Zeolites naturally occurring

Voids The space between the resinous particles in an ion-exchange bed. Zeolite Naturally occurring hydrous silicates exhibiting limited base exchange. 

Aluminate sodalites belong to a structural family which, in principle, has been known for quite a long time and which is closely related to various zeolites. Naturally occurring sodalites belong to the class of aluminosilicates (e.g., sodalite in the proper sense has the idealized formula Nag [A1 pSig02i ] CI2) 5 whereas aluminate sodalites have a composition Mg [a1 2 24 2 (pseudo-)cubic cell... 

Zeolites are tire product of a hydrotliennal conversion process [28]. As such tliey can be found in sedimentary deposits especially in areas tliat show signs of fonner volcanic activity. There are about 40 naturally occurring zeolite types. Types such as chabazite, clinoptilolite, mordenite and phillipsite occur witli up to 80% phase purity in quite large... 

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

Over the years, thousands of compounds have been tried as cracking catalysts. These compounds fall into two general categories natural and synthetic. Natural catalyst, as the name denotes, is a naturally occurring clay that is given relatively mild treating and screening before use. The synthetic catalysts are of more importance because of their widespread use. Of the synthetic catalysts, two main types are amorphous and zeolitic. 

Molecular sieves are an adsorbent that is produced by the dehydration of naturally occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates). The dehydration leaves inter-crystalline cavities into which normal paraffin molecules are selectively retained and other molecules are excluded. This process is used to remove normal paraffins from gasoline fuels for improved combustion. Molecular sieves are used to manufacture high-purity solvents. 

Zeolites employed in the manufacture of the FCC catalyst are synthetic versions of naturally occurring zeolites called faujasites. There are about 40 known natural zeolites and over 150 zeolites that have been synthesized. Of this number, only a few have found commercial applications. Table 3-1 shows properties of the major synthetic zeolites. 

Faujasite is a naturally occurring mineral, having a specific crystalline, alumina-silicate structure, used in the manufacturing of the FCC catalyst. Zeolite faujasite is a synthetic form of the mineral. 

Zeolites are naturally occurring hydrous aluminum-sodium silicates in porous granule form. They are capable of exchanging their sodium base for calcium or magnesium and of expelling these alkaline earth metals for sodium by treatment with salt. Thus, they are a type of ion-exchange media. (Some zeolites act as molecular sieves by adsorption of water and polar compounds.)... 

Most home water softeners are based on ion-exchange resins. The first ion-exchange materials used in softening water were naturally occurring polymeric aluminum silicates called zeolites. At present, synthetic zeolites are also used for this purpose. Today, most... 

It follows that the thermodynamic potentials of the hydrogen and sodium ions in the glass must be affected by the concentrations in the solution or ionic replacement in the glass occurs. This power of ionic interchange is shared in common with glass by the naturally occurring hydrated silicates the zeolites. 

In a 1996 study by the U.S. Department of Energy (U.S. DOE), researchers claimed that one reason that clinoptilolite was chosen over other materials in PNNL s studies is that it is a naturally occurring zeolite and is available at a relatively low cost (approximately 200 per ton). Because this type of system is passive, operational costs are minimal, and external energy is not required to operate the systems once they are installed (D132230, p. 140). 

The zeolite component belongs to a broad class of minerals composed of crystalline hydrous alumino silicates containing one or more alkali or alkaline earth metals. Some of the naturally occuring zeolites are analcite, chabasite, mordenite, natrolite and faujasite, all of which are generally found in small deposits. 

The structures of the zeolite frameworks have been determined by X-ray and neutron diffraction techniques. Some of the naturally occurring minerals were characterized in the 1930s, and the synthetic zeolites have been investigated from 1956 onward. Unfortunately, it is extremely difficult for diffraction techniques to determine a structure unequivocally because A1 and Si are next to each other in the Periodic Table and thus have very similar atomic scattering factors (Chapter 2). It is possible to determine the overall shape of the framework with accurate atomic positions but not to locate the Si and A1 atoms precisely. 

Some zeolites have a strong affinity for particular cations. Clinoptilolite (HEU) is a naturally occurring zeolite which sequesters caesium, and is used by British Nuclear Fuels (BNFL) to remove Cs from radioactive waste, exchanging its own Na ions for the radioactive Cs cations. Similarly, zeolite A can be used to recover radioactive strontium. Zeolites were heavily used in the clean up operations after the Chernobyl and Three Mile Island incidents. 

Lest you be muttering, So out with phosphate pollution, in with zeolite pollution , zeolites seem to be one or the Tew things we can add to the ecosystem without negative consequences. The very structures of zeolites make them thermodynamically unstable, and they degrade readily to more stable aluminosilicates that are naturally occurring clays. But that raises other interesting questions If they are melaslabie, why do they form, rather than their more stable decomposition products How can wc synthesize them ... 

In some instances attempts are mude to synthesize naturally occurring zeolites. Boggsite, shown on the cover of this book and discussed in Chapter I. is one such example. Its low abundance in nature restricts studies which could demonstrate us usefulness. 

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