Silicon Compounds - Production - Metallurgical Grade

Metallurgical Grade

Elemental silicon not alloyed with significant quantities of other elements, and usually > 95%, is often referred to loosely as silicon metal. It makes up about 20% of the world total elemental silicon production, with less than 1 to 2% of total elemental silicon (5–10% of metallurgical grade silicon) ever purified to higher grades for use in electronics. Metallurgical grade silicon is commercially prepared by the reaction of high-purity silica with wood, charcoal, and coal in an electric arc furnace using carbon electrodes. At temperatures over 1,900 °C (3,450 °F), the carbon in the aforementioned materials and the silicon undergo the chemical reaction SiO₂ + 2 C → Si + 2 CO. Liquid silicon collects in the bottom of the furnace, which is then drained and cooled. The silicon produced this manner is called metallurgical grade silicon and is at least 98% pure. Using this method, silicon carbide (SiC) may also form from an excess of carbon in one or both of the following ways: SiO₂ + C → SiO + CO or SiO + 2 C → SiC + CO. However, provided the concentration of SiO₂ is kept high, the silicon carbide can be eliminated by the chemical reaction 2 SiC + SiO₂ → 3 Si + 2 CO.

As noted above, metallurgical grade silicon "metal" has its primary use in the aluminum casting industry to make aluminum-silicon alloy parts. The remainder (about 45%) is used by the chemical industry, where it is primarily employed to make fumed silica.

As of September 2008, metallurgical grade silicon costs about US$1.45 per pound ($3.20/kg), up from $0.77 per pound ($1.70/kg) in 2005.