JPH0664927A - Production of acicular hematite particle powder - Google Patents

Abstract

PURPOSE:To provide a process for the industrial production of an essentially high-density acicular hematite particle having improved crystallinity while keeping the acicular form of the starting material. CONSTITUTION:The objective acicular hematite particle having an average major axis diameter of 0.1-0.5gm and a specific surface area of 15-60m<2>/g measured by BET method can be produced by heating and dehydrating acicular goethite particle at 250-400 deg.C to obtain acicular hematite particle having an average length of 0.1-0.5mum and a specific surface area of 70-150m<2>/g measured by BET method and subjecting an aqueous dispersion containing the hematite particles to hydrothermal treatment at 150-250 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing needle-shaped hematite particle powder as a starting material for producing magnetic particle powder for magnetic recording. It is an object of the present invention to obtain a substantially dense acicular hematite particle powder having a high degree of crystallinity on the surface and inside of the particle.

[0002]

2. Description of the Related Art In recent years, with the progress of miniaturization and weight reduction of magnetic recording / reproducing devices, there is an increasing need for higher performance of magnetic recording media such as magnetic tapes and magnetic disks.
That is, high density recording characteristics, high output characteristics, high sensitivity characteristics,
Improvements in various characteristics such as frequency characteristics are required, and for that purpose, it is necessary to increase the coercive force of the magnetic recording medium and improve the residual magnetic flux density Br.

In order to increase the coercive force of the magnetic recording medium, it is necessary that the magnetic particle powder has a high coercive force. Further, the residual magnetic flux density Br of the magnetic recording medium depends on the dispersibility of the magnetic particle powder in the vehicle, the orientation in the coating film, and the filling property, and the magnetic particle powder has a needle-like shape. It is necessary that sintering does not occur between each other, no pores exist on the surface of the particle and inside the particle, and the particle has a substantially high density.

At present, acicular magnetite particle powder and acicular maghemite particle powder are mainly used as magnetic particle powder for magnetic recording. These magnetic particle powders are generally acicular goethite particles obtained by air-oxidizing ferrous hydroxide colloid in an aqueous solution obtained by reacting an aqueous ferrous salt solution with an alkali (usually referred to as wet reaction). In air at 300 ° C
It is heated and dehydrated in the vicinity to form acicular hematite particles, and further reduced in a reducing gas such as hydrogen at 300 to 400 ° C. to acicular magnetite particles, which are then heated to 200 to 30 in air.
It is obtained by oxidation at 0 ° C. into needle-shaped maghemite particles.

The coercive force Hc of the magnetic particle powder can be expressed by the following relational expression. Hc = K. (Nb-Na) .Ms In this relational expression, K is the degree of crystallinity of the particles, and (N
b-Na) relates to the shape (acicularity) of the particles, and Ms relates to the chemical composition of the particles. As is clear from this relational expression, in order to improve the coercive force of the magnetic particle powder, it is required to maintain and inherit the acicular shape of the acicular goethite particles and increase the degree of crystallinity. The properties of these magnetic particle powders are required not only for improving the coercive force, but also for improving the dispersibility of the magnetic particle powder in the vehicle, the orientation in the coating film, and the filling property.

Conventionally, in the production of magnetic particle powder, heating and dehydration have been carried out at a relatively low temperature in order to retain and inherit the acicular shape of goethite particles as described above. The needle-shaped hematite particles thus obtained are needle-shaped skeleton particles leaving the outer shape of the needle-shaped goethite particles, on the other hand, the skeleton particles consist of agglomerated particles in which a large number of single particles are linked, Since the grain growth of one grain is not sufficient, the degree of crystallinity is small.

When needle-like hematite particles having a low degree of crystallinity are used as a starting material and subjected to heat reduction, the particle growth of the single particles, that is, the physical change is rapid during the heat reduction process. Growth does not occur easily, and therefore, in the portion where single particle particle growth occurs rapidly, sintering occurs between particles and between particles, and the particle shape is likely to collapse.

Further, the acicular goethite particles are made of NaOH or F.
Needle-shaped goethite particles containing a large amount of impurities such as Na ⁺ and SO ₄ ^{2-, which} are derived from raw materials such as eSO _4, are inevitably contained, but these impurities promote sintering in the heating and firing process. In the case of using, the particles and the particles are more likely to sinter in the heating and reduction process, and the particle shape is likely to collapse.

In the heating and oxidizing process in which the acicular magnetite particles are converted into acicular maghemite particles, single particle growth does not occur. This phenomenon is described in “γ-F” on page 19 of the 7th Ferrite Summer Seminar Lecture Summary (1977).
e ₂ O ₃ does not grow grains due to sintering ”.

[0010] Conventionally, as a method for obtaining needle-like hematite particles of substantially high density which retain and inherit the needle-like shape and have a high degree of crystallinity, for example, Japanese Patent Publication No. 55-4.
There is a method described in Japanese Patent No. 2934. JP-B-55-42
In the method described in Japanese Patent No. 934, needle-shaped hematite particles obtained by heating and dehydrating needle-shaped goethite particles are heated to 350 to 70 in an atmosphere consisting of heated steam and a non-reducing gas.
It is heated and baked at 0 ° C.

[0011]

Although the needle-like hematite particles which retain and inherit the needle-like shape and have a high degree of crystallinity and which are substantially dense, are currently most demanded, In the case of the method described in Japanese Patent Publication No. 55-42934, the higher the treatment temperature, the easier the particle growth of single particles to occur and the higher the degree of crystallinity. On the other hand, at the same time as the growth of the single particles, the particles and the sintering between the particles easily occur, and it becomes difficult to maintain and inherit the acicular shape.

[0012] Therefore, it is a technical object of the present invention to obtain needle-like hematite particles having a needle-like shape while maintaining and inheriting the needle-like shape and having a substantially high degree of crystallinity.

[0013]

The above technical problems can be achieved by the present invention as follows. That is, the present invention has an average major axis length of 0.1 to 0.5 μm obtained by heating and dehydrating acicular goethite particles in a temperature range of 250 to 400 ° C., and has a specific surface area by the BET method. 70-150m ²
150 g of an aqueous liquid medium containing needle-like hematite particles having an amount of / g.
By hydrothermal treatment in the temperature range of ~ 250 ° C, the needle-like shape having an average major axis length of 0.1 to 0.5 µm and a BET specific surface area of 15 to 60 m ² / g is inherited. The method for producing a needle-shaped hematite particle powder, which comprises obtaining substantially high-density needle-shaped hematite particles.

Next, various conditions for carrying out the present invention will be described. The acicular goethite particles in the present invention are prepared by mixing a well-known ferrous salt aqueous solution and an alkaline solution, and then suspending a suspension containing ferrous hydroxide particles at a temperature of 80 ° C. or lower with an oxygen-containing gas. And a Fe obtained by reacting an aqueous solution of ferrous salt with an alkali carbonate
It can be obtained by any method such as a method in which an oxygen-containing gas is passed through a suspension containing CO ₃ to carry out an oxidation reaction, and the major axis is 0.1 to 0.5 μm, the axial ratio (average major axis length). : Average short axis length) Needle-like particles having a diameter of 5: 1 to 20: 1, as well as particles in any shape such as spindle-shaped particles, rice particles, and spheroids can be used. In the above-mentioned goethite particle formation reaction, Co, Ni, Zn, Al, Mn, and C which are usually added to improve the characteristics of the target magnetic particle powder.
Different elements other than Fe such as u and Si may be present.

The particles to be treated in the present invention are obtained by heating and dehydrating acicular goethite particles in the temperature range of 250 to 400 ° C., the average major axis length is 0.1 to 0.5 μm, and the specific surface area is 70. Needle-like hematite particles of 150 m ² / g. When the heating dehydration temperature is lower than 250 ° C, it is difficult to obtain acicular hematite particles. If it exceeds 400 ° C., the needle-like shape of the obtained hematite particles will be broken due to the sintering between the particles. Considering the acicular shape of acicular hematite particles, the temperature is preferably lower than 350 ° C. If the average major axis length is less than 0.1 μm, 0.5 μm
When it exceeds, it is hard to say that the needle-like shape is retained and inherited. The specific surface area of the acicular hematite particle powder in which sintering between particles and particles is prevented is 50 to 150 m ² / g.

As the aqueous liquid medium in the present invention, a neutral aqueous solution such as water or an alkaline aqueous solution such as NaOH or KOH can be used. When an alkaline aqueous solution is used, the particle growth of single particles can be effectively caused at a lower temperature than when a neutral aqueous solution such as water is used.

The hydrothermal treatment in the present invention can be carried out by treating it in a temperature range of 150 to 250 ° C. using an apparatus such as an autoclave. When the temperature is lower than 150 ° C., pores remain on the surface of the acicular hematite particles and inside the particles, and the degree of crystallinity is insufficient. 25
When the temperature exceeds 0 ° C, it cannot be said that the needle-like shape is maintained and inherited due to fusion between particles and particles. Considering the degree of crystallinity and the needle shape, 150 to 200 ° C. is preferable.

The acicular hematite particles after hydrothermal treatment in the present invention have an average major axis length of 0.1 to 0.5 μm,
Moreover, the specific surface area is 15 to 60 m ² / g. If the average major axis length is less than 0.1 μm, the average major axis length is 0.5 μm
If it exceeds m, it cannot be said that the needle-like shape is retained and inherited. The specific surface area of the needle-like hematite particles in which fusion between particles and particles is prevented and the degree of crystallinity is increased is 1
It is 5 to 60 m ² / g.

[0019]

The most important point in the present invention is that the average major axis length obtained by heating and dehydrating acicular goethite particles in the temperature range of 250 to 400 ° C. is 0.1 to 0.5 μm, and Aqueous liquid medium containing acicular hematite particles having a specific surface area of 70 to 150 m ² / g by the BET method is 150 to 25
When hydrothermally treated in the temperature range of 0 ° C., the needle-like shape having an average major axis length of 0.1 to 0.5 μm and a BET specific surface area of 15 to 60 m ² / g is inherited. The fact is that it is possible to obtain acicular hematite particles of substantially high density.

In the present invention, the inventor of the present invention has a reason for obtaining needle-like hematite particles which retain and inherit the acicular shape and have a substantially high degree of crystallinity.
It is considered that it is because the temperature is low and the treatment is carried out in an aqueous medium, so that almost no fusion between particles and particles occurs, and particle growth of single particles occurs preferentially. .

In the present invention, the acicular hematite particles obtained by hydrothermal treatment contain few impurities such as Na ⁺ and SO ₄ ²⁻ . Therefore, since the particles and the sintering between the particles can be suppressed or prevented in the subsequent heating and reduction process, the needle-like shape is easily retained and inherited.

In the present invention, the reason why the acicular hematite particles obtained by hydrothermal treatment contain a small amount of impurities such as Na ⁺ and SO ₄ ²⁻ is that the present inventor sufficiently grows single particles. It is believed that this is because impurities that are contained inside the particles and are difficult to clean by normal cleaning are pushed to the surface of the particles or in the vicinity of the surfaces of the particles to facilitate cleaning.

[0023]

EXAMPLES Next, the present invention will be described with reference to Examples and Comparative Examples. The average major axis length and axial ratio (average major axis length: average minor axis length) in the following examples and comparative examples are shown by values observed with an electron microscope. Also. The degree of crystallinity of the particles is determined by the BET specific surface area value and multi-volume densitometer type 1305 (Micromeritics).
The density of the particles is shown by the measured value.

Example 1 Needle-like α-F having an average major axis length of 0.35 μm, an axial ratio (average major axis length: average minor axis length) of 8: 1 and a specific surface area of 48 m ² / g.
The eO (OH) particle powder was pyrolyzed in air at 320 ° C. to give an average major axis length of 0.35 μm and an axial ratio (average major axis length:
Average minor axis length) 8: 1 and specific surface area 85 m ² /
A needle-like hematite particle powder (Na: 970 ppm, SO ₄ : 680 ppm) having g and a density of 4.3 g / cc was obtained. 8.0 g of the acicular hematite particle powder was dispersed in water to obtain a water suspension having a total volume of 100 ml. This water suspension was placed in a closed container and hydrothermally treated at a temperature of 200 ° C. for 5 hours, and then the precipitate was separated by filtration, washed with water and dried.

As a result of electron microscopic observation, the obtained acicular hematite particles had an average major axis length of 0.35 μm and an axial ratio (average major axis length: average minor axis length) of 8: 1. Surface area 3
It was acicular hematite particle powder having a density of 2 m ² / g and a density of 4.6 g / cc, retained and inherited acicular shape, and was a substantially high density particle having an increased degree of crystallinity. Impurities in the hematite particles were Na ⁺ 90p.
It was pm and SO ₄ ^2- 130 ppm, and had few impurities.

Example 2 An acicular α-F having an average major axis length of 0.2 μm, an axial ratio (average major axis length: average minor axis length) of 7: 1 and a specific surface area of 105 m ² / g.
The eO (OH) particle powder was pyrolyzed in air at 300 ° C. to give an average major axis length of 0.2 μm and an axial ratio (average major axis length: average minor axis length) of 7: 1. Surface area 130 m ² /
A needle-like hematite particle powder (Na: 300 ppm, SO ₄ : 80 ppm) having a density of 4.2 g / cc and a density of 4.2 g / cc was obtained.
8.0 g of the acicular hematite particle powder was dispersed in water to obtain a water suspension having a total volume of 100 ml. This water suspension was placed in a closed container and subjected to hydrothermal treatment at a temperature of 150 ° C. for 5 hours, and then the precipitate was filtered off, washed with water and dried.

As a result of electron microscopic observation, the obtained acicular hematite particles had an average major axis length of 0.2 μm and an axial ratio (average major axis length: average minor axis length) of 7: 1. Surface area 55
The acicular hematite particle powder had m ² / g and a density of 4.5 g / cc, and retained and inherited the acicular shape, and was a substantially high-density particle having an increased degree of crystallinity. Impurities in the hematite particles are Na ⁺ 190
ppm and SO ₄ ²⁻⁵⁰ ppm, and there were few impurities.

Example 3 Needle-like α-F having an average major axis length of 0.4 μm, an axial ratio (average major axis length: average minor axis length) of 10: 1 and a specific surface area of 20 m ² / g.
The eO (OH) particle powder was pyrolyzed in air at 340 ° C. to give an average major axis length of 0.4 μm and an axial ratio (average major axis length: average minor axis length) of 10: 1. Surface area 100 m ² /
A needle-shaped hematite particle powder (Na: 970 ppm, SO ₄ : 4800 ppm) having a density of 4.3 g / cc and a density of 4.3 g / cc was obtained. 8.0 g of the acicular hematite particle powder was dispersed in water to obtain a water suspension having a total volume of 100 ml. This water suspension was placed in a closed container and hydrothermally treated at a temperature of 200 ° C. for 5 hours, and then the precipitate was separated by filtration, washed with water and dried.

As a result of electron microscopic observation, the obtained acicular hematite particles had an average major axis length of 0.4 μm and an axial ratio (average major axis length: average minor axis length) of 10: 1. Surface area 1
It was acicular hematite particle powder having a density of 8 m ² / g and a density of 4.7 g / cc, maintained and inherited acicular shape, and was a substantially high-density particle having an increased degree of crystallinity. Impurities in the hematite particles were Na ⁺ 90p.
It was pm and SO ₄ ²⁻ 730 ppm, and contained few impurities.

[0030]

EFFECTS OF THE INVENTION According to the present invention, as shown in the above-mentioned Examples, the needle-like shape is retained and inherited, and substantially dense needle-like hematite particles having an increased degree of crystallinity are obtained. be able to. The obtained needle-like shape is retained and inherited, and the needle-like magnetite particle powder obtained by heat reduction using substantially high-density needle-like hematite particles with enhanced degree of crystallinity, heat-reduction , The acicular maghemite particle powder obtained by oxidation also retains the acicular shape and is inherited, and since it is a substantially high density magnetic particle powder having an increased degree of crystallinity, high density recording, It is suitable as a magnetic particle powder for high output. Since the acicular hematite particles in the present invention have few impurities, they are easy to wash and are industrially and economically advantageous.

Claims (1)

[Claims] 1. The needle-shaped goethite particles are heated to 250 to 400 ° C.
The average major axis length obtained by heating and dehydrating in the temperature range of 0.
The average length is obtained by hydrothermally treating an aqueous liquid medium containing acicular hematite particles having a specific surface area of 1 to 0.5 μm and a BET method of 70 to 150 m ² / g in a temperature range of 150 to 250 ° C. The axial length is 0.1 to 0.5 μm, and the specific surface area by the BET method is 15 to 60 m ² /
A method for producing a needle-like hematite particle powder, which comprises obtaining a needle-like hematite particle having a substantially high density which inherits the needle-like shape of g.