Effect of fluoride additive on alpha-Alumina Formation from Active Alumina

a-Al2O3 is the most important raw material for synthesizing modern ceramics and high alumina refractories. Active Alumina (also known as Hydratable Alumina and r-Al2O3) is a kind of transition alumina which is produced by flash-calcining of Gibbsite at 500-600 oC. In this research, the effect of Ammonium bi-fluoride additive (NH4HF2), on the formation of a-Al2O3 from active alumina was investigated. Suspensions containing active alumina and different amounts of fluoride additive (0-3 wt%) were stirred for 1 hour. The water of each suspension was removed by filter paper and the resultant powders were dried at 110 oC for 24 hrs. Powders were calcined up to 1400 oC for DSC and XRD analysis. The DSC test results showed that by having 1 wt% fluoride additive, a sharp peak appeared at 1100 oC indicating the formation of a-Al2O3. Increasing the additive amount to 3 wt%, shifted the transformation peak to as low temperatures as 925 oC. FESEM studies showed that in samples containing fluoride additive, the particles possessed hexagonal morphology. However, the ordering of the platelets and their thickness was a function of additive content. The higher the fluoride additive, the more ordered hexagonal shape as well as more thickness of the platelets.