CBSE CHEMISTRY X - Extraction of metals

OCCURRENCE OF METALS

The earth’s crust is the major source of metals. Seawater also contains some soluble salts such as sodium chloride, magnesium chloride, etc. The elements or compounds, which occur naturally in the earth’s crust, are known as minerals. At some places, minerals contain a very high percentage of a particular metal and the metal can be profitably extracted from it. These minerals are called ores.

Minerals:-The compounds of an element that occurs naturally in the earth’s crust.

Ores: - The minerals from which an element can be profitably and conveniently extracted.

Enrichment of ores: - It is done to remove earthy impurities like clay, sand etc. from an ore is called enrichment or concentration of the ore. 

Gangue: - The earthy impurities that is found with ores.

Extraction of Metals

Extracting metal low in activity series:- The metals low in the reactive series are very less reactive. The oxides of such metals can be reduced to metals by heating alone e.g. mercury is obtained from its ore, cinnabar (HgS), by the process of heating.Extraction of metal using Heat is called Pyrometallurgy.

                              Heat

2HgS(s) + 3O2 -------------->  2HgO(s) + 2SO2(g)

2HgO(s)           -------------->  2Hg(l) + O2(g)

Copper can also be obtained in a similar manner from its sulphide ore (Cu₂S).

                               Heat

2Cu₂S + 3O (g) ------------------->  2u₂O(s) + 2S2O (g)

                               Heat

2Cu2O + Cu₂S -------------------------> 6Cu(s) + SO2 (g)

Extracting Metals in the Middle of the Activity series

Metals like iron, zinc, lead etc. occur as sulphides or carbonates in nature. First the ore is converted into its oxide form This is done by either roasting or by calcination. 

The sulphide ores are converted into oxides by heating strongly in the presence of excess air. This process is known as roasting.

The carbonate ores are changed into oxides by heating strongly in limited air. This process is known as calcination.

1.Roasting:- 

2Zns(s) + 3O2 (g)---- heat --------> 2ZnO +2SO2 (g)

2Pbs (s) + 3O2 (g)---- heat --------> 2PbO +2SO2 (g)

4Fes (s) +7O2 (g)---- heat --------> 2Fe2O3 +4SO2 (g)
2.Calcination:-

ZnCO3(s) -----heat--------> ZnO(s) +CO2(g) 

Al2O3.2H20(s) -----> Al2O3(s) + 4 H2O(g)

CaCO3   ---------> CaO + CO2

3.SMELTING 

The metal oxide is reduced to metal by using suitable reducing agent such as Carbon. 

For example, when zinc oxide is heated with carbon, it is reduced to metallic zinc.

ZnO(s) + C(s) → Zn(s) + CO(g)

Besides using carbon (coke) to reduce metal oxides to metals, sometimes displacement reactions can also be used. The highly reactive metals such as sodium, calcium, aluminium, etc., are used as reducing
agents because they can displace metals of lower reactivity from their compounds.

For example, when manganese dioxide is heated with aluminium powder, the following reaction takes place –
3MnO2(s) + 4Al(s) → 3Mn(l) + 2Al2O3(s) + Heat

The reaction of iron(III) oxide (Fe2O3) with aluminium is used to join railway tracks or cracked machine parts. This reaction is known as the thermit reaction.

Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s) + Heat

Extracting metals towards the top of activity series                                                                                       The metals high up in the activity series are very reactive. They cannot obtain by reduction of their oxides by carbon. These metals are obtained by the electrolysis of their molten chlorides e.g. Extraction of Na from NaCl

At cathode Na⁺ + e^- -----àNa

At anode 2Cl^- --à Cl₂ + 2e^-