JPH0341403B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing acicular particles containing iron carbide.

(Prior Art) Generally, in order to increase the recording density of a recording medium using a magnetic material for magnetic recording, it is better for the magnetic material to have a higher coercive force. However, magnetic materials having various coercive forces are required depending on the capabilities of the head of the playback device.

Acicular particles containing iron carbide have high coercive force, electrical conductivity, and high hardness, and are excellent magnetic powders for magnetic recording media. Such acicular particles containing iron carbide can be produced by heating, for example, acicular iron oxyhydroxide particles or acicular iron oxide particles at 250 to 400°C using a reducing carbonizing agent.
produced by contacting with This acicular particle containing iron carbide is the main component of iron carbide.
It is known to contain Fe ₅ C ₂ and also Fe ₃ O ₄ (magnetite) and free carbon. However, since the acicular particles containing iron carbide have an extremely high coercive force, their use may be limited depending on the capacity of the head. Therefore, there has been a demand for the development of acicular particles containing iron carbide having a relatively low coercive force.

Furthermore, magnetic powder for magnetic recording media is required to have a large amount of magnetization in order to increase output.

There are two methods to control the coercive force of acicular particles containing iron carbide: (1) stopping the carbonization reaction midway through, and (2) increasing the temperature of the carbonization reaction. However, in the case of (1), a large amount of Fe ₃ O ₄ is present, and therefore a suitable magnetic recording material with a small amount of magnetization cannot be obtained. In the case of (2), a large amount of carbon is precipitated as a side reaction due to the decomposition of the reducing carbonizing agent, so there is a problem that only acicular particles with a low amount of magnetization can be obtained as a whole.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for producing acicular particles containing iron carbide having a relatively low coercive force and a large amount of magnetization.

(Means for Solving the Problems) The present invention involves heating an aqueous suspension of iron hydroxide (2) in the presence of a water-soluble compound having a coordinating ability to iron under alkaline conditions to form acicular α- Iron oxide () containing iron carbide, characterized in that it is brought into contact with a carbon-containing reducing agent or a mixture thereof with a carbon-free reducing agent, with or without contacting with a carbon-free reducing agent. This relates to a method for producing acicular particles.

The iron hydroxide () used in the present invention may be produced by any method, but generally it is an iron () salt, such as iron chloride (), iron sulfate (),
Add an alkali such as sodium hydroxide, potassium hydroxide, or ammonia to an aqueous solution of iron nitrate (),
This gives iron hydroxide () as an amorphous precipitate. Such methods are already well known. Depending on the reaction conditions, the resulting precipitate may still contain the constituent anions of the iron (2) salt as its constituent atoms; however, such a precipitate may also be used as the iron (2) hydroxide in the present invention. Can be done.

The concentration of iron hydroxide () in the aqueous suspension of iron hydroxide () used is such that the suspension is mixed with a water-soluble compound having coordinating ability for iron and preferably with seed crystals, as will be explained later. When stirring in the presence of
It is sufficient as long as it is not difficult to stir, usually 1.5 mol/or less in terms of iron, preferably 0.1 to 1
mole/.

The water-soluble compound having the ability to coordinate iron used in the present invention acts as a crystallization control agent,
A water-soluble product that controls the growth direction and speed of α-iron oxide crystals (α-iron oxide) to form needle-shaped crystals when an aqueous suspension of iron hydroxide (2) is heated through its coordination ability. selected from organic or inorganic compounds. Such a compound has at least one coordinating group in its molecule containing an atom capable of coordinating iron, such as an oxygen, nitrogen and/or sulfur atom. Specific examples of such coordination groups include -OH, -
COOH, -O-, >C=C, -SO ₃ H, -PO ₃ H ₂ , -
NH ₂ ,=N−OH, 〓N, −SH, −S−, >C=S,
-CS ₂ H, -COSH, -OCN, etc. can be mentioned. Preferably, in the present invention, the water-soluble compound capable of coordinating iron has two or more of the above-mentioned coordinating groups of the same type or different types in the molecule.

Specific examples of water-soluble compounds having the ability to coordinate with iron that are preferably used in the present invention include polycarboxylic acids such as succinic acid, maleic acid, and nitrilotriacetic acid, particularly di- and tricarboxylic acids, citric acid, tartaric acid, and glycolic acid. , hydroxycarboxylic acids such as malic acid, α-methylmalic acid, α-hydroxyglutaric acid, dihydroxyglutaric acid, and salicylic acid, aminocarboxylic acids such as lysine and glycine, polyamines such as ethylenediamine, hydroxylamine, aminotri(methylenephosphonic acid) ,
Ethylene diaminotetra (methylene phosphonic acid),
Organic phosphonic acids such as ethylene-1,1'-diphosphonic acid and 1-hydroxyethylene-1,1'-diethylenephosphonic acid, thiocarboxylic acids such as cysteine and mercaptoacetic acid, polyhydric alcohols such as mannitol and pentaerythritol, acetylacetone,
β-dicarbonyl compounds such as ethyl acetoacetate,
Aromatic sulfonic acids such as sulfophenyliminodiacetic acid and the like can be mentioned. Further, these water-soluble salts and esters can also be used in the present invention as long as they have the ability to coordinate with iron. For example, sodium citrate, sodium tartrate, 1
Examples include sodium -hydroxypropyl-1,1'-diphosphonate, triethyl citrate, dimethyl hydroxysuccinate, and ethyl mercaptoacetate. Further, phosphates can also be used in the present invention. Specific examples include sodium phosphate, potassium phosphate, ammonium phosphate, and the like.

In the present invention, various compounds can be used as described above, but particularly preferred are the aforementioned aliphatic hydroxycarboxylic acids, the following organic phosphonic acids, and their salts and esters.
That is, the general formula (However, n represents an integer of 2 to 6, and m represents an integer of 0 or 1 to 5.) Organic phosphonic acid, its salt, or its ester, general formula (However, X and Y are hydrogen, hydroxyl group, amino group,
It represents an alkyl group or an aryl group, and q represents an integer of 1 to 6. ), an organic phosphonic acid, its salt or its ester, and the general formula (However, R represents hydrogen or an alkyl group.) An organic phosphonic acid, a salt thereof, or an ester thereof.

Specific examples of the organic phosphonic acid represented by the above formula ( ), Tetraethylenepentaaminohepta (methylenephosphonic acid), Pentaethylenehexaaminoocta (methylenephosphonic acid)
etc. can be mentioned. In the organic phosphonic acid represented by the above formula (), the alkyl group preferably has 1 to 6 carbon atoms, and the aryl group preferably has 6 to 14 carbon atoms. Specific examples include methylene diphosphonic acid, ethylene-1,1'-diphosphonic acid, ethylene-1,2-diphosphonic acid, propylene-1,
1'-diphosphonic acid, propylene-1,3-diphosphonic acid, hexamethylene-1,6-diphosphonic acid, 2,6-dihydroxypentamethylene-2,
4-diphosphonic acid, 2,5-dihydroxyhexamethylene-2,5-diphosphonic acid, 2,3-dihydroxybutylene-2,3-diphosphonic acid, 1-
Hydroxybenzyl-1,1'-diphosphonic acid, 1
-aminoethylene-1,1'-diphosphonic acid and the like.

In the above formula (), the alkyl group preferably has 1 to 5 carbon atoms, and specific examples of such organic phosphonic acids include hydroxymethylene diphosphonic acid,
1-hydroxyethylene-1,1'-diphosphonic acid, 1-hydroxypropylene-1,1'-diphosphonic acid, 1-hydroxybutylene-1,1'-diphosphonic acid, 1-hydroxyhexamethylene-1,
Examples include 1'-diphosphonic acid.

The amount of the water-soluble compound having the ability to coordinate with iron is not particularly limited as long as it is sufficient to control the direction and speed of crystal growth of α-iron oxide () in the hydrothermal reaction, but usually, The amount is 1×10 ⁻⁵ to 3 mol, preferably 1×10 ⁻⁴ to 1×10 ⁻¹ mol per gram atom of iron atom of iron hydroxide (). in general,
If the amount of the water-soluble compound having the ability to coordinate with iron is too small, it is difficult to obtain α-iron oxide with excellent needle-like properties, whereas if it is too large, the reaction takes a long time, which is not preferable.

When an aqueous suspension of iron hydroxide (2) is heated under alkaline conditions in the presence of a water-soluble compound capable of coordinating iron as described above, it forms acicular α-Fe ₂ O with virtually no pores. ₃ is obtained, but during this hydrothermal reaction, in addition to water-soluble compounds that have coordination ability for iron, α
By further coexisting with −Fc ₂ O ₃ seed crystals, acicular α-Fe ₂ O ₃ having a narrower particle size distribution can be obtained.

The shape of the particles of this seed crystal α-Fe ₂ O ₃ is not particularly limited and may be arbitrary, such as acicular, spherical, cubic, etc., but the average diameter of the shortest axis is required to be 0.4 μm or less. , preferably 0.2 μm or less. When the average diameter of the shortest axis of the seed iron oxide exceeds 0.4 μm, the resulting acicular α-iron oxide has a small acicular ratio defined as the ratio of the long axis to the short axis, and/or Average particle size is too large. The lower limit of the shortest axis average diameter of the seed crystal α-iron oxide is not particularly limited, but usually,
It is about 100 Å. α-Iron oxide having the shortest axis average diameter within such a range can be obtained as a commercial product or by a known production method.

Further, the amount of seed crystals used is 0.1 mol% to 25 mol%, preferably 0.5 to 15 mol% in terms of iron, based on the raw material iron hydroxide (). When this usage amount is less than 0.1%, the obtained acicular α-iron oxide particles are too large, while when it is more than 25 mol%, the obtained acicular α-iron oxide particles have a small acicular ratio. ,as well as/
Or the particles are too small.

In the present invention, it is sufficient that the water-soluble compound (and seed crystal) having the ability to coordinate iron is present during the heat treatment of the iron hydroxide () aqueous suspension, and the order of addition thereof is arbitrary. . Therefore, for example, when precipitating iron () hydroxide from an iron () salt aqueous solution, a water-soluble compound and/or seed crystals having the ability to coordinate with iron are added to the iron () salt aqueous solution in advance. Good too. At this time, the water-soluble compound that has the ability to coordinate with iron will coordinate with the iron atom in iron hydroxide () and will be included in the precipitate.
It is not necessary to newly add it during the heat treatment of the iron hydroxide () suspension. However, it is usually preferable to add seed crystals and a water-soluble compound capable of coordinating to iron to the aqueous suspension of iron hydroxide.

The heating reaction is carried out at a pH of 7 or more, preferably in the range of 8 to 12.5. The alkali used is not particularly limited, but it is usually sodium hydroxide, potassium hydroxide, ammonia, etc., and iron hydroxide () is used before or after adding a water-soluble compound having coordinating ability to iron or a seed crystal. Added to an aqueous suspension.

In the present invention, the reaction temperature is generally preferably 100°C or higher. In reactions at temperatures lower than 100℃,
Formation of cross-shaped or T-shaped branched particles and α-FeOOH particles with excellent acicularity was observed, making it difficult to obtain α-Fe ₂ O ₃ with excellent acicularity. By performing the reaction at a temperature higher than 100℃, branched particles and α
Although the formation of -FeOOH is eliminated, the upper limit of the reaction temperature is a temperature that does not cause thermal decomposition of the water-soluble compound having the ability to coordinate with iron used. Although pressure may be applied during the reaction, it is usually not necessary to apply pressure intentionally, and the reaction mixture may simply be heated and stirred in a closed container. In this case, the reaction temperature is usually about 100 to 250°C, preferably about 130 to 200°C. The reaction time is not particularly limited, but is usually several tens of minutes to several hours.

Acicular α-iron oxide is obtained in this way, but its shape and dimensions depend on the heating temperature during the above reaction, as well as the type and amount of water-soluble compounds that have coordination ability for iron, and the seed crystals. It can also be controlled within a desired range by selecting the dimensions, amounts, etc.

In the present invention, this acicular α-Fe ₂ O ₃ is then brought into contact with a carbon-containing reducing agent or a mixture thereof with a carbon-free reducing agent without or without contacting with a carbon-free reducing agent. The desired acicular particles containing iron carbide are manufactured by this method.

Examples of the reducing agent that does not contain carbon include hydrogen, and the contact is preferably carried out at about 200 to 700°C.

As the reducing agent containing carbon (hereinafter referred to as reducing carbonizing agent), at least one of the following compounds can be used.

CO Aliphatic, linear or cyclic, saturated or unsaturated hydrocarbons, such as methane, propane, butane, cyclohexane, methylcyclohexane, acetylene, ethylene, propylene, butadiene, isoprene, town gas, etc.

Aromatic hydrocarbons, such as benzene, toluene, xylene, and their alkyl and alkenyl derivatives with a boiling point of 150°C or less.

Aliphatic alcohols such as methanol, ethanol, propanol, cyclohexanol.

Esters, such as methyl formate, ethyl acetate, etc., with a boiling point of 150°C or less.

Ethers, such as lower alkyl ethers and vinyl ethers, with a boiling point of 150°C or less.

Aldehydes, such as formaldehyde, acetaldehyde, etc., with a boiling point of 150°C or less.

Ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, etc., with a boiling point of 150 or less.

Particularly preferred reducing carbonizing agents are CO, CH ₃ OH,
HCOOCH ₃ is a saturated or unsaturated aliphatic hydrocarbon having 1 to 5 carbon atoms.

In the present invention, the reducing carbonizing agent can be used diluted or undiluted, and examples of the diluent include N ₂ , argon, helium, and the like. Further, the dilution ratio can be selected arbitrarily, but it is preferable to dilute to more than 1 and up to 10 times (volume ratio). Contact conditions such as contact temperature, contact time, and flow rate are determined based on, for example, the manufacturing history of acicular iron oxide,
Since it varies depending on the average axial ratio, average particle diameter, specific surface area, etc., it is best to select it appropriately. Preferred contact temperature is about 250-400°C, more preferably about 300-400°C
℃, the preferred contact time is about 0.5 to 6 hours.
The preferred flow rate is about 1 to 1000 ml STP/min, preferably about 5 to 500 ml STP/min, excluding diluent, per gram of raw acicular iron oxide. The contact pressure, including the diluent, is usually 1 to 2 atmospheres, but is not particularly limited.

When observed with an electron microscope, the particles obtained in the present invention are averagely uniform acicular particles, and have the same shape as the acicular particles of the raw material. are doing. Further, it is clear that the obtained acicular particles contain carbon according to elemental analysis, and further contain iron carbide according to the X-ray diffraction pattern. The X-ray diffraction pattern shows interplanar spacings of 2.28, 2.20, 2.08, 2.05 and 1.92 Å. Such a pattern corresponds to Fe ₅ C ₂ , and the main component of the iron carbide of the present invention consists of Fe ₅ C ₂ .

The acicular particles obtained according to the present invention often also contain iron oxide, mainly Fe ₃ O ₄ .

Further, the acicular particles of the present invention often contain free carbon, but the content must be 20% by weight or less. If it exceeds 20% by weight, the amount of magnetization decreases, which is inappropriate.

Further, the average axial ratio and average particle diameter of the obtained acicular particles are slightly smaller than those of the raw material acicular particles, but there is almost no difference. Therefore, the average axis ratio of the acicular particles obtained by this manufacturing method is usually 3 or more, preferably 3 to 20, and the average particle diameter (long axis) is usually 3 or more, preferably 3 to 20.
2 μm or less, preferably 0.1 to 2 μm, optimally 0.1 to 2 μm
It is 1.0 μm.

As is clear from the above characteristics, the iron carbide-containing acicular particles of the present invention can be used as a magnetic material for magnetic recording, but are not limited thereto. It can also be used as a catalyst for synthesis from _H2 .

(Effects of the Invention) The iron carbide-containing acicular particles obtained by the present invention are characterized by a relatively low coercive force and a large amount of magnetization. That is, the acicular particles of the present invention generally have a relatively low coercive force of about 600 to 750 Oe, and a large saturation magnetization of about 85 to 95 emu/g.

(Example) Examples and comparative examples will be given below to explain in detail. Incidentally, the magnetic properties were determined by the following method.

Magnetic properties: Determine by the following method unless otherwise specified. The magnetization properties are measured using a BHH-50 DC magnetization property automatic recorder manufactured by Riken Denshi Co., Ltd. with a maximum magnetic field of 2500 Oe.

Example 1 Iron chloride () aqueous solution (concentration 0.2 mol/) 10
After dropping concentrated ammonia water into the solution with vigorous stirring until the pH reached 8, the precipitate was separated and washed with water to obtain iron hydroxide particles. This was dispersed in water to obtain slurry 2. 13 g of sodium citrate was added to this, and sodium hydroxide was added until the pH reached 12. The mixture thus obtained is placed in a closed container.
The mixture was stirred at 150°C for 3 hours. Then, separate the precipitate,
It was washed with water and dried to obtain a reddish-orange powder. The obtained powder was confirmed to be α-Fe ₂ O ₃ by X-ray diffraction, and the average particle size (long axis) was determined by electron microscopy.
was 0.3 μm, and the average axial ratio (major axis/minor axis) was 4.

Next, 100 g of this α-Fe ₂ O ₃ powder was dispersed in 2 ml of water, and 6 g of a silane coupling agent was added and adsorbed, and then separated and dried.

2 g of the obtained powder was placed in a porcelain boat, and this was placed in a tube furnace at 340°C and heated with CO at a flow rate of 200 ml/min for 5 minutes.
Time passed. The obtained powder has a coercive force of 725 Oe,
The saturation magnetization amount was 92 emu/g.

Comparative Example 1 Example 1 α-Fe ₂ O ₃ obtained by heating and dehydrating α-iron oxyhydroxide powder according to the conventional method
The powder obtained by contacting with CO in the same manner as
The coercive force was 950 Oe and the saturation magnetization was 75 emu/g.

Claims (1)

[Claims] 1. Heating an aqueous suspension of iron hydroxide () under alkaline conditions in the presence of a water-soluble compound capable of coordinating iron to form acicular α-iron oxide (); A method for producing acicular particles containing iron carbide, which comprises contacting with a carbon-containing reducing agent or a mixture thereof with a carbon-free reducing agent, with or without contacting with a carbon-free reducing agent. 2. The method according to claim 1, wherein the acicular particles have a coercive force of about 600 to 750 Oe and a saturation magnetization of about 85 to 95 emu/g.