JPS61270315A - Production of ferromagnetic iron powder - Google Patents

Abstract

PURPOSE:To obtain ferromagnetic iron powder having high dispersion orientation and adequate coercive force by reducing ferric oxide to iron by gaseous hydrogen then subjecting the iron to a heating treatment in a gaseous mixture composed of NH3+N2 then to a stabilizing treatment. CONSTITUTION:An oxidizing gas is brought into contact with the iron hydroxide obtd. by neutralizing an aq. ferrous salt soln. to form needle-like iron oxyhydroxide. Such iron oxyhydroxide is dehydrated and calcined to form ferric oxide which is then reduced by hydrogen. The reduced iron is cooled and is then subjected to the heating treatment in the gaseous mixture composed of NH3+N2 to solutionize about 0.5-3wt% nitrogen in the iron-component. The iron powder after the treatment is immersed in toluene, etc. and filtered and thereafter the iron powder is dried in air so as to be given atm. stability. The surface of the particles is thereby coated with the thin oxide film. The ferromagnetic iron powder having the adequately decreased coercive force is obtd. by the above-mentioned method.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is a magnetic powder used as a raw material for magnetic recording media such as magnetic tapes, magnetic disks, and magnetic sheets, and is said to be particularly suitable for high-density magnetic recording. The present invention relates to a method for producing ferromagnetic iron powder.

The greatest feature of the present invention is the production of ferromagnetic iron powder that has highly dispersed orientation, which was conventionally said to be a contradictory characteristic of ferromagnetic iron powder, and has an appropriate coercive force that matches the saturation magnetic flux density of the head. It is related to.

(Prior art) Acicular magnetic iron powder can be used in the conventional γ-Ire, O, or CO-
Unlike γ-Fe2O3, it has a high coercive force and saturation magnetic flux density, making it ideal for high-density magnetic recording, and it is also smaller and lighter81)1)! Metal tapes using magnetic iron powder are indispensable for magnetic tapes for VTRs.

Among the characteristics of magnetic powder for magnetic recording, coercive force is particularly important for increasing recording density due to short wavelength recording, preventing transfer, etc.
It plays such an important role that it is even said that the increase in coercive force is the history of the development of magnetic materials.

Conventional A-inch VT is used as raw material for magnetic tape for 8as VTR.
The main reason why metal tape is used instead of R magnetic tape is to take advantage of this high coercive force.

The performance of heads that can handle metal tapes has gradually been developed, and recently the synthesis and processing technology for amorphous or thin film sendust has improved, resulting in high saturation magnetic flux density and magnetic permeability. Coercive force is also about 1
Now it can tolerate up to 5000e.

However, originally metal iron was 1-Fe, O, or Pe.
, 04, has a high spontaneous saturation magnetization, and has a higher
By combining this method with the method utilizing acicularity, that is, shape anisotropy, invented by Camras, it has become possible to easily obtain iron powder having a high coercive force of 15,000 or more. In particular, when manufacturing magnetic tape using the coated in-plane recording method, by increasing the axial ratio (major axis/minor axis ratio), so-called dispersion orientation improves, and when a hysteresis loop is drawn, a high squareness ratio is achieved. Furthermore, with the recent trend of development to improve output and sensitivity through noise reduction, magnetic particles have become finer. While the cor-r-F's, O, used in tapes is about 45 m'/g at most, magnetic iron powder is about 55 m'7 g.
In the near future, it is predicted that ultrafine iron powder with a size of 60d19 or more and a long axis of 12 μm or less will appear.

The problem here is that increasing the axial ratio and making the grains finer further increases the coercive force.

If the coercive force greatly exceeds 15,000e, it is higher than the saturation magnetic flux density of current heads, and this causes some practical problems, such as recording signals not being recorded sufficiently or being unable to be erased. Therefore, in the production of magnetic iron powder, it is important to match the coercive force to the performance of the head. Generally, iron powder with fine particles and a shape with excellent dispersion orientation has a large coercive force. The challenge is how to reduce only the coercive force without degrading these characteristics to better match the performance of the head.

As a method to reduce the coercive force of magnetic iron powder, TIQe
It has been investigated by reducing the factors shown in the formula ll:, g(Nb-Na)40 (He: coercive force, g8: spontaneous magnetization, Wb-1) a: shape anisotropy coefficient, ：Constant representing crystallinity]. As this method, the following methods have been used so far.

■ A large amount of nickel etc. is deposited and F'e is removed during the heat treatment process.
- Alloy Ni to reduce spontaneous magnetization.

■ Reduce the coefficient of shape anisotropy by reducing the axial ratio.

■ A method by suppressing crystal growth during the heat treatment process.

However, method (1) reduces the above-mentioned dispersion orientation and is not used unless the purpose is to manufacture magnetic iron powder with a special shape.
．． This is a method that cannot be applied; even if it were applied, the effect of reducing coercive force would not be noticeable unless the axial ratio was set to about 5 or less, and in this case, the dispersion orientation would be greatly reduced, making it impractical.

Next, method (2) is a method that is often used in practice because the coercive force can be relatively easily reduced by combining various operating conditions for heat treatment. However, the growth of a single crystal within the grain and the sealing phenomenon in which the pores within the grain (dehydration pores and deoxidation pores) are diffused and expelled from the grain proceed simultaneously, and during the dehydration and firing process, iron hydroxide α-Fe00
H) is sufficiently sintered to produce ferric oxide (α-we, o,) with advanced crystal growth and pore sealing, and then completely reduced with hydrogen to form a needle-like, pore-free product. Iron particles are obtained, and suppressing crystal growth in these two processes means leaving vacancies within the particles, and reducing the amount of magnetization due to the Lorentz effect, that is, reducing the coercive force. Although this is true in the end, the dispersion and orientation of the particles is also reduced due to the pores remaining in the particles. In practice, this is a method that is often used under the condition that emphasis is placed on the effect of reducing coercive force, and that the reduction in dispersion orientation is brought as close to an acceptable value as possible.

Finally, in method ①, nickel or other elements are
After adhering to the surface of eOOH particles, it is diffused into the inside of the particles through heat treatment, and then formed into a solid solution alloy with metallic iron.
This method aims to reduce the coercive force by reducing the spontaneous saturation magnetization of the iron particles themselves.

The problem with this treatment method is that, for example, when nickel is used as an adhesion/solid solution component, the weight ratio of nickel to iron is 15.
A large reduction in coercive force cannot be obtained unless nickel is dissolved in solid solution in a large amount, preferably 20 or more, and it is generally very difficult to dissolve nickel in such a large amount as iron particles. The shape of the iron particles themselves changes due to the presence of nickel that is not deposited on the surface, that is, free nickel particles, and due to the large amount of nickel deposited. When used for these productions, the coercive force is reduced and at the same time the dispersion orientation, which is an important property for magnetic paints, is reduced, and the overall evaluation often results in extremely low-quality magnetic iron powder.

As a countermeasure to this problem, it is possible to predict the shape of highly dispersed and oriented iron powder in advance and make it take the same shape after nickel deposition and solid solution, but in reality, α-FθOOH
There are also differences in the crystal structure of the particles and the nickel salt, so it is difficult and impractical to freely control the adhesion state.

The present invention is a method for reducing coercive force by trapping a component other than metal elements such as nickel, that is, nitrogen, as a solid solution in iron, but unlike the case of metal components such as nickel, there are almost no other properties. The feature is that there is no change, and only the coercive force can be freely controlled.

The technology related to the nitriding method using ammonia gas is disclosed in JP-A-5
No. 7-59504, this method directly reduces and nitrides iron oxide with ammonia gas at a temperature of 400 to 700°C to reduce iron nitride (Fe, N) from 10 to 700°C.
By containing 70%, the coercive force is 450-7000
The present invention relates to a method for producing magnetic iron powder. The object of this method is the raw material for existing audio or 1/2-inch VTR tapes, that is, cobalt-containing iron oxide, which has a low coercive force. In view of this, the object of the present invention is that the raw material for 8m vTR tape is magnetic iron powder, which has a high coercive force of approximately 15,000e. Because the treatment is performed with a mixed gas of ammonia and nitrogen at a low temperature below ℃, the concentration of nitrogen solidly dissolved in the iron particles is also low, and even in X-ray diffraction, the peak of iron nitride (Ire, N) is not clearly fixed. belongs to.

(Means for Solving the Problems) The present invention relates to a treatment method for reducing the coercive force of magnetic iron powder by reducing the above-mentioned spontaneous magnetization. I considered it. In other words, as a processing method that suppresses changes in the shape of acicular iron powder with good dispersion and orientation as much as possible, and has uniform characteristic values with no variation between particles, ■ Reacting the additive components in a gaseous state. .

■ Processing reduced iron powder that is acicular and has good dispersion and orientation.

■ In order to suppress the shape change of iron powder,
To be effective even when the amount of solid solution components is small.

conditional titration, addition of Concerning ingredients and processing conditions.

The present invention has found that the coercive force can be reduced by about 15,000 eK by treating the iron particles with nitrogen gas containing a small amount of ammonia gas at 100 to 300° C. and dissolving α5 to 54 wt of nitrogen into the iron particles. In this treatment method, there is almost no change in the shape of the iron particles before and after the treatment, and therefore there is no change in the specific surface area (BIT value), which is one indicator of the particle size of the iron particles. While the reaction is relatively slow, the amount of solid solute nitrogen is determined by the set temperature, and in the temperature range of 100 to 300°C, the amount of solute nitrogen increases almost linearly as the processing temperature rises. This can be said to be a highly practical treatment method.

The basic principle of this invention is to reduce the coercive force by reducing spontaneous magnetization, and although a slight decrease in the saturation magnetic flux density (δa) of iron particles is unavoidable, it is generally 8%.
1 20 em required as raw material for VTR tape
A saturation magnetic flux density of u/g or higher is fully achievable.
There are no practical problems.

The feature of the reaction of the present invention is that due to the catalytic action of iron particles,
NH, is dissociated, and the generated nitrogen diffuses from the surface of the iron particles to the inside, and a small amount of nitrogen solid solution has the effect of reducing the coercive force. When using nitrogen gas alone, there is almost no solid solution of nitrogen or the formation of iron nitride, so there is no reduction in the coercive force.If nitrogen gas is used alone, the coercive force will not be reduced.In the case of nitrogen gas alone, the coercive force will not be reduced. Although formation occurs, at such high temperatures, the shape of the iron particles themselves changes, the needle shape collapses, and the dispersion orientation rapidly decreases. In addition, when using gas (■) as a gas to transport Nil, denitrification (NH) by ■.

The efficiency of nitriding is poor because of the reaction (formation of Also, when using an inert gas such as argon gas (Ar),
Not only does the reaction proceed quickly and is difficult to control, but it is also expensive, making it less industrially practical than N or gas.

Next, the reaction mechanism of the present invention will be explained. For convenience, the reaction formula can be considered using the following reaction formula: 2Fa (solla) + 12NHs (Gas
) +-A172 Fe4N(soxta)+5/4H
, (G!Lll) Standard free energy ΔG'-5100+2.26TtOgT-14,52T
Dissociation of ammonia gas (N1)n(G)→'A N!
(a) +5/! 1(t(G)) is about 200°C or higher, and the reaction for producing Fe4N by NH in the above formula is about 4
Nitrogen progresses actively at temperatures above 00°C, but in reality, nitrogen enters the inside of iron particles at temperatures above about 150°C due to both the partial pressure reduction effect of the carrier gas and the catalytic effect of ultrafine reduced iron powder. A reduction in practical coercive force due to solid solution is observed, and when the concentration of ammonia is high or the processing temperature is high, Fe4N can be identified by X-ray diffraction. As a carrier gas to accompany NH gas, it is more effective to use horse gas than H gas because gas is produced as a by-product.
H! In the case of gas, it is necessary to increase the concentration of ME and gas.

Figures 1 and 2 have a long axis α of 29 μm and a long axis/short axis ratio of 1 &
40 Needle-shaped iron oxyhydroxide (a-pθ0OH) is used as a raw material, and after dehydration and calcination and hydrogen reduction, it is cooled in a nitrogen atmosphere, further immersed in toluene, filtered and dried, and then dried and oxidized in an air stream at room temperature. Stabilized magnetic iron powder (long axis (L2)
Qμm, major axis/minor axis ratio 152. Nitrogen concentration aaS chi wt)
was subjected to the same heat treatment and held at a temperature of 100℃ or higher for about 2 hours during the cooling process to reduce the ammonia gas concentration α5 Yuma 01.
(Remaining: Nitrogen gas) This shows the coercive force when nitriding is performed at a concentration higher than (remaining: nitrogen gas).

Currently, the coercive force of magnetic iron powder used as raw material for 8% VTR tape on the market is approximately 150, commensurate with head performance.
Although it is strictly regulated to be within 0±300e, according to the present invention, not only can the coercive force be controlled relatively accurately,
No deterioration of other properties occurs, and it is possible to do this at a low temperature of around 200° C. under almost atmospheric pressure, so it has high industrial practicality. Details will be explained below in Examples.

Example 1 Unbranched acicular α-7eOO crystallized on the alkali side
The surface of H particles (long axis a, 29 ttm, axial ratio 1) 1L4) was coated with a commercially available water glass aqueous solution (using J Engineering No. 83, the concentration of Si0. was diluted to cL5 mol/l in N). After 5 hours of coating, it is filtered and washed with water to obtain an α-IFeOOH cake. The composition of the cake is 51=544% w with respect to Fs
It was t.

Dry this cake using a general-purpose disc-type spray dryer (
The mixture was granulated and dried into spherical granules with an average particle size of 100 μm using a hot air temperature of 250°C, and further dehydrated and fired in the air at 725°C for 5 hours to obtain hematite (α-Ha^). This Fs
1/9 portions of lOl were collected and placed in a fluidized bed reactor (inner diameter 150 m/
Hydrogen reduction was carried out at 400°C.

After the reduction, the retort was taken out of the furnace and cooled to 200°C in about 1 hour in Hou gas, and then Z5%N
Hs-% mixed gas was introduced and kept in another processing furnace for 2 hours. Thereafter, the gas was switched to phosphorus gas, and the mixture was cooled to room temperature.

The treated iron powder was immersed in toluene, taken out into the air, filtered, and dried in the air to impart atmospheric stability to obtain magnetic iron powder.

A portion of the recovered iron powder (approx. The results are shown in Table 1. NA of Comparative Example 1
-Compared to the case without nitriding with Nit mixed gas, there are almost no other characteristic values except that the coercive force decreased by about 5100e from 14900e and the saturation magnetic flux density (δ8) decreased by 12 emu/9. No difference was observed. Further, the content of solid-dissolved nitrogen was 1.90%wt. Further, FIG. 5 is an electron micrograph showing the crystalline state of the iron powder, and no deformation of the acicular particles due to the nitriding treatment is observed.

Moreover, the X-ray diffraction diagram of this iron powder is shown in FIG.

Furthermore, Table 2 shows the results of the magnetic sheet, and no deterioration was observed in terms of dispersion and orientation.In the above points, the nitriding treatment of the present invention has an acicular property with excellent dispersion and orientation. This shows that it is possible to accurately reduce the coercive force of iron particles to an appropriate value.

Example 2 The same α-7θOOH as in Example 1 was used, and after heat treatment, it was nitrided at 175° C. for 2 hours with a 2-5% N-hydrogen mixed gas to recover iron powder.

The coercive force is 15500s, which is about 2500s compared to Comparative Example 1.
This was a reduction in The nitrogen content was 1.28%.

Example 5 This is a case in which nitriding treatment was similarly performed at 150° C. for 1 hour using a 5% MaolN%-% mixed gas.

The coercive force was 15,700· and the nitrogen content was 195% wt.

As described above, no deterioration in characteristic values other than a slight decrease in coercive force and saturation magnetic flux density was observed in any of the cases.

Comparative Example 1 Using α-FeOOH as a raw material in Example 1, heat treatment was performed under the same conditions, omitting the nitriding treatment, immersion in toluene, and then gradual drying in air to recover iron powder.

The characteristics of the recovered iron powder are shown in Table 1, with a coercive force of 180
00s, which is a value that cannot be used as a raw material for current 8 ml VTR tape, but other characteristic values are good and it is an acicular iron powder similar to the electron micrograph in Figure 5. be.

In Examples 1 to 3, this α-7θOOH was heat treated and then subjected to the nitriding treatment of the present invention to reduce the coercive force to the respective values.

Comparative Example 2 The same α-FeOOH as in Example 1 was used, and after heat treatment, it was nitrided with a 20% vol NHa-4 mixed gas at 200°C for 2 hours, and the coercive force and saturation magnetic flux density were respectively 1) It was low at 500θ and 104 emu/9 and could not be put to practical use. This is NH.

This is because the concentration is high, the reaction is rapid, and the diffusion rate of N is high.
Under these conditions, a uniform composition cannot be obtained and is not practical.

Comparative Example 5 After heat treatment with the same α-7θOOH as in Example 1, 20%v
The nitrogen concentration in the iron powder was 1.14% after being cooled from 460°C to 200°C for 15 minutes in a MB, -4 mixed gas, immersed in toluene, and then dried in air.
wt, but the diffusion into the interior of the iron particles was insufficient and the coercive force was f 6200e, meaning that the reduction effect was small and not practical.

Comparative Example 4 After heat treatment in the same manner as Comparative Example 3, 20% vo was added to the reaction gas.
The mixture was maintained at 460° C. for 2 hours in a 1N%% mixed gas, further cooled to 200° C. for 15 minutes under that atmosphere, and then cooled to room temperature in horse gas.

The nitrogen concentration in the iron particles recovered by drying in the air was 5.83.
%wt, the coercive force was 10800n, and the saturation magnetic flux density was low at 107 emu/9 (8, the quality was such that it could not be used as a magnetic powder raw material for VTR tape. Figure 5 shows this iron powder. It can be seen that the F@4N peak is clear in the X-ray diffraction diagram of FIG.

Table 2 Magnetic sheet characteristics (Note) 1. Magnetic sheet creation conditions (1) Paint composition: Iron powder: 4 (parts by weight) vAllH: 1 Solvent: z5 Dispersant: 0. Q64 Electrosolvent: MethiAζri9retetonρApaVshiNsenanone==1/IVAGH: Salt-vinyl acetate copolymer 2, lI & material Dispersion time: 48 hours (paint shaker) 5 Orienting magnetic field; 5 4KOe

[Brief explanation of the drawing]

FIG. 1 shows the relationship between nitrogen concentration in iron particles and coercive force, and FIG. 2 shows the relationship between nitrogen treatment temperature and coercive force. FIG. 3 is an electron micrograph (magnification: 5 aooo) showing the state of crystal grains of ferromagnetic iron powder obtained by processing according to an embodiment of the present invention. Moreover, FIG. 4 shows the X@ diffraction diagram (OuKα) of the ferromagnetic iron powder obtained in one example of the present invention, and FIG. 5 shows the X-ray diagram of the magnetic powder obtained in the comparative example of the present invention. It shows a diffraction diagram (OuKα). Patent applicant Toyo Soda Kogyo Co., Ltd. Processing temperature (0C)

Claims (5)

[Claims] (1) Iron hydroxide obtained by neutralizing a ferrous salt aqueous solution with an alkaline aqueous solution is brought into contact with an oxidizing gas, oxidized to produce acicular iron oxyhydroxide, and further dehydrated and calcined. In the method of manufacturing ferromagnetic iron powder, the surface of which is coated with a thin oxide film in a stabilization process, ferric oxide is reduced to iron using hydrogen gas, and then ammonia and nitrogen are added. A method for producing ferromagnetic iron powder characterized by heat treatment in a mixed gas of (2) The manufacturing method according to claim (1), wherein the heat treatment temperature is 100 to 300°C. (3) The manufacturing method according to claim (1) or (2), wherein the ammonia gas concentration is 0.1 to 10% by volume. (4) The manufacturing method according to any one of claims (1) to (3), wherein the heat treatment time is 0.5 to 5 hours. (5) The manufacturing method according to any one of claims (1) to (4), wherein the solid solution amount of nitrogen in the ferromagnetic iron powder is 0.5 to 3% by weight.