Reduction of silica

After oxygen, silicon is the most abundant element in the earth s crust, It occurs extensively as the oxide, silica, in various forms, for example, flint, quartz, sand, and as silicates in rocks and clays, but not as the free element, silicon. Silicon is prepared by reduction of silica, Si02- Powdered amorphous silicon can be obtained by heating dry powdered silica with either powdered magnesium or a... 

If an excess of magnesium is used, magnesium silicide, Mg2Si, is also produced.) The silicon obtained is a light brown hygroscopic powder. Crystalline or metallic silicon is obtained industrially by the reduction of silica with carbon in an electric arc furnace ... 

Silicon has an atomic number of 14 and an atomic weight of 28.06. It is a hard, brittle substance crystallising in a diamond lattice and has a specific gravity of 2.42. The elemental material is prepared commercially by the electrothermal reduction of silica. 

The equilibrium levels of the reaction products are very small, but both can dissolve in liquid sodium, and sodium oxide can form compounds with silica. As a consequence, the reaction moves to the right, leading to further reduction of silica. Nevertheless, vitreous silica crucibles have been used sucessfully for containing molten antimony (850°C), copper (1 210°C), gallium (1 100°C), germanium (1 100°C), lead (500°C) and tin (900 C). 

Figure 1. Temperature prograimned reduction of silica- and zirconia-supported catalysts containing nickel and sulfate, a) silica-series b) zirconia-series.

A step by step process of nucleation allows the good selectivities obtained in the controlled reduction of silica-supported [Ru(CO)3Cl2]2 or [Ru(CO)3Cl2(HOSi ] in the presence of alkali carbonates (Scheme 16.4) [11]. 

The ferrosilicon alloy required in the above process is produced by thermal reduction of silica with carbon in the presence of iron ... 

Figure 1. Temperature progiammed reduction of silica supported M0O3 and metal molybdates.

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Silicones are made by hydrolysis of organochlorosilanes RnSiCl4-n, which are produced from elemental silicon (obtainable from the carbon reduction of silica, i.e., sand, Section 17.7) by the Rochow process ... 

Crude elemental silicon can be obtained by reduction of silica sand with coke in the electric furnace (reaction 17.33) and may be adequate for making ferrosilicon alloys (Section 16.7.5) or silicones (Section 3.5). The high purity silicon used for electronic chips can be made from silica via silicon tetrachloride, which, like TiCU, is a volatile liquid (bp 57 °C) susceptible to hydrolysis but readily purifiable by fractional distillation. Indeed, the procedure for silicon resembles the Kroll process for titanium, except that an argon atmosphere is not necessary ... 

Cochran GA, Conner CL, Eismann GA, Weimer AW, Carroll DF, Dunmead SD, Hwang CJ (1994) The Synthesis of a High Quality, Low Cost Silicon Nitride Powder by the Carbothermal Reduction of Silica. In Hoffmann MJ, Becher PF, Petzow G (eds) Silicon Nitride 93. Key Eng Mater 89-91. Trans Tech Publications Ltd, Switzerland, p 3... 

Hnatko, M., Galusek, D., Sajgalik, P., Low-cost preparation of Si3N4-SiC micro/ nano composites by in-situ carbothermal reduction of silica in silicon nitride matrix, J. Eur. Ceram. Soc., (2), 24 2004, 189-195. 

Roberto, D., Cariati, E., Ugo, R., and Psaro, R., Surface-mediated organometallic synthesis High-yield preparations of neutral and anionic osmium carbonyl clusters by controlled reduction of silica-supported [Os(CO)3Cl2]2 and OsClj in the presence of Na2C03 or K2C03. Inorg. Chem. 35,2311 (1996). 

As an element, silicon is a hard brittle metallic-looking substance which crystallizes in the diamond lattice. It is produced commercially by the electrothermal reduction of silica, resulting in a product which contains about 97 per cent silicon. The element shows no visible oxidation or corrosion at ordinary temperatures and oxidizes very slowly below red heat. Halogens attack it more readily, and chlorination proceeds satisfactorily at 250° C. to form the silicon chlorides which are perhaps the best-known volatile compounds of silicon. Whenever the element is exposed to nascent hydrogen, or its metallic compounds are treated with acids, some hydrides usually are formed, as may be noticed by the odor which such hydrides impart to the gas evolved when cast iron is dissolved in dilute acids. A mixture of concentrated nitric and hydrofluoric acids will dissolve pure silicon, but mineral acids singly will not do so. Hot concentrated solutions of alkalies will dissolve it, however, with the evolution of hydrogen. 

If attention at first is confined to the production of methyl silicone from the previously accepted raw materials, the chemical processes must include reduction of silica to silicon, preparation of the methyl chloride from methane or methanol, reaction of the methyl chloride with silicon, and hydrolysis of the methylchlorosilanes. If the same conventions are used as in the discussion of the,Grignard method, and the methanol process for methyl chloride is elected, the steps are ... 

The carbothermic reduction of silica is believed to follow [25] a reaction mechanism given by... 

Silicon differs from carbon in that all of the element existing in nature is in an oxidized form, as SiCb or silicate minerals. The first step in the synthesis of sihcones then becomes the reduction of silica to an active reduced form of sihcon. The usual starting point is the electrothermal reduction of Si02 with carbon (equation 1). The elemental sihcon can be converted to reactive SiCLi by chlorination (equation 2) or to hydrochlorosilanes by reaction with HCl (equation 3). The importance of the latter materials in silicone technology will be described later. 

Silicon nitride powders have also been prepared from more readily available starting materials. The preparations involve the reduction of silica by carbon in the presence of nitrogen (2) or of silicic acid reacting with ammonia (4-7) ... 

The formation of silicon nitride whiskers was observed in several different reactions, including vapor deposition, CVD, and growth from a melt. However, only the following techniques are considered to have commercial significance nitriding of metallic silicon or silicon-silica mixture, carbothermal reduction of silica with simultaneous nitridation, and thermal decomposition of silicon halides. 

Elemental silicon is usually prepared by the high-temperature reduction of silica (sand) with coke. Excess Si02 prevents the formation of silicon carbide. 

Some ceramic materials are not found widely or at all in nature, and thus are synthesized for use. To prepare more complex ceramic compositions such as perovskites of general structural formula ABO3, and ferrites, of formula MFc204, the individual oxides or salts of the cations A, B, and M are often combined as powders and then reacted at high temperature by a solid-state diffusion mechanism. Silicon nitride (Si3N4) can be manufactured from either the nitridation of silicon metal or from the reaction of silicon tetrachloride with ammonia. Silicon carbide (SiC) is obtained from the reduction of silica with a carbon containing source. 

Compared to the Grignard route, the direct process reaction requires only the reduction of silica to silicon and the production of methyl chloride. A form of the overall reaction (related to equation 2) is shown in equation 3. On the basis of this equation there... 

The temperatures encountered in normal combustion conditions are generally not high enough to volatilise any silica, but it is possible that where higher temperatures do exist a small degree of silica vaporisation could occur, via the reduction of silica with carbon to SiO which later reoxidises to SiO creating a fiime. 

The reduction of silica surface at high temperature by the so-called spillover process leading to the formation of exposed silicon atoms and Si-H groups, is finally studied. 

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