O Level Notes : Chemistry - Extraction of Metals

We will start off with ores. Ores are natural compounds of metals found in the ground, often oxides, sulphides or carbonates. Obtaining metals from their ores is a reduction process as the metal ion has to gain electrons. Ores of metals high in reactivity series are difficult to extract and electrolysis is used. Middle order metals like iron are extracted by reducing with carbon in the blast furnace.

Blast Furnace:

The main ore of iron is called haematite. We extract iron from this.

The furnace is charged from the top, with our three raw materials: haematite, coke and limestone. It is heated by blowing hot air in at the base of the furnace, through pipes. What happens now is that the coke present in the furnace burns, and as it is a very exothermic reaction, we get all the heat we need. The coke also burns to provide carbon monoxide, which is the reducing agent for the ore. The limestone present in the furnace removes the main impurity in the ore, which is sand (silicon dioxide). The iron produced in the blast furnace is called pig iron and contains about 4% carbon.

Steel:

It is produced by purifying the pig iron from the blast furnace by burning off impurities with oxygen gas. Controlled amounts of carbon are then added (0.5% carbon is mild steel, 1.5% carbon is high carbon steel). Mild steel is an example of steel that is low in carbon content, and is soft and easy to mould. It is used in car bodies and machinery.

Rusting:

It is the reaction of iron with air (oxygen) and water to form rust (hydrated iron(III) oxide). Rusting is a redox process. Many a times, this rusting needs to be prevented, and for that some methods have been devised, for example, rusting can be prevented by painting, coating with oil or grease, metal planting or electroplating, and also by sacrificial protection.

Stainless steel is an example of an alloy of iron that has been made specifically resistant to corrosion.

Aluminium Extraction:

The main ore of aluminium is bauxite (aluminium oxide). After it is mined, it is first purified by adding it in sodium hydroxide. The impurities do not dissolve and are filtered off. The dissolved aluminium oxide is then precipitated out as aluminium hydroxide by diluting with water. This is then heated to form pure white aluminium oxide, or alumina. The aluminium is then extracted by electrolysis.

The alumina is added to and electrolytic cell and melted. A compound called cryolite (another aluminium compound) is added to lower the melting point of the alumina. The electrodes are made of graphite. Oxygen gas collects at the anode. The cathode is very interesting; it is the lining of the electrolytic cell that is made the cathode. Molten aluminium collects here.

At cathode: Al³⁺ (l) + 3e^- Al (l)

At anode: 2O^2- (l) O₂ (g) + 4e^-

Aluminium is a more reactive metal than its apparent lack of reactivity is explained by a thin protective oxide coating. This coating (unlike iron oxide) does not flake off and makes aluminium corrosion resistant.

Aluminium metal is relatively light, corrosion resistant, and a good conductor of heat and electricity. When alloyed, it becomes quite resistant.

Aluminium can be used in making kitchen foils due to its malleability and heat conducting ability, in overhead power cables due to its light weight and electrical conducting abilities, and in aircraft construction due to its lightness and resistance to corrosion.