Effect of inorganic acid on the phase transformation of alumina

This paper focused on the effect of hydrochloric acid and nitric acid on phase transformation of gamma to alpha alumina. The microstructure, phase composition of specimens and crystallite sizes of α-Al2O3 are investigated by X-ray diffraction, SEM, Scherrer formula and external standard method. The results showed that the phase transitions of alumina was affected by the kinds and concentrations of inorganic acid. Hydrochloric acid was more advantageous to promote the transformation of γ-alumina to α-alumina than nitric acid above 900 °C. In addition, the α-fraction of specimens obtained by Al2O3-HCl gels increased from 71.0% to 87.1% after calcination at 1000 °C when acid concentration increased from 0.1*10-4 mol/L to 0.1mol/L. On the contrary, the α-fraction of specimens obtained by Al2O3-HNO3 gels decreased from 79.4% to 73.9%. And at 1100 °C up to nearly 100% of α-alumina in the specimens was obtained. Moreover, both hydrochloric acid and nitric acid were beneficial to the growth of the crystallite sizes of prepared α-Al2O3. Meanwhile the effect of hydrochloric acid on the phase transitions of alumina was further promoted by employing a two-step heat-treatment method. XRD and SEM confirmed that the first-step heat-treatment played an important role in the course of the calcination processes for low-temperature of α-alumina formation.