JP7599695B2 - Filtration filter and method for producing same - Google Patents

Description

The present invention relates to a filter for filtration and a method for manufacturing the same.

Conventionally, a filtration filter is known in which a porous hollow cylindrical substrate is immersed in a suspension in which a solid powder is dispersed, the inside of the substrate is sucked to deposit the solid powder on the outer peripheral surface of the substrate, and the deposited solid powder is sintered together with the substrate (see, for example, Patent Document 1).

JP 2000-185209 A

However, there is a risk of cracks occurring in the sintered layer, which is formed by sintering solid powder, due to sintering shrinkage, etc. In a filtration filter in which cracks have occurred in the sintered layer, the fluid to be filtered tends to concentrate in the cracked areas (fissures) where the filtration resistance is relatively low, and the substances to be captured in the target fluid pass through the cracked areas through the sintered layer and ultimately the base material, resulting in a substantial decrease in filtration accuracy.

On the other hand, it is also possible to suppress sintering shrinkage of the solid powder by compressing the deposited layer of solid powder before sintering to increase the density in advance and reduce the amount of volume loss before and after sintering. However, if the density is increased in advance by compressing the deposited layer of solid powder, the porosity of the sintered layer in the filter may decrease, leading to a significant increase in filtration resistance.

In view of the above problems, the present invention aims to provide a filtration filter and a manufacturing method thereof that suppresses a substantial decrease in filtration accuracy due to cracks in the sintered layer while suppressing a significant increase in filtration resistance.

For this reason, the filtration filter according to the present invention comprises a porous and hollow substrate, a first sintered layer of solid powder laminated on the outer surface side of the substrate, and further comprises a second sintered layer of repair particles laminated on the first sintered layer to fill at least the cracks caused by the cracks in the first sintered layer.

The method for manufacturing a filtration filter according to the present invention includes the steps of dispersing a solid powder in a solvent to generate a first suspension, filtering the first suspension with a porous and hollow substrate capable of capturing the solid powder and depositing the solid powder on the outer surface side of the substrate, sintering the solid powder deposited on the substrate to form a first sintered layer, dispersing repair particles that can be captured by the substrate and the first sintered layer in a solvent to generate a second suspension, passing the second suspension through cracks in the first sintered layer caused by cracking to deposit repair particles at least in the cracks, and sintering the repair particles deposited at least in the cracks to form a second sintered layer.

The filtration filter and manufacturing method thereof of the present invention can prevent a significant increase in filtration resistance while suppressing a substantial decrease in filtration accuracy due to cracks in the sintered layer.

1 is a cross-sectional view that typically shows a vertical cross section of a filtration filter according to a first embodiment. 2 is a cross-sectional view showing a schematic cross section taken along line AA in FIG. 1. FIG. 1 is a perspective view illustrating an example of a substrate assembly. FIG. 2 is a block diagram showing an example of a method for forming a deposition layer of solid powder. FIG. 4 is a cross-sectional view illustrating a schematic crack in the first sintered layer. 4 is a cross-sectional view showing a second sintered layer laminated on the cracked portion. FIG. 10 is a cross-sectional view showing an example of forming a sintering precursor of a second sintered layer in a cracked portion. FIG. FIG. 5 is a cross-sectional view showing the internal structure of a filtration filter according to a second embodiment. 1 is a table showing the specifications of specimens for bubble point testing. 1 is a table showing the results of a bubble point test.

Below, an embodiment for carrying out the present invention will be described in detail with reference to the attached drawings.

First Embodiment
(Basic structure of a filtration filter)
The basic configuration of a filtration filter according to a first embodiment will be described with reference to Figures 1 to 3. Figure 1 shows a schematic vertical cross section of the filtration filter, and Figure 2 shows a schematic horizontal cross section of the filtration filter.

The filtration filter 1 is a filter that filters a target fluid (liquid, gas) and separates solids contained in the target fluid. The filtration filter 1 is used for a variety of filtering applications, such as filtering river water, lake water, or seawater, raw material fluids in various industries, fluids in intermediate processes, product fluids, and cooling fluids for various devices.

The filtration filter 1 is equipped with a hollow cylindrical filter element 2. A blocking member 3 is provided at an opening at one end of the filter element 2 to block this opening. A connecting member 4 is provided at an opening at the other end of the filter element 2 to connect this opening to the outside. With this configuration of the filtration filter 1, the target fluid flows from the outside to the inside of the hollow cylindrical filter element 2 and is filtered, and the filtered filtrate inside flows out to the outside via the connecting member 4 (white arrow in Figure 1).

The filter element 2 includes a hollow cylindrical substrate 5 and a first sintered layer 6 laminated on the outer peripheral surface 5a side of the substrate 5. The first sintered layer 6 is formed by sintering a solid powder deposited on the outer peripheral surface 5a side of the substrate 5 as a sintering precursor. The solid powder deposition layer is formed when the solid powder is captured by the substrate 5 when a suspension (first suspension) in which the solid powder is dispersed in a solvent is filtered through the substrate 5. Therefore, the substrate 5 is a porous body having a filtration accuracy capable of filtering and capturing the solid powder deposited thereon. Here, the filtration accuracy indicates the minimum particle size of particles that can be captured with a certain efficiency (e.g., 99.9%) or higher.

In this specification, "on the outer peripheral surface (outer surface) side of the substrate" is used to mean "from the outer peripheral surface (outer surface) side of the substrate toward the substrate," and "on the inner peripheral surface (inner surface) side of the substrate" is used to mean "from the inner peripheral surface (inner surface) side of the substrate toward the substrate." Therefore, the expressions "on the outer peripheral surface (outer surface) side of the substrate" and "on the inner peripheral surface (inner surface) side of the substrate" are not limited to the outer peripheral surface (outer surface) 5a and the inner peripheral surface (inner surface) 5b of the substrate 5, but may also include voids inside the substrate 5 and positions separated from the substrate 5 via an intermediate layer (e.g., a spacer, etc.).

Although not shown in the figure, when the target fluid of the filtration filter flows from the inside to the outside of the filter element and is filtered, the first sintered layer 6 is formed by sintering a layer of solid powder deposited on the inner circumferential surface 5b of the substrate 5 instead of the outer circumferential surface 5a of the substrate 5.

Figure 3 shows an example of a substrate assembly. The substrate assembly 9 is a semi-finished product or work-in-progress in which the blocking member 3 and the connecting member 4 are connected to the substrate 5. In the substrate 5, the blocking member 3 fitted into the opening at one end is joined by welding, crimping, or the like around the entire circumference of the opening to form a one-end joint 7 (first joint). In addition, in the substrate 5, the connecting member 4 fitted into the opening at the other end is joined by welding or the like around the entire circumference of the opening to form a other-end joint 8 (first joint). In the one-end joint 7 and the other-end joint 8, the internal porous structure of the substrate 5 is discontinuous. The substrate type of the substrate assembly 9 can be appropriately selected depending on the filtration accuracy required for the filtration filter 1 and the properties of the target fluid.

As the substrate 5 of the substrate assembly 9 shown in FIG. 3, a sheet end joint substrate is used. The sheet end joint substrate is a hollow cylindrical substrate type in which a porous filter sheet 10 is rolled into a cylindrical shape (see FIG. 3(a)), and the seams at both ends in the circumferential direction are joined by welding, crimping, or the like over the entire length in the axial direction (opening direction) to form a joint 11 (second joint) (see FIG. 3(b)). At the joint 11, the porous structure inside the filter sheet 10 is discontinuous. Examples of the filter sheet 10 include punching plates, sintered nonwoven fabrics (so-called metal fibers) made by stacking and sintering micron-sized metal fiber felts, and sintered wire mesh sintered bodies (for example, the applicant's product "Fuji Plate (registered trademark)"). Alternatively, the filter sheet 10 may be formed by stacking at least two of the punching plates, sintered nonwoven fabrics, and sintered wire mesh sintered bodies. The weaving methods of the wire mesh of each layer in the sintered wire mesh body include plain weave, twill weave, satin weave, tatami weave, twill tatami weave, etc. Austenitic stainless steel is a typical example of the metal used for these filter media sheets 10, but other sinterable metals such as nickel alloys and copper alloys can be used regardless of the type.

After the sheet end joining substrate is formed, the blocking member 3 and the connecting member 4 are fitted into the openings at both ends of the substrate 5 (see FIG. 3(a)), and are joined to the openings at both ends of the sheet end joining substrate around the entire circumference (see FIG. 3(b)), forming a substrate assembly 9. Note that in FIG. 3(b), a portion of the blocking member 3 is cut away to clarify the joining state between the blocking member 3 and the sheet end joining substrate.

As another example of the substrate assembly 9, although not shown in the figures, a substrate 5 can be made by simply drilling holes in a metal pipe in the radial direction, with a blocking member 3 joined to one of the openings at both ends and a connecting member 4 joined to the other end.

When the above suspension is filtered through the substrate 5 to deposit the solid powder that will be the sintering precursor of the first sintered layer 6 on the substrate 5, if it is difficult for the substrate 5 to maintain its own shape against the filtration pressure, a support structure for supporting the substrate 5 can be used. For example, it is assumed that the filter sheet 10, such as a sintered nonwoven fabric or sintered wire mesh used as the substrate for joining the sheet ends, will have difficulty maintaining its own shape. In this case, a thick punched plate rolled into a cylindrical shape or a metal pipe with radial holes may be used as the support structure, and the filter sheet 10 may be wrapped around and joined to it.

On the other hand, there may be cases where it is difficult to provide fine filter holes in the punched plate or metal pipe used as the substrate 5 that can filter the suspension in which the solid powder is dispersed and capture the solid powder. In this case, the substrate 5 can be made by wrapping a porous filter sheet 10 such as a sintered nonwoven fabric or sintered wire mesh as a filter aid around the outer surface of a punched plate rolled into a cylindrical shape or a metal pipe with radial holes.

As the solid powder to be deposited on the substrate 5 as the sintering precursor of the first sintered layer 6, for example, particles such as stainless steel powder (hereinafter referred to as "SUS powder") having a particle size in microns are used. However, as long as the solid powder does not affect the substrate 5 during sintering to form the first sintered layer 6, it is not limited to stainless steel and can be used regardless of the type. For example, for the sheet end joining substrate formed of the filter medium sheet 10 of a sintered body such as a nonwoven fabric sintered body or a wire mesh sintered body, a metal with a lower sintering temperature than these sintered bodies can be used as the solid powder to be deposited on the substrate 5. The solid powder can be obtained by various manufacturing methods such as atomization, pulverization, oxide reduction, and rotating electrode methods.

The solid powder deposited on the substrate 5 as the sintering precursor of the first sintered layer 6 is classified in advance according to the filtration accuracy required for the filtration filter 1, and the solid powder is obtained as a solid powder group with a certain size distribution. In the case of SUS powder, the solid powder is obtained as a particle group with a certain particle size distribution. If the sintering conditions are the same, the filtration accuracy of the filtration filter 1 decreases as the median size of the solid powder group (the median or average value in the size distribution of the solid powder group) increases, while the filtration accuracy of the filtration filter 1 increases as the median size of the solid powder group decreases.

(Basic manufacturing method for filtration filters)
Next, a basic manufacturing method of the filtration filter 1 will be described with reference to Fig. 4. Fig. 4 shows an example of a method for depositing a solid powder on the outer peripheral surface side of a substrate as a sintering precursor of a first sintered layer. The deposited layer of the solid powder is formed by suction filtration in which a suspension in which the solid powder is dispersed in a solvent is sucked from the outside to the inside of the substrate 5, and the solid powder is captured on the outer peripheral surface 5a side of the substrate 5.

The suspension is obtained by adding the solid powder to be deposited on the outer peripheral surface 5a of the substrate 5 to a solvent (e.g., isopropanol or a diluted solution thereof) in the stirring tank 100 and stirring with the stirrer 101 to disperse the solid powder in the solvent. Then, a substrate assembly 9 equipped with a substrate 5 capable of capturing the dispersed solid powder is immersed in the suspension in the stirring tank 100.

The inside of the substrate assembly 9 is connected to the sealed suction bell 102 via the connection member 4. The suction bell 102 is connected to the suction port 103b of the vacuum pump 103, whose discharge port 103a is open to the atmosphere, and the internal pressure of the suction bell 102 is reduced to a negative pressure below atmospheric pressure by driving the vacuum pump 103. When the internal pressure of the suction bell 102 is reduced to a negative pressure, a pressure difference occurs between the atmospheric pressure applied to the suspension in the mixing tank 100 and the internal pressure of the suction bell 102, and the suspension is filtered by the substrate 5 and sucked from the outside to the inside of the substrate assembly 9. When the suspension is filtered by the substrate 5, the solid powder in the suspension is captured and the solid powder is deposited on the outer peripheral surface 5a of the substrate 5. The filtrate produced by filtering the suspension by the substrate 5 is led to the filtrate tank 104 arranged inside the suction bell 102 via the connection member 4.

Although not shown, when solid powder is deposited on the inner circumferential surface 5b of the substrate 5 as a sintering precursor of the first sintered layer 6, the substrate assembly 9 is placed in the filtrate tank 104 in the suction bell 102, and the inside of the substrate assembly 9 is opened to the suspension in the stirring tank 100 via the connection member 4. Then, the vacuum pump 103 is driven to suck the suspension in the stirring tank 100 from the inside to the outside of the substrate assembly 9, and suction filtration is performed to capture the solid powder on the inner circumferential surface 5b of the substrate 5, thereby forming a deposition layer of solid powder on the inner circumferential surface 5b of the substrate 5.

When it is estimated that the deposition thickness of the solid powder has reached the desired thickness according to the filtration accuracy required for the filtration filter, the substrate assembly 9 is removed from the stirring tank 100, and then the operation of the vacuum pump 103 is stopped. The timing for removing the substrate assembly 9 can be set, for example, based on the suction flow rate determined from the suction pressure of the vacuum pump 103 and the amount of suspension filtrated until the deposition layer of the solid powder reaches the desired thickness. Then, the deposition layer of the solid powder deposited on the outer peripheral surface 5a of the substrate 5 is dried to obtain a sintered precursor of the first sintered layer 6.

The solid powder deposited on the outer peripheral surface 5a of the substrate 5 tends to crumble during drying as the solid powder used becomes coarser. Therefore, a binder may be added to the suspension in the stirring tank 100 according to the coarseness of the solid powder. Examples of binders that can be used include lacquer, polyvinyl alcohol, and water-soluble cellulose ether. In addition, the solid powder obtained by the atomization method has a nearly spherical shape, and as the particle size increases, the fluidity increases. Therefore, in order to improve the shape retention of the solid powder deposited on the outer peripheral surface 5a of the substrate 5, the solid powder may be compressed in advance by cold pressing at a pressure according to the particle size before sintering. However, the condition is that when the solid powder is compressed and sintered at this pressure, the filtration resistance does not increase significantly due to a decrease in the porosity of the sintered layer.

The solid powder deposition layer deposited and dried on the outer peripheral surface 5a of the substrate 5 is sintered together with the substrate assembly 9 on which it is formed in an atmospheric furnace (e.g., a hydrogen furnace or a vacuum furnace) at a predetermined sintering temperature and sintering time. This forms a first sintered layer 6 on the outer peripheral surface 5a of the substrate 5. If the suspension contains a binder, it may be thermally decomposed by performing a heat treatment once before sintering. Note that even when a solid powder deposition layer is formed on the inner peripheral surface 5b of the substrate 5, a first sintered layer 6 is formed on the inner peripheral surface 5b of the substrate 5 in the same manner as when a solid powder deposition layer is formed on the outer peripheral surface 5a of the substrate 5.

The outer shape (layer thickness) of the first sintered layer 6 formed on the outer peripheral surface 5a of the substrate 5 can be adjusted by applying pressure to the surface of the first sintered layer 6. When the substrate 5 is a hollow cylinder, the outer diameter of the first sintered layer 6 is adjusted to a predetermined dimension by, for example, swaging, which continuously strikes the surface of the first sintered layer 6 from the periphery with a rotating die in a cold state. Since the processing machine that performs the swaging process has a maximum outer diameter that can be processed, if the outer diameter of the first sintered layer 6 exceeds the maximum dimension, a rolling process is performed as a pre-processing to reduce the outer diameter of the first sintered layer 6. At least one of the swaging process and the rolling process can be replaced by static pressing, stamping, etc.

The filtration filter 1 is basically manufactured as described above, but defects such as those shown in Figure 5 may occur in the first sintered layer 6.

Figure 5 is an enlarged view of part B in Figure 2, showing defects in the first sintered layer when particles such as SUS powder are used as the solid powder. When the first sintered layer 6 is formed by sintering, the volume of the solid powder deposit layer serving as the sintering precursor is significantly reduced (e.g., halved) by sintering shrinkage compared to the base material 5, which only slightly reduces in volume, and the circumferential tensile force acting on the solid powder deposit layer increases from the inner peripheral surface to the outer peripheral surface. For this reason, cracks 12 are likely to occur in the first sintered layer 6 in the form of cracks from the outer peripheral surface, where the circumferential tensile force is at its maximum, toward the inner peripheral surface. In particular, the solid powder obtained by the atomization method has high fluidity due to its shape that is close to a sphere, so that the first sintered layer 6 formed by sintering such a solid powder deposit layer tends to have more cracks 12. In addition, if the surface of the first sintered layer 6 is subjected to external shape adjustment processing by applying pressure after sintering, cracks 12 may occur in the first sintered layer 6. Furthermore, there is a risk that cracks may have already occurred in the solid powder deposition layer before sintering for some reason, and these cracks may expand in the first sintered layer 6 after sintering. Furthermore, when the sheet end joining substrate is used as the substrate 5 in the substrate assembly 9, the cracks in the first sintered layer 6 caused by sintering shrinkage and the external shape adjustment process after sintering, and in turn the solid powder deposition process before sintering, are particularly noticeable at the boundaries between the one end joining portion 7, the other end joining portion 8, or the seam joining portion 11 in the filter sheet 10 and the rest, that is, at the parts where the internal porous structure of the filter sheet 10 is discontinuous.

In a filtration filter 1 in which cracks have occurred in the first sintered layer 6, the fluid to be filtered tends to concentrate in the cracked area (the above-mentioned crack) where the filtration resistance is relatively low, resulting in a substantial decrease in filtration accuracy.

The cracks 12 in the first sintered layer 6 occur because the volume reduction of the solid powder deposit layer due to sintering shrinkage is greater than the volume reduction of the base material 5. For this reason, as described in International Publication No. 1993/06912, for example, it is possible to reduce the volume reduction caused by the sintering shrinkage of the solid powder by compressing the solid powder deposit layer before sintering to increase the density of the deposit layer in advance. However, compressing the solid powder deposited on the base material 5 may be difficult depending on the shape of the base material 5, and may reduce the porosity of the first sintered layer 6, resulting in an increase in filtration resistance. For this reason, in the filtration filter 1, in order to suppress a significant increase in filtration resistance while suppressing a substantial decrease in filtration accuracy due to cracks in the sintered layer, the cracks in the first sintered layer 6 are repaired with the second sintered layer, and the filter element 2 is formed by the base material 5, the first sintered layer 6, and the second sintered layer.

(Second sintered layer)
The second sintered layer is formed by depositing particles such as SUS powder (hereinafter referred to as "repair particles") in the cracked portion of the first sintered layer 6, and sintering the deposited layer of the repair particles as a sintering precursor. The repair particles can be obtained by various manufacturing methods, such as the atomization method, the pulverization method, the oxide reduction method, the rotating electrode method, etc., in the same manner as the solid powder.

The repair particles must be sintered at a temperature equal to or lower than the sintering temperature of the solid powder that is the sintering precursor of the first sintered layer 6. This is because if the sintering temperature of the repair particles is higher than the sintering temperature of the solid powder that is the sintering precursor of the first sintered layer 6, the sintering of the first sintered layer 6 will proceed further, causing the porosity to decrease significantly and cracks 12 to expand or new cracks to appear, making it difficult to achieve the filtration accuracy required of the filtration filter 1.

Figure 6 shows a second sintered layer that repairs the cracks in the first sintered layer in Figure 5. The second sintered layer 13 is laminated on the first sintered layer 6 by filling the cracks caused by the cracks 12, because the filtration resistance during filtration is low and the target fluid is likely to concentrate in the cracks caused by the cracks 12. For example, when the second sintered layer 13 is formed, the surface of the first sintered layer 6 and the surface of the second sintered layer 13 are approximately flush with each other (see dashed line), or the second sintered layer 13 may be laminated on the first sintered layer 6 so as to cover the entire first sintered layer 6. Even when the second sintered layer 13 is laminated so as to cover the entire first sintered layer 6, it is formed with a thickness thinner than that of the first sintered layer 6.

The repair particles, which serve as the sintering precursor of the second sintered layer 13 that repairs the cracks 12 that have occurred in the first sintered layer 6, have a particle size that satisfies all of the following conditions:

First, the repair particles have a particle size that allows them to penetrate into the cracks caused by the cracks 12 in the first sintered layer 6. For example, the repair particles have a particle size that does not protrude from the surface of the first sintered layer 6 when they penetrate into the cracks. The size of the crack can be determined by an actual measurement value that is measured each time the first sintered layer 6 is formed, or an estimated value that is estimated based on an actual measurement value that is previously measured on the first sintered layer 6 of one or more reference filtration filters 1.

Secondly, the repair particles have a particle size that allows them to be captured by the first sintered layer 6 when the suspension in which they are dispersed is filtered through the first sintered layer 6. This is because if the repair particles are not captured by the first sintered layer 6, they cannot be deposited as sintering precursors for the second sintered layer 13.

Third, the repair particles have a central (average) particle size equal to or smaller than the central (average) particle size of the particles that will be the sintering precursors of the first sintered layer 6. This is based on the above premise that the repair particles are sintered at a sintering temperature equal to or lower than the sintering temperature of the solid powder that is the sintering precursor of the first sintered layer 6, and the fact that the smaller the particle size, the lower the temperature at which sintering is promoted. When repair particles having such a particle size are sintered at a temperature equal to or lower than the sintering temperature of the solid powder that is the sintering precursor of the first sintered layer 6, sintering of the repair particles is promoted while sintering of the first sintered layer 6 is suppressed. In addition, the smaller the central particle size of the repair particles is compared to the central particle size of the particles that will be the sintering precursors of the first sintered layer 6, the lower the sintering temperature of the repair particles, which suppresses the expansion of cracks in the first sintered layer 6 and the occurrence of cracks in the second sintered layer 13.

(Method of forming second sintered layer)
The deposition layer of repair particles, which serves as the sintering precursor of the second sintered layer 13, can be formed in the same manner as the deposition layer of solid powder, which serves as the sintering precursor of the first sintered layer 6, as shown in Fig. 4, for example. That is, the deposition layer of repair particles can be formed by suction filtration in which a suspension (second suspension) in which repair particles are dispersed in a solvent is sucked from the outside to the inside of the filtration filter 1 before repair, thereby capturing the repair particles.

First, the suspension is obtained by adding the repair particles to a solvent (e.g., isopropanol) in the stirring tank 100 and stirring with the stirrer 101 to disperse the repair particles in the solvent. As described above, a binder may be added to the suspension as necessary. Then, the filtration filter 1 before repair is immersed in the suspension in the stirring tank 100.

The inside of the filtration filter 1 before repair is connected to the suction bell 102 via the connection member 4. When the internal pressure of the suction bell 102 is reduced to a negative pressure below atmospheric pressure by driving the vacuum pump 103, a pressure difference occurs between the atmospheric pressure applied to the suspension in the stirring tank 100 and the internal pressure of the suction bell 102, and as a result, the suspension is sucked into the inside of the filtration filter 1 before repair through the first sintered layer 6 and the base material 5. Since the filtration resistance is reduced in the cracked parts of the first sintered layer 6, the flow rate of the suspension passing through the cracked parts increases. For this reason, the repair particles are preferentially deposited in the cracked parts of the first sintered layer 6. The filtrate generated by filtering the suspension through the first sintered layer 6 and the base material 5 is sucked into the filtrate tank 104 arranged inside the suction bell 102 via the connection member 4.

When it is estimated that the repair particles have filled at least the cracks in the first sintered layer 6, the substrate assembly 9 is removed from the stirring tank 100, and then the operation of the vacuum pump 103 is stopped. The timing for removing the substrate assembly 9 can be set, for example, based on the suction flow rate determined from the suction pressure of the vacuum pump 103 and the amount of filtered suspension required to fill the largest of the cracks. The deposited layer of repair particles is then dried to obtain a sintered precursor for the second sintered layer 13.

After drying, the deposited layer of repair particles is sintered together with the filtration filter 1 on which it is formed in an atmospheric furnace (e.g., a hydrogen furnace or a vacuum furnace) at a predetermined sintering temperature and for a predetermined sintering time. As described above, this sintering temperature is equal to or lower than the sintering temperature at which the first sintered layer 6 was formed. In this way, the second sintered layer 13 is formed on the substrate 5. If the suspension contains a binder, it may be thermally decomposed by carrying out a heat treatment once before sintering.

The outer shape (layer thickness) of the second sintered layer 13 may be adjusted by applying pressure to the surface of the second sintered layer 13 by rolling or swaging, as with the first sintered layer 6.

In this way, a sediment layer of repair particles, which serve as the sintering precursor of the second sintered layer 13, is formed in the cracked portion of the first sintered layer 6 by suction filtration, and the sediment layer is sintered to form the second sintered layer 13 that repairs the crack in the first sintered layer 6.

The deposition layer of repair particles, which serves as the sintering precursor of the second sintered layer 13, may be formed by selectively dripping a suspension of repair particles dispersed in a solvent onto the cracked portion of the first sintered layer 6, as shown in FIG. 7, rather than by suction filtration as described above.

First, a small amount of suspension in which repair particles are dispersed in a solvent in a stirring tank 100 or the like is sucked up by a liquid moving means 105 such as a pipette, and dropped onto the crack (crack) in FIG. 7 caused by the crack 12 in the first sintered layer 6. Then, the solvent in the attached suspension passes through the first sintered layer 6 and the base material 5, but the repair particles are captured by the first sintered layer 6 and the base material 5 on their surfaces. As a result, the repair particles are deposited on the crack in the first sintered layer 6. The deposited layer of repair particles is dried and sintered at a temperature lower than the sintering temperature when the first sintered layer 6 was formed. Note that the inside of the filtration filter 1 before repair may be made negative pressure from the time of dropping or until a predetermined time has passed from this time, which makes it easier for the suspension to concentrate on the crack, and therefore easier for the solvent in the suspension to pass through the crack.

In addition, in both the suction filtration and the selective dripping onto the cracked portion of the first sintered layer 6, the repair particles are deposited by masking the portion of the first sintered layer 6 other than the cracked portion, thereby reducing the wasteful use of repair particles.

According to this type of filtration filter 1, at least the cracked parts of the first sintered layer 6 are provided with a second sintered layer 13 in which repair particles are sintered. This makes it difficult for the fluid to be filtered to concentrate in the cracked parts of the first sintered layer 6, reducing the amount of material to be captured in the target fluid passing through the sintered layer, making it possible to suppress a substantial decrease in the filtration accuracy of the filtration filter 1.

If cracks still remain even after the second sintered layer 13 is formed, repair particles may be deposited again into the cracks by suction filtration or natural filtration to form a further sintered layer.

Second Embodiment
Next, a filtration filter according to a second embodiment will be described. Note that the same reference numerals are used to designate configurations similar to those of the first embodiment, and descriptions thereof will be simplified or omitted. The same applies to the following embodiments.

The filtration filter 1 described above suppresses substantial deterioration of filtration accuracy by repairing cracks that occur in the first sintered layer 6 mainly due to shrinkage during the sintering process with the second sintered layer 13. In contrast, the filtration filter 1A according to the second embodiment further enhances the effect of suppressing substantial deterioration of filtration accuracy by preventing defects from occurring before and after sintering.

The substrate 5 of the filtration filter 1A is a sheet end joining substrate using a sintered wire mesh, which is a filter medium sheet 10. Specifically, this substrate 5 has a seam joining section 11 in which a sintered wire mesh is rolled into a cylindrical shape and the seams at both ends in the circumferential direction are joined by welding or the like over the entire length in the axial direction (opening direction).

Figure 8 shows a cross section of a filter element having a substrate and a first sintered layer. The sintered wire mesh used in the substrate 5 has multiple layers from the outer peripheral surface 5a to the inner peripheral surface 5b. In the example shown in Figure 8, the sintered wire mesh used in the substrate 5 has five layers, the first wire mesh layer 14, the second wire mesh layer 15, the third wire mesh layer 16, the fourth wire mesh layer 17, and the fifth wire mesh layer 18, in that order from the outer peripheral surface 5a to the inner peripheral surface 5b. Of these five layers, the first wire mesh layer 14 is the outermost layer that covers the outer peripheral surface 5a of the substrate 5, and the fifth wire mesh layer 18 is the innermost layer that covers the inner peripheral surface 5b of the substrate 5. In addition, when a suspension containing dispersed solid powder is filtered through the substrate 5 to form a deposition layer of solid powder that serves as a sintering precursor for the first sintered layer 6 on the outer peripheral surface 5a side, the second wire mesh layer 15 has a filtering accuracy that makes it easier to capture solid powder compared to the other wire mesh layers, while the first wire mesh layer 14 and the third to fifth wire mesh layers have a filtering accuracy that makes it harder to capture solid powder compared to the second wire mesh layer 15.

By immersing the substrate assembly 9 having such a substrate 5 in the suspension in the stirring tank 100 and performing suction filtration, the suspension is sucked from the outside to the inside of the substrate assembly 9, and the solid powder in the suspension passes through the first wire mesh layer 14 and is captured by the second wire mesh layer 15. The solid powder then accumulates outside the second wire mesh layer 15, i.e., in the voids in the first wire mesh layer 14, forming a deposition layer of solid powder. However, the suction filtration is completed in a state where the first wire mesh layer 14 protrudes outward from the deposition layer of solid powder in order to protect the deposition layer of solid powder, which has relatively low shape retention, with the first wire mesh layer 14.

The solid powder deposit layer that has been deposited and dried on the substrate 5 is sintered together with the substrate assembly 9 on which it is formed in an atmospheric furnace for a predetermined sintering time and sintering time. As a result, as shown in FIG. 8, a first sintered layer 6 is formed on the outer peripheral surface 5a of the substrate 5, but since the solid powder deposit layer shrinks due to sintering, at least the first wire mesh layer 14 protrudes outward from the first sintered layer 6.

Although not shown, when the first sintered layer 6 is formed on the inner peripheral surface 5b of the base material 5, the first wire mesh layer 14 is the innermost layer of the base material 5, and the fifth wire mesh layer 18 is the outermost layer of the base material 5, and the order of the wire mesh layers of the wire mesh sintered body from the outer peripheral surface 5a to the inner peripheral surface 5b is reversed when the first sintered layer 6 is formed on the outer peripheral surface 5a. Then, the suction filtration is completed in a state where the first wire mesh layer 14 protrudes inward from the solid powder deposition layer, in order to protect the solid powder deposition layer, which has a relatively low shape retention, with the first wire mesh layer 14. By performing sintering in this state, the first sintered layer 6 is formed on the inner peripheral surface 5b of the base material 5, but since the solid powder deposition layer shrinks due to sintering, at least the first wire mesh layer 14 protrudes inward from the first sintered layer 6.

According to such a filtration filter 1A, the solid powder deposit layer formed on the base material 5 before sintering is protected by the first wire mesh layer 14, so that the possibility of defects occurring in the first sintered layer 6 due to peeling or deformation of the solid powder deposit layer due to contact with an external object can be reduced. In addition, since the first sintered layer 6 formed after sintering is also protected by the first wire mesh layer 14, the possibility of defects such as cracks occurring in the first sintered layer 6 due to collision with an external object can be reduced. Therefore, according to the filtration filter 1A, defects before and after sintering are suppressed, so that the effect of suppressing a substantial decrease in filtration accuracy can be further enhanced.

As described above, even when the filtration filter 1A is formed by making the first wire mesh layer 14, which is the outermost layer, protrude outward from the first sintered layer 6, or by making the first wire mesh layer 14, which is the innermost layer, protrude inward from the first sintered layer 6, it is expected that a crack 12 will occur in the first sintered layer 6. This crack 12 can be repaired by forming the second sintered layer 13, as described in the first embodiment.

As described above, the second wire mesh layer 15 captures the solid powder to form a solid powder deposit layer, but it is conceivable that the first sintered layer 6 formed by sintering the deposit layer thus formed may have an insufficient layer thickness for the filtration accuracy required for the filtration filter 1A. In this case, the filtration accuracy of each wire mesh layer may be set so that the solid powder can be captured by any of the third to fifth wire mesh layers, which are located deeper than the second wire mesh layer 15.

In order to evaluate the filtering accuracy based on the results of a bubble point test for a filtering filter in which the solid powder serving as the sintering precursor for the first sintered layer is SUS powder, which is particles obtained by an atomization method, two filtering filters with the specifications shown in Figure 9 were produced as test specimens. In the bubble point test, the filtering filter was immersed in a diluted isopropanol solution, and air pressure was applied from the inside of the filtering filter to measure the pressure difference between the inside and outside when bubbles were generated on the outer surface of the filter element.

The base material of the test filter was a sintered wire mesh "Fujiplate (registered trademark)" made by stacking and sintering five wire meshes, rolled into a cylinder with a diameter of 12.5 mm and a length of 150 mm, and welded both ends (joints) in the circumferential direction along the entire length in the axial direction (opening direction) of the cylinder to form a sheet end joining base material. A blocking member 3 was then welded to the opening at one end of the sheet end joining base material, and a connecting member 4 was welded to the opening at the other end to create a base material assembly.

In the sintered wire mesh used as the substrate, from the filtering side, the first wire mesh layer had a mesh count of 60 mesh, the second wire mesh layer had a mesh count described below, the third wire mesh layer had a mesh count of 100 mesh, the fourth wire mesh layer had a mesh count of 60 mesh, and the fifth wire mesh layer had a mesh count of 28 mesh. Here, "mesh" refers to the number of meshes per inch.

The second wire mesh layer of the sintered wire mesh used as the substrate was made of "fine mesh" wire mesh with a twill weave of 325 mesh length and 2400 mesh width and wire diameter of 0.035 mm length and 0.024 mm width.

For reference, a bubble point test was conducted in the substrate assembly state. The results are shown in Figure 10. The pressure difference when bubbles first appear on the outer surface of the filter element (substrate in the substrate assembly state) (initial bubble point, hereinafter referred to as "IBP") was 5.10 [kPa] on average. The pressure difference when uniform bubbles appear from the entire outer surface of the filter element (substrate in the substrate assembly state) (burst bubble point, hereinafter referred to as "BBP") was 5.74 [kPa] on average. Note that IBP and BBP were converted to pressure difference values at a water temperature of 20 [°C] and a water depth of 0 [mm] so that the filtration performance can be appropriately evaluated regardless of the water temperature and water depth when the substrate assembly is actually immersed. The same applies to IBP and BBP for filtration filters below.

Next, the test substrate assembly was immersed in the suspension and suction filtration was performed at a suction pressure of 0.15 MPa for a suction time of 1 to 2 minutes. The suspension in which the test substrate assembly was immersed was prepared by dispersing SUS powder classified to a median particle size of 2 μm as a solid powder to be the sintering precursor of the first sintered layer in a diluted isopropanol solution in a stirring tank.

The suction filtration was completed when the first wire mesh layer of the substrate protruded from the deposited layer of SUS powder. After drying the deposited layer of SUS powder on the substrate, the deposited layer of SUS powder was sintered together with the substrate assembly in an atmospheric furnace for 1 hour to form a first sintered layer, and a filtration filter was produced. The sintering temperature was 820°C. After the sintering process, neither swaging nor rolling was performed on either sample number 1 or 2.

A bubble point test was conducted to confirm the filtering accuracy of the filter thus prepared. The results are shown in Figure 10. The IBP and BBP of the sample filter increased in all cases compared to those in the substrate assembly state, with the average IBP being 8.91 [kPa] and the average BBP being 17.0 [kPa].

It was confirmed that the first sintered layer of the filtration filter produced as described above had cracks. Therefore, in order to repair the cracks in the filtration filter, a second sintered layer was formed as follows. First, the sample filtration filter was immersed in a suspension and suction filtration was performed at a suction pressure of 0.15 MPa for a suction time of 3 to 10 minutes. The suspension in which the sample filtration filter was immersed was produced by dispersing SUS powder classified to a median particle size of 2 μm as a solid powder that would serve as a sintering precursor for the second sintered layer in a diluted isopropanol solution in a stirring tank.

After drying the layer of repair SUS powder deposited on the first sintered layer by suction filtration, the layer of repair SUS powder was sintered for one hour to form a second sintered layer, repairing the filtration filter. The sintering temperature was 820°C.

A bubble point test was conducted to confirm the filtering accuracy of the repaired filter. The results are shown in Figure 10. In the repaired filter, both the IBP and BBP increased compared to the unrepaired filter, with the average IBP being 17.60 [kPa] and the average BBP being 48.53 [kPa].

The results of the bubble point test confirmed that the filtration accuracy of the repaired filtration filter was improved compared to the unrepaired filtration filter. Therefore, it was found that the repaired filtration filter can suppress a substantial decrease in filtration accuracy caused by cracks in the first sintered layer.

In addition, it was predicted that the BBP value would increase if the thickness of the first sintered layer in the repaired filtration filter was increased by depositing SUS powder from a wire mesh layer deeper than the second wire mesh layer.

In the filtration filter 1A according to the second embodiment, the first wire mesh layer 14, which is the outermost layer, is protruding outward from the first sintered layer 6 laminated on the outer peripheral surface 5a side of the substrate 5. Alternatively, the first wire mesh layer 14, which is the innermost layer, is protruding inward from the first sintered layer 6 laminated on the inner peripheral surface 5b side of the substrate 5. However, since the first sintered layer 6 is formed in the voids in the first wire mesh layer 14, the first sintered layer 6 can be structurally reinforced by the first wire mesh layer 14. In addition, the solid powder deposit layer, which is the sintering precursor of the first sintered layer 6 formed in this way, is also formed in the voids in the first wire mesh layer 14, so that the shape retention of the solid powder deposit layer is improved by the first wire mesh layer 14, and thus defects in the first sintered layer 6 are suppressed. Therefore, the first wire mesh layer 14 may not protrude from the first sintered layer 6, and the first wire mesh layer 14 may be entirely covered by the first sintered layer 6.

In addition, in the first to third embodiments described above, the shape of the substrate 5 is not limited to a hollow cylinder, and various shapes such as a candle, a leaf disk, and disk pleats can be used as long as they are hollow. Even if the first sintered layer 6 is formed on substrates of such various shapes, defects that occur in the first sintered layer 6 can be repaired with the second sintered layer 13.

The present invention has been specifically described above with reference to preferred embodiments, but it is obvious that a person skilled in the art can adopt various modified forms based on the basic technical ideas and teachings of the present invention. Furthermore, the technical ideas described in the first to third embodiments above can be used in appropriate combinations as long as no contradictions arise.

1, 1A... Filtration filter, 2... Filter element, 5... Substrate, 5a... Outer circumferential surface, 5b... Inner circumferential surface, 6... First sintered layer, 9... Substrate assembly, 10... Filter medium sheet, 11... Seam joint, 12... Crack, 13... Second sintered layer, 14... First wire mesh layer, 15... Second wire mesh layer, 16... Third wire mesh layer, 17... Fourth wire mesh layer, 18... Fifth wire mesh layer

Claims (11)

A porous and hollow substrate;
a first sintered layer of solid powder laminated on an outer surface side of the substrate;
Equipped with
Further comprising a second sintered layer of repair particles laminated on the first sintered layer by filling at least the cracks caused by the cracks in the first sintered layer ,
The substrate has a porous filter sheet, a first joint portion in which both end openings of the filter sheet rolled into a hollow cylinder are joined to other members over the entire circumference, and a second joint portion in which the seams of both end portions in the circumferential direction of the filter sheet rolled into a hollow cylinder are joined over the entire length in the opening direction, and the internal structure of the filter sheet is discontinuous at the first joint portion and the second joint portion .
Filter for filtration. A porous and hollow substrate;
a first sintered layer of solid powder laminated on an outer surface side of the substrate;
Equipped with
Further comprising a second sintered layer of repair particles laminated on the first sintered layer by filling at least the cracks caused by the cracks in the first sintered layer,
The substrate has a plurality of layers from the outer surface to the inner surface,
The first sintered layer is formed from an inner layer that is located inside the outermost layer of the plurality of layers toward the outside.
Filter for filtration. The filter according to claim 1 or 2, wherein the solid powder is particles. The median particle size of the repair particles is equal to or smaller than the median particle size of the particles;
The filter according to claim 3. The outermost layer is in a state of protruding outward from the first sintered layer.
The filter according to claim 2. dispersing a solid powder in a solvent to form a first suspension;
filtering the first suspension through a porous and hollow substrate capable of capturing the solid powder, and depositing the solid powder on an outer surface side of the substrate;
sintering the solid powder deposited on the substrate to form a first sintered layer;
dispersing repair particles that can be captured by the base material and the first sintered layer in a solvent to form a second suspension;
passing the second suspension through cracks in the first sintered layer to deposit the repair particles at least in the cracks;
sintering at least the repair particles deposited in the crack to form a second sintered layer;
Including,
A method for manufacturing a filtration filter, further comprising a step of applying pressure to a surface of the first sintered layer to adjust its outer shape after the step of forming the first sintered layer and before the step of depositing the repair particles. dispersing a solid powder in a solvent to form a first suspension;
filtering the first suspension through a porous and hollow substrate capable of capturing the solid powder, and depositing the solid powder on an outer surface of the substrate;
sintering the solid powder deposited on the substrate to form a first sintered layer;
dispersing repair particles that can be captured by the base material and the first sintered layer in a solvent to form a second suspension;
passing the second suspension through cracks in the first sintered layer to deposit the repair particles at least in the cracks;
sintering at least the repair particles deposited in the crack to form a second sintered layer;
Including,
The substrate comprises a porous filter material sheet, a first joint portion where both end openings of the filter material sheet rolled up into a hollow cylinder are joined to other members around their entire circumference, and a second joint portion where the seams of both end circumferential ends of the filter material sheet rolled up into a hollow cylinder are joined along their entire length in the opening direction, and the internal structure of the filter material sheet is discontinuous at the first joint portion and the second joint portion . dispersing a solid powder in a solvent to form a first suspension;
filtering the first suspension through a porous and hollow substrate capable of capturing the solid powder, and depositing the solid powder on an outer surface side of the substrate;
sintering the solid powder deposited on the substrate to form a first sintered layer;
dispersing repair particles that can be captured by the base material and the first sintered layer in a solvent to form a second suspension;
passing the second suspension through cracks in the first sintered layer to deposit the repair particles at least in the cracks;
sintering at least the repair particles deposited in the crack to form a second sintered layer;
Including,
The substrate has a plurality of layers from the outer surface to the inner surface,
a first suspension liquid that is formed on a first surface of the first substrate and a second suspension liquid that is formed on a second surface of the first substrate and a third suspension liquid that is formed on a second surface of the first substrate; dispersing a solid powder in a solvent to form a first suspension;
filtering the first suspension through a porous and hollow substrate capable of capturing the solid powder, and depositing the solid powder on an outer surface side of the substrate;
sintering the solid powder deposited on the substrate to form a first sintered layer;
dispersing repair particles that can be captured by the base material and the first sintered layer in a solvent to form a second suspension;
passing the second suspension through cracks in the first sintered layer to deposit the repair particles at least in the cracks;
sintering at least the repair particles deposited in the crack to form a second sintered layer;
Including,
A method for manufacturing a filtration filter, wherein the step of depositing repair particles is performed by masking an area other than the crack. The method for producing a filtration filter according to any one of claims 6 to 9, wherein the solid powder is in the form of particles. The step of depositing a solid powder comprises:
completing the deposition with the outermost layer protruding outwardly from the deposited layer of solid powder;
A method for producing the filtration filter according to claim 8.