Method for producing 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, and more particularly to a technique for producing alumina having high crystallinity through a short hydrothermal treatment. [Background technology]

[0002] There have been many reports on the production methods of boehmite alumina particles having high crystallinity.

[0003] Patent Document 1 describes hydrothermal treatment of other forms of alumina (boehmite alumina precursors) in the presence of appropriate amounts 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 producing suitable microcrystalline boehmite and microcrystalline sols and gels.

[0004] Patent Document 2 describes a method of preparing a solid, mainly consisting of alumina powder such as alumina hydrate, by mixing it with water or an inorganic compound and an aqueous solution of an oxygen-containing organic compound, and then filling a reaction vessel with the wet powder in a three-phase state of solid, liquid, and gas, including air, to carry out a hydrothermal reaction. It is claimed that this method improves the heat transfer rate, ensures temperature uniformity, and also enables the production of an alumina composition with good fluidity.

[0005] Patent Document 3 describes crystalline alumina layered particles having a structure in which a plurality of tetragonal plate-like alumina fine particles are layered with at least two sides not overlapping, and a method for producing the crystalline alumina layered particles. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 01-069511 [Patent Document 2] International Publication No. 2007 / 135977 [Patent Document 3] Special Publication No. 2014-058438 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the conventional technology, the method for producing crystalline boehmite alumina using hydrothermal treatment requires a relatively long treatment time, and there is room for further improvement in industrialization where mass production is required. It is useful in industrial processes to continuously obtain a large amount of crystalline boehmite alumina in a short period of time.

[0008] An object of the present invention is to provide boehmite alumina having high crystallinity using a production method that can obtain boehmite alumina having high crystallinity by using a short-term hydrothermal treatment. [Means for solving the problem]

[0009] The present invention, which has been developed to solve the above problems and achieve the above objects, is as follows. a. providing a dispersion comprising an alumina hydrogel; b. adjusting the pH of the alumina hydrogel to a range of 10.6 to 12.6 with a basic compound to prepare a dispersion containing a basic alumina hydrogel, and then performing a hydrothermal treatment to obtain a dispersion containing crystalline boehmite alumina; The present invention proposes a method for producing crystalline boehmite alumina, comprising:

[0010] The method for producing crystalline boehmite alumina according to the present invention is as follows: (1)Furthermore, c1. heat-treating the dispersion containing the crystalline boehmite alumina to obtain a dried crystalline boehmite alumina product; c2. adjusting the pH of the dispersion containing the crystalline boehmite alumina to a range of 3.6 to 5.0 with an acidic compound, and then spray-drying the dispersion to obtain crystalline boehmite alumina powder; and c3. removing residual salts contained in the crystalline boehmite alumina, and then spray-drying the crystalline boehmite alumina to obtain crystalline boehmite alumina powder; The method comprises any one of the steps selected from the following: (2) the alumina hydrogel used in the 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 the step b is 0.05 to 0.20; (3) The dried crystalline boehmite alumina has a specific surface area of ​​80 to 125 m 2 / g, crystallite diameter: 18 to 30 nm (180 to 300 Å), and crystal transition temperature from boehmite to γ-alumina: 470 to 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. This is thought to be a more preferable solution. [Effects of the Invention]

[0011] According to the present invention, by adopting the above-described configuration, boehmite alumina having high crystallinity can be produced by using a short-time hydrothermal treatment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the method for producing crystalline boehmite alumina according to the present invention will be described.

[0013] [Crystalline boehmite alumina] The crystalline boehmite alumina according to this embodiment has a specific surface area of ​​80 to 125 m 2 / g range. 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 (boehmite to γ) is in the range of 470 to 490°C.

[0014] The crystalline boehmite alumina according to the present embodiment is usually in a powder form when filtered and dried. Since the crystalline boehmite alumina becomes lumpy after filtering, it may be crushed appropriately.

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

[0016] When dried by spray drying, the powdered crystalline boehmite alumina preferably has an average particle size of 5 to 25 μm, more preferably 10 to 20 μm, as measured by laser diffraction / scattering, and a particle size ratio (D90 / D50) of 1.5 to 3.0, more preferably 1.5 to 2.8.

[0017] [Method for producing crystalline boehmite alumina] The method for producing crystalline boehmite alumina according to this embodiment includes the following steps: a. providing a dispersion comprising an alumina hydrogel; b. adjusting the pH of the alumina hydrogel to a range of 10.6 to 12.6 with a basic compound to prepare a dispersion containing a basic alumina hydrogel, and then performing a hydrothermal treatment to obtain a dispersion containing crystalline boehmite alumina; The present invention is characterized in that it includes:

[0018] The crystalline boehmite alumina of this embodiment can be obtained by hydrothermal treatment of 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, or treated as waste generated during the production of anodized aluminum. The average particle size of the alumina hydrogel (aluminum hydroxide) is also not particularly limited, but is preferably in the range of 0.1 to 3.5 μm, and more preferably 0.15 to 2.5 μm, as measured by laser diffraction / scattering. 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] The particle size ratio (D90 / D50) of the alumina hydrogel is preferably in the range of 1.1 to 2.5, more preferably 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% volume accumulation, and D50 represents the particle size at 50% volume accumulation.

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

[0021] The amount of basic compound added in step b is an amount that can adjust the pH to a 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, resulting in a decrease in particle size. The pH is preferably in the range of 11.0 to 12.0, more preferably 11.3 to 11.8.

[0022] The content of a metal, expressed as metal oxide (NaO equivalent for sodium), or ammonia, in the alumina-equivalent content of the alumina hydrogel contained in the dispersion is preferably added in a molar ratio of (basic compound) / Al2O3 of 0.05 to 0.20. A more preferred range is 0.07 to 0.18, and most preferred range is 0.08 to 0.16. At or above the lower limit, the boehmite crystal growth rate is industrially suitable. At or 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 pressure vessel such as an autoclave at a temperature in the range of 140°C or higher but lower than 350°C, more preferably in the range of 150 to 220°C. The reaction time can be in the range of 1 to 50 hours, more preferably in the range of 1 to 10 hours. At a length equal to or greater than the lower limit, sufficient boehmite crystals are produced by the hydrothermal reaction, and the hydrothermal reaction continues up to the upper limit, so the treatment can be carried out without wasting energy. In addition, the applied pressure is 5 to 18 x 10 5 It is preferable to set the pressure to about 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 a desired shape, and then subjected to hydrothermal treatment. Alternatively, commercially available aluminum hydroxide can be crushed 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. Alternatively, crystalline boehmite alumina can be produced by adding a thickener to a mixture of aluminum hydroxide, a basic compound, and water and subjecting the mixture to hydrothermal treatment. The thickener is not particularly limited, and examples include commonly used thickeners such as polyvinyl alcohol, methyl cellulose, gum arabic, diatomaceous earth, bentonite, polyacrylamide, polyethylene oxide, polyacrylic esters, and locust bean gum. Strong acidity of the thickener tends to inhibit the hydrothermal reaction, so a weakly acidic to alkaline thickener is preferred. Polyacrylic esters (emulsions) are particularly preferred because they promote the growth of boehmite crystals, thereby improving strength and porosity.

[0025] The crystalline boehmite alumina of this embodiment is presumably formed by plate-like or needle-like boehmite crystals connecting to each other through an intergrowth structure. Furthermore, because continuous pores are formed, the alumina is highly porous and has excellent gas permeability, making it suitable for a variety of applications, particularly those requiring maximum porosity. Energy can also be saved because a highly porous molded body can be obtained without firing. Furthermore, because the alumina is plate-like or needle-like, rather than granular or lumpy like conventional porous bodies, it also has a high specific surface area.

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

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

[0028] Alternatively, after the hydrothermal treatment, an acidic compound may be added to the dispersion containing crystalline boehmite alumina to adjust the pH to a range of 3.6 to 5.0, followed by spray drying to obtain a powdered crystalline boehmite alumina powder (step c2). Examples of the acidic compound that can be used include sulfuric acid, nitric acid, hydrochloric acid, formic acid, and acetic acid. Alternatively, a step (step c3) may be performed in which residual salts contained in the crystalline boehmite alumina are removed, followed by spray drying to obtain a crystalline boehmite alumina powder. To remove the residual salts, the dispersion containing crystalline boehmite alumina can be dehydrated using a degassing plate filter or the like, and the filter cake can be washed with warm ion-exchanged water. The spray drying conditions 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 to 40°C), classified, and then subjected to heat treatment.

[0030] Furthermore, after spray drying, calcination may be further carried out by heat treatment.

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

[0032] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples in any way. [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) Mass analysis of each element was performed using an atomic absorption spectrophotometer for Na and an inductively coupled plasma spectrometer for elements other than Na. Specifically, the sample was heated to dryness after adding sulfuric acid and hydrofluoric acid, and the dried product was dissolved in concentrated hydrochloric acid and diluted with water to a concentration of 10 to 100 ppm by mass. This solution was 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 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 to 95%, and measurements were performed under the following conditions: circulation rate: 2.8 L / min, ultrasonic irradiation: 3 minutes, repetition number: 30 times. In the particle size distribution, the 50% cumulative diameter (D50) was used as the average particle diameter, and the 90% cumulative diameter (D90) was used. The particle size ratio was calculated as D90 / D50.

[0035] (specific surface area of ​​alumina) The specific surface area (SA) was measured using a Microtrac-Bell BELSORP-mini Ver. 2.5.6. Specifically, the catalyst was pretreated at 500°C for 1 hour, and nitrogen was used as the adsorbed 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 measured using an X-ray diffractometer (Rigaku Corporation: RINT2100). The measurement sample was pressed onto a non-reflective measurement plate and used as the observation sample. 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 crystalline structure of the sample was determined by comparing the diffraction peaks attributed to boehmite, γ-alumina, etc. The diffraction peaks representing the boehmite (020) and (120) crystal structures were measured at 2θ = 14° and 2θ = 28°, respectively, and the diffraction peak representing the aluminum crystal structure attributed to the γ-alumina (440) plane was measured at 2θ = 67°. The diffraction peaks representing the bayerite (002) and (−132) planes were measured at 2θ = 19° and 2θ = 40°, respectively.

[0037] (Measurement of crystal phase transition temperature) The crystal phase transition temperature from boehmite alumina to γ-alumina was determined by differential thermogravimetry (TG-DTA) and was taken as the endothermic peak temperature accompanying the weight change.

[0038] [Example 1] a-1. Step of preparing a dispersion containing alumina hydrogel 1 3411 g of aluminum sulfate 14-18 hydrate (Kanto Chemical Co., Ltd., special grade, 17% by mass in terms of alumina) was added to 16,000 g of ion-exchanged water and thoroughly stirred to dissolve. The mixture was then heated to 80°C, and a sodium hydroxide solution prepared by diluting 48% by mass of sodium hydroxide aqueous solution (Kanto Chemical Co., Ltd., special grade) 10 times was added until the pH reached 3.8-4.0, and the mixture was maintained for 1 hour. The resulting slurry was dehydrated using a degassing plate filter. 30-60 L of 60°C ion-exchanged water was passed through the prepared alumina hydrogel cake obtained on the plate filter under reduced pressure to obtain a washed alumina hydrogel cake. The alumina concentration of the washed alumina hydrogel cake was 11.3% by mass.

[0039] The washed alumina hydrogel cake was diluted with ion-exchanged water to an alumina concentration of 1% by mass, and the pH was adjusted to 0.8 with nitric acid 1.38 (Kanto Chemical Co., Ltd., Grade 1). The particle size distribution was measured, revealing an average particle size 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 of obtaining a dispersion containing crystalline boehmite alumina 1 Ion-exchanged water was added to the washed alumina hydrogel 1 cake obtained in the step a, and the solid content was diluted to 7.8% by mass. After thorough stirring, the sodium hydroxide solution was added to adjust the pH to 11.6. The mixture was then placed in an autoclave reactor, heated to 170°C with stirring, and held under pressure for 4 hours to obtain a dispersion containing crystalline boehmite alumina 1. The pressure applied was 8×10 5 The pressure was set to about Pa.

[0041] c1. A step of heat-treating 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 degassing 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 at 120°C for 8 hours in a fan-type box dryer to obtain dried crystalline boehmite alumina product 1. Various property analyses of dried crystalline boehmite alumina product 1 were carried out, and the results are shown in Table 1.

[0042] c2. A step of adjusting the pH of the dispersion containing the crystalline boehmite alumina 1 to a range of 3.6 to 5.0 with an acidic compound, and then spray-drying the dispersion to obtain crystalline boehmite alumina powder 1. The pH of the dispersion containing crystalline boehmite alumina 1 obtained in step b was adjusted to 4.0 with an acidic compound aqueous solution prepared by diluting 96% by mass of sulfuric acid aqueous solution (special grade, manufactured by Kanto Chemical Co., Inc.) 10 times. After preparing the raw slurry in this manner, the raw slurry was dried 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-exchange water to a solids concentration of approximately 1% by mass, and particle size distribution measurement was performed. The average particle diameter was 12 μm. The results of various property analyses are shown in Table 1.

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

[0044] [Example 3] c3. A process of removing residual salts contained in the crystalline boehmite alumina and then spray-drying the crystalline boehmite alumina to obtain crystalline boehmite alumina powder 3. In Example 1, the dispersion containing crystalline boehmite alumina 1 obtained was dehydrated using a degassing plate filter. Then, 35 L of ion-exchanged water at 60°C was passed through, and the remaining washed cake was slurried with ion-exchanged water to a solids concentration of 8.5 mass%. The raw slurry was then dried in droplets using 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 same procedure was carried out as in step b of Example 1, except that the pH of the alumina hydrogel dispersion before being placed in the autoclave was adjusted to 11.1. 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 same procedure was carried out as in step b of Example 1, except that the pH of the alumina hydrogel dispersion before being placed in the autoclave was adjusted to 12.4. 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 (Catal-A, manufactured by Sasol, concentration 78% by mass) was added to 18,800 g of ion-exchanged water and stirred thoroughly. The mixture was then pulverized in an attritor for 1 hour to obtain a slurry. A 96% by mass aqueous sulfuric acid solution (special grade, manufactured by Kanto Chemical Co., Ltd.) diluted 10-fold was added to the slurry to adjust the pH to 3.4. The mixture was then heated to 80°C and maintained for 1 hour. The resulting slurry was dehydrated using a degassing plate filter, and 30 to 60 L of 60°C ion-exchanged water was passed through the prepared hydrogel cake obtained on the plate filter under reduced pressure to obtain a washed cake. The alumina concentration of the washed cake was 13.5% by mass. From step b onwards, the same procedures as in Example 1 were carried out to obtain a dispersion containing crystalline boehmite alumina 6, a dried crystalline boehmite alumina product 6 and a crystalline boehmite alumina powder 6. 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 (Kanto Chemical Co., Ltd., Grade 1, 39% by mass in terms of alumina) was added to 18,000 g of ion-exchanged water and thoroughly stirred. The mixture was then heated to 80°C, and a 96% by mass aqueous sulfuric acid solution (Kanto Chemical Co., Ltd., Special Grade) diluted 10 times was added until the pH reached 10.3 to 10.5, and the mixture was then maintained for 1 hour. The same procedure as in Example 1 was repeated, except for this. A dispersion containing crystalline boehmite alumina R1, a dried crystalline boehmite alumina product R1, and a crystalline boehmite alumina powder R1 were obtained. The results of various property analyses are shown in Table 1.

[0049] Comparative Example 2 The same procedure was followed as in step b of Example 1, except that the pH of the alumina hydrogel dispersion before being placed in the autoclave was adjusted to 13.0. A dispersion containing crystalline boehmite alumina R2, a dried crystalline boehmite alumina R2, and a crystalline boehmite alumina powder R2 were obtained. The results of various property analyses are shown in Table 1.

[0050] Comparative Example 3 The same procedure was followed as in step b of Example 1, except that the pH of the alumina hydrogel dispersion before being placed in the autoclave was adjusted to 10.1. A dispersion containing crystalline boehmite alumina R3, a dried crystalline boehmite alumina product R3, and a crystalline boehmite alumina powder R3 were obtained. The results of various property analyses are shown in Table 1.

[0051] Comparative Example 4 The same procedure as in Example 6 was repeated, except that the attritor pulverization, pH adjustment, heating to 80°C, and holding for 1 hour were omitted. A dispersion containing crystalline boehmite alumina R4, a dried crystalline boehmite alumina product R4, and a 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 method for producing crystalline boehmite alumina according to the present invention uses a short-term hydrothermal treatment to provide boehmite alumina with high crystallinity, and as a result, the resulting crystalline boehmite alumina can be used in the fields of catalysts, abrasives, cosmetics, fillers to be blended into adhesives and paints, filters, oxygen sensor partitions, easily processable ceramics, sliding members (impregnated with wax, etc.), transpiration agents such as aromatics, hydrogen storage members, and fire-resistant coating materials.

Claims

1. A first step of preparing a dispersion containing an alumina hydrogel having a particle size ratio (D90 / D50) of a 90% cumulative diameter (D90) to a 50% cumulative diameter (D50) of 1.1 to 2.5, and the D50 being 0.1 to 3.5 μm; a second step of preparing a dispersion containing a basic alumina hydrogel by adjusting the pH of the dispersion containing the alumina hydrogel to 10.6 to 12.6 using a basic compound; and a third step of hydrothermally treating the dispersion of basic alumina hydrogel.

2. The method for producing a dispersion containing crystalline boehmite alumina according to claim 1, characterized in that in the second step, a molar ratio (basic compound / Al 2 O 3 ) of the basic compound to the alumina hydrogel (calculated as Al 2 O 3 ) is 0.05 to 0.

20.

3. A method for producing crystalline boehmite alumina, comprising a step of drying a dispersion liquid containing the crystalline boehmite alumina obtained as claimed 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 to the dispersion containing crystalline boehmite alumina; and then spray-drying the dispersion containing crystalline boehmite alumina.