Silicon Metal Manufacturing
Silicon metal is produced in a number of complex and highly technical steps. First, the raw materials (quartz sand and charcoal) are placed in an electric arc furnace and heated to 2,000°-2,500°C. This forms silicon and carbon monoxide, takes between six and eight hours. The metal is then treated with oxygen and air to reduce the calcium and aluminium impurities. This produces metallurgical silicon.
Metallurgical silicon can be processed in a number of ways, and the current most popular method is using the Siemens process. The Siemens process involves exposing the previously purified silicon rods being exposed to a gas, usually trichloridesilane, at high temperatures. The high temperatures cause the gas to deposit pure silicon onto the rods, while the chlorine gas is removed from the chamber (Wikipedia and Peak Sun Silicon). However, the Siemens process is very expensive, and requires a large amount of energy to be successful.
A cheaper option is to upgrade the metallurgical silicon using high purity quartz in slag refining. A slag mixture containing SiO2 and CaO is often used for this purpose, as this mixture is suitable for removing boron and phosphorous, which are both essential to control when manufacturing solar wafers. More information on the chemistry behind slag refining can be found on our blog.