Classification of metallic silicon

The classification of metallic silicon is usually based on the content of the three main impurities, iron, aluminum, and calcium, contained in the composition of metallic silicon. According to the content of iron, aluminum, and calcium in metal silicon, it can be divided into different grades such as 553, 441, 411, 421, 3303, 3305, 2202, 2502, 1501, 1101, etc.
In industry, silicon metal is usually produced by reducing silicon dioxide by carbon in an electric furnace. The chemical reaction equation is SiO2+2C → Si+2CO. The purity of silicon obtained by this method is 97~98%, which is called metallic silicon. After melting and recrystallizing, impurities are removed with acid to obtain metallic silicon with a purity of 99.7-99.8%.

The main component of metallic silicon is silicon, therefore it has similar properties to silicon. Silicon has two allotropy, amorphous silicon and crystalline silicon. Amorphous silicon is a gray black powder, which is actually a microcrystalline material. Crystalline silicon has the crystal structure and semiconductor properties of diamond, with a melting point of 1410 ℃, boiling point of 2355 ℃, Mohs hardness of 7, and brittleness. Amorphous silicon has active chemical properties and can burn violently in oxygen. It can react with nonmetals such as halogen, nitrogen and carbon at high temperature, and can also react with metals such as magnesium, calcium and iron to form silicide. Amorphous silicon is almost insoluble in all inorganic and organic acids, including hydrofluoric acid, but can dissolve in mixed acids of nitric acid and hydrofluoric acid. A concentrated sodium hydroxide solution can dissolve amorphous silicon and release hydrogen gas. Crystalline silicon is relatively inactive and does not combine with oxygen even at high temperatures. It is not soluble in any inorganic or organic acid, but can be dissolved in mixed acids of nitric acid and hydrofluoric acid, as well as concentrated sodium hydroxide solutions.