JPH0455979B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention is directed to the production of hematite particles used as a starting material in the production of magnetic particles for magnetic recording, particularly magnetic iron oxide particles for rigid disks, floppy disks, and digital recording. It is related to the manufacturing method, and in detail, it has substantially high density with no pores on the particle surface or inside the particle, has uniform particle size, does not contain dendritic particles, and The present invention relates to a method for producing hematite particle powder consisting of spindle-shaped hematite particles with a small axis ratio (long axis: short axis). [Prior Art] In recent years, as magnetic recording and reproducing equipment has become longer recording time and has become smaller and lighter, both these magnetic recording and reproducing equipment and magnetic recording media such as magnetic tapes and magnetic disks have become more efficient and sophisticated. Demand for density recording is increasing. This fact can be seen, for example, in "Development of Magnetic Materials and Highly Dispersed Magnetic Powder Technology (1982)" published by the General Technology Center.
On p. 134 of ``Maghemite (γ-Fe <sub>2</sub> O <sub>3</sub> ) iron oxide Despite using powder as a magnetic material, the recording density has been dramatically improved by more than <sub>two</sub> orders of magnitude over the past <sub>20</sub> years. These include thinning the coating film, increasing the precision of the thin film surface, employing magnetic field alignment technology, lowering the flying height of the head, and improving head characteristics.'' In order to improve the performance and recording density of magnetic recording media, it is necessary to improve dispersibility, filling properties, residual magnetic flux density Br and coercive force Hc, improve the smoothness of the tape surface, and make the coating film thinner. It is. This fact is based on page 140 of the aforementioned ``Development of magnetic materials and high dispersion technology of magnetic particles'', ``Increasing recording density...
...It is necessary to increase Br to ensure a constant output. In order to increase Br, it is necessary to increase not only the magnetic field orientation but also the filling rate of the magnetic powder. '', the statement on page 141 of the document, ``For high-density recording, thinning of the coating film is an important factor'', and the same in the case of floating heads such as rigid disks. On page 143 of the document, ``The flying height of the head is the controlling factor for high-density recording.
By making this smaller, higher density becomes possible. ...When the flying height is lowered, if the surface of the disk is poor, the reproduction output decreases due to chipping of the head, and stable flying is disturbed, resulting in head crash. Therefore, high-precision finishing of the coating film surface is important. It is clear from the statement ``. These various properties of magnetic recording media are closely related to the magnetic iron oxide particles used in magnetic recording media, and it is strongly desired to improve the properties of magnetic iron oxide particles. The relationship between the various characteristics of the magnetic recording medium and the characteristics of the magnetic iron oxide particles used will now be detailed as follows. First, the residual magnetic flux density Br of a magnetic recording medium depends on the dispersibility of the magnetic iron oxide particles in the vehicle, the orientation and filling properties in the coating film. In order to improve the separability in the vehicle, the orientation and filling properties in the coating film, it is necessary to prevent the presence of pores on the surface and inside of the magnetic iron oxide particles to be dispersed in the vehicle. It has a substantially high density, has a uniform particle size, does not contain dendritic particles, and in terms of the shape of the particles,
Particles exhibiting a spindle shape are required. Next, in order to improve the surface properties of magnetic recording media,
A magnetic iron oxide particle powder with good dispersibility, good orientation, and a small particle size is preferable, and such a magnetic iron oxide particle powder has no pores on the particle surface or inside the particle, and has a substantially high particle size. The density is
In addition, the particle size is uniform, dendritic particles are not mixed, and in terms of particle shape, particles are required to be spindle-shaped. Furthermore, in order to thin the coating film of magnetic recording media,
On page 141 of the above document, ``To make the coating film thinner, it is necessary to reduce the size of the magnetic powder and improve its orientation in the coating film thickness direction.In the end, forming a thin coating film requires 2.3 As mentioned in , it is possible to use magnetic powder with low oil absorption to make magnetic paint with good coating properties.''As is clear from the statement, magnetic iron oxide particle powder with good dispersibility and orientation is used. Well, as such magnetic iron oxide particle powder,
As mentioned above, 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, dendritic particles are not mixed, and in terms of particle shape, spindle-shaped particles are required. As mentioned above, the coercive force Hc of the magnetic recording medium is
For high-density recording, it is necessary that the coercive force Hc of the magnetic particles dispersed in the vehicle be as high as possible. As is well known, the magnitude of the coercive force of magnetic iron oxide particles depends on shape anisotropy, crystal anisotropy, strain anisotropy, and exchange anisotropy, or their interaction. Acicular magnetite particles or acicular maghemite particles, which are currently used as magnetic particles for magnetic recording, can obtain relatively high coercive force by utilizing the anisotropy derived from their shape. It is something that exists. On the other hand, one of the methods for improving recording density in magnetic recording and reproducing equipment is to narrow the magnetic head gap width. This fact is based on the statement on page 15 of the above-mentioned document that states, ``An important index expressing performance in magnetic recording is...recording density.'' Until now, its increase has been mainly due to improvements in magnetic heads and recording media. To summarize the directions of improvement so far in this field:
This is clear from the description "...magnetic head; narrow gap width and narrow track width...". The recording principle of the recording medium and magnetic head in the conventionally employed longitudinal recording method (a method of recording signals in the longitudinal direction of the magnetic layer) is as described on page 18 of the aforementioned document, ``In the ring head (Figure 2a), the winding The signal current creates an arc-shaped magnetic field near the gap in the magnetic core.This has a strong longitudinal component at the center of the gap, so the medium is mainly magnetized in the longitudinal (in-plane) direction.'' It is as follows. In recent years, the gap width of magnetic heads has become narrower and narrower for the purpose of high-density recording. However, when the gap width of magnetic heads is narrowed, the magnetic field near the gap of the magnetic core has a strong vertical component along the longitudinal direction. will be included. Therefore, in the surface layer of the magnetic recording medium that is in contact with the head, the magnetic flux distribution in the direction perpendicular to the medium increases significantly. Therefore, in order to achieve high-density recording, it is preferable that the magnetic recording medium have an easy magnetization direction perpendicular to the medium. Typical magnetic particles conventionally used are acicular magnetite particles or maghemite particles. In this case, due to shape anisotropy, the direction of easy magnetization is the longitudinal direction of the acicular shape, so acicular oxide particles are coated. It is preferable to increase the vertical component by vertically oriented in the film or randomly oriented three-dimensionally. This fact is explained in Japanese Unexamined Patent Publication No. 183626/1983, "Also, in recent years, the concept of perpendicular magnetization recording has been introduced.
There is also a proposal to effectively use the residual magnetization component in the direction perpendicular to the surface of the magnetic recording medium. This perpendicular magnetization recording increases the recording density defined above, and...
..." and "A coated magnetic layer that uses diagonal or perpendicular magnetization components that are not parallel to the magnetic surface..."
It is clear from the statement "...". In order to randomly orient the magnetic iron oxide particles three-dimensionally in the coating film and increase the vertical component,
As mentioned above, in addition to having uniform particle size and no dendritic particles mixed therein, it is effective to make the axial ratio (long axis: short axis) of the magnetic iron oxide particles as small as possible. This fact is based on the above-mentioned Japanese Unexamined Patent Publication No. 57-183626, which states, ``The present invention... has a major diameter of 0.4 to 2μ or 0.3 to 1μ and an aspect ratio of 5 to 20, which is used in the above-mentioned prior art.
Instead of regular acicular particles, the particle size is 0.30μ
By making it as small as below and having a short shape with an aspect/width ratio of more than 1 and less than 3, it is possible to achieve orientation within the image by reducing the thickness of the coating film in the thickness direction during application and drying, and flow during application. It is characterized by suppressing the tendency of the particles to be oriented in the direction of the casting and lying in the plane, and to actively increase the perpendicular residual magnetization if necessary. . It is clear from the statement ``. Currently, acicular magnetite particles or acicular maghemite particles are mainly used as magnetic particles for magnetic recording. These are generally performed 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 (usually called a "wet reaction"). The obtained acicular γ-Fe ₂ OOH particles are heated and dehydrated in air at around 300°C to form hematite particles, and then heated in a reducing gas such as hydrogen at 300°C.
Reduced to acicular magnetite particles at ~400℃,
Alternatively, this is then oxidized in air at 200 to 300°C to obtain acicular 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. , spindle-shaped magnetic iron oxide particles with a small axis ratio (major axis: minor axis) are currently in greatest demand, but the above-mentioned known method for producing acicular goethite particles as a starting material is The particle powder obtained by
It is a particle with an acicular shape with an axial ratio (major axis: short axis) of 10:1 or more, contains dendritic particles, and has a uniform particle size. It's hard to say. 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 involves the reaction of a ferrous salt solution with an alkali carbonate.
This is a method of oxidizing an aqueous solution containing FeCO ₃ by passing an oxygen-containing gas through it. When this method is used, goethite particles with uniform particle size, no dendritic particles mixed therein, and 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 spindle-shaped 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 become particles by dehydration. A large number of pores are created on the surface and inside the particles, and then the hematite particles are reduced, or
It is observed that magnetite particles or maghemite 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 Unexamined Patent Publication No. 55-47220, which states that γ-Fe ₂ O ₃ is used as a magnetic powder for magnetic recording.
(maghemite) particles have been widely used, and efforts have been made to increase the coercive force, but in order to achieve this, the produced γ-Fe ₂ O ₃ has to be mixed with its mother salt iron oxyhydroxide (goethite) ( α, β, γ−
It is necessary to maintain the needle-like form of FeOOH) and eliminate the dehydration pores (vacancies). These dehydration pores (vacancies) are generated during the dehydration of iron oxyhydroxide, which is the mother salt, and these are the γ-
If it remains in Fe ₂ O ₃ particles, it reduces the coercive force. ” and the description in Electrochemistry and Industrial Physical Chemistry Vol. 38, No. 7 (1970), p. 544, “...Although various dispersion techniques in vehicles have been considered, γ-Fe ₂ produced The dispersion in the O ₃ tape is poor.... γ-Fe ₂ O ₃ has holes (vacancies) as shown in Figure 17, and magnetic poles appear at the edges of the holes, creating a Lorentz magnetic field. As a result, the needle-like bodies that were initially well dispersed during the ball milling process are attracted, lowering the static magnetic energy, and forming a chestnut burr-shaped agglomerate, which is clear from the statement "...". As mentioned above, magnetic iron oxide particles suitable for disks, floppy disks, and digital recording are required to have a small axial ratio (long axis: short axis); Magnetic iron oxide particles with a small axis (minor axis) cannot utilize the anisotropy derived from the shape, so the coercive force
Only particles with Hc of about 300 Oe or less can be obtained, and therefore there is a strong demand to improve the coercive force 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 higher and 1000°C or lower in a vacuum, in a hydrogen gas flow, or in a carbon gas flow to prevent the metal from becoming damaged. In addition, Element 2-6 of the lecture outline for the Spring Conference of the Powder and Powder Metallurgy Association 1961 states that as the dehydration temperature of acicular goethite increases, the number of pores on the surface and inside of the acicular hematite particles decreases. 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, instead of eliminating 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 particle surface or inside the particles as a starting material. ing. In this method, acicular crystalline magnetic iron oxide particles are obtained by reducing and oxidizing acicular crystalline hematite particles directly from an aqueous solution as a starting material. 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 crystalline 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 magnetic iron oxide particles obtained by reduction and oxidation also have no pores 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
Because it requires high temperatures above ℃ and requires a special device called an "autoclave," it is not suitable for industrial use.
It's not economical. 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. In addition, as for generating granular hematite particles, JP-A-49-52800 and JP-A-56-
There are methods described in Publication No. 17929, but when using these methods, the axial ratio (major axis: short axis) is approximately 1.
can only generate hematite particles of
The magnetic iron oxide particles obtained by thermally reducing the hematite particles or further oxidizing them if necessary are:
Shape anisotropy cannot be utilized, resulting in a small coercive force. 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. In order to obtain spindle-shaped magnetic iron oxide particles with a small (long axis: short axis), the particles must be uniform, without dendritic particles mixed, and the axis ratio (long axis: short axis) must be small. There is a strong demand for a method for directly producing small spindle-shaped hematite particles from an aqueous solution at temperatures below 100°C. [Means for solving the problem] The present inventor has developed hematite particles having a spindle shape with a uniform particle size, no dendritic particles, and a small axis ratio (long axis: short axis). As a result of various studies on methods of directly producing the product from an aqueous solution at temperatures below ℃, we found that a ferrous salt aqueous solution and an alkali carbonate aqueous solution with a ratio of 1.8 equivalents or more of Fe in the ferrous salt aqueous solution in terms of CO ₃ were combined. FeCO ₃ obtained by reaction
Before oxidizing the aqueous solution containing FeCO ₃ by passing an oxygen-containing gas through it, Fe When citric acid or its salt is added in an amount of 0.1 to 1.5 mol%, the particle size is uniform, dendritic particles are not mixed, and spindle-shaped particles with a small axis ratio (long axis: short axis) are formed. We found that the hematite particles shown above can be directly produced from an aqueous solution at temperatures below 100℃. That is, the present invention provides an aqueous solution of an alkali carbonate obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution having a ratio of 1.8 equivalents or more 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. Spindle-shaped hematite particles are produced by adding 0.1 to 1.5 mol% of citric acid or its salt to Fe, and then oxidizing it by passing oxygen-containing gas through the aqueous solution to produce spindle-shaped hematite particles. This is a method for producing hematite particle powder consisting of hematite particles exhibiting the following. [Function] First, the hematite particles according to the present invention are spindle-shaped hematite particles that are uniform in particle size, do not contain dendritic particles, and have a small axis ratio (long axis: short axis). Since it is produced directly from an aqueous solution at a temperature of 0.degree. Further, according to the method for producing hematite particles according to the present invention, only spindle-shaped hematite particles can be produced from an aqueous solution at 100° C. or lower. In the case of 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 determined that a ferrous salt aqueous solution and an alkali carbonate aqueous solution can be produced. When oxidizing an aqueous solution containing FeCO ₃ obtained by reacting with FeCO 3 by passing an oxygen-containing gas through the aqueous solution, the condition is such that the amount of the alkaline carbonate aqueous solution is less than 1.8 equivalents in terms of CO ₃ relative to the Fe in the ferrous salt aqueous solution. When citric acid or its salt is added as described below, granular magnetite particles are mixed in the spindle-shaped hematite particles, so it is thought that this is due to the synergistic effect of alkali carbonate and citric acid or its salt. . 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 used in the present invention is
It is 1.8 equivalent or more in terms of CO ₃ compared to Fe. If the amount is less than 1.8 equivalents, granular magnetite particles will be mixed in the spindle-shaped hematite particles. 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% relative to Fe. If it is less than 0.1 mol%, spindle-shaped goethite particles and granular hematite particles will coexist. 1.5 mol%
If it exceeds 100%, 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 affects the type and form of the produced particles through a synergistic action with the alkali carbonate, and therefore, the production reaction of spindle-shaped hematite particles is initiated. It must be added before oxidation, and it can be added to any of the ferrous salt aqueous solution, the aqueous alkali carbonate solution, and the aqueous solution containing FeCO ₃ before being oxidized by bubbling oxygen-containing gas. [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. Example 1 Ferrous sulfate obtained by adding 3.3 g of sodium citrate to contain 0.5 mol% based on Fe
1.5 mol/aqueous solution 1.5 and 1.54 mol/aqueous solution 3.0 of Na ₂ CO ₃ previously prepared in the reactor.
In addition, FeCO ₃ was generated at a temperature of 80°C (corresponding to CO ₃ /Fe = 2.0 equivalent). Particles were generated by blowing 20 air per minute into the aqueous solution containing FeCO ₃ at a temperature of 80° C. for 4.5 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 0.25 μm and an axial ratio (long axis: short axis) of 3.5:1. 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 12 Except that the type of ferrous salt, the type, concentration and equivalent ratio of alkali carbonate, the type of citric acid or its salt, the amount added, addition time, and temperature were varied,
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 12 were only hematite particles. Table 1 shows the main manufacturing conditions and characteristics of the produced hematite particles. Comparative Example 1 1.12 mol/Na ₂ CO ₃ aqueous solution 3.0 (CO ₃ /Fe
= corresponds to 1.4 equivalent. ) Particles were produced in the same manner as in Example 1, except that Particles were used. 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. 3, this particulate powder was a mixture of granular particles 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 13.2 g of sodium citrate (corresponding to 2.0 mol % based on Fe) was 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. 4, the powder particles were fine, amorphous particles. Comparative Example 3 Particles were produced in the same manner as in Example 1 except that the temperature was 65°C. The resulting particles were separated, washed with water, dried, and pulverized using a conventional method. As is clear from the X-ray diffraction diagram shown in FIG. 5, this particle powder was a mixture of hematite particles and goethite particles. In FIG. 5, peak A indicates hematite;
Peak B is a peak indicating goethite.

〔effect〕

According to the method for producing hematite particles according to the present invention, as shown in the previous example, there are no pores on the particle surface or inside the particles, and the particle size is substantially high. It is possible to obtain hematite particle powder consisting of spindle-shaped hematite particles that are uniform, do not contain dendritic particles, and have a small axis ratio (long axis: short axis). Magnetite particles and maghemite particles obtained by heating reduction or further oxidation using hematite particles consisting of spindle-shaped hematite particles obtained in this manner as a starting material can also be used to improve the particle surface and particle size. A spindle-shaped material with no internal pores, substantially high density, uniform particle size, no dendritic particles, and a small axis ratio (major axis: short axis). Since the powder is composed of particles exhibiting the above characteristics, it is suitable as a magnetic material for high-density recording, which is currently most in demand. In addition, when the above-mentioned magnetite particle powder or maghemite particle powder is used in the production of a magnetic material, the dispersibility in the vehicle is good,
It is possible to obtain a preferred magnetic recording medium with extremely excellent orientation and filling properties in the coating film.

[Brief explanation of the drawing]

FIGS. 1 and 2 are an electron micrograph (×20,000) and an X-ray diffraction diagram, respectively, showing the particle structure of spindle-shaped hematite particles obtained by the present invention. Figures 3 and 4 are electron micrographs (x20000), respectively, of Comparative Example 1.
and shows the particle structure of the particle powder obtained in Comparative Example 2. FIG. 5 is an X-ray diffraction diagram of the particles obtained in Comparative Example 3.

Claims (1)

[Claims] 1 Ferrous salt aqueous solution and Fe in the ferrous salt aqueous solution
When oxygen-containing gas is passed through the aqueous solution containing FeCO _{3 obtained by reacting FeCO 3} with an aqueous alkali carbonate solution having a proportion of 1.8 equivalents or more in terms of CO ₃ , the ferrous salt aqueous solution and the carbonic acid Add Fe to either the alkaline aqueous solution or the _FeCO3 -containing aqueous solution before oxidation by bubbling oxygen-containing gas.
0.1 to 1.5 mol% of citric acid or its salt is added to the solution, and then oxidized by passing oxygen-containing gas in a temperature range of 70 to 100°C to generate spindle-shaped hematite particles from the aqueous solution. A method for producing hematite particle powder consisting of hematite particles exhibiting a spindle shape.