Kaolin, or china clay, is defined as a white, claylike material composed mainly of kaolinite, a hydrated aluminum silicate (Al2O3·2SiO2·2H2O), and other kaolin-group minerals. Kaolin has a wide variety of industrial applications including paper coating and filling, refractories, fiberglass and insulation, rubber, paint, ceramics, and chemicals.
Kaolin is both dry- and wet-processed. The dry process is more simple and produces a lower quality product than the wet process. Dry-processed kaolin is used mainly in the rubber industry, and to a lesser extent, for paper filling and to produce fiberglass and sanitary ware. Wet-processed kaolin is used extensively in the paper manufacturing industry.
In the dry process, the raw material is crushed to the desired size, dried in rotary dryers, pulverized and air-floated to remove most of the coarse grit. Wet processing of kaolin begins with blunging to produce a slurry, which is then fractionated into coarse and fine fractions using centrifuges, hydrocyclones, or hydroseparators. At this step in the process, various chemical methods such as bleaching, physical and magnetic methods may be used to refine the material. Chemical processing includes leaching with sulfuric acid, followed by the addition of a strong reducing agent such as hydrosulfite. Before drying, the slurry is filtered and dewatered by means of a filter press, centrifuge, rotary vacuum filter, or tube filter. The filtered dewatered slurry material may be shipped or further processed by drying in apron, rotary, or spray dryers. Following the drying step, the kaolin may be calcined for use as filler or refractory material. Multiple hearth furnaces are most often used to calcine kaolin. Flash and rotary calciners are also used.
Kaolin is used in a variety of agricultural applications including seed coatings and pesticide formulations.
Paint and Coatings Industry
In the paint and coatings industry, kaolin is widely used due to its consistency, performance and costs.
The main use of the mineral kaolinite is the production of paper - its use ensures the presence of gloss on some grades of coated paper. Kaolins also enhance specialty paper products including thermal paper, carbonless paper, label grades and inkjet paper. Calcined kaolin also supports papermakers in their thermal, packaging and graphical applications.
Rubber and Plastic Industry
In the rubber and plastics industry, the bonds between the kaolin and the polymers provide enhanced mechanical strength, improved impact and flex strength, and a high degree of hydrophobicity.
Using calcined and surface treated kaolin has helped improve electrical and physical performance in wires and cable, even at high stress with elevated moisture and temperature levels. Kaolin is also known for its capabilities to induce and accelerate blood clotting.