JP5381781B2 - Cleaning method of ultrapure water production system - Google Patents

Description

  The present invention relates to a method for cleaning an ultrapure water manufacturing system used in a semiconductor manufacturing process or the like, and more particularly, to a cleaning method for an ultrapure water manufacturing system capable of quickly reducing the metal concentration in ultrapure water after cleaning. .

In a cleaning process in the field of semiconductor manufacturing or the like, ultrapure water is used as cleaning water. The ultrapure water is required not to contain fine particles that cause cleaning trouble, organic matter, and inorganic matter, for example, resistivity: 18.2 MΩ · cm or more, 0.05 μm diameter fine particles: 1 piece / mL or less, Live bacteria: 1 / L or less, TOC (Total Organic Carbon)
The required water quality is 1 μg / L or less, metals: 1 ng / L or less, and ions: 1 ng / L or less.

  The use location (use point) of ultrapure water is connected to the ultrapure water production equipment by piping (flow path), and the remaining ultra pure water not used at this use point passes through the other flow path. By returning to the pure water production apparatus, a circulation system is formed, and an ultrapure water production system is configured as a whole.

  When an ultrapure water production system is newly constructed or suspended for a long period of time, impurities are mixed into the system and the quality of the ultrapure water is deteriorated. Therefore, it is necessary to perform appropriate cleaning.

  In particular, when the above system is newly installed in connection with the construction of a factory, fine particles and dirt (organic matter, metal, etc.) are mixed into the system during the construction, and therefore the cleaning work for removing this is prolonged (for example, 1 month), the factory utilization rate will drop.

  For this reason, there is a demand for shortening the time from cleaning the ultrapure water production system to obtaining ultrapure water that satisfies the required water quality (vertical startup of the ultrapure water production system). In order to improve the cleaning efficiency, for example, a basic cleaning solution such as tetramethylammonium hydroxide (TMAH) or hydrogen peroxide water is used as the cleaning water (Japanese Patent Laid-Open No. 2000-317413, specially disclosed). JP 2004-122020, JP 2007-260211). Japanese Patent Application Laid-Open No. 7-195073 proposes a cleaning method using alcohol having a large cleaning power.

  However, these cleaning methods are not suitable for sufficiently removing the metal.

  In paragraphs 0043 and 0045 of the above Japanese Patent Application Laid-Open No. 2004-122020, it is described that an aqueous solution of an acid such as hydrochloric acid, sulfuric acid, nitric acid or the like having a pH of 4 or less is used as a cleaning liquid for an ultrapure water production system. In the publication, it is described that the cleaning liquid passes through the ion exchange apparatus so as to bypass the ion exchange apparatus at the time of cleaning with the basic cleaning liquid or the acid cleaning liquid. This is to prevent the base or acid from being ion-exchanged with the ion-exchange resin (paragraph 0050 of the same publication).

JP-A-7-195073 JP 2000-317413 A Japanese Patent Laid-Open No. 2004-122020 JP 2007-260211 A

  As described above, cleaning with hydrogen peroxide, a basic cleaning solution, or alcohol is not suitable for removing metal impurities from an ultrapure water production system.

  When an acid aqueous solution is used as in JP-A-2004-122020, the metal can be efficiently removed from the ultrapure water production system. However, as can be seen in the test examples in the Examples section described later, the turbidity membrane device such as an ultrafiltration membrane device (UF device) can be subjected to rinsing (extrusion cleaning with pure water) after acid cleaning. Is difficult to remove, and it takes a very long time to completely wash out the acid, and the total cleaning time becomes extremely long.

  The present invention solves such problems and provides a cleaning method for an ultrapure water production system that can efficiently remove metal impurities from the ultrapure water production system and that requires a short total cleaning time including rinsing. The purpose is to do.

A cleaning method for an ultrapure water production system of the present invention (claim 1) includes a subsystem for processing primary pure water to produce ultrapure water, a use point, and a flow path connecting the subsystem and the use point. The sub-system is a method for cleaning an ultra-pure water production system comprising a turbidity-filtering device and an ion exchange device. A sub-system acid cleaning step for cleaning with the contained liquid, wherein the sub-system acid cleaning step passes the mineral acid-containing liquid through the turbidity membrane filtration device. .

The method of cleaning an ultrapure water production system according to claim 2, in claim 1, in the previous SL subsystem acid washing step, passing liquid a mineral acid-containing solution to bypass the removal Nigomaku filtration device and said ion exchange device It is characterized by doing.

  The method for cleaning an ultrapure water production system according to claim 3 is the method according to claim 2, wherein the subsystem includes a degassing device, and in the subsystem acid cleaning step, the mineral acid-containing liquid is removed from the turbidity membrane filtration device. The ion exchange device and the degassing device are bypassed to pass through the liquid.

  The method of cleaning an ultrapure water production system according to claim 4 is characterized in that in any one of claims 1 to 3, at least one of the use point and the flow path is further subjected to acid cleaning. .

  The ultrapure water production system cleaning method according to claim 5 is the method for cleaning ultrapure water according to claim 4, wherein the mineral acid-containing liquid is passed through the subsystem in the subsystem acid cleaning step and flows out of the subsystem. The liquid is circulated through the use point and the flow path.

  The method for cleaning an ultrapure water production system according to claim 6 is characterized in that, in any one of claims 1 to 5, the mineral acid is hydrochloric acid and the concentration thereof is 0.0001 to 10 wt%. It is.

  In the cleaning method of the ultrapure water production system of the present invention, the subsystem is cleaned with the mineral acid-containing liquid, so that metal impurities can be efficiently removed from the subsystem. In the present invention, since the mineral acid-containing liquid is not passed through the turbidity membrane filtration device in this subsystem acid cleaning step, the extrusion cleaning time of the mineral acid-containing liquid after the mineral acid cleaning is short, and the total cleaning time is reduced. It is enough in a short time. Further, it is possible to prevent the membrane of the turbidity membrane filtration device from being deteriorated by acid.

  In the present invention, the mineral acid-containing liquid may be bypassed between the ion exchange device or the ion exchange device and the degassing device in the subsystem acid cleaning step. By the mineral acid-containing liquid bypassing the ion exchange device, the mineral acid is prevented from being ion-exchanged. In addition, the mineral acid-containing liquid bypasses the degassing device, thereby eliminating the retention of the mineral acid-containing liquid in the degassing device, reducing the extrusion cleaning time of the mineral acid-containing liquid, and reducing the total cleaning time. Time is enough. Further, the degassing device is prevented from being deteriorated by acid.

  In the present invention, in the subsystem acid cleaning step, the mineral acid-containing liquid is passed in the direction of passing the subsystem, and the mineral acid-containing liquid flowing out from the subsystem is circulated through the use channel and the connection channel with the use point. It is preferable to do. Thereby, the metal impurities in the use point and the channel are also efficiently removed.

It is a systematic diagram which shows embodiment of the washing | cleaning method of the ultrapure water manufacturing system of this invention. It is a graph which shows the result of an Example and a comparative example. It is a graph which shows the result of a test example.

  Embodiments of a cleaning method for an ultrapure water production system according to the present invention will be described below with reference to the drawings. In this embodiment, an ultrafiltration membrane device is used as a turbidity membrane device as a final filter, but a microfiltration membrane device or the like may be used.

  FIG. 1 is a system diagram showing an embodiment of a cleaning method for an ultrapure water production system according to the present invention. The ultrapure water production system 1 includes a subsystem 2, a use point 4 of ultrapure water, and ultrapure water channels 6a and 6b connecting them. Then, the ultrapure water produced by the subsystem 2 is sent to the use point 4 through the flow path 6a and a part thereof is used at the use point 4, and the unused ultrapure water passes through the flow path 6b. Return to subsystem 2.

  The subsystem (secondary pure water device) 2 includes a primary pure water tank 21, a pump 22, a heat exchanger 23, an ultraviolet oxidation device 24, a degassing device 25, an ion exchange device 26, an ultrafiltration membrane device 27, and these. The primary pure water from the primary pure water tank 21 is pumped out by a pump 22, heated by a heat exchanger 23, and then subjected to an ultraviolet irradiation treatment by an ultraviolet oxidation device 24. Then, the organic substance is decomposed into an organic acid, degassed by the degassing device 25, deionized by the ion exchange device (ion exchange resin tower in this embodiment) 26, and then subjected to an ultrafiltration membrane device (UF device). 27), for example, ultrapure water that satisfies the above-mentioned required water quality is produced by removing the fine particles. As the degassing device 25, a vacuum degassing device, a membrane degassing device, a nitrogen degassing device, or the like can be used.

  The primary pure water 10 supplied to the tank 21 is obtained by treating raw water with, for example, a reverse osmosis membrane, sequentially performing treatment with an anionic and cationic ion exchange resin, and further performing a reverse osmosis membrane treatment. However, the manufacturing method of primary pure water is not limited to this. For example, vacuum degassing treatment may be performed, and the reverse osmosis membrane treatment may be performed once.

  In this embodiment, a primary pure water tank 21 is provided on the inlet side of the subsystem 2 and accommodates the primary pure water 10 and unused ultrapure water returned from the use point 4. The pure water stored in the tank 21 is sent to the heat exchanger 23 via the pump 22 and the above-described series of processing is performed. In some cases, a reverse osmosis membrane or other membrane processing apparatus is incorporated in the subsystem 2.

  The use point 4 indicates a place where ultrapure water is used, and may include pipes, nozzles, and the like as appropriate in addition to a cleaning device (cleaning tank) 4a for cleaning an object (for example, a semiconductor). Note that the ultrapure water used at the use point 4 is appropriately collected as drainage.

  The ultrapure water flow paths 6a and 6b connecting the subsystem 2 and the use point 4 are basically composed of pipes or tubes. In the present invention, tanks, pumps, joints, and The term “flow path” includes valves and other equipment. The material used for the channels 6a and 6b may be any material that does not elute into ultrapure water. For example, PVC (polyvinyl chloride), PPS (polyphenylene sulfide), PVDF (polyvinyl difluoride), FRP (fiber reinforced plastic), PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer), stainless steel and the like can be used.

  In this embodiment, the bypass line 30 that bypasses the degassing device 25, the bypass line 31 that bypasses the ion exchange resin tower 26, and the UF device 27 are bypassed as bypass lines that pass water during the acid cleaning of the subsystem 2. A bypass line 32 is provided. Although not shown, each bypass line and connection line are provided with a valve for switching between the flow path selection for flowing the liquid through the bypass line and the flow path selection through the device and its connection line without passing through the bypass line. ing.

  When acid cleaning of the ultrapure water production system is performed, an acid is preferably added to the tank 21 to obtain a predetermined pH. As the acid, a mineral acid such as hydrochloric acid, sulfuric acid, nitric acid, preferably hydrochloric acid or sulfuric acid, particularly preferably hydrochloric acid is used, and is added so that the pH in the tank 21 is 5 or less, for example, 5-1. In the case of hydrochloric acid, it is preferably added so that the concentration in the tank 21 is 0.0001 to 10 wt%, particularly 0.01 to 1 wt%.

  In addition to the acid, a surfactant or the like may be further added. As the surfactant, any of an anionic surfactant, a cationic surfactant, an amphoteric surfactant and a nonionic surfactant can be used. Anionic surfactants, particularly alkyl sulfates, can be used. Among these, those having 10 to 14 carbon atoms, particularly those having 12 carbon atoms (sodium dodecyl sulfate (SDS)), sodium cholate (SC) and the like are preferable. These surfactants can be used by mixing them, and can also be used in combination with other surfactants. The surfactant used in combination may be an anionic surfactant, a cationic surfactant, an amphoteric surfactant, a nonionic surfactant, or a dispersing agent such as a polymer, DNA or protein.

  The acid-containing liquid in the tank 21 is sent out toward the heat exchanger 23 using the pump 22 in the same manner as in the production of ultrapure water, and the acid cleaning process of the ultrapure water cleaning system 1 is performed.

  In this acid cleaning step, an acid-containing liquid is passed so that at least the ion exchange resin tower 26 and the UF device 27 are bypassed by the bypass lines 31 and 32. This prevents the acid from being ion exchanged. Further, since the ultrafiltration membrane of the UF apparatus does not come into contact with the acid, the cleaning time of the entire system (particularly, the extrusion cleaning time after the acid cleaning) can be shortened. Further, in this embodiment, since the degassing device 25 is also bypassed by the bypass line 30 at the time of this acid cleaning, the effect of shortening the cleaning time can be obtained as in the case of UF. However, in the acid cleaning step, an acid-containing liquid may be passed through the degassing device 25. In this case, pure water is also passed through the degassing device 25 in the subsequent extrusion cleaning step.

  In the acid cleaning step, it is possible to increase the cleaning power by heating the acid-containing liquid by the heat exchanger 23 so that the temperature becomes as high as possible without exceeding the heat resistance temperature of the members and pipes constituting the ultrapure water production system. It is preferable at the point which carries out, and it is good to heat to 20-100 degreeC specifically ,. For example, when PVC having a heat-resistant temperature of about 45 ° C. is used as a constituent material, the temperature of the acid-containing liquid is about 40 ° C., and in the case of PVDF having a heat-resistant temperature of about 80 ° C., the temperature of the acid-containing liquid is 75-80. It may be set to ° C. Further, when stainless steel is used as the constituent material, it can be cleaned with an acid-containing liquid having a temperature of about 100 ° C.

  In the acid cleaning step, the acid-containing liquid in the tank 21 is sent out by the pump 22 and circulated through the subsystem 2, the use point 4 and the flow paths 6a and 6b. After the liquid existing in these is replaced with the acid-containing liquid, It is preferable that the pump 22 is stopped and the wetted part is kept in contact with the acid-containing liquid for a predetermined time (for example, about 1 to 24 hours, particularly about 3 to 6 hours). At the beginning of cleaning, the acid-containing liquid is sent out from the tank 21 and ultrapure water flows into the tank 21 from the flow path 6b, so that the acid-containing liquid in the tank 21 is diluted. It is preferable to add an acid to 21 to keep the acid-containing liquid in the tank 21 at a pH or acid concentration within the above range.

  Thereafter, the residual liquid in the ultrapure water production system is extruded and washed with primary pure water or ultrapure water. Specifically, preferably, the acid-containing liquid in the tank 21 is discharged, the tank 21 is washed with primary pure water, the primary pure water is stored in the tank 21, and then the primary pure water is removed. The pump 22 sends the heat to the heat exchanger 23, and the subsystem 2, the use point 4, and the flow paths 6a and 6b are pushed and washed. Also at this time, the primary pure water passes through the bypass lines 30, 31 and 32. The extruded cleaning waste water from the flow path 6b is preferably discharged after being brought into contact with an ion exchange resin (not shown). Extrusion cleaning is performed sufficiently until the acid in the extrusion cleaning wastewater falls below the detection limit or below the specified concentration.

  As described above, by acid cleaning the ultrapure water production system, metals (for example, Ca, Al, Fe, Ni, Zn), organic substances, fine particles, and viable bacteria in the flow path of the system are removed. In the present invention, since the cleaning liquid is an acid-containing liquid, metal impurities in the flow path can be efficiently removed.

  In the above description, the subsystem 2, the flow paths 6a and 6b, and the use point 4 are all cleaned with acid, but only the subsystem 2 may be cleaned with acid, and only the subsystem 2 and the flow path 6a are cleaned with acid. Alternatively, only the subsystem 2 and the use point 4 may be acid-washed, or only the subsystem 2 and the flow paths 6a and 6b may be acid-washed. The subsystem 2, the flow path 6a, and the use point 4 may be used. Only the acid may be washed. However, as in this embodiment, it is preferable that the subsystem 2, the flow paths 6a and 6b, and the use point 4 are acid cleaned.

  In the above description, the acid is added to the tank 21, but the acid may be added at other locations.

  In the present invention, sterilization cleaning may be performed during the above-described extrusion cleaning or before the extrusion cleaning, by injecting hydrogen peroxide or ozone into the system and circulating the liquid in the system.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. In the following examples and comparative examples, a vacuum degassing apparatus was used as the degassing apparatus.

Example 1
The ultrapure water production system shown in FIG. 1 was cleaned as follows.

  First, an acid-containing liquid (hydrochloric acid aqueous solution) having a pH of 3.5 containing 1000 mg / L of hydrochloric acid is accommodated in the tank 21 of the subsystem 2, and the heat exchanger 23, the ultraviolet oxidizer 24, the bypass lines 30, 31, 32 are pumped by the pump 22. Then, 1 Hr circulation water was passed in the order of the flow path 6 a, the use point 4, the flow path 6 b, and the tank 21. The temperature after heating by the heat exchanger 23 was 25 ° C. In this acid cleaning step, the acid-containing liquid was passed through the bypass lines 30, 31, and 32, and was not passed through the degassing device 25, the ion exchange tower 26, and the UF device 27. Thereafter, the pump 22 was stopped and immersion was performed for 12 hours, and the system was washed with hydrochloric acid (circulation: 1 hour, immersion 12 hours).

  Next, primary pure water is accommodated in the tank 21, and this primary pure water is pumped by the heat exchanger 23, the ultraviolet oxidizer 24, the bypass lines 30, 31, 32, the flow path 6a, the use point in the same manner as described above. 4 and the flow path 6b were passed through 6Hr in order to extrude the acid-containing liquid. The washing waste water was discharged out of the system from the end of 6b, passed through a strongly basic anion exchange resin, and discharged after adsorption treatment.

  After the cleaning was completed, normal operation was performed, and the Ca concentration in the ultrapure water at the use point 11 was measured. The change with time is shown in FIG.

Example 2
The tank 21 of the subsystem 2 contains an acid-containing liquid (hydrochloric acid aqueous solution) containing hydrochloric acid at 10 mg / L and having a pH of 5.5, and the pump 22 heats the heat exchanger 23, ultraviolet oxidizer 24, bypass lines 30, 31, 32. Then, 1 Hr circulation water was passed in the order of the flow path 6 a, the use point 4, the flow path 6 b, and the tank 21. The temperature after heating by the heat exchanger 23 was 25 ° C. In this acid cleaning step, the acid-containing liquid was passed through the bypass lines 30, 31, and 32, and was not passed through the degassing device 25, the ion exchange tower 26, and the UF device 27. Thereafter, the pump 22 was stopped and immersion was performed for 12 hours, and the system was washed with hydrochloric acid (circulation: 1 hour, immersion: 12 hours).

Example 3
The ultrapure water production apparatus was cleaned using nitric acid instead of hydrochloric acid under the same conditions as in Example 1 (circulation: 1 Hr, immersion 12 Hr).

Comparative Example 1
In Example 1, the system was washed using a carbon dioxide gas solution instead of the hydrochloric acid aqueous solution. That is, hydrochloric acid is not added to the tank 21 and primary pure water is kept stored. Instead, carbon dioxide gas is supplied to the vacuum side of the degassing device 25 with the pump 22 operated, and the carbon dioxide concentration is 100 mg / day. The carbon dioxide aqueous solution of L is circulated for 1 hour in the order of the bypass lines 31 and 32, the flow path 6a, the use point 4, the flow path 6b, the tank 21, the pump 22, the heat exchanger 23, the ultraviolet oxidation device 24, and the degassing device 25. The pump 22 and the carbon dioxide gas supply were stopped, immersion was performed for 12 hours, and the system was cleaned (circulation: 1 hour, immersion 12 hours). Thus, in Comparative Example 1, the carbonic acid solution was bypassed by the bypass lines 31 and 32 through the ion exchange resin tower 26 and the UF device 27, respectively.

  Next, the carbonic acid solution was extruded in the same manner as in Example 1. The carbonic acid-containing wastewater from the inside of the system was discharged after passing through a strongly basic anion exchange resin to perform an adsorption treatment.

  After the cleaning was completed, normal operation was performed, and the Ca concentration in the ultrapure water at the use point 11 was measured. The change with time is shown in FIG.

Comparative Examples 2 and 3
The cleaning liquid was replaced with primary pure water at 25 ° C. (Comparative Example 2) or 40 ° C. (Comparative Example 3), and the ultrapure water production system was cleaned. In this case, the primary pure water is passed through the tank 21, the pump 22, the heat exchanger 23, the ultraviolet oxidation device 24, the bypass lines 30, 31, 32, the flow path 6a, the use point 4 and the flow path 6b in this order. It discharged | emitted from the terminal of the flow path 6b. The primary pure water was heated by the heat exchanger 23 so as to reach the above temperature.

  After the cleaning was completed, normal operation was performed, and the Ca concentration in the ultrapure water at the use point 11 was measured. The change with time is shown in FIG.

Comparative Example 4
The tank 21 of the subsystem 2 contains a liquid of pH = 11 containing 500 mg / L of TMAH, and the heat exchanger 23, the ultraviolet oxidizer 24, the bypass lines 30, 31, 32, the flow path 6a, the use point 4 by the pump 22 And 1Hr circulation water flow was carried out in order of channel 6b and tank 21. The temperature after heating by the heat exchanger 23 was 25 ° C. In this alkali cleaning step, the TMAH-containing liquid was passed through the bypass lines 30, 31, and 32, and was not passed through the degassing device 25, the ion exchange tower 26, and the UF device 27. Then, the pump 22 was stopped and 1Hr immersion was performed. Thereafter, the extrusion cleaning of TMAH was performed for 3 hours (pH <9 was confirmed), and a solution containing 0.1 wt% of hydrogen peroxide was stored in the tank 21 of the subsystem 2, and the heat exchanger 23, the ultraviolet oxidizer 24, The bypass lines 30, 31, 32, the flow path 6 a, the use point 4, the flow path 6 b, and the tank 21 were circulated through 1 Hr in order. The temperature after heating by the heat exchanger 23 was 25 ° C. In this hydrogen peroxide cleaning step, the hydrogen peroxide-containing liquid was passed through the pipelines 30, 31, and 32, and was not passed through the degassing device 25, the ion exchange tower 26, and the UF device 27. Then, the pump 22 was stopped and 12Hr immersion was performed.

  As shown in FIG. 2, the Ca concentration in the ultrapure water at the use point 4 decreases to below the detection limit (0.1 ng / L) in Example 1 in about one day. 2 days, 8 days for Comparative Example 2 and 5 days for Comparative Example 3 are required.

[Test example (Cl ^- ion removal test after hydrochloric acid cleaning of UF membrane)]
After passing hydrochloric acid through the UF device, ultrapure water was passed through, and the change over time in the Cl ⁻ ion concentration in the permeated water was measured. The UF apparatus used was OLT-6036 manufactured by Asahi Kasei, the hydrochloric acid concentration was 5 wt%, 1 wt% or 0.1 wt%, the liquid passing time was 3 hours, and the liquid passing speed was 15 m ³ / Hr / book. The flow rate of ultrapure water was 15 m ³ / Hr / tube. The results are shown in FIG.

As shown in FIG. 3, when the UF apparatus is washed with hydrochloric acid, it takes about 20 days to completely remove Cl ⁻ ions even when the hydrochloric acid concentration is 0.1 wt%. . The same was true when sulfuric acid was used instead of hydrochloric acid.

DESCRIPTION OF SYMBOLS 1 Ultrapure water production system 2 Subsystem 4 Use point 6a, 6b Flow path 21 Tank 22 Pump 23 Heat exchanger 24 Ultraviolet oxidation apparatus 25 Degassing apparatus 26 Ion exchange resin tower 27 Ultrafiltration membrane separation apparatus (UF apparatus)
30, 31, 32 Bypass line

Claims (6)

A method of cleaning an ultrapure water production system comprising a subsystem for treating primary pure water to produce ultrapure water, a use point, and a flow path connecting the subsystem and the use point, In the cleaning method of the ultrapure water production system, the subsystem is equipped with a turbidity membrane filtration device and an ion exchange device .
A subsystem acid cleaning step for cleaning the subsystem with a mineral acid-containing liquid;
In the sub-system acid cleaning step, the mineral acid-containing liquid is passed through the turbidation membrane filtration device and passed through the ultrapure water production system. According to claim 1, in the previous SL subsystem acid washing step, the cleaning method of the ultrapure water production system, which comprises passing liquid mineral acids-containing liquid to bypass the removal Nigomaku filtration device and said ion exchange device . The sub-system according to claim 2, wherein the sub-system includes a degassing device,
In the sub-system acid cleaning step, a mineral acid-containing liquid is passed through the turbidity membrane filtration device, the ion exchange device, and the degassing device, and passed through the ultrapure water production system.   4. The method for cleaning an ultrapure water production system according to claim 1, further comprising acid cleaning at least one of the use point and the flow path. 5.   5. The mineral acid-containing liquid is passed through the subsystem in the subsystem acid cleaning step in the subsystem flow direction, and the mineral acid-containing liquid flowing out from the subsystem is circulated through the use point and the flow path. A method for cleaning an ultrapure water production system.   6. The method for cleaning an ultrapure water production system according to any one of claims 1 to 5, wherein the mineral acid is hydrochloric acid and the concentration thereof is 0.0001 to 10 wt%.

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