JP2011051833A - Method for purification of alkaline aqueous solution - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for purification of an alkaline aqueous solution, which can remove metal impurities contained in an alkaline solution simply at a high rate, and sufficiently reduce residual metal impurities. <P>SOLUTION: The metal impurities are removed from the alkaline aqueous solution by making the alkaline aqueous solution containing metal impurities contact with a material to which at least either one selected from a N-methyl-glucamine group reacting as a chelating material to a fiber base material and an iminodiethanol group reacting as a weakly anion exchanger and a chelating material is introduced. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for purifying an alkaline aqueous solution capable of purifying the concentration of metal impurities contained in the alkaline aqueous solution to a very low concentration.

  The alkaline aqueous solution is used in the polishing process in combination with an etching process for removing a work-affected layer generated when a silicon wafer is produced, or an abrasive such as colloidal silica for pH adjustment and buffering purposes. Further, the alkaline aqueous solution is often used in wet cleaning after the polishing step. In the etching process, sodium hydroxide or potassium hydroxide, in the polishing process, sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, ammonia, sodium carbonate, sodium hydrogen carbonate, potassium carbonate and potassium hydrogen carbonate, in the cleaning process, Organic alkalis such as ammonia, tetramethylammonium hydroxide, choline, and choline derivatives are used.

  These alkaline aqueous solutions include a heating reaction step and a concentration step in the production process, and stainless steel members are generally used from the viewpoints of heat resistance and corrosion resistance. Iron, nickel, copper, chromium, etc., which are components of this stainless steel member, are mixed as metal impurities in the manufacturing process of the alkaline aqueous solution. Therefore, purification treatment is performed, and the mixed metal impurities are reduced to a concentration on the order of ppb, for example. It will be necessary.

  For example, sodium hydroxide is produced by electrolysis of sodium chloride, and the produced sodium hydroxide contains various metal impurities on the order of several ppm. Among these metal impurities, for example, copper and nickel permeate and remain in the silicon wafer to change its electrical characteristics and to deteriorate its surface flatness. Therefore, the alkaline aqueous solution containing these metal impurities cannot be used in the manufacturing process of the silicon wafer. Metals such as iron and chromium are difficult to diffuse inside the silicon wafer, but remain as residues on the surface, which also impedes electrical characteristics and the like.

  Therefore, in order to obtain an alkaline aqueous solution that can be used in an actual process such as etching, it is necessary to carry out a purification treatment to remove mixed metal impurities and reduce the metal impurity concentration to, for example, the order of ppb. become.

  As a method for purifying the metal impurities, for example, a method is proposed in which a high-concentration sodium hydroxide aqueous solution containing metal impurities is brought into contact with activated activated coconut shell activated carbon after treatment with nitric acid, and the metal impurities are adsorbed and removed. (Japanese Patent Laid-Open No. 2004-344715). In this case, the alkali can be easily purified at low cost. However, in this case, since it is necessary to prepare a dedicated activated carbon, the cost increases.

  Further, iron can be refined to about 100 ppb, but copper and nickel can only be refined to about 100 ppb, and the metal impurity concentration is not practical enough. Moreover, activated carbon elutes metal impurities such as zinc and aluminum contained in the material, and these are difficult to reduce even if activated by nitric acid. Accordingly, it is sufficient that the metal impurity removal requirement item is limited to iron, but it becomes difficult to use when there are many items such as copper and nickel.

  Furthermore, as a method for reducing metal impurities contained in a sodium hydroxide aqueous solution, a purification method by electrolysis of a sodium hydroxide aqueous solution using a cation exchange membrane is known (Japanese Patent Laid-Open No. 2002-317285). According to this method, it is said that the metal impurities in the sodium hydroxide aqueous solution can be 10 ppb or less. However, this method has the disadvantages of high capital investment, complicated equipment, high purification costs, and difficult operation management.

  On the other hand, when transporting a high-purity alkaline aqueous solution in which metal impurities are reduced to 100 ppb or less to a point of use in the same high-purity state, a transportation means such as an alkaline aqueous solution manufacturing plant or a tank truck, an alkaline aqueous solution is used. In the wafer manufacturing factory and semiconductor device manufacturing factory to be used, containers such as connection / supply piping and drums / tanks must be lined with expensive materials such as fluororesin, which has little metal elution, or manufactured using these materials. I must. Also, even if you want to recycle the alkaline aqueous solution used in the wafer manufacturing factory or semiconductor device manufacturing factory, it must be discarded because it is contaminated by the metal generated as a by-product from the manufacturing process and cannot be reused. Absent.

  Further, in the conventional purification method of an aqueous solution using a cation exchange resin used for removal of metal impurities, when an alkali component is contained in a high concentration, the alkali component is also separated from the resin in the same manner as the metal impurity. Due to the reaction, metal impurities cannot be selectively removed. Therefore, the concentration of the alkali component as the main component may change unless further appropriate treatment is performed.

JP 2004-344715 A JP 2002-317285 A

  In view of such conventional problems, the present invention provides a method for purifying an alkaline aqueous solution that can easily remove metal impurities contained in an alkaline aqueous solution at a high rate and sufficiently reduce residual metal impurities. With the goal.

  In order to achieve the above-mentioned object, the present invention brings an alkaline aqueous solution containing metal impurities into contact with at least one of a fibrous weakly basic anion exchanger and a chelating agent, and the metal impurities are removed from the alkaline aqueous solution. The present invention relates to a method for purifying an alkaline aqueous solution, which is characterized by removing.

  According to the present invention, when the alkaline aqueous solution is purified, the base material, that is, the weakly basic anion exchanger and the chelating material are in the form of fibers, so that the contact efficiency with the alkaline aqueous solution to be purified is increased, thereby The purification rate of the aqueous solution can be improved. Therefore, the flow rate SV of the alkaline aqueous solution used for the treatment can be 10 (1 / h) or more. Currently, sufficient processing can be performed even if the maximum value of the flow rate SV is increased to 50 (1 / h). On the other hand, in the prior art as described above, the flow rate SV of the alkaline aqueous solution used for the treatment can be increased only to about 2 (1 / h) at the maximum.

  In addition, as a weak base anion exchanger and a chelating material, what combined the weak basic anion exchange group or the chelate functional group with the alkali-resistant synthetic resin fiber can be used.

  The synthetic resin used as the base of the alkali-resistant synthetic resin fiber includes polyvinyl alcohol, styrene-divinylbenzene copolymer, polysulfone, polyphenylsulfone, polyhydroxymethacrylate, polyethylene, polypropylene, aramid, PFA (tetrafluoroethylene par Fluoroalkyl vinyl ether copolymer), PTFE (polytetrafluoroethylene) resin, etc. are exemplified, but in terms of versatility and price, polyvinyl alcohol, styrene-divinylbenzene copolymer, polyamide, polysulfone, polyphenylsulfone. Polyethylene and the like are preferred. These synthetic resins are not particularly limited as long as they can be used stably with respect to a high concentration aqueous alkali solution. These materials can be used alone or in combination of a plurality of types.

  In addition, the synthetic resin fiber mentioned above can be comprised as a nonwoven fabric, The average fiber diameter is 0.2 micrometers-20 micrometers, Comprising: A fiber filling factor can be 5%-45%. The nonwoven fabric can be formed by, for example, a melt blow method.

  Moreover, since the weakly basic anion exchanger and the chelating material are based on the synthetic resin as described above, when they are made into a fibrous form, a nonwoven fabric is formed from the synthetic resin, Thus, N-methylglucamine group and iminodiethanol group can be synthesized by introducing by radiation graft polymerization.

  Therefore, since the material used for the purification of the alkaline aqueous solution of the present invention can contain at least one of an N-methylglucamine group and an iminodiethanol group, metal impurities in the alkaline aqueous solution can be remarkably removed. This has been clarified through extensive experiments and is not clarified theoretically. At present, it is considered that the anion exchange reaction and hydrogen bond of the hydroxyl group or amine group of the functional group and the chelate reaction of amine group and imino group are involved.

  For example, metal impurities in an aqueous alkali solution are thought to form negatively charged hydroxide complex ions, and these complex ions exist sterically and electrically adsorb with weakly basic anion exchangers. This is because it is considered that the metal impurities are removed by the binding by the chelating action of the amine group. In addition, metals derived from stainless steel members used in the industrial synthesis process of alkaline aqueous solutions are expected to exist as a mixture of metal oxide species and metal oxide / metal hydroxide that are electrically neutral. It is thought that it is adsorbed and removed by a chelating reaction of a functional group that exerts a specific adsorption action on a semimetal such as a camin group or hydrogen bonding.

  The N-methylglucamine group and the iminodiethanol group can be used alone, or both can be used in combination. The N-methylglucamine group is effective mainly for the purification (removal) of iron, chromium, etc., but the degree of purification (removal) for copper, nickel, etc. is not so high. On the other hand, the iminodiethanol group is effective mainly for purification (removal) of copper, nickel, etc., but the degree of purification (removal) of iron, chromium, etc. is not so high. Therefore, by using both in combination, the degree of purification (removal) of heavy metals such as iron, chromium, copper, and nickel can be increased.

  Specifically, by using the functional group, the concentration of metal impurities such as iron and chromium in the alkaline aqueous solution after purification of metal impurities can be reduced to 5 ppb or less. In particular, by using an iminodiethanol group, the concentration of the metal impurity in the alkaline aqueous solution after purification of the metal impurity such as copper and nickel can be 0.5 ppb or less, further 0.1 ppb or less.

  The alkaline component in the alkaline aqueous solution of the present invention includes at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonia, sodium carbonate, sodium bicarbonate, potassium carbonate and potassium bicarbonate, and organic alkali. It can be illustrated. Examples of the organic alkali include tetramethylammonium hydroxide, choline, and choline derivatives (such as trimethylhydroxy 2-ethoxyammonium hydroxide and 2- (2-hydroxyethoxy) ethoxyammonium hydroxide). The illustrated alkaline component can be contained alone, or two or more can be mixed and contained. In addition to those exemplified here, any strong alkaline aqueous solution can be used without particular limitation.

  In the present invention, the material to be purified can be used after being processed into a cartridge filter shape or a membrane shape. In this case, for example, when a high-concentration alkali such as 50% by weight NaOH or 50% by weight KOH is used, or when the material is formed in a bead shape, for example, the synthetic resin constituting the material is It shrinks about 30%. Therefore, when the material is packed in a column or the like and used for purification of the alkaline aqueous solution, voids are generated in the column, and the alkaline aqueous solution is short-passed through the voids without passing through the material. Therefore, the aqueous alkali solution cannot be sufficiently purified.

  On the other hand, as described above, if the material to be refined is processed into a cartridge filter shape or a membrane shape, the material is fixed to the cartridge filter in the former case, and the membrane-like material is predetermined in the latter case. By fixing to a column by this method, or by setting the size to have a margin that takes into account the degree of shrinkage, it is possible to purify an aqueous alkali solution without producing voids even when high concentration alkali is used. It becomes like this.

  Therefore, according to the present invention, it is possible to purify an arbitrary aqueous alkali solution from a low concentration to a high concentration.

  The concentration of the aqueous alkaline solution in which the present invention is effective is 0.01 to 50% by weight for sodium hydroxide, 0.01 to 50% by weight for potassium hydroxide, 0.01 to 23% by weight for sodium carbonate, 0.01 to 50% by weight, sodium bicarbonate 0.01 to 8% by weight, potassium bicarbonate 0.01 to 50% by weight, ammonia 0.01 to 30% by weight, and tetramethylammonium hydroxide 0.8%. A concentration of 01% by weight or more, preferably 0.01% by weight or more for choline, more preferably 0.1 to 50% by weight for sodium hydroxide, 0.1 to 50% by weight for potassium hydroxide, and 0.1% for sodium carbonate. 1 to 23 wt%, potassium carbonate 0.1 to 50 wt%, sodium bicarbonate 0.1 to 50 wt%, potassium bicarbonate 0.01 to 50 wt% 0.1 to 30 wt% in ammonia, the tetramethylammonium hydroxide 0.1% by weight or more, the choline at a concentration of more than 0.1 wt%. In particular, 5-50% by weight for sodium hydroxide, 5-50% by weight for potassium hydroxide, 5-23% by weight for sodium carbonate, 5-50% by weight for potassium carbonate, 5-8% by weight for sodium bicarbonate, A high concentration region of 5 to 50% by weight for potassium hydrogen carbonate, 5 to 30% by weight for ammonia, 5% by weight or more for tetramethylammonium hydroxide, and 5% by weight or more for choline is preferable, and the present invention exhibits the most effect.

  As described above, according to the present invention, there is provided a method for purifying an alkaline aqueous solution that can easily remove metal impurities contained in an alkaline aqueous solution at a high rate and sufficiently reduce residual metal impurities. Can do.

  Next, examples of the present invention will be described.

  As the alkaline aqueous solution for the sample, a 48% sodium hydroxide aqueous solution manufactured by Tosoh Corporation and a 48% potassium hydroxide aqueous solution manufactured by Toagosei Co., Ltd. were used for purification by the following method. Prior to purification, a washing operation as described in detail below was also performed.

  Measurement of metal impurities was performed by ICP-MS (ELANDRC-II manufactured by PerkinElmer). In addition, the processing apparatus and processing body which were used by the following Examples and comparative examples regarding purification are as follows.

Processing equipment:
Processing body packed column 3/4 inch PFA column 200mm
(PFA: tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer)
Test system and tank material PTFE (Polytetrafluoroethylene)

Used treatment body:
(A) Chelate fiber
Base material: Polyethylene
Functional group: N-methylglucamine group
Shape: Fiber diameter 18μm Non-woven fabric
Introducing N-methylglucamine groups into polyethylene fibers by graft polymerization

(B) Weakly basic anion exchange fiber
Base material: Polyethylene
Functional group: Tertiary amine group
Shape: Fiber diameter 18μm Non-woven fabric
Introduction of iminodiethanol groups as tertiary amine groups into polyethylene fibers by graft polymerization

(C) Chelate resin:
Manufacturer: Mitsubishi Chemical Corporation
Product Name: Diaion CRB-02
Base material: Styrene
Functional group: N-methylglucamine group
Shape: Bead shape with a diameter of 350-550 μm

(D) Polyethylene fiber
Base material: Polyethylene
Functional group: None
Shape: Fiber diameter 18μm Non-woven fabric

(Chemicals used for cleaning)
Alkaline cleaning solution 1: 48 mass% sodium hydroxide aqueous solution (Asahi Glass Co., Ltd., trade name: Super Alkali)
Alkaline cleaning solution 2: 2.0N aqueous sodium hydroxide solution (prepared by mixing alkaline cleaning solution 1 and ultrapure water)
Alkaline cleaning solution 3: 48% by mass potassium hydroxide aqueous solution (manufactured by Toagosei Co., Ltd., trade name: Super Cali)
Alkaline cleaning solution 4: 2.0N potassium hydroxide aqueous solution (prepared by mixing alkaline cleaning solution 3 and ultrapure water)
Acid cleaning solution 1: 1.0 N (6% by mass) nitric acid
(Prepared by mixing 70 mass% nitric acid (manufactured by Kanto Chemical Co., Inc .; grade for electronic industry) and ultrapure water)
Acid cleaning solution 2: 1.0 N (4% by mass) hydrochloric acid
(Prepared by mixing 35 mass% hydrochloric acid (Kanto Chemical Co., Ltd .; special grade) and ultrapure water)

  In the following examples and comparative examples, the used treatment bodies are indicated by the abbreviations (A), (B), (C), and (D).

(Washing equipment used)
A PTFE tank (1000 ml in volume), a PFA column (φ3 / 4 inch, length 200 mm), and a sampling PE (polyethylene) container (300 ml in volume) are all 1.0N nitric acid (acid) in advance to eliminate metal contamination. After being immersed in the cleaning liquid 1) for 1 hour or longer, it was washed with running ultrapure water. The ultrapure water used has a metal content of 1 ppt for each metal, 50 ppt for Si, and 10 ppb or less for inorganic carbon produced by an ultrapure water production system.

  The appliances cleaned by the above-described cleaning method were connected by PFA piping in the order of a PTFE tank and a PFA column. Table 1 shows the analysis results of the test system blank when ultrapure water was passed through this system and received in the PE container at the PFA column outlet. As can be seen from Table 1, there is no contamination by this system.

(Washing operation)
The metal impurities contained in the processed bodies (A), (B), (C) and (D), unreacted organic substances in the synthesis process, and the like were removed by the following washing operation. The washing operation was performed at a temperature of 20 to 25 ° C.

  A test module was prepared by packing (A), (B), (C) and (D) as treatment bodies into a PFA column (φ3 / 4 inch, length 200 mm) densely. Ultrapure water was passed through the test module at 10 mL / min for 12 hours or longer to remove air and clean impurities.

(Cleaning operation of Examples 1-3 and Comparative Examples 1-2)
This ultrapure water cleaned test module was subjected to acid cleaning by passing 40 mL of 1.0 N hydrochloric acid (acid cleaning solution 2) at 0.5 mL / min (SV = 3 1 / h). After removing the solution from the test module, ultrapure water was supplied at 10 mL / min, and water was washed until the effluent water had a pH of 4 to 5. Next, 40 mL of a 2.0N aqueous sodium hydroxide solution (alkaline cleaning solution 2) was passed at 0.5 mL / min (SV = 3 1 / h).

  40 mL of 48 mass% sodium hydroxide aqueous solution (alkaline washing | cleaning liquid 1) was flowed through the said processed test module at 0.5 mL / min (SV = 3 1 / h). Thereafter, ultrapure water was supplied to the test module at 10 mL / min, and water was washed until the effluent water reached pH 9-10.

  Then, a series of cleaning operations using the above 1.0N hydrochloric acid (acid cleaning solution 2), 2.0N aqueous sodium hydroxide solution (alkali cleaning solution 2), and then 48% by mass sodium hydroxide aqueous solution (alkali cleaning solution 1) are performed again. The processed bodies (A), (B), (C) and (D) were obtained.

(Cleaning operation of Examples 4 to 6 and Comparative Examples 3 to 4)
Acid cleaning was performed by passing 40 mL of 1.0 N hydrochloric acid (acid cleaning solution 2) through the ultrapure water cleaned test module at 0.5 mL / min (SV = 3 1 / h). After extracting the solution from the test module, ultrapure water was supplied at 10 mL / min, and water was washed until the effluent water had a pH of 4 to 5. Next, 40 mL of a 2.0 N aqueous potassium hydroxide solution (alkali cleaning solution 4) was passed at 0.5 mL / min (SV = 3 1 / h).

  40 mL of 48 mass% potassium hydroxide aqueous solution (alkaline washing | cleaning liquid 3) was passed through the said processed test module at 0.5 mL / min (SV = 3 1 / h). Thereafter, ultrapure water was supplied to the test module at 10 mL / min, and water was washed until the effluent water reached pH 9-10.

  Then, a series of washing operations using the above 1.0N hydrochloric acid (acid washing solution 1), 2.0N potassium hydroxide aqueous solution (alkali washing solution 4), and then 48% by mass potassium hydroxide aqueous solution (alkali washing solution 3) are performed again. The processed bodies (A), (B), (C) and (D) were obtained.

(Examples 1-6 and Comparative Examples 1-4)
A sample alkaline aqueous solution was put into the PTFE tank. A test module filled with a PTFE tank filled with an alkaline aqueous solution and the above-described cleaned processing body (A), (B), (C) or (D) is connected by a 1/4 inch PFA tube, and the test module A PE container for sample collection was installed at the outlet.

  Nitrogen gas is introduced from the top of the PTFE tank, the pressure inside the container is increased to 0.2 MPa, the flow rate adjustment valve is operated, and the flow rate of the alkaline aqueous solution flowing out from the outlet of the test module is 1.7 ml / min ( SV = 10 1 / h) or less. Moreover, the liquid temperature of the alkaline aqueous solution at this time was used after adjusting to 20-25 degreeC previously. The pH of the alkaline aqueous solution after passing through the test module was measured, and when the same value as the supplied alkaline aqueous solution was shown, a sample was collected with a PE container.

  In addition, each Example and the comparative example were performed on condition of the following.

Example 1: Test module: treated body (A), sample: 48 mass% sodium hydroxide aqueous solution Example 2: Test module: treated body (B), sample: 48 mass% sodium hydroxide aqueous solution Example 3: Test module : The processing body (A) and the processing body (B) are connected in series in this order.
Sample: 48 mass% sodium hydroxide aqueous solution Example 4: Test module: treated body (A), sample: 48 mass% potassium hydroxide aqueous solution Example 5: Test module: treated body (B), sample: 48 mass% water Potassium oxide aqueous solution Example 6: Test module: The treatment body (A) and the treatment body (B) are connected in series in this order.
Sample: 48 mass% potassium hydroxide aqueous solution Comparative example 1: Test module: treated body (C), sample: 48 mass% sodium hydroxide aqueous solution Comparative example 2: Test module: treated body (D), sample: 48 mass% water Sodium oxide aqueous solution Comparative example 3: Test module: treated body (C), sample: 48 mass% potassium hydroxide aqueous solution Comparative example 4: Test module: treated body (D), sample: 48 mass% potassium hydroxide aqueous solution

  The alkaline aqueous solution samples treated in Examples 1-6 and Comparative Examples 1-2 were analyzed for the amount of metal by ICP-MS. The metal analysis items were chromium, iron, nickel, and copper, which are considered to be mixed from stainless steel members used in the manufacturing process of sodium hydroxide and potassium hydroxide. In the analysis by ICP-MS, sodium and potassium components were removed by a solid phase extraction method as needed as a treatment before introducing the analyzer.

  The results obtained in Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Tables 2 and 3.

  In Example 1, 48 mass% sodium hydroxide was passed through the test module filled with the treated body (A), and as a result, iron, nickel, and copper could be effectively removed. It was also confirmed that chromium was decreased.

  In Example 2, as a result of passing 48 mass% sodium hydroxide through the test module filled with the treated body (A), nickel and copper could be removed more effectively than in Example 1. However, a decrease in chromium and iron could not be confirmed.

  In Example 3, the test modules filled with the processing body (A) and the processing body (B) were connected in series, and 48 mass% sodium hydroxide was passed through the liquid. As a result, compared with Example 1 and Example 2, nickel was used. Copper and copper could be removed more effectively. Moreover, the decreasing tendency similar to Example 1 was confirmed about chromium and iron.

  In Example 4, as a result of passing liquid mass of 48 mass% potassium hydroxide through the test module filled with the treated body (A), iron, nickel and copper could be effectively removed. It was also confirmed that chromium was decreased.

  In Example 5, as a result of liquid-contacting 48 mass% potassium hydroxide to the test module filled with the treatment body (A), nickel and copper could be removed more effectively than in Example 1. However, a decrease in chromium and iron could not be confirmed.

  In Example 6, the test modules filled with the treatment body (A) and the treatment body (B) were connected in series, and 48 mass% potassium hydroxide was passed through as a result of contact. As a result, nickel was compared with Examples 4 and 5. Copper and copper could be removed more effectively. However, the same decreasing tendency as in Example 4 was confirmed for chromium and iron.

  In Comparative Example 1, reduction of chromium, iron, nickel and copper was confirmed, but sufficient results were not obtained.

  In Comparative Example 2, the metal removal effect was not confirmed.

  In Comparative Example 3, reduction of chromium, iron, nickel and copper was confirmed, but sufficient results were not obtained.

  In Comparative Example 4, the metal removal effect was not confirmed.

  Here, it is known that the chelate fiber having an N-methylglucamine group of the treated body (A) conventionally captures a semimetal such as boron. However, in the examples, since the capturing ability of chromium, iron, nickel and copper was confirmed, these heavy metals mixed in the synthesis process of the 48 mass% sodium hydroxide aqueous solution and the potassium hydroxide aqueous solution are the presence of metalloids in the aqueous solution. It exists as an oxo acid or hydroxide similar to the form, and is presumed to have been captured by chelate reaction or hydrogen bonding adsorption action, respectively.

  On the other hand, the weakly basic anion exchange fiber (B) having an iminodiethanol group in the treated body is made of nickel and copper by the action of electrostatically adsorbing a metal present as a hydroxide anion and the chelating action by the donor property of the amine part. Is thought to be specifically captured.

  Further, in the bead-like form like the treated body (C), the functional group is the same as that of the treated body (A), but the heavy metal capturing performance is remarkably lowered. The reason for this is that in the bead-like form of the treated body (C), the treated body (C) contracts due to the strong osmotic pressure generated when it comes into contact with the high-concentration aqueous alkali solution, and voids are generated in the test module. Therefore, it is considered that the alkaline aqueous solution to be treated short-passed and did not contact efficiently. In addition, the fibrous form such as the treated bodies (A) and (B) can be in contact with the metal by the surface layer, but in the bead-like form of the treated body (C), the metal to be removed is brought into the interior. The fact that the metal cannot be sufficiently contacted unless it is diffused is considered to be a factor that the metal could not be captured efficiently.

  As shown in Comparative Examples 2 and 4, in the treated body (D), since no functional group is introduced into the polyethylene fiber, it can be seen that there is no metal capturing performance.

From the above results, if the treated bodies (A) and (B) used in the present invention are in a fibrous form, the salt has a high viscosity like a 48 mass% sodium hydroxide aqueous solution or potassium hydroxide aqueous solution. Even an aqueous solution containing a large amount can capture heavy metals efficiently. Further, as shown in Examples 3 and 6, it was possible to further reduce nickel and copper by combining the processed bodies (A) and (B). Although not specifically illustrated, the same effects as those of the present invention can be obtained with other strong alkaline aqueous solutions.

  The present invention has been described in detail based on the above specific examples. However, the present invention is not limited to the above specific examples, and various modifications and changes can be made without departing from the scope of the present invention.

Claims (10)

  An alkaline aqueous solution containing a metal impurity is brought into contact with at least one of a fibrous weakly basic anion exchanger and a chelating material, and the metal impurity is removed from the alkaline aqueous solution. Purification method.   The method for purifying an alkaline aqueous solution according to claim 1, wherein the fibrous weakly basic anion exchanger and the chelating agent contain an N-methylglucamine group and an iminodiethanol group.   3. The alkaline aqueous solution according to claim 2, wherein the functional group of the fibrous weakly basic anion exchanger is an iminodiethanol group, and the functional group of the chelating agent is an N-methylglucamine group. Purification method.   The fibrous weakly basic anion exchanger and chelating material are nonwoven fabrics of the weakly basic anion exchanger and chelating material, according to any one of claims 1 to 3. Purification method of alkaline aqueous solution.   The method for purifying an alkaline aqueous solution according to any one of claims 1 to 4, wherein the material is used after being processed into a cartridge filter shape or a membrane shape.   The alkaline aqueous solution contains at least one alkaline component selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonia, sodium carbonate, sodium bicarbonate, potassium carbonate and potassium bicarbonate, and organic alkali. The method for purifying an alkaline aqueous solution according to any one of claims 1 to 5.   The method for purifying an alkaline aqueous solution according to any one of claims 1 to 6, wherein the metal impurity is at least one selected from the group consisting of iron, nickel, copper and chromium.   The method for purifying an alkaline aqueous solution according to claim 7, wherein the concentration of the metal impurity in the alkaline aqueous solution after purification of the metal impurity is 5 ppb or less.   8. The metal impurity according to claim 7, wherein the metal impurity is at least one of copper and nickel, and the concentration of the metal impurity in the alkaline aqueous solution after purification of the metal impurity is 0.5 ppb or less. Purification method of alkaline aqueous solution.   The method for purifying an alkaline aqueous solution according to any one of claims 1 to 9, wherein the flow rate SV of the alkaline aqueous solution is 10 (1 / h) or more.