While neutral on the scale of industry and nature, my first load of iron, which was only 20 ore and not even a full smelter, yielded me 6 cast iron bars. I turned them into bloom and only 1 slagged. The 5 remaining bloom I handed to nearly full industry friend and he turned them into 5 wrought iron.
Since then I've run into the occasional ironless load, but usually those are made up for by the loads with 5 or 6 cast iron. The more smelters you fill the more consistent the results you'll see. With a single smelter full you run the risk of getting nothing for your work, or getting a lot.