Method for producing iota-alumina, and iota-alumina

Abstract

The present invention produces iota-alumina by a method having a simple configuration. This method for producing iota-alumina includes a reaction step in which solid starting materials comprising an alkali metal compound, a fluoride, and at least one material selected from among aluminum compounds and aluminum are mixed together and heated at a temperature of 560-720°C.

Description

Method for producing iota alumina and iota alumina

[0001] The present invention relates to a method for producing iota alumina and iota alumina.

[0002] Conventional methods for producing iota-alumina have required, for example, a step of mixing aluminum nitric oxide and carboxymethyl cellulose and heating the mixture to 1000°C or higher by a sol-gel method, which requires equipment that can withstand high temperatures (Non-Patent Document 1). Patent Document 1 also discloses a method for producing plate-shaped iota-alumina by oxidizing a melt of an alkali metal haloaluminate with oxygen gas or an oxygen-containing gas at a temperature of 400°C to 800°C (Patent Document 1). However, because an oxygen-containing gas needs to be introduced into the melt during heating, a reaction device that allows gas exchange during heating is required.

[0003] Japanese Patent Application Publication No. 05-201723

[0004] Mullite-type Na0.67Al6O9.33 and a discussion of iota-alumina, Journal of the European Ceramic Society 40 (2020) 4276-4280.

[0005] An object of the present invention is to provide a method for producing iota alumina having a simple configuration and iota alumina having a rectangular crystal habit obtained by the method.

[0006] The method for producing iota alumina according to the first aspect is characterized by comprising a reaction step of mixing solid raw materials containing an alkali metal compound, a fluoride, and at least one material selected from an aluminum compound and aluminum, and heat-treating the mixture at a temperature of 560°C to 720°C, and a washing step of introducing the product produced in the reaction step into a polar solvent.

[0007] In the first aspect, the fluoride is sodium hexafluoroaluminate (Na ３ AlF ６ ), potassium hexafluoroaluminate (K ３ AlF ６), potassium fluoride (KF), potassium aluminum fluoride (KAIF ４ ), aluminum fluoride (AlF ３ ) and lithium fluoride (LiF).

[0008] In the first aspect, the reaction step may be performed by heat treatment at a temperature of 580° C. or higher.

[0009] In a first embodiment, the alkali metal compound is potassium aluminum dioxide (KAiO ２ ), potassium borates, potassium hydride (KH), potassium borohydride (KBH ４ ), potassium carbonate (K ２ CO ３ ), potassium hydroxide (KOH), potassium fluoride (KF), potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium aluminum fluoride (KAIF ４ ) and potassium oxide (K ２ O) may be at least one selected from the group consisting of:

[0010] In the first aspect, the potassium borates are potassium metaborate (KBO). ２ ), potassium tetraborate (K ２ B ４ O ７ ) and potassium diborate (K ４ B ２ O ５ ) may be at least one selected from the above.

[0011] In a first embodiment, the aluminum compound is sodium aluminum dioxide (NaAlO ２ ), potassium aluminum dioxide (KAIO ２ ), alumina (Al ２ O ３ ), aluminum hydroxide (Al(OH) ３ ) and aluminum fluoride.

[0012] In the first aspect, the method for producing iota alumina may further include a washing step of introducing the product produced in the reaction step into a polar solvent.

[0013] In the first aspect, the polar solvent may be water.

[0014] The iota alumina of the first embodiment comprises a cubic crystalline habit.

[0015] According to the first aspect, it is possible to provide a method for producing iota alumina having a simple configuration and iota alumina having a rectangular crystal habit.

[0016] FIG. 1 is a SEM backscattered electron image of an iota alumina crystal according to a comparative example. FIG. 2 is a SEM backscattered electron image of an iota alumina crystal according to a comparative example. FIG. 3 is a SEM backscattered electron image of an iota alumina crystal according to an example. FIG. 4 is a SEM backscattered electron image of an iota alumina crystal according to an example. FIG. 5 is a SEM backscattered electron image of an iota alumina crystal according to an example. FIG. 6 is a SEM backscattered electron image of an iota alumina crystal according to an example. FIG. 7 is a SEM backscattered electron image of a wide area of ​​an iota alumina crystal according to an example. FIG. 8 is a process diagram showing a first manufacturing method of iota alumina. FIG. 9 is a process diagram showing a second manufacturing method of iota alumina. FIG. 10 is a process diagram showing a third manufacturing method of iota alumina. FIG. 11 is a process diagram showing a fourth manufacturing method of iota alumina. FIG. 12 is a partial cross-sectional view showing an example of a sealed container used in the first to fourth manufacturing methods. FIG. 13 is a partial cross-sectional view showing another example of a sealed container used in the first to fourth manufacturing methods. FIG. 14 is a partial cross-sectional view showing another example of a sealed container used in the first to fourth manufacturing methods. FIG. 15 is a partial cross-sectional view showing another example of a sealed container used in the first to fourth manufacturing methods. FIG. 16A is a front view of the appearance of a stirrer equipped with a plate-shaped (blade) stirrer using the sealed container shown in FIG. 13. FIG. 16B is a side view of the appearance of a stirrer equipped with a plate-shaped (blade) stirrer using the sealed container shown in FIG. 13. FIG. 17A is a photograph of the plate-shaped (blade) stirrer used in the sealed container shown in FIG. 13, in which the reaction product was removed from the container after stirring for a predetermined period of time. FIG. 17B is a photograph of the reaction product removed from the container after stirring for a predetermined period of time using the sealed container shown in FIG. 13. FIG. 18A is a photograph of the sealed container shown in FIG. 15 before rotating the horizontal wide paddle (WP) stirrer to produce iota alumina. Figure 18B is a photograph after iota alumina has been produced by rotating the horizontal wide paddle (WP) stirrer of the sealed vessel shown in Figure 15. Figure 19 is a reaction schematic diagram showing the iota alumina production reaction. Figure 20 is a reaction schematic diagram showing the iota alumina production reaction. Figure 21 is a reaction schematic diagram showing the iota alumina production reaction. Figure 22 is a reaction schematic diagram showing the iota alumina production reaction.Fig. 23 is a reaction schematic diagram showing an iota alumina production reaction. Fig. 24 is a reaction schematic diagram showing an iota alumina production reaction. Fig. 25 is a process diagram showing a fifth production method of iota alumina.

[0017] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings, but the present disclosure is not limited thereto. The components of the embodiments described below can be combined as appropriate. In addition, some components may not be used. Furthermore, the components in the embodiments described below include those that can be easily imagined by a person skilled in the art, those that are substantially the same, and those that are within the so-called equivalent range.

[0018] The method for producing iota-alumina according to this embodiment includes a reaction step and a washing step. ０．６７ Al ６ O ９．３３ (A is an alkali metal).

[0019] The reaction step is a step in which solid raw materials are mixed and heated.

[0020] The solid raw material contains at least one material selected from an alkali metal compound, a fluoride, an aluminum compound, and aluminum.

[0021] The alkali metal compounds contained in the solid raw materials include potassium aluminum dioxide (KAIO ２ ), potassium borates, potassium hydride (KH), potassium borohydride (KBH ４ ), potassium carbonate (K ２ CO ３ ), potassium hydroxide (KOH), potassium fluoride (KF), potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium aluminum fluoride (KAIF ４ ) and potassium oxide (K ２ At least one selected from the group consisting of hydroxybenzoates, ...

[0022] The potassium borates contained in the solid raw materials include potassium metaborate (KBO２ ), potassium tetraborate (K ２ B ４ O ７ ) and potassium diborate (K ４ B ２ O ５ ) can be used, but is not limited to these.

[0023] When potassium metaborate is used as the potassium borate, powdered potassium metaborate powder (potassium metaborate powder) is used. The maximum particle size of the potassium metaborate powder is preferably 100 μm or less. The potassium metaborate powder is obtained by grinding the powder to a certain degree and then passing it through a sieve with a mesh size of 100 μm. If the maximum particle size of the potassium metaborate powder exceeds 100 μm, the production efficiency of potassium borohydride may decrease. The maximum particle size of the potassium metaborate powder is more preferably less than 100 μm. In other words, the potassium metaborate powder is preferably obtained by passing it through a sieve with a mesh size of less than 100 μm (for example, a sieve with a mesh size of 50 μm or less). By setting the maximum particle size of the potassium metaborate powder to 100 μm or less, the production efficiency of iota alumina can be further improved.

[0024] Since potassium metaborate powder contains moisture, the amount of potassium metaborate powder is preferably increased by the mass of moisture relative to the amount required for synthesizing potassium borohydride.

[0025] The aluminum contained in the solid raw material can be fragments such as powder material or scrap material. The aluminum fragments can be, for example, scrap material such as cutting chips or waste materials. It is preferable that the aluminum fragments have as little content of metals nobler than aluminum as possible, which is an impurity. Furthermore, the aluminum contained in the solid raw material can be aluminum with an oxide coating (anodized aluminum) formed by electrolytic processing or the like.

[0026] The amount of aluminum contained in the solid raw material is preferably 110% or more in molar ratio relative to the amount required for synthesizing iota-alumina. Part of the excess aluminum is consumed by reaction with water, but this also contributes to increasing the opportunity for contact with potassium metaborate when the amount of aluminum as raw material decreases as the reaction progresses, thereby improving the yield. The excess aluminum can be recovered and reused as metallic aluminum in the washing step described below.

[0027] The average particle size of the aluminum powder contained in the solid raw material is preferably 1 μm or more, and the maximum particle size is preferably 10 mm or less. If the average particle size of the aluminum powder is less than 1 μm, it is prone to dust explosions and becomes difficult to handle, and the particles may easily adhere to each other and clump together. If the average particle size is greater than 10 mm, the specific surface area per mass becomes small, the reaction area decreases, and the initial reaction rate may be significantly reduced. The average particle size of the aluminum powder is more preferably 5 μm or more and 5 mm or less. In the present disclosure, the average particle size of the powder is the average particle size obtained as the particle size of the spherical equivalent diameter using a laser diffraction particle size distribution analyzer.

[0028] The aluminum compound contained in the solid raw material is sodium aluminum dioxide (NaAlO ２ ), potassium aluminum dioxide (KAIO ２ ), alumina (Al ２ O ３ ), aluminum hydroxide (Al(OH) ３ ) and aluminum fluoride (AlF ３ At least one selected from the following can be used.

[0029] The fluorides contained in the solid raw materials include sodium fluoride (NaF), sodium hexafluoroaluminate (Na ３ AlF ６ ), potassium hexafluoroaluminate (K ３ AlF ６ ), potassium fluoride (KF), potassium aluminum fluoride (KAIF ４ ), aluminum fluoride (AlF３ At least one selected from potassium fluoride (KF) and potassium aluminum fluoride (KAIF) can be used. ４ ) and potassium hexafluoroaluminate (K ３ AlF ６ ) is preferred, and potassium fluoride (KF) is most preferred. Potassium fluoride generates potassium fluoride (KF) in the reaction system and can react with the oxide film of aluminum particles. Therefore, by using potassium fluoride as the fluoride contained in the solid raw material, the yield of iota-alumina can be improved. Furthermore, since potassium fluoride (KF) does not contain aluminum, aluminum fluoride (AlF ３ Since the acidification of the reaction system due to the iota alumina can be suppressed, the yield of iota alumina can be further improved by using potassium fluoride (KF) as the fluoride contained in the solid raw material.

[0030] The addition of fluoride promotes the crystallization of iota-alumina. Specifically, fluoride reacts with the oxide film on the surface of the aluminum particles, improving the yield of iota-alumina, which is the reaction product, and promoting the crystallization of iota-alumina.

[0031] In the following description, the molar ratio of the total of the alkali metals and alkaline earth metals contained in the solid raw material to the boron (B) contained in the solid raw material may be referred to as A / B (molar ratio). In this embodiment, the A / B (molar ratio) of the raw materials is preferably 1.0 or more and 1.3 or less. When A / B exceeds 1.3, the amount of alkali metal ions in the reaction system becomes excessive, resulting in a reduced yield. When A / B is less than 1.0, the amount of boron in the reaction system becomes excessive, resulting in a reduced yield.

[0032] In the following description, the molar ratio of boron (B) contained in the solid raw material to the total of alkali metals and alkaline earth metals contained in the solid raw material may be referred to as B / A (molar ratio). That is, B / A is the reciprocal of A / B. In this embodiment, the B / A (molar ratio) of the raw material is preferably 0.76 or more, 1.0 or less, and 1.3 or less. If B / A is less than 0.76, the amount of alkali metal ions in the reaction system becomes excessive, resulting in a reduced yield. If B / A exceeds 1.0, the amount of boron in the reaction system becomes excessive, resulting in a reduced yield.

[0033] In the reaction step, a product containing iota alumina is obtained by heat treating the solid raw materials. In this disclosure, heat treatment refers to a process of heating. In the following description, heating temperature refers to the temperature during the heat treatment. The heating temperature can be the temperature measured by contacting a thermocouple with the outside of a sealed container into which the raw materials are placed.

[0034] The heating temperature in the reaction step is 560° C. or higher. If the heating temperature is lower than 560° C., the intermediate product potassium aluminum fluoride (KAIF ４ ) does not melt, and therefore potassium borohydride is not sufficiently produced. The heating temperature in the reaction step is preferably higher than 560°C, and more preferably 580°C or higher. This can improve the reaction rate and the production efficiency of potassium borohydride. The heating temperature in the reaction step is further preferably 620°C or higher. This can further improve the reaction rate without stirring in the reaction step and further improve the production efficiency of potassium borohydride.

[0035] The heating temperature in the reaction step is 720°C or lower. If the heating temperature is higher than 720°C, iota-alumina begins to change to beta-alumina, resulting in a decrease in yield. The heating temperature in the reaction step is more preferably 660°C or lower. By setting the heating temperature to 660°C or lower, aluminum can exist in a solid state, so that the aluminum does not become droplets but remains in a powder state, increasing the specific surface area of ​​the aluminum and accelerating the reaction. This ensures a sufficient reaction rate and excellent efficiency in producing iota-alumina.

[0036] The reaction step is carried out in a hydrogen-containing gas atmosphere. When a hydrogen atmosphere is used, the maximum value of the hydrogen gas pressure (maximum hydrogen pressure) in the reaction step is preferably in the range of 0.5 MPa to 10 MPa, more preferably in the range of 0.5 MPa to 1 MPa. By setting the maximum hydrogen pressure in this range, the production efficiency of potassium borohydride is excellent, and an increase in equipment costs can be suppressed because a reaction vessel or equipment with excellent pressure resistance is not required.

[0037] The washing step involves pouring the mixture of solid raw materials into a polar solvent. This step increases the concentration of iota alumina in the reaction product, resulting in iota alumina with fewer impurities.

[0038] In the washing step, the mixture of solid raw materials is stirred with a polar solvent, which can improve the dissolution rate of substances other than iota alumina.

[0039] The polar solvent is, for example, water, and preferably pure water such as ion-exchanged water. This allows potassium borohydride (KBH ４ ), potassium hydride (KH), potassium fluoride (KF), potassium borates, potassium aluminum dioxide (KAIO ２ Water-soluble impurities such as methyl methyl acrylate can be removed from the reaction product.

[0040] The water used as the polar solvent is not limited to pure water, and may be an acidic or alkaline aqueous solution. The pH of the water used as the polar solvent is preferably 2 or more and 11 or less, and more preferably 9 or more. When the pH of the water is 9 or more, potassium hexafluoroaluminate (K ３ AlF ６ Impurities that are insoluble in neutral water, such as fluorides, aluminum, etc., can be further eluted and removed from the reaction product.

[0041] When the polar solvent is water, the washing step is preferably carried out at a temperature of 80°C or higher and 100°C or lower. This temperature range can improve the rate at which impurities are eluted. The washing step may also be carried out at room temperature. In the following description of the washing step, unless otherwise specified, high temperature refers to a temperature of 80°C or higher and 100°C or lower, and room temperature refers to a temperature of 5°C or higher and 35°C or lower.

[0042] The polar solvent is not limited to water, and dimethyl sulfoxide may also be used. In this case, potassium borohydride (KBH ４ ) can be preferably removed.

[0043] The iota alumina according to this embodiment has a rectangular crystal habit. Here, "iota alumina has a rectangular crystal habit" means that at least a portion of the iota alumina has an average aspect ratio of 1 or more and 3 or less. The average aspect ratio of iota alumina refers to the arithmetic average of the aspect ratios of multiple iota alumina crystals. In calculating the average aspect ratio, multiple structures that clearly appear among the crystal habits appearing in a predetermined range of an SEM backscattered electron image are extracted as iota alumina crystals. Here, the imaging range of the SEM backscattered electron image used to calculate the average aspect ratio is a range in which at least 10 iota alumina crystals are extracted. The aspect ratio of the iota alumina crystals is measured as follows: The maximum length and the minimum length of the iota alumina crystals are measured in the SEM backscattered electron image. The aspect ratio is then calculated by dividing the maximum length by the minimum length. When the average aspect ratio is calculated for a plurality of imaging ranges, it can be said that the iota alumina has a cubic crystal habit if the average aspect ratio calculated for at least one imaging range is equal to or greater than 1 and equal to or less than 3. In other words, the iota alumina according to this embodiment may have a crystal habit of a shape other than cubic.

[0044] The crystal habit of iota alumina will be described in detail below with reference to the drawings.

[0045] 1 and 2 are SEM backscattered electron images of iota alumina crystals according to a comparative example. Here, FIGS. 1 and 2 are SEM backscattered electron images obtained by observing different regions of the product according to Test Example 2, which will be described later. As shown in FIGS. 1 and 2, in this case, the tabular iota alumina crystals shown in FIG. 1 and the rod-shaped iota alumina crystals shown in FIG. 2 were obtained. The tabular iota alumina crystals according to FIG. 1 had a maximum length of 0.499 μm or more and 1.111 μm or less, a minimum length of 0.018 μm or more and 0.047 μm or less, and an aspect ratio of 12.8 or more and 60.5 or less. The average aspect ratio in the imaged area of ​​FIG. 1 was calculated to be 25.4. The rod-shaped iota alumina crystals shown in Figure 2 had a maximum length of 0.879 μm or more and 1.444 μm or less, a minimum length of 0.298 μm or more and 0.400 μm or less, and an aspect ratio of 2.4 or more and 4.0 or less. The average aspect ratio in the image capture area of ​​Figure 2 was calculated to be 3.34.

[0046] Figure 3 is an SEM backscattered electron image of iota alumina crystals according to an example. The reaction product according to Figure 3 is the product according to Test Example 6, which will be described later. The rectangular iota alumina crystals according to Figure 3 had a maximum length of 0.58 μm or more and 3.78 μm or less, a minimum length of 0.43 μm or more and 1.30 μm or less, and an aspect ratio of 1.10 or more and 3.78 or less. The width of the rectangular iota alumina refers to the maximum or minimum length of the iota alumina. The average aspect ratio within the imaging range of Figure 3 was calculated to be 1.34.

[0047] Figure 4 is an SEM backscattered electron image of iota alumina crystals according to an example. The reaction product shown in Figure 4 is the product of Test Example 6, which will be described later. The rectangular iota alumina crystals shown in Figure 4 had a maximum length of 0.60 μm or more and 2.32 μm or less, a minimum length of 0.36 μm or more and 1.97 μm or less, and an aspect ratio of 1.01 or more and 2.97 or less. The average aspect ratio in the image capture area of ​​Figure 4 was calculated to be 1.69.

[0048] Figure 5 is an SEM backscattered electron image of iota alumina crystals according to an example. The reaction product shown in Figure 5 is the product of Test Example 6, which will be described later. The rectangular iota alumina crystals shown in Figure 5 had a maximum length of 1.21 μm or more and 4.65 μm or less, a minimum length of 0.58 μm or more and 3.69 μm or less, and an aspect ratio of 1.03 or more and 3.43 or less. The average aspect ratio in the imaged area of ​​Figure 5 was calculated to be 1.74.

[0049] Figure 6 is an SEM backscattered electron image of iota alumina crystals according to an example. The reaction product shown in Figure 6 is the product of Test Example 6, which will be described later. The rectangular iota alumina crystals shown in Figure 6 had a maximum length of 61.22 μm or more and 271.18 μm or less, a minimum length of 41.18 μm or more and 156.1 μm or less, and an aspect ratio of 1.00 or more and 3.32 or less. The average aspect ratio in the image capture area of ​​Figure 6 was calculated to be 1.86.

[0050] FIG. 7 is a wide-area SEM backscattered electron image of iota-alumina crystals according to the example. Here, the reaction product according to FIG. 7 is the product according to Test Example 6, which will be described later. Table 1 shows the results of measuring the composition ratio by EDX at measurement positions 1 to 5 shown in FIG. 7. From Table 1, it can be seen that at measurement positions 1, 2, and 5, the ratio of aluminum to potassium (Al / K) was 8.0 or more and 8.7 or less, which was close to the Al / K of iota-alumina, 9.0, and therefore it was confirmed that iota-alumina had been formed. The fluorine detected at measurement positions 1 to 5 was potassium aluminum fluoride (KAIF ４ ), potassium hexafluoroaluminate (K ３ AlF ６ These fluorine compounds are presumably the residues left behind when neutral water is used in the washing process of the product, which will be described later.

[0051]

[0052] Hereinafter, first to fourth manufacturing methods, which are specific examples of the manufacturing method of iota alumina according to the present embodiment, will be described in detail. In the following steps, unless otherwise specified, potassium fluoride is used as the fluoride, and potassium metaborate (KBO) separated as crystals is used as the potassium borate. ２ ) will be described. The first to fourth production methods described below are merely examples, and the present invention is not limited to these. For example, the method for producing iota alumina according to this embodiment may include only the third step (reaction step).

[0053] [First Manufacturing Method] FIG. 8 is a process diagram showing the first manufacturing method of iota alumina. As shown in FIG. 8, the manufacturing method of iota alumina according to the first manufacturing method includes steps from the first step (S-11) to the fourth step (S-14). The first step (S-11) is a step of mixing aluminum powder, fluoride powder, and potassium borate and charging the mixture into a sealed container. The second step (S-12) is a step of heating the mixture at 400°C to 610°C after the first step (S-11) and removing moisture with a vacuum pump. Here, the potassium borates and aluminum powder react while remaining in their solid phases. The third step (S-13) is a step of reacting the mixture at 560°C to 660°C in a sealed container filled with hydrogen gas after the third step (S-13). The potassium borates and aluminum powder react while remaining in their solid phases. The fourth step (S-14) is a step of washing the reaction product obtained in the third step (S-13). Here, the third step (S-13) is an example of a "reaction step," and the fourth step (S-14) is an example of a "washing step."

[0054] (a) First Step As a pretreatment step, after or before aluminum powder, potassium fluoride, and potassium borate powder are charged into a sealed container, a non-oxidizing gas is introduced into the sealed container to fill the interior with a non-oxidizing gas atmosphere. These steps constitute the first step (S-11). That is, in the first step, all raw materials are mixed and charged into the sealed container. After the treated powder and potassium borate powder are mixed and charged into the sealed container, the timing for filling the sealed container with a non-oxidizing gas atmosphere may be either after or before the raw materials are charged into the sealed container.

[0055] In the first step, the temperature inside the sealed container when the raw materials are charged is not particularly limited, but is preferably less than 100°C from the viewpoint of workability.

[0056] The sealed container used in the first step is a container that has heat resistance and pressure resistance that can withstand high temperatures (e.g., 630°C) and high pressures (e.g., 10 MPa) and can ensure a sealed space for filling with gas. The sealed container is equipped with a stirring means. Details of the sealed container will be described later.

[0057] (b) Second Step The second step (S-12) shown in FIG. 8 is a step in which all raw materials are mixed and charged into a sealed container, and the inside of the sealed container is heated to 400°C or higher and 610°C or lower. The second step is a step in which residual moisture contained in the raw materials is degassed, and moisture that could not be degassed is reacted with aluminum to convert it into hydrogen gas and an aluminum oxide film, thereby removing the moisture. The second step is a step in which vaporized moisture (i.e., residual moisture contained in the raw materials in the sealed container) is reacted with aluminum, or a step in which moisture is removed from the reaction system by degassing using a vacuum pump.

[0058] (c) Third Step The third step (S-13) shown in FIG. 8 is a step of heating the inside of the sealed container to 560°C or higher and 660°C or lower, and introducing hydrogen gas. By achieving this temperature, the cyclical reaction described above can occur. In the third step, the stirring means in the sealed container was rotated to stir. In this way, a reaction product was obtained. Note that when the inside of the sealed container is heated to 620°C or higher in the third step, stirring is not required because the reaction proceeds without stirring.

[0059] In the third step, the sealed container used in the first and second steps may be used as is, or a different sealed container may be used. That is, the first to third steps may be carried out as steps in a single sealed container, or may be steps in different sealed containers.

[0060] (d) Fourth Step The fourth step (S-14) shown in FIG. 8 is a step of washing the reaction product with a polar solvent. Here, the polar solvent is water. In the fourth step, the reaction product is stirred with water at 50°C or higher and 100°C or lower for 30 minutes or longer. This allows potassium borohydride (KBH ) to be extracted from the reaction product into the polar solvent. ４ ), potassium hydride (KH), potassium fluoride (KF), potassium borates and potassium aluminum dioxide (KAIO ２ ) is eluted. Then, the reaction product is removed from the solvent and dried under reduced pressure. In the fourth step, when an alkaline aqueous solution is used as the polar solvent, potassium hexafluoroaluminate (K ３ AlF ６ When an alkaline aqueous solution is used as the polar solvent, iota-alumina and aluminum hydroxide remain as solids, making it easier to extract the iota-alumina.

[0061] Iota alumina can be produced by the above first to fourth steps.

[0062] Another manufacturing method will be described below, and a description of the same points as in the first manufacturing method will be omitted.

[0063] [Second Manufacturing Method] FIG. 9 is a process diagram showing the second manufacturing method of iota alumina. As shown in FIG. 9, the second manufacturing method of iota alumina includes steps from step 1 (S-21) to step 4 (S-24). In step 1 (S-21), aluminum powder 52, potassium hydroxide powder 55, and fluoride powder 54 are mixed and preheated at a temperature of 100°C to 330°C, and then mixed with potassium borate powder 51 and charged into a sealed container. In step 2 (S-22), after step 1 (S-21), the mixture is heated to a temperature of 400°C to 610°C and moisture is removed using a vacuum pump. In step 3 (S-23), hydrogen gas is introduced after step 2 (S-22) and reacted at a temperature of 560°C to 660°C in a sealed container filled with hydrogen gas. The potassium borate powder 51 and aluminum powder 52 react while remaining in their solid phases. The fourth step (S-24) is a step of washing the reaction product obtained in the third step (S-23). ​​Here, the third step (S-23) is an example of a "reaction step," and the fourth step (S-24) is an example of a "washing step."

[0064] (a) First Step: In a pretreatment step, aluminum powder 52, fluoride powder 54, and potassium hydroxide powder 55 are mixed and rubbed together. After the treatment, the mixture is placed in a container, the lid is closed, and the atmosphere is kept at a humidity of 10% or less. The mixture is then placed in a furnace and heated in an atmosphere of 100°C to 330°C for 30 minutes or more. This causes some of the moisture contained in the raw materials to be released, and an aluminum oxide film is formed on the particle surfaces of the aluminum (Al) powder 52. These steps constitute the first step (S-21). That is, in the first step, aluminum and potassium hydroxide are premixed and heat-treated, and then all the raw materials are mixed and placed in a sealed container. In this first step, treatment in an atmosphere of 10% or less humidity prevents moisture from adhering to the aluminum, potassium hydroxide, and fluoride in the air, and also prevents oxidation of the aluminum.

[0065] (b) Second Step In the second step (S-22) shown in FIG. 9 , all of the mixed raw materials are loaded into a sealed container and the sealed container is heated to 400°C or higher and 610°C or lower. In the second step (S-22), the powder heat-treated in the first step (S-21) is cooled and then mixed with potassium borate powder. After or before the mixed powder is loaded into the sealed container, a non-oxidizing gas is introduced into the sealed container to fill the interior with a non-oxidizing gas atmosphere. Note that the timing for filling the sealed container with a non-oxidizing gas atmosphere after mixing the treated powder and potassium borate powder may be either after or before the raw materials are loaded into the sealed container. Thereafter, the sealed container is heated to 400°C or higher and 610°C or lower to degas the remaining moisture contained in the raw materials and react with aluminum to convert the moisture that could not be degassed into hydrogen gas and an aluminum oxide film, thereby removing the moisture. That is, the second step (S-22) is a step of reacting aluminum with residual moisture contained in the raw materials in the sealed container, or a step of removing moisture from the reaction system by degassing using a vacuum pump.

[0066] (c) Third Step The third step (S-23) shown in FIG. 9 is a step of heating the inside of the sealed container to 560°C or higher and 660°C or lower, and introducing hydrogen gas. In the third step, the stirring means in the sealed container was rotated to stir. In this way, a reaction product was obtained. Note that when the inside of the sealed container is heated to 620°C or higher in the third step, stirring is not required because the reaction proceeds without stirring.

[0067] (d) Fourth Step The fourth step (S-24) shown in FIG. 9 is a step of washing the reaction product with a polar solvent. Here, the polar solvent is water. In the fourth step, the reaction product is stirred with water at 80°C or higher and 100°C or lower for 30 minutes or longer. This allows potassium borohydride (KBH ) to be extracted from the reaction product into the polar solvent. ４ ), potassium hydride (KH), potassium fluoride (KF), low-grade aluminum fluoride (AlF １．５ ), potassium borates and potassium aluminum dioxide (KAIO ２ The reaction product is then removed from the solvent and dried under reduced pressure.

[0068] [Third Manufacturing Method] Figure 10 is a process diagram showing a third manufacturing method of iota alumina. As shown in Figure 10, the method includes steps from the first step (S-31) to the fourth step (S-34). The first step (S-31) is a step of mixing aluminum powder 52 and potassium hydroxide powder 55 and pre-heating them at a temperature of 100°C to 330°C. The second step (S-32) is a step of mixing the mixture with fluoride powder 54 and potassium borate powder 51 after the first step (S-31) and charging the mixture into a sealed container. The third step (S-33) is a step of heating the mixture after the second step (S-32) at a temperature of 400°C to 610°C, removing moisture with a vacuum pump, and then introducing hydrogen gas into a sealed container filled with hydrogen gas to react the mixture at a temperature of 560°C to 660°C. The potassium borate powder 51 and aluminum powder 52 react in their respective solid phases. The fourth step (S-34) is a step of washing the reaction product obtained in the third step (S-33). Here, the third step (S-33) is an example of a "reaction step," and the fourth step (S-34) is an example of a "washing step."

[0069] (a) First Step: In a pretreatment step, aluminum powder 52 and potassium hydroxide powder 55 are mixed and rubbed together. After the treatment, the mixture is placed in a container, the lid is closed, and the atmosphere is kept at a humidity of 10% or less. The mixture is then placed in a furnace and heated in an atmosphere of 100°C to 330°C for 30 minutes or more. This causes some of the moisture contained in the potassium hydroxide to be released into the aluminum (Al), and an aluminum oxide coating is formed on the particle surfaces of the aluminum (Al) powder 52. The above steps constitute the first step (S-31). That is, in the first step, aluminum and potassium hydroxide are premixed and heat-treated, and then all the raw materials are mixed and placed in a sealed container. In this first step, treatment in an atmosphere of 10% or less humidity can prevent moisture from the air from adhering to the aluminum, potassium hydroxide, and fluoride, and oxidation of the aluminum.

[0070] (b) Second Step The second step (S-32) shown in FIG. 10 is a step in which all mixed raw materials are charged into a sealed container and the sealed container is heated to 400°C or higher and 610°C or lower. In the second step (S-32), the powder heat-treated in the first step (S-31) is cooled, and then potassium borate powder and fluoride powder are mixed. After or before the mixed powder is charged into the sealed container, a non-oxidizing gas is introduced into the sealed container to fill the interior with a non-oxidizing gas atmosphere. After mixing the treated powder and potassium borate powder, the sealed container can be filled with a non-oxidizing gas atmosphere either before or after the raw materials are charged into the sealed container. The sealed container is then heated to 400°C or higher and 610°C or lower to degas residual moisture contained in the raw materials. That is, the second step (S-32) is a step of reacting aluminum with residual moisture contained in the raw materials in the sealed container, or a step of removing moisture from the reaction system by degassing using a vacuum pump.

[0071] (c) Third Step In the third step (S-33) shown in FIG. 10, the inside of the sealed container was heated to 560°C or higher and 660°C or lower, and hydrogen gas was introduced. In the third step, the stirring means in the sealed container was rotated to stir. In this way, a reaction product was obtained. Note that when the inside of the sealed container is heated to 620°C or higher in the third step, stirring is not required because the reaction proceeds without stirring.

[0072] (d) Fourth Step The fourth step (S-34) shown in FIG. 10 is a step of washing the reaction product with a polar solvent. Here, the polar solvent is water. In the fourth step, the reaction product is stirred with water at 80°C or higher and 100°C or lower for 30 minutes or longer. This allows potassium borohydride (KBH ) to be extracted from the reaction product into the polar solvent. ４ ), potassium hydride (KH), potassium fluoride (KF), potassium borates and potassium aluminum dioxide (KAIO ２ The reaction product is then removed from the solvent and dried under reduced pressure.

[0073] [Fourth Manufacturing Method] FIG. 11 is a process diagram showing the fourth manufacturing method of iota alumina. As shown in FIG. 11, the fourth manufacturing method of iota alumina includes steps from a first step (S-41) to a fourth step (S-44). The first step (S-41) involves mixing aluminum powder 52, potassium hydroxide powder 55, fluoride powder 54, and potassium borate powder 51, placing the mixture in a sealed container, filling it with a non-oxidizing gas, and then sealing it. The second step (S-42) involves heating the sealed container after the first step (S-41) to a temperature of 400°C to 610°C to react moisture generated from the raw materials with fluoride, generating gaseous hydrogen fluoride, which then reacts with aluminum to produce aluminum fluoride on the aluminum surface. The third step (S-43) involves carrying out the reaction at a temperature of 560°C to 660°C in a sealed container filled with hydrogen gas after the second step (S-42). The potassium borates and aluminum powder react in a solid phase. The fourth step (S-44) is a step of washing the reaction product obtained in the third step (S-43). Here, the third step (S-43) is an example of a "reaction step," and the fourth step (S-44) is an example of a "washing step."

[0074] (a) First Step As a pretreatment step, aluminum powder 52, potassium hydroxide powder 55, and fluoride powder 54 are mixed and rubbed together in an atmosphere with a humidity of 10% or less, and then potassium borate powder 51 is added. These are then charged into a sealed container. After or before charging, a non-oxidizing gas is introduced into the sealed container to fill the interior with a non-oxidizing gas atmosphere. This completes the first step (S-41). The timing for introducing the non-oxidizing gas may be after or before charging the raw materials into the sealed container. Note that the method for mixing the raw materials is not limited to this; all of the raw materials, i.e., aluminum powder 52, potassium hydroxide powder 55, fluoride powder 54, and potassium borate powder 51, may be mixed and rubbed together at the same time.

[0075] 11 is a step of heating the inside of the sealed container to 400° C. or more and 610° C. or less after sealing the sealed container. The second step is a step of removing residual moisture contained in the raw material by reacting it with aluminum (Al).

[0076] (c) Third Step The third step (S-43) shown in FIG. 11 is a step of heating the inside of the sealed container to 560°C or higher and 660°C or lower, and introducing hydrogen gas. In the third step, the stirring means in the sealed container was rotated to stir. In this way, a reaction product was obtained. Note that when the inside of the sealed container is heated to 620°C or higher in the third step, stirring is not required because the reaction proceeds without stirring.

[0077] (d) Fourth Step The fourth step (S-44) shown in FIG. 11 is a step of washing the reaction product with a polar solvent. Here, the polar solvent is water. In the fourth step, the reaction product is stirred with water at 80°C or higher and 100°C or lower for 30 minutes or longer. This allows potassium borohydride (KBH ) to be extracted from the reaction product into the polar solvent. ４ ), potassium hydride (KH), potassium fluoride (KF), potassium borates, and potassium aluminum dioxide (KAIO ２ The reaction product is then removed from the solvent and dried under reduced pressure.

[0078] (Sealed Container) Here, an example of a sealed container that can be used in the first to fourth manufacturing methods will be described.

[0079] Fig. 12 is a partial cross-sectional view showing an example of a sealed container used in the first to fourth manufacturing methods. As shown in Fig. 12, the sealed container 10A has a cylindrical container body 12 with a round bottom and a removable disk-shaped lid 14 that seals the container body 12. A temperature-controllable heater 16 is disposed on the outside of the lower part of the container body 12, and the contents of the container body 12 are heated by the heater 16. In addition, an O-ring 18 is disposed on the upper end surface of the container body 12, which is in close contact with the lid 14 to ensure airtightness of the interior. When the lid 14 is closed, the lid 14 is in close contact with the O-ring 18 against the container body 12.

[0080] The lid 14 has an opening in its center, a cylindrical portion erected near the opening, and a motor 20 disposed above the cylindrical portion. The motor 20 powers the stirring means of the sealed container 10A. The stirring means includes the motor 20, a stirring rod 22 connected to the rotation shaft of the motor 20, and a plurality of pin-shaped stirring bars 22A arranged in a direction perpendicular to the axis of the stirring rod 22. When the lid 14 is attached to the container body 12, the tip of the stirring rod 22 reaches a lower region inside the container body 12. In other words, when the motor 20 is driven, the pin-shaped stirring bars 22A rotate together with the stirring rod 22, stirring the contents of the container body 12.

[0081] The lid 14 is further provided with a first pipe 24 and a second pipe 30 that communicate with the inside of the container body 12. The first pipe 24 is connected to a hydrogen gas supply source (not shown) via a hydrogen gas supply valve 26 and to a vacuum pump (not shown) via an exhaust valve 28. That is, when the hydrogen gas supply valve 26 is opened, hydrogen gas is supplied into the container body 12, and when the exhaust valve 28 is opened, the inside of the container body 12 is degassed. The second pipe 30 is also connected to a pressure gauge 32, which allows the pressure inside the container body 12 to be measured.

[0082] Fig. 13 is a partial cross-sectional view showing another example of a sealed container used in the first to fourth manufacturing methods. The sealed container 10B of Fig. 13 differs from the sealed container 10A of Fig. 12 in the stirring means. In the sealed container 10B of Fig. 13, a stirring rod 22 that rotates inside the container body 12 is provided at the lower end with multiple plate-shaped scrapers 35, forming a multi-blade scraper (MB). That is, the stirring means of Fig. 13 includes the stirring rod 22 and multiple plate-shaped scrapers 35.

[0083] 16A is a front view of a stirring bar equipped with a plate-shaped (blade) stirring bar when using the sealed container shown in FIG. 13. FIG. 16B is a side view of a stirring bar equipped with a plate-shaped (blade) stirring bar when using the sealed container shown in FIG. 13. FIG. 17A is a photograph of a plate-shaped (blade) stirring bar removed from the sealed container shown in FIG. 13 after stirring for a predetermined period of time. FIG. 17B is a photograph of the reaction product removed from the sealed container shown in FIG. 13 after stirring for a predetermined period of time. As shown in FIG. 17A, the reaction product in the form of balls resting on the blade and an adhesion layer adhering to the container wall can be seen.

[0084] Fig. 14 is a partial cross-sectional view showing another example of a sealed container used in the first to fourth manufacturing methods. The sealed container 10C of Fig. 14 differs from the sealed container 10A of Fig. 12 in the stirring means. In the sealed container 10C of Fig. 14, a ribbon-shaped scraper 36 is provided at the lower end of the stirring rod 22 that rotates inside the container body 12. That is, the stirring means of Fig. 14 includes the stirring rod 22 and a plurality of plate-shaped scrapers 36.

[0085] FIG. 15 is a partial cross-sectional view showing another example of a sealed container used in the first to fourth manufacturing methods. The sealed container 10D of FIG. 15 differs from the sealed container 10A of FIG. 12 in that the container body 12 is horizontally placed and the stirring rod 22 rotating inside the container body 12b is provided with a wide paddle stirrer 37 via a support. That is, the stirring means of FIG. 15 includes the stirring rod 22 and the wide paddle stirrer 37. When the wide paddle stirrer 37 of the sealed container 10D of FIG. 15 is used for stirring, the raw material is scooped up by the paddle surface as the wide paddle stirrer 37 rotates. While the raw material is being scooped up by the paddle surface or while the scooped raw material falls, raw material particles collide with each other, resulting in a good reaction.

[0086] Fig. 18A is a photograph before iota alumina was produced by rotating the horizontal wide paddle (WP) stirrer of the sealed container shown in Fig. 15. Fig. 18B is a photograph after iota alumina was produced by rotating the horizontal wide paddle (WP) stirrer of the sealed container shown in Fig. 15.

[0087] Although an example of a sealed container that can be used in the first to fourth manufacturing methods has been described above, the sealed container that can be used in this embodiment is not limited to the sealed containers 10A to 10D described above. For example, a ball mill that can be heated and introduced with hydrogen gas may also be used as the sealed container. Furthermore, if stirring is not performed, the sealed container does not need to have a stirring means.

[0088] (Iota-alumina Production Reaction) The iota-alumina production reaction according to the present invention will be described in detail below.

[0089] In the reaction step, a production reaction occurs due to heat treatment of the solid raw material, and iota-alumina is produced. Figures 19 to 24 are reaction schematic diagrams that illustrate the iota-alumina production reaction. The production process of iota-alumina will be described in detail with reference to the reaction schematic diagrams (Figures 19 to 24). Note that the action described below is not limited to potassium fluoride. That is, for example, potassium aluminum fluoride (KAlF ４ Fluorides containing potassium, such as potassium fluoride (KF), are double salts containing potassium fluoride (KF), and therefore can supply potassium fluoride to the reaction system. Therefore, fluorides other than potassium fluoride can also have the same effect as potassium fluoride.

[0090] As shown in Fig. 19, in the early stage of the reaction, potassium fluoride (KF) 103 comes into contact with aluminum (Al) particles 101 due to stirring, and potassium fluoride (KF) adheres to the aluminum (Al) particles 101. This adhesion causes the aluminum oxide film 101b on the surface 101a of the aluminum (Al) particle to react with potassium fluoride (KF) 103 through the reactions of the following formulas (1) and (2), to form potassium aluminum fluoride (KAIF) ４ ) 105 and potassium oxide (K ２ O)113 is generated in the reaction process. ４ ), the generated potassium aluminum fluoride (KAIF ４ The reaction mixture 105 becomes liquid and adheres to the surface 101a of the aluminum (Al) particles. The reactions of the following formulas (1) and (2) proceed even in an atmosphere without hydrogen: KF + Al２ O ３ →K ２ O+AlF ３ ... (1) K ２ O+AlF ３ → KAlF ４ ... (2)

[0091] As shown in FIG. 20, potassium aluminum fluoride (KAIF) formed on the surface of the aluminum (Al) particle 101 ４ ) 105 is converted to potassium oxide (K ２ O)113 to form needle-shaped iota alumina (K ０．６７ Al ６ O ９．３３ ) 101c and potassium hexafluoroaluminate (K ３ AlF ６ ) is generated. ４ +28K ２ O→3K ０．６７ Al ６ O ９．３３ +K ３ AlF ６ ... (3) Potassium hexafluoroaluminate (K ３ AlF ６ ) is liquid potassium aluminum fluoride (KAIF ４ ) 105, it can come into contact with the aluminum (Al) particles 101. As a result, potassium hexafluoroaluminate (K ３ AlF ６ ) reacts with aluminum (Al) to form potassium aluminum fluoride (KAIF) via lower aluminum fluoride. ４ ) 105 and elemental potassium (K). Elemental potassium reacts with hydrogen to form potassium hydride (KH) according to the following formula (5): ３ AlF ６ +Al →Al2F ３ +3KF+K ３ AlF ６ → 3K + 3KAlF ４ ...(4) 2K+H ２ →2KH...(5)

[0092] As shown in FIG. 21, potassium hydride (KH) produced by the reaction of formula (5) is a gas and can move between particles. This allows the gaseous potassium hydride (KH) to be converted into potassium metaborate (KBO). ２ ) particle 102 and potassium metaborate (KBO ２ ) and potassium borohydride (KBH ４ ) 111 and potassium oxide (K ２ O) 113. Potassium borohydride (KBH ４ ) 111 and potassium oxide (K ２ O)113 is potassium metaborate (KBO ２ ) is generated so as to permeate the surface of the particles 102. ２ →KBH ４ +2K ２ O...(6)

[0093] As shown in FIG. 22, potassium aluminum fluoride (KAIF) produced by the reaction of formula (4) ４ ) also has vapor pressure above its melting point, so it can move between particles. This allows gaseous potassium aluminum fluoride (KAIF ４ ) is potassium metaborate (KBO ２ ) particle 102 and potassium metaborate (KBO ２ ) potassium oxide (K) on the surface of the particle 102 ２ O)113 reacts with iota alumina (K ０．６７ Al ６ O ９．３３ ) 101c and potassium hexafluoroaluminate (K ３ AlF ６ ) is generated. At this time, potassium metaborate (KBO ２ The droplet-like product 115 is formed on the surface of the particle 102. The droplet-like product 115 is formed by the liquid potassium aluminum fluoride (KAIF ４ ) 105, potassium hexafluoroaluminate (K ３ AlF ６ ) is dissolved in the solution. Here, potassium hexafluoroaluminate (K ３ AlF６ ) concentration increases, the liquid potassium aluminum fluoride (KAIF ４ ) is difficult to evaporate, so the gas potassium aluminum fluoride (KAIF ４ ) is more easily absorbed. ４ +28K ２ O→3K ０．６７ Al ６ O ９．３３ +K ３ AlF ６ ... (7)

[0094] As shown in FIG. 23, the above reaction produces potassium hexafluoroaluminate (K ３ AlF ６ As a result, the concentration of potassium aluminum fluoride (KAIF) on the surface 101a of the aluminum (Al) particle decreases. ４ ) 105 becomes easy to evaporate, and potassium metaborate (KBO) is evaporated from the surface 101a of the aluminum (Al) particle. ２ ) particles 102, potassium aluminum fluoride (KAIF ４ ) gases move more easily. On the other hand, liquid potassium aluminum fluoride (KAIF ４ ) 105 and potassium hexafluoroaluminate (K ３ AlF ６ The droplet-like product 115 containing the aluminum (Al) moves as a liquid to the surface 101 a of the aluminum (Al) particle, whereby the reactions of formulas (4) and (5) occur again.

[0095] Furthermore, potassium borohydride (KBH ４ ) also has a large vapor pressure at temperatures above 620°C, allowing it to move through space, and becomes potassium borohydride (KBH ４ ) is a mixture of potassium hydride (KH) and potassium oxide (K ２ O) to the surface of the aluminum.

[0096] As shown in FIG. 24, the above mechanism results in a cyclic reaction, and iota alumina (K ０．６７Al ６ O ９．３３ ) is produced. Here, iota alumina (K ０．６７ Al ６ O ９．３３ The inner part of the crystal grows needle-like, while the inner part of the iota alumina (K ０．６７ Al ６ O ９．３３ The outer part of the crystal grows in a rectangular shape. This results in the formation of iota alumina (K) crystals, whose inner part has an acicular habit and whose outer part has a rectangular habit. ０．６７ Al ６ O ９．３３ ) can be obtained.

[0097] [Fifth Manufacturing Method] Figure 25 is a process diagram showing the fifth manufacturing method of iota alumina. As shown in Figure 25, the fifth manufacturing method of iota alumina includes steps from a first step (S-51) to a fourth step (S-54). The first step (S-51) is a step of mixing a potassium compound, an aluminum oxide compound, and an aluminum potassium fluoride compound and placing the mixture in a sealed container in an atmosphere with a humidity of 10% or less. The second step (S-52) is a step of heating the mixture at 300°C to 560°C and removing moisture using a vacuum pump after the first step (S-51). The third step (S-53) is a step of reacting the mixture in a sealed container at 560°C to 720°C after the second step (S-52). The fourth step (S-54) is a step of washing the reaction product obtained in the third step (S-53). Here, the third step (S-53) is an example of a "reaction step", and the fourth step (S-54) is an example of a "washing step".

[0098] (a) First Step As a pretreatment step, a potassium compound, an aluminum oxide compound, and a potassium aluminum fluoride compound are mixed. After the treatment, the mixture is immediately placed in a container and the lid is closed. These steps constitute the first step (S-51). The aluminum oxide compound refers to a compound containing aluminum and oxygen, and is, for example, at least one selected from alumina and aluminum hydroxide. Note that alumite may be used in addition to or instead of the aluminum oxide compound. The potassium compound refers to an alkali metal compound containing potassium, and is, for example, potassium aluminum dioxide (KAIO ２ ), potassium borates, potassium hydride (KH), potassium borohydride (KBH ４ ), potassium carbonate (K ２ CO ３ ), potassium oxide (K ２ O), potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium aluminum fluoride (KAIF ４ Here, the potassium aluminum fluoride compound refers to a compound containing fluorine, aluminum, and potassium, and is, for example, potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium aluminum fluoride (KAIF ４ The aluminum potassium fluoride compound is an example of the fluoride and alkali metal compound according to the present disclosure.

[0099] (b) Second Step The second step (S-52) shown in Figure 25 is a step in which all of the mixed raw materials are charged into a sealed container and the inside of the sealed container is heated to 300°C or higher and 720°C or lower in an air atmosphere. In the second step (S-52), the inside of the sealed container is heated to 300°C or higher and 560°C or lower to reduce residual moisture contained in the raw materials. Here, the sealed container can be any of the sealed containers that can be used in the first to fourth manufacturing methods described above.

[0100] (c) Third Step In the third step (S-53) shown in Fig. 25, the raw materials are melted by heating the sealed container to 560°C or higher and 720°C or lower in an air atmosphere. In this way, a reaction product is obtained. Note that stirring is not required in the third step.

[0101] (d) Fourth Step The fourth step (S-54) shown in FIG. 25 is a step of washing the reaction product with a polar solvent. Here, the polar solvent is water. In the fourth step, the reaction product is stirred with water at 80°C or higher and 100°C or lower for 30 minutes or more. As a result, potassium hydride (KH), potassium fluoride (KF), and lower aluminum fluoride (AlF) are extracted from the reaction product and mixed with the polar solvent. １．５ ), potassium borates and potassium aluminum dioxide (KAIO ２ ) and the like are eluted. Thereafter, the reaction product is removed from the solvent and dried under reduced pressure. Note that the fourth step is not an essential step and may not be performed.

[0102] (Test Examples) Hereinafter, the present embodiment will be described in more detail with reference to test examples that illustrate the effects of the present embodiment, but the present embodiment is not limited to these.

[0103] Table 2 shows the raw materials, production conditions, and results of Test Examples 1 to 17. In the tests, products according to Test Examples 1 to 17 were synthesized using the materials and production conditions shown in Table 2. In Test Examples 1 to 17, products were produced using the first, third, and fourth steps of the first production method described above, except for the conditions specifically described below. That is, for example, Test Example 1 did not include the second step. In the following description, K / B refers to the A / B (molar ratio) when the alkali metal and alkaline earth metal contained in the raw materials is potassium (K). That is, in Table 2, B / K refers to the B / A (molar ratio) when the alkali metal and alkaline earth metal contained in the raw materials is potassium (K). In Table 2, "Al" refers to aluminum that has not been anodized, and "anodized" refers to aluminum that has been anodized. Furthermore, in Table 2, "hydrogen pressure" refers to the maximum hydrogen partial pressure (maximum hydrogen pressure) in the sealed container during the reaction process. The stirring speed refers to the peripheral speed of the stirring means described above. In Table 2 and the following examples, "pure water" refers to ion-exchanged water, and the pH of the ion-exchanged water is 5.5 or more and 7.5 or less.

[0104]

[0105] The PBH reaction rates shown in Table 2 were calculated based on the amount of hydrogen consumed in the reaction step, i.e., the difference between the maximum hydrogen pressure and the minimum hydrogen pressure under the reaction conditions. In Table 2, when the crystal habit of the obtained iota alumina contained tetragonal iota alumina, the crystal habit was recorded as "A," and when the crystal habit of the obtained iota alumina did not contain tetragonal iota alumina, the crystal habit was recorded as "B."

[0106] Table 3 is a list of substances detected in the products obtained by the production methods of Test Examples 1 to 17. Powder X-ray diffraction measurements were performed on the products of Test Examples 1 to 17, and the substances shown in Table 3 were identified. Here, unless otherwise noted, powder X-ray diffraction measurements were performed at room temperature and normal pressure, i.e., at 5°C to 35°C and 1 atm (101.325 kPa) in air. The intensity of the peaks attributable to inclusions in the diffraction pattern of the product depends on the component ratio of the inclusions in the product, so inclusions that appear as strong peaks and are clearly present were rated "A." Inclusions that appear as smaller peaks than in inclusions rated A and are estimated to have a small content were rated "B." Inclusions that appear as even smaller peaks than in inclusions rated B and are estimated to have a slight content were rated "C." Inclusions that appear even smaller than in inclusions rated C and are recognizable as peaks and are estimated to have a trace content were rated "D."

[0107]

[0108] Test Example 1 Test Example 1 is a comparative example. In the first step (pretreatment step), potassium metaborate (KBO) was used as a raw material, as shown in Table 2. ２ 7.24 g of potassium fluoride (KF) powder, 1.00 g of potassium fluoride (KF), and 3.817 g of aluminum (Al) powder were charged into a sealed container equipped with a stirring means. After charging, hydrogen gas was introduced into the sealed container as a non-oxidizing gas. Here, the K / B ratio of Test Example 1 was 1.19. That is, the B / K ratio of Test Example 1 was 0.84. In the third step (reaction step), the sealed container was heated to 522°C, and hydrogen gas was introduced and stirred at a stirring speed of 30 rpm using the stirring means in the sealed container. The maximum partial pressure of the hydrogen gas in the reaction step was 0.78 MPa. The fourth step (cleaning step) was not performed. Through the above steps, a product according to Test Example 1 was obtained.

[0109] As shown in Table 3, potassium metaborate (KBO) was found to be a component rated A in the product of Test Example 1. ２ ) and aluminum (Al) were detected, and iota alumina (K) was detected as a component of the B grade. ０．６７ Al ６ O９．３３ ) and potassium borohydride (KBH ４ ), potassium hexafluoroaluminate (K ３ AlF ６ ) was detected.

[0110] Test Example 2 is a comparative example. In Test Example 2, as shown in Table 2, a product according to Test Example 2 was obtained in the same manner as Test Example 1, except that washing with pure water at room temperature was performed as the fourth step (washing step).

[0111] As shown in Table 3, the product of Test Example 2 contained aluminum (Al) and iota alumina (K) as components rated A. ０．６７ Al ６ O ９．３３ ) was detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) and aluminum hydroxide (Al(OH) ３ ) was detected.

[0112] Test Example 3 is an example. As shown in Table 2, in Test Example 3, a product according to Test Example 3 was obtained in the same manner as Test Example 1, except that the heating temperature and maximum hydrogen pressure were as shown in Table 2.

[0113] As shown in Table 3, the product of Test Example 3 contained potassium metaborate (KBO) as a component rated A. ２ ) and aluminum (Al) were detected, and potassium borohydride (KBH ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) and potassium hexafluoroaluminate (K ３ AlF ６ ) was detected.

[0114] Test Example 4 Test Example 4 is an example. As shown in Table 2, in Test Example 4, a product according to Test Example 4 was obtained in the same manner as Test Example 3, except that the heating temperature was as shown in Table 2.

[0115] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 4. ４) and iota alumina (K ０．６７ Al ６ O ９．３３ ) and potassium metaborate (KBO ２ ) and aluminum (Al) were detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) was detected.

[0116] Test Example 5 Test Example 5 is an example. As shown in Table 2, in Test Example 5, a product according to Test Example 5 was obtained in the same manner as Test Example 3, except that the heating temperature was as shown in Table 2.

[0117] As shown in Table 3, potassium borohydride (KBH ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) and potassium metaborate (KBO ２ ) and aluminum (Al) were detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) was detected.

[0118] Test Example 6 is an example. As shown in Table 2, in Test Example 6, the heating temperature was as shown in Table 2, stirring was not performed, and washing with pure water at room temperature was performed as a washing step after the reaction step. Except for this, a product according to Test Example 6 was obtained in the same manner as Test Example 3.

[0119] As shown in Table 3, from the product of Test Example 6, iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and aluminum (Al) was detected as a component rated B.

[0120] Test Example 7 is an example. As shown in Table 2, in Test Example 7, a product according to Test Example 7 was obtained in the same manner as Test Example 3, except that the heating temperature in the reaction step was as shown in Table 2 and stirring was not performed.

[0121] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 7.４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium metaborate (KBO ２ ) and aluminum (Al) were detected.

[0122] Test Example 8 is an example. As shown in Table 2, in Test Example 8, a product according to Test Example 8 was obtained in the same manner as Test Example 3, except that the heating temperature in the reaction step was as shown in Table 2 and stirring was not performed.

[0123] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 8. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium metaborate (KBO ２ ) and aluminum (Al) were detected.

[0124] Test Example 9 is an example. As shown in Table 2, in Test Example 9, the product according to Test Example 8 was obtained in the same manner as Test Example 3, except that the heating temperature in the reaction step was as shown in Table 2.

[0125] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 9. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and potassium metaborate (KBO) was detected as a component of grade B. ２ ) and aluminum (Al) were detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) was detected.

[0126] Test Example 10 is an example. As shown in Table 2, in Test Example 10, a product according to Test Example 10 was obtained in the same manner as Test Example 6, except that the maximum hydrogen pressure was set as shown in Table 2.

[0127] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 10. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium metaborate (KBO ２ ) and aluminum (Al) were detected.

[0128] [Test Example 11] Test Example 11 is a comparative example. As shown in Table 2, in Test Example 11, a product according to Test Example 11 was obtained in the same manner as Test Example 6, except that no fluoride was added and the reaction step was carried out by stirring at a stirring speed of 30 rpm using a stirring means in a sealed container. In Test Example 11, K / B was 1.00. That is, B / K in Test Example 11 was 1.00.

[0129] As shown in Table 3, potassium metaborate (KBO) was found to be a component rated A in the product of Test Example 11. ２ ) and aluminum (Al) were detected.

[0130] Test Example 12 Test Example 12 is an example. As shown in Table 2, in Test Example 12, a product according to Test Example 12 was obtained in the same manner as in Test Example 6.

[0131] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 12. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium metaborate (KBO ２ ) and aluminum (Al) were detected.

[0132] Test Example 13 is an example. As shown in Table 2, in Test Example 13, 0.611 g of KAlF was used as the fluoride instead of potassium fluoride (KF) powder. ４ A product according to Test Example 13 was obtained in the same manner as Test Example 6, except that powder was used. In Test Example 13, K / B was 1.05. That is, B / K in Test Example 13 was 0.95.

[0133] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 13. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and potassium metaborate (KBO) was detected as a component of grade B. ２ ) and aluminum (Al) were detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) was detected.

[0134] Test Example 14 Test Example 14 is an example. As shown in Table 2, in Test Example 14, 0.481 g of aluminum fluoride (AlF) was used instead of potassium fluoride (KF) powder as the fluoride. ３ A product according to Test Example 14 was obtained in the same manner as in Test Example 6, except that the powder was used. In Test Example 14, K / B was 1.00. That is, B / K in Test Example 14 was 1.00.

[0135] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 14. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and potassium metaborate (KBO) was detected as a component of grade B. ２ ) and aluminum (Al) were detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) was detected.

[0136] Test Example 15 Test Example 15 is an example. As shown in Table 2, in Test Example 15, potassium metaborate (KBO) was used as the alkali metal compound.２ ) powder 4.43 g and potassium tetraborate (K ２ B ４ O ７ A product according to Test Example 15 was obtained in the same manner as in Test Example 6, except that 2.01 g of the powder was used. In Test Example 15, K / B was 1.00. That is, B / K in Test Example 15 was 1.00.

[0137] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 15. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, aluminum (Al) was detected as a component of the B grade, and potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium metaborate (KBO ２ ) and potassium tetraborate (K ２ B ４ O ７ ) was detected.

[0138] Test Example 16 Test Example 16 is an example. As shown in Table 2, in Test Example 16, potassium metaborate (KBO) was used as the alkali metal compound. ２ ) powder 3.04 g and potassium tetraborate (K ２ B ４ O ７ A product according to Test Example 16 was obtained in the same manner as Test Example 6, except that 2.98 g of the powder was used. In Test Example 16, K / B was 0.91. That is, B / K in Test Example 16 was 1.10.

[0139] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 16. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, aluminum (Al) was detected as a component of the B grade, and potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium metaborate (KBO ２ ) and potassium tetraborate (K２ B ４ O ７ ) was detected.

[0140] Test Example 17 is an example. As shown in Table 2, in Test Example 17, a product according to Test Example 17 was obtained in the same manner as Test Example 6, except that the amount of potassium fluoride (KF) powder was 1.50 g. In Test Example 17, K / B was 1.29. That is, B / K in Test Example 17 was 0.78.

[0141] As shown in Table 3, potassium borohydride (KBH) was found to be a component rated A in the product of Test Example 17. ４ ) and iota alumina (K ０．６７ Al ６ O ９．３３ ) was detected, and potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium metaborate (KBO ２ ) and aluminum (Al) were detected.

[0142] Test Example 18 Test Example 18 is an example. In the first step (pretreatment step), as shown in Table 2, 1.02 g of potassium hydroxide (KOH) powder and 1.02 g of potassium carbonate (K ２ CO ３ ) powder 2.10 g, and potassium aluminum fluoride (KAIF ４ ) powder 1.25 g and aluminum hydroxide (Al(OH) ３ ) powder and 5.20 g were charged into a sealed container. After charging, air was introduced into the sealed container. Here, B / K of Test Example 18 was 0. In the third step (reaction step), the sealed container was heated at 580°C. The maximum partial pressure of air in the reaction step was 1 atm (101.325 kPa). The fourth step (washing step) was not performed. Through the above steps, a product according to Test Example 18 was obtained.

[0143] As shown in Table 3, from the product of Test Example 18, iota alumina (K) was found to be a component rated A. ０．６７ Al ６ O ９．３３ ) was detected, and potassium hexafluoroaluminate (K３ AlF ６ ), alumina (χ-Al ２ O ３ ) and potassium aluminum oxide fluoride (K ２ Al ２ O ３ F ２ ) was detected.

[0144] Test Example 19 Test Example 19 is an example. As shown in Table 2, in Test Example 19, potassium aluminum fluoride (KAlF ４ A product according to Test Example 19 was obtained in the same manner as in Test Example 18, except that the amount of the 2.25 g powder was 0.53 g and the sealed container was heated to 650° C. in the third step (reaction step). In Test Example 19, B / K was 0.

[0145] As shown in Table 3, from the product of Test Example 19, iota alumina (K) was found to be a component rated A. ０．６７ Al ６ O ９．３３ ) was detected, and potassium hexafluoroaluminate (K ３ AlF ６ ), alumina (χ-Al ２ O ３ ) and potassium aluminum oxide fluoride (K2Al ２ O ３ F2) was detected.

[0146] Test Example 20 Test Example 20 is an example. As shown in Table 2, in Test Example 20, potassium aluminum fluoride (KAlF ４ A product according to Test Example 20 was obtained in the same manner as in Test Example 18, except that the amount of the powder was 1.34 g and the sealed container was heated to 720° C. in the third step (reaction step). In Test Example 20, B / K was 0.

[0147] As shown in Table 3, potassium hexafluoroaluminate (K ３ AlF ６ ) was detected, and iota alumina (K) was detected as a component of B grade. ０．６７ Al ６ O ９．３３ ) and alumina (χ-Al ２ O ３) was detected.

[0148] Test Example 21 Test Example 21 is a comparative example. In the first step (pretreatment step), aluminum hydroxide (Al(OH) ３ 5.20 g of the powder was charged into a sealed container. After charging, air was introduced into the sealed container. Here, B / K in Test Example 21 was 0. In the third step (reaction step), the sealed container was heated to 650°C. The maximum partial pressure of air in the reaction step was 1 atm (101.325 kPa). The fourth step (cleaning step) was not performed.

[0149] As shown in Table 3, the product of Test Example 21 contained alumina (χ-Al ２ O ３ ) was detected.

[0150] Test Example 22 Test Example 22 is an example. In the first step (pretreatment step), as shown in Table 2, potassium hexafluoroaluminate (K ３ AlF ６ ) powder 2.30 g, and potassium aluminum fluoride (KAIF ４ 1.08 g of bismuth oxide powder, 10.25 g of aluminum powder, and 8.56 g of anodized aluminum powder were charged into a sealed container. After charging, air was introduced into the sealed container. Here, B / K in Test Example 22 was 0. In the third step (reaction step), the sealed container was heated to 720°C. The maximum partial pressure of air in the reaction step was 1 atm (101.325 kPa). The fourth step (cleaning step) was not performed.

[0151] As shown in Table 3, from the product of Test Example 22, iota alumina (K) was found to be a component rated A. ０．６７ Al ６ O ９．３３ ) and potassium hexafluoroaluminate (K ３ AlF ６ ) was detected, and aluminum (Al) was detected as a component rated B.

[0152] As shown in Tables 2 and 3, iota-alumina was not produced in Test Examples 11 and 21, in which no fluoride was added, whereas iota-alumina was produced in Test Examples 1 to 10, 12 to 20, and 22, in which fluoride was added. This shows that iota-alumina can be produced by adding fluoride.

[0153] As shown in Table 2, iota alumina having a rectangular crystal habit was obtained in Test Examples 3 to 10, 12 to 20, and 22, in which the heating temperature was 560° C. or higher. On the other hand, iota alumina not having a rectangular crystal habit was obtained in Test Examples 1 and 2, in which the heating temperature was less than 560° C. This shows that iota alumina having a rectangular crystal habit can be obtained by setting the heating temperature to 560° C. or higher.

[0154] As shown in Table 2, in Test Examples 4 to 10 and 12 to 17 in which the heating temperature was 580° C. or higher, the PBH reaction rate was improved compared to Test Examples 1 to 3 in which the heating temperature was less than 580° C. This shows that iota alumina can be efficiently obtained by setting the heating temperature to 580° C. or higher.

[0155] As shown in Table 2, in Test Examples 6 to 8, 10, and 12 to 17 in which the heating temperature was 620° C. or higher, the PBH reaction rate was further improved without stirring compared to Test Examples 1 to 5 and 9 in which the heating temperature was less than 620° C. This shows that by setting the heating temperature to 620° C. or higher, iota alumina can be obtained more efficiently even without stirring.

[0156] As shown in Table 2, in Test Examples 15 and 17 where K / B was 1.00 or more, the PBH reaction rate was further improved compared to Test Example 16 where K / B was less than 1.00. This shows that iota alumina can be efficiently obtained by setting K / B to 1.00 or more.

[0157] As shown in Table 2, in Test Examples 15 and 17 where B / K was 1.00 or less, the PBH reaction rate was further improved compared to Test Example 16 where B / K was greater than 1.00. This shows that iota alumina can be efficiently obtained by setting B / K to 1.00 or less.

[0158] Iota alumina (K０．６７ Al ６ O ９．３３ ) has never been mass-produced, and as of January 2024, it is not even sold as a reagent. This is because there has been no inexpensive and highly pure production method. According to the method disclosed herein, potassium hexafluoroaluminate (K ３ AlF ６ ) to produce potassium aluminum fluoride (KAIF ４ ) between aluminum particles and potassium borates particles, and by heat treating inexpensive materials such as aluminum, potassium borates, and hydrogen gas, rectangular iota alumina (K ０．６７ Al ６ O ９．３３ ) into the valuable potassium borohydride (KBH ４ ) can be produced together with inexpensive iota alumina (K ０．６７ Al ６ O ９．３３ ) can be used as a raw material for producing beta alumina (β Alumina, β" Alumina). ０．６７ Al ６ O ９．３３ ) can be used as a composite material to improve the strength of resins and metals.

[0159] 10A to 10D Sealed container 12, 12b Container body 14 Lid 16 Heater 18 O-ring 20 Motor 22 Stirring rod 22A Stirring bar 24 First pipe 26 Hydrogen gas supply valve 28 Exhaust valve 30 Second pipe 32 Pressure gauge 35 Scraper 36 Ribbon-shaped scraper 37 Wide paddle stirrer 51 Potassium borate powder 52 Aluminum powder 54 Fluoride powder 55 Potassium hydroxide powder 101 Aluminum (Al) particle 101a Surface of aluminum (Al) particle 101b Aluminum oxide coating 101c Iota alumina (K ０．６７ Al ６ O ９．３３ ) 102 Potassium metaborate (KBO ２) particles 103 potassium fluoride (KF) 105 potassium aluminum fluoride (KAIF ４ ) 111 Potassium borohydride (KBH ４ ) 113 Potassium oxide (K ２ O) 115 product

Claims

1. A method for producing iota alumina, characterized by comprising a reaction step of mixing solid raw materials containing an alkali metal compound, a fluoride, and at least one material selected from an aluminum compound and aluminum, and then subjecting the mixture to a heat treatment at a temperature of 560°C to 720°C.

2. The fluoride is sodium hexafluoroaluminate (Na ３ AlF ６ ), potassium fluoride (KF), potassium aluminum fluoride (KAIF ４ ), potassium hexafluoroaluminate (K ３ AlF ６ ), aluminum fluoride (AlF ３ 2. The method for producing iota alumina according to claim 1, wherein the iota alumina is at least one selected from the group consisting of iota alumina (IOTA), ... and lithium fluoride (LiF).

3. The method for producing iota alumina according to claim 2, wherein the reaction step involves heat treatment at a temperature of 580°C or higher.

4. The alkali metal compound is potassium aluminum dioxide (KAIO ２ ), potassium borates, potassium hydride (KH), potassium borohydride (KBH ４ ), potassium carbonate (K ２ CO ３ ), potassium hydroxide (KOH), potassium fluoride (KF), potassium hexafluoroaluminate (K ３ AlF ６ ) and potassium aluminum fluoride (KAIF ４ ) and potassium oxide (K ２ The method for producing iota alumina according to any one of claims 1 to 3, characterized in that the iota alumina is at least one selected from the group consisting of iota alumina, ...

5. The potassium borates are potassium metaborate (KBO ２ ), potassium tetraborate (K ２ B ４ O ７ ) and potassium diborate (K ４ B ２ O ５ 5. The method for producing iota alumina according to claim 4, wherein the iota alumina is at least one selected from the group consisting of iota alumina, ...

6. The aluminum compound is sodium aluminum dioxide (NaAlO ２ ), potassium aluminum dioxide (KAIO ２ ), alumina (Al ２ O ３ ), aluminum hydroxide (Al(OH) ３ ) and aluminum fluoride (AlF ３ 4. The method for producing iota alumina according to claim 1, wherein the iota alumina is at least one selected from the group consisting of iota alumina, ...

7. A method for producing iota alumina according to any one of claims 1 to 3, further comprising a washing step of introducing the product produced in the reaction step into a polar solvent.

8. The method for producing iota alumina according to claim 7, wherein the polar solvent is water.

9. Iota alumina characterized by having a rectangular crystal habit.

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