Clays
Clays are a diverse group of fine-grained minerals and rocks, primarily composed of aluminum silicates containing water. These materials, often less than four micrometers in particle size, are formed through the weathering of other minerals and can be found globally, although economically significant deposits are more limited. Major types of clays include kaolin, bentonite, ball clay, and refractory clay, each classified based on their composition or functional properties. Kaolin, known for its purity and whiteness, is widely used in ceramics and paper manufacturing. Bentonite, a clay with significant swelling properties, is utilized in drilling muds and as a binder in various applications, while fuller's earth serves as a bleaching and absorbent material. Clays are considered industrial minerals due to their commercial value and non-metallic character, and they play crucial roles in many industries, from construction to cosmetics. Mining methods typically involve open pit extraction, followed by processing to enhance quality and reduce impurities. Overall, clays offer a range of applications and are vital to numerous manufacturing processes.
Clays
The term “clay” may be used to describe a group of fine-grained minerals, a type of rock, or a range of particle size, generally less than four micrometers. As a rock term, clay is generally understood to mean an earthy, fine-grained material formed largely of crystalline minerals known as the clay minerals.
Background
Clays can be found throughout the world, but economically valuable deposits are limited in extent and distribution. Majorkaolin deposits in the United States are found in Georgia and South Carolina. The world’s major bentonite deposits are found in Wyoming and Montana, and large fuller’s earth deposits can be found in Georgia and Florida. Ball and refractory clays are abundant in Kentucky and Tennessee.
Clays are used in a number of applications requiring the incorporation of fine-grained materials that contribute to a product’s physical or chemical properties. Uses include fillers in paint, paper, and plastics, additives to drilling muds, the manufacture of and brick, carriers for pesticides and insecticides, the manufacture of catalysts, and cosmetic and pharmaceutical uses.
Clays are considered “industrial minerals,” a group of minerals composed of geological materials having commercial value and of a nonmetallic, nonfuel character. They may be marketed in a natural, as-mined state or as processed materials. Clays can vary widely in composition and physical characteristics. Certain similarities exist among a number of clays, however, and they can be categorized in broad terms as kaolin, bentonite or fuller’s earth, ball clay, and refractory clay based on similarities in either composition or functional performance. Clays that do not fall into any of the major categories are generally referred to as common clay or shale.
Mineralogy and Chemistry
Clays are hydrous (water-containing) aluminum silicates containing alkalies or alkaline earth elements. Magnesium or iron may substitute wholly or partially for aluminum in the clay mineral structure. Clay minerals are composed of alternating layers of two different atomic structures. The first is an aluminum-bearing octahedral sheet structure, and the second is a layer of silica tetrahedrons. The aluminum and silicon atoms are chemically bonded to oxygen in these layers, which are held to one another by weaker electrostatic bonds. Interlayer sites in many clays contain water molecules or cations such as calcium, sodium, potassium, magnesium, lithium, or hydrogen. The presence or absence of interlayer molecules affects both the physical and chemical properties of the clay.
Kaolin
Kaolin is a clay consisting predominantly of pure kaolinite or related clay minerals. Most major deposits of kaolin are referred to as either primary (residual) or secondary. Primary deposits are formed in place as the products of or other feldspar-rich rocks. Other minerals associated with deposits of this type include quartz, micas, amphiboles, tourmaline, and unweathered feldspars. Primary deposits are irregular in shape, grading downward into unaltered parent (source) rock.
Secondary deposits of kaolin are accumulations of kaolinitic material that has been transported from its source area. Deposits of this type may contain up to 95 percent kaolinite; in contrast, primary deposits may contain as little as 10 percent. Associated minerals may include quartz, micas, other clay minerals, and a variety of high-density “heavy minerals.” Secondary deposits are generally lenticular or tabular in shape, with thicknesses up to sixty meters and areal dimensions of up to about two kilometers.
Kaolin is also found as a product of hydrothermally altered rocks. Deposits of this nature are of limited size and extent. They occur as irregularly elongated pods or pipelike bodies along faults, joints, and other conduits along which hot solutions have flowed.
Kaolin is generally soft and plastic, although harder silica-bearing varieties also exist. Crystals of kaolinite are hexagonal, composed of individual platelets stacked in an accordion-like manner. There is little ionic substitution in the lattice.
Kaolin has numerous industrial uses and is perhaps best known for its use in the manufacture of china and porcelain. Its chemical inertness, high brightness, white color (either naturally or resulting from processing and beneficiation), and crystal shape make it useful in other applications as well. Kaolin is used as a filler or coating in the manufacture of paper, as a filler in paint, plastics, and pharmaceuticals, and in the manufacture of rubber, tile, brick, ink, adhesives, detergents, cosmetics, pencils, pastes, and other consumer products.
Ball Clay and Refractory Clay
Ball clays are composed of up to 70 percent kaolin. They generally occur in secondary sedimentary deposits characterized by the presence of organic matter along with varying amounts of other clays, quartz, feldspar, calcite, and heavy minerals. Sedimentary deposits of ball clay represent accumulations of clay materials that were derived from a number of sources and that were deposited in nonmarine environments. Most deposits are lenticular, with areal dimensions of up to 850 meters and thicknesses of up to 10 or more meters.
Ball clays are plastic or semi-plastic and are used to provide strength and to ceramic bodies prior to firing. They fuse during firing, also acting as a “cement” to bind together the refractory, nonshrinking component of a ceramic body. Ball clay is used to manufacture tableware, stoneware, tiles, plumbing fixtures, and bricks. It is also used as a sealant in landfills.
Refractory clays are generally kaolin, containing only small quantities of mica or iron-bearing minerals that might combine with other materials during firing to form low-melting-point glasses. Refractory clays have a high heat resistance. Other properties that affect overall quality include shrinkage, warping, cracking, and abrasion. Refractory clays can be soft and plastic or hard like flint. They generally occur as sedimentary deposits that are lenticular or tabular in shape. They are mined for use in the manufacture of firebrick, insulating brick, and other heat-resistant clay products.
Bentonite and Fuller’s Earth
“Bentonite” is generally understood to mean a clay consisting of minerals from the montmorillonite group, regardless of origin or occurrence. The most important commercial montmorillonites are the sodium and calcium varieties. Sodium montmorillonite (Wyoming or western bentonite) has high swelling capabilities when added to water. Calcium montmorillonite (southern bentonite) has a lower swelling capacity, and it generally crumbles when added to water. Other montmorillonites include those rich in lithium (hectorite), magnesium (saponite), or iron (nontronite).
Bentonite can be both physically and chemically reactive. It shrinks or swells as it releases or absorbs interstitial water or organic molecules, and it has important exchange and chemical sorption properties. Bentonite’s physical and chemical properties account for its usefulness in modifying fluid viscosity or plasticity; it also has a variety of other uses. Bentonites can be modified through chemical treatment to enhance selected physical or chemical properties.
Wyoming bentonites are suitable for use in an as-mined condition. They are used as an additive in drilling mud to increase viscosity and aid in the removal of drill hole cuttings. The clay also helps maintain cuttings in suspension and creates an impervious coating on the wall of the drill hole to prevent fluid loss during drilling. Southern bentonites can be modified to have properties similar to Wyoming bentonites, but their use is generally restricted to other applications such as binding iron during pelletizing and the manufacture of catalysts and no-carbon-required (NCR) copy papers. Bentonites are also used to refine, decolor, and purify oils and beverages; to manufacture fire retardants; and as hydraulic barriers.
“Fuller’s earth” refers to clays (generally bentonitic) suitable for bleaching and absorbent or other special uses. The term was first used to describe materials used for cleansing or fulling wool (removing lanolin and dirt), but it is now used more broadly to include decolorizers or purifiers in filtering applications. Fuller’s earth products include cat litter, and carriers, soil conditioners, lightweight aggregate, and pharmaceuticals.
Mining and Processing
Most clay deposits are mined from open pits, although some are mined by underground methods. generally involves the stripping of overburden, excavation of the clay, and transport of mined material to the processing plant. Some operations may require blasting.
The simplest operations involve excavation, transport to the plant, drying, and shipment to the customer. More complex operations may require that the mined material first be put into a form for removal of grit or sand, with transport to the plant by pipeline. Clay slurries can be chemically or physically treated to remove contaminants that contribute to discoloration or poor chemical or physical performance. They can then be filtered and dried prior to packaging and shipment to the customer. Some clays are put back into slurry form prior to shipment, depending upon a customer’s needs.
Bibliography
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