Kaolin

What is Kaolin?

Kaolinite is a mineral belonging to the group of aluminosilicates. It is commonly referred to as “China Clay” because it was first discovered in Kao-Lin, China. The term kaolin is used to describe a group of relatively common clay minerals dominated by kaolinite and derived primarily from the alteration of alkali feldspar and micas.

Kaolin is a white, soft, plastic clay mainly composed of fine-grained plate-like particles. It is formed when the anhydrous aluminium silicates found in feldspar-rich rocks, like granite, are altered by weathering or via hydrothermal processes. The process that converted the hard granite into the soft matrix found in kaolin pits is known as kaolinization. The quartz and mica of the granite remain relatively unchanged while the feldspar is transformed into kaolinite.

The refining and processing of the fine fraction of the kaolinised granite yields predominantly kaolinite with minor amounts of mica, feldspar, traces of quartz and, depending on the origin, organic substances and/or heavy minerals.

Individual kaolins vary in many physical aspects, which in turn influence their end use. Of commercial interest is the degree of crystallinity, which influences the brightness, whiteness, opacity, gloss, film strength, and viscosity.

Kaolin’s whiteness and plasticity make it extremely suitable for extensive use as a filler, extender, ceramic raw material, and pigment. It is also a raw material important to refractories and to catalyst, cement, and fiberglass industries.

Kaolin is a unique essential mineral that remains chemically inert over a relatively wide pH range, and it offers excellent coverage when used as a pigment or extender in coated films and filling applications. In addition, it is soft and non-abrasive and has a low conductivity of heat and electricity.

The two largest applications of kaolin are the coating of paper to hide the pulp strands and the production of high-grade ceramic products.

Uses for Kaolin

Paper Kaolin is used both as a filler in the bulk of paper and to coat its surface. Kaolin’s whiteness, opacity, large surface area, and low abrasiveness make it an ideal raw material for paper production. Its use allows a reduction in the amount of expensive woodpulp required, enhances the optical properties of the paper, and improves its printing characteristics. When used as a coating on the surface of the paper, kaolin’s whiteness improves paper brightness and opacity, while the size and the shape of the individual kaolin particles give the gloss and printed paper quality required for many kinds of paper, such as paper for magazines and brochures, art paper, cartons, and boxes.

Ceramics Kaolin converts to mullite and glass when fired to temperatures exceeding 1,800° F. It is used to make whitewares, which consists of tableware, sanitary ware, and wall and floor tiles.

Filters When kaolin is used as a pigment, it is divided broadly into filler and paper-coating grade clays based on their brightness and viscosity. Its main properties, especially its whiteness or near whiteness, make it very suitable as a filler or pigment. In addition, it remains inert over a wide pH range, is nonabrasive, has low heat and electrical conductivity, and offers brightness and opacity.

Paint In its hydrous or calcined forms, kaolin can improve the optical, mechanical, and rheological properties of a paint. Calcined kaolins are widely used in satin and matte paints where they can deliver increased opacity, whiteness, and scrub resistance. Kaolin is particularly useful as a partial replacement for titanium dioxide (TiO₂) pigment.

Rubber Kaolin adds strength, abrasion resistance, and rigidity to rubber. Calcined kaolin, with or without a silane chemical surface treatment, finds extensive use in high value thermoplastic elastomers for a variety of applications and in rubber insulation on high voltage power lines.

Plastics Kaolin is used in plastics to provide smooth surfaces, dimensional stability, and resistance to chemical attack; to conceal fiber reinforcement patterns; and to reduce shrinkage and cracking during polymer compounding and shape forming. It is also used as a rheological modifier and a functional filler to improve mechanical, electrical, and thermal properties. A major application is in PVC cables where its main function is to improve electrical properties.

Refractories Refractories are produced from natural materials, combinations of compounds and minerals, such as kaolin, which are used to build structures subjected to high temperatures, ranging from simple to sophisticated products, e.g., from fireplace brick linings to re-entry heatshields for spacecraft. In industry, they are used to line boilers and furnaces of all types: reactors, ladles, stills, kilns, etc.

Fiberglass Fiberglass that is used as a strengthener in a multitude of applications requires the use of kaolin, which allows for the strengthening of fibers integrated into the material. It also improves the integration of fibers in products requiring strengthened plastics such as cars, boats, and marine products, sporting goods and recreational products, aviation and aerospace products, circuit boards, fiberglass insulation, air filters, tanks and pipes, and building and construction products.

Cosmetics & Pharmaceuticals Kaolin is used in both human and veterinary medicinal products, for example, to treat digestion problems and as a constituent of poultices. It helps protect the epithelial lining in human digestive tracts, giving the body space from the food and internal waste it needs to heal. It can also be used as an excipient in personal care products, including in cosmetics. It is also found in several dietary products, plasters, foot-powders, and in the specialized treatment of some lung disorders.

Agriculture Kaolin-based fertilizers are EPA approved to protect agricultural crops, act as an insecticide, and reduce environmental stresses caused by the sun and heat.

Learn All About Kaolin

What is Kaolin?

Fun Facts

Where to Find Kaolin

Uses for Kaolin

Related Resources

Keep Learning