JPS6333509A - Production of fine hexagonal cobalt particle - Google Patents

Abstract

PURPOSE:To produce fine hexagonal cobalt particles having uniform grain sizes at a high yield by subjecting cobalt acetate contg. water of crystallization to a dehydration heat treatment, then heating the same in a hydrocarbon medium and reducing the cobalt acetate by pressurized hydrogen. CONSTITUTION:The cobalt acetate tetrahydrate is heated for about 10-20hr at about 100-250 deg.C under a reduced pressure to eliminate part or the whole of the water of crystallization contained therein. The cobalt acetate is then dispersed into the hydrocarbon medium (heptane, octane, etc.) until the initial concn. attains about 1-50wt%, then the cobalt acetate is reduced in the heated state by the hydrogen dissolved under the pressurization. The pressure of the hydrogen at this time is maintained under about 0.5-300kg/cm<2>, the reaction temp. at about 150-350 deg.C and the reaction time for about 0.5-20hr. The reaction is carried out under stirring. The hexagonal cobalt is thereby obtd. at approximately 10% reaction yield and the particles thereof are as fine as 0.01-1mu grain size. The prepd. hexagonal cobalt is adequately usable as a magnetic material for perpendicular magnetic recording medium.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a magnetic material, particularly for use in recording media (tapes, disks, drums, etc.) for perpendicular magnetic recording systems that enable high-density magnetic recording. The present invention relates to a method for producing hexagonal cobalt fine particles useful as magnetic powder.

[Conventional technology]

A perpendicular method has been proposed as an ultimately high-density magnetic recording method different from the current longitudinal method, and its high performance has been experimentally demonstrated. In order to put this new method into practical use, coated magnetic powder with new characteristics, such as barium ferrite fine particle powder, is currently being developed.

However, if cobalt fine particles having a higher saturation magnetization (100 emu/g or more) than the barium ferrite fine particles are formed in a hexagonal crystal lattice, have an appropriate submicron particle size, uniform dispersibility, and coercive force ( 200-2,000
If it can be produced as a magnetic powder exhibiting 0e), it is expected that it will be a far superior perpendicular recording material.

Conventional methods for producing metallic cobalt powder or sol can be roughly divided into (1) a method in which cobalt oxide is reduced with gas phase hydrogen at high temperature, and (2) a method in which cobalt carbonyl or cobalt salt is thermally decomposed in a liquid phase. , (3) A method of reducing cobalt salts such as oxalates and carbonates with gas phase hydrogen at high temperatures (for example, Japanese Patent Publication No. 47-38417, Japanese Patent Application Laid-open No. 50-26750)
(No. 43), +41 A method of reducing cobalt salts such as carbonates with hydrogen or a reducing agent in a liquid phase medium (for example, Japanese Patent Publication No. 43-22395), +51 A method of electrolyzing water with a cobalt electrode in the presence of a reducing agent, ( 6) A method in which metallic cobalt is evaporated at high temperature in an inert gas is known.

The manufacturing method according to the present invention belongs to method (4) among the above methods. This is because the other methods (11 to (6)) cannot produce the material for perpendicular recording that is the object of the present invention in terms of wood quality in terms of crystal type, particle size, or dispersibility.

[Problem that the invention seeks to solve]

The first major problem to be solved by the present invention is how to produce pure hexagonal cobalt fine particles from the raw material cobalt salt.

The crystal lattice of cobalt has a hexagonal system that is stable at low temperatures and a face-centered cubic system that is stable at high temperatures (about 300 degrees Celsius or higher). This is because the powder must be a pure crystal system of the former type in order to exhibit -axial high magnetic anisotropy4, and the latter or one mixed with the latter is useless. All of the hexagonal cobalt fine particles obtained by the conventionally known production method are face-centered cubic crystals or are mainly composed of face-centered cubic crystals,
Basically, it is completely unsuitable for use as a magnetic powder for perpendicular magnetic recording as intended by the present invention.

The second problem contemplated by the present invention is how to obtain particles with a particle diameter of submicron or less, which tend to aggregate, in a good dispersed state.

Since the above-mentioned slurry hydrogen reduction method in water can only obtain particles of several microns, it is necessary to further refine the particles. Further, cobalt fine particles, which are ferromagnetic substances, obtained by conventional known methods are arranged in needles, lines, rings, or irregularly agglomerated or aggregated, and are completely unsuitable for the use of the present invention.

The third problem addressed by the present invention is how to maintain high saturation magnetization and appropriate coercive force in the fine particles.

Regarding static magnetic properties, saturation magnetization and coercive force are important, and pure metallic cobalt itself has a saturation magnetization of 16
It has 1 emu/g (20° C.), which is more than twice that of barium ferrite, so it should be considered an extremely superior material in this respect. On the other hand, the coercive force is usually 200 to 2,0000e,
especially! , 0000e is desirable. Therefore, it is desirable to obtain stable magnetic particles that exhibit an appropriate coercive force without reducing the high saturation magnetization.

[Means to solve the problem]

In order to solve the above problems, the present inventors have proposed that all of the above problems can be solved by subjecting a cobalt salt of a specific carboxylic acid to a specific treatment and then reducing it with hydrogen under specific conditions. They discovered this and completed the present invention.

The present invention will be explained in detail below.

The present invention is a method for producing cobalt fine particles, comprising:
After dehydrating cobalt acetate containing water of crystallization, it is heated in a hydrocarbon medium and reduced with pressurized hydrogen, that is, two reaction steps: (1) dehydration heat treatment of the raw material cobalt acetate, and (2) hydrogen reduction of the dehydration heat treatment product. This is a method for producing hexagonal cobalt fine particle powder characterized by comprising:

The raw material cobalt acetate used in the present invention is tetrahydrate Co(OCOC)Is)! 4H20 and is easily available commercially. This tetrahydrate does not dissolve in hydrocarbons as it is, and even if crystals obtained by pulverizing the tetrahydrate are attempted to be reduced with hydrogen by the method of the present invention, they do not react at all as shown in the comparative example below. However, surprisingly,
According to studies conducted by the present inventors, the tetrahydrate subjected to dehydration heat treatment can easily undergo hydrogen reduction under similar conditions, and the cobalt fine particles that are the object of the present invention can be obtained. The dehydration heat treatment in the present invention refers to the raw material 4
Heating the aqueous salt to a temperature of approximately 100°C to 250°C under atmospheric pressure or reduced pressure to cause part or all of the crystal water contained in the raw material to be separated from the raw material and taken out of the heating reaction system. says. At this time, acetic acid roots are also usually partially detached. The dehydration heat-treated product is usually 20 to 50% by weight, preferably 25 to 40% by weight of the original mold, from which water-based liquid has been removed. These usually occur as stalks.

The heating time to obtain such a dehydration heat-treated product may vary depending on the form of the raw material, the type and size of the heating reactor, heating temperature and pressure, but is usually about 10 minutes to 20 hours, preferably about 1 hour to 10 hours. range. Note that the raw material tetrahydrate can be used in various forms, from coarse crystals as they are to pulverized ones. The dehydration heat treatment reaction may be carried out in the atmosphere, but nitrogen, hydrocarbons,
- Preferably carried out under an atmosphere of an inert gas or reducing gas such as carbon oxide, carbon dioxide or hydrogen.

These atmospheric gases are preferably used as carrier gases for the vapor generated in the dehydration reaction to the outside of the system.

The reaction apparatus for carrying out the dehydration heat treatment is one that is equipped with a means for heating the solid or powder and a means for separating and condensing the generated vapor components, and from which the cobalt compound that is the reaction product can be easily taken out. There are no particular restrictions on the format. The reaction can of course be carried out in one stage or in multiple stages, either batchwise or continuously.

The cobalt compound obtained as described above is used as it is or after being pulverized in the next hydrogen reduction step.

In the hydrogen reduction step, the dehydrated product is reduced by heating under hydrogen pressure in a hydrocarbon medium.

The hydrocarbon medium used in the present invention includes typical examples of organic compounds such as pentane, hexane, heptane, octane, decane, dodecane, 2゜2-dimethylbutane, and hexadecane. , petroleum ether, petroleum benzene, ligroin,
Aliphatic saturated hydrocarbons such as gasoline, kerosene, petroleum spirit, petroleum naphtha, liquid paraffin, other aliphatic heating media and petroleum fractions; cyclopenkune, cyclohexane, methylcyclohexane, ethylcyclohexane, cyclooctane, decalin, P- Alicyclic saturated hydrocarbons such as menthene, cyclododecane, naphthenic heat carriers and petroleum fractions; benzene, toluene, ethylbenzene, dodecylbenzene, xylene, dibutylbenzene,
Preferred examples include aromatic hydrocarbons such as methylnaphthalene, tetralin, diphenylmethane, terphenyl, and aromatic petroleum fractions, and particularly preferred are heptane, octane, dodecane, cyclohexane, toluene, ethylbenzene, xylene, etc. I'm happy. These hydrocarbon media may be used alone or in combination. The amounts used are such that the initial concentration of the dehydrated cobalt acetate to be dispersed therein is about 1 to 5%, preferably about 5 to 20%.

The cobalt acetate dehydration product is dispersed in the above-mentioned hydrocarbon medium and subjected to reduction under heating by dissolved hydrogen under pressure under effective mixing, preferably in a fluidized dispersion state, to obtain the desired hexagonal cobalt fine particles. generate.

In the present invention, hydrogen is used as a reducing agent, but this does not necessarily have to be of particularly high purity, and of course it may be used in combination with an inert gas.

In addition, other reducing agents include sodium hypophosphite,
There are also chemical reagents such as sodium borohydride and hydrazine (for example, JP-A No. 48-44194, JP-B No. 49-Sho.
-1997, JP-A No. 59-17209, etc.), in this case, the reducing agent is expensive and the obtained cobalt fine particles cannot satisfy the above-mentioned characteristics that are the object of the present invention, and contaminating impurities It is also difficult to remove.

In the present invention, the reaction is carried out under pressure of hydrogen,
The pressure of hydrogen is preferably in the range of 0.5 to 300 kg/c, more preferably in the range of 2 to 200 kg/c, particularly preferably in the range of 5 to 150 kg/c. The reaction temperature is usually in the range of 150 to 350°C, particularly preferably 180 to 300°C. If the reaction temperature is lower than this, the reaction will not substantially proceed, and if the temperature is higher than this, the transition from the hexagonal system to the face-centered cubic system cannot be ignored, making it impossible to obtain the desired hexagonal cobalt system. It disappears. Note that, in reality, an appropriate value is selected as the reaction temperature depending on the pressure of hydrogen. Further, the reaction time may vary depending on the raw material concentration, reaction temperature, reaction pressure, etc., but is usually about 0.5 to 20 hours, preferably about 1 to 10 hours.

Basically, in a reduction reaction, the raw materials and products are suspended in a slurry state in a medium, and hydrogen gas is absorbed while the reaction takes place! In order to make this reaction proceed smoothly, consideration should be given to chemical engineering, and the reaction slurry should be prepared to improve the gas diffusion and the dispersion of the produced particles during the heating reaction. It is desirable to mix or stir thoroughly. In this way, hexagonal cobalt fine particles can be produced with almost 100% reaction yield. In addition, as a reaction method, either a batch reaction method or a continuous reaction method can be adopted.

In the present invention, the reduction reaction can basically be carried out without using a particular surfactant, but the presence of a surfactant improves particle size, particle dispersion, stabilization of the generated magnetic powder, coercive force, etc. can be controlled more easily.

Various surfactants can be selected and used for this purpose. For example, as an ionic type, for example, sodium dodecylbenzenesulfonate,
Sulfonates such as dioctyl sulfone succinic acid sorta, ammonium perfluoroalkyl sulfonate, etc.; carboxylates such as sodium stearate, sodium perfluoroalkyl carboxylate, etc.; sulfuric esters such as sodium lauryl sulfate, etc. Anionic surfactants of phosphates, such as di(polyethylene glycol-p-nonylphenyl) soda phosphate, sodium perfluoroalkyl phosphate, etc.; such as trioctylmethylammonium chloride, perfluoroalkylmethylammonium chloride, etc. ammonium salts; cationic surfactants such as pyridinium salts and imidazolinium salts; amphoteric surfactants such as betaines, perfluoroalkylbetaines, and aminocarboxylate salts. Nonionic ones include ether type ones such as polyethylene glycol lauryl ether, polyethylene glycol nonylphenyl ether, perfluoroalkyl polyoxyethylene ethanol, etc.; for example, polyethylene glycol monostearate, sorbitan oleate, fluoroalkyl ester, etc. Ester-type surfactants such as , and nonionic surfactants such as nitrogen-containing surfactants such as polyoxyethylene ethylamine, perfluoroalkyl amine oxide, etc. can be used.

The amount of surfactant used is not particularly limited, but is usually about 0.001 to 1% by weight based on the medium.

In the present invention, the cobalt fine particles obtained as a reaction product are separated from the medium by known methods such as decanting, filtration, and centrifugation, and if necessary, they are washed by known methods, slow oxidation drying, etc. Through various processes, it becomes a highly pure cobalt magnetic powder that is stable in the air. In addition, unreacted hydrogen gas,
The recovered medium and, if necessary, the used washing liquid can be purified in situ or by conventional methods and reused.

Furthermore, without separating and extracting the produced fine powder from the slurry, it is possible to use it as a raw material for manufacturing magnetic recording media in the form of a concentrated slurry in which only an appropriate amount of the medium is separated.

The cobalt fine powder obtained by the present invention is composed of highly pure cobalt fine particles and exhibits a hexagonal lattice with good crystallinity, so that it can be suitably used as a magnetic fine particle powder for perpendicular magnetic recording materials. Its magnetic properties are saturation magnetization 10
It is a stable ferromagnetic material of 100e/g or more, and has a coercive force of 20
It is a useful magnetic powder of 00e or higher. Although some differences may occur depending on the reaction conditions, the cobalt m particles obtained by the present invention have a particle size of 0.01 to several μm, particularly 0.01 to 1 μm.
It consists of nearly spherical fine particles with a relatively uniform particle size of μm and uniformly dispersed.

〔Example〕

Hereinafter, the present invention will be specifically explained with reference to Examples and Comparative Examples.

Example 1 (81 Cobalt acetate (OCOC) I3) z4H1
o 494.2 g to 17! The mixture was placed in a flask and heated in an oil bath at 140° C. while dispersing and continuously supplying nitrogen from the bottom. The water vapor containing acetic acid generated during this time is drawn out from the top of the flask together with nitrogen and condensed.
The dehydration heat treatment was continued for an hour. The obtained solid consisting of reddish-orange crystals was pulverized in nitrogen without absorbing moisture, and a pale reddish-purple powder of 329.0% was obtained as a dehydration product of cobalt acetate.
I got g.

(bl) 15.3 g of this dehydrated product, 0.8 g of sodium dioctyl sulfone succinate as a surfactant, and 200 ml of toluene were charged into a 500 ml stainless steel autoclave. After nitrogen substitution, 30 kg of hydrogen was charged.
/cm" and heated at 200°C for 5 hours with stirring. The hydrogen pressure decreased, indicating that a reduction reaction had occurred. After cooling and releasing the pressure, the contents were taken out and the black fine powder was filtered out. Then, it was washed with 5% aqueous methanol and dried to obtain 4.9 g of reduced cobalt fine particles.

According to X-ray diffraction analysis, these Kobal+-a particles have a hexagonal crystal lattice with good crystallinity, and according to electron microscopic observation, the particle size ranges from well-dispersed, nearly spherical particles with a diameter of 0.2 to less than 1 mm. It had become. In addition, 10
The saturation magnetization when magnetized to KOe was 118 emu/g, and the coercive force was 4180e.

Comparative Example I Cobalt acetate (OCOCH3) z4)! A hydrogen reduction reaction was carried out in the same manner as in Fl'i Example 1(b) except that 23.3 g of zo was pulverized and used instead of the dehydrated product.

However, substantially no hydrogen absorption was observed and reduced cobalt was not obtained.

This shows that reduced cobalt, which is the object of the present invention, cannot be obtained unless a product obtained by heat-dehydrating cobalt acetate is used.

Examples 2 to 4 The same experiments as in Example 1(b) were conducted except that the type of medium, temperature, pressure, or reaction time for the reduction reaction were changed as shown in Table 1. A reduced cobalt fine powder exhibiting similar crystal type and properties was obtained. The magnetic properties are summarized in Table 1.

Table 1 Example 5 (200.0 g of cobalt acetate was packed into a JLL tube reactor and heated to 120°C for 3 hours under reduced pressure of 100 mmHH while flowing a small amount of nitrogen. During this time, the vapor components generated were reacted. The 0 reaction product, which was extracted from the outlet of the tube end, cooled and condensed, and was separated, was treated in the same manner as in Example 1(a) to obtain 146.0 g of reddish-purple powder.

7.7g of this dehydrated product and toluene Loom
l, 200m j! The hexagonal reduced cobalt powder mainly consists of particles with a submicron size. 2.1g was obtained.Saturation magnetization was 1
The coercive force was 23 emu/g and 2360 e.

Examples 6 to 10 0 surfactant was added to the reducing medium Loom 1 shown in Table 2.
．． The same experiment as in Example 5(b) was conducted except that 4 g was added, and cobalt soot particles exhibiting the magnetism shown in the table were obtained.

Example 11 (al Cobalt acetate (OCOCHt) z4Hzo
150. 500 mj of Og! The mixture was placed in a cylindrical flask and heated at 180° C. for 1 hour while dispersing and continuously introducing nitrogen from the bottom. Water and a portion of acetic acid were removed by dehydration heat treatment to obtain 87.0 g of pale reddish-purple powder.

Cb) 7.7 g of this dehydrated product, 0.4 g of sodium dioctyl sulfone succinate and 200 g of toluene.
m j! was placed in a 200 m 12 stainless steel autoclave and heated at 200°C and a hydrogen pressure of 100 kg/cm".
After the time reaction, 2.9 g of hexagonal reduced cobalt powder consisting of spherical particles with a particle size of 0 submicron was obtained. The saturation magnetization was 120 emu/g and the coercive force was 3950 e.

Table 2 Example 12 The same experiment as in Example 11thl was conducted, except that 30.7 g of dehydrated cobalt acetate was used, and 11.8 g of the same fine cobalt powder was obtained. Saturation magnetization is 116 emu/g,
The coercive force was 2920e.

Example 13 280°C using 15.3g of cobalt acetate dehydrated product
The same experiment as in Example 11 (bl) was carried out except that the reaction was carried out for 2.5 hours with a saturation magnetization of 112 emu/g and a coercive force of 32
5.1 g of a similar cobalt i powder of 80e was obtained.

〔Effect of the invention〕

According to the method of the present invention, from cobalt acetate a particle size of 0.01
For the first time, it has become possible to produce hexagonal cobalt fine particles mainly having a size of ~1 μm with a good yield. These fine particles have a high saturation magnetization of 10,100 e/g or more, and a coercive force of 2,000 e/g or more.
e or more, and has the basic properties necessary as a magnetic material, especially as a magnetic powder for perpendicular magnetic recording, and therefore opens up useful applications for it.

Claims (3)

[Claims] (1) A method for producing fine cobalt particles, which comprises subjecting cobalt acetate containing water of crystallization to a dehydration heat treatment, followed by heating in a hydrocarbon medium and reducing with pressurized hydrogen. Manufacturing method. (2) The particle size of the obtained hexagonal cobalt fine particles is 0.01~
The method according to claim 1, wherein the hexagonal cobalt fine particles are 1 μm in size. (3) The method according to claim 1 or 2, in which the obtained hexagonal cobalt fine particles can be suitably used as a magnetic material for perpendicular magnetic recording media.