Efficient separation of hazardous trace metals and improvement of the filtration properties of green liquor dregs by a hydrocyclone

The treatment of green liquor dregs (GLD), the main inorganic solid residues of kraft pulp mills, is a major concern in the industrial scale. In this study, the effect of hydrocyclone classification on the filtration properties of GLD and the separation of hazardous trace metals, such as Cd, Ni, Pb and Zn, rare earth elements, and other trace metals was investigated. The experiments were designed to find the influence of parameters such as the overflow to the underflow outlet diameter ratio and inlet pressure on the separation efficiency of a hydrocyclone. The results showed that a lower total filtration area was required for the separation of underflow and overflow fractions than for the original GLD sludge. Also, hazardous trace metals were effectively separated into finer overflow fractions, enhancing the possibilities to utilize the purified underflows e.g. in fertilizers and soil amendment. By using the diameter ratio of 3.70 and the inlet pressure of 1 bar, 90.1 wt-% of Cd, 70.1 wt-% of Ni and 91.4 wt-% of Zn were separated into the overflow, collecting 30 wt-% of the dregs in this fraction. The concentrations of rare earth elements (REEs) in the underflow solids were lower than the ones in the original sludge. Unlike the trace metals and REEs, Ca was accumulated in coarser particles that were separated by underflow fractions. Therefore, a majority of the dregs, enriched by Ca, was separated by underflow fractions. The possibility of categorizing the underflow fractions in CE-marked fertilizing products was also studied.