Abstract
A viable route for dealing with non-recyclable municipal solid waste is energy via incineration. This route, at the same time, reduces the mass of material to be landfilled and mitigates power generation emissions. The Waste-to-Energy (WtE) sector still faces a challenge in managing the fly ash generated during the process. The tightening of environmental regulations in Europe, especially the costs associated with landfilling, intensifies the search for viable valorisation strategies. One emerging alternative is using ash as an additive to cement clinker. Before use, chlorides, sulphates, and heavy metals such as Zn and Pb, which could otherwise compromise cement integrity or risk soil and water contamination, must be removed. The present study presents the results obtained by the fly-ash treatment, combining aqueous and high-temperature unit operations. Initially, chlorides are removed with an efficiency greater than 95% via water leaching (at 60°C, with a liquid-to-solid ratio of 5, and a 60-minute residence time). The now-low Cl-content fly ash can be directed to the high-temperature step.
To it, ca. 16 wt.% of sand (as SiO2 source) is added. The added silica reduces the fly ash melting point and assists in sulphur removal. While the material is heated to 1300 °C, the sulphates decompose, leaving the ash sulphur-free. Following that, a reducing agent, such as diesel used in a Top Submerged Lance (TSL) reactor, promotes the removal of heavy metals. Both processes were scaled up from experiments performed with lower amounts, i.e., below 1 kg. In the aqueous process step, 35 kg batches treated under the optimum parameters demonstrated high chloride removal efficiency, as in the lab scale. For the high temperature treatment, a Top Submerged Lance (TSL) reactor was utilised, accommodating batches up to 10 kg. These initial TSL trials, conducted under a reductive heating atmosphere, confirmed the complete removal of S, Pb, and Cl, with a final content of 0.04 wt% of Zn. The resulting treated slag is compositionally similar to blast furnace slag, a common additive in cement clinker. Future investigations are scheduled for early 2026, comprising a parametric study to evaluate the influence of various reduction regimes on compound removal. Experimental variations in air-to-fuel ratios, residence times, and the controlled addition of chlorination agents are intended to elucidate the mechanisms of impurity elimination and are expected to be submitted at the conference paper. Subsequently, a demonstration-scale TSL reactor, designed to process 150 kg/h of dechlorinated fly ash, is planned for implementation. This phase aims to recover and characterise Zn and Pb-bearing fumes and to assess the feasibility of generating H2SO4 from off-gas sulphur dioxide. The proposed treatment also offers opportunities for minor-element recovery, specifically Ti and Sb, to enhance the process flowsheet. Consequently, this work establishes WtE fly-ash as a viable secondary source for Pb, Zn, S, and Cl recovery, while simultaneously yielding a safe and valuable Supplementary Cementitious Material (SCM).