Removal of hazardous trace elements from green liquor dregs by mechanical separation methods

Abstract

Green liquor dregs represent the most important inorganic residue of chemical pulp mills. The dregs are usually settled in thickeners, washed and deliquored with lime mud precoat filters, and transported to the landfill. The utilization of dregs is challenging, due to the high concentration of hazardous trace elements (HTE) in their solid phase. There are basically two potential strategies for the reduction of the HTE content of dregs: mechanical classification according to differences in the size and density of particles, and removal of HTE by various chemical treatments. The objective of this study was to evaluate the applicability of straightforward mechanical separation methods for the purification of dregs from HTE. The evaluated separation methods included particle size-based classification by sieving, and classification on the basis of differences in the settling properties of particles in gravitational and centrifugal separation. It can be concluded that all the evaluated separation methods could be used to reduce the HTE content of dregs, although the separation efficiency was not very high in most cases. Centrifugation had clearly the best performance of the investigated techniques. The fractions consisting of large particles contained consistently lower concentrations of HTE, compared to fractions containing a lot of fines.