JPS6313938B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides acicular crystal α-
It relates to a method for producing FeOOH particle powder,
Average value of major axis 0.3 to 2.0 μm, axial ratio (major axis: short axis)
Acicular crystal α with excellent needle crystals of 20:1 or more
- Aim to obtain FeOOH particle powder. 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 Os. As is well known, the magnitude of the coercive force of magnetic particles depends on shape anisotropy, crystal anisotropy, strain anisotropy, 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. Acicular crystals α- obtained by this method
FeOOH particles are particles with a needle-like shape with a length of about 0.5 to 1.5μ, but the axial ratio (long axis: short axis) is about 10:1 at most, and particles with excellent needle crystals. It is difficult to say that it is. 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.
In a method for producing acicular α-FeOOH particle powder by passing an oxygen-containing gas through a suspension of 11 or more particles to oxidize it, the above suspension is preliminarily injected into the suspension before being oxidized by passing an oxygen-containing gas through the suspension. Water-soluble magnesium salt is 0.5 to 7.0 atomic% as Mg based on Fe.
After that, by aerating oxygen-containing gas and performing a needle-shaped α-FeOOH production reaction for at least 12 hours, the average value of the long axis is 0.3 to 2.0 μm, and the axial ratio (long axis: short axis) Acicular crystal α− with a ratio of 20:1 or more
Acicular crystals α− consisting of obtaining FeOOH particles
This is a method for producing FeOOH particle powder. 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, abundance, and period of presence in the reaction system of different metal ions other than Fe, which would improve the axial ratio without causing ultrafine particle size, we found that a ferrous salt aqueous solution and Contains Fe(OH) ₂ obtained by reacting with an aqueous alkaline solution
In a method for producing acicular α-FeOOH particle powder by passing an oxygen-containing gas through a suspension having a pH of 11 or higher for oxidation, the suspension is preliminarily injected into the suspension before being oxidized by passing an oxygen-containing gas through the suspension. In the case where a water-soluble magnesium salt is present in an amount of 0.5 to 7.0 atomic % based on Fe in terms of Mg, and then an oxygen-containing gas is aerated to carry out a reaction for producing needle-shaped α-FeOOH for at least 12 hours. , the axial ratio can be improved without causing ultra-fineness of the acicular α-FeOOH particles, and the average value of the long axis is 0.3 to 2.0 μm, and the axial ratio (long axis: short axis) is 20:1 or more. Needle crystal α−FeOOH
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 nuclear particle generation and the acicular α-FeOOH
The process consists of two steps: the growth of nuclear particles, and this method involves pre-existing a water-soluble magnesium salt in a suspension containing Fe(OH) ₂ before oxidizing it by passing in an oxygen-containing gas. According to the method of the invention, magnesium ions form needle-shaped α-
At the stage of FeOOH core particle generation, acicular α-FeOOH core particles with excellent axial ratio are generated, and at the growth stage of the acicular α-FeOOH core particles, growth in the short axis direction of the particles is suppressed. and promotes growth in the long axis direction of the particles, resulting in acicular crystals α- with an excellent axial ratio.
FeOOH particles can be obtained. Although the theoretical elucidation of the effect of water-soluble magnesium salts on this phenomenon has not yet been fully elucidated, the present inventors have demonstrated that magnesium ions are absorbed into the short axis of the acicular α-FeOOH core particles during their growth stage. The reason why magnesium ions have the action and effect of suppressing growth in the long axis direction of the particle and promoting growth in the long axis direction of the particle is that magnesium ions have the effect of suppressing growth in the long axis direction of the particle and promoting growth in the long axis direction of the particle. We believe that one of the reasons is that it easily sticks to surfaces. In order to exhibit the above-mentioned functions and effects of magnesium ions, it is important to slowly carry out the reaction for producing acicular α-FeOOH particles over a period of 12 hours or more, as shown in the Examples below. In other words, by slowly performing the reaction to generate acicular α-FeOOH particles, magnesium ions are selectively adsorbed to the plane parallel to the long axis of the acicular α-FeOOH particles, and the acicular α-FeOOH particles are This promotes growth in the longitudinal direction. Conventionally, in the production of α-FeOOH particles, there is a method described on page 108 of the collection of lecture 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, and α-
FeOOH particles are generated, 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. has an average value of 1.0 μm, but when 2% sodium hexametaphosphate is added, the average value becomes 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. In addition, water-soluble calcium salts, which are also alkaline earth metals, are similar to water-soluble magnesium salts.
In the present invention, even if water-soluble calcium salt is used instead of water-soluble magnesium salt, needle-shaped α-
Promotes the growth of FeOOH particles in the long axis direction and increases the axial ratio.
The effect of increasing the ratio to 20:1 or higher cannot be obtained. Next, from the numerous experimental examples conducted by the present inventor,
Some of them can 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;
The relationship between the axial ratio of the acicular α-FeOOH particles and the amount of magnesium sulfate obtained by oxidizing the suspension by passing 100 air per minute at a temperature of 45°C for 12 hours or more was shown. It is something. 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, when the amount of water-soluble magnesium salt is up to 2 atomic % based on Fe (calculated as Mg), as the amount of water-soluble magnesium salt increases, the long axis of the acicular α-FeOOH particle changes. Shows a tendency to grow in the direction. 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 considered to be due to the increase in the amount of water-soluble magnesium salt present, which caused magnesium ions to be adsorbed also 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 (X20000) of acicular α-FeOOH particles obtained by the method of the present invention (Example 2 described later). 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 . Incidentally, even if the water-soluble magnesium salt is present at a stage when some acicular 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. Considering the axial ratio and long axis of the resulting acicular α-FeOOH particles, the content is preferably 1.0 to 3.0 at %. The reaction time for producing acicular α-FeOOH particles in the present invention is 12 hours or more. If it is 12 hours or less, the axial ratio (long axis: short axis) targeted by the present invention
Acicular α-FeOOH particles with a ratio of 20:1 or more cannot be obtained. 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. The present invention configured as described above has the following effects. That is, according to the present invention, a water-soluble magnesium salt is made to exist in a suspension containing Fe(OH) 2 before oxidation by passing an oxygen-containing gas through the solution, and then a water-soluble magnesium salt is made to exist in the suspension containing Fe(OH) ₂ before being oxidized by passing an oxygen-containing gas through the suspension. Acicular crystals α obtained in an alkaline region with a pH of 11 or higher by carrying out a needle-shaped α-FeOOH production reaction for at least 12 hours or more.
-The axial ratio can be improved without causing ultra-fineness of FeOOH particles, and the average value of major axis is 0.3~
It is possible to obtain acicular α-FeOOH particles having excellent acicular crystals with a diameter of 2.0 μm and an axial ratio (long axis: short axis) of 20:1 or more. Acicular crystal magnetite particles and acicular crystal maghemite particles obtained by thermally reducing and oxidizing the thus obtained acicular α-FeOOH particle powder as a starting material also have excellent acicular crystals. Therefore, it can be used as a magnetic material for high-output, high-sensitivity, and high-density recording, which are currently most required. In addition, when the above-mentioned acicular magnetite particles or acicular maghemite particles are used in the production of a magnetic coating material, the orientation and filling properties in the coating film are extremely excellent, and a desirable magnetic recording medium can be 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. Example 1 Ferrous sulfate 1.00 mol/20.0 aqueous solution obtained by making 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,
4.75-N prepared in advance in the reactor
In addition to NaOH aqueous solution 20.0, Fe(OH) ₂ production reaction was performed at pH 13.5 and temperature 45°C. 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 the presence or absence of blue coloring reaction of Fe ²⁺ using a red blood salt bath after a portion of the reaction solution was extracted and acidified with hydrochloric acid. 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 time 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. An electron micrograph (X20000) of the acicular α-FeOOH particles obtained in Example 2 is shown in FIG. 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. An electron micrograph (X20000) of the obtained acicular α-FeOOH particles is shown in FIG. 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-like crystals. 【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 and 4 are electron micrographs (X20000), and FIG. 3 is the acicular α-FeOOH particle powder obtained in Example 2, and FIG. 4 is Comparative Example 1.
This is the acicular α-FeOOH particle powder obtained in .

Claims (1)

[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 higher is oxidized by passing an oxygen-containing gas through it to form a acicular shape. In the method for producing crystalline α-FeOOH particles, a water-soluble magnesium salt is added to the suspension in advance before oxidation by passing oxygen-containing gas through the suspension.
The average value of major axis is 0.3 to 2.0 by making the presence of Fe 0.5 to 7.0 at% in terms of Mg, and then aerating oxygen-containing gas to carry out a reaction to produce needle-shaped α-FeOOH for at least 12 hours. 1. A method for producing acicular α-FeOOH particles, the method comprising obtaining acicular α-FeOOH particles having an axial ratio (long axis: short axis) of 20:1 or more. 2. Acicular crystal α- 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.
Production method of FeOOH particle powder.