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• Overview

  Kaolin is a significant industrial clay that mainly contains a hydrated aluminum silicate mineral named as kaolinite (Al2Si2O5(OH)4)^{[1, 2]}. Other kaolinminerals include dickite, nacrite, and halloysite. Pure forms of these minerals are not as ubiquitous as kaolinite, and are usually found along with kaolinite in hydrothermal deposits^[1]. Kaolin may form in residual or sedimentary modes. In the former type, kaolinite is created by in-situ weathering or hydrothermal alteration of aluminosilicate parent rocks like granite; though, in the latter, the mineral is produced by the deposition of kaolinite formed elsewhere^{[2, 3]}.
  Unique mineralogy, morphology, chemical and physical specifications of kaolin make it a versatile raw material appropriate for many different applications [1, 4], such as ceramic, paper coating and fillers, pigment extender in water-based interior latex paints and oil-based exterior industrial primer. In addition, kaolin is applied in non-black rubber, medicines and pharmaceuticals, cosmetics, crayons, fertilizers, detergents, pesticides, white cement, ink, catalysts, and many other products^{[1, 3, 4]}. These properties are greatly affected by the mode of clay formation which controls the kaolin quality through varying the kaolinite and impurity contents. For instance, kaolinite content of the residual and sedimentary kaolins differs from 20% to 60%, respectively^[3]. High quality kaolins are also low in iron-bearing minerals.
  The existence of iron oxides in kaolin adversely affects the clay color, and reduces its brightness and refractoriness ^{[4, 5]}.
  These cause a dramatic decrease in its commercial price^[6]. Even an amount of 0.4% of oxides, hydroxides and hydrated oxides of ferric iron may be enough to impart a red to yellow pigmentation to clay deposits. These iron oxide/hydroxides may be hematite (red), maghemite (reddish brown), goethite (brownish yellow), lepidocrocite (orange), ferrihydrate (brownish red), etc.^[7] Similarly, iron ores such as hematite may contain clays like kaolin as contamination which cause problems in the operation of blast furnaces. Therefore, the first beneficiation step to make these raw materials commercially valuable is to effectively eliminate iron oxides from kaolinite clays and vice versa. ;

• Structure and physical properties

  Kaolin is a plastic raw material, particularly consisting of the clay mineral kaolinite. The chemical formula is Al2O3.2SiO2.2H2O (39.5% Al2O3, 46.5% SiO2, 14.0% H2O). In systemic mineralogy, kaolinite ranks among phyllosilicates, which are stratified clay minerals formed by a net of tetrahedral and octahedral layers. Phyllosilicates are classified into the main groups according to the type of the layers, interlayer content, charge of the layers and chemical formulas. Besides kaolinite groups, serpentine, halloysite, pyrofylite, mica, and montmorillonite groups also rank among phyllosilicates. Group of kaolinites includes di-octahedral minerals (1:1) with two layers, one silica[SiO4] tetrahedral layer and one aluminium[Al2(OH)4] octahedral layer. The layers are bonded together by sharing oxygen anion between Al and Si. Together, these two layers are called platelets^{[8, 9]}.
  The 1:1 platelets of kaolinite are held together strongly via hydrogen bonding between the OH of the octahedral layer and the O of the tetrahedral layer. Due to this strong attraction, these platelets do not expand when hydrated and kaolinite only has external surface area. Also, kaolinite has very little isomorphic substitution of Al for Si in the tetrahedral layer. Accordingly, it has a low cation exchange capacity. Kaolinite easily adsorbs water and forms a plastic, paste-like substance^{[8, 9]}. ;

• Availability, mining and processing

  Kaolin is formed under acidic conditions through weathering or hydrothermal changes of feldspars, and – to a lower extent – also other aluminosilicates. It can form independent weathered kaolin deposits, kaolinite clays or may be a compound of kaolinite sandstones and oolitic ironstones, and less frequently also of pegmatites and hydrothermal deposits. The most significant kaolin deposits were formed through intensive weathering of rocks rich in feldspar (granite, arkose, certain types of ortho-gneisses, and migmatites). Millions of years ago, original material was decomposed by weathering, giving rise to kaolin and silica combined with higher or lower amounts of admixtures. Mechanical erosion formed the rock under the tropical climate of that era and at increased temperatures, chemical corrosion occurred under the activity of water saturated with CO2 and humic acids which eluted from water^{[9, 10]}.
  World renowned deposits in the Czech Republic are especially situated in the district of Karlovy Vary (Sedlec, Podlesi, and Otovice). Kaolin deposits in the area of Karlovy Vary are primary, i.e. kaolin remained in the place of its formation. Extracted raw material contains 20 to 30% kaolin; the remainder is silica sand which is an integral part of the raw material. Deeper deposits tend to be less kaolinized. Larger areas with kaolin material contents of 15 to 35% formed through weathering of arkoses are found in the vicinity of Horni Briza, Kaznejov, and Chotikov. Lower quality kaolin deposites are near Nova Role, Vidnava, Kadan, Podborany, Znojmo, and Veverska Bityska^{[9, 10]}.
  Kaolin was obtained from extracted kaolinite or kaolinite-illitic gritstone or pudding-stone from the "mine U" in southern Moravia using the following procedure (unpublished data):
  1. Superficial soil layer (about 50 cm) was removed, and the raw material was floated to a suction pump by water cannon (water source was a pond formed on the surface of the mine after kaolin extraction).
  2. It was transported in the form of dense slurry through about 150 m long pipeline with about 20 cm in diameter to the processing plant halls.
  3. After kaolin washing, classification and separation steps according to the particle size, kaolin sedimentation, addition of colloid agent, and kaolin drying in wire baskets, the product was finished and could be dispatched to customers.
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• Kaolin added to diet fed to farm animals

  Due to its adsorbent capability and lack of primary toxicity, kaolin is considered a simple and effective means to prevent or ameliorate the adverse effects exerted by many toxic agents, not only those from the environment, but also those from the living organisms. Kaolin based medication often combined with pectin is commonly used as a palliative for diarrhea and digestive problems in humans[11]. Kaolin, given to the animals in the diet, firmly and selectively binds compounds present in the diets which are noxious to the intestine and thus decreases their absorption through intestinal mucosa into the organism and subsequently prevents their toxic mode of action. A number of studies confirmed
  kaolin capability to decontaminate aflatoxins^[12], plant metabolites (alkaloids, tannins), diarrhea causing enterotoxins^[13], pathogenic microorganisms, heavy metals^[14] and poisons^[15]. In contrast, vitamin B12 adsorption by kaolinite clays is very low^[16].
  Few studies have investigated the effects of kaolin-feeding on farm animals. Savory (1984)^[17]investigated the effect of kaolin feeding on adult roosters. That author did not register any change in the live body weight gain when 100 and 200 g kaolin/kg of the diet was fed. Although in the initial phase of the experiment when animals ingested 300 g kaolin/kg of the diet their weight did not increase, and when the animals ingested 400 g kaolin/kg of the diet, the live body weight of the animals even decreased; the differences were gradually compensated, primarily by increased feed intakes. Although it was not possible to completely compensate the body weight differences by increased feed intake, compensation was reached by significant increase in digestibility of basal diet. That effect was observed in a group fed 100 g kaolin/kg of the diet within a couple of days when more than 200 and 300 g of kaolin per week and 400 g per three weeks, was consumed, respectively. Sakata (1986)^[18] registered stimulation of the live weight gain in rats given kaolin combined with the diet (100 mg/g) with concurrent proportional increase in the weight of the tissues of some digestive organs. ;

• References

  1. Murray, H.H., 2006b. Chapter 5 — kaolin applications. In: Haydn, H.M. (Ed.), Developments in Clay Science. Elsevier, pp. 85–109.
  2. Zegeye, A., Yahaya, S., Fialips, C.I., White, M.L., Gray, N.D., Manning, D.A.C., 2013. Refinement of industrial kaolin by microbial removal of iron-bearing impurities. Appl. Clay Sci. 86, 47–53.
  3. Bloodworth, A.J., Highley, D.E., Mitchell, C.J., 1993. Industrial minerals laboratory manual: kaolin, mineralogy and petrology series. British Geological Survey, Nottingham.
  4. Ryu, H.W., Cho, K.S., Chang, Y.K., Kim, S.D., Mori, T., 1995. Refinement of low-grade clay by microbial removal of sulfur and iron compounds using Thiobacillus ferrooxidans. J. Ferment. Bioeng. 80, 46–52.
  5. de Mesquita, L.M.S., Rodrigues, T., Gomes, S.S., 1996. Bleaching of Brazilian kaolins using organic acids and fermented medium. Miner. Eng. 9, 965–971.
  6. Guo, M.R., Lin, Y.M., Xu, X.P., Chen, Z.L., 2010. Bioleaching of iron from kaolin using Fe (III)-reducing bacteria with various carbon nitrogen sources. Appl. Clay Sci. 48, 379–383.
  7. Ambikadevi, V.R., Lalithambika, M., 2000. Effect of organic acids on ferric iron removal from iron-stained kaolinite. Appl. Clay Sci. 16, 133–145
  8. Klein C., Hurlbut C.S. (1993): Manual of Mineralogy. 21st ed. John Wiley and Sons, Inc., New York. 681 pp.
  9. Slivka V. (2002): Mining and treatment of silicate (in Czech). Silikatovy Svaz, Praha. 443 pp.
  10. Bernard J.H., Rost R. (1992): Encyclopaedic knowledge of minerals (in Czech). 1st ed. Academia, Prague. 704 pp.
  11. Heimann G. (1984): Pharmacotherapy of acute infant enteritis (in German). Montss??r. Kinderheilkd., 132, 303–305.
  12. Phillips T.D. (1999): Dietary clay in the chemoprevention of aflatoxin-induced disease. Toxicol. Sci., 52, 118– 126.
  13. Dominy N.J., Davoust E., Minekus M. (2004): Adaptive function of soil consumption: an in vitro study modelling the human stomach and small intestine. J. Exp. Biol., 207, 319–324.
  14. Katsumata H., Kaneco S., Inomata K., Itoh K., Funasaka K., Masuyama K., Suzuki T., Ohta K. (2003): Removal of heavy metals in rinsing wastewater from plating factory by adsorption with economical viable materials. J. Environ. Manage., 69, 187–191.
  15. Knezevich D.L., Tadic V. (1994): Decontamination with clay or alcoholate of pigs percutaneously poisoned with VX and soman (in Croatian). Vojnosanit. Pregl., 51, 488–491.
  16. Hashsham S.A., Freedman D.L. (2003): Adsorption of vitamin B12 to alumina, kaolinite, sand and sandy soil. Water. Res., 37, 3189–3193.
  17. Savory C.J. (1984): Regulation of food intake by brown Leghorn cockerels in response to dietary dilution with kaolin. Brit. Poult. Sci., 25, 253–258.
  18. Sakata T. (1986): Effects of unpalatable dietary bulk and short chain fatty acids on the tissue weight and epithelial cell proliferation rate of the digestive tract in rats. J. Nutr. Sci. Vitaminol. (Tokyo), 32, 355–362.
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