JP2744641B2 - Method for producing ferromagnetic metal powder - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a ferromagnetic metal powder used for magnetic recording, and particularly to a method comprising iron as a main component, which is excellent in stability, dispersibility at the time of coating preparation, and the like. And a method for producing a ferromagnetic metal powder.

[Conventional technology and its problems]

In recent years, various recording methods have developed remarkably,
Above all, progress in reducing the size and weight of the magnetic recording / reproducing apparatus is remarkable. Accordingly, there is an increasing demand for higher performance of magnetic recording media such as magnetic tapes and magnetic disks.

In order to satisfy such requirements for a magnetic recording medium, a magnetic powder having a high coercive force and a high saturation magnetization is required. Conventionally, so-called cobalt-containing iron oxide obtained by modifying acicular magnetite or maghemite or these magnetic iron oxide powders with cobalt has been used as a magnetic powder for magnetic recording. , Ferromagnetic metal powder with higher coercivity and saturation magnetization,
So-called metal powder has begun to be used.

Various methods have been proposed as a method for producing the ferromagnetic metal powder.However, from the economic advantage, generally, needle-like goethite or iron oxide obtained by dehydrating the same is used as a raw material, A method of reducing this has been used. Specifically, first, heat dehydration in a non-reducing atmosphere to form iron oxide, and after baking pores generated by dehydration, heat-reducing in a reducing atmosphere such as hydrogen or iron oxide A method is used in which goethite is directly heated and reduced under a reducing atmosphere such as hydrogen without being used. These methods cause sintering of acicular particles and collapse of acicularity during heating dehydration or reduction, leading to a decrease in magnetic properties and dispersibility. In many cases, various processes are performed.

By the way, this ferromagnetic metal powder is chemically unstable and has a drawback that it undergoes oxidation and its magnetic properties deteriorate with time. In order to solve this drawback, attempts have been made to form an oxide film on the surface of the metal powder to stabilize the metal powder, and various methods have been proposed.

One example is a method of suspending a metal powder in a solution and blowing an oxygen-containing gas (for example, JP-A-60-128202 and JP-A-60-128202).
9-16904 and 55-164001), a method of gradually oxidizing a metal powder with a diluted oxygen gas and gradually increasing the oxygen concentration while forming an oxide film (for example, JP-A-61-216306, JP-A-61-216306).
However, oxidation in a solvent oxidizes the solvent, which adversely affects the coating film and poses a problem in handling the solvent. Oxidation with diluted oxygen does not have the above-mentioned problems, but tends to cause agglomeration and sintering of particles during the formation of an oxide film, making it difficult to disperse during the preparation of paints, and the surface properties of the finally obtained coating film And a decrease in the squareness ratio.

In addition, a method has been proposed in which a metal powder and carbon dioxide gas are contacted at a temperature of 200 ° C. or lower to stabilize the carbon dioxide gas by adsorbing the same (Japanese Patent Application Laid-Open No. 62-156201). It merely forms an adsorption film that is chemically very fragile. Therefore, the metal powder obtained by this technique is not different from the above-mentioned metal powder at all, and has not completely solved the essential deficiency of the conventional technique that the magnetic properties rapidly decrease with time.

[Object of the invention]

The present invention has been made in order to solve such problems of the prior art, and in particular, provides a production method for obtaining a ferromagnetic metal powder having excellent dispersibility and excellent oxidation resistance. The purpose is to:

[Means for solving the problem]

In order to solve the problems of the prior art as described above, the present inventors have repeated careful observation and analysis on the process of oxidizing the reduced fresh iron surface, and have variously studied the formation mechanism of the surface oxide film. As a result of repeated trial and error based on a hypothesis, carbon dioxide or a mixed gas of carbon dioxide and inert gas, which was conventionally considered chemically inert gas, is brought into contact with freshly reduced iron at high temperature. It has been found that by doing so, an oxide film having excellent stability is formed.
Further, the present inventors have unexpectedly found that the metal powder having a surface oxide film formed in such a manner exhibits an extremely good dispersibility when it is formed into a coating material, thereby completing the present invention.

That is, the present invention reduces the oxide powder mainly composed of iron oxide with a reducing gas to obtain metallic iron powder,
A gas consisting of carbon dioxide alone or a mixed gas consisting of carbon dioxide and an inert gas selected from argon and nitrogen at 50 ° C. An object of the present invention is to provide a method for producing a ferromagnetic metal powder containing iron as a main component, characterized by forming a layer.

In the present invention, the oxide powder mainly containing iron oxide,
It is a generic term for acicular goethite or various iron oxides having different oxidation states or crystal states.

This oxide powder may contain metal elements for crystal modification reflecting its manufacturing process, for example, elements other than iron such as Co, Ni, Cr, Al, Ti, Si, Zn, Cu, Ca, Mg, etc. .

For the purpose of the present invention, particularly preferred oxide powders are those having a spinel structure such as magnetite or maghemite. In particular, in the case of magnetite obtained by dehydrating and reducing acicular goethite in a reducing gas atmosphere, particularly good results can be obtained. In this case, it is preferable to perform a surface treatment for maintaining the shape and preventing sintering before the dehydration and reduction of the goethite. The surface treatment is Si, Al, Zr
And the like. Specifically, for example, a method of adding a water-soluble compound solution such as water glass or sodium aluminate to a goethite slurry and then adjusting the pH of the system to precipitate an insoluble compound, or adding triisopropoxy to the goethite slurry A surface treatment such as adding a metal alkoxide such as aluminum or tetraethoxysilane to precipitate a hydrolyzate may be performed. The surface-treated goethite thus obtained is dehydrated and reduced in a reducing atmosphere such as a hydrogen stream, and iron oxide having a spinel structure is preferable as the oxide powder of the present invention.

In the present invention, various oxide powders mainly containing iron oxide as described above can be used, and when a compound layer of a transition metal element other than iron is formed on the surface thereof, The ferromagnetic metal powder obtained as described above is preferable in terms of oxidation resistance stability. In particular, an oxide powder obtained by applying a compound of Co or Ni as a transition metal element and further performing a treatment for maintaining the shape and preventing sintering is preferable.
As a method of depositing a transition metal element compound such as Co or Ni on the oxide powder, a solution of a water-soluble salt of a transition metal element to be deposited is added to an alkaline slurry of the oxide powder,
After the hydroxide is precipitated, a method of performing an appropriate post-treatment (for example, reflux of a slurry, heat treatment of a dry powder, or the like) is used. Furthermore, when performing this shape maintenance treatment and sintering prevention treatment, in addition to treatment with compounds such as Si, Al, Zr,
Treatment with a thermosetting resin such as a phenol resin or a furan resin is also effective. As the specific method, the same method as described above for the game site is used. An example of the treatment with a thermosetting resin is performed by adding a solution of these resins in a water-soluble organic solvent (acetone, ethanol, etc.) to a magnetite slurry to make it insoluble. Of course, these treating agents may be used alone or in combination.

The reducing gas referred to in the present invention is specifically a hydrogen gas or a mixed gas of hydrogen gas and an inert gas such as nitrogen, argon and helium. For the purposes of the present invention and for safety reasons, it must be substantially free of oxidizing gases.

The reduction of the oxide powder mainly composed of iron oxide is performed by heating in a retort furnace, a fluidized bed furnace or the like while maintaining a reducing atmosphere, to obtain metallic iron powder.

Next, an oxide film is formed on the surface by bringing the obtained metallic iron powder into contact with carbon dioxide gas or a mixed gas of carbon dioxide gas and an inert gas. The temperature at this time needs to be 250 ° C. or higher, preferably 300 ° C. or higher. At a lower temperature than this, substantially no oxide film is formed on the surface. The higher the reaction temperature, the easier the formation of an oxide film, which is preferable from the viewpoint of shortening the reaction time. However, if the temperature is too high, sintering and fusing of the metal powder particles are likely to occur. And the upper limit is about 450 ° C.

Carbon dioxide gas may be used alone, or may be used by mixing with an inert gas such as argon or nitrogen depending on the conditions. In this case, if the concentration of carbon dioxide is too low, the oxidation is slowed down. Therefore, the concentration of carbon dioxide is preferably 10% by volume or more in practical use. The contact time depends on the temperature and the concentration of carbon dioxide gas.
About 30 minutes to 20 hours are preferable. If the contact time is too short, an effective oxide film will not be formed sufficiently. If the contact time is too long, the oxidation will be excessive and the magnetic properties will be reduced.

The contact reaction between metal powder and carbon dioxide or a mixed gas of carbon dioxide and inert gas uses, for example, a retort furnace, fluidized bed reactor, rotary kiln, multi-stage vertical furnace, etc. You can do it. If a fluidized bed reactor is used as an example, the flow rate of carbon dioxide gas flowing through the fluidized bed may be any flow rate that can maintain a good fluidized state, for example, a good gas linear velocity of about 5 to 25 cm / sec. Give the result.

〔The invention's effect〕

When the method for producing a ferromagnetic metal powder of the present invention is used, it is possible to obtain a ferromagnetic metal fine powder having excellent magnetic properties in general and a good shape ratio of a coating film which is a measure of dispersibility of the magnetic powder. .

In the production method of the present invention, if formation of an oxide film of an oxide powder mainly containing iron oxide is performed using carbon dioxide gas or a mixed gas of carbon dioxide gas and an inert gas, fusion of particles during stabilization is unlikely to occur. Although the reason for this is not clear, it is presumed to be related to the fact that the reaction between carbon dioxide and metallic iron is an endothermic reaction and different from the reaction between metallic iron and oxygen (exothermic reaction).

〔Example〕

Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples.

Example 1 1 kg of goethite (major axis diameter 0.18 μm, axis ratio 8) was dispersed in 10 l of a 3% solution of Poise 530 (polycarboxylic acid oligomer: dispersant, manufactured by Kao Corporation), and TK homomixer SL type was used. After dispersion and stirring for about 1 hour (manufactured by Tokushu Kika Kogyo Co., Ltd.), 70 g of No. 3 Keiso (SiO ₂ content: 29%) was added, and stirring was continued for another hour. After that, dilute nitric acid was added to adjust the pH of the system to 6.5.
After stirring for 1 hour, filtration, washing and drying were performed to obtain goethite having a silicon compound layer.

This silicon-containing goethite was placed in a retort furnace (with an internal volume of 30).
In l), while flowing a hydrogen / nitrogen = 1/1 mixed gas at 50 l / min.
After the temperature was raised to 300 ° C. at 5 ° C./min, the temperature was kept at 300 ° C., and reduction was performed until the goethite turned into magnetite. The completion of the reaction with magnetite was confirmed by X-ray diffraction.

Then, 500 g of this magnetite was dispersed in 5 liters of a 3% solution of poise 530, and the TK homomixer was used in the same manner as in the case of goethite.
After dispersing and stirring for about 1 hour with SL type (manufactured by Tokushu Kika Kogyo Co., Ltd.), add 35 g of No. 3 Keiso (SiO ₂ min. 29%) and add 1
Stirring was continued for hours. Then, dilute nitric acid was added to raise the pH of the system to 6.
After stirring for 1 hour, the mixture was filtered, washed and dried to attach a silicon compound layer.

The ferromagnetic metal precursor obtained as above is sized to 60 to 200 mesh, and the gas linear velocity is 7 cm / in a fluidized bed furnace having an inner diameter of 62 mm.
7 hours at 330 ° C, 10 hours at 350 ° C,
It was reduced at 450 ° C. for 3 hours to obtain a ferromagnetic metal powder.

After completion of the above reduction, the mixture was cooled in a nitrogen stream to 350 ° C., and a mixed gas of carbon dioxide / nitrogen = 3/7 was applied at a linear velocity of 7 cm / sec.
After oxidizing by flowing carbon dioxide for 30 hours, the reaction system
After cooling to 500 ° C., the oxygen concentration was gradually increased from the oxygen concentration of 500 ppm, and finally the atmosphere was changed to the atmosphere, whereby air oxidation was further performed to obtain metal powder 1.

 Table 1 shows the properties of the obtained metal powder 1.

Comparative Example 1 The goethite used in Example 1 was reduced to metal powder in the same manner as in Example 1, cooled to 30 ° C. in a nitrogen stream without contact with a mixed gas of carbon dioxide / nitrogen, and an oxygen concentration of 50%.
The oxygen concentration was gradually increased from 0 ppm, and air oxidation was performed to obtain metal powder 11.

 Table 1 shows the properties of the obtained metal powder 11.

Comparative Example 2 A metal powder 12 was obtained in the same manner as in Example 1 except that the contact with a mixed gas of carbon dioxide / nitrogen was performed at 100 ° C.

 Table 1 shows the properties of the obtained metal powder 12.

Example 2 In the same manner as in Example 1 except that 3 l of a sulfuric acid band aqueous solution (Al ₂ O, ₃ %, 2.1%) was added in place of No. 3 Keiso in Example 1, and the pH of the system was adjusted to 7.0 with an aqueous ammonia solution. After obtaining a goethite having an aluminum compound layer and reducing it in a retort furnace to magnetite in the same manner as in Example 1, a silicon compound layer was formed in the same manner as in Example 1, and after reduction, carbon dioxide oxidation and air oxidation were performed. Metal powder 2 was obtained.

 Table 1 shows the properties of the obtained metal powder 2.

Comparative Example 3 After reducing the aluminum compound layer-containing goethite obtained by the same operation as in Example 2, air oxidation was performed without performing carbon dioxide gas oxidation to obtain metal powder 13.

 Table 1 shows the properties of the obtained metal powder 13.

Example 3 A goethite having a silicon compound layer was obtained by the same operation as in Example 1, and reduction was performed until the content became magnetite.

Then, while blowing nitrogen gas into a slurry in which 500 g of this magnetite was dispersed in 2.7 L of an aqueous solution containing 370 g of caustic soda, 175 g of ferrous sulfate heptahydrate, 70 g of cobalt sulfate heptahydrate
After adding 1.3 liters of an aqueous solution containing, a reaction was performed at 40 ° C. for 6 hours, and then the temperature was increased and reflux was performed for 6 hours to obtain magnetite having a cobalt compound layer on the surface.After washing, 15 g of poise 530 and water glass were removed. After adding 35 g and sufficiently dispersing, the pH of the system was adjusted to 5.5 with dilute nitric acid, and the anti-fusing agent S
iO ₂ was deposited.

The ferromagnetic metal precursor obtained as described above was reduced again in the same manner as in Example 1. After completion of reduction, cool in nitrogen stream
After setting the temperature to 0 ° C, a mixed gas of carbon dioxide / nitrogen = 5/5 was used for linear velocity
Oxidation reaction was performed by flowing at m / sec for 6 hours. Thereafter, the reactor was cooled to 30 ° C., and the oxygen concentration was gradually increased from the oxygen concentration of 500 ppm to perform air oxidation to obtain metal powder 3.

 Table 1 shows the properties of the obtained metal powder 3.

Comparative Example 4 Reduction to metal powder was performed in the same manner as in Example 3, and air oxidation was performed without performing carbon dioxide gas oxidation to obtain metal powder 14.

 Table 1 shows the properties of the obtained metal powder 3.

Example 4 A goethite having an aluminum compound layer was obtained by the same operation as in Example 2, and reduction was performed until the content became magnetite.

Then, ferrous sulfate heptahydrate 200 g, cobalt sulfate heptahydrate 100 g while blowing nitrogen gas into a slurry in which 500 g of the magnetite was dispersed in 2.7 l of an aqueous solution containing 370 g of caustic soda.
After adding 1.3 liters of an aqueous solution containing, a reaction was performed at 40 ° C. for 6 hours, and then the temperature was increased and reflux was performed for 6 hours to obtain magnetite having a cobalt compound layer on the surface.After washing, 15 g of poise 530 and water glass were removed. After adding 40 g and sufficiently dispersing, the pH of the system was adjusted to 5.5 with diluted nitric acid, and SiO ₂ as an anti-fusing agent was adhered.

The ferromagnetic metal precursor obtained as described above was reduced, carbon dioxide oxidized, and air oxidized in the same manner as in Example 3 to obtain a metal powder 4.

 Table 1 shows the properties of the obtained metal powder 4.

Comparative Example 5 A metal powder was produced in the same manner as in Example 4, and air oxidation was performed without performing carbon dioxide gas oxidation to obtain a metal powder 15.

 Table 1 shows the properties of the obtained metal powder 15.

[Reference Example] Preparation of Magnetic Tape Using metal powders 1 to 4 and 11 to 15 obtained in Examples 1 to 4 and Comparative Examples 1 to 5, the following coating composition was mixed for 6 hours in a batch type sand mill. Thereafter, 2.5 parts by weight of Coronate L (manufactured by Nippon Polyurethane Industry Co., Ltd.) was added to the mixture, and the mixture was further mixed for 15 minutes, and then filtered to separate glass beads to prepare a magnetic paint.

Paint composition Ferromagnetic metal 100 parts by weight Lecithin 2 カ ー ボ ン Carbon black 3 γ γ-alumina 5 〃 VAGH ^{* 1} 15 〃 Nipporan 2304 ^{* 2} 10 メ チ ル Methyl ethyl ketone 150 ト ル エ ン Toluene 50 〃 Cyclohexanone 75 〃 (Note) * 1: manufactured by Union Carbide Vinyl chloride / vinyl acetate / polyvinyl alcohol copolymer * 2: Polyurethane resin manufactured by Nippon Polyurethane Industry Co., Ltd. This paint has a dry film thickness of 3 on a 10 μm thick PET film.
μm, dry after magnetic field alignment treatment
A magnetic layer was formed on the film. Then, the tapes were mirror-finished by calendering to obtain magnetic tapes 1 to 4 and 11 to 15.

 Table 2 shows the magnetostatic properties of each of the obtained tapes.

Claims (2)

(57) [Claims] An oxide powder mainly composed of iron oxide is reduced with a reducing gas to obtain a metallic iron powder.
A metal oxide layer is formed on the surface by contacting with a gas consisting of only carbon dioxide or a mixed gas consisting of carbon dioxide and an inert gas selected from argon and nitrogen and containing carbon dioxide at least 10% by volume. A method for producing a ferromagnetic metal powder containing iron as a main component. 2. The ferromagnetic metal according to claim 1, wherein the oxide powder mainly containing iron oxide is an oxide powder having a surface containing a layer containing a transition metal element compound other than iron. Powder manufacturing method.