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API Bentonite

Iran Bentonite Co. is a pioneer specialized manufacturer, supplier and exporter of all grades of Bentonite. Having several Bentonite Mines in different areas of Iran, well-equipped Mineral Processing Plants and a knowledgeable and experienced technical team, Iran Bentonite Co., managed to pave its own way to the international markets promptly.

The term Bentonite was first used for a clay found in about 1890 in upper cretaceous tuff near Fort Benton, Wyoming. Bentonite clay is a fine-grained rock composed of clay minerals, predominantly montmorillonite with minor amounts of other smectite group minerals. The formation of bentonite is an in-situ alteration of rhyolitic volcanic ash. Pyroclastic material was ejected into the atmosphere by volcanic activity and deposited as sediment in a marine environment.

Bentonite clay is composed of microscopic platelets consisting of layers of aluminium hydroxide held between layers of silicate atoms. These platelets are stacked one on top of the other. If a gram of bentonite were spread out only one particle layer thick, its billions of particles would cover an area greater than a football field. There are two basic types of bentonite, sodium type (Na-bentonite) and calcium type (Ca-bentonite).

  • Sodium Type – This type has sodium in the crystal lattice and is sometimes referred to as Wyoming Bentonite. This type swells when wet and can increase as much as fifteen times its original volume when wetted.
  • Calcium Type – This type has calcium in the crystal lattice and is sometimes referred to as Southern Bentonite. This type will swell only 2 times their unwetted volume.

The colour of bentonite ranges from white to black and includes shades of grey, light olive green, cream, yellow, earthy red, brown, sky blue and purple. Bentonite feels and appears greasy or waxy and when wet it is highly plastic and slippery.

The industry uses bentonite for its inherent physical properties or for the physical properties it can develop in another material or product. The major applications of bentonite are those that involve mixture with water. By adjusting the water content, properties of the mixture can be developed which are utilized by industry for bonding, plasticizing, and suspending applications.

What is API Bentonite?

Bentonite represents one of the main players in the stability of the drilling mud. Drilling fluid is a very important element for drilling operation to be completed safely. Its main functions are removing cuttings produced by drilling, cooling the bit, and adjusting formation pressure. These are achieved by controlling drilling mud weight and minimizing formation damage. The types of drilling mud are water-based mud (WBM), oil-based mud (OBM), and gas-based mud (GBM). The most commonly used is the WBM. Bentonite is an important drilling mud component. It is used as an additive in WBM to enhance the stability of the slurry by improving its rheological properties such as plastic viscosity and gel strength.

API bentonite is a sodium bentonite that represents the major component of clay drilling fluid. It consists mainly of montmorillonite that meets the specifications of American Petroleum Institute (API) for bentonite. Therefore, it is used in the drilling mud industry to control borehole stability, increase viscosity of the slurry, reduce fluid loss, and form a thin filter cake with low permeability.

Bentonite is naturally occurring industrial rock, characterized by the property of absorbing water and by capacity for base exchange; both properties are significantly greater than that of plastic clays and kaolin; in certain bentonites water absorption is accompanied by a considerable increase in volume and formation of gelatinous mass.

Bentonite is processed to give sodium and calcium montmorillonite, activated clays and organo-clays all of which have different properties that can be applied in wide range of uses and application. Foundry, pelletizing, construction – civil engineering, environmental markets, drilling, oils and food markets, agriculture, pharmacy, cosmetic and medical markets, detergents, paints, dyes and polishes, cat litter, paper, ceramics, catalyst are uses and applications reported by European Bentonite Association.

Special use of bentonite, where its absorbing properties are employed to provide water-tight sealing is for an underground repository in granites.

In repository, compacted bentonite is used in buffer zone which is designed for containment of radioactive materials over long time period, as protection against minor rock displacements, impermeable barrier to the mass transport by ground water, protection of chemical buffering

and retardation capacity, conduction of heat from the radioactive canister to the surrounding rock. In mixtures with other materials, bentonite is used as backfill that is low in permeability and its purpose is to maintain mechanical stability of underground constructions (tunnel, buffer).

Aim of this paper is to describe the bentonite beneficiation process.

Production Process of API Bentonite

Bentonite deposits are usually exploited by quarrying. Extracted bentonite is distinctly solid, even with a high moisture content of approximately 30%. The material is initially crushed or sliced, and, if necessary, activated by addition of soda ash (Na2CO3). Bentonite is subsequently dried (air and/or forced drying) to reach the required moisture content.

In relation to the final application, bentonite is either sieved (granular form) or milled (into powder and superfine powder form). For special applications, bentonite is purified by removing the associated gangue minerals, or treated with acids to produce acid activated bentonite (bleaching earths), or treated with organics to produce organoclays. Figures 1 shows the process flow diagram for bentonite processing.

Bentonite is processed first by drying in the rotary kilns to moisture content 5-8%, and then grounded in high-speed roller mills followed by classification into the size grades using cyclones and screens.

Ring roller mills are size reduction machines embodying functions of size reduction, drying, conveying and beneficiation. Feed size 50 mm is grinded to various size products as fine as 0.037 mm, or to granulation 3.36 mm containing minimum of fines. Wet fed bentonite, moisture content 15-32%, is fluid bed dried to 7-11% of moisture content. Fuller’s earth of moisture content 47% is fluid bed dried to 30-35% moisture. Negative pressure air system results in dust free conveying. Liberation and removal of foreign materials such as silica via auxiliary bottom discharge is achieved simultaneously with grinding.

The most important property of the montmorillonite – to regain interlayer water and to expand – is lost if dried (heated) above the certain temperature which are: 105-125°C; 300-390°C; and 390-490°C for Li montmorillonite, H or Ca montmorillonite, and Na montmorillonite, respectively. For many sodium bentonites drying temperatures up to 205°C are safe and bentonite may be dried and wetted any number of times repeatedly without losing swelling (colloidal) properties.

Experience in drying the bentonite commercially has shown that, regaining of interlayer water is difficult in practice if the last trace of interlayer water is removed, but as long as some water remains between the layers, swelling is generally relatively easy.



Figure 1. Process flow diagram for bentonite processing

Applications of API Bentonite

The main application of Bentonite is in drilling fluids. The bentonite in the flush fluid lubricates and cools the cutting tools while protecting against corrosion. As the drilling fluid generates hydrostatic pressure in the borehole, it hinders fluid and gas penetration. The drilling liquid conveys the drill cuttings to the surface. When the purge pumps have idle periods, the thixotrophy of the bentonite hinders the stone material from dropping back into the drilled shaft.

Any drilling fluid must have common properties, which facilitate safe and satisfactory completion of the well such as:

  • Bottom hole cleaning and removal of cuttings to the surface.
  • Controlling high pressure zones.
  • Cooling and lubricating down hole drilling equipment.
  • Forming bore hole wall supporting filter cake.
  • Supporting the drill string and casing weight.
  • Allowing the interpretation of electric logs.

These functions are controlled by the reheological and filtration properties of mud. The main component of water base mud is clay (mostly bentonite). One definition of a clay is that it will break upon dispersion into particles smaller than 2 microns when placed in water. Bentonite is a geological term for a rock which contains montmorillonite as its major component.

A good bentonite for a drilling fluid requires montmorillonite with sodium and calcium as the minor-cations associated with its exchange sites. A drilling grade bentonite must readily disperse in water to produce a thixotropic or shear thinning fluid which possesses gel strength and a low fluid loss to the formation.

As a flushing fluid, bentonite seals the drilled shaft from water ingress downwards and at the sides of the shaft. The mineral forms a firm sludge cake on the bore wall, which provides the borehole with additional stability. The fine bentonite particles enter into the bore wall where they swell and harden. Depending on the constitution of the ground structure, fine and coarse particles are needed.

In petroleum engineering, drilling muds are used to drill boreholes deep into the earth. The boreholes are mainly used for the production of natural gas and oil.

The main purposes of these drilling muds are to cool and clean the drill bit, to provide hydrostatic pressure in the drilling column so that fluids will not enter the borehole, to carry out the drill cuttings and to suspend these cuttings when the drilling operation is in static state. For these last two purposes it is important to choose a good clay with the right properties. Bentonite is often used in the drilling industry to create a so called ‘water-based mud’. This is often referred to as ‘gel’ in the petroleum industry.

The term ’gel’ is used since bentonite muds have the properties of being a free-flowing liquid in dynamic state and a gel in static state. This means that this gel has a certain yield point. These kinds of fluids are called ’Bingham plastics’.

A bentonite drilling mud is qualified as a good drilling mud if it meets the API standards (Appendix B.1). There exist 3 API standards for drilling muds, these are presented in table 1 and Appendix A.1.


The Fann Viscosity is a value for the viscosity measured by a Fann viscometer, or a V-G meter. The API definition of the filtrate volume is the volume of a mud filtrate measured after a 30-minute API static filtration test. Such a test consists of measuring the process of separating components of a slurry by leaving the suspended solids (dispersed particles from the drilling process) as a filter cake on a filter medium while the liquid passes through the filter. The moisture content is the percentage of water in the end state of the semi-finished product.