A simplified mass balance

Nearly all the iron in the ore leaves as hot metal, which is roughly 94.5 percent iron with about 4.5 percent carbon picked up from the coke. So the model divides the iron charged by 0.945 to estimate hot metal tonnes.

If the iron is present as hematite (Fe2O3), each tonne of iron carries about 0.43 tonnes of oxygen, and pure hematite is 69.9 percent iron. Whatever is left after the hematite is gangue, the sand and clay that limestone turns into slag. These are teaching estimates; real furnaces also lose some iron to slag and dust and add pellets, sinter and fluxes.

Why it runs nonstop

A blast furnace campaign can last fifteen years or more without a shutdown. Cooling it down would freeze the burden into a solid mass, so operators tap iron and slag around the clock while new charges enter the top.