JPH0122968B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing ferromagnetic metal powder for magnetic recording, which is useful for magnetic tapes, magnetic disks, magnetic cards, and the like. Specifically, the present invention provides a ferromagnetic metal powder that has a high coercivity suitable for high-density magnetic recording media, a large amount of saturation magnetization, and particularly excellent moisture resistance and oxidation resistance. Recently, with the increasing density of magnetic recording, magnetic metal powders with high coercive force and large saturation magnetization have begun to be used in place of conventional magnetic oxide powders. This magnetic metal powder is usually pure iron or an iron alloy containing cobalt, nickel, chromium, etc. as elements other than iron. The conventional method for producing magnetic iron powder or magnetic iron alloy powder is to reduce iron oxide, iron oxyhydroxide, or these oxides containing other metals (Co, Ni, Cr, etc.). A method of reducing metal powder with a gas is known. (For example, IEEE Trans.
Magnetics Vol Mag-5, No.3, 317-320
(1969), Vol Mag-8, No. 3, 430-432
(1972), Japanese Patent Publication No. 35-3862, Japanese Patent Publication No. 53-11512, Japanese Patent Publication No. 54-22838, Japanese Unexamined Patent Publication No. 122663-1982, etc.) However, these methods do not allow sintering between powder particles during reduction. The drawback was that the particle shape changed significantly, resulting in a decrease in coercive force. In order to improve this, a method has been disclosed in which the surface of these oxides is coated in advance with bismuth or bismuth and a silicide, and then reduced with hydrogen gas (JP-A-46-7153, JP-B-Sho. 52-
Publication No. 19541, etc.). However, since all of these methods use soluble salts or emulsions, the particle surfaces cannot be sufficiently covered, making it difficult to obtain sufficient effects. Alternatively, a method is known in which molybdenum trioxide is dissolved in an aqueous sodium hydroxide solution, cobalt-containing magnetic iron oxide powder is immersed in this solution, filtered, washed with water, dried, and then reduced. Publication No. 54299). However, in this method, most of the soluble molybdenum salt flows out, and the effect of preventing sintering is weak. Also, a method of attaching silicone oil and ammonium borate
76960) has also been proposed, but this also has a weak sintering prevention effect. In addition to the above known matters,
In order to improve the magnetic properties of metal powder obtained by thermal reduction, there are documents showing treatments with magnesium silicate, calcium metasilicate, magnesium phosphate, etc. ) is also known. However, in the above conventionally known techniques, no consideration is given to the moisture resistance and oxidation resistance of the obtained metal powder, and therefore, their effects are not recognized or are not suggested at all. The present inventors conducted intensive research to improve the above points, and as a result, established the method of the present invention. That is, the present invention provides at least one of borates and molybdates of elements belonging to group a of the periodic table on the surface of powder of iron or iron-based metal oxides or hydrated oxides. It is characterized in that one type of powder is deposited thereon, and such powder is then reduced in a reducing gas. To describe this more specifically, first, powder of the metal oxide or hydrated oxide is added to an aqueous solution containing at least one salt (A) of water-soluble borates and molybdates. For example, iron oxide particles or goethite particles are dispersed, an aqueous solution of a water-soluble salt (B) of an element belonging to group A of the periodic table is added, and an insoluble or poorly soluble salt of a group A element is added to the surface of the particles. (e.g., calcium borate, calcium silicate, co-precipitation of both water-soluble salts (A) and (B), etc.), thus producing a uniform coating of these salts. Afterwards, these particles are separated, washed with water, and dried. The metal oxide particles etc. treated in this way are heated and reduced in a reducing gas to form metal powder. Or, conversely, a powder such as a metal oxide is dispersed in an aqueous solution of the water-soluble salt (B) of the Group A element of the periodic table, and the other aqueous solution (A) is added to the resulting dispersion. In particular, in the present invention, water-insoluble or poorly water-soluble salts (both water-soluble salts (A), (B)) containing the characteristic group a element and B, P, Si, Mo, etc.
Also includes coprecipitates. ) can be attached to a powder such as a metal oxide and then reduced to obtain the desired metal powder. In the present invention, the group a elements include Be,
Examples include Mg, Ca, Sr and Ba. According to the method of the present invention, powders such as metal oxides and reduced metals are prevented from sintering between particles during thermal reduction, and as a result, metal powders with desirable particle shapes can be obtained. Therefore, the obtained metal powder is
It has high coercive force and large saturation magnetization, as well as excellent moisture resistance and oxidation resistance, making it extremely valuable as a material for high-density magnetic recording. Examples of water-soluble borates used in the present invention include NaBO ₂ , NaBO ₂ 4H ₂ O,
Na ₂ B ₄ O ₇ , Na ₂ B ₄ O ₇・5H ₂ O, NaB ₄ O ₇・10H ₂ O,
Examples include NH ₄ HB ₄ O ₇ , NH ₄ B ₅ O ₈ .5H ₂ O, NaBO ₃ and the like. In addition, as a water-soluble molybdate,
For example, Na ₂ MoO ₄・2H ₂ O, Na ₂ Mo ₄ O ₁₃・6H ₂ O,
(NH ₄ ) ₆・Mo ₇ O ₂₄・4H ₂ O, etc. are listed, respectively. Similarly, water-soluble salts of group a elements include, for example, Be(CH ₃ COO) ₂ , Be(HCOO) ₂ , Be(NO ₃ ) ₂ ,
Be(NO ₃ ) ₂・3H ₂ O, Mg(CH ₃ COO) ₂ , Mg
(HCOO) ₂・2H ₂ O, Mg(NO ₃ ) ₂・6H ₂ O,
_MgCl2・_6H2O ,Ca( _CH3COO ) ₂ ,Ca
(CH ₃ COO) ₂・H ₂ O, Ca(HCOO) ₂ , Ca(NO ₃ ) ₂・
4H ₂ O, Ca(NO ₂ ) ₂・H ₂ O, CaCl ₂・H ₂ O, Ca
(ClO ₃ ) ₂・2H ₂ O, Sr(CH ₃ COO) ₂ , Sr
(HCOO) ₂・2H ₂ O, Sr(NO ₃ ) ₂ , Sr(NO ₂ ) ₂ ,
_SrCl2・_6H2O , Ba( _CH3COO ) ₂ , Ba(HCOO) ₂ ,
Ba(NO ₃ ) ₂ , Ba(NO ₂ ) ₂・H ₂ O, BaCl ₂ , Ba
(ClO ₃ ) ₂ and the like. Among these combinations of water-soluble salts (A) and water-soluble salts of group a elements (B), when borate or molybdate is used as (A) and acetate or nitrate is used as (B), Since the obtained metal powder has particularly excellent moisture resistance and oxidation resistance, this combination is suitable. EXAMPLES The contents of the present invention will be explained in more detail with reference to Examples below. (Note that % means weight basis unless otherwise specified.) Example ₁ Goethite (length _0.5-0.8μ , needle After soaking 10 g of powder and dispersing it by irradiating it with ultrasonic waves, 250 c.c. of a 0.1% solution of ammonium molybdate [(NH ₄ ) ₆ Mo ₇ O ₂₄・4H ₂ O] , to deposit calcium molybdate or a mixture of calcium acetate and ammonium molybdate on the surface of the goethite. After passing the goethite through a suction port, drying it at about 80°C and pulverizing it, the mixture was heated with a hydrogen stream of 3
/min at 450℃ for 30 minutes to obtain iron powder.
After cooling this to room temperature, it was immersed in toluene to stabilize it. The magnetic properties of the obtained iron powder were as follows: saturation magnetization σ _n =166 emu/g coercive force Hc = 1290 Oe, and squareness ratio σ _r /σ _n =0.51. When this was left in a humid box at 60℃ and 90%RH (relative humidity) for 40 days,
σ _n continued to decrease to 112 emu/g. For comparison, goethite (0.25% Si) coated with K ₂ SiO ₃ was reduced under the same conditions, and the following results were obtained. The value 20 hours after reduction is σ _n =
The values were 162emu/g, Hc = 1200Oe, and squareness ratio 0.52, but after being left at 60℃ and 90%RH for 40 days, the values were as follows.
It had decreased to σ _n =50 emu/g. Example ₂ Goethite (length _0.5-0.8μ , acicular ratio
15) Soak 10g of powder and apply ultrasound (17KC)
After being dispersed by irradiation, borax (Na ₂ B ₄ O ₇ )
250 c.c. of a 0.43% solution of is added to deposit calcium borate or a mixture of calcium acetate and sodium borate on the goethite surface, and after filtering, drying and pulverizing the goethite, hydrogen (3/min) was passed at 450°C.
Reduced for 30 minutes. As a result, σ _n =171emu/g, Hc =1260Oe,
Iron powder with magnetic properties with a squareness ratio of 0.5 was obtained. When this was left in a humid chamber at 60° C. and 90% RH for 40 days, σ _n =142 emu/g, Hc = 1260 Oe, and the squareness ratio was 0.5, which remained almost constant thereafter. Example 3 Various iron powders were produced using the same operating method as in Example 1 or 2, varying the type and concentration of the treatment agent, and their magnetic properties were measured. The results are summarized in Table 1. Nos. 28 to 31 are comparative examples in which Ca and B compounds were added alone, and the magnetic properties after reduction were poor. In Table 1, immediately after reduction means iron powder is placed in toluene.
This is the value immediately after being stabilized by immersion for 20 hours, and the percentage of the treatment agent means the weight ratio of the treatment agent added to the goethite.

【table】

[Table] Example 4 10 g of goethite in Example 1 was immersed in 250 c.c. of a 0.2% solution of barium acetate, the powder was dispersed by ultrasonic waves, and sodium perborate (NaBO ₃ ) was added to the powder.
Example 1
In the same manner as above, it is reduced to iron powder, which is stabilized by immersing it in water. The properties of the obtained magnetic iron powder are σ _n =
It was 170emu/g, Hc=1260Oe, and squareness ratio 0.50. This was tested for stability in a humid chamber at 60°C and 90% RH. The relationship between the standing time and the decrease in σm was measured, and a graph of the results is shown in FIG. 1 together with a comparative sample. Example 5 10 g of the goethite in Example 1 was dispersed in 200 c.c. of a 0.6% aqueous solution of calcium chloride, and a 1.8% aqueous solution of sodium perborate was added thereto with stirring to produce calcium perborate. Deposit on the particle surface. Next, this is suction filtered and dried. Next, the dried and crushed goethite was heated in an electric furnace while flowing hydrogen gas at a flow rate of 3/min.
It was reduced to iron powder at 400°C for 2 hours. The surface of the powder particles was gradually oxidized in toluene. The properties of the obtained magnetic powder are Hc = 1220Oe, σm = 168emu/
g, σr/σ _n =0.5. When the same humidification conditions as in Example 1 were applied, σ _n =138 emu/g. Example 6 Iron powder was produced using the same treatment method and reduction conditions as in Example 5. The water-soluble salts of Group A elements to be added were MgCl ₂ , CaCl ₂ , SrCl ₂ , and BaCl ₂ , and the concentrations thereof were varied. The results are summarized in Table 2. Incidentally, when the iron powder was subjected to the same humidification conditions as in Example 1, the decrease in the saturation magnetization amount σ _n was approximately within 20%.

[Table] Example 7 In producing iron powder using the same treatment method as in Example 5, calcium acetate was used instead of calcium chloride to make it poorly soluble with boric acid or molybdic acid to prevent sintering. I went there. Furthermore, the obtained magnetic iron powder was heated in air at 200° C. for 1 hour, and the rate of decrease in coercive force and saturation magnetization after heating was measured to compare heat resistance. The film was also kept at 60°C and 90% relative humidity for 7 days to test its humidity resistance. The results are shown in Table 4. For comparison, the characteristics of iron powder obtained by surface treatment and reduction with only Na ₂ SiO ₃ are also shown.

【table】

[Table] From the above results, the magnetic metal powder produced according to the present invention not only has excellent magnetic properties but also excellent heat resistance and moisture resistance stability, and is industrially very useful.

[Brief explanation of drawings]

The drawing is a graph showing the change in σ _n over time of the iron powder produced in Example 4 and a comparative sample under wet conditions.

Claims (1)

[Scope of Claims] 1. A method for producing iron or a magnetic metal powder mainly composed of iron by reducing an oxide or hydrated oxide of iron or a metal mainly composed of iron, wherein said oxide or hydrated oxide At least one of borates and molybdates of elements belonging to group a of the periodic table is deposited on the surface of a powder of an object, and then the powder is reduced in a reducing gas. A method for producing magnetic metal powder. 2 Metal oxidation in either an aqueous solution containing at least one water-soluble salt of borates and molybdates (A) or an aqueous solution containing a water-soluble salt of a Group A element of the periodic table (B). The method according to claim 1, characterized in that the aqueous solution of the other compound is added to the resulting dispersion. 3. The method according to claim 2, wherein the water-soluble salt (B) of the group a element is an acetate or a nitrate.