JPH0446899B2 - - Google Patents

Description

Detailed Description of the Invention The present invention provides α-
The present invention relates to a method for producing iron oxyhydroxide, and particularly to a method for producing α-iron oxyhydroxide, which is suitable as a raw material for magnetic materials.

In recent years, there has been a strong need for improved performance in recording media, and high-density recording, high output characteristics, and lower noise levels are required. The characteristics of a magnetic material that satisfies these requirements include fine powder properties, large surface area, and excellent acicularity, and magnetic properties such as large saturation magnetization and the desired optimum properties. It has a coercive force.

In the case of a ferromagnetic iron compound, since it is necessary to have excellent needle-like properties, it is common to use iron oxyhydroxide, which tends to form needle-like crystals, as a starting material. Therefore, many methods for producing iron oxyhydroxide have been reported, and among them, a method in which acicular α-iron oxyhydroxide is produced by wet oxidation of ferrous salt in an alkaline region has been actually adopted.

There are many particle properties required for α-iron oxyhydroxide, such as no branching, no aggregates, narrow particle size distribution, appropriate length and large axial ratio. The α-iron oxyhydroxide crystallization step is considered to be important, and many efforts have been made for this purpose, such as adding various elements to expect a moderation effect and specifying crystallization conditions.

For example, as a modifier, JP-A-56-165302 discloses that an alkaline earth metal is present in an alkaline suspension of ferrous hydroxide having a pH of 11 or higher.

In addition, as for the reaction conditions, α-iron oxyhydroxide is produced at 40°C or less according to JP-A No. 56-22638, and 40
A two-step reaction method in which crystals are grown by raising the temperature to ~60°C is disclosed, but as can be seen from the photos in the example, α-iron oxyhydroxide is formed in which individual particles are completely dispersed. Not yet.

Japanese Patent Publication No. 54-7280 discloses the use of phosphate ions, but the intended product is different and non-acicular magnetite is produced.

JP-A-55-149138 discloses adding citrate and phosphate, but the pH of the reaction solution
is required to be maintained between 5.5 and 7.5, which is different from the PH range of the present invention.

Many other reports have been made, but in all of them, the dispersibility of α-iron oxyhydroxide is insufficient;
Instead of particles being dispersed one by one, the particles become agglomerated particles that aggregate several or more, which adversely affects the subsequent surface treatment, firing, oxidation, and reduction processes, and the properties of the ferromagnetic iron compound cannot be fully demonstrated. Not yet.
The requirements for high-performance magnetic materials currently in demand are becoming even more stringent, and complete dispersion of particles, especially dispersion of iron oxyhydroxide as a starting material, is an essential condition.

The present inventors have repeatedly conducted research on a method for producing α-iron oxyhydroxide, which is a starting material for a ferromagnetic iron compound having a high surface area, but have found that the dispersibility of α-iron oxyhydroxide can be improved. For example, branching becomes noticeable, making it quite difficult to produce acicular particles with good dispersibility and no branching.

Therefore, as we continued to study the reaction method and conditions in detail and deepened our crystallization technology, we decided to strictly limit the reaction conditions. It was discovered that α-iron oxyhydroxide exhibits a specific effect, and it became possible to produce α-iron oxyhydroxide with good dispersibility.

Furthermore, by increasing the alkali hydroxide concentration in the second stage of the two-stage oxidation reaction compared to the first stage, it becomes possible to increase the acicularity, that is, to increase the axial ratio, without impairing the dispersibility. I was able to complete it.

The crystal modifier used in the present invention, calcium cation, is combined with chlorine anion, and is effective only under specific conditions. The specific conditions here include the concentration of ferrous chloride, the concentration of calcium salt, the concentration of alkali hydroxide, the order of addition of raw material aqueous solutions, the reaction temperature, etc., and it is necessary to strictly control the reaction. If even one of these conditions is lacking, α-iron oxyhydroxide with good dispersibility, which is the object of the present invention, cannot be obtained.

An unexpected effect was obtained by combining both calcium and chloride ions, but the reason for this effect is not clear.

In addition, it was common practice to adjust the alkali hydroxide concentration only at the start of the reaction, but by changing the alkali hydroxide concentration during the reaction, it is possible to improve not only the dispersibility but also the acicularity. This makes it possible to produce α-iron oxyhydroxide suitable as a magnetic material.

That is, the present invention provides a method for producing α-iron oxyhydroxide by an oxidation reaction of an aqueous ferrous salt solution, which includes preparing an aqueous solution consisting of ferrous chloride, an alkali hydroxide, and a calcium salt as an aqueous ferrous salt solution. However, for the preparation, an aqueous ferrous chloride solution is added to an aqueous alkali hydroxide solution with an amount of 6 to 16 moles relative to iron at a liquid temperature of 25°C or less, and an iron concentration of 0.03 to 0.3 mol/min is added.
In addition, calcium salt is added in an amount of 0.5 to 2 mol% relative to iron in the ferrous salt aqueous solution.
Then, after supplying an oxidizing gas so that the oxidation rate is 50% or less, the temperature is raised in an inert gas atmosphere to 40 to 80 %.
℃, and after adjusting the concentration of the alkali hydroxide aqueous solution to be 12 to 30 times the molar amount of iron, the oxidizing gas is supplied again to complete the oxidation reaction. A method for producing iron oxide is provided.

According to the method of the present invention, the particles are dispersed one by one without agglomeration or sticking in the direction parallel to the long axis, which tends to occur in the case of acicular particles, and α-oxy water with excellent acicularity Iron oxide is obtained, which allows for uniform surface treatment in subsequent steps, resulting in a high-performance magnetic material.

Further, the present invention will be explained in detail.

The ferrous salt used in the present invention is ferrous chloride, and other mineral salts such as sulfates, carbonates, and nitrates which are commonly used cannot be used. The present invention is based on the expectation of a mediocrystal effect of anions during crystallization, and the combination of chloride ions and calcium ions is an essential condition, and other anions cannot provide a sufficient mediocrystal effect. Therefore, the crystallizing agent to be added is also limited to calcium salts. As the calcium salt, various salts such as chloride, nitrate, carbonate, and sulfate can be used. Since it contains less calcium salt than ferrous salt, various salts can be used, but chlorides and nitrates are preferably used. Examples of methods for adding calcium salts include adding calcium salts to an aqueous ferrous chloride solution, adding calcium salts to an aqueous alkali hydroxide solution, and adding calcium salts to an aqueous ferrous chloride solution before supplying the oxidizing gas. Examples include a method in which a calcium salt is added to each of the ferrous chloride aqueous solution and an alkali hydroxide aqueous solution, and a method in which a calcium salt is added after mixing the ferrous chloride aqueous solution and the alkali hydroxide aqueous solution. This is done when preparing an aqueous iron solution, and the amount added is 0.5 to 2 mol% relative to iron. Even if the calcium salt is added during the oxidation reaction using an oxidizing gas, at the completion of the oxidation reaction, or in a subsequent step, α-iron oxyhydroxide without agglomeration, which is the object of the present invention, cannot be obtained.

If the amount added is less than 0.5 mol %, the medicinal effect of the calcium salt will not be sufficient and aggregates of several α-iron oxyhydroxides will be present. Moreover, if the amount exceeds 2 mol%, magnetite tends to precipitate, which is not preferable.

The alkali used in the present invention is
Use an alkali hydroxide such as KOH or NaOH.
Aqueous solutions of alkali carbonate NH ₃ such as Na ₂ CO ₃ , urea, etc. cannot be used because they cancel the medicinal effect of chlorine ions.

The amount of alkali hydroxide used is 6 to 16 times the amount of iron. If the amount used is less than 6 times the mole amount, magnetite will be easily formed and the acicularity of α-oxyiron hydroxide will deteriorate, and if the amount used is more than 16 times the amount, α-oxy Aggregates of iron hydroxide are present, and the desired α-iron oxyhydroxide cannot be obtained.

The neutralization reaction of a ferrous salt aqueous solution with an alkali hydroxide is a ferrous chloride aqueous solution containing calcium salt.
This is done by adding it to an aqueous alkali hydroxide solution kept below 25°C while stirring.

If the temperature at the time of addition exceeds 25°C, or if an aqueous alkali hydroxide solution is added to an aqueous ferrous chloride solution, the crystals of ferrous hydroxide produced by the neutralization reaction will become larger or agglomerate. The α-iron oxyhydroxide obtained tends to have a wide particle size distribution and to be branched.

Furthermore, the iron concentration in the reaction solution after neutralization reaction is 0.03~
Adjust the concentrations of the ferrous chloride aqueous solution and the alkali hydroxide aqueous solution so that the concentration is 0.3 mol/.

When the iron concentration is less than 0.03 mol/mol, magnetite is likely to be generated. In addition, if it exceeds 0.3 mol/ml, the slurry concentration in the liquid will increase, the viscosity will increase, and it will be difficult to carry out the reaction uniformly.
- Aggregates of iron oxyhydroxide become present.

Furthermore, replacing the inside of the reaction system and the raw material aqueous solution with an inert gas is a desirable method in terms of preventing oxidation of ferrous ions during the neutralization reaction.

The oxidizing gas used in the present invention is a gas containing oxygen, such as air or oxygen-enriched gas,
Alternatively, a gas obtained by mixing oxygen and an inert gas is used.

In the present invention, when α-iron oxyhydroxide is produced by an oxidation reaction using an oxidizing gas, the oxidation reaction is clearly divided into two stages. In the first stage, oxidation is carried out at a temperature of 25° C. or less to an oxidation rate of 50% or less, preferably in the range of 5 to 50%. The oxidation rate is expressed as [(Fe ₃ /total Fe) x 100] and represents the oxidized proportion of ferrous iron.

When the first stage reaction temperature exceeds 25°C, the crystal growth rate of iron oxyhydroxide becomes faster, and the precipitated α-iron oxyhydroxide grows irregularly, resulting in a broader particle size distribution and α-oxyhydroxide. Iron aggregates become present. Since the lower limit of the reaction temperature is related to the supply rate of the oxidizing gas, it is necessary to set a temperature suitable for the reaction tank and gas blowing method, but the lower limit is generally 5°C.

If the oxidation rate is less than 5%, new nuclei will be generated in the second stage of oxidation, resulting in a wide particle size distribution, which is not preferable. On the other hand, if it exceeds 50%, the α-iron oxyhydroxide is too fine and the acicular ratio is also poor, which is not preferable.

In the present invention, it is also preferable to add a phosphoric acid compound in addition to the calcium salt until the oxidation rate is 30%. As the phosphoric acid compound, phosphoric acid or an inorganic phosphate such as sodium phosphate is preferred. The phosphoric acid compound can be added in various ways, such as into an aqueous ferrous salt solution, into an aqueous alkali hydroxide solution, into an alkaline suspension of ferrous hydroxide after a neutralization reaction, or after the start of an oxidation reaction. There is a method, but it is essential to add it when the oxidation rate is below 30%. If the addition time is at a stage where the oxidation rate exceeds 30%, branching of the α-iron oxyhydroxide produced cannot be prevented.

The amount of the phosphoric acid compound used is 1 to 80 mol%, preferably 5 to 30 mol%, based on iron.

If the amount of the phosphoric acid compound used is less than 1 mol %, the effect will be insufficient, and if it exceeds 80 mol %, crystal growth will be strongly suppressed and α-iron oxyhydroxide particles will become too small, which is not preferable.

Addition of a phosphoric acid compound has the effect of further reducing branching without impairing the effect of improving dispersibility and branching due to the combination of calcium and chloride ions.

After the first stage oxidation reaction, the oxidizing gas is switched to an inert gas and the temperature is raised to 40-80°C.

The reason why the reaction temperature in the second stage was set at 40 to 80°C was to allow the iron oxyhydroxide generated in the first stage to grow and to achieve the desired acicularity and particle length.

Furthermore, in the second stage, it is important to adjust the alkali hydroxide concentration to 12 to 30 times the molar amount of iron.

By increasing the alkali hydroxide concentration, α-iron oxyhydroxide with excellent acicular properties can be produced without agglomeration. The reason why acicularity improves when the alkali concentration is increased is unknown, but since the solubility of ferrous hydroxide and α-iron oxyhydroxide increases when the alkali concentration is increased, they are necessary for crystal growth. It is speculated that stable supersaturation is likely to be maintained.

If the second stage oxidation reaction is carried out at the same alkali hydroxide concentration as the first stage, the particles will have good dispersibility but will only have needle-like properties with an axial ratio of about 20.

Furthermore, if the oxidation reaction is carried out at a high alkali hydroxide concentration from the first stage, aggregation or sticking in the direction parallel to the long axis tends to occur, making it impossible to obtain α-iron oxyhydroxide with good dispersibility.

After setting the reaction temperature and alkali hydroxide concentration,
The oxidizing gas is supplied again to complete the oxidation reaction.

The α-iron oxyhydroxide obtained in this way is particles with no agglomeration and excellent acicular properties, which are processed by conventional methods to be oxidized and reduced to become a high-performance magnetic recording material. I can do it.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 A NaOH aqueous solution 10 with a concentration of 1.6 mol/contains 0.002 mol/CaCl ₂ while being stirred at 20°C.
Add 0.2 mol/FeCl ₂ aqueous solution 10,
A suspension mainly consisting of (OH) ₂ was obtained.

Air was added to this suspension at a rate of 4/min at 20°C.
Bubbling for 10 minutes oxidized some of the ferrous iron to ferric iron. Oxidation rate at this time [Fe ³⁺ / (Fe ²⁺ + Fe ³⁺ ) ×
100] was 8%.

Thereafter, the air was changed to nitrogen gas, and the system was made into an inert gas atmosphere, and the temperature of the suspension was set at 50°C. At that time, NaOH was added to increase the NaOH concentration to 18 times the molar amount of Fe.

Next, the nitrogen gas was replaced with air, which was blown in at a rate of 1/min to complete the reaction and obtain α-FeOOH.

The obtained α-FeOOH was observed using a transmission electron microscope, and the crystal shape, aggregation state, length, and axial ratio were measured. α-FeOOH was rod-shaped with an average length of about 0.4μ and an axial ratio (major axis/minor axis) of about 30, and was a dispersed particle with almost no agglomeration.

Example 2 1.6 mol/H ₃ PO ₄ added at 5 mol % based on Fe
NaOH aqueous solution of 10 to 20 °C with stirring
0.2mol/ _FeCl2 including 0.002mol/ _CaCl2
10 of the aqueous solution was added to obtain a suspension containing Fe(OH) ₂ .

Air was blown into this suspension under the same conditions as in Example 1 to obtain α-FeOOH.

The obtained α-FeOOH was rod-shaped with an average length of about 0.35μ and an axial ratio of about 40, and was dispersed particles without agglomeration. In addition, there was even less branching than in Example 1.

Comparative Example 1 In the method of Example 1, the conditions were the same as in Example 1, except that the oxidation reaction was carried out with the NaOH concentration unchanged at the initial concentration. α-FeOOH
I got it.

The obtained α-FeOOH was in the form of well-dispersed particles with an average length of about 0.3μ and an axial ratio (major axis/minor axis) of about 15.

Comparative Example 2 In the method of Example 1, instead of CaCl ₂
α-FeOOH was obtained using MgCl ₂ and other conditions being the same as in Example 1.

The obtained α-FeOOH has many branches,
Moreover, fine particles were present.

Claims (1)

[Claims] 1. A method for producing α-ferrous oxyhydroxide by an oxidation reaction of an aqueous ferrous salt solution, wherein the aqueous ferrous salt solution is an aqueous solution consisting of ferrous chloride, an alkali hydroxide, and a calcium salt. During the preparation, a ferrous chloride aqueous solution is added to an alkali hydroxide aqueous solution with an amount of 6 to 16 moles relative to iron at a liquid temperature of 25°C or below, and an iron concentration of 0.03 to 0.3 mol/min is added.
In addition, calcium salt is added in an amount of 0.5 to 2 mol% relative to iron in the ferrous salt aqueous solution.
Then, after supplying an oxidizing gas so that the oxidation rate is 50% or less, the temperature is raised in an inert gas atmosphere to 40 to 80 %.
℃, and after adjusting the concentration of the alkali hydroxide aqueous solution to be 12 to 30 times the molar amount of iron, the oxidizing gas is supplied again to complete the oxidation reaction. Method of producing iron oxide.