Introduction
Kaolinite (/ˈkeɪ.ələˌnaɪt, -lɪ-/ KAY-ə-lə-nyte, -lih-; also called kaolin)[5][6][7] is a clay mineral, with the chemical composition Al2Si2O5(OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO4) linked through oxygen atoms to one octahedral sheet of alumina (AlO6).[8]
Kaolinite is a soft, earthy, usually white, mineral (dioctahedral phyllosilicate clay), produced by the chemical weathering of aluminium silicate minerals like feldspar. It has a low shrink–swell capacity and a
low cation-exchange capacity (1–15 meq/100 g).
Rocks that are rich in kaolinite, and halloysite, are known as kaolin (/ˈkeɪ.əlɪn/) or china clay.[9] In many parts of the world kaolin is colored pink-orange-red by iron oxide, giving it a distinct rust hue. Lower concentrations of iron oxide yield the white, yellow, or light orange colors of kaolin. Alternating lighter and darker layers are sometimes found, as at Providence Canyon State Park in Georgia, United States.
Kaolin is an important raw material in many industries and applications.
Commercial grades of kaolin are supplied and transported as powder, lumps, semi-dried noodle or slurry. Global production of kaolin in 2021 was estimated to be 45 million tonnes,[10] with a total market value of US $4.24 billion.[11]
Names
The English name kaolin was borrowed in 1727 from François Xavier d’Entrecolles’s 1712 French reports on the manufacture of Jingdezhen porcelain.[12] D’Entrecolles was transcribing the Chinese term 高嶺土, now romanized as gāolǐngtǔ in pinyin, taken from the name of the village of Gaoling (“High Ridge”) near Ehu in Fuliang County, now part of Jiangxi Province’s Jingdezhen Prefecture.[13][14] The area around the village had become the main source of Jingdezhen’s kaolin over the course of the Qing dynasty.[14] The mineralogical suffix -ite was later added to generalize the name to cover nearly identical minerals from other locations.
1. Chemistry
Notation
The chemical formula for kaolinite as written in mineralogy is Al2Si2O5(OH)4,[4] however, in ceramics applications the same formula is typically written in terms of oxides, thus giving Al2O3·2SiO2·2H2O.[16]
Structure
Compared with other clay minerals, kaolinite is chemically and structurally simple. It is described as a 1:1 or TO clay mineral because its crystals consist of stacked TO layers. Each TO layer consists of a tetrahedral (T) sheet composed of silicon and oxygen ions bonded to an octahedral (O) sheet composed of oxygen, aluminium, and hydroxyl ions.
The T sheet is so called because each silicon ion is surrounded by four oxygen ions forming a tetrahedron. The O sheet is so called because each aluminium ion is surrounded by six oxygen or hydroxyl ions arranged at the corners of an octahedron. The two sheets in each layer are strongly bonded together via shared oxygen ions, while layers are bonded via hydrogen bonding between oxygen on the outer face of the T sheet of one layer and hydroxyl on the outer face of the O sheet of the next layer.
2. Occurrence
Kaolinite is one of the most common minerals; it is mined, as kaolin, in Australia, Brazil, Bulgaria, China, Czech Republic, France, Germany, India, Iran, Malaysia, South Africa, South Korea, Spain, Tanzania, Thailand, United Kingdom, United States and Vietnam.[2]
Mantles of kaolinite are common in Western and Northern Europe. The ages of these mantles are Mesozoic to Early Cenozoic.[23]
Kaolinite clay occurs in abundance in soils that have formed from the chemical weathering of rocks in hot, moist climates; for example in tropical rainforest areas. Comparing soils along a gradient towards progressively cooler or drier climates, the proportion of kaolinite decreases, while the proportion of other clay minerals such as illite (in cooler climates) or smectite (in drier climates) increases. Such climatically related differences in clay mineral content are often used to infer changes in climates in the geological past, where ancient soils have been buried and preserved.[24]
A kaolin processing plantn the Institut National pour l’Étude Agronomique au Congo Belge (INEAC) classification system, soils in which the clay fraction is predominantly kaolinite are called kaolisol (from kaolin and soil).[25]
3. Synthesis and genesis
Difficulties are encountered when trying to explain kaolinite formation under atmospheric conditions by extrapolation of thermodynamic data from the more successful high-temperature syntheses.[31] La Iglesia and Van Oosterwijk-Gastuche (1978)[32] thought that the conditions under which kaolinite will nucleate can be deduced from stability diagrams, based as they are on dissolution data.
Because of a lack of convincing results in their own experiments, La Iglesia and Van Oosterwijk-Gastuche (1978) had to conclude, however, that there were other, still unknown, factors involved in the low-temperature nucleation of kaolinite. Because of the observed very slow crystallization rates of kaolinite from solution at room temperature Fripiat and Herbillon (1971) postulated the existence of high activation energies in the low-temperature nucleation of kaolinite.
At high temperatures, equilibrium thermodynamic models appear to be satisfactory for the description of kaolinite dissolution and nucleation, because the thermal energy suffices to overcome the energy barriers involved in the nucleation process. The importance of syntheses at ambient temperature and atmospheric pressure towards the understanding of the mechanism involved in the nucleation of clay minerals lies in overcoming these energy barriers.
As indicated by Caillère and Hénin (1960)[33] the processes involved will have to be studied in well-defined experiments, because it is virtually impossible to isolate the factors involved by mere deduction from complex natural physico-chemical systems such as the soil environment. Fripiat and Herbillon (1971),[34] in a review on the formation of kaolinite, raised the fundamental question how a disordered material (i.e., the amorphous fraction of tropical soils) could ever be transformed into a corresponding ordered structure.
This transformation seems to take place in soils without major changes in the environment, in a relatively short period of time, and at ambient temperature (and pressure).
4. Laboratory syntheses
Laboratory syntheses of kaolinite at room temperature and atmospheric pressure have been described by DeKimpe et al. (1961).[59] From those tests the role of periodicity becomes convincingly clear. DeKimpe et al. (1961) had used daily additions of alumina (as AlCl3·6 H2O) and silica (in the form of ethyl silicate) during at least two months.
In addition, adjustments of the pH took place every day by way of adding either hydrochloric acid or sodium hydroxide. Such daily additions of Si and Al to the solution in combination with the daily titrations with hydrochloric acid or sodium hydroxide during at least 60 days will have introduced the necessary element of periodicity.
Only now the actual role of what has been described as the “aging” (Alterung) of amorphous alumino-silicates (as for example Harder, 1978[60] had noted) can be fully understood. As such, time is not bringing about any change in a closed system at equilibrium; but a series of alternations of periodically changing conditions (by definition, taking place in an open system) will bring about the low-temperature formation of more and more of the stable phase kaolinite instead of (ill-defined) amorphous alumino-silicates.
5. Applications
In 2009, up to 70% of kaolin was used in the production of paper. Following reduced demand from the paper industry, resulting from both competing minerals and the effect of digital media, in 2016 the market share was reported to be: paper, 36%; ceramics, 31%; paint, 7% and other, 26%.[61][62] According to the USGS, in 2021 the global production of kaolin was estimated to be around 45 million tones
6. Safety
Kaolin is generally recognized as safe, but may cause mild irritation of the skin or mucous membranes. Kaolin products may also contain traces of crystalline silica, a known carcinogen if inhaled.[84]
In the US, the Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for kaolin exposure in the workplace as 15 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 10 mg/m3 total exposure TWA 5 mg/m3 respiratory exposure over an 8-hour workday.[85]
7. Evaluation of Swat kaolin deposits of Pakistan for industrial uses
Muhammad Ashraf Siddiqui a, Zulfiqar Ahmed a b, Akhter Ali Saleemi
Abstract
The Swat kaolin deposits are characterized and evaluated for their potential as industrial raw materials. Their particle size, colour measurement, viscosity, plastic limits, liquid limits, drying, firing shrinkage, water absorption, oil absorption, pH, water soluble matter, moisture and bulk density are tested for <63, <10, <5 and <2 μm size fractions. Due to low plasticity index and high Fe2O3, TiO2 and CaO contents, the “water-washed” <63-μm fraction of kaolin is unsuitable for ceramic use. The yellowness of this fraction, 1.3–9.8%, can be reduced by 20.4–61.3%, by chemical bleaching. The <2-μm fraction can be used for high-solids formulation for publication-grade paper and medium-finish lower cost enamels. The <5-μm fraction is useable in paper filling. The <10-μm fraction is suitable for the ceramics, paint, plastic and rubber-filler industries.
Introduction
The industrial applications of kaolin or china clay are diverse and depend largely on the physical properties, such as particle size, whiteness, platyness, etc., specific for each kaolin deposit (Bundy, 1993, Murray and Keller, 1993, Chandrasekhar and Ramaswamy, 2002, Murray, 2003). The kaolin deposits of Swat District of Pakistan (GPS: 34° 53′ 30ʺ N; 72° 53′ 30″ E), are amongst the oldest known kaolins from Pakistan.
Their industrial attributes are previously poorly characterized.
Previous studies of these deposits include those by Kyotani et al. (1967), Faruqi and Ahmad (1967), Faruqi et al., 1970a, Faruqi et al., 1970b, Moosvi et al. (1974), Ahmed et al. (1978), and Fujii (1995), though none of these focussed on the details of kaolin’s physical properties. Kyotani et al. (1967) estimated about 2.5 million tons reserves of Swat kaolin, revisable upwards with working of the deposits.
They categorized the clay into two grades on the basis of grain size (<44 and <5 μm fractions). They carried out mineralogical and chemical studies and determined the physical properties, which are used in ceramics. Relatively more elaborate account of physical properties was given by Faruqi and Ahmad (1967) andFaruqi et al., 1970a, Faruqi et al., 1970b.
They studied water-washed samples of the English kaolin and of the −200 mesh (or 74 μm) Swat kaolin; to determine the chemical and mineralogical composition and the following physical properties: specific gravity, colour and brightness, dry and fired shrinkage, particle size distribution, reversible thermal change, transverse strength, viscosity and casting rate, plasticity and workability.
Section snippets
1. Geology of Swat kaolin
These primary kaolin deposits of northern Pakistan occur within the amphibolites, sensu lato, of the Cretaceous age Kohistan island arc terrane (Fig. 1a) , close to its southern boundary marked by the Main Mantle Thrust (MMT). These ortho-amphibolites form the “country rocks” around certain felsic zones enriched in small leucocratic igneous bodies and dikes that constitute the “host rocks” of kaolin, and lie a few kilometers northwards of the east–west-trending part of MMT exposed near Mingora
2. Methods and material studied
Four size fractions were concentrated. The <63-μm fraction was extracted from raw material by wet-sieve screening, the <5- and <10-μm fractions were prepared by hydrocyclone technique, while the <2-μm fraction was obtained by the procedure of settling in a bucket following Stoke’s law. Mineralogical and chemical compositions and kaolin-specific physical properties of each product were determined. The physical properties evaluated are: particle size analysis, colour measurement of washed and
Results.
The industrial properties determined from samples of the Swat kaolin are described below, and evaluated by comparisons with the limits fixed by the industrial users, and also with the respective properties of better-known commercial kaolin deposits.
3. Discussion
Kaolin finds diversified industrial uses, which are determined mainly by its physical properties (Bundy, 1993), which in turn, may depend on its chemical and mineral composition. Individual kaolin deposits possess characteristics specific for each (Murray and Keller, 1993, Murray, 2003). The study of Swat kaolin by Kyotani et al. (1967) found that its 2nd grade (<44 μm) kaolin still contains fairly large quantities of remnant CaO and falls somewhat behind the English kaolin in respect to
4. Industrial use evaluation
Proper industrial utilization of kaolin is constrained by its attributes, which should conform to the standard specifications by the kaolin-consuming industries.
5. Conclusions
This evaluation of the Swat kaolin deposits is based on the kaolin-specific physical properties including the particle size, colour measurement, viscosity, plastic limit, liquid limit, drying, firing shrinkage, water absorption, oil absorption, volatile matter and water soluble matter. The deposits are found to be suitable for certain industrial applications evaluated in this study for different size fractions. The <2-μm fraction is suitable for paper coating; fraction <5 μm is suitable for…
8. Kaolin: processing, properties and applications
Kaolins are white raw materials, their essential constituent being fine grained white clay, which are amenable for beneficiation that make them ideal for an assortment of industrial applications. Kaolin deposits can be classified into two types, primary (residual) and secondary (sedimentary).
The main commercially important kaolin resources at the present time are the primary deposits of Cornwall in England and the sedimentary deposits in Georgia and South Carolina in the USA
This review outlines the geology and occurrence of kaolins in the world and the state of the art in processing high-quality kaolins.
The physical and chemical characteristics of kaolin are also reviewed with respect to their use for specific applications in paper, ceramics and refractories, plastics, rubber, adhesives and paint industries.
The market outlook for world kaolin is outlined with respect to product specifications, markets and prices and the competition from other fillers such as calcium carbonate.
9. Uses
1. Industrial Applications
Kaolin is used in a multiplicity of industries because of unique physical and chemical properties. Shape, particle size, color, softness, and non-abrasiveness are physical properties that are especially important. Chemical properties, such as comparatively low base exchange capacity, as well as other chemical properties of the kaolin surface, and relative insolubility, are governing in many uses.
2. Paper Industry
The primary use of kaolin is in the paper industry. It serves as a paper coating which improves appearance by contributing to brightness, smoothness and gloss. It also improves printability. Additionally, it is used by the paper industry as a filler reducing cost and the use of tree-based resources. The largest single user of kaolin is the paper industry because when kaolin is used, paper products print better and are made whiter and smoother.
Kaolin used as a filler in the interstices of the sheet adds ink receptivity and opacity to the paper sheet. Kaolin used to coat the surface of the paper sheet makes possible sharp photographic illustrations and bright printed colors. Kaolin constitutes nearly onethird the weight of today’s slick sheet magazines.
3. Ceramic Industry
Kaolin is used in ceramic whiteware products, insulators, and refractories. In whitewares, kaolin aids accurate control of molding properties, and adds dry and fired strength, dimensional stability, and a smooth surface finish to the ware. The excellent dielectric properties and chemical inertness of kaolin make it well suited for porcelain electrical insulators. In refractory applications, the dimensional stability, high fusion point, and low water content, along with high green strength, make kaolin an important constituent.
- Paint Industries
Kaolin is used in paint industry because it is chemically inert and insoluble in the paint system, has a high covering power, gives the paint desirable flow properties and is low in cost.
5. Rubber Industries
Kaolin is used as a filler in many rubber goods. It adds strength, abrasion resistance, and rigidity to both natural and synthetic rubber products. In general, most rubber products extrude more easily after kaolin filler is added. The major reason that kaolin is used in rubber compounds is its whiteness and low cost. Although kaolin costs less than most other rubber pigments, it has excellent functional properties.
- Other Uses
Kaolin has a variety of other uses in products including cable insulation, specialty films and fertilizers, glass fiber, white cement and refractory insulation bricks.. New uses are being discovered frequently, and ensure that the mineral will remain in demand for a long time.
Question
1. What is the purpose of kaolin?
Kaolin is used in ceramics, medicine, coated paper, as a food additive, in toothpaste, as a light diffusing material in white incandescent light bulbs, and in cosmetics. Until the early 1990s it was the active substance of anti-diarrhoea medicine Kaopectate.
2. What is a kaolin blood test?
The Kaolin Clotting Time (KCT) has often been regarded as the most sensitive test for the detection of circulating anticoagulants. The KCT detects all class of inhibitors, including those directed against Factor VIII but is also sensitive to the presence of Unfractionated Heparin [UFH].27-Sept-2022
3. What is kaolin solution used for?
Kaolin is used for mild-to-moderate diarrhea, severe diarrhea (dysentery), and cholera. In combination products, kaolin is used to treat diarrhea and to relieve soreness and swelling inside the mouth caused by radiation treatments.
4. Is kaolin good for skin?
Kaolin clay is a natural & detoxifying cleanser that is safe for all skin types. Kaolin clay is a naturally occurring mineral – Layered Silicate (Silica). It can deeply clean your skin without being too harsh, while Silica promotes collagen production, promoting the skin’s elasticity and making it more firm.04-Aug-2023
5. What is the pH of kaolin?
around 4.5
Kaolin clay is known for being very finely powdered, soft and usually off-white or pink in color. The clay is composed of tiny minerals and crystals (including feldspar, quartz, silica, copper, magnesium and zinc). It has a relatively neutral pH level of around 4.5, which is close to the skin’s natural pH of 5.5.28-Mar-2022
6. What is kaolin BP?
BP Light Kaolin is a natural, purified, light, white to white-grayish powdered hydrated aluminium silicate. It’s odourless and unctuous to the touch and contains a dispersing chemical. Tested in accordance with specifications of the British Pharmacopoeia monograph.
7. Is kaolin good for hair?
Other than detoxing your hair of chemicals and pollutants, kaolin clay is an excellent choice for dry, brittle hair. Whether your hair is in this condition because of your tap water, chemical processing or continued use of chemical hair products, you can use this to help restore the moisture.