"Since the demand for recycled aluminum continues to increase, the discarded aircraft provide a large source of valuable metal. However cost-effective recycling of aircraft alloys is complex because aircraft alloys are: (a) typically relatively high in alloying elements and (b) contain very low levels of impurities to optimize toughness and other performance characteristics."
https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=...
Also
"The separation of scrap into specific aluminum alloy groups can be checked by Laser Induced Breakdown Spectroscopy (LIBS) or color sorting, but economic and technological challenges limit the use of these processes (Capuzzi and Timelli 2018; Gaustad et al., 2012). All sorting methods offer less than 100 % efficiency. This leads to significant contamination of the aluminum, in particular with iron and copper"
https://www.sciencedirect.com/science/article/pii/S092134492...