InfoPoint No. 9
The so-called ‘Faberschlemm’ was the site of the first kaolin-processing plant in the district, established in 1835. Unprocessed soil and water were placed in two-by-two-metre wooden trays, and then stirred with wooden paddles until the kaolin slurry floated to the surface to form a thin layer there. The slurry was then drained off through an approx. 30-metre-long channel and thickened. It was then dried at once, either by the sun or in a kiln. Although nowadays the unprocessed soil continues to be separated in large vats of water, the finer particles of kaolin and feldspar now flow into a piece of equipment called a hydrocyclone.
Model of a classifier: the process of separating the components in the unprocessed soil can be observed here.
Hydrocyclones are now part of the standard equipment used in processing raw materials. They are centrifuges with vertical walls in which a suspension is rotated. The diagram shows the movement of the liquid inside this piece of equipment. The suspension, which is introduced tangentially by means of a pressure pump, describes a downwards spiral movement along the inner wall. Some of the solid matter is already dragged downwards into the underflow in this primary spiral. The Unterlaufdüse (pump injector) has a throttling function, thus forcing most of the liquid to change direction and flow upwards in an inner spiral (secondary spiral). While the larger particles are again flung outwards, only the finest solid particles leave the system via the Überlauf (overflow). By setting up several hydrocyclones to form cyclone series, the various components can be separated in the most efficient way possible, thus increasing yields. The kaolin subsequently flows into sedimentation ponds, where it is thickened and dried.
General flow sheet for the refining of kaolin: