ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 35, Number 4
JKCSEZ 35(4)
August 20, 1991 

High-purity alumina powder was prepared by extracting the natural alumino-silicate mineral (halloysite) in H2SO4 solution. For the selective precipitation of alum and aluminum hydroxide, the solubility diagram was prior calculated by also considering the formation of hydroxides and carbonates for all the metal ions in an aqueous solution, which allow us to control the contamination of impurities envolved in the natural minerals. Ammonium aluminum sulfate (alum) and alumium hydroxide could be successfully prepared at pH = 1.5∼2.5 and pH = 6∼8, respectively according to our solubility diagrams. The purity of alum-and hydroxide-derived α-Al2O3 was determined to be 99.7% and 99.0%, respectively, which indicates the former route would be more desirable for the large scale application. It is also worthy to note that the impurities like Na and Si were strongly reduced in the former (Na = 0.05%, Si = 0.09%) compared to the latter (Na = 0.29%, Si = 0.12%).