Process for producing an acidic aqueous alumina sol

A process for producing a stable acidic aqueous alumina sol containing elongate secondary particles having a dimension of 50 to 300 nm and which are elongated in only one plane and formed by edge-to-edge coagulation of rectangular plate-like primary particles having a length of one side of 10 to 30 nm when observed through an electron microscope, the process comprising the steps of:

• (a) adding an alkali to an aqueous alumina sol containing fibrous colloidal particles of an amorphous alumina hydrate to produce a reaction mixture having a pH of 9 to 12;
• (b) subjecting the reaction mixture obtained in the step (a) to a hydrothermal treatment at a temperature of 100 to 250.degree. C. to produce an aqueous suspension containing an alumina hydrate having a boehmite structure;
• and
• (c) contacting the aqueous suspension obtained in the step (b) to a hydrogen-type acid cation-exchange resin and a hydroxyl-type strong-base anion-exchange resin to form an acidic aqueous alumina sol having a pH of 3 to 6.

A process for producing a stable acidic aqueous alumina sol containing elongate secondary particles having a dimension of 50 to 300 nm and which are elongated in only one plane and formed by edge-to-edge coagulation of rectangular plate-like primary particles having a length of one side of 10 to 30 nm when observed through an electron microscope, the process comprising the steps of:

• (A') adding an alkali to an aqueous alumina sol containing fibrous colloidal particles of an amorphous alumina hydrate to produce a reaction mixture having a pH of 9 to 12;
• (B') desalting the reaction mixture obtained in the step (A') by adding water by cake filtration to produce a desalted reaction mixture having a pH of 9 to 12;
• (C') subjecting the desalted reaction mixture obtained in the step (B') to a hydrothermal treatment at a temperature of 110 to 250.degree. C. to produce an aqueous suspension containing an alumina hydrate having a boehmite structure;
• and
• (D') adding an acid to the aqueous suspension obtained in the step (C') to form an acidic aqueous alumina sol having a pH of 3 to 6.