Simulation of acid washing of municipal solid waste incineration fly ashes in order to remove heavy metals

Fly ash from municipal waste incinerators constitutes an environmental problem, as it is polluted with heavy metals. By extracting the heavy metals from fly ashes with an acid solution, they can be partly removed and possibly recovered. The remaining fly ash can be landfilled or used as construction material. In this paper, the results of experimental leaching tests of fly ash are compared with computer calculations of the thermodynamic equilibrium of the leaching solution - fly ash system. The computer program MINTEQA2, used for this purpose, allows to predict the metal concentrations in the leaching solution, the minerals that precipitate, and the pH of the leaching solution at equilibrium. A calculation approach was chosen, whereby the simulation of the complex leaching process was divided into five subproblems. A leaching diagram with linear scales was introduced to visualise the equilibria as a function of pH. The computer simulation was used in two applications: calculation of the impact of addition of a complexing agent on the leaching behaviour of lead, and optimisation of the volume of liquid to be used in the leaching process for a given amount of fly ash. (C) 1998 Elsevier Science B.V.