JP2007246367A - Method for recovering silicon-containing material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for recovering a silicon-containing material from which a coolant is removed. <P>SOLUTION: In the method for recovering a silicon-containing material, a water-soluble coolant is preliminarily removed from a used slurry in manufacturing processes of silicon wafers, the slurry containing at least a water-soluble coolant, abrasive grains and silicon grains to obtain a solid portion, the water-soluble coolant remaining in the solid portion is extracted from the solid portion by using a low boiling point organic solvent having compatibility with the water-soluble coolant and a boiling point lower than that of the coolant, the low boiling point organic solvent used for the extraction is removed by centrifugal separation, and a solid portion obtained by the centrifugal separation is recovered. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention uses a multi-wire saw (hereinafter referred to as “MWS”) and a wrapping apparatus, and used slurry used for producing silicon wafers for polycrystalline silicon for solar cells or semiconductor materials, for example. In this process, the present invention relates to a method for recovering a silicon-containing material from a used slurry.

  As a method of slicing a silicon ingot into a wafer (thin plate), there is a method using a wire saw. In that case, a method of supplying a mixture of abrasive grains called a slurry and coolant to a cutting site is common. . Slurries are generally used repeatedly, but silicon chips and the like are mixed in each slicing process, and the cutting performance of the wire saw is gradually lowered.

The used slurry contains silicon chips, but these silicon scraps have been disposed of without being used so far, or, as shown in Patent Document 1, using HF or inorganic acid, and filtered. It was recovered after being subjected to many treatments such as drying and drying.
JP 2001-278612 A

However, the conventional technology for recovering silicon contained in the used slurry requires a large amount of equipment and a lot of man-hours. In particular, when mineral oil is used in the slurry, petroleum-based organic solvents such as kerosene and light oil, and inorganic acids such as HF are required, which is very expensive for safety equipment and environmental pollution prevention treatment. Was needed. In addition, since a filtration device is required, the cost of the filtration filter is also a factor in increasing the cost.
For this reason, there has been a demand for an effective treatment method for silicon scrap in the used slurry. However, when a used slurry is used as a silicon-containing material, the coolant that is an organic substance contained in the slurry may cause various problems. Problems include, for example, generation of organic harmful substances and contamination due to adhesion of organic substances in the refining furnace. Therefore, in order to use the used slurry as a silicon-containing material, it is important to reliably remove the coolant.
This invention is made | formed in view of the situation which concerns, and provides the method of collect | recovering the silicon-containing material from which the coolant was removed.

Means for Solving the Problems and Effects of the Invention

  The method for recovering a silicon-containing material according to the present invention obtains a solid content by previously removing a water-soluble coolant from a used slurry in a silicon wafer manufacturing process containing at least a water-soluble coolant, abrasive grains, and silicon grains. The water-soluble coolant remaining in the solid is extracted from the solid using a low-boiling organic solvent that is compatible with the water-soluble coolant and has a lower boiling point than the water-soluble coolant. Boiling point organic solvent is removed by centrifugation, and solid content obtained by centrifugation is recovered.

In the present invention, a low-boiling organic solvent that is relatively easy to evaporate is used to extract the water-soluble coolant remaining in the solid content, and the low-boiling organic solvent used for the extraction is removed by centrifugation to The silicon-containing material from which the coolant has been removed is recovered. The solid content obtained by centrifugation contains a low boiling point organic solvent, but the low boiling point organic solvent has a relatively low boiling point and can be easily removed.
The recovered silicon-containing material contains a large amount of silicon and can be used in various ways, such as a raw material for producing hydrogen gas and sodium silicate, or a raw material for producing polysilicon. Furthermore, it can be used as a raw material for producing halosilane by the method disclosed in JP-A-2005-330149.
Further, since the water-soluble coolant is removed from this material, problems such as generation of organic harmful substances and contamination due to adhesion of organic substances in the refining furnace can be avoided.

  A method for recovering a silicon-containing material according to an embodiment of the present invention is a method for removing a water-soluble coolant from a used slurry in a silicon wafer manufacturing process, which contains at least a water-soluble coolant, abrasive grains, and silicon grains. And extract the water-soluble coolant remaining in the solid content from the solid content using a low-boiling organic solvent that is compatible with the water-soluble coolant and has a lower boiling point than the water-soluble coolant. The low-boiling organic solvent used in the above is removed by centrifugation, and the solid content obtained by centrifugation is recovered.

Hereinafter, each process is explained in full detail.
1. Solid content acquisition process First, solid content is obtained by previously removing a water-soluble coolant from the used slurry in the manufacturing process of a silicon wafer which contains water-soluble coolant, an abrasive grain, and a silicon grain at least.

The used slurry is a slurry in which silicon particles are mixed.
The slurry consists of abrasive grains and a water-soluble coolant that disperses them. The type of abrasive grains is not limited, but is made of, for example, SiC, diamond, CBN, alumina, or the like. The type of the water-soluble coolant is not limited, but includes, for example, a water-soluble organic solvent such as ethylene glycol, propylene glycol, or polyethylene glycol. The water-soluble coolant may contain about 5% to 15% water. In this case, it can be avoided that this coolant becomes a dangerous material under the Fire Service Law. Further, a coolant (bentonite) or the like (approximately several percent) for dispersing abrasive grains or Si chips is usually added to the coolant. In this specification, “%” means “% by weight”.

  Examples of steps included in the silicon wafer manufacturing process include a silicon ingot slicing step and a silicon wafer lapping step. The silicon grains are, for example, silicon chips generated when a silicon wafer is formed by slicing a silicon ingot, or polishing chips generated when lapping the silicon wafer. The silicon concentration in the used slurry is, for example, 10% or more.

  The solid content is obtained by previously removing the water-soluble coolant from the used slurry. Although the removal method of a water-soluble coolant is not limited, For example, it is distillation. The distillation method is not limited as long as the water-soluble coolant is evaporated from the used slurry to obtain a solid content. Distillation may be any of atmospheric distillation, vacuum distillation, and vacuum distillation, but vacuum (10 Torr or less) distillation is preferred from the viewpoint of energy saving and safety. The solid content is usually in powder form. The evaporated water-soluble coolant is preferably recovered and used for slurry regeneration.

  The water-soluble coolant may be removed by centrifugation or filtration in addition to distillation. (1) A combination of centrifugation and distillation, or (2) a combination of filtration and distillation. The solid content obtained by distillation or centrifugation may be distilled, or the liquid content may be distilled. Only a part of the solid or liquid obtained by distillation or centrifugation may be distilled. Centrifugation or filtration may be primary or multiple.

More specifically, the water-soluble coolant can be removed by the following method, for example.
1-1. First Method The first method of removing water-soluble coolant is to remove the water-soluble coolant from the used slurry in advance by first centrifuging the used slurry to obtain a solid containing abrasive grains as a main component. The liquid fraction obtained by primary centrifugation is recovered by secondary centrifugation, so that a part of the liquid component whose main component is water-soluble coolant is recovered and the remaining liquid content obtained by secondary centrifugation is recovered. And a step of previously removing the water-soluble coolant by distillation from a sample comprising at least one of sludge.

Hereinafter, the steps included in the first method will be described in detail.
(1) Primary Centrifugation Step In this step, the spent slurry is primarily centrifuged to recover the solids whose main component is abrasive grains.

  The primary centrifugation is preferably performed at 100 to 1000G. The used slurry is separated into a first solid content and a first liquid content by primary centrifugation. The first solid content is mainly composed of abrasive grains. Abrasive grains generally have a specific gravity greater than that of silicon grains, and therefore settle faster than silicon grains. For this reason, the abrasive grains are selectively settled when performing low-speed centrifugation. Since many abrasive grains are contained in the first solid content, the first solid content can be used to regenerate the slurry. On the other hand, the first liquid mainly contains water-soluble coolant and silicon particles.

(2) Secondary Centrifugation Step In this step, the liquid component obtained by the primary centrifugation is subjected to secondary centrifugation to recover a portion of the liquid component whose main component is a water-soluble coolant.
The secondary centrifugation is preferably performed at 2000 to 5000G. When such high-speed centrifugation is performed, solid components that have not settled in the primary centrifugation also settle. Therefore, by the secondary centrifugation, the first liquid is separated into a liquid mainly composed of water-soluble coolant (second liquid) and sludge that is a solid. The sludge contains silicon grains and abrasive grains that have not settled in the primary centrifugation. The second liquid component also contains abrasive grains and silicon grains. The second liquid component is usually used for slurry regeneration. However, if the entire amount is used as it is for slurry regeneration, the silicon mass ratio of the regenerated slurry becomes too large, which is not preferable. Therefore, in this method, only a part of the second liquid is collected. The recovered liquid can be used for slurry regeneration.

(3) Distillation step In this step, a sample consisting of at least one of the remainder of the liquid obtained by secondary centrifugation and sludge is distilled. The sample to be distilled may be, for example, (1) only the remainder of the liquid, (2) only the sludge, or (3) a mixture of the remainder of the liquid and sludge. Only a portion of the sludge may be included in the sample. The sludge may be dried before distillation. The method for drying is not particularly limited.

1-2. Second Method The second method of removing the water-soluble coolant is to remove the water-soluble coolant from the used slurry in advance by first centrifuging the used slurry to obtain a solid containing abrasive grains as a main component. This is a method including a step of recovering a water-soluble coolant in advance by distillation from a sample comprising at least a part of a liquid obtained by primary centrifugation.

  Hereinafter, steps included in the second method will be described. The primary centrifugation method and the distillation method are the same as the first method. In the second method, the sample to be distilled is different from the first method. In the second method, a sample consisting of at least a part of the liquid obtained by primary centrifugation is distilled to obtain a solid content. The sample to be distilled may be, for example, any one of (1) a part of the liquid and (2) the whole of the liquid. When a part of the liquid is distilled, the remainder of the liquid may be subjected to secondary centrifugation, and the resulting sludge (total amount or part) may be included in the sample to be distilled.

2. Extraction Step Next, the water-soluble coolant remaining in the solid content is extracted from the solid content using a low-boiling organic solvent that is compatible with the water-soluble coolant and has a boiling point lower than that of the water-soluble coolant. “Extraction” means that the water-soluble coolant remaining in the solid content is dissolved in a low-boiling organic solvent.

The extraction of the water-soluble coolant can be performed, for example, by adding the low boiling point organic solvent to the solid content and stirring both.
The kind of the low-boiling organic solvent is not limited as long as it is compatible with the water-soluble coolant and has a boiling point lower than that of the water-soluble coolant. For example, the number of carbon atoms is 1 to 6 (preferably 1 , 2, 3, 4, 5 and 6). Specific examples of such alcohol include methanol, ethanol, isopropyl alcohol, and butyl alcohol. Specific examples of such ketones include acetone and methyl ethyl ketone. The low boiling point organic solvent may be a mixture of a plurality of types of organic solvents. From another viewpoint, the low-boiling organic solvent preferably has a boiling point lower by 50 ° C. or more (preferably 60 ° C., 70 ° C., 80 ° C., 90 ° C. or 100 ° C. or more) than the water-soluble coolant. In this case, when the low-boiling organic solvent is recovered by distilling the liquid obtained by centrifugation, the advantage that it is easy to evaporate the low-boiling organic solvent remaining in the solid content obtained by centrifugation described below. , There is an advantage that the purity of the low boiling point organic solvent obtained by distillation is easily increased.

  In order to efficiently dissolve the water-soluble coolant remaining in the solid content in the low-boiling organic solvent, a step of pulverizing the solid content before extraction or at the time of extraction may be further provided.

3. Centrifugation step Next, the low-boiling organic solvent used for extraction is removed by centrifugation. The centrifugation method and the apparatus used therefor are not limited. Centrifugation is preferably performed at 3000 G or more (preferably 4000 G, 5000 G, 6000 G, 7000 G, or 8000 G or more).

  The water-soluble coolant remaining in the solid content is extracted from the solid content obtained by centrifugation using the same or different kind of low-boiling organic solvent as the low-boiling organic solvent, and the low-boiling organic solvent used for the extraction is centrifuged. You may further provide the process of removing by solidification and collect | recovering solid content obtained by centrifugation once or more. In other words, the extraction and centrifugation steps may be repeated. Although there is no upper limit on the number of repetitions, it is considered that the water-soluble coolant remaining in the solid content is almost completely removed if extraction and centrifugation are performed 10 times. Multiple extractions and centrifugations may be performed under different conditions, or may be performed under the same conditions.

  It is preferable to further comprise a step of recovering the low boiling point organic solvent by distilling the liquid obtained by centrifugation. The recovered low-boiling organic solvent can be used again for the extraction of residual water-soluble coolant.

4). Next, it is preferable to provide a step of evaporating the low-boiling organic solvent remaining in the solid content and drying the solid content.
Low boiling organic solvents are relatively easy to evaporate because of their relatively low boiling points. The method for drying the solid content is not limited as long as it is a method capable of evaporating the low boiling point organic solvent in the solid content. Drying may be natural drying or drying by heating or reduced pressure.

  Various features shown in the above embodiments can be combined with each other. When a plurality of features are included in one embodiment, one or a plurality of features can be appropriately extracted and used in the present invention alone or in combination.

A first embodiment of the present invention will be described with reference to FIG. FIG. 1 shows a silicon-containing material recovery system of this embodiment.
In this example, silicon for solar cells was selected as an object for slicing using slurry. The MWS for solar cells used in this example focuses on the production capacity, and in one process, four silicon ingots (125W × 125D × 400L) are processed at one time to produce a wafer (125W × 125D × 0.3L) can be processed about 3200 sheets.

  A slurry tank having a size of about 200 L was used at the time of processing, and abrasive grains (specific gravity: 3.21) and water-soluble coolant (specific gravity: 1) were mixed at a mass ratio of 1: 1. A water-soluble coolant having a composition of 80% propylene glycol, 15% water, and 5% dispersant was used. A solid mainly composed of about 20 kg of silicon chips is mixed into the slurry by one processing.

1. Primary and secondary centrifugation steps First, 500 kg (specific gravity: 1.72, 290 L) of the used slurry 1 is separated into liquid 3a and solid content by decanter-type centrifugation (referred to as primary centrifugation, 500G). Separated into 3b.
Next, the entire amount of the liquid component 3a was further separated into the liquid component 5a and the sludge 5b by decanter type centrifugation (referred to as secondary centrifugation, 3500G).
The total amount of the solid content (abundant of abrasive grains) 3b generated by the primary centrifugation is recovered as regenerated abrasive grains 4, and a part (30%) of the liquid content 5a after the secondary centrifugation is used as the regenerative coolant 6. Recovered, mixed, adjusted specific gravity and viscosity, and added new abrasive grains and new coolant to create a regenerated slurry. When the regeneration and slicing of the slurry were repeated, the concentrations of the silicon contained in the used slurry and the regenerated slurry were about 12% and 6%, respectively.

  The generated amounts of sludge 5b and liquid component 5a that is not used as regenerated coolant 6 (surplus coolant) 7 were 100 kg and 80 kg, respectively. The amount and components of sludge 5b and surplus coolant 7 were measured. The results are shown in Table 1.

2. Distillation Step Next, a sample in which the sludge 5b and the excess coolant 7 were mixed was vacuum distilled to obtain a liquid component 9a and a solid component 9b. The vacuum distillation was performed using a vacuum distillation apparatus (temperature: 160 ° C., final ultimate vacuum 10 Torr). The liquid fraction 9a was used as a distillation coolant 11 for slurry regeneration. The generation amount and components of the solid content 9b were measured. The results are shown in Table 2.

The particle size distribution of the solid content 9b was measured. The particle size measurement was performed by first separating particles of 10 mm or more, 1 mm or more, and 0.1 mm or more in order using a sieve, and then measuring the particle size of the remaining particles using a particle size distribution meter. The results are shown in Table 3. When Table 3 is seen, it turns out that many large grains about 1-10 mm exist. This is considered to be because fine particles are bonded to each other by the coolant remaining in the solid content 9b to form large particles.

3. Next, 10 times the amount (980 kg) of isopropyl alcohol (hereinafter referred to as “alcohol”) 12 was added to the solid content 9b and stirred for 30 minutes. Thereafter, solid-liquid separation was performed by centrifugation (8000 G). As a result, liquid 15a and solid 15b were obtained. This solid content 15b was again stirred and centrifuged under the same conditions. The stirring and centrifugation steps were performed a total of 5 times.
The solid content 15b after the stirring and centrifuging process was completed five times had an overall weight of 111.8 kg and contained 28 kg of alcohol.

The liquid fraction 15a after centrifugation was distilled. The distilled liquid was taken out as recycled alcohol 19. Regenerated alcohol 19 was reused as stirring alcohol. Distillation was performed at 90 ° C., and the evaporated liquid was collected using a condenser. In order to increase the yield, the condenser temperature was set to 0 ° C. The alcohol (isopropyl alcohol) contained in the liquid 15a has a boiling point of 82.4 ° C, and propylene glycol has a temperature of 187.85 ° C. Therefore, in distillation at 90 ° C., propylene glycol hardly evaporates, and high-purity alcohol can be recovered by distillation.
The total weight of the liquid fraction 15a recovered by the five times of centrifugation was 4872 kg. The weight of recycled alcohol 19 was 4500 kg. The solid content remaining after the distillation was mainly composed of PG as a coolant component, and contained SiC, Si, metal impurities and others as trace components.

4). Next, drying (heating at 100 ° C. for 1 hour) was performed on the solid content 17b, and the liquid content 17a was evaporated to take out the solid content 17b. The evaporated liquid component 17a was recovered with a condenser. The condenser conditions were set at 0 ° C. at normal pressure. The recovered amount was 25 kg. The recovered liquid 17a was directly used as regenerated alcohol 19. The generation amount and components of the solid content 17b were measured. The results are shown in Table 4.

Furthermore, the particle size distribution of the solid content 17b was measured. The particle size was measured using a particle size distribution meter. The results are shown in Table 5. Comparing Table 3 and Table 5, by adding alcohol 12 to the solid content 9b after distillation and stirring, particles with a particle size of 0.1 mm or more disappear, and the particle size of most particles is 0.1 mm. It turns out that it became less than. This is considered to be because the coolant remaining in the solid content 9b after the distillation was dissolved in the alcohol and the fine particles adhered by the coolant were separated.

  Through the above steps, the silicon-containing material 21 substantially free of coolant could be recovered. The silicon-containing material 21 contains a large amount of silicon and can be used as a raw material for producing polysilicon or halosilane.

  In this embodiment, alcohol is recovered from the liquid portion 15a after centrifugation, and alcohol evaporated in the drying step is also recovered. Therefore, most of the alcohol used can be recovered and reused.

  In the above embodiment, the secondary centrifugation is performed on the entire amount of the liquid fraction 3a of the primary centrifugation. However, as shown in FIG. 2, only the amount necessary for obtaining the regenerated coolant 6 is secondary. Centrifugation is performed, and the remainder of the liquid 3a may be mixed with the sludge 5b and distilled. In this case, the load in the secondary centrifugation can be reduced.

  Further, in the above embodiment, the sludge 5b is mixed with the surplus coolant 7 without drying, but as shown in FIG. 3, the sludge 5b and the surplus coolant 7 may be mixed after the sludge 5b is dried. Even in this case, the same effect as in the above embodiment can be obtained.

  Further, in the above-described embodiment, primary and secondary centrifugation are performed. However, as shown in FIG. 4, only the primary centrifugation may be performed and the liquid 3a may be used as a sample. In this case, there is an advantage that the equipment cost can be reduced.

  Embodiment 2 of the present invention will be described with reference to FIG. This example differs from Example 1 in that distillation is performed on a sample consisting only of excess coolant.

1.1 Primary and Secondary Centrifugation Steps Since this step is common to Example 1, description thereof is omitted. However, the sludge 5b was discarded and only the surplus coolant 7 was used as a sample to be distilled.

2. Distillation step Next, a sample comprising the excess coolant 7 was vacuum distilled to obtain a liquid content 9a and a solid content 9b. Vacuum distillation was performed under the same conditions as in Example 1. The liquid fraction 9a was used as a distillation coolant 11 for slurry regeneration. The generation amount and components of the solid content 9b were measured. The results are shown in Table 6.

The particle size distribution of the solid content 9b was measured. The results are shown in Table 7. When Table 7 is seen, it turns out that many large grains about 1-10 mm exist. This is considered to be because fine particles are bonded to each other by the coolant remaining in the solid content 9b to form large particles.

3. Next, 10 times the amount (128 kg) of isopropyl alcohol (hereinafter referred to as “alcohol”) 12 was added to the solid content 9b and stirred for 30 minutes. Thereafter, solid-liquid separation was performed by centrifugation (8000 G). As a result, liquid 15a and solid 15b were obtained.
This solid content 15b was again stirred and centrifuged under the same conditions. This stirring and centrifuging step was performed 5 times in total.
The solid content 15b after the stirring and centrifuging process was completed five times had an overall weight of 14 kg and contained 3 kg of alcohol.

The liquid fraction 15a after centrifugation was distilled. The distilled liquid was taken out as recycled alcohol 19. Regenerated alcohol 19 was reused as stirring alcohol. Distillation was performed at 90 ° C., and the evaporated liquid was collected using a condenser. In order to increase the yield, the condenser temperature was set to 0 ° C.
The total weight of the liquid fraction 15a recovered by the five times of centrifugation was 632 kg. The weight of recycled alcohol 19 was 550 kg. The solid content remaining after the distillation was mainly composed of PG as a coolant component, and contained SiC, Si, metal impurities and others as trace components.

4). Next, drying (heating at 100 ° C. for 1 hour) was performed on the solid content 17b, and the liquid content 17a was evaporated to take out the solid content 17b. The evaporated liquid component 17a was recovered with a condenser. The condenser conditions were set at 0 ° C. at normal pressure. The recovered amount was 2.5 kg. The recovered liquid 17a was directly used as regenerated alcohol 19. The generation amount and components of the solid content 17b were measured. The results are shown in Table 8.

Furthermore, the particle size distribution of the solid content 17b was measured. The results are shown in Table 9.

  Through the above steps, the silicon-containing material 21 substantially free of coolant could be recovered.

Example 3 was performed under the same conditions as Example 2 except that the weight of the alcohol added during stirring was halved (that is, 5 times the amount) and the number of stirring and centrifugation was changed. The weight of the coolant contained in each of the solid content 9b after distillation and the solid content 15b after each centrifugation was measured. The residual coolant weight is measured by heating each solid to 100 ° C. to remove the low-boiling organic solvent, and then measuring the weight after heating at 300 ° C. to remove the coolant. The amount of change in weight before and after heating in was used as the weight of the residual coolant. The results are shown in Table 10. According to Table 10, it can be seen that the weight of the remaining coolant is reduced by repeating the stirring and centrifugation. What is necessary is just to determine suitably the frequency | count of performing stirring and centrifugation according to the use of a silicon containing material with reference to Table 10. FIG. Usually it is enough to do it twice.

1 shows a silicon-containing material recovery system according to Example 1 of the present invention. The modification of the silicon-containing material collection | recovery system of Example 1 of this invention is shown. The modification of the silicon-containing material collection | recovery system of Example 1 of this invention is shown. The modification of the silicon-containing material collection | recovery system of Example 1 of this invention is shown. The The silicon-containing material collection | recovery system of Example 2 of this invention is shown.

Explanation of symbols

1: Used slurry 3a: Liquid content after primary centrifugation 3b: Solid content after primary centrifugation 4: Regenerated abrasive grains 5a: Liquid content after secondary centrifugation 5b: Solid content after secondary centrifugation 6: Regenerated coolant 7: Excess coolant 9a: Liquid content after distillation 9b: Solid content after distillation 11: Distillation coolant 12: Alcohol 15a: Liquid content after centrifugation 15b: Solid content after centrifugation 17a: After drying Liquid 17b: Solid content after drying 19: Recycled alcohol 21: Silicon-containing material

Claims (9)

A solid content is obtained by previously removing the water-soluble coolant from the used slurry in the silicon wafer manufacturing process, which contains at least water-soluble coolant, abrasive grains, and silicon grains.
From the solid content, the water-soluble coolant remaining in the solid content is extracted using a low-boiling organic solvent that is compatible with the water-soluble coolant and has a boiling point lower than that of the water-soluble coolant. A method for recovering a silicon-containing material, comprising removing an organic solvent by centrifugation and recovering a solid content obtained by centrifugation. The process of removing the water-soluble coolant from the used slurry in advance is as follows:
The used slurry is subjected to primary centrifugation to recover the solids mainly composed of abrasive grains.
The liquid obtained by the primary centrifugation is subjected to secondary centrifugation to recover a part of the liquid mainly composed of water-soluble coolant,
The method of Claim 1 including the process of removing a water-soluble coolant previously by distillation from the sample which consists of the remainder of the liquid obtained by secondary centrifugation, and at least one of sludge. The process of removing the water-soluble coolant from the used slurry in advance is as follows:
The used slurry is subjected to primary centrifugation to recover the solids mainly composed of abrasive grains.
The method of Claim 1 including the process of removing a water-soluble coolant previously by distillation from the sample which consists of at least one part of the liquid obtained by primary centrifugation. The water-soluble coolant remaining in the solid content is extracted from the solid content obtained by centrifugation using the same or different kind of low-boiling organic solvent as the low-boiling organic solvent, and the low-boiling organic solvent used for the extraction is centrifuged. The method according to claim 1, further comprising one or more steps of removing by separation and recovering a solid content obtained by centrifugation. The method according to claim 1, wherein the centrifugation is performed at 3000G or more. The method according to claim 1, wherein the low-boiling organic solvent comprises an alcohol or ketone having 1 to 6 carbon atoms. The method according to claim 1, wherein the low-boiling organic solvent comprises methanol, ethanol, isopropyl alcohol, or acetone. The method of Claim 1 provided with the process of drying the solid content obtained by centrifugation. The method according to claim 1, further comprising a step of recovering the low-boiling organic solvent by distilling the liquid obtained by centrifugation.

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