Treatment of Energy from Waste Plant fly-ash for blast furnace slag substitution as a Supplementary Cementitious Material

Abstract

Energy from Waste (EfW) via incineration is a state-of-the-art process for managing non-recyclable waste, with landfill disposal of produced ashes being standard practice. However, chlorides, sulfates, and heavy metals can leach into water and soil. These must be first removed, especially if the use as a Supplementary Cementitious Material (SCM) is envisaged. Consequently, a process combination utilizing aqueous and high-temperature steps is proposed for the first time. Firstly, chlorides are removed by water dissolution (60 °C, 60 minutes, liquid/solid ratio of 5 L/kg) leading a to a Cl-lean fly ash (< 1 wt % Cl-content corresponding to a Cl-concentration reduction of approx. 95 %). The liquid/solid ratio is improved by reusing the leach solution for 3 fly-ash batches, leading to an effective liquid/solid ratio of approx. 1.7 L/kg. The leached ash is directed to a first high-temperature stage to remove sulfates and partly heavy metals/ chlorine as gaseous chlorides (i.e., ZnCl 2 and PbCl 2 ). The second stage almost entirely removes heavy metals through the gas phase by adding graphite as a reducing agent (10 wt % to the initial fly-ash mass). Both steps were realized at 1300 °C with a duration of 3 h. An important finding is that the proposed process combination achieves near-complete impurity removal, with reductions in chlorine, sulfur, zinc, and lead concentrations by at least 98% in the final product compared to the feed fly ash. Utilization routes for removed impurities are proposed to produce HCl, H 2 SO 4 and Zn, Pb fumes. The principal process product – purified ash – is compositionally similar to iron blast-furnace slag, which has been intensively utilized as an SCM. Thereby, a potential process route for EfW fly-ash utilization is proposed. • Municipal solid waste incineration fly ash can be valorized • Chlorides can be removed through the hydrometallurgical route • Sulfur and heavy metals are removed through pyrometallurgical route • Treated material similar to blast furnace slag