Geological controls on kaolin particle shape and consequences for mineral processing

Abstract

The kaolinized granites of St. Austell, England, are worked to produce a range of china clay products, for some of which the kaolin has to meet stringent particle shape and size specifications. Systematic petrographic study indicates that kaolin occurs in the form of two textural types: (i) finely crystalline kaolin (typically <5 µm in average diameter), which infills dissolution porosity of granitic feldspars, and (ii) coarsely crystalline vermiform aggregates (up to 100 µm or more in length), which are closely associated with expanded micas. The vermiform aggregates are characterized by an intergrowth of mica and kaolin crystals, which can be observed at scales of resolution offered by TEM. Textural and chemical evidence suggest that the expanded mica texture is probably the result of preferential precipitation of kaolin along mica cleavage planes and is not simply a process of chemical replacement.Petrographic examination of kaolin slurries sampled at different points in a typical refinery circuit indicates that platy products with high aspect ratio are derived exclusively from raw materials rich in vermiform aggregates. The fine scale intergrowth of kaolin and mica within the aggregates results in circumstances where mica persists through to fine grained products. Furthermore, the absence of Fe or other chemical components in the kaolin structure suggests that any iron reported for the final products may be a consequence of the presence of Fe-bearing mica within a very fine grained intergrowth.

Assessment of Industrial Clays

“…Kaolin group minerals may form euhedral to subhedral pseudo-hexagonal crystals in kaolinite (Pruett and Murray, 1993;Beaufort et al, 1998;Psyrillos et al, 1999), to blocky pseudo-hexagonal or trapezoidal crystals in dickite (Beaufort et al, 1998), and tubular, spherical, or platy in halloysite (Giese, 1988;Pruett and Murray, 1993). Kaolin group minerals may form euhedral to subhedral pseudo-hexagonal crystals in kaolinite (Pruett and Murray, 1993;Beaufort et al, 1998;Psyrillos et al, 1999), to blocky pseudo-hexagonal or trapezoidal crystals in dickite (Beaufort et al, 1998), and tubular, spherical, or platy in halloysite (Giese, 1988;Pruett and Murray, 1993).…”

Mineralogical characteristics and micromorphological observations of brittle/soft Fe/Mn concretions from Okinawan soils

“…Hexagonal crystals of soil kaolinite have been reported by numerous researchers (Dixon, 1999;Zbiku and Smart, 1998;Psyrillos et al, (1999). Kaolinite pseudomorphs derived from primary phyllosilicates might have implications for the behavior of elements and the process of formation of kaolinite in weathering profiles (Jeong, 1998).…”

14. Brick and other ceramic products

“…The coarse particle size is related to stacks of individual kaolinite platelets. Over-blunging of the china clay slurry will lead to de-lamination of these stacks, a closer packing of the platelets and reduce the casting rate (Psyrillos et al 1999). The proportion of china clay in a whiteware ceramic body can range from 5 to 55%.…”