Method for producing a dispersion containing crystalline boehmite alumina and method for producing crystalline boehmite alumina

Abstract

To provide a method for producing boehmite alumina having high crystallinity by utilizing short-time hydrothermal treatment.SOLUTION: A method for producing crystalline boehmite alumina includes: (a) a step of preparing a dispersion containing an alumina hydrogel; and (b) a step of adjusting the alumina hydrogel to a pH range of 10.6-12.6 with a basic compound, preparing a dispersion containing the basic alumina hydrogel, and then performing hydrothermal treatment to obtain a dispersion containing crystalline boehmite alumina.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing boehmite alumina having high crystallinity. In particular, it relates to a technique capable of producing alumina having high crystallinity by short-time hydrothermal treatment.

Background Art

[0002] Numerous reports have been made regarding the manufacturing method of boehmite alumina particles having high crystallinity.

[0003] Patent Document 1 describes that other forms of alumina (boehmite alumina precursors) are hydrothermally treated in the presence of an appropriate amount of boehmite seeding agents, preferably at a pH of about 5 or less, or for certain applications, at a pH of 8 or more. Thereby, it is said that appropriate microcrystalline boehmite, as well as microcrystalline sols and gels, can be produced.

[0004] Patent Document 2 describes that a solid mainly composed of alumina powder such as alumina hydrate is mixed with water or an aqueous solution of an inorganic compound and an oxygen-containing organic compound, and a wet powder in a three-phase state of solid-liquid-gas containing air is filled into a reaction vessel to perform a hydrothermal reaction. Thereby, it is said that the heat transfer rate can be improved and the temperature uniformity can be ensured, and furthermore, an alumina composition with good fluidity can be produced.

[0005] Patent Document 3 describes a crystalline alumina laminated particle in which a plurality of square plate-shaped alumina fine particles are laminated without at least two sides overlapping, and a method for producing the crystalline alumina laminated particle.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

[0007] However, conventional methods for producing crystalline boehmite alumina using hydrothermal treatment require relatively long processing times, leaving room for further improvement in industrial applications where mass production is required. Continuously obtaining large quantities of crystalline boehmite alumina in a short time is beneficial for industrial processes.

[0008] The object of the present invention is to provide boehmite alumina with high crystallinity using a manufacturing method that allows for obtaining boehmite alumina with high crystallinity by using short-time hydrothermal treatment. [Means for solving the problem]

[0009] The present invention, developed to solve the aforementioned problems and achieve the above objectives, is as follows. That is, the present invention is a. A step of preparing a dispersion containing alumina hydrogel, b. The alumina hydrogel is adjusted to a pH range of 10.6 to 12.6 with a basic compound to prepare a dispersion containing the basic alumina hydrogel, and then subjected to hydrothermal treatment to obtain a dispersion containing crystalline boehmite alumina. We propose a method for producing crystalline boehmite alumina, including [the specified ingredient].

[0010] Furthermore, regarding the method for producing crystalline boehmite alumina according to the present invention, (1) Furthermore, c1. A step of heating the dispersion containing the crystalline boehmite alumina to obtain a dried crystalline boehmite alumina product, c2. The process of adjusting the pH of the dispersion containing the crystalline boehmite alumina to a range of 3.6 to 5.0 with an acidic compound, followed by spray drying to obtain crystalline boehmite alumina powder, and c3. A step of removing residual salts contained in the crystalline boehmite alumina, followed by spray drying to obtain crystalline boehmite alumina powder, It must include one of the following processes: (2) The alumina hydrogel used in step a has an average particle size of 0.1 to 3.5 μm and a particle size ratio (D90 / D50) of 1.1 to 2.5, and the molar ratio of the basic compound to alumina in step b is 0.05 to 0.20. (3) The crystalline boehmite alumina dried product has a specific surface area of ​​80 to 125 m². 2 The requirements are: / g, crystallite size: 18~30 nm (180~300 Å), and crystal transition temperature from boehmite to γ-alumina: 470~490°C. (4) The crystalline boehmite alumina powder has an average particle size of 5 to 25 μm and a particle size ratio (D90 / D50) of 1.5 to 3.0. These are considered to be more desirable solutions. [Effects of the Invention]

[0011] By adopting the above configuration, the present invention makes it possible to produce boehmite alumina with high crystallinity by using short-time hydrothermal treatment. [Modes for carrying out the invention]

[0012] The following describes embodiments of the method for producing crystalline boehmite alumina according to the present invention.

[0013] [Crystalline boehmite alumina] The crystalline boehmite alumina according to this embodiment has a specific surface area of ​​80 to 125 m². 2It is in the range of / g. The crystallite size is in the range of 18 to 30 nm (180 to 300 Å). The phase transition temperature from boehmite to γ-alumina (from boehmite to γ) is in the range of 470 to 490 °C.

[0014] The crystalline boehmite alumina according to this embodiment usually has a powdery form when filtered and dried. Since it becomes a lump by filtration, it may be appropriately pulverized.

[0015] The average particle size of the powdery crystalline boehmite alumina measured by the laser diffraction / scattering method under the conditions adopted in the examples described later is preferably in the range of 5 to 25 μm, more preferably in the range of 10 to 20 μm.

[0016] When dried by the spray drying method, the average particle size of the powdery crystalline boehmite alumina measured by the laser diffraction / scattering method is preferably in the range of 5 to 25 μm, more preferably in the range of 10 to 20 μm. Also, the particle size ratio (D90 / D50) is in the range of 1.5 to 3.0, more preferably in the range of 1.5 to 2.8.

[0017] [Manufacturing method of crystalline boehmite alumina] The manufacturing method of the crystalline boehmite alumina according to this embodiment is a. A step of preparing a dispersion containing an alumina hydrogel; b. Adjusting the alumina hydrogel to a pH range of 10.6 to 12.6 with a basic compound, preparing a dispersion containing a basic alumina hydrogel, and then performing hydrothermal treatment to obtain a dispersion containing crystalline boehmite alumina. It is characterized by including the above.

[0018] The crystalline boehmite alumina of this embodiment can be obtained by hydrothermally treating a mixture of alumina hydrogel (hereinafter also referred to as aluminum hydroxide), a basic compound, and water. The aluminum hydroxide is not particularly limited and can be prepared from mineral acid aluminum (chloride, sulfate, nitrate) or aluminate salts, or treated as waste generated during anodizing production. The average particle size of the alumina hydrogel (aluminum hydroxide) is also not particularly limited, but a range of 0.1 to 3.5 μm, measured by laser diffraction and scattering, is preferred, and a range of 0.15 to 2.5 μm is more preferred. Within this range, the boehmite crystal growth rate during hydrothermal synthesis is industrially suitable. Here, the average particle size is defined as the particle size D50 at 50% cumulative volume.

[0019] Furthermore, the particle size ratio (D90 / D50) of the alumina hydrogel is preferably in the range of 1.1 to 2.5, and more preferably in the range of 1.2 to 2.2. Within this range, the boehmite crystal growth rate during hydrothermal synthesis is industrially suitable. Here, D90 represents the particle size at 90% accumulation by volume, and D50 represents the particle size at 50% accumulation.

[0020] The basic compound used in step b can be one or more selected from ammonia and alkali metals or alkaline earth metals such as sodium, potassium, calcium, barium, and strontium, including hydroxides, oxides, chlorides, sulfates, nitrates, phosphates, acetates, formates, and other organic acid salts. Among these, alkali metal or alkaline earth metal hydroxides are preferred, and sodium hydroxide is particularly preferred.

[0021] Furthermore, the amount of basic compound added in step b should be such that the pH can be adjusted within the range of 10.6 to 12.6. If the pH is below the lower limit, the crystal growth rate may decrease. On the other hand, if the pH is above the upper limit, excessive dissolution may occur, potentially reducing the particle size. Preferably, the pH is in the range of 11.0 to 12.0, and more preferably, in the range of 11.3 to 11.8.

[0022] It is preferable to add the basic compound in a molar ratio of 0.05 to 0.20 in the range of (basic compound) / Al2O3, calculated in terms of metal oxides (Na2O in the case of sodium) or ammonia content, relative to the alumina hydrogel contained in the dispersion. More preferably, the range is 0.07 to 0.18, and most preferably 0.08 to 0.16. Above the lower limit, the boehmite crystal growth rate becomes industrially suitable. On the other hand, below the upper limit, the boehmite particle size becomes sufficiently large.

[0023] The hydrothermal treatment used in step b is preferably carried out in a pressurized vessel such as an autoclave at a temperature of 140°C to less than 350°C, and more preferably in the range of 150°C to 220°C. The reaction time can be in the range of 1 to 50 hours, and more preferably in the range of 1 to 10 hours. Boehmite crystals are sufficiently formed by the hydrothermal reaction above the lower limit, and the hydrothermal reaction continues up to the upper limit, so the process can be carried out without wasting energy. The pressurized pressure is 5 to 18 × 10⁻⁶. 5 It is preferable to set it to around Pa.

[0024] The crystalline boehmite alumina of this embodiment can be subjected to hydrothermal treatment in various ways. Specifically, alumina hydrogel (aluminum hydroxide), a basic compound, and water can be mixed, filled into a container of the desired shape, and subjected to hydrothermal treatment as is. Alternatively, commercially available aluminum hydroxide can be pulverized to a desired particle size to prepare an aqueous dispersion (slurry), which can then be mixed with a basic compound and subjected to hydrothermal treatment. Furthermore, crystalline boehmite alumina can be produced by adding a thickener to a mixture of aluminum hydroxide, a basic compound, and water and subjecting it to hydrothermal treatment. There are no particular limitations on the thickener, but commonly used examples include polyvinyl alcohol, methylcellulose, gum arabic, diatomaceous earth, bentonite, polyacrylamide, polyethylene oxide, polyacrylic acid ester, and locust bean gum. Since the hydrothermal reaction is easily inhibited if the thickener is too acidic, weakly acidic to alkaline thickeners are preferred. Polyacrylic acid esters (emulsions) are particularly preferred because they promote the growth of boehmite crystals and can contribute to improved strength and porosity.

[0025] The crystalline boehmite alumina of this embodiment is presumed to be formed by the intercalation of plate-like or needle-like boehmite crystals, creating a interconnected structure. Furthermore, because continuous pores are formed, it is extremely porous, exhibiting excellent gas permeability, and can be suitably used in a variety of applications, particularly those requiring maximum porosity. At the same time, it is possible to save energy because it becomes an extremely porous molded body without firing. In addition, since it is plate-like or needle-like rather than granular or lumpy like conventional porous materials, it has a high specific surface area.

[0026] Here, the crystalline boehmite alumina dried product obtained by heat treatment (drying) (step c1) has a specific surface area of ​​80-125 m² as measured by the BET adsorption method. 2 The range is per g, with a crystallite size of 18-30 nm (180-300 Å) and a crystallization transition temperature from boehmite to γ-alumina of 470-490°C.

[0027] Furthermore, crystalline alumina (γ-alumina) can also be produced by calcining the crystalline boehmite alumina of this embodiment. By performing heat treatment at a temperature in the range of 500 to 750°C, preferably 550 to 700°C, for a period of 0.2 to 5.0 hours, and more preferably 0.5 to 2.0 hours, crystalline alumina (γ-alumina) powder can be obtained. In addition, since the boehmite molded body of the raw material itself has a porous structure of plate-like or needle-like intercrystals, the crystalline alumina obtained by calcination is extremely porous and can be used for a variety of applications similar to those of the crystalline boehmite alumina described above.

[0028] Alternatively, after hydrothermal treatment, an acidic compound may be added to a dispersion containing crystalline boehmite alumina to adjust the pH to a range of 3.6 to 5.0, and then spray-dried to obtain powdered crystalline boehmite alumina (step c2). Examples of acidic compounds include sulfuric acid, nitric acid, hydrochloric acid, formic acid, and acetic acid. Alternatively, residual salts contained in the crystalline boehmite alumina may be removed, followed by spray-drying to obtain crystalline boehmite alumina powder (step c3). To remove residual salts, the dispersion containing crystalline boehmite alumina can be dehydrated using a degassing plate filter, and the filter cake can be washed with ion-exchanged hot water. The conditions for spray drying are, for example, as follows: Spray inlet temperature: 200~450℃ Outlet temperature: 110~350℃

[0029] The powder obtained by spray drying may be allowed to cool to room temperature (for example, 0-40°C), then classified, and finally subjected to heat treatment.

[0030] Furthermore, after spray drying, the area may be further heated and fired.

[0031] The crystalline boehmite alumina powder obtained by heat treatment (drying) here preferably has an average particle size (D50) in the range of 5 to 25 μm, and more preferably in the range of 10 to 20 μm, as measured by laser diffraction and scattering. Furthermore, its particle size ratio (D90 / D50) is in the range of 1.5 to 3.0, and more preferably in the range of 1.5 to 2.8. [Examples]

[0032] The present invention will be described in detail below with reference to examples, but the present invention is not limited in any way to these examples. [Measurement method or evaluation method] The measurement methods and evaluation test methods used in the examples are as follows.

[0033] (Method for measuring the content of each element) For the mass spectrometry analysis of each element, sodium (Na) was analyzed using an atomic absorption spectrophotometer, while all other elements were analyzed using an inductively coupled plasma spectrometer. Specifically, the samples were heated with sulfuric acid and hydrofluoric acid until dry, the dried material was dissolved in concentrated hydrochloric acid, and then diluted with water to prepare solutions with a concentration of 10-100 ppm by mass. These solutions were then analyzed using an atomic absorption spectrophotometer (Z-2310) manufactured by Hitachi High-Tech Science Corporation and an inductively coupled plasma spectrometer (ICPS-8100) manufactured by Shimadzu Corporation.

[0034] (Average particle size of alumina) The particle size distribution of the sample was measured using a laser diffraction / scattering particle size distribution analyzer (LA-300) manufactured by Horiba, Ltd. Specifically, the sample was placed in a solvent (water) so that the light transmittance was in the range of 70-95%, and measurements were taken under the conditions of circulation speed: 2.8 L / min, ultrasonic irradiation: 3 minutes, and number of repetitions: 30. In the particle size distribution, the 50% cumulative diameter (D50) was adopted as the average particle size, and the 90% cumulative diameter (D90) was adopted. The particle size ratio was calculated as D90 / D50.

[0035] (Specific surface area of ​​alumina) Specific surface area (SA) was measured using BELSORP-mini Ver2.5.6 manufactured by Microtrac-Bell Co., Ltd. Specifically, samples pretreated with catalyst at 500°C for 1 hour were used, and nitrogen was used as the adsorbent gas. The specific surface area (SA) of alumina was calculated using the BET method.

[0036] (Measurement of crystal morphology and crystallite size) The crystallite size was determined using an X-ray diffractometer (RINT2100, manufactured by Rigaku Denki Co., Ltd.). The sample was compacted onto a non-reflective plate for measurement, and the crystalline state was measured by X-ray diffraction. The crystallite size of the sample was calculated using the Scherrer method from the diffraction peak of the (020) plane attributed to boehmite. The crystal structure of the sample was determined by comparing the diffraction peaks attributed to boehmite, γ-alumina, etc. Here, the diffraction peaks indicating the boehmite (020) and (120) plane crystal structures were measured at 2θ=14° and 2θ=28°, respectively, while the diffraction peak indicating the aluminum crystal structure attributed to the γ-alumina (440) plane was measured at 2θ=67°. The diffraction peaks indicating the byerlite (002) and (-132) planes were measured at 2θ=19° and 2θ=40°, respectively.

[0037] (Measurement of crystal phase transition temperature) The crystalline phase transition temperature from boehmite alumina to γ-alumina was determined by differential thermogravimetric analysis (TG-DTA), and the peak temperature of the endothermic reaction accompanied by weight change was used.

[0038] [Example 1] a-1. Step to prepare a dispersion containing alumina hydrogel 1. 3411g of aluminum sulfate 14-18 hydrate (manufactured by Kanto Chemical Co., Ltd., special grade, equivalent to 17% by mass of alumina) was added to 16000g of deionized water and stirred well until dissolved. Then, the mixture was heated to 80°C, and a sodium hydroxide solution prepared by diluting a 48% by mass sodium hydroxide aqueous solution (manufactured by Kanto Chemical Co., Ltd., special grade) 10 times was added until the pH reached 3.8-4.0, and the mixture was held for 1 hour. The resulting slurry was dehydrated using a deaeration plate filter. 30-60 L of deionized water at 60°C was passed through the prepared alumina hydrogel cake obtained on the plate filter under reduced pressure to obtain a washed alumina hydrogel 1 cake. The alumina concentration of the washed alumina hydrogel 1 cake was 11.3% by mass.

[0039] One cake of the washed alumina hydrogel was diluted with deionized water to an alumina concentration of 1% by mass, and the pH was adjusted to 0.8 with 1.38 nitric acid (manufactured by Kanto Chemical Co., Ltd., Grade 1). The particle size distribution measured showed an average particle diameter D50 of 0.17 μm and a particle size ratio D90 / D50 of 1.33. The results of various property analyses are shown in Table 1.

[0040] b. Step to obtain a dispersion containing crystalline boehmite alumina 1. Deionized water was added to the washed alumina hydrogel 1 cake obtained in step a above, and the mixture was diluted to a solid content concentration of 7.8% by mass. After thorough stirring, the sodium hydroxide solution was added, and the pH was adjusted to 11.6. Then, the mixture was placed in an autoclave reactor, heated to 170°C under stirring, and held under pressure for 4 hours to obtain a dispersion containing crystalline boehmite alumina 1. The pressurized pressure was 8 × 10⁻⁶. 5 It was set to approximately Pa.

[0041] c1. A step of heating the dispersion containing the crystalline boehmite alumina to obtain crystalline boehmite alumina powder 1. The dispersion containing crystalline boehmite alumina 1 obtained in step b above was dehydrated using a deaeration plate filter, and 50 L of ion-exchanged water at 60°C was passed through to obtain a washed crystalline boehmite alumina cake. The crystalline boehmite cake was dried in a forced-air box-type dryer at 120°C for 8 hours to obtain dried crystalline boehmite alumina product 1. Various property analyses were performed on dried crystalline boehmite alumina product 1, and the results are shown in Table 1.

[0042] c2. A dispersion containing the crystalline boehmite alumina 1 is adjusted to a pH range of 3.6 to 5.0 with an acidic compound, and then subjected to spray drying to obtain crystalline boehmite alumina powder 1. Furthermore, the dispersion containing crystalline boehmite alumina 1 obtained in step b above was adjusted to a pH of 4.0 with an acidic compound aqueous solution obtained by diluting a 96% by mass sulfuric acid aqueous solution (manufactured by Kanto Chemical Co., Ltd., special grade) 10 times. After preparing the raw material slurry in this manner, the raw material slurry was dried as droplets in a spray dryer with an inlet temperature of 250°C and an outlet temperature of 120°C to obtain crystalline boehmite alumina powder 1. The obtained crystalline boehmite alumina powder 1 was suspended in ion-exchanged water to a solid content concentration of approximately 1% by mass, and particle size distribution measurement was performed. The average particle size was 12 μm. The results of various property analyses are shown in Table 1.

[0043] [Example 2] In Example 1, the procedure was carried out similarly except that the holding time in the autoclave was 8 hours in step b, to obtain a dispersion containing crystalline boehmite alumina 2, dried crystalline boehmite alumina 2, and crystalline boehmite alumina powder 2. The results of various property analyses are shown in Table 1.

[0044] [Example 3] c3. After removing residual salts contained in crystalline boehmite alumina, the process involves spray drying to obtain crystalline boehmite alumina powder 3. In Example 1, the dispersion containing the obtained crystalline boehmite alumina 1 was dehydrated using a deaeration plate filter. Then, 35 L of ion-exchanged water at 60°C was passed through, and the remaining washing cake was slurryed with ion-exchanged water to a solid content concentration of 8.5% by mass. Subsequently, the raw material slurry was dried as droplets in a spray dryer with an inlet temperature of 250°C and an outlet temperature of 120°C to obtain crystalline boehmite alumina powder 3. The results of various property analyses are shown in Table 1.

[0045] [Example 4] The procedure was carried out similarly to that of Example 1, except that the pH of the alumina hydrogel dispersion before autoclaving was set to 11.1 in step b. A dispersion containing crystalline boehmite alumina 4, dried crystalline boehmite alumina 4, and crystalline boehmite alumina powder 4 were obtained. The results of various property analyses are shown in Table 1.

[0046] [Example 5] The procedure was carried out similarly to that of Example 1, except that the pH of the alumina hydrogel dispersion before autoclaving was set to 12.4 in step b. A dispersion containing crystalline boehmite alumina 5, dried crystalline boehmite alumina 5, and crystalline boehmite alumina powder 5 were obtained. The results of various property analyses are shown in Table 1.

[0047] [Example 6] a-2. 744 g of microcrystalline boehmite alumina powder (Sasol, Catapal-A, 78% by mass) was added to 18800 g of deionized water and thoroughly mixed. Then, the mixture was ground in an attritor for 1 hour to obtain a slurry. A 10-fold diluted 96% by mass sulfuric acid aqueous solution (Kanto Chemical Co., Ltd., special grade) was added to the slurry to adjust the pH to 3.4. The mixture was then heated to 80°C and held for 1 hour. The obtained slurry was dehydrated using a deaeration plate filter, and 30-60 L of deionized water at 60°C was passed through the prepared hydrogel cake obtained on the plate filter under reduced pressure to obtain a wash cake. The alumina concentration of the wash cake was 13.5% by mass. From step b onward, a dispersion containing crystalline boehmite alumina 6, a dried crystalline boehmite alumina product 6, and a crystalline boehmite alumina powder 6 were obtained in the same manner as in Example 1. The results of various property analyses are shown in Table 1.

[0048] [Comparative Example 1] In step a-1 of Example 1, 1487 g of sodium aluminate (manufactured by Kanto Chemical Co., Ltd., Grade 1, equivalent to 39% by mass of alumina) was added to 18000 g of deionized water and stirred well. Then, the mixture was heated to 80°C, and a 96% by mass aqueous sulfuric acid solution (manufactured by Kanto Chemical Co., Ltd., special grade), diluted 10 times, was added until the pH reached 10.3-10.5. The mixture was then held for 1 hour, and the procedure was carried out in the same manner as in Example 1. A dispersion containing crystalline boehmite alumina R1, dried crystalline boehmite alumina R1, and crystalline boehmite alumina powder R1 were obtained. The results of various property analyses are shown in Table 1.

[0049] [Comparative Example 2] The procedure was carried out similarly to that of Example 1, except that the pH of the alumina hydrogel dispersion before autoclaving was set to 13.0 in step b. A dispersion containing crystalline boehmite alumina R2, dried crystalline boehmite alumina R2, and crystalline boehmite alumina powder R2 were obtained. The results of various property analyses are shown in Table 1.

[0050] [Comparative Example 3] The procedure was carried out similarly to that of Example 1, except that the pH of the alumina hydrogel dispersion before autoclaving was set to 10.1 in step b. A dispersion containing crystalline boehmite alumina R3, dried crystalline boehmite alumina R3, and crystalline boehmite alumina powder R3 were obtained. The results of various property analyses are shown in Table 1.

[0051] [Comparative Example 4] In Example 6, the procedure was carried out similarly except that attritor grinding, pH adjustment, heating to 80°C, and holding for 1 hour were omitted. A dispersion containing crystalline boehmite alumina R4, dried crystalline boehmite alumina R4, and crystalline boehmite alumina powder R4 were obtained. The results of various property analyses are shown in Table 1.

[0052] [Table 1] [Industrial applicability]

[0053] The present invention provides a method for producing crystalline boehmite alumina, which makes it possible to provide boehmite alumina with high crystallinity by using a short-time hydrothermal treatment. As a result, the obtained crystalline boehmite alumina can be used in the fields of catalysts, polishing, cosmetics, fillers and filters blended into adhesives and paints, oxygen sensor partitions, easily processable ceramics, sliding members (impregnated with wax, etc.), evaporators such as fragrances, hydrogen storage members, and refractory coatings.

Claims

1. A first step of preparing a dispersion containing an alumina hydrogel in which the particle size ratio (D90 / D50) of the 90% cumulative diameter (D90) to the 50% cumulative diameter (D50) is 1.1 to 2.5 and the D50 is 0.1 to 3.5 μm, A second step involves preparing a dispersion containing basic alumina hydrogel by adjusting the pH of the dispersion containing the alumina hydrogel to 10.6 to 12.6 using a basic compound. A method for producing a dispersion containing crystalline boehmite alumina, comprising a third step of hydrothermally treating the dispersion of the basic alumina hydrogel.

2. A method for producing a dispersion containing crystalline boehmite alumina according to claim 1, characterized in that, in the second step, the molar ratio of the basic compound to the alumina hydrogel (in terms of Al₂O₃) (basic compound / Al₂O₃) is 0.05 to 0.

20.

3. A method for producing crystalline boehmite alumina, comprising the step of drying a dispersion containing crystalline boehmite alumina obtained in Claim 1.

4. A method for producing crystalline boehmite alumina, comprising the steps of adjusting the pH of a dispersion containing crystalline boehmite alumina obtained in Claim 1 to 3.6 to 5.0 by adding an acidic compound, and then spray-drying the dispersion containing crystalline boehmite alumina.

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