Chapter 5 Kaolin Applications

Publisher Summary

This chapter discusses the kaolin applications. Kaolin is one of the more important industrial clay minerals. Kaolin is comprised predominantly of the mineral kaolinite, a hydrated aluminum silicate. Kaolinite, which is the dominant mineral in kaolin deposits, is a common clay mineral, but the relatively pure and commercially useable deposits are few in number. Kaolinite has physical and chemical properties which make it useful in a great number of applications. One of the most important applications of kaolin is coating and filling paper. As filler, the kaolin is mixed with the cellulose fibers in wood pulp and as a coating; the kaolin is mixed with water, adhesives, and various additives and coated onto the surface of the paper. The coating makes the paper sheet smoother, brighter, glossier, more opaque, and most importantly, improves the printability. Paper that is not coated is made up of cellulose fibers interwoven in a random and open configuration.

Section snippets

Paper

One of the most important applications of kaolin is coating and filling paper. As a filler, the kaolin is mixed with the cellulose fibers in wood pulp and as a coating, the kaolin is mixed with water, adhesives, and various additives and coated onto the surface of the paper. The coating makes the paper sheet smoother, brighter, glossier, more opaque, and most importantly, improves the printability (Bundy, 1993). Paper that is not coated is made up of cellulose fibers interwoven in a random and

Paint

Paint is a significant market for kaolin, although it is considerably less than the market for paper coating and filling. About 600,000 tons annually are used worldwide as extender pigments in paint. The largest use is as a pigment extender in water-based interior latex paints. It is also used in oil-based exterior industrial primers. Calcined and delaminated kaolins are used extensively in interior water-based paints. These paints have moderate to high pigment volume concentrations ranging

Ceramics

Ceramics includes a wide range of products in which kaolins are utilized. These include dinnerware, sanitaryware, tile, electrical porcelain, pottery, and refractories. Kaolins and ball clays, which are kaolinitic clays, are both used as major ingredients in many ceramic products. The term ceramic refers to the manufacture of products from earthen materials by the application of high temperatures (Grim, 1962). Ceramics historically goes back to prehistoric times when early man used earthenware

Rubber

Kaolin is used in rubber because of its reinforcing and stiffening properties and it is relatively low cost in comparison with other pigments. In rubber goods which are black, the favored pigment is carbon black, but in non-black rubber goods, kaolin is used (Anonymous, 1955). As mentioned previously, there are hard clays which are fine in particle size and soft clays which are relatively coarse in particle size. Hard clays are used in non-black rubber goods where wear resistance is important.

Plastics

Kaolin is used as a filler in plastics because it aids in producing a smooth surface finish, reduces cracking and shrinkage during curing, obscures the fiber pattern when fiberglass is used as reinforcement, improves thermal stability, contributes to a high impact strength, improves resistance to chemical action and weathering, and helps control the flow properties. Filler loading in various plastic compositions varies from about 15% to as high as 60%.

The most important use of kaolin is in

Ink

The major inorganic pigment used in ink is kaolin. This is a small but important use of kaolin. Ink formulations are similar to those of paper coating and paint with vehicles (binders) and pigments as the basic components. The most important uses of kaolin in inks are to improve ink holdout and to extend both colored and white pigments (Stoy, 1989). To preserve the gloss of ink films, the kaolin extender must not be so coarse in particle size that the particle protrudes above the film surface.

Catalysts

The most important mineral used in the manufacture of carriers for catalysts is kaolin. The largest use of kaolin is in catalyst substrates in the catalytic cracking of petroleum. Because many catalysts are used at high temperatures and pressures, the refractory character of kaolin is appropriate for these applications. The purity of the kaolin is critical in this petroleum cracking operation so a processed kaolin with low iron, titanium and alkali, and alkaline earth compounds is preferred.

Fiberglass

Kaolin is a major component used in the production of fiberglass. Fiberglass has a large number of applications, including insulation, reinforcement of plastics, textile yarn, electronic circuit board substrates, paper, cloth, and roofing shingles. The basic component materials used to make fiberglass are silica, kaolin, and limestone, along with small amounts of boric acid, soda ash, and sodium sulfate. The kaolin must meet rather stringent chemical specifications (Watkins, 1986): Al2O3

Portland cement

Cement is made by mixing materials containing lime, silica, alumina, and iron oxide. This mixture is sintered and then pulverized at which time a retardant, gypsum is added. Kaolin is an ideal source of alumina and silica and also makes the cement whiter. Relatively recently, a metakaolin (partially calcined) product is used as a pozzolanic additive in certain cements where high strength is needed. The reactive amorphous alumina and silica in the metakaolin reacts with excess calcium to produce

Miscellaneous uses

There are a multitude of uses of kaolin which are briefly described. The kaolin surface is acidic, but as shown by Solomon and Murray (1972), the acidic surface of kaolinite is largely neutralized in the presence of water, but dry surfaces show a large increase in acidity. At 1% surface moisture, the acidity is equivalent to 48% sulfuric acid and at approximately 0% moisture surface acidity is equivalent to 90% sulfuric acid. This surface acidity and activity must be considered in many of its