JPS5947003B2 - Method for producing magnetic metallic iron particles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the production of magnetic metallic iron particles.

More specifically, the present invention relates to a method of producing acicular magnetic metallic iron fine particles by thermally reducing goethite in a reducing atmosphere. Magnetic powder useful as magnetic recording media such as magnetic tapes has traditionally been mainly γ-Fe2O magnetic powder, but in recent years, high-density magnetic recording media for VTRs and high-end audio have become desirable, and with that. Many ferromagnetic powders have been proposed.

Among these, powders mainly composed of metallic iron obtained by thermal reduction of powders mainly composed of iron oxide or α-iron oxyhydroxide are known to have high coercive force. Generally, goethite is produced by adding excess sodium hydroxide to an aqueous solution of a ferrous salt such as ferrous sulfate to form an aqueous ferrous hydroxide solution, and then oxidizing the solution. This is heated and reduced to obtain acicular magnetic metallic iron fine particles.
However, in this heating step, sintering between needle-like crystal particles is likely to occur. To prevent this sintering, etc., Si, Ai, C
It is known to pre-coat the surface of the goethite particles as a starting material with R or the like. In particular, coating with Si is common, and water glass is often used as the Si source. That is, a predetermined amount of water glass is added to an aqueous suspension of goethite and stirred, and then the pH is adjusted to 3 to 5 with sulfuric acid or the like.
After depositing SiO2 on the goethite surface, the aqueous suspension is filtered, washed and dried. However, in this method, the dispersion of the water suspension of goethite is insufficient, so it is difficult to uniformly deposit the Si source, and it is also difficult to remove impurities such as Na+ and 504'-. This can lead to condensation and make recovery difficult. Therefore, the magnetic properties of the magnetic metallic iron fine particles obtained are not necessarily favorable. An object of the present invention is to provide a method for producing acicular magnetic metallic iron fine particles having excellent magnetic properties by improving the drawbacks of the conventional techniques.

The method for producing acicular magnetic metallic iron fine particles of the present invention involves adding an organic acid to an aqueous suspension of goethite consisting of acicular particles, and adjusting the pH of the solution to 5 or less (preferably 3 to 3.5). A goethite aqueous dispersion is prepared by adding an aqueous silicic acid solution.Ammonia is then added to the liquid to adjust the pH to 9 to 10.5 to form flocs, and the flocs are separated and dried. 400~
It consists of reduction at 600°C. The aqueous suspension of goethite in the present invention can be obtained by suspending goethite powder consisting of acicular particles in pure water and thoroughly stirring the suspension, or by
An aqueous alkali solution is added to an aqueous iron salt solution to produce a precipitate of ferrous hydroxide, which is oxidized in the air, then filtered, washed with water, etc., and the resulting wet cake of goethite is poured into pure water and stirred. It can be obtained by

Next, an organic acid is added to the aqueous suspension. This addition significantly reduces the viscosity of the suspension, resulting in an aqueous goethite dispersion in which individual goethite particles are completely dispersed. A silicic acid aqueous solution is added to this completely dispersed goethite aqueous dispersion to obtain a desired goethite aqueous dispersion in which Si is uniformly deposited on the surfaces of the goethite acicular particles. In the present invention, the addition of an organic acid and an aqueous silicic acid solution improves the Si deposition efficiency (
The amount of deposited Si/the amount of added Si can be increased, the reproducibility of Si deposition is good, and the amount of Si deposited can be easily controlled. Sulfuric acid instead of the organic acid of the present invention,
The use of nitric acid, hydrochloric acid, phosphoric acid, etc. promotes sintering and inhibits reduction. Further, even if sodium silicate, water glass, or the like is used instead of the silicic acid aqueous solution, sintering will be similarly promoted, and the reduction rate will decrease, making it impossible to achieve the object of the present invention. The silicic acid aqueous solution used in the present invention refers to various silicic acid aqueous solutions such as orthosilicic acid and H4SiO4, aqueous silica sol, aqueous silica sol stabilized with ammonia, or aqueous silica sol modified with aluminum.

The Si concentration of these aqueous solutions or sols is usually 0.01~
It is 10%. The organic acid in the present invention may be any of acetic acid, formic acid, oxalic acid, citric acid, etc., but acetic acid can be advantageously used industrially in view of the boiling point of the acid and the boiling point of the salt produced. In addition, the ammonia addition method in the present invention can be achieved by adding ammonia water, blowing ammonia gas, or adding a substance such as urea that is thermally decomposed by heating in an aqueous solution state and has substantially the same effect as ammonia. But that's fine. Preferably, ammonia water is used. It may be added so that the pH of the suspension becomes 9 to 10.5. When acetic acid is used as the organic acid as in the present invention, ammonium acetate is decomposed and volatilized by drying at 120 to 125°C after filtration, and goethite with few impurities and uniformly coated with Si can be obtained. . If a mineral acid such as sulfuric acid, phosphoric acid, or hydrochloric acid is used instead of the organic acid of the present invention, it is difficult to remove anions, which causes sintering and a decrease in the reduction rate, which causes a decrease in magnetic properties. The treated goethite of the present invention is reduced in a reducing atmosphere such as a hydrogen stream to obtain acicular magnetic metallic iron fine particles.

The reduction temperature is 400-600°C. The impurity-free treated goethite obtained by the method of the invention can withstand reduction at higher temperatures than conventional reduction temperatures. Moreover, products with excellent magnetic properties can be manufactured. The characteristics of the production method of the present invention are that the goethite used contains almost no impurities due to the treatment unique to the present invention, Si is uniformly deposited on the particle surface, and the amount of Si deposited is controlled. . In the present invention, by using an organic acid, a liquid in which individual goethite particles are completely dispersed and Si is uniformly deposited can be obtained. For example, by using aqueous ammonia, the salt produced, ie, ammonium acetate, and the remaining acetic acid are removed by decomposition and evaporation at a drying temperature of 120 to 125°C. In other words, this is thought to be because Na+, SO4-, which cause sintering, and PO4-, which cause reduction, do not adhere as in the conventional method.The method of the present invention is effective in controlling the adhesion of Si to goethite. The Si adhesion efficiency is 90% or more, and the uniform adhesion significantly improves the sintering prevention effect.Furthermore, the magnetic metal iron fine particles of the present invention have extremely good dispersibility in magnetic paints. The present invention will be explained below with reference to Examples and Comparative Examples.

Examples 1-10 Wet cake of goethite (dry standard 3
30g) was put into 6600g of pure water and suspended.

11 ml of 99.5% acetic acid was added to this to adjust the pH to 3.3. The slurry viscosity decreased rapidly to several Cp. 3
After stirring for 0 minutes, a predetermined amount of a silicic acid aqueous solution (Si concentration 1%) is added, mixed, and sufficiently stirred to uniformly deposit Si.

After that, add 28% ammonia water (1ml) and adjust the pH to 9.3.
A flock was generated as follows. The flock is sent door to door, 1
After drying at 20-125°C, treated dry goethite as shown in Table 1 was obtained. This treated dry goethite 310θ was heated at a hydrogen flow rate of 101/min for 12 hours, Table 1
The mixture was reduced using a rotary reduction furnace under the treatment temperature conditions shown in Figure 2, to obtain acicular magnetic metal iron fine particles. Table 1 shows its characteristic values.
It was shown to. Comparative Examples 1 to 8 In Comparative Examples, acetic acid and aqueous ammonia were not used in the Examples.

Claims (1)

[Claims] 1. In a method for producing acicular magnetic iron fine particles by thermally reducing goethite in a reducing atmosphere, an organic acid is added to an aqueous suspension of goethite to bring the pH to 5 or less, and an aqueous silicic acid solution is added to the solution. , an acicular magnetic metal iron fine particle characterized in that a goethite aqueous dispersion is adjusted, then ammonia is added to adjust the pH to 9 to 10.5 to generate flocs, and treated goethite consisting of the flocs is used. manufacturing method.