The biggest limitation in modern iron ore extraction is the removal of impurities from the extracted metal. Since a metal can be damaged and rendered unsafe if it contains impurities, blast furnace smelting has always used methods of extracting the impurities without remixing them into the molten metal. Early methods involved resmelting the Pig Iron extracted from the ore in a finery forge, which allowed the impurities to either evaporate as the iron was stirred or move up or down in the liquid metal, separating it.
Today, impurities are extracted using the Bessemer Process, which resmelts pig iron in a rotating container and uses oxygen to chemically separate carbon and other impurities. This method is much more efficient and easier to control. Still, since blast furnace smelting uses coke (coal processed into a hotter-burning substance) for its fuel, significant carbon remains in the metal after the initial smelting, and so the Bessemer process is forced to extract much more carbon than if a hypothetical carbon-free fuel were used.