Silica gel

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Silica gel

 

Silica gel is a dried gel formed from supersaturated solutions silicic acid(nSiO2·mH2O) at pH 5-6. Solid hydrophilic sorbent.

It turns out upon acidification of solutions of silicates of alkali metals, followed by washing and drying of the formed gel.

Silica gel has a huge surface area (800 m2/g 1), consisting of groups —SiOH, located at a distance of 0.5 nm from each other. These groups are active centers, and the activity of a particular batch of silica gel depends on the number and the activity of such centers. In the active adsorbent, i.e. that from which removed adsorbed on the surface of the water, many centres will be active. Such activation occurs by heating the gel to 150-200 °C.

When heated to a higher temperature interval 200-400 °C the activity lost in the formation of relations Si-O occurring with elimination water. This stage is, however, reversible. When heated above 400 °C the size the surface of the silica is irreversibly reduced.

Active the centers interact with dissolved polar substances mainly through the formation of hydrogen bonds.

Silicagel distinguish:

• in the form of granules (granular or lump);

• grain size (large and small);

• the pore size (macroporous and fine-pored).

There four-letter designation of the type of silica gel:

• 1st letter describes the size of the granules (K — major, M — minor, A — activated, sh — charge);

• 2nd letter is always C (siliconegel);

• 3rd letter of the pore size (K — large-pore, M — microporous);

• 4th letter form of particles (G — granular, lump).

Distinguish hydrogels (Aquagel), in which the pores are filled with water, alcogel (alcohol gel), xerogels, in which the gel is removed a liquid medium aerogels (dried in the autoclave when heated above the critical point).

Also there are silikagele special purpose, for example, silica gel indicator humidity (saturated with cobalt salts, as they absorb moisture and reduce activity granules change their color from blue to pink), silicagel for chromatography, silicagel for household refrigerators and several others.

Ways received:

• Silica gel obtained by interaction sodium silicate with an acid, an alkaline silicate with an ammonium salt of concentrated sols of colloidal silica;

• Dilution of solutions at low pH values and temperatures below room;

• Hydrolysis of silicon compounds (chloride silicon, orthosilicic esters).

Application

Commodity silica gel is produced in the form of grains or spherical granules of size from 5-7 to 10-2 mm. Various grades and silica gels have an average effective pore diameter of 20—150Å and the specific surface area of 100-1000 m2/g.

Main application silicagel find for drying air, carbon dioxide, hydrogen, oxygen, nitrogen, chlorine and other industrial gases.

Ability silica gel to absorb significant amounts of water are used for drying various liquids, in particular in the case where the dehydration fluid poorly soluble water (drying of halogenated liquids such as freon). Silicagel also serve as dehumidifiers for preservation of the equipment to protect it from corrosion.

Along with water silicagel pairs well absorb many organic substances. These property used for the capture (recovery) of gasoline vapors, benzene, ether, acetone, etc. from the air, benzene from coke oven gas and gasoline from natural gases.

Property silica gel to absorb many substances from the liquid phase is used in industrial the purification of various oils, in the desulfurization of oil cuts and removal of oil high-polymeric resinous substances.

Also silica gel is used as adsorbent in chromatography as a scavenger of water vapour (desiccant) and organic solvents (e.g., assicurarselo air cleaning adsorption of non-polar liquids for the separation of alcohols, amino acids, vitamins, antibiotics, etc.).

Macroporous silicagel used as catalyst carriers. Also silica gel is well adsorbs the nucleus of radon-222 (and its isotopes). Silica gel is widely used for the production of cat litter. Dehumidifiers begin to act as soon as they are removed from protective packaging, so to the the last moment they must remain closed. To improve the efficiency dehumidifiers them would be best, if possible, to hang freely in the upper part of the package. It is desirable that the packaging was sealed. "Container desiccant" more volume in the big mass relative to the surface are slower, which increases the service life this product. Norms bookmark "container desiccants" in transit goods that are sensitive to humidity, set at the rate of 1 kg/m3. When transportation and storage of products, products in a tropical humid climate rules bookmark the silica gel needs to be increased 3.5 times.

Technique security

Silica gel pozharo - and explosion-proof, the degree of impact on the body relates to substances of the 3rd hazard class. Dust silica gel contains from 10 to 70 % free silica. When working with silica gel it is recommended to use means of respiratory protection. Silica gel is a moisture indicator may contain cobalt chloride(II), which