How efficient is iron smelting ?
since you are an astrophysics student, I'm going to make a big assumption here, if I'm wrong well, plan "B"... I'm assuming you are trying to figure out how much mass and energy you'll need to launch in order to smelt an asteroid. consider another thought. if it is an asteroid and not something planetary like say, mars....
you don't NEED carbon to tie up the oxygen. all you need is heat and a vacuum. and lo and behold what do you have all around you in space? a vacuum of a quality that most semiconductor manufacturers envy.... save the carbon to add back to the refined iron to make steel with instead.
and by the way. you wouldn't need a very massive furnace, since you won't have any conduction, only radiative heating, you don't need a particularly strong vessel. vacuum alone is a very good insulator. you could concievably use something akin to a solar sail to focus solar energy onto your furnace load thereby making the energy required free. perhaps a magnetic field to contain your furnace load
again assuming I'm understanding what you are after, you need to re-think the process. processes that work in a gravity field and/or atmosphere don't work the same way outside one.
what is the goal. and how do you arrive at that goal. doing things like you would on earth probably won't work the same in space.