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Sodium is the 7th most abundant element by weight found in earth’s crust.
Minerals of sodium
(i) Rock salt – NaCl
(ii) Chile salt petre – NaNO3
(iii) Glauber’s salt – Na2SO4.10H2O
(iv) Borax or sodium borate – Na2B4O7.10H2O
(v) Albite or soda feldspar – Na2O.Al2O3.6SiO2 or NaAlSi3O8
Extraction of sodium
Sodium is extracted by Down’s process through electrolysis of fused sodium chloride.
In this method, sodium is obtained by the electrolysis of mixture of sodium chloride (40%) and calcium chloride (60%) in fused state. The function of calcium chloride is to lower the operating temperature from 1080 K (m.pt. of NaCl) to about 850 K. The main reason for lowering the temperature are:
(i) The melting point of sodium chloride is very high it is very difficult to maintain it in the molten state during electrolysis.
(ii) Sodium is considerably volatile at the temperature needed for the electrolysis and therefore, a part of the metal produced is vapourised.
(iii) Molten sodium gets dispersed in molten chloride to form a metallic fog (colloidal solution) at high temperature.
(iv) Both sodium and chlorine, the two products of the electrolysis, have a corrosive action on the material of the vessel employed for the electrolysis at such a high temperature.
The cell consists of a steel tank lined with fire bricks. A circular graphite anode is placed in the centre of the cell which is surrounded by a cylindrical iron cathode. The anode and cathode are separated by a steel gauge cylinder through which molten sodium chloride can pass but molten sodium cannot. The purpose of using steel gauge is to keep sodium separate from chlorine which would otherwise react with chlorine. The anode is covered by a dome – shaped steel hood which provides the out – let for the escape of chlorine gas. The molten metal liberated at the cathode moves up and flow into the receiver containing kerosene oil. The following reactions take place:
Alt text: Down’s process to extract sodium