Kaolin Occurrence and Extraction
Clay deposits which are rich in kaolinite are widely distributed as hydrothermal, residual, and sedimentary deposits around the world, with the most important resources being in Cornwall in SW England and in South Carolina and Georgia in the USA. Large deposits are also found, and are being exploited, in Russia, Ukraine, Brazil, Spain, Australia, Bavaria, and Bohemia.
Kaolins are formed by the hydrothermal alteration and weathering of feldspathic igneous and metamorphic rocks (especially granite) under relatively low temperatures and pressures. The most common parent minerals are feldspars and muscovite micas. Kaolins can be found as primary deposits (i.e., in the same place as where the alteration took place) or as secondary deposits, such as sedimentary layers which have been transported by water to another location. Because of their relatively complex geological origins, no two deposits are exactly the same, and the type of deposit determines the impurities present and also influences the production process. Extraction and processing of clays for polymer applications is a complex subject much influenced by the nature and location of the deposit and is only covered in outline here. More details can be found in Hancock (2003) and Duca (2010).
Primary deposits will usually be mixed with unaltered granite, mica, feldspar, and quartz. Secondary deposits are usually found in “layered qualities” with very variable purity. The main primary deposits in SW England and Brazil are exploited by hydraulic mining, in which the clay is washed out of the granite matrix using high-pressure jets of water or by dry mining, where the kaolinitic rock is extracted at source and stockpiled into grades of different physical characteristics. The dry-mined rock is crushed and the quartz-containing fraction separated before the remainder is diluted into a slurry with water for further beneficiation. Refining into different particle size fractions is carried out by sedimentation of this aqueous slurry, using the principle of Stokes law to select the required particle size. To achieve good separation, the particles must be deflocculated (separated from each other); this is usually achieved at a neutral pH and by treatment with a polyanion, which can often carry over into the final product. Mineralogical separation is also achieved in the refining step, with ancillary minerals (mostly feldspar, quartz, and mica) remaining in the coarse fractions. During aqueous processing, products may also be reductively or oxidatively bleached to reduce or remove colored inorganic impurities (usually hydrated iron oxides) and organic matter (humus type materials) which are often present as coatings on the particle surface. The clays are then filtered, dried, and, for the polymer industries, pulverized to break down agglomerates, which form during drying.
The sedimentary deposits are mined by a variety of techniques depending on the nature and extent of the impurities. The simplest and cheapest production route involves dry mining, crushing, and milling. More sophistication is used for air-floated products, where the clay after crushing and grinding passes into an air stream of constant velocity and grit and coarse particles remain behind.
More controlled, purer products are produced by wet refining with the dry-mined clay being dispersed in water, degritted and refined using hydrocyclones or centrifuges.
Aspect ratio of the particles in any given product is determined by the degree to which the individual plates are separated from the stacks. For some deposits and products, special grinding is carried out to maximize this. The aim is to separate the stacks as much as possible without too much fracturing of the plates, as this would reduce the aspect ratio.
A stack of kaolinite crystals is shown in Fig. 1 and typical platy particles obtained after processing are shown in Fig. 2.
Fig. 1 A stack of kaolinite crystals (reproduced with kind permission from Imerys)
Fig. 2 Typical kaolinite plates (reproduced with kind permission from Imerys)