JPS6323136B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing cobalt-containing ferromagnetic iron oxide useful as a material for magnetic recording media. Cobalt-containing magnetic iron oxide has a high coercive force, and magnetic tapes made using it are capable of high-density recording and have excellent sensitivity in the high frequency range, and have been widely used in fields such as videotapes in recent years. In fact, many methods have been proposed for incorporating cobalt into magnetic iron oxide, but it has been pointed out that one of their shortcomings is a wide distribution of coercive force, and as a result, for example, transfer characteristics, erasing characteristics, etc. I have a problem with. The present inventors have made a proposal for obtaining a high coercive force by treating a dispersion containing a metal salt including a cobalt salt, an alkali having a chemical equivalent or more, and magnetic iron oxide powder at a temperature below its boiling point. Then, this treated product is washed with water, filtered, and dispersed in water without drying and heat treated (Japanese Patent Application Laid-Open No. 55-56016
After conducting various studies, (1) performing the above cobalt deposition treatment in a non-oxidizing atmosphere, and (2)
When the above heat treatment was carried out under pressure at a temperature exceeding the boiling point, we obtained the knowledge that not only a high coercive force but also a product with a surprisingly small coercive force distribution could be obtained, and we developed the present invention. completed. In the present invention, a magnetic iron oxide powder is treated with an aqueous solution of a metal salt containing at least a cobalt salt and an alkali to deposit a metal compound containing at least cobalt on the surface of the powder particles in a non-oxidizing atmosphere. A cobalt-containing product characterized by filtering and washing a slurry containing magnetic iron oxide with a metal compound containing at least cobalt, dispersing the slurry in water, and heat-treating the slurry at a temperature exceeding the boiling point under pressure. This is a method for producing ferromagnetic iron oxide. The magnetic iron oxide used in the present invention includes γ-
Examples include bertolide compounds obtained by partially reducing Fe ₂ O ₃ , Fe ₃ O ₄ or γ-Fe ₂ O ₃ . Examples of cobalt salts include cobalt chloride, cobalt sulfate, and cobalt acetate. Other metal salts that may be used as necessary include ferrous salts, manganic salts, zinc salts, chromium salts, and nickel salts. is selected appropriately. As the alkali, sodium hydroxide, potassium hydroxide, etc. are appropriately selected. Methods for depositing a metal compound containing at least cobalt include (1) dispersing magnetic iron oxide powder in an aqueous solution of a metal salt containing at least a cobalt salt, and adding an alkaline aqueous solution thereto; (2) magnetic oxidation. A method of dispersing iron powder in a mixture of an aqueous solution of a metal salt containing at least a cobalt salt and an aqueous alkali solution, (3) dispersing magnetic iron oxide powder in water, and adding to the aqueous solution of a metal salt containing at least a cobalt salt. (4) A method of dispersing magnetic iron oxide powder in an alkaline aqueous solution and adding thereto an aqueous solution of a metal salt containing at least cobalt salt; (5) A method of dispersing magnetic iron oxide powder in an aqueous alkaline solution containing at least cobalt salt; There are methods such as dispersing a metal salt in an aqueous solution containing a salt and dropping this dispersion into an alkaline aqueous solution, and also treating some or all of the cobalt salt and other metal salts simultaneously or sequentially. An appropriate method can be adopted. This deposition treatment must be carried out in a non-oxidizing atmosphere, that is, in an atmosphere in which cobalt and other metals are not substantially oxidized. Specifically, for example, bubbling an inert gas into the reaction solution, It is preferable to carry out the reaction by replacing the air in the reaction vessel with an inert gas. This treatment is usually carried out at 0°C.
~ boiling point, preferably carried out at a temperature of 10 to 50 °C,
The OH group concentration in the system is usually 0.01 to 3 mol/,
If this concentration is too low, the desired effect will not be obtained, while if it is too high, the cobalt compound once deposited will partially dissolve, which is not desirable. Also, this processing time is usually
It is 0.1 to 10 hours. The amount of cobalt deposited is 0.5 to 30%, preferably 1 to 10%, based on the weight of magnetic iron oxide, 1 to 30%, preferably 2 to 20% in the case of ferrous iron, and 1 to 30%, preferably 2 to 20% in the case of ferrous iron. 0-10 if
%, and in order to obtain a high coercive force, it is preferable to combine a cobalt salt and a ferrous salt. Next, the slurry containing the magnetic iron oxide subjected to the above treatment is usually filtered and washed with water, then dispersed again in water to form a slurry, and this slurry is placed in a closed container and heated under pressure at a temperature exceeding the boiling point. That is, heat treatment is performed at a temperature of 105 to 200°C, preferably 110 to 150°C. If this temperature is too low than the above range, the desired effect will not be obtained, while if it is too high, problems will occur on the equipment, which is undesirable. The pressure at this time is usually 1.1 to 20 atm, preferably about 1.4 to 5 atm, and the treatment time is usually 1 to 10 hours. Although drying may be performed in a non-oxidizing atmosphere before this heat treatment, it is preferable not to perform drying from an industrial perspective. Further, the slurry subjected to the heat treatment is usually filtered or washed with water, and then dried at as low a temperature as possible or in a non-oxidizing atmosphere to obtain the desired cobalt-containing ferromagnetic iron oxide. Further, this object may be subjected to dry heat treatment in a non-oxidizing or oxidizing atmosphere. The cobalt-containing ferromagnetic iron oxide obtained by the method of the present invention has a high coercive force, and the magnetic tape manufactured using this material has a high coercive force and a small spread of coercive force distribution, so it has a high coercive force. It can be said that it is excellent in electromagnetic conversion characteristics, such as transfer characteristics and erasing characteristics, which are said to be caused by magnetic force distribution. The reason for this is not fully clear, but when using a cobalt compound as a deposit, (1) a uniform cobalt compound layer is formed on the surface of the magnetic iron oxide by deposition under specific conditions, and (2) Next, by heat treatment, the cobalt compound changes to cobalt ferrite near the surface of the magnetic iron oxide, and as the cobalt ferrite crystallization reaction progresses, a uniform crystal growth reaction is promoted, and this cobalt ferrite crystal is further stabilized. In addition, when a ferrous compound is combined with a cobalt compound as a deposit, (1) due to deposition under specific conditions, a uniform cobalt containing ferrous ion is formed on the surface of the magnetic iron oxide. A ferrite layer is formed, and (2) the cobalt ferrite crystallization reaction proceeds in this cobalt ferrite layer by heat treatment, promoting a uniform crystal growth reaction, and further stabilizing this cobalt ferrite crystal. It is estimated that the The present invention will be illustrated by the following examples and comparative examples. Example 1 Acicular γ-Fe ₂ O ₃ [Coercive force (Hc): 393 Oe, axial ratio (L/W): 7, P content (P conversion to γ);
Disperse 100 g of 0.4% by weight in 1 part of water to make a slurry, add 70 ml of a 0.85 mol/cobalt sulfate aqueous solution while blowing _N2 gas into the liquid, and then
Add 175 ml of a 10 mol/aqueous sodium hydroxide solution and stir, then add 125 ml of a 1 mol/aqueous ferrous sulfate solution, and stir at room temperature (30°C) for 5 hours.
Next, the slurry after the reaction was filtered and washed with water, the resulting cake was redispersed in 1 part of water, this slurry was placed in an autoclave, and the autoclave was replaced with N ₂ gas.
Heat treatment was performed at 130°C for 3 hours. Furthermore, after filtering and washing the slurry after this reaction, it was heated to 60°C in the atmosphere.
was dried for 15 hours to obtain the desired cobalt-containing ferromagnetic iron oxide (A). Comparative Example 1 The slurry obtained in Example 1 after the adhesion treatment was filtered and washed with water, and the resulting cake was dried in the atmosphere at 60°C for 15 hours to obtain cobalt-containing magnetic iron oxide (B). Ta. Comparative Example 2 The redispersed slurry obtained in Example 1 was placed in a normal reactor and replaced with N ₂ gas.
Heat treatment was performed at 100°C for 5 hours. The obtained slurry was filtered and washed with water, and then dried in the atmosphere at 60°C for 15 hours to obtain cobalt-containing magnetic iron oxide (C). Regarding the magnetic iron oxides (A) to (C) obtained in Example 1 and Comparative Examples 1 to 2, the coercive force was measured by a conventional method, and the thermal properties were measured by the following method. The results are shown below. It is shown in Table 1. (Thermal characteristics) This relates to the temperature dependence of coercive force and is calculated by the following formula. Thermal properties (Tp; %) = Coercive force at 125°C / Coercive force at room temperature x 100 Furthermore, for samples (A) to (C), compounds were prepared according to the blending ratios below and kneaded in a ball mill. Manufactured magnetic paint. (1) Cobalt-containing (strong) magnetic iron oxide 100 parts by weight (2) Soybean lecithin 1 (3) Surfactant 4 (4) PVC-vinyl acetate copolymer resin 15 (5) Dioctyl phthalate 5 (6) ) Methyl ethyl ketone 111 〃 (7) Toluene 122 〃 Next, each magnetic coating material was applied to a polyester film by a conventional method, oriented and dried to produce a magnetic tape having a magnetic coating film about 9μ thick. Coercive force (Hc), squareness ratio (Br/Bm), orientation (OR), saturation magnetic flux density (Bm), reversal magnetic field distribution (SFD), anisotropic magnetic field distribution (AFD) were determined for each tape using the usual method. ) are shown in Table 1 below. The volume % of the anisotropic magnetic field distribution was calculated by the following method, and in the present invention, a volume % of 4.5 kOersted or more was expressed as a representative value of AFD as a guideline for erasing characteristics. (Measurement of anisotropic magnetic field distribution) Using a vibrating magnetometer (manufactured by Toei Kogyo, model VSM-3), apply a magnetic field of 10 kOersted in the tape orientation direction, then return the magnetic field to 0, and then Measure the residual magnetization σr ₁ in the direction perpendicular to the direction.
Next, it is rotated 10 degrees from the initial magnetic field direction, a magnetic field of 100 oersted is applied, the magnetic field is returned to 0, and σr ₂ in a direction perpendicular to the initial magnetic field direction is measured again. In this way, the operation of applying a magnetic field in the 10° direction and measuring σrm in the direction perpendicular to the initial magnetic field direction is repeated at intervals of 100 oersteds up to 10 kilooersteds.
Subtract σrn _-1 from each measured value of σrn to obtain Σ[σrn−σrn _-1 ]
seek. Each σr ₂ −σr ₁ , σr ₃ −σr ₂ , σr ₄ −σr ₃ ,…
...σrn−σrn ₋₁ is divided by Σ[σrn−σrn ₋₁ ], and this value is taken as the volume % of the particles whose magnetization is reversed by the corresponding applied magnetic field.

[Table] As is clear from the above results, AFD becomes worse with underwater heat treatment at 100℃ than when no underwater heat treatment is performed, whereas unexpectedly in the case of the present invention, that is, treatment at 130℃. It's also getting better. From this, it can be seen that the coercive force distribution has less spread in the coercive force distribution obtained by the method of the present invention.

Claims (1)

[Claims] 1. Magnetic iron oxide powder is treated with an aqueous solution of a metal salt containing at least a cobalt salt and an alkali to deposit a metal compound containing at least cobalt on the surface of the powder particles in a non-oxidizing atmosphere, and then A cobalt-containing ferromagnetic material that is characterized in that a slurry containing magnetic iron oxide is filtered and washed with water, and then this slurry is dispersed in water and heat-treated under pressure at a temperature exceeding the boiling point. Method for producing iron oxide.