JP4632184B2 - Magnetite particles and method for producing the same - Google Patents

Description

  The present invention relates to magnetite particles and a method for producing the same. More specifically, the present invention relates to a magnetite particle, in particular, an electrostatic copying magnetic toner having a depression on the particle surface, a basic shape of 10 to 18-hedron and containing a silicon compound. The present invention relates to magnetite particles used for applications such as material powders, electrostatic latent image developing carrier material powders, and black pigment powders for paints, and a method for producing the same.

  Recently, magnetite particles produced by an aqueous solution reaction have been widely used as materials for magnetic toners such as electronic copying machines and printers. Various general development characteristics are required for magnetic toners, but in recent years, due to the development of electrophotographic technology, especially copiers and printers using digital technology have rapidly developed, and the required characteristics have become more advanced. I came.

  That is, in addition to conventional characters, output of graphics and photographs is also required, and some copiers and printers have a capacity of 1200 dots or more per inch, and the latent image on the photoconductor is more precise. It is becoming. For this reason, there are strong demands for high reproducibility of fine lines in development, and that they can be used without problems even in various environments.

  In addition, recent copying machines and printers have been increased in speed, and the toner particles filled in the machine body are subjected to considerable mechanical and electrical stress. Since problems such as magnetite particles falling off have occurred, an improvement in binding properties with resin has been strongly demanded as an effect of preventing magnetite particles from being detached.

  For such purposes, various functions have been imparted to iron oxide. For example, as means for obtaining magnetite particles having various shapes, magnetite particles having octahedron (Patent Document 1), magnetite particles having spherical shape (Patent Document 2), magnetite particles having hexahedron ( Patent documents 3) are each disclosed.

Of these various shapes of magnetite particles, octahedral particles are most preferred for improving the binding properties with the resin, followed by hexahedral particles, and spherical particles having the worst binding properties. This is because the spherical particles are not only easily removed from the resin due to the shape but also the surface area in contact with the resin is the smallest in the spherical shape.
However, spherical particles are superior in dispersibility and fluidity compared to octahedral particles and hexahedral particles, such as electrostatic powder magnetic toner material powder, electrostatic latent image developing carrier material powder, paint black pigment powder, etc. It has suitable characteristics required for the application.

  In recent years, various improvements have been made to such basic particle shapes. For example, magnetite particles having a polyhedron (Patent Document 4) and magnetite particles having a depression on the particle surface (Patent Document 5) are disclosed.

However, the magnetite particles disclosed in Patent Document 4 are for the purpose of improving dispersibility and do not require binding properties with a resin. Moreover, although patent document 5 is improving the binding property with resin, since it does not contain a silica, it is inadequate in a dispersibility.
Japanese Patent Publication No. 44-668 Japanese Examined Patent Publication No. 62-51208 JP-A-3-201509 Japanese Patent Laid-Open No. 10-101339 JP 2001-89155 A

  Accordingly, an object of the present invention is to provide magnetite particles that can eliminate the various drawbacks of the above-described prior art.

  In other words, it does not impair various properties required for applications such as electrostatic copying magnetic toner material powder, electrostatic latent image developing carrier material powder, and black pigment powder for paints, and it can bind to resin. An object is to provide excellent magnetite particles.

The present invention has a depression on the particle surface, the basic shape is a 10-18 plane, and contains a silicon compound ,
The object is achieved by providing magnetite particles having 2 or more and less than 8 indentations on the particle surface .

The present invention also provides water-soluble silicon component-containing water obtained by neutralizing and mixing an aqueous solution whose main component is a ferrous salt, a water-soluble silicon salt, and a mixed aqueous solution of an alkali hydroxide aqueous solution and an alkali carbonate aqueous solution. An oxidation reaction is performed on the ferrous oxide slurry while maintaining the pH of 8 to 11, and when the oxidation reaction proceeds 50% to 95%, a water-soluble silicon salt is converted into silicon with respect to the magnetite particles to be generated. The present invention provides a method for producing magnetite particles characterized by adding 0.05 to 1% by weight and continuing the oxidation reaction.

  The magnetite particle of the present invention has a depression on the particle surface, the basic shape is a 10-18 plane, and contains a silicon compound. Such a magnetite particle has a remarkable binding force with a resin. It is suitable for applications such as electrostatic copying magnetic toner material powder, electrostatic latent image developing carrier material powder, and black pigment powder for paint.

  Hereinafter, the present invention will be described based on preferred forms thereof.

The magnetite particles referred to in the present invention are preferably composed mainly of magnetite (Fe ₃ O ₄ ), and an intermediate composition beltride compound (FeOx · Fe _2).
O ₃ , 0 <X <1), and a single or composite compound of these other than Fe, Si, Al, Mn, Ni, Zn, Cu, Mg, Ti, Co, Zr, W, Mo, P, etc. A spinel ferrite particle containing at least seeds may be included depending on the required characteristics.

  It is important that the shape of the magnetite particles of the present invention is a 10-18 plane shape, preferably a 12-16 plane shape. The reason is to reduce detachment of magnetite particles from the resin when used for toner of a high-speed type copying machine or printer. The 10- to 18-hedron shape referred to here refers to a polygonal shape with missing corners centering on a 14-hedron shape having eight corners of a hexahedron.

  Specifically, when the target particles are assumed to be hexahedral (cubic) particles, particles having a ratio of the length of the corner missing portion to the length of one side of the hexahedral particles of 0.1 to 0.4. It is defined as 10 to 18-hedron particles (particles with excessive and insufficient corner defects compared to 14-hedron particles shall be handled in the same manner as described above).

  When this value is less than 0.1, the particles are almost the same as hexahedral particles, and when a large number of such particles are present in the particle powder, it is possible to achieve the balance of various properties, which is the intended purpose. Have difficulty. In addition, when it exceeds 0.4, the particles are infinitely close to octahedral particles, and when many such particles are present in the particle powder, it is difficult to achieve the intended purpose as in the case of the above shortage. is there.

Further, in the case of magnetite particles having a surface number of less than 10, the dispersibility in the resin is inferior, and conversely, in the case of magnetite particles having a surface number of more than 18, the shape is very similar to a spherical shape, so the dispersibility is good. However, it tends to be detached from the resin.
In addition, the number of faces of each particle is counted by counting the number of faces of the particle surface in the SEM photograph and using a double conversion value.

  Moreover, the magnetite particle | grains of this invention have a hollow part in the particle | grain surface. Specifically, this dent refers to what exists on the single particle surface or the boundary between the particles without regularity with respect to the particle ridgeline, and the shape of the dent is a linear shape cut into a V shape. There are a dimple shape such as a semicircular shape, a depression obtained by hollowing out a rectangular parallelepiped, and a shape such as a combination thereof. When many particles having no dent are present in the particle powder, the binding property with the resin is not sufficient.

  The magnetite particles of the present invention contain a silicon compound. When the silicon compound is not contained, it is difficult to exist as magnetite particles having a depression on the particle surface and a basic shape of 10 to 18 planes.

In the present invention, the number of depressions per particle is preferably 2 locations / particle or more and less than 8 locations / particle. When the number of dents is small, the binding property with the resin is not sufficient, and conversely, when the number of dents is 8 places / particles or more , particles having protrusion-shaped irregularities are mixed on the particle surface. This is not preferable because the magnetite particles are easily broken.

  Moreover, it is preferable that the magnitude | size of a hollow part is a magnitude | size of 1/20 or more with a projection area with respect to the magnitude | size of a magnetite particle. When this size is small, the resin can sufficiently enter during kneading with the resin and cannot be bonded to the interface of the magnetite particles, so that the effect of the present invention cannot be sufficiently obtained. On the contrary, it is preferable that the upper limit of the size of the hollow portion is 1/5 or less in terms of the projected area with respect to the size of the magnetite particles. When this size is larger than this, the shape of the magnetite particles is easily broken, and the breakage progresses when kneaded with the resin, which is not preferable. From the viewpoint that the effect of improving the binding property with the resin is sufficiently recognized and the magnetite particles are not destroyed during the kneading with the resin, the size of the recess is 20 minutes in terms of the projected area relative to the size of the magnetite particles. 1 or more and 1/5 or less are preferable.

  Moreover, it is preferable that content of the said silicon compound contained in the magnetite particle | grains of this invention is 0.1 to 2 weight%. When the amount of the silicon compound contained is less than 0.1% by weight, the effect of adding the silicon compound cannot be obtained, and magnetite particles having 10-sided hexahedrons having depressions cannot be obtained. Insufficient sex. On the other hand, when the content of the silicon compound exceeds 2% by weight, the magnetic properties are deteriorated and the hygroscopicity is increased, which is not preferable.

  The magnetite particles of the present invention preferably contain a titanium compound. By containing the titanium compound, the saturation magnetization is particularly increased. The amount of the titanium compound contained is 0.1 to 1% by weight. When the amount of titanium compound contained is less than 0.1% by weight in terms of titanium, the effect of saturation magnetization is not sufficient, and conversely, when the content of titanium compound exceeds 1% by weight, saturation magnetization This is uneconomical because it cannot be expected to improve further.

  Furthermore, in the present invention, it is a magnetite particle powder containing magnetite particles having dents as described above, and preferably the particles having dents contain 5% by number or more based on the total number of particles. . When the content ratio is less than 5% by number, the characteristics of the particles cannot be sufficiently exhibited, such as insufficient binding property with a resin described later, and it is difficult to achieve the desired effect. In order to sufficiently enhance the binding property with the resin, it is more preferable that the particles having the dents contain 15% by number or more based on the total number of particles.

  The average particle size of the magnetite particles of the present invention may correspond to the level of magnetite particles produced by a normal wet oxidation method, and the particle size is 0.05 to 1 μm in ferret diameter. More preferably, it is 0.1-0.5 micrometer, More preferably, it is 0.1-0.3 micrometer.

  The magnetite particles of the present invention preferably contain 18% by mass or more of FeO with respect to the entire particles. More preferably, it is 22% or more. When FeO is less than 18% by mass, the blackness is lowered and redness is exhibited, and the saturation magnetization is relatively low.

  Moreover, it is preferable that the silicon compound contained in the magnetite particles of the present invention is partially exposed on the particle surface. The reason is that fluidity and dispersibility are better when silicon is exposed.

  The amount of the silicon compound exposed on the particle surface can be determined by suspending the obtained magnetite particles in 1 N sodium hydroxide for a certain time and measuring the eluted silicon component amount by ICP. The exposure amount is preferably 0.05 to 1% by weight with respect to the whole magnetite particles. When the exposed silicon compound is less than 0.05% by weight, it tends to be magnetite having no depression. On the other hand, if the exposed silicon compound exceeds 1% by weight, the specific surface area increases and the hygroscopicity increases, such being undesirable.

Next, the preferable manufacturing method of the magnetite particle | grains of this invention is demonstrated.
The method for producing magnetite particles of the present invention comprises a water-soluble substance obtained by neutralizing and mixing an aqueous solution whose main component is a ferrous salt, a water-soluble silicon salt, and a mixed solution of an alkali hydroxide aqueous solution and an alkali carbonate aqueous solution. An oxidation reaction is performed while maintaining the silicon component-containing ferrous hydroxide slurry at a pH of 8 to 11, and when the oxidation reaction proceeds 50% to 95%, a water-soluble silicon salt is further added and the oxidation reaction is continued. It is characterized by.

  As the ferrous salt, water-soluble salts such as ferrous sulfate, ferrous chloride, and ferrous nitrate can be used. Examples of the alkali hydroxide include sodium hydroxide and potassium hydroxide, and examples of the alkali carbonate include sodium carbonate. Moreover, as water-soluble silicon salt to add, silicates, such as sodium silicate, etc. can be used.

  When the ferrous hydroxide slurry at the beginning of the oxidation reaction does not contain a water-soluble silicon component, magnetite particles having a 10-to-18-hedron shape are hardly generated. Therefore, the ferrous hydroxide slurry used for the oxidation reaction needs to contain a water-soluble silicon component, and preferably contains the water-soluble silicon salt with respect to the number of moles of iron ions in the ferrous hydroxide slurry. The number of moles of silicon in the film is preferably 0.1 to 5% by mole, and more preferably 0.2 to 3% by mole in consideration of the balance between the shape and various properties. When this amount is less than 0.1 mol%, it is difficult to uniformly obtain magnetite particles having a 10-18 plane shape even if the pH during the oxidation reaction is maintained from 8 to 11. On the other hand, if it exceeds 5 mol%, magnetite particles having a 10- to 18-hedron shape can be obtained, but this is not preferable because the magnetic properties are deteriorated.

  In addition, when magnetite is produced without maintaining the oxidation reaction at pH 8 to 11, it is difficult to obtain magnetite particles having a 10- to 18-hedron shape. This is because, when the pH is less than 8, spherical particles are likely to be formed, and the pH is more than 11 regardless of the water-soluble silicon component contained in the ferrous hydroxide slurry at the beginning of the oxidation reaction described above. In this case, octahedral particles are easily generated.

  In order to maintain this pH from 8 to 11, alkali hydroxide and alkali carbonate are used in combination. The reason why the alkali carbonate is used in combination is to maintain the state of 8 to 11 which is the pH of the weak alkali region more stably.

Specifically, the acid-base equivalent of the total amount of alkali hydroxide and alkali carbonate with respect to the amount of Fe ²⁺ ions contained in the ferrous salt aqueous solution is preferably 1 to 1.5, more preferably 1 to 1.5. Let 1.2. The acid-base equivalent herein means 1 equivalent of 2 moles when Fe ²⁺ is hydroxide ion with respect to 1 mole, and 1 mole is 1 equivalent when Fe ²⁺ is alkali carbonate with respect to 1 mole. It means to be a quantity. That is, it is 1 equivalent to just neutralize by acid-base reaction, and when it is larger than 1 equivalent, it means that the alkali is excessive, and when it is smaller than 1 equivalent, it is Fe ²⁺ excessive. When this equivalent amount is less than 1 or exceeds 1.5, it is difficult to control the pH of the reaction slurry from 8 to 11. The molar ratio of the hydroxide ion of alkali hydroxide to the carbonate ion of alkali carbonate is preferably 2 to 40 for the former / the latter because 10-18 plane particles can be easily formed. More preferably, it is 5-30.

  Moreover, the slurry temperature in the case of reaction has preferable 70 degreeC or more, and 70-100 degreeC is still more preferable. When this temperature is less than 70 ° C., iron oxide other than magnetite is also generated, which is not preferable. Moreover, when it exceeds 100 degreeC, although the magnetite particle | grains of this invention are obtained, cost starts and it is not industrially preferable.

  Further, in the method for producing magnetite particles of the present invention, after the oxidation reaction is started, the consumption of ferrous hydroxide is observed, and when the reaction proceeds 50% to 95%, a water-soluble silicon salt is further added. It is also important to continue the oxidation reaction.

  In order to intentionally form a depression in the magnetite particles of the present invention, it is necessary to add a water-soluble silicon salt to the reaction slurry when the magnetite particles are produced to some extent. It has not been clearly clarified why the addition of a water-soluble silicon salt in the reaction process produces magnetite particles having depressions. The inventors adsorbed the water-soluble silicon salt added in the middle of the reaction to a specific part of the magnetite particles that are being produced, and that part stopped the growth of the magnetite particles, except for the part where the silicon compound was adsorbed. It is presumed that a depression is generated due to the progress of growth.

  Incidentally, although the above-mentioned Patent Document 5 discloses magnetite particles having depressions, the manufacturing method does not mention addition of a silicon component. As shown in a comparative example described later, in the production method of Patent Document 5, when the silicon component is contained only at the beginning of the reaction, the magnetite particle shape becomes a polyhedron, but no depression is generated. Further, in this method, when the silicon component is added only when the reaction proceeds from 50 to 95%, the magnetite particle shape is a hexahedron and no dent is generated. As can be seen from the above, the mechanism in which the magnetite particles of the present invention exhibit a 10- to 18-hedron shape and a dent is generated is a peculiar one that is different from that of Patent Document 5.

  In the present invention, when the water-soluble silicon salt is further added when the reaction proceeds from 50% to 95%, and the water-soluble silicon salt is added at a stage where the reaction proceeds at an earlier stage than 50%, the silicon compound is adsorbed. Magnetite begins to grow again at a unique site where growth has stopped, and the indentation does not become clear. When a water-soluble silicon salt is added at a stage where the reaction progress rate is slower than 95%, a unique site where the silicon compound is adsorbed and the growth stops is secured, but sufficient hydroxylation is sufficient for subsequent growth. A sufficient amount of iron does not remain, and the dents are unlikely to exist in the finished particles. About the said addition time, Preferably a water-soluble silicon salt should just be added between 70 to 95%, More preferably, it is 80 to 95%.

Moreover, it is preferable that the water-soluble silicon salt added in the middle of the reaction is 0.05 to 1% by weight in terms of silicon with respect to the generated magnetite particles. If it is less than 0.05% by weight, it is not sufficient to give a dent, and if it is 1% by weight or more, the silicon compound is adsorbed to the entire magnetite, resulting in the absence of growing sites and new nuclei. The particle size distribution may be inferior.
From the above, it is considered that the generation of dents depends on the timing of adding the water-soluble silicon salt, and the generation rate of the dents is controlled by the amount added. When it is desired to increase the number of depressions, it is preferable to add as much water-soluble silicon salt as possible within the above-mentioned preferred range at an early stage.

  Moreover, in the manufacturing method of the magnetite particle | grains of this invention, a water-soluble titanium salt can also be contained at the time of neutralization mixing with the water-soluble silicon salt and the mixed aqueous solution of alkali hydroxide aqueous solution and alkali carbonate aqueous solution. By this operation, as described above, the saturation magnetization of the obtained magnetite particles can be increased.

  Further, the magnetite particles of the present invention may be further coated with an aluminum compound on the particle surface. This is preferable because dispersibility, fluidity, electrical resistance and the like are improved.

  In addition, the magnetite particles of the present invention may be coated with an organic treating agent such as a silicone oil, silane-based or titanate-based coupling agent on the particle surface. Thereby, since the further improvement of binding property with resin is obtained, it is preferable.

  In addition, it is preferable that the particle powder composed of the magnetite particles of the present invention is processed by a dry type crusher that works by compression, shearing, or spatula, because dispersibility is improved.

  Hereinafter, the present invention will be specifically described with reference to examples and the like. However, the scope of the present invention is not limited to such examples.

[Example 1]
Ferrous sulfate, sodium hydroxide, sodium carbonate, and sodium silicate were mixed at a ratio shown in Table 1 to produce ferrous hydroxide containing a silicon component. At this time, the pH was 10.3 and the liquid temperature was 80 ° C.

Air was blown into the slurry to initiate the oxidation reaction. Simultaneously with the start of air blowing, the unreacted Fe ²⁺ concentration was measured, and oxidation was continued to a reaction progress rate of 70%.

  When the reaction progress rate reached 70%, the oxidation reaction was stopped, and the amount of sodium silicate aqueous solution shown in Table 1 was added. After the addition, the pH of the slurry was 10.1.

Further, air was blown in and the oxidation reaction was continued until unreacted Fe ²⁺ was consumed. The obtained magnetite particles were washed, dried and crushed by ordinary methods to obtain magnetite particles. The obtained magnetite particles were evaluated by the following method.

<Evaluation method>
(A) Observation of particle shape, particle diameter, and dent part Using a scanning microscope (magnification of 50,000 times), the particle shape, particle diameter, and the number of particles having a dent part among 100 particles are obtained, and the dent part is obtained. The proportion of particles having Since a photograph with a magnification of 50,000 cannot capture 100 particles in the same field of view, a sufficient photograph was taken to obtain 100 particles that can be measured. In addition, the number of hollow parts measured the number of reflected hollow parts about all the particles which have a hollow part, and made the value which doubled the number the number of hollow parts which exist in a particle | grain. The reason why the magnification is doubled is that the SEM photograph shows only the front side of the particle, and it is assumed that the same degree of depression is present on the back side of the particle, so that the conversion is performed.
(B) Content of Si and Ti with respect to the whole particle Magnetite particle | grains were melt | dissolved completely in the acid, and content of Si and Ti was calculated | required by ICP.
(C) The amount of Si exposed on the particle surface with respect to the whole particle Magnetite particles were dispersed in a 1N-sodium hydroxide aqueous solution at a concentration of 10 g / l, stirred at 50 ° C. for 4 hours, and the magnetite particles were separated by filtration. The component concentration of the aqueous sodium hydroxide solution from which the silicon component was eluted was measured by ICP, and the amount of silicon dissolved from the particle surface was determined by calculation.
(D) FeO
Magnetite particles were dissolved in sulfuric acid and analyzed by redox titration using a potassium permanganate standard solution.
(E) Saturation magnetization A vibration type magnetometer VSM-P7 manufactured by Toei Industry Co., Ltd. was used, and measurement was performed under an external magnetic field of 796 kA / m.
(F) Evaluation of binding property with resin Magnetite particles, styrene-acrylic thermoplastic resin (manufactured by Sanyo Chemical Co., Ltd., TB-1000F), charge control agent (manufactured by Orient Chemical Co., Ltd., Bontron S-34) and wax ( Sanyo Chemical Co., Ltd., Viscol 550P) was weighed at a ratio of 100: 100: 1: 2 by weight ratio, mixed with a Henschel mixer, and melt kneaded at 180 ° C. using a biaxial kneader. Went. The obtained kneaded material was coarsely pulverized and finely pulverized, followed by air classification to obtain a powder having an average particle size of 7 μm.
20 g of the obtained powder was put into a glass container and shaken with a paint shaker for 1 hour and 10 hours, and then the presence or absence of dropped particles was examined with a scanning electron microscope. The level at which no desorption was confirmed was Level 1, the level at which about 3 magnetite particles were dropped per powder particle was Level 1, and the level at which more than 3 was confirmed was Level 2.

[Examples 2 to 6]
Under the conditions shown in Table 1, magnetite particles were produced according to Example 1 and evaluated in the same manner as in Example 1. In Examples 2, 4, and 6, titanyl sulfate was contained in the iron hydroxide slurry at the start of the reaction at the ratio shown in Table 1.

[Comparative Example 1]
As shown in Table 1, magnetite particles were produced according to Example 1 except that no sodium silicate was added to the iron hydroxide slurry at the start of the reaction or the slurry in the middle of the reaction. The obtained magnetite particles were evaluated in the same manner as in Example 1.

[Comparative Example 2]
As shown in Table 1, magnetite particles were produced according to Example 1 except that sodium silicate addition during the reaction was not performed. The obtained magnetite particles were evaluated in the same manner as in Example 1.

[Comparative Example 3]
As shown in Table 1, magnetite particles were produced according to Example 1 except that sodium silicate was not contained in the iron hydroxide slurry at the start of the reaction. The obtained magnetite particles were evaluated in the same manner as in Example 1.

[Comparative Examples 4 and 5]
As shown in Table 1, magnetite particles were produced according to Example 1 except that the amount of sodium hydroxide and sodium carbonate were adjusted and the reaction pH was changed. The obtained magnetite particles were evaluated in the same manner as in Example 1.

[Comparative Example 6]
As shown in Table 1, although the acid-base equivalent is adjusted in the same manner as in Example 1, the concentration of sodium silicate added to the iron hydroxide slurry at the start of the reaction is changed while changing the mixing amount of sodium hydroxide and sodium carbonate Magnetite particles were produced in the same manner as in Example 1 except that the above was changed. The obtained magnetite particles were evaluated in the same manner as in Example 1.

[Comparative Example 7]
As shown in Table 1, magnetite particles were produced according to Example 1 except that the concentration of sodium silicate added during the reaction was changed. The obtained magnetite particles were evaluated in the same manner as in Example 1.

  As is apparent from the results shown in Table 2, the magnetite particles obtained in each example are magnetite particles having a basic shape of 10 to 18 planes and having depressions, and the binding evaluation results with the resin are as follows. It was good.

On the other hand, since the magnetite particles of Comparative Example 1 did not contain a silicon component at the beginning of the reaction, the generated particles were hexahedral particles, not 10-18. In addition, although the magnetite particles of Comparative Example 1 have the dents of the present invention on the particle surface, the number of the dents is small and the shape is not a 10 to 18-hedron. It was inferior to the binding property.
In addition, the comparative example 1 is a magnetite particle manufactured based on the manufacturing method of patent document 5. FIG.

  Moreover, the magnetite particles of Comparative Examples 2 and 3 produced magnetite particles having no dents, and as a result, the binding properties with the resin were inferior.

  In addition, since the magnetite particles of Comparative Examples 4 and 5 had a pH of less than 8 or more than 11 at the time of reaction, the generated particles were spherical and octahedral, respectively, and there were no depressions. As a result, the binding property with the resin was poor.

  In Comparative Example 6, although the acid-base equivalent is adjusted to substantially the same value as Comparative Example 1, the ratio of sodium carbonate is high, and the amount of sodium silicate added at the beginning of the reaction is large. Thus, the number of faces of the generated particles was icosahedron. In addition, since sodium silicate was added in the middle of the reaction as in Comparative Example 1, the dents were present at almost the same ratio as in Comparative Example 1. However, since the number of faces is icosahedron, the binding property with the resin is inferior.

  Further, in Comparative Example 7, the amount of sodium silicate added during the reaction was large, so the number of depressions was larger than that in Example 1, and the binding property with the resin was poor. The reason for this was presumed that there were too many indentations, the number of particles having protrusion-shaped irregularities increased on the particle surface, the particles were destroyed by the stress during kneading, and as a result, the magnetite particles were dropped from the resin. .

FIG. 1 is a scanning electron micrograph of magnetite particles having depressions in Example 1. FIG. 2 is a scanning electron micrograph of magnetite particles having depressions in Example 2. FIG. 3 is a scanning electron micrograph of magnetite particles having depressions in Example 3.

Claims (7)

It has a depression on the particle surface, the basic shape is a 10-18 plane, and contains a silicon compound ,
Magnetite particles characterized in that the number of depressions on the particle surface is 2 or more and less than 8 per particle. Magnetite particles of claim 1, wherein the silicon compound content, characterized in that from 0.1 to 2% by weight in terms of silicon. The magnetite particles according to claim 1 or 2 , comprising a titanium compound. The magnetite particles according to claim 3 , wherein the titanium compound content is 0.1 to 1% by weight in terms of titanium. A magnetite particle powder comprising 5% by number or more of the magnetite particles having a depression according to any one of claims 1 to 4 . A water-soluble silicon component-containing ferrous hydroxide slurry obtained by neutralizing and mixing an aqueous solution whose main component is ferrous salt, a water-soluble silicon salt, and a mixed aqueous solution of an alkali hydroxide aqueous solution and an alkali carbonate aqueous solution. Then, the oxidation reaction is carried out while maintaining the pH of 8 to 11, and when the oxidation reaction proceeds from 50% to 95%, the water-soluble silicon salt is further converted to silicon with respect to the generated magnetite particles by 0.05 to 1 weight. The method for producing magnetite particles according to any one of claims 1 to 5 , wherein the oxidation reaction is continued. The water-soluble titanium salt is contained during neutralization mixing of the aqueous solution in which the main component is a ferrous salt, a water-soluble silicon salt, and a mixed aqueous solution of an alkali hydroxide aqueous solution and an alkali carbonate aqueous solution. Item 7. A method for producing magnetite particles according to Item 6 .