JP3164355B2 - Method for producing acicular alloy magnetic powder - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a needle-like alloy magnetic powder useful as a recording element of a magnetic recording medium such as a magnetic tape or a magnetic disk.

[Conventional technology]

Metal magnetic powders are superior in magnetic properties such as saturation magnetization as compared with oxide magnetic powders such as conventional general-purpose γ-Fe ₂ O ₃ powders. is there. In recent years, as such a metal magnetic powder, an alloy magnetic powder obtained by alloying iron with another magnetic metal such as cobalt has been used for the purpose of further improving magnetic properties and corrosion resistance more than general metal iron magnetic powder. Various attempts have been made to obtain them.

For example, in the case of alloy magnetic powder mainly composed of iron and cobalt, the following production method is conventionally known as a typical one.

(A) A method of heating and reducing a coprecipitate obtained from an iron salt and a cobalt salt added to an aqueous solution of oxalic acid.

(B) A method of adding a reducing agent to a solution containing an iron salt and a cobalt salt.

(C) A method in which iron and cobalt are evaporated in an inert gas and alloyed by collision of gas molecules.

(D) A method of reducing and alloying by flowing a vapor of iron and cobalt chlorides in a mixed gas of hydrogen and nitrogen or argon.

(E) A ferrous salt and a cobalt salt are reacted with an alkali in an aqueous suspension in which needle-like particles of iron oxyhydroxide are dispersed to form a cobalt-containing layer on the surface of the particles, and then heat-reduced. Method.

[Problems to be solved by the invention]

However, in the above method (a), it is difficult to control the composition of the alloy particles to be produced, and it is difficult to obtain the alloy particles having the desired magnetic properties.
In the method (d), since the alloy particles become granular or beaded, the orientation and the magnetic properties are inferior.

On the other hand, in the alloy powder obtained by the method (e), both the magnetic properties and the corrosion resistance are improved to some extent as compared with the ordinary metallic iron powder, but it is still enough to cope with the recent high performance of the magnetic recording medium. However, it cannot be said that these properties are further improved.

The present invention has been made in view of the above circumstances, and has as its object to provide a method for producing an alloy magnetic powder having excellent magnetic properties and corrosion resistance.

[Means for solving the problem]

In the process of studying to achieve the above object, the present inventors first pursued the cause of not being able to obtain sufficient magnetic properties and corrosion resistance in the conventional method (a). When the thickness of the cobalt-containing layer to be attached to the surface of the particles of the alloy is increased, the layer does not adhere uniformly and densely to the surface.Therefore, considering the particle shape after reduction, the cobalt content of the alloy powder obtained by this method is maximum. It was found that there was the above-mentioned cause of being limited to a low degree of 7% by weight.

Therefore, based on this finding, as a result of further intensive studies on a method of obtaining an alloy magnetic powder having a larger cobalt content, the surface of the needle-like particles of iron oxyhydroxide or iron oxide was obtained using an aqueous solution containing specific ions. When a cobalt-containing layer is formed on the surface, the cobalt-containing layer becomes uniform and dense, and it is possible to increase the amount of adhesion and, eventually, the cobalt content of the final alloy powder, to improve magnetic properties and corrosion resistance. The present inventors have found that an extremely excellent alloy magnetic powder can be obtained, and have accomplished the present invention.

That is, the present invention disperses needle-like particles of iron oxyhydroxide or iron oxide in an alkaline aqueous solution, and in this suspension, together with Fe ³⁺ ions and Co ²⁺ ions, Cr ²⁺ , Ni ²⁺ , Zn
A needle-like alloy characterized by adding an aqueous solution containing at least one other divalent metal ion selected from ²⁺ to form a coating layer of a compound containing these ions on the surface of the particles, and then reducing by heating. The present invention relates to a method for producing a magnetic powder.

According to the present invention, the aqueous solution to be added to the suspension is 30 to 100 mol% of Co ²⁺ ions with respect to Fe ³⁺ ions and 1 to 10 mol% of the other divalent metal ions with respect to Fe ³⁺ ions. Is a preferred embodiment.

[Structure and operation of the invention]

The most important feature of the method of the present invention is that, when forming a cobalt-containing coating layer on the surface of the needle-like particles of iron oxyhydroxide or iron oxide, Cr is ^added together with Fe ³⁺ ions and Co ²⁺ ions.
^{It is characterized in} that an aqueous solution containing at least one other divalent metal ion selected from ²⁺ , Ni ²⁺ and Zn ²⁺ is used. In other words, by using such a trivalent iron ion and using the above-mentioned other divalent metal ion, a comparison can be made between the case of using the divalent iron ion as in the conventional method (e). As a result, a uniform and dense cobalt-containing coating layer can be formed thick on the surface of the needle-like particles, and the alloy powder produced through heat reduction has a high cobalt content and is very excellent in magnetic properties and corrosion resistance. Can be obtained. The above-mentioned cobalt-containing coating layer has been found to have a spinel-type crystal structure by X-ray diffraction.

In the method of the present invention, first, needle-like particles of iron oxyhydroxide or iron oxide are dispersed in an alkaline aqueous solution. The alkaline aqueous solution contains an aqueous solution of an alkali metal or an alkaline earth metal, particularly preferably sodium hydroxide or hydroxide. An aqueous solution of potassium is used, and the pH of the liquid after the dispersion of the acicular particles is generally set to be 8 or more.

The needle-like particles of iron oxyhydroxide and iron oxide as the raw materials preferably have an average axis ratio of about 7 to 15 and an average major axis diameter of about 0.12 to 0.3 μm. These raw material particles may contain a metal element other than iron, such as cobalt or nickel, in the inside or on the surface of the particle in advance.

Fe ³⁺ ions added to the above-described suspension of needle-like particles and
The aqueous solution containing Co ²⁺ ions includes ferric chloride, ferric sulfate,
It is prepared by dissolving a ferric salt such as ferric nitrate, a cobalt salt such as cobalt chloride, cobalt sulfate and cobalt nitrate and water. Here, the ratio of Fe ³⁺ ions and Co ²⁺ ions, Co ²⁺ ions 30 with respect to Fe ³⁺ ions
A range of 100 mol% is suitable, and if it is outside this range, the cobalt-containing coating layer deposited on the particle surface is unlikely to be uniform and dense in crystal structure.

In the present invention, Fe ³⁺ ion and Co
It is important to include at least one divalent metal ion selected from Cr ²⁺ , Ni ²⁺ and Zn ²⁺ together with the ²⁺ ions. That is, such other divalent metal ions are Fe ³⁺
Co-precipitation with Co ²⁺ to form a coating layer on the particle surface,
It will be introduced as an alloy component into the final alloy powder. In this case, the obtained alloy powder has better magnetic properties and corrosion resistance than the alloy powder composed of iron and cobalt. These other divalent metal ions are
It is contained in the form of chlorides, sulfates, nitrates and the like, and is preferably used in the range of 1 to 10 mol% with respect to Fe ³⁺ ions. ,
Conversely, if the amount is too large, the above two characteristics will be degraded.

The reaction is carried out in an alkaline suspension in which the raw material particles are dispersed,
The above Fe ³⁺ ions and Co ²⁺ ions are removed under agitation.By gradually adding an aqueous solution containing the other divalent metal ions, these metal ions are co-precipitated on the surface of the raw material particles. However, at this time, it is desirable that the pH of the suspension is maintained at a constant value on the alkali side by adding an alkaline aqueous solution together with the above aqueous solution. The aqueous solution containing the metal ions may be simultaneously added as a different aqueous solution for each ionic species. Furthermore, it is also possible to add an appropriate buffer salt to the above alkaline suspension in advance.

The coating amount of the cobalt compound deposited on the surface of the needle-like particles of iron oxyhydroxide or iron oxide by this reaction corresponds to the amount of the metal ion used. Therefore, Co
By increasing the amount of ²⁺ ions used, the cobalt content in the final alloy powder can be set as high as, for example, 10% by weight or more. However, if the coating amount of the above-mentioned cobalt compound is too large, the needle-like shape of the particles is broken and magnetic properties such as coercive force are reduced, so that the cobalt content in the final alloy powder is usually 25% by weight or less. It is desirable to set the amount of Co ²⁺ ions to be used.

The raw material particles on which the coating layer of the predetermined amount of the cobalt compound has been formed are reduced by heating according to a conventional method to obtain an alloy magnetic powder mainly composed of iron and cobalt.

In this case, it is recommended to perform a surface treatment for providing a coating of a silicon compound and / or an aluminum compound on the surface of the raw material particles prior to the above-mentioned heat reduction. These coatings have the function of preventing sintering between powder particles during heat reduction and maintaining the needle-like shape of the raw material particles, and give good results to the magnetic properties of the finally obtained alloy magnetic powder.

As a means for such a surface treatment, for example, the raw material particles provided with the cobalt-containing coating layer are redispersed in water, and a water-soluble silicate or / and a water-soluble aluminate are added to the suspension. After mixing, a method of blowing carbon dioxide gas for neutralization, etc. may be mentioned.

The amount of the silicon compound and the aluminum compound deposited is preferably such that the atomic weight of Si and Al is about 0.2 to 5.0% by weight based on the metal Fe of the raw material particles.

Further, in the present invention, regardless of whether the raw material particles are iron oxyhydroxide or iron oxide, it is desirable to perform a heat treatment at about 400 to 700 ° C. in the air prior to the above-mentioned heat reduction. By this heat treatment, the cobalt-containing coating layer becomes a complete oxide, and iron and cobalt, as well as chromium, nickel, zinc, and the like are easily alloyed during heat reduction.

Heat reduction after performing such surface treatment or heat treatment may be generally performed at a temperature of about 350 to 550 ° C. in a hydrogen gas stream.

〔The invention's effect〕

According to the method of the present invention, it is possible to obtain, as an alloy magnetic powder mainly composed of iron and cobalt, one having needle-like particles having a high cobalt content and having extremely excellent magnetic properties and corrosion resistance. ^By setting the use ratio of the Co ²⁺ ion and the other divalent metal ion to the ³⁺ ion in an appropriate range, the above-mentioned effects can be surely exerted without deteriorating other characteristics.

〔Example〕

Next, examples of the present invention will be described in comparison with comparative examples.

Example 1 Goethite powder (average major axis diameter 0.22 μm, average axis ratio 10)
100 g was dispersed in water 3 and adjusted to pH 11 by adding a 5 mol / concentration aqueous sodium hydroxide solution. To this suspension, 500 ml of an aqueous solution in which 0.4 mol of ferric nitrate, 0.18 mol of cobalt sulfate and 0.02 mol of nickel nitrate were added with stirring over 3 hours, and the suspension was mixed at 5 mol / mol. The reaction was maintained at pH 11 by the addition of an aqueous solution of sodium hydroxide having a concentration, and the mixture was further stirred for 1 hour, washed with water and dried to obtain a goethite powder having a cobalt-containing coating layer on the particle surface.

Next, this goethite powder was redispersed in a 0.2 mol / concentration sodium hydroxide aqueous solution 5 and the 1 mol / concentration sodium orthosilicate aqueous solution 26 was added to the suspension.
Add carbon dioxide gas into the solution under stirring to neutralize the solution until pH 8 is reached, and apply a silicon compound to the particle surface.
Washed and dried. After that, this goethite powder was added to 0.
Dispersed in a 2 mol / concentration sodium hydroxide aqueous solution 5 to form a 0.5 mol / concentration sodium aluminate aqueous solution 1
35 ml was added, and carbon dioxide gas was blown into the solution under stirring to adjust the pH to 8
The particles were neutralized to give an aluminum compound on the particle surface, washed with water and dried.

Then, the goethite powder after the aluminum compound was applied was calcined in air at 500 ° C. for 4 hours, and then reduced by heating at 450 ° C. for 8 hours in a stream of hydrogen gas to obtain Fe—Co—Ni.
Acicular magnetic particles made of an alloy were obtained.

Example 2 A needle-like magnetic powder made of an Fe-Co-Zn alloy was obtained in the same manner as in Example 1 except that 0.02 mol of zinc sulfate was used instead of nickel nitrate.

Example 3 An acicular magnetic powder made of an Fe-Co-Cr alloy was obtained in the same manner as in Example 1, except that 0.02 mol of chromium sulfate was used instead of nickel nitrate.

Example 4 A needle-like magnetic powder made of an Fe-Co-Ni-Cr alloy was prepared in the same manner as in Example 1 except that the amount of nickel nitrate was changed to 0.01 mol and 0.01 mol of chromium sulfate was additionally used. Obtained.

Example 5 Fe-Co-Ni was prepared in the same manner as in Example 1, except that the amount of ferric nitrate used was changed to 0.8 mol, the amount of cobalt nitrate was changed to 0.36 mol, and the amount of nickel nitrate was changed to 0.04 mol.
A needle-like magnetic powder made of an alloy was obtained.

Comparative Example 1 An acicular magnetic powder made of metallic iron was obtained in the same manner as in Example 1, except that the cobalt-containing coating layer was not formed on the particle surface of the goethite powder.

Comparative Example 2 A needle-like magnetic powder made of an Fe-Co-Ni alloy was obtained in the same manner as in Example 1 except that ferrous nitrate was replaced by 0.4 mol of ferrous nitrate.

Reference Example 1 An acicular magnetic powder made of an Fe-Co alloy was obtained in the same manner as in Example 1 except that nickel nitrate was not used.

Each of the magnetic powders obtained in the above Examples, Comparative Examples and Reference Examples was gradually oxidized in air at 60 ° C. for 2 hours, and then saturated using a sample vibration type magnetometer (manufactured by Toei Kogyo Co., Ltd.). Measure magnetization s and coercive force Hc,
The saturation magnetization s 'after storage for 7 days under the condition of% RH was measured in the same manner as described above, and the saturation magnetization reduction rate (σs-σs') / σs
I asked. The results were measured for Co measured by X-ray fluorescence analysis.
It is shown in the table below together with the weight% of / Fe.

From the results in the above table, the alloy magnetic powders obtained by the method of the present invention (Examples 1 to 5) are very excellent in magnetic properties and corrosion resistance. This result is based on the fact that nickel and chromium are mainly composed of iron and cobalt. Alternatively, it is found that the effect is more remarkable as compared with the magnetic powder containing no zinc (Reference Example 1).

In contrast, when the shape of the cobalt-containing coating layer was changed to Fe ²⁺
The alloy magnetic powder obtained by the conventional method using ion (Comparative Example 2) has improved saturation magnetization and corrosion resistance as compared with the metallic iron magnetic powder (Comparative Example 1), but has a uniform coating layer formation. The coercive force is low due to the lack of precision, and it is clear that each characteristic is significantly inferior to the alloy magnetic powder obtained by the method of the present invention.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-59-173209 (JP, A) JP-A-3-174704 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01F 1/06 G11B 5/842 B22F 9/22

Claims (2)

(57) [Claims] 1. A needle-like particle of iron oxyhydroxide or iron oxide is dispersed in an alkaline aqueous solution, and Cr ²⁺ , Ni ²⁺ , Zn ^{2 +} and Fe ³⁺ ion and Co ²⁺ ion are dispersed in the suspension. A needle-like alloy magnetism characterized by adding an aqueous solution containing at least one other divalent metal ion selected from ⁺ , forming a coating layer of a compound containing these ions on the surface of the particles, and then reducing by heating. Powder manufacturing method. 2. The aqueous solution to be added to the suspension contains 30 to 100 mol% of Co ²⁺ ions and 1 to 10 mol% of other divalent metal ions based on Fe ³⁺ ions. The method for producing a needle-shaped alloy magnetic powder according to 1).