Infrastructure and Construction Materials Guide — Bentonite

Commodity Description

Bentonite is a type of clay rock that is widely used in industrial processes and products and in consumer products. When wetted it holds water and swells significantly and, upon drying, it shrinks significantly. When mixed with sufficient water to create a slurry it forms materials that become more solid when compressed. It is both adhesive and highly slippery when wetted and impedes aqueous fluid movement. These properties, individually or together, make bentonite a highly useful industrial mineral.

Geology and Mineralogy

Most bentonite mined and sold in North America was formed 90 to 110 million years ago when ash from volcanos, in what is now Idaho and northern Utah, fell into an inland sea. The ash dissolved in the seawater and re-precipitated, primarily as the clay mineral montmorillonite, along with accessory minerals such as feldspar, biotite mica, quartz, opal and other minor minerals which, together, are known by the rock name “bentonite.” In the US, the largest deposits of bentonite are found in Wyoming, Montana and South Dakota. More localized deposits also occur in California, Colorado, Oregon, Utah, Texas and Mississippi. Some bentonites were formed when volcanic ash deposits were altered in place by hydrothermal ground water. Most bentonite found in the US is of the high swelling, sodium-containing type, also known as Western or Wyoming bentonite. Deposits of low swelling, calcium-containing type, also known as Southern bentonite, are commonly found in Mississippi.

Production and Pricing

For U.S. production statistics and pricing, “Clays; Bentonite,” go to p. 60 of the USGS Commodity Summaries 2024.

For World Mine Production and Reserves, “Clays; Bentonite,” go to p. 61 of the USGS Commodity Summaries 2024.

Uses

Bentonite is called the “Clay of a Thousand Uses” because of its unique properties. It is used to increase the viscosity of drilling liquids, reacts with oil to form gels, lubricates machines, and controls odor and moisture and is antimicrobial and antifungal in numerous products.
Bentonite is used in a number of construction applications:
• Cement, Building Blocks (Cinder Blocks)
• Cement Tile
• Clay Brick
• Insulation Blocks
• Roofing Paper
• Slag Cement
• Stucco Cement
• Tar Preparations

Cinder block structure

The use of bentonite for pet litter, in oil, gas, water well and other drilling, as a binder in sand molds used to form metal castings and in iron ore pelletizing accounts for more than 80% of the bentonite produced in North America.

Bentonite is used in oil well drilling when added to freshwater or to freshwater muds for one or more of the following purposes (British Geological Survey):
• increase hole cleaning capability
• reduce water seepage or filtration into permeable formations
• form a thin filter cake of low permeability
• promote hole stability in poorly cemented formations to avoid or overcome the loss of circulation.

Substitutes

In most of the applications where bentonite is used there are no economical substitutes that provide the performance required by the user.

Major Producers

U.S.: Bentonite Performance Minerals, Black Hills Bentonite, MI-Swaco, MTI, Wyo-Ben, Inc.
International: IMERYS, numerous Chinese, Indian and Japanese companies

Mining Methods

All significant commercial bentonite mining in North America is by surface mining operations using scrapers, bulldozers and excavators to remove the relatively soft overburden that covers the clay deposit. When removed from the ground the moisture content of the bentonite may range from about 15 to 35%. Depending upon need, the bentonite may be partially dried at the mine site before hauling, using the sun and wind, or hauled directly to the production facility for processing. The bentonite is typically stockpiled at the processing plant according to predefined, market driven performance characteristics.

Reclamation

All bentonite mine sites are backfilled, re-contoured, top soiled and reseeded following removal of the bentonite from the site. In all cases, bentonite mine reclamation must meet applicable State and Federal standards, which means that the post mine condition of the land must be equal to or better than the land condition that existed prior to mining disturbance. Financial bonds to State and Federal agencies must be in place for all mining disturbances prior to mining.

Mineral Processing

Bentonite produced in the US is typically processed by drying to a finished moisture of about 8 to 10% using natural gas or coal-fired rotary driers. It is then sized for final use by either milling it to a powder of varying fineness or by screening to produce granular products of different sizes. The finished product is then placed in 50- or 100-pound paper bags, or their metric equivalent, bulk sacks of various sizes or shipped by bulk truck or rail car to the end user.

Environmental Factors

Bentonite, like all clays, contains naturally occurring crystalline silica in the form of quartz at concentrations of about 0.5% to 6%. Appropriate engineering controls and personal protection devices, such as MSHA approved dust masks, should be used when handling bentonite products where airborne dust may be generated.

References

British Geological Survey (BGS) World Mineral Production 2016-2020. https://www.bgs.ac.uk/.

SME Industrial Minerals & Rocks 7th Edition.  SME Books. Editors: Jessica Elzea Kogel, Nikhil C. Trivedi, James M. Barker, Stanley T. Krukowski, 2006.

United States Geological Survey (USGS) Commodity Summaries 2023.  https://pubs.usgs.gov/publication/mcs2023. The 2023 summary reports 2022 statistics.

United States Geological Survey (USGS) Commodity Summaries 2024.  https://pubs.usgs.gov/publication/mcs2024. The 2024 summary reports 2023 statistics.