The industrial utility of kaolin covers a diverse range of applications which can be classified into six categories according to primary functions: kaolin in film formation, kaolin in fiber extension, kaolin in polymer extension and reinforcement, kaolin use by virtue of chemical composition, kaolin as a carrier, adsorbent, diluent, etc., and kaolin as a polishing agent. By far the most important use of kaolin is in film formation, especially paper coating, which commands the most stringent kaolin specifications. These include brightness, particle size, particle size distribution, particle shape, and rheology. Along with fine particle size, the most important property of kaolin for film formation is the thin platelet shape which for Georgia kaolins generally do not exceed about 0.15 μm in average thickness. Calcined kaolins for paper applications must be bright, fine in particle size, and have aggregate structures with high internal void volume for enhanced light scatter. Excluding rheology, the same specifications are necessary for kaolin in fiber extension such as paper filling.
For polymer extension and reinforcement, such as in plastics and rubber, surface chemistry and particle size are of primary importance. The finer the particle size the better the reinforcement, but dispersion is essential to realize fully this benefit. For best dispersion, the polarity of the kaolin surface should approximate that of the polymer and can be achieved by surface treatment. In some systems silanes act as coupling agents giving a strong covalent bond between the kaolin surface and the polymer, thereby offering maximum reinforcement to the composite.
Where kaolin use involves reconstitution, as in catalyst supports, cement, fiber glass, and for the production of aluminum compounds, chemical composition is most critical. As a carrier for pesticides and pharmaceuticals, catalytic activity promoted by Lewis acid and Brõnsted acid sites on kaolin surfaces can affect transformation of some organic substances. Fine particle calcined clays, subangular to subrounded, are efficient polishing agents for teeth, automobiles, and soft metals such as gold and silver. The wide utility of kaolin is a function of a broad composite of characteristics but the most important are low cost, high brightness, fine particle size, platelet shape, and hydrophilic surface chemistry.
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Kaolin is an important industrial mineral in several world markets including uses in paper coating and filling, ceramics, paint, plastics, rubber, ink, fiberglass, cracking catalysts and many other uses (Murray, 1991). The kaolin minerals kaolinite, halloysite, dickite, and nacrite have essentially similar chemical composition but each has important structural and stacking differences. The most common kaolin mineral and the one that is the most important industrially is kaolinite [Al2Si205(OH)4]. Kaolinite can be formed as a residual weathering product, by hydrothermal alteration, and as an authigenic sedimentary mineral. The residual and hydrothermal occurrences are classed as primary and the sedimentary occurrences as secondary. Primary kaolins are those that have formed in situ usually by the alteration of crystalline rocks such as granites and rhyolites. The alteration results from surface weathering, groundwater movement below the surface or action of hydrothermal fluids. Secondary kaolins are sedimentary which were eroded, transported and deposited as beds or lenses associated with other sedimentary rocks. Most kaolin deposits of secondary origin were formed by the deposition of kaolinite which had been formed elsewhere. Some secondary deposits were formed from arkosic sediments that were altered after deposition, primarily by groundwater. There are far more deposits of primary kaolins in the world than secondary kaolin deposits because special geologic conditions are necessary for both the deposition and preservation of secondary kaolins.