Removal of hazardous trace elements from recovery boiler fly ash with an ash dissolution method

Abstract

The presence of hazardous trace elements (HTE) in the chemical recovery cycle of Kraft pulp mills is the main obstacle for the utilization of the inorganic residues of the process. Electrostatically precipitated (ESP) recovery boiler fly ash (RBFA), consisting mainly of sodium sulfate Na2SO4, is a solid side stream where HTE are concentrated. Unlike most other ashes, RBFA is to a great extent water-soluble. A novel reverse leaching method, based on the dissolution behavior of RBFA in water, is introduced in this paper. The method is founded on the use of an appropriate liquid/solid mass ratio and a favorable pH, which together contribute to the formation of a small and readily settling solid residue where almost all HTE are concentrated. This paper focuses on evaluating the influence of the treatment conditions on HTE removal, energy consumption and material losses. The results of this study show that the removal efficiency of the investigated HTE and other analyzed metals was excellent under alkaline conditions, the apparently suitable pH range for the removal of most of these metals being approximately 11.7–12.2. Lead was observed to be the most difficult HTE to remove: the highest obtained degree of removal of Pb was 89%. The removal rate of Cd and Zn was approx. 100% within the mentioned pH range.