Production of Metallic Silicon
Industrially, metallic silicon is usually produced by reducing silica with carbon in an electric furnace. The chemical reaction is: SiO2 + 2C gives Si + 2CO. The purity of the silicon produced in this way is 97-98 percent, 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 99.7-99.8 percent.
Classification by Purity
Metallic silicon is classified by purity and impurity profile: chemical grade, about 98-99 percent, for silicones and chemical products, metallurgical grade for aluminum alloys and ferroalloys, and higher-purity grades of 99.7 percent and above for electronics and photovoltaic applications.
The Two Allotropes of Silicon
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 that is actually a microcrystal. It is chemically active, can burn violently in oxygen, reacts with non-metals such as halogens, nitrogen and carbon at high temperatures, and can react with metals such as magnesium, calcium and iron to form silicides. It is almost insoluble in most acids including hydrofluoric acid, but soluble in a mixed acid of nitric and hydrofluoric acid; concentrated sodium hydroxide solution dissolves it with hydrogen release.
Crystalline silicon has the crystal structure and semiconductor properties of diamond, with a melting point of 1410 degrees Celsius, a boiling point of 2355 degrees Celsius, Mohs hardness of 7, and brittleness. It is relatively inactive and does not combine with oxygen even at high temperatures; it is insoluble in most acids but soluble in mixed nitric-hydrofluoric acid and concentrated sodium hydroxide solution.
Frequently Asked Questions
How is metallic silicon made? By reducing silica with carbon in an electric furnace, SiO2 + 2C gives Si + 2CO, followed by refining to higher purity.
What is the difference between amorphous and crystalline silicon? amorphous silicon is a gray-black microcrystalline powder, chemically active; crystalline silicon has diamond structure and semiconductor properties, with high melting point and low chemical activity.
What purity is metallic silicon? Direct furnace product is 97-98 percent; after refining, 99.7-99.8 percent and higher.
What are the key properties of crystalline silicon? Crystalline silicon has a melting point of 1410 degrees Celsius, a boiling point of 2355 degrees Celsius, Mohs hardness of 7, and diamond-type semiconductor properties.
What is metallic silicon used for? Chemical grade serves silicones and chemical products, metallurgical grade serves aluminum alloys and ferroalloys, and high-purity grades serve electronics and photovoltaic applications.
