JP2017122035A - Coated magnetite particle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetite particle having an octahedron shape and high dispersibility and high resistance.SOLUTION: A coated magnetite particle has a core particle of magnetite and a coating layer positioned with neighboring a surface of the core particle as an outermost layer. The coating layer positioned on each surface of an octahedron consist of oxide, the coating layer contains iron, silicon and aluminum and has a plurality of salients projecting from surfaces thereof. Average particle diameter of a primary particle measured by an electron microscope observation is 50 nm to 150 nm. Dust resistance is preferably 1.0×10Ω cm to 5.0×10Ω cm.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a coated magnetite particle in which the surface of a magnetite core particle is coated with a coating layer.

  Coated magnetite particles are known in which a coating layer containing a silicon compound or an aluminum compound is formed on the surface of magnetite particles. For example, in Patent Document 1, magnetite particles whose surfaces are coated with a silicon compound and an aluminum compound are heated and oxidized in an oxidizing atmosphere to generate maghemite-hematite composite particles containing oxides of these compounds. Subsequently, a method for producing magnetite particles by heating and reducing the maghemite / hematite composite particles in a reducing atmosphere is described. The literature describes that the obtained magnetite particles are octahedral in shape. Patent Document 2 also describes magnetite particles produced by the same method.

  Patent Document 3 describes magnetite particles that contain silicon inside the particles, and on which the coprecipitates of silica and alumina adhere or adhere to the surface of the particles. Patent Document 4 describes magnetite particles having Si and Al on the particle surface and having a polyhedron shape such as an octahedron. The magnetite particles are oxidized by aeration of air in an aqueous ferrous salt solution containing alkali hydroxide, and the water-soluble silicate and water-soluble aluminum when the iron reaction rate is 50% or more. Produced by adding salt followed by the rest of the reaction.

  Patent Document 5 describes iron oxide particles having a coating layer of a composite iron oxide of Al and Fe as a lower layer and a coating layer of a composite iron oxide of Si and Fe as an upper layer. Patent Document 6 describes iron oxide particles having a coating layer of a composite iron oxide of Si and Fe as a lower layer and a coating layer of a composite iron oxide of Al and Fe as an upper layer.

JP-A-4-170325 JP-A-5-281778 JP-A-7-110598 JP 2008-230960 A JP 2001-72424 A JP 2002-128525 A

  In the magnetite particles described in Patent Documents 1 and 2, the raw material magnetite particles are oxidized and once converted into maghemite / hematite composite particles, and then reduced to return to magnetite particles. Therefore, it is considered that the dispersibility is deteriorated and the resistance is lowered because the particles are aggregated in the reduction heat treatment step. Further, it is unclear what compound state the silicon compound and aluminum compound contained in the raw magnetite particles are after the reduction of the maghemite / hematite composite particles.

  Since the magnetite particles described in Patent Document 3 have a coprecipitate of silica and alumina on the surface thereof, silica and alumina do not exist separately. Similarly, in the magnetite particles described in Patent Document 4, magnetite particles are generated by oxidation by aeration of air in the presence of a water-soluble silicate and a water-soluble aluminum salt. It is thought that is generated.

  In the iron oxide particles described in Patent Documents 5 and 6, a coating layer of a composite iron oxide of Al and Fe and a coating layer of a composite iron oxide of Si and Fe are formed on the surface of the iron oxide particle located at the center. It has a two-layer structure in order or in the opposite order, and does not contain Si, Al, and Fe in a single coating layer.

  By the way, magnetite particles are generally known to be spherical and those of polyhedron such as octahedron as described in Patent Document 1 and the like described above. Whether to use a spherical or octahedral one is selected depending on the specific application of the magnetite particles. In recent years, high-speed printing of electronic copying machines and printers has been progressing, and magnetite used as a raw material for magnetic toner is required to have high residual magnetization so that fog does not occur during high-speed printing. Further, from the viewpoint of reducing power consumption, magnetic toners are required to have low temperature fixability. By reducing the magnetite in the resin, the low-temperature fixability is improved, but the coloring power of the magnetic toner is reduced. Therefore, it is required to highly disperse the fine magnetite in the resin so as to prevent the coloring power from being lowered. The magnetite particles described in the above-mentioned patent documents do not satisfy such requirements. In order to satisfy the required characteristics of such high remanent magnetization, it is preferable to use octahedral magnetite. However, since octahedral magnetite has a smooth particle surface, the particles are in contact with each other and the agglomerates are difficult to break. High-resistance magnetite is dispersed by dispersing finely-magnetized magnetite in the resin. It has been difficult to produce particles.

  Accordingly, an object of the present invention is to provide octahedral magnetite particles having good dispersibility and high resistance.

The present invention has magnetite core particles and a coating layer as an outermost layer located adjacent to the surface of the core particles,
It consists of an octahedron shape,
The covering layer located on each face of the octahedron is made of an oxide, the covering layer includes iron, silicon, and aluminum, and the covering layer has a plurality of protrusions protruding from the surface;
The present invention provides coated magnetite particles in which the average particle diameter of primary particles measured by electron microscope observation is 50 nm or more and 150 nm or less.

  According to the present invention, magnetite particles having an octahedral shape, high dispersion, and high resistance are provided.

FIG. 1 is a schematic view showing a method for measuring the height of a convex portion in the coated magnetite particle of the present invention. FIG. 2 is a scanning electron microscope image of the coated magnetite particles obtained in Example 1. 3A to 3C are transmission electron microscope images of the coated magnetite particles obtained in Example 1. FIG. 4A to 4C are transmission electron microscope images of the coated magnetite particles obtained in Example 2. FIG. 5A to 5C are transmission electron microscope images of the magnetite particles obtained in Comparative Example 1. FIG.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The coated magnetite particles of the present invention have a basic structure of magnetite core particles and a coating layer located on the surface thereof. The coating layer is the outermost layer of the coated magnetite particle of the present invention. That is, no other layer exists outside the coating layer. The coating layer is located adjacent to the surface of the core particle. That is, no other layer is interposed between the surface of the core particle and the coating layer. Therefore, the coated magnetite particle of the present invention has a basic structure consisting of two parts, a core particle and a coating layer. However, depending on the specific application of the coated magnetite particles, one layer or two or more other layers may be laminated on the outer surface of the coating layer. Alternatively, one layer or two or more other layers may be interposed between the core particles and the coating layer.

  The core particle is composed of magnetite. As the core particles, those whose main peak coincides with the peak of magnetite when XRD measurement is performed are used. In this case, only the magnetite peak may be observed, or a peak such as maghemite may be observed in addition to the main peak of magnetite. Depending on the specific application of the coated magnetite particles, the core particles may be composed of one or more elements such as Si, Ti, Al, Zr, Mn, Zn, Mg, etc. It may be contained in the particles in the state of an oxide or the like.

  It is preferable to use an octahedral shape as the core particle. In the octahedral core particle, each ridgeline may be clear, or the portion of the ridgeline may be rounded. As will be described later, since the coating layer covering the surface of the core particle is thin, the shape of the coated magnetite particle of the present invention is an octahedral shape reflecting the shape of the octahedral shape of the core particle. The coated magnetite particles of the present invention have an octahedral shape. When 100 coated magnetite particles are arbitrarily selected and the shape thereof is observed, 80 or more particles have an octahedral shape. That means. Therefore, it is not necessary that all coated magnetite particles have an octahedral shape.

  The size of the core particle reflects the size of the target coated magnetite particle. The size of the core particle is preferably 50 nm or more and 150 nm or less, and more preferably 60 nm or more and 130 nm or less, expressed as a ferret diameter by observation with a scanning electron microscope (SEM).

  The coating layer located on the surface of the core particle includes iron, silicon, and aluminum as constituent elements in a single layer. The coating layer contains an oxide. Hereinafter, this oxide is referred to as “iron-containing oxide”. The iron-containing oxide is a composite oxide of iron and silicon, a composite oxide of iron and aluminum, or a composite oxide of iron, silicon, and aluminum. The iron-containing oxide is deposited on the surface of the core particle in the form of fine particles, and a single coating layer is formed from the deposited layer of the fine particles. The iron-containing oxide fine particles contain a composite oxide of iron and silicon and a composite oxide of iron and aluminum, a composite oxide of iron, silicon and aluminum, or iron and silicon. Or a composite oxide of iron, silicon, and aluminum, or a composite oxide of iron, aluminum, and a composite oxide of iron, silicon, and aluminum. Therefore, the coating layer made of the deposit of fine particles also contains a composite oxide of iron and silicon and a composite oxide of iron and aluminum, or a composite oxide of iron, silicon, and aluminum. Or a composite oxide of iron and silicon and a composite oxide of iron, silicon and aluminum, or a composite oxide of iron and aluminum and a composite oxide of iron, silicon and aluminum. ing. In the coating layer, the iron-containing oxide fine particles are substantially uniformly dispersed. “Single coating layer” means that the coating layer has a single layer structure which is not a multilayer structure.

  Adhering a silicon compound or an aluminum compound to the surface of magnetite particles is a technique often employed in the technical field as described in the above-mentioned patent documents. However, it has not been known so far to use octahedral magnetite particles as the core particles and to form the iron-containing oxide on the surface of the magnetite particles having such a shape. On the other hand, the present inventors can increase the resistance of the target coated magnetite particles by forming a single coating layer containing iron-containing oxide on the surface of octahedral magnetite core particles. I found it possible. In particular, in the coated magnetite particle of the present invention having an octahedral shape, when the coating layer located on each surface of the octahedron has a plurality of convex portions protruding from the surface, the coated magnetite particle It has been found that the surface becomes rough, the particles come into point contact, and the contact resistance between the particles increases. This convex part is also comprised from the iron containing oxide. It has also been found that the dispersibility of the coated magnetite particles is improved when the coating layer located on each face of the octahedron has a plurality of protrusions protruding from the surface. The dispersibility here refers to the dispersibility in a resin when the coated magnetite particles of the present invention are used as a raw material for a magnetic toner, for example.

  The convex part mentioned above is comprised from 1 or 2 or more of iron-containing oxide fine particles. As shown in FIG. 3 described later, when the coated magnetite particles are observed with a TEM, the convex portions are observed with a size of about several nm. Specifically, the size is 1 nm to 40 nm, particularly 7 nm to 20 nm. The size of the convex portion is the height protruding from the base surface of the coating layer when the coated magnetite particles are observed with a TEM. The convex portion may be present on the surface of the coating layer without any gap, or may be present so loosely that the base surface of the coating layer (that is, the reference surface on which the convex portion rises) is observed by TEM. Good. The size of the convex portion is obtained by TEM observation of the coated magnetite particles and measuring the height h from the base surface of the coating layer to the top of the convex portion as shown in FIG. Obtained by arithmetically averaging the measured values.

  The coating layer covers the entire surface of the core particle evenly or covers the surface of the core particle in a partially exposed state. In any of the coating modes, the coating layer covers the surface of the core particle thinly. When the coated magnetite particles are observed with a TEM, the thickness of the coating layer is preferably 1 nm or more and 50 nm or less, and more preferably 2 nm or more and 20 nm or less. The thickness of the coating layer is obtained by TEM observation of the coated magnetite particles, measuring the thickness of the coating layer at 15 or more positions, and averaging the measured values. The thickness of the covering layer does not include the size of the convex portion.

  The respective ratios of silicon, aluminum and iron contained in the coating layer are preferably 5% by mass or more and 30% by mass or less with respect to the total mass of silicon, aluminum and iron contained in the coating layer, More preferably, it is 7 mass% or more and 20 mass% or less. Regarding aluminum, it is preferably 10% by mass or more and 45% by mass or less, more preferably 13% by mass or more and 42% by mass or less, based on the total mass of silicon, aluminum and iron contained in the coating layer. More preferably, it is at least 37% by mass. Regarding iron, it is preferably 40% by mass or more and 83% by mass or less, more preferably 42% by mass or more and 80% by mass or less, based on the total mass of silicon, aluminum and iron contained in the coating layer. More preferably, it is at least 78% by mass. In order for the coated magnetite particles of the present invention to have a plurality of convex portions on the surface of the coating layer, the molar ratio of silicon to iron in the coating layer is 0.2 or more and 1.0 or less, particularly 0.3 or more. The molar ratio of aluminum to iron is preferably 0.5 or more and 2.0 or less, and particularly preferably 0.5 or more and 1.8 or less. The ratio of these elements was determined by suspending 25 g of the coated magnetite particles of the present invention in 5 L of 5% by mass dilute sulfuric acid at 50 ° C. 15, 25, 35, 45, 60, 75, 90, 105, 120 minutes), and the concentration of silicon, aluminum, and iron contained in the filtrate obtained by filtering this with a membrane filter is determined by ICP. It can be measured by quantification. In this measurement, the total amount of iron eluted up to the point where the detected amounts of silicon and aluminum become constant is regarded as the amount of iron in the coating layer.

  The ratio of the total mass of silicon, aluminum and iron in the coating layer in the coated magnetite particles is preferably 1.0% by mass or more and 5.5% by mass or less, and more preferably 1.8% by mass or more and 4.1% by mass. More preferably, it is as follows. By forming the coating layer in this range, the resistance of the coated magnetite particles can be made sufficiently high. Further, the dispersibility in the resin can be sufficiently increased. The ratio of the total mass of silicon, aluminum, and iron in the coating layer in the coated magnetite particles was obtained by suspending 25 g of the coated magnetite particles of the present invention in 5 L of 5% by weight diluted sulfuric acid at 50 ° C. to obtain a measurement solution. A part of the measurement solution is sampled every 25 ml at regular time intervals (5, 15, 25, 35, 45, 60, 75, 90, 105, 120 minutes), and this is filtered through a membrane filter. The concentration of Si, Al and Fe contained can be measured by quantifying with ICP. In this measurement, the ratio of each element is calculated by dividing the total amount (g) of silicon, aluminum, and iron up to the time when the detected amounts of silicon and aluminum become constant and dividing by 25 g and multiplying by 100.

The coated magnetite particles having the above structure similarly have an octahedral shape because the core particles have an octahedral shape. The coated magnetite particles having the octahedral shape are characterized by high dispersion and high resistance. Specifically, the value of the dust resistance is 1.0 × 10 ⁴ Ω · cm to 5.0 × 10 ⁶ Ω · cm, particularly 3.0 × 10 ⁴ Ω · cm to 2.0 × 10 ⁶ Ω. -It is cm or less. On the other hand, the dust resistance of normal magnetite particles (magnetite particles having no coating layer) is on the order of 10 ² Ω · cm. The dust resistance is measured by the following method. 10 g of coated magnetite particles are prepared, and this is pressed at a pressure of 60 MPa to prepare a measurement sample made of a cylindrical pellet having a diameter of 25 mm. The electric resistance of the pellet of the measurement sample is measured by a four-terminal method using a digital multimeter (2400 Source Meter manufactured by Toyo Technica).

  When the coated magnetite particles are used as a raw material for magnetic toner, for example, the particles are preferably fine particles from the viewpoint of improving image definition. Specifically, in the coated magnetite particles, the average particle diameter of the primary particles is preferably 50 nm or more and 150 nm or less, more preferably 60 nm or more and 130 nm or less, expressed as a ferret diameter by SEM observation. In the measurement of the particle diameter by SEM observation, 200 or more particles are measured and the average value is obtained.

The degree of dispersibility of the coated magnetite particles of the present invention, which is characterized by high dispersibility, can be expressed by the value of D ₅₀ reduction rate represented by the following formula.
D ₅₀ decrease rate (%) = {D ₅₀ (A) −D ₅₀ (B)} / D ₅₀ (A) × 100
(In the formula, D ₅₀ (A) represents a volume cumulative particle size at a cumulative volume of 50 vol% by a laser diffraction scattering particle size distribution measurement method measured after ultrasonically dispersing magnetite particles at an output of 10 W, and D ₅₀ (B) represents a volume cumulative particle size at a cumulative volume of 50 vol% by a laser diffraction scattering particle size distribution measurement method measured after ultrasonically dispersing magnetite particles at an output of 30 W.)

Cumulative volume particle diameter D ₅₀ in the cumulative volume 50% by volume by laser diffraction scattering particle size distribution measuring method, the smaller the value, it means that aggregates of the coated magnetite particles are smaller, the dispersibility of the coated magnetite particles In other words, it means good aggregation. The D ₅₀ reduction rate defined by the above formula means that the larger the value is, the more easily the agglomerates are broken, and the higher the dispersibility in the resin when the coated magnetite particles are used as the raw material of the magnetic toner. To do. Specifically, coated magnetite particles of the present invention, the D ₅₀ percent reduction, preferably 4% or more, still more preferably 5% or more, more preferably not less than 6%. The method for measuring the D ₅₀ reduction rate will be described in detail in Examples described later.

  Next, the suitable manufacturing method of the covering magnetite particle | grains of this invention is demonstrated. The coated magnetite particles are roughly classified into (1) a process for producing core particles and (2) a process for forming a coating layer. Hereinafter, each process will be described.

  In the manufacturing process of the core particles of (1), wet oxidation of iron ions is performed by passing an oxidizing gas through the aqueous solution under conditions in which the pH of the aqueous solution of the iron source compound is adjusted. As the iron source compound, it is preferable to use a water-soluble compound of divalent iron. Examples of such compounds include ferrous sulfate and ferrous chloride. The pH of the aqueous solution is preferably adjusted to 9.5 or higher. By setting the pH above this value, octahedral core particles can be successfully produced. For adjusting the pH, for example, an alkali metal hydroxide such as sodium hydroxide can be used. As the oxidizing gas, it is preferable to use an oxygen-containing gas such as air. It is preferable to blow the oxidizing gas until no divalent iron ions exist in the liquid. When air is used as the gas, the amount of the oxidizing gas blown is preferably 1 to 80 L / min, particularly 2 to 50 L / min with respect to a 200 L magnetite production tank. It is preferable from the point of obtaining an appropriate reaction rate to heat the liquid during blowing of the oxidizing gas and keep it at 60 to 100 ° C., particularly 80 to 95 ° C.

  When octahedral core particles are obtained in the liquid in this way, the coating layer forming step (2) is performed. In this step, a silicon source compound, an aluminum source compound, and an iron source compound are added to the aqueous dispersion containing the core particles. These compounds may be added all at once or sequentially. From the viewpoint of successfully forming the target coating layer, it is advantageous to add them sequentially. In the case of sequential addition, first, (i) a silicon source compound and an iron source compound are added to an aqueous dispersion containing octahedral core particles, and an oxidizing gas is blown into the aqueous dispersion to form the core particles. Silicon oxide fine particles and iron oxide fine particles are adhered to the surface. Next, (b) an aluminum source compound and an iron source compound are added to the aqueous dispersion, an oxidizing gas is blown into the aqueous dispersion, and aluminum oxide fine particles and iron oxide fine particles are formed on the surfaces of the core particles. To form precursor particles. By forming the coating layer by such a procedure, the coating layer can be composed of an aggregate of fine particles made of an oxide containing iron and silicon or aluminum. In addition, a plurality of convex portions can be formed on the coating layer. In addition, in Patent Document 6 described in the background art section, the above-described procedures (a) and (b) are employed. At that time, the molar ratio of silicon to iron, and the molar ratio of aluminum to iron. However, this is different from the conditions employed in the present production method, and as a result, the iron oxide particles described in the document do not form a single coating layer having convex portions in the present invention.

  On the other hand, when the silicon source compound, the aluminum source compound and the iron source compound are added all at once, after adding these compounds all at once into the aqueous dispersion containing the core particles, an oxidizing gas is blown into the aqueous dispersion. On the surface of the core particle, it is possible to form a coating layer made of an oxide containing iron and silicon or aluminum and having a plurality of convex portions protruding from the surface of the coating layer.

  Examples of the silicon source compound used for forming the coating layer include sodium silicate and potassium silicate. These compounds can be used individually by 1 type or in combination of 2 or more types. Examples of the aluminum source compound include aluminum sulfate, sodium aluminate, and aluminum nitrate. These compounds can be used individually by 1 type or in combination of 2 or more types. As the iron source compound, the same compounds as those used for the production of magnetite core particles can be used. As the oxidizing gas, the same gas as that used for producing the magnetite core particles can be used.

  When employing the sequential addition method described above, the silicon source compound used in the step (a) is preferably added in an amount of 0.1% by mass or more and 1.0% by mass or less with respect to the core particles. More preferably, it is added in an amount of 0.2% by mass or more and 0.8% by mass or less. On the other hand, the iron source compound used in the step (a) is preferably added in an amount of 0.3% by mass or more and 1.5% by mass or less, and 0.6% by mass or more and 1. More preferably, it is added in an amount of 1% by mass or less.

  The amount of the oxidizing gas blown in the step (a) is preferably 1 L / min or more and 80 L / min or less, particularly 2 L / min or more and 50 L / min or less with respect to the 200 L magnetite production tank. During the blowing of the oxidizing gas, the aqueous dispersion is preferably heated and maintained at 60 ° C. or higher and 100 ° C. or lower, particularly 80 ° C. or higher and 95 ° C. or lower. The blowing of the oxidizing gas is preferably continued until substantially no iron ions are present in the aqueous dispersion.

  In the step (b) after the step (b), the aluminum source compound used in the step is added in an amount of 0.3% by mass to 1.5% by mass with respect to the core particles. It is more preferable that it is added in an amount of 0.4% by mass or more and 1.1% by mass or less. On the other hand, the iron source compound used in the step (b) is preferably added in an amount of 0.3% by mass to 1.5% by mass with respect to the core particles. More preferably, it is added in an amount of 1% by mass or less.

  The amount of the oxidizing gas blown in step (b) is preferably 1 L / min or more and 80 L / min or less, particularly 2 L / min or more and 50 L / min or less with respect to the 200 L magnetite production tank. During the blowing of the oxidizing gas, the aqueous dispersion is preferably heated and maintained at 60 ° C. or higher and 100 ° C. or lower, particularly 80 ° C. or higher and 95 ° C. or lower. The blowing of the oxidizing gas is preferably continued until substantially no iron ions are present in the aqueous dispersion.

  Through the above steps, a coating layer containing iron, silicon, and aluminum and made of an oxide can be formed on the surface of the octahedral magnetite core particles. Moreover, the some convex part protruded from the surface of this coating layer can be formed. This coating layer contains a composite oxide of iron and silicon and a composite oxide of iron and aluminum, or a composite oxide of iron, silicon and aluminum, depending on the reaction conditions. Or a composite oxide of iron, silicon, and aluminum, or a composite oxide of iron and aluminum, and a composite oxide of iron, silicon, and aluminum. The ratio of silicon and aluminum to iron in the coating layer is one of the important factors in forming the coating layer and the plurality of protrusions protruding from the surface of the coating layer. When the ratio of silicon and aluminum to iron is too large, the convex portions formed by the iron-containing oxide may be covered with these elements, resulting in a coating layer having no convex portions on the surface. Similarly, when the ratio of silicic acid and aluminum to iron is small, the convex portion formed by the iron-containing oxide may be covered with the iron oxide, resulting in a coating layer having no convex portion on the surface. The molar ratio of silicon to iron in the coating layer is preferably 0.2 to 1.0, more preferably 0.3 to 0.9, and the molar ratio of aluminum to iron is preferably 0.5 or more. The coated magnetite particles obtained by coating the iron-containing oxide at a ratio of 2.0 or less, more preferably 0.5 or more and 1.8 or less, resulted from the formation of the coating layer and the convex portions. As a result, the dispersibility is improved, and as a result, the resistance of the coated magnetite particles is about two to three orders of magnitude higher than the resistance of the core particles before the coating layer is formed.

  The coated magnetite particles obtained in this way, for example, take advantage of their properties such as high resistance and high tap density, as well as high dispersibility and high oxidation resistance. It is suitably used as a raw material for carriers for developing latent images.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples.

[Example 1]
(1) Production of core particles 92 L of a ferrous sulfate aqueous solution having a Fe ²⁺ concentration of 1.79 mol / L and 88 L of a 3.74 N aqueous sodium hydroxide solution were added and mixed and stirred. The pH of this solution was 6.5. While maintaining this solution at a temperature of 89 ° C. and a pH of 9 to 12, air of 20 L / min was blown to cause an oxidation reaction to generate core particles. When ferrous hydroxide was completely consumed, air blowing was stopped and the oxidation reaction was terminated. The obtained magnetite core particles had an octahedral shape.
(2) Formation of coating layer 5 L of aqueous solution obtained by mixing 2 L of 0.7 mol / L sodium silicate aqueous solution and 2 L of 0.9 mol / L ferrous sulfate aqueous solution into the slurry after reaction containing 13500 g of core particles It was added while maintaining pH 7-9. Thereafter, 10 L / min of air was blown until Fe ²⁺ in the slurry no longer remained.
Subsequently, 5 L of an aqueous solution obtained by mixing 2 L of a 1.5 mol / L aluminum sulfate aqueous solution and 2 L of a 0.9 mol / L ferrous sulfate aqueous solution was added to the post-reaction slurry containing core particles while maintaining pH 7-9. did. Thereafter, 10 L / min of air was blown until Fe ²⁺ in the slurry no longer remained. The temperature of the slurry was maintained at 89 ° C. After mixing and stirring for 30 minutes, the slurry was filtered, washed and dried to obtain the desired coated magnetite particles.

[Examples 2 to 5 and Comparative Examples 1 to 5]
After producing core particles in the same manner as in Example 1, magnetite particles were obtained under the conditions shown in Table 1.

[Performance evaluation]
About the magnetite particle | grains obtained by the Example and the comparative example, using a scanning electron microscope (JEOL JSM-6330F) and a transmission electron microscope (JEOL JEM-2100), the particle shape and the presence or absence of a convex part were determined. Observed. Moreover, the ratio of Si, Al, and Fe contained in a coating layer, Si / Fe molar ratio, and Al / Fe molar ratio were measured by the above-mentioned method. Further, the average particle diameter and the dust resistance of the primary particles were measured by the method described above. Further, the D ₅₀ reduction rate was evaluated by the following method. These results are shown in Table 2.

_{[D 50} decrease rate]
In a 100 mL beaker, 100 mL of 5% sodium hexametaphosphate aqueous solution and 0.1 g of magnetite particles were charged. BRANSON ultrasonic homogenizer SONIFIER 450 was immersed in the liquid, and the magnetite particles were dispersed in the liquid for 3 minutes. The ultrasonic homogenizer was immersed in the liquid in the vertical direction so that the lower end of the vibrator almost coincided with the position of the 30 mL marked line of the beaker. The particle size distribution of the obtained magnetite dispersion was measured using Microtrac MT3000 manufactured by Nikkiso, and the value of D ₅₀ was recorded. The dispersion strength of the ultrasonic homogenizer was two types, 10W and 30W. The dispersion strength increases as the value increases.

As is clear from Table 2, the coated magnetite particles of the examples have higher resistance than the magnetite particles of the comparative example. Further, the magnetite particles of Comparative Example, since the plurality of protruding portions protruding in the coating layer surface is not present, the value of D ₅₀ percent reduction compared with Example is small, it can be seen that the poor dispersibility.

Claims (3)

A magnetite core particle, and a coating layer as an outermost layer located adjacent to the surface of the core particle;
It consists of an octahedron shape,
The covering layer located on each face of the octahedron is made of an oxide, the covering layer includes iron, silicon, and aluminum, and the covering layer has a plurality of protrusions protruding from the surface;
Coated magnetite particles having an average particle size of primary particles measured by electron microscope observation of 50 nm or more and 150 nm or less. The coated magnetite particle according to claim 1, wherein the value of the _D50 reduction rate represented by the following formula is 4% or more.
D ₅₀ decrease rate (%) = {D ₅₀ (A) −D ₅₀ (B)} / D ₅₀ (A) × 100
(In the formula, D ₅₀ (A) represents a volume cumulative particle size at a cumulative volume of 50 vol% by a laser diffraction scattering particle size distribution measurement method measured after ultrasonically dispersing magnetite particles at an output of 10 W, and D ₅₀ (B) represents a volume cumulative particle size at a cumulative volume of 50 vol% by a laser diffraction scattering particle size distribution measurement method measured after ultrasonically dispersing magnetite particles at an output of 30 W.) The coated magnetite particle according to claim 1, wherein the dust resistance is 1.0 × 10 ⁴ Ω · cm or more and 5.0 × 10 ⁶ Ω · cm or less.