The term Bentonite was first used for a clay found in about 1890 in upper cretaceous tuff near Fort Benton, Wyoming. The main constituent, which is the determinant factor in the clay’s properties, is the clay mineral montmorillonite. This in turn, derives its name from a deposit at Montmorillon, in Southern France.
Bentonite is a clay generated frequently from the alteration of volcanic ash, consisting predominantly of smectite minerals, usually montmorillonite. Other smectite group minerals include hectorite, saponite, beidelite and nontronite. Smectites are clay minerals, i.e.
they consist of individual crystallites the majority of which are <2µm in largest dimension. Smectite crystallites themselves are three-layer clay minerals. They consist of two tetrahedral layers and one octahedral layer.
In montmorillonite tetrahedral layers consisting of [SiO4] – tetrahedrons enclose the [M(O5,OH)]-octahedron layer (M = and mainly Al, Mg, but Fe is also often found). The silicate layers have a slight negative charge that is compensated by exchangeable ions in the intercrystallite region. The charge is so weak that the cations (in natural form, predominantly Ca2+, Mg2+ or Na+ ions) can be adsorbed in this region with their hydrate shell. The extent of hydration produces intercrystalline swelling.
Depending on the nature of their genesis, bentonites contain a variety of accessory minerals in addition to montmorillonite. These minerals may include quartz, feldspar, calcite and gypsum. The presence of these minerals can impact the industrial value of a deposit, reducing or increasing its value depending on the application.
Bentonite presents strong colloidal properties and its volume increases several times when coming into contact with water, creating a gelatinous and viscous fluid.
PHYSICAL PROPERTIES OF BENTONITE :
- Strong colloidal properties
- Swells in contact with water
- Water absorption
- High viscosity
- High plasticity
The special properties of bentonite (hydration, swelling, water absorption, viscosity, thixotropy) make it a valuable material for a wide range of uses and applications.
Bentonite deposits are normally exploited by quarrying. Extracted bentonite is distinctly solid, even with a moisture content of approximately 30%.
The material is initially crushed and, if necessary, activated with the addition of soda ash (Na2CO3). Bentonite is subsequently dried (air and/or forced drying) to reach a moisture content of approximately 15%.
For special applications, bentonite is purified by removing the associated gangue minerals, or treated with acids to produce acid-activated bentonite (bleaching earths), or treated with organics to produce organoclays.
Two types of bentonite are recognized, and the uses of each depend on specific physical properties.
- sodium bentonite
- calcium bentonite
Sodium bentonites absorb large quantities of water, swelling to many times their original volume, and give rise to permanent suspensions of gellike masses. These have been used to seal dams; in bonding foundry sands, asbestos, and mineral wool; as drilling muds; in portland cements and concrete, ceramics, emulsions, insecticides, soaps, pharmaceuticals, and paints; in the manufacture of paper; for clarifying water, juices, and liquors; and as a water softener to remove calcium from hard water.
Calcium bentonites are nonswelling and break down to a finely granular aggregate that is widely used as an absorbent clay sometimes called fuller’s earth.