JPS6138129B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing cobalt-coated ferromagnetic iron oxide that is useful as a magnetic recording material, and more specifically, to produce ferromagnetic iron oxide coated with cobalt, phosphorus is preliminarily added to the crystal at the precursor stage. The present invention relates to a method for producing cobalt-coated ferromagnetic iron oxide, in which a compound containing at least cobalt is coated on the surface of the ferromagnetic iron oxide obtained by using the ferromagnetic iron oxide. Ferromagnetic iron oxide coated with a cobalt compound has a high coercive force, so it is widely used in magnetic recording fields such as video and audio. Many methods have also been proposed for obtaining magnetic iron oxide. There are also some proposals for incorporating phosphorus into iron oxide before depositing cobalt. For example, Japanese Patent Application Laid-open No. 49-69588 describes magnetite obtained by depositing phosphate on α-FeOOH before being reduced to magnetite and reducing it, or γ-Fe derived from this. A method has been described in which a cobalt compound is deposited on ₂ O ₃ and then tempered, but the method has the drawback that the cobalt compound is not easily deposited uniformly. Separately, JP-A-55-149137
The publication describes a method for adsorbing and diffusing a cobalt compound into crystals containing silicon and phosphorus, but the method mainly involves increasing the specific surface area by adding silicon and phosphorus and increasing the coercive force accordingly. However, improvements are still desired in terms of variations in coercive force. The present invention differs from the above method in that the phosphorus compound is present in the crystal rather than on the particle surface, and α-FeOOH containing no silicon is used, and a cobalt compound is deposited on the ferromagnetic iron oxide derived from the α-FeOOH. Features. The present inventors have found that the phosphorus contained in the crystal does not interfere with the adhesion of the cobalt compound and that a good and uniform cobalt coating can be obtained, and that the phosphorus prevents cobalt from diffusing into the crystal. , with high coercive force,
It was confirmed that ferromagnetic iron oxide with excellent thermal properties and stability over time can be produced. The present invention enables the production of a ferrous salt aqueous solution containing α-FeOOH nucleus crystals suspended in the total α-
In the presence of a phosphorus compound in an amount of 0.1 to 1% by weight in p terms based on the amount of FeOOH, the nucleic crystals are grown by neutralization and oxidation to generate phosphorus-containing α-FeOOH, which is obtained. This is a method for producing cobalt-coated ferromagnetic iron oxide, which comprises inducing α-FeOOH into ferromagnetic iron oxide, and then coating the surface of the ferromagnetic iron oxide particles with a compound containing at least cobalt. α-FeOOH can be produced by, for example, partially neutralizing a ferrous salt solution with an alkali to precipitate some of the Fe, oxidizing it to produce α-FeOOH nucleus crystals, and then further neutralizing this solution with an alkali. It is produced by oxidizing and growing the nucleus crystals. In the method of the present invention, a phosphorus compound is present in the liquid during the nucleic crystal growth reaction. The ferrous salts used include ferrous salts of mineral acids such as ferrous sulfate, ferrous nitrate, and ferrous chloride, with ferrous sulfate being industrially preferred. Examples of alkalis include hydroxides, oxides, or carbonates of alkali metals or alkaline earth metals, such as sodium hydroxide, potassium hydroxide, sodium oxide, and calcium carbonate. Potassium is preferred. Examples of the oxidizing agent used for oxidation include air, oxygen, and other oxidizing agents, with air being generally preferred.
Examples of phosphorus compounds include phosphoric acids such as orthophosphoric acid, metaphosphoric acid, polyphosphoric acid, and phosphorous acid, or water-soluble salts of these with alkali metals, ammonium, etc., and orthophosphoric acid or its salts are usually used. . The above α-FeOOH nucleation reaction is usually performed for 30~
Fe in the mother liquor is added to a ferrous salt solution with a concentration of 100 g/
Add the alkali necessary to precipitate 5 to 25 g of ions, maintain the reaction temperature between 50 and 100°C and pH between 3 and 8 while blowing air,
It lasts ~80 minutes. In this step, when adding a phosphorus compound to obtain well-shaped nucleus crystals, it is necessary to add a phosphorus compound in a p-equivalent amount of 0.1 to
Although it is recommended that a phosphorus compound be added to the mother liquor at a concentration of 0.4% by weight, this phosphorus is encapsulated in the core crystals and does not exhibit the effect of phosphorus according to the present invention. The subsequent α-FeOOH nucleus crystal growth reaction is usually carried out by adding an alkali to the ferrous salt solution in which α-FeOOH nuclei are suspended, and increasing the reaction temperature to 50 to 90°C and pH to 3 while blowing air. The growth rate of the nucleus crystals is controlled to be about 5 to 15 g/hour, and the growth rate is adjusted to about 1.5 to 6. In this step, by adding a phosphorus compound to the mother liquor, α-FeOOH can be made to contain phosphorus almost uniformly. The amount of phosphorus compound added is determined by the amount of total α-
0.1 to 1% by weight in p equivalent amount based on FeOOH amount
shall be. If this amount is too small, the desired effect cannot be obtained, while if it is too large, it is difficult to obtain good thermal properties, stability over time, etc. Phosphorus-containing α-FeOOH obtained in this way
α-
FeOOH powder is obtained, and this powder is converted into ferromagnetic iron oxide in the method of the present invention, that is, γ-
Fe ₂ O ₃ , Fe ₃ O ₄ or bertolide compounds can be obtained. Dehydration, e.g. 300-700 in air
Reduction is carried out by heating to a temperature of 300 to 500 °C, for example with hydrogen or hydrogen containing water vapor, and oxidation is carried out by heating to a temperature of 300 to 500 °C, for example in oxygen or air.
This is done by heating to a temperature of 200-400°C. In the method of the present invention, the phosphorus-containing ferromagnetic iron oxide obtained by the method described above is coated with a compound containing at least cobalt. This coating is carried out by alkali neutralization of the metal salt in the iron oxide slurry. Examples of the cobalt salt used here include inorganic and organic acid salts of cobalt, such as cobalt sulfate, cobalt chloride,
Examples include cobalt acetate, and the amount of the coating is usually 0.5 to 10%, preferably 1 to 6%, based on the weight of iron oxide. At this time, examples of metal salts other than cobalt that may be coated at the same time or before or after the coating include ferrous salts, manganous salts, zinc salts, nickel salts, etc., and the coating amount is usually smaller than that of iron oxide. On the other hand, in the case of ferrous iron, it is 0.5 to 20%, preferably 5 to 20%.
15%, and 0-10% for other metals. Examples of methods for depositing include (1) dispersing ferromagnetic iron oxide in a metal aqueous solution containing at least cobalt and adding an alkaline solution thereto; (2) dispersing ferromagnetic iron oxide in a metal salt containing at least cobalt; Dispersion in a mixture of aqueous and alkaline solutions, (3)
A method of dispersing ferromagnetic iron oxide in water and adding thereto an aqueous metal salt solution containing at least cobalt and an alkaline solution; (4) dispersing ferromagnetic iron oxide in an aqueous alkaline solution containing at least cobalt; (5) A method in which ferromagnetic iron oxide is dispersed in an aqueous metal salt solution containing at least cobalt, and this dispersion is added dropwise into an alkaline solution. 1. Part or all of iron and other metals can be treated simultaneously or sequentially, or any suitable method can be adopted. It's okay. The cobalt-coated ferromagnetic iron oxide obtained by the above-mentioned method is filtered and then dried by a conventional method or heated in a non-oxidizing or oxidizing atmosphere for
Heat treatment may be performed at about 300°C. The cobalt-coated ferromagnetic iron oxide obtained by the method of the present invention has a high coercive force and good thermal properties and stability over time. The reason for this is not clear, but ferromagnetic iron oxide with few vacancies is easily obtained by heating phosphorus-containing α-FeOOH at relatively high temperatures, and there is no large amount of phosphorus on the surface of ferromagnetic iron oxide. It is also presumed that uniform and good cobalt deposition is obtained, and that phosphorus contained in the ferromagnetic iron oxide prevents cobalt from diffusing into the crystal, resulting in a preferable structure. The present invention will be understood in more detail by the following Examples and Comparative Examples. Examples 1-2 and Comparative Examples 1-2 In a reactor equipped with an air blowing pipe and a stirrer,
Pour 20 g of an aqueous solution containing 3800 g of FeSO4, raise the temperature to 43°C, add 2.14 g of NaOH aqueous solution (concentration 200 g/) with stirring (precipitated Fe 15 g/), and add 600 g of FeSO4 to this solution.
Air was blown into the mixture at a rate of /hour and the reaction was carried out at 45°C for 40 minutes to obtain α-FeOOH nucleus crystals. Further, while blowing air at a rate of 85/hour, a predetermined amount of orthophosphoric acid and NaOH aqueous solution (concentration 200g/hour) 5.4 shown in Table 1 below was gradually added to maintain the pH at 5.1, and at 50℃ The reaction was carried out for a period of time, and the nucleus crystals grew approximately 3.5 times. Each α-FeOOH obtained by the above method
After passing through the usual filtering, water washing, drying and grinding, α
- Obtained as FeOOH powder, which was dehydrated (in air, 650°C), reduced (in hydrogen containing steam, 420°C) and oxidized (in air, 280°C).
Each γ-Fe ₂ O ₃ was obtained. Each γ- obtained by the above method
Disperse 200g of Fe ₂ O ₃ in water 2 to make a slurry,
While blowing N ₂ gas into the liquid, a mixed solution of 1 mol of cobalt sulfate/120 ml of solution and 1 mol of ferrous sulfate/250 ml of solution was added and stirred. Furthermore, 5 mol of sodium hydroxide/780 ml of solution was added, and stirring was continued at room temperature (28°C) for 5 hours. After the reaction, the slurry was filtered and washed with water, and the resulting slurry was heated to 110℃.
The powder was dried for 10 hours in an inert atmosphere to obtain the desired cobalt-coated ferromagnetic iron oxide powders A to D. Comparative Examples 3 to 4 For α-FeOOH obtained by the method of Comparative Example 1, after washing with filtrate and water, orthophosphoric acid was applied to the α-FeOOH in a predetermined amount shown in Table 1 below, and then the method of Example 1 was applied. γ-Fe ₂ O ₃ was obtained in the same manner as above, and the desired cobalt-coated ferromagnetic iron oxide powders E and F were obtained in the same manner. The coercive force of the obtained samples A to F was measured by a conventional method, and the thermal properties and stability over time were calculated by the following method, and the results shown in Table 1 were obtained. (Measurement method for thermal identification) Measure coercive force at room temperature and 125°C, and calculate thermal characteristics using the following formula. Thermal properties (%) = Coercive force (Hc) at 125°C/room temperature 〃 (〃) × 100 (Measurement of stability over time) Initial coercive force and coercive force after being left at 60°C and relative humidity 80% for 10 days is measured, and the stability over time is determined using the formula below. Stability over time (△Hc) = (initial coercive force) - (temperature
Coercive force after being left at 60℃ and 80% relative humidity for 10 days)

[Table] Example 3 200 g of γ-Fe ₂ O ₃ obtained by the method of Example 1 was dispersed in 2 water to make a slurry, and while blowing N ₂ gas into the liquid, 1 mol of cobalt sulfate/solution was added.
120 ml was added and stirred. Furthermore, 5 mol of sodium hydroxide/590 ml of solution was added, and stirring was continued at 90°C for 5 hours. After the reaction, the slurry was filtered and washed with water, and the resulting slurry was dried at 110°C for 10 hours.
The desired cobalt-coated ferromagnetic iron oxide powder H was obtained. Example 4 Add 1 mole of cobalt sulfate/120 ml of solution, stir, and then add 5 mole of sodium hydroxide/solution.
Add 590 ml of cobalt sulfate 1 mole/
120 ml of solution and 1 mol of manganese sulfate/solution
The desired cobalt-coated ferromagnetic iron oxide I was obtained in the same manner as in Example 3, except that 120 ml was added, stirred, and further 5 mol of sodium hydroxide/680 ml of solution was added. Example 5 γ- Depositing Cobalt and Ferrous Compounds
The desired cobalt-coated ferromagnetic iron oxide J was obtained in the same manner as in Example 1 except that Fe ₂ O ₃ was replaced with magnetite. For Samples H, I, and J obtained above, the coercive force was measured by a conventional method, and the thermal properties and stability over time were calculated by the above method, and the results shown in Table 2 were obtained.

【table】

Claims (1)

[Claims] 1. A ferrous salt aqueous solution containing suspended α-FeOOH nucleus crystals is prepared in the presence of an amount of phosphorus compound such that the amount is 0.1 to 1% by weight in terms of P based on the total amount of α-FeOOH to be finally produced. Then, the nucleic crystals are grown by neutralization and oxidation to generate phosphorus-containing α-FeOOH, and the obtained α-FeOOH is induced into ferromagnetic iron oxide. A method for producing cobalt-coated ferromagnetic iron oxide, the method comprising depositing a compound containing cobalt.