Carbothermic silica

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. 

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. 

There are several reports on the preparation of SiC nanowires in the literature but fewer on the preparation of SisKi nanowires.38-39 The methods employed for the synthesis of SiC nanowires have been varied. Since both SiC and Si3N, are products of the carbothermal reduction of SI02, it should be possible to establish conditions wherein one set of specific conditions favor one over the other. We have been able to prepare SijN nanowires,40 by reacting multiwalled carbon nanotubes produced by ferrocene pyrolysis with ammonia and silica gel at 1360... 

The ionic liquid l-n-butyl-3-methylimidazolium tetrafluorob-orate has been used to synthesize nanoneedlcs and nanorods of manganese dioxide (MnO,).192 Crystalline silica nanowires were prepared by Deepalc et al.m by a carbothermal procedure. Crystalline SiO, nanowires have also been prepared by a low-temperature iron assisted hydrothermal procedure.1,5... 

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

Silica is reduced via a carbothermic process to silicon, which is converted to a variety of chlorosilanes. The major monomer, dimethyldichlorosilane, is produced in well over a billion pounds per year by several basic producers... 

However, the synthesis process most extensively studied by solid-state NMR is that of carbothermal reduction of aluminosilicate minerals such as kaolinite, which are mixed with finely divided carbon and heated in nitrogen at > 1400°C (Neal et al. 1994, MacKenzie et al. 1994a). Under carbothermal conditions the clay decomposes to a mixture of mullite and amorphous silica (MacKenzie et al. 1996b), the latter forming SiC which reacts with the mullite to form P-sialon, in some cases via other sialon phases such as X-sialon (see below). The precise reaction sequence and the nature of the intermediates has been shown by the NMR studies to depend on various factors including the nature of the aluminosilicate starting mineral (MacKenzie er a/. 1994a). 

Figure 5.38. Effect of hot-pressing on the MAS NMR spectra of yttrium-a-sialon synthesised by carbothermal and silicothermal synthesis using Si02 as the silica source (top) and Si as the silica source (bottom). Asterisks denote spinning side bands. From Ekstrdm et al. (1998).

Hence, by using the correct carbon source with the silica gel under carbothermal conditions it is possible to obtain nitride or carbide nanowires. With the carbon nanotubes the reaction follows one step (Step III shown below) to produce Si3N4 nanowires. 

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. 

M. Sopicka-Lizer, R. A. Terpstra and R. Metselaar, Carbothermal Production of P-Sialon from Alumina Silica and Carbon Mixture. J. Mater. Soc., 30, 6363-9 (1995). 

N2-ASSISTED CARBOTHERMAL REDUCTION OF CARBON AND ALUMINUM NANOCASTED MESOPOROUS SILICA... 

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. 

Carbothermal Reduction of Silica This process is based on the following equation... 

S.3.2.2.3 Vapor-Solid Reaction At present the principal commercial method for SiC whisker production is the carbothermic reduction of low cost silica sources at temperatures of 1500-1700°C. The reaction for the formation of VS-cubic (3SiC whiskers occurs in two steps ... 

Durham BG, Murtha MJ, Burnet G (1988) Si3N4 by the carbothermal ammonolysis of silica. Adv Ceram Mater 3 45-48... 

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... 

Narisawa M, Okabe Y, Iguchi M, Okamura K (1998) Synthesis of ultrafine SiC powders from carbon-silica hybridized precursors with carbothermic reduction. J Sol-Gel Sci Technol 12 143-152... 

Suri J, Shaw LL, Zawrah ME (2012) Tailoring the relative SiaN4 and SiC contents in Si3N4/ SiC nanopowders through carbothermic reduction and nitridation of silica fume. Int J Appl Ceram Technol 9 291-303... 

Shin, Y., Wang, C., Exarhos, G.J. Synthesis of SiC ceramics by tbe carbothermal reduction of mineralized wood with silica. Adv. Mater. 17, 73-77 (2005)... 

Carbothermal Reduction of Silica The primary method to synthesize [i-SiC is by carbothermal reduction reaction below 1900 °C that is, carbon reacts with silica (Si02) to produce SiC and CO gas. The chemical equation for this reaction is commonly written as ... 

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