ABSTRACT
Bentonites are naturally occurring rocks or soils with a high clay fraction content. Generally, this clay fraction consists mainly of montmorillonite, a clay mineral capable of swelling. Montmorillonite has a large specific surface area, a high cation exchange capacity, excellent plasticity, a high capacity for swelling and water adsorption, as well as low hydraulic conductivity and low apparent diffusion coefficients for cations. Because of these properties, bentonites are used for numerous purposes, for example as foundry clay, in the chemical and pharmaceutical industry, in food processing, for pets (e.g. cat litter), in drilling and underground workings, and last as sealing materials. Often, bentonites result from the transformation of volcanic ashes or rocks. They are found world-wide. There are large deposits of natural sodiumbentonite in Wyoming and Montana, USA. However, compared with calciumbentonites, natural sodiumbentonites are relatively rare. Deposits of calciumbentonites are to be found, for example, in the Slovakian Republic, on the island of Milos in Greece, in Turkey, Iraq, Syria, Italy, France and the south of Germany. Almost half of the world’s bentonite production takes place in the USA (Jasmund & Lagaly 1993). As sodiumbentonite is better suited for many applications, calciumbentonites are converted into sodiumbentonites by adding soda. These are then distributed as so-called soda-activated sodiumbentonites.
