Two-step Harmless Treatment of Secondary Aluminum Dross (SAD) by Vacuum Distillation and Alkaline Roasting

Secondary aluminum dross (SAD) is classified as hazardous waste by many countries in the world because it contains a large number of toxic and leachable fluorides and chlorides, and AlN which easily reacts with water and releases ammonia gas. In this study, a method combined vacuum distillation with alkaline roasting is developed to realize the harmless treatment of SAD. First, the fluorides and chlorides in SAD were separated and recovered under high temperature and vacuum condition, and then the residue was roasted with alkaline reagents to remove AlN, finally harmless aluminate products were obtained. The results show that the salts could be completely separated from SAD under optimal conditions of 750 ℃, 3 h and below 5 Pa. However, direct distillation of SAD powder sample in vacuum furnace will lead to SAD particles spraying out, and the reduced metal Na is also found in the condensate salts. Then, the salts-free SAD is roasted at 800–1400 ℃ by adding Na2CO3 or CaCO3, the results show that SAD with Na2CO3 could form homogeneous NaAlO2 at 1200 ℃, while SAD and CaCO3 could form various mineral phases calcium aluminate products at 1400 ℃, and the removal rates of AlN are 95.59% and 94.37%, respectively.