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During the extraction of limestone in quarries, so-called pre-screening debris are yielded. Up to 90 % of this pre-screened material consists of good limestone rock, which is frequently heavily interstratified with clay and loam impurities. Therefore, it could only be sold as a low-grade product without being cleaned correspondingly.
Usually wet washing processes are used, e.g. rock washing plants known from sand and gravel processing, to remove the pollutants adherent to the limestone. Then the washed materials have to be dried before further processing (crushing, screening, classifying, burning). The use of rock washing plants requires an expensive sewage and sludge treatment and, only for this reason, cannot be used unproblematically everywhere. The required capital spending and power for such plants, above all combined with drying, is considerable, also because the washing of the rock leads to the a higher water content exceeding the original rock moisture. It would not be reasonable to operate rock washing plants in regions with heavy frost or arid areas, from the technical and economic point of view.
The delivery program of Allgaier includes a special rotary dryer, which renders superfluous the use of rock washing plants. The patented drying and cleaning drum system TRH combines the process steps of drying and cleaning in one unit. After a quick pre-drying of the rock in the first part of the dryer, the loam and clay components adherent to the limestone are separated by means of a carefully controlled combination of retention time and mechanical stress in the second part of the dryer with an extended diameter drum. Fine components of the separated pollutants are discharged to the filter plant already with the waste air of the dryer, while the remaining pollutants, which, however, are separated from the rock, can be separated from the cleaned limestone rock by means of screening plants.