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Extraction of Silicon

More than a million tonnes of silicon , Si , are produced annually . The element Si is obtained by reducing silica, SiO2 , with high purity graphite (as coke) in an electric
furnace .

SiO2 + 2C àSi + 2CO

Under this conditions , silicon carbide , SiC , can form . However , SiC can be elimalited by keeping the amount of SiO2 high .

2SiC + SiO2 à3Si + 2CO

For extracting high purity Si for the semiconductor industry , Si is converted to SiCl4 which is a volatile compound , and then is purified by exhaustive fractional distillation , and then the chloride is reduced with Mg or Zn . The resulting semiconductor-grade Si is melted and grown into single crystal .

SiO2 + 2C àSi + 2CO

Si + 2Cl2 à SiCl4

SiCl4 + 2Mg à Si + MgCl2



Reactivity of silicon and related compounds


The element Si is an unreactive element .It is unreactive towards oxygen , water , acids ( except HF ) , but dissolves in hot alkaline solution .

The following table shows some reactions of silicon

Reaction of silicon with Result
1 Water No reaction
2 Dilute acids No reaction
3 Concentrated acids Silicon is oxidized and fluorinated by

concentrated HF / HNO3

4 Aqueous solution of sodium


Cold solution :

Silicon reacts slowly

Hot solution :

Silicon reacts readily and gives solution

of silicates [ SiO4 ]4-

5 Halogens Silicon reacts readily , forming volatile

halides SiX4



Silica in any form is unreactive and it is an acidic oxides . The following table shows some reactions of silica :
Reaction of silica with Result
1 Acids Silica does not react with acid except

with HF , forming silicon tetrafluoride


2 Aqueous alkali Silica dissolves slowly in aqueous alkali ,

and more rapidly in fused alkalis MOH

or fused carbonates M2CO3 , forming


SiO2 + NaOH à ( Na2SiO3 )n and


3 Halogens Only fluorine attacks SiO2

SiO2 + 2F2 à SiF4 +O2


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