JPH0445561B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention uses acicular iron oxide or Ni, Co, Zn, Mn
This invention relates to a method for producing metal magnetic powder containing Fe as a main component, which comprises heating and reducing modified acicular iron oxide containing elements such as hydrogen gas or the like using a reducing gas such as hydrogen gas. More specifically, a colloidal solution of Si or Al hydroxide is generated under strictly controlled conditions, and this colloidal solution is used to treat the raw material, acicular iron oxide, to form an acicular iron oxide surface. Si or
By forming a uniform adhesion layer of Al and then reducing it, the shape anisotropy derived from the acicular shape of acicular iron oxide is maintained, and Hc is 1400~
1600Oe, or if desired, further producing the metal magnetic powder.
Provides a method for producing metal magnetic powder that has excellent acicularity and dispersibility and is stable in the air by stabilizing the surface of the powder as an oxide by treating it with a mixed gas of H ₂ and CO ₂ . It is something to do. Furthermore, the above
The method of pretreatment and stabilization with a mixed gas of H ₂ and CO ₂ is also effective for stabilizing metal magnetic powders made by reducing acicular iron oxide by any other method. [Prior Art] Metal magnetic powder obtained by reducing iron oxyhydroxide or iron oxide with a reducing gas such as H ₂ is essentially inferior to oxide-based magnetic powder such as γ-Fe ₂ O ₃ . It has a high coercive force (Hc) and a large saturation magnetic moment (σs), so it has excellent properties as a material for high-density magnetic recording.In recent years, it has begun to be used for 8mm video tapes and DAT tapes.
It is being put into practical use. A gas phase reduction method is generally used to produce metal magnetic powder particles containing Fe as a main component, which are called metal powders. However, in this gas phase reduction method, the powder particles are α−FeOOH→α−Fe ₂ O ₃ →Fe ₃ O ₄ →α
Since it goes through a reaction process called Fe, a change in crystal structure and an accompanying volume contraction occur. This volumetric shrinkage ultimately reaches about 47%, so that during the above transformation process, the particles fuse together and the particles themselves sinter, causing the shape to collapse. As a result, metal magnetic powders obtained by conventional gas phase reduction methods generally have low shape magnetic anisotropy, and therefore tend to have low Hc and poor dispersibility. Therefore, various methods have been researched to obtain metal magnetic powder with predetermined characteristics by preventing the particles from fusing together and sintering the particles themselves, and the results include:
Fe obtained by surface treating raw iron oxide with metal salts such as P, Si, Al, Zn, Zr, Ti, Bi or hydroxides of these metals, and then reducing it.
Various methods have been disclosed for favorably retaining the shape of magnetic powder containing magnetic powder as a main component (for example, Japanese Patent Laid-open No.
79153, Tokuko Sho 51-5608, and many others). These disclosed methods generate mutual fusion of particles by forming salts of various metals such as P, Si, Al, Zn, Zr, Ti, Bi, or hydroxides of these metals on the surface of iron oxide. These methods attempt to retain the acicular shape by preventing sintering of the particles themselves, and have achieved some success, but the acicularity of the metal magnetic powder obtained by these methods is not sufficient. It could hardly be said to be good, and was not fully satisfactory as a metal magnetic powder for high-density magnetic recording media, which has been increasingly sought after in recent years. Furthermore, these metal magnetic powders are extremely susceptible to oxidation, and their handling requires great care. As a countermeasure, the metal powder after reduction is taken out in an organic solvent such as toluene and air-dried, or the surface of the metal magnetic particles is oxidized by blowing air into the toluene while immersed in the toluene. After stabilization, the organic solvent is removed by drying and taken out into the air, or the metal magnetic powder obtained by reduction is cooled to room temperature and then brought into contact with a gas mixture of N ₂ and air and mixed gradually. Relying on methods such as the method of gradually oxidizing the surface of metal magnetic particles by increasing the oxygen partial pressure (Po ₂ ) of the gas, and then extracting it into the air without burning it (Japanese Patent Publications No. 47-30477, No. 51-5608). This is commonly done. However, it has been recognized that the method of stabilizing the surface of metal magnetic particles using an organic solvent such as toluene is accompanied by a phenomenon in which a modified product of the organic solvent is generated due to the catalytic action of the metal magnetic particles. Depending on the composition of the dispersant and binder, agglomeration may be significant when forming a coating to create a medium.
It has been pointed out that this method is undesirable because the product obtained by coating has poor squareness and surface properties. On the other hand, using a mixed gas of N ₂ and air, the Po ₂
Unlike the method of using organic solvents such as toluene, the method of gradually forming an oxide film on the particle surface by solid-gas reaction while controlling the oxygen partial pressure (oxygen partial pressure) is called the formation of modified organic solvents. No problems occur, and there is also no need to use large amounts of organic solvents, which is advantageous in terms of cost. However, on the other hand, since it is a method of introducing oxygen in the gas phase into metal magnetic powder particles that are extremely active against oxygen,
The heat balance is greatly influenced by a variety of factors, such as the specifications of the equipment used, the properties of the oxidized material (powder, granule, crumb, etc.), the schedule for increasing the oxygen partial pressure, and the flow rate of the _N2 -air mixture gas.
It may burn during oxidation or when taken out.
Requires delicate control. In addition, in this method, the number of surface active sites in the metal magnetic powder must be reduced as much as possible in order to ensure the desired stability over time, so it is not possible to reduce the metal magnetic powder at a temperature that produces a slightly sintered metal magnetic powder. Alternatively, heat treatment in an inert gas atmosphere such as N ₂ kept at a temperature higher than the reduction temperature (Japanese Patent Application Laid-Open No. 61-56201) is required, which may sacrifice the dispersibility and acicularity of the resulting product. The current situation is that there are cases where this is the case. [Problems to be Solved by the Invention] The present invention improves the above-mentioned drawbacks of the prior art, and reduces the occurrence of shape collapse and fusion of particles during reduction, and improves the squareness ratio when used as a paint. The present invention provides a method for producing metal magnetic powder that has excellent dispersibility and stability over time. [Means for solving the problems] In order to solve these problems, the present inventors
As a result of intensive research into surface treatment using mainly Al-based treatment agents, we have succeeded in preventing sintering and deformation of acicular iron oxide when heat-reducing it, and have improved the particle shape of acicular iron oxide, which is the starting material. We developed a method using aluminum hydroxide colloid as a processing agent as a method to obtain metal magnetic powder that has been well inherited.
According to the method of the present invention, aluminum hydroxide colloid is prepared by strictly controlling its production conditions, thereby producing fine and uniformly distributed aluminum hydroxide colloid, which is then combined with acicular iron oxide slurry. If the mixture is adsorbed or deposited on the surface of acicular iron oxide by an electrochemical method, the aluminum ions dissolved in the conventional slurry of acicular iron oxide will be neutralized and the aluminum ions will be adsorbed or deposited on the surface of acicular iron oxide. Compared to the method of adsorbing or depositing aluminum hydroxide, the adsorption layer of aluminum hydroxide becomes more dense and uniform, making it possible to produce metal powder with better squareness and dispersibility than those previously obtained. Served. In the present invention, as the acicular iron oxide used as a raw material, acicular α-FeOOH is generally used, but α-Fe ₂ O ₃ and γ-FeOOH can also be used. Even if these acicular iron oxides contain Co, Ni, Zn, Mn, etc., any conventionally known method (for example, Japanese Patent Publication No. 53-27719, Japanese Patent Publication No. 53-27719,
23773). Generally, α-FeOOH with a major axis length of 0.1 to 0.5μ and an axial ratio of 10 to 20 is produced by reacting an aqueous iron sulfate solution and an aqueous sodium hydroxide solution, and then blowing an oxidizing gas such as air into it. In particular, acicular iron oxide, which is a raw material for magnetic powder for 8 mm video tapes and DAT tapes with a coercive force (Hc) of 1400 to 1600 Oe, is preferably prepared by adding sulfates, chlorides, etc. to an aqueous solution of iron sulfate. α- produced by reacting an aqueous solution containing Ni or Co with an aqueous sodium hydroxide solution and then performing an oxidation reaction.
FeOOH is preferred. Furthermore, in the present invention, after the acicular iron hydroxide is separated from the reaction mother liquor after the oxidation reaction, it is particularly important to thoroughly wash and remove miscellaneous salts such as Na ₂ SO ₄ and the like. The present inventors conducted various treatments to deposit aluminum hydroxide on acicular iron oxide, and as a result of detailed studies on the effects of impurity elements on the properties of metal magnetic powder, we found that oxidation Na and on the surface or inside of iron particles
It was found that SO ₃ causes the fusion of particles, collapse of particle morphology, and decrease in axial ratio during thermal reduction, and has a significant effect on Hc of metal magnetic powder. In particular, it has been found that when an aluminum compound is deposited on acicular iron oxide, the above-mentioned influence appears more greatly than when Si is deposited. The content of Na and SO ₃ is set to 0.07% or less, preferably 0.05% or less, and 0.5% or less to SO ₃ .
Preferably 1400~1600Oe if controlled to 0.3% or less
A metal magnetic powder with extremely good dispersibility and squareness ratio while maintaining the desired Hc can be obtained. Therefore, according to the above guidelines, after thoroughly cleaning and removing Na and SO ₃ , an aluminum compound is applied to the acicular iron oxide, which is then sized, dried, fired, reduced, and oxidized to produce metal magnetic powder. It should be made.
One of the important features of the present invention is that when applying an aluminum compound, a colloid of aluminum hydroxide is used to treat the surface of iron oxide particles, which are to be reduced. A colloidal solution of aluminum hydroxide can be prepared as follows.
That is, an aqueous solution of a water-soluble aluminum compound is
By completely neutralizing at a temperature of 20° C. or lower, the water-soluble aluminum compound is precipitated as aluminum hydroxide, and this precipitate is washed and peptized with HCl to form a colloidal solution. Also, as another method,
Create an aqueous solution of aluminum compounds such as Al ₂ (SO ₄ ) ₃ , NaAlO ₂ , PAC, etc., adjust the temperature to below 20°C, and add an alkali or acid equivalent to 60% of the neutralization equivalent of each. The solution can also be partially neutralized and peptized with residual acid or alkali to form a colloidal solution. In any of the above cases, if the temperature of the solution is 20° C. or higher, it is not preferable because the aluminum hydroxide produced is difficult to completely turn into a colloid. Treatment of iron oxide with the colloidal solution thus obtained is carried out by adding and mixing a predetermined amount of the colloidal solution to a slurry of acicular iron oxide and heating the mixture to 60°C. This allows the aluminum compound to be uniformly deposited on the surface of the acicular iron oxide. As the water-soluble aluminum compound, sodium aluminate, aluminum sulfate, polyaluminum chloride, etc. can be used. According to the method of the present invention, an aluminum compound is deposited on the surface of iron oxide particles by adding a water-soluble aluminum compound to a slurry of acicular iron oxide and then neutralizing the slurry. In comparison, a metal magnetic powder with excellent acicularity of skeleton particles and less presence of fused aggregate particles can be obtained. According to the above method of the present invention, in the coexistence of Ni,
When producing metal magnetic powder using an aluminum compound as the main treatment agent, the amount of aluminum compound used is 1 to 10% by weight, preferably 3 to 6% by weight as Al based on the amount of Fe in the raw material iron oxide. After applying an aluminum compound under conditions within a range of C., preferably around 350.degree. C., a metal magnetic powder having a coercive force (Hc) of 1400 to 1600 Oe can be obtained. In this case, if the firing temperature in a non-reducing atmosphere is too high, sintering of α-Fe ₂ O ₃ will occur at this point, so the firing temperature is preferably in the range of 650 to 800°C. Further, in some cases, the same effect as the above-mentioned calcination can be achieved by adjusting the temperature in the next reduction step without performing the above-mentioned heat treatment. When the reduction temperature is as high as 450°C, it becomes difficult to avoid sintering of Fe crystals during thermal reduction even with the colloidal solution treatment method of the present invention. ,
As a result, the resulting powder has a reduced axial ratio and coercive force (Hc). Next, the stabilization method will be explained. The metal magnetic powder obtained by completing the reduction is usually cooled to room temperature, and then gradually forms an oxide layer on the surface of the metal magnetic particles while adjusting the oxygen partial pressure in a mixed gas of N ₂ and air. The surface of the magnetic powder is stabilized by a method such as taking the metal powder into an organic solvent such as toluene and introducing an oxidizing gas such as air into the solvent to oxidize the surface of the magnetic powder. is commonly practiced. Among these methods, the method of oxidizing in an organic solvent such as toluene is known to cause polymerization and modification of organic substances due to the catalytic action of iron magnetic powder during oxidation, which has an adverse effect on the coating process.
Although the method of controlling the oxygen partial pressure using a mixed gas of N ₂ and air is a preferable method, the oxidation reaction sometimes proceeds rapidly and deteriorates the characteristics of the metal magnetic powder. The present inventors developed the following method in order to solve the above problems. i.e. 200~450℃
Metal powder is brought into contact with a mixed gas of H ₂ and CO ₂ at a temperature similar to the reduction temperature of H ₂ and CO ₂ in this case.
By controlling the mixing ratio of CO ₂ to H ₂ to an appropriate value within the range of 0.001 to 0.05 based on thermodynamic calculations, is in the oxide state,
Thereafter, by contacting this metal powder with a gas mixture of N ₂ and air and changing the mixing ratio of N ₂ and air as described below, i.e., the oxygen partial pressure is reduced at a relatively low temperature. (Po ₂ )
This is a method in which the surface oxidation progresses gradually by gradually increasing the concentration from 10 ^-3 atm to form a dense α-Fe ₂ O ₃ layer. The surface oxidation reaction of metal magnetic particles by a mixed gas of H ₂ and CO ₂ is expressed by the following equation. H ₂ + CO ₂ = H ₂ O + CO (1) 3Fe + 4H ₂ O = Fe ₃ O ₄ + 4H ₂ (2) The equilibrium relationship of equations (1) and (2) is calculated by thermodynamic calculation.
H 2 and H ₂ correspond to the region where the presence of Fe ₃ O ₄ is stable.
Determine the composition ratio of CO ₂ . However, the oxidation reaction rate is controlled by mass transfer phenomena, so in order to obtain a controllable oxidation rate, careful experiments are performed to determine the actual H ₂ :CO ₂ composition ratio and the absolute value of the gas flow rate. Decided. For example, in the processing example above, 350
At °C, oxidation is too rapid at Pco ₂ =0.1 atm for a total pressure of 1 atm, making it difficult to control the degree of surface oxidation to an appropriate level, but the oxidation rate becomes controllable at Pco ₂ =0.002 atm. At 350℃, _Pco2 is 0.001~
0.05 atm is preferable, and the oxidation time is suitably 5 minutes to 300 minutes. 350℃−H ₂ Flow rate 4.5N/min−CO ₂
At a flow rate of 0.5 N/min (corresponding to Pco ₂ = 0.1 atm), the oxidation reaction progresses rapidly, and σs (Fe) increases from 200 emu/g-Fe to 120 emu/g-Fe in 15 minutes.
Hc also decreased from 1480 Oe to 860 Oe. The metal magnetic powder, on which an appropriate amount of oxide layer has been formed on the surface in this way, is cooled to near room temperature in an inert gas atmosphere such as N ₂ gas, and then
By mixing N ₂ and air, oxidation starts from Po ₂ = 10 ^-3 atm, and Po ₂ is gradually increased using the same operation as the known technique (Japanese Patent Publications No. 47-30477, No. 51-5608).
Fe ₃ O ₄ is oxidized to α-Fe ₂ O ₃ and stabilized, which can be taken out into the air. According to this method, the processing time required to stabilize the metal magnetic powder is short, and σs
It was observed that the change over time of Po 2 was smaller than that of the conventional Po ₂ control method. [Example] The present invention will be specifically described below with reference to Examples. Example 1 A mixed aqueous solution of ferrous sulfate and nickel sulfate (Ni/Fe = 3%) was reacted with an aqueous sodium hydroxide solution, and the neutralized product was further oxidized with air to produce α-FeOOH (long). Average axial length 0.3μ, average axial ratio
18) The slurry is filtered, washed and repulped to remove Na.
and SO ₃ were sufficiently removed. α-FeOOH slurry (20g/100g,
A predetermined amount (an amount of Al/Fe=6%) of aluminum hydroxide colloid prepared separately in advance was added to the solution at 25° C., and stirring was continued for 30 minutes. Then 1N−
Adjust the pH to 8-9 with NaOH aqueous solution and 60℃
The temperature was raised to 150°C and aged for 1 hour to produce acicular goethite treated with aluminum hydroxide colloid. <Preparation of aluminum hydroxide colloid> Aqueous solution of sodium aluminate (as Al ₂ O ₃
After taking 100ml of 299g/) and adding 200ml of pure water,
Add 920 ml of 1N-HCl aqueous solution while maintaining at 15℃.
The pH was adjusted to 7.5. Next, after adding 500 ml of pure water, stirring was stopped and the mixture was allowed to stand, the supernatant was discarded, the mixture was washed by decantation, and the mixture was suction-filtered and washed using a Nutssie filter. Next, pulp the cake with 300ml of pure water to 1N
-176 ml of aqueous HCl solution was added to peptize the colloid. By addition of aluminum hydroxide colloid,
Ni-containing α− coated with aluminum hydroxide
FeOOH was thoroughly washed with pure water to remove Na and SO ₃ . The Na content of α-FeOOH after washing is
The SO ₃ content was 0.02%. This α-FeOOH was granulated into cylindrical crumbs with a length of about 7 mm and a diameter of 3 mm, and then dried. In addition, 700
After baking at ℃ for 30 minutes, transfer to a vertical fixed bed reduction apparatus.
Insert 25g and heat to 350℃ with _H2 gas flow rate of 5N/min.
Reduction was performed for 10 hours. When a part of the powder was taken out in toluene and the powder magnetic properties were measured using VSM (applied magnetic field 10KOe), PHc = 1480Oe, σs (Fe) =
195emu/g-Fe, σr/σs=0.512, indicating good reduction. This metal magnetic powder was oxidized at a temperature of 350° C. for 15 minutes with a mixed gas flow of H ₂ 5N/min and CO ₂ 0.01N/min. When a part of the obtained oxide was taken out into toluene and the powder magnetic properties were measured using VSM, PHc=1450Oe, σs−Fe=
170emu/g-Fe, σr/σs=0.514, indicating proper oxidation. After cooling to 20 °C in a stream of _N2 ,
With a mixed gas flow of N ₂ 2N/min and air 0.01N/min, oxidation begins with Po ₂ = 10 ^-3 atm, and Po ₂ gradually increases until finally air alone becomes 4N.
The airflow was set to /min. When the product was taken out into the air outside the furnace and its magnetic properties were measured, PHc=
1450 Oe, σs=128emu/g, σr/σs=0.515. 30,000 of the metal magnetic powder particles obtained in this example
A magnified electron micrograph is shown in Figure 1. It can be seen that the acicularity is well maintained and there are few fused particles. The σs of this powder on the third day at 60℃ in air is
It was 123 emu/g, which was good. Also, 60℃−
σs on the 7th day of 90% RH was 105 emu/g, which was good. As a result of measuring the sheet properties, Sq=0.78, 60°−
60° gloss = 70%. The measurement results of these characteristic values are shown in Table 1 together with the measurement results of the comparative examples. The coating conditions were as follows.

【table】

[Table] Glass beads
63.3g used)
Comparative Example 1 By adjusting the washing after colloid addition
An experiment was conducted under the same conditions as in Example 1 except that the Na content was 0.27%. After reduction, it was sampled into toluene and the powder magnetic properties were measured using VSM. PHc=905Oe, σs(Fe)=200emu/g-Fe,
σr/σs=0.382. In the obtained metal magnetic powder particles, the particles were fused to each other, and the axial ratio of the skeleton was reduced, giving it a rounded shape. Figure 2 shows an electron micrograph of the magnetic powder particles magnified 30,000 times. The properties of the powder were as shown in Table 1. Comparative Example 2 By adding ammonium sulfate to the washed α-FeOOH after adding colloid, only SO ₃ was removed as an impurity.
The reduction was carried out under the same conditions as in Example 1 except that 1.1% remained. The reduced metal magnetic powder was sampled into toluene and its magnetic properties were measured. PH
c=1230Oe, σs(Fe)=196emu/g−Fe, σr/
σs=0.491. These results are shown in Table 1. Comparative Example 3 Example except that the aluminum compound was not added by colloid, but Al was added in the form of adding sodium aluminate to the aqueous slurry of α-FeOOH, and then neutralized with 1N-HCl at room temperature. The experiment was conducted under the same conditions as in 1.
After reduction, the powder was sampled into toluene and its magnetic properties were measured. PHc=1350Oe, σs(Fe)=182emu/
g-Fe, σr/σs = 0.490. Next, it was oxidized and stabilized under the same conditions as in Example 1, taken out into the air, and examined for powder magnetic properties, sheet properties, and stability over time. PHc=1340Oe, σs=128emu/g, σr/σs
= 0.492. Also, Sq = 0.74, gross = 60%
It was hot. σs on the third day at 60℃−air is
113emu/g, deterioration rate is 12%, 30℃ - 90%
σs on the 7th day of RH was 98 emu/g. These measurement results are shown in Tables 1 and 2 in comparison with the results of Example 1. Comparative Example 4 The same procedure as in Example 1 was repeated except that after reduction, the sample was taken out into toluene and oxidized by blowing air into the toluene. The results were as shown in Table 2. Comparative Example 5 Without converting the particle surface to Fe ₃ O ₄ using a mixed gas of H ₂ and CO ₂ , the particle surface was cooled to room temperature after reduction and then
The same procedure as in Example 1 was repeated except that oxidative stabilization was performed by gradually increasing Po ₂ with N ₂ and air. The results were as shown in Table 2.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 is an electron micrograph at 30,000 times magnification showing the particle structure of the metal magnetic powder produced by the method of the present invention. FIG. 2 is an electron micrograph at 30,000 times magnification showing the particle structure of metal magnetic powder produced by a conventional method.

Claims (1)

[Claims] 1. Metal magnetic powder mainly composed of Fe produced by reduction of iron oxide is mixed with H ₂ and CO 2 at a temperature of 200 to 450°C, with a ratio of CO ₂ to H ₂ of 0.001 to 0.05 _. After preliminary oxidation treatment of the surface of the powder particles by contacting with a mixed gas of an inert gas and air, the treated powder particles are brought into contact with a mixed gas of an inert gas and air at around room temperature,
A method for producing metal magnetic powder for magnetic recording, comprising stabilizing powder particles by surface oxidation by gradually increasing the oxygen partial pressure (Po ₂ ) of the mixed gas. 2. Use acicular iron oxide or modified acicular iron oxide containing one or more elements selected from Ni, Co, Zn, and Mn as a raw material, and use the raw material as
Si or Al by contacting with a colloidal solution of Si or Al hydroxide prepared in advance.
The acicular iron oxide is prepared by depositing hydroxide on its surface, and the deposited acicular iron oxide is reduced to a metal magnetic powder containing Fe as the main component.
At a temperature of 450 ° C, the ratio of CO ₂ to H ₂ is from 0.001 to
After preliminary oxidation treatment of the surface of the powder particles by contacting them with a mixed gas of 0.05 H ₂ and CO ₂ , the treated powder particles are brought into contact with a mixed gas of an inert gas and air at around room temperature. and stabilizing the powder particles by surface oxidation by gradually increasing the oxygen partial pressure (Po ₂ ) of the mixed gas.
A method for producing metal magnetic powder for magnetic recording.