Various colloidal particles of alumina hydrates have already been produced by various processes for production. The colloidal particles are a generic term for particles having a particle size of 1 to 1000 nm, that is, those having a particle size of colloidal level. Most of these produced colloidal particles are composed of alumina hydrates having a boehmite structure or pseudoboehmite structure, or amorphous alumina hydrates, and it is known that they are plate-like, ribbon-like, spindle-like, needle-like or fibrous in shape.
JP 57-111237 A (1982) discloses a process for producing an alumina sol comprising hydrothermally treating an alumina hydrate obtained by reacting a water-soluble aluminum salt with carbon dioxide or a carbonate to obtain a processed product, and then drying it, or drying the processed product, and then mixing it with a monobasic acid.
JP 02-243512 A (1990) discloses a process for isolating a fine and plate-like α-aluminum monohydrate (alumina hydrate having a boehmite structure) product by controlling molar ratio of a caustic aluminate liquor at a temperature below 100° C. and cooling the alkaline solution with vigorous stirring.
U.S. Pat. No. 4,666,614 discloses a process for producing α-aluminum monohydrate comprising reacting an acid or alkali reaction solution controlled to pH of 7.5 to 10 at 60 to 100° C. for 2 to 7 hours and drying it to 35 mass % or more.
JP 57-111237 A (1982) states that as to production condition of alumina hydrate, in case where an aluminate of alkali metal is used as a water-soluble aluminum salt, it is sufficient to follow a usual production process in which gaseous carbon dioxide is bubbled into the salt and the carbon dioxide is desirably introduced until the reaction system reaches pH near 7, and then the produced alumina hydrate is washed for removing impurities. Further, the publication states that smaller amount of remaining impurities is preferable from viewpoint of the production or use of alumina sol.
JP 02-243512 A (1990) states that the α-aluminum monohydrate of the invention is plate-like crystal having a size (average particle size is 0.4 μm, or in particle size distribution, 90% or more of particles are in a range from 0.2 to 0.8 μm) suitable for pigment filler, and that boehmite separating maximum rate from sodium aluminate solution is obtained at a molar ratio Na2O/Al2O3 of 1.3.
U.S. Pat. No. 4,666,614 states that an aluminum compound is reacted with an acid or an alkali, pH is adjusted to 7.5 to 10, and then heating is carried out at 60 to 100° C., thereafter by-products formed by neutralization are removed to obtain a sol having an alumina hydrate concentration of 10 to 35 mass %.
In the production processes disclosed in JP 57-111237 A (1982) and JP 02-243512 A (1990), the need of washing step carried out after the neutralization with an alkali and an acid is stated. However, the washing is carried out with pure water, not only it is time-consuming until completion but also the intermediates allowed to stand after washing and filtration tend to cause aluminum hydroxide. The aluminum hydroxide is produced due to aging of the aimed product (amorphous alumina gel). And, alumina hydrate having a boehmite structure originally aimed is not obtained at a normal temperature. In order to prevent the aging, carbonate or gaseous carbon dioxide, etc. is generally added. However, as carbon dioxide is weak acid, a large amount of it is required for neutralization, therefore it is economically disadvantageous to adjust to pH around neutrality.
In addition, in case where the alumina obtained by the process disclosed in JP 02-243512 A (1990) is used as a filler for filling pigment, it cannot show its sufficient performance for any application purpose as the size of the particles is large. Therefore, those having further small particle size are required.
In the production processes disclosed in JP 57-111237A (1982) and U.S. Pat. No. 4,666,614, a large amount of acid or alkali used at pH range in neutralization step is required and thereby leading to high production cost, and further there are some problems that a large amount of pure water or chemicals and time are required for washing by-products, and efficiency is not good.
Further, the required physical properties such as viscosity and thixotropic property of the aqueous alumina sol are different according to each application purpose. It is desired to provide a further improved aqueous alumina sol for specific application purposes.