RESUMO
Two municipal solid waste incineration fly ashes were selected for washing by deionized water and Na2CO3 solution for comparison. Results showed that the benefits of washing were two folds: (1) Washing was able to reduce the contents of Cl- and SO42- while increased the contents of CaO, SiO2, Al2O3 etc.; (2) Washing by Na2CO3 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. SO42- and Ca2+ removal was highly dependent on the dissolution and precipitation equilibriums. Na2CO3 washing promoted the formation of CaCO3. Thus SO42- was washed off instead of precipitating as CaSO4 and retained in fly ash solid. SO42- removal was raised to more than 80% by Na2CO3 washing as compared with about 30% by deionized water. At the same time, Ca removal by Na2CO3 dropped to 1-2%. In addition, the basicity of fly ash was important as high basicity helped SO42- removal. Overall, washing by Na2CO3 appears to be a promising option for fly ash treatment.
