Calcined Kaolin

Calcined Kaolin is produced by the controlled thermal treatment of high-purity kaolin clay (hydrated aluminium silicate, Al₂Si₂O₅(OH)₄) at temperatures typically between 650°C and 1050°C. The calcination process drives off the chemically bound hydroxyl groups from the kaolin crystal structure, producing an anhydrous aluminium silicate with significantly different physical and optical properties compared with the raw clay feedstock.

At calcination temperatures around 600 to 700°C, kaolin converts to metakaolin, a highly reactive amorphous phase. At higher temperatures of 900 to 1050°C, further phase transformation produces a more stable, partially crystalline product with enhanced brightness, hardness, and electrical insulation properties. The specific calcination temperature profile is carefully controlled to achieve the desired balance of brightness, oil absorption, and reactivity for the intended end use.

Calcination produces a significant increase in brightness compared with raw kaolin. The removal of hydroxyl groups and disruption of the layered crystal structure scatter light more effectively, giving ISO brightness values of 90 to 94%, considerably higher than the 80 to 88% typical of washed and refined raw kaolin. This brightness improvement, combined with the material's platelet morphology and moderate refractive index, makes calcined kaolin an effective extender for titanium dioxide in paint and coatings formulations.

Calcined kaolin also exhibits improved electrical insulation properties compared with hydrated kaolin, as the removal of bound water reduces ionic conductivity. This makes it the preferred filler in wire and cable insulation compounds where high dielectric strength and low dissipation factor are critical.

Calcined kaolin is used as a functional extender and performance filler across the coatings, plastics, rubber, and cable industries. In architectural and industrial paints, it extends titanium dioxide at loadings of 10 to 30% by weight of TiO₂, exploiting its platelet morphology and moderate refractive index to maintain opacity and hiding power whilst reducing pigment costs. The platelet shape creates a layered structure within the dried paint film that improves scrub resistance and reduces gloss, making it particularly useful in matt and eggshell interior finishes. In wire and cable insulation compounds, calcined kaolin is used in polyethylene and PVC compounds where its low moisture content, high electrical resistivity, and platelet morphology contribute to improved dielectric strength and reduced water absorption in the cured insulation. In rubber compounds, it improves dimensional stability, reduces compression set, and contributes to improved surface finish in moulded goods. In ceramic applications, it provides a controlled source of alumina and silica in porcelain bodies and technical ceramics.