JPS6313943B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing acicular magnetic iron oxide particles for magnetic recording, and the average value of the long axis
Acicular α-FeOOH particles having excellent acicular crystals of 0.3 to 2.0 μm and an axial ratio (long axis: short axis) of 20:1 or more were obtained, and the acicular α-FeOOH particles were heated and reduced. The object of the present invention is to obtain acicular magnetite particles and acicular maghemite particles having excellent acicular crystals by oxidation. In recent years, as magnetic recording and reproducing equipment has become smaller and lighter, there has been an increasing need for higher performance magnetic recording media such as magnetic tapes and magnetic disks. In other words, high density recording characteristics, high output characteristics,
Improvements in various characteristics such as high sensitivity characteristics and frequency characteristics are required. The characteristics of magnetic materials suitable for satisfying the above requirements for magnetic recording media are high coercive force.
Hc and a large saturation magnetic flux density σs. As is well known, the magnitude of the coercive force of magnetic particles depends on any one of shape anisotropy, crystal anisotropy, strain anisotropy, and exchange anisotropy, or their interaction. In addition, the output characteristics and sensitivity characteristics of magnetic recording media such as magnetic tapes and magnetic disks depend on the residual magnetic flux density Br, which is determined by the dispersibility of magnetic particles in the vehicle and the orientation in the coating film. It depends on the properties and filling properties. In order to improve the orientation and filling properties in the coating film, it is required that the magnetic particles dispersed in the vehicle have acicular crystals as excellent as possible. Currently, acicular magnetite particles or acicular maghemite particles are mainly used as magnetic recording materials. These are generally produced by air oxidizing a colloidal aqueous solution with a pH of 11 or higher containing ferrous hydroxide particles obtained by reacting an aqueous ferrous salt solution with an alkali (usually called a "wet reaction"). The obtained acicular α-FeOOH particles are reduced to acicular magnetite particles in a reducing gas such as hydrogen at 300 to 400°C, or are then oxidized in air at 200 to 300°C to form acicular crystals. It is obtained by making it into maghemite particles. Acicular magnetite particles or acicular maghemite particles, which are currently used as magnetic particles for magnetic recording, utilize their shape magnetic anisotropy to obtain a relatively high coercive force, and their orientation It takes advantage of its excellent properties to obtain a relatively large square shape (Br/Bm) and degree of orientation.
Furthermore, research and development efforts are underway to improve the properties of acicular magnetite particles and acicular maghemite particles. As mentioned above, acicular magnetic particles with excellent acicular crystals are currently in high demand, and in order to obtain magnetic particles with such characteristics, starting materials are required. Needle crystal α−
It is necessary that the FeOOH particles have excellent acicular crystal structure. Conventionally, acicular crystals α-
The most typical known method for producing FeOOH particles is to add an equivalent or more alkaline solution to a ferrous salt solution to obtain a solution containing ferrous hydroxide particles at a pH of 11 or higher and a temperature of 80°C or lower. Acicular α-FeOOH particles are obtained by performing an oxidation reaction. The acicular α-FeOOH particles obtained by this method have a needle-like shape with a length of about 0.5 to 1.5μ, but the axial ratio (long axis: short axis) is at most 10:
1, and it is difficult to say that the particles have excellent acicular crystals. In this way, the acicular crystal α− with an axial ratio of about 10:1
When FeOOH particles are made into acicular crystal magnetic iron oxide particles through a reduction process and an oxidation process, the particles shrink in each reduction and oxidation process, so the axial ratio of the obtained acicular crystal magnetic iron oxide particles is The ratio is about 6:1 at most. On the other hand, the present inventor has been working on needle-like α-
We are involved in the production and development of FeOOH particle powder, but in the process, acicular crystals α
-When producing FeOOH particles, when some kind of dissimilar metal ion other than Fe is added to the ferrous salt aqueous solution, which is the raw material iron salt, the particles generally grow more easily in the long axis direction and the axial ratio decreases. big α−
We have discovered a phenomenon in which FeOOH particles are obtained. Certain types of foreign metal ions other than Fe include
For example, Co, Ni, Cr, Mn, Cd, etc. However, the addition of these metal ions other than Fe generally causes the acicular α-FeOOH particles to become extremely fine, and this tendency becomes more pronounced as the amount added increases. Are known. These extremely fine acicular α-FeOOH particles are not suitable as a starting material for magnetic particles for magnetic recording. In view of the above-mentioned prior art, the present inventor has discovered that acicular crystals α-FeOOH obtained in an alkaline region with a pH of 11 or higher
The present invention was achieved as a result of various studies aimed at improving the axial ratio without making the particles extremely fine. That is, the present invention provides a PH containing Fe(OH) ₂ obtained by reacting a ferrous salt aqueous solution with an alkaline aqueous solution.
Acicular crystal α-FeOOH particles are generated by passing oxygen-containing gas through the suspension of 11 or more to oxidize it,
The acicular crystal α-FeOOH particles are washed with water, separated, dried, and then thermally reduced to obtain acicular crystal magnetite particles, or further oxidized to obtain acicular crystal maghemite particles.Acicular crystal magnetic iron oxide particle powder In the production method, a water-soluble magnesium salt is added to the suspension before oxidation by passing oxygen-containing gas through
By making the presence of 0.5 to 7.0 at% in terms of Mg relative to Fe, acicular α-
Generating FeOOH particles, washing the acicular α-FeOOH particles with water, separating them, drying, and then heating and reducing them to produce acicular magnetite particles, or further oxidizing them to produce acicular maghemite particles. This is a method for producing acicular magnetic iron oxide particles. First, the technical background that led to the completion of the present invention and the structure of the present invention will be described. In view of the prior art described above, the present inventor has discovered that acicular crystals α-FeOOH obtained in an alkaline region with a pH of 11 or higher
As a result of repeated studies on the type, amount, and period of presence in the reaction system of different metals other than Fe that would improve the axial ratio without causing ultra-fine particle size, we found that a ferrous salt aqueous solution In a method for producing acicular α-FeOOH particle powder by passing an oxygen-containing gas through a suspension containing Fe(OH) ₂ and having a pH of 11 or higher obtained by reacting Fe(OH) 2 with an aqueous alkali solution to oxidize the powder. If a water-soluble magnesium salt is pre-existing in the above suspension in an amount of 0.5 to 7.0 atomic % based on Fe in terms of Mg before oxidation by passing oxygen-containing gas, the polarization of the acicular α-FeOOH particles Acicular α-FeOOH with an average long axis of 0.3 to 2.0 μm and an axial ratio (long axis: short axis) of 20:1 or more, which can improve the axial ratio without causing miniaturization.
We obtained new knowledge that it is possible to obtain particles. In other words, the reaction for producing acicular α-FeOOH particles is as follows:
Acicular α−FeOOH from a suspension containing Fe(OH) ₂
The stage of generation of core particles and the formation of the acicular crystal α-
The process consists of two steps: the growth of FeOOH core particles, and water-soluble magnesium salts are made to exist in a suspension containing Fe(OH) ₂ before oxidation by passing in an oxygen-containing gas. According to the method of the present invention, magnesium ions are formed into acicular crystals α
- At the stage of generation of FeOOH core particles, acicular α-FeOOH core particles with an excellent axial ratio are generated, and furthermore, at the stage of growth of the acicular α-FeOOH core particles, growth in the short axis direction of the particles is caused. As it suppresses the growth of grains in the long axis direction, the acicular crystal α-
FeOOH particles can be obtained. Although the theoretical explanation of the action of water-soluble magnesium salts in this phenomenon has not yet been fully elucidated, the present inventors believe that magnesium ions
- At the growth stage of FeOOH core particles, magnesium ions have the effect of suppressing the growth in the short axis direction of the particle and promoting growth in the long axis direction of the particle. We believe that one reason is that it is easier to adsorb on surfaces parallel to the long axis than on vertical surfaces. Conventionally, in the production of α-FeOOH particles, there is a method described on page 108 of the collection of presentation summaries of the 1971 Spring Conference of the Powder and Powder Metallurgy Association, which involves the presence of magnesium. The reaction system of this method uses alkali carbonate as the alkali to produce α-FeOOH particles in the pH range of 7 to 11, and the resulting particles have a spindle shape. In this reaction system, ``sodium salts such as hexametaphosphoric acid, pyrophosphoric acid, tartaric acid, etc. are added to an alkali carbonate, or Zn, Cu, Mg, etc. are added to a ferrous salt.
Sulfate of Mn, Cr, Al, etc. is 0.2 to 2 to ferrous iron.
% by weight, the reaction product has a fine particle size.'' For example, in this reaction system, the long axis of α-FeOOH particles obtained at a temperature of 50°C is about 1.0 μm on average, but when sodium hexametaphosphate is
When added, the average value is about 0.15 μm. Therefore, in this reaction system, when the above-mentioned additive is added, particles become finer, which is completely different from the effect of the water-soluble magnesium salt in the present invention. Next, some of the many experimental examples conducted by the present inventor will be extracted and explained as follows. FIG. 1 is a diagram showing the relationship between the amount of water-soluble magnesium salt present and the axial ratio of acicular α-FeOOH particles. In other words, Fe(OH) ₂ with a pH of 13 is prepared by using a 1.0 mol/aqueous solution of ferrous sulfate obtained in the presence of magnesium sulfate so as to contain 0.1 to 10.0 atomic % of Fe in terms of Mg, and an aqueous solution of caustic soda. obtain a suspension;
Acicular crystals α obtained by oxidizing the suspension by passing 100 air per minute at a temperature of 45°C.
This figure shows the relationship between the axial ratio of −FeOOH particles and the amount of magnesium sulfate present. As shown in FIG. 1, the axial ratio of the acicular α-FeOOH particles tends to improve as the amount of water-soluble magnesium salt increases. FIG. 2 shows the relationship between the amount of water-soluble magnesium salt present and the long axis of acicular α-FeOOH particles produced under the same reaction conditions as in FIG. As shown in Figure 2, as the amount of water-soluble magnesium salt increases to 2 atomic % in Mg terms relative to Fe, needle-like crystals α
-The long axis of FeOOH particles shows an increasing trend. Abundance of water-soluble magnesium salt relative to Fe
When the amount increases by more than 2 atomic % in terms of Mg, the long axis gradually decreases. This phenomenon is thought to be due to an increase in the amount of water-soluble magnesium salt present, which caused magnesium ions to also be adsorbed on the plane perpendicular to the long axis of the particles, thereby suppressing growth in the long axis direction of the particles. However, at the same time, adsorption of magnesium ions also increases on planes parallel to the long axis of the particle, so growth in the short axis direction is increasingly suppressed, and the axial ratio of the particle itself is as shown in Figure 1. As shown in , even if the amount of water-soluble magnesium salt present exceeds 2 atomic % in terms of Mg based on Fe, it does not decrease. FIG. 3 shows an electron micrograph (×20,000) of acicular α-FeOOH particles obtained by the method of the present invention (Example 2 described below). As is clear from FIG. 3, the acicular α-FeOOH particles obtained by the method of the present invention have excellent acicular crystals. Next, various conditions for carrying out the method of the present invention will be described. Examples of the ferrous salt aqueous solution used in the present invention include a ferrous sulfate aqueous solution and a ferrous chloride aqueous solution. As the water-soluble magnesium salt used in the present invention, magnesium sulfate, magnesium chloride, magnesium nitrate, etc. can be used. The presence of the water-soluble magnesium salt in the present invention is required prior to bubbling oxygen-containing gas into the Fe(OH) _2- containing suspension to produce α-FeOOH particle powder; It can be present in any of a salt aqueous solution, an alkaline aqueous solution, or a suspension containing Fe(OH) ₂ . Note that even if the water-soluble magnesium salt is present at a stage when some needle-like α-FeOOH core particles have already been formed after the start of oxygen-containing gas ventilation, a sufficient effect cannot be obtained. In the present invention, the amount of water-soluble magnesium salt present is 0.5 to 7.0 atomic % based on Fe in terms of Mg. Abundance of water-soluble magnesium salt relative to Fe
If it is less than 0.5 atomic % in terms of Mg, the object of the present invention cannot be fully achieved. The purpose of the present invention can also be achieved when the concentration is 7.0 at% or more, but the effect in that case is not remarkable, and the obtained acicular α-FeOOH particles are heated and oxidized. The resulting acicular crystalline magnetic iron oxide particles are undesirable because their saturation magnetic flux density is greatly reduced due to a decrease in purity. The resulting acicular crystals α−
1.0 ~ when considering the axial ratio and long axis of FeOOH particles
3.0 at% is preferred. The obtained acicular α-FeOOH particles have an average long axis of 0.3 to 2.0 μm and an axial ratio (long axis: short axis) of 20:1.
That's all. If the long axis has an average value of 0.3 μm or less and 2.0 μm or more, it is not preferred as a starting material for magnetic recording. When the axial ratio (major axis: short axis) of the acicular α-FeOOH particles is 20:1 or less, the acicular α-
The acicular magnetic iron oxide particles obtained by reducing and oxidizing FeOOH particles cannot be said to have an excellent axial ratio because the particles shrink in each reduction and oxidation process. The axial ratio of the acicular α-FeOOH particles as a starting material for magnetic recording is preferably 20:1 or more. Acicular α-FeOOH obtained by the method of the present invention
The particles may be reduced and oxidized by well-known means, but it goes without saying that these must be carried out in a manner that does not destroy the particle form of the acicular α-FeOOH particles, particularly the acicular crystals. The present invention configured as described above has the following effects. That is, according to the present invention, by pre-existing a water-soluble magnesium salt in a suspension containing Fe(OH) ₂ before oxidizing by passing an oxygen-containing gas, it can be used in an alkaline region with a pH of 11 or higher. The axial ratio can be improved without causing the resulting acicular α-FeOOH particles to become extremely fine, with an average value of 0.3 to 2.0 μm for the long axis and an axial ratio (long axis: short axis) of 20:1 or more. It is possible to obtain acicular α-FeOOH particles having excellent acicular crystals, and the acicular α-FeOOH
By using particle powder as a starting material and thermally reducing and oxidizing it, it is possible to obtain excellent acicular crystal magnetite particles and acicular crystal maghemite particles. It can be used as a magnetic material for sensitive, high-density recording. Furthermore, when the above-mentioned acicular crystal magnetite particle powder or acicular crystal maghemite particle powder is used in the production of a magnetic coating material, the orientation and filling properties in the coating film are extremely excellent, and a magnetic recording medium is preferably obtained. be able to. Next, the present invention will be explained with reference to Examples and Comparative Examples. In addition, the axial ratio (long axis: short axis) and long axis of the particles in the above experimental examples, the following examples, and comparative examples are as follows:
All values are shown as average values of values measured from electron micrographs. <Production of needle-shaped α-FeOOH particle powder> Examples 1 to 8, Comparative Example 1; Example 1 49.7 g of magnesium sulfate (MgSO ₄ 7H ₂ O) so as to contain 1.0 atomic % in terms of Mg based on Fe Ferrous sulfate 1.00 mol/aqueous solution 20.0 obtained in the presence of
4.75-N prepared in advance in the reactor
In addition to NaOH aqueous solution 20.0, PH13.5, temperature 45℃
The production reaction of Fe(OH) ₂ was carried out at Acicular α-FeOOH particles were produced by passing 100 air per minute through the Fe(OH) ₂ -containing suspension for 16.5 hours at a temperature of 45°C. The end point of the oxidation reaction was determined by extracting a portion of the reaction solution and acidifying it with hydrochloric acid, and then using a red blood salt solution to determine the presence or absence of a blue coloring reaction of Fe ²⁺ . The generated particles are washed with water, separated, dried, and
Shattered. As a result of electron microscopic observation, the acicular α-FeOOH particles had an average long axis of 0.70 μm and an axial ratio (long axis:short axis) of 31:1, indicating that the particles were excellent in acicular crystals. Examples 2 to 8 Type of ferrous salt aqueous solution, concentration of NaOH aqueous solution,
Acicular α-FeOOH particles were produced in the same manner as in Example 1, except that the type, amount, and period of presence of the water-soluble magnesium salt were varied. Table 1 shows the main manufacturing conditions and characteristics at this time. As a result of electron microscopic observation, all of the acicular crystal α-FeOOH particles obtained in Examples 2 to 8 were found to have excellent acicular crystals. FIG. 3 shows an electron micrograph (×20,000) of the acicular α-FeOOH particles obtained in Example 2. Comparative Example 1 Acicular α-FeOOH particles were produced in the same manner as in Example 1 except for the absence of magnesium sulfate. Table 1 shows the main manufacturing conditions and characteristics at this time. FIG. 4 shows an electron micrograph (×20,000) of the obtained acicular α-FeOOH particles. As is clear from FIG. 4, the obtained acicular crystals α-
The FeOOH particles had an average long axis of 0.50 μm and an axial ratio (long axis:short axis) of 10:1, and had bad needle crystals. <Manufacture of acicular crystal magnetite particle powder> Examples 9 to 16, Comparative Example 2; Example 9 300 g of acicular crystal α-FeOOH particle powder obtained in Example 1 was put into a retort container with one end open in 3. Then, while driving and rotating, H ₂ gas was passed through at a rate of 3 per minute, and reduction was carried out at a reduction temperature of 400° C. to obtain 260 g of acicular magnetite particle powder. As a result of electron microscopy observation, the obtained acicular magnetite particles inherited the particle shape of the starting material acicular α-FeOOH particles, had an average value of 0.58 μm on the long axis, and an axial ratio (long axis: short axis). axis) was 23:1, with excellent needle-like crystals. In addition, as a result of magnetic measurement, the coercive force Hc is 473Oe,
The saturation magnetization σs was 84.1 emu/g. Examples 10 to 16, Comparative Example 2 Acicular magnetite particles were obtained in the same manner as in Example 9, except that the type of starting material and the reduction temperature were varied. Table 2 shows the main manufacturing conditions and characteristics of the particles at this time. The acicular crystal magnetite particles obtained in Examples 10 to 16 were all found to have excellent acicular crystals as a result of electron microscopic observation. An electron micrograph (×20,000) of the acicular magnetite particle powder obtained in Example 10 is shown in FIG. An electron micrograph (×20,000) of the acicular magnetite particle powder obtained in Comparative Example 2 is shown in FIG. As is clear from Figure 6, the average value of the acicular magnetite particles obtained in Comparative Example 2 is
It was 0.31 μm, the axis ratio (long axis: short axis) was 6:1, and the needle crystals were poor. <Production of needle-shaped maghemite particles> Examples 17 to 24, Comparative Example 3; Example 17 120 g of the needle-shaped magnetite particles obtained in Example 9 was oxidized in air at 270°C for 90 minutes to form needle-shaped particles. Crystalline maghemite particle powder was obtained. As a result of electron microscopic observation, the obtained acicular maghemite particles had an average long axis of 0.58 μm and an axial ratio (long axis:short axis) of 23:1, indicating that the acicular crystals were excellent. In addition, as a result of magnetic measurement, the coercive force Hc is 437Oe,
The saturation magnetization σs was 73.2 emu/g. Examples 18 to 24, Comparative Example 3: Acicular maghemite particles were obtained in the same manner as in Example 17, except that the type of acicular magnetite particles was varied. Table 3 shows various properties of this particulate powder. The acicular maghemite particles obtained in Examples 18 to 24 were all found to have excellent acicular crystals as a result of electron microscopic observation. An electron micrograph (×20,000) of the acicular maghemite particles obtained in Example 18 is shown in FIG. An electron micrograph (×20,000) of the acicular maghemite particles obtained in Comparative Example 3 is shown in FIG. As is clear from Fig. 8, the average value of the acicular maghemite particles obtained in Comparative Example 3 has a long axis.
It was 0.31 μm, the axis ratio (long axis: short axis) was 6:1, and the needle crystals were poor. <Manufacture of magnetic tape> Examples 25 to 40, Comparative Examples 4 and 5; Example 25 After blending the acicular magnetite particle powder obtained in Example 9 with the binder composition shown below, The mixture was mixed and dispersed in a ball mill for 8 hours to obtain a magnetic paint. Acicular magnetite particle powder 100g Vinyl resin (vinyl acetate: vinyl chloride =
3:91 copolymer) 20g Nitrile rubber (acrylonitrile copolymer)
100g Toluene 100g Methyl ethyl ketone 75g Methyl isobutyl ketone 75g Dispersant (lecithin) 0.2g Add a solvent (toluene: methyl ethyl ketone: methyl isobutyl ketone = 1:1:
Add 1) and adjust to the appropriate paint viscosity.
A magnetic tape was manufactured by coating and drying it on a polyester resin film using a conventional method. The coercive force Hc of this magnetic tape is 431Oe, and the residual magnetic flux density Br is
1420 Gauss, square Br/Bm was 0.86, and degree of orientation was 3.2. Examples 26 to 40, Comparative Examples 4 and 5: Magnetic tapes were produced in exactly the same manner as in Example 25, except that the type of acicular magnetic iron oxide particles was varied. Table 4 shows the characteristics of this magnetic tape.

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【table】 [Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the amount of water-soluble magnesium salt present and the axial ratio of acicular α-FeOOH particles.
Figure 2 shows the abundance of water-soluble magnesium salts and Figure 1
This figure shows the relationship between the long axis of acicular α-FeOOH particles produced under the same reaction conditions as in the case of . 3 to 8 are electron micrographs (×20000), and FIGS. 3 and 4 are acicular crystals α-obtained in Example 2 and Comparative Example 1, respectively.
It is FeOOH particle powder. 5 and 6 show acicular magnetite particles obtained in Example 10 and Comparative Example 2, respectively. 7 and 8 show acicular maghemite particles obtained in Example 18 and Comparative Example 3, respectively.

Claims (1)

[Scope of Claims] 1. A suspension containing Fe(OH) ₂ obtained by reacting a ferrous salt aqueous solution with an alkaline aqueous solution and having a pH of 11 or above is oxidized by passing an oxygen-containing gas through it to form a acicular shape. crystalline α-FeOOH particles are produced, and the acicular α-
In the method for producing acicular crystal magnetic iron oxide particles, FeOOH particles are washed with water, separated, dried, and then thermally reduced to produce acicular crystal magnetite particles, or further oxidized to produce acicular crystal maghemite particles. By pre-existing a water-soluble magnesium salt in the above suspension in an amount of 0.5 to 7.0 atomic % based on Fe in terms of Mg before aerating the contained gas and oxidizing it, the average value of the major axis is 0.3 to 2.0 μm, and the axis is 0.3 to 2.0 μm. Ratio (major axis:minor axis) 20:
producing acicular α-FeOOH particles having a particle size of 1 or more;
A needle crystal characterized in that the needle crystal α-FeOOH particles are washed with water, separated, dried, and then heated and reduced to produce needle crystal magnetite particles, or further oxidized to produce needle crystal maghemite particles. Method for producing magnetic iron oxide particle powder. 2. The method for producing acicular crystalline magnetic iron oxide particles according to claim 1, wherein the water-soluble magnesium salt present in the suspension is 1.0 to 3.0 atomic % based on Fe in terms of Mg.