We came to some conclusions after the first attempt, so we changed the shape of the furnace to make it thinner but taller. And before putting the first portion of iron ore we put more charcoal to the furnace to make sure that all the iron ore can go through a good amount of charcoal.
At the end of the process the result was very much similar to the first attempt. No slug flew out, there were some bigger peaces of some strange mixture in the furnace.
You can find photos here.