Artificial carbonation for controlling the mobility of critical elements in bottom ash

Abstract

In municipal solid waste incineration (MSWI), bottom ash, generated at a stoker grate type incinerator, the critical elements were identified in terms of EU regulation. The stabilizing effect of moderate carbonation (pH 8.28 ± 0.03) on critical contaminants was studied through availability and diffusion leaching protocols. Data from the performed tests were evaluated with the goal of reusing MSWI bottom ash as secondary construction material. To investigate the mobilizing effect of CO2, suspended MSWI bottom ash was severely carbonated (pH 6.40 ± 0.07). The effect of CO2 and its interaction with other leaching factors, such as liquid/solid (L/S) ratio, leaching time, pH, ultrasound treatment, and leaching temperature, were examined using a reduced 26-1 experimental design. Contaminants identified as critical were Cr, Cu, Mo, Sb, Cl−, and SO42−. Although moderate carbonation decreased the release of Cr, Cu, Mo, and Sb from compacted bottom ash, the main disadvantage remains its inability to demobilize Cl− and SO42−. The hypothesized mobilizing effect of severe carbonation was proven. The treatment enhanced the separation of critical components (α = 0.05) (except for Cl−), i.e., about fivefold for Sb and about twofold for Cr, Cu, and S. Nevertheless, the prospect is good that severe carbonation could constitute the deciding key parameter to facilitate the technical feasibility of a future washing process for MSWI bottom ash.