Accelerated carbonation of incineration fly ash (IFA) and its impact on inhibiting heavy metals leaching and removal of soluble salts

This study analysed the composition and leaching results for 12 IFA samples from a local incineration plant by XRF and leaching tests based on BS EN12457-1:2002. XRF results show that the main elements are Ca, Cl and S in all IFA samples. Leaching test results shows a high leachability of Na, Cl, Br, Pb, Zn and Cu. The accelerated carbonation with (NH4)2CO3 was attempted to inhibit heavy metals leaching and remove soluble salts. The effect of (NH4)2CO3 concentration on carbonation efficiency and inhibition of heavy metal leaching was systematically investigated. TGA and ICP-MS were used to determine carbonation capacity and heavy metal leaching, respectively. TGA results show the maximum carbonation capacity is achieved in one hour with 10 wt% (NH4)2CO3 and 76% carbonation occurs -in the first 10 min. Leaching test results reveal that Cu and Pb are efficiently immobilised when the concentration of (NH4)2CO3 is as low as 2 wt% with a s/l ratio of 1:5 within 1 hr. Increasing the concentration of (NH4)2CO3 to 4 wt%, the leaching rate of Zn is also below NEA RVs (National Environment Agency of Singapore). Carbonation is also effective to immobilise Cd and Ni, but not for Cr since it exists as CrO42- and cannot be carbonated. XRF results from a larger scale reaction confirm the significant removal of soluble salts, and lower leachability (except SO42-) is substantiated by leaching tests. By integrating the processes of CO2 capture with ammonia and accelerated carbonation, our work contributes to CO2 sequestration and IFA detoxification.