Abstract:The chemical thermodynamic equilibrium calculations were performed to reveal the migration and transformation of the heavy metals, such as Pb, Ni, Mn, Cr, Zn and Cu, during incineration of four sewage sludges. The actual sewage sludge compositions and combustion conditions were utilized in all simulations. The results show that the heavy metals Ni, Mn and Zn are affected greatly by the minerals, such as Al₂O₃, CaO, SiO₂, Fe₂O₃, and form some stable solids easily which exist in the residual slag at low temperature. Under the same condition, Cr is easy to form its oxides and does not affected by the minerals, but Pb is transformed to other volatile forms easily and affected by many other factors. With increasing temperature, the heavy metals Pb, Cu and Zn transfer from their solid salts, oxides to gas metal chlorides，oxides，and finally to gas elements, which enter into the fly ash or the atmosphere. The elements Mn, Ni and Cr are easy to form some stable, high melting point and non-volatile oxides existing in their residual slags. During the incineration, the element S reacts with Ni, Cu and Pb, and forms sulfates, while the transformation of Mn, Cr and Zn affected by sulfur is less. When there is Cl in the incineration system only, the effect of Cl on the volatilization of Pb is the greatest, followed by the volatilizations of Cu and Zn, Ni, Cr and Mn has less effect or no effect.

Key words: sewage; sludge; incineration; heavy metal; speciation; thermodynamic analysis