JP2013193173A - Method for regenerating cerium oxide abrasive - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for regenerating cerium oxide abrasives, capable of efficiently separating, recovering and regenerating hydrated Si adhered to the surfaces of the cerium oxide abrasives from the cerium oxide abrasives which are materials essential as high-quality and highly efficient abrasives in free abrasive polishing for glass and quartz.SOLUTION: A method for regenerating cerium oxide abrasives includes: a first step of bringing abrasives into physical contact or collision with each other by performing any one of high-speed stirring, bubbling, ultrasonic vibration and high-pressure jetting for slurry which includes soiled abrasives with hydrated Si adhered to the surfaces of used cerium oxide abrasives, and is stored in a separation/precipitation storage tank; and a second process for sorting the hydrated Si floating after the separation in the first step and regenerated abrasives.

Description

  The present invention efficiently removes Si hydrate adhering to the surface from cerium oxide abrasive grains, which are indispensable materials for high-quality and high-efficiency abrasive grains in polishing of free abrasive grains for glass materials and quartz. The present invention relates to a method for regenerating cerium oxide abrasive that can be separated and recovered and regenerated.

In various fields, for example, manufacture and adjustment of inspection materials used for various materials and various analyzes, methods and apparatuses for processing while applying an electric field to slurry in which abrasive grains are dispersed have been proposed.
In particular, when free abrasive polishing for glass material is performed, unused abrasive grains that do not contribute to polishing, glass processing waste, and glass processing waste are abrasive grains in the polishing slurry after use. Used abrasive grains adhering to.
Patent Document 1 discloses a method of separating polishing scraps using a suction force generated by an electric field that acts more strongly on the outer side in the radial direction while rotating centrifugal force on a used polishing slurry containing polishing scraps and abrasive grains. Proposed.

A cerium oxide-based abrasive (abrasive grain) that is particularly excellent as an abrasive grain for a glass material is known as an indispensable material as a high-grade and highly efficient abrasive grain in polishing.
So far, cerium oxide abrasive grains have been inexpensive, and thus have generally been treated as industrial waste with a reduction in polishing ability.
However, this abrasive grain is a kind of rare earth, and at present, the price is soaring and difficult to obtain due to the global situation.
In view of the above-mentioned social background, it is desired to develop a technique for regenerating used cerium oxide abrasive grains at high speed and an inexpensive apparatus. As one means, attempts have been made to peel glass processing waste (Si hydrate) adhering to the surface of cerium oxide abrasive grains using a cleaning agent or the like.

JP 2012-16796 A

However, even in the latest research (regeneration by chemical cleaning or the like), there is a problem that the regeneration process requires more than two days. On the other hand, regenerators using centrifugal separation are also commercially available, but they are expensive and difficult to introduce into small and medium businesses. In all studies, the polishing rate of regenerated abrasive grains tended to be significantly lower than that of unused ones.
In addition, in the case of regeneration using a cleaning agent or the like, the cleaning agent or the like is dissolved in the polishing liquid, and thus a treatment for removing the cleaning agent itself is required.

Therefore, the present invention has developed a filterless electric field filtering technique as a pretreatment step, and the unused abrasive grains and used abrasive grains contained in the slurry (Si hydrate, which is glass processing waste, is ground). Ideally, an operation to separate and separate the particles around the grain) is introduced.
And as a 1st process of this invention, the abrasive grain (Si hydrate which is glass processing waste adheres to the circumference | surroundings of an abrasive grain) in the used slurry obtained by the said pre-processing is made a physical contact. The object is to recover the regenerated abrasive grains by a treatment operation to desorb Si hydrate by collision.
As the subsequent second step, an operation of separating the Si hydrate floating after desorption in the first step and the recycled abrasive grains is performed. That is, the regenerated abrasive grains are allowed to settle, and the re-adhesion of Si hydrate is suppressed and discharged using the difference in specific gravity with floating Si hydrate. For this reason, sedimentation techniques are important. Here, it aims at developing the waste-liquid processing technique of the solution containing Si hydrate to discharge | emit.

  The present invention has been proposed in view of the above situation, and a slurry containing fouling abrasive grains in which Si hydrate is adhered to the surface of the used cerium oxide abrasive grains is stirred at 90 to 1400 rpm using a stirrer. The first step of contacting or colliding the abrasive grains so that the abrasive grains are rubbed together by high-speed stirring in a range, and the Si hydrate floating after desorption in the first step and the recycled abrasive grains are separated A cerium oxide abrasive grain regenerating method (hereinafter referred to as the first invention).

  In addition, the present invention injects air from an air pipe provided inside a desorption / sedimentation storage tank containing fouling abrasive grains with Si hydrate adhering to the surface of the used cerium oxide abrasive grains, A first step in which abrasive grains are rubbed into contact or collide with each other by bubbling generated, and a second step in which Si hydrate floating after desorption in the first step and regenerated abrasive grains are separated. The present invention also proposes a method for regenerating cerium oxide abrasive grains (hereinafter referred to as the second invention).

  Furthermore, the present invention injects slurry containing fouling abrasive grains with Si hydrate adhering to the surface of the used cerium oxide abrasive grains by a high-pressure pump toward the collision wall provided inside the desorption / sedimentation storage tank. By doing so, the first step of contacting or colliding the abrasive grains so as to be rubbed together, and the second step of separating the Si hydrate and regenerated abrasive grains floating after desorption in the first step And a process for regenerating cerium oxide abrasive grains (hereinafter referred to as third invention).

  In addition, the present invention provides an ultrasonic vibrator inside the desorption / sedimentation storage tank containing the fouling abrasive grains with Si hydrate adhering to the surface of the used cerium oxide abrasive grains, and ultrasonic cavitation is performed. A first step of contacting or colliding the abrasive grains so as to be rubbed together, and a second step of separating the Si hydrate floating from the reclaimed abrasive grains after desorption in the first step. The present invention also proposes a cerium oxide abrasive grain regeneration method (hereinafter referred to as a fourth invention) characterized by comprising:

  Further, in the present invention, as a pretreatment, while a used polishing slurry containing polishing scraps and cerium oxide abrasive grains is housed in a separation cylinder, a centrifugal force is applied in the radial direction by rotating around a shaft. In addition, by applying an electric field in a state where the electrodes are faced up and down in the axial direction and the arrangement interval outside the inside in the radial direction is narrowed, polishing dust and After separating the combined cerium oxide abrasive grains and polishing scraps and recovering the cerium oxide abrasive grains from the recovery port provided outside in the radial direction, any one of the first, second, third, and fourth The present invention also proposes a cerium oxide abrasive grain regeneration method characterized by implementing the regeneration method of the present invention.

  Furthermore, the present invention is the post-treatment, after the regeneration method, as a post-treatment, as a post-treatment, the flocculant is added, the abrasive grains are coagulated and settled, and separated from the supernatant liquid containing Si hydrate, or centrifuged The present invention also proposes a method for regenerating cerium oxide abrasives, characterized in that the abrasive grains and the Si hydrate are separated by collecting the recovered abrasive grains and discarding the supernatant liquid containing the Si hydrate. .

In the cerium oxide abrasive grain regeneration method of the present invention, it adheres to the surface of the cerium oxide abrasive grain by four contact / collision methods: high-speed stirring treatment, bubbling using high-pressure air, collision by high-pressure jetting, and cavitation using an ultrasonic vibrator Since the Si hydrate can be easily and sufficiently removed in a short time, and the apparatus configuration can be simplified by any of the methods, the practical value is extremely high.
And especially by each method of bubbling treatment, high-pressure jetting treatment, and ultrasonic treatment, the recovered abrasive grains obtained recovered to 97% when the unused abrasive grains were used, demonstrating a high regeneration effect. It was confirmed that it was possible.
In addition, when using high-speed agitation that is expected to have the lowest collision physical energy, repeated cleaning is necessary, a large number of aggregated large-diameter abrasive grains are contained, and sufficient desorption results cannot be obtained. Moreover, since the polishing rate of the regenerated abrasive grains was low, the regenerative effect as a single process was low compared, but this agitating process has the advantage that a general-purpose agitating device can be applied and combined with other processes Thus, other processing time can be shortened.

  Also, as a pretreatment, by electric field filtering technology, polishing dust and used cerium oxide abrasive grains and abrasive waste combined are separated, and after collecting the cerium oxide abrasive grains, bubbling using high-pressure air, high-pressure injection By performing the three contact / collision methods of collision and cavitation using an ultrasonic vibrator, Si hydrate adhering to the surface of the used cerium oxide abrasive grains can be desorbed and effectively reused. The effectiveness of the treatment itself is also very large due to the synergistic effect.

  In addition, as post-treatment, by adding a flocculant or performing centrifugation, the regenerated abrasive grains can be recovered and the supernatant liquid containing Si hydrate can be discarded.

It is a schematic diagram which shows the electric field filtering technique used as pre-processing. It is a general view (photocopy) of an electric field filtering part. It is sectional drawing of the separation tank of an electric field filtering part. It is a copy of three types of electrode appearance photographs made as a prototype, and a cross-sectional view. It is a small high voltage low frequency power supply circuit diagram. Small electric field controller (a) Frequency generator, (b) High voltage generator, (c) Device overview. Small high-voltage power supply waveforms (a) 4 kV, 18 Hz, (b) 4 kV, 200 Hz. It is a graph which shows each abrasive grain measurement result by a particle size distribution meter. It is a particle size comparison in the SEM image using the abrasive grain B. It is the result of the slurry discharge | emission amount by each rotation speed at the time of 1 kV application electric field at the time of no electric field. It is analysis evaluation by SEM photograph. It is a graph which shows polishing rate transition. It is a SEM image in each condition. It is an enlarged image of the abrasive grain obtained from the inner side and the outer side when E = 1 kV is applied. It is a Si hydrate desorption processing image figure. This is a simple stirring experimental device. This is a polishing evaluation experimental apparatus. Two types of abrasive grain regeneration procedures. It is the stirring speed comparison by turbidity. It is the stirring time comparison by grinding | polishing evaluation experiment. It is a polishing rate comparison according to the number of cleanings. It is a Si hydrate detached part whole view (photocopy). It is a processing flow figure of Si hydrate desorption part. It is a reproduction of a photograph showing each apparatus that implements a high-speed stirring method, an air bubbling method, a high-pressure pump method, and an ultrasonic method. It is a SEM image of the reproduction | regeneration abrasive grain obtained by each contact collision method. FIG. 5 is a copy diagram showing an electron micrograph and an EDX image (left) SEM image and (right) EDX image (Si plot). EDX analysis result It is a copy figure which shows Si residual amount in reproduction | regeneration slurry. It is the grinding | polishing experiment result (polishing rate) by four types of reproduction | regeneration processes, stirring process, bubbling, high pressure injection, and ultrasonic dispersion | distribution. It is a general view (photocopy) showing a white microscopic interferometer (New View 6300 manufactured by ZYGO). The surface roughness by various regeneration processes is shown. It is a schematic diagram about the separation process of the detached Si hydrate. It is a comparative experimental example of the pH value after adding the flocculant and the abrasive particles for 1 minute to stand. This is an experimental procedure for determining the amount of flocculant added. It is a graph which shows the relationship between pH value and optimal coagulant addition amount. This is a redispersibility experimental procedure. It is a residue comparison photograph. Results of residual substance% comparison experiment It is a flocculant for waste liquid treatment. Experimental procedure. It is an experimental result which shows the BOD value before and behind waste liquid treatment. It is the processing flow shown including the pre-processing and post-processing concerning this invention. It is the processing flow shown including the post-process regarding this invention.

The cerium oxide abrasive grain regeneration method of the present invention has developed a filterless electric field filtering technique as a pretreatment step, and unused abrasive grains and used abrasive grains contained in the slurry (Si, which is glass processing waste). Ideally, an operation for separating and separating hydrates from adhering around the abrasive grains) is introduced. This technique is a method proposed in Patent Document 1 as described above, and is stronger to the outer side in the radial direction to be rotated while applying centrifugal force to the used polishing slurry containing polishing scraps and abrasive grains. A method of separating polishing debris by using an attractive force generated by a working electric field is adopted.
As a target, among the unused abrasive grains remaining in the used slurry, a separation rate of 50% or more / time is increased to achieve separation of 80% or more.

  In the field filtering technique of this pre-processing step, as shown in an example described later, a field applying device dedicated for field filtering (commercially available is a function generator and a high voltage amplifier that generate basic voltage and waveform). It is the next purpose to develop an inexpensive device by developing a structure (which is designed to reduce the size by integrating the functions and limiting the functions).

And as a 1st process of this invention, the abrasive grain (Si hydrate which is glass processing waste adheres to the circumference | surroundings of an abrasive grain) in the used slurry obtained by the said pre-processing is made a physical contact. The object is to recover the regenerated abrasive grains by a treatment operation to desorb Si hydrate by collision (giving physical contact collision energy). By this contact collision, Si hydrate adhering to the abrasive grains is desorbed into the solvent.
As a specific method for giving physical contact collision energy to the abrasive grains around which Si hydrate adheres, as described in detail in the examples described later, high-speed stirring treatment, bubbling treatment, Sonication and high pressure injection were performed.
Further, in the Si hydrate desorption technique of the first step, the next object is to determine an optimum apparatus specification by minimizing the desorption energy used in each contact collision technique.

Establishing a desorption simulation technology for Si hydrates, which combines the quantum molecular dynamics method, molecular dynamics method, and first-principles calculations developed at Kubo Laboratory, Graduate School of Engineering, Tohoku University, and elucidating the desorption mechanism And desorption conditions based on computational science were optimized.
As a result, the optimum conditions for desorbing ideal Si hydrate were proposed as follows.
(A) When the abrasive grains are brought into contact collision, the 0 abrasive grains are caused to collide with each other without colliding from the front.
(A) In order to desorb Si hydrate, increase the rotation speed and the stirring time so as to make a contact collision not only once but multiple times.
(C) Raising the temperature to increase the chemical reactivity of the Ce-O bond between the abrasive grains and the Si hydrate and water molecules.
Thus, from the theoretical standpoint, we succeeded in deriving the optimum conditions for desorbing Si hydrate from the Si hydrate desorption simulation.

Based on the simulation results, experiments were constructed as follows for each of high-speed stirring processing, bubbling processing, ultrasonic processing, and high-pressure injection.
In the high-speed stirring treatment, stirring is performed in the range of 90 to 1400 rpm using a stirrer, and contact with and colliding makes it possible to desorb Si hydrate.
In the air bubbling process, Si hydrate is desorbed by injecting air at a pressure of 0.1 to 2.0 Mpa from an air pipe arranged inside the desorption / sedimentation storage tank. Here, the air nozzle arrangement in the tank was arranged at an angle of 10 ° to 120 °. The air nozzle arrangement was variable in the range of 10 ° to 120 °.
In the high-pressure injection, dehydration of Si hydrate is attempted by injecting slurry at an angle of 10 ° to 90 ° with a high-pressure pump toward a collision wall provided inside the desorption / sedimentation storage tank.
In the ultrasonic treatment, an ultrasonic vibrator is provided inside the desorption / sedimentation storage tank, and Si hydrate is desorbed using cavitation by ultrasonic waves of 25 kW or more and 0.2 kW to 1.5 kW.
In these high-speed stirring treatments, bubbling treatments using high-pressure air, high-pressure jets using high-pressure pumps, and cavitation treatments using ultrasonic waves, all were able to efficiently desorb Si hydrates. It was confirmed that a high Si desorption rate can be realized particularly in a short time by bubbling, high-pressure injection, and ultrasonic treatment.

As the subsequent second step, an operation of separating the Si hydrate floating after desorption in the first step and the recycled abrasive grains is performed. That is, the regenerated abrasive grains are settled and separated from the suspended Si hydrate by utilizing the difference in specific gravity to suppress reattachment of Si hydrate. For this reason, sedimentation techniques are important.
As a subsequent post-treatment technique, the next object is to develop a waste liquid treatment technique for a solution containing discharged Si hydrate.

  In the polishing waste liquid after use after polishing the glass material with cerium oxide abrasive grains, unused abrasive grains and used abrasive grains with glass processing waste adhering to the abrasive grains are mixed. . In order to improve the efficiency of the regeneration process, pretreatment is performed to separate the above-mentioned unused abrasive grains and used abrasive grains using the gradient of centrifugal force, gravity, and electric field strength, and the target remains in the used slurry. Aiming at a separation rate of 50% or more of the unused abrasive grains, the separation was aimed at 80% by increasing the number of stages.

[Development of electric field filtering unit to separate unused abrasive grains]
1) Structure of the electric field filtering unit Electric field filtering technology separates unused abrasive grains from used slurry and used abrasive grains with Si hydrate attached to the abrasive grains using the gradient of centrifugal force, gravity, and electric field strength. Technology. FIG. 1 shows a schematic diagram of this electric field filtering technique.
As shown in the figure, the used abrasive grains having a heavy mass are discharged from the inner discharge port by the action of gravity. Moreover, unused abrasive grains having a light mass are discharged from the outer discharge port by the action of the centrifugal force due to the rotation of the separation tank and the suction force due to the electric field that increases toward the outer periphery due to the inclination provided in the tank. By optimizing these forces, we establish an electric field filtering technique that exhibits a good separation effect.
FIG. 2 shows an overall view of a reproduction experiment apparatus (electric field filtering unit) for which trial development was performed.
This unit consists of a slurry charging unit, a separation tank for performing separation processing, a slip ring system for supplying an electric field to the separation tank, a separation tank rotating motor for rotating the separation tank, an unused abrasive discharge port, and glass processing waste discharge. It consists of two outlets of the outlet, and the separation tank can change the rotation speed in the range of 5.3 to 267 rpm. In addition, since this apparatus uses an electric field, the electrode and the slurry flow path were insulated.
FIG. 3 shows a sectional view of the inside of the separation tank.
The separation tank has a diameter of 141 mm and can be provided with electrodes on the upper and lower sides.

2) Electrode configuration of electric field filtering unit In this electric field filtering, the gradient of the electric field strength greatly affects the separation rate. Since the electric field strength depends on the distance between the upper and lower electrodes, three types of electrodes with different inclination angles are developed and the optimum inclination is obtained.
FIG. 4 shows three types of electrode appearance photographs and cross-sectional views. The electrodes were made of anodized aluminum in consideration of corrosion resistance and insulation.
The tilt angle of the electrode (1) is 10 °, the tilt angle of the electrode (2) is 8 °, and the tilt angle of the electrode (3) is 5 °. Experiments can be performed with different electric field strengths. In addition, each electrode is provided with a discharge hole for discharging glass processing waste of used abrasive grains, and a discharge hole system that increases separation efficiency can be selected.

3) Small high-voltage low-frequency application device a) Circuit integration of signal generator and voltage amplifier When the electric field filtering unit is put into practical use, this device, which is an electric field supply source, can be downsized and greatly reduced in cost. If the possibility is not opened, it becomes a big obstacle. (Traditionally, it costs over 2 million yen only with a high-voltage supply source.) While maintaining functions such as high insulation and good cooling, it is customized with a dedicated CPU circuit equipped with embedded software, and low frequency Miniaturization was achieved by forming the signal generation circuit on the same substrate. Fig. 5 shows the developed high-voltage low-frequency power supply circuit diagram.
b) Establishment of control method for small high voltage low frequency high voltage In this electric field filtering, in order to change the frequency and confirm the effect of filtering, it is developed as a power supply device capable of controlling the output voltage and frequency. However, existing circuit schemes are complicated and difficult to implement. Therefore, by introducing built-in software, generating a low-frequency signal waveform and controlling the optimum applied frequency, the circuit is simplified and the number of components used is reduced to minimize the hardware configuration. We succeeded in reducing the size to 1/13, and led to the development of a lighter power supply. The developed electric field controller is shown in FIG. The obtained waveform is shown in FIG. The applied frequency can be adjusted in the range of 0.5 to 200 Hz with repeated square waves.

[Optimization of electric field filtering processing conditions]
1) Preliminary experiment for electric field filtering 1) -1 experimental sample In order to obtain the basic conditions of the separation vessel rotation speed and applied voltage at first using the experimentally developed electric field filtering section, particle size selection of unused abrasive grains as a preliminary experiment Experiments were performed.
In order to determine the abrasive grains used in this experiment, three kinds of unused abrasive grains of abrasive grains A, abrasive grains B, and abrasive grains C were measured using a particle size distribution meter (SALD-1100 manufactured by Shimadzu Corporation). FIG. 8 shows each measurement result. From this result, it was decided to use “abrasive grain B” having a wide particle size distribution and having a particle size distribution that can be approximated to unused abrasive grains contained in the used slurry assumed by us.

1) -2 Experimental conditions The preliminary separation experimental conditions are shown below.
(1) Abrasive grain / concentration: abrasive grain B / 5 wt%
(2) Separation tank rotation speed: 40, 90, 120, 150 rpm
(3) Applied voltage: (1) 0 kV, (2) 1 kV
(4) Frequency: 20Hz
(5) Waveform: Rectangular wave The laboratory basic experiment was carried out under the applied electric field conditions described above.

1) -3 Preparation for evaluation As an evaluation method, in the implementation plan, the particle size grasp using a white microscopic interferometer is planned for the separation evaluation of glass processing waste (abrasive grains to which Si hydrate adheres). However, in order to perform measurement / measurement with higher definition, evaluation was performed using an electron microscope (SEM). First, in order to evaluate the balance of the amount of slurry discharged from the internal and external discharge ports, the weight of the discharged slurry is measured. Secondly, as a method for confirming the separation effect, the abrasive grains obtained from the respective outlets described in the section 2-1-1 were subjected to particle size comparison by SEM. To observe abrasive grains with SEM, it is necessary to dry the slurry.
As shown in FIG. 9, the abrasive grains are dispersed in pure water and then dried, and when changes such as agglomeration are confirmed, an image similar to that in an unused state is obtained. Therefore, this evaluation method is applicable. It was judged. Using this method, the abrasive grain sizes obtained with and without an electric field are compared.

1) -4 Preliminary Experimental Results FIG. 10 shows the amount of slurry obtained from the discharge port of each rotation. When the rotation speed of the separation tank was 40 rpm when no electric field was applied, the centrifugal force applied to the unused abrasive grains was weak, and the discharge of the slurry could not be confirmed. On the other hand, since the centrifugal force generated at the separation tank rotational speed of 120 or 150 rpm is strong, a lot of slurry is discharged from the glass processing waste (Si hydrate-attached abrasive grains) discharge port, and the slurry from the unused abrasive discharge port. Was only available in small quantities. At a separation tank rotational speed of 90 rpm, the same level of slurry was discharged from the unused abrasive grain outlet and the glass processing waste outlet.
At an applied electric field of 1 kV, at a separation tank rotational speed of 40 rpm, slurry discharge could be confirmed though a small amount from the unused abrasive grain discharge port. This is thought to be due to the attraction force due to the electric field. Moreover, at the separation tank rotation speed of 120 or 150 rpm, the slurry discharge from the unused abrasive grain discharge port is more remarkable than when there is no electric field, and it is considered that the suction force by the electric field is likely to act like 40 rpm. At a separation tank rotational speed of 90 rpm, as in the case of no electric field, almost the same amount of slurry was discharged from the unused abrasive grain outlet and the glass processing waste outlet. From this, it was found that 90 rpm was appropriate as the amount of slurry discharged.

FIG. 11 shows SEM images of the abrasive grains discharged under the above conditions.
When there was no electric field, it was discharged from each outlet at any number of revolutions. A large difference in the respective abrasive grain sizes could not be confirmed, and no separation effect was obtained. On the other hand, when an electric field of 1 kV was applied, the difference in the abrasive grain size could not be confirmed at the separation tank rotation speed of 40 or 120 rpm, but at 90 or 150 rpm, the abrasive grains discharged from the unused abrasive grain discharge port It was confirmed that the large-diameter abrasive grains were contained in the slurry discharged from the glass processing waste discharge port, and a good separation effect was obtained.
From the above results, the same amount of slurry can be obtained from each discharge port, and 90 rpm and 1 kV at which a good separation effect is obtained are set as basic conditions, and an electric field filtering experiment using used slurry is performed.

2) Electric field filtering technology optimization experiment using used abrasive grains 2) -1 Production of used slurry The used slurry used in the electric field filtering experiment was prepared by polishing under the following conditions.
(1) Polishing machine: 4-way double-side polishing machine (2) Processing pressure: 105 gf / cm ²
(3) Rotation speed: 40rpm
(4) Workpiece: BK-7
(5) Number of samples: 9 sheets / time (6) Slurry: Abrasive grain B + pure water (7) Slurry concentration / Abrasive grain amount used: 5 wt% / 1.25 kg
As in the preliminary experiment, abrasive grain B was adopted as the abrasive grain, and the slurry concentration was similarly set to 5 wt%. Polishing was repeated under the above conditions, and a decrease in polishing rate was defined as used. FIG. 12 shows the transition of the polishing rate. Since the polishing time has decreased by about 10% after the polishing time of 2280 minutes, it is determined that the slurry is a used slurry, and a separation experiment is performed using the present abrasive grains.

2) -2 Electric field filtering experimental conditions The state of the used slurry is shown below.
(1) Abrasive grain / concentration: abrasive grain B / 6.8 wt% (specific gravity 1.06)
(2) Usage time: 2280 minutes (38 hours)
(3) Workpiece: BK-7
(4) pH 9.3
The electric field filtering experimental conditions are shown below.
(1) Separation tank rotation speed: 90 rpm
(2) Applied voltage: 0, 0.5, 1.0, 2.0, 3.0 kV
(3) Frequency: 20Hz
(4) Waveform: Rectangular wave The rotation speed of the separation tank was set to 90 rpm obtained in the preliminary experiment, the voltage was changed to 4 levels to obtain the optimum electric field application conditions, and the correlation with the rotation speed of the separation tank was evaluated. The evaluation by the SEM image was adopted for the evaluation of the filtering effect as in the separation preliminary experiment.

2) -3 Electric field filtering experiment results FIG. 13 shows SEM images under various conditions. When the applied voltage was set to 2 or 3 kV, large abrasive grains were discharged from the unused abrasive discharge port. This is considered to be because the suction force by the electric field is strong and the glass processing waste is also drawn to the unused abrasive grain outlet side. Moreover, when there was no electric field, large-diameter glass processing waste (Si hydrate-attached abrasive grains) was discharged from the unused abrasive discharge port as in the preliminary separation experiment, and a filtering effect was not obtained.
Therefore, when the applied voltage was 0.5 and 1.0 kV, small-diameter unused abrasive grains were discharged from the unused abrasive grain discharge port, and large-diameter glass processing waste was discharged from the inner glass processing waste outlet. It was confirmed.
Thus, a good separation effect was obtained by applying a voltage of 0.5, 1.0 kV.
FIG. 14 shows enlarged images of abrasive grains obtained from the inside and outside when E = 1 kV is applied.
There were no abrasive grains having a large particle diameter in the SEM image of the abrasive grains discharged from the outside, and a separation rate of 90% or more was obtained. In addition, the abrasive grains obtained from the inside formed large aggregates compared to when no electric field was applied, and interesting data were obtained. These details will be clarified in future complementary studies.

  In order to introduce a separation technique using an electric field, an electric field filtering unit was developed. In a separation experiment using a used slurry, an unused abrasive grain was removed at a separation cylinder rotation speed of 90 rpm and an applied voltage of 0.5 and 1.0 kV. An excellent separation effect was obtained from glass processing waste. From this, by applying an electric field, agglomeration of abrasive grains was suppressed, and there was a possibility that the subsequent desorption process would be easy. A separation rate of 90% was obtained.

[Development of desorption technology for Si hydrate adhering to glass processing waste]
This apparatus part is an apparatus part which gives physical contact collision energy to the glass processing waste separated by the electric field filtering part which is a pretreatment process, and desorbs and removes Si hydrate. FIG. 15 shows an image diagram of the Si hydrate desorption treatment. By applying physical energy to glass processing waste (used abrasive grains to which Si hydrate is adhered), they are brought into contact with each other. By this contact collision, Si hydrate adhering to the abrasive grains is desorbed into the solvent. Furthermore, the recovery of the abrasive grains is realized by removing the detached Si hydrate.

・ Lab level Si hydrate desorption experiment (1) Si hydrate desorption experiment by high-speed agitation treatment First of all, in the desorption technique by bubbling treatment, ultrasonic treatment and high-speed agitation treatment, the lowest collision physical energy Si hydrate desorption experiments were conducted on the expected high-speed stirring treatment, and evaluated and compared through polishing experiments. In a laboratory level experiment, the sample volume initially planned was changed from 2 L to 500 ml for the purpose of increasing the number of experimental samples in the course of research.

a) Stirring regeneration experiment and polishing experiment apparatus The regenerated abrasive grains were regenerated by a simple stirring experiment apparatus shown in FIG. Table 1 shows the equipment used for the equipment. Further, an outline of the polishing evaluation experimental apparatus is shown in Table 2, and the experimental polishing machine is shown in FIG.

b) Abrasive Grain Reproduction Experiment Procedure The abrasive grain regeneration experiment procedure is shown in FIG. The pH value after the addition of the flocculant in the Si hydrate separation step (1) is obtained by conducting a preliminary experiment. Details will be described later in “Establishment of Rapid Grain Settling Technology”.
c) Experimental Results FIG. 19 shows the results of using the turbidity as an index for the comparative experiment for the stirring speed. Here, turbidity is a numerical value of the degree of turbidity of water, and the smaller the measured value, the lower the degree of turbidity. From this result, it became clear that the rotational speed of 200 rpm or more is good as the desorption treatment. However, in the course of the experiment, the abrasive on the bottom surface of the beaker may not be sufficiently stirred at 200 rpm, so the optimum stirring speed was set to 300 rpm or more. Further, FIG. 20 shows the result of the polishing experiment depending on the stirring time. Here, when the polishing rate of the virgin slurry (unused abrasive grains) was set to 100, it was found that the polishing rate was recovered most in the stirring time of 2 minutes.

(2) Si hydrate desorption experiment by multi-stage treatment If the desorption step (high-speed stirring treatment) in (1) is called washing, the removal rate of Si hydrate is improved by repeated washing. It is thought to do. Therefore, the relationship between the number of cleanings and the polishing rate is investigated.
a) Regeneration conditions TH-1300 (manufactured in China) was used as abrasive grains, and virgin slurry, used slurry, and regenerated slurry with 1 to 3 washings were subjected to experiments under the same conditions as in Table 2 and compared. The procedure for regenerating abrasive grains was the same as in the previous section, with a stirring time of 2 minutes, a stirring speed of 300 rpm, a pH value of 6.2 after adding the flocculant, and a standing time of 5 minutes.
b) Experimental Results FIG. 21 shows the polishing experimental results after multistage cleaning. It was confirmed that the polishing rate was improved by discharging the Si hydrate by repeating the cleaning, and approaching the virgin slurry. The knowledge that Si hydrate can be desorbed and discharged by repeated washing was obtained. In addition, since the desorption method using other collision energy supplies the abrasive grains with higher energy than the present plan, the evaluation in the laboratory experiment is shifted to the evaluation at the actual machine level.
c) Summary In the desorption method using bubbling, ultrasonic treatment, high-pressure jetting, and high-speed agitation, high-speed agitation, which is expected to have the lowest collision physical energy, is used as an optimum abrasive grain regeneration condition at the laboratory level. It was found that 2 minutes, the stirring speed was 300 rpm or more and the number of washings was 3 times or more.

-Configuration of Si Hydrate Desorbed Part FIG. 22 shows an overall view of the Si hydrate desorbed part that was experimentally developed. This device is mainly (1) a processing waste storage tank for storing used abrasive grains obtained from the electric field filtering unit, that is, a slurry containing processing waste, (2) Si hydrate desorption treatment, and subsequent steps. It consists of a desorption / sedimentation storage tank that performs sedimentation of abrasive grains, (3) a regenerated abrasive storage tank that stocks the obtained regenerated abrasive grains, and (4) a waste liquid storage tank that stocks the desorbed Si hydrate. .
Each storage tank bottom is provided with an inclination of 30 ° to 80 ° in order to suppress residual abrasive grains in the tank, and the tank material is made of stainless steel in consideration of corrosion resistance.

FIG. 23 shows a flow diagram of the Si hydrate desorbing part.
Glass processing waste is transferred from the processing waste storage tank to a desorption / sedimentation storage tank by a motor pump, and Si desorption processing is performed by each contact collision method installed. Thereafter, using a pH-linked type chemical injection pump provided in the tank, a precipitating agent is charged by a pH control method, and the abrasive grains are rapidly settled.
As a method for separating the precipitated abrasive and the supernatant liquid containing Si hydrate, a photoelectric sensor is provided in the pipe, and the motorized valve is switched by sensing the difference in transmittance. The supernatant liquid containing Si hydrate is sent to the waste liquid storage tank.
Furthermore, the regenerated abrasive grain storage tank is connected to a motor pump, and can be sent again to the desorption / sedimentation storage tank for multistage processing.
As with the desorption / sedimentation storage tank, the waste liquid storage tank is equipped with a pH-linked chemical injection pump, enabling optimization of waste liquid treatment technology.
The pipes connecting the tanks and the water supply pipes are equipped with an electric valve and a water supply electromagnetic valve so that the desorption treatment process can be automatically operated.

-Each desorption processing method of Si hydrate desorption part FIG. 24 shows each apparatus which implements the high-speed stirring method, the air bubbling method, the high-pressure pump method, and the ultrasonic method provided in the desorption treatment sedimentation tank.
In the high-speed stirring method, stirring is performed in the range of 90 to 1400 rpm using a stirrer to realize contact collision.
In the air bubbling method, air is injected at a pressure of 0.5 Mpa from an air pipe arranged inside the desorption / sedimentation storage tank. Here, based on the simulation results of Tohoku University that it is effective to contact the abrasive grains so that they are rubbed together, the air nozzles in the tank are arranged so that they face each other.
In the high-pressure pump system, the slurry was ejected at high pressure toward the collision wall provided inside the desorption / sedimentation storage tank to desorb Si.
In the ultrasonic method, an ultrasonic transducer was installed inside the desorption / sedimentation storage tank, and desorption was attempted using ultrasonic cavitation.
In this study, it is necessary to establish a method with high desorption effect and low cost.

[Optimization of Si hydrate desorption technology at a practical level]
1) Desorption experiment conditions in the Si hydrate desorption part In order to determine the optimal contact collision method using the developed Si hydrate desorption part, the above-mentioned desorption experiment results at the laboratory level and the above-mentioned Si Based on the simulation results obtained from the elucidation of the mechanism of hydrate desorption, desorption treatment experiments were conducted.
In the desorption treatment experiment, used abrasive grains obtained in the electric field filtering technique optimization experiment, that is, a slurry containing glass processing waste was used. The state of the used slurry used in the desorption experiment is shown below.
(1) Abrasive grain: Abrasive grain B
(2) Concentration: about 6.8 wt% (specific gravity 1.060)
(3) Amount of slurry: 1.192L
(4) Usage time: 2280 minutes (38 hours)
(5) pH: 9.3

Moreover, Si hydrate desorption treatment experimental conditions are shown below.
(1) Contact collision method:
-Example 1: High-speed stirring (1200 rpm)
-Example 2; Air bubbling (0.5 Mpa)
The air nozzle arrangement was variable in the range of 10 ° to 120 °.
Example 3 Collision by slurry injection using a high-pressure pump Injection was performed at an angle of 10 ° to 90 ° toward the collision wall.
-Example 4: Cavitation by an ultrasonic vibrator It implemented at 25 kW or more and 0.2 kW-1.5 kW.
(2) Desorption treatment time: 2 min (set from the desorption experiment results at the laboratory level described above)
(3) Precipitating agent: flocculant A
(4) Number of treatment stages: 1 stage The desorption treatment time and the precipitating agent conditions were set from the desorption experiment at the laboratory level described above.
Evaluation of the obtained reclaimed abrasive grains employed abrasive grain shape evaluation by SEM images and polishing evaluation using regenerated abrasive grains, as in the electric field filtering experiment.

2) Desorption experiment result in Si hydrate desorbing portion FIG. 25 shows SEM images of regenerated abrasive grains obtained by each contact collision method.
Example 1 It was confirmed that the regenerated abrasive grains obtained by using high-speed stirring contained a large number of aggregated large-diameter abrasive grains. On the other hand, Example 2; Air bubbling, Example 4 Cavitation by ultrasonic vibration, Example 3 Regenerated abrasive grains obtained by injection collision with a high-pressure pump contain many small-diameter abrasive grains. It was confirmed.
The remaining amount of Si was initially planned to be evaluated by ICP MS (Induction Plasma Mass Spectrometer), but there was a concern that information would be closed by a filter in the apparatus, and sufficient evaluation could not be performed. Therefore, evaluation was performed by energy dispersive X-ray analysis (EDX). An example of the measurement is shown in FIG. 26, and the remaining amount of Si is shown in FIG. In any of the methods, Si resulting from the main component SiO ₂ of glass remained. These results suggest that good Si desorption may have been performed in the contact collision method except high-speed agitation, but there is still room for study in the sedimentation / separation process.

Further, a polishing evaluation experiment was conducted using the above-mentioned recycled abrasive grains under the conditions shown in Table 3.

FIG. 28 shows the results of the polishing experiment.
When the polishing rate of unused abrasive grains was 100%, the polishing rate of used abrasive grains decreased to 65%. Thus, in Example 1; a polishing experiment using regenerated abrasive grains obtained by high-speed stirring, the polishing rate was recovered to 77%. Therefore, a clear abrasive grain regeneration effect was confirmed.
Further, Example 2: Bubbling using high-pressure air, Example 3; Collision by high-pressure injection, Example 4; Polishing experiment using regenerated abrasive grains obtained by three contact collision methods of cavitation using an ultrasonic vibrator The polishing rate recovered to 97% when the unused abrasive grains were used, and a high regeneration effect could be demonstrated.
At this time, the rate of decrease of the polishing rate provided by the used abrasive grains was 35%, while the percentage of the recovery rate of the polishing rate by regeneration was defined as the Si desorption rate, and 91% was achieved.
In addition, among the above three methods that obtained a high regeneration effect, it was found that the most inexpensive, Example 2; bubbling method was the most suitable desorption method considering the polishing rate.

〔reaserch result〕
Based on the results of desorption experiments at the laboratory level, we developed a prototype of the Si hydrate desorption site. Moreover, based on the simulation result of the desorption mechanism of Si hydrate, desorption treatment experiments were performed using each contact collision method of the Si hydrate desorption part.
From the experimental results, Example 1; high-speed stirring method, Example 2; bubbling method using high-pressure air, Example 3; slurry high-pressure injection method using a high-pressure pump, Example 4; cavitation method using ultrasonic waves In all methods, the effect of regenerating abrasive grains was confirmed. Moreover, in Examples 2 to 4, a Si desorption rate of 91% was achieved, the target desorption rate of 30% or more was achieved, and a high regeneration effect was obtained.

[Implementation of polishing evaluation experiment using recycled abrasive grains]
1) Recycled abrasive polishing experiment by Si hydrate desorption experiment As described above, with respect to the abrasive grains regenerated by the developed abrasive regenerator, Example 1; stirring process, Example 2; bubbling, implementation Example 3: High pressure injection, Example 4: Polishing rates by four types of regeneration processes of ultrasonic dispersion were obtained as shown in FIG.

2) Surface Roughness by Recycled Abrasive Polishing Experiment Using a white microscopic interferometer (New View 6300 manufactured by ZYGO) shown in FIG. 29, the polished surface quality of the regenerated slurry was compared with unused slurry and used slurry.
The result is shown in FIG.
The polishing quality of the unused slurry was 0.741 nm in terms of Ra value, but the 38 hour spent slurry was deteriorated to 0.840 nm and the number of scratches was increased.
On the other hand, it was found that in Example 1; stirring process, the roughness was 0.839 nm and did not recover and was insufficient. Further, in Example 2 in which the polishing rate was recovered; in the bubbling process, Ra was 0.744 nm, which was improved to the level of the unused slurry, but fine scratches could be confirmed. Example 3 It was found that in the high pressure injection process, Ra was also improved and generation of scratches was suppressed. Example 2: Although the same level of roughness as that of an unused slurry was obtained even in the bubbling process, Example 2; high pressure injection is considered necessary for sufficient regeneration including scratches.

・ Research results Regarding the abrasive grains regenerated by the developed abrasive regenerator, Example 1; Stirring process, Example 2; Bubbling, Example 3; High-pressure injection, Example 4; Evaluation was carried out through polishing experiments. As a result, when the regeneration process was compared by evaluation based on the polishing rate and surface roughness, Example 1; the stirring process was insufficient in both rate and roughness, and Example 1 was regenerated by physical collision energy higher than the bubbling process. I found it necessary.

[Establishment of rapid abrasive sedimentation technology]
In order to efficiently discharge and remove Si contained in the slurry liquid after the desorption treatment, it is necessary to quickly settle the abrasive grains. In the basic experiment, it took 1 hour to settle, but we aim to optimize the sedimentation technique and set the sedimentation time within 30 minutes. As a sedimentation technique, the flocculant is administered while stirring the slurry. Reattachment is prevented by quickly precipitating abrasive grains having a large specific gravity and quickly discharging Si hydrate floating after desorption into the solution. A flocculant that has good redispersibility and does not adversely affect the polishing characteristics is selected, and a sedimentation technique optimization experiment is performed.

・ Investigation of Lab Level Flocculant Addition Amount and Sedimentation Characteristics 1) Outline of Separation Process by Addition of Flocculant FIG. 31 shows a schematic diagram of the separation process of desorbed Si hydrate. For rapid sedimentation of the abrasive grains, it is necessary to add a flocculant that can efficiently separate the detached Si hydrate. It is considered that the abrasive grains have a pH value that easily aggregates, that is, an equipotential point, and if the flocculant is excessively added or insufficient, rapid settling is hindered. Therefore, it is considered that the abrasive grains after the addition of the flocculant has a faster sedimentation rate as the amount of abrasive grains contained in the supernatant liquid after the slurry is allowed to stand is smaller, that is, as the turbidity of the supernatant liquid becomes smaller. An experimental example is shown in FIG. Experiments were conducted using a flocculant, and the amount added and sedimentation characteristics were investigated.
2) Experimental conditions Using the stirring experimental apparatus of FIG. 16, the abrasive grains were subjected to the experiment under the conditions of MIREK E-40, the stirring conditions and the stationary time of the abrasive grains, and compared with the turbidity of the supernatant. . The procedure is shown in FIG.

3) Experimental results and summary The experiment was conducted on 7 types of used slurries. Paying attention to the pH value at which the turbidity of the supernatant liquid became the minimum value for each slurry and the addition amount of the flocculant at that time, the optimum addition amount and the optimum pH value were set, respectively. The results are shown in Table 5. From this result, the optimum pH value at which all the used slurries are set to the optimum sedimentation rate was around 6.0. Further, FIG. 34 shows the relationship between the pH value and the optimum flocculant addition amount.

-Examination of flocculant addition amount and redispersibility at lab level 1) Outline of experiment When used scraps increase in processing waste in the liquid by repeated polishing, precipitates with high viscosity may be generated. As a result, the abrasive grains do not spread sufficiently in the liquid, which may lead to a decrease in the polishing rate. Therefore, an experiment was conducted to compare the redispersibility of used abrasive grains and recycled abrasive grains.
2) Experimental procedure and results An experiment was conducted using 200 g each of the used slurry and regenerated slurry of MIREK E-21. The experimental procedure is shown in FIG. The experimental photograph is shown in FIG. 36, and the experimental result is shown in FIG. It is considered that the redispersibility of the abrasive grains regenerated at the optimum pH value is very good. The application experiment on redispersibility at the actual machine level will continue to be pursued as a complementary study.

-Actual machine level (Evaluation at slurry liquid volume 20L The results of application experiments at the actual machine level are in the process of the abrasive grain regeneration procedure of FIG. 18 in [Optimization of Si hydrate desorption technology at practical level]. Thus, sedimentation in 5 minutes was achieved, and the target within 30 minutes was achieved.

・ Research results In order to efficiently discharge and remove Si contained in the slurry liquid after the desorption treatment, we optimized the sedimentation technique to quickly settle the abrasive grains by administering a flocculant. As a result, the optimum flocculant addition amount depended on the pH value of the used slurry, and an exponential curve was drawn. In addition, the optimum pH value was found to be around pH 6.0. As a result of an application experiment at the actual machine level, sedimentation in 6 minutes was achieved, and the target was within 30 minutes.

-Establishment of recycled slurry adjustment technology and waste liquid treatment technology (a) Purpose and target In order to improve the polishing rate of the obtained recycled abrasive grains, it is necessary to adjust the pH and the individual concentration of abrasive grains. Therefore, the addition amount of the pH adjusting agent is optimized and the pH is within ± 0.5. The regeneration process developed in this study includes chemical treatment. Accordingly, in the treatment of waste liquid generated by regeneration, it is essential to establish a treatment method that complies with relevant laws and regulations. The BOD emission standards are 160 mg / L for the country and 30 mg / L for the Akita Prefecture Ordinance, and the processing technology that makes it possible to achieve this standard value will be established. Specifically, a flocculant is added to the waste liquid, and the waste is separated from water. Sato Giken Co., Ltd. optimizes the amount of addition by coagulation experiment and evaluates BOD measurement.

・ Establishment of regenerative slurry adjustment technology that contributes to improvement of polishing rate In the regeneration process of used slurries, abrasive grains are allowed to settle at around pH 6.0, and fresh water is added after discharging the supernatant liquid. Accordingly, it was found that the pH value of the regenerated slurry was around pH 6.0 to 7.0, which was equivalent to that of the virgin slurry. From this, it is thought that a pH adjuster is unnecessary.

・ Examination and proposal of disposal method of Si hydrate in waste liquid considering environment 1) Outline of experiment For the treatment of waste liquid generated in the regeneration process, the flocculant shown in Fig. 38 is used to separate water and waste. . An experiment is performed on the waste liquid before treatment and water after treatment, and BOD values are compared. For the waste liquid treatment, the flocculant shown in FIG. 19 and caustic soda for adjusting the pH value were used. The experimental procedure is shown in FIG.
2) Experimental results and summary The experimental results are shown in FIG. The BOD value after waste liquid treatment decreased from 34.1 mg / L before treatment to 3.5 mg / L, and achieved the business target of 30 mg / L or less in the Akita Prefecture Ordinance.

・ Research results In the used slurry regeneration process, the abrasive grains are allowed to settle at around pH 6.0, and fresh water is added after the supernatant is discharged. Therefore, it was found that the pH value of the regenerated slurry was around pH 6.0 to 7.0, which was equivalent to the virgin slurry. From this, it is thought that a pH adjuster is unnecessary. In addition, as a result of examining the waste liquid treatment process, the BOD value decreased from 34.1 mg / L before treatment to 3.5 mg / L, and achieved the business target of 30 mg / L or less in the Akita Prefecture Ordinance.

  The processing flow shown in FIG. 41 includes pre-processing and post-processing related to the present invention, and the processing flow shown in FIG. 42 includes post-processing related to the present invention.

Claims (6)

  The abrasive grains are rubbed together by stirring the slurry containing contaminated abrasive grains with Si hydrate adhering to the surface of the used cerium oxide abrasive grains at a high speed in the range of 90 to 1400 rpm using a stirrer. And a second step of separating Si hydrate floating after desorption in the first step and reclaimed abrasive grains. Cerium abrasive grain regeneration method.   The abrasive grains are rubbed together by bubbling that injects air from the air piping provided inside the desorption / sedimentation storage tank containing fouling abrasive grains with Si hydrate adhering to the surface of the used cerium oxide abrasive grains. And a second step of separating the Si hydrate floating after the desorption in the first step and the recycled abrasive grains. Cerium oxide abrasive grain regeneration method.   By injecting a slurry containing fouling abrasive grains with Si hydrate adhering to the surface of the used cerium oxide abrasive grains onto a collision wall provided inside the desorption / sedimentation storage tank by a high pressure pump, the abrasive grains A first step of contacting or colliding with each other so that they are rubbed together, and a second step of separating Si hydrate floating after desorption in the first step and recycled abrasive grains. A method for regenerating cerium oxide abrasive grains.   An ultrasonic vibrator is installed inside the desorption / sedimentation storage tank containing the fouling abrasive grains with Si hydrate adhering to the surface of the used cerium oxide abrasive grains, and the abrasive grains are separated from each other using ultrasonic cavitation. It comprises a first step of contacting or colliding so as to be rubbed together, and a second step of separating Si hydrate and regenerated abrasive particles that float after desorption in the first step. A method for regenerating cerium oxide abrasive grains.   As pretreatment, in a state in which used polishing slurry containing polishing scraps and cerium oxide abrasive grains is housed in a separation cylinder, while rotating around the shaft and applying centrifugal force in the radial direction, it is moved up and down in the axial direction. By applying an electric field with the electrode facing and narrowing the arrangement interval outside the inside in the radial direction, polishing dust and used cerium oxide abrasive grains are discharged from the discharge port provided on the inside in the radial direction. And the scraps are separated from each other, and the cerium oxide abrasive grains are recovered from a recovery port provided on the outer side in the radial direction, and then the regeneration method according to any one of claims 1 to 4 is performed. A method for regenerating cerium oxide abrasive grains.   After carrying out the regeneration method according to any one of claims 1 to 5, as a post-treatment, the flocculant is added to aggregate and settle the abrasive grains, or separated from the supernatant liquid containing Si hydrate, Alternatively, a method for regenerating cerium oxide abrasive grains, comprising separating abrasive grains and Si hydrate by centrifugation, collecting the regenerated abrasive grains, and discarding the supernatant liquid containing Si hydrate.