Naturally occurring kaolin ore can be classified into three industrial types based on its origin, quality, plasticity, and content of sandy minerals (quartz, feldspar, mica, etc., with a particle size greater than 50μm): hard kaolinite, soft kaolin, and sandy kaolin.
Hard kaolinite: Kaolinite rock, including large quantities of coal-bearing kaolinite occurring as the roof, floor, or interbedded rock of coal seams, or found close to coal seams. Due to the presence of organic matter and impurities, it is blackish-gray, brown, light green, or grayish-green in color. It is dense, massive, or sandy, with a porcelain-like or conchoidal fracture, a dull to waxy luster, a gray-white streak, and a hardness of 3-4. It only becomes plastic after fine grinding.
Soft kaolin: Earthy kaolin, soft in texture, generally with strong plasticity, and a sand content <50%.
Sandy kaolin: Loose in texture, generally with weak plasticity, but stronger after sand removal, and a sand content ≥50%.
