Intensified Roasting at Low-Temperature and Alkaline Leaching to Efficiently Remove Harmful Elements for Green Utilization of Secondary Aluminum Dross

Secondary aluminum dross (SAD) from the aluminum industry is a hazardous residue that restricts sustainable aluminum production. Residual aluminum nitride (AlN) and insoluble fluorides after conventional pretreatment are major barriers to the safe utilization of SAD. In this study, a low-temperature intensified roasting–alkaline leaching process was developed to remove harmful elements by using reaction heat and waste heat to expose enveloped AlN and fluoride phases, form soluble sodium-bearing compounds, and intensify AlN oxidation. Without additives, roasting at 750 ◦C for 3 h converted 91.65% of AlN, leaving 1.37% AlN in the roasted SAD. With Na2O2 as an oxygen donor in situ, NaF as a mineralizer to reduce roasting temperature, and sodium-bearing species as reactants for soluble NaAlO2/Na2SiO3 formation, the AlN conversion increased up to 97.01% under 5% NaF and 2.5% Na2O2 at 750 ◦C for 3 h. Afterwards, higher temperature, longer duration, lower roasted SAD dosage, and higher caustic soda concentration all improved fluorine removal in the subsequent alkaline leaching. Under 100 g/L Na2O, 20 g/L roasted SAD, and 100 ◦C, fluorine and chlorine removal efficiencies reached 92.52% and 99.47%, respectively. The final high-alumina residue contained 74% Al2O3, mainly in the forms of α-Al2O3, NaAl11O17, and MgAl2O4, with only 0.19% F and 0.03% Cl, making it suitable for the preparation of various alumina-bearing materials and alumina production.