Particle recovery method and particle recovery device

Abstract

To provide: a particle recovery method which can make one device continuously perform a recovering step and a cleaning step for particles in slurry; and a particle recovery device.SOLUTION: A particle recovery device according to the present invention recovers particles dispersed in liquid by attaching the particles to the vicinities of electrodes by electrophoresis, and comprises: a processing tank for storing the liquid; a pair of the electrodes arranged in the processing tank; and a power source that applies voltages to the pair of electrodes, which is further provided with relative movement means that can move the particles attaching to the vicinities of the electrodes and liquid around the particles relatively to each other, so as to apply shear force to the particles attaching to the vicinities of the electrodes to redisperse the particles.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for recovering particles from a slurry and a particle recovery device used therefor, and is particularly suitable for use in recovering nanoparticles. [Background technology]

[0002] Conventionally, methods for recovering particles from slurries have been known, such as filtration and centrifugation. However, with advances in nanotechnology, nanoparticles with diameters of 100 nanometers or less are increasingly being used in industrial processes. However, the above-mentioned filtration and centrifugation methods have the problem of making it difficult to separate and recover nanoparticles.

[0003] To solve these problems, methods for recovering particles from a slurry by electrophoresis have been developed. For example, Patent Document 1 describes a method in which a pair of electrodes is immersed in the slurry, a voltage is applied between the electrodes, and the particles are concentrated near the electrodes by electrophoresis to form a paste, which is then recovered. This method makes it possible to recover nanoparticles even from a nanoparticle slurry.

[0004] However, the particles recovered by electrophoresis are in a paste state, contain a large amount of the liquid used in electrophoresis, and various substances are dissolved in the particles. Therefore, the recovered paste-like particles need to be washed. Therefore, when using the particle recovery method described in Patent Document 1, it is necessary to scrape off the paste-like particles concentrated near the electrodes with a scraper or the like, transfer them to another container, redisperse them in wash water, and then transfer them back to the electrophoresis device for recovery. This process must be repeated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] WO2011 / 007820 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0006] The present invention has been made in consideration of the above-mentioned conventional situation, and an object of the present invention is to provide a particle recovery method that can continuously perform the recovery process and washing process of particles in a slurry in a single device, and a particle recovery device used therefor. [Means for solving the problem]

[0007] The particle recovery device of the present invention is a particle recovery device that recovers particles dispersed in a liquid by adhering them to the vicinity of an electrode through electrophoresis, and is characterized by comprising a treatment tank that contains the liquid, a pair of electrodes arranged in the treatment tank, and a power source that applies a voltage between the pair of electrodes, and is provided with relative movement means that can apply shear force to the particles adhered to the vicinity of the electrode and redisperse them by causing relative movement between the particles adhered to the vicinity of the electrode and the liquid surrounding the particles.

[0008] In the particle recovery device of the present invention, a liquid containing dispersed particles is placed in a treatment tank, and a voltage is applied between a pair of electrodes with the power turned on. The particles dispersed in the liquid migrate toward the electrodes by electrophoresis, concentrating near the electrodes and adhering to them in a paste-like state. With the particles thus adhering to the electrodes, the liquid in the treatment tank is removed and a cleaning liquid is introduced into the treatment tank. The relative motion means is then driven to cause relative motion between the particles adhering to the electrodes and the liquid surrounding the particles. This applies shear force to the particles adhering to the electrodes in a paste-like state, causing them to be redispersed in the cleaning liquid. Furthermore, if the particles redispersed in the cleaning liquid are subjected to electrophoresis again, the particles washed with the cleaning liquid will adhere to the electrode, and the re-adhered particles can be scraped off and the washed particles can be collected. Furthermore, by repeating electrophoresis and washing with a washing liquid multiple times, the washing effect can be further improved.

[0009] Therefore, according to the particle recovery device of the present invention, the recovery step and washing step of particles in a slurry can be carried out continuously in one device.

[0010] The relative movement means may be, for example, an electrode movement means that moves the electrode on the side where the particles are concentrated in the liquid, or a liquid flow means that flows the liquid. Either means can apply shear force to the particles adhering to the electrode to redisperse them. Furthermore, the relative movement means and the liquid flow means may be used in combination.

[0011] The electrode surface on the side where the particles adhere is preferably provided with irregularities. If the electrode surface is irregular, the amount of particles that adhere to the electrode can be increased, and the adhered particles are less likely to fall off during electrophoresis.

[0012] By using the particle recovery device of the present invention, the recovery of particles in a slurry can be carried out continuously in one device by the following method. That is, the particle recovery method of the present invention includes an electrophoresis step of placing a particle dispersion liquid in which particles are dispersed in a liquid into a treatment tank, immersing a pair of electrodes in the particle dispersion liquid, and applying a voltage between the electrodes to cause particles to adhere to the electrodes by electrophoresis; a liquid replacement step of replacing the liquid with a cleaning liquid; and a redispersion step of redispersing the particles adhered to the electrode in the cleaning liquid by applying shear force to the particles adhered to the electrode by causing relative movement between the particles adhered to the electrode and the cleaning liquid.

[0013] Alternatively, the particles in the slurry can be made to form a monolayer of particles on the electrode by the following method. That is, the monolayer forming method of the present invention includes an electrophoresis step of placing a particle dispersion liquid in which particles are dispersed in a liquid into a treatment tank, immersing a pair of electrodes in the particle dispersion liquid, and applying a voltage between the electrodes to cause particles to adhere to the electrodes by electrophoresis; a liquid replacement step of replacing the liquid with a cleaning liquid; and a monolayer forming step of applying shear force to the particles adhering to the electrode by moving the particles adhering to the electrode and the cleaning liquid relative to each other, thereby forming a monolayer of particles on the electrode.

[0014] In the monolayer formation method of the present invention, if a metal sample stage for an electron microscope is used as an electrode, the particles spread in a monolayer on the sample stage can be observed with an electron microscope, thereby preventing particle overlap and enabling clear observation of the particle structure. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a process diagram of a particle recovery method according to an embodiment. [Figure 2] 2A is a schematic diagram showing the structure of the particle recovery device of the first embodiment, and FIG. 2B is a cross-sectional view taken along the line XX. [Figure 3] 2A is a schematic diagram showing the structure of a particle recovery device of a second embodiment, and FIG. 2B is a cross-sectional view taken along the line XX. [Figure 4] FIG. 10 is a cross-sectional view of a cylindrical container 20 of a particle recovery device according to a third embodiment. [Figure 5] Photographs are taken before (FIG. 5(a)) and after (FIG. 5(b)) electrophoresis tests. [Figure 6] FIG. 2 is a side view of the particle recovery device of the first embodiment. [Figure 7] 3 is a cross-sectional view of an electrode rod 34 in the particle recovery device of the first embodiment. FIG. [Figure 8] FIG. 1 is a front view of a particle recovery device according to a first embodiment. [Figure 9] 1A is a photograph of the entire particle recovery device of Example 1, and FIG. 1B is a photograph of the inside of a frame 31. [Figure 10] 10 is a cross-sectional view of electrode rods 50a, 50b, and 50c in a particle collection device according to a second embodiment. [Figure 11] 1 is a graph showing the particle size distribution measurement results of the solid nanosilica used in Example 2. [Figure 12] 10 is a graph showing the amount of solid nanosilica attached to each of the electrode rods 50a, 50b, and 50c in Example 2. [Figure 13] 10 is a cross-sectional view of electrode rods 50a, 50b, and 50c and an auxiliary anode 51 in a particle recovery device according to a third embodiment. [Figure 14] 10 is a graph showing the amount of solid nanosilica attached to each of the electrode rods 50a, 50b, and 50c in Example 3. [Figure 15] FIG. 10 is a schematic diagram of a particle recovery device according to a fourth embodiment. [Figure 16] FIG. 10 is an exploded perspective view of the tip portion of the electrode rod 60 of the fourth embodiment. [Figure 17] FIG. 10 is a schematic diagram showing how hollow nano-silica particles attached to a carbon support film 60c form a monolayer by stirring in Example 4. [Figure 18] 10 is an electron microscope photograph of nanohollow silica particles adhered to a carbon support film 60c in Example 4 and Comparative Example 1. [Figure 19] 10 is an electron microscope photograph of hollow silica nanospheres of an example deposited on a carbon support film 60c. DETAILED DESCRIPTION OF THE INVENTION

[0016] <Particle collection method> FIG. 1 shows a flow chart of the particle recovery method according to the embodiment. In this particle recovery method, a slurry in which particles to be recovered are dispersed in a liquid is first prepared and placed in a container. The particles to be recovered are not particularly limited, and ceramic particles such as silica, alumina, mullite, clay, glass, fly ash, and zirconia can be used. From the same perspective, the specific gravity of the particles is preferably close to that of the liquid in which they are dispersed. Hollow particles with an apparent specific gravity close to that of the liquid are also preferred.

[0017] The liquid used for the slurry is not particularly limited, but examples include water, organic solvents such as alcohol, and water-alcohol mixed solvents. It is desirable that the liquid contains a low concentration of ions or substances that undergo electrochemical oxidation or reduction during electrophoresis. To regulate particle migration during the electrophoresis process, agents for adjusting the charge state of the particle surfaces (e.g., cationic surfactants, anionic surfactants, aluminum sulfate, aluminum chloride, etc.) may be added.

[0018] (Electrophoresis step S1) In the electrophoresis step S1, a pair of electrodes is placed in the slurry, and a voltage is applied between the electrodes. The electrodes can be made of conductive, insoluble materials that do not dissolve during electrophoresis (e.g., various precious metals such as platinum and gold, carbon, graphite, stainless steel, titanium, precious metal-coated titanium, niobium, tantalum, etc.). The voltage applied between the electrodes can be direct current, alternating current, or pulsed current. The applied voltage can be appropriately selected so that the particles electrophoretically migrate at a practical speed. However, it is preferable to consider the effects of electrochemical reactions of the liquid used in the slurry and the ions dissolved in the slurry. For example, electrolysis of water can generate oxygen and hydrogen, which can cause particles attached to the electrodes to fall off, or the generation of chlorine gas when chloride ions are present, which can lead to environmental degradation. To adjust the speed of such electrochemical reactions, electrophoresis can be performed at a constant current.

[0019] In the electrophoresis step 1S, particles dispersed in the liquid move to the negative electrode if the zeta potential is positive, and move to the positive electrode if the zeta potential is negative, and become concentrated near the electrode, forming a paste and adhering to it. The mechanism behind this phenomenon can be explained as follows. The interaction potential (total interaction) V acting between particles dispersed in a liquid is the sum of the Van del Waals interaction VV and the electric double layer interaction VE (V = VE + VV), and reaches a maximum value Vmax at a certain interparticle distance h1, and as the particles approach further, the Van del Waals interaction VV becomes dominant. The electric double layer interaction VE is correlated with the zeta potential of the particles; the higher the zeta potential of the particles, the greater the electric double layer interaction VE. Particles dispersed in a liquid move by electrophoresis toward the electrode with the opposite sign to the zeta potential, increasing the particle concentration near the electrode and reducing the distance between particles. At this time, if the particles cannot approach each other by overcoming the potential barrier Vmax, they do not aggregate but remain near the electrode. This state can also be expressed as a balance between the approach due to electrophoresis and the repulsion due to the interaction potential V. By having particles remain near the electrode in this way, the particles become concentrated and dispersed (paste-like). In other words, if the potential barrier Vmax of the interaction potential V is greater than the kinetic energy of the electrophoretic particles, the particles cannot approach each other beyond the potential barrier Vmax and do not aggregate, so the particles become concentrated and dispersed (paste-like).

[0020] The kinetic energy of electrophoretic particles is correlated with the particle's zeta potential, the potential difference between the electrodes, the electrode spacing, and the viscosity of the slurry. As is clear from the Helmholtz-Smoluchowski basic equation of electrophoresis, the particle migration speed due to electrophoresis is proportional to the zeta potential and potential difference, and inversely proportional to the distance between the electrodes. Also, the higher the viscosity of the slurry, the greater the resistance to particle movement. In the electrophoresis test of Example 1 using the nanohollow silica slurry described below, the viscosity was measured using a rotational viscometer, and the viscosity of the paste-like slurry concentrated by applying an electric field increased by 100,000 times compared to the viscosity of the slurry before concentration.

[0021] (Liquid replacement process S2) After the particles in a paste form are attached to the electrode as described above, the liquid in which the particles have been dispersed is removed and a cleaning liquid is introduced.

[0022] (Redispersion step S3) Then, by rotating the electrode and stirring the cleaning liquid, shear force is applied to the particles that have adhered to the electrode in a paste-like state, which causes the particles to peel off and redisperse in the cleaning liquid, resulting in a slurry. This makes it possible to perform the particle recovery process and cleaning process in a single device.

[0023] If it is desired to recover the washed particles in a concentrated paste state, the electrophoresis step S1 can be carried out again in the slurry obtained in the redispersion step S3, the electrode can be pulled up together with the particles attached thereto, and the particles attached to the electrode in a paste state can be peeled off. Furthermore, by further repeating the electrophoresis step S1, the liquid substitution step S2, and the re-dispersion step S3, a more refined slurry or particles can be obtained.

[0024] <Particle collection device> The above particle recovery method can be carried out using a particle recovery device according to the following embodiment. (Embodiment 1) As shown in Fig. 2(a), the particle collection device of the first embodiment has a rotating electrode 2 and a fixed electrode 3 erected in a treatment tank 1. The rotating electrode 2 is bifurcated downward, and the cross section facing each other has an arc-shaped shape (see Fig. 2(b)). The electrodes are electrically connected to each other at their upper ends and rotatably connected to a motor 5 via a rotating shaft 4. The rotating shaft 4 is electrically connected to a power source 6 via a brush electrode (not shown). The fixed electrode 3 is erected and fixed at the center of the opposing electrodes 2, and is electrically connected to the power source 6.

[0025] A method of using the particle recovery device of the first embodiment configured as above will be described. Electrophoresis step S1 A slurry in which particles to be collected (for example, silica) are dispersed in a liquid (for example, a water-alcohol mixed solvent) is prepared and placed in the treatment tank 1 . Then, in the electrophoresis step S1, the power supply 6 is turned on to apply a voltage between the rotating electrode 2 and the fixed electrode 3. The motor 5 is kept off during electrophoresis. The positive and negative polarities of the applied voltage are determined so that the particles move toward the rotating electrode 2 by electrophoresis (for example, in the case of silica particles with a negative zeta potential, the voltage is applied so that the rotating electrode 2 is positive). The applied voltage must be such that the particles electrophorese at a practical speed. However, the applied voltage is adjusted appropriately so that gases generated by the electrolysis of water, etc., do not inhibit electrophoresis. Electrophoresis may be performed at a constant current to adjust the speed of the electrochemical reaction.

[0026] ·Liquid replacement process S2 After the particles have been sufficiently attached to the surface of the rotating electrode 2 by electrophoresis, the power supply 6 is turned off, the liquid in the treatment tank 1 is drained, and the treatment tank 1 is then filled with a cleaning liquid.

[0027] ·Redispersion process S3 Then, the motor 5 is turned on to rotate the rotating electrode 2. As a result, the particles that had adhered to the rotating electrode 2 in a paste state are subjected to shearing force by the flow of the cleaning liquid and are redispersed in the cleaning liquid (redispersion step S3). In this way, a slurry in which the particles are dispersed after being washed with the cleaning liquid is obtained. With this particle recovery device, the recovery step and cleaning step of the particles in the slurry can be carried out continuously in one device.

[0028] If the washed particles are to be recovered as a paste, the electrophoresis step S1 is carried out again in the slurry obtained in the redispersion step S3, the electrode is pulled up together with the particles attached thereto, and the particles attached to the electrode are peeled off, whereby the particles can be recovered. Furthermore, by further repeating the electrophoresis step S1, the liquid substitution step S2, and the re-dispersion step S3, a more refined slurry or particles can be obtained.

[0029] (Embodiment 2) As shown in FIG. 3(a), the particle collection device of the second embodiment has a cylindrical container 11 placed in a treatment tank 10. The cylindrical container 11 has no bottom and is closed at the top by a top plate 11a. An inlet pipe 12 is connected to the center of the top plate 11a and is connected to a circulation pump 14 via an on-off valve 13. The circulation pump 14 is connected to an outlet pipe 15 connected to the bottom of the treatment tank 10 via an on-off valve 16. A honeycomb electrode 17 is provided inside the cylindrical container 11, and a square rod-shaped insertion electrode 18 is inserted into the center of each hole of the honeycomb electrode 17 without contacting it (see FIG. 3(b)). The honeycomb electrode 17 and the insertion electrode 18 are electrically connected to a power source 19.

[0030] The particle recovery device of the second embodiment configured as above is used as follows. Electrophoresis step S1 A slurry containing particles to be collected dispersed in a liquid is prepared and placed in the treatment tank 10. Then, in the electrophoresis step S1, the power supply 19 is turned on to apply a voltage between the honeycomb electrode 17 and the insertion electrode 18. This causes the particles in the slurry to adhere in a paste-like state to the electrode on the opposite side of the zeta potential. The applied voltage is appropriately adjusted while taking the same precautions as those described in the first embodiment.

[0031] ·Liquid replacement process S2 After the particles are attached in a paste state to the surface of the honeycomb electrode 17 or the insertion electrode 18 by electrophoresis, the power supply 19 is turned off, the liquid in the treatment tank 10 is drained, and then the treatment tank 10 is filled with a cleaning liquid.

[0032] ·Redispersion process S3 Then, the circulation pump 14 is turned on to allow the cleaning liquid to flow into each hole of the honeycomb electrode 17. As a result, the particles adhering to the honeycomb electrode 17 or the inserted electrode 18 are subjected to shear force by the flow of the cleaning liquid and are redispersed in the cleaning liquid. In this way, a slurry in which the particles are dispersed after being washed with the cleaning liquid is obtained. Thus, according to the particle recovery device of the second embodiment, the process of recovering particles in the slurry and the process of washing can be carried out continuously in a single device.

[0033] (Embodiment 3) 4, in the particle collection device of the third embodiment, spiral cylindrical electrodes 21 and 22 are inserted into a cylindrical container 20 so that they face each other without touching each other. The spiral cylindrical electrodes 21 and 22 are connected to a power source. The other structures are the same as those of the particle collection device of the second embodiment, and the same structures are denoted by the same reference numerals and will not be described again.

[0034] In the particle recovery device of the third embodiment, particles adhere in a paste state to either of the spiral cylindrical electrodes 21 and 22 in the electrophoresis step 1S, and then a slurry in which the particles washed with the washing liquid are redispersed can be obtained by driving the circulation pump 14. In other words, the recovery step and washing step of the particles in the slurry can be performed continuously in one device.

[0035] In the redispersion step S3 in the first to third embodiments, it is also preferable to apply ultrasonic vibrations to the cleaning liquid in order to promote the redispersion of particles that have adhered to the electrode in a paste form. [Example]

[0036] (Electrophoresis test) In preparation for the construction of a particle collection device, an electrophoresis test of nano-hollow silica slurry was carried out. Synthesis of nano-hollow silica slurry The nanohollow silica slurry was synthesized according to the method described in JP 2005-263550 A. Carbon dioxide gas was introduced into 2.0 L of calcium hydroxide slurry with a solid content of 7.5 wt % and adjusted to a liquid temperature of 15° C. at a rate of 1.5 L / min for 2 hours while stirring, to precipitate calcium carbonate. The liquid temperature was then raised to 80° C., and the mixture was aged with stirring for 24 hours.

[0037] The resulting calcium carbonate slurry was centrifuged to produce a wet cake with a water content of 65% by weight. 22 g of this wet cake was then added to 450 g of ethanol and ultrasonicated for 1 minute to disperse the calcium carbonate in the ethanol. 21 g of 28% aqueous ammonia and 7.5 g of tetraethoxysilane were added (tetraethoxysilane / ethanol volume ratio: 0.01, NH3 in the aqueous ammonia was 9.3 moles per mole of tetraethoxysilane, and water was 30 moles per mole of tetraethoxysilane). Stirring was continued for 12 hours to produce silica-coated calcium carbonate. Transmission electron microscopy of this preparation revealed a 5-10 nm thick silica shell on a 40-80 nm thick calcium carbonate surface.

[0038] Next, the silica-coated calcium carbonate slurry was dewatered by suction filtration, washed with ethanol and then with water, and then dispersed in water again. 200 mL of 2.5 mol / L HCl was added thereto (total acid concentration of the liquid: 0.5 mol / L), and the mixture was stirred for 1 hour to dissolve the calcium carbonate, yielding a nano-hollow silica slurry.

[0039] Observation of the product under a transmission electron microscope confirmed the presence of hollow silica particles with a primary particle diameter of 45 to 90 nm, and static light scattering (Malvern Zetasizer 3000HS) determined the particle diameter to be 350 nm.

[0040] Electrophoresis of nano-hollow silica slurry An electrophoresis test was conducted on the nanohollow silica slurry obtained as described above. A 2-L beaker was prepared, and a stainless steel wire mesh electrode was placed along the inside of the beaker in a cylindrical shape. 1500 mL of nanohollow silica slurry was then poured into it. Six stainless steel rods were then inserted into a stainless steel plate with numerous holes, screwed into place, and placed on the beaker. A DC power supply was used to apply a voltage of 20 V, with the wire mesh electrode acting as the negative electrode and the stainless steel rod electrode acting as the positive electrode. Figure 5 shows the electrophoresis test results before (Figure 5(a)) and after (Figure 5(b)). After the test, a large amount of nanohollow silica particles was found to adhere to the stainless steel rod (see Figure 5(b)). Because such nanohollow silica particles have a primary particle diameter of 45–90 nm and are hollow, their recovery by centrifugation or filtration is difficult, demonstrating that electrophoresis is a superior recovery method.

[0041] Example 1 In Example 1, a particle recovery device was produced that was capable of performing the electrophoresis step S1 of the silica slurry, followed by the liquid substitution step S2 and the re-dispersion step S3. 6 and 8 show the particle collection device of Example 1. This particle collection device has a motor 32 attached to the top of a frame 31 in the shape of a bottomless rectangular parallelepiped container with the lower half of one side open, with the motor shaft 32a facing downward. A rotating shaft 33 is joined to the motor shaft 32a and extends downward, and an electrode rod 34 made of a total of 102 SUS304 bolts hangs concentrically from the lower end of the motor shaft 32a in a detachable manner (see FIG. 7). The electrode rod 34 is connected to a DC power supply 37 via the rotating shaft 33 and a brush 36 in contact with the rotating shaft 33. Two ring-shaped counter electrodes 35a and 35b made of SUS304 are provided at the top and bottom of the periphery of the electrode rod 34, close to but separated from the electrode rod 34. The counter electrodes 35a and 35b are connected to the DC power supply 37.

[0042] A cylindrical treatment tank 38 is provided below the electrode rod 34, and treatment tank 38 can be raised and lowered by a jack 40 mounted on a movable cart 39. An inlet 38a is attached to the upper end of the side of treatment tank 38, an outlet 38b is attached to the lower end of the side, and a drainage port 38c is attached to the bottom. Inlet 38a and outlet 38b are connected to a circulation pump 41, allowing the slurry in treatment tank 38 to circulate.

[0043] 8, a frame 42 is provided adjacent to the frame 31, and like the frame 31, the frame 42 is a bottomless rectangular parallelepiped container with the lower half of one side open. A stirrer 43 and a cooling panel 45 connected to a cooler 44 are attached to the top of the frame 31.

[0044] 9 shows a photograph of the particle recovery device of Example 1. By using this device, the nanohollow silica slurry can be purified and recovered by the method shown below.

[0045] Synthesis of nano-hollow silica slurry The treatment tank 38 is placed on a jack 40 attached to a movable cart 39, which is then placed under a frame 42, and the above-described method for synthesizing nanohollow silica slurry can be carried out in the treatment tank 38. During the liquid mixing and reaction, the slurry is stirred with a stirrer 43, and cooled as needed with a cooler 44. The compounding ratio of the chemicals and other procedures are the same as those described above, and therefore further explanation will be omitted. In this way, a nano-hollow silica slurry can be obtained with a primary particle size of 45 to 90 nm, which is 350 nm when measured by static light scattering (Malvern Zetasizer 3000HS).

[0046] Electrophoresis step S1 The jack 40 is lowered, and the treatment tank 38 containing the nanohollow silica slurry is lowered from the agitator 43 and the cooling panel 45. Then, the cart 39 carrying the treatment tank 38 is placed on another cart 50 (see FIG. 8) and positioned below the frame 31, and then the jack 40 is raised to immerse the electrode rod 34 and counter electrodes 35a, 35b in the nanohollow silica slurry in the treatment tank 38. Then, electrophoresis can be performed using a DC power supply 37, with the electrode rod 34 as positive and the counter electrodes 35a, 35b as negative.

[0047] ·Liquid replacement process S2 After confirming that the particles have been sufficiently attached to the surface of the electrode rod 34 by electrophoresis, the DC power supply 37 is turned off, the remaining liquid in the treatment tank 38 is drained from the drain outlet 38c, and then the treatment tank 38 is filled with cleaning water.

[0048] ·Redispersion process S3 Then, the motor 32 is turned on to rotate the electrode rod 34 to which the hollow nanosilica particles are attached. As a result, the hollow nanosilica particles attached to the electrode rod 34 are subjected to shearing force by the flow of the cleaning liquid, and are redispersed in the water (redispersion step S3). In this way, a slurry in which the particles washed with the cleaning water are dispersed can be obtained.

[0049] As described above, according to the particle recovery device of Example 1, recovery of particles in the slurry in the electrophoresis step S1 and washing of the hollow silica nanoparticles in the redispersion step S3 can be carried out continuously in one device. Furthermore, by repeating the electrophoresis step S1 and the re-dispersion step S3, a more refined nanohollow silica slurry can be obtained.

[0050] Example 2 In Example 2, a recovery experiment from solid nanosilica slurry was carried out using the particle recovery device used in Example 1. However, of the 102 attachable electrode rods 34 (see FIG. 7), as shown in FIG. 10, electrode rod 50a was attached at a position 16 mm axially from counter electrodes 35a and 35b, electrode rod 50b at a position 36 mm, and electrode rod 50c at a position 56 mm, forming 12 concentric rows, for a total of 36 electrodes, and the remaining electrode rods were removed.

[0051] Using the particle recovery device of Example 2, an electrophoresis test was carried out on a solid nanosilica slurry (AEROSIL (registered trademark) OX50, manufactured by Nippon Aerosil Co., Ltd., primary particle diameter 40 nm). ·Particle size distribution measurement of solid nanosilica The particle size distribution of a 5 wt% solid nanosilica slurry was measured using a particle size analyzer (Microtrac MT3000II series). The refractive index of silica was 1.77, and the refractive index of ethanol used as the dispersion medium was 1.36. The measurement conditions were set to "transparent" for "transmittance" and "shape" for "non-spherical." The results are shown in Figure 11. The horizontal axis of the graph represents particle diameter Dp [μm], and the vertical axis represents particle frequency Qf [%]. The measurement results showed that the average particle size Dp was 0.731 μm. The primary particle diameter of the solid nanosilica dispersed in the slurry was 40 nm, indicating that the solid nanosilica in the slurry had undergone secondary aggregation.

[0052] Electrophoresis test 17 L of 2.83 wt % solid nanosilica slurry was placed in the treatment tank 38, and a DC power supply 37 was connected so that the electrode rods 50a, 50b, and 50c were on the positive side and the counter electrodes 35a and 35b were on the negative side.The voltage was set to 20 V, and electrophoresis was performed for 20 minutes. After the electrophoresis was completed, the jack 40 was lowered, the electrodes 50a, 50b, and 50c were removed, and the sample was allowed to dry naturally before being weighed using an electronic balance. The results are shown in FIG. This graph shows that the amount of silica deposited per unit area increases as the distance from the counter electrodes 35a, 35b decreases. This is thought to be because the current concentrates on the electrode rod 50a, which is located closer to the counter electrodes 35a, 35b, increasing the potential gradient between the counter electrodes 35a, 35b and the electrode rod 50a. In addition, the energy consumption during electrophoresis was calculated to be 0.440 [kWh / kg], based on the voltage during electrophoresis being 20 V, the current being 10 mA, the electrophoresis time being 20 minutes, and the weight of the recovered silica being 151 g.

[0053] Example 3 In Example 3, as shown in Fig. 13, twelve rectangular plate-shaped auxiliary anodes 51 were added radially between adjacent electrode rods 50a, 50b, and 50c arranged in a row in the axial direction. The other conditions were the same as in Example 2, and therefore a description thereof will be omitted.

[0054] The results are shown in Figure 14. From this graph, it can be seen that the amount of silica deposited per unit area was electrode rod 50a > electrode rod 50c > electrode rod 50b. Furthermore, compared to Example 2, in which auxiliary electrode 51 was not provided, the difference in the amount of deposited silica depending on the position was smaller. This is thought to be because the presence of auxiliary electrode 51 made it easier for current to flow even in positions close to the axis of treatment tank 38, thereby reducing the difference in potential gradient depending on the position. The energy consumption during electrophoresis was calculated to be 0.221 kWh / kg, based on the voltage during electrophoresis of 20 V, the current of 10 mA, the electrophoresis time of 20 minutes, and the weight of the recovered silica of 302 g. This energy consumption value was about half that of Example 2, and it was found that the provision of the auxiliary electrode 51 could significantly reduce energy consumption. Example 4 Example 4 is a particle collection device for preparing a sample to form a single particle layer on the sample stage of an electron microscope. As shown in Fig. 15, this particle collection device is equipped with an electrode rod 60, a counter electrode 61, and a DC power supply 62. As shown in Fig. 16, at the tip of the electrode rod 60, three layers are fixed by axial pressure using an annular screw 60d, and the three layers are: 1) a disk-shaped copper grid 60a with many holes, 2) a microgrid 60b which is a carbon-reinforced porous Triahole (cellulose acetate butyrate) membrane, and 3) a 4-nm-thick carbon support membrane 60c.

[0055] Monolayer formation Preparation of nano-hollow silica particle slurry We used polyacrylic acid (PAA) nanoparticles as a mold, coated them with silica, and then dissolved the PAA nanoparticles to prepare a hollow silica nanoparticle slurry. The details of the manufacturing method are described below. 0.12 g of a 25 wt % aqueous solution of polyacrylic acid (PAA) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., number average molecular weight M = 25,000) was mixed with 2.0 ml of a 25% aqueous ammonia solution and stirred for 24 hours to prepare a polyacrylic acid / ammonia water mixed solution, which was then stirred for 2 minutes to prepare a polyacrylic acid / ammonia water mixed solution, which was added to 35 ml of ethanol and stirred to obtain an emulsion. After stirring this emulsion for 2 minutes, 2 ml of tetraethoxysilane (TEOS) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added at a rate of 40 μL / min, and the emulsion was stirred for one day. The powder was then subjected to solid-liquid separation using a refrigerated / high-speed centrifuge (H-9R, Kokusan Co., Ltd.) at 15,000 rpm for 10 minutes, yielding a powder. After washing with water, the powder was dried at 180°C for 24 hours. When the powder was observed under a scanning electron microscope, it was found to be hollow nanosilica with particle diameters of several tens of nanometers.

[0056] The resulting hollow nanosilica dispersion was diluted with water and placed in a beaker 63. An electrode rod 60 and a counter electrode 61 were immersed in the slurry. A voltage of 15 V was applied from a DC power supply 62 so that the electrode rod 60 was positive and the counter electrode 61 was negative, resulting in the deposition of hollow nanosilica particles onto the carbon support film 60c (see Figure 17(a)). The voltage application was then stopped, the liquid in the beaker 63 was removed, and washing water was added. The mixture was then stirred with a magnetic stirrer 64 to apply shear force to the hollow nanosilica particles deposited on the carbon support film 60c (see Figure 17(b)). The annular screw 60d was then removed, and the grid 60a, microgrid 60b, and carbon support film 60c were removed. Carbon deposition was performed, and the mixture was observed using a transmission electron microscope. The results are shown in Figure 18 (top) and Figure 19. The hollow nanosilica particles were clearly observed in a single layer, without contact with each other, and the observation of the particles was not hindered by particle overlap. The reason why the particles did not come into contact with each other is thought to be due to the repulsive force caused by the electric double layer interaction (i.e., the repulsive force due to the same zeta potential), and the reason why only a monolayer was adsorbed on the electrode is thought to be due to the attractive force caused by the electric double layer interaction (i.e., the electrostatic attraction between the electrode and the hollow silica nanoparticles).

[0057] (Comparative Example 1) In Comparative Example 1, a water-diluted nanohollow silica slurry was dropped onto the specimen stage of an electron microscope and dried, followed by carbon deposition and observation with a transmission electron microscope. As a result, as shown in Figure 18 (bottom), aggregated nanohollow silica was observed, and the overlapping of particles made it difficult to observe the details of each particle.

[0058] The present invention is not limited to the above-described embodiments and examples. Various modifications within the scope of the claims and within the scope that can be easily conceived by a person skilled in the art are also included in the present invention. [Industrial Applicability]

[0059] The particle recovery method and particle recovery device of the present invention can recover particles from a slurry in which ceramic particles are dispersed, or can generate a slurry, and therefore can be suitably used for preparing ceramic raw materials. Furthermore, the single layer formation method of the present invention can be suitably used for preparing samples for electron microscopes. [Explanation of symbols]

[0060] S1...electrophoresis step, S2...liquid replacement step, S3...redispersion step, 1, 10...treatment tank, 2...rotating electrode, 3...fixed electrode, 4...rotating shaft, 5...motor, 6...power source, 7...container, 8...partition plate, 9...glass plate, 11, 20...cylindrical container, 11a...top plate, 14...circulation pump, 15...outlet pipe, 16...opening / closing valve, 17...honeycomb electrode, 18...insertion electrode, 19...power source, 21, 22...spiral cylindrical electrode, 31...frame, 32...motor, 32a...motor shaft, 34, 50a, 50b, 50c...electrode rods, 35a, 35b...counter electrodes, 36...brush, 37...DC power supply, 38...treatment tank, 38a...inlet, 38b...outlet, 38c...drainage port, 39...cart, 40...jack, 41...circulation pump, 42...frame, 43...mixer, 44...cooler, 60...electrode rods, 61...counter electrodes, 60a...grid, 60b...microgrid, 60c...carbon support film, 60d...annular screw

Claims

1. an electrophoresis step of placing a particle dispersion liquid in which particles are dispersed in a liquid into a treatment tank, immersing a pair of electrodes in the particle dispersion liquid, and applying a voltage between the electrodes to cause particles to adhere to the electrodes by electrophoresis; a liquid replacement step of replacing the liquid with a cleaning liquid; and a redispersion step of redispersing the particles adhering to the electrode in the cleaning liquid by applying shear force to the particles adhering to the electrode by moving the particles adhering to the electrode and the cleaning liquid relative to each other.

2. an electrophoresis step of placing a particle dispersion liquid in which particles are dispersed in a liquid into a treatment tank, immersing a pair of electrodes in the particle dispersion liquid, and applying a voltage between the electrodes to cause particles to adhere to the electrodes by electrophoresis; a liquid replacement step of replacing the liquid with a cleaning liquid; and a monolayer formation step of applying shear force to the particles adhered to the electrode by moving the particles adhered to the electrode and the cleaning liquid relative to each other, thereby forming a monolayer of particles on the electrode.

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