What is the role of ferrosilicon in the Pidgeon process?

In the Pidgeon process, ferrosilicon serves as the reducing agent used to extract magnesium from calcined dolomite.1

During this thermal reduction method, ground and calcined dolomite—which contains magnesium oxide—is mixed with finely ground ferrosilicon and formed into briquettes. When these briquettes are heated to approximately 1,200 °C (2,200 °F) under a vacuum, the silicon within the ferrosilicon reacts with the magnesium oxide to produce magnesium gas and a dicalcium silicate slag. The magnesium gas is subsequently collected and condensed into solid crystals, known as crowns.1