Silicon silica, carbothermal reduction

Production. Silicon is typically produced in a three-electrode, a-c submerged electric arc furnace by the carbothermic reduction of silicon dioxide (quartz) with carbonaceous reducing agents. The reductants consist of a mixture of coal (qv), charcoal, petroleum coke, and wood chips. Petroleum coke, if used, accounts for less than 10% of the total carbon requirements. Low ash bituminous coal, having a fixed carbon content of 55—70% and ash content of <4%, provides a majority of the required carbon. Typical carbon contribution is 65%. Charcoal, as a reductant, is highly reactive and varies in fixed carbon from 70—92%. Wood chips are added to the reductant mix to increase the raw material mix porosity, which improves the SiO (g) to solid carbon reaction. Silica is added to the furnace in the form of quartz, quartzite, or gravel. The key quartz requirements are friability and thermal stability. Depending on the desired silicon quality, the total oxide impurities in quartz may vary from 0.5—1%. 

Carbothermic Reduction. Silicon carbide is commercially produced by the electrochemical reaction of high grade silica sand (quartz) and carbon in an electric resistance furnace. The carbon is in the form of petroleum coke or anthracite coal. The overall reaction is... 

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... 

Ishii T, Sano A, Imai I (1987) ot-Si3N4 powder produced by nitriding silica using carbothermal reduction. In Somiya S, Mitomo M, Yoshimura M (eds) Silicon Nitride I. Elsevier, London, p 59... 

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. 

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. 

A fourth ahoy separation technique is fractional crystahization. If silica is co-reduced with alumina, nearly pure silicon and an aluminum silicon eutectic can be obtained by fractional crystahization. Tin can be removed to low levels in aluminum by fractional crystallization and a carbothermic reduction process using tin to ahoy the aluminum produced, fohowed by fractional crystahization and sodium treatment to obtain pure aluminum, has been developed (25). This method looked very promising in the laboratory, but has not been tested on an industrial scale. 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

While gaseous SiO is the most abundant silicon oxide in the universe, solid SiO does not exist naturally on earth. It was first prepared by Potter in 1905 by reduction of Si02 with carbon, silicon or SiC. Today, several tons of solid SiO are produced industrially each year by comproportionation of silica and silicon at low pressure (10 -10 " mbar) and high temperatures (1250-1400 °C). The gaseous SiO formed under these conditions is then condensed at colder surfaces. However, SiO is always present whenever silica or silicates are reduced at high temperatures, such as in the carbothermal reduction of Si02 (production of SiC) or in blast furnaces. The local structure at the interface between Si02 films on elemental Si, or the nature of nanocrystalline SiO particles,is also related to that of solid SiO. 

It has already been well known for more than two decades that it is feasible to produce SiC powders and/or whiskers by pyrolyzing rice husk [178] under an inert atmosphere, since a considerable content of colloidal amorphous silica is incorporated in husks and straws of this vegetable. It means that nature by itself is supplying a similar intimate mixture of carbon-containing molecules (cellulose) and colloidal silica like those artificially made mixtures we discussed in the last section. Thus, also in this case, silicon carbide may be formed by a carbothermal reduction process. This process starts in the temperature range 1100-1400 °C ... 

The preparation of a porous silicon carbide has been described by Fox and co-workers (10). The synthesis is based on heating the organosilicon pol3nmer (0 1158iOj 5) at 1600 C under argon (see Fig. 1). The pyrolysis reaction results in an intramolecular carbothermic reduction, i.e. the carbon bonded to silicon is used to remove oxygen and to form the carbide (the commercial manufacture of silicon carbide uses an external source of carbon for example, by mixing quartz sand and petroleum cokes). The product is purified by oxidation to remove excess carbon, followed by treatment with HF to remove silica. 

Moshtaghioun BM, Monshi A, Abbasi MH, Karimzadeh F (2011) A study on the effects of silica particle size and milling time on synthesis of silicon carbide nanoparticles by carbothermic reduction. Int J Refract Metal Hard Mater 29 645-650... 

Other markets for char include iron, steel, and sili-con/ferro-silicon industries. Char can be used as a reducing agent in direct reduction of iron. Ferro-silicon and metallurgical-grade silicon metal are produced carbothermally in electric furnaces. Silica is mixed with coke, either iron ore or scrap steel (in the case of ferro-silicon), and sawdust or charcoal in order to form a charge. The charge is then processed by the furnace to create the desired product. Char can be substituted for the coke as a source of reducing carbon for this process. Some plants in Norway are known to have used coal-char in the production of silicon-based metal products as late as mid-1990.5 The use of char in this industry is not practiced due to lack of char supply. 

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