Extraction of Magnesium

Extraction of Magnesium

Magnesium Atomic Number: 12
Magnesium Electronic configuration: [Ne]3s²

Magnesium is highly electropositive and strong reducing agent. Therefore it is extracted by electrolysis of their fused metal halides containing alkali metal halides. The function of alkali metal halides is

1. To lower the operating temperature.
   
2. To increase the electrical conductivity.

1. From Magnesite: (MgCO₃)
   The magnesite ore is calcinated into magnesium oxide.
   
   MgCO₃  MgO + CO₂
   
   Magnesium from magnesium oxide is obtained either by thermal reduction or electrolytic reduction. The magnesium oxide is dissolved in a mixture of molten fluorides of magnesium, bairum and sodium. The electrolysis is done by using carbon rods as anodes suspended in molten mass and cast iron rods as cathodes at 650^oC. The magnesium is obtained in molten state.
   
2. By Electrolysis of anhydrous magnesium chloride:
   
   Step 1: Preparation of anhydrous MgCl₂
   
   1. From carnallite: (KCl.MgCl₂.6H₂O)
      Carnallite can not be dehydrated by heating alone. Because the water of crystallization reacts chemically with magnesium chloride to form oxychloride and finally to magnesium oxide.
      
      MgCl₂.6H₂O MgCl₂.2H2O Mg₂OCl₂ MgO
      
      So, carnallite is heated in a current of HCL gas and air.
      
      MgCl₂.6H₂O    MgCl₂ + 6H₂O
      
   2. From MgO:
      Chlorine is passed over red hot mixture of MgO and carbon.
      
      MgO + C + Cl₂    MgCl₂ + CO
      
   3. From Dow's process: Sea water containing magnesium compound is treated with calcium hydroxide when magnesium hydroxide get precipitated. It is dissolved in dil. HCL. The solution on concentration gives hydrated magnesium chloride, which is hydrated as
      
      
      

Electrolysis of MgCl₂

Anhydrous magnesium chloride is fused with NaCl and anhydrous calcium chloride in the ratio of 35% (MgCl₂); 50% (NaCl) and 15% (CaCl₂) to lower the fusion temperature and to increase the conductivity. The mixture is electrolysed at 700^oC in presence of an inert gas to protect it from reaction of N₂, O₂ and CO₂ present in air.

Magnesium is discharged at cathode. Magnesium being lighter than the fused mass floats on the surface from where it is removed periodically.