JPH0544414B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing magnetic oxide particle powder for magnetic recording, which has substantially high density with no pores on the particle surface or inside the particle, and It is an object of the present invention to provide magnetic iron oxide particles consisting of magnetite particles or maghemite particles, which have a uniform particle size, do not contain dendritic particles, and exhibit a spindle shape with a small axis ratio (long axis: short axis). purpose. [Prior Art] In recent years, as magnetic recording and reproducing equipment has become longer recording time, smaller and lighter, there has been an increasing need for higher performance recording media such as magnetic tapes and magnetic disks. That is, high recording density, high sensitivity characteristics, high output characteristics,
and low noise characteristics are required. In order to satisfy the above-mentioned requirements for magnetic recording media, the characteristics of magnetic material particles required are excellent dispersibility, high coercive force, and a small axial ratio (long axis: short axis). It is. This fact can be seen, for example, in "Development of magnetic materials and high dispersion technology of magnetic particles" published by Sogo Gijutsu Center Co., Ltd.
(1982), p. 312, ``The conditions for high-density recording on coated tape are:
It is possible to maintain high output characteristics with low noise, but for this purpose it is necessary that both the coercive force Hc and the residual magnetization Br be large, and that the thickness of the coating film be thinner. ” and Japanese Patent Application Laid-Open No. 183626/1983, “Also, in recent years, the idea of perpendicular magnetization recording has been introduced, and there is also a proposal to effectively use the residual magnetization component in the direction perpendicular to the surface of the magnetic recording medium. According to magnetization recording, the recording density defined above increases,
...""...the particle size...its longitudinal/axial ratio is 1
By making it a short shape exceeding 3 and below 3,
...Suppresses the tendency of particles to lie in-plane and become oriented, such as in-plane orientation due to thinning of the coating film in the thickness direction during application and drying, and orientation in the casting direction due to flow during application. Moreover, it is characterized by being able to actively increase the perpendicular residual magnetization if necessary. ” is as stated. In addition, the residual magnetization Br of a magnetic recording medium depends on the dispersibility of the magnetic particles in the vehicle, the orientation and filling properties in the coating film, and in order to improve these characteristics, it is necessary to The magnetic particles to be dispersed in the particles have substantially high density with no pores on the particle surface or inside the particles, have uniform particle size, do not contain dendritic particles, and have a high density of particles. In terms of shape, spindle-shaped particles are required. As is well known, the magnitude of the coercive force of magnetic particles depends on shape anisotropy, crystal anisotropy, strain anisotropy, exchange anisotropy, or their interaction. Currently, acicular magnetite particles or acicular maghemite particles, which are currently used as magnetic particles for magnetic recording, utilize the anisotropy derived from their shape, that is, the axial ratio (long axis : A relatively high coercive force is obtained by increasing the short axis). These known acicular magnetite particle powders,
Alternatively, acicular maghemite particles utilize their shape anisotropy to obtain a relatively high coercive force, but by adding Co to these particles,
It is generally known that the coercive force can be further improved by utilizing the crystal anisotropy. Known acicular magnetite particles or maghemite particles are generally produced by air oxidizing a colloidal aqueous solution with a pH of 11 or higher containing ferrous hydroxide particles obtained by reacting an aqueous ferrous salt solution with an alkali ( The acicular goethite particles obtained by this process (usually called a "wet reaction") are heated in air at around 300°C, dehydrated to form hematite particles, and further heated at 300 to 400°C in a reducing gas such as hydrogen. The acicular crystal magnetite particles are obtained by reducing the acicular crystal magnetite particles, or by subsequently oxidizing them in air at 200 to 300°C to obtain the acicular crystal maghemite particles. [Problems to be solved by the invention] There are no pores on the particle surface or inside the particle, and the particle has a substantially high density, and the particle size is uniform and there are no dendritic particles mixed therein. Magnetic iron oxide particles exhibiting a spindle shape with a small axis ratio (major axis: short axis) are currently most in demand, and in order to obtain magnetic particles with such characteristics, naturally, The starting material particles themselves have substantially high density with no pores on the particle surface or inside the particles, have a uniform particle size, are free of dendritic particles, and have an axial ratio ( long axis: short axis)
It is necessary that the particles exhibit a small spindle shape, but the particle powder obtained by the above-mentioned known method for producing acicular goethite particles as a starting material has an axial ratio (long axis: short axis) of The particles exhibit a needle-like morphology with a ratio of 10:1 or more, and dendritic particles are mixed, and judging from the particle size, it is difficult to say that the particles have a uniform particle size. On the other hand, as a method for producing goethite particles, there is a method described in JP-A-50-80999. That is,
The method described in Japanese Patent Application Laid-Open No. 50-80999 is based on the method described in JP-A No. 50-80999.
This is a method of oxidizing an aqueous solution containing FeCO <sub>3</sub> by passing an oxygen-containing gas through it. When using this method, the particle size is uniform, dendritic particles are not mixed,
Goethite particles exhibiting a spindle shape are obtained. However, the above-mentioned known method or the above-mentioned Japanese Patent Application Laid-Open No.
When magnetic iron oxide particles are obtained by a conventional method using the acicular goethite particles obtained by the method described in Publication No. 80999 as a starting material, the hematite particles obtained by heating and dehydrating the goethite particles are A large number of pores are generated on the particle surface and inside the particle, and then the hematite particle is reduced, or
It is observed that the magnetite particles or maghematite particles obtained by further oxidation if necessary also have a large number of pores distributed on the particle surface and inside the particles. In this way, magnetic iron oxide particles having a large number of pores on the particle surface and inside the particles have a coercive force Hc
Moreover, the dispersion in the vehicle is poor. This fact is evidenced, for example, in Japanese Patent Application Laid-open No. 55-47227, which states, ``As a magnetic powder for magnetic recording, γ-Fe ₂ O ₃
(maghemite) particles have been widely used, and efforts have been made to increase the _{coercive force} . , γ-FeOOH), it is necessary to maintain the needle-like morphology and eliminate the dehydration pores (vacancies). These dehydration pores (vacancies) are generated during dehydration of iron oxyhydroxide, which is the mother salt, and if they remain in the product γ-Fe ₂ O ₃ particles, they reduce the coercive force. ”, and
Electrochemistry and Industrial Physical Chemistry Volume 38, No. 7 (1970
2007), p. 544, ``...Although various dispersion techniques in vehicles have been considered, the dispersion in the manufactured γ-Fe ₂ O ₃ tape is still poor.
γ-Fe ₂ O ₃ has holes (vacancies) as shown in Figure 17, and magnetic poles appear on the edges of the holes, creating a Lorentz magnetic field. is attracted, lowering the static magnetic energy, and forming a chestnut burr-shaped agglomerate, which is clear from the description "...". As mentioned above, magnetic iron oxide particles suitable for high-density recording are required to have a small axial ratio (long axis: short axis); Because small magnetic iron oxide particles cannot utilize the anisotropy derived from their shape,
Coercivity Hc of only about 200 Oe or less can be obtained, and there is therefore a strong demand to improve coercivity as much as possible by eliminating pores generated on the particle surface and inside the particles. Attempts have been made to eliminate pores generated on the surface and inside of magnetic iron oxide particles. For example, in Japanese Patent Publication No. 38-26156, there is a method for reducing the composition to magnetite at low temperatures and then changing its composition. A method is described in which annealing is performed at a temperature of 800°C or more and 1000°C or less in a vacuum, in a hydrogen stream, or in a carbon dioxide gas stream so as not to damage the steel. In addition, the Powder and Powder Metallurgy Association Spring Conference 1960 Conference Abstracts 2-6 states that as the dehydration temperature of acicular goethite increases, the pores on the surface of the acicular hematite particles and inside the particles increase. However, it has been reported that when the dehydration temperature is higher than 700°C, the pores disappear, but sintering progresses and the acicular crystal grains collapse. In both of the above methods, it is necessary to heat the particles at high temperatures in order to eliminate pores generated on the surface and inside the particles.As a result, sintering occurs between the particles and between the particles, which is reduced and oxidized. The coercive force of the magnetic iron oxide particles obtained by this process is extremely low, and the dispersibility in a vehicle when producing a magnetic paint is also poor. On the other hand, this method does not eliminate the pores once generated on the particle surface and inside the magnetic iron oxide particles.
Attempts have also been made to obtain magnetic iron oxide particles using particles without pores on the surface or inside the particles as a starting material. In this method, acicular hematite particles are directly generated from an aqueous solution, and the acicular hematite particles are used as a starting material to be reduced and oxidized to obtain acicular magnetic iron oxide particles. That is, as mentioned above, the pores on the particle surface and inside the particles are generated by dehydration when acicular goethite particles are heated and dehydrated to form acicular hematite particles. If crystalline hematite particles are produced, the dehydration step can be omitted, and therefore acicular hematite particles with no pores on the particle surface or inside the particles can be obtained, and the hematite particles can be used as a starting material. The acicular crystalline magnetic iron oxide particles obtained by reduction and oxidation as acicular crystals also have no pores at all on the particle surface or inside the particle. As a method for directly producing acicular hematite particles from an aqueous solution, for example, Japanese Patent Publication No. 55-22416
There is a method described in the publication. In other words, the special public official 1977-
The method described in Publication No. 22416 involves the use of ferric hydroxide, citric acid or/and its salt, and an alkali compound.
In the method of heat treating an aqueous slurry in which the components coexist, the amount of alkali compound is set to an amount equivalent to setting the pH of the aqueous slurry in which the three components coexist at 25°C to 10 to 13, and the heat treatment temperature is set to 100 to 250°C. Acicular hematite particles are obtained by this method. However, when using this method, 100
It is not industrially or economically viable because it requires a high temperature of ℃ or higher and requires a special device called an autoclave. As a method for producing hematite particles from an aqueous solution at a temperature of 100°C or lower, there is a method described in JP-A-51-8193. That is, JP-A-51-8193
The method described in the publication involves adding alkali hydrogen carbonate alone or both alkali hydrogen carbonate, alkali carbonate, and alkali hydroxide to a ferrous salt solution, at a pH of 7 to 11, and a temperature of 65 to 100 °C. The oxidation reaction is carried out at a temperature of °C. In the case of this method, the shape of the hematite particles produced is spherical, and other types of particles other than hematite particles are produced and mixed. As is clear from the above, there are no pores on the particle surface or inside the particle, and the density is substantially high, the particle is uniform, there are no dendritic particles mixed in, and the axial ratio is low. There is a strong desire to establish a method for producing magnetic iron oxide particles having a spindle shape with a small (long axis: short axis). [Means for Solving the Problems] The present inventors have developed particles that have substantially high density with no pores on the surface or inside the particles, have uniform particle sizes, and have dendritic particles mixed therein. I haven't...
Moreover, as a result of various studies to obtain spindle-shaped magnetic iron oxide particles with a small axis ratio (long axis: short axis), we found that
When oxidizing an aqueous solution containing FeCO ₃ obtained by reacting Fe with an alkali carbonate at a ratio of 1.1 to 1.7 equivalents in terms of CO ₃ ,
Into any of the ferrous salt aqueous solution, the alkali carbonate aqueous solution, and the FeCO ₃ -containing aqueous solution before oxidation by passing oxygen-containing gas through the solution.
When 0.1 to 5.0 atomic % of water-soluble magnesium salt and 0.1 to 1.5 mol % of citric acid or its salt are added to Fe in terms of Mg, the particle size of the starting material is uniform and dendritic particles are added. We have obtained the new knowledge that it is possible to directly produce spindle-shaped hematite particles with a small axis ratio (long axis: short axis) from an aqueous solution at 100°C or lower, and we have developed the present invention. It is completed. That is, the present inventor obtained an alkali carbonate obtained by reacting an aqueous ferrous salt solution with an alkali carbonate in an amount of 1.1 to 1.7 equivalents in terms of CO ₃ to Fe in the aqueous ferrous salt solution.
When the aqueous solution containing FeCO ₃ is oxidized by passing an oxygen-containing gas through the aqueous solution, either the ferrous salt aqueous solution, the alkali carbonate aqueous solution, or the aqueous solution containing FeCO ₃ before being oxidized by passing the oxygen-containing gas through the aqueous solution. 0.1 to 5.0 in terms of Mg to Fe in the liquid
For atomic% water-soluble magnesium salts and Fe
Adding 0.1 to 1.5 mol% citric acid or its salt,
Next, spindle-shaped hematite particles are generated from the aqueous solution by passing oxygen-containing gas through oxidation, and the spindle-shaped hematite particles are heated and reduced in a reducing gas to produce spindle-shaped magnetite. A method for producing magnetic iron oxide particle powder consisting of spindle-shaped magnetic iron oxide particles, which is formed into particles or further oxidized to form spindle-shaped maghemite particles, and a ferrous salt aqueous solution. In oxidizing the aqueous solution containing FeCO ₃ obtained by reacting Fe in the ferrous salt aqueous solution with an aqueous alkali carbonate solution at a ratio of 1.1 to 1.7 equivalents in terms of CO ₃ , an oxygen-containing gas is passed through the aqueous solution. The ferrous salt aqueous solution, the alkali carbonate aqueous solution, and the aqueous solution containing FeCO ₃ before oxidation by passing oxygen-containing gas in advance contain 0.1 to 0.1 to Mg equivalent to Fe in the aqueous solution containing FeCO3.
Add 5.0 at % of water-soluble magnesium salt, 0.1 to 1.5 mol % of citric acid or its salt to Fe, and 0.5 to 10.0 at % of water-soluble cobalt salt in terms of Co to Fe, and then add oxygen-containing gas. By aeration and oxidation, spindle-shaped hematite particles containing Co are generated from the aqueous solution, and the Co
The spindle-shaped hematite particles containing Co are thermally reduced in a reducing gas to produce spindle-shaped magnetite particles containing Co, or they are further oxidized to become spindle-shaped containing Co. This is a method for producing magnetic oxide particle powder consisting of spindle-shaped magnetic iron oxide particles by forming maghemite particles. [Operation] First, the most important point in the present invention is that the particles have substantially high density with no pores on the surface or inside the particles, have uniform particle sizes, and have dendritic particles mixed therein. Not only that, but also the axial ratio (long axis: short axis)
In order to produce magnetic iron oxide particles having a small spindle shape, the starting raw material is one that has substantially no pores on the particle surface or inside the particle, has a substantially high density, has a uniform particle size, and has a dendritic structure. The point is to use hematite particle powder that does not contain mixed particles and has a spindle shape with a small axis ratio (long axis: short axis). In the present invention, hematite particles with no pores on the particle surface or inside the particles are obtained by directly generating spindle-shaped hematite particles from an aqueous solution at 100°C or lower. It does not require special equipment such as an "autoclave". Further, in the present invention, only spindle-shaped hematite particles can be produced from an aqueous solution at 100° C. or lower. In the present invention, it is not yet clear why only spindle-shaped hematite particles with a small axial ratio can be produced, but the present inventor has developed a ferrous salt aqueous solution and a ferrous salt aqueous solution. In
When oxidizing an aqueous solution containing FeCO ₃ obtained by reacting Fe with an aqueous alkali carbonate solution at a ratio of 1.1 to 1.7 equivalents in terms of CO ₃ , water-soluble magnesium salt is used alone. When citric acid or its salt is added alone, a mixture of spindle-shaped goethite particles and spherical hematite is produced, and when citric acid or its salt is added alone, granular magnetite is formed in the spindle-shaped hematite particles. Since particles were mixed, it is thought that this is due to a synergistic effect between the water-soluble magnesium salt and citric acid or its salt. In fact, as shown in the Examples below, according to the method of the present invention, spindle-shaped hematite particles with an extremely small axis ratio of about 4:1 or less of major axis to minor axis can be obtained. In the present invention, magnetic iron oxide particles having a higher coercive force are produced by adding a water-soluble cobalt salt in the reaction for producing spindle-shaped hematite particles, thereby producing spindle-shaped hematite particles containing Co. and reducing the Co-containing spindle-shaped hematite particles or, if necessary, further oxidizing them to obtain Co-containing spindle-shaped magnetic iron oxide particles,
The obtained spindle-shaped magnetic iron oxide particles containing Co are not only isotropic in shape but also magnetically isotropic, making them suitable as magnetic iron oxide particles for magnetic recording. be. Next, various conditions for implementing the present invention will be described. Examples of the ferrous salt aqueous solution used in the present invention include a ferrous sulfate aqueous solution and a ferrous chloride aqueous solution. As the alkali carbonate used in the present invention, sodium carbonate, potassium carbonate, ammonium carbonate, etc. can be used alone or in combination. The ferrous salt aqueous solution and the alkali carbonate may be added either first or simultaneously. The reaction temperature in the present invention is 70 to 100°C.
If the temperature is lower than 70°C, goethite particles will be mixed in the hematite particles. Although the purpose of the present invention can be achieved even when the temperature exceeds 100°C,
It requires special equipment such as an autoclave and is not economical. The amount of alkali carbonate in the present invention can be used in a ratio of 1.1 to 1.7 equivalents based on CO ₃ based on Fe. If the amount is less than 1.1 equivalent, granular magnetite particles will be mixed in the spindle-shaped hematite particles. Although the object of the present invention can be achieved even if the amount exceeds 1.7 equivalents, there is no point in adding more than necessary and it is not economical. As the water-soluble magnesium salt in the present invention, magnesium sulfate, magnesium chloride, etc. can be used. The amount of water-soluble magnesium salt in the present invention is 0.1 to 5.0 atomic % based on Fe in terms of Mg.
If it is less than 0.1 atomic %, granular magnetite particles will be mixed in the spindle-shaped hematite particles. Although the purpose of the present invention can be achieved even when the content exceeds 5.0 at %, there is no point in adding more than necessary. The water-soluble magnesium salt in the present invention has a synergistic effect with citric acid or its salt, which affects the type and morphology of the produced particles, and therefore, the production reaction of spindle-shaped hematite particles is It must be added before starting and can be added to any of the ferrous salt aqueous solution, the aqueous alkali carbonate solution, and the aqueous solution containing FeCO ₃ before oxidation by bubbling the oxygen-containing gas. In the present invention, citric acid or a salt thereof can be used. Here, the salt includes sodium citrate, potassium citrate, lithium citrate, ammonium citrate, and the like. The amount of citric acid or its salt added in the present invention is 0.1 to 1.5 mol% based on Fe. When the amount is less than 0.1 mol%, spindle-shaped goethite particles and granular hematite particles coexist. 1.5 mol%
If it exceeds this, fine amorphous particles will be produced. Considering the particle morphology of the produced hematite particles, the content is preferably 0.1 to 1.0 mol%. The citric acid or its salt in the present invention has a synergistic effect with the water-soluble magnesium salt, which affects the type and morphology of the particles produced. Therefore, the reaction for producing spindle-shaped hematite particles is It must be added before starting, and can be added to any of the ferrous salt aqueous solution, alkali carbonate aqueous solution, and the aqueous solution containing FeCO ₃ before oxidation by bubbling oxygen-containing gas. In the present invention, the water-soluble magnesium salt and citric acid or its salt may be added in either order or simultaneously. The water-soluble cobalt salt in the present invention includes:
Cobalt sulfate, cobalt chloride, etc. can be used. The amount of water-soluble cobalt salt added is
In terms of conversion, it is 0.5 to 10.0 atomic%. If it is less than 0.5 at %, the effect of improving the coercive force of the resulting spindle-shaped magnetite particles or maghemite particles cannot be sufficiently achieved. If it exceeds 10.0 at %, the temperature stability of the resulting spindle-shaped magnetite particles or maghemite particles will deteriorate. The water-soluble cobalt salt in the present invention needs to be contained in the spindle-shaped hematite particles to be produced, and therefore, it should be added before the production reaction of the spindle-shaped hematite particles starts. It is necessary to prepare the ferrous salt aqueous solution, alkaline carbonate aqueous solution and oxygen-containing gas before oxidation.
It can be added to any aqueous solution containing _FeCO3 . In addition, in the present invention, Fe such as Mg, Al, Cr, Zn, Ni, etc., which have been conventionally added during the production of starting material particles to improve various properties of magnetic iron oxide particles, are used.
Other metals can be added. The heating reduction treatment and oxidation treatment in a reducing gas in the present invention can be performed by conventional methods. In addition, the hematite particles, which are the starting material, can be coated with a substance that has a sintering prevention effect, such as Si, Al, or P compounds, which is usually carried out prior to heat treatment, so that they have better dispersibility. Magnetic iron oxide particles having a spindle shape can be obtained. [Example] Next, the present invention will be explained with reference to Examples and Comparative Examples. In addition, the axial ratio (long axis: short axis) and long axis of the particles in the following Examples and Comparative Examples are both shown as average values of numerical values measured from electron micrographs. The magnetic properties of the magnetic iron oxide particles were measured using a vibrating sample magnetometer VSM P-1 type (manufactured by Toei Kogyo).
Measurements were made with a measurement magnetic field of 5KOe. <Production of spindle-shaped hematite particles> Examples 1 to 17, Comparative Examples 1 to 3; Example 1 Magnesium sulfate (MgSO ₄ 7H ₂ O) containing 3.0 atomic % in terms of Mg based on Fe Sodium citrate 3.3 to contain 16.8g and 0.5 mol% for Fe
Ferrous sulfate obtained by adding g
aqueous solution 1.5, 1.12 mol/Na ₂ CO ₃ aqueous solution 3.0
(corresponding to CO ₃ /Fe = 1.4 equivalent), and FeCO ₃ was generated at a temperature of 80°C. Particles were generated by blowing 20 air per minute into the aqueous solution containing FeCO ₃ at a temperature of 80° C. for 3.3 hours. At the end point of the oxidation reaction, a part of the reaction solution is extracted and adjusted to acidity with hydrochloric acid, and then Fe ²⁺ is added using red blood salt solution.
Judgment was made based on the presence or absence of a blue color reaction. The produced particles were separated, washed with water, dried, and crushed by a conventional method. As is clear from the electron micrograph (×20000) shown in Figure 1, this particle powder consists of spindle-shaped particles with an average long axis of 0.30 μm and an axial ratio (long axis: short axis) of 4.0:1. There were no pores on the particle surface or inside the particle, the particle size was uniform, and dendritic particles were not mixed. Moreover, the X-ray diffraction diagram of this particle is shown in FIG. As is clear from FIG. 2, peak A is a peak indicating hematite, and it can be seen that it consists only of hematite. Examples 2 to 17 Type of ferrous salt, type of alkali carbonate, concentration and equivalent ratio, type of water-soluble magnesium salt, amount and time of addition, type of citric acid or its salt, amount and timing of addition, Co Except for various changes in the presence or absence of addition of ions, the amount of addition, and the reaction temperature,
Particles exhibiting a spindle shape were produced in the same manner as in Example 1. As a result of X-ray diffraction, it was confirmed that all the particles obtained in Examples 2 to 17 were only hematite particles. Table 1 shows the main manufacturing conditions and characteristics of the produced hematite particles. Electron micrograph of spindle-shaped hematite particles containing Co obtained in Example 16 (×
20000) is shown in FIG. 3, and the X-ray diffraction diagram is shown in FIG. Comparative Example 1 Particles were produced in the same manner as in Example 1, except that magnesium sulfate was not added. The produced particles were separated, washed with water, dried, and crushed by a conventional method. As is clear from the electron micrograph (×20,000) shown in FIG. 5, this powder powder contained granular particles mixed in spindle-shaped particles. Also,
The X-ray diffraction results showed hematite and magnetite peaks. Comparative Example 2 Particles were produced in the same manner as in Example 1, except that sodium citrate was not added. The produced particles were separated, washed with water, dried, and crushed by a conventional method. As is clear from the electron micrograph (×20,000) shown in FIG. 6, this particle powder had spindle-shaped particles mixed with spherical particles. Also,
The results of X-ray diffraction showed goethite and hematite peaks. Comparative example 3 Sodium citrate 6.5g (2.0 mol% based on Fe)
Applies to. ) Particles were produced in the same manner as in Example 1 except for adding . The produced particles were separated, washed with water, dried, and crushed by a conventional method. As is clear from the electron micrograph (×20,000) shown in FIG. 7, the powder particles were fine amorphous particles. Comparative Example 4 Particles were produced in the same manner as in Example 1 except that the reaction temperature was 65°C. The produced particles were separated, washed with water, dried, and crushed by a conventional method. As is clear from the X-ray diffraction diagram shown in FIG. 8, this powder was a mixture of hematite particles and goethite particles. In FIG. 8, peak A is a peak indicating hematite, and peak B is a peak indicating goethite. <Production of spindle-shaped magnetite particle powder> Examples 18 to 35; Example 18 80 g of the spindle-shaped hematite powder obtained in Example 1 was charged into a 1.0 retort reduction container and driven. While rotating, H ₂ gas was passed through at a rate of 0.2 per minute, and reduction was performed at a reduction temperature of 330° C. to obtain spindle-shaped magnetite particle powder. As a result of electron microscopic observation, the obtained spindle-shaped magnetite powder was found to have substantially high density with no pores on the particle surface or inside the particle, and the particle size was uniform and dendritic. There are no particles mixed in, the average value is 0.28 μm on the long axis, and the axial ratio (long axis: short axis
The ratio was 3.5:1. In addition, as a result of the magnetic properties, the coercive force Hc is 302Oe,
The saturation magnetization σs was 81.3 emu/g. Examples 19-35 Spindle-shaped magnetite particles were obtained in the same manner as in Example 18, except that the type of starting material and the reduction temperature were varied. Table 2 shows the main manufacturing conditions and characteristics of the particles at this time. As a result of electron microscopic observation, all of the spindle-shaped magnetite particles obtained in Examples 19 to 35 had substantially high density with no pores on the particle surface or inside the particles, and The grain size was uniform and no dendritic particles were present. <Production of spindle-shaped maghemite particle powder> Examples 36 to 53; Example 36 75 g of the spindle-shaped magnetite particle powder obtained in Example 18 was oxidized in air at 300°C for 60 minutes to form a spindle shape. A maghemite particle powder exhibiting the following was obtained. As a result of electron microscopy, the obtained spindle-shaped maghemite particles were found to have substantially high density with no pores on the particle surface or inside the particles, and the particle size was uniform and dendritic. No particles mixed, average value of major axis 0.28 μm, axial ratio (major axis: short axis)
The ratio was 3.5:1. In addition, as a result of the magnetic properties, the coercive force Hc is 253Oe,
The saturation magnetization σs was 73.5 emu/g. Examples 37 to 53 Spindle-shaped maghemite particles were obtained in the same manner as in Example 36, except that the type of spindle-shaped magnetite particles was varied. Table 3 shows the main manufacturing conditions and characteristics of the particles at this time. As a result of electron microscopic observation, all of the spindle-shaped maghemite particles obtained in Examples 37 to 53 had substantially high density with no pores on the particle surface or inside the particles, In addition, the particle size was uniform and no dendritic particles were present.

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〔effect〕

According to the method for producing spindle-shaped magnetic iron oxide particles according to the present invention, as shown in the above example,
There are no pores on the particle surface or inside the particle, the particle size is substantially high, the particle size is uniform, there are no dendritic particles mixed, and the axial ratio (long axis: short axis) It is possible to obtain magnetic iron oxide particles consisting of small spindle-shaped magnetite particles or maghemite particles, which is currently the most demanded magnetic material particle powder for high recording density, high sensitivity, high output, and low noise. It is suitable as In addition, when the above-mentioned magnetite particle powder or maghemite particle powder is used in the production of magnetic paint, it has good dispersibility in vehicles,
It is possible to obtain a preferred magnetic recording medium with extremely excellent orientation and filling properties in the coating film.

[Brief explanation of drawings]

1 and 2 are an electron micrograph (×20,000) and an X-ray diffraction diagram showing the particle structure of the spindle-shaped hematite particles obtained in Example 1, respectively. 3 and 4 are electron micrographs (×
20000) and an X-ray diffraction diagram. Figures 5 to 7 are
All are electron micrographs (×20,000) and show the particle structures of the powder particles obtained in Comparative Examples 1 to 3, respectively. FIG. 8 shows comparative example 4.
It is an X-ray diffraction diagram of the particle powder obtained in .

Claims (1)

[Claims] 1. Ferrous salt aqueous solution and Fe in the ferrous salt aqueous solution
When oxygen-containing gas is passed through an aqueous solution containing FeCO _{3 obtained by reacting FeCO 3} with an aqueous alkali carbonate solution at a ratio of 1.1 to 1.7 equivalents in terms of CO ₃ , the ferrous salt aqueous solution, the A water-soluble magnesium salt of 0.1 to 5.0 atomic % (calculated as Mg with respect to Fe) and 0.1 atomic % with respect to Fe in any of the aqueous solutions containing FeCO ₃ before oxidation by passing through an aqueous alkali carbonate solution and an oxygen-containing gas. ~1.5 mol% of citric acid or its salt is added and then oxidized by passing oxygen-containing gas to generate spindle-shaped hematite particles from the aqueous solution, and the spindle-shaped hematite particles are reduced. Consisting of spindle-shaped magnetic iron oxide particles characterized by being thermally reduced in a magnetic gas to form spindle-shaped magnetite particles, or further oxidized to form spindle-shaped maghemite particles. Method for producing magnetic iron oxide particle powder. 2. Oxygen-containing gas is bubbled through the FeCO ₃ -containing aqueous solution obtained by reacting the ferrous salt aqueous solution with an alkali carbonate aqueous solution at a ratio of 1.1 to 1.7 equivalents in terms of CO ₃ to Fe in the ferrous salt aqueous solution. During oxidation, the ferrous salt aqueous solution, the alkali carbonate aqueous solution, and the aqueous solution containing FeCO ₃ before oxidation are mixed with 0.1 in terms of Mg relative to Fe in any of the aqueous solutions containing FeCO3 before oxidation. ~5.0 atom% of water-soluble magnesium salt and 0.1 to 1.5 mol% of citric acid or its salt to Fe and 0.5 to 10.0 of Co in terms of Fe
atomic percent of a water-soluble cobalt salt, and then oxidize by passing oxygen-containing gas to produce spindle-shaped hematite particles containing Co from the aqueous solution. The obtained hematite particles are thermally reduced in a reducing gas to form spindle-shaped magnetite particles containing Co, or further oxidized to form spindle-shaped maghemite particles containing Co. A method for producing magnetic iron oxide particle powder consisting of magnetic iron oxide particles exhibiting a characteristic spindle shape.