Preparation of high purity α-Al₂O₃ particles derived from industrial waste solution via microwave calcining and seeding technique

Anodic aluminium oxidation course is an important method for metal surface finishing, which produces large quantity of waste acid solutions containing ${{SO}_4}^{2-}$ and Al³⁺. In this study, high purity α-Al₂O₃ particles were synthesized from this waste solution via recrystallization and microwave calcination with seeds addition. Effects of pH value, recrystallization times, seeding technique and calcination process on the properties of products were investigated. Phase composition, microstructure, crystal grain size and purity of products were studied by XRD, TG-DSC, SEM, BET, laser particle size analysis and ICP-AES. The results show that when pH value is 3.0 and recrystallization time is twice, high purity NH₄Al(SO₄)₂ · 12H₂O (≥99.999%) precursor is produced. Addition of seeds and microwave technique can lower the calcination temperature and improve the performance of products. Well dispersed ultrafine α-Al₂O₃ powders (≥99.99% mass fraction), which have short rod-like shape with 150 nm in length and 100 nm in diameter, can be obtained at 1050 °C for 30 min, which is 150 °C lower than conventional calcination, through microwave heating the precursor with 5 wt% seeds content. Seeds can improve the phase transformation rate of α-Al₂O₃, and microwave can increase the rate of precursor decomposition. (paper)