JPS5932882B2 - Method for producing acicular magnetic iron oxide particles - 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. prevent,
By causing only the production reaction of acicular α-FeOOH particles from an aqueous ferrous salt solution, acicular crystals with acicular crystals, uniform particle size, and no dendritic particles are produced. α-FeOOH particles are obtained, and the acicular α-FeOOH
By thermally reducing and oxidizing the particles, it has needle-like crystals,
Another object of the present invention is to obtain acicular magnetite particles and acicular maghemite particles whose particle size is uniform and in which dendritic particles are not mixed.

When acicular magnetite particles or acicular maghemite particles obtained by the method of the present invention are used in the production of magnetic recording media, they have acicular crystals, are uniform in particle size, and have no dendrites. Due to the absence of acicular magnetic iron oxide particles, the dispersibility of the acicular magnetic iron oxide particles in the vehicle is low.
A magnetic recording medium with extremely excellent orientation and filling properties in the coating film can be obtained. 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.

In other words, high density recording characteristics, high output characteristics, high sensitivity characteristics,
Improvements in various characteristics such as frequency characteristics are required. The characteristics of a magnetic material suitable for satisfying the above requirements for a magnetic recording medium are that it has a high coercive force Hc and a large saturation magnetization σs.

As is well known, the magnitude of the coercive force of magnetic particles depends on shape anisotropy, crystal anisotropy, strain anisotropy, exchange anisotropy, or their interaction. 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 B
r depends on the dispersibility of the magnetic particles in the vehicle, 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, the magnetic particles to be dispersed in the vehicle should have acicular crystals and have a uniform particle size. Further, it is required that dendritic particles are not mixed therein.

Currently, acicular magnetite particles or acicular maghemite particles are mainly used as magnetic recording materials.

These are generally performed by air oxidizing a suspension containing Fe(0H)2, obtained by reacting an aqueous ferrous salt solution with an aqueous alkaline solution and having a pH of 1 or more (usually referred to as a "wet reaction"). The acicular α-FeOOH particles obtained are reduced to acicular magnetite particles at 300 to 400°C in a reducing gas such as hydrogen, and then reduced to acicular magnetite particles in air at 200 to 400°C.
It is obtained by oxidizing it at 300°C to form acicular maghemite particles. Acicular magnetite particles or acicular maghemite particles, which are currently used as magnetic particles for magnetic recording, have relatively high storage stability by making maximum use of the anisotropy derived from their shape. Obtain magnetic force and utilize its orientation to create a relatively large square shape (Br/
Bm), however, research and development efforts are underway to further improve the properties of acicular magnetite particles and acicular maghemite particles.

As mentioned above, acicular magnetic iron oxide particles having acicular crystals, uniform particle size, and no dendritic particles are currently in high demand. In order to obtain magnetic iron oxide particles with such characteristics, the starting material, acicular α-FeOOH particles, must have acicular crystals, be uniform in particle size, and have no dendritic particles mixed therein. It is necessary.

Conventionally, in the alkaline region above PHll, acicular crystal α-FeO
The most typical known method for producing OH particles is to add an aqueous alkali solution of an equivalent or more amount to a ferrous salt solution, and then add an aqueous solution containing Fe(0H)2 to a ferrous salt solution at a temperature of 80° C. or less at a temperature of PHll or more. Acicular α-FeOOH particles are obtained by performing an oxidation reaction.

The acicular α-FeOOH particles obtained by this method have a needle-like shape with a length of about 0.5 to 1.5μ, but they also contain dendritic particles, and the particle size Therefore, it is difficult to say that the particles have a uniform particle size.
The reason for the formation of acicular α-FeOOH particles having asymmetric particle sizes and containing dendritic particles will be discussed below.

The aqueous ferrous salt solution used to produce acicular α-FeOOH particles generally contains small amounts of ferric ions.

That is, in the industry, ferrous sulfate, ferrous chloride, etc., which are by-products from the steel pickling process and the titanium white manufacturing process, are used as raw materials for ferrous salt aqueous solutions, but these are used as impurities. It contains ferric ions, and when the ferrous salt aqueous solution is oxidized by oxygen in the air, ferric ions are generated in the solution.

The above is clear from, for example, the following description in Japanese Patent Publication No. 43-24770.

[Pickling waste solution is usually judged to contain iron in the form of ferrous ions, but in reality, there are many that contain ferric ions. Pickling waste liquid from which a portion of ammonia has been removed as salt crystals, Pickling waste liquid used to absorb ammonia in industrial gases, Sulfuric acid or (Also, oxidized and
Many of them have come to contain iron ions. ” As mentioned above, when acicular α-FeOOH particles are produced using a ferrous salt aqueous solution containing ferric ions, two different reactions occur, namely, needle formation from a ferrous salt aqueous solution. crystal α-F
eOOH particle formation reaction and needle crystal α from ferric ions
It is considered that the reaction for producing -FeOOH particles occurs simultaneously, and as a result, acicular α-FeOOH particles with asymmetric particle sizes and a mixture of branched particles are obtained. The reaction process for producing acicular α-FeOOH particles from a ferrous salt aqueous solution is that the ferrous salt aqueous solution and an alkaline aqueous solution react to produce Fe(0H)2, and the Fe(0H)2 is oxidized. This produces acicular α-FeOOH particles.

On the other hand, in the reaction process for producing acicular α-FeOOH particles from ferric ions, ferric ions react with an alkali and acicular α-FeOOH particles are generated at the same time.

This fact is shown, for example, in 333 of ``Electrochemistry and Industrial Physical Chemistry (published by the Electrochemical Society), Vol. 37, No. 5, 1969''.
The page states:

``If less than the equivalent amount of alkali is added to the ferric salt solution,
......When more than o equivalent of alkali is added, α-FeOOH is mainly produced. When an alkali is added to a ferrous salt, a colloid of mainly Fe(0H)2 is produced. Acicular crystal magnetite particles or acicular crystal maghemite particles obtained by thermal reduction and oxidation of acicular α-FeOOH particles having asymmetric particle sizes and mixed with dendritic particles are also used. The particle size is asymmetric, and
The result is a mixture of dendritic particles.

When a magnetic recording medium is manufactured using such magnetic iron oxide particles, the dispersibility in the vehicle, the orientation and filling properties in the coating film are poor, and the residual magnetic flux density (Br
) will decrease. The present inventor (well, in view of the above-mentioned prior art, it has acicular crystals and has uniform particle size,
Further, as a result of various studies in order to obtain acicular crystal α-FeOOH particle powder in which dendritic particles are not mixed, the present invention was arrived at. That is, the present invention produces acicular crystals by passing an oxygen-containing gas through a suspension containing Fe(0H)2, which is obtained by reacting a ferrous salt aqueous solution with an alkaline aqueous solution and having a pH of PH11 or more. Acicular crystal magnetic iron oxide to produce α-FeOOH particles, and then thermally reduce the acicular α-FeOOH particles to produce acicular magnetite particles, or further oxidize to produce acicular maghemite particles. In the method for producing particulate powder, a water-soluble stannous salt of 0.1 to 2.0 atomic % based on Fe, calculated as Sn, is pre-existing in the aqueous alkaline solution, or further, a water-soluble stannous salt of 0.1 to 2.0 atomic % based on Sn is present in the aqueous alkali solution, or Acicular crystals α-F are formed by the presence of a calcium compound of 0.1 to 2.0 atomic % in terms of Ca based on Fe.
generate eOOH particles, then the acicular α-FeOOH
This is a method for producing acicular crystal magnetic iron oxide particle powder, which comprises heating and reducing the particles to obtain acicular crystal magnetite particles, or further oxidizing the particles to obtain acicular crystal maghemite particles.

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

The present inventor has discovered that the particle size of the acicular α-FeOOH particles obtained by the conventional method in the alkaline region of PHll or higher is asymmetric, and the reason why the dendritic particles are mixed is due to the fact that It was thought that this was due to the fact that since the monoferrous salt aqueous solution contained ferric ions, two different reaction processes occurred during the production of the acicular α-FeOOH particle powder.

Then, the present inventor has discovered that acicular crystals α-F from ferric ions
If the formation of eOOH particles is prevented, the particle size will be uniform, and acicular α-FeOO without dendritic particles will be produced.
It was thought that H particle powder could be obtained.

Therefore, the present inventor has developed a method for reducing ferric ions in a ferrous salt aqueous solution to ferrous ions without adversely affecting the needle crystals of the product α-FeOOH particles. As a result of various studies, P containing Fe(0H)2 obtained by reacting a ferrous salt aqueous solution with an alkaline aqueous solution was found.
In a method for producing acicular α-FeOOH particle powder by passing an oxygen-containing gas through a suspension of Hll or more for oxidation, the above alkaline aqueous solution is preliminarily mixed with 0.1 to 2. When 0 atomic % water-soluble stannous salt is present, ferric ions in the ferrous salt aqueous solution can be reduced to ferrous ions, and therefore ferric ions As a result, the particle size is uniform and acicular α-FeOOH particles are not mixed with dendritic particles.
It was found that OH particle powder can be obtained. The effects of the water-soluble stannous salt present in the alkaline aqueous solution will be explained below. Water-soluble stannous salt reacts in an alkaline aqueous solution as follows to form stannous ion [Sn(0H)]
4] 2-.

This stannite ion has a strong reducing power and reduces ferric ions to ferrous ions when a ferrous salt aqueous solution containing ferric ions reacts with an alkaline aqueous solution.

As a result, the formation reaction of acicular α-FeOOH particles from ferric ions is suppressed, and acicular α-FeOOH particles with uniform particle size and no dendritic particles are obtained. it is conceivable that.

By the method described above, the acicular crystals of the product acicular α-FeOOH particles are uniform without adversely affecting the acicular crystals, and the particle size is uniform, and acicular crystals of α-FeOOH particles are not mixed with dendritic particles.
Although it is possible to obtain H particle powder, the present inventor has further conducted studies to improve the axial ratio of the above-mentioned acicular α-FeOOH particle powder.

The present inventors have studied metal ions that can improve the axial ratio without impairing the properties of water-soluble stannous salts, such as equalizing the particle size and preventing the presence of dendritic particles. We have repeatedly found that Ca is effective among many different metal ions. That is, the present inventors oxidized the needle by passing an oxygen-containing gas through a suspension containing Fe(0H)2 obtained by reacting a ferrous salt aqueous solution with an alkaline aqueous solution and having a pH of PH11 or more. In the method for producing crystalline α-FeOOH particle powder, 0.0% of Fe is added to the above alkaline aqueous solution in terms of Sn in advance.
1 to 2.0 atomic % of water-soluble stannous salt is present, and before oxidation by passing oxygen-containing gas into the suspension, 0.1 to 2.0 at. 0 atomic%
When a calcium compound of It was discovered that it is possible to obtain acicular α-FeOOH particles having a special periphery.

Some of the many examples carried out by the present inventor will be extracted and explained as follows.

Figure 1 shows the amount of Ca present and the axial ratio of the acicular α-FeOOH particles when the amount of water-soluble stannous salt in the alkaline aqueous solution is kept constant at 0.5 atomic % in Sn terms relative to Fe. (long axis: short axis).

That is, using a 1.5 m01/l aqueous solution of ferrous sulfate and an aqueous solution of caustic soda obtained in the presence of stannous sulfate so as to contain 0.5 atomic % of Fe in terms of Sn, the pH was adjusted to about 1.
A suspension containing Fe(0H)2 of 3 was obtained, and Fe(0H)2 was added to the suspension.
Acicular shape obtained by stirring and mixing in the presence of calcium chloride so as to contain 0.1 to 3.0 at% in terms of Ca, and then oxidizing the mixture by blowing air at a rate of 150 l/min at a temperature of 45°C. This figure shows the relationship between the axial ratio (long axis: short axis) of crystalline α-FeOOH particles and the amount of Ca present.

As shown in the figure, when the amount of water-soluble stannous salt is kept constant, as the amount of Ca increases, the acicular α-Fe
The axial ratio of OOH particles shows a tendency to improve.

FIG. 2 shows the relationship between the amount of Ca present and the long axis of acicular α-FeOOH particles produced under exactly the same reaction conditions as in FIG. 1.

As shown in the figure, as the Ca content increases, the acicular α-FeOOH particles tend to grow in the long axis direction of the particles.

FIGS. 5 and 6 respectively show an electron micrograph (X2OOOO) and particle size distribution of acicular α-FeOOH particles obtained by the method of the present invention (Example 7 described below).

As is clear from FIGS. 5 and 6, the acicular α-FeOOH particles obtained by the method of the present invention have excellent acicular crystals, uniform particle size, and dendritic particles. They are not mixed. As can be seen from FIGS. 1, 2, 5, and 6, Sn and Ca in the present invention have a synergistic effect to equalize the particle size of the acicular α-FeOOH particles, and the presence of dendritic particles. This has the effect of inhibiting the grain size and increasing the axial ratio by growing the particles in the long axis direction.

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

Examples of the ferrous salt aqueous solution used in the present invention include a ferrous sulfate aqueous solution and a ferrous chloride aqueous solution.

As the water-soluble stannous salt used in the present invention, stannous sulfate and stannous chloride can be used.

In the present invention, the water-soluble stannous salt must be present in an alkaline aqueous solution.

This is because the water-soluble stannous salt is in a solution state in the alkaline aqueous solution and exists as stannous ions with strong reducing power, so that when the alkaline aqueous solution and the ferrous salt aqueous solution react, This is to produce the effect of reducing ferric ions in the ferrous salt aqueous solution.

When a water-soluble stannous salt is present in the ferrous salt aqueous solution, the ferrous salt aqueous solution used to generate the acicular α-FeOOH particles is used at a temperature of at most about 50°C, generally around room temperature. Therefore, the reducing power of the water-soluble stannous salt is not sufficient at this temperature.

On the other hand, although it is not yet theoretically clear that an alkaline aqueous solution exhibits sufficient reducing power at around this temperature, sufficient evidence has been obtained as a result of confirmation experiments. In addition, when a water-soluble stannous salt is present in a suspension containing Fe(0H)2 obtained by reacting a ferrous salt aqueous solution and an alkaline aqueous solution and has a pH of PH11 or higher, it is necessary to Since the α-FeOOH production reaction from diiron ions is occurring, there is no point in the presence of the water-soluble stannous salt. In the present invention, the amount of water-soluble stannous salt present in the alkaline aqueous solution is
It is 0.1 to 2.0 atomic % based on Fe in terms of Sn.

The amount of water-soluble stannous salt is 0 relative to Fe in terms of Sn.
．． If it is 1 atomic % or less, the object of the present invention cannot be fully achieved, and if it is 2.0 atomic % or more, F
The viscosity of the suspension containing e(0H)2 increases and the oxidation reaction is extremely suppressed, so that granular magnetite particles become mixed in. The particle size of the obtained acicular α-FeOOH particle powder,
When considering the axial ratio, 0.2 to 0.8 atomic % is preferable. Calcium compounds used in the present invention include calcium chloride, calcium nitrate, calcium acetate,
Calcium hydroxide, calcium oxide, calcium sulfate,
Calcium carbonate and the like can be used, but from an industrial standpoint, calcium hydroxide and calcium chloride are preferred.

The presence of calcium compounds in the present invention means that Fe(0H
)2 by passing oxygen-containing gas into the suspension containing α-Fe.
It is necessary before producing OOH particle powder,
In ferrous salt aqueous solution, alkaline aqueous solution, or Fe(0H
)2 in an aqueous suspension.

In either case, the desired effects of the present invention can be obtained. The reaction for producing acicular α-FeOOH particles is as follows:
Generation of acicular α-FeOOH core particles and the acicular α-Fe
It consists of the growth of OOH core particles, but needle crystal α
The axial ratio of -FeOOH particles is determined at the stage of generation of acicular α-FeOOH particles. Therefore, some acicular α-FeOOH core particles are already generated after the start of oxygen-containing gas ventilation. There is no point in having calcium compounds present at this stage.

In the present invention, the amount of the calcium compound present is 0.1 to 2.0 at% in terms of Ca based on Fe.

If the amount of calcium compound present is less than 0.1 atomic % based on Fe, the object of the present invention cannot be fully achieved. If the content is 2.0 atomic % or more, the object of the present invention can be achieved, but the effect is not significant.

Considering the axial ratio and particle size of the resulting acicular α-FeOOH particles, 0.5 to 1.5 atomic % is preferable. The present invention configured as described above has the following effects. That is, according to the present invention, the production reaction of acicular α-FeOOH particles from ferric ions in the reaction system is prevented, and only the production reaction of acicular α-FeOOH particles from the ferrous salt aqueous solution is prevented. By causing , it has needle-like crystals,
In addition, it is possible to obtain acicular α-FeOOH particles with uniform particle size and no dendritic particles, and by heating and oxidizing the acicular α-FeOOH particles, acicular α-FeOOH particles can be obtained. It is possible to obtain acicular magnetite particles and acicular maghemite particles that have uniform particle size and do not contain dendritic particles. It can be used as a magnetic material for high-density recording.

In addition, when the acicular magnetite particles or acicular maghemite particles obtained according to the present invention are used in the production of magnetic paint, the dispersibility in the vehicle is good, and the orientation in the coating film is good. Excellent properties and filling properties,
A preferred magnetic recording medium can be obtained. Next, the present invention will be explained using actual examples and comparative examples.

In addition, the axial ratio (long axis: short axis) and long axis of the particles in the above experimental examples, the following actual foam examples, and comparative examples are all shown as average values of numerical values measured from electron micrographs.

In addition, the particle size distribution is shown in a histogram based on the measurement results from electron micrographs.

Generation of acicular α-FeOOH particle powder〉Examples 1 to 1
4. Comparative Examples 1 to 4; Actual Example 1 Ferrous sulfate aqueous solution containing Fe2+1.5m01/l 13
．． 31 was added to stannous sulfate 11 in advance so that it contained 0.25 atomic % in terms of Sn based on the Fe prepared in the reactor.
3.75-N NaOH aqueous solution in the presence of 9 26.7
In addition to 1, Fe(0
H) A reaction for producing 2 was carried out.

Acicular crystals α-
FeOOH particles were produced. The end point of the oxidation reaction was determined by extracting a portion of the reaction solution and acidifying it with hydrochloric acid, and then using a red blood salt solution to determine the presence or absence of a blue coloring reaction of Fe2+.

The produced particles were washed with water, separated from P, dried, and crushed by a conventional method.

As a result of electron microscopic observation, these acicular α-FeOOH particles had an average long axis of 0.401 tm and a long axis:short axis ratio of 9:1, and the particle size was uniform and dendritic particles were not mixed. . Practical Examples 2 to 5 Acicular α-FeOOH particles were produced in the same manner as Practical Example 1, except that the type of ferrous salt aqueous solution and the type and amount of water-soluble stannous salt were varied. .

Table 1 shows the main manufacturing conditions and characteristics at this time. Actual example 2
As a result of electron microscopic observation, all of the acicular α-FeOOH particles obtained in Steps 5 to 5 were found to have uniform particle size and no dendritic particles were present. Electron micrograph of the acicular α-FeOOH particles obtained in Practical Example 2 (×
20000) is shown in FIG. 3, and the measurement results of the particle size distribution are shown in FIG.

Actual example 6 Ferrous sulfate aqueous solution containing Fe2+1.5m01/l 13
．． 3.75-N(7)NaOll 7l<1.31 was prepared in advance in the presence of 22g of stannous sulfate so as to contain 0.5 at% of Sn based on the Fe prepared in the reactor. In addition to solution 26.71, a reaction for producing Fe(0H)2 was carried out at a pH of 3.5 and a temperature of 45°C.

Calcium chloride (CaCl) was added to the suspension containing Fe(0H)2 so that the concentration was 0.3 at.
After stirring and mixing in the presence of 2.2H20) 8.89, air was passed through at a rate of 150 l/min for 12.5 hours at a temperature of 45°C to produce acicular α-FeOOH particles.

The end point of the oxidation reaction was determined by the presence or absence of a blue coloring reaction of Fe2+ using a red blood salt solution after extracting a portion of the reaction mixture and acidifying it with hydrochloric acid.

The produced particles were washed with water, separated from P, dried, and crushed by a conventional method.

As a result of electron microscopy observation, the average value of these acicular α-FeOOH particles was 0.50 μm on the long axis, long axis: short axis, 13:1.
The particle size was uniform and dendritic particles were not mixed. Actual examples 7 to 14 Varying the type and amount of ferrous salt aqueous solution, the concentration and amount of NaOH aqueous solution, the type and amount of water-soluble stannous salt, and the type, amount and time of presence of calcium compounds Acicular α-FeOOH particles were produced in the same manner as in Practical Example 6, except that

Table 1 shows the main manufacturing conditions and characteristics at this time.

As a result of electron microscopic observation, the acicular α-FeOOH particles obtained in Practical Examples 7 to 14 all had excellent acicular crystals, uniform particle size, and dendritic particles. were not mixed together. Acicular crystal α- obtained in Example 7
Electron micrograph of FeOOH particle powder (×20000)
FIG. 5 shows the measurement results of the particle size distribution, and FIG. 6 shows the measurement results of the particle size distribution. Comparative Example 1 Acicular α-FeOOH particle powder was produced in the same manner as in Example 1 except that stannous sulfate was not present.

Table 1 shows the main manufacturing conditions and characteristics at this time.

An electron micrograph (×20,000) of the obtained acicular α-FeOOH particles is shown in FIG. 7, and the measurement results of the particle size distribution are shown in FIG. As is clear from FIGS. 7 and 8, the obtained acicular α-FeOOH particles had asymmetric particle sizes and contained dendritic particles.

Comparative Example 2 Acicular α-FeOOH particles were produced in the same manner as in Example 1 except that stannous sulfate was present in the ferrous sulfate aqueous solution.

As a result of electron microscopy observation, the obtained acicular α-FeOOH particles had an average length of 0.40 μm on the long axis, and a length of 7 on the long axis.
:1, the particle size was asymmetric, and dendritic particles were present.

An electron micrograph (×20,000) of the obtained acicular α-FeOOH particles is shown in FIG. 9, and the measurement results of the particle size distribution are shown in FIG.

Comparative Example 3 Acicular α-FeOOH particle powder was produced in the same manner as in Practical Example 1, except that stannous sulfate was present in the suspension containing Fe(0H)2.

As a result of electron microscopy observation, the obtained acicular α-FeOOH particles had an average length of 0.45 μm on the long axis and 8 on the short axis.
:1, the particle size was asymmetric, and dendritic particles were present.

An electron micrograph (X2OOOO) of the obtained acicular α-FeOOH particles is shown in FIG. 11, and the measurement results of the particle size distribution are shown in FIG.

Comparative Example 4 The amount of stannous sulfate present in the NaOH water bath solution was
The production reaction was carried out in the same manner as in the actual example except that the amount of Sn was 2.2 atomic % based on e.

As a result of electron microscopy observation of the obtained powder particles, it was found that acicular crystal particles and granular particles were mixed together. Moreover, as a result of X-ray analysis, this particle powder was a mixed particle powder of α-FeOOH and magnetite. Manufacturing method of acicular crystal magnetite particle powder〉Examples 15 to 28, Comparative Examples 5 to 7; Actual example 15 Acicular crystal α-FeOOH particle powder obtained in Actual example 1 30
09 was put into a retort container with one end open in 31, and H2 gas was vented at a rate of 32 per minute while rotating it for one drive.
The mixture was reduced at a reduction temperature of 350° C. to obtain acicular magnetite particle powder 2609.

As a result of electron microscopic observation, the obtained acicular magnetite particles inherited the particle shape of the acicular α-FeOOH particles that were the starting material, and had an average long axis of 0.30 μm and a uniaxial ratio (
The ratio (long axis: short axis) was 8:1, the particle size was uniform, and dendritic particles were not mixed. In addition, as a result of magnetic measurement, the coercive force Hc is 4500e, and the saturation magnetization σs is 79.7em.
It was u/9. Actual Examples 16-28, Comparative Examples 5-7 Acicular magnetite particle powder was obtained in the same manner as Actual Example 15, except that the type of starting material and the reduction temperature were varied.

Table 2 shows the main manufacturing conditions and characteristics of the particles at this time. As a result of electron microscopy, the acicular magnetite particles obtained in Examples 16 to 28 were all uniform in particle size and free of dendritic particles.

An electron micrograph (×20,000) of the acicular magnetite particles obtained in Practical Example 16 is shown in FIG. 14, and the measurement results of the particle size distribution are shown in FIG. 15.

An electron micrograph (×20,000) of the acicular magnetite particles obtained in Example 21 is shown in FIG. 16, and the measurement results of the particle size distribution are shown in FIG. 17. As a result of electron microscopy, all of the acicular magnetite particles obtained in Comparative Examples 5 to 7 had asymmetric particle sizes and had dendritic particles mixed therein. <Production method of acicular crystal maghemite particles> Actual examples 29 to 42, comparative examples 8 to 10; Actual example 29 Acicular crystal magnetite particles obtained in Actual example 15 12
09 was oxidized in air at 270° C. for 90 minutes to obtain acicular maghemite particles.

As a result of electron microscopic observation, the obtained acicular maghemite particles inherit the shape of the acicular α-FeOOH particles that are the starting material, have an average long axis of 0.30 μm, and an axial ratio (long axis: short axis). ) 8:1, the particle size was uniform, and dendritic particles were not mixed. In addition, as a result of magnetic measurement, the coercive force Hc is 4150e, and the saturation magnetization σs is 71.7emu/9
It was hot.

Examples 30 to 42, Comparative Examples 8 to 10; Example 29 except that the type of acicular magnetite particle powder was variously changed.
Acicular maghemite particles were obtained in the same manner as above.

Table 3 shows various properties of this particulate powder. Definite examples 30-42
As a result of electron microscopy, the obtained acicular maghemite particles were found to have excellent acicular crystals, uniform particle size, and no dendritic particles.

An electron micrograph (X2OOOO) of the acicular maghemite particles obtained in Blood Example 30 is shown in FIG. 18, and the measurement results of the particle size distribution are shown in FIG. 19.

An electron micrograph (X2OOOO) of the acicular maghemite particles obtained in Example 35 is shown in FIG. 20, and the measurement results of the particle size distribution are shown in FIG. As a result of electron microscopy, all of the acicular maghemite particles obtained in Comparative Examples 8 to 10 had asymmetric particle sizes and had dendritic particles mixed therein. An electron micrograph (X2OOO) of the acicular maghemite particles obtained in Comparative Example 8 is shown in FIG. 22, and the measurement results of the particle size distribution are shown in FIG.

[Brief explanation of the drawing]

Figure 1 shows the amount of Ca present and the axial ratio of acicular α-FeOOH particles ( It is a relationship diagram between the long axis and the short axis. FIG. 2 shows the relationship between the amount of Ca present and the long axis of acicular α-FeOOH particles produced under the same reaction conditions as in FIG. 1. 3 and 4 respectively show an electron micrograph (X2OOOO) of the acicular α-FeOOH particles obtained in Example 2 and a histogram of the particle size distribution. FIGS. 5 and 6 respectively show actual example 7.
2 shows an electron micrograph (X2OOOO) of the acicular α-FeOOH particles obtained in the above and a histogram of the particle size distribution. 7 and 8 show an electron micrograph (X2OOOO) and a histogram of the particle size distribution of the acicular α-FeOOH particles obtained in Comparative Example 1, respectively. 9 and 10 respectively show an electron micrograph (X2OOOO) and a histogram of the particle size distribution of the acicular α-FeOOH particle powder obtained in Comparative Example 2. FIG. 11 and FIG. 12 show Comparative Example 3, respectively.
2 shows an electron micrograph (×20,000) and a histogram of particle size distribution of the acicular α-FeOOH particles obtained in FIG. FIG. 13 shows an electron micrograph (×20,000) of the particles obtained in Comparative Example 4. 14 and 15 are electron micrographs (×2
0000) and a histogram of particle size distribution. 16 and 17 are electron micrographs of the acicular magnetite particles obtained in Example 2'1 (
X2OOOO) and a histogram of particle size distribution. 18 and 19 respectively show an electron micrograph (X2OOOO) of the acicular maghemite particles obtained in Example 30 and a histogram of the particle size distribution. 20 and 21 respectively show Example 35.
2 shows an electron micrograph (X2OOOO) of the acicular maghemite particles obtained in the above and a histogram of the particle size distribution.

Claims (1)

[Claims] 1. A suspension containing Fe(OH)_2 obtained by reacting an aqueous ferrous salt solution with an aqueous alkali solution and having a pH of 11 or above is oxidized by passing an oxygen-containing gas through it to form a needle. Akira α-
FeOOH particles are generated, and then the acicular α-FeOO
In the method for producing acicular magnetic iron oxide particles in which H particles are heated and reduced to obtain acicular magnetite particles, the above alkaline aqueous solution is preliminarily added with 0.1 to 2 % of Fe in terms of Sn.
．． Acicular α-FeOOH particles are produced by the presence of 0 atomic % water-soluble stannous salt, and then the acicular α-FeOOH particles are thermally reduced to obtain acicular magnetite particles. A method for producing characteristic acicular magnetic iron oxide particles. 2 The amount of water-soluble stannous salt pre-existing in the alkaline aqueous solution is 0.2 to 0.8 in terms of Sn relative to Fe.
A method for producing acicular magnetic iron oxide particles according to claim 1, wherein the acicular magnetic iron oxide particles are in atomic %. 3 Acicular crystals α-
FeOOH particles are generated, and then the acicular α-FeOO
In the method for producing acicular crystal magnetic iron oxide particle powder in which H particles are thermally reduced and oxidized to obtain acicular crystal maghemite particles, 0.0% of Fe is added to the above alkaline aqueous solution in terms of Sn in advance.
Acicular α-FeOOH particles are produced by the presence of 1 to 2.0 at % of a water-soluble stannous salt, and then the acicular α-FeOOH particles are thermally reduced and oxidized to form acicular crystals. A method for producing acicular magnetic iron oxide particle powder, characterized by obtaining maghemite particles. 4 The amount of water-soluble stannous salt pre-existing in the alkaline aqueous solution is 0.2 to 0.8 in terms of Sn relative to Fe.
A method for producing acicular magnetic iron oxide particles according to claim 3, wherein the acicular magnetic iron oxide particles are in atomic %. 5 Acicular crystals α-
FeOOH particles are generated, and then the acicular α-FeO
In a method for producing acicular magnetic iron oxide particles in which OH particles are thermally reduced to obtain acicular magnetite particles, 0.1 to 0.1 to 0.1 in terms of Sn relative to Fe is added to the above alkaline aqueous solution in advance.
In the presence of 2.0 atom % of a water-soluble stannous salt, and before oxidation by passing an oxygen-containing gas into the suspension, 0.1 to 2.0 atoms of Fe (calculated as Ca) were added to the suspension. % of calcium compound in the presence of acicular crystals α-
FeOOH particles are generated, and then the acicular α-FeO
A method for producing acicular crystal magnetic iron oxide particles, the method comprising heating and reducing OH particles to obtain acicular magnetite particles. 6 The amount of water-soluble stannous salt pre-existing in the alkaline aqueous solution is 0.2 to 0.8 in terms of Sn relative to Fe.
atomic %, and the amount of calcium compound pre-existing in the suspension is 0.5 in terms of Ca relative to Fe.
The method for producing acicular crystalline magnetic iron oxide particles according to claim 5, wherein the content is 1.5 atomic %. 7 Acicular crystals α-
FeOOH particles are generated, and then the acicular α-FeO
In the method for producing acicular crystal magnetic iron oxide particle powder in which OH particles are thermally reduced and oxidized to obtain acicular crystal maghemite particles, the above alkaline aqueous solution is preliminarily added with 0% Fe in terms of Sn.
．． 1 to 2.0 atomic % of water-soluble stannous salt is present, and before oxidation by passing oxygen-containing gas into the suspension, 0.1 to 2.0 atomic % of Fe to Ca is added to the suspension. By the presence of 0 atomic % calcium compound, acicular crystals α
-FeOOH particles, and then the acicular α-Fe
A method for producing acicular magnetic iron oxide particle powder, which comprises heating reducing and oxidizing OOH particles to obtain acicular maghemite particles. 8 The amount of water-soluble stannous salt pre-existing in the alkaline aqueous solution is 0.2 to 0.8 in terms of Sn relative to Fe.
atomic %, and the amount of calcium compound pre-existing in the suspension is 0.5 in terms of Ca relative to Fe.
8. The method for producing acicular magnetic iron oxide particles according to claim 7, wherein the content is 1.5 atomic %.