JP3006633B2 - Iron oxide superparamagnetic fine particle powder and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an iron oxide superparamagnetic fine particle powder having a large magnetization value and better oxidation stability and exhibiting a black color, and a method for producing the same. The main use of the iron oxide superparamagnetic fine particle powder according to the present invention is as a material particle powder for a magnetic toner.

[0002]

2. Description of the Related Art Conventionally, as one method of developing an electrostatic latent image,
A development method using a so-called one-component magnetic toner using a composite particle in which magnetic particles such as magnetite particles are mixed and dispersed in a resin without using a carrier is widely known and widely used.

In recent years, along with high performance such as high speed and high image quality of copying machines, there has been a strong demand for improved characteristics of magnetic toner as a developer. It is required that the particle powder has a large magnetization value, excellent kneading properties, and a black color.

[0004] This fact is described in Japanese Patent Application Laid-Open No. 55-65406, entitled "Generally, such magnetic powder for a magnetic toner in the one-component system is required to have the following properties."
i) The magnetic flux density in a magnetic field of about 10 ³ Oe is as high as possible. For example, in an external magnetic field of 1000 Oe,
It is necessary to have a maximum magnetization value σm of about 40 emu / g or more. Iv) Blackness that can withstand practical use. The magnetic toner may contain a colorant, but it is preferable that the powder itself has a black color and no colorant is used. ‥‥ vii) Good mixing with resin.微 The degree of microscopic mixing in the toner is important for the characteristics of the toner. ‥‥ ”.

[0005] In order to improve the mixing property between the magnetic particle powder and the resin, it is necessary that the magnetic particle powder has excellent dispersibility. For this purpose, the residual magnetization is as small as possible. It is required that the magnetic cohesion is small.

[0006] The blackness of magnetic particles such as magnetite particles is described in "Powder and Powder Metallurgy", Vol. 26, No. 7, No. 239-2 (1979), published by the Japan Powder and Powder Metallurgy Association.
On page 40, "The degree of blackness of the sample depends on the Fe (II) content and the average particle size. ‥‥. When the Fe (II) content is 10% or more, there is a slight difference in the degree of blackness, Each of the samples is black, and when the Fe (II) content is reduced to 10% or less, each sample changes from black to reddish brown. It is known that it depends on the ²⁺ content, and it is required that the Fe ²⁺ content be 0.16 or more in molar ratio with respect to Fe ³⁺ .

Conventionally, as a magnetic particle powder used for a magnetic toner, a pH 10 obtained by reacting a ferrous salt aqueous solution with an alkaline aqueous solution having an equivalent or more to F ²⁺ in the ferrous salt aqueous solution is used. Octahedral magnetite particle powder obtained by aerating an oxygen-containing gas into the above suspension containing ferrous hydroxide colloid (JP-B-44-66)
No. 8), a ferrous hydroxide colloid obtained by reacting an aqueous ferrous salt solution with 0.80 to 0.99 equivalents of alkali hydroxide relative to Fe ²⁺ in the aqueous ferrous salt solution The first step in which spherical magnetite particles are generated by aerating an oxygen-containing gas into a ferrous salt reaction aqueous solution containing, and after the first step reaction, 1.00 equivalent or more of hydroxylation relative to the residual Fe ²⁺ There is a magnetite particle powder having a spherical shape obtained by heating and oxidizing at pH 10 or more by adding an alkali (Japanese Patent Publication No. Sho 62-51208).

[0008]

The present invention has a large magnetization value,
In addition, the magnetic particles having a small magnetic cohesion due to the remanence as small as possible and exhibiting black color are the most required at present, but the octahedron described above. The magnetite particle powder exhibiting a high magnetization value is as large as about 80 emu / g, and is excellent in blackness. However, the residual magnetization is as large as 9 emu / g or more and magnetic aggregation is likely to occur. However, kneadability is a problem. The magnetite particle powder having a spherical shape as described above has a large magnetization value of about 80 emu / g or more, but is slightly brownish black and inferior in blackness. The remanent magnetization is 4 to 6 emu /
g, which is relatively small compared to the octahedral magnetite particles, but is not yet sufficient.

Heretofore, iron oxide superparamagnetic fine particle powder has been known as a magnetic particle powder having a residual magnetization of substantially zero.

However, since the iron oxide superparamagnetic fine particles are very fine particles, they are easily oxidized by oxygen in the air and are extremely unstable chemically and magnetically. That is, when taken out into the air, an oxidation reaction occurs due to oxygen in the air and the magnetization value decreases. Further, the oxidation reaction proceeds during storage in the air, and the magnetization value decreases over time and over time. . Further, Fe ²⁺ is oxidized to Fe ³⁺ and turns reddish brown.

The present inventor has been involved in the production and development of iron oxide superparamagnetic fine particles for many years. In the process, the present inventors have a large magnetization value and have excellent oxidation stability. The technology to obtain iron oxide superparamagnetic fine particle powder has already been established.

That is, an unsaturated fatty acid or a salt thereof is added to an aqueous suspension containing iron oxide superparamagnetic fine particles, and the pH is adjusted to 9 or more.
Stirring at a temperature in the range of 50 to 100 ° C. to form a dispersion;
An unsaturated fatty acid is adsorbed on the particle surface of the iron oxide superparamagnetic fine particles by adding an acid to the dispersion to adjust the pH to 7 or less, followed by washing with water and dehydration, followed by organic miscible with water. It is made of iron oxide superparamagnetic fine particles having an unsaturated fatty acid adsorbed on the particle surface by wet drying in a solvent and then vacuum drying, and has a magnetization value of 50 emu / g or more and a change rate of the magnetization value of 10%. Is less than and
Fe ²⁺ content is 0.16 to 0.5 in molar ratio with respect to Fe ^3+.
(Patent Application No. 2-42177).

However, there is no end to the demand for improvement in oxidative stability, and there is a strong demand for iron oxide superparamagnetic fine particles having better oxidative stability.

Therefore, the present invention has a large magnetization value,
Another object of the present invention is to obtain iron oxide superparamagnetic fine particle powder having more excellent oxidation stability.

[0015]

The above technical object can be achieved by the present invention as described below.

That is, according to the present invention, Al or Si or any oxide of Al and Si or Al or Si is provided between the surface of the iron oxide superparamagnetic fine particles and the coating layer of the unsaturated fatty acid coating the surface of the particles. It is composed of iron oxide superparamagnetic fine particles in which a hydroxide is present, and has a magnetization value of 50 emu / g or more, a change rate of the magnetization value of 5% or less, and Fe ²⁺
Iron oxide superparamagnetic fine particle powder having a molar ratio of 0.16 to 0.5 with respect to Fe ^3+, and Al or Si or any oxide of Al and Si or water on the particle surface. An unsaturated fatty acid or a salt thereof is added to an aqueous suspension containing iron oxide superparamagnetic fine particles in which an oxide is present, and the mixture is stirred at a pH of 8 or more and at a temperature of 50 to 100 ° C. to form a dispersion. By adding an acid to the dispersion to adjust the pH to 7 or less, Al or Si or A
The unsaturated fatty acid is adsorbed on the iron oxide superparamagnetic fine particles in which an oxide or hydroxide of either l or Si is present, and then washed with water and dehydrated, and then washed with an organic solvent miscible with water. This is a method for producing iron oxide superparamagnetic fine particle powder by wet-drying and vacuum drying.

Next, various conditions for implementing the present invention will be described.

The production of the iron oxide superparamagnetic particles in the present invention can be performed by a conventional method. For example, a mixed aqueous solution containing a ferrous salt and a ferric salt in a molar ratio of 1: 2 is used. A method in which an alkali is added to adjust the pH to 9 or more, and then an aging reaction is performed at an appropriate temperature. And then add alkali
Oxidation at a temperature of from 70 to 70 ° C.
734) and the like.

In the present invention, the presence of Al or Si or an oxide or hydroxide of Al or Si on the particle surface of the iron oxide superparamagnetic particles is determined by a conventional method, for example, by using iron oxide superparamagnetic particles. After adding an alkaline aqueous solution to the suspension containing to adjust the pH to 10 or more, adding a compound containing Al or a compound containing Si or both the compounds,
Then, the suspension can be adjusted by adjusting the pH to 6.5 to 8.5.

The reason for setting the pH to 10 or more is to make the iron oxide superparamagnetic fine particles in a dispersed state and to uniformly mix with the added compound.

As the alkaline aqueous solution for adjusting the pH to 10 or more, sodium hydroxide, potassium hydroxide, aqueous ammonia and the like can be used.

The pH is adjusted to 6.5 to 8.5 so that Si or Al or an oxide or hydroxide of any of Si and Al is present on the surface of the iron oxide superparamagnetic fine particles.

As the acid for adjusting the pH, sulfuric acid, hydrochloric acid, acetic acid, nitric acid and the like can be used.

As the compound containing Al, alkali aluminates such as sodium aluminate and potassium aluminate, aluminum salts such as aluminum sulfate, aluminum chloride and aluminum nitrate can be used.

The amount of the compound containing Al is 0.01 to 10% by weight, preferably 0.05 to 5.0% by weight, in terms of Al, based on the iron oxide superparamagnetic fine particle powder. If the amount is less than 0.01% by weight, the effects of the present invention cannot be obtained. When the content exceeds 10% by weight, the desired effects of the present invention can be obtained, but the compound containing Al alone which does not exist on the particle surface of the iron oxide superparamagnetic particles increases, which is more than necessary. There is no point in adding to

The adsorption amount of the compound containing Al is 0.01 to 5.0 in terms of Al with respect to the iron oxide superparamagnetic fine particle powder.
% By weight. If it is less than 0.01% by weight, the effects of the present invention cannot be obtained. When the content exceeds 5.0% by weight, the desired effects of the present invention can be obtained, but the magnetization value decreases due to an increase in the Al-containing compound which does not contribute to magnetism, which is not preferable. In consideration of the dispersibility and the magnetization value of the iron oxide superparamagnetic fine particles, the content is preferably 0.05 to 3.0% by weight.

As the compound containing Si, sodium silicate, potassium silicate, colloidal silica and the like can be used.

The amount of the compound containing Si is 0.01 to 10% by weight, preferably 0.05 to 5.0% by weight, in terms of Si, based on the iron oxide superparamagnetic fine particle powder. If the amount is less than 0.01% by weight, the effects of the present invention cannot be obtained. Even when the content exceeds 10% by weight, the desired effect of the present invention can be obtained, but the number of compounds containing Si that are present alone without being present on the particle surface of the iron oxide superparamagnetic particles increases, and this is more than necessary. There is no point in adding to

The adsorption amount of the compound containing Si is 0.01 to 5.0 in terms of Si with respect to the iron oxide superparamagnetic fine particle powder.
% By weight. If it is less than 0.01% by weight, the effects of the present invention cannot be obtained. If the content exceeds 5.0% by weight, the desired effects of the present invention can be obtained, but the magnetization value decreases due to an increase in the amount of Si-containing compounds not involved in magnetism, which is not preferable. In consideration of the dispersibility and the magnetization value of the iron oxide superparamagnetic fine particles, the content is preferably 0.05 to 3.0% by weight.

The amount of adsorption when both the Si compound and the Al compound are contained is expressed as A to the iron oxide superparamagnetic fine particle powder.
0.01 to 5.0% by weight in total in terms of 1 and Si
Preferably it is 0.05 to 3.0% by weight.

The compound containing Si or the compound containing Al added to the suspension having a pH of 10 or more is Si or Al or an oxide or hydroxide of any of Si and Al, and the surface of the particles of the iron oxide superparamagnetic fine particles as hydroxide. Precipitates and deposits on the surface.

In the present invention, the unsaturated fatty acid or salt thereof is a suspension containing iron oxide superparamagnetic fine particles having Al or Si or any oxide or hydroxide of Al and Si present on the particle surface. PH 8 or more,
The dispersion is stirred at a temperature in the range of 50 to 100 ° C.

When the temperature is lower than 50 ° C., it is difficult to uniformly monodisperse the iron oxide superparamagnetic fine particle powder. When the temperature exceeds 100 ° C., the iron oxide superparamagnetic fine particle powder can be uniformly and monodispersed, but it is not industrial and economical because a special device such as an autoclave is required.

When the pH is less than 8, it becomes difficult to uniformly monodisperse the iron oxide superparamagnetic fine particles and to adsorb the unsaturated fatty acid on the surface of each particle.

As unsaturated fatty acids, monoene unsaturated fatty acids such as oleic acid and erucic acid; polyene unsaturated fatty acids such as linoleic acid and linoleic acid can be used.

The amount of the unsaturated fatty acid or salt added is 200% by weight or less based on the iron oxide superparamagnetic fine particles. 2
If the content exceeds 00% by weight, the amount of unsaturated fatty acids not adsorbed on the surface of the iron oxide superparamagnetic fine particles increases, and the efficiency of removing these fatty acids deteriorates.

The coating amount of the unsaturated fatty acid or salt thereof is 0.1 to 100% by weight based on the iron oxide superparamagnetic particles.
If it is less than 0.1% by weight, it is not possible to obtain superparamagnetic fine particles having more excellent oxidation stability. 100% by weight
If it exceeds 3, the amount of unsaturated fatty acids not adsorbed on the particle surface increases, so that the particles become hydrophilic powder and are easily oxidized.

In the present invention, an acid is added to the suspension to give p
H is set to 7 or less. As the acid, H ₂ SO ₄ , HCl,
CH ₃ COOH or the like can be used.

When the pH exceeds 7, it becomes difficult to convert the bimolecular unsaturated fatty acid adsorbed on the particle surface into a monomolecular layer in a dispersion having a pH of 8 or more, and the resulting particle powder is a hydrophilic powder. Becomes unstable to oxidation. In addition, the iron oxide superparamagnetic fine particle powder is in a dispersed state in water, and it is difficult to remove it as a precipitate.

If the pH is less than 4, the iron oxide superparamagnetic fine particles may be dissolved, so the lower limit is preferably 4.

The washing in the present invention may be performed by a conventional method, for example, by a method such as decantation.

As the organic solvent miscible with water in the present invention, methanol, ethanol, acetone, ether and the like can be used, and these low-boiling organic solvents are preferred.

The vacuum drying in the present invention may be performed by a method using an ordinary vacuum dryer or the like.

[0044]

First, the most important point in the present invention is that an unsaturated fatty acid or a salt thereof is added to an aqueous suspension containing iron oxide superparamagnetic fine particles, and the mixture is stirred at pH 8 or higher and at a temperature in the range of 50 to 100 ° C. To obtain a dispersion, and an acid is added to the dispersion to obtain p.
When the unsaturated fatty acid is adsorbed on the iron oxide superparamagnetic fine particles by setting H to 7 or less, then washed with water, dehydrated, then wetted with a water-miscible organic solvent, and then dried under vacuum. If Al or Si or any oxide or hydroxide of Al and Si is present on the surface of the iron oxide superparamagnetic fine particles before the unsaturated fatty acid is adsorbed, it is extremely stable against oxidation. As a result, the iron oxide superparamagnetic fine particles can be obtained, and as a result, the magnetization value is 50 emu / g or more, the change rate of the magnetization value is 5% or less, and the Fe ²⁺ content is reduced to Fe ³⁺ . This is a fact that iron oxide superparamagnetic fine particles having a molar ratio of 0.16 to 0.5 can be obtained.

Regarding the reason that the iron oxide superparamagnetic fine particles according to the present invention have a large magnetization value and more excellent oxidation stability, the present inventor has shown that the iron oxide superparamagnetic fine particles as described in the comparative examples described later. After adsorbing unsaturated fatty acids on the surface,
When dried by heating in air or vacuum, or when wetted in a water-miscible organic solvent without adsorbing unsaturated fatty acids on the surface of the iron oxide superparamagnetic fine particles, and then dried in vacuum. Is easily oxidized during drying or storage, so that the unsaturated fatty acid is adsorbed on the surface of the particles and wet-dried in an organic solvent miscible with water and then vacuum-dried. As a result, oxidation during drying or during storage can be suppressed, and in particular, Si or Al
Or, when any oxide or hydroxide of Si and Al is present, the unsaturated fatty acid is strongly adsorbed on the particle surface of the superparamagnetic fine particles via the oxide or hydroxide, It is believed that oxidation during storage is further suppressed.

The iron oxide superparamagnetic fine particles according to the present invention
Since the residual magnetization is substantially zero, there is almost no magnetic aggregation, and since the surface of each particle is coated with the unsaturated fatty acid, the kneading property with the resin is more excellent.

Conventionally, there is a method described in Japanese Patent Publication No. 53-4078 as a method for coating the particle surface of iron oxide superparamagnetic fine particles with oleate ion, which is a kind of unsaturated fatty acid. A lipophilic group is adsorbed on the particle surface in a monomolecular layer for the purpose of improving dispersibility in nonpolar and polar solvents. The present invention for obtaining magnetic fine particle powder is completely different from the present invention in its purpose, constitution and effect.

[0048]

Next, the present invention will be described with reference to examples and comparative examples.

The average particle diameter of the particles in the following Examples and Comparative Examples was determined from the half-value width of the X-ray diffraction line (311) using the Schierr formula. The magnetic properties were measured using an "oscillating sample magnetometer VSM-3S-15" (manufactured by Toei Kogyo Co., Ltd.) with an external magnetic field of up to 10 KOe. The rate of change of the magnetization value was represented by the rate of decrease (%) of the magnetization value after one month at room temperature.

The amount of Fe ²⁺ was measured by a chelate titration method. The L ^* value (brightness) and the a ^* value were obtained by measuring the colorimetric sample piece with a Hunter Lab space using a multi-source spectrophotometer MSC-IS-2D (manufactured by Suga Test Instruments Co., Ltd.).
^* Value, a ^* value, and b ^* value were measured, and the International Commission on Illumination (Commission International)
le de l'Eclairage, CIE) 197
6 (L ^* , a ^* , b ^* ) The values are displayed according to the uniform perceived color space. As the a ^* value representing reddishness approaches 0 and the value of the L ^* value is smaller, the blackness is better. The gloss of the coating film surface was indicated by a value measured using a digital gloss meter UGV-50 (manufactured by Suga Test Instruments Co., Ltd.) at incident angles of 45 ° and 60 °. The higher the gloss, the smoother the surface and the better the kneadability with the resin.

The measurement specimen was prepared by kneading 0.5 g of iron oxide superparamagnetic particles and 1.0 cc of castor oil with a Hoover type muller into a paste, adding 4.5 g of clear lacquer to the paste, kneading the mixture, and forming a paint. Then, it was obtained by coating on a cast-coated paper using a 6 mil applicator.

<Production of Iron Oxide Superparamagnetic Fine Particle Powder> Sample A: 3.3 mol / l Na heated to a temperature of 80 ° C.
1.4 mol / l of Fe in 34.5 l of OH aqueous solution
7.75 l of SO ₄ aqueous solution and 2.8 mol / l FeC
15.5 l of a mixed solution with 7.75 l of l ₃ aqueous solution
The solution was added dropwise over 5 minutes with stirring. Then, the mixture was stirred for 30 minutes while maintaining the temperature at 80 ° C. to produce a black precipitate.

Sample B: The slurry containing the black precipitate produced in the same manner as in Sample A was sufficiently washed with decantation, and water was added to obtain a 5 wt% slurry solution. NaOH was added to 500 g of the slurry solution to adjust the pH to 10, then the temperature was raised to 60 ° C., and the mixture was stirred for 30 minutes. Then
2.5 g of a 10 wt% sodium aluminate aqueous solution (corresponding to 0.33 wt% in terms of Al with respect to the black fine particles) was added, and the mixture was stirred at the same temperature for 10 minutes.
By adjusting to H8 and stirring for 30 minutes, aluminum hydroxide was deposited on the surface of the black fine particles.

Sample C: A hydroxide of aluminum was deposited on the surface of the black fine particles in the same manner as in the preparation of Sample B, except that the amount of the 10 wt% sodium aluminate aqueous solution was changed to 12.5 g.

Sample D: No. 3 water glass (manufactured by Tokuyama Soda Co., Ltd., Si
The amount corresponds to 29.0% by weight in terms of SiO ₂ . )
0.63 g (0.35 wt% of black fine particles in terms of Si)
%. ) Was added, except that silicon oxide was deposited on the surface of the black fine particles in the same manner as in the production of Sample B.

Sample E: The amount of No. 3 water glass added was 3.12.
Silicon oxide was deposited on the surface of the black fine particles in the same manner as in Sample D, except that g was used.

<Adsorption of Unsaturated Fatty Acid on Particle Surface> Examples 1 to 4; Example 1 500 ml of a slurry of sample B (solid content of 2 as black particles)
This corresponds to 5.0 g. ) Was added with 50 g of a 10 wt% aqueous sodium oleate solution (corresponding to 20 wt% of oleic acid with respect to the solid content of black particles), and the pH was 8 and the temperature was 6.
The mixture was stirred at 0 ° C. for 30 minutes to obtain a dispersion. Add 3N to the dispersion
HCl was added to bring the pH to 6. After cooling to room temperature,
Water was sufficiently washed by decantation and dehydrated.
Next, after being moistened with 10 ml of acetone, vacuum drying was performed at room temperature.

The obtained black powder was a spinel-type iron oxide as a result of X-ray diffraction, and had a particle size of 97 ° calculated from the half width of the reflection peak from the 311 plane in X-ray diffraction.
The amount of Al existing on the surface of the black fine particles is determined by the fluorescent X
As a result of a line analysis, it was 0.17% by weight.

As for the magnetic characteristics, the magnetization value is 55.7 em.
u / g, the residual magnetization is approximately 0, the coercive force is approximately 0, and Fe ²⁺
The content was 0.184 in molar ratio to Fe ³⁺ . With respect to the coating properties, the L ^* value was 16.95, the a ^* value was 0.21, and the glossiness was 61.0 when the reflection angle was 45 ° and 70.1 when the reflection angle was 60 °.

Examples 2 to 4, Reference Example 1, Comparative Examples 1 to 2 Type and amount of slurry containing black precipitate, type and amount of unsaturated fatty acid or salt thereof, pH and temperature during stirring, addition of acid Black particle powder was obtained in the same manner as in Example 1 except that the kind of acid, the pH after addition, and the kind of organic solvent in the process were variously changed.

The main production conditions at this time are shown in Table 1, and various characteristics of the black particle powder and the coating film containing the particles are shown in Table 2.

[0062]

[Table 1]

[0063]

[Table 2]

[0064]

The iron oxide superparamagnetic fine particle powder according to the present invention has a large magnetization value and more excellent oxidation stability,
In addition, the particles are excellent in blackness, and have low magnetic cohesion due to the fact that the residual magnetization is substantially zero, and have excellent kneading properties with a resin. Suitable as a powder.

The iron oxide superparamagnetic particle powder according to the present invention has a black color, has a small magnetic cohesion, and has excellent dispersibility due to the unsaturated fatty acid adsorbed on the particle surface. Therefore, it can be naturally used as a well-known pigment powder for paint or pigment powder for coloring resin.

Further, carbon black which has been conventionally used as a black pigment is problematic in terms of safety and health, such as carcinogenicity. However, the iron oxide superparamagnetic fine particle powder according to the present invention is an alternative to carbon black. Can be expected to be used.

[0067]

[Brief description of the drawings]

FIG. 1 is an electron micrograph (× 200000) showing the structure of iron oxide superparamagnetic fine particle powder obtained in Example 2.

FIG. 2 is an electron micrograph (× 200000) showing the structure of the iron oxide superparamagnetic fine particle powder obtained in Example 4.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-247514 (JP, A) JP-A-63-17222 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C01G 49/00-49/16 H01F 1/11 G03G 9/083

Claims (2)

(57) [Claims] 1. The method according to claim 1, wherein the surface of the iron oxide superparamagnetic fine particles and the unsaturated fatty acid coating layer covering
1 or iron oxide superparamagnetic fine particles in which an oxide or hydroxide of Si or Al and Si is present, and has a magnetization value of 50 emu / g or more and a change rate of the magnetization value of 5% or less. And the Fe ²⁺ content is Fe ³⁺
Iron oxide superparamagnetic fine particle powder having a molar ratio of 0.16 to 0.5 with respect to 2. An unsaturated fatty acid or a salt thereof is added to an aqueous suspension containing iron oxide superparamagnetic fine particles having Al or Si or an oxide or hydroxide of Al and Si on the particle surface. Add, pH 8 or more, temperature 50-100
The dispersion was stirred at a temperature in the range of 0 ° C., and an acid was added to the dispersion to adjust the pH to 7 or less.
Alternatively, the unsaturated fatty acid is adsorbed on the iron oxide superparamagnetic fine particles in which an oxide or a hydroxide of Si or Al and Si is present, then washed with water, dehydrated, and subsequently miscible with water. A method for producing iron oxide superparamagnetic fine particle powder, comprising drying in an organic solvent and vacuum drying.