Resource recovery from MSW incineration residues through the 'Ash-by-Ash Treatment Method' (AATM): optimization of water and bottom ash contents

Abstract

Fine-fraction of Municipal Solid Waste (MSW) incineration bottom ash (IBA) contains amorphous silica, known as pozzolan is one of the potential heavy metal stabilizers in MSW incineration fly ash (IFA) by forming the cementitious compounds of calcium silicate hydrates (C - S - H) and calcium aluminate hydrates (C-A-H). The technique can be called the 'Ash-by-Ash Treatment Method' (AATM). To optimize the AATM, effects of water (L/S ratios) and IBA amounts (IFA/IBA ratios) were examined in this study. IFA and IBA were provided by two distinct incineration plants; IBA was utilized at fractions d < 250μm because of its high reactive amorphous pozzolan. The testing involved mixing ashes at different L/S (ml/g) and IFA/IBA ratios, leaving the air dry at ambient temperature, afterward settling for periods of 4, 8, 16, 30, and 60 days. The results confirmed that various L/S (ml/g) ratios have no discernible influence on the immobilization of heavy metals. At 60 days, even a lowered moisture content could maintain more than 99% of Pb immobilization efficiency and 100% of Cr and Cu immobilization efficiency. In another experiment, using up to 30% of IBA with L/S (ml/g) ratios of 0.6 also demonstrated >99% of Pb immobilization. No discernible change has been observed between the different L/S (ml/g) and IFA/IBA. Consequently, increasing IBA and decreasing water could assist in optimizing AATM. The immobilization process involves carbonation and pozzolanic reactions, which are substantiated by the leaching solution's pH decrease, the formation of calcite, the appearance of multiple new minerals, and the cementitious products C-S-H gels.

Keywords: MSW incineration; bottom ash; calcium-silicate-hydrate; fly ash; pozzolanic.