JPS5951725B2 - Method for producing acicular magnetic iron oxide particle powder - Google Patents

Description

Detailed Description of the Invention The present invention relates to a method for producing acicular magnetic iron oxide particles for magnetic recording, which have excellent dispersibility in organic solvents and orientation in coating films, and are suitable for magnetic recording. The purpose of the present invention is to obtain acicular magnetic iron oxide particles or cobalt-containing acicular magnetic iron oxide particles suitable for use as materials.

As is well known, acicular magnetic iron oxide particles, which are currently widely used as magnetic recording materials, are obtained by a wet reaction between a ferrous salt aqueous solution such as a ferrous sulfate aqueous solution and an alkali such as NaOH. The starting material is acicular hydrated ferric oxide particles, which are heated and dehydrated in air at 200 to 300°C to form hematite particles, and then heated to 300 to 400°C in a hydrogen gas atmosphere.
It is reduced to acicular magnetite particles at 0°C.

In addition, if necessary, the acicular magnetite particles may be added to
It is obtained by reoxidizing at 00°C to obtain acicular maghemite particles. The above-mentioned "wet reaction" refers to adding an alkali to a ferrous salt solution to generate ferrous hydroxide,
This is a well-known method in which acicular hydrated ferric oxide is produced by blowing air at a predetermined temperature and oxidizing it.

In addition, when obtaining acicular hydrated ferric oxide in the above-mentioned "wet reaction" in order to increase magnetic properties, especially coercive force, after incorporating cobalt into the acicular hydrated ferric oxide, heating dehydration, It is well known that cobalt-containing acicular magnetite particles or cobalt-containing acicular maghemite can be obtained by reduction and further oxidation.

Acicular magnetic iron oxide particles such as acicular magnetite particles, cobalt-containing acicular magnetite particles, acicular maghemite particles, or cobalt-containing acicular maghemide particles obtained by the well-known method described above can be used as a magnetic coating material for magnetic recording materials, etc. In order to obtain this, it requires a great deal of time and effort to disperse it in an organic solvent such as toluene or methyl ethyl ketone.

This is due to the fact that the iron oxide particles themselves are inherently aqueous and are therefore difficult to disperse in organic solvents, and in addition, the acicular magnetic oxide particles obtained by the well-known method described above Iron particles undergo remarkable particle growth during the reduction and oxidation heat treatment processes, especially during the heat reduction process in reducing gases such as hydrogen, and a single particle grows to a size exceeding that of a skeleton particle. The outer shape of the skeletal particles gradually disappears, causing deformation of the particle shape and sintering of the particles and each other, causing them to aggregate, making it extremely difficult to uniformly disperse them in an organic solvent. It is necessary. In addition, 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 materials.

In order to obtain such high-performance magnetic recording materials, such as magnetic tapes and magnetic disks, it is necessary to orient the acicular magnetic iron oxide particles in the coating film, but as mentioned above, each heat treatment process Acicular magnetic iron oxide particles, which are aggregated together due to deformation of the particle shape and sintering of the particles and each other, cannot obtain a satisfactory degree of orientation. For this reason, in the industry, when dispersing acicular magnetic iron oxide particles in an organic solvent, a surfactant is added to make the acicular magnetic iron oxide particles oily, and at the same time, a powerful mixer is used to disperse the acicular magnetic iron oxide particles for a long time. It is dispersing the information.

However, although it is possible to make the above-mentioned acicular magnetic iron oxide particles somewhat oily by adding a surfactant, the acicular magnetic iron oxide particles are agglomerated by sintering etc. The surfactant added does not have the function of loosening this aggregation. Therefore, in order to loosen the agglomeration of the acicular magnetic iron oxide particles, it is necessary to rely on physical force such as the above-mentioned strong mixer or crusher. In this way, when attempting to loosen and disperse agglomerations caused by sintering etc. using physical force, the acicular magnetic iron oxide particles are inevitably damaged or crushed, resulting in acicular magnetic iron oxide particles. The needle-like nature, which is the lifeblood of the body, is lost. Based on the above, the present inventor has discovered that when attempting to obtain a magnetic paint using acicular magnetic iron oxide particles, in order to improve the dispersibility, it is necessary to simply add a surfactant to the acicular magnetic iron oxide particles. We have learned that simply trying to disperse particles by making them oily is insufficient.

In order to eliminate agglomeration of acicular magnetic iron oxide particles due to sintering between particles, the present inventor has developed a manufacturing process for acicular magnetic iron oxide particles used for magnetic recording materials, or For many years, we have conducted extensive research into the timing at which agglomeration occurs due to sintering between particles.In the past, when producing acicular magnetic iron oxide particles, the particles generated during each heat treatment process of iron oxide particles In addition, various methods have been proposed for obtaining acicular magnetic iron oxide particles by coating the surfaces of iron oxide particles with an organic compound as a sintering inhibitor to prevent agglomeration due to sintering between particles.

For example, by coating the particle surface of acicular hydrated ferric oxide particles with a higher fatty acid such as an oil fatty acid, the acicular magnetic iron oxide particles are thermally reduced or reoxidized if necessary, thereby reducing sintering between the particles. There is a way to get the powder. By coating with a higher fatty acid such as an oil fatty acid alone in this way, it is possible to obtain an acicular magnetic iron oxide particle powder with less sintering between the particles even after subsequent heat treatments. Although this slightly contributes to improving the orientation in the coating film, the surface of the acicular magnetic iron oxide particles is coated with the residue of fatty acids decomposed during heat treatment, which is necessary when making magnetic paints. It becomes difficult to adsorb various dispersants, and due to this phenomenon, the effect of using the dispersant is weakened, and the magnetic tape characteristics, particularly the squareness ratio, are not improved, and therefore, no significant improvement in the degree of orientation can be expected. On the other hand, a method is also known in which a large amount of an organic compound is used as a sintering inhibitor to increase the effect of preventing agglomeration by sintering iron oxide particles.

If a large amount of organic compound is used as an anti-sintering agent in this way, the effect of preventing agglomeration due to sintering of iron oxide particles will be noticeable, but on the other hand, it will reduce the reactivity of iron oxide particles during reduction and oxidation reactions. This also reduces the effectiveness of the dispersant used in the production of magnetic coating materials, resulting in deterioration of various magnetic properties of the acicular magnetic iron oxide particles.
From the above, in order to obtain acicular magnetic iron oxide particles that fully satisfy various magnetic properties, it is necessary to reduce the amount of organic compounds that have an agglomeration prevention effect by sintering iron oxide particles, etc. As a result, expectations for the agglomeration prevention effect of sintering iron oxide particles have diminished, and it still takes a great deal of time and effort to disperse acicular magnetic iron oxide particles in an organic solvent or orient them in a coating film. need.
In view of the current state of the prior art described above, and as a result of various considerations, the inventors of the present invention have determined that a small amount of an organic compound with a high thermal decomposition temperature and a large amount of residue after thermal decomposition can be converted into acicular hydrated ferric oxide particles. We have already developed a technology that allows us to obtain acicular magnetic iron oxide particles that do not agglomerate by sintering the iron oxide particles even after heat treatment such as reduction oxidation by uniformly coating the surface of the particles. (Special Publication No. 56-17291)
.

In other words, the invention of Japanese Patent Publication No. 17291/1986 provides an anti-sintering effect when coating the particle surface of acicular hydrated ferric oxide particles with an organic compound before being subjected to reduction and oxidation heat treatments. A mixture of a phenolic resin and a higher fatty acid is used as the organic compound. As mentioned above, a method for coating the particle surface of acicular hydrated ferric oxide particles with higher fatty acids is already known, but the present inventors dissolved a mixture of a phenolic resin and a higher fatty acid to cover the particle surface. It was thought that if a uniform and dense coating was formed on the surface of the particles, the deformation of the particle shape during the heat treatment process and the prevention of sintering between particles and particles would be more effective.

The present inventor then obtained an aqueous suspension of 8 or more in which a mixture of a phenolic resin and a higher fatty acid was dissolved in an aqueous suspension in which acicular hydrated ferric oxide particles were dispersed. Then, the phenolic resin and higher fatty acid are precipitated by neutralization with an acid, etc., and the surface of the acicular hydrated ferric oxide particles is coated with the precipitate, and then filtered and dried. obtained the knowledge that it was possible to form a uniform and dense coating of phenolic resin and higher fatty acid on the particle surface, and published
He completed the invention of No. 17291. On the other hand, while conducting further research on the technology related to the invention of Japanese Patent Publication No. 56-17291, the present inventor discovered that the acicular hydrated ferric oxide was used instead of the acicular hydrated ferric oxide as a starting material. ,
When the iron oxide dehydrated so as not to lose its shape, that is, the acicular hydrated ferric oxide is α-FeOOH, α-Fe.

O. , in the case of γ-FeOOH, even if it is an iron oxide represented by γ-Fe2O3,
At the same time as the invention of No. 1, it was confirmed that the same effect could be obtained by dispersing and suspending the iron oxide in water and then coating it with a mixture of phenolic resin and higher fatty acid, and completed the present invention. That's what I did. That is, in the present invention, in an aqueous suspension in which acicular iron oxide particles or cobalt-containing acicular iron oxide particles are dispersed and suspended, 0.5 ~5.0% by weight of a mixture of phenolic resin and higher fatty acid (phenolic resin: higher fatty acid = (95:5) ~ (5:95)) was dissolved to obtain an aqueous suspension of...8 or more. Next, by adjusting the volume value of this liquid, the phenolic resin and higher fatty acid are precipitated, and the surface of the acicular iron oxide or cobalt-containing acicular iron oxide particles is coated with the precipitate. , filtered,
The acicular magnetite particles are dried and then reduced by heating in a reducing gas at a temperature range of 300 to 550°C to obtain a cobalt-containing acicular magnetite particle powder, or further oxidized at a temperature range of 200 to 350°C if necessary. This is a method for producing acicular magnetic iron oxide particles, characterized in that acicular maghemite particles or cobalt-containing acicular maghemite particles are obtained by doing so.

In the present invention, the acicular iron oxide particles or cobalt-containing acicular iron oxide particles used as the starting material are acicular α-Fe2O3.
Or γ-Fe2O3, or cobalt-containing! Acicular α-
It refers to Fe2O3 or γ-Fe2O3.

Next, the configuration and effects of the method of the present invention will be explained. First, the reason for using a mixture of a phenolic resin and a higher fatty acid in the present invention will be described. When the particle surface of acicular iron oxide particles or cobalt-containing acicular iron oxide particles is coated with a small amount of a phenolic resin that has a high thermal decomposition temperature and carbonizes during thermal decomposition and leaves a large amount of residue. The effect of preventing particles and particles from sintering during the subsequent heat treatment processes of reduction and oxidation is better than when higher fatty acids are used alone. However, when phenolic resin is used alone, phenolic resin generally has an extremely good coating property on the surface of acicular iron oxide particles or cobalt-containing acicular iron oxide particles, so hydrogen gas used during thermal reduction Since the rate at which the reducing agent diffuses into the particles 5 is suppressed, the reduction reaction of the treated particles becomes insufficient, resulting in acicular magnetic iron oxide particles or cobalt-containing acicular magnetic iron oxide particles. magnetic performance may deteriorate. On the other hand, when a higher fatty acid is used alone, it generally has a low thermal decomposition temperature, so it easily decomposes during thermal reduction after coating, and the coating film is destroyed. Therefore, the present inventor focused on the fact that phenolic resins and higher fatty acids have complementary properties, and used acicular iron oxide particles or cobalt-containing needles to form a mixture of a certain proportion of phenolic resins and higher fatty acids. After dissolving in an aqueous suspension in which iron oxide particles are dispersed, the precipitates of the phenolic resin and higher fatty acids are deposited on the surface of the acicular iron oxide particles or the cobalt-containing acicular iron oxide particles. The acicular magnetic iron oxide particles obtained by heat-treating the coated particles have properties that are superior to those obtained by using phenolic resin or higher fatty acid alone, especially in the degree of orientation of magnetic tape. Furthermore, it has been found that in this case, a sufficient degree of orientation can be obtained even if the amount of the phenolic resin and higher fatty acid mixed is smaller than the amount when each is used alone. Although the mechanism of these actions is not clear, it is thought that the synergistic effect between the phenolic resin and the higher fatty acid contributes greatly. The phenolic resins used in the present invention include phenol, resorcinol,
Water-soluble resol-type phenolic resins or polyparavinylphenol obtained by condensing phenols such as alkylresorcinol, xylenol, and cresol individually or in combination with formaldehyde can be mentioned.

As the higher fatty acid, aliphatic monocarboxylic acids having 8 to 24 carbon atoms are preferred.

The amount of the mixture of phenolic resin and higher fatty acid added is
If the amount is 0.5% by weight or less based on the acicular iron oxide particles or cobalt-containing acicular iron oxide particles that are the object to be treated, the effect of addition, that is, the sintering prevention effect will not be noticeable.

In addition, if it is added in an amount of 5.0% by weight or more, a long time is required for the thermal reduction treatment, or the reduction temperature has to be made higher, which promotes sintering between particles.
This is not preferable because it deteriorates the magnetic properties. For the purpose of the present invention, a sufficient effect can be obtained with 0.5 to 5.0% by weight. Furthermore, from the purpose of the present invention, the mixing ratio of phenolic resin and higher fatty acid is 95:5.
) to (5:95).

In addition, if the mixing ratio is outside the above range, only the characteristics (for example, degree of orientation) can be obtained when the phenolic resin or higher fatty acid is used alone, but even if the mixing ratio is within the above range, each A sufficient effect can be obtained with a smaller amount than when used alone.

Next, the film forming means in the present invention will be explained in detail.

In the present invention, in order to uniformly and densely coat the particle surface of the acicular iron oxide particles or cobalt-containing acicular iron oxide particles with the phenolic resin and higher fatty acid, it is necessary to The aqueous suspension of acicular iron oxide particles or cobalt-containing acicular iron oxide particles must be properly controlled.

First, the phenolic resin and higher fatty acid must be uniformly dissolved in an aqueous suspension in which acicular iron oxide particles or cobalt-containing acicular iron oxide particles are dispersed or suspended.

For this purpose, a phenolic resin and a higher fatty acid are dissolved in water in advance, either singly or as a mixture, and these solutions are added to an aqueous suspension of acicular iron oxide particles or cobalt-containing acicular iron oxide particles. It is preferable to adopt a method of adding and mixing.
In this case, a caustic alkali such as sodium hydroxide, an alkali carbonate such as sodium carbonate, an amine such as morpholine, or an alkaline substance such as ammonia can be used as a dissolution aid. Furthermore, it is necessary to control the peak temperature and maintain the dissolved state of the phenolic resin and higher fatty acids. It is necessary that the aqueous suspension of iron particles has a ... value of 8 or more, and preferably should be maintained at IfIIO or more.

In addition, when using a phenolic resin, the temperature should be at least room temperature, but when using a higher fatty acid, it needs to be at least the melting point of the higher fatty acid. Preferably, it is retained. Therefore, acicular iron oxide particles or cobalt-containing acicular acid before adding phenolic resin or higher fatty acid. Regarding the aqueous suspension of iron oxide particles, acicular iron oxide particles or cobalt-containing acicular iron oxide particles can be formed by adding an aqueous solution of a phenolic resin and a higher fatty acid alone or a mixture of them later, depending on fluctuations and temperature fluctuations. Aggregation of particles or phenolic resin and higher fatty acids. Does not cause precipitation of The temperature of the phenolic resin and the higher fatty acid alone or the mixed aqueous solution thereof must be similarly considered.

In addition, the temperature is not appropriate, but the acicular iron oxide particles. Alternatively, if aggregation of cobalt-containing acicular iron oxide particles and precipitation of phenolic resin and higher fatty acids occur, a method can be adopted in which an alkaline substance is added to raise the mountain and the temperature is also raised.

Next, in precipitating the dissolved phenolic resin and higher fatty acid, acicular iron oxide particles or cobalt-containing acicular iron oxide particles in which the phenolic resin and higher fatty acid are dissolved are While maintaining the suspension at a temperature above the melting point of the higher fatty acid, an oxidizing substance such as an inorganic acid or an organic acid is gradually added to adjust the IiI4 of the dispersion to 8 or less.

In order to achieve more complete precipitation, it is preferable to adjust Iil to 3.5 to 5, and further below the melting point of the higher fatty acid.
Next, a method for forming acicular magnetic iron oxide particles from acicular iron oxide particles or cobalt-containing acicular iron oxide particles coated with a phenolic resin and a higher fatty acid will be described.

As mentioned above, the acicular iron oxide particles or cobalt-containing acicular iron oxide particles coated with a phenolic resin and higher fatty acid are produced by acicular iron oxide particles or cobalt-containing acicular iron oxide particles obtained by a conventional method. Heat treatment may be carried out at a temperature higher than the usual heat treatment temperatures used to form magnetic iron oxide particles, for example, at 300 to 550°C in a reducing atmosphere such as hydrogen for reduction, and at 200 to 350°C for oxidation.

In addition, the temperature for heating reduction in a reducing atmosphere such as hydrogen is 300°C.
If it is below, the reduction reaction progresses slowly and takes a long time.

Further, if the temperature is 550° C. or higher, the reduction reaction rapidly progresses, causing deformation of the particle shape and sintering of the particles and the particles themselves. On the other hand, when the oxidation temperature is 200° C. or lower, the oxidation reaction takes a long time. Also 350℃
If this is the case, the oxidation reaction will rapidly proceed, resulting in deformation of the particle shape and sintering of the particles and each other. The effects of the method of the present invention, which have been described in detail above, are as follows.

By implementing the present invention, agglomeration due to sintering of iron oxide particles and particles during heat treatment processes such as reduction and oxidation is extremely efficiently prevented, so the acicularity of the starting particles is maintained. The desired product, that is, acicular magnetite particle powder, cobalt-containing acicular magnetite particle powder, acicular maghemite particle powder, or cobalt-containing acicular maghemite particle powder is obtained.

Furthermore, as a result, when the acicular magnetite particles, cobalt-containing acicular magnetite particles, acicular maghemite particles, or cobalt-containing acicular maghemite particles are used in producing a magnetic coating material, it is difficult to knead them with an organic solvent such as toluene or methyl ethyl ketone. It sometimes exhibits excellent dispersibility, and when used in magnetic recording materials such as magnetic tapes, it is particularly rich in orientation, resulting in magnetic tapes with extremely excellent electromagnetic properties. Next, the present invention will be explained with reference to Examples and Comparative Examples.

Example 1 Acicular ferric oxide (α-Fe2O3) with an average long axis length of 0.65 μm and an average axis ratio (major axis: short axis) of 8:1
1 kg was dispersed and suspended in 131 water, and then heated to 70°C.
A suspension of acicular ferric oxide particles was obtained by raising the temperature to .

Next, polyparavinylphenol (“Resin M” manufactured by Maruzen Sekiyu Co., Ltd.) was added to the suspension of acicular ferric oxide particles under stirring.
10g (equivalent to .1.0% by weight relative to α-Fe2O3 particles) and 10g stearic acid (equivalent to 1.0% by weight relative to α-Fe2O3 particles). Phenol resin: higher fatty acid = 50:50 corresponds to Sodium hydroxide 2
．． 5g of morpholine and 1.5g of morpholine as a solubilizing agent in water 11, and then heated to 75°C and added with a mixed aqueous solution of polyparavinylphenol and stearic acid. value 106
, an aqueous suspension of acicular ferric oxide particles was obtained at a temperature of 70°C. Subsequently, while stirring the aqueous suspension of acicular ferric oxide particles, a 1.8-N sulfuric acid aqueous solution was gradually added,
An aqueous suspension of acicular ferric oxide particles having a ratio value of 4.5 was obtained in which polyparavinylphenol and stearic acid were precipitated.

Next, the dried acicular ferric oxide particles were separated by filtration by a conventional method, and the dried acicular ferric oxide particles were mixed with polyparavinylphenol (1.0% by weight/α-
Fe2O3) and stearin, acid (1.0% by weight/α-F
The particles were coated with e2O3). Examples 2 to 6,
Comparative Examples 1 to 5 The type and amount of phenolic resin, higher fatty acid used, amount of solubilizing agent used, production of an aqueous suspension of acicular ferric oxide particles to which mixed organic compound solution was added, and temperature were varied. Except for this, organic compound coated particles were produced in the same manner as in Example 1.

The main manufacturing conditions are shown in Table 1. As the phenolic resin in Example 2, a water-soluble resol type phenolic resin ("Dianol No. 17" manufactured by Oshika Shinko Co., Ltd.) was used. Further, for the acicular ferric oxide particles in Example 6, α-Fe2O3 obtained by dehydrating αFeOOH particles containing 1.0 atomic % of cobalt based on Fe was used as a starting material.

Production Examples 7 to 9 of acicular magnetite particle powder Comparative Examples 6 to 7; 1.0 kg of acicular α-Fe2O3 particles obtained in Example 1
was placed in a retort container with one end open at 61, and N2 gas was aerated at a rate of 21 per minute while driving and rotating.
The temperature was raised to 0.degree. C., and then H2 gas was passed through at a rate of 21 degrees per minute, and the mixture was reduced at a reduction temperature of 420.degree. C. to obtain acicular magnetite particle powder.

As a result of electron microscopic observation, the obtained acicular magnetite particles were found to retain the shape of the starting material particles, have a specific surface area of 28.8 m/g by the BET method, and have a coercive force Hc of 28.8 m/g as a result of magnetic measurement. 4060e, saturation magnetic flux density σS is 80
It was 9 emu/g. Examples 8 and 9 Comparative Examples 6 and 7 Except for varying the type of particles to be treated and the reduction temperature,
Acicular magnetite particle powder or cobalt-containing acicular magnetite particle powder was obtained in the same manner as in Example 7.

Table 2 shows the main manufacturing conditions and various properties of the particles.
Production of acicular maghemite particle powder Examples 10 to 17 Comparative Examples 8 to 14; Example 10 The acicular ferric oxide particle powder obtained in Example 1 was reduced under the reducing conditions in Example 7. The obtained acicular magnetite particles were oxidized in air at 250° C. to obtain acicular maghemite particles.

The obtained acicular maghemite particles retain the shape of the starting material particles as observed by electron microscopy. The specific surface area according to the BET method is 284 m2/g, and as a result of magnetic measurement, the coercive force Hc is 3720e, and the saturation magnetic flux density σS is 69.
It was 8 emu/g. Examples 11 to 17, Comparative Example 8 to
14 Acicular maghemite particle powder or cobalt-containing acicular maghemite particle powder was obtained in the same manner as in Example 10, except that the type of particles to be treated, reduction temperature, and oxidation temperature were variously changed.

Table 3 shows various properties of this particulate powder. Manufacture of magnetic tape
Examples 18-28, Comparative Examples 15-23;
Example 18 Using the acicular magnetite particle powder obtained in Example 7, the composition was blended as shown below, and the powder was milled using a ball mill for 5 minutes.
A magnetic paint was obtained by time mixing and dispersion.

Acicular magnetite particle powder Vinyl chloride Vinyl acetate copolymer rubber (7:3) Toluene Methyl ethyl ketone Methyl isobutyl ketone Lecithin 15 g : Nitri 5.4 g 12 g 12 g 12 g 0.6 g The obtained magnetic paint was placed on a 30 μm polyethylene terephthalate base. A coating film is formed by applying the material to a thickness of 4 μm, and then running in a magnetic field by a known method.
After drying, it was cut into a 625 mm width to produce a magnetic tape.

The coercive force Hc of the obtained magnetic tape was 3660e, the residual magnetic flux density Br was 1510 gauss, and the squareness ratio Br/
The Bm was 0.832 and the degree of orientation was 2.80. Examples 19 to 28, Comparative Examples 15 to 23 Magnetic tapes were manufactured in the same manner as in Example 18, except that the type of acicular magnetic iron oxide particles was variously changed.

Claims (1)

[Scope of Claims] 1. In an aqueous suspension in which acicular iron oxide particles or cobalt-containing acicular iron oxide particles are dispersed or suspended, 0. A mixture of 5 to 5% by weight of phenolic resin and higher fatty acid (phenolic resin: higher fatty acid = (95:5) to (5:95)) is dissolved to obtain an aqueous suspension with a pH of 8 or higher, and then , p of this liquid
By adjusting the H value, the phenolic resin and higher fatty acid are precipitated, and the surface of the acicular iron oxide particles or cobalt-containing acicular iron oxide particles is coated with the precipitate, followed by filtration and drying. and then 300-550℃ in reducing gas.
A method for producing acicular magnetic iron oxide particles, the method comprising obtaining acicular magnetite particles or cobalt-containing acicular magnetite particles by thermal reduction in a temperature range of . 2. The method for producing acicular magnetic iron oxide particle powder according to claim 1, wherein the mixture of phenolic resin and higher fatty acid is 1.5 to 3.0% by weight based on the acicular iron oxide. 3. The method for producing acicular magnetic iron oxide particles according to claim 1, wherein the mixture of phenolic resin and higher fatty acid is 1.5 to 3.0% by weight based on cobalt-containing acicular iron oxide. . 4 In an aqueous suspension in which acicular iron oxide particles or cobalt-containing acicular iron oxide particles are dispersed and suspended, 0.5 to 5% by weight of the acicular iron oxide or cobalt-containing acicular iron oxide is added. A mixture of phenolic resin and higher fatty acid (phenolic resin: higher fatty acid = (95:5) to (5:95)) is dissolved to obtain an aqueous suspension with a pH of 8 or higher, and then the pH of this liquid is
By adjusting the H value, the phenolic resin and higher fatty acid are precipitated, and the surface of the acicular iron oxide particles or cobalt-containing acicular iron oxide particles is coated with the precipitate, followed by filtration and drying. and then 300-550℃ in reducing gas.
Production of acicular magnetic iron oxide particle powder characterized by obtaining acicular maghemite particles or cobalt-containing acicular maghemite particles by heating reduction in a temperature range of 200 to 350 °C and then oxidizing in a temperature range of 200 to 350 °C. Law. 5. The method for producing acicular magnetic iron oxide particles according to claim 4, wherein the mixture of phenolic resin and higher fatty acid is 1.5 to 3.0% by weight based on the acicular iron oxide. 6. The method for producing acicular magnetic iron oxide particle powder according to claim 4, wherein the mixture of phenolic resin and higher fatty acid is 1.5 to 3.0% by weight based on cobalt-containing acicular iron oxide. .