Ultrasound-assisted extraction of Sn from tinplate scraps by alkaline leaching: Novel acoustoelectric synergy effect underlying intensifying mechanism

Clean and fast extraction of tin from the surface of tinplate scraps is of great significance for the efficient utilization of waste resources. However, the dense tin layer causes the low efficiency of conventional leaching process. To improve Sn leaching efficiency, the ultrasound technique was adopted to extract Sn from tinplate scraps by alkaline leaching in this study. In the NaOH-H2O2 leaching system, metallic tin and alloyed tin in Fe-Sn alloy located on the surface of tinplate scraps can be oxidized and transferred to soluble Na2SnO3, while the iron in Fe-Sn alloy was oxidized to oxides which were chemically inert in alkaline solution. The differences in chemical solubility of Sn and Fe, and solubleness of stannate and iron oxides gave rise to the selective separation of Sn from the tinplate scraps. The effects of the leaching parameters on the Sn leaching behaviors in conventional and ultrasound-assisted leaching processes were compared. The conventional leaching temperature and time were significantly reduced during the ultrasound-assisted leaching process. Almost all of Sn can be extracted after conventional leaching at 1 mol/L NaOH, temperature of 80 ℃ and time of 60 min, however the same Sn leaching effect can be achieved by ultrasound-assisted leaching at 60 ℃ for 30 min with ultrasound power of 60% (360 W). Sn leaching kinetics based on the plate model demonstrated the reaction rate constant of the ultrasound-assisted leaching was 70% higher than that of the conventional leaching. A novel acoustoelectric synergy effect underlying intensifying mechanism by ultrasound irradiation was proposed in this study. Eventually, this work provided a rapid and clean tin extraction method from tinplate scraps via the ultrasound-assisted alkaline leaching treatment.