JP3045207B2 - Production method of plate-like iron oxide particles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing plate-like iron oxide particles having a uniform particle size, excellent orientation and a controlled particle thickness. The main uses of the plate-like iron oxide particles according to the present invention include coatings, printing inks, cosmetics, coloring pigments for resin moldings, electromagnetic wave absorption, shielding, vibration suppression, soundproofing, magnetic For toner,
It is a material for magnetic recording and the like.

[0002]

_2. Description of the Related Art As iron oxide pigments, hematite (α-Fe ₂ O ₃ ) particles powder exhibiting red to reddish brown, and magnetite ( FeO _x .Fe ₂ O ₃ 0 <x ≦
1) Particle powder, brown-colored maghemite (γ-F
e ₂ O ₃ ) particle powders are known, and all of these particle powders are mixed and dispersed in a vehicle to prepare paints, printing inks,
It is widely used as a coloring pigment for producing cosmetics and the like, or kneading and dispersing in a resin, followed by molding to produce a resin molded product. In addition, among iron oxide pigments, iron oxide pigments having a plate-like shape are used as rust preventive pigments by taking advantage of the property of being excellent in coverage because plate-like particles are easily oriented due to the shape. It is also used as a coloring pigment for top coats of automobiles and the like by utilizing its metallic luster.

[0003] Further, as described above, magnetite particles and maghemite particles are used as color pigments and take advantage of their magnetic properties to absorb electromagnetic waves.
It is used in various fields as a material for shielding, vibration damping, soundproofing, magnetic toner, and magnetic recording.Especially, plate-like magnetite particles and maghemite particles are effective in their properties. It is expected to be used for

This fact is described, for example, in JP-B-63-418.
No. 53, "Opening the possibility of using flaky magnetic iron oxide particles for magnetic recording supports such as magnetic tapes, cards or discs containing one or more magnetic layers", "Magnetite or maghemite. There is another application for hexagonal flake iron oxide having the following crystal structure.
極 め て A very pronounced parallel orientation of the individual particles occurs in the coating material. Therefore, it is possible to achieve a significantly higher packing density, which results in, for example, an increased corrosion protection effect, more effective shielding against electromagnetic interference fields and higher conductivity. And Japanese Patent Application Laid-Open No. 61-138959, "When platelet-like or scale-like magnetizable particles are used as the magnetic component of a one-component toner, the color of the toner is determined by the magnetic component itself. Was found to be slightly and mainly due to the strong colorant added. "," These particles then consume any special classification energy with a simple stirring. Without a rapid conversion to a good dispersion in the binder or binder solution. "

As described above, plate-like iron oxide particles are used in various fields. In any of the above-mentioned fields, the iron oxide particle powders can sufficiently exhibit their original functions such as color and magnetism, as well as covering properties and shielding properties. In order to sufficiently exhibit various morphological properties such as dispersibility, it is excellent in dispersibility and orientation when mixing and dispersing in a vehicle, kneading and dispersing in a resin, and at the same time, depending on the type of resin and the type of solvent, There is a strong demand that the thickness can be freely controlled to a suitable thickness that is easy to conform to.

[0006] In order to improve the dispersibility of the plate-like iron oxide particles, it is desirable that the particle size be as uniform as possible. It is preferable that the particles overlap in the direction and are continuously arranged.

Conventionally, as a method of producing plate-like hematite particles, a method of hydrothermally treating an alkaline suspension containing ferric hydroxide particles or a ferric salt (Japanese Patent Publication No. 43-124).
No. 35, JP-B-45-54, JP-B-48-2
9718, JP-B-49-44878, JP-B-63-41853) The size of plate-like hematite particles formed in an alkaline suspension containing ferric hydroxide particles or a ferric salt is uniformized. Method of adding plate-like hematite particles as seed crystals and subjecting them to hydrothermal treatment
16978, JP-A-50-34000, JP-A-61-174120) and the like.

The plate-like magnetite particles are formed by reducing plate-like hematite particles generated from an aqueous solution in a reducing atmosphere to 300 to 4 times.
By heating and reducing in a temperature range of 50 ° C., the plate-like maghemite particles further reduce the plate-like magnetite particles by 200 to
It is obtained by oxidizing in a temperature range of 400 ° C. The obtained plate-like magnetite particles and plate-like maghemite particles retain and inherit the size, shape, and aggregation state of the plate-like hematite particles.

[0009]

Plate-like iron oxide particles having a uniform particle size and excellent orientation and having a controlled particle thickness are the most demanded at present. The obtained plate-like iron oxide particles do not sufficiently satisfy these properties.

That is, as shown in the electron micrograph of Comparative Example 1 described above, the particle size is relatively uniform in the above-mentioned known method as compared with the above-mentioned known method. The direction of the plate-like particles is not uniform, and
Since the individual particles exist independently, the orientation is not sufficient.

According to the above-mentioned known method, as shown in the electron micrograph of Comparative Example 2 below, the individual plate-like particles are oriented in a substantially constant direction, but the reaction of the plate-like hematite particles added as seed crystals. Due to the low property, the seed crystal growth reaction does not occur uniformly, and the particle size tends to be uneven.

In the above-mentioned methods, it is known that the thickness of the plate-like hematite particles to be produced can be controlled by the reaction temperature or the alkali concentration during the hydrothermal treatment. Plate-like hematite particles are easily generated, and plate-like hematite particles having a thickness of less than about 0.5 μm, which is generally said to have good orientation, are difficult to generate, and since the rate of change in particle thickness is small, It is difficult to control the thickness of the particles within a range of less than about 0.5 μm.

Accordingly, an object of the present invention is to provide a plate-like iron oxide particle powder having a uniform particle size, excellent orientation, and a controlled particle thickness.

[0014]

The above technical problems can be achieved by the present invention as described below. That is, the present invention
In a method of producing plate-like hematite particle powder by hydrothermally treating an alkaline suspension having a pH of 11 or more containing hydrous ferric oxide particles or a ferric salt, a needle-like form is previously prepared as a seed crystal in the alkaline suspension. Needle-like hematite particles obtained by heat-treating the hydrous ferric oxide particles are added , the pH is adjusted to 11 or more, and
Degree in the range 0.3-0.7N , then 150-33
Hydrothermal treatment in a temperature range of 0 ° C has a thickness of 0.5
μm or less plate-like hematite particles, if necessary, the plate-like hematite particles are reduced by heating to plate-like magnetite particles, or further oxidized to plate-like maghemite particles This is a method for producing iron oxide particles.

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

The hydrous ferric oxide particles in the present invention may be in the form of any of needles, spindles, and the like obtained by various known reactions. A wet cake obtained by washing and filtering the reaction mother liquor and a dried product of the wet cake can be used.

As the ferric salt in the present invention, ferric nitrate, ferric chloride, ferric sulfate and the like can be used.

The pH of the alkaline suspension of the present invention is 11 or more. If it is less than 11, acicular ferric hydroxide oxide particles or ferric hydroxide are mixed in the plate-like hematite particles. In addition, the reaction time is long, which is not industrial.

The hydrothermal treatment in the present invention is carried out usually in a temperature range of 150 to 330 ° C. using an autoclave apparatus. Generally, as the alkali concentration becomes higher, the reaction temperature at which plate-like hematite particles are formed tends to be lower. 150
When the temperature is lower than ° C, hydrous ferric oxide and acicular hematite particles are present as they are, and plate-like hematite particles are not generated. Although it is possible to generate plate-like hematite particles even when the temperature exceeds 330 ° C., considering safety on the device,
The upper limit of the temperature is 330 ° C.

The acicular hematite particles to be added as seed crystals in the present invention can be obtained by heat-treating acicular hydrous ferric oxide particles. Heat treatment temperature is 260
~ 700 ° C is preferred. If it is less than 260 ° C,
The dehydration reaction of the needle-like hydrous ferric oxide particles is insufficient, the effect as a seed crystal is small, and the plate-like hematite particles intended in the present invention cannot be obtained. When the temperature exceeds 700 ° C., sintering between the acicular particles and the particles occurs, and it is difficult to produce the plate-like hematite particles having good orientation, which is the object of the present invention.

The acicular hematite particles include Ti other than Fe,
Ni, Co, Zr or the like may be contained, and by containing these elements, plate-like hematite particles having a thinner particle thickness and excellent orientation can be easily generated, which is more effective. Thus, a plate-like hematite particle powder intended for the present invention can be obtained.

Ti is 0.1 to 20 mol%, Ni, C
As for o and Zr, 0.1 to 10 mol% can be contained in the acicular hematite particles, and in order to effectively obtain the plate-like hematite particle powder intended for the present invention, the Ti-containing acicular hematite particles are seeded. It is preferably used as a crystal.

The amount of the acicular hematite particles added is
It is 0.005 to 20 mol% based on mol. 0.0
When the content is less than 05 mol%, the effect as a seed crystal is insufficient, and the plate-like iron oxide particles having good orientation intended in the present invention cannot be obtained. If it exceeds 20 mol%, the hematite particles to be formed become acicular in shape, and the orientation of the particles is poor.

The heat reduction temperature of the plate-like magnetite particles in the present invention is 300 to 450 ° C. 300
If the temperature is lower than 0 ° C, the progress of the reduction reaction is extremely slow, and it takes a long time. If it exceeds 450 ° C., sintering occurs between the particles.

The oxidation temperature for forming plate-like maghemite particles in the present invention is 200 to 400 ° C. 200 ° C
If it is less than 1, the progress of the oxidation reaction is extremely slow, and it takes a long time. When the temperature exceeds 400 ° C., a rapid oxidation reaction occurs, transformation to hematite occurs, and hematite particles are mixed.

[0026]

First, the most important point in the present invention is that a plate-like hematite particle powder is produced by hydrothermally treating an alkaline suspension containing ferric hydroxide particles or a ferric salt having a pH of 11 or more. The needle-like hematite particles obtained by heat-treating the needle-like hydrous ferric oxide particles as seed crystals in advance in the alkaline suspension are added .
At the same time, the pH is adjusted to 11 or more, and the alkali concentration is adjusted to
0.3-0.7N range, then 150-330 ° C
This is a fact that when the hydrothermal treatment is performed in the above temperature range , the particle size is uniform, the orientation is excellent, and the thickness of the particles can be controlled.

The present inventor has described the reason why plate-like hematite particles having a uniform particle size can be obtained, as will be described in Examples below.
Since the reaction of forming plate-like hematite particles is completed in a short time, the needle-like hematite particles obtained by heating the needle-like hydrous ferric oxide particles have high reactivity, so that the needle-like hematite particles can be seeded. It is thought that this is because the growth reaction starts at the same time.

The reason why the orientation is excellent is as follows.
The present inventors have found that the thickness of the particles is appropriately thin as about 0.5 μm or less, that the particles are continuously arranged in an in-plane direction overlapping each other, and as shown in the electron micrographs of Examples described later, This is considered to be due to the structural characteristics of the resulting plate-like hematite particles, which have a large amount of holes near the center of the plate surface. It is considered that the pores near the center of the particles were generated due to the dissolution reaction of acicular hematite particles having good reactivity among the acicular hematite particles used as seed crystals.

In the present invention, the thickness of the plate-like hematite particles formed depending on the reaction temperature, the alkali concentration and the thickness of the acicular hematite particles is 0.5 μm or less, especially 0.05 to 0.05 μm.
It can be controlled freely within the range of 0.5 μm.

Now, a description will be given of a part of a number of experimental examples conducted by the present inventor as described below.

FIGS. 1 to 3 show the relationship between the thickness of the produced plate-like hematite particles, the reaction temperature, the alkali concentration, and the thickness of the acicular hematite particles. In FIG. 1, curves A, B and C are respectively shown in Example 4 and Comparative Example 1 described later (Japanese Patent Publication No.
This corresponds to JP-A-3-12435. ) And Comparative Example 2 (corresponding to Japanese Patent Publication No. 55-16978 among the known methods described above) in which the reaction temperature was varied under the same conditions. As shown in FIG. 1, in the present invention, the thickness of the particles can be effectively controlled in a thin range of 0.5 μm or less.

FIG. 2 shows a case where the alkali concentration is varied under the same conditions as in Example 4 described later. As shown in FIG. 2, in the present invention, the thickness of the particles is said to have good orientation. When the thickness is less than about 0.5 μm, more effective control can be achieved. FIG. 3 shows the case where the thickness of the acicular hematite particles as seed crystals was variously changed under the same conditions as in Example 4 described later, and as shown in FIG. Can be more effectively controlled at about 0.5 μm or less, which is said to be good.

[0033]

Next, the present invention will be described with reference to examples and comparative examples. In the following Examples and Comparative Examples, the average diameter of the particles is indicated by an average value of numerical values measured from an electron micrograph, and the thickness and plate ratio are the specific surface area value measured by the BET method and the average diameter. It was shown by the numerical value obtained from. The quality of the particle size distribution was shown by the result of observation with an electron microscope.

The degree of orientation is shown by the ratio of the X-ray peak intensity I _{(006) on} the 006 plane of hematite to the X-ray peak intensity I ₍₁₁₀₎ on the 110 plane of hematite obtained by X-ray diffraction using the following test piece. Was. The larger this value is, the better the degree of orientation is.

The measurement sample piece was prepared by using a hematite particle powder of 0.1%.
5 g and 1.0 cc of castor oil are kneaded with a Hoover-type muller to form a paste.
Add 5 g, knead it to make a paint, and put 6 g on cast coated paper.
Obtained by application using a mil applicator.

<Method for Producing Plate-like Hematite Particle Powder> Examples 1-5 Comparative Examples 1-2; Example 1 Acicular α-FeOOH particles (major axis 1 μm, axis ratio 20: 1,
Acicular hematite particles (major axis 1μm polycrystalline obtained a BET specific surface area of 60 m ² / g) for 1 hour heated and dehydrated at a temperature 400 ° C., axial ratio 20: 1, a BET specific surface area of 80 m ² /
g, with a crystallite size of 80 °) as a seed crystal.
44 mol (corresponding to 0.55 mol% with respect to α-FeOOH) and α-FeOOH (long axis 3 μm, axial ratio 1)
0, BET specific surface area 20 m ^{2 /} g) 0.8 mol and Na
0.3 N alkaline suspension obtained by mixing with 0.3 mol of OH was heated to 245 ° C. in an autoclave and kept at this temperature for 0.5 hour while mechanically stirring,
A dark purple precipitate formed. After cooling to room temperature, the dark purple precipitate was filtered off, washed thoroughly with water and dried. The obtained dry particle powder is a hematite particle as a result of X-ray diffraction, and as a result of observation with an electron microscope, a plate surface diameter of 6 μm and a thickness of 0.31 μm
In this case, the plate-like ratio was 19, and the particles overlapped each other in the in-plane direction and were continuously arranged. Further, the particle size distribution was narrow, and the degree of orientation was 40, which was excellent in orientation.

Example 2 The amount of the seed crystal used was 0.0019 mol (corresponding to 0.27 mol% based on α-FeOOH), α-FeOOH
0.7mol, NaOH 0.4m
ol, and a dark purple particle powder was obtained in the same manner as in Example 1 except that the hydrothermal treatment was performed at a temperature of 240 ° C. for 0.75 hours. The obtained dry particle powder was a hematite particle as a result of X-ray diffraction.
m, the thickness was 0.30 μm, the plate-like ratio was 26.7, and the particles overlapped in the in-plane direction and were continuously arranged. The particle size distribution is narrow, and the degree of orientation is 4
5 and excellent orientation.

Example 3 Acicular α-FeOOH particles (long axis 1.5 μm, axial ratio 15,
BET specific surface area 35m ^Two/ G) at 400 ° C for 1 hour
Polycrystalline needle-like hematite particles (length
1.5 μm axis, axial ratio 15, BET specific surface area 45 m^Two/
g, crystallite size of 110 °) as a seed crystal.
0018 mol (0.023 mol based on α-FeOOH)
1%. ) And α-FeOOH (long axis 2.5μ)
m, axial ratio 12, BET specific surface area 22m^Two/ G) 0.8m
ol and 0.2 mol of NaOH obtained by mixing.
2N alkaline suspension at 255 ° C. in autoclave
Heat to this temperature for 0.5 hour with mechanical stirring
Retained, forming a dark purple precipitate. After cooling to room temperature,
The dark purple precipitate was filtered off, washed thoroughly with water and dried. Obtained
X-ray diffraction shows that the dried powder
As a result of observation with an electron microscope, the plate surface diameter was 7 μm and the thickness was 0.
39 μm, the plate-like ratio is 18, and particles in the in-plane direction
They overlapped each other and were arranged continuously. Also, the particle size
The distribution is narrow and the degree of orientation is 35, which is excellent in orientation.
Was something.

Example 4 Acicular α-FeOOH particles (major axis: 0.5 μm, axial ratio: 10:
1. BET specific surface area 110 m ² / g, Ti containing Ti /
Polycrystalline Ti-containing needle-like hematite particles (major axis: 0.5 μm, axis ratio: 10: 1, BET specific surface area: 50 m ² / g, crystal obtained by heating and dehydrating Ti + Fe = 15 mol%) at a temperature of 500 ° C. for 30 minutes. (Crystal size 90 °) was used as a seed crystal, and the seed crystal was 0.0044 mol (0.5 with respect to α-FeOOH).
This corresponds to 5 mol%. ) And α-FeOOH (long axis 3)
μm, axial ratio 10: 1, BET specific surface area 20 m ² / g)
A 0.3 N alkaline suspension obtained by mixing 0.8 mol with 0.3 mol of NaOH is heated to 245 ° C. in an autoclave and kept at this temperature for 0.5 hour while mechanically stirring to obtain a dark purple A precipitate formed. After cooling to room temperature, the dark purple precipitate was filtered off, washed thoroughly with water and dried. As a result of X-ray diffraction, the obtained dry particle powder was a hematite particle, and was observed by an electron micrograph (× 150) shown in FIG.
As shown in (0), the plate surface diameter was 6 μm, the thickness was 0.21 μm, the plate ratio was 29, and the particles were overlapped and arranged continuously in the in-plane direction. Further, the particle size distribution was narrow, and the degree of orientation was 53, which was excellent.

Example 5 Acicular α-FeOOH particles (major axis: 0.5 μm, axial ratio: 10:
1. BET specific surface area 110 m ² / g, Ti containing Ti /
Polycrystalline Ti-containing needle-like hematite particles (major axis: 0.5 μm, axis ratio: 10: 1, BET specific surface area: 50 m ² / g, crystal obtained by heating and dehydrating Ti + Fe = 15 mol%) at a temperature of 500 ° C. for 30 minutes. (Crystal size 90 °) was used as a seed crystal, and the seed crystal was 0.0018 mol (0.2% based on α-FeOOH).
This corresponds to 7 mol%. ) And FeCl ₃ 0.7mo
and 0.1 mol of NaOH obtained by mixing 0.4 mol of NaOH with 0.4 mol of NaOH.
The 7N alkaline suspension was heated to 230 ° C. in an autoclave and held at this temperature for 1 hour with mechanical stirring to produce a dark purple precipitate. After cooling to room temperature, the dark purple precipitate was filtered off, washed thoroughly with water and dried. The obtained dry particle powder was a hematite particle as a result of X-ray diffraction, and as a result of observation with an electron microscope, the plate surface diameter was 7 μm and the thickness was 0.2.
The particle size was 8 μm, the plate ratio was 25, and the particles overlapped in the in-plane direction and were continuously arranged. In addition, the particle size distribution was narrow, and the degree of orientation was 41, which was excellent.

Comparative Example 1 A dark purple precipitate was formed in the same manner as in Example 4 except that no seed crystal was added. After cooling to room temperature, the dark purple precipitate was filtered off, washed thoroughly with water and dried. As a result of X-ray diffraction, the obtained dry particle powder was a hematite particle.
As shown in the electron micrograph (× 1000) shown in FIG. 1, the plate surface diameter was 8 μm, the thickness was 1.0 μm, and the plate ratio was 8.
In addition, the particle size distribution is hard to say excellent, due to the large thickness of the particles, the direction of the individual plate-like particles is not uniform, and furthermore, the individual particles exist independently, as a result, The degree of orientation was 5 and the orientation was poor.

Comparative Example 2 A dark purple precipitate was formed in the same manner as in Example 4, except that plate-like hematite particles (plate diameter: 5 μm, thickness: 0.6 μm, plate ratio: 8) were used as seed crystals. After cooling to room temperature,
The dark purple precipitate was filtered off, washed thoroughly with water and dried. As a result of X-ray diffraction, the obtained dry particle powder was a hematite particle, and had a plate surface diameter of 9 μm, a thickness of 0.7 μm and a plate ratio of 1 as shown in the electron micrograph (× 1000) shown in FIG.
It was 3. In addition, the particle size was uneven and individual particles were present independently. As a result, the degree of orientation was 7 and the orientation was poor.

<Method for Producing Plate-like Magnetite Particle Powder> Example 6 30 g of the plate-like hematite particle powder obtained in Example 3 was H
_Two gases were introduced at a rate of 0.5 l / min to reduce the temperature to 380.
It was reduced at ℃ to obtain a plate-like magnetite particle powder. As a result of observation with an electron microscope, the obtained plate-like magnetite particles had a plate surface diameter of 7 μm, a thickness of 0.39 μm, and a plate ratio of 18, and the particles were overlapped in the in-plane direction and continuously arranged. Was. The particle size distribution was narrow, and the orientation was excellent as a result of observation with a scanning electron microscope.

Example 7 30 g of the plate-like hematite particle powder obtained in Example 1 was mixed with H
_Two gases were introduced at a rate of 0.5 l / min to reduce the temperature to 380.
It was reduced at ℃ to obtain a plate-like magnetite particle powder. As a result of observation with an electron microscope, the obtained plate-like magnetite particles had a plate surface diameter of 6 μm, a thickness of 0.30 μm, and a plate ratio of 20, and the particles were overlapped in the in-plane direction and arranged continuously. Was. The particle size distribution was narrow, and the orientation was excellent as a result of observation with a scanning electron microscope.

<Production method of plate-like maghemite particle powder> Example 8 20 g of the plate-like magnetite particle powder obtained in Example 7 was oxidized in air at 280 ° C for 30 minutes to obtain plate-like maghemite particle powder. As a result of observation with an electron microscope, the obtained plate-like maghemite particles had a plate surface diameter of 6 μm, a thickness of 0.31 μm, and a plate-like ratio of 19, and were continuously arranged with the particles overlapping in the in-plane direction. Was. The particle size distribution was narrow, and the orientation was excellent as a result of observation with a scanning electron microscope.

[0046]

[Table 1]

[0047]

According to the method for producing plate-like iron oxide particles according to the present invention, the particle size is uniform, the orientation is excellent, and the thickness of the particles is controlled, as shown in the above examples. The obtained plate-like iron oxide particle powder has excellent dispersibility, orientation and conformability to vehicles and resins, and is suitable for paints, resins, printing inks, cosmetics, resin moldings, etc. It is suitable as a coloring pigment, a material for electromagnetic wave absorption, shielding, vibration suppression, soundproofing, magnetic recording for magnetic toner, and the like.

Further, the plate-like iron oxide particles according to the present invention, as shown in the above-described embodiment, have elongated small holes near the center of the plate-like particles. When a film is formed or a molded product is produced, the resin is mixed into the pores of the particles, thereby strengthening and preventing peeling of the coating film. It does not impair the luminosity of the camera.

[Brief description of the drawings]

FIG. 1 shows the relationship between the thickness of the produced plate-like hematite particle powder and the reaction temperature.

FIG. 2 shows the relationship between the thickness of the produced plate-like hematite particles and the alkali concentration.

FIG. 3 shows the relationship between the thickness of the produced plate-like hematite particle powder and the thickness of acicular hematite particles used as seed crystals.

FIG. 4 is an electron micrograph (× 1500) showing the particle structure of the plate-like hematite particle powder obtained in Example 4.

FIG. 5 is an electron micrograph (× 1000) showing the particle structure of the plate-like hematite particle powder obtained in Comparative Example 1.

FIG. 6 is an electron micrograph (× 1000) showing the particle structure of the plate-like hematite particle powder obtained in Comparative Example 2.

Claims (2)

(57) [Claims] 1. A method for producing a plate-like hematite particle powder by hydrothermally treating an alkaline suspension having a pH of 11 or more containing hydrous ferric oxide particles or a ferric salt. Needle-like hematite particles obtained by heat-treating needle-like hydrous ferric oxide particles as seed crystals are added , the pH is adjusted to 11 or more, and
First, a plate-like hematite particle having a thickness of 0.5 μm or less is formed by setting the alkali concentration in the range of 0.3 to 0.7 N and then performing a hydrothermal treatment in a temperature range of 150 to 330 ° C. A method for producing a plate-like iron oxide particle powder, comprising: 2. The heat treatment of the plate-like hematite particles obtained in claim 1 to form plate-like magnetite particles, or
A method for producing plate-like iron oxide particles, which is further oxidized into plate-like maghemite particles.