Latest revision as of 03:57, 5 March 2018

Metallurgical-grade silicon (MSG or MG-Si) is silicon of relatively high purity in the order of 98% or higher which is used extensively in the metallurgical industry. MGS is not considered pure enough to be used for electronics and must be further purified into either extremely pure electronic-grade silicon which can be used for integrated circuit fabrication or slightly purer upgraded metallurgical-grade silicon which can be used in cheaper electronic devices such as solar cells and liquid crystal displays.

Overview[edit]

Despite being the second most abundant element in the Earth's crust, silicon is rarely found as the pure element. It is most commonly found as silica (silicon dioxide), e.g., from sand. A mixture of quartzite gravel and carbon are heated to around 1500-2000 °C in an electrode arc furnace. The SiO₂ is reduced to around 98% pure silicon by the carbon taking the oxygen.

SiO₂ + 2C → Si + 2CO

It's worth noting that this equation masks a more complex process. In reality, the reaction involves the formation of solid silicon carbide (SiC) and gaseous silicon monoxide (SiO) intermediates. At the bottom of the furnace (where temperatures can get as high as 1900 °C), two gases are produced as a result of the reduction of molten silica with carbon.

SiO₂ + C → SiO + CO

The two gases then flow to the cooler area of the furnace where the silicon monoxide is further reacts with the carbon to form silicon carbide (SiC).

SiO + CO → SiC + CO

The silica then reacts with the silicon carbide to form silicon monoxide and the desired silicon.

SiC + SiO₂ = Si + SiO + CO

The molten silicon is drained from the furnace via the tap hole where it is taken to the casting area and solidified. The final product is the metallurgical-grade silicon which is 98% pure silicon or higher.

Typical impurities[edit]

Metallurgical-grade silicon is roughly 98-99% pure silicon with aluminium and iron being the major source of impurities.