I'd crash them into the moon (no atmosphere, so they wouldn't burn up on entry) for about a decade, then send ore-processing robots to smelt them down, then figure out later how to bring the results back.
Good idea at first look, but it would be tricky to get the trajectory right, though. The Moon's sphere of influence is tiny compared to the Earth's. You would need a guidance package and, perhaps more importantly in terms of cost, onboard rockets to change delta-v (and only one-use because they would be destroyed on impact).