JPH0434902A - Preparation of needle alloy magnetic powder - Google Patents

Abstract

PURPOSE:To improve the magnetic characteristic and anti-corrosiveness of magnetic powder by using water solution containing certain specific ion in order to form cobalt containing layer on the surface of needle particles of iron oxyhydroxide or iron oxide. CONSTITUTION:Needle particle of iron oxyhydroxide or iron oxide are distributed in alkali water solution, and in this soil suspension the water solution containing Fe<3+> ion and Co<2+> ion is added in order to form compound coating layer containing these types of ions on the particles, and then thus final resultant soil suspension is heated for dioxidation. It is further preferred that the water solution which is to be added to the soil suspension contains 30-100mol percent Co<2+> ion or Fe<3+> ion and furthermore preferred that the water solution contains at least one type of other divalent metal ion selected from Cr<2+>, Ni<2+>, and Zn<2+> and that this water solution contains 1-10mol percent of another divalent metal ions just like the aforementioned 30-100mol percent of Co<2+> ion for aforementioned amount Fe<3+> ion.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing acicular alloy magnetic powder useful as a recording element of a magnetic recording medium such as a magnetic tape or a magnetic disk.

[Conventional technology]

Metal magnetic powder is in increasing demand as a material suitable for high-density recording because it has superior magnetic properties such as saturation magnetization compared to conventionally general-purpose oxide-based magnetic powders such as 7 FezO powder. In recent years, alloy magnetic powder, which is made by alloying iron with other magnetic metals such as cobalt, has been developed as such metal magnetic powder, with the aim of further enhancing magnetic properties and corrosion resistance than general metal iron magnetic powder. Various attempts have been made to obtain this.

For example, the following manufacturing method has been known as a representative method for producing alloy magnetic powder mainly composed of iron and cobalt.

(a) A method of heating and reducing a coprecipitate obtained from an iron salt and a cobalt salt added to an oxalic acid aqueous solution.

(b) A method of adding a reducing agent to a solution containing an iron salt and a cobalt salt.

(C1 A method in which iron and cobalt are evaporated in an inert gas and alloyed by collisions of gas molecules.

(d) A method of reducing and alloying iron and cobalt chloride vapor by flowing them through a mixed gas of hydrogen and nitrogen, argon, etc.

(e) After forming a cobalt-containing layer on the surface of the particles by reacting a ferrous salt and a cobalt salt with an alkali in an aqueous suspension in which acicular particles of iron oxyhydroxide are dispersed,
A method of heating and reducing.

[Problems to be Solved by the Invention] However, in the method (a) above, it is difficult to control the composition of the alloy particles produced, and it is difficult to obtain particles with the desired magnetic properties. )
In this method, the alloy particles become spherical or bead-shaped, resulting in poor orientation and magnetic properties.

On the other hand, the alloy powder obtained by method (e) has improved magnetic properties and corrosion resistance to a certain extent compared to ordinary metallic iron powder, but it is still insufficient to cope with the high performance of magnetic recording media in recent years. However, it is desired to further improve these characteristics.

In view of the above-mentioned circumstances, it is an object of the present invention to provide a method for producing an alloy magnetic powder having excellent magnetic properties and corrosion resistance.

[Means to solve the problem]

In the process of conducting studies to achieve the above object, the inventors first investigated the reason why sufficient magnetic properties and corrosion resistance could not be obtained in the conventional method (8), and found that iron oxyhydroxide If you try to thicken the cobalt-containing layer to be attached to the particle surface, it will not adhere uniformly and densely to the surface, so if you take into account the particle shape after reduction, the cobalt content of the alloy powder obtained by this method will be at most 7. It has been found that the above-mentioned cause is due to the fact that the amount is limited to about % by weight.

Based on this knowledge, we conducted further studies on how to obtain alloy magnetic powder with a higher cobalt content, and found that the surface of acicular particles of iron oxyhydroxide or iron oxide was When a cobalt-containing layer is formed on a steel, this cobalt-containing layer becomes uniform and dense, and it is possible to increase the amount of cobalt deposited and the cobalt content of the final alloy powder, which improves magnetic properties and corrosion resistance. It was discovered that an extremely excellent alloy magnetic powder can be obtained, and the present invention was completed.

That is, in the present invention, acicular particles of iron oxyhydroxide or iron oxide are dispersed in an alkaline aqueous solution, and an aqueous solution containing Fe3+ ions and Co2+ ions is added to this suspension to form these ions on the surface of the particles. The present invention relates to a method for producing an acicular alloy magnetic powder, which comprises forming a coating layer of a compound containing the compound and then reducing it by heating.

In addition, in this invention, the aqueous solution added to the above suspension has Cot"" ions of 30 to 1
00 mol%, the same aqueous solution further contains Cr”・,N
i"", Zn g 4, and this aqueous solution contains Fe
30-100 mol% Coz+ for 3+ ions
A configuration containing 1 to 10 mol % of other divalent metal ions as well as ions is a preferred embodiment.

[Structure and operation of the invention]

The most distinctive feature of this invention method is that an aqueous solution containing Fe 3 + ions and Co 2+ ions is used to form a cobalt-containing coating layer on the surface of acicular particles of iron oxyhydroxide or iron oxide. .

That is, by using trivalent iron ions in this way, compared to the case where divalent iron ions are used as in the conventional method (e), the surface of the acicular particles is uniformly and densely coated. It becomes possible to form a thick cobalt-containing coating layer, and it is possible to obtain an alloy powder produced through thermal reduction with a high cobalt content and excellent magnetic properties and corrosion resistance. Note that the above-mentioned cobalt-containing coating layer has been found to have a spinel-type crystal structure by X-ray diffraction.

In the method of this invention, acicular particles of iron oxyhydroxide or iron oxide are first dispersed in an aqueous alkaline solution, and this aqueous alkaline solution contains an aqueous solution of an alkali metal or an alkaline earth metal, particularly preferably sodium hydroxide or iron hydroxide. An aqueous solution of potassium is used, and the pH of the solution after dispersing the acicular particles is generally set to 8 or higher.

The acicular particles of iron oxyhydroxide and iron oxide used as raw materials have an average axial ratio of about 7 to 15 and an average major axis diameter of 0812 to 0.3.
A thickness on the order of μm is suitable. Further, these raw material particles may previously contain a metal element other than iron, such as cobalt or nickel, inside or on the surface of the particles.

The aqueous solution containing Fe3 ions and C0Z- ions added to the above suspension of acicular particles contains ferric salts such as ferric chloride, ferric sulfate, and ferric nitrate, cobalt chloride, and sulfuric acid. It is prepared by dissolving cobalt and cobalt salts such as cobalt nitrate in water. Here, the ratio of Fe3+ ions and Co"4 ions is preferably in a range where CO2" ions are 30 to 100 mol% relative to Fe"4 ions, and outside this range, cobalt-containing particles precipitate on the particle surface. It is difficult for the coating layer to have a uniform and dense crystal structure.

In the present invention, Cr g + , Ni''z
It is recommended to include at least one divalent metal ion selected from n g +. That is, such other divalent metal ions co-precipitate on the particle surface with Fe 1+ and co'' to form a coating layer, and are introduced as alloy components into the final alloy powder. In this case, the resulting alloy powder has better magnetic properties and corrosion resistance than alloy powders made of iron and cobalt.These other divalent metal ions are present in the form of chlorides, sulfates, nitrates, etc. It is best to use it in the range of 1 to 10 mol % based on Fe3+ ion. If it is too small, the above effects will not be fully exhibited, and if it is too large, both of the above properties will deteriorate. will be invited.

The reaction is carried out by gradually adding, under stirring, an aqueous solution containing the above Fe 3+ ions, CO'l ions, and, if necessary, other divalent metal ions above, to an alkaline suspension in which raw material particles are dispersed. These metal ions are co-precipitated on the surface of the raw material particles. At this time, it is desirable to add an alkaline aqueous solution together with the above aqueous solution to maintain the pH of the suspension at a constant value on the alkaline side. Further, the aqueous solutions containing the metal ions described above may be added simultaneously as different aqueous solutions for each type of ion. Furthermore, it is also possible to add appropriate buffer salts to the above alkaline suspension in advance.

The amount of the cobalt compound deposited on the surface of the acicular particles of iron oxyhydroxide or iron oxide by this reaction corresponds to the amount of the metal ion used. Therefore, Co
"By increasing the amount of 4-on used, the cobalt content in the final alloy powder can be set to a high value of, for example, 10% by weight or more. However, if the amount of coating of the above cobalt compound is too large, the particles may become Because the shape collapses and causes a decline in magnetic properties such as coercive force, CO2 is added so that the cobalt content in the final alloy powder is usually 25% by weight or less.
・It is desirable to set the amount of ions used.

The raw material particles on which a coating layer of a predetermined amount of cobalt compound has been formed are heated and reduced in accordance with a conventional method to obtain an alloy magnetic powder mainly composed of iron and cobalt.

In this case, it is recommended to perform a surface treatment to form a coating of a silicon compound and/or an aluminum compound on the surface of the raw material particles prior to the thermal reduction described above. These coatings prevent sintering between powder particles during thermal reduction,
It shows the ability to maintain the acicular shape of the raw material particles, giving good results to the magnetic properties of the finally obtained alloy magnetic powder.

As a means for such surface treatment, for example, the raw material particles provided with the above-mentioned cobalt-containing coating layer are redispersed in water, and a water-soluble silicate or/and a water-soluble aluminate is added to this suspension. Examples include a method of neutralizing by blowing carbon dioxide gas after mixing.

The amount of these silicon compounds and aluminum compounds deposited is Si and A! It is preferable that the atomic weight of is about 0.2 to 5.0% by weight.

In addition, in this invention, regardless of whether the raw material particles are iron oxyhydroxide or iron oxide, it is preferable that the raw material particles be heat-treated in the air at about 400 to 700°C prior to the above-mentioned thermal reduction. The containing coating layer becomes a complete oxide, and iron and cobalt as well as the above-mentioned chromium, nickel, zinc, etc. are easily alloyed during thermal reduction.

Thermal reduction after such surface treatment and heat treatment may generally be carried out at a temperature of about 350 to 550° C. in a hydrogen gas stream.

〔Effect of the invention〕

According to the method of the present invention, it is possible to obtain an alloy magnetic powder mainly composed of iron and cobalt, which consists of acicular particles with a high cobalt content and excellent magnetic properties and corrosion resistance. By setting the ratio of Co2+ ions to ions within an appropriate range, alloy magnetic powder having the above-mentioned excellent properties can be reliably produced.

In addition, in the method of this invention, Fe"4 on and C
o" ion, Cr1, N i "z n z
By using at least one other divalent metal ion selected from +, it is possible to further improve the magnetic properties and corrosion resistance of the obtained alloy magnetic powder.

Furthermore, if the ratio of Co''''- ions and other divalent metal ions mentioned above to Fe'' ions is set within an appropriate range, the above-mentioned effects can be reliably exhibited without deteriorating other properties. be able to.

〔Example〕

Next, examples of the present invention will be explained in comparison with comparative examples.

Example 1 Goethite powder (average major axis diameter 0.22 μm, average axial ratio 1
0) Disperse 100g in water 31, 5mol/l! The pH was adjusted to 1) by adding a concentrated aqueous sodium hydroxide solution.

In this suspension, 0.4 mol of ferric nitrate, cobal nitrate)
500 ml of an aqueous solution containing 0.18 mol of nickel nitrate and 0.02 mol of nickel nitrate was added and mixed under stirring over 3 hours, and the suspension was brought to pH 1 by adding a 5 mol/l aqueous sodium hydroxide solution. After the mixture was maintained at a temperature of 100.degree. C. for reaction, stirring was continued for an additional hour, followed by washing with water and drying to obtain goethite powder having a cobalt-containing coating layer on the surface of the particles.

Next, this goethite powder is redispersed in a sodium hydroxide aqueous solution 51 with a concentration of 0.2 mol/1, 26 mj+ of a 1 morph 128 degree sodium orthosilicate aqueous solution is added to this suspension, and the mixture is stirred. The particles were neutralized by blowing carbon dioxide gas into them until the pH reached 8, and a silicon compound was deposited on the particle surface, followed by washing with water and drying. After that, this goethite powder was again dispersed in a sodium hydroxide aqueous solution 51 with a concentration of 0.2 mol/l, and 135 ml of an aqueous sodium aluminate solution with a concentration of 0.5 mol/l was added, and carbon dioxide gas was introduced into the liquid under stirring. The particles were neutralized by blowing until the pH reached 8, an aluminum compound was deposited on the particle surface, and the particles were washed with water and dried.

Next, the getite powder coated with the aluminum compound was calcined in air at 500°C for 4 hours, and then heated and reduced in a hydrogen gas stream at 450°C for 8 hours to form Fe-Co.
Acicular magnetic powder made of -Ni alloy was obtained.

Example 2 Acicular magnetic powder made of Fe--Co--Zn alloy was obtained in the same manner as in Example 1, except that 0.02 mol of zinc sulfate was used instead of nickel nitrate.

Example 3 Acicular magnetic powder made of Fe--Co--Cr alloy was obtained in the same manner as in Example 1, except that 0.02 mole of chromium sulfate was used instead of nickel nitrate.

Example 4 Example 1 except that the amount of nickel nitrate used was changed to 0.01 mol and 0.01 mol of chromium sulfate was additionally used.
Acicular magnetic powder made of Fe-Co-Ni-Cr alloy was obtained in the same manner as above.

Example 5 Same as Example 1 except that the amount of ferric nitrate used was changed to 0.8 mol, the amount of cobalt nitrate was changed to 0.36 mol, and the amount of nickel nitrate used was changed to 0.04 mol. TeF
Acicular magnetic powder made of e-Co-Ni alloy was obtained.

Comparative Example 1 Acicular magnetic powder made of metallic iron was obtained in the same manner as in Example 1, except that a cobalt-containing coating layer was not formed on the surface of the goethite powder particles.

Comparative Example 2 Acicular magnetic powder made of an Fe-Co-Ni alloy was obtained in the same manner as in Example 1, except that 0.4 mol of ferrous nitrate was used instead of ferric nitrate.

Example 6 Acicular magnetic powder made of Fe--Co alloy was obtained in the same manner as in Example 1, except that nickel nitrate was not used.

Each of the magnetic powders obtained in the above Examples and Comparative Examples was slowly oxidized in air at 60°C for 2 hours, and then the saturation magnetization σS was determined using a sample vibrating magnetometer (manufactured by Toei Kogyo Co., Ltd.). While measuring the coercive force HC, 60℃, 90%
The saturation magnetization σS'' after storage for 7 days under RH conditions was measured in the same manner as above, and the decrease rate of saturation magnetization (σS - σS゛)/σS
I asked for

The results are shown in the table below along with the weight percent of Co/Fe measured by X-ray fluorescence analysis.

From the results shown in the above table, the alloy magnetic powder obtained by the method of this invention (Examples 1 to 6) has excellent magnetic properties and corrosion resistance, and is particularly composed of mainly iron and cobalt and at least one of nickel, chromium, and zinc. It can be seen that both of the above properties of the magnetic powders (Examples 1 to 5) containing the above properties are extremely high.

On the other hand, when forming a cobalt-containing coating layer,
The alloy magnetic powder obtained by the conventional method using ions (Comparative Example 2) has improved saturation magnetization and corrosion resistance compared to the metal iron magnetic powder (Comparative Example 1), but the coating layer is formed uniformly and It is clear that the coercive force is low due to the lack of precision, and that each property is significantly inferior to that of the alloy magnetic powder obtained by the method of the present invention.

Patent applicant: Hitachi Maxell, Ltd.

Claims (4)

[Claims] (1) Acicular particles of iron oxyhydroxide or iron oxide are dispersed in an alkaline aqueous solution, and an aqueous solution containing Fe^3^+ ions and Co^2^+ ions is added to this suspension to disperse the above particles. A method for producing an acicular alloy magnetic powder, which comprises forming a coating layer of a compound containing these ions on the surface of the powder, and then reducing the powder by heating. (2) The method for producing an acicular alloy magnetic powder according to (1), wherein the aqueous solution added to the suspension contains 30 to 100 mol% of Co^2^+ ions relative to Fe^3^+ ions. . (3) Cr^2^+, Ni^2^+, and Z are further added to the aqueous solution containing Fe^3^+ ions and Co^2^+ ions.
The method for producing an acicular alloy magnetic powder according to claim 1 or 2, further comprising at least one other divalent metal ion selected from n^2^+. (4) The aqueous solution added to the suspension contains 30 to 100 mol% of Co^2^+ ions relative to Fe^3^+ ions,
Claims containing 1 to 10 mol% of other divalent metal ions (
3) The method for producing an acicular alloy magnetic powder.