JP6953814B2 - Manufacturing method of porous filter - Google Patents

Description

The present invention relates to a method for producing a porous filter for molding and firing a raw material soil containing silica, talc, and an Al source.

Exhaust gas emitted from internal combustion engines such as diesel engines and gasoline engines, and heat engines such as boilers contains fine particles called particulates. Particulate is hereinafter appropriately referred to as "PM". A porous filter is used to collect PM in the exhaust gas.

As the porous filter, a porous body made of ceramics such as cordierite having excellent thermal stability is used. The porous filter is produced, for example, by molding and firing a clay containing a cordierite-forming raw material such as silica, talc, or an Al source.

Porous filters are required to improve the wall permeability coefficient, increase the collection rate, and reduce the pressure loss. For that purpose, it is effective to prevent the change in the pore-forming characteristics during firing and to reduce the variation in the pore size distribution. The pressure loss is hereinafter appropriately referred to as "pressure loss".

Therefore, for example, Patent Document 1 proposes a method for producing a porous body using porous silica having a bulk density reduced to a predetermined value or less. As a result, it is possible to avoid an increase in firing time and generation of harmful gas due to the combustible component of the pore-forming material in the raw material soil, and prevent changes in pore-forming characteristics and deformation of the molded product.

International Publication No. 2005/090263

However, it cannot always be said that the change in the pore-forming characteristics can be sufficiently prevented only by reducing the bulk density. Specifically, due to the diffusion of the liquid phase component generated from silica during heating at a high temperature during firing, the positional flow of the raw material particles becomes large, and the raw material particles may be rearranged. Due to this rearrangement, the pore-forming characteristics may change from the raw powder-filled structure before firing, and large pores may be formed.

Further, due to cooling during firing, crystal phase displacement occurs due to a change in the stable crystal phase in the transient temperature region, but the crystal grain structure may be destroyed or refined in this phase displacement process. As a result, the fracture interface becomes pores, and the pore-forming characteristics may change.

As described above, if the pore-forming characteristics change due to the diffusion of the liquid phase component or the destruction of the crystal grain structure, the pore size distribution may vary. As a result, it becomes difficult to manufacture a porous filter having a high collection rate due to low pressure loss.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a method for manufacturing a porous filter capable of preventing a change in pore-forming characteristics during firing and reducing variations in pore size distribution.

One aspect of the present invention is to mold and bake a raw material clay (2) containing silica (21), talc (22) and an Al source (23) (except when the Al source is boehmite). Thereby, in the method for producing the porous filter (1) having the composition of talc (24),
As the Al source, silica and aluminum hydroxide having an average particle size smaller than that of talc are used.
The ratio Ws / Wt of the mass Ws of the silica to the mass Wt of the talc in the raw material clay, the ratio Ss / St of the specific surface area Ss of the silica to the specific surface area St of the talc, and the bulk density Dt of the talc. There is a method for producing a porous filter in which the raw material clay is adjusted so that the ratio Ds / Dt of the bulk density Ds of silica satisfies the following formula (I).
Ws / Wt> Ss / St × Ds / Dt ・ ・ ・ (I)

In the above manufacturing method, the raw material soil is adjusted so as to satisfy the above formula (I), and the raw material soil is formed and fired. At the time of this firing, a part of each raw material component becomes liquid phase toward the composition of cordierite, and the raw material components are mutually diffused by atomic diffusion due to diffusion of the liquid phase component with respect to the solid phase component to obtain the target crystal structure. Cordellite crystals are formed.

By adjusting the raw material soil so as to satisfy the formula (I), it is possible to prevent the number of atoms required for the composition of cordierite from becoming silica-rich between the adjacent talc and silica particles during multiphase formation. .. As a result, it is possible to prevent the excess liquid phase component generated from silica from diffusing over a long distance during heating at a high temperature during firing. Therefore, it is possible to prevent the raw material particles such as silica, talc, and Al source from flowing. That is, it is possible to prevent the raw material particles from being rearranged from the packed structure of the raw material particles before firing. As a result, the pore size distribution according to the distribution of the starting material particles can be easily maintained, and the variation in the pore size distribution of the porous filter can be reduced.

Further, by adjusting the raw material soil satisfying the formula (I), it is possible to prevent the crystal grain structure from being destroyed in the phase displacement due to cooling during firing. That is, for example, the formation of a Si-rich phase, which is an intermediate phase composed of _{MgSiO 4, is suppressed, and local grain fracture during the cooling process during sintering is suppressed.} As a result, it is possible to prevent the expansion of the stomatal structure and the unintended change in the stomatal characteristics due to the rearrangement of the particles. Therefore, the variation in the pore size distribution of the porous filter can be reduced.

The above formula (I) will be described. The mass Ws of silica, the mass Wt of talc, the volume Vs of silica, the volume Vt of talc, the bulk density Ds of silica, and the bulk density Dt of talc have the relationship of the following formula (i).
Vs / Vt = Ws / Wt ÷ Ds / Dt = Ws / Wt × Dt / Ds ... (i)

The ideal condition is that the volume ratio Vs / Vt between the silica and talc particles is larger than the ratio of the specific surface area that is the contact region of these particles. In this case, the formation of the excess liquid phase of silica is suppressed. Furthermore, the formation of the intermediate phase is suppressed. That is, assuming that the specific surface area of silica is Ss and the specific surface area of talc is St, the ideal condition is that the following equation (ii) is satisfied.
Vs / Vt> Ss / St ... (ii)

Substituting equation (i) into equation (ii) derives the following equation (iii).
Ws / Wt × Dt / Ds> Ss / St ・ ・ ・ (iii)

By modifying equation (iii), the above equation (I) is derived. That is, the above-mentioned ideal condition is to adjust the raw material soil so as to satisfy this formula (I).

As described above, according to the above aspect, it is possible to provide a method for producing a porous filter that can prevent changes in pore-forming characteristics during firing and reduce variations in pore size distribution.
The reference numerals in parentheses described in the scope of claims and the means for solving the problem indicate the correspondence with the specific means described in the embodiments described later, and limit the technical scope of the present invention. It's not a thing.

The perspective view of the porous filter in Embodiment 1. FIG. FIG. 5 is an enlarged partial cross-sectional view of the porous filter in the axial direction according to the first embodiment. The explanatory view which shows the change of the powder-filled structure at the time of firing in Embodiment 1. FIG. The explanatory view which shows the change of the powder-filled structure by the liquid phase formation at the time of firing in the comparative form 1. FIG. The explanatory view which shows the change of the powder-filled structure by the destruction of the crystal grain structure at the time of firing in the comparative form 1. FIG. The figure which shows the pore diameter distribution of each porous filter in an experimental example.

(Embodiment 1)
An embodiment relating to a method for producing a porous filter will be described with reference to FIGS. 1 to 3.
In this embodiment, the porous filter 1 illustrated in FIGS. 1 and 2 is manufactured. The porous filter 1 is made of cordierite. That is, the porous filter 1 has a cordierite composition.

As illustrated in FIGS. 1 and 2, the porous filter 1 has an outer skin 11, a cell wall 12, and a cell 13. The outer skin 11 has a tubular shape such as a cylindrical shape. The axial direction X of the tubular outer skin will be described below as the axial direction X of the porous filter 1. Further, the arrow in FIG. 2 indicates the flow of exhaust gas when the porous filter 1 is arranged in the path of exhaust gas such as an exhaust gas pipe.

As illustrated in FIGS. 1 and 2, the cell wall 12 partitions the inside of the outer skin 11. The cell walls 12 are provided, for example, in a grid pattern. The porous filter 1 is a porous body, and pores are formed in the cell wall 12.

The cell 13 is surrounded by the cell wall 12 to form a gas flow path. The extension direction of the cell 13 usually coincides with the axial direction X. As illustrated in FIG. 1, the cell shape in the filter cross section in the direction orthogonal to the axial direction X is, for example, a quadrangle, but is not limited thereto. The cell shape may be a polygon such as a triangle, a quadrangle, or a hexagon, or a circle. Further, it may be a combination of two or more different shapes.

The porous filter 1 is, for example, a columnar body such as a columnar body, and has a first end surface 14 and a second end surface 15 at both ends in the axial direction X. When the porous filter 1 is arranged in an exhaust gas path such as an exhaust gas pipe, the first end face 14 becomes, for example, the upstream end face, and the second end face 15 becomes, for example, the downstream end face.

The cell 13 may have a first cell 131 and a second cell 132. As illustrated in FIG. 2, the first cell 131 opens in the first end surface 14 and is closed by the plug portion 16 in the second end surface 15. The second cell 132 opens to the second end surface 15 and is closed by the plug portion 16 at the first end surface 14. The stopper 16 is made of ceramics such as cordierite.

The first cell 131 and the second cell 132 are formed alternately side by side so as to be adjacent to each other, for example, in the horizontal direction orthogonal to the axial direction X and in the vertical direction orthogonal to both the axial direction X and the horizontal direction. Will be done. That is, when the first end surface 14 or the second end surface of the porous filter 1 is viewed from the axial direction X, the first cell 131 and the second cell 132 are arranged in a check pattern, for example.

Next, an embodiment of a method for producing the porous filter 1 will be described. First, silica, talc, and Al sources are mixed to produce raw material soil. A kneader can be used for mixing.

As the silica, talc, and Al sources, for example, granular ones can be used. As the silica, for example, porous silica can be used. As the Al source, a compound containing Al can be used. Examples of such a compound include aluminum hydroxide. Binder, lubricating oil, water and the like can be added to the raw material clay as needed.

The adjustment of the raw material soil will be described using the mass ratio Ws / Wt, the specific surface area ratio Ss / St, and the bulk density ratio Ds / Dt. The mass ratio Ws / Wt is the ratio of the mass Ws of silica to the mass Wt of talc in the raw material clay. The specific surface area ratio Ss / St is the ratio of the specific surface area Ss of silica to the specific surface area St of talc. The specific surface area referred to here is an apparent specific surface area that does not include internal pores. The bulk density ratio Ds / Dt is the ratio of the bulk density Ds of silica to the bulk density Dt of talc.

The specific surface area can be measured by the Brunauer-Emmett-Teller method (that is, the BET method) of 0.1 g of the raw material powder. For the measurement, for example, "Tristar II 3020" manufactured by Shimadzu Corporation can be used.

The bulk density can be measured by, for example, a tap density method fluidity adhesive force measuring device (Tap Densor, manufactured by Seishin Enterprise Co., Ltd.). Specifically, the cylinder of the tap density method fluidity adhesive force measuring device is filled with the powder to be measured, and the powder to be measured is compressed by tapping, and the mass of the powder to be measured and the volume of the cylinder in the compressed state are used. The bulk density can be calculated.

In adjusting the raw material clay, the product of the specific surface area Ss / St and the bulk density ratio Ds / Dt is adjusted to be larger than the mass ratio Ws / Wt for silica and talc. That is, the adjustment is made so as to satisfy the following formula (I).
Ws / Wt> Ss / St × Ds / Dt ・ ・ ・ (I)

In the case of Ws / Wt ≦ Ss / St × Ds / Dt, a surplus silica liquid phase is generated during the heating of the firing described later, and particle rearrangement occurs due to the long-distance movement of the liquid phase, resulting in a powder-filled structure. It is easy to change from the state before sintering. Therefore, after firing, the pores separated by the crystal grains due to the rearrangement of the particles are bonded to each other, so that coarse air pores are likely to be formed, and there is a possibility that the variation in the pore size distribution becomes large. Further, in this case, a silica-rich intermediate crystal phase is generated during cooling during firing, and the grain crystals may be destroyed in the process of phase displacement in the cooling step. This may also lead to the formation of coarse air pores due to the integration of the pores isolated by the crystal grains due to the breakage of the crystal grains, or the increase of minute pores with respect to the target pore diameter. As a result, the variation in the pore size distribution may increase. The above-mentioned particle rearrangement and crystal grain destruction will be described in Comparative Form 1 described later. By satisfying the above formula (I), the rearrangement of particles and the destruction of crystal grains can be sufficiently suppressed, and the variation in pore size distribution can be sufficiently suppressed.

The raw material soil is formed into a honeycomb structure by, for example, extrusion molding. The molded product is made of raw material soil and is cut after drying, for example. Then, the molded product is fired. As a result, a sintered body having a honeycomb structure can be obtained. Although not shown, the honeycomb-structured sintered body has the same configuration as the porous filter 1 illustrated in FIGS. 1 and 2 except that the stopper is not formed.

Firing is performed while appropriately adjusting the rate of temperature rise and cooling. In order to sufficiently generate the cordierite crystal phase, the maximum temperature at the time of firing can be set to, for example, 1350 to 1450 ° C. The holding time at the maximum temperature can be adjusted, for example, in the range of 1 to 50 hours. After holding at the maximum temperature, it is cooled to room temperature. That is, the calcination is performed by raising the temperature, holding the cordierite crystal phase at the formation temperature, and cooling.

Next, the plug portion 16 is formed. The stopper portion 16 is formed by filling the first end surface 14 or the second end surface 15 of the cell 13 with a slurry containing a ceramic raw material of the same type as the sintered body having a honeycomb structure and firing using a dispenser, printing, or the like. NS. The method for forming the plug portion 16 is not particularly limited, and other methods can be used.

As described above, the porous filter 1 can be manufactured. In the production method of this embodiment, the raw material soil is adjusted so that silica and talc satisfy the relationship of the above formula (I). For example, the raw material soil can be adjusted by adjusting the particle size and bulk density of the raw material particles. In the present specification, the "particle size" is the particle size at a volume integrated value of 50% in the particle size distribution obtained by the laser diffraction / scattering method. That is, it is the average particle size.

Next, the effect of this embodiment will be described. As shown by the arrows in FIG. 3, in the raw material clay 2 during firing, a liquid phase is generated from silica 21 and incorporated into other solid phase components such as talc 22 and Al source 23. When the raw material clay 2 is adjusted so as to satisfy the formula (I) as in the present embodiment, the silica 21 is present in a volume ratio equal to or higher than that of the other components. As a result, it is possible to prevent the number of atoms required for forming cordierite 24 from becoming silica-rich between the particles of the adjacent silica 21 and talc 22 during the formation of a mixed phase during firing and heating. Therefore, as shown in Comparative Form 1 described later, it is possible to prevent the excess liquid phase component from being generated and diffused during heating at a high temperature during firing, and to prevent the raw material particles from flowing and rearranging.

Therefore, the pore size distribution according to the distribution of the raw material particles 21, 22, and 23 in the raw material clay 2 before firing is easily maintained, and the formation of coarse air pores is suppressed. As a result, the variation in the pore size distribution of the porous filter 1 can be reduced. Further, since the silica 21 is uniformly dispersed throughout, a stable crystal phase of cordierite 24 is rapidly formed in the entire region of the raw material clay 2. The raw material particles 21, 22, and 23 are representatives of the particles of silica 21, talc 22, and Al source 23.

Further, since the raw material clay 2 is adjusted so that the above-mentioned ratios of silica 21 to talc 22 satisfy the formula (I), the crystal grain structure is destroyed by the phase displacement caused by cooling during firing. Can be prevented. This is because, for example, the formation of a Si-rich phase, which is an intermediate phase composed of _{MgSiO 4, is suppressed.} By satisfying the formula (I), the formation of an intermediate phase made of _{MgSiO 4 can be prevented.} In other words, it is preferable to adjust the raw material so that _{MgSiO 3} which is a Si-rich phase is not generated during the sintering reaction. As a result, it is possible to prevent the expansion of the pore structure and the unintended change in the pore characteristics due to the rearrangement of the raw material particles.

Therefore, by satisfying the formula (I), as illustrated in FIG. 3, pores 25 are formed in the portion where the silica 21 that was uniformly dispersed in the raw material clay 2 after sintering was present. Will be done. After the precipitation of the crystal phase composed of cordierite 24, particle flow due to the formation of a new liquid phase does not occur, a space defect 251 remains at the initial position of the silica 21 before firing, and the space defect 251 becomes a pore 25. Is. Therefore, pores 25 are formed according to the particle size of silica 21, which is a starting material. Therefore, the variation in the pore size distribution of the porous filter 1 can be reduced, and the porous filter 1 having a sharper pore size distribution can be obtained.

The raw material soil 2 satisfying the formula (I) is preferably adjusted by adjusting the particle size and bulk density of silica 21 and talc 22. In this case, the raw material clay 2 satisfying the formula (I) can be easily obtained.

The ratio Ds / Dt of the bulk density Ds of silica to the bulk density Dt of talc is preferably less than 0.65. In this case as well, a soil satisfying the formula (I) can be easily obtained. Further, the variation in the pore size distribution can be made smaller, and the porous filter 1 having a sharper pore size distribution can be obtained. As a result, it becomes possible to realize a porous filter 1 that has both improved high collection performance and low pressure loss at a higher level. This is because the required number of atoms between adjacent particles is suppressed from becoming silica-rich during the formation of a mixed phase for the composition of cordierite, and the formation of a surplus silica liquid phase is suppressed. From the viewpoint of further enhancing this effect, the bulk density ratio Ds / Dt is more preferably less than 0.5.

As the Al source 23, it is preferable to use aluminum hydroxide having an average particle size smaller than that of silica 21 and talc 22. In this case, a porous filter 1 having a sharper pore size distribution can be obtained. This is because the influence of the Al source 23 on the pore formation is reduced by using aluminum hydroxide having a small particle size. That is, in the raw material soil 2, the particles of the Al source 23 made of aluminum hydroxide having a small particle size are arranged between the particles of the silica 21 and the talc 22, and do not significantly affect the pore formation process. Therefore, the above-mentioned effect obtained by satisfying the formula (I) becomes more remarkable.

As described above, according to the present embodiment, it is possible to manufacture the porous filter 1 which can prevent the change in the pore-forming characteristics at the time of firing and reduce the variation in the pore size distribution.

(Comparison form 1)
Next, a case where a raw material soil that does not satisfy the above formula (I) is used will be described with reference to FIGS. 4 and 5. In this embodiment, the Al source 93 made of silica 91, talc 92, and aluminum hydroxide is used as the raw material particles, which is the same as that of the first embodiment. Not satisfied with (I). Other manufacturing methods are the same as those in the first embodiment.

When the formula (I) is not satisfied, the ratio of the number of raw material particles 91, 92, 93 to the number of particles is determined by the volume ratio. As illustrated in FIG. 4, when silica 91 is incorporated into talc 92 or Al source 93 in the firing process, stable cordierite 94 is formed, but since the formula (I) is not satisfied, Si is locally formed. Become rich. As a result, the surplus liquid phase 911 is generated from the silica 91, and the surplus liquid phase 911 remains. Further, although crystals of cordierite 94 are precipitated in a part of the region, another reaction proceeds between particles in which silica 91 having a low reaction start temperature is not present nearby.

As a result, in the firing process, the arrangement of the initial raw material particles 91, 92, 93 is changed and rearranged. Therefore, coarse pores 951 are formed due to the influence of aggregation of particles and the like, and pores 95 having large variations in pore diameter are formed. As a result, in the porous filter obtained after sintering, the pore size distribution width is widened.

Further, in the firing process, a liquid phase is formed from silica, and this liquid phase is incorporated into talc 92 and Al source 93 to form a crystal phase according to the composition. When the formula (I) is not satisfied, as illustrated in FIG. 5, protoenstatite which is a Si-rich phase in a part of the region adjacent to the silica 91, for example, in the holding process at a high temperature in the firing step (protoenstatite). An intermediate phase 941 consisting of Protoenstatite) is formed. The intermediate phase 941 is made of, for example, MgSiO ₃ .

The intermediate phase 941 undergoes a phase displacement from the high temperature stable phase protoenstatite to the low temperature stable phase Clinoenstatite during the cooling process of calcination. In the process of this phase displacement, as illustrated in FIG. 5, the crystal grain structure is damaged by the destruction of the crystal structure. As a result, the coarse pores 951 are formed, and the pores 95 having a large variation in pore diameter are formed.

As described above, when the formula (I) is not satisfied, the powder-filled structure of the raw material particles 91, 92, 93 before firing changes during firing, and the pore-forming characteristics change. As a result, as described above, the variation in the pore size distribution of the porous filter after sintering becomes widespread.

(Experimental example)
In this example, a plurality of porous filters 1 are actually prepared using the raw material soil satisfying the formula (I) and the raw material soil not satisfying the formula (I), and their pore size distributions are compared. That is, four types of porous filters, Example 1, Example 2, Comparative Example 1, and Comparative Example 4, are produced and these are comparatively evaluated. In addition, among the codes used in the experimental examples and thereafter, the same codes as those used in the above-described embodiments represent the same components and the like as those in the above-mentioned embodiments, unless otherwise specified.

_{First, talc Mg 3} Si ₄ O ₂₀ (OH) ₈ , silica SiO ₂ , and aluminum hydroxide Al (OH) are used as starting materials so that the theoretical composition formula of corderite is Mg ₂ Al ₃ (AlSi ₅ O _18). Mix ₃ Specifically, the mass ratio is silica: talc: aluminum hydroxide = 17.85: 46.46: 35.70.

Bulk density Ds of silica, bulk density Dt of talc, particle size of silica, particle size of talc, specific surface area Ss of silica, specific surface area St of talc, mass ratio Ws of silica used in the production of each Example and Comparative Example. The mass ratio Wt of talc is shown in Table 1. In addition, from these relationships, the suitability of the above formula (I) was investigated, and the results are shown in Table 1. As the aluminum hydroxide, those having a particle size smaller than that of silica and talc were adopted.

The porous filters of each Example and Comparative Example were produced in the same manner as in the first embodiment except that the raw materials shown in Table 1 were used. For the porous filters of each example and comparative example, the pore size distribution was measured using a mercury porosimeter using the principle of the mercury intrusion method. The result is shown in FIG. As the mercury porosimeter, Autopore IV9500 manufactured by Shimadzu Corporation was used.

As is known from FIG. 6, in Examples 1 and 2 in which the raw material soil was adjusted so as to satisfy the formula (I), the pore size distribution was sharp. On the other hand, in Comparative Examples 1 and 2 which do not satisfy the formula (I), the pore size distribution is broad. In these comparative examples, the abundance ratio of the region having a large pore diameter is large.

The reason why the porous filter 1 of the example shows a sharp pore size distribution as described above is that, as shown in the first embodiment, particle rearrangement and crystal grain fracture during firing are suppressed. As a result, the particle-filled structures of the raw material particles 21, 22, and 23 in the raw material clay 2 in the molded body before firing are transferred to the porous filter 1 after sintering, and pores are present at the positions where the silica 21 particles are present. It is considered that 25 is formed (see FIG. 3). That is, since it is possible to prevent changes in the pore-forming characteristics during firing, it is possible to manufacture the porous filter 1 having a small variation in the pore size distribution as in the examples.

Further, as can be seen by comparing Example 1 and Example 2, by making the ratio Ds / Dt of the bulk density Ds of the silica to the bulk density Dt of talc smaller, the variation in the pore size distribution becomes smaller. There is. Ds / Dt is preferably 0.65 or less, but more preferably Ds / Dt is 0.5 or less as in Example 2.

The present invention is not limited to each of the above embodiments and examples, and can be applied to various embodiments without departing from the gist thereof.

1 Porous medium filter 2 Raw material soil 21 Silica 22 Talc 23 Al source

Claims (3)

Corgerite (24) is formed by molding and firing a raw material clay (2) containing silica (21), talc (22) and an Al source (23) (except when the Al source is boehmite). ) In the method for producing a porous filter (1) having a composition.
As the Al source, silica and aluminum hydroxide having an average particle size smaller than that of talc are used.
The ratio Ws / Wt of the mass Ws of the silica to the mass Wt of the talc in the raw material clay, the ratio Ss / St of the specific surface area Ss of the silica to the specific surface area St of the talc, and the bulk density Dt of the talc. A method for producing a porous filter, wherein the raw material clay is adjusted so that the ratio Ds / Dt of the bulk density Ds of silica satisfies the following formula (I).
Ws / Wt> Ss / St × Ds / Dt ・ ・ ・ (I) The method for producing a porous filter according to claim 1, wherein the average particle size and bulk density of the silica and talc are adjusted so as to satisfy the formula (I). The method for producing a porous filter according to claim 1 or 2, wherein the ratio Ds / Dt of the bulk density Ds of the silica to the bulk density Dt of the talc is less than 0.65 .

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