Kaolins: their properties and uses

Abstract

The main commercially important kaolin resources are the primary deposits in Cornwall and the sedimentary deposits in South Carolina and Georgia in the U.S.A. Annual world production is about 18 Mt (1 t = 10 3 kg). In 1981 the U.K. production was 2.6 x 10 6 t of which about 77 % represented paper filler and paper coating grades. About 15% of the U.K. production is used in ceramics. The kaolin extracted from the commercial deposits contains kaolinite as the major component together with ancilliary minerals. The kaolinite particles take the form of pseudo-hexagonal platelets with widths that vary from 10 to 0.1 pm and less. Surface chemical properties have been widely researched. There is no uniformity of view on the origin of the cation exchange capacity. Aqueous suspensions of kaolin are flocculated at low pH. This can be understood in terms of the positively charged edges and the negatively charged faces of the platelets. The deflocculation of kaolin under alkaline conditions is important, and is assisted by adding a polyanion. The Kubelka-Munk equations have provided a useful basis for interpreting the optical properties of kaolins in terms of an absorption coefficient ( k ) and a scattering coefficient ( s ). The parameter k ; is a bulk property and is interpreted in terms of light absorption by ancilliary minerals, by coatings of hydrated iron oxides on the kaolinite particles and by transition metals in the kaolinite structure. The parameter s depends on the size distribution of the platelets and the manner in which they are packed together. The rheological properties of deflocculated suspensions of kaolin in water are important in modern paper coating applications. The relation between the bulk physical properties of kaolin and rheology is complex. Viscosity at low rates of shear increases with increasing specific surface area of the kaolin. Viscosity at high rates of shear depends on the shapes of the particles and their packing. The extraction and production of kaolins are described for both primary and sedimentary deposits. The main use of kaolin is as a paper filler and a paper coating pigment. The various filler and coating grades are indicated, and effect of kaolin properties on coated sheet properties is discussed. Kaolin is a major component in most ceramic bodies. Examples of formulations are given. The manufacture of sanitaryware and tableware is discussed in terms of kaolin properties. Kaolin is used as an extender in aqueous based paints and as a filler in rubber and synthetic polymers.

Surface Charge Properties Of Kaolinite

“…Prominent among these characteristics are the density of surface charge created by isomorphic substitutions in a mineral structure (denoted tr0) and that created by proton adsorption and desorption reactions with a contiguous aqueous solution (denoted ~). The sign of ~r 0 is always negative for kaolinite, arising either directly from AI(III) substitution for Si(IV) in the tetrahedral sheet of the mineral (Bolland et al 1976), or indirectly from isomorphic substitutions in 2:1 layer type clay mineral inclusions (Lim et al 1980;Jepson 1984;Kim et al 1996). The sign of tr H varies with aqueous solution pH, taking on a zero value at the p.z.n.p.c.…”

The Effect of Surface Modification by an Organosilane on the Electrochemical Properties of Kaolinite

“…Kaolinite is a mineral that has a wide variety of applications in industry, particularly as a paper filler and a coating pigment (Jepson, 1984). It is used as an extender in water based paints and ink, as a functional additive in polymers and is a major component of ceramics (Jepson, 1984).…”

Determination of Shape of Kaolin Pigment Particles

“…In his review paper of kaolins, Jepson (1984) reported the range of aspect ratios for Cornish kaolins from 10 : I for coarse particles to 50 : 1 for the fine ones, also determined by transmission electron microscopy. Although measuring the aspect ratio by microscopy is direct and fundamentally simple, it has two serious drawbacks: the method is tedious and becomes subjective when individual kaolin particles cannot be clearly distinguished from clusters and spacial overlaps.…”