JPS6369714A - Production of ferromagnetic iron oxide powder - Google Patents

Abstract

PURPOSE:To prevent occurrence of sintering phenomenon in the stage of heating of water-contg. iron oxide powder when water-contg. iron oxide powder is reduced after it is sintered, by forming film of product generated by oxidizing a compd. contg. Si, P or B in the sintering stage on the surface of the powder particles. CONSTITUTION:A compd. contg. at least a kind of components among Si, P and B is oxidized basing on the principle of the CVD process in the sintering stage when ferromagnetic iron oxide powder is obtd. by the reduction of water- contg. iron oxide powder after it is sintered, and the film of the oxidized compd. is formed on the surface of the powder particles. Such compds. are hydrides, carbohydrides, etc., for example, Si compds., such as SiH4, SiH2(CH3)2, etc., B compds., such as B2H6, B(CH3)3, P compds., such as PH3, P(C2H5)3, etc. The oxidation of these compds. is carried out by the oxidation reaction of each single component or the combination of each single reaction. In this case, it is necessary to flow >=equivalent amt. of gaseous O2, and it is preferred to carry out the reaction in a temp. range between 350 deg.C and 450 deg.C.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to the production of ferromagnetic iron oxide powder, and more specifically, heat treatment of hydrated iron oxide powder. Regarding how to prevent it.

(Prior Art) In general, iron oxide powder for magnetic recording and iron powder for magnetic recording obtained by reducing iron oxide powder used for video tapes, audio tapes, floppy disks, cards, etc. are goethite (α
-FeOOH), lepidocrocite (γ-FeOOH)
) etc. as a starting material and is manufactured by subjecting it to heat treatment such as calcination and reduction.

That is, first, an aqueous solution of iron sulfate or iron chloride is neutralized by adding an alkali such as caustic soda or ammonia, and an oxygen-containing gas (air or oxygen) is blown into it to form a nail-like hydrated material such as goethite or lepidocrocite. Synthesize iron oxide powder. Next, α-Fe203 is obtained by filtering, washing with water, drying, and firing in air, and reducing this in a hydrogen stream produces magnetite 1-(Fe, O,) and iron powder. Furthermore, maghemite (γ-Fe2O3) is produced by oxidizing magnetite in air.

When the particle surface of maghemite is subjected to a cobalt modification treatment, maghemite (Co-γ-Fe, 03) coated with cobalt is obtained. Furthermore, iron powder can be obtained by reducing the iron oxide powder obtained through this step using a reducing air stream.

The needle-shaped or grain-shaped iron oxide powder (maghemite) produced by this process is made smaller in particle size in order to improve recording density and improve magnetic properties such as coercive force, squareness ratio, and orientation ratio. However, as particles become finer, sintering phenomena tend to occur between particles during heat treatments such as firing, reduction, and oxidation, and the sintering phenomenon is particularly noticeable during the reduction process. This sintering phenomenon progresses more easily as the particles are made finer, and as a result, the dispersibility deteriorates, and the magnetic properties such as coercive force, saturation magnetization, squareness ratio, and orientation ratio deteriorate, especially the coercive force decreases.
/N ratio deteriorates, making the material unsuitable for high-density recording.

For this reason, various measures have been attempted to prevent interparticle sintering. For example, during or after the synthesis reaction of hydrated iron oxide powder, an aqueous solution containing Si and/or P (e.g., sodium metasilicate, phosphoric acid, etc.) is added to an aqueous solution slurry containing particles, so that Si oxide is formed on the particle surface. Methods for producing Si compounds such as
161, 008, 56-26730), or a similar method of producing P compounds such as P oxide (e.g., JP-A-55-115902, JP-A-55-149138, JP-A-56-26-
38405, 56-41835), etc.

However, these methods for preventing interparticle sintering have the disadvantage that magnetic properties deteriorate as the amount of added Sl, P, etc. increases, and the sintering preventing effect is not necessarily sufficient.

The present invention solves the drawbacks of the prior art described in 1. regarding interparticle sintering, reduces the occurrence of sintering phenomenon even by making particles finer, prevents deterioration of S/N ratio and dispersibility, and improves magnetic properties. The object of the present invention is to provide an efficient method for producing ferromagnetic iron oxide powder or ferromagnetic iron powder that does not cause deterioration.

(Means for Solving the Problems) In order to achieve the above object, the present inventor conducted an analysis and study on the cause of the deterioration of coercive force and saturation magnetization, especially among magnetic properties, due to the conventional interparticle sintering prevention method. In addition, it cannot be explained simply by an increase in non-magnetic substances such as Si oxide and P oxide, but compounds such as Sj and P are produced during or after the synthesis reaction of hydrated iron oxide powder. Therefore, it is thought that these Si compounds and P compounds have a negative effect on crystal growth, pore movement, and particle w and @ formation in the firing process, and furthermore, they are causing a decrease in the reduction reaction rate in the reduction process. .

On the other hand, we used a transmission electron microscope to investigate how the particle shape of hydrated iron oxide particles changes during heat treatment of calcination, reduction, and oxidation.
Although sintering occurs during each oxidation step, it was found that sintering was the most severe during the reduction step.

To prevent sintering in this reduction process, measures such as lowering the reduction temperature, shortening the reduction time, and reducing the hydrogen gas flow rate can be taken, but on the other hand, the magnetic properties, especially the coercive force and saturation magnetization, This cannot be said to be a fundamental solution as it causes deterioration and a decrease in efficiency.

Therefore, in view of the fact that the conventional method for preventing interparticle sintering is a so-called wet method, the inventors of the present invention have conducted intensive research on measures to prevent interparticle sintering using a dry method. Using the principle of vapor deposition (CVD), Si, P
, B, etc. on the surface of the particles to reduce interparticle sintering as much as possible in the subsequent steps such as reduction, and hereby the present invention has been made. be.

The present invention will be explained in detail below based on examples.

As mentioned above, the firing process is a process in which acicular or grain-shaped hydrated iron oxide particles are transformed into iron oxide (α-Fe20.) particles, and is generally carried out during heating periods A to A as shown in Figure 1. Heating and cooling are performed according to a profile consisting of C (including a temporary holding period B due to the transformation point), a maximum temperature holding period, and a cooling period E.

CVD in the present invention can be carried out at any stage in the above profile, and Si,
A compound gas containing at least one component of P and B is flowed together with oxygen or an oxygen-containing gas to cause an oxidation reaction and form a coating of the oxide on the surface of the hydrated iron oxide particles.

The above compounds include hydrides, hydrocarbons, etc.
For example, SiH4,Sj,zHI,,5j3H8,5
i (CH3) S jH3 (CN3), 5i (CH3)
3 (C3HS), S I H2 (CH3) 2.5j
(C2H,)4, S jH(CzHs)3, (CH3
)30Si(CH3)3 or other Si compounds, or B2
H6, B4H,. , B(CH3)3, B(C2H5)3
B compounds such as PH,, P2H4, P(CH3)3
, P(C2H5)3, P(CH3)N2, P(C2Hs
) H2, P(CH,)2H, P(C2H,)2H,
P(C.

H5) Examples include P compounds such as H2.

Oxidation of these compounds is, for example, 5jH4+202→S i O2+ 2 FI 20S
iH4+4PH3+1002→SjO□+2P205+
8H20 2B2H6+602 →2B203+6H,,0 This is carried out by an oxidation reaction of each component alone or by a combination of these reactions. At this time, it is necessary to flow an equivalent amount or more of 02 gas, and it is preferable to carry out the reaction in a temperature range of 350 to 450°C.

The reaction gas for the above compounds is usually a carrier gas (N2
etc.) and flow a very small amount to the extent that an extremely thin oxide film is formed on the particle surface, and the treatment time can be adjusted.

Note that if this CVD is carried out during the maximum temperature holding period, it is effective in preventing interparticle sintering, but since there is a risk of insufficient sintering compaction (densification of iron oxide), it is usually carried out during the first half of the temperature rise or during the temperature fall. Implemented during the process. In particular, if it is carried out during the temperature-falling process, the problem of insufficient sintering will be eliminated since sufficient sintering will occur during the temperature rising period and the maximum temperature holding period. Of course, it goes without saying that the reaction gas may be allowed to flow through all stages of the firing process.

(Example) Next, examples of the present invention will be shown.

The BET method specific surface area SSA synthesized by the conventional method is 91 r
After filtering, washing with water, and drying rr/g of lepidocrocite, 200 g of this dried product was charged into a rotary sintering furnace, and 5 f.
The furnace was heated at a temperature increase rate of 6° C./min while flowing 1/n+in of air into the furnace.

When the powder temperature in the furnace reached 470°C, the heater was stopped, and the introduced air was switched to a mixed gas having the following composition, which was allowed to flow into the furnace for 15 minutes. However, S i H4 was used after being diluted with carrier gas N2.

Concentration Flow rate SjH, 10% 0, 5 Q/m1nO□
100% Q, 5Q/mjnN2 100
% 20.0 (1/min then 5 Q/mi
It was allowed to cool naturally while flowing air of n. The iron oxide (α-Fe203) powder obtained by firing in this manner was reduced and oxidized in a conventional manner to produce maghemite (γ-Fe2O3) powder.

200 g of dried lepidocrocite, the same as that used in Example 1, was charged into a rotary kiln and heated at 5Q/mid.
It was heated at a temperature increase rate of 6° C./min while flowing air.

When the powder temperature in the furnace reached 470° C., the heater was stopped, and the introduced air was changed to a mixed gas having the following composition, which was allowed to flow into the furnace for 10 minutes. However, PH3 was used after being diluted with carrier gas N2.

Concentration Flow rate PH310% 0,5 n/m1nO210
0% 0, 5 Q/m1nN2 100
% 20. Off/mjn then 5Ω/mi again
The furnace was cooled to room temperature while flowing n air into the furnace. The iron oxide (α-Fe203) powder obtained by firing in this way is reduced and oxidized by the usual method to form maghemite (γ-Fe2O
3) A powder was produced.

*@1゜The same lepidocrosite as in Example 1 was obtained by firing without flowing a mixed gas of PH 3,0□ and N2 during heating and under the same firing conditions. Iron oxide (α-
Fe20. ) The powder was reduced and oxidized in the same manner as in Example 1 to produce maghemite powder.

After the synthesis reaction of the same lepidocrocite as in Example 1 was completed, the particle surface was coated with silica co-1 by a conventional method.

That is, an aqueous solution of sodium metasilicate was added in an Si content of 0.3 wt % based on lepidocrocite, and the particle surfaces were coated while the slurry was mixed and stirred.

This lepidocrocite was fired under the same conditions as in Comparative Example 1, reduced and oxidized to produce maghemite powder.

The BET specific surface area SSA and magnetic properties of each maghemite powder obtained in Example 1.2 and Comparative Example 1.2 were investigated. The results are shown in Table 1.

As is clear from the table, when particle coating is performed using CVD during the temperature cooling process of firing as in the example of the present invention,
The large specific surface area SSA promotes atomization while preventing sintering, and therefore maghemite with excellent magnetic properties such as coercive force He, saturation magnetization 6s, and squareness ratio SR can be obtained.

On the other hand, in the conventional particle coating (Comparative Example 2), although the specific surface area SSA is larger than in the case without particle coating (Comparative Example), the saturation magnetization is deteriorated, and as a result of the improvement in the specific surface area. It cannot be said that the effect of improving magnetic properties is sufficient.

Loss of cold resistance - BET method specific surface area SSA synthesized by conventional method is 79 r
After filtering, washing and drying ri'/g of goethite, 200g of this dried product was charged into a rotary kiln, and 511! /
The furnace was heated at a temperature increase rate of 6° C./min while flowing air at a rate of 6° C./min into the furnace.

When the powder temperature in the furnace reached 400'C, the introduction of air was stopped and the introduced air was switched to a mixed gas with the following composition, which was allowed to flow in the furnace for 11 minutes, and then switched to air at 5 (!/min). However, 5JH4 was used after being diluted with carrier gas N2.

Concentration Meteor SiH, 10% 0, 5 Q/m1n02
100% 0, 5 Q/m1nN2 1
00% 20. On/min powder temperature in the furnace is 47
When the temperature reached 0° C., the heater was stopped, and the sample was allowed to cool naturally while flowing air at a rate of 5 Q/min. The iron oxide (α-Fe203) powder obtained by firing in this manner was reduced and oxidized in a conventional manner to produce maghemite powder.

Example 4 200 pieces of dried goethite same as that used in Example 3
g was placed in a rotary firing furnace and heated at a temperature increase rate of 6° C./min while flowing air at 5 Q/min.

When the powder temperature in the furnace reached 420° C., the introduction of air was stopped, and the introduced air was changed to a mixed gas having the following composition, which was flowed into the furnace for 8 minutes, and then changed to air at 5 Q/min. However, PH3 was used after being diluted with carrier gas N2.

Concentration Flow rate PH310% 0, 5 (1/m1n0□
100% 0, 5 Q/m1nN2 1
00% 20. The powder temperature in the OA/min furnace is 47
When the temperature reached 0° C., the heater was stopped, and the sample was allowed to cool naturally while flowing air at a rate of 5 n/min. The iron oxide (α-Fe203) powder obtained by firing in this manner was reduced and oxidized in a conventional manner to produce maghemite powder.

Comparative Example 3 The same goethite as in Example 3 was fired without flowing a mixed gas of PH3. Fe2
03) The powder was reduced and oxidized in the same manner as in Example 3 to produce maghemite powder.

After the synthesis reaction of the same goethite as in Example 3, the particle surfaces were coated with silica using a conventional method.

That is, an aqueous solution of sodium metasilicate was added in an amount of 0.3 wt% Si based on goethite, and the particle surfaces were coated while the slurry was mixed and stirred.

This goethite was fired under the same conditions as Comparative Example 3, reduced,
Maghemite powder was produced by oxidation.

The specific surface area and magnetic properties of each of the maghemite powders obtained in Example 3.4 and Comparative Example 3.4 were investigated. The results are shown in Table 2.

As is clear from the table, when particles are coated using CVD during heating during firing as in the example of the present invention, the specific surface area SSA is large and atomization is promoted while preventing the sintering phenomenon. Maghemite with excellent magnetic properties such as magnetic force He, saturation magnetization σS, and squareness ratio SR can be obtained. On the other hand, in the conventional particle coating (Comparative Example 4), although the specific surface area SSA is larger than in the case without particle coating (Comparative Example 3), the saturation magnetization is deteriorated, and as a result of the improvement in the specific surface area. It cannot be said that the effect of improving magnetic properties is sufficient.

[Blank 1 below] In each of the above examples, an example of the production of iron oxide powder (maghemite) was shown, but it goes without saying that similar effects can be expected with iron powder obtained by reducing this. .

(Effects of the Invention) As detailed above, according to the present invention, when producing ferromagnetic iron oxide powder and ferromagnetic iron powder, the particle surface is coated with Si, P or B by utilizing the principle of CVD in the firing process. Since it is coated with an oxide film of 4.5%, interparticle sintering in the next reduction process can be effectively prevented, and the specific surface area SSA is 4. Orr? /g
With the above, it is possible to efficiently produce magnetic powder having excellent magnetic properties. In particular, problems such as deterioration of the S/N ratio and dispersibility, or deterioration of coercive force and saturation magnetization, which occur in conventional particle coating methods, do not occur. Furthermore, since the method of the present invention is a dry method, it can be carried out without the trouble of the firing step of the usual continuous heat treatment process, and therefore has great economic effects.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a temperature profile of a general firing process.

Claims (1)

[Claims] After firing the hydrated iron oxide powder, when obtaining ferromagnetic iron oxide powder through a reduction process, at least one component of Si, P, and B is added in the firing process using the principle of chemical vapor deposition. A method for producing ferromagnetic iron oxide powder, which comprises subjecting a compound contained therein to an oxidation reaction to form an oxide film of the component on the surface of the particles.