8. Metallurgy

The science and technology of extracting metals from their ores and purifying them for use is called metallurgy. It includes concentration of the ore, extraction of the metal and refining of the metal.

The minerals from which a metal can be separated economically are called ores. Example: bauxite is the ore of aluminium.

The compounds of metals that occur in nature along with impurities are called minerals. All ores are minerals, but all minerals are not ores.

The impurities such as soil, sand and rocky substances present in an ore along with the metal compound are called gangue. Example: silica and iron oxide in bauxite.

1. Carbon dioxide in moist air reacts with copper and forms a greenish layer of copper carbonate on the vessel. This is called patination.
2. Lemon contains citric acid and tamarind contains tartaric acid.
3. These acids react with the basic copper carbonate layer and dissolve it. The layer is removed and the vessel shines again.

1. Ionic compounds are made of positive and negative ions.
2. There is a very strong electrostatic force of attraction between the oppositely charged ions.
3. A large amount of energy is needed to overcome this force. Hence ionic compounds have high melting points. Example: NaCl melts at 801 °C.

1. Sodium is a highly reactive metal. It reacts with oxygen in the air even at room temperature and catches fire.
2. It also reacts vigorously with moisture (water) and gives hydrogen and heat.
3. Sodium does not react with kerosene. Keeping it under kerosene cuts off its contact with air and moisture and prevents accidents.

1. Froth flotation is based on the hydrophobic and hydrophilic properties of particles.
2. Particles of metal sulphide ores are hydrophobic; they get wetted mainly by oil. Gangue particles are hydrophilic; they get wetted by water.
3. Pine oil forms a froth with water and blown air. The sulphide ore particles stick to this oily froth and rise to the surface, while the gangue settles down.
4. So pine oil helps to separate the ore from the gangue.

1. During the electrolysis of alumina, graphite (carbon) rods are used as anodes.
2. Oxygen gas is liberated at the anode: 2O²⁻ → O₂ + 4e⁻.
3. At the high temperature, this oxygen reacts with the carbon of the anodes and forms carbon dioxide gas.
4. So the anodes get oxidised and wear away gradually. Hence they have to be replaced from time to time.

1. Copper is more reactive than silver.
2. So copper displaces silver from the silver nitrate solution. This is a displacement reaction.
3. The silver metal set free gets deposited on the surface of the coin. This thin layer of silver gives the coin a glitter.
4. The solution slowly turns bluish because copper nitrate is formed.

Metal A (2, 8, 1) has atomic number 11, so it is sodium (Na). Metal B (2, 8, 2) has atomic number 12, so it is magnesium (Mg).

Metal A (sodium) is more reactive. It has only one valence electron, which it loses very easily. Magnesium has to lose two electrons, which needs more energy. Also, sodium lies above magnesium in the reactivity series.

Powdered ore is poured on a conveyor belt moving over a nonmagnetic roller and an electromagnetic roller. Magnetic particles stick to the magnetic roller and fall into the collector near it. Nonmagnetic particles are thrown farther and fall into the other collector. Example: cassiterite (tin ore), where magnetic ferrous tungstate is separated from nonmagnetic stannic oxide.

Finely ground ore is put in a tank of water with pine oil. Air is blown in and the agitator rotates. The oily froth carries the sulphide ore particles to the surface, where they are collected. The gangue settles at the bottom. Example: zinc blende (ZnS), copper pyrite (CuFeS₂).

A molten mixture of alumina, cryolite and fluorspar is electrolysed in a steel tank. The graphite lining of the tank is the cathode, and graphite rods dipped in the electrolyte are the anodes. Aluminium is deposited at the cathode and collects at the bottom. Oxygen is liberated at the anodes.

Finely ground ore is released into a tapering tank. A forceful jet of water is sent in from the lower side. The lighter gangue particles flow out with the water from the upper outlet. The heavier ore particles settle and are collected at the bottom.

A thin protective layer of aluminium oxide forms on the surface.

Iron is more reactive, so it displaces copper. The blue solution turns pale green and reddish copper is deposited.

This is the thermit reaction. So much heat is produced that iron is formed in molten state; it is used for welding rails.

During the extraction of aluminium, the ore used is bauxite (Al₂O₃·nH₂O). It contains 30% to 70% alumina (Al₂O₃). The rest is gangue, made of sand, silica (SiO₂), ferric oxide (Fe₂O₃) and titanium oxide (TiO₂).

During the extraction of aluminium, bauxite is concentrated by leaching. The ore is soaked in aqueous NaOH or aqueous Na₂CO₃. The alumina dissolves and forms water-soluble sodium aluminate, while most of the gangue does not dissolve and is filtered off.

During the extraction of aluminium by Hall’s process, powdered bauxite is heated with aqueous sodium carbonate to form sodium aluminate. The insoluble impurities are filtered out. The filtrate is warmed and CO₂ is passed through it, which precipitates aluminium hydroxide. This is heated at 1000 °C to obtain alumina.

During the extraction of aluminium by Bayer’s process, ground bauxite is heated with concentrated caustic soda (NaOH) at 140–150 °C under high pressure for 2 to 8 hours in a digester. Alumina, being amphoteric, forms water-soluble sodium aluminate. Iron oxide does not dissolve and is filtered off. The sodium aluminate solution is diluted and cooled to 50 °C, and aluminium hydroxide precipitates.

This grouping follows the reactivity series in the textbook (Fig. 8.5).

Write a note on each article (metal, use, how it has become dull). Some laboratory methods to restore glitter: