JPH07138022A - Preparation of needle-like magnetic iron oxide containing cobalt - Google Patents

Abstract

PURPOSE: To obtain an acicular cobalt-modified iron oxide having stable magnetic performance by adding an aq. soln. of Al salt and silicate to an alkaline dispersion liquid of cobalt-modified iron oxide, precipitating the salts on the particle surface, heating, filtering and drying.

CONSTITUTION: Iron oxide is dispersed in an alkaline aq. soln. and FeSO₄.7H₂O and CoSO₄.7H₂O are added to react to prepare an alkaline dispersion liquid of cobalt-modified iron oxide. Then an aq. soln. of aluminum salt (e.g. aluminum nitrate) and an aq. soln. of alkali metal silicate (e.g. sodium silicate) are added to the dispersion liquid, and if necessary, the liquid is heated to lower than the boiling point to precipitate the aluminum salt and silicate on the particle surface. Then the liquid is heated to react, filtered and dried to obtain acicular cobalt-modified iron oxide comprising an iron oxide nuclei expressed by the formula, FeOx ((x) ranges 1.33 to 1.5) and a Co ion-rich surface layer, and a coating film comprising an Al compd. and a Si compd. on the particle surface.

COPYRIGHT: (C)1995,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention comprises a core of iron oxide represented by the formula FeOx, where x is 1.33 to 1.5, and a surface layer rich in cobalt ions. The present invention relates to a method for producing needle-shaped cobalt-modified iron oxide, which is characterized in that the stability of magnetic performance is improved by coating the surface of particles.

[0002]

2. Description of the Related Art Needle-shaped cobalt-modified magnetic iron oxide is widely used for manufacturing magnetic recording media. Due to the increasing demand for storage efficiency of such recording media,
The pigments used in its manufacture also have
Improvements are constantly underway. In addition, efforts are also being made to enhance the specific magnetization of the pigment. For logical reasons and from experimental investigations, as is becoming more common in efficient recording media in recent years, especially in the recording of short wavelength, if the switching field distribution (SFD) is lowered, the signal It is also revealed that the level will be greatly improved.

Attempts have always been made to improve the long-term stability of pigments and the magnetic recording media produced using these pigments. For this reason, many authors have used cobalt-modified magnetic iron oxides for silicon, aluminum, titanium,
It is proposed to coat with boron or a compound of other elements. Examples of such post-treatments are described in the patent applications given below.

In JP-A-2-030625, a magnetic pigment is redispersed in water, and at a pH value of 8, a silicon compound,
Then, there is a description that a post-treatment is performed with an aluminum compound.

According to JP 62-158301 A, an alkaline dispersion of a magnetic pigment is neutralized and then a titanium compound is added to deposit the titanium compound on the pigment.

JP-A-63-064306 proposes to disperse a solution of an aluminum salt and a silicate in an alkaline dispersion of a magnetic pigment, then neutralize the dispersion and attach the added salt to the surface of the pigment. Has been done.

Japanese Patent Laid-Open No. 1-061324 discloses a pH value of 6.
5 to 8.5 and relates to applying aluminum and silicon by precipitation.

Japanese Patent Application Laid-Open No. 1-239819 discloses the coating of a boron compound and then an aluminum compound.

European Patent No. 0421213 discloses p
It is disclosed to apply a magnetic iron oxide which may contain cobalt with a chromium (III) salt having an H value of 3 to 10.

JP-A-3-209625 describes that an alkali metal silicate is added to an alkali pigment dispersion, CO ₂ is blown for neutralization, and then an alkaline earth metal is added.

In the method disclosed in JP-A-1-290534, the cobalt modification reaction is carried out in the presence of aluminum. In this way, the added aluminum is not directly present on the pigment surface, but a product is obtained which is incorporated in the cobalt-containing surface layer, and as a result the effect of adjusting the chemical performance of the pigment surface is lost. Moreover, according to this method, the SFD is increased.

Regarding the cohesive force, a method for producing magnetic iron oxide of the general type having high aging resistance is disclosed in German Patent Application Publication No. 3912976. According to the above publication, the iron oxide produced in this way must first be filtered and washed, and then the redispersed pigment is treated with alkali metal silicic acid in order to carry out the reaction at a pH value of 8 to 9. It is said that it can be treated with salt.

In all known processes for producing improved cobalt-modified iron oxides, the disadvantage is that weakly alkaline reaction conditions are essential for carrying out the coating reaction. On the other hand, in all known methods, the surface cobalt modification must be carried out under strongly alkaline conditions, often with a pH value above 14. Therefore, in the coating method known in the prior art, it is necessary to filter or wash the pigment again to remove the excess alkali, or to add a chemical to increase the salt content of the mother liquor. Must be done, resulting in a prolonged filtration time.

[0014]

That is, the first object of the present invention is known, in particular, in a method of directly carrying out a precipitation reaction in a strong alkaline mother liquor without further intermediate step after the completion of the cobalt modification reaction. It is to urgently simplify the method.

A further object of the invention is to reduce the oxidative sensitivity of cobalt-modified iron oxides with a high content of iron (II), which has the advantage of specific magnetization.

A further object of the present invention is to modify the surface properties of the modified magnetic iron oxide pigments in a controlled manner similar to the prior art method in order to favorably affect the dispersibility of the pigment in the production of magnetic recording media. It is to let.

[0017]

The present inventors have surprisingly found that the concentration is kept constant after a conventional cobalt modification process using an alkaline mother liquor with a high pH value of 12 or higher after the end of the doping process. It has been found that carrying out precipitation with a combination of silicon and aluminum compounds, while maintaining, achieves these objects of the invention. This result is even more surprising since it is known that both soluble silicates and aluminum salts readily dissolve in alkali in excess.

Using the modified iron oxide prepared by the novel process according to the invention, surprisingly, less than 1% S
It was revealed that even if the i / Al coating liquid was added, the oxidation process was significantly slowed down, and as a result, the aging resistance of the specific magnetization was substantially improved. When the modified iron oxide of the present invention is used in a VHS video system, it has a darker natural color, which is advantageous compared to conventional cobalt modified iron oxide, and the color is maintained during storage.

The dispersion characteristics of the pigment produced in this way are
It is corrected by the adjustment method by the coating film applied by the new method.
Furthermore, the production of the pigment by a new process accelerates the filtration step for isolating the product from the strongly alkaline mother liquor which is generally used during precipitation.

As described in detail in the Examples below, the novel process of the present invention consists of the following steps directly following the cobalt modification reaction in the mother liquor. That is, while selecting and maintaining the gas atmosphere required for the cobalt doping method used, the pigment dispersion at room temperature is kept below the boiling point,
It is preferably heated to 60 ° C. or higher. -Add the aluminum salt aqueous solution with sufficient stirring. -Add the alkali metal silicate aqueous solution with sufficient stirring. -Then, the two components applied to the surface of the pigment are allowed to react for about 1 hour, preferably after heating.

The method according to the present invention can also be carried out by changing the order of addition of the aluminum salt solution and the silicate solution, or by adding both solutions simultaneously if necessary. The method of the present invention comprises:
Minimum concentration of Si and Al depending on the pH value of the dispersion
It only needs to exist. In each case a constant p
Determining the concentrations of Si and Al required to cause precipitation in H-valued alkaline solutions is necessary for the new method to be feasible.

The present invention will be described with reference to the following examples, but the present invention is not limited to these examples.

In the examples, European Patent No. 0246650
A cobalt-modified iron oxide dispersion produced by the method disclosed in Japanese Patent No. 1 is used as a starting material. This is suitable for the treatment of conventional iron oxide with γ-iron and cobalt-doped iron oxide, and also for the base material of further developed beltrite iron oxide, which is suitable for precipitation of cobalt-doped iron oxide. . Cobalt modification is also carried out by methods other than those described in EP 0246501.

[0024]

Example 1 Beltrid having a specific surface area of 32.9 m ² / g and a cohesive force of 25.1 kA / m in a stirred reactor equipped with a stirrer, a dispersing means, a gas feeding system, and a heating means. Iron oxide 890
g 5200 ml H ₂ O and 1160 ml concentration 5
Disperse in a mixture of 0% NaOH. Under a nitrogen atmosphere,
The resulting dispersion is heated to 50 ° C. with sufficient stirring,
178.5 g FeSO ₄ .7H ₂ O 500 ml H
Add to ₂ O solution and react with iron oxide in 1 hour.
After that, the dispersion liquid was cooled to 30 ° C., and further stirred sufficiently, first, 223 g of Fe dissolved in 700 ml of H ₂ O was dissolved.
The SO ₄ · 7H ₂ O, followed by addition of CoSO ₄ · 7H ₂ O in 171g dissolved in H ₂ O in 300 ml. After the addition was complete, the feed gas was switched to an air / nitrogen mixture with a ratio of 1:10 and the dispersion was stirred under the above conditions for 3 hours and finally at room temperature under nitrogen atmosphere for another hour.

[0025] 9.6g of which was dissolved in of H ₂ O _{50ml Al (NO 3) 3 ·} 9H 2 O and 50ml of H ₂ O 5.59 g of sodium silicate solution dissolved in (SiO ₂
The content (26.5%) was sequentially added to 350 ml of the alkali dispersion thus produced with stirring and heated to 80 ° C. The dispersion is stirred for another hour at the above temperature,
It was then filtered, washed and dried as above.

[0026] Example 2 9.6 g of _{Al (NO 3) 3 · 9H} 2 O and 6.52
The same operation as in Example 1 was performed except that g of sodium silicate solution was used.

[0027] Example 3 11.2 g of _{Al (NO 3) 3 · 9H} 2 O and 6.5
Example 1 except that 2 g of sodium silicate solution was used
The same operation was performed.

[0028] Example 4 19.2 g of _{Al (NO 3) 3 · 9H} 2 O and 3.7
Example 1 except 3 g of sodium silicate solution was used
The same operation was performed.

[0029] Example 5 8.0 g of _{Al (NO 3) 3 · 9H} 2 O and 13.0
The same operation as in Example 1 was performed, except that 6 g of sodium silicate solution was added at room temperature and the stirred dispersion was heated to 80 ° C. for 1 hour. The product obtained could be filtered and washed very easily.

Example 6 An aluminum salt solution and a sodium silicate solution were treated at 80 ° C.
The same operation as in Example 5 was performed, except that the dispersion was added to the dispersion.

Example 7 A specific surface area of 31.4 m ² / g in a reactor of 75 liters capacity equipped with heating means, gas feed system, metering feed means, stirrer and laterally connected disperser. Cohesive force 23
8 kg of kA / m beltride iron oxide was dispersed in 38 liters of H ₂ O. Under a nitrogen atmosphere, 9.22 liters of concentrated sodium hydroxide solution was added and the dispersion was added to 5
It was heated to 0 ° C., but after reaching this temperature 1.522 g FeSO ₄ · dissolved in 3.5 liter H ₂ O.
7H ₂ O was added while the disperser was operating, and the dispersion was kept in this state for 1 hour. Then, the dispersion liquid
Chilled to 4.5 ° C. and dissolved in 4.5 liters of H ₂ O 1.9
1.526 kg CoSO ₄ .7H dissolved in 91 kg FeSO ₄ .7H ₂ O and 3.1 liters H ₂ O
₂ O was added sequentially. The nitrogen gas feed was then switched to gas feed using an air / nitrogen mixture in the ratio 1: 4 and the oxidation was carried out at 35 ° C. for 3 hours.

Then, the dispersion liquid thus obtained was heated to 80 ° C. under a nitrogen atmosphere and kept at this temperature for 1 hour.
Then, first use 1.993 with 5 liters of H ₂ O.
kg of _{Al (NO 3) 3 · 9H} 2 O, then similarly supplemented with 5 liters of H ₂ O and 1.159g of sodium silicate solution used (SiO ₂ content 26.5%), under a nitrogen atmosphere, Stirring was continued at 80 ° C. for another hour. The resulting dispersion was cooled, filtered using a chamber membrane filter press, washed and dried in a laboratory under nitrogen atmosphere.

Example 8 By the method of Example 7, a cobalt-modified iron oxide was produced. 600 m of the resulting suspension, after the cobalt modification reaction yielded, with a theoretical cobalt modified iron oxide content of 91.2 g.
1 was post-treated for 1 hour at 85 ° C. under a nitrogen atmosphere in a glass reactor having a volume of 1 liter equipped with a dispersing means, and then cooled to room temperature. While the reaction vessel contents were sufficiently dispersed, initially Al (NO ₃₎ of 25.31g was dissolved in H ₂ O in 50ml ₃ · 9H ₂ O, followed by 50ml of H ₂ O
14.72 g of sodium silicate dissolved in was added. The product obtained is filtered, washed and dried in a conventional manner under a nitrogen atmosphere.

Example 9 Example 8 except that a beltride-based oxide having a cohesive force of 22.6 kA / m, a specific surface area of 31.6 m ² / g and an iron (II) content of 11.3% was used. The same operation was performed.

Example 10 Specific surface area 39.7 m ² / g and cohesive force 26.7 kA /
m γ-iron oxide was dispersed in 313 ml H ₂ O. 8
7 ml of 50% strength by weight NaOH were added to the obtained dispersion and the reaction batch was heated to 50 ° C. under nitrogen atmosphere. At this temperature, dissolved in 40 ml H ₂ O 1
It was added to 4.93 g of FeSO ₄ .7H ₂ O, the mixture was kept at the above temperature for 1 hour and then cooled to 35 ° C. After reaching this temperature, firstly 18.75 g FeSO ₄ .7H ₂ O dissolved in 60 ml H ₂ O and then 3
12.53 g CoSO ₄ dissolved in 0 ml H ₂ O
• The 7H ₂ O was added. From this point, the vigorously stirred dispersion is passed through a mixture with an air / nitrogen ratio of 1:10,
Oxidized for 3 hours. After that, the gas feeding was returned to the nitrogen feeding, and the aging was continued at 80 ° C. for 1 hour.

Then dissolved in 50 ml of H ₂ O 2
4.53g of _{Al (NO 3) 3 · 9H} 2 O and 50m
14.28 g of water glass dissolved in 1 H ₂ O (Si
An O ₂ content of 26.5%) was added to the obtained cobalt-modified iron oxide dispersion and the mixture was brought to 1
Held for hours. The product obtained was then worked up.

Comparative Example 1 Neither an aluminum solution nor a sodium salt solution was added to the alkaline dispersion, but instead, after the cobalt modification was completed, the dispersion was filtered with a suction filter, and the residue was washed to neutralize the mixture in a nitrogen atmosphere. The same operation as in Example 1 was performed except that the film was dried below.

Comparative Example 2 65 g of oxidized γ-iron having a specific surface area of 97 m ² / g and a cohesive force of 26.7 kA / m was dispersed in 350 ml of H ₂ O.
Then 50 ml of 50% strength by weight NaOH was added, and the resulting solution was heated to 50 ° C. under nitrogen atmosphere.
12.94 g dissolved in 30 ml H ₂ O at this temperature
FeSO ₄ .7H ₂ O was added and the resulting dispersion was held at the above temperature for 1 hour. Then 10, which was cooled to 35 ° C., after reaching this temperature, the addition of FeSO ₄ · 7H ₂ O in 16.18g dissolved in H ₂ O in 30 ml, and then dissolved in H ₂ O in 30 ml. 86g CoSO
The ₄ · 7H ₂ O was added. The feed gas was changed to an air / nitrogen mixture with a ratio of 1: 4 and oxygen was allowed to act for 3 hours. next,
The dispersion was heated to 80 ° C. for 1 hour under a nitrogen atmosphere and cooled to room temperature again. At this temperature, 5.24 g of sodium silicate solution diluted with 45 ml of H ₂ O (SiO ₂ content 2
6.5%) was added and dispersion was continued for another 15 minutes. It is filtered, made neutral by washing and dried in a conventional manner under a nitrogen atmosphere.

Comparative Example 3 The same operation as in Example 8 was carried out except that the pigment dispersion was prepared without using either the silicon solution or the aluminum salt solution.

Comparative Example 4 Example 10 was repeated except that the base oxide used was replaced with iron oxide (II) having a content of 13.9%, a specific surface area of 31.4 m ² and a cohesive force of 23.0 kA / m. The same operation was performed. Further, 7.14 g of water glass having a SiO ₂ content of 26.5% was added to the alkaline dispersion of the cobalt-doped product to perform surface stabilization treatment using only Si.

The following tests were carried out in order to determine the properties of the products obtained according to the examples of the invention and the products obtained according to the comparative examples. The test results are specified in the examples or in the table below.

The specific surface area (SSA) of the pigment was measured by the BET single point difference method by Haul and Duembgen. The magnetic properties of the pigment were measured using a vibrating sample magnetometer. This measurement was performed with the maximum magnetic field strength of 380 kA / m, and the cohesive force was measured after the tap density of the measurement sample reached 1.2 g / cm ² .

The amounts of silicon and aluminum deposited on the pigment were determined by conventional analytical methods.

The stability of the cohesive strength of the pigment is 50 ° C. in the atmosphere.
It was stored for a certain period of time and measured. In addition, the magnetic polarity was measured by taking samples from the powder during storage from time to time.

The oxidative stability of the product based on ferric oxide bertride was determined by measuring the Fe ²⁺ content before and after performing the aging test.

The pH value of the pigment is DIN ISO 787,
It was measured in an aqueous dispersion having a concentration of 10% by a method similar to Part 9.

[0047]

[Table 1]

Example 11 The improved oxidative stability of the pigments post-treated according to the invention is evident from the data in the above table, which is related to the Fe (II) content in the sample. However, the improvement in the oxidative stability of the pigment will be further demonstrated in the experiments shown below.

0.71% by weight of Si and 0.53% by weight of Al were applied, and the Fe (II) content was 13.6% by weight.
The cobalt-doped iron oxide was heated at 150 ° C. for 2 hours in the air while being thoroughly mixed. Here, the Fe (II) content was measured again. F at this time
The e (II) content was still 6.1% by weight.

COMPARATIVE EXAMPLE 5 Iron oxide having a Fe (II) content of 13.6% by weight and not subjected to coating and having cobalt doping treatment was sufficiently mixed and heated at 150 ° C. for 2 hours in the air. Where Fe
(II) The content was measured again. In the case of the uncoated pigment, the Fe (II) content was only 0.4% by weight.

[Brief description of drawings]

FIG. 1 shows a pH value of 14.3 in a heat-treated pigment dispersion liquid.
3 is a graph showing the minimum required concentrations of Si and Al in FIG.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] August 24, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0002

[Correction method] Change

[Correction content]

[0002]

2. Description of the Related Art Needle-shaped cobalt-modified magnetic iron oxide is widely used for manufacturing magnetic recording media. Due to the increasing demand for storage efficiency of such recording media,
Pigments used in their preparation may, for example the coercive force,
Improvements are constantly underway. In addition, efforts are also being made to enhance the specific magnetization of the pigment. For logical reasons and from experimental investigations, as is becoming more common in efficient recording media in recent years, especially in the recording of short wavelength, if the switching field distribution (SFD) is lowered, the signal It is also revealed that the level will be greatly improved.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

Regarding the coercive force , a method for producing magnetic iron oxide of the above-mentioned general type having high resistance to aging is disclosed in German Patent Application Publication No. 3912976. According to the above publication, the iron oxide produced in this way must first be filtered and washed, and then the redispersed pigment is treated with alkali metal silicic acid in order to carry out the reaction at a pH value of 8-9. It is said that it can be treated with salt.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0024

[Correction method] Change

[Correction content]

[0024]

Example 1 A belt ride having a specific surface area of 32.9 m ² / g and a coercive force of 25.1 kA / m in a stirred reactor equipped with a stirrer, a dispersing means, a gas feeding system, and a heating means. Iron oxide 890
g 5200 ml H ₂ O and 1160 ml concentration 5
Disperse in a mixture of 0% NaOH. Under a nitrogen atmosphere,
The resulting dispersion is heated to 50 ° C. with sufficient stirring,
178.5 g FeSO ₄ .7H ₂ O 500 ml H
Add to ₂ O solution and react with iron oxide in 1 hour.
After that, the dispersion liquid was cooled to 30 ° C., and further stirred sufficiently, first, 223 g of Fe dissolved in 700 ml of H ₂ O was dissolved.
The SO ₄ · 7H ₂ O, followed by addition of CoSO ₄ · 7H ₂ O in 171g dissolved in H ₂ O in 300 ml. After the addition was complete, the feed gas was switched to an air / nitrogen mixture in the ratio 1:10 and the dispersion was stirred under the above conditions for 3 hours and finally at room temperature under nitrogen atmosphere for another hour.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0031

[Correction method] Change

[Correction content]

EXAMPLE 7 A specific surface area of 31.4 m ² / g in a reactor with a capacity of 75 liters equipped with heating means, gas feed system, metering feed means, stirrer and laterally connected disperser, and Coercive force 23
8 kg of kA / m beltride iron oxide was dispersed in 38 liters of H ₂ O. Under a nitrogen atmosphere, 9.22 liters of concentrated sodium hydroxide solution was added and the dispersion was added to 5
It was heated to 0 ° C., but after reaching this temperature 1.522 g FeSO ₄ · dissolved in 3.5 liter H ₂ O.
7H ₂ O was added while the disperser was operating, and the dispersion was kept in this state for 1 hour. Then, the dispersion liquid
Chilled to 4.5 ° C. and dissolved in 4.5 liters of H ₂ O 1.9
1.526 kg CoSO ₄ .7H dissolved in 91 kg FeSO ₄ .7H ₂ O and 3.1 liters H ₂ O
₂ O was added sequentially. The nitrogen gas feed was then switched to gas feed using an air / nitrogen mixture in the ratio 1: 4 and the oxidation was carried out at 35 ° C. for 3 hours.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0034

[Correction method] Change

[Correction content]

Example 9 Example 8 except that a beltride-based oxide having a coercive force of 22.6 kA / m, a specific surface area of 31.6 m ² / g and an iron (II) content of 11.3% was used. The same operation was performed.

[Procedure correction 6]

[Document name to be amended] Statement

[Correction target item name] 0035

[Correction method] Change

[Correction content]

Example 10 Specific surface area 39.7 m ² / g and coercive force 26.7 kA /
m γ-iron oxide was dispersed in 313 ml H ₂ O. 8
7 ml of 50% strength by weight NaOH were added to the obtained dispersion and the reaction batch was heated to 50 ° C. under nitrogen atmosphere. At this temperature, dissolved in 40 ml H ₂ O 1
It was added to 4.93 g of FeSO ₄ .7H ₂ O, the mixture was kept at the above temperature for 1 hour and then cooled to 35 ° C. After reaching this temperature, firstly 18.75 g FeSO ₄ .7H ₂ O dissolved in 60 ml H ₂ O and then 3
12.53 g CoSO ₄ dissolved in 0 ml H ₂ O
• The 7H ₂ O was added. From this point, the vigorously stirred dispersion is passed through a mixture with an air / nitrogen ratio of 1:10,
Oxidized for 3 hours. After that, the gas feeding was returned to the nitrogen feeding, and the aging was continued at 80 ° C. for 1 hour.

[Procedure Amendment 7]

[Document name to be amended] Statement

[Correction target item name] 0038

[Correction method] Change

[Correction content]

Comparative Example 2 65 g of oxidized γ-iron having a specific surface area of 97 m ² / g and a coercive force of 26.7 kA / m was dispersed in 350 ml of H ₂ O.
Then 50 ml of 50% strength by weight NaOH was added, and the resulting solution was heated to 50 ° C. under nitrogen atmosphere.
12.94 g dissolved in 30 ml H ₂ O at this temperature
FeSO ₄ .7H ₂ O was added and the resulting dispersion was held at the above temperature for 1 hour. Then 10, which was cooled to 35 ° C., after reaching this temperature, the addition of FeSO ₄ · 7H ₂ O in 16.18g dissolved in H ₂ O in 30 ml, and then dissolved in H ₂ O in 30 ml. 86g CoSO
The ₄ · 7H ₂ O was added. The feed gas was changed to an air / nitrogen mixture with a ratio of 1: 4 and oxygen was allowed to act for 3 hours. next,
The dispersion was heated to 80 ° C. for 1 hour under a nitrogen atmosphere and cooled to room temperature again. At this temperature, 5.24 g of sodium silicate solution diluted with 45 ml of H ₂ O (SiO ₂ content 2
6.5%) was added and dispersion was continued for another 15 minutes. It is filtered, made neutral by washing and dried in a conventional manner under a nitrogen atmosphere.

[Procedure Amendment 8]

[Document name to be amended] Statement

[Correction target item name] 0040

[Correction method] Change

[Correction content]

Comparative Example 4 Example 10 was repeated, except that the substrate oxide used was replaced with a belt oxide iron oxide having an iron (II) content of 13.9%, a specific surface area of 31.4 m ² and a coercive force of 23.0 kA / m. The same operation was performed. Further, 7.14 g of water glass having a SiO ₂ content of 26.5% was added to the alkaline dispersion of the cobalt-doped product to perform surface stabilization treatment using only Si.

[Procedure Amendment 9]

[Document name to be amended] Statement

[Correction target item name] 0042

[Correction method] Change

[Correction content]

The specific surface area (SSA) of the pigment was measured by the BET single point difference method by Haul and Duembgen. The magnetic properties of the pigment were measured using a vibrating sample magnetometer. This measurement was performed with the maximum magnetic field strength of 380 kA / m, and the coercive force was measured after the tap density of the measurement sample reached 1.2 g / cm ² .

[Procedure Amendment 10]

[Document name to be amended] Statement

[Correction target item name] 0044

[Correction method] Change

[Correction content]

The stability of the coercive force of the pigment is 50 ° C. in the atmosphere.
It was stored for a certain period of time and measured. In addition, the magnetic polarity was measured by taking samples from the powder during storage from time to time.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Bernd, Hepner Germany, 67434, Neustadt, Am, Heuselberg, 24 (72) Inventor Reinhardt, Kerner Germany, 67227, Frankenthal, Ott-Dill-Strasse, 4 (72) Inventor Amir, Pfanebecker Germany, 67105, Schifferstadt, Robert-Schumann-Strasse, 55

Claims (1)

[Claims] 1. A coating comprising an iron oxide nucleus of the formula FeOx, where x is 1.33 to 1.5, and a surface layer enriched with cobalt ions, the coating comprising aluminum and a silicon compound. A method for producing acicular cobalt-modified iron oxide applied to the surface of particles, which comprises immediately after producing cobalt-modified iron oxide present in an alkaline dispersion liquid, an aluminum salt and an alkali metal silicate in an aqueous solution on the particle surface. After further precipitating at room temperature to a temperature below the boiling point with further stirring, the temperature of the two kinds of components applied on the particle surface is raised,
A production method characterized by reacting while sufficiently stirring, filtering and drying.