Recycling of secondary aluminum dross to make alumina by hydrometallurgy: A review

Secondary aluminum (Al) dross (SAD) is a hazardous waste discharged from Al production, processing and recycling. Over 6.7 million tons of SAD was discharged on the planet in 2023. SAD is consisted of 40–60 wt% of alumina (AlO), 10–30 wt% of AlN, 5–15 wt% of salts and 3–10 wt% of heavy metal oxides. Currently, recycling of SAD to make AlO by hydrometallurgy is a promising method for disposal of SAD. Hydrometallurgy method is mainly divided into acid leaching and alkali leaching. In acid leaching, Al, AlN and AlO react with acid to form aluminum sulfate and aluminum chloride. In alkali leaching, Al, AlN and AlO react with alkali to form sodium aluminate. High-purity AlO is obtained after precipitation, washing, drying and calcination from the leachate. Resource consumption and emission was calculated to evaluate the economic and environmental benefits. About 147.9 and 172.6 dollars was earned after making AlO from a ton of SAD by alkali and acid leaching process, respectively. And carbon emissions of a ton of AlO was risen about 596.5 and 2216.0 kg CO, respectively, compared with the Bayer process with bauxite. We proposed a calcination pre-treatment with quicklime on SAD to reduce the carbon emission. The Al and AlN are oxidized into AlO after calcination, and the AlO reacts with CaO to form CaO·AlO. The Al in CaO·AlO can be leached out easily with a low concentration of alkali. This review provides a guidance for the recycling of SAD by hydrometallurgy, and proposes a novel idea for the energy and consumption reduction in alumina (Al₂O₃) production.