JPS6053444B2 - Manufacturing method of acicular iron magnetic particle powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing acicular iron magnetic particles for magnetic recording, and more specifically, the present invention relates to a method for producing acicular iron magnetic particles for magnetic recording. In producing crystalline β-iron oxyhydroxide particles, a ferric salt was added to the above ferrous chloride aqueous solution in an amount of 0.1 to 0.1 to 0.1 to 0.1 in terms of Fe'゛ based on total iron.
By adding 15 mol% and then oxidizing, acicular β-iron oxyhydroxide particles with a large specific surface area are produced, and the morphology of the Biao φ-iron oxyhydroxide particles, especially the acicular crystals, is maintained. It is characterized by obtaining acicular ferromagnetic particles with a large specific surface area by heating, firing and thermal reduction while inheriting the powder.

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, high-density recording characteristics, high output characteristics, high sensitivity characteristics, improvement in frequency characteristics, and reduction in noise level are required. 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, which is determined by the dispersibility of magnetic particles in the vehicle and the orientation in the coating film. and filling properties.

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. It is required that there is no mixture of

Conventionally, acicular magnetite particles, acicular maghemite particles, and acicular magnetic iron oxide particles containing cobalt have been mainly used as magnetic recording materials, but in recent years, With the above-mentioned demand for higher performance in magnetic recording media, acicular iron magnetic particles having higher coercive force and larger saturation magnetization than acicular magnetic iron oxide particles have been put into practical use.

These acicular iron magnetic particles are generally produced by heating and baking acicular iron oxyhydroxide particles in air to produce acicular iron α-F particles.
It is obtained by converting the e2α particles into e2α particles and then heating and reducing them in a reducing gas at about 35°C.

The coercive force of the acicular ferromagnetic particles largely depends on the shape anisotropy, and the acicular structure of the acicular ferromagnetic particles is one of the most important properties.

As mentioned above, acicular ferromagnetic particles with acicular crystals, uniform particle size, and no twin or dendritic particles are currently in high demand. In order to obtain acicular iron magnetic particle powder with such characteristics,
First, the starting materials. It is necessary for the iron oxyhydroxide particles to have acicular crystals, uniform particle size, and no twins or dendritic particles.
In particular, it is important to maintain and inherit the acicular crystals while heating and reducing them to obtain acicular ferromagnetic particles. This is becoming a major issue. First, iron oxyhydroxide particles as a starting material will be described.

As iron oxyhydroxide, α-iron oxyhydroxide, β-iron oxyhydroxide, and γ-iron oxyhydroxide, which have different crystal structures, are known.

Compared to α-iron oxyhydroxide particles and γ-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.

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

One method is to hydrolyze an aqueous ferric chloride solution, and the other method is to perform an oxidation reaction by passing an oxygen/element-containing gas through the aqueous ferric chloride solution.

In the first method, since the shape of the β-iron oxyhydroxide obtained is spindle-shaped, it is difficult to say that the ferromagnetic particles obtained by reduction using the particles have an axial ratio. Therefore, since it is difficult to obtain a high coercive force, it is not preferred as a starting material for ferromagnetic particles for magnetic recording.

The present invention belongs to the second method.

Conventionally, the most typical method for producing acicular β-iron oxyhydroxide by passing an oxygen-containing gas through an aqueous solution of ferrous chloride to perform an oxidation reaction was disclosed in Japanese Patent Publication No. 47-25.
This is a method described in the 95 Wani Publication, in which acicular β-iron oxyhydroxide particles are produced in a reaction solution with a pH of 2 or less by performing an oxidation reaction by passing an oxygen-containing gas through an aqueous ferrous salt solution. It is something that generates. Next, the major issue is how to maintain and inherit the particle morphology of the acicular β-iron oxyhydroxide particles, which is the starting material, and in particular, how to heat-sinter and heat-reduce the acicular crystal iron magnetic particles while retaining and inheriting the acicular crystals. It's coming.

Regarding the heating and calcination process, the starting material acicular crystal β
- When the iron oxyhydroxide particles are heated and fired at 300° C. or higher to form α-Fe2O3 particles, the needle-like crystals break down and become lump-like particles.

The reason why the acicular crystals of the α-Fe2O3 particles break down and become lump-like particles during heating and firing is due to the Cl- roots contained in the acicular β-iron oxyhydroxide particles. .

As mentioned above, the reason why the acicular β-oxyhydroxide particles contain one C1 root is because the acicular β-oxyhydroxide particles are contained in the acicular β-oxyhydroxide particles in both the first and second methods. This is because iron chloride aqueous solution is used as the iron raw material when producing iron oxide particles, and the Cl contained in the particles cannot be completely removed even after repeated washing, resulting in needle-shaped β crystals.
- The iron oxyhydroxide particles contain 2-8% by weight of C1 roots.

The present inventor has been engaged in the production and development of acicular β-oxyhydroxide particles for many years, and during the research process, the starting material acicular β-oxyhydroxide particles We have already developed a method of heating and firing iron oxide particles to obtain acicular α-Fe2O particles without destroying the acicular crystals.

For example, as described below.

That is, the acicular α-Fe2O3 particles that retain and inherit the needle crystals of the acicular β-iron oxyhydroxide particles that are the starting material are 0.5 in terms of SO,-2 for crystalline β-iron oxyhydroxide particles
300-5 in air after impregnation with ~5.0% by weight of sulfate
It can be obtained by heating and firing in a temperature range of 00'C.

This method will be explained as follows.

Acicular β-iron oxyhydroxide particles are heated and fired to form Fe2O
Observing the process of forming three particles in more detail, first, the acicular β-iron oxyhydroxide particles are heated and dehydrated to form acicular β-
Fe2O3 particles, and then the acicular β-Fe2O3
The particles undergo crystal transformation into α-Fe2O particles.

Needle crystal β-Fe2O from needle crystal β-iron oxyhydroxide particles
Since the change to 3 is a topotactic reaction, there is no change in particle shape. Therefore, the generated p-Fe2O3 particles retain and inherit the needle-like crystals of the needle-like β-iron oxyhydroxide particles. During the subsequent crystal transformation from the acicular β-Fe2O3 particles to the α-Fe2O3 particles, the carbon root acts and the acicular crystals break down and become particles on a lump.

The reason why the needle-like crystals of the obtained α-Fe2OJ particles collapse and become lump-like particles is because one Cl root contained in the needle-like β-Fe2O3 particles reacts with β-Fe2O during heating and sintering, and FeCl3 Then, Biao Cecl3 is thermally decomposed to produce lumpy α-Fe2O3 particles, and at the same time, the decomposed Cl reacts with the acicular β-Fe2O3 particles to produce a liquid phase of FeCl3. ,F
This is thought to be due to the so-called dissolution precipitation process in which the acicular β-Fe2O3 particles are dissolved and the α-Fe2O3 particles are precipitated through a liquid phase of eCl. Therefore, the particle morphology of the acicular crystal β-iron oxyhydroxide particles, especially the acicular crystal α-Fe2O3 that retains and inherited the acicular crystals.
In order to obtain particles, it is first necessary to stably exist acicular β-Fe2O3 particles, and then, during crystal transformation from acicular β-Fe2O3 particles to acicular α-Fe2O3 particles, They believed that it was necessary to prevent the formation of a liquid phase of FeCl3, and as a result of repeated studies on substances that have such effects, they 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 500C, the acicular β-oxyhydroxide particles are reduced due to the presence of the sulfate. Acicular β-Fe2O3 particles can be stably produced from iron particles, and FeC
It is thought that since the liquid phase of l3 is not generated, the needle crystals do not collapse, and it is possible to obtain needle-shaped α-Fe2OJ particles that retain and inherit the particle morphology of the needle-shaped β-iron oxyhydroxide particles. .

As the sulfate used in the above method, sodium sulfate, ferric sulfate, cobalt sulfate, aluminum sulfate, nickel sulfate, etc. can be used.

The sulfate 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 amount of sulfate is S
It is 0.5% to original tumor volume in terms of O,-2. The amount of sulfate is 0.0% in terms of SO,-2 relative to the acicular β-iron oxyhydroxide particles.
If the heel weight is less than %, the particle morphology of the acicular β-iron oxyhydroxide particles, especially the effect of obtaining acicular α-Fe2O3 particles that retain and inherit the acicular crystals, is not sufficient. do not have. Even if the amount is more than % of the original weight, it is possible to obtain acicular α-Fe2O particles that retain and inherit the particle morphology of the acicular β-iron oxyhydroxide particles, especially the acicular crystals. The acicular iron magnetic particles obtained by thermal reduction of the acicular α-Fe2O3 particles are undesirable because the saturation magnetization is greatly reduced due to a decrease in purity.

As for the treatment temperature with the sulfate, the desired purpose can be sufficiently achieved even at room temperature.

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

The heating and firing temperature is in a temperature range of 300 to 500°C.

If the heating and firing temperature is below 30s, acicular crystals α
-If it takes a long time to obtain Fe2O3 particles and the temperature is above 500°C, the growth of single particles in the generated α-Fe2O3 particles will be excessive, resulting in deformation of the particles and sintering between the particles and each other. bring up

By the way, it is generally well known that the noise level caused by magnetic recording media is greatly affected by the specific surface area of the magnetic particles, and that the noise level tends to decrease as the specific surface area of the magnetic particles increases. It is being

That is, this phenomenon is explained in the Institute of Electronics and Communication Engineers technical research report MR8l.
-11 It is clear from "Fig 3" of the second vertex 23-9.

"Fig. 3" is a diagram showing the relationship between the specific surface area of the particles and the noise level in CO-coated acicular maghemite particle powder, and the noise level decreases linearly as the specific surface area of the particles increases.

This phenomenon also applies to acicular iron magnetic particles.

As mentioned above, as the ferromagnetic particle powder for magnetic recording,
Furthermore, there is a need to develop magnetic particles with a lower noise level, and it is necessary to develop magnetic particles with a larger specific surface area.

In order to obtain magnetic particles with a large specific surface area, it is necessary that the specific surface area of the starting material, acicular β-iron oxyhydroxide particles, be as large as possible. Ru.

According to the known method described in the aforementioned Japanese Patent Publication No. 47-2595, B of the obtained acicular β-iron oxyhydroxide particle powder is
The ET specific surface area is 34Tft as shown in Example 1.
It is about If.

Further, the method does not disclose any method for increasing the specific surface area of the obtained acicular β-iron oxyhydroxide particles.

In view of the above, the present inventor has conducted various studies on methods for further increasing the specific surface area of acicular β-iron oxyhydroxide particles with uniform particle size and no twin or dendritic particles. The present invention has been achieved.

That is, the present invention involves adding a ferric salt to the ferrous chloride aqueous solution in producing acicular β-iron oxyhydroxide particles by passing an oxygen-containing gas through the ferrous chloride aqueous solution to oxidize it. 0.1 to 15 mol% calculated as Fe3+ based on total iron is added, and then oxidized to produce acicular β-iron oxyhydroxide particles. 0.5 in terms of SO4-2 for the particles
300-5 in air after impregnation with ~5.0% by weight of sulfate
By heating and firing in a temperature range of 0C)C, the acicular α-Fe2O3 particles that retain and inherit the acicular crystals of the acicular β-oxyiron hydroxide particles are formed, and then the acicular α-
Fe2O, the particles may be thermally reduced in a reducing gas to form acicular iron magnetic particles, or if necessary, the acicular crystals α
-Fe2O particles are coated with an anti-sintering agent and then heated and reduced in a -reducing gas to form acicular iron magnetic particles.

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

In order to further increase the specific surface area of acicular β-iron oxyhydroxide particles having a uniform particle size and containing no twins or dendritic particles, the inventors have developed various types and amounts of additives. As a result of our investigation, we found that in producing acicular β-iron oxyhydroxide particles by passing an oxygen-containing gas through the ferrous chloride aqueous solution to oxidize it, we added ferric salt to the ferrous chloride aqueous solution. When 0.1 to 15 mol% is added in terms of Fe3+ and then oxidized, acicular crystal β-
We have obtained a new finding that the specific surface area of iron oxyhydroxide particles can be further increased.

Specific surface area of acicular β-iron oxyhydroxide particles obtained by adding 0.1 to 15 mol% of ferric salt (calculated as Fe3+ based on total iron) to a ferrous chloride aqueous solution and then oxidizing Although the theoretical explanation of the phenomenon of the increase in the size of acicular β-iron oxyhydroxide particles has not yet been theoretically elucidated, the present inventors believe that, in general, the formation of acicular β-iron oxyhydroxide particles is due to the formation of acicular β-iron oxyhydroxide nuclei. The generation process consists of two stages of growth of acicular β-iron oxyhydroxide nuclei, and by adding Fe3+ to the ferrous chloride aqueous solution in advance, the acicular β-iron oxyhydroxide nuclei can be grown. It is thought that this is because the formation of acicular crystal β-iron oxyhydroxide is promoted and a large number of nuclei of acicular β-iron oxyhydroxide are formed.

The above-mentioned phenomenon will be explained as follows by extracting some of the many experimental examples conducted by the present inventor. FIG. 1 is a diagram showing the relationship between the amount of Fe3+ added and the specific surface area of the produced acicular β-iron oxyhydroxide particles.

That is, a ferrous chloride aqueous solution (1.75 to 1.98n1011e) with a total volume of 51 obtained by adding 0 to 25.0 mol% of ferric chloride in terms of Fe3+ to the total iron was heated at a temperature of 70C.
This figure shows the relationship between the specific surface area of acicular β-iron oxyhydroxide particles obtained by blowing air at a rate of 10 e/min to carry out an oxidation reaction and the amount of Fe3+ added.

As shown in FIG. 1, the specific surface area of the acicular β-iron oxyhydroxide particles tends to increase as the amount of Fe3+ added increases.

Next, conditions for carrying out the method of the present invention will be described.

As the ferric salt used in the present invention, ferric oxide, ferric sulfate, ferric nitrate, etc. can be used.

In the present invention, the amount of ferric salt added is Fe based on total iron.
It is 0.1 to 15 mol% in terms of 3+.

When the amount of the ferric salt added is 0.1 mol % or less in terms of Fe3+ based on the total iron, the effect of increasing the specific surface area of the acicular β-iron oxyhydroxide particles is not sufficient. 15
Even if the amount is more than mol%, the acicular crystal β-
Iron oxyhydroxide particle powder can be obtained, but its effect is not significant.

In addition, when adding 0.1 to 15 mol% of ferric salt, as mentioned above, the specific surface area of the obtained acicular β-iron oxyhydroxide particles tends to increase as the amount added increases. It is in.

Therefore, by adjusting the amount added, acicular β-iron oxyhydroxide particles having a desired specific surface area can be obtained. The present invention configured as described above has the following effects.

That is, according to the present invention, acicular β-oxyhydroxide particles having a large specific surface area, uniform particle size, and no twin or dendritic particles are produced, and the acicular β-oxyhydroxide particles are The particle morphology of iron hydroxide particles, in particular, is achieved through heat firing and heat reduction while retaining acicular crystals, resulting in a large specific surface area, uniform particle size, and an acicular shape with no twins or dendritic particles. A crystalline iron magnetic particle powder can be obtained.

In the thermal reduction step of the present invention, if desired, the particle form of the acicular β-iron oxyhydroxide particles, particularly the acicular crystals, is retained and inherited while the acicular α-Fe particles are heated and fired.
The 2O3 particles can be coated with an anti-sintering agent in advance, and in this case, acicular iron magnetic particles with even better acicular crystals can be obtained. This fact will be explained below.

Acicular β-iron oxyhydroxide particles with uniform particle size and no twin or dendritic particles are heated and calcined to form α-Fe2O3 particles while preserving and inheriting the particle morphology, especially the acicular crystals, and then When obtaining acicular ferromagnetic particles by heating reduction in a reducing gas, when the heating reduction temperature becomes high, the acicular ferromagnetic particles of the acicular ferromagnetic particles are deformed and the particles and their mutual interactions are sintered. The coercive force of the obtained acicular iron magnetic 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 shape obtained by heating and firing while retaining and inheriting the acicular crystals. In order not to destroy the needle-like crystalline α-Fe2O3 particles, the thermal reduction is usually carried out at 300 to 4500C.

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-Fe2O3 particles are not part of the particle growth force, and therefore, the crystallinity of the particles is small, so the growth of a single particle of the produced particles is rapid in the thermal reduction process. Uniform particle growth of a single particle 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 the grain is likely to collapse.

Therefore, in order to prevent particle shape deformation and sintering between particles and particles in the thermal reduction process, it is necessary to prepare particles with uniform particle size and a mixture of twins and dendritic particles prior to the thermal reduction process. There is a method in which acicular α-Fe2O3 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 acicular α-Fe2O3 particles, which are coated with an anti-sintering agent and have a uniform particle size and are not mixed with twins or dendritic particles, can be heated and reduced at 350C to 50C to make the particle size uniform. It is possible to obtain acicular ferromagnetic particles that do not contain twins or dendritic particles.

If the temperature is below 350C, the reduction reaction progresses slowly and takes a long time.

Furthermore, if the temperature is 500° C. or higher, the reduction reaction rapidly progresses, resulting in deformation of the acicular crystal particles and sintering of the particles and each other.

The well-known organic and inorganic compounds having anti-sintering properties include Si. ．． A], Cr, . Mn,. Cu. ．． Compounds such as NilP, such as sodium silicate, colloidal silica, aluminum sulfate, sodium aluminate, chromium sulfate,
One or more of nickel sulfate, sodium metaphosphate, etc. can be used.

Although a method of coating the sintering inhibitor by adding and mixing acicular α-Fe2O3 particle powder into an aqueous solution containing the sintering inhibitor is effective, it is preferable that the particle surface is coated uniformly.

As mentioned above, when a magnetic tape is manufactured using the acicular iron magnetic particles obtained by the method of the present invention, the dispersibility in the vehicle, the orientation in the coating film, and the filling properties are extremely high. Because of its excellent properties, it is possible to obtain a magnetic recording medium that has a low noise level, high output, high sensitivity, and is capable of high-density recording, which are currently most required.

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

In addition, the specific surface area of the particles in the above experimental examples and the following examples and comparative examples is shown by the BET method, and the long axis and axial ratio are shown as the average value of the values measured from electron micrographs.

Further, the acicular β-iron oxyhydroxide particles were identified by two X-ray diffraction method, and the SO and Cl single root contents were measured by fluorescent X-ray analysis.

(Production of acicular β-iron oxyhydroxide particles) Example 1
~8 Comparative Example 1; Example 1 Ferrous chloride aqueous solution of 3m01'e 3.3e and 2m01'
e of ferric chloride aqueous solution 0.05e (Fe3+
Equivalent to 1.0 mol%.

) was further added with water to adjust the total volume to 5e, and air was passed through the mixed aqueous solution at a rate of 10e/min for 2m at a temperature of 70C to produce particulate powder. The resulting particles were washed with water, separated, dried, and crushed by a conventional method. The produced particles were identified by X-ray diffraction and were found to be 100% acicular β-iron oxyhydroxide particles.

The obtained acicular β-iron oxyhydroxide particles have a specific surface area of 40.1rt1', and as a result of electron microscopy observation, the average long axis is 0.6PTn, and the particle size is uniform and twin-shaped. It did not contain any crystals or dendritic particles. Example 2~
8 Particle powder was produced in the same manner as in Example 1, except that the amount of the ferrous chloride aqueous solution used, the type and amount of ferric salt used, and the reaction temperature were varied.

All produced particles were identified by X-ray diffraction method, and as a result, 10
The powder was 0% acicular β-iron oxyhydroxide particles. Table 1 shows the main manufacturing conditions and characteristics at this time. Examples 2-8
As a result of electron microscopy, the acicular β-iron oxyhydroxide particles obtained in the above were all uniform in particle size and free of twins and dendritic particles. Comparative Example 1 Acicular crystal β-iron oxyhydroxide particles were produced in the same manner as in Example 1 except that ferric chloride was not added.

Table 1 shows the main manufacturing conditions and characteristics at this time. <Production of acicular iron magnetic particles> Examples 9 to 16 Comparative Example 2; Example 9 The acicular β-oxyiron hydroxide particles 125y obtained in Example 1 were added to 1 mol/e of sodium sulfate. After adding to the aqueous solution 51 and stirring and mixing the chrysanthemum mixture, it was separated, washed with water, and dried.

The obtained acicular β-iron oxyhydroxide particles have a particle diameter of 7.0
Contains chlorine of % by weight, and 0.0% in terms of SO,-2. % sodium sulfate by weight.

This acicular β-iron oxyhydroxide particle powder 125f was heated and calcined in air at 44°C to obtain α-Fe2O3 particle powder.

As a result of electron microscopic observation, the obtained α-Fe2O3 particles had a long axis of 0.6 PW1, and retained and inherited the needle-like crystals of the starting material, the needle-like β-iron oxyhydroxide particles.
The particle size was uniform and twinned dendritic particles were not mixed.

The above acicular α-Fe2O3 particle powder 100f was put into a 3e retort container with one end open, and H2 gas was aerated at a rate of 25e per minute while driving and rotating.
Heat reduction was carried out at ℃.

The acicular iron magnetic particles obtained by reduction are immersed in a toluene solution and evaporated to prevent rapid oxidation when taken out into the air. A stable oxide film has been applied.

The obtained acicular iron magnetic particles have a specific surface area of 34.1.
As a result of electron microscopic observation, it was found that the major axis was 0.6 PTr1, the particle size was uniform, and there were no twins or dendritic particles mixed together.

As for the magnetic properties, the coercive force was 7430e, and the saturation magnetization σs was 172 emu'f.

Examples 10 to 14 Acicular iron magnetic particles were obtained in the same manner as in Example 9, except that the type of starting material, the type and amount of sulfate, the heating firing temperature, and the heating reduction temperature were varied.

Table 2 shows the main manufacturing conditions and various properties of the obtained acicular iron magnetic particles.

All of the acicular iron magnetic particles obtained in Examples 10 to 14 had a large BET specific surface area, and as a result of electron microscopy observation, it was found that the acicular β-iron oxyhydroxide particles, which were the starting materials, had a large BET specific surface area. It retained and inherited acicular crystals, had uniform grain size, and did not contain twins or dendritic particles.

Example 15 Acicular crystal α-F obtained in the same manner as in Example 9 except that the type of starting material, the amount of sulfate, and the heating and calcination temperature were changed.
After suspending 100f of e2O3 particles in 4' water, an aqueous NaOH solution was added to adjust the pH of the suspension to 10.0.

Next, 10.0 f of sodium silicate (No. 3 water glass) (S
This corresponds to 2.8111% as iO2.

) was added and stirred, the pH value of the suspension was 7.0.
After adding 10% sulfuric acid so as to give the following properties, the acicular α-Fe2O3 particles were separated using a breath filter and dried to obtain acicular α-Fe2O3 particles coated with a Si compound. Table 2 shows the main manufacturing conditions at this time.

Acicular ferromagnetic particles were obtained in the same manner as in Example 9, except that the acicular α-Fe2O3 particles were used and the heating reduction temperature was 460°C.

The obtained acicular iron magnetic particles have a specific surface area of 70.4W.
tIy, and as a result of electron microscopy observation, the long axis was 0.3 Bpm, and the particle size was uniform, retaining the acicular crystals of the starting material, acicular β-iron oxyhydroxide particles. It did not contain twins or dendritic particles.

The magnetic properties were a coercive force of 9660e and a saturation magnetization σS of 151 emuIV.

Example 16 Acicular α-Fe2O3 particles obtained in the same manner as in Example 9 were used, except that the type of starting material, the type and amount of sulfate, and the heating and calcination temperature were changed, and the type of sintering inhibitor was changed. Acicular iron magnetic particles were produced in the same manner as in Example 15, except that the amounts were changed.

Table 2 shows the main manufacturing conditions at this time.

The acicular iron magnetic particles obtained in Example 16 had a large specific surface area of 73.2 If, and as a result of electron microscopic observation, the long axis was 0.30 PTrL, and it was a raw material. Acicular crystals The acicular crystals of β-iron oxyhydroxide particles were preserved, the particle size was uniform, and twins and dendritic particles were not mixed.

Comparative Example 2 Acicular iron magnetic particles were obtained in the same manner as in Example 9, except that the acicular β-iron oxyhydroxide particles of Comparative Example 1 were used.

Table 2 shows the main manufacturing conditions and various characteristics at this time.

l The obtained acicular iron magnetic particles had a small specific surface area.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the amount of Fe3+ added and the specific surface area of the produced acicular β-iron oxyhydroxide particles.

Claims (1)

[Scope of Claims] 1. In producing acicular β-iron oxyhydroxide particles by oxidizing an aqueous ferrous chloride solution by passing an oxygen-containing gas through the aqueous ferrous chloride solution, a ferric salt is added to the aqueous ferrous chloride solution. is added in an amount of 0.1 to 15 mol% in terms of Fe^3^+ based on the total iron, and then oxidized to produce acicular β-iron oxyhydroxide particles. - 0.5 in terms of SO_4^-^2 for the iron oxyhydroxide particles
300-5 in air after impregnation with ~5.0% by weight of sulfate
By heating and firing in a temperature range of 00°C, the acicular α-Fe_2O_3 particles retaining and inheriting the acicular crystals of the acicular β-iron oxyhydroxide particles are formed, and then the acicular α-
A method for producing acicular ferromagnetic particles, which comprises heating and reducing Fe_2O_3 particles in a reducing gas to obtain acicular ferromagnetic particles. 2. In producing acicular β-iron oxyhydroxide particles by passing an oxygen-containing gas through the ferrous chloride aqueous solution and oxidizing it, a ferric salt was added to the ferrous chloride aqueous solution to increase the amount of Fe relative to the total iron. 0.1 to 15 mol% in terms of ^3^+ is added, and then oxidized to produce acicular β-iron oxyhydroxide particles, and then the acicular β-iron oxyhydroxide particles 0.5 to 0.5 in terms of SO_4^-^2 for the particles.
300-50 in air after containing 5.0% by weight of sulfate
By heating and firing in a temperature range of 0° C., the needle crystals α-Fe_2O_3 particles are formed which retain and inherit the needle crystals of the needle crystal β-iron oxyhydroxide particles, and then the needle crystal α-F
A method for producing acicular ferromagnetic particles, which comprises coating e_2O_3 particles with an anti-sintering agent and then heating and reducing the particles in a reducing gas to obtain acicular ferromagnetic particles. 3. The method for producing acicular iron magnetic particles according to claim 2, wherein the sintering inhibitor is one selected from sodium silicate, chromium sulfate, and sodium aluminate.