Characterization of leaching from waste incineration air-pollution-control residues

Waste incineration generates solid residues with high contents of salts and heavy metals that may leach to the surrounding environment upon disposal. Residues from flue gas cleaning, i.e. air-pollution-control (APC) residues, are highly alkaline and have potentials for leaching in very long periods of time. This leaching needs to be quantified and evaluated in a long-term perspective in order to manage the residues in a sustainable manner, however the time frames involved render this unfeasible using traditional leaching tests. This thesis provides a methodology for estimating long-term leaching from APC residues. A relation between pH of a residue leaching system and the liquid-to-solid (L/S) ratio can be produced based on data for leaching of alkalinity as a function of pH. By relating alkalinity leaching at specified pH-values to losses in residue alkalinity, it is possible by a simple mass balance approach to associate pH of a leaching system with the L/S ratio. This may further be combined with relationships between metal leaching and pH to provide estimated leaching as a function of L/S until depletion from the solid phase. Typically, APC residues have natural pH-values of 11-12.5 and may maintain alkaline pH-levels for L/S ratios above 2000 l/kg. For a typical landfill, this corresponds to about 100,000 years. This means that metal leaching is most important with respect to high pH. Elements like Ca, Cl, K, Na, Pb and S typically leach in very high concentrations at low L/S, i.e. in mg/l and g/l. Although leaching may continue for very long time, concentrations generally decrease significantly compared to the initial level. Within L/S 5000 l/kg, only Al, Mg and Zn are predicted to have higher leachate concentrations than what can be observed at L/S 2-10 l/kg. Uptake of atmospheric CO2 may somewhat increase the leaching of Ca and S. These two elements can be completely removed from the solid phase within L/S 300-800 l/kg, whereas other elements generally require leaching for more than L/S 2000 l/kg to remove about 20-30 % of the solid content. In a long-term perspective, leaching of most elements can be considered controlled by mineral solubility, however, specific elements such as Cr may possibly be affected by other mechanisms. In batch leaching experiments, Cr is highly affected by the redox conditions of the test and reaction kinetics. Cr(VI) reduction capacity provided by Al(0) in the residues may significantly lower Cr concentrations within a few hours of experiment, whereas Cr(VI) dissolution may require a day to complete. Al-O₂-Cr interactions appear to control the release of Cr in batch experiments.