JPH02192103A - Manufacture of ferromagnetic metal powder having oxide film - Google Patents

Abstract

PURPOSE:To obtain ferromagnetic metal powder having uniform and highly densed oxide film and excellent magnetic characteristics by a method wherein the surface of the metal powder is subjected to slow oxidation while the increase in partial pressure of oxygen is being conducted at a fixed speed. CONSTITUTION:Ferromagnetic metal powder mainly composed of iron is fluidized in a fluidized bed at a temperature of 30 deg.C or higher using inert gas. Oxygen gas is mixed therein at an introducing speed with the fixed increment rate of 10ml/nin<2> or less per 1 gram of metal magnetic powder. When an oxide film, which is necessary for stabilization of the ferromagnetic metal powder, is formed by continuously conducting a slow oxidation treatment uniformly, an oxidative reaction is automatically stopped. This oxidative reaction can be traced by monitoring the temperature of the fluidized bed. To be more precise, when the reaction temperature comes down and the oxygen density on the outlet side is detected, the fluidized bed is cooled down to the room temperature, the oxidative reaction is completely stopped, and the stabilization treatment is finished. As a result, an oxide film is formed uniformly and in high density on the grain surface of the ferromagnetic metal powder, and the ferromagnetic metal powder, having no danger of ignition, excellent magnetic characteristics and lesser deterioration with time, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a stabilization treatment of magnetic powder for magnetic recording mainly composed of iron, and in particular to a method for producing a ferromagnetic metal powder having an oxide film.

[Prior Art] In recent years, the trend in magnetic recording has been toward smaller size and higher performance, as typified by 8 mm VTRs and DATs. Along with this, recording media are also required to have higher density and higher output.
Metal iron-based tapes are becoming mainstream in place of the iron oxide-based tapes that have traditionally been used. This metal tape is characterized by high-density recording and reproduction output due to high coercive force and high residual magnetic flux density, and improved signal-to-noise ratio, but the performance of this recording medium is greatly influenced by the characteristics of the magnetic material. The material adopted as this magnetic material is a ferromagnetic metal powder mainly composed of iron with excellent coercive force and saturation magnetization. It is produced by heating and reducing with hydrogen gas, etc.

However, the major drawback of the ferromagnetic metal powder mainly composed of iron used in this magnetic recording medium is that it has poor oxidation resistance. That is, it has a large specific surface area and is chemically extremely active, and when it is taken out into the atmosphere after reduction, heat generation and ignition occur due to a rapid oxidation reaction, and the inherent coercive force and saturation magnetization are significantly lost. Furthermore, deterioration over time due to oxidation is significant, and reliability remains a problem.

Therefore, in order to prevent ignition even if taken out into the atmosphere and to suppress deterioration over time, the surface of the metal particles is slowly oxidized with an oxygen-containing gas in the liquid phase or gas phase after reduction to form an oxide film. A method has been proposed in which stabilization treatment is performed by forming. Conventional stabilization methods include the liquid phase method (Japanese Patent Laid-Open No. 52-850) in which metal magnetic powder is slowly oxidized in an organic solvent.
54), a gas phase method in which gradual oxidation is carried out directly with a gas whose oxygen partial pressure is adjusted (Japanese Patent Application Laid-Open No. 48-79153), and a method combining the two (Japanese Patent Application Laid-open No. 170201-1983) are known.

However, in the liquid phase method, oxidized products of the organic solvent and other impurities remain on the surface of the metal magnetic powder, which may not have a negative effect on the dispersibility during the coating process. The phase law is inside the membrane. Gas phase methods include a fluidized bed method and a fixed bed method, but the former is more advantageous for uniform gradual oxidation. However, this method is also difficult to set flow conditions and control saturation magnetization, and has not yet been completed as a satisfactory method.

[Problems to be Solved by the Invention] When stabilizing ferromagnetic metal powder by gradual surface oxidation using a fluidized bed, it is extremely difficult to set the flow conditions and oxygen partial pressure to uniformly generate an oxide film. Furthermore, if the fluidity is improved, particles collide with each other, causing powdering and agglomeration, which not only forms an oxide film non-uniformly, but also deteriorates the magnetic properties.

The present invention solves these problems using a practical method that can be easily implemented on an industrial scale. A method for producing ferromagnetic metal powder is provided.

[Means for Solving the Problems] The present invention involves fluidizing ferromagnetic metal powder mainly composed of iron using an inert gas at a temperature of 30°C or higher in a fluidized bed, and introducing oxygen gas into the powder at a rate of introduction. are mixed at a constant increase rate of 10 mA/m1n2 or less per 1 g of metal magnetic powder, and when the temperature in the fluidized bed reaches its peak and/or when the oxygen concentration on the outlet side is detected, the temperature is lowered to room temperature and the mixture is uniformly mixed.・This is a method for producing ferromagnetic metal powder with a dense oxide film.

In order to obtain a good fluidization state when performing stabilization treatment in a fluidized bed, it is necessary to use granules with an appropriate particle size range. In the present invention, the particle size range of the ferromagnetic metal powder granules is preferably 0.35 to 5 m [I+ is suitable. That is, if the particle size is less than 0.355 mm, scattering outside the fluidized bed will be significant, whereas if the particle size is 5 mm, unfluidized particles will remain at the bottom of the fluidized bed.

Next, a specific method of stabilization processing will be described. First, this granulated ferromagnetic metal powder having a particle size of 0.355 to 5 mm is brought into a good fluid state with an inert gas. The inert gas used here includes He, Ne, Ar, 110□N2, etc., but it is usually practical to use N2 gas. Next, the temperature of the flowing ferromagnetic metal powder granules is 30~
The temperature of the fluidized bed is controlled to be constant at 120°C.

The temperature at this time determines the amount of oxidation on the surface of the ferromagnetic metal powder,
This is extremely important in controlling saturation magnetization. If the temperature at this time is less than 30°C, the surface oxidation will not be sufficiently performed, and it will catch fire when taken out into the atmosphere. Further, if the temperature at this time exceeds 120° C., the surface oxidation proceeds to the required amount -F, and high saturation magnetization cannot be obtained.

Granules of ferromagnetic metal powder are fluidized with an inert gas, and when the temperature becomes constant, oxygen gas or air is mixed with the inert gas at a constant rate to gradually increase the oxygen partial pressure. At this time, the oxygen increase rate is 10m per gram of ferromagnetic metal powder.
It must be less than l/min2. However, the oxygen increase rate is 0.01 mfl/m1n2 per 1 g of ferromagnetic metal powder.
If it is less than that, it will take a long time to stabilize and is not practical. Conversely, the oxygen increase rate is 10+nQ/min per gram of ferromagnetic metal powder.
If it exceeds min, gradual oxidation becomes uneven and effective stabilization cannot be achieved, making it impossible to obtain high saturation magnetization. In this way, as the oxygen partial pressure is gradually raised from zero, in the extremely active state immediately after stabilization begins, the oxygen is slowly oxidized with very dilute oxygen, and once a certain amount of oxide film has been formed, it becomes even more Oxidized by gas with high oxygen partial pressure, the oxide film becomes stronger. In other words, by gradually increasing the oxygen partial pressure at a constant rate, it is possible to uniformly and constantly oxidize the surface at any point during the stabilization process.

If gradual oxidation is continued uniformly in this manner, the oxidation reaction will automatically stop when the oxide film necessary to stabilize the ferromagnetic metal powder at that temperature is formed. This oxidation reaction can be tracked by monitoring the temperature within the fluidized bed. In other words, if you plot the temperature in the fluidized bed against time, the temperature will rise linearly as shown in Figure 1, which shows that the oxidation reaction is always occurring at a constant rate. . It can also be seen that the oxidation reaction has stopped because the temperature begins to drop rapidly at a certain point, and at the same time the oxygen concentration on the outlet side, which had previously been zero, begins to be detected. As described above, according to the present invention, the end point of the oxidation reaction can be determined very easily by monitoring the temperature and the outlet oxygen concentration.

When the reaction temperature drops and the outlet oxygen concentration is detected,
The stabilization process is completed by cooling the fluidized bed to room temperature to completely stop the oxidation reaction. Also, the end point of the oxidation reaction changes depending on the initial temperature in the fluidized bed, so this temperature is
By controlling the temperature within the range of 0 to 120°C, the saturation magnetization of the ferromagnetic metal powder can be easily controlled. An example is shown in FIG. 2, and the relationship between temperature and saturation magnetization shifts depending on the type and amount of the sintering inhibitor.

By gradually oxidizing the surface while increasing the oxygen partial pressure at a constant rate, it is possible to produce ferromagnetic metal powder with a uniform and dense oxide film, excellent magnetic properties, and good corrosion resistance. . Furthermore, by increasing the oxygen partial pressure at a constant rate, the end point of the stabilization treatment becomes clear, and by preventing unnecessary oxidation, high saturation magnetization can be obtained.

[Examples] Example 1 The fluidized bed used for stabilization was made of Pyrex glass with an inner diameter of 40++, and a glass filter was used for the perforated plate. This fluidized bed has a double pipe structure, and a jacket is installed on the outside to circulate hot water from a constant temperature water tank to control the temperature inside the fluidized bed.

Immediately after reduction, 23 g of granulated ferromagnetic metal powder with a particle size range of 0.355 to 1 mm was transferred to a fluidized bed used for stabilization without exposing it to the atmosphere. The ferromagnetic metal powder used here was AQ203 coated with 3vit96 and reduced at 460°C. The magnetic properties of this ferromagnetic metal powder before stabilization treatment are: coercive force (Hc) of 14200e and saturation magnetization (Is) of 145e when immersed in toluene and air-dried.
mu/g and squareness ratio (SR) were 0.53. The temperature in the fluidized bed was maintained at 40° C., and nitrogen gas was blown in at 1017 ml to bring the ferromagnetic metal powder into a good fluidized state.

Air was continuously mixed with this nitrogen gas at a rate of 10 mQ/in/min, and the oxygen partial pressure was increased at a constant rate. It can be seen that the temperature within the fluidized bed increases with time, indicating that the oxidation reaction is progressing. 54 minutes after stabilization started, the temperature inside the fluidized bed was 52℃
The oxygen concentration on the outlet side was detected. The inlet oxygen concentration at this time was 0.5*. At this point, the temperature of the jacket water was lowered all at once to room temperature (15℃), and after confirming that the oxygen concentration on the inlet and outlet sides were the same, the mixed gas flow rate was increased at 1 (Horn in) every minute. 1
The stabilization process was completed when the air flow rate was increased to 21% and the oxygen concentration of the mixed gas was 21%. Table 1 shows the magnetic properties of the ferromagnetic metal powder having an oxide film after the stabilization treatment and the deterioration rate of saturation magnetization in a one-week corrosion resistance test at 60° C. and 90% relative humidity.

Example 2 Stabilization was carried out in the same manner as in Example 1 except that the temperature in the fluidized bed was 60°C. At this time, the temperature within the fluidized bed reached a peak at 77° C. 61 minutes after the start of stabilization. At this point, the inlet oxygen concentration was 0.6 mm. Table 1 shows the magnetic properties of the ferromagnetic metal powder having an oxide film after the stabilization treatment and the deterioration rate of saturation magnetization in a corrosion resistance test at 60° C. and relative humidity 90*1 week.

Example 3 Stabilization was carried out in the same manner as in Example 1 except that the temperature in the fluidized bed was 80°C. At this time, the temperature within the fluidized bed reached a peak at 94° C. 66 minutes after the start of stabilization. The inlet oxygen concentration at this point was 0.65%. Table 1 shows the magnetic properties of the ferromagnetic metal powder having an oxide film after the stabilization treatment and the deterioration rate of saturation magnetization in a one-week corrosion resistance test at 60° C. and 90% relative humidity.

Comparative Example 1 Magnetic properties and 60
Table 1 shows the deterioration rate of saturation magnetization after a one week corrosion resistance test at 90° C. relative humidity.

Comparative Example 2 Stabilization was carried out in the same manner as in Example 1 except that the temperature in the fluidized bed was 20°C. At this time, the temperature within the fluidized bed reached a peak at 33° C. 47 minutes after the start of stabilization. The inlet oxygen concentration at this point is 0.38! It was lli. However, it caught fire several minutes after being removed from the fluidized bed.

Table 1 shows the magnetic properties of ferromagnetic metal powders with oxide films obtained by the method of each example and comparative example at a maximum applied magnetic field of 10 kiloersteds by VSM and saturation at 60°C, relative humidity 90%, and one week. The deterioration rate of magnetization is shown. Furthermore, from this result, if the temperature is stabilized at a temperature higher than 40° C. in Comparative Example 1, it is easily predicted that the saturation magnetization will become less than 120 emu/g, which is not preferable.

Table 1 [Effects of the Invention] By the method of the present invention, a uniform and dense oxide film is formed on the particle surface of the ferromagnetic metal powder. Magnetic metal powder particles are obtained. Furthermore, the method according to the present invention is extremely easy to determine the end point of the stabilization treatment, and moreover, the treatment can be completed in a short time, making it a practical method even when implemented on an industrial scale.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the temperature in the fluidized bed and the change in the oxygen concentration on the outlet side when the oxygen partial pressure is increased at a constant rate, which is the greatest feature of the present invention. continues to rise and reaches a peak at a certain point, but at the same time the outlet oxygen concentration begins to be detected, indicating that this is the end point of the stabilization process. Further, FIG. 2 is a graph showing the relationship between processing temperature and saturation magnetization in Example 1.2.3, and it can be seen from this that the saturation magnetization can be controlled by the processing temperature. Figure 1 sunny day

Claims (2)

[Claims] 1. When the surface of ferromagnetic metal powder mainly composed of iron is slowly oxidized with oxygen-containing gas in a fluidized bed, the temperature inside the fluidized bed is 3.
Oxygen is introduced into the fluidized bed at a temperature of 0°C or higher and with zero oxygen partial pressure, and the introduction speed is set at 10 m/g of ferromagnetic metal powder.
The temperature increases at a constant rate of increase of l/min^2 or less, and the temperature inside the fluidized bed is lowered to room temperature when the temperature within the fluidized bed reaches a peak and/or when the outlet oxygen concentration is detected. A method for producing a ferromagnetic metal powder having an oxide film. 2. The production of ferromagnetic metal powder having an oxide film according to claim 1, wherein saturation magnetization is controlled by setting the temperature in the fluidized bed at an arbitrary temperature of 30 to 120°C during the gradual oxidation. Method.