Iron

Iron is extracted from its naturally occurring compounds such as haematite and magnetite. These mainly contain iron III oxide, which are reduced at high temperature with carbon in the blast furnace.

Reactions in a blast furnace

Raw Materials : Haematite (or Magnetite)
Coke
Limestone
Hot air

Coke is obtained from coal by heating it in a very limited supply of air. It has a high energy value per kg, but a troublesomely high sulphur content. The air which is blown in at the bottom of the blast furnace s heated by a process using heat from the hot waste gases - see above diagram. The 3 solid raw materials are mixed together thoroughly and added from the top of the furnace from time to time.

Reaction : Coke burns in hot air.

C_(s) + O_2(g) à CO_2(g)

This reaction is highly exothermic and provides much of the energy for the later reactions. CO₂ is also used in later reactions. Temperatures over 140^oC are achieved in the furnace.

Reaction 2 : Carbon Monoxide is formed with 2 reactions.

C + CO₂ à 2CO

Incomplete combustion of the Coke - the Coke burns in the limited supply of Oxygen.

Reaction 3 : the Iron(iii) Oxide in the ore is Reduced mainly by CO.

Fe₂O_3(s) + 3CO_(g) à 2Fe_(l) + 3CO_2(g)
Fe₂O_3(s) + 3C_(s) à 2Fe_(l) + 3CO_(g)

At the high temperatures, the limestone thermally decomposes.

CaCO₃ à CaO + CO₂

Calcium Oxide is a metal oxide and therefore basic. The main impurity of the ore is sand, a form of Silicon Dioxide, which is a non-metal oxide and is therefore acidic. It is removed by a reaction with the basic CaO.

CaO_(s) + SiO_2(s) à CaSiO_3(l) (a neutralisation reaction)

At the temperature of the blast furnace, calcium silicate is a low density liquid which floats on top of the molten iron. It is called Slag and it can be removed by scraping. It is used in roadmaking and insulating houses.
We notice that the conversion of Iron(iii) Oxide to iron is a Reduction, and that the formation of the slag is an acid based neutralisation.
The Iron coming out of the blast furnace contains about 4% carbon impurity. It's called 'Pig Iron'.
Pure iron is not useful for construction purposes as it is too soft and weak.

Steels are Alloys of Iron, containing up to 1% Carbon, and very small amounts of metals such as Manganese or Chromium or Nickel, which are added to different Steels according to what they are needed for.
Not enough Carbon and the steel is too weak; too much, and the Steel is too brittle.

Basic Oxygen Steel-Making Process (BOP)

The carbon and other non-metal impurities in the Pig Iron are removed by Oxidation by Oxygen gas to form Acidic Oxides. Those which are gases, e.g. CO₂ bubble out of the mixture. Those which remain in the mixture are removed by an Acid-Base reaction with added lime, CaO.

Depending on the ore, the molten Iron from the blast furnace can containing considerable Sulphur impurity. This is removed by blasting a jet of Magnesium powder into the molten Iron down a lance. The reaction between Magnesium and Sulphur is vigorous and exothermic.

Mg + S à MgS

This Magnesium Sulphide is a low density liquid at these temperatures which floats on top of the Iron. It forms a slag which can be removed by scraping.
The molten iron is now poured into a container which contains carefully selected and separated pieces of scrap iron. The scrap iron is necessary:

The blast of pure oxygen is forced into the iron down a lance. The carbon burns, and bubbles out of the mixture, and is so removed.

C + O₂ à CO₂

Other non-metal impurities also burn but remain in the mixture.

P₄ + 5O₂ à P₄O₁₀
Si + O2 à SiO2

These are non-metal oxides and are therefore acidic. They are removed by a neutralisation reaction with the added base, calcium oxide

6CaO + P₄O₁₀ à 2Ca3(PO₄)₂ MMiCalcium Phosphate
CaO + SiO₂ à CaSiO₃ MMMMMiCalcium Silicate

This is the point at which the small amounts of carbon and other metals are added. The final mixture is stirred by bubbling the noble gas Argon through the mixture

Different steels have different properties. E.g. High Carbon Steel is strong but brittle. Low carbon steel (mild steel) is soft and easily shaped. Stainless Steel contains Chromium and Nickel and is Hard and Resistant to Corrosion.