A closed-loop and multi-product recycling strategy for secondary aluminum dross via microwave-gradient heating and directed conversion of fluorine, chlorine, and nitrogen

The treatment of secondary aluminum dross (SAD), a hazardous solid waste from the aluminum industry, urgently requires the development of efficient and environmentally friendly recycling methods. Conventional processes are plagued by high energy consumption, low product purity, and inefficient management of fluorine, chlorine, and nitrogen components. This study presents a novel multi-product coupled recycling strategy for the directed conversion and comprehensive utilization of SAD. The integrated process consists of three key stages: (1) Microwave-assisted gradient heating (600, 750, and 800 °C) coupled with a Na/K carbonate modifier achieves synchronous separation of high-purity KCl (>97%) and NaF (>95%) salts, reducing energy consumption by over 15% compared to conventional two-stage roasting and eliminating acid-washing wastewater. (2) A closed-loop ammonia conversion system, where hydrolyzed NH₃ is fixed as ammonium sulfate and subsequently synthesized into urea (yield >90%), achieves an NH₃ recovery rate > 94%. The synthesized urea serves as a multi-functional agent for low-temperature sintering (1350–1450 °C) of Al₂O₃ ceramics, enhancing flexural strength to > 324 MPa via in situ formation of nano-AlN. (3) Plasma activation or mechano-chemical activation enables one-step synthesis of nano-core-shell Al₂O₃-SiO₂ composite powder, which is used to smelt high-hardness Al-Si-Fe alloy (≥132 HV). This work establishes a sustainable, closed-loop paradigm for the harmless and resource-efficient treatment of SAD, demonstrating significant advantages in energy efficiency, economic benefit, and environmental friendliness.