Nanodiamond powder manufacturing method

Abstract

To provide a method for producing nanodiamond powder which carries out speedy water washing and drying of crude nanodiamond, the nanodiamond powder having high dispersibility and a little remaining water content, while suppressing generation of loss.SOLUTION: A production method of nanodiamond powder includes carrying out the following oxidation treatment step, separation step, water washing step, and drying step to obtain the nanodiamond powder having a particle diameter D50 of 500 μm or less: the oxidation treatment step of adding an oxidant to an aqueous crude nanodiamond dispersion and heating the resultant; the separation step of filtering reaction liquid obtained after termination of the oxidation treatment step with a filter to obtain a filtration product; the water washing step of carrying out water washing of the filtration product obtained through the separation step and filtering the filtration product with a filter; and the drying step of loading the filtration product obtained through the water washing step and a spherical medium into a drying container and carrying out drying by heating at pressure with -0.1 to 0.1 MPa G and at 60 to 150°C with agitation in the drying container with the spherical medium.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing nanodiamond powder. [Background technology]

[0002] Nanodiamonds are ultrafine diamond particles with a very large specific surface area, with at least one of the major axis, minor axis, and thickness being 100 nm or less. They have high mechanical strength, electrical insulation, and excellent thermal conductivity. They also have deodorizing and antibacterial effects. For this reason, they are used as abrasives, conductivity enhancers, insulating materials, deodorizers, antibacterial agents, etc.

[0003] Nanodiamonds are produced by methods such as detonation, but the crude nanodiamonds obtained by these methods are contaminated with large amounts of by-products such as metal oxides and graphite that originate from the vessels used in the reaction.These contaminants reduce the dispersibility of the nanodiamonds and cause them to form aggregates, which reduces the above-mentioned properties.

[0004] Therefore, crude nanodiamonds are reacted with an oxidizing agent to separate by-products such as metal oxides and graphite from the nanodiamonds, which are then washed with water and dried using a spray dryer or the like.

[0005] A known method of washing with water involves adding water to a reaction vessel containing nanodiamonds after reacting with an oxidizing agent, stirring the mixture, and then removing the supernatant liquid after the nanodiamonds have settled, separating the solid from the liquid, and then redispersing the mixture in water (see, for example, Patent Document 1). However, when producing, for example, 1 kg of nanodiamond powder, the washing process takes about 4 to 5 days, and drying also takes more than 24 hours, resulting in poor work efficiency.

[0006] Centrifugal separation can also be considered as a method for solid-liquid separation. However, this method has problems such as the difficulty in recovering small particles, resulting in a large amount of loss, and the difficulty in redispersing nanodiamonds once they have aggregated. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2018-118883 Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, the object of the present disclosure is to provide a method for producing nanodiamond powder that is highly dispersible and has a low residual moisture content by quickly washing and drying crude nanodiamonds while suppressing the generation of loss. [Means for solving the problem]

[0009] As a result of extensive research into solving the above problems, the present inventors have discovered the following. 1. After oxidation, if the nanodiamonds containing impurities are washed with water and filtered using a filter with a specific air permeability, the impurities can be quickly removed along with the washing water while minimizing the loss of nanodiamonds. 2. When washing nanodiamonds with water, adding a specific medium to the mixture of nanodiamonds and washing water and stirring the mixture improves the removal of impurities contained in the nanodiamonds. 3. When filtering a mixture of nanodiamonds and washing water, stirring the mixture with a specific medium can prevent the nanodiamonds from forming a cake layer, which can prevent the filter from clogging and the filtrate permeation rate from decreasing. 4. In conventional drying methods, nanodiamonds containing water are compacted during the drying process to form agglomerates. However, the water trapped in the agglomerates is very difficult to dry, making it difficult to shorten the drying time. 5. After washing and filtering, the residue is placed in a drying container containing a specific medium, and then dried under reduced pressure and heat while stirring with the medium. This prevents moisture from being trapped inside the nanodiamond mass, and nanodiamond powder with an extremely low residual moisture content can be obtained. The present disclosure has been completed based on these findings.

[0010] That is, the present disclosure provides a method for producing nanodiamond powder, which obtains nanodiamond powder having a particle size D50 of 500 μm or less through the following oxidation treatment step, separation step, water washing step, and drying step. Oxidation treatment process: An oxidizing agent is added to the crude nanodiamond aqueous dispersion and heated. Separation step: After the oxidation treatment step is completed, the reaction liquid is filtered to obtain a residue. Washing step: The residue obtained through the separation step is washed with water and filtered through a filter. The air permeability of the filter is 2 cm 3 / cm 2 ·Min or more, 10cm 3 / cm 2 Less than min. Drying process: The filtered material obtained through the water washing process and the specific gravity of 4 to 9 g / cm 3 The spherical media (diameter: 5 to 50 mm) are charged into a drying vessel in an amount of 1 to 100 times the weight of the filtered material, and the content of the spherical media is stirred in the drying vessel by the spherical media while the dried material is heated and dried at a pressure of -0.1 to 0.1 MPa·G and at a temperature of 60 to 150°C.

[0011] The present disclosure also provides a method for producing the nanodiamond powder, wherein the water washing step is the following water washing step: Washing process: The filtered material obtained through the separation process and the specific gravity of 4 to 9 g / cm 3 The spherical media (diameter: 5 to 50 mm) are charged in a water washing vessel in an amount of 1 to 100 times the weight of the filtered material. The filtered material is washed with water while stirring the contents of the water washing vessel with the spherical media, and then filtered through a filter. The air permeability of the filter is 2 cm. 3 / cm 2·Min or more, 10cm 3 / cm 2 Less than min.

[0012] The present disclosure also provides the method for producing the nanodiamond powder, wherein the oxidizing agent is a mixed acid [sulfuric acid / nitric acid (weight ratio) = 60 / 40 to 95 / 5].

[0013] The present disclosure also provides a method for producing the nanodiamond powder, wherein the graphite content of the nanodiamond powder is 40% by weight or less.

[0014] The present disclosure also provides a method for producing the nanodiamond powder, wherein the metal element content of the nanodiamond powder is 0.5% by weight or less.

[0015] The present disclosure also provides a method for producing the nanodiamond powder, wherein the water content of the nanodiamond powder is 10% by weight or less.

[0016] The present disclosure also provides a method for producing the nanodiamond powder, which comprises obtaining crude nanodiamonds by a detonation method, and subjecting the obtained aqueous dispersion of crude nanodiamonds to an oxidation treatment step. [Effects of the Invention]

[0017] According to the method for producing nanodiamond powder disclosed herein, nanodiamond powder with excellent dispersibility due to its low impurity content and low residual moisture content can be produced quickly while suppressing the generation of waste. The nanodiamond powder obtained in this manner has excellent mechanical strength, electrical insulation, thermal conductivity, deodorizing effect, and antibacterial effect. Therefore, the nanodiamond powder obtained by the manufacturing method disclosed herein is suitable for use as an abrasive, conductivity imparting material, insulating material, deodorizer, antibacterial agent, etc. [Brief explanation of the drawings]

[0018] [Figure 1]FIG. 1 is a schematic cross-sectional view showing an example of a filter dryer that can be used in the nanodiamond powder manufacturing method of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0019] [Method of manufacturing nanodiamond powder] The method for producing nanodiamond powder disclosed herein is a method for obtaining nanodiamond powder with a particle size D50 of 500 μm or less through the following [oxidation treatment process], [separation process], [water washing process], and [drying process]. [Oxidation treatment process] This is a process in which an oxidizing agent is added to the aqueous dispersion of crude nanodiamonds and heated. [Separation step] This is a step in which the reaction liquid after the oxidation treatment step is filtered to obtain the residue. [Water washing step] This is a step in which the filtered material obtained through the separation step is washed with water and filtered. [Drying process] The filtered material obtained through the water washing process and the specific gravity of 4-9 g / cm 3 a step of heating and drying the filtered material at a pressure of -0.1 to 0.1 MPa·G and at 60 to 150°C while stirring the contents of the drying vessel with the spherical media.

[0020] The method for producing nanodiamond powder of the present disclosure may include other steps in addition to the steps described above, as necessary.

[0021] <Oxidation treatment process> The oxidation treatment process is a process of adding an oxidizing agent to a crude nanodiamond aqueous dispersion and heating it. The crude nanodiamond is a nanodiamond containing impurities, and in this process, by adding an oxidizing agent and heating it, the impurities contained in the crude nanodiamond can be changed into a form that is easy to remove from the nanodiamond.

[0022] Examples of the oxidizing agent include chromic acid, chromic anhydride, dichromic acid, permanganic acid, perchloric acid, nitric acid, mixed acids (mixtures of sulfuric acid and nitric acid), and salts thereof. Of these, mixed acids (mixtures of sulfuric acid and nitric acid) are preferred as the oxidizing agent because of their excellent impurity removal efficiency.

[0023] The mixing ratio of sulfuric acid to nitric acid (former / latter; mass ratio) in the mixed acid is preferably, for example, 60 / 40 to 95 / 5, since this allows the reaction temperature to be raised to, for example, 130°C or higher (preferably 130 to 200°C, particularly preferably 150 to 200°C) even under near atmospheric pressure (e.g., 0.5 to 2 atm), and graphite can be efficiently oxidized and removed. The lower limit of the mixing ratio of sulfuric acid to nitric acid is preferably 65 / 35, particularly preferably 70 / 30. The upper limit of the mixing ratio of sulfuric acid to nitric acid is preferably 90 / 10, particularly preferably 85 / 15, and most preferably 80 / 20.

[0024] If the proportion of nitric acid in the mixed acid exceeds the above range, the content of sulfuric acid, which has a high boiling point, is small, and the reaction temperature becomes, for example, 120°C or lower under pressure near atmospheric pressure, which tends to reduce the graphite removal efficiency. On the other hand, if the proportion of nitric acid in the mixed acid is below the above range, the content of nitric acid, which greatly contributes to the oxidation of graphite, decreases, which tends to reduce the graphite removal efficiency.

[0025] The amount of oxidizing agent (particularly the mixed acid) used is, for example, 10 to 50 parts by mass, preferably 15 to 40 parts by mass, and particularly preferably 20 to 40 parts by mass, per part by mass of crude nanodiamonds. The amount of sulfuric acid used in the mixed acid is, for example, 5 to 48 parts by mass, preferably 10 to 35 parts by mass, and particularly preferably 15 to 30 parts by mass, per part by mass of crude nanodiamonds, and the amount of nitric acid used in the mixed acid is, for example, 2 to 20 parts by mass, preferably 4 to 10 parts by mass, and particularly preferably 5 to 8 parts by mass, per part by mass of crude nanodiamonds.

[0026] Here, the crude nanodiamonds contained in the crude nanodiamond aqueous dispersion liquid to be subjected to the oxidation treatment step are crude nanodiamonds produced by, for example, a detonation method.

[0027] The detonation method involves sealing an explosive in a pressure-resistant container (for example, a metal container such as an iron container) in the presence of atmospheric gas at normal pressure, causing the explosive to detonate, and using the carbon liberated by the partial incomplete combustion of the explosive as a raw material, crude nanodiamonds are produced by the action of the pressure and energy of the shock wave generated by the explosion.

[0028] The explosive may be a mixture of trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX), with the weight ratio of TNT to RDX (TNT / RDX) ranging from 40 / 60 to 60 / 40, for example.

[0029] The crude nanodiamonds obtained in this way contain metallic impurities such as oxides of metals such as Al, Fe, Co, Cr, and Ni (e.g., Fe2O3, Fe3O4, Co2O3, Co3O4, NiO, Ni2O3, etc.) from the reaction vessel, which cause the nanodiamonds to aggregate. In addition, by-products such as graphite may also be present, which also cause the nanodiamonds to aggregate.

[0030] Therefore, the impurities contained in the crude nanodiamonds subjected to the oxidation treatment process are by-products such as graphite and metal oxides, and when the crude nanodiamonds are subjected to the above-mentioned oxidation treatment, they can be changed into a shape that makes it easier to remove the impurities.

[0031] More specifically, metal oxides contained in crude nanodiamonds are ionized (becoming ionic impurities) by oxidation with an oxidizing agent and dissolved. Also, by-products such as graphite contained in crude nanodiamonds are oxidized to carbon dioxide by the oxidizing agent, making them easily expelled from the system.

[0032] <Separation process> The separation step is a step in which the reaction liquid after the oxidation treatment step is filtered to obtain a residue.

[0033] The reaction solution contains nanodiamonds, excess oxidizing agent, and ionic impurities derived from metal oxides.

[0034] The filter has an air permeability of, for example, 10 cm 3 / cm 2 A filter having an air permeability of less than 2 cm 3 / cm 2 ·Min or more, 10cm 3 / cm 2 Less than min, preferably 2-8cm 3 / cm 2 min, particularly preferably 2 to 7 cm 3 / cm 2 min, most preferably 2-6cm 3 / cm 2 min, preferably 2 to 5 cm 3 / cm 2 By using a filter with an air permeability in the above range, the loss of nanodiamonds can be suppressed.

[0035] Examples of the filter include a centrifugal filtration membrane, a filter press filtration membrane, an ultrafiltration membrane, a microfiltration membrane, a nanofilter, and a reverse osmosis membrane.

[0036] The membrane shape of the filtration filter may be, for example, any of a hollow fiber filtration membrane, a tubular membrane, a spiral membrane, a flat membrane, and the like.

[0037] The material of the filtration filter is not particularly limited and may be any common material such as cellulose acetate, polyacrylonitrile, polysulfone, polyethersulfone, polyacrylonitrile, aromatic polyamide, polyvinylidene fluoride, polyvinyl chloride, polyethylene, polypropylene, polyimide, ceramic, etc. In the present disclosure, cellulose acetate, polysulfone, polyethersulfone (PES), polyacrylonitrile, polypropylene, and aromatic polyamide are particularly preferred.

[0038] When the reaction solution is filtered through a filter, most of the excess oxidizing agent and ionic impurities derived from the metal oxide are removed as a filtrate, leaving only those adhering to the nanodiamonds. The filtered residue contains the nanodiamonds, the oxidizing agent remaining attached to the nanodiamonds, and the ionic impurities derived from the metal oxides.

[0039] <Water washing process> The water washing step is a step in which the filtered material obtained through the separation step is washed with water and filtered through a filter, thereby removing impurities contained in the filtered material together with the filtrate.

[0040] The impurities contained in the residue subjected to the water washing process are ionic impurities derived from oxidizing agents and metal oxides, and these impurities are easily dissolved in water and can be easily removed by washing with water. After this process, wet nanodiamonds are obtained from which the impurities have been removed.

[0041] The filter has a permeability of 2 cm 3 / cm 2 ·Min or more, 10cm 3 / cm 2 A filter with an air permeability of less than 2 to 8 cm 3 / cm 2 min, particularly preferably 2 to 7 cm 3 / cm 2 min, most preferably 2-6cm 3 / cm 2min, preferably 2 to 5 cm 3 / cm 2 ·min.

[0042] The preferred shape and material of the filter are the same as those of the filter used in the separation step.

[0043] The filtrate can be washed with water by, for example, placing the filtrate in a water washing vessel, adding water thereto, and stirring the mixture. The filtrate after washing can be filtered with a filter by passing a mixture of the filtrate and washing water in the water washing vessel through a filter.

[0044] The water washing vessel is preferably made of a material with excellent corrosion resistance (for example, an alloy of Ni, Cr, Mo, etc.).

[0045] In addition, the washing step of placing the filtered material in a washing vessel, adding water thereto and stirring the mixture is preferably carried out by placing spherical media together with the filtered material in the washing vessel and stirring the contents of the washing vessel with the spherical media, since this has the effect of significantly improving the efficiency of removing impurities from the filtered material.

[0046] Furthermore, the operation of filtering the mixture of the filtrate and wash water through a filter is preferably carried out while adding spherical media to the water washing vessel and stirring the mixture with the spherical media, because this prevents the filtrate from forming a cake layer, and the cake layer suppresses clogging of the filtration filter, thereby suppressing a decrease in the permeation rate of the filtrate or improving the permeation rate of the filtrate.

[0047] The method of stirring the inside of the washing vessel with the spherical media includes, for example, rotating the washing vessel in which the spherical media are placed. The rotation speed of the washing vessel is, for example, about 1 to 10 rpm.

[0048] The filtrate permeation rate when using spherical media is, for example, 15 kg / Hr m 2More than 30 kg / Hr·m 2 More than 40 kg / H·m is preferable. 2 More than 45 kg / H·m is preferable. 2 More than 50 kg / H·m is preferable. 2 More preferably, 55 kg / Hr·m 2 The upper limit of the filtrate permeation rate is, for example, 100 kg / Hr m 2 is.

[0049] The washing and filtering steps are preferably repeated until the pH or electrical conductivity of the filtrate falls within the ranges described below in order to obtain nanodiamond powder with an extremely low level of impurities.

[0050] The washing and filtering process is preferably repeated until the pH of the filtrate changes from a more acidic side to 2.0 or higher (e.g., 2.0 to 7.0, preferably 3.0 to 7.0, and particularly preferably 3.5 to 7.0) in order to obtain nanodiamond powder with extremely low impurity content.

[0051] Furthermore, the water washing-filtration process may be repeated until the electrical conductivity of the filtrate is 3000 μS / cm or less (preferably 2000 μS / cm or less, more preferably 1000 μS / cm or less, even more preferably 500 μS / cm or less, even more preferably 300 μS / cm or less, even more preferably 200 μS / cm or less, particularly preferably 100 μS / cm or less, and most preferably 50 μS / cm or less). Repeated membrane filtration until the electrical conductivity of the filtrate falls within the above range allows for the production of nanodiamond powder with extremely low impurity content.

[0052] The specific gravity of the spherical media is, for example, 4 to 9 g / cm 3 , preferably 5 to 8 g / cm 3 The spherical media may be, for example, iron spherical media. By using spherical media having the above specific gravity, impact stress can be efficiently applied to the filtered material, preventing the filtered material from forming lumps, thereby improving the efficiency of removing impurities contained in the filtered material.

[0053] When the spherical media are made of a hard material such as iron, it is preferable that the surfaces of the spherical media are coated with a plastic such as nylon, in order to prevent damage to the inside of the washing container.

[0054] The diameter of the spherical media is 5 to 50 mm, preferably 5 to 30 mm, and particularly preferably 5 to 20 mm. By using spherical media having a diameter within this range, impact stress can be efficiently applied to the filtered material, preventing the filtered material from forming lumps, thereby improving the efficiency of removing impurities contained in the filtered material.

[0055] The amount of the spherical media charged is 1 to 100 times the weight of the filtered material. The upper limit of the charged amount is preferably 50 times, more preferably 20 times, even more preferably 10 times, particularly preferably 8 times, most preferably 6 times, and especially preferably 5 times. The lower limit of the charged amount is preferably 1.5 times, particularly preferably 2 times, and most preferably 4 times.

[0056] The amount of spherical media used is such that the ratio of the projected area of ​​the spherical media (or the total area when multiple spherical media are used) to the surface area of ​​the filter (former / latter) is, for example, 0.05 to 0.8. The lower limit is preferably 0.07, particularly preferably 0.10, and most preferably 0.13. The upper limit is preferably 0.7, particularly preferably 0.6, most preferably 0.5, and particularly preferably 0.3.

[0057] By adding the spherical media in the above range, it is possible to efficiently apply impact stress to the filtered material, while suppressing a decrease in the filtrate permeation rate and the occurrence of loss, thereby preventing the filtered material from forming lumps, and thereby preventing moisture from remaining in the lumps of the filtered material.

[0058] The thickness of the cake layer of the filter cake (when spherical media are used, the total thickness of the cake layer of the filter cake and the spherical media) is preferably set within a range of, for example, 0.5 to 10.0 cm (preferably 0.5 to 5.0 cm, particularly preferably 0.5 to 3.0 cm) from the viewpoint of achieving both impurity removal efficiency and work efficiency.

[0059] <Drying process> In the drying process, the filtered material obtained after the water washing process is placed in a drying container together with spherical media, and the container is rotated while the material is dried under reduced pressure and heat. Through this process, nanodiamond powder is obtained.

[0060] The pressure in the drying vessel is -0.1 to 0.1 MPa·G, preferably -0.1 to 0.05 MPa·G, and particularly preferably -0.1 to 0 MPa·G.

[0061] The heating and drying temperature in the drying vessel is 60 to 150°C, preferably 60 to 120°C, particularly preferably 70 to 100°C, and most preferably 70 to 90°C.

[0062] Therefore, it is preferable to use a container made of a material with excellent heat resistance (for example, an alloy of Ni, Cr, Mo, etc.) as the drying container.

[0063] The water-washing step and the drying step may be carried out continuously using, for example, a filter dryer (i.e., a device capable of performing filtering, washing, and drying treatments within a single machine) equipped with a water-washing / drying container made of a material having excellent corrosion resistance and heat resistance.

[0064] An example of a filter dryer is shown in Figure 1. In the filter dryer shown in Figure 1, a water-washing / drying vessel 1 can rotate around a rotation axis 5. A pressure port 6 and a filter 2 are installed in the water-washing / drying vessel 1, and the reaction liquid, spherical media, and washing water can be charged into the water-washing / drying vessel 1 through a charging port 3, and the filtrate that passes through the filter 2 can be discharged through a discharge port 4.

[0065] The rotation speed of the drying container is, for example, 1 to 10 rpm, preferably 5 to 10 rpm. By rotating the drying container at this speed, the spherical media can be caused to collide with the filtered material at an appropriate speed inside the drying container, thereby applying impact stress to the filtered material. This allows the filtered material to be crushed if it is consolidated during drying to form lumps, and prevents moisture trapped in the lumps from remaining undried.

[0066] The preferred ranges of specific gravity and diameter of the spherical media are the same as those of the spherical media used in the water washing step. The use of such spherical media can efficiently impart impact stress to the filtered material, preventing the filtered material from forming lumps, and thereby preventing moisture from remaining in the lumps of the filtered material.

[0067] Furthermore, the spherical media are preferably iron spherical media, and in particular, the surfaces of the iron spherical media are preferably coated with a plastic such as nylon, in order to prevent damage to the inside of the drying vessel.

[0068] The amount of the spherical media added is 1 to 100 times the weight of the filtered material, preferably 1 to 10 times, and particularly preferably 1 to 5 times. By adding the spherical media in this range, impact stress can be efficiently applied to the filtered material, preventing the filtered material from forming lumps, and thereby preventing moisture from remaining in the lumps of the filtered material.

[0069] The water content of the nanodiamond powder obtained through the above process is, for example, 10% by weight or less, preferably 7% by weight or less, and particularly preferably 5% by weight or less.

[0070] The graphite content of the nanodiamond powder obtained through the above process is, for example, 40% by weight or less, preferably 30% by weight or less, and particularly preferably 25% by weight or less.

[0071] The metal element content of the nanodiamond powder obtained through the above process is, for example, 0.5% by weight or less, preferably 0.2% by weight or less, and particularly preferably 0.1% by weight or less.

[0072] As described above, the nanodiamond powder has a low content of graphite and metal elements. Therefore, the nanodiamond powder has high dispersibility. The particle size D50 of the nanodiamond powder is 500 μm or less, preferably 1 to 300 μm, and particularly preferably 1 to 150 μm.

[0073] The nanodiamond powder obtained by the manufacturing method of the present disclosure has low impurity and moisture content and is highly dispersible, allowing it to fully exhibit the properties of nanodiamonds (high mechanical strength, electrical insulation, excellent thermal conductivity, deodorizing effect, antibacterial effect). Therefore, the nanodiamond powder can be suitably used as an abrasive, conductivity imparting material, insulating material, deodorizer, antibacterial agent, etc.

[0074] The above-described configurations and combinations thereof of the present disclosure are merely examples, and additions, omissions, substitutions, and modifications of the configurations can be made as appropriate without departing from the spirit of the present disclosure. [Example]

[0075] The present disclosure will be described in more detail below with reference to examples. It is not intended to be limited, but rather to be limited only by the terms of the claims appended hereto. Here, Examples 3 and 4 should be read as Reference Examples 1 and 2, respectively.

[0076] Example 1 (generation process) The explosives were fitted with electric detonators and placed inside a pressure-resistant container for detonation. The container was sealed in a state where atmospheric gas coexisted with the explosives. The container was made of steel and had a volume of 15 m. 3The explosive used was a mixture of TNT and RDX (TNT / RDX (weight ratio) = 50 / 50) of 0.50 kg. Next, an electric detonator was detonated, causing the explosive to explode inside the container. The container and its interior were then allowed to cool at room temperature for 24 hours. After cooling, the crude nanodiamonds adhering to the inner wall of the container were scraped off with a spatula and collected.

[0077] (Oxidation treatment process) 100 g of crude nanodiamonds were added to 5 L of a mixed acid solution consisting of 6 L of 98 wt% sulfuric acid aqueous solution and 1 L of 69 wt% nitric acid to form a slurry, and then this slurry was heated at 140-160°C for 48 hours under reflux at atmospheric pressure.

[0078] (Washing process) Filtration filter (product name "P2704C", manufactured by Okawara Manufacturing Co., Ltd., air permeability: 3 cm 3 / cm 2 After the oxidation treatment process, 3 L of the reaction liquid was charged into a filter dryer (Okawahara Manufacturing Co., Ltd., FVD-lab, internal volume 4.2 L) equipped with a flow rate of 1000 kJ / min. The inside of the filter dryer was pressurized to 0.19 MPaG using compressor air, and the filtrate was then discharged outside the system through the filter. The filter residue remained in the filter dryer, and 3 L of Milli-Q water and nylon-coated iron balls (diameter: 6.3 mm, specific gravity: approximately 8 g / cm) were added. 3 ) (an amount equivalent to three times the weight of the filtered material, with a ball projected area / filter surface area (ratio) of 0.33) were added, and the filter dryer body was rotated at 10 rpm for 5 minutes (washing). Next, the inside of the filter dryer was pressurized to 0.19 MPaG using compressor air, and the filtrate was discharged (drainage). After the filtrate had been discharged, the pressure was released, and the same amount of Milli-Q water as that discharged was added to wash again, then the pressure was applied again and the filtrate was discharged. This washing and draining process was repeated until the pH of the filtrate reached 5 and the electrical conductivity of the filtrate reached 100 μS / cm or less. The water washing process took 20 hours.

[0079] (Drying process) Next, 80°C hot water was passed through the jacket of the filter dryer (containing the filtered material and nylon-coated iron balls inside the machine) to heat the filter dryer, and the pressure inside the machine was reduced to -0.1 MPaG.The filter dryer was then rotated at a speed of 10 rpm for 10 hours to obtain nanodiamond powder 1.

[0080] The median diameter (D50) of nanodiamond powder 1 was measured by the following method, and the result was 500 μm or less. <Median diameter measurement> Measurements were performed using a laser diffraction method using a HORIBA device (product name "Partica LA-960") The measurement sample was a nanodiamond dispersion obtained by diluting the nanodiamond powder with ultrapure water to a concentration of 0.5 to 2.0 wt % and irradiating it with ultrasonic waves using an ultrasonic cleaner.

[0081] The graphite content of nanodiamond powder 1 was determined by Raman spectroscopy using the graphite peak area / nanodiamond peak area (ratio), and was found to be 25% by weight or less.

[0082] Furthermore, the metal element content of nanodiamond powder 1 was measured by the following method, and the result was 0.1 wt % or less. <Measuring Metal Element Content> Nanodiamond powder 1 was placed in a magnetic crucible and subjected to dry decomposition in an electric furnace. This dry decomposition was carried out in three stages: 1 hour at 450°C, 1 hour at 550°C, and 1 hour at 650°C. After this dry decomposition, 0.5 mL of concentrated sulfuric acid was added to the residue in the magnetic crucible and evaporated to dryness. The resulting dry matter was then dissolved in 20 mL of ultrapure water. In this way, an analytical sample was prepared. The obtained analytical sample was subjected to ICP emission spectrometry using an ICP emission spectrometry analyzer (product name "CIROS120", manufactured by Rigaku Corporation). In this analysis, the measured value obtained by similar operation and analysis using an empty crucible was subtracted from the measured value for the measurement sample to determine the concentration of metal components in the measurement sample.

[0083] Furthermore, the moisture content of nanodiamond powder 1 was determined by a heat drying method. That is, the nanodiamond powder was heated and dried by infrared irradiation, and the moisture content was measured from the mass change due to evaporation of moisture. The results are shown in the table below.

[0084] Example 2 Nylon-coated iron ball (diameter: 6.3 mm, specific gravity: approx. 8 g / cm 3 ) were replaced with nylon-coated iron balls (diameter: 19.0 mm, specific gravity: approx. 8 g / cm 3 The procedure was the same as in Example 1, except that 10 pieces of spheres (equivalent to 5 times the weight of the filtered material, with a sphere projected area / filter surface area ratio of 0.14) were used to obtain nanodiamond powder.

[0085] Example 3 Nylon-coated iron ball (diameter: 6.3 mm, specific gravity: approx. 8 g / cm 3 ) were replaced with nylon-coated iron balls (diameter: 12.7 mm, specific gravity: approx. 8 g / cm 3 The procedure was the same as in Example 1, except that 100 pieces of spheres (equivalent to 15 times the weight of the filtered material, with a ratio of the projected area of ​​the spheres to the surface area of ​​the filtration filter of 0.67) were used to obtain nanodiamond powder.

[0086] Example 4 Nylon-coated iron ball (diameter: 6.3 mm, specific gravity: approx. 8 g / cm 3 The procedure was the same as in Example 1, except that the amount of spheres used was changed from 200 to 80 (an amount equivalent to 1.2 times the weight of the filtered material, with a ratio of projected area of ​​spheres to surface area of ​​filtration filter of 0.125), and nanodiamond powder was obtained.

[0087] Comparative Example 1 The filter installed in the filter dryer was a product name "T89-1C" (manufactured by Okawara Manufacturing Co., Ltd., air permeability: 10 cm 3 / cm 2 / min), the water washing step was carried out in the same manner as in Example 1. As a result, the nanodiamond powder passed through the filtration filter, resulting in a large loss.

[0088] Comparative Example 2 The same procedure as in Example 1 was carried out to obtain nanodiamond powder, except that the water washing process and drying process were carried out without using nylon-coated iron balls.

[0089] The water content of the nanodiamond powders obtained in Examples 2 to 4 and Comparative Examples 1 and 2 was determined in the same manner as in Example 1. The results are shown in the table below.

[0090] In addition, the dispersibility of the nanodiamond powders obtained in the examples and comparative examples was evaluated by the following method. <Evaluation method> The nanodiamond powder was placed in a transparent container, and the approximate volume ratio of lumps of 5 mm or larger was visually observed and evaluated according to the following criteria. <Evaluation criteria> ○: 5% or less △: Over 5% and 20% or less ×: Over 20%

[0091] [Table 1] [Explanation of symbols]

[0092] 1 Washing / drying container 2. Filtration filter 3 Preparation 4 Outlet 5 Rotation Axis 6 Pressure port 7 Bottom lid

Claims

1. A method for producing nanodiamond powder, which involves obtaining nanodiamond powder having a particle size D50 of 500 μm or less through the following oxidation treatment process, separation process, water washing process, and drying process. Oxidation treatment step: An oxidizing agent is added to the aqueous dispersion of crude nanodiamonds and heated. Separation step: After the oxidation treatment step, the reaction mixture is filtered to obtain a residue, the filter having an air permeability of 2 to 8 cm 3 / cm 2 ·min. Washing step: The filtered residue obtained through the separation step and a solution of 4 to 9 g / cm ３ The amount of spherical media (diameter 5 to 50 mm) charged in a water washing vessel is 2 to 10 times the weight of the filtered material, and the filtered material is washed with water while stirring the inside of the water washing vessel with the spherical media, and then filtered through a filter. The air permeability of the filter is 2 to 8 cm ３ / cm ２ ・min. Drying step: The filtered product obtained through the water washing step and the specific gravity of 4 to 9 g / cm ３ The amount of spherical media (diameter: 5 to 50 mm) charged into a drying vessel is 2 to 10 times the weight of the filtered material, and the dried material is heated and dried at a pressure of −0.1 to 0.1 MPa G and 60 to 150° C. while stirring the contents of the drying vessel with the spherical media.

2. 2. The method for producing nanodiamond powder according to claim 1, wherein the oxidizing agent is a mixed acid [sulfuric acid / nitric acid (weight ratio) = 60 / 40 to 95 / 5].

3. 3. A method for producing nanodiamond powder according to claim 1 or 2, wherein the graphite content of the nanodiamond powder is 40% by weight or less.

4. A method for producing nanodiamond powder according to any one of claims 1 to 3, wherein the metal element content of the nanodiamond powder is 0.5% by weight or less.

5. A method for producing nanodiamond powder according to any one of claims 1 to 4, wherein the water content of the nanodiamond powder is 10% by weight or less.

6. A method for producing nanodiamond powder according to any one of claims 1 to 5, wherein crude nanodiamonds are obtained by a detonation method, and the aqueous dispersion of the obtained crude nanodiamonds is subjected to an oxidation treatment process.

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