JP2025010610A - Manufacturing method of anisotropic magnetic powder - Google Patents

Abstract

To provide a method for producing an anisotropic magnetic powder excellent in magnetic properties.SOLUTION: There is provided a method for producing an anisotropic magnetic powder including the steps of: mixing iron oxide particles having an average particle diameter of 0.1 μm or more and 0.4 μm or less with oxide particles of R having an average particle diameter of 0.5 μm or more and 0.8 μm or less (R is at least one kind selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) to obtain a mixture; heat-treating the mixture under a reducing gas atmosphere to obtain partial oxide; obtaining alloy particles by reducing the partial oxide; and obtaining an anisotropic magnetic powder by nitriding the alloy particles.SELECTED DRAWING: None

Description

The present invention relates to a method for producing anisotropic magnetic powder.

Patent Document 1 discloses a method for producing anisotropic magnetic powder by dropping an acidic aqueous solution of rare earth elements and iron into an alkaline solution to obtain a coprecipitate, oxidizing the coprecipitate, and then reducing and nitriding the resulting mixture.

Patent Document 2 discloses a method for producing anisotropic magnetic powder by reducing a mixture of iron oxide particles and rare earth element oxide particles through hydrogen heat treatment, further reducing the mixture to obtain an alloy, and then nitriding the alloy.

JP 2014-080653 A JP 2010-270379 A

The present invention aims to provide a method for producing anisotropic magnetic powder with excellent magnetic properties.

The method for producing the anisotropic magnetic powder of the present invention comprises the steps of:
A step of mixing iron oxide particles having an average particle size of 0.1 μm or more and 0.4 μm or less with oxide particles of R (R is at least one selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) having an average particle size of 0.5 μm or more and 0.8 μm or less to obtain a mixture;
a step of heat-treating the mixture in a reducing gas atmosphere to obtain a partial oxide;
obtaining alloy particles by reducing the partial oxide; and
The method includes a step of obtaining an anisotropic magnetic powder by nitriding the alloy particles.

We can provide a method for producing anisotropic magnetic powder with excellent magnetic properties.

1 is an electron microscope image of the anisotropic magnetic powder obtained in Example 1. 1 is an electron microscope image of the anisotropic magnetic powder obtained in Comparative Example 1. 4 is an electron microscope image of the anisotropic magnetic powder obtained in Comparative Example 2. 1 is an electron microscope image of the anisotropic magnetic powder obtained in Comparative Example 3. 1 is an electron microscope image of the anisotropic magnetic powder obtained in Comparative Example 4. 1 is an electron microscope image of the anisotropic magnetic powder obtained in Comparative Example 5. 1 is an electron microscope image of the anisotropic magnetic powder obtained in Comparative Example 6.

The following describes in detail the embodiments of the present invention. However, the embodiments shown below are merely examples for embodying the technical concept of the present invention, and the present invention is not limited to the following. In this specification, the term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. Furthermore, a numerical range indicated using "~" indicates a range that includes the numerical values written before and after "~" as the minimum and maximum values, respectively. Furthermore, when multiple substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the multiple substances present in the composition, unless otherwise specified.

The method for producing the anisotropic magnetic powder in this embodiment is as follows:
A step of mixing iron oxide particles having an average particle size of 0.1 μm or more and 0.4 μm or less with oxide particles of R (R is at least one selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) having an average particle size of 0.5 μm or more and 0.8 μm or less to obtain a mixture (mixing step);
a step of heat-treating the mixture in a reducing gas atmosphere to obtain a partial oxide (pretreatment step);
A step of obtaining alloy particles by reducing the partial oxide (reduction step); and
A step of obtaining anisotropic magnetic powder by nitriding the alloy particles (nitriding step).
According to this embodiment, by mixing iron oxide particles having the above-mentioned average particle size with oxide particles of R (R is at least one selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) having the above-mentioned average particle size, it is possible to reduce uneven distribution of the iron oxide particles and the oxide particles of R in the mixture, which is believed to improve the magnetic properties.

<Mixing step>
The mixing step is a step of mixing iron oxide particles with oxide particles of R (R is at least one selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu).

The average particle size of the iron oxide particles is 0.1 μm or more and 0.4 μm or less, preferably 0.13 μm or more and 0.35 μm or less, and more preferably 0.15 μm or more and 0.25 μm or less. If the average particle size exceeds 0.4 μm, the magnetic properties tend to deteriorate due to a decrease in the mixability with the oxide particles of R. If the average particle size is less than 0.1 μm, the iron oxide particles contain a large amount of impurities, which act as a flux during the heat treatment described below to produce coarse particles, and the magnetic properties tend to deteriorate. Here, the average particle size in the present invention is the particle size measured under dry conditions using a laser diffraction type particle size distribution measuring device.

The oxide particles of R are at least one oxide selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu, but from the viewpoint of magnetic properties, Sm oxide _Sm2O3 is preferred. The average particle size of the oxide particles of R is 0.5 μm or more and 0.8 _μm or less, preferably 0.53 μm or more and 0.75 μm or less, and more preferably 0.55 μm or more and 0.70 μm or less. If the average particle size is less than 0.5 μm, hydration reaction is likely to occur, which causes rapid heat generation, making handling difficult, and if the average particle size exceeds 0.8 μm, the mixability with iron oxide particles decreases, so that abnormal growth of iron oxide particles is likely to occur in the pretreatment process described below, and magnetic properties tend to decrease.

The average particle size of the R oxide particles is preferably 4 times or less than the average particle size of the iron oxide particles. If it exceeds 4 times, the mixing ability of the iron oxide particles and the R oxide particles tends to deteriorate.

The amount of R oxide particles is preferably 32.2 parts by weight to 36.8 parts by weight, and more preferably 33.2 parts by weight to 35.3 parts by weight, per 100 parts by weight of iron oxide particles. If it exceeds 36.8 parts by weight, the proportion of R-rich non-magnetic phase increases and the magnetic properties tend to deteriorate, while if it is less than 32.2 parts by weight, α-Fe is generated and the magnetic properties, especially the coercive force, tend to deteriorate.

Examples of iron oxide particles include magnetite (Fe ₃ O ₄ ) and hematite (Fe ₂ O ₃ ). Among these, magnetite is preferred because it is easily magnetically agglomerated and allows solid-liquid separation even in fine particles.

As the iron oxide particles, commercially available ones may be used as long as they are within the above-mentioned particle size range. For example, magnetite iron oxide particles can be synthesized by blowing an oxygen-containing gas such as air into a slurry containing a precipitate of iron hydroxide (Fe(OH) ₂ ) prepared by adding an alkaline agent to an aqueous iron solution under conditions of a temperature of 70 to 90° C. and a pH of 7.0 to 8.5 (wet oxidation method). As an iron raw material for producing the iron hydroxide precipitate, iron sulfate can be mentioned in terms of ease of availability. As the alkaline agent, an alkali metal hydroxide or ammonia can be mentioned, but an alkali metal hydroxide is preferable, and sodium hydroxide is particularly more preferable in terms of its low manufacturing cost and environmental load. In addition, according to the wet oxidation method, recrystallization occurs when magnetite iron oxide particles are synthesized from the precipitate of iron hydroxide particles, so it is easy to reduce the concentration of residual alkali metals described below.

The alkali metal concentration in the iron oxide particles is preferably 200 ppm or less, and more preferably 100 ppm or less. If the alkali metal concentration in the iron oxide particles exceeds 200 ppm, it acts as a flux during the reduction process described below, and coarse particles are generated, which tends to deteriorate the magnetic properties. In addition, in the above-mentioned wet oxidation method, the alkali metal concentration can be reduced by penetrating and washing the slurry of synthesized magnetite iron oxide particles with pure water. Penetrating and washing here refers to the operation of washing the dehydrated cake in the filter without opening the filter.

The mixing of iron oxide particles and oxide particles of R can be carried out by mixing a slurry containing iron oxide particles with a slurry containing oxide particles of R. Mixing may be carried out by stirring and mixing, but precision mixing is preferred because it can reduce uneven distribution of iron oxide particles and oxide particles of R in the mixture in a small area (4 μm). Methods of precision mixing include mixing using a bead mill and mixing using a beadless line mixer. When using a bead mill, the bead diameter is preferably 0.05 mm or more and 0.5 mm or less.

The coefficient of variation (CV) of the mixed distribution, which indicates the uneven distribution of iron oxide particles and R oxide particles in the mixture, is preferably 23% or less, more preferably 22% or less, and particularly preferably 21% or less. If the coefficient of variation exceeds 23%, the magnetic properties of the anisotropic magnetic powder tend to deteriorate. The coefficient of variation of the mixed distribution is obtained by taking a backscattered electron image of the mixture at 5000x magnification, dividing the image into squares with corners equivalent to 4 μm, calculating the area ratio of iron oxide and R oxide particles for each square from these squares, and calculating the variation in the area ratio.

After mixing the iron oxide particles and the R oxide particles, it is preferable to wash and dry the mixture. Washing can be performed with pure water until the effluent conductivity is 30 μS/cm or less, and drying can be performed by spray drying at approximately 200°C or higher and 250°C or lower.

<Addition of calcium hydroxide>
In addition to the iron oxide particles and the oxide particles of R, calcium hydroxide may be added. When calcium hydroxide is added to a mixture of iron oxide particles and oxide particles of R, the presence of calcium hydroxide between the iron oxide particles can suppress contact between the iron oxide particles. As a result, sintering between the iron oxide particles can be suppressed, and the dispersibility of the anisotropic magnetic powder is improved. Calcium hydroxide is a substance that is resistant to high temperature heating in the pretreatment step, reduction step, and nitriding step, reduction by reducing gas, and reduction by metallic Ca, and is preferable because it does not generate gas other than water in the pretreatment step and is easy to remove in the water washing step.

The average particle size of calcium hydroxide is preferably 0.05 μm or more and 0.8 μm or less, and more preferably 0.3 μm or more and 0.5 μm or less. If the average particle size exceeds 0.8 μm, the effect of suppressing sintering of iron oxide particles tends to be reduced, and if it is less than 0.05 μm, it is difficult to produce by grinding with a bead mill.

The average particle size of calcium hydroxide is preferably 4 times or less than the average particle size of iron oxide particles. If it exceeds 4 times, the mixing ability of iron oxide particles and R oxide particles tends to deteriorate.

When calcium hydroxide is added, the amount is preferably 8 parts by weight or more and 48 parts by weight or less, and more preferably 18 parts by weight or more and 32 parts by weight or less, per 100 parts by weight of iron oxide particles. If it exceeds 48 parts by weight, the particle size of the magnetic powder tends to become small and the magnetic properties, especially the residual magnetization, tend to decrease, and if it is less than 8 parts by weight, the effect of suppressing sintering of the iron oxide particles tends to decrease.

The calcium hydroxide may be in a state where it is mixed with the iron oxide particles before the pretreatment step, and the mixing method is not particularly limited, but it is preferable to mix a slurry of calcium hydroxide with a slurry containing iron oxide particles and oxide particles of R. The mixing may be performed by stirring and mixing, but precision mixing is preferable because it can reduce uneven distribution of the iron oxide particles, oxide particles of R, and calcium hydroxide in the mixture.

<Pretreatment process>
The pretreatment step is a step in which a mixture of iron oxide particles and R oxide particles is heat-treated in a reducing gas atmosphere to obtain a partial oxide in which most of the contained iron oxide is reduced.

The reducing gas is appropriately selected from hydrocarbon gases such as hydrogen (H ₂ ), carbon monoxide (CO), and methane (CH ₄ ), but hydrogen gas is preferred from the viewpoint of cost. The flow rate of the gas is appropriately adjusted within a range in which oxides do not fly off. The heat treatment temperature in the pretreatment step (hereinafter, pretreatment temperature) is preferably 300° C. or higher and 950° C. or lower, more preferably 400° C. or higher, even more preferably 750° C. or higher, and preferably lower than 900° C. When the pretreatment temperature is 300° C. or higher, the reduction of iron oxide proceeds efficiently. Furthermore, when the pretreatment temperature is 950° C. or lower, particle growth and segregation of iron particles generated by reduction are suppressed, and the desired particle size can be maintained. Furthermore, when hydrogen is used as the reducing gas, it is preferable to adjust the thickness of the oxide layer used to 20 mm or less, and further adjust the dew point in the reaction furnace to −10° C. or less.

<Reduction step>
The reduction step is a step of reducing the obtained partial oxide, for example, by mixing the partial oxide with metallic calcium and heat-treating the mixture in an atmosphere of an inert gas other than nitrogen, such as argon, or in vacuum, to obtain alloy particles containing iron and R.

Reduction is carried out by contacting the oxide with calcium melt or calcium vapor. The heat treatment temperature in the reduction step (hereinafter, reduction temperature) is in the range of 700°C to 1200°C, and preferably in the range of 800°C to 1100°C. From the viewpoint of carrying out the reduction reaction more uniformly, the heat treatment time can be in the range of 10 minutes to 10 hours, and is preferably in the range of more than 10 minutes to 2 hours.

Metallic calcium is used in granular or powder form, with the particle size preferably being 10 mm or less. This makes it possible to more effectively suppress necking of particles during the reduction reaction. Metallic calcium can be added in an amount between 1.1 and 3.0 times the reaction equivalent (the stoichiometric amount required to reduce rare earth oxides, including the amount required to reduce iron oxides), and is preferably added in an amount between 1.5 and 2.5 times the reaction equivalent.

In the reduction step, a disintegration promoter can be used as necessary together with metallic calcium as a reducing agent. This disintegration promoter is used as appropriate to promote disintegration and granulation of the product in the water washing step described below, and examples of such disintegration promoters include alkaline earth metal salts such as calcium chloride, and alkaline earth oxides such as calcium oxide. These disintegration promoters are used in a ratio of 1% by mass to 30% by mass, preferably 5% by mass to 30% by mass, per rare earth oxide used as the rare earth source.

<Nitriding process>
The nitriding process is a process for obtaining anisotropic magnetic particles by nitriding the alloy particles obtained in the reduction process. Since a porous sintered mass is obtained in the reduction process, the nitriding process can be performed uniformly by immediately subjecting the sintered mass to heat treatment in a nitrogen atmosphere without pulverization.

The heat treatment temperature for the nitriding of alloy particles (hereinafter referred to as the nitriding temperature) is 300°C or higher and 600°C or lower, particularly 400°C or higher and 550°C or lower, and is performed by replacing the atmosphere with a nitrogen atmosphere within this temperature range. The heat treatment time may be set to a time sufficient for the nitriding of the alloy particles to be sufficiently uniform, for example, 2 hours or higher and 30 hours or lower.

<Water washing process>
The water washing step is a step in which the sintered body obtained in the nitriding step is put into cold water to disintegrate the sintered body and separate the anisotropic magnetic particles from impurities. The product obtained after the nitriding step may contain, in addition to the magnetic particles, by-produced CaO, unreacted metallic calcium, etc., and these may be in a composite sintered lump state. In this case, the product is put into cooling water to separate CaO and metallic calcium from the magnetic particles as a calcium hydroxide (Ca(OH) ₂ ) suspension. Furthermore, the remaining calcium hydroxide may be sufficiently removed by washing the magnetic particles with acetic acid or the like. When the product is put into water, the oxidation of metallic calcium by water and the hydration reaction of by-produced CaO cause the composite sintered lump-like reaction product to disintegrate, i.e., to become fine powder. In addition, when calcium hydroxide is added in the mixing step, it can be removed in the water washing step. When performing surface treatment, a phosphoric acid solution may be put into the range of 0.10% by mass to 10% by mass as PO ₄ relative to the magnetic particle solid content obtained in the nitriding step as a surface treatment agent. After appropriate separation from the solution and drying, anisotropic magnetic powder is obtained.

The anisotropic magnetic powder obtained as described above typically has the following general formula R _x Fe _(100-x-y) N _y (wherein R represents at least one selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu, x is 3 or more and 30 or less, and y is 5 or more and 15 or less).
From the viewpoint of magnetic properties, R is preferably Sm.

In the general formula, x is specified to be 3 to 30 because if it is less than 3, the unreacted portion of the iron component (α-Fe phase) separates, reducing the coercive force of the nitride and making it no longer a practical magnet, and if it exceeds 30, at least one element selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu precipitates, making the magnetic powder unstable in the air and reducing the residual magnetic flux density. Also, y is specified to be 5 to 15 because if it is less than 5, almost no coercive force is expressed, and if it exceeds 15, nitrides of at least one element selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu and of iron itself are formed.

<Composite materials>
A composite material can be produced from the anisotropic magnetic powder of the present invention and a resin. By including the anisotropic magnetic powder of the present invention, a composite material having high magnetic properties can be formed.

The resin contained in the composite material may be a thermosetting resin or a thermoplastic resin, but is preferably a thermoplastic resin. Specific examples of thermoplastic resins include polyphenylene sulfide resin (PPS), polyether ether ketone (PEEK), liquid crystal polymer (LCP), polyamide (PA), polypropylene (PP), polyethylene (PE), etc.

When obtaining a composite material, the mixing ratio of the anisotropic magnetic powder and the resin (resin/magnetic powder) is preferably 0.10 or more and 0.15 or less, and more preferably 0.11 or more and 0.14 or less.

The composite material can be obtained, for example, by mixing anisotropic magnetic powder and resin at 280°C or higher and 330°C or lower using a kneader.

<Bonded magnet>
The composite material described above can be used to manufacture a bonded magnet. Specifically, for example, the composite material can be heat-treated while aligning the axis of easy magnetization in an orienting magnetic field (orientation process), and then pulse-magnetized in a magnetizing magnetic field (magnetization process) to obtain a bonded magnet.

The heat treatment temperature in the orientation process is preferably, for example, 90°C or higher and 200°C or lower, and more preferably, 100°C or higher and 150°C or lower. The magnitude of the orientation magnetic field in the orientation process can be, for example, 720 kA/m. In addition, the magnitude of the magnetization magnetic field in the magnetization process can be, for example, 1500 kA/m or higher and 2500 kA/m or lower.

The following are examples. Unless otherwise specified, "%" is by weight.

(1) Example 1
(Mixing process)
2000 kg of ferrous sulfate ( _FeSO4 ) with an iron concentration of 8% was added to 988 kg of pure water, and 25% caustic soda was added with stirring until the pH reached 10.5 to prepare an iron hydroxide slurry. After heating this iron hydroxide slurry to 70-80°C, aeration was performed while controlling the pH to 7.5-8.5 to prepare a slurry containing 221.3 kg of magnetite ( _{Fe3O4 )} with an average particle size of 0.20 μm as iron oxide particles.

While stirring 74.7 kg of pure water, 74.7 kg of samarium oxide (Sm ₂ O ₃ ) was dissolved little by little so as not to cause aggregation, to prepare a samarium oxide slurry. The samarium oxide particles in the slurry were then pulverized until the average particle size was 0.70 μm.

The samarium oxide slurry was added to the magnetite slurry and stirred, then precision mixed in a bead mill (bead diameter 0.1 mm) to create an oxide slurry in which iron and samarium were mixed uniformly at a micro level. This oxide slurry was washed with pure water until the effluent conductivity was 30 μS/cm or less, and then dehydrated to a solids concentration of 30-50%, increasing the concentration of the slurry. The increased concentration of the oxide slurry was pumped and spray-dried at approximately 250°C, yielding 296 kg of black Fe, Sm mixed oxide.

(Pretreatment process)
The oxide powder (296 kg) obtained above was reduced in a hydrogen reduction furnace at about 800° C. to reduce the oxygen concentration in the powder to 5.8 mass % or less. 228 kg of hydrogen-reduced powder was recovered.

(Reduction process)
Metal calcium (particle size: about 6 mm) was prepared in an amount 2.25 times equivalent to the amount of oxygen contained in the powder obtained in the pretreatment process, and mixed with the hydrogen-reduced powder. Specifically, 103.8 g of hydrogen-reduced powder and 29.8 g of metal calcium were mixed, press-molded, and placed in a furnace. After evacuating the furnace, argon gas (Ar gas) was introduced. The temperature was raised to 1030°C and held there for 2 hours to obtain Fe-Sm-M alloy particles.

(Nitriding process)
After the reduction step, the mixture was cooled to 100° C., evacuated, and then the temperature was raised to 430° C. while introducing nitrogen gas. The mixture was then held at that temperature for 23 hours to obtain a reaction product containing anisotropic magnetic powder.

(Water washing process)
The lump-shaped reaction product obtained in the nitriding step was put into 3 kg of pure water and stirred for 30 minutes. After standing, the supernatant was drained by decantation. The process of putting into pure water, stirring and decantation was repeated 10 times. Then, 4.0 g of 99.9% acetic acid was put in and stirred for 15 minutes. After the obtained slurry was separated into solid and liquid, it was vacuum dried at 80°C for 3 hours to obtain an anisotropic magnetic powder. The obtained _anisotropic magnetic powder is represented _{by Sm2Fe17N3} .

(Vibration mill process)
The anisotropic magnetic powder obtained above was placed in a 500 ml metal pot containing 900 g of Φ2.5 mm beads (SUJ2) and dry-pulverized in a vibration mill. After that, the anisotropic magnetic powder was sieved to recover only the anisotropic magnetic powder.

(2) Example 2
Anisotropic magnetic powder was prepared in the same manner as in Example 1, except that in the mixing step, a magnetite slurry having an average particle size of 0.31 μm and a samarium oxide slurry having an average particle size of 0.63 μm were prepared.

(3) Example 3
(Mixing process)
2000 kg of ferrous sulfate ( _FeSO4 ) with an iron concentration of 8% was added to 988 kg of pure water, and 25% caustic soda was added with stirring until the pH reached 10.5 to prepare an iron hydroxide slurry. After heating this iron hydroxide slurry to 70-80°C, aeration was performed while controlling the pH to 7.5-8.5 to prepare a slurry containing 221.3 kg of magnetite ( _{Fe3O4 )} with an average particle size of 0.20 μm as iron oxide particles.

While stirring 74.7 kg of pure water, 74.7 kg of samarium oxide (Sm ₂ O ₃ ) was dissolved little by little so as not to cause aggregation, to prepare a samarium oxide slurry. The samarium oxide particles in the slurry were then pulverized until the average particle size was 0.70 μm.

The samarium oxide slurry was added to the magnetite slurry and stirred, then precision mixed using a bead mill (bead diameter 0.1 mm) to create an oxide slurry in which iron and samarium were mixed uniformly at the micro level. This oxide slurry was washed with pure water until the effluent conductivity was 30 μS/cm or less.

While stirring 400 kg of pure water, 80 kg of calcium hydroxide was dissolved in small amounts to avoid clumping, creating a calcium hydroxide slurry. The calcium hydroxide particles in this slurry were then pulverized until the average particle size was 0.50 μm.

This calcium hydroxide slurry was added to the previously washed oxide slurry and stirred, then precision mixed in a bead mill (bead diameter 0.1 mm) to create an oxide slurry in which iron, samarium, and calcium hydroxide were mixed uniformly at a microscopic level. The slurry was then dehydrated to a solids concentration of 30-50%, increasing the concentration. The highly concentrated oxide slurry was pumped and spray-dried at approximately 250°C, yielding 376 kg of black Fe, Sm, Ca mixed oxide.

(Pretreatment process)
The oxide powder thus obtained (376 kg) was reduced in a hydrogen reduction furnace at about 800° C. to reduce the oxygen concentration in the powder to 5.8 mass % or less. 315 kg of hydrogen-reduced powder was recovered.

Thereafter, the mixture was subjected to a reduction process, a nitriding process, a water washing process, and a vibration mill process under the same conditions as in Example 1 to prepare anisotropic magnetic powder of Sm ₂ Fe ₁₇ N ₃ .

(4) Comparative Example 1
Anisotropic magnetic powder was prepared in the same manner as in Example 1, except that in the mixing step, a magnetite slurry having an average particle size of 0.15 μm and a samarium oxide slurry having an average particle size of 1.1 μm were prepared.

(5) Comparative Example 2
Anisotropic magnetic powder was prepared in the same manner as in Example 1, except that in the mixing step, a magnetite slurry having an average particle size of 0.22 μm and a samarium oxide slurry having an average particle size of 1.6 μm were prepared.

(6) Comparative Example 3
1000 kg of ferrous sulfate with an iron concentration of 8% and 1000 kg of ferric sulfate (Fe ₂ (SO ₄ ) ₃ ) with an iron concentration of 8% were added to 988 kg of pure water, and 25% caustic soda was added while stirring until the pH reached 9.2, and the mixture was aged at 70 to 80° C. to produce a magnetite slurry with an average particle size of 0.05 μm. An anisotropic magnetic powder was produced in the same manner as in Example 1, except that the mixture was mixed with a samarium oxide slurry with an average particle size of 0.59 μm.

(7) Comparative Example 4
While stirring 550 kg of pure water, 221 kg of magnetite with an average particle size of about 5 μm was dissolved little by little so as not to form lumps, to prepare a magnetite slurry. This slurry was pulverized in a bead mill until the average particle size became 1.3 μm. Then, anisotropic magnetic powder was prepared in the same manner as in Example 1, except that a slurry of samarium oxide with an average particle size of 0.72 μm was prepared.

(8) Comparative Example 5
Anisotropic magnetic powder was prepared in the same manner as in Comparative Example 4, except that in the mixing step, a magnetite slurry having an average particle size of 0.68 μm and a samarium oxide slurry having an average particle size of 0.77 μm were prepared.

(9) Comparative Example 6
(Dissolving process)
1500 kg of pure water was heated to 35°C, 2000 kg of ferrous sulfate with an iron concentration of 8% and 20 kg of 70% sulfuric acid were added, and then 74.7 kg of samarium oxide was added and mixed to dissolve. Ammonia water was further added until the pH was 2.0 and the mixture was thoroughly stirred to dissolve completely. Pure water was added to adjust the concentration so that the total amount of the solution was 4766 kg, and this was used as the metal solution.

(Reaction step)
Next, 4766 kg of the 35°C metal solution obtained above was added to 2500 kg of pure water kept at a temperature of 35°C over a period of 70 minutes, while adjusting the pH to 7-8 with 18% ammonia and carbon dioxide gas. This resulted in the production of precipitates of Fe and Sm carbonates. The precipitate obtained was in the form of a slurry.

(Washing process)
The obtained slurry was thoroughly washed with pure water until the effluent conductivity reached 150 μS/cm or less, and then vacuum dried at about 90° C. to obtain granular powders of Fe and Sm carbonates.

(Air firing process)
The resulting Fe and Sm carbonate (448 kg) was calcined in air at 900 to 1100° C. to obtain 309 kg of a reddish brown Fe, Sm composite oxide.

The above oxide was subjected to the pretreatment process of Example 1 and subsequent processes to produce anisotropic magnetic powder.

(10) Evaluation of magnetic properties The anisotropic magnetic powders obtained in the above examples and comparative examples were filled into a sample container together with paraffin wax, and the paraffin wax was melted in a dryer, after which the easy magnetic domains were aligned in an orientation magnetic field of 16 kA/m. The magnetically oriented sample was pulse-magnetized in a magnetizing magnetic field of 32 kA/m, and the residual magnetic flux density (σr), coercive force (iHc), and squareness ratio (HK) were measured using a VSM (vibrating sample magnetometer) with a maximum magnetic field of 16 kA/m.

(11) Evaluation of the mixed distribution of iron oxide and samarium oxide The coefficient of variation of the mixed distribution was determined by dividing the backscattered electron image (5000x) of the mixture obtained in the mixing process into 24 squares with corners equivalent to 4 μm, calculating the Fe and Sm area ratio for each square from these squares, and determining the variation in the area ratio. A field emission scanning electron microscope (SU8230, Hitachi High-Technologies, 3.0 KV, 5000x) was used for the imaging.
The variation in the area ratio was calculated by dividing the brightness of the obtained image into three values within the range of 0 to 255 degrees using a macro created in Excel: less than 70 degrees, 70 degrees or more and less than 170 degrees, and 170 degrees or more. The images were color-coded in descending order of brightness with white, light blue, and black, and identified as samarium oxide particles, iron oxide, and background, respectively. These were cut into 4 μm squares, and the area of each color was measured using image analysis software ImageJ, and the variation in the area ratio of white to light blue was quantified using CV (%) for evaluation.

(12) SEM images of anisotropic magnetic powders The magnetic powders obtained in Example 1 and Comparative Examples 1 to 6 were photographed with a scanning electron microscope (SU3500, Hitachi High-Technologies, 5KV, 5000x). The results are shown in Figures 1 to 7. Compared to Comparative Examples 1 to 6, the magnetic powder of Example 1 had round particles with uniform particle size and good magnetic properties.

(13) Magnetic Properties The magnetic properties of the anisotropic magnetic powders obtained in the Examples and Comparative Examples, the particle size of the raw material particles, the mixed state of the powder particles, and the residual salt concentration in the raw material are shown in Table 1.

From Table 1, the magnetic powders of Examples 1 and 2 obtained using iron oxide particles and samarium oxide particles of specific particle sizes had improved residual magnetic flux density (σr), coercive force (iHc), and squareness ratio (HK) compared to the magnetic powders of Comparative Examples 1 to 5. The magnetic powders of Examples 1 and 2 also had improved residual magnetic flux density (σr), coercive force (iHc), and squareness ratio (HK) compared to the magnetic powder of Comparative Example 6 obtained by oxidizing the coprecipitate of samarium and iron, followed by reduction and nitridation. The magnetic powder of Example 3, in which calcium hydroxide was mixed in the mixing process, had significantly improved coercive force (iHc) and squareness ratio (HK) compared to the magnetic powders of Comparative Examples 1 to 5. Although the variation in the mixed distribution was small for Comparative Example 3, the reason for the deterioration of the magnetic properties is thought to be that the particle size of the iron oxide was small, and therefore the reduction of residual sodium salts during washing was insufficient.

The method for producing anisotropic magnetic powder of the present invention can produce anisotropic magnetic powder with excellent magnetic properties.

Claims (6)

A step of mixing iron oxide particles having an average particle size of 0.1 μm to 0.4 μm with oxide particles of R (R is at least one selected from the group consisting of Sc, Y, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu) having an average particle size of 0.5 μm to 0.8 μm to obtain a mixture;
a step of heat-treating the mixture in a reducing gas atmosphere to obtain a partial oxide;
obtaining alloy particles by reducing the partial oxide; and
A method for producing anisotropic magnetic powder, comprising the step of nitriding the alloy particles to obtain anisotropic magnetic powder. The method for producing anisotropic magnetic powder according to claim 1, wherein the coefficient of variation of the mixture distribution of the mixture is 23% or less. The iron oxide particles are _{Fe3O4 particles} ;
A method for producing the anisotropic magnetic powder according to claim 1 or 2. The alkali metal concentration in the iron oxide particles is 200 ppm or less.
A method for producing the anisotropic magnetic powder according to any one of claims 1 to 3. A method for producing anisotropic magnetic powder according to any one of claims 1 to 4, in which R is Sm. In the step of obtaining the mixture, calcium hydroxide is further mixed.
A method for producing the anisotropic magnetic powder according to any one of claims 1 to 5.

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