Silicates scale deposition

Silica limits are based on the avoidance of silicate scale deposition and the limitation of silica solubility in steam. 

Silica limits are based on the avoidance of silicate scale deposition and the limitation of silica solubility in steam. The BW silica levels in this table have been set to achieve a level of 0.002 mg/kg in the steam, the normal level specified for modem steam turbines. 

XAFS studies of the crystal chemistry of iron silicate scale deposits in the Salton Sea geothermal field (Manceau et al. 1995)... 

Manceau A, Ildefonse Ph, Hazemann J-L, Fland A-M, Gallup D (1995) Crystal chemistry of hydrous iron silicate scale deposits at the Salton Sea geothermal field. Clays Clay Minerals 43 304-317 Manceau A, Lanson B, Drits VA (2002b) Structure of heavy metal sorbed bimessite. Part III Results from powder and polarized extended X-ray absorption fine structure spectroscopy. Geochim Cosmochim Acta 66 2639-2663... 

Scale formation Controlled scale deposition by the Langelier approach or by the proper use of polyphosphates or silicates is a useful method of corrosion control, but uncontrolled scale deposition is a disadvantage as it will screen the metal surfaces from contact with the inhibitor, lead to loss of inhibitor by its incorporation into the scale and also reduce heat transfer in cooling systems. Apart from scale formation arising from constituents naturally present in waters, scaling can also occur by reaction of inhibitors with these constituents. Notable examples are the deposition of excess amounts of phosphates and silicates by reaction with calcium ions. The problem can be largely overcome by suitable pH control and also by the additional use of scale-controlling chemicals. 

Silicate scales are particularly adherent and extremely difficult to remove, and they are among the most heat-transfer-resistant of all scales. Where scale-based deposits have been found in cooling systems, analyses show they almost always contain some silica or silicate. Fortunately, the silica found is typically less than 5 to 6% unless there is a specific silica problem (under these circumstances the deposit may contain more than 20 to 30% Si02). 

The other major source - dissolved solids - is common to practically all aqueous systems and will result in the formation of calcium, magnesium, or iron scales. Tightly adherent calcium carbonate or phosphate, magnesium hydroxide or silicate, or deposits of iron compounds are laid down on the... 

Uses Multifunctional prod, for pretreatment of cellulosics and its blends peroxide stabilizer in silicate systems to protect goods from catalytic damage rapid wetting agent wax/oil emulsifier disperses impurities in goods reduces scale deposition on equip. 

Localized pre-boiler scale and corrosion debris deposits. Combination of New phosphate, iron, copper, and silica deposition Old re-deposited debris Transport of Fe, Cu, Ni, Zn, Cr oxides to HP boiler section, leading to deposition, fouling, and possible tube failures Transport of minerals and debris including malachite, ammonium carbamate, basic ferric ammonium carbonate Precipitation in FW line of phosphates, iron, and silicates... 

Apart from calcium and magnesium bicarbonates, most natural sources of MU water commonly contain some small amounts of silica and other dissolved minerals, salts, and contaminants. Under a wide variety of operational circumstances, every one of these common materials may contribute to complex boiler scales and deposits, especially the silicates. Thus, it is necessary to ensure that water chemistries are properly balanced and controlled. 

Table 7.4 Summary notes silica and silicate crystalline scales and deposits affecting boiler section waterside surfaces.

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. 

Is the deposit primarily a mineral scale, such as calcium carbonate (relatively easy to remove), or phosphate, or sulfate (more difficult) Does the deposit contain a significant percentage of silicate (which is extremely difficult to remove, exhibits a high resistance to heat transfer, and its presence can ultimately result in heat exchanger failure) ... 

In some pipe deposits in geothermal power plants, arsenic is associated with clays or other silicate minerals rather than sulfides or (oxy)(hydr)oxides. Pascua et al. (2005) found that about 80 % of the arsenic in pipe scales from a Japanese geothermal power plant was associated with Mg-rich smectite clays. The arsenic (mostly III) was probably located in the crystalline structures of the clays and/or present as submicron inclusions. 

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