Method for producing aqueous alumina dispersion

Abstract

To provide a method for producing an aqueous alumina dispersion that forms a highly stable aqueous alumina dispersion utilizing only the protective stability of water-soluble aliphatic amino acids and minimizes the content of alkali metal components and halogen elements.SOLUTION: In the aqueous alumina dispersion, (a) an average particle diameter of dispersed particles is in the range of 5 to 500 nm, (b) alumina particles when the alumina-containing aqueous dispersion is dried at 100°C are composed of one or more of boehmite and pseudo-boehmite, (c) a pH of the aqueous alumina dispersion is in the range of 2.0 to 5.3, (d) the aqueous alumina dispersion comprises a water-soluble aliphatic amino acid, (e) an alumina (Al2O3) equivalent concentration of aluminum contained in the aqueous alumina dispersion is in the range of 0.3 to 11.5 mass%, and (f) a time until the viscosity of the aqueous alumina dispersion becomes 100,000 mPa s or more when a base is added to the aqueous alumina dispersion to adjust the pH to 5.4 to 10 is 24 hours or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing an aqueous alumina dispersion. [Background technology]

[0002] Dispersions of solid fine particles such as alumina in liquids such as water are used in many applications. Alumina aqueous dispersions, in which alumina fine particles are dispersed in water, have a wide range of uses, but have many problems to be solved, such as problems caused by thickening of the aqueous dispersion, aggregation and sedimentation of alumina, and gelation after dispersion. In addition, their production methods often involve treating various aluminum salts with acid or alkali, but it is necessary to reduce the presence of residual components.

[0003] Japanese Patent Laid-Open Publication No. 112299 / 1978 (Patent Document 1) describes a method for obtaining an alumina sol by hydrothermal synthesis in the presence of a monovalent organic acid from an alumina gel obtained by neutralizing a water-soluble basic aluminum salt with an alkali. Japanese Patent Laid-Open Publication No. 111237 / 1982 (Patent Document 2) also proposes a method for producing an alumina sol by hydrothermally treating an alumina hydrate obtained by reacting a water-soluble aluminum salt with carbonic acid or a carbonate, and then either mixing the treated product with a monovalent organic acid and drying it, or drying the treated product and then mixing it with a monovalent organic acid. Both of these production methods require hydrothermal treatment, which requires high pressure at temperatures exceeding 120°C, requiring a dedicated pressure-resistant reactor, resulting in poor productivity.

[0004] Japanese Patent No. 4502133 (Patent Document 3) discloses a method for producing an acidic alumina sol, which includes: (A) a step of adding liquid or gaseous carbon dioxide to an aqueous alkali aluminate solution at a liquid temperature of 5 to 35°C to produce a reaction mixture having a pH of 10.5 to 11.2; (B) a step of hydrothermally treating the reaction mixture obtained in step (A) at a temperature of 110 to 250°C to produce an aqueous suspension containing alumina hydrate having a boehmite structure; and (C) a step of desalination of the aqueous suspension obtained in step (B) by adding water and an acid using an ultrafiltration method to form an acidic aqueous alumina sol having a pH of 3 to 7. Furthermore, Japanese Patent Laid-Open Publication No. 59-223223 (Patent Document 4) proposes a method for producing an alumina sol, which comprises neutralizing an aqueous solution of an alkali metal aluminate with an aqueous solution of an organic hydroxyl acid represented by the general formula: R(OH)(COOH)... (I), R(OH)(POH)... (II) (wherein m and n are 1 or greater, R is a saturated or unsaturated aliphatic group having 1 to 10 carbon atoms, and the molecular weight per acid group is 200 or less). Both of the production methods described in Patent Documents 3 and 4 use an alkali metal aluminate (e.g., sodium aluminate or potassium aluminate), which leaves a large amount of alkali metal remaining in the alumina sol after synthesis. This can cause a decrease in sintering temperature and abnormal crystal growth in the sintered compact when used in ceramic or refractory applications, potentially resulting in defects in the sintered compact. Furthermore, in Patent Document 3, the hydrothermal treatment temperature exceeds 140° C. in the examples and is high pressure, so that, as in Patent Documents 1 and 2, a dedicated pressure-resistant reactor is required, which results in poor productivity.

[0005] As a method for producing an aqueous alumina dispersion without using high temperatures and high pressures, Japanese Patent Laid-Open Publication No. 2000-109315 (Patent Document 5) describes a method for producing an aqueous alumina dispersion by mixing δ-alumina with a nitrogen-containing monocarboxylic acid and ultrasonically dispersing the mixture. The nitrogen-containing monocarboxylic acid used in Patent Document 5 contains an amino acid, and Patent Document 5 also describes an aqueous dispersion containing an amino acid and alumina. However, the technology in Patent Document 5 does not use an amino acid alone, but also contains other acid components (specifically, nicotinic acid, lactic acid, and acetic acid) and alcohol components in addition to the amino acid, which provide storage stability to the alumina particles (see, for example, the Examples of Patent Document 5). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 53-112299 [Patent Document 2] Japanese Patent Application Publication No. 57-111237 [Patent Document 3] Patent No. 4502133 [Patent Document 4] Japanese Patent Publication No. 59-223223 [Patent Document 5] Japanese Patent Application Laid-Open No. 2000-109315 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a method for producing a highly stable alumina aqueous dispersion by utilizing only the protective stability of a water-soluble aliphatic amino acid, and for use in refractories and ceramics, with minimal alkali metal and halogen element content. [Means for solving the problem]

[0008] That is, the present invention provides the following aspects: [1] A method for producing an aqueous alumina dispersion, comprising heating and concentrating an aqueous solution of amorphous aluminum hydroxide, a water-soluble aliphatic amino acid, and water at a temperature of 90 to 120°C for 5 to 36 hours, When the volume of the aqueous solution obtained by mixing the amorphous aluminum hydroxide, the water-soluble aliphatic amino acid, and water during production is defined as C, and the volume of the aqueous alumina dispersion obtained by heating and concentrating the dispersion at 90 to 120°C is defined as D, the value of D / C is controlled to be 0.1 to 0.8, The obtained aqueous alumina dispersion has the following characteristics: (a) the average particle size of the dispersed particles is in the range of 5 to 500 nm; (b) When the alumina-containing aqueous dispersion is dried at 100°C, the alumina particles are composed of one or more of boehmite and pseudo-boehmite, (c) the pH of the alumina aqueous dispersion is in the range of 2.0 to 5.3; (d) the alumina aqueous dispersion contains a water-soluble aliphatic amino acid; (e) The concentration of aluminum contained in the alumina aqueous dispersion in terms of alumina (Al2O3) is in the range of 0.3 to 11.5 mass %, and (f) A method for producing an aqueous alumina dispersion, characterized in that when a base is added to the aqueous alumina dispersion to adjust the pH to 5.4 to 10, it takes 24 hours or more for the viscosity of the aqueous alumina dispersion to reach 100,000 mPa·s or more. [2] The method for producing an aqueous alumina dispersion according to [1], wherein the water-soluble aliphatic amino acid is added in an amount of 1.5 to 5 moles per mole of aluminum in the amorphous aluminum hydroxide. [3] The method for producing an aqueous alumina dispersion according to [1] or [2], wherein the water-soluble aliphatic amino acid is glycine, alanine, valine, leucine, isoleucine, or a mixture thereof. [4] The method for producing an alumina aqueous dispersion according to [1] or [2], wherein the alumina aqueous dispersion has a thixotropic index (TI value) of 2 to 10, which is expressed as [viscosity at a rotation speed of 6 rpm] / [viscosity at a rotation speed of 60 rpm] measured with a Brookfield viscometer. [5] The method for producing an alumina aqueous dispersion according to [1] or [2], wherein the alumina aqueous dispersion contains 0 to 0.02 mass% of chlorine, bromine, or iodine, and 0 to 0.05 mass% of sodium or potassium. [6] The amorphous aluminum hydroxide can be prepared by adjusting an aqueous solution of an inorganic aluminum salt selected from aluminum nitrate and / or aluminum sulfate to a pH of 5 to 8 with aqueous ammonia to form a precipitate of amorphous aluminum hydroxide; washing the produced amorphous aluminum hydroxide with water; separating the amorphous aluminum hydroxide from water; The method for producing an aqueous alumina dispersion according to [1] or [2], which is obtained by drying the amorphous aluminum hydroxide so that the moisture content is 60 to 80 mass %. [Effects of the Invention]

[0009] The alumina aqueous dispersion of the present invention contains a water-soluble aliphatic amino acid. Since the amino acid has an amino group and a carboxyl group, it becomes a zwitterion in water. This acts as a protecting group to prevent the alumina in the alumina aqueous dispersion from suddenly flocculating or gelling.

[0010] Generally, the isoelectric point of alumina (the pH at which the zeta potential becomes 0) is considered to be approximately pH 9, and with ordinary acidic alumina sols, alumina particles rapidly aggregate and the aqueous alumina dispersion rapidly thickens when the pH approaches 9. However, the aqueous alumina dispersion of the present invention uses a water-soluble aliphatic amino acid, so even when the pH is changed to 5.4 to 10 using a base such as ammonia, gelation and other problems caused by thickening due to a sudden change in pH are prevented, and unlike ordinary protective agents, it is possible to ensure that the time required for the viscosity of the aqueous alumina dispersion to reach 100,000 mPa s or higher is at least 24 hours, thereby expanding the range of use. [Brief explanation of the drawings]

[0011] [Figure 1]1 is an electron microscope photograph of a dried alumina aqueous dispersion obtained in Example 1. [Figure 2] 1 is an X-ray diffraction image of the dried alumina aqueous dispersion obtained in Example 1. [Figure 3] 1 is an X-ray diffraction image of only glycine crystallized by drying the alumina aqueous dispersion used in Example 1. [Figure 4] 1 is an X-ray diffraction image of powder obtained by calcining the dried alumina aqueous dispersion obtained in Example 1 at 700° C. for 2 hours. [Figure 5] 1 is a graph showing the particle size distribution, based on the number of particles, of the aqueous alumina dispersion obtained in Example 1, determined by dynamic light scattering. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in detail below based on preferred embodiments, but the present invention is not limited to the following embodiments and various modifications are possible within the scope of the claims. In the present invention, the expression "numeric value 1 to numerical value 2" in a numerical range indicates that numerical value 1 is the lower limit and numerical value 2 is the upper limit. This means a range including both numerical values ​​1 and 2, and is synonymous with "numerical value 1 or more and numerical value 2 or less."

[0013] The present invention provides a method for producing an aqueous alumina dispersion containing alumina as dispersed particles. The aqueous alumina dispersion of the present invention can be obtained by heating and concentrating an aqueous solution of amorphous aluminum hydroxide, a water-soluble aliphatic amino acid, and water at a temperature of 90 to 120°C for 5 to 36 hours. In this process, the D / C value is controlled to be 0.1 to 0.8, where C is the volume of the aqueous solution obtained by mixing amorphous aluminum hydroxide, a water-soluble aliphatic amino acid, and water, and D is the volume of the aqueous alumina dispersion after heating and concentrating it at 90 to 120°C. Furthermore, the obtained aqueous alumina dispersion has the following characteristics: (a) the average particle size of the dispersed particles is in the range of 5 to 500 nm; (b) When the alumina-containing aqueous dispersion is dried at 100°C, the alumina particles are composed of one or more of boehmite and pseudo-boehmite, (c) the pH of the alumina aqueous dispersion is in the range of 2.0 to 5.3; (d) the alumina aqueous dispersion contains a water-soluble aliphatic amino acid; (e) The concentration of aluminum contained in the alumina aqueous dispersion in terms of alumina (Al2O3) is in the range of 0.3 to 11.5 mass %, and (f) When a base is added to the alumina aqueous dispersion to adjust the pH to 5.4 to 10, it takes 24 hours or more for the viscosity of the alumina aqueous dispersion to reach 100,000 mPa·s or more. It has.

[0014] The alumina aqueous dispersion of the present invention can be synthesized by heating and concentrating an aqueous solution of amorphous aluminum hydroxide, a water-soluble aliphatic amino acid, and water at a temperature of 90 to 120°C for 1 to 24 hours. In this case, the D / C ratio, where C is the volume of the aqueous solution of amorphous aluminum hydroxide, a water-soluble aliphatic amino acid, and water, and D is the volume after heating and concentrating at 90 to 120°C, is preferably 0.1 to 0.8, more preferably 0.2 to 0.7. If the D / C ratio is less than 0.1, insoluble precipitates will form in the alumina aqueous dispersion, making the alumina aqueous dispersion unstable. On the other hand, if the D / C ratio exceeds 0.8, the heating and concentrating will be insufficient, and the desired alumina aqueous dispersion will not be obtained.

[0015] (amorphous aluminum hydroxide) The amorphous aluminum hydroxide used to produce the alumina aqueous dispersion of the present invention can be obtained through a precipitation step in which an aqueous solution of an aluminum inorganic acid salt selected from aluminum nitrate and aluminum sulfate is adjusted to pH 5 to 8 with ammonia water to produce an amorphous aluminum hydroxide precipitate, a water washing step in which the obtained amorphous aluminum hydroxide is washed with water, a water separation step in which the amorphous aluminum hydroxide is separated from the water, and a drying step in which the amorphous aluminum hydroxide is dried to a moisture content of 60 to 80%.

[0016] In the first precipitation step, an amorphous aluminum hydroxide precipitation step for producing amorphous aluminum hydroxide involves adjusting the pH of an aqueous solution of an aluminum inorganic acid salt selected from aluminum nitrate and aluminum sulfate to a pH of 5 to 8 with aqueous ammonia. The pH in this step is preferably controlled to a pH of 6 to 7. If the pH is below 5, it takes a very long time for the amorphous aluminum hydroxide precipitate to settle, resulting in poor productivity. On the other hand, if the pH exceeds 8, poor dissolution occurs when adding water to the amorphous aluminum hydroxide, adding a water-soluble aliphatic amino acid, and heating to concentrate.

[0017] The precipitation step for forming an amorphous aluminum hydroxide precipitate is performed by stirring at a temperature in the range of 15 to 45°C for 0.5 to 3 hours. Preferably, the temperature is 20 to 35°C for 1 to 2 hours. If the temperature in the precipitation step is less than 15°C, the reactivity of aluminum nitrate or aluminum sulfate with ammonia will be poor, and the reaction will take a long time. If the temperature exceeds 45°C, the ammonia will volatilize rapidly, and the pH will not stabilize. Furthermore, if the stirring time is less than 0.5 hours, the reaction time will be insufficient and amorphous aluminum hydroxide will not precipitate sufficiently. If the stirring time exceeds 3 hours, amorphous aluminum hydroxide can be synthesized, but the reaction time will be too long, resulting in poor productivity.

[0018] After obtaining the amorphous aluminum hydroxide precipitate from the precipitation step, the amorphous aluminum hydroxide precipitate is washed with water (water washing step), and then the water is separated (water separation step). The amorphous aluminum hydroxide is then subjected to a drying step in which it is dried to a moisture content of 60 to 80% by mass. The moisture content in the drying step is preferably 65 to 75% by mass. If the moisture content is less than 60% by mass, dispersibility will be poor when adding a water-soluble aliphatic amino acid and water to the amorphous aluminum hydroxide and reacting them. If the moisture content exceeds 80% by mass, the amorphous aluminum hydroxide will have too much moisture content, making it difficult to handle and reducing productivity. The moisture content is measured using an MX50 manufactured by A&D Co., Ltd. at 105°C for 40 minutes.

[0019] The drying method for controlling the moisture content of amorphous aluminum hydroxide to 60 to 80 mass % is not particularly limited, and the heating temperature and heating time are also not particularly limited as long as they do not cause deterioration of the amorphous aluminum hydroxide. Specifically, the heating temperature used in the drying step is 70 to 110°C, preferably 80 to 100°C, for 1 to 12 hours, preferably 2 to 8 hours.

[0020] (Water-soluble aliphatic amino acids) The water-soluble aliphatic amino acid used in the production of the alumina aqueous dispersion of the present invention can be specifically represented by R-CH(NH2)COOH (chemical formula 1). In chemical formula 1, R is C n H 2n+1 where n is an integer selected from 0 to 3. The water-soluble aliphatic amino acid is specifically one or more selected from glycine, alanine, valine, leucine, and isoleucine. In Chemical Formula 1 of the water-soluble aliphatic amino acid, if n is 4 or more, the thermal decomposition property at 600°C is poor, and the effect of the invention cannot be achieved.

[0021] (Method of producing aqueous alumina dispersion) In the present invention, the alumina aqueous dispersion is obtained by mixing the amorphous aluminum hydroxide, water-soluble aliphatic amino acid, and water to form an aqueous solution, which is then heated and concentrated at a temperature of 90°C to 120°C for 5 to 36 hours. The heating method for the heating and concentration is not particularly limited, and conventional heating methods or an autoclave can be used. Heating is performed with stirring. Conventional stirring methods are acceptable. The reaction temperature and reaction time are preferably 95°C to 110°C. The reaction time is preferably 5 to 24 hours. If the temperature is below 90°C, the reaction will not proceed sufficiently to produce an alumina aqueous dispersion. On the other hand, if the temperature exceeds 120°C, a special reaction environment, such as applying pressure, will be required to increase the temperature, which will result in reduced productivity. If the heating time is less than 1 hour, concentration will be insufficient and an alumina aqueous dispersion may not be produced. If the heating time exceeds 36 hours, the heating and concentration time will be too long, resulting in reduced productivity.

[0022] The water-soluble aliphatic amino acid is added in an amount of 1.5 to 5 moles, more preferably 1.9 to 3 moles, per mole of aluminum in the amorphous aluminum hydroxide. If the amount of water-soluble aliphatic amino acid added is less than 1.5 moles per mole of aluminum, the amount of water-soluble aliphatic amino acid added will be insufficient, causing alumina to aggregate. If the amount exceeds 5 moles, the amount of water-soluble aliphatic amino acid added will be excessive, causing the excess water-soluble aliphatic amino acid to precipitate during heating and concentration.

[0023] (Characteristics of alumina water dispersion) The alumina particles contained in the alumina aqueous dispersion of the present invention have the following six characteristics: (a) The average particle size of the dispersed particles is in the range of 5 to 500 nm. (b) When the alumina-containing aqueous dispersion is dried at 100°C, the alumina particles are composed of one or more of boehmite and pseudo-boehmite, (c) The pH of the alumina aqueous dispersion is in the range of 2.0 to 5.3. (d) The alumina aqueous dispersion contains a water-soluble aliphatic amino acid. (e) The alumina (Al2O3) equivalent concentration contained in the alumina aqueous dispersion is in the range of 0.3 to 11.5 mass %. (f) When a base is added to the aqueous alumina dispersion to adjust the pH to 5.4 to 10, it takes 24 hours or more for the viscosity of the aqueous alumina dispersion to reach 100,000 mPa·s or more.

[0024] Feature (a) The average particle size of the alumina particles is preferably 5 to 500 nm, more preferably 10 to 200 nm. If the average particle size is less than 5 nm, the binding strength with inorganic particles will be poor, resulting in poor shape retention during molding. On the other hand, if the average particle size exceeds 500 nm, the average particle size will be too large, and when inorganic powders with primary particles of several hundred nm are bound together and fired, they will become foreign matter and become the starting point for fracture. Here, the "average particle size" refers to the average particle size measured by number using a Zetasizer Nano ZS (manufactured by Spectris Inc.) in an aqueous solvent by dynamic light scattering.

[0025] Feature (b) The crystalline system of alumina in the aqueous alumina dispersion of the present invention is preferably such that when the aqueous alumina dispersion is dried at 100°C, the particles are composed of one or more of boehmite and pseudo-boehmite, because they undergo a phase transition to become γ-alumina or α-alumina depending on the firing conditions during high-temperature sintering of ceramics, etc.

[0026] Feature (c) The pH of the aqueous alumina dispersion of the present invention is 2.0 to 5.3, more preferably 3.0 to 5.0. When the pH of the aqueous alumina dispersion is less than 2.0, it becomes strongly acidic, which can alter the inorganic particles when mixed with them or cause corrosion when the aqueous alumina dispersion comes into contact with metals. On the other hand, when the pH of the aqueous alumina dispersion is between 5.3 and 10, it takes 24 hours or more for the viscosity to reach 100,000 mPa·s or higher, making it possible to mold refractories and prepare catalyst carriers. However, when the pH of the aqueous alumina dispersion exceeds 10, it takes less than 24 hours for the viscosity to reach 100,000 mPa·s or higher, making the dispersion unsuitable for storage stability in terms of molding refractories or preparing catalyst carriers. The pH of the aqueous alumina dispersion is measured using a portable pH meter "HM-40P" and a pH combination electrode "GST-2739C" (manufactured by DKK Toa Corporation).

[0027] Feature (d) As described above, the alumina aqueous dispersion of the present invention contains the water-soluble aliphatic amino acid used in forming the alumina aqueous dispersion. As described above, the water-soluble aliphatic amino acid is one or more selected from glycine, alanine, valine, leucine, and isoleucine. It is designed to have good thermal decomposition properties at low temperatures. Alumina aqueous dispersions containing lactic acid, malic acid, etc. have poor thermal decomposition properties and leave carbon residues, making them unsuitable for thermal decomposition at relatively low temperatures (e.g., 700°C). The water-soluble aliphatic amino acid crystallizes when the water content of the alumina aqueous dispersion is evaporated. Therefore, when checking the crystal system of the alumina described in feature (b) above, the alumina is dried at 100°C, and in this case, the water-soluble aliphatic amino acid may crystallize.

[0028] Feature (e) The Al2O3-equivalent concentration of aluminum in the alumina in the alumina aqueous dispersion of the present invention is in the range of 0.3 to 11.5 mass%, preferably 2 to 10 mass%. If the Al2O3-equivalent concentration is less than 0.3 mass%, the alumina concentration in the alumina aqueous dispersion is too low, resulting in poor adhesiveness when used as a binder for inorganic particles. On the other hand, if the Al2O3-equivalent concentration exceeds 11.5 mass%, the storage stability of the alumina aqueous dispersion will be poor, and precipitates may form in the alumina aqueous dispersion over time. The Al2O3-equivalent concentration in the alumina aqueous dispersion of the present invention is calculated as follows.

[0029] The aluminum concentration in the alumina aqueous dispersion is measured using an aluminum concentration meter (Agilent Technologies: ICP-OCS5110) and converted into alumina (Al2O3) using the following formula: The atomic weight of aluminum is 27, the atomic weight of oxygen is 16, and the molecular weight of Al2O3 is 102. The Al2O3 equivalent concentration (mass%) = Al concentration (mass%) × (102 / 27)

[0030] Feature (f) When the aqueous alumina dispersion of the present invention is adjusted to a pH of 5.4 to 10 by adding a base (e.g., ammonia), it takes 24 hours or more for the viscosity of the aqueous alumina dispersion to reach 100,000 mPa·s or greater. The water-soluble aliphatic amino acid described in feature (d) above contains both an amino group and a carboxyl group, and thus forms a zwitterion in water. Therefore, even when the pH is adjusted to 5.4 to 10 using a base such as ammonia, gelation due to a sudden pH change in the aqueous dispersion is prevented, and the time required for the viscosity to reach 100,000 mPa·s or greater can be ensured to be 24 hours or greater, preferably 24 to 720 hours, and more preferably 30 to 240 hours, unlike conventional protective agents. This feature broadens the usable pH range of the aqueous alumina dispersion and extends the time it maintains a usable viscosity, enabling the formation of ceramics and refractories by adding a slurry at neutral to basic pHs.

[0031] (Other features) The aqueous alumina dispersion of the present invention has a thixotropic index (TI value) of 2 to 10. The thixotropic index, measured using a Brookfield viscometer (using a TVB10M TM-3 spindle rotor manufactured by Toki Sangyo Co., Ltd.), is expressed as TI value = [viscosity at a rotation speed of 6 rpm] / [viscosity at a rotation speed of 60 rpm]. The thixotropic index is preferably 2 to 10, and more preferably 3 to 6. If the TI value is less than 2, the liquid will lack structural viscosity, making it difficult to maintain its shape when a mixture of inorganic particles and the aqueous alumina dispersion is placed in a mold for molding, resulting in deformation. On the other hand, if the TI value exceeds 10, the structural viscosity will be too strong, making it difficult to deform unless the shear rate is increased, making it difficult to perform pouring or other molding operations using a mold.

[0032] The alumina aqueous dispersion of the present invention has a low content of halogen elements and alkali metal elements. The halogen element and alkali metal element contents were measured by drying the alumina aqueous dispersion at 80°C for 30 minutes and measuring the powder obtained by drying using a scanning electron microscope (JEOL Ltd. JCM-7000 equipped with an energy dispersive X-ray fluorescence (EDX) microscope). The halogen element (specifically, chlorine, bromine, or iodine) content is preferably 0 to 500 ppm, more preferably 0 to 200 ppm. If the halogen element content exceeds 500 ppm, when used in refractories, ceramics, etc., gases derived from the elements are generated during firing, which tends to cause corrosion inside the furnace. Furthermore, the alkali metal element (specifically, sodium or potassium) content is preferably 0 to 500 ppm, more preferably 0 to 200 ppm. If the alkali metal content exceeds 500 ppm, when used in refractories, ceramics, etc., the sintering temperature decreases during firing, resulting in defects in the sintered body.

[0033] The alumina aqueous dispersion of the present invention must be designed to select materials that do not contain halogen gases such as chlorine and bromine that can damage furnaces when fired at high temperatures for ceramics or refractories; alkali metal elements such as sodium and potassium that can lower the sintering temperature of ceramics or refractories; or phosphorus, silica, etc. that can create different crystal phases.

[0034] The alumina aqueous dispersion of the present invention thermally decomposes at low temperatures without leaving any organic residue. To confirm that the alumina aqueous dispersion of the present invention thermally decomposes at low temperatures, a sample of the alumina aqueous dispersion was dried at 100°C for 1 hour and heated in a platinum pan (TG-DTA60 manufactured by Rigaku Corporation) at an air flow rate of 100 ml / min at a temperature increase rate of 10°C / min over a set range of 25 to 650°C to confirm the state of thermal decomposition. The temperature at which weight loss is complete is preferably in the range of 400 to 700°C, more preferably 450 to 650°C. The firing time is preferably in the range of 2 to 300 minutes, more preferably 5 to 60 minutes. If the temperature at which weight loss is complete is less than 400°C, decomposition will begin when resins or the like are blended and molding is performed at a temperature of around 300°C, resulting in an unstable molded product. On the other hand, if the temperature at which weight loss is complete exceeds 700°C, residual carbon is likely to remain inside when firing thick refractories or ceramics, posing the issue of defects and cracks. If the temperature at which weight loss is complete is less than two minutes, the rate of thermal decomposition is too fast when firing large refractories or ceramics at the specified temperature, which can lead to firing variations between the edges and center. On the other hand, if the temperature at which weight loss is complete exceeds 300 minutes, productivity can be reduced.

[0035] In the present invention, amorphous aluminum hydroxide gel as a main raw material is hydrated, a water-soluble aliphatic amino acid is added, and the resulting aqueous solution is heated and concentrated at 90 to 120°C for 5 to 36 hours to obtain an aqueous alumina dispersion having a pH of 2.0 to 5.3, an average particle size measured on a number basis in the range of 5 to 500 nm, an alumina (Al2O3) equivalent concentration of 0.3 to 11.5 mass%, and a thixotropic index (TI value) in the range of 2 to 10. The aqueous alumina dispersion of the present invention can be calcined at a temperature of 400 to 700°C for 2 to 300 minutes to complete the thermal decomposition of organic matter, and can be provided as an aqueous alumina dispersion for ceramics and refractories applications in which the time required for the viscosity of the aqueous alumina dispersion to reach 100,000 mPa s or higher is 24 hours or more when a base such as ammonia is added to adjust the pH to 5.4 to 10.

[0036] The alumina aqueous dispersion of the present invention is designed to have a reduced content of halogen elements (i.e., chlorine, bromine, and iodine) and a reduced content of alkali metals (i.e., sodium and potassium) that lower the sintering temperature of ceramics and refractories, thereby making it possible to provide an alumina aqueous dispersion that can reduce sinter defects caused by the addition of sodium and potassium to the sintering temperature without generating harmful halogen-based gases during firing.

[0037] The alumina aqueous dispersion of the present invention can thermally decompose organic components at relatively low temperatures of 400 to 700°C, and because it contains a water-soluble aliphatic amino acid, it does not reach a viscosity of 100,000 mPa·s for more than 24 hours in neutral to weakly alkaline conditions, which is difficult for acidic alumina sols to achieve. This allows for the time required for mixing with inorganic fillers such as ceramics and refractories before molding, making it possible to use it as an inorganic binder over a wide pH range. Furthermore, because the alumina particles in the alumina aqueous dispersion of the present invention have an average particle size of 5 to 500 nm, they have excellent transparency and sinterability, making it suitable for use in alumina coatings on glass and metals, surface modification of catalyst supports, and as a fine abrasive. [Example]

[0038] The present invention will be described in more detail with reference to examples. The present invention should not be construed as being limited to these examples. In the examples, "parts", "%", etc. are based on mass unless otherwise specified.

[0039] [Example 1] The aqueous alumina dispersion of the present invention was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.1 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65.5%.

[0040] After adding 6695 g of water to 1137.5 g of the dried amorphous aluminum hydroxide gel, 1000 g of glycine was added, and the mixture was heated and concentrated at 200 rpm at 120°C for 8 hours to obtain an aqueous alumina dispersion. The amount of glycine added per mole of aluminum in the amorphous aluminum hydroxide was 2.6 moles.

[0041] [Example 2] The aqueous alumina dispersion of the present invention was synthesized as follows. 2250 g of 0.17 mol / L aluminum sulfate aqueous solution was placed in a 20 L separable flask, and then 1260.1 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 7.0. The precipitated amorphous aluminum hydroxide gel was washed by centrifuging at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and adding 4 L of water three times. The amorphous aluminum hydroxide gel was then dried in a tray dryer set to 100°C for 1 hour, and the moisture content was 65.1%.

[0042] To 1137.5 g of the dried amorphous aluminum hydroxide gel was added 6695 g of water, followed by the addition of 1000 g of glycine, which was then heated and concentrated at 200 rpm at 120°C for 8 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 2.6 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0043] [Example 3] The aqueous alumina dispersion of the present invention was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1060.1 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.2. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65.1%.

[0044] To 1,137.5 g of the dried amorphous aluminum hydroxide gel was added 6,695 g of water, followed by the addition of 1,550 g of glycine, and the mixture was heated and concentrated at 200 rpm at 110°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 4.1 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0045] [Example 4] The aqueous alumina dispersion of the present invention was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.2. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65.1%.

[0046] To 1137.5 g of the dried amorphous aluminum hydroxide gel was added 6695 g of water, followed by the addition of 1000 g of glycine. The mixture was heated and concentrated at 200 rpm at 120°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 2.6 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0047] [Example 5] The aqueous alumina dispersion of the present invention was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, which was then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65.5%.

[0048] After adding 6695 g of water to 1137.5 g of dried amorphous aluminum hydroxide gel, 1300 g of valine was added and the mixture was heated and concentrated at 200 rpm at 110°C for 20 hours to obtain an aqueous alumina dispersion. The amount of valine added per mole of aluminum in the amorphous aluminum hydroxide was 2.2 moles.

[0049] [Example 6] The aqueous alumina dispersion of the present invention was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 71%.

[0050] To 1137.5 g of the dried amorphous aluminum hydroxide gel was added 6695 g of water, followed by the addition of 1000 g of glycine, followed by heating and concentration at 200 rpm at 110°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 3.1 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0051] [Example 7] The aqueous alumina dispersion of the present invention was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, which was then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65.5%.

[0052] After adding 6695 g of water to 1137.5 g of dried amorphous aluminum hydroxide gel, 500 g of glycine and 500 g of valine were added, and the mixture was heated and concentrated at 200 rpm at 110°C for 20 hours to obtain an aqueous alumina dispersion. The total amount of glycine and valine added per mole of aluminum in the amorphous aluminum hydroxide was 2.6 moles.

[0053] [Example 8] The aqueous alumina dispersion of the present invention was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was washed by centrifuging at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and adding 4 L of water, three times. The amorphous aluminum hydroxide gel was then dried in a tray dryer at 100°C for 1 hour, and the moisture content was 67%.

[0054] After drying, 6695 g of water was added to 1137.5 g of amorphous aluminum hydroxide gel and the mixture was stirred to disperse, after which 1000 g of glycine was added and the mixture was heated and concentrated at 100°C for 24 hours at 200 rpm to obtain an aqueous alumina dispersion. The amount of glycine added was 2.7 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0055] [Comparative Example 1] The alumina aqueous dispersion in Comparative Example 1 was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1960.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 8.9. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65%.

[0056] To 1137.5 g of the dried amorphous aluminum hydroxide gel was added 6695 g of water, followed by the addition of 1000 g of glycine, and the mixture was heated and concentrated at 200 rpm at 110°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 2.9 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0057] Comparative Example 2 The alumina aqueous dispersion in Comparative Example 2 was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was washed by centrifuging at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and adding 4 L of water three times. The amorphous aluminum hydroxide gel was then dried in a tray dryer at 100°C for 8 hours, resulting in a moisture content of 45%.

[0058] To 1,137.5 g of the dried amorphous aluminum hydroxide gel was added 6,695 g of water, followed by the addition of 1,000 g of glycine. The mixture was heated and concentrated at 200 rpm at 110°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 1.9 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0059] Comparative Example 3 The alumina aqueous dispersion in Comparative Example 3 was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65%.

[0060] To 1137.5 g of the dried amorphous aluminum hydroxide gel was added 6695 g of water, followed by the addition of 3500 g of glycine, which was then heated and concentrated at 200 rpm at 110°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 9.0 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0061] Comparative Example 4 The aqueous alumina dispersion in the comparative example was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65%.

[0062] To 1137.5 g of the dried amorphous aluminum hydroxide gel was added 6695 g of water, followed by the addition of 1000 g of glycine, which was then heated and concentrated at 200 rpm at 45°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 2.9 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0063] Comparative Example 5 The aqueous alumina dispersion in the comparative example was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65%.

[0064] To 1137.5 g of the dried amorphous aluminum hydroxide gel was added 6695 g of water, followed by the addition of 100 g of glycine, and the mixture was heated and concentrated at 200 rpm at 110°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 0.3 mol per mol of aluminum in the amorphous aluminum hydroxide.

[0065] Comparative Example 6 The aqueous alumina dispersion in the comparative example was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65%.

[0066] After adding 6695 g of water to 1137.5 g of dried amorphous aluminum hydroxide gel, 1000 g of glycine was added and the mixture was heated and concentrated at 200 rpm at 110°C for 1 hour to obtain an aqueous alumina dispersion. The amount of glycine added was 2.9 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0067] Comparative Example 7 The aqueous alumina dispersion in the comparative example was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.6 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set at 100°C, resulting in a moisture content of 65%.

[0068] 6695g of water was added to 1137.5g of dried amorphous aluminum hydroxide gel and stirred to disperse, after which 2386.6g of 23.1mol / L lactic acid was added and stirred at 45°C and 250rpm for 1 hour to synthesize an aqueous solution of basic aluminum lactate. This aqueous solution of basic aluminum lactate was heated and concentrated at 250rpm at 100°C for 20 hours to obtain an aqueous alumina dispersion. The amount of glycine added per mole of aluminum in the amorphous aluminum hydroxide was 2.5 moles.

[0069] [Comparative Example 8] The aqueous alumina dispersion in the comparative example was synthesized as follows. 2250 g of 0.5 mol / L aluminum nitrate aqueous solution was placed in a 20 L separable flask, and then 1160.1 g of 13.3 mol / L ammonia water was added while stirring at 250 rpm. After stirring for 1 hour at 30°C, the pH was 6.6. The precipitated amorphous aluminum hydroxide gel was centrifuged at 2000 rpm for 1 minute in a centrifuge (Kansai Centrifugal Separation Manufacturing Co., Ltd., KBS-12) and 4 L of water were added three times to wash the amorphous aluminum hydroxide gel, and then dried for 1 hour in a tray dryer set to 100°C, resulting in a moisture content of 65.5%.

[0070] To 1137.5 g of the dried amorphous aluminum hydroxide gel was added 6695 g of water, followed by the addition of 1000 g of acetic acid. The mixture was heated and concentrated at 200 rpm at 120°C for 8 hours to obtain an aqueous alumina dispersion. The amount of glycine added was 3.3 moles per mole of aluminum in the amorphous aluminum hydroxide.

[0071] The alumina aqueous dispersions obtained in Examples 1 to 8 and Comparative Examples 1 to 8 were subjected to the following measurements, and the results are shown in Tables 1 to 4.

[0072] Al 2 O 3 (%) The Al concentrations in Examples 1 to 8 and Comparative Example 7 were measured using an aluminum concentration measuring instrument (ICP-OCS5110 manufactured by Agilent Technologies), and the Al concentrations were converted to Al2O3 concentrations, which are shown in Tables 1 to 4.

[0073] pH The pH of the alumina aqueous dispersion was measured using a portable pH meter (manufactured by Toa DKK Corporation: HM-40P, pH composite electrode "GST-2739C") The results are shown in Tables 1 to 4.

[0074] D / C The volume of the aqueous solution obtained by mixing amorphous aluminum hydroxide, a water-soluble aliphatic amino acid, and water was defined as C, and the volume of the alumina aqueous dispersion obtained by heating and concentrating it at 90 to 120°C was defined as D, and D / C was calculated. The results are shown in Tables 1 to 4.

[0075] Crystal system when dried at 100℃ The crystal systems of Examples 1 to 8 and Comparative Example 7 when dried at 100° C. were measured using an X-ray diffractometer (MiniFlex600, manufactured by Rigaku Corporation). The results are shown in Tables 1 to 4.

[0076] TI value (thixotropic index) The thixotropic index (TI value) was measured using a Brookfield viscometer (using a TVB10M TM-3 spindle rotor manufactured by Toki Sangyo Co., Ltd.), and calculated as TI value = [viscosity at a rotation speed of 6 rpm] / [viscosity at a rotation speed of 60 rpm]. The results are shown in Tables 1 to 4.

[0077] Average particle size The average particle diameters of Examples 1 to 8 and Comparative Example 7 were determined by measuring the average particle diameter of alumina in an aqueous solvent using a Zetasizer Nano ZS (manufactured by Spectris Inc.) by dynamic light scattering, and the average particle diameters measured by number are shown in Tables 1 to 4.

[0078] Liquid viscosity check results after 24 hours 25% aqueous ammonia was added to the aqueous alumina dispersions of Examples 1 to 8 and Comparative Example 7 to adjust the pH to 9, and then the dispersions were sealed to prevent evaporation. After leaving the dispersions at room temperature for 24 hours, a B-type viscometer (Toki Sangyo Co., Ltd.: TVB10M TM-3 spindle rotor) was used to check whether the viscosity exceeded 100,000 mPa s after 24 hours, and the results are shown in Tables 1 to 4. Evaluation criteria Good: Viscosity of the liquid (aqueous dispersion) after 24 hours is less than 100,000 mPa·s ×: Viscosity of the liquid (aqueous dispersion) after 24 hours is 100,000 mPa·s or more

[0079] Chlorine, bromine, iodine, sodium, and potassium concentrations The concentrations of chlorine, bromine, iodine, sodium, and potassium in Examples 1 to 8 and Comparative Example 7 were measured using EDS (energy dispersive fluorescent X-ray) with a scanning electron microscope (JEOL Ltd. JCM-7000 equipped with energy dispersive fluorescent X-ray (EDX)) and are shown in Tables 1 to 4. nd: not detected.

[0080] Thermal decomposition evaluation The thermal decomposition properties of Examples 1 to 8 and Comparative Example 7 were evaluated by dropping 5 g of the alumina aqueous dispersions of Examples 1 to 8 and Comparative Example 7 into a 50 ml alumina crucible (manufactured by Nikkato Corporation), drying the mixture at 100°C for 5 minutes using a hot air circulation dryer, and then firing the mixture at 650°C for 15 minutes in an electric furnace (FO200 manufactured by Yamato Scientific Co., Ltd.). The appearance (color) of the material remaining in the alumina crucible was confirmed, and the results are shown in Tables 1 to 4.

[0081] [Table 1]

[0082] [Table 2]

[0083] [Table 3]

[0084] [Table 4]

[0085] The alumina aqueous dispersion of Example 1 was dried in a glass evaporating dish at 100°C for 10 minutes, and the result of observing the dried alumina aqueous dispersion at 100,000 magnifications with a field emission scanning electron microscope (JEOL Ltd., JSM-7001F) is shown in Figure 1.

[0086] The alumina aqueous dispersion of Example 1 was placed in a glass evaporating dish and dried in a hot air circulation dryer at 100°C for 20 minutes, and the powder and glycine were measured using an X-ray diffractometer (Mini Flex 600, manufactured by Rigaku Corporation). The results are shown in Figures 2 and 3.

[0087] The alumina aqueous dispersion of Example 1 was placed in a glass evaporating dish and dried in a hot air circulation dryer at 100°C for 20 minutes, and the resulting powder was placed in an alumina crucible and fired in an electric furnace (Yamato Scientific Co., Ltd.: FO200) at 700°C for 2 hours. The powder was measured using an X-ray diffractometer (Rigaku Corporation: Mini Flex600), and the evaluation results are shown in Figure 4.

[0088] The alumina aqueous dispersion of Example 1 was measured in an aqueous solvent by dynamic light scattering using a Zetasizer Nano ZS (manufactured by Spectris Inc.), and the results measured on a number basis are shown in FIG.

[0089] As shown in Tables 1 and 2, in Examples 1 to 5, 7, and 8, an aqueous alumina dispersion of boehmite was prepared, and the Al2O3 value, pH, average particle size, and TI value all fell within the ranges of the present invention. After 15 minutes of firing at 600°C, white matter was observed, and no organic matter was found to remain after firing, confirming that the thermal decomposition of the organic matter was complete. Furthermore, when the pH was adjusted to 10 with ammonia, the viscosity did not exceed 100,000 mPa·s after 24 hours. Furthermore, in Example 6, an aqueous alumina dispersion of boehmite and pseudo-boehmite was prepared, and the Al2O3 value, pH, average particle size, and TI value all fell within the ranges of the present invention. After 15 minutes of firing at 650°C, white matter was observed, and no organic matter was found to remain after firing, confirming that the thermal decomposition of the organic matter was complete. Furthermore, when the pH was adjusted to 10 with ammonia, the viscosity did not exceed 100,000 mPa·s after 24 hours.

[0090] In Comparative Example 1, as shown in Table 3, the pH of the amorphous aluminum hydroxide was too high at 8.9, resulting in some amorphous aluminum hydroxide remaining undissolved in the next step. In Comparative Example 2, the moisture content of the amorphous aluminum hydroxide was low at 45%, resulting in some amorphous aluminum hydroxide remaining undissolved. In Comparative Example 3, the amount of glycine was in excess of 9.0 moles relative to the aluminum, resulting in glycine precipitation during heating and concentration. In Comparative Example 4, the heating temperature during formation of the aqueous alumina dispersion was too low at 45°C, preventing heating and concentration, and therefore no aqueous alumina dispersion was obtained. Furthermore, in Comparative Example 4, the D / C was high at 0.98, and the pH of the aqueous alumina dispersion was also high at 6.5.

[0091] In Comparative Example 5, the amount of glycine added was too low (0.3 moles relative to the aluminum), resulting in an insufficient amount of protective agent when the alumina aqueous dispersion was heated and concentrated, resulting in aggregation of the alumina particles. In Comparative Example 6, the heating time was too short (1 hour), resulting in insufficient time for heating and concentration, resulting in the failure to produce an alumina aqueous dispersion. Furthermore, in Comparative Example 6, the pH of the alumina aqueous dispersion was high (5.6) and the D / C ratio was also high (0.91). In Comparative Example 7, a boehmite-based alumina aqueous dispersion was produced, but because it contained lactic acid, thermal decomposition was insufficient when heated at 650°C for 15 minutes, resulting in a black film surface and no alumina. Furthermore, low-temperature thermal decomposition was an issue. In Comparative Example 8, acetic acid was used instead of a water-soluble aliphatic amino acid, so an alumina aqueous dispersion was apparently formed, but the protective effect of the water-soluble aliphatic amino acid was not effective, resulting in the formation of a white precipitate during storage (poor shelf life).

[0092] FIG. 1 is an electron microscope photograph of the alumina aqueous dispersion of Example 1 dried at 100°C for 10 minutes. The majority of particles have a primary particle size of 100 nm or less, confirming that the desired alumina aqueous dispersion was synthesized.

[0093] FIG. 2 shows the X-ray diffraction of the alumina aqueous dispersion of Example 1 dried at 100°C for 20 minutes, and FIG. 3 shows the X-ray diffraction of glycine (crystallized glycine) alone when the alumina aqueous dispersion was dried at 100°C for 20 minutes. In FIG. 2, the peaks of boehmite are indicated by →, and the peaks of glycine are indicated by the blackened arrow tips, confirming the presence of a mixture of the peaks of crystallized glycine and boehmite. It was confirmed that an alumina particle dispersion containing a mixture of boehmite particles and crystallized glycine, a water-soluble aliphatic amino acid, was prepared. Removing the crystallized glycine in FIG. 3 from FIG. 2 results in the X-ray diffraction of boehmite.

[0094] FIG. 4 shows the X-ray diffraction pattern of the alumina aqueous dispersion of Example 1, which was dried at 100°C for 20 minutes and then calcined at 750°C for 2 hours. The pattern shows a gamma-alumina peak pattern, and it was confirmed that by increasing the calcination temperature of the alumina particle dispersion in which boehmite particles and glycine coexist, glycine is thermally decomposed and the boehmite particles undergo a phase transition to gamma-alumina.

[0095] FIG. 5 shows the particle size distribution of the alumina aqueous dispersion of Example 1 measured on a number basis. It was confirmed that the average particle size was 22 nm and that the alumina aqueous dispersion did not contain coarse particles exceeding 100 nm.

[0096] The present invention also encompasses the following aspects: [1] A method for producing an aqueous alumina dispersion, comprising heating and concentrating an aqueous solution of amorphous aluminum hydroxide, a water-soluble aliphatic amino acid, and water at a temperature of 90 to 120°C for 5 to 36 hours, When the volume of the aqueous solution obtained by mixing the amorphous aluminum hydroxide, the water-soluble aliphatic amino acid, and water during production is defined as C, and the volume of the aqueous alumina dispersion obtained by heating and concentrating the dispersion at 90 to 120°C is defined as D, the value of D / C is controlled to be 0.1 to 0.8, The obtained aqueous alumina dispersion has the following characteristics: (a) the average particle size of the dispersed particles is in the range of 5 to 500 nm; (b) When the alumina-containing aqueous dispersion is dried at 100°C, the alumina particles are composed of one or more of boehmite and pseudo-boehmite, (c) the pH of the alumina aqueous dispersion is in the range of 2.0 to 5.3; (d) the alumina aqueous dispersion contains a water-soluble aliphatic amino acid; (e) The concentration of aluminum contained in the alumina aqueous dispersion in terms of alumina (Al2O3) is in the range of 0.3 to 11.5 mass %, and (f) A method for producing an aqueous alumina dispersion, characterized in that when a base is added to the aqueous alumina dispersion to adjust the pH to 5.4 to 10, it takes 24 hours or more for the viscosity of the aqueous alumina dispersion to reach 100,000 mPa·s or more. [2] The method for producing an aqueous alumina dispersion according to [1], wherein the water-soluble aliphatic amino acid is added in an amount of 1.5 to 5 moles per mole of aluminum in the amorphous aluminum hydroxide. [3] The method for producing an aqueous alumina dispersion according to [1] or [2], wherein the water-soluble aliphatic amino acid is glycine, alanine, valine, leucine, isoleucine, or a mixture thereof. [4] The method for producing an alumina aqueous dispersion according to any one of [1] to [3], wherein the alumina aqueous dispersion has a thixotropic index (TI value) of 2 to 10, which is expressed as [viscosity at a rotation speed of 6 rpm] / [viscosity at a rotation speed of 60 rpm] measured with a Brookfield viscometer. [5] The method for producing an alumina aqueous dispersion according to any one of [1] to [4], wherein the alumina aqueous dispersion contains 0 to 0.02 mass% of chlorine, bromine, or iodine, and 0 to 0.05 mass% of sodium or potassium. [6] The amorphous aluminum hydroxide can be prepared by adjusting an aqueous solution of an inorganic aluminum salt selected from aluminum nitrate and / or aluminum sulfate to a pH of 5 to 8 with aqueous ammonia to form a precipitate of amorphous aluminum hydroxide; washing the produced amorphous aluminum hydroxide with water; separating the amorphous aluminum hydroxide from water; The method for producing an aqueous alumina dispersion according to any one of [1] to [5], which is obtained by drying the amorphous aluminum hydroxide so that the moisture content is 60 to 80 mass %.

Claims

1. A method for producing an aqueous alumina dispersion, comprising heating and concentrating an aqueous solution of amorphous aluminum hydroxide, a water-soluble aliphatic amino acid, and water at a temperature of 90 to 120°C for 5 to 36 hours; the volume of the aqueous solution obtained by mixing the amorphous aluminum hydroxide, the water-soluble aliphatic amino acid, and water during production is defined as C, and the volume of the aqueous alumina dispersion obtained by heating and concentrating the aqueous alumina dispersion at 90 to 120°C is defined as D, the value of D / C is controlled to be 0.1 to 0.8, The obtained aqueous alumina dispersion has the following characteristics: (a) the average particle size of the dispersed particles is in the range of 5 to 500 nm; (b) when the alumina-containing aqueous dispersion is dried at 100°C, the alumina particles are composed of one or more of boehmite and pseudo-boehmite, (c) the pH of the alumina aqueous dispersion is in the range of 2.0 to 5.3; (d) the alumina aqueous dispersion contains a water-soluble aliphatic amino acid; (e) Alumina (Al) of aluminum contained in the alumina aqueous dispersion ２ O ３ ) the converted concentration is in the range of 0.3 to 11.5% by mass, and (f) A method for producing an aqueous alumina dispersion, characterized in that when a base is added to the aqueous alumina dispersion to adjust the pH to 5.4 to 10, it takes 24 hours or more for the viscosity of the aqueous alumina dispersion to reach 100,000 mPa·s or more.

2. 2. The method for producing an aqueous alumina dispersion according to claim 1, wherein the water-soluble aliphatic amino acid is added in an amount of 1.5 to 5 moles per mole of aluminum in the amorphous aluminum hydroxide.

3. 3. The method for producing an aqueous alumina dispersion according to claim 1, wherein the water-soluble aliphatic amino acid is glycine, alanine, valine, leucine, isoleucine, or a mixture thereof.

4. 3. The method for producing an alumina aqueous dispersion according to claim 1, wherein the alumina aqueous dispersion has a thixotropic index (TI value) of 2 to 10, which is expressed as [viscosity at a rotation speed of 6 rpm] / [viscosity at a rotation speed of 60 rpm] measured with a Brookfield viscometer.

5. The method for producing an alumina aqueous dispersion according to claim 1 or 2, wherein the alumina aqueous dispersion contains 0 to 0.02 mass% of chlorine, bromine, or iodine, and 0 to 0.05 mass% of sodium or potassium.

6. The amorphous aluminum hydroxide can be prepared by adjusting an aqueous solution of an aluminum inorganic acid salt selected from aluminum nitrate and / or aluminum sulfate to a pH of 5 to 8 with aqueous ammonia to form a precipitate of amorphous aluminum hydroxide; washing the produced amorphous aluminum hydroxide with water; separating the amorphous aluminum hydroxide from water; 3. The method for producing an alumina aqueous dispersion according to claim 1, wherein the alumina aqueous dispersion is obtained by a step of drying the amorphous aluminum hydroxide so that the moisture content is 60 to 80 mass %.

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