JPS6349722B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing ferromagnetic powder, and in particular to a method for producing ferromagnetic metal or alloy powder for magnetic recording media with high coercive force and high saturation magnetic flux density suitable for high-density recording. Ferromagnetic powders conventionally used in magnetic recording materials include γ-Fe ₂ O ₃ , Co-containing γ-Fe ₂ O ₃ , Fe ₃ O ₄
Co-containing Fe ₃ O ₄ , Fe ₃ O ₄ −γ−Fe ₂ O ₃ , CrO _{2 ,} etc. were found. However, these ferromagnetic powders cannot be said to be suitable for the future trend toward higher densification in which the recording signal per unit volume of the magnetic layer is increased.
In other words, coercive force (Hc) is required for use in high-density recording.
Also, the magnetic properties such as magnetic flux density (σ) are insufficient, and it is not very suitable for magnetic recording of signals with short recording wavelengths. Therefore, recently, ferromagnetic media suitable for high-density recording have been actively developed. One of the target materials is powder of ferromagnetic metals and alloys. In other words, the saturation magnetic flux density per unit volume of γ-Fe ₂ O ₃ is approximately 5000 Gauss, but that of Fe, Fe-Co
For alloys, etc., it is about 20,000~, which is about four times that of γ-Fe ₂ O ₃ .
It is said to have a saturation magnetic flux density as high as 25,000 gauss, and in very simple terms, it is expected that the playback output will be about four times as high, making even higher density recording possible. Conventionally, the following methods have been studied as methods for producing such ferromagnetic metal and alloy powders. (1) A method in which organic acid salts (mainly oxalates) of ferromagnetic metals and alloys are thermally decomposed and reduced with a reducing gas. For example, Tokuko Sho 36-11412, Tokko Sho 36-22230,
Tokuko Sho 40-8027, Tokuko Sho 42-24032, Tokko Shou 43
−22394, JP 1977-38417, JP 47-38523
No. 48-29280, 22346-2013, 22994-1984, U.S. Patent No. 3186829, U.S. Patent No.
No. 3190748, Japanese Patent Publication No. 47-4286 for using phthalates, (2) Iron oxyhydroxide or these doped with other metals (e.g. Co),
Alternatively, a method of reducing iron oxide or ferrite composition oxide with a reducing gas. For example, Tokuko Sho 35-3862, Tokko Sho 37-11520,
Tokuko Sho 39-20939, Tokuko Sho 47-29706, Tokuko Sho 47
−30477, Japanese Patent Publication No. 47-39477, Japanese Patent Publication No. 46-5057,
Japanese Patent Publication No. 1973-7153, Special Publication No. 48-24952, Japanese Patent Publication No. 1977-24952
-79153, Japanese Patent Publication No. 1983-82393, Special Publication No. 49-7313,
JP 49-135867, JP 51-5608, U.S. Patent No. 3598568, U.S. Patent No. 3607220, U.S. Patent No.
3702270, British Patent No. 640438, (3) Method for vaporizing ferromagnetic metals and alloys in an inert gas. For example, Tokuko Sho 47-27718, Tokuko Sho 48
-964, JP 1977-42780, JP 1977-25662~
25665, Japanese Patent Publication No. 48-31166, Japanese Patent Publication No. 48-55400,
JP-A-48-81092, JP-A-49-52134, (4) Method for decomposing metal carbonyl compounds. For example, Japanese Patent Publication No. 45-16868, U.S. Patent No.
2983997, U.S. Patent No. 3172776, U.S. Patent No.
3200007, US Patent No. 3228882, (5) A method of electrodepositing ferromagnetic metal powder by mercury electrolysis and then decomposing and removing mercury. For example, Tokuko Sho 39-15525, Tokuko Sho 40-8123,
Japanese Patent Publication No. 47-6007, U.S. Patent No. 3156650, U.S. Patent No. 3262812, (6) A method of wet reduction of a ferromagnetic metal salt with sodium hypophosphite, sodium borohydride, etc. in its solution. . For example, Tokuko Sho 38-20520, Tokuko Sho 38-
26555, Special Publication No. 43-20116, Special Publication No. 45-9869, Special Publication No. 47-7820, Special Publication No. 47-16052, Special Publication No. 47-
41718, JP 47-41719, JP 47-1353, JP 47-42252, JP 47-42253, JP 48-
7585, JP-A 1987-25896, JP-A 48-44194, JP-A 48-79754, JP-A 1972-82396, JP-A 1972-
28999, Japanese Unexamined Patent Publication No. 48-1998, US Patent No. 3607218,
U.S. Patent No. 3494760, U.S. Patent No. 3535104, U.S. Patent No. 3661556, U.S. Patent No. 3663318, U.S. Patent No. 3669643, U.S. Patent No. 3672867; How to generate it. For example, Japanese Patent Application Laid-Open No. 47-33857. As mentioned above, various methods have been investigated, but each method has its own advantages and disadvantages, and none of them has yet been applied in an industrial or practical sense. It cannot be a definitive method. For example, method (1) by decomposing oxalate is a method that has been studied for a long time, but the magnetic powder obtained by this method generally has a large particle size of 5 to 10 μm.
To some extent, if left as is, when used as a magnetic tape, the surface of the tape would be rough and the noise level would be poor.
Another disadvantage is that the magnetic head is worn out and the adhesion between the head and the tape surface is hindered, making it difficult to achieve the desired high-density recording. Furthermore, the evaporation method (3) requires complicated equipment and operations, and its large-scale implementation has disadvantages in terms of industrial efficiency and economy. In addition, in the mercury electrolysis method (5), the electrodeposited product is dendritic particles containing 4 to 6% mercury, which is removed from the dendritic particles by heat treatment. It is difficult to completely remove it, and there are also problems such as mercury pollution that make it difficult to put it into practical use. Regarding the method using wet reduction reaction (6),
The ferromagnetic metals and alloy powders obtained by this method have a strong surface activity, making them highly natural, and are sensitive to oxygen and moisture in the air. Even at room temperature and humidity, they gradually oxidize and deteriorate their magnetic properties. easy to imitate. In addition, the individual particles obtained by this method are thread-like, but during the process of mixing and dispersing with the binder, the particle shape is destroyed and the magnetic field orientation becomes poor.
This has the disadvantage that the magnetic properties of the magnetic recording medium, especially the squareness ratio, deteriorate. Now, the present invention belongs to the method (2) of dry reducing iron oxyhydroxide or iron oxide with a reducing gas, but some drawbacks have been pointed out in the conventional methods related thereto. First, the reduction process is usually carried out in a high-temperature hydrogen stream, which results in volume reduction, porosity, shape changes, and sintering, resulting in less-than-expected magnetic properties even if starting from iron oxide in the desired shape. is difficult to obtain. This is true even in the normal manufacturing process of γ-Fe ₂ O ₃ , in the process of α-Fe ₂ O ₃ → Fe ₃ O ₄ , that is, the process of removing 1/9 of the oxygen from α-Fe ₂ O ₃ with hydrogen gas. ,
It is said that shape change and sintering are the most likely to occur, and considerable efforts are being made to prevent this. In comparison, in the case of metals and alloys, all the oxygen is removed, so it can be said that shape changes and sintering become even more severe, resulting in poor coercive force (Hc) and squareness ratio. Due to incomplete dispersion during tape production, it has not been practical as a recording material, although it has promising properties. Furthermore, the metal and alloy powders obtained by this method also have the disadvantage of being flammable, which hinders their practical use. The present invention eliminates the above drawbacks. In other words, by treating acicular iron oxyhydroxide with water glass, filtering, washing with water, firing, and then reducing, the above-mentioned drawbacks of shape collapse and sintering can be prevented, and the holding force (Hc) and squareness ratio can be improved. (σr/σs), a metal powder with excellent dispersibility and suppressed ignitability that can be used as a stable ferromagnetic material for magnetic recording is obtained. The acicular iron oxyhydroxide used in the present invention includes α-FeOOH, β-FeOOH and γ-FeOOH, and may also be a mixture thereof.
The acicular iron oxyhydroxide used in the present invention includes the above-mentioned acicular iron oxyhydroxide doped with metals such as Co, Ni, Ti, Bi, Mo, and Ag. Treatment with a silicon compound involves suspending the above raw materials in water, dissolving the silicon compound, stirring well,
This is done by mixing. The silicon compound used in the treatment may be water-soluble or colloidal; for example,
Coiates such as Na ₂ SiO ₃ , Na ₂ Si ₂ O ₅ etc., Si
Examples include (OH) ₄ , colloidal silica, and water glass is particularly suitable. The amount of water glass treated is 0.1 to 10% by weight in terms of Fe ₂ O ₃ based on the raw material acicular iron oxyhydroxide. It is undesirable to use a large amount of water glass, as the magnetic properties of the metal or alloy powder will be diluted and the characteristic values will be lowered. Next, the treated product is filtered and washed with water. The acicular iron oxyhydroxide that has been subjected to these treatments, filtering, and water washing is then calcined at 600 to 800°C, and then reduced in a hydrogen atmosphere at a temperature not exceeding 600°C, preferably not exceeding 500°C. . There is practically no lower limit for concentration, but since the reaction proceeds very slowly at low temperatures, from a practical point of view a temperature of at least 250°C should be used to avoid long reaction times that become impractical. Should. However, it is also possible at temperatures as low as 200°C. After reduction, the reducer is cooled and a gas mixture of 1% air and 99% nitrogen is introduced into the reducer, doubling the air content of this gas at approximately 30 minute intervals.
After 4-5 hours, it is possible to switch to air only and remove the magnetic iron powder from the reducer. Then, it can be a magnetic tape or other magnetic recording medium. Next, embodiments of the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. Example 1, Comparative Example 1 100g of acicular goethite (α-FeOOH) was
9 g of JIS-1 water glass (α-
10% by weight (calculated as Fe ₂ O ₃ ) was dissolved therein, stirred for 40 minutes, filtered, and thoroughly washed with water. After drying, the cake was baked at temperatures of 400, 500, 600, 700, and 800°C, and then reduced with hydrogen gas. The magnetic properties of the obtained metal magnetic powder are shown in Table 1. In addition, the case where no firing was performed is also shown in the comparative example.
Incidentally, the calcination was performed in an air atmosphere for 2 hours, and the reduction was performed under the following conditions. That is, in a nitrogen stream
Raise the temperature to 450℃ over about 1 hour, then switch nitrogen to hydrogen, set the hydrogen flow rate to 2/min, and heat for 3 hours.
The temperature was maintained at 450°C for an hour. After reduction, it is cooled in a stream of nitrogen, then a gas mixture of 1% air and 99% nitrogen is introduced, the air content of this gas is doubled at intervals of about 30 minutes, and after 4 hours it is reduced to air only. I switched.

[Table] Example 2, Comparative Example 2 Metal magnetic powder was obtained in the same manner as in Example 1, except that 4.5 g of water glass (5% by weight based on α-Fe ₂ O ₃ ) was used. The magnetic properties of the metal magnetic powder are shown in Table 2.

[Table] Example 3, Comparative Example 3 Metal magnetic powder was obtained in the same manner as in Example 1, except that 0.9 g of water glass (1% by weight based on α-Fe ₂ O ₃ ) was used. The magnetic properties of the metal magnetic powder are shown in Table 3.
The case where no firing was performed was also shown as Comparative Example 2.

【table】

[Table] Example 4, Comparative Example 4 Water glass 0.09g (0.1% by weight based on α-Fe ₂ O ₃ )
A metal magnetic powder was obtained in the same manner as in Example 1 except that . The magnetic properties of the metal magnetic powder are shown in Table 4.

[Table] Comparative Example 5 Metal magnetic powder was obtained in the same manner as in Example 4 except that the water glass treatment was not performed. The magnetic properties of the metal magnetic powder are shown in Table 5.

【table】

Claims (1)

[Scope of Claims] 1. Acicular iron oxyhydroxide is treated with water glass in an amount of 0.1 to 10% by weight calculated as Fe ₂ O ₃ based on the acicular iron oxyhydroxide, and then filtered and washed with water. A method for producing metal magnetic powder, which is characterized by firing at a temperature of 800°C and then reducing. 2 Acicular iron oxyhydroxide is α-FeOOH, β-
The manufacturing method according to claim 1, wherein the method is one or a mixture of two or more of FeOOH and γ-FeOOH. 3. The production method according to claim 1, wherein the reduction temperature is 200 to 600°C. 4. The production method according to claim 1, wherein the reduction is carried out in a hydrogen stream.