Silica scales

Silica—Generally, limit silica to 150 ppm as SiOg to prevent silica scale. 

The raw water silica is 22mg/l as Si02, and therefore becomes a major constituent of the treated water. Silica scale must now be avoided by raising the boiler water pH and letting silica rather than the TDS control the necessary blowdown. Silica scale not only has a tenth of the heat conductivity of calcium carbonate scale but it is glassy, adherent, and extremely resistant to boilercleaning chemicals. 

All-polymer/all-organic programs are relatively thermally stable and multifunctional. Typically, they replace coagulation and precipitation programs, flocculation improvers, deflocculators, and sludge conditioners and also provide control over very specific and troublesome BW problems, such as silica scaling and iron transport. 

A. C. "Study of Silica Scaling from Geothermal Brines," EIC Corp. Prog. Rep. March-September 1976, C00-2607-3. 

Moller, N., Greenberg, J. P. Weare, J. H. 1998. Computer modeling for geothermal systems predicting carbonate and silica scale formation, CO2 breakout, and H2S exchange. Transport in Porous Media, 33, 173-204. 

Klein, C. W. 1995. Management of fluid injection in geothermal wells to avoid silica scaling at low levels of silica oversaturation. In Proceedings World Geothermal Congress, Florence, Italy, 18-31 May, 2451-2456. 

Silica scale in the form of serpentine, 3MgO 2Si02 2H20, is a common constituent of waterside deposits. 

Silica can be a particularly troublesome sealant because no effective antisealant or dispersant is available. The solubility of silica is a strong function of pH and temperature, but in general the brine should not exceed 120 ppm silica. Once formed, silica scale is difficult to remove. 

Semiat, R Sutzcover, I. and Hasson, D. (2001) Technique for evaluating silica scaling and its inhibition in RO desalting. Desalination, 140, 181-193. 

Soluble silica often limits the recovery of an RO system because of the potential for scaling and the difficulty in removing silica scale from membranes. Silica antisealants that can handle up to about 200 ppm silica (depending on the conditions and antisealant manufacturer) are available. 

Silica fouling causes high pressure drop and low productivity. Silica scaling causes low rejection of silica and perhaps other ions as well. 

Colloidal silica can be removed or reduced in RO feed water using coagulation/clarification (see Chapter 8.1.1). Soluble silica is typically dealt with using high pH, elevated temperature, antisealants (see Chapter 8.2.3), or the HERO process. Sometimes, the recovery of the RO system is adjusted to minimize the potential for forming silica scale. 

Sodium hydroxide used for microbial fouling and silica scale... 

Reduced membrane cleaning due to the elimination of bacterial growth on the membranes and reduced silica scaling... 

Figure 20 (A) SEM images of Celtis Occidentalis (Hackberry) endocarp. Aragonite deposited in a (a) honeycomb pattern, (b) the inner silica scaffolding, and (c) organic matter occluded within the silica (scale bar = 10 p,m) (source A. H. Jahren, unpublished collection). (B) Petrographic thin section of Celtis Occidentalis (Hackberry) endocarp, note honeycomb pattern (scale bar = 100 p,m). Photograph taken with crossed polars and quartz plate inserted...

Use of scale-inhibiting chemicals to reduce carbonate scaling of flashing wells. Development of pH modification to control silica scaling in power plants. 

Silicoflagellates - Heterokonts (Chrysophceae, Am 40 (11) 1 + ability to lose silica scales or skeleton... 

A number of intermetallic compounds, which form protective alumina or silica scales at high temperature, undergo accelerated degradation at intermediate temperatures. This subject has been recently reviewed [29]. The observations actually involve several different, but related, phenomena which may be subdivided into accelerated oxidation , internal oxidation , intergranular oxidation and disintegration . The following definitions will be used throughout this paper. 

The process described above in which a solute oxidizes preferentially to the parent element and forms a continuous layer on the surface is referred to as selective oxidation. The selective oxidation of elements which form a slowly growing, protective layer is the basis for the oxidation protection of all alloys and coatings used at high temperature. The only elements which consistently result in protective scales are Cr (chromia scale), Al (alumina scale), and Si (silica scale). Therefore, much research has been directed at finding alloy and coating compositions, which meet other property (e.g., mechanical) requirements and also form one of these scales. 

The use of SiC as a hard material in wear application, can thus be endangered by a reaction with the metal if the silica scale is eroded by the process and contact temperatures are high enough. 

As is described in more detail for Si3N4, recent studies suggest that free carbon is an enhancing factor but not a necessary condition [73,74]. SiC may act as the reducing agent to promote sulfate dissociation either directly or via an oxygen gradient in the silica scale consumed by reaction (20). 

The influence of water on the corrosion of SiC seems to be threefold It enhances the oxidation rate slightly it transports impurities (e.g. A1 and alkalis) which can strongly enhance oxidation and it is able to evaporize the silica scale inducing para-linear behavior under appropriate flow conditions [23]. This may impair some applications. 

In mixed and oxidizing atmospheres Si3N4 has a superior resistance to Cl-containing environments relative to SiC materials [135-137]. A small water content in H2S-H2 mixtures is also capable of producing a silica scale, which slows down... 

Figure 15. Calculated critical times (years) for 0.1 mm homogeneous recession by parabolic silica scale growth on Si3N4-based materials using data from the collection of [53],...

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