Clays utilized as raw material in manufacturing, which are among the most important nonmetallic mineral resources. The value of clays is related to their mineralogical and chemical composition, particularly the clay mineral constituents kaolinite, montmorillonite, illite, chlorite, and attapulgite. The presence of minor amounts of mineral or soluble salt impurities in clays can ®restrict their use. The more common mineral impurities are quartz, mica, carbonates, iron oxides and sulfides, and feldspar. In addition, many clays contain some organic material. See also Clay minerals.
Kaolinitic clays
Clays containing a preponderance of the clay mineral kaolinite are known as kaolinitic clays. Several commercial clays are composed predominantly of kaolinite. These are china clays, kaolines, ball clays, fireclays, and flint clays. The terms china clay and kaolin are used interchangeably in industry. See also Kaolinite.
China clays are high-grade white kaolins found in the south-eastern United States, England, and many other countries. Many grades of kaolin are used in the manufacture of ceramics, paper, rubber, paint, plastics, insecticides, adhesives, catalysts, and ink. By far the largest consumer of white kaolins is the paper industry, which uses them to make paper products smoother, whiter, and more printable. The kaolin is used both as a filler in the sheet to enhance opacity and receptivity to ink and as a thin coating on the surface of the sheet to make it smoother and whiter for printing. See also Ceramics; Porcelain; Pottery.
Ball clays are composed mainly of the mineral kaolinite but usually are much darker in color than kaolin. The term ball clay is used for a fine-grained, very plastic, refractory bond clay. Most ball clays contain minor amounts of organic material and the clay mineral montmorillonite, and are finer grained than china clays. This fineness, together with the montmorillonite and organic material, gives ball clays excellent plasticity and strength. For these reasons and because they fire to a light-cream color, ball clays are commonly used in whitewares and sanitary ware.
The term fireclay is used for clays that will withstand temperatures of 2730°F (1500°C) or higher. Such clays are composed primarily of the mineral kaolinite. Fireclays are generally light to dark gray in color, contain minor amounts of mineral impurities such as illite and quartz, and fire to a cream or buff color. Most fireclays are plastic, but some are nonplastic and very hard; these are known as flint clays. Fireclays are used primarily by the refractories industry. The foundry industry uses fireclay to bind sands into shapes in which metals can be cast. See also Refractory.
Diaspore clay
This clay is composed of the minerals diaspore and kaolinite. Diaspore is a hydrated aluminum oxide with an Al2O3 content of 85% and a water content of 15%. Diaspore clay is used almost exclusively by the refractories industry in making refractory brick. However, after calcination, it is sometimes used as an abrasive material.
Mullite
Mullite is a high-temperature conversion product of many aluminum silicate minerals, including kaolinite, pinite, topaz, dumortierite, pyrophyllite, sericite, andalusite, kyanite, and sillimanite. Mullite is used in refractories to produce materials of high strength and great refractoriness. Mullite does not spall, withstands the shock of heating and cooling exceptionally well, and is resistant to slag erosion. It is used in making spark plugs, laboratory crucibles, kiln furniture, saggars, and other special refractories.
Bentonites
Those clays that are composed mainly of the clay mineral montmorillonite and are formed by the alteration of volcanic ash are known as bentonites. The term bentonite is a rock term, but in industrial usage it has become almost synonymous with swelling clay. Bentonites are used in many industries; the most important uses are as drilling muds and catalysts in the petroleum industry, as bonding clays in foundries, as bonding agents for taconite pellets, and as adsorbents in many industries. See also Bentonite; Montmorillonite; Oil and gas well drilling.
Attapulgite clays
Attapulgite is a hydrated magnesium aluminum silicate with a needlelike shape. Each individual needle is exceedingly small, about 1 micrometer in length and approximately 0.01 micrometer across. Attapulgite is used as a suspending agent, and gives high viscosity because of the interaction of the needles. Some commercial uses are as an oil well drilling fluid, in adhesives as a viscosity control, in oil base foundry sand binders, as thickeners in latex paints, in liquid suspension fertilizers, and as a suspending agent and thickener in pharmaceuticals.
Properties
Most clays become plastic when mixed with varying proportions of water. Plasticity of a material can be defined as the ability of the material to undergo permanent deformation in any direction without rupture under a stress beyond that of elastic yielding. Clays range from those which are very plastic, called fat clay, to those which are barely plastic, called lean clay. The type of clay mineral, particle size and shape, organic matter, soluble salts, adsorbed ions, and the amount and type of nonclay minerals are all known to affect the plastic properties of a clay.
Green strength and dry strength properties are very important because most structural clay products are handled at least once and must be strong enough to maintain shape. Green strength is the strength of the clay material in the wet, plastic state. Dry strength is the strength of the clay after it has been dried.
Both drying and firing shrinkages are important properties of clay used for structural clay products. Shrinkage is the loss in volume of a clay when it dries or when it is fired. Drying shrinkage is high in most very plastic clays and tends to produce cracking and warping. It is low in sandy clays or clays of low plasticity and tends to produce a weak, porous body. Firing shrinkage depends on the volatile materials present, the types of crystalline phase changes that take place during firing, and the dehydration characteristics of the clay minerals.
The temperature range of vitrification, or glass formation, is a very important property in structural products. Vitrification is due to a process of gradual fusion in which some of the more easily melted constituents begin to produce an increasing amount of liquid which makes up the glassy bonding material in the final fired product. The degree of vitrification depends on the duration of firing as well as on the temperature attained.
Color is important in most structural clay products, particularly the maintenance of uniform color. The color of a product is influenced by the state of oxidation of iron, the state of division of the iron minerals, the firing temperature and degree of vitrification, the proportion of alumina, lime, and magnesia in the clay material, and the composition of the fire gases during the burning operation.
Structural uses
All types of clay and shale are used in the structural products industry but, in general, the clays that are used are considered to be relatively low grade. Clays that are used for conduit tile, glazed tile, and sewer pipe are underclays and shales that contain large proportions of kaolinite and illite. The semirefractory plastic clays found directly beneath the coal seams make the best raw material for the above mentioned uses. Brick and drain tile can be made from a wide variety of clays depending on their location and the quality of product desired. Clays used for brick and drain tile must be plastic enough to be shaped. In addition, color and vitrification range are very important. For common brick, drain tile, and terra-cotta, shales and surface clays are usually suitable, but for high-quality face bricks, shales and underclays are used. Geographic location is a prime factor in the type of clay used for structural clay products because, in general, these products cannot be shipped great distances without excessive transportation costs. Many raw materials of questionable quality are utilized in certain areas because no better raw material is available nearby. See also Brick.
The cement industry uses large quantities of impure clays and shales. Clays are used to provide alumina and silica to the charge for the cement kiln. Generally, a suitable clay can be found in the area in which the cement is being manufactured. See also Cement; Pottery; Refractory.
