Method for producing a silicon carbide-containing honeycomb structure

Abstract

A method for producing a silicon carbide-containing honeycomb structure, comprising mixing a recycled raw material derived from a material forming a first silicon carbide-containing honeycomb structure, in a post-firing process, as a component of a starting material for a second silicon carbide-containing honeycomb structure. wherein the starting material comprises silicon carbide and metallic silicon; and the recycled raw material is a powder obtained from the material that forms the first silicon carbide-containing honeycomb structure in the process after firing, and whose particle size has been adjusted after recovery such that a 10% diameter (D10) is 10 µm or more and a 50% diameter (D50) is 35 µm or less when the cumulative volume-weighted particle size distribution is measured by a laser diffraction or laser scattering method.

Description

AREA OF INVENTION

[0001] The present invention relates to a method for producing a silicon carbide-containing honeycomb structure. In particular, the present invention relates to a method for producing a silicon carbide-containing honeycomb structure that is used for a filter or a catalyst support for cleaning automotive exhaust gases or the like. BACKGROUND OF THE INVENTION

[0002] Silicon carbide (SiC) is used in various ceramic products such as heat sinks, exhaust filters, catalyst supports, sliding components, nozzles, heat exchangers, and semiconductor components, utilizing its positive properties such as high heat resistance, high hardness, excellent chemical resistance, and outstanding abrasion resistance. Among these, a Si-impregnated SiC material, in which a porous silicon carbide body is impregnated with metallic silicon, is used as a heat sink material due to its excellent thermal conductivity.Furthermore, a Si-bonded SiC material in which silicon carbide particles are bound by metallic silicon has excellent heat resistance, thermal shock resistance and oxidation resistance and is known as a typical material component of a honeycomb structure used as a filter for trapping fine particles in exhaust gas from an internal combustion engine, a boiler or the like, or used for a catalyst support of an exhaust gas purification catalyst or the like.

[0003] The silicon carbide-containing honeycomb structure can be produced, for example, by adding metallic silicon (silicon), an organic binder, and an alkaline earth metal to a silicon carbide powder; mixing and kneading the materials to obtain a green body; extruding the green body into a honeycomb mold with a predetermined honeycomb structure; calcining the resulting honeycomb mold to remove the organic binder from the mold; and then firing the mold.

[0004] Such a silicon carbide-containing honeycomb structure can be supplied as a single-piece molded object, but to improve thermal shock resistance, multiple honeycomb structure segments can be joined with an adhesive after firing. In the latter case, the outer circumferential section of the segmented assembly, formed by joining multiple honeycomb structure segments to obtain a desired shape (for example, a cylindrical shape), is ground. Then, the outer circumferential surface of the segmented assembly is coated with a coating material and subsequently heat-cured to form the outer circumferential wall.

[0005] For several years, there have been calls to recycle waste material generated during the manufacturing process of silicon carbide-containing honeycomb structures in order to improve raw material utilization and reduce waste. Typically, tests are conducted to collect defective honeycomb molds before firing and reuse them as raw material. However, it is also assumed that waste materials such as defective products and waste generated after firing negatively impact the quality of the honeycomb structure. This has resulted in limited technological development for recovering waste material generated after firing and reusing it as raw material for honeycomb structures.However, as an example of the state of the art for the use of waste material after firing the honeycomb molded body, which contains silicon carbide as raw material, there is a process described in patent literature 1 (Japanese patent application, publication no. JP 2011-168438 A).

[0006] Patent literature 1 proposes a method for producing a silicon carbide honeycomb structure, in which a recycled raw material is recovered from the material of which the silicon carbide honeycomb structure is made, after firing the initial raw material of the silicon carbide honeycomb structure, the average particle diameter of the recycled raw material is adjusted to 5 to 100 µm and it is then added as part of the initial raw material of the silicon carbide honeycomb structure, such that its proportion of the total initial raw material is 50 percent by mass or less.Furthermore, an alternative method is proposed in which the recycled raw material is added as part of the starting material of the silicon carbide honeycomb structure, such that its proportion of the total starting material is 50% by mass or less, and then the average particle diameter is adjusted to 5 to 100 µm, and the process for producing a silicon carbide honeycomb structure is used. According to the process for producing a silicon carbide honeycomb structure described in patent literature 1, a honeycomb structure with properties such as thermal conductivity, strength, and porosity can be produced that are comparable to those that occur when no recycled materials are used. LIST OF LITERATURES Patent Literature

[0007] [Patent Literature 1] Japanese patent application, publication no. JP 2011-168438 A SUMMARY OF THE INVENTION

[0008] The process described in patent literature 1 is significant insofar as it provides a method for recovering and reusing waste material generated in the process of firing a silicon carbide-containing honeycomb body to produce a silicon carbide-containing honeycomb structure. However, this technique is still under development and there is room for improvement. In particular, the investigations of the inventors of the present application have revealed the problem that the silicon carbide-containing honeycomb structure produced using recycled raw material according to the process of patent specification 1 cannot achieve consistently good thermal conductivity. If the thermal conductivity decreases, the service life of the silicon carbide-containing honeycomb structure as a product is shortened, which should be avoided.Furthermore, higher thermal conductivity allows the soot to burn off more easily during filter regeneration, thus enabling faster filter regeneration. Additionally, heating the honeycomb structure reduces the temperature difference between the inner and outer surfaces.

[0009] Furthermore, the silicon carbide-containing honeycomb structure, which is produced using a recycled raw material according to the process of patent specification 1, is still capable of improvement with regard to achieving a consistently good separation efficiency when used as a filter.

[0010] The present invention was developed in consideration of the above statements, and in one embodiment the objective is to provide a method for the constant production of a silicon carbide-containing honeycomb structure in which the decrease in thermal conductivity and the decrease in deposition efficiency are suppressed when a recycled raw material derived from the material that forms a silicon carbide-containing honeycomb structure in the post-firing process is reused as the starting raw material for a silicon carbide-containing honeycomb structure.

[0011] The inventors of the present application have carried out careful investigations to solve the above problems and have found that, although patent literature 1 indicates that the average particle diameter of the recycled raw material powder is adjusted to 5 to 100 µm, this is insufficient, and that by adjusting D10 and D50 of the recycled raw material powder according to specified conditions, the decrease in thermal conductivity is effectively suppressed. The present invention has been further developed based on the above findings and is described below by way of example. [1] A process for producing a silicon carbide-containing honeycomb structure, comprising mixing a recycled raw material derived from a material forming a first silicon carbide-containing honeycomb structure, in a post-firing process, as a component of a starting material for a second silicon carbide-containing honeycomb structure, wherein the starting material comprises silicon carbide and metallic silicon; and the recycled raw material is a powder obtained from the material that forms the first silicon carbide-containing honeycomb structure in the process after firing, and whose particle size has been adjusted after recovery such that a 10% diameter (D10) is 10 µm or more and a 50% diameter (D50) is 35 µm or less when the cumulative volume-weighted particle size distribution is measured by a laser diffraction / laser scattering method. [2] Method according to claim 1, wherein the recycled raw material is the powder in which the particle size has been adjusted such that the 10% diameter (D10) is 15 µm or more when the cumulative particle size distribution by volume weighted is measured by the laser diffraction / laser scattering method. [3] Method according to claim 1 or 2, wherein the recycled raw material is the powder in which the particle size has been adjusted such that a 90% diameter (D90) is 60 µm or less when the cumulative volume weighted particle size distribution is measured by the laser diffraction / laser scattering method. [4] Method according to any one of claims 1 to 3, comprising mixing the recycled raw material into the starting raw material, such that the proportion of the recycled raw material to the total mass of the silicon carbide, the metallic silicon and the recycled raw material is 20 to 80% by mass.

[0012] According to one embodiment of the present invention, it is possible to consistently produce a silicon carbide-containing honeycomb structure in which the decrease in thermal conductivity is suppressed when using recycled raw material derived from the material that forms a honeycomb structure containing silicon carbide in the post-firing process. Therefore, according to one embodiment of the present invention, it is possible to contribute to better utilization of the raw material and a reduction in the amount of waste in the industrial production of the silicon carbide-containing honeycomb structure. DETAILED DESCRIPTION OF THE INVENTION

[0013] The following sections describe in detail embodiments of the present invention with reference to the drawings. It is understood that the present invention is not limited to the following embodiments and that any modification, improvement, or the like can be added to the embodiment based on the general knowledge of a person skilled in the art, without departing from the purpose of the present invention.

[0014] According to one embodiment of the present invention, a method for producing a silicon carbide-containing honeycomb structure is provided in which a recycled raw material originating from a material forming a first silicon carbide-containing honeycomb structure is mixed in a post-firing process as part of a starting raw material for a second silicon carbide-containing honeycomb structure. (1. Recycled raw material)

[0015] The recycled raw material is not particularly limited as long as it originates from the material forming an initial honeycomb structure containing silicon carbide from a post-firing process. However, it typically originates from a defective-fired product of a silicon carbide-containing honeycomb structure obtained by firing a silicon carbide-containing honeycomb body. This fired product may be fired using only the pure raw material, or it may be fired using a mixture of the pure raw material and recycled raw material. If a silicon carbide-containing honeycomb structure is produced by joining and integrating multiple honeycomb structure segments with an joining material, the recycled raw material may originate from: (1) a defective product of a honeycomb structure segment (a fired product);(2) a defective product of a segmented joining body formed by joining a plurality of honeycomb structure segments using an joining material (in addition to the honeycomb structure segment, which is a fired product, the joining material, which is not a fired product, is included); (3) grinding powder produced when the outer circumferential section of the segmented joining body is ground to obtain a desired shape (for example, a cylindrical shape) (in addition to the honeycomb structure segment, which is a fired product, the joining material, which is not a fired product, is included);(4) a defective end product obtained by a process of applying a coating material to the outer circumferential side surface of the segment joining body and subsequent drying and heat treatment to form an outer circumferential wall (in addition to the honeycomb structure segment, which is a fired product, the joining material and the coating material, which are not fired products, are included).

[0016] Therefore, the recycled raw material can comprise not only a calcined product forming the main body of the silicon carbide-containing honeycomb structure, but also uncalcined material such as an adhesive and a coating material for the outer perimeter. With regard to impurities, it is preferred that 80% or more by weight of the recycled raw material is the calcined product, and more preferably that 90% or more by weight of the recycled raw material is the calcined product. In particular, the proportion of the silicon carbide-silicon composite material that is a calcined product in the recycled raw material is preferably 70% or more by weight and more preferably 80% or more by weight.

[0017] It is desirable that the recycled raw material be a powder recovered from the material forming the first silicon carbide-containing honeycomb structure in the post-firing process, and that its particle size after recovery be adjusted so that the 10% diameter (D10) is 10 µm or greater and the 50% diameter (D50) is 35 µm or less, as measured by volume-weighted cumulative particle size distribution using laser diffraction / scattering. Adjusting D10 and D50 to the above range enables the consistent production of a second silicon carbide-containing honeycomb structure, suppressing the decrease in thermal conductivity when using the recycled raw material. Furthermore, if the second silicon carbide-containing honeycomb structure is used as an exhaust gas filter, it is possible to suppress the decrease in separation efficiency.The particle size can be adjusted, for example, by carrying out known techniques such as pulverizing and sieving after recovery of the material that forms the first silicon carbide-containing honeycomb structure in the post-firing process.

[0018] The lower limit of D10 of the recycled raw material is preferably 10 µm or more, more preferably 15 µm or more, and particularly preferably 20 µm or more, with a view to increasing the thermal conductivity of the second silicon carbide-containing honeycomb structure. The upper limit of D10 of the recycled raw material is not particularly limited and is naturally 35 µm or less, since the upper limit of D50 is 35 µm. The upper limit of D10 of the recycled raw material is typically 30 µm or less, and even more typically 25 µm or less.

[0019] The D50 of the recycled raw material correlates with the pore volume fraction of ≥ 40 µm, and the smaller the D50, the smaller the pore volume fraction tends to be. The pore volume fraction of 40 µm or larger pores refers to the volume fraction of the total pore volume in the partitions. Furthermore, the pore volume fraction of ≥ 40 µm correlates with the particulate matter (PM) separation efficiency of the filter, and the smaller the pore volume fraction of ≥ 40 µm, the better the PM separation. Therefore, the D50 of the recycled raw material is preferably 35 µm or less, more preferably 30 µm or less, and most preferably 25 µm or less, with a view to improving the separation efficiency. The lower limit of D50 of the recycled raw material is not particularly limited, but it is naturally 10 µm or more, since the lower limit of D10 is 10 µm.The lower limit of D10 is typically 15 µm or more, and even more typically 20 µm or more.

[0020] Therefore, in a preferred embodiment, the recycled raw material is a powder in which the particle size is adjusted such that D10 is 15 µm or more and D50 is 30 µm or less. In another preferred embodiment, the recycled raw material is a powder in which the particle size is adjusted such that D10 is 20 µm or more and D50 is 30 µm or less. In a further preferred embodiment, the recycled raw material is a powder in which the particle size is adjusted such that D10 is 15 µm or more and D50 is 25 µm or less.

[0021] Furthermore, it is desirable that the recycled raw material be a powder in which the particle size is adjusted such that a 90% diameter (D90) is 60 µm or less when the cumulative volume-weighted particle size distribution is measured by laser diffraction / laser scattering. Adjusting the D90 of the recycled raw material to 60 µm or less, preferably 50 µm or less, ensures good formability and achieves good productivity. Although the lower limit of the D90 of the recycled raw material is not particularly restrictive, it is typically 35 µm or more, and even more typically 40 µm or more. (2. Starting material)

[0022] The recycled material can be mixed as part of the starting material for the production of a second silicon carbide-containing honeycomb structure. In one embodiment, the starting material for the second silicon carbide-containing honeycomb structure comprises silicon carbide and metallic silicon, as well as the recycled raw material. The proportion of the recycled raw material in the total mass of silicon carbide, metallic silicon, and recycled raw material is preferably 20 to 80 percent by mass. Considering the general amount of waste material generated in the industrial production of silicon carbide-containing honeycomb structures, the waste material produced in the manufacturing process of the silicon carbide-containing honeycomb structure can be almost completely consumed if the proportion of recycled raw material is 20 percent by mass or more, preferably 30 percent by mass or more.Furthermore, if the proportion of recycled raw material is 80% by mass or less, good formability and productivity can be ensured, and it is also possible to prevent a deterioration of the properties of the second silicon carbide-containing honeycomb structure. With regard to ensuring good properties and good formability of the second silicon carbide-containing honeycomb structure, the proportion of recycled raw material is preferably 70% by mass or less, more preferably 60% by mass or less, and particularly preferably 50% by mass or less.

[0023] In one embodiment, the silicon carbide added to the starting material for the production of the second silicon carbide-containing honeycomb structure is in powder form. In this case, D50 of the silicon carbide particles forming the silicon carbide powder is preferably 5 µm or more, more preferably 10 µm or more, and particularly preferably 15 µm or more, with a view to reducing the pressure drop. Furthermore, D50 of the silicon carbide particles forming the silicon carbide powder is preferably 60 µm or less, more preferably 50 µm or less, and particularly preferably 40 µm or less, with a view to improving the separation efficiency as a filter. In the present invention, D50 of the silicon carbide particles refers to the 50% diameter of the silicon carbide powder when the cumulative particle size distribution is measured by volume weighted using a laser diffraction / laser scattering method.

[0024] By mixing metallic silicon into the initial raw material for producing the second silicon carbide-containing honeycomb structure, a composite material of silicon carbide and metallic silicon can be obtained after firing. In one embodiment, the metallic silicon added to the initial raw material is in powder form. In this case, the D50 of the metallic silicon particles forming the metallic silicon powder is preferably 10 µm or less, more preferably 8 µm or less, and particularly preferably 6 µm or less, with a view to increasing the strength of the fired body. Since the finer metallic silicon particles are preferred, the lower limit of D50 is not particularly restricted, but from the point of view of availability, D50 of the metallic silicon particles is normally 3 µm or more.In the present invention, D50 of the metallic silicon particles refers to the 50% diameter of the metallic silicon powder when the cumulative particle size distribution is measured by volume weighted using a laser diffraction / laser scattering method.

[0025] If the total mass of silicon carbide and metallic silicon (excluding the silicon carbide and metallic silicon contained in the recycled raw material) among the above starting materials is set to 100 parts by weight, the concentration of metallic silicon is preferably 14 parts by weight or more, and more preferably 16 parts by weight or more, to increase the strength of the fired body. If the total mass of silicon carbide and metallic silicon (excluding the silicon carbide and metallic silicon contained in the recycled raw material) is set to 100 parts by weight, the concentration of metallic silicon is preferably 24 parts by weight or less, and more preferably 22 parts by weight or less, to suppress deformation during firing.

[0026] The starting material for the second silicon carbide-containing honeycomb structure may also contain an organic binder. Examples of organic binders include, but are not limited to, methylcellulose, hydroxypropylmethylcellulose, hydroxypropoxylcellulose, hydroxyethylcellulose, carboxymethylcellulose, polyvinyl alcohol, and the like. The organic binder may consist of a single agent or two or more agents in combination.

[0027] To improve the dimensional stability of the molded part, the concentration of the organic binder in the starting material is preferably 3 parts by weight or more, more preferably 4 parts by weight or more, and particularly preferably 5 parts by weight or more, based on a total of 100 parts by weight of silicon carbide, metallic silicon, and recycled raw material. Furthermore, to reduce drying shrinkage, the concentration of the binder in the starting material is preferably 10 parts by weight or less, more preferably 9 parts by weight or less, and particularly preferably 8 parts by weight or less, based on a total of 100 parts by weight of silicon carbide, metallic silicon, and recycled raw material.

[0028] The starting material for the second silicon carbide-containing honeycomb structure may further include a pore-forming agent. For example, if the silicon carbide-containing honeycomb structure is used as an exhaust gas filter, a pore-forming agent may be added to the starting material to increase its porosity. The amount of pore-forming agent added may be, for example, 40 parts by weight or less, typically 1 to 25 parts by weight, based on a total of 100 parts by weight of silicon carbide, metallic silicon, and recycled raw material.

[0029] The type of pore-forming agent used is not precisely limited, but examples include graphite, malleable resin, foamed resin, wheat flour, starch, phenolic resin, polymethyl methacrylate, polyethylene, polymethacrylate, polyethylene terephthalate, and the like. The pore-forming agent can be a single substance or a combination of two or more.

[0030] The starting material for the second silicon carbide-containing honeycomb structure may additionally contain an alkaline earth metal to improve the wettability of the metallic silicon during firing. The amount of alkaline earth metal added may be, for example, 5 parts by weight or less, typically 1 to 3 parts by weight, based on a total of 100 parts by weight of silicon carbide, metallic silicon, and recycled raw material. The type of alkaline earth metal used is not precisely limited, but examples include calcium and strontium. The alkaline earth metal may be a single metal, or two or more metals may be included in combination. (3. Method for producing a second silicon carbide-containing honeycomb structure)

[0031] The second silicon carbide-containing honeycomb structure can be produced, for example, by extruding a green body formed by mixing and kneading the aforementioned starting material, which includes the recycled raw material, to obtain a honeycomb mold with a predetermined honeycomb structure; subsequently calcining the resulting honeycomb mold to remove the organic binder from the mold; and then firing. Each step is described below as an example.

[0032] In one embodiment, it is possible to produce a honeycomb-shaped body by extruding a green body. This honeycomb structure comprises a columnar honeycomb section with an outer circumferential sidewall and a plurality of partitions arranged on the inner circumferential sidewall. The plurality of partitions divides into a plurality of cells, forming flow paths for a fluid from one end face to the other. During extrusion, a die with the desired shape, cell shape, partition thickness, cell density, and the like can be used.

[0033] Next, the resulting undried molded body is dried to remove water. Drying can be achieved, for example, by hot gas treatment of the molded body at approximately 120 to 160°C. Care must be taken to prevent the organic substances from decomposing during drying.

[0034] The shape of the cells in cross-section perpendicular to the flow direction of the cells is not limited, but is preferably a square, a hexagon, an octagon, or a combination thereof. Among these, a square and a hexagon are preferred. By producing the cell shape in this way, it is possible to reduce the pressure drop when a gas is passed through the honeycomb structure after firing.

[0035] The shape of the honeycomb body is not limited, but it can be, for example, a column shape with a circular end face (cylindrical shape), a column shape with an oval end face, a column shape with a polygonal end face (square, pentagon, hexagon, heptagon, octagon and the like), and the like.

[0036] It is possible to perform a step on the dried, honeycomb-shaped body to obtain a degreased body by removing organic substances, such as the binder, by heat (a degreasing step) after sealing sections have been formed on both end surfaces as required. The method for forming the sealing sections on both end surfaces is not strictly limited, and a known technique such as filling the cell openings with sealing slurry while simultaneously applying a predetermined mask to the end surface can be used. The firing temperature of the molded body in the degreasing step can be, for example, 400 to 500°C, and the firing time at firing temperature can be, for example, 1 to 3 hours.

[0037] The degreasing step can be carried out, for example, under an atmosphere of air, an inert atmosphere, or a reduced-pressure atmosphere. Of these, an inert atmosphere and a reduced-pressure atmosphere are preferred because they prevent insufficient bonding between the silicon carbide and the metallic silicon due to silicon carbide oxidation and facilitate the reduction of oxides present in the raw material. However, if the degreasing step is performed under an inert atmosphere or a reduced-pressure atmosphere, it is very time-consuming. Furthermore, since the degreasing temperature is not very high, oxidation of the molded part is unlikely, even when this step is carried out under an atmosphere of air. Therefore, considering the balance between productivity and quality, it is advisable to perform the degreasing step under an atmosphere of air.

[0038] The production of a silicon carbide-containing honeycomb structure is achieved by firing the dried or degreased molded body under an inert atmosphere. It is also possible to perform the degreasing and firing steps simultaneously in a continuous furnace. The firing temperature can be, for example, 1400 to 1500°C, and the firing time at this temperature can be, for example, 0.1 to 3 hours.

[0039] The porosity of the partition walls of the second silicon carbide-containing honeycomb structure obtained by the firing step is not precisely limited, but with regard to reducing pressure loss, it is preferably 35% or more, and more preferably 40% or more. However, with regard to service life, the porosity is preferably 70% or less, and more preferably 65% ​​or less. In this description, the porosity refers to a value measured by the mercury intrusion method.

[0040] With regard to ensuring service life, the thermal conductivity of the partitions of the second silicon carbide-containing honeycomb structure obtained by the firing step is preferably 15 W / (m·K) or more, more preferably 17 W / (m·K) or more, particularly preferably 19 W / (m·K) or more, and can, for example, be 15 to 23 W / (m·K). In this description, the thermal conductivity of the partitions of the second silicon carbide-containing honeycomb structure refers to a value at 50°C measured using a steady-state method.

[0041] The pore volume fraction of the partitions of the second silicon carbide-containing honeycomb structure, obtained by performing the firing step, is preferably 7% or less, and more preferably 6% or less, with a view to increasing the particulate matter separation efficiency when the honeycomb structure is used as a filter. The pore volume fraction of the partitions of the second silicon carbide-containing honeycomb structure refers to a value measured from the cumulative pore diameter distribution curve obtained by the mercury intrusion process specified in JIS R1655:2003.

[0042] The second silicon carbide-containing honeycomb structure, obtained by the firing step, can be used as a finished product. In another embodiment, multiple silicon carbide-containing honeycomb structures can be used individually as honeycomb segments, and the side surfaces of these segments can be joined together using an adhesive. The segmented assembly obtained by heat drying can then be used as the finished product of the second silicon carbide-containing honeycomb structure. In a further embodiment, the outer circumferential region of the segmented assembly can be ground to form a desired shape (e.g., a cylindrical shape), and a finished product can be obtained by applying a coating material to the outer circumferential side surface and subsequently drying and heat-treating it to form an outer circumferential wall.The temperature of the heat treatment can be, for example, 400 to 700°C.

[0043] A known joining material can be used. For example, a joining material prepared by mixing a ceramic powder, a dispersion medium (e.g., water or the like), and optionally an additive such as a binder, a peptizing agent, and a foamed resin can be used. Examples of the ceramic include cordierite, mullite, zirconium phosphate, aluminum titanate, silicon carbide, silicon-silicon carbide composite (e.g., Si-bonded SiC), cordierite-silicon carbide composite, zirconium dioxide, spinel, indialite, sapphire, corundum, titanium dioxide, silicon nitride, and the like, with a silicon-silicon carbide composite being preferred. Examples of the binder include polyvinyl alcohol and methylcellulose.

[0044] A known external coating material can be used. This external coating material can be obtained, for example, by adding additives such as an organic binder, a foamed resin, and a dispersant, as well as water, to inorganic raw materials such as inorganic fiber, colloidal silicon dioxide, clay, and ceramic particles, and kneading them into a slurry. Furthermore, the coating process for the external coating material is not particularly restricted, and a known method can be used.

[0045] If the second silicon carbide-containing honeycomb structure is used as an exhaust filter, a suitable catalyst can be applied, depending on the application. One method for applying the catalyst to the filter involves introducing a catalyst slurry into the cells using a conventional suction method or similar technique. The slurry adheres to the surface and pores of the partition walls, after which a high-temperature treatment is performed to fuse the catalyst contained in the slurry to the partition walls.

[0046] Examples of catalysts include a DOC oxidation catalyst for the oxidative combustion of hydrocarbons (HC) and carbon monoxide (CO) to increase exhaust gas temperature, a PM combustion catalyst that supports the combustion of PM such as soot, an SCR catalyst, and a NO x - Catalyst for the removal of nitrogen oxides (NOx)x ), as well as a three-way catalyst that simultaneously converts hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) x ) can remove. The catalyst can, in a suitable manner, contain, for example, precious metals (Pt, Pd, Rh and the like), alkali metals (Li, Na, K, Cs and the like), alkaline earth metals (Ca, Ba, Sr and the like), rare earths (Ce, Sm, Gd, Nd, Y, Zr, Ca, La, Pr and the like), transition metals (Mn, Fe, Co, Ni, Cu, Zn, Sc, Ti, V, Cr and the like) and the like. EXAMPLES (1. Production of the first silicon carbide-containing honeycomb structure)

[0047] Eighty parts by weight of silicon carbide powder with a D50 of 30 µm, 20 parts by weight of metallic silicon powder with a D50 of 5 µm, 2 parts by weight of starch (pore former), 5 parts by weight of hydroxypropyl methylcellulose (organic binder), and 2 parts by weight of strontium carbonate were prepared and mixed in powder form. After adding water, the mixture was kneaded. The resulting kneaded mass was extruded through a predefined die into a rectangular, cuboid honeycomb body using an extrusion die. The honeycomb body had a columnar honeycomb structure with an outer circumferential sidewall and a plurality of partitions arranged on the inner circumferential sidewall. These partitions subdivided into a plurality of cells, forming flow paths for fluid from one end face to the other.

[0048] After microwave drying of the honeycomb mold, it was dried at 120°C for 2 hours using a hot gas dryer, and the necessary processing, such as cutting both end faces to a predetermined length, was carried out to produce a dried rectangular, cuboid honeycomb mold with a length of 35 mm, a width of 35 mm, a height (the direction in which the cells extend) of 160 mm, a septum thickness of 300 µm and a cell density of 47 cells / cm³. 2To produce the honeycomb, the dried honeycomb body was placed in an electric continuous oven after alternating closure sections had formed at the cell ends to create a checkerboard pattern on both end faces. It was then heated at 450°C or lower for 2 hours in an air atmosphere to remove the binder. The defatted honeycomb body was subsequently fired at 1450°C for 2 hours in an air atmosphere to produce a silicon carbide-containing honeycomb structure (a honeycomb segment).

[0049] Next, 16 honeycomb segments, obtained through the manufacturing process described above, were produced. Their side surfaces were joined together in an arrangement of 4 segments vertically × 4 segments horizontally using a bonding material containing silicon carbide and ceramic fibers. Heat-drying at 140°C resulted in a segmented connector body. The outer circumference of the segmented connector body was ground into a cylindrical shape, and a silicon carbide-containing coating material was applied to the entire outer circumferential side surface. This coating was then heat-dried at 600°C to form a 0.2 mm thick outer circumferential coating layer, thus creating an initial silicon carbide-containing honeycomb structure. (2. Production of a second silicon carbide-containing honeycomb structure)

[0050] Defective honeycomb segments after firing, defective segment connectors, external circumferential grinding powder, and defective finished products generated during the industrial production of the first silicon carbide-containing honeycomb structure were collected and recycled by crushing with a roller mill and sieving. These recycled materials had different particle size distributions (D10, D50, D90), as shown in the test results listed in Table 1. The proportion of silicon carbide-silicon composite (fired product) in the recycled raw material was 90% by mass.

[0051] Silicon carbide powder with a D50 of 30 µm and metallic silicon powder with a D50 of 5 µm were prepared in a mass ratio of silicon carbide powder to metallic silicon powder of 4:1. Recycled raw material was then prepared such that the ratio of the recycled raw material to the total mass of silicon carbide powder, metallic silicon powder, and recycled raw material corresponded to the values ​​listed in Table 1. The proportion of recycled material was varied from 0% to 90% by mass in each test. Next, 5 parts by weight of hydroxypropyl methylcellulose (organic binder), 2 parts by weight of starch (pore former), and 2 parts by weight of strontium carbonate were prepared, based on a total of 100 parts by weight of silicon carbide, metallic silicon, and recycled raw material.These were dry-mixed together with the silicon carbide powder, the metallic silicon powder, and the recycled raw material. After adding water, the mixture was kneaded with a kneader. The resulting putty was extruded through a predefined die into a rectangular, cuboid honeycomb body using an extrusion die. The honeycomb body had a columnar honeycomb structure section with an outer circumferential sidewall and a plurality of partitions arranged on the inner circumferential side of the outer circumferential sidewall. The plurality of partitions subdivided into a plurality of cells, forming flow paths for fluid from one end face to the other.

[0052] After microwave drying of the honeycomb mold, it was dried at 120°C for 2 hours using a hot gas dryer, and the necessary processing, such as cutting both end faces to a predetermined length, was carried out to produce a dried rectangular, cuboid honeycomb mold with a length of 35 mm, a width of 35 mm, a height (the direction in which the cells extend) of 160 mm, a septum thickness of 300 µm and a cell density of 47 cells / cm³. 2To produce a second honeycomb structure, the dried honeycomb body, after the alternating formation of closure sections to create a checkerboard pattern at the cell ends on both end faces, was placed in an electric continuous furnace and degreased (removed from the binder) by heating at 450°C or lower for 2 hours in an air atmosphere. Subsequently, the degreased honeycomb body was fired at 1450°C for 2 hours in an air atmosphere to produce a second silicon carbide-containing honeycomb structure (a honeycomb segment). (3. Porosity)

[0053] A sample of approximately 1 cm² was cut from the second silicon carbide-containing honeycomb structure obtained above, and the porosity (%) was measured at a mercury intrusion pressure of 0.6 to 10,000 psia using the mercury intrusion method (available from Shimadzu Corporation, model AUTOPORE) according to JIS R1655:2003. The results are shown in Table 1. The measurement was performed on a large number of second silicon carbide-containing honeycomb structures, and essentially the same measurement results were obtained in the second silicon carbide-containing honeycomb structures prepared under the same conditions. (4. Thermal conductivity)

[0054] A sample of the second silicon carbide-containing honeycomb structure obtained above was taken by cutting it open in a direction perpendicular to the flow path, with a thickness of approximately 20 mm, and its thermal conductivity (W / (m·K)) at 50°C was measured using a steady-state method (available from ULVAC-RIKO, model GH-1S). The results are shown in Table 1. The measurement was performed on a large number of second silicon carbide-containing honeycomb structures, and essentially the same measurement results were obtained for the second silicon carbide-containing honeycomb structures produced under the same conditions. (5. ≥ 40 µm pore volume fraction)

[0055] From the cumulative pore diameter distribution curve obtained by measuring the porosity, the volume of pores 40 µm or larger was determined, and its proportion relative to the total pore volume was calculated. The measurement was performed on a variety of second silicon carbide-containing honeycomb structures, and essentially the same measurement results were obtained for all second silicon carbide-containing honeycomb structures produced under identical conditions. (6. Transformation indicator)

[0056] During the production of the second silicon carbide-containing honeycomb structure, the forming speed was measured using a laser speedometer, and the forming coefficient (seconds / piece) for extruding the aforementioned green body was calculated by dividing the length of each honeycomb body by the forming speed. The results are shown in Table 1. Table 1 D10 D50 D90 Recycled raw material percentage (%) 0 20 50 80 90 Comparison Example 1 5 30 50 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 21 18 13 12 > 40 pore volume fraction (%) 5 6 6 7 7 Forming coefficient (seconds / piece) 6 6 6 6 6 Example 1 10 30 50 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 20 17 16 ≥ 40 pore volume fraction (%) 5 6 6 7 7 Forming coefficient (second / piece) 6 6 6 6 6 Example 2 15 30 50 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 21 19 18 ≥ 40 pore volume fraction (%) 5 6 6 7 7 Forming coefficient (seconds / piece) 6 6 6 6 6 Example 3 20 30 50 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 23 22 20 20 ≥ 40 pore volume fraction (%) 5 6 6 7 7 Forming coefficient (seconds / piece) 6 6 6 6 6 Example 4 15 20 60 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 21 19 18 ≥ 40 pore volume fraction (%) 5 5 5 6 6 Forming coefficient (seconds / piece) 6 6 6 6 7 Example 5 15 30 60 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 21 19 18 ≥ 40 pore volume fraction (%) 5 6 6 7 7 Forming coefficient (seconds / piece) 6 6 6 6 7 Continuation of Table 1 D10 D50 D90 Recycled raw material percentage (%) 0 20 50 80 90 Example 6 15 35 60 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 21 19 18 ≥ 40 pore volume fraction (%) 5 6 6 7 7 Forming coefficient (seconds / piece) 6 6 6 6 7 Comparison Example 2 15 40 60 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 21 19 18 ≥ 40 pore volume fraction (%) 5 6 7 8 8 Forming coefficient (seconds / piece) 6 6 6 6 6 Example 7 15 25 50 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 21 19 18 ≥ 40 pore volume fraction (%) 5 5 6 6 6 Forming coefficient (seconds / piece) 6 6 6 6 6 Example 8 15 25 60 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 21 19 18 ≥ 40 pore volume fraction (%) 5 5 6 6 6 Forming coefficient (seconds / piece) 6 6 6 6 7 Example 9 15 25 70 Porosity (%) 41 41 41 41 41 Thermal conductivity (W / m·K) 23 22 21 19 18 ≥ 40 pore volume fraction (%) 5 5 6 6 6 Forming coefficient (seconds / piece) 6 6 6 9 9 (7. Discussion)

[0057] As can be seen from Table 1, the thermal conductivity in all test examples tended to decrease when the proportion of recycled raw material was increased. However, since the D10 and D50 values ​​of the recycled raw material were adjusted accordingly in examples 1 to 9, compared to examples 1 and 2, the decrease in thermal conductivity was suppressed. Furthermore, the increase in the ≥ 40 µm pore volume fraction, which is related to the deposition efficiency, was also suppressed. Additionally, a comparison of examples 7 to 9 shows that the increase in the forming coefficient was suppressed by appropriately adjusting the D90 value of the recycled raw material. (8. Relationship between ≥ 40 µm pore volume fraction and filter separation efficiency)

[0058] In the production of the first silicon carbide-containing honeycomb structure, honeycomb structures with varying pore volume fractions (≥ 40 µm) ranging from 6% to 9% were obtained by varying the average particle size of the pore-forming agent. When using the silicon carbide-containing honeycomb structure as a diesel particulate filter, the number of particles (PN) emitted in the exhaust gas was measured during operation according to European regulations for the New European Driving Cycle (NEDC) using a measurement method in accordance with the European Particle Measurement Programme (PMP). The results showed a high correlation with the coefficient of determination (R). 2 ) = 0.9551 between “≥ 40 µm pore volume fraction” and PN. That is, it was found that there is a high correlation between the ≥ 40 µm pore volume fraction and the filter separation efficiency.

Claims

[1] A method for producing a silicon carbide-containing honeycomb structure, comprising mixing, in a post-firing process, a recycled raw material derived from a material constituting a first silicon carbide-containing honeycomb structure as a component of a starting raw material for a second silicon carbide-containing honeycomb structure, wherein the starting raw material comprises silicon carbide and metallic silicon; and the recycled raw material is a powder obtained from the material forming the first silicon carbide-containing honeycomb structure in the process after firing, and whose particle size has been adjusted after recovery so that a 10% diameter (D10) is 10 µm or more and a 50% diameter (D50) is 35 µm or less when the volume-weighted cumulative particle size distribution is measured by a laser diffraction method or laser scattering method. [2] The method according to claim 1, wherein the recycled raw material is the powder in which the particle size has been adjusted so that the 10% diameter (D10) is 15 µm or more when the cumulative particle size distribution by volume weight is measured by the laser diffraction method or laser scattering method. [3] The method according to claim 1 or 2, wherein the recycled raw material is the powder in which the particle size has been adjusted so that a 90% diameter (D90) is 60 µm or less when the cumulative particle size distribution by volume weight is measured by the laser diffraction method or laser scattering method. [4] A method according to any one of claims 1 to 3, comprising mixing the recycled raw material into the starting raw material so that the proportion of the recycled raw material to the total mass of the silicon carbide, the metallic silicon and the recycled raw material is 20 to 80 mass%.