Release of soluble ions and heavy metal during fly ash washing by deionized water and sodium carbonate solution.

Two municipal solid waste incineration fly ashes were selected for washing by deionized water and NaCO solution for comparison. Results showed that the benefits of washing were two folds: (1) Washing was able to reduce the contents of Cl and SO while increased the contents of CaO, SiO, AlO etc.; (2) Washing by NaCO solution showed increased stability of heavy metals (Cr, Ni, Cd, Cu, Zn and Pb) and fly ash was safe for later reuse. Release of Cl was high at more than 90% regardless of washing solution. SO and Ca removal was highly dependent on the dissolution and precipitation equilibriums. NaCO washing promoted the formation of CaCO. Thus SO was washed off instead of precipitating as CaSO and retained in fly ash solid. SO removal was raised to more than 80% by NaCO washing as compared with about 30% by deionized water. At the same time, Ca removal by NaCO dropped to 1-2%. In addition, the basicity of fly ash was important as high basicity helped SO removal. Overall, washing by NaCO appears to be a promising option for fly ash treatment.