JPH0248601B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a stabilization treatment for ferromagnetic metal powder, and particularly to a stabilization treatment for ferromagnetic metal powder whose main component is iron. [Prior Art] In recent years, ferromagnetic metal powders containing iron as a main component have been attracting attention and being used as magnetic materials for magnetic recording media. This ferromagnetic metal powder is obtained by heating, dehydrating, and reducing goethite or iron oxide, and has superior coercive force (Hc) and saturation magnetization (σs) compared to conventional iron oxide-based magnetic materials, making it suitable for high-density recording. Although it is achievable, it has the biggest drawback of poor oxidation resistance. Ferromagnetic metal particles used in magnetic recording media have a large specific surface area and are extremely chemically active, and if they are taken out into the atmosphere, heat generation and ignition occur due to rapid oxidation reactions. For this purpose, metal particles are brought into contact with an oxygen-containing gas in a liquid phase or a gas phase to gradually form an oxide film on the surface of the metal particles (hereinafter referred to as "gradually forming an oxide film on the surface of metal particles"). A method of stabilization treatment using slow acid treatment has been proposed. Conventional stabilization treatments for ferromagnetic metal powder include the so-called liquid phase slow acidification method (for example, JP-A-52-85054), in which the powder is immersed in an organic solvent and oxidized in the solution through an oxygen-containing inert gas. , the so-called gas-phase gradual oxidation method (for example, the method of
48-79153) etc. have been proposed. However, since the former reaction occurs in the liquid phase, the diffusion of dissolved oxygen in the organic solvent to the surface layer of the ferromagnetic metal powder particles is insufficient, and therefore the oxidation reaction occurs non-uniformly, resulting in unevenness of the oxide film. However, since it is a liquid phase reaction, the reaction for forming an oxide film is insufficient, and the magnetic properties deteriorate quickly over time. In addition, since the latter reaction occurs in the gas phase, the activity of the ferromagnetic metal powder particles is very strong, making it extremely difficult to control the oxidation reaction, which tends to result in non-uniform slow oxidation, and the coercive force and saturation magnetization. It has drawbacks such as a decrease in As an alternative method, a method has been proposed in which the ferromagnetic metal powder is brought into contact with an oxygen-containing inert gas gradually from the liquid phase to the gas phase (Japanese Patent Application Laid-Open No. 170201/1983). However, the current situation is that it has not yet been completed as a satisfactory method. [Problems to be solved by the invention] In the method proposed in JP-A-59-170201, the reaction temperature is relatively high and the oxidation reaction is rapid in a very short time. The film tends to become uneven, which leads to deterioration of the coercive force, and the saturation magnetization is extremely difficult to control. The present inventors have improved these shortcomings and created a practical material that is easy to implement on an industrial scale, has a uniform and dense oxide film on the surface, has excellent magnetic properties, and has a small change in saturation magnetization over time. ,
The present invention has been arrived at as a result of intensive studies aimed at providing a method for producing ferromagnetic metal powder containing iron as a main component. [Means and effects for solving the problems] The present invention provides granules of ferromagnetic metal powder mainly composed of iron soaked in an organic solvent and having a minimum diameter of 0.25 to 0.25.
In a horizontal cylindrical rotary reactor with a scraper plate attached to the inner wall parallel to its generatrix, the granules are removed from the organic solvent as the reactor rotates. The surface of the primary powder is brought into contact with an oxygen-containing gas while it is scraped up on a raising plate and falls down. The surface of the powder is first deoxidized at 30 to 90°C for 4 to 25 hours. After the primary deacidification is completed, the organic solvent is separated and removed, and then This is a method for producing ferromagnetic metal powder having an oxide film by subjecting it to slow acidification in the gas phase at 10 to 80°C for 10 to 50 hours. The present invention is based on a ferromagnetic metal whose main component is iron.
When the granules of ~10 mm are subjected to primary deacidification in the horizontal cylindrical rotary reactor equipped with the above-mentioned scraping plate, the granules are scraped up from the organic solvent by the scraping plate, and then transferred to the organic solvent layer. This method utilizes the fact that the granules come into contact with the oxygen-containing gas before falling and react with the granules, which have a decreasing organic solvent content. For this reason, it is completely different from conventional liquid phase slow acidification, and the oxygen diffusion reaction is extremely easy.On the other hand, since it contains a small amount of organic solvent, the oxidation reaction is suitably controlled and uniform slow acidification is possible. becomes. In addition, since the granules fall from the scraping plate to the organic solvent layer within a certain time of scraping,
Oxidation reactions are not excessively promoted, and local heat generation does not occur at all. Such primary slow acid is considered to be a mixed slow acid of liquid phase and gas phase. In the present invention, in order to shift to gas phase slow acid after sufficiently performing such primary slow acid, unlike conventional gas phase slow acid, particles of ferromagnetic metal powder are used in gas phase slow acid. The greatest feature is that the surface activity is significantly stabilized by the primary slow acidification, and the transition from the primary slow acidification to the gaseous slow acidification occurs very smoothly without any heat generation. In the end, mild and stable gas phase slow acidification, which is unimaginable with conventional gas phase slow acidification, is achieved, making it possible to form a uniform and dense oxide film with excellent dispersibility. The significance of the present invention lies in the fact that a high quality ferromagnetic metal powder with a small change in saturation magnetization over time can be obtained. In addition, since the oxidation reaction can be easily controlled in this way, it is also suitable for implementation on an industrial scale. The method of the present invention can be applied to all iron-based ferromagnetic powders used as magnetic recording media.
This ferromagnetic powder can contain only iron or in addition to iron, Ni,
Si, Al, Mn, Cu, Cr, Ti, Mg, Co, Zn, Ba,
At least one component of metal elements other than iron such as Sn and compounds of those metals is 0 based on iron.
It may contain up to 50% by weight.
Examples of ferromagnetic metal powders containing iron as a main component include ferromagnetic metal powders obtained by reducing iron oxyhydroxide, hematite, maghemite, magnetite, etc., which may or may not contain metal elements other than iron. can be mentioned. The ferromagnetic metal powder used in the present invention has a minimum diameter to prevent it from scattering outside the reactor system due to oxygen-containing gas during the slow acid reaction.
It is desirable to use granules of 0.25 mm or more. On the other hand, the minimum diameter of the granules is preferably 10 mm or less in order to uniformly oxidize the surfaces of the ferromagnetic metal powders that constitute the granules. In the primary slow acid, the reaction temperature is 30-90°C.
℃, preferably 30 to 70℃. If the temperature is lower than 30°C, the oxidation reaction will proceed very slowly and it will take a long time to obtain the effects of the present invention, which is uneconomical.If the temperature exceeds 90°C, the oxidation reaction will proceed rapidly. In addition, the oxide film becomes uneven, resulting in deterioration of magnetic properties. The reaction time is preferably 4 to 25 hours. If the time is less than 4 hours, the oxidation will be incoherent, and the subsequent gas phase gradual acidification will generate intense heat, making temperature control difficult. If it exceeds 25 hours, it is uneconomical. Examples of organic solvents that can be used for this primary slow acid include aromatic hydrocarbons such as benzene, toluene, and xylene, fluorine-based solvents such as trifluoroethanol and perfluorooctane, and lower alcohols such as methanol and ethanol. The organic solvent distilled out along with the oxygen-containing gas during this primary slow acidification may be recovered and refluxed in a heat exchanger, or may be fed separately or continuously or intermittently. It is preferable that the amount of organic solvent in the horizontal cylindrical rotary reactor is kept at least twice the weight of the ferromagnetic metal powder. If the amount is less than 2 times by weight, it tends to be difficult to control the oxidation reaction during the primary slow acidification. More preferably 2~
A range of 5 times the weight is suitable. As a method for separating and removing the organic solvent, the organic solvent may be passed mechanically or may be entrained in a gas and then distilled off. The vapor phase slow acidification refers to slow acidification in a state in which no organic solvent exists other than the organic solvent impregnated into the ferromagnetic metal powder granules, or in a state in which the impregnated organic solvent is completely dried. When the ferromagnetic metal powder granules are impregnated with an organic solvent at the beginning of the vapor phase slow acidification, the ferromagnetic powder granules may be completely dried during the vapor phase slow acidification process. In the gas phase slow acid, the reaction temperature is 10 to 80℃.
℃, preferably 10 to 60℃. If the temperature is less than 10°C, it will take a long time to form the desired oxide film, which is uneconomical. If the temperature exceeds 80°C, the oxidation reaction will be rapidly accelerated, making it difficult to control and causing unevenness of the oxide film, which is unfavorable in terms of quality.
The reaction time is preferably 10 to 50 hours. If the time is less than 10 hours, the thickness of the oxide film will be insufficient, and the stabilization-treated ferromagnetic metal powder will not be sufficiently stabilized. If it exceeds 50 hours, it is uneconomical. As the oxygen-containing gas used in the present invention, any gas containing oxygen can be used, but
An inert gas containing 10% by volume or less oxygen is preferred. If it exceeds 10% by volume, it may fall within the explosive range when using organic solvents, which is not desirable from a safety standpoint. In addition, He, Ne, Ar,
There are CO ₂ , N _{2 ,} etc., but it is usually cheaper and more practical to use N ₂ . In the horizontal cylindrical rotary reactor used in the present invention, which has a scraping plate attached to its inner wall parallel to its generatrix, a jacket may be provided around the outer periphery of the reactor to adjust the reaction temperature. [Example] Next, the present invention will be explained with reference to Examples, but the present invention is not limited thereto. Example 1 Coercive force (Hc) 1495Oe, saturation magnetization (σs)
Iron-based ferromagnetic powder granules (diameter 3
mm, cylindrical shape with a length of 5 mm) 15 kg, toluene
It is a horizontal cylindrical rotary reactor in which the reactor is immersed in 45 kg, and a scraping plate is attached to the inner wall parallel to the generatrix, and a jacket is installed around the outer circumference of the reactor to control the reaction temperature. The reactor was placed in a reactor equipped with an oxygen-containing gas aeration device and diluted with _N2 while stirring at a rotational speed of 2.0 rpm while maintaining the internal temperature (temperature of the granules in the reactor; hereinafter the same) at 45°C. 20 m ³ / of mixed gas containing 5% oxygen by volume
The toluene distilled out along with the mixed gas was condensed in a condenser and refluxed into the reactor for 12 hours of reaction (primary gradual acidification). The magnetic properties of the ferromagnetic powder at this time are Hc: 1511Oe,
σs: 1447emu/g, Rs: 0.511. Thereafter, the toluene in the reactor was overseparated, and the reaction was stopped for 30 hours by aerating the mixed gas while stirring at a rotation speed of 2.0 rpm while maintaining the internal temperature inside the reactor at 30°C (gas phase slow acid). The ferromagnetic metal powder granules taken out were completely dry and their magnetic properties were
Hc: 1518 Oe, σs: 135emu/g, Rs: 0.513. Additionally, the oxidation stability of this powder is 60% in air.
Evaluation was made based on the rate of decrease in σs (△σs) after standing for 3 days under conditions of ℃ and 90% RH (evaluation was made in the same manner in the following examples), and the value was 5.0%. Next, to 55 g of this ferromagnetic metal powder, 12.4 g of a binder made of vinyl chloride acetate and polyurethane, 0.7 g of a hardening agent,
3.8 g of abrasive, 2.8 g of dispersant, and 171 g of a solvent consisting of toluene, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone were placed in a sand mill and stirred at 1850 rpm for 2 hours to obtain a magnetic paint.
Apply this paint to polyester film,
Create a tape by orienting it with a magnetic field of 3000G to produce 5KOe
Its magnetostatic properties were measured under the magnetic field of The magnetostatic properties of this tape were a coercive force (Hc) of 1455 Oe, a residual magnetic flux density (Br) of 2990 G, and a squareness ratio (SQ) of 0.83. Table 1 shows their magnetic properties. Example 2 The same ferromagnetic powder granules and equipment as in Example 1 were used, and the same treatment as in Example 1 was carried out except that the internal temperature during the primary slow acidification was set at 60°C. Its magnetic properties etc. are shown in Table 1. Example 3 Using the same ferromagnetic metal powder granules and equipment as in Example 1, the same treatment as in Example 1 was carried out except that the internal temperature during gas phase slow acidification was set at 50°C. Its magnetic properties etc. are shown in Table 1. Example 4 Using the same ferromagnetic metal powder granules and equipment as in Example 1, the same treatment as in Example 1 was carried out except that the primary slow acid reaction time was changed to 20 hours. Its magnetic properties etc. are shown in Table 1. Comparative Example 1 5 kg of the same ferromagnetic metal powder granules as in Example 1 were immersed in 15 kg of toluene and placed in a reactor equipped with a stirring blade, a heating device, and an organic solvent recovery device.
While stirring at a rotation speed of 6.0 rpm, the internal temperature was maintained at 90°C, and a mixed gas containing 5% by volume of oxygen diluted with N ₂ was aerated at a rate of 1.2 m ³ /hr, followed by distillation along with the mixed gas. The toluene was collected and the reaction was carried out for 4.5 hours without refluxing into the reactor. Table 1 shows the magnetic properties of the ferromagnetic metal powder etc. taken out. Comparative Example 2 15 kg of the same ferromagnetic metal powder granules as in Example 1 were immersed in 45 kg of toluene and placed in the same horizontal cylindrical rotary reactor as in Example 1, and toluene was added without any primary slow acidification. was over-separated, and the same gas phase slow acidification as in Example 1 was performed for 30 hours. This reaction is extremely exothermic, and the internal temperature was kept at 25°C at the beginning and end of the reaction.
However, the internal temperature rose to 50°C during the reaction. Table 1 shows the magnetic properties of the ferromagnetic metal powder taken out.

【table】

〔Effect of the invention〕

As is clear from the Examples and Comparative Examples in Table 1, according to the method of the present invention, due to the effect of primary slow acidification, heat generation during the subsequent gas phase slow acidification is completely suppressed, resulting in a mild and stable gas phase. In order to achieve slow acidification, the tape has a uniform and dense oxide film, which is the objective of the present invention, and has excellent dispersibility and other magnetic properties (the tape has a high squareness ratio Rs in the magnetostatic properties, so it has excellent dispersibility). ferromagnetic metal powder with a small change in saturation magnetization over time can be produced.

Claims (1)

[Scope of Claims] 1. Granules of ferromagnetic metal powder mainly composed of iron immersed in an organic solvent and having a minimum diameter of 0.25 to 10 mm.
The granules are removed from the organic solvent as the reactor rotates in a horizontal cylindrical rotary reactor with a scraping plate attached to the inner wall parallel to its generatrix. Gradual oxidation (hereinafter referred to as "slow acidification") of the surface of the primary powder is carried out at 30 to 90°C for 4 to 25 hours by bringing it into contact with oxygen-containing gas while it is being scraped up on a raising board and falling to complete the primary slow acidification. After that, the organic solvent is separated and removed, and then the
1. A method for producing a ferromagnetic metal powder having an oxide film, comprising slow acidification in a gas phase at ℃ for 10 to 50 hours. 2. The method according to claim 1, wherein the temperature of the primary slow acid is 30 to 70°C. 3. The method according to claim 1 or 2, wherein the temperature of the gas phase slow acid is 10 to 60°C. 4. Claims 1 to 3, characterized in that the amount of the organic solvent during the primary slow acidification is 2 to 5 times the weight of the ferromagnetic metal powder containing iron as a main component. The method described in any of the above. 5 Claims characterized in that the oxygen-containing gas is a mixture of air or oxygen gas and an inert gas consisting of at least one of nitrogen, carbon dioxide, helium, argon, and neon as a diluent gas. The method according to any one of paragraphs 1 to 4. 6. The method according to any one of claims 1 to 5, wherein the oxygen concentration of the oxygen-containing gas is 10% by volume or less.