Silica fouling

Silica fouling is the accumulation of insoluble silica on anion resins. It is caused by improper regeneration which allows the silicate (ionic form) to hydrolyze to soluble silicic acid which in turn polymerizes to form colloidal silicic acid with the beads. Silica fouling occurs in weak-base anion resins when they are regenerated with silica-laden waste caustic from the strongbase anion resin unless intermediate partial dumping is done. 

Reverse osmosis plants also are not immune from silica fouling, and where the raw water source naturally contains relatively high levels of silica, good pretreatment of the RO FW is a prerequisite. To reduce fouling of RO membranes by silica, pretreatment by acid adjustment, alum coagulation, and filtration usually is provided. 

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

It had a monomodal pore distribution in the range of 4.4 to 12 nm pore diameter, with a most probable pore size of 5.4 nm. The pore volume and surface area of the catalyst were 0.17 cnr/g and 106 mz/g respectively. The spent catalyst also contained significant amounts of silica fouling, which had deposited on the catalyst from de-foaming agents added to the oil during the industrial use of the catalyst. 

Sahachaiyunta et al. [38] conducted dynamic tests to investigate the effect of silica fouling of reverse osmosis membranes in the presence of minute amounts of various inorganic cations such as iron, manganese, nickel, and barium, which are present in industrial and mineral processing wastewaters. Experimental results showed that the presence of iron greatly affected the scale structure on the membrane surface when compared to the other metal species. 

Sahachaiyunta P., Koo T., and Sheikholeslami R., Effect of several inorganic species on silica fouling in RO membranes. Desalination 144 2002 373-378. 

Limited seawater application — energy consumption is 3—5 times SWRO Requires less pretreatment than RO Not limited by osmotic pressure Not affected by silica fouling... 

Ionic silica is not totally removable by DI. Colloidal silica is difficult to remove by both DI and reverse osmosis (RO) it may cause some resin fouling as well as leaking into the treated water. Where the cation effluent is maintained at a pH of 2.0 to 3.0, however, silica tends to both depolymerize and ionize thus enabling its effective removal in strongly basic, anion resin beds. 

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... 

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... 

Although the risk of scale deposition and fouling in the boiler section is related to several factors such as the FW volume demands, boiler pressure, and heat flux density at various boiler surfaces, it is equally a function of the level of FW contaminants such as residual hardness, sulfates, silica, and iron. Thus, as a generality, the higher the quality of FW (reduced levels of contaminants), the lower the risk of deposition on boiler surfaces. 

The type of membrane cleaning required depends on both the type and degree of fouling experienced, but typically it is either organic (bacterial slimes, natural organics, or process foulants and nutrients) or inorganic (silica, carbonate, sulfate, or phosphate deposits). 

As discussed under boiler feedwater treatment, boiler blowdown is required to prevent the build up of solids in the boiler that would otherwise cause fouling and corrosion in the boiler. Carry over of solids from the boiler to the steam system via tiny water droplets should also be avoided. Total dissolved solids (TDS) and silica (SiC>2), as measured by the conductivity of water, are both important to be controlled in the boiler3. Dissolved solids carried over from the boiler will be a problem to all components of the steam system. Silica is a particular problem because of its damaging effect on steam turbines, particularly the low-pressure section of steam turbines where some condensation can occur. Blowdown... 

To increase the solubility of calcium carbonate and reduce its potential for precipitation and hence membrane fouling, the pH of the feed water is lowered through the addition of acid (Greenlee et al. 2009). Sulphuric acid and hydrochloric acid are most commonly used. Acid treatment can also help to improve the coagulation of colloids and slightly increase the solubility of silica (Bergman 2007). 

The compositions of biomass among fuel types are considerably varied, especially with respect to inorganic constituents important to the critical problems of fouling and slagging. Alkali and alkaline earth metals, in combination with other fuel elements such as silica and sulfur, and facilitated by the presence of chlorine, are responsible for many imdesirable reactions in combustion furnaces and power boilers. 

New advances in the l.c. of carbohydrates are likely to come from three general areas. The first is in the development of more-durable and stable, stationary phases. At present, a major limitation on the use of commercial columns, especially those of the aminopropyl-bonded silica-gel variety, is their short life-time and ease of fouling. More-durable, resin-based columns that operate with the same solvent system and selectivity as aminopropyl silica-gel columns are currently available, and will see further use and development. The development of improved phases for supercritical, fluid-type l.c. will allow this method to be of use for analysis of various carbohydrates. ... 

Mixed-media filters, the subject of the last paper in this section, use three or more materials of different sizes and densities that intermix to form a filter grading from coarse at the top to line at the bottom in the direction of flow. Typical media used are coal, silica sand, and garnet sand. High oil- and solids-rcmoval efficiencies are obtained Design and application criteria are presented, and media fouling or mud-balling" is discussed... 

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