JPH033614B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention is directed to γ-iron oxide (γ-iron oxide), which is useful as a magnetic material.
-Fe ₂ O ₃ ) This relates to a method for producing fine particles.
More specifically, the present invention has excellent dispersibility in water and various aerobic solvents, and has excellent thermal and magnetic stability, and is particularly suitable as a magnetic material used for the separation and recovery of magnetic fluids and various useful substances. Excellent γ
- It relates to a method for effectively producing iron oxide fine particles. Conventional technology Since magnetite and γ-iron oxide, which are iron oxides, have ferromagnetic properties, their powders have been used as magnetic materials, such as magnetic recording materials such as magnetic tapes, magnetic disks, and magnetic tickets. It is widely used in ferrite materials such as magnetic cores, permanent magnets, and memory elements. In particular, γ-iron oxide has better chemical and magnetic stability at room temperature than magnetite, so its demand has increased with the development of audio equipment such as stereos, tape decks, and car stereos, and video tape recorders. is growing rapidly. Recently, the use of magnetite particles and γ-iron oxide particles as magnetic fluids has attracted attention as a new application. Applications are being considered for sensors, medicines, agricultural chemicals, etc. Furthermore, magnetite particles and γ-iron oxide particles are also used for the separation and recovery of various useful substances. For example, heavy metal adsorbents or various ion adsorbents are immobilized on the surface of the particles to selectively capture desired useful substances, which are then used for purposes such as efficient recovery and separation using a magnetic field. There is. By the way, magnetite particles and γ-
Iron oxide particles have so far been produced by heating and oxidizing acicular iron oxyhydroxide obtained by a wet method in an electric furnace. For example, as a general method for producing magnetite particles, ferric salt is dissolved in water, an alkali is added to this solution, the solution is heated and oxidized to produce needle-shaped α-iron oxyhydroxide particles, and the particles are washed with water. After drying, place it in an electric oven.
A method is used to heat and dehydrate at a temperature of 600°C for about 3 hours to produce α-iron oxide (α-Fe ₂ O ₃ ) particles, and then reduce them to obtain magnetite particles (Japanese Patent Application Laid-Open No. 1983-1993).
-60632). In addition, γ-iron oxide particles are produced by heating the magnetite particles obtained in this way and gradually increasing the temperature.
It is manufactured by holding it at a temperature of 200°C for about 1 hour. By the way, the magnetite particles and γ-iron oxide particles produced by such a method have a length in the major axis direction of 0.3 to 0.5 μm and a length ratio of major axis to minor axis of 8 to 10.
It has an acicular shape and has excellent magnetic properties, but magnetite particles obtained by the above method and γ
- When iron oxide particles are used as a magnetic fluid or as a carrier for a separation and recovery material for useful substances, there are problems such as the particles are too large, have poor dispersion stability in solvents, and have a small specific surface area. On the other hand, iron oxide fine particles (particle size of several 100 Å) are used as carriers for separation and recovery materials for magnetic fluids and useful substances.
As a manufacturing method, a coprecipitation method (Japanese Patent Application Laid-open No. 175754/1983) is known, in which fine magnetite particles are obtained by adding an alkali to an aqueous solution containing a ferrous salt and a ferric salt. However, the magnetite fine particles produced in this way dispersed in an aqueous solution have poor dispersibility in low polar solvents because hydration water and adsorbed water cannot be completely removed, and they also have poor dispersibility in low polar solvents. It has drawbacks such as being magnetically unstable. Therefore, it has been desired to develop ferromagnetic iron oxide fine particles that have good magnetic stability and excellent dispersibility in various solvents. Problems to be Solved by the Invention The purpose of the present invention is to meet these demands and to obtain particles having a particle size of approximately 100 Å, excellent dispersibility in water and various organic solvents, and good thermal and magnetic stability. An object of the present invention is to provide ferromagnetic iron oxide fine particles. Means for Solving the Problems As a result of intensive research, the present inventors have found that γ-
It was discovered that iron oxide fine particles are suitable for the above purpose, and based on this knowledge, the present invention was completed. That is, in the present invention, after uniformly dispersing dehydrated magnetite fine particles in a non-aqueous solvent,
The present invention provides a method for producing γ-iron oxide fine particles, which is characterized by carrying out thermal oxidation at a temperature of 100°C or higher. The magnetite particles used in the method of the present invention need to be fine particles with a particle size of about several hundred angstroms,
Usually, as mentioned above, it is obtained by a coprecipitation method from an aqueous solution containing a ferrous salt and a ferric salt. These magnetite fine particles are used after being washed with ion-exchanged water, but before being heated and oxidized in a non-aqueous solvent, it is necessary to remove the moisture adsorbed on the surface of the fine particles. , the following two methods are suitable. That is, the first method is to remove adsorbed water by washing the magnetite fine particles after washing with ion-exchanged water with acetone or a highly polar lower alcohol such as methanol, ethanol, or propanol. The magnetite fine particles from which the adsorbed water has been removed in this manner are uniformly dispersed in a non-aqueous solvent and heated and oxidized. The second method is to add magnetite fine particles washed with ion-exchanged water to a non-aqueous solvent, and then heat and dehydrate the particles. The magnetite fine particles heated and dehydrated in this manner are heated to a higher temperature and oxidized while being uniformly dispersed in the non-aqueous solvent. In the method of the present invention, a nonaqueous solvent having a boiling point of 100° C. or higher is preferably used as a medium for uniformly dispersing magnetite fine particles and heating and oxidizing them. Suitable examples of this nonaqueous solvent include decyl alcohol, kerosene, and silicone oil. Particularly, a solvent having a hydrophilic group such as decyl alcohol is preferable because the hydrophilic group adsorbs to the hydroxyl group present on the surface of the iron oxide fine particles, thereby increasing the dispersion stability of the particles. Also,
A solvent such as silicone oil, which has a higher oxygen solubility than kerosene, accelerates the oxidation rate, thereby shortening the time for producing γ-iron oxide. In this way, the desired γ-iron oxide particles can be obtained by uniformly dispersing the magnetite fine particles in a non-aqueous solvent and then heating and oxidizing them. At this time, heating oxidation may be performed while blowing oxygen or air if necessary. The reaction temperature is selected to be 100°C or higher, preferably in the range of 100 to 180°C. In addition, the heating time until γ-iron oxide fine particles are generated depends on the particle size of the magnetite particles, the type of dispersion medium, the degree of stirring, the heating temperature, and the presence or absence of oxygen or air blowing. In general, the smaller the particle size of the magnetite particles, the higher the oxygen solubility of the solvent, the stronger the stirring, and the higher the heating temperature, the shorter the heating time until the formation of γ-iron oxide fine particles. Furthermore, magnetite particles obtained by heating and oxidizing in the air using an electric furnace are thermally stable;
It is difficult to convert it into γ-iron oxide using the method of the present invention. The γ-iron oxide fine particles obtained in this way are
The solvent may be separated and taken out using a conventional method, or the solvent may be removed by agglomeration using a magnet. These γ-iron oxide fine particles are not only uniformly dispersed in the above-mentioned non-aqueous solvent, but also uniformly dispersed in an aqueous medium by washing with acetone. Effects of the Invention According to the method of the present invention, γ of magnetite fine particles
- Since the oxidation reaction to the iron oxide fine particles is carried out in a non-aqueous solvent, stirring of the magnetite fine particles and temperature control of the reaction system are facilitated, and as a result, the oxidation reaction can proceed uniformly. Furthermore, since the oxidation reaction is carried out with the particles uniformly dispersed in the medium, it is possible to prevent non-uniformity and loss of particles due to aggregation of particles or adhesion to the reaction vessel. The particle morphology of the γ-iron oxide fine particles obtained by the method of the present invention is achieved by using fine magnetite particles obtained by, for example, a coprecipitation method, in order to maintain the shape of the magnetite fine particles dispersed in the solvent. γ-iron oxide particles can be obtained. The γ-iron oxide fine particles obtained by the method of the present invention have excellent dispersibility in water and various organic solvents, and excellent thermal and magnetic stability, and can be used as magnetic materials such as magnetic recording materials and ferrite materials. It is particularly useful as a carrier for magnetic fluids and materials for separating and recovering useful substances. Examples Next, the present invention will be explained in more detail with reference to Examples. Example 1 Approximately 20 g of magnetite particles produced by the coprecipitation method were washed with acetone by repeating decantation several times, then forcibly precipitated with a magnet to remove acetone, and then uniformly dispersed in 500 ml of decyl alcohol. I let it happen. Next, this dispersion was stirred while being heated and maintained at a temperature of 150°C, and after 30 minutes, the black particles dispersed in decyl alcohol changed to brown particles. The particles were taken out, washed with acetone, and dried in vacuum to obtain a powder sample. According to a scanning electron microscope, this material retained the particle morphology of magnetite particles before heat treatment, and it was found that fine particles aggregated to form lumpy particles with a particle size of 700 Å or more. In addition, X-ray diffraction analysis was performed and the results are shown in the table. From this table, it is clear that the sample obtained is γ-iron oxide. Example 2 Magnetite fine particles from which oleic acid has been partially removed were recovered by washing a commercially available magnetic object with acetone, 20 g of these fine particles were uniformly dispersed in 500 ml of decyl alcohol, and then heated.
While stirring while maintaining the temperature at 170℃,
It passed through the air at a speed of 50 per hour. After 1 hour, the color of the fine particles changed from black to brown, so the fine particles were taken out, washed with acetone, and vacuum dried to obtain a powder sample. This product had a particle morphology similar to that obtained in Example 1, and formed lumpy particles with a particle size of 700 Å or more. An X-ray diffraction analysis was also performed, and the results are shown in the table. As can be seen from this table, the obtained sample is γ
- Confirmed to be iron oxide. Example 3 Magnetite particles obtained in the same manner as Example 2
After uniformly dispersing 20 g in 300 ml of kerosene, it was heated and stirred while maintaining the temperature at 170°C. After 1 hour, the black particles dispersed in the kerosene turned brown. The particles were taken out, washed with acetone, and dried in vacuum to obtain a powder sample. This product had a particle morphology similar to that obtained in Example 1, and formed lumpy particles with a particle size of 700 Å or more. In addition, X-ray diffraction analysis was performed, and the results are shown in the table. From this table, it is clear that this is γ-iron oxide.

[Table] In the X-ray diffraction analysis values in this table, the diffraction image from the (311) reflecting surface is observed most strongly, and when this intensity is taken as 100, the relative intensity of the diffraction images from other reflecting surfaces is , respectively (440) is 53, (220) is 34,
(333) and (511) were 33, and (400) was 24. Therefore, in Examples 2 and 3, the values for the four points with high intensity matched the literature values and were identified, so the intensity measurement for (400) was omitted.

Claims (1)

[Claims] 1. A method for producing γ-iron oxide fine particles, which comprises uniformly dispersing dehydrated magnetite fine particles in a non-aqueous solvent and then heating and oxidizing them at a temperature of 100°C or higher. 2. The manufacturing method according to claim 1, wherein the dehydration is carried out by washing with a hydrophilic solvent such as acetone or alcohol. 3. The manufacturing method according to claim 2, wherein the alcohol is methanol, ethanol, or propanol. 4. The manufacturing method according to claim 1, wherein the dehydration is carried out by heating in a non-aqueous solvent.