JPS58151333A - Manufacture of needlelike iron oxide particle - Google Patents

Abstract

PURPOSE:To manufacture needlelike iron oxide particles giving a magnetic recording medium having high output and high sensitivity and capable of increasing the recording density by adding sulfate to needlelike beta-iron oxyhydroxide particles and by calcining and particles by heating in air. CONSTITUTION:To needlelike beta-iron oxyhydroxide particles as a starting material is added 0.5-5.0wt% as SO4<2-> of sulfate basing on the amount of the particles, and the particles are calcined by heating at 300-500 deg.C in the air to form needlelike alpha-Fe2O3 particles holding the needlelike crystal form of the starting material. The alpha-Fe2O3 particles are reduced under heating in a reducing gas and further oxidized to obtain needlelike maghemite particles. Said alpha-Fe2O3 particles are optionally coated with a sintering inhibitor and reduced under heating in a reducing gas to obtain needlelike magnetite particles.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to obtaining a powder of acicular α-ha^ particles that retains and inherits the acicular crystals of acicular β-iron oxyhydroxide particles, and to generate magnetic particles using this as a precursor. The purpose of the present invention is to obtain acicular magnetite particles and acicular maghemite particles, which are recording material powders.

Specifically, 24 ttl of 0.5 to 5.0% by weight of sulfate (calculated as 30.2) based on the acicular β-iron oxyhydroxide particles which are the starting materials. .

After that, by heating and firing in the air at a temperature range of 300 to 500°C, the needle crystals α that retain the needle crystals of the starting material are produced.
-F~, heating and reducing the acicular crystals a-Fe, O, particles in a reducing gas to obtain acicular magnetite particles, and reducing the acicular α-F~ pseudo particles. After being heated and reduced in a neutral gas, the acicular crystals α-? −
After coating the 03 particles with an anti-sintering agent, they were heated and reduced in a reducing gas to obtain acicular magnetite particles, and the particles were coated with an anti-sintering agent to form the acicular crystals α-p~. After that, it is characterized by being heated and reduced in a reducing gas and further oxidized to form acicular maghemite particles.

In recent years, as magnetic recording and reproducing equipment has become smaller and lighter, there has been an increasing need for higher performance magnetic recording media such as magnetic tapes and magnetic disks. That is, improvements in high-density recording properties, high output properties, high sensitivity properties, and frequency properties are required.

The characteristics of a magnetic material suitable for satisfying the above requirements for magnetic recording media include having high coercive force in terms of magnetic properties, and having acicular crystals in terms of powder properties. , the particle size is uniform, and twins and dendritic particles are not mixed.0 In other words, when magnetic particle powder with high coercive force is used as a magnetic recording medium, it has a large residual magnetic flux density Br and a high coercive force. Because of this, high density recording and high output characteristics can be obtained.

In addition, the output characteristics and sensitivity characteristics of magnetic recording media such as magnetic tapes and magnetic disks depend on the residual magnetic flux density Br, and the residual magnetic flux density Br depends on the dispersibility of magnetic particles in the vehicle,
It depends on the orientation and filling properties in the coating film.

In order to improve the dispersibility in the vehicle, the orientation and filling properties in the coating film, it is necessary that the magnetic particles dispersed in the vehicle have acicular crystals, uniform particle size, and dendritic particles. is required not to be &fE.

Conventionally, acicular magnetite particles have mainly been used as magnetic recording materials.1. Alternatively, acicular maghemite particle powder or the like is used.

These acicular magnetic iron oxide particles are generally produced by heating and baking acicular iron oxyhydroxide particles in air to produce acicular magnetic iron oxide particles.
- Obtained by heating and reducing Fe203 particles in a reducing gas to obtain acicular magnetite particles, which are then heated and oxidized in air at 200 to 600°C to obtain acicular maghemite particles. There is. The coercive force of the acicular crystal magnetic iron oxide particles largely depends on the shape anisotropy, and the acicular structure of the acicular crystal magnetic iron oxide particles is one of the most important properties.

As mentioned above, acicular magnetic iron oxide particles with acicular crystals, uniform particle size, and no twin or dendritic particles are currently in greatest demand. In order to obtain acicular crystal magnetic iron oxide particles having such characteristics, first, iron oxyhydroxide particles as a starting material must have acicular crystals, have uniform particle size, and have no twin or dendritic structure. It is necessary that the particles are not mixed together, and then how to maintain and inherit the particle morphology, especially the acicular crystals, and conduct heat sintering, heat reduction, and heat oxidation to form acicular crystal magnetic iron oxide particle powder. The big question is whether to do so.

First, iron oxyhydroxide particles as a starting material will be described.

As iron oxyhydroxide, α-iron oxyhydroxide, β-iron oxyhydroxide, r-iron oxyhydroxide, etc., which have different crystal structures, are known.

Compared to α-iron oxyhydroxide particles and l-iron oxyhydroxide particles, β-iron oxyhydroxide particles have a uniform particle size and an acicular morphology without twins or dendritic particles. Since it has the characteristic that it is easy to obtain particles exhibiting the same characteristics, it is very preferable as a starting material for magnetic iron oxide particles for magnetic recording.

Next, the particle morphology of the acicular crystal β-oxyhydride iron particles, which is the starting material, is heated and oxidized into acicular crystal magnetic oxides by heating and calcination while retaining and inheriting the acicular crystals. The question is whether to use iron particles.

Regarding the heating and sintering process, acicular β-iron oxyhydroxide particles with uniform particle size and no twin or dendritic particles are heated and sintered at 500°C or higher to form α-Fe2os particles. The needle-like crystals are broken down and are in the form of a lump.

This fact is based on the article in Japanese Patent Publication No. 47-39477 that states, ``When the acicular shape collapses due to surface heating and large lumps of α-Fe2os are produced, there is always a sudden generation of gas, and analysis shows that it is a type of chloride. I learned that.・It is clear from the description that song-J.

The reason why the acicular β-iron oxyhydroxide particles contain C/- roots will be explained below.

Conventionally, two methods are known for producing acicular β-iron oxyhydroxide particles.

One of them is a method of hydrolyzing a ferric chloride aqueous solution, and the other is a method of performing an oxidation reaction by passing an oxygen-containing gas through the ferrous chloride aqueous solution.

In any of the above methods, since an aqueous iron chloride solution is used as the iron raw material, a large amount of at-roots is contained in the produced β-iron oxyhydroxide particles.

The OZ-roots contained in the particles cannot be completely removed even after repeated washing, and the acicular β-iron oxyhydroxide particles contain 2 to 8 weight meters of CI-roots. .

Conventionally, methods for obtaining acicular α-Fr0g particles by heating and baking acicular β-iron oxyhydroxide particles without breaking the acicular crystals have been disclosed in Japanese Patent Publication No. 42-24662 and the aforementioned Japanese Patent Publication No. There is a method described in Japanese Patent No. 47-39477.

Special Public Service 1977-. In the method described in 24662, acicular β-iron oxyhydroxide particles are heated at 205 to 275°C.
This method attempts to suppress the action of OZ-roots by gradually heating and firing at a low temperature of isn't it.

In the method described in Japanese Patent Publication No. 47-Zf14ff7, acicular β-iron oxyhydroxide particles are heated at 85 to 100°C in an aqueous solution of sodium hydroxide, ammonium acetate, sodium carbonate, aqueous ammonia, etc. By heating for 2 to 10 hours, chlorides are removed and acicular β-iron oxyhydroxide particles containing no C/- roots are obtained.

In view of the above, the present inventor has developed an α-F~03
When making powder particles, the action of OZ-roots contained in the particles is suppressed to maintain the particle morphology of the acicular crystal β-iron oxyhydroxide particles, especially the acicular crystals that have inherited the acicular crystal structure. Crystal α-I
As a result of various studies on how to obtain I'% Oa particles,
The present invention has been achieved.

That is, the present invention provides Sq for the acicular β-iron oxyhydroxide particles as a starting material.
By heating and baking in the air at a temperature range of 500 to 500°C, the needle crystals α- 1
``e203 particles, the acicular α-Ff32- particles are heated and reduced in a reducing gas to obtain acicular magnetite particles, and the acicular α-1'%Os particles are reduced by heating in a reducing gas. After heating and reducing the particles in a reducing gas, the particles are further oxidized to form acicular maghemite particles, or if necessary, the acicular α-1lPe2as particles are coated with an anti-sintering agent and then heated in a reducing gas. The particles are reduced to acicular crystal magnetite particles, and the particles are coated with an anti-sintering agent to form the acicular crystal α-F, heated and reduced in a reducing gas, and further oxidized to form acicular crystal maghemite. It is characterized by being in the form of particles.

Next, the technical background that led to the completion of the present invention and the configuration of the present invention will be described.

The present inventor has developed the method of heating and calcining acicular β-iron oxyhydroxide particles with uniform particle size and no twin or dendritic particles (
In order to prevent the acicular crystals of the particles from collapsing when forming Z-F l3203 particles, it is necessary to perform some kind of treatment on the acicular β-iron oxyhydroxide particles prior to heating and firing. We thought that it was necessary, and as a result of various studies on treatment methods, we found that the starting material, acicular β-iron oxyhydroxide particles, was
When calcined in air at a temperature range of 600 to 500°C after containing 0.5 to 5.0% by weight of sulfate epsilon, the particle size is uniform and twins and dendritic particles are mixed. We have obtained a new finding that it is possible to obtain acicular α-Fe, O, particles that retain and inherit the morphology of acicular crystal β-iron oxyhydroxide particles that do not have acicular crystals.

Acicular crystal β-iron oxyhydroxide particles powder t Sulfate coating of 0.5 to 5.0% by weight in terms of Sq to the needle crystal iron oxyhydroxide particles When calcined in a temperature range of Although the theoretical elucidation of the phenomenon in which particles are obtained is still unclear, the inventor of the present invention thinks as follows.

Acicular β-iron oxyhydroxide particles are heated and fired to form α-Fe.
, Q, when observing the process of forming particles in more detail, first,
The acicular β-iron oxyhydroxide particles are heated and dehydrated to become acicular β-Fe203 particles, and then the acicular β-Fe,
Crystal transformation of O, particles to α-Fe, 03 particles occurs.

This fact is shown in [......β-IFeO(OH
) particles (β-iron oxyhydroxide particles) are thermally decomposed,
Although they are very fine particles, they become β-F%Os particles that are three-dimensionally oriented with each other, and it is thought that these particles are further transformed into the final product α-Fe, 03 by thermal decomposition. It is clear from the statement ``.

Needle crystal β-Pe20 from needle crystal β-iron oxyhydroxide particles
Since the change to 8 is a tobotactic reaction, there is no change in shape, so the generated β-Pros particles are acicular β-Pros particles.
Although the needle-like crystals of iron oxyhydroxide particles are retained and inherited, α-F is formed from the subsequently generated needle-like β-ha^ particles.
During the crystal transformation to e2% particles, the at-root acts to break down the needle-like crystals and produce a liquid phase of lump-like particles, and then the Fe
When O13 is thermally decomposed, particles are generated into lumpy a-IF~, and at the same time, the decomposed O/ is acicular crystal β-F.
Acicular β-Fe,%
This is thought to be due to the so-called dissolution precipitation process in which dissolution of particles and precipitation of α-Fe)% particles occur.

Therefore, the present inventor investigated the particle morphology of the acicular β-iron oxyhydroxide particles, in particular, the acicular crystals α-I' that retain and inherit the acicular crystals.
In order to obtain %Qa particles, first, acicular β-Fe20
It is necessary for the three particles to exist stably, and then, during the crystal transformation from the acicular β-ri particles to the acicular α-ha particles, a liquid phase of FeO/, must not be generated. As a result of repeated studies on substances that have such effects, we found that sulfates are effective.

That is, when acicular β-oxyhydroxide particles are impregnated with sulfate and then heated and calcined in the air at a temperature range of 300 to 500°C, the acicular β-oxyhydroxide particles are dissolved due to the presence of the sulfate. Acicular β-Pe208 particles can be stably produced from iron oxide particles, and during crystal transformation from acicular β-Pe208 particles to acicular hematite particles, F e O1B
It is thought that since the liquid phase of the acicular crystals is not generated, the acicular crystals do not collapse, and particles can be obtained into acicular crystals α-F~ that retain the particle morphology of the acicular crystal β-iron oxyhydroxide particles. It will be done.

Furthermore, the fact that F e O/3 generates a liquid phase when heated and fired at 300°C or higher is explained in Physical and Chemistry Dictionary 168-16.
9 pages (Iwanami Shoten, 1967) “...F”
This is clear from the description "e04...melting point 300°C...".

Because of the heating and firing at a low temperature, we were able to obtain acicular α-Fe, 03 particles that retained the acicular crystals of the acicular β-iron oxyhydroxide particles without causing the acicular crystals to collapse. considered to be a thing.

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

The sulfate used in the present invention can be used in either a solid state or a solution state, but it is preferably used in a solution state in order to mix uniformly.

The sulfate in the present invention is 0.5 to 5.0% by weight based on 5O4-1 based on the acicular β-iron oxyhydroxide particles, and the sulfate is so , - If the amount is 0.5% by weight or less in terms of acicular crystal β-iron oxyhydroxide particles, the particle morphology of the acicular crystal β-iron oxyhydroxide particle powder, especially the acicular crystal α-r~pseudo particles that retain and inherit the acicular crystal. The effect of obtaining powder is not sufficient.

Even when the weight is 5.0 weight or more, it is possible to obtain acicular α-Fe, O, particles that retain and inherit the particle morphology of the acicular β-iron oxyhydroxide particles, especially the acicular crystals. However, the acicular magnetite particles obtained by thermal reduction of the acicular α-Fe 2% particle powder or the acicular maghemite particles obtained by further thermal oxidation have a lower purity. The saturation magnetization is greatly reduced, which is not preferable.

The temperature at which the sulfate is used in the present invention can sufficiently achieve the desired purpose even at room temperature.

It goes without saying that similar effects can be obtained when heated.

The heating and firing temperature in the present invention is in the temperature range of 300 to 500°C.

If the heating and firing temperature is 300°C or less, needle crystal α
- It takes a long time to obtain Fe203 particles, and the temperature is 500°C.
If it is above, the growth of a single particle in the particles to the generated α-F~ becomes excessive, causing deformation of the particles and sintering of the particles and each other.

According to the above-mentioned conditions, the particle morphology of acicular β-iron oxyhydroxide particles with uniform particle size and no twin or dendritic particles, especially acicular crystals that retain and inherit acicular crystals. α-1
lI'F3203 particles can be obtained by heating and reducing the acicular α-F%Qa particles in a reducing gas or further oxidizing them, or if necessary, the acicular α-h ^ After coating the particles with an anti-sintering agent, the particles are reduced by heating in a reducing gas or further oxidized to produce acicular magnetite particles with uniform particle size and no twin or dendritic particles. A powder or acicular maghemite particle powder can be obtained.

In the thermal reduction step of the present invention, the acicular crystal α-F82 obtained by heating and firing the acicular crystal β-iron oxyhydroxide particles while retaining and inheriting the particle morphology, especially the acicular crystal, as desired.
03 particles can be coated with an anti-sintering agent in advance, and in this case, acicular crystal magnetite particles with even better acicular crystals can be obtained, and maghemite obtained by oxidizing the magnetite particles The particles are also preferably acicular.

This fact will be explained below.

Acicular β-iron oxyhydroxide particles with uniform particle size and no twin or dendritic particles are heated and fired while retaining the particle morphology, especially the acicular crystals, and when the α-element temperature increases, The deformation of the acicular crystal particles of the acicular magnetite particles and the sintering between the particles and the particles become significant, and the coercive force of the obtained acicular magnetite particles becomes extremely low.

In particular, when the atmosphere is reducing, the shape of the particles is easily affected by the heating temperature, and particle growth is significant, with single particles growing to a size exceeding the size of the shell particles, and the outer shape of the shell particles becoming smaller. It gradually disappears, causing deformation of the particle shape and sintering of the particles and each other.

As a result, the coercive force decreases.

Therefore, the particle morphology of acicular β-iron oxyhydroxide particles with uniform particle size and no twin or dendritic particles, especially the acicular crystals obtained by heating and firing while preserving and inheriting the acicular crystals. α−
In order not to destroy the needle-like crystals of the P%On particles, the thermal reduction is usually carried out at 300 to 450°C.

As mentioned above, deformation of the particle shape and sintering between particles in a reducing gas occur when acicular β-iron oxyhydroxide particles are heated and calcined, as is well known. The acicular α-IF8203 particles do not grow sufficiently, and therefore, due to the small crystallinity of the particles, the growth of a single particle of the produced particles is rapid in the thermal reduction process. Uniform particle growth is difficult to occur,
Therefore, it is considered that in the portion where the grain growth of a single grain has rapidly occurred, sintering of grains and grains occurs, and the shape of grains tends to collapse.

Therefore, in order to prevent particle shape deformation and sintering between particles and particles in the heat-reduction process, it is necessary to make sure that the grain size is uniform and that twins and dendritic particles are not mixed together before the heat-reduction process. There is a method in which acicular α-Fe203 particles are coated with an organic or inorganic compound that has a sintering prevention effect.

Note that the sintering prevention effect here refers to the effect of suppressing the rapid particle growth of single particles in the particles produced during the heating reduction process, and substances that have this effect are hereinafter referred to as sintering inhibitors. .

As is well known, the particles are heated and reduced at 50°C to 500'C to form acicular crystals α-F~, which is coated with an anti-sintering agent and has a uniform particle size and is free of twins and dendritic particles. Acicular magnetite particles with uniform particle size and no twin or dendritic particles can be obtained.

If the temperature is 550°C or lower, the reduction reaction progresses slowly and requires a long time.

In addition, if the temperature is 500°C or higher, the reduction reaction will rapidly proceed, causing deformation of the acicular crystal particles and sintering of the particles and each other.
Inorganic compounds include well-known Sl, 11%, OrXMn,
One or more compounds such as Ou, Ni, and P, such as sodium silicate, colloidal silica, aluminum sulfate, sodium aluminate, chromium sulfate, nickel sulfate, and sodium metaphosphate, can be used.

A method for coating the sintering inhibitor by adding powder particles to the acicular crystals α-F and mixing them into an aqueous solution containing the sintering inhibitor is also effective, but it is difficult to coat the particle surface uniformly. Preferably, it is deposited as a hydroxide of the sintering inhibitor.

As mentioned above, when a magnetic tape is manufactured using the acicular magnetite particles or the acicular maghemite particles obtained by the method of the present invention, the dispersibility in the vehicle and the orientation in the coating film are improved. Because the properties and filling properties are extremely hard,
It is possible to obtain a magnetic recording medium that has high output, high sensitivity, and is capable of high recording density, which are currently most required.

Next, the present invention will be explained with reference to Examples and Comparative Examples.

The long axes and axial ratios of the particles in Examples and Comparative Examples are shown as average values of values measured from electron micrographs.

Example 1 Acicular β-iron oxyhydroxide particle powder 100C1 containing 18% by weight of chlorine and having a long axis of 0.6 μm and a monoaxial ratio (long axis: short axis) of 25:1 was mixed with 1 mol/4 sodium sulfate aqueous solution 40 / After stirring and mixing for 30 minutes, separate p, wash with water,
Dry.

The obtained acicular β-iron oxyhydroxide particle powder contains 16% by weight of chlorine and α in terms of 5O4-”
There was 62% by weight of sodium sulfate in t3fl.

This acicular β-iron oxyhydroxide particle powder 10009 was heated and calcined in air at 450°C to form α-Fe10. A particulate powder was obtained.

As is clear from the electron micrograph (xsoooo) shown in FIG. 1, the obtained α-F~ particles are acicular β-iron oxyhydroxide with a long axis of 06 μm and a monoaxial ratio (long axis: short axis) of 25:1. It retained the acicular structure of the particles, had a uniform particle size, and did not contain twins or dendritic particles.

100 g of the above needle-shaped α-Frog particle powder was put into a retort container with one end open in 64, and the retort container was rotated.
Gas was passed through the reactor at a rate of 21/min, and reduction was carried out by heating at a reduction temperature of 650° C. to obtain acicular magnetite particle powder.

As a result of electron microscopic observation, the obtained acicular magnetite particles had a long axis of 0.6 μm and a uniaxial ratio (long axis: short axis) of 25
: 1, the particle size was uniform, and twins and dendritic particles were not mixed.

Further, regarding the magnetic properties, the coercive force was 4200 s, and the saturation magnetization σ8 was B5.5 emu/.

Next, the acicular magnetite particles 909 were heated and oxidized in air at 300° C. for 60 minutes to obtain acicular maghemite particles.

As a result of electron microscopic observation, the obtained acicular maghemite particles had a long axis of 0.6μ and an axial ratio (long axis: short axis) of 25.
: 1, the particle size was uniform, and twins and dendritic particles were not mixed.

Further, regarding the magnetic properties, the coercive force was 5900e, and the saturation magnetization σB was 76.50 m''/9.

Examples 2 to 8 The same acicular β-iron oxyhydroxide particles as in Example 1 were used as the starting material, except that the type and amount of sulfate, heating calcination temperature, and heating reduction temperature were varied. In the same manner as in Example 1, acicular crystal α-F~ particle powder, acicular crystal magnetite particle powder, and acicular crystal maghemite particle powder were obtained.

Table 1 shows the main manufacturing conditions and properties of the obtained acicular crystal α-F~ particle powder, acicular crystal magnetite particle powder, and acicular crystal maghetite particle powder.

As a result of electron microscopy observation, the long axis (
1, (S ttm, axial ratio (major axis: short axis) 25:
The acicular crystal β-iron oxyhydroxide particles of No. 1 were retained and inherited, the particle size was uniform, and twins and dendritic particles were not mixed.

Example 9 The pH of the suspension was adjusted to 17.5.

Next, 1.4 q of IJum (No. 3 water glass) (corresponding to 0.4 weight % as 5iQ2 with respect to the acicular crystal α-F~ particle powder) was added to the above suspension. After adding and stirring for 60 minutes, 10% sulfuric acid was added so that the pH value of the suspension became 45, and the particles were separated into acicular crystals α-F through a press filter and dried to form an Si compound. A powder of particles coated with acicular crystals α-F~ was obtained.

Acicular crystal magnetite particle powder and acicular crystal maghemite particle powder were obtained in the same manner as in Example 1 except that the above-mentioned acicular crystal a-Fe2Q and particle powder were used and the heating reduction temperature was set to 450°C.

Table 1 shows the properties of the obtained acicular magnetite particles and acicular maghemite particles.

As a result of electron microscopic observation, both the acicular crystal magnetite particle powder and the acicular crystal maghemite particle powder have a long axis of 0.
6 μm uniaxial ratio (major axis: minor axis) was 25:1.0 Examples 10 to 14 Implemented except that the type of acicular α-Fe203 particles used, the type of sintering inhibitor, and the base were variously changed. Acicular magnetite particles and acicular maghemite particles were obtained in the same manner as in Example 9.

Table 1 shows the properties of the obtained acicular magnetite particles and acicular maghemite particles.

As a result of electron microscopic observation, the acicular magnetite particles and the acicular maghemite particles obtained in Examples 10 to 14 both have a long axis of 0.6 μm and a uniaxial ratio (long axis: short axis).
The ratio was about 25:1.

Comparative Example 1 α-F~08 particle powder was obtained in the same manner as in Example 1 except that sodium sulfate τJ [was not added.

As is clear from the electron micrograph (X30000) shown in FIG. 2, the obtained α-I'e2Qa particle powder had a distorted particle shape and had a lumpy shape.

[Brief explanation of the drawing]

Figures 1 and 2 are electron and electron micrographs (X500
00), and FIG. 1 shows the acicular crystal tL'-Fern 5 particles obtained in Example 1, and FIG. It is a powder. Patent applicant: Toda Kogyo Co., Ltd.

Claims (1)

[Scope of Claims] 1) Starting material acicular β-iron oxyhydroxide particles Q 0.5 to 5.0 weight in terms of so; 30% in air after containing gold sulfate
1. A method for producing acicular iron oxide particles, the method comprising heating and firing in a temperature range of 0 to 500° C. to obtain acicular α-Fe20B particles that retain and inherit the acicular crystals of the starting material. 2) The starting material, acicular β-iron oxyhydroxide particles, has a carbon
L5~50% by weight of sulfate and 300~ in air after incubation
By heating and firing in a temperature range of 500°C, the needle crystals of the starting material are retained and transformed into particles into needle crystal α-F~,
Next, the acicular α-F%Qa particles are heated and reduced in a reducing gas to form acicular magnetite particles, or
A method for producing acicular iron oxide particles, which comprises further oxidizing to obtain acicular maghemite particles. 3) The starting material, acicular β-iron oxyhydroxide particles, contains 0 in terms of 5042 relative to the acicular β-iron oxyhydroxide particles.
．． 5-5.0% by weight of sulfate 'l'akB in air.
By heating and firing at a temperature range of 00 to 500℃,
Acicular crystals α-1' e that retain and inherit the needle crystals of the starting material,
O, particles and then the acicular crystals α-? %Oa particles are coated with an anti-sintering agent, and then heated and reduced in a reducing gas to form acicular crystal magnetite particles, or further oxidized to form acicular crystal maghemite particles. A method for producing acicular iron oxide particle powder. 4) The method for producing acicular iron oxide particles according to claim 3, wherein the sintering inhibitor is one selected from sodium silicate, chromium sulfate, and sodium aluminate.