JP3020374B2 - Method for producing cobalt-containing magnetic iron oxide powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cobalt-containing magnetic iron oxide powder useful as a recording element of a magnetic recording medium, particularly an image recording element, and a method for producing the same.
The present invention relates to a cobalt-containing magnetic iron oxide powder which is extremely suitable for producing a magnetic recording medium having excellent video noise characteristics, and a method for producing the same.

[0002]

[Technical background of the invention and its problems] Audio tape,
Magnetic iron oxide powder used for magnetic recording media such as video tapes, magnetic disks, and magnetic cards can be produced by various methods.However, iron sulfate solutions, various iron salt solutions such as iron chloride are neutralized with alkali, Oxidized needle-like goethite (α
-FeOOH) and lepidocrocite (γ-FeOOH)
Then, these precursors are sequentially subjected to heat treatment such as calcination (dehydration, densification), reduction, and oxidation to obtain acicular magnetite (Fe ₃ O ₄ ) or maghemite (γ-Fe _2). O ₃ ), a method for producing a magnetic iron oxide powder mainly suitable for an audio recording medium, or a method for applying a cobalt compound to the magnetic iron oxide powder to further form a Co-F
The most common method is to use e ₃ O ₄ or Co-γ-Fe ₂ O ₃ to produce a cobalt-containing magnetic iron oxide powder having a high coercive force and suitable mainly for an image recording medium.

[0003] Thus, goethite (α-FeOOH)
In producing cobalt-containing magnetic iron oxides using as a precursor or lepidocrocite (γ-FeOOH), for example, in the case of α-FeOOH, it is dehydrated at 250 ° C. to 800 ° C. to convert into α-Fe ₂ O ₃ , and then Reducing at 300-500 ° C. to produce Fe ₃ O ₄ , or
Oxidation is performed at 0 ° C. to 300 ° C. to produce γ-Fe ₂ O ₃ , and then a cobalt-containing metal compound is deposited on their surface to produce cobalt-containing magnetic iron oxide. Note that α-Fe
The dehydration reaction of OOH is completed at 250 ° C to 300 ° C and α-
It is converted to Fe ₂ O ₃ , but at a relatively high temperature of 600
A heat treatment at a temperature of from 800 ° C. to 800 ° C. to grow the α-Fe ₂ O ₃ crystal, reduce vacancies generated by a dehydration reaction, and improve the magnetic properties of the cobalt-containing magnetic iron oxide derived therefrom. Is commonly done.

On the other hand, γ-FeOOH is α-FeOOH
It has the characteristic that almost no twins are generated during the precipitation process. Therefore, the magnetic powder finally obtained by using γ-FeOOH as a precursor is straight needle-like particles without branching, and thus has a low dispersibility in a synthetic resin medium when processed into a magnetic tape. Good and excellent orientation,
It is said that it is easy to obtain a better product such as a large coercive force. When γ-FeOOH is heated and dehydrated at a relatively low temperature of 250 ° C. to 300 ° C., γ-Fe ₂ O ₃ is directly obtained. However, γ-Fe ₂ O ₃ is inferior in crystallinity. Not enough, so that γ-FeOOH
And then oxidized (JP-A-47-40097,
Nos. 57-12988 and 58-84127), and γ-FeOOH is dehydrated, then reduced and oxidized (JP-B-39-20939, JP-A-54-2829).
No. 9, No. 58-84407 etc.) γ for use in audio recording media
Method for producing -fe ₂ O ₃ has been proposed. At this time, as in the case of α-FeOOH, the material is heated and dehydrated at a temperature of 500 ° C. or more, and α-Fe ₂ once formed of dense crystals.
After converting into O _3, attempts have been made to reduction and oxidation. Further, in the method for producing γ-Fe ₂ O ₃ , γ-FeOOH particles may be formed by adding Ti, B, Si,
It has also been proposed to treat organic compounds such as P (JP-A-57-129828, JP-A-58-74529, JP-A-58-84407, JP-A-63-69714, and JP-A-63-69714.
69715). However, γ-FeOOH is
Compared to FeOOH, sintering between particles and collapse of the particle shape are more likely to occur during these heat treatments, and the resulting γ-Fe ₂
In general, the properties of O ₃ are generally inferior to those using α-FeOOH as a precursor in many cases.

In the case of image recording which requires a higher recording density than that of audio recording, the recording medium element used for the recording is improved in magnetic characteristics such as coercive force and the like, and is particularly excellent in noise characteristics. Is required.
This noise characteristic is closely correlated with the particle volume of the magnetic powder that is the recording medium element. The smaller the particle volume, the better the noise characteristic. In general, the BET specific surface area is simply used as an index representing the particle volume. It is also known that the noise characteristic improves as the value of the BET specific surface area increases.
Therefore, if the recording medium element is made finer, the noise characteristic tends to be improved, but on the other hand, when the magnetic recording medium is manufactured due to the increase in particle surface activity accompanying the finer recording medium element. When they are made into paints, they are liable to agglomerate, so that good dispersibility cannot be obtained, and it is difficult to achieve desired noise characteristic improvement.

When γ-FeOOH is used as a precursor, sintering between particles and particle shape during the heat treatment process during dehydration and especially during reduction are accompanied by the reduction in the size of particles compared to the case of α-FeOOH. Collapse is more likely to occur, and as a result, the advantages of γ-FeOOH as a precursor have not been fully exploited. For this reason, in general, α-FeOOH is often used as a precursor as an image recording medium element, and γ-FeOOH is used as a precursor to produce a cobalt-containing magnetic iron oxide suitable for an image recording medium element. However, there are still many problems that need to be resolved.

In recent years, the most commonly used particle specific surface area (specific surface area value by the BET method) is used as an index as a characteristic value representing the particle size of magnetic powder which causes noise characteristics of the magnetic recording medium. It is pointed out that it is not always appropriate to perform the above operation, and as one of the measures, a magnetic switching unit (magnetic switching unit) is used.
It has been proposed to use the physical quantity t).

The present inventors have aimed at developing a cobalt-containing magnetic iron oxide having an optimum magnetization reversal volume capable of satisfying the noise characteristics and the like. Investigations have been made to produce a cobalt-containing magnetic iron oxide particularly suitable for image recording by using γ-FeOOH as a precursor, from which magnetic particles having good branch-like needle-like properties are easily obtained.

As a result, a cobalt-containing magnetic iron oxide powder having a specific range of a specific surface area obtained by applying a cobalt compound to an iron oxide powder obtained by performing a specific heat treatment using γ-FeOOH as a precursor. The magnetization reversal volume of BE is
In relation to the T specific surface area, the magnetization reversal of the cobalt-containing magnetic iron oxide powder once produced through α-Fe ₂ O ₃ by using α-FeOOH as a precursor or γ-FeOOH at a high temperature. Extremely small compared to the volume,
In relation to the magnetization reversal volume (V) and the BET specific surface area (SSA), log [V] ≦ −3.05 log [SS
A] It was found that there was a clear difference according to the formula of -11.35.

That is, iron oxide powder mainly composed of magnetite, which is produced by directly reducing γ-FeOOH or by heating and dehydrating at a temperature lower than the temperature at which α-Fe ₂ O ₃ is transformed into α-Fe ₂ O ₃ , is obtained by: The shape of the particles is well maintained, and the magnetite crystals constituting the particles are small with growth suppressed, and as a result, the magnetization reversal volume is reduced. Further, when the iron oxide powder mainly composed of magnetite is then oxidized into an iron oxide powder composed of beltride or maghemite, the crystal growth is extremely small, and as a result, the change in magnetization reversal volume is extremely small. It has been found that the change in magnetization reversal volume when the surface of the iron oxide powder is subjected to cobalt transformation by applying at least a cobalt compound to the iron oxide powder is extremely small. Then, the noise characteristic of the luminance signal (Y−S /
N) was found to correlate more strongly with the magnetization reversal volume than with the particle volume of the cobalt-containing magnetic iron oxide powder.

The present inventors have further studied on the improvement of the noise characteristics (CS / N) of the color signal when the cobalt-containing magnetic iron oxide powder is used as an image recording medium based on the above findings. . CS / N is caused by the relatively long-period smoothness of the surface of the magnetic recording medium, and the surface property of the recording medium is closely related to the coating technique at the time of manufacturing the recording medium.
In order to improve CS / N, the influence of the dispersibility and orientation of the magnetic powder is extremely large, and it has been found that the improvement is strongly desired. By the way, in order to improve the dispersibility and orientation, γ-FeOO as a precursor is used.
It is very important to suppress as much as possible the shape collapse, inter-particle sintering, and increase of the magnetization reversal volume during the heat reduction treatment and further the heat oxidation treatment process of the H particles. The effect of the deposition treatment of various metal compounds on γ-FeOOH was studied.
As a result, the cobalt-containing magnetic iron oxide obtained by subjecting the γ-FeOOH treated with the Al compound to the above-mentioned heat treatment and the treatment of depositing the cobalt compound is more dispersible than the case where the Al compound is not treated. And orientation are greatly improved, and when this is used as an image recording medium, the S
The inventors have found that the magnetic properties such as Q, OR, and SFD are excellent, and that the CS / N is extremely excellent, thereby completing the present invention. Incidentally, the production method of the present invention has excellent magnetic properties such as saturation magnetization and can reduce the light transmittance of a magnetic recording medium as compared with γ-Fe ₂ O ₃ -based cobalt magnetic iron oxide. This is more desirable for producing a magnetite-based or belt-ride-based cobalt-containing magnetic iron oxide, which is rapidly progressing.

[0012]

DISCLOSURE OF THE INVENTION That is, the present invention provides: After applying an aluminum compound to the surface of the lepidocrocyte particles, the aluminum compound is directly reduced, or reduced after heating and dehydration at a temperature lower than the temperature at which it is transformed into hematite, or after the reduction, A cobalt-containing magnetic iron oxide powder satisfying the following formulas (1) and (2), comprising oxidizing to obtain an iron oxide powder, and applying a cobalt compound or a cobalt compound and another metal compound to the surface thereof. Manufacturing method Formula (1) 20 ≦ SSA ≦ 50 Formula (2) −16.70 ≦ log [V] ≦ −3.05 l
og [SSA] -11.35 [wherein SSA is a BET specific surface area (m ² / g),
V is the magnetization reversal volume (cm ³ ). 〕and,

2. The amount of the aluminum compound to be deposited is 0.05 to 3.0 as Al with respect to the weight of lepidocrocite.
%. 12. A method for producing a cobalt-containing magnetic iron oxide powder as described in the above item.

In this specification, the magnetization reversal volume (V)
Is M. P. Sherlock (MP Sharo)
ck) et al. (IEEE Tr
actions on Magnetics, v
ol. MAG-17, No. 6, Nov. 1981
pp. 3020-3022), the magnetization reversal volume (V) of each sample was determined as follows.

The sample powder is packed in a container, and set in a vibrating sample magnetometer (VSM-P7, manufactured by Toei Kogyo), the maximum applied magnetic field is set to 10 kOe, and the magnetic field sweep speed is 10 kOe.
e / x minutes, (1) 20 minutes, (2) 100 minutes, (3) 2
00 minutes and each coercive force Hc (i) (i = 1, 2, 2,
3) and saturation magnetization Ms (Gauss) were measured.

Next, the magnetization reversal time t (i) was determined by the following equation.

(Equation 1)

The obtained three sets of (t (i), Hc (i))
Was used to determine the following regression equation, and Ha and K · V were determined.

[0018]

(Equation 2) Ha: anisotropic magnetic field (Oe) k: Boltzmann constant (1.38 × 10 ⁻¹⁶ erg / d)
eg) T: measurement temperature (measured at 300 K) K: anisotropy constant (erg / cm ³ ) V: magnetization reversal volume (cm ³ ) A: spin precession frequency (2 × 10 ⁹ sec ⁻¹ )

Finally, using the relational expression Ha = 2K / Ms, the magnetization reversal volume V was obtained from V = (K · V) / (Ha · Ms / 2).

The lepidocrocite (γ-FeOOH) used as a precursor in the present invention may be one produced by a known method. In particular, acicular γ-FeOO produced by neutralizing and oxidizing an aqueous ferrous chloride solution in a wet manner.
The use of H is preferred. In the production of γ-FeOOH, a small amount of P, S is used for the purpose of controlling the shape of γ-FeOOH.
Metal compounds such as i and Al can be contained.
Aluminum chloride, aluminum sulfate, aluminum nitrate, sodium aluminate and the like are used as the Al compound to be applied to the surface of the γ-FeOOH particles. The treatment amount of these Al compounds is 0.05 to 3.0% by weight, preferably 0.05 to 1.0% by weight as Al, based on the weight of γ-FeOOH. If the treatment amount is less than this range, a satisfactory treatment effect cannot be obtained, and if it is too large, the desired saturation magnetization of the cobalt-containing magnetic iron oxide powder is decreased, which is not preferable. A
The l compound can be applied to the surface of the γ-FeOOH particles by various methods. For example, the γ-FeOOH reaction slurry obtained by wet-neutralizing and oxidizing an aqueous ferrous chloride solution is used to add the Al compound to the slurry. An aqueous solution having an appropriate concentration and an alkaline aqueous solution are added to neutralize and adhere, or γ-FeO
An aqueous solution of an Al compound and an aqueous alkali solution are added to a slurry in which OH powder is dispersed in water, and the slurry is neutralized and adhered. In addition,
In the deposition treatment of the Al compound, a compound such as Si, P, or B can be further treated. After the Al compound deposition treatment, a normal filtration, water washing, and drying steps can be performed to obtain a γ-FeOOH powder containing Al on the surface.

In the present invention, the step of dehydrating γ-FeOOH coated with an Al compound is not particularly required,
Dehydration is preferred in terms of accelerating the reaction rate in the reduction step and improving the H ₂ utilization when H ₂ gas is used as the reducing agent. Dehydration temperature is α-FeOOH is α-
It is essential that the temperature is lower than the transition to Fe ₂ O _3, and it is not more than about 450 ° C., preferably 250 ° C. to 350 ° C.
° C. When dehydrating at a temperature higher than the above temperature,
γ-FeOOH is transferred to α-Fe ₂ O ₃ and the α-Fe
₂ O ₃ crystals are easy to grow, and iron oxide powders such as magnetite obtained by reducing these α-Fe ₂ O ₃ , beltride and maghemite obtained by further oxidation, and
Furthermore, a cobalt-containing magnetic iron oxide powder obtained by applying a cobalt compound or the like to these materials cannot obtain a target having a small magnetization reversal volume. Furthermore, γ-Fe
Α-Fe ₂ O ₃ transferred from OOH is α-FeOOH
Interparticle sintering is more likely to occur than α-Fe ₂ O ₃ transferred from iron oxide, and iron oxides such as magnetite obtained by reducing these α-Fe ₂ O ₃ , and beltride or maghemite obtained by further oxidization Powders, and cobalt-containing magnetic iron oxides obtained by further applying a cobalt compound or the like thereto, have inferior magnetic properties and dispersibility during coating when manufacturing a magnetic recording medium using the same. Become. The atmosphere during the dehydration treatment may be performed in an oxidizing or inert gas atmosphere. However, when γ-FeOOH is dehydrated in an inert gas atmosphere such as N ₂ gas, sintering between particles and deformation of the particle shape may occur. This tends to be easy, and an oxidizing gas atmosphere such as air is preferable.

The reduction may be performed by a usual method. For example, 250 ° C. under the flow of hydrogen gas containing hydrogen or steam
There is a method of reducing at a temperature of 250 to 500 ° C. using a decomposition gas of fatty acids such as soybean oil, coconut oil, oleic acid, and stearic acid at a temperature of 250 to 500 ° C. Also in the reduction, the magnetization reversal volume of the generated magnetite tends to slightly increase as the processing temperature increases, and it is preferable that the reduction is performed at a temperature as low as possible even in the above-mentioned temperature range as long as the reduction proceeds substantially. The magnetized reversal volume (V) and the BET of the magnetized reversal volume (V) of the iron oxide powder mainly composed of magnetite and the iron oxide powder obtained by further oxidizing the iron oxide powder mainly composed of magnetite obtained by the reduction are obtained. In relation to the specific surface area (SSA), log [V] ≦ −3.0
It is preferable to satisfy the formula of 5 log [SSA] -11.35.

The surface of the iron oxide powder obtained as described above is coated with a cobalt compound or a cobalt compound and another metal compound to obtain a cobalt compound satisfying the following formulas (1) and (2) of the present invention. Containing magnetic iron oxide powder. Formula (1) 20 ≦ SSA ≦ 50 Formula (2) −16.70 ≦ log [V] ≦ −3.05 l
og [SSA] -11.35 [wherein SSA is a BET specific surface area (m ² / g),
V is the magnetization reversal volume (cm ³ ). ]

In the present invention, the conditions represented by the above formulas (1) and (2) need to be satisfied at the same time, and if the specific surface area is smaller than the above range, the image recording medium may not be used. However, if the recording density is too high, it is difficult to achieve a desired dispersion of the magnetic powder in the magnetic recording medium. Furthermore, if the magnetization reversal volume is smaller than the above range, it is difficult to exhibit characteristics as a ferromagnetic material, and if it is larger than the above range, desired excellent noise characteristics cannot be obtained.

Other metal compounds used together with the cobalt compound include metal compounds such as Fe, Mn, Zn, Ni, and Cr, which can be used as appropriate for the purpose of improving various properties of the cobalt-containing magnetic iron oxide powder.

As a method of depositing a cobalt compound or a cobalt compound and another metal compound, iron oxide powder is dispersed in an aqueous solution of cobalt or cobalt and another metal salt, and an alkali aqueous solution is added thereto. There is a method in which iron oxide powder is dispersed in an aqueous alkaline solution, and cobalt or an aqueous solution of cobalt and another metal salt is added thereto. The atmosphere at the time of deposition may be any of oxidizing and inert, and the reaction temperature may be from room temperature to the boiling point or above the boiling point, and may be appropriately performed according to the chemical properties and purpose of the compound to be deposited. I can do it.

Further, in order to improve the dispersibility of the obtained cobalt-containing magnetic iron oxide powder as a coating material, a cobalt compound or the like is coated thereon, and then Si, Al
Can be surface-treated. Hereinafter, the present invention will be described in more detail with reference to examples.

[0028]

【Example】

Example 1 A reactor having an internal volume of 30 liters equipped with an air blowing tube and a stirrer was charged with 22 liters of a 25 g / liter ferrous chloride aqueous solution as Fe ²⁺ and 400 g / liter while maintaining the liquid temperature at 21 ° C. with stirring. 1.4 liter of aqueous NaOH solution
After adding 8 liters, the mixture was kept at 21 ° C. for 2 hours while blowing air at a rate of 6 liters / minute, and γ-FeOOH
Of nuclei. Then, after raising the liquid temperature to 41 ° C., 400 g / liter of NaO
An aqueous H solution (0.49 liters) was added over 2.5 hours to grow nuclei, and a needle-like γ having a BET specific surface area of about 71 m ² / g.
-The reaction for producing FeOOH was completed. Next, after stopping the blowing of air, 162 ml of a 1 mol / L aluminum chloride aqueous solution was added with stirring, and then 1N-NaO was added.
486 ml of an aqueous H solution was added over 30 minutes, and 1N
While maintaining the pH of the slurry at 8 while adding an aqueous solution of NaOH, stirring was continued for 30 minutes, and Al to γ-FeOOH was added.
The deposition process has been completed. The obtained α-FeOOH slurry was filtered, washed and dried to obtain acicular γ-FeOOH powder containing 0.5% by weight of Al. This acicular γ-FeOOH
500 g of powder is put into a small rotary kiln and 3 liters /
For 1 hour at 280 ° C. under the flow of air for one minute. After replacing the air in the kiln with N ₂ gas, the mixture was heated and reduced at 350 ° C. for 1 hour under a hydrogen gas flow of 3 L / min. After cooling, the reduced powder was put into 3 liters of water and collected. This reduced powder was analyzed by chemical analysis to find that Fe ²⁺ / total Fe = 0.340. As a result of measuring various characteristics of a dried product obtained by sampling a small amount and drying at 60 ° C. under an N ₂ atmosphere, SSA = 33.8 m ² / g and Hc = 2
960 e, V = 0.38 × 10 ⁻¹⁶ cm ³ .
The collected slurry was subjected to a dispersion treatment with a mixer for 20 minutes, and then diluted with water to adjust the solid concentration to 100 g / liter. Two liters of this slurry was charged into a four-necked flask, and then 368 ml of a 10N-NaOH aqueous solution was added. Next, 336 ml of a 1 mol / l ferrous chloride aqueous solution was added over 45 minutes with stirring, and 98 ml of a 1 mol / l cobalt sulfate aqueous solution was added over 60 minutes. Thereafter, the temperature was raised to 45 ° C., and stirring was continued for 5 hours to complete the cobalt deposition treatment. The slurry was filtered and washed, and N ₂ was added.
Drying at 120 ° C. in a gas atmosphere gave the desired cobalt-containing magnetic iron oxide powder. (Sample A)

Example 2 γ-FeOOH was produced in the same manner as in Example 1. Next, the amounts of the added 1 mol / liter aluminum chloride aqueous solution and 1N-NaOH aqueous solution were 97 m each.
The Al coating treatment was performed in the same manner as in Example 1 except that the amounts were changed to 1 and 292 ml. The obtained γ-FeOOH slurry was filtered, washed and dried to obtain an acicular γ-FeOOH powder containing 0.3% by weight of Al. This acicular γ-Fe
500 g of the OOH powder was put into a small rotary kiln, heated and dehydrated and reduced in the same manner as in Example 1, and the obtained reduced powder was put into water and collected. The first embodiment
Various characteristics of the reduced powder measured in the same manner as in the case of
²⁺ / total Fe = 0.332, SSA = 30.9 m ² /
g, Hc = 327 Oe, V = 0.50 × 10 ⁻¹⁶ cm
It was ³ . Hereinafter, the recovered slurry was used in Example 1 described above.
Dispersion treatment and cobalt deposition treatment as well as
The desired cobalt-containing magnetic iron oxide powder was obtained. (Sample B)

Example 3 γ-FeOOH was produced in the same manner as in Example 1. Next, the amounts of the added 1 mol / liter aluminum chloride aqueous solution and 1N-NaOH aqueous solution were each 32 m.
The Al coating treatment was performed in the same manner as in Example 1 except that the amount was changed to 1 and 97 ml. The obtained γ-FeOOH slurry was filtered, washed and dried to obtain an acicular γ-FeOOH powder containing 0.1% by weight of Al. This acicular γ-FeO
500 g of OH powder was put into a small rotary kiln, and was heated and dehydrated at 280 ° C. for 1 hour under a flow of air of 3 L / min. After replacing the air in the kiln with N ₂ gas, the mixture was heated and reduced at 280 ° C. for 2 hours under a hydrogen gas flow of 3 L / min. After cooling, the reduced powder was put into 3 liters of water and collected. The properties of the reduced powder measured in the same manner as in Example 1 were Fe ²⁺ / total Fe = 0.31.
1, SSA = 32.9 m ² / g, Hc = 318 Oe, V
= 0.61 × 10 ⁻¹⁶ cm ³ . Hereinafter, the recovered slurry was subjected to a dispersion treatment and a cobalt deposition treatment in the same manner as in Example 1 to obtain a target cobalt-containing magnetic iron oxide powder. (Sample C)

Example 4 γ-FeOOH was produced in the same manner as in Example 1, and then Al was applied to obtain a needle-like γ-FeOOH powder containing 0.5% by weight of Al. This acicular γ-OOH
500 g of powder is put into a small rotary kiln and 3 liters /
For 1 hour at 280 ° C. under the flow of air for one minute. After replacing the air in the kiln with N ₂ gas, the mixture was heated and reduced at 350 ° C. for 1 hour under a hydrogen gas flow of 3 L / min. Then, the mixture is allowed to cool while replacing the hydrogen gas in the kiln with N ₂ gas, and then, in a mixed gas flow of air and N ₂ gas at 3 L / min (air / N ₂ = 1/1: volume ratio), 1
Oxidation treatment was performed at 50 ° C. for 1.5 hours to obtain a beltride powder. Various properties of the beltride powder collected after cooling are Fe
^{2 +} / total Fe = 0.105, SSA = 32.9 m ² /
g, Hc = 340 Oe, V = 0.90 × 10 ⁻¹⁶ cm
It was ³ . 200 g of this beltride powder and 2 liters of water were charged into a mixer and dispersed for 20 minutes to form a slurry. This slurry is charged into a four-necked flask,
384 ml of a 10N-NaOH aqueous solution was added. Then, 376 m of 1 mol / l ferrous chloride aqueous solution was stirred.
was added over 45 minutes, and then 106 ml of a 1 mol / liter aqueous solution of cobalt sulfate was added over 60 minutes. Thereafter, the temperature was raised to 45 ° C., and stirring was continued for 5 hours to complete the cobalt deposition treatment. The slurry was filtered, washed, and dried at 120 ° C. in an N ₂ gas atmosphere to obtain the desired cobalt-containing magnetic iron oxide powder. Obtained. (Sample D)

Example 5 Acicular γ-FeOOH powder 50 having a specific surface area of about 56 m ² / g
After suspending 0 g in 10 liters of water, 93 ml of a 1 mol / l aqueous solution of aluminum chloride was added with stirring, 278 ml of a 1N aqueous solution of NaOH was added over 30 minutes, and a 1 aqueous solution of NaOH was added dropwise. While maintaining the pH of the slurry at 8, while stirring for 30 minutes, the γ-
The Al deposition on FeOOH was completed. The obtained γ-F
The eOOH slurry was filtered, washed, and dried to remove Al to 0.1.
Acicular γ-FeOOH powder containing 5% by weight was obtained. 500 g of this needle-like γ-FeOOH powder was put into a small rotary kiln, and was heated and dehydrated at 280 ° C. for 1 hour under a flow of air of 3 L / min. After replacing the air in the kiln with N ₂ gas, the mixture was heated and reduced at 360 ° C. for 1 hour under a hydrogen gas flow of 3 L / min. Then, after cooling while replacing the hydrogen gas in the kiln with _{N 2} gas for 3 l / mixed airflow min air and _{N 2} gas (air _/ N 2 = 1/1: volume ratio) at 280 Oxidized at ℃ for 1.5 hours
Fe ₂ O ₃ powder was used. Γ-Fe ₂ O ₃ recovered after cooling
Various properties of the powder were as follows: SSA = 24.0 m ² / g, Hc = 3
78 Oe, V = 3.02 × 10 ⁻¹⁶ cm ³ .
After the γ-Fe ₂ O ₃ powder was subjected to dispersion treatment in the same manner as in Example 4, the amounts of the 10 N-NaOH aqueous solution, 1 mol / l ferrous chloride aqueous solution, and 1 mol / l cobalt sulfate aqueous solution were added. 342ml, 276ml respectively
Cobalt deposition treatment was performed in the same manner as in Example 4 except that the amount was changed to 84 ml, and the desired cobalt-containing magnetic iron oxide powder was obtained. (Sample E)

Comparative Example A cobalt-containing magnetic iron oxide powder of a comparative sample was obtained in the same manner as in Example 3, except that the Al deposition step was omitted. (Sample F) The characteristics of the reduced powder were Fe
^{2 +} / total Fe = 0.358, SSA = 27.7 m ² /
g, Hc = 335 Oe, V = 0.60 × 10 ⁻¹⁶ cm
³ was shown.

The BET specific surface areas of the cobalt-containing magnetic iron oxides [Samples A to F] obtained in the above Examples and Comparative Examples,
Table 1 shows the measurement results of coercive force, Fe ²⁺ / total Fe, and magnetization reversal volume. A video tape was manufactured in the following manner, and the measured tape characteristics and noise characteristics (YS / N, CS /
N) is shown in Table 2. From these, the cobalt-containing magnetic iron oxide powder of the present invention is excellent in dispersibility and orientation, and the noise characteristics (CS / N) of the color signal due to the excellent surface properties of the video tape manufactured using the same. ) Is excellent. Further, in the relationship between the BET specific surface area and the magnetization reversal volume, the magnetization reversal volume is small and log [V] ≦ −
It can be seen that the equation (3.05 log [SSA] -11.35) has a clear characteristic, and that the noise characteristic (YS / N) of the luminance signal is excellent due to the small magnetization reversal volume. .

[Manufacture of Video Tape and Measurement of Noise Characteristics] A paint was prepared with the following composition. Cobalt-containing magnetic iron oxide powder 100 parts by weight Vinyl chloride / vinyl acetate copolymer 6.0 {Polyurethane resin 14.2} Dispersant 3.6 << Conductive agent 3.2 >> Abrasive 2.0 >> Lubricant 1.2 〃 Curing agent 2.4 溶 剤 Solvent 249 磁性 A magnetic paint composed of the above composition is applied on a polyester base film in a thickness of 5 μm, dried, calendered,
Curing was performed at 60 ° C. for 16 hours. Then, it was cut into a 0.5-inch width with a slitter and assembled into a VHS cassette half. Video noise characteristics (YS / N, CS /
N) was measured using a video noise meter (925D / 1, manufactured by Shibasoku Co., Ltd.).

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

The cobalt-containing magnetic iron oxide powder obtained by the method of the present invention is suitable as an element for a magnetic recording medium, has a good dispersibility in a medium, and has particularly good noise characteristics (Y- (S / N, C-S / N), it is extremely suitable for an image recording medium. Further, the method of the present invention is a method capable of extremely easily producing a cobalt-containing magnetic iron oxide powder suitable for an element for a magnetic recording medium by using γ-FeOOH as a precursor and making it extremely efficient. It is.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Koichi Tamura 1 Ishiharacho, Yokkaichi-shi, Mie Ishihara Sangyo Co., Ltd. (72) Inventor Tokuo Suita 1 Ishiharacho, Yokkaichi-shi, Mie Ishihara Sangyo Co., Ltd. Examiner Yoshizo Hiratsuka (58) Field surveyed (Int.Cl. ⁷ , DB name) H01F 1/11 C01G 49/00

Claims (2)

(57) [Claims] The present invention relates to a method for applying an aluminum compound to the surface of lepidocrocite particles, and then directly reducing the aluminum compound.
Alternatively, it is heated and dehydrated at a temperature lower than the temperature at which hematite is transformed, and then reduced, or after the reduction, further oxidized to obtain an iron oxide powder, and a cobalt compound or a cobalt compound and another metal compound on the surface thereof. And producing a cobalt-containing magnetic iron oxide powder satisfying the following formulas (1) and (2). Formula (1) 20 ≦ SSA ≦ 50 Formula (2) −16.70 ≦ log [V] ≦ −3.05 l
og [SSA] -11.35 [wherein SSA is a BET specific surface area (m ² / g),
V is the magnetization reversal volume (cm ³ ). ] 2. The method for producing a cobalt-containing magnetic iron oxide powder according to claim 1, wherein the amount of the aluminum compound deposited is 0.05 to 3.0% by weight as Al with respect to the weight of lepidocrocite.