How silicon metal (industrial silicon) is made

Silicon metal, also known as industrial silicon or crystalline silicon, is usually produced by reducing silica from carbon in an electric furnace. Silicon metal is a product smelted by quartz and coke in an electric furnace, the content of the main component silicon element is about 98% (in recent years, the Si content of 99.99% is also contained in silicon metal), and the remaining impurities are iron, aluminum, calcium, etc. Its main use is as an additive for non-ferrous alloys and as a starting material for the production of semiconductor silicon and silicones.

In China, silicon metal is usually classified according to the content of the three main impurities it contains iron, aluminum and calcium. According to the percentage content of iron, aluminum and calcium in silicon metal, silicon metal can be divided into 553, 441, 411, 421, 3303, 3305, 2202, 2502, 1501, 1101 and other different grades. (Regarding the source of silicon metal numbers: the codes 1st and 2nd are the percentage content of iron and aluminum, and the third and fourth digits represent the calcium content.) For example, 553 represents that the content of iron, aluminum and calcium is 5%, 5%, and 3%; 3303 represents that the content of iron, aluminum and calcium is 3%, 3%, 0.3%)

The production of silicon metal adopts the carbon thermal method, that is, the method of smelting silica and carbonaceous reducing agent in a mineral heat furnace. The purity of the silicon obtained in this way is 97%~98%, and such silicon can generally be used for metallurgical purposes. If you want to obtain a higher grade of silicon, you need to refine it to remove impurities to obtain silicon metal with a purity of 99.7%~99.8%.

Quartz sand is used as raw material to smelt silicon metal, including quartz sand blocking, preparation of charge and mineral furnace smelting.

Generally speaking, high-quality quartz sand will be directly used to produce high-grade quartz glass products, and even processed into gem-level products such as crystal, tourmaline and so on. The grade is slightly worse but the reserves are larger, the mining conditions are slightly better, and the surrounding electricity is cheaper and suitable for the production of silicon metal.

At present, the production process route of the carbon thermal method for the production of silicon metal in China is generally used to use silica as raw material, petroleum coke, charcoal, wood chips, low ash coal, etc. as reducing agent, smelting at high temperature in the mine thermoelectric furnace, and reducing silicon metal from silica, which is a slag-free submerged arc high-temperature melting process.

Principles of chemical reactions

Chemistry reaction mechanism

It is generally believed that smelting silicon metal is such a reaction:

SiO2 + C -> Si + CO2 ?

But there are actually at least 10 reactions and side reactions involved:

SiO2 + 3 C -> SiC + 2 CO ........ (1)

2SiO2 + SiC -> 3 SiO + CO ........ (2)

SiO2 + 2 SiC -> 3 Si + 2 CO ........ (3)

2SiO + O2 -> 2SiO2 ........ (9)

Therefore, although silicon metal is extracted from silica, not all silica is suitable for making silicon metal. The ordinary sand we see daily is not the real raw material of silicon metal, but the quartz sand used in the above-mentioned industrial production, and has undergone a multi-step reaction to complete the transformation from sand to silicon metal.