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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
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Reactivity of silicon and
related compounds
Silicon
- 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 Hydroxide |
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 |
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Silica
- 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
SiF4 |
| 2 |
Aqueous alkali |
Silica
dissolves slowly in aqueous alkali , and
more rapidly in fused alkalis MOH
or fused carbonates M2CO3 ,
forming
silicates
SiO2 + NaOH à ( Na2SiO3
)n and
Na4SiO4 |
| 3 |
Halogens |
Only fluorine
attacks SiO2 SiO2 +
2F2 à SiF4 +O2 |
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