Transport of silica

Evidence for the Transport of Silica from the Cracking Catalyst to Additive R ... 

The action of steam may have a deleterious effect on catalytic materials. For example, transport of silica can lead to loss of support material or to the encapsulation of the active phase. Steaming may also change the pore structure of the support. As somewhat lower temperatures the action of water vapour may result in an irreversible decrease in the surface area. 

Cracks heal due to transport of silica from wallrock to cracks wallrocks become depleted in silica adjacent to cracks... 

Transport of Silica along a Temperature Gradient. This example simulates diffusion of silicic acid into a domain where temperature decreases with increasing distance from the inlet at X = 0. The temperature field is steady in time. Quartz precipitates according to the reaction... 

When the rate of quartz cementation becomes significant at temperatures exceeding 80-100°C, the porosity loss is mainly due to quartz cementation. Calculations show that the advective transport of silica in the pore water can be ignored as the low flow rates due to compaction are so low and because of the low solubility and solubility gradients of silica. The rate of import or export of silica is a function of the water flux (F), the solubility of quartz as a function of temperature (at) and the geothermal gradient (VT)... 

Even if the sandstones of the basins investigated are nearly always intercalated with silty-argillaceous rocks of a thickness well above that of the sandstones themselves, the water supplied during their dehydration will not suffice to transport such a vast amount of silica mobilized by pressure solution or derived from another source. Under this situation the polycyclical circulation of fluids by convection could be a highly probable mechanism explaining the transport of silica (Fig. 4.15), in particular by the... 

The polycyclical circulation of fluids by convection under these circumstances appears to represent a highly probable mechanism for the transport of silica, in particular in the case of waters the alkalinity of which is controlled by the decomposition of mica schist fragments and feldspars. 

This result means that the transport of silica to the quartz surface by dispersion and diffusion is very fast and is not rate limiting. 

Silica is most dangerous in combination with other species that can form insoluble silicates. Any specification for silica presupposes that these other species (e.g calcium and aluminum) also are under control. Because of its mechanism of transport of silica, its concentration within the membrane actually goes through a maximum (Section 4.8.5). 

In a study of transport of silica through membranes during electrodialysis, Boari et al. (92) found that no transport or silica deposition occurred unless the pH was such that HSiOj" ions were present. This is consistent with the observation that it is necessary to carry out electrodialysis at less than pH 9.5 (88) in order not to deposit silica in the membrane. 

The anionic forms of polyol complexes (derived from alcohob bearing three or more OH groups) are stabilized thermodynamically and are not prone to hydrolysis in aqueous solutions. They can easily be obtained at room temperature via interaction of sodium silicate with polyols, for example, sorbitol [26]. Formation of polyol complexes has been identified as possible mechanism in uptake and transport of silica for formation of endoskeletons in marine single-cell organisms such as diatoms. 

Large yields of polymer seem to be obtained only when polymerization proceeds on the outer catalyst surface, because the transport of high molecular polyethylene from catalyst pores is impossible (112). The working part of the specific surface of the catalyst can be expected to increase with diminishing strength of links between catalyst particles (112). Therefore, to obtain a highly active catalyst a support with large pore volume should be used (e.g. silica with pore volume >1.5 cm8/g). 

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... 

Also, basic factors such as the transport of materials, residual hardness, ion leakage, soluble iron, colloidal silica and clays, and other contaminants, which can produce scales and deposits in the FW lines and other parts of the pre-boiler section, may also produce similar detrimental effects in the boiler section. In the boiler itself, however, the buildup rate may be quicker and the results may be more devastating. 

Takeno, N., Ishido, T. and Pritchelt, J.W. (2000) Dissolution, transportation, and precipitation of silica in geothermal systems. Rept. Geol. Surv. Japan, 284, 235-248. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

This category includes a large variety of silica, zirconia, and alumina mesoporous films. Although the inorganic scaffold of such layers does not transport electric current, the pore architecture, which can be also used as a host matrix for incorporation of functional molecules, can alter electron transport to and from the conducting surface, thus influencing electronic properties of the complete system. 

In a first reactive transport model (Bethke, 1997), we consider the reaction of silica as rainwater infiltrates an aquifer containing quartz (SiC>2) as the only mineral. Initially, groundwater is in equilibrium with the aquifer, giving a SiC>2(aq) concentration of 6 mg kg-1. The rainwater contains only 1 mg kg-1 Si02(aq), so as it enters the aquifer, quartz there begins to dissolve,... 

Aplin, A. C. and E. A. Warren, 1994, Oxygen isotopic indications of the mechanisms of silica transport and quartz cementation in deeply buried sandstones. 

The first consideration was the speciation and distribution of the metal in the sediment and water. Benthic organisms are exposed to surface water, pore water and sediment via the epidermis and/or the alimentary tract. Common binding sites for the metals in the sediment are iron and manganese oxides, clays, silica often with a coating of organic carbon that usually accounts for ca. 2% w/w. In a reducing environment contaminant metals will be precipitated as their sulfides. There is not necessarily a direct relationship between bioavailability and bioaccumulation, as digestion affects the availability and transport of the metals in animals, in ways that differ from those in plants. 

Transparent vitreous silica chemical durability of, 22 417 density of, 22 422 devitrification of, 22 421 manufacture of, 22 412-415 viscosity of, 22 424t Transpeptidases, 3 27 Transport. See also Transportation of ascorbic acid, 25 771 of hydrated lime, 15 56-57 of quicklime, 15 56 of radioactive waste, 25 855-856 in waste collection, 25 869-870 Transportation, 25 322-348. See also Shipping Transport aluminum applications, 2 340-341 cost of, 25 323... 

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