Bentonite is a clay mineral, whose name goes back to the American geologist Knight. Around the year 1890 he explored in Wyoming (USA) near Fort Benton a clay deposit. After the place of discovery this material was named bentonite. Decisive for the versatile properties is the montmorillonite, a clay mineral named after its deposit at Montmorillon in Southern France. Bentonite is the name given to an industrial mineral that can contain besides the main component montmorillonite also accompanying minerals such as quartz, feldspar, mica, or calcite.

Important bentonite deposits are found not only in the United States, but worldwide. In Europe it is mined around the Mediterranean, for example in Sardinia, Spain, Greek Islands, Turkey and Morocco. In Germany there are economically interesting bentonite deposits in Bavaria right and left of the river Isar, in the region of Mainburg, Moosburg and Landshut.

In 2007 the bentonite production has been 5.1 million tons in the United States, 3.1 million tons in China, 1 million tons each in Turkey and Greece.

Formation of bentonite – using the example of the Bavarian deposit

The Bavarian bentonites are weathered materials of acidic volcanic glass tuffs. These glass tuffs originate from a massive volcanic activity in the Carpathian arc about 14 million years ago. By soil drifts the volcanic ashes were transported over a distance of about 3,000 kilometres to Bavaria. Here several hundreds of meters of huge sedimentations in river depressions were formed by ash drifts. Due to the influence of water they were quickly covered by sand and marl.

  In the course of time under appropriate hydrothermal conditions, i.e. under the influence of water and temperature bentonite was formed. For a successful bentonite extraction exploratory drilling has to be carried out to find the mostly lenticular enclosed bentonite. The clay mineral is extracted in surface mining. After the closure of mines the resulted pits are re-cultivated. The countryside destroyed by mining is restored to its original state and returned to its owners. The new area again can be used for forestry and agriculture.

 Modifications of bentonite

Montmorillonite is a crystalline, layer-formed aluminium hydro silicate (4SiO ₂ * Al ₂ O ₃ * H ₂ O + N * H ₂ O). The silicate lamellae consist of three layers: a SiO4 tetrahedron, an aluminium hydrate octahedron and another SiO4 tetrahedron.

Each crystal consists of 15 to 20 silicate lamellae. By replacing trivalent aluminium in the octahedral layer by divalent ions (e.g. magnesium) the individual lamellae have negative excess charge, which is compensated by the incorporation of exchangeable cations such as calcium and/or sodium ions into the interlayer. The characteristics of the montmorillonite can be enhanced by chemical post-treatment.

Raw clay from the pit has a moisture content of 30 to 40%. If used as natural bentonite it will be dried to approximately 10 to 15% moisture content. With suitable crushing devices products with defined particle sizes are produced. Natural bentonites are commonly used in food, cosmetics and animal feed production. The alkaline activation is based on ion-exchange reaction, where the alkaline earth metals of montmoril

lonite are replaced by alkali metal ions. These particles are located at the edges and in the intermediate layers. In the presence of water they have a tendency to hydrate. This results in the incorporation of water between the layers and subsequently in strong expansion of interlayer distances. The intracrystalline swelling results in expansion of the interlayers which helps to explain the high adsorption and water retention.

To achieve maximum adsorption alkaline activated bentonites are mainly used in pre-dispersed, extended form.

This product group is characterized by thixotropic behaviour. This means: when products containing these additives are shaken or stirred they change from gelatinous to liquid state. At rest again gel is formed.

In the case of acid activation bentonites are treated with mineral acids (hydrochloric or sulphuric acid). The exchangeable cations are replaced by hydrogen ions. Depending on reaction conditions aluminium and magnesium ions are also dissolved out of the octahedron layers. The voluminous silicic acid remains, which is connected via SiO ₄ tetrahedrons to the unaffected montmorillonite. The acid treatment leads to the loose of crystal structure and to a strong increase of the specific surface area (m² /g). After dispersing in water the crystal structure remains intact with a stable pore structure and a high content of macro pores.

For the production of organophilic bentonites quaternary alkyl ammonium compounds are used. The exchange of interlayer cations results in products, which are used to modify rheology (thixotropic) of organic solvents, oils, fats, ointments and plastics.

By the organic modification the polarity of the bentonite surface is reversed.

The originally hydrophilic bentonite surface is made hydrophobic. This will allow the complete dispersion of silicate platelets in organic solvents.

Bentonite for the application in paper industry

Carbonless multiple form sets contain sheets of paper with different coatings: The coated back paper (top sheet) has a releasing layer and the coated front paper (final sheet) has a receiving layer. By pressing the top sheet the dye capsules on the back are destroyed. The colourless pre-stage is adsorbed by the absorbent receiving layer. The acid activated bentonite within the receiver layer reacts as a colour developer to a coloured final stage.

Even small quantities of coating-bentonites provide a good coverage of paper surface. In coating formulations binder content can be reduced and the production of paper qualities for various printing methods is possible.

Fast running paper machines require micro particle systems in order to set the following parameters: retention, drainage and formation. Combined with synthetic and natural polymers alkaline activated bentonites provide the desired solutions.

The closure of water and stock loops and the increasing load of impurities from raw materials lead inevitably to a build-up of stickies. Bentonite is used for the passivation of sticky components.

In peroxide bleaching heavy metals are bonded to bentonite and the bleaching agent is destroyed in an environmentally friendly manner.

Bentonite for the application in detergents

The extremely soft and thin silicate platelets are deposited on textile fibre surfaces and ensure soft hand of fabrics.

They offer an environmentally friendly alternative to organic fabric softeners.

In agglomerate form bentonites are used as a substrate for different detergent additives (optical brighteners, dyes, surfactants).

Use of bentonite as cat litter

 Usually cat litter consists of clay minerals (e.g. bentonite). Due to their porous structure they are able to absorb a lot of moisture. The advantage of a clumping cat litter is that the litter can be sieved and therefore sparingly used.

Bentonite for the application in iron ore industry

 In iron ore industry bentonite is used as a binder in ore pelletisation. In the processing very fine-grained ore is produced, which has to be pelletized for further processing in the blast furnace. This is done with a drum process by adding water and bentonite.

Bentonites for the application in foundry industry

 Anyone who has ever built a sand castle on the beach knows that it breaks down, when the sun dries the sand. By adding only small quantities of bentonite the castle is preserved, because bentonite binds water and this ensures sufficient cohesion of sand grains.

To produce foundry moulds from quartz sand and water for the liquid metal bentonite must be added to the mixture. The silica grains are bonded and well integrated. The moulding sand become sufficiently plastic and receives the necessary stability to withstand the temperature of the molten metals.

Bentonite for the application in building industry  and drilling

Bentonite suspensions are used to stabilize drill holes and to transport the drilled material. This feature enables a wide variety of works, from cable laying in horizontal drilling up to the construction of tunnels. Bentonite suspensions support the excavated slots of slot-off walls and make additional boardings redundant.

Bentonite for landfill sealing

When it is raining, at landfills there is always a danger that stored pollutants are leached out into the groundwater.

Bentonite prevents this by swelling in contact with rainwater and forming a layer impermeable to water. These seals are extremely insensitive to temperature fluctuations. They cracks and leaks are not formed such as is the case of using asphalt.

Use of bentonite in bleaching clays

For refining of edible and mineral oils, fats and waxes, acid-activated bentonites (so-called bleaching clays) with a specific surface 200-400 m \ g are used.

By removing unwanted accompanying substances such as dyes and oxidation products besides brightening also taste stabilization is achieved.

Use of bentonite In  beverage preparation

Bentonite adsorbs proteins and other impurities in wine, beer and fruit juices.

The improvement of durability and compatibility is called fining, stabilization and clarification. After clarification the bentonite is

removed without any residue from the liquid.

Use of bentonite in soil improvement

 Bentonite mixed with soil prevents premature drying and seeping away of rainwater into the ground. Nitrogen, phosphorus and calcium fertilizers, which are flushed out from the soil by the rain, are adsorbed by bentonite.

Use of bentonite for soil improvement

Instead of grass, fresh grains and field crops increasingly prepared feed is fed to farm animals.

Bentonite is a versatile natural product, which contributes to improve quality of life in many areas

Frequently animal feed is mould infested with so-called mycotoxins releasing dangerous toxic substances. Mycotoxin adsorbers are based on bentonite. They are added to the feed to bind the mycotoxins. Thus, mycotoxines do no longer affect the metabolism of the animals but they are excreted.

Use of bentonite in waste water treatment

In the activated sludge biology bentonite is used as a nursery area for bacteria. Thus, the purification capacity of the activated sludge is stabilized in regard to carbon and nitrogen degradation. Bentonite has a high adsorption and buffering capacity for cations (eg NH ₄ + – ions). The addition of bentonite improves sludge weighting and sludge thickening.

Use of bentonite in drying agents

Bentonites are used for the packaging of high-quality and sensitive goods. Despite adverse conditions it ensures that quality is maintained during storage and transportation. Bentonite adsorbs occurring moisture and the goods will remain dry and free flowing, even if fully loaded. The water vapour absorption capacity is approximately ten percent by weight in dry environment and up to 30 weight percent in humid environment.

Conclusion 

 Bentonite has a variety of properties: high adsorption and absorption capacity, high pore volume, high specific surface area, high intra-crystalline swelling capacity, high water retention capacity, high aspect ratio, it contains small particles (nanoparticles), and is a charge carrier. Therefore, it is an extremely versatile natural product contributing to improve life quality in many areas, and, moreover, bentonite is an environmentally friendly product.

Education:

 Based on a broad scientific engineering education– ideally obtained during a Bachelor of Science degree in “Chemical and Process Engineering” (Verfahrenstechnik) at TU Graz – the Institute for Paper, Pulp and Fiber Technology (IPZ) is one of the few places in Europe and the only one in Austria offering a Master of Science degree (Dipl.Ing.) in “Paper and Pulp Technology”. In the compulsory courses the students get to know in depth the processes, the technology and the equipment involved in pulp and paper production and converting. This theoretical education is complemented by a number of laboratory courses and several of industry excursions to paper and pulp producers and to converters. In the field of elective subjects the students can choose from a variety of topics – taught by industry professionals – to deepen their education in areas as e.g. modeling and simulation, minerals, lignocellulosic biorefineries, environmental technology and so on.

A six month diploma thesis – usually carried out in the pulp and paper industry – marks the end of the studies. Interested graduates have the possibility to continue their education and to obtain a Ph.D. degree.

For graduates from other disciplines or for technicians with some years of industry experience interested in deepening their theoretical knowledge the institute also offers a three-semester course in “Paper and Pulp Technology”, which leads to the title “Certified Academic Paper Engineer”.

Services:

The IPZ also operates the “Institute for Testing

and Research for Paper, Pulp and Fiber Technology” which holds an accreditation according to ISO/IEC 17 025. In this framework testing of raw materials and paper and fiber products according to international standards is performed for a variety of customers. An overview of possible tests and prices is available under www.ipz.tugraz.at/Dienstleistungen.

Besides standard tests also unique testing routines developed in research projects are available as e.g. 3D coating thickness distribution maps, fiber cross sectional properties in the dry state, single fiber flexibility, single fiber damage due to refining, advanced image analytical routines for printability evaluation, refractive index maps of paper surfaces, local compressibility

Local coating thickness distribution on a WFC paper sample

maps etc. The examination of laboratory instruments represents an additional area of activities. Also in the frame of the accreditation according to ISO/IEC 17025 pulp, paper and board testing instruments are checked regarding their compliance to international standards.

These routine checks are performed on site at the customer and detailed reports concerning the performance of the instruments are issued, which are used in the certification of the quality management system of the customer.

The institute is independent from testing equipment suppliers and besides the check of the equipment also detailed advice concerning all questions regarding testing equipment is offered.

In general service is of high importance for the staff of institute since experience has shown that advice offered to the paper and pulp industry in solving current problems often is the basis for a long lasting and trustful cooperation.

Research:

The main challenge in papermaking is to achieve a uniform distribution of the highly inhomogeneous raw materials in the final product. High resolution measurement routines and modelling of paper and fiber structures and production processes therefore are some of the keys in the development of improved and novel paper products at optimized costs. At the IPZ an interdisciplinary team of paper and process engineers, mechanical engineers, image analysis specialists and mathematicians work on these topics and a few are presented in the following: High resolution analysis of the 3D material structure of paper Knowledge of the spatial arrangement of the main components (fibers, fillers…) in paper products is the basis for a deeper understanding of paper and board properties. A detailed analysis of a 3D-paper structure requires a high spatial resolution and a sufficient sample size. A serial sectioning technique has been developed which allows the digitization of paper samples up to a size of 1 cm2 at a resolution of 0.6 x 0.6 x 1 μm3. The result is a sequence of images which are further processed using specially developed image analysis algorithms. A three dimensional coating layer analysis tool allows the characterisation of coating layer structures. Other tools allow the in situ characterisation of fiber morphological parameters such as fiber width, fiber wall thickness and degree of collapse of the fibers. Furthermore, three dimensional reconstructions of single fibers in the paper network are possible. Ongoing work is focused on the prediction of paper properties based on the measured data as well as the development of new evaluation routines for paper and other materials. Image analytical evaluation of papermaking fibers and fiber suspensions.

Automated image analysis routines have been developed to characterise the morphological parameters of pulp fibers in suspension on a single fiber basis. Besides standard parameters (fiber length, width, curl etc.) several new parameters like single fiber flexibility, fibrillation, degree of damage of the S1 fiber wall and drag coefficient of fibers in suspension are measured in highly diluted fiber suspensions.

Another focus is set on the image analytical characterisation of fiber suspensions at a consistency of 0.5 to 1.5% which corresponds to the consistency normally used on industrial paper machines. The main parameter of interest is the degree of fiber flocculation in the suspension under various flow conditions. Correlations of fiber morphological parameters to the degree of flocculation and the influence of chemical additives on fiber flocculation are also evaluated.

Modelling the printability behavior of paper based on local measurements Research is aimed at predicting the evenness of the printing ink layer transferred to paper.

Uneven printing ink transfer and acceptance caused by inhomogeneities in the paper structure lead to a mottled appearance of the print. Multivariate statistical modelling is used to determine which of the local paper properties (e.g. surface topography, liquid penetration, chemical composition, pore structure) is mainly responsible for uneven ink transfer and the resulting poor print quality.