A method for removing iron ions from a solution containing a carboxyl compound

Abstract

The present application relates to a method for removing iron ions from a solution containing a carboxyl compound, comprising the following steps: (1) pre-treating an anion resin until the anion concentration in the resin eluent is less than 10 ppb; and (2) passing a solution containing iron ions and carboxyl compounds through the anion resin at a flow rate of 0.1 to 10 BV / h, collecting and detecting the iron ion content in the solution containing the carboxyl compound after passing through the anion resin, and stopping the collection when the iron ion content relative to the carboxyl compound is greater than 10 ppb. The iron ion content in the solution containing the carboxyl compound finally obtained is less than 10 ppb, and the operation is simple, the yield is high, the repeatability is good, the production is easy, the impurities introduced are few, and the anion resin will not be penetrated.

Description

Technical Field

[0001] The present application relates to the field of chemical purification, and in particular to a method for removing iron ions from a solution containing a carboxyl compound. Background Art

[0002] Carboxyl compounds are widely used in semiconductor cleaning and polishing because carboxyl compounds can form stable water-soluble complexes with soluble metal ions to prevent metal ion residues. They also have stable functions, ensure storage stability and stable performance, and can prevent or reduce metal corrosion. For example, a low-etching photoresist residue cleaning solution disclosed in CN108121176A comprises (a) alcohol amine (b) hydroxylamine (c) water (d) chelating agent (e) polyamine compound (f) surfactant, wherein the mass percentage of the chelating agent is 1-20%. We know that hydroxylamine is very easy to decompose under the catalysis of metal ions (such as iron ions). The most direct role of the chelating agent is to effectively stabilize the composition of the cleaning solution and ensure its storage stability. CN101463227B discloses a chemical mechanical polishing liquid for polishing a barrier layer, which contains: abrasive particles, an oxidant, a film-forming agent, an organic chelating agent, a stabilizer and water, wherein the chelating agent content is 0.2-1.5%. The chelating agent can effectively chelate metal ions to prevent the generation of metal ion residues during polishing, thereby affecting electrical properties.

[0003] It is known from the public literature that water-soluble carboxylic acid compounds are widely used in the semiconductor industry. For example, CN104745085B discloses that the complexing agent in the polishing liquid is acetic acid, propionic acid, oxalic acid, malonic acid, succinic acid, citric acid, tartaric acid, etc. CN103809394B discloses that the organic acid in the cleaning liquid is oxalic acid, malonic acid, succinic acid, glutaric acid, citric acid, tartaric acid, malic acid, etc. We know that metal ions have a great influence on the subsequent electrical properties of the chip. Generally speaking, the metal ions need to be less than 1ppm, and for advanced processes, the metal ions need to reach the ppb level, which is tantamount to a great challenge to the raw materials.

[0004] Taking electronic grade citric acid as an example, a method for preparing electronic grade citric acid and its device disclosed in CN109972167A, through ultra-clean electrolysis, the content of trace metal ions (Na+, K+, Ca2+, Mg2+, Zn2+) in the final product is 20ppb, but the method is strict in the selection of membranes, and the metal ions processed at the same time are limited to the 5 types mentioned. CN101973871A discloses an electronic grade citric acid and its production method, which treats citric acid with anionic and cationic resins, and finally each metal ion is <0.5μg / g, heavy metals are <0.1μg / g, chlorine is <1μg / g, and sulfate is <1μg / g. However, we found in actual operation that iron ions are very easy to penetrate, because citric acid has strong chelating properties and is also highly acidic. For example, CN101973871A mentions an electronic-grade citric acid and a production method thereof, in which citric acid is passed through a cation exchange column equipped with a special chelating resin to reduce the cationic content of the citric acid solution, and then the mixed anion content of the citric acid solution is reduced by an anion exchange column equipped with a macroporous weakly alkaline anion resin. However, in the actual production process, when the chelating resin is used to treat citric acid, iron ions can be chelated and removed, but because citric acid is acidic, the iron ions in the resin will be dissociated and enter the solution, and finally the resin will penetrate, which ultimately increases the difficulty of production and the final cost.

[0005] There are many patents reporting on the removal of iron ions from organic carboxylic acids, such as a method for removing iron ions from oxalic acid disclosed in CN106866411A, in which citric acid is added to an oxalic acid solution, and then recrystallized to precipitate oxalic acid, and finally the iron ions are reduced to less than 0.0002% (2ppm). A method for removing iron from a sodium citrate mother liquor disclosed in CN110143868A, in which sodium hydroxide is used to adjust the pH of the sodium citrate mother liquor, and then calcium citrate is added, and the calcium citrate first reacts with sodium hydroxide to generate sodium citrate and calcium hydroxide, and then the calcium hydroxide and the iron ions undergo a flocculation reaction, and the generated floccules can be directly removed by filtering, and the iron ion content is less than 10ppm. A sodium citrate mother liquor iron removal purification process disclosed in CN103864604A, the sodium citrate mother liquor is added with water to obtain solution A, sodium hydroxide solution, sodium sulfide, activated carbon and hydrogen peroxide are added in sequence, and after two filtrations, the high iron ions, a small amount of protein colloids and organic impurities contained in the sodium citrate mother liquor are removed, and the iron ions in the obtained sodium citrate solution are less than 1ppm. However, these iron removal methods all introduce impurities, and trace impurities will affect the electrical properties of cleaning liquid or polishing liquid. If these methods are used in the preparation of semiconductor-grade citric acid, the subsequent removal of impurities is a huge project.

[0006] In summary, the methods for removing iron ions from carboxyl compounds in the prior art all introduce impurities, and the resin treatment faces the problem of penetration. Summary of the invention

[0007] In order to solve the problems of introducing impurities and resin penetration in the prior art method for removing iron ions from a carboxyl compound solution, the present application provides a method for removing iron ions from a carboxyl compound solution, comprising the following steps:

[0008] (1) pretreating anion resin; and

[0009] (2) A solution containing iron ions and carboxyl compounds is passed through the anion resin at a flow rate of 0.1 to 10 BV / h, and the iron ion content in the solution containing carboxyl compounds after passing through the anion resin is collected and detected, and the collection is stopped when the content of the iron ions relative to the carboxyl compounds is >10 ppb. At the beginning, the iron ion content in the anion resin receiving liquid is extremely low, and the iron ion content in the receiving liquid tends to gradually increase. Subsequently, as the anion resin is saturated, its adsorption amount decreases, but the iron ion content in the anion resin receiving liquid gradually increases.

[0010] Furthermore, the anion resin in step (1) is a weakly basic anion resin.

[0011] Furthermore, the weakly basic anionic resin in step (1) comprises primary amine, secondary amine or tertiary amine functional groups.

[0012] Furthermore, the pretreatment described in step (1) includes heat treatment or alkaline treatment, wherein the heat treatment is to remove anions adsorbed on the resin with a hot aqueous solution, and the alkaline treatment is to remove anions adsorbed on the resin with an alkaline solution until the anion concentration in the resin eluate is less than 10 ppb, and the anions are one or more of sulfate, chloride, nitrate, or phosphate.

[0013] Furthermore, in the step (1), the resin is subjected to an acid treatment before the alkali treatment, and the acid treatment is to remove metal ions in the resin by an acid solution. The acid is one or more of hydrochloric acid, sulfuric acid, or nitric acid. Because acid treatment such as sulfuric acid, hydrochloric acid, etc. will introduce anions, such as sulfate and chloride ions, it is necessary to first treat with acid and then with alkali, and the alkali treatment allows the resin to be completely transformed into the OH type.

[0014] Furthermore, after the pretreatment is completed, the anion resin is rinsed with ultrapure water until the metal ion in the eluent is less than 1 ppb. The metal ion is one or more of Na, Mg, Al, K, Ca, Cr, Mn, Fe, Ni, Cu, Zn, Ag, or Pb. Generally speaking, in the semiconductor industry, the anion can be appropriately relaxed, and the lower detection limit of the instrument is generally <10 ppb; while the requirements for metal ions are more stringent, and metal ions can be <1 ppb.

[0015] Furthermore, the temperature of the hot aqueous solution in the heat treatment is 40-100° C.; the alkali in the alkali treatment is one or more of sodium hydroxide, potassium hydroxide or ammonia water.

[0016] Furthermore, the carboxyl compound in step (2) is an organic carboxylic acid and / or an organic carboxylate.

[0017] Furthermore, the organic carboxylic acid and / or the organic carboxylate in the organic carboxylic acid salt refers to one or more of oxalic acid, malonic acid, succinic acid, glutaric acid, citric acid, tartaric acid, or malic acid, and the organic carboxylate salt refers to one or more of ammonium carboxylate, sodium carboxylate, or potassium carboxylate.

[0018] Furthermore, the solvent of the solution in step (2) is an aqueous solution and / or an organic solution.

[0019] Furthermore, the organic solution is one or more of a methanol solution, an ethanol solution or an isopropanol solution.

[0020] Furthermore, the metal ion content in the hot water solution and the alkaline solution is less than 1 ppm, preferably, the metal ion content is less than 10 ppb.

[0021] Furthermore, the metal ion content in the acid solution is less than 1 ppm, preferably, the metal ion content is less than 10 ppb.

[0022] All reagents and equipment in this application are commercially available.

[0023] Compared with the prior art, the advantages of the present invention are: the conventional technical means in the art is to remove iron with chelating resin, but under acidic conditions, the penetration of the resin is inevitable. In addition, in the purification of carboxyl compounds, there is no report on the removal of iron ions with anionic resin. The present invention discloses a method for removing iron ions from a solution containing a carboxyl compound. The carboxyl compound and the iron ions exist in the form of a chelate through treatment with a weakly basic anion resin. The iron ions in the carboxyl compound solution with chelation can be removed through the weakly basic anion resin. At the same time, the method uses an anion resin, and cations have no effect on the method, and will not cause the anion resin to be penetrated. The iron ion content in the carboxyl compound solution finally obtained is <10ppb. In addition, the anion resin is specially pretreated, so it has a high degree of cleanliness, and less impurities are introduced in the subsequent purification process of the carboxyl compound. The method is simple to operate, has great operational flexibility (a larger range of column flow rates can be selected), has a wider range of applications (can be applied to organic carboxylic acids or organic acid salts, rather than a single substance), has a high yield (compared with chelating resins, it is not easy to be penetrated, and the processing volume can be large), has good repeatability (the experiment is repeatable), and is easy to produce. DETAILED DESCRIPTION

[0024] The advantages of the present invention are described in detail below in conjunction with specific embodiments.

[0025] All "%" in the following examples refer to mass percentage.

[0026] Embodiment 1:

[0027] (1) Filling 1 L of primary amine-based weakly basic anion resin into an ion exchange column;

[0028] (2) eluting the primary amine-based weakly basic anion resin with a 5% electronic grade HCl aqueous solution at 0.1 BV / h until the metal ion concentration in the receiving solution is less than 1 pbb;

[0029] (3) Elute the primary amine-based weakly alkaline anion resin with 5% electronic grade KOH at 0.1 BV / h until the anion concentration in the receiving solution is <10 ppb;

[0030] (4) Rinse with ultrapure water until the metal ion content of the primary amine-based weakly alkaline anion resin is less than 1 ppb and the anion content is less than 10 ppb;

[0031] (5) A 50% citric acid aqueous solution is passed through a primary amine-based weakly basic anion resin at a rate of 0.1 BV / h, and the metal ion content in the citric acid solution is measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion in the citric acid is greater than 10 ppb, the collection is stopped, and the treatment amount of the primary amine-based weakly basic anion resin and the iron ion concentration after treatment are counted;

[0032] Embodiment 2:

[0033] (1) Filling 1 L of secondary amine weakly basic anion resin into an ion exchange column;

[0034] (2) eluting the secondary amine weakly basic anion resin with 5% electronic grade H2SO4 aqueous solution at 2BV / h until the metal ion concentration in the receiving solution is less than 1pbb;

[0035] (3) Wash the secondary amine weakly basic anion resin with hot water at 90°C until the anion concentration in the receiving solution is less than 10 ppb;

[0036] (4) Rinse with ultrapure water until the metal ion content of the secondary amine weakly alkaline anion resin is less than 1 ppb and the anion content is less than 10 ppb;

[0037] (5) A 50% sodium citrate aqueous solution was passed through a secondary amine weak base anion resin at a rate of 0.5 BV / h, and the metal ion content of the product was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion in citric acid was greater than 10 ppb, the collection was stopped, and the treatment amount of the secondary amine weak base anion resin and the iron ion concentration after treatment were counted;

[0038] Embodiment 3:

[0039] (1) Fill 1 L of tertiary amine weakly basic anion resin into an ion exchange column;

[0040] (2) eluting the tertiary amine weakly basic anion resin with a 5% electronic grade HNO3 aqueous solution at 10 BV / h until the metal ion concentration in the receiving solution is less than 1 pbb;

[0041] (3) Using 5% electronic grade NaOH at 10 BV / h to elute the tertiary amine weakly basic anion resin until the anion concentration in the receiving solution is less than 10 ppb;

[0042] (4) Rinse with ultrapure water until the metal ion content of the tertiary amine weakly alkaline anion resin is less than 1 ppb and the anion content is less than 10 ppb;

[0043] (5) A 50% ammonium citrate aqueous solution was passed through a tertiary amine weakly basic anion resin at a rate of 10 BV / h, and the metal ion content of the product was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion in the citric acid was greater than 10 ppb, the collection was stopped, and the treatment amount of the tertiary amine weakly basic anion resin and the iron ion concentration after treatment were counted;

[0044] Embodiment 4:

[0045] (1) Filling 1 L of primary amine-based weakly basic anion resin into an ion exchange column;

[0046] (2) Elute the primary amine-based weakly alkaline anion resin with a 5% electronic grade HNO3 aqueous solution at 1 BV / h until the metal ion concentration in the receiving solution is less than 1 pbb;

[0047] (3) Elute the primary amine-based weakly alkaline anion resin with 5% electronic grade ammonia water at 1 BV / h until the anion concentration in the receiving solution is less than 10 ppb;

[0048] (4) Rinse with ultrapure water until the metal ion content of the primary amine-based weakly alkaline anion resin is less than 1 ppb and the anion content is less than 10 ppb;

[0049] (5) A 30% malic acid aqueous solution was passed through the resin at a rate of 1 BV / h, and the metal ion content of the product was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion in citric acid was greater than 10 ppb, the collection was stopped, and the treatment amount of the primary amine-based weakly basic anion resin and the iron ion concentration after treatment were counted;

[0050] Embodiment 5:

[0051] (1) Fill 1 L of primary amine weakly basic anion resin into an ion exchange column;

[0052] (2) eluting the primary amine weakly basic anion resin with 2% electronic grade HCl aqueous solution at 1 BV / h until the metal ion concentration in the receiving solution is less than 1 pbb;

[0053] (3) Elute the primary amine weakly basic anion resin with 2% electronic grade KOH at 1 BV / h until the anion concentration in the receiving solution is <10 ppb;

[0054] (4) Rinse with ultrapure water until the metal ion content of the primary amine weakly alkaline anion resin is less than 1 ppb and the anion content is less than 10 ppb;

[0055] (5) A 50% sodium tartrate aqueous solution was passed through a primary amine weakly basic anion resin at a rate of 1 BV / h, and the metal ion content of the product was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion in citric acid was greater than 10 ppb, the collection was stopped, and the treatment amount of the primary amine weakly basic anion resin and the iron ion concentration after treatment were counted;

[0056] Embodiment 6:

[0057] (1) Filling 1 L of secondary amine weakly basic anion resin into an ion exchange column;

[0058] (2) eluting the secondary amine weakly basic anion resin with 5% electronic grade HCl aqueous solution at 1 BV / h until the metal ion concentration in the receiving solution is less than 1 pbb;

[0059] (3) Elute the secondary amine weakly basic anion resin with 5% electronic grade KOH at 1 BV / h until the anion concentration in the receiving solution is <10 ppb;

[0060] (4) Rinse with ultrapure water until the metal ion content of the secondary amine weakly alkaline anion resin is less than 1 ppb and the anion content is less than 10 ppb;

[0061] (5) A 10% aqueous solution of disodium ethylenediaminetetraacetate was passed through a secondary amine weakly basic anion resin at a rate of 5 BV / h, and the metal ion content of the product was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion content in citric acid was greater than 10 ppb, the collection was stopped, and the treatment amount of the secondary amine weakly basic anion resin and the iron ion concentration after treatment were counted;

[0062] Comparative Example 1:

[0063] (1) Filling 1 L of primary amine-based weakly basic anion resin into an ion exchange column;

[0064] (2) Rinse with ultrapure water until the metal ion content of the primary amine-based weakly alkaline anion resin is less than 1 ppb and the anion content is less than 10 ppb;

[0065] (3) A 50% citric acid aqueous solution was passed through a primary amine-based weakly basic anion resin at a rate of 1 BV / h, and the metal ion content of the product was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion content in the citric acid was greater than 10 ppb, the collection was stopped, and the treatment amount of the primary amine-based weakly basic anion resin and the iron ion concentration after treatment were counted;

[0066] Comparative Example 2:

[0067] (1) Filling 1 L of primary amine-based weakly basic anion resin into an ion exchange column;

[0068] (2) Elute the primary amine-based weakly alkaline anion resin with 5% electronic grade KOH at 1 BV / h until the anion concentration in the receiving solution is less than 10 ppb;

[0069] (3) Rinse with ultrapure water until the metal ion content of the primary amine-based weakly alkaline anion resin is less than 1 ppb and the anion content is less than 10 ppb;

[0070] (4) A 50% citric acid aqueous solution was passed through a primary amine-based weakly basic anion resin at a rate of 1 BV / h, and the metal ion content of the product was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion content in the citric acid was greater than 10 ppb, the collection was stopped, and the treatment amount of the primary amine-based weakly basic anion resin and the iron ion concentration after treatment were counted;

[0071] Comparative Example 3:

[0072] (1) 1L D412 chelating resin was filled into an ion exchange column;

[0073] (2) washing the D412 chelating resin with 5% electronic grade HCl aqueous solution at 1 BV / h until the metal ion concentration in the receiving solution is less than 1 pbb;

[0074] (3) Elute the D412 chelating resin with 5% electronic grade KOH at 1 BV / h until the anion concentration in the receiving solution is <10 ppb;

[0075] (4) Rinse with ultrapure water until the metal ion content of the D412 chelating resin receiving liquid is less than 1 ppb and the anion content is less than 10 ppb;

[0076] (5) A 50% citric acid aqueous solution was passed through a D412 chelating resin at a rate of 1 BV / h. The metal ion content of the product was measured by an inductively coupled plasma optical emission spectrometer (ICP-OES). When the iron ion content in the citric acid was greater than 10 ppb, the collection was stopped. The treatment amount of the D412 chelating resin and the iron ion concentration after treatment were counted.

[0077] Table 1 Iron ion content of citric acid-containing solution before and after treatment with the resins of Examples 1-6 and Comparative Examples 1-3, and the treatment amount of the citric acid-containing solution by the resins

[0078]

[0079]

[0080] From Examples 1-6 in Table 1, it can be found that the weakly basic anion resin has a good performance for both carboxyl compounds and salts of carboxyl compounds, the metal ions can be stabilized at a concentration of <10 ppb, and the processing volume is extremely large.

[0081] Comparative Example 1 is the result of washing the resin with ultrapure water without acid or alkali treatment. It can be found that the resin can hardly remove metal ions. The raw material is 562ppb, and after treatment it is 560ppb, which is almost unchanged. This is because if the resin cannot be completely transformed, the active sites will be occupied by other substances and it will not be able to play a purification role.

[0082] Comparative Example 2 is the result of the resin being treated with alkali only without acid treatment. When the resin is not treated with acid, the iron ion concentration in the solution is about 80 ppb, which is relatively high. This is because if the resin is not treated with acid, the metal ions can exist in the form of oxides or hydroxides, and the metal ions cannot be removed by subsequent alkali treatment or water washing. The substances we need to purify, such as citric acid, not only have chelating properties but also have acidity. Therefore, the cleanliness of the resin is low, and the impurities between the subsequent resins will be eluted by citric acid, thereby affecting the purity of the final product.

[0083] Comparative Example 3 is a traditional chelating resin. From the treatment results of D412 chelating resin, it can be found that D412 is a chelating resin that chelates iron in ionic form. Because the solution containing acidic compounds is acidic, the acidity will cause the chelated iron ions in the chelating resin to dissociate, so it will penetrate quickly and the chelating resin is very easy to deactivate. Finally, there are only 2kg of raw materials with less than 10ppb. However, the present application uses a weakly basic anionic resin, and the carboxyl compound in the solution dissociates H + The anion part and the iron ion exist in the form of a chelate to form an anion chelate, rather than free iron ions, and the anion chelate is selectively adsorbed by the weakly basic anion resin. Because the carboxyl compound in the present application has chelating properties, even if the carboxyl compound solution is acidic, the iron ions in the weakly basic anion resin will not be dissociated from the anion chelate, that is, the weakly basic anion resin will not be penetrated. And the processing capacity of the weakly basic anion resin of the present application is very large.

[0084] The method disclosed in this patent can effectively reduce metal ions to the 10ppb level through anion resin treatment. The purification process is simple, highly operable, flexible and has a wider range of applications.

[0085] The specific embodiments of the present invention are described in detail above, but they are only examples, and the present invention is not limited to the specific embodiments described above. For those skilled in the art, any equivalent modifications and substitutions made to the present invention are also within the scope of the present invention. Therefore, the equalization changes and modifications made without departing from the spirit and scope of the present invention should be included in the scope of the present invention.

Claims

1. A method for removing iron ions from a solution containing a carboxyl compound, characterized in that: The following steps are involved: (1) Pretreating the anion resin; as well as (2) passing a solution containing a carboxyl compound through the anion resin at a flow rate of 0.1 to 10 BV / h, collecting and detecting the iron ion content in the solution containing the carboxyl compound after passing through the anion resin, and stopping the collection when the content of the iron ion relative to the carboxyl compound is greater than 10 ppb; The anion resin in step (1) is a weakly alkaline anion resin; the pretreatment in step (1) includes heat treatment or alkali treatment, wherein the heat treatment is to remove anions adsorbed on the resin with a hot aqueous solution, and the alkali treatment is to remove anions adsorbed on the resin with an alkali solution until the anion concentration in the resin eluate is less than 10 ppb; in step (1), the resin is subjected to an acid treatment before the heat treatment or alkali treatment, wherein the acid treatment is to remove metal ions in the resin with an acid solution, wherein the acid in the acid treatment is one or more of hydrochloric acid, sulfuric acid, or nitric acid; and the alkali in the alkali treatment is one or more of sodium hydroxide, potassium hydroxide, or ammonia water; The carboxyl compound in step (2) is an organic carboxylic acid and / or an organic carboxylate; The organic carboxylic acid in the organic carboxylic acid and / or the organic carboxylate refers to one or more of oxalic acid, malonic acid, succinic acid, glutaric acid, citric acid, tartaric acid, or malic acid, and the organic carboxylate refers to one or more of ammonium carboxylates, sodium carboxylates, or potassium carboxylates of the above-mentioned organic carboxylic acids.

2. The method for removing iron ions from a solution containing a carboxyl compound as claimed in claim 1, wherein: The weakly basic anionic resin in step (1) comprises primary amine, secondary amine or tertiary amine functional groups.

3. The method for removing iron ions from a solution containing a carboxyl compound as claimed in claim 1, wherein: After the pretreatment is completed, the anion resin is rinsed with ultrapure water until the metal ions in the eluent are less than 1 ppb.

4. The method for removing iron ions from a solution containing a carboxyl compound as claimed in claim 1, wherein: The temperature of the hot aqueous solution in the heat treatment is 40-100°C.

5. The method for removing iron ions from a solution containing a carboxyl compound as claimed in claim 1, wherein: The solvent of the solution in step (2) is an aqueous solution and / or an organic solution.

6. The method for removing iron ions from a solution containing a carboxyl compound as claimed in claim 5, wherein: The organic solution is one or more of a methanol solution, an ethanol solution, or an isopropanol solution.

7. The method for removing iron ions from a solution containing a carboxyl compound as claimed in claim 1, wherein: The metal ion contents in the hot aqueous solution and the alkaline solution are both less than 1 ppm.

8. The method for removing iron ions from a solution containing a carboxyl compound as claimed in claim 1, wherein: The metal ion content in the acid solution is less than 1 ppm.

9. A method for removing iron ions from a solution containing a carboxyl compound as described in any one of claims 7-8, characterized in that: The metal ion content in the hot water solution, alkaline solution and acid solution is less than 10 ppb.

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