> Either way your going to need fuel to move the material to earth. Energy to process the material ect..

There is unlimites solar power in the asteroid belt, 24/7.

If you were really desparate, a solar furnace could vaporize mine tailings and accelerate it through a nozzle for thrust. It wouldn't be efficient, but it is pretty much free, (except for shipping and handling). Kind of like that classic black and white movie where they are breaking up the wood of a box car to feed the steam engine pulling the train.