JPH09188520A - Production of fusiform goethite - Google Patents

Abstract

PROBLEM TO BE SOLVED: To profitably obtain fusiform goethite particles uniform in particle distribution and having a high axial ratio by lowering an oxidation rate to a specific range in a time specific in the oxidation degree of a ferrous compound in a suspension, when a suspension containing FeCO3 is subjected to an oxidation reaction treatment comprising blowing an oxygen-containing gas in the suspension. SOLUTION: A suspension containing FeCO3 is oxidized with an oxygencontaining gas to produce the fusiform goethite. Therein, the following means is employed: the oxidation reaction of the suspension is started at an oxidation rate (γ1), and subsequently lowered to 20-80% of the (γ1), when the oxidation degree of the ferrous compound in the suspension is 4-20%. The suspension containing FeCO3 is obtained by reacting a ferrous salt aqueous solution with an alkali carbonate aqueous solution or its mixture with an alkali hydroxide. A means for lowering the oxidation rate includes a method for controlling an oxygen-supplying rate and/or a method for controlling a stirring rate.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an axial ratio (long axis) which is suitable as a starting material for producing magnetic particle powders for magnetic recording and has excellent particle size distribution and does not contain dendritic particles. The present invention relates to a method for industrially obtaining goethite particles having a high spindle shape (diameter / minor axis diameter).

[0002]

2. Description of the Related Art In recent years, as the output and the recording density of a magnetic recording medium have been increased, the magnetic particle powder used for the magnetic recording medium is required to have characteristics such as high coercive force and excellent dispersibility. ing. The magnitude of the coercive force of the magnetic particle powder does not depend on any of shape magnetic anisotropy, crystal magnetic anisotropy, strain magnetic anisotropy, and exchange magnetic anisotropy, or their interaction. Widely known. At present, magnetic particle powders for magnetic recording use shape magnetic anisotropy to increase coercive force by increasing the axial ratio. In addition, in order to improve the properties such as the dispersibility of the magnetic particle powder in the paint and the orientation and filling properties in the coating film, the particle size distribution of the magnetic particle powder should be as uniform as possible, It is desirable that they are not mixed.

These magnetic particle powders are mainly composed of goethite particles as a starting material or hematite particles obtained by heat-treating the goethite particles by reducing with a reducing gas such as hydrogen, and magnetite particles or iron as a main component. It is also possible to obtain a magnetic powder of the above-mentioned metal, or to oxidize the magnetite particles with an oxygen-containing gas to obtain maghemite particles. It is widely known that the shape of these magnetic particle powders is derived from the shape of the goethite particles as the starting material, and in order to obtain magnetic particle powders having high coercive force and excellent dispersibility, the starting material It is necessary that the particle size distribution of the goethite particles is uniform, that the dendritic particles are not mixed, and that the goethite particles have a high axial ratio.

As an industrial method for manufacturing goethite,
Broadly, there are the following methods a and b. a. A method of oxidizing an aqueous solution containing ferrous hydroxide produced by adding an equivalent amount or more of alkali to an aqueous solution of ferrous salt at a pH of 11 or more (Japanese Patent Publication No. 39-5610). b. A method of oxidizing at a pH of 7 to 11 by using an equivalent amount or more of alkali carbonate alone or a mixture of alkali carbonate and alkali hydroxide as an aqueous alkali solution added to an aqueous solution of ferrous salt (JP-A-50-80999).

As described above, the goethite particles suitable as a starting material for producing the magnetic particle powder for magnetic recording have a uniform particle size distribution, do not contain dendritic particles, and have a high axial ratio. Although it is most required that the method a) can obtain particles having a high axial ratio, it has a drawback that dendritic particles are likely to be mixed and the width of the particle size distribution is wide and the uniformity is poor. doing. As a method of improving this drawback, a method of adding a different element (for example, Japanese Patent Laid-Open No. Hei.
No. 317421), and a manufacturing method in which the device or operating conditions are devised (for example, Japanese Patent Laid-Open No. 5-170452), it is hard to say that the particle size distribution is uniform.

The method b) has a more uniform particle size distribution than the method a and produces goethite particles in which dendritic particles are not mixed, but on the other hand, it has a high axial ratio. There is a drawback that particles are difficult to obtain, and further, when it is attempted to generate fine goethite particles, the axial ratio tends to be significantly reduced. As a method of increasing the axial ratio, a method of adding a different element or a different compound (for example, Japanese Patent Application Laid-Open No. 6-16425) is disclosed, but it becomes difficult to control the major axis diameter and the manufacturing cost is high. It is hard to say that it is an industrially advantageous method such as increasing the cost.

[0007]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for industrially producing spindle-shaped goethite particles having a uniform particle size distribution and a high axial ratio.

[0008]

[Means for Solving the Problems] As a result of intensive studies for solving the above problems, the present inventors have blown an oxygen-containing gas into a suspension containing FeCO ₃ to cause an oxidation reaction to produce spindle-shaped goethite. In the method, ferrous iron (2
The present invention has been completed by finding that the axial ratio of goethite particles can be increased by reducing the oxidation rate at a specific time when the oxidation rate of (valence) is specific.

That is, the gist of the present invention is: (1) When a suspension containing FeCO ₃ is oxidized with an oxygen-containing gas to produce spindle-shaped goethite, the suspension has an oxidation rate (r1). After the oxidation reaction is started by the above, when the oxidation rate of ferrous iron in the suspension is between 4 and 20%, the oxidation rate is set to 2 of (r1).
A method for producing a spindle-shaped goethite, which is characterized by reducing the amount to 0 to 80%, and (2) a suspension containing FeCO ₃ obtained by reacting an aqueous solution of an alkali carbonate with an aqueous solution of a ferrous salt. The method for producing a spindle-shaped goethite according to (1) above, and (3) a suspension containing FeCO ₃ is obtained by reacting a mixed aqueous solution of alkali carbonate and alkali hydroxide with a ferrous salt aqueous solution. (1) The method for producing the spindle-shaped goethite according to (1) above, wherein the means for reducing the oxidation rate is to control the oxygen supply rate.
(3) The method for producing a spindle-shaped goethite according to any one of
(5) The method for producing a spindle-shaped goethite according to any one of (1) to (3) above, wherein the means for reducing the oxidation rate is controlling the stirring rate.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
The suspension containing FeCO ₃ is usually prepared by reacting an aqueous solution containing an alkali carbonate with an aqueous solution of a ferrous salt, or by reacting a mixed aqueous solution of an alkali carbonate and an alkali hydroxide with a ferrous salt. can get. The latter is more preferable because the axial ratio can be increased. When obtaining the suspension,
It is desirable to remove the dissolved oxygen from the ferrous iron salt aqueous solution and the alkaline aqueous solution in advance and react them in a non-oxygen atmosphere to prepare the suspension.

The ferrous salt is not particularly limited,
One or more selected from the group consisting of ferrous sulfate, ferrous chloride, ferrous nitrate, and ferrous acetate can be used. In the present invention, the above-mentioned ferrous salt is usually made into an aqueous solution prior to the reaction. The ferrous iron concentration at this time is usually 0.5 to 2.0 mol / L.
And preferably 1.0 to 1.7 mol / L, and more preferably 1.3 to 1.5 mol / L. If the concentration of ferrous iron is less than 0.5 mol / L, productivity is poor and it is difficult to say that it is industrially advantageous.
If it exceeds ol / L, it is difficult to obtain goethite having a uniform particle size distribution.

On the other hand, as the alkali carbonate used,
Sodium carbonate, sodium bicarbonate, potassium carbonate,
Examples thereof include potassium hydrogen carbonate, ammonium carbonate, ammonium hydrogen carbonate and the like. Examples of alkali hydroxides include sodium hydroxide, potassium hydroxide, ammonium hydroxide and the like. Usually, the alkali is also an aqueous solution.

The amount of alkali used is preferably 1 to 4 equivalents, more preferably 1.5 to 3 equivalents, based on ferrous iron. When the amount of alkali used is less than 1 equivalent, it is difficult to generate goethite having a uniform particle size distribution, and when it is used in excess of 4 equivalents, it becomes costly and it is not industrially advantageous.

When a mixed aqueous solution of alkali carbonate and alkali hydroxide is used as the alkaline aqueous solution, the ratio of the amounts of the alkali carbonate and the alkali hydroxide to be used is the alkali mixing ratio ([OH ⁻ ] / 2 [CO ₃ ²⁻ ]) Is preferably 0.5 or less, and more preferably 0.3 or less. When the ratio of alkali hydroxide is more than 0.5, dendritic particles are mixed and it becomes difficult to obtain spindle-shaped goethite particles having a uniform particle size distribution. The concentration of ferrous iron in the suspension containing FeCO ₃ is
Usually 0.05 to 0.9 mol / L, preferably 0.3 to 0.7 mol / L
And more preferably 0.4 to 0.65 mol / L. If the ferrous iron concentration is lower than 0.05 mol / L, the productivity is poor, and 0.9 mol / L
At a concentration higher than / L, the viscosity of the suspension becomes too high, making it difficult to carry out a uniform oxidation reaction, and the particle size distribution tends to be non-uniform.

When the suspension is obtained, the temperature of the ferrous salt aqueous solution and the alkaline aqueous solution is preferably set to be equal to the oxidation reaction temperature of the subsequent oxidation reaction step, and the suspension is aged. When the oxidation reaction step is carried out later, it is preferable to make it the same as the aging temperature. Setting the aging step after the preparation of the suspension until the start of the oxidation reaction is effective in performing stable and reproducible particle size control. The aging temperature is usually 30 to 55 ° C., preferably the same temperature as the oxidation reaction. The aging temperature is preferably the same as the oxidation reaction temperature in terms of operation. Aging time is usually
It is 30 minutes or more, preferably 60 to 240 minutes, and more preferably 120 to 180 minutes. If the aging is within 30 minutes, the effect of aging is not exhibited, and there is no reason to ripen longer than necessary.

As the oxygen-containing gas for promoting the oxidation reaction of the suspension, air, oxygen, a mixed gas of air and oxygen, a mixed gas of oxygen and nitrogen, a mixed gas of air and nitrogen, or the like is used. You can The flow rate of the oxygen-containing gas to be supplied varies depending on the volume of the suspension, the size of the reactor, the size of the target goethite particles, etc., but is 0 for 1 L of the suspension.
It is about 001-10L / min.

The temperature of the oxygen-containing gas is preferably between 0 ° C. and the oxidation reaction temperature. When the temperature of the oxygen-containing gas exceeds the oxidation reaction temperature, the effect of increasing the axial ratio intended by the present invention becomes difficult to be exhibited. Further, it is not necessary to lower the temperature below 0 ° C. The oxidation reaction in the present invention is performed at
7-11. As the reactor used for the oxidation reaction, a gas-liquid contact reactor such as an ordinary bubble column reactor or a stirring bubble column reactor can be used. The use of a stirred bubble column reactor equipped with a draft tube, a reactor equipped with an external circulation line, and a reactor equipped with an external circulation line passing through a disperser is not limited.

The oxidation rate is obtained by measuring the oxygen concentration in the gas discharged from the suspension with an oxygen concentration meter (for example, manufactured by Fujikura Ltd.,
(FCX-SK) and measured over time, and back calculated from the amount of oxygen consumed in the reaction per unit time. The oxidation rate (r) can be calculated by equation (1). (R) = (Qin · Cin−Qout · Cout) / N · V · Vm (1) (r): Oxidation rate [mol / Lhr] Qin: Oxygen-containing gas inlet flow rate [L / hr] Cin: Inlet Concentration of oxygen [vol%] Qout: outlet flow rate of oxygen-containing gas [L / hr] Cout: oxygen concentration of outlet [vol%] N: stoichiometric ratio (= 1/4) V: volume of suspension [ L] Vm: molar volume of ideal gas at 0 ° C, 1 atm = 22.4 [L
/ mol] The stoichiometric ratio N was set to 1/4 according to the reaction formula (2) of the main oxidation reaction. FeCO ₃ + 1/4 O ₂ + 3/2 H ₂ O → FeOOH + H ₂ CO ₃ (2) Oxidation rate is the iron content before oxidation of the iron concentration of the oxidized portion obtained by time integration of the oxidation rate. It can be calculated as a ratio to the concentration of ferrous iron.

The present inventors started the oxidation reaction at an oxidation rate (r1) as described above, and at the time when the oxidation rate of ferrous iron in the suspension was between 4 and 20%. , Oxidation rate (r1)
It was found that the spindle-shaped goethite particles having a high axial ratio can be obtained by reducing it to 20 to 80%. In order to increase the axial ratio, the oxidation rate may be relatively reduced with respect to (r1) at the time when the oxidation rate becomes 4 to 20%, and the absolute value of (r1) is particularly limited. Not a thing. The reason for this is that in the present invention, the growth rate in the major axis direction and the minor axis direction of the generated crystal nuclei is changed by changing the oxidation rate during the nucleation period and particle growth period in the oxidation reaction, and the axial ratio is increased. It is thought that this is possible, but the absolute value of (r1) mainly affects only the size of crystal nuclei generated during the nucleation stage.

The timing of nucleation varies depending on the conditions such as the alkali equivalent ratio, the concentration of ferrous iron, the initial oxidation rate, and the oxidation reaction temperature, but it occurs when the oxidation rate reaches 5 to 10%. It is estimated that In the present invention, the time when the oxidation rate is lowered in order to obtain the effect of increasing the axial ratio is a time when the oxidation rate is between 4 and 20%, more preferably 5 to 15%. If the oxidation rate is less than 4%,
It is presumed that nucleation did not occur, and it is difficult to achieve the high axial ratio aimed at by the present invention. Further, even if the oxidation rate is reduced after the oxidation rate exceeds 20%, the effect of increasing the axial ratio is not noticeable. In the present invention, "when the oxidation rate is between 4% and 20%" means that the oxidation rate may be decreased at any time during this period, and is not particularly limited, but after the oxidation rate is decreased. Preferably maintains a reduced oxidation rate by at least 20%.

If the oxidation rate reduced at the time when the oxidation rate of ferrous iron is 4 to 20% is 20 to 80% of the oxidation rate (r1) at which the oxidation reaction is started, the high axial ratio in the present invention can be obtained. The effect is remarkably obtained. The reduced oxidation rate is (r
If it is more than 80% of 1), it is difficult to obtain the effect of increasing the axial ratio, and if the oxidation rate is reduced to less than 20% of (r1) too much, the axial ratio cannot be increased and the reaction time becomes long, which is an industrial problem. It is hard to say that it is advantageous. The oxidation rate after the decrease is
30 to 70% of the initial oxidation rate (r1) is more preferable,
40-60% is the most preferable.

After the rate of oxidation of ferrous iron is reduced to 4 to 20% and the rate of oxidation is reduced to 20 to 80% of (r1), the rate of oxidation may be increased, but until the rate of oxidation reaches 50%. Preferably has an oxidation rate not exceeding (r1). More preferably, the oxidation rate does not exceed (r1) until the oxidation rate reaches 75%. More preferably,
The oxidation rate remains the same until the oxidation rate reaches 100% (r1)
Is between 20 and 80%.

Next, the means for reducing the oxidation rate will be described. The oxidation reaction in the present invention is a gas-liquid contact heterogeneous reaction that proceeds by blowing an oxygen-containing gas into a suspension. In this case, the oxidation rate is affected by the suspension temperature (= oxidation reaction temperature), the solubility of oxygen, the oxygen partial pressure, the oxygen supply rate, the gas-liquid interface area, and the like.

The solubility of oxygen depends on the concentration of ions in the suspension,
Depends on suspension temperature and gas temperature. It is operationally difficult to raise the suspension temperature to lower the oxygen solubility and lower the oxidation rate, and it cannot be said to be an industrially advantageous method. The oxygen partial pressure method is possible by operating the pressure in the reactor in a pressurized system.

As a method for controlling the oxygen supply rate, a method of changing only the oxygen concentration with the gas supply rate kept constant, a method of changing the gas supply rate with the oxygen concentration kept constant, and changing both the oxygen concentration and the gas supply rate. There is a method, and any method is possible.

The gas-liquid interface area can be controlled by changing the stirring speed. That is, it is possible to easily reduce the oxidation rate by slowing the stirring rate, and it is possible to perform the method simultaneously with the method based on the oxygen supply rate. In the method of the present invention, one or more of the above means are used to reduce the oxidation rate to the desired value, thereby increasing the goethite axial ratio.

Next, the oxidation reaction temperature is set to a suspension temperature and may be in a temperature range in which goethite is produced, preferably 80 ° C. or lower, more preferably 30 to 55 ° C., and the desired goethite. Determined by particle size. The goethite particle size generally means the major axis diameter, and is strongly influenced by the suspension temperature. The lower the suspension temperature, the finer the goethite particles are formed.

The major axis diameter, the minor axis diameter, and the axial ratio of goethite are as follows, after washing the goethite slurry obtained by the reaction with water.
Take a picture with a transmission electron microscope (TEM)
The major axis diameter and the minor axis diameter of the primary particles are read and obtained from the photograph. The average major axis diameter and the average axial ratio are determined by averaging the measured values of 300 primary particles. The particle size distribution is represented by the coefficient of variation σ Y / Y of the major axis diameter. σ Y is the standard deviation of the major axis diameter, and Y is the average major axis diameter. The smaller the coefficient of variation is, the more uniform the particle size distribution is. It is difficult to produce goethite having a uniform particle size distribution in which the variation coefficient of the major axis diameter is 0.3 or less by the method of oxidizing at pH 11 or more like the method of a) described in the section of the prior art. On the other hand, in the manufacturing method of the present invention, the variation coefficient of the major axis diameter is 0.3
It is possible to produce goethite having the following uniform particle size distribution.

As the magnetic recording medium has a higher output and a higher recording density, the particle size of goethite, which is a raw material of the magnetic particle powder, is required to be finer. The average major axis diameter of goethite particles is 0.
Fine particles of about 05-0.4 μm are required, but the smaller the particles, the smaller the goethite axial ratio. Therefore, the present invention provides an industrially advantageous method for producing fine-grain goethite having a high axial ratio.

In the present invention, in order to improve the characteristics of the magnetic particles, a different element other than Fe such as Ni, Co, Zn, P, Al and Si, which is usually added during the production of goethite particles, is included. It is also possible to add a compound and / or an organic substance, and in this case, the effect aimed at by the present invention can be obtained.

[0031]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples and the like.

Example 1 In a stirred bubble column reactor having an internal volume of 10 L, 450 g of sodium carbonate (purity 100%) and sodium hydroxide (purity 9
6%) and 62.5 g were dissolved in deionized water 3.33.
An alkaline aqueous solution of L was prepared, and nitrogen was blown into the alkaline aqueous solution to expel dissolved oxygen. Ferrous sulfate (heptahydrate) 6 in ion-exchanged water from which dissolved oxygen has been sufficiently removed
A 1.67 L ferrous sulfate aqueous solution in which 95 g was dissolved was introduced into the above alkaline aqueous solution in a nitrogen atmosphere for 3 minutes to prepare a suspension 5 L (38 ° C.) containing FeCO ₃ (suspension). Ferrous iron concentration = 0.5 mol / L, alkali equivalent ratio ([CO ₃ ²⁻ ] / [Fe ²⁺ ] = 1.7, [OH ⁻ ] / 2
[Fe ²⁺ ] = 0.3)). The suspension was sparged with 5 L / min of nitrogen and stirred at a stirring speed of 600 rpm while
Aged for 120 minutes at ° C.

Next, the stirring speed was set to 700 rpm (peripheral edge speed 2.9 m / s), the blowing of nitrogen was stopped, and air (room temperature) was blown at 5 L / min to start the oxidation reaction.
The change with time of the oxygen concentration in the gas discharged from the upper part of the reactor was measured and recorded, and the oxidation rate was calculated from the amount of oxygen consumed in the oxidation reaction by using the above formula (1). The oxidation rate (r1) at the initial stage of this reaction was 0.53 mol / Lhr.
Then, 6 minutes after starting the oxidation reaction, the stirring speed was increased to 5
It was reduced to 00 rpm. The oxidation rate at this time is 0.3
It was 2 mol / Lhr. The oxidation rate after 6 minutes is 11%,
The oxidation rate after lowering the stirring rate is 60 which is the initial oxidation rate (r1)
%Met. The subsequent oxidation reaction was performed at the same oxidation rate as that after the reduction at 38 ° C. until the oxidation rate reached 100%. The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced. Further, the major axis diameter and the minor axis diameter of 300 primary particles were read from the TEM photograph to determine the average major axis diameter and the average axial ratio. These are shown in Table 1.

Example 2 The procedure of Example 1 was repeated, except that the stirring speed was reduced 10 minutes after the oxidation reaction was started. The oxidation rate after 10 minutes was 18%, and the oxidation rate after reducing the stirring rate was 60% of the initial oxidation rate (r1). After washing the obtained goethite slurry with water, a transmission electron microscope (T
It was confirmed by EM) that spindle-shaped goethite was generated. The average major axis diameter and the average axial ratio are shown in Table 1.

[0035]

[Table 1]

Example 3 The procedure of Example 1 was repeated, except that the stirring speed was reduced 4 minutes after the oxidation reaction was started. The oxidation rate after 4 minutes was 7%, and the oxidation rate after reducing the stirring rate was 60% of the initial oxidation rate (r1). After washing the obtained goethite slurry with water, a transmission electron microscope (TEM)
It was confirmed by observation that the spindle-shaped goethite was generated. The average major axis diameter and the average axial ratio are shown in Table 1.

Example 4 Oxidation reaction was carried out at an aging temperature and an oxidation temperature of 45 ° C., a stirring rate at the start of oxidation of 600 rpm, an air blowing rate of 1 L / min, and an oxidation rate (r1) of 0.18 mol / Lhr. Was started, and then, after 8 minutes, the procedure was performed in the same manner as in Example 1 except that the air flow rate was reduced to 0.5 L / min as a means for reducing the oxidation rate. The oxidation rate when the oxidation rate was lowered was 5%, and the lowered oxidation rate was 65% of the initial oxidation rate (r1). The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced.
The average major axis diameter and the average axial ratio are shown in Table 1.

Example 5 The same as Example 4 except that the air flow rate was reduced 16 minutes after the initiation of the oxidation reaction. The oxidation rate after 16 minutes was 10%, and the oxidation rate after the decrease was 65% of the initial oxidation rate (r1). The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced.
The average major axis diameter and the average axial ratio are shown in Table 1.

Example 6 The stirring speed at the start of oxidation was 600 rpm, the amount of blown air was 5 L / min, and the oxidation speed (r1) was 0.29 mol / Lhr.
The oxidation reaction was started at 12 minutes, then the stirring speed was reduced to 500 rpm after 12 minutes, and at the same time, the air flow rate was 1 L.
It was carried out in the same manner as in Example 4 except that the oxidation rate was decreased to 0.1 / mol / Lhr. The oxidation rate when the oxidation rate was lowered was 12%, and the oxidation rate after the reduction was 41% of the initial oxidation rate (r1). After washing the obtained goethite slurry with water, a transmission electron microscope (TE
M), and it was confirmed that spindle-shaped goethite was generated. The average major axis diameter and the average axial ratio are shown in Table 1.

Example 7 Example 6 was carried out in the same manner as in Example 6 except that the stirring speed was decreased 5 minutes after the oxidation reaction was started. The oxidation rate after 5 minutes was 5%, and the oxidation rate after the decrease was the initial oxidation rate (r
It was 41% of 1). The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM).
It was confirmed that spindle-shaped goethite was generated. The average major axis diameter and the average axial ratio are shown in Table 1.

Example 8 384 g of sodium carbonate (purity 100%) was dissolved in ion-exchanged water in a stirred bubble column reactor having an internal volume of 10 L.
An alkaline aqueous solution of L was prepared, and nitrogen was blown into the alkaline aqueous solution to expel dissolved oxygen. Then, 1 L of an aqueous ferrous sulfate solution in which 348 g of ferrous sulfate (heptahydrate) was dissolved in ion-exchanged water from which dissolved oxygen was sufficiently removed,
In a nitrogen atmosphere, the mixture was poured into the above alkaline aqueous solution for 3 minutes to prepare a suspension 5 L (47 ° C.) containing FeCO ₃ (concentration of ferrous iron in suspension = 0.25 mol / L, alkali equivalent). Ratio ([CO ₃ ²⁻ ] / [Fe ²⁺ ] = 2.9)). Nitrogen was blown into the suspension at 5 L / min and the mixture was aged at 47 ° C. for 230 minutes while stirring at a stirring speed of 600 rpm.

Next, at a stirring speed of 600 rpm, blowing of nitrogen was stopped and air (room temperature) was blown at 15 L / min to start the oxidation reaction. The change with time of the oxygen concentration in the gas discharged from the upper part of the reactor was measured and recorded, and the oxidation rate was calculated from the amount of oxygen consumed in the oxidation reaction by using the above formula (1). The oxidation rate (r1) at the beginning of this reaction is
It was 0.09 mol / Lhr. Then, 17 minutes after starting the oxidation reaction, the stirring speed was reduced to 500 rpm. At this time, the oxidation rate was 0.05 mol / Lhr. 1
The oxidation rate after 7 minutes was 10%, and the oxidation rate after the decrease was 60% of the initial oxidation rate (r1). The subsequent oxidation reaction was performed at the same oxidation rate as that after the reduction at 47 ° C. until the oxidation rate reached 100%. The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced. Table 2 shows the average major axis diameter and the average axial ratio.

[0043]

[Table 2]

Comparative Example 1 Goethite was manufactured in the same manner as in Example 1 except that the stirring rate during the oxidation reaction was not lowered and was kept constant at 700 rpm, except that the oxidation rate was not lowered. After washing the obtained goethite slurry with water, a transmission electron microscope (T
It was confirmed by EM) that spindle-shaped goethite was generated. The average major axis diameter and the average axial ratio are shown in Table 1.

Comparative Example 2 The procedure of Example 4 was repeated, except that the air flow rate was reduced 5 minutes after the oxidation reaction was started. The oxidation rate after 5 minutes was 3%, and the oxidation rate after the decrease was 65% of the initial oxidation rate (r1). The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced.
The average major axis diameter and the average axial ratio are shown in Table 1.

Comparative Example 3 The same procedure as in Example 6 was performed except that the stirring rate and the air flow rate during the oxidation reaction were not lowered, and the stirring rate was 600 rpm and the air flow rate was 5 L / min. did. The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced. The average major axis diameter and the average axial ratio are shown in Table 1.

Comparative Example 4 51 minutes after starting the oxidation reaction, the same procedure as in Example 4 was performed except that the air flow rate was reduced. The oxidation rate after 51 minutes was 31%, and the oxidation rate after the decrease was 65% of the initial oxidation rate (r1). The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced. The average major axis diameter and the average axial ratio are shown in Table 1.

Comparative Example 5 The procedure of Example 6 was repeated except that the stirring speed was reduced 24 minutes after the oxidation reaction was started. The oxidation rate after 24 minutes was 23%, and the oxidation rate after the decrease was 41% of the initial oxidation rate (r1). The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced. The average major axis diameter and the average axial ratio are shown in Table 1.

Comparative Example 6 The stirring speed at the start of oxidation was 600 rpm, the blowing air amount was 5 L / min, and the oxidation speed (r1) was 0.27 mol / Lhr.
The oxidation reaction was started at 20 minutes, and then 20 minutes later, the procedure was carried out in the same manner as in Example 4 except that the stirring rate was lowered to 550 rpm as a means for reducing the oxidation rate. The oxidation rate when the oxidation rate was lowered was 18%, and the oxidation rate after the reduction was 84% of the initial oxidation rate (r1). After washing the obtained goethite slurry with water, a transmission electron microscope (T
It was confirmed by EM) that spindle-shaped goethite was generated. The average major axis diameter and the average axial ratio are shown in Table 1.

Comparative Example 7 The stirring rate at the start of oxidation was 750 rpm, the amount of blown air was 5 L / min, the oxidation reaction was started at an oxidation rate (r1) of 0.6 mol / Lhr, and 6 minutes later, the stirring rate was increased. 30
Example 1 was repeated except that the speed was reduced to 0 rpm. The oxidation rate when the oxidation rate was stopped was 12%, and the oxidation rate after the reduction was 15% of the initial oxidation rate (r1). The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced. The average major axis diameter and the average axial ratio are shown in Table 1.

Comparative Example 8 Example 8 was repeated except that the stirring rate during the oxidation reaction was kept constant at 600 rpm and the oxidation rate was not lowered.
Was performed in the same manner as described above. The obtained goethite slurry was washed with water and then observed with a transmission electron microscope (TEM) to confirm that spindle-shaped goethite was produced. Table 2 shows the average major axis diameter and the average axial ratio.

The axial ratios of the goethite major axis diameters of Examples 1 to 7 and Comparative Examples 1 to 7 in Table 1 are shown in FIG. From FIG.
It can be seen that the axial ratio tends to decrease as the major axis diameter of goethite decreases. With the increase in output and recording density of magnetic recording media, the particle size of goethite, which is a raw material of the magnetic particle powder, is required to be finer. According to the method of the present invention, As shown in Examples 1 to 7, it is possible to manufacture spindle-shaped goethite having a higher axial ratio than the comparative example. That is, comparing the axial ratios of the example and the comparative example with the same major axis diameter, the method of the present invention makes it possible to produce spindle-shaped goethite having a clearly high axial ratio. Further, the goethite obtained by the method of the present invention is a goethite having a uniform particle size distribution with a major axis diameter variation coefficient σ Y / Y of 0.3 or less, which is a characteristic of the spindle-shaped goethite.

The present invention also contemplates the case where the alkali used is only alkali carbonate and the concentration of ferrous iron in the suspension containing FeCO ₃ is 0.25 mol / L as in Example 8 of Table 2. However, it is possible to manufacture spindle-shaped goethite with a high axial ratio.

[0054]

Industrial Applicability According to the production method of the present invention, spindle-shaped goethite particles having a uniform particle size distribution and a high axial ratio, which are suitable as a starting material for producing magnetic particle powder for magnetic recording, are industrially advantageous. It is possible to manufacture

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship of the axial ratio to the major axis diameter of goethite obtained in Examples and Comparative Examples.

Claims (5)

[Claims] 1. When producing a spindle-shaped goethite by oxidizing a suspension containing FeCO ₃ with an oxygen-containing gas, after starting an oxidation reaction of the suspension at an oxidation rate (r1), A method for producing spindle-shaped goethite, which comprises reducing the oxidation rate to 20 to 80% of (r1) when the oxidation rate of ferrous iron in the suspension is between 4 and 20%. . 2. The method for producing spindle-shaped goethite according to claim 1, wherein the suspension containing FeCO ₃ is obtained by reacting an aqueous solution of an alkali carbonate with an aqueous solution of a ferrous salt. 3. The spindle-shaped goethite according to claim 1, wherein the suspension containing FeCO ₃ is obtained by reacting a mixed aqueous solution of alkali carbonate and alkali hydroxide with a ferrous iron salt aqueous solution. Production method. 4. The method according to claim 1, wherein the means for decreasing the oxidation rate is controlling the oxygen supply rate.
Item 3. The method for producing the spindle-shaped goethite according to the item. 5. The method for producing a spindle-shaped goethite according to claim 1, wherein the means for lowering the oxidation rate is controlling the stirring rate.