Silica fume, production

Aggregate, alumiiia (alumiiium oxide), cement raw mix, clay, dolomite, fluorspar, kaolin, light weight aggregate, lime, limestone, magnesia (magnesium oxide), pigments, silica (silicon dioxide), silica fume Products... 

Cheyrezy, 1995). The particle size and specific surface area of the silica fume are secondary factors, and products with specific surface areas ranging between 10 m /g and 20 m /g may be considered acceptable. The most injurious impurities that may be present in silica fume products are residual carbon and alkalis (de Larrard, 1989). 

A 75% ferrosilicon furnace operating at 90% silicon recovery would give 0.24 kg of by-product silica fume per kg of silicon produced. A silicon metal furnace operating at 85% recovery (typical of the industry) would give 0.38 kg of fume silica per kg silicon produced. 

The fluffy product is extremely finely divided (particle size typically 7-40 nm, powder density 0.03-0.06 g cm-3) but nonporous (surface area ca. 200-400 m2 g-1). Like fused silica, silica fume is amorphous, and this, together with the small particle size, gives it relatively high solubility. It is used as a thixotropic thickening agent for epoxy resins, in special high strength... 

Silicon Dioxide occurs as an amorphous substance that shows a noncrystalline pattern when examined by X-ray diffraction. It is produced synthetically, either by a vapor-phase hydrolysis process, yielding fumed silica, or by a wet process, yielding precipitated silica, silica gel, colloidal silica, or hydrous silica. Fumed silica is produced in an essentially anhydrous state, whereas the wet-process products are obtained as hydrates or contain surface-adsorbed water. 

C45 Cook, D. J. and Suwanvitaya. P., in Fly Ash, Silica Fume, Slay and Other Mineral By-Products in Concrete (ed. V. M. Malhotra). Sp. Publ. SP79, Vol. 2, p. 831, American Concrete Institute, Detroit (1983). 

Silica fume which is a by-product in the manufacture of ferrosilicon was used as a dispersant for carbon fibers in all the mixes. The chemical and physical properties of the silica fume is listed in Table 1. Polycarboxylie acid-type superplasticizer and hydroxy propyl methyl cellulose (HPMC, viscosity 3500-5600mPa s at 20°C as 2% aqueous solution) were employed as chemical admixtures. 

The production of silica fume for 1985 was 1.2 million metric tons per year (18). If an annual increase rate of 4% is calculated, this figure should have risen to 1.46 million metric tons per year by 1990. Fly ashes and silica fume were given off into the atmosphere on a virtually uncontrolled basis from about 175 production plants in 1985 (19). In addition, plasma processes have been used since 1968 to produce... 

Until 1985 silicas were usually divided into two groups natural and synthetic. Most of the naturally occurring SiC>2 modifications are crystalline, but this group also includes amorphous or mostly amorphous products, such as diatomaceous earth or kieselguhr. However, when the observations made at the end of the introduction are considered, this classification no longer proves sufficient, as fly ashes and silica fume are also synthetically produced, although not deliberately. These airborne dusts are not harmless (22). 

Another important reinforcement application is in silicone rubber. Historically, fumed silicas have played the major role here, but recently precipitated silicas have been developed that possess the characteristics required for this application (6). Compared to conventional precipitated silicas, a product designed for this end use must have higher purity (to impart acceptable electrical properties, because silicone rubbers are often used as insulating materials) and lower water adsorption (to prevent bubbles from forming during extrusion and to impart resistance against moisture pickup). Good dispersibility is also important. 

Easily Dispersible Products Easily dispersible products are very similar to instant products. The only difference is in the fact that the primary particles are not soluble. Typical examples are pigments, carbon black, silica fume, etc. 

Fig. 7.84 Typical silica fume particle size distribution (courtesy Norchem Concrete Products, Fort Pierce, FL, USA).

J. Wolsiefer, Sr., The measurement and analysis of silica fume particle size distribution and dispersion , Norchem Concrete Products, Inc. (see Section 14.1), Paper at 5th CANMET/ACI Int l Conf on Durability of Concrete, Barcelona, Spain, June 4, 2000. 

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