The classification of metallic silicon is usually based on the content of the three main impurities of iron, aluminum, and calcium contained in the metallic silicon component. According to the content of iron, aluminum, and calcium in metallic silicon, metallic silicon can be divided into different grades such as 553, 441, 411, 421, 3303, 3305, 2202, 2502, 1501, and 1101.
Industrially, metallic silicon is usually produced by reducing silica with carbon in an electric furnace. Chemical reaction equation: SiO2 + 2C → Si + 2CO The purity of the silicon produced in this way is 97~98%, which is called metallic silicon. It is then melted and recrystallized, and impurities are removed with acid to obtain metallic silicon with a purity of 99.7 to 99.8 percent.
The main component of metallic silicon is silicon, so it has similar properties to silicon. Silicon has two allotropes: amorphous silicon and crystalline silicon. Amorphous silicon is a gray-black powder, which is actually a microcrystal. Crystalline silicon has the crystal structure and semiconductor properties of diamond, with a melting point of 1410°C, a boiling point of 2355°C, Mohs hardness of 7, and brittleness. Amorphous silicon is chemically active and can burn violently in oxygen.
It reacts with non-metals such as halogen, nitrogen, and carbon at high temperatures, and can also react with metals such as magnesium, calcium, and iron to form silicides. Amorphous silicon is almost insoluble in all inorganic and organic acids, including hydrofluoric acid, but is soluble in a mixed acid of nitric acid and hydrofluoric acid. 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 acid or organic acid, but is soluble in mixed acids of nitric acid and hydrofluoric acid and concentrated sodium hydroxide solution.
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