Sanyo Electric Co., Ltd. (Osaka, JP)
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CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional of Ser. No. 10/831,585, filed Apr. 23, 2004.
This application claims the benefit of priority to Japanese Patent Applications Nos. 2003-122583, 2003-122584, 2003-122585 and 2003-122586 filed on Apr. 25, 2003, contents thereof being hereby incorporated by reference.
1. A filtration apparatus comprising: a tank for housing a fluid containing removables, a filter apparatus for filtering the fluid, a pump connected to the filter apparatus via a pipe, and a peel cistern connected to the pipe and operable to store filtered fluid filtered by the filter apparatus, wherein the peel cistern is located at a level above a fluid level of the fluid contained in the tank, an air diffuser located under the filter apparatus and generating air bubbles inside the fluid, an air pump for supplying gas via an air pipe connected to the air diffuser, wherein the air pipe comprises a first path and a second path which are parallel to each other, the first path being provided with a first adjustment valve and a first stop valve and the second path being provided with a second adjustment valve and a second stop valve, wherein each stop valve is operable to block or release the gas that passes inside the air pipe and each adjustment valve is regulated in advance so that a predetermined amount of gas is caused to pass there through, wherein an amount of gas flowing through the first adjustment valve is less than an amount of gas flowing through the second adjustment valve, and a controller to control operation modes of the filtration apparatus, wherein in a filtration mode, in which the fluid is pumped from the tank through the filter apparatus by the pump, the controller is operable to control the first stop valve to be open and the second stop valve to be closed, and wherein in a regeneration mode, in which the filtered fluid within the peel cistern flows from the peel cistern through the filter apparatus into the tank, the controller is operable to control the second stop valve to be open and the first stop valve to be closed.
2. The filtration apparatus of claim 1, wherein the air pipe is further provided with a parallel third path, the third path being provided with a third adjustment valve and a third stop valve.
3. The filtration apparatus of claim 2, wherein when any one of the stop valves is in an open state, the other stop valves are in a close state.
4. The filtration apparatus of claim 2, wherein the removables are CMP slurry.
5. The filtration apparatus of claim 2, wherein the removables comprise CMP slurry and process waste generated during the CMP process.
6. The filtration apparatus of claim 2, further comprising a valve at the bottom of the tank.
7. The filtration apparatus of claim 1 being operable to recover precipitated removables from the filtration apparatus.
8. The filtration apparatus of claim 7 wherein the precipitated removables are a gelled material.
9. A filtration apparatus comprising: a tank for housing a fluid containing colloidal removables, a filter apparatus formed of a first filter immersed inside the tank, wherein the filtration apparatus is operable to form during operation a second filter comprising a gel film adhered to a surface of the first filter, a pump connected to the filter apparatus via a pipe, and a peel cistern connected to the pipe and operable to store filtered fluid filtered by the filter apparatus, wherein the peel cistern is located at a level above a fluid level of the fluid contained in the tank, an air diffuser located under the filter apparatus and generating air bubbles inside the fluid, an air pump for supplying gas via an air pipe connected to the air diffuser, wherein the air pipe comprises a first path and a second path which are parallel to each other, the first path being provided with a first adjustment valve and a first stop valve and the second path being provided with a second adjustment valve and a second stop valve, wherein each stop valve is operable to block or release the gas that passes inside the air pipe and each adjustment valve is regulated in advance so that a predetermined amount of gas is caused to pass there through, wherein an amount of gas flowing through the first adjustment valve is less than an amount of gas flowing through the second adjustment valve a controller to control operation modes of the filtration apparatus, wherein in a filtration mode, in which the fluid is pumped from the tank through the filter apparatus by the pump, the controller is operable to control the first stop valve to be open and the second stop valve to be closed, and wherein in a regeneration mode, in which the filtered fluid within the peel cistern flows from the peel cistern through the filter apparatus into the tank, the controller is operable to control the second stop valve to be open and the first stop valve to be closed.
10. The filtration apparatus of claim 9, wherein the air pipe is further provided with a parallel third path, the third path being provided with a third adjustment valve and a third stop valve, so that when filtration by the filter apparatus is halted, only the third stop valve is in an open state.
11. The filtration apparatus of claim 9, wherein when any one of the stop valves is in an open state, the other stop valves are in a close state.
12. The filtration apparatus of claim 9, being operable to perform filtration using the filter apparatus by keeping the first valves only in an open state and mixing the fluid inside the tank by the air bubbles generated from the air diffuser.
13. The filtration apparatus of claim 9, being operable to separate the second filter comprising a gel film from the first filter by keeping the second valves only in an open state and generating air bubbles inside the tank by means of the air diffuser.
14. The filtration apparatus of claim 9 being operable to recover precipitated removables from the filtration apparatus.
15. The filtration apparatus of claim 14 wherein the precipitated removables are a gelled material.
FIELD OF THE INVENTION
The present invention relates to a filtration apparatus, and more particularly to a filtration apparatus which is easy to maintain and manage and which enables preservation of filtration characteristics.
DESCRIPTION OF THE BACKGROUND ART
At present, companies are faced with very important and challenging ecological issues such as reducing industrial waste, sorting out industrial waste and recycling or avoiding discharge of industrial waste into the nature. One type of industrial waste comprises various fluids containing contaminants.
These fluids have different denominations, such as for instance sewage water, drainage, effluent, etc., but in the description to follow, fluids, like for example water, chemicals, etc. comprising materials which are contaminants are called wastewater. Such wastewater is filtered using very expensive filtration apparatuses to remove the contaminants therein. The resulted clean water is recycled, whereas the separated contaminants or the residues which could not be filtered are disposed of as industrial waste. In particular, water is filtered to a level of purity which meets the environmental standards and is then discharged into the nature, in rivers, seas, or the like, or is otherwise recycled.
However, the usage of these filtration apparatuses is extremely difficult and becomes an environmental issue in itself due to a high running cost and equipment cost, etc. of the filtration process, etc.
As can be understood from the above, technology and techniques for wastewater treatment raise an important issue in terms of environmental pollution and recycling and therefore a system with a low initial cost and low running cost is promptly required.
An example of wastewater treatment in the semiconductor field is described in the following. When a plate-like metal, semiconductor or ceramic, etc. is polished or ground, a liquid such as water, etc. is showered on the polishing (grinding) jig and the plate due to considerations such as prevention of temperature rise of the polishing (grinding) jig, etc. caused by friction, improvement of lubricating property, prevention of polishing waste or cutting waste adherence, etc. to the plate.
In more detail, pure water is caused to flow during the process of dicing or backgrinding a semiconductor wafer comprising a plate-like semiconductor material. In a dicing apparatus, a flow of pure water is provided on the semiconductor wafer or pure water is showered by means of a discharge nozzle such that the blade is exposed to the pure water in order to prevent a rise in the temperature of the dicing blade and prevent adherence of dicing waste to wafer. Pure water is also used due to similar considerations in a process of thinning the wafer by backgrinding.
The wastewater containing polishing waste or grinding waste discharged from the above dicing apparatus or backgrinding apparatus is filtered and the clean water obtained thereby is returned into nature or is recycled, whereas the concentrated wastewater is recovered.
At present, two methods are employed in the semiconductor manufacturing process for the treatment of wastewater containing contaminants (waste) comprising mainly Si: the flocculation method and a method combining filter filtration and a centrifugal separator.
In the flocculation method, PAC (polychlorinated aluminum) or Al 2 (SO 4 ) 3 (aluminum sulfate), or the like is mixed in the wastewater as flocculant to generate a reaction with the Si and filtration is carried out by removing the reaction.
In the method combining filter filtration and a centrifugal separator, after the wastewater is filtered, the concentrated wastewater is fed to the centrifugal separator and silicon waste is collected as sludge, whereas the clean water resulted from the filtration process is discharged into the nature or is recycled.
For instance, wastewater generated in the dicing process is collected in a raw water tank 201 and is fed to a filtration apparatus 203 via pump 202 as shown in FIG. 16. The filtration apparatus 203 is provided with a ceramic and an organic filter F and the water filtered thereby is fed to a recycled water tank 205 via a pipe 204 , to be further recycled or discharged into the nature.
The filtration apparatus 203 is periodically washed due to clogging of the filter F. The filter F is back-washed with the water inside the recycled water tank 205 by closing valve B 1 provided in the raw water tank 201 and opening valve B 3 and valve B 2 which feed washing water from the raw water tank 201 . The wastewater thus generated and having a high concentration of Si waste mixed therein is returned to the raw water tank 201 . The concentrated water in the concentrated water tank 206 is fed to the centrifugal separator 209 by means of a pump 208 and the centrifugal separator 209 separates the sludge and the separate liquid. The sludge comprising Si waste is collected in a sludge recovery tank 210 , whereas the separate liquid is collected in a separate liquid tank 211 . The wastewater in the separate liquid tank 211 containing the separate liquid is fed to the raw water tank 201 by means of a pump 212 .
These methods are also employed when collecting waste generated during polishing and grinding of, for instance, solids mainly comprising metal materials such as Cu, Fe, Al, etc., or solids comprising inorganic materials such as plates, ceramic, etc., or plates, etc.
On the one hand, the CMP (Chemical-Mechanical Polishing) method has emerged as a novel semiconductor process technology. The CMP technology was brought about by the achievement of plane insulating film devices and by the implementation of embedded structures comprising a material different from the material of the substrate.
Plane insulating film devices are formed by forming a highly accurate micropattern using the lithography technology. Together with the Si wafer attachment technology, etc., this achievement provides the potential for the implementation of three-dimensional ICs.
With respect to the implementation of embedded structures, conventionally, a technology is used where tungsten (W) is embedded in the multilayer wiring of an IC. Here, W is embedded in the grooves of an interlayer film by the CVD method and a surface thereof is planarized by etch-back. However, recently, the CMP method is used in the planarization process. The embedding technology can be applied in the Damascene process and elements separation. The CMP technology and its applications are described in detail in “CMP Science” published by Science Forum.
Next, the mechanism of the CMP technology is described. As shown in FIG. 17, a semiconductor wafer 252 is placed on an abrasive cloth 251 provided on a rotating table 250 and the uneven surface of the wafer 252 is leveled by lapping, polishing and chemical etching while causing a polishing material (slurry) 253 to flow. Planarization is obtained by a mechanical polishing process between a chemical reaction of a solvent included in the polishing material 253 and a polishing abrasive coating included in the abrasive cloth and the polishing material. Foamed polyurethane, non-woven cloth, etc. can be used as the abrasive cloth 251 , whereas the polishing material is a material comprising polishing abrasive coating such as silica, alumina, etc. mixed with water comprising a pH adjuster, and is generally known as slurry. Lapping is carried out by applying a constant pressure while rotating the wafer 252 in the abrasive coat and causing the slurry 253 to flow. A dresser 254 has the function of maintaining the polishing capabilities of the abrasive cloth 251 and always keeps a surface of the abrasive cloth 251 in a dressed state. Motors 202 , 208 and 212 and belts 255 , 256 and 257 are also provided.
The above-described mechanism is constructed as a system, as shown in FIG. 18. This system can be divided in a wafer cassette, loading/unloading station 260 , a wafer reprint mechanical section 261 , a polishing mechanical section 262 as described with reference to FIG. 17, a wafer washing mechanical section 263 and a control system controlling all these elements.
First, the cassette 264 comprising wafers is placed in the wafer cassette loading/unloading station 260 and a wafer inside the cassette 264 is removed. Next, the wafer reprint mechanical section 261 holds the wafer with, for instance, a manipulator 265 and places it on the rotating table 250 provided in the polishing mechanical section 262 . Planarization of the wafer is carried out using the CMP technology. When the planarization process is finished, the wafer is fed to the washing mechanical section 263 by the manipulator 266 and is washed in order to wash away the slurry. The washed wafer is housed in a wafer cassette 266 .
The amount of slurry used in one process is, for instance, around 500 cc to 1 liter/wafer. Also, pure water is caused to flow in the polishing mechanical section 262 and wafer washing mechanical section 263 . Thus, the total amount of wastewater, at drainage, discharged in one planarization process is around 5 liters to 10 liters/wafer. For example, in case of a 3-layered metal, the planarization process is carried out about 7 times for metal planarization and interlayer dielectric film planarization, accordingly, the wastewater amount discharged until one wafer is completed is 5 to 10 liters multiplied by 7. Thus, it can be understood that by using the CMP apparatus, the amount of slurry diluted by pure water and discharged is quite large. The wastewater is then treated by the flocculation method.
However, with the flocculation method, chemicals are injected as flocculants. Nevertheless, it is extremely difficult to specify the amount of chemicals that fully react and a large amount of chemicals which do not react is left. By contrast, if the amount of chemicals is low, not all of the contaminants are coagulated and are left un-separated. In particular, if the amount of the chemicals is large, chemicals are left in the clear supernatant liquid and therefore, because chemicals remain in the filtered liquid, reuse thereof is impossible at when further chemical reactions need to be carried out.
Flocks which are reactions of the chemicals and contaminants are generated by the suspension of algae. The formation of flocks requires very strict pH conditions, an agitator, a pH measurement device, a flocculating agent implantation device, and a controlling device, for controlling all of these elements, etc. Also, in order to stabilize and cause precipitation of the flocks, a huge precipitation tank is required. For instance, for a wastewater processing capability of 3 m 3 /1 h, a tank having about 3 meters in diameter and about 4 meters in depth (around 15 tones) is needed, so that the entire system becomes a huge system requiring a compound of about 11 meters by 11 meters.
However, there are also flocks that keep floating without precipitating in the precipitation tank. It is therefore possible that they are discharged to the exterior and recovery thereof is very difficult. Due to size considerations, initial cost of this system is expensive, reuse of water is difficult and the running cost generated by the use of chemicals is very expensive.
In a method combining filter filtration (5 m 3 /1 h) and a centrifugal separator, as shown in FIG. 16, a filter F (it is called a UF module and comprises polysulfone fiber or ceramic filter) is used in the filtration apparatus 203 which enables reuse of water. Four filters F are attached in the filtration apparatus 203 , one filter costing around 50000 yen and having a life span of no more than a year. Due to the fact that filter F is a pressurized filtration method, the filter clogs and the load to the motor of pump 202 increases thus requiring a high-capacity pump 202 . Moreover, ⅔ of the wastewater passing through the filter F are returned into the raw water tank 201 and because the wastewater containing removables is supplied using pump 202 , the inner walls of the pump 202 are chipped and the life span of the pump 202 is extremely short.
To summarize, running cost, including extremely high power consumption by the motor, high costs associated with replacement of the pump P and filter F, becomes extremely high.
Moreover, with the CMP method, in the dicing process, a very large amount of wastewater is discharged. Colloid slurry is distributed in the fluid, but due to a Brownian motion, they do not precipitate. Particles of the abrasive coating mixed in the slurry have a diameter of 10 thru 200 nm, in other words, they are very fine particles. Consequently, when the slurry comprising very fine abrasive coating is filtered through the filter, particles enter the holes provided in the filter causing clogging. The filter clogs very frequently so that it is impossible to filter a large amount of wastewater.
As can be understood from the above description, in order to remove as much as possible all materials that may cause damage to the environment and to recycle filtered fluid and removables separated in the filtration process, the wastewater filtration system becomes a huge system due to various additional apparatuses, thus triggering extremely high initial cost and running cost. Accordingly, wastewater treatment apparatuses until now, could not be easily installed and used for wastewater treatment.
When using a self-generated film (pre-coat filter) to filter the wastewater, control of the pump carrying out filtration is very difficult.
Furthermore, in order to recover the removables precipitated at the bottom of the tank, filtration process is temporarily halted in order to discharge the fluid inside the tank. This led to a reduction of filtration efficiency.
Also, when filtration is carried out in a filtration apparatus having a self-generated filter, there is no appropriate method of removing the self-generated film which clogged.
SUMMARY OF THE INVENTION
A first aspect of the present invention thereby provides a tank for housing a fluid containing removables, a filter apparatus for filtering the fluid, an air diffuser located under the filter apparatus and generating air bubbles inside the fluid, an air pump for supplying gas via an air pipe connected to the air diffuser, wherein the air pipe is provided with an adjustment valve regulated in advance so that a predetermined amount of gas is caused to pass, and a stop valve for blocking or releasing the gas that passes inside the air pipe.
A further aspect of the present invention provides a tank for housing a fluid containing colloidal removables, a filter apparatus formed of a first filter immersed inside the tank and a second filter comprising a gel film adhered to a surface of the first film, an air diffuser located under the filter apparatus and generating air bubbles inside the fluid, an air pump for supplying gas via an air pipe connected to the air diffuser, wherein the air pipe is provided with an adjustment valve regulated in advance so that a predetermined amount of gas is caused to pass, and a stop valve for blocking or releasing the gas that passes inside the air pipe.
The present invention enables to set the amount of gas generated from the air pipe to a desired value by using in combination the adjustment valve having the amount of gas passing therethrough regulated in advance and the stop valve for blocking of releasing the gas flow.
Another aspect of the present invention provides a tank for housing a fluid containing removables, a filter apparatus formed of a first filter immersed inside the tank and a second filter comprising removables deposited on a surface of the first filter, a pump connected to the filter apparatus via a pipe, wherein the second filter is formed by the passage of the fluid through the first filter by applying a suction pressure of the pump and the filter apparatus thus having the second filter formed filters the fluid by causing the fluid to pass by applying a suction pressure of the pump, wherein the suction pressure of the pump in a process of forming the second filter is larger than in a process of filtering the fluid.
A further aspect of the present invention provides a tank for housing a fluid containing colloidal removables, a filter apparatus formed of a first filter immersed inside the tank and a second filter comprising a gel film adhered to a surface of the first layer, a pump connected to the filter apparatus via a pipe, wherein the second filter is formed by the passage of the fluid through the first filter by applying a suction pressure of the pump and the filter apparatus thus having the second filter formed filters the fluid by causing the fluid to pass by applying a suction pressure of the pump, wherein the suction pressure of the pump in the process of forming the second filter is larger than in the process of filtering the fluid.
Accordingly, the present invention enables a smooth formation of a second filter which is a self-generated film and a smooth filtration by means of the second filter by regulating the suction pressure of a pump.
Another aspect of the present invention provides a tank for housing a fluid containing removables, a filter apparatus immersed inside the tank, a recovery tank for removables precipitation which communicates with a lower part of the tank via a valve, wherein the recovery tank is detachable from the tank by closing the valve and detaching the recovery tank from the tank to thus recover removables precipitated in the recovery tank.
A further aspect of the present invention provides a tank for housing a fluid containing colloidal removables, a filter apparatus formed of a first filter immersed inside the tank and a second filter comprising a gel film adhered to a surface of the first filter, a recovery tank for removables precipitation which communicates with a lower part of the tank via a valve, wherein the recovery tank is detachable from the tank by closing the valve and detaching the recovery tank from the tank to thus recover removables precipitated in the recovery tank.
Accordingly, in the present invention, a detachable recovery tank is provided in lower part of a tank carrying out a filtration process to enable recovery of the precipitated removables by simply detaching the recovery tank from the tank. It is also possible to recover the precipitated removables without halting the filtration operation.
Another aspect of the present invention provides a tank for housing a fluid containing removables, a filter apparatus formed of a first filter immersed inside the tank and a second filter comprising removables deposited on a surface of the first filter, a pump connected to the filter apparatus via a pipe, a peel cistern connected to the pipe and storing filtered water filtered by the filter apparatus, wherein the peel cistern is located at a level above the fluid level of the fluid contained in the tank and when the second filter clogs and the flow rate of the filtered water decreases, the second filter is removed by causing the filtered water accumulated in the peel cistern to flow back to the filter apparatus via the pipe.
A further aspect of the present invention provides a tank for housing a fluid containing colloidal removables, a filter apparatus formed of a first filter immersed inside the tank and a second filter comprising a gel film adhered to a surface of the first film, a pump connected to the filter apparatus via a pipe, a peel cistern connected to the pipe and storing filtered water filtered by the filter apparatus, wherein the peel cistern is located at a level above the fluid level of the fluid contained in the tank and when the second filter clogs and the flow rate of the filtered water decreases, the gelatinous second filter is removed by causing the filtered water accumulated in the peel cistern to flow back in the filter apparatus via the pipe.
Accordingly, when the second filter which is a self-generated film clogs, the second filter is removed by causing the filtered water accumulated in the peel cistern to flow back to the filter apparatus. Thus, the present structure enables the removal of the second filter by a very simple structure.
Typically, in order to remove particles equal to or smaller than 200 nm like the abrasive coating contained in CMP slurry, a filter film comprising holes smaller in size than the size of the particles is generally adopted. However, in the present invention, a gel film comprising removables is used as a filter and the numerous spaces formed in the filter are used as passages for the fluid. Here, the filter itself is an aggregation of removables particles and filtration characteristics thereof can be preserved by removing the removables causing the clogging from the filter. Even if the filter comprising the gel film clogs after prolonged filtration, the filter is regenerated and filtration can be continued for a long time.
With this invention, an air pipe connecting an air diffuser 54 that generates air bubbles inside raw water with an air pump 55 is provided with an adjustment valve and a stop valve. The adjustment valve is configured so that a desired predetermined amount of gas is caused to pass therethrough. Accordingly, the desired amount of gas can be provided to the air diffuser by opening or closing the stop valve.
The air pipe is provided with a plurality of paths, each of the paths being provided with an adjustment valve and a stop valve, respectively, for allowing a different amount of gas to pass therethrough. Thus, by opening only one of the stop valves and closing the other stop valves, a desired amount of gas can be provided to a raw water tank.
In a filtration apparatus where filtration is carried out by using a second filter which is a self-generated film, filtered water from a filter apparatus 53 is removed by applying the suction pressure of a pump that has an adjustable motor. In a process of forming the second filter which is a self-generating film, the motor is caused to rotate at a high speed, whereas in a process of filtration using the self-generated film thus formed, the motor can be caused to rotate at a low speed. Thus, in the process of forming the second filter, a second filter can be formed promptly, whereas in the process of filtration, destruction of the second filter by applying an excessive suction pressure is prevented.
Furthermore, in case the second filter is a gelatinous self-generating film, the gel can be kept at a desired degree of swelling by causing the motor to rotate at a low speed during the filtration process. Also, the gelatinous second filter can be prevented from entering in the holes provided in the first filter.
Removables concentrated in the filtration process can be precipitated in a recovery tank 15 which communicates with a lower part of a raw water tank 50 where filtration is carried out. Moreover, the recovery tank 15 is detachable from the filtration apparatus thus enabling recovery of the removables precipitated inside the recovery tank 15 while he filtration process is ongoing.
The raw water tank 50 is formed of a material having excellent water-shedding qualities and thus, the gelatinous removables consolidate by a tensile force of a surface thereof and are moved into the recovery tank 15 . The gelatinous removables can be prevented from adhering to inner walls of the raw water tank 50 .
The filtered water filtered by the filter apparatus 53 is accumulated in a peel cistern 70 and when the second filter which is a self-generated film clogs, the second filter is removed by causing the filtered water from the peel cistern 70 to flow back. A mechanism is thus provided for removing the second filter without the need for another pump, etc.
In order to remove microparticles mainly equal to or smaller than 0.15 μm like the abrasive coating contained in CMP slurry, a filter film comprising holes smaller in size than the size of the microparticles is generally adopted. However, the filtration apparatus hereby provided forms a gel film filter comprising removables of a colloidal solution and filtration is carried out without the need of a filter film comprising mainly holes equal to or smaller than 0.15 μm.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a filtration apparatus according to an embodiment of the invention.
FIG. 2 shows a filtration apparatus according to an embodiment of the invention.
FIG. 3 shows a filtration apparatus according to an embodiment of the invention.
FIG. 4 shows a filtration apparatus according to an embodiment of the invention.
FIG. 5 shows a concrete filtration apparatus according to an embodiment of the invention.
FIG. 6 illustrates the operation of a concrete filtration apparatus according to an embodiment of the invention.
FIG. 7 shows a filter apparatus according to an embodiment of the invention.
FIG. 8 shows a concrete filter apparatus according to an embodiment of the invention.
FIG. 9 illustrates a filter according to an embodiment of the invention.
FIG. 10 illustrates the operation principle of the filter.
FIG. 11A is a cross-sectional view and FIG. 11B is a characteristics diagram showing the conditions for the formation of the second filter film according to an embodiment of the invention.
FIG. 12 shows the characteristics of the second filter.
FIG. 13 shows regeneration of the filter apparatus according to an embodiment of the invention.
FIG. 14 shows the operation of a filtration apparatus according to an embodiment of the invention.
FIG. 15 shows filtration characteristics according to an embodiment of the invention.
FIG. 16 illustrates a conventional filtration system.
FIG. 17 shows a CMP apparatus.
FIG. 18 shows a CMP apparatus system.
DETAILED DESCRIPTION OF THE INVENTION
FIRST EMBODIMENT
Configuration, etc. of a filtration apparatus 20 according to a first embodiment of the invention is described referring to FIG. 1. The filtration apparatus 20 comprises a tank 50 for housing a fluid comprising removables, a filter apparatus 53 for filtering the fluid, an air diffuser 54 as an air diffusion means, located at a lower part of the filter apparatus 53 and generating bubbles inside the fluid, an air pump 55 connected to the air diffuser 54 via an air pipe 40 and supplying gas, wherein the air pipe 40 is provided with an adjustment valve 41 regulated in advance so that a predetermined amount of gas is caused to pass, and a stop valve 42 for blocking or releasing the gas that passes inside the air pipe 40 . A more detailed description of the configuration of the filtration apparatus 20 is next provided.
A raw water tank 50 as shown in FIG. 1 is provided in an upper part thereof with a pipe 51 as means for supplying wastewater. The pipe 51 supplies the fluid containing removables to the tank 50 . For instance, to better describe this using semiconductor-related terms, it is the pipe that supplies wastewater (raw water) containing removables of a colloidal solution which flows from dicing apparatuses, backgrinding apparatuses, mirror polishing apparatuses or CMP apparatuses. A further description is next given of the wastewater as wastewater comprising abrasive coating flowing from the CMP apparatus and waste resulted from polishing or grinding by the abrasive coating. As shown in FIG. 1, the fluid temporarily stored in a wastewater reservoir 17 may also be supplied to the raw water tank 50 via the pipe 51 .
A plurality of filter apparatuses 53 comprising a second filter are placed in the raw water 52 accumulated in the raw water tank 50 . An air diffuser 54 , similar for instance to a bubbling apparatus used in fish aquariums and having small holes opened in a pipe thereof, is placed at a lower part of the filter apparatus 53 and the position thereof is adjusted so that air bubbles generated therefrom can pass through a surface of the filter apparatus 53 . The air diffuser 54 is located along the entire bottom of the filter apparatus 53 so that air bubbles can be uniformly supplied in the entire filter apparatus 53 . An air pump 55 and the air diffuser 54 are connected via an air pipe 40 .
The raw water 52 supplied from the pipe 51 is accumulated in the raw water tank 50 and is filtered by the filter apparatus 53 . The bubbles pass through a surface of a second filter 2 that adheres to the filter apparatus 53 thus generating a parallel flow by the climbing power and the burst of the air bubbles, which moves the gelatinized removables adhered to the second filter 2 so that they adhere uniformly to the entire filter apparatus 53 , thus preserving filtration capabilities.
The air pipe 40 connecting the air pump 55 and the air diffuser 54 is provided with an adjustment valve 41 and a stop valve 42 . The adjustment valve 41 is configured so that a desired amount of gas is allowed to pass therethrough, for example, a needle valve, etc. can be adopted as an adjustment valve. The stop valve 42 controls releasing and blocking of the gas flowing inside the air pipe 40 . More concretely, a valve, etc. using for instance solenoid, etc., can be employed as the stop valve 42 . A desired amount of gas can be supplied to the air diffuser by using in combination the adjustment valve 41 A and the stop valve 42 A, that is, open only stop valve 42 A while fixing an output of the air pump 55 .
Furthermore, a plurality of parallel paths branch off from the air pipe 40 . In more detail, a first path 40 A, a second path 40 B and a third path 40 C parallel to each other branch off from the air pipe 40 . Each of these paths is provided with adjustment valve 41 and stop valve 42 , respectively.
A first adjustment valve 41 A and a first stop valve 42 A are provided in the first path 40 A. The first adjustment valve 41 A is regulated so that an appropriate amount of gas is allowed to pass during the filtration operation of the filter apparatus 53 . During the filtration process using the filter apparatus 53 or during the formation process of the gelatinous second filter, the first stop valve 24 A is opened. Also, when the first stop valve 42 A is in an open state, the second stop valve 42 B and the third stop valve 42 C are all in a close state. It is thereby possible, in the filtration process, to supply a moderate amount of gas from the air diffuser 54 via the first adjustment valve 41 A thus regulated. Accordingly, the raw water in the raw water tank 50 is mixed by the air bubbles rising from the air diffuser 54 , thus enabling a smooth filtration.
The second adjustment valve 41 B and the second stop valve 42 B are provided in the second path 40 B. The second adjustment valve 41 B is set to allow the passage of a larger amount of gas than the first adjustment valve 41 A. The second stop valve 42 B is opened during the process of removing (regeneration process) the gelatinous second filter from the first filter, both filters forming the filter apparatus 53 . The second filter can be removed by supplying a large amount of gas from the air diffuser 54 into the raw water. When the second stop valve 42 B is in an open state, the first stop valve 42 A and the third stop valve 42 C are in a close state.
The third adjustment valve 41 C and the third stop valve 42 C are provided in the third path 40 C. The third adjustment valve 41 C is set to allow the passage of a smaller amount of gas than the first adjustment valve 41 A and the second adjustment valve 41 B. The third stop valve 42 C is opened when the operation of the entire filtration apparatus 20 is halted. When the third stop valve 42 C is in an open state, the first stop valve 42 A and the second stop valve 42 B are in a close state. Clogging of the air diffuser 54 can be prevented by maintaining the third stop valve 42 C in an open state when the filtration operation of the entire filtration apparatus is halted.
SECOND EMBODIMENT
Configuration of a filtration apparatus according to a second embodiment of the invention is basically the same as that described in the first embodiment, consequently, description is given only of the differences therebetween.
FIG. 2A is a schematic diagram of a filtration apparatus 20 according to a second embodiment of the present invention and FIG. 2B is a characteristics diagram showing variation in time of revolution speed of the motor and the suction pressure inside a pipe 56 .
The filtration apparatus 20 shown in FIG. 2A comprises a raw water tank 50 for housing a fluid containing removables, a filter apparatus 53 comprising a first filter immersed inside the raw water tank 50 and a second filter comprising removables deposited on a surface of the first filter, a pump 57 connected to the filter apparatus 53 via a pipe 56 , wherein the second filter is formed by the passage of the fluid through the first filter by applying a suction pressure from the pump 57 and the filter apparatus 53 thus having the second filter formed, filters the fluid by causing the fluid to pass by applying a suction pressure from the pump 57 , wherein the suction pressure of the pump 57 in the process of forming the second filter is larger than in the process of filtering the fluid.
The pump 57 is coupled with the filter apparatus 53 via the pipe 56 so that the fluid filtered in the filter apparatus 53 is discharged to the exterior by the suction power generated by the pump 57 . In the process of forming the second filter or in case the filtered fluid does not have the desired transparency, the fluid discharged from the filter apparatus 53 is returned to the raw water tank 50 . In other words, the filtration apparatus 20 according to this embodiment has a process allowing circulation of filtered water. A barometer 59 for detecting the pressure inside the pipe 56 is provided therein. The suction pressure of the pump is controlled by controlling the revolutions of the motor provided in the pipe, in response to the pressure inside the pipe 56 which is measured by means of the barometer 59 . A peel cistern 70 for temporarily storing the filtered liquid communicates with the pipe 56 .
Devices comprising an alternating-current motor having a revolution speed thereof controlled by an inverter circuit, or a direct-current motor having a revolution speed thereof controlled by a voltage applied thereto, etc. can be employed as the pump 57 . The use of a motor having a controllable revolution speed enables variation of the suction pressure applied to the filter apparatus 53 in the second filter formation process and in the filtration process. Particularly, in the present embodiment, the fluid is filtered using the gel film formed on the surface of the first filter. The use of a motor having a controllable revolution speed enables prevention of the second filter destruction which may be caused by the gel entering the holes provided in the first filter. A more detailed description of the gelatinous second filter is provided later.
Means other than variation of the motor revolution speed described above can be used as adjustment means for the suction pressure applied to the filter apparatus 53 . For instance, the suction pressure applied to the filter apparatus 53 can be adjusted by fixing the revolution speed of the motor that drives the pump 57 and providing a release valve in the pipe 56 . More concretely, the suction pressure can be decreased by opening the release valve or can be increased by closing the release valve. The suction pressure applied to the filter apparatus 53 can also be varied by moving the position of the pump 57 on a vertical direction.
Next, the relationship between the suction pressure applied to the filter apparatus 53 and the revolution speed of the motor provided in the pump 55 is described with reference to FIG. 2B. The abscissa axis shows the lapsed time, whereas the vertical axis on the left shows the suction pressure applied to the filter apparatus 53 , in other words, the increase in negative pressure. The vertical axis on the right shows the revolution speed of the motor operating the pump 57 . The broken line shows the variation of suction pressure whereas the continuous line shows variation of the motor revolution speed.
The filter apparatus 53 is immersed in the raw water 52 and by operating the pump 57 , a second filter which is a self-generated film is formed on a surface of the first filter. This is the process in which the second filter is formed. In this process, the revolution speed of the motor driving the pump 57 is increased as much as possible in order to allow a prompt formation of the second filter. Also, here, the discharged filtered water may be returned into the raw water tank 50 .
In the above process, the second filter which is a self-generated filter is gradually formed on the surface of the first filter of the filter apparatus 53 . The second filter comprises removables contained in the fluid and is provided with very fine holes compared to the holes provided in the first filter. Along with the formation of the second filter, the suction pressure gradually decreases. This suction pressure is monitored by the barometer 59 .
When the suction pressure has reached a predetermined value, it is determined that the formation of the second filter is complete and the filtration process is started. The revolution speed of the motor in the filtration process is slower than the revolution speed in above process where the second filter is formed. Accordingly, the suction pressure can be set to be equal or less than a predetermined value and destruction of the second filter by applying an excessive suction pressure thereto can be prevented.
Filtration capabilities of the second filter decrease with the progress of the filtration process. The second filter having decreased filtration capabilities is removed by causing the filtered water from the peel cistern 70 to flow back into the filter apparatus 53 . The filtered water which is caused to flow back into the filter apparatus 53 is then returned into the peel cistern 70 by operating the pump 57 . The suction pressure of the pump 57 at that time is set to be larger compared to that during the filtration process.
When the filtration operation is completed, before halting operation of the filtration apparatus 20 , the suction pressure is reduced compared to that during the filtration process and the pump is thus operated for a predetermined period of time. The load to the filtration apparatus 20 when operation thereof is restarted can thus be reduced.
THIRD EMBODIMENT
Configuration of a filtration apparatus according to a third embodiment of the invention is basically the same as that described in the first embodiment, consequently, description is given only of the differences therebetween.
The configuration, etc. of a filtration apparatus 20 according to a third embodiment of the invention is described with reference to FIG. 3, where FIG. 3A is a schematic diagram of a filtration apparatus 20 and FIG. 3B is a cross-sectional enlarged view of a tapered lower part of a raw water tank 50 .
The filtration apparatus 20 comprises a raw water tank 50 housing a fluid containing removables, a filter apparatus 53 immersed inside the raw water tank 50 , a recovery tank 15 that communicates with a lower part of the raw water tank 50 via a valve V and where removables precipitate, wherein the recovery tank 15 is detachable from the raw water tank 50 and the removables precipitated in the recovery tank 15 are recovered by closing the valve V and separating the recovery tank 15 from the raw water tank 50 .
The bottom of the raw water tank 50 is tapered and a lowermost part of the bottom communicates with the recovery tank 15 . By forming the bottom of the raw water tank 50 to have a tapered shape, it is possible to efficiently move the removables precipitated therein into the recovery tank 15 . It is preferable to employ resins such as polyvinyl chloride, etc. as the material of the raw water tank 50 . The steeper the tapered inclination, the greater the efficiency in recovering the precipitated removables in the recovery tank 15 . That is, the steeper the tapered inclination, the easier the removables move below the recovery tank 15 without adhering to the inner walls of the tapered parts of the raw water tank 50 .
Air diffusers S are provided in the vicinity of the tapered inner walls of the raw water tank 50 and are connected with an air pump 55 via an air pipe 40 . An air diffuser 54 has the function of generating air bubbles inside the raw water tank 52 and is operated during recovery of the removables accumulated in the recovery tank 15 . The removables that adhere to the flat walls of the raw water tank 50 are removed by the air bubbles generated by the air diffuser 54 and are moved inside the raw water tank 50 .
The recovery tank 15 communicates with a lower part of the raw water tank 50 via a valve V. That is, they communicate with each other when the valve V is in an open state and the communication thereinbetween is blocked when the valve V is in a close state.
A feed section K and a discharge section H are provided in the recovery tank 15 , each being provided with a valve. The feed section K is used when a large amount of removables are precipitated inside the recovery tank 15 . Air and fluid are supplied from the feed section K into the recovery tank 15 and the fluid containing removables can be discharged via the discharge section H. The valve provided in the discharge section H is opened to enable recovery of the fluid containing removables.
While the filter apparatus 53 is operating, the valve V is in an open state, so that when raw water 52 is supplied in the raw water tank 50 , the inside of the recovery tank 15 is also filled with raw water 52 . The recovery tank 15 may be formed of transparent materials so that the amount of removables precipitated inside the recovery tank 15 can be checked visually. The recovery tank 15 is detachable from the filtration apparatus 20 .
The recovery tank 15 is mounted on a transfer vehicle D, similar, for instance, to a carriage having wheels.
An air pump 55 and the air diffuser 54 are connected via an air pipe 40 . The recovery tank 15 communicating with a lower part of the raw water tank 50 is also provided.
The pump 57 is coupled with the filter apparatus 53 via pipe 56 so that the suction pressure generated by the pump 57 causes discharge of the fluid filtered by the filter apparatus 53 to the exterior.
The peel cistern 70 is coupled with the pipe 56 that extracts the filtered fluid from the filter apparatus 53 and stores it.
Next, operation of the filtration apparatus 20 according to this embodiment is described with reference to FIG. 3A and FIG. 3B.
First, raw water 52 is supplied into the raw water tank 50 . Then, the second filter which is a self-generating film is formed on the filtration surface of the first filter by allowing the passage of raw water 52 into the filter apparatus 53 by applying a suction pressure from the pump 55 . At this stage, the fluid that passes through the pipe 56 is not sufficiently filtered and may therefore be returned to raw water 52 . At this stage, the valve V is in an open state so that raw water is also supplied in the recovery tank 15 .
Next, the filtration process of the raw water 52 is started using the filter apparatus 53 which has a second filter 2 sufficiently formed thereon. At this stage, the filtered water obtained from the filter apparatus 53 has sufficient transparency and accordingly, it can be discharged to the exterior of the filtration apparatus 20 . A part of the filtered water is stored in the peel cistern 70 .
As the filtration process progresses, the second filter 2 gradually clogs and the amount of the filtered water that can be obtained is reduced. Next, a process of removing the second filter 2 is carried out. First, the pump 55 which applies a suction pressure to the filter apparatus 53 is halted. Then, the filtered water accumulated in the peel cistern 70 is caused to flow back to the filter apparatus 53 via the pump 56 . The filtered water is caused to flow back into center (hollow area) 5 of the filter apparatus 53 so that the pressure acting from inside to outside is applied to the filter apparatus 53 . This pressure removes the second filter 2 from the filter 1 and moves it downward. In order to facilitate the movement of the second filter, a large amount of air bubbles are generated from the air diffuser 54 .
The removed second filter is moved to the recovery tank 15 via the bottom of the tapered raw water tank 50 , as shown in FIG. 3B. As described, the raw water tank 50 is made of a material which has excellent water-shedding qualities. Accordingly, residues of the second filter comprising gelatinous removables solidify based on the surface tensile force. The so-solidified removables are moved to the recovery tank 15 sliding down the inner walls of the tapered raw water tank 50 .
When a certain amount of removables are accumulated inside the recovery tank 15 , they are recovered. First, air bubbles are generated from the air diffusers S to cause removables adhered to the inner walls of the raw water tank 50 to precipitate in the recovery tank 15 . Next, the valve V is closed and the recovery tank 15 is detached from the raw water tank 50 . The recovery tank 15 is then detached from the filtration apparatus 20 and raw water 52 accumulated therein and containing a large amount of removables is discharged. After that, the recovery tank 15 is attached to the filtration apparatus 20 and the valve V is closed so that raw water inside the raw water tank 50 is supplied inside the recovery tank 15 and the recovery tank 15 is once again filled with raw water 52 . The transfer vehicle D is used to move the recovery tank 15 when the raw water inside the recovery tank 15 is discharged. The recovery operation of the removables can be carried out while the filter apparatus 53 is in operation.
FOURTH EMBODIMENT
Configuration of a filtration apparatus according to a fourth embodiment of the invention is basically the same as that described in the first embodiment, consequently, description is given only of the differences therebetween.
FIG. 4A is a schematic diagram of a filtration apparatus 20 according to this embodiment and FIG. 4B is a cross-sectional enlarged view of a filter apparatus.
The filtration apparatus 20 as shown in FIG. 4A comprises a raw water tank 50 housing a fluid containing removables, a recovery tank communicating with a lower part of the raw water tank 50 , a filter apparatus 53 immersed inside the raw water tank 50 and comprising a first filter and a second filter containing removables deposited on a surface thereof, a pump 57 connected to the filter apparatus 53 via a pipe 56 , a peel cistern 70 connected with the pipe 56 and storing filtered water filtered by the filter apparatus 53 , wherein the peel cistern 70 is located at a level above the fluid level of the fluid contained in the raw water tank and when the second filter 2 clogs and the flow rate of the filtered water decreases, the second filter 2 is removed by causing the filtered water accumulated in the peel cistern 70 to flow back into the filter apparatus 53 via the pipe 56 .
The filter apparatus 53 as shown in FIG. 4B comprises a flat membrane first filter 1 and a second filter 2 which is a self-generated film formed on a surface thereof. Here, a fluid containing very fine colloidal removables is employed as raw water so that the second filter 2 has a gelatinous aspect. A more detailed description of the filter apparatus 53 is given later.
Raw water 52 supplied from a pipe 51 is accumulated in the raw water tank 50 where it is filtered by the filter apparatus 53 . A parallel flow which is generated by the climbing power and the bursting of air bubbles passing through a surface of the second filter 2 attached to the filter apparatus 53 , moves the gelatinous removables adhered to the second filter, thus causing the removables to attach uniformly the entire filter apparatus 53 so that filtration capabilities thereof are maintained.
Pump 57 is connected to the filter apparatus 53 via the pipe 56 . The fluid filtered in the filter apparatus 53 is discharged to the exterior by applying a suction pressure from the pump 57 . The fluid removed from the filter apparatus 53 is returned into the raw water tank 50 during the second filter formation process or in case the filtered fluid does not have a desired degree of transparency. In other words, the filter apparatus 53 of this embodiment has a process of causing the filtered water to circulate. A cistern 70 for peel purposes temporarily storing filtered fluid is provided so as to communicate with the pope 56 .
The peel cistern 70 is coupled to the pipe 56 which extracts filtered fluid from the filter apparatus 53 and stores it. The capacity of the peel cistern 70 is equal or more than the sum of the halves of the inner product of each filter apparatus 53 immersed in the raw water 52 . More explicitly, in the process of removing the second filter, the capacity of the peel cistern 70 is configured so that a quantity of filtered water equal to or larger than half of the inner product of each filter apparatus 52 can be caused to flow back to each filter apparatus 53 . The peel cistern 70 is located at a level above the fluid level of the raw water 52 . The filtered fluid accumulated in the peel cistern 70 is caused to flow back inside the filter apparatus 53 by the hydraulic pressure generated by the positional relationship of the peel cistern 70 and the raw water 52 .
Next, operation of the filtration apparatus 20 according to this embodiment is described with reference to FIG. 4A and FIG. 4B.
First, raw water 52 is supplied into the raw water tank 50 . The second filter 2 which is a self-generated film is formed on the filtration surface of the first filter 1 by causing the raw water 52 to pass through the filter apparatus 53 by applying a suction power from the pump 55 . At this stage, the fluid that passes through the pipe 56 is not sufficiently filtered and is therefore turned to raw water 52 . Also, the filtered water at this stage is not stored in the peel cistern 70 .
Next, the raw water 52 is filtered using the filter apparatus 53 having a second filter 2 sufficiently formed. At this stage, the filtered water obtained from the filter apparatus 53 has a sufficient degree of transparency and can therefore be discharged outside the filtration apparatus 20 . Also, a part of the filtered water is stored in the peel cistern 70 .
As the filtration process progresses, the second filter 2 gradually clogs, as shown in FIG. 4B, and the amount of filtered water decreases. A process of removing the second filter 2 is then carried out. First, the pump 55 which inputs a suction power to the filter apparatus 53 is halted so that the filtered water stored in the peel cistern 70 is caused to flow back into the filter apparatus via the pipe 56 . The filtered water is caused to flow back into center 5 of the filter apparatus 53 so that the pressure acting from inside to outside is applied to the filter apparatus 53 . This pressure removes the second filter 2 from the first filter 1 and moves it downward. In order to facilitate the movement of the second filter, a large amount of air bubbles are generated from an air diffuser 54 .
Merits of the above described configuration are described below. In this embodiment, the peel cistern 70 is positioned at a level above the liquid level of the raw water 52 where the filter apparatus 53 is immersed. The filtered water accumulated in the peel cistern 70 can thus be caused to flow back inside the filter apparatus 53 by the hydraulic pressure generated by the positional relationship of the peel cistern 70 and the raw water 52 , without the need to use additional pumps.
Furthermore, clogging inside the first filter 1 can be prevented by causing pure filtered water filtered by the filter apparatus 53 to flow back. The holes provided in the first filter 1 are very fine, therefore, in case tap water or the like is used in the flow-back process, removables contained in the tap water cause the first filter 1 to clog from the inside. In this embodiment, clogging of the first filter 1 during the flow-back process can be prevented and cost of filtration can be reduced by using, in the flow-back process, filtered water generated by the filter apparatus 53 itself. Distilled water, etc. can also be supplied into the peel cistern, instead of filtered water.
FIFTH EMBODIMENT
A concrete example of a filtration apparatus is next described with reference to FIG. 5, wherein configuration elements which are same as those of the filtration apparatuses shown in FIGS. 1 thru 4 are denoted by the same numerical symbols.
In FIG. 5, a pipe 51 is provided as a means for wastewater supply above a raw water tank 50 . The pipe 51 supplies a fluid containing removables into the raw water tank 50 . For instance, to better describe this using semiconductor-related terms, it is the pipe that supplies wastewater (raw water) containing removables of a colloidal solution which flows from dicing apparatuses, backgrinding apparatuses, mirror polishing apparatuses or CMP apparatuses. A further description is next given of the wastewater as wastewater comprising abrasive coating flowing from the CMP apparatus and waste resulted from polishing or grinding by the abrasive coating.
An adjustment valve 41 and a stop valve 42 are provided in an air pipe that connects an air pump 55 and an air diffuser 54 . The adjustment valve 41 is configured so that a desired amount of gas is allowed to pass therethrough, for example, a needle valve, etc. can be adopted as an adjustment valve. The stop valve 42 controls releasing and blocking of the gas flowing inside the air pipe 40 . More concretely, a valve, etc. using for instance solenoid, etc., can be employed as the stop valve 42 . A desired amount of gas can thus be supplied to the air diffuser 54 by using in combination an adjustment valve 41 A and the stop valve 42 , that is, only open/close stop valve 42 while fixing an output of the air pump 55 .
Furthermore, a plurality of parallel paths branch off from the air pipe 40 . In more detail, a first path 40 A, a second path 40 B and a third path 40 C parallel to each other branch off from the air pipe 40 . Each of these paths is provided with the adjustment valve 41 and stop valve 42 , respectively.
A first adjustment valve 41 A and a first stop valve 42 A are provided in the first path 40 A. The first adjustment valve 41 A is regulated so that an appropriate amount of gas is allowed to pass during the filtration operation of the filter apparatus 53 . During the filtration process using the filter apparatus 53 or during the formation process of the gelatinous second filter, the first stop valve 24 A is opened. Also, when the first stop valve 42 A is in an open state, the second stop valve 42 B and the third stop valve 42 C are both in a close state. It is thereby possible, in the filtration process, to supply a moderate amount of gas from the air diffuser 54 via the first adjustment valve 41 A thus regulated. Accordingly, the raw water in the raw water tank 50 is mixed by the air bubbles rising from the air diffuser 54 , so that a smooth filtration is enabled.
The second adjustment valve 41 B and the second stop valve 42 B are provided in the second path 40 B. The second adjustment valve 41 B is set to allow the passage of a larger amount of gas than the first adjustment valve 41 A. The second stop valve 42 B is opened during the process of removing (regeneration process) the gelatinous second filter from the first filter, both filters forming the filter apparatus 53 . The second filter can be removed by supplying a large amount of gas from the air diffuser 54 into the raw water. When the second stop valve 42 B is in an open state, the first stop valve 42 A and the third stop valve 42 C are in a close state.
The third adjustment valve 41 C and the third stop valve 42 C are provided in the third path 40 C. The third adjustment valve 41 C is set to allow the passage of a smaller amount of gas than the first adjustment valve 41 A and the second adjustment valve 41 B. The third stop valve 42 C is opened when the operation of the entire filtration apparatus 20 is halted. When the third stop valve 42 C is in an open state, the first stop valve 42 A and the second stop valve 42 B are in a close state. Clogging of the air diffuser 54 can be prevented by maintaining the third stop valve 42 C in an open state when the filtration operation of the entire filtration apparatus 20 is halted.
The pipe 56 , wherein filtered fluid filtered in the filter apparatus 53 flows, is connected to a magnet pump 57 which suctions via a valve V 1 . A pipe 58 connects the magnet pump 57 with a valve V 3 and a valve V 4 via a control valve CV 1 . A first barometer 59 is provided after the valve V 1 of the pipe 56 for measuring a suction pressure Pin. A flowmeter F and a second barometer 60 are provided after the control valve CV 1 of the pipe 58 ,wherein the flowmeter 61 controls a constant flow rate. A control valve CV 2 controls the flow rate of the air from the air pump 55 .
A plurality of filter apparatuses 53 comprising a second filter are placed in the raw water 52 accumulated in the raw water tank 50 . An air diffuser 54 , similar for instance to a bubbling apparatus used in fish aquariums and having small holes opened in a pipe thereof, is placed at a lower part of the filter apparatus 53 and the position thereof is adjusted so that air bubbles generated therefrom can pass through a surface of the filter apparatus 53 . An air pump 55 supplies gas to an air diffuser 54 via an air flowmeter 69 . An air pump 40 connecting the air diffuser 54 and the air pump 55 is provided with the above-described first path 40 A, the second path 40 B and the third path 40 C. Also, the first stop valve 42 A, the second stop valve 42 B and the third stop valve 42 C are electrically connected with the controller 68 .
The pipe 56 fixed in the filter apparatus 53 and circulated by filtered fluid filtered by the filter apparatus 53 is connected to the magnet pump 57 which suctions via a valve V 1 . The pipe 58 connects the magnet pump 57 and valve V 3 and valve V 4 via the control valve CV 1 . A first barometer 59 is provided after the valve V 1 of the pipe 56 for measuring the suction pressure Pin. A flowmeter 61 and a second barometer 60 are provided after the control valve CV 1 of the pipe 58 ,wherein the flowmeter 61 controls a constant flow rate of the filtered water.
The pipe 58 is connected to an optical sensor 62 , and is conducted to pipes 63 and 64 branching off from the optical sensor 62 . Pipes 63 and 64 contain valves V 3 and V 4 , respectively, which are opened/closed based on a detection signal from the optical sensor 62 , wherein pipe 63 returns filtered water to the raw water tank 50 and pipe 64 discharges filtered water to the exterior. The optical sensor 62 monitors the concentration of the microparticles contained in the filtered water to determine that the mix rate of the microparticles is smaller than a desired value, after which filtration is started. When the filtration process is started, the valve V 3 is closed based on a detection signal from the optical sensor 62 and the valve V 4 is opened to discharge pure water to the exterior.
The peel cistern 70 is connected to the pipe 58 via a valve V 5 and has a function of storing the filtered water, which, when exceeding a constant value, overflows and is returned into the raw water tank 70 via a pipe 71 . A valve V 2 is provided at a lower part of the pipe 71 which is connected to the pipe 56 . The peel cistern is positioned about 10 to 20 cm above the fluid surface of the raw water tank 50 and is used during the regeneration process of the second filter.
A pH regulator 65 and a temperature adjusting device 66 are provided in the raw water tank 70 for adjusting in particular the pH of the CMP wastewater to a range of 6 to 7 and for adjusting the temperature of the wastewater for facilitating the gel formation process. A liquid level meter 67 monitors the level of the wastewater in the raw water tank 50 to prevent overflow thereof and adjusts the inlet flow of the wastewater.
Furthermore, a controlling device 68 for controlling the operation of the filtration apparatus is provided. This controls the control valve CV 1 , the flowmeters 61 and 69 , the pump 57 , the barometers 59 and 60 , the optical sensor 62 , etc. in the processes where they are used, respectively, as shown by the dashed line.
In the second filter formation process, the filtration process, the second filter regeneration process, the re-filtration process and the maintenance process, the controlling device 68 opens/closes the valves, etc. and controls the operation of the pump 57 , etc. The operation status thereof in each of the processes is described below. FIG. 6 shows the operation status of the pump 57 , the optical sensor 62 , the air pump 55 and each of the valves in each respective process.
First, wastewater containing removables of a colloidal solution is supplied into the raw water tank 50 via a pipe 51 . A number of filter apparatuses comprising only a first filter 1 without a second filter 2 formed thereon are immersed in the raw water tank leaving a space thereamongst so that a desired filtration flow rate can be obtained. Concretely, figures here do not illustrate around 10 to 40 filter apparatuses but these apparatuses are mounted in a support means. The number of the filter apparatuses 53 differs depending on the filtration surface of one filter apparatus 53 , and the total filtration surface of the filter apparatuses 53 depends on the size of the raw water tank 50 .
Next, the second filter 2 formation process is carried out. The wastewater is caused to circulate inside the raw water tank 50 while suctioning with a weak suction pressure from the pump 57 via the pipe 56 . The circulation path includes the filter apparatus 53 , the pipe 56 , the valve V 1 , the pump 57 , the pipe 58 , the control valve CV 1 , the flowmeter 61 , the optical sensor 62 , and the valve V 3 . The wastewater is thereby suctioned from the raw water tank 50 and is returned to the raw water tank. Air bubbles supplied by the air pump 55 to the air diffuser 54 via the first path 40 A rise up to the surface of the filter apparatus 53 . In other words, the first stop valve 42 A is opened so that a desired amount of gas is supplied to the air diffuser 54 via the first adjustment valve 41 A. At this time, valves V 2 , V 4 , V 5 , V 6 and D are closed.
A second filter 2 is formed in the first filter 1 of the filter apparatus 53 by causing the wastewater to circulate so that the removables of a colloidal solution are finally captured (the concrete principle thereof is further described). When the wastewater is suctioned by the pump 57 through the first filter 1 using a weak suction pressure, the microparticles of removables are gelatinized as they get closer to the first filter and adhere to the surface of the first filter 1 . The gelatinous microparticles which are larger than filter holes 11 provided in the first filter 1 are absorbed thereby and therefore are gradually deposited on the surface of the first filter 1 , thus forming the second filter 2 which is a gel film. Gelatinous microparticles having a smaller diameter than the filter holes 11 pass through the first filter, but, together with the formation of the second filter 2 , the water inside the wastewater is suctioned using these spaces as passages, and then passes through the first filter 1 to be finally discharged to the outside as filtered, pure water.
At this time, the optical sensor 62 monitors the concentration of the microparticles contained in the filtered water and after it determines that the mix rate of the microparticles is lower than a desired value, the filtration process is started.
When the formation of the second filter 2 is completed, the filtration process is started. The valve V 3 is closed based on the detection signal from the optical sensor 62 , the valve V 4 is opened and the above-described circulation path is closed so that filtered water can be discharged from valve V 4 . In this process, the controlling device 68 controls the flowmeter 61 so that a constant filtration flow rate is set and clogging of the second filter 2 is prevented, thereby preserving filtration capabilities thereof for a long time. As shown in FIG. 12, the suction pressure Pin of the pump 57 is gradually increased to keep a constant filtration flow rate. The other elements have the same operation as in the filter formation process. In this process, gas is supplied to the air diffuser 54 via the first path 40 A.
When the second filter is damaged due to different causes, the optical sensor 62 detects the mix rate of the microparticles and the filtered water is returned into the raw water tank 50 by closing the valve V 4 and opening the valve V 3 . In other words, the second filter 2 is restored by returning to the filter formation process and after that, carrying out the re-filtration process.
The filtration process is continuously carried out and water inside the wastewater contained in the raw water tank 50 is discharged to the outside of the raw water tank 50 as filtered water thus raising concentration of the removables in the wastewater. More precisely, the colloidal solution is concentrated and the viscosity thereof is increased. Due to this, wastewater is supplied into the raw water tank 50 via a pipe 51 in order to suppress the rise in the wastewater concentration and improve filtration efficiency. However, a thick gel film adheres to the surface of the second filter 2 of the filter apparatus 53 and eventually causes clogging of the second filter 2 thus blocking filtration.
When a thick gel film adheres to the surface of the second filter 2 of the filter apparatus 53 , a decrease in the filtration flow rate is detected at the flowmeter 61 and the process of regenerating the second filter is started by the controlling device 68 .
In the filter regeneration process, the pump 57 is halted thus canceling the negative suction pressure applied to the filter apparatus 53 . At the same time, the valve V 2 is opened and the filtered water stored in the peel cistern 70 in advance is sent back into center 5 of the filter apparatus 53 via valve V 1 by reversing the flow in the pipe 56 .
Consequently, in the filter regeneration process, the weak suction pressure is stopped, thus returning to atmospheric pressure and the first filter 1 of the filter apparatus 53 is restored from a concave shape caused by the suction pressure to its original shape. The second filter 2 and the gel film adhered to a surface thereof similarly return to their original shape. As a result, the suction pressure causing the gel film to adhere disappears, thus causing the gel film to lose adsorbability to the filter apparatus 53 and to be influenced by a force triggering swelling of the film towards the exterior. Furthermore, the peel cistern is positioned at a level above the fluid level in the raw water tank 50 so that a hydrostatic pressure generated by the difference of elevation threbetween is applied by the back flow of the filtered water from the peel cistern 70 so that the first filter 1 and the second filter 2 of the filter apparatus 53 are caused to swell to the exterior. Accordingly, the gel film adhered thereto starts separating from the filter apparatus 53 by its own weight and by the hydrostatic pressure. Experiments show that separation begins from a lower end of the filter apparatus 53 and continues with the separation of the second filter 2 adhered to the surface of the first filter 1 , similar to an avalanche. Separated parts precipitate to the bottom of the raw water tank 50 . Next, water is caused to circulate in the circulation path described above and the second filter 2 is re-formed. In this regeneration process, the second filter 2 is restored to its original state enabling re-filtration of wastewater. At this time, the valve V 2 is closed and the valve V 5 is opened so that filtered water can be accumulated into the peel cistern 70 in view of the next filter regeneration process.
In order to facilitate the removal of the filter, the amount of air bubbles generated by the air diffuser 54 may be doubled. More precisely, the second stop valve 42 B is opened whereas the first stop valve 42 A and the third stop valve 42 C are closed.
The re-filtration process is then started and wastewater is filtered again. Operation is similar with the filtration process. After repeating the filtration process for a number of times while regenerating the second filter 2 , the concentration of the removables in the wastewater contained in the raw water tank 70 increases and eventually, the wastewater becomes more and more viscous. When the concentration of the removables of the wastewater exceeds a desired concentration value, filtration is halted and the maintenance process is carried out.
The maintenance process comprises the step of discharging the filtered water contained in the pipes 56 and 58 and in the peel cistern 70 and the step of discharging the wastewater contained in the raw water tank 50 and the gel accumulated at the bottom thereof. In the former step, pump 57 and air pump 55 are halted and control valve CV 1 , valves V 1 , V 2 and V 5 are opened so that the filtered water inside pipes 56 and 58 and in the peel cistern 70 is discharged to the outside via a discharge valve D provided in the pipe 56 .
In the latter stage, the concentrated slurry at the bottom of the raw water tank 50 is left to precipitate for flocculation purposes and is then recovered by opening valve V 6 . The so-recovered concentrated slurry is heat-dried to further concentrate it by vaporizing the water contained therein. Consequently, the amount of slurry handled as industrial waste can be substantially reduced. The supernatant wastewater is similarly discharged via valve V 6 and is fed back to the filtration process or is returned to the raw water tank 50 .
Next, an embodiment of the filter apparatus 53 as immersed inside the raw water tank 50 is described with reference to FIG. 7 and FIG. 8.
A frame 30 having a frame-like shape as shown in FIG. 7A is provided on both surfaces thereof with filter films 31 and 32 which form the first filter. Inner spaces 33 surrounded by the frame 30 , the filter films 31 and 32 which filtrate the wastewater by applying a pressure from the pipe 34 . The filtered water is then discharged via a pipe 34 sealed to the frame 30 . The filter films 31 and 32 and the frame 30 are perfectly sealed so that wastewater does not enter the spaces 33 .
Filter films 31 and 32 shown in FIG. 7A are thin resin films and may be warped or damaged when suctioned. Thus, in order to decrease this space as much as possible and increase filtration capabilities, it is necessary that these spaces 33 be formed larger. This is solved by the mechanism shown in FIG. 7B. In this figure, only 9 spaces 33 are illustrated, but in reality, a larger number of spaces 33 may be formed. Also, the filter film 31 actually used is a polyolefin polymer membrane having a thickness of about 0.1 mm, as shown in FIG. 7B by the thin pouched filter film FT. The frame 30 integrated with the pipe 34 is inserted in the pouched filter FT and these two are then secured together. The frame having the filter FT attached thereto is held from both sides by holding support RG. Filter FT is exposed from an opening OP provided in the holding support RG. A more detailed description is provided with reference to FIG. 8.
FIG. 7C shows a filter apparatus 53 having a cylindrical shape. The frame attached to the pipe 34 is cylindrical and is provided on a side surface thereof with openings OP 1 and OP 2 . Due to the fact that parts of the side surface corresponding to openings OP 1 and OP 2 are removed, support SUS is provided between the openings for supporting the filter film 31 . Next, the filter film 31 is attached to the side surface.
The filter apparatus 53 illustrated in FIG. 7B is described in more detail with reference to FIG. 8. A part 30 a corresponding to the frame 30 from FIG. 7B is described referring to FIG. 8A and FIG. 8B. At a first sight, part 30 a is similar to a cardboard. Thin resin sheets SHT 1 and SHT 2 having about 0.2 mm each are stacked and a plurality of sections SC are provided thereinbetween in a vertical direction so that a space 33 surrounded by the resin films SHT 1 , SHT 2 and the section SC is formed. The cross-section of this space 33 is a rectangle sized 3 mm×4 mm, in other words, a plurality of straws having a rectangular cross-section are aligned and integrated. Part 30 a is called a spacer in the description below because it maintains the filter film FT on both sides at a certain interval.
A plurality of holes HL of 1 mm in diameter are provided on the surface of the thin resin sheet SHT 1 and SHT 2 forming the spacer 30 a and the filter film FT is attached to the surface. Accordingly, the filtered water filtered by the filter film FT passes through the holes HL and the space 33 and is finally discharged using the pipe 34 .
The filter film FT is also attached to both surfaces SHT 1 and SHT 2 of the spacer 30 a . Surfaces SHT 1 and SHT 2 of the spacer 30 a comprise parts where no holes HL are formed, so that when filter film FT 1 is attached directly thereto, filter film FT 1 corresponding to parts where such holes HL are not formed has no filtration capabilities and therefore does not allow the passage of wastewater. Consequently, parts which do not capture removables are formed. In order to prevent this phenomenon, at least 2 filter films FT are provided. The outermost filter film TF 1 is a filter film that captures removables. A filter film FT 2 is also provided comprising holes which are formed to be larger than the holes provided in the filter film FT 1 as it draws near the surface SHT 1 of the spacer 30 a . Consequently, the entire filter film FT 1 is enabled with filtration capabilities even in parts where holes HL of spacer 30 are not formed, because of the existence of filter film FT 2 allowing removables to be captured on the entire surface of the filter film FT 1 , so that a second filter film is formed on the entire surfaces SH 1 and SH 2 of both surfaces. In this figure, for the sake of convenience, filter film SHT 1 and SHT 2 are formed as a rectangular sheet, but in reality they are pouched as shown in FIG. 7B.
Next, a description is given of how the pouched filter films SHT 1 , SHT 2 , the spacer 30 a and the holding support RG are attached referring to FIG. 8A, FIG. 8C and FIG. 8D.
FIG. 8A is a completion drawing, FIG. 8C is a figure showing the apparatus described in FIG. 8A along the A-A line, that is from the head of the pipe 34 in the extending direction (vertical direction) of the pipe, FIG. 8D is a sectional view taken along the B-B line, that is, in a horizontal direction of the filter apparatus 35 .
As can be understood from FIG. 8A, FIG. 8C and FIG. 8D, spacer 30 a inserted in the pouched filter film FT, including the filter film FT are held on 4 sides thereof by the holding support RG. The pouched 3 sides and the remaining side are secured by an adhesive agent AD 1 applied to the holding support RG. A space SP is formed between the remaining side (openings) and the holding support RG so that filtered water generated in the space 33 is suctioned by the pipe 34 via the space SP. An adhesive AD 2 is applied in the entire opening OP of the holding support RG so that it is perfectly sealed and fluid is prevented from entering therein.
Space 33 communicates with pipe 34 , so that when suction is applied through pipe 34 , the fluid passes via the holes of the filter film FT and the holes HL of the spacer 30 a and is directed to the spaces 33 . The filtered water can then be discharged to the exterior via the pipe 34 .
The filter apparatus 53 used here has a configuration as the one described in FIG. 8 and the size of the frame (holding support RG) for attaching the filter film is A4 size, more precisely, about 19 cm by 28.2 cm, and 5 to 10 mm in thickness. The filter apparatus 53 is actually on both sides of the frame so that the surface doubles (surface: 0.109 m 2 ). However, the number and size of the filter apparatus is freely selected depending on the size of the raw water tank 50 and on the filtration rate required.
Next, the principle of filtering raw water using the gelatinous second filter is described. At first, terms to be used in the following description are defined.
A colloidal solution refers to a medium having microparticles with a diameter of 1 nm to 1 μm dispersed therein. These microparticles have a Brownian motion and can pass through a common filter paper but cannot pass through a semipermeable membrane. Because electrostatic repulsive force works between very fine particles having an extremely slow coagulation rate, chances that they come close to each other are reduced.
Sol is used as a substantial synonym for colloidal solution, but sol is different from gel in that it is dispersed in the medium and shows mobility, so that the microparticles have an energetic Brownian motion.
Gel refers to the colloidal particles which have lost their independent mobility and gather together to solidify. For instance, when agar or gelatin is melted down in warm water, they disperse and form a gel. When this gel is cooled down, the resulted gel loses mobility. Some types of gel can include hydrogel comprising a large amount of liquid and xerogel dried to some degree.
Removal and drying of water from a dispersion medium, addition of electrolyte salts to silica slurry (pH 9 to 10) and adjustment of pH up to pH 6 or 7, loss of mobility due to cooling down are just a few of the causes leading to gelatinization.
Slurry refers to the colloidal solution or the sol used in polishing and contains particles, fluid and chemicals. Abrasives used in the CMP process described above are called CMP slurry. CMP slurry is known to include silica abrasive, aluminum oxide (alumina) abrasive, cerium oxide (ceria) abrasive, etc. The abrasive used most frequently is silica abrasive, one type thereof which is largely used is colloidal silica. Colloidal silica refers to a dispersion liquid, called silica sol, wherein very fine silica particles with a colloid size of 7 to 30 nm are dispersed homogeneously in a water or organic liquid medium without precipitating. Particles of the colloidal silica are monodispersed in water so that even if the colloidal silica is left unattended for more than one year, the colloid particles do not precipitate almost at all due to their mutual force of repulsion.
The resent invention provides a method of removing removables from wastewater by filtering the removables from wastewater which comprises removables in a colloid fluid or sol.
Removables are a colloid fluid (sol) containing a large amount of microparticles having a particle size distribution of 3 nm to 2 μm, such as for example, silica and alumina used in the CMP process or abrasive coating such as ceria, etc. and semiconductor material waste, metal waste and/or insulation film material waste generated by grinding with abrasive coating. Here, slurry (W 2000) from tungsten polishing from Cabot Micro Electronics Co. is used as CMP slurry. The slurry mainly comprises silica having a pH of 2.5 and a particle size distribution of 10 to 200 nm.
The basic principle of the present invention is described with reference to FIG. 9. With the present invention, a fluid (wastewater) containing removables of a colloid solution (sol) is filtered using a filter comprising a gel film formed of removables.
More concretely, a gel film as a second filter 2 is formed of CMP slurry comprising removables of a colloid solution on a surface of a first filter 1 formed of organic polymers. Filters 1 and 2 are immersed inside a tank containing fluid 3 where they carry out filtration of wastewater containing removables.
Organic polymers or ceramic can be used in principle as materials for the first filter 1 considering that a gel film is to adhere thereto. Here, a polyolefin polymer membrane having a thickness of 0.1 mm and an average hole diameter of 0.25 μm is used. FIG. 10B shows a picture of a surface of the polyolefin filter film.
The first filer 1 , which is a flat membrane provided on both sides of a frame 4 , is immersed perpendicularly in the fluid and wastewater is suctioned by a pump 6 from center 5 of the frame 4 . Filtrate 7 is then discharged.
Next, a second filter 2 , which is a gel film, adheres to the entire surface of the first filter 1 by suctioning and gelatinizing the sol containing removables. Typically, a gel film is a gel which is believed to have no filtration capabilities. However, with this invention, the gel film is endowed with filtration capabilities by selecting the gel film formation conditions. These conditions are described later.
After the second filter 2 which is a gel film made of removables of a colloid solution (sol) is formed, filtration of the removables is carried out. This is described with reference to FIG. 9 and FIG. 10A.
The first filter 1 is provided with filter holes 11 . The film formed as a layer in the openings of the filter holes 11 and on the surface of the first filter 1 is a gel film formed of removables 13 . Removables 13 are absorbed via the first filter by applying a suction pressure from a pump, moisture from the fluid 3 is removed by drying (dehydration) so that microparticles of the removables from the colloid fluid merge together and gelatinize. This gel film corresponds to the second filter 2 which cannot pass through the filter holes 11 .
When the second filter 2 reaches a predetermined film thickness, spaces are formed therein which do not allow the passage of gel removables and filtration of removables of colloid substance is started using this second filter 2 . After the filtration process is performed a number of times while suctioning by pump 6 , the gel film on the surface of the second filter 2 gradually stacks and thickens, so that eventually the second filter 2 clogs and filtration is impeded. While the colloid solution is gelatinized, water from the colloid solution that adhered to the surface of the second filter passes through the first filter and is discharged as filtered water.
FIG. 10A shows wastewater containing colloid fluid with removables, on one side of the first filter 1 and filtered water which passed through the first filter 1 , on the other side thereof. Wastewater is suctioned and flows in the direction shown by the arrows. Microparticles in the colloid solution lose their electrostatic repulsive force as they get closer to the first filter 1 and are gelatinized. The second filter 2 is formed by the gel film comprising some microparticles that merged together and adheres to the surface of the first filter 1 . Wastewater is filtered by means of the second filter 2 while gelatinizing removables inside the colloidal solution. Filtered water is suctioned from the opposite side of the first filter 1 .
The water inside the wastewater is thus discharged by smoothly suctioning wastewater of the colloid solution via the second filter 2 and removables are captured to form the gel film by drying, gelatinizing them so that they stack on the surface of second filter.
Next, second filter 2 formation conditions and the subsequent filtration rate are described with reference to FIG. 11.
This invention provides a process for forming a second filter 2 and a process for re-filtration. The purified water filtration rate during filtration largely varies depending on the second filter 2 formation conditions. Accordingly, it was determined that when purification conditions of the second filter 2 are not properly selected, filtration using the gelatinous second filter 2 is almost impossible. This fact coincides with the conventional art, where it was known that filtration of colloid solution is impossible.
The characteristics shown in FIG. 11B are all proved by experiments carried out according to the method shown in FIG. 11A. A first filter 1 is provided at the bottom of a cylindrical container 21 and a concentrate solution 50 cc of a tungsten polishing slurry 22 (W 2000) from Cabot Micro Electronics Co. is supplied thereto. Formation of the gel film is carried out by varying the suction pressure. Next, the slurry 22 that remained is removed and after supplying 100 cc of purified water 23 , filtration is carried out at a very low suction pressure. It is thus possible to determine the filtration characteristics of the gelatinous second filter 2 . The first filter 1 used at this time has a diameter of 47 mm and has a surface of 1734 mm 2 .
In the gel film formation process shown in FIG. 11B, gel film characteristics when the suction pressure is varied among −55 cm Hg, −30 cm Hg, −10 cm Hg, −5 cm Hg, −2 cm Hg and the formation process lasts for 120 minutes were examined. Results show that filtration rate is best when suction pressure is set to −55 H, so that in 2 hours the filtered amount is 16 cc, then decreasing in turn to 12.5 cc, 7.5 cc, 6 cc, 4.5 cc.
Next, purified water is supplied and filtration is carried out using this gel film. The suction pressure at this time is set to −10 cm Hg. With the gel film formed at a suction pressure of −55 cm Hg, only 0.75 cc/h can be filtered, with a gel film formed at a suction pressure of −30 cm Hg, filtration rate is about 1 cc/h. However, with a gel film formed at −10 cm Hg, filtration rate is 25 cc/h, with a gel film at −5 cm Hg, filtration rate is 3.25 cc/h and with a gel film at −2 cm Hg, filtration rate is 3.1 cc/h. Accordingly, even with a gel film formed at a very low suction pressure, filtration is stable. From these experimental results it is determined that if the suction pressure in the second filter 2 formation process is set so that the filtration rate is about 3 cc/h, the filtration rate in the subsequent filtration process is the largest.
The reason for this is that, if the suction pressure is strong, the so-formed gel film has a low degree of swelling and solidifies due to compactness because the film is formed to be concentrated and thus contains a very small amount of water. This is probably because paths used by pure water to pass disappear.
When the suction pressure is decreased, the so-formed gel film has a high degree of swelling and it is soft due to a decreased density because the film contains a large amount of water and is formed in a state of swelling. Thus, a large number of paths allowing the purified water to pass therethrough can be obtained. The principle is more easily understood when imagining the snow powder slowly falling and thickening on the ground. One of the aspects of this invention is to provide a filtration method using a gel film formed at a very low suction pressure and having a high degree of swelling by causing moisture to pass through the gel film.
Characteristics of the gel film are described with reference to FIG. 12. FIG. 12 A shows the relationship between the sol amount contained in the gel film and the filtration rate. The amount of removed sol shows the sol amount captured by the first filter from the filtered amount during the gel film formation process when purified water having a slurry concentration of 3% is used. It is believed that the sol amount is an amount that gelatinizes and adheres as the second filter by drying by suction. It is also determined that a