My goal was to build an aluminum can smelter. At the time I had this idea that I could crush a diet Dr. Pepper can with my hands (this I often do) and then slide it down into a crucible at the focal point of the lens, and have it melt and drip into a bucket of water below, creating aluminum bb's which would be much denser than even crushed cans. The problem however was that aluminum oxidizes before it melts, so in the ideal set up you really want it running in an oxygen free environment. Way more work than I wanted to invest.
Later I discovered induction heating (which you can do with a windmill, some magnets and a coil of wire around a piece of steel (or aluminum).
Did you watch those videos of guys melting aluminum in their backyards? :)
But watching folks melting alumninum is instructive.