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Bentonite is an absorbent aluminium phyllosilicate, generally impure clay consisting mostly of montmorillonite. There are a few types of bentonites and their names depend on the dominant elements, such as K, Na, Ca, and Al. As noted in several places in the geologic literature, there are some nomenclatorial problems with the classification of bentonite clays. Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as a similar clay called tonstein, have been used for clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite. In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite. Other common clay species, and sometimes dominant, are montmorillinite and kaolinite. Kaolinite dominated clays are commonly referred to as tonsteins and are typically associated with coal.

Sodium bentonite

Sodium bentonite expands when wet, possibly absorbing several times its dry mass in water. Because of its excellent colloidal properties (see Odom ref below) it is often used in drilling mud for oil and gas wells and for geotechnical and environmental investigations.

The property of swelling also makes sodium bentonite useful as a sealant, especially for the sealing of subsurface disposal systems for spent nuclear fuel [31043] [31044] and for quarantining metal pollutants of groundwater. Similar uses include making slurry walls, waterproofing of below-grade walls and forming other impermeable barriers: e.g., to seal off the annulus of a water well, to plug old wells, or as a liner in the base of landfills to prevent migration of leachate.

Sodium bentonite can also be "sandwiched" between synthetic materials to create geo-synthetic clay liners for the aforementioned purposes. This technique allows for more convenient transport and installation and it greatly reduces the volume of sodium bentonite required.

Various surface modifications to sodium bentonite improve some rheological or sealing performance in geoenviromental applications, for example the addition of polymers .

Calcium bentonite

Calcium bentonite is a useful adsorbent of ions in solution. as well as fats and oils, being a main active ingredient of fuller's earth, probably one of the earliest industrial cleaning agents.Calcium bentonite may be converted to sodium bentonite (termed sodium beneficiation or sodium activation) to exhibit many of sodium bentonite's properties by a process known as "ion exchange" (patented in 1935 by Germans U Hofmann and K Endell). Commonly this means adding 5-10% of a soluble sodium salt such as sodium carbonate to wet bentonite, mixing well, and allowing time for the ion exchange to take place and water to remove the exchanged calcium. Some properties, such as viscosity and fluid loss of suspensions, of sodium beneficiated calcium bentonite (or sodium activated bentonite) may not be fully equivalent to natural sodium bentonite. For example, residual calcium carbonates (formed if exchanged cations are insufficiently removed) may result in inferior performance of the bentonite in geosynthetic liners

Pascalite is a commercial name for the calcium bentonite clay.

Potassium bentonite

Also known as potash bentonite or K-bentonite, potassium bentonite is a potassium rich illitic clay formed from alteration of volcanic ash.

Uses

Much of bentonite's usefulness in the drilling and geotechnical engineering industry comes from its unique rheological properties. Relatively small quantities of bentonite suspended in water form a viscous, shear thinning material. Most often, bentonite suspensions are also thixotropic, although rare cases of rheopectic behavior have also been reported. At high enough concentrations (~60 grams of bentonite per litre of suspension), bentonite suspensions begin to take on the characteristics of a gel (a fluid with a minimum yield strength required to make it move). For these reasons it is a common component of drilling mud used to curtail drilling fluid invasion by its propensity for aiding in the formation of mud cake.

Bentonite can be used in cement, adhesives, ceramic bodies, and cat litter. Bentonite is also used as a binding agent in the manufacture of taconite pellets as used in the steelmaking industry. Fuller's earth, an ancient dry cleaning substance, is finely ground bentonite, typically used for purifying transformer oil. Bentonite, in small percentages, is used as an ingredient in commercially designed clay bodies and ceramic glazes. Bentonite clay is also used in pyrotechnics to make end plugs and rocket nozzles.

The ionic surface of bentonite has a useful property in making a sticky coating on sand grains. When a small proportion of finely ground bentonite clay is added to hard sand and wetted, the clay binds the sand particles into a moldable aggregate known as green sand used for making molds in sand casting. Some river deltas naturally deposit just such a blend of such clay silt and sand, creating a natural source of excellent molding sand that was critical to ancient metalworking technology. Modern chemical processes to modify the ionic surface of bentonite greatly intensify this stickiness, resulting in remarkably dough-like yet strong casting sand mixes that stand up to molten metal temperatures.

The same effluvial deposition of bentonite clay onto beaches accounts for the variety of plasticity of sand from place to place for building sand castles. Beach sand consisting of only silica and shell grains does not mold well compared to grains coated with bentonite clay. This is why some beaches are much better for building sand castles than others.

The self-stickiness of bentonite allows high-pressure ramming or pressing of the clay in molds to produce hard, refractory shapes, such as model rocket nozzles. Indeed, to test whether a particular brand of cat litter is bentonite, simply ram a sample with a hammer into a sturdy tube with a close-fitting rod; bentonite will form a very hard, consolidated plug that is not easily crumbled.

Bentonite also has the interesting property of adsorbing relatively large amounts of protein molecules from aqueous solutions. It is therefore uniquely useful in the process of winemaking, where it is used to remove excessive amounts of protein from white wines. Were it not for this use of bentonite, many or most white wines would precipitate undesirable flocculent clouds or hazes upon exposure to warmer temperatures, as these proteins denature. It also has the incidental use of inducing more rapid clarification of both red and white wines.

Bentonite has been prescribed as a bulk laxative, and it is also used as a base for many dermatologic formulas.

History and natural occurrence

Bentonite output in 2005.
Click the image for the details.
In 2005, U.S. was the top producer of bentonite with almost one-third world share followed by China and Greece, reports the British Geological Survey.

The absorbent clay was given the name bentonite by Wilbur C. Knight in 1898 — after the Cretaceous Benton Shale near Rock River, Wyomingmarker. Other modern discoveries include montmorillonite discovered in 1847 in Montmorillonmarker in the Viennemarker prefecture of Francemarker, in Poitou-Charentesmarker, South of the Loire Valleymarker.

Most high-grade natural sodium bentonite is produced from the western United States in an area between the Black Hillsmarker of South Dakota and the Big Horn Basin of Wyoming. Mixed sodium/calcium bentonite is mined in Greecemarker, Australia, India, Russia and the Ukraine. In the United States, calcium bentonite is primarily mined in Mississippi and Alabama. Other major locations producing calcium bentonite include Germany, Greece, Turkey, and China.

It should be noted that in some countries like the UK and US, calcium bentonite is known as fuller's earth, a term which is also used to refer to attapulgite, a mineralogically distinct clay mineral but exhibiting similar properties.

References



External links



Further reading

  • Brady, G.S., Clauser, H.R., & Vaccari, J.A. (2002). Materials handbook. (15th ed.) New York: McGraw-Hill.
  • Hosterman, J.W. and S.H. Patterson. (1992). Bentonite and Fuller's earth resources of the United States [U.S. Geological Survey Professional Paper 1522]. Washington, D.C.: United States Government Printing Office.
  • Milne, G.W.A. (Ed.). (2005). Gardner's commercially important chemicals: Synonyms, trade names, and properties. Hoboken, N.J.: Wiley-Interscience.



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