JPS6369715A - 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 the 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 O2-contg. gas on the surface of the powder particles. CONSTITUTION:A compd. contg. at least a kind of components among Si, P, and B is oxidized in O2-contg. gas and the film of said component is formed on the surface of the powder particles basing on the principle of the CVD process in the reducing stage when ferromagnetic iron oxide powder is obtd. by reducing water-contg. iron oxide powder after it is sintered. Such compds. are hydrides, or carbohydrides, for example, Si compds. such as SiH4, SiH2(CH3)2, etc., B compds. such as B2H6, B(CH3)3, etc., 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 by the combination of the each single oxidation reaction. In this case, it necessary to flow >=equivalent amt. of O2 and the reaction is preferred to be carried out in the temp. range between 350 deg.C and 450 deg.C.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to the production of ferromagnetic iron oxide powder, and more particularly, to a method for preventing sintering during a heat treatment process of hydrated iron oxide powder. Regarding.

(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
-F e OOH), lepidocrocite (γ-FeO
It is manufactured by using needle-shaped or grain-shaped hydrated iron oxide powder such as OH) as a starting material and subjecting it to heat treatment such as calcination and reduction.

That is, first, an alkali such as caustic soda or ammonia is added to an aqueous solution of iron sulfate or iron chloride to neutralize it, and an oxygen-containing gas (air or oxygen) is blown into it to form hydrated iron oxides such as goethite and lepidocrocite. Synthesize powder.

Next, this is filtered, washed with water, dried, and then calcined in air to obtain α-Fe203, which is reduced in a hydrogen stream to obtain magnetite (Fe304) and iron powder.

Furthermore, maghemite (γ-Fe2O3) is produced by oxidizing magnetite in air. When the surface of maghemite particles is subjected to a cobalt modification treatment, maghemite (Co-γ-Fe2O3) coated with cobalt is obtained. Furthermore, iron powder can be obtained by reducing the iron oxide powder obtained through this step in a reducing gas flow.

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 Sj and/or P (e.g., sodium metasilicate, phosphoric acid, etc.) is added to the particle-containing aqueous solution slurry to form Si oxide on the particle surface. Methods for producing Si compounds such as
161008, 56-26730), or a similar method of producing P compounds such as P oxide (e.g., JP-A-5
5-115902, 55-149138, 56-3
8405, 56-44835), etc.

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

The present invention eliminates the drawbacks of the above-mentioned prior art regarding interparticle sintering, and reduces the occurrence of sintering phenomena even by making the particles finer.
Moreover, it is an object of the present invention to provide an efficient method for producing ferromagnetic iron oxide powder or ferromagnetic iron powder, which can prevent deterioration of the S/N ratio and dispersibility and does not cause deterioration of magnetic properties.

(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 S] oxide and P oxide, but compounds such as Sj and P during the synthesis reaction of hydrated iron oxide powder or after the synthesis reaction. It is thought that these Si compounds and P compounds have a negative effect on crystal growth, pore movement, and particle densification during the calcination process, and also cause a decrease in the reduction reaction rate during 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. Sj using the principle of chemical vapor deposition (CVD)
We have discovered that this is possible by forming and coating the surfaces of particles with oxide films of , P, B, etc., and have accomplished the present invention.

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 rice-grain-shaped hydrated iron oxide particles are modified into iron oxide (α-Fe203) particles, and it is necessary to sufficiently dehydrate and sinter the particles. In the sintering prevention method, particle surfaces are coated during or after the synthesis reaction of hydrous iron oxide particles, which is thought to inhibit intraparticle sintering, which is necessary for improving magnetic properties.

Therefore, in the present invention, the sintering step is carried out according to normal conditions at a temperature that promotes intragranular sintering (generally between 430 and 48
Taking note of the fact that there is little interparticle sintering even when heat-treated at 0° C., the particle surfaces are coated using the principle of CVD in the next reduction step.

Generally, in the reduction process, iron oxide (α-
Fe, 03) particles are charged into a reduction furnace and heat-treated at 300 to 550°C while flowing hydrogen gas into the furnace.

In order to carry out the oxidation reaction using the principle of CVD in this reduction process, a mixed gas of a gas containing a compound of Sj, P or B and a gas containing oxygen (oxygen or air) is added to the furnace during temperature rise. is passed through the particles, and an oxidation reaction forms these oxide films on the particle surfaces for coating treatment.

Examples of the compound described in item 1 include hydrides, hydrocarbons, etc., such as 5in4. Si2H6, 5i3HIl, Si
(CH3) 4, S 1H3 (CN3), S j, (CN
3) 3(C3H5), s j, H2(c H3)2.
5j(C2H5)4, Sj, H(C,N5)3, (C
H3) Sj compounds such as 30Si(CH3)3, or B2H6, B4H□. , B(CH3)3, B(C2H5
) B compounds such as 3, PH3, P2H P(CH
3) 3, P(zHs)a, P(CH3)N7, P (C
2H5)N2, P(CH3)2H, P(C2H5)2H
, P(C,N5)N2 and the like.

Oxidation of these compounds is, for example, SiH4+20□→Si○2 +2H20SiH4+
4PH3+1002→SiO, +2P205+8H20 2B2H, +602→2B20. It is carried out by oxidation reaction of each component alone as in +6H20 or by a combination of these reactions. At that time, an equivalent amount or more of 02 gas is required, and it is preferable to carry out the reaction at 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.

After the particle surface coating treatment by oxidation reaction, N2 gas is flowed to purge the 02 gas in the furnace, and then hydrogen gas is immediately flowed into the furnace and heat treatment is performed under the specified reduction conditions.Coating and reduction can be performed continuously. can be done.

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

Example 1 BET method specific surface area SSA synthesized by conventional method is 89 m2
200g of this dried product was charged into a rotary kiln and heated at 6℃/g while flowing 5Q/min of air. After heating to 470° C. at a temperature increase rate of min, baking was performed by allowing natural cooling.

The obtained iron oxide (α-Fe304) powder was transferred to a reduction furnace and heated at 5°C/mj, n while flowing N2 gas at 5Q/min.
When the temperature inside the furnace reached 350° C., a mixed gas having the following composition was flowed into the furnace for 5 minutes to perform coating treatment. However, SiH4 uses carrier gas N2
It was diluted and used.

Concentration Flow rate SiH410% 0, 5 Q/m1n02
100% 0, 50/mjnN2 10
0% 20.0 Q /mj, n Then, after flowing N2 gas and purging the 02 gas in the furnace, 52
Heating to 0℃, cooling and magnetite 1~(F(130
4) A powder was obtained. This powder was oxidized in a conventional manner to produce maghemite-8=(γ-Fe2O3) powder.

Example 2 The same lepidocrocite (γ-FeO
Charge 200g of dried product of OH) into a rotary kiln,
After heating to 470° C. at a temperature increase rate of 6° C./min while flowing air at Q/min, the product was baked by being allowed to cool naturally.

The obtained iron oxide (α-Fe203) powder was transferred to a reduction furnace and heated at 5°C/mj while flowing 5Q/n+in of N2 gas.
, n, and when the temperature inside the furnace reached 350° C., a mixed gas having the following composition was flowed into the furnace for 3 minutes to perform coating treatment. However, PH3 is carrier gas N
It was diluted with 2 and used.

Concentration Flow rate PH310% 0, 50 /m1n02 1
00% 0.5fl/m1nN2 100%
20. ○Q/min Then, after flowing N2 gas and purging the 02 gas in the furnace, according to the normal reduction conditions,
While flowing N2 gas diluted with N2 gas, it was heated to 520°C and cooled to obtain magnetite (Fe304) powder. This powder was oxidized in a conventional manner to produce magnetite powder.

As a comparative example, the same recipe as in Example 1 and 1 black rhinoceros were used.
The iron oxide (Fe30.) powder obtained was heat-treated by not flowing a mixed gas of PH3.02 and N during the temperature rise in the reduction process, and using the same firing and reduction conditions as Example J-. Maghemite powder was produced by oxidation in the same manner as in Example 1.

Comparative Example 2 After completing the synthesis of the same lepidocrocite as in Example 1, the particle surface was coated by a conventional method.

That is, an aqueous solution of sodium metasilicate was added with an Si content of Q and 3 wt % per lepidocrocite, and the particle surfaces were coated while the slurry was mixed and stirred.

This Lepido black rhinoceros 1~ was calcined under the same conditions as in Comparative Example 1, and was reduced and oxidized to produce maghemite powder.

The specific surface area SSA and magnetic properties of the maghemite powders 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 particles are coated using CVD in the reduction process as in the example of the present invention, the specific surface area SSA is large and atomization is promoted while preventing the sintering phenomenon, and therefore the coercive force Maghemite with excellent magnetic properties such as He, saturation magnetization σS, and squareness ratio SR can be obtained. On the other hand, in the conventional particle coating method (Comparative Example 2), although the specific surface area SSA is larger than in the case without particle coating (Comparative Example 1), there is a problem in that the saturation magnetization deteriorates.

Although the above example shows the case of manufacturing maghemite, it goes without saying that similar effects can be expected with ferromagnetic iron powder obtained by reduction.

(Effects of the Invention) As detailed above, according to the present invention, in the production of 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 reduction step. Since it is coated with an oxide film of
It is possible to efficiently produce ferromagnetic powder with a particle size of -0 m2/g or more and excellent magnetic properties. In particular, problems such as deterioration of S/N ratio, dispersibility, 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 in the reduction step of a normal heat treatment process, which is not a hindrance, and therefore it has great economic effects.

Claims (1)

[Claims] After firing the hydrated iron oxide powder, when obtaining ferromagnetic iron oxide powder through a reduction step, at least one component of Si, P, and B is added in the reduction step using the principle of chemical vapor deposition. 1. A method for producing ferromagnetic iron oxide powder, which comprises subjecting a compound containing the compound to an oxidation reaction in an oxygen-containing gas to form an oxide film of the component on the particle surface.