JP5640260B2 - Coolant recovery method - Google Patents

Description

  The present invention relates to a method for recovering coolant discharged from an ingot slicing step.

  During the manufacturing process of semiconductors and solar cells, there is a process of slicing ingots such as silicon, SiC, and sapphire with a wire saw. In this slicing step, cutting is performed while spraying a coolant called coolant on the wire. Abrasive grains such as SiC are used for the purpose of assisting the cutting with the wire, and the free abrasive grain method in which the abrasive grains are dispersed in the coolant and cut by the wire, and the fixed abrasive using the wire to which the abrasive grains are fixed Grain method is adopted. This fixed abrasive method has the advantage of being sharper than the free abrasive method, reducing the working time, and reducing the variation in the thickness of the cut wafer.

  Although a large amount of coolant is discharged from such an ingot slicing step, it is not preferable to discard the coolant as it is from the viewpoints of economy and environmental conservation. Therefore, it is desired to recover and reuse the discharged coolant.

  In this coolant, a large amount of ingot cutting powder is mixed in addition to abrasive grains. For this reason, it is necessary to separate and remove these inclusions and recover only the coolant component. Therefore, Patent Document 1 proposes a method in which the recovered coolant is applied to a centrifugal separator and further processed with a filter made of a nonwoven fabric. Patent Document 2 also discloses a method using a centrifuge.

  However, in these conventional methods, the separation efficiency of abrasive grains and chips is poor, and in particular, silicon chips have a particle size in the range of 0.1 to 10 μm and cannot be removed sufficiently. There was a problem of low clarity.

Therefore, the present inventor tried to separate and remove silicon chips in the coolant using a ceramic membrane having a membrane pore diameter of about 0.1 μm. If such a ceramic film is used, it should be possible to almost completely remove 0.1 to 10 μm of silicon chips. However, during the coolant while others molecular weight contains 10,000 to 1,000,000 thickeners, filtration rate for thickening when you try to filtration such coolant can not pass through the membrane It was found that the practicality was poor.

JP 9-168971 A JP-A-11-33913

  Therefore, the object of the present invention is to solve the above-mentioned conventional problems, and to sufficiently remove the mixed silicon powder even when the thickener is contained in the coolant discharged from the ingot slicing process. It is another object of the present invention to provide a coolant recovery method capable of maintaining a practical filtration rate.

The present invention made in order to solve the above-mentioned problems is a clear coolant obtained by filtering the coolant containing the thickener discharged from the ingot slicing step through a ceramic membrane and separating the chips into a membrane. A coolant recovery method for recovering the coolant, and by using a monolithic ceramic membrane having a membrane pore diameter of 1 to 10 μm, while ensuring the filtration speed of the coolant containing the thickener , periodically during the filtration operation By repeating air backwashing for 1 to 5 seconds, crossflow filtration is performed while suppressing a decrease in filtration rate.

Incidentally, in a preferred embodiment, a slicing process is fixed abrasive grain system, in which the coolant is mainly composed of polyethylene glycol, the molecular weight contains 10,000 to 1,000,000 thickeners.

  In addition, it is possible to add a step of adding a new coolant to the recovered clear coolant and returning it to the ingot slicing step.

  According to the coolant recovery method of the present invention, the coolant containing the thickener discharged from the ingot slicing step is cross-flow filtered through a ceramic membrane having a membrane pore diameter of 1 to 10 μm to separate the chips. Since such a ceramic membrane having a large membrane pore diameter is used, a thickener having a molecular weight of 10,000 to 1,000,000 contained in the coolant can also pass through the membrane pores. Further, as described above, the chip has a particle size distribution of 0.1 to 10 μm, but the membrane pore size is 1 to 2 because of the trapping effect inside the ceramic film or the trapping powder deposited on the primary surface of the film. Even with a 10 μm ceramic membrane, the chips can be sufficiently separated and removed.

In addition, according to the coolant recovery method of the present invention, the air backwashing is periodically repeated for 1 to 5 seconds during the continuation of the filtration operation , thereby suppressing a decrease in the coolant filtration rate. For this reason, since it is not necessary to use a filtrate for backwashing of a ceramic membrane, the recovery efficiency of a coolant can be improved.

It is a block diagram which shows embodiment of this invention. It is a graph which shows other embodiment of this invention. It is a graph which shows the Example of this invention.

  Embodiments of the present invention will be described below. In the following description, the ingot is a silicon ingot, but the present invention can also be applied to coolant discharged from an ingot slicing process such as SiC or sapphire.

FIG. 1 is a block diagram showing an embodiment of the present invention, in which 1 is a silicon ingot slicing step, and 2 is a recovery tank for coolant discharged from the slicing step 1. In the slicing step 1, cutting is performed while spraying a coolant called coolant on the wire. As described above, the fixed abrasive method and the free abrasive method are known as the cutting method, but the present invention can correspond to any method. Wire saw coolant for slicing a silicon ingot is mainly composed of polyethylene glycol, molecular weight contains 10,000 to 1,000,000 thickeners, many of them having 15 to 20 centipoise. As cutting progresses, silicon chips are gradually accumulated in the coolant and the performance deteriorates, so that the used coolant is discharged batchwise into the collection tank 2. As described above, the silicon chips have a particle size distribution of 0.1 to 10 μm.

The discharged used coolant is sent from the collection tank 2 to the ceramic membrane 4 by the filtration pump 3 and subjected to cross flow filtration. The ceramic membrane 4 has a membrane pore diameter of 1 to 10 μm, and a ceramic monolith membrane is used . The membrane pore size of the ceramic membrane 4 to be used is 1 to 10 μm. If the membrane pore size is smaller than this, the molecules of the thickening agent are difficult to permeate the membrane pores, and the filtration rate is slow, which is not practical. Conversely, if it exceeds 10 μm, part of the silicon chips having a particle size distribution of 0.1 to 10 μm cannot be separated and removed, and a clear coolant cannot be obtained. If the membrane pore diameter is in the range of 1 to 10 μm, the silicon chips can be separated and removed almost 100%. A more preferable range of the pore diameter of the ceramic membrane 4 is 1 to 3 μm.

  The permeated liquid of the ceramic membrane 4 from which the silicon chips and the like are removed in this way is sent to the filtration coolant tank 5. The concentrated liquid that has passed through the primary side of the ceramic membrane 4 is returned to the recovery tank 2 through the pipe 6. This concentrated liquid is mixed with the used coolant and sent again to the ceramic membrane 4 by the pump 3 and subjected to cross flow filtration, so that the coolant inside the recovery tank 2 is gradually concentrated. At the same time, the filtration rate gradually decreases, but in the present invention, the reduction in the filtration rate of the coolant is suppressed by periodically repeating backwashing with only air.

  Air backwashing is performed by introducing compressed air from the backwashing air source 7 to the secondary side of the ceramic membrane 4. In the present invention, the reason why back washing with only air is effective is that the ceramic membrane 4 has a large membrane pore diameter of 1 to 10 μm, so that relatively low-pressure air blows out from the primary side through the membrane surface, and the membrane surface is blocked. This is because the object can be peeled off. At this time, the thickener remaining in the film is also blown out to the primary side. The frequency of air backwashing is once every 5 to 30 minutes, and the time required for one air backwashing is about 1 to 5 seconds. This air backwashing is performed during the continuation of the filtration operation, and the pressure may be about 0.2 to 0.5 MPa higher than the liquid pressure on the primary side.

  When cross-flow filtration is performed while repeating such air backwashing, the SS concentration of the used coolant in the recovery tank 2 reaches 25 to 40 wt%. When this state is reached, the concentrated used coolant is taken out to the waste tank 8, and the ceramic membrane 4 is cleaned with chemicals using chemicals such as sodium hydroxide and nitric acid. Since the ceramic film 4 is superior in chemical resistance compared to the polymer film, it does not deteriorate even after repeated chemical cleaning using sodium hydroxide, nitric acid or the like.

  On the other hand, coolant from which silicon chips and the like are removed is collected in the filtration coolant tank 5. Since this coolant is a clear coolant from which almost 100% of silicon chips are removed, a new coolant is added to the preparation tank 9 and then returned to the slicing step 1 for reuse.

  In the embodiment described above, the used coolant is directly sent from the recovery tank 2 to the ceramic membrane 4 and subjected to cross-flow filtration. It can also be installed in the front stage of the ceramic film 4. Although the separation and recovery of the silicon chips by the centrifugal separator 10 is incomplete, if a part of the silicon chips mixed in the coolant is removed by the centrifugal separator 10, the load on the ceramic membrane 4 is reduced. It is possible to maintain the filtration rate at a higher speed. It is also possible to provide the centrifuge 10 in parallel with the ceramic membrane 4.

  Thus, according to the present invention, from the coolant containing the thickener discharged from the ingot slicing step, it is possible to sufficiently remove chips at a practical filtration rate and recover a clear coolant. it can. Examples of the present invention are shown below.

  Using a monolithic ceramic membrane having a membrane pore diameter of 2 μm (manufactured by the applicant company: 19 holes), four types of used coolant discharged from the slicing process of the silicon ingot were filtered. The filtration temperature is room temperature (25 ° C.). Cross flow filtration was continued for 24 hours while repeating air backwashing for 2 seconds every 10 minutes immediately after the start of filtration.

The result is as shown in the graph of FIG. 3, and the used coolant A had an SS concentration of about 10 wt% at the start of filtration, but could be concentrated to about 35 wt%. Note filtration rate at the start filtration was 45L / ^{m 2} h, but filtration completion was 20L / ^{m 2} h. The used coolant B had an SS concentration of about 10 wt% at the start of filtration, but could be concentrated to about 30 wt%. The filtration rate was 50 L / m ² h at the start of filtration, but 15 L / m ² h at the end of filtration. The used coolant C had an SS concentration of about 8 wt% at the start of filtration, but could be concentrated to about 25 wt%. The filtration rate was 30 L / m ² h at the start of filtration, but 10 L / m ² h at the end of filtration. The used coolant D had an SS concentration of about 6 wt% at the start of filtration, but could be concentrated to about 25 wt%. The filtration rate was 20 L / m ² h at the start of filtration, but 10 L / m ² h at the end of filtration. Thus, although there is a large variation in the characteristics of the used coolant, according to the present invention, it was possible to greatly concentrate the used coolant containing silicon chips and recover a clear coolant. .

On the other hand, when the used coolant E having an SS concentration of about 10 wt% was filtered using a UF membrane having a molecular weight cut off of 20,000, the SS concentration at the end of filtration was 20 wt% . The filtration rate was considerably lower than 10 L / m ² h, and the filtrate quality was not practical because the thickener was significantly removed and the quality of the regenerated coolant was changed.

The SS concentration in this example was measured by the following method.
(1) First, the weight W1 (mg) of a 0.1 μm membrane filter stored dry in a desiccator is measured.
(2) Filter the sample of V (mL) with a 0.1 μm membrane filter.
(3) In order to prevent precipitation from the solvent component, the suspended matter deposited on the 0.1 μm membrane filter is sufficiently washed with pure water.
(4) After sufficiently drying the 0.1 μm membrane filter and the suspended substance, the weight C (mg) is measured.
(5) The SS concentration (mg / L) is calculated by the equation SS concentration = (W2−W1) / (V / 1000).

  As described above, according to the present invention, it is possible to separate and remove the chips from the coolant containing the thickener discharged from the ingot slicing step, and to recover a clear coolant. Excellent results can be obtained in terms of environmental conservation.

DESCRIPTION OF SYMBOLS 1 Ingot slicing process 2 Coolant collection tank 3 Filtration pump 4 Ceramic membrane 5 Filtration coolant tank 6 Piping 7 Backwash air source 8 Disposal tank 9 Preparation tank 10 Centrifuge

Claims (3)

A coolant recovery method in which the coolant containing the thickener discharged from the ingot slicing step is filtered through a ceramic membrane to separate the chips, and the recovered clear coolant is recovered.
By using a monolithic ceramic membrane having a membrane pore diameter of 1 to 10 μm, the filtration speed of the coolant containing the thickener is secured, and air backwashing is periodically repeated for 1 to 5 seconds during the filtration operation. Thus, the coolant recovery method characterized by carrying out crossflow filtration, suppressing the fall of a filtration rate.   The coolant recovery method according to claim 1, wherein the ingot slicing step is a fixed abrasive method. The coolant recovery method according to claim 1, further comprising a step of adding and mixing a new coolant to the recovered clear coolant and returning it to the ingot slicing step .

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