JPH0415601B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention particularly relates to a method for producing acicular α-FeOOH for magnetic recording materials with improved particle size distribution. [Prior art and its problems] Maghemite (γ-Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), and bertholide compounds (FeOX, 1.33
<X<1.5), magnetic iron oxides modified with metal compounds such as cobalt, or acicular metals (α-
Magnetic powders such as Fe, ferroalloys) are usually produced by heat-treating hydrated iron oxide powders (α, β, γ-FeOOH) to dehydrate, reduce, or reduce and then oxidize. Magnetic powder produced by such a method often has uneven particle size distribution, which makes it difficult for the magnetic powder to be uniformly dispersed when, for example, kneading it with a resin binder to prepare a magnetic paint. It is considered extremely difficult to obtain a magnetic recording medium with a high filling degree and good magnetic properties. However, in conjunction with the recent increase in the density of magnetic recording media, there is a desire for magnetic powders with smaller particles, a sharper particle size distribution, and greater filling properties, and there is a strong demand for a solution to the above-mentioned problems. In order to solve the above problems, various manufacturing methods have been proposed for improving the particle size distribution of α-FeOOH, which is a precursor of the magnetic powder. For example, a method of adding phosphate ions, Zn ions, Sn ions, etc. during α-FeOOH nucleation crystal formation (Japanese Patent Publications No. 39-25546, No. 55-23217, No.
-155024), A method in which a certain amount of ammonium salt is pre-existing in an aqueous solution of ferrous salt, and then the solution is partially neutralized with ammonia to generate α-FeOOH nucleus crystals (Japanese Patent Publication No. 51-17518), α− under alkaline conditions
Examples include a method for producing FeOOH (Japanese Unexamined Patent Publication No. 53-127400). However, the α-FeOOH obtained by these methods is not necessarily satisfactory in terms of particle size distribution, particle shape, etc., and the magnetic properties of the magnetic powder produced using these methods are also insufficient, so further improvements are desired. ing. [Object of the Invention] An object of the present invention is to solve the problems of the prior art described above and to provide a method for producing α-FeOOH for magnetic recording materials, which has a sharp particle size distribution and excellent particle shape. [Summary of the Invention] As a result of various studies to achieve the above object, the present inventors partially neutralized and oxidized a ferrous salt aqueous solution while maintaining it at a specific temperature to generate α-FeOOH nucleus crystals. The resulting suspension is aged under specific conditions, and then the solution or the solution to which a ferrous salt aqueous solution is added is neutralized and oxidized to grow the nucleus crystals, resulting in a particle size distribution. It is possible to obtain α-FeOOH, which has a sharp shape, less branching, and a clear crystal shape, and the various magnetic properties of the magnetic powder produced using this material are also good, and all of the above problems can be solved. The present invention was completed based on this knowledge. That is, the present invention provides a suspension in which α-FeOOH nucleus crystals are produced by partially neutralizing and oxidizing an aqueous ferrous salt solution while maintaining the temperature at 40 to 70°C. The solution is aged for 30 minutes or more while being maintained at a pH of 2 to 6 at a temperature higher than the above temperature, and then the solution or a solution prepared by adding a ferrous salt aqueous solution is neutralized and oxidized to grow the nucleus crystals, and α −
This is a method for producing acicular α-FeOOH for magnetic recording materials, which is characterized by obtaining FeOOH. Ferrous salts used include ferrous salts of mineral acids such as ferrous sulfate, ferrous chloride, and ferrous nitrate, and ferrous carbonate.Industrially, ferrous sulfate is desirable. The alkali used for neutralizing ferrous salts includes hydroxides, oxides, or carbonates of alkali metals or alkaline earth metals, such as sodium hydroxide, potassium hydroxide, sodium oxide, and sodium carbonate. It will be done. Also, ammonia gas,
Ammonia aqueous solution, ammonium carbonate, etc. can also be used as the alkali. Industrially, sodium hydroxide, potassium hydroxide, and ammonia are preferred.
Examples of the oxidizing agent include air, oxygen, sodium peroxide, hydrogen peroxide, and sodium chlorate, but air is generally preferred. In the method of the present invention, first, an aqueous ferrous salt solution is partially neutralized and oxidized to remove part of the Fe content in the solution from α-
The concentration of the ferrous salt aqueous solution is usually 30 to 90 g, and the amount of alkali added is 5 to 50% of the Fe ion concentration in the mother liquor, which is 5 to 25 g in terms of FeOOH nucleation crystals. This is the amount necessary to precipitate /. If the concentration of the generated nuclei crystals is too low than the above range, the production efficiency will decrease, making it unsuitable for industrial (economic) implementation, while if it is too high, the viscosity of the nucleic crystal reaction solution will increase, which will prevent a uniform oxidation reaction. The particle size distribution is no longer sharp, which in turn leads to a decrease in the magnetic properties of the magnetic powder derived therefrom. The reaction temperature in this nucleation stage is usually 30-80℃
The temperature is preferably 40 to 70°C. If this temperature is too lower than the above range, the reaction time will be long and the particle size distribution will not be sharp. On the other hand, if it is too high, granular magnetite tends to be generated, which is not desirable as a nucleus crystal. It is best to complete this production reaction in as short a time as possible in order to sharpen the particle size distribution of the nucleus crystals, but it is preferably adjusted to about 20 to 180 minutes, for example.
The obtained nucleus crystals preferably have a BET specific surface area of about 40 to 140 m ² /g. The liquid after the above-mentioned nucleation reaction is α-
It is an aqueous solution of ferrous salt in which FeOOH nucleus crystals are suspended,
In the method of the present invention, this liquid is then subjected to aging treatment. The ripening method is to ripen the suspension as it is under predetermined conditions, or to filter the suspension and disperse the resulting nucleic crystals in water or fresh mother liquor and then to ripen the suspension under predetermined conditions. There is a method of ripening, but the effect of ripening can be obtained even if you use it. However, from an industrial point of view, it is preferable to subject the liquid after the nucleation reaction to an aging treatment as it is. The ripening temperature is 50°C or higher and higher than the nucleation temperature, preferably 60°C or higher and higher than the nucleation temperature. If this temperature is less than 50℃, the dissolution precipitation reaction of the nucleus crystals will be suppressed, and it will be difficult to produce particles with a sharp particle size distribution and less branching, which will require a longer ripening time, which is industrially disadvantageous. Become. The higher this temperature is, the more effective it is in improving particle size distribution and particle shape, and it is also possible to shorten the ripening time. Therefore, the effect can be obtained even by aging at 100℃ or higher using an autoclave, but considering the ease of industrial implementation and manufacturing cost, it is usually done at 50 to 95℃ under normal pressure.
A range of is desirable. Regarding aging time, at least
30 minutes is necessary, and although there is no particular upper limit, 10 hours or less is sufficient from an industrial perspective. The pH during ripening is usually between 2 and 6, preferably
kept between 2.5 and 5.5. If this PH is too lower than the above range, the nucleus crystals tend to aggregate, resulting in formation of aggregates or burr-like particles, which is not preferable. On the other hand, if the temperature is too high, the magnetite formation becomes stable, and
This is undesirable because magnetite is generated from the coexisting iron hydroxide and green rust, and fine nuclear crystals are transferred to magnetite. In addition, the pH at the time of ripening
It is important to keep the fluctuation range as small as possible to obtain α that has a sharp particle size distribution and excellent particle shape.
-Preferable in obtaining FeOOH. If air is drawn into the suspension due to stirring during ripening,
As the oxidation reaction progresses, the pH decreases, so in order to prevent air from entering as much as possible, it is recommended to slow down the stirring speed or replace the air in the reaction vessel with _N2 gas to keep the pH constant. From good. The α-FeOOH nucleus crystals subjected to the above-mentioned aging treatment are
In order to obtain α-FeOOH with the desired particle size,
Usually, it is subsequently subjected to a nucleation crystal growth reaction. This growth reaction is carried out by oxidizing the aged suspension or a solution obtained by adding an aqueous ferrous salt solution by adding an oxidizing agent such as air while adding an alkali. . The reaction temperature is usually 50°C or higher, preferably the same as or higher than the ripening temperature. If this temperature is less than 50°C, the growth rates of α-FeOOH in the three crystal axis directions become different, causing disorder in the crystal, which is not good in terms of particle size distribution and particle shape. The growth ratio is set to be 1 to 5, preferably 1.5 to 3 in terms of weight ratio to the nucleus crystals. If this magnification is too high than the above range, the width of the particle size distribution will become large and the particles will become more branched. In this step, it is preferable that the BET specific surface area of the generated α-FeOOH is, for example, 25 to 100 m ² /g. α-
After normal filtration, water washing, and drying, FeOOH is
- Obtained as FeOOH powder. This α-
FeOOH powder is dehydrated by heat treatment in the usual way.
By reduction or oxidation after reduction, maghemite (γ-Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ),
Bertolide compound (FeOx1.33<X<1.5),
It can be made into magnetic powder such as acicular metal (α-Fe, iron alloy). Among the above magnetic powders, magnetic iron oxides such as maghemite, magnetite, and bertholide compounds can be modified with a metal compound such as cobalt to form a cobalt-containing magnetic iron oxide. During the heat treatment of the above α-FeOOH powder,
In order to prevent particle sintering and improve the magnetic properties of the product, P, Si, Ti, Al, Mg, Ca, B,
Compounds such as Zn are used as heat-resistant agents. [Example] The present invention will be described below with reference to Examples. (1) Nucleic crystal formation reaction In a reaction vessel equipped with an air blowing tube and a stirrer.
Add 1.50 mol/FeSO ₄ aqueous solution 20 and heat at 40℃.
Alternatively, raise the temperature to 60℃ and maintain this temperature.
2.14 mol/ml of NaOH aqueous solution was added under stirring (precipitated Fe: 15 g/min), air was blown into the mixture at a rate of 10/min, and the mixture was reacted for 80 to 100 minutes to obtain α-FeOOH nucleus crystals. (2) Ripening of Nucleic Crystals Regarding the suspension obtained in the above-mentioned nuclear crystal production reaction, the liquid temperature and pH were promptly adjusted to predetermined values, and the suspension was aged for a predetermined time. Adjustment of PH is done with a trace amount of
This was done by adding an aqueous NaOH solution or an aqueous H ₂ SO ₄ solution to the suspension to maintain the PH within ±0.2 of the set value during aging. Note that the stirring speed was kept low in order to prevent as much as possible air from being drawn into the liquid by stirring during ripening. The specific surface area (SSA) of the α-FeOOH nuclei obtained in the above ripening process was measured by the BET method, and the particle size distribution (σL/
L) was measured and the results shown in Tables 1 to 3 were obtained. Method for measuring particle size distribution (σL/) Using a well-dispersed α-FeOOH nucleus crystal as a sample, read the major axis particle diameter of about 500 particles using an electron microscope, and calculate the arithmetic mean axial length (μ) and standard deviation σL. (μ) and obtain the particle size distribution (L distribution) according to the following formula. L distribution = σL/ The smaller the value of this L distribution, the sharper the particle size distribution, and this value was also used as an index of particle size distribution improvement.

【table】

【table】

【table】

[Table] (3) Nucleic crystal growth distribution The suspension after the aging process was adjusted to a predetermined temperature, and 5 mol/min of NaOH aqueous solution was added to the reaction solution while blowing air at a rate of approximately 10/min. Add gradually to keep the pH between 3.5 and 5.5.
The reaction was allowed to occur until the nucleus crystals grew to a predetermined ratio (based on weight). For the obtained α-FeOOH, the arithmetic mean axial length (μ), standard deviation σL (μ), particle size distribution (σL/), and specific surface area (SSA) were determined in the same manner as in the previous case and are shown in Table 4. . For each sample obtained in the above nucleic crystal growth reaction, after filtration and water washing, orthophosphoric acid was added to α-
After depositing 0.2% by weight (P equivalent amount) on FeOOH, dehydration (in air at 650℃ for 3 hours), reduction (in a hydrogen stream for 3 hours at 400℃), and reoxidation (in air at 300℃ for 1 hour) time) to obtain γ-Fe ₂ O ₃ . A blend of each γ-Fe ₂ O ₃ was prepared according to the blending ratio shown below, and the mixture was kneaded in a ball mill to produce a magnetic paint. (1) γ-Fe ₂ O ₃ powder 100 parts by weight (2) Soybean lecithin 1.6 〃 (3) Surfactant 4 〃 (4) Vinyl acetate-PVC copolymer resin 10.5 〃 (5) Dioctyl phthalate 4 〃 (6 ) Methyl ethyl ketone 84 〃 (7) Toluene 93 〃 Next, each magnetic coating material was applied to a polyester film by a conventional method, oriented and dried to produce a magnetic recording medium having a magnetic coating film about 7μ thick. For these magnetic recording bodies, coercive force (Hc), magnetic flux density (Br), squareness ratio (Br/Bm), orientation (OR), switching field distribution (SFD), 300Hz low frequency output (EL) and transcription (PT)
were measured and shown in Table 4.

【table】

【table】

〔Effect of the invention〕

By virtue of the above-described structure, the present invention achieves the following various excellent effects. That is, this aging treatment sharpens the particle size distribution of α-FeOOH nucleus crystals and makes the shape of the crystals clear. When the α-FeOOH nucleus crystals subjected to this aging treatment are grown under specified conditions, the generation of branched particles and fine particles is reduced, and α-FeOOH with a sharp particle size distribution and excellent particle shape can be obtained. I can do it. γ- obtained by dehydrating, reducing or reducing this α-FeOOH and then oxidizing it
Magnetic powders such as Fe ₂ O ₃ , Fe ₃ O ₄ , bertholide compounds, magnetic iron oxides modified with metal compounds such as cobalt, or acicular metals have good particle size distribution and particle shape. When a magnetic paint is prepared using this magnetic powder, it has good dispersibility, and it is possible to shorten the mixing and dispersion time with the paint resin and improve the magnetic substance content ratio of the magnetic paint film. The magnetic powder derived from this α-FeOOH has good filling properties, high magnetic flux density (Br), improved squareness ratio (Br/Bm), and improved orientation (OR), as well as coercive force (Hc), Switching field distribution (SFD),
Improvements can also be seen in low frequency output (EL) and transcription (PT).

Claims (1)

[Claims] 1 While maintaining a ferrous salt aqueous solution at a temperature of 40 to 70°C, partially neutralize and oxidize the suspension to generate α-FeOOH nucleus crystals at a temperature of 50°C or higher and higher than the nucleation temperature. Aging is carried out for 30 minutes or more while maintaining the pH at 2 to 6, and then the solution or a solution to which a ferrous salt aqueous solution is added is neutralized and oxidized to grow the nucleic crystals.
A method for producing acicular α-FeOOH for magnetic recording materials, characterized by obtaining FeOOH.