Overview
Calcined kaolin is produced by heating natural kaolin clay to temperatures between 650°C and 1,050°C, altering its crystalline structure and enhancing its functional properties. This process removes chemically bound water, increasing porosity and reactivity. The resulting material is whiter, harder, and more chemically inert than raw kaolin, making it ideal for industrial applications requiring durability and performance. Unlike hydrous kaolin, calcined kaolin exhibits superior opacity and scatter efficiency, which is critical in coatings and paints. Its thermal stability also makes it a preferred filler in plastics and rubber, where it improves mechanical strength and resistance to deformation.
Physical and Chemical Properties
Calcined kaolin’s key physical properties include a Mohs hardness of ~3.5–4.5 and a refractive index of 1.56, contributing to its light-scattering ability. Its particle size typically ranges from 0.2 to 10 microns, with surface areas of 8–25 m²/g after calcination. The material is chemically inert, with no solubility in acids or alkalis under normal conditions. Thermal treatment eliminates hydroxyl groups, converting kaolinite into metakaolin (at ~450–700°C) or mullite (above 900°C). This phase change reduces plasticity but enhances abrasion resistance and electrical insulation properties, broadening its utility in high-temperature applications like refractory ceramics.
Main Applications
In paints and coatings, calcined kaolin acts as an extender pigment, improving opacity and reducing TiO₂ usage. Its platelet structure enhances barrier properties in packaging films. The ceramics industry relies on it for vitrified tiles and sanitaryware, where its low shrinkage and high strength are advantageous. Rubber and plastics manufacturers use it as a reinforcing filler to improve tensile strength and reduce costs. In paper, it increases smoothness and printability. Emerging applications include catalysts and cementitious materials, where its pozzolanic activity (in metakaolin form) enhances concrete durability.
Safety and Storage
While calcined kaolin is non-hazardous, prolonged inhalation of dust may cause respiratory irritation. OSHA recommends a permissible exposure limit (PEL) of 15 mg/m³ for total dust. Use NIOSH-approved N95 masks and ensure adequate ventilation in processing areas. Store in moisture-resistant packaging to prevent clumping. Avoid contamination with organic materials, which may affect performance in sensitive applications like electronics. Bulk storage silos should incorporate vibration or aeration systems to prevent compaction.
B2B Procurement Guide
Buyers should prioritize suppliers offering consistent particle size distribution (PSD) and brightness (measured by ISO 2470). For paint applications, seek grades with ≤2% residue on 325-mesh sieves. Request technical datasheets detailing loss on ignition (LOI, ideally <1%) and oil absorption values. Regional sourcing considerations include transportation costs—China dominates production, but local calcination plants may reduce logistics expenses. Audit suppliers for ISO 9001 certification and batch-to-batch consistency. Sample testing under actual use conditions (e.g., in polymer compounds) is recommended before large-scale procurement.
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