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Kaolin Clay

  • Kaolin (china clay) is a hydrated alumina silicate (Al2O3.2SiO2.2H2O) crystalline mineral (kaolinite), formed by the decomposition of minerals such as Granite & Feldspar. The name kaolin derives from the Chinese and means high ridge.
  • Kaolin, also known as china clay is a soft, white clay mineral primarily composed of kaolinite. It is used extensively in various industries due to its desirable properties, such as whiteness, absorbency, and fine particle size.
  • Kaolin, or china clay, has specific properties and specifications that vary depending on its intended use.

Formation of Kaolin:

1. Primary Kaolin (Residual Kaolin):

  • Forms directly from the weathering of parent rocks, such as granite and remains at the site of its formation.
  • Typically purer and contains fewer impurities.
  • High-quality ceramics, porcelain, and specialized applications.

2. Secondary Kaolin (Sedimentary Kaolin):

  • Transported from the original weathering site by water or wind and deposited elsewhere.
  • May contain more impurities and be less pure than primary Kaolin.
  • Common in paper coating, ceramics, and fillers.

Application of Kaolin:

  1. Ceramics body, Refractory & Insulation 
  2. Printing Ink 
  3. White Cement, Gray Cement etc. 
  4. Detergent & Soap 
  5. Rubber, Plastic and cable 
  6. Sanitary ware

Physical Properties of Kaolin

  • Color: Typically white, but may have slight variations due to impurities. White to off-white, with brightness typically above 80% (ISO brightness).
  • Texture: Fine and smooth, with a silky feel.
  • Chemical Composition: Primarily composed of kaolinite (Al2Si2O5(OH)4), often with traces of other minerals such as quartz, mica, and feldspar.
  • Particle Size: Typically fine, with a high surface area. Particle sizes typically ranging from 1 to 2 microns.
  • Plasticity: Exhibits good plasticity when wet, making it moldable. High plasticity when wet, which is important for molding and shaping.
  • Moisture Content: Usually 10-15% in raw form.
  • PH: Typically around 4.5-6.5 in a water suspension.
  • Bulk Density: Approximately 2.5-2.7 g/cm3.
  • Specific Gravity: Around 2.6-2.7 g/cm3.
Chemical Properties
S.No List of Parameters Result in (%)
1 Silica Si2O3 49
2 Iron Oxide Fe2O3 0.5
3 Alumina Oxide Al2O3 37
4 Sodium Na2O 0.151
5 Magnesium Oxide MgO 0.055
6 Titanium TiO2 0.872
7 Potassium K2O 0.054
8 Calcium Oxide CaO 0.164
9 Loss on Ignition LOI 12.21

Types of Kaolin Clay:

1. Raw Kaolin:

  • Raw kaolin, or crude kaolin, is the natural form of kaolin directly extracted from the earth without any processing.
  • Typically grayish to white, with a coarse texture.
  • Requires further processing to remove impurities and enhance properties for specific uses.
  • Can be used as is in some applications or as the starting material for more refined products.

2. Hydrous Kaolin (Hydrated Kaolin):

  • Hydrous kaolin is kaolin that contains water of hydration, meaning it has not been subjected to high temperatures during processing.
  • White and powdery with a fine texture.
  • Produced by refining raw kaolin through processes like blunging (mixing with water), hydrocycloning (separating), and refining. It retains chemically bound water.
  • Commonly used in paper coatings, ceramics, and fillers due to its plasticity and smoothness.

3. Calcined Kaolin:

  • Calcined kaolin is kaolin that has been heated to high temperatures (above 600°C) in a kiln. This process removes water of hydration and changes the mineral structure.
  • Typically white and fine, with altered physical properties compared to hydrous kaolin.
  • The raw kaolin is calcined to enhance properties such as brightness, hardness, and thermal stability.
  • Suitable for high-temperature applications, ceramics, paint formulations, and as an additive in plastics due to its improved hardness and thermal resistance.

4. Met kaolin:

  • Metakaolin is a product of the calcination of kaolin at higher temperatures (between 600°C and 800°C). It is an anhydrous, amorphous form of kaolin.
  • Fine, white, and powdery with a high surface area.
  • Produced by heating kaolin to a temperature that dehydrates and partially dehydrates it, converting it into a reactive form.
  • Primarily used as a pozzolanic material in concrete and cement applications, where it reacts with lime to form compounds that contribute to the strength and durability of the concrete.

Application of Kaolin Industrial & Others

1. Paper Grade Kaolin

  • High brightness and fine particle size, used as a filler and coating agent in paper production.
  • Improves the smoothness and printability of paper products.

2. Ceramic Grade Kaolin

  • High purity and plasticity, suitable for making ceramics and porcelain.
  • Used in the production of tiles, dinnerware, and sanitary ware.

3. Refractory Grade Kaolin

  • High resistance to high temperatures, used in refractory materials.
  • Essential in the manufacture of refractory bricks and other materials that withstand high temperatures.

4. Pharmaceutical and Cosmetic Grade Kaolin

  • Pure and non-toxic, used in medical and cosmetic applications.
  • Used in medications (e.g., antacids) and cosmetics (e.g., face masks).

5. Industrial Grade Kaolin

  • Lower purity, used in a variety of industrial applications.
  • Includes use in rubber, plastics, and as a filler in various products.

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