179.

How does magnesium occur in nature? How is magnesium obtained by electrolysis method ?

Magnesium docs do not occur in the free state in nature. In the combined state (minerals) it occurs as,
(i) Magnesite MgCO₃
(ii) Dolomite MgCO₃.CaCO₃
(iii) Carnallite KCl MgCl₂.6H₂O
(iv) Epsom salt MgSO₄.7H₂O

All plants and animal tissues contain small amounts of magnesium, it is contained in chlorophyll, the green colouring matter of plants. Sea water contains an appreciable amount of magnesium chloride.

Extraction of magnesium: Magnesium is usually extracted by the electrolysis of fused magnesium chloride or carnallite. It is isolated form sea water by Dow process. The various steps employed are

(i) Precipitation of magnesium hydroxide: Sea water is treated with lime water when magnesium hydroxide gets precipitated.


(ii) Conversion of magnesium hydroxide into magnesium chloride: The precipitate of magnesium hydroxide is dissolved in hydrochloric acid to get a clear solution of magnesium chloride.

The solution is concentrated when MgCl₂.6H₂O crystallises out.

(iii) Preparation of anhydrous magnesium chloride: A current of HCl gas is passed through magnesium chloride hexahydrate when anhydrous magnesium chloride is obtained.



(iv) Electrolysis of anhydrous magnesium chloride: Anhydrous magnesium chloride is added to a molten mixture of sodium chloride and calcium chloride (973 -1023 K). The mixture is then electrolyzed in an electrolytic cell which consists of the iron vessel which acts as a cathode. The anode consists of a graphite rod enclosed in a porcelain hood. The cell is heated externally to about 973 -1073 K. A stream of an inert gas such as coal gas is passed through the cell to check the oxidation of liberated magnesium by atmospheric oxygen. On passing electric current, the following reaction takes place:


At cathode. 

Thus, magnesium is liberated at the cathode while chlorine gas is liberated at the anode.