Fumed silicas manufacture

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. 

For TSCA Inventory purposes, the EPA distinguishes silica gel (CASRN 63231-67-4), and silica fumes, (CASRN 69012-64-2), as unique chemical substances. The EPA takes the position that manufacturers are free to describe these types of substances in promotional literature and in Material Safety Data Sheets (MSDS) in any way they wish. For example, the substance silica, crystalline, has no CASRN assigned to it and does not appear on the TSCA Inventory, but it has been listed by California under Proposition 65 as a carcinogen. The list of various forms of silica in Table 4.1 does not contain forms that are specifically described as being crystalline, but two of them (CASRN 112945-52-5 and 112926-00-8) clearly state that they are not crystalline (crystalline-free). Some suppliers may use these CAS numbers on their MSDS to assure California customers that their products are not subject to a Proposition-65 warning, even though the correct CASRN for TSCA purposes is 76310-86-9. 

Silica fume. Silica fume (SF) is a waste product of manufacturing ferro-sihcon alloys. It consists of an extremely fine powder of amorphous silica. Average particle diameter is about 100 times smaller than that of Portland cement and the specific surface area is enormous 13000-30000 m /kg compared to 300-400 m /kg for common Portland cements. Silica fume shows an elevated pozzolanic activity and is also a very effective filler. For these reasons, addition of silica fume to Portland cement may lead to a very low porosity of the cement paste, increasing the strength and lowering the permeability. It is usually added in the proportion of 5 to 10 % and it is combined with the use of a superplasticizer in order to maintain adequate workability of the fresh concrete. 

Silica fume is a byproduct from manufacturing of metallic silicon or various silicon alloys with iron (ferro-silicon) or other metals in electrical arc furnaces. They are formed by oxidation of SiO vapour in the gaseous phase and are precipitated in the dust collectors. 

An inorganic impact modifier has been marketed by the Norwegian waste recovery company, Elkem Materials. It is called Sidistar and made of amorphous silica spheres with a median particle size of about 150 nm. They are derived from the silica fume accumulating during the manufacture of ferro-silicon alloys and silicon metal. Elkem claim that their silica modifiers can impart good impact strength at low temperatures in thermoplastic pipe, and can replace conventional acrylic, MBS and chlorinated polyethylene modifiers. 

Silica fume - a residual product from the manufacture of silicon or ferrosilicon - is used as an additive for concrete, in quantities up to 10 % of the cement weight. Addition of silica fume results in a significant change of the strength properties of the concrete, as well as a dense and durable concrete. Mainly, silica fume consists of amorphous siUcon dioxide SiOg(amorphous) in the strongly alkaline environment - pH 13.5 -of the concrete, silica fume reacts with calcium hydroxide Ca(OH)2 derived from the cement hydration, and thus forms calcium silicate hydrates with binder properties. The following reaction is an imperfect, but useful approximation of the reaction that occurs ... 

Fumed silicas have an amorphous nature probably caused by the fast cooling during the manufacturing process. It is an important benefit because it does not cause silicosis. 

To meet the 2001 U.S. energy standards and the 2003 phase-out of HCFCs, there is a great incentive to develop a significantly better thermal insulation. The most dramatic approach would use vacuum panels for insulating the cabinet. A number of U.S. and Japanese manufacturers have developed such panels and placed these kinds of refrigerators in homes. The panels consist of multilayer plastic envelopes filled with precipitated (fumed) silica. The claimed thermal conductivity is one-fourth that of polyurethane foam. The two major obstacles are cost and the maintenance of vacuum for twenty years. 

These processes are very rapid and allow the preparation of inorganic supports in one step. This technique allows large-scale manufacturing of supports such as titania, fumed silica, and aluminas. Sometimes the properties of the material differ from the conventional preparation routes and make this approach unique. Multicomponent systems can be also prepared, either by multimetallic solutions or by using a two-nozzle system fed with monometallic solutions [22]. The as-prepared powder can be directly deposited onto substrates, and the process is termed combustion chemical vapor deposition [23]. 

We have recently tested the Tx model described above by obtaining T, measurements in powder samples with known S/V. Samples used were constructed from fumed silica (CAB-O-SIL M-5 and TS-500, Cabot Corp.), and were either hydrophilic (M-5) or treated by the manufacturer to be hydrophobic (TS-500). Powder of each type was pressed into a polycarbonate cylinder, with a degree of compression controlling the pore space volume of each sample. These materials have a very high specific surface area (200 m2 g 1 for M-5, 212 m2 g-1 for TS-500), which is not expected to change significantly even at the maximum compaction pressure used. 

According to the Illinois Environmental Protection Agency (IEPA), a series of gas blowouts has occurred at two waste injection wells in the state (Brower el al., 1989). In each case, well operators were injecting concentrated hydrochloric acid into a dolomite bed. At its plant near Tuscola, the Cabot Corporation injects acid waste from the production of fumed silica into the Cambrian Eminence and Potosi Formations below 5 000 ft (1 500 m) depth. Allied Chemical Corporation injects acid into the Potosi formation below about 3 600 ft (1 100 m). The acid, which is contaminated with arsenic, is a byproduct of the manufacture of refrigerant gas. Since some of the blowouts have caused damage such as fish kills, there is environmental interest as well as operational concern in preventing such accidents. 

The chemistry of silicone halides was recently reviewed by Collins.13 The primary use for SiCU is in the manufacturing of fumed silica, but it is also used in the manufacture of polycrystalline silicon for the semiconductor industry. It is also commonly used in the synthesis of silicate esters. T richlorosilane (another important product of the reaction of silicon or silicon alloys with chlorine) is primarily used in the manufacture of semiconductor-grade silicon, and in the synthesis of organotrichlorosilane by the hydrosilylation reactions. The silicon halohydrides are particularly useful intermediate chemicals because of their ability to add to alkenes, allowing the production of a broad range of alkyl- and functional alkyltrihalosilanes. These alkylsilanes have important commercial value as monomers, and are also used in the production of silicon fluids and resins. On the other hand, trichlorosilane is a basic precursor to the synthesis of functional silsesquioxanes and other highly branched siloxane structures. 

A number of reagents containing oxide components are used in zeolite manufacture [19]. Silica is provided by addihon of sodium or other alkali silicate solutions, precipitated, colloidal, or fumed silica, or tetraalkylorthosihcate (alkyl = methyl, ethyl) and certain mineral silicates such as clays and kaolin. Alumina is provided as sodium aluminate, aluminum sulfate soluhon, hydrous aluminum oxides such as pseudo boehmite, aluminum nitrate, or aluminum alkoxides. Additional alkali is added as hydroxide or as halide salts, while organic amines and/or... 

The nuisance dust aspect of bauxite is in sharp contrast to the limited industrial situation where lung injury was reported in Canadian workers, who in the 1940s engaged in the manufacture of alumina abrasives in the virtual absence of fume control. Fusing of bauxite at 2000°C gave rise to a fume composed of freshly formed particles of amorphous silica and aluminum oxide. Despite the poor choice of the term—bauxite fume pneumoconiosis—sometimes used to describe the disease, scientific opinion favors the silica component as the probable toxic agent. It should be emphasized that bauxite from some sources may contain small amounts of silica. 

Dusts, Mists, Aerosols and Fumes. The P-5 Digital Dust Indicator is another sensor currently available for use as a component of the Chronotox System. Suitable for the measurement of silica, lead fumes, pharmaceutical powders as well as many other types of particulates found in manufacturing or laboratory situations, the battery-operated P-5 uses the light scattering technique to measure dusts over a range of either 0.01-100 mg/m or 0.001-10 mg/m (Figure 6). 

In the fifteenth and sixteenth centuries the alchemists obtained sulphuric acid in this manner, frequently aiding the process by the addition of silica, and the process was worked on a commercial scale until the end of the nineteenth century. From the fact that much of the fuming acid was distributed from Nordhausen in Prussia, the name Nordhausen sulphuric acid originated, but little if any of the acid was manufactured in this town.6... 

The hand lay-up or spray-up process, used universally for the production of laminar composites incorporating glass fiber reinforcement, is most efficient for the manufacture of large parts, such as boats, bathtubs, tanks, architectural shapes, and recreational accessories. Resins intended for spray-up processes are usually modified with thixotropic additives, such as fumed silica (1%), to reduce the risk of drainage when applied over large vertical mold surfaces. Molds are also made from FRP for short-run products usually surfaced with a tooling gel coat to provide consistent surface quality and appearance. 

For commercial processes, formed supports are more useful. Compared with other supports, fumed oxide supports showed new catalytic effects [41]. Some intensively investigated applications for these supports are abstracted in the following. SiC>2 pellets have been successfully introduced in a new generation of precious metal supports in vinylacetate monomer production [42]. This resulted in better selcctivities and an up to 50% higher space-time yield compared with supports based on natural alumo-silicates. In alkene hydration fumed silica pellets serve as a support for phosphoric acid. In this case, an increased catalyst lifetime and a higher space-time yield were observed [43]. Pyrogenic TiC>2 powder can be used as a starting material for the manufacture of monolithic catalysts [44] for the selective reduction of NOv with ammonia. 

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