JP4989024B2 - Magnetite powder and method for producing the same - Google Patents

Description

The present invention relates to a magnetite (Fe ₃ O ₄ ) powder obtained by reducing hematite (Fe ₂ O ₃ ), and more particularly to an improved radio wave absorptivity in the GHz band.

  In recent years, due to the rapid development of communication means, electronic devices using a high frequency in the GHz band are rapidly increasing. Along with this, from the viewpoint of preventing malfunction of electronic devices and the like, the spread of radio wave absorbers that exhibit good characteristics in the GHz band is strongly desired.

  Conventionally, soft ferrite has been almost exclusively used for radio wave absorbers for radio waves in the microwave region (from the MHz band to the GHz band) except for special applications. However, soft ferrite is expensive because it is manufactured by adding metal oxides for compounding to hematite, which contributes to an increase in the cost of the radio wave absorber and thus the electronic equipment. In addition, soft ferrite has a radio wave absorption limit called “the limit of snake”, and the absorption capacity for high frequency radio waves of 10 GHz or more is rapidly attenuated, so that it cannot sufficiently cope with high frequency radio waves for communication. There is a problem.

On the other hand, magnetite (Fe ₃ O ₄ ) is one of the few black pigments that absorbs electromagnetic waves in the visible light region and can be expected to absorb radio waves in the high frequency region, but is widely used for paint and black toner applications. However, it has not been widely used as a radio wave absorber.

  As an industrial production method of magnetite, a method of producing magnetite directly from a Fe ion-containing aqueous solution without using hematite by a “wet method” (for example, Patent Document 1) is known. In addition, a method of obtaining magnetite from a mixed material of ferrous chloride and hematite by utilizing thermal decomposition and solid phase reaction (for example, Patent Document 2) is also known.

  There are a “wet method” and a “dry method” as methods for producing the intermediate hematite. The “wet method” neutralizes the Fe-containing acid solution, and the accuracy of the particle size can be increased by carefully setting the neutralization conditions. However, since there are many processes, cost becomes high. On the other hand, the “dry method” is a method of dehydrating and oxidizing the Fe-containing acid solution, and there are a “low temperature drying method” in which the Fe-containing acid solution is once dried at a low temperature and then fired at a high temperature and a “thermal decomposition method” in which it is fired at a high temperature at once . The dry method is advantageous in terms of cost because there are few steps.

JP 7-257930 A JP-A-9-71423

  As described above, although magnetite is expected to have radio wave absorption characteristics in a high frequency region, it has not yet been widely distributed in place of soft ferrite. The present invention seeks to develop and provide magnetite with significantly improved radio wave absorption performance.

  As a result of various studies, the inventor uses hematite obtained by thermally decomposing an Fe-containing acid solution by a dry method, and when this is reduced to magnetite, it is compared with the existing magnetite produced mainly by the wet method. It has been found that the radio wave absorption performance for high frequencies in the GHz band is significantly improved. As a result of investigations on the structure of hematite, an intermediate substance, it was found that hematite obtained by the pyrolysis method has a surface property that is significantly different from that obtained by the wet method. The present invention has been completed based on such novel findings.

That is, in the present invention, as the magnetite powder with significantly improved radio wave absorption performance, hematite in which layered uneven patterns with an interval of 5 to 80 nm are observed in an AFM (Atomic Force Microscope) image is used. 98 mass% or more formed by cooling and heating at 400 to 650 ° C. with a rotary kiln equipped with a mechanism and a cooling mechanism using a water cooling device in one core tube and not lined inside the core tube An electromagnetic wave absorbing magnetite powder is provided which comprises magnetite.
Here, the layered unevenness is an uneven shape or a stepped uneven shape in which bowl-shaped protrusions are arranged substantially in parallel. The interval between the layered concavo-convex patterns is a projection distance in an AFM image between adjacent convex portions (or concave portions).

Such electromagnetic wave absorbing magnetite powder is obtained by reducing and cooling hematite (hereinafter referred to as “pyrolytic hematite”) obtained by pyrolyzing an Fe-containing acid solution in an air atmosphere at 450 ° C. or higher by the above-described method. Obtainable. For example, it obtained by reducing and cooling hematite obtained by pyrolysis by spraying the steel pickling waste liquid in the air atmosphere at 450 to 900 ° C. in the above manner.

Among such magnetite powders, those having an average particle size D ₅₀ of 1 to 10 μm and D ₉₀ of 10 to ₅₀ μm are suitable targets.
Here, D ₅₀ is the average particle diameter in the particle size distribution measured by laser diffraction method. D ₉₀ is a particle size corresponding to 90% number abundance in the particle size distribution measured by laser diffraction method.

The magnetite powder obtained by reducing and cooling the pyrolytic hematite may be further subjected to wet grinding. The powder obtained by wet-grinding the pyrolytic hematite can be reduced to magnetite. As the magnetite thus refined, a magnetite powder pulverized to have an average primary particle size of 0.1 to 1.0 μm is a suitable target.

  According to the present invention, as will be described later, it has become possible to provide a magnetite that has significantly improved radio wave absorption performance in the GHz band compared to conventional magnetite. Since this magnetite can use hematite obtained by thermally decomposing iron pickling waste liquid as a raw material, the cost can be reduced as compared with conventional soft ferrite. In addition, since pyrolytic hematite usually has a broad particle size distribution, the magnetite produced based on the particle size distribution also has a broad particle size distribution compared to the wet method. A magnetite powder having a broad particle size distribution is expected to have a large filling rate as a filler.

  In FIG. 1, the AFM (atomic force microscope) image of the hematite used for manufacture of the sample E (magnetite of the example of this invention) of the below-mentioned Example is illustrated. This hematite is obtained by spraying a steel pickling waste liquid into an air atmosphere of about 700 ° C. and thermally decomposing it. AFM observation was performed in the dynamic mode using SFT-9800 manufactured by Shimadzu Corporation. Hematite powder was evenly applied to the flat cleaved surface of mica at the atomic level and observed. As can be seen from FIG. 1, a layered uneven pattern is seen on the surface of the hematite particles. The part where the space | interval of the uneven | corrugated pattern (For example, the projection distance on the photograph of the convex parts which look white) is 5-80 nm is observed everywhere.

  FIG. 2 illustrates an AFM (Atomic Force Microscope) image of hematite used for the manufacture of Sample D (magnetite of Comparative Example) in Examples described later. This hematite is a hematite (hereinafter referred to as “wet hematite”) manufactured by adding sodium hydroxide to an aqueous ferrous sulfate solution to produce iron oxyhydroxide and aeration of it with air. AFM observation was performed in the same manner as described above. No clear surface irregularities are seen in the AFM image of FIG.

  The layered surface irregularities seen in FIG. 1 are presumed to reflect the structure in which a large number of planar defects exist inside the crystal of the hematite particles. That is, it can be considered that the hematite particles have a structure including many faults. Such a fault structure is not found in wet hematite (FIG. 2), and is unique to hematite obtained by a dry process involving thermal decomposition. Although the mechanism of its formation has not been fully elucidated, it is considered that a large number of defects are introduced into the crystal by applying a large thermal shock due to thermal decomposition during the crystal growth process.

  FIG. 3 shows a TEM (transmission electron microscope) photograph of magnetite particles (sample E described later) obtained by reducing the pyrolytic hematite of FIG. Contours of the irregularities are observed, which are considered to be derived from the layered irregularities seen on the surface of the hematite particles. As will be described later in Examples, this magnetite has significantly improved radio wave absorption characteristics as compared with magnetite obtained directly by a wet method or magnetite obtained by reducing wet hematite. The improvement mechanism is still unclear, but it is presumed that the structural difference due to internal defects is acting effectively.

Pyrolytic hematite, which is an intermediate substance for obtaining the magnetite powder of the present invention, can be produced, for example, as follows.
First, an Fe-containing acid solution is prepared as a raw material. A solution mainly composed of ferrous chloride is preferred. For example, the waste acid from the steel pickling line is mainly ferrous chloride and can be used in the present invention. In particular, the waste acid that comes out of the pickling line of ordinary steel contains Fe (mostly 95% by mass or more) of the metal content. It can be subjected to a reduction treatment to magnetite. Care must be taken because the waste acid from the pickling line of high alloy steel such as stainless steel contains a large amount of metal such as Cr and Ni in addition to Fe.

  Next, pyrolyzed hematite is obtained by baking the Fe-containing acid solution in a high-temperature atmosphere. The atmosphere temperature is desirably 450 ° C. or higher. If the temperature is lower than that, it is difficult to stably obtain hematite having a fault structure. This is thought to be due to the lack of thermal shock. Practically, the temperature is preferably about 500 to 900 ° C, and more preferably 550 to 850 ° C. As the atmospheric temperature increases, the average particle size of the obtained hematite generally tends to increase. Such roasting can be suitably carried out by using an acid-resistant rotary kiln described later. In the case of using a steel pickling waste liquid, it is possible to divert the treatment by the Lusner method, which is mainly intended for the recovery of hydrochloric acid.

  In this way, hematite is obtained in which layered uneven patterns with an interval of 5 to 80 nm are observed in an AFM (atomic force microscope) image. When this pyrolytic hematite is reduced to magnetite, it becomes a magnetite with improved radio wave absorption characteristics in the high frequency band. Reduction to magnetite is basically performed by heating in a reducing atmosphere such as hydrogen. However, it should be noted that pyrolytic hematite generally has a broad particle size distribution, and the powder contains submicron ultrafine particles. That is, when using a general furnace (rotary kiln, etc.) used for powder drying and roasting treatment, submicron ultrafine particle hematite enters the lining joint, and when the residence time increases, it is excessively reduced to metallic iron. There is a risk that. If this fine metallic iron powder is mixed in the product, it may ignite in the atmosphere, which is dangerous. In addition, when the furnace is opened, it may ignite in the furnace and damage the lining.

As a result of various studies, the inventor uses a reduction furnace having the following configuration in order to safely finish the reduction at the magnetite stage without causing such trouble and to obtain the target magnetite powder product. I found out that I should do.
That is, in a rotary kiln in which reducing gas is introduced and the inside of the furnace is kept in a reducing atmosphere, i) to prevent scattering of fine hematite particles, heating is an external heating radial method, and ii) the core tube is lined Made of stainless steel and other highly corrosion-resistant and heat-resistant alloys, iii) Provide a stirring mechanism so that the entire powder contacts the gas as uniformly as possible, and iv) To prevent ignition when metallic iron powder is generated A powerful cooling mechanism (such as a water cooling device) is provided on the rear surface (powder take-out side) of the core tube.
The target magnetite powder could be obtained safely and stably by the rotary kiln having such a special structure.

As the reducing atmosphere, for example, an atmosphere in which the temperature is maintained at about 400 to 650 ° C. while continuously introducing a reducing gas such as H ₂ or CO can be adopted. If it is less than 400 degreeC, a reductive reaction will not advance easily. If the temperature exceeds 650 ° C., it becomes difficult to control the residence time for preventing excessive reduction. That is, it becomes easy to reduce to wustite or even metallic iron beyond magnetite due to a delay in residence time. In the above temperature range, an appropriate reduction residence time can be set in a range of approximately several minutes to 30 minutes, which is suitable for industrial production.

When considering the application of the wave absorber, the average particle diameter D ₅₀ of 1 to 10 [mu] m, the powder of magnetite D ₉₀ of having a particle size distribution of 10~50μm is preferred. Since magnetite is a magnetic particle, it usually does not exist in a form separated into primary particles, and exists as secondary particles in which primary particles are aggregated. The D ₅₀ is a value measured as the particle size of the secondary particles. However, the inventors' research has shown that the radio wave absorption performance is further improved when the average particle diameter D ₅₀ of the primary particles constituting the secondary particles is reduced to 1 μm or less. Specifically, finely pulverized magnetite having an average particle diameter D ₅₀ of primary particles of magnetite powder of 0.1 to 1.0 μm is extremely effective in improving the radio wave absorption characteristics in the GHz band. In this case, the D ₉₀ of the primary particles is desirably about 0.4 to 3.0 μm.

In the usual pyrolysis method, pyrolyzed hematite having a D ₅₀ of about 1 to several μm is usually obtained. However, since granulation and slight sintering occur during the reduction treatment, the D ₅₀ of the reduced magnetite powder is It becomes bigger than that. In the state after the reduction, when such D ₅₀ of the magnetite powder is greater than 10μm, it is possible to adjust the D ₅₀ to 1~10μm by wet grinding the powder. D ₉₀ can also be adjusted to 10 to 50 μm at the same time. Moreover, the magnetite powder which refined the primary particle to 0.1-1.0 micrometer by fully wet-pulverizing the magnetite powder obtained by reduction | restoration can be obtained. As such a wet grinding means, it is preferable to use a high-speed bead mill. However, when pulverizing after reduction, it is necessary to pay attention to the drying temperature and atmosphere so that the magnetite is not oxidized during drying.

[Example 1]
After concentrating the iron pickling waste liquid from the pickling line of ordinary steel, spray roasting was performed by the Lusner method to obtain pyrolytic hematite. This concentrated waste acid was composed of Fe for 97% of the contained metal. In the spray roasting, the concentrated waste acid was sprayed from the top of the roasting furnace (roaster) into several air nozzles at a pressure of 50 to 80 N / cm ² in an air atmosphere of about 700 ° C. As a result of measuring the particle size distribution of the obtained pyrolytic hematite with a laser diffraction particle size distribution analyzer (SALD-2000 manufactured by Shimadzu Corporation), the average particle size D ₅₀ = 2.93 μm and D ₉₀ = 19.40 μm. . Moreover, as a result of observing this pyrolysis hematite with an AFM (atomic force microscope), as shown in FIG. 1 described above, layered uneven patterns with an interval of 5 to 80 nm were observed. The observation method of AFM is as described above.

This pyrolytic hematite was reduced with the rotary kiln having the special structure described above to obtain a magnetite powder. In this rotary kiln, the inner diameter of the core tube is about 250 mm, and has the above-described structures i) to iv). The core tube and the stirring blade are made of stainless steel, and the interior is not lined. H ₂ gas was introduced at a flow rate of 150 L / min, and reduction treatment was performed at a reduction temperature of 500 ° C. for 15 minutes. As a result of X-ray diffraction, the obtained powder was 98% by mass or more of magnetite. The particle size distribution of the magnetite powder was measured by the above method. As a result, D ₅₀ = 6.2 μm and D ₉₀ = 35.3 μm. The difference in the particle size between pyrolytic hematite and magnetite obtained by reducing it is thought to be due to granulation due to reduction by a rotary kiln system and slight sintering.
This magnetite powder was used as sample E and subjected to the radio wave absorption characteristic test described later and the pulverization treatment of Example 3.

[Example 2]
An aqueous ferrous chloride solution with an Fe concentration of 20% was prepared, and spray roasting was performed by the method of dropping into the rotary kiln furnace used for the reduction of hematite in Example 1 to obtain pyrolytic hematite. The roasting atmosphere was an air atmosphere of about 800 ° C. As a result of measuring the particle size distribution of the obtained pyrolyzed hematite by the above method, the average particle size D ₅₀ = 6.61 μm and D ₉₀ = 19.40 μm. Moreover, as a result of observing this pyrolytic hematite with an AFM (atomic force microscope) in the same manner as in Example 1, layered uneven patterns with an interval of 5 to 80 nm were observed as in FIG.

This pyrolytic hematite was reduced by the rotary kiln used in Example 1 to obtain a magnetite powder. H ₂ gas was introduced at a flow rate of 150 L / min, and reduction treatment was performed at a reduction temperature of 600 ° C. for 20 minutes. As a result of X-ray diffraction, the obtained powder was 98% by mass or more of magnetite. The particle size distribution of the magnetite powder was measured by the above method. As a result, D ₅₀ = 9.4 μm and D ₉₀ = 39.0 μm.
This magnetite powder was used as Sample F and subjected to a radio wave absorption characteristic test described later.

Example 3
The magnetite powder of Sample E obtained in Example 1 was pulverized for 60 minutes by a wet high-speed bead mill to obtain a refined magnetite powder. The particle size distribution of the magnetite powder was measured by the above method. As a result, D ₅₀ = 1.61 μm and D ₉₀ = 3.42 μm. The measured value of the particle size distribution is that of the secondary particles in which the primary particles are aggregated. When a pulverization treatment experiment was performed using hematite powder under the same conditions as described above, the particle size distribution of the finely pulverized hematite was D. ₅₀ = 0.37 μm and D ₉₀ = 0.64 μm. Since hematite is a particle that does not have magnetism (that is, the primary particles can exist separately), the measured value is considered to be a measured value of the primary particles, and the primary of the finely pulverized magnetite obtained here It can be seen that the particle size distribution of the particles is about the same. This is affirmed from the results of observation of the finely pulverized magnetite particles by a scanning electron microscope.
This refined magnetite powder was used as a sample G for a radio wave absorption characteristic test described later.

(Radio wave absorption characteristics test)
(1) Microwave exposure test The following magnetite powder was prepared as a sample.
Sample C: Magnetite powder obtained directly by reducing iron oxyhydroxide by a wet method. Average particle diameter D ₅₀ = 2.3 μm, D ₉₀ = 4.2 μm.
Sample D: Magnetite powder obtained by reducing hematite obtained by the method of neutralizing the Fe-containing acid solution by a wet method by the same method as in Example 1. Average particle diameter D ₅₀ = 2.8 μm, D ₉₀ = 5.3 μm.
Sample E: Magnetite powder of Example 1 obtained by reducing pyrolytic hematite. D ₅₀ = 6.2 μm, D ₉₀ = 35.3 μm.
Sample F: Magnetite powder of Example 2 obtained by reducing pyrolytic hematite. D ₅₀ = 9.4 μm, D ₉₀ = 39.0 μm
Sample G: Example 3 magnetite powder obtained by pulverizing Sample E. D ₅₀ = 1.61 μm, D ₉₀ = 3.42 μm. The average particle size of the primary particles is in the range of 0.1 to 1.0 μm.

  A radio wave absorber absorbs radio waves by converting radio wave energy into thermal energy. Therefore, in order to easily compare the radio wave absorption characteristics of each sample in the GHz band, the reaction of the sample was examined using a microwave oven. A microwave oven is a cooking utensil that emits a powerful radio wave of 2.45 GHz. Here, a microwave oven with an output of 1000 W that is commercially available for home use was used. 5 g of the sample powder was put in an evaporating dish, exposed to a powerful radio wave of 2.45 GHz with a microwave oven, and the time from the start of the exposure until it became red heat or the start of discharge was measured. It is considered that the shorter the reaction time, the greater the action of converting the radio wave energy into heat, that is, the radio wave absorption action is greater. The experiment was performed with a repetition number n = 5 for each sample. The results are shown in Table 1.

  As can be seen from Table 1, the magnetite (samples E, F, and G) obtained through pyrolysis hematite has a longer time to start red heat and discharge than the magnetite (samples C and D) obtained through the wet method. Was significantly shortened, and the absorption characteristics for radio waves of 2.45 GHz were significantly improved. This is presumed that the defect structure inside the crystal introduced in the process of generating pyrolytic hematite has some effect. In addition, finely pulverized magnetite (sample G) having an average primary particle size in the range of 0.1 to 1.0 μm is further absorbed as compared with non-pulverized particles (samples E and F). The characteristics have been improved.

(2) Radio wave absorption measurement test using powder alone Generally, magnetic powder for radio wave absorbers is kneaded with other molding materials such as polymers, and the radio wave absorption is based on the type of polymer, filler filling rate, compound It is affected by the thickness. Here, in order to grasp the absorption characteristics inherent in the powder, only a powder that has not been compounded was prepared to a thickness of 2 mm or 6 mm by a natural deposition / leveling method, and used as a measurement sample. Samples C and E of the magnetite powder and a reagent carbonyl iron powder (average particle size 8.5 μm) were used. For each measurement sample, the radio wave absorption at 0.1 to 3 GHz was measured by the near electric field absorption measurement method. The measurement was performed by the microstrip line method at the organization to which the electromagnetic wave absorption characteristics JIS committee belongs. The radio wave absorption amount was evaluated by the absorption rate (%) based on the absorption amount of carbonyl iron powder (100%) for each green compact thickness. The results are shown in Table 2.

  As can be seen from Table 2, the magnetite powder (sample E) of the present invention obtained by reducing pyrolytic hematite is compared with that of the conventional magnetite powder (sample C) obtained through the wet method, Absorption characteristics are significantly improved.

The AFM (atomic force microscope) image about the pyrolysis hematite used for manufacture of the magnetite of the sample E (invention example). The AFM (atomic force microscope) image about the wet hematite used for manufacture of the magnetite of the sample D (comparative example). The transmission electron micrograph which image | photographed a part of magnetite particle | grains of the sample E (invention example).

Claims (9)

Hematite whose layer surface unevenness pattern is observed on the particle surface in the AFM (Atomic Force Microscope) image is combined with an external heating type heating mechanism, a stirring mechanism, and a cooling mechanism using a water cooling device in one core tube. A magnetite powder for radio wave absorption comprising 98% by mass or more of magnetite , which is provided and is heated and reduced at 400 to 650 ° C. by a rotary kiln that is not lined inside the furnace tube and then cooled. Hematite obtained by pyrolyzing an Fe-containing acid solution in an air atmosphere at 450 ° C. or higher is equipped with an external heating type heating mechanism, a stirring mechanism, and a cooling mechanism by a water cooling device in one core tube, and the core tube Electromagnetic wave absorbing magnetite powder comprising 98% by mass or more of magnetite, which is reduced by heating and reducing at 400 to 650 ° C. with a rotary kiln without lining inside, and then cooling. Hematite obtained by thermal decomposition by spraying steel pickling waste liquid into an air atmosphere at 450 to 900 ° C. is combined with an external heating type heating mechanism, a stirring mechanism, and a cooling mechanism with a water cooling device in one core tube. A magnetite powder for radio wave absorption comprising 98% by mass or more of magnetite , which is provided and is heated and reduced at 400 to 650 ° C. by a rotary kiln that is not lined inside the furnace tube and then cooled. 4. The electromagnetic wave absorbing magnetite powder comprising 98% by mass or more of magnetite according to claim 1, having an average particle size D ₅₀ of 1 to 10 μm and D ₉₀ of 10 to ₅₀ μm.
However, D ₅₀ is the average particle diameter measured by a laser method, D ₉₀ is the particle diameter corresponding to 90% the number existence ratio in the particle size distribution measured by a laser method. A magnetite powder for radio wave absorption in which 98% by mass or more obtained by wet-grinding the magnetite powder according to any one of claims 1 to 3 comprises magnetite. 6. An electromagnetic wave absorbing magnetite powder comprising 98% by mass or more of magnetite according to claim 5, wherein the primary particles have an average particle size of 0.1 to 1.0 μm.   Hematite whose layer surface unevenness pattern is observed on the particle surface in the AFM (Atomic Force Microscope) image is combined with an external heating type heating mechanism, a stirring mechanism, and a cooling mechanism using a water cooling device in one core tube. A method for producing magnetite powder for radio wave absorption, which is provided with a rotary kiln that is not lined inside the furnace tube and is heated and reduced at 400 to 650 ° C. to form magnetite and then cooled.   Hematite obtained by pyrolyzing an Fe-containing acid solution in an air atmosphere at 450 ° C. or higher is equipped with an external heating type heating mechanism, a stirring mechanism, and a cooling mechanism by a water cooling device in one core tube, and the core tube A method for producing a magnetite powder for radio wave absorption, which is heated and reduced at 400 to 650 ° C. to form magnetite and then cooled by a rotary kiln without lining inside.   Hematite obtained by thermal decomposition by spraying steel pickling waste liquid into an air atmosphere at 450 to 900 ° C. is combined with an external heating type heating mechanism, a stirring mechanism, and a cooling mechanism with a water cooling device in one core tube. A method for producing magnetite powder for radio wave absorption, which is provided with a rotary kiln that is not lined inside the furnace tube and is heated and reduced at 400 to 650 ° C. to form magnetite and then cooled.