crystal nuclei exist in the crystal nucleation temperature range during heating during heating in the subsequent firing step, crystal growth of mullite crystals is suppressed, fine powder does not fall off from the filter surface, and strength reduction of the filter is prevented. It will be possible.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An exhaust gas filter according to an embodiment of the present invention will be described below with reference to the drawings.

First, the ceramic fiber used as the constituent material of the exhaust gas filter of this embodiment will be described. The exhaust gas filter of the present embodiment collects the fine particles in the exhaust gas and heats and incinerates the accumulated combustible fine particles to regenerate it, so that the constituent material thereof is required to have high heat resistance. Typical high heat resistant ceramic fibers include alumina fibers, aluminosilicate fibers, aluminoborosilicate fibers, and mullite fibers. Of these, alumina fibers have a large thermal expansion coefficient of alumina themselves, and thus are inferior in thermal shock resistance (strength against a sudden temperature gradient). Mullite fibers are polycrystalline and weak in strength, so they are not suitable as constituent materials for exhaust gas filters. For the above reasons, aluminosilicate fiber and aluminoborosilicate fiber were selected as constituent materials in this example. Among these, for example, aluminosilicate fibers having a composition of about 50 wt% for both Al ₂ O ₃ and SiO ₂ are commercially available as heat resistant fibers having a temperature of 1260 ° C.

This heat-resistant 1260 ° C. aluminosilicate
When the differential thermal analysis was performed on the fiber,
It shows a strong exothermic peak. This exothermic peak is mullite (3
Al ₂O₃・ 2SiO₂) At the exothermic peak during crystallization of crystals
Hit Therefore, heating the aluminosilicate fiber
In case of normal temperature, it is in amorphous state at room temperature, but 950 ℃
Mullite crystal nuclei have already begun to precipitate at the following temperature,
Crystal growth peaks at around 0 ° C. Mullite crystal is A
l₂O₃: SiO₂Since the ratio of Al is 72:28, Al₂O
₃: SiO₂With an aluminosilicate fiber of almost 50:50
Therefore, the amount of crystal precipitation is limited. Therefore, 980 ℃
SiO above temperature₂Rich aluminosilicate moth
Needle-shaped mullite crystals are dispersed in the lath
ing. For example, the aluminosilicate fiber of this example is
When crystallizing into light, the change in crystal structure causes
A volume reduction of 3% occurs. Therefore, with mullite crystals
Tensile stress is generated between the aluminosilicate fiber
Does not cause softening of aluminosilicate glass components
In this case, the larger the grain size of the mullite crystal, the more
Draw between light crystals and aluminosilicate glass components
The tensile stress increases, and eventually the mullite crystals are exfoliated or removed.
Drops occur.

When the temperature is raised from this amorphous state to deposit the crystal, the crystal is grown through the following two temperature regions. One is a crystal nucleation temperature range, and minute crystal nuclei are formed in this temperature range. The other is a crystal nucleus growth temperature region in which minute crystal nuclei grow into large crystals. At this time, the temperature at which crystal growth is most prominent appears as a peak of crystallization heat when a differential thermal analysis is performed.

It was confirmed from the differential thermal analysis that the aluminosilicate fiber having heat resistance of 1260 ° C. had a crystallization peak at 980 ° C. In this example, from the results of X-ray diffraction and electron microscope observation, this fiber was tested at 930 ° C. to 97 °
If heat treatment is performed at 0 ° C. for 1 hour, crystal nuclei are already generated,
It was found that 0 ° C to 970 ° C is the optimum heat treatment temperature for suppressing crystallization. Similarly, for aluminoborosilicate fibers, it was found that 800 ° C to 850 ° C is the optimum heat treatment temperature for suppressing crystallization.

If the fibers are preheated in these temperature regions, a large amount of crystal nuclei are generated in the fibers, so that even if the fibers are heated to a temperature above the crystal nuclei growth temperature region, the crystal nuclei become large. It will never grow. When the ceramic fiber of this example is heated and fired without heat treatment as in the conventional case, the crystal size of mullite is about 0.
Although it was about 5 μm, it was possible to suppress the growth to an average size of about 0.2 μm by preheating in the above temperature range. By producing the exhaust gas filter under these heat treatment conditions, it was possible to suppress the growth and separation of the mullite crystals from the fibers. From this result, it was found that the size of the mullite crystals in the fiber should be about 0.2 μm or less on average in the length direction. This heat treatment does not make the aluminosilicate fibers brittle. Therefore, this heat treatment may be performed either before or after forming the exhaust gas filter.

Next, the inorganic binder used in this embodiment will be described. When an exhaust gas filter is composed of aluminosilicate fiber, sufficient strength cannot be obtained by the fiber alone, so it is necessary to add an inorganic binder. The inorganic binder is required to have high heat resistance, a coefficient of thermal expansion close to that of ceramic fibers, and affinity with ceramic fibers. As the inorganic binder satisfying this condition, glass or ceramic containing Al ₂ O ₃ or SiO ₂ is suitable. Moreover, in order to realize high-strength bonding without breaking the shape of the ceramic fiber, it is necessary to use a material having a melting point (or softening point) lower than the melting point (or softening point) of the ceramic fiber. For that purpose, those containing a small amount of alkali metal oxides and alkaline earth metal oxides are desirable. In particular, when the step of aggregating the binder in an aqueous solution is included as in the method of Example 1 below, the inorganic binder is required to have hydration property, and therefore it is desirable to use clay mineral as the binder. Typical clay minerals for this purpose include kaolinite, pyrophyllite, montmorillonite, and sericite. Among them, sericite best satisfies the above conditions. The course improving the cohesiveness, alkali metal oxides (alkaline earth metal oxides) -SiO ₂ -Al ₂ and O ₃ based glass, potash feldspar, also be used as feldspars minerals such as plagioclase Needless to say. The ceramic fibers are dispersed in these inorganic binders and fired. The firing temperature in this step is higher than the temperature at which the inorganic binder vitrifies (the temperature at which the glass softens) and lower than the temperature at which the ceramic fibers melt. You have to choose the temperature.

An exhaust gas filter according to an embodiment of the present invention will be described below with reference to the drawings. 1 and 2 show the structure of the exhaust gas filter of this embodiment. Cell 1a with inflow side open and outflow side closed with outflow plug as shown.
The exhaust gas filter 5 is composed of the core 1 whose inlet side is closed by the inlet plug 2 and which is alternately wound and laminated and whose center is closed on the inlet side. The exhaust gas flows into the cell 1 from the left side as shown by the arrow in FIG. 2, passes through the ceramic wall of the cell 1 and flows into the cell b. At this time, the fine particles contained in the exhaust gas are collected on the ceramic wall.

[0027]

Example 1 20 parts by weight of aluminosilicate fibers chopped to have an average fiber diameter of about 3 μm and a length of 0.1 to 10 mm are sufficiently dispersed and suspended in 1000 parts by weight of water. This aluminosilicate fiber has SiO _{2 of} 40 to 60 wt.
%, Al ₂ O ₃ range 40～60Wt% is desirable, more and SiO ₂ is much Al ₂ O ₃ is less lowered heat resistance temperature of the filter, Al ₂ O ₃ is more SiO ₂ is less and a coefficient of thermal expansion It becomes large and causes thermal destruction at the time of heat regeneration of the filter. On the other hand, 15 parts by weight of sericite clay as a ceramic raw material powder is suspended in 50 parts by weight of water. The fiber suspension and the suspension of the ceramic raw material powder were mixed with stirring. Next, 1 part by weight of an emulsion liquid of a vinyl acetate-acrylic acid ester copolymer was added as an organic binder, and after sufficiently stirring and mixing, a polymer coagulant was added to add an aluminosilicate fiber, sericite clay, organic binder. Are flocculated with each other to form a floc suspension. The flocculated suspension thus obtained is diluted with water to 3000 parts by weight, and then made into a sheet by an ordinary Fourdrinier paper machine to produce a sheet.

On the other hand, crushed aluminosilicate fiber 2
0 parts by weight and 15 parts by weight of sericite clay are made into a paste with polyvinyl alcohol to prepare a plug raw material.

The sheet obtained as described above is divided into two parts, one of which is formed into a corrugated shape by a corrugating machine having two gear-shaped rolls, and at the same time, a plug raw material is injected into one end and crushed aluminosilicate. An adhesive obtained by kneading fibers and sericite clay with an organic sizing agent is applied to the top of the corrugate, and the other flat sheet is attached. The plug raw material injected at this time becomes the inflow side plug 2 after firing. The corrugated board-shaped molded product obtained here is applied to the corrugated top portion and the other flat sheet is attached. The adhesive is applied to the corrugated top of the corrugated board-shaped molded body obtained here, and the plug raw material is injected into the other end and rolled up into a cylindrical shape to obtain a honeycomb structured molded body. The plug raw material injected at this time becomes the outflow side plug 3 after firing. Here, at the time of winding, since the core is wound up, the center of the molded body becomes hollow when removed from the winding machine. Therefore, the plug raw material is injected into the inflow side. This plug material forms the core 4 after firing. 950 this molded body in an electric furnace
After pre-baking at 1 ° C. for 1 hour and then raising the temperature to 1350 ° C. and baking at 1350 ° C. for 2 hours, organic substances disappear,
The aluminosilicate fibers are fused by the K ₂ O—SiO ₂ —Al ₂ O ₃ -based glass sericite clay to obtain an exhaust gas filter having a fiber ceramic honeycomb structure. At this time, the same effect can be obtained by performing the main calcination after lowering the temperature to room temperature after the preliminary calcination. Is more efficient.

As the clay, sericite clay is used,
By firing at 1350 ° C. or higher at which the sericite clay is sufficiently vitrified, the generated mullite crystals are sealed with the vitrified sericite clay, and the effect of preventing the mullite powder from peeling can be further enhanced.

When a conventional exhaust gas filter was cut with a saw, a large amount of white powder fell off when the filter was hit after cutting, but almost no powder fell off with the exhaust gas filter of this example.

[0032]

Example 2 40-60 wt% silica chopped to a length of 0.1-10 mm with an average fiber diameter of about 3 μm, and alumina 4
Aluminosilicate fibers having a composition of 0 to 60 wt% are heat-treated at 950 ° C. for 1 hour. Here, when the aluminoborosilicate fiber is used, the heat treatment may be performed at 830 ° C. for 1 hour. Next, 20 parts by weight of this aluminosilicate fiber is sufficiently dispersed and suspended in 1000 parts by weight of water. On the other hand, 15 parts by weight of sericite clay as a ceramic raw material powder is suspended in 50 parts by weight of water. The fiber suspension and the suspension of the ceramic raw material powder were mixed with stirring. Next, as an organic binder, 1 part by weight of a vinyl acetate-acrylic acid ester copolymer emulsion liquid was added and sufficiently stirred and mixed, and then a polymer flocculant was added to add aluminosilicate fiber, sericite clay, and an organic binder. Coagulate with each other to form a floc suspension. The flocculated suspension thus obtained is diluted with water to 3000 parts by weight and then made into a sheet by a conventional Fourdrinier paper machine.

On the other hand, crushed aluminosilicate fiber 2
A plug material is prepared by making 0 parts by weight and 15 parts by weight of sericite clay into a paste with polyvinyl alcohol.

The above-obtained sheet was divided into two parts, one of which was formed into a corrugated shape by a corrugating machine having two gear-shaped rolls, and the crushed aluminosilicate fiber and serine were injected while the plug raw material was injected into one end. An adhesive obtained by kneading site clay with an organic sizing agent is applied to the top of the corrugate, and the other flat sheet is attached. The plug raw material injected at this time becomes the inflow side plug 2 after firing. The corrugated board-shaped molded product obtained here is applied to the corrugated top portion and the other flat sheet is attached. The adhesive is applied to the corrugated top of the corrugated board-shaped molded body obtained here, and the plug raw material is injected into the other end and rolled up into a cylindrical shape to obtain a honeycomb structured molded body. The plug raw material injected at this time becomes the outflow side plug 3 after firing. Here, at the time of winding, since the core is wound up, the center of the molded body becomes hollow when removed from the winding machine. Therefore, the plug raw material is injected into the inflow side. This plug material forms the core 4 after firing. When this molded body was baked in an electric furnace at 1350 ° C. for 2 hours, organic substances disappeared, and the aluminosilicate fiber was fused by the K ₂ O—SiO ₂ —Al ₂ O ₃ -based glass sericite clay, An exhaust gas filter having a fiber ceramic honeycomb structure is obtained.

The method of Example 1 is very effective when the aluminosilicate fiber or the aluminoborosilicate fiber is used alone, but when both are used as a mixture, the method of Example 2 is used. Is more practical.

For example, when it is desired to reduce the coefficient of thermal expansion of the fiber, it is possible to use a mixture of 15 parts by weight of aluminosilicate fiber heat-treated at 950 ° C. for 1 hour and 5 parts by weight of aluminoborosilicate fiber heat-treated at 830 ° C. Becomes

By using the method of Example 2, the same crystal growth suppressing effect as that of the method of Example 1 can be obtained.

[0038]

As is apparent from the above description of the embodiments, according to the present invention, ceramic fibers mainly composed of aluminum oxide and silicon oxide are bound by an inorganic binder mainly composed of aluminum oxide and silicon oxide. In the honeycomb structure formed as described above, the ceramic fiber is heat-treated to contain mullite crystals having an average length of 0.2 μm or less in the fiber to form an exhaust gas filter. Crystal growth is suppressed, and it becomes possible to prevent mullite crystal powder from falling out of the exhaust gas filter. As a result, it is possible to prevent the ceramic powder from being scattered in the atmosphere, improve the strength of the exhaust gas filter, and achieve a long service life of the exhaust gas filter.

[Brief description of drawings]

FIG. 1 is a sectional view showing a schematic configuration of an exhaust gas filter according to an embodiment of the present invention.

FIG. 2 is a perspective view of the exhaust gas filter.

[Explanation of symbols]

 1a, 1b Cell 2 Inflow side plug 3 Outflow side plug 4 Core 5 Exhaust gas filter

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location F01N 3/02 301 C 7910-3G

Claims (6)

[Claims] 1. A honeycomb structure formed by bonding ceramic fibers mainly composed of aluminum oxide and silicon oxide with an inorganic binder mainly composed of aluminum oxide and silicon oxide, wherein the ceramic fibers have an average length. An exhaust gas filter containing mullite crystals with an upper limit of 0.2 μm. 2. The exhaust gas filter according to claim 1, wherein the ceramic fiber is an aluminosilicate type or an aluminoborosilicate type. 3. An amorphous ceramic fiber composed mainly of aluminum oxide and silicon oxide is previously heat-treated in a crystal nucleation temperature region of a mullite crystal, and then a binder composed mainly of an inorganic binder and an organic sizing material is used to form the non-crystalline material. A method for producing an exhaust gas filter, comprising: forming a honeycomb structure by binding crystalline ceramic fibers to each other, followed by heating and firing at a temperature equal to or higher than the crystal nucleus generation temperature. 4. A honeycomb structure formed by bonding an amorphous ceramic fiber composed mainly of aluminum oxide and silicon oxide and an inorganic binder with an organic paste material is heat-treated in a crystal nucleation temperature region of mullite crystals. A method for producing an exhaust gas filter, which comprises heating and firing at a temperature equal to or higher than the crystal nucleation temperature. 5. The method for producing an exhaust gas filter according to claim 3, wherein the ceramic fiber is an aluminosilicate type or an aluminoborosilicate type. 6. The method for producing an exhaust gas filter according to claim 3, wherein clay having sericite as a main crystal component is used as the inorganic binder, and heating and firing is performed at a temperature of 1350 ° C. as a lower limit.