JP7806987B2 - Particle recovery method and particle recovery device - Google Patents

Description

The present invention relates to a method for recovering particles from a slurry and a particle recovery device used therein, and is particularly suitable for use in recovering nanoparticles.

Traditionally, methods for recovering particles from slurries have been known to involve separation by filtration or using a centrifuge. 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 posed a problem in that it is difficult to separate and recover nanoparticles.

To solve these problems, methods have been developed for recovering particles from slurry using electrophoresis. 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 nanoparticle slurry.

However, the particles recovered by electrophoresis are in a paste-like state, contain a large amount of the liquid used in electrophoresis, and have various substances dissolved in them. For this reason, it is necessary to wash the recovered paste-like particles. 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.

WO2011/007820

The present invention was made in consideration of the above-mentioned conventional situation, and aims to solve the problem of providing a particle recovery method that can continuously perform the recovery and washing processes of particles in a slurry using a single device, as well as a particle recovery device used therein.

The particle recovery device of the present invention recovers particles dispersed in a liquid by electrophoresis, causing them to adhere to the vicinity of an electrode. It is characterized by comprising a treatment tank that contains the liquid, a pair of electrodes disposed 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 near the electrodes and redisperse them by causing relative movement between the particles adhered near the electrodes and the liquid surrounding the particles.

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.

Therefore, the particle recovery device of the present invention makes it possible to continuously perform the recovery and cleaning processes of particles in slurry using a single device.

The relative movement means can 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 moves the liquid. Either means can apply shear force to the particles attached to the electrode, causing them to be redispersed. It is also possible to use a combination of a relative movement means and a liquid flow means.

It is preferable that the electrode surface on which the particles adhere is uneven. If the electrode surface is uneven, a larger amount of particles can adhere to the electrode, and the adhered particles are less likely to fall off during electrophoresis.

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.

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.

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

1 is a process diagram of a particle recovery method according to an embodiment. 1A is a schematic diagram showing the structure of a particle recovery device of embodiment 1, and FIG. 1B is a cross-sectional view taken along the line X-X. 1A is a schematic diagram showing the structure of a particle recovery device of a second embodiment, and FIG. 1B is a cross-sectional view taken along the line X-X of FIG. FIG. 10 is a cross-sectional view of a cylindrical container 20 of a particle recovery device according to a third embodiment. 5(a) and 5(b) are photographs taken before and after the electrophoresis test. FIG. 2 is a side view of the particle recovery device of the first embodiment. 3 is a cross-sectional view of an electrode rod 34 in the particle recovery device of Example 1. FIG. FIG. 2 is a front view of the particle recovery device of the first embodiment. 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. 10 is a cross-sectional view of electrode rods 50a, 50b, and 50c in a particle recovery device according to a second embodiment. 1 is a graph showing the particle size distribution measurement results of the solid nanosilica used in Example 2. 10 is a graph showing the amount of solid nanosilica attached to each of the electrode rods 50a, 50b, and 50c in Example 2. 10 is a cross-sectional view of electrode rods 50a, 50b, 50c and an auxiliary anode 51 in a particle recovery device according to a third embodiment. FIG. 10 is a graph showing the amount of solid nanosilica attached to each of the electrode rods 50a, 50b, and 50c in Example 3. FIG. 10 is a schematic diagram of a particle recovery device according to a fourth embodiment. FIG. 10 is an exploded perspective view of the tip portion of the electrode rod 60 of the fourth embodiment. 10 is a schematic diagram showing how nano-hollow silica particles attached to a carbon support film 60c are formed into a single layer by stirring in Example 4. FIG. 10 is an electron microscope photograph of nano-hollow silica particles adhered to a carbon support film 60c in Example 4 and Comparative Example 1. 10 is an electron microscope photograph of hollow silica nanospheres of an example attached to a carbon support film 60c.

<Particle recovery 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.

The liquid used in the slurry is not particularly limited, but examples include water, organic solvents such as alcohol, and water-alcohol mixed solvents. It is desirable for the liquid to have a low concentration of ions or substances that undergo electrochemical oxidation or reduction during electrophoresis. In addition, to adjust the particle movement during the electrophoresis process, agents for adjusting the charge state of the particle surface (e.g., cationic surfactants, anionic surfactants, aluminum sulfate, aluminum chloride, etc.) may be added.

(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, causing particles attached to the electrodes to fall off, or environmental degradation due to the generation of chlorine gas when chloride ions are present. To adjust the rate of such electrochemical reactions, electrophoresis can be performed at a constant current.

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 are concentrated near the electrode, becoming 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 der 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. When the particles approach each other further, the Van der 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 the 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 the particles remain near the electrode in this way, the particles become concentrated and dispersed (paste-like).
That is, if the potential barrier Vmax of the interaction potential V is larger than the kinetic energy of the electrophoretic particles, the particles cannot approach each other over the potential barrier Vmax and do not aggregate, so the particles become concentrated and dispersed (paste-like).

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.

(Liquid replacement step 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.

(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.

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, it is possible to obtain a more refined slurry or particles.

<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.

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 water electrolysis, etc., do not inhibit electrophoresis. Electrophoresis may be performed at a constant current to adjust the speed of the electrochemical reaction.

・Liquid replacement step 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.

・Redispersion step 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 shear 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.

If it is desired to recover the washed particles 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.

(Embodiment 2)
As shown in FIG. 3( a), the particle collection device of the second embodiment has a cylindrical container 11 installed in a treatment tank 10. The cylindrical container 11 has no bottom and is closed at its upper end 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 square rod-shaped insertion electrodes 18 are inserted into the centers of each hole of the honeycomb electrode 17 without contacting them (see FIG. 3( b)). The honeycomb electrode 17 and insertion electrodes 18 are electrically connected to a power source 19.

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 described in the first embodiment.

・Liquid replacement step S2
After the particles are attached in a paste state to the surface of the honeycomb electrode 17 or the inserted electrode 18 by electrophoresis, the power supply 19 is turned off, the liquid in the treatment tank 10 is drained, and the tank is filled with a cleaning liquid.

・Redispersion step 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.

(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.

In the particle recovery device of embodiment 3, particles adhere in a paste state to one of the spiral cylindrical electrodes 21, 22 during the electrophoresis process 1S, and then by driving the circulation pump 14, a slurry can be obtained in which the particles are redispersed after being washed with the cleaning liquid. In other words, the process of recovering particles in the slurry and the process of cleaning can be carried out continuously in a single device.

In the redispersion step S3 in embodiments 1 to 3, 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.

(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 nanohollow silica slurry The nanohollow silica slurry was synthesized by the method described in JP 2005-263550 A. That is, carbon dioxide gas was introduced at a rate of 1.5 L/min for 2 hours 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. while stirring, to precipitate calcium carbonate. Thereafter, the liquid temperature was raised to 80° C., and the mixture was aged with stirring for 24 hours.

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 content in the aqueous ammonia was 9.3 moles per mole of tetraethoxysilane, and water content 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.

Next, the silica-coated calcium carbonate slurry was drained 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 (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 nanohollow silica slurry.

When the product was observed under a transmission electron microscope, hollow silica particles with a primary particle diameter of 45 to 90 nm were confirmed. Furthermore, static light scattering (Malvern Zetasizer 3000HS) determined the particle diameter to be 350 nm.

Electrophoresis of Nanohollow Silica Slurry An electrophoresis test was conducted on the nanohollow silica slurry obtained as described above. A 2-L beaker was prepared, a stainless steel wire mesh electrode was placed along the inside of the beaker in a cylindrical shape, and 1500 ml of nanohollow silica slurry was 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 voltage of 20 V was applied using a DC power supply, 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 before (Figure 5(a)) and after (Figure 5(b)). After the test, it was found that a large amount of nanohollow silica had adhered to the stainless steel rod (see Figure 5(b)). Because such nanohollow silica has a primary particle diameter of 45 to 90 nm and is hollow, it is difficult to recover by centrifugation or filtration, demonstrating that electrophoresis is a superior recovery method.

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 redispersion step S3.
Figures 6 and 8 show the particle collection device of Example 1. This particle collection device has a motor 32 mounted with its motor shaft 32a facing downward on the top of a frame 31, which is a bottomless rectangular parallelepiped container with the lower half of one side open. A rotating shaft 33 extends downward and is joined to the motor shaft 32a. An electrode rod 34, consisting of a total of 102 SUS304 bolts, hangs concentrically from the lower end of the motor shaft 32a in a detachable manner (see Figure 7). The electrode rod 34 is connected to a DC power supply 37 via the rotating shaft 33 and brushes 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, adjacent to but separated from the electrode rod 34. The counter electrodes 35a and 35b are connected to the DC power supply 37.

A cylindrical treatment tank 38 is provided below the electrode rod 34, and 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 the 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. The inlet 38a and outlet 38b are connected to a circulation pump 41, allowing the slurry in the treatment tank 38 to circulate.

As shown in Figure 8, frame 42 is provided adjacent to frame 31. Like frame 31, frame 42 is a bottomless rectangular container with the lower half of one side open. Attached to the top of frame 31 are a stirrer 43 and a cooling panel 45 connected to a cooler 44.

A photograph of the particle recovery device of Example 1 is shown in Figure 9. Using this device, nanohollow silica slurry can be purified and recovered using the method described below.

Synthesis of nanohollow silica slurry The treatment tank 38 is placed on a jack 40 attached to a movable cart 39, and 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 mixing and reaction of the liquids, 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 nanohollow silica slurry can be obtained having a primary particle size of 45 to 90 nm, which is 350 nm as measured by static light scattering (Zetasizer 3000HS manufactured by Malvern Instruments).

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 35 a, 35 b 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 35 a, 35 b as negative.

・Liquid replacement step 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.

・Redispersion step S3
Then, the motor 32 is turned on to rotate the electrode rod 34 to which the nanohollow silica particles are attached. As a result, the nanohollow silica 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.

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 redispersion step S3, a more refined nanohollow silica slurry can be obtained.

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 axially, and electrode rod 50c at a position 56 mm axially, for a total of 36 electrodes, and the remaining electrodes were removed.

Using the particle recovery device of Example 2, an electrophoresis test was carried out on a solid nanosilica slurry (AEROSIL (registered trademark) OX50, primary particle diameter 40 nm, manufactured by Nippon Aerosil Co., Ltd.).
Measurement of particle size distribution 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 "transparent" and "shape" for "aspherical." 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 was undergoing secondary aggregation.

Electrophoresis test 17 L of 2.83 wt % solid nanosilica slurry was placed in the treatment tank 38, and the DC power supply 37 was connected so that the electrode rods 50 a, 50 b, and 50 c were on the positive side and the counter electrodes 35 a and 35 b 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.
From this graph, it can be seen that the amount of silica deposited per unit area increases as the distance from the counter electrodes 35 a, 35 b decreases. This is thought to be because the current concentrates on the electrode rod 50 a, which is located closer to the counter electrodes 35 a, 35 b, increasing the potential gradient between the counter electrodes 35 a, 35 b and the electrode rod 50 a.
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.

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 further explanation will be omitted.

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 the 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 the auxiliary electrode 51 made it easier for current to flow even in positions close to the axis of the 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 Figure 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 Figure 16, three layers are fixed to the tip of the electrode rod 60 by being pressed axially with an annular screw 60d: 1) a disk-shaped copper grid 60a with multiple 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.

- Monolayer formation Preparation of nanohollow silica particle slurry Using polyacrylic acid (PAA) nanoparticles as a mold, silica was coated around them, and then the polyacrylic acid (PAA) nanoparticles were dissolved out to prepare nanohollow silica particle 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 cooling/high-speed centrifuge (H-9R, Kokusan Co., Ltd.) at 15,000 rpm for 10 minutes, yielding a powder. After washing with water and drying at 180°C for 24 hours, the powder was observed under a scanning electron microscope and found to be hollow nanosilica with particle diameters of several tens of nanometers.

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 15 V DC power supply 62 was applied, with the electrode rod 60 serving as the positive electrode and the counter electrode 61 serving as the negative electrode. This resulted in the deposition of nanohollow silica 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, applying shear force to the nanohollow silica 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 particles were observed using a transmission electron microscope. The results are shown in Figures 18 (top) and 19. The nanohollow silica 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).

(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.

This invention is in no way 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 this invention.

The particle recovery method and particle recovery device of the present invention can recover particles from a slurry containing dispersed particles such as ceramics, or can generate a slurry, making them suitable for use in preparing ceramic raw materials. Furthermore, the single-layer formation method of the present invention can be used to prepare samples for electron microscopes.

S1...electrophoresis process, S2...liquid replacement process, S3...redispersion process, 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, 3 2a...motor shaft; 34, 50a, 50b, 50c...electrode rod; 35a, 35b...counter electrode; 36...brush; 37...DC power supply; 38...treatment tank; 38a...inlet; 38b...outlet; 38c...drain; 39...cart; 40...jack; 41...circulation pump; 42...frame; 43...stirrer; 44...cooler; 60...electrode rod; 61...counter electrode; 60a...grid; 60b...microgrid; 60c...carbon support film; 60d...annular screw

Claims (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 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. 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.