Metallurgical-grade silicon

Metallurgical-grade silicon is a shiny, gray, brittle solid that's about 98 to 99% silicon. It's pure enough for metal alloys and chemistry, but far too impure for chips. The rest is mostly iron, aluminum and calcium.

It's made by pulling the oxygen out of quartz. Lumps of quartz are loaded into a submerged-arc furnace together with sources of carbon: coal, coke, charcoal and wood chips. The wood chips keep the mix loose so gases can escape. Huge carbon electrodes carry an electric current that heats the heart of the furnace to around 2,000 °C. There, the carbon takes the oxygen from the silicon dioxide, leaving liquid silicon and carbon monoxide gas. The molten silicon is tapped from the bottom, cooled and crushed. The process uses a lot of electricity, so plants are often built near cheap power.

Most metallurgical silicon goes into aluminum alloys. In this build it has two jobs. Reacted with hydrogen chloride, it becomes trichlorosilane, the liquid that's purified into chip-grade polysilicon. Reacted with chloromethane over a copper catalyst, it becomes dimethyldichlorosilane, the starting point for silicone.