JPH0159724B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an industrial method for producing magnetic recording powder containing reduced metallic iron as a main component. Conventionally, magnetic recording powders include iron compound powders such as α-Fe ₂ O ₃ , β-FeOOH, and iron formate, whose primary particles (individual particles without agglomeration) are fine needle-like particles; Furthermore, these compounds contain cobalt,
Powders containing effective amounts of additives such as nickel and boron are in practical use. Such iron compound powder (for example, commercially available γ-
Comparing the magnetic properties of Fe ₂ O ₃ ) and metallic iron powder (Fe) obtained by reducing it with hydrogen, listing Hc, σr, and σs of the hysteresis loop in Figure 1, As shown in the table, the values of Hc, σr, and σs of reduced metal iron are larger than those of γ-Fe ₂ O ₃ , and the magnetic recording performance is greatly improved.
Based on these facts, in order to further improve recording density and increase output in the high frequency range, recently reduced metallic iron obtained by reducing various iron compound powders using reducing gas has been mainly used. The acicular particle material (hereinafter abbreviated as "reduced metal iron powder") used as a component is about to be put into practical use. Practically desirable reduced metallic iron powder is required not only to have large values for Hc, σr, and σs, but also to have a dispersion value (variation) of these magnetic properties within an allowable range, and also to have a variation between lots. It is also required that the variations fall within an acceptable range. However, even if the raw material iron oxide powder is reduced with a reducing gas under appropriate reducing conditions using known techniques, it is rare to obtain the magnetic properties shown in Table 1. In other words, reduced metallic iron powder with the desired magnetic properties can be obtained only when reduction is carried out carefully on a small scale of several hundred grams or less in the laboratory, and it is still possible to obtain reduced metallic iron powder with the desired magnetic properties.
The technology for mass production on an industrial scale beyond units has not been perfected. The inventors investigated the cause of this problem and conducted numerous experiments. As a result, the inventors found that in order to obtain reduced metal iron powder with predetermined magnetic property values, it is necessary to maintain the shape of the primary particles of the raw material and sinter the powder. The present invention was completed by discovering that the reduction should be carried out without causing any oxidation. That is, the purpose of the present invention is to reduce fine particles of iron compound powder as a raw material using a reducing gas containing hydrogen as a reducing component to obtain reduced metal iron powder, which is a high-performance magnetic recording powder. The goal is to provide technology for mass production on an industrial scale. The raw material iron compound powder used in the present invention is α-
Fe ₂ O ₃ , α-FOOH, β-FeOOH, γ-Fe ₂ O ₃ ,
Fine powder of compounds of iron and nonmetallic elements such as γ-FeOOH, iron oxalate, iron formate, etc., as well as cobalt, nickel, tin, silver, copper, titanium, silicon, etc.
It means a powder to which an effective amount of at least one of aluminum arsenic, phosphorus, and boron is added. Particularly preferred are iron oxides and iron hydroxides. The reducing gas used in the present invention is pure hydrogen gas or hydrogen with nitrogen, carbon dioxide gas, etc. as an inert component.
A mixed gas containing lower hydrocarbon gas etc. can be used, but in the latter case, hydrogen needs to be at least 50% by volume. In the case of any reducing gas, the water content (H ₂ O) in the gas is always below 10,000 ppm, especially
It is preferably 6000ppm or less. In the method of the present invention, raw material powder such as iron oxide and reducing gas are heated at a pressure of 0.05 to 50 kg/cm ² (gauge) and a temperature of
Reduction is carried out by contacting under reducing conditions at 150 to 500°C, preferably 200 to 450°C. At this time, the raw material must be stirred while a reducing gas is aerated at a superficial linear velocity of 1 to 20 cm/sec. The reducing gas is held dispersed in the powder, and a homogeneous mixture of the powder and the reducing gas is formed throughout the reaction period, the viscosity of which is significantly lower than that of the powder alone. However, if the aeration speed of the reducing gas exceeds 20 cm/sec, the gas passing through the powder will form a state of bubbles.
Moreover, coalescence of bubbles occurs and the scattering of fine particles becomes noticeable. Furthermore, when the ventilation rate is less than 1 cm/sec, the amount of the reducing gas dispersed in the powder decreases, and a homogeneous mixture of the powder and the reducing gas is not formed. The reason why such reduction conditions are essential in the present invention is to complete the reduction reaction while preventing the particle shape of the raw material powder from being damaged due to sintering or disintegration of the raw material during reduction. It is. In other words, using iron oxide as an example, the reaction of reducing iron oxide to iron consists of two steps: 3Fe ₂ O ₃ +H ₂ →2Fe ₃ O ₄ +H ₂ O (1) Fe ₃ O ₄ +4H ₂ →3Fe＋4H ₂ O (2) When reduction begins, reaction (1) occurs first. (1) is an exothermic reaction that has a relatively fast reaction rate and proceeds in a short period of time. Moreover, a large volume change (expansion) occurs in the powder at this stage. Next, reaction (2) occurs. (2) is an endothermic reaction, the reaction rate is slow, it takes a long time to complete the reduction, and the volume contracts, contrary to (1). Since sintering is likely to occur in step (1), it must be carried out under relatively low temperature conditions or under conditions where the reducing gas flow rate is increased to facilitate the removal of reaction heat. However, since the particles of the raw material iron oxide powder are fine, the particles tend to scatter when the flow rate of the reducing gas is increased. In most cases, the primary particle size of the raw material is 0.05 to 5 microns in length and 0.01 to 0.2 microns in diameter. In reality, secondary particles are aggregated primary particles, but approximately 99% by weight are 40 microns or less. Therefore, in order to prevent particle sintering during reduction, for example, if a conventional fluidized bed method is used and the reducing gas flow rate is set to about 0.5 cm/sec, particles with a particle size of 10 microns or less will be reduced. Scattered outside the reaction system. Alternatively, in a rotary furnace where the flow direction of the reducing gas and the flow direction of the powder intersect, the powder is scattered due to the gas flow, making it difficult to continue the reduction reaction.
On the other hand, when aerating using the conventional fixed bed method to suppress particle scattering, the reactor must be equipped with a function that can follow volumetric changes such as expansion and contraction. Such a reactor has a complicated structure, which causes problems in filling raw materials and taking out products, and is not suitable for mass production technology, which is the object of the present invention. Furthermore, since sintering is likely to occur even in step (2), in Japanese Patent Application No. 12874/1987, the hydrogen-containing gas space velocity (SV) is given as at least SV=3000/θ−3.7+1200 (where θ is time). It states that the amount of gas required is
However, the layer thickness of the fixed layer that can be implemented with the above SV is only 80 mm at most. When reducing with a layer thickness greater than this, local particle movement occurs due to expansion and contraction of the particle volume, creating a constant gas passage.
As a result, there are places where the temperature distribution within the layer is uneven and places where the H ₂ O partial pressure increases abnormally, causing sintering. The state of sintering can be easily confirmed by looking at changes in shape, size, and agglomeration status in electron micrographs.
Of course, this can also be determined by measuring magnetic properties. In the reducing conditions of the present invention, by stirring the raw material powder in an aerated state within a range in which it does not start flowing due to reducing gas, hydrogen is supplied, reaction heat is removed and supplied, and the partial pressure of the produced water is The technically difficult problem of securing the reducing gas supply rate required to prevent particle sintering and preventing particle scattering due to factors such as reduction can be solved on an industrial scale. In the method of the present invention, the reducing gas is supplied by blowing it into the raw material powder through a dispersion plate composed of a sintered plate, a perforated plate, a wire mesh, or the like. Alternatively, the air may be blown from a sparger with a large number of holes installed near the stirring blades for stirring the powder, or by making the shaft and arm to which the stirring blades are attached hollow and allowing the reducing gas to flow through them. It is also possible to adopt a method in which the reducing gas is ejected from a large number of holes provided in the pores. In the latter case, where gas is not blown through a distribution plate, a non-porous bottom plate may be used instead of a distribution plate. For stirring used in the method of the present invention, a method commonly used for stirring powder is used. for example,
Mixed stirring arm types such as chi type, ribbon type, and squid type are suitable. In this case, it is better to install the stirring blade as close to the above-mentioned dispersion plate or bottom plate as possible. Note that the blade shape, number of blades, rotation speed, etc. can be appropriately selected depending on the properties of the raw material powder. In the method of the present invention, the lower limit of the pressure of the reduction reaction
0.05Kg/cm ² (gauge) means that the reducing gas is released into the atmosphere after passing through a series of devices such as a semi-fluidized bed reactor, dust collectors such as cyclones and bag filters, a heat exchanger, and conduits that connect them. This is the minimum pressure required to be released. When most of the used reducing gas is processed and recycled for reuse, it passes through a series of devices such as a reactor, heat exchanger, dust collector, dehumidifier, etc. from the circulation blower outlet to the circulation blower inlet. This is the minimum pressure required to compensate for the pressure loss required for one cycle. The higher the reaction pressure, the faster the movement speed of hydrogen molecules diffusing from the surface of the powder particle to the inside, giving the desired effect of promoting the reduction reaction. However, on the other hand, water (H ₂ O)
This has the undesirable effect of slowing the rate of movement of molecules from inside the particle through the surface and into the body of gas. At pressures exceeding 50Kg/cm ² (gauge), the speed of movement of water molecules from inside the particles becomes noticeably lower. Therefore, the practical reduction pressure is 0.05~50Kg/
cm ² (gauge), especially 2 to 25
Kg/cm ² (gauge) is a preferred range. Regarding the reaction temperature used in the method of the present invention, the lower the temperature, the slower the reduction reaction progresses, and conversely, the higher the temperature, the faster the reduction reaction progresses. However, if the reaction proceeds rapidly, sintering of the particles easily occurs and the magnetic properties deteriorate. Therefore, the practical range of reaction temperature is
It is necessary to keep the temperature between 150 and 500℃, especially
The preferred range is 200-380°C. By using the method of the present invention, it becomes possible to produce reduced metallic iron powder from iron compound fine powder on an industrial scale, which was previously possible only on a laboratory scale.
Moreover, the magnetic properties of the obtained reduced metal iron are excellent, and there is little variation in quality. Therefore, the method of the present invention has high industrial value as a method for producing magnetic recording powder. The present invention will be explained in more detail with reference to Examples. Example 1 In a reactor with an inner diameter of 250 mm, a stirrer blade diameter of 190 mm, and a sintered metal plate as a gas dispersion plate, commercially available acicular γ-
3Kg of Fe ₂ O ₃ was charged. This has a temperature of 350℃ and a pressure of 2
Under conditions of Kg/cm ² (gauge), hydrogen gas with a purity of 99% by volume was introduced as a reducing gas, and the mixture was stirred.
It was reduced for 7.5 hours. The magnetic property values after completion of the ring element are shown in Experiment No. 1 in Table 2.
Shown below. Comparative Example 1 The same amount of the same raw material iron oxide powder as in Example 1 was charged into the same reactor as in Example 1 but without a stirring blade, and reduction was performed under the same conditions for the same time. Table 2 shows the magnetic properties. The values shown in Experiment No. 2 were obtained. Not only were the characteristic values lower than in Example 1, but considerable variation was observed depending on the location within the reactor. Comparative Example 2 A fixed bed reactor (no stirring) with a diameter of 250 mm and using a sintered metal plate as a gas dispersion plate was filled with 5.2 kg (layer thickness 150 mm) of the same raw material iron oxide powder as in Example 1, and reduced. Reduction was carried out under the same reducing conditions as in Example 1 at a gas flow rate of 5 cm/sec. At this flow rate, the partial pressure of water (H ₂ O) increased in the gas flow direction, and there appeared to be places where it reached 10,000 ppm or more, and the water (H 2 O) partial pressure increased in the direction of the gas flow. The obtained magnetic property values vary depending on the material, and the lower layer with the lowest water pressure has the best magnetic properties. The results are shown above, middle, and below of Experiment No. 3 in Table 2. Example 2 An experiment was conducted in which three types of raw material iron oxide powder (lot numbers A, B, and C) were charged into the apparatus used in Example 1. The reduction conditions are exactly the same as in Example 1. The experimental results are shown in experiment numbers 1 to 9 in Table 3. Clearly, the best magnetic properties are obtained when a reactor with stirring blades is used. Comparative Example 3 The above-mentioned A, B,
Each raw material of C was charged and reduced under the same conditions as in Example 1. The results are shown in Table 3. In the case of the apparatus used in Experiment No. 3, the measured values are for the top layer sample. Example 3 The raw material iron oxide powder A used in Example 2 was placed in a reactor having a diameter of 1,000 mm, a total height of 4,000 mm, a structure in which a perforated plate and a wire mesh were stacked as a gas distribution plate, and a stirring blade with a blade diameter of 800 mm. 30Kg charged, reducing gas flow rate 10cm/
sec, temperature 280℃, pressure 11Kg/cm ² (gauge),
Reduction was performed for 10 hours using hydrogen containing 2% by volume of CH ₄ and 0.1% by volume of CO ₂ . Experiment No. 1 in Table 4 shows the magnetic properties without reduction, and Experiment No. 2, 3, and 4 show the magnetic properties after reduction in different batches. Reproducible data were obtained.

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[Table] Comparative Example 4 Into the same reactor as in Example 3, 30 kg of the raw material iron oxide powder A used in Example 2 was charged, and hydrogen gas with a purity of 99% was used as the reducing gas at a flow rate of 0.5 cm/sec. temperature
Reduction was carried out for 20 hours at 280° C. and a pressure of 11 Kg/cm ² (gauge). In the case of this comparative example, the hydrogen gas flow rate was lower than the specified value, and the amount of dispersed reducing gas in the powder was significantly reduced, resulting in significant reduction unevenness that was unfavorable in terms of product quality. As shown in Table 5, it was found that the magnetic properties of the obtained products differed greatly depending on the top, middle, and bottom (equally divided into three in the layer thickness direction) positions in the layer thickness direction.

[Table] Comparative Example 5 30 kg of raw material iron oxide powder A used in Example 2 was charged into the same reactor as in Example 3, and hydrogen gas with a purity of 99% was used as the reducing gas at a flow rate of 10 cm/sec. I tried to carry out the reduction reaction under the conditions of temperature 280℃ and pressure 0.03Kg/cm ² (gauge), but the pressure drop due to the reactor, cyclone dust collector, heat exchanger, these conduits, etc. was 0.045Kg. /cm ² (gauge), gas could not be circulated satisfactorily, and the reduction reaction could not actually be carried out.

[Brief explanation of drawings]

Figure 1 shows a hysteresis loop showing the magnetic properties of magnetic powder, where σr represents residual magnetic susceptibility, σs represents saturation magnetic susceptibility, and Hc represents coercive force. FIG. 2 is a diagram showing the main parts of a reactor in which a dispersion plate is used as an embodiment of the present invention.
Figure 3 is a diagram showing the main parts of a reactor when a hollow stirring shaft and stirring blades are used as an embodiment of the present invention, and Figure 4 is a diagram showing the main parts of a reactor when a hollow stirring shaft and stirring blade are used as an embodiment of the present invention.
It is a diagram showing the main parts of a reactor when a sparger is used. 1... Reactor main body, 2... Stirring blade, 3, 3'...
...Reducing gas conduit, 4...Gas distribution plate, 5...Reducing gas outlet, 6...Gas spargeer, 7
...Bottom plate, 8...Stirring shaft.

Claims (1)

[Scope of Claims] 1. In a method for producing magnetic recording powder containing metallic iron as a main component by reducing iron compound powder using a reducing gas, applying a gauge pressure of 0.05 to 50 kg/cm ² While stirring the iron compound powder with a stirring blade under pressure reaction pressure and heating, the reducing gas is added at a superficial linear velocity of 1 to 20 cm/sec, at which the powder does not start flowing with gas alone. A magnetism that substantially prevents both sintering and scattering of the powder, characterized in that the powder is vented from the lower part of the powder, and the powder is reduced while forming a homogeneous mixture of the powder and the aerated reducing gas. Method for producing recording powder. 2. The method for producing magnetic recording powder according to claim 1, wherein the reduction reaction pressure is a gauge pressure of 2 to 25 Kg/cm ² . 3. The method for producing magnetic recording powder according to claim 1 or 2, wherein the reduction reaction temperature is 150 to 500°C.