JPS63120404A - Manufacture of magnetic iron powder - Google Patents

Abstract

PURPOSE:To obtain magnetic iron powder having improved and stable resistance to oxidation, by suspending magnetic metal powder in an organic solvent in hich a modifying metal salt has been dissolved, and stirring the suspension while heating it in contact with gas containing oxygen so that a dense oxide coat film is formed on the surface of the magnetic metal powder. CONSTITUTION:Magnetic metal powder is suspended in an organic solvent in which a modifying metal salt has been dissolved. This suspension is stirred while it is heated with the top face of the suspension contacted with gas containing oxygen. Said modifying metal may be aluminium salt and/or chromium salt. Said organic solvent may be alcohol or the like. For example, 100g of magnetic metal powder obtained by heating and reducing alpha-Fe2O3 containing SiO2, Ni and Ca within flowing hydrogen gas is suspended in 2l of ethylene glycol solution in which 12g of AlCl3 has been dissolved. This suspension is heated to 180 deg.C within air while it is stirred by means of a propeller stirrer, so that the suspension is subjected to oxidation for five hours. Then the suspension is cooled to room temperature to obtain magnetic iron powder.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing magnetic iron powder for magnetic recording having excellent oxidation resistance stability.

In order to measure high output and low noise in a wide recording wavelength range, the magnetic material for plate rice thorn strain magnetic recording has highly uniform fine-shaped particles, high coercive force (Hc), and saturation magnetization ( σ1)
, residual magnetization (σ, ,) are both large, and the squareness ratio (Rs=
σr/σ1) is basically required to have as large a magnetic property as possible. Among these magnetic materials, magnetic iron powder (hereinafter referred to as metal magnetic powder or magnetic iron powder) made of ferromagnetic metal powder is first used as a material for audio magnetic tapes due to its excellent magnetic properties. It has also been put into practical use as a material for 8+am video.

Magnetic iron powder is generally produced by a method of heating and reducing acicular iron oxyhydroxide. Acicular iron oxyhydroxide is known to have α-, β-, and γ-transformations, and the manufacturing method differs depending on each transformation type. However, as a starting material for magnetic iron powder for magnetic recording, α- FeOOH is excellent because it has fewer twins and resinous crystals and has a large needle ratio of around 10. More specifically, the α- The FeOOH□ synthesis method is particularly excellent, and α-FeOOH synthesized exclusively on the alkali side is used as the starting material for magnetic iron powder.

In the synthesis of α-FeOOH, Ni, Mn, Go, Cr, A are added to the ferrous salt aqueous solution or alkaline aqueous solution.
I, Si.

By adding salts such as Zn, Mg, Ca, Cu, Zr, etc., α-FeO is co-precipitated with these salts.
OH can be obtained, and these coprecipitated α-FeOOl+
is often used as a starting material for magnetic iron powder.

To obtain magnetic iron powder mainly composed of metallic iron by thermally reducing acicular iron oxyhydroxide, first, iron oxyhydroxide is heated and dehydrated in a non-reducing atmosphere such as air to form iron oxide. 1. Afterwards, α-FeO
A method is known in which 1+ is directly reduced by heating in a reducing atmosphere such as hydrogen.

However, depending on the size and composition of the starting material iron oxyhydroxide, during the thermal dehydration or thermal reduction described in Section 2,
The magnetic properties of the finally obtained magnetic iron powder, which is mainly composed of metallic iron, may be significantly deteriorated due to sintering of the acicular particles or collapse of the acicular particles. For this reason, before reducing the iron oxyhydroxide, Si, P, B, AI, Cr are added to the surface of the iron oxyhydroxide or the particles containing 113 α-Feze3 particles obtained by heating and dehydrating the iron oxyhydroxide.
, Ni, Mn, Co, Zn, Mg, C
By depositing compounds such as a, Cu, Ti, etc. singly or in combination, it is possible to prevent the collapse of the acicular particles and the sintering of the acicular particles during the subsequent reduction or thermal reduction, resulting in excellent magnetic properties. Methods for obtaining magnetic iron powder having the following properties are also known.

Magnetic iron powder is usually a fine particle with a trigonometric axis of 11III+ or less and a short axis of about one-tenth of the long axis, so its oxidation activity against air is extremely strong and it cannot be used as a magnetic recording medium if it remains reduced. It cannot be used as

Therefore, after reduction, the surface of magnetic powder mainly composed of reduced metallic iron is oxidized to form an oxide film on the surface of the magnetic powder, and the oxidation activity against air is suppressed. used.

As described above, methods for oxidizing the surface of magnetic powder mainly composed of reduced metallic iron to form an oxide film on the surface of the magnetic powder include a method in which the oxidation is carried out by a gas phase catalytic reaction, and a method in which the oxidation is carried out by a gas phase catalytic reaction, and a method in which the oxidation is carried out by a gas phase catalytic reaction, and a method in which the oxidation is carried out by a gas phase catalytic reaction, and a liquid phase reaction. There are known methods to do this, for example, the
6-28961, JP-A-55-164001, JP-A-57-219305, and JP-A-58-135102.

In this case, the thicker the oxide film formed on the surface of the magnetic iron powder, the higher the corrosion resistance (oxidation resistance stability) of the magnetic iron powder. As the thickness of the oxide film increases, the saturation magnetization (σ), which is one of the important characteristics of magnetic iron powder, decreases, so it is impossible to form a thick oxide film without thinking about it. Can not.

This can be understood from the fact that one of the characteristics of magnetic iron powder is that σ is higher than that of oxide-based magnetic iron oxide powder.

The value of σ of magnetic iron powder varies depending on the size, composition, and purpose of use of the magnetic iron powder.3 Although it cannot be generalized, for example, if the specific surface area used for audio applications is 3(l
For magnetic iron powder of about nf/g, σ5 is about 150 emu/g, and the specific surface area used for 8 mm video is 5Or+f/
For magnetic iron powder of about 1.5 g, σ needs to be about 120 emu/g, and values less than this are not preferable because the properties of the final product magnetic tape deteriorate.

Therefore, in order to obtain magnetic iron powder with high oxidation resistance stability that can be produced within a limited range of oxide film thickness, the film quality of the oxide film provided on the surface of the iron powder must be as dense as possible. It needs to be coated.

The corrosion resistance of magnetic iron powder can be quantitatively evaluated from the change in σ before and after exposing the magnetic iron powder to an oxidizing atmosphere at high temperature and high humidity for a certain period of time.

For example, when magnetic iron powder is exposed continuously for weeks at 60'C in high humidity air with a relative humidity of 90%, the magnetic iron powder is It is possible to quantitatively evaluate the oxidation resistance stability of

The initial σ9 of magnetic iron powder for audio is 150 emu.
/g has an oxidation resistance stability under the above conditions when this ratio is 0.10 or less, and magnetic iron powder for 8mm video has an initial σ8 of about 120 emu/g. Regarding the oxidation stability, it is desired that this ratio is 0.15 or less, but conventional oxidation removal methods have not been able to form a dense oxide film with such high oxidation resistance stability.

In order to achieve a p-value of 7, the present inventors have worked diligently on a method to obtain magnetic iron powder with excellent oxidation resistance stability by forming a dense oxide film on the surface of metal magnetic powder mainly composed of iron. As a result of our investigation, we found that metal magnetic powder was suspended in an organic solvent in which a specific metamorphic metal salt was dissolved, and the suspension was heated while the upper surface of the suspension was in contact with an oxygen-containing gas. The present invention was achieved by discovering that an extremely dense oxide film can be formed on the surface of metal magnetic powder by stirring the powder.

That is, in the present invention, metal magnetic powder is suspended in an organic solvent in which a metamorphic metal salt is dissolved to form a suspension, and the upper surface of the suspension is in contact with an oxygen-containing gas. , a method for oxidation stabilization of metal magnetic powder characterized by stirring the suspension under heating.

The present invention will be explained in detail below.

In the present invention, a metal magnetic powder is suspended in an organic solvent in which a metal salt for modification is dissolved to form a suspension, and the metal salt for modification used herein is an aluminum salt and/or a chromium salt.

Preferred examples of such aluminum salts include aluminum nitrate such as aluminum chloride salts such as AlCl3, and aluminum sulfates such as Alz(SO2)s.
Chromium bromide such as rBr3, chromium oxide such as Cry, chromium chloride such as CrCl2, CrC15, (:r(
Preferred examples include metal salts such as chromium nitrate and chromium acetate, such as NO3)s H9H,O, but those used in the present invention include those that can be dissolved in the following organic solvents under heating. For example, any material can be used.

The organic solvent used in the present invention is one that dissolves the metamorphic metals mentioned above under heating, does not itself undergo substantial deterioration when brought into contact with oxygen-containing gas at high temperatures, and is one that dissolves under normal pressure. Polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin; decyl alcohol,
Particularly preferred are trihydric alcohols such as 1-nonanol.

Metal magnetic powder suspended in an organic solvent in which metamorphic metals are dissolved needs to be appropriately agitated using a propeller stirrer or the like to prevent agglomeration and sedimentation. However, if this suspension of metal magnetic powder in an organic solvent is heated and moderately stirred in an atmosphere of an oxygen-containing gas such as air, that is, in a state in which it is in contact with the oxygen-containing gas,
The metal magnetic powder undergoes oxidation near the interface where it is in contact with the gas phase of the suspension. In the present invention, in order to form a dense oxide film, the temperature of the suspension is preferably 100°C or more and 300°C or less. If the temperature is less than 100°C, a dense oxide film cannot be formed, while if it exceeds 300°C, non-magnetic α-FezOi will be mixed in the oxide film, which is not preferable.

In the present invention, the oxygen concentration of the oxygen-containing gas is
It is preferably 0.1vo1% or more and 25vo1% or less.

If the oxygen concentration is less than this, the time required to form the oxide film becomes extremely long, making it uneconomical. On the other hand, if the oxygen concentration exceeds 25 vol%, the density of the oxide film formed will be impaired. Air can be used as is as the oxygen-containing gas, but of course non-oxidizing inert gas,
For example, a gas mixture of nitrogen, helium, neon, argon, etc. and oxygen may be used.

Of course, the oxygen-containing gas can be forcibly blown into the suspension, but it is also possible to blow it into the suspension but into the upper part of the suspension, that is, through the free surface of the suspension. It is more preferable to adopt a method of absorption into a cloudy liquid because a denser oxide film can be formed. By this operation, a dense film in which metamorphic metal atoms are incorporated into the oxide film is formed, and magnetic iron powder having excellent oxidation resistance stability can be obtained.

Further, if necessary, it is also possible to heat-treat the oxide film by switching the gas phase to a non-oxidizing gas such as nitrogen to make the oxide film even more dense.

According to the findings of the present inventors, in order to form a dense oxide film, it is possible to oxidize at an appropriate oxidation rate and rearrange the crystals of the oxide film layer to become more dense. A moderate temperature is required.

Therefore, in the method of the present invention, only the part of the suspension near the gas-liquid interface in contact with the upper part of the suspension is used for oxidizing the metal magnetic powder, and the metal magnetic powder itself is covered with an organic solvent. Therefore, it is presumed that a dense oxide film is formed at an appropriate speed even at high temperatures of 100'C or higher.

Further, in the present invention, since the suspension is appropriately stirred, the metal magnetic powder oxidized at the gas-liquid interface continues to undergo surface renewal and move into the main body of the suspension. While in this body, oxidation is virtually negligible. In other words, it is thought that a dense oxidized liquid can be formed because only the crystal rearrangement of the oxide film formed on the liquid surface occurs until it moves into the suspension body and reaches the gas-liquid interface again. It will be done.

Furthermore, as mentioned in Section 2, oxidation progresses relatively slowly even at high temperatures, so the metal atoms of the metamorphic metal salt dissolved in the suspension are absorbed into the oxide film through the process of forming the oxide film. As a result, a dense oxide film with excellent oxidation resistance and stability is formed.

The method and effects of the present invention will be explained in more detail below using Examples and Comparative Examples.

Practicability - 5 parts by weight of SiO2 per 100 parts by weight of Fe, N
100 g of metal magnetic powder obtained by thermally reducing α-Fe203, a precursor of magnetic iron powder for 8 mm video, containing 3 parts by weight of i and 0.05 parts by weight of Ca, in an AlC
The suspension was suspended in a 2N ethylene glycol solution in which 1312g of the suspension was dissolved, and the suspension was heated to 180°C in the air while stirring using a propeller stirrer, and the oxidation reaction was carried out for 5 hours, after which it was cooled to room temperature. Magnetic iron powder was collected.

When the composition of the obtained magnetic iron powder (abbreviated as magnetic powder A) was analyzed using a fluorescent X-ray analyzer, it was found that A
1 was contained in an amount of 2.6 parts by weight. 6. Part into magnetic powder
It was left continuously for one week at 0°C under high humidity air with a relative humidity of 90%. The powder characteristics of the obtained magnetic iron powder (abbreviated as magnetic powder B) were measured and the values shown in the table below were obtained. The magnetic properties were measured using a vibrating sample magnetometer (VSM) with a magnetic field of 10 KO
Measured at e. The results are shown in Table 1.

Table 1 From the table, the ratio ΔσS/σ5 value (hereinafter referred to as oxidation stability ratio) indicating the oxidation resistance stability of 8mm video magnetic powder A was calculated to be 0.081, which is excellent. I understand that there is something.

Example 2 100g of the metal magnetic powder obtained by reduction in Example 1 was converted into Cr(
NOs)s Suspend 145 g of H9H20 in 2p ethylene glycol solution, heat the suspension to 180'C in air while stirring using a propeller stirrer, maintain the reaction for 5 hours, and then cool to room temperature. It was cooled and the magnetic iron powder was collected.

When the composition of the obtained magnetic iron powder (abbreviated as magnetic powder C) was analyzed using a fluorescent X-ray analyzer, it was found that C
It contained 2.0 parts by weight of r. Part of magnetic powder C is 6
It was left to stand continuously for one week under high humidity air with a relative humidity of 90% at 0° C. to obtain magnetic iron powder (abbreviated as magnetic powder). magnetic powder C,
The powder characteristics of D were measured and the values shown in Table 2 were obtained.

Table-2 From the table, the powder properties of magnetic powders C and D are oxidation resistance stability ratios of 0 and 0.
74, which was recognized as being excellent.

Comparative Example 1 100 g of the metal magnetic powder obtained in Example 1 was suspended in 12 parts of toluene, and while stirring using a propeller stirrer, a 1:1 mixed gas of air and N2 was blown into the suspension at 40°.
After performing an oxidation reaction with C for 4 hours, toluene was removed to obtain a magnetic powder (abbreviated as magnetic powder E). Magnetic powder E at 60°C
1. Magnetic powders E and F were obtained by leaving them continuously for one week under high humidity air with a relative humidity of 90% (abbreviated as magnetic powder F).
Table 3 was obtained by measuring the powder properties.

Table 3 The oxidation resistance stability ratio of magnetic powder E is 0.161 and Example 1
It can be seen that this is far inferior to Example 2.

Claims (3)

[Claims] (1) Metal magnetic powder is suspended in an organic solvent in which a metal salt for metamorphism is dissolved to form a suspension, and the upper surface of the suspension is brought into contact with an oxygen-containing gas under heating. A method for stabilizing oxidation of metal magnetic powder, which method comprises stirring the suspension. (2) The method according to claim 1, wherein the metal for metamorphosis is an aluminum salt and/or a chromium salt. (3) Claim 1 in which the organic solvent is alcohol
The method described in section.