JP2008201668A - Magnetic iron oxide particle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide magnetic iron oxide particles having high heat conductivity and heat resistance. <P>SOLUTION: The magnetic iron oxide particles are prepared by forming a coated layer containing silicon and aluminum on the surface of core particles of magnetic iron oxide containing silicon. The particles is characterized in that, when Fe in an amount of 10 wt.% of the whole amount of Fe contained in the particles is dissolved, during the process of dissolving the particles having an octahedron form from the surface thereof, the ratio X of the amount of Fe(II) contained in the dissolved total Fe to an amount of the total Fe (the former/the latter) is within the range expressed by 0.34≤X≤0.50. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to magnetic iron oxide particles.

  The present applicant has previously proposed magnetic iron oxide (magnetite) particles containing a silicon component inside and exposing the silicon component on the surface (see Patent Document 1). The magnetic iron oxide particles have low residual magnetization, high electrical resistance, and excellent workability and fluidity. However, in order to improve the heat resistance of the magnetic iron oxide particles only with the silicon component, it is necessary to add a large amount of the silicon component. Therefore, it is difficult to improve the performance in a balanced manner.

  With regard to the content of FeO, the present applicant has previously described an octahedron in which Ti is contained inside, and the amount of FeO from the particle surface to a depth of 10% is 0.7 to 1.0 of the amount of FeO inside. Proposed magnetic iron oxide particles having a shape (see Patent Document 2). However, the magnetic iron oxide particles do not contain Si inside. Further, magnetic iron oxide containing Ti is slightly inferior in saturation magnetization and heat resistance. Regarding the content of FeO contained in the magnetic iron oxide, there are further descriptions related to Patent Documents 3 and 4.

  A technique of coprecipitation of silicon and aluminum on the surface of magnetic iron oxide particles is also known. For example, in Patent Document 5, an aqueous solution of an alkali metal carbonate and an alkali metal hydroxide is added to a ferrous salt aqueous solution, and the reaction temperature is maintained at 70 to 100 ° C., and an oxidizing gas is passed through to form a polyhedron. That the magnetic iron oxide is formed and then the slurry pH is kept in the region of 10 to 13 and Si and Al are co-precipitated on the surface of the magnetic iron oxide particles while dropping the Si compound and Al compound and the aqueous ferric chloride solution. Has been. By depositing the Si component and the Al component on the surface of the magnetic iron oxide particles, the dispersibility and heat resistance of the particles are enhanced. However, the core particles of the magnetic iron oxide particles are inferior in the basic characteristic level due to the fact that the FeO content is not high and the saturation magnetization is not high.

  By the way, magnetic iron oxide particles are used as materials for magnetic toners in electronic copying machines and printers. When magnetic toner containing magnetic iron oxide particles is heat-fixed on paper, the toner can be used at a lower temperature. Adhesion is advantageous from the viewpoint of energy efficiency. However, in each patent document mentioned above, the characteristic of a magnetic iron oxide particle is not examined from a viewpoint of thermal conductivity.

JP-A-5-213620 JP 2003-192352 A Japanese Patent Laid-Open No. 4-141664 JP-A-4-338971 JP-A-5-286723

  An object of the present invention is to provide magnetic iron oxide particles having various performances further improved than the above-described conventional magnetic iron oxide particles.

  The present invention relates to a magnetic iron oxide particle having an octahedral shape in which a coating layer containing silicon and aluminum is formed on the surface of a core particle of magnetic iron oxide containing silicon, the magnetic iron oxide particle The amount of Fe (II) contained in the dissolved total Fe at the time when 10% by weight of Fe with respect to the total amount of Fe contained in the particles is dissolved, and the total Fe The magnetic iron oxide particles are characterized in that the ratio X to the amount X (the former / the latter) is 0.34 ≦ X ≦ 0.50.

  According to the present invention, magnetic iron oxide particles having high thermal conductivity and good low-temperature fixability when used as a toner are provided. The magnetic iron oxide particles of the present invention have a good hue. In particular, even when fine particles are used, the hue is good. Furthermore, since the magnetic iron oxide particles of the present invention originally have a high FeO content and high saturation magnetization and are excellent in heat resistance, the degree of blackness even after being exposed to high temperatures during toner preparation. And saturation magnetization is maintained at a high level. The magnetic iron oxide particles of the present invention have few secondary aggregations (agglomerates) and are excellent in dispersibility, and also have excellent environmental stability.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The magnetic iron oxide particles of the present invention are composed of core particles and a coating layer that covers the core particles. The core particles are made of magnetic iron oxide (magnetite) containing silicon. The coating layer is a layer containing silicon and aluminum.

  The magnetic iron oxide particles of the present invention are characterized by the amount of divalent iron Fe (II) contained therein. Specifically, the magnetic iron oxide particles are dissolved from the surface thereof, and Fe (II) contained in the dissolved total Fe at the time when 10 wt% of Fe is dissolved with respect to the total amount of Fe contained in the particles. ) And the total Fe amount, the ratio X (the former / the latter) is set to 0.34 ≦ X ≦ 0.50, preferably 0.34 ≦ X ≦ 0.45, more preferably 0.34 ≦ X ≦ 0.40 is set. As a result of the examination by the present inventors, the ratio X of Fe (II) / total Fe greatly affects the heat resistance and thermal conductivity of the magnetic iron oxide particles. The high heat resistance of the magnetic iron oxide particles is advantageous from the viewpoint that there is little deterioration in blackness and saturation magnetization when exposed to high temperatures, for example, when the particles are used as a raw material for toner. Further, the high thermal conductivity of the magnetic iron oxide particles is advantageous from the viewpoint of allowing the resin to adhere at a lower temperature when the particles are used as, for example, a toner raw material. When the ratio X of Fe (II) / total Fe is less than 0.34, the thermal conductivity of the magnetic iron oxide particles is low. On the other hand, it is possible to produce magnetic iron oxide particles having a ratio X exceeding 0.5 by using a gas phase reduction method. However, magnetic iron oxide particles produced in such a manner are unstable in the atmosphere and practical. Not.

  A specific method for measuring the Fe (II) / total Fe ratio X is as follows. Add 25 g of the sample magnetic iron oxide particles to 3.8 liters of deionized water. Stir at a stirring speed of 200 rpm while maintaining at 40 ° C. in a water bath. To this slurry is added 1250 mL of an aqueous hydrochloric acid solution obtained by dissolving 424 mL of a special grade hydrochloric acid reagent (concentration 35%) in deionized water. This initiates dissolution of the magnetic iron oxide particles. From the start of dissolution of the magnetic iron oxide particles until the particles are completely dissolved and the slurry becomes transparent, 50 mL of liquid is sampled every 10 minutes. The sampled solution is filtered through a 0.1 μm membrane filter, and the filtrate is collected. 25 mL of the collected filtrate is used to determine the amount of iron by plasma emission analysis (ICP). And iron element dissolution rate (weight%) is computed from the following formula | equation.

  The amount of Fe (II) is measured using the remaining 25 mL of the filtrate. A sample is prepared by adding about 75 mL of deionized water to this 25 mL solution. Add sodium diphenylamine sulfonate as an indicator to the sample. The sample is then subjected to redox titration using 0.1N potassium dichromate. The titration amount is obtained with the point where the sample is colored blue-violet, and the concentration (mg / L) of Fe (II) is calculated from the titration amount. Using the concentration of iron element (mg / L) when the iron element dissolution rate was 10% by weight obtained by the above method and the concentration (mg / L) of Fe (II) obtained from the titration at that time , Fe (II) / total Fe ratio X is determined.

  In the present invention, in determining the ratio X of Fe (II) / total Fe in the magnetic iron oxide particles, the reason is that 10% by weight of Fe is dissolved with respect to the total amount of Fe contained in the particles. This is because the portion until 10% by weight of Fe is dissolved corresponds to a thickness of about 3.5% from the particle surface, and this portion has a great influence on the thermal conductivity.

  In order to make the ratio X of Fe (II) / total Fe in the magnetic iron oxide particles within the above-mentioned range, the particles include the core particles of magnetic iron oxide containing silicon, and the silicon and aluminum coatings thereon. It is necessary to have a structure composed of a coating layer containing, and that the particles are octahedral as described below. And according to the magnetic iron oxide particles having such a structure and shape, the core particles are characteristic in that they contain FeO in a high ratio and have high saturation magnetization. This characteristic core particle is protected by coating with a coating layer comprising silicon and aluminum. By using a coating layer containing silicon and aluminum for protecting the core particles, the specific surface area does not increase and the heat conduction is not hindered compared to the case where a coating layer containing silicon or aluminum alone is used. The coating can effectively protect the core particles. By protecting the core particles having a high thermal conductivity with a coating layer containing silicon and aluminum, magnetic powder particles having excellent heat resistance can be obtained without deteriorating the thermal conductivity. The magnetic iron oxide particles having such a structure and shape are suitably manufactured according to, for example, a manufacturing method described later.

  In order for the magnetic iron oxide particles of the present invention to achieve the desired characteristics, it is essential that the shape is octahedral. Commonly known shapes of magnetic iron oxide particles include octahedral, spherical, hexahedral, and octahedral polyhedrons. The ratio of Fe (II) / total Fe in magnetic iron oxide particles X As a result of the examination by the present inventors, it is necessary for the particles to be octahedral in order to increase the thermal conductivity of the particles and increase the heat resistance. did. When the particles have a shape other than octahedron, both or one of thermal conductivity and heat resistance cannot be increased to a desired level.

  In the magnetic iron oxide particles of the present invention, the ratio (the former / the latter) of the amount of Fe (II) contained in the total Fe in which 90% by weight of all remaining Fe is dissolved and the total Fe amount is determined. When Y is defined, the ratio X of Fe (II) / total Fe and Y (X / Y) is preferably more than 1.00 and not more than 1.30. The value of X / Y is the ratio of the ratio of Fe (II) in the region close to the surface of the magnetic iron oxide particle to the ratio of Fe (II) in the region close to the center of the particle. The value of is within the above range means that the ratio of Fe (II) in the region near the surface of the magnetic iron oxide particles is higher than that in the region near the center. That is, the proportion of Fe (II) is not uniform over the depth direction of the magnetic iron oxide particles, but has a gradient that tends to increase from the center toward the surface. As a result, the magnetic iron oxide particles of the present invention have high coloring power and blackness in addition to high thermal conductivity and heat resistance. It is preferable that the X / Y value is 1.03 to 1.30, particularly 1.10 to 1.30, since coloring power and blackness are further increased. It is difficult to produce magnetic iron oxide particles having an X / Y value exceeding 1.3. The value of Y itself is preferably 0.25 to 0.50, particularly 0.27 to 0.38, provided that the value of X / Y is in the above range.

  The value of Y is measured by the following method. That is, in the above X measurement, the difference between the total concentration of iron element (mg / L) when the iron element is completely dissolved and the concentration of iron element (mg / L) when the iron element dissolution rate is 10% by weight The iron element concentration (mg / L) in the remaining 90% by weight. Separately from this, the concentration of Fe (II) when the iron element is completely dissolved is obtained by the same method as the measurement of the ratio X described above. And the difference between the concentration of Fe (II) when the iron element is completely dissolved (mg / L) and the concentration of Fe (II) when the iron element dissolution rate is 10% by weight (mg / L), The concentration of Fe (II) in the remaining 90% by weight (mg / L). Y is calculated by dividing the concentration (mg / L) of Fe (II) in the remaining 90% by weight obtained in this way by the concentration of iron element concentration in the remaining 90% by weight.

  As already described, the magnetic iron oxide particles of the present invention are composed of core particles and a coating layer covering the core particles, and the core particles are composed of magnetic iron oxide containing silicon. Silicon is present continuously and substantially uniformly from the center of the core particle to the surface. And it is preferable to set the weight of silicon contained in the core particles to 0.30 to 1.50% by weight, particularly 0.40 to 1.00% by weight as Si with respect to the weight of the whole magnetic iron oxide particles. Thereby, the thermal conductivity of the magnetic iron oxide particles is further improved. Further, the magnetic properties of the magnetic iron oxide particles, particularly the saturation magnetization can be increased. The ability to increase the value of saturation magnetization is particularly advantageous when the magnetic iron oxide particles of the present invention are used, for example, as a raw material for toner for electrostatic copying or a carrier for electrostatic latent image development. If the amount of silicon is too large, it is not easy to increase the saturation magnetization.

  The magnetic iron oxide particles of the present invention preferably have an average primary particle size of 0.10 to 0.30 μm, particularly 0.10 to 0.20 μm. By setting the primary particle diameter within this range, the blackness and coloring power of the magnetic iron oxide particles are sufficiently high. The primary particle diameter is an average value obtained by observing magnetic iron oxide particles with a scanning electron microscope (magnification 40000 times) and measuring the ferret diameter of 200 particles. As illustrated in the examples described later, the magnetic iron oxide particles of the present invention are fine but have good hue. However, if the primary particle diameter becomes too small, the particles tend to be reddish. Conversely, if it is too large, the coloring power tends to decrease.

Regarding the saturation magnetization, the magnetic iron oxide particles of the present invention have a value at an external magnetic field of 795.8 kA / m (10 kOe) of not less than 86.0 Am ² / kg, particularly as high as 87.0-90.0 Am ² / kg. . The iron oxide particles of the present invention having such a high saturation magnetization are particularly suitable as a toner for a high speed copying machine. Saturation magnetization is measured at a temperature of 25 ° C. and an external magnetic field of 795.8 kA / m using a vibrating sample magnetometer VSM-P7 manufactured by Toei Kogyo.

Although associated with the saturation magnetization of the magnetic iron oxide particles of the present invention, the residual magnetization in the external magnetic field 795.8 kA / m is 9.0~17.0Am ² / kg, in particular 11.0~15.0Am ² / kg is preferred. The coercive force in the external magnetic field is preferably 6.0 to 16.0 A / m, particularly preferably 8.0 to 15.0 A / m. The residual magnetization and the coercive force are measured in the same manner as the saturation magnetization measurement method described above.

  Since the amount of silicon contained in the core particle is very small as described above with respect to the entire magnetic iron oxide particle, it is not clear what form silicon is present in the core particle. It is speculated that it may be incorporated in the crystal of magnetic iron oxide in the form of silicate ions. The core particles preferably do not contain various heavy metals such as copper, nickel, cobalt, and zinc. Since there is a concern that these heavy metals may adversely affect the human body, it is advantageous from the viewpoint of safety that these heavy metals are not included. As used herein, heavy metal refers to a metal element having an atomic number of 21 or more (except for iron, which is the main component of magnetic iron oxide particles). It should be noted that a trace amount of heavy metal is allowed as an inevitable impurity derived from the raw material.

  In addition to the silicon contained in the core particles, the magnetic iron oxide particles of the present invention also contain silicon in the coating layer. Then, the weight of silicon contained in the coating layer may be set to 0.05 to 0.50 wt%, particularly 0.05 to 0.35 wt% as Si with respect to the total weight of the magnetic iron oxide particles. preferable. A method for measuring the amount of silicon contained in the coating layer will be described later. The coating layer contains aluminum in addition to silicon. The weight of aluminum contained in the coating layer can be set to 0.05 to 0.50% by weight, particularly 0.05 to 0.35% by weight as Al with respect to the total weight of the magnetic iron oxide particles. preferable. The amount of aluminum contained in the coating layer is measured by a method similar to the method for measuring the amount of silicon contained in the coating layer.

  By covering the core particles with a coating layer containing silicon and aluminum, it is possible to prevent deterioration of the core particles having a high FeO content and a high saturation magnetization. Specifically, by including silicon and aluminum, high thermal conductivity and high saturation magnetization can be ensured with the minimum necessary coating amount. In particular, by keeping the amount of silicon and aluminum contained in the coating layer within the above range, the thermal conductivity, saturation magnetization, and electrical conductivity are maintained at high levels while maintaining good heat resistance of the magnetic iron oxide particles. It becomes possible to do. In order to improve the heat resistance by containing a silicon component or an aluminum component alone in the coating layer, the content of these components needs to be excessively increased. This leads to a rough surface of the magnetic iron oxide particles. When the surface of the magnetic iron oxide particle becomes rough, when the particle is used as a raw material for toner, for example, it becomes a cause of a decrease in kneadability with the resin.

  Although it is as above-mentioned that a coating layer contains silicon and aluminum, it is preferable that iron is not contained substantially in this coating layer. The reason is not clear, but when iron is contained in the coating layer containing silicon and aluminum, the heat resistance of the magnetic iron oxide particles is slightly deteriorated. “Substantially not contained” means that a trace amount of iron inevitably mixed in the coating layer is allowed in the production process of the magnetic iron oxide particles.

  Due to the small amount of silicon and aluminum contained in the coating layer, the coating layer may coat the surface of the core particles discontinuously. However, it is not impeded that the coating layer continuously covers the surface of the core particle. From the viewpoint of reducing the electric resistance of the magnetic iron oxide particles, the coating layer preferably coats the surface of the core particles discontinuously. Although it is difficult to measure the thickness of the coating layer, the present inventors presume that it is roughly on the order of 5 to 20 mm.

  The amount of silicon and aluminum contained in the coating layer and the amount of silicon contained in the core particles are measured by the following method. First, the amounts of silicon and aluminum contained in the coating layer are measured by the following method. That is, 0.900 g of magnetic iron oxide particles as a sample are weighed, and 25 mL of 1N NaOH solution is added thereto. The solution is warmed to 45 ° C. with stirring. This dissolves the coating layer and dissolves the silicon and aluminum components contained therein. After filtering undissolved matter (= core particles), the eluate is made up to 125 mL with pure water. Next, silicon and aluminum contained in the eluate are quantified by ICP, and the concentration (g / L) of silicon and aluminum contained in the eluate is obtained. By multiplying this concentration by 0.125, the weight (g) of silicon and aluminum contained in the coating layer is calculated. By dividing the weight of silicon and aluminum by 0.900 g which is the weight of the sample and multiplying by 100, the ratio of the weight of silicon and aluminum contained in the coating layer to the total weight of the magnetic iron oxide particles is Calculated.

  The amount of silicon contained in the core particles is determined based on the amount of silicon contained in the coating layer measured by the above-described method (however, the sample weight is 1.00 g) from the amount of silicon contained in the entire magnetic iron oxide particles. Calculated by subtracting the amount. The amount of silicon contained in the entire magnetic iron oxide particle is calculated by the following method. That is, 26 mL of an aqueous hydrochloric acid solution in which 16 mL of a special grade hydrochloric acid reagent (concentration 35%) is dissolved is added to 1.00 g of magnetic iron oxide particles as a sample. Dissolve by heating until the powder is gone, then cool to room temperature. Next, after adding 4 mL of a hydrofluoric acid aqueous solution in which 2 mL of a special grade hydrofluoric acid reagent (concentration 4%) is dissolved, it is allowed to stand for 20 minutes. Subsequently, 10 mL of Triton X (concentration 10%, manufactured by ACROS ORGANICS, Japan distributor: Kanto Chemical Co., Inc.) is added, and then transferred to a 100 mL polymeas flask. And pure water is added and the whole solution is made up to 100 mL. The silicon contained in the solution sample thus obtained is quantified by ICP, and the concentration (g / L) of silicon contained in the solution sample is obtained. By multiplying this concentration by 0.1, the weight (g) of silicon contained in the entire magnetic iron oxide particle is calculated. By subtracting the amount of silicon contained in the coating layer measured by the above-described method (however, the sample weight is assumed to be 1.00 g) from the weight (g) of silicon thus measured, The amount (g) of silicon contained in is calculated. By multiplying the amount (g) of silicon contained in the core particles by 100, the ratio of the weight of silicon contained in the core particles to the total weight of the magnetic iron oxide particles is calculated.

The magnetic iron oxide particles of the present invention have high thermal conductivity as already described. Specifically, the thermal conductivity of the magnetic iron oxide particles of the present invention is as high as 0.15 to 0.30 W / (m · K), particularly 0.20 to 0.30 W / (m · K). . The thermal conductivity is measured by a thermal conductivity measuring device (LFA447 manufactured by NETZSCH) using magnetic iron oxide particles as a pressure molded body and using the molded body. The molded body can be obtained by putting 0.5 g of magnetic iron oxide particles into a molding machine having a diameter of 12.5 mm and pressurizing for 1 minute at a load of 100 kgf / cm ² .

  Next, the preferable manufacturing method of the magnetic iron oxide particle of this invention is demonstrated. In this production method, (i) oxidizing gas is blown into a slurry of ferrous hydroxide containing a silicon source and having a pH of 9.5 or higher, and hydroxylation is performed until no divalent iron ions are present in the liquid. A slurry in which iron iron is oxidized to produce magnetic iron oxide core particles containing silicon is obtained. (Ii) A silicon source and an aluminum source are added to the slurry, and the pH is adjusted to 5 to 9 to obtain core particles. A coating layer containing silicon and aluminum is formed on the surface of the substrate. In other words, this production method is roughly divided into (a) a process for producing core particles and (b) a process for forming a coating layer. Hereinafter, each process will be described.

  In the production process of the core particles, oxidizing gas is blown into the ferrous hydroxide slurry to perform wet oxidation of the ferrous hydroxide. This wet oxidation produces core particles. Specifically, oxidizing gas is blown into a slurry of ferrous hydroxide containing a silicon source and having a pH of 9.5 or higher, and the ferrous hydroxide is wet until no divalent iron ions are present in the liquid. Oxidize. As a result, magnetic iron oxide core particles containing silicon are produced. The slurry of ferrous hydroxide can be obtained by adding an alkali to an aqueous solution containing a divalent iron source and a silicon source.

  As the divalent iron source, a water-soluble compound containing ferrous iron is used. Examples thereof include ferrous sulfate and ferrous chloride. As the silicon source, a water-soluble compound containing silicon is used. An example is sodium silicate. The amount of silicon source and divalent iron source used is preferably such that Si is 0.002 to 0.050 mol, particularly 0.005 to 0.030 mol, per 1 mol of Fe (II). The concentration of the divalent iron source in the aqueous solution is preferably 0.5 to 2.5 mol / L in terms of Fe. On the other hand, the concentration of the silicon source in the aqueous solution is preferably 0.01 to 0.06 mol / L in terms of Si.

  An aqueous solution containing a divalent iron source and silicon and an alkali aqueous solution of an alkali metal hydroxide such as sodium hydroxide are mixed to make the solution alkaline. This gives a slurry of ferrous hydroxide. This slurry contains the initially added silicon source. However, this slurry does not contain any heavy metal sources (except for inevitable impurities). For example, when an alkali metal hydroxide is used as the alkali, the alkali is added in an amount of 2.00 to 3.00 mol, especially 2.02 to 2 mol per 1 mol of Fe (II). It is preferable to be 50 mol.

  The ferrous hydroxide slurry thus obtained is wet-oxidized to produce magnetic iron oxide core particles. In this case, the pH of the slurry needs to be 9.5 or higher. When the pH is less than 9.5, silicon of the silicon source contained in the slurry is less likely to be taken into the magnetic iron oxide crystals. In addition, it is difficult to generate octahedral core particles. In order to make the pH of the slurry 9.5 or more, for example, an appropriate amount of alkali such as an alkali metal hydroxide may be added to the slurry. On the condition that the pH of the slurry is 9.5 or more, if the pH is set lower within this pH range, octahedral particles having a relatively small primary particle diameter can be obtained. Conversely, when the pH is set high, octahedral particles having a relatively large primary particle diameter can be obtained.

  For wet oxidation of the ferrous hydroxide slurry, blowing of oxidizing gas is used. As the oxidizing gas, for example, oxygen gas or oxygen-containing gas such as air is used. 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. During the blowing of the oxidizing gas, it is preferable to heat the slurry and keep it at 60 to 100 ° C., particularly 80 to 95 ° C. from the viewpoint of obtaining an appropriate reaction rate.

In order to bring the Fe (II) / total Fe ratio X described above into a desired range, it is important to control the degree of wet oxidation. Specifically, as the oxidation of ferrous hydroxide progresses, the amount of the oxidizing gas blown is gradually reduced, and the portion closer to the surface of the core particle becomes deficient in oxygen relative to Fe (II). It is preferable to do. By blowing the oxidizing gas in this way, silicon can be more uniformly taken into the magnetic iron oxide crystal. From these viewpoints, when air is used as the oxidizing gas, it is preferable to control the blowing amount as follows on condition that the blowing amount is gradually reduced.
-Until 50% of the ferrous hydroxide is oxidized:
10-80 L / min, especially 10-50 L / min
-Until ferrous hydroxide is oxidized above 50% and below 75%:
5-50 L / min, especially 5-30 L / min
Until ferrous hydroxide is oxidized above 75% and below 90%:
1-30L / min, especially 2-20L / min
-State in which ferrous hydroxide is superoxidized by 90%:
1-15L / min, especially 2-8L / min

  If the air blowing amount is increased on condition that the air blowing amount is controlled as described above, particles having a relatively small primary particle diameter can be obtained. Conversely, if the amount of air blown is reduced, particles having a relatively large primary particle size can be obtained. Thus, the core particles of magnetic iron oxide containing silicon produced by controlling the amount of blowing of oxidizing gas have a characteristic that the abundance of Fe (II) is high at a site close to the surface.

  The wet oxidation of ferrous hydroxide is performed until no divalent iron ions are present in the liquid. That is, complete oxidation is performed. In this manner, a slurry in which core particles of magnetic iron oxide containing silicon are generated is obtained. The generated core particles are octahedral in shape.

  Next, a silicon source and an aluminum source are added to the slurry to form a coating layer. As the silicon source, the same compounds as those described above can be used. As the aluminum source, a water-soluble compound containing aluminum, for example, aluminum sulfate can be used. The concentration of the silicon source in the slurry is preferably 0.001 to 0.050% by weight, particularly 0.002 to 0.020% by weight in terms of Si. On the other hand, the concentration of the aluminum source in the slurry is preferably 0.001 to 0.050% by weight, particularly 0.002 to 0.020% by weight in terms of Al.

  The silicon source and the aluminum source may be added simultaneously to the slurry of the core particles, or may be added sequentially. In the case of sequential addition, the order of addition of the silicon source and the aluminum source is not particularly limited. As described above, no divalent iron ions are present in the slurry of the core particles, but no divalent iron source is added to the slurry. Also, no heavy metal source other than the silicon source and the aluminum source is added to the slurry.

  Once the silicon source and aluminum source are added to the core particle slurry, the pH of the slurry is adjusted to 5-9, preferably 5-7. By this pH adjustment, a coating layer is formed on the surface of the core particles from the silicon source and the aluminum source. The temperature at this time can be between room temperature and 90 ° C. In the coating layer thus formed, it is presumed that silicon and aluminum are present in the form of their hydroxides as described above. The coating layer does not contain heavy metals. Even after the coating layer is formed, the octahedral shape of the core particles is maintained.

  In this way, magnetic iron oxide particles are obtained in which a coating layer containing silicon and aluminum is formed on the surface of core particles made of magnetic iron oxide containing silicon. The obtained particles are preferably subjected to wet mechanical treatment for the purpose of stabilizing the coating layer. Moreover, by subjecting to wet mechanical treatment, the aggregate can be reduced and magnetic iron oxide particles having good dispersibility can be obtained. For the wet mechanical treatment, for example, a wet jet mill, a wet media mill, an emulsifying disperser, or the like can be used.

  The magnetic iron oxide particles obtained in this way, for example, by utilizing characteristics such as heat resistance and thermal conductivity, high blackness, and good dispersibility, a raw material for toner for electrostatic copying, It is suitably used as a raw material for a carrier for developing an electrostatic latent image.

  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]
(I) Generation of core particles Ferrous sulfate was used as a divalent iron source. Sodium silicate was used as a silicon source. 10.0 L of an aqueous solution containing 0.23 mol / L of Si ⁴⁺ was added to 50 L of an aqueous solution containing 2.0 mol / L of Fe ²⁺ . This aqueous solution was stirred and mixed with 42 liters of an aqueous solution containing 5.0 mol / L of sodium hydroxide to obtain a ferrous hydroxide slurry. The pH of this ferrous hydroxide slurry was adjusted to 12 using an aqueous sodium hydroxide solution. Next, the slurry was heated to 90 ° C., and air was blown at 30 L / min to conduct wet oxidation of ferrous hydroxide. When the oxidation of ferrous hydroxide exceeded 50%, the air blowing amount was reduced to 20 L / min. Furthermore, when the oxidation of ferrous hydroxide exceeded 75%, the amount of air blown was reduced to 10 L / min. When the oxidation of ferrous hydroxide exceeded 90%, the amount of air blown was reduced to 5 L / min, and wet oxidation was performed until no divalent iron ions existed in the liquid. In this way, a slurry of octahedral core particles made of magnetic iron oxide containing silicon was obtained.

(B) Formation of coating layer 120 g of an aqueous solution of sodium silicate (Si grade 13.4 wt%) and 380 g of an aqueous solution of aluminum sulfate (Al grade 4.2 wt%) were added simultaneously to the obtained slurry. Next, the pH of the slurry was adjusted to 5-9 with dilute sulfuric acid at 80 ° C. Thereby, a coating layer containing silicon and aluminum was formed on the surface of the core particle. The slurry containing the magnetic iron oxide particles thus obtained was subjected to conventional dehydration, washing, filtration, drying and crushing steps to obtain the desired magnetic iron oxide particles.

[Examples 2 to 4 and Comparative Examples 1 to 5]
Magnetic iron oxide particles were obtained in the same manner as in Example 1 except that the conditions for (a) generation of core particles and (b) formation of the coating layer in Example 1 were as shown in Table 1 below.

[Evaluation 1]
Various characteristics of the magnetic iron oxide particles obtained in Examples and Comparative Examples were measured according to the above-described methods. The results are shown in Table 2 below.

[Evaluation 2]
About the magnetic iron oxide particle obtained by the Example and the comparative example, the heat conductivity was measured by the above-mentioned method. Further, the coloring power and hue and the Fe (II) reduction rate were measured by the method described below. The results are shown in Table 3 below. The Fe (II) reduction rate is a measure of the heat resistance of magnetic iron oxide.

[Coloring power and hue]
1.3 cc of castor oil is added to 0.5 g of magnetic iron oxide particles and 1.5 g of titanium oxide (R800 manufactured by Ishihara Sangyo Co., Ltd.) and kneaded with a Hoover type Mahler. 4.5 g of lacquer is added to 2.0 g of this kneaded sample, and after further kneading, this is applied onto mirror-coated paper using a 4 mil applicator. After drying, the coloring power (L value) and hue (a value, b value) are measured with a color difference meter (Color Analyzer TC-1800 manufactured by Tokyo Denshoku Co., Ltd.).

[Fe (II) reduction rate]
5 g of magnetic iron oxide particles are placed on a watch glass and exposed for 1 hour under an environment of 160 ° C. in a ventilated dryer (Tabai Espec oven, PH-201 type). The amount of Fe (II) before and after exposure is determined by redox titration. As a titration indicator, sodium diphenylamine sulfonate is used. As a titration reagent, 0.1N potassium dichromate is used. The titration amount is obtained with the end point where the sample is colored blue-violet by titration, and the Fe (II) (mg / l) concentration is calculated from the titration amount. Then, the Fe (II) reduction rate (%) is calculated from the following equation.

  As is apparent from the results shown in Table 3, it can be seen that the magnetic iron oxide particles of each example have higher thermal conductivity than that of each comparative example. It can also be seen that the Fe (II) reduction rate is low, that is, the heat resistance is high. Furthermore, it turns out that coloring power is high and color is favorable. In particular, as is clear from the comparison between Example 3 and Example 4, when the amount of Si contained in the core particles is large, the values of Fe (II) ratio X and X / Y increase. Further, as is clear from the comparison between Example 2 and Example 3, when the amount of air blown is large and the pH is low, the primary particle size of the particles becomes small.

Claims (6)

  A magnetic iron oxide particle having an octahedral shape in which a coating layer containing silicon and aluminum is formed on the surface of a core particle of magnetic iron oxide containing silicon, and the magnetic iron oxide particle is separated from the surface thereof. The ratio of the amount of Fe (II) contained in the total dissolved Fe and the total amount of Fe at the time when 10 wt% Fe is dissolved with respect to the total amount of Fe contained in the particles. Magnetic iron oxide particles, wherein X (the former / the latter) is 0.34 ≦ X ≦ 0.50.   When the ratio of the amount of Fe (II) contained in the total Fe in which the remaining 90% by weight of Fe is dissolved and the total amount of Fe (the former / the latter) is Y, the ratio of the above X and Y ( The magnetic iron oxide particles according to claim 1, wherein X / Y) is more than 1.00 and 1.30 or less.   The magnetic iron oxide particles according to claim 1 or 2, wherein the weight of silicon contained in the core particles is 0.30 to 1.50 wt% as Si with respect to the total weight of the magnetic iron oxide particles. The weight of silicon contained in the coating layer is 0.05 to 0.50% by weight as Si with respect to the total weight of the magnetic iron oxide particles,
The magnetic iron oxide particles according to any one of claims 1 to 3, wherein the amount of aluminum contained in the coating layer is 0.05 to 0.50 wt% as Al with respect to the entire magnetic iron oxide particles.   The magnetic iron oxide particles according to any one of claims 1 to 4, wherein the primary particle diameter is 0.10 to 0.30 µm. 6. The magnetic iron oxide particle according to claim 1, which has a saturation magnetization of 86.0 Am ² / kg or more in an external magnetic field of 795.8 kA / m (10 kOe).