Pidgeon process

The Pidgeon process is one of the methods of magnesium metal production, via a silicothermic reduction. Practical production requires roughly 35–40 MWh/ton of metal produced, which is on par with the molten salt electrolytic methods of production, though above the 7 MWh/ton theoretical minimum.

The basic chemical equation of this process is:

Silicon and magnesia react to produce silica and magnesium.

Though, according to Ellingham diagrams, this reaction is thermodynamically unfavorable, in accordance with the Le Chatelier's principle of equilibria, it can still be driven to the right by continuous supply of heat, and by removing one of the products, namely distilling out the magnesium vapor. The atmospheric pressure boiling point of magnesium metal is very low, only 1090 °C, and even lower in vacuum. Vacuum is preferred, because it allows lower temperatures.

The most commonly used and cheapest form of silicon is as a ferrosilicon alloy. The iron from the alloy is but a spectator in the reactions.

The magnesium raw material of this reaction is magnesium oxide, which can be obtained by several ways. In all cases the raw materials have to be calcined to remove both water and carbon dioxide, which would be gaseous at reaction temperatures, and follow the magnesium vapor around, and revert the reaction.

One way is by sea or lakewater magnesium chloride hydrolyzed to hydroxide, which is then calcined to magnesium oxide by removal of water. Another way is using mined magnesite (MgCO₃) that has been calcined to magnesium oxide by carbon dioxide removal. By far the most used raw material is mined dolomite, a mixed (Ca,Mg)CO₃, where the calcium oxide present in the reaction zone scavenges the silica formed, releasing heat and consuming one of the products, thus helping push the equilibrium to the right.

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