How does thermal reduction extract magnesium from calcined dolomite?

In thermal reduction, calcined dolomite—consisting of magnesium oxide (MgO) and lime (CaO)—is reduced using silicon, typically in the form of ferrosilicon, to produce magnesium gas and a dicalcium silicate slag. Because this reaction is endothermic, it requires significant heat input. To lower the necessary reaction temperatures, industrial processes are conducted under vacuum.1

There are three primary methods for this extraction:

• Pidgeon process: Calcined dolomite and ferrosilicon are briquetted and heated in nickel-chromium-steel retorts at 1,200 °C under a vacuum of 13 pascals. Magnesium is collected as solid crystals (crowns) in condensers.1
• Bolzano process: Briquettes are heated using internal electric conduction at 1,200 °C and pressures below 400 pascals.1
• Magnetherm process: Alumina is added to the charge to lower the slag's melting point, allowing it to remain liquid. This enables direct heating via electric current at 1,600 °C. The magnesium vapor is condensed in a separate system, while the molten slag is tapped periodically.1

These methods are energy-intensive, reflecting the stable nature of magnesium compounds.2