JP6007006B2 - Method for producing hexagonal ferrite magnetic powder - Google Patents

Description

  The present invention relates to a manufacturing method for obtaining a ferrite magnetic powder suitable for a high-density magnetic recording medium, particularly a hexagonal ferrite magnetic powder.

  In order to further increase the density of the coating type magnetic recording medium, a fine magnetic powder is required. Conventionally used metal magnetic powder is a magnetic powder having excellent magnetic properties such as high saturation magnetization and large coercive force. However, as it becomes finer, it has become difficult to maintain both its magnetic properties for a long period of time (weather resistance) and to achieve excellent magnetic properties. In addition, the recent improvement in characteristics of magnetic heads (particularly reproducing heads) has made it possible to sufficiently use magnetic powders that do not have saturation magnetization as high as that of metal magnetic powders.

  Against this background of technology, ferrite magnetic powders that have been used only for specific applications due to their low saturation magnetization, but excellent stability of magnetic properties, especially hexagonal ferrite magnetic powders, are the next generation of high density. Attention has been focused on magnetic powder for magnetic recording media.

  In addition, in the process of studying hexagonal ferrite magnetic powders suitable for high-density magnetic recording, to obtain hexagonal ferrite magnetic powders that are fine and have high magnetic properties and can be suitably used for high-density magnetic recording media. Has been found to be preferred to use the glass crystallization method.

  Here, the glass crystallization method is a method in which a ferrite component and a glass-forming component (also referred to as “non-magnetic component”) are dissolved at a high temperature, and the resulting amorphous body is subjected to heat treatment in the amorphous body. This is a method of depositing hexagonal ferrite magnetic powder. Naturally, in order to use as a magnetic powder, a nonmagnetic component is unnecessary, and therefore, an operation for separating the nonmagnetic component and the hexagonal ferrite magnetic powder (magnetic material) is required. Usually, for such a separation operation, a method utilizing the property that glass, which is a nonmagnetic component, dissolves in an acid is used (for example, see Patent Documents 1 and 2).

  Patent Document 3 discloses a method in which the metal magnetic powder is washed with water in the form of aggregates in order to remove soluble calcium remaining in the metal magnetic powder.

JP 2005-340673 A International publication WO2011 / 125633A1 pamphlet Japanese Patent No. 4336932 (Japanese Patent Laid-Open No. 2003-037004)

  Conventionally, the separation operation of the liquid in which the nonmagnetic component is dissolved and the hexagonal ferrite magnetic powder has generally been performed using filtration. However, as the primary particle diameter of the hexagonal ferrite magnetic powder becomes finer, the time required for separation becomes longer.

  In addition, the separated hexagonal ferrite magnetic powder needs to be removed by washing the liquid in which the nonmagnetic component adhering to the magnetic powder is dissolved. Conventionally, this cleaning has used a filter press method. However, in the filter press method, the hexagonal ferrite magnetic powder where water easily flows is washed well, but the portion where water does not easily flow is difficult to wash. That is, there is a problem that uneven cleaning tends to occur.

  Insufficient washing leaves the acid component (and non-magnetic component) used in the separation in the hexagonal ferrite magnetic powder. As disclosed in Patent Document 2, the hexagonal ferrite magnetic powder in which the acid component remains may deteriorate the magnetic characteristics when used as a recording medium. Therefore, it is necessary to remove such an acid component as much as possible.

  However, as pointed out in Patent Document 3, when the primary particles become fine, clogging with particles tends to occur and the efficiency of filtration decreases. As a result, the ratio of the time required for cleaning the hexagonal ferrite magnetic powder to the time required for the manufacturing process of the hexagonal ferrite magnetic powder increases. Therefore, in order to efficiently produce fine hexagonal ferrite magnetic powder, shortening the time for solid-liquid separation such as separation and washing has been recognized as an urgent issue.

  Patent Document 3 discloses a technique relating to a metal magnetic powder, but it is disclosed that a calcium salt can be efficiently removed if the particles have a certain degree of agglomerated diameter. That is, as a method for efficient cleaning, if primary particles of hexagonal ferrite magnetic powder can be collected and handled as a unit of agglomerated particles of a certain size, excess components can be efficiently removed even with normal cleaning. I can expect that.

  Therefore, the present inventors should solve the problem of establishing a technique for efficiently producing fine hexagonal ferrite magnetic powder by reducing the time required for solid-liquid separation and washing in the production of hexagonal ferrite magnetic powder. It was determined as an issue.

When the inventors diligently studied the above problem,
A step of melting the hexagonal ferrite magnetic powder and the raw material of the nonmagnetic component to obtain a molten metal;
Cooling the molten metal to obtain a precursor;
Heat treating the precursor to precipitate the hexagonal ferrite magnetic powder, and obtaining a precursor after heat treatment;
A step of immersing the precursor after the heat treatment in an acidic solution, dissolving a nonmagnetic component covering the hexagonal ferrite magnetic powder, and obtaining a treatment liquid comprising aggregated particles of the hexagonal ferrite magnetic powder and an acidic solution; ,
A separation and recovery step of concentrating the liquid to be processed to obtain a concentrated slurry by a liquid cyclone processing step from the liquid to be processed;
The concentrated slurry was added water as a secondary treated liquid, the hydrocyclone process the concentrated the secondary treatment liquid, possess a cleaning step of obtaining a new concentrated slurry, and
The separation and recovery step and the washing step can solve the above-mentioned problems by providing a method for producing a hexagonal ferrite magnetic powder, wherein the treatment is continuously performed in a hydrocyclone apparatus having different classification points. And the present invention was completed.

  As a more preferable form, for example, the average particle diameter of the aggregated particles recovered in the separation is 20 μm or more and 100 μm or less, and the average particle diameter of the primary particles constituting the aggregate particle is 10 to 30 nm. Become.

When configured to sequentially process a plurality of hydrocyclone as this, classification point of the particles in the cyclone of the second stage is such that less than 20 [mu] m, it is desirable to design the cyclone. This is because the classification point of the first-stage cyclone is 20 μm or more.

  Further, in order to improve the degree of cleaning, a re-washing process is performed in which the new concentrated slurry is replaced with the concentrated slurry and the washing process is repeated again. Since the cleaning process can be completed in a short time, the cleaning process can be efficiently repeated and the impurity components can be separated and removed more preferably.

  According to the above method, since the separation and washing of the solid can be performed efficiently, the time required for producing the particles can be shortened.

It is a block diagram of a hydrocyclone apparatus. It is a block diagram of the liquid cyclone apparatus which can perform the washing | cleaning process of multiple times. It is a block diagram of the liquid cyclone apparatus which connected the liquid cyclone apparatus from which a classification point differs in series. It is a transmission electron microscope (TEM) photograph of the hexagonal ferrite magnetic powder produced according to the manufacturing method of the example (using a liquid cyclone apparatus). It is a transmission electron microscope (TEM) photograph of the hexagonal ferrite magnetic powder produced according to the manufacturing method of the comparative example (conventional method: using a filter press apparatus).

  The production method of the hexagonal ferrite magnetic powder obtained by using the glass crystallization method according to the present invention will be described in detail as follows. First, a molten metal in which a hexagonal ferrite magnetic powder and nonmagnetic component raw materials (boric acid, iron oxide, alkaline earth elements and various additive components) are dissolved in a platinum crucible is rapidly cooled to obtain a precursor. A material obtained by dissolving and quenching the hexagonal ferrite magnetic powder and the raw material of the nonmagnetic component is called a “precursor”. For example, when rapidly cooled using the atomizing method, the size of the precursor substantially matches the size of the aggregated particles of the finished hexagonal ferrite magnetic powder. Therefore, in the atomization method, it can be said that the molding of the precursor affects the effects of the subsequent separation and recovery step and the washing step. Here, as the atomizing method, either a water atomizing method or a gas atomizing method can be adopted.

  By heat-treating the precursor thus obtained, hexagonal ferrite magnetic powder (primary particles) is precipitated in the precursor, and a substance in which hexagonal ferrite magnetic powder is formed in a nonmagnetic component can be obtained. it can. This material is called “post-heat treatment precursor”. Thereafter, an acidic solution (especially acetic acid) is added to the precursor after this heat treatment and stirred to obtain a solution in which aggregates of hexagonal ferrite magnetic powder are present in a solution in which the nonmagnetic component is dissolved in the acidic solution. Can do. The solution thus obtained becomes the liquid to be treated. Next, this liquid to be processed is processed by a hydrocyclone apparatus.

  FIG. 1 shows a configuration diagram of the hydrocyclone apparatus. The hydrocyclone apparatus 1 has a shape in which an upper nozzle 14 and a bottom nozzle 12 are provided above and below an inverted conical cylindrical liquid cyclone main body 10. The pipe 24 from the dissolution tank 20 in which the liquid to be treated is produced communicates with the input port 16 formed on the side surface of the liquid cyclone main body 10.

  A collection tank 28 is disposed on the downstream side of the bottom nozzle 12. The upper nozzle 14 communicates with the drainage tank 26. Further, a pump 22 is provided in the middle of the pipe 24 to send the liquid to be treated from the dissolution tank 20 to the liquid cyclone body 10 of the liquid cyclone apparatus 1 at a predetermined pressure.

  Here, when the liquid to be treated is supplied into the apparatus, particles that are somewhat large due to centrifugal force are directed toward the peripheral wall portion of the hydrocyclone main body 10 and are generated along a taper on the side surface of the liquid cyclone main body 10. Then, it is guided to the bottom nozzle 12 and discharged. The bottom nozzle 12 communicates with the collection tank 28. Further, since an upward flow is generated in the central portion, the liquid in which the nonmagnetic component is dissolved is discharged from the upper nozzle 14 at the top and is collected in the drainage tank 26.

  In this way, the liquid to be treated is separated into liquid and particles in which the nonmagnetic component is dissolved. In other words, the aggregated particles of the concentrated hexagonal ferrite magnetic powder pass through the bottom nozzle 12 and are discharged and collected from the lower portion as a concentrated slurry, and the acidic solution containing a large amount of boron and the like is discharged from the upper upper nozzle 14. To be separated. A process of treating a liquid to be treated (including an n-order liquid to be treated, which will be described later) with the hydrocyclone apparatus 1 and separating it into a solution component and hexagonal ferrite magnetic powder (including the aggregated particles) and concentrating the liquid to be treated This is called a hydrocyclone treatment process. This separation / recovery mechanism is called a separation / recovery step.

According to this separation and recovery step, it is preferable that the hexagonal ferrite magnetic powder to be separated is agglomerated particles aggregated to a certain size because the separation efficiency is further improved. The agglomerated particles to be separated herein, the average particle diameter _{(D 50} diameter), 20 [mu] m or more, is preferably not more than 100 [mu] m, more preferably as long as 80μm or less, and most preferably a 75μm or less. These values may be calculated from the volume-based particle size distribution using a laser diffraction particle size distribution measuring device (Heros-Rodos) which is a dry particle size distribution measuring device.

  In the glass crystallization method, contaminants such as acids and barium salts are attached to the surface of the aggregated particles after the cleaning operation. According to the knowledge of the inventors, if the acid remains, the adsorption of the binder when the magnetic coating material is made is inhibited, and the magnetic properties when the medium is made deteriorate. Further, if barium ions remain, it is not preferable because it causes an adverse effect over time such as barium carbonate crystallizes and precipitates on the surface of the recording medium.

  Therefore, the aggregated particles are preferably further washed with pure water or boiled pure water to remove the adhering components. In order to use boiled pure water, a heating tank may be provided in the recovery tank 28 provided downstream of the bottom nozzle 12 of the hydrocyclone apparatus 1 or the dissolution tank 20 for producing the liquid to be treated.

  In some cases, it is also preferable to wash while neutralizing acetic acid adhering to the dissolution of the nonmagnetic component with aqueous ammonia, aqueous sodium hydroxide, aqueous potassium hydroxide, or the like. If it is sodium hydroxide aqueous solution, it is good to set it as 0.01-1.5 mol / L, Preferably it is 0.05-1.2 mol / L, More preferably, it is 0.1-1.0 mol / L. If the concentration is lower than 1.0 mol / L, the cleaning effect is small, and if the concentration is high, the cleaning effect is saturated and the risk of mixing impurities is not preferable.

  These aqueous solutions may be supplied as a cleaning liquid from the dissolution tank 20 or the pipe 24 between the dissolution tank 20 and the hydrocyclone body 10. In addition, when using such an aqueous solution, it is necessary to give the chemical | medical agent used the surface treatment etc. which had corrosion resistance at least in the part which these aqueous solutions (cleaning liquid) contact.

  In order to determine whether or not the washing has been sufficiently performed, the residual amount of acid attached to the hexagonal ferrite magnetic powder is a guide. However, it is not preferable to perform the evaluation for each cleaning process because it increases the takt time. According to previous studies by the inventors, it has been found that there is a correlation between the conductivity of the filtrate after filtration and the degree of washing.

  In order to obtain a hexagonal ferrite magnetic powder having no practical problem, washing should be performed until the filtrate or slurry has a conductivity of 1.0 mS / m or less, preferably 0.8 mS / m or less. . In the present invention, the conductivity can be confirmed by the concentrated slurry discharged from the lower bottom nozzle 12. Further, when the desired level of cleaning cannot be achieved by a single cleaning, the cleaning may be performed a plurality of times or may be used in combination with a filter press or the like after a plurality of cleanings.

  In FIG. 2, the structure of the hydrocyclone apparatus 2 which can be washed several times is shown. In addition to the configuration of FIG. 1, a water tank 25 for supplying water to the recovery tank 28 and a pump 21 for feeding liquid from the recovery tank 28 to the dissolution tank 20 are provided. Water is added from the water tank 25 to the concentrated slurry recovered from the bottom nozzle 12 of the hydrocyclone main body 10 to produce secondary treated water again. This is called a secondary liquid to be treated.

  Thereafter, when the liquid to be processed is processed by a hydrocyclone apparatus to obtain a concentrated slurry, the liquid to be processed is called an n-th liquid to be processed (n is an integer of 1 or more) and distinguished. For example, the first liquid to be processed is a primary liquid to be processed. And what diluted the pure slurry to the concentrated slurry obtained from the first to-be-treated liquid (primary to-be-treated liquid) is a secondary to-be-treated liquid. The secondary liquid to be treated is again passed through the liquid cyclone main body 10 and the new concentrated slurry obtained is diluted with pure water to obtain the tertiary liquid.

  The secondary liquid to be treated is transferred again to the dissolution tank 20 by the pump 21. And it is collect | recovered as a new concentrated slurry again from the bottom nozzle 12 of the liquid cyclone main body 10 of the liquid cyclone apparatus 2. This means that the concentrated slurry has been passed through pure water. Therefore, every time the concentrated slurry (or new concentrated slurry) is diluted and subjected to the hydrocyclone treatment step, the agglomerated particles are repeatedly washed. The liquid cyclone apparatus 2 shown in FIG. 2 can be used to perform repeated cleaning multiple times.

  In addition, the cleaning step can be rephrased as a step of applying a hydrocyclone treatment step to the secondary liquid to obtain a new concentrated slurry. More specifically, water is added to the concentrated slurry obtained in the separation and recovery step to obtain a secondary treatment liquid, and the secondary treatment liquid is concentrated in the liquid cyclone treatment step to obtain a new concentrated slurry.

  Repeating the washing step a plurality of times means replacing the new concentrated slurry with the concentrated slurry and performing the washing step again. This is called a re-cleaning process. At this time, what is processed in the hydrocyclone processing step is an n-order liquid to be processed in which n is 3 or more.

  In FIG. 3, the structure of the hydrocyclone apparatus 3 is shown. In addition to the configuration shown in FIG. 2, the hydrocyclone device 3 further includes a one-stage hydrocyclone main body 30. From the upper nozzle 14 of the liquid cyclone main body 10, a solution containing nonmagnetic components and particles smaller than the classification point of the liquid cyclone main body 10 is collected in the drainage tank 26. Therefore, the solution in the drainage tank 26 is further pumped to the liquid cyclone main body 30 by the pump 31.

  The hydrocyclone main body 30 has an outer diameter smaller than that of the hydrocyclone main body 10, and the classification point is further smaller. Therefore, aggregated particles having a size that could not be collected by the liquid cyclone main body 10 can be collected from the bottom nozzle 32. As described above, a plurality of hydrocyclone apparatuses may be operated in series. The classification points of the connected hydrocyclone devices may be sequentially reduced, or the hydrocyclone devices having the same classification point may be connected. By doing so, the recovery rate becomes higher, and the time required for solid-liquid separation and washing becomes shorter.

  The obtained aggregated particles after the washing treatment can be obtained as a dry powder by subjecting them to moisture removal treatment under conditions of 100 ° C. or higher in the atmosphere. Then, in order to improve the dispersibility with respect to a binder, you may make a water | moisture content adhere to the dry magnetic powder surface about 0.5-5.0 mass% in the wet environment of about 80% RH.

<Evaluation of conductivity>
The electrical conductivity of the filtrate and slurry was measured using an electrical conductivity meter ES-51 manufactured by Horiba, Ltd.

<Evaluation of magnetic powder>
The physical properties of the obtained magnetic powder were evaluated by the following methods.

<Calculation of specific surface area by nitrogen adsorption>
The calculation of the specific surface area using nitrogen was measured using the BET single point method, and the measuring device was measured using 4 Sorb US made by Your Sonics Co., Ltd.

<Powder magnetic property evaluation>
Hexagonal ferrite magnetic powder is packed in a φ6 mm plastic container and using a VSM device (VSM-P7-15) manufactured by Toei Kogyo Co., Ltd. with an external magnetic field of 795.8 kA / m (10 kOe) and a coercive force Hc. (KA / m, Oe), saturation magnetization σs (Am ² / kg), squareness ratio SQ, and SFD (SFD value in the bulk state) of the powder were measured.

<Evaluation of magnetic powder yield>
The yield of the hexagonal ferrite magnetic powder recovery was calculated from the weight of the hexagonal ferrite magnetic powder actually obtained with respect to the weight of the hexagonal ferrite magnetic powder calculated from the composition design.

(Example)
As raw materials, iron oxide (HRT made by Tetgen), barium carbonate (manufactured by Solvay / industrial use), boric acid (manufactured by Borax / industrial use) and auxiliary materials (bismuth oxide (manufactured by Yensho Industrial Co., Ltd./industrial use)) Niobium oxide (manufactured by High-Purity Chemical Laboratories / reagent) was mixed so as to have a predetermined ratio, and the composition at this time was implemented in a design in which 44.9% of ferrite was contained in the precursor. The mixture was put into a crucible and stirred and melted for 1 hour in a furnace at 1400 ° C. Thereafter, the melt was quenched by a gas atomization method to prepare a precursor.

  This precursor was heat-treated at 630 ° C. for 1 hour to prepare 20 kg of a precursor after heat treatment in which hexagonal ferrite magnetic powder was precipitated. The precursor after this heat treatment is added to 200 kg of acetic acid adjusted to a concentration of 10% by mass and then stirred at 60 ° C. for 1 hour to obtain an acetic acid solution (treated liquid) in which aggregated particles of hexagonal ferrite magnetic powder are dispersed. (Ferrite concentration: about 4% by mass). The average particle diameter D50 of the agglomerated particles of the hexagonal ferrite magnetic powder was 60 μm.

  Next, by supplying the liquid to be treated into a liquid cyclone main body (classification point: 10 μm) by a pump (primary pressure supplied to the liquid cyclone main body is 0.3 MPa), a concentrated slurry recovered from the bottom nozzle, The acidic solution containing fine particles recovered from the upper nozzle was separated (separation recovery step).

  To the concentrated slurry recovered from the bottom nozzle, the same volume of pure water as the acidic solution recovered from the upper nozzle is added, and the ferrite concentration of the concentrated slurry is almost the same as the solid concentration of the liquid to be treated first. Dilute to This is a secondary liquid to be treated. This secondary liquid to be treated was stirred again and then supplied again into the liquid cyclone to separate it into a new concentrated slurry and an acidic solution (cleaning step).

  In the acidic solution recovered from the upper nozzle, fine particles below the classification point of the liquid cyclone main body and nonmagnetic components such as an acetic acid component and boric acid are separated. In addition, you may process this acidic solution again with the liquid cyclone main body with a smaller classification point shown in FIG.

  Thereafter, a new concentrated slurry was used as a concentrated slurry, and the same concentration and dilution processes were performed again using a liquid cyclone apparatus to obtain a tertiary liquid to be treated. The time required to obtain the third liquid to be treated (time for performing the two washing steps) was 30 minutes.

The new concentrated slurry thus obtained was subjected to a commercially available filter press (model number: TFO-10-9 (13), manufacturer: manufactured by Ataca Daiki Co., Ltd., filter: PP201A, filtration capacity: 6.8 L, filtration area: 1 .36 m ² ) and collected and washed. Since the agglomerates below the classification point were separated by the hydrocyclone apparatus, the filter press apparatus was hardly clogged and was washed until the conductivity reached 1.0 mS / m, and the time required for that was 50 minutes. . The yield was 88.0%.

(Comparative example)
An acetic acid solution in which aggregated particles (average particle diameter D50: 60 μm) of hexagonal ferrite magnetic powder produced in the same manner as in Example was dispersed was used as a commercially available filter press device (model number: TFO-10-9 (13), manufacturer: Ataka Univ.). Using an apparatus manufactured by Kikai Co., Ltd., filter: PP201A, filtration capacity: 6.8 L, filtration area: 1.36 m ² ), an operation for removing the acidic solution by filtration was performed. At this time, particles passing through the eyes of the filter cloth were present and the yield was lowered. Therefore, an operation of recharging the collected filtrate into the filter press apparatus (resupply of the filtrate to the filter press apparatus) was performed.

  Thereafter, pure water was supplied (supply pressure: 0.5 MPa), and an operation of removing non-magnetic components such as acetic acid components and boric acid attached to the ferrite magnetic powder aggregated particles was performed in a filter press apparatus.

  At this time, it took 200 minutes to reach the same conductivity (1.0 mS / m or less) as in the example. In addition, since the time required for resupply of the filtrate to the filter press apparatus implemented for yield improvement was 30 minutes, the time required for the treatment was 230 minutes in total. The yield was 87.9%.

  When the change in the filtrate flow rate during filtration according to the comparative example was monitored, the measured flow rate tended to decrease as the cumulative amount of liquid passing through the filter increased during filtration.

  The agglomerated particles of the hexagonal ferrite magnetic powder washed by the method of the example and the agglomerated particles of the hexagonal ferrite magnetic powder washed by the method of the comparative example are not so different in physical properties as the hexagonal ferrite magnetic powder. It was. There was no significant difference between the magnetic characteristics and the evaluation of the magnetic characteristics when the medium was formed.

  FIG. 4 shows a TEM photograph of hexagonal ferrite magnetic powder (Example 1) that has been subjected to the separation and recovery step and the washing step with a hydrocyclone device and a filter press device, and FIG. The TEM photograph of the hexagonal ferrite magnetic powder (Comparative Example 1) is shown. Each magnification is 174,000. Even if it confirmed with the TEM photograph, the difference in shape size was not seen.

  Table 1 shows the magnetic properties, specific surface area, and yield values of the hexagonal ferrite magnetic powders of Example 1 and Comparative Example 1. All values were almost the same.

  From the above, if the production method according to the present invention is used, the time required for production can be greatly shortened even though the same substance is obtained as the final product. It was confirmed that it contributes greatly to the establishment of a simple manufacturing method.

  By using the production method of the present invention, a fine hexagonal ferrite magnetic powder can be obtained efficiently. This contributes to an increase in the supply amount of the high-density magnetic recording medium.

1, 2, 3 Hydrocyclone apparatus 10, 30 Hydrocyclone main body 12, 32 Bottom nozzle 14, 34 Upper nozzle 16 Input port 20 Dissolution tank 21, 22, 31 Pump 23 Valve 24 Pipe 25 Water tank 26, 36 Drain tank 28, 38 Collection tank

Claims (4)

A step of melting the hexagonal ferrite magnetic powder and the raw material of the nonmagnetic component to obtain a molten metal;
Cooling the molten metal to obtain a precursor; and
Heat treating the precursor to precipitate the hexagonal ferrite magnetic powder, and obtaining a precursor after heat treatment;
A step of immersing the precursor after the heat treatment in an acidic solution, dissolving a nonmagnetic component covering the hexagonal ferrite magnetic powder, and obtaining a treatment liquid comprising aggregated particles of the hexagonal ferrite magnetic powder and an acidic solution; ,
A separation and recovery step of concentrating the liquid to be processed to obtain a concentrated slurry by a liquid cyclone processing step from the liquid to be processed;
The concentrated slurry was added water as a secondary treated liquid, the hydrocyclone process the concentrated the secondary treatment liquid, possess a cleaning step of obtaining a new concentrated slurry, and
The method for producing a hexagonal ferrite magnetic powder is characterized in that the separation and recovery step and the washing step are continuously performed in a liquid cyclone apparatus having different classification points .   2. The method for producing a hexagonal ferrite magnetic powder according to claim 1, wherein an average particle diameter of the aggregated particles is 20 μm or more and 100 μm or less.   3. The method for producing a hexagonal ferrite magnetic powder according to claim 1, wherein an average particle diameter of primary particles constituting the aggregated particles is 10 to 30 nm. The method for producing a hexagonal ferrite magnetic powder according to any one of claims 1 to 3 , further comprising a rewashing step of replacing the new concentrated slurry with the concentrated slurry and repeating the washing step again.