Kaolin is an important industrial mineral because it exhibits desirable properties in many applications and because of its relatively low cost. Approximately 2,000,000 tons of kaolin from Georgia and South Carolina are processed and used commercially in the United States each year. Strictly defined, the name kaolin is applied to a group of hydrous aluminum silicates of which the mineral kaolinite is the most common. Kaolinite has such desirable properties as natural whiteness, chemical stability, non-abrasiveness, and very fine particle size. Kaolinite particles are characteristically thin, flat, and hexagonal in outline. The particles range in size from submicron to as much as 40 microns (equivalent spherical diameter).
The kaolin is mined with diesel or electric shovels or draglines and loaded in trucks for transport to the processing plant or dropped directly from the dragline into a blunger. The blunger chews the kaolin into small lumps and mixes it with water and dispersing chemical to form a clay-water slip or slurry. This slurry is pumped from the blunger to settling boxes and screens to remove the coarse material which is called “grit” in the industry, but is usually designated as material coarser than about 44 microns. After the kaolin slurry is degritted it is collected in large storage tanks and pumped several miles to the processing plant.
The kaolin slurry is pumped from the mines to the plant into large terminal tanks in a dispersed state. The common dispersants used are the sodium polyphosphates and sodium silicate. The first step in the wet process is to fractionate the kaolin slurry into a coarse and fine fraction through continuous centrifuges or by settling in large hydroseparators or vats. The coarse fraction can be used as filler in paper, plastics, paint, and adhesives and for many of the same applications as the dry processed kaolin. The fine fractions are used in paper coatings, high gloss paints, inks, special ceramics, and rubber. The fine particle kaolins undergo the most rigorous and advanced treatment in the processing. After fractionation the kaolin is bleached, if necessary, to remove ferruginous coloring components. In the bleaching operation the kaolin slip is acidified with sulfuric acid to a pH of approximately 3.0 to solubilize the iron. Zinc or sodium hydrosulfite is added and the iron forms a soluble sulfate and is removed from the kaolin during the dewatering process.
