JP4621911B2 - Method for producing magnetite fine particles - Google Patents

Description

  The present invention relates to a method for producing magnetite fine particles, and more particularly, to a method for producing magnetite fine particles having good crystallinity and a uniform crystal shape and particle size by a method of producing from an alkaline solution.

  Magnetite fine particles are chemically stable and relatively large magnetic fine particles, and thus have been widely used for various applications such as magnetic recording media, magnetic fluids, and magnetic toners. In recent years, magnetite fine particles have been attracting attention in the fields of medical and biotechnology, such as being used as a magnetic concentration / separation carrier in immunoassays.

  As the application field of magnetite fine particles expands, various properties that are not regarded as important in conventional magnetite fine particles have come to be required for magnetite fine particles. For example, properties such as better particle shape and size and better dispersibility in water are required compared to conventional products, and when used in vivo, such as amino acids and proteins Properties such as having affinity for organic substances and not having harmful effects on living organisms have been demanded. Therefore, it has been desired to develop magnetite fine particles that can meet such various requirements and a method for producing the same.

  Various methods have been developed and used for the production of magnetite fine particles. The method for producing magnetite fine particles can be broadly classified into two methods: a method of firing and pulverizing raw material powders after being prepared and calcinated, and a wet method of generating in aqueous solution. Among these, the calcination / pulverization method is excellent in productivity, but becomes fine particles having irregular particle shapes and particle size distributions. On the other hand, the wet method for producing magnetite fine particles in an aqueous solution is not as good as the firing / grinding method in terms of productivity, but the production of magnetite fine particles having a good particle shape and particle size distribution that cannot be obtained by the firing / grinding method. Is possible.

  As a method for producing magnetite fine particles and related ferrite fine particles by a wet method, the results of research by Sato, Sugihara and Saito are described in Non-Patent Document 1 (Industrial Chemical Journal 65 (1962) p.1748). Further, a technical prospect regarding the wet method is described in Non-Patent Document 2 (Proc. ICF (1980)) p.3. In these wet methods, an aqueous solution containing divalent iron ions or an aqueous solution containing divalent iron ions and trivalent iron ions is precipitated with an alkali to obtain a suspension. By keeping the divalent iron ions in the precipitate, the divalent iron ions in the precipitate are oxidized by oxygen in the air, thereby generating magnetite fine particles.

  As compared with magnetite fine particles produced by such a wet method, those having a uniform shape and size and those having a large saturation magnetization have been desired. In order to meet these demands, as a means for obtaining magnetite fine particles having high saturation magnetization using a wet method, a mixture of an aqueous solution containing divalent iron ions and trivalent iron ions and an alkaline solution, divalent iron ions, A mixed solution of an aqueous solution containing an oxidizing agent that oxidizes divalent iron ions and an alkaline solution is performed at a liquid temperature of 0 ° C. to 15 ° C. to generate colloidal particles of magnetite particles, and then the temperature is raised to, for example, about 80 ° C. A method has been developed for obtaining magnetite fine particles by raising and aging (Patent Document 1: JP-A-9-169525). By this method, magnetite fine particles having higher saturation magnetization and higher initial magnetization characteristics than before can be obtained. However, the crystallinity and particle size distribution of the magnetite fine particles obtained by this method have many points to be improved.

  Ferrite plating is a method for forming a ferrite film such as magnetite from an aqueous solution having a substantially neutral pH and a temperature close to room temperature. In ferrite plating, a ferrite film having excellent magnetism is formed by oxidizing a part of divalent iron ions in an aqueous solution containing divalent iron ions while keeping the pH of the solution constant with a buffer solution. . It has been developed as a new wet manufacturing method for ferrite fine particles such as magnetite fine particles to which this ferrite plating method is applied (Patent Document 2: JP-A-2002-128523). By using this wet method, various ferrite fine particles including magnetite fine particles can be obtained from an aqueous solution having a pH close to neutral.

  In addition, in the production of ferrite fine particles such as magnetite by a wet method, there is a soap method in which a surfactant is added as an effective means for obtaining magnetite fine particles having a uniform shape and size and well dispersed in water. However, the magnetite fine particles produced by the soap method have a problem that it is difficult to sufficiently remove the adsorbing surfactant by washing. Further, the method of obtaining magnetite fine particles from iron alkoxide is an excellent method for obtaining magnetite fine particles, but this method also has a problem in that organic matter remains.

Furthermore, as a method for producing magnetite fine particles having a uniform particle size, there is a method by, for example, mass culture of magnetic bacteria (Non-patent Document 3). If this method is used, magnetite fine particles suitable for use in biotechnology can be obtained because the shapes, properties, and particle sizes of the fine particles are well aligned. However, in order to use this method, there is a problem that detailed condition setting and control for satisfying the conditions for mass culture of magnetic bacteria are required.
: JP-A-9-169525 : Japanese Patent Application Laid-Open No. 2002-128523 : (Industrial Chemical Journal 65 (1962) p.1748) : (Prosheetings of the International Conference of Ferrite (1980), p. 3 ： Participation of fine particle engineering, Volume II, Applied technology (Fuji / Techno System) 687 (2002)

  An object of the present invention is to provide new magnetite fine particles that can be produced by simple means to produce magnetite fine particles having a more uniform shape and size and better crystallinity than the above-described methods for producing various magnetite fine particles. It is in providing the manufacturing method of.

  The inventors of the present invention have studied the production method for obtaining magnetite fine particles having well-shaped and well-shaped particles and excellent crystallinity, and drastically solved the problems in the conventional production methods of various magnetite fine particles described above. As a result of repeated research to solve this problem, we have reached the following method for producing magnetite fine particles.

In the method for producing magnetite fine particles of the present invention, an alkaline aqueous solution in which an alkali is dissolved in pure water is deoxygenated, and a part of divalent iron ions is oxidized in the deoxygenated alkaline aqueous solution to form divalent iron ions. A step of containing an oxidizing agent having nitrate ions forming a coexistence state with trivalent iron ions to obtain an oxidizing agent-containing alkaline aqueous solution, deoxidizing the water, and divalent iron ions to the deoxygenated water. Adding a divalent iron ion aqueous solution, mixing these oxidizing agent-containing alkaline aqueous solution and divalent iron ion aqueous solution to obtain a mixed solution, and stirring the mixed solution at a predetermined temperature of 40 ° C. or lower. And a step of generating magnetite fine particles while maintaining the temperature.

In the present invention, oxygen dissolved in an alkaline aqueous solution is previously removed by deoxygenation of the alkaline aqueous solution, and then a part of the divalent iron ion is oxidized to coexist with the divalent iron ion and the trivalent iron ion. By adding an oxidizing agent having nitrate ions to form a divalent iron ion aqueous solution, the divalent iron ions in the aqueous solution are gently oxidized to generate magnetite fine particles. By the magnetite fine particle production method of the present invention, it is possible to obtain magnetite fine particles having a good particle shape and particle size, good crystallinity and well separated from each other, and obtained by the conventional method for producing magnetite fine particles. The production of magnetite fine particles that could not be performed became possible.

  In the method for producing magnetite fine particles of the present invention, an alkaline aqueous solution is subjected to deoxygenation treatment, and a part of divalent iron ion is oxidized to form a coexistence state of divalent iron ion and trivalent iron ion. Since an oxidizing agent is added and contained, divalent iron ions and trivalent iron ions coexist in the alkaline aqueous solution, and an oxidation state suitable for the production of magnetite fine particles can be obtained. By maintaining the temperature at a low temperature of ℃ or less, it is considered that conditions suitable for the magnetite fine particles to grow with good crystallinity can be obtained.

If the operation of deoxidizing the alkaline aqueous solution in the present invention is omitted, generation of particles other than magnetite is observed. This is thought to be due to the rapid oxidation of divalent iron ions in the aqueous solution by residual oxygen. The generation of particles other than magnetite should be prevented by deoxidizing the alkaline aqueous solution in advance. Can do. In the present invention, pure water is used to prevent the generation of particles other than magnetite, and the specific resistance of pure water is preferably 1 × 10 ⁶ Ω · cm or more, and is preferably 10 × 10 ⁶ Ω · cm or more. More desirably.

  Patent Document 1 described above describes that mixing of an aqueous solution containing divalent iron ions and an oxidizing agent that oxidizes divalent iron ions with an alkaline solution is performed at a liquid temperature of 0 ° C. to 15 ° C. Yes. However, the method of Patent Document 1 generates magnetite particle colloidal particles by mixing at a liquid temperature of 0 ° C. to 15 ° C., and then raises the temperature to about 80 ° C. and ripens to magnetite. Fine particles are obtained. Patent Document 1 describes that high saturation magnetization cannot be obtained by aging fine particles at a low temperature.

  The present invention breaks the common sense of these prior arts that it is difficult to obtain good crystallinity and high saturation magnetization in a low temperature aqueous solution, and produces magnetite fine particles in a low temperature aqueous solution. As a result, it has been found that magnetite fine particles having a uniform particle shape and particle diameter and good crystallinity can be obtained, and the present invention has been achieved.

  According to the present invention, it is possible to obtain magnetite fine particles having a uniform particle shape and particle diameter, good crystallinity, and well separated from each other without using a surfactant that is frequently used in conventional production methods. As described above, when a surfactant is used in the production of the magnetite fine particles, it is difficult to sufficiently wash and remove the surfactant after the production. Magnetite fine particles produced by such a method are, for example, There is a problem that it is not suitable for use in applications where the presence of a surfactant is not preferred, such as in vivo. In the present invention, since it is not necessary to use a surfactant, such problems can be avoided by producing magnetite fine particles by the production method of the present invention.

  According to the present invention, it is possible to produce magnetite fine particles having a uniform particle shape and particle diameter and well separated from each other and having good crystallinity by a simple method.

  FIG. 1 shows a specific example of the flow of steps in the method for producing magnetite fine particles of the present invention. In FIG. 1, an alkali 11 is added to pure water 10 to obtain an alkaline aqueous solution 12, which is deoxygenated by a deoxygenation treatment step 13 to obtain a deoxygenated alkaline aqueous solution 14, and then divalent iron. A slow-acting oxidant 15 that oxidizes part of the ions to form a coexistence state of divalent iron ions and trivalent iron ions is added to obtain an alkaline aqueous solution 16 containing the oxidant. On the other hand, after the pure water 17 is deoxygenated in the deoxygenation process 18 to obtain the deoxygenated pure water 19, a divalent iron salt 20 is added to obtain a divalent iron ion aqueous solution 21. The divalent iron ion aqueous solution 21 and the alkaline aqueous solution 16 to which the oxidant is added are mixed in the mixing step 22, and the mixture is stirred and held while continuing stirring to enter the step 23 of generating magnetite fine particles. . In this mixing, the two aqueous solutions are mixed at a predetermined temperature, for example, both at 4 ° C. After this mixing and holding is performed for a predetermined time to generate magnetite fine particles in the aqueous solution, the magnetite fine particles 25 are obtained by collecting them by a method such as magnetic separation 24.

  In the present invention, the divalent iron ion is slowly oxidized using the slow-acting oxidant 15 that oxidizes a part of the divalent iron ion to form a coexistence state of the divalent iron ion and the trivalent iron ion. It is particularly important. In order to produce magnetite from an aqueous solution having divalent iron ions, a part of the divalent iron ions are oxidized to form trivalent iron ions, and the divalent iron ions and the trivalent iron ions coexist. : It is necessary to be incorporated into the spinel structure at a ratio of 2.

  In the present invention, the desirable rate of divalent iron ion oxidation for oxidizing divalent iron ions is 5 hours or more at room temperature, that is, 2/3 of divalent iron ions in an alkaline aqueous solution are oxidized to be trivalent. It is desirable that the time required to become iron ions is 5 hours or more. If the time required to oxidize divalent iron ions to trivalent iron ions is shorter than this, it will be difficult to obtain magnetite fine particles with good particle shape and particle size that are well aligned. The oxidation speed of such divalent iron ions is more preferably 10 hours or more at room temperature. In the case of a temperature lower than room temperature, a longer time is required than in the case of room temperature. On the other hand, if the time required to oxidize divalent iron ions is too long, the productivity of magnetite fine particle production will be reduced. For example, the time required for the above oxidation should be kept within 50 hours, for example. Is desirable.

As a slow-acting oxidant that oxidizes a part of such divalent iron ions to form a coexistence state of divalent iron ions and trivalent iron ions, NaNO ₃ , KNO _3, NH ₄ NO _3, etc. Nitric acid compounds that supply nitrate ions to can be particularly preferably used. The concentration of the nitrate ion is desirably an amount exceeding the equivalent with respect to the divalent iron ion to be oxidized, and is preferably 2 to 4 times the equivalent.

In the present invention, a slow-acting oxidant that oxidizes a part of divalent iron ions (Fe ²⁺ ) in an aqueous solution and forms a coexistence state of Fe ²⁺ and trivalent iron ions (Fe ³⁺ ) is thus obtained. Used, part of Fe ²⁺ in the aqueous solution is oxidized to produce Fe ³⁺ , and these are incorporated into the spinel structure together with oxygen to form magnetite Fe ³⁺ (Fe ³⁺ Fe ²⁺ ) O ₄ .

The generated fine particles have a little more Fe ³⁺ than magnetite, and while maintaining the same spinel structure as this magnetite, maghemite Fe ³⁺ (Fe ³⁺ _5/3 □ _1/3 ) O ₄ (here □ may be in the form of a part of the cation vacancies). Further, other metal ions may be added to the solution to form ferrite fine particles in which iron ions in the spinel are partially substituted.

In the present invention, an aqueous alkali metal hydroxide solution such as NaOH or KOH can be used as the alkaline aqueous solution used to obtain such magnetite fine particles. An NH ₄ OH aqueous solution can also be used. In order to obtain magnetite fine particles having a uniform particle size and good crystallinity, it is desirable that the pH value of the mixed solution is in the range of more than 9 and less than 13.5, and the pH value is more than 11 and 13 It is further desirable to be in a range of less than.

  As the aqueous solution having divalent iron ions used in the present invention, various ferrous aqueous solutions that can have divalent iron ions in the aqueous solution such as ferrous chloride, ferrous nitrate, and ferrous sulfate are used. Can do.

  When an aqueous solution of ferrous nitrate is used as an aqueous solution having divalent iron ions, nitrate ions, which are oxidizing agents that gently oxidize divalent iron ions in an alkaline solution in the present invention, are already present as ferrous nitrate. Therefore, an additional amount of nitrate ions may be added as an oxidizing agent.

  The concentration of the alkaline aqueous solution having divalent iron ions is a concentration at which magnetic microparticles are generated by partial oxidation of the divalent iron ions, and this value can be appropriately adjusted depending on the pH value of the alkaline aqueous solution to be added. In order to obtain magnetite fine particles having a uniform particle size distribution and good crystallinity, the concentration of the aqueous solution having divalent iron ions is preferably 0.1 M or less, more preferably 0.05 M or less. Desirably, 0.01M or less is more desirable. On the other hand, if this concentration is too low, the yield decreases and productivity decreases. From this, for example, 0.001M can be selected as the lower limit of the concentration of the aqueous solution having divalent iron ions.

A strong oxidizing agent such as hydrogen peroxide is not suitable as the oxidizing agent used in the present invention. Hydrogen peroxide tends to form weakly magnetic α-Fe ₂ O ₃ formed with only trivalent iron ions, even when used at a reduced concentration. It is difficult to use as a slow-acting oxidant that oxidizes only a part of it and forms a coexistence state of divalent iron ions and trivalent iron ions.

  In the magnetite fine particle production step of the present invention in which an aqueous alkaline solution containing a divalent iron ion and an oxidizing agent that gently oxidizes the divalent iron ion is held while stirring to produce magnetite fine particles, the aqueous solution is continuously stirred. In addition, the uniform state of the aqueous solution can be maintained, and magnetite fine particles having a uniform particle diameter can be generated.

  In the present invention, magnetite fine particles are generated by maintaining a stirring state of an alkaline aqueous solution containing divalent iron ions and an oxidizing agent that gently oxidizes divalent iron ions at a predetermined temperature of 40 ° C. or lower. By maintaining the stirring state and setting the temperature at which the magnetite fine particles are generated to be 40 ° C. or less, it is possible to obtain magnetite fine particles having a uniform particle shape and particle diameter and good crystallinity. This temperature is more desirably a temperature of room temperature or less, and further desirably a temperature of 15 ° C. or less.

  In the present invention, when the magnetite fine particles are produced by maintaining the stirring state of an alkaline aqueous solution containing divalent iron ions and an oxidizing agent that gently oxidizes divalent iron ions at a predetermined temperature of 40 ° C. or lower, It is desirable to shield the light. By shielding the light, decomposition of the oxidant by light can be prevented, so that oxidation of divalent iron ions and accompanying generation and growth of magnetite fine particles can be stabilized.

  In the present invention, the temperature of the alkaline aqueous solution containing divalent iron ions and the oxidizing agent that gently oxidizes the divalent iron ions is set by adjusting the alkaline aqueous solution and the divalent iron ion aqueous solution to which the oxidizing agent is added before mixing. It is desirable to mix those previously set at a predetermined temperature of 40 ° C. or less, and to maintain the mixed solution at the predetermined temperature as it is to generate magnetite fine particles. It is desirable that the set temperature is room temperature or lower, and it is further desirable that this temperature be 15 ° C. or lower.

  In the present invention, since magnetite fine particles are slowly generated and grown from an aqueous solution, the magnetite fine particles can be obtained in the form of a single crystal.

  Further, in the present invention, although the average particle size is as small as 200 nm or less or 150 nm or less, magnetite fine particles having good crystallinity can be obtained, and the saturation magnetization value is as great as 85 emu / g or more. Magnetite fine particles having a large value can be obtained.

  Since the magnetite fine particles produced in the present invention have magnetism, the magnetite fine particles generated in the aqueous solution can be recovered by so-called magnetic separation using a magnetic field gradient. Using magnetic separation, even when non-magnetic particles other than magnetite fine particles are generated at the same time as magnetite fine particles, it is easy to select and recover magnetite fine particles, or wash them to remove impurities. Can be done.

  Next, based on an Example, this invention is demonstrated still in detail.

  A 500 ml flask was filled with 465 ml of pure water, and 10 ml of NaOH was added thereto as an alkali so that a final 20 mM NaOH aqueous solution was obtained.

This aqueous NaOH solution was bubbled with N ₂ gas for 3 hours to remove residual oxygen, and then 0.75 g of NaNO ₃ was added as an oxidizing agent to obtain an aqueous NaOH solution with a well-adjusted oxidizing power.

On the other hand, 25 ml of pure water was similarly bubbled with N ₂ gas to remove residual oxygen, and then 0.5 g of FeCl ₂ was added to obtain a 0.1 M FeCl ₂ aqueous solution.

On cooling to both 4 ° C. of added NaOH solution and FeCl ₂ solution of the oxidizing agent was added FeCl ₂ aqueous NaOH solution. The alkaline aqueous solution containing this oxidizing agent and divalent iron ions was sealed, kept at a temperature of 4 ° C. and kept for 40 hours while continuing to stir in a light-shielded state.

Next, an aqueous solution containing 0.1 M FeCl ₂ and an aqueous NaOH solution were prepared under the same conditions as above, and then the temperature of these aqueous solutions was 10 ° C. (Example 2-1) , 15 ° C. (Example 2-2). The temperature was kept at room temperature (25 ° C.) (Example 2-3) and 37 ° C. (Example 2-4) , and the mixture was held at each temperature for 40 hours while stirring was continued.

  Magnetite fine particles produced by magnetic separation were recovered from the aqueous solution kept at each temperature while continuing the stirring.

  As a result of performing X-ray diffraction on the collected magnetite fine particles, a diffraction line having a spinel structure is observed in any of the magnetite fine particles, and the lattice constant calculated from the diffraction line corresponds to a known value of the lattice constant of the magnetite. It was a thing. Further, the half width of the diffraction line showed a value as expected from the particle diameter of the magnetite fine particles. Further, as a result of chemical analysis of the obtained magnetite fine particles, a value of about 1: 2 corresponding to magnetite was obtained as a ratio of divalent iron ions to trivalent iron ions in any magnetic fine particles. From these results, it was possible to determine that the obtained fine particles were substantially magnetite fine particles.

  (A) to (e) in FIG. 2 are transmission types of magnetite fine particles obtained by stirring and holding an aqueous solution at 4 ° C., 10 ° C., 15 ° C., room temperature (25 ° C.) and 37 ° C., respectively. An electron microscope (TEM) photograph is shown respectively.

  In each TEM photograph of FIG. 2, the octahedral particle shape in which the magnetite fine particles are seen as crystal particles having a spinel structure was recognized.

  FIG. 3 (c) shows a part of the TEM image of the magnetite fine particles shown in FIG. 2 (c) at 15 ° C. The magnification is further increased so that the lattice image of the crystal particles can be observed. In the two crystal particles in FIG. 3, a linear lattice image that is parallel to one crystal plane and is not disturbed is observed, and the produced magnetite fine particles have good crystallinity. I was able to get the backing of.

  FIG. 4 shows the average particle diameter (bar graph) obtained by measuring the diameter of about 200 magnetic fine particles prepared at each temperature using the TEM photograph of FIG. And the standard deviation (described above the bar graph). From this figure, it was found that the average particle size of the magnetite fine particles changed with the holding temperature, and that the particle size increased as the temperature decreased.

  From the results of FIG. 4, it can be seen that the average particle size of the magnetite fine particles obtained can be adjusted by the temperature to be produced. In addition, the value of the standard deviation in this case shows a value of around 5 nm regardless of the average particle diameter, and it was found that the sizes are very well aligned. Since the value of the standard deviation is about 5 nm regardless of the average particle diameter, the lower the temperature produced, the smaller the standard deviation / average particle diameter value of the obtained magnetite fine particles. In the following, it was found that the particle size can be made extremely well when manufactured at a low temperature of 15 ° C. or less.

  Thus, the magnetite fine particles obtained by the production method of the present invention have an average particle size that increases as the temperature decreases, and the standard deviation of the particle size does not vary greatly with temperature. It was found that the fine particles had a very small ratio between the standard deviation of the particle size and the average particle size, and the particle sizes were very well aligned. For this reason, when the temperature of the liquid in the production process of the magnetite fine particles is set to room temperature or lower, particularly excellent magnetite fine particles can be obtained, and if the temperature of the liquid in the production process of the magnetite fine particles is 15 ° C. or less, Further, it was found that excellent magnetite fine particles can be obtained. Note that the lower limit of this temperature is the freezing point of the aqueous solution.

  Magnetite fine particles were produced under exactly the same conditions as in Examples 1 and 2, except that the NaOH in the alkaline aqueous solution in Examples 1 and 2 was changed to KOH. As a result, almost the same results as in Examples 1 and 2 could be obtained.

  As an aqueous solution having divalent iron ions in Examples 1 and 2, ferrous nitrate was used instead of ferrous chloride, and the amount of oxidizing agent used was the amount of nitrate ions contained in ferrous nitrate. Magnetite fine particles were produced under exactly the same conditions as in Example 1 except that the amount was smaller than in the cases of 1 and 2. As a result, in this case, almost the same results as in Examples 1 and 2 could be obtained.

  According to the present invention, a complicated process is not required, the particle shape and the particle diameter are well aligned in a very simple process, the dispersibility is excellent, the magnetic property is excellent, and a substance such as a surface activity is added during the process. Magnetite fine particles can be obtained without addition. Therefore, it can be expected that the present invention is widely used for producing magnetite fine particles suitable for use in a wide range of fields such as medicine and biotechnology.

It is the figure which showed the flow of the process in one Embodiment of the manufacturing method of the magnetite fine particle which concerns on this invention. It is a transmission electron micrograph of the magnetite fine particle manufactured by the Example of this invention. FIG. 2C is a transmission electron micrograph of the magnetite particles shown in the transmission electron micrograph at 15 ° C. in which the magnification is further increased to a magnification at which a lattice image of the crystal particles can be observed. It is the figure which showed the average particle diameter of the magnetite fine particle manufactured at each temperature by the Example of this invention, and its standard deviation.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Pure water, 11 ... Alkali, 12 ... Alkaline aqueous solution, 13 ... Deoxygenation treatment, 14 ... Deoxygenation alkaline aqueous solution, 15 ... Oxidizing agent, 16 ... Alkaline aqueous solution to which oxidizing agent was added, 17 ... Pure water, DESCRIPTION OF SYMBOLS 18 ... Deoxygenation treatment, 19 ... Deoxygenation pure water, 20 ... Divalent iron salt, 21 ... Divalent iron ion aqueous solution, 22 ... Stirring holding, 23 ... Magnetic separation, 24 ... Magnetite fine particle.

Claims (5)

An alkaline aqueous solution in which alkali is dissolved in pure water is deoxygenated, and a part of divalent iron ions is oxidized in the deoxygenated alkaline aqueous solution to form a coexistence state of divalent iron ions and trivalent iron ions. A step of containing an oxidizing agent having nitrate ions to obtain an oxidizing agent-containing alkaline aqueous solution;
Deoxygenating pure water, adding a divalent iron ion to the deoxygenated pure water to obtain a divalent iron ion aqueous solution, mixing the oxidizing agent-containing alkaline aqueous solution and the divalent iron ion aqueous solution, Obtaining a liquid mixture;
And a step of producing magnetite fine particles by maintaining the mixture at a predetermined temperature of 40 ° C. or lower while stirring. A step of mixing the oxidant-containing alkaline aqueous solution and the divalent iron ion aqueous solution to obtain a mixed solution, and a step of maintaining the mixed solution at a predetermined temperature while stirring to produce magnetite fine particles at room temperature or lower. The method for producing magnetite fine particles according to claim 1, wherein the method is performed at a temperature. 3. The magnetite according to claim 1, wherein the divalent iron ion concentration contained in the mixed solution of the oxidizing agent-containing alkaline aqueous solution and the divalent iron ion aqueous solution is 0.01 M or less. A method for producing fine particles. The method for producing magnetite fine particles according to any one of claims 1 to 3 , wherein single-crystal magnetite fine particles are produced. The method for producing magnetite fine particles according to any one of claims 1 to 4 , wherein magnetite fine particles having a saturation magnetization value of 85 emu / g or more are generated.