JPH0760071A - Filtration of suspension - Google Patents

Abstract

PURPOSE:To filter a liquid containing a fine suspended substance over a long period of time with high flux under low pressure by guiding the liquid containing the suspended substance in a membrane separation tank to a dehydrator to separate the same into the sludge of the suspended substance and a liquid reduced in the concn. of the suspended substance and feeding back the low conc. liquid to the membrane separation tank. CONSTITUTION:A liquid containing a suspended substance (e.g. a cutting fluid containing the cut layer of a semiconductor wafer) is guide to a membrane separation tank 1 and filtered through the porous hollow fiber membranes 2 arranged in the tank 1 to recover a liquid from which the suspended substance is removed. At this time, the liquid containing the suspended substance in the membrane separation tank 1 is guided to be separated into the sludge of the suspended substance and a liquid reduced in the concn. of the suspended substance and the low conc. liquid is fed back to the membrane separation tank 1. That is, since the concn. of the suspended substance in the membrane separation tank is lowered to the concn. suitable for the membrane filtration due to porous hollow fiber membranes, the liquid containing the fine suspended substance can be stably filtered over a long period of time with high flux under low pressure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for filtering a suspension containing fine suspended matter.

[0002]

2. Description of the Related Art Hitherto, a suspension containing a fine suspended substance such as a grinding fluid for semiconductor wafers has been filtered by diatomaceous earth, filtration using a cartridge filter made of non-woven fabric, or the like. However, it was not satisfactory in terms of the amount of suspended substances and the particle size that can be captured. In addition, although turbidity removal using various filtration membranes has been attempted, a reverse osmosis membrane cannot cope with a high-concentration suspension and requires a pretreatment step. On the other hand, in the case of the ultrafiltration membrane, the plate and tubular membranes have a large equipment cost for the treatment amount, and in the pressure filtration using the porous hollow fiber membrane,
Clogging occurred in an extremely short time.

As a method for solving these problems, recently, a method of arranging a porous hollow fiber membrane in a membrane separation tank in a migrating state and performing suction filtration has been studied. But,
When the operation was continued for a long time and the suspended solids were accumulated at a high concentration in the tank, the hollow fiber membrane was likely to be clogged, and the suction pressure and the filtration flow rate were unavoidably increased. Therefore, the high-concentration suspension was either discarded or introduced into a settling tank to remove the suspended matter by sedimentation. However, when the particle size of the suspended solids is small, the sedimentation rate is slow, so an extremely large-capacity settling tank is required to reduce the suspended solids concentration to a level that does not hinder suction filtration, and therefore, in practice In many cases, the concentration of suspended substances in the membrane separation tank exceeded the proper range, which sometimes caused trouble.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a filtration method capable of filtering a liquid containing a fine suspended substance at a high flux and a low pressure for a long period of time.

Another object of the present invention is to provide a method for filtering a suspension, which has a small amount of waste liquid and can reduce the amount of chemicals added to the suspension.

[0006]

That is, according to the present invention, a liquid containing a suspended substance is introduced into a membrane separation tank, and the suspended substance is filtered through a porous hollow fiber membrane arranged in the tank. In the method of recovering the liquid after removing the liquid, the liquid containing the suspended substance in the membrane separation tank is introduced into a dehydrator, and separated into sludge of suspended substance and liquid with reduced suspended substance concentration, It is a method of filtering a suspension, which has a step of returning the concentrated liquid to a membrane separation tank.

[0007]

In the centrifuge, even if a low-concentration fine particle suspension on the order of several hundred ppm is treated, the suspended substance cannot be removed very effectively. However, the concentration of suspended matter is several thousand p
It is effective for reducing the concentration of the suspension of about pm or more. Therefore, the liquid in the membrane separation tank containing a high concentration of suspended substances is guided to a dehydrator such as a centrifugal separator, and separated into sludge and liquid with a reduced concentration of suspended substances, and the low-concentration liquid is separated into a membrane separation tank. When it is returned to, the concentration of suspended matter in the membrane separation tank is prevented, the suspended matter concentration can be maintained at a low level suitable for filtration by the porous hollow fiber membrane, and stable suspension filtration is possible. I can do it.

Hereinafter, the suspension filtration method of the present invention will be described in more detail with reference to the drawings. FIG. 1 is a flow sheet showing a method for filtering a suspension of the present invention.

In the filtration method of the present invention, a liquid containing a suspended substance is introduced into a membrane separation tank 1 and filtered through a porous hollow fiber membrane 2 arranged in the tank to remove the suspended substance. The method of recovering the liquid is applicable.

The liquid containing a suspended substance as referred to in the present invention means a liquid containing a suspended solid substance which can be separated by a porous hollow fiber membrane, and the liquid phase is not limited to an aqueous liquid.

The porous hollow fiber membranes are known to be typically produced by a solvent extraction method or a stretch opening method depending on the manufacturing method. However, the stretch opening method has a slit-like pore shape. Because of this, clogging is less likely to occur, and therefore it is preferable as the hollow fiber membrane used in the method of the present invention. The fractionation performance of the porous film is preferably 1.0 μm or less, and particularly when applied to a grinding fluid containing grinding dust of semiconductor wafers, the fractionation performance is preferably 0.05 μm or less. The value of the fractionation performance here means that the permeation inhibition rate of the polystyrene latex particles having the particle diameter is 90% or more.

The porous hollow fiber membrane to be arranged in the membrane separation tank is not particularly limited, but as shown in FIG. 2, the hollow fiber membrane is spread as a knitted fabric into a sheet shape to form a hollow fiber membrane. A module is exemplified as a preferable one. In such a hollow fiber membrane module, the suspended substance is unlikely to be clogged in the spaces between the hollow fiber membranes, and the suspended substance adhering to the surface of the hollow fiber membrane can be easily shaken off.

In the filtration method of the present invention, a pressure filtration method can be adopted as the filtration through the hollow fiber membrane in the membrane separation tank, but a suction filtration method is preferred. For a suspension having a relatively high concentration such that a dehydrator such as a centrifuge can effectively function, the pressure filtration method tends to cause clogging of the porous hollow fiber membrane. In contrast,
Since the pressure difference between the membranes in suction filtration is not so large, physically moving the hollow fiber membrane allows the suspension adsorbed and attached to the membrane surface to be easily separated from the membrane surface. In addition, when the hollow fiber membrane is clogged with time and the flux is reduced, the flux can be easily recovered by performing intermittent backwashing for about one second.

When the filtration using the porous hollow fiber membrane is continued, the hollow fiber membrane impermeable suspended substance is concentrated and accumulated in the membrane separation tank with the passage of time. When the ss in the membrane separation tank reaches the order of tens of thousands ppm even by the suction filtration method,
Since it is inevitable that the suction pressure rises and the filtration flow rate decreases, and the hollow fiber membranes are likely to become clogged, conventionally, the concentrated suspension in the membrane separation tank is discarded at an appropriate cycle, or a separate precipitation tank is used. Was provided to precipitate and process the suspended matter.

In the filtration method of the present invention, the membrane separation tank 1
The liquid containing the suspended substance therein is guided to the dehydrator 3, where it is separated into sludge in which the suspended substance is concentrated and liquid in which the concentration of the suspended substance is reduced.

Examples of the dehydrator 3 used in the present invention include a centrifugal separator, a belt press, a filter press, a vacuum dehydrator and a screw press.
Of these, a centrifugal separator is preferable. There are various types of centrifugal separators such as a separator type, a cylinder type, and a screw decanter type, and a separator type capable of expressing a high centrifugal force and capable of continuously discharging sludge is exemplified as a particularly preferable one. The required centrifugal force depends on the properties of the suspension and the processing flow rate, but is usually 2000 G or more,
It is preferably 4000 G or more.

The sludge of the suspended solids separated by the dehydrator 3 is collected in the sludge receiving tank 4 and then discarded, while the suspension having a low suspended solid concentration is returned to the membrane separation tank 1. The reduction rate of the concentration of suspended matter in the dehydrator is not particularly limited, but it is preferably 20% or more. As a result, the concentration of suspended solids in the membrane separation tank can be reduced or maintained at a low level. Separation efficiency in a dehydrator, particularly a centrifugal separator, depends on the concentration, particle size, difference in specific gravity, etc. of suspended substances, and the larger these values, the more efficient the separation. Therefore, even if the suspension is led to the dehydrator at a stage where the concentration of the suspended substance in the membrane separation tank is low, more specifically, at a stage where ss is less than 1,000 ppm, the separation efficiency is not so good, and therefore, the concentration cannot be increased to some extent. It is preferable to operate the dehydrator after the progress ss reaches several thousands ppm. The dehydrator may be operated continuously, or when the ss in the membrane separation tank falls to a predetermined level, the dehydrator is temporarily stopped and the ss is removed.
It is also possible to carry out an intermittent operation such that the operation is restarted at the time when the value becomes equal to or higher than a predetermined level.

Depending on the fractionation performance of the porous membrane used, the liquid from which the suspended substances have been removed is recovered and reused via the circulation tank 7 or directly. In addition, the suspension supplied to the membrane separation tank is not limited to be passed through various pretreatment devices.

The suspension to which the filtration method of the present invention can be applied includes polishing and grinding liquid of precision industry (grinding liquid containing grinding dust of semiconductor wafer, lens polishing liquid, electric discharge machining cooling water, etc.), iron rust and the like. Examples include brine and lubricating oil.

[0020]

【Example】

Example FIG. 1 shows a circulating grinding fluid of a surface grinding apparatus for a silicon wafer.
It processed with the turbidity removal apparatus which consists of the flow shown by. That is, the grinding liquid in the circulation tank 7 was used by the grinding device 8 and returned to the circulation tank as a grinding liquid containing grinding dust (hereinafter, abbreviated as suspension). On the other hand, the suspension in the circulation tank was introduced at a flow rate of 2 liters / minute into a 0.3 m ³ membrane separation tank 1, where a polyethylene porous hollow fiber membrane (EHF-390A, manufactured by Mitsubishi Rayon Co., Ltd., Product name, fractionation performance 0.03 μm) 2
Hollow fiber membrane module (total membrane area 10m
² ) was suction-filtered with a suction pump 5 at a flow rate of 2 liters / minute at a pressure of 0.15 kg / cm ² , and the filtrate was returned to the circulation tank. The hollow fiber membrane module is of the type shown in FIG. 2, and the hollow fiber membrane was constantly rocked using the crank 6. The concentration of grinding debris in the suspension after passing through the separation membrane was 9 ppm, and the concentration of grinding debris in the suspension in the circulation tank at the time of the test was about 300 ppm.

After 250 hours from the start of the treatment, the concentration of grinding debris in the suspension in the membrane separation tank increased to 30,000 ppm and the suction pressure rose slightly to 0.20 kg / cm ² , so that the suspension in the membrane separation tank was suspended. The liquid is guided to a centrifugal separator (SJ 400, manufactured by Mitsubishi Koki Co., Ltd.) 3 at a rate of 0.2 liter / min, where grinding dust is concentrated and removed as sludge and discharged into a sludge receiving tank 4, and at the same time, grinding dust. The process of returning the liquid (concentration of grinding debris of 1200 ppm) whose concentration was reduced to the membrane separation tank 1 was started.

After this treatment was continued for 1000 hours, there was almost no change in the suction pressure and the filtration flow rate in the hollow fiber membrane module, and the filtration could be continued smoothly. Further, the concentration of grinding debris in the membrane separation tank finally decreased to 2500 ppm, and was in a substantially steady state.

Comparative Example The same suspension as above was treated in a similar manner except that no treatment with the centrifuge 3 was carried out. After 500 hours from the start of the treatment, the suction pressure of the hollow fiber membrane module increased to 0.4 kg / cm ² and the membrane permeation flow rate also decreased to 1.8 liters / minute, so the suspension in the membrane separation tank was All was discarded. The concentration of grinding dust in the waste liquid is 60000.
It was ppm.

[0024]

According to the suspension filtration method of the present invention, by operating the dehydrator, the level of the concentration of the suspended substance in the membrane separation tank is suitable for the membrane filtration by the porous hollow fiber membrane. Since the concentration is lowered, the liquid containing the fine suspended substance can be stably filtered with high flux and low pressure for a long period of time. Moreover, since the suspended solids are simply discharged as sludge, the suspension is almost never discarded, and the amount of the drug used in the suspension can be reduced.

[Brief description of drawings]

FIG. 1 is a flow sheet of an example of the filtration method of the present invention.

FIG. 2 is a partially cutaway perspective view showing an example of a hollow fiber membrane module suitable for use in the filtration method of the present invention.

[Explanation of symbols]

 1 Separation Tank 2 Hollow Fiber Membrane Module 3 Centrifugal Separator 4 Sludge Receiving Tank 5 Suction Pump 6 Crank 7 Grinding Fluid Tank 8 Grinding Process 11 Hollow Fiber Membrane 12 Structural Material 13 Potting Material 14 Water Collection Port

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yutaka Onuki 3-18-3 Kanda, Chiyoda-ku, Tokyo Koryu Kogyo Co., Ltd. Tokyo Branch

Claims (3)

[Claims] 1. A method for introducing a liquid containing a suspended substance into a membrane separation tank, and collecting the liquid from which the suspended substance is removed by filtering through a porous hollow fiber membrane arranged in the tank, A step of introducing a liquid containing suspended substances in a membrane separation tank to a dehydrator, separating the sludge of suspended substances and a liquid with a reduced concentration of suspended substances, and returning the low-concentration liquid to the membrane separation tank. A method for filtering a suspension, comprising: 2. The method for filtering a suspension according to claim 1, wherein the filtration through the porous hollow fiber membrane is suction filtration. 3. The method for filtering a suspension according to claim 1, wherein the suspension-containing liquid is a grinding liquid containing semiconductor wafer grinding dust.