Method of manufacture of spherical alumina particles

Claim 1

In a method of manufacturing substantially spherical alumina particles comprising commingling alumina hydrosol with a weak base which has a strong buffering action and is thermally decomposable at elevated temperature, dispersing the resulting mixture as droplets in a suspending medium to effect gelation thereby forming substantially spherical alumina hydrogel particles, thereafter ageing said hydrogel particles in said suspending medium, then ageing said hydrogel particles in aqueous ammonium hydroxide solution, then washing said particles in water, then drying and calcining same to obtain spherical alumina particles, the improvement which comprises: the step of ageing said hydrogel particles in aqueous ammonium hydroxide solution is performed by contacting said particles with an aqueous ammonium hydroxide solution having an ammonia concentration in the range of from about 0.05 wt.% to about 0.5 wt.% for a first time period of at least one hour and then subjecting the thus aged particles to further ageing by contacting said particles with aqueous ammonium hydroxide solution for a second time period of at least about six hours and during said second time period continuously increasing the concentration of ammonia in said solution from an initial ammonia concentration in the range of from about 0.05 wt.% to about 0.5 wt.% up to a final ammonia concentration in the range of from about 0.8 wt.% to about 2.5 wt.%.