Silica deposition

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Sohre, J. S., Causes and Cures for Silica Deposits in Steam Turbines, Hydrocarbon Processing p. 87, Dec. (1972). 

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. 

Where MU is intended for power boiler FW, particularly strict limits are imposed on sodium leakage (breakthrough) and silica leakage to prevent FW contamination and subsequent downstream caustic gouging corrosion, caustic cracking corrosion, or silica deposition. 

Hardness breakthrough with ion-exchange (base exchange, BX) softening NOTE Caused by Fe/Mn fouling, resin breakdown/loss, or inadequate regeneration. Increased risk of carbonate scale or phosphate sludge Loss of alkalinity and hence an increased silica deposition risk... 

No specific action usually is taken to remove silica (Si02) from MU water sources where the boiler plant operates at below 250 to 300 psig. Nevertheless, particularly undesirable FW line silica deposits may occur, and when this happens, often there is no simple, low-cost, practical way to remove them. 

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 the risk of silica problems within the boiler section, at pressures above 400 to 500 psig silica volatilization and distillation occurs, resulting in silica deposition in superheaters and on turbine blades. Under these conditions the maximum concentration of silica permitted in steam is 0.02 ppm Si02. 

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. 

NOTE For lower pressure boilers, the maximum permitted level of boiler water silica (as ppm Si02), typically is 0.4 x caustic alkalinity (as ppm CaCOf). This method of control is satisfactory as far as it goes, but it does not solve the problem of the risk of silica deposits in the pre-boiler section. 

Essentially, the higher the pressure or temperature, the greater the potential for vaporous silica carryover and steam dissolution. When pressure or temperature falls, the steam becomes supersaturated and various forms of crystalline silica deposition begin to occur. As temperatures continue to fall, the deposits become increasingly amorphous in nature and also more insoluble. 

Silica deposition in the temperature range of 500 to 700 °F (260-371 °C) is primarily sodium disilicate [Na6(Si207)2 alternatively given the formula fJ-Na Oj]... 

Vaporous silica, present as silicic acid and silicate ion Crystalline silica deposits Amorphous silica deposits... 

Softening and raising the pH level of the RW tends to increase the rejection rate of silica, boron, and total organic carbon (TOC). It also tends to increase silica solubility, thus reducing the risk of silica deposition on the membrane. 

Polymaleic acid (PMA). The use of chemicals based on PMA and some derivatives has become standard practice for very brackish waters and seawater distillation processes around the world, where the TDS may reach 50,000 ppm TDS, or where total hardness levels exceed 500 to 1,000 ppm CaC03. Its use in RO systems is growing. However, PMA has limited dispersing properties and may need to be formulated with a dispersant chemical to provide satisfactory performance with some RO designs. It is claimed that PMA is also a successful silica deposit control agent and therefore may be incorporated into formulations where this is a problem. 

When silica levels increase, hydroxide alkalinity measurements become increasingly important to ensure maximum silica solubilization and the reduction of risk of silica deposition. This is especially so for coil boilers because, more than any other boiler type, the use of hydroxide is critical in keeping the waterside surface of the coiled tube clean and deposit free. 

Boiler having 1/32 inch iron or silica deposit This type and thickness of deposit translates into an 8% reduction in heat transfer, or a loss of 10,318 million Btu per year. To produce the same volume of steam requires an additional 72,664 gallon of fuel, at a cost of 61,800 per year. 

Hydrothermal clay-silica deposits (kaolinite, halloysite, sericite, montmorillonite and silica) and zeolite deposits occur in Tertiary-Quaternary volcanic regions. These deposits are distributed in areas of epithermal gold mineralization. 

Advanced arigillic alteration is found at the upper horizon than the sites of potassic and intermediate argillic alterations where the Au-Ag mineralization occurs (e.g., Seigoshi, Yatani, Kushikino, Hishikari). This type of alteration takes blanket-form in upper part and vein-form in lower part (Iwao, 1962 Shikazono, 1985a). The conspicuous zonation from upper to lower horizon is known at the Ugusu silica deposit, namely, silica zone, alunite zone, kaolinite zone and montmorillonite zone (Iwao, 1949, 1958, 1962). 

For example, Shikazono (1985a) has shown that advanced argillic alteration (Ugusu silica deposit in west Izu Penninsula, central Honshu) occurs at the centre and... 

Iwao, S. (1949) The alunite deposits in Japan (app. silica deposit of the Ugusu Mine). Geol. Surv. Japan, Rept. No. 130 (in Japanese with English abst.). 

Degens, E. T. Molecular Mechanisms on Carbonate, Phosphate, and Silica Deposition in the Living Cell. 64, 1-112(1976). 

Baral, S. and Schoen P. (1993) Silica-deposited phospholipids tubules as a precursor to hollow submicron-diameter silica cylinders. Chemistry of Materials, 5, 145-147. 

Rouse JH, Lillehei PT, Sanderson J, Siochi EJ (2004). Polymer/Single-walled carbon nanotube films assembled via donor-acceptor interactions and their use as scaffolds for silica deposition. Chem. Mat. 16 3904-3910. 

However, " Sn Mossbauer spectroscopy allows even much better characterization of Pt/Sn bimetallic nanoparticles. For example, with silica-deposited platinum-tin particles of homogeneous size (1.7 and 4.0nm, respectively, for two different samples) it has been demonstrated that (Figure 2.17) ... 

The rates of hydrolysis of siliceous materials will be affected by several factors. For instance, the rate will be directly related to surface area, explaining the low rates observed for silica deposition from the Vycor apparatus. Also, the composition of the siliceous material will Influence the rate of hydrolysis, explaining the differing amounts of silica transferred from pure silica, silica alumina, zeolite, and the high alumina cracking catalyst. 

Fig. 1. Increase of accumulated surface concentration of silica deposited by successive deposition on NaM-10 (O). NaM-15 ( ), NaM-20 (A). NaDM-10 ( ) and SIO2 ( ).

On a highly siliceous external surface, the pore-opening is not readily narrowed because of the similarity between the zeolite and silica deposited. With increasing the aluminum content, silica grows in a somewhat different manner, with different bond length and bond angle from those of zeolites. As a result, the protrudent slloxane bonds reduce the pore-opening size. 

---