JP2011051836A - Iron oxyhydroxide sol and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an iron oxyhydroxide sol excellent in stability at extremely simple steps and the iron oxyhydroxide sol produced by this production method. <P>SOLUTION: The method for producing the iron oxyhydroxide sol comprises the steps of: adding an alkaline agent to a water-soluble inorganic compound of iron by 0.5-0.9 (the alkaline agent)/(an inorganic acid radical) (molar ratio) to obtain an iron hydroxide solution; adding hydroxycarboxylic acid to the iron hydroxide solution by 0.05-0.20 (the hydroxycarboxylic acid)/(iron (Fe)) (molar ratio); again adding the alkaline agent to the hydroxycarboxylic acid-added solution and adjusting the pH of the hydroxycarboxylic acid-added solution to 6-12 to obtain a crude iron oxyhydroxide sol; and washing the crude iron oxyhydroxide sol to the extent that the filtrate, which is obtained by filtering the crude iron oxyhydroxide sol by using an ultrafiltration membrane having ≤20,000 molecular weight cutoff, has ≤1 mS/cm electric conductivity. The iron oxyhydroxide sol produced by this production method is also provided. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention provides a method for producing an iron oxyhydroxide sol and an iron oxyhydroxide sol produced by this method, in particular, an iron oxyhydroxide sol excellent in stability by skillfully using an alkali agent and a hydroxycarboxylic acid. The present invention relates to a method of producing in a process and an iron oxyhydroxide sol produced by this method.

Iron oxide is well known as a magnetic material or pigment, and is one of the industrially useful substances used for catalysts and the like because of its valence change characteristics.
In the field of magnetic materials and catalysts, a stable iron oxide sol that does not contain inorganic acid groups such as chlorine, nitrate, and sulfate has been desired. As a method for producing such a stable iron oxide sol containing no ionic inorganic acid radical, Patent Document 1 discloses that an iron hydroxide gel is prepared by adding an alkaline aqueous solution to iron chloride in the presence of citric acid. There has been proposed a method in which it is produced, ripened after washing, washed again, and then hydrothermally treated by adding citric acid. In this method, ionic inorganic acid radicals are removed in the washing step, and the sol is stabilized by the remaining citric acid. However, this method has a large number of steps associated with reaction and washing, and is not an industrial production method.

  On the other hand, the applicant of the present invention discloses a method for producing a titanium oxide colloidal solution by adding an alkali agent to a titanium oxychloride solution, washing the generated gel, and then adding a hydroxycarboxylic acid and hydrothermally treating it. (See Patent Document 2). Furthermore, a method for producing a rare earth oxide sol by the same method is disclosed (see Patent Document 3). However, when these methods are used to produce an iron oxyhydroxide sol from a water-soluble inorganic compound of iron, the resulting product is a gel that settles over time, resulting in an iron oxyhydroxide sol. There was no problem.

JP 2006-182604 A JP 2002-136869 A JP 200645015 A

  An object of the present invention is to provide a method for producing an iron oxyhydroxide sol excellent in stability by an extremely simple process and an iron oxyhydroxide sol produced by this method.

The inventors of the present invention have intensively studied about a method for producing a sol of an iron compound having excellent stability, and that a sol can be obtained in an extremely simple process if an alkali agent and a hydroxycarboxylic acid are skillfully used. The present invention has been completed based on such findings. That is, the present invention
(1) A step of adding an alkali agent to a water-soluble inorganic compound of iron in a range of alkali agent / inorganic acid radical (molar ratio) = 0.5 to 0.9 to obtain an iron hydroxide solution.
(2) A step of adding hydroxycarboxylic acid to the iron hydroxide solution of step (1) in a range of hydroxycarboxylic acid / iron (Fe) (molar ratio) = 0.05 to 0.20.
(3) A step of adding an alkali agent to the solution added with the hydroxycarboxylic acid in step (2) to adjust the pH to 6 to 12 to obtain a crude iron oxyhydroxide sol.
(4) When the crude iron oxyhydroxide sol of step (3) is filtered through an ultrafiltration membrane having a molecular weight cut-off of 20000 or less, the filtrate is washed until the electrical conductivity of the filtrate becomes 1 mS / cm or less. Obtaining a sol;
The present invention relates to a method for producing an iron oxyhydroxide sol produced by the steps (1) to (4) and an iron oxyhydroxide sol produced by this method.

Since the present invention is a method for producing an iron oxyhydroxide sol in a very simple process by skillfully using an alkali agent and a hydroxycarboxylic acid, it is extremely economical. The resulting sol has excellent stability. Furthermore, the sol of the present invention is stable even when the concentration is increased by a concentration operation such as a membrane filtration method or heating, and has excellent stability even when diluted with water, and its industrial significance is tremendous. is there.

Hereinafter, the manufacturing method of the iron oxyhydroxide sol of this invention is demonstrated in detail.
In the step (1), an alkali agent is added to an acidic aqueous solution of a water-soluble inorganic compound of iron in the range of alkali agent / inorganic acid radical (molar ratio) = 0.5 to 0.9, and an iron hydroxide solution is added. obtain.
Examples of the water-soluble inorganic compound of iron used in the present invention include inorganic water-soluble iron compounds such as iron chloride, iron nitrate, and iron sulfate, and iron may be either ferrous or ferric iron. good. In the case of a ferrous compound, it is preferably used as a ferric compound with an oxidizing agent such as hydrogen peroxide.

In the present invention, an acidic aqueous solution of the water-soluble inorganic compound of iron is first prepared. The iron concentration at this time can be appropriately selected without any particular limitation. If the iron concentration is low, the concentration of the iron oxyhydroxide sol obtained is low and not economical, and if it is too high, the viscosity of the liquid after the reaction with the alkali agent becomes high, handling properties are deteriorated, and production efficiency is lowered. Generally, a concentration of about 0.2 to 5% by mass as ferric oxide (Fe ₂ O ₃ ) is preferable. Next, an alkali agent is added to the prepared acidic aqueous solution of iron. As the alkali agent to be used, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide can be used, but depending on the intended use of the sol. Since there is a case where it is not preferable to contain sodium or potassium, it is preferable to use ammonia, ammonium hydrogen carbonate or urea. Now, what is most important in the present invention is that the alkali agent is added first and the amount of the alkali agent added. The amount of the alkali agent added to the water-soluble inorganic compound of iron is in the range of alkali agent / inorganic acid radical (molar ratio) = 0.5 to 0.9. The purpose of adding the alkali agent is to use the iron hydroxide solution. There is to get. The solution pH at this time is approximately 1.5 to 2.5 although it varies slightly depending on the type of alkaline agent. By adding an alkali agent, the aqueous solution once exhibits a gel state, but this gel is gradually peptized without any steps and becomes a transparent colloidal particle dispersion solution. This fact is the first factor for simplification of the process in the present invention. In addition, when an alkaline agent is added after adding a hydroxycarboxylic acid to an acidic aqueous solution of a water-soluble inorganic compound of iron, this colloidal particle dispersion solution cannot be obtained, and finally the oxywater excellent in stability of the present invention. An iron oxide sol cannot be obtained.
This colloidal particle dispersion solution can be obtained even when the addition amount of the alkaline agent is lower than the lower limit of the above molar ratio, but ionic iron is generated when the hydroxycarboxylic acid is added in the next step to obtain a highly stable sol. Step (4) for cleaning, this reduces the leakage yield, and the ionic iron remaining in the particles after being taken into the particles or the like has lost the charge balance between the particles and has excellent stability. The sol cannot be obtained. If the upper limit is exceeded, even if the stirring time is increased, a colloidal particle dispersion solution with good dispersibility cannot be obtained, and as a result, the iron oxyhydroxide sol of the present invention cannot be produced. It is not clear why a colloidal particle dispersion solution with good dispersibility cannot be obtained, but because the inorganic acid radical that temporarily functions as a dispersant partially reacts with an alkaline agent and the functional inorganic acid radical is insufficient. Presumed.

There are no particular restrictions on the use mode and addition mode of the alkaline agent and the temperature of the acidic solution, but the alkaline agent is an aqueous solution, and the concentration varies depending on the type of alkaline agent, but is generally 30% by mass or less. It is preferable to use at a concentration of Although the sol can be obtained even when the concentration of the alkaline agent is low, a concentration of 0.2 to 30% by mass is preferable in consideration of productivity. Moreover, the addition aspect may be continuous or intermittent. When the temperature of the acidic aqueous solution is high, the particles tend to become abnormally large due to hydrolysis of the water-soluble inorganic compound of iron, and therefore the temperature is usually preferably 10 to 60 ° C, more preferably 10 to 40 ° C.
Immediately after adding the alkali agent while stirring the acidic aqueous solution of the water-soluble inorganic compound of iron, as described above, the entire solution once becomes a gel state, but gradually peptizes over time, and finally has a transparent feeling. A brown colloidal particle dispersion solution is obtained.

Next, in the step (2), hydroxycarboxylic acid is added to the colloidal particle dispersion solution, which is the iron hydroxide solution obtained in the step (1), and hydroxycarboxylic acid / iron (Fe) (molar ratio) = 0.05. Add in the range of ~ 0.20. The purpose of adding the hydroxycarboxylic acid is to stabilize the produced colloidal particles in the first stage. When this molar ratio is less than 0.05, the sol of the present invention cannot be obtained.
On the other hand, if it exceeds 0.20, part of the iron component existing as colloidal particles is ionized, and not only the sol of the present invention can be produced with high efficiency but also an unstable sol. This is presumed that the charge balance between particles is broken by ionic iron and amplifies the instability of the sol.

  As the hydroxycarboxylic acid used in the present invention, citric acid, malic acid, tartaric acid, lactic acid, hydroxyvaleric acid, glyceric acid, tropic acid, benzylic acid and the like can be used. It is preferable because the iron oxyhydroxide sol of the present invention can be produced with a small content of the above hydroxycarboxylic acid having a carboxyl group. Although there is no special restriction | limiting about the aspect of addition of hydroxycarboxylic acid, It is preferable to set it as aqueous solution and add slowly, stirring.

Next, in the step (3), an alkaline agent is added to the hydroxycarboxylic acid-containing solution obtained in the step (2) to adjust the pH to 6 to 12 to obtain a crude iron oxyhydroxide sol. The alkali agent is the same as the alkali agent and can be appropriately used, but ammonia is most preferable.
If the pH is lower than 6, the sol targeted by the present invention cannot be obtained. On the other hand, adding an alkaline agent exceeding pH 12 is not economical because a further cleaning effect, that is, stabilization cannot be obtained in the cleaning step (4). Although it is not clear why the addition of an alkali agent can stabilize the structure, it is not possible to understand the mechanism of inorganic acid radicals such as chlorine, nitrate, and sulfate radicals derived from iron raw materials that stabilize colloidal iron oxyhydroxide. It is presumed that the substitution with hydroxycarboxylic acid is promoted.

  Next, in the step (4), the crude iron oxyhydroxide sol of (3) is washed to obtain the iron oxyhydroxide sol of the present invention. The purpose of this process is to complete the solation of iron oxyhydroxide and its stabilization. That is, by removing ionic inorganic acid radicals derived from iron raw materials and salts by-produced by adding an alkaline agent by washing, the crude iron oxyhydroxide sol is completely solated and stabilized. As a washing method, there are methods using ion exchange resin, membrane filtration, etc., but a method using an ultrafiltration membrane is simple and is most suitable and recommended for the purpose of the present invention. In addition, in the step (3), the crude iron oxyhydroxide sol is referred to as fine particles of iron oxyhydroxide generated in this step. In addition to this, impurities such as ionic inorganic acid radicals and soluble salts This is because it contains impurities. The state of the solution in this step becomes a gel state partially containing sol when the hydroxycarboxylic acid is added, and even if an alkali agent is added next, a color change is recognized, but no significant change is observed in the state. That is, although it can be said that it is almost a gel state, in this invention, the solution obtained at the process of (3) was called crude iron oxyhydroxide sol.

Regarding the degree of washing, washing is carried out until the electrical conductivity of the filtrate is 1 mS / cm or less when filtered through an ultrafiltration membrane having a fractional molecular weight of 20000 or less. That is, by setting the electric conductivity of the filtrate to 1 mS / cm or less, the crude iron oxyhydroxide sol is completely solated, and the inorganic acid radicals contained in the iron oxyhydroxide sol are almost inorganic with respect to iron. The molar ratio of acid radical / iron (Fe) can be 0.02 or less. By reducing the inorganic acid radicals in the sol to such a limit, the solification of the crude iron oxyhydroxide sol that is the object of the present invention is completed, it has long-term storage stability, and it can withstand concentration and dilution. Sol can be obtained. The reason why inorganic acid radicals can be easily reduced to such a limit by an ultrafiltration membrane or the like is due to the addition of an appropriate alkali agent in step (3).
The iron oxyhydroxide sol of the present invention thus obtained is a brown and transparent solution. Most of the fine particles in the sol are fine crystals of iron oxyhydroxide. This can be confirmed by the powder X-ray diffraction of the dried sol having a broad diffraction pattern unique to iron oxyhydroxide (see FIG. 1).

Since the obtained iron oxyhydroxide sol of the present invention has excellent stability, it is optimal for concentration and concentration, and is also optimal for use by diluting with water or the like. In particular, when it is used after being concentrated, it can be concentrated with low viscosity by heating after washing in the step (4). The heating temperature and time conditions at this time may be appropriately selected depending on the intended degree of concentration, but it is usually preferable to carry out at 60 to 200 ° C. More preferably, the hydrothermal treatment is performed at 100 to 140 ° C. for about 2 to 10 hours. In the sol of the present invention, the particle size is uniformized and the dispersibility is improved by the heat treatment. Therefore, for example, the concentration can be increased to 10% by mass or more as iron oxide (Fe ₂ O ₃ ).

  The sol of the present invention thus produced has a feature that it contains almost no ionic iron. This feature avoids problems such as corrosion and poisoning and can be used in various industrial applications. Further, the sol of the present invention is not only diluted or concentrated, but also difficult to thicken or gel when mixed with other substances such as acids and alkalis and amphiphilic organic solvents such as alcohols. It has extremely excellent properties that it is difficult to do.

  The stable pH range of the sol of the present invention is 3 to 10, and more stable in the range of pH 6 to 8. Usually, the pH of the sol obtained after the step (4) is 6 to 12, but if necessary, the pH can be adjusted by adding acid or alkali within a stable range of the sol. For example, the pH can be lowered by adding hydroxycarboxylic acid to the obtained sol, and it can be made alkaline by adding ammonia or the like.

  EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto. In Examples, “%” means “% by mass” unless otherwise specified.

<analysis>
(1) The iron oxide concentration was calculated from the firing residue after firing the sol at 800 ° C. according to the old JIS K-5407-8. The iron concentration was calculated from the iron oxide concentration.
(2) Hydroxycarboxylic acid is measured using total organic carbon (TOC) analyzer TOC-Vc (manufactured by Shimadzu Corporation) because there is no carbon source other than hydroxycarboxylic acid in the raw materials used. It was converted to the molecular weight of hydroxycarboxylic acid.
(3) Chlorine was quantified by back titrating silver nitrate with an ammonium thiocyanate solution. Nitric acid was measured using PROGRAM KJELDALH 3 (manufactured by Foss Japan Co., Ltd.) by the Kjeldahl method.
(4) An X-ray diffractometer XRD-7000 (manufactured by Shimadzu Corporation) was used to identify the substance of the dried sol.
(5) The electric conductivity was measured using an electric conductivity meter CM-14S (manufactured by TOA ELECTRON Ltd.).
(6) The median diameter was measured using a dynamic light scattering color particle size distribution analyzer LB-500 (manufactured by Horiba, Ltd.).

<Filtration method>
As an ultrafiltration membrane, filtration was performed by an ultrafiltration apparatus using a model SLP-1053 (manufactured by Asahi Kasei Co., Ltd.) having a molecular weight cut-off of 10,000.
<Filtration leakage rate of iron components>
The filtration leakage rate of the iron component was calculated by the percentage of the mass of the iron component in the filtrate with respect to the mass of the iron component before filtration.
<Storage stability test>
The storage stability test was conducted by placing the sample in a 50 mL sample bottle and enclosing it in a 35 ° C. constant temperature bath.

[Example 1]
To a 2% ferric chloride aqueous solution 10000 g (pH 1.3) in terms of iron oxide (Fe ₂ O ₃ ), a 28% ammonia aqueous solution 365 g (NH ₃ / Cl molar ratio = 0.8) was added with stirring, and the gel A liquid in a state was obtained. After stirring for 2 hours, this solution became a transparent brown colloidal particle dispersion solution. The pH at this time was 2.1. Next, 481 g of a 10% citric acid aqueous solution (citric acid / Fe molar ratio = 0.10) was added to this solution with stirring, and immediately the pH was adjusted to 8.7 with a 10% aqueous ammonia solution. Thereafter, using an ultrafiltration device, the electrical conductivity of the filtrate is washed until below 100 .mu.S / cm, to obtain a 10.2% sol as Fe ₂ O _3. Since the analytical value of the iron component in the filtrate at this time was below the detection limit, the filtration leakage rate of the iron component was set to 0%. The obtained sol had a pH of 6.6, a median diameter of 6 nm, and citric acid / Fe (molar ratio) = 0.10. The powder X-ray diffraction pattern of the dried sol showed a diffraction pattern of FeO (OH), confirming that the obtained sol was an iron oxyhydroxide sol. 3 months storage stability of the sol obtained by concentrating the iron oxyhydroxide sol obtained in the present invention to an Fe ₂ O ₃ concentration of 15.0% by an evaporator and the sol obtained by diluting the iron oxyhydroxide sol to 5.0% with ion-exchanged water As a result of the sex test, the stable state was maintained. This indicates that the sol of the present invention is extremely stable.

[Example 2]
To 40000 g of 0.5% ferric nitrate aqueous solution (pH 1.3) in terms of iron oxide (Fe ₂ O ₃ ), add 766 g of 10% ammonium carbonate solution (NH ₃ / NO ₃ molar ratio = 0.6) with stirring. A gel-like liquid was obtained. After stirring for 2 hours, this solution became a transparent brown colloidal particle dispersion solution. The pH at this time was 1.8. Next, 504 g of a 10% malic acid aqueous solution (molar ratio of malic acid / Fe = 0.15) was added to this solution with stirring, and the pH was adjusted to 9.1 with 10% ammonia aqueous solution after stirring for 30 minutes. Thereafter, using an ultrafiltration device, the electrical conductivity of the filtrate is washed until below 50 [mu] S / cm, to obtain a 15.0% sol as Fe ₂ O _3. At this time, the filtration leakage rate of the iron component detected in the filtrate was 1%.
The obtained sol had a pH of 7.2, a median diameter of 10 nm, and malic acid / Fe (molar ratio) = 0.14. The powder X-ray diffraction pattern of the dried sol showed a diffraction pattern of FeO (OH), confirming that the obtained sol was an iron oxyhydroxide sol. In addition, as a result of conducting a storage stability test for 3 months, the stable state was maintained.

[Example 3]
To 6667 g (pH 1.2) of 3% ferric nitrate aqueous solution in terms of iron oxide (Fe ₂ O ₃ ), 892 g of 1% aqueous ammonia (NH ₃ / NO ₃ molar ratio = 0.7) was added with stirring. A gel-like liquid was obtained. After stirring for 2 hours, this solution became a transparent brown colloidal particle dispersion solution. The pH at this time was 1.9. Next, 752 g of a 10% tartaric acid aqueous solution (tartaric acid / Fe molar ratio = 0.20) was added to this solution under stirring, and the pH was adjusted to 10.0 with a 10% aqueous ammonia solution after 30 minutes of stirring. Thereafter, using an ultrafiltration device, the electrical conductivity of the filtrate is washed until below 400 [mu] S / cm, to obtain a 5.1% sol as Fe ₂ O _3. At this time, the filtration leakage rate of the iron component detected in the filtrate was 3%.
The obtained sol had a pH of 7.5, a median diameter of 11 nm, and tartaric acid / Fe (molar ratio) = 0.18. The powder X-ray diffraction pattern of the dried sol showed a diffraction pattern of FeO (OH), confirming that the obtained sol was an iron oxyhydroxide sol. In addition, as a result of conducting a storage stability test for 3 months, the stable state was maintained.

[Comparative Example 1]
To 10000 g of 2% ferric chloride aqueous solution (pH 1.3) in terms of iron oxide (Fe ₂ O ₃ ), add 502 g of 28% aqueous ammonia (NH ₃ / Cl molar ratio = 1.1) under stirring and gel A liquid was obtained. The pH at this time was 8.6, and even after stirring for 24 hours, this liquid was in a gel state with no clearness. Thereafter, as in Example 1, 481 g of a 10% citric acid aqueous solution (citric acid / Fe molar ratio = 0.10) was added with stirring, and the pH was immediately adjusted to 8.7 with a 10% aqueous ammonia solution. Thereafter, using an ultrafiltration device, the filtrate was washed until the electric conductivity of the filtrate reached 100 μS / cm or less, but the resulting product was a gel that settled with time and had excellent dispersibility. An iron oxide sol was not obtained. That is, it can be seen that the iron oxyhydroxide sol of the present invention cannot be obtained when the alkali agent / inorganic acid radical (molar ratio) departs from the scope of the present invention.

[Comparative Example 2]
After obtaining a transparent colloidal particle dispersion solution having a transparent feeling in the same manner as in Example 1, 4812 g of a 10% citric acid aqueous solution (citric acid / Fe molar ratio = 1.00) was added to this liquid under stirring. The pH was immediately adjusted to 8.7 with a 10% aqueous ammonia solution. Thereafter, using an ultrafiltration device, the electrical conductivity of the filtrate is washed until below 100 .mu.S / cm, to obtain a 0.2% sol as Fe ₂ O _3. However, the filtration leakage rate of the iron component detected in the filtrate at this time was 98%. That is, it can be seen that the sol is hardly formed unless the amount of hydroxycarboxylic acid added to iron is too large and appropriate.

[Comparative Example 3]
10% aqueous citric acid solution 481 g (citric acid / Fe molar ratio = 0.10) was added to 10000 g of 2% ferric chloride aqueous solution in terms of iron oxide (Fe ₂ O ₃ ) with stirring, and immediately 10% aqueous ammonia To pH 8.7. Thereafter, using an ultrafiltration device, the electrical conductivity of the filtrate is washed until below 100 mS / cm, to obtain a 5.1% sol as Fe ₂ O _3. However, the filtration leakage rate of the iron component detected in the filtrate was 9%. That is, it turns out that it becomes a partly ionic iron and a yield deteriorates. When this iron oxyhydroxide sol was concentrated to an Fe ₂ O ₃ concentration of 15% by an evaporator, the viscosity increased during the concentration. In addition, when the sol before concentration was subjected to a storage stability test for 3 months, the viscosity increased and the properties of the sol of the present invention differed significantly, and it was not a sol.

[Comparative Example 4]
After obtaining a transparent brown colloidal particle dispersion solution in the same manner as in Example 1, 481 g of a 10% aqueous citric acid solution (citric acid / Fe molar ratio = 0.10) was added to this liquid under stirring. Immediately the pH was adjusted to 3.0 with 10% ammonia solution. After that, using an ultrafiltration device, the filtrate was washed until the electric conductivity of the filtrate reached 100 mS / cm or less. The resulting product was a gel that settled with time, and had excellent dispersibility. An iron oxide sol was not obtained. That is, it can be seen that the iron oxyhydroxide sol of the present invention cannot be obtained if the pH after addition of the alkaline agent in the step (3) is out of the range of the present invention. Moreover, since the filtration leakage rate of the iron component detected in the filtrate was 27%, when the pH after addition of the alkaline agent deviated from the scope of the present invention, ionic iron was produced and washed. It can be seen that it is sometimes removed from the system.
The above results are summarized in Table 1.

It is a figure which shows the powder X-ray diffraction of the dried material of this invention sol of Example 1, Example 2, and Example 3. FIG.

Claims (3)

The manufacturing method of the iron oxyhydroxide sol manufactured by the following process (1)-(4).
(1) A step of adding an alkali agent to a water-soluble inorganic compound of iron in a range of alkali agent / inorganic acid radical (molar ratio) = 0.5 to 0.9 to obtain an iron hydroxide solution.
(2) A step of adding hydroxycarboxylic acid to the iron hydroxide solution of step (1) in a range of hydroxycarboxylic acid / iron (Fe) (molar ratio) = 0.05 to 0.20.
(3) A step of obtaining a crude iron oxyhydroxide sol by adding an alkaline agent to the solution obtained by adding the hydroxycarboxylic acid in step (2) to adjust the pH to 6 to 12.
(4) When the crude iron oxyhydroxide sol of step (3) is filtered through an ultrafiltration membrane having a molecular weight cut-off of 20000 or less, the filtrate is washed until the electrical conductivity of the filtrate becomes 1 mS / cm or less. Obtaining a sol; The method for producing an iron oxyhydroxide sol according to claim 1, wherein the hydroxycarboxylic acid in step (2) is at least one selected from citric acid, malic acid and tartaric acid. Iron oxyhydroxide sol produced by the production method according to claim 1 or 2.

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