If your end product isn't salable, your job isn't done yet :-) For some components of "garbage", simple mechanical separation may be sufficient, and there may

If your end product isn't salable, your job isn't done yet :-) For some components of "garbage", simple mechanical separation may be sufficient, and there may already be buyers happy to buy your product if you can reach a sufficient level of purity and offer it at a sufficiently low cost. Simply separating out those components of garbage efficiently enough is one huge unsolved problem, and once you have solved that, you can get to work on the fun part, which is how to further refine every unwanted component until you have broken it down or built it up into something someone wants.