Size dependence of the composition of cement kiln dust

In cement mills large amounts of dust are separated from the kiln off-gas by the de-dusting filters. The specific generation of cement kiln dust (CKD) is said to be in the range of 50-200 kg per tonne of cement clinker produced. In some mills this dust can be mixed into the product, while in other mills it has to be utilized elsewhere or is sent to landfill, depending on the chemical composition of the dust. In this study, the CKDs from three different cement mills were investigated with the focus on the size dependence of the composition. Prior to chemical analysis the CKDs were split into four size fractions by air classification. After digestion, the concentration of various metals (Ca, Cd, Co, Fe, K, Mg, Mn, Pb and Zn), of total carbon (TC) and anions (Cl- and SO42-) was determined. For the heavy metals Cd, Pb and Zn pronounced size dependence was found with considerably higher concentrations in the fine size fractions. Similar but less pronounced size dependence was found also for the anions and for K. The concentration of Ca, Fe and TC showed no distinct size dependence. Thus, air classification might be applied for CKD pre-processing in CKD-utilization, for example to produce heavy-metal-reduced or chloride-and-sulfate reduced fractions.