JPS6334608B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improved method for producing acicular α-FeOOH. The product obtained by this method is acicular α-FeOOH with a sharp particle size distribution, less branching, and a good axial ratio.
γ-Fe ₂ O ₃ derived from this α-FeOOH, that is, γ-Fe ₂ O ₃ obtained by dehydrating, reducing, and oxidizing this α-FeOOH by a conventional method, has good magnetic properties, such as high retention. It has good magnetic force, saturation magnetization, and squareness ratio, and magnetic tapes manufactured using it have good coercive force, squareness ratio, residual magnetic flux density, orientation, reversal magnetic field distribution, electromagnetic conversion characteristics, etc. One well-known method for producing α-FeOOH is to neutralize a portion of a ferrous salt solution with an alkali and then oxidize it. At that time, attempts have also been made to include phosphate ions. ing. For example, Japanese Patent Publication No. 39-25546 describes the precipitation of ferrous hydroxide and its oxidation in the presence of phosphate ions.
℃ to generate nucleated crystals, and add metallic iron and an oxidizing agent to this nucleic crystal suspension to grow the nucleated particles until they reach 6 to 30 times their total weight, producing slightly stubby particles. It is described that short columnar α-FeOOH can be obtained, and Japanese Patent Publication No. 55-3295 describes the use of phosphoric acid or its water-soluble salt, α-FeOOH nucleus crystals, basic coagulants, or proton acceptors. It is described that α-FeOOH is obtained by oxidizing a ferrous salt solution in the presence of ferrous salts. However, the α-
FeOOH has a wide particle size distribution and is difficult to obtain with good shape, so γ-Fe ₂ O ₃ derived from FeOOH and magnetic tapes made from it do not necessarily have satisfactory magnetic properties. Not a kimono. The method of the present invention is different from the prior art described above. (1) The nucleation reaction is carried out in the presence of a certain phosphate ion, by adding an alkali in an amount of Fe5 to 20 g/precipitate, and at a certain reaction temperature and reaction time. , (2) Nucleus crystal growth reaction is
Keep the PH of the reaction solution between 3 and 6 to prevent the growth of nuclei from 1.5 to 3.5.
They are different in that they have twice the control. The obtained α-FeOOH is acicular particles with a narrow particle size distribution, little branching, and a high axial ratio, and the γ-Fe ₂ O ₃ derived from this has good magnetic properties. In the present invention, a ferrous salt aqueous solution is partially neutralized with an alkali to precipitate a part of the Fe content, and then oxidized to form α-
FeOOH nucleus crystals are generated, and then the liquid is neutralized with alkali and oxidized to grow the nucleus crystals and form acicular α-
A method for producing FeOOH, wherein the nucleation reaction involves precipitating 5 to 20 g of Fe as an alkali in the presence of 0.05 to 0.6% by weight of phosphoric acid or phosphate salt in terms of P based on the amount of generated nucleic crystals. using an amount of alkali metal or alkaline earth metal hydroxide, oxide or carbonate, reaction temperature 50-70℃, reaction time 10
The nucleus crystal growth reaction is carried out for ~100 minutes, and the reaction solution is
This is a method for producing acicular α-FeOOH for magnetic recording materials, which is characterized in that the pH is maintained at 3 to 6, and the process is carried out so that the nucleus crystals grow 1.5 to 3.5 times in terms of weight ratio. The ferrous salt solutions used include ferrous sulfate,
Examples include ferrous salt solutions of mineral acids such as ferrous nitrate and ferrous chloride, and ferrous sulfate is preferred industrially. Examples of hydroxides, oxides or carbonates of alkali metals or alkaline earth metals include sodium hydroxide, potassium hydroxide, sodium oxide, calcium carbonate, etc. Industrially, sodium hydroxide, potassium hydroxide is preferred. Phosphoric acids or phosphates include phosphoric acids such as orthophosphoric acid, metaphosphoric acid, polyphosphoric acid, and water-soluble salts of these with alkali metals, ammonium, etc.
Orthophosphoric acid or its salts are usually used. Examples of the oxidizing agent include air, oxygen, and other oxidizing agents, but air is generally preferred. In the method of the present invention, first, a ferrous salt solution is partially neutralized with an alkali and oxidized to convert a part of the Fe content in the solution into α-FeOOH nucleus crystals. An acid or phosphate is present. The concentration of ferrous salt solution is usually 30g/~100g/,
The amount of phosphoric acid added is 0.05 to 0.6% by weight, preferably 0.1 to 0.4% by weight in terms of P, based on the α-FeOOH nucleus precipitate produced. If the amount of P is too small than the above range, the addition effect will not be exhibited and the particle size distribution will become wide or branching will occur, while if it is too large, the axial ratio of the acicular particles will decrease and they will become short columnar. The amount of alkali added is approximately 5% Fe ion in the mother liquor.
The amount necessary to precipitate ~25g/preferably 10-15g/. If the concentration of the generated nuclei crystals is too low than the above range, the production efficiency will decrease and it will become unsuitable for industrial (economic) implementation, and α-FeOOH with an unfavorable burr-chestnut shape will be produced, and on the other hand, if it is too high. This increases the viscosity of the mother liquor, which prevents a uniform oxidation reaction and causes the particle size distribution to become unsharp, which in turn leads to a decrease in the magnetic properties of the γ-Fe ₂ O ₃ derived therefrom. In this nucleation stage, it is also important to keep the reaction temperature at 50°C or higher. Since the reaction is usually carried out in an aqueous system, the temperature is 50 to 100°C, preferably 55 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. However, it is known that granular magnetite is likely to be produced at reaction temperatures of 50°C or higher, but in this method, as mentioned above, there is no fear of magnetite being produced because phosphate ions are present. , high purity α-FeOOH can be obtained.
PH is usually kept between 3 and 8. In this nucleation, the neutralization precipitation rate is preferably 70% or less in order to prevent γ-FeOOH from being mixed. Further, this production reaction is preferably completed in as short a time as possible, for example, adjusted to about 10 to 80 minutes. The obtained nuclear crystal is
It is desirable that the BET specific surface area is about 50 to 70 m ² /g. The liquid after the above-mentioned nucleation reaction is α-
It is a ferrous salt solution in which FeOOH nucleus crystals are suspended, and the added phosphoric acid or phosphate is incorporated into the nucleus crystals or strongly adsorbed to the surface of the nuclei crystals, and in the free state, it becomes liquid. It is normal that no residue remains inside. In the method of the present invention, this solution is then oxidized as it is, that is, under conditions in which free phosphoric acid or phosphate is substantially absent, while adding an alkali to grow nucleus crystals and obtain the desired α- FeOOH
get. In this case, the presence of very small amounts of free phosphoric acid is acceptable, but the amount is
It is limited to 0.045% by weight or less in terms of P based on the total amount of FeOOH. If the amount of P exceeds the above range, the axial ratio will not be large and the desired acicular α-FeOOH will not be obtained. This reaction is also preferably carried out at a temperature of 50° C. or higher, as in the case of nucleation crystal formation, and oxidation is carried out while adding the same alkali as above so as to maintain the pH at a substantially constant value of 3 to 6. The growth rate of the nucleus crystals is desirably adjusted to about 5 to 15 g/hour in order to narrow the width of the particle size distribution of the product and obtain acicular particles with less branching. In addition, α-FeOOH nuclei can be generated by adjusting the concentration of the mother liquor and the amount of nuclei crystals produced in advance, by replenishing ferrous salt after nucleation crystal formation, and then carrying out the growth reaction, or by appropriately aborting the growth reaction. The growth rate based on the weight of the nucleus crystal is 1.5 to 3.5, preferably 2.
~3. If this magnification is too low than the above range, the axial ratio will not be large and a good needle-like α
-FeOOH cannot be obtained; on the other hand, if it is too high, the particle size distribution width becomes large and the particles become branched. In this process, the generated α-FeOOH
It is preferable that the BET specific surface area is, for example, 35 to 45 m ² /g. α-FeOOH obtained by the method of the present invention is
It has a sharp particle size distribution, less branching, and a good axial ratio, and furthermore, the γ-Fe ₂ O ₃ induced by this and the magnetic tape made from it have good magnetic properties. be. α-FeOOH is obtained as α-FeOOH powder through conventional filtering, water washing, drying and pulverization.
From this α-FeOOH powder, γ-
Fe ₂ O ₃ can be obtained. i.e. first 300
Dehydration in air at a temperature of ~700°C, then reduction of this dehydrate with hydrogen or hydrogen containing water vapor at a temperature of 300-500°C to obtain Fe ₂ O ₄ , and further reduction of this Fe ₂ O ₄ to 200 ~ γ-Fe ₂ O ₃ can be obtained by oxidation with oxygen or air at a temperature of 400°C. As an effective means to further enhance the magnetic properties of γ-Fe ₂ O ₃ , for example, α-FeOOH obtained by the method of the present invention may be treated with phosphoric acid, phosphate, silicate, etc. before dehydration treatment. One example is the processing of The present invention will be explained below with reference to Examples and Comparative Examples. Examples 1-2 and Comparative Examples 1-2 In a reactor equipped with an air blowing pipe and a stirrer,
Pour aqueous solution 20 containing 3800g of FeSO ₄ and raise the temperature to 60℃, and while maintaining this temperature, add a predetermined amount of orthophosphoric acid and NaOH aqueous solution (concentration 200g/).
2.15 was added under stirring (precipitated Fe 15 g/hour), and air was blown into the mixture at a rate of 100 to 60 minutes per hour.
The reaction was carried out at 60°C for 70 minutes to obtain α-FeOOH nucleus crystals. Furthermore, while blowing air at a rate of 120 g/hour, a NaOH aqueous solution 4.2 with a concentration of 200 g/hour was gradually added to maintain the pH between 3.5 and 5.5, and the reaction was carried out at 60°C for 72 hours to grow the nucleus crystals approximately three times as much. Ta. The axial ratio (L/w) of the α-FeOOH obtained in the above reaction was measured by the usual method, and the particle size distribution (σL/) and branching incidence were also measured by the following method. The results were obtained. Method for measuring particle size distribution (σL/) Using well-dispersed α-FeOOH as a sample, read the major axis particle diameter of more than 3000 particles using an electron microscope, and calculate the arithmetic mean axial length (μ) and standard deviation σL (μ ) and calculate the particle size distribution according to the formula below. Particle size distribution = σL / Method for measuring branching Using the same method as for measuring particle size distribution, determine the number nd of branched crystals in the particles and the total number n, and calculate the branching rate according to the formula below. seek. Branching rate (%) = nd/n×100

[Table] Examples 3 to 4 and Comparative Example 3 In a reactor equipped with an air blowing pipe and a stirrer, 5165
Add 20 g of an aqueous solution containing FeSO ₄ and raise the temperature to 60°C, and while maintaining this temperature, generate α-FeOOH.
Add 0.2% by weight of orthophosphoric acid (calculated as P based on the amount of nuclear crystals) and a predetermined amount of NaOH aqueous solution (concentration 200g/) with stirring, and heat at 60℃ for 70 minutes while blowing air.
After reacting for ~100 minutes, α-FeOOH nucleus crystals were obtained.
Furthermore, while blowing air approximately 600/hour,
An aqueous NaOH solution was gradually added to the reaction solution to maintain a pH of 3.5 to 5.5, and the reaction was allowed to occur until the nucleus crystals grew three times as much. The particle size distribution (σL/
L) and branching were measured and the results in Table 2 below were obtained.

[Table] Example 5 and Comparative Example 4 α-
FeOOH was obtained, and the particle size distribution (σL/) and branching were measured in the same manner as in Example 1, and the results shown in Table 3 below were obtained.

[Table] Example 6 and Comparative Example 5 α-FeOOH was obtained in the same manner as in Example 1 except that free orthophosphoric acid was present in the mother liquor during the growth of α-FeOOH nucleus crystals. . The axial ratio (L/
W) was measured, and the particle size distribution (σL/) and branching were measured in the same manner as in Example 1, and the results shown in Table 4 below were obtained.

[Table] Examples 7 and 8 and Comparative Examples 6 and 7 In a reactor equipped with an air blowing pipe and a stirrer, 3800
Add 20 g of an aqueous solution containing 20 g of FeSO ₄ and raise the temperature to 60°C. While maintaining this temperature, add orthophosphoric acid (in terms of P to the α-FeOOH nucleus precipitate formed).
0.3% by weight) and NaOH aqueous solution (200g/) 2.14
was added under stirring, air was blown at a rate of 60°C/hour, and the reaction was carried out at 60°C for 70 minutes to obtain α-FeOOH nucleus crystals (15g/hour in terms of Fe). While blowing air at a speed of 120/hour, the specified amount of
Gradually add NaOH aqueous solution (200g/) to 80°C.
The reaction was carried out for 4.5 hours. For α-FeOOH obtained in the above reaction, the axial ratio (L/W) was measured by a normal method, and the particle size distribution (σL/) was determined in the same manner as in Example 1.
and branching were measured and the results shown in Table 5 below were obtained.

[Table] For each sample obtained in the above examples and comparative examples, orthophosphoric acid was added after filtering and washing with water.
After depositing 0.2% by weight (P equivalent amount) on FeOOH, dehydration (in air at 650℃), reduction (in hydrogen containing water vapor at 420℃), and reoxidation (in air at 280℃) are carried out using the usual methods. γ-Fe ₂ O ₃ was obtained. For each γ-Fe ₂ O ₃ , the coercive force (Hc) was measured by a normal method, and for each γ-Fe ₂ O ₃ , a mixture was prepared according to the following blending ratio,
A magnetic paint was produced by kneading in a ball mill. (1) γ-Fe ₂ O ₃ powder 100 parts by weight (2) Soy lecithin 1.6 〃 (3) Surfactant 4 〃 (4) Vinegar vinyl chloride 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 using a conventional method, oriented and dried, thereby producing a magnetic recording medium having a magnetic coating film with a thickness of about 7 μm. The coercive force (Hc), saturation magnetization (Bm), squareness ratio (Br/Bm), and switching field distribution (SFD) of these magnetic recording bodies were measured by conventional methods, and the results shown in Table 6 were obtained.

【table】

Claims (1)

[Claims] 1 Partially neutralize the ferrous salt aqueous solution with an alkali.
After precipitating a part of the Fe content, it is oxidized to form α-FeOOH.
Nucleic crystals are generated, and then the solution is neutralized with an alkali and oxidized to grow the nuclear crystals and form acicular α-FeOOH.
A method for producing, the nucleation reaction comprising:
As an alkali in the presence of 0.05 to 0.6% by weight of phosphoric acid or phosphate salt in terms of P based on the amount of generated nucleus crystals.
Using an amount of alkali metal or alkaline earth metal hydroxide, oxide or carbonate to precipitate Fe5~20g/, reaction temperature 50~70℃, reaction time 10~100℃.
The nucleic crystal growth reaction was carried out for 3 minutes, and the PH of the reaction solution was 3 minutes.
6. A method for producing acicular α-FeOOH for magnetic recording materials, characterized in that the process is carried out so that the nucleus crystals grow by 1.5 to 3.5 times in terms of weight ratio.