JPS5950607B2 - Method for producing acicular magnetic iron oxide particles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing acicular magnetic iron oxide particles for magnetic recording.

More specifically, it is a substantially high-density particle powder that retains acicular crystals and particle size, does not contain dendritic particles, and has an increased degree of crystallinity on the particle surface and inside the particle. Due to this, it is a magnetic material for magnetic recording that has high saturation magnetic flux density Vs and high coercive force Hc in terms of magnetic properties, and has high dispersibility, high orientation, and high packing property in terms of powder properties. It is an object of the present invention to provide a new technical means by which acicular crystal magnetite particles and acicular crystal maghemite particles powder particularly suited for the production of acicular crystal maghemite particles can be easily produced.

In recent years, as magnetic recording and reproducing equipment has become smaller and lighter, there has been an increasing need for higher performance recording media such as magnetic tapes and magnetic disks.

That is, high recording density, high sensitivity characteristics, high output characteristics, and especially improvement in frequency characteristics are required. The characteristics of a magnetic material suitable for satisfying the above-mentioned requirements for a magnetic recording medium are that it has a large saturation magnetic flux density and a high coercive force. Currently, acicular maghemite particles are mainly used as a magnetic recording material.

Acicular maghemite particles are generally produced by air oxidizing ferrous hydroxide colloid in an aqueous solution obtained by reacting an aqueous ferrous salt solution with an alkali (usually referred to as a wet reaction).
The obtained acicular goethite particles are heated and dehydrated in air at around 300°C to form acicular hematite particles, and further reduced at 300 to 400°C in a reducing gas such as hydrogen to form acicular magnetite particles. , then heat it in air for 200~3
It is obtained by oxidizing at 00°C to form acicular maghemite particles. The coercive force Hc of the acicular maghemite particles can be expressed by the following relational expression.

Hc-K・(Nb-Na)・Ms In this relational expression, K is the degree of crystallinity of the particles (Nb-Na) is the shape (acicularity) of the particles, and Ms is the chemical composition of the particles. This is a related matter.

As is clear from this relational expression, in order to improve the coercive force of the acicular crystal maghemite particles, it is necessary to maintain and inherit the acicular crystals of the acicular crystal goethite particles, and to increase the degree of crystallinity of the product maghemite particles. It is necessary to increase the

Conventionally, in the production of acicular crystal maghemite particles, heat treatments such as thermal dehydration, reduction, and oxidation are performed at relatively low temperatures as described above. The first consideration was how to preserve and pass on the legacy.

However, even though it is possible to retain and inherit the acicular crystals of the particles relatively well by employing heat treatment at a low temperature, the acicular maghemite particles produced have a small degree of crystallinity; The coercive force Hc also has a small value. on the other hand,
Output characteristics of magnetic recording media such as magnetic tapes and magnetic disks,
The sensitivity characteristics depend on the residual magnetic flux density Br, and the residual magnetic flux density Br depends on the dispersibility of the magnetic particles in the vehicle, the orientation and filling properties in the coating film.

In order to improve the dispersibility in the vehicle, the orientation in the coating film, and the filling property, the magnetic particles dispersed in the vehicle should have acicular crystals and be uniform in particle size. Further, it is required that dendritic particles are not mixed therein.

In order to obtain magnetic particles with such characteristics, the starting material, acicular goethite particles, must have acicular crystals, be uniform in particle size, and be free of dendritic particles. It is necessary. As mentioned above, in the manufacturing process of acicular magnetite particles and acicular maghemite particles, first, the starting material is acicular crystals, the particle size is uniform, and the dendritic particles are It is necessary to generate unmixed acicular goethite particles, and then how to inherit the acicular crystals and particle size while applying heat treatments such as reduction and oxidation to increase the degree of crystallinity. Enhanced substantially dense acicular magnetite particles powder and acicular maghemite particle powder.
``This is becoming a big issue.

The present inventor has been involved in the production and development of acicular crystal goethite particles for many years, and in the course of his research, he discovered that the particles have acicular crystals and are uniform in particle size. We have already developed a method to obtain acicular goethite particles that do not contain dendritic particles.

For example, as described below. That is, it has acicular crystals,
In addition, the acicular goethite particles, which have uniform particle size and do not contain dendritic particles, are prepared by adding a water-soluble silicate in advance to an alkaline aqueous solution used to obtain an aqueous solution containing Fe(0H)2 of PH11 or more. Add
Next, by reacting the ferrous salt aqueous solution with the alkali aqueous solution, P containing fine and uniform Fe(0H)2 is produced.
It can be obtained by obtaining an aqueous solution of Hll or higher and then oxidizing it. This method will be explained as follows.

Conventionally, acicular goethite particles obtained in the alkaline region above PHll generally have asymmetric particle sizes and contain dendritic particles, but this is due to Fe(0H )2 is asymmetric, and at the same time, the Fe(0H)2 plotter is
H) 2 particles themselves are asymmetric, and furthermore, Fe(
When producing acicular goethite particles from an aqueous solution containing 0H)2, the generation of acicular goethite core particles and the growth of the acicular goethite core particles occur simultaneously, and several times are repeated until the goethite production reaction is completed. This is also caused by the generation of new nuclear particles. As mentioned above, Fe(0H
) 2 containing Fe(0H ) 2 into sufficiently fine and uniform flocs, and at the same time, Fe(0H
) The Fe(0H)2 particles constituting the floc 2 can be made into sufficiently fine and uniform particles, and furthermore,
Due to the fact that water-soluble silicate has the effect of suppressing the oxidation reaction when generating acicular goethite particles from an aqueous solution containing Fe(0H)21, it is possible to prevent the generation of acicular goethite core particles and the acicular goethite particles. Since the growth of the acicular goethite core particles can be carried out in stages, it is possible to obtain acicular goethite particles which have uniform particle size and do not contain dendritic phylum particles.

The obtained acicular goethite particles contain a trace amount of Si. The water-soluble silicates used in the above method include sodium and potassium silicates.

The amount of water-soluble silicate added to the alkaline aqueous solution is Fe
It is 0.1 to 1.7 atomic % in terms of Si.

If it is 0.1 atomic % or less, the effect of obtaining acicular grains with uniform particle size and no dendritic particles is not sufficient, and if it is 1.7 atomic % or more, Magnetite particles are mixed in.

Figure 1 shows the amount of water-soluble silicate added to an alkaline aqueous solution and the amount of acicular goethite particles under conditions where the reaction iron (Fe2+) concentration, the pH of the reaction solution, the reaction temperature, and the amount of air aeration are constant. It is a relationship diagram of production reaction time.

In the figure, curves A, B, and C represent the reaction iron (two Fe concentrations: 0.3 m01/I, 0.4 m01/l, 0.7 m01
/1.

5 As shown in Figure 1, even though the reaction iron (Fe2+) concentration, the pH of the reaction solution, the reaction temperature, and the amount of air aeration are constant.
However, as the amount of water-soluble silicate added increases, the formation reaction time of acicular goethite particles is significantly shortened due to Fe(0H)2 flocculation and Fe(0H)2 flocculation. This is thought to be because the contact reaction between Fe(0H)2 particles constituting the oxide and dissolved oxygen was carried out very efficiently.

This means that as the amount of water-soluble silicate added increases, the Fe(0H)2 flocs become sufficiently fine and homogenized, and at the same time, the Fe(0H)2 flocs become finer and homogenized.

Fe (0H) makes up the flock. This is thought to indicate that the particles themselves are also refined and homogenized. Figure 2 shows the relationship between the amount of water-soluble silicate added to an aqueous alkaline solution and the specific surface area of acicular goethite particles produced under exactly the same reaction conditions as in Figure 1. be.

In the figure, curves A, B, and C each have a reaction iron concentration of 0.3 m
01/l, 0.4m01/l, and 0.7m01/l. Generally, when the reaction iron concentration is kept constant, the specific surface area of the particles tends to increase as the reaction time for forming acicular goethite particles becomes shorter. However, in the above method, the specific surface area of the particles remains almost constant despite the shortening of the formation reaction time of the acicular goethite particles, as shown in Figure 1. The water-soluble silicate is Fe(0H). It has the effect of suppressing the oxidation reaction when acicular goethite particles are produced by oxidizing an aqueous solution containing It is thought that this is to wake up. Figure 3 shows the relationship between the axial ratio of acicular goethite particles obtained under constant conditions except for the amount of water-soluble silicate added to the alkaline aqueous solution and the amount of water-soluble silicate added. It is.

As a result of repeated studies to improve the axial ratio of acicular goethite particles that have uniform particle size and do not contain dendritic particles, the inventors have developed Fe(0H).

and Zn(0H). A mixed aqueous solution of PHll or more containing (
However, Zn (0H). is 0.1 in terms of Zn with respect to Fe.
~2.3 at%)), a water-soluble silicate was added in advance to the alkaline aqueous solution used to obtain 0.1 to 1.7 at% of Fe in terms of Si (however, the amount of Zn(0H)2 mixed The ferrous salt aqueous solution and the water-soluble Zn salt are added so that the total amount of the water-soluble silicate added is 2.5 atomic % or less based on Fe (calculated as Zn and Si, respectively), and then the ferrous salt aqueous solution and the water-soluble Zn salt By reacting an aqueous solution with the alkali aqueous solution, fine and uniform Fe(0H) is produced. and Zn (0H)
. Obtain a mixed aqueous solution containing PHll or more, and then,
It has been found that when oxidized, it is possible to obtain acicular goethite particles having excellent acicular crystals, uniform particle size, and no dendritic particles. Figure 4
Figure 5 shows the Zn mixing amount and the axial ratio of needle crystals and goethite particles ( FIG. 3 is a relationship diagram between the long axis and the short axis.

Zn used in the above method includes water-soluble Zn salts such as zinc sulfate, zinc chloride, and zinc nitrate.

The water-soluble Zn salt aqueous solution may be mixed into the ferrous salt solution in advance, or the water-soluble Zn salt aqueous solution may be mixed with the ferrous salt solution.

The same effect can be obtained in either case. Zn(0H). The mixing amount and the addition amount of water-soluble silicate are 0.1 to 2.3 atomic % based on Fe in terms of Zn, and 0.1 to 2.3 atomic % in terms of Si based on Fe.
1 to 1.7 at%, provided that Zn(OH). The total amount of the mixed amount of the water-soluble silicate and the added amount of the water-soluble silicate may be set to 2.5 atomic % or less in terms of Zn and Si, respectively, based on Fe. ' The amount of water-soluble silicate added is 0.1 to 1.7 atomic % based on Fe in terms of Si, and Zn (0H).

and the sum of the added amount of water-soluble silicate is 2.5 atomic % or less in terms of Zn and Si, respectively, based on Fe, and Zn(OH). The mixing amount is 0.0 in terms of Zn with respect to Fe.
When it is less than 1 atomic %, the effect of improving the axial ratio of the particles is not sufficient, and when it is 2.3 atomic % or more, magnetite particles are mixed. Zn(0H).

When the sum of the mixed amount and the added amount of water-soluble silicate is 2.5 atomic % or more in terms of Zn and Si, respectively, granular magnetite particles will be mixed in. This will be explained with reference to FIG.

In FIG. 6, the shaded area is the area where the acicular goethite particles used in the present invention are generated, and the triangular area surrounded by points A, b, and c with straight line A as one side is the area where the acicular goethite particles used in the present invention are formed. This is the area where madanetite particles are generated.

That is, Zn(0H).

The production area of acicular goethite particles is determined by straight line A indicating that the sum of the mixed amount of The regions where granular magnetite particles are produced are distinguished. The acicular goethite particles obtained by the above method contain trace amounts of Si and Zn. Next, the needle-like crystals of the needle-like goethite particles obtained by the method described in detail above, which have uniform particle size, and are free of dendritic particles. By applying heat treatments such as heat dehydration, reduction, and oxidation while maintaining the particle size,
The problem is to obtain substantially dense acicular magnetite particles and acicular maghemite particles with an increased degree of crystallinity. As mentioned above, even if the acicular crystal structure and particle size of the particles can be maintained relatively well by applying heat treatments such as reduction and oxidation at low temperatures, the acicular crystal magnetite particle powder and the acicular crystal structure that are produced Crystalline maghemite particles have a low degree of crystallinity.

The higher the heat treatment temperature, the higher the degree of crystallinity. However, on the other hand, the deformation of the acicular crystal particles of the acicular magnetite particles and the acicular maghemite particles and the inter-particles Sintering becomes significant and the coercive force is extremely reduced. In particular, the shape of the particles is easily affected by the heating temperature, and when the atmosphere is particularly reducing, particle growth is significant, and a single particle grows beyond the size of the shell particle, and the outer shape of the shell particle becomes It gradually disappears, causing deformation of the particle shape and sintering of the particles and each other. As a result, the coercive force decreases. The present inventor has discovered that the acicular goethite particles containing a trace amount of Si used in the present invention are uniform in particle size and are not mixed with dendritic particles, and are obtained by heating and dehydrating the particles at around 300°C. Details of the deformation of particle shape and the sintering phenomenon between particles and particles when acicular hematite particles are used as a starting material and the starting material is heated and reduced in a reducing gas to produce acicular magnetite particles. We considered this.

That is, FIG. 7 shows the needle crystals used in the present invention that contain a trace amount of Si (Si content 1.1 atomic % relative to Fe), have uniform particle sizes, and do not contain dendritic particles. Acicular crystalline skeleton particles with an average major axis length of 0.50 μm and a specific surface area of 130 m2/g, which are made up of a group of fine hematite single particles obtained by heating and dehydrating goethite particles, are heated at 400°C in a hydrogen stream. This figure shows the relationship between the degree of reduction X (FeOx, 1.5>x>1.33) and the specific surface area of particles produced by thermal reduction in the thermal reduction process of heating and reducing them into acicular magnetite particles.

As can be seen from Figure 7, the specific surface area of the generated particles rapidly decreases as thermal reduction progresses, which is due to the rapid occurrence of deformation of the particle shape and sintering of the particles and their mutual particles. It shows.

The same tendency as in FIG. 7 was also observed when acicular goethite particles containing trace amounts of Si and Zn were used.

This phenomenon will be explained in detail below. The acicular goethite particles used in the present invention contain a trace amount of Si, and the acicular hematite particles used as a starting material also contain Si.

In general, acicular hematite particles obtained from acicular goethite particles containing Si have many pores generated by dehydration on the particle surface and inside the particles, and these pores are exposed to an increase in heating temperature. On the other hand, it is known that when the heating temperature exceeds 800° C., sintering progresses and the acicular crystal grains break down.

This is described in Japanese Unexamined Patent Publication No. 48-83100 as follows. Acicular goethite particles containing a trace amount of Si are produced during dehydration treatment or subsequent tempering (
High temperature heat treatment of acicular crystal hematite particles) During the work, high temperatures up to 800°C can be used without sintering of the acicular crystals. Acicular hematite particles, which have traditionally been commonly used as a starting material, are obtained by heating and dehydrating acicular goethite particles at a temperature of around 300°C.
These are acicular crystal shell particles that retain the outer shape of acicular crystal goethite particles, and these shell particles are composed of aggregated particles in which a large number of single particles are connected.

In this case, the reason why the acicular goethite particles are heated and dehydrated at a relatively low temperature of around 300° C. is because the primary consideration is how to retain and inherit the acicular crystals of the acicular goethite particles. However, the acicular hematite particles obtained by heating at a relatively low temperature around 300°C retain and inherit the acicular crystals, but on the other hand, the grain growth of single particles is not sufficient. Therefore, the degree of crystallinity of the particles is small. In particular, acicular goethite particles containing a trace amount of Si are heated and dehydrated at a low temperature around 300°C by a conventional method.
In this case, as is well known, the degree of crystallinity becomes even smaller due to the grain growth inhibiting effect of Si.

Therefore, acicular hematite particles containing a trace amount of Si have many pores on the particle surface and inside the particle, and only particles with a large specific surface area can be obtained. Figure 8 shows that the average major axis length is 0.55 μm and the specific surface area is 38 m·
Acicular goethite particles containing a small amount of Si (Si content 1.1 atomic % relative to Fe), uniform in particle size, and containing no dendritic particles are heated and dehydrated. This figure shows the relationship between the dehydration rate and specific surface area of particles produced under conditions of different dehydration rates during the process of forming acicular hematite particles. In the figure, curve A,
B and C have dehydration rates of 7.2 mol/min and 2.0 mol/min, respectively.
This is the case of 0.25 mol/min. As is clear from FIG. 8, the specific surface area of the acicular hematite particles containing a small amount of Si obtained by changing the dehydration rate differs; the slower the dehydration rate, the smaller the acicular hematite particle powder. Although it is possible to obtain crystalline hematite particles, the amount is about 50 to 80 m2/g at most. Trace amounts of Si and Z
When acicular goethite particles containing n were used, a similar tendency to that shown in FIG. 8 was observed.

In this way, when acicular hematite particles containing a trace amount of Si with insufficient particle growth and a low degree of crystallinity are thermally reduced in a reducing gas, the Since the growth of single particles, that is, the physical change is rapid, uniform growth of single particles is difficult to occur. It is thought that mutual sintering occurs, making particle formation more likely to dull.

Furthermore, since the atmosphere in the heat treatment in the thermal reduction process is reducing, a chemical change called a reduction reaction occurs at the same time as a physical change such as growth of a single particle.

Therefore, in order to obtain acicular magnetite particles and acicular maghemite particles that retain and inherit acicular crystals, it is necessary to simultaneously control physical and chemical changes.
Therefore, the thermal reduction treatment required a very long time and also required a large amount of reducing gas. The fact that the heat reduction treatment requires a long time causes deformation of the particle shape of the generated particles and further progresses sintering of the particles and the particles themselves. As mentioned above, the reason for the deformation of particle shape and sintering between particles and particles during the thermal reduction process is that the uniform particle growth of single particles occurs because the particle growth of single particles is rapid. It is conceivable that the physical change of particle growth of a single particle and the chemical change of reduction reaction occur simultaneously.

Next, in the heating oxidation process for converting acicular magnetite particles into acicular maghemite particles, no single particle growth occurs.

Regarding this, it is stated on page 19 of the 7th Ferrite Summer Seminar Abstracts (1977) that "γ-Fe.O. does not grow grains through sintering." Therefore,
In view of the above-mentioned phenomenon, the present inventor conducted heating and firing in a non-reducing atmosphere in which the physical change of particle growth of single particles and the chemical change of reduction reaction do not occur simultaneously, prior to the thermal reduction process. By achieving sufficient and uniform particle growth of single particles, the starting material can be used as a starting material that has a substantially high density with an increased degree of crystallinity and retains needle-shaped crystals. For example, we thought that it would be possible to prevent particle deformation and sintering between particles and particles during the thermal reduction process, since it is sufficient to mainly perform chemical changes during the thermal reduction process.

Then, the present inventors heated and dehydrated the needle-like goethite particles containing a trace amount of Si used in the present invention, and fired the nail-like crystal hematite particles in a non-reducing gas to form a single particle. Various methods are used to obtain starting material hematite particles that have a substantially high density with an increased degree of crystallinity and retain and inherit acicular crystals by ensuring sufficient and uniform particle growth of the particles. As a result of study, the present invention was arrived at.

That is, the present invention uses Fe(0H)2 or Fe(0H).

and Zn(0H). A water-soluble silicate is added in advance to an alkaline aqueous solution used to obtain an aqueous solution or a mixed aqueous solution containing PHll or more (however, Zn(0H) is 0.1 to 2.3 at% in terms of Zn with respect to Fe). 0.1 to 1.7 atomic % in Si terms relative to Fe (however, in the case of a mixed aqueous solution, Zn(0H).The sum of the mixed amount and the added amount of water-soluble silicate is 2.5 at. Fine and uniform Fe(0H)2 or Fe(0H)2 and Zn(0H). A mixed aqueous solution of PHll or higher containing
The average major axis length obtained by heating and dehydration is 0.3
~2.0 μm or 0.3 to 2.5 μm, and B
Acicular hematite particles having a specific surface area of 50 to 300 m2/g by the ET method and maintaining the long axis length and axial ratio of acicular goethite particles are placed in an atmosphere consisting of heated steam and non-reducing gas. Under the water vapor partial pressure -11-(P
By heating and firing at a temperature range of 350 to 700°C at a temperature of 350 to 700°C, the average major axis length is 0.1
~1.5 μm or 0.1 to 2.0 μm, and B
After forming substantially high-density acicular hematite particles inheriting acicular crystals with a specific surface area of 10 to 30 m2/g by the ET method, the acicular hematite particles were immersed in a reducing gas for 30 minutes.
This is a method for producing acicular magnetic iron oxide particle powder, which comprises heating and reducing the powder in a temperature range of 00 to 450° C. to obtain acicular magnetite particles, or further oxidizing it to obtain acicular maghemite particles.

The structure and effects of the present invention will be explained as follows. First, various findings on which the present invention is based will be described.

Acicular hematite particles containing a trace amount of Si, which are obtained by heating and dehydrating acicular goethite particles containing a trace amount of Si at around 300°C, retain the needle crystals as described above. However, on the other hand, the grain growth of single grains is not sufficient, and therefore the degree of crystallinity is very small.

Even with such acicular hematite particles that have a small degree of crystallinity and contain a trace amount of Si, it is possible to grow single particles by further heating and firing such as tempering. , the degree of crystallinity can also be increased. As mentioned above, the higher the temperature at which acicular hematite particles containing a trace amount of Si are fired in a non-reducing gas, the more effectively the growth of single particles can be achieved. Acicular hematite particles with an increased degree of crystallinity can be obtained, but at temperatures above 800°C, single particles grow beyond the size of the skeleton particles, causing deformation of the acicular crystal particles and the formation of particles. It is known that they cause mutual sintering.

Furthermore, grain growth of single grains is achieved by heating and firing at a temperature as high as possible within a temperature range of 800°C or less that can retain and inherit the acicular crystals of the starting material containing a trace amount of Si. Therefore, a method for obtaining acicular hematite particles containing a trace amount of Si and having an increased degree of crystallinity is known.

For example, Japanese Patent Application Laid-Open No. 52-95097 describes the following. "α- that has been adsorbed or mixed with Si
By heating and firing FeOOH or α-Fe2O3 particles under appropriate heat treatment conditions, sintering between particles is suppressed and acicularity is maintained, while dehydration and pore-sealing properties are promoted. Acicular hematite particles with "high crystal integrity" can be obtained.

According to the description in the examples, "appropriate heat treatment conditions" in this method mean that acicular goethite particles containing a trace amount of Si are heated at 70°C in a non-reducing atmosphere such as argon or air.
It is heated and fired at a temperature of 0 to 800°C.

That is, in order to increase the degree of crystallinity by heating and firing acicular hematite particles containing a small amount of Si to grow single particles and thereby increase the degree of crystallinity, a temperature of 700° C. or higher is required. At temperatures below 700° C., the effect of suppressing Si particle growth actually inhibits the growth of single particles, and only a very small degree of crystallinity can be obtained.

In this way, heating and firing at a high temperature of 700°C or higher requires highly accurate equipment and advanced technology, and it is not suitable for industrial use.
I can't say it's economical.

Therefore, in view of the above-mentioned facts, the present inventors heated and fired the acicular hematite particles containing a trace amount of Si used in the present invention at a temperature as low as 700°C or lower in a non-reducing atmosphere. Therefore, further studies were conducted to obtain acicular hematite particles with an increased degree of crystallinity by achieving sufficient and uniform particle growth of a single particle. As a result, the average major axis length obtained by heating and dehydrating the acicular goethite particles containing trace amounts of Si or Si and Zn used in the present invention was 0.
Acicular hematite particles having a size of 3 to 2.0 μm, a specific surface area of 50 to 300 m2/g by the BET method, and maintaining the long axis length and axial ratio of acicular goethite particles are heated with steam. In an atmosphere consisting of Ps and a non-reducing gas, the partial pressure of water vapor -J and iPS is 1...
- Ps+Pi water vapor partial pressure, Pi is non-reducing gas partial pressure) 30 to 100%, and the average major axis length is 0.1 to 1.5 by heating and firing in the temperature range of 350 to 700°C.
μm or 0.1 to 2.0 μm and a specific surface area of 10 to 30 m/g by the BET method, the crystallinity is substantially increased. It was found that it is possible to obtain acicular hematite particles that are highly dense and retain and inherit acicular crystals.

This will be explained in more detail as follows.

The process in which acicular goethite particles containing a trace amount of Si undergo heating dehydration and become acicular hematite particles consists of the generation of a single hematite particle and the growth of the single particle. If this dehydration reaction occurs rapidly, uniform particle growth of a single particle of produced hematite becomes difficult to occur. Therefore, rapid grain growth of a single grain causes sintering of the grains and between grains, resulting in deformation of the grain shape of the skeleton grain, making it difficult to retain and inherit the acicular crystals. Therefore, the present inventor discovered that the Si material has a substantially high density with an increased degree of crystallinity, and a trace amount of Si that retains and inherits acicular crystals.
In order to obtain acicular hematite particles containing
We considered that it is necessary to separately control the timing of the generation of the nucleus of a single hematite particle and the timing of the growth of the nucleus of the single particle. That is, first of all, it is necessary to control the growth of the nucleus during the generation of the nucleus of a single particle of hematite. Strictly speaking, the time when the nucleus of a single hematite particle occurs is when the dehydration rate of the acicular goethite particle reaches 100%.
On an industrial scale, it is impossible to stop the reaction at this point, and its determination is very difficult. However, according to the ordinary method for obtaining acicular crystal hematite particles, as described in the above-mentioned Japanese Patent Publication No. 15759/1983, hematite skeleton particles that retain and inherit acicular crystals have a large specific surface area. Therefore, it consists of a single fine and uniform hematite particle group.

The present inventor conducted a detailed study on this phenomenon, and as explained in detail in the explanation of FIG. Confirm the relationship by experiment, and control the dehydration rate as a result. From the value of the specific surface area (BET method) of the formed hematite skeleton particles containing a trace amount of Si,
This made it possible to determine when the nucleus of a single hematite particle was generated.

In addition, the heating temperature for the dehydration reaction of goethite particles is approximately 250°C.
Although it is known that dehydration occurs in the above conditions, the rate of dehydration generally increases as the heating temperature increases, but it also changes depending on the heating rate, atmospheric pressure, etc.

Next, the acicular crystal skeleton particles, which are made up of numerous nuclei of fine hematite single particles containing a trace amount of Si used in the present invention, are heated and calcined to retain the acicular crystals of the skeleton particles. In order to achieve sufficient growth of a large number of nuclei in a single particle while inheriting the particles, it is necessary to control the growth of the single particle within a range that does not exceed the size of the skeleton particle.

Therefore, the present inventor proposed that 70%
Acicular crystal skeleton particles consisting of a large number of nuclei of fine hematite single particles containing a trace amount of Si are heated and fired at a temperature as low as possible below 0 ° C. to obtain a sufficient number of single particles and
We investigated the possibility of producing acicular crystal grains with an increased degree of crystallinity by achieving uniform grain growth.

As a result, acicular crystal skeleton particles with uniform particle size consisting of numerous nuclei of fine hematite single particles containing a trace amount of Si were heated in an atmosphere consisting of heated steam and a non-reducing gas or Ps. Pressure. When firing in the range of 30 to PS + PllOO%, a trace amount of S is removed at a temperature of 700°C or less.
a sufficient amount of hematite single particles containing i, and
It has been found that it is possible to achieve uniform grain growth and thus to obtain substantially dense acicular hematite grains with an increased degree of crystallinity.

The following is an explanation of some of the many experimental examples conducted by the present inventor.

FIG. 9 is a diagram showing the relationship between the specific surface area and heating temperature of fired particles obtained by heating and firing the acicular hematite particles containing a trace amount of Si used in the present invention under different heating and firing atmospheres. It is. That is, the average major axis length is 0.
300 g of acicular crystalline particle powder consisting of many nuclei of fine hematite single particles containing a trace amount of Si (Si content relative to Fe: 1.1 at%) with a specific surface area of 130 m/g and a diameter of 55 μm. Ratio of acicular hematite particles obtained by heating and firing at each temperature of 300 to 800°C for 90 minutes under different heating and firing atmospheres while driving and rotating the containers. This figure shows the relationship between surface area and heating and firing temperature. In the figure,
Curve A uses air, curve B uses N2 gas as a non-reducing gas, and when water vapor partial pressure -b-2 ゛ ″゛゛ Ps+Pi. is 75%, curve C uses PsN2 gas as a non-reducing gas. Used and water vapor partial pressure -
--Ding is 952... P8+Pi%
This is the case.

As can be seen from the figure, when the partial pressure of water vapor in the heating and firing atmosphere - - - τ is 75% and 95%, it is below 700°C Ps + P
Acicular hematite particles containing a trace amount of Si and having a specific surface area of 30 m·/g or less can be obtained at a heating and firing temperature of i.

That is, a substantially dense trace amount of Si with an increased degree of crystallinity due to sufficient and uniform grain growth of a single grain.
It is possible to obtain needle-shaped hematite particles containing . From this, it is considered that the water vapor partial pressure in the heating and firing atmosphere worked very effectively on the growth of single particles of acicular hematite particles containing a trace amount of Si. By the way, as a conventional technique for growing hematite particles, for example, Japanese Patent Publication No. 39-1973 is known as a technique for heating and firing in a non-reducing gas at a temperature of 500°C to 600°C or higher at the stage of forming acicular hematite particles.
Publication No. 20939, Japanese Patent Publication No. 11733/1973, Publication No. 30037/1983, Publication No. 28120/1986
There are methods described in US Pat. No. 4,052,326 and US Pat.

However, none of these takes into account the partial pressure of water vapor in the heating and firing atmosphere. In addition, methods for causing particle growth of acicular hematite particles using water vapor include, for example, methods (1) and (2) described in Powder Metallurgy Association 1960 Autumn Lecture Abstracts 2-1. There is.

In method (1), acicular goethite particles are placed in water vapor (N
In this method, acicular hematite particles are obtained by heating the mixture at 350° C. for 30 minutes (by passing two gases through water maintained at temperatures of 25° C., 50° C., 70° C., and 90° C.) for 30 minutes.

This method does not concern the preparation of acicular hematite particles, but rather the generation of acicular hematite particles.Moreover, this method does not involve the production of acicular hematite particles from acicular goethite particles. Since the timing of the generation of the nucleus of a single particle and the timing of the growth of the nucleus of the single particle occur at the same time, it is difficult to uniformly grow the many nuclei of a single particle, which is difficult to control. For this reason, it is difficult to maintain and inherit needle-like crystals.

In method (2), acicular goethite particles are
Acicular hematite particles obtained by heating at 0°C for 30 minutes are heated in an autoclave to a state of high water vapor pressure, and the water vapor pressure changes in response to changes in heating temperature in a closed container. This is an observation of the influence of acicular hematite particles on particle growth.

To elaborate on this method, 150
This is a method of heating acicular hematite particles at a temperature of ~350°C, and as is clear from the well-known water phase diagram, it is a so-called "hydrothermal treatment" in which acicular hematite particles are treated in the presence of water and steam. This method does not include the process of controlling the timing of the generation of nuclei of hematite single particles, making it difficult to maintain and inherit needle-like crystals.

Furthermore, the same document states that in this method, when acicular goethite particles are used as the object to be treated, the produced hematite particles become granular particles. This phenomenon is caused by the fact that when hematite particles are generated from acicular goethite particles under high temperature and pressure in an autoclave, the timing of the generation of the nucleus of a single hematite particle and the timing of the growth of the nucleus of a single particle are simultaneous and rapid. It is thought that the hematite produced becomes granular particles as a result of particle growth exceeding the size of the acicular crystal skeleton particles.

Next, in the thermal reduction process in the conventional method, when hydrogen is used as the reducing gas, the ferric acid particles and hydrogen gas react to generate water vapor.

In this way, the reducing atmosphere containing water vapor has a significant effect on the particle growth of single particles, and therefore, single particles are caused to grow excessively, causing sintering and deformation of the particles and each other. It is becoming. Therefore, conventional efforts have been made to minimize the amount of water vapor generated by the reaction between iron oxide particles and hydrogen gas. For example, there are examples in which carbon monoxide, which does not generate water vapor, is used as the reducing gas. That is, Japanese Patent Publication No. 39-5009 describes the following. ``We found that the partial pressure of water vapor is extremely important to prevent jitter between needle particles, and that the partial pressure and flow rate of hydrogen in the reducing atmosphere are also important.
” ``It is desirable to keep the water vapor partial pressure low.'' Therefore, ``In order to lower the water vapor partial pressure, it is necessary to increase the flow rate when using hydrogen.'' ``If the water vapor partial pressure in the reducing atmosphere is 1
It has been found that when the pressure exceeds 0.55 atm (water vapor partial pressure 5%) for more than an hour, significant agglomeration of particles tends to occur. "It is best to use carbon monoxide gas as a reducing gas to prevent particles from agglomerating due to water vapor partial pressure. This is because no effect has been observed.
Next, various conditions for carrying out the method of the present invention will be described.

The acicular hematite particles obtained by heating and dehydrating the acicular goethite particles containing minute amounts of Si or Si and Zn used in the present invention have an average major axis length of 0.3.
~2.0 μm, or 0.3 to 2.5 μm, specific surface area 5
0 to 300 m/g, which maintains and inherits the long axis length and axial ratio of the acicular goethite particles.

Particles having an average major axis length of 0.3 μm or less and 2.5 μm or more are not preferred as raw materials for magnetic powder for magnetic recording. Normally, it is difficult to obtain hematite particles containing a trace amount of Si and having a specific surface area of 50 m·/g or less. This is because dehydration must be performed at a slow rate in order to retain the needle-like crystals of the skeleton particles, which results in a long dehydration process, which is not industrially preferable. On the other hand, under severe dehydration conditions, hematite particles with a specific surface area of 50 m·/g or less can be obtained, but they can no longer be said to maintain the particle shape of needle-like crystals. Although it is possible to carry out the method of the present invention even when the specific surface area is 300 m/g or more, the specific surface area of the hematite particles obtained even if the dehydration rate is stopped early is about 300 m/g at most.

The acicular hematite particles that maintain the long axis length and axial ratio of the acicular goethite particles are shell particles consisting of many nuclei of fine hematite single particles, and this is done in consideration of the preservation and inheritance of the acicular crystals. This is what I did. 7, length 7,':'@da:}
Park: 3 convex? - High temperature is required to obtain acicular hematite particles containing barbed Si, and control is difficult because the control range is narrow.

In order to stably and effectively obtain acicular hematite particles containing a small amount of Si with a specific surface area of 30 m2/g or less, the water vapor partial pressure, -Edancho, must be 50 to 100%. It is preferable that there be Ps+Pi.

The steam partial pressure can be controlled by controlling the flow rate of heating steam using a steam flow meter. As the non-reducing gas in the present invention, air and nitrogen gas can be used.

If the heating and firing temperature in the present invention is 350° C. or lower, it will take a long time to obtain acicular hematite particles containing a trace amount of Si and have a specific surface area of 30 m 2 /g or lower, which is not effective.

If the temperature is 700°C or higher, highly accurate equipment and advanced technology are required, which is not industrially or economically viable.

When economic efficiency is considered in terms of the material of the industrial material and the structure of the equipment, a temperature range of 450 to 650° C. is preferable. The average major axis length of the acicular hematite particles containing trace amounts of Si or Si and Zn obtained by heating and firing in the present invention is 0.1 to 1.5 μm or 0.1 to 2.0 μm. And the specific surface area is 10 to 30 m2/g.

Considering the acicular nature and high density of hematite particles, the average major axis length is preferably 0.1 to 2.0 μm. Particles with a specific surface area of 10 m/g or less are acicular crystal particles with a distorted particle shape, and magnetic iron oxide particles obtained using such particles are also not suitable for magnetic recording because the acicular crystals are defective. Not preferred as a magnetic material. When the specific surface is 30 m/g or more, it is difficult to say that the growth of a single particle of acicular hematite particles is sufficient, and therefore, it is difficult to say that the particle growth of a single particle of acicular hematite particles is sufficient. I can't say it. In the present invention, when the temperature for heating reduction in a reducing gas is 300° C. or lower, the reduction reaction progresses slowly and takes a long time.

Furthermore, if the temperature is 450° C. or higher, the reduction reaction rapidly progresses, resulting in deformation of the acicular crystal particles and sintering of the particles and each other. Moreover, 450℃ in reducing gas
Reduction by heating at such high temperatures requires highly accurate equipment and advanced technology, and cannot be said to be industrial or economical. Next, the effects of the present invention will be described.

According to the present invention as described above, the acicular crystals and particle size of the starting material particles are maintained and inherited, and there are no dendritic particles mixed in, and the sufficient and uniformity of single particles is obtained. It is possible to obtain acicular magnetite particles and acicular maghemite particles having a substantially high density with an increased degree of crystallinity on the surface of the particles as well as inside the particles due to the particle growth.

The thus obtained acicular magnetite particles or acicular maghemite particles have magnetic properties such as a large saturation magnetic flux density σS and a high coercive force Hc, and powder properties such as high dispersibility, Since it has high orientation and high filling properties, it is suitable as a magnetic particle powder for high output, high sensitivity, and high recording density, which are currently most required. Furthermore, when the above-mentioned acicular magnetite particles or acicular maghemite particles are used in the production of magnetic paints, the dispersibility in the vehicle is good, and the orientation and filling properties in the coating film are improved. A magnetic recording medium having extremely excellent and desirable electromagnetic conversion characteristics can be obtained.

Furthermore, by carrying out the method of the present invention, it is possible to sufficiently cause the physical change of particle growth of a single particle prior to the thermal reduction process by the conventional method, so that the chemical change called reduction reaction occurs during the thermal reduction process. Because it only needs to be carried out mainly in There is no adverse effect on particle morphology.

Next, the present invention will be explained with reference to Examples and Comparative Examples.

In addition, the specific surface area in the experimental examples, examples, and comparative examples was determined by the BET method, and the amount of Si was determined according to JIS G-11212.
The amount of Zn was measured by fluorescent X-ray analysis using the Si analysis method.

<Manufacture of acicular goethite particles> Examples 1 to 13, Comparative Example 1; Example 1 Ferrous sulfate aqueous solution 611 containing Fe2+1.15m01
8.26-N(7) NaOH aqueous solution 3 obtained by adding sodium silicate (No. 3) to the Fe prepared in the reactor in advance to contain 1.0 atomic % in terms of Si.
In addition to 91, PHL3. O, Fe(
0H)2 was produced.

Acicular goethite particles were produced by passing 15 quartz air per minute through the aqueous solution containing Fe(0H)2 at a temperature of 45° C. for 7.5 hours.

The end point of the oxidation reaction was determined by extracting a portion of the reaction solution and adjusting it to be acidic with hydrochloric acid, and then using a red blood salt solution to determine the presence or absence of a Fe2+ blue coloring reaction.

The resulting particles were washed with water, separated, dried, and crushed by a conventional method.
As a result of electron microscopic observation, the obtained acicular goethite particles had an average long axis length of 0.60 μm and an axial ratio (long axis: short axis) of 10.
:1, the particle size was uniform, dendritic particles were not mixed, and the specific surface area was 28.2 m2/g.
Examples 2 to 4 Acicular goethite particles were prepared in the same manner as in Example 1, except that the type of ferrous salt aqueous solution, the Fe2+ concentration, the concentration of NaOH aqueous solution, and the amount of water-soluble silicate added were varied. was generated.

Table 1 shows the main manufacturing conditions and characteristics at this time.

The acicular goethite particles obtained in Examples 2 to 4 were
As a result of electron microscopy, the particle size of each sample was uniform and no dendritic particles were present. Comparative Example 1 Acicular goethite particles were produced in exactly the same manner as in Example 2 except that no water-soluble silicate was added.

Table 1 shows the main manufacturing conditions and characteristics at this time. As a result of electron microscopic observation, the obtained acicular goethite particles were found to be
The particle size was asymmetric and dendritic particles were present. Example 5 A ferrous sulfate 1.68 m01/1 aqueous solution 501 obtained by mixing zinc sulfate so as to contain 1.5 atomic % in terms of Zn with respect to Fe was added to Fe prepared in advance in a reactor. 5.83-N NaOH obtained by adding sodium silicate (No. 3) to contain 0.50 at% in terms of Si
In addition to aqueous solution 7, a mixed aqueous solution containing Fe(0H)2 and Zn(0H)2 was obtained at a pH of 2.8 and a temperature of 50°C.

1501 air per minute was passed through the mixed aqueous solution containing Fe(0H)2 and Zn(0H)2 at a temperature of 50° C. for 17 hours to produce acicular goethite particles.

The end point of the oxidation reaction was determined by taking out a portion of the reaction solution and adjusting the acidity with hydrochloric acid, and then using a red blood salt solution to determine the presence or absence of a blue-tinting reaction of Fe2+. The generated particles are washed with water and
It was filtered, dried and ground.

As a result of electron microscopic observation, the obtained acicular goethite particles had an average long axis length of 0.80 μm and an axial ratio (long axis: short axis) of 2.
The ratio was 5:1, and the particle size was uniform and dendritic particles were not mixed. In addition, Zn was added to Fe by 1.4
The content was 3 at %, and the specific surface area was 30.3 m2/g. Examples 6 to 13 The following examples were carried out except that the type of ferrous salt aqueous solution, the type of water-soluble Zn, the amount and timing of mixing of water-soluble Zn salt, the amount of water-soluble silicate added, and the reaction time were varied. Acicular goethite particles were produced in exactly the same manner as in Example 5.

Table 1 shows the main manufacturing conditions and characteristics at this time. Example 6
As a result of electron microscopy, all of the acicular goethite particles obtained in Steps 1 to 13 had excellent axial ratios, uniform particle sizes, and no dendritic particles.
<Production of raw material acicular hematite particle powder> Example 14~
26, Comparative Example 2; Example 14 Acicular goethite particles obtained in Example 1 5000
g in air at 350°C (dehydration rate 2.2 mol/
minute) to obtain acicular hematite particle powder.

The obtained acicular hematite particles have an average major axis length of 0.
．． Acicular crystal skeleton particles consisting of a group of fine hematite single particles that maintain and inherit the long axis length and axial ratio of acicular goethite particles with a diameter of 60 μm and an axial ratio (long axis: short axis) of 10:1, and The specific surface area was 134 m2/g. Example 15~
26. Comparative Example 2 Acicular hematite particles were obtained in exactly the same manner as in Example 14, except that the type of acicular goethite particles, the rate of heating and dehydration, and the heating temperature were varied.

Table 2 shows the main manufacturing conditions and various properties of the obtained acicular hematite particles. <Preparation of acicular hematite particle powder> Examples 27 to 57, Comparative Examples 3 to 9; Example 27 500 g of acicular hematite particle powder of Example 14 was charged into a retort container with a volume of 71 and one end open, Air and water vapor are passed through the retort while rotating the drive, and the water vapor partial pressure P8+Pi in the retort is maintained at 85%.37
It was heated and baked at a temperature of 0° C. for 180 minutes.

The obtained acicular hematite particles have an average major axis length of 0.
．． It had a diameter of 55 μm, an axial ratio (long axis: short axis) of 10:1, and a specific surface area of 29 mt/g. Examples 28-57
, Comparative Examples 4-5, 8-9 Acicular hematite particles were produced in exactly the same manner as in Example 27, except that the type of raw material, type of non-reducing gas, water vapor partial pressure, calcination temperature, and calcination time were variously changed. A powder was obtained.

Tables 3 and 4 show the main manufacturing conditions and various properties of the obtained acicular hematite particles. Comparative Examples 3 and 7 Example 2 except that air at a temperature of 30°C and humidity of 80% was used without blowing in steam, and the type of raw material was changed.
Acicular hematite particles were obtained in exactly the same manner as in Example 9.

Table 4 shows various properties of the obtained acicular hematite particles.
Comparative Example 6 Acicular hematite particles were obtained in exactly the same manner as in Example 29, except that the acicular goethite particles obtained in Comparative Example 1 were used as they were.

The obtained acicular hematite particles have an average major axis length of 0.
．． 60 μm and an axial ratio (long axis: short axis) of 4:1, which caused deformation of the particle shape and sintering of the particles and each other, and the specific surface area was 10 m·/g. <Manufacture of acicular crystal magnetic iron oxide particle powder> Examples 58 to 88, Comparative Examples 10 to 19; Example 58 ゛Acicular crystal hematite particle powder 30 obtained in Example 27
0 g was heated and reduced in a hydrogen stream at 430° C. for 90 minutes to obtain acicular crystal magnetite granular powder.

The obtained acicular magnetite particles had an average major axis length of 0.50 μm, an axial ratio (major axis: minor axis) of 10:l, and a specific surface area of 18 m 2 /g. Also, the coercive force H
c is 5070e, saturation magnetic flux density σS is 90.3emu/
It was hot at g. Next, 150 g of the acicular magnetite particles were heated in air at 300° C. for 60 minutes to obtain acicular maghemite particles.

The obtained acicular maghemite particles have a coercive force Hc of 4780e and a saturation magnetic flux density σS of 78.7emu.
/g. Examples 59 to 88, Comparative Examples 11 to 18 Acicular hematite particles A crystalline magnetite particle powder and an acicular crystalline maghemite particle powder were obtained.

Tables 5 and 6 show various properties of the obtained acicular magnetite particles and acicular maghemite particle powders. Comparative example 1
0 Magnetite particles and maghemite particles were obtained in exactly the same manner as in Example 58, except that the acicular goethite particles obtained in Comparative Example 1 were used as they were.

As a result of electron microscopy observation of the obtained magnetite particles and maghemite particles, it was found that the particles were deformed and sintered between the particles and between the particles. The obtained magnetite particles had an average long axis length of 0.3 μm, an axial ratio (long axis: short axis) of 5:1, and a coercive force Hc of 3
500e, and the saturation magnetic flux density σS was 89.3 emu/g. Moreover, the maghemite particle powder has an average major axis length of 0
．． 3 μm, axial ratio (long axis: short axis) 5:1, and
Coercive force Hc is 3350e, saturation magnetic flux density σS is 77.3
It was emu/g. Comparative Example 19 Acicular hematite particles obtained in Example 14 350
g into a retort container with a volume of 71 and one end open, and while driving and rotating it to aerate hydrogen gas and water vapor at 2.21/min. ) was heated and reduced at \350°C while maintaining the concentration of 40% to obtain magnetite particle powder.

Next, 150 g of the obtained magnetite particles were heated in air at 300° C. for 60 minutes to obtain maghemite particles. As a result of electron microscopy observation of both the obtained magnetite particles and maghemite particles, it was found that the particles were deformed and the particles and the particles were sintered. Table 6 shows various properties of the magnetite particles and maghemite particles.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the amount of water-soluble silicate added to an aqueous alkaline solution and the reaction time for producing acicular goethite particles. In the figure, curves A, B, and C have reactive iron (Fe2+) concentrations of 0.3 m01/1, 0.4 m01/1, and 0.7 m0, respectively.
This is the case of 1ha. FIG. 2 shows the relationship between the amount of water-soluble silicate added to an aqueous alkaline solution and the specific surface area of acicular goethite particles. In the figure, curves A, B, and C represent reactive iron (Fe2 concentration 0.3m01/1.0.
This is the case of 4m01/1.0.7m01c. Figure 3 shows
Axial ratio of acicular goethite particles obtained under constant conditions except for the amount of water-soluble silicate added to the alkaline aqueous solution,
It is a relationship diagram with the amount of water-soluble silicate added. Figures 4 and 5 show Zn when the amount of water-soluble silicate added is constant at 0.5 atomic % in terms of Si with respect to 5Fe.
FIG. 2 is a diagram showing the relationship between the mixing amount, the axial ratio (long axis: short axis), and the long axis of acicular goethite particles. FIG. 6 shows the amount of Si added (atomic %) and the amount of Zn mixed (
It shows two production regions in the relationship of atomic%),
In the figure, the shaded area is the production area of the acicular goethite particles used in the present invention, and the area A with the straight line A as one side,
The triangular portion surrounded by points b and c is a region where the acicular goethite particles and granular magnetite particles are mixed and formed. Figure 7 shows a graph containing Si (Si vs. Fe).
Acicular crystal skeleton particle powder with a specific surface area of 130 m/g consisting of a group of fine hematite single particles obtained by heating and dehydrating acicular crystal goethite particles having a content of 1.1 at%) was placed in a hydrogen stream. FIG. 2 is a diagram showing the relationship between the degree of reduction and the specific surface area of particles produced by thermal reduction in a thermal reduction process in which particles are subjected to addition reduction at 400° C. to obtain magnetite particle powder. Figure 8 shows a graph containing Si (Fe
Dehydration rate and specific surface area of particles produced under conditions of different dehydration rates in the process of heating and dehydrating acicular goethite particles with a Si content of 1.1 atomic %) to obtain acicular hematite particles. It is a relationship diagram. In the figure, curves A, B, and C have dehydration rates of 7.2 mol/min and 2 mol/min, respectively.
．． 0. {-l/min, 0.25 mol/min. Figure 9 shows the graphs of Si-containing (F) under different heating and firing atmospheres.
FIG. 2 is a diagram showing the relationship between the specific surface area of fired particles obtained by heating and firing acicular hematite particles having a Si content of 1.1 atomic % (Si content relative to e) and heating and firing temperature. ~ In the figure, A is in air, B uses PsN2 gas as a non-reducing gas, and when water vapor partial pressure P8+P1 is 75%, C uses N2 gas as a non-reducing gas, and water vapor partial pressure - 'If Σ1 is 95%...
Ps + Piru.

Claims (1)

[Claims] 1. Water-soluble silicate is added in advance to an alkaline aqueous solution used to obtain an aqueous solution containing Fe(OH)_2 with a pH of 11 or more at 0.1 to 1.7 atomic % based on Fe in terms of Si. Then, by reacting the ferrous salt aqueous solution with the alkali aqueous solution, an aqueous solution containing fine and uniform Fe(OH)_2 with a pH of 11 or more is obtained, and then oxidized to form needles. The average major axis length obtained by generating crystalline goethite particles, then filtering the goethite particles, washing them with water, drying them, and dehydrating them by heating.
．． Acicular hematite particles having a size of 3 to 2.0 μm, a specific surface area of 50 to 300 m^2/g by the BET method, and retaining the long axis length and axial ratio of acicular goethite particles, In an atmosphere consisting of heated steam and non-reducing gas, water vapor partial pressure [Ps/Ps+Pi] (Ps is water vapor partial pressure, Pi is non-reducing gas partial pressure) 30 to 100%
By heating and firing at a temperature in the range of 350 to 700°C, it inherits acicular crystals with an average major axis length of 0.1 to 1.5 μm and a specific surface area of 10 to 30 m^2/g by BET method. After forming the acicular hematite particles into substantially high-density acicular hematite particles, the acicular hematite particles were heated to
1. A method for producing acicular magnetic iron oxide particles, the method comprising heating and reducing the powder in a temperature range of ~450° C. to obtain acicular magnetite particles, or further oxidizing the powder to obtain acicular maghemite particles. 2. Claim No. 2, wherein the water vapor partial pressure [Ps/Ps+Pi] (Ps is the water vapor partial pressure, Pi is the non-reducing gas partial pressure) is 50 to 100% in an atmosphere consisting of heated steam and a non-reducing gas. A method for producing acicular magnetic iron oxide particles according to item 1. 3. The method for producing acicular crystalline magnetic iron oxide particles according to claim 1 or 2, wherein the heating and firing temperature is in the range of 450 to 650°C. 4 pH containing Fe(OH)_2 and Zn(OH)_2
A mixed aqueous solution of 11 or more (however, Zn(OH)_2 is F
Add water-soluble silicate in advance to the alkaline aqueous solution used to obtain 0.1 to 2.3 atomic % (calculated as Si) of Fe to
However, the sum of the mixed amount of Zn(OH)_2 and the added amount of water-soluble silicate is 2.5 atomic % or less based on Fe, calculated as Zn and Si, respectively. Next, by reacting the ferrous salt aqueous solution and water-soluble Zn salt aqueous solution with the alkali aqueous solution, fine and uniform F
An aqueous solution containing e(OH)_2 and Zn(OH)_2 with a pH of 11 or higher is obtained, and then oxidized to produce acicular goethite particles.
The average major axis length obtained by washing with water, drying, and heating dehydration is 0.3 to 2.5 μm, and the specific surface area by BET method is 50 to 300 m^2/g, and it is a needle-shaped crystal. Acicular hematite particles, which maintain the long axis length and axial ratio of goethite particles, are exposed to water vapor partial pressure [Ps/Ps+Pi] in an atmosphere consisting of heated steam and non-reducing gas.
] (Ps is water vapor partial pressure, Pi is non-reducing gas partial pressure) 30
~100%, by heating and firing at a temperature in the range of 350 to 700°C, the needles have an average long axis length of 0.1 to 2.0 μm and a specific surface area of 10 to 30 m^2/g by the BET method. After forming substantially high-density acicular hematite particles inheriting the crystal structure, the acicular hematite particles are heated and reduced in a reducing gas in a temperature range of 300 to 450°C to obtain acicular magnetite particles. A method for producing acicular magnetic iron oxide particles, the method comprising: or further oxidizing to obtain acicular maghemite particles. 5 The amount of Zn(OH)_2 mixed with respect to Fe(OH)_2 is 1.0 to 2.0 at% in terms of Zn, and the amount of water-soluble silicate added is 0.0% in terms of Si with respect to Fe. 3
~0.7 atomic%, however, the sum of the mixed amount of Zn(OH)_2 and the added amount of water-soluble silicate is
and 2.5 atomic % or less in terms of Si. The method for producing acicular magnetic iron oxide particles according to claim 4. 6. Claim No. 6, wherein the water vapor partial pressure [Ps/Ps+Pi] (Ps is the water vapor partial pressure, Pi is the non-reducing gas partial pressure) is 50 to 100% in an atmosphere consisting of heated steam and a non-reducing gas. A method for producing acicular magnetic iron oxide particles according to item 4 or 5. 7. The method for producing acicular crystal magnetic iron oxide particle powder according to any one of claims 4 to 6, wherein the heating and firing temperature is in the range of 450 to 650°C.