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).

AM30 was inoculated by MgCO3 powder with different holding time. The influence of MgO decomposed by MgCO3 on the grain refinement effect was mainly discussed in the present study. Three sets of comparative samples were prepared. They were AM30 alloy inoculated by MgO and pure Mg inoculated by MgO and MgCO3. The possible nucleating particles were observed and analyzed by EPMA and SEM. AM30 alloy could be effectively refined by either MgCO3 or MgO inoculation. Grain refining efficiency and fading effect of MgO inoculation were better than those of MgCO3 inoculation. However, pure Mg could not be refined by these two inoculants. Al is an indispensable element to determine the grain refinement of Mg alloys inoculated by either MgCO3 or MgO. MgO should not be the effective substrates for α-Mg phase. A novel grain refining mechanism of MgCO3 inoculation on AM30 alloy was proposed by combining experimental results with theoretical calculation, i.e., MgAl2O4 should be the potent nuclei of α-Mg grain for the AM30 alloy in addition to Al4C3