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The Extraction of Iron

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    Extracting iron from iron ore using a Blast Furnace

    The common ores of iron are both iron oxides, and these can be reduced to iron by heating them with carbon in the form of coke. Coke is produced by heating coal in the absence of air. Coke is cheap and provides both the reducing agent for the reaction and also the heat source. The most commonly used iron ores are hematite, \(Fe_2O_3\), and magnetite, \(Fe_3O_4\).

    The Blast Furnace


    At 500 °C
    3Fe2O3 +CO → 2Fe3O4 + CO2
    Fe2O3 +CO → 2FeO + CO2

    At 850 °C
    Fe3O4 +CO → 3FeO + CO2
    At 1000 °C
    FeO +CO → Fe + CO2
    At 1300 °C
    CO2 + C → 2CO
    At 1900 °C
    C+ O2 → CO2
    FeO +C → Fe + CO

    The air blown into the bottom of the furnace is heated using the hot waste gases from the top. Heat energy is valuable, and it is important not to waste any. The coke (essentially impure carbon) burns in the blast of hot air to form carbon dioxide - a strongly exothermic reaction. This reaction is the main source of heat in the furnace.

    \[ C + O_2 \rightarrow CO_2 \tag{1}\]

    The reduction of the ore


    \[ C + CO_2 \rightarrow 2CO \tag{2}\]


    \[ Fe_2O_3 +3CO \rightarrow 2Fe + 3CO_2 \tag{3}\]


    \[ Fe_2O_3 + 3C \rightarrow 2Fe + 3CO \tag{4}\]


    The function of the limestone

    Iron ore is not pure iron oxide - it also contains an assortment of rocky material that would not melt at the temperature of the furnace, and would eventually clog it up. The limestone is added to convert this into slag which melts and runs to the bottom. The heat of the furnace decomposes the limestone to give calcium oxide.

    \[ CaCO_3 \rightarrow CaO + CO_2 \tag{5}\]

    This is an endothermic reaction, absorbing heat from the furnace. It is therefore important not to add too much limestone because it would otherwise cool the furnace. Calcium oxide is a basic oxide and reacts with acidic oxides such as silicon dioxide present in the rock. Calcium oxide reacts with silicon dioxide to give calcium silicate.

    \[ CaO + SiO_2 \rightarrow CaSiO_3 \tag{6}\]

    The calcium silicate melts and runs down through the furnace to form a layer on top of the molten iron. It can be tapped off from time to time as slag. Slag is used in road making and as "slag cement" - a final ground slag which can be used in cement, often mixed with Portland cement.

    Cast iron

    The molten iron from the bottom of the furnace can be used as cast iron. Cast iron is very runny when it is molten and doesn't shrink much when it solidifies. It is therefore ideal for making castings - hence its name. However, it is very impure, containing about 4% of carbon. This carbon makes it very hard, but also very brittle. If you hit it hard, it tends to shatter rather than bend or dent. Cast iron is used for things like manhole covers, guttering and drainpipes, cylinder blocks in car engines, Aga-type cookers, and very expensive and very heavy cookware.



    Steel-making: the basic oxygen process

    Impurities in the iron from the Blast Furnace include carbon, sulfur, phosphorus and silicon. These have to be removed.

    • Removal of sulfur: Sulfur has to be removed first in a separate process. Magnesium powder is blown through the molten iron and the sulphur reacts with it to form magnesium sulfide. This forms a slag on top of the iron and can be removed.

    \[ Mg + S \rightarrow MgS \tag{7}\]

    • Removal of carbon: The still impure molten iron is mixed with scrap iron (from recycling) and oxygen is blown on to the mixture. The oxygen reacts with the remaining impurities to form various oxides. The carbon forms carbon monoxide. Since this is a gas it removes itself from the iron! This carbon monoxide can be cleaned and used as a fuel gas.

    Types of iron and steel


    • Mild steel: Mild steel is iron containing up to about 0.25% of carbon. The presence of the carbon makes the steel stronger and harder than pure iron. The higher the percentage of carbon, the harder the steel becomes. Mild steel is used for lots of things - nails, wire, car bodies, ship building, girders and bridges amongst others.
    • Special steels: These are iron alloyed with other metals. For example:

    iron mixed with special properties uses include
    stainless steel chromium and nickel resists corrosion cutlery, cooking utensils, kitchen sinks, industrial equipment for food and drink processing
    titanium steel titanium withstands high temperatures gas turbines, spacecraft
    manganese steel manganese very hard rock-breaking machinery, some railway track (e.g. points), military helmets

    Some environmental considerations


    Environmental problems in mining and transporting the raw materials

    Think about:


    Extracting iron from the ore

    Think about:

    • Noise.
    • Atmospheric pollution from the various stages of extraction. For example: carbon dioxide (greenhouse effect); carbon monoxide (poisonous); sulfur dioxide from the sulfur content of the ores (poisonous, acid rain).
    • Disposal of slag, some of which is just dumped.
    • Transport of the finished iron.


    Think about:
