JPH07101731A - Granular magnetite particle powder and its production - Google Patents

Abstract

PURPOSE:To industrially obtain a granular magnetite particle powder black in color and small in magnetization value. CONSTITUTION:The granular magnetite particle powder, which is of a granular magnetite particle containing 3-6wt.% aluminum element in a spinel type crystal lattice and 8.340-8.375Angstrom in lattice constant and has 30-50emu/g magnetization value in 1kOe magnetic field, or the granular magnetite particle powder, which is, if necessary, of the granular magnetite particle with hydroxides of aluminum stuck to the surface of the granular magnetite particle and has 20-45emu/g in magnetization value in 1kOe magnetic field, is selected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a granular magnetite particle powder which is black and has a small magnetization value, and a method for producing the same.

The main uses of the granular magnetite particles according to the present invention are for coatings, printing inks, colored pigment powders for rubber and plastics, magnetic toners, magnetic carriers,
Examples of the material include powder particles for magnetic cards.

[0003]

2. Description of the Related Art Magnetite particles have a black color and are widely used as black color pigment powders for paints, printing inks, rubbers and plastics.

Since the magnetite particle powder is a ferromagnetic particle, it is mixed and dispersed in a resin to form a composite particle, so that a magnetic toner material particle powder and a magnetic carrier material particle for electrostatic copying are obtained. Used as a powder.

Further, the magnetite particle powder is provided with a magnetic stripe by directly coating a magnetic paint obtained by dispersing it in a vehicle on a card base or by adhering a magnetic tape coated on a base film to the card base. It is also used as a powder of particles for magnetic cards represented by credit cards, railway tickets, commuter passes, road tickets, telephone cards, orange cards and the like.

In any of the above fields, there is no end to the demands for higher performance and higher quality, and the improvement of the characteristics of the magnetite particle powder, which is the material particle powder, and in particular, the excellent dispersibility, makes it compatible with the resin. There is a strong demand for easy kneading and mixing with the vehicle.

This fact is true in the case of coloring pigments for paints, printing inks, rubbers and plastics, "... coating film," on page 8 of the Journal of Coloring Materials Vol. 49, No. 1 (1976). It seems to be no exaggeration to say that most of the characteristics to be possessed are largely determined by the dispersibility of the pigment in the coating film if they are the same pigment. The theory teaches that if the dispersibility of the pigment is good, the color tone becomes clear, and the basic basic properties of the pigment, such as coloring power and swelling power, are improved. In addition, the air resistance of the coating film is improved, which results in improving the durability of the coating film.In this way, the dispersibility of the pigment in the coating film is an extremely important factor that determines various characteristics of the coating film. It can be understood that it is. "

Regarding the magnetite particle powder as the material particle powder for magnetic toner, JP-A-55-6540 is known.
Good compatibility with resin of "6 ... VII)"
Usually, the particle size of the toner is several tens of μm or less, and the degree of microscopic mixing in the toner is important for the characteristics of the toner. ...
... "is as described.

Further, regarding magnetite particle powder as a material particle powder for magnetic carrier, Japanese Patent Laid-Open No. 61-
No. 53660, "In accordance with the present invention, the dispersibility of primary particles is improved, the magnetic powder is uniformly dispersed, and the difference in charging property and magnetic property between magnetic developer particles is reduced." Is the street.

Speaking of magnetite particle powder as a material particle powder for magnetic cards, "... Magnetic" of "Magnetic Card and Chemical Technology" on pages 68 to 72 of "Chemical Technology Magazine MOL" (1985) published by Ohmsha, Ltd. The characteristics greatly change depending on the state of dispersion of the body .... It is important to obtain a good quality dispersion system. "

Conventionally, magnetite particle powders are generally easy to obtain particles having excellent dispersibility due to the fact that the particles are scattered one by one. Therefore, the so-called wet method in which the particles are directly produced from an aqueous solution. Is manufactured by.

That is, in the wet method, Fe (OH) ₂ or F obtained by mixing an aqueous ferrous salt solution such as ferrous sulfate with an alkaline aqueous solution such as sodium hydroxide or sodium carbonate.
60 to 10 in an aqueous solution containing Fe-containing precipitates such as eCO ₃
This is a method of ventilating an oxidizing gas in a temperature range of 0 ° C. (Japanese Patent Publication No. 44-668).

However, the granular magnetite particle powder obtained by the above-mentioned wet method has a large magnetization value of 55 to 70 emu / g, though the particles are scattered one by one, and magnetic aggregation is likely to occur. , Was an obstacle to improving the dispersibility.

Therefore, there is a strong demand for magnetite particle powders having a small magnetization value in order to further improve the dispersibility in vehicles and resins.

This requirement is particularly strong when the granular magnetite particle powder is used as the magnetic toner material particle powder. This fact will be explained below.

As the performance of electrostatic copying machines is reduced, such as downsizing and speeding up, there is a strong demand for improving the characteristics of magnetic toner as a developer, that is, magnetic toner capable of high density development and high resolution.

The characteristics of the magnetic toner are closely related to the characteristics of the magnetic particle powder contained in the resin. In order to enable high density development, the content of the magnetic particle powder contained in the resin is required. However, if the content of the magnetic particle powder is increased, magnetic toner remains as agglomerates on the latent image after development due to magnetic agglomeration of the magnetic particle powder, and a fine latent image is faithfully reproduced. It is difficult to reproduce, and high resolution cannot be obtained.

Therefore, in order to obtain a magnetic toner capable of both high density development and high resolution, even if the content of the magnetic particle powder contained in the resin is increased, magnetic cohesive force is not generated. It is required to make the magnetization value of the magnetic particle powder as small as possible.

This fact is described in Japanese Unexamined Patent Publication No. 4-184354, "Toner faithful to signals and originals, that is, faithful to latent image and developing at high density is required. There is.

However, it is difficult to satisfy the above high requirements by using a toner containing a magnetic material.

For example, if the content of the magnetic substance is simply increased in order to increase the coloring power of the magnetic toner to obtain a high density, the image quality deteriorates and it becomes impossible to satisfy the high resolution. Because the size of the ears is related to the magnetic force of the toner. As the amount of magnetic material increases, the ears grow larger. Such ears tend to remain as agglomerates due to the aggregating force even on the latent image after the toner is developed, and it becomes difficult to faithfully reproduce a fine latent image.

On the contrary, in order to satisfy the high resolution, it is considered that the content of the magnetic material is reduced to reduce the magnetic cohesive force, but in addition to the reduction of the coloring power, the developing property is lowered due to the increase of the charge amount, and the toner is reduced. The production efficiency will be reduced. In that case, it may be improved by using a colorant such as carbon black in combination, but it may be difficult to put it into practical use because it may cause deterioration of environmental dependency. ‥‥‥

Therefore, it has been proposed to adjust the magnetic force to obtain high-performance toner. ‥‥"as well as"‥
By containing an appropriate amount of a magnetic material having an appropriately small magnetic force, fine line reproducibility is improved, and an image quality can be obtained by developing with high fidelity to the latent image or signal. "..." is as described.

Conventionally, as a magnetic particle powder having a small magnetization value, a method of using zinc ferrite particles or zinc-manganese ferrite particles having the same spinel type crystal structure as that of magnetite particle powder has been proposed (JP-A-4-184354). ing.

Various attempts have been made to improve various properties of the magnetite particle powder. For example, a method of adding an aluminum compound in the reaction for producing the magnetite particle powder (Japanese Patent Publication No. 1-38684, JP-A-4-3684).
No. 190242) is known.

[0026]

Granular magnetite particles having a small magnetization value are currently most demanded, but such granular magnetite particles have not been obtained yet.

That is, although the zinc ferrite particles and the zinc-manganese ferrite particles described in the above-mentioned JP-A-4-184354 have small magnetization values, the hue is not black but brown, so that high density development is performed. It is not possible to obtain a magnetic toner capable of

The granular magnetite particle powder obtained by the method described in Japanese Patent Publication No. 1-36864 and Japanese Patent Application Laid-Open No. 4-190242 has a black color peculiar to magnetite particles, but is shown in Comparative Example 1 below. As described above, the magnetization value is large and a magnetic cohesive force is easily generated.

Therefore, the present invention has a technical object to obtain a granular magnetite particle powder which is black and has a small magnetization value.

[0030]

The above technical problems can be achieved by the present invention as follows.

That is, according to the present invention, the spinel type crystal lattice contains 3 to 6 wt% of aluminum element, and
Granular magnetite particles having a lattice constant of 8.340 to 8.375Å and having a magnetization value of 30 to 50 emu / g in a magnetic field of 1 kOe, and 3 to 6 of aluminum element in a spinel type crystal lattice.
wt% and the lattice constant is 8.340
Magnetite particles in which aluminum hydroxide is deposited on the surface of granular magnetite particles having a particle size of 8.375Å and having a magnetization value of 20 to 45 in a magnetic field of 1 KOe.
Aluminum obtained by adding 15 to 70 mol% of aluminum compound in terms of Al to ferrous iron in granular magnetite particle powder of emu / g and alkaline aqueous solution, and then adding ferrous salt aqueous solution And granular magnetite particles are generated by bubbling an oxygen-containing gas into an alkaline suspension having a pH of 10 or more containing an iron-containing precipitate, or if necessary, the pH in the alkaline suspension is further adjusted to a range of 5 to 9. The method for producing a granular magnetite particle powder, which comprises adjusting aluminum hydroxide to the particle surface of the granular magnetite particle.

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

In the granular magnetite particle powder according to the present invention, it is necessary that the aluminum element is contained in the spinel type crystal lattice, and whether or not the aluminum element is contained in the spinel type crystal lattice is the lattice constant. It can be confirmed by measuring.

When the spinel type crystal lattice does not contain aluminum element, it is not possible to obtain granular magnetite particles having a small magnetization value.

The content of aluminum element in the granular magnetite particles according to the present invention is 3 to 6 wt%. 3 wt
When it is less than%, it is not possible to obtain granular magnetite particles having a small magnetization value, so that the magnetic cohesive force is large and the dispersibility in the vehicle and the kneadability in the resin are poor.
Even if a magnetic toner is manufactured using such granular magnetite particles, high resolution cannot be satisfied.

In the present invention, the upper limit value of the aluminum element that can be contained in the spinel type crystal lattice is 6
wt%. The larger the amount of aluminum element contained in the spinel type crystal lattice, the smaller the magnetization value of the obtained granular magnetite particles.

The granular magnetite particles used in the present invention have a lattice constant of 8.340 to 8.375Å. The larger the amount of aluminum element contained in the spinel type crystal lattice, the smaller the lattice constant tends to be. When the content of aluminum element is 3 wt%, the lattice constant is 8.375.
Granular magnetite particles of about Å are obtained, and when the content of aluminum element is 6 wt%, the lattice constant is 8.340 Å
Some granular magnetite particles are obtained.

The granular magnetite particles according to the present invention have a magnetization value of 30 to 50 emu / g. In the present invention, the magnetization value tends to decrease as the amount of aluminum element contained in the spinel type crystal lattice increases, and when the content of aluminum element is 6 wt%, the magnetization value is 30 em.
Granular magnetite particles of about u / g can be obtained. 50e
When it exceeds mu / g, the magnetic cohesive force is large and the dispersibility in the vehicle and the kneadability in the resin are poor.
Even if a magnetic toner is manufactured using such granular magnetite particles, high resolution cannot be satisfied.

With the granular magnetite particle powder according to the present invention, particles having a BET specific surface area of 3 to 15 m ² / g can be obtained.

The granular magnetite particles according to the present invention may be coated with aluminum hydroxide on the surface of the particles, if necessary. In this case, since the aluminum hydroxide is non-magnetic, Value is 20 to 45 emu / g
It is possible to obtain smaller granular magnetite particles. In the present invention, magnetite particles having a deposition amount of aluminum hydroxide of 1 to 9 wt% can be obtained.

The granular magnetite particles according to the present invention are obtained by adding 15 to 70 mol% of an aluminum compound in terms of Al to ferrous iron in an alkaline aqueous solution, and then adding a ferrous salt aqueous solution. Granular magnetite particles are produced by bubbling an oxygen-containing gas into an alkaline suspension having a pH of 10 or more containing a precipitate containing aluminum and iron, and if necessary, the pH in the suspension is further adjusted to a range of 5 to 9. And aluminum hydroxide is deposited on the surface of the granular magnetite particles.

As the alkaline aqueous solution in the present invention,
Sodium hydroxide, potassium hydroxide and the like can be used.

As the aluminum compound in the present invention, aluminum sulfate, sodium aluminate, aluminum chloride, aluminum nitrate and the like can be used.

The amount of the aluminum compound added is 15 to 70 mol% in terms of Al based on ferrous iron. 15mo
If it is less than 1%, 3 wt% or more of the aluminum element in the spinel type crystal lattice cannot be contained, so that magnetite particle powder having a small magnetization value cannot be obtained. Even when it exceeds 70 mol%, the aluminum element in the spinel type crystal lattice can be contained, but the aluminum element cannot be contained in excess of 6 wt%, so that it is meaningless to add more than necessary.

As the ferrous salt aqueous solution in the present invention,
A ferrous sulfate aqueous solution, a ferrous chloride aqueous solution or the like can be used.

In the present invention, it is important to add the aluminum compound to the alkaline aqueous solution and then the ferrous salt aqueous solution. In the case of reversing the order of addition or simultaneously adding them, spinel type The elemental aluminum cannot be contained in the crystal lattice, and as a result, the granular magnetite particle powder having a small magnetization value, which is the object of the present invention, cannot be obtained. Moreover, the above-mentioned publicly known special fair 1
In the inventions described in JP-A-36864 and JP-A-4-190242, acicular goethite particles are easily generated and mixed with granular magnetite particles, but in the present invention, generation of acicular goethite particles is suppressed and granular. Magnetite particles can be easily generated.

The pH value of the oxidation reaction is 10 or more. If the pH value is less than 10, acicular goethite particles are likely to be generated and aluminum element is preferentially contained in acicular goethite particles, so that it is difficult to include granular magnetite particles in the spinel type crystal lattice. Becomes

The oxidizing means in the present invention is performed by passing an oxygen-containing gas (for example, air) through the liquid.

The temperature during the oxidation reaction in the present invention is 70
Is in the range of -100 ° C If the temperature is lower than 70 ° C, acicular goethite particles are likely to be mixed. Even when the temperature exceeds 100 ° C., the elemental aluminum can be contained in the spinel type crystal lattice, but it is not industrial because it requires a special device such as an autoclave.

The deposition of the aluminum hydroxide in the present invention may be carried out by adjusting the pH of the alkaline suspension within the range of 5-9. When the pH is less than 5 or more than 9, the hydroxide of aluminum is hard to form, and it remains as aluminate ion in the alkaline suspension without adhering to the surface of the granular magnetite particles.

[0051]

First, the most important point in the present invention is to add an aluminum compound and a ferrous iron aqueous solution to an alkaline aqueous solution in this order, and to set a specific amount of the aluminum compound to be added. It is a fact that when the range is set to a specific range, aluminum element can be contained in the spinel type crystal lattice of the granular magnetite particles, and as a result, granular magnetite particle powder having a small magnetization value can be obtained.

Although the reason why the aluminum element can be contained in the spinel type crystal lattice of the granular magnetite particles has not been clarified yet, the present inventor, as will be shown in the comparative example to be described later, has an alkaline aqueous solution, an aluminum compound, and When the order of addition of the aqueous ferrous salt solution was changed, the amount of the aluminum compound added was 15 mol% with respect to the ferrous iron.
In the case where the pH is less than 10 and the pH during the oxidation reaction is less than 10, granular magnetite particles having a small magnetization value cannot be obtained. Therefore, the order of addition of the alkaline aqueous solution, the aluminum compound and the ferrous salt aqueous solution, It is considered to be due to the synergistic effect of the addition amount and pH during the oxidation reaction.

Regarding the reason why granular magnetite particles having a small magnetization value can be obtained, the present inventor has found that the granular magnetite particle powder according to the present invention contains an aluminum element in the spinel type crystal lattice, as will be shown in the examples below. The lattice constant of magnetite particles that are not formed is about 8.4 Å, whereas the lattice constant is small as 8.349-8.367 Å, and as shown in FIG. Since the lattice constant tends to be small and it is recognized that the aluminum element strongly influences the formation of the spinel type crystal lattice, the magnetic moment in which the aluminum element having no magnetic moment is contained in the spinel type crystal lattice. It is believed that this is due to the fact that it is contained by substituting the iron element with.

The following is a partial description of a large number of experimental examples conducted by the present inventor.

FIG. 1 shows the relationship between the amount of aluminum element contained in the spinel type crystal lattice and the lattice constant of the granular magnetite particle powder. That is, A to be added
except that the amount of the l ₂ (SO ₄ ) ₃ aqueous solution was variously changed.
2 is a graph in which the content of aluminum element and the lattice constant of the granular magnetite particle powder obtained in the same manner as in Example 1 below are measured and plotted. As shown in FIG. 1, the lattice constant of the granular magnetite particles tends to decrease as the content of aluminum element increases.

The granular magnetite particles according to the present invention in which the aluminum hydroxide is adhered to the surface thereof can have a smaller magnetization value because the aluminum hydroxide is non-magnetic. , The charge amount of the granular magnetite particles can be controlled in the range of -30 to 20 μC / g by adjusting the amount of the adhered particles, so that it should be appropriately used for various kinds of magnetic toners according to the application. You can

In the granular magnetite particles according to the present invention, the hue of the previously known zinc ferrite particles or zinc manganese ferrite particles is 13 ≦ L ^* ≦ 20, a ^* > 0.7, −3 ≦.
As shown by b ^* ≦ 0, a ^* is larger than 0.7 and is brownish, whereas 13 ≦ L ^* ≦ 20, −1 ≦
a ^* ≦ 0.7, indicates the hue of ^{-3 ≦ b * ≦ 0, a} * is enough to hold black inherent small magnetite particles less than 0.7.

[0058]

The present invention will be described below with reference to examples and comparative examples.

The shapes of the particles in the following Examples and Comparative Examples were observed with a transmission electron microscope and a scanning electron microscope.

The magnetic characteristics of the magnetic particle powder are values measured under a magnetic field of 1 kOe using "Vibration sample magnetometer VSM-3S-15" (manufactured by Toei Industry Co., Ltd.).

The amount of Al contained in the granular magnetite particles was determined by "high frequency plasma emission spectroscopic analyzer ICAP-57".
5 ”(manufactured by Nippon Jarrell Ash Co., Ltd.).

The lattice constant of the granular magnetite particles is "X
It is a value measured using a line diffraction device RAD-2A "(manufactured by Rigaku Denki Co., Ltd.).

The hue is represented by L ^* (brightness), a ^* value and b ^* value, which are obtained by using a colorimetric test piece as a light source spectrocolorimeter M.
Hu using SC-IS-2D (manufactured by Suga Test Instruments Co., Ltd.)
The L ^* value, the a ^* value, and the b ^* value are measured by the Lab space of the computer, and the
n International de 1'Ecl
image, CIE) 1976 (L ^* , a ^* , b ^* )
The values are shown according to the uniform perceptual color space.

A sample piece for measuring hue was formed into a paste by kneading 0.5 g of granular magnetite particle powder and 0.5 cc of castor oil with a Hoover-type Mahler, and adding 4.5 g of clear lacquer to this paste and kneading to form a paint. , Cast coated paper using a 6 mil applicator.

Example 1 To 22.07 l of 4.3N aqueous NaOH solution, 0.672
5 l of a mol / l Al ₂ (SO ₄ ) ₃ aqueous solution was added,
Then, 1.8 mol / l ferrous sulfate aqueous solution 14.9
3 l were added to give a total of 42 l of an alkaline suspension containing the aluminum and iron containing precipitate. The pH of this alkaline suspension was 10.6. Then, the temperature of the alkaline suspension is raised to 100 l / mi at a temperature of 90 ° C.
Air was bubbled through at a flow rate of n for 110 minutes to form a black precipitate. The black precipitate was separated by filtration, washed with water, and dried at 60 ° C. to obtain black particle powder.

The obtained black particle powder was granular particles as a result of electron microscope observation, and as a result of X-ray diffraction, only peaks of spinel type crystals were recognized. The BET specific surface area was 8.5 m ² / g.

This black particle powder had an aluminum content of 3.54 wt%, a lattice constant of 8.36Å, and magnetic characteristics of a magnetization value of 43.9 emu / g.

Examples 2 to 3 and Comparative Examples 1 to 4 The order of addition of the aqueous alkali solution, the aluminum compound and the aqueous ferrous salt solution, the type and amount of the aluminum compound, and the pH and temperature of the alkaline suspension were variously changed. Particles were generated in the same manner as in Example 1 except for the above.

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

In the black particle powders obtained in Examples 2 and 3, only the peak of spinel type crystal was observed.

The particle powders produced in Comparative Examples 2 and 4 were found to have spinel type crystal peaks and goethite peaks as a result of X-ray diffraction, and as a result of electron microscope observation, granular particles and acicular particles. Since the particles are recognized, it is a mixed powder of granular magnetite particles and non-magnetic acicular goethite particles, and as a result, it is recognized that the magnetization value is small.

Example 4 A part of the reaction mother liquor containing the black precipitate obtained in Example 1 was sampled, and sulfuric acid was added to this sampled solution while stirring to obtain p.
The H was adjusted to 7, and aluminum hydroxide was deposited on the surface of the granular magnetite particles. The black precipitate was separated by filtration, washed with water, and dried at 60 ° C. to obtain black particle powder.

As a result of the measurement, the obtained black particle powder had an aluminum content of 6.74 wt%, and the aluminum element contained in the spinel type crystal lattice was as described above.
Since it is 3.54 wt%, the amount of deposited aluminum is 3.20 wt% (6.74-3.54 = 3.20).
It is recognized that

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

Examples 5 to 6 Aluminum hydroxide was deposited on the surface of granular magnetite particles in the same manner as in Example 4 except that the reaction mother liquor containing the black precipitate and the pH were changed variously.

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

[0077]

[Table 1]

[0078]

[Table 2]

[0079]

The granular magnetite particle powder according to the present invention is black and has a small magnetic cohesive force due to its small magnetization value. Therefore, it can be used for paints, printing inks, rubbers and plastics. It is suitable for use as a coloring pigment powder, a magnetic toner, a magnetic carrier, and a magnetic card material particle powder.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of aluminum element contained in the spinel crystal lattice of the granular magnetite particles according to the present invention and the lattice constant.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI technical display location H01F 1/36 (72) Inventor Hiromitsu Misawa 4-1-2 Fununairi Minami, Naka-ku, Hiroshima-shi, Hiroshima Toda Kogyo Co., Ltd. Creation Center (72) Inventor Minoru Yoshizawa 4-1-2, Funarinami, Naka-ku, Hiroshima City, Hiroshima Prefecture Toda Kogyo Co., Ltd. Creation Center (72) Inventor Naoki Uchida, Hiroshima City, Hiroshima Prefecture 4-1-2 Fununairi Minami-ku, Toda Kogyo Co., Ltd. Creative Center

Claims (4)

[Claims] 1. A spinel type crystal lattice containing 3 to 6 wt% of aluminum element and having a lattice constant of 8.
Granular magnetite particles having a size of 340 to 8.375Å and a magnetization value of 30 to 50 em in a magnetic field of 1 kOe.
A granular magnetite particle powder characterized by having u / g. 2. A spinel type crystal lattice containing 3 to 6 wt% of aluminum element and having a lattice constant of 8.
Magnetite particles having a particle size of 340 to 8.375Å and aluminum hydroxide deposited on the surface of the particles, characterized by having a magnetization value of 20 to 45 emu / g in a magnetic field of 1 kOe. Granular magnetite particles powder. 3. An aluminum- and iron-containing precipitate obtained by adding 15 to 70 mol% of an aluminum compound in terms of Al to ferrous iron in an alkaline aqueous solution, and then adding a ferrous salt aqueous solution. The method for producing granular magnetite particle powder according to claim 1, wherein the granular magnetite particles are produced by aerating an oxygen-containing gas in an alkaline suspension having a pH of 10 or more containing. 4. An aluminum- and iron-containing precipitate obtained by adding an aluminum compound in an alkaline aqueous solution in an amount of 15 to 70 mol% in terms of Al with respect to ferrous iron, and then adding a ferrous salt aqueous solution. Granules of magnetite are produced by aerating an oxygen-containing gas in an alkaline suspension having a pH of 10 or more containing, and then the pH of the alkaline suspension is adjusted to a range of 5 to 9 to obtain the above-mentioned granular magnetite. The method for producing granular magnetite particle powder according to claim 2, wherein aluminum hydroxide is deposited on the surface of the particles.