Tin sludge produced during tin electroplating of steel sheet is an interesting secondary source of tin. Dried sludge usually contains more than 50% tin. Hydrometallurgical sludge treatment consists of several steps, including leaching in hydrochloric acid and electrolytic recovery of tin.

http://www.journalssystem.com/ppmp/Sustainable-development-in-the-tinplate-industry-refining-tinplate-leachate-with,115270,0,2.html