JPH0840723A - Production of barium ferrite fine particle - Google Patents

Abstract

PURPOSE:To obtain barium ferrite fine particles excellent in magnetic characteristics by mixing a ferric compound, a barium compound and an alkali material with each other and rapidly raising temperature of the mixture from around room temperature up to a temperature range from sub-critical temperature to super-critical temperature of water in two steps by using a flow type reacting tube. CONSTITUTION:(A) Aqueous solutions of each of a ferric compound (e.g. ferric nitrate), a barium compound (e.g. barium nitrate) and an alkali material (e.g. potassium hydroxide) are homogeneously mixed with each other at 10-60 deg.C to prepare a reaction solution. (B) The temperature of the reaction mixture is rapidly raised up to 250-300 deg.C within 5sec under a constant pressure of 20-50MPa in a flow-type reacting tube. (C) Subsequently, the temperature is further rapidly raised up to 350-450 deg.C within 5sec and this temperature of the reaction solution is kept for 10-1000sec in the reaction tube. In the reaction solution, a molar ratio of the total of anions of the ferric compound and the barium compound to the hydroxide ion of the alkali material is 1-4. Concentrations of the ferric compound, the barium compound and the alkali material are each 0.0001-1.0mol/L.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous production method of barium ferrite fine particles useful as a magnetic recording material. More specifically, in the subcritical or critical state of water,
The present invention relates to a continuous production method of barium ferrite fine particles in which an aqueous solution of an iron compound, a barium compound and an alkaline substance is reacted using a flow-type reaction tube.

[0002]

_2. Description of the Related Art In recent years, along with the demand for higher density magnetic recording, the development of perpendicular magnetic recording media using hexagonal barium ferrite (BaO.6Fe ₂ O ₃ ) particles has been advanced. As a manufacturing method, a glass crystallization method and a hydrothermal synthesis method are mainly known. For example, in JP-A-56-60002, an alkali and an alkali carbonate are mixed in an aqueous solution containing a ferric salt and a barium salt while stirring to obtain a coprecipitate of ferric hydroxide and barium carbonate. After that, a method of obtaining barium ferrite particles by performing filtration, washing with water, drying and then heat treatment is disclosed. Japanese Patent Application Laid-Open No. 56-160328 discloses an alkaline solution in which a ferric salt and a barium salt are dissolved in an autoclave.
A method of obtaining a barium ferrite powder by treating at -250 ° C to form a precipitate of a barium ferrite precursor and then firing at a temperature of 800 ° C or higher is disclosed. JP 56
-67904, after mixing the components forming the barium ferrite such as barium oxide and iron oxide and the components forming the glass such as boron oxide, after quenching and solidifying the melted,
After precipitating barium ferrite in the glass by heat treatment, the fine powder obtained is treated with a dilute acid, barium ferrite particles are extracted by dissolving and removing the glass component, washed with water, and a method of drying is disclosed. ing.

[0003]

The method of Japanese Patent Laid-Open No. 56-60002 is a simple process, but it is difficult to obtain monodisperse fine particles, and the coprecipitate has poor filterability. .
The method of JP-A-56-160328 can solve the problem of coprecipitate filtration, but the reaction time is long and the batch type hydrothermal treatment using an autoclave results in poor productivity and economical efficiency. There is a problem of lacking. JP-A-56-67904
The method of No. 1 can obtain fine powder having a sharp particle size distribution, but it requires a high temperature to melt the raw material and requires heat treatment again, resulting in a complicated manufacturing process and extremely low productivity. .

The inventors of the present invention previously disclosed a method of continuously producing metal oxide fine particles in a subcritical or supercritical state of an aqueous solution of a metal salt using a flow-type reaction tube in JP-A-4-50105. Proposed. This method was applied to the production of barium ferrite. That is, it was found that by passing a mixed aqueous solution of an iron compound, a barium compound and an alkaline substance through a flow-type reaction tube, the above problems can be overcome, and barium ferrite fine particles can be selectively and efficiently produced. Filed as Japanese Patent Application No. 5-115425. However, the magnetic properties of the obtained barium ferrite fine particles are
It was not always satisfactory. Therefore, the crystal formation conditions of barium ferrite were examined in detail, including the change of the temperature raising method of the flow reactor. As a result, it was discovered that the temperature at the time of mixing the raw material aqueous solution is around room temperature, and that the rapid temperature increase in two steps from that is remarkably effective in improving the magnetic properties of the barium ferrite fine particles. Was completed.

[0005]

That is, according to the present invention, the following consecutive aqueous solutions of (a) step, (b) step and (c) step, (a) iron compound, barium compound and alkaline substance are used. Step of uniformly mixing at ~ 60 ℃ to prepare the reaction solution, then (b) 20 ~ in the flow-type reaction tube
250 to 300 ℃ within 5 seconds at a constant pressure of 50MPa
A step of rapidly raising the temperature to 350 to 450 ° C. within 5 seconds and then circulating the reaction liquid at the temperature for a residence time of 10 to 1000 seconds. And a method for producing barium ferrite fine particles.

The present invention will be described in more detail below. (
In the step (a), the temperature at which the respective raw material aqueous solutions are uniformly mixed is 10 to 60 ° C., and particularly preferably a low temperature of 10 to 30 ° C. Needless to say, the iron compound, the barium compound and the alkaline substance can be directly dissolved in water and used as a reaction liquid, but the temperature at that time is 60
It is important not to exceed ℃. Next, in the step (b), hot water is supplied to the reaction solution in the flow-type reaction tube and / or the reaction solution is heated from the outside, and the temperature is 250 to 250 seconds instantaneously in 5 seconds or less, preferably 1 second or less. 300 ° C, preferably 260
By rapidly raising the reaction to ~ 290 ° C,
Generate uniform barium ferrite crystal nuclei, and
Lead to the process. In the step (c), supercritical water is supplied to the reaction solution at 250 to 300 ° C. in which the crystal nuclei of barium ferrite are generated, and / or the reaction solution is externally heated for 5 seconds or less, preferably Instantly 350-450 in less than 1 second
The reaction solution is rapidly heated to ℃, preferably 380 to 420 ℃, and is allowed to flow at the temperature for a residence time of 10 to 1000 seconds to carry out crystal growth of barium ferrite. (
After the step (c), the reaction solution is cooled and filtered, and barium ferrite fine particles are collected on the filter. The essence of the present invention resides in that the temperature is rapidly raised in two steps from step (a) to step (b) and step (c).

Explaining the raw materials used in the present invention, iron compounds include chlorides, bromides, iodides, nitrates, sulfates, carbonates, organic acid salts and complex salts. For example, ferrous chloride and chloride. Ferric iron, ferrous bromide, ferric bromide, ferric iodide
There are iron, ferrous nitrate, ferric nitrate, ferrous sulfate, ferric sulfate and ferrous carbonate. Furthermore, oxalic acid, acetic acid, glycolic acid, glyceric acid, lactic acid, malic acid, tartaric acid,
Examples include iron salts of citric acid, mandelic acid, and salicylic acid, and iron chelate complexes. These iron compounds are one or two
More than one type can be used. Among these iron compounds,
From the viewpoint of solubility in water, corrosiveness of equipment, and economical efficiency, ferric nitrate is most preferable. The concentration of these iron compounds is 0.00
It is from 01 to 1.0 mol / liter, preferably from 0.001 to 0.2 mol / liter.

The barium compound used in the present invention includes halides, nitrates, carbonates, organic acid salts and complex salts, for example, barium chloride, barium bromide, barium iodide, barium nitrate and barium carbonate. Further, a chelate complex with an organic acid salt such as barium oxalate or an organic substance having a hydroxyl group or a carboxyl group in the molecule can be mentioned. These barium compounds may be used alone or in combination of two or more. Among these barium compounds, barium nitrate and barium iodide are more preferable from the viewpoints of solubility in water, corrosiveness of equipment, and economical efficiency. The concentration of these barium compounds is 0.0001 to 1.0 mol / liter, preferably
It is 0.001 to 0.2 mol / liter.

The alkaline substances used in the present invention include alkali metal hydroxides, alkaline earth hydroxides and quaternary ammonium hydroxides, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, There are magnesium hydroxide, barium hydroxide, ammonia and tetramethylammonium hydroxide. These alkaline substances may be used alone or in combination of two or more. Among the above alkaline substances, potassium hydroxide and sodium hydroxide are preferred. The concentration of these alkaline substances is 0.0001 ~
It is 1.0 mol / liter, preferably 0.01 to 2.0 mol / liter.

The ratio of the iron compound and the barium compound in the aqueous solution used in the present invention is represented by the Fe / Ba mol ratio.
Barium ferrite can be generated in the range of 5/1 to 1/2, but barium ferrite fine particles are selectively generated,
In order to increase the values of the saturation magnetization σ _s and the coercive force H _c , which are parameters of the magnetic characteristics of the generated barium ferrite, it is preferable that the molar ratio of Fe / Ba be 2/1 to 1/2.

The aqueous solution used in the present invention has a ratio of the number of moles of hydroxide ions of an alkaline substance to the total number of moles of anions of an iron compound and a barium compound, that is, an alkali mole ratio (R) of 1 or more. It is preferably 1 to 4. Further, a typical reaction apparatus used in the present invention will be described with reference to FIG. A circular tube made of stainless steel is used as a flow-type reaction tube, and liquid is fed to the reaction tube by a high-pressure pump having no pulsating flow. The pressure in the system can be controlled by the back pressure valve 18. Further, the liquid feeding line and the reaction tube are heated by a sheath heater, and their temperatures are controlled by PID. In the step (a), the pump 4 feeds the mixed aqueous solution of the iron compound and the barium compound contained in the tank 1. The aqueous solution of the alkaline substance contained in the tank 2 is sent by the pump 5 and mixed with the above mixed aqueous solution to form a reaction solution. The temperature at this time is measured by the thermometer 10 and is set to 10 to 60 ° C. On the other hand, in the step (b), the pure water from the tank 3 is pumped by the pump 6
And the sheath heater 8 heats it into hot water (
A) Supply and mix with the reaction solution of step. The temperature at this time is measured by the thermometer 11 and is set to 250 to 300 ° C. This temperature is the crystal nucleus generation temperature of barium ferrite. Immediately thereafter, in the step (c), the pure water in the tank 3 is sent by the pump 7 and heated by the sheath heater 9 to be hot water, which is supplied and mixed with the reaction solution in the step (b). The temperature at this time is measured by the thermometer 12 and is set to 350 to 450 ° C. The temperature at the outlet of the reaction tube is measured by the thermometer 15, and the temperature is usually set to the temperature measured by the thermometer 12. (B), (b) and (c)
The aqueous solution containing barium ferrite produced through the above step is cooled to room temperature by the cooler 16, and then the filter 1
Collect at 7. The aqueous solution after the barium ferrite is collected is returned to normal pressure by the back pressure valve 18 and enters the container 19.

[0012]

[Operation] In the (a) step, the formation of impurities is suppressed by thoroughly mixing at around room temperature of 10 to 30 ° C, and in the subsequent (b) step
Rapidly raise the temperature to 250-300 ℃ to generate homogeneous crystal nuclei of barium ferrite, and in the subsequent step (c),
After rapidly raising the temperature to 350-450 ° C, crystal growth of barium ferrite is performed at that temperature. These (a), (b)
By continuously performing the step (c) and (c), barium ferrite particles having a small particle size distribution and excellent magnetic properties can be obtained.

In the step (a), when the temperature of the mixed solution exceeds 60 ° C., impurities such as hematite are produced, and a side reaction of producing amorphous fine particles other than barium ferrite progresses. Saturation magnetization decreases. Although it is possible to cool the mixed solution to below 10 ° C, the saturation magnetization of the produced barium ferrite does not rise further. In the step (b), if the temperature is raised to 250 to 300 ° C. for a long time exceeding 5 seconds, the crystal nuclei of barium ferrite are likely to become heterogeneous. Even if the temperature is rapidly raised in 5 seconds or less, the crystal nucleus of barium ferrite is not sufficiently generated at 250 ° C or less. Further, when the temperature exceeds 300 ° C, some crystallization progresses and fine crystal grains of barium ferrite are precipitated. (C) In the process, 350 to 450 in a long time exceeding 5 seconds
When the temperature is raised to ℃, the particle size of barium ferrite becomes uneven. Even if the temperature is rapidly raised in 5 seconds or less, crystal growth does not proceed sufficiently at 250 ° C or less. It is possible to exceed 450 ° C, but the load on the heating device and the reaction device becomes large, and it becomes impractical. If the residence time is less than 10 seconds, the crystal growth does not proceed sufficiently, and if the residence time exceeds 1000 seconds, the reaction tube becomes longer than necessary and is not efficient.

When the Fe / Ba mol ratio of the starting reaction aqueous solution exceeds 5, hematite is likely to be produced, and the average particle diameter of the produced barium ferrite reaches about 1 μ or more, which is not preferable. If the Fe / Ba mol ratio is less than 1/2, BaO ・ 6Fe ₂
This is not preferable because the phase change from O ₃ to BaO ₂ Fe ₂ O ₃ tends to proceed. Alkali molar ratio R of the starting reaction aqueous solution
When is less than 1, barium ferrite is hardly generated. Further, as R becomes larger, the number of particles produced increases, and the average particle diameter of the barium ferrite produced becomes smaller. It is possible for R to exceed 4, but the average grain size of barium ferrite does not decrease even if the amount of alkaline substance is increased further.

The hydrothermal synthesis method is a method for producing oxide fine particles by subjecting a precipitate of a metal hydroxide produced by hydrolysis or the like to high temperature and high pressure treatment with water or an aqueous solution. Since it is used, the reaction is faster than normal temperature and pressure, and the crystallinity and uniformity of the product are excellent. In addition to the general features of the hydrothermal synthesis method, the method of the present invention is a continuous process and has a very short residence time and high production efficiency. Compact size, the particle size and shape of the produced particles can be changed by the reaction temperature, pressure, metal salt concentration, residence time, anion species, etc. Supercritical water can be uniformly mixed with other gases, so the raw material aqueous solution At the same time, it has an advantage that a reactive gas can be introduced into the reaction tube to control the reaction atmosphere.

[0016]

【Example】

Example 1 will be described with reference to FIG. A mixed solution (tank 1) consisting of an aqueous ferric nitrate solution having a concentration of 0.01 mol / liter and an aqueous barium nitrate solution having a concentration of 0.005 mol / liter (tank 1) was pressurized and sent to 30 MPa from a pump 4 at a flow rate of 3.0 ml / min. On the other hand, from pump 5, 0.16 mol / liter potassium hydroxide aqueous solution (Tank 2)
Was pressurized and sent to 30 MPa with a pump 5 at a flow rate of 3.0 ml / min, and both aqueous solutions were mixed. When the temperature of the aqueous solution immediately after mixing was measured with a thermometer 10, it was 25 ° C. The molar ratio of Fe / Ba in the mixed aqueous solution was 2, and the molar ratio (R) of hydroxide in potassium hydroxide to the total amount of nitrate ions contained in ferric nitrate and barium nitrate was 4. It was

Pure water pressurized to 30 MPa by the pump 6 is 6.0 m
The liquid was sent at a flow rate of l / min, and the temperature was adjusted by the heater 8 of the liquid sending line so that the thermometer 11 showed 280 ° C. (corresponding to the crystal nucleus generation temperature of barium ferrite). On the other hand, pure water pressurized to 30 MPa from the pump 7 was sent at a flow rate of 6.0 ml / min of water, and the sheath heater 9 in the solution sending line was used to adjust the heat so that the thermometer 12 showed 380 ° C. It is mixed with the reaction solution. The mixed reaction solution is continuously supplied to a SUS316 circular tube (outer diameter 3/8 inch, thickness 1.65 mm). The pure water feed line and the flow reactor are sheath heaters (outer shape)
1.6 mm) and the temperature was controlled by PID. In the flow-type reaction tube, the pressure was 30 MPa, the temperature was 380 ° C, and the residence time was
The reaction was carried out under the condition of 53 seconds. The effluent from the flow-type reaction tube is cooled to room temperature through the double-tube cooling pipe 16, and the in-line filter 17 (pore size) installed in front of the back pressure valve 18 is used.
The produced particles were collected at 0.1 μm). The experimental conditions are collectively shown in Table 1, including Example 2 and Comparative Examples 1 to 3 described later. In Table 1, the nucleation temperature is
It is an abbreviation for the crystal nucleus generation temperature of barium ferrite.

Example 2 The same procedure as in Example 1 was carried out except that the temperature was raised by the thermometer 11 in FIG. 1 to indicate 300 ° C. (corresponding to the crystal nucleus generation temperature of barium ferrite). Comparative Example 1 Similar to the example, description will be given according to FIG. 3.0 ml of a mixture (tank 1) consisting of a ferric nitrate aqueous solution with a concentration of 0.01 mol / l and a barium nitrate aqueous solution with a concentration of 0.005 mol / l
At a flow rate of / minute, the pressure was increased by pump 4 to 30 MPa. On the other hand, 0.16 mol / liter potassium hydroxide aqueous solution (tank 2) from pump 5 was pumped at 30 MPa from pump 5 at a flow rate of 3.0 ml / min.
Then, the solution was pressurized and fed, and both aqueous solutions were mixed. When the temperature of the aqueous solution immediately after mixing was measured with a thermometer 10, it was 25 ° C. The Fe / Ba mol ratio in the mixed aqueous solution and the nitric acid second
The mol ratio of hydroxide in potassium hydroxide to the total amount of nitrate ions contained in iron and barium nitrate is the same as in Example 1. Pump 6 remains stopped, pump 30
Pure water pressurized to MPa is sent at a flow rate of 6.0 ml / min of water, and the sheath heater 9 in the solution sending line is used to heat and adjust the temperature.
12 was mixed with the reaction solution so as to show 380 ° C., and the solution was sent to a flow-type reaction tube. In the flow-type reaction tube, the reaction was carried out under the same conditions as in Example 1, and the produced particles were collected in the same manner as in Example 1.

Comparative Example 2 An explanation will be given with reference to FIG. A mixed solution (tank 1) consisting of an aqueous ferric nitrate solution having a concentration of 0.01 mol / liter and an aqueous barium nitrate solution having a concentration of 0.005 mol / liter (tank 1) was pressurized and sent to 30 MPa from a pump 4 at a flow rate of 3.0 ml / min. Meanwhile, pump 5
0.16 mol / liter potassium hydroxide aqueous solution (tank
2) was pressurized and sent to 30 MPa with pump 5 at a flow rate of 3.0 ml / min, and both aqueous solutions were mixed. However, the solution feeding line of the potassium hydroxide aqueous solution was heated by a sheath heater (not shown in FIG. 1), and the temperature of the aqueous solution immediately after mixing was measured with a thermometer 10 to be 200 ° C. Note that m of Fe / Ba in the mixed aqueous solution
ol ratio and m of hydroxide in potassium hydroxide with respect to the total amount of nitrate ions contained in ferric nitrate and barium nitrate
The ol ratio is the same as in Example 1.

Subsequently, the pump 6 remains stopped and the pump
Pure water pressurized from 7 to 30 MPa is sent at a flow rate of 6.0 ml / min, and the sheath heater 9 in the solution sending line heats and adjusts it.
The mixture was mixed with the reaction solution so that the thermometer 12 showed 380 ° C., and the solution was sent to the flow-type reaction tube. In the flow-type reaction tube, Example 1 was used.
The reaction was conducted under the same conditions as above, and the produced particles were collected in the same manner as in Example 1.

Comparative Example 3 Ferric nitrate aqueous solution having a concentration of 0.01 mol / liter and a concentration of 0.005 m
20m mixed solution consisting of ol / liter barium nitrate aqueous solution
l and 20 ml of 0.16 mol / liter potassium hydroxide aqueous solution
Mixed at 5 ° C. 9 ml of this mixed aqueous solution is the effective internal volume of 9 ml.
The SUS304 reactor was filled up and tightly capped, and then immersed in a metal salt bath heated to 380 ° C. Although the temperature reached 380 ° C. in 145 seconds, it was immersed in the metal salt bath for 60 seconds as it was, and then the reactor was taken out and cooled with water to collect the produced particles.

Evaluation method The particle diameters of the particles obtained in the examples and comparative examples were measured by photographing with a transmission electron microscope. At the same time, the particle size distribution was also observed. As for the magnetic properties of the powder, the coercive force Hc and the saturation magnetization s were measured with a magnetic balance. In the powder X-ray diffraction method, all the particles were made of barium ferrite (BaO ・ 6F).
Only the crystalline peak of e ₂ O ₃ ) was detected. The results of the above measurements are shown together in Table 2.

[0023]

[Table 1] Table 1 ─────────────────────────────────── Example Mixing temperature Nucleation temperature / Time Inlet temperature Outlet temperature Residence time / Comparative example (℃) (℃) (sec) (℃) (℃) (sec) ─────────────────────── ───────────── Example 1 25 280 <1 380 380 53 Example 2 25 300 <1 380 380 53 Comparative example 1 25 380 <1 380 380 73 Comparative example 2 200 380 <1 380 380 60 Comparative Example 3 25 380 145 (380) (380) 90 ─────────────────────────────────── ──

[0024]

[Table 2] Table 2 ─────────────────────────────────── Example Average particle size Particle size distribution Magnetic force Saturation magnetization / Comparative example (μm) H _c (Oe) σ _s (emu / g) ─────────────────────────────── ────── Example 1 0.15 Very small 2026 47.1 Example 2 0.20 Small 2075 48.1 Comparative example 1 0.20 Large 1700 49.4 Comparative example 2 0.35 Very large 1819 38.6 Comparative example 3 0.20 Large 1253 37.0 ───── ─────────────────────────────── Table 2 shows the following. That is, it can be seen that in Examples 1 and 2, the barium ferrite particles are excellent in that the particle size distribution of the particles is small, and that they have high coercive force and high saturation magnetization. On the other hand, in Comparative Example 1 (nucleation temperature 380 ° C) in which the temperature was rapidly raised from 25 ° C in one step, the saturation magnetization was high, but large particles and small particles were mixed and the particle size distribution was large. Further, in Comparative Example 2 and Comparative Example 3, spherical particles of 50 nm estimated to be hematite are mixed, and the particle size distribution is large. In Comparative Example 2 in which the mixing temperature is 200 ° C.,
It can be seen that the saturation magnetization is low and the particle size distribution is large. Even if the mixing temperature is 25 ° C., in Comparative Example 3 in which the time to reach 380 ° C. is as long as 145 seconds, the saturation magnetization is low and the particle size distribution is large, and it is possible to generate a homogeneous crystal nucleus by rapid heating. It turns out to be important.

[0025]

INDUSTRIAL APPLICABILITY According to the present invention, hexagonal barium is obtained by instantaneously rapidly raising the temperature of an aqueous reaction solution from near room temperature to a subcritical temperature or supercritical temperature of water by using a flow-type reaction tube. This is a method for continuously producing fine particles of ferrite (BaO · 6Fe ₂ O ₃ ). According to the method of the present invention, hydrolysis and crystallization simultaneously occur in a short time from a non-equilibrium state, and uniform fine particles are obtained, so that barium ferrite fine particles having excellent magnetic properties can be produced. The hexagonal barium ferrite fine particles have a hexagonal plate shape and have an easy axis of magnetization in a direction perpendicular to the plate surface. Therefore, by arranging the plate surface of the particles parallel to the substrate surface, a perpendicular magnetic recording medium is produced. be able to. Further, since it is a coating medium, it is suitable for mass production, and conventional techniques can be applied in terms of ensuring reliability such as durability. Therefore, the industrial significance of the present invention is extremely large.

[Brief description of drawings]

FIG. 1 is a schematic view of an apparatus of the present invention using a flow-type reaction tube.

2 is a barium ferrite (Ba) obtained in Example 1. FIG.
An electron micrograph showing the particle structure of O · 6Fe ₂ O ₃ ).
The magnification is 80,000 times.

3 is a barium ferrite (Ba) obtained in Example 2. FIG.
An electron micrograph showing the particle structure of O · 6Fe ₂ O ₃ ).
The magnification is 80,000 times.

4 is a barium ferrite (Ba) obtained in Comparative Example 1. FIG.
An electron micrograph showing the particle structure of O · 6Fe ₂ O ₃ ).
The magnification is 80,000 times.

5 is a barium ferrite (Ba) obtained in Comparative Example 2. FIG.
An electron micrograph showing the particle structure of O · 6Fe ₂ O ₃ ).
The magnification is 80,000 times.

6 is a barium ferrite (Ba) obtained in Comparative Example 3. FIG.
An electron micrograph showing the particle structure of O · 6Fe ₂ O ₃ ).
The magnification is 80,000 times.

[Explanation of symbols]

1 Tank containing mixed aqueous solution of iron compound and barium compound 2 Tank containing aqueous solution of alkaline substance 3 Pure water tank 4 High pressure pulseless pump 5 High pressure pulseless pump 6 High pressure pulseless pump 7 High pressure pulseless Flow pump 8 Sheath heater for liquid feed line 9 Sheath heater for liquid feed line 10 Thermometer for reaction solution mixing section 11 Thermometer for 1st step rapid temperature rise section 12 Thermometer for 2nd step rapid temperature rise section 13 Reaction Tube sheath heater 14 Reaction tube 15 Thermometer at the outlet of the reaction tube 16 External water-cooled cooler 17 Filter (switch between two) 18 Back pressure valve 19 Container for receiving the aqueous solution after collecting the product

Claims (5)

[Claims] 1. The following consecutive steps (a), (b) and (c), (a) an iron compound, a barium compound and an aqueous solution of an alkaline substance are uniformly mixed at 10 to 60 ° C. A step of preparing a reaction solution, and then (b) a step of rapidly raising the reaction solution to 250 to 300 ° C. within 5 seconds at a constant pressure of 20 to 50 MPa in a flow-type reaction tube,
And (c) a step of rapidly raising the temperature to 350 to 450 ° C. within 5 seconds and flowing the reaction solution at the temperature for a residence time of 10 to 1000 seconds,
A method for producing barium ferrite fine particles, which comprises: 2. The method for producing barium ferrite fine particles according to claim 1, wherein the iron compound is iron nitrate. 3. The method for producing fine barium ferrite particles according to claim 1, wherein the barium compound is barium nitrate. 4. The method for producing barium ferrite fine particles according to claim 1, wherein the alkaline substance is potassium hydroxide and / or sodium hydroxide. 5. The barium ferrite according to claim 1, wherein an aqueous solution having a molar ratio of hydroxide ion of an alkaline substance to 1 to 4 with respect to a total anion of an iron compound and a barium compound is used. Method for producing fine particles.