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 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 SO₄^2- 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. SO₄^2- and Ca²⁺ removal was highly dependent on the dissolution and precipitation equilibriums. Na₂CO₃ washing promoted the formation of CaCO₃. Thus SO₄^2- was washed off instead of precipitating as CaSO₄ and retained in fly ash solid. SO₄^2- 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 SO₄^2- removal. Overall, washing by Na₂CO₃ appears to be a promising option for fly ash treatment.