Zeolite

Zeolites (Greek, zein,"to boil";lithos,"a stone") are minerals that have a porous structure. The term was originally coined in the 18th century by a Swedish mineralogist named Axel Fredrick Cronstedt who observed, upon rapidly heating a natural mineral, that the stones began to dance about as the water evaporated. Using the Greek words which mean "stone that boils," he called this material zeolite.

More than 150 zeolite types have been synthesized and 48 naturally occurring zeolites are known. They are basically hydrated alumino-silicate minerals with an "open" structure that can accommodate a wide variety of positive ions, such as Na⁺, K⁺, Ca²⁺, Mg²⁺ and others. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Some of the more common mineral zeolites are: analcime, chabazite, heulandite, natrolite, phillipsite, and stilbite. An example mineral formula is: Na₂Al₂Si₃O₁₀-2H₂O, the formula for natrolite.

Natural zeolites form where volcanic rocks and ash layers react with alkaline groundwater. Zeolites also crystallized in post-depositional environments over periods ranging from thousands to millions of years in shallow marine basins. Naturally occurring zeolites are rarely pure and are contaminated to varying degrees by other minerals metals,quartz or other zeolites. For this reason, naturally occurring zeolites are excluded from many important commercial applications where uniformity and purity are essential.

Sources

Conventional open pit mining techniques are used to mine natural zeolites. The overburden is removed to allow access to the ore. The ore may be blasted or stripped for processing by using tractors equipped with ripper blades and front-end loaders. In processing, the ore is crushed, dried, and milled. The milled ore may be air-classified as to particle size and shipped in bags or bulk. The crushed product may be screened to remove fine material when a granular product is required, and some pelletized products are produced from fine material. Producers also may modify the properties of the zeolite or blend their zeolite products with other materials before sale to enhance their performance.

Currently, the world’s annual production of natural zeolite is about 4 million tons. Of this quantity, 2.6 million tons are shipped to Chinese markets to be used in the concrete industry. Eastern Europe, Western Europe, Australia, and Asia are world leaders in supplying the world’s demand for natural zeolite. By comparison, only 57,400 metric tons (source: U.S. Geological Survey, 2004) of zeolite (only 1% of the world’s current production) is produced in North America; only recently has North America realized the potential for current and future markets.

There are several types of synthetic zeolites that form by a process of slow crystallization of a silica-alumina gel in the presence of alkalis and organic templates. One of the important process to carry out zeolite synthesis is sol-gel processing. The product properties depend on reaction mixture composition, pH of the system, operating temperature, reaction time as well as the templates used. In sol-gel process, other elements (metals, metal oxides) can be easily incorporated. The silicalite sol formed by the hydrothermal method is very stable. Also the ease of scaling up this process makes it favorite route for zeolite synthesis.

Synthetic zeolites hold some key advantages over their natural analogs. The synthetics can, of course, be manufactured in a uniform, phase-pure state. It is also possible to manufacture desirable zeolite structures which do not appear in nature. Zeolite A is a well-known example. Since the principal raw materials used to manufacture zeolites are silica and alumina, which are among the most abundant mineral components on earth, the potential to supply zeolites is virtually unlimited. Finally, zeolite manufacturing processes engineered by man require significantly less time than the 50 to 50,000 years prescribed by nature.

Uses

Commercial and Domestic

Zeolites are widely used as ion-exchange beds in domestic and commercial water purification, softening, and other applications. In chemistry, zeolites are used to separate molecules (only molecules of certain sizes and shapes can pass through), as traps for molecules so they can be analyzed, or as catalysts by confining molecules in small spaces, which causes changes in their structure and reactivity.

Zeolites have the potential of providing precise and specific separation of gases including the removal of H₂O, C0₂ and SO₂ from low-grade natural gas streams. Other separations include: HCHO, Noble gases, N₂, Freon and Formaldehyde. However at present, the true potential to improve the handling of such gases in this manner remains unknown.

Agriculture

In agriculture, clinoptilolite (a naturally occurring zeolite) is used as a soil treatment. It provides a source of slowly released potassium. If previously loaded with ammonium, the zeolite can serve a similar function in the slow release of nitrogen. Cuban studies in the emerging field of "zeoponics" suggest that some crops may be grown in 100% zeolite or zeolite mixtures in which the zeolite is previously loaded or coated with fertilizer and micronutrients.

Medical

Zeolite-based oxygen generation systems are widely used to produce medical grade oxygen The zeolite is used as a molecular sieve which extracts oxygen from air, in a process involving adsorption of atmospheric nitrogen.

Their use is also being explored for quickly clotting severe bleeding under the brand name "QuikClot" or "Hemosorb". The manufacturer claims that the biologically and botanically inert granulated material can be poured directly on the wound to stop high-volume bleeding almost instantaneously.

Heating and refrigeration

Zeolites can be used as solar thermal collectors and for adsorption refrigeration. In these applications, their high heat of adsorption and ability to hydrate and dehydrate while maintaining structural stability is exploited. This hygroscopic property coupled with an inherent exothermic reaction when transitioning from a dehydrated to a hydrated form (heat adsorption), make natural zeolites effective in the storage of solar and waste heat energy.

Detergents

The largest outlet for synthetic zeolite is the global laundry detergent market. This amounted to 1.44 million metric tons per year of anhydrous zeolite A in 1992.

= Food

It has been proven that zeolites have been used as thickening agents in McDonalds thickshakes.

Zeolite mineral species

The Zeolite family includes:

Refererences

Zeolites in Sedimentary Rocks. Ch. in United States Mineral Resources, Professional Paper 820, 1973.
Natural and Synthetic Zeolites. U.S. Bureau of Mines Information Circular 9140, 1987.

External links

Zeolite FAQ
Z-Medica the company that produces QuikClot
Zeolite descriptions and pictures.
Database of Zeolite Structures
The Synthesis Commission of the International Zeolite Association
British Zeolite Association
U.S. Geological Survey, References on Zeolites

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