Industrial synthesis of technical-grade silicon
On an industrial scale, silicon is obtained using high temperatures in specialized electric furnaces. Silicon dioxide ($SiO_2$) serves as the starting raw material, and carbon (typically in the form of coal) acts as the reducing agent.
The essence of the process is that carbon, under intense heating, strips oxygen from silicon dioxide. This results in the formation of free silicon and the release of carbon monoxide, which leaves the reaction zone:
$$SiO_2 + 2C = Si + 2CO\uparrow$$
This method is economically profitable; however, the product obtained in this manner contains a significant amount of impurities and is unsuitable for high-tech manufacturing.
Synthesis of high-purity silicon
Manufacturing semiconductor devices requires silicon with a minimal content of foreign atoms. The industrial method of reduction with coal is completely unsuitable for these purposes.
High-purity silicon is obtained by an indirect route via halides. The starting substance is silicon tetrachloride ($SiCl_4$). An alkali metal, sodium ($Na$), is used as a powerful reducing agent. During the chemical interaction, sodium binds with chlorine, releasing pure silicon:
$$SiCl_4 + 4Na = Si + 4NaCl$$
The resulting element possesses the necessary semiconductor properties and can be used in electronics.
Laboratory production and purification
In a chemical laboratory setting, free silicon is obtained by the magnesiothermic method. This utilizes accessible reagents: pure white sand (which is silicon dioxide, $SiO_2$) and metallic magnesium powder ($Mg$).
Procedure:
- The starting substances are thoroughly mixed.
- The reaction is initiated by ignition (a burning magnesium ribbon is convenient to use as a fuse).
- A vigorous exothermic reaction takes place, in which magnesium reduces silicon:
$$SiO_2 + 2Mg = Si + 2MgO$$
Product purification: Upon completion of the reaction, a mixture is formed containing the target silicon and a byproduct, magnesium oxide ($MgO$). To isolate the pure element, the resulting mass is treated with a hydrochloric acid ($HCl$) solution. The acid dissolves the magnesium oxide, leaving the silicon untouched. Through filtration and washing, silicon is isolated in the form of a characteristic dark-gray powder. In this state, it is referred to as amorphous silicon.
Side processes and silane formation
Laboratory synthesis using magnesium rarely proceeds perfectly smoothly. Alongside the main silicon dioxide reduction reaction, a portion of the reducing agent interacts with the newly formed silicon. This leads to the appearance of an undesirable impurity: magnesium silicide ($Mg_2Si$).
The presence of magnesium silicide is detected during the acid purification stage of the mixture. When hydrochloric acid contacts $Mg_2Si$, a decomposition reaction occurs, accompanied by the release of gas:
$$Mg_2Si + 4HCl = SiH_4\uparrow + 2MgCl_2$$
The released gas is called silane ($SiH_4$). This is a hydrogen compound of silicon that, in its chemical structure, is a complete structural analogue of methane ($CH_4$), with the central carbon atom replaced by a silicon atom.
Briefly: answers to topical questions
What allotropic modifications of free silicon exist?
Free silicon does not have allotropic modifications; it forms only a single simple substance. Unlike carbon, silicon is represented by a single substance that has a diamond-type crystal lattice. Externally, it is a brittle, dark-gray material with a metallic luster. When obtained in the laboratory by reducing silicon oxide with magnesium, a dark-gray powder is formed, traditionally called amorphous silicon.
What occupational lung disease develops from prolonged inhalation of silicon dioxide dust?
Prolonged inhalation of silicon dioxide dust leads to the development of silicosis. This occupational disease belongs to the group of pneumoconioses. All types of pneumoconiosis are characterized by the development of pulmonary pneumofibrosis. Silicon dioxide exhibits pronounced immunogenic properties, and crystalline silicon dioxide is classified as an occupational carcinogen.
If silicon dioxide is not in a free state but bound to other elements, silicatoses develop—pneumoconioses caused by dust containing silicates. These include, for example, asbestosis and talcosis.
Class and exam questions
Why is semiconductor-grade silicon not produced by coal reduction in furnaces?
The process in electric furnaces involving coal yields technical-grade silicon contaminated with carbon and other impurities. Microelectronics require ideal purity, which is provided exclusively by reducing silicon tetrachloride with sodium.
Why is hydrochloric acid used in the laboratory production of silicon?
Hydrochloric acid is necessary for purifying the final product. It dissolves the byproduct magnesium oxide ($MgO$) and magnesium silicide impurities, leaving behind an insoluble dark-gray powder of pure amorphous silicon.
What is silane and under what conditions is it formed in the laboratory?
Silane ($SiH_4$) is a hydrogen compound of silicon, an analogue of methane. It is released as a gas as a byproduct during the purification stage when hydrochloric acid destroys the formed magnesium silicide impurity ($Mg_2Si$).
What else to study on this topic
- Structure and properties of amorphous silicon compared to crystalline silicon
- Comparative characteristics of silane ($SiH_4$) and methane ($CH_4$)
- Chemical properties and applications of silicon tetrachloride ($SiCl_4$)
- The nature of the chemical bond in metal silicides
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