Silicon
The history of silicon (Si) began shortly after 1800 when Davy concluded that silica was a compound, not an element. Silicon is second only to oxygen in occurrence in nature. It constitutes 27.6% of the Earth’s crust. Man knows more than 200 different var
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Silicon
14.1
Introduction
The history of silicon (Si) began shortly after 1800 when Davy concluded that silica was a compound, not an element. Silicon is second only to oxygen in occurrence in nature. It constitutes 27.6% of the Earth’s crust. Man knows more than 200 different varieties of silicon: quartz, quartzite, chalcedony, rock crystal, opal, sand and many others. Silicon is a strong deoxidizer; this property determines basically its use in metallurgy. Commercially, pure silicon is used for the production of semiconductor, silicon bronzes, etc. It is used as a deoxidizer, in the forms of ferroalloy, in the production of all kind of steels. High amounts of ferrosilicon in the form of powder are used as a slag deoxidizer; it is also used for the reduction of various oxides in ferroalloy production.
14.2
Sources
Silicon occurs widely over the Earth’s surface; minerals having a high content in silica, such as quartz, quartzite and chalcedony. Quartz is a compact mineral with crystal structure, specific gravity of 2.59–2.65, hardness 7, mostly colourless, white, grey or reddish, depending on impurities. Quartz is a relatively costly mineral and is used for the manufacture of crystalline silicon. SiO2 content in quartz is 98% and over. Iron oxide in quartz should not exceed 0.3%. Quartzite is a rock composed of quartz grains, cemented by a substance mainly containing silicon. Quartzites are widely distributed and their reserves are inexhaustible. Good grades of quartzite contain 96–97% SiO2, 1% Al2O3, roughly 1% CaO and MgO, and not more than 0.02% P2O5. They do not require upgrading, and are readily available.
© Springer Nature Singapore Pte Ltd. 2018 S.K. Dutta and D.R. Lodhari, Extraction of Nuclear and Non-ferrous Metals, Topics in Mining, Metallurgy and Materials Engineering, https://doi.org/10.1007/978-981-10-5172-2_14
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Chalcedony is a thin fibrous, sometimes porous, mineral, of various colours. Its behaviour in metallurgical processes is somewhat worse than that of quartz and quartzite as it contains a lesser amount of SiO2 (about 95%); its use is limited.
14.3
Extraction
14.3.1 Metallic Silicon The quartzite is reduced by charcoal in an electric furnace to produce silicon. SiO2 þ 2C ¼ 2Si þ 2CO
ð14:1Þ
SiO2 þ 3C ¼ SiC þ 2CO
ð14:2Þ
Any SiC formed is removed by secondary smelting with quartzite: 2SiC þ SiO2 ¼ 3Si þ 2CO
ð14:3Þ
In the production of silicon or ferrosilicon, it appears that the major gaseous reagent is silicon monoxide (SiO), which can form through the reaction of silicon or silicon carbide with SiO2 in the high-temperature region of the furnace. The silicon monoxide (SiO) product rises through the furnace and reacts with the reductant in the upper part of the furnace, to yield silicon carbide and carbon monoxide as products: Si þ SiO2 ¼ 2SiO
ð14:4Þ
SiC þ 2SiO2 ¼ 3SiO þ CO
ð14:5Þ
or
By production of SiO gas in the higher temperature (1650 °C) region, the reduction of SiO to silicon or silicon carbide is higher in the furnace: SiO þ C ¼ Si þ CO
ð14:6Þ
SiO
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