JPH0257122B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Fields> In recent years, with the spread of digital recording of audio, images, and information, the demand for higher density magnetic recording has been increasing. In fact, even in audio tapes, video tapes, floppy disks, etc.
Recording media using magnetic metal powder as a recording element instead of iron oxide particles have been developed and are being commercialized. The present invention relates to metal magnetic powder for high-density magnetic recording, and is used in the field of magnetic recording media such as magnetic tapes, magnetic disks, and magnetic cards. <Prior art> Currently, metal tapes for audio are in practical use as magnetic recording media using magnetic metal powder, but these tapes have relatively large magnetic metal tapes with a length of about 0.5 μm and an axial ratio of 10 to 15. powder is used. On the other hand, the recently developed metal tape for 8mm video has a length of
Small magnetic metal powder with a diameter of 0.2 μm and an axial ratio of approximately 10 is used. Conventionally, the method for manufacturing such magnetic metal powder is to coat the surface of iron oxide, iron oxyhydroxide, or powder containing other metals (Co, Ni, etc.) with bismuth or bismuth and silicon compounds in advance. Method of reducing with hydrogen gas after coating
Publication No. 19541) has been started. Also, in addition,
A method using an inorganic silicate compound or an organosilicon compound as a surface treatment substance (Japanese Patent Application Laid-Open No. 52-30758),
A method using an Al compound or a titanium compound (JP-A-52-122213), a method using silicone oil (JP-A No. 55-85605), a method using silicone oil and ammonium borate (JP-A-53-1999)
76960), a method using Co, Ni, Mn, and Sb-based compounds (Japanese Patent Application Laid-Open No. 122664/1983), and the like.
However, in all of these methods, if the metal iron oxide or iron oxyhydroxide particles have a length of 0.2 μm or less and a specific surface area of 70 m ² /g or more, the sintering prevention effect is not sufficient, and the sintering prevention effect is insufficient during surface treatment. Due to insufficient dispersion of the coated particles, some particles remain bundled together on the surface, resulting in sintering between particles during reduction, resulting in deformation of the particle shape, and The particles themselves may become enlarged. In order to improve the method of preventing sintering, the present inventors added borates, phosphates, silicates, molybdates of elements belonging to group a of the periodic table to the surface of the iron oxide or iron oxyhydroxide powder. method of depositing salt, etc. (Japanese Unexamined Patent Publication No. 58-46607)), or Mg, Ca, Sr, Ba,
A method of attaching peroxoborates of at least one element among Co, Ni, Zn, Zr, and Al (Japanese Patent Application Laid-Open No. 59-5603) has been proposed. <Problems to be Solved by the Invention> In recent years, magnetic metal powder for magnetic recording has become increasingly smaller in particle size, highly dispersible, and There is a growing demand for highly filling materials. Conventional methods have not been satisfactory for producing ultrafine magnetic metal powder for such purposes. This is due to the fact that the powder needs to be dispersed into primary particles before it is applied with a treatment agent, and the surface treatment agent has a sufficient effect of preventing sintering during reduction, and the resulting metal This is because the requirement to maintain the magnetic properties of magnetic powder at good values was not satisfied. In addition, spindle-shaped goethite with small particle size and large axial ratio (length: 0.05 to 0.50 μm, axial ratio of 4 or more) proposed by the present inventors in JP-A-59-232922
When reduced by a conventional method to prevent sintering, particle decomposition and sintering occur during reduction, resulting in complete loss of acicularity. The present invention aims to solve these problems. <Means for Solving the Problems> The present invention is based on the invention proposed by the present applicant, namely, the surface of iron oxide, iron oxyhydroxide, or powder containing these and other metals is subjected to a periodic rule. This is an improved method of coating with a borate or peroxyborate of an element belonging to group a in the table and then reducing it with hydrogen gas (see JP-A-58-46607 and JP-A-59-5603). In these methods, the surface treatment of iron oxide powder or iron oxyhydroxide powder is first carried out by
After dispersing in an aqueous solution of magnesium acetate or magnesium nitrate, the borate or peroxoborate is added, or the reverse order is applied. However, although this method has an excellent sintering prevention effect when the powder becomes very small, there is a phenomenon in which several particles are bundled together, which is not always satisfactory from the viewpoint of paint dispersibility. There was nothing I could do. Therefore, in order to improve this point, when coating the surface of iron oxide particles or iron oxyhydroxide particles with an anti-sintering treatment agent, the powder is first dispersed in the primary particles, and then the treatment agent is applied. I thought about putting it on. To do this, first make these powders acidic (PH2~
5) The particles are immersed in the aqueous metal salt solution and mechanically dispersed to adsorb positively charged metal ions onto the surfaces of these particles, so that the individual particles become independent of each other due to Coulomb repulsion. Next, a processing agent with a strong sintering prevention effect is added and adhered to each particle, and then a flocculant is added and the particles are separated by filtration. The present inventors investigated various metal salts and found that salts that form trivalent metal ions in an aqueous solution, ie, chromium salts, cerium salts, neodymium salts, etc., are suitable. So, like this
A suspension of iron oxide particles or iron oxyhydroxide particles dispersed by adhesion of metal ions such as Cr ³⁺ , Ce ³⁺ , Nd ³⁺ , etc., contains elements of group A of the periodic table, which have a strong anti-sintering effect. A water-soluble salt and a boron compound or a boron compound are added individually and stirred. Next, aqueous ammonia is added to adjust the pH of the suspension to about 8 to coagulate the slurry, followed by filtration and drying. That is, the main part of the present invention is to reduce iron or a metal oxide containing at least one of Ni and Co, or a hydrated oxide thereof, to produce iron or a magnetic metal mainly composed of iron. In the method for producing a powder, the oxide or hydrated oxide powder is dispersed in an aqueous solution containing a water-soluble salt of Cr, Ce or Nd as a dispersant, thereby adding Cr, Ce or Nd to the oxide. ions are attached, then a water-soluble boric acid compound or perborate compound is further attached as an anti-sintering agent to the surface of the dispersed particles of the oxide or hydrated oxide, and then the oxide is heated in a reducing gas A method for producing a ferromagnetic metal powder, which is the first invention of the present invention, is characterized in that the powder is reduced with The case is the second invention. The present invention will be described in more detail. first,
Aqueous solution of water-soluble salt of Cr, Ce or Nd (PH=2~
5) Disperse iron oxide, iron oxyhydroxide, or a powder of these containing Ni or Co. In this case, the temperature of the solution may be room temperature, but increasing the temperature lowers the pH of the solution and is more effective. However, raising and lowering the temperature requires energy and time, and is not industrially advisable. Dispersion can be achieved by simply stirring the suspension, but it is more effective to use an ultrasonic disperser. Next, while stirring, an aqueous solution of a water-soluble salt of a Group A element of the periodic table is added, followed by an aqueous solution of boric acid, a borate, or a peroxoborate. Alternatively, an aqueous solution of boric acid, borate or peroxoborate may be simply added. Next, add 1N ammonia water little by little.
Adjust the pH of this suspension to approximately 8. This results in
The suspension becomes a bit agglomerated, making it easier to filter the slurry. After drying the filtered slurry at approximately 100℃,
It is heated and reduced in hydrogen gas to produce metal grain powder. According to the present invention, sintering of iron oxide or iron oxyhydroxide itself or each other is effectively prevented during thermal reduction, and the metal powder (length = 0.02
~0.2 μm, width = 0.01-0.03 μm) It can be obtained as acicular particles dispersed in primary particles. Furthermore, since the surfaces of these metal particles are protected by a stable coating derived from borate or peroxoborate, a metal iron powder that is resistant to corrosion can be obtained. Therefore,
The obtained metal powder has high coercive force and saturation magnetization, as well as excellent paint dispersibility, and is extremely valuable as a material for high-density magnetic recording. The water-soluble salts of Cr, Ce or Nd used in the present invention include nitrates, hydrochlorides, sulfates, acetates, and formates. Here, when a hydrochloride or a sulfate is used, chlorine and sulfur elements are adsorbed and remain on the surface of the iron powder, which is not preferable from the viewpoint of corrosion resistance. Furthermore, since the aqueous solution pH of acetate and formate is around 6 and does not become small, it is difficult to disperse the surface-treated substance down to the primary particles. On the other hand, nitrates are most suitable from the viewpoint of dispersion and corrosion resistance. In the present invention, the group a elements include Mg,
Ca, Sr, Ba, etc., and their water-soluble salts (X) include formates, acetates, hydrochlorides, sulfates, nitrates (for example, Mg
(HCOO) ₂・2H ₂ O, Mg (CH ₃ COO) ₂・4H ₂ O,
_MgCl2・_6H2O , _MgSO4・_7H2O , Mg( _NO3 ) ₂・
6H ₂ O, Ca(CH ₃ COO) ₂ H ₂ O, etc.).
Here, it is preferable to use acetate or nitrate because the resulting metal powder has excellent corrosion resistance. Further, regarding the amount of each of these salts to be used, for example, in the case of goethite (α-FeOOH), the amount of chromium nitrate, cerium nitrate, or neodymium nitrate to be used is preferably in the range of 5 to 30% by weight based on goethite. Next, the amount of the water-soluble salt of the group a element (for example, magnesium acetate) used is preferably in the range of 0 to 15 percent by weight based on goethite. Further, the amount of boric acid, borate or peroxoborate used is preferably in the range of 1 to 30 weight percent based on goethite. For example, the amount of Nd, Mg, and B attached to goethite after filtration varies slightly depending on the concentration of goethite in water, but when [water]/[goethite] = 20, Nd is 100% and Mg is about 35%. %, B is approximately 30%. If the amount of adhesion is small, sintering between powder particles cannot be prevented and the shape of the particles deteriorates, resulting in a decrease in coercive force of the obtained metal powder. If the amount of adhesion is too large, the reduction of the raw material powder will be insufficient even if you try to promote reduction by increasing the heating temperature during reduction or extending the heating time for a long time, and the saturation magnetization or coercive force of the product will decrease. will decrease. The temperature at which the material to be reduced having Cr, Ce or Nd, an element of group a of the periodic table, and boron deposited thereon is reduced by heating is preferably in the range of 300 to 500°C. Here, the reduction may be carried out after the substance to be reduced is heated and dehydrated in air before the reduction. This heat treatment has the effect of making the internal structure of the reduced iron particles homogeneous. Therefore, as the temperature increases, the coercive force of iron powder decreases, but the saturation magnetization amount and squareness ratio tend to increase. However, if the temperature is raised too high above the sintering temperature of the metal particles (600° C. or higher), the shape of the particles deteriorates due to the sintering effect, and the magnetic properties also deteriorate. <Examples> Examples and comparative examples will be given below to further specifically explain the content of the present invention. (Note that % means weight basis) Example 1 1700 ml of an aqueous solution in which 240 g of caustic soda and 28 mg of potassium silicate were dissolved was placed in a bubble column type reaction tank while N ₂ gas was flowing. Next, ferrous sulfate
Ferrous ions were precipitated by adding 300 ml of an aqueous solution containing 166.7 g of FeSO ₄ .7H ₂ O. After setting the slurry temperature to 40℃, switch the _N2 gas to air and
The reaction was carried out for 6 hours by flowing an amount of air for 6 hours. Next, the slurry was filtered and thoroughly washed with water until the washing liquid became neutral to obtain goethite (α-FeOOH) having a length of about 0.2 μm and a needle ratio of about 8. Next, 3.0g of this goethite was taken as a solid content, and chromium nitrate [Cr] was added to the goethite.
(NO ₃ ) ₃ ·9H ₂ O] was immersed in 200 ml of an aqueous chromium nitrate solution (containing chromium nitrate x/100 x 30 g) containing x%, and dispersed using a stirrer and an ultrasonic disperser. Next, magnesium acetate [Mg
Add 100 ml of a magnesium acetate aqueous solution (containing y/100 x 30 g of magnesium acetate) in which the amount of (CH ₃ COO) _{2.4H 2} O] is y% relative to goethite,
After stirring the powder, 150 ml of a boric acid aqueous solution (containing z/100 x 30 g of boric acid) was added, and the powder was stirred and dispersed again. Thereafter, this suspension was filtered to form a wet cake with a solid content of about 30%, which was then dried at about 100°C. Take 20g of this dry cake, put it in a fixed bed tubular reduction furnace, replace the air with nitrogen gas, and then
The temperature was raised in a hydrogen gas atmosphere of min., and reduction was performed at 420°C for 3 hours to obtain metallic iron. After lowering the temperature to room temperature and purging with nitrogen gas again, it was immersed in toluene for 20 hours. Thereafter, this metallic iron powder was filtered and dried in air to obtain stabilized iron powder. Table 1 shows the magnetic properties of the iron powder thus obtained with respect to the concentration (x, y, z) of the surface treatment agent relative to goethite. Here, the magnetic measurement was performed using a vibrating sample magnetometer manufactured by Toei Kogyo Co., Ltd. at a maximum measurement magnetic field of 10 kOe. As shown in the table, as the amount of chromium nitrate or boric acid added increases, the coercive force Hc of the iron powder increases. In particular, the effect of boric acid is remarkable. On the other hand, when magnesium acetate was added at around 5 wt%, Hc increased and then
Hc gradually decreases. Examples 2 and 3 Iron powder was produced in the same manner as in Example 1 except that cerium nitrate or neodymium nitrate was used instead of chromium nitrate. Table 1 shows the magnetic properties of the obtained iron powder.
Shown below. Comparative Example 1 Various iron powders were produced in the same manner as in Example 1 except that chromium nitrate was not used and the types and concentrations of water-soluble magnesium salt and boric acid compound were changed. The magnetic properties of the obtained iron powder are shown in Table 2.
Compared to the results of Example 1, the coercive force and squareness ratio are smaller. Comparative Examples 2 and 3 This is a case where a water-soluble calcium salt or a strontium salt was used in place of the water-soluble magnesium salt in Comparative Example 1. Hc is slightly smaller than when magnesium salt is used. Comparative Example 4 Iron powder was produced in the same manner as in Example 1 except that boric acid was not used. The magnetic properties of the iron powder are shown in Table 2, and the coercive force and squareness ratio are significantly smaller than the results of Example 1. As a result of electron microscopic observation, it was found that the iron powder particles were heavily sintered and the acicularity was extremely poor. Comparative Example 5 Iron powder was produced in the same manner as in Example 1 except that zinc nitrate was used instead of chromium nitrate. The magnetic properties of the obtained iron powder are shown in Table 2, and the coercive force is higher than that of chromium.
Smaller than 100Oe. Example 4 After dissolving 570g of soda carbonate in 6.12Kg of water,
The mixture was introduced into a bubble column reactor while flowing N ₂ gas.
Next, 500 g of ferrous sulfate was dissolved in 2.8 Kg of water and added to the reaction vessel to precipitate ferrous ions. After the slurry temperature was set to 40° C., the N ₂ gas was changed to air, and the reaction was carried out for 2 hours by flowing the air at a flow rate of 10/min. Next, the slurry is filtered, washed thoroughly with water until the washing liquid becomes neutral, and then
A "spindle-shaped goethite" with a diameter of 0.15 μm and an acicular ratio of about 6 was obtained. Next, 30g of this goethite was taken as a solid content, and Cr, Mg and B were added using the same method as in Example 1.
Alternatively, after obtaining goethite to which Cr and B elements were attached, reduction was performed to obtain acicular metallic iron powder. Table 3 shows the magnetic properties of the iron powder thus obtained with respect to the concentration of the goethite surface treatment agent. In other words, even when using the spindle-shaped goethite described above, the coercive force is reduced according to this sintering prevention treatment method.
Iron powder with a high saturation magnetization of 1400 Oe or more can be obtained. Comparative Example 6 Goethite obtained in Example 4 without surface treatment or surface-treated with various treatment agents other than the three-element system of Cr-Mg-B was reduced to metal by the same method as in Example 1. Obtained iron powder. The magnetic properties of such iron powder are shown in Table 3 in comparison with Example 4, and the coercive force does not reach 1000 Oe. Further, when observing the particle shape with an electron microscope, the goethite particles are decomposed and sintered to have a rectangular or nearly spherical shape. Example 5 The spindle-shaped goethite obtained in Example 4 was surface-treated in the same manner as in Example 1 using a system using chromium nitrate, cerium nitrate, and neodymium nitrate, and reduced to obtain metallic iron powder. . Table 4 shows the magnetic properties of the iron powder with respect to the type and concentration of the surface treatment agent. Next, take 9 g of each of the above iron powders and add polyurethane resin (Pandex-B, trade name of Dainippon Ink Chemical Co., Ltd.)
Mix 1.5 g of vinyl chloride and vinyl acetate copolymer (VAGH, trade name of Union Carbide), 39 g of a 1:1 mixture of methyl isobutyl ketone and toluene, and 0.27 g of lecithin, and mix in a paint shaker for 4 hours. A magnetic paint was produced by dispersion. This paint is applied to polyester film and approximately 5kOe is applied.
It was dried at a temperature of 50°C by applying a magnetic field of . Next, a magnetic coating film was obtained by calendering at a temperature of 80° C. and a linear pressure of 100 kg/cm. The magnetic properties of the obtained coating film were measured using a VSM magnetometer to determine the maximum applied magnetic field.
Table 4 shows the results measured at 10 kOe. From the table,
It can be seen that the iron powder obtained by this method has good paint dispersibility and filling properties, has a high saturation magnetic flux density Bs and a squareness ratio Br/Bs, and therefore has a residual magnetic flux density Br of more than 2500 Gauss. Comparative Example 7 Iron powder was produced in the same manner as in Example 5 using various treatment agents shown in Table 4 other than chromium nitrate-magnesium acetate-boric acid as surface treatment agents for spindle-shaped goethite, and a magnetic coating film was further applied. Obtained. The magnetic properties of these iron powders or magnetic coatings are shown in Table 4, and Example 5
The coercive force is small compared to that of the tape, and the tape magnetic properties (Bs, Br or Br/Bs) are poor.

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[Table] <Effect> As described above, the method for producing ferromagnetic metal powder of the present invention can produce very small magnetic metal powder (length of 0.2 μm or less,
Specific surface area of 40 m ² /g or more) can be obtained in the form of acicular particles that do not sinter or lose their shape. Above all,
It has become possible to obtain spindle-shaped goethite, which is synthesized from an alkali carbonate and ferrous salt as raw materials and has an excellent particle size distribution and a small particle size, as metal iron particles that have good acicular properties and are dispersed in each other. As a result, it is possible to increase the degree of filling of the ferromagnetic metal powder in the magnetic coating film, so it is thought that a magnetic recording medium with low noise and high output can be obtained.

Claims (1)

[Claims] 1. Powder of iron or a magnetic metal mainly composed of iron by reducing iron or an oxide of a metal mainly composed of iron and containing at least one of Ni and Co, or a hydrated oxide thereof In the method for producing oxide or hydrated oxide powder, Cr, Ce or Nd
Cr, Ce or Nd ions are attached to the oxide by dispersing it in an aqueous solution containing a water-soluble salt as a dispersant, and then the surface of the dispersed particles of the oxide or hydrated oxide is further sintered. A method for producing ferromagnetic metal powder, which comprises depositing a water-soluble boric acid compound or perboric acid compound as an anti-caking agent, and then reducing the powder in a reducing gas. 2. The water-soluble salt of Cr, Ce or Nd is a nitrate, and the water-soluble boric acid compound or perborate compound is boric acid, perborate, or their ammonium salt, sodium salt, or potassium salt. The method according to claim 1. 3. Iron, oxides of metals mainly containing iron and at least one of Ni and Co, or their hydrated oxides are oxidized in a mixed solution of an aqueous alkali carbonate solution and an aqueous ferrous salt solution at a temperature above room temperature. The major axis diameter obtained by introducing a reactive gas and performing an oxidation reaction is 0.05
3. The method according to claim 1 or 2, characterized in that the anti-weight type goethite has a diameter of 0.3 .mu.m and an axial ratio of 3 to 15, or it is dehydrated by heat treatment to obtain iron oxide. 4. A method for producing iron or a magnetic metal powder mainly consisting of iron by reducing iron or an oxide of a metal mainly composed of iron and containing at least one of Ni and Co, or a hydrated oxide thereof, By dispersing the oxide or hydrated oxide powder in an aqueous solution containing a water-soluble salt of Cr, Ce or Nd as a dispersant, Cr, Ce or Nd ions are attached to the oxide, and then Furthermore, on the surface of the dispersed particles of the oxide or hydrated oxide, a water-soluble salt of a group A element of the periodic table and a water-soluble boric acid compound or perborate compound are attached as an anti-sintering agent,
A method for producing a ferromagnetic metal powder, which comprises then reducing the powder in a reducing gas. 5. The method according to claim 4, wherein the water-soluble salt of the Group A element is an acetate or a nitrate.