Silica Removal

FIG. 19-57 TriiKK lectric separators, -S tat electrostatic separator for silica removal from industrial minerals, (Courtesy of Carpeo., Inc.)... 

Typically, high-pressure WT boiler plants (say, over 650-900 psig) require some degree of MU water silica removal. Most commercial and general industrial facilities however, operate boiler plants at pressures below 300 psig, and irrespective of whether FT or WT boilers are employed, a silicate removal process usually is nor provided or deemed necessary. 

Where MU water sources contain relatively high natural silica levels (i.e., above 30-40 ppm Si02) or a high ratio of silica to TDS (say, > 25-30% of TDS present as Si02), this reasoning is often invalid. And in those boiler plants that use high-silica waters but operate without benefit of external silica removal equipment, extremely careful control of BW chemistry with very tight control parameters is vital to avoid risks of serious silicate deposition. 

If BW silica levels increase above 180 to 200 ppm, it may not, in fact, be possible to totally control silica deposition by water chemistry means alone. Also, as boiler pressures, temperatures, and heat-flux densities increases, so does the need for external silica removal equipment. 

All boiler plants providing steam for electricity generation must be provided with silica removal pretreatment equipment. Pretreatment technologies for silica reduction and removal include ... 

It should be noted that, in practice, where lower pressure industrial process boiler plants are operated, the problem of overcoming risks of silica deposition is seldom tackled by the installation of external silica removal pretreatment equipment. Instead, control centers primarily around ensuring an adequate ratio of caustic alkalinity to silica in the BW and limiting the TDS. 

Where silica removal from raw water is not practiced, it is commonly managed by raising the alkalinity of the FW. However, high alkalinity water may lead to BW carryover, but simply reducing this high alkalinity without concern for silica removal is a recipe for disaster. Glassy silicate scales undoubtedly will occur. 

Adsorbents that remove proteins or polyphenols are used to treat a number of beverages to delay the onset of haze formation. Protein adsorbents include bentonite and silica. Bentonite removes protein nonspecifically (see Fig. 2.19) and so is unsuitable for stabilizing beverages where foam is desirable (beer and champagne). Silica, on the other hand, has remarkable specificity for HA proteins while virtually sparing foam-active proteins in beer (Siebert and Lynn, 1997b) (see Fig. 2.20). Silica removes approximately 80% of the HA protein from unstabilized beer, while leaving foam-active protein nearly untouched at commercial treatment levels. 

This was shown to occur because silica binds to the same features in polypeptides that polyphenols do (peptidically linked proline Siebert and Lynn, 1997b) (see the concept in Fig. 2.21). In contrast, in unstabilized apple juice, silica removes only on the order of 20% of the HA protein... 

Silica removal to this concentration may require the additional feed of magnesium oxide with sludge recirculation. 

Ryoo et al. reported the first ordered mesoporous carbon, CMK-1, using cubic MCM-48 as template and sucrose as carbon source. CMK-1 exhibited a highly ordered cubic structure, as confirmed by transmission electron microscopy (TEM). However, x-ray powder diffraction patterns indicated that CMK-1 underwent a structural transformation upon the silica removal due to the two disconnected porous systems separated by the silica wall. ... 

Figure 13.23a shows the first mechanism, which is mainly chemical in nature. The silica removal rate is accelerated by the ceria-silica interactions, which results in the improved dissolution of the silica substrate during polishing. Figure 13.23b shows the second mechanism, which is based on physicochemical particle-surface interactions in which the ceria-silica bonding does not result in direct modification of the silica substrate but enhances the... 

FIGURE 13.23 Schematic depiction of possible ceria-silica removal mechanisms (a) primarily chemical, (b) surface chemical, and (c) enhanced mechanical. 

This effect (not seen up to now) can be explained as follows The brush represents a much lower (5 times) electron density than the silica body nevertheless the potential step vacuum-brash is sufficiently high (more than kT) to be a trap for positronium, and o-Ps is confined in the empty part. The radius of this part is smaller, but Ps wavefunction penetrates a thick brash layer only, and depending on electron density (see Eq.(3)), is much smaller than in pure silica, so the lifetime increases. If the potential step at vacuum-brash boundary is small the penetration range 1/x increases, but the alkane layer is sufficiently thick to prevent Ps reaching silica. Removal of the hydrocarbon phase increases the radius, but increases too, giving as a result lifetime shortening. 

In the UKF process hexafluorosilicic acid is reacted with ammonia to ammonium fluoride and silica. After silica removal, the ammonium fluoride solution is reacted with double its molar quantity of aluminum oxide hydrate to a mixture of ammonium aluminum hexafluoride (ammonium... 

Use Descaling of heat-treated and hot-work oxides and scales deglassing (removal of glass-drawing lubricants), investment, and silica removal removal of bumed-in carbon deposits cleaning of oils, greases, and organic materials from the surface of metals. 

A flow diagram of the weathering sequences that can be expected for the most important rock-forming minerals is depicted in Figure 6.9. This diagram incorporates the processes of alteration, neoformation by addition (silica added to Fe and A1 oxides in confined environments), and neoformation by subtraction (silica removed... 

Because the chemistry of stream water reflects its source in the soil profile, detailed studies of runoff chemistry from small basins should be very useful in delineating the ways and rates at which various elements are released in weathering. In the Mattole basin, it is apparent that groundwater inflow to the stream at the end of the dry season has a relatively high pH ( 8), relatively high dissolved salts (Spec. Cond. 300), and rather low silica (7-8 mg/liter). Thus, the alkalis and alkaline earths are preferentially removed as compared with silica from rock minerals at or near the water table. In surface soils, however, during storm runoff, quite different conditions prevail. The pH of the water may at first be low (5=t), silica release is relatively rapid (8-12 mg/ liter), and alkalis and alkaline earths are removed rather slowly (Spec. Cond. 70-150). The result is that the rate of silica removal compared... 

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