Silica physical precipitation

Physical Form. Fine white powder with particle sizes generally below Ipm. This is not the same as the commercial products fumes silica, silica gel, precipitated silica, or fused silica. It is formed during the electric arc production of elemental silicon from... 

A review of chemical databases reveals that different CASRN have been assigned to the different physical forms of Si02- The assignment of these CASRN was unrelated to TSCA. The EPA has taken the position that, for TSCA purposes, the different physical forms of Si02 are considered to be included under the TSCA Inventory listing for silica, CASRN 7631-86-9. For example, the EPA instructs the regulated community to treat silica, amorphous, fumed, crystalline-free (CASRN 112945-52-5), and silica gel, precipitated, crystalline-free CASRN (112926-00-8), as silica (CASRN 7631-86-9) for TSCA Inventory purposes. 

An example of the physical impregnation method is provided by U.S. Patent 4,302,565, which was discussed above. The dissolution of a magnesium compound and a titanium compound in a suitable solvent, usually an electron donor solvent such as tetrahydrofuran (THF), can lead to the formation of a new species (catalyst precursor) that provides a highly-active Ziegler system when activated. Removing the solvent at elevated temperatures in the presence of a support material such as silica will precipitate the precursor into the pores of the silica. In this method it is necessary that... 

This chapter reports the results of studies on the physical, dynamic mechanical, and rheological behavior of zinc oxide neutralized m-EPDM, particularly in the presence of stearic acid and zinc stearate, with special reference to the effects of precipitated silica filler. 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

Journal of Applied Polymer Science 83, No.2, 10th Jan.2002, p.357-66 MORPHOLOGY AND PHYSICAL PROPERTIES OF CLOSED CELL MICROCELLULAR ETHYLENE-OCTENE COPOLYMER EFFECT OF PRECIPITATED SILICA FILLER AND BLOWING AGENT Nayak N C Tripathy D K Indian Institute of Technology... 

Pottery is produced by the conversion of sedimentary clay (produced by the weathering of rocks) into hard rocklike objects. The clay minerals, which were formed by the chemical decomposition of certain rock-forming minerals, contain trace elements. The sediments in which these clays are found, however, also contain fragments of the primary minerals from the parent rock (including grains of silica sand). These detrital components, which result from the physical and chemical breakdown of minerals, are often accompanied by authigenic minerals that are chemically precipitated from aqueous solutions. In some ceramics, additional components were added as temper during production. 

Silicate solutions of equivalent composition may exhibit different physical properties and chemical reactivities because of differences in the distributions of polymer silicate species. This effect is keenly observed in commercial alkali silicate solutions with compositions that lie in the metastable region near the solubility limit of amorphous silica. Experimental studies have shown that the precipitation boundaries of sodium silicate solutions expand as a function of time, depending on the concentration of metal salts (29,58). Apparendy, the high viscosity of concentrated alkali silicate solutions contributes to the slow approach to equilibrium. 

Therefore, the aim of the present work is to deepen into the accepted exclusive mesoporous character of MCM-41 by carefiilly analyzing the physical adsorption data of N2 at 77 K and CO2 at 273 K, covering in both cases a wide range of relative pressures (from 10 to near unity) and comparing the adsorption data of a sample of MCM-41 with a precipitated silica, prepared under identical conditions but without the templating agent. 

A brief description of the processes involved in biogenic-silica precipitation is useful in understanding many of the physical properties of this amorphous material. There have been significant advances in our understanding of the biochemical... 

Second step preparation of silica-alumina gels. Aluminium sulfate solution (33 wt% Al2(S04)3-18H20) is added to the silica gel slurry. A further addition of an ammonia solution (20 wt% NHj) leads to the precipitation of alumina at pH 6. The obtained slurries of silica-alumina hydrogels are successively filtered under vacuum and washed several times to remove impurities, and spray-dryed in well defined conditions. Spray drying leads to solid spherical particles with reproducible physical characteristics, in particular pore size distribution, pore texture and particle size distribution. 

Solubility (in the molecular sense, rather than in the sense of forming dispersions and sols) opens up a number of possibilities. The first and perhaps most important, is that it allows size-selective precipitation [10], permitting monodisperse nanoparticles to be prepared. It is only when particles are monodisperse that their size-dependent physical properties can be studied in detail [6j. It is also possible to organize these monodisperse nanoparticles via slow evaporation to yield superlattices [11-13]. Superlattices of nanocrystals can rightly be described as a new class of materials, comprising crystals of crystals as opposed to most crystalline solids which are crystals of atoms [14]. In contrast, naturally occurring opals are crystals of amorphous silica spheres [15]. 

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