About bentonite
Bentonite is commonly referred to as clay containing at least 70% of the mineral montmorillonite. Montmorillonite is a highly dispersed layered aluminosilicate, the specific surface area of which can reach 800 m2/g. The crystal lattice of montmorillonite consists of silicate sheets formed by two tetrahedral silica-oxygen meshes connected with each other by octahedral alumino-oxygen mesh by fusion with it through common apical oxygen atoms. Moreover, a characteristic feature of montmorillonite is that in the octahedral layer a part of trivalent aluminium atoms is replaced by atoms of divalent cations due to which silicate sheets acquire excessive negative charge, which is compensated by cations located between the sheets (Fig.1).
Due to such ‘cross-linking’ by cations, the sheets form tactoids (Fig.2a). The tactoids, in turn, are assembled into aggregates (Fig.2b).
This structure of montmorillonite particles causes its high hydrophilicity and adsorption properties. When interacting with water, interlayer cations are hydrated, which causes an increase in the distance between the sheets of montmorillonite. This phenomenon is called swelling, in which bentonite can increase its volume several times. If swelling occurs in a limited volume, bentonite forms a dense watertight mass. In the free state, however, if the amount of water is further increased, the montmorillonite aggregates separate and a highly dispersed, stable suspension is formed. Tactoids can swell up to disintegration into separate particles (Fig.2c), and the suspension becomes practically a semi-colloid with thixotropic properties. In the hydrated state montmorillonite acquires the ability to substitute and exchange interlayer charge-compensating cations, as the force of their interaction with silicate sheets weakens.
Due to the above-mentioned properties, bentonite has found wide application in more than 200 different fields and applications. It is used as a binder in moulding mixtures and iron ore pellets, for obtaining optimum rheological properties and reducing the filtration index of drilling fluids in well drilling, horizontal directional drilling and tunnelling.
Besides, bentonite is effectively used in agriculture – in production of mixed fodder, as animal bedding, mineral-food and regulatory additives, for soil amelioration, as well as for clarification of wines and juices. Bentonite is used to produce the best quality clumping cat litter.
In the production of ceramic products and porcelain, bentonite is used to give plasticity and moisture retention properties to ceramic masses, helps to increase the mechanical strength of semi-finished products after drying, allows to reduce the firing temperature. Besides, bentonites can be used as a rheological additive used in compositions of engobes and glazes to make them resistant to settling.
In the pulp and paper industry, bentonites help to retain fine fibres on the net, promote drainage and improve the uniformity of the paper web.
In the paint and varnish industry bentonites can be used both pure and as a part of complex rheological additives on their basis and are intended for use as a suspending additive, allowing to provide sedimentation and aggregative stability of pigment fillers, i.e. they prevent rapid separation of suspensions and formation of hard-to-mix precipitates.
When purifying, clarifying and stabilising vegetable and animal oils and fats, mineral oils, natural bleaching or acid-activated bentonites are used.
Bentonites are used in dry construction mixtures as water-retaining, water-regulating rheological additives.
Bentonites can be used as adsorbents of excess moisture (dehumidifiers).
Bentonites with montmorillonite containing mainly exchangeable sodium cations have the best technological properties. Deposits of natural sodium bentonites are found in Transcaucasia and Kazakhstan. Unlike them, almost all Russian bentonites are calcium-magnesium. Consumer properties of such bentonites containing at least 60-70% of montmorillonite, after activation with soda ash can approach the properties of natural sodium bentonites.