This article was written by Executive Consultant, Andrew Scogings.
Bentonite is sometimes called the ‘mineral of a thousand uses’ and is characterised by properties such as its ability to swell in water, to act as a 'natural glue' or 'bond', special rheological properties when added to water (e.g. thixotropy), high surface area and exchangeable cations. These properties may be modified by treatment with acids, soda ash or other compounds such as polymers.
Bentonite finds application across a wide range of markets including foundry (metal casting in sand moulds), clumping cat litter, iron ore pelletising, edible oil, wine and beverage purification, oil and other exploration drilling, civil engineering, animal feed and paper manufacture.
Approximately 16 to 20 million tonnes of bentonite is estimated to be produced annually and leading producing countries include China, Greece, India, USA and Turkey (Scogings, 2016). Other significant producing countries include Australia, Brazil, Germany, Indonesia, Italy, Japan, Mexico, Morocco, Russia and South Africa (note that countries are listed alphabetically, not in order of production tonnages).
Geology and Mineralogy
Bentonite is a clay consisting mainly of smectite minerals which are generally considered to have been formed by the alteration of volcanic ash. Bentonite deposits often occur as bedded deposits (e.g. western USA), though they may be irregular bodies associated with rhyolite or perlite / obsidian (e.g. China and Mozambique).
Bentonite deposits range in age, with most of Cretaceous or younger. For example, the Australian deposits in Queensland occur within sandstones of Upper Jurassic / Lower Cretaceous age (~145 million years 'Ma') while the western USA bentonite deposits are Upper Cretaceous (~90 Ma) and the Koppies deposits in South Africa are associated with Permian shales(~280 Ma). The rhyolite hosted bentonites of southern Mozambique are Jurassic (~180 Ma).
Bentonite is characterised by exchangeable cations such as Ca2+, Mg2+, Na+, K+ or Li+ which affect its performance including bonding, swelling, sorptive, sealing and rheological properties. Sodium bentonite swells more in water than calcium bentonite. The USA is well known as a source of high-quality sodium bentonite mainly mined in South Dakota, Montana and Wyoming; this is also known as ‘western bentonite’ which has Na+ as the dominant exchangeable cation. Calcium bentonite is sometimes known as 'southern bentonite' from the deposits in states such as Alabama and Mississippi.
Bentonite deposits are diverse in their mineralogy and chemistry, hence properties vary between sources. Accessory minerals in bentonite may include quartz, feldspar, kaolinite, illite, gypsum, zeolite and opaline silica.
Bentonite is often weathered or oxidised close to outcrop - such processes cause changes in properties, e.g. oxidised bentonite may swell more and have higher viscosity than oxidised bentonite. This variation was described by Williams et al. (1954) "as a function of the overburden" - when they noted that blue (fresh) clay at depth changed to yellow (oxidised) clay near surface and that rheological performance decreased with increasing depth.
Bentonite exploration generally follows a similar path to other minerals, often from discovery of an outcrop, which is then explored by methods such as field mapping, trenching, drilling and lab testing. The data generated could ultimately be used for the estimation of a Mineral Resource.
Drilling methods vary from country to country, for example auger drilling is common in the USA where the overburden is generally soft shale. Diamond core drilling (DD) is often used in locations such as China and Turkey, especially when there is hard andesite or rhyolite overburden, while rotary air blast (RAB) may also be appropriate in other geological settings.
Mining and Production
Bentonite is usually mined opencast, followed by drying, crushing, grinding or screening. Drying may be on drying pads (solar drying) and / or in rotary dryers. Different bentonites are often blended to achieve desired final specifications and performance.
Blending of high and low grade clays helps to improve bentonite utilisation by extending reserves and reducing natural variability so that a more consistent product can be produced. Blends may have improved performance compared with the individual components.
Although bentonites are generally described as being either calcium or sodium based on their dominant exchangeable cations, a complete range of calcium to sodium varieties exist throughout the world. Sodium and calcium bentonite may be treated (activated, or peptised) with soda ash to modify properties such as rheology, fluid loss, swelling index, water absorption and thermal durability. The addition of sodium carbonate (soda ash) replaces exchangeable Ca and Mg cations with Na cations. If bentonite requires additional sodium, soda ash may be added as powder or a solution to bentonite drying pads, or added to bentonite in mixing and shearing equipment such as pan mills, paddle mixers or extruders.
There is no optimum level of sodium that meets all requirements. Therefore as a general rule, bentonites require different levels of sodium activation (typically less than ~ 4% soda ash), depending on the amount of exchangeable calcium and magnesium cations present in the initial clay (or clay blend) and the desired final properties.
The addition of too much sodium, also known as ‘over-activation’, can result in decreased thermal durability in foundry clays and in increased fluid loss in drilling muds. On the other hand, too little soda ash may result in low swelling, low viscosity and low water absorption which could adversely affect performance in drilling muds, civil engineering applications and iron ore pelletising.
Organic polymers can be added in small amounts (e.g. 0.1%) as extenders (to increase viscosity) or to improve (reduce) fluid loss.
Calcium bentonite used for edible oil purification can be treated with hydrochloric acid or sulphuric acid; this is known as acid activation. This treatment removes impurities, replaces exchangeable cations with hydrogen (protonation) and increases the specific surface area by 'opening up' clay platelets.
Bentonite is produced and sold as either dried crudes (sometimes described as ‘dried & crushed’), or granular or powdered and delivered as loose bulk on rail or by ship, in road tankers, bulk bags or small bags on pallets.
Bentonite is a typical industrial mineral and is sold according to market specifications such as: viscosity, yield point / plastic viscosity ratio, fluid loss, residue or grit content (oil drilling muds); bond strength, thermal durability (foundry sand); particle size, colour, clump strength (cat litter); water absorption (iron ore pelletising); colour, drainage performance - the rate at which water is released from wet pulp, grit, abrasion (paper); clarification and lees formation (wine); moisture, swelling index, viscosity, fluid loss, permeability (geosynthetic clay liners); moisture, Marsh Funnel viscosity, fluid loss, syneresis (also known as 'bleed') and compressive strength for civil engineering applications e.g. slurry trenches and tunnelling where the bentonite may be mixed with cement and other additives.
As an example of specifications, the American Petroleum Institute (API) specifies Section 9 drilling-grade bentonite as follows for a slurry made of 22.5g bentonite in 350 ml water: viscometer dial reading at 600 RPM to be more than 30, yield point / plastic viscosity ratio to be a maximum 3, filtrate volume a maximum of 15.0 ml and maximum of 4.0% residue greater than 75 micron.
Reporting bentonite Mineral Resources and Ore Reserves
Bentonite is an industrial mineral that is produced to meet customer specifications and therefore ASX and NZX listed companies are required to report Mineral Resources and Ore Reserves in accordance with Clause 49 of the JORC Code.
Clause 49 of the JORC Code 2012 requires that: "For minerals that are defined by a specification, the Mineral Resource or Ore Reserve estimation must be reported in terms of the mineral or minerals on which the project is to be based and must include the specification of those minerals."
The map below shows an example of a sodium bentonite deposit where the Free Swell (FS) is highest in the oxidised domain at shallow depths. This deposit could be reported according to FS if this deemed to be a product specification; for example <16 ml for the deeper domain and >16 ml for the shallow domain.
American Petroleum Institute (2010). Specification for Drilling-Fluid Materials, API Specification 13A. Eighteenth edition, February 2010.
Eisenhour, D. and Reisch, F. (2006). Bentonite, in Industrial Minerals & Rocks, 7th edition, 1487-1494. Society for Mining, Metallurgy, and Exploration, Inc. Colorado, USA.
Harben, P.W. and Kuzvart, M. (1996). Clays: Bentonite and Hectorite, in Industrial Minerals - A Global Geology, 33-42. Industrial Minerals information Ltd.
Joint Ore Reserves Committee (2012). Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves. The JORC Code, 2012 Edition.
Scogings, A.J. (2014). AMCOL Australia’s Gurulmundi mine – a leading supplier of high quality sodium bentonites. Industrial Minerals Magazine, June 2014, 39-43.
Scogings, A. J. (2016). Soaking it up: Bentonite’s global reach. Industrial Minerals Magazine, June 2016, 37-42.
Williams, F.J., Elsley, B.C. and Weintritt, D.J. (1954). The variations of Wyoming bentonite beds as a function of the overburden. Proceedings of the Second National Conference on Clays and Clay Minerals, 141-151. Eds. Swineford, A. and Plummer, N.