Sources of silica

Industrial by-products are becoming more widely used as raw materials for cement, eg, slags contain carbonate-free lime, as well as substantial levels of silica and alumina. Fly ash from utility boilers can often be a suitable feed component, because it is already finely dispersed and provides silica and alumina. Even vegetable wastes, such as rice hull ash, provide a source of silica. Probably 50% of all industrial by-products are potential raw materials for Pordand cement manufacture. 

NaY zeolite is produced by digesting a mixture of silica, alumina, and caustic for several hours at a prescribed temperature until crystallization occurs (Figure 3-8). Typical sources of silica and alumina are sodium silicate and sodium aluminate. Crystallization of Y-zeolite typically takes 10 hours at about 210°F (100°C). Production of a quality zeolite requires proper control of temperature, time, and pH of the crystallization solution. NaY zeolite is separated after filtering and water-washing of the crystalline solution. 

To overcome the limitations of natural zeolites a whole range of synthetic zeolites have been manufactured since the 1950s. These have tailored pore sizes and tuned acidities, as well as often incorporating other metal species. The basic synthesis involves mixing a source of silica, usually sodium silicate or colloidal Si02, with a source of alumina, often sodium aluminate, and a base such as sodium hydroxide. The mixture is heated at temperatures up to 200 °C under autogenous pressure for a period of a few days to a few weeks to allow crystallization of the zeolite. 

Spent foundry sand can be used as a good source of silica in manufacturing portland cement. Also, the clay fraction of foundry sand is an additional source of iron and aluminum oxides. According to the portland cement industry, spent foundry sand can be beneficially used in the manufacture of Portland cement, as sand possesses the following characteristics ... 

The main source of silica in antiquity, as today, was sand and, very occasionally, crushed rock or pebbles. The most common modifier was soda, which was obtained from natron lakes, as in ancient Egypt, for example, or from vegetable ash. Most lime was derived from limestone, although some lime could also enter the mixture of glass raw materials together with soda... 

A preformed chitosan-silica composite with 60% weight inorganic part [7] is used as the source of silica for the zeolite synthesis. An alkaline solution of sodium aluminate (Na 2.1 M, Al 1 M) was used in three methods of preparation (A) beads of the chitosan-silica composite were stirred overnight in the aluminate solution, extracted and submitted to a hydrothermal treatment at 80 °C during 48h (B) beads of the chitosan-silica composite were immersed in the aluminate solution and the system underwent a hydrothermal treatment at 80 °C for 48h (C) beads of the chitosan-silica composite were stirred overnight in the aluminate solution, extracted, dried at 80 °C and exposed to water vapour at 80°C during 48h. 

The existence of two distinct mechanisms describing the synthesis of ZSM-5 type materials has been proposed and experimentally confirmed. The control of primary synthesis variables such as the source of silica and the composition of the reaction mixtures enables us to prepare ZSM-5 particles with specific properties desired for various catalytic applications large single crystals having a Si-rich core and am Al-enriched outer rim, smaller and homogeneous... 

Source of Silica. Silica can migrate either from free silica present in the cracking catalyst or from the silica alumina matrix but not as readily from the zeolite. Figure 11 shows SEM-EDAX silicon scans of cerium/alumina steamed in the presence of these three sources of silica. Again, the bright dots represent silicon. Qualitatively the sample steamed with pure silica contains more silicon than the sample steamed with silica-alumina. The sample steamed with zeolite shows silicon at the surface of the cross-sectioned particle but little in the interior. The surface silicon comes from dusting of the particle with very finely divided zeolite. 

Zeolite X. Analytical results are reported in Table I (11). The compositions of solid and liquid phases of freshly prepared hydrogel depended on the source of silica. However, after crystallization and prolonged aging... 

To study the influence of the silica source on the mesoporous materials formation, we have performed synthesis by adding different source of silica neutral TMOS and ionic sodium silicate to the micellar solution. Syntheses were made either under acidic (pH = 2) or basic (pH = 10) conditions for TMOS. Table 1 contains essential information of the obtained products. 

Synthesis conditions, specific surface area (Sbet), pore volume (V) and pore diameter (0) of products obtained by adding different source of silica to the micellar solution ... 

Ceramic glazes have three basic components silica, a flux used to lower the melting point of the silica, and a colorant. In Mesopotamia, sources of silica include crushed and sifted mineral quartz or quartz-rich sands. Two different types of fluxes are used. One is based on the use of the ashes from alkali-rich plants and the other on lead oxide. Colorants were derived from metallic oxides, the sources of which include native ores and debris from metal-crafting (2, <5, 7). 

Starting materials for the direct synthesis of (Si,B)-ZSM-5 were fumed silica (Cabosil), tetrapropylammonium bromide (TPA-Br), ammonium fluoride and orthoboric acid (16). The source of silica was mixed with TPA-Br and water and then a mixed solution of NH4F and H3BO3 was added under vigorous stirring. The resultant gel was homogenized for 1.5 h and transferred into a Teflon-lined stainless steel autoclave which was then heated at 200°C for 17 days. Zeolite crystals were washed, dried at 60°C and hydrated in a desiccator. 

Results compiled in Table V show that a decrease of the initial TEAOH concentration automatically favours the crystallization of zeolite Beta, irrespectively to the source of silica used. 

Figure 11 shows the kinetics of crystallization of samples 2,4,and 5. Obviously die nucleation time for sample 5 is shorter than that characterizing samples 2 and 4, prepared in presence of larger TEAOH concentrations after only 5 days heating at 100°C. It also appears that for a TEA+/Al203 ratio of 17.5, the source of silica does not affect the nucleation time of samples 2 and 4. However, sample 2 (Si(OEt)4) achieves more rapidly a 100% crystallinity than sample 4 (Aerosil), suggesting a more efficient utilization of the Si(OH)4 monomers stemming from a slow hydrolysis, to build up the final framework. 

Sl/Al < 1.25 The two modifications of the method leading to successful preparations of pure materials with a low Si/Al ratio involved changing the source of silica to silica gel ( 8), and "cold-aging" of the parent gels at room temperature for 2 days prior to heating at 100°C (( 9), Table II). 

The main source of the alpha particles is trace quantities of uranium and thorium in the silica filler. Because silica fillers that did not contain these radioactive elements were not available, other methods for preventing alpha particles from reaching the active DRAM cells were devised. These early methods consisted of cov-vering the active cells with either a silicone or polyimide chip coat or with Rapton tape. These methods added extra steps to the manufacturing process which were cumbersome and labor intensive and, if not done precisely, had a negative reliability impact. These processes were not widely used once "low alpha fillers" became commercially available in 1982/1983. Initially, these "low alpha fillers", which contain <1 ppb uranium, were only available from one or two natural sources. Now, however, there are additional natural and synthetic sources of silica, all of which contain <1 ppb of uranium and have an alpha particle emission rate of less than. 001 alpha particles/hr-cm. Figure 9 shows where the industry was in 1980 and where it stands today. An improvement by a factor of 30-50 has been achieved with "lower alpha" filler and compound manufacturing. 

That nucleation and growth rate are the limiting steps in quartz cementation has no particular imphcations with respect to the ultimate source of the sihca or the mechanism of transport. Potential sources of silica for quartz cementation are numerous (McBride, 1989) and include all documented silicate dissolution reactions in sandstones and shales. 

Towe K. M. (1962) Clay mineral diagenesis as a possible source of silica cement in sedimentary rocks. J. Sedim. 

The synthesis of aluminum containing MCM-41 molecular sieves was reported in both the patent [117-119] and the open [48,49,92,113,115,120-129] literature. There are also some reports on Al-HMS [130] and Al-FSM-16 [77]. MCM-41 aluminosilicates were prepared under hydrothermal conditions, typically at 70-150 °C during 1 to 10 days. Various sources of silica and alumina were used. Janicke et al. [121] reported that aluminum isopropoxide is a much better precursor than Catapal B. hi the presence of isopropoxide, they prepared samples with Si/Al ratios down to 16, with aluminum being entirely in tetrahedral positions. 

The basalt formulation employs Columbia River-type basalt as a source of silica. Extensive deposits of this basalt underlie the Hanford Reservation so that it is readily available as a raw material for large-scale glass-making. The chemical composition of typical Columbia River basalt is shown in Table VI physical properties of this type of basalt have been determined by Krupka (8) and Leibowitz, Williams, and Chasanov (9). We also note that incorporation of radioactive waste material into melted basalt was studied briefly earlier in Czechoslovakia by Saidl and Ralkova (10). 

Aluminum for the manufacture of aluminum-rich zeolites is obtained from sodium aluminate solutions, which are obtained by dissolving aluminum oxide hydrate in sodium hydroxide. Silica is used in the form of water glass, fine particulate silica (e.g. silica fillers) or silica sols. The cheaper water glass is preferred, but exhibits the lowest activity of the above-mentioned sources of silica. The reaction has therefore to be carried out in a special way to... 

Any attempt to understand silcrete genesis must take into account three key factors (a) potential sources of silica (b) routes by which this silica is transported to the site of silicification and (c) factors leading to the precipitation of the various silica species. 

---