The general reaction that occurs in the Pidgeon process is:
For industrial use, ferrosilicon is used because its cheaper and more readily available than silicon. The iron from the alloy is a spectator in the reaction. CaC may also be used as an even cheaper alternative for silicon and ferrosilicon, but is disadvantageous because it decreases the magnesium yield slightly.
The magnesium raw material of this type of reaction is magnesium oxide, which is obtained in many ways. In all cases, the raw materials have to be calcined to remove both water and carbon dioxide. Without doing so, the reaction would be gaseous at reaction temperatures and may even revert the reaction. Magnesium oxide can be obtained by sea or lake water magnesium chloride hydrolyzed to hydroxide. It is calcined to magnesium oxide by removing water. Another option is to use mined magnesite (MgCO) calcined to magnesium oxide by carbon dioxide removal.
The most used raw material is mined dolomite, a mixed (Ca,Mg)CO, where the calcium oxide present in the reaction zone scavenges the silica formed, releasing heat and consuming one of the products, ultimately helping push the equilibrium to the right.
(1) Dolomite calcination
(2) Reduction
The Pidgeon process is an endothermic reaction (H° ~183.0kJ/mol Si). Thermodynamically speaking, the temperatures decrease when the vacuum is used for both MgO and calcined dolomite.