Iron compounds. Cast iron

Cast iron is an iron alloy containing more than 2.14 % carbon (usually 2.14–4.5 %), together with silicon, manganese, sulfur and phosphorus. It is made in a blast furnace: iron ore (Fe₂O₃, Fe₃O₄), coke and flux (limestone) are fed in at the top, and heated air or oxygen-enriched air is blown in from below. Coke burns: C + O₂ → CO₂, and CO₂ with hot coke gives CO: CO₂ + C → 2CO; CO reduces the ore oxides step by step: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂, Fe₃O₄ + CO → 3FeO + CO₂, FeO + CO → Fe + CO₂. The flux turns the waste rock of the ore into easily melting slag: CaCO₃ → CaO + CO₂, CaO + SiO₂ → CaSiO₃. In the hearth liquid cast iron collects at the bottom and the lighter slag, which protects it from oxidation, on top, and they are tapped separately. In grey (foundry) cast iron the carbon is graphite, and pipes, railings and machine parts are cast from it; in white cast iron the carbon is Fe₃C, it is hard and brittle and mostly converted into steel. In Uzbekistan ferrous metallurgy is mainly in Bekabad (the Uzbek Metallurgical Plant), which mostly makes steel from scrap in electric furnaces. The blast-furnace process creates dust, gases and waste and pollutes the environment, so methods of making iron without coke using hydrogen or natural gas (sponge iron) are being developed.