JPS5916904A - Processing method protecting metal powder from oxidation - Google Patents

Abstract

PURPOSE:To obtain stable ferromagnetic metal powder with little deterioration of magnetic property, by a method wherein homogeneously oxidized film is formed on the surface of metal powder by allowing the pressure of gas involving O2 adjacent to the solution where metal powder is suspended in the organic solvent inactive thereto to be not less than atmospheric pressure. CONSTITUTION:Metal powder, for example iron or iron base alloy powder, is suspended in the organic solvent inactive thereto by using a pressure vessel and the gas involving O2 is injected into the space in contact with this suspension to a pressure of not less than atmospheric pressure. Thus, the oxidized film is formed on the surface of metal powder by reacting the O2 dissolved in solvent with the metal, this reaction proceeding very gently, excess oxidizing reaction being controlled and the homogeneous minute oxidized film is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing oxidation of metal powder, and more particularly to a method for forming an oxidation-resistant oxide film on the surface of metal powder.

Metal powders, especially ferromagnetic powders made of iron or iron-based alloys, are widely used as magnetic materials for magnetic recording media. However, during handling, there are risks of ignition due to rapid oxidation and deterioration of magnetic properties due to gradual oxidation. This has become a problem. Conventionally,
A method of stabilizing metal powder is to form an oxidation-resistant oxide film on the surface of the metal powder. Specifically, wet metal powder that has been immersed in an organic solvent is immersed in an organic solvent and then immersed in dry air. An air-drying method in which an oxide film is formed by gradual volatilization in an organic solvent, and a method in which oxygen-containing gas is blown into a suspension of metal powder in an organic solvent (Special Publication Publication No. 1973).
-28961) is known and established.

Individual particles of ferromagnetic gold powder used for magnetic recording media are ideally composed of undisturbed single crystals, but they are generally thin, flat particles with a thickness of about 100 to several angstroms. In many cases, the powder consists of particles with many defects such as irregular grain boundaries of crystallites and vacancies. When such particles are oxidized, areas with a high density of crystal defects are more likely to be oxidized, and if oxidation conditions persist, oxidation of such areas will induce oxidation of areas with fewer defects. Even on the surface of particles, the intended oxidation tends to occur not only in the shallow surface layer but also throughout the entire thickness of the particles. This makes it difficult to stabilize metal powder by forming an oxide film.

That is, when there are many particles that cause excessive oxidation, this causes agglomeration or ignition phenomena, and these phenomena often occur in the air drying method.

On the other hand, in the oxygen-containing gas injection method, the heat of oxidation of the metal powder is dissipated by the organic solvent surrounding the particles, so excessive oxidation is stopped at individual particles, and the chain reaction is sealed, which is effective in suppressing the ignition phenomenon. However, it is insufficient to prevent excessive oxidation of individual particles. %, it becomes extremely difficult to adjust the reaction conditions to form a homogeneous oxide film, such as the volatilization rate of the solvent in the air drying method, the gas blowing speed in the oxygen-containing gas blowing method, etc. .

The purpose of the present invention is to provide a method for forming a homogeneous oxide film on the surface of metal powder. Another objective is to provide a stable ferromagnetic metal powder.

As a result of intensive research to achieve the above object, the present inventors focused on the fact that the solubility of a gas component in a solvent depends on the pressure of the gas in contact with the solution, that is, "Henry's Law", and completed the present invention.

In the present invention, metal powder is suspended in an inert organic solvent, and the pressure of an oxygen-containing gas in contact with the suspension is increased to a level higher than atmospheric pressure. This is a method for preventing oxidation of metal powder, which is characterized by forming a film.

In the present invention, metal powders include iron, cobalt, nickel, chromium, zinc, tin, aluminum, titanium, etc. alone, or copper, manganese, magnesium, etc. in addition to the above metals.
It is particularly important to use ferromagnetic powder, which is an alloy powder consisting of two or more selected from silicon, tungsten, etc., and which is an alloy containing iron alone or iron as a base metal. In addition, alkali metal or alkaline earth metal salts such as phosphoric acid or boric acid are added to water-soluble inorganic or organic silicon and/or aluminum compounds, or the mixture is used as a coating material and a precursor of metal powder. Also included are metal powders obtained by further reducing the heated and dehydrated precursor coated with anion.

In the present invention, the organic solvent that is inert to the metal powder is one that does not alter the quality of the metal powder, or one that does not react with the metal powder to form a compound, or one that does not react with the metal powder to form a compound, or forms a compound only in the surface layer of the metal powder. It does not change the basic properties required for water-insoluble organic solvents, and it is a water-insoluble organic solvent alone or a mixture of two or more, or a solution of a solid or liquid organic compound in an organic solvent. Preferably, it is a single or mixed solution of an organic solvent in which the solubility of oxygen is 〒〒〒〒〉〒〉〉〉〉〉〉〉〉〉〉〉〉〒〉〉〉〒〉〉〒〉〉〒〉〒〉〒〉〒〉〒〉〒〉〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒〒 preferably 〒〒〒〒 〒 〒 〒 〒 〒 〒 . A mixed solution containing a fluorocarbon having a high oxygen solubility can also be preferably used.

In the present invention, metal powder is suspended in an inert organic solvent using a pressurized container, and oxygen-containing gas is injected into the space in contact with the suspension at a pressure higher than atmospheric pressure. , a homogeneous and dense oxide film is formed on the surface of the metal powder. The pressurized container to be used is preferably one with a large cross-sectional area that allows for a large contact area between the carcass suspension and the oxygen-containing gas. The pressure of the oxygen-containing gas to be injected varies depending on the oxygen concentration in the oxygen-containing gas, the oxygen concentration in the suspension, the type of organic solvent, etc., but usually 1 to 10 atmospheres is sufficient, and preferably 1.5 to 10 atmospheres. It is 5 atmospheres.

In the present invention, the oxide film on the surface of the metal powder is formed by the reaction between the metal and dissolved oxygen in the suspension.

Therefore, the reaction proceeds extremely mildly, excessive oxidation reaction is suppressed, and a homogeneous and dense oxide film is formed. In particular, it is effective for metal powders containing many crystal defects, so the magnetic properties of metal powders are: - The metal powder is made into a metal state through a complicated process to be suitable for magnetic recording media, and the temperature after it becomes a metal state is low. It is also effective for powders of iron or iron-based alloys, which have many crystal defects due to insufficient diffusion of metal atoms.

The present invention prevents excessive oxidation of metal powder by utilizing dissolved oxygen, forms an extremely homogeneous and dense oxide film on the surface of the metal powder, and prevents further oxidation of the metal powder, especially iron or iron. The present invention provides a method for preventing oxidation of a metal powder, which can prevent changes in magnetic properties over time due to oxidation of magnetic powder made from a base alloy, and its industrial significance is extremely large.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

However, the present invention is not limited to the following examples.

Example 1 Acicular ferromagnetic material with a specific area of 31 tri/El obtained by reducing acicular α-Fe, O, coated with 30% by weight of silicon oxide based on iron in a hydrogen flow 80ON of iron powder 809
The suspension was suspended in toluene, and placed in an autoclave having a capacity of 21 mm and equipped with a stirrer, one air was introduced under pressure, and the pressure was maintained at room temperature for 3 hours with stirring. After the treatment, the ferromagnetic iron powder was dried in the air with a phosphorus oxide, and an oxidation resistance test was conducted by leaving it in an atmosphere of 60% volume at 30°C. The oxidation resistance is shown in Table 1 with the change in saturation magnetization (δ#) over time. In addition, 1. The frequency of generation of excess oxidized particles was determined by the αFe (110
) Qualitative determination is made based on the presence or absence or extent of the presence of a spot equivalent to the clear diffraction spot seen in the Debye-Scherrer ring, and the results are shown in Table 1.

Comparative Example 1 An oxide film was formed by gradually volatilizing toluene from a toluene suspension of the acicular ferromagnetic iron powder produced in Example 1 and Lot 4 at room temperature by a so-called air drying method. Table 1 shows the results of the oxidation resistance test of the obtained ferromagnetic iron powder under the same conditions as in Example 1 and the observation results of the frequency of generation of excess oxidized particles.

Comparative Example 2 A toluene suspension of acicular ferromagnetic iron powder produced in the same lot as Example 1 was blown with air through a bubbler for 3 hours under the conditions of so m4/, 9 (Fe) in, while stirring at room temperature. Then, oxide film formation treatment was performed. This ferromagnetic iron powder was overdried in air, and an oxidation resistance test was conducted under the same conditions as in Example 1. The oxidation resistance test results and the observation results of the frequency of occurrence of excessive oxidized particles are shown in Table 1.

Applicant: Nippon Soda Co., Ltd. Agent Haruyuki Ito Horizontal Mountain Yoshi Beauty

Claims (1)

[Claims] ``1 Metal powder is suspended in an organic solvent inert to the metal powder, and the pressure of an oxygen-containing gas in contact with the suspension is increased to above atmospheric pressure, thereby imparting acid-resistant properties to the surface of the metal powder. A method for preventing oxidation of metal powder, characterized by forming a oxidized oxide film. 2. The method according to claim 1, wherein the metal powder is iron or an iron-based alloy. Claim 1 The method described in section.