Safe comprehensive utilization of the hazardous secondary aluminum dross: Mechanism and technology

Secondary aluminum dross (SAD) is a hazardous waste generated from aluminum electrolytes, processing, and regeneration. To comprehensively utilize SAD, this review critically evaluated various approaches to transforming SAD into value-added products, reaction mechanisms and treatments of harmful elements. The changeable composition and inhomogeneous phases of SAD were clearly identified, leading to the low extraction efficiency of alumina, the poor quality of alumina-bearing materials and difficult operation in practice. Reaction mechanism of Al, AlN, alpha-Al2O3 and salts has been carefully summarized. The aluminum-bearing substances embedded by Al2O3 layer or Al(OH)3 layer notably reduced the reaction efficiency. The mutually embedded phases and the rich bubbles in the aqueous solution notably changed the reaction behavior of the active aluminum-bearing substances and salts. Afterwards, technologies for comprehensive utilization of SAD were summarized according to pyrometallurgy, hydrometallurgy and combination of pyro/hydrometallurgy. Calcium aluminate and sodium aluminate by roasting process, preparation of alumina-bearing materials after wet- pretreatment for impurity removal, and production of alumina and water purificant from combination of pyro-hydrometallurgy were further discussed. In addition, harmful gases, detrimental ions (F-, Cl-and NH4+), and salts in alumina-bearing materials all limited SAD utilization owing to the environmental risk. With the integration of safety, efficiency and performance, economical comprehensive utilization of SAD was proposed by integrating alumina production, industrial ceramics, and cements on the basis of aluminum industry chain. The bottleneck of environmental risk and roadmap of SAD utilization were finally provided for the safe comprehensive utilization of SAD.