Facts & Figures Book - KPI-JCI

While a particle is in suspension, one force acts on it tomake it fall, while others act to limit the fall. The force thatacts downward is that of gravity (g). It has been broughtout that viscosity of the liquid may slow the fall. The differencebetween free settling and hindered settling is arelative one between the factors causing a particle to falland those restricting the fall. In free settling, the downwardcomponent is much greater than those slowing upthe fall are. In hindered settling, the downward componentis only slightly greater than those slowing the fallare.Apart from the multiple sizing, the scalping tank servesto eliminate the surplus water prior to discharge of productto a screw-type classifier. By so doing, the amountof water handled by the screw classifier can be regulatedbetter for the mesh size of fines to be retained. Itbecomes apparent, then, that a water scalping tank willbe followed by as many screw classifiers as there aresizes of sand products to be made.Adjustable weirs on the scalping tank regulate the rateand velocity of overflow to provide the size separationsrequired. Clays, silt and slime, which are lighter than thefinest mesh sand, remain suspended in the water andare washed out over the tank weirs for discharge into asettling pond.In order to re-blend sand fractions into a specificationproduct, settling stations are located along the bottomlength of the tank. The best classifying occurs with morelength to the classifying tank. It is recommended to use aminimum of a 28’ tank. Shorter tanks will work when thematerial is very consistent in gradation and close to theproduct specification to be made.Build up or “silting in” of the classifying tank will occur asthe specific gravity of the overflow slurry goes beyond1.065. The ideal slurry is between 1.025 and 1.030. Atthis point, maximum efficiency occurs. Additional waterwill carry away more fines unless the tank area is oversized.Washin g/Classifyin g117