JP2003236384A - Silicon nitride filter and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon nitride filter which has low pressure loss, and in which exhaust gas containing particulates such as diesel exhaust gas can effectively be treated at a low cost and to provide a method for manufacturing the filter. <P>SOLUTION: The silicon nitride filter is a silicon nitride porous body having continuous open pores and contains catalyst particle in the porous body. The silicon nitride filter is manufactured by preparing a mixture containing 30 to 90 mass% of metal silicon particles having 1 to 100 μm average particle diameter, 9 to 60 mass% of a pore forming agent and 0.01 to 15 mass% of the catalyst or the catalyst precursor, adding water as a molding assistant to the mixture, and kneading, extruding the kneaded material by using a die to obtain an extruded body having through-holes with each hole having a 1 to 100 mm<SP>2</SP>cross section, and then heat treating the extruded body in a nitrogen atmosphere to nitride. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride filter suitable for removing dust and harmful substances contained in high-temperature exhaust gas and a method for manufacturing the same.

[0002]

Cordierite honeycomb bodies are widely used as carriers for automobile exhaust gas purifying catalysts, porous bodies for removing high-temperature gas containing dust, etc., and porous bodies for removing particulates discharged from diesel engines. ing.
However, in terms of material, although cordierite is excellent in thermal shock resistance, it is not always sufficient in terms of corrosion resistance, heat resistance, etc., so silicon carbide and silicon nitride, which are excellent in corrosion resistance, heat resistance, and mechanical strength, have attracted attention. There is.

For example, Japanese Unexamined Patent Application Publication No. 2001-293315 proposes a method of heat-treating a molded body using a mixed powder of metal silicon particles and metal oxide hollow particles as a starting material to obtain a silicon nitride porous body. However, although these can remove particulates and the like generated from the diesel engine, they cannot process the supplemented particulates themselves (hereinafter referred to as a purification function). Although sometimes referred to as soot, the particulates are mainly composed of solid carbon particles called soot and organic solvent-soluble hydrocarbon particles called SOF. However, in the present specification, these are used without distinction.

As a method of imparting a purifying function to a ceramics filter (hereinafter, simply referred to as a filter), Japanese Patent Laid-Open No. 28528/1990 discloses a honeycomb filter made of cordierite or silicon carbide with a wash coat or the like. -A method of supporting a catalyst such as platinum or palladium via alumina or the like is disclosed. But,
In this method, during washcoat treatment for supporting the catalyst, the rate of blocking open pores in the filter increases, and pressure loss (hereinafter referred to as pressure loss), which is an important factor for the filter, increases. There is a problem of letting it go.

Further, Japanese Patent Laid-Open No. 7-133713 proposes a method of directly supporting the catalyst particles on the filter surface without supporting them on the washcoat layer. However, in this method, since the catalyst is directly supported on the surface of the filter, there is a problem in terms of durability such that the catalyst particles easily fall off.

On the other hand, as an exhaust gas purifying method other than the method of supporting a catalyst on a filter, there is disclosed in Japanese Unexamined Patent Publication No. 8-218884.
No. 9 discloses a method in which diesel fuel as a fuel is mixed with a rare earth metal fuel additive to purify and regenerate accumulated particulates in a diesel particulate filter (hereinafter, simply referred to as DPF) by engine control. There is. However, this method requires a fuel additive tank, which requires an extra system, which is costly and consumes valuable space, especially for applications such as mounting on a vehicle. Furthermore, it is necessary to periodically replenish the fuel additive to the tank, which is troublesome in maintenance.

In other words, there is a strong demand for a DPF that does not block the formed pores as much as possible, does not require an extra system such as a fuel additive tank, requires no space, and is easy to maintain.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a silicon nitride filter capable of effectively treating exhaust gas containing fine particles such as diesel exhaust gas with low pressure loss and low cost, and a method for producing the same. To do.

[0009]

DISCLOSURE OF THE INVENTION The present invention is a silicon nitride porous material having open pores communicating with each other, wherein catalyst particles are contained in the porous material. I will provide a.

Further, the present invention has an average particle diameter of 1 to 1.
30 to 90% by mass of metallic silicon particles having a size of 00 μm, 9 to 60% by mass of a pore forming agent, and 0.01 to 15% by mass of a catalyst precursor having a function of purifying diesel engine exhaust gas.
A molding aid and water are added to a mixture containing and kneaded to form a kneaded product, and the kneaded product is extruded using a mold, and a through-hole having a cross-sectional area of 1 to 100 mm ² per hole. The present invention provides a method for producing a silicon nitride filter, characterized in that an extruded product having the above is obtained, and then the extruded product is heat treated in a nitrogen atmosphere to be nitrided.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A silicon nitride-based filter of the present invention (hereinafter referred to as the present filter) is a silicon nitride-based porous body having communicating open pores, and catalyst particles are contained inside the porous body. It is characterized by The silicon nitride porous body having open pores communicating with each other is preferably obtained by nitriding metal silicon particles because a large number of columnar substances are deposited therein.

The present filter is particularly suitable as a DPF, but is preferably used for the purpose of treating dust-containing gas. The catalyst particles are preferably those that promote combustion of the soot collected on the filter or those that are active with respect to exhaust gas, for exhaust gas from a diesel engine (hereinafter abbreviated as diesel exhaust gas) It is more preferable that the compound has activity.

The shape of the present filter is not particularly limited, but a honeycomb shape having a plurality of parallel through holes in the axial direction is preferable. A typical method of using this filter is to seal both end faces of a DPF having a honeycomb shape in a checkered pattern so that one side of each through hole is opened and install it in the middle of the flow path of diesel exhaust gas. Can be mentioned.

The porosity of the filter is 30.
-80% is preferable because the mechanical strength and the pressure loss are well balanced, and the porosity is more preferably 40-80%. In the present invention, the porosity is measured by the Archimedes method.

It is preferable that the average pore diameter of the present filter measured by the mercury porosimetry is 5 to 40 μm, because it is possible to efficiently collect particulates while suppressing an increase in pressure loss. More preferably, the average pore diameter is 8 to 25 μm.

As for the mechanical characteristics of the filter, the Young's modulus is preferably 20 to 100 GPa, and 30 to 60.
More preferably GPa. Bending strength is 1-100
It is preferably GPa, more preferably 10 to 50 GPa.

In the present filter, the catalyst particles are not supported and used, but contained in the filter. That is, usually, after the filter is produced, the catalyst particles are supported on the filter surface by using a wash coat typified by γ-alumina, but in the present invention, the catalyst particles are included in the raw material when the filter is produced. It is a waste.

In this filter, the catalyst particles contained are not particularly limited as long as they have the desired activity, but lanthanum, cerium, neodymium, praseodymium, vanadium, titanium, chromium, manganese,
Iron, nickel, cobalt, copper, platinum, rhodium, palladium, iridium, potassium, sodium, lithium,
Diesel exhaust gas that is a metal, alloy, metal oxide or sulfate containing at least one element selected from the group consisting of rubidium, cesium, strontium, barium, calcium, indium, arsenic, antimony, molybdenum and tungsten Is preferable because it can be rapidly processed at a relatively low temperature.

The method for producing the present filter (hereinafter referred to as the present production method) comprises 30 to 90 mass% of metal silicon particles having an average particle diameter of 1 to 100 μm (hereinafter, simply abbreviated as%),
Pore forming agent 9 to 60% and catalyst or catalyst precursor 0.0
A molding aid and water are added to a mixture containing 1 to 15% to knead to form a kneaded product, and the kneaded product is extrusion-molded using a mold, and the cross-sectional area per hole is 1 to 100 mm. An extruded product having ² through holes is obtained, and then the extruded product is heat treated in a nitrogen atmosphere to be nitrided. The present production method uses a mixture containing metallic silicon particles, a pore-forming agent and a catalyst or a catalyst precursor, which will be described.

In this manufacturing method, the average particle diameter of the metal silicon particles is 1 to 100 μm. If the average particle diameter of the metal silicon particles is less than 1 μm, many closed pores that do not contribute to the filter function are formed, or the diameter of the pores becomes too small, resulting in a decrease in the filter function and an increase in pressure loss. The average particle diameter of the metal silicon particles is 10
If it exceeds 0 μm, metallic silicon particles that are not nitrided remain inside the sintered body and the filter characteristics deteriorate. It is preferable that the average particle diameter of the metal silicon particles is 5 to 50 μm from the viewpoint of molding stability and mechanical strength of the filter. The purity of the metal silicon particles is not particularly limited, but a purity of 95% or more is preferable because the nitriding treatment in nitrogen is promoted.

In the present production method, the pore-forming agent is not particularly limited, but organic polymer particles or Si, Al,
The metal oxide particles of one or more elements selected from the group consisting of Y, Mg and Yb are preferable. The metal oxide particles form pores during the heat treatment process of the extruded product, and at the same time, they serve as a sintering aid for the silicon nitride particles produced by nitriding, and the effect of improving the strength can be expected. The organic polymer particles may be particles such as acrylic and polyvinyl alcohol that do not leave a carbon component after thermal decomposition.

These pore-forming agents are preferably hollow or spherical particles. The average particle diameter of the pore forming agent is preferably 10 to 100 μm. If it is 10 μm or less, the pore diameter of the obtained porous body becomes too small, and if it is 100 μm or more, the pore diameter of the porous body becomes too large, so that particulates pass through, which is not preferable. More preferably, the pore-forming agent is hollow particles or spherical particles, and the average particle diameter thereof is 10 to 100 μm.

In the present manufacturing method, the catalyst precursor is a substance capable of producing catalyst particles in the filter manufacturing process. Any catalyst that has a function of purifying exhaust gas, especially diesel exhaust gas, in the present filter may be used.

Examples of the catalyst precursor include metals, alloys, oxides, sulfates, nitrates, carbonates, hydroxides, acetates containing at least one element selected from the above-mentioned element group contained in the catalyst. Phosphates, ammonium salts, oxalates, halides or complexes thereof are preferred. As the precursor, these powders may be used, but alkoxides that decompose after thermal decomposition, organic polymers or aqueous solutions thereof, metal silicon particles when an organic solvent is used, a pore-forming agent,
It is preferable for homogeneously and effectively dispersing when kneading with a molding aid, and for exhibiting a catalytic function.

As the catalyst precursor, for example, in the case of cerium element, ultrafine cerium oxide powder, cerium oxide powder dispersion aqueous solution, cerium nitrate powder, cerium nitrate aqueous solution, cerium ethylene glycol solution, cerium acetylacetonate, triethoxycerium. In the case of platinum element, platinum black, hexachloroplatinic acid hexahydrate, tetraammineplatinum (II) chloride aqueous solution, platinum acetylacetonate and the like are specifically mentioned. In the following description, the term catalyst precursor shall include a catalyst.

In the present production method, the average particle diameter is 1 to 1
A mixture containing 30 to 90% of metallic silicon particles having a size of 00 μm, 9 to 60% of a pore forming agent, and 0.01 to 15% of a catalyst precursor is prepared.

In the above mixture, the proportion of metallic silicon particles is 3.
If it is less than 0%, sufficient strength cannot be obtained and 90
If it exceeds%, sufficient pores may not be secured. Further, if the proportion of the pore-forming agent is less than 9%, sufficient pores may not be secured, and if it exceeds 60%, sufficient strength may not be secured, which is not preferable. Further, if the proportion of the catalyst precursor is less than 0.01%, a sufficient catalytic function may not be exhibited, and if it exceeds 15%, the heat resistance of the substrate is impaired, which is not preferable.

In the present production method, a molding aid and water are added to the above mixture and kneaded to obtain a kneaded product. Ion-exchanged water is preferably used as water. Further, as the molding aid, a binder, a plasticizer, a dispersant, a viscosity modifier, a wetting agent and the like can be used. Specific examples of the molding aid include polyvinyl alcohol or a modified product thereof, starch or a modified product thereof, methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose, polyethylene glycol, propylene glycol, various organic substances such as glycerin, alone or Used as a mixture of two or more kinds.

The blending ratio of the molding aid is preferably in the range of 10 to 40% in the external ratio with respect to the mixture containing the metal silicon particles, the pore-forming agent and the catalyst precursor. If the blending ratio is less than 10%, it becomes difficult to extrude the molded product, while if it exceeds 40%, the amount of the molding aid is too large, and the mechanical strength of the porous body may be reduced.

The proportion of water to be added together with the molding aid is preferably in the range of 5 to 30% externally with respect to the mixture containing the metal silicon particles, the pore-forming agent and the catalyst precursor. If the mixing ratio is less than 5%, the composition containing the metal silicon particles, the pore-forming agent and the catalyst precursor and the organic component cannot be uniformly kneaded. On the other hand, if the mixing ratio exceeds 30%, the viscosity of the raw kneaded material decreases, This causes molding problems such as poor shape retention. For kneading these, a kneading machine or the like usually used in the ceramics field such as a ribbon mixer, a Henschel mixer, and a kneader can be used.

In the present manufacturing method, the kneaded product obtained above is extruded into a columnar shape having through holes using a mold. As the mold, a mold having a cross-sectional area of 1 to 100 mm ² per through hole of the extrusion molded body is used. In the present specification, the cross-sectional area per through-hole is the cross-sectional area of a portion corresponding to one cell in the case of a honeycomb structure, for example.

If the cross-sectional area of each through hole is less than 1 mm ² , it becomes difficult to manufacture or mold a die. Further, when the cross-sectional area of each through hole exceeds 100 mm ² , columnar crystals are less likely to precipitate, and the surface area of the through hole per volume of the porous body becomes smaller. There is a possibility that the number will increase and it will be a problem in practical use. It is preferable that the flow channels have a honeycomb structure because the surface area of the through holes per volume of the porous body can be increased.

In the present production method, the kneaded product is extruded using an extruder. As the extruder, a single-screw extruder, a twin-screw extruder or the like used in the field of ceramics can be appropriately used.

In the present manufacturing method, the extruded product obtained is heat-treated in nitrogen to nitride silicon metal. The heat treatment conditions are 1000 to 1800 ° C. in a nitrogen atmosphere, 1 to
It is preferable to hold for 12 hours. If the temperature is lower than 1000 ° C, nitriding or sintering does not proceed sufficiently, which is not preferable, while if it exceeds 1800 ° C, the silicon nitride particles are decomposed, which is not preferable. Further, if the temperature holding time is less than 1 hour, the bonding between the particles does not proceed sufficiently, which is not preferable. On the other hand, if the temperature holding time exceeds 12 hours, the silicon nitride is easily decomposed particularly at a high temperature, which is not preferable.

The rate of temperature rise during the heat treatment is appropriately selected depending on the size and shape of the molded body, the number of heat treatments and the like.
It is preferably from 600 to 600 ° C./h in terms of nitriding rate and control of pore diameter. Further, the nitrogen atmosphere means an atmosphere containing substantially only nitrogen and not oxygen, but may contain other inert gas. The nitrogen partial pressure is preferably 50 kPa or more. When the catalyst precursor is composed of only a metal element, it is preferable to perform heat treatment at 500 to 800 ° C. for 1 to 10 hours in an atmosphere containing 5 to 100% hydrogen, in addition to the above heat treatment. When the catalyst precursor is an oxide, instead of heat treatment in a hydrogen-containing atmosphere, the temperature is 500 to 1000 ° C. in the atmosphere.
It is preferable to heat-treat for 1 to 10 hours.

[0036]

EXAMPLES Examples of the present invention (Examples 1 and 2) and comparative examples (Examples 3, 4 and 5) are shown below.

Example 1 A honeycomb-shaped ceramic filter was prepared as the present filter. Average particle diameter 10μ
m metal silicon particles (Si: 97%, SiO ₂ : 2.1)
%) 74 parts by mass (hereinafter, abbreviated as “part”) contains 20% of Al ₂ O ₃ as a pore-forming material, and has an average particle diameter of 50 μm.
m Al ₂ O ₃ · SiO ₂ composite oxide particles (manufactured by Taiheiyo Cement Co., trade name: Espheres), 24 parts, and a platinum ammonium salt containing 55% of platinum as a catalyst precursor expressing a catalytic function (N.E. Chemcat, trade name: Tetraamminedichloroplatinum hydrate) is mixed in 2 parts to form a mixture, and 10% of methylcellulose, 1% of glycerin and 10% of ion-exchanged water are added to each of the mixtures to form a kneader. Was sufficiently kneaded to prepare a kneaded material for extrusion molding.

The kneaded material thus obtained was extrusion-molded by a vacuum extrusion molding machine having a honeycomb-shaped mold to obtain a honeycomb-shaped molded body. The outer shape of the molded body is 14.38 cm in diameter,
The length is 15.24 cm, the cell wall thickness is 0.25 mm, and the number of cells is 31 cells / cm ² . The cross-sectional area of one cell, that is, one through hole is about 3.2 mm ² . After drying, the obtained molded body was alternately sealed in a checkered pattern on both end surfaces with a sealing material of the same material as the kneaded material used for the molded body, and then in an electric furnace under a nitrogen atmosphere (pressure: 0. 1 MPa), the temperature is raised from room temperature to 1300 ° C at 200 ° C / h, and
After holding at ℃ for 10 hours, 60 ℃ up to 1550 ℃
/ H and the temperature was raised at 1550 ° C. for 5 hours for heat treatment to obtain a sintered body, and a silicon nitride filter was obtained. Then, reduction treatment was performed for 4 hours in an electric furnace under a hydrogen atmosphere of 20 vol% nitrogen (pressure 0.1 MPa).

The obtained silicon nitride filter maintained a honeycomb shape, and cracks and the like were not observed on the surface or inside. The porosity of this sintered body was 55%, the average pore diameter was 11 μm, and β-type silicon nitride was identified by X-ray diffraction.

Example 2 In Example 1, Al ₂ O ₃ .SiO
Example 2 except that the addition amount of ₂ composite oxide particles was changed from 24 parts to 22 parts, and 2 parts of platinum ammonium salt was changed to 7 parts of cerium nitrate (manufactured by Junsei Chemical Co., Ltd.) and no reduction treatment was performed. Similarly, a silicon nitride filter was produced. The obtained silicon nitride filter maintained the honeycomb shape, and cracks and the like were not observed on the surface or inside. The porosity of this sintered body was 55%, the average pore diameter was 11 μm, and silicon nitride was identified by X-ray diffraction.

Example 3 A silicon nitride filter was produced in the same manner as in Example 1 except that the platinum ammonium salt as the catalyst precursor was not added. The obtained silicon nitride filter maintained the honeycomb shape, and no cracks or the like were observed on the surface or inside. The porosity of this sintered body was 55%, and the average pore diameter was 11 μm.
The sample had the same porosity and pore diameter, and β-type silicon nitride was identified by X-ray diffraction.

Example 4 The silicon nitride filter obtained in Example 3 was further immersed in a slurry in which platinum ammonium salt and γ-alumina particles were mixed in an amount of 50% each, and the temperature was 500 ° C. in an electric furnace at 10% by volume of hydrogen. Heat treatment was carried out for 2 hours in argon gas to support the catalyst.

Example 5 In Example 1, silicon nitride particles (manufactured by Ube Industries, trade name: SN) were used instead of the metallic silicon particles.
The same procedure as in Example 1 was carried out except that E-10) was used. No columnar substance was visually observed on the surface of the through hole. The porosity of this sintered body is 45% and the average pore diameter is 8μ.
m, and β-type silicon nitride was identified by X-ray diffraction.

[Evaluation Method] In order to compare the characteristics when the silicon nitride filter obtained above is used as a DPF, diesel exhaust gas is passed through the silicon nitride filters of Examples 1 to 4 to change pressure loss and particulates. The combustion start temperature was evaluated.

That is, the characteristics of this filter are as follows: diesel engine (displacement 4000 cc, 140 PS / 400
The exhaust gas from 0 rpm (outlet temperature 250 ° C., outlet particulate amount Bosch concentration 2) was used for evaluation. First, regarding the pressure loss of each filter, after selecting the operating condition of the engine so that the soot emission amount is 30 g in advance, the DPF is mounted and the operation is restarted, the exhaust gas flow rate is 400 m ³ / hour, the exhaust gas temperature is Was about 270 ° C., the pressure difference between the inlet and the outlet of the filter was monitored and used as the initial pressure loss. After measuring the initial pressure loss, the engine is operated under the selected engine operating conditions, the pressure loss at the same flow rate and the same exhaust gas temperature is monitored again, and then the heater arranged around the DPF is energized to further raise the exhaust gas temperature, At that time, the temperature rise of the DPF and the change of the differential pressure are measured in parallel,
The temperature at which the increase in pressure loss disappeared was measured as the particulate combustion start temperature. The results are shown in Table 1.

[0046]

[Table 1]

From Table 1, the pressure loss of Example 1 and the pressure loss of Example 2 were almost the same. Furthermore, the particulate combustion start temperatures of Example 1 and Example 2 were lower by 50 ° C. and 40 ° C. than in Example 3, respectively. Example 4 has a pressure loss of about 30 × 10 ² P as compared with Examples 1, 2 and 3.
In addition to the increase in pressure, there was a variation in the pressure loss among individuals due to a slight variation in the coat layer. Also, Example 1 is Example 5
The particulate combustion temperature is reduced by about 20 ° C. compared to the above, but the reason is not clear, but this is because the DPF of Example 1, which is the present filter, is obtained by directly nitriding metallic silicon particles, and thus columnar substances are present on the filter surface. It is thought to be related to having the structural feature of being precipitated.

[0048]

According to the present filter, the particulate combustion start temperature is lowered and the regeneration efficiency of the filter is improved without impairing the pressure loss characteristic. Further, the durability of the filter is improved by lowering the particulate combustion start temperature, and the durability of the catalyst itself is also improved. Further, depending on the regeneration system, there is an advantage that the heating of the required exhaust gas is unnecessary or only a slight heating is required.

Further, according to this manufacturing method, the step of supporting the catalyst can be omitted, and the manufacturing is simplified and the cost is reduced. Further, problems due to loading the catalyst, such as fluctuations in pressure loss at the time of collection due to the thickness of the washcoat layer, can be reduced, so it is possible to estimate the amount of collected matter calculated based on the pressure loss. It is accurate and can prevent defective regeneration, filter melting and damage that occur during regeneration.

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Claims (8)

[Claims] 1. A silicon nitride-based filter, which is a silicon nitride-based porous body having open pores communicating with each other, wherein catalyst particles are contained inside the porous body. 2. The catalyst is lanthanum, cerium, neodymium, praseodymium, vanadium, titanium, chromium, manganese, iron, nickel, cobalt, copper, platinum, rhodium, palladium, iridium, potassium, sodium,
Lithium, rubidium, cesium, strontium, barium, calcium, indium, arsenic, antimony,
The silicon nitride filter according to claim 1, which is a metal, alloy, metal oxide, or sulfate containing at least one element selected from the group consisting of molybdenum and tungsten. 3. The porosity of the filter is 30-80%,
And the average pore diameter measured by mercury porosimetry is 5-40μ
The silicon nitride filter according to claim 1, wherein m is m. 4. A mixture containing 30 to 90% by mass of metal silicon particles having an average particle diameter of 1 to 100 μm, 9 to 60% by mass of a pore-forming agent, and 0.01 to 15% by mass of a catalyst or catalyst precursor. In addition, a molding aid and water are added and kneaded to form a kneaded product, and the kneaded product is extrusion-molded using a mold, and the extrusion molding has a through hole having a cross-sectional area of 1 to 100 mm ² per hole. A method for producing a silicon nitride filter, comprising obtaining a body, and then subjecting the extruded body to heat treatment in a nitrogen atmosphere for nitriding. 5. The catalyst precursor is lanthanum, cerium, neodymium, praseodymium, vanadium, titanium, chromium, manganese, iron, nickel, cobalt, copper, platinum, rhodium, palladium, iridium, potassium, sodium, lithium, rubidium, A metal or alloy containing at least one element selected from the group consisting of cesium, strontium, barium, calcium, indium, arsenic, antimony, molybdenum and tungsten,
Oxides, sulfates, nitrates, carbonates, hydroxides, acetates,
The method for producing a silicon nitride filter according to claim 4, which is a phosphate, an ammonium salt, an oxalate, a halide or a complex thereof. 6. The pore-forming agent is organic polymer particles or S
The method for producing a silicon nitride filter according to claim 4 or 5, which is one or more kinds of metal oxide particles selected from the group consisting of i, Al, Y, Mg, and Yb. 7. The method for producing a silicon nitride filter according to claim 4, 5 or 6, wherein the pore forming agent is hollow particles or spherical particles, and the average particle diameter thereof is 10 to 100 μm. 8. The heat treatment is performed at a temperature of 1000 to 1800 ° C.
The method for producing a silicon nitride filter according to claim 4, which is held for 1 to 12 hours.