JPH0257321B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing γ-iron oxide magnetic powder used for magnetic recording purposes, and more specifically, a method for producing γ-iron oxide powder surface-treated with cobalt and divalent iron. It is related to. [Prior Art] Conventionally, acicular chromium dioxide and cobalt-containing acicular magnetic iron oxide have been widely used as high-density magnetic recording media. However, tapes using acicular chromium dioxide have the disadvantage that the head wears out more than conventional tapes using iron oxide, resulting in deterioration of characteristics with repeated use. There are also problems with manufacturing costs and pollution. On the other hand, cobalt-containing acicular magnetic iron oxides can be roughly divided into the following two types. Specifically, there are two types: a so-called cobalt-doped magnetic iron oxide type in which cobalt is uniformly dispersed within the particles, and a type in which cobalt is deposited only on the surface of the magnetic iron oxide particles. In general, the cobalt-doped type and the cobalt-doped type have a higher particle coercivity when the cobalt content is the same, and the cobalt-doped type has a higher particle coercive force than the cobalt-doped type. However, the cobalt-doped type has problems with demagnetization characteristics and changes over time when made into a tape, and also has the drawback of poor transfer characteristics. In the type in which cobalt is deposited only on the particle surface, these drawbacks are improved and excellent magnetic particles can be obtained. In the case of particles in which cobalt is deposited only on the surface of the γ-iron oxide particles, the coercive force of the particles increases almost linearly as the amount of deposited cobalt increases. The addition of cobalt also improves the transfer characteristics when made into tape by 2 to 8 dB compared to the original γ-iron oxide. However, if the amount of cobalt is too large, there are disadvantages in that the magnetic properties change over time and stability against pressure and temperature deteriorates. Also, from the viewpoint of manufacturing costs, it is desirable to reduce the amount of cobalt. Furthermore, as a powder magnetic material, there is a demand for a material that has not only high coercive force but also high saturation magnetization and lower electrical resistance. In addition, VHS-type VTR tapes must be made black, and a magnetic material that has a high degree of blackening when made into a tape is required, and it is necessary to make the magnetic material as black as possible. Magnetic tape especially VTR
As tapes run at high speeds, they become charged due to friction, which makes them run unsmoothly and causes problems such as dropouts due to the accumulation of dust. Furthermore, the S/N ratio also decreases due to the generation of discharge noise. These problems caused by the generation of static electricity can be solved by making the magnetic tape conductive. Adding divalent iron to the particle surface is effective in giving powdered magnetic materials high saturation magnetization and low electrical resistance. However, the addition of divalent iron has the disadvantage that the transfer effect of the tape deteriorates. As a method for improving the coercive force and saturation magnetic properties of magnetic powder, a method has been proposed in which cobalt is deposited on iron oxide powder in the presence of an aqueous salt solution of barium or strontium (Japanese Patent Laid-Open No. 198607, 1982). At this time, a method of further adding an aqueous ferrous salt solution (Japanese Patent Application Laid-Open No. 181102/1983) has also been proposed. In both of these methods, an aqueous solution of cobalt salt, barium salt, etc. is added to a dispersion of iron oxide particles, and in the latter case, an aqueous ferrous salt solution is added, and finally an alkali is added and a heating reaction is carried out. It undergoes metamorphosis using cobalt, etc. However, in these methods, the reaction rate is fast, for example, the reaction is completed in 3 to 4 hours at a reaction temperature of 100°C, but the coercive force of the final product is lower than that of the present invention, and moreover, the coercive force of the magnetic powder is lower than that of the present invention. The drawback is that the magnetic force distribution is wide. The reason is that cobalt is γ−
It is difficult to deposit uniformly on the surface of iron oxide particles, so a larger amount of cobalt is required than in the present invention to obtain the required coercive force, and furthermore, cobalt is not uniformly deposited, resulting in a wide distribution of cobalt. There is. [Object of the invention] The object of the present invention is to provide a magnetic iron oxide powder that has excellent coercive force, conductivity, saturation magnetic properties, and transfer properties,
Another object of the present invention is to provide a method for producing magnetic iron oxide powder having a narrow coercive force distribution. [Structure of the Invention] The present invention comprises dispersing γ-iron oxide particles in an alkaline aqueous solution, adding at least one of a calcium salt, a strontium salt, and a barium salt and a cobalt salt,
It is characterized by carrying out cobalt modification of γ-iron oxide, then adding divalent iron salt (ferrous salt) to further carry out divalent iron modification, and then separating and drying. The present invention will be explained in detail below. In the present invention, γ is dispersed in an alkaline solution.
- Iron oxide particles are first subjected to cobalt modification in the presence of an alkaline earth metal salt, and then a divalent iron salt is added thereto to perform modification with divalent iron. This series of steps is the key point of the present invention, and through this, γ-
It has been surprisingly discovered that iron oxide magnetic powder not only has improved conductivity and saturation magnetic properties, but also has an increased coercive force. If cobalt metamorphosis and then divalent iron metamorphosis are performed without adding an alkaline earth metal salt, the coercive force will decrease, which will have an effect completely opposite to that of the present invention. These conditions are shown in Figure 1. Furthermore, in the metamorphism reaction step, adding an alkaline aqueous solution at the end to simultaneously deposit (transform) cobalt and divalent iron has drawbacks such as a wide distribution of coercive force, as described in the conventional example. Figure 1 shows that γ-iron oxide particles (100g/) are dispersed in a 6N aqueous solution of caustic soda, and the chloride of each alkaline earth metal shown in Figure 1 is added to the solution in amounts of 0.5% by weight and cobalt chloride. in the amount of cobalt
Ferrous chloride was added at a concentration of 3.0% by weight and treated at 100°C to deposit (transform) cobalt.The relationship between the reaction time and coercive force at that time was investigated, and after a further 5 hours of reaction, ferrous chloride The graph shows the results of measuring the coercive force of the powder obtained by adding 5% by weight of iron and heating at 100° C. for 1 hour to transform (deposit) divalent iron. In the figure, white circles indicate the case where an alkaline earth metal salt is added, and black circles indicate the case where no addition is made, for comparison. When the reaction time for cobalt modification had elapsed for 5 hours (arrow in the figure), ferrous chloride was added, and a heating reaction was carried out to perform modification with divalent iron. For this last one, the cobalt deposition amount (transformed amount) is about 3% compared to iron oxide.
%, and the amount of divalent iron deposited was approximately 5% relative to iron oxide. In this invention, when expressed in this way, the number of parts of cobalt per 100 parts of iron oxide and the number of parts of divalent iron per 100 parts of iron oxide are expressed, and the ratios are expressed on a weight basis unless otherwise specified. The same applies to the following. As can be seen from the figure, the coercive force increases slightly with the addition of alkaline earth metal salts, but when ferrous chloride is subsequently added, the effect is completely opposite depending on the presence or absence of alkaline earth metal salts. . That is, it can be seen that in the absence of alkaline earth metal salt, the coercive force decreases, whereas in the presence of alkaline earth metal salt, the coercive force increases rapidly. This makes it possible to further reduce the amount of cobalt required to obtain the desired coercive force. Another major feature of the method of the present invention is that magnetic powder with a very narrow coercive force distribution can be obtained as shown in the Examples. In the present invention, the effects of adding alkaline earth metal salts and divalent iron salts as described above have not yet been fully elucidated, but the alkaline earth metals act as catalysts, and cobalt is uniformly distributed on the surface of the γ-iron oxide particles. It is thought that the coercive force increases because a magnetite or cobalt ferrite layer is uniformly formed on the surface by divalent iron, or a crystal alignment layer similar to the magnetoplumbite structure known from ferrite magnets is formed. It is also possible to assume that it is formed. Furthermore, it is thought that cobalt and divalent iron are deposited uniformly on each particle, so that the coercive force distribution becomes narrower. Regarding the transfer effect, the addition of divalent iron generally worsens the transfer characteristics, but in the present invention, the transfer improvement due to the cobalt deposition effect by the addition of alkaline earth metal is superior, so the final transfer characteristics are improved. The value is obtained. In the present invention, γ dispersed in an alkaline aqueous solution
-Iron oxide particles are usually used in particle sizes of 0.2~
The diameter is about 0.8 μm, and the appropriate amount of dispersion is 80 to 150 g/. A suitable alkali concentration is 1 to 8 mol/OH group concentration. The higher the alkali concentration, the faster the reaction rate, but this has the disadvantage of increasing the cost of the alkali and requiring a large amount of water and time to wash the reaction product. For these reasons, the OH concentration is preferably 2 to 8 mol/. As the alkali, caustic potash, caustic soda, lithium hydroxide, etc. can be used, but caustic soda is industrially advantageous in terms of manufacturing cost. Regarding the calcium salt, strontium salt, and barium salt used in the present invention, all water-soluble salts can be used. That is, chloride, bromide, iodide, acetate, formate, nitrate, etc. can be used, but chloride is industrially advantageous from the viewpoint of manufacturing cost. The appropriate amount of addition is 0.01 to 3% of alkaline earth metal based on γ-iron oxide. If it is less than 0.01%, the effect will not be sufficient, and if it exceeds 3%, the amount of components that do not contribute to magnetization will increase, which is not preferable. Next, as for the cobalt salt, the same salts as those normally used for cobalt modification of iron oxide particles, such as cobalt chloride, cobalt nitrate, and cobalt sulfate, can be used. In the present invention, the amount of cobalt metamorphosed (deposited) can be made considerably low; for example, even 0.1% is effective for γ-iron oxide. The upper limit is preferably about 10%. The amount of cobalt chloride, etc. added is determined so that the amount of cobalt transformation falls within these ranges. The amount added remains almost the same as the metamorphic amount. The alkaline earth metal salt and cobalt salt are added as solids or their aqueous solutions. There is no particular restriction on the order of addition of these. The above mixed solution is heated and stirred to react. The appropriate temperature is 50° to the boiling point. The higher the temperature, the faster the reaction rate, but it is necessary to pressurize the reaction system to raise the temperature above the boiling point, which is industrially disadvantageous. The boiling point varies depending on the concentration of salt dissolved in the solution, but within the above concentration range, the upper limit of the boiling point is approximately 104°C. Therefore, the heating temperature is 50
It is appropriate to choose a temperature within the range of ~104°C. The reaction by heating is preferably carried out with stirring; therefore, if the atmosphere in the reaction system is oxidizing, divalent iron and cobalt will be oxidized, so it is preferably non-oxidizing. The reaction time is suitably 4 hours or more, preferably 4 to 8 hours. Through the above treatment, the cobalt salt turns into cobalt hydroxide and is deposited on the surface of the γ-iron oxide particles.
In this way, cobalt transformation of the γ-iron oxide particles is carried out. At this time, most of the alkaline earth metal salts are deposited on the particle surfaces. Once the cobalt transformation has been completed, divalent iron salt is then added to the solution. As a divalent iron salt, sulfuric acid is the first
Iron, ferrous chloride, etc. are used. This ferrous salt-added solution is treated at the same temperature, time, and atmosphere as the above-mentioned cobalt modification to perform modification with divalent iron. The suitable metamorphic amount of divalent iron is 0.1 to 10% as metallic iron relative to γ-iron oxide. Therefore, the amount of divalent iron salt added is determined so that the amount of metamorphosis falls within this range. The normal addition amount becomes the metamorphic amount. In this way, the precipitate of γ-iron oxide particles modified by cobalt and divalent iron is separated from the solution,
The product is washed repeatedly and finally dried. Even if alkaline earth metals remain in the product, there is no harm to the magnetic properties within the scope of the present invention. [Effects of the Present Invention] According to the present invention, iron oxide magnetic powder having excellent conductivity, saturation magnetic properties, transfer properties, and high coercive force can be obtained. Moreover, while the coercive force is not normally increased by metamorphosis of divalent iron, the method of the present invention makes it possible to significantly increase the coercive force. Therefore, the amount of cobalt metamorphosis can be reduced, and drawbacks such as stability problems due to an increase in cobalt are therefore eliminated. The present invention will be specifically explained below using Examples. Example 1 γ-iron oxide (long axis average 0.4μ, long axis/short axis = 8/1, coercive force) in 6N caustic soda aqueous solution 4
HC = 390 oersted, saturation magnetization (σ _S =
73.4emu/g) was dispersed, and while heating and stirring in a nitrogen stream, an aqueous solution of 11g of CaCl ₂ 6H ₂ O (0.5% as Ca relative to γ-iron oxide) dissolved in 100ml of distilled water was added, and the mixture was heated and stirred. Continue. Reaction solution is 80
When the temperature reaches ℃, 57.2 g of C ₀ SO ₄・7H ₂ O (γ-
Add an aqueous solution of 300 ml of distilled water containing 3% C ₀ based on iron oxide, raise the reaction temperature to 100°C, and continue stirring for 5 hours. After 5 hours, _FeSO4・
7H ₂ O97.5g (5% as Fe based on γ-iron oxide)
Add an aqueous solution of 500ml of distilled water and heat to 100℃.
The reaction was continued for an additional hour. After the reaction was completed, it was thoroughly washed with water and dried at 100°C. Table 1 shows the properties of the obtained magnetic powder. In the table, HC is coercive force and σ _S is magnetization strength. The transfer characteristics were determined by making a magnetic paint using a sand mill with the following binder composition, applying it to a dry thickness of 10μ on a 20μ polyethylene terephthalate film, orienting it in a 2500 Gauss magnetic field, and drying the film. Slit to /4 inch and JIS C
-5542 measurement method. Binder composition Magnetic powder 100 parts Vinyl chloride vinyl acetate copolymer (VAGH)
25 parts rosin 3 parts silicone oil 1 part lecithin 0.2 parts toluene 150 parts MIBK 150 parts Coercive force distribution is expressed as ΔB/Bm. Here, ΔB is the difference between the magnetic flux density value on the magnetization rising curve and the magnetic flux density value on the magnetization falling curve at 1500 O¨e of the magnetic hysteresis curve, and Bm is the saturation magnetic flux density. The larger the value of ΔB/Bm, the better the transfer characteristics, and the smaller the value of ΔB/Bm, the narrower the distribution and the better. Example 2 SrCl ₂ − instead of calcium chloride in Example 1
6H ₂ O6.1g (0.5% as Sr based on γ-iron oxide)
The reaction was carried out in the same manner as in Example 1, except that an aqueous solution of 100 ml of distilled water was added. Table 1 shows the magnetic properties of the obtained γ-iron oxide powder. Example 3 BaCl ₂ was used instead of calcium chloride in Example 1.
2H ₂ O3.56g (0.5% as Ba based on γ-iron oxide)
It was used. The rest is exactly the same as in Example 1. The magnetic properties of the obtained γ-iron oxide powder are shown in Table 1. Comparative Example 1 The reaction was carried out in the same manner as in Example 1 except that calcium chloride was not used. Table 1 shows the characteristics of the obtained results. Comparative Example 2 57.2 g of cobalt sulfate (CoSO ₄ 7H ₂ O) and 97.5 g of ferrous sulfate (FeSO ₄ 7H ₂ O) (3% and 5% as Co and Fe, respectively, based on γ-iron oxide) Dissolve in distilled water 3, add 400 g of γ-iron oxide, the same as that used in Example 1, and continue heating and stirring.
When the reaction solution reaches 80℃, add caustic soda 960.
Add an aqueous solution of 1 g dissolved in 1 part distilled water. The temperature of the reaction solution rose to 100°C due to the heat of dilution. Stirring was continued for 4 hours while maintaining the reaction solution at 100°C. After the reaction was completed, it was thoroughly washed with water and dried at 100°C. Table 1 shows the properties of the obtained γ-iron oxide magnetic powder. Comparative Example 3 Strontium chloride 6.1g in Comparative Example 2
A γ-iron oxide magnetic powder was obtained in the same manner except that γ-iron oxide (0.5% based on γ-iron oxide) was added together with caustic soda. Its characteristics are shown in Table 1.

[Table] Addition amount is weight% based on γ-iron oxide
In Example 4, a mixed solution of CaCl ₂ 〓6H ₂ O and CoCl ₂ 〓6H ₂ O was added.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the effects of addition of alkaline earth metal salts and ferrous salts. The vertical axis of the figure is the coercive force of the product
The horizontal axis shows Hc and the reaction time.

Claims (1)

[Claims] 1. γ-iron oxide particles are dispersed in an alkaline aqueous solution, at least one of calcium salt, strontium salt, or barium salt and a cobalt salt are added,
- carrying out cobalt transformation of iron oxide particles, then adding divalent iron salt to carry out divalent iron transformation, and then
Separately, a method for producing magnetic powder for magnetic recording, characterized by drying. 2. The method for producing magnetic powder for magnetic recording according to claim 1, wherein the amount of cobalt metamorphosis is 0.1 to 10% by weight as metallic cobalt based on γ-iron oxide. 3. The magnetic recording according to claim 1, wherein the amount of at least one of calcium salt, strontium salt, or barium salt added is 0.01 to 3% by weight as calcium, strontium, or barium metal relative to γ-iron oxide. Manufacturing method of magnetic powder for use. 4. The method for producing magnetic powder for magnetic recording according to claim 1, wherein the amount of metamorphosed divalent iron is 0.1 to 10% by weight as metallic iron relative to γ-iron oxide. 5. A method for producing magnetic powder for magnetic recording in which the OH concentration in the alkaline aqueous solution is 1.0 to 8.0 mol/ and the reaction temperature is 50 to 104°C in the reaction according to claim 1.