Release of salts from municipal solid waste combustion residues

Residues from fluidized bed combustion of municipal solid waste were investigated with respect to their leaching behavior and possible extraction of salts. The total water extractable amounts of Na, K, Ca, Cl(-), Br(-), F(-) and SO(4)(2-) along with the total dissolved solids of bottom, hopper, cyclone and bag house filter ashes were determined. A simple multistage washing process (using water as the extraction medium) was tested in lab scale experiments. The effect of variations in parameters, such as water to ash weight ratio, contact time, temperature and number of extraction steps was investigated. The leaching behavior of untreated and washed cyclone and bag house filter ashes was evaluated by a two-step batch-leaching test, i.e. the CEN test. The ashes investigated in this study can be arranged according to their decreasing water extractable contents and total dissolved solids as follows: filter ash > cyclone ash > hopper ash > bottom ash. A triple extraction with water at liquid to solid ratio 2 and extraction time 5 min gave the best results for the extraction of Ca, Na, K, Cl(-) and SO(4)(2-) from the cyclone as well as from the filter ashes. The leached amounts of salts in the CEN test performed on the washed cyclone ash were considerably lower than the corresponding amounts released from the unwashed ash. Thus, the washed cyclone ash was made more stable with respect to salt leachability. On the other hand, large amounts of salts were leached from the washed filter ashes as well as from unwashed filter ashes. Therefore, it can be concluded that three stage water extraction is not a suitable stabilization method for this type of filter ashes.