Silica precipitation

The thickening mechanism of precipitated silicas is similar to that of the fumed silicas. Precipitated silicas have more silanol groups than fumed silicas, but they have a lower concentration of silica (precipitated silicas generally contain some sodium sulphate). 

Filler Carbon black Natural amorphous silica, precipitated silica, nonblack nanofiller Solvent Organic solvent Aqua-based solvent... 

Gold-rich silica precipitates at the Osorezan volcano, which is located in the most northern part of Honshu, have features very similar to epithermal Te-bearing gold vein-type deposits of the Plio-Pleistocene. 

Amorphous silica and barite precipitate simultaneously from white smoker in midoceanic ridge hydrothermal system (Edmond et al., 1979). It is inferred that amorphous silica precipitates in the chimney at a later stage than sulfides and sulfates (anhydrite and barite) which constitute chimneys from which black smoker is emerging. 

Formation permeability damage caused by precipitation of dissolved minerals such as colloidal silica, aluminum hydroxide, and aluminum fluoride can reduce the benefits of acidizing (132-134). Careful treatment design, particularly in the concentration and amount of HF used is needed to minimize this problem. Hydrofluoric acid initially reacts with clays and feldspars to form silicon and aluminum fluorides. These species can react with additional clays and feldspars depositing hydrated silica in rock flow channels (106). This usually occurs before the spent acid can be recovered from the formation. However, some workers have concluded that permeability damage due to silica precipitation is much less than previously thought (135). 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

Naik, R.R., Brott, L.L., Clarson, S.J. and, Stone, M.O. (2002) Silica-precipitating peptides isolated from a combinatorial phage display peptide library. Journal of Nanoscience and Nanotechnology, 2, 95-100. 

In contrast to the behaviour of pyrogenic silica precipitated silica can only be flocculated with PEG by lowering the pH value below pH = 3.5 or by addition of an electrolyte. 

The pH value and the electrolyte concentration show decisive influences on the flocculation and the sedimentation behaviour of precipitated silica. Precipitated silica could not be flocculated at high pH values without the addition of an electrolyte. At pH> 3.5 no change in the state of the suspension was observed, e.g. the sedimentogramms of the suspensions with polymer addition up to saturation (0 1) have the same shapes as that of the sedi-mentogram of the polymer free suspension. At pH 2.3 and even at a very low polymer concentration (c(40,000/PEG) > 3 10-5 mg/ml,... 

Z>) Preparation of Ore Solution Weigh accurately 0.1 g of powdered and dried ore on a clean watch glass and transfer it quantitatively into a 100 ml-volumetric flask. Add 15 ml of concentrated hydrochloric acid, warm the contents of the flask carefully over a sand-bath until most of the dark grains of ore get dissolved completely and only a whitish silica precipitate settles at the bottom of the flask. 

Amorphous Silica The term amorphous silica refers to aggregate of smaU particles with high specific surface area. They lack crystal structure and do not form a sharp x-ray diffraction pattern. They are known in several forms such as colloidal silica, precipitated silica, silica gels, and fumed sdica. The surface of such amorphous silica may contain silanol (SiOH) groups or can be anhydrous. 

With slight variations, the Stober silica precipitation process proceeds from the same chemicals. The starting material is TEOS, tetraethoxysilane, Si(OC2H5)4 the solvent is an alcohol (preferably ethanol) water is added and ammonia acts as the catalyst to initialiate the hydrolysis and condensation reaction. In a very schematic way the reaction could be described as follows ... 

LaMer, Monomer Addition Growth Model. Most of the recent publications (13,18,37,43-45) concerning the Stober silica precipitation describe a first-order hydrolysis of TEOS as the rate-limiting process in the silica particle precipitation. The second reaction step, the condensation reaction, was found to be faster by at least a... 

The diversity of silica forms and their properties leads to a broad range of applications. Reviews on usage include those on crystalline and vitreous silicas (94), on silica sols (95), on silica gels and precipitated silicas (96), and on pyrogenic silicas, precipitated silicas, and silica gels (97). 

The forms of SiC found in sediments and sedimentary rocks are quite varied but those which could be suspected of near surface origin are generally as follows quartz, chalcedony, opal, amorphous gels and ionic forms in solution. Natural occurrences indicate that the solid forms of silica precipitate which has crystallized after the time of initial deposition (Siever, 1962). 

The solution is then cooled to 150 °C and 3 ml of nitric acid 25% v/v sulfuric acid 25% v/v solution is added. Subsequent reheating at 550 °C under oxygen produces a white ash, which is extracted with 10 ml 5.5 M hydrochloric acid. This suspension is diluted to 50 ml and the clear solution decanted from the silica precipitate prior to instrumental analysis. 

A large amount of water is usually contained in the dictyo-structure of the hydrated silica precipitated, including both free and combined moisture. The precipitate is separated out by filtering and washed to remove impurities, mainly Na and acid radical ions. The cake is made into slurry again by stirring and the latter is then spray-dried to yield the powdery product of white carbon black. 

The addition of asbestos fibers at one time provided excellent thixotropic adhesive formulations, especially at elevated temperatures. However, health and environmental regulations have severely limited the use of this material. Today, fumed silica, precipitated calcium carbonate, certain clays, and cellulose and other fibers offer thixotropic properties at relatively low levels of loading. 

Silica, as insoluble silicates and as soluble or "reactive" silica, can cause problems for an RO system. Insoluble silicates form when silica precipitates. When iron and aluminum are present, silicates of... 

Silica Precipitation from Vapour Pyrogenic Silica... 

Resorcinol, /w-C6H4(OH)2 Salicylic acid, C6H4(OH)COOH Silica (precipitated), Si02... 

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