JPS6241722A - Lamellar hematite powder and production thereof - Google Patents

Abstract

PURPOSE:To produce fine lamellar hematite powder having uniform particle diameters, by heating an aqueous slurry composition which comprises a ferric salt, tartaric acid or its salt and an alkali compound and has a composition to satisfy a specific pH condition, at a specific temperature. CONSTITUTION:Aqueous slurry comprising a ferric salt such as FeCl3, etc., tartaric acid and/or its salt and a third component of an alkali compound such as NaOH, etc., is processed into a composition to satisfy a condition shown by formulas pH>=8.2 and (pH-6.6)/600<=gamma<=(pH-2.6)/600 [gamma is molar ratio of (tartaric acid or its salt)/iron salt]. The slurry composition is heated to >=120 deg.C and treated for about 10hr. By this treatment, lamellar hematite powder consisting of lamellar particles having an ogive wherein particle diameter of median is 0.01-1mum, particle diameter ratio of first quartile point to third quartile point is 0.67-1.50, particle diameter ratio of 1/10 point to 9/10 point is 0.-2.0 and lamellar ratio is >=2 is obtained.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to plate-like hematite, which is a precursor of magnetic powder for magnetic recording media, and a method for producing the same. The present invention relates to fine plate-shaped hematite (α-FezO:+) powder with uniform particle size, which is a precursor of plate-shaped magnetic powder suitable for two-dimensional magnetic recording media such as disks, and a method for producing the same.

[Conventional technology and problems]

Acicular magnetic iron oxide has conventionally been used as magnetic powder for magnetic recording media such as audio and video. These are synthesized using goethite as a starting material, paying particular attention to particle size, axial ratio, and distribution thereof.

The need for the acicular shape is, in part, to increase the coercive force due to shape magnetic anisotropy, and in part, when this is applied as a magnetic paint, the squareness ratio is increased due to particle orientation. This is to make it bigger.

All of these work to increase the signal/noise ratio of the recording medium.

In addition, regarding the particle size, 0.8-0.5-0.3 la
Gradually, smaller particle sizes were sought and manufactured.

This is to improve high frequency recording characteristics. For the same reason, powder with small particle size is essential for high recording density.

A wide distribution of particle size and shape also increases noise, so it is extremely important to make these uniform.

On the other hand, with the development of microcomputers in recent years, disk media such as floppy disks have begun to be widely used as external recording media that are simple and quick to access.

Conventional acicular magnetic powder is used for this, but the orientation of the particles in the coating direction, which was advantageous in tapes, produces disadvantageous results here.

That is, particle orientation in one direction in a disk medium causes non-uniformity along the circumference of the disk, resulting in output unevenness (modulation). To avoid this, a non-orientation treatment is performed in a magnetic field before the coating surface is cured, but even this is not sufficient.

Furthermore, even if a non-orientation treatment has been applied, the acicular powder oriented so that the angle of intersection between the direction of magnetization to be recorded and the axis of easy magnetization is within 45° is only half of the applied powder. do not have.

Other particles not only have little involvement in recording, but also tend to peel off from the coating surface because they are rubbed laterally by the magnetic head.

As described above, since the conventional acicular powder was not developed for use in disk media, several disadvantages are inevitable. Moreover, these problems cannot be avoided unless a special application method such as spin coating is used.
Regardless of whether it is rod-shaped or spindle-shaped, it is common to magnetic powders that have a shape that extends in one dimension. Here, if the magnetic powder can be made into a plate shape and oriented flat (te) within the coating surface,
It would be extremely advantageous if it could be magnetized in-plane.

That is, if the advantages of a coating film of plate-shaped magnetic powder compared to needle-shaped magnetic powder are explained individually in detail, they can be mainly summarized into the following three points.

■ Plate-shaped magnetic powder with small modulation tends to be oriented parallel to the coating surface, and is not oriented in a specific direction within the plane. Therefore, even if a disk is cut out from the coating surface and rotated, the intensity of the output signal picked up by the magnetic head does not depend on the angle of rotation. In other words, modulation can be reduced.

■ Output improvement can be expected.

When magnetic powders that basically belong to the cubic system, such as magnetite and T-ferrite, form hexagonal or square plate crystals,
The axis of easy magnetization is such that the projection lines onto the plate surface intersect with each other by 6
It is estimated that there are three main axes that make an angle of 0° or two main axes that make an intersecting angle of 90°. When such plate-like powder is oriented parallel to the disk surface, the angle between the direction of the magnetization vector to be recorded, that is, the tangential direction of the circumference of the disk medium, and the axis of easy magnetization of the plate-like powder is at most 45°. It's nothing more than that. This shows that, unlike in the case of needle-shaped powder, in the case of plate-shaped powder, all the powder on the coating surface can greatly contribute as a carrier of recorded information. This is convenient for improving output.

■ Less damage to the magnetic head and coating surface.

Since a floppy disk rotates at high speed in direct contact with a magnetic head when writing or reading data, the frictional force is extremely large.

Therefore, the unevenness on the surface of the coating abrades the head and causes magnetic powder to peel off.

Since plate-shaped powder has a higher surface smoothness than needle-shaped powder, it is less susceptible to head abrasion and coating peeling force. Moreover, the greater the adhesion area between the powder and the paint surface, the less chance of paint peeling off.
Plate-shaped powder is more preferable than needle-shaped powder.

However, plate-shaped magnetic powders that have been developed as magnetic recording media in the prior art are hexaferrites, such as barium ferrite and strontium ferrite, which tend to form plate-shaped crystals due to their original crystal structure.

The magnetization of hexaferrite is based on magnetocrystalline anisotropy, and its axis of easy magnetization is perpendicular to the plate surface.

Taking advantage of this property, it has been proposed to use minute plate-like crystals of hexaferrite as a recording medium for perpendicular magnetic recording. However, hexaferrite platelet crystals cannot be used for conventional in-plane magnetized disk media.

Moreover, even if it were possible to use it, it would be difficult to take advantage of the advantages of the plate-shaped powder mentioned above. That is, because of its perpendicular magnetization, hexaferrite tends to form long laminated particles with overlapping plate surfaces between the particles, which causes considerable difficulty in forming a coating film. Furthermore, the smoothness of the coating surface is not guaranteed.

On the other hand, plate-shaped hematite, which is a precursor of plate-shaped magnetic powder, has been conventionally known as mica-like iron oxide (so-called MIO), and many methods have been proposed for its production. For example, Japanese Patent Publication No. 43-12435 relates to a method of hydrothermally treating goethite or trivalent iron salt in an alkali at 250° C. or higher to obtain mica-like iron oxide, which is excellent as a rust-preventing pigment, decorative material, etc. Also, for example, Japanese Patent Application Laid-open No. 54
No. 151598 and No. 55-15431 and No. 8 disclose a method for producing large-grained mica-like iron oxide in a system in which phosphoric acid, boric acid, or a salt thereof is added.

In these methods, large particle size M
Efforts were focused solely on obtaining IO. From the viewpoint of use as Zunawa anti-corrosion pigment, the larger the particle size, the better it is to prevent water from penetrating under the paint film, and as a decorative material to increase brightness.For this reason, conventional studies The focus was on how to increase the particle size.

Furthermore, in JP-A No. 58-69730, there is an example of a 2- and smaller particle size compared to the previous MIO, but this is characterized by using T-Fe203 as a raw material. Based on the disclosed data, 17
It is difficult to obtain plate-shaped iron oxide of 1 inch or less.

On the other hand, there is a description in the literature (Color Materials, 57.602 (1984)) of plate iron oxide of 0.1 to 0°5 ttm, but the particle size distribution is extremely wide. There is also no disclosure of specific manufacturing conditions.

As mentioned above, the average particle size is 11M or less, and
Until now, there has been no known method for synthesizing plate-shaped hematite particles with extremely uniform particle sizes.

[Means for solving problems]

In view of this situation, the present inventors conducted intensive research on plate-shaped magnetic powder for floppy disks, and found that it consists of plate-shaped particles, and in the cumulative distribution curve of the area of the plate-shaped surface, the median particle size ( equivalent diameter) is 0.01 to 1-1, the particle size ratio at the first and third quartiles is 0.67 to 1.5, and the ratio of particle sizes at the 1710th and 9710th minutes. is 0.
5 to 2.0, has a plate ratio (equivalent diameter/thickness) of 2 or more, and has found that a plate-shaped magnetic powder made of crystals belonging to the cubic system satisfies the above various performances, and the magnetic powder We have arrived at the present invention as a precursor for.

That is, in the present invention, in the cumulative distribution curve of the area of the plate-like surface,
The median particle size (equivalent diameter) is 0.01 to 1-1, the particle size ratio at the first and third quartiles is 0.67 to 1.50, and the 1710th and 9710th minute This relates to a plate-shaped matite powder consisting of plate-shaped particles having a particle size ratio of 0.5 to 2.0 and a plate-like ratio (equivalent diameter/thickness) of 2 or more and a method for producing the same. be.

The plate-shaped magnetic powder obtained by reducing the plate-shaped hematite powder of the present invention, and optionally reoxidizing it and/or treating it with a cobalt salt has less lamination,
Not only is the coating film extremely smooth, but it can also be used as is as a recording medium in current recording methods. It also has advantages over acicular powders in terms of modulation and particle orientation. In this way, the above-mentioned advantages of the plate-like powder can only be exhibited when the magnetization vector mainly has an in-plane magnetization component.

The particle size of the magnetic powder must be 0.01 to 1-φ.

Here, the particle size referred to in the present invention refers to the diameter of the plate surface or equivalent diameter (diameter of a circle having the same area as the plate surface).

The smaller the particle size, the higher the coercive force, which is preferable, but if the particle size is too small, the coercive force will decrease due to superparamagnetism.

Further, the plate ratio (equivalent diameter/thickness) needs to be 2 or less in consideration of the smoothness and orientation of the coating film.

If you look at the particle size distribution curve, the particle size distribution is as follows:
In the area distribution cumulative curve of the plate surface, the particle size ratio at the first and third quartiles is 0.67 to 1.50, and the particle size ratio at the 1/10th and 9/1° Particle size ratio is 0.5
Must be in the range ~2.0.

There is a strong correlation between particle size and magnetic properties, and the smaller the particle size, the higher the coercive force, so the broadening of the particle size distribution means the broadening of the coercive force.

This lowers the squareness ratio of the BH curve and at the same time widens the reversal magnetic field distribution (the half width of the BH differential curve), which is an important hindrance to the sharpness of recording. In particular, broadening of the particle size distribution should be avoided for high-density recording.

Furthermore, the presence of particularly coarse particles or particularly fine particles causes serious defects such as dropouts, so it is essential to keep them below a certain limit.

The plate-shaped hematite powder of the present invention can be produced by the following method.

That is, in a method of hydrothermally treating an aqueous slurry in which three components, ferric salt, tartaric acid or its salt, and an alkali compound coexist, the slurry composition is adjusted to pH 8.2 or higher, and
The molar ratio r of tartaric acid and its salt to iron salt is (pH-6,
6)/600 or more, and (pl+-2,6)/6
By setting the temperature to a value of 00 or less and heating at 120° C. or higher, plate-shaped hematite having the desired particle size and particle size distribution can be obtained.

The fine plate-shaped hematite particles with uniform diameter obtained by the method 2 are further reduced in a hydrogen stream, and if necessary, further reoxidized and/or treated with Co salt to produce an excellent material for use in floppy disks. It is possible to obtain plate-shaped magnetic powder having the following properties. Here, plate-like is a general term for hexagonal flake-like, disc-like, hexagonal plate-like, rhombic plate-like, square plate-like, etc., and the plate surface has a (major axis/minor axis) ratio of 2 or less. means.

In the present invention, the most important factors that determine the particle size and shape of the particles to be produced are the pH and the molar ratio r of tartaric acid or its salt added to the iron salt.

If the p)I of the slurry before hydrothermal treatment is less than 8.2, plate-shaped hematite cannot be obtained regardless of whether tartaric acid is added or not. What is obtained is hematite with small particle size and nearly spherical shape.

When r exceeds (pH-2,6)/600, what is produced is lumpy magnetite;
)/600, octahedrons or oblate octahedrons with large grain sizes are produced.

None of these meet the objectives of the present invention.

The production of hematite that satisfies the above three requirements, which is the object of the present invention, requires pH≧8.2 and r between the above values, that is, (1
)H-6,6)/600≦r≦(pi+ -2,6)
It is essential that it be in the area surrounded by /600.

Within this region, the particle size becomes smaller as r decreases, so particle size control is possible.

Hydrothermal treatment temperature and treatment time are also important factors.

At temperatures below 120° C., the rate of hematite formation is slow, and the coexistence of some goethite is unavoidable even after 10 hours of treatment. This appears as an intermediate when crystal rearrangement occurs from iron hydroxide to hematite, and its remaining is extremely problematic from the viewpoint of the purpose of the present invention.

At 140°C, the production of hematite is completed in 10 hours, and at 160°C, it is completed in 40 minutes.

As the starting iron salt, sulfate, hydrochloride, nitrate, etc. can be used. Further, iron hydroxide oxides such as iron hydroxide and goethite can also be used.

The iron salt concentration in the slurry is preferably 0.6 mol/dm3 or less.

A high concentration is preferable from a production technology perspective, but if the concentration is too high, magnetite is likely to be generated, and 0.9 mol/
It becomes difficult to form hematite at di3 or higher.

The order of mixing and addition for slurry preparation can be completely arbitrary;
The effect on particle size and shape is not a problem. Furthermore, there is almost no effect of the maturation time of the slurry.

The content of the present invention will be explained below using examples.

〔Example〕

Example 1 5.0 g of sodium hydroxide was added to 30 c of a solution of 8 g of FeC1z-682010 dissolved in 60 cffl of water.
After adding a solution of No. J in water and stirring, a solution of 90 ml of tartaric acid dissolved in 30 C♂ water was further added and stirred thoroughly. The pH of this slurry was 9.70. Adjust the pH of the slurry to 11.7 using sodium hydroxide solution
The mixture was heated to 80°C, charged into an autoclave, and heated to 160°C in 2 hours.

After stirring and holding at 160° C. for 3 hours, the mixture was cooled to room temperature over 1 hour. The precipitate was taken out, washed, and dried at 80°C.

x of the obtained reddish brown powder! The v1 diffraction results revealed that this crystal powder was hematite.

From photographic observation using a scanning electron microscope (hereinafter abbreviated as electron microscope), it was found that these were plate-shaped crystals with well-uniformed particle sizes, and were suitable as a precursor for magnetic powder for floppy disks. The results of these particle sizes and particle size distributions are shown in Table 1.

Example 2 FeC1+ 6 HzO], 0.8 g was 5Qcn
A solution of 60 μm of tartaric acid dissolved in 3 QcJ of water was added to the solution of 5. A solution of Og dissolved in 30 crA water was added and stirred well. The pH of the slurry was adjusted to 9.92 three times using a sodium hydroxide solution, and the slurry was charged into an autoclave and heated to 140° C. over 2 hours.

After being maintained at 140° C. for 10 hours, it was cooled to room temperature in 1 hour. The precipitate was taken out, washed, and dried at 80°C.

The results of X-ray diffraction of the obtained reddish-brown powder revealed that this crystalline powder was hematite.

Photographic observation using a scanning electron microscope revealed that these were plate-shaped crystals with well-uniformed particle sizes, and were suitable as a precursor for magnetic powder for floppy disks. The results of these particle sizes and particle size distributions are shown in Table 1.

Example 3 FeC1+ ・6 Hz 10.8 g to 60 cf
After stirring, a solution of 5.0 g of sodium hydroxide dissolved in 30 cal of water was added to the solution of fl of water, and then a solution of 48 cal of tartaric acid dissolved in 30 cal of water was added. Stir well. The pH of this slurry was 9.70. The pH of the slurry was adjusted to 8 using sodium hydroxide solution.
．． Thereafter, a hydrothermal reaction was carried out under the same conditions as in Example 1 to obtain hematite powder crystals.

Photographic observation using a scanning electron microscope revealed that these were plate-shaped crystals with well-uniformed particle sizes, and were suitable as a precursor for magnetic powder for floppy disks. The results of these particle sizes and particle size distributions are shown in Table 1.

Example 4 Fe(NO3):+・91 (2016 g dissolved in 60 cJ of water) was added with a solution of 90 μl of tartaric acid dissolved in 39 cal of water and stirred. A solution obtained by dissolving .0 g of 3Qcn! A crystalline powder was obtained.

Photographic observation using a scanning electron microscope revealed that these were plate-shaped crystals with well-uniformed particle sizes, and were suitable as a precursor for magnetic powder for floppy disks. The results of these particle sizes and particle size distributions are shown in Table 1.

Example 5 240 μg of tartaric acid and 130 μg of sodium hydroxide were added to a solution in which 1.6 g of FeC13.6 HzO2 was dissolved in 120 g of water.
After adding a solution of (1) dissolved in 50 cm of water and stirring, the pH of the slurry was adjusted to 11.8 using sodium hydroxide solution.
Afterwards, the same hydrothermal treatment as in Example 1 was carried out,
A hematite crystal powder was obtained.

Photographic observation using a scanning electron microscope revealed that these were plate-shaped crystals with well-uniformed particle sizes, and were suitable as a precursor for magnetic powder for floppy disks. The results of these particle sizes and particle size distributions are shown in Table 1.

Table 1 * Fifty particles were randomly selected from the field of view of the electron micrograph, an area distribution curve was drawn, and the median value was taken as the particle size. Further, the particle size ratio (■) is the particle size ratio between the first and third quadrants of the distribution curve, and the particle size ratio (■) is the particle size ratio between the 1/10th and 9/10th points.

Example 6 8.1 g of tartaric acid and 45 thorium hydroxide were added to a solution in which 20,973 g of FeCIa-6H was dissolved in 8 kg of water.
After adding a solution of 0g dissolved in 3kg of water and stirring,
Reduce the plJ of the slurry to 11 using sodium hydroxide solution.
．． Adjust to 7 and set 1 in 20ff autotra 1 knob.
Hydrothermal reaction was carried out at 60° C. for 4 hours with stirring.

It took 1.5 hours to raise the temperature, and 2.5 hours to cool down after the reaction. The reaction slurry was centrifuged, and the resulting cake was washed with water.
Repeat the separation 5 times, and finally replace the water with acetone for 60 minutes.
It was dried at 110°C for 2 hours and then at 110°C for 2 hours.

X-ray diffraction and scanning electron microscopy revealed that this material had a particle size of 0.
487/[111 thickness 0.06μ, particle size ratio I 1.31
It was confirmed that the hematite particles were composed of well-aligned hexagonal plate-shaped hematite particles with a particle size ratio of 1.63.

31 g of the plate-like particles were placed in a tubular rotary furnace and reduced in a hydrogen stream containing water vapor at 370° C. for 4 hours. This material had a black color and was confirmed to be magnetite by X-rays.

Further, this was oxidized in air at 200°C for 2 hours to obtain reddish brown magnetic particles. As a result of electron microscopic observation of this material, its shape was almost the same as that of the precursor hematite plate-like particles.

'' Using two pairs of hematite as raw materials, the same reduction and oxidation treatment was performed again to obtain reddish-brown magnetic particles, and from a homogeneous mixture of these particles, 45 g was weighed out and CO3O4.7 t (
A suspension of 207.4 g in 274 g of water was prepared and hydrothermally treated at 200° C. for 7 hours in an autoclave to obtain a dark brown powder.

This material contains 3.3χ Co, has a coercive force of 6350e,
Saturation magnetization 63.3 emu/g, residual magnetization 40.2 emu
/g, and from its electron microscopy observation, the particle size,
It was found that the particle size distribution was almost the same as that of the hematite described above.

105 parts of the thus obtained magnetic powder was coated on a RET film with 30 parts of polyurethane and 20 parts of salt-vinyl acetate resin, and its magnetic properties were investigated. The coercive force was 6550.
The eS saturation magnetic flux density was 1030G and the residual magnetic flux density was 740G.

In addition, the modulation of this coating film is less than 1%,
Commercially available acicular powder (major axis diameter 0.50 mm, axial ratio 7, extremely superior value compared to 3.6z when non-orientation treatment is performed on a coating film using Co-coated mafelite) Met.

Example 7 3 kg of a solution of 68 g of FeC1z-6H2011 dissolved in 8800 g of water, a solution of 6.48 g of tartaric acid and 2500 g of water, and 528 g of sodium hydroxide
After adding the solution dissolved in water and stirring, adjust the pt+ of the slurry to 9.96 using sodium hydroxide solution,
The hydrothermal reaction was carried out in a No. 201 autoclave at 160° C. for 2.5 hours with stirring. It took 40 minutes to start raising the temperature, 1 hour and 20 minutes to raise the temperature, and 2 hours to let it cool after the reaction. The reaction slurry was centrifuged, and the resulting cake was washed and separated four times, and finally the water was replaced with acetone and dried at 60°C for 2 hours and then at 110°C for 2 hours.

As a result, 289 g of reddish brown powder was obtained. By X-ray diffraction,
This material was confirmed to be hematite. Also,
Electron microscopic observation confirmed that it consisted of well-aligned disc-shaped particles with a particle size of 0.25 cape, a thickness of 0.06 p, a particle size ratio I of 1.28, and a particle size ratio I of 1.61. .

50 g of the plate-shaped particles were placed in a tubular rotary furnace and reduced at 370° C. for 4 hours in a hydrogen stream containing water vapor. This material had a black color and was confirmed to be magnetite by X-rays.

Further, this was oxidized in air at 200°C for 2 hours to obtain reddish brown magnetic particles. As a result of electron microscopic observation of this material, it was found that it was almost the same as the plate-like particles of hematite that was the precursor.

Using the above-mentioned hematite as a raw material, similar reduction and oxidation treatments were performed to obtain reddish-brown magnetic particles, and from a homogeneous mixture of these, 45g was weighed out.
g, a suspension of 274 g of water in an autoclave at 200°C.
The mixture was hydrothermally treated for 7 hours to obtain a blackish brown powder.

This one contains 2.1z Go, coercive force 5700e,
Saturation magnetization 69.5 emu/g, residual magnetization 40.3 emu
/g, and from its electron microscopy observation, the particle size,
It was found that the particle size distribution was almost the same as that of the hematite described above.

105 parts of the magnetic powder thus obtained was coated on a PET film together with 30 parts of polyurethane and 20 parts of salt-vinyl acetate resin, and its magnetic properties were investigated.
The saturation magnetic flux density was 1320G and the residual magnetic flux density was 870G.

In addition, the modulation of this coating film is less than 1%,
It was extremely superior to the 366z value obtained when non-orientation treatment was performed on a coating film using commercially available acicular powder (major axis diameter 0.50-1 axis ratio 7, Co-adhered rougherite). Ta.

Comparative Example 1 In Example 1, the same amount of water was added instead of the tartaric acid solution and the suspension was adjusted to 11.78. When the suspension was hydrothermally treated in exactly the same manner, rod-shaped particles of 0.5 to 1 ρ were formed. Particles were obtained, and X-ray diffraction revealed that they were goethite crystal particles (Figure 5).Furthermore, the hydrothermally treated suspension whose pH was adjusted to 9.0 was
It was hematite consisting of polyhedral particles of 0.06 to 0.2 tm.

Comparative Example 2 In Example 1, the dissolved amount at the time of preparing the sodium hydroxide solution was changed to 4.0 g, and a solution in which 118 μm of trisodium citrate was dissolved instead of tartaric acid was used, and the pl of the suspension was adjusted to 10.0 g. A reddish-brown powder was obtained in exactly the same manner except that the powder was adjusted to 0.1 to 2.0. This consisted of spherical particles of 0.1 to 2.0, and was confirmed to be hematite by X-ray (Fig. 6). ) Comparative Example 3 In Example 2, a solution containing 52 g of sodium phosphate dihydrate i was used instead of tartaric acid, and the p of the suspension was
A reddish brown powder was obtained in exactly the same manner except that H was adjusted to 12.2. This material was found to consist of 0.01 particles of amorphous particles.

Comparative Example 4 In Example 2, glucose 144 was used instead of tartaric acid.
A black powder was obtained in exactly the same manner except that the p)I of the suspension was adjusted to 11.7 using a solution containing (2). This material was found to be composed of ultrafine aggregates of magnetite.

Comparative Example 5 A black powder was obtained in exactly the same manner as in Example 2, except that a solution containing 600 μl of tartaric acid was used instead of 60 μl, and the pH of the suspension was adjusted to 11.9. This material was found to be composed of ultrafine aggregates of magnetite. (Fig. 7) Similarly, tartaric acid 90■, p) 113.0 has a grain size of 0.6 Xo, 25- planooctahedral he7tite
Figure), but with tartaric acid 30cm, p) 110.0, the particle size is 0.1
5-0.45-hexagonal bipyramidal hematite was obtained.

Comparative Example 6 20g of each of the hematites obtained in Examples 6 and 7 was weighed, thoroughly wet-mixed in a mortar, and then dried and ground. Of this, 30 g was reduced and oxidized in a tubular rotary furnace in the same manner as in Example 6, and subsequently treated with Co to obtain magnetic powder containing 2.8χ Co.

The average particle size of the hematite mixture was 0.21 mm, the particle size ratio I was 1.92, and the particle size ratio I was 2.30. The magnetic powder has a coercive force of 6100e and a saturation magnetization of 62. Oemu/g, residual magnetization was 37.2 emu/g, and the particle size distribution was similar to that of the raw material hematite.

When the magnetic powder thus obtained was formed into a coating film in the same manner as in Example 6, the coercive force was 6450e, and the saturation magnetic flux density was 970G.
, 8 left! The magnetic flux density was 490G, and the output was too low to be used as a magnetic recording medium.

[Brief explanation of the drawing]

Figures 1 to 4 are electron micrographs of plate-shaped hematite 1 powder obtained in Examples of the present invention, and Figures 5 to 8 are electron micrographs of respective particles obtained in Comparative Examples. It's a photo. Applicant's agent Kaoru Furuya-FA-1--, ---f'C, Helip...S) No. 51y1 Condolences
・(A1
Article ε Interim Procedures Chief Official Seal Seal) 2 September 10, 1986

Claims (1)

[Claims] 1. In the cumulative distribution curve of the area of the plate-like surface, the median particle size (equivalent diameter) is 0.01 to 1 μm, and the first and third quadrants are
The particle size ratio at the quarter point is 0.67 to 1.50,
and the ratio of particle sizes at the 1/10 minute point and the 9/10 minute point is 0.
．． 5 to 2.0, and plate-like particles having a plate-like ratio (equivalent diameter/thickness) of 2 or more. 2 An aqueous slurry in which three components, ferric salt, tartaric acid or/and its salt, and an alkali compound coexist, with a pH≧8.2 and (pH-6.6)/600≦r≦(pH-2 .8)/6
00 (however, r = [tartaric acid and its salt] / [iron salt] (molar ratio)) By heating the slurry composition satisfying the condition of 120 ° C. or higher, in the cumulative distribution curve of the area of the plate-like surface,
The median particle size (equivalent diameter) is 0.01 to 1 μm, the particle size ratio at the 1st and 3rd quartiles is 0.67 to 1.50, and the 1/10th and 9th The particle size ratio at the /10 minute point is 0.5 to 2.0, and the plate ratio (equivalent diameter/thickness) is 2.
A method for producing plate-shaped hematite powder, which comprises obtaining plate-shaped hematite powder consisting of plate-shaped particles having the above.