Process for producing sodium borohydride

Abstract

The present invention relates to a method for producing sodium borohydride. In the method, when a sodium borate compound, aluminum powder, and fluoride powder are mixed in a closed container filled with hydrogen and reacted at a temperature of 560°C or lower, the mixing is performed using a stirrer in the closed container, and a stirring height ratio (X) represented by the following mathematical expression (I) determined from a minimum gap (a) between the stirrer and the lowest part of the closed container in the direction of gravity and a raw material feeding height (b) at which the raw material is fed into the closed container is 75% or greater, X = [(b - a) / b] x 100 Expression (I).

Description

Technical Field

[0001] This invention relates to a method for manufacturing sodium borohydride, and more specifically, to a method for manufacturing sodium borohydride from sodium metaborate. Background Technology

[0002] Among hydrogen fuels, which have garnered significant attention as an alternative energy source to fossil fuels, sodium borohydride (SBH) is a promising hydrogen carrier for storing, transporting, and generating hydrogen. To facilitate the widespread adoption of sodium borohydride as a hydrogen carrier, it is essential to establish an optimal manufacturing method that prioritizes large-scale production.

[0003] As a conventional method for manufacturing sodium borohydride, for example, Patent Document 1 discloses a method for manufacturing sodium borohydride by reacting a trialkyl borate ester with sodium aluminum hydride.

[0004] In addition, Patent Document 2 discloses a method for manufacturing sodium borohydride, which involves rolling and pulverizing sodium metaborate and granular aluminum using a stirring medium in a hydrogen environment, and simultaneously reacting the two to obtain sodium borohydride.

[0005] In addition, Patent Document 3 discloses a method for manufacturing sodium borohydride by using ceramic balls as a pulverizing medium for calendering in a sealed container.

[0006] Furthermore, Non-Patent Document 1 discloses a method for producing sodium borohydride by reacting sodium diborate (Na4B2O5) and sodium oxide (Na2O) with aluminum and hydrogen in a high-temperature molten state (855K (581°C), preferably 873K (599°C)).

[0007] Prior art literature

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 2809666

[0010] Patent Document 2: International Publication No. 2015 / 190403

[0011] Patent Document 3: Japanese Patent Application Publication No. 2019-189483

[0012] Non-patent literature

[0013] Non-patent literature 1: Sodium Borohydride Synthesis by Reaction of Na₂O Contained Sodium Borate with Al and Hydrogen, Bin Hong, LIU et al., *Energy & Fuels*, 2007, Vol. 21, No. 3, pp. 1707-1711 Summary of the Invention

[0014] The problem that the invention aims to solve

[0015] However, in the scheme of Patent Document 1, boric acid needs to be converted into a trialkyl borate beforehand, and sodium, aluminum and hydrogen need to be reacted in advance to generate sodium aluminum hydride, which presents a problem of complex manufacturing process.

[0016] In the scheme of Patent Document 2, dry anhydrous sodium metaborate (NaBO2) and aluminum particles with a diameter of about 3 mm are loaded into a rotatable reaction section arranged in a cylindrical reaction vessel. Sodium borohydride is produced by calendering and crushing using a stirring medium (steel balls with a diameter of about 30 mm) in the reaction vessel. However, there is a problem that the calendering time is too long, and the crushed aluminum particles become too small, making crushing difficult and the reaction difficult to advance.

[0017] In the scheme of Patent Document 3, when manufacturing sodium borohydride, ceramic balls are used as the pulverizing medium in a closed container. Therefore, not only does the calendering and pulverization cause the reaction product to become a high-density state, but the ceramic balls also continuously damage the reaction product. As a result, once the aluminum particle size becomes too small and forms a shape and size that cannot be pulverized by calendering and pulverization, the new surface required for the reaction cannot be generated, the reaction stops, and the reaction rate decreases.

[0018] In the scheme of Non-Patent Document 1, sodium hydroxide is added to sodium metaborate and heated to form an aqueous solution, thereby producing sodium diborate (Na4B2O5) containing sodium oxide (Na2O) or sodium metaborate synthesized by heating and dehydration. The reaction then occurs in the molten state under a high pressure of 2.3 MPa. To melt the sodium diborate, a high temperature (around 855 K (581 °C), preferably 873 K (599 °C)) is required. To achieve a high reaction rate of 65.8%, the molar ratio of sodium metaborate to sodium oxide needs to be 3:2. However, there is a drawback that once the above molar ratio is reduced, the reaction rate decreases sharply, and at the above temperature, the reaction rate of solid sodium metaborate alone becomes zero.

[0019] The present invention was made in view of the above-mentioned existing problems, and its purpose is to provide a method for producing sodium borohydride with a simple structure.

[0020] Technical solutions for solving the problem

[0021] The method for manufacturing sodium borohydride according to the first embodiment is characterized in that, when sodium borate compound, aluminum powder and fluoride powder are mixed in a closed container filled with hydrogen and reacted at a temperature of 560°C or below, the mixture is stirred in the closed container using a stir bar, and the stirring height ratio (X) expressed by the following mathematical formula (I), which is determined by the minimum gap (a) between the stir bar and the bottom of the closed container in the direction of gravity and the raw material input height (b) into the closed container, is 75% or more.

[0022] X=[(b-a) / b]×100 Formula (I)

[0023] In the first embodiment, the aforementioned sodium borate compound may be selected from one or more of sodium metaborate and sodium tetraborate.

[0024] In the first embodiment, the fluoride may be selected from one or more of sodium fluoride (NaF), sodium hexafluoroaluminate (Na3AlF6), potassium fluoride (KF), potassium aluminum fluoride (KAlF4), aluminum fluoride (AlF3), and lithium fluoride (LiF).

[0025] Furthermore, in the first embodiment...

[0026] Regarding the aluminum in the above-mentioned aluminum powder, the molar ratio of aluminum to boron in the above-mentioned sodium borate compound is 4 / 3 or more.

[0027] In addition, in the first embodiment, alkali metal oxides or alkaline earth metal oxides may be further added. The molar ratio of the alkali metal, alkaline earth metal, and the alkali metal and alkaline earth metal contained in the sodium borate compound to the molar amount of boron contained in the sodium borate compound may be 1.0 or more and 1.4 or less.

[0028] In addition, in the first embodiment, the interior of the sealed container may be heat-treated at a temperature above 280°C and below 560°C before stirring.

[0029] The effects of the invention

[0030] According to the first embodiment, a method for manufacturing sodium borohydride without the need for large-scale equipment can be provided. Attached Figure Description

[0031] Figure 1 This is a process diagram for producing sodium borohydride (SBH) from sodium borate compounds.

[0032] Figure 2AThis is a schematic diagram of the process for generating sodium borohydride (SBH).

[0033] Figure 2B This is a schematic diagram of the process for generating sodium borohydride (SBH).

[0034] Figure 2C This is a schematic diagram of the process for generating sodium borohydride (SBH).

[0035] Figure 2D This is a schematic diagram of the process for generating sodium borohydride (SBH).

[0036] Figure 2E This is a schematic diagram of the process for generating sodium borohydride (SBH).

[0037] Figure 2F This is a schematic diagram of the process for generating sodium borohydride (SBH).

[0038] Figure 3 This is a partial cross-sectional schematic diagram of an example of a sealed container used in an embodiment of the present invention.

[0039] Figure 4 This is a partial cross-sectional schematic diagram of an example of a sealed container used in an embodiment of the present invention.

[0040] Figure 5 This is a partial cross-sectional schematic diagram of another example of a sealed container used in an embodiment of the present invention.

[0041] Figure 6 This is a partial cross-sectional schematic diagram of another example of a sealed container used in an embodiment of the present invention.

[0042] Figure 7 This is a partial cross-sectional schematic diagram of another example of a sealed container used in an embodiment of the present invention.

[0043] Figure 8 This is a partial cross-sectional schematic diagram of another example of a sealed container used in an embodiment of the present invention.

[0044] Figure 9 This is a SEM backscattered electron image of the product at a reaction temperature of 510℃.

[0045] Figure 10 This is a SEM backscattered electron image of the product at a reaction temperature of 478℃.

[0046] Figure 11 The X-ray diffraction pattern of the product is obtained by conducting the experiment under the same conditions as in Experimental Example 18, and stopping the reaction when the reaction rate was 8.1% after the initial reaction.

[0047] Figure 12AThe SEM image of the product was obtained under the same conditions as in Experimental Example 18, and the reaction was stopped when the reaction rate was 8.1% after the initial reaction.

[0048] Figure 12B The SEM image of the product was obtained under the same conditions as in Experimental Example 18, and the reaction was stopped when the reaction rate was 8.1% after the initial reaction.

[0049] Figure 13 The image shows the X-ray diffraction pattern of the product with a reaction rate of 91.1% in Experimental Example 20.

[0050] Figure 14 This is an X-ray diffraction pattern of the product of Comparative Example 10 with a reaction rate of 14.1% after the addition of NaOH.

[0051] Figure 15 This is a SEM image of the product of Comparative Example 10 with a reaction rate of 14.1% after the addition of NaOH.

[0052] Figure 16 This is a schematic diagram showing the relationship between the height ratio ((b-a) / b) and the reaction time.

[0053] Figure 17 This is a schematic diagram showing the relationship between the reaction rate (SBH rate) and the reaction time.

[0054] Figure 18 This is a schematic diagram of the manufacturing conditions for Experimental Example 22.

[0055] Figure 19 This is a schematic diagram of the manufacturing conditions for Experimental Example 23.

[0056] Figure 20A Is using Figure 3 Photograph of the appearance of the sealed container (gap a: 4mm) equipped with a needle-shaped stir bar.

[0057] Figure 20B This is a photograph of the product after sodium borohydride is produced by rotating a stirring rod.

[0058] Figure 20C This is a photo of the product being separated from the container after the scheduled stirring time has ended.

[0059] Figure 20D It is a photograph of the thickness of the measured product.

[0060] Figure 20E This is a magnified photograph of the obtained product.

[0061] Figure 20F This is a cross-sectional SEM image of the product block with a reaction rate of 91.1%.

[0062] Figure 21A Is using Figure 3 Photograph of the product after sodium borohydride is produced by rotating a stirring rod in a sealed container (gap a: 5 mm).

[0063] Figure 21B This is a magnified photograph of the product's state after stirring for the predetermined time has ended. Detailed Implementation

[0064] <First Implementation Method>

[0065] Figure 1 This is a process diagram for producing sodium borohydride (SBH) from sodium borate compounds.

[0066] The method for manufacturing sodium borohydride according to the first embodiment involves mixing sodium borate compound, aluminum powder, and fluoride powder in a sealed container filled with hydrogen gas, and reacting the mixture at a temperature of 490°C to 560°C. The sodium borate compound and aluminum powder participate in the reaction while remaining in a solid state. Figure 1 As shown, the method for manufacturing sodium borohydride according to the first embodiment includes steps from the first step (S-11) to the third step (S-13).

[0067] Sodium borate compounds 51 used as boric acid raw materials in the first process can be specifically exemplified, but are not limited to, borax (sodium tetraborate: Na2B4O7), sodium metaborate (NaBO2 (=Na2B2O4)), or combinations thereof.

[0068] In this invention, the Na / B (molar ratio) of the sodium borate compounds in the embodiments of this invention, as shown in the experimental examples described later, is preferably in the range of greater than 0.85 and less than 1.5, and more preferably in the range of greater than 1.0 and less than 1.4.

[0069] The following details each step. In the following steps, sodium metaborate (NaBO2) separated from crystals will be used as the example of a sodium borate compound.

[0070] [First Process]

[0071] The first step (S-11) is to introduce a non-oxidizing gas 53 into a sealed container after or before loading sodium borate compound 51 (with a particle size of less than 100 μm), aluminum powder 52, and fluoride 54 into the sealed container, so that the inside of the container is filled with a non-oxidizing gas environment.

[0072] The first process mainly involves preparing raw materials and loading them into the container.

[0073] This first step prevents moisture in the air from adhering to the oxide coating on the surfaces of sodium borate and aluminum by filling the sealed container with a non-oxidizing gas. The timing of filling the sealed container with the non-oxidizing gas can be either after or before the raw materials are loaded into the container.

[0074] In this invention, examples of non-oxidizing gas 53 include hydrogen, rare gases (e.g., helium, argon, etc.). Furthermore, the first step may involve creating a vacuum state inside a sealed container after or before loading sodium borate and aluminum particles with a particle size of 100 μm or less into the sealed container.

[0075] As a sealed container, a container with heat resistance and pressure resistance that can withstand high temperatures (e.g., 560°C) and high pressures (e.g., 10 MPa) can be used to ensure a sealed space for filling gas.

[0076] It should be noted that containers with at least a stirring element can be used. Details of such sealed containers will be provided later.

[0077] Regarding the sodium borate compound used as a raw material, it is preferably selected from one or more of sodium metaborate, sodium tetraborate, and sodium diborate.

[0078] In the following text, sodium metaborate powder is used as an example of sodium borate compounds.

[0079] Furthermore, in this embodiment of the invention, the particle size of the sodium metaborate powder is 100 μm or less. When the particle size of the sodium metaborate powder exceeds 100 μm, it may lead to a decrease in the manufacturing efficiency of sodium borohydride. It should be noted that the sodium metaborate powder is the raw material obtained by feeding it through a sieve with a mesh size of 100 μm after it has been pulverized and ground to a certain degree. To further improve the manufacturing efficiency of sodium borohydride, it is preferable to use sodium metaborate powder with a smaller particle size. For this purpose, the sodium metaborate powder can be powder sieved by a sieve with a mesh size of less than 100 μm (e.g., a sieve with a mesh size of less than 50 μm).

[0080] The mass of sodium metaborate added in the first process can be determined based on the required sodium borohydride yield. However, since sodium metaborate contains water, the mass loss of water needs to be taken into account, and the estimated charge amount should be increased accordingly.

[0081] Aluminum as a raw material can be obtained from powdered materials, scrap, or other materials. Aluminum scrap can be obtained from waste materials such as shavings and waste, and it is preferable to select aluminum with the lowest possible content of metallic impurities that are more reactive than aluminum.

[0082] The average particle size of the aluminum added is, for example, 1 μm or more, and the maximum particle size is preferably 10 mm or less. When the average particle size of the aluminum is less than 1 μm, it is prone to dust explosions, making it difficult to use, and the particles tend to adhere to each other and solidify easily. When the average particle size is greater than 10 mm, the specific surface area per unit mass decreases, the reaction area decreases, and this may lead to a significant reduction in the initial reaction rate. The average particle size is more preferably 5 μm or more and 5 mm or less. It should be noted that the average particle size is the equivalent sphere diameter obtained by a laser diffraction particle size distribution measurement device.

[0083] Furthermore, the fluoride used as a raw material is selected from one or more of sodium fluoride (NaF), sodium hexafluoroaluminate (Na3AlF6), potassium fluoride (KF), potassium aluminum fluoride (KAlF4), aluminum fluoride (AlF3), and lithium fluoride (LiF). Among these, sodium fluoride, which is an alkali metal fluoride, is particularly preferred.

[0084] In this invention, fluoride is added to improve the reaction rate of sodium borohydride and enhance the efficiency of particle stirring reaction. As a fluoride in the reaction process, it reacts with alkali metals during the reduction reaction of alkali metal oxides on the aluminum surface to form an alkaline aluminum fluoride layer. Because sodium fluoride has a low and stable formation free energy, the fluoride layer ensures that the interior of the aluminum particles is in a fluoride environment.

[0085] The "particle stirring reaction" triggered by the addition of fluoride is explained.

[0086] In the initial stage of the reaction, aluminum (Al) is brought into contact with sodium metaborate (NaBO2) through stirring, and the sodium metaborate adheres to the aluminum (Al) particles. Through this adhesion, the oxide coating on the aluminum (Al) surface adsorbs sodium oxide (Na2O), and needle-like ι-alumina (0.67Na·6Al·9.33O) is formed by the catalytic action of fluoride ions. ι-alumina is known to be a precursor for β-alumina in low-temperature β-alumina synthesis methods such as the sol-gel method. ι-alumina possesses ion permeability based on its structure and that of β-alumina. This allows sodium ions to pass easily through the oxide coating on the aluminum (Al) surface. The ι-alumina forms a layered structure on the aluminum (Al) surface.

[0087] Inside the thin (5nm to 10nm) oxide coating layer of ι-alumina that can be formed in the early stage of the reaction, sodium oxide (Na2O) is reduced to sodium (Na) through the reduction of aluminum (Al), and sodium (Na) is converted to sodium hydride (NaH) with the help of hydrogen (H2).

[0088] On the other hand, due to the high concentration of Na ions, the simultaneously generated aluminum oxide (Al₂O₃) immediately neutralizes with sodium oxide (Na₂O) to form sodium alumina (NaAlO₂). In the presence of fluoride ions, sodium alumina (NaAlO₂) crystallizes through its catalytic action to form ι-alumina (0.67Na·6Al·9.33O), releasing sodium oxide (Na₂O). This ι-alumina is in the form of needle-like crystals.

[0089] The inventors hypothesize that the reason ι-alumina forms needle-like crystals is that the crystal growth reaction occurs simultaneously with the release of sodium oxide (Na₂O) during the crystallization process. That is, the inventors hypothesize that since the crystal growth direction preferentially follows the direction of lower sodium oxide (Na₂O) concentration, ι-alumina extends in one direction, causing it to form needle-like crystals.

[0090] Sodium hydride (NaH) passes through the ι-alumina layer and is transferred to the surface of the aluminum particles. It then contacts and reacts with sodium metaborate (NaBO2) on the surface of the aluminum particles to form sodium borohydride (SBH) and sodium oxide (Na2O).

[0091] At temperatures above 490°C, which is a necessary condition for the reaction, sodium borohydride (SBH) is in a molten state. Therefore, sodium borohydride (SBH) becomes a solvent for sodium hydride (NaH) and sodium oxide (Na2O), forming a medium for material transfer filled with an ι-alumina layer.

[0092] Furthermore, because sodium borohydride (SBH) is a molten substance, it readily adheres to sodium metaborate (NaBO2). Moreover, the sodium hydride (NaH) contained in sodium borohydride (SBH) reacts with sodium metaborate (NaBO2) to further generate sodium borohydride (NaBH4) and sodium oxide (Na2O). The sodium oxide (Na2O) is brought into contact with the particles through stirring and returns to the aluminum particles, becoming a raw material for sodium hydride.

[0093] On the other hand, although it is speculated that sodium ions can transfer most rapidly within the needle-like ι-alumina layer, when the interstices between the needle-like crystals mainly contain sodium borohydride (SBH), these interstices become channels for sodium ions, boron ions, oxygen ions, aluminum ions, and fluoride ions to pass through. Thus, because sodium ions, boron ions, oxygen ions, aluminum ions, and fluoride ions pass through the ι-alumina layer, a conversion reaction to sodium borohydride (SBH) can be initiated even within the ι-alumina layer.

[0094] As described above, by satisfying the conditions for reaction on the inner and outer sides of the ι-alumina layer of aluminum (Al) particles and on the surface of sodium metaborate (NaBO2) particles, aluminum (Al) particles containing products and product intermediates and sodium metaborate (NaBO2) particles are brought into contact with each other through stirring. As a result, the high-speed transfer and uniform mixing of sodium borohydride (SBH), sodium hydride (NaH), sodium oxide (NaO2), sodium metaborate (NaBO2), and aluminum ions to each particle, which cannot be achieved by diffusion reaction alone, are realized, thus promoting the formation reaction of sodium borohydride (SBH).

[0095] In addition to promoting the crystallization of 1-alumina as described above, fluorides also form a NaAlF (a system of cryolite mixed with sodium fluoride and aluminum fluoride) layer on the aluminum surface, and react with aluminum to form a low-grade aluminum fluoride (presumably AlF) that is a strong reducing agent. 1.5 Lower aluminum fluoride, together with hydrogen, reduces sodium oxide to produce sodium hydride, aluminum oxide, and sodium fluoride. Lower aluminum fluoride can be transferred via sodium borohydride. Furthermore, with increasing concentration and temperature, lower aluminum fluoride and sodium hydride partially evaporate and can also be transferred in the gas phase. Due to these effects, sodium hydride transfer or formation also occurs in areas other than the aluminum surface, thus promoting the formation of sodium borohydride and contributing to an increased reaction rate.

[0096] 2AlF 1.5 +3Na2O+3 / 2H2→3NaF+3NaH+Al2O3 (1)

[0097] In these cases, although the sodium hydride (NaH) formation reaction mainly occurs on the surface of aluminum (Al) particles, the sodium borohydride (SBH) formation reaction with alumina can also occur inside the ι-alumina layer, outside the ι-alumina layer, and on sodium metaborate (NaBO2) particles.

[0098] Sodium metaborate (NaBO2), as a raw material, also forms a cyclical reaction through simultaneous contact with intermediate products sodium oxide (Na2O) and aluminum (Al) particles.

[0099] In this embodiment, the following state will be referred to as "particle stirring reaction" in which, as described above, the particles come into contact with each other, causing repeated material transfer not only inside the aluminum (Al) particles, but also between the raw material sodium metaborate (NaBO2) and the intermediate product sodium borohydride, resulting in a high-speed reaction on the surface and inside each particle.

[0100] Next, a schematic diagram of the reaction process for generating sodium borohydride (SBH) using sodium fluoride (NaF) as a fluoride is shown in the model. Figures 2A-2F (This will be explained in detail.) Figures 2A-2FThis is a schematic diagram of the process for the formation of sodium borohydride (SBH). As mentioned above, the addition of a fluoride (e.g., sodium fluoride: NaF) is used to increase the reaction rate of sodium borohydride. Fluorides have two effects on aluminum: firstly, they promote alumina crystallization; secondly, they form a NaAlF layer on the aluminum surface, supplying a strong reducing agent to the outside of the aluminum particles and promoting reduction.

[0101] 1) Initial stage of stirring - 1

[0102] like Figure 2A As shown, the three components (aluminum (Al) 101, sodium metaborate (NaBO2) 102, and sodium fluoride (NaF) 103) particles come into contact with each other, causing separation and adhesion between the particles.

[0103] 2) Initial stage of stirring - 2

[0104] like Figure 2B As shown, a dense Al oxide coating (Al2O3·H2O)101b formed by natural oxidation is formed on the surface 101a of aluminum (Al)101 particles.

[0105] The dense Al oxide coating 101b formed by natural oxidation has a thickness of approximately 0.01 μm. At high temperatures (~300°C), the dense Al oxide coating 101b on the surface of the aluminum particles releases water, and sodium fluoride (NaF) 103 promotes crystallization, further forming ι-alumina, which transforms into a needle-like ι-alumina (0.67Na·6Al·9.33O) layer 101c.

[0106] 3) Initial stage of reaction - 3

[0107] like Figure 2C As shown, inside the needle-like ι-alumina layer 101c, the main components are hydrogen ions (H+) from hydrogen (H2) in the ambient gas. + Sodium ions of sodium metaborate (Na) + The ions can be transferred. Then, the transferred hydrogen and sodium ions initiate a reaction inside the needle-like ι-alumina layer 101c, causing sodium borohydride (SBH) 111 and sodium hydride (NaH) 112, as reaction products, to diffuse out of the needle-like ι-alumina layer 101c. Then, the diffusion of sodium oxide (Na2O) 113 further progresses inside and outside the ι-alumina (0.67Na·6Al·9.33O) layer 101c.

[0108] 4) Mid-reaction period

[0109] like Figure 2D As shown, through particle stirring reaction, hydrogen ions (H+) are respectively... + Sodium ions (Na) + ), aluminum ions (Al)3+ ), metaborate ions (BO2) from sodium metaborate (NaBO2)102 － ), oxygen ions (O 2－ The product layer (sodium borohydride (SBH) 111, sodium hydride (NaH) 112) 115 is formed by transferring the product through the needle-like ι-alumina layer 101c and initiating a reaction on the inside and outside of the needle-like ι-alumina layer 101c.

[0110] 5) Peak reaction period

[0111] like Figure 2E As shown, through particle stirring reaction, the three components of particles are repeatedly brought into contact with each other. Thus, through particle stirring, the supply of sodium metaborate (NaBO2)102 to aluminum 101 particles and the transfer of reaction products (sodium borohydride (SBH)111 and sodium hydride (NaH)112) from aluminum 101 particles to sodium metaborate (NaBO2)102 are achieved at high speed.

[0112] 6) Reaction terminated

[0113] like Figure 2F As shown, aluminum 101 particles are transformed into the reaction products sodium borohydride (SBH) 111 and needle-like ι-alumina layers 101c.

[0114] In the first step (S-11), there are no special restrictions on the temperature inside the sealed container when the raw materials are fed; it can be below 100°C. Since no special heating is required inside the sealed container, it can be set to room temperature. It should be noted that, in order to avoid the reaction between sodium metaborate and moisture in the air, the container needs to be sealed quickly after the raw materials are loaded.

[0115] Furthermore, aluminum is consumed due to oxidation during the manufacture of sodium borohydride. Therefore, in the first step, aluminum is loaded into a sealed container in a ratio such that the molar ratio of aluminum to boron in sodium metaborate is 4 / 3 or more.

[0116] The amount of aluminum powder relative to the amount required for the synthesis of sodium borohydride is preferably 120% or more in molar ratio. A portion of the excess aluminum is consumed through reaction with water, but as the reaction proceeds and the amount of aluminum as a raw material decreases, it also helps to increase the probability of contact with sodium metaborate, thus improving the reaction rate.

[0117] [Second Process]

[0118] like Figure 1 As shown, the second process (S-12) is a process that, after the first process, involves heat treatment of a sealed container at a temperature between 280°C and 560°C without stirring, so that the residual moisture contained in the sodium metaborate and aluminum powder reacts with the aluminum to be converted into hydrogen and aluminum oxide.

[0119] That is, this process involves reacting vaporized moisture, i.e., residual moisture in a sealed container, with aluminum, or removing gas using a vacuum pump to remove moisture from the reaction system.

[0120] In this process, when aluminum reacts with water, aluminum oxide and hydrogen are generated. More specifically, vaporized water vapor passes through an aluminum oxide coating, causing aluminum to react with water to form aluminum oxide and hydrogen. The reaction between water and aluminum is represented by the following reaction formula (2).

[0121] 2Al + 3H₂O → Al₂O₃ + 3H₂ (2)

[0122] Typically, defects allow gas to pass through the aluminum oxide coating. Water vapor reaches the aluminum substrate and oxidizes the aluminum, producing hydrogen gas.

[0123] Regarding the heating conditions in the gasification process of the second step, as long as the temperature is above 280°C, sodium metaborate hydrate can completely release water. However, the pulverized sodium borate powder is prone to adsorbing moisture, and the desorption temperature is increased. Therefore, it is preferable to carry out dehydration and drying at a temperature above 400°C, which can remove moisture from the environment in a shorter time.

[0124] Furthermore, as a second step, pre-baking can be performed without stirring, in a non-oxidizing environment, at temperatures above 400°C and below 560°C for 1 hour. By performing this pre-baking, the reaction rate in the latter half of the subsequent third step can be increased.

[0125] Alternatively, after the first process, without stirring and before pre-baking, the water can be removed by dehydration in the air at a temperature above 280°C and below 560°C, and this water removal treatment and pre-baking can be carried out in parallel.

[0126] [Third Process]

[0127] like Figure 1 As shown, in this embodiment of the invention, after completing the second step (S-12), sodium borohydride can be generated by setting the next third step (S-13).

[0128] In this embodiment of the invention, the reaction of sodium borohydride involves contacting the powders together while maintaining a solid state. After reacting on the surface of aluminum, the product and raw materials are transferred through diffusion, and the reaction continues to achieve synthesis. However, to further facilitate the transfer of these substances, stirring can also be used to increase kinetic energy.

[0129] As sodium borohydride is generated, the amount of hydrogen in the reaction vessel decreases, but the reaction rate will increase by increasing the hydrogen pressure. The reaction in this process is represented by the following reaction formula (3).

[0130] 4Al+6H2+3NaBO2→3NaBH4+2Al2O3 (3)

[0131] In the third process, the sealed container can be the same as the sealed container used in the first and second processes, or it can be another sealed container. That is, the first to third processes can be carried out as processes within the same sealed container, or as processes within different sealed containers.

[0132] The hydrogen pressure maintained in the third step is preferably in the range of 0.4 MPa to 10 MPa, more preferably in the range of 0.5 MPa to 10 MPa. Even more preferably, it is set to the range of 0.7 MPa to 10 MPa. By setting the hydrogen pressure to 0.3 MPa to 10 MPa, not only is the formation efficiency of sodium borohydride high, but it also eliminates the need for reaction vessels and equipment with excellent pressure resistance, thus suppressing the increase in equipment costs.

[0133] In the third step, to ensure the reaction proceeds fully, the heating temperature is preferably above 490℃ and below 560℃. By setting the heating temperature to above 490℃ and below 560℃, not only can a sufficient reaction rate be obtained, resulting in high sodium borohydride formation efficiency, but the sublimation of the generated sodium hydride and sodium borohydride can also be suppressed, achieving a sufficient recovery rate.

[0134] In this process, the reaction temperature is set above 490°C to promote the formation of needle-like ι-alumina and to ensure that the sodium borohydride (SBH) product is in a molten state, thus accelerating the reaction. That is, at temperatures below 490°C, even if a reaction occurs and an oxide coating of aluminum (Al) forms ι-alumina, the formation rate of ι-alumina becomes very slow, changing from the elongated, so-called needle-like ι-alumina formed at temperatures above 490°C to shorter ι-alumina. Furthermore, since sodium borohydride (SBH) does not melt at temperatures below 490°C, it remains solid, slowing down the process through the ι-alumina layer and significantly reducing the amount of sodium borohydride that seeps out to the outside of the aluminum (Al) particles along with sodium hydride (NaH). Therefore, the reaction is mostly a reaction between boron ions that have passed through the alumina layer and sodium hydride (NaH) inside the aluminum (Al) particles, failing to sufficiently promote the product reaction.

[0135] in, Figure 9 and Figure 10 The comparison shows the state of the needle-like alumina layer caused by the difference in reaction temperature with that of sodium borohydride (SBH). Figure 9 and Figure 10 The black substance on the outside of the particles is carbon, which acts as a dispersant. Figure 9 This is a SEM backscattered electron image of the product at a reaction temperature of 510℃. Figure 10This is a SEM backscattered electron image of the product at a reaction temperature of 478℃. (Example:) Figure 9 As shown, a needle-like ι-alumina layer is formed around the central aluminum (Al), and sodium borohydride (SBH) fills the inner, outer, and inner layers of the ι-alumina layer. When the reaction temperature is set to 510°C, sodium borohydride (SBH) permeates the ι-alumina layer in molten form and seeps to the outside, adhering to the surrounding sodium metaborate (NaBO2) particles.

[0136] On the other hand, such as Figure 10 As shown, when the reaction temperature is set to 478℃, since the reaction temperature is lower than 490℃, sodium borohydride (SBH) exists in large quantities inside the ι-alumina layer.

[0137] Furthermore, numerous voids exist within the outer needle-like ι-alumina layer. The inventors hypothesize that these voids exist in a state where diffusion and transfer cannot occur, resulting in an extremely slow reaction rate. Figure 10 Only a very small amount of sodium borohydride was observed on the outer side. The inventors believe that this sodium borohydride is formed by the reaction of sodium hydride gas or low-grade aluminum fluoride gas with boric acid after gas diffusion.

[0138] Sodium borohydride (SBH) can be generated through the first and even the third processes described above.

[0139] In addition, a fourth process (hereinafter referred to as post-baking) can be carried out after the third process to maintain the temperature of the third process as a post-treatment. This fourth process can be carried out either by continuing in the closed container with stirring of the third process, or by transferring to a closed container without stirring and continuing.

[0140] Next, the time lag (lag time) from the start of stirring to the start of the reaction in the third step will be explained. In the third step, the time until the reaction begins is called the lag time. The minimum combined amount of sodium hydride (NaH) and sodium borohydride (SBH) required to start the particle-stirred reaction relative to the total amount of raw materials is a hydrogen reduction of 2.5% to 3.5% of the boron (B) content in the raw materials. The proportions of these hydrides can be measured by the hydrogen reduction.

[0141] The lag time described in this invention refers to the time required from the start of stirring until the reaction rate reaches 3%. The reaction rate is very slow until the particle stirring reaction begins. This is due to the presence of an aluminum oxide coating formed on the surface of the aluminum (Al) particles. That is, the aluminum oxide coating (estimated to be 5 nm to 10 nm) is a very strong barrier for mass transfer. When sodium oxide (Na₂O) diffuses into the interior of the aluminum particles, the aluminum oxide coating becomes an oxide coating containing sodium oxide (Na₂O). By allowing sodium oxide (Na₂O) to reach the inside of this oxide coating layer and react with aluminum (Al), the sodium oxide (Na₂O) is reduced to generate sodium (Na) and aluminum oxide (Al₂O₃). Sodium (Na) reacts with hydrogen (H₂) to generate sodium hydride (NaH), which then transfers to the outside of the oxide coating layer and reacts with sodium metaborate (NaBO₂) to generate sodium borohydride (SBH).

[0142] The aluminum oxide (Al2O3) on the inner side of the oxide coating reacts with sodium oxide (Na2O) to form sodium alumina (NaAlO2). In the presence of fluoride ions, the formation of needle-like crystals of ι-alumina (0.67Na·6Al·9.33O) is promoted, transforming into a thick needle-like ι-alumina layer.

[0143] This indicates that when the change in aluminum (Al) particle ratio relative to the raw material, the reaction rate, calculated by the reduction in hydrogen, reaches approximately 3%, the "particle stirring reaction" proceeds at a very rapid rate.

[0144] In the above embodiments, sodium metaborate and aluminum powder can be separately and sequentially loaded into a sealed container, or they can be loaded into the sealed container as a mixture of the two. When loading in the form of a mixture, it is preferable to include a step of mixing sodium metaborate and aluminum powder to obtain a mixture before the first step, in which sodium metaborate and aluminum powder are loaded into the reaction vessel in the form of this mixture. By pre-mixing the aluminum powder and sodium metaborate, the initial reaction rate can be improved by using the pre-mixed raw materials.

[0145] In addition, when aluminum powder and sodium metaborate are mixed, they can be pre-dispersed and mixed, then introduced into a mold and pressure applied to form granules. Compared with powder, granules have advantages such as less moisture absorption and excellent workability.

[0146] Fluoride, sodium metaborate, and aluminum powder can be loaded separately into a sealed container, or the fluoride can be mixed with aluminum first, and after the fluoride adheres to the surface of the aluminum, sodium metaborate can be added. Alternatively, the mixture of all three can be used for loading.

[0147] By adding sodium fluoride (NaF) as a fluoride to sodium metaborate powder, the molar ratio of alkali metal in sodium borate and sodium to boron (hereinafter referred to as Na / B (molar ratio)) is greater than 1, which can improve the reaction rate of sodium borohydride.

[0148] The Na / B (molar ratio) is preferably greater than 1 and less than 4.

[0149] The method of this invention is a reaction system of aluminum powder (solid), sodium metaborate powder (solid), fluoride powder (solid), and hydrogen (gas). In this reaction system, two points need to be addressed: first, to ensure the reaction proceeds smoothly, the oxide coating on the surface of the aluminum particles needs to be removed; second, to ensure the reaction proceeds, energy transfer needs to be supplied to ensure a uniform concentration of the raw materials or products in the solid.

[0150] To eliminate the effects of the initial aluminum oxide coating, it is necessary to create scratches on the aluminum surface through stirring, forming a new surface. This allows the raw materials to come into contact with the aluminum and begin the reaction, or sodium oxide to diffuse and permeate through the oxide coating to form ι-alumina, enabling the diffusion of metal ions. The period until the reaction begins is called the induction period (incubation period).

[0151] Once the reaction begins, sodium oxide (Na₂O) is supplied to the surface of the aluminum, initiating a reduction reaction to form sodium hydride. Sodium borohydride then acts as a binder, facilitating the exchange of sodium hydride and sodium metaborate between the aluminum and sodium particles. This causes sodium hydride to react with sodium metaborate, further initiating the formation of sodium borohydride. Thus, this particle-stirred reaction, which proceeds very rapidly through stirring and mixing in a granular state, requires no stirring medium; the reaction is driven solely by the contact between the particles.

[0152] Below is an example of a sealed container that can be used in embodiments of the present invention, but the embodiments of the present invention are not limited to the following.

[0153] Figure 3 This is a partial cross-sectional schematic diagram of an example of a sealed container used in an embodiment of the present invention.

[0154] like Figure 3 As shown, the sealed container 10A has a bottomed cylindrical container body 12 with a rounded bottom and a removable disc-shaped lid 14 that seals the container body 12. A temperature-adjustable heater 16 is disposed on the lower outer side of the container body 12, and the contents of the container body 12 are heated by the heater 16. Additionally, an O-ring 18 is provided on the upper end face of the container body 12, which is in close contact with the lid 14 to ensure internal airtightness. When the lid 14 is closed, the lid 14 is in a state of close contact with the O-ring 18 relative to the container body 12.

[0155] The cover 14 has an opening in its center, and a cylindrical part stands upright near the opening. A motor 20 is arranged on the upper side of the cylindrical part. The stirring device includes the motor 20, a stirring rod 22 connected to the rotation shaft of the motor 20, and a plurality of needle-shaped stirring elements 22A arranged in a direction perpendicular to the axis of the stirring rod 22. When the cover 14 is assembled to the container body 12, the tip of the stirring rod 22 reaches the lower region inside the container body 12. That is, after the motor 20 is driven, the stirring rod 22 and the needle-shaped stirring elements 22A rotate together to stir the contents of the container body 12.

[0156] The cover 14 also includes a first pipe 24 and a second pipe 30 communicating with the interior of the container body 12. The first pipe 24 is connected to a hydrogen supply source (not shown) via a hydrogen supply valve 26 and to a vacuum pump (not shown) via an exhaust valve 28. That is, when the hydrogen supply valve 26 is open, hydrogen is supplied to the container body 12, and when the exhaust valve 28 is open, the gas inside the container body 12 is evacuated. In addition, the second pipe 30 is connected to a pressure gauge 32, through which the pressure inside the container body 12 can be determined.

[0157] In addition, such as Figure 3 As shown, the minimum gap (a) between the stir bar 22A and the inner circumferential surface of the container body 12 of the sealed container 10A is 0 mm to 4 mm, as shown in the test example described later. This is to ensure that the stirring height ratio, determined by the container size, raw material quantity, and bulk density of the test example, is 75% or more. When the gap (a) between the bottom part 22a and the inner circumferential surface 12b of the container bottom is 4 mm or less, the raw material 39 passes through the needle-shaped stir bar 22A of the stir bar 22, causing the minimum amount of aluminum (Al) particles required to start the reaction to combine with sodium metaborate (NaBO2) fragments, so that the oxide coating of the aluminum (Al) particles becomes 1-alumina, generating sodium hydride (NaH) and sodium borohydride (SBH) required for the reaction of all aluminum (Al) particles. Until the reaction begins, there is a long start-up period, referred to as the lag time. However, once the sodium hydride (NaH) reaction begins, sodium borohydride (SBH) acts as a mass transfer medium for sodium hydride (NaH) on the aluminum (Al) surface, initiating a very rapid sodium borohydride (SBH) formation reaction in all particles.

[0158] Furthermore, as the reaction rate increases from 40% to 60%, the proportion of sodium borohydride (SBH) increases, acting as a binder. This makes the particles covered by the product more prone to agglomeration, forming clumps. In the case of a short-needle stir bar, since there is no force pressing against the container wall, the formed clumps become like dough, completely coalescing together.

[0159] Furthermore, due to the small and short cross-sectional area of ​​the needle-shaped stir bar, very little product adheres to the stir bar. The product moves like dough due to the stirring needle, forming a material transfer state that is slower than the stirring state of particles but faster than the diffusion transfer state of the static state, allowing the reaction to continue and resulting in a faster reaction rate and reaction ratio.

[0160] In comparison, such as Figure 3 In the sealed container 10A shown, when the gap a between the lowest part 22a side of the stirring rod 22 and the inner circumferential surface 12b side of the bottom 12a of the container body 12 is more than 5 mm, the stirring intensity decreases, the proportion of aluminum (Al) particles with oxide coating becoming 1-alumina decreases, only a small amount of sodium hydride (NaH) is generated and 1-alumina layer is formed, the reaction lag time increases, the reaction cannot be started within the actual production time, and the reaction rate drops sharply.

[0161] Furthermore, in the sealed container, the stirring height ratio (hereinafter also referred to as "height ratio") is preferably 75% or more. In this invention, the stirring height ratio refers to the ratio when the minimum gap between the stir bar 22A and the bottom of the sealed container 10A in the direction of gravity is set as a, and the material input height after the material 39 is put into the sealed container 10A is set as b, and is expressed by the following mathematical formula (I).

[0162] X=[(b-a) / b]×100 Formula (I)

[0163] As shown in the experimental examples described later, when the stirring height ratio (X) is above 75%, the particle stirring reaction is good; when the stirring height ratio (X) is below 75%, the particle stirring reaction does not occur. It should be noted that the reaction rate and reaction ratio are highest when the stirring height ratio (X) is 100%.

[0164] For example, when the minimum gap between the stir bar 22A and the inner circumferential surface of the container body of the sealed container 10A is set to a = 4 mm, and the material input height after the material is put into the sealed container is set to b = 17.7 mm, X is 77% (refer to Test Example 3 described later).

[0165] Figures 20A to 20F Indicates the use Figure 3 The photograph shows an example of sodium borohydride (SBH) manufactured in a sealed container 10A (gap a = 4 mm) with a height ratio (X) of 77%. Figure 20A A photograph of a stir bar after sodium borohydride (SBH) was manufactured to make the stir bar rotate. Figure 20B A photograph of the product after sodium borohydride (SBH) was manufactured to make the stir bar rotate. Figure 20C This is a photo of sodium borohydride (SBH) after it has been stirred for the predetermined time and then removed from the container. Figure 20DThis is a photograph of the measured product thickness. The product thickness at the bottom is shown as 4mm. Figure 20E This is a magnified photograph of the obtained product. Figure 20F This is a cross-sectional SEM image of the product block with a reaction rate of 91.1%. The cross-section of the block product shows that the particulate reaction products are partially bonded to each other, and there are gaps between the particles of the reaction products.

[0166] Figures 21A-21B Is using Figure 3 The photograph shows an example of sodium borohydride (SBH) manufactured in a sealed container 10A (gap a = 5 mm) with a height ratio (X) of 72%. Figure 21A This is a photograph of the product after the stir bar is rotated to produce sodium borohydride (SBH). Figure 21B This is a magnified photograph of the product after stirring for the predetermined time has ended.

[0167] like Figure 16 As shown, when using Figure 3 In the case of the sealed container 10A shown (height ratio X = 77%), the formation of sodium borohydride (SBH) proceeded smoothly. This exceeded the 3% reaction rate required for the feedstock to begin reacting at the start of the reaction with stirring. By initiating particle stirring, the formation of sodium borohydride was advanced, as... Figure 20C As shown, several wrinkles were observed on the bottom side of the product after the experiment, such as... Figure 20F As shown, voids were observed in the cross-section of the product. This indicates that due to the adhesive force of the sodium borohydride melt, the product becomes dough-like, and the stirring needle of stirrer 22A causes the product to be formed into a whole, inducing material transfer within the product. The material transfer rate in this dough-like stirring (hereinafter referred to as "dough stirring") is slower than the material transfer rate in particle stirring reactions, but faster than the diffusion transfer rate of material in a static state. Therefore, by first initiating a particle stirring reaction, sodium borate and aluminum particles are bonded, resulting in localized mixing of the product. Then, by mixing the product as a whole through dough stirring, the sodium borohydride, as the target product, can achieve a reaction rate of over 90%.

[0168] In comparison, although the needle-shaped stirrer has the same length, when using... Figure 3 When the sealed container 10A shown is a sealed container 10A (height ratio X = 72%), if Figure 21A As shown, the diameter of the void in the central part after stirring is the same as the length of the stir bar, and the diameter of the void is smaller than... Figure 20BThe reduced stirring cross-sectional area and intensity meant that at the start of the reaction, the amount of raw material in the stirring cross-section could not exceed the required reaction rate of 3% for the initial particle stirring reaction. Therefore, the product could not form a dough-like structure, and the formation of sodium borohydride (SBH) was insufficient. As shown in the comparative examples described later, the reaction rate of the target product, sodium borohydride, was less than 2% (refer to Comparative Example 1 described later). Figure 21B It can be seen that the product with a gap a of 5mm is in a loose powder state similar to the raw material.

[0169] Figure 5 This is a partial cross-sectional schematic diagram of another example of a sealed container used in an embodiment of the present invention. Figure 3 The difference between the sealed container 10A and the other type is that, for example... Figure 5 As shown, the sealed container 10B has a J-shaped stirring section 22B at the lower end of the stirring rod 22 that rotates inside the container body 12. The J-shaped stirring section 22B is formed to bend along the inner circumferential surface of the bottom 12a of the container body 12. Moreover, when raw materials are added to the container body 12 and the stirring rod 22 is rotated, the J-shaped stirring section 22B bends along the inner circumferential surface of the bottom 12a, thus preventing the raw materials from adhering to the inner wall of the container during stirring. Figure 5 Similarly, in a sealed container, the gap a between the bottom 12a of the J-type stirring section 22B and the inner circumferential surface is set to 0mm to 4mm.

[0170] Figure 6 This is a partial cross-sectional schematic diagram of another example of a sealed container used in an embodiment of the present invention. Figure 3 The difference between the sealed container 10A and the sealed container 10C is that the lower end of the stirring rod 22 rotating inside the container body 12 is provided with multiple scrapers 35, forming a multi-blade type.

[0171] in, Figure 6 In the sealed container 10C, the gap a between the top of the inner circumferential side of the scraper 35 and the inner circumferential side of the bottom 12a is 0mm to 2mm.

[0172] Figure 7 This is a partial cross-sectional schematic diagram of another example of a sealed container used in an embodiment of the present invention. Figure 3 The difference between the sealed container 10A and the sealed container 10D is that, in the sealed container 10D, the lower end of the stirring rod 22 rotating inside the container body 12 is provided with a spiral ribbon-shaped scraper 36. Among these, Figure 7 Similarly, in the sealed container 10D, the gap a between the top of the inner circumferential side of the strip scraper 36 and the inner circumferential side of the bottom 12a is 0mm to 2mm.

[0173] Figure 8 This is a partial cross-sectional schematic diagram of another example of a sealed container used in an embodiment of the present invention. Figure 3The difference between the sealed container 10A and the sealed container 10E is that, in the sealed container 10E, the container body 12 is horizontal, and the stirring rod 22 rotating inside the container body is provided with wide-faced blades 37 by means of a support. Figure 8 Similarly, in the sealed container 10E, the gap a between the top of the inner circumferential surface of the wide-faced paddle 37 and the bottom of the inner circumferential surface 12b is 2 mm. When the wide-faced paddle 37 is used for stirring according to the sealed container 10E, the raw material is scooped up from the paddle surface when the wide-faced paddle 37 rotates. During the scooping of the raw material by the paddle surface, or during the falling of the scooped raw material, the raw material particles collide with each other, resulting in a good particle stirring reaction.

[0174] <Second Implementation Method>

[0175] The method for manufacturing sodium borohydride according to the second embodiment includes the first to third steps described in the method for manufacturing sodium borohydride according to the first embodiment. For each step, descriptions identical to those in the first embodiment will be omitted, and only the differences will be described.

[0176] In the second embodiment of the sodium borohydride manufacturing method, in the first step, an alkali metal hydroxide or alkali metal oxide is placed in a sealed container. Then, in the second step, in a sealed container filled with hydrogen, the mixed sodium borate powder, aluminum powder, and alkali metal hydroxide are reacted and dehydrated at a temperature of 490°C to 560°C. Alternatively, after dehydration by heating and vacuum suction, in the third step, in a sealed container filled with hydrogen, sodium metaborate powder, aluminum powder, and alkali metal oxide are mixed and reacted at a temperature of 490°C to 560°C.

[0177] Examples of metal hydroxides with this alkalinity include sodium hydroxide (NaOH), potassium hydroxide (KOH), and lithium hydroxide (LiOH).

[0178] In addition, examples of alkaline metal oxides include sodium oxide (Na₂O), lithium oxide (Li₂O), sodium aluminum dioxide (NaAlO₂), calcium oxide (CaO), and strontium oxide (SrO). The addition of calcium oxide is to increase alkalinity.

[0179] When sodium hydroxide (NaOH) is used as a hydroxide of an alkaline metal, as shown in reaction formula (3) below, sodium hydroxide consumes aluminum and instead supplies hydrogen to the sealed container. Therefore, it is preferable to increase the amount of aluminum added in proportion to the amount lost due to aluminum consumption.

[0180] 6NaOH+2Al→2NaAlO2+2Na2O+3H2 (3)

[0181] [Experimental and Comparative Examples]

[0182] In the following, embodiments of the present invention will be described in more detail by means of test examples that demonstrate the effects of the embodiments of the present invention, but the embodiments of the present invention are not limited thereto.

[0183] It should be noted that in the following text, Test Examples 1-11, Test Examples 13-26, and Comparative Examples 1, 3, 4, 7, 8-11 are examples in which sodium metaborate (NaBO2) powder is used as sodium borate and fluoride is added. Additionally, Test Example 12 and Comparative Example 2 use both sodium metaborate (NaBO2) and borax (sodium tetraborate: Na2B4O7) as sodium borate. Comparative Examples 5 and 6 use sodium diborate (Na4B2O5) as a sodium borate compound. Furthermore, although sodium fluoride (NaF) is used as a fluoride, Test Example 10 uses sodium hexafluoroaluminate (Na3AlF6) as a fluoride, while Test Example 11 uses aluminum fluoride (AlF3). It should be noted that Test Examples 18-23 and Comparative Examples 8-10 are additives; sodium hydroxide is added along with the fluoride.

[0184] [Experimental Example 1]

[0185] (a) First process

[0186] Sodium metaborate powder was used as the raw material for boric acid in Experimental Example 1.

[0187] 5.82 g of sodium metaborate (NaBO2), 3.82 g of aluminum powder (Aluminum (Al) with an average particle size of 30 μm), and 0.72 g of sodium fluoride (NaF) were pulverized and sieved through a 100 μm mesh. The mixture was then placed in a container at room temperature. Figure 3 The needle-shaped stirrer 22A is placed inside a sealed container 10A. Next, a vacuum pump is connected to the inside of the sealed container. The gap a between the stirrer 22A and the inner circumferential surface 12b of the container body 12 is set to 0 mm.

[0188] (b) Second process

[0189] Moisture removal from the raw materials is achieved by connecting to a vacuum pump and heating the inside of the sealed container to 520°C. The time from reaching 500°C to starting the third process is 0.4 hours.

[0190] (c) Third process

[0191] After filling the sealed container with hydrogen gas to 0.74 MPa, the stirring mechanism inside the container was rotated at a circumferential speed of 4.87 cm / s (30 rpm) while maintaining a heating temperature of 520°C for 17.9 hours. It should be noted that the third step ended when the stirring time reached 20 hours or when the reaction rate, calculated from the rate of pressure reduction within the sealed container, fell below 0.5% / hour, and stirring was stopped followed by cooling. At this point, the maximum hydrogen pressure was 0.76 MPa, the minimum was 0.69 MPa, and the final hydrogen pressure was 0.70 MPa. Following the above procedure, sodium borohydride (SBH) was obtained.

[0192] After the third step was completed, the volume (molar amount) of hydrogen consumed was calculated based on the standard volume of hydrogen introduced, the average temperature and volume inside the container, and the hydrogen pressure. As a result, the reaction rate (SBH rate) was 93.0%. The reaction rate measured by iodine titration was 89.7%.

[0193] It should be noted that the Na / B (molar ratio) in Test Example 1 is 1.19. Additionally, the height ratio in Test Example 1 is 100%.

[0194] Furthermore, as shown below, the sodium borohydride content in the reaction product is the same as that obtained by iodine titration (89%). The sodium borohydride yield by titration is lower than the above-mentioned yield calculated based on hydrogen reduction because sodium hydride, as a hydride, coexists. Since sodium hydride can be converted into sodium borohydride upon association with boric acid through diffusion or stirring, the reaction rate calculated from hydrogen consumption is used uncorrected. It should be noted that, unless otherwise stated, in embodiments of the present invention, "reaction rate" is described as the reaction rate (SBH rate) calculated from hydrogen consumption, not the reaction rate obtained by iodine titration.

[0195] ~Iodine Titration Method~

[0196] (1) Weigh 50 mg of the sample (reaction product), accurate to the level of 0.1 mg, and collect it into a weighing bottle.

[0197] (2) Transfer the sample collected in (1) to a 200ml stoppered conical flask. Add 40ml of 20g / L NaOH solution to the stoppered conical flask and heat in a water bath to completely decompose the unreacted aluminum powder.

[0198] (3) After cooling the decomposition product of (2) to room temperature, add 20.0 ml of 0.05 mol / L iodine solution with a pipette, cap the bottle, and let it stand in the dark for 15 minutes.

[0199] (4) Add 3 ml of hydrochloric acid to the standing mixture in (3) and shake well. Then titrate with 0.1 mol / L sodium thiosulfate.

[0200] (5) The titration endpoint is set at the moment when the purple color of iodine turns colorless.

[0201] (6) A blank test was conducted without adding a sample, and the sodium borohydride content was calculated. The formula used to calculate the content is shown below.

[0202] <Formula for determining the sodium borohydride content>

[0203] NaBH4 (mass%) = {(A-B)×0.1×f×37.83 / 8} / C×100

[0204] The variables and constants in the above formula are shown below.

[0205] A: Titration value (ml) of 0.1 mol / L sodium thiosulfate solution in the blank test.

[0206] B: Titration value (ml) of the sample solution with 0.1 mol / L sodium thiosulfate solution

[0207] f: Coefficient of 0.1 mol / L sodium thiosulfate solution

[0208] C: Sample volume (mg)

[0209] 37.83: Molecular weight of sodium borohydride (g / mol)

[0210] 8: Equivalent concentration (N) of 1 mol / L sodium borohydride solution

[0211] [Experimental Example 2]

[0212] Except for changing the gap a between the container body 12 and the inner circumferential surface 12b side to 2 mm in Experimental Example 1, sodium borohydride (SBH) was obtained by operating in the same manner as in Experimental Example 1. As a result, the reaction rate (SBH rate) was 92.9%. The Na / B (molar ratio) of Experimental Example 2 was 1.19. It should be noted that the height ratio of Experimental Example 2 was 89%.

[0213] [Experimental Example 3]

[0214] Except for changing the gap a between the container body 12 and the inner circumferential surface 12b side to 4 mm in Experimental Example 1, sodium borohydride (SBH) was obtained by operating in the same manner as in Experimental Example 1. As a result, the reaction rate (SBH rate) was 93.2%. It should be noted that the height ratio of Experimental Example 3 was 77%.

[0215] [Experimental Example 4]

[0216] In addition to the experimental example 1 Figure 3 The sealed container 10A shown is replaced by Figure 5A sealed container 10C equipped with a J-type stirrer 22B was used, with the gap a changed to less than 1 mm (partially in contact with the inner circumferential surface of the bottom), the heating temperature of the third step changed to 518°C, and the stirring time changed to 20.5 hours, operating in the same manner as in Test Example 1, to obtain sodium borohydride (SBH). As a result, the reaction rate (SBH rate) was 95.1%. It should be noted that the height ratio of Test Example 4 was 100%.

[0217] [Experimental Example 5]

[0218] In addition to the experimental example 1 Figure 3 The sealed container 10A shown is replaced by Figure 8 A horizontal container with a wide-faced paddle (WP) 37 and a sealed container 10E was used. A mixture of 17.46 g of sodium metaborate (NaBO2), 11.45 g of aluminum powder (with an average particle size of 30 μm), and 2.17 g of sodium fluoride (NaF) was placed in the sealed container 10E. In the third step, the heating temperature was changed to 517°C, and the stirring time was changed to 13 hours. The operation was the same as in Example 1, and sodium borohydride (SBH) was obtained. As a result, the reaction rate (SBH rate) was 94.1%. It should be noted that the height ratio of Example 5 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b was set to 2 mm.

[0219] [Experimental Example 6]

[0220] Except that in Experimental Example 5, the amount of sodium fluoride (NaF) was changed to half the amount of Experimental Example 5 (1.08 g), and the heating temperature was changed to 510°C and the stirring time was changed to 23.4 hours in the third step, the operation was the same as in Experimental Example 1, and sodium borohydride (SBH) was obtained. As a result, the reaction rate (SBH rate) was 90.4%. The Na / B (molar ratio) of Experimental Example 6 was 1.10. It should be noted that the height ratio of Experimental Example 6 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0221] [Experimental Example 7]

[0222] Except for changing the heating temperature to 549°C and the stirring time to 23.3 hours in the third step of Experimental Example 5, the procedure was the same as in Experimental Example 1, and sodium borohydride (SBH) was obtained. The Na / B (molar ratio) of Experimental Example 7 was 1.19. As a result, the reaction rate (SBH rate) was 81.7%. It should be noted that the height ratio of Experimental Example 7 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0223] [Experimental Example 8]

[0224] Except for changing the aluminum (Al) particle size to 5 μm, the heating temperature to 510 °C, and the stirring time to 6.5 hours in Experimental Example 5, sodium borohydride (SBH) was obtained by operating the same procedure as in Experimental Example 5. As a result, the reaction rate (SBH rate) was 75.6%. It should be noted that the height ratio of Experimental Example 8 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0225] [Experimental Example 9]

[0226] Except for stirring at a stirring speed of 6 rpm (circumferential speed 1.54 cm / s) for 16.9 hours in the third step of Experimental Example 6, sodium borohydride (SBH) was obtained by operating in the same manner as in Experimental Example 6. The Na / B (molar ratio) of Experimental Example 9 was 1.19. As a result, the reaction rate (SBH rate) was 86.6%. It should be noted that the height ratio of Experimental Example 9 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0227] [Experimental Example 10]

[0228] Except for changing the additive from sodium fluoride (NaF) to sodium hexafluoroaluminate (Na3AlF6) and the amount added to 1.81 g in Experimental Example 6, the procedure was the same as in Experimental Example 6 to obtain sodium borohydride (SBH). The Na / B (molar ratio) of Experimental Example 10 was 1.10. As a result, the reaction rate (SBH rate) was 85.3%. It should be noted that the height ratio of Experimental Example 10 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0229] [Experimental Example 11]

[0230] Except for changing the additive from sodium fluoride (NaF) to aluminum fluoride (AlF3) and the amount added to 1.44 g in Experimental Example 6, the procedure was the same as in Experimental Example 6 to obtain sodium borohydride (SBH). The Na / B (molar ratio) in Experimental Example 11 was 1.00. As a result, the reaction rate (SBH rate) was 73.4%. It should be noted that the height ratio in Experimental Example 11 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0231] [Experimental Example 12]

[0232] Except that in Example 5, 6.98 g of sodium metaborate (NaBO2) and 8.01 g of borax were used as the boric acid raw material, and the heating temperature was changed to 510 °C, the procedure was the same as in Example 6 to obtain sodium borohydride (SBH). The Na / B (molar ratio) of Example 12 was 0.89. As a result, the reaction rate (SBH rate) was 56.0%. It should be noted that the height ratio of Example 12 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0233] [Experimental Example 13]

[0234] Except for changing the sodium fluoride (NaF) to 1.08 g in Example 2, the procedure was the same as in Example 2 to obtain sodium borohydride (SBH). As a result, the reaction rate (SBH rate) was 95.5%. The Na / B (molar ratio) in Example 13 was 1.29. It should be noted that the height ratio in Example 13 was 89%. The gap a between the container body 12 and the inner circumferential surface 12b was set to 2 mm.

[0235] The results of these test examples 1 to 13 are shown in Table 1.

[0236] [Table 1]

[0237]

[0238] As shown in Table 1, in Experimental Examples 1 to 13, through particle stirring reaction, material transfer occurred not only within the aluminum (Al) particles but also through contact between the particles, resulting in good SBH reaction rate. Furthermore, when the gap (a) within the sealed container was set to 4 mm or less, the particle stirring reaction proceeded effectively, leading to high sodium borohydride (SBH) production efficiency.

[0239] [Comparative Example 1]

[0240] In addition to the first step in Experiment Example 1, using Figure 3 In a sealed container 10A with a gap a of 5 mm, 17.46 g of sodium metaborate (NaBO2), 11.45 g of aluminum powder, and 0.72 g of sodium fluoride (NaF) were added. Except for changing the heating temperature in the third step to 520°C, the procedure was the same as in Experimental Example 1, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Comparative Example 1 was 1.19. As a result, the reaction rate (SBH rate) was 1.6%. It should be noted that the height ratio of Comparative Example 1 was 72%.

[0241] [Comparative Example 2]

[0242] In addition to the first step in Experimental Example 1, Figure 3 Ceramic balls (5 mm in diameter) were placed inside the sealed container 10A shown. The weight ratio of stirring medium to raw materials was changed to 4:8. Sodium metaborate (NaBO2) 1.94 g, aluminum powder 1.27 g, and sodium fluoride (NaF) 0.206 g were added. The heating temperature in the third step was changed to 558 °C, and the circumferential speed was changed to 78.5 cm / s (300 rpm). Stirring was performed for 13.8 hours, and the procedure was the same as in Experimental Example 1, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Comparative Example 2 was 1.17. The results showed that the reaction rate at a 10% reaction rate was 90% / Hr, which was good, but the reaction rate at a 50% reaction rate decreased to 2.1% / Hr, and the final reaction rate (SBH rate) was 71.9%. It should be noted that the height ratio of Comparative Example 2 was 100%.

[0243] [Comparative Example 3]

[0244] In addition to the first step in Experimental Example 1, Figure 3 Ceramic balls (5 mm in diameter) were placed inside the sealed container 10A shown. The weight ratio of stirring medium to raw materials was changed to 4:8. Sodium metaborate (NaBO2) 1.95 g and aluminum powder 1.12 g were added. Except for the heating temperature in the third step being changed to 520 °C and the circumferential speed being changed to 301 cm / s (1150 rpm), the operation was the same as in Experimental Example 1, and sodium borohydride (SBH) was obtained. The Na / B (molar ratio) of Comparative Example 3 was 1.00. The result was that the lag time was 0.33 hours, which was the shortest, but the reaction rate (SBH rate) was 21.9%. It should be noted that the height ratio of Comparative Example 3 was 100%.

[0245] [Comparative Example 4]

[0246] Except for changing the particle size of aluminum (Al) in Experimental Example 5 to 90 μm and setting the heating temperature to 510 °C, sodium borohydride (SBH) was obtained by operating in the same manner as in Experimental Example 5. As a result, the reaction rate (SBH rate) was 37.8%. It should be noted that the height ratio of Comparative Example 4 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0247] [Comparative Example 5]

[0248] In addition to the first step of Experimental Example 5, the following was adopted Figure 6The multi-blade (MB) sealed container 10C with multiple scrapers 35 shown was prepared by adding 5.88 g of sodium diborate (Na4B2O5), 2.55 g of aluminum powder, and 1.65 g of sodium fluoride (NaF). Except for changing the heating temperature of the third step to 530°C and the circumferential speed to 78.5 cm / s (300 rpm), the operation was the same as in Experimental Example 1, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Comparative Example 5 was 2.65. The results showed that the reaction rate at a reaction rate of 10% was 12.3% / Hr, and the reaction rate (SBH rate) was 72.6%. It should be noted that the height ratio of Comparative Example 5 was 89%. The gap a between the container body 12 and the inner circumferential surface 12b was set to 2 mm.

[0249] [Comparative Example 6]

[0250] In addition to the first step of Experimental Example 5, the following was adopted Figure 6 The sealed container 10C shown was prepared by adding 5.88 g of sodium diborate (Na4B2O5), 2.54 g of aluminum powder, and 0.22 g of sodium fluoride (NaF). Except for changing the heating temperature of the third step to 530°C and the circumferential speed to 78.5 cm / s (300 rpm), the operation was the same as in Experimental Example 1, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Comparative Example 6 was 2.09. The results showed that the reaction rate at a reaction rate of 10% was 13.2% / Hr, and the reaction rate (SBH rate) was 73.2%. It should be noted that the height ratio of Comparative Example 6 was 89%. The gap a between the container and the inner circumferential surface 12b of the container body 12 was set to 2 mm.

[0251] The results of these comparative examples 1 to 6 are shown in Table 2.

[0252] [Table 2]

[0253]

[0254] In Comparative Example 1, since the gap (a) was more than 5 mm and the height ratio was 72%, the particle stirring reaction with a height ratio of more than 75% could not be carried out smoothly as in Experimental Examples 1 to 4, and the product could not be obtained.

[0255] In Comparative Example 2, during the production of sodium borohydride, ceramic balls were used as the pulverizing medium for gentle stirring within a sealed container. Therefore, the initial reaction was close to a particle-stirring reaction. However, due to the disruption of the product by the ceramic balls, the product was continuously compacted within the sealed container, forming a compacted layer containing the product, which dominated the diffusion reaction. As a result, the reaction rate and the sodium borohydride (SBH) reaction rate decreased. In Comparative Example 3, due to the high circumferential speed, calendering and pulverization occurred, deforming the aluminum particles and shortening the lag time to 0.33 Hr, but the reaction rate immediately decreased. Furthermore, since no fluoride was added, the final sodium borohydride (SBH) reaction rate was also significantly reduced.

[0256] In Comparative Example 4, because the aluminum (Al) particles used in the production of sodium borohydride had a particle size of 90 μm, the reaction surface area was reduced, resulting in a significant decrease in the reaction rate of sodium borohydride (SBH). The reaction rate at 10% was as low as 17% / Hr, and the reaction ended at a rate as low as 0.7% / Hr after 22 hours of stirring. The reaction rate after the reaction ended was significantly reduced to 37.8%, and the iodometric titration reaction rate was significantly reduced to 37.5%.

[0257] In Comparative Examples 5 and 6, sodium diborate was used as the sodium fluoride and sodium borate compounds, with a Na / B ratio of 2 or higher. Therefore, the aluminum oxide coating on the surface of the aluminum (Al) particles became a sodium alumina (NaAlO2) layer. Even at a high Na / B ratio of 1.5 or higher, the sodium alumina (NaAlO2) did not transform into ι-alumina in the presence of fluorides, but maintained its sodium alumina (NaAlO2) form, with some even transforming into xNa2O·Al2O3 and Na2O, where the sodium oxide ratio was higher than that of ι-alumina. When the Na / B ratio of the raw material was high, in addition to a large amount of free sodium oxide (Na2O), the product also contained a melt formed by the generated sodium boride (SBH), sodium oxide (Na2O), and sodium hydride (NaH), which became slushy after the reaction began. As a result, the reaction rate at 10% slowed down due to the diffusion and mass transfer within this slushy, high-density solid-liquid mixture.

[0258] In Comparative Example 5, the reaction rate at a 10% reaction rate was 12.3% / Hr, with a final reaction rate of 72.6%. In Comparative Example 6, the reaction rate at a 10% reaction rate was 13.2% / Hr, with a final reaction rate of 73.2%. The final reaction rates of Comparative Examples 5 and 6 are both lower than the 94.1% final reaction rate of Experimental Example 5 with a Na / B ratio of 1.19.

[0259] Figure 16 This is a schematic diagram showing the relationship between the height ratio and the reaction time.

[0260] exist Figure 16In the diagram, symbol I represents a height ratio of 100% (a = 0 mm), symbol II represents a height ratio of 89% (a = 2 mm), symbol III represents a height ratio of 77% (a = 4 mm), and symbol IV represents a height ratio of 72% (a = 5 mm). For example... Figure 16 As shown, the particle stirring reaction begins when the stirring height ratio is above 75%, but fails to start when the height ratio is below 75% (72%). The results indicate that the reaction rate and efficiency are highest when the height ratio is 100% (a = 0 mm), and the sodium borohydride (SBH) production efficiency is good when the height ratio is above 75%.

[0261] [Experimental Examples 14–17, Comparative Example 7]

[0262] In the third step of Experimental Example 2, the hydrogen pressure was set to 0.4 MPa to 0.8 MPa, and the operation was the same as in Experimental Example 1, yielding sodium borohydride (SBH). As a result, the reaction rate (SBH rate) was 68.1% in Experimental Example 14 at a hydrogen pressure of 0.5 MPa, 90.0% in Experimental Example 15 at a hydrogen pressure of 0.6 MPa, 92.9% in Experimental Example 16 at a hydrogen pressure of 0.7 MPa, and 95.5% in Experimental Example 17 at a hydrogen pressure of 0.8 MPa, all of which were good. It should be noted that the reaction rate (SBH rate) in Comparative Example 7 was 20.4% at a hydrogen pressure of 0.4 MPa. The results of Experimental Examples 2, 14-17, and Comparative Example 7 are shown in Table 3 and... Figure 17 The gap between the container body 12 and the inner circumferential surface 12b is 2mm.

[0263] [Table 3]

[0264]

[0265] Figure 17 This is a schematic diagram showing the relationship between the reaction rate (SBH rate) and the reaction time. Figure 17 In the diagram, the symbol V represents a hydrogen pressure of 0.4 MPa, VI represents a hydrogen pressure of 0.5 MPa, VII represents a hydrogen pressure of 0.6 MPa, VIII represents a hydrogen pressure of 0.7 MPa, and IX represents a hydrogen pressure of 0.8 MPa. For example... Figure 17As shown, when the hydrogen pressure is above 0.5 MPa, the reaction rate (SBH rate) of sodium borohydride (SBH) is above 60%, which is considered good. In contrast, when the hydrogen pressure is 0.4 MPa, the reaction rate (SBH rate) of sodium borohydride (SBH) is only 10.0%. When the hydrogen pressure decreases, the chemical equilibrium stabilizes the hydrogen gas, leading to a decrease in the stability of the hydrides. At 0.4 MPa, the formation of sodium hydride and sodium borohydride (SBH) is low, and particle adhesion and aggregation are less likely to occur. Areas that the stirring needle cannot reach are not stirred, resulting in partial reaction stagnation and a reduced reaction rate.

[0266] The results show that the necessary hydrogen pressure is preferably at least 0.5 MPa. It should be noted that in the graphs of the experimental examples with hydrogen pressures of 0.5 MPa to 0.7 MPa, the increase in reaction rate starting from the inflection point of 40% to 60% indicates a change from a stirring reaction in the form of particles near the needle-shaped stir bar 22A to a whole-body stirring reaction with the particles entangled like dough.

[0267] Furthermore, at a hydrogen pressure of 0.8 MPa, due to the initially rapid reaction rate, there is a possibility that the inflection point cannot be determined, or that the reaction may begin very early, or that the reaction rate may be similar to that under particle stirring conditions. Additionally, to achieve a reaction rate of up to 90% sodium borohydride (SBH) with the addition of fluoride, at least 0.6 MPa is required. It should be noted that at a hydrogen pressure of 0.8 MPa, the reaction rate of sodium borohydride (SBH) (SBH rate) increases to approximately 90%. The inventors speculate that this is because the chemical equilibrium tilts towards the hydride side, increasing the yield of sodium hydride, thereby achieving the effect of increased sodium borohydride yield.

[0268] [Experimental Example 18]

[0269] In addition to the first step in Experimental Example 1, the following was adopted: Figure 6 The multi-blade (MB) sealed container 10C with multiple scrapers 35 shown was used to add 5.82 g of sodium metaborate (NaBO2), 3.18 g of aluminum powder, 0.62 g of sodium fluoride (NaF), and 0.1 g of sodium hydroxide powder. Except for changing the heating temperature of the third step to 531°C and the circumferential speed to 15.7 cm / s (60 rpm), the operation was the same as in Example 5, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Example 18 was 1.19. The result was a lag time of 1.14 Hr. The reaction rate at 10% was 31.3% / Hr. The reaction rate (SBH rate) was 62.2% at the end of a 5-hour stirring period. It should be noted that the height ratio of Example 18 was 89%. The gap a between the container body 12 and the inner circumferential surface 12b was set to 2 mm.

[0270] [Experimental Example 19]

[0271] Except for the addition of 5.82 g of sodium metaborate (NaBO2), 3.18 g of aluminum powder, 1.86 g of sodium fluoride (NaF), and 0.1 g of sodium hydroxide powder in the first step of Experimental Example 18, and the change of the heating temperature in the third step to 530 °C, the procedure was the same as in Experimental Example 17 to obtain sodium borohydride (SBH). The Na / B (molar ratio) of Experimental Example 19 was 1.50. The result was a lag time of 2.33 Hr. The reaction rate at 10% was 47% / Hr. The reaction rate (SBH rate) was 84.5% at the end of the stirring time of 19.1 hours. It should be noted that the height ratio of Experimental Example 19 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0272] [Experimental Example 20]

[0273] In addition to the first step of Experimental Example 5, the following was adopted Figure 7 In the sealed container 10D with the strip scraper 36 shown, 17.46 g of sodium metaborate (NaBO2), 11.45 g of aluminum powder, 1.86 g of sodium fluoride (NaF), and 0.31 g of sodium hydroxide powder were added. Except for the third step, where the heating temperature was changed to 530°C, the circumferential speed was changed to 15.76 cm / s (60 rpm), and stirring was carried out for 18.6 hours, the operation was the same as in Example 5, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Example 20 was 1.20. As a result, the reaction rate (SBH rate) was 91.17%. It should be noted that the height ratio of Example 20 was 94%. The gap a between the sample and the inner circumferential surface 12b of the container body 12 was set to 2 mm.

[0274] [Experimental Example 21]

[0275] Except for the addition of 17.46 g of sodium metaborate (NaBO2), 11.45 g of aluminum powder, 1.86 g of sodium fluoride (NaF), and 0.31 g of sodium hydroxide powder in the first step of Experimental Example 5, and the change of the heating temperature to 490°C and the circumferential speed to 7.8 cm / s (30 rpm) in the third step, the operation was the same as in Experimental Example 5, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Experimental Example 21 was 1.17. The result was a reaction rate of 131% / Hr at a reaction rate of 10%. The reaction rate (SBH rate) was 86.7%. It should be noted that the height ratio of Experimental Example 21 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b was set to 2 mm.

[0276] [Comparative Example 8]

[0277] In Experimental Example 18, sodium borohydride (SBH) was obtained by the same procedure as in Experimental Example 17, except that 5.82 g of sodium metaborate (NaBO2), 3.81 g of aluminum powder, 0.62 g of sodium fluoride (NaF), and 1.06 g of sodium hydroxide powder were added. The Na / B (molar ratio) of Comparative Example 8 was 1.47. As a result, the reaction rate at a reaction rate of 10% was 8.3% / Hr. The reaction rate (SBH rate) was 32.4%. It should be noted that the height ratio of Comparative Example 8 was 89%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0278] [Comparative Example 9]

[0279] Except for the addition of 5.82 g of sodium metaborate (NaBO2), 3.81 g of aluminum powder, 0.62 g of sodium fluoride (NaF), and 0.29 g of sodium hydroxide powder in Experimental Example 20, and the change of the heating temperature in the third step to 478°C, the procedure was the same as in Experimental Example 19 to obtain sodium borohydride (SBH). The Na / B (molar ratio) of Comparative Example 9 was 1.19. As a result, the reaction rate at a reaction rate of 10% was 2.0% / Hr. The reaction rate (SBH rate) was 19.6%. It should be noted that the height ratio of Comparative Example 9 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0280] The results of these experimental examples 18–20 and comparative examples 8 and 9 are shown in Table 4.

[0281] [Table 4]

[0282]

[0283] As shown in Table 4, sodium hydroxide was added in Examples 18-20, but the Na / B ratio was below 1.5. In Example 19, when the Na / B ratio was as high as 1.50, the SBH reaction rate was slightly lower at 84.9%, but when the Na / B ratio was around 1.2, it was 86.7%–91.1%, showing good results. In Example 18, when the stirring time was as short as 5.0 hours, the SBH reaction rate decreased.

[0284] On the other hand, in Comparative Example 8, the Na / B ratio was 1.47, but the reaction rate decreased significantly to 32%. This is because the amount of sodium hydroxide added was high. Under these experimental conditions, sodium borate formed hydrates, and the removal of water was insufficient. Furthermore, the use of sodium hydroxide increased the Na / B (molar ratio) to 1.47, resulting in a slush-like diffusion and mass transfer, which slowed down the reaction and reduced the sodium boroide (SBH) reaction rate. Additionally, at the lower heating temperature of 478°C described in Comparative Example 9, sodium borohydride, being a solid, could not serve as a transfer medium for the mass transfer of various ions. Relying solely on diffusion within the solid, the reaction rate decreased significantly, and the sodium boroide (SBH) reaction rate also decreased.

[0285] In this case, when a small amount of sodium hydroxide is added, the aluminum surface is oxidized by the generated water, forming a new alumina film. Sodium oxide is also formed, thus the alumina film transforms into sodium aluminate (NaAlO2), which is then easily converted to ι-alumina by fluoride ions. Additionally, some water is converted into hydrogen fluoride, forming a NaAlF layer on the aluminum surface. However, when a large amount of sodium hydroxide is mixed, as in Comparative Example 8, a thicker sodium alumina (NaAlO2) layer is formed, and the continuous generation of moisture leads to the formation of fine crystal hydrates. This keeps the oxygen partial pressure in the system at a high level, competing with the oxidation reaction of aluminum and the formation of hydrides, resulting in a decrease in reaction rate and reaction volume.

[0286] Figure 11 The X-ray diffraction pattern of the product is obtained by conducting the experiment under the same conditions as in Experimental Example 18, and stopping the reaction when the reaction rate was 8.1% after the initial reaction. Figure 12A , Figure 12B The SEM image of the product was obtained under the same conditions as in Experimental Example 18, and the reaction was stopped when the reaction rate was 8.1% after the initial reaction.

[0287] In the initial stage of the particle stirring reaction, according to Figure 11 X-ray diffraction analysis confirmed the formation of Na3AlF6 and sodium borohydride. This indicates that Na3AlF6 was formed in the initial stage of the reaction. Figure 12B The SEM image, specifically the "F-spectrum," also reveals a fluoride layer on the surface of the aluminum particles. The inventors speculate that this is a fluoride layer primarily containing Na3AlF6. This fluoride layer is referred to as the "NaAlF layer."

[0288] Around the NaAlF layer, multiple thick layers of aluminum and sodium oxides were observed. Upon magnification of the oxides, needle-like crystals were visible. The inventors believe these needle-like crystals are ι-alumina. Sodium metaborate was also observed adhering to the oxides.

[0289] The inventors speculate that when the reaction begins around the NaAlF layer and forms SBH with the ι-alumina layer, sodium borate further adheres to the ι-alumina layer. Sodium borate supplies sodium oxide and boric acid through the ι-alumina layer, and the synthesis reaction of sodium borohydride occurs through aluminum and hydrogen.

[0290] Figure 13 The image shows the X-ray diffraction pattern of the product with a reaction rate of 91.1% in Experimental Example 20.

[0291] X-ray diffraction analysis of the products with a reaction rate of 91.1% indicating that the reaction was essentially complete was performed. Figure 13 It can be determined that the product is sodium borohydride and has a composition of Na. 0.67 ·Al6·O 9.33 Sodium alumina was the main component. Excess aluminum relative to the reaction mixture, the addition of aluminum fluoride, and cryolite (Na3AlF6) observed in the initial products of the reaction were confirmed. 0.67 ·Al6·O 9.33 It has a diffraction pattern almost identical to mullite, and its crystal structure should be considered the same. This crystal was identified as ι-alumina. Fluorides are known catalysts for promoting alumina crystallization and are believed to have the effect of promoting ι-alumina crystal growth. Observations show that a portion of the fluoride exists in the form of cryolite, exhibiting a diffraction intensity that remains constant relative to the initial stage, while the majority exists in the form of sodium fluoride, showing diffraction intensity.

[0292] [Experimental Example 22]

[0293] In the second step of Experiment 17, a vacuum pump was connected to a sealed container containing the raw material. After heating at 530°C for 0.73 hours to remove moisture, hydrogen gas was introduced to 0.76 MPa. Except for performing the third step at 530°C, stopping stirring, and then baking at 530°C, the operation was the same as in Experiment 18, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Experiment 22 was 1.21. The result was that the reaction rate, calculated from the slope of the reaction rate curve, was 133% / hr at a reaction rate of 10%. The lag time was 1.21 hr. The reaction time was 11.3 hours. The reaction rate (SBH rate) was 89.9%. It should be noted that the height ratio of Experiment 22 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b was set to 2 mm.

[0294] [Experimental Example 23]

[0295] Except for Experiment 21, where, as a second step prior to the third step, a pre-baking process was performed in a sealed container under hydrogen atmosphere at 530°C for 2.4 hours, followed by a third step at 531°C with stirring for 5.5 hours, sodium borohydride (SBH) was obtained by operating in the same manner as in Experiment 21. The Na / B (molar ratio) of Experiment 21 was 1.21. As a result, the lag time was 1.5 hours. The reaction rate at a reaction rate of 10% was 144% / hour. Since the reaction rate at a reaction rate of 30% was as high as 211% / hour, the stirring time for the third step was shortened to 5.5 hours. The reaction rate (SBH rate) of Experiment 23 was 95.5%. It should be noted that the height ratio of Experiment 23 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0296] The results of these experiments, Examples 22 and 23, are shown in Table 5. Figure 18 , Figure 19 . Figure 18 This is a schematic diagram of the manufacturing conditions for Experimental Example 22. Figure 19 This is a schematic diagram of the manufacturing conditions for Experiment 23. After the second step of Experiment 22 was performed at 530°C for 0.73 hours, hydrogen was introduced, and stirring only began after the pressure reached 0.76 MPa. The hydrogen pressure dropped sharply after a lag time of 1.2 hours, initiating a particle-mixing reaction. The reaction rate decreased briefly when the reaction rate was around 23%, but then recovered. This indicates a shift from a granular reaction to a state of particle aggregation and dough-like mixing. Stirring was stopped at 6.4 hours after the start of heating, when the reaction rate was 78%, and the reaction continued with post-baking in a static state. Heating was stopped at approximately 13 hours after the start of heating, terminating the reaction. Even with stirring stopped, the reaction continued, ultimately reaching a reaction rate of 89.9%. Furthermore, it was observed that the reaction rate increased with increasing hydrogen pressure.

[0297] Example 23 was identical to Example 22 except that the vacuum removal of moisture in the second step was replaced with the introduction of hydrogen gas and carried out for 2.3 hours. Although the lag time was slightly longer, reaching 1.5 hours, the reaction rate of the particle stirring reaction in the third step was greatly improved, the reaction rate at which the reaction rate was above 25% was increased, and the reaction time of the third step was shortened from 11.3 hours to 5.5 hours. In Example 23, similarly, even when the reaction rate was above 90%, the reaction continued under post-baking.

[0298] [Table 5]

[0299]

[0300] The results of Experiment 22 show that even after the third step, the reaction continues without stirring but with post-baking, achieving a reaction rate of approximately 90%. Compared to Experiment 20, pre-baking increases the reaction rate and shortens the stirring time.

[0301] The results of Experiment 23 show that when hydrogen heating is performed in the second step and post-baking is performed after the third step, the reaction rate and reaction rate (SBH rate) are improved compared to Experiment 20. It can be inferred that the longer particle stirring reaction is carried out, the better.

[0302] In addition, such as Figure 18 and Figure 19 As shown, in Test Examples 22 and 23, it was confirmed that the reaction rate was increased by post-baking even when the reaction in the third step was completed and no further stirring was required. The inventors believe that after sufficient particle stirring during the reaction, the required amount of sodium borate particles adheres to the aluminum particles, thus promoting the reaction even during post-baking in a static state.

[0303] [Experimental Example 24]

[0304] Except for the second step in Example 2, where moisture removal was performed at 510°C for 1.5 hours under vacuum, followed by the introduction of hydrogen gas to 0.82 MPa, and the third step, where stirring was carried out at 510°C, the procedure was the same as in Example 2 to obtain sodium borohydride (SBH). The Na / B (molar ratio) of Example 24 was 1.19. The resulting lag time was 1.98 Hr. The reaction rate (SBH rate) was 101.9%. It should be noted that the height ratio of Example 24 was 89%. The gap a between the container body 12 and the inner circumferential surface 12b was set to 2 mm.

[0305] [Experimental Example 25]

[0306] Except that in Example 24, before the third step, a second step was performed to remove moisture at 510°C under vacuum for 2.4 hours, followed by heating with stirring at 510°C in the third step, the procedure was the same as in Example 24 to obtain sodium borohydride (SBH). The Na / B (molar ratio) of Example 25 was 1.19. The resulting lag time was 1.1 Hr. The reaction rate (SBH rate) was 103.6%. It should be noted that the height ratio of Example 25 was 89%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0307] [Experimental Example 26]

[0308] Except for Experiment 24, where, as a second step prior to the third step, a moisture removal treatment was performed at 350°C for 1.2 hours under atmospheric conditions, followed by heating with stirring at 510°C in the third step, the procedure was the same as in Experiment 2, yielding sodium borohydride (SBH). The Na / B (molar ratio) of Experiment 26 was 1.19. The resulting lag time was 3.1 hours. The reaction rate (SBH rate) was 102.4%. It should be noted that the height ratio of Experiment 26 was 89%. The gap a between the container body 12 and the inner circumferential surface 12b was set to 2 mm.

[0309] [Comparative Example 10]

[0310] Except for Experimental Example 21, in which NaF was not added, but 17.46 g of sodium metaborate (NaBO2), 12.5 g of aluminum powder, and 2.00 g of sodium hydroxide powder were added, and as a second step before the third step, a pre-baking treatment was performed at 530°C in a vacuum environment for 4.0 hours, followed by heating with stirring at 520°C in the third step, the procedure was the same as in Experimental Example 21 to obtain sodium borohydride (SBH). The Na / B (molar ratio) of Comparative Example 10 was 1.19. As a result, the reaction rate (SBH rate) was 14.1%. It should be noted that the height ratio of Comparative Example 10 was 87%. The gap a between the container body 12 and the inner circumferential surface 12b side was set to 2 mm.

[0311] [Comparative Example 11]

[0312] Except in Experimental Example 21, where sodium fluoride and sodium hydroxide were not added, but 4.69 g of sodium metaborate powder (NaBO2), 1.66 g of sodium diborate powder, and 3.82 g of aluminum powder were added, and a pre-baking treatment was performed at 530°C for 1.3 hours under vacuum as a second step before the third step, followed by heating with stirring at 530°C in the third step, and the treatment was completed without a fourth post-baking step, sodium borohydride (SBH) was obtained by operating in the same manner as in Experimental Example 21. The Na / B (molar ratio) of Comparative Example 11 was 1.19. The results showed that the reaction rate at a reaction rate of 10% was 3.9% / Hr, and the final reaction rate (SBH rate) was 42.5%.

[0313] The results of these test examples 22 to 26, comparative example 10 and comparative example 11 are shown in Table 5.

[0314] In Experimental Examples 22 and 23, a pre-baking process involving prolonged heating at conditions requiring moisture removal (heating at approximately 500°C for 0.5 hours) and a post-baking process involving continued reaction under static conditions were performed. Compared to Experimental Example 20, which did not undergo pre-baking, this resulted in an increased reaction rate and a shorter stirring time. The product from the particle-stirred reaction could be further processed by post-baking without stirring.

[0315] Based on the results of Experiments 24-26, by performing pre-baking as the second step, setting the temperature of the third step to 510°C, and increasing the hydrogen pressure, the reaction rate (SBH rate) increased by approximately 1.1 times compared to Experiment 2. For pre-baking, vacuum heating treatment shortened the pre-baking lag time compared to treatment under atmospheric conditions, thus reducing the overall synthesis time. It should be noted that since the reaction rates in these experiments exceeded 100%, the SBH reaction rate was verified by iodine titration, with Experiment 24 showing a rate of 97.3%. This is a very high reaction rate. This is because it indicates that hydrogen is consumed in the form of sodium hydride. The formation of sodium hydride in the high reaction rate range is considered not to be due to the reduction reaction of sodium oxide (Na₂O) caused by aluminum and hydrogen, but rather to the reduction reaction in the NaAlF layer leading to the formation of lower aluminum fluoride.

[0316] Figure 14 This is an X-ray diffraction pattern of the product of Comparative Example 10 with a reaction rate of 14.1% after the addition of NaOH. Figure 15 This is a SEM image of the product of Comparative Example 10, after the addition of 2.0 g of NaOH, showing a reaction rate of 14.1%. Figure 14 As shown, in the particle stirring method using only a stir bar without a stirring medium, when the Na / B ratio increases, the reaction rate is very low (14.1%) when alkalinity is increased by NaOH alone without adding NaF. According to... Figure 14 NaOH was added to make the Na / B ratio 1.2, and the mixture was stirred for 35 hours. The X-ray diffraction pattern of the product with a reaction rate of 14.1% detected sodium borohydride and sodium aluminate (NaAlO2) as products. However, ι-alumina was not detected. Furthermore, sodium aluminum hydrate and sodium borohydride hydrate were detected, indicating residual moisture in the product. The inventors speculate that the low reaction rate and yield are due to the moisture supplied by these hydrates causing oxidation of the alumina surface. From Comparative Example 10... Figure 15 The SEM-mapped composite image shown confirms that the surface of the aluminum particles is covered with sodium / alumina.

[0317] AlB2 was formed in the aluminum, and a small amount of sodium borohydride was also formed in the oxide portion inside the aluminum cracks. The outer side of the sodium aluminum oxide was bonded to sodium metaborate. This sodium aluminum oxide is considered to be sodium alumina (NaAlO2) detected by X-ray diffraction analysis. Although the oxide has a dense surface, cracks exist in some areas, some of which are present in the fractured sodium metaborate particles. From these states, it can be inferred that although this oxide is dense, it is more easily cut and fractured than deformed.

[0318] From the perspective of ion conduction, even with the adhesion of sodium metaborate, the amount of sodium borohydride formed after a 35-hour reaction is very small. The inventors speculate that this is because sodium alumina (NaAlO2) has almost no ion transfer capability. Therefore, they hypothesize that Na2O, as the intermediate medium, cannot transfer, resulting in very little NaH formation on the aluminum surface. The insufficiency of Na2O on the aluminum surface also supports the phenomenon of AlB2 formation from aluminum.

[0319] Symbol Explanation

[0320] 10A~10E: Sealed container; 12: Container body; 12a: Bottom; 14: Lid; 16: Heater; 18: O-ring;

[0321] 20: Motor; 22: Stirring rod; 22a: Bottom part; 22b: Inner circumferential surface; 22A: Stirring element; 22B: J-type stirring section; 24: First pipe; 26: Hydrogen supply valve; 28: Exhaust valve;

[0322] 30: Second pipe; 32: Pressure gauge; 35: Scraper; 36: Strip scraper; 37: Wide blade; 39: Raw material;

[0323] 51: Sodium borate compounds; 52: Aluminum powder; 53: Non-oxidizing gas; 54: Fluorides;

[0324] 101: Aluminum; 101a: Surface; 101b: Aluminum oxide coating; 101c: Needle-shaped ι-alumina (0.67Na·6Al·9.33O) layer; 102: Sodium metaborate (NaBO2); 103: Sodium fluoride (NaF); 111: Sodium borohydride (SBH); 112: Sodium hydride (NaH); 113: Sodium oxide (Na2O); 115: Product layer (sodium borohydride (SBH), sodium hydride (NaH)).

Claims

1. A method for manufacturing sodium borohydride, characterized in that, When sodium borate compounds, aluminum powder, and fluoride powder are mixed in a sealed container filled with hydrogen gas and reacted at a temperature below 560°C, The mixture is stirred using a stir bar within the sealed container. The stirring height ratio (X), expressed by the following mathematical formula (I) determined by the minimum gap (a) between the stir bar and the bottommost part of the sealed container in the direction of gravity, and the material input height (b) of the material into the sealed container, is 75% or more. X=[(b-a) / b]×100 Formula (I).

2. The method for manufacturing sodium borohydride as described in claim 1, characterized in that, The sodium borate compound is selected from one or more of sodium metaborate and sodium tetraborate.

3. The method for manufacturing sodium borohydride as described in claim 1 or 2, characterized in that, The fluoride is selected from one or more of sodium fluoride (NaF), sodium hexafluoroaluminate (Na3AlF6), potassium fluoride (KF), potassium aluminum fluoride (KAlF4), aluminum fluoride (AlF3), and lithium fluoride (LiF).

4. The method for manufacturing sodium borohydride as described in claim 1 or 2, characterized in that, Regarding the aluminum in the aluminum powder, the molar ratio of aluminum in the aluminum powder to boron in the sodium borate compound is 4 / 3 or more.

5. The method for manufacturing sodium borohydride as described in claim 1 or 2, characterized in that, Further addition of alkali metal oxides or alkaline earth metal oxides, The molar ratio of the alkali metal, alkaline earth metal, and the alkali metal and alkaline earth metal contained in the sodium borate compound to the molar amount of boron contained in the sodium borate compound is 1.0 or more and 1.4 or less.

6. The method for manufacturing sodium borohydride as described in claim 1 or 2, characterized in that, Furthermore, before stirring, the sealed container is subjected to heat treatment at a temperature above 280°C and below 560°C.

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