Thermodynamics equilibrium analysis of partitioning of cadmium during sludge incineration

Key words: 
• sludge /
• incineration /
• cadmium(Cd) /
• speciation /
• thermodynamic equilibrium

Abstract: The thermodynamic equilibrium calculations were performed to reveal the distribution and transformation of the element Cd during incineration of the typical sewage sludge．The actual sewage sludge compositions and the combustion conditions were utilized in all simulations. The effects of major minerals and the different contents of sulphur and chloride on the Cd speciation transformation were considered during the calculations. The results showed that during the incineration of sewage sludge Cd was mainly in the form of solid carbonates at low temperatures, which decomposed into CdO with the increasing of temperature. The composite gases of Cd(OH)2 , Cd and CdO came into being at higher temperatures,in which the gas of Cd was the main form. During incinerations of sewage sludge with major minerals, the impact of SiO2 on Cd transformation was stronger than the others,which shows that the stable solid CdSiO3 combined easily between Cd and SiO2, and the presence of SiO2 can restrain the emissions of gaseous Cd pollutants. The CdCl2 was formed easier by the presence of chloride during incineration，and the volatilization of Cd was advanced with the increasing of chloride.At low temperatures, the volatilization of Cd was restrained for the forming of the refractory solid metal sulfate.At high temperatures, the speciations of Cd were not affected with the presence of element S ，but S could affect the formation temperature of gaseous metals. The pollution control of heavy metal Cd could be realized during incineration of the typical sewage sludge，for different speciations of Cd were formed at different incineration temperatures，major minerals and the contents of sulphur and chloride .