JPS58161725A - Production of magnetic metallic iron powder - Google Patents

Abstract

PURPOSE:To produce magnetic metallic iron powder having excellent dispersibility in binders and oxidation stability, by forming films of silicic acid and aluminum oxide on the particle surfaces of metallic iron powder by the use of an aq. soln. of silicate and an aluminum salt. CONSTITUTION:An alkaline suspension contg. metallic iron powder, silicate and an aluminum salt is neutralized with an acid of carbon dioxide or the like to form films of silicic acid and aluminum oxide on the particle surfaces of said metallic iron powder. Caustic soda or the like is used as said alkali to control pH to about >= 12. The metallic iron powder is obtd. by reducing iron oxyhydroxide or iron oxide under heating. Sodium orthosilicate or the like is used as the silicate and aluminum sulfate or the like is used as the aluminum salt. The combining ratio thereof is about 1/10-10/1 (based on weight), and the total amt. thereof is about 0.5-10pts.wt. in 100pts. the metallic iron powder. After the iron powder is neutralized with the acid, the powder is washed and is wetted with an org. solvent, whereafter the powder is dried by heating.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing magnetic metallic iron powder, which has excellent dispersibility in a binder and excellent oxidation stability when used as a recording element, etc. The purpose is to provide

Due to its surface activity, magnetic metal iron powder has poor oxidation stability, and also has the disadvantage that it is difficult to disperse well in the binder when it is dispersed and bound in the binder for use in recording elements. For this reason, conventional efforts have been made to forcibly form a dense and stable oxide film on the surface of metallic iron powder, but it has not been possible to sufficiently improve both oxidation stability and dispersibility.

The inventors neutralized an alkaline suspension containing metallic iron powder, silicate, and aluminum salt with an acid such as carbon dioxide to form a coating of silicic acid and aluminum oxide on the particle surface of the metallic iron powder. It was discovered that magnetic metal iron powder with highly improved oxidation stability and dispersibility in a binder could be obtained by forming a magnetic iron powder, which led to the completion of this invention.

In this invention, first, an alkaline suspension containing metal iron powder, silicate, and aluminum salt is prepared.

Caustic soda, caustic potash, etc. are used as the alkali, and the pH of the liquid is generally adjusted to about 12 or higher. The metallic iron powder may be produced by thermally reducing iron hydroxide or iron oxide, or may or may not use an anti-sintering agent during the thermal reduction. However, when a silicon compound or an aluminum compound is particularly selected as the above-mentioned sintering inhibitor, a portion of silicon oxide or aluminum oxide adheres to the surface of the iron powder, so the coating of silicic acid and aluminum oxide of this invention It has been found that the adhesion and compactness of the oxidation stability and dispersibility are improved.

Silicates include water-soluble silicates such as sodium orthosilicate, sodium metasilicate, potassium metasilicate, and various water glasses. These salts dissolve well in alkaline aqueous solutions. Examples of aluminum salts include water-soluble salts such as aluminum sulfate, aluminum nitrate, and aluminum chloride, and water-soluble aluminates such as sodium aluminate. These salts dissolve well in alkaline aqueous solutions.

The total amount of the silicate and aluminum salt used is usually 0.5 to 10 parts by weight per 100 parts by weight of the metal powder.
If this amount is too small, the effect of the present invention cannot be obtained; on the other hand, if this amount is too large, the saturation magnetization of the obtained magnetic iron powder will be too low, making it unsuitable for use in recording elements, etc. In addition, the combined ratio of silicate and aluminum salt can be within the range of 1/10 to 10/1 (weight ratio), but generally it is better to use less aluminum salt than silicate. can. That is, compared to silicate, aluminum salt has the effect of improving dispersibility in a binder even when used in a small amount.

Therefore, it is preferable to reduce the amount of the aluminum salt and use a large amount of silicate to ensure dispersion stability.

Next, in this invention, the above alkaline suspension is neutralized. As a result, the silicate and aluminum salt dissolved in the liquid become silicate sol and aluminum hydroxide and precipitate on the surface of the iron powder. . This neutralization should be carried out as gradually as possible; if it is too rapid, the above-mentioned compounds will coarsely precipitate on the surface of the iron powder, making it impossible to obtain good oxidation stability, dispersibility, and formation of cavities.

The above-mentioned gradual neutralization reaction is suitably carried out by blowing carbon dioxide gas into the alkaline suspension.

However, it is also possible to neutralize the alkaline suspension by gradually adding a dilute aqueous solution of a weak acid such as acetic acid, carbonic acid, oxalic acid, or formic acid to the alkaline suspension. The neutralization reaction is carried out at room temperature), but may be carried out under heating if necessary.The reaction end point is preferably carried out until the final pH reaches about 8 or less.

In the above explanation, a method was explained in which a silicate sol and an aluminum salt were added together to an alkaline suspension, and this was neutralized to precipitate a silicate sol and aluminum hydroxide at the same time. The method is not limited, and for example, a method in which silicic acid sol is first deposited on one surface of the iron powder and then aluminum hydroxide is deposited thereon, or a two-step precipitation method opposite to the above method may be adopted.

Among these methods, the most suitable method from the viewpoint of both oxidation stability and dispersibility is a method in which a λ acid sol is precipitated, and then aluminum hydroxide is precipitated.
The above-mentioned simultaneous precipitation method is also a preferred method. The reason for this is, as mentioned above, that it is preferable to provide aluminum salts on the outer surface of the powder because they particularly contribute to improving dispersibility, and silicates particularly contribute to oxidative stability. Therefore, it is desirable to keep the powder in close contact with the inner surface of the powder.

Needless to say, in the two-step precipitation method, first, an alkaline suspension is prepared in which either the silicate or the aluminum salt is dissolved, and this is neutralized with an acid such as carbon dioxide gas. The hydroxide of the corresponding oxide (G) is precipitated, and then the powder after heating and drying is resuspended in an alkaline aqueous solution in which the other salt is dissolved, and the same precipitation as described above is carried out.

After carrying out the neutralization reaction in this manner and washing with water, the magnetic metal iron powder of the present invention is obtained by wet treatment with an organic solvent and drying by heating. This iron powder has a dense coating of silicon oxide and aluminum oxide on its surface, which greatly contributes to improving oxidation stability and dispersibility in the binder.

In the following, embodiments of the present invention will be explained.

Example 1 A solution prepared by dissolving 15.29 g of aluminum sulfate in 500 ffi/water and a solution prepared by dissolving 330 g of sodium silicate in 2 e water were added to an alkaline suspension containing 1 kq of α-Fe00H. They were added in this order, and while stirring, carbon dioxide gas was blown into the solution to neutralize it until the pH of the solution reached 8, thereby adhering aluminum hydroxide and silicate sol to the surface of the powder particles. After washing with water and drying, use a Matsufuru furnace at 900°C.
After dehydrating by heating for 2 hours in a
The mixture was heated and reduced in a hydrogen stream for 8 hours.

Iron powder containing aluminum oxide and silicon oxide (particle size 0.25μ) originating from the sintering inhibitor obtained by the above method.
, coercive force 1460 oersted, saturation magnetization 151 e
Mu/g) 620g was suspended in an alkaline aqueous solution of 201, to which was added 20g of an aqueous solution containing 1659.1mk aluminum dissolved in 409% of sodium silicate, and while stirring, carbon dioxide gas was blown into the solution until the pH of the solution was 8. The magnetic metal iron powder according to the present invention has a coating of silicon oxide and aluminum oxide on the particle surface by neutralizing the above powder until it becomes silicic acid and aluminum hydroxide on the particle surface and drying it. I got it.

Comparative Example Iron powder containing silicon oxide and aluminum oxide, which originated from the sintering inhibitor obtained by the method of Example 1, was forcibly oxidized in the gas phase to form a magnetic material with an oxide film. Metallic iron powder was obtained.

Example 2 Using commercially available metallic iron powder (particle size 0.25μ, coercive force 1460 oersted, saturation magnetization 151 en1u/9i, iron powder obtained by thermal reduction without using an anti-sintering agent), an example was carried out. By processing in the same manner as in Example 1, magnetic metallic iron powder according to the present invention having a coating of silicon oxide and aluminum oxide on the particle surface was obtained.

The oxidation stability and dispersibility in binders of each of the magnetic metal iron powders of Examples 1 and 2 and Comparative Example were examined by the following method, and the results were as shown in the table below. In addition, Reference Example 1゜2 in the table refers to Example 1 and Example 1, respectively.
These are the test results for the untreated iron powder used in Example 2, that is, the iron powder that does not have a coating of silicon oxide and aluminum oxide.

<Oxidation stability> Based on the saturation magnetization after being left under 90"C, 60% R, H for 24 hours and the initial saturation magnetization, the deterioration rate (%) of the saturation magnetization after being left is determined as the initial value. Calculated with 100%.

Dispersibility> The amount of binder adsorption was used as an index of dispersibility. That is, V.A.
GH (hydroxyl group-containing vinyl chloride manufactured by U.S.A.C., Inc.)
Add magnetic metal iron powder 1y to 20cc of a 1% by weight solution prepared by dissolving 1 yen + [vinyl copolymer] in a 1:1 mixed solvent of methyl isobutyl ketone and toluene, and disperse it with ultrasonic waves for more than 2 hours. , leave for 3 days and then centrifuge. The dissolved binder concentration in the liquid was actually measured, and from the difference between this and the initial concentration,
The amount of VAGH (#) adsorbed to the magnetic powder 1f was determined.

As is clear from the above test results, according to the present invention, it is possible to obtain a magnetic metal iron powder that is excellent in both oxidation stability and dispersibility in a binder.

Patent applicant: Hitachi Maxell, Ltd.

Claims (1)

[Claims] (1) Neutralize an alkaline suspension containing metal iron powder, silicate, and aluminum salt with acid such as carbon dioxide to form a film of silicic acid and aluminum oxide on the surface of the metal iron powder particles. A method for producing magnetic metal iron powder characterized by: