Abstract
Secondary aluminum dross (SAD) is regarded as a solid waste of aluminum recycling process that creates serious environmental and health concerns. However, SAD can also be used as a good source of aluminum, so that utilizing the SAD for the production of valuable products is a promising approach of recycling such waste. In the present work, a novel eco-friendly three-step process was proposed for the synthesis of cryolite (Na3AlF6) from the SAD, and it consisted of (1) water-washing pretreatment of SAD, (2) extraction of Al component via pyro-hydrometallurgy, including low-temperature alkaline smelting, water leaching and purification of leachate in sequence, (3) precipitation of cryolite from the purified NaAlO2 solution using the carbonation method. By analysis of the parameter optimization for each procedure, it was found that the maximum hydrolysis efficiency of aluminum nitride (AlN) in the SAD was around 68.3% accompanied with an extraction efficiency of Al reaching 91.5%. On this basis, the cryolite of high quality was synthesized under the following optimal carbonation conditions: reaction temperature of 75 °C, NaAlO2 concentration of 0.11 mol/L, F/(6Al) molar ratio of 1.10, and 99.99% CO2 gas pressure, and flow rate of 0.2 MPa and 0.5 L/min respectively. The formation of Na3AlF6 phase can be detected by XRD. The morphological feature observed by SEM revealed that the as-synthesized cryolite had a polyhedral shape (~1 μm size) with obvious agglomeration. The chemical composition and ignition loss of the as-synthesized cryolite complied well with the requirements of the Chinese national standard (GB/T 4291-2017).
Highlights
With the extensive development of secondary aluminum industry, secondary aluminum dross (SAD) is widely generated during aluminum recycling process through remelting with salt flux.In general, the SAD primarily contains Al2 O3, aluminum metal, aluminum nitride (AlN), nine crystalline phases: sodium chloride (NaCl), KCl, SiO2, Si, and MgAl2 O4 .As a rough estimation in a recent report [1], more than a million tons of the SAD are produced throughout the world each year
Of the as-received SAD consisted of Al and O elements, and the rest of them were composed of salts constituents like Na, K, F, and Cl, and other alloying elements (Mg, Si, Ca, Fe, Ti, Mn, Cu, etc.,)
According to the diffraction peaks, it can be confirmed that the as-received SAD was mainly constituted of nine crystalline phases: sodium chloride (NaCl), potassium chloride (KCl), pure aluminum (Al), aluminum nitride (AlN), corundum (Al2 O3 ), silicon (Si), silica (SiO2 ), calcium fluoride (CaF2 ), and spinel (MgAl2 O4 )
Summary
With the extensive development of secondary aluminum industry, secondary aluminum dross (SAD) is widely generated during aluminum recycling process through remelting with salt flux.In general, the SAD primarily contains Al2 O3 , aluminum metal, AlN, NaCl, KCl, SiO2 , Si, and MgAl2 O4 .As a rough estimation in a recent report [1], more than a million tons of the SAD are produced throughout the world each year. With the extensive development of secondary aluminum industry, secondary aluminum dross (SAD) is widely generated during aluminum recycling process through remelting with salt flux. The SAD primarily contains Al2 O3 , aluminum metal, AlN, NaCl, KCl, SiO2 , Si, and MgAl2 O4. It should be noted that nearly 95% of the SAD is dumped in landfills, which imposes a severe menace to the ecosystem [2]. F− , Cl− , and other toxic metal ions might leak into ground water resulting in serious water pollution [3]. Inhalation of the SAD particles suspended in the air may cause some respiratory diseases [4]. When the AlN in the SAD comes in touch with water, water vapor, or moisture, hazardous ammonia gas (NH3 ) is released, which pollutes the atmosphere [5]
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