JPH0523612A - Method for screening fine fragments - Google Patents

Abstract

PURPOSE:To effectively screen fine fragments like powder. CONSTITUTION:Inside a pressure vessel 10, fine fragments 28 to be screened pass through a screen 20 while suspended in high pressure solvent consisting of material whose b. p. is lower than ordinary temp. such as CO2 in the supercritical state. When the solvent becomes in the state of ordinary temp. and pressure, it turns into ordinary gas having much smaller surface tension compared with liquid solvent, such as water. Thereby the screened fine fragments are not aggregated again by the surface tension of solvent and there is no need for drying the fine fragments.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for sieving fine pieces such as powder, whiskers and fibers, and more specifically, a method for sieving fine pieces using a solvent composed of a substance having a boiling point lower than room temperature. Pertain to.

[0002]

2. Description of the Related Art As one of methods for sieving fine powder pieces such as fine powder, for example, as described in Japanese Utility Model Application Laid-Open No. 2-61484 filed by the same applicant as the present applicant,
A method of suspending fine powder pieces to be sieved in a liquid solvent such as water or alcohol and removing coarse particles and agglomerates of fine powder pieces by passing the suspension through a sieve is well known in the art. There is.

[0003]

However, in the conventional wet sieving method as described above, since the liquid solvent is used at room temperature and normal pressure with high surface tension, fine particles after sieving are used. When the solvent is removed from the pieces, the surface tension of the solvent causes the formation of aggregates again, which makes it difficult to screen the fine pieces effectively.

When the sieved fine pieces are subjected to sintering or the like, the fine pieces must be dried by sieving and then heated to a predetermined temperature for a long time. The process requires a long time, for example, several hours, and thus it is difficult to efficiently obtain fine pieces that can be sieved and used for sintering or the like.

In view of the above-mentioned problems in the conventional wet sieving method, the present invention is an improved fine piece capable of sieving fine pieces such as powder effectively and efficiently. The purpose of the present invention is to provide a sieving method.

[0006]

According to the present invention, the above-described object is to suspend fine particles to be sieved in a high-pressure solvent composed of a substance having a boiling point lower than room temperature and pass the fine particles through a sieve. This is achieved by the piece sieving method.

[0007]

According to the method of the present invention, the solvent used is a high-pressure solvent composed of a substance having a boiling point lower than room temperature, and the state of the solvent at the time of sieving is a supercritical state, a high-pressure gas state, Or, whether it is in a high-pressure liquid state or not, at normal temperature and pressure, it changes into a normal gas with a surface tension that is much smaller than that of a liquid solvent such as water. It is possible to obtain fine particles in a dried state which have been sieved without causing the fine particles to reaggregate due to the tension and without drying the fine particles.

[0008]

[Supplementary Explanation of Means for Solving the Problem] The solvent in the method of the present invention may be any substance which has a boiling point lower than room temperature and can be pressurized to high pressure, but the price and safety In terms of easiness of handling, for example, CO ₂ , air, N
It is preferably ₂ or the like.

Further, substances having a boiling point lower than room temperature generally have smaller surface tension and viscosity than liquid solvents such as water and alcohol used in the conventional wet sieving method, but they are particularly supercritical. -State and high-pressure gas-state solvents have smaller surface tension and viscosity than high-pressure liquid-state solvents, and are more likely to enter the agglomerates of fine particles.Moreover, supercritical solvents tend to absorb water adsorbed on the surface of fine particles. The solvent in the method of the present invention is preferably a solvent in a supercritical state or a high-pressure gas state, particularly a solvent in a supercritical state, because it has the action of extracting and collapsing the aggregates.

[0010]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

Embodiment 1 FIG. 1 is a schematic longitudinal sectional view showing one embodiment of a sieving apparatus suitable for carrying out a method for sieving fine pieces according to the present invention, and FIG. 2 is shown in FIG. It is a partial longitudinal cross-sectional view which expands and shows a part of shown sieving apparatus.

In these figures, the sieving device comprises a pressure vessel 10, which is supported by and is vibrated by a shaker 12. The pressure vessel 10 includes a vessel body 14 and a fine piece collection vessel 16. The container body 14 has a flange 14a at the lower end,
The fine piece collection container 16 has a flange 16a at the upper end. The flanges 14a and 16a are detachably fastened to each other by fastening means 18 such as a bolt.

A sieve 20 is fixedly arranged in the pressure vessel 10 in a state of being sandwiched between the flanges 14a and 16a at the peripheral portion thereof. As shown in FIG. 2, a sieve 20
A seal ring 22 is interposed between the flanges 14a and 16a to seal between them. An inlet port 26 having an opening / closing valve 24 is provided on the ceiling wall of the container body 14, so that fine particles to be sieved into the pressure vessel 10 above the sieve 20 and a substance having a boiling point lower than room temperature are used. It is possible to introduce a high-pressure solvent.

The fine particle collection container 16 integrally has a solvent discharge conduit 30 at a position close to the upper end thereof, and the communication of the conduit is controlled by an opening / closing valve 32. Conduit 3
A filter 34, which allows passage of the solvent but prevents passage of the fine particles after sieving, is arranged between 2 and the fine particle collecting container 16.

Next, one embodiment of the sieving method of the present invention performed by using the sieving apparatus formed as described above will be explained.

First, as fine pieces to be sieved, 96
00g of silicon nitride powder (average particle size 0.2μm) and 20
0 g of yttrium oxide (average particle size 0.2 μm) and 20
A raw material powder 28 mixed with 0 g of alumina powder (average particle size 0.1 μm) was prepared, and this was put into the pressure vessel 10 above the sieve 20. Then temperature 35 ℃, pressure 10
A supercritical fluid of 0 kg / cm ² of CO ₂ is introduced into the pressure vessel 10, whereby the raw material powder 28 is suspended in the supercritical fluid, and the vibration vessel 12 causes the pressure vessel 10 to vibrate at a frequency of 60 Hz and an amplitude of 1 The raw material powder 28 was passed through the sieve 20 by vibrating at 0.0 mm. The opening size of the used sieve is 4 μm
Met.

When one hour has passed after the start of vibration, the vibration exciter 12 is stopped and the opening / closing valve 32 is slightly opened to slowly move the supercritical fluid in the pressure vessel 10 to the outside of the pressure vessel. It was discharged. In this case, although it could not be visually observed, the pressure of the supercritical fluid in the pressure vessel 10 gradually decreased in the process of passing through the solvent discharge conduit 30,
It is presumed that CO _{2 in a} normal gaseous state was released into the atmosphere. When the pressure in the pressure vessel 10 becomes substantially atmospheric pressure, the fastening means 18 is removed to disassemble the pressure vessel, whereby the pressure vessel passes through the sieve 20 and is collected in the fine piece collection vessel 16. The obtained raw material powder 28a was taken out.

25 g of the raw material powder thus sieved is weighed and pressed at a pressure of 200 kg / cm ² for 1 minute to dry-form a 5 × 6 × 45 mm prism. Pressureless sintering was performed by heating at 1760 ° C. for 5 hours in an N ₂ atmosphere. Next, 50 bending test pieces were formed from the sintered body based on JIS standard R1601, and a 4-point bending bending test was performed on each test piece to measure the strength (MPa) of each test piece. These test results are shown in Table 1 below.
Shown in.

Comparative Example 1 For the purpose of comparison, sieving was carried out under the same procedure and conditions as in Example 1 except that water was used as a solvent, and the sieving raw material powder was wetted. Then, a prism having the same size was formed by pressure molding. Then, the prism was dried at room temperature for 48 hours, and then heated at a rate of 10 ° C./hour for 15
Drying was completed by raising the temperature to 0 ° C. and then holding it at 150 ° C. for 5 hours. The thus dried prisms were then pressure-sintered under the same conditions and conditions as in Example 1 to form 50 bending test pieces from the sintered body, and 4-point bending bending test was performed on each test piece. The test was conducted. The results of these tests are shown in Table 1 below.

Comparative Example 2 Sieve screening, pressure molding, normal pressure sintering, and 4-point bending bending test were conducted in the same manner and conditions as in Example 1 except that no solvent was used. It was The results of these tests are shown in Table 1 below.

[Table 1]Average strength Weibull coefficient Minimum strength Highest strength  Example 1100 16 970 1240 Comparative Example 1 980 12 870 1100 Comparative Example 2 870 7 690 970

From Table 1 above, the average strength of the sintered body of Example 1 is higher than that of Comparative Examples 1 and 2, and the variation in strength is small. Therefore, in the raw material powder after sieving of Example 1, It is speculated that the amounts of aggregates and coarse particles were smaller than those of the raw material powders of Comparative Examples 1 and 2. It is also presumed that the fact that the silicon nitride powder and the auxiliary agent powder were uniformly mixed due to the small amount of agglomerates also contributed to the improvement in strength.

In the above embodiment, the solvent is a supercritical fluid, but the temperature of the solvent is 35 ° C. and the pressure is 50 kg / cm.
Even when CO ^{2 in} the high-pressure gas state of ₂ was used, it was confirmed that the obtained sintered body had higher strength and higher Weibull coefficient than those of the comparative examples.

Example 2 Zirconium oxide powder in which 3 mol% of yttrium oxide was dissolved (average particle size 0.024 μm, specific surface area 17 m ^2).
/ G, density 6.05 g / cm ³ ) was weighed and 10 kg was placed in the pressure vessel 10 above the sieve 20. Then, liquid CO ₂ having a temperature of 20 ° C. and a pressure of 100 kg / cm ² is introduced into the pressure vessel 10, whereby the zirconium oxide powder is suspended in the liquid CO ₂ , and the vibration vessel 12 causes the pressure vessel 10 to vibrate at a frequency of 60 Hz. Then, the yttrium oxide was passed through the sieve 20 by vibrating with an amplitude of 1.0 mm. The opening size of the used sieve was 4 μm.

When 10 minutes have passed after the start of vibration, the vibration exciter 12 is stopped and the opening / closing valve 32 is slightly opened to slowly move the supercritical fluid in the pressure vessel 10 to the outside of the pressure vessel. It was discharged. In this case, although it could not be visually observed, the pressure of the supercritical fluid in the pressure vessel 10 gradually decreased in the process of passing through the solvent discharge conduit 30,
It is presumed that CO _{2 in a} normal gaseous state was released into the atmosphere. When the pressure vessel was disassembled by removing the fastening means 18 when the pressure inside the pressure vessel 10 became substantially atmospheric pressure, CO ₂ in the pressure vessel did not remain in the liquid state and was oxidized. The zirconium powder was dry.

Next, 47 g of the sieved zirconium oxide powder collected in the fine piece collection container 16 was weighed, and this was pressurized at a pressure of 200 kg / cm ² for 1 minute, and further 3
1 minute cold isostatic pressing at a pressure of ton / cm ² (CI
P) to form a 5 × 6 × 45 mm prism,
Pressureless sintering was performed by heating the prisms at 1460 ° C. for 5 hours in an N ₂ atmosphere. Then from the sintered body to JIS standard R16
Based on 01, 50 bending test pieces were formed, and a 4-point bending bending test was performed on each test piece to measure the strength of each test piece.

As a result, the average strength was 1200 MPa, the Weibull coefficient was 15, and the minimum strength was 1020 M.
It is Pa and the maximum strength is 1320 MPa. Therefore, it is speculated that the amounts of aggregates and coarse particles in the zirconium oxide powder after sieving in this example were also small values. It is also presumed that the zirconium oxide powder and the auxiliary powder were uniformly mixed with each other.

Embodiment 3 FIG. 3 is a schematic longitudinal sectional view showing another embodiment of a sieving apparatus suitable for carrying out the method for sieving fine pieces according to the present invention. Note that, in FIG. 3, the substantially same parts as those shown in FIG. 1 are designated by the same reference numerals as those shown in FIG.

In this sieving apparatus, the container body 14 of the pressure container 10 has a solvent inlet 38 having an opening / closing valve 36 in addition to the inlet 26. Further, the solvent discharge conduit 30 and the filter 34 are provided at the lower end of the fine particle collection container 16.

The sieving was performed as follows using the sieving apparatus configured as described above. First, 1000 g of the same raw material powder 28 as the raw material powder used in Example 1 was weighed and charged into the pressure vessel 10 above the sieve 20 through the inlet 26. Next, temperature 35 ℃, pressure 100kg
/ Cm ² of CO ₂ supercritical fluid was introduced into the pressure vessel 10 through the introduction port 26, whereby the raw material powder 28 was suspended in the supercritical fluid. Next, temperature 35 ℃, pressure 100kg /
40 liters / mi of CO ₂ supercritical fluid 40 cm ²
While introducing into the pressure vessel 10 from the solvent introduction port 38 at a flow rate of n, and discharging the supercritical fluid 40 from the discharge conduit 30 to the outside of the pressure vessel at a flow rate of 40 liters / min,
The pressure vessel 10 is vibrated at a vibration frequency of 60 Hz and an amplitude of 1.0 mm by the vibrating machine 12 to obtain the raw material powder 28 and the supercritical fluid 4
0 was passed through sieve 20. The opening size of the used sieve is 4
It was μm.

When 30 minutes had passed after the start of the vibration, the supply of the supercritical fluid 40 was stopped and the vibrator 12 was stopped. Although it could not be visually observed in this case, the pressure of the supercritical fluid in the pressure vessel 10 gradually decreased in the process of passing through the pressure vessel, the filter 34, and the solvent discharge conduit 30, and the normal pressure was reduced. It is presumed that CO _{2 in a} gaseous state was released into the atmosphere. When the pressure in the pressure vessel 10 becomes substantially atmospheric pressure, the fastening means 18 is removed to disassemble the pressure vessel, whereby the pressure vessel passes through the sieve 20 and is collected in the fine piece collection vessel 16. The obtained raw material powder 28a was taken out.

Observation of the raw material powder immediately after sieving revealed that the raw material powder contained almost no agglomerates and was very well sieved.
Further, from this example, it is understood that the sieving can be performed more efficiently by passing the solvent through the sieve.

Embodiment 4 FIG. 4 is a schematic vertical sectional view showing still another embodiment of a sieving apparatus suitable for carrying out the method for sieving fine pieces according to the present invention. Note that, in FIG. 4, portions substantially the same as the portions shown in FIG. 1 are given the same reference numerals as the reference numerals given in FIG.

The sieving device 42 not only sifts fine pieces, but also, for example, Japanese Patent Application No. 3-83613 (reference number AT-470), which has the same application as the applicant of the present application.
7), Japanese Patent Application No. 3-169233 (reference number AT-47)
20), Japanese Patent Application No. 3-169234 (reference number AT-4)
722) and is suitable for directly molding fine pieces after sieving according to the method described in 722).

In this sieving device, the container body 14 of the pressure container 10 and the fine particle collecting container 16 are integrally formed with each other. The inlet 26 is connected to the mixing chamber 48 of the mixing tank 46 by a conduit 44, and the solvent discharge conduit 30
Is connected to one end of a conduit 52 having a pump 50 in the middle. No filter is provided between the solvent discharge conduit 30 and the fine particle collecting container 16, and the conduit 30 communicates directly with the inside of the fine particle collecting container without a filter.

The mixing chamber 48 of the mixing tank 46 is connected by a conduit 54 to a cylinder for storing CO ₂ therein, which is not shown in the drawing. Further, the mixing chamber 48 is heated to a predetermined temperature by a heater 56, and the fine pieces (raw material powder) 28 charged into the mixing chamber 48 to be sieved and formed are formed in the mixing chamber 48. A stirrer 58 for stirring is provided.

The other end of the conduit 52 is communicatively connected to the slurry introduction port 62 of the molding die 60. In the illustrated embodiment, the mold 60 comprises an upper mold 64 and a lower mold 66 which, when mated with each other, define a slurry inlet 62 and a product cavity 68 communicating with the inlet. It is like this. The product cavity 68 is an upper mold 64 and a lower mold 66.
A minute gas discharge passage 70 formed as a minute gap between the parting planes communicates with the atmosphere.

The raw material powder was sieved and shaped as follows using the sieving apparatus and shaping apparatus configured as described above. First, 10 kg of the same raw material powder 28 as the raw material powder used in Example 1 was weighed and charged into the mixing chamber 48 of the mixing tank 46. Then supplying the CO ₂ 72 at a pressure of 5 kg / cm ² from the conduit 54 in a state that closes the on-off valve 24 into the mixing chamber 48, CO ₂ by the heater 56
CO ₂ is heated at a temperature of 80 ° C. and a pressure of 120
It brought to a supercritical state of kg / cm ² .

Then, the stirrer 58 is rotated for 3 hours to stir and mix the raw material powder and CO _{2 in a} supercritical state,
As a result, a slurry of raw material powder using CO ₂ in a supercritical state as a solvent was formed. Next, the on-off valve 30 is opened to introduce the slurry into the container body 14 of the sieving device 42, and after several minutes have elapsed, the pump 50 is operated so that 300 kg / cm of the sieved raw material powder and the supercritical fluid are discharged. ^While pressurizing to ² , it was injected into the product cavity 68 of the mold 60. When the raw material powder is completely filled in the cavity 68, the on-off valve 30 is closed, and the pump 50 and the stirrer 5 are closed.
8, the molding die 60 was disassembled, and the powder compact 74 molded in the product cavity was taken out.

Although it could not be visually observed, C in a supercritical state introduced into the product cavity 68.
Since the cavity is in communication with the atmosphere through the gas discharge passage 70, the pressure of O ₂ is gradually reduced in the cavity and is released into the atmosphere as a gas of substantially atmospheric pressure from the supercritical state. It is supposed to be. Further, in the product cavity 68, a pressure gradient in which the pressure gradually decreases from the vicinity of the slurry introduction port 62 toward the gas discharge passage is generated, and due to this pressure gradient, the raw material powder is formed in a portion near the gas discharge passage of the molding die. It is speculated that it was gradually pressed while being pressed against the inner wall surface.

The compact thus formed was heated at 1760 ° C. for 5 hours in an N ₂ atmosphere to carry out pressureless sintering, and 10 sintered pieces were obtained from the obtained sintered body based on JIS standard R1601. Fold test pieces were formed, and a 4-point bending bending test was performed on each test piece. As a result, the average strength of each test piece was 1095 MPa, the Weibull coefficient was 14, and the minimum strength and the maximum strength were 970 MPa and 1210, respectively.
It was MPa.

From the above results, it is presumed that the raw material powder was effectively sieved also in this example. Further, from this example, it is understood that the sifting and molding of the fine pieces can be carried out even more efficiently by introducing the sifted raw material powder together with the solvent into the molding die.
Furthermore, even if the powder from which the agglomerates are removed by sieving is stored in the air for a long time, the agglomerates may be formed again due to the moisture in the air. According to this, since molding is performed immediately after sieving, such a risk can be completely eliminated.

Although the present invention has been described above in detail with reference to specific embodiments, the present invention is not limited to these embodiments, and various other embodiments within the scope of the present invention. It will be apparent to those skilled in the art that

For example, the fine pieces to be molded are not limited to powder as in the above-mentioned embodiment, but may be in other forms such as whiskers and short fibers. Further, in each of the above-mentioned embodiments, a stirring device such as a propeller for stirring and mixing the fine particles and the solvent in the container body 14 may be provided in the container body. Further, in Example 4, the sieving device 4
2 may be incorporated between the pump 50 and the molding die 60, and the solvent used is a high-pressure solvent composed of a substance having a boiling point lower than room temperature and is in a high-pressure gas state or a high-pressure liquid state. It may be a solvent.

[0044]

As is apparent from the above description, according to the present invention, the solvent used is a high-pressure solvent composed of a substance having a boiling point lower than room temperature, and the state of the solvent at the time of sieving is Regardless of whether it is in a supercritical state, a high-pressure gas state, or a high-pressure liquid state, it becomes a normal gas that has a very small surface tension compared to a liquid solvent such as water at normal temperature and pressure. As a result, the sieved fine pieces are not re-aggregated by the surface tension of the solvent, and there is no need to dry the fine pieces after sieving, thus effectively sieving fine pieces such as powder. It can be implemented efficiently.

[Brief description of drawings]

FIG. 1 is a schematic longitudinal sectional view showing one embodiment of a sieving apparatus suitable for carrying out a method for sieving fine pieces according to the present invention.

FIG. 2 is a partial vertical cross-sectional view showing an enlarged part of the sieving apparatus shown in FIG.

FIG. 3 is a schematic longitudinal sectional view showing another embodiment of a sieving apparatus suitable for use in carrying out the method for sieving fine pieces according to the present invention.

FIG. 4 is a schematic longitudinal sectional view showing still another embodiment of the sieving apparatus suitable for carrying out the method for sieving fine pieces according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Pressure container 12 ... Vibrating machine 14 ... Container body 16 ... Fine particle collection container 20 ... Sieve 26 ... Inlet port 28 ... Raw material powder 30 ... Solvent discharge conduit 34 ... Filter 38 ... Solvent inlet port 42 ... Sieving device 46 ... Mixing tank 50 ... Pump 60 ... Mold 68 ... Product cavity 74 ... Powder compact

Claims (1)

Claim: What is claimed is: 1. A method for sieving fine particles, wherein the fine particles to be sieved are suspended in a high-pressure solvent consisting of a substance having a boiling point lower than room temperature and passed through a sieve.