I doubt that earth has the resources to serve as a processing facility for space asteroids. Even if we get metals from asteroids we would still have to smelt and refine them on Earth, which is a polluting and resource demanding process.
Would it not be possible to create a nuclear powered robotic installation on that asteroid as well? I guess the initial costs are high, but it would pay off in the long term (just move the factory once you are done, the low gravity requires little fuel)
If you're only going to the asteroid belt, you can probably get away with photovoltaics instead of nuclear power, too. And yeah, it seems it would be better to do it in place. But if it's really quite pure metal, there's not much refining that would need to be done.
Refining almost-pure metals is fortunately NOT as polluting as refining ore. It may come to the point where it becomes illegal to refine anything BUT roid-metal.
