Mixing and pelletizing sewage sludge ash (SSA) or municipal solid waste (MSW) fly ash with CaCl2 and treating these pellets in a rotary reactor at approximately 1000 degrees C lead to a significant decrease of the heavy metal concentration. Experiments were carried out in an indirectly-heated laboratory-scale rotary reactor. From SSA, after 10 min at 1050 degrees C, more than 95% of Cu, Pb and Zn could be removed. The heavy metal removal from municipal solid waste (MSW) fly ash proceeded more slowly and often less efficiently. After 45 min at 1050 degrees C, 95% Cd, 60% Cu, 98% Pb and 80% Zn could be removed. Thermodynamic equilibrium calculations for possible single reactions were carried out. It was found that SiO2, a main component in sewage sludge ash, accelerates the formation of HCl and Cl-2, CaO, which is contained in CaO-bearing phases in MSW fly ash. decreases the rate of this reaction. Al2O3, also contained in MSW fly ash-phases, leads to the formation of aluminates. Cd-aluminate can be affected by HCl and Cl-2, leading to a lower removal rate. Zn-aluminate can only be affected by Cl-2; Cu-aluminate is stable under chlorination conditions. I.e.. Cu and Zn can easily be incorporated into the MSW fly ash matrix. (C) 2011 Elsevier B.V. All rights reserved.