Silicon from sand

The First Step toward Silicones – Silicon from Sand

Silicon can be made in the laboratory by igniting a mixture of sand and magnesium powder with an ignition pellet:

SiO₂(s) + 2 Mg (s) ® Si (s) +2 MgO (s) DH°_R = -342 kJ/mol

In industry, a cheaper reducing agent – carbon (coke) – is used to convert the sand into silicon. This so-called electrothermal process is carried out in gigantic furnaces in which the following endothermic reaction occurs:

SiO₂(s) + 2 C (s) ® Si (s) + 2 CO (g) DH°_R = + 695 kJ/mol

The electrothermal process for the manufacture of silicon is illustrated in the diagram.

The production of one kilogram of silicon consumes 14 kWh electricity.

The liquid product mostly consists of silicon carbide SiC. Iron is added to suppress the formation of carbide. This leads to silicon-iron alloys.

The maximum possible purity of the raw silicon made by this process is 98 %. This level of purity is sufficient for conversion to silicones.

But it is far too low for the 99.9999999 % purity level required for semiconductor chips!