Process for preparing iron carbide fine particles

Abstract: The present invention provides a process for preparing iron carbide fine particles having an average particle size (long axis) of 0.1 to 2 µ m and an average axial ratio of 3 to 20, which comprises, (a) optionally contacting an iron compound selected from iron oxyhydroxide fine particles and iron oxide fine particles with a reducing agent which does not contain a carbon atom, the iron compound being, prior to said contact, coated with an iron compound or an iron compound and a cobalt compound, and with an aluminium compound or a silicon compound, and (b) contacting the iron compound of (a) with a reducing-and-carburizing agent containing a carbon atom or a mixture thereof with a reducing agent which does not contain a carbon atom.

Description: The present invention relates to a process for preparing iron carbide fine particles. It is known that acicular particles containing iron carbide are prepared by contacting acicular iron oxyhydroxide or acicular iron oxide with CO or a mixture of CO and H₂ at 250 to 400 °C and that the resulting particles are stable chemically, have a high coercive force and are useful as a magnetic material for magnetic recording media. (For example, refer to JP-B-1-43683, JP-A-60-108309, JP-B-1-35772 and JP-A-60-155522. The term "JP-B" as used herein means an "examined published Japanese patent application" and "JP-A" as used herein means an "unexamined published Japanese patent application".) Since iron carbide fine particles are generally used as a coating-type magnetic recording medium, they are required to have characteristics as a coating material like a pigment. However, iron carbide fine particles would be sintered or deformed during reduction or carburizing step. Therefore, the starting particles are coated with a silicon compound or aluminum compound and then reduced and carburized. Although the particles are prevented from sintering or deformation, a large amount of carbon deposits on the resulting particles to cause deterioration of the magnetic characteristics. JP-A-1-192713 (USSN 302636 & EP-A-0326165) discloses a process for preparing iron carbide fine particles which are less in deterioration of the magnetic characteristics. In this process, iron carbide fine particles are obtained by contacting at 250 to 400 °C an iron compound selected from the group consisting of iron oxyhydroxide fine particles and iron oxide fine particles with a reducing-and-carburizing agent containing carbon atom such as CO, the iron compound being, prior to the above contact, coated with nickel compound, copper compound or manganese compound and then coated with silicon compound or aluminum compound. With this process are obtained fine particles which are improved in coercive force and saturation magnetization. However, it is still desired to obtain fine particles having more improved magnetic characteristics. Among coercive force and saturation magnetization, since the former has to be adjusted to the capacity of a magnetic head and is restricted to the existing magnetic head, an improvement of the latter is especially desired. The larger the saturation magnetization, the greater is the magnetic recording capacity without restricted by the capacity of the magnetic head and will achieve high recording density and power. An object of the invention is to provide a process for preparing iron carbide fine particles which are small in deposition of free carbon on the resulting iron carbide particles and high in saturation magnetization. The above and other objects of the invention will become apparent from the following description. The present invention provides a process for preparing iron carbide fine particles having an average particle size (long axis) of 0.1 to 2 µ m and an average axial ratio of 3 to 20, which comprises, (a) coating an iron compound selected from iron oxyhydroxide fine particles and iron oxide fine particles, said fine particles having an average particle size of up to 2 µm and an average axial ratio of at least 3, with (1) an iron compound or an iron compound and a cobalt compound and (2) with an aluminium compound or a silicon compound and, optionally, reducing said coated iron compound with a reducing agent which does not contain carbon atoms, and (b) contacting the iron compound of (a) with a reducing-and-carburizing agent containing carbon atoms or a mixture thereof with a reducing agent which does not contain carbon atoms. It is presumed that the aluminum compound or silicon compound, which is added to stabilizing the shape of the starting iron compound, acts as a catalyst for depositing carbon in the reduction with CO. In the present invention, it is found that the iron compound or cobalt compound is effective to prevent the above deposition of carbon. Examples of preferred iron oxyhydroxides are α - FeOOH (goethite), β -FeOOH (akaganite) and γ -FeOOH (lepidocrosite). Examples of preferred iron oxides are α - Fe₂O₃ (hematite) γ -Fe₂O₃ (maghemite) and Fe₃O₄ (magnetite). α -Fe₂O₃ or γ -Fe₂O₃ is, for example, one prepared by heating α -FeOOH, β -FeOOH or γ -FeOOH at about 200 to about 350 °C, followed by dehydrating it, α -Fe₂O₃ or γ -Fe₂O₃ prepared by heating the resulting product further at about 350 to about 900 °C to compact the crystals, and others. β -FeOOH is preferably treated with an aqueous solution of alkaline compound. The above-mentioned Fe₃O₄ can be prepared by contacting an iron oxide other than Fe₃O₄ or iron oxyhydroxide with a reducing-and-carburizing agent containing carbon atom or a reducing agent which does not contain carbon atom, or a mixture thereof, although the Fe₃O₄ is not limited to one prepared in those methods. When a reducing-and-carburizing agent containing carbon atom or a mixture thereof with a reducing agent which does not contain carbon atom, is contacted with an iron oxyhydroxide or iron oxide other than Fe₃O₄ to prepare Fe₃O₄, the same contact conditions as those in the process (b) of the invention can also be employed except for the time condition. In this case, the Fe₃O₄ formed can be subsequently held in contact with the gas under the same conditions as in the preceding reaction without any interruption to obtain the desired particles of the invention. The starting iron oxyhydroxides or iron oxides can be those at least 3, preferably 3 to 20, in average axial ratio and having an average particle size (long axis) of up to 2 µ m, preferably 0.1 to 2 µ m, most preferably 0.1 to 1.0 µ m. As will be described later, the particles produced are slightly smaller than, but almost unchanged from, the starting material in average axial ratio and in average particle size, so that the particles of the invention in general preferably have such sizes. The iron oxyhydroxide or iron oxide to be used as a starting material in the process for producing particles of the invention may contain a small amount or small amounts of a compound, such as oxide or carbonate of copper, magnesium, manganese or nickel; silicon oxide, potassium salt, sodium salt, etc., which are added when preparing the iron oxyhydroxide or iron oxide, insofar as the starting material chiefly comprises an iron oxyhydroxide or iron oxide. The starting iron oxyhydroxide preferably has a pH of at least 5 on the surface thereof as disclosed in JP-A-60-108309. In this case are produced particles having a higher coercive force. The starting iron oxyhydroxide having a pH of less than 5 can be used after it is increased to at least 5 in pH by contacting with an aqueous solution of an alkali compound (eg, sodium hydroxide, potassium hydroxide, ammonium hydroxide). Further, alkali-treated iron oxide can also be used. The starting material can be treated with an alkaline compound by contacting the material with an aqueous solution of sodium hydroxide, potassium hydroxide, ammonium hydroxide and like alkaline compound (e.g., with an aqueous solution having a pH of at least 8, preferably at least 10), stirring for 30 minutes to 1 hour when required, filtering the mixture and drying. In the present invention, the surface pH is defined as a value which is obtained by boiling 5g of a sample in 100cc of distilled water for one hour, cooling the solution to room temperature, allowed to stand for one hour and measuring a pH of the resulting supernatant with a pH meter. In the present invention, the particles of the above iron oxyhydroxide or iron oxide (hereinafter both compounds may be referred to as "starting iron compound") is coated with iron compound or iron compound and cobalt compound, and with aluminum compound or silicon compound. Examples of iron compounds are iron chloride, iron nitrate and iron sulfate. Cobalt compounds include cobalt chloride, cobalt nitrate and cobalt sulfate. Further, examples of silicon compounds are sodium orthosilicate, sodium metasilicate, potassium metasilicate and water glass, which includes various compositions. Aluminum compounds include aluminum sulfate, aluminum nitrate, aluminum chloride, various aluminum alum, sodium aluminate and potassium aluminate. Each of those metal compounds can be coated on the surface of the starting iron compound, for example, by adding an aqueous solution of the above compound to a dispersion of the starting iron compound and then neutralizing the mixture. The amount for coating of each of the metal compound is 0.01 to 10 % by weight in terms of the metal element based on the weight of the iron oxyhydroxide or iron oxide in terms of Fe₂O₃. Then the obtained starting iron compound fine particles are subjected to a contact reaction. In the invention, typical examples of the reducing agent which does not contain carbon atom are H₂, NH₂NH₂, etc. As the reducing-and-carburizing agent containing carbon atom, at least one of the following compounds can be used. 1 ○ CO 2 ○ aliphatic, linear or cyclic, saturated or unsaturated hydrocarbons such as methane, propane, butane, cyclohexane, methylcyclohexane, acetylene, ethylene, propylene, butadiene, isoprene, town gas, etc. 3 ○ aromatic hydrocarbons such as benzene, toluene, xylene, alkylated or alkenylated derivatives thereof having a boiling point up to 150 °C. 4 ○ aliphatic alcohols such as methanol, ethanol, propanol, cyclohexanol, etc. 5 ○ esters such as methyl formate, ethyl acetate and like ester having a boiling point up to 150 °C 6 ○ ethers such as lower alkyl ether, vinyl ether and like ether having a boiling point up to 150 °C 7 ○ aldehydes such as formaldehyde, acetaldehyde and like aldehyde having a boiling point up to 150 °C 8 ○ ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and like ketone having a boiling point up to 150 °C Particularly preferable reducing-and-carburizing agent containing carbon atom are CO, CH₃ OH, HCOOCH₃, saturated or unsaturated aliphatic hydrocarbons having 1 to 5 carbon atoms. In the process (a) of the invention, the reducing agent which does not contain carbon atom can be used as it is or as diluted. Examples of diluents are N₂, CO₂, argon, helium, etc. The dilution ratio is suitably selected but is preferably up to about 10 times (by volume). The contacting temperature, contacting time, gas flow rate and other conditions depend, for example, on the production history, average axial ratio, average particle size and specific surface area of the starting iron compound. The preferred contacting temperature is about 200 to about 700 °C, preferably about 300 to about 400 °C. The preferred contacting time is about 0.5 to about 6 hours. The preferred gas flow rate (excluding diluent) is about 1 to about 1000 ml S.T.P./min per gram of the starting iron compound. The contacting pressure inclusive of that of the diluent is usually 0,1 to 0,2 MPa (1 to 2 atm). although not particularly limitative. In the process (b) of the invention, the reducing-and-carburizing agent containing carbon atom or a mixture thereof with the reducing agent which does not contain carbon atom can be used as it is or as diluted. When the mixture is used, the mixing ratio of the reducing-and-carburizing agent to the reducing agent is suitably selected but is preferably up to 1 to 5 by volume. Contact conditions are also suitably selected but the preferred contacting temperature is about 250 to about 400 °C, more preferably about 300 to about 400 °C. The preferred contacting time is about 0.5 to 6 hours when the contact in (a) is conducted, and about 1 to about 12 hours when the contact in (a) is not conducted. The preferred gas flow rate (excluding diluent) is about 1 to about 1000 ml S.T.P./min per gram of the starting iron compound. The contacting pressure inclusive of that of the diluent is usually 0,1 to 0,2 MPa (1 to 2 atm). although not particularly limitative. The particles obtained by the present invention are in the form of generally uniform particles when observed under an electron microscope. The particles are present as primary acicular particles and have the same particulate form as the particles of the starting iron compound. The particles obtained by the process are found to contain carbon by elementary analysis and to contain an iron carbide by its X-ray diffraction pattern, which exhibits plane spacings at 2.28Å , 2.20Å , 2.08Å , 2.05Å and 1.92Å . Such pattern corresponds to Fe₅C₂ . The iron carbide component of the present invention chiefly comprises Fe₅C₂ , with Fe₂C, Fe₂₀C₉ (Fe₂.₂C), Fe₃C, Fe₇C₃, etc. present conjointly therewith in some cases. It is suitable to represent the iron carbide as FexC (2≦x<3). The iron carbide fine particles of the present invention are useful as a magnetic material for magnetic recording as is apparent from the foregoing characteristics, etc., while the use thereof is not limited thereto. For example, the particulate material is usable as a catalyst for preparing lower aliphatic hydrocarbons from CO and H₂. Further, the present iron carbide fine particles are useful as a starting material for producing metal iron fine particles containing carbon. In the present invention, it is possible to produce iron carbide fine particles which are small in deposition of free carbon on the particles and high in saturation magnetization. The invention will be described below in detail by showing reference examples, examples and comparison examples. In the following examples, characteristics, etc. are determined in the methods described below. (1) Magnetic characteristics. Determined in the following method unless otherwise specified. Coercive force (Hc, Oe), saturation magnetization (σs, e.m.u./g) and square ratio (Sq) are measured in a magnetic field with a strength of 5 kOe, using a gauss meter equipped with a Hall-effect element and the powder sample with a packing ratio of 0.2. (2) Elementary analysis for C, H and N The sample is subjected to elementary analysis in the conventional method using MT2 CHN CORDER Yanaco, Yanagimoto Mfg. Co., Ltd, with passage of oxygen (helium carrier) at 900 °C. Reference Examples 1 to 4 and 11 to 14 Sixteen grams (14.4g in terms of Fe₂O₃) of goethite particles, 0.4 µ m in average particle size and 10 in average axial ratio, were dispersed in 0.6ℓ of pure water. The dispersion was adjusted to pH 3 with an aqueous solution of 1N-HCℓ and thereto was added an aqueous solution obtained by dissolving a specific amount of the metal compound listed in Table 1 in 10 ml of pure water. The mixture was stirred and adjusted to pH 8.5 with 0.2N-NaOH to perform the coating of the metal compound. Finally, the particles were filtered from the dispersion, washed with water, dried and pulverized in a mortar to obtain goethite particles coated with the metal compounds. Although a combination of iron compound, cobalt compound and silicon compound is not shown in Table 1, silicon compound and aluminum compound are alternatively used and silicon compound can be used in place of aluminum compound with the same effect. Reference Examples 5 and 15 The same amount of the goethite particles used in Reference Example 1 were dispersed in the same manner as in Reference Example 1. Similarly, to the dispersion was added an aqueous solution obtained by dissolving a specific amount of the firstly coated metal compound listed in Table 2 in 10 ml of pure water. The metal compound was similarly coated on the goethite particles. The dispersion was adjusted to pH 10.5 with an aqueous solution of 1N-NaOH and thereto was added an aqueous solution obtained by dissolving a specific amount of the secondly coated metal compound listed in Table 2 in 10 ml of pure water. The mixture was stirred and adjusted to pH 8.5 with 0.2N-HCℓ to perform the coating of the metal compound. Finally, in the same manner as in Reference Example 1 were obtained goethite particles coated firstly with the metal compounds and secondly with silicon compound or aluminum compound. Reference Examples 21 to 25 and 31 to 35 The goethite particles obtained in each of Reference Examples were heated and dehydrated in air at 350 °C for 60 minutes to obtain α -Fe₂O₃ particles coated with compounds shown in Tables 1 and 2. Examples and Comparison Examples Two grams of each of α -Fe₂O₃ particles obtained in Reference Examples were contacted with CO at 365 °C for 3.5 hours at a flow rate of 200 mℓ/min. The particles were thereafter allowed to cool to room temperature to obtain a black powder. The resulting iron carbide was analyzed qualitatively and quantitatively by X-ray diffraction pattern of the product. In the qualitative analysis, Fe₅C₂ was detected by the peaks having plane spacings at 2.05Å , 2.08Å and 2.20Å , and Fe₇C₃ was detected by the peaks having plane spacings at 2.02Å , 2.12Å and 2.26Å (plane spacings having high intensity were selected in Fe₅C₂ Iron Carbide on ASTM X-Ray Powder Data File 20-509 and Fe₇C₃ Iron Carbide on ASTM X-Ray Powder Data File 17-333). In the quantitative analysis, ratio of Fe₅C₂ and Fe₇C₃ was measured from ratio of their intensity in 2.05Å and 2.02Å respectively. Table 3 gives magnetic characteristics and the results of the analysis. As apparent from Table 3, the iron carbide fine particles of Example 1 obtained from the starting iron compound coated with iron compound have a small amount of free carbon, and the iron carbide fine particles of Examples 2 to 4 obtained from the starting iron compound coated with both of iron compound and cobalt compound have a small amount of free carbon and are excellent in coercive force and saturation magnetization. Although iron carbide fine particles of Comparison Example 4 are small in free carbon and large in saturation magnetization, the particles deform in shape by sintering and are low in orientation and remanent flux density when made into a magnetic tape. Ref. Ex. Metal compound [metal content (wt% )] Amount (g) [in terms of element] 1 Fe₂(SO₄)₃ · nH₂O (Fe 16.76) 0.477 (0.080) Aℓ₂ (SO₄)₃ · nH₂O (Aℓ 10.72) 0.224 (0.024) 2 Fe₂ (SO₄)₃ · nH₂O (Fe 16.76) 0.239 (0.040) CoSO₄ · 7H₂ O (Co 20.96) 0.191 (0.040) Aℓ₂ (SO₄)₃ · nH₂O (Aℓ 10.72) 0.224 (0.024) 3 Fe₂ (SO₄)₃ · nH₂O (Fe 16.76) 0.239 (0.040) CoSO₄ · 7H₂O (Co 20.96) 0.191 (0.040) Aℓ₂ (SO₄)₃ · nH₂O (Aℓ 10.72) 0.149 (0.016) 4 Fe₂ (SO₄)₃ · nH₂O (Fe 16.76) 0.239 (0.040) CoSO₄ · 7H₂O (Co 20.96). 0.191 (0.040) Aℓ₂ (SO₄)₃ · nH₂O (Aℓ 10.72) 0.075 (0.008) 11 Aℓ₂ (SO₄)₃ · nH₂O (Aℓ 10.72) 0.448 (0.048) 12 CoSO₄ · 7H₂O (Co 20.96) 0.382 (0.080) Aℓ₂ (SO₄)₃ · nH₂O (Aℓ 10.72) 0.224 (0.024) 13 NiSO₄ · 7H₂O (Ni 20.91) 0.383 (0.080) Aℓ₂ (SO₄)₃ · nH₂O (Aℓ 10.72) 0.224 (0.024) 14 CoSO₄ · 7H₂O (Co 20.96) 0.382 (0.080) Ref. Ex. Firstly coated metal compound Secondly coated compound kind amount(g) kind amount(g) 5 Fe₂(SO₄)₃ · nH₂O (Fe 16.76) 0.477 water glass No.3 [Na₂O· nSiO₂ (n=2∼4)] (SiO₂ 29) 0.177 15 CoSO₄ · 7H₂O (Co 20.96) 0.382 water glass No.3 (SiO₂ 29) 0.177

Claims: A process for preparing iron carbide fine particles having an average particle size (long axis) of 0.1 to 2 µm and an average axial ratio of 3 to 20, which comprises: (a) coating an iron compound selected from iron oxyhydroxide fine particles and iron oxide fine particles, said fine particles having an average particle size of up to 2 µm and an average axial ratio of at least 3, with (1) an iron compound or an iron compound and a cobalt compound and (2) with an aluminium compound or a silicon compound and, optionally, reducing said coated iron compound with a reducing agent which does not contain carbon atoms, and (b) contacting the iron compound of (a) with a reducing-and-carburizing agent containing carbon atoms or a mixture thereof with a reducing agent which does not contain carbon atoms. A process defined in claim 1 wherein the coated iron compound in step (a) is heated at about 200 to 700°C. A process as defined in claim 1 wherein the contacting temperature in step (a) is about 200 to 700°C and the contacting temperature in step (b) is about 250 to 400°C. A process as defined in any of claims 1-3 wherein the amount of coating of each compound in step (a) is 0.01 to 10% by weight in terms of the metal element based on the weight of the iron oxyhydroxide or iron oxide in terms of Fe₂O₃. Iron carbide fine particles obtainable by the process of any of claims 1-4.

Inventors: DAIMON SHIGEO C O DAIKIN INDUS, MIYAMOTO NAOKI C O DAIKIN INDU, SHIBUYA YOSHIYUKI C O DAIKIN I, DAIMON, SHIGEO, C/O DAIKIN INDUSTRIES LTD., MIYAMOTO, NAOKI, C/O DAIKIN INDUSTRIES LTD., SHIBUYA, YOSHIYUKI, C/O DAIKIN INDUSTRIES LTD.
Applicants: DAIKIN IND LTD, DAIKIN INDUSTRIES, LIMITED
Application year: 1991
Main classification: C01B31
IPC codes: G11B5, H01F1, C01B31