Silicate deposits

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. 

Treatment chemicals should preferably be fed to the FW tank to minimize sludge deposits in the coils. Hydroxide alkalinity in ppm (mg/1) CaC03 must be maintained at a sufficient concentration to keep silica soluble and avoid complex silicate deposits. These precautions are necessary because scale-control internal treatment chemicals usually are not employed to assist in the prevention of such deposits in coil-type steam generators. 

Most common silicate deposits, such as analcite (NajOAljO SiO HjO), do not dissolve when tested with concentrated HC1. 

For one thing, steam produced from hot-lime-softened water will have some amount of silicates. These silicates tend to deposit on the rotor blades of turbines, which use the motive steam as a source of energy. The silicate fouling of the turbine blades reduces the turbine s efficiency. But, more importantly, from an operator s point of view, the silicate deposits eventually break off of the blades. This unbalances the rotor. An unbalanced rotor is the fundamental cause of vibration, Vibrations lead to damage of the shaft bearings and seals. Eventually, vibrations will destroy the turbine s internal components. 

A useful rule for the prevention or minimization of magnesium silicate deposition within the typical pH range of a cooling system (which therefore tends to limit the calcium hardness component in the cooling water as well) is as follows ... 

A useful rule for prevention of magnesium silicate deposition is ... 

A. IIJIM A, J.R. HEIN and R. SIEVER. Editors SILICEOUS DEPOSITS IN THE PACIFIC REGION... 

Alkaline Permanganate. This material is good for removing siliceous deposits and acidic crud. 

There is no risk of sedimentation occurring as in the case when such bleach liquors are stabilized with sodium silicate. Many problems related to the formation of silicate deposits on the fabric or on the equipment are thus eliminated. 

Proprietary blend. Extremely effective liquid stabilizer for hydrogen peroxide bleach baths. It is a complete replacement for sodium silicate, eliminating the problems of silicate deposits on equipment and fabric. 

Hydrofluoric add Sulfamic acid Dissolves silicate deposits Dissolves calcium salts As an additive to HCI (as ammonium bifluoride) As an additive to HCI ... 

Origin of the amorphous silica in stalagmites may be related to organic materials. The incorporation of silica in fragments of invertebrate parts seems to support an organic-related origin. Amorphous silicate deposition associated with plants (Jones et ah, 1998)... 

When silica-depositing organisms die, the oi anic constituents of their cells decompose, and the polymeric silica originally deposited within and around these cells is released, usually in a particulate form. The sources, nature, and ultimate fate of this biogenic silica are the subjects of this chapter. The first section deals with biogenic siliceous deposits on land and the second with such deposits in the sea. 

In 1 of these plants, silica is taken up through the roots as dissolved Si(OH)4 and is precipitated within and around the cells as hydrated opaline deposits which often replicate the shapes of the associated cellular structures. Upon death and decomposition of a plant, these siliceous deposits are released to the soil as discrete and generally microscopic structures known as phytoliths or plant opal . Most phytoliths in soils are derived from the aerial parts of plants. However, silicification can be extensive also in the roots and rhizomes of certain grasses, so that soils developed under this kind of vegetation receive significant quantities of biogenic silica from the underground portions of the plants. A rarer type of siliceous deposit in some vascular plants is tabashir (Jones et al., 1966). This material apparently is restricted to the bamboos, where it occurs within the hollow stems as solid, transluscent, opaline masses up to several cm thick. 

A final factor influencing the rates of diagenetic reactions (solution, reprecipitation, recrystallization) in marine siliceous deposits is the porosity and permeability of the sediment. In studies of radiolarian cherts from Italy, Thurston (1972) found that the effects of diagenetic processes were least in... 

Sil-Free. [Sybrm] Additive to prevent silicate deposit in peroxide/silicate bleach systems. 

When the samples of the glassy substances with P/Si ratios more than 6 are treated with water, almost all the silicate deposits as silicic acid. All phosphate species and trace amounts of silicate dissolve. The compositions of the phosphate species in the solutions were determined by paper chromatography. Solvents A and B reported by Bernhart and Chess (41) were used in this investigation. Solvent A was made by mixing 25 ml. of a 20% trichloroacetic acid solution, 10 ml. of water, and 65 ml. of acetone. Solvent B was made by mixing 25 ml. of a 20% trichloroacetic acid solution, 17 ml. of water, and 58 ml. of acetone. 

Bartel, F. H., Dahlkamp, F. J., Ditz, R. B., Schubert, P., Topper, W., Gmelin s handbook of inorganic and organometallic chemistry. Thorium, Minerals (silicates). Deposits. Mineral Index, supplement Alb, p. 440. Springer-Verlag, Berlin, (1991). 

FIGURE 51.5 High resolution transmission electron micrograph of a 9 nm silver particle coated with a thin silica layer by silicate deposition for 48 h. 

FIGURE 51.20 Time traces of the absorbance at 525 mn of Au—Si02 sols with increasing shell thickness. Upper plot shells formed from silicate deposition lower plot shells formed hy growth with TES. 

If more than 10% siliceous deposits are present, mention in description. Examples calcareous ooze (Ca Oz) with siliceous diatoms and clayey foram calcareous ooze (C Ca Oz) with siliceous material... 

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