BENTONITE

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Bentonite clay is an aluminum phyllosilicate, which consists mostly of the mineral montomorillonite. It has the incredible ability to increase as much as 14 times its original volume.

Bentonite is the commercial name of a whole range of natural clays with a high water absorption capacity causing it to expand and swell. Bentonite predominantly consists of montmorillonite: a clay mineral belonging to a class of phyllosilicates called smectites. Bentonites may contain a variety of accessory minerals in addition to montmorillonite. These may include lesser amounts of other clay minerals such as kaolin, mica, illite, as well as non-clay minerals like quartz, feldspar, calcite, and gypsum. Bentonite quality, and, consequently, its applications, depend on whether it contains any of these other minerals.

Its density when dry varies depending on the quality, and may range from 2.2 to 2.8 g/cm3. Bentonite apparent density, when quarried and piled under natural moisture conditions, ranges from 1.5 to 1.8 g/cm3. The apparent density of milled bentonite products varies depending on mill fineness, ranging from 0.7 to 0.9 g/cm3.

 

FORMATION of BENTONITE

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Bentonite is a material derived from the alteration, over geological time periods, of glassy material emitted from volcanoes (tuff or ash), or from the alteration of silica bearing rocks such as granite and basalt. Bentonite only forms in the presence of water. Depending on the nature of formation, Bentonite can have a variety of accessory minerals in addition to its constituent mineral Montomorillonite. These minerals may include attapulgite, kaolin, mica, and illite as well as minerals like quartz, feldspar, calcite and gypsum. The presence of these minerals may affect the value of a deposit.

 

STRUCTURE of BENTONITE

 

While sharing a common elementary structure, the various types of bentonite are very different with regard to their chemical composition, as well as to the physical state of their constituents, which account for bentonite different properties and determine its various technological applications.

Montmorillonite is an aggregate of lamellar platelets, packed together by electrochemical forces and containing interposition water.

Each platelet consists of three sandwich-arranged layers: a central octahedral alumina (Al2O3) layer, and two tetrahedral silica (SiO2) layers. The silicon ion and the aluminium ion often undergo isomorphous substitutions by lower valence metals, such as magnesium and iron. In turn, these substitutions lead to a charge imbalance, compensated by exchangeable cations, in particular calcium (Ca2+), magnesium (Mg2+) and sodium (Na+) ions, together with water molecules bonded together by ion-dipole forces. These ions, with no more place inside the reticular structure, migrate to the external silica layers and are the main cause of hydration in the crystal lattice. Therefore, each platelet can be assumed to have the following general formulation:

 

Water absorption and swelling

A fundamental property of bentonite is to absorb water and expand. However, not all bentonites have the same absorption capacity. Its level of hydration and swelling depends on the type of exchangeable ions contained, with different hydrophilic and solvating power. Swelling is mainly due to two factors: 1) water absorption at platelet surface level, and 2) osmotic repulsive forces, forcing platelets to detach and open up like a “stack of cards”. Sodium bentonite, with sodium cation prevalence (Na+) allows water to penetrate through the platelets, forcing them apart, thus leading to swelling. Conversely, calcium bentonite, with calcium cation prevalence (Ca2+), while getting hydrated in much the same way, due to its strong positive charge, has lower absorption properties, not permitting water to penetrate through the platelets. In this latter case, platelets flake off rather than swell.

Viscosity and thixotropy of aqueous suspensions

when bentonite is dispersed in water, highly stable colloidal suspensions are formed with high viscosity and thixotropy. At high enough concentrations, these suspensions begin to take on the characteristics of a gel. Suspensions are formed when water molecules penetrate into platelet interlayers. Here, hydrogen bridge bonds are formed by the hydrogen atoms contained in the water molecules. Platelets become isolated from each other, while bonded through interposition water. When left still, a mesh is formed which, by incorporating water, jellifies. Conversely, under mechanical stress, these bonds partially break, thus allowing platelets to move more freely. Viscosity under these conditions is lower than at rest. This reversible sol-gel-sol process is known as thixotropy. These properties shown by bentonite aqueous suspensions are mainly exploited in drilling slurries.

Colloidal and waterproofing properties

when water is absorbed by bentonite, a semisolid gel is formed with strong hydrostatic pressure resistance. A montmorillonite platelet can be figured out as a thin packet of negatively charged layers. Due to their negative charge, they repel each other while letting water through. In this way, while the packet swells, a stable shell is formed around the platelet. When saturated, this shell will repel water, even under pressure. For all these properties, bentonite is employed in ponds and docks, to seal off soil infiltrations, and line the base of landfills.

Binding property

This bentonite property is mainly exploited to produce green molding sand. In this application, bentonite with a suitable moisture content covers quartz sand grains and acts as a connective tissue to the entire mass. Under this homogenous coating, even at maximum compression, water will remain in a highly “rigid” state, binding the sand grains and lending maximum resistance to the sand mould. Bentonite vitrification temperature is higher than other clays. Therefore, when used as an additive, it makes green sand more durable, and, in particular, more resistant to heat stress.

Surface properties (coagulation– absorption – adsorption)

Bentonite absorption – adsorption properties are determined by the high specific surface and free charges present on each micelle. Coagulation occurs through the adsorption of ions of opposite charge to that of colloidal particles.

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[ (Si Al)4 (Al Fe . Mg)2 O10 (OH)2 ]2 ∙ Mn ∙ mH2O

 

where the first member in brackets refers to isomorphic constitutions in the tetrahedral layers, the second member refers to isomorphic constitutions in the octahedral layer; and M and mH2O symbols refer to exchangeable cations and interposition water, respectively.

Every clay has a constant, maximum amount of exchangeable cations, as indicated by its cation exchange capacity (CEC), measured in milliequivalents per gram (meq/g) or, more frequently, per 100 grams (meq/100g). Bentonite CEC varies depending on the level of isomorphous substitutions occurred within the lattice. From a chemical point of view, bentonites can be distinguished depending on the quantity and quality of exchangeable bases: in particular, we have calcium bentonites and sodium bentonites, when the prevailing exchangeable cation is calcium or sodium, respectively. The largest deposits on earth contain calcium bentonites, which, however, have less hydration and swelling capacity than sodium bentonites.